PITUITARY ADENYLATE CYCLASE-ACTIVATING POLYPEPTIDE INDUCES EXPRESSIONOF CORTICOSTEROID-BINDING GLOBULIN IN CULTURED FETAL HEPATOCYTES - SYNERGY WITH TRIIODOTHYRONINE

The purpose of the present study was to determine whether functional r eceptors for pituitary adenylate cyclase-activating polypeptide (PACAP ) are expressed in cultured rat fetal hepatocytes and eventually play a role in regulating gene expression of corticosteroid-binding globuli n (CBG). We found PACAP38 and PACAP27 to elevate cAMP levels in hepato cytes in a dose-dependent manner, with a plateau being achieved at 10 nM and EC(50) values of about 0.5-1 nM. PACAP failed to alter the turn over of inositol phosphates, whereas PACAP and VIP stimulated cAMP acc umulation in an equipotent manner, suggesting the presence in these ce lls of type II receptor isoforms. As revealed by measurements of both CBG mRNA levels and concentrations of binding sites, long-term treatme nt of fetal cells with 10 nM PACAP, although resulting in partial dese nsitization of peptide-induced cAMP accumulation, caused a significant 3-fold elevation in CBG synthesis. This stimulatory influence of PACA P was mimicked by the cell permeant N-6,2'-O-dibutyryladenosine 3',5'- phosphate (dbcAMP). Treatment of hepatocytes with tri-iodothyronine (T -3) enhanced CBG expression and, most interestingly, appeared to syner gize with PACAP to elicit a 2-3-fold amplification of CBG synthesis. T his study thus provides first evidence for the up-regulation by PACAP and cAMP of CBG expression in fetal hepatocytes and for T-3's playing a synergistic role in enhancing PACAP-induced synthesis of the binder.