Mitophagy is a selective form of macro-autophagy in which unwanted or damaged mitochondria are preferentially targeted for degradation at the auto-phago-lysosome, as an adaptative response to physiological stressors, such as hypoxia, nutrient deprivation, and/or DNA damage. After cellular stress, such as hypoxia and nutrient deprivation, BNIP3 and BNIP3L form stable homo-dimerization complexes that co-localize to the outer membrane of the mitochondria. As such, these proteins play important roles in tissue differentiation and the stress responses. Mitophagy is promoted by specific mitochondrial outer membrane receptors, or ubiquitin molecules conjugated to proteins on the mitochondrial surface, which interact directly with processed LC3/GABARAP proteins leading to the formation of auto-phagosomes surrounding the mitochondria.
To date, four outer mitochondrial membrane (OMM)-anchored proteins—FUNDC1, BCL2L13, NIX and BNIP3—have been identified to serve as direct receptors for mitochondrial autophagy. Sequestosome/SQSTM1/p62 may also be regarded as one of them. Through their conserved LIR motifs, they link with LC3 for autophagic degradation of mitochondria in mammalian cells. These autophagy receptors are required for multiple mitophagy programs that operate either independently or are linked by crosstalk. FUNDC1 is a mitochondrial receptor for hypoxia-induced mitophagy, which can be negatively regulated by Src tyrosine kinase and casein kinase 2 (CK-II). BCL2L13 is a newly identified mitophagy receptor in mammalian somatic cells, which can induce mitochondrial fragmentation and Parkin-independent mitophagy.
BNIP3 and its homolog NIX are atypical members of the pro-apoptotic Bcl-2 subfamily, They can not only promote cellular apoptosis via their BH3 domains but can also induce mitophagy via their LIR motifs. While NIX was demonstrated to be required for mitochondrial clearance during reticulocyte development, BNIP3 was identified to mediate mitochondrial autophagy in cardiomyocytes and liver cells. By screening these OMM-anchored proteins, scientists identified that mitophagy mediated by BNIP3 only is required for both maintenance and acquisition of pluripotency. In contrast, a recent study has defined that mitochondrial autophagy mediated by NIX but not BNIP3 is critical for three factor-induced reprogramming (Oct4, Sox2, and KLF4). The same study also found that NIX is not involved in four factor induced reprogramming (Oct4, Sox2, KLF4 and c-Myc).
However, the mechanisms that underlie the functional divergence between NIX and BNIP3 in pluripotency regulation need further investigation. The OMM proteins p62, NBR1 and OPTN serve as adaptors that link signals from depolarized mitochondria, which are sensed by PINK1/PARK2, with autophagic signals to trigger mitochondrial autophagy. The fact that Park2 depletion does not affect mitochondrial clearance and reprogramming efficiency supports the view that PINK1/PARK2-mediated selective mitophagy is not involved in pluripotency regulation. There is also relationship between mitochondria turnover, metabolism and cancer stem cell renewal. Cancer stem cells (CSCs) are a small sub-population of cells with stem cell-like features, such as self-renewal, high proliferation rates and/or drug-resistance. CSCs are responsible for cancer recurrence, treatment failure, and metastatic dissemination.
Accordingly, the elimination of CSCs represents one of the most important new therapeutic approaches in cancer treatment. For aggressive tumors like breast cancer, lung cancer, pancreatic cancer or melanoma, mitophagy could, be a valuable option to trat these incurable forms. For example, BNIP3(L)-high MCF7 breast cancer cells showed a significant increase in their mammosphere forming ability, as well as in their expression levels of a well-known epithelial stem cell marker (i.e. CD44), indicating that this sub-population is functionally enriched in CSC activity. In further support of the idea that mammosphere forming activity is mitophagy-dependent, mitophagy-high cells were more resistant to two known mitophagy inhibitors chloroquine and cyclosporin A. This last, a known immune suppressor, induces mitophagy by inhibiting mitochondrial cyclophilin D (CypD).
Cancer cells use glycolysis to support macromolecular biosynthesis and energy production (“Warburg effect”); however, mitochondrial oxidative phosphorylation has been shown to be still active during carcinogenesis and even exacerbated in drug-resistant and stem cancer cells. This metabolic rewiring is associated with alterations of ROS production and redox biology, a fine-tuned balance between anti-/proapoptotic proteins and mutations in genes encoding mitochondrial metabolic enzymes. This situation lead the production of the so-called “oncometabolites”, like fumarate, 2-hydroxy-glutarate, succinate, lactate and asparagine. They suppress aìcellular apoptosis and rewire the epigenetic control over certain chromatine regions. For example, 2-hydroxy-glutarate inhibits ATP synthesis and may enhance DNA methylation while fumarate inhibits histone demethylation.
Finally, it is widely accepted that somatic cells rely heavily on oxidative phosphorylation while pluripotent stem cells favor glycolysis for energy production. Accordingly, it has been observed that a transition from somatic mitochondrial oxidative metabolism to glycolysis is required for successful reprogramming. These results indicate that mitochondria must be remodeled during the reprogramming of somatic cells and that pluripotent stem cells must employ unique molecular mechanisms to regulate mitochondrial homeostasis. In support of this view, recent studies have shown that both canonical and noncanonical autophagy mediate mitochondrial remodeling during somatic cell reprogramming. Furthermore, high autophagic flux has been identified as an intrinsic characteristic of embryonal stem cells to maintain mitochondrial homeostasis and pluripotency. These data suggest that mitochondria are precisely regulated by autophagy during the induction and maintenance of pluripotency.
Edited by Dr. Gianfrancesco Cormaci, PhD; specialist in Clinical Biochemistry.
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