Abstract 4829: Monitoring metabolic plasticity in the tumor microenvironment in vivo
Authors: Howard Marissa Ashton 
Published: April 2023

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 in the accumulation of damaged mitochondria that is harmful to the cell, and can suppress biogenesis of healthy mitochondria. 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 sustains 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. A set of proteins required for sequential steps of fission-induced mitophagy preferentially populated the CD81+/PD-L1+ IF EVs; including PINK1 and ARIH1 E3 ubiquitin ligase (required for Parkin-independent mitophagy), DRP1 and FIS1 (mitochondrial pinching), VDAC-1 (ubiquitination state triggers mitophagy away from apoptosis), and VPS35, SEC22b, and Rab33b (vacuolar sorting). Comparing in vivo IF EVs to in vitro EVs revealed 40% concordance, with an elevation of mitophagy proteins in the CD81+ EVs for both murine and human cell lines subjected to metabolic stress. The export of cellular mitochondria proteins to CD81+ EVs was confirmed by density gradient isolation from the bulk EV isolate followed by anti-CD81 immunoprecipitation, MitoTracker export into CD81+ EVs, and ultrastructural characterization. Further, we stimulated mitochondrial oxidative stress and blocked the fusion of the mitophagosome with the lysosome, which markedly stimulated the export of the secretory mitophagy unit. We also found that mitophagy inducer 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. 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 state of mitophagic flux within the tumor microenvironment.

Citation Format: Marissa Ashton Howard. Monitoring metabolic plasticity in the tumor microenvironment in vivo. [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 4829.