Search

Mitosis is a highly dynamic process, aimed at separating identical copies of genomic material into two daughter cells. A failure of the mitotic process generates cells that carry abnormal chromosome numbers. Such cells are predisposed to become tumorigenic upon continuous cell division and thus need to be removed from the population to avoid cancer formation. Cells that fail in mitotic progression indeed activate cell death or cell cycle arrest pathways; however, these mechanisms are not well understood. Growing evidence suggests that the formation of de novo DNA damage during and after mitotic failure is one of the causal factors that initiate those pathways. Here, we analyze several distinct malfunctions during mitosis and cytokinesis that lead to de novo DNA damage generation. [ABSTRACT FROM AUTHOR]

In response to ionizing radiation, the MRE11/RAD50/NBN complex re-distributes to the sites of DNA double-strand breaks (DSBs) where each of its individual components is phosphorylated by the serine-threonine kinase, ATM. ATM phosphorylation of NBN is required for the activation of the S-phase checkpoint, but the mechanism whereby these phosphorylation events signal the checkpoint machinery remains unexplained. Here, we describe the use of direct protein transduction of the homing endonuclease, I-PpoI, into human cells to generate site-specific DSBs. Direct transduction of I-PpoI protein results in rapid accumulation and turnover of the endonuclease in live cells, facilitating comparisons across multiple cell lines. We demonstrate the utility of this system by introducing I-PpoI into isogenic cell lines carrying mutations at the ATM phosphorylation sites in NBN and assaying the effects of these mutations on the spatial distribution and temporal accumulation of NBN and ATM at DSBs by chromatin immunoprecipitation, as well as timing and extent of DSB repair. Although the spatial distribution of NBN and ATM recruited to the sites of DSBs was comparable between control cells and those expressing phosphorylation mutants of NBN, the timing of accumulation of NBN and ATM was altered. Serine-to-alanine mutations that blocked phosphorylation resulted in delayed recruitment of both NBN and ATM to DSBs. Serine-to-glutamic acid substitutions that mimicked the phosphorylation event resulted in both increased and prolonged accumulation of both NBN and ATM at DSBs. The repair of DSBs in cells lacking full-length NBN was significantly delayed compared with control cells, whereas blocking phosphorylation of NBN resulted in a more modest delay in repair. These data indicate that following the induction of DSBs, phosphorylation of NBN regulates its accumulation, and that of ATM, at sites of DNA DSB as well as the timing of the repair of these sites. [ABSTRACT FROM AUTHOR]

Chromatin remodeling factors are becoming known as crucial facilitators of recruitment of repair proteins to sites of DNA damage. Multiple chromatin remodeling protein complexes are now known to be required for efficient double strand break repair. In a screen for microRNAs (miRNAs) that modulate the DNA damage response, we discovered that expression of the miR-99 family of miRNAs correlates with radiation sensitivity. These miRNAs were also transiently induced following radiation. The miRNAs target the SWI/SNF chromatin remodeling factor SNF2H/SMARCA5, a component of the ACF1 complex. We found that by reducing levels of SNF2H, miR-99a and miR-100 reduced BRCA1 localization to sites of DNA damage. Introduction of the miR-99 family of miRNAs into cells reduced the rate and overall efficiency of repair by both homologous recombination and non-homologous end joining. Finally, induction of the miR-99 family following radiation prevents an increase in SNF2H expression and reduces the recruitment of BRCA1 to the sites of DNA damage following a second dose of radiation, reducing the efficiency of repair after multiple rounds of radiation, as used in fractionated radiotherapy. [ABSTRACT FROM AUTHOR]

We synthesized several novel bifunctional alkylating derivatives of 3a-aza-cyclopenta[a]indene (BO-1012, BO-1005, BO-1099 and BO-1101) that are potent DNA interstrand crosslinking agents. In in vitro cytotoxicity assay, these compounds were more cytotoxic to multidrug-resistant (MDR) cells, such as KBvin10, KBtax50 and CEM/VBL, than their parental cells. Using a xenograft model, BO-1012, at a dose of 5 mg/kg, partially suppressed the growth of parental KB cells but completely suppressed the growth of KBvin10 cells in nude mice. In exploring the possible mechanism, we found that DNA double-strand break (DSB) repair activity in MDR cells, KBvin10 and CEM/VBL, was significantly reduced compared with their parental cells, KB and CEM. Reduced DSB repair activity in KBvin10 cells was likely due to a defect in nuclear translocation of DNA-dependent protein kinase (DNA-PK), a component of the non-homologous end-joining repair machinery. Furthermore, BO-1012-induced DNA-PK translocation from the cytosol into the nucleus in KB cells is associated with the activation of the Src/nuclear epidermal growth factor receptor (EGFR) cascade, which is defective in MDR cells. As knockdown of P-glycoprotein (P-gp) by siRNA reactivated the Src/nuclear EGFR cascade, DNA-PK translocation and DNA repair activity in MDR cells, overexpression of P-gp attenuates the activity of DNA DSB repair through suppression of Src/nuclear EGFR cascade. Therefore, DNA interstrand crosslinking agents may have potential therapeutic use against P-gp-overexpressing MDR cells. [ABSTRACT FROM AUTHOR]

BCR-ABL transforms bone marrow progenitor cells and promotes genome instability, leading to development of chronic myelogenous leukemia (CML). The tyrosine kinase inhibitor imatinib effectively treats CML, but acquired resistance can develop because of BCR-ABL mutations. Mechanisms for acquisition of BCR-ABL mutations are not fully understood. Using a novel culture model of CML acquired resistance, we show that inhibition of SIRT1 deacetylase by small molecule inhibitors or gene knockdown blocks acquisition of BCR-ABL mutations and relapse of CML cells on tyrosine kinase inhibitors. SIRT1 knockdown also suppresses de novo genetic mutations of hypoxanthine phosphoribosyl transferase gene in CML and non-CML cells upon treatment with DNA damaging agent camptothecin. Although SIRT1 can enhance cellular DNA damage response, it alters functions of DNA repair machineries in CML cells and stimulates activity of error-prone DNA damage repair, in association with acquisition of genetic mutations. These results reveal a previously unrecognized role of SIRT1 for promoting mutation acquisition in cancer, and have implication for targeting SIRT1 to overcome CML drug resistance. [ABSTRACT FROM AUTHOR]

DNA-damaging therapies represent the most frequently used non-surgical anticancer strategies in the treatment of human tumors. These therapies can kill tumor cells, but at the same time they can be particularly damaging and mutagenic to healthy tissues. The efficacy of DNA-damaging treatments can be improved if tumor cell death is selectively enhanced, and the recent application of poly-(ADP-ribose) polymerase inhibitors in BRCA1/2-deficient tumors is a successful example of this. DNA damage is known to trigger cell-cycle arrest through activation of DNA-damage checkpoints. This arrest can be reversed once the damage has been repaired, but irreparable damage can promote apoptosis or senescence. Alternatively, cells can reenter the cell cycle before repair has been completed, giving rise to mutations. In this review we discuss the mechanisms involved in the activation and inactivation of DNA-damage checkpoints, and how the transition from arrest and cell-cycle re-entry is controlled. In addition, we discuss recent attempts to target the checkpoint in anticancer strategies. [ABSTRACT FROM AUTHOR]

5-Fluorouracil (5-FU), together with other drugs such as oxaliplatin, is one of the most important pharmacological agents in the treatment of colorectal cancer. Although mitogen-activated protein kinases (MAPKs) have been extensively connected with resistance to platinum compounds, no role has been established in 5-FU resistance. Here we demonstrate that p38MAPK activation is a key determinant in the cellular response to 5-FU. Thus, inhibition of p38MAPKα by SB203580 compound or by short-hairpin RNA interference-specific knockdown correlates with a decrease in the 5-FU-associated apoptosis and chemical resistance in both HaCaT and HCT116 cells. Activation of p38MAPK by 5-FU was dependent on canonical MAP2K, MAPK kinase (MKK)-3 and MKK6. In addition, ataxia telangiectasia mutated (ATM) and ataxia telangiectasia and Rad3 related (ATR) showed a redundancy of function for the final activation of p38MAPK. Resistance associated with p38MAPK inhibition correlates with an autophagic response that was mediated by a decrease in p53-driven apoptosis, without effect onto p53-dependent autophagy. Moreover, the results with colorectal cancer-derived cell lines with different p53 status and patterns of resistance to 5-FU suggest that de novo and acquired resistance was controlled by similar mechanisms. In summary, our data demonstrate a critical role for the p38MAPK signaling pathway in the cellular response to 5-FU by controlling the balance between apoptosis and autophagy. [ABSTRACT FROM AUTHOR]

Wild-type p53-induced phosphatase 1 (WIP1) is a serine/threonine phosphatase that dephosphorylates proteins in the ataxia telangiectasia mutated (ATM)-initiated DNA damage response pathway. WIP1 may have a homeostatic role in ATM signaling by returning the cell to a normal pre-stress state following completion of DNA repair. To better understand the effects of WIP1 on ATM signaling, we crossed Atm-deficient mice to Wip1-deficient mice and characterized phenotypes of the double knockout progeny. We hypothesized that the absence of Wip1 might rescue Atm deficiency phenotypes. Atm null mice, like ATM-deficient humans with the inherited syndrome ataxia telangiectasia, exhibit radiation sensitivity, fertility defects, and are T-cell lymphoma prone. Most double knockout mice were largely protected from lymphoma development and had a greatly extended lifespan compared with Atm null mice. Double knockout mice had increased p53 and H2AX phosphorylation and p21 expression compared with their Atm null counterparts, indicating enhanced p53 and DNA damage responses. Additionally, double knockout splenocytes displayed reduced chromosomal instability compared with Atm null mice. Finally, doubly null mice were partially rescued from gametogenesis defects observed in Atm null mice. These results indicate that inhibition of WIP1 may represent a useful strategy for cancer treatment in general and A-T patients in particular. [ABSTRACT FROM AUTHOR]

In the presence of sustained DNA damage occurring in S-phase or G2, normal cells arrest before mitosis and eventually become senescent. The checkpoint kinases Chk1/Chk2 and the CDK inhibitor p21 are known to have important complementary roles in this process, in G2 arrest and cell cycle exit, respectively. However, additional checkpoint roles have been reported for these regulators and it is not clear to what extent their functions are redundant. Here we compared the respective roles of Chk1, Chk2 and p21 in DNA damage-induced G2 arrest in normal human fibroblasts, normal epithelial cells and frequently used p53 proficient cancer cells. We show that in normal cells, Chk1, but not Chk2, is involved in G2 arrest whereas neither are essential. In contrast, p21 is required. However, Chk1, but not Chk2, becomes necessary for arrest in U2OS osteosarcoma cells. We find that their ATM/p53/p21 response in G2 phase is defective, like in other cancer cells with wild-type p53, and conclude that cross-talk between the Chk1 and p21 pathways allows them to switch dependency for G2 arrest onto Chk1. Using the specific ATM inhibitor KU-55933 we confirm the essential role of ATM in the induction of p21 for G2 arrest of normal cells. Efficient p21 induction is required for nuclear sequestration of inactive cyclin B1-Cdk1 complexes preceding irreversible cell cycle exit in G2. Our results demonstrate that p21 is able to fulfill the Chk1 functions in G2 arrest under continuous genotoxic stress, which has important implications for cancer chemotherapy. [ABSTRACT FROM AUTHOR]

An elevated DNA-repair capacity in cancer cells leads to radiation resistance and severely limits the efficacy of radiation therapy. Activation of Akt is tightly associated with resistance to radiotherapy, and Mre11 protein has important role during the repair of DNA double-strand breaks (DSBs). In this report, our results showed that inhibition of Akt activity impaired the repair of DSBs in CNE2 cells, whereas activated Akt promoted the repair of DSBs in HeLa cells. Knockdown of Mre11 also impaired the process of DSB repair in both these two cell lines. More importantly, we found that Akt could regulate Mre11 expression. Inhibition of Akt activity by small interfering RNA or LY294002 efficiently downregulated the Mre11 expression in CNE2 cells, and transfection with myr-Akt plasmid in HeLa cells upregulated the Mre11 expression. In addition, luciferase reporter analysis revealed that Mre11 reporter activity increased after transfection with myr-Akt1 plasmids, and this myr-Akt1-induced transcriptional activity was blocked in the presence of LY294002. Further study showed GSK3β/β-catenin/LEF-1 pathway was involved in this regulation. Knockdown of β-catenin or LEF-1 led to the downregulation of Mre11, whereas overexpression of β-catenin led to upregulation of Mre11. The chromatin immunoprecipitation assay assay showed β-catenin/LEF-1 heterodimer could directly bind to the promoter of Mre11 in vivo. And the luciferase activity of the pGL3-Mre11 and pGL3-Lef increased in HeLa cells following β-catenin plasmid co-transfected, but was abolished when the LEF-1-binding conserved sequences of Mre11 promoter were mutated. These results together support Akt can upregulate the expression of Mre11 through GSK3β/ β-catenin/LEF pathway to elevate DSB-repair capacity in cancer cells. [ABSTRACT FROM AUTHOR]

Cells experiencing DNA damage undergo a complex response entailing cell-cycle arrest, DNA repair and apoptosis, the relative importance of the three being modulated by the extent of the lesion. The observation that Abl interacts in the nucleus with several proteins involved in different aspects of DNA repair has led to the hypothesis that this kinase is part of the damage-sensing mechanism. However, the mechanistic details underlying the role of Abl in DNA repair remain unclear. Here, I will review the evidence supporting our current understanding of Abl activation following DNA insults, while focusing on the relevance of these mechanisms in protecting DNA-injured germ cells. Early studies have shown that Abl transcripts are highly expressed in the germ line. Abl-deficient mice exhibit multiple abnormalities, increased perinatal mortality and reduced fertility. Recent findings have implicated Abl in a cisplatin-induced signaling pathway eliciting death of immature oocytes. A p53-related protein, TAp63, is an important immediate downstream effector of this pathway. Of note, pharmacological inhibition of Abl protects the ovarian reserve from the toxic effects of cisplatin. This suggests that the extent of Abl catalytic outputs may shift the balance between survival (likely through DNA repair) and activation of a death response. Taken together, these observations are consistent with the evolutionary conserved relationship between DNA damage and activation of the p53 family of transcription factors, while shedding light on the key role of Abl in dictating the fate of germ cells upon genotoxic insults. [ABSTRACT FROM AUTHOR]

The cellular response to DNA damage is a crucial surveillance mechanism that maintains genomic integrity and prevents cancer progression. Previous studies identified multiple Ser/Thr protein kinases that have pivotal roles in the activation of this response. It is interesting that a growing body of evidence suggests that these kinases and their substrates are under tight modulation by numerous Ser/Thr phosphatases. In this study, we review recent reports that reveal new functions and regulation of these phosphatases. Similar to the kinases in this pathway, phosphatases may also be intimately involved in cancer progression and present valuable targets for cancer therapy. [ABSTRACT FROM AUTHOR]

Although post-translational modifications by the small ubiquitin-like modifiers (SUMO) are known to be important in DNA damage response, it is unclear whether they have a role in double-strand break (DSB) repair by non-homologous end joining (NHEJ). Here, we analyzed various DSB repair pathways upon inhibition of SUMO-mediated protein–protein interactions using peptides that contain the SUMO-interaction motif (SIM) and discriminate between mono- and SUMO-chain modifications. The SIM peptides specifically inhibit NHEJ as shown by in vivo repair assays and radio-sensitivity of cell lines deficient in different DSB repair pathways. Furthermore, mono-SUMO, instead of SUMO-chain, modifications appear to be involved in NHEJ. Immunoprecipitation experiments also showed that the SIM peptide interacted with SUMOylated Ku70 after radiation. This study is the first to show an important role for SUMO:SIM-mediated protein–protein interactions in NHEJ, and provides a mechanistic basis for the role of SIM peptide in sensitizing genotoxic stress of cancer cells. [ABSTRACT FROM AUTHOR]

Poly (ADP-ribose) polymerase-1 (PARP-1) has an important role in the cellular response to a broad spectrum of DNA lesions. PARP-1 is strongly activated in response to double-stranded DNA breaks (DSBs), yet its contribution to the DSB response is poorly understood. Here we used bleomycin, a radiomimetic that generates DSBs with high specificity to focus on the response of PARP-1 to DSBs. We report that the induction of PARP-1 activity by bleomycin depends on the Ku antigen, a nonhomologous-DNA-End-Joining factor and protein phosphatase 5 (PP5). PARP-1 activation in response to bleomycin was reduced over 10-fold in Ku-deficient cells, whereas its activation in response to U.V. was unaffected. PARP-1 activation was rescued by reexpression of Ku, but was refractory to manipulation of DNA-dependent protein kinase or ATM. Similarly, PARP-1 activation subsequent to bleomycin was reduced 2-fold on ablation of PP5 and was increased 5-fold when PP5 was overexpressed. PP5 seemed to act directly on PARP-1, as its basal phosphorylation was reduced on overexpression of PP5, and PP5 dephosphorylated PARP-1 in vitro. These results highlight the functional importance of Ku antigen and PP5 for PARP-1 activity subsequent to DSBs. [ABSTRACT FROM AUTHOR]

The authors reflect on the discoveries on how telomeres and enzyme telomerase protect the chromosomes by Elizabeth H. Blackburn, Carol W. Greider, and Jack W. Szostak that led them to receive the 2009 Nobel Prize in Physiology or Medicine. It states that the discoveries revealed a direction to understand the molecular bases underlying cancer, stem cell biology, aging, and other human diseases. It relates that Blackburn and Greider continuously study the pathways controlling telomere maintenance.

One of the pivotal functions of endogenous tumor suppression is to oppose aberrant cell survival, but the molecular requirements of this process are not completely understood. Here, we show that caspase 2, a death effector with largely unknown functions, represses transcription of the survivin gene, a general regulator of cell division and cytoprotection in tumors. This pathway involves caspase 2 proteolytic cleavage of the nuclear factor κB (NFκB) activator, RIP1. In turn, loss of RIP1 abolishes transcription of NFκB target genes, including survivin, resulting in deregulated mitotic transitions, enhanced apoptosis and suppression of tumorigenicity in vivo. Therefore, caspase 2 functions as an endogenous inhibitor of NFκB-dependent cell survival and this mechanism may contribute to tumor suppression in humans. [ABSTRACT FROM AUTHOR]

V(D)J recombination is essential for the maturation of lymphocytes. Because of the involvement of cutting and joining DNA double strands, this recombination activity is strictly contained within the noncycling phases of the cell cycle. Such containment is crucial for the maintenance of genomic integrity. The ataxia telangiectasia mutated (ATM) gene is known to have a central role in sensing general DNA damage and mediating cell-cycle checkpoint. In this study, we investigated the role of ATM and its downstream targets in the cell-cycle control of V(D)J recombination in vivo. Our results revealed the persistence of double-strand breaks (DSBs) throughout the cell cycle in ATM−/− and p53−/− thymocytes, but the cell-cycle regulation of a V(D)J recombinase, Rag-2, was normal. The histone variant H2AX, which is phosphorylated during normal V(D)J recombination, was dispensable for containing DSBs. H2AX was still phosphorylated at V(D)J loci in the absence of ATM. Therefore, V(D)J recombination, a physiological DNA rearrangement process, activates the ATM/p53 pathway to contain DNA breaks within the noncycling cells and surprisingly this pathway is not important for containing Rag-2 activity. This study shows the dynamic multiple functions of ATM in maintaining genomic stability and preventing tumorigenesis in developing lymphocytes. [ABSTRACT FROM AUTHOR]

Mice transgenic for MUC1 (mucin 1) and polyomavirus middle T (PyMT) develop mammary carcinomas within 15 weeks with 100% penetrance. PyMT-induced mammary tumorigenesis is closely correlated with robust telomerase expression and activity. To assess the role of telomerase activation and telomere maintenance in mammary carcinoma tumorigenesis, we generated mice expressing MUC1 and PyMT (MMT mice) but deficient in the telomerase RNA component, mTerc, on the C57BL/6 background. Successive generational intercrosses of mTerc−/−MMT mice produced cohorts with progressively shorter telomeres that were audited for mammary tumor formation. Relative to MMT (N=14) and G0 mTerc+/− female controls (G0=14), mTerc−/−MMT females (G1=11, G2=15, G3=15 and G4=5) showed decreased tumor volumes and increased tumor latency—MMT=95.6 days; G0 mTerc+/−MMT=98.6 days versus G1, G2, G3 and G4 mTerc−/−MMT mice with latencies of 122.6, 138.9, 140.7 and 220.9 days, respectively (controls versus G1–G4, P<0.005). The progressive impairment of lung metastasis was also observed with each successive mTerc−/−MMT generation. The impairment of tumorigenesis was associated with decreased proliferation of mammary epithelial and tumor cells and increased apoptosis of tumor cells. Together, these results indicate that, in the setting of viral oncoprotein mammary tumorigenesis, telomerase-dependent telomere maintenance facilitates the formation and metastatic progression of mammary tumors. [ABSTRACT FROM AUTHOR]

The failure of conventional therapies in glioblastoma (GBM) is largely due to an aberrant activity of survival cascades, such as PI3 kinase (PI3K)/Akt-mediated signaling. This study is the first to show that the class I PI3K inhibitor, PI-103, enhances chemotherapy-induced cell death of GBM cells. Concurrent treatment with PI-103 and DNA-damaging drugs, in particular doxorubicin, significantly increases apoptosis and reduces colony formation compared with chemotherapy treatment alone. The underlying molecular mechanism for this chemosensitization was shown by two independent approaches, that is, pharmacological and genetic inhibition of PI3K, DNA-PK and mTOR, to involve inhibition of DNA-PK-mediated DNA repair. Accordingly, blockage of PI3K or DNA-PK, but not of mTOR, significantly delays the resolution of doxorubicin-induced DNA damage and concomitantly increases apoptosis. Importantly, not only are several GBM cell lines chemosensitized by PI-103 but also GBM stem cells. Clinical relevance was further confirmed by the use of primary cultured GBM cells, which also exhibit increased cell death and reduced colony formation on combined treatment with PI-103 and doxorubicin. By identifying class I PI3K inhibitors as powerful agents in enhancing the lethality of DNA-damaging drugs, to which GBMs are usually considered unresponsive, our findings have important implications for the design of rational combination regimens in overcoming the frequent chemoresistance of GBM. [ABSTRACT FROM AUTHOR]

Despite the early discovery of caspase-2, its physiological function has long remained an enigma. A number of recent publications now suggest not just one, but multiple functions, including roles in apoptosis, DNA repair and tumor suppression. How can one enzyme have so many talents? Considering the diversity of interaction partners and the specific mode of pro-apoptotic action proposed in these studies, caspase-2 might in fact represent a primordial protease serving numerous pathways, established before the advent of a more elaborate functionally diversified caspases system. [ABSTRACT FROM AUTHOR]

Inositol hexakisphosphate kinase 2 (IP6K2), a member of the inositol hexakisphosphate kinase family, functions as a growth suppressive and apoptosis-enhancing kinase during cell stress. We created mice with a targeted deletion of IP6K2; these mice display normal embryogenesis, development, growth and fertility. Chronic exposure to the carcinogen 4-nitroquinoline 1-oxide (4-NQO, a UV-mimetic compound) in drinking water resulted in fourfold increased incidence of invasive squamous cell carcinoma (SCC) formation in the oral cavity and esophagus of the knockout (KO) mice compared to the wild-type (WT) littermates. Paradoxically, KO mice displayed relative resistance to ionizing radiation and exhibit enhanced survival following 8–10 Gy total body irradiation. Primary KO fibroblasts displayed resistance to antiproliferative effects of interferon-β and increased colony forming units following ionizing radiation. Radioresistance of KO fibroblasts was associated with accelerated DNA repair measured by comet assay. Direct microinjection of 5-PP-Ins(1,2,3,4,6)P5 (the enzymatic product of IP6K2), but not InsP6 (the substrate of IP6K2) induced cell death in SCC22A squamous carcinoma cells.Oncogene (2009) 28, 2383–2392; doi:10.1038/onc.2009.113; published online 11 May 2009 [ABSTRACT FROM AUTHOR]

Ku70 forms a heterodimer with Ku80, called Ku that is well known for repairing DNA double-strand breaks through non-homologous end joining. As a result, deletion of either causes a very similar phenotype in mice that includes hypersensitivity to clastogens and early aging. In addition, deletion of Ku80 along with the cell cycle checkpoint protein, p53, dramatically increases the incidence of pro-B-cell lymphoma. Even though Ku70- p53-mutant mice have not been analysed, a logical assumption is they would exhibit the same cancer phenotype. Here, we test this assumption by comparing p53-mutant littermates deleted for either Ku70 or Ku80 or both. We find this assumption to be incorrect as p53-mutants live significantly longer when deleted for Ku70 rather than Ku80 or Ku70+Ku80. We also find the former cohort displays much lower levels of pro-B-cell lymphoma than the latter two cohorts. As pro-B-cell lymphoma is caused by a translocation between chromosomes 12 and 15, we tested fibroblasts for DNA repair capacity, and found that p53-mutant fibroblasts are more sensitive to streptonigrin and paraquat when deleted for Ku80 as compared with Ku70. Thus, Ku80 may function outside the Ku heterodimer to influence DNA damage repair presenting the possibility that Ku80 influenced the open coding ends in a manner that suppressed a cancer-causing translocation.Oncogene (2009) 28, 1875–1878; doi:10.1038/onc.2009.57; published online 30 March 2009 [ABSTRACT FROM AUTHOR]

The Akt serine/threonine kinase (also called protein kinase B) has emerged as a critical signaling molecule within eukaryotic cells. Significant progress has been made in clarifying its regulation by upstream kinases and identifying downstream mechanisms that mediate its effects in cells and contribute to signaling specificity. Here, we provide an overview of present advances in the field regarding the function of Akt in physiological and pathological cell function within a more generalized framework of Akt signal transduction. An emphasis is placed on the involvement of Akt in human diseases ranging from cancer to metabolic dysfunction and mental disease.Oncogene (2008) 27, 6473–6488; doi:10.1038/onc.2008.313 [ABSTRACT FROM AUTHOR]

P53 wild-type and p53-null or mutant cells undergo a G2-phase cell-cycle arrest in response to ionizing radiation (IR). In this study we examined the effect of heat-shock protein 90 (HSP90) inhibitor, geldanamycin (GA), on IR-induced G2 arrest in human colon adenocarcinoma cells with different p53 status. We show that GA treatment abrogates IR-induced G2-phase arrest in cells null or mutant for p53. Specifically, GA treatment pushed irradiated p53 signaling-defective cells into a premature mitosis characterized by aberrant mitotic figures, increased γH2AX expression and formation of micronucleated cells. Cells expressing wild-type p53 were resistant to GA-induced G2 checkpoint abrogation. Notably, GA treatment decreased levels of G2 regulatory proteins Wee1 and Chk1, and inhibitory phosphorylation of Cdc2, independent of p53 status. Further investigation identified p21 as the potential downstream effector of p53 that mediates resistance to G2 checkpoint abrogation. Clonogenic survival studies demonstrated higher sensitivity to GA alone or combination IR plus GA treatment in p53 and p21-null cells. Collectively, these data demonstrate potential mechanisms through which HSP90 inhibition can enhance the effects of ionizing radiation in p53-compromised cancer cells. Combination IR plus HSP90 inhibitor therapies may be particularly useful in treating cancers that lack wild-type p53.Oncogene (2008) 27, 5567–5577; doi:10.1038/onc.2008.172; published online 26 May 2008 [ABSTRACT FROM AUTHOR]

The removal of DNA interstrand cross-links (ICLs) has proven to be notoriously complicated due to the involvement of multiple pathways of DNA repair, which include the Fanconi anemia/BRCA pathway, homologous recombination and components of the nucleotide excision and mismatch repair pathways. Members of the SNM1 gene family have also been shown to have a role in mediating cellular resistance to ICLs, although their precise function has remained elusive. Here, we show that knockdown of Snm1B/Apollo in human cells results in hypersensitivity to mitomycin C (MMC), but not to IR. We also show that Snm1B-deficient cells exhibit a defective S phase checkpoint in response to MMC, but not to IR, and this finding may account for the specific sensitivity to the cross-linking drug. Interestingly, although previous studies have largely implicated ATR as the major kinase activated in response to ICLs, we show that it is activation of the ATM-mediated checkpoint that is defective in Snm1B-deficient cells. The requirement for Snm1B in ATM checkpoint activation specifically after ICL damage is correlated with its role in promoting double-strand break formation, and thus replication fork collapse. Consistent with this result Snm1B was found to interact directly with Mus81-Eme1, an endonuclease previously implicated in fork collapse. In addition, we also show that Snm1B interacts with the Mre11-Rad50-Nbs1 (MRN) complex and with FancD2 further substantiating its role as a checkpoint/DNA repair protein.Oncogene (2008) 27, 5045–5056; doi:10.1038/onc.2008.139; published online 12 May 2008 [ABSTRACT FROM AUTHOR]

p53 plays critical roles in tumor suppression and the loss of its function is required for cancer progression. In this context, the p53 gene is the most commonly mutated tumor suppressor gene in human cancers. The majority of the p53 gene mutations in human cancers are missense mutations, leading to the expression of the full-length mutant p53 protein in cancer cells. In addition to the loss of tumor suppression activity, p53 cancer mutants gain new oncogenic activities to promote tumorigenesis and drug resistance. Recent studies have identified a novel gain-of-function of p53 cancer mutants in inducing genetic instability by inactivating critical tumor suppressors such as ATM. Genetic instability is a common mechanism by which cancer cells efficiently accumulate genetic mutations to promote their growth, survival and metastatic potential. Therefore, this gain-of-function of p53 cancer mutants could play important roles in tumorigenesis and drug resistance of various types of human cancers. In addition, because many cancer therapies such as radiation therapy suppress or kill cancer cells by activating ATM-dependent responses to DNA double-stranded break damage, elucidation of this gain-of-function of p53 cancer mutants will have important implications on cancer therapy.Oncogene (2008) 27, 3501–3507; doi:10.1038/sj.onc.1211023; published online 28 January 2008 [ABSTRACT FROM AUTHOR]

We used intrachromosomal substrates to directly monitor the effect of the cell cycle on the efficiency and the accuracy of nonhomologous end joining (NHEJ) in mammalian cells. We show that both KU and KU-independent (KU-alt) pathways are efficient when maintaining cells in G1/S, in G2/M or during dynamic progression through S phase. In addition, the accuracy of NHEJ is barely altered when the cells are blocked in G1/S or in G2/M. However, progression through S phase increases the frequency of deletions, which is a hallmark of the KU-alt pathway. Moreover, we show that the intermediates that are generated by the KU-dependent end joining of non-fully complementary ends, and which contain mismatches, nicks or gap intermediates, are less accurately processed when the cells progress through S phase. In conclusion, both KU and KU-alt processes are active throughout the cell cycle, but the repair is more error prone during S phase, both by increasing the mutagenic KU-alt pathway and decreasing the accuracy of the repair of the intermediates generated by the KU-dependent pathway.Oncogene (2008) 27, 1726–1736; doi:10.1038/sj.onc.1210807; published online 24 September 2007 [ABSTRACT FROM AUTHOR]

The recognition and repair of DNA double-strand breaks (DSBs) is a complex process that draws upon a multitude of proteins. This is not surprising since this is a lethal lesion if left unrepaired and also contributes to genome instability and the consequential risk of cancer and other pathologies. Some of the key proteins that recognize these breaks in DNA are mutated in distinct genetic disorders that predispose to agent sensitivity, genome instability, cancer predisposition and/or neurodegeneration. These include members of the Mre11 complex (Mre11/Rad50/Nbs1) and ataxia-telangiectasia (A-T) mutated (ATM), mutated in the human genetic disorder A-T. The mre11 (MRN) complex appears to be the major sensor of the breaks and subsequently recruits ATM where it is activated to phosphorylate in turn members of that complex and a variety of other proteins involved in cell-cycle control and DNA repair. The MRN complex is also upstream of ATM and ATR (A-T-mutated and rad3-related) protein in responding to agents that block DNA replication. To date, more than 30 ATM-dependent substrates have been identified in multiple pathways that maintain genome stability and reduce the risk of disease. We focus here on the relationship between ATM and the MRN complex in recognizing and responding to DNA DSBs.Oncogene (2007) 26, 7749–7758; doi:10.1038/sj.onc.1210880 [ABSTRACT FROM AUTHOR]

The ataxia–telangiectasia-mutated (ATM) protein kinase is rapidly and specifically activated in response to DNA double-strand breaks in eukaryotic cells. In this review, we summarize recent insights into the mechanism of ATM activation, focusing on the role of the Mre11/Rad50/Nbs1 (MRN) complex in this process. We also compare observations of the ATM activation process in different biological systems and highlight potential candidates for cellular factors that may participate in regulating ATM activity in human cells.Oncogene (2007) 26, 7741–7748; doi:10.1038/sj.onc.1210872 [ABSTRACT FROM AUTHOR]

Double-strand breaks (DSBs) are intermediates in several physiological processes including V(D)J and class switch recombination. They are also potent substrates for chromosomal translocations that arise as by-products of antigen receptor gene assembly in lymphocytes. ATM is one among several key proteins involved in the detection, signaling and repair of DNA breaks. Despite redundancies in DSB signaling pathways, it has recently been demonstrated that ATM deficient lymphocytes can survive and proliferate several generations in vitro and in vivo despite harboring terminally deleted chromosomes produced by V(D)J recombination. In this review, we discuss how two complementary genome maintenance functions mediated by ATM prevent lymphocytes from adapting to persistent DNA damage.Oncogene (2007) 26, 7759–7764; doi:10.1038/sj.onc.1210873 [ABSTRACT FROM AUTHOR]

Normal development of an organism requires the ability to respond to DNA damage. A particularly deleterious lesion is a DNA double-strand break (DSB). The cellular response to DNA DSBs occurs via an integrated sensing and signaling network that maintains genomic stability. The outcomes of defective DNA DSB repair are related to the developmental stage of an organism, and often show striking tissue specificity. Many human diseases are associated with deficiencies in DNA DSB repair and can be characterized by neuropathology, immune deficiency, growth retardation or predisposition to cancer. This review will focus on the requirements of the DNA DSB response that function to maintain homeostasis during mammalian development.Oncogene (2007) 26, 7799–7808; doi:10.1038/sj.onc.1210877 [ABSTRACT FROM AUTHOR]

Both DNA double- and single-strand break repair are highly coordinated processes utilizing signal transduction cascades and post-translational modifications such as phosphorylation, acetylation and ADP ribosylation. ‘Drugable’ targets within these networks have been identified that could potentially lead to novel therapeutic approaches within the oncology arena. Key regulators within these signalling cascades, such as DNA-dependent protein kinase, ataxia-telangiectasia mutated, checkpoint kinase 1 (CHK1), checkpoint kinase 2 (CHK2) and poly(ADP-ribose) polymerase, use either ATP or nicotinamide adenine dinucleotide for their enzymatic functions and are therefore readily accessible to small molecule inhibition at their catalytic sites. A range of highly potent and selective inhibitors of these DNA damage response pathways has now been identified through drug discovery efforts, with candidate molecules either approaching or already in clinical trials. This review will describe the small molecule inhibitors and drug discovery activities that focus on DNA break repair, along with the therapeutic rationale behind chemosensitization and the concept of synthetic lethality. We will also describe the emerging clinical data coming from this exciting new approach to targeted cancer therapy.Oncogene (2007) 26, 7816–7824; doi:10.1038/sj.onc.1210879 [ABSTRACT FROM AUTHOR]

The proteins responsible for radiation sensitive disorders, NBS1, kinase ataxia-telangiectasia-(A-T)-mutated (ATM) and MRE11, interact through the C-terminus of NBS1 in response to the generation of DNA double-strand breaks (DSBs) and are all implicated in checkpoint regulation and DSB repair, such as homologous recombination (HR). We measured the ability of several NBS1 mutant clones and A-T cells to regulate HR repair using the DR-GFP or SCneo systems. ATM deficiency did not reduce the HR repair frequency of an induced DSB, and it was confirmed by findings that HR frequencies are only slightly affected by deletion of ATM-binding site at the extreme C-terminus of NBS1. In contrast, The HR-regulating ability is dramatically reduced by deletion of the MRE11-binding domain at the C-terminus of NBS1 and markedly inhibited by mutations in the FHA/BRCT domains at the N-terminus. This impaired capability in HR is consistent with a failure to observe MRE11 foci formation. Furthermore, normal HR using sister chromatid was completely inhibited by the absence of FHA/BRCT domains. These results suggested that the N- and C-terminal domains of NBS1 are the major regulatory domains for HR pathways, very likely through the recruitment and retention of the MRE11 nuclease to DSB sites in an ATM-independent fashion.Oncogene (2007) 26, 6002–6009; doi:10.1038/sj.onc.1210428; published online 26 March 2007 [ABSTRACT FROM AUTHOR]

Chromatin modifications at core histones including acetylation, methylation, phosphorylation and ubiquitination play an important role in diverse biological processes. Acetylation of specific lysine residues within the N terminus tails of core histones is arguably the most studied histone modification; however, its precise roles in different cellular processes and how it is disrupted in human diseases remain poorly understood. In the last decade, a number of histone acetyltransferases (HATs) enzymes responsible for histone acetylation, has been identified and functional studies have begun to unravel their biological functions. The activity of many HATs is dependent on HAT complexes, the multiprotein assemblies that contain one HAT catalytic subunit, adapter proteins, several other molecules of unknown function and a large protein called TRansformation/tRanscription domain-Associated Protein (TRRAP). As a common component of many HAT complexes, TRRAP appears to be responsible for the recruitment of these complexes to chromatin during transcription, replication and DNA repair. Recent studies have shed new light on the role of TRRAP in HAT complexes as well as mechanisms by which it mediates diverse cellular processes. Thus, TRRAP appears to be responsible for a concerted and context-dependent recruitment of HATs and coordination of distinct chromatin-based processes, suggesting that its deregulation may contribute to diseases. In this review, we summarize recent developments in our understanding of the function of TRRAP and TRRAP-containing HAT complexes in normal cellular processes and speculate on the mechanism underlying abnormal events that may lead to human diseases such as cancer.Oncogene (2007) 26, 5358–5372; doi:10.1038/sj.onc.1210605 [ABSTRACT FROM AUTHOR]

Octamer transcription factor-1 (Oct-1) has recently been shown to function as a stress sensor that promotes cell survival subsequent to DNA damage. Here, we show that the survival signal imparted by Oct-1 following exposure to ionizing radiation (IR) is dependent upon DNA-dependent protein kinase (DNA-PK)-dependent phosphorylation of a cluster of 13 specific ser/thr residues within the N-terminal transcriptional regulatory domain of Oct-1. Although IR treatment did not affect the recruitment of Oct-1 to the histone H2B promoter, the recruitment of RNA polymerase II, TATA-binding protein and histone H4 acetylation were strongly reduced, consistent with a decrease in Oct-1 transcriptional regulatory potential following IR exposure. Ser/Thr-Ala substitution of 13 sites present in Oct-1 transcriptional regulatory domain eliminated Oct-1 phosphorylation subsequent to IR exposure. Further, these substitutions prevented Oct-1 from rescuing the survival of IR-treated Oct-1/ murine embryonic fibroblasts, providing a direct link between DNA-PK-dependent phosphorylation and the contribution of Oct-1 to cell survival. These results implicate Oct-1 as a primary effector in a DNA-PK-dependent cell survival pathway that is activated by double-stranded DNA breaks.Oncogene (2007) 26, 3980–3988. doi:10.1038/sj.onc.1210165; published online 8 January 2007 [ABSTRACT FROM AUTHOR]

The Werner syndrome protein (WRN) and chromatin assembly factor 1 (CAF-1) are both involved in the maintenance of genome stability. In response to DNA-damaging signals, both of these proteins relocate to sites where DNA synthesis occurs. However, the interaction between WRN and CAF-1 has not yet been investigated. In this report, we show that WRN interacts physically with the largest subunit of CAF-1, hp150, in vitro and in vivo. Although hp150 does not alter WRN catalytic activities in vitro, and the chromatin assembly activity of CAF-1 is not affected in the absence of WRN in vivo, this interaction may have an important role during the cellular response to DNA replication fork blockage and/or DNA damage signals. In hp150 RNA-mediated interference (RNAi) knockdown cells, WRN partially formed foci following hydroxyurea (HU) treatment. However, in the absence of WRN, hp150 did not relocate to form foci following exposure to HU and ultraviolet light. Thus, our results demonstrate that WRN responds to DNA damage before CAF-1 and suggest that WRN may recruit CAF-1, via interaction with hp150, to DNA damage sites during DNA synthesis.Oncogene (2007) 26, 3811–3822. doi:10.1038/sj.onc.1210150; published online 18 December 2006 [ABSTRACT FROM AUTHOR]

Ionizing radiation exposure results in the activation of several tyrosine kinase receptors that participate in radiation-induced DNA damage response and radioresistance. We previously showed that insulin-like growth factor 1 receptor (IGF-1R) inhibition enhanced radiosensitivity of non-small-cell lung cancer (NSCLC) cells. In this paper, we demonstrate that in U1810 NSCLC cells γ-radiation activates IGF-1R within 10 min, with a maximal activation effect 2 h post-irradiation. Impairment of IGF-1R tyrosine kinase activity enhances human lung cancer cells radiosensitivity by a mechanism that involves phosphatidylinositol 3-kinase (PI3-K) and p38 kinase. In an active form, IGF-1R binds and activates p38 kinase, promoting receptor signaling. Conversely, inhibition of IGF-1R phosphorylation results in IGF-1R/p38 complex disruption and p38 kinase inactivation. We have also demonstrated that in insulin-like growth factor-1-stimulated cells, Ku-DNA-binding activation is induced by ionizing radiation within 4 h, reaches a maximum level at 12 h and remains active up to 72 h. Blockade of IGF-1R activity or its downstream signaling through p38 kinase induces a decrease in radiation-mediated Ku-DNA-binding activation and downregulates the level of Ku86, without affecting Ku70 expression in the nucleus of U1810 cells. The IGF-1R signaling via PI3-K does not interfere with the p38 signaling, the Ku-DNA-binding activity or the level of Ku86. Our present study demonstrates for the first time that ionizing radiation activates IGF-1R. Inhibition of IGF-1R signaling via p38 kinase induces radiosensitivity by a novel mechanism involving nuclear Ku86.Oncogene (2007) 26, 2423–2434. doi:10.1038/sj.onc.1210037; published online 9 October 2006 [ABSTRACT FROM AUTHOR]

The inactive transcription factor NF-κB is localized in the cytoplasm and rapidly responds to a variety of extracellular factors and intracellular stress conditions to initiate multiple cellular responses. While the knowledge regarding NF-κB signaling pathways initiated by extracellular ligands is rapidly expanding, the mechanisms of activation by intracellular stress conditions are not well understood. We recently described a critical role for a small ubiquitin-like modifier (SUMO) modification of NF-κB essential modulator (NEMO), the regulatory subunit of the IκB kinase, in response to certain genotoxic stress conditions. One important unanswered question is whether the role of this modification is limited to the genotoxic agents or some other signaling pathways also employ SUMOylation of NEMO to regulate NF-κB activation. Here, we report that a variety of other stress conditions, including oxidative stress, ethanol exposure, heat shock and electric shock, also induce NEMO SUMOylation, thus demonstrating that DNA damage per se is not necessary for this NEMO modification to occur. Moreover, combinations of certain SUMO stress and ATM (ataxia telangiectasia mutated) activation conditions lead to NF-κB activation without inducing DNA damage. Our study helps to conceptualize how individual or a combination of different stress conditions may funnel into this previously unappreciated signal transduction mechanism to regulate the activity of the ubiquitous NF-κB transcription factor.Oncogene (2007) 26, 641–651. doi:10.1038/sj.onc.1209815; published online 24 July 2006 [ABSTRACT FROM AUTHOR]

Reciprocal chromosomal translocations may arise as a result of unfaithful repair of spontaneous DNA double-strand breaks, most probably induced by oxidative stress, radiation, genotoxic chemicals and/or replication stress. Genes encoding tyrosine kinases are targeted by these mechanisms resulting in the generation of chimera genes encoding fusion tyrosine kinases (FTKs). FTKs display transforming activity owing to their constitutive kinase activity causing deregulated proliferation, apoptosis, differentiation and adhesion. Moreover, FTKs are able to facilitate DNA repair, prolong activation of G2/M and S cell cycle checkpoints, and elevate expression of antiapoptotic protein Bcl-XL, making malignant cells less responsive to antitumor treatment. FTKs may also stimulate the generation of reactive oxygen species and enhance spontaneous DNA damage in tumor cells. Unfortunately, FTKs compromise the fidelity of DNA repair mechanisms, which contribute to the accumulation of additional genetic abnormalities leading to the resistance to inhibitors such as imatinib mesylate and malignant progression of the disease.Oncogene (2007) 26, 11–20. doi:10.1038/sj.onc.1209756; published online 19 June 2006 [ABSTRACT FROM AUTHOR]

Ku80 maintains the genome by repairing DNA double-strand breaks (DSBs) through nonhomologous end joining (NHEJ), a pathway that repairs nonspecific DSBs and Rag-1 Rag-2 (Rag)-specific DSBs. As a result, Ku80 deletion results in phenotypes characteristic of defective repair for both nonspecific DSBs (γ-radiation hypersensitivity and genomic instability) and Rag-specific DSBs (immunodeficiency). ku80−/− mice also exhibit neuronal apoptosis, but we do not know the type of DSBs responsible for this response. In spite of genomic instability and immunodeficiency, cancer incidence is not increased in ku80−/− mice. However, deletion of the tumor suppressor, p53 greatly increases pro-B-cell lymphoma in ku80−/− mice due to IgH/c-Myc translocations suggesting that responses to Rag-specific DNA DSBs suppress cancer. Like suppression of pro-B-cell lymphoma, neuronal apoptosis requires p53 presenting the intriguing possibility that Rag-specific DSBs mediate neuronal development as they do lymphocyte development. Here we delete Rag-1 from ku80−/−p53−/− mice to differentiate the impact nonspecific vs Rag-specific DSBs have on ku80−/− mice. We find that deleting Rag-1 prevents pro-B cell lymphoma confirming Rag-induced DSBs induce this form of cancer. Both the triple mutant mice and the p53−/−rag-1−/− mice exhibit T-cell lymphoma and medulloblastoma; incidence of T-cell lymphoma is the same for both cohorts whereas incidence of medulloblastoma is higher for the triple-mutant cohort. Thus, p53-mediated neuronal apoptosis likely suppresses medulloblastoma in Ku80-deleted mice and Ku80 likely suppresses medulloblastoma by repairing nonspecific DNA DSBs instead of Rag-specific DSBs. Our observations are the first to show that Ku80 suppresses cancer caused by nonspecific DNA damage and we present a novel mouse model for medulloblastoma.Oncogene (2006) 25, 7159–7165. doi:10.1038/sj.onc.1209704; published online 5 June 2006 [ABSTRACT FROM AUTHOR]

The BRCA1 and BRCA2 proteins are important in maintaining genomic stability by promoting efficient and precise repair of double-strand breaks. The main role of BRCA2 appears to involve regulating the function of RAD51 in the repair by homologous recombination. BRCA1 has a broader role upstream of BRCA2, participating in various cellular processes in response to DNA damage. The DNA repair defect associated with mutations in BRCA1 or BRCA2 could be exploited to develop new targeted therapeutic approaches for cancer occurring in mutation carriers.Oncogene (2006) 25, 5864–5874. doi:10.1038/sj.onc.1209874 [ABSTRACT FROM AUTHOR]

Common fragile sites are specific regions of the genome that form gaps and breaks on metaphase chromosomes when DNA synthesis is partially inhibited. Fragile sites and their associated genes show frequent deletions and other rearrangements in cancer cells, and may be indicators of DNA replication stress early in tumorigenesis. We have previously shown that the DNA damage response proteins ATR, BRCA1 and FANCD2 play critical roles in maintaining the stability of fragile site regions. To further elucidate the pathways regulating fragile site stability, we have investigated the effects of depletion of the cell cycle checkpoint kinases, CHK1 and CHK2 on common fragile site stability in human cells. We demonstrate that both CHK1 and CHK2 are activated following treatment of cells with low doses of aphidicolin that induce fragile site breakage. Furthermore, we show that depletion of CHK1, but not CHK2, using short-interfering RNA (siRNA) leads to highly destabilized chromosomes and specific common fragile site breakage. In many cells, CHK1 depletion resulted in extensive chromosome fragmentation, which was distinct from endonucleolytic cleavage commonly associated with apoptosis. These findings demonstrate a critical role for the CHK1 kinase in regulating chromosome stability, and in particular, common fragile site stability.Oncogene (2006) 25, 4381–4388. doi:10.1038/sj.onc.1209466; published online 29 May 2006 [ABSTRACT FROM AUTHOR]

Histone deacetylase (HDAC) inhibitors are emerging as a new class of targeted cancer chemotherapeutics. Several HDAC inhibitors are currently in clinical trials and promising anticancer effects at well-tolerated doses have been observed for both hematologic and solid cancers. HDAC inhibitors have been shown to induce cell-cycle and growth arrest, differentiation and in certain cases apoptosis in cell cultures and in vivo. However, it is known that these compounds induce varying responses in different cells and biological settings, and identifying their precise mechanisms of action is an area of great interest. Important findings are continually expanding our understanding of the cellular effects of HDAC inhibitors and recent studies will be briefly outlined in this review. In addition to their intrinsic anticancer properties, numerous studies have demonstrated that HDAC inhibitors can modulate cellular responses to other cytotoxic modalities including ionizing radiation, ultraviolet radiation and chemotherapeutic drugs. Hence, there is a growing interest in potential clinical use of HDAC inhibitors in combination with conventional cancer therapies. In this review, the interaction of HDAC inhibitors with other anticancer agents is discussed. The focus of the article is on the different mechanisms by which HDAC inhibitors enhance the sensitivity of cells to the effects of ionizing radiation.Oncogene (2006) 25, 3885–3893. doi:10.1038/sj.onc.1209417; published online 6 February 2006 [ABSTRACT FROM AUTHOR]

Mutations in the NF1 tumor-suppressor gene underlie neurofibromatosis type 1 (NF1), in which patients are predisposed to certain tumors such as neurofibromas and may associate with vascular disorder. Plexiform neurofibromas are slow growing benign tumors that are highly vascular and can progress to malignancy. The development of neurofibromas requires loss of both Nf1 alleles in Schwann cells destined to become neoplastic and may be exacerbated by Nf1 heterozygosity in other non-neoplastic cells. This study tested the hypothesis that Nf1 heterozygosity exaggerates angiogenesis. We found that Nf1 heterozygous mice showed increased neovascularization in both the retina and cornea in response to hypoxia and bFGF, respectively, compared to their wild-type littermates. The increase in corneal neovascularization was associated with heightened endothelial cell proliferation and migration, and increased infiltration of inflammatory cells. In addition, Nf1 heterozygous endothelial cell cultures showed an exaggerated proliferative response to angiogenic factors, particularly to bFGF. These findings support the conclusion that Nf1 heterozygosity in endothelial cells and perhaps inflammatory cells augments angiogenesis, which may promote neurofibroma formation in NF1.Oncogene (2006) 25, 2297–2303. doi:10.1038/sj.onc.1209264; published online 14 November 2005 [ABSTRACT FROM AUTHOR]

DNA viruses promote cell cycle progression, stimulate unscheduled DNA synthesis, and present the cell with an extraordinary amount of exogenous DNA. These insults elicit vigorous responses mediated by cellular factors that govern cellular homeostasis. To ensure productive infection, adenovirus has developed means to inactivate these intracellular antiviral responses. Among the challenges to the host cell is the viral DNA genome, which is viewed as DNA damage and elicits a cellular response to inhibit replication. Adenovirus therefore encodes proteins that dismantle the cellular DNA damage machinery. Studying virus–host interactions has yielded insights into the molecular functioning of fundamental cellular mechanisms. In addition, it has suggested ways that viral cytotoxicity can be exploited to offer a selective means of restricted growth in tumor cells as a therapy against cancer. In this review, we discuss aspects of the intracellular response that are unique to adenovirus infection and how adenoviral proteins produced from the early region E4 act to neutralize antiviral defenses, with a particular focus on DNA damage signaling.Oncogene (2005) 24, 7686–7696. doi:10.1038/sj.onc.1209063 [ABSTRACT FROM AUTHOR]

Ten years ago, it was observed that the Akt kinase is activated by phosphorylation via a phosphoinositide 3-kinase (PI-3K)-dependent process. This discovery generated enormous interest because it provided a link between PI-3K, an enzyme known to play a critical role in cellular physiology, and its downstream targets. Subsequently, it was shown that the activity of the core components of the ‘PI-3K/Akt pathway’ is modulated by a complex network of regulatory proteins and pathways. Some of the Akt-binding partners modulate its activation by external signals by interacting with different domains of the Akt protein. This review focuses on the Akt interacting proteins and the mechanisms by which they regulate Akt activation.Oncogene (2005) 24, 7401–7409. doi:10.1038/sj.onc.1209099 [ABSTRACT FROM AUTHOR]

Aberrant genome-wide hypomethylation is thought to be related to tumorigenesis by promoting genomic instability. Since DNA methylation is considered an important mechanism for the silencing of retroelements, hypomethylation in human tumors may lead to their reactivation. However, the role of DNA hypomethylation in chronic myeloid leukemia (CML) remains to be elucidated. In this study, the methylation status of the LINE-1 (L1) retrotransposon promoter was analysed in CML samples from the chronic-phase (CP, n=140) and the blast crisis (BC, n=47). L1 hypomethylation was significantly more frequent in BC (74.5%) than in CP (38%) (P<0.0001). Furthermore, L1 hypomethylation led to activation of both ORF1 sense transcription (P<0.0001) and c-MET gene antisense transcription (P<0.0001), and was significantly associated with high levels of BCR–ABL (P=0.02) and DNMT3b4 (P=0.001) transcripts. Interestingly, in CP-CML, extensive L1 hypomethylation was associated with poorer prognosis in terms of cytogenetic response to interferon (P=0.004) or imatinib (P=0.034) and progression-free survival (P=0.005). The above results strongly suggest that activation of both sense and antisense transcriptions by aberrant promoter hypomethylation of the L1 elements plays a role in the progression and clinical behavior of the CML.Oncogene (2005) 24, 7213–7223. doi:10.1038/sj.onc.1208866; published online 19 September 2005 [ABSTRACT FROM AUTHOR]

Nonhomologous end-joining (NHEJ) of DNA double-strand breaks (DSBs) entails two principal mechanisms: modification of DNA ends prior to ligation (error-prone rejoining) or precise ligation without modification if the DNA ends are complementary (error-free repair). Error-prone rejoining is mutagenic, because it can lead to destruction of coding sequence or to chromosomal aberrations, and therefore must be tightly regulated. Previous studies on the role of the p53 tumor suppressor in the regulation of NHEJ have yielded conflicting results, but a rigorous analysis of NHEJ proficiency and fidelity in a purely chromosomal context has not been carried out. To this end, we created novel repair plasmid substrates that integrate into the genome. DSBs generated by the I-SceI endonuclease within these substrates were repaired by either error-prone rejoining or precise ligation. We found that the expression of wild-type p53 inhibited any repair-associated DNA sequence deletion, including a more than 250-fold inhibition of error-prone rejoining events compared to p53-null cells, while any promoting effect of p53 on precise ligation could not be directly evaluated. The role of p53 in NHEJ appeared to involve a direct transactivation-independent mechanism, possibly restricting DNA end-modification by blocking the annealing of single strands along flanking stretches of microhomology. The inhibition of error-prone rejoining by p53 did not apply to the rejoining of DSBs induced by ionizing radiation. In conclusion, our data suggest that p53 restricts the mutagenic effects of NHEJ without compromising repair proficiency or cell survival, thereby maintaining genomic stability.Oncogene (2005) 24, 1663-1672. doi:10.1038/sj.onc.1208396 Published online 31 January 2005 [ABSTRACT FROM AUTHOR]

Although PS-341 (bortezomib) is a promising agent to improve multiple myeloma (MM) patient outcome, 65%of patients with relapsed and refractory disease do not respond. We have previously shown that heat shock protein (Hsp)27 is upregulated after PS-341 treatment, that overexpression of Hsp27 confers PS-341 resistance, and that inhibition of Hsp27 overcomes PS-341 resistance. Since Hsp27 is a downstream target of p38 mitogen-activated protein kinase (MAPK)/MAPK-mitogen-activated protein kinase-2 (MAPKAPK2), we hypothesized that inhibition of p38 MAPK activity could augment PS-341 cytotoxicity by downregulating Hsp27. Although p38 MAPK inhibitor SCIO-469 (Scios Inc, CA, USA) alone did not induce significant growth inhibition, it blocked baseline and PS-341-triggered phosphorylation of p38 MAPK as well as upregulation of Hsp27, associated with enhanced cytotoxicity in MM.1S cells. Importantly, SCIO-469 enhanced phosphorylation of c-Jun NH2-terminal kinase (JNK) and augmented cleavage of caspase-8 and poly(ADP)-ribose polymerase. Moreover, SCIO-469 downregulated PS-341-induced increases in G2/M-phase cells, associated with downregulation of p21Cip1 expression. Importantly, SCIO-469 treatment augmented cytotoxicity of PS-341 even against PS-341-resistant cell lines and patient MM cells. These studies therefore provide the framework for clinical trials of SCIO-469 to enhance sensitivity and overcome resistance to PS-341, thereby improving patient outcome in MM.Oncogene (2004) 23, 8766-8776. doi:10.1038/sj.onc.1208118 Published online 11 October 2004 [ABSTRACT FROM AUTHOR]

DNA interstrand crosslinks (ICLs) are critical lesions for the mammalian cell since they affect both DNA strands and block transcription and replication. The repair of ICLs in the mammalian cell involves components of different repair pathways such as nucleotide-excision repair and the double-strand break/homologous recombination repair pathways. However, the mechanistic details of mammalian ICL repair have not been fully delineated. We describe here the complete coding sequence and the genomic organization of hSNM1B, one of at least three human homologs of the Saccharomyces cerevisiae PSO2 gene. Depletion of hSNM1B by RNA interference rendered cells hypersensitive to ICL-inducing agents. This requirement for hSNM1B in the cellular response to ICL has been hypothesized before but never experimentally verified. In addition, siRNA knockdown of hSNM1B rendered cells sensitive to ionizing radiation, suggesting the possibility of hSNM1B involvement in homologous recombination repair of double-strand breaks arising as intermediates of ICL repair. Monoubiquitination of FANCD2, a key step in the FANC/BRCA pathway, is not affected in hSNM1B-depleted HeLa cells, indicating that hSNM1B is probably not a part of the Fanconi anemia core complex. Nonetheless, similarities in the phenotype of hSNM1B-depleted cells and cultured cells from patients suffering from Fanconi anemia make hSNM1B a candidate for one of the as yet unidentified Fanconi anemia genes not involved in monoubiquitination of FANCD2.Oncogene (2004) 23, 8611-8618. doi:10.1038/sj.onc.1207895 Published online 4 October 2004 [ABSTRACT FROM AUTHOR]