Adrenomedullin augments nitric oxide and tetrahydrobioptein synthesis in cytokine-stimulated vascular smooth muscle cells.

Abstract

Immunostimulants increase nitric oxide (NO) and tetrahydrobiopterin (BH4) synthesis in vascular smooth muscle cells (VSMC) by coinducing expression of an isoform of NO synthase (iNOS) and GTP cyclohydrolase I (GTPCH). GTPCH is the first and rate-limiting enzyme in the synthesis of BH4, a cofactor of NO synthases. Given the adrenomedullin (AM) increases NO production, this effect of AM may involve modulation of BH4 synthesis in cytokine-stimulated VSMC. We investigated the effects of AM on the synthesis of NO and BH4, the expression of iNOS and GTPCH mRNA, and the promoter activity of iNOS and GTPCH genes in rat VSMC stimulated with interleukin-1 (IL-1). IL-1 increased both NO and BH4 synthesis as well as the abundance of iNOS and GTPCH mRNA. AM significantly increased both NO and BH4 synthesis caused by IL-1 stimulation. AM also augmented the IL-1-induced increase in the abundance of iNOS and GTPCH mRNA. IL-1 activated the iNOS promoter activity as well as the GTPCH promoter activity in VSMC. AM alone had no effect on the activity of either the iNOS or the GTPCH promoter, nor did it potentiate the activation by IL-1 of either of these promoters. These results suggest that AM increases IL-1-induced NO and BH4 synthesis by enhancing the expression of iNOS and GTPCH genes at the post-transcriptional level. Thus, the potentiating effect of AM on NO synthesis appears to be associated with an increased expression of both genes necessary for cellular NO synthesis in VSMC.