| Quinones are ubiquitous and naturally occurring compounds present in food, vehicle emissions, dyes, cosmetics, and anticancer drugs. Due to the ubiquity and reactivity of quinones and potential for human exposure, it is important to understand the mechanism of action of this class of compounds. As a model of quinone-mediated toxicities, we have been studying the adverse effects of 2-bromohydroquinone (2-BrHQ) which causes severe renal proximal tubular necrosis in rats following oxidation to the corresponding quinone, giving rise to several glutathione (GSH) conjugates. The formation of GSH conjugates, in general, has been regarded as an important cellular defense mechanism against a variety of toxic xenobiotics. More recently however, it has become apparent that GSI-I conjugation of certain types of chemicals leads to more reactive and toxic derivatives. However, the precise cellular and molecular mechanism(s) of action remain unclear. 2-Bromo-bis-(glutathion-S-yl)hydroquinone (2-Br-bis- (GSyl) HQ) causes rapid and extensive karyorrhexis and karyolysis following administration to rats, and rapidly induces DNA single strand breaks in cultured renal proximal tubular epithelial cells (LLC-PK{dollar}sb1{dollar}). In addition, hydroxyl radicals appear to be involved in quinone-thioether mediated cytotoxicity, and the in vivo generation of quinone-thioethers from 2-Br- ({dollar}sp{lcub}14{rcub}{dollar}C) HQ resulted in the formation of covalent adducts with cellular macromolecules. Because a variety of physical, chemical, and physiological insults induce stress response genes we investigated changes in stress gene expression in response to cell injury and death caused by quinone-thioethers. Quinone-thioethers induced the expression of the growth-arrest and DNA damage inducible gene (gadd153), c-myc, heat shock protein 70 (hsp70), and glucose regulated protein 78 (grp78). In contrast, quinone-thioethers downregulated both linker and core histone mRNA expression. Altered gene expression, as well as cytotoxicity, were prevented by catalase, and the iron chelator deferoxamine, suggesting that oxidative stress plays a role in quinone-thioether mediated changes in gene expression and cytotoxicity. In addition, DNA synthesis as measured by the incorporation of ({dollar}sp3{dollar}H) -thymidine into newly synthesized DNA in LLC-PK{dollar}sb1{dollar} cells was inhibited by quinone-thioethers within 10 min of exposures. Moreover the effects of quinone-thioethers on gene expression were cell cycle dependent. Thus, histone gene expression was downregulated in subconfluent cells but not in confluent cells. Although the relationship between altered gene expression in response to quinone-thioethers and cytotoxicity remains to be determined, data obtained from this study provide initial insights into the role of altered gene expression in chemical-induced toxicity. Moreover, chemical induced alterations in the expression of the histone genes, which are important chromosomal structural proteins, may contribute to non-genotoxic chemical-induced carcinogenesis. |
