Regulation of cardiac autophagy in response to lipid overload

Obesity is an independent risk factor for several cardiac pathologies. Sustained exposure to nutrient overload in obesity overwhelms cellular homeostatic apparatus, leading to metabolic disorders and organelle dysfunction. The integrity of homeostatic machinery is crucial for the heart to meet its energy need and for cardiomyocytes to survive. Recently, autophagy has emerged as a major catabolic process in maintaining energy and organelle homeostasis.;Growing evidence suggest a role for autophagy in obesity related cardiac pathologies. Yet, the regulation of cardiac autophagy in obesity is unclear. In this study, we explored mechanisms regulating cardiac autophagy in an invivo mouse model of diet-induced obesity and an invitro cell culture model of lipid overload. 12 weeks of high-fat diet (HFD) (45% kcal fat) significantly increased the autophagy marker protein LC3-II and autophagosome number in the murine heart independently of canonical upstream signaling through mTORC1 and AMPK. Interestingly, high-fat fed mice displayed a defective autophagosome turnover that may have led to autophagosome accumulation. Alteration in Beclin1 expression had no effect on HFD-induced autophagy, ruling out a major contribution of autophagy initiation pathways in this process. In vitro, cultured H9C2 cardiomyocytes displayed a biphasic autophagic response to palmitate. To explore the mechanism of early autophagic response to lipid overload, we performed most experiments after 4 h of treatment when features of ER stress and cell death were absent. Similar to HFD, palmitate increased autophagosome accumulation primarily via an impairment in autophagosome turnover. Oleate alone had no effect on autophagy but cotreatment normalized the palmitate-induced autophagosome accumulation. Moreover, palmitate treatment led to a massive accumulation of superoxide which correlated with impaired lysosomal acidification and pH-dependent lysosomal enzyme activity. Using specific inhibitors and siRNA mediated gene silencing, we identified Nox2 as the major source of superoxide production. The activation of Nox2 was dependent on the palmitate-induced activation of classical PKCs. Together, our study has identified a novel mechanism wherein palmitate-induced activation of PKC-Nox2 pathway led to impaired lysosomal enzyme activity and diminished autophagic turnover in cardiomyocytes. The Nox2-mediated inhibition of autophagic flux might contribute to other known pathological roles of Nox2-mediated oxidative stress in obesity.