REGULATION OF TYROSINE-HYDROXYLASE AND TETRAHYDROBIOPTERIN BIOSYNTHETIC-ENZYMES IN PC12 CELLS BY NGF, EGF AND IFN-GAMMA

The regulation of catecholamine and tetrahydrobiopterin synthesis was investigated in cultured rat pheochromocytoma PC12 cells following tre atments with nerve growth factor (NGF), epidermal growth factor (EGF) and interferon-gamma (IFN-gamma). NGF and EGF, but not IFN-gamma, caus ed an increase after 24 h in the levels of BH4 and catecholamines, and the activities of tyrosine hydroxylase and GTP cyclohydrolase, the ra te-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-gamma did not affect the activity of sepiapterin reduc tase, the final enzyme in BH, biosynthesis. Rp-cAMP, an inhibitor of c AMP-mediated responses, blocked the induction of tyrosine hydroxylase by NGF or EGF; inhibition of protein kinase C partially blocked the EG F 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 cyclohyd rolase in a protein kinase C-independent manner without affecting tyro sine hydroxylase activity. Our results suggest that both tyrosine hydr oxylase and GTP cyclohydrolase are induced in a coordinate and transcr iption-dependent manner by NGF and EGF, while conditions exist where t he induction of tyrosine hydroxylase and GTP cyclohydrolase is not coo rdinately regulated.