Regulation of tyrosine hydroxylase and tetrahydrobiopterin biosynthetic enzymes in PC12 cells by NGF, EGF and IFN-γ

The regulation of catecholamine and tetrahydrobiopterin synthesis was investigated in cultured rat pheochromocytoma PC12 cells following treatments with nerve growth factor (NGF), epidermal growth factor (EGF) and interferon-γ (IFN-γ). NGF and EGF, but not IFN-γ, caused an increase after 24 h in the levels of BH₄ and catecholamines, and the activities of tyrosine hydroxylase and GTP cyclohydrolase, the rate-limiting enzymes in catecholamine and BH₄ synthesis, respectively. Actinomycin D, a transcriptional inhibitor, blocked treatment-induced elevations in tyrosine hydroxylase and GTP cyclohydrolase activities. NGF, EGF or IFN-γ did not affect the activity of sepiapterin reductase, the final enzyme in BH₄ biosynthesis. Rp-cAMP, an inhibitor of cAMP-mediated responses, blocked the induction of tyrosine hydroxylase by NGF or EGF; inhibition of protein kinase C partially blocked the EGF effect, but not the NGF effect. NGF also induced GTP cyclohydrolase in a cAMP-dependent manner, while the EGF effect was not blocked by Rp-cAMP or protein kinase C inhibitors. Sphingosine induced GTP cyclohydrolase in a protein kinase C-independent manner without affecting tyrosine hydroxylase activity. Our results suggest that both tyrosine hydroxylase and GTP cyclohydrolase are induced in a coordinate and transcription-dependent manner by NGF and EGF, while conditions exist where the induction of tyrosine hydroxylase and GTP cyclohydrolase is not coordinately regulated.