PINK-1 containing extracellular vesicles: A new molecular measurement of mitochondrial health and tumor growth

Mitophagy is an important survival mechanism to eliminate damaged mitochondria via shuttling to the lysosome for destruction.Tumor hypoxia, chemotherapy, and immunotherapy damage mitochondria. Consequently, mitophagy plays a guardian role in cancer pathogenesis. PINK1 is a mitochondrial membrane sensor to initiate mitophagy. Healthy mitochondria break down PINK1 into fragments by protease cleavage of the full length (FL)-PINK1. In contrast, dysfunctional mitochondria fail to cleave PINK1, thereby accumulating FL-PINK1 triggering mitophagy. We hypothesize PINK1 cleavage status is a novel measure of a cell’s mitochondrial health. We discovered that PINK1, during high mitophagy demand, exports damaged mitochondria within EVs. We found that tumor interstitial fluid(IF) EVs contain different ratios of FL-PINK1 to c-PINK1. We compared ratios of the PINK1 levels in tumor IF vs normal IF and found non-tumor IF contained only c-PINK1 whereas tumor IF contained more FL-PINK1 indicative of tumor oxidative stress. Applying mitophagy-inducing drugs on 4T1 breast cancer cells generated greater levels of FL-PINK1 EV export. EV-associated PINK1 fragmentation is a sensitive indicator of cellular mitochondrial health. It has been shown that the tumor suppressor, p53, locates to PINK1 during mitochondrial stress. We hypothesize that PINK1+ EVs contain p53 to support tumor growth. Using purified 4T1 EV populations, PINK1 was immunoprecipitated and probed for CD81, p-p53, and Alix. We found that p-p53 co-located with PINK1+ EVs, therefore the export of tumor suppressors via secretory mitophagy is a novel pro-tumor mechanism. Future work would include developing a rapid PINK1 fragmentation assay and further analysis of p53-PINK1 localization intracellularly.