Abstract 16517: G Beta Gamma Independent Role Of GRK2 in TNF Alpha Induced Beta-AR Dysfunction
Cytokines contribute to left ventricular dysfunction and cardiomyopathy by altering cardiac mechanical function by negative inotropic effects. Although cytokine-induced β-adrenergic receptor (βAR) desensitization is well known, little is known about events proximal to βAR. We hypothesize that cytokines contribute to cardiac dysfunction by regulating βAR desensitization machinery. To determine the mechanism, we used transgenic mice with cardiac expression of myotrophin (a pro-hypertrophic molecule, Myo-Tg) which is characterized by significant upregulation of inflammatory cytokines that is accompanied by cardiac hypertrophy. A direct correlation between the progressive loss in the ability of βARs to activate cardiac adenylyl cyclase and deteriorating cardiac function was observed in Myo-Tg mice without changes in the plasma membrane βAR density. The progressive βAR desensitization observed in Myo-Tg mice is coupled with specific upregulation of G-protein coupled receptor kinase 2 (GRK2), PI3Kγ and GRK2-associated PI3K activity. Interestingly, the circulating epinephrine/norepinephrine levels were unaltered in the Myo-Tg mice despite significant cardiac remodeling indicating mechanisms other than sympathetic overdrive as the causative mechanism for βAR desensitization. Plasma inflammatory cytokines (TNFα, IL-6, IL-10 & TGFβ) are significantly elevated and precede the initiation of cardiac dysfunction in Myo-Tg mice. Cells treated with each of these cytokines showed that TNFα pre-treatment alone is sufficient to desensitize βARs. Consistently, significant βAR desensitization is observed in TNFα overexpressing transgenic mice (TNFα-Tg) that is associated with significant GRK2 upregulation preceding the onset of the heart failure phenotype. siRNA depletion of GRK2 resulted in significant loss of βAR phosphorylation upon TNFα pre-treatment. Interestingly, βAR phosphorylation upon TNFα was not altered by Gβγ sequestering C-terminal GRK2 peptide (βARKct) overexpression. In conclusion, our studies show that TNFα alone is sufficient for inducing βAR desensitization via GRK2 upregulation indicating the presence of a yet unidentified cross-talk mechanism between TNFα and βAR desensitization machinery.
- © 2011 by American Heart Association, Inc.