Xenobiotic induction of quinone oxidoreductase activity in lens epithelial cells

Xenobiotic regulatory elements have been identified for enzymes which ameliorate oxidative damage in cells. Zeta (zeta)-crystallin, a taxon-specific enzyme/crystallin shown to be a novel NADPH-dependent quinone reductase, is found in a number of tissues and cell types. This study shows that zeta-crystallin is present in mouse lens epithelium, as well as in the alpha TN4 mouse lens epithelial cell line. To determine whether zeta-crystallin is an inducible quinone reductase, cell cultures were exposed to the xenobiotics, 1,2-naphthoquinone and beta-naphthoflavone. Assays of cellular homogenates showed that quinone reductase activity was stimulated greater than 70% and 90%, respectively, over the control cells. This observed activity was sensitive to dicumarol, a potent inhibitor of quinone reductase activity. 1,2-Naphthoquinone- and beta-naphthoflavone-exposed cells were found to exhibit 1.47- and 1.68-fold increases, respectively, in zeta-crystallin protein concentration. A comparable increase in zeta-crystallin mRNA was indicative of an induction in zeta-crystallin expression in response to naphthalene challenge. Lens epithelial cells were also checked for DT-diaphorase, a well-known cellular protective enzyme which can catalyze the two-electron reduction of quinones. Slot blot analyses indicated that alpha TN4 cells exposed to 1,2-naphthoquinone and beta-naphthoflavone exhibited 2.71- and 6.81-fold increases in DT-diaphorase concentration when compared to the control cells. The data suggest that while DT-diaphorase is most likely responsible for the majority of the observed increase in quinone reductase activity, the zeta-crystallin gene also undergoes activation which is apparently mediated by a xenobiotic-responsive element.