The identification and function of the mitochondrial pyruvate carrier: lessons from yeast, cancer, and stem cells

The discovery of the Mitochondrial Pyruvate Carrier (MPC) began with investigating the role of highly conserved uncharacterized mitochondrial proteins. This was accomplished using diverse approaches to ensure the discovery stood up to the harshest scrutiny. Yeast genetics, biochemical assays using radiolabeled tracers, identifying human disease causing mutations, and metabolomics all supported this discovery. Synthetic lethal screens in yeast showed that the loss of a redundant pathway for producing pyruvate in yeast mitochondria exacerbated the phenotype. Most strikingly, deletion of genes in this complex blocked all radiolabeled C14-pyruvate entry into the mitochondria. Building on decades of work validating a specific MPC inhibitor, a mutagenic screen isolated a variant that could survive on the previously identified inhibitor and maintained the ability to transport C14-pyruvate, while the wild-type form did not. The identification of two patient families that had defects in pyruvate transport but preserved pyruvate metabolism, and were traced to highly conserved mutations in the MPC, showed the essential function of this complex. Following this discovery the focus of this work has been linking reductions in MPC to aerobic glycolysis and supporting cancer metabolism. Starting with publically available bioinformatics databases showing that MPC is reduced in many cancers, we had reason to suspect it may be an important mediator of the Warburg effect. Reexpressing the MPC in colon cancer cell lines altered the cancer-initiating cell capacity and reduced tumor growth. Functional measures of cancer-initiating cell capacity further specified the growth defects to the stem-like compartment. The defects in MPC-expressing cells were rescued by addition of the specific inhibitor of mitochondrial pyruvate transport, showing that MPC-mediated pyruvate entry into the mitochondria was the cause of these differences. The work on cancer stem cells led to exploring adult tissue stem cell systems. This is being carried out to better understand the role of metabolism in stem cell biology, with the goal of extending this to cancer initiation and progression. Observations in normal intestinal stem cells have largely mirrored what we observed in cancer cell lines. Inhibition or loss of the MPC increases stem cell proliferation, while reexpression results in loss of stem cell properties.