βAR downregulation and desensitization are hallmarks of heart failure. Agonist occupied βARs undergo desensitization through phosphorylation by G-protein coupled receptor kinases leading to βAR internalization. Phosphorylated βAR becomes resensitized following dephosphorylation by PP2A in the endosomes. In contrast to this paradigm our recent studies have shown that resensitization of βARs can occur at the plasma membrane through the inhibition of PP2A activity by I2PP2A. We generated a stable HEK cell line expressing short hairpin RNA targeting I2PP2A (shRNA-I2PP2A). Confocal microscopy studies show cells expressing shRNA-I2PP2A resulted in significant loss of receptor phosphorylation despite the presence of the agonist. Radioligand binding and confocal imaging also showed marked inhibition of receptor internalization upon depletion of I2PP2A with significant PP2A activation. We also observed preservation of βAR function despite the presence of agonist measured by cAMP generation and adenylyl cyclase activity. To further dissect the interaction of I2PP2A-PP2A we generated mutants of PP2A from amino acids 263-309. Over expression of these PP2A mutant peptides significantly reversed receptor phosphorylation upon isoproterenol (ISO) stimulation compared to full length PP2A. Also, expression of PP2A mutant showed marked increase in adenylyl cyclase activity in response ISO stimulation suggesting that this mutant I2PP2A competes out inhibitory I2PP2A interaction with PP2A. To test whether alteration in βAR resensitization contributes to cardiac dysfunction with stress we generated transgenic (Tg) mice with cardiac specific over expression of wt I2PP2A and mutant I2PP2A (phopspho- and dephospho-). ISO treatment of wt I2PP2A Tg and littermate controls showed that wt I2PP2A Tg mice had significant βAR dysfunction and cardiac hypertrophy. Assessment of age dependent cardiac function of these mice showed that wt I2PP2A Tg and phospho-I2PP2A Tg mice have cardiac hypertrophic response followed by dilated cardiomyopathy; expression of dephospho-I2PP2A mutant reversed this phenotype. These studies suggest targeting I2PP2A alters receptor function and may have implications in cardiac remodeling and hypertrophy with cardiac stress.
Agonist-induced desensitization of dopamine D1 receptor-stimulated adenylyl cyclase activity is temporally and biochemically separated from D1 receptor internalization.
