Mechanisms underlying induction of heme oxygenase-1 by nitric oxide in renal tubular epithelial cells

We examined whether nitric oxide-generating agents influence expression of heme oxygenase-1 (HO-1) in renal proximal tubular epithelial cells, LLC-PK1 cells, and the mechanisms underlying any such effects. In sublytic amounts , the nitric oxide donor sodium nitroprusside induced HO-1 mRNA and protein and HO activity in a dose-dependent and time-dependent fashion; this induc tion was specific for nitric oxide since the nitric oxide scavenger carboxy -2-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl-3- oxide significantly redu ced such induction. The induction of HO activity by sodium nitroprusside, o r by another nitric oxide donor, spermine NONOate, was markedly reduced by the iron chelator deferoxamine. Two different thiol-containing agents, N-ac etylcysteine and dithiothreitol, blunted such induction of HO by nitric oxi de. Downstream products of nitric oxide, such as peroxynitrite or cGMP, wer e not involved in inducing HO. In higher concentrations (millimolar amounts ), sodium nitroprusside induced appreciable cytotoxicity as assessed by lac tate dehydrogenase (LDH) release and lipid peroxidation, and both of these effects were markedly reduced by deferoxamine. Inhibition of HO did not aff ect the cytotoxic effects (measured by LDH release) of sodium nitroprusside . We thus provide the novel description of the induction of HO-1 in renal p roximal tubular epithelial cells exposed to nitric oxide donors and provide the first demonstration in kidney-derived cells for the involvement of a r edox-based mechanism in such expression. We also demonstrate that, in LLC-P K1 cells exposed to nitric oxide donors, chelatable iron is involved in eli citing the HO-1 response observed at lower concentrations of these donors, and in mediating the cytotoxic effects of these donors when present in high er concentrations.