VASOACTIVE-INTESTINAL-PEPTIDE INDUCES BOTH TYROSINE-HYDROXYLASE ACTIVITY AND TETRAHYDROBIOPTERIN BIOSYNTHESIS IN PC12 CELLS

Vasoactive intestinal peptide plays an important role in the trans-syn aptic activation of tyrosine hydroxylase in sympathoadrenal tissues in response to physiological stress. Since tyrosine hydroxylase is thoug ht to be subsaturated with its cofactor, tetrahydrobiopterin, we teste d the hypothesis that up-regulation of tyrosine hydroxylase gene expre ssion following vasoactive intestinal peptide treatment is accompanied by a concomitant elevation of intracellular tetrahydrobiopterin biosy nthesis. We also investigated the second messenger systems involved in vasoactive intestinal peptide's effects on tetrahydrobiopterin metabo lism. Our results demonstrate that treatment of PC12 cells for 24 h wi th vasoactive intestinal peptide induced intracellular tetrahydrobiopt erin levels 3.5-fold. This increase was due to increased expression of the gene encoding GTP cyclohydrolase, the initial and rate-limiting e nzyme in tetrahydrobiopterin biosynthesis, which was blocked by the tr anscriptional inhibitor, actinomycin D. Activation of tyrosine hydroxy lase and GTP cyclohydrolase by vasoactive intestinal peptide was media ted by cyclic-AMP. Furthermore, stimulation of cyclic-AMP-mediated res ponses or protein kinase C activity induced the maximal in vitro activ ities of both tyrosine hydroxylase and GTP cyclohydrolase; the respons es were additive when both treatments were combined. Induction of sphi ngolipid metabolism had no effect on the activation of tyrosine hydrox ylase, while it induced GTP cyclohydrolase in a protein kinase C-indep endent manner. Our results support the hypothesis that intracellular t etrahydrobiopterin levels are tightly linked to tyrosine hydroxylation and that tetrahydrobiopterin bioavailability modulates catecholamine synthesis. (C) 1998 IBRO. Published by Elsevier Science Ltd.