Regulation of the murine inducible nitric oxide synthase gene by dexamethasone involves a heterogeneous nuclear ribonucleoprotein I (hnRNPI) dependent pathway

Glucocorticoids down regulate the inducible nitric oxide synthase (iNOS) gene both transcriptionally and post-transcriptionally. The post-transcriptional events are suggested to involve destabilization of the iNOS transcript although the molecular mechanisms for this effect are not known. Recently, our laboratory demonstrated a lipopolysaccharide (LPS)/interferon-γ (IFNγ)-induction-dependent interaction of heterogeneous nuclear ribonucleoprotein (hnRNP) I and hnRNPL with a destabilizing element contained in the 3′untranslated region (UTR) of iNOS mRNA. The aim of this study was to investigate if dexamethasone, which down regulates iNOS, is able to modulate this protein–mRNA interaction. As expected, dexamethasone inhibited the induction of iNOS by LPS and IFNγ in RAW 264.7 cells, and destabilized the iNOS mRNA. Dexamethasone also counteracted the LPS/IFNγ-induced disappearance of a gel shifted iNOS mRNA–protein complex containing hnRNPI and hnRNPL. UV cross-linking and Western blot analyses revealed that the RNA-binding and levels of hnRNPI, which decreased by LPS/IFNγ treatment, were restored by dexamethasone. The results support our hypothesis that hnRNPI is pivotal in the post-transcriptional regulation of iNOS and strongly suggest that hnRNPI is one of the trans-acting factors mediating the post-transcriptional effects of dexamethasone.