TRANSCRIPTIONAL DOWN-REGULATION BY INSULIN OF THE BETA(3)-ADRENERGIC RECEPTOR EXPRESSION IN 3T3-F442A ADIPOCYTES - A MECHANISM FOR REPRESSING THE CAMP SIGNALING PATHWAY

Modulation of the three beta-adrenergic receptor subtypes (beta-ARs) b y insulin was investigated in mouse 3T3-F442A adipocytes. Saturation a nd competition experiments measuring binding of I-125-labeled (-)-cyan opindolol to adipocyte membranes demonstrated that cell exposure to in sulin for 4 days caused a 3.5-fold decrease in the density of the majo r beta-AR component of the adipocyte, the beta(3)-AR, while beta(1)-AR sites remained unchanged and beta(2)-ARs were undetectable. This corr elated with a lower potency of the beta(3)-AR-selective agonists CGP12 177, ICI201651, and BRL37344 in stimulating adenylate cyclase. Norther n blotting analysis indicated that insulin induced a rapid and sharp d ecrease in beta(3)-AR mRNA levels. This effect was detectable at low i nsulin concentrations (EC(50) = 3 nM) and was not observed in the pres ence of insulin-like growth factor I, suggesting an insulin receptor-m ediated phenomenon. Reverse transcriptase-PCR analysis showed that, in contrast to its dramatic down-regulatory effect on beta(3)-AR mRNA, i nsulin did not modify the levels of beta(1)- and beta(2)-AR transcript s. As assessed by nuclear run-on assays, insulin inhibited the beta(3) -AR gene transcription rate by 90% within 30 min. mRNA turnover experi ments showed that the half-life of beta(3)-AR mRNA was short (90 min) and remained unaffected by insulin. These findings demonstrate the gen etic control of a beta-AR subtype expression by insulin and reveal a m echanism for the regulation by this hormone of cAMP-dependent biologic al processes in adipocytes.