DEFICIENCY OF ATAXIA-TELANGIECTASIA MUTATED KINASE AFFECTS AUTOPHAGY AFTER MYOCARDIAL INFARCTION

Background: Autophagy is a conserved physiological process in the body that functions to maintain homeostasis via degradation and recycling of dysfunctional proteins and even entire organelles. It is typically triggered by nutritional stress and/or growth factor deprivation and ultimately results in the packaging of cellular components into autophagosomes. These autophagosomes then fuse with lysosomes to be degraded. Autophagy is suggested to play a significant role in cardiac remodeling, particularly following myocardial infarction (MI). Ataxia-telangiectasia mutated kinase (ATM) is a cell cycle checkpoint protein activated in response to DNA damage. Mutations in ATM cause a multi-systemic disease known as Ataxia-telangiectasia (AT). The present study aims to investigate the relationship between ATM and autophagy in the heart, particularly post-MI. Methods: Wild-type (WT) and ATM heterozygous (hKO; aged ~4 months) were injected with either bafilomycin (Baf; autophagy inhibitor) or rapamycin (Rap; autophagy activator) for 30 minutes. MI was then induced mice by ligation of the left anterior descending coronary artery. Heart function was measured using M-mode echocardiography 4 hours post-MI. For cellular analysis of autophagy, confluent cultures of cardiac fibroblasts were isolated from adult male rats and treated with KU-55933 (KU; specific ATM inhibitor) in serum-free media for 4 hours. Cardiac fibroblasts were also isolated from ATM WT, heterozygous (hKO), and knockout (KO) mice, grown to confluency, and serum-starved for 4 hours. Levels of microtubule-associated protein light chain 3-II (LC3-II), a marker for autophagy, was examined in the heart and cell lysates using western blots. Results: M-mode echocardiography revealed that MI decreases heart function in both genotypes as measured by decreased %FS and EF. No change in heart function was observed between WT-MI and hKO-MI groups following Baf treatment. Rap treatment resulted in the functional recovery of the heart in WT-MI, not in hKO-MI group. Levels of LC3-II protein were higher in hKO-sham versus WT-sham hearts. MI decreased LC3-II protein in hKO-MI, not in WT-MI group. Baf treatment further decreased LC3-II protein levels in hKO-MI group. LC3-II levels were lower in KU-treated rat cardiac fibroblasts when compared to control. Cardiac fibroblasts isolated from hKO and KO hearts exhibited decreased LC3-II levels versus those isolated from WT hearts. Conclusion: Although further investigations are needed to confirm our findings, these data provide evidence that ATM deficiency hinders improvement in heart function post-MI following activation of autophagy. ATM deficiency results in reduced autophagy post-MI, an effect that appears to be exaggerated following autophagy inhibition. ATM deficiency also reduces autophagy in rat and mouse cardiac fibroblasts.