Abstract P129: G Protein--Coupled Receptor Kinase-2 (GRK2) Is a Novel Regulator of Collagen Synthesis in Adult Human Cardiac Fibroblasts

Abstract

Cardiac fibroblasts (CF) make up 70% of the total cell number in the heart and play a critical role in regulating normal myocardial function and in adverse remodeling following myocardial infarction. Recent studies have shown that increased intracellular cAMP can inhibit CF transformation and collagen synthesis in adult rat CF; however, mechanisms by which cAMP production is regulated in CF have not been elucidated. The objective of this study was to investigate the potential role of GRK2 in modulating CF transformation to myofibroblasts and collagen synthesis in adult human CF isolated from normal and failing left ventricles. CF isolated from failing ventricles showed a significant increase in expression of collagen I, III and VI compared with controls. α-SMA was increased 2-fold over controls, consistent with CF transformation to myofibroblasts. Baseline collagen synthesis was elevated 2-fold in failing CF and was not inhibited by isoproterenol (ISO)-stimulation in contrast to normal controls. β-adrenergic receptor (β-AR) signaling was markedly uncoupled in failing CF as assessed by basal and ISO-stimulated cAMP production. The primary mechanism appears to be a 2.5-fold increase in GRK2 activity as GRK2 phosphorylates and uncouples agonist-occupied β-ARs. Overexpression of GRK2 in normal CF recapitulated a heart failure phenotype with minimal inhibition of collagen synthesis following ISO stimulation. In contrast, siRNA-mediated knockdown of GRK2 expression in normal CF enhanced cAMP production and led to greater β-agonist-mediated inhibition of basal and TGFβ-stimulated collagen synthesis versus control. Inhibition of GRK2 activity by adenoviral-mediated βARKct expression or GRK2 knockdown in failing CF led to a significant decline in collagen and α-SMA expression. GRK2 inhibition restored β-AR signaling and ISO-stimulated inhibition of collagen synthesis and also significantly decreased collagen synthesis in response to TGFβ stimulation. In conclusion, GRK2 appears to play a significant role in regulating CF transformation and collagen synthesis in adult human CF and increased activity of this kinase may be an important mechanism of maladaptive ventricular remodeling as mediated by cardiac fibroblasts.