REGULATION OF THE RAT PROOPIOMELANOCORTIN GENE-EXPRESSION IN ATT-20 CELLS .2. EFFECTS OF THE PITUITARY ADENYLATE CYCLASE-ACTIVATING POLYPEPTIDE AND VASOACTIVE INTESTINAL POLYPEPTIDE

Pituitary adenylate cyclase-activating polypeptide (PACAP) and vasoact ive intestinal polypeptide (VIP), members of the glucagon-secretin fam ily, have recently been suggested to be involved in the regulation of corticotropin (ACTH) secretion. In this study, we examined the effects of both peptides on POMC gene expression. Using AtT20PL, a clone of t he AtT20 mouse corticotroph tumor cells stably transfected with 0.7 kb of the rat POMC 5' promoter-luciferase fusion gene, the effects of bo th peptides on the POMC promoter activity were estimated by a lucifera se assay. PACAP stimulated POMC 5' promoter activity as well as cAMP g eneration and ACTH secretion in a dose- and time-dependent manner, wit h the maximal effect being observed 3 h after the start of incubation. A similar effect was observed with VIP. Although the combined effects of PACAP/CRH or VIP/CRH were greater than that of either hormone alon e, no such. effect was observed between PACAP and VIP. Furthermore, RT -PCR analysis showed the presence of only the PVR3 receptor subtype in this cell line, which is known to have a similar affinity to PACAP an d VIP, indicating that both peptides exert their effects through the s ame receptor. In contrast to the effect of CRH, which was completely a bolished by a protein kinase A inhibitor H89, the effects of PACAP/VIP on POMC expression persisted during H89 treatment, suggesting the inv olvement of alternative intracellular signaling pathway(s) distinct fr om the protein kinase A system. Our results suggest that PACAP and VIP have positive effects on POMC gene expression and that multiple signa ling pathways are involved in the transcriptional event.