Nitric oxide-inducible expression of heme oxygenase-1 in human cells. Translation-independent stabilization of the mRNA and evidence for direct action of nitric oxide.

Expression of heme oxygenase-1 (HO-1) in mammalian cells contributes to resistance to various types of free radical damage. Nitric oxide (NO) induces HO-1 in many cell types, but the specific contribution of transcriptional or post-transcriptional effects to this induction have remained unresolved. Here we show that the extent of HO-1 mRNA expression in IMR-90 and HeLa cells depends on the rate of NO delivery, and that the induction occurs more slowly in HeLa than in human fibroblast (IMR-90) cells. We used a specific NO scavenger (2-(4-carboxylphenyl)-4,4,5,5-tetramethylimidazolin-1-oxyl 3-oxide) that completely prevented the inducible expression of HO-1 by NO, pointing to direct signaling action of NO in this induction. By inhibiting transcription during the NO exposure, we have confirmed that NO treatment activates a mechanism that stabilizes HO-1 mRNA. The increase in the HO-1 mRNA half-life in IMR-90 cells was directly correlated with increasing rates of NO release. We also show here that the stabilization of the HO-1 message does not require de novo protein synthesis. Collectively, these results show that stabilization of HO-1 mRNA can be finely tuned to the NO exposure, and that the effect in human fibroblasts is mediated by a pre-existing protein.