Epac is required for exogenous and endogenous stimulation of adenosine A2B receptor for inhibition of angiotensin II-induced collagen synthesis and myofibroblast differentiation.

Angiotensin II (Ang II) plays an important role on the pathogenesis of cardiac fibrosis. Prolong and overstimulation of angiotensin II type 1 receptor with Ang II-induced collagen synthesis and myofibroblast differentiation in cardiac fibroblasts, leading to cardiac fibrosis. Although adenosine and its analogues are known to have cardioprotective effects, the mechanistic by which adenosine A2 receptors (A2Rs) inhibit Ang II-induced cardiac fibrosis is not clearly understood. In the present study, we examined the effects of exogenous adenosine and endogenous adenosine on Ang II-induced collagen and myofibroblast differentiation determined by α-smooth muscle action (α-SMA) overexpression and their underlying signal transduction. Elevation of endogenous adenosine levels resulted in the inhibition of Ang II-induced collagen type I and III and α-SMA synthesis in cardiac fibroblasts. Moreover, treatment with exogenous adenosine which selectively stimulated A2Rs also suppressed Ang II-induced collagen synthesis and α-SMA production. These antifibrotic effects of both endogenous and exogenous adenosines are mediated through the A2B receptor (A2BR) subtype. Stimulation of A2BR exhibited antifibrotic effects via the cAMP-dependent and Epac-dependent pathways. Our results provide new mechanistic insights regarding the role for cAMP and Epac on A2BR-mediated antifibrotic effects. Thus, A2BR is one of the potential therapeutic targets against cardiac fibrosis.

Stimulation of Adenosine A2B Receptor Inhibits Endothelin-1-Induced Cardiac Fibroblast Proliferation and α-Smooth Muscle Actin Synthesis Through the cAMP/Epac/PI3K/Akt-Signaling Pathway.

Background and Purpose: Cardiac fibrosis is characterized by an increase in fibroblast proliferation, overproduction of extracellular matrix proteins, and the formation of myofibroblast that express α-smooth muscle actin (α-SMA). Endothelin-1 (ET-1) is involved in the pathogenesis of cardiac fibrosis. Overstimulation of endothelin receptors induced cell proliferation, collagen synthesis, and α-SMA expression in cardiac fibroblasts. Although adenosine was shown to have cardioprotective effects, the molecular mechanisms by which adenosine A2 receptor inhibit ET-1-induced fibroblast proliferation and α-SMA expression in cardiac fibroblasts are not clearly identified. Experimental Approach: This study aimed at evaluating the mechanisms of cardioprotective effects of adenosine receptor agonist in rat cardiac fibroblast by measurement of cell proliferation, and mRNA and protein levels of α-SMA. Key results: Stimulation of adenosine subtype 2B (A2B) receptor resulted in the inhibition of ET-1-induced fibroblast proliferation, and a reduction of ET-1-induced α-SMA expression that is dependent on cAMP/Epac/PI3K/Akt signaling pathways in cardiac fibroblasts. The data in this study confirm a critical role for Epac signaling on A2B receptor-mediated inhibition of ET-1-induced cardiac fibrosis via PI3K and Akt activation. Conclusion and Implications: This is the first work reporting a novel signaling pathway for the inhibition of ET-1-induced cardiac fibrosis mediated through the A2B receptor. Thus, A2B receptor agonists represent a promising perspective as therapeutic targets for the prevention of cardiac fibrosis.

Sustained ßAR Stimulation Mediates Cardiac Insulin Resistance in a PKA-Dependent Manner.

Insulin resistance is a condition in which cells are defective in response to the actions of insulin in tissue glucose uptake. Overstimulation of ß-adrenergic receptors (ßARs) leads to the development of heart failure and is associated with the pathogenesis of insulin resistance in the heart. However, the mechanisms by which sustained ßAR stimulation affects insulin resistance in the heart are incompletely understood. In this study, we demonstrate that sustained ßAR stimulation resulted in the inhibition of insulin-induced glucose uptake, and a reduction of insulin induced glucose transporter (GLUT)4 expression that were mediated by the ß2AR subtype in cardiomyocytes and heart tissue. Overstimulation of ß2AR inhibited the insulin-induced translocation of GLUT4 to the plasma membrane of cardiomyocytes. Additionally, ßAR mediated cardiac insulin resistance by reducing glucose uptake and GLUT4 expression via the cAMP-dependent and protein kinase A-dependent pathways. Treatment with ß-blockers, including propranolol and metoprolol antagonized isoproterenol-mediated insulin resistance in the heart. The data in this present study confirm a critical role for protein kinase A in ßAR-mediated insulin resistance.