Abstract
Homologous (agonist-specific) desensitization of beta-adrenergic receptors
(beta ARs) is accompanied by and appears to require phosphorylation
of the receptors. We have recently described a novel protein kinase,
beta AR kinase, which phosphorylates beta ARs in vitro in an agonist-dependent
manner. This kinase is inhibited by two classes of compounds, polyanions
and synthetic peptides derived from the beta 2-adrenergic receptor
(beta 2AR). In this report we describe the effects of these inhibitors
on the process of homologous desensitization induced by the beta-adrenergic
agonist isoproterenol. Permeabilization of human epidermoid carcinoma
A431 cells with digitonin was used to permit access of the charged
inhibitors to the cytosol; this procedure did not interfere with
the pattern of isoproterenol-induced homologous desensitization of
beta 2AR-stimulated adenylyl cyclase. Inhibitors of beta AR kinase
markedly inhibited homologous desensitization of beta 2ARs in the
permeabilized cells. Inhibition of desensitization by heparin, the
most potent of the polyanion inhibitors of beta AR kinase, occurred
over the same concentration range (5-50 nM) as inhibition of purified
beta AR kinase assessed in a reconstituted system. Inhibition of
desensitization by heparin was accompanied by a marked reduction
of receptor phosphorylation in the permeabilized cells. Whereas inhibitors
of beta AR kinase inhibited homologous desensitization, inhibitors
of protein kinase C and of cyclic-nucleotide-dependent protein kinases
were ineffective. These data establish that phosphorylation of beta
ARs by beta AR kinase is an essential step in homologous desensitization
of the receptors. They further suggest a potential therapeutic value
of inhibitors of beta AR kinase in inhibiting agonist-induced desensitization.
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