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

The human aldo-keto reductase AKR1C1 (20alpha(3alpha)-hydroxysteroid dehydrogenase) is induced by electrophilic Michael acceptors and reactive oxygen species (ROS) via a presumptive antioxidant response element (Burczynski, M. E., Lin, H. K., and Penning, T. M. (1999) Cancer Res. 59, 607-614). Physiologically, AKR1C1 regulates progesterone action by converting the hormone into its inactive metabolite 20alpha-hydroxyprogesterone, and toxicologically this enzyme activates polycyclic aromatic hydrocarbon trans-dihydrodiols to redox-cycling o-quinones. However, the significance of its potent induction by Michael acceptors and oxidative stress is unknown. 4-Hydroxy-2-nonenal (HNE) and other alpha,beta-unsaturated aldehydes produced during lipid peroxidation were reduced by AKR1C1 with high catalytic efficiency. Kinetic studies revealed that AKR1C1 reduced HNE (K(m) = 34 microm, k(cat) = 8.8 min(-1)) with a k(cat)/K(m) similar to that for 20alpha-hydroxysteroids. Six other homogeneous recombinant AKRs were examined for their ability to reduce HNE. Of these, AKR1C1 possessed one of the highest specific activities and was the only isoform induced by oxidative stress and by agents that deplete 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 approximately 6 microm, K(m)(app) NADH >6 mm) suggested that it is primed for reductive metabolism of HNE. Isoform-specific reverse transcription-polymerase chain reaction showed that exposure of HepG2 cells to HNE resulted in elevated levels of AKR1C1 mRNA. Thus, HNE induces its own metabolism via AKR1C1, and this enzyme may play a hitherto unrecognized role in a response mounted to counter oxidative stress. AKRs represent alternative GSH-independent/NADPH-dependent routes for the reductive elimination of HNE. Of these, AKR1C1 provides an inducible cytosolic barrier to HNE following ROS exposure.