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The special AT-rich DNA binding protein (SATB2) is a nuclear matrix-associated protein and an important transcription factor for biological development, gene regulation and chromatin remodeling. Aberrant regulation of SATB2 has been found to highly correlate with various types of cancers including lung, colon, prostate, breast, gastric and liver. Recent studies have revealed that a subset of small non-coding RNAs, termed microRNAs (miRNAs), are important regulators of SATB2 function. As post-transcriptional regulators, miRNAs have been found to have fundament importance maintaining normal cellular development. Evidence suggests that multiple miRNAs, including miR-31, miR-34, miR-182, miR-211, miR-599, are capable of regulating SATB2 in cancers of the lung, liver, colon and breast. This review examines the molecular functions of SATB2 and miRNAs in the text of cancer development and potential strategies for cancer therapy with a focus on systemic miRNA delivery.

Cadmium is highly toxic to almost all forms of life. It is released into the environment through natural and anthropogenic activities. Natural sources originate from volcanic activity, forest fires, fossil fuel burning, and soil particles contaminated with cadmium (Joseph, 2009). Cadmium is extensively used in industries such as nickel-cadmium battery manufacturing, electroplating, welding, smelting and refining, pigments, and plastic stabilizers (Joseph, 2009). Inhalation through occupational exposure, cigarette smoking, and indoor inhalable particles contaminated with cadmium is one of the main routes of exposure to cadmium in humans, with more than 90% of a dose being absorbed through the lung after inhalation of cadmium (Nawrot et al., 2010; Waalkes, 2003). Both epidemiological and experimental studies have identified cadmium as a lung carcinogen in humans (IARC, 2012). Due to persistence in the environment and a long biological half-life in humans, cadmium has always been a serious public health concern. Even though much work has been carried out to elucidate the molecular mechanisms of cadmium-induced carcinogenesis, the exact mechanisms still remain unclear. The ability of cadmium to induce gene mutations in bacteria was limited (Beyersmann and Hartwig, 1994), and no clear association between cadmium exposure and cytogenetic endpoint in humans has been found (Verougstraete et al., 2002), both of which implied that cadmium-induced carcinogenesis may be mediated through nongenotoxic or indirect genotoxic mechanisms (Beyersmann and Hechtenberg, 1997; Bolognesi et al., 1999). It has been proposed that epigenetic mechanisms may play a role in cadmium-induced carcinogenesis (Waalkes, 2003). Over the last decade or two, accumulating evidences have shown that cadmium is able to alter DNA methylation both at the global and gene-specific levels, which may play a role in carcinogenesis. Takiguchi et al. (2003) reported that after 10 weeks of exposure to cadmium, TRL1215 rat liver cells showed indications of transformation and significant increases in genomic DNA methylation and DNA methyltransferase (DNMT) activity. In cadmium-transformed human prostate epithelial RWPE-1 cells, the tumor suppressor genes RASSF1A and p16 were inactivated due to DNA hypermethylation at their promoter regions (Benbrahim-Tallaa et al., 2007). Another study showed that chronic exposure to cadmium in human embryo lung fibroblast cells resulted in increases in global DNA methylation and DNMTs activities (Jiang et al., 2008). A recent study reported that in cadmium-transformed human bronchial epithelial (16HBE) cells, global DNA methylation, and DNA methylation at the promoter regions of DNA repair genes (hMSH2, ERCC1, XRCC1, and hOGG1) were increased (Zhou et al., 2012). These findings suggested that cadmium disrupts DNA methylation, which may be involved in cadmium-induced carcinogenesis. To date, much of the work has been focused on DNA methylation. Whether cadmium induces aberrant histone methylation has yet to be investigated. In general, DNA methylation may act as a template for some histone modifications following DNA replication but more likely histone methylation can aid in directing DNA methylation patterns (Cedar and Bergman, 2009). Therefore, it’s likely that cadmium is able to induce aberrant histone methylation. Histone H3 on lysine 4 (H3K4) and H3K9 are the regular sites of lysine methylations, both of which can be mono-, di-, or trimethylated. In general, trimethylated H3K4 (H3K4me3) is always found at the promoter regions of transcriptionally activated genes (Santos-Rosa et al., 2002), whereas dimethylated H3K9 (H3K9me2) is located in the regulatory regions of transcriptionally silent genes (Rice et al., 2003). Aberrant modifications of H3K4me3 and H3K9me2 have been found to be closely associated with carcinogenesis. H3K4me3 has been found to be increased at the promoter region of MT-3 in cadmium-transformed human urothelial cells compared with that in parental human urothelial cells (Somji et al., 2011). Activation of H3K4me3 has also been found to be associated with overexpression of LAMB3 and LAMC2 genes in gastric cancer cell line, which may play an important role in gastric carcinogenesis (Kwon et al., 2011). Gain of H3K9me2 has been observed in silencing RASSF1A in prostate cancer (Kawamoto et al., 2007). However, to the best of our knowledge, no one has investigated the effects of cadmium on global H3K4me3 and H3K9me2 and their potential roles in cadmium-induced carcinogenesis. In this present study, we investigated the effects of cadmium on global H3K4me3 and H3K9me2 in immortalized normal human bronchial epithelial (BEAS-2B) cells and whether histone demethylases played a role in cadmium-induced histone modifications. BEAS-2B cells are similar to normal human lung cells in characteristics and cellular responses to carcinogens (Jing et al., 2012; Son et al., 2012; Wang et al., 2011), thus they are often used to establish the model of cell transformation. Since cell transformation assay is considered as a predictive test for carcinogenicity (Barrett et al., 1984), we further studied whether cadmium modulated global H3K4me3 and H3K9me2 during cadmium-induced transformation of BEAS-2B cells. Our work would further contribute to the understanding of the mechanisms of cadmium-induced carcinogenesis.

The special AT-rich sequence-binding proteins 1 and 2 (SATB1/2) are nuclear matrix associated proteins that are transcription factors involved in chromatin remodeling and gene regulation. Expression of the SATB2 gene is tissue-specific, and the only epithelial cells expressing SATB2 are the glandular cells of the lower gastrointestinal tract where its expression is regulated by microRNA-31 (miR-31) and miR-182. SATB2, along with its homolog SATB1, are thought to be involved in various cancers with their roles in this disease being specific to the type of cancer. Colorectal cancer (CRC) provides the largest association of SATB2 with cancer and the roles of SATB2 are better defined and more studied in CRC than in any other cancer type. SATB1 displays a negative association with SATB2 in CRC. The various studies that have investigated the involvement of SATB1 and 2 in CRC have produced consistent findings. Here, we form four major conclusions regarding the role of these proteins in CRC and their potential clinical value: (i) SATB2 is a sensitive marker to distinguish CRC from other cancer types, (ii) Reduced expression of SATB2 in CRC is associated with poor prognosis, (iii) High levels of SATB1 expression facilitate CRC and are associated with poor prognosis and (iv) Overexpression of miR-31 and -182 in CRC leads to more aggressive cancer. This review will describe several of the key investigations that established these conclusions and highlight results that offer opportunities for future research in the treatment and diagnosis of CRC.

Epigenetic silencing of tumor suppressor genes commonly occurs in human cancers via increasing DNA methylation and repressive histone modifications at gene promoters. However, little is known about how pathogenic environmental factors contribute to cancer development by affecting epigenetic regulatory mechanisms. Previously, we reported that both hypoxia and nickel (an environmental carcinogen) increased global histone H3 lysine 9 methylation in cells through inhibiting a novel class of iron- and α-ketoglutarate-dependent histone demethylases. Here, we investigated whether inhibition of histone demethylase JMJD1A by hypoxia and nickel could lead to repression/silencing of JMJD1A-targeted gene(s). By using Affymetrix GeneChip and ChIP-on-chip technologies, we identified Spry2 gene, a key regulator of receptor tyrosine kinase/extracellular signal-regulated kinase (ERK) signaling, as one of the JMJD1A-targeted genes in human bronchial epithelial BEAS-2B cells. Both hypoxia and nickel exposure increased the level of H3K9me2 at the Spry2 promoter by inhibiting JMJD1A, which probably led to a decreased expression of Spry2 in BEAS-2B cells. Repression of Spry2 potentiated the nickel-induced ERK phosphorylation, and forced expression of Spry2 in BEAS-2B cells decreased the nickel-induced ERK phosphorylation and significantly suppressed nickel-induced anchorage-independent growth. Taken together, our results suggest that histone demethylases could be targets of environmental carcinogens and their inhibition may lead to altered gene expression and eventually carcinogenesis.

Arsenic (As) is a well-characterized human carcinogen but is generally not mutagenic. The evidence that As induces both loss of global DNA methylation and gene promoter DNA hypermethylation has suggested that epigenetic mechanisms may play an important role in As-induced carcinogenesis. In the present study, we examined the change in histone methylation by As exposure. In human lung carcinoma A549 cells, exposure to inorganic trivalent As (arsenite) increased H3K9 dimethylation (H3K9me2) and decreased H3K27 trimethylation (H3K27me3), both of which represent gene silencing marks, while increasing the global levels of the H3K4 trimethylation (H3K4me3), a gene-activating mark. The increase in H3K9me2 was mediated by an increase in the histone methyltransferase G9a protein and messenger RNA levels. We also observed strikingly significant altered histone modifications induced by very low-dose (0.1 microM) arsenite. Taken together, these results suggest a potential mechanism by which As induces carcinogenesis through the alteration of specific histone methylations that represent both gene silencing and activating marks. Furthermore, these marks are known to affect DNA methylation, and it is likely that arsenic's effect is not limited to histone modifications alone, but extends, perhaps by them, to DNA methylations as well. Future studies in our laboratory will address the genomic location of these silencing and activating marks using ChIP-on-chip technology.

Activation of the signaling pathways leading to gene expression regulation is critical in the carcinogenic effects of nickel exposure. In the present study, we found nickel exposure could induce cyclooxygenase-2 (COX-2) expression at transcriptional and protein levels in both human bronchoepithelial cells (Beas-2B) and murine embryonic fibroblasts (MEFs). We further provided direct evidence for the specific involvement of the JNK1 signaling pathway in the COX-2 induction using specific gene knockout approaches. Our results demonstrated that COX-2 induction by nickel was impaired in JNK1(-/-) MEFs, but not in JNK2(-/-) MEFs. Moreover, re-constitutional expression of JNK1 restored COX-2 induction, confirming the specific requirement of JNK1 in COX-2 induction. Further investigation revealed that JNK1 mediated the nickel-induced COX-2 expression in a c-Jun/AP-1-dependent manner. Ectopic expression of TAM67, a c-Jun dominant negative mutant, also suppressed the COX-2 induction. Our results demonstrate that the JNK1/c-Jun/AP-1 pathway, but not the JNK2 pathway, plays a critical role in nickel-induced COX-2 expression.

Although it has been well established that insoluble nickel compounds are potent carcinogens and soluble nickel compounds are less potent, the mechanisms remain unclear. Nickel compounds are weakly mutagenic, but cause epigenetic effects in cells. Previous studies have shown that insoluble nickel compounds enter cells by phagocytosis and silence gene expression, but the entry of soluble nickel compounds and their effects on gene silencing have not been well studied. Here, we have demonstrated, using a dye that fluoresces when nickel ions bind, that soluble nickel compounds were taken up by cells. Nickel ions localized initially in the cytoplasm, but later entered the nucleus and eventually silenced a transgene. In addition, we described three major changes in histone modification of cells exposed to soluble nickel compounds: (i) loss of acetylation of H2A, H2B, H3 and H4; (ii) increases of H3K9 dimethylation; and (iii) substantial increases of the ubiquitination of H2A and H2B. These effects were observed at nickel exposure conditions that had minimum effects on cell cytotoxicity. Moreover, we demonstrated that nickel-induced transgene silencing was associated with similar changes of histone modifications in their nuclesomes. This study is the first to show that nickel compounds increase histone ubiquitination in cells. These new findings will further our understanding of the epigenetic mechanisms of nickel-mediated carcinogenesis.

Hexavalent chromium [Cr(VI)] is a known etiological factor in human lung cancer. Cr(VI) exposure-related lung cancer has a high mutation incidence in the p53 gene. Upon intake in human cells Cr(VI) is reduced to Cr(III), which is able to conjugate with amino acids and consequently form either binary Cr(III)-DNA or ternary Cr(III)-amino acid-DNA adducts. Both binary and ternary Cr(III)-DNA adducts are mutagenic. We have found that the Escherichia coli nucleotide excision enzyme UvrABC nuclease is able to incise Cr(III)- and Cr(III)-histidine-modified plasmid DNA and the extent of incision is proportional to the amount of Cr(III)-DNA adducts in the plasmid. In order to determine the role of Cr(III)-DNA adducts in the mutagenesis of the p53 gene in human cancer using the UvrABC nuclease incision method, we have mapped the Cr(III)-DNA distribution in PCR DNA fragments amplified from exons 5, 7 and 8 of the p53 gene. We have found that the sequence specificities of Cr(III)-DNA and Cr(III)-histidine-DNA adducts in the p53 gene sequence are identical and that both types of adducts are preferentially formed at -NGG- sequences, including codons 245, 248 and 249, the mutational hotspots in human lung cancer. It has been found that Cr(III)-DNA adducts induce mainly G to T mutations. Therefore, these results suggest that Cr(III)-DNA adduct formation contributes to the p53 gene mutations in lung carcinogenesis.

Chromium(VI) [Cr(VI)] is a ubiquitous environmental and industrial contaminant. Cr(VI) exposure is strongly associated with a higher incidence of human lung cancer, but the mechanism of Cr(VI) carcinogenicity remains unclear. Cigarette smoking has been known as the prominent cause of lung cancer, and polycyclic aromatic hydrocarbons (PAHs), the major carcinogens in cigarette smoke, have been suggested as being responsible for the initiation and development of lung cancer. It has been reported that lung cancer from workers exposed to Cr(VI) has a high percentage of G to T transversion mutations in the non-transcribed strand of the p53 gene, a hallmark of PAH-induced mutation. Cr(VI) is a weak mutagen although it can induce a high percentage of G to T transversion mutations. These results raise the possibility that Cr(VI) may enhance PAH binding at the p53 gene in lung tissue. To test this possibility, we have determined the effect of Cr(VI) exposure on benzo[a]pyrene diol epoxides (BPDE)-DNA binding at total genomic DNA level and at the p53 gene in normal human lung fibroblast cells. We found that in lung cells Cr(VI) pre-exposure does not affect the BPDE-DNA binding at the total genomic DNA level or at exons 5, 6 and 9 of the p53 gene; however, it greatly enhances BPDE-DNA binding at exons 7 and 8 of the p53 gene, especially at mutational hotspots of lung cancer: codons 248, 273 and 282 of the p53 gene. No enhancement of BPDE-DNA binding in the p53 was observed when naked genomic DNA isolated from Cr(VI)-exposed cells was modified with BPDE in vitro. These results suggest that Cr(VI) exposure may enhance chromatin structure-dependent carcinogen-DNA binding. This effect may contribute to the synergism of Cr(VI) and BPDE on mutagenesis and cell transformation, and may also contribute to the higher incidence of lung cancer in Cr(VI)-exposed populations.

7,8-Dihydro-8-oxo-2'-deoxyguanosine (8-oxo-dG) was measured as an indicator of nickel-induced oxidative base damage in the presence of H 2 O 2 . Heterochromatic proteins isolated from Chinese hamster liver cells enhanced the formation of 8-oxo-dG induced by NiCl 2 and H 2 O 2 in vitro, whereas euchromatic proteins inhibited this reaction. The inhibitory effect of euchromatic proteins on dG oxidation may be due to the oxygen radical scavenging effects of low molecular weight protein-rich fractions. Gel electrophoresis confirmed that histone H 1 was present at a higher concentration in heterochromatin than in euchromatin. It is believed that the presence of nickel-protein complexes in cells is crucial for the formation of reactive oxygen species (ROS). We found that Ni 2+ binds to histone H 1 and core histones as determined by 63 Ni autoradiography of proteins on nitrocellulose membranes. In vitro studies showed that commercially purified histone H 1 , and to a considerably lesser extent core histones, enhanced the NiCl 2 and H 2 O 2 catalyzed formation of 8-oxo-dG in a reaction containing free dG base. Since histone H 1 is a lysine- and alanine-rich protein, the levels of 8-oxo-dG induced by NiCl 2 and H 2 O 2 were studied in the presence of these amino acids and found to be enhanced by them. These results suggest that nickel may specifically produce oxidative DNA damage in heterochromatin because of the nature of its binding to histone H 1 and core histones. This selective oxidation of genetically inactive heterochromatin may explain why nickel compounds which generate oxygen radicals and oxidize DNA bases are inactive in most gene mutation assays.

Chinese hamster ovary cells were incubated with radioactive amino acids, the DNA was isolated by standard proteinase K/phenol/chloroform extraction and residual amino acids complexed to the DNA were examined as an index of metal induced DNA-protein crosslinks. Using this method, both chromate and nickel caused residual histidine and cysteine to be complexed with the DNA isolated from metal-treated cells. In the case of chromate, a number of amino acids were studied and Tyr, Thr and Cys were found to be complexed to DNA at a level (above the untreated control) that was statistically significant. Stability studies indicated that some of the chromate-induced DNA-protein complexes were mediated by direct participation of chromium(III), whereas others that were resistant to dissociation by EDTA and mercaptoethanol did not seem to involve direct chromium(III) participation. A significant portion of the cysteine complexed to DNA by chromate was believed to involve glutathione since treatment of cells with cycloheximide did not decrease chromate-induced cysteine-DNA crosslinks. In the case of nickel, most of the stable DNA-protein crosslinks did not involve direct metal participation and were probably oxidatively mediated by Ni(II)/Ni(III) redox cycling. These findings present new methodology for analysis of DNA-protein crosslinks by examination of residual amino acids associated with the DNA. This method should be highly sensitive and will yield important information about the mechanism of metal-induced DNA-protein crosslinks.

Cellular senescence is the genetically programmed cessation of cellular proliferation. We have recently mapped a putative senescence gene(s) on the X chromosome of Chinese hamster embryo (CHE) cells. In the present study, we have utilized microcell-mediated chromosome transfer (microcell fusion) to test whether: (i) the human X chromosome exhibits similar genetic potential to induce senescence and (ii) the deletion or inactivation of the X-linked senescence gene(s) in CHE cells is associated with nickel-induced immortalization. A normal CHE or human X chromosome was first introduced into mouse-cell hybrids, then transferred by microcell fusion into a nickel-transformed, immortal male CHE cell line (Ni-2/TGR) with an X deletion (Xq1). Microcell fusion of the normal CHE X chromosome into tumorigenic Ni-2/TGR cells yielded senescence of all X recipient clones. The normal human X chromosome induced dominant senescence of tumorigenic Ni-2/TGR cells in only 17% of the resulting microcell hybrids (14/81). Karyotypic analyses of 13 non-senescing human X chromosome-derived microcell hybrid clones revealed that none of these clones retained the complete X. A normal CHE X chromosome induced senescence of 75% of hybrids obtained with another immortal and tumorigenic nickel-transformed male CHE cell line (Ni-6/TGR), which exhibited no visible deletion of the X chromosome, while the normal human X chromosome, only induced senescence in 19% of these hybrids. Transfer of the normal CHE or human X chromosome into spontaneously transformed and tumorigenic cell lines, CHO/TGR or V79/TGR, had little or no effect on their growth. These data suggest that both human and CHE cells possess similar X-linked genetic activities that regulate the process of cellular senescence, and that in Chinese hamster cells nickel-induced immortalization but not that of CHO or V79 cells is associated with inactivation of an X-linked senescence gene.

Addition of potassium chloride to sodium dodecyl sulfate (SDS) resulted in the formation of an insoluble precipitate that was easily recovered by low-speed centrifugation. Since SDS tightly binds to proteins but not DNA, all proteins and detergent-resistant DNA--protein complexes were also effectively co-precipitated in the presence of potassium--SDS leaving free DNA in the supernatant. The amount of SDS-precipitable DNA represented a measure of DNA--protein crosslinks. We have adapted this method for determining DNA--protein crosslinks formed in cells following their exposure in culture or in vivo to crosslinking agents such as chromate, cis-Pt(II) diammine dichloride and formaldehyde. The critical parameters for application of the K--SDS assay to cells were rigorously reproducible shearing of chromosomal DNA and effective washing steps. We have found that freeze-thawing SDS lysed cells followed by vortexing and repeated resuspensions of the precipitate by pipeting resulted in a low background and high reproducibility of the assay. The method detected in a dose-dependent manner DNA--protein crosslinks induced in CHO cells by chromate, cis-platinum and formaldehyde, with sensitivity similar to the alkaline elution procedure. The K--SDS assay was also successfully utilized to determine DNA--protein crosslinks in rat and mouse white blood cells exposed in vivo to chromate. Its sensitivity and simplicity in sample handling and DNA--protein complex isolation potential allows wide application of the assay to measure formation of DNA--protein crosslinks. The ease of recovery of DNA--protein complexes allows for a more thorough investigation of this lesion.

DNA--protein complexes isolated from CHO cells treated with at least 10-30 microM potassium chromate exhibited an alteration in the degradation of the DNA by restriction enzymes compared to DNA--protein complexes isolated from untreated cells. This alteration in restriction enzyme digestion of DNA--protein complexes induced by chromate was shown to depend upon the binding of trivalent chromium to the DNA and upon the protein associated with the DNA, since both EDTA pretreatment and protease K reversed the inhibition of restriction enzyme degradation of the DNA. The inhibition of restriction enzyme degradation of DNA--protein complexes may be utilized as an indirect way to detect DNA--protein complexing induced by chromate and perhaps other agents.

The complex process of carcinogenesis begins with transformation of a single cell to favor aberrant traits such as loss of contact inhibition and unregulated proliferation – features found in every cancer. Despite cancer’s widespread prevalence, the early events that initiate cancer remain elusive, and without knowledge of these events cancer prevention is difficult. Here we show that exposure to As, Cr, Ni, or Vanadium (V) promotes changes in gene expression that occur in conjunction with aberrant growth. We exposed immortalized human bronchial epithelial cells to one of four metals/metalloid for four to eight weeks and selected transformed clonal populations based upon anchorage independent growth of single cells in soft agar. We detected a metal-specific footprint of cancer-related gene expression that was consistent across multiple transformed clones. These gene expression changes persisted in the absence of the progenitor metal for numerous cell divisions. Our results show that even a brief exposure to a carcinogenic metal may cause many changes in gene expression in the exposed cells, and that from these many changes, the specific change(s) that each metal causes that initiate cancer likely arise.

Although carcinogenic metals have been known to disrupt a wide range of cellular processes the precise mechanism by which they exert their carcinogenic effects is not known. Over the last decade or two, studies in the field of metal carcinogenesis suggest that epigenetic mechanisms may play a role in metal-induced carcinogenesis. In this review we summarize the evidence demonstrating that exposure to carcinogenic metals such as nickel, arsenic, chromium, and cadmium can perturb DNA methylation levels as well as global and gene specific histone tail posttranslational modification marks. We also wish to emphasize the importance of understanding that gene expression can be regulated by both genetic and epigenetic mechanisms and both these must be considered when studying the mechanism underlying the toxicity and cell transforming ability of carcinogenic metals and other toxicants, as well as aberrant changes in gene expression that occur during disease states such as cancer.

Ten nickel oxides and nickel-copper oxides, which all contained NiO (bunsenite) as the predominant crystalline phase, were assayed as follows: in vitro dissolution tests in water and body fluids; in vitro phagocytosis tests in Chinese hamster ovary and C3H-10T1/2 cells; morphological transformation and cytotoxicity tests in cultured Syrian hamster embryo (SHE) cells; erythropoiesis stimulation assay by intrarenal administration to Fischer-344 rats; and scoring the renal histopathologic responses in rats killed 3 months post-injection. The test compounds differed substantially in their biological effects when tested in the various experimental systems. Based upon highly significant concordance of ranked results in the assays (P less than 0.001), six colligative biological attributes of the compounds were identified: (i) dissolution half-times in rat serum and renal cytosol; (ii) phagocytosis by C3H-10T1/2 cells; (iii) morphological transformation of SHE cells; (iv) erythropoiesis stimulation in rats; (v) induction of tubular hyperplasia in rat kidneys; and (vi) induction of arteriosclerosis in rat kidneys. Strong rank correlation (P less than 0.01) between results of the cell transformation and erythropoiesis stimulation assays is especially notable, since the compounds were tested by blind protocols in independent laboratories. The presence of high surface area and demonstrable Ni(III) were two physicochemical characteristics that were associated with the greatest biological effects of nickel oxides.

Raising the extracellular level of magnesium ions inhibited nickel-induced DNA strand breaks, DNA-protein crosslinks, sister chromatid exchanges, chromosomal aberrations and cell transformation. Carcinogenic nickel ions preferentially damaged centromeres and other heterochromatic regions of Chinese hamster ovary cell chromosomes. Elevation of extracellular magnesium levels prevented the effects of nickel on heterochromatin and inhibited cell transformation, but did not substantially reduce the DNA damage induced by nickel in euchromatic regions. This study suggests that heterochromatic DNA damage may be important to the nickel-induced neoplastic transformation process.

CaCrO4 was shown to induce alkali labile sites in the DNA of Chinese hamster ovary cells by analysis of the lack of linearity in the alkaline elution curves, and by a study of the dependence upon pH in the elution of DNA from filters. The effect of CaCrO4 on these parameters was compared with N-methyl-N-nitrosourea, an agent known to produce alkali labile sites. Based upon the aforementioned parameters HgCl2, formaldehyde and X-rays caused the formation of frank single strand breaks with little or no induction of alkaline labile sites. These findings demonstrate differences in the production of alkali sensitive sites by agents that cause DNA single strand breaks.