Utilization of exogenous tetrahydrobiopterin in nitric oxide synthesis in human neuroblastoma cell line

We and others have previously reported that neurons expressing neuronal nitric oxide synthase (nNOS) do not co-express GTP cyclohydrolase I, the enzyme that synthesize its cofactor tetrahydrobiopterin (BH4). BH4 is released from catecholaminergic cells and nNOS-expressing cells are located close to BH4-producing catecholaminergic nerve terminals. We show that BH4 is taken up into the nNOS-expressing human neuroblastoma cells TGW-I-nu in a linear, dose-dependent manner and elevates NO production. Direct exposure to BH4, dihydrobiopterin or biopterin, or coculture with catecholaminergic CATH.a cells increases NO production by TGW-I-nu. Thus, BH4-requiring nNOS cells may obtain BH4 from neighboring catecholaminergic cells or terminals and an intercellular crosstalk may exist between the two cells in vivo.