Bnip3 impairs mitochondrial bioenergetics and stimulates mitochondrial turnover

Bnip3 (Bcl-2/adenovirus E1B 19-kDa-interacting protein 3) is a mitochondrial BH3-only protein that contributes to cell death through activation of the mitochondrial pathway of apoptosis. Bnip3 is also known to induce autophagy, but the functional role of autophagy is unclear. In this study, we investigated the relationship between mitochondrial dysfunction and upregulation of autophagy in response to Bnip3 in cells lacking Bax and Bak. We found that Bnip3 induced mitochondrial autophagy in the absence of mitochondrial membrane permeabilization and Bax/Bak. Also, co-immunoprecipitation experiments showed that Bnip3 interacted with the autophagy protein LC3 (microtubule-associated protein light chain 3). Although Bax-/Bak-deficient cells were resistant to Bnip3-mediated cell death, inhibition of mitochondrial autophagy induced necrotic cell death. When investigating why these mitochondria had to be removed by autophagy, we discovered that Bnip3 reduced both nuclear- and mitochondria-encoded proteins involved in oxidative phosphorylation. Interestingly, Bnip3 had no effect on other mitochondrial proteins, such as Tom20 and MnSOD, or actin and tubulin in the cytosol. Bnip3 did not seem to reduce transcription or translation of these proteins. However, we found that Bnip3 caused an increase in mitochondrial protease activity, suggesting that Bnip3 might promote degradation of proteins in the mitochondria. Thus, Bnip3-mediated impairment of mitochondrial respiration induces mitochondrial turnover by activating mitochondrial autophagy.