Abstract 11320: Ablation of Sam68 in Cardiomyocytes Attenuates Cardiac Hypertrophy but Exacerbates Heart Failure

Abstract

Background & Hypothesis: Src-associated-in-mitosis-of-68-kDa (Sam68) is a pleotropic adaptor protein shown to regulate signal transduction, RNA splicing, cell cycle, differentiation, and energy metabolism. Its function in the heart has never been studied. We hypothesize that Sam68 regulates cardiac hypertrophy and heart failure. Methods & Results: Pressure overload was induced by Ang II infusion (via osmotic pump) in 12-week-old male Sam68 -/- , Sam68 +/- , and WT littermates, followed by weekly assessments with echocardiography; Sam68 -/- mice displayed attenuated left ventricular (LV) wall thickness compared to Sam68 +/- or WT littermates. We then generated Sam68 floxed mice and, by crossing with α-MHC-Cre transgenic (Cre Tg ) mice, obtained cardiomyocyte-specific Sam68 knockout (Sam68 CM-KO ) mice. Consistently, Ang II-induced LV wall thickening was attenuated in Sam68 CM-KO mice compared to Cre Tg littermates, confirming an autonomous role of Sam68 in promoting cardiac hypertrophy. Interestingly, in the surgically-induced transverse-aortic-constriction (TAC) model, Sam68 CM-KO mice, while had an overall lesser LV wall hypertrophy over the course of 6 weeks, displayed higher rates of immediate post-surgery mortality and later-stage LV dilation and ejection-fraction drop, than Cre Tg littermates. The Sam68 CM-KO mouse hearts showed smaller cross section area, fibrosis area, and cardiomyocyte size than Cre Tg mouse hearts. To gain mechanistic insights, we engineered a lentiviral vector coding for a Sam68 shRNA, which knocked down Sam68 protein expression in H9C2 cells (H9C2 Sam68KD ) by more than 90% (after selection). Sam68 knockdown led to a marked reduction in the expression of Raptor (by >50%), Rictor (by >50%), and ATG13 (by >80%) proteins, but no change in the level of mTOR protein or the mRNA expression of Raptor, Rictor, and ATG13. Furthermore, in H9C2 Sam68KD , starvation-induced autophagy flux was increased as shown by elevated LC3II/LC3I ratio with Bafilomycin A treatment. Conclusions: Sam68 promotes cardiac hypertrophy and protects against heart failure likely through the activity of mTOR complexes.