BACKGROUND: GRKs (GPCR [G protein-coupled receptor] kinases) are key regulators of GPCR signaling. Because GPCRs are targeted by approximately 40% of clinical drugs, GRKs also represent therapeutic targets for combating cardiovascular diseases. However, the predominant GRK that responds to diabetic heart injury and the underlying mechanisms remain largely unknown.
METHODS: Human and mouse diabetic heart tissues were used to assess the expression pattern of the predominant GRK. Cardiac conditional knockout mice, heart-restricted adeno-associated virus 9, CRISPR (clustered regularly interspaced short palindromic repeats)/Cas9 (CRISPR-associated protein 9)-mediated gene editing, and pharmacological modulators were used for functional investigation. Heart histopathology and function were evaluated by hematoxylin-eosin staining, wheat germ agglutinin staining, Sirius red staining, Masson trichrome staining, and echocardiography. Immunoprecipitation, electrophoretic mobility shift assays, site-specific mutation, autodocking, and molecular dynamics simulation were conducted for mechanism studies. Computer-aided virtual screening was conducted to identify inhibitors of the predominant GRK.
RESULTS: GRK3 was the predominant GRK that was significantly upregulated in cardiomyocytes from both human and mouse diabetic hearts. Expression of GRK3 positively correlated with cardiac fibrosis and hypertrophy in human diabetic hearts. Heart-restricted overexpression of GRK3 aggravated, whereas conditional knockout of GRK3 in cardiomyocytes significantly relieved, heart injuries elicited by hyperglycemia. Mechanistically, hyperglycemia upregulated GRK3 expression by inhibiting the transcriptional repressor YY1 (Yin-Yang 1). GRK3 then essentially targeted CB2R (cannabinoid receptor 2) to phosphorylate CB2R at serine 335 (S335) and promote β-arrestin 2-mediated internalization and ubiquitin-dependent degradation of CB2R. Modulation of either YY1 or CB2R perturbed GRK3 functions in the diabetic hearts. Virtual screening identified a small molecule, isochlorogenic acid A, as an efficient GRK3 inhibitor that protects the heart from diabetic injury without causing additional side effects.
CONCLUSIONS: GRK3 is the predominant GRK that aggravates diabetic heart injury. YY1-dependent upregulation of GRK3 functions through direct phosphorylation of CB2R. Small molecules targeting GRK3 may represent a promising avenue for curing diabetic heart injury in the clinic.
