Search

p16 is a tumor suppressor encoded by the CDKN2A gene whose expression is lost in approximately 50% of all human cancers. In its canonical role, p16 inhibits the G1-S-phase cell cycle progression through suppression of cyclin-dependent kinases. Interestingly, p16 also has roles in metabolic reprogramming, and we previously published that loss of p16 promotes nucleotide synthesis via the pentose phosphate pathway. However, the broader impact of p16/CDKN2A loss on other nucleotide metabolic pathways and potential therapeutic targets remains unexplored. Using CRISPR knockout libraries in isogenic human and mouse melanoma cell lines, we determined several nucleotide metabolism genes essential for the survival of cells with loss of p16/CDKN2A. Consistently, many of these genes are upregulated in melanoma cells with p16 knockdown or endogenously low CDKN2A expression. We determined that cells with low p16/CDKN2A expression are sensitive to multiple inhibitors of de novo purine synthesis, including antifolates. Finally, tumors with p16 knockdown were more sensitive to the antifolate methotrexate in vivo than control tumors. Together, our data provide evidence to reevaluate the utility of these drugs in patients with p16/CDKN2Alow tumors as loss of p16/CDKN2A may provide a therapeutic window for these agents., Significance: Antimetabolites were the first chemotherapies, yet many have failed in the clinic due to toxicity and poor patient selection. Our data suggest that p16 loss provides a therapeutic window to kill cancer cells with widely-used antifolates with relatively little toxicity., (© 2024 The Authors; Published by the American Association for Cancer Research.)

Neutrophils act as a "double-edged sword" in the tumor microenvironment by either supporting or suppressing tumor progression. Thus, eliciting a neutrophil antitumor response remains challenging. Here, we showed that tumor cell-derived microparticles induced by methotrexate (MTX-MP) acts as an immunotherapeutic agent to activate neutrophils, increasing the tumor-killing effect of the cells and augmenting T-cell antitumor responses. We found that lactate induced tumor-associated neutrophils to elevate expression of programmed cell death protein 1 (PD-1) and that PD-1+ neutrophils had the properties of N2 neutrophils and suppressed T-cell activation through PD-1/programmed death-ligand 1 (PD-L1) signaling. By performing ex vivo experiments, we found that MTX-MPs-activated neutrophils had reduced surface expression of PD-1 as a result of PD-1 internalization and degradation in the lysosomes, leading to the cells showing a decreased capacity to suppress T-cell responses. In addition, we also found that MTX-MP-activated neutrophils released neutrophil elastase which could kill tumor cells and disrupt tumor stroma, leading to increased T-cell infiltration. Furthermore, using a combination of anti-PD-L1 and MTX-MPs, we observed that long-term survival increased in a mouse model of lung cancer. Collectively, these findings highlight the potential use of a combination of anti-PD-L1 and MTX-MPs to enhance the therapeutic effect of anti-PD-L1 alone., (©2023 American Association for Cancer Research.)

Previous studies have documented the roles of transport via the reduced folate carrier, retention via polyglutamylation, and increased levels of the target enzyme, dihydrofolate reductase in sensitivity to methotrexate. Recent studies have shown that the mitochondrial enzymes in the cellular metabolism of serine, folate, and glycine are overexpressed in a subset of human cancers and that their expression is required for tumor maintenance. In this Perspective article, we propose that the expression of mitochondrial enzymes in the metabolism of serine and glycine, in addition to those involved in folate metabolism, are determinants of the response to methotrexate. Furthermore, we show that myc activation in tumors is associated with upregulation of these enzymes. We propose that patients whose tumors show this phenotype will be sensitive to folate antagonists targeting thymidylate or purine biosynthesis.

DNA strand breakage and perturbation of cell-cycle progression contribute to gene amplification events that can drive cancer. In cells lacking p53, DNA damage does not trigger an effective cell-cycle arrest and in this setting promotes gene amplification. This is also increased in cells harboring oncogenic Ras, in which cell-cycle arrest is perturbed and ROS levels that cause DNA single strand breaks are elevated. This study focused on the effects of v-K-ras and p53 on Methotrexate (MTX)-mediated DHFR amplification. Rat lung epithelial cells expressing v-K-ras or murine lung cancer LKR cells harboring active K-ras continued cell-cycle progression when treated with MTX. However, upon loss of p53, amplification of DHFR and formation of MTX-resistant colonies occurred. Expression levels of cyclin A, Geminin, and Cdt1 were increased in v-K-ras transfectants. Geminin was sufficient to prevent the occurrence of multiple replications via interaction with Cdt1 after MTX treatment, and DHFR amplification proceeded in v-K-ras transfectants that possess a functional p53 in the absence of geminin. Taken together, our findings indicate that p53 not only regulates cell-cycle progression, but also functions through geminin to prevent DHFR amplification and protect genomic integrity.

Sildenafil is a potent and selective inhibitor of the type 5 cGMP (cyclic guanosine 3',5'-monophosphate)-specific phosphodiesterase that is used clinically to treat erectile dysfunction and pulmonary arterial hypertension. Here, we report that sildenafil has differential effects on cell surface ABC transporters such as ABCB1, ABCC1, and ABCG2 that modulate intracompartmental and intracellular concentrations of chemotherapeutic drugs. In ABCB1-overexpressing cells, nontoxic doses of sildenafil inhibited resistance and increased the effective intracellular concentration of ABCB1 substrate drugs such as paclitaxel. Similarly, in ABCG2-overexpressing cells, sildenafil inhibited resistance to ABCG2 substrate anticancer drugs, for example, increasing the effective intracellular concentration of mitoxantrone or the fluorescent compound BODIPY-prazosin. Sildenafil also moderately inhibited the transport of E(2)17βG and methotrexate by the ABCG2 transporter. Mechanistic investigations revealed that sildenafil stimulated ABCB1 ATPase activity and inhibited photolabeling of ABCB1 with [(125)I]-iodoarylazidoprazosin (IAAP), whereas it only slightly stimulated ABCG2 ATPase activity and inhibited photolabeling of ABCG2 with [(125)I]-IAAP. In contrast, sildenafil did not alter the sensitivity of parental, ABCB1-, or ABCG2-overexpressing cells to non-ABCB1 and non-ABCG2 substrate drugs, nor did sildenafil affect the function of another ABC drug transporter, ABCC1. Homology modeling predicted the binding conformation of sildenafil within the large cavity of the transmembrane region of ABCB1. Overall, we found that sildenafil inhibits the transporter function of ABCB1 and ABCG2, with a stronger effect on ABCB1. Our findings suggest a possible strategy to enhance the distribution and potentially the activity of anticancer drugs by jointly using a clinically approved drug with known side effects and drug-drug interactions., (©2011 AACR.)

Metastasis and drug resistance are major problems in cancer chemotherapy. The purpose of this work was to analyze the molecular mechanisms underlying the invasive potential of drug-resistant colon carcinoma cells. Cellular models included the parental HT-29 cell line and its drug-resistant derivatives selected after chronic treatment with either 5-fluorouracil, methotrexate, doxorubicin, or oxaliplatin. Drug-resistant invasive cells were compared with noninvasive cells using cDNA microarray, quantitative reverse transcription-PCR, flow cytometry, immunoblots, and ELISA. Functional and cellular signaling analyses were undertaken using pharmacologic inhibitors, function-blocking antibodies, and silencing by retrovirus-mediated RNA interference. 5-Fluorouracil- and methotrexate-resistant HT-29 cells expressing an invasive phenotype in collagen type I and a metastatic behavior in immunodeficient mice exhibited high expression of the chemokine receptor CXCR4. Macrophage migration-inhibitory factor (MIF) was identified as the critical autocrine CXCR4 ligand promoting invasion in drug-resistant colon carcinoma HT-29 cells. Silencing of CXCR4 and impairing the MIF-CXCR4 signaling pathways by ISO-1, pAb FL-115, AMD-3100, monoclonal antibody 12G5, and BIM-46187 abolished this aggressive phenotype. Induction of CXCR4 was associated with the upregulation of two genes encoding transcription factors previously shown to control CXCR4 expression (HIF-2alpha and ASCL2) and maintenance of intestinal stem cells (ASCL2). Enhanced CXCR4 expression was detected in liver metastases resected from patients with colon cancer treated by the standard FOLFOX regimen. Combination therapies targeting the CXCR4-MIF axis could potentially counteract the emergence of the invasive metastatic behavior in clonal derivatives of drug-resistant colon cancer cells., (Copyright 2010 AACR.)

MDM2 is a ubiquitin ligase that is best known for its essential function in the negative regulation of p53. In addition, MDM2 expression is associated with tumor progression in a number of common cancers, and in some cases, this has been shown to be independent of p53 status. MDM2 has been shown to promote the degradation of a number of other proteins involved in the regulation of normal cell growth and proliferation, including MDM4 and RB1. Here, we describe the identification of a novel substrate for the MDM2 ubiquitin ligase: dihydrofolate reductase (DHFR). MDM2 binds directly to DHFR and catalyses its monoubiquitination and not its polyubiquitination. In addition, MDM2 expression reduces DHFR activity in a p53-independent manner, but has no effect upon the steady-state level of expression of DHFR. We show that changes in MDM2 expression alter folate metabolism in cells as evidenced by MDM2-dependent alteration in the sensitivity of cells to the antifolate drug methotrexate. Furthermore, we show that the ability of MDM2 to inhibit DHFR activity depends upon an intact MDM2 RING finger. Our studies provide for the first time a link between MDM2, an oncogene with a critical ubiquitin ligase activity and a vital one-carbon donor pathway involved in epigenetic regulation, and DNA metabolism, which has wide ranging implications for both cell biology and tumor development.

It has been reported that gefitinib, an epidermal growth factor receptor (EGFR) tyrosine kinase inhibitor (TKI), has the ability to modulate the function of certain ATP-binding cassette (ABC) transporters and to reverse ABC subfamily B member 1 (ABCB1; P-glycoprotein)- and ABC subfamily G member 2 (ABCG2; breast cancer resistance protein/mitoxantrone resistance protein)-mediated multidrug resistance (MDR) in cancer cells. However, it is unknown whether other EGFR TKIs have effects similar to that of gefitinib. In the present study, we have investigated the interaction of another EGFR TKI, erlotinib, with selected ABC drug transporters. Our findings show that erlotinib significantly potentiated the sensitivity of established ABCB1 or ABCG2 substrates and increased the accumulation of paclitaxel or mitoxantrone in ABCB1- or ABCG2-overexpressing cells. Furthermore, erlotinib did not significantly alter the sensitivity of non-ABCB1 or non-ABCG2 substrates in all cells and was unable to reverse MRP1-mediated MDR and had no effect on the parental cells. However, erlotinib remarkably inhibited the transport of E(2)17 beta G and methotrexate by ABCG2. In addition, the results of ATPase assays show that erlotinib stimulated the ATPase activity of both ABCB1 and ABCG2. Interestingly, erlotinib slightly inhibited the photolabeling of ABCB1 with [(125)I]iodoarylazidoprazosin (IAAP) at high concentration, but it did not inhibit the photolabeling of ABCG2 with IAAP. Overall, we conclude that erlotinib reverses ABCB1- and ABCG2-mediated MDR in cancer cells through direct inhibition of the drug efflux function of ABCB1 and ABCG2. These findings may be useful for cancer combinational therapy with erlotinib in the clinic.

Folylpolyglutamate synthase (FPGS) catalyzes the polyglutamation of folic acid, methotrexate, and pemetrexed to produce highly active metabolites. To characterize genetic variation in the FPGS gene, FPGS, have resequenced the gene in four different ethnic populations. Thirty-four single nucleotide polymorphisms were identified including five nonsynonymous coding single nucleotide polymorphisms that altered the FPGS protein sequence: F13L and V22I polymorphisms in the mitochondrial isoform of FPGS, and R466/424C, A489/447V, and S499/457F polymorphisms, which exist in both the mitochondrial and cytosolic isoforms. When expressed in AuxB1 cells, the A447V cytosolic variant was functionally similar to the wild-type cytosolic (WT Cyt) allozyme, whereas the R424C and S457F cytosolic variants were reduced by approximately 2-fold in protein expression compared with WT Cyt (P < 0.01). The intrinsic clearance of glutamate was reduced by 12.3-fold (R424C, P < 0.01) and 6.2-fold (S457F, P < 0.01), whereas the intrinsic clearance of methotrexate was reduced by 4.2-fold (R424C, P < 0.05) and 5.4-fold (S457F, P < 0.05) in these two cytosolic variants when compared with the WT Cyt isoform. Additionally, the in vitro enzyme velocity at saturating pemetrexed concentrations was reduced by 1.6-fold (R424C, P < 0.05) and 2.6-fold (S457F, P < 0.01) compared with WT Cyt. AuxB1 cells harboring these same cytosolic variant allozymes displayed significant increases in the EC(50) for folic acid and in the IC(50) values for both methotrexate and pemetrexed relative to the WT Cyt form of FPGS. These observations suggest that genetic variations in FPGS may alter the efficacy of antifolate therapy in cancer patients.

Syndecans are transmembrane heparan sulfate proteoglycans controlling cell adhesion, migration, and proliferation. We previously showed that syndecan-2 is involved in the control of apoptosis in cultured osteosarcoma cells. These data led us to the hypothesis that syndecan-2 may play a role in the apoptotic signaling in bone tumors. We immunohistochemically analyzed tissue sections from biopsies from 21 patients with well-characterized osteosarcoma. These tissues expressed low levels of syndecan-2 compared with osteoblasts and osteocytes in normal bone. Cultured human osteosarcoma cells also produced lower mRNA levels of syndecan-2 than normal osteoblastic cells. Moreover, the presence of syndecan-2 correlated with spontaneous apoptosis in osteosarcoma tissues as assessed by detection of DNA fragmentation in situ. Overexpression of syndecan-2 resulted in decreased number of migrating and invading U2OS osteosarcoma cells in Matrigel. In addition, overexpression of syndecan-2 sensitized human osteosarcoma cells to chemotherapy-induced apoptosis, increasing the response to methotrexate, doxorubicin, and cisplatin. Consistently, knockdown of the proteoglycan using stable transfection with a plasmid coding small interfering RNA resulted in inhibition of chemotherapy-induced apoptosis. Analysis of syndecan-2 expression both in biopsies and in corresponding postchemotherapy-resected tumors, as well as in cells treated with methotrexate or doxorubicin, showed that the cytotoxic action of chemotherapy can be associated with an increase in syndecan-2. These results provide support for a tumor-suppressor function for syndecan-2 and suggest that dysregulation of apoptosis may be related to abnormal syndecan-2 expression or induction in osteosarcoma. Moreover, our data identify syndecan-2 as a new factor mediating the antioncogenic effect of chemotherapeutic drugs.

ABCG2 is an ATP-binding cassette transporter that confers resistance to various chemotherapeutic agents. Recent studies have established that an Arg (wild-type) to Gly mutation at amino acid 482 in ABCG2 alters substrate specificity. Here, we explored the role of this G482 mutation in antifolate resistance using a clinically relevant 4-hour drug exposure. Stable transfectants overexpressing the mutant G482 transporter displayed 120-, 1,000-, and >6,250-fold resistance to the antifolates methotrexate, GW1843, and Tomudex, respectively, relative to parental human embryonic kidney cells. Moreover, although overexpressing equal transporter levels at the plasma membrane, G482-ABCG2 cells were 6-, 23-, and >521-fold more resistant to methotrexate, GW1843, and Tomudex, respectively, than R482-ABCG2 cells. In contrast, upon a continuous (72-hour) drug exposure, both the G482- and R482-ABCG2 cells lost almost all their antifolate resistance; this result was consistent with the inability of ABCG2 to extrude long-chain antifolate polyglutamates. Ko143, a specific and potent ABCG2 inhibitor reversed methotrexate resistance in both G482- and R482-ABCG2 cells. Consistently, whereas the pool of free methotrexate in parental human embryonic kidney cells was prominent after 4 hours of transport with 1 micromol/L [3H]methotrexate, in R482- and G482-ABCG2 cells, it was minimal. Furthermore, G482-ABCG2 cells contained marked decreases in the di- and triglutamate species of [3H]methotrexate at 4 hours of incubation with methotrexate and in the tetra- and pentaglutamates at 24 hours. These changes were not associated with any significant decrease in folylypoly-gamma-glutamate synthetase activity. These results provide the first evidence that the G482-ABCG2 mutation confers high-level resistance to various hydrophilic antifolates.

We propose that the lethality of commonly used anticancer drugs, e.g., methotrexate and cis-platinum are due, at least in part, to an increase of the E2F-1-mediated apoptotic cascade. The drugs directly or indirectly decrease deoxynucleoside triphosphates. The E2F family acts to provide control of S phase by transcribing genes required for deoxynucleoside triphosphate and DNA synthesis. Thus, a mechanism for control of E2F-1 is essential, a signal safeguarding against aberrant or uncontrolled cell proliferation. We have proposed a feedback control by NTPs that down-regulates E2F-1. Here, we provide evidence in support of this hypothesis.

Patients with Fanconi anemia (FA) are prone to develop malignancies at an early age. Besides hematologic malignancies, squamous cell carcinomas in the anogenital region and head and neck are also frequently found in these patients. The aim of this study was to generate a panel of head and neck squamous cell carcinoma (HNSCC) cell lines and xenografts of FA HNSCC, and to characterize these cell lines in comparison with a panel of seven cell lines from patients with sporadic HNSCC. Analyses have been done on sensitivity to DNA cross-linking agents, loss of heterozygosity profile, TP53 mutations, TP53 polymorphisms and the presence of human papillomavirus. Four FA HNSCC cell lines were established. Sensitivity to DNA cross-linking agents (cisplatin) in the FA HNSCC cell lines was on average 10 times higher as compared with the sporadic HNSCC cell lines. Human papillomavirus was not detected in any of the FA or sporadic cell lines. No differences were found in loss of heterozygosity pattern, TP53 mutation frequency and TP53 polymorphism between FA and sporadic HNSCC cell lines. This is the first report on the generation of squamous cell lines of FA patients. The FA HNSCC cell lines we have generated may be utilized for future studies and might aid in the development of new preventive therapies for FA patients. The genetic characteristics of these cell lines suggest that FA HNSCC are not very different from sporadic HNSCC, except for the sensitivity to cisplatin which is consistent with the known cellular FA phenotype.

The antifolate drug methotrexate (MTX) is transported by breast cancer resistance protein (BCRP; ABCG2) and multidrug resistance-associated protein1-4 (MRP1-4; ABCC1-4). In cancer patients, coadministration of benzimidazoles and MTX can result in profound MTX-induced toxicity coinciding with an increase in the serum concentrations of MTX and its main metabolite 7-hydroxymethotrexate. We hypothesized that benzimidazoles interfere with the clearance of MTX and/or 7-hydroxymethotrexate by inhibition of the ATP-binding cassette drug transporters BCRP and/or MRP2, two transporters known to transport MTX and located in apical membranes of epithelia involved in drug disposition. First, we investigated the mechanism of interaction between benzimidazoles (pantoprazole and omeprazole) and MTX in vitro in membrane vesicles from Sf9 cells infected with a baculovirus containing human BCRP or human MRP2 cDNA. In Sf9-BCRP vesicles, pantoprazole and omeprazole inhibited MTX transport (IC50 13 microm and 36 microm, respectively). In Sf9-MRP2 vesicles, pantoprazole did not inhibit MTX transport and at high concentrations (1 mm), it even stimulated MTX transport 1.6-fold. Secondly, we studied the transport of pantoprazole in MDCKII monolayers transfected with mouse Bcrp1 or human MRP2. Pantoprazole was actively transported by Bcrp1 but not by MRP2. Finally, the mechanism of the interaction was studied in vivo using Bcrp1-/- mice and wild-type mice. Both in wild-type mice pretreated with pantoprazole to inhibit Bcrp1 and in Bcrp1-/- mice that lack Bcrp1, the clearance of i.v. MTX was decreased significantly 1.8- to 1.9-fold compared with the clearance of i.v. MTX in wild-type mice. The conclusion is as follows: benzimidazoles differentially affect transport of MTX mediated by BCRP and MRP2. Competition for BCRP may explain the clinical interaction between MTX and benzimidazoles.

The p14(ARF) protein, the product of an alternate reading frame of the INK4A/ARF locus on human chromosome 9p21, disrupts the ability of MDM2 to target p53 for proteosomal degradation and causes an increase in steady-state p53 levels, leading to a G(1) and G(2) arrest of cells in the cell cycle. Although much is known about the function of p14(ARF) in the p53 pathway, not as much is known about its function in human tumor growth and chemosensitivity independently of up-regulation of p53 protein levels. To learn more about its effect on cellular proliferation and chemoresistance independent of p53 up-regulation, human HT-1080 fibrosarcoma cells null for p14(ARF) and harboring a defective p53 pathway were stably transfected with p14(ARF) cDNA under the tight control of a doxycycline-inducible promoter. Induction of p14(ARF) caused a decrease in cell proliferation rate and colony formation and a marked decrease in the level of dihydrofolate reductase (DHFR) protein. The effect of p14(ARF) on DHFR protein levels was specific, because thymidylate kinase and thymidylate synthase protein levels were not decreased nor were p53 or p21WAF1 protein levels increased. The decrease in DHFR protein was abolished when the cells were treated with the proteasome inhibitor MG132, demonstrating that p14(ARF) augments proteasomal degradation of the protein. Surprisingly, induction of p14(ARF) increased resistance to the folate antagonists methotrexate, trimetrexate, and raltitrexed. Depletion of thymidine in the medium reversed this resistance, indicating that p14(ARF) induction increases the reliance of these cells on thymidine salvage.

Induced chemoresistance leads to the reduction of apoptotic responses. Although several drugs are in development that circumvent or decrease existing chemoresistance, none has the potential to prevent or reduce its induction. Here, we present data from a drug that could perhaps fill this gap. Cotreatment of chemotherapy with (E)-5-(2-bromovinyl)-2'-deoxyuridine (BVDU, RP101) prevented the decrease of apoptotic effects during the course of chemotherapy and reduced nonspecific toxicity. Amplification of chemoresistance genes (Mdr1 and Dhfr) and overexpression of gene products involved in proliferation (DDX1) or DNA repair (UBE2N and APEX) were inhibited, whereas activity of NAD(P)H: quinone oxidoreductase 1 (NQO1) was enhanced. During recovery, when treatment was with BVDU only, microfilamental proteins were up-regulated, and proteins involved in ATP generation or cell survival (STAT3 and JUN-D) were down-regulated. That way, in three different rat tumor models, the antitumor efficiency of chemotherapy was optimized, and toxic side effects were reduced. Because of these beneficial properties of BVDU, a clinical pilot Phase I/II study with five human tumor entities has been started at the University of Dresden (Dresden, Germany). So far, no unwanted side effects have been observed.

Previously, we have reported that a multidrug-resistant, mitoxantrone (MX)-selected cell line, MCF7/MX, is highly cross-resistant to the antifolate methotrexate (MTX), because of enhanced ATP-dependent drug efflux (E. L. Volk et al., Cancer Res., 60: 3514-3521, 2000). These cells overexpress the breast cancer resistance protein (BCRP), and resistance to MTX as well as to MX was reversible by the BCRP inhibitor, GF120918. These data indicated that BCRP causes the multidrug-resistance phenotype. To further examine the role of this transporter in MTX resistance, and in particular the role of amino acid 482, we analyzed a number of BCRP-overexpressing cell lines. MTX resistance correlated with BCRP expression in all of the cell lines expressing the wild-type transporter, which contains an Arg at position 482. In contrast, little or no cross-resistance was found in the MCF7/AdVp1000 and S1-M1-3.2 and S1-M1-80 cell lines, which contain acquired mutations at this position, R482T and R482G, respectively. Concomitantly, the greatest reduction in MTX accumulation was observed in the MCF7/MX cells (BCRP(Arg)) as compared with cells expressing the Thr and Gly BCRP variants. Furthermore, the reduction in drug accumulation was sensitive to BCRP inhibition by GF120918. In conclusion, we have demonstrated a novel role for BCRP as a mediator of MTX resistance and have provided further evidence for the importance of amino acid 482 in substrate specificity.

Magnetic resonance spectroscopy (MRS) methods have provided valuable information on cancer cell metabolism. In this study, we characterized the 31P-MR spectra of breast cancer cell lines exhibiting differences in hormonal response, estrogen receptors (positive/negative), and metastatic potential. A correlation was made between the cytotoxic effect of antimitotic drugs and changes in cell metabolism pattern. Because most anticancer drugs are more effective on proliferating cells, our study attempted to elucidate the metabolic profile and specific metabolic changes associated with the effect of anticancer drugs on proliferating breast cancer cell lines. Accordingly, for the 31P-MRS experiments, cells were embedded in Matrigel to preserve their proliferation profile and ability to absorb drugs. The MRS studies of untreated cells indicated that the levels of phosphodiesters and uridine diphosphosugar metabolites were significantly higher in estrogen receptor-positive and low metastatic potential cell lines. 31P-MRS observations revealed a correlation between the mode of action of anticancer drugs and the observed changes in cell metabolic profiles. When cells were treated with antimicrotubule drugs (paclitaxel, vincristine, colchicine, nocodazole), but not with methotrexate and doxorubicin, a profound elevation of intracellular glycerophosphorylcholine (GPC) was recorded that was not associated with changes in phospholipid composition of cell membrane. Remarkably, the rate of elevation of intracellular GPC was much faster in cell population synchronized at G2-M compared with the unsynchronized cells. The steady-state level of GPC for paclitaxel-treated cells was reached after approximately 4 h for synchronized cells and after approximately 24 h (approximate duration of one cell cycle) for the unsynchronized ones. These observations may indicate a correlation between microtubule status and cellular phospholipid metabolism. This study demonstrates that 31P-MRS may have diagnostic value for treatment decisions of breast cancer and reveals new aspects of the mechanism of action of antimicrotubule drugs.

Intracellular metabolism of methotrexate (MTX) to MTX-polyglutamates (MTXPG) is one determinant of cytotoxicity. Steady-state accumulation of MTXPG seems to depend on the activity of two enzymes: folylpolyglutamate synthetase (FPGS), which adds glutamate residues, and gamma-glutamyl hydrolase (GGH), which removes them. Overexpression of GGH would be expected to decrease intracellular MTXPG, thereby increasing efflux of MTX and decreasing cytotoxicity. Increased expression of GGH has been shown to be associated with resistance to MTX in human sarcoma cell lines and a rat hepatoma cell line. To clarify the specific role of GGH in determining MTX sensitivity, we investigated the phenotype produced by forced GGH overexpression in two cell types. Furthermore, because MTX and folic acid share metabolic pathways, we measured the effects of GGH overexpression on folic acid metabolism. The full-length cDNA for GGH, subcloned into a constitutive expression vector, was transfected into a human fibrosarcoma (HT-1080) and a human breast carcinoma (MCF-7) cell line. Compared with the clones containing an empty vector, the GGH-overexpressing cells express 15- to 30-fold more GGH mRNA, more GGH protein, and 15- to 90-fold more GGH enzyme activity. GGH overexpression altered MTX accumulation and metabolism to long-chain polyglutamates. In contrast to expectations, however, GGH overexpression did not confer resistance to short MTX exposures in either cell line. Changes in MTX metabolism were found to be balanced by alterations in accumulation and metabolism of folic acid. The ratio of MTX:folate accumulation may be a better predictor of MTX cytotoxicity than the accumulation of either alone. We conclude that, at least for these two cell lines, GGH overexpression alone is insufficient to produce clinical resistance to MTX.

Caveolin-1 expression and function were investigated in human colon cancer. Low levels of caveolin-1 mRNA and protein were detected in several colon carcinoma cell lines. Moreover, caveolin-1 protein levels were significantly reduced in human tumor epithelial mucosa (3.6 +/- 1.4-fold) when compared with normal colon mucosa for a majority (10 of 15) of the patients characterized. To directly assess the role of caveolin-1 in tumor development, caveolin-1 was reexpressed in the HT29 and DLD1 colon carcinoma cells, and the resulting HT29-cav-1 or DLD1-cav-1 cells were tested for tumorigenicity in nude mice. In most experiments, tumor formation was either blocked or retarded for HT29-cav-1 cells (10 of 13 mice) and DLD1-cav-1 cells (5 of 7 mice), as compared with both mock-transfected and parental HT29 or DLD1 cells. Interestingly, basal caveolin-1 levels were significantly reduced in HT29-cav-1 and DLD1-cav-1 cells isolated from tumors. Likewise, endogenous caveolin-1 mRNA and protein levels were found to be reduced in NIH-3T3 cells recovered from tumors after injection into nude mice. Thus, reexpression of caveolin-1 in colon carcinoma lines reduced the probability of tumor formation in vivo, and when tumors did develop from either HT29-cav-1, DLD1-cav-1, or NIH-3T3 cells, lower basal levels of caveolin-1 were detected. Finally, evidence was obtained indicating that initial caveolin-1 down-regulation in colon cancer cells need not be an entirely irreversible process because cell survival on selection for either drug resistance or increased metastatic potential correlated with increased caveolin-1 expression levels.

Physiological cell conditions, such as glucose deprivation and hypoxia, play a role in developing drug resistance in solid tumors. These tumor-specific conditions cause decreased expression of DNA topoisomerase IIalpha (topo IIalpha), rendering cells resistant to topo II-targeted drugs, such as etoposide and doxorubicin. We show here that inhibition of proteasome attenuated drug resistance by inhibiting topo IIalpha depletion induced by glucose starvation and hypoxia. topo IIalpha restoration was seen only at the protein levels, indicating that the topo IIalpha protein depletion occurred through a proteasome-mediated degradation mechanism. The stress-induced etoposide resistance was effectively prevented in vitro by the proteasome inhibitor lactacystin in both intrinsically resistant and sensitive tumor cells (colon cancer HT-29 and ovarian cancer A2780 cells, respectively). Furthermore, lactacystin effectively enhanced the antitumor activity of etoposide in the refractory HT-29 xenograft. These results indicate that lactacystin could serve as a new therapeutic agent to circumvent resistance to topo II-targeted chemotherapy in solid tumors.

The anthracyclines daunorubicin and doxorubicin were shown to induce apoptosis of hematopoietic cell lines. Here we report that they induce apoptosis of both nonactivated and phytohemagglutinin-activated human peripheral blood lymphocytes. Apoptosis demonstrated by surface expression of phosphatidylserine and typical nuclear alterations reached a maximum after 48 h of incubation with these agents. In contrast to topoisomerase inhibitors (etoposide and camptothecin) and antimetabolites (methotrexate and 5-fluorouracil) that induced apoptosis of activated cells only, daunorubicin and doxorubicin triggered apoptosis of cells in the G0-G1 phases of the cell cycle. In agreement with in vitro data, a single i.p. injection of daunorubicin or doxorubicin in BALB/c mice induced T- and B-cell depletion in spleen, lymph nodes, and to a lesser extent in the thymus. Soluble Fas-Fc, CD95 antagonistic antibodies, as well as the p55 tumor necrosis factor receptor-immunoglobulin fusion protein, did not inhibit drug-induced apoptosis. The level of reactive oxygen species was significantly increased in the presence of daunorubicin or doxorubicin only in nonactivated lymphocytes. However, antioxidants such as N-acetyl-L-cysteine or glutathione did not prevent apoptosis. Activation of caspase-3 after daunorubicin or doxorubicin treatment of either nonactivated or activated lymphocytes was demonstrated by the cleavage of poly(ADP-ribose) polymerase, which was, as apoptosis, inhibited by the peptide benzyloxycarbonyl-Val-Ala-Asp-fluoromethylketone. Finally, daunorubicin and doxorubicin induced a rapid production of ceramides. These data indicate that anthracyclines may induce major peripheral T-cell deletion, a property not shared by many cytotoxic agents.

Methotrexate (MTX) is one of the most commonly used agents in the treatment of solid malignancies; however, the toxicities of MTX to bone marrow and gastrointestinal tract complicate this therapy. We, therefore, propose a gene-dependent enzyme prodrug therapy to limit these toxicities by localizing the production of MTX to the site of the tumor. The combination of MTX-alpha-peptide prodrugs, which cannot be internalized by the cellular reduced folate carrier, with carboxypeptidase A (CPA), which can remove the blocking peptide, has been demonstrated previously in vitro using antibody-dependent enzyme prodrug therapy. CPA is normally synthesized as a zymogen that is inactive without proteolytic removal of its propeptide by trypsin. Therefore, to adapt this system to gene-dependent enzyme prodrug therapy, a mutant form of CPA was engineered, CPA(ST3), that does not require trypsin-dependent zymogen cleavage but is instead activated by ubiquitously expressed intracellular propeptidases. Purification, peptide sequencing, and kinetic analysis indicated that mature CPA(ST3) is structurally and functionally similar to the trypsin-activated, wild-type enzyme. In addition, CPA(ST3)-expressing tumors cells were sensitized to MTX prodrugs in a dose- and time-dependent manner. To limit diffusion of CPA, a cell surface localized form was generated by constructing a fusion protein between CPA(ST3) and the phosphatidylinositol linkage domain from decay accelerating factor. SDS-PAGE and flow cytometric analysis of infected tumor cells indicated that CPA(DAF) was cell surface localized. Finally, after retroviral transduction, this enzyme/prodrug strategy exhibited a potent bystander effect, even when <10% of the cells were transduced, because extracellular production of MTX sensitized both transduced and nontransduced cells.

Transfection of multidrug resistance proteins (MRPs) MRP1 and MRP2 in human ovarian carcinoma 2008 cells conferred a marked level of resistance to short-term (1-4 h) exposure to the polyglutamatable antifolates methotrexate (MTX; 21-74-fold), ZD1694 (4-138-fold), and GW1843 (101-156-fold). Evidence for MRP-mediated antifolate efflux relies upon the following findings: (a) a 2-3.3-fold lower accumulation of [3H]MTX and subsequent reduced formation of long-chain polyglutamate forms of MTX; (b) reversal of MTX resistance by probenecid in both transfectants, and (c) ATP-dependent uptake of [3H]MTX in inside-out vesicles of MRP1 and MRP2 transfectants. This report provides a mechanistic basis for resistance to polyglutamatable antifolates through an MRP-mediated drug extrusion.

Cervical carcinoma is an AIDS-defining illness. The expression of folate receptors (FRs) in cervical carcinoma (HeLa-IU1) cells was modulated by stable transduction of FR cDNA encapsidated in recombinant adeno-associated virus-2 in the sense and antisense orientation (sense and antisense cells, respectively). Although sense cells proliferated slower than antisense or untransduced cells in vivo and in vitro in 2% (but not 10%) FCS, [methyl-3H]thymidine incorporation into DNA was significantly increased in sense cells in 10% serum; therefore, the basis for this discrepancy was investigated. The activity of thymidine kinase (TK) was subsequently directly correlated with the extent of FR expression in single cell-derived clones of transduced cells. This elevated TK activity was not a result of recruitment of the salvage pathway based on the presence of adequate dTTP pools, normal thymidylate synthase (TS) activity, persistence of increased thymidine incorporation despite the exogenous provision of excess 5,10-methylene-tetrahydrofolate, and documentation of adequate folates in sense cells. The increase in TK activity conferred significant biological properties to sense cells (but not antisense or untransduced cells) as demonstrated by augmented phosphorylation of 3'-azido-3'-deoxythymidine (AZT) and concomitantly greater sensitivity to the cytotoxic effects of AZT. Conversely, sense cells were highly resistant to methotrexate, but this was reversed by the addition of AZT. The direct correlation of FR expression and TK activity indicates a previously unrecognized consequence of FR overexpression.

The combined action among polyglutamylatable and nonpolyglutamylatable antifolates, directed against dihydrofolate reductase (DHFR), glycinamide ribonucleotide formyltransferase (GARFT), 5-aminoimidazole-4-carboxamide ribonucleotide formyltransferase (AICARFT), and thymidylate synthase (TS), in human ileocecal HCT-8 cells was examined in a 96-well plate growth inhibition assay (96-h continuous drug exposure). An interaction parameter, alpha, was estimated for each of 95 experiments by fitting a seven-parameter model to data with weighted nonlinear regression. In a representative experiment, raising the folic acid concentration in the medium dramatically increased the Loewe synergy for the combination of trimetrexate (TMTX) and the GARFT inhibitor AG2034 (from a mean alpha +/- SE of 1.50 +/- 0.25 at 2.3 microM folic acid to 146 +/- 20 at 78 microM folic acid). Enhancements were also found for combinations of TMTX with the GARFT inhibitors AG2032, Lometrexol, and LY309887, the AICARFT inhibitor AG2009, and the TS inhibitors LY231514 and Tomudex but not with the GARFT inhibitor LL95509 or with the TS inhibitors AG337, ZD9331, and BW1843U89. Replacing TMTX with methotrexate in two-drug mixtures decreased the intensity of Loewe synergy. Examination of isobolograms at different effect levels revealed informative reproducible changes in isobol patterns. No two-drug combinations among inhibitors of GARFT, AICARFT, and TS exhibited Loewe synergy at either 2.3 or 78 microM folic acid. Thus, the ideal requirement for the folic acid-enhanced synergy is that a nonpolyglutamylatable DHFR inhibitor be combined with a polyglutamylatable inhibitor of another folate-requiring enzyme. A hypothesis to explain this general phenomenon involves the critical role of folylpoly-gamma-glutamate synthetase and the effect of the DHFR inhibitor in decreasing the protection by folic acid of cells to the other antifolates.

Ribonucleotide reductase is a highly regulated, cell cycle-controlled activity that plays an important role in DNA synthesis and repair. Recent studies have shown that elevated expression of the rate-limiting R2 component of ribonucleotide reductase increases Raf-1 protein activation and mitogen-activated protein kinase activity and acts as a novel malignancy determinant in cooperation with activated oncogenes like H-ras. We show that hydroxyurea-resistant mouse L cells with elevated R2 gene expression and increased ribonucleotide reductase activity exhibit significantly decreased sensitivities to the chemotherapeutic compounds N-(phosphonacetyl)-L-aspartate (PALA) and methotrexate (MTX). Furthermore, BALB/c 3T3 cells containing a retroviral expression vector encoding the R2 sequence also showed decreased sensitivity to PALA and MTX when compared to cells containing the same vector but without the R2 coding region. Colonies that developed in the presence of PALA or MTX contained amplifications of the CAD or dihydrofolate reductase genes and exhibited wild-type p53 function as determined in sequence-specific p53 binding activity assays. NIH-3T3 cells containing the R2 retroviral expression vector also showed significantly decreased sensitivity to hydroxyurea and MTX but not to PALA. Furthermore, NIH-3T3 cells transfected with a vector containing the R2 sequence in antisense orientation exhibited increased sensitivity to hydroxyurea, PALA, and MTX. Similarly, mouse 10T1/2 cells that are highly transformed and drug resistant due to alterations in H-ras and a mutant oncogenic form of p53 exhibited significant increases in sensitivity to hydroxyurea, PALA, and MTX when transfected with a vector containing the R2 sequence in antisense orientation and compared to cells containing the same vector without the antisense sequence. These results indicate that altered expression of the R2 component is capable of significantly modifying drug sensitivity properties of tumor cells. We hypothesize that this occurs, at least in part, through a mechanism of increased genetic instability that is independent of direct p53 mutation or loss and involves R2 stimulation of the mitogen-activated protein kinase signal pathway.

ID50 and ID90 values for L-prolyl-L-m-[bis(chloroethyl)amino]-phenylalanyl-L-norvaline ethyl ester HCl (MF13), were determined in four murine (leukemia, lymphoma, melanoma, and lung) and eight human cancer cell lines (two leukemia, prostate, kidney, colon, two melanoma, and breast). Cytotoxic activity was 2-5 times higher than that of sarcolysin [(L-3-[bis(2-chloroethyl)amino]-L-phenylalanine] against all leukemias and lymphomas, ID50 0.5-0.9 microM, and against human solid tumors, ID50 0.4-2.1 microM. Sensitivities of L-phenylalanine mustard-resistant and methotrexate-resistant L1210 cells were the same as the naive lines, ID50 0.5 microM. Apoptosis was confirmed by: (a) morphology, revealing chromatin condensation and nuclear fragmentation; (b) flow cytometry, showing changes in cell size and DNA integrity; and (c) DNA electrophoresis, demonstrating multiples of 180-200-bp DNA units. MF13 had no cytotoxicity against human peripheral blood lymphocytes at concentrations lethal to tumor cells (ID50, 13.3 microM without and 11 microM with phytohemagglutinin stimulation) and failed to induce apoptosis. s.c. MF13 treatment of mice with advanced EL4 leukemic ascites yielded extensive apoptosis, with DNA degradation identical to that seen in vitro, and resulted in complete tumor regression in all treated mice. These results suggest MF13 as a potential chemotherapeutic agent.

In this study, we tested the hypothesis that insulin-like growth factor-1 (IGF-1) modulates apoptosis in human breast cancer cells, HBL100, induced by diverse chemotherapeutic drugs. IGF-1 increased cell survival of HBL100 cells treated with 5-fluorouracil (antimetabolite), methotrexate (antimetabolite), tamoxifen (antiestrogen/antiproliferative), or camptothecin (topoisomerase 1 inhibitor) and after serum withdrawal. Elevated cell survival was not due to an increase in cell proliferation by IGF-1, but rather to an inhibition of apoptosis. Evidence for death by apoptosis was supported by cellular morphology and DNA fragmentation. There were no changes observed in Bcl-2 protein or bax mRNA levels. Extracellular matrix (ECM) is known to influence the apoptotic response of cells; therefore, the antiapoptotic effect of IGF-1 on breast cancer cells was examined using different ECMs: laminin, collagen IV, or Matrigel. IGF-1 protected cells from apoptosis induced by methotrexate on all ECMs tested, providing the first evidence that IGF-1 protects against apoptosis in three-dimensional culture systems. These data provide the rationale to search for drugs that lower serum IGF-1 in an effort to improve the efficacy of chemotherapeutic drugs for the treatment of breast cancer.

The effect of overexpression of p21waf1 on drug sensitivity was studied in an osteosarcoma cell line (SaOs-2) lacking both p53 and functional retinoblastoma protein using a tetracycline (TC)-inducible expression system. p21waf1 expression was barely detectable in SaOS-2 cells incubated in the presence of TC. After TC withdrawal, high levels of p21waf1 were induced in these cells. These p21waf1-induced cells showed increased sensitivity to doxorubicin, tomudex, and methotrexate as compared to uninduced cells; this condition is associated with increased apoptosis. Expression of p21waf1 reduced cyclin A-associated kinase activity and, surprisingly, resulted in inhibition of phosphorylation of E2F-1 and increased E2F-1 binding activity. An S-G2 cell cycle arrest/delay and an increase in expression of E2F-responsive genes (dihydrofolate reductase and thymidylate synthase) was correspondingly observed. Overexpression of p21waf1 in cells lacking functional retinoblastoma protein may mediate sensitivity to anticancer drugs by inhibiting E2F-1 phosphorylation, which may contribute to increased S-G2 cell cycle delay and increased cell susceptibility to apoptosis.

We report the establishment of stable, high-level green fluorescent protein (GFP)-expressing cell lines in vitro that permit the detection and visualization of distant micrometastases when they are implanted orthotopically in nude mice. Chinese hamster ovary cells were transfected with the dicistronic expression vector containing the humanized GFP cDNA. A stable GFP-expressing clone was selected in 1.5 microM methotrexate in vitro and injected s.c. in nude mice. Stable high-level expression of GFP was maintained in the s.c. growing tumors. To use GFP expression for metastasis studies, fragments of s.c. growing tumor, which are comprised of GFP-expressing cells, were implanted by surgical orthotopic implantation in the ovary of nude mice. Subsequent micrometastases were detected in systemic organs and could be visualized by GFP fluorescence in the lung, liver, and other organs down to the single-cell level. With this fluorescent tool, we detected and visualized for the first time tumor cells at the microscopic level in fresh viable tissue in their normal host organ. Confocal microscopy further enabled us to study physiologically relevant patterns of invasion and micrometastasis.

N-[4-[2-(2-amino-3,4-dihydro-4-oxo-7H-pyrrolo[2,3-d]pyrimidin-5-yl)ethyl ]-benzoyl]-L-glutamic acid (LY231514) is a novel pyrrolo[2,3-d]pyrimidine-based antifolate currently undergoing extensive Phase II clinical trials. Previous studies have established that LY231514 and its synthetic gamma-polyglutamates (glu3 and glu5) exert potent inhibition against thymidylate synthase (TS). We now report that LY231514 and its polyglutamates also markedly inhibit other key folate-requiring enzymes, including dihydrofolate reductase (DHFR) and glycinamide ribonucleotide formyltransferase (GARFT). For example, the Ki values of the pentaglutamate of LY231514 are 1.3, 7.2, and 65 nM for inhibition against TS, DHFR, and GARFT, respectively. In contrast, although a similar high level of inhibitory potency was observed for the parent monoglutamate against DHFR (7.0 nM), the inhibition constants (Ki) for the parent monoglutamate are significantly weaker for TS (109 nM) and GARFT (9,300 nM). The effects of LY231514 and its polyglutamates on aminoimidazole carboxamide ribonucleotide formyltransferase, 5,10-methylenetetrahydrofolate dehydrogenase, and 10-formyltetrahydrofolate synthetase were also evaluated. The end product reversal studies conducted in human cell lines further support the concept that multiple enzyme-inhibitory mechanisms are involved in cytotoxicity. The reversal pattern of LY231514 suggests that although TS may be a major site of action for LY231514 at concentrations near the IC50, higher concentrations can lead to inhibition of DHFR and/or other enzymes along the purine de novo pathway. Studies with mutant cell lines demonstrated that LY231514 requires polyglutamation and transport via the reduced folate carrier for cytotoxic potency. Therefore, our data suggest that LY231514 is a novel classical antifolate, the antitumor activity of which may result from simultaneous and multiple inhibition of several key folate-requiring enzymes via its polyglutamated metabolites.

Methylthioadenosine phosphorylase (MTAP), an enzyme involved in purine and methionine metabolism, is present in all normal tissues but is frequently deficient in a variety of cancers. It has been suggested that this metabolic difference between normal and cancer cells may be exploited to selectively treat MTAP-negative cancers by inhibiting de novo purine synthesis and by depleting L-methionine. However, these therapeutic strategies have only been tested in naturally occurring MTAP-positive and -negative cell lines, which might have additional genetic alterations that affect chemotherapeutic sensitivity. Therefore, it is of importance to examine the feasibility of enzyme-selective treatment using paired cell lines that have an identical genotype except for MTAP status. MTAP-negative A549 lung cancer cells were transfected with eukaryotic expression vectors encoding MTAP cDNA in sense and antisense orientations. The resultant stable transfectomas were treated with inhibitors of de novo purine synthesis such as methotrexate, 5,10-dideazatetrahydrofolate, and L-alanosine and by methionine depletion. The A549 cells transfected with an antisense construct (antisense transfectoma) expressed no MTAP protein and were more sensitive to both purine and methionine depletion than were cells expressing MTAP protein (sense transfectoma). Methylthioadenosine was able to completely rescue the sense transfectoma but not the antisense transfectoma from growth inhibition by depletion of purine and methionine. These results prove that MTAP deficiency contributes directly to the sensitivity of cancer cells to purine or methionine depletion. Inhibition of de novo purine synthesis, combined with methionine depletion in the presence of methylthioadenosine, is a highly selective treatment for MTAP-negative cancers.

Phosphoprotein p18 was identified originally on the basis of its very high level of expression in leukemic cells of different lineages. Changes in the level of p18 accumulation and phosphorylation associated with induction of differentiation of leukemic cells suggested a potential role for this phosphoprotein in cellular proliferation and differentiation and possibly in malignant transformation. Recent studies have demonstrated that p18 plays an important role in cell cycle progression by serving as a substrate for p34(cdc2) kinase. These studies showed that inhibition of p18 expression in leukemic cells results in growth retardation and accumulation of cells in G(2)-M. In this study, we explore the potential role of p18 in cellular transformation by investigating the effects of inhibition of p18 expression on the malignant phenotype of K562 erythroleukemia cells. These studies show that antisense inhibition of p18 expression in leukemic cells results in growth arrest at a lower saturation density, loss of serum independence, and loss of anchorage-independent growth in vitro. In addition, inhibition of p18 expression results in a marked inhibition of tumorigenicity of leukemic cells in vivo in the severe combined immune deficiency mouse model. These studies demonstrate that the high level of p18 expression in leukemic cells is necessary for the maintenance of the transformed phenotype and suggest p18 as a potential target for antileukemic interventions.

Pancreatic carcinoma cells lines are known to have a high incidence of homozygous deletion of the candidate tumor suppressor gene p16 (MTS1/CDKN2), which resides in the chromosome 9p21 region. Here we: (a)examined a series of these cell lines for the incidence of codeletion of genes located near p16, in particular, the gene for the enzyme 5'-deoxy-5'-methylthioadenosine phosphorylase (MTAP) and the genes of the IFN-alpha and -beta cluster (IFNs); and (b) investigated whether therapeutic strategies could be developed that target malignant cells that have undergone the codeletion of such genes. Five of the eight pancreatic carcinoma cell lines were p16(-), MTAP was codeleted in all five cases. Because MTAP phosphorolyzes 5'-deoxy-5'-methylthioadenosine (MTA), generated as a byproduct of polyamine synthesis, to the salvageable purine base adenine, loss of this pathway in p16(-), MTAP(-) cells might sensitize these cells to methotrexate (MTX), the mechanism of action of which involves, in part, an inhibition of purine de novo synthesis. MTAP(+) normal keratinocytes and pancreatic carcinoma lines had relatively poor sensitivity, in terms of efficacy, to the purine nucleotide-starving actions of MTX. This may be in part due to the MTAP-dependent salvage of adenine moieties from endogenously generated MTA, because the MTAP inhibitor 5'-chloro-5'-de- oxyformycin A potentiates the antipurine actions of MTX in some of these MTAP(+) lines. Also, exogenous MTA (10 microM) reverses the growth-inhibitory actions of MTX in these lines. In contrast, MTAP(-) cell lines, which cannot recycle purines from endogenous MTA, have a relatively high sensitivity to the antipurine actions of MTX, which is not modulated by 5'-chloro-5'-deoxyformycin A or exogenous MTA. Thus the MTAP loss in malignant cells may be an example of gene deletion chemoselectivity, in which genetic deletions that occur as part of the oncogenic process render these cells more sensitive to particular anticancer agents than normal cells, which have not undergone such deletions. We also examined whether the loss of IFN genes sensitize cells to the growth-inhibitory actions of these cytokines. Three of the five p16(-) cell lines bore homozygous deletions of IFNA1 and IFNB1 genes, representing each end of the IFN-alpha,-beta gene cluster; one cell line bore a codeletion of the IFNA1 gene but retained the IFNB1 locus. Whereas the cell lines that were most sensitive to the growth-inhibitory effects of IFN-beta or IFN-alpha(2b), tended to be those with IFN deletions, there were enough exceptions to this pattern to indicate that the IFN genotype does not reliably predict IFN responsiveness.

Pulmonary vascular leak induced in mice by interleukin 2 (IL-2) was attenuated by pretreatment with single or multiple doses of oral methotrexate. Methotrexate also attenuated pulmonary vascular leak when either larger doses of IL-2 or when lymphokine-activated killer (LAK) cells or LAK cells plus IL-2 were administered. Lymphoid infiltrates in the lungs of mice treated with IL-2 and methotrexate were significantly lower. The number of mice surviving treatment with high doses of IL-2 was also significantly increased when these mice were treated with methotrexate. Methotrexate prevented the IL-2-induced increase in the number of splenocytes that were asialo GM1+ but had no effect on Lyt 2+ or L3T4+ cell content. A marginal but significant inhibition in the generation of effector splenocytes that were cytolytic to either YAC or MCA-205 tumor targets was observed in mice treated with methotrexate and IL-2. In vivo studies indicated that methotrexate did not compromise the anti-tumor efficacy of treatment regimens that contained IL-2, LAK cells, or IL-2 and LAK cells. These results demonstrate the potential clinical utility of methotrexate in attenuating pulmonary vascular leak induced by IL-2 without compromising its efficacy. One potential mechanism of action of methotrexate is related to its ability to stimulate the release of adenosine followed by the inhibition of the adhesion of leukocytes to the IL-2-activated endothelium.

The role of a membrane-associated folate binding protein (mFBP) in transport of folate analogues was investigated in three epithelial cell lines that were grown in high folate medium and folate-conditioned medium and express different levels of mFBP: human nasopharyngeal KB cells, monkey kidney MA104 cells, and IGROV-I ovarian carcinoma cells. Folate analogues were selected for which mFBP exhibits a low affinity, i.e., methotrexate (MTX) and 10-ethyl-10-deazaaminopterin (10-EdAM) or a (moderately) high affinity as compared to folic acid, i.e., N-(5[N-(3,4-dihydro-2-methyl-4-oxoquinazolin-6-ylmethyl(-N-m ethylamino]-2-theonyl)-L-glutamic acid (ZD1694), N10-propargyl-5,8-dideazafolic acid (CB3717), and 5,10-dideazatetrahydrofolic acid. Regardless of the medium folate status, growth inhibition studies with IGROV-I and MA104 cells demonstrated a lack of correlation between the affinity of mFBP for the antifolate drugs and their sensitivity profile; both cell lines were highly sensitive to growth inhibition by MTX, 10-EdAM, ZD1694 and 5,10-dideazatetrahydrofolic acid, but were insensitive for CB3717. The same drug sensitivity profile was observed for KB cells, with the exception that these cells were also sensitive to growth inhibition by CB3717 but only in folate-conditioned medium. This overall drug sensitivity profile appeared to correlate with the differential efficiency of drug transport via the "classical" reduced folate/MTX carrier (RFC), rather than by mFBP. Characteristics that further supported functional RFC activity in KB, IGROV-I, and MA104 cells included: (a) the growth inhibitory effects of the drugs could be prevented by the reduced folate leucovorin rather than by folic acid; (b) rates for uptake of [3H]10-EdAM were 2-4-fold higher than for [3H]MTX at 1 microM extracellular concentrations and coincided with the affinity of the RFC for these drugs, rather than those of the mFBP; (c) uptake of [3H]10-EdAM and [3H]leucovorin was markedly inhibited by leucovorin and 10-EdAM, respectively, or by an N-hydroxysuccinimide ester of MTX (irreversibly labeling RFC) but only to a minor extent by folic acid or an N-hydroxysuccinimide ester of folic acid (irreversibly labeling mFBP); and, finally, (d) labeling with an N-hydroxysuccinimide ester of [3H]MTX identified a protein with a molecular weight within the range of that reported for the RFC in human leukemic cells. Altogether, these results indicate that both RFC and mFBP are coexpressed in three epithelial cell lines and that RFC is the preferential route of entry for antifolate compounds, even when mFBP is expressed to very high levels.

Chinese hamster ovary (and many rodent cell lines) transiently delay mitosis and progress into a second cell cycle without undergoing cytokinesis when treated with Colcemid, whereas HeLaS3 (and most human cell lines) arrest permanently in mitosis. We have discussed these differences and their consequences for cell survival under cell cycle-perturbing conditions within the context of mitotic checkpoint control (Schimke et al., Cold Spring Harbor Symp. Quant. Biol., 56: 417-425, 1991). Here, we report studies with mouse BALB/3T3 cell populations which, by the criterion of response to Colcemid, constitute a heterogeneous population with respect to mitotic checkpoint control. Clonal and subclonal populations retain population heterogeneity but with a bias for enrichment of cell populations that respond as do HeLaS3 cells. We have analyzed clones for their propensity for gene amplification as assessed by a stepwise increment selection protocol in methotrexate and report that there are significant differences in amplification propensities that correlate with differences in mitotic checkpoint control properties.

High-resolution 19F nuclear magnetic resonance spectroscopy at 7 T was used to study the effect of modulators on the metabolism of 5-fluorouracil (5-FUra, 115 mg/kg i.p.) in C38 murine colon tumors grown in C57BL/6 mice. Distinct 5-FUra metabolite patterns were found in perchloric acid extracts of these tumors after treatment. The 19F nuclear magnetic resonance spectra exhibited resonances representing 5-FUra, the catabolites alpha-fluoro-beta-ureidopropionic acid and alpha-fluoro-beta-alanine, as well as four distinct fluoronucleotide anabolites. Using this model system the effect of several modulators on 5-FUra tumor metabolite patterns was investigated: methotrexate (300 mg/kg); alpha-interferon (10(5) IU/animal); N-(phosphonacetyl)-L-aspartate (100 and 250 mg/kg); and leucovorin (300 and 750 mg/kg). A significant increase in the anabolite:catabolite ratio was observed for the groups treated with 5-FUra in combination with the modulators methotrexate (n = 8), alpha-interferon (n = 7), and high-dose leucovorin (n = 14), but not for low-dose leucovorin (n = 7). Cotreatment with high-dose N-(phosphonacetyl)-L-aspartate (n = 8) resulted in a significant decrease in the anabolite: catabolite ratio compared to treatment with 5-FUra alone (n = 16). Possible correlations of metabolite profiles with therapy response are discussed.

Six novel antifolates with 2,4-diaminopyrimidine-fused five-membered rings containing either pyrrole or cyclopentene rings were characterized at the cellular and biochemical level. Five of these antifolates were more growth inhibitory to the CCRF-CEM human leukemia cell line than methotrexate [MTX; drug concentration effective at inhibiting cell growth by 50% relative to untreated control (EC50), 12 nM], the antifolate used in the clinic, and two were more potent than 10-ethyl-10-deazaaminopterin (EC50, 2.7 nM); similar patterns of response were obtained in the FaDu and A253 squamous carcinoma cell lines. In addition, the growth inhibitory potency of these antifolates was generally less dependent on exposure time than was MTX. Growth inhibitory effects could be reversed by leucovorin, indicating an antifolate mechanism. These antifolates targeted dihydrofolate reductase (DHFR) based on direct human DHFR inhibition assays [drug concentration inhibiting enzyme activity by 50% (IC50), 0.6-28 nM; MTX IC50, 0.8 nM] and the cross-resistance of MTX-resistant CCRF-CEM cells containing elevated DHFR. Inhibition of human thymidylate synthase was generally weak. These 6,5-fused ring heterocyclic antifolates utilized the reduced folate/MTX transporter for uptake, based on the cross-resistance of MTX uptake-impaired CCRF-CEM cells, and were efficient substrates for this uptake system, based on inhibition of [3H]MTX uptake (IC50, 0.3-5.8 microM; aminopterin IC50, 2.6 microM). These analogues were substrates for CCRF-CEM folylpolyglutamate synthetase, with several being among the most active substrates now known (highest Vrel/Km 0.73; MTX and 10-ethyl-10-deazaaminopterin, 0.013 and 0.24, respectively). Substrate activity for murine intestinal folylpolyglutamate synthetase was also assayed, and a different specificity pattern was observed. These new antifolates are apparently not substrates for aldehyde oxidase. Analogues containing the fused cyclopentene ring are preferred to those containing the fused pyrrole ring based on growth inhibitory potency, effectiveness against decreased uptake mutants and apparent affinity for transport, and inhibition of DHFR. In addition, fused cyclopentene-containing analogues are efficiently polyglutamylated. The data indicate that antifolates with 2,4-diaminopyrimidine-fused five-membered rings, especially those containing the fused cyclopentene ring, are an important new class of antifolates which warrant further exploration at the synthetic and preclinical levels.

Oncogene activation and loss of tumor suppressor genes are known to play a role in tumor initiation as well as its progression. The potential roles of these genes in perturbation of genome stability has become a major interest. To better understand the relationship between expression of an oncogene and genetic instability, we have studied a cell line expressing an activated human Ha-ras under the control of bacterial lactose operon regulatory elements for changes in methotrexate resistance and dihydrofolate reductase (dhfr) gene amplification following mutant Ha-ras induction. In these cells mutant Ha-ras is directed by an inducible SV40 promoter containing a bacterial lac operator sequence which is repressed due to constitutive expression of bacterial lac repressor gene. The expression of this Ha-ras is specifically induced by the addition of isopropyl-1-thio-beta-D-galactopyranoside (IPTG), a lactose analogue, to the culture medium. During single-step methotrexate selection, these cells showed an increased frequency of methotrexate resistance in the presence of IPTG. More than 60% of the methotrexate-resistant colonies showed a 2-6-fold amplification of the dhfr gene. One clone with rearranged dhfr had about 100-fold amplification of the gene. The increased capacity to amplify DNA in response to mutant Ha-ras induction was not locus specific since cells also displayed an increased frequency of resistance to N-(phosphonacetyl)-L-aspartic acid in the presence of ITPG. Four of the methotrexate-resistant clones with amplified dhfr gene were cultured further in the presence or absence of IPTG and subsequently compared for their ability to grow in soft agar as a measure of transformation. In medium containing methotrexate but no IPTG, the clones were unable to grow in soft agar, indicating that methotrexate resistance due to gene amplification is separable from transformation.

We have studied the cytotoxicity of 5,10-dideazatetrahydrofolate (DDATHF) and of D-1694 to human WiDr colonic carcinoma cells as a model system for the effects of pure inhibitors of either the de novo purine synthesis pathway or thymidylate synthesis. The growth of this cell line was inhibited by very low concentrations of either agent and the lethality of DDATHF and D-1694 was completely prevented by continuous exposure to either hypoxanthine or thymidine, respectively, indicating that these compounds were very potent metabolic inhibitors, each specific for one of these pathways. D-1694 was highly cytotoxic (> 3 logs of kill) after a 4-h exposure to 1 microM drug, or a 24-h exposure to very low concentrations (0.04 microM). On the other hand, the cytotoxicity of DDATHF was substantially lower, with 2 logs of cell kill requiring >> 100 microM with 4 h of exposure or approximately 40 microM for 72 h of exposure. Maximal cell kill induced by D-1694 was 5-6 logs, consistent with elimination of all viable cells except preexisting mutants. A maximum of 2-3 logs of cell kill was observed with DDATHF. Exposure of WiDr cells to either D-1694 or DDATHF caused striking cellular changes, but the morphologies of cells treated with the two drugs were remarkably different. D-1694-treated cells detached from the dish within 1-2 days after a megaloblastosis, whereas DDATHF-treated cells remained adherent to the dishes for at least 10 days after treatment. The addition of thymidine to D-1694-treated cultures or hypoxanthine to DDATHF-treated cells after up to 20 h of drug exposure completely prevented cytotoxicity of either drug. With longer exposures, cytotoxicity of both drugs progressively increased in spite of such rescue. Our results indicate that substantial (99-99.9%) tumor cell kill can be induced by a pure inhibitor of purine synthesis, but that the rate of commitment to cell death and the extent of cell kill is greater with a pure inhibitor of thymidylate synthesis.

10-Ethyl-10-deazaaminopterin (EDX, edatrexate) exhibits therapeutic activity against methotrexate (MTX)-resistant tumors in animals and patients. In an effort to improve its efficacy among more chemoresistant tumors, studies were initiated in murine models of advanced metastatic disease comparing EDX and MTX at their maximum tolerated dose alone and in a high-dose regimen incorporating low-dose, delayed Ca leucovorin (LCV) rescue. Both twice-weekly x 3 and weekly x 3 schedules of administration were used with LCV given 16, 20, and 24 h after EDX. The LCV dose required to protect mice was 1/40 and 1/20 of the EDX or MTX dose, respectively, on either schedule. Therapy was initiated 5 or 6 days following i.v. implant of 5 x 10(5) cells of the E0771 mammary adenocarcinoma, T241 fibrosarcoma, Lewis lung carcinoma, B16 melanoma, or C38 colon carcinoma. MTX was essentially ineffective (increase in life span = < 30%) when given alone and either ineffective or only modestly effective (increase in life span = 20-80%) in increasing survival when given in the high-dose regimen to tumor-bearing mice. EDX alone was more effective than MTX when it was given in either regimen of therapy. Also, EDX given in the high-dose regimen (either twice-weekly or weekly x 3) was markedly more effective than EDX alone. Increased survival with this regimen was 2-3-fold greater than EDX alone against all 5 tumors, and long-term survivors were obtained with E0771 (20%), T241 (30-40%), Lewis lung (10-15%), B16 (20%), and C38 (40%) tumors. The administration of 6 doses rather than 3 doses on the twice-weekly schedule against T241 and Lewis lung tumors required a modest increase in the LCV dose but substantially improved efficacy, with as much as 70% long-term survivors (T241 tumor). We conclude that the use of a high-dose regimen with delayed LCV rescue markedly improved the therapeutic effectiveness of EDX against advanced metastatic disease in tumor-bearing mice. These studies should provide a framework for further clinical work with EDX, using this modality of therapy.

Clofazimine, a riminophenazine antimicrobial agent, and its analogue B669 were investigated for their effects on FaDu cells, a human squamous carcinoma cell line. These agents, at concentrations within the therapeutic range (0.25-2 micrograms/ml), caused a dose-dependent tumor cell cytotoxicosis which was greatly enhanced in the presence of human neutrophils. The neutrophil-mediated increment in tumoricidal activity, but not the direct antitumor effects of the drugs per se, was inhibited by catalase. The effects of these drugs on three more cell carcinoma lines as well as on two primary cultures and a noncarcinoma cell line were also investigated and compared with the activity of the standard antitumor chemotherapeutic agents bleomycin, cisplatin, and methotrexate. All seven cultures were sensitive to clofazimine and B669 compared to six that were sensitive to cisplatin, three that were sensitive to bleomycin, and one that was sensitive to methotrexate. The treatment of FaDu cells with clofazimine and B669 was associated with enhanced activity of phospholipase A2, as evidenced by increased release of radiolabeled arachidonate and lysophosphatidylcholine from membrane phospholipids. Inhibitors of arachidonic acid metabolism, protein kinase C inhibitors, as well as water and lipid soluble antioxidants failed to protect the cells against the cytotoxic activity of clofazimine and B669. However, alpha-tocopherol, a lysophospholipid-complexing agent, completely blocked the antiproliferative effects of the riminophenazines and also protected the cells against the direct cytotoxic effect of lysophosphatidylcholine, while the lysophospholipid-neutralizing enzyme lysophospholipase protected against the riminophenazines. These observations demonstrate that the tumoricidal properties of clofazimine and B669 are probably due to increases in the lysophospholipid content of cell membranes.

A human lymphoblastoid line (RPMI-1788), a methotrexate-sensitive human fibrosarcoma cell line (HT-1080), and a naturally resistant mixed mesodermal human sarcoma cell line with impaired methotrexate polyglutamylation (HS-42), recently established in our laboratory, were used to compare the ability of leucovorin to prevent trimetrexate cytotoxicity. Growth inhibition and an in situ thymidylate synthesis activity assay showed that inhibitory effects of trimetrexate (1 to 10 microM), 24-h exposure, were prevented by 10 microM leucovorin in the RPMI-1788 and HT-1080 cell lines but not in the HS-42 cell line. Total intracellular reduced folates increased about 2-fold in the three cell lines after exposure to leucovorin (10 microM) for 4 h, and after a 6-hour efflux remained elevated (1.5- and 1.3-fold of control levels) in RPMI-1788 and HT-1080 cells but decreased to 80% of control levels in HS-42 cells. Although uptake of leucovorin and levels of N5,N10-methylenetetrahydrofolate achieved after leucovorin administration were similar in RP-MI-1788 and HS-42 cells, polyglutamylate forms of this coenzyme were less in the HS-42 cells as compared to RPMI-1788 cells. Based on these studies, the combination of trimetrexate with leucovorin should be further investigated as a way to increase the therapeutic index in some patients with soft tissue sarcomas.

DNA single-strand breaks and associated growth inhibition induced by the thymidylate synthase inhibitor N-(5-[N-(3,4-dihydro-2-methyl-4-oxoquinazoline-6-ylmethyl)-N -methylamino]-2 - thenoyl)-L-glutamic acid (ICI D1694) were quantitated using the human ileocecal adenocarcinoma cell line, HCT-8. The effects of different concentrations and schedules of [6R,S]-5-formyltetrahydrofolate ([6RS]LV) and 2'-deoxy-thymidine (dThd) on drug growth inhibition and DNA damage were also evaluated. The drug concentrations for 50% inhibition of cell growth in culture following 2-h and 72-h exposures were 0.073 and 0.003 microM, respectively. After a 2-h drug exposure, the occurrence of DNA single-strand breaks (SSBs) was time dependent. It was detectable at 8 h and reached a maximum at about 24 h, 34 +/- 3 (SD) and 305 +/- 34 rad equivalents with 0.1 microM (50% inhibition concentration) and 1.0 microM (90% inhibition concentration) ICI D1694, respectively. A significant level of DNA SSBs (101 +/- 13 rad equivalents) was still detectable at 72 h after the 2-h treatment with 1 microM ICI D1694. No significant level of DNA SSBs was detected when cells were exposed simultaneously to ICI D1694 and 20 microM [6RS]LV. Complete rescue of drug-induced DNA SSBs could be achieved when cells were exposed to 10 microM dThd starting no later than 4 h after drug treatment. The growth inhibition of ICI D1694 was abrogated by [6RS]LV in a concentration-dependent manner. Complete protection was achieved when cells were exposed simultaneously to 1 microM ICI D1694 and 5 microMs [6RS]LV or to 3 microMs dThd immediately after drug treatment. The results demonstrate that: (a) the growth inhibition of ICI D1694 is a function of time and schedule; (b) the growth inhibition is accompanied by extensive DNA single-strand breaks and slow repair; (c) at 1 microM ICI D1694, 3 microMs dThd and 5 microMs [6RS]LV can completely rescue cells from drug effects when dThd is added up to 4 h following drug treatment or when [6RS]LV is given in combination with the drug; (d) interference of [6RS]LV with ICI D1694 action may be occurring at the level of drug uptake and at intracellular targets, while dThd interferes with the drug action at intracellular targets.

A series of experiments were conducted to study synthesis and secretion of mucin in mucus-secreting subpopulations of HT29 human colonic adenocarcinoma cells selected by resistance to methotrexate (MTX). Mucin was quantitated by [3H]glucosamine labeling and chromatography on Sepharose CL-4B. The mucinous nature of the labeled high molecular weight glycoprotein was verified by alkaline borohydride treatment, cesium chloride density gradient ultracentrifugation, and sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The results of these experiments demonstrated that MTX-treated cells have increased amounts of mucin in medium, cytosol, and membrane fractions. This was associated with the increase in the activities of polypeptidyl-N-acetylgalactosaminyltransferase and beta-1,3-galactosyltransferase compared to control cells. DEAE-Sephacel chromatography of [3H]glucosamine-labeled high molecular weight glycoproteins suggest that MTX-treated cells are less acidic compared to controls. Using complementary DNA probes for two distinct human intestinal mucins (MUC2 and MUC3) and one mammary mucin (MUC1), it was found that MTX-treated cells expressed more mucin messages compared to untreated cells. These results were consistent with immunoblots using anti-MRP (MUC2 repeat peptide), anti-M3P (MUC3 repeat peptide), 139H2 (MUC1 peptide), anti-T (peanut lectin), anti-Tn (91S8), and anti-sialosyl Tn (JT10e) antibodies. These data indicate that MTX-resistant HT29 cells show enhanced secretion and synthesis of mucin as well as expression of MUC1-, MUC2-, and MUC3-related mucin polypeptide epitopes.

We have reported that 5-fluorouracil can increase the cytotoxic and antineoplastic activity of 3'-azido-3'-deoxythymidine (AZT). To further evaluate the antineoplastic utility of AZT we now have assessed its effect in combination with methotrexate (MTX) in the human colon tumor model HCT-8. Incubation of these cells for 5 days in AZT and MTX caused a reduction in the 50% inhibitory concentration of AZT and isobologram analysis revealed additive effects which were reversed by the addition of 50 microM thymidine to the incubation media. This enhanced cytotoxicity appeared not to be related to an effect of AZT on MTX activity; in whole-cell assays the ability of MTX to inhibit de novo dTMP synthesis and deplete intracellular pools of dTTP was not affected by AZT. In contrast, although MTX did not alter AZT triphosphate production, it did affect AZT triphosphate utilization in DNA synthesis. Incubation of cells for 24 h in [3H]AZT alone (5 microM, 3 microCi/ml) resulted in 6.6 pmol AZT incorporated into cellular DNA/10(6) cells. Coincubation of these cells in [3H]AZT (5 microM) plus 5 or 15 nM MTX increased AZT incorporation into DNA to 8.0 and 20.5 pmol/10(6) cells, respectively. Biochemically, this effect appeared to correlate with the concentration-dependent ability of 5 or 15 nM MTX to deplete intracellular dTTP pools, which were reduced by 25 and 49%, respectively. Further evidence of the relationship between intracellular dTTP pools and AZT cytotoxicity was that, in the presence of both MTX and 50 microM thymidine, intracellular dTTP pools remained near normal levels and the incorporation of 5 microM AZT into DNA was not enhanced. Therapeutically, studies conducted in athymic (nude) mice bearing HCT-8 xenografts that received six weekly cycles of MTX (87.5 mg/kg) and AZT (300 mg/kg) revealed that the two-drug regimen exerted superior antineoplastic effects compared to either drug alone (treated versus control approximately 0.9 for AZT or MTX and approximately 0.3 for MTX plus AZT). In addition, the combination did not increase toxicity compared to therapy with MTX alone. These findings are discussed in light of their biochemical and clinical implications.

A human fibrosarcoma cell line, HT-1080, and four new cell lines (HS-16, HS-28, HS-30, and HS-42) were established from untreated patients with mesenchymal chondrosarcoma, peripheral nerve sheath sarcoma, malignant hemangiopericytoma, and mixed mesodermal tumor, respectively, and were used for analysis of mechanisms of intrinsic resistance to methotrexate. All four new cell lines were resistant to methotrexate as determined by inhibition of thymidylate synthase in whole cells and by growth inhibition, as compared with HT-1080, a methotrexate sensitive cell line. Methotrexate uptake, level of dihydrofolate reductase, and inhibition of this enzyme by methotrexate in the four cell lines were comparable to HT-1080 cells. However, levels of long chain polyglutamates (glu3-5) of methotrexate achieved after a 24-h incubation with this drug were much lower in the four new cell lines as compared to the HT-1080 cell line (5- to 20-fold lower). The low levels of methotrexate polyglutamates formed is likely the major cause of intrinsic methotrexate resistance in these new sarcoma cell lines.

Choriocarcinoma, a highly malignant tumor arising from the trophoblast, comprises a heterogenous population of cells including cytotrophoblasts, intermediate trophoblasts, and syncytiotrophoblasts. In order to investigate trophoblast differentiation, we used centrifugal elutriation to separate cells from the JAr choriocarcinoma cell line according to their size and to further show that the resultant cell populations differ in their stage of differentiation. Two % of the cell population consists of large, multinuclear cells, which display the highest level of chorionic gonadotropin (CG) mRNAs. The increase in the CG beta mRNA with cell size is a consequence of the transcriptional mechanism, since agents which induce differentiation in JAr cells, i.e., methotrexate, increase the level of CG alpha and CG beta transcripts, cause a shift in cell size, and result in the formation of multinuclear cells. The multinuclear cells in the JAr population arise, at least partly, from kariokinesis without cytokinesis.