Tumor necrosis factor receptor-associated factor 2 mediates mitochondrial autophagy

Tumor necrosis factor (TNF) signaling protects against ischemia/reperfusion-induced cardiomyocyte death, in vitro, ex vivo, and in vivo. TNF-receptor-associated factor 2 (TRAF2), an E3 ubiquitin ligase, coordinates cytoprotective signaling downstream of both TNF receptors, via unclear mechanisms. Noting that TRAF2 is recruited to mitochondria, and that autophagic removal of ubiquitin-tagged damaged mitochondria is cytoprotective, we tested the hypothesis that TRAF2 mediates mitochondrial autophagy.

TRAF2 localizes to the mitochondria in neonatal rat cardiac myocytes, and TNF treatment transcriptionally upregulates TRAF2 abundance in the mitochondrial subfraction. TRAF2 colocalizes with ubiquitin, p62 adaptor protein, and mitochondria within LC3-bound autophagosomes; and exogenous TRAF2 enhances autophagic removal of mitochondria. TRAF2 knockdown with adenoviral shRNA transduction induces accumulation of depolarized mitochondria in resting neonatal rat cardiac myocytes, as well as in those treated with TNF or uncoupling agent carbonyl cyanide m-chlorophenyl hydrazone, suggesting an essential role for TRAF2 in homeostatic and stress-induced mitochondrial autophagy. TRAF2 also colocalizes and interacts with PARKIN, a previously described E3 ubiquitin ligase and mitophagy effector, on depolarized mitochondria in neonatal rat cardiac myocytes. Exogenous expression of TRAF2, but not its E3 ligase-deficient mutants, is sufficient to partially restore mitophagy in the setting of PARKIN knockdown, suggesting redundancy in their ubiquitin ligase roles. TRAF2 abundance increases in the mitochondrial subfraction of ischemia/reperfusion-modeled hearts; and exogenous TRAF2, but not its E3 ligase-deficient mutants, reduces depolarized mitochondria and rescues cell death in neonatal rat cardiac myocytes subjected to hypoxia/reoxygenation.

Taken together, these data indicate an essential role for TRAF2 in concert with PARKIN as a mitophagy effector, which contributes to TRAF2-induced cytoprotective signaling.