Modulation of DNA repair and glutathione levels in human keratinocytes by micromolar arsenite

Arsenic (As) is a human carcinogen, bur not a mutagen, although it inhibits DNA repair and is a comutagen. Human AG06 keratinocytes treated with micromolar arsenic exhibit dose and time-dependent loss of DNA ligase function. However, purified human DNA ligase I, ligase III, and other repair enzymes such as DNA polymerase beta, are not inhibited by less than millimolar arsenite, As(III), the most toxic form of As found in the environment. DNA ligase activity in extracts from untreated keratinocytes is also insensitive to less than millimolar As. Pyruvate dehydrogenase, on the other hand, is inhibited by micromolar As and probably determines As-induced cytotoxicity. Simultaneous treatment of AG06 cells with an alkylating agent, 1-methyl-3-nitro-1-nitrosoguanidine (MNNG), plus As produces a synergistic increase in viability (dye uptake) at low doses and a synergistic increase in toxicity at high doses. Micromolar As also modulates cellular redox levels and induces a variety of cellular stress response genes. Keratinocytes treated with As exhibit both a time-and dose-dependent increase in cellular GSH levels and alterations in the relative activity of several GSH-dependent enzymes. These As-induced changes in cellular redox capacity and DNA repair activity are not directly related to toxicity. Maximal induction of GSH and DNA repair occurs after treatment with sub-toxic concentrations of As. At submicromolar concentrations, arsenic also induces hyperproliferation of keratinocytes, both in vivo and in vitro. Our results suggest that As modulates DNA repair and redox levels primarily through post-translational or transcriptional mechanisms.