Local Mitochondrial-Endolysosomal Microfusion Cleaves Voltage-Dependent Anion Channel 1 To Promote Survival in Hypoxia

Abstract

The oxygen-limiting (hypoxic) microenvironment of tumors induces metabolic reprogramming and cell survival, but the underlying mechanisms involving mitochondria remain poorly understood. We previously demonstrated that hypoxia-inducible factor 1 mediates the hyperfusion of mitochondria by inducing Bcl-2/adenovirus E1B 19-kDa interacting protein 3 and posttranslational truncation of the mitochondrial ATP transporter outer membrane voltage-dependent anion channel 1 in hypoxic cells. In addition, we showed that truncation is associated with increased resistance to drug-induced apoptosis and is indicative of increased patient chemoresistance. We now show that silencing of the tumor suppressor TP53 decreases truncation and increases drug-induced apoptosis. We also show that TP53 regulates truncation through induction of the mitochondrial protein Mieap. While we found that truncation was independent of mitophagy, we observed local microfusion between mitochondria and endolysosomes in hypoxic cells in culture and in patients' tumor tissues. Since we found that the endolysosomal asparagine endopeptidase was responsible for truncation, we propose that it is a readout of mitochondrial-endolysosomal microfusion in hypoxia. These novel findings provide the framework for a better understanding of hypoxic cell metabolism and cell survival through mitochondrial-endolysosomal microfusion regulated by hypoxia-inducible factor 1 and TP53. Hypoxia is a natural occurring stress that results in compensatory changes in metabolism and cell survival during embryonic development and tumor growth. Hypoxia stabilizes and activates the transcription factor hypoxia-inducible factor (HIF) through inhibition of oxygen-dependent hydroxylases that earmark the alpha subunit of HIF for proteasomal degradation (1). HIF induces or represses the expression of genes implicated in a myriad of functions, including those regulating metabolism and resistance to drug-induced cell death. Genes coding for the enzymes of the glycolytic pathway, including hexokinase, are highly induced by HIF-1, and this is in part responsible for the switch in metabolism from mitochondrial respiration to glycolysis in cancer cells. Considerable studies have pointed to the Warburg effect, also termed aerobic glycolysis, as the major adaptive response of cancer cells, but mitochondrial metabolism and mitochondrial dynamics are also starting to be recognized as important adaptive strategies of cancer cells (2). Mitochondria are critical organelles that regulate both metabolism and cell death. They are dynamic organelles that continuously undergo fission and fusion during cell growth (3, 4). Under stress conditions, such as nutrient depletion or hypoxia, mitochondria either fragment or are degraded by HIF-dependent mitophagy (mitochondrial removal by autophagy) (5) or hyperfuse together to form elongated or rounded structures that optimize ATP production and promote cell survival (6-11).We reported previously that certain cell lines exposed to hypoxia contained enlarged mitochon...

Knockout of Vdac1 activates hypoxia-inducible factor through reactive oxygen species generation and induces tumor growth by promoting metabolic reprogramming and inflammation

“…The role of VDAC1 in cancer has not been extensively investigated [ 7 , 8 , 4 ]. We recently reported the presence in tumor cells of a novel hypoxia-induced form of VDAC1 lacking the C-terminus (VDAC1-ΔC) [ 7 , 9 ]. This new form was produced after long-term exposure to hypoxia.…”

Causative role for defective expression of mitochondria‐eating protein in accumulation of mitochondria in thyroid oncocytic cell tumors

“…Forced overexpression of exogenous MIEAP induced MIEAP‐induced vacuole and apoptotic cell death in some breast cancer cell lines, 19 and a MIEAP‐nonexpressing gastric cancer cell line produced more ROS and was more invasive in hypoxic conditions than a MIEAP‐expressing one 33 . By contrast, it is also reported that MIEAP participates in the hypoxic process of and chemoresistance 34 …”

Co‐culture of human fibroblasts, smooth muscle and endothelial cells promotes osteopontin induction in hypoxia

“…Protein immunodetection was performed as previously ///described . Briefly, cells were lysed in 1.5X SDS buffer, and if exposed to hypoxia, they were lysed while inside the hypoxic chamber.…”

“…Protein immunodetection was performed as previously ///described. 49 Briefly, cells were lysed in 1.5X SDS buffer, and if exposed to hypoxia, they were lysed while inside the hypoxic chamber. Proteins (40 μg) were separated on 7.5% SDSpolyacrylamide gels and transferred onto polyvinylidene difluoride membranes (Millipore).…”