Vasoactive Intestinal Polypeptide‐Related Peptides Modulate Tyrosine Hydroxylase Gene Expression in PC12 Cells Through Multiple Adenylate Cyclase‐Coupled Receptors

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Abstract
We investigated the receptor mechanisms by which vasoactive intestinal polypeptide (VIP) and related peptides exert their effects on tyrosine hydroxylase (TH) gene expression. VIP, secretin, and peptide histidine isoleucine (PHI) each produced increases in TH gene expression, as measured by increases in TH mRNA levels and TH activity. The concentrations at which the effects of these peptides were maximal differed for TH activity and TH mRNA. Moreover, maximal increases in TH activity were 130-140% of control, whereas maximal increases in TH mRNA were 250% of control. The concentration dependence of the increases in TH mRNA in response to the three peptides was analyzed by fitting the data to nonlinear regression models that assume either one or two components to the response. The data for secretin fit best to a model that assumes a single component to the increase in TH mRNA levels. The data derived for PHI and VIP fit best to models that assumed two components to the TH mRNA response. These data suggested that there may be more than one receptor or signal transduction mechanism involved in the response to the various peptides. We examined whether the peptides exerted their effects through common or multiple second messenger systems. The ability of maximally active concentrations of these peptides to stimulate increases in TH mRNA was not additive, indicating that the peptides work through a common receptor or signal transduction pathway. Each peptide stimulated increases in protein kinase A (PKA) activity. Secretin and VIP were ineffective in increasing TH mRNA levels in a PKA-deficient mutant PC12 cell line (A 126-1B2). Moreover, the adenylate cyclase antagonist 2′,5′-dideoxyadenosine prevented the increase in TH mRNA produced by each peptide. Thus, each peptide requires an intact cyclic AMP second messenger pathway to produce changes in TH gene expression, suggesting that the complex pattern of response to VIP and PHI revealed by concentration-response analysis was due to the actions of these peptides at multiple receptors. To evaluate this possibility, we examined the effect of several peptide receptor antagonists on the increase in TH gene expression elicited by VIP, PHI, and secretin. The secretin antagonist secretin (5–27) (20 μM) had no significant effect on VIP or PHI stimulation of TH gene expression, but reduced the effect of secretin. The VIP antagonist VIP (10–28) (20 μM) reduced the effect of VIP on increasing TH mRNA, but had no significant effect on the response of TH mRNA to secretin or PHI. Interestingly, the VIP antagonist [Ac-Tyr1,D-Phe2]-growth hormone releasing factor [GRF(1–29)] amide (20 μM) potentiated the effect of VIP on elevating TH mRNA levels, but had no effect on secretin-stimulated TH mRNA induction. To determine whether this response was mediated through the cyclic AMP pathway, we examined the effects of the VIP antagonist [Ac-Tyr1,D-Phe2]-GRF(1–29) amide on VIP stimulation of PKA activity. Although the antagonist had no effect alone, it enhanced stimulation of PKA activity by VIP. Taken together, these findings indicate that VIP and secretin stimulate TH mRNA through different adenylate cyclase-linked receptors and that a second VIP receptor may modulate TH induction by inhibiting VIP stimulation of PKA.

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