Desensitization of the Y1 cell adrenocorticotropin receptor - Evidence fora restricted heterologous mechanism implying a role for receptor-effector complexes

Receptor desensitization provides a potential mechanism for the regulation of adrenocortical adrenocorticotropin (ACTH) responsiveness. Using the mous e adrenocortical Y1 cell line we demonstrate that ACTH effectively desensit izes the cAMP response of its own receptor, the melanocortin 2 receptor (MC 2R), in these cells with a maximal effect between 30 and 60 min. Neither fo rskolin nor isoproterenol (in Y1 cells stably transfected with the beta (2) -adrenergic receptor) desensitize this ACTH response. ACTH desensitizes its receptor at concentrations at which only a fraction of receptors are occup ied, implying that this mechanism acts on agonist-unoccupied receptors. Y1 cells express G protein-coupled receptor kinase (GRK) 2 and 5, but stable e xpression of a dominant negative GRK2 (K220W) only marginally reduces the d esensitization by ACTH. The protein kinase A (PKA) inhibitor, H89, extingui shes almost the entire desensitization response over the initial 30-min per iod at all concentrations of ACTH. A mutant MC2R in which the single consen sus PKA phosphorylation site has been mutated (S208A) when expressed in MC2 R-negative Y6 cells is also unable to desensitize. These data imply a heter ologous, PKA-dependent, mode of desensitization, which is restricted to ago nist-occupied and -unoccupied MC2R, possibly as a consequence of receptor/e ffector complexes that functionally compartmentalize this receptor.