Autophagy, TERT, and mitochondrial dysfunction in hyperoxia

Ventilation with gases containing enhanced fractions of oxygen is the cornerstone of therapy for patients with hypoxia and acute respiratory distress syndrome. Yet, hyperoxia treatment increases free reactive oxygen species (ROS)-induced lung injury, which is reported to disrupt autophagy/mitophagy. Altered extranuclear activity of the catalytic subunit of telomerase, TERT, plays a protective role in ROS injury and autophagy in the systemic and coronary endothelium. We investigated interactions between autophagy/mitophagy and TERT that contribute to mitochondrial dysfunction and pulmonary injury in cultured rat lung microvascular endothelial cells (RLMVECs) exposed in vitro, and rat lungs exposed in vivo to hyperoxia for 48 hours. Hyperoxia induced mitochondrial damage in rat lungs (TOMM20, MTT), which was paralleled by increased markers of inflammation (MPO, IL-1β, TLR9), impaired autophagy signaling (Beclin-1, LC3B-II/1, p62), and decreased the expression of TERT. Mitochondrial specific autophagy (mitophagy) was not altered as hyperoxia increased expression of Pink1 but not Parkin. Hyperoxia-induced mitochondrial damage (TOMM20) was more pronounced in rats that lack the catalytic subunit of TERT, and resulted in a reduction in cellular proliferation rather than cell death in RLMVECs. Activation of TERT or autophagy individually offset mitochondrial damage (MTT). Combined activation/inhibition failed to alleviate hyperoxic-induced mitochondrial damage in vitro, while activation of autophagy in vivo decreased mitochondrial damage (MTT) in both WT and rats lacking TERT. Functionally, activation of either TERT or autophagy preserved transendothelial membrane resistance. Altogether, these observations show that activation of autophagy/mitophagy and/or TERT mitigate loss of mitochondrial function and barrier integrity in hyperoxia.