Abstract 12632: Disruption of Epac1 Decreases Phosphorylation of Phospholamban and Protects the Heart Against Stresses
Background Protein kinase A (PKA), or cyclic AMP (cAMP)-dependent protein kinase, mediates the phosphorylation of multiple molecules involved in calcium (Ca2+) handling in cardiac myocytes, and this is the direct mechanism for β-adrenergic receptor (AR)-mediated enhancement of cardiac contractility. This paradigm may be challenged, however, with the recent identification of Epac (exchange protein activated by cAMP), which is also regulated by cAMP independent from PKA.
Methods and Results We found that mice with genetic disruption of Epac1 (Epac1KO) showed decreased left ventricular ejection fraction (LVEF: wild-type (WT) vs. Epac1KO 70+0.8 vs. 60+1.1%, n=18-31, P<0.01) with decreased phospholamban (PLN) phosphorylation at serine 16 (WT vs. Epac1KO 100+6.6 vs. 57+7.1%, n=6, P<0.01) . Although low cardiac contractility in Epac1KO was readily compensated by β-AR stimulation with acute isoproterenol (ISO) infusion, PKA expression was unaltered in Epac1KO. In contrast, direct activation of Epac with 8-CPT-AM (10μM for 2 hours), an Epac-selective cAMP analogue, led to increased PLN phosphorylation by 4.4-fold in cultured cardiac myocytes in wild type. The phospholamban phosphorylation was made most likely via Epac/PLC/PKCε as demonstrated by using various inhibitors and gene silencing techniques. Sarcoplasmic reticulum-Ca2+-uptake is regulated via PLN phosphorylation. We thus examined the intracellular Ca2+-concentration in isolated cardiac myocytes of Epac1KO. The basal and peak Ca2+-concentration was significantly decreased in Epac1KO than that in WT controls (Basal WT vs. Epac1KO 139+6.8 vs. 99+11.8nM, n=32, P<0.01; Peak WT vs. Epac1KO 883+35.3 vs. 559+33.2nM, n=32, P<0.01). More important, Epac1 disruption protected the heart from various stresses. Chronic ISO infusion (60mg/kg/day for 7 days) induced a similar degree of cardiac hypertrophy between Epac1KO and WT. However, subsequent cardiac dysfunction was prevented in Epac1KO (WT vs. Epac1KO: from 70+0.8 to 60+1.1 vs. from 62+1.4 to 60+0.9%, n= 14-31).with decreased cardiac myocyte apoptosis and thus fibrosis.
Conclusion Epac1, independent from PKA, plays an important role in regulating cardiac contractility and responsiveness to various pathological stresses to the heart.
- © 2012 by American Heart Association, Inc.