Induction of tetrahydrobiopterin synthesis in human umbilical vein smooth muscle cells by inflammatory stimuli

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Tetrahydrobiopterin (BH 4) is an obligatory cofactor and regulator of nitric oxide synthases (NOS). We evaluated the biosynthesis of BH 4 in human umbilical vein smooth muscle cells (HUVSMC). Trace amounts of BH 4 were found intra- and extracellularly in untreated cells. When HUVSMC were activated by individual inflammatory stimuli (IL-1 β, TNF α, IFN γ or LPS), both intra- and extracellular levels of BH 4 increased significantly, with TNF α being the most potent single stimulus. Combined inflammatory cytokines synergized in the induction of an up to 600-fold increase of BH 4 synthesis. Addition of LPS to the cytokine mixture led to a further increase of BH 4 synthesis. Neopterin, a product of the first intermediate in BH 4 biosynthesis, was also raised, but to a much lesser extent. The increase of BH 4 synthesis was paralleled by an enhanced expression of isoform-1 (the only isoform coding for the active enzyme) of GTP cyclohydrolase I in cytokine treated cells. Our results show for the first time that BH 4 biosynthesis is strongly induced by combinations of inflammatory stimuli in HUVSMC. The importance of BH 4-dependent NO synthesis in HUVSMC needs, however, additional detailed studies.