GROWTH FACTORS AND INFLAMMATORY MEDIATORS IN ARSENIC-INDUCED TOXICITY

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Arsenic has been classified as a known human carcinogen of the skin, lung and urinary bladder, however, the mechanisms involved in carcinogenesis by arsenic are not known, due in part to a lack of appropriate animal models. Ongoing studies in our laboratory have suggested that arsenic serves as a tumor enhancer or progressor in the skin via stimulation of the chronic production of cytokines and growth factors, including granulocyte macrophage colony stimulating factor, transforming growth factor alpha and tumor necrosis factor alpha. We are currently investigating inter-individual variation in responses to arsenic, to address the question of why some individuals or populations appear to be less sensitive to arsenic-induced toxicities. We have examined the cytokine profiles produced by several different keratinocyte donors in response to arsenic treatment and have shown that while cytokine responses are qualitatively similar, in that similar cytokines are produced, sensitivity to dose and quantitative cytokine secretion vary from donor to donor. In collaboration with Dr. Miroslav Styblo at UNC we are examining whether the differential sensitivity seen with various donors may be a reflection of their ability to methylate arsenic. To date these studies have indicated that keratinocytes have very little ability to methylate arsenicals, as compared with cells from other tissues such as the bladder and liver. However, differences in the uptake of inorganic arsenic and in the yield and distribution of monomethylarsenic suggest that interindividual differences could occur in human skin in vivo. We have evaluated the effects of arsenic exposure on alterations in cytokine profiles in p53 mice fed choline deficient and choline sufficient diets to alter their ability to methylate (and detoxify) arsenals. p53 mice fed a choline deficient diet exhibit higher levels of TNFalpha, TGFalpha and GM-CSF mRNAs in their skin than corresponding controls. These studies suggest that deficiencies in methylation may influence the cytokines produced in individual tissues in response to arsenic. We have also examined the toxicity of both ArsenicIII and ArsenicV and their monomethyl and dimethyl metabolites in keratinocyte cultures. As expected, inorganic trivalent arsenic is the most toxic to all donors. Surprisingly, monomethyl arsenicals are also highly toxic to keratinocytes. We have characterized the dose response curves for toxicity and examined cytokine secretion patterns and proliferative activity after exposure to the trivalent and pentavalent metabolites. As hypersensitivity responses are highly dependent on cytokine secretion and regulation, and as we have previously demonstrated that growth and inflammatory cytokines are modulated by arsenic exposure, we have investigated how low level arsenic exposure modulates hypersensitivity responses. Antigen-specific proliferative responses induced by moderate and strong sensitizing agents are significantly reduced in the draining lymph nodes of arsenic-exposed animals, particularly at low doses. In contrast, mitogen-stimulated proliferative responses in the draining lymph nodes were not affected by arsenic exposure. Arsenic also reduced challenged-induced responses in previously sensitized mice. To investigate the potential mechanisms of these alterations we examined the effects of arsenic on Langerhans cell migration and antigen presentation. Langerhans cell migration out of the ear was enhanced in mice exposed to arsenic via the drinking water. As TNFalpha is reported to be a primary stimulator of Langerhans cell migration, this is consistent with our previous findings that TNFalpha levels are elevated in the skin of arsenic treated mice. Quantitation of the expression of regulatory cytokines in the skin of these animals is currently under investigation. - arsenic, skin, keratinocytes, cancer, cell proliferation, cytokines, growth factors, Tg.AC transgenic mice, PCR