Downregulation of G protein-coupled receptor kinase 4 protects against kidney ischemia-reperfusion injury

Ischemia/reperfusion injury of the kidney is associated with high morbidity and mortality, and treatment of this injury remains a challenge. G protein-coupled receptor kinase 4 (GRK4) plays a vital role in essential hypertension and myocardial infarction, but its function in kidney ischemia/ reperfusion injury remains undetermined. Among the GRK subtypes (GRK2-6) expressed in kidneys, the increase in GRK4 expression was much more apparent than that of the other four GRKs 24 hours after injury and was found to accumulate in the nuclei of injured mouse and human renal tubule cells. Gain-and loss-of-function experiments revealed that GRK4 overexpression exacerbated acute kidney ischemia/reperfusion injury, whereas kidney tubule-specific knockout of GRK4 decreased injury-induced kidney dysfunction. Necroptosis was the major type of tubule cell death mediated by GRK4, because GRK4 significantly increased receptor interacting kinase (RIPK)1 expression and phosphorylation, subsequently leading to RIPK3 and mixed lineage kinase domain-like protein (MLKL) phosphorylation after kidney ischemia/reperfusion injury, but was reversed by necrostatin-1 pretreatment (an RIPK1 inhibitor). Using co-immunoprecipitation, mass spectrometry, and siRNA screening studies, we identified signal transducer and activator of transcription (STAT)1 as a GRK4 binding protein, which co-localized with GRK4 in the nuclei of renal tubule cells. Additionally, GRK4 phosphorylated STAT1 at serine 727, whose inactive mutation effectively reversed GRK4-mediated RIPK1 activation and tubule cell death. Kidney-targeted GRK4 silencing with nanoparticle delivery considerably ameliorated kidney ischemia/reperfusion injury. Thus, our findings reveal that GRK4 triggers necroptosis and aggravates kidney ischemia/reperfusion injury, and its downregulation may provide a promising therapeutic strategy for kidney protection.