LONG-TERM AGONIST EXPOSURE INDUCES UP-REGULATION OF BETA-3-ADRENERGIC RECEPTOR EXPRESSION VIA MULTIPLE CAMP RESPONSE ELEMENTS

During continuous stimulation by agonist, beta-1- and beta-2-adrenergic receptors (ARs) undergo processes that lead to decreases in receptor expression. This receptor down-regulation serves to limit the cellular cAMP response during chronic agonist exposure. In the recently described third subtype of the beta-AR, denoted beta-3AR, we found four potential cAMP response elements in the 5' flanking region, suggesting that expression of this receptor might be positively regulated by agonists. These elements were cloned into the vector pA10CAT2, which contains a chloramphenicol acetyltransferase reporter gene, and transiently expressed in VERO cells. Three of these elements, TGACTCCA, TGAGGTCT, and CGAGGTCA (located 518, 622, and 1125 bases upstream of the beta-3AR coding block, respectively) were found to increase transcription of the chloramphenicol acetyltransferase gene in response to cAMP analogues and agents that increase intracellular cAMP. 3T3-F442A cells, when differentiated into the adipocyte phenotype by insulin, expressed beta-3AR, and nuclear runoff studies from such cells confirmed cAMP enhancement of beta-3AR mRNA transcription. In these cells, beta-3AR mRNA increased in response to exposure to the beta-3AR agonist isoproterenol and remained elevated during exposures of up to 24-30 hr. During prolonged exposure to agonist, no downregulation of beta-3AR expression in 3T3-F442A cells occurred. Indeed, beta-3AR expression increased during agonist exposure to almost-equal-to 165% of basal expression. In marked contrast, beta-1AR expression declined by almost-equal-to 70% in response to chronic agonist exposure. These studies reveal a subtype-specific prolonged transcriptional regulation of a beta-AR gene by the end product of its signal transduction pathway. Thus, the beta-3AR undergoes a paradoxical increase in receptor expression during chronic agonist exposure.