John J Gildea, Jonathan A Israel, Andrew K Johnson, Jin Zhang, Pedro A Jose, Robin A Felder
In moderate sodium-replete states, dopamine 1-like receptors (D1R/D5R) are responsible for regulating >50% of renal sodium excretion. This is partly mediated by internalization and inactivation of NaKATPase, when associated with adapter protein 2. We used dopaminergic stimulation via fenoldopam (D1-like receptor agonist) to study the interaction among D1-like receptors, caveolin-1 (CAV1), and the G protein-coupled receptor kinase type 4 in cultured human renal proximal tubule cells (RPTCs). We compared 2 groups of RPTCs, 1 of cell lines that were isolated from normal subjects (nRPTCs) and a second group of cell lines that have D1-like receptors that are uncoupled (uncoupled RPTCs) from adenylyl cyclase second messengers. In nRPTCs, fenoldopam increased the plasma membrane expression of D1R (10.0-fold) and CAV1 (1.3-fold) and markedly decreased G protein-coupled receptor kinase type 4 by 94+/-8%; no effects were seen in uncoupled RPTCs. Fenoldopam also increased the association of adapter protein 2 and NaKATPase by 53+/-9% in nRPTCs but not in uncoupled RPTCs. When CAV1 expression was reduced by 86.0+/-8.5% using small interfering RNA, restimulation of the D1-like receptors with fenoldopam in nRPTCs resulted in only a 7+/-9% increase in association between adapter protein 2 and NaKATPase. Basal CAV1 expression and association with G protein-coupled receptor kinase type 4 was decreased in uncoupled RPTCs (58+/-5% decrease in association) relative to nRPTCs. We conclude that the scaffolding protein CAV1 is necessary for the association of D1-like receptors with G protein-coupled receptor kinase type 4 and the adapter protein 2-associated reduction in plasma membrane NaKATPase.
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