IRF-1 Regulates Mitochondrial Respiration and Intrinsic Apoptosis Under Metabolic Stress through ATP Synthase Ancillary Factor TMEM70

Mitochondrial dysfunction, which can be caused by metabolic stressors such as oxidized low-density lipoprotein (oxLDL), sensitizes the endothelium to pathological changes. The transcription factor interferon regulatory factor 1 (IRF-1) is a master regulator of inflammation, previously shown to promote oxLDL-induced inflammatory pyroptosis in human aortic endothelial cells (HAEC). However, a presumed role for IRF-1 in regulating the intrinsic apoptotic pathway in response to metabolic stress has not been demonstrated. Here targeted deletion of IRF-1 by siRNA in HAEC aggravated oxLDL-induced, mitochondria-mediated intrinsic apoptosis, as evidenced by increased Caspase-3 and Caspase-9 activation, and chromosomal DNA breakage. The increased apoptosis was concomitant with accumulation of mitochondrial ROS, decrease in intracellular ATP production and respiratory oxygen consumption, and abnormal mitochondrial structure. RNA profiling of endothelial cells isolated from wild type and Irf1 knockout mice, followed by quantitative PCR, luciferase activity assay and chromatin immunoprecipitation (ChIP), revealed that IRF-1 directly regulated the expression of transmembrane protein 70 (TMEM70), an ancillary factor required for the assembly of ATP synthase and conversion of an electrochemical gradient to ATP synthesis. Mirroring the effect of IRF1 knockdown, depletion of TMEM70 in HAEC resulted in impaired mitochondrial function and enhanced cell apoptosis. In contrast, overexpression of TMEM70 rescued ATP biosynthesis and suppressed apoptosis in oxLDL-treated, IRF-1-deficient HAEC. These results reveal a novel homeostatic role for IRF-1 in the regulation of mitochondrial function and associated stress-induced apoptosis.