Transcriptional and post-transcriptional regulation of Tyrosine Hydroxylase messenger RNA in PC12 cells during persistent stimulation by VIP and PACAP38: Differential regulation by protein kinase A and protein kinase C-dependent pathways

VIP and PACAP38 are closely related peptides that are released in the adrenal gland and sympathetic ganglia and regulate catecholamine synthesis and release. We used PC12 cells as a model system to examine receptor and second messenger pathways by which each peptide stimulates transcriptional and post-transcriptional mechanisms that regulate the level of the mRNA for tyrosine hydroxylase (TH), the rate-limiting enzymatic step in catecholamine synthesis. Concentration-response studies revealed that PACAP38 had both greater efficacy and potency than VIP. The specific PAC1 receptor antagonist PACAP[6-38] blocked the effects of each peptide on TH mRNA content while the PACAP/VIP type II receptor antagonist (N-AC-Tyr¹-D-Phe²)-GRF-(1-29)-NH₂ was without effect. At equipotent concentrations, each peptide stimulated a transient increase in TH gene transcription lasting less than 3 h. Continuous VIP treatment stimulated a transient increase in TH mRNA lasting less than 24 h. In contrast, continuous exposure to PACAP38 stimulated a stable increase in TH mRNA that persisted for 2 days in the absence of elevated transcription, pointing to different post-transcriptional effects of the two peptides. PACAP38 alone had no effect on the magnitude of TH gene transcription or TH mRNA in A126-1B2 PKA-deficient PC12 cells. However, when combined with dexamethasone, PACAP38 produced a synergistic increase in TH mRNA in the absence of PACAP38-stimulated TH gene transcription. In contrast, VIP had no effect on either TH mRNA content or TH gene transcription in this model. PACAP38, but not VIP, stimulated PKC activity. Calphostin C antagonized the effect of PACAP38 on the persistent post-transcriptional elevation in TH mRNA. Thus, the results support the conclusion that VIP and PACAP38 each stimulate PAC1 receptors to increase TH gene transcription through a PKA-controlled pathway, but their divergent post-transcriptional effects result at least partly from differing abilities to stimulate PKC.