Inhibition of Protein Kinase C Prevents Rapid Desensitization of Type 1B Angiotensin II Receptor

Abstract

Abstract The type 1B angiotensin II (AT 1B ) receptor cloned from rat kidney was stably expressed in Chinese hamster ovary cells. The stably expressed receptor was characterized by radioligand binding studies and functional coupling to inositol 1,4,5-triphosphate (IP 3 ) formation. Exposure of cells expressing the AT 1B receptor to angiotensin II (Ang II) resulted in a rapid and dose-dependent homologous desensitization of receptor-mediated production of IP 3 , with an essentially complete desensitization at an agonist concentration >10 nmol/L. Binding studies revealed no significant change in the number of AT 1B receptors in transfected cells exposed to 1 nmol/L Ang II, whereas exposure to 100 nmol/L Ang II caused a rapid decrease of cell surface receptors, with a 75% loss of receptor number seen at 1 hour. Rapid desensitization occurred in the absence of receptor internalization. Blockade of receptor internalization with concanavalin A had at most only a slight effect on the agonist-induced desensitization. This indicates that factors other than internalization are chiefly responsible for the rapid agonist-induced desensitization. Phorbol 12-myristate 13-acetate (PMA), a protein kinase C (PKC) activator, caused rapid desensitization of the receptor-mediated IP 3 response. Neither tyrosine kinase inhibitors nor a protein kinase A activator affected the receptor-mediated IP 3 response. The specific PKC inhibitor GF109203X or PKC depletion by prolonged treatment with 1 μmol/L PMA completely blocked the PMA-dependent desensitization. Desensitization evoked by a low Ang II agonist concentration (1 nmol/L) was reversed by the PKC-specific inhibitor GF109203X or PKC depletion, whereas the desensitizing effect at a high agonist concentration (100 nmol/L) is only partially prevented by PKC inhibitory treatment. These results demonstrate that PKC plays a crucial role in the desensitization of the AT 1B receptor. They also suggest that receptor internalization and an additional PKC-independent pathway also contribute to desensitization of the AT 1B receptor in transfected cells.