G-protein-coupled receptor kinase 4 causes renal angiotensin II type 2 receptor dysfunction by increasing its phosphorylation

Activation of the angiotensin II type 2 receptor (AT₂R) induces diuresis and natriuresis. Increased expression or/and activity of G-protein-coupled receptor kinase 4 (GRK4) or genetic variants (e.g., GRK4γ142V) cause sodium retention and hypertension. Whether GRK4 plays a role in the regulation of AT₂R in the kidney remains unknown. In the present study, we found that spontaneously hypertensive rats (SHRs) had increased AT₂R phosphorylation and impaired AT₂R-mediated diuretic and natriuretic effects, as compared with normotensive Wistar-Kyoto (WKY) rats. The regulation by GRK4 of renal AT₂R phosphorylation and function was studied in human (h) GRK4γ transgenic mice. hGRK4γ142V transgenic mice had increased renal AT₂R phosphorylation and impaired AT₂R-mediated natriuresis, relative to hGRK4γ wild-type (WT) littermates. These were confirmed in vitro; AT₂R phosphorylation was increased and AT₂R-mediated inhibition of Na⁺-K⁺-ATPase activity was decreased in hGRK4γ142V, relative to hGRK4γ WT-transfected renal proximal tubule (RPT) cells. There was a direct physical interaction between renal GRK4 and AT₂R that was increased in SHRs, relative to WKY rats. Ultrasound-targeted microbubble destruction of renal GRK4 decreased the renal AT₂R phosphorylation and restored the impaired AT₂R-mediated diuresis and natriuresis in SHRs. In vitro studies showed that GRK4 siRNA reduced AT₂R phosphorylation and reversed the impaired AT₂R-mediated inhibition of Na⁺-K⁺-ATPase activity in SHR RPT cells. Our present study shows that GRK4, at least in part, impairs renal AT₂R-mediated diuresis and natriuresis by increasing its phosphorylation; inhibition of GRK4 expression and/or activity may be a potential strategy to improve the renal function of AT₂R.