Abstract 379: Inhibition of Bcl-x Phosphorylation Prevents Cardiomyocyte Death, Cardiac Remodeling and Heart Failure After Myocardial Infarction

Abstract

Cardiac remodeling promotes heart failure (HF). Cardiomyocyte (CM) death is one of the mechanisms to develop cardiac remodeling. We recently reported that Mst1 phosphorylates Bcl-xL at Ser14, which promotes apoptosis by inducing dissociation of Bcl-xL from Bax and consequent activation of Bax in CMs. Its phosphorylation is increased in response to ischemia-reperfusion (IR) in an Mst1-dependent manner. However, the functional significance of endogenous Bcl-xL phosphorylation remains unclear in vivo. To address this question, knock-in (KI) mice with alanine mutation at Ser14 in Bcl-x were generated. At baseline, cardiac function was similar between wild-type (WT) and heterozygous KI (HKI) mice (EF 76% and 79%, respectively). HKI mice exhibited smaller % infarct area (30%) than WT (43%) (p=0.016) upon IR, suggesting that phosphorylation of endogenous Bcl-xL at Ser14 plays an essential role in mediating IR injury. In order to test the role of Bcl-xL phosphorylation in the development of HF, HKI and WT mice were subjected to permanent ligation of LAD for 4 weeks. During progression of cardiac remodeling, Mst1 was activated in both WT and HKI mice. Phosphorylation of Bcl-xL and Bcl-xS, an alternative transcriptional variant of Bcl-x, both at Ser14, were increased in WT mice, which were abrogated in HKI mice. The infarct area evaluated with TTC staining at Day 1 was similar in WT and HKI mice (59.1% and 61.2%, p=0.65). Four weeks after myocardial infarction (MI), WT mice exhibited lower cardiac contraction (EF 46.5%) and higher LVEDP (10.8mmHg) than those in HKI mice (EF 68.9% and LVEDP 7.0mmHg) (both p<0.05). Scar area and TUNEL-positive CMs were greater in WT (49.0% and 1.6%, respectively) than in HKI mice (29.2% and 0.4%, respectively) (both p<0.05). Cleaved caspase 3 and 9 were significantly increased (3.2- and 5.7-fold, respectively) in WT but not in HKI mice. In vitro experiments with overexpression of phospho-mimicking mutant (Bcl-xS-S14D) showed 13% reduction in cell viability compared with that of phospho-resistant mutant (Bcl-xS-S14A) (p=0.01%). Our results suggest that phosphorylation of Bcl-xL and Bcl-xS at Ser14 contributes to CM death in response to IR and chronic MI in vivo, thereby promoting cardiac remodeling and HF.