Statin enhances cytokine-mediated induction of nitric oxide synthesis in vascular smooth muscle cells

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Abstract
Objective: We investigated the effects of the statins, cerivastatin and fluvastatin, on the induction of nitric oxide (NO) production in vascular smooth muscle cells (VSMC) stimulated by interleukin-1β (IL-1) or in combination with interferon-γ (IFN). Methods: We measured NO release, inducible NO synthase (iNOS) mRNA and protein levels, iNOS gene transcription rates, and iNOS mRNA stabilities in cytokine-activated VSMC. We also evaluated nuclear factor (NF)-κB activity and tetrahydrobiopterin (BH4) synthesis. Results: NO production induced by cytokines was dose-dependently enhanced by both statins. Incubating VSMC with IL-1/IFN stimulated iNOS mRNA and protein expression. Both statins significantly upregulated IL-1/IFN-stimulated iNOS mRNA and protein expression, and enhanced iNOS gene transcription as shown by nuclear run-on assays. However, they did not alter the stability of iNOS mRNA. Both statins slightly modulated IL-1/IFN-induced NF-κB activation, which was not associated with their effect on NO production. Cytokines induce the de novo synthesis of BH4 in VSMC. This event is essential for the induction of NO synthesis, which requires transcriptional induction of the genes that encode not only iNOS but also guanosine triphosphate cyclohydrolase I (GTPCH), the first and rate-limiting enzyme in de novo BH4 synthesis. The synthesis of BH4 and GTPCH mRNA induced by IL-1/IFN were enhanced by both statins. Exogenous mevalonate significantly prevented and geranylgeranylpyrophosphate reversed the stimulatory effect of both statins. Furthermore, the geranylgeranyltransferase I inhibitor GGTI-298 significantly increased IL-1/IFN-induced NO production. Conclusion: Our data demonstrated that statins enhance immunostimulants-induced NO production by increasing iNOS gene expression at the transcriptional level via an NF-κB-independent pathway. The effect of statins on NO production is due at least partly through blocking the biosynthesis of mevalonate, which prevents isoprenoid biosynthesis. In addition to augmenting iNOS expression, statins potentiate GTPCH gene expression and BH4 synthesis, thereby preventing a relative shortage of BH4 which may shift the balance between NOS-catalyzed generation of protective NO and deleterious reactive oxygen species.