Arsenic-Glutathione Interactions and Skin CancerEPA Grant Number: R826135
Title: Arsenic-Glutathione Interactions and Skin Cancer
Investigators: Snow, Elizabeth T. , Frenkel, Krystyna , Klein, Catherine B. , Mirochnitchenko, Oleg I. , Bosland, Maartin , Steinberg, Mark
Institution: New York University Medical Center
EPA Project Officer: Hahn, Intaek
Project Period: October 1, 1997 through September 30, 2000 (Extended to September 30, 2001)
Project Amount: $822,293
RFA: Arsenic Health Effects Research (1997) RFA Text | Recipients Lists
Research Category: Drinking Water , Human Health , Water
The main goal of this project is to test the hypothesis that arsenic-induced cancer is the result of changes in cellular redox control mediated by altered glutathione (GSH) levels. The hypothesis will be tested by exploring the effect of As on glutathione regulating enzymes in human keratinocytes in vitro, and in mice in vivo. Current evidence suggests that As is primarily a tumor promoter or co-promoter (progressor). Two different, but related, cellular responses appear to be central to As-induced genotoxicity and carcinogenesis: (a) oxidative stress and signal transduction and (b) DNA repair. It is not clear how As affects these processes. One prevailing theory is that AsIII binds indiscriminately to protein sulfhydryls, inactivating and denaturing the proteins. However, this does not reflect the true specificity of enzyme inhibition by As. Whereas 1 to 10 µM arsenite [AsIII] is cytotoxic; most purified enzymes are insensitive to less than millimolar AsIII. However, when assayed in whole cells or in cell extracts, some enzymatic reactions, including DNA ligase and glutathione S-transferase (GST), are significantly more sensitive to As than the purified enzymes. GSH concentrations and protein phosphorylation can also be altered by exposure to low concentrations of As. Physiologically relevant, low doses of As are expected to result in the activation or inactivation of enzymes such as p21-ras, GST, glutathione reductase (GR) and glutathione peroxidase (GPx) due to changes in cellular phosphorylation and/or redox status, and thereby potentiate the induction of cellular stress responses. Subsequent to an oxidative stress response in certain cell types, such as keratinocytes, arsenic can induce hyperproliferation and inhibit DNA repair. This can contribute to carcinogenesis, as well as certain other aspects of arsenic toxicity such as hyperkeratosis and cardiovascular disease.
Cultured human keratinocytes will be exposed to chronic or acute low As doses, and the enzyme activity and phosphorylation status of several key proteins: p21-ras, GST, GR, and GPx, will be examined. The activity of these enzymes will be assayed in purified form and in extracts from either untreated cells or cells pretreated with As. The role of GSH and As(GS)x complexes in mediating these responses will be examined by measuring cellular GSH and GSSG levels, by assessing the formation of As(GS)x complexes, and by modulating GSH levels. The role of GSH in As carcinogenesis will be assessed by examining the effect of varied GSH levels on the rate of papilloma induction in a mouse skin tumorigenesis model using normal mice and mice which overexpress human GPx.
Chronic exposure to micromolar doses of As is expected to activate mitogenesis and stress response pathways by altering the expression and activation of proteins involved in signal transduction, and by disrupting cellular redox balance. These effects may be modulated by the formation of As complexes with important cellular thiols such as GSH. Positive results would suggest that people with decreased cellular GSH levels, or with altered GSH-dependent metabolism may be abnormally sensitive to As carcinogenesis. This hypothesis also predicts that diets high in sulfhydryls, such as cruciferous vegetables and garlic, might protect against As toxicity.