Secretory Mitophagy in Cancer Metabolic Reprogramming - Tumor metabolic reprogramming is a hallmark of cancer progression, survival, and therapeutic
resistance. A targetable class of cancer metabolic adaptation exploits mitophagy, a specialized
autophagy pathway known to be linked to the cancer phenotype. Mitophagy selectively eliminates
dysfunctional mitochondria by targeting them, via autophagosome shuttling, to the lysosome for
degradation. Cancer cell mitophagy is triggered by elevated oxidative stress and mitochondria DNA
damage caused by hypoxia, radiotherapy, molecular therapy, and immunotherapy. A high mitophagy
demand can overwhelm the lysosome capacity resulting accumulated damaged mitochondria that is
harmful to the cell. We hypothesize that the newly discovered process of secretory mitophagy
exports damaged mitochondrial fission-released segments to reduce the overload pressure on the
lysosomal system, and thereby sustain cancer cell survival in the face of therapeutic mitochondrial
stress. We have discovered a form of secretory mitophagy occurring in vivo in a growing solid tumor.
Our molecular analysis of the full repertoire of extracellular vesicles (EV) shed into the resident tumor
interstitial fluid (IF) in vivo yielded a rich set of information about the functional state of mitochondria
within the tumor cells, and the host cells. Within tumor IF, and within the culture media of cancer cells
undergoing oxidative stress, we identified an EV-packaged full set of mitochondria molecules
comprising the peripheral fission pinched-off segment of the mitochondrial organelle. It has recently
been found that peripheral mitophagy fission, regulated by mitochondrial fission 1 protein (FIS1), is a
key essential regulator of mitophagy, distinct from mid-zone mitochondria biogenesis. We also found
that mitophagy inducer PTEN induced kinase 1 (PINK1) cleavage status (full length versus cleaved),
is prominently reflected in the set of mitochondrial proteins exported within IF EVs, and may constitute
a new quantitative measurement tool to monitor the real-time state of tumor intracellular mitophagy.
Our findings raise important mechanistic questions, that we will explore in the Aims. Under Aim 1 we
will test the hypothesis that peripheral fission secretory mitophagy is induced by high mitophagy
demands that overwhelm lysosomal engulfment, and explore if secretory mitophagy is distinct from
secretory autophagy within the murine 4T1 and human MDA-MB-231 triple negative breast cancer
cell lines. Under Aim 2 we will test the hypothesis that the ratio of full length to cleaved PINK1 is
elevated (mitochondria damage sensor) within the exported murine syngeneic breast tumor IF EVs,
following treatment with mitophagy inducers Mito-CP and Rapamycin. The outcome is new
understanding of the importance of secretory mitophagy that can constitute an important therapeutic
target, and a new clinically relevant means of monitoring the in vivo cancer state of mitophagic flux.
