THE CATECHOLAMINE STIMULATED ADENYLATE CYCLASE OF DIFFERENTIATING 3T3-L1 CELLS
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The development of 3T3-L1 preadipocytes into adipocytes was found to be associated with the acquisition of catecholamine stimulated adenylate cyclase activity. Preadipocyte adenylate cyclase is stimulated only 20% by (beta)-adrenergic agonists while the adipocyte enzyme in stimulated 6- to 10- fold by isoproterenol. Studies were begun to elucidate the basis for the appearance of hormone responsiveness. Preadipocytes were found to possess 65% of the number of receptors observed in the catecholamine responsive adipocytes. The receptors in the two cell types had the same affinity for ('125)I-HYP (K(,D) = 0.2 nM). In contrast to the unaltered properties of the (beta)-receptor, the regulation of adenylate cyclase activity by GTP changes markedly during adipocyte development. GTP (10 (mu)M) stimulates the preadipocyte enzyme equally well in the absence or presence of isoproterenol, wile the GTP-mediated enhancement of adipocyte membrane adenylate cyclase activity is highly dependent on the presence of the (beta)-adrenergic agonist. In addition, hydrolysis resistant analogs of GTP activate preadipocyte adenylate cyclase without the lag period which is observed in the activation of the adipocyte enzyme. This lag period is essentially eliminated in the presence of isoproterenol. These observations suggested that the G/F protein exhibited different sets of regulatory (coupling) properties in undifferentiated and differentiated cells.;In complementary experiments, the cholera toxin catalyzed ('32)P-ADP-ribosylation of 3T3-L1 membranes revealed dramatic differences between the G/F subunits of preadipocytes and adipocytes. In adipocyte membranes, 13-fold more ('32)P was incorporated into the 42,000-dalton component and 4-fold more labeling was observed in the 49,000- to 50,000-dalton doublet than in the corresponding G/F subunits in preadipocyte membranes. Furthermore, determinations of the relative proportions of the two ('32)P-labeled components of G/F disclosed a preponderance (4.2:1) of the 49,000- to 50,000-dalton species in preadipocyte membranes and nearly equal amounts of the 42,000-dalton and 49,000- to 50,000-dalton polypeptides in adipocyte membranes. These findings raise the possibility that differentiation associated changes in the coupling properties and regulation of catecholamine-stimulated adenylate cyclase may be determined principally by the modulation of the levels, proportions and/or properties of the constituents of G/F.;The subtype of the (beta)-adrenergic receptor expressed in 3T3-L1 preadipocytes and adipocytes differentiated with dexamethasone and methylisobutylxanthine was determined by comparing the affinity of the receptors for epinephrine, norepinephrine and (beta)-1 and (beta)-2 selective drugs. . . . (Author's abstract exceeds stipulated maximum length. Discontinued here with permission of author.) UMI.
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