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Virtually all known biological actions stimulated by beta-adrenergic and other adenylate cyclase coupled receptors are mediated by cAMP-dependent protein kinase. Nonetheless, "homologous" or beta-adrenergic agonist-specific desensitization does not require cAMP. Since beta-adrenergic receptor phosphorylation may be involved in desensitization, we studied agonist-promoted receptor phosphorylation during homologous desensitization in wild-type S49 lymphoma cells (WT) and two mutants defective in the cAMP-dependent pathway of beta-agonist-stimulated protein phosphorylation (cyc- cannot generate cAMP in response to beta-adrenergic agonists; kin- lacks cAMP-dependent kinase). All three cell types demonstrate rapid, beta-adrenergic agonist-promoted, stoichiometric phosphorylation of the receptor which is clearly not cAMP mediated. The amino acid residue phosphorylated is solely serine. These data demonstrate, for the first time, that catecholamines can promote phosphorylation of a cellular protein (the beta-adrenergic receptor) via a cAMP-independent pathway. Moreover, the ability of cells with mutations in the adenylate cyclase-cAMP-dependent protein kinase pathway to both homologously desensitize and phosphorylate the beta-adrenergic receptors provides very strong support for the notion that receptor phosphorylation may indeed be central to the molecular mechanism of desensitization.
Strasser et al. (Sat,) studied this question.