Regulation of Substrate Utilization by the Mitochondrial Pyruvate Carrier

Abstract

SUMMARY Pyruvate lies at a central biochemical node connecting carbohydrate, amino acid, and fatty acid metabolism, and the regulation of pyruvate flux into mitochondria represents a critical step in intermediary metabolism impacting numerous diseases. To characterize changes in mitochondrial substrate utilization in the context of compromised mitochondrial pyruvate transport, we applied 13C metabolic flux analysis (MFA) to cells after transcriptional or pharmacological inhibition of the mitochondrial pyruvate carrier (MPC). Despite profound suppression of both glucose and pyruvate oxidation, cell growth, oxygen consumption, and tricarboxylic acid (TCA) metabolism were surprisingly maintained. Oxidative TCA flux was achieved through enhanced reliance on glutaminolysis through malic enzyme and pyruvate dehydrogenase (PDH) as well as fatty acid and branched chain amino acid oxidation. Thus, in contrast to inhibition of complex I or PDH, suppression of pyruvate transport induces a form of metabolic flexibility associated with use of lipids and amino acids as catabolic and anabolic fuels.

Requirement for the Mitochondrial Pyruvate Carrier in Mammalian Development Revealed by a Hypomorphic Allelic Series

“…9D). Increased oxidative glutaminolysis and reductive carboxylation upon MPC inhibition were observed in transformed cell lines, although the relative contributions of these fates of glutamine were dependent on the cell line and growth conditions (59,60). Glutamine anaplerosis supports cellular proliferation in transformed cells and inhibition of glutaminase-or glutamate dehydrogenase-sensitized cells to growth inhibition upon treatment with UK-5099 (60).…”

“…We found that alanine provides a robust anaplerotic substrate in several cellular models of MPC deficiency, as well as in vivo in embryonic brain and liver. It has been demonstrated that Mpc2-deficient mitochondria exhibit no alterations in alanine uptake (62), and decreased steady-state alanine concentrations have been observed in several MPC-deficient cell models and in vivo (59,60,62). The decrease in steadystate alanine has been interpreted as a result of most cellular alanine being derived from mitochondrial pyruvate, but we sought to test the flux of alanine itself.…”

The Mitochondrial Pyruvate Carrier and Metabolic Regulation

“…A recent study by Vacanti et al using 13 C metabolic flux analysis of cells after genetic or pharmacological ablation of MPC activity revealed a surprising degree of cellular metabolic flexibility and adaptation [29]. In MPC-deficient cells, both glucose and pyruvate oxidation were suppressed.…”

Treating fatty liver disease by modulating mitochondrial pyruvate metabolism

“…It should be noted that knockout or knockdown of the MPC results in significant compensatory changes in metabolism,7, 34 and thus compensatory metabolic pathways (increased amino acid and/or fatty acid metabolism) could reduce the efficacy of these compounds for treating NASH. However, knockout of the MPC proteins is likely to have different effects than attenuation of function by TZDs.…”

“…Clearly, metabolomics studies should be performed after chronic MPC inhibition with TZDs. Another possible limitation of inhibiting MPC activity is that the MPC proteins are significantly down‐regulated in cancer cell lines, and regulation of pyruvate use is likely important in stem cells and solid tumor expansion 34, 35. In this respect, it is reassuring to know that in over 20 years of clinical use of pioglitazone, which also works through attenuation of the MPC,24 hepatocellular cancer incidence appears to be reduced 36, 37…”