Abstract 72: Ca 2+ and Calmodulin-Dependent Protein Kinase II Mediate a Temporal Switch Between Cardiotoxic and Cardioprotective Signaling by β2-Adrenergic Receptors

Background: ß-adrenergic receptors (AR) play a critical role in cardiac function but also mediate adverse remodeling associated with chronic sympathetic stimulation in heart failure. Some studies have suggested cardiotoxic signaling through ß1-AR and cardioprotective through ß2-AR. However, ß2-AR can signal through both Gs & Gi, partially dictated by the time course of activation. We sought to determine whether this temporal effect influenced the differential roles of ß1- & ß2-AR in the pathogenesis of cardiomyopathy. Methods: We compared two models of oxidative stress-induced cardiomyopathy: acute doxorubicin (AD) 15 mg/kg in a single dose vs. chronic doxorubicin (CD) 2 mg/kg/wk x 8 wks in WT, ß1-AR-/- & ß2-AR-/- mice. Endpoints were survival and cardiac size and function. Activation of AR signaling pathways was assessed by Western blot. Results: With AD, 100% of ß2-/- die within 20 min, an effect not seen in ß1-/- or WT. With CD, in contrast, ß2-/- were protected: WT survival t½ 42 d, 100% mortality 70 d, n=33; ß2-/- t½ 56 d, 100% mortality 109 d, n=44; ß1-/- t½ 54 d, 100% mortality 99 d, n=31 (log rank p<0.0001). Fractional shortening declined and LV diastolic dimension increased by 2 wks in WT (13.9±2.5, 10.3±2.2%) whereas both were preserved in ß2-/- (5.0±1.7, p<0.01; 4.8±1.5%) and ß1-/- (2±1.6, p<0.001; 0.8±1.8, p<0.01) at least until 4 wks. Total Ca2+ and calmodulin-dependent protein kinase II (CaMKII) was increased at baseline in both ß1-/- & ß2-/- vs. WT. However, in ß2-/-, CaMKII was decreased with AD (0.74±0.1; p<0.01), and increased with CD (1.3±0.05; p<0.01). Similarly, CaMKII phosphorylation was decreased with AD (1.27±0.16) vs. baseline (1.95±0.19) and increased with CD (2.35±0.18, p<0.001). With AD, p-p38 increased 20-fold in ß2-/-, but was normal in CD. In contrast, p-JNK was increased in CD in both ß2-/- (0.34±0.09) and ß1-/- vs. WT (0.1±0.04; p<0.001). Conclusion: The differential role of ß2-AR in mediating cardioprotection/cardiotoxicity are more complex than initially thought. ß2-AR signaling exhibits a temporal switch depending on whether a stress is acute (cardioprotective) vs. chronic (cardiotoxic). This switch is mediated by differential activation of CaMKII and JNK (mediating cardioprotection) and p38 (mediating cardiotoxicity).