Non-canonical regulation of GRK2 by TNFalpha impairs BetaAR function

Cleveland Clinic Lerner Com-Cwru, Cleveland OH

Investigators

Linked publications & trials

Abstract

? DESCRIPTION (provided by applicant): Tumor necrosis factor alpha (TNF?) is a major pro-inflammatory cytokine that is significantly elevated in Type 2 diabetes and obesity which are known co-morbid cardiovascular risk factors. It is known that TNF? exposure leads to cardio-depressant negative inotropic effects. Despite this observation, little is understood about the underlying mechanisms. We have recently shown that TNF? causes ?-adrenergic receptor (?AR) dysfunction through G-protein coupled receptor kinase 2 (GRK2) which may underlie cardio-depressant negative inotropic effects of TNF?. ?ARs are one of the most powerful regulators of cardiac function and ?AR desensitization (i.e., diminished catecholamine) response is a hallmark of heart failure. Reduced ?AR response to catecholamines is due to phosphorylation of ?ARs that is predominantly mediated by GRK2 which is markedly elevated in cardiac stress. Interestingly, our studies show that TNF? up-regulates GRK2 mediating ?AR desensitization as siRNA knock down of GRK2 normalizes ?AR function despite TNF?. TNF? pre-treatment of murine cardiomyocytes inhibits contractility to ?AR agonist isoproterenol (ISO) that is remarkably preserved in GRK2 null myocytes indicating a role for GRK2 in TNF? mediated ?AR function. Since GRK2 recruitment is mediated by G?? subunits of G-proteins, we determined whether TNF? mediated ?AR function could be rescued by expression of G?? sequestering peptide GRK2-ct (?ARK-ct). Although GRK2-ct preserved ?AR function to ISO, it could not preserve ?AR function to TNF?. Furthermore, TNF? administration in GRK2-ct transgenic mice with cardiac expression of GRK2-ct resulted in cardiac dysfunction associated with ?AR desensitization despite no changes in catecholamines. Surprisingly, TNF? treatment resulted in marked ?2AR phosphorylation even in the presence of ?AR antagonist propranolol. These data suggest that TNF? mediates ?AR desensitization by GRK2 in an agonist- and G??-independent manner contrary to the current paradigm of ?AR desensitization. In addition to elevation in TNF?, mouse models of obesity (mice on high fat-diet or lipolysis deficient adipose triglyceride lipase null mice (ATGL-/-)) are characterized by increased cardiac ?2AR phosphorylation and upregulation of GRK2. Based on these exciting observations, we hypothesize that TNF? contributes to cardiac ?AR desensitization by non-traditional GRK2 recruitment to the ?AR complex suggesting a yet, unidentified cross-talk between TNF? and ?AR signaling. Thus, to mechanistically understand TNF?- induced ?AR desensitization, we have designed the following studies: Specific Aim 1: To determine whether TNF?-TNFR1/2-TRAF2 axis facilitates G??-independent GRK2-mediated ?AR desensitization. Specific Aim 2: To identify the mechanism of TRAF2-dependent GRK2 recruitment to ?AR complex. Specific Aim 3: To demonstrate whether conditional cardiomyocyte GRK2 ablation (GKR2 del) in mice will ameliorate cardiac dysfunction/hypertrophy in mouse models of obesity. Understanding this pathway may provide novel therapeutic targets/strategies and importantly could also be a universal phenomenon of desensitizing other GPCRs in response to TNF?. 1