Nuclear Localization of Biliverdin Reductase in the Rat Kidney: Response to Nephrotoxins That Induce Heme Oxygenase-1

摘要

Biliverdin reductase catalyzes the reduction of biliverdin, the product of heme oxygenase (HO) activity, to bilirubin. The re- ductase is unique among all enzymes characterized to date in being dual pH/cofactor-dependent. Until now the enzyme was assumed to be a non inducible cytosolic protein. This report, for the first time, demonstrates induction and nuclear localization of reductase in rat kidney in response to HO-1 inducers: bac- terial lipopolysaccharide (LPS) and bromobenzene. The study also demonstrates that nuclear localization requires an intact nuclear localization signal and is responsive to cGMP. Specif- iCally 16 h after treatment of rats (i.p.) with LPS (5 mg/kg), there was an increase in nuclear biliverdin reductase as determined by immunostaining, Western blotting, and activity analysis. Induction ad nuclear localization of the reductase in kidney was also observed in bromobenzene-treated rats (2 mmol/kg, s.c., 24 h). The reductase message levels, however, were not in- creased in response to either treatment, suggesting post-tran- scriptional activation of the reductase by LPS and bromoben- zene. The mechanism of nuclear transport of the reductase was examined using HeLa cells transfected with the hemagglutinin- tagged reductase construct. When cells were treated with 8-Br- cGMP the protein translocated into the nucleus. Mutation of the putative nuclear localization signal domain of the reductase blocked nuclear transport of the protein. We suggest the sig- nificance of nuclear localization of the reductase may relate to: 1) chain-breaking antioxidant activity of bilirubin; 2) inhibition of superoxide formation by bilirubin; and 3) modulation of the siqnal transduction pathways.