W P Hausdorff

Turning off the signal: desensitization of beta-adrenergic receptor function.

Cellular responses to many hormones and neurotransmitters wane rapidly despite continuous exposure of cells to these stimuli. This phenomenon, termed desensitization, has been particularly well studied for the stimulation of cAMP levels by plasma membrane β‐adrenergic receptors (βAR). The molecular mechanisms underlying rapid βAR desensitization do not appear to require internalization of the receptors, but rather an alteration in the functioning of βAR themselves that uncouples the receptors from the stimulatory G protein Gs. This uncoupling phenomenon involves phosphorylation of βAR by at least two kinases, PKA and the βAR kinase (βARK), which are activated under different desensitizing conditions. Receptor phosphorylation by the two kinases leads to desensitization of the receptor response via distinct biochemical mechanisms, and additional cytosolic factors appear to be involved in the case of βARK. Numerous experimental approaches have been used recently to elucidate the molecular details of this ubiquitous biological process.— Hausdorff, W. P.; Caron, M. G.; Lefkowitz, R. J. Turning off the signal: desensitization of β‐adrenergic receptor function. FASEB J. 4: 2881‐2889; 1990.

Role of phosphorylation in desensitization of the β-adrenoceptor

Abstract Regulation of receptors allows their responses to be modified rapidly and appropriately according to the needs of the environment. Multiple mechanisms are involved in the loss of sensitivity that follows exposure to agonists. Receptor sequestration, a rapid and transient event, and receptor downregulation, which requires more prolonged agonist exposure, contribute to this effect. However, in this article Bob Lefkowitz and colleagues focus primarily on the recent developments in understanding mechanisms of rapid desensitization involving receptor phosphorylation. Various molecular biological techniques have been used to demonstrate the important roles of two particular kinases — βARK and protein kinase A — in this regard .

Mechanisms of Ligand-Induced Desensitization of Beta-Adrenergic Receptors

Attenuation of responsiveness to extracellular signal molecules (neurotransmitters, hormones, growth factors, and so on) is a cellular regulatory mechanism commonly observed in organisms from microbes to mammals. In the slime mold Dictyostelium, desensitization to an extracellular signal, cyclic AMP, is programmed to facilitate a periodic synchronous behavior that fosters aggregation within the cellular population (Devreotes and Zigmond, 1988). In the mammalian nervous and endocrine systems, desensitization to the effects of neurotransmitters and hormones may be a mechanism for maintenance of target cell function within normal limits (Perkins et al., 1982; Harden, 1983; Sibley and Lefkowitz, 1985; Benovic et al., 1988)

Overexpression of pp60c-src is associated with altered regulation of adenylyl cyclase

The ability of activators of the beta-adrenergic receptor to elevate intracellular cAMP levels in murine fibroblasts is enhanced upon overexpression of avian c-src [Bushman et al. (1990) Proc. natn. Acad. Sci. U.S.A. 87, 7462-7466]. To investigate the molecular basis for this effect, we prepared particulate fractions from control and pp60c-src overexpressing C3H10T1/2 fibroblasts and assessed the relative abilities of several activators of the beta-adrenergic receptor-Gs-adenylyl cyclase (AC) signal transduction pathway to stimulate the enzymatic response. Two- to three-fold increases in both the sensitivity and maximum responsiveness of AC to the beta-adrenergic agonist isoproterenol were consistently observed in fractions prepared from the c-src overexpressing cells. Interestingly, the AC response to two agents believed to act directly at the level of the G protein were either enhanced (NaF) or unaffected (GTP gamma S) by c-src overexpression. Finally, overexpression of c-src was associated with a reduced ability of both Mn2+ and forskolin to activate AC directly. These results suggest that overexpression of wild type c-src may affect two distinct steps in the regulation of AC exerting a positive effect at the level of Gs activation and a negative effect on AC itself. As no differences in the relative number or affinity of beta-adrenergic receptors, or in the level of AC, Gs alpha or G beta, were detected between control cells and those overexpressing c-src, we propose that pp60c-src overexpression results in a modification of one or more components in this signal transduction pathway.