Abstract
Cardiac β
2
-adrenergic receptors (ARs) are known to inhibit collagen production and fibrosis in cardiac fibroblasts and myocytes. β
2
AR is a Gs protein-coupled receptor (GPCR) and, upon its activation, stimulates generation of cyclic 3`, 5`-adenosine monophosphate (cAMP). cAMP has two effectors: protein kinase A (PKA) and the exchange protein directly activated by cAMP (Epac). Epac1 has been shown to inhibit cardiac fibroblast activation and fibrosis. Osteopontin (OPN) is a ubiquitous pro-inflammatory and pro-fibrotic cytokine, including in the heart. We report here that the cardiotoxic hormone aldosterone upregulates OPN via the mineralocorticoid receptor (MR) in H9c2 cardiomyoblasts. This is prevented by β
2
AR-activated GPCR-kinase (GRK)-5. GRK5 directly phosphorylates and inhibits the MR in cardiomyocytes. Additionally, CRISPR-mediated OPN deletion enhances β
2
AR-dependent cAMP generation in H9c2 cardiomyocytes and upregulates Epac1. OPN deletion also enables the β
2
AR to completely abrogate transforming growth factor (TGF)-β-dependent fibrosis in H9c2 cardiomyocytes. Mechanistically, OPN interacts with G
α
s subunits in H9c2 cardiomyocytes to facilitate recruitment of GRK2, the major GRK phosphorylating and desensitizing the cardiac β
2
AR. This, in turn, augments the GRK2-dependent functional desensitization of the β
2
AR, thereby opposing this receptor`s anti-fibrotic cAMP/Epac1 signaling. In conclusion, we have uncovered a direct inhibitory effect of OPN in cardiac β
2
AR`s anti-fibrotic signaling via facilitation of GRK2-mediated receptor desensitization. Thus, OPN blockade could be of value in the treatment and/or prevention of cardiac fibrosis.