Induction of Tetrahydrobiopterin Synthesis in Cardiac Myocytes
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Cardiac myocytes are now known to express the high-capacity inducible isoform of nitric oxide (NO) synthase (iNOS). Induction of iNOS by soluble inflammatory mediators, including cytokines, causes a marked depression in myocyte contractile responsiveness to b-adrenergic agonists and has been implicated as a contributor to the pathogenesis of heart failure. Since tetrahydrobiopterin (BH4) is an essential cofactor for NO formation, we investigated whether BH4 synthesis is required for cytokine-induced NO production in cultured rat cardiac myocytes. Activation of NO formation by cytokines in cardiac myocytes requires transcriptional induction of the genes that encode iNOS and GTP cyclohydrolase I (GTPCH), the first and rate-limiting enzyme in de novo BH4 synthesis. Given that nuclear factor κB (NF-κB) mediates the induction of iNOS gene expression in various cell types, the role of NF-κB in the induction of iNOS in cytokine-stimulated rat neonatal cardiac myocytes was assessed by examining the effects of pyrrolidine dithiocarbamate (PDTC). an inhibitor of NF-κB activation, on iNOS mRNA expression and subsequent NO production. The effects of PDTC on GTPCH mRNA expression and pterin synthesis were also examined. We here demonstrate that BH4 synthesis is an absolute requirement for induction of NO synthesis by cytokines in cardiac myocytes. We show that PDTC inhibited in a dose-dependent manner both NO and BH4 synthesis induced by a combination of interleukin-1α (IL-1) and interferon-γ (IFN) and that PDTC also prevented the accumulation of iNOS and GTPCH mRNAs induced by IL-1 and IFN. The induction of both genes necessary for NO synthesis in cardiac myocytes appears to be regulated, at least in part, by a common mechanism: NF-κB activation. Our findings also suggest that regulation of pterin synthesis may be an important target for pharmacologic interventions for NO overproduction within the myocardium in cytokine-related cardiac dysfunction.
Key Wordsnitric oxide (NO) nitric oxide synthase cardiac myocyte tetrahydrobiopterin (BH4) GTP cyclohydrolase I (GTPCH)
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