The reactive oxygen species- and Michael acceptor-inducible human aldo-keto reductase AKR1C1 reduces the alpha,beta-unsaturated aldehyde 4-hydroacy-2-nonenal to 1,4-dihydroxy-2-nonene

The human aldo-keto reductase AKR1C1 (20 alpha (3 alpha)-hydroxysteroid deh ydrogenase) is induced by electrophilic Michael accepters and reactive oxyg en species (ROS) via a presumptive antioxidant response element (Burczynski , M. E., Lin, H. K., and Penning, T. M. (1999) Cancer Res. 59, 607-614). Ph ysiologically, AKR1C1 regulates progesterone action by converting the hormo ne into its inactive metabolite 20 alpha -hydroxyprogesterone, and toxicolo gically this enzyme activates polycyclic aromatic hydrocarbon trans-dihydro diols to redox-cycling o-quinones, However, the significance of its potent induction by Michael accepters and oxidative stress is unknown. 4-Hydroxy-8 -nonenal (HNE) and other alpha,beta -unsaturated aldehydes produced during lipid peroxidation were reduced by AKR1C1 with high catalytic efficiency. K inetic studies revealed that AKR1C1 reduced HNE (K-m = 34 muM, k(cat) = 8.8 min(-1)) with a k(cat)/K-m similar to that for 20 alpha -hydroxysteroids. Six other homogeneous recombinant AKRs were examined for their ability to r educe HNE. Of these, AKR1C1 possessed one of the highest specific activitie s and was the only isoform induced by oxidative stress and by agents that d eplete glutathione (ethacrynic acid). Several hydroxysteroid dehydrogenases of the AKR1C subfamily catalyzed the reduction of HNE with higher activity than aldehyde reductase (AKR1A1). NMR spectroscopy identified the product of the NADPH-dependent reduction of HNE as 1,4-dihydroxy-2-nonene. The K-m of recombinant AKR1C1 for nicotinamide cofactors (K-m NADPH similar to6 muM , K-m(app) NADH >6 mM) suggested that it is primed for reductive metabolism of HNE. Isoformspecific reverse transcription-polymerase chain reaction sh owed that exposure of HeppG2 cells to HNE resulted in elevated levels of AK R1C1 mRNA. Thus, HNE induces its own metabolism via AKR1C1, and this enzyme may play a hitherto unrecognized role in a response mounted to counter oxi dative stress. AKRs represent alternative GSH-independent/NADPH-dependent r outes for the reductive elimination of HNE. Of these, AKR1C1 provides an in ducible cytosolic barrier to HNE following ROS exposure.