Endothelial Stanniocalcin 1 Maintains Mitochondrial Bioenergetics and Prevents Oxidant-Induced Lung Injury via Toll-Like Receptor 4

Oxidant-induced endothelial injury plays a critical role in the pathogenesis of acute lung injury (ALI) and subsequent respiratory failure. Our previous studies revealed an endogenous antioxidant and protective pathway in lung endothelium mediated by heat shock protein 70 (Hsp70)-toll-like receptor 4 (TLR4) signaling. However, the downstream effector mechanisms remained unclear. Stanniocalcin 1 (STC1) has been reported to mediate antioxidant responses in tissues such as the lungs. However, regulators of STC1 expression as well as its physiological function in the lungs were unknown. We sought to elucidate the relationship between TLR4 and STC1 in hyperoxia-induced lung injury in vitro and in vivo and to define the functional role of STC1 expression in lung endothelium. We identified significantly decreased STC1 expression in TLR4 knockout mouse lungs and primary lung endothelium isolated from TLR4 knockout mice. Overexpression of STC1 was associated with endothelial cytoprotection, whereas decreased or insufficient expression was associated with increased oxidant-induced injury and death. An Hsp70-TLR4-nuclear factor kappa-light-chain-enhancer of activated B cells (NFκB) signal mediates STC1 induction in the lungs and endothelial cells. We also demonstrated a previously unrecognized role for mitochondrial-associated STC1, via TLR4, in maintaining normal glycolysis, mitochondrial bioenergetics, and mitochondrial calcium levels. To date, a physiological role for STC1 in oxidant-induced ALI has not been identified. In addition, our studies show that STC1 is regulated by TLR4 and exerts lung and endothelial protection in response to sterile oxidant-induced lung injury. Our studies reveal a novel TLR4-STC1-mediated mitochondrial pathway that has homeostatic as well as oxidant-induced cytoprotective functions in lung endothelium.