Desensitization of Gs-coupled receptor signaling by constitutively active mutants of the human lutropin/choriogonadotropin receptor

Activating mutations of the human lutropin/choriogonadotropin receptor (hLHR), a Gs-coupled receptor, have been identified in young boys with gonadotropin-independent precocious puberty (testotoxicosis). The properties of these mutants have typically been characterized in heterologous cells transfected with recombinant mutant receptor and compared with those expressing wild-type (wt) receptor. The affected individuals, however, are heterozygous and, therefore, express wt receptor in addition to the mutant receptor. The present studies were undertaken to determine what effects, if any, coexpression of a constitutively active hLHR might have on hLHR(wt). HEK 293 cells were cotransfected with hLHR(wt) and hLHR(L457R), a mutant that we have previously shown to be both constitutively active and unresponsive to further hormonal stimulation as determined in both intact cells and isolated membranes. When coexpressed at submaximal concentrations, L457R does not decrease the cell surface expression of hLHR(wt). Coexpression of L457R, however, causes an attenuation of human choriogonadotropin-stimulated cAMP production by hLHR(wt). We show that this attenuation is caused by an activation of the phosphodiesterase (PDE)4D3. Additional experiments demonstrate that the coexpression of L457R with the human beta(2)-adrenergic receptor causes an attenuation of isoproterenol-stimulated cAMP and that other activating mutations of the hLHR also induce PDE activation. Taken together, these data demonstrate that the activation of PDE is a compensatory mechanism common to hLHR constitutively active mutants and that cellular responses to agonists that stimulate Gs-coupled receptors may be blunted in tissues expressing these activating mutants. This novel desensitizing effect of constitutively active hLHRs on hormone-stimulated cAMP production has not been noticed before and would typically not be detected because of the routine inclusion of PDE inhibitors in experiments determining cAMP accumulation. Importantly, however, this mechanism of desensitization would be expected to occur in a physiological context in which PDE inhibitors are not present and thus may influence hormonal signaling in cells expressing the activating hLHR mutant.