Sodium arsenite-induced dysregulation of proteins involved in proliferative signaling

Effect of arsenic on transcription factor AP-1 and NF-kappaB DNA binding activity and related gene expression

Both acute (24 h) and chronic (10-20 week) exposure of human fibroblast cells to low dose sodium arsenite (As(III)) significantly affects activating protein-1 (AP-1) and nuclear factor kappa B (NF-kappa B) DNA binding activity. Short-term treatment with 0.1-5 microM As(III) up-regulates expression of c-Fos and c-Jun and the redox regulators, thioredoxin (Trx) and Redox factor-1 (Ref-1) and activates both AP-1 and NF-kappa B binding. Chronic exposure to 0.1 or 0.5 microM As(III) decreased c-Jun, c-Fos and Ref-1 protein levels and AP-1 and NF-kappa B binding activity, but increased Trx expression. Short term exposure to phorbol 12-myristate 13-acetate (TPA), a phorbol ester tumour promoter, or hydrogen peroxide (H(2)O(2)) also activates AP-1 and NF-kappa B binding. However, pre-treatment with As(III) prevents this increase. These results suggest that As(III) may alter AP-1 and NF-kappa B activity, in part, by up-regulating Trx and Ref-1. The different effects of short- versus long-term As(III) treatment on acute-phase response to oxidative stress reflect changes in the expression of Ref-1, c-Fos and c-Jun, but not Trx.

The paradox of arsenic: molecular mechanisms of cell transformation and chemotherapeutic effects

Arsenic is a well-documented carcinogen that also appears to be a valuable therapeutic tool in cancer treatment. This creates a paradox for which no unified hypothesis has been reached regarding the molecular mechanisms that determine whether arsenic will act as a carcinogen or as an effectual chemotherapeutic agent. Much of our knowledge with respect to the actions of arsenic has been drawn from epidemiological or clinical studies. The actions of arsenic are likely to be related to cell type, arsenic species, and length and dose of exposure. Arsenic unquestionably induces apoptosis and may specifically target certain tumor cells. Research data strongly suggest that arsenic influences distinct signaling pathways involved in mediating proliferation or apoptosis, including mitogen-activated protein kinases, p53, activator protein-1 or nuclear factor kappa B. The primary purpose of this review is to examine recent findings, from this laboratory and others, that focus on the molecular mechanisms of arsenic's actions in cell transformation and as a therapeutic agent.

Arsenic-induced Mre11 phosphorylation is cell cycle-dependent and defective in NBS cells

Cancer-prone diseases ataxia-telangiectasia (AT), Nijmegen breakage syndrome (NBS) and ataxia-telangiectasia-like disorder (ATLD) are defective in the repair of DNA double-stranded break (DSB). On the other hand, arsenic (As) has been reported to cause DSB and to be involved in the occurrence of skin, lung and bladder cancers. To dissect the repair mechanism of As-induced DSB, wild type, AT and NBS cells were treated with sodium arsenite to study the complex formation and post-translational modification of Rad50/NBS1/Mre11 repair proteins. Our results showed that Mre11 went through cell cycle-dependent phosphorylation upon sodium arsenite treatment and this post-translational modification required NBS1 but not ATM. Defective As-induced Mre11 phosphorylation was rescued by reconstitution with full length NBS1 in NBS cells. Although As-induced Mre11 phosphorylation was not required for Rad50/NBS1/Mre11 complex formation, it might be required for the formation of Rad50/NBS1/Mre11 nuclear foci upon DNA damage.

Arsenite is a cocarcinogen with solar ultraviolet radiation for mouse skin: an animal model for arsenic carcinogenesis

Although epidemiological evidence shows an association between arsenic in drinking water and increased risk of skin, lung, and bladder cancers, arsenic compounds are not animal carcinogens. The lack of animal models has hindered mechanistic studies of arsenic carcinogenesis. Previously, this laboratory found that low concentrations of arsenite (the likely environmental carcinogen) which are not mutagenic can enhance the mutagenicity of other agents, including ultraviolet radiation (UVR). This enhancing effect appears to result from inhibition of DNA repair by arsenite. Recently we found that low concentrations of arsenite disrupted p53 function and upregulated cyclin D1. These results suggest that the failure to find an animal model for arsenic carcinogenesis is because arsenite is not a carcinogen per se, but rather acts as an enhancing agent (cocarcinogen) with a genotoxic partner. We tested this hypothesis with solar UVR as carcinogenic stimulus in hairless Skh1 mice. Mice given 10 mg/l sodium arsenite in drinking water for 26 weeks had a 2.4-fold increase in yield of tumors after 1.7 KJ/m(2) UVR three times weekly compared with mice given UVR alone. No tumors appeared in mice given arsenite alone. The tumors were mostly squamous cell carcinomas, and those occurring in mice given UVR plus arsenite appeared earlier and were much larger and more invasive than in mice given UVR alone. These results are consistent with the hypothesis that arsenic acts as a cocarcinogen with a second (genotoxic) agent by inhibiting DNA repair and/or enhancing positive growth signaling.

Association of c-myc overexpression and hyperproliferation with arsenite-induced malignant transformation

Numerous studies link arsenic exposure to human cancers in a variety of tissues, including the liver. However, inorganic arsenic has never been unequivocally shown to be an animal carcinogen, and its carcinogenic mechanism remains undefined. Our previous studies indicate that chronic (> or =18 weeks), low-level (125 to 500 nM) exposure to arsenite induces malignant transformation in the normally nontumorigenic rat liver epithelial cell line (TRL 1215), and these chronic arsenic-exposed (CAsE) cells produce invasive and metastatic tumors upon inoculation into nude mice. In addition, a prior microarray screening analysis of aberrant gene expression showed several oncogenes were overexpressed in CAsE cells exposed to 500 nM arsenite, including a prominent overexpression of the protooncogene c-myc, as well as genes related to cell proliferation. Thus, to better understand the mechanism of arsenic carcinogenesis, we studied the role of c-myc overexpression in arsenite-induced cell transformation. The upregulation of c-myc was confirmed by RT-PCR at the transcription level and by Western blot analysis for the translation product. Further analysis showed that arsenite produced significant increases in the steady-state expression of c-myc in a time- and concentration-dependent manner during the malignant transformation process. The level of c-myc expression was highly correlated (r = 0.988) with tumor formation after inoculation of CAsE cells into nude mice and was also highly correlated (r = 0.997) with genomic DNA hypomethylation. CAsE cells showed a high cell proliferation rate in a fashion related to the level of arsenic exposure. The expression of c-myc was highly correlated with cellular hyperproliferation (r = 0.961). Consistent with the enhanced proliferation both proliferating cell nuclear antigen and cyclin D1 were overexpressed in CAsE cells. In summary, a prominent overexpression of c-myc, a gene frequently activated during hepatocarcinogenesis, is strongly correlated with several events possibly associated with arsenic-induced malignant transformation, including hyperproliferation, DNA hypomethylation and tumor formation upon inoculation into nude mice. These correlations provide convincing evidence c-myc overexpression is mechanistically important in arsenic-induced malignant transformation in this model system.

Keratinocyte differentiation marker suppression by arsenic: mediation by AP1 response elements and antagonism by tetradecanoylphorbol acetate

Culture models of target cells are anticipated to help elucidate the mechanism by which inorganic arsenic acts as a carcinogen in humans. Present work characterizes the response of human keratinocytes, a target cell type, to arsenic suppression of their differentiation program. Four representative differentiation marker mRNAs (involucrin, keratinocyte transglutaminase, small proline-rich protein 1, and filaggrin) were suppressed by both arsenate and arsenite in normal, spontaneously immortalized (premalignant), and malignant keratinocytes with EC50 values in the low micromolar range. The suppression was almost completely reversed 9 days after removal of arsenate from the culture medium. In the case of the involucrin gene, suppression was mediated primarily by two functional AP1 response elements in the gene promoter. Both glucocorticoid and serum stimulation of differentiation occurred to a similar extent in the presence and absence of arsenic, indicating neither stimulation was a specific target of arsenic action and neither agent could overcome arsenic suppression. In contrast, 12-O-tetradecanoylphorbol-13-acetate prevented the suppression of keratinocyte transglutaminase, suggesting that arsenic acts upstream of protein kinase C.

Effects of arsenite on p53, p21 and cyclin D expression in normal human fibroblasts -- a possible mechanism for arsenite's comutagenicity

Arsenite, the most likely environmental carcinogenic form of arsenic, is not significantly mutagenic at non-toxic concentrations, but is able to enhance the mutagenicity of other agents. Evidence suggests that this comutagenic effect of arsenite is due to inhibition of DNA repair, but no specific repair enzyme has been found to be sensitive to low (<1 microM) concentrations of arsenite. To determine whether arsenite affects signaling which might alter DNA repair, this study assesses the effect of arsenite on p53-related signal transduction pathways after ionizing radiation. Long-term (14 day) low dose (0.1 microM) arsenite caused a modest increase in p53 expression in WI38 normal human fibroblasts, while only toxic (50 microM) concentrations increased p53 levels after short-term (18 h) exposure. When cells were irradiated (6 Gy), p53 and p21 protein concentrations were increased after 4h, as expected. Both long-term, low dose and short-term, high dose exposure to arsenite greatly suppressed the radiation-induced increase in p21 abundance. In addition, long-term, low dose (but not short-term, high dose) exposure to arsenite resulted in increased expression of cyclin D1. These results show that in cells treated with arsenite, p53-dependent increase in p21 expression, normally a block to cell cycle progression after DNA damage, is deficient. At the same time, low (non-toxic) exposure to arsenite enhances positive growth signaling. We suggest that the absence of normal p53 functioning, along with increased positive growth signaling in the presence of DNA damage may result in defective DNA repair and account for the comutagenic effects of arsenite.

Genetic events associated with arsenic-induced malignant transformation: applications of cDNA microarray technology

Arsenic is a human carcinogen. Our recent work showed that chronic (>18 wk), low-level (125-500 nM) arsenite exposure induces malignant transformation in normal rat liver cell line TRL1215. In these arsenic-transformed cells, thecellular S-adenosylmethionine pool was depleted from arsenic metabolism, resulting in global DNA hypomethylation. DNA methylation status in turn may affect the expression of a variety of genes. This study examined the aberrant gene expression associated with arsenic-induced transformation with the use of Atlas Rat cDNA Expression microarrays. Poly(A(+)) RNA was prepared from arsenic-transformed cells and passage-matched control cells, and (32)P-labeled cDNA probes were synthesized with Clontech Rat cDNA Synthesis primers and moloney murine leukemia virus reverse transcriptase. The hybrid intensity was analyzed with AtlasImage software and normalized with the sum of the four housekeeping genes. Four hybridizations from separate cell preparations were performed, and mean and SEM for the expression of each gene were calculated for statistical analysis. Among the 588 genes, approximately 80 genes ( approximately 13%) were aberrantly expressed. These included genes involved in cell-cycle regulation, signal transduction, stress response, apoptosis, cytokine production and growth-factor and hormone-receptor production and various oncogenes. These initial gene expression analyses for the first time showed potentially important aberrant gene expression patterns associated with arsenic-induced malignant transformation and set the stage for numerous further studies. Mol. Carcinog. 30:79-87, 2001. Published 2001 Wiley-Liss, Inc.

Sodium arsenite inhibits terminal differentiation of murine C3H 10T1/2 preadipocytes

Cancer represents an imbalance between cell proliferation and differentiation, two processes that are coordinately and antagonistically regulated. Aberrant cell proliferation is considered to be an important etiological factor in the development of arsenic-induced cancer, suggesting that arsenic also dysregulates differentiation. Based on evidence that arsenic modulates mitogenic events that antagonize the process of differentiation, this study addresses the hypothesis that sodium arsenite inhibits insulin/dexamethasone-induced differentiation of C3H 10T1/2 preadipocytes; it was further postulated that arsenic-treated cells retain mitogenic responsiveness under differentiating conditions. To test this hypothesis, the differentiation capacity of C3H 10T1/2 preadipocytes was examined in control cells and cells treated with sodium arsenite. Differentiation was assessed morphologically and quantified by Oil Red-O staining of accumulated lipids. The effect of long-term arsenic exposure on mitogenic competence was quantified by flow cytometry, [(3)H]thymidine incorporation, and cell counting under conditions favorable for adipocyte differentiation. Results indicate that arsenic inhibits morphological differentiation of wild-type C3H 10T1/2 preadipocytes. Short-term arsenic exposure inhibits differentiation in a dose-dependent manner, with arsenic concentrations > or = 3 microM producing a significant inhibition of dexamethasone/insulin-induced lipid accumulation. Furthermore, arsenic-treated cells exhibit an accentuated response to mitogenic stimulation under differentiating conditions. These data suggest that arsenic exposure results in the inhibition of cellular programming required for terminal differentiation of C3H 10T1/2 preadipocytes and that cells acquire mitogenic hyperresponsiveness. The ability of arsenic to dysregulate the balance between proliferation and differentiation is proposed to be one mechanism by which this metalloid causes cancer in humans.