| Inorganic arsenic has been a known human carcinogen for nearly a century, and the carcinogenic effects of arsenic can be observed among numerous populations. The proposed carcinogenic mechanisms of action of arsenic include enhancement of mitogenesis, modulation of gene expression, and potentiation of chemical- and viral-induced cellular transformation. Arsenic-induced modulation of mitogenesis may be the result of alterations in the expression or activity of genes involved in cancer development such as cellular oncogenes or tumor suppressor genes. Because arsenic is known to modulate the process of cell division, it was hypothesized that arsenic contributes to the formation of cancerous lesions by deregulating gene expression involved in mitogenesis and cell cycle progression. This dissertation addresses the hypothesis that arsenic exposure disrupts the normal expression of both positive and negative regulators of cell proliferation, thereby contributing to aberrant cell proliferatory responses. The hypothesis consists of three specific aims that examine (1) the effect of short-term sodium arsenite exposure on mitogen-stimulated proliferative events in wild-type and ras-transformed fibroblasts, (2) the effect of long-term sodium arsenite exposure on the activity and expression of proteins involved in signal transduction that contributes to cell cycle progression and (3) the effect of both short- and long-term sodium arsenite exposure on cellular differentiation using a preadipocyte model. In wild-type fibroblasts, [3H]-thymidine and flow cytometry assays indicate that short-term arsenic exposure enhances the effect of suboptimal doses of mitogens that stimulate cell proliferation. In contrast, ras-transformed fibroblasts display a reduced response to mitogens following arsenic exposure. Northern and Western blotting indicate that c-myc and E2F-1, both positive transcriptional regulators of cell cycle progression, expression increases following long-term arsenic exposure. In contrast, expression of p27Kip1 and MKP-1, negative regulators of cell cycle progression, decreases following long-term arsenic exposure. Oil Red-O accumulation experiments indicate that arsenic inhibits preadipocyte differentiation. Flow cytometry and [3H]-thymidine assays also indicate that cells remain mitogenically competent following inhibition of differentiation. It is anticipated that results from this study will contribute to a better understanding of arsenic carcinogenicity. |
