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) by insulin was investigated in mouse 3T3-F442A adipocytes. Saturation and competition experiments measuring binding of I-125-labeled (-)-cyanopindolol to adipocyte membranes demonstrated that cell exposure to insulin for 4 days caused a 3.5-fold decrease in the density of the major beta-AR component of the adipocyte, the beta(3)-AR, while beta(1)-AR sites remained unchanged and beta(2)-ARs were undetectable. This correlated with a lower potency of the beta(3)-AR-selective agonists CGP12177, ICI201651, and BRL37344 in stimulating adenylate cyclase. Northern blotting analysis indicated that insulin induced a rapid and sharp decrease in beta(3)-AR mRNA levels. This effect was detectable at low insulin concentrations (EC(50) = 3 nM) and was not observed in the presence of insulin-like growth factor I, suggesting an insulin receptor-mediated phenomenon. Reverse transcriptase-PCR analysis showed that, in contrast to its dramatic down-regulatory effect on beta(3)-AR mRNA, insulin did not modify the levels of beta(1)- and beta(2)-AR transcripts. As assessed by nuclear run-on assays, insulin inhibited the beta(3)-AR gene transcription rate by 90% within 30 min. mRNA turnover experiments showed that the half-life of beta(3)-AR mRNA was short (90 min) and remained unaffected by insulin. These findings demonstrate the genetic control of a beta-AR subtype expression by insulin and reveal a mechanism for the regulation by this hormone of cAMP-dependent biological processes in adipocytes.