No abstract

Mitochondria are primarily responsible for providing the contracting cardiac myocyte with a continuous supply of ATP. However, mitochondria can rapidly change into death-promoting organelles. In response to changes in the intracellular environment, mitochondria become producers of excessive reactive oxygen species and release pro-death proteins, resulting in disrupted ATP synthesis and activation of cell death pathways. Interestingly, cells have developed a defense mechanism against aberrant mitochondria that can cause harm to the cell. This mechanism involves selective sequestration and subsequent degradation of the dysfunctional mitochondrion before it causes activation of cell death. Induction of mitochondrial autophagy, or mitophagy, results in selective clearance of damaged mitochondria in cells. In response to stress such as ischemia/reperfusion, pro-survival and pro-death pathways are concomitantly activated in cardiac myocytes. Thus, there is a delicate balance between life and death in the myocytes during stress, and the final outcome depends on the complex crosstalk between these pathways. Mitophagy functions as an early cardioprotective response, favoring adaptation to stress by removing damaged mitochondria. In contrast, increased oxidative stress and apoptotic proteases can inactivate mitophagy, allowing for the execution of cell death. Herein, we discuss the importance of mitochondria and mitophagy in cardiovascular health and disease, and provide a review of our current understanding of how these processes are regulated in the myocardium.

Bnip3 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. 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 appear 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.