INDUCTION OF TETRAHYDROBIOPTERIN SYNTHESIS IN HUMAN UMBILICAL VEIN SMOOTH-MUSCLE CELLS BY INFLAMMATORY STIMULI

Tetrahydrobiopterin (BH4) is an obligatory cofactor and regulator of n itric oxide synthases (NOS). We evaluated the biosynthesis of BH, in h uman umbilical vein smooth muscle cells (HUVSMC). Trace amounts of BH4 were found intra-and extracellularly in untreated cells. When HUVSMC were activated by individual inflammatory stimuli (IL-1 beta, TNF alph a, IFN gamma or LPS), both intra-and extracellular levels of BH4 incre ased significantly, with TNF alpha being the most potent single stimul us. Combined inflammatory cytokines synergized in the induction of an up to 600-fold increase of BH4 synthesis. Addition of LPS to the cytok ine mixture led to a further increase of BH4 synthesis. Neopterin, a p roduct of the first intermediate in BH4 biosynthesis, was also raised, but to a much lesser extent. The increase of BH4 synthesis was parall eled by an enhanced expression of isoform-l (the only isoform coding f or the active enzyme) of GTP cyclohydrolase I in cytokine treated cell s. Our results show for the first time that BH4 biosynthesis is strong ly induced by combinations of inflammatory stimuli in HUVSMC. The impo rtance of BH4-dependent NO synthesis in HUVSMC needs, however, additio nal detailed studies. (C) 1998 Elsevier Science B.V.