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Direct mitochondrial import of lactate supports resilient carbohydrate oxidation

ABSTRACT: Lactate is the most abundant circulating metabolic intermediate in mammals and an important energy source for many organs. To use lactate as a fuel, it must be oxidized to pyruvate for entry into the TCA cycle. It is usually described that this reaction occurs in the cytosol and requires the mitochondrial pyruvate carrier (MPC) for pyruvate transport into the mitochondria. Here, using 13C stable isotope tracing, we report that lactate can be oxidized in the heart tissue of mice even when the MPC is genetically deleted. MPC-independent lactate import and oxidation within mitochondria is dependent upon the monocarboxylate transporter 1 (MCT1/ Slc16a1). Mitochondria isolated from Mct1iCKO hearts have impaired respiration on lactate but not pyruvate. Lactate import coupled to mitochondrial lactate dehydrogenase activity functions as an electron shuttle which can produce NADH sufficient for respiration even when the TCA cycle is blocked. Cardiac-specific loss of MCT1 leads to rapid decompensation into heart failure with reduced ejection fraction in response to diverse cardiac injuries. Thus, we identify a new mitochondrial electron shuttle that enables the oxidation of lactate and is required to support cardiac energetics under stress conditions.

ORGANISM(S): Mus musculus

PROVIDER: GSE276036 | GEO | 2024/11/22

REPOSITORIES: GEO

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Project description:Many cancers rely on glycolytic metabolism to fuel rapid proliferation. This has spurred interest in designing drugs that target tumor glycolysis such as AZD3965, a small molecule inhibitor of Monocarboxylate Transporter 1 (MCT1) currently undergoing Phase I evaluation for cancer treatment. Since MCT1 mediates proton-linked transport of monocarboxylates such as lactate and pyruvate across the plasma membrane (Halestrap and Meredith, 2004), AZD3965 is thought to block tumor growth through disruption of lactate transport and glycolysis. Here we show that MCT1 inhibition impairs proliferation of glycolytic breast cancer cells that express MCT4 via disruption of pyruvate rather than lactate export. We found that MCT1 expression is elevated in glycolytic breast tumors and cell lines as well as in malignant breast and lung tissues. High MCT1 expression predicts poor prognosis in breast and lung cancer patients. Stable knockdown and AZD3965-mediated inhibition of MCT1 promote oxidative metabolism. Acute inhibition of MCT1 reduces pyruvate export rate but does not consistently alter lactate transport or glycolytic flux in breast cancer cells that also express MCT4. Despite the lack of glycolysis impairment, MCT1 loss-of-function decreases breast cancer cell proliferation and blocks growth of mammary fat pad xenograft tumors. Our data suggest that MCT1 expression is elevated in glycolytic cancers to promote pyruvate export, which when inhibited enhances oxidative metabolism and reduces proliferation. This study presents an alternative molecular consequence of MCT1 inhibitors that further supports their use as anti-cancer therapeutics. Since MCT1 levels are elevated in glycolytic and malignant breast tumors, we hypothesized that MCT1 may contribute to the Warburg effect metabolic phenotype. To test this hypothesis, we generated whole genome microarray data from breast cancer cell lines either a) expressing a short hairpin (sh)RNA-mediated stable knockdown of MCT1; or b) treated for 24 hours with an MCT1 inhibitor (AZD3965). Scramble shRNA or DMSO were used as controls, and all conditions were analzed in triplicate. The cell lines used â HS578T, SUM149PT, and SUM159PT â are among the most glycolytic in a panel of 31 breast cancer cell lines.

Project description:Background: Heart failure involves metabolic alterations including increased glycolysis despite unchanged or decreased glucose oxidation. The mitochondrial pyruvate carrier (MPC) regulates pyruvate entry into the mitochondrial matrix, and cardiac deletion of the MPC in mice causes heart failure. How MPC deletion results in heart failure is unknown. Methods: We performed targeted metabolomics and isotope tracing in wildtype (fl/fl) and cardiac-specific Mpc2-/- (CS-Mpc2-/-) hearts after in vivo injection of 13C-glucose. Cardiac glycogen was measured biochemically and by transmission electron microscopy. Cardiac glucose uptake of 2-deoxyglucose was measured and western blotting performed to analyze insulin signaling and enzymatic regulators of glycogen synthesis and degradation. Isotope tracing and glycogen analysis was also performed in hearts from mice fed either low-fat diet or a ketogenic diet previously shown to reverse the heart failure in CS-Mpc2-/- mice. Cardiac glycogen was also assessed in mice infused with angiotensin-II that were fed either low-fat or ketogenic diet. Results: Failing CS-Mpc2-/- hearts contained normal levels of ATP and phosphocreatine, suggesting their heart failure is not caused by energetic stress. These hearts displayed increased enrichment from 13C-glucose and increased glycolytic metabolite pool sizes. 13C enrichment and pool size was also increased for the glycogen intermediate UDP-glucose, as well as increased enrichment of the glycogen pool. Glycogen levels were increased ~6-fold in the failing CS-Mpc2-/- hearts, and glycogen granules were easily detected by electron microscopy. This increased glycogen synthesis occurred despite enhanced inhibitory phosphorylation of glycogen synthase and reduced expression of the priming enzyme glycogenin-1. In young, non-failing CS-Mpc2-/- hearts, increased glycolytic 13C enrichment occurred, but glycogen levels remained low and unchanged compared to fl/fl hearts. Feeding a ketogenic diet to CS-Mpc2-/- mice reversed the heart failure and normalized the cardiac glycogen and glycolytic metabolite accumulation. Cardiac glycogen levels were also elevated in mice infused with angiotensin II, and both the cardiac hypertrophy and glycogen levels were improved by ketogenic diet. Conclusions: Our results indicate that loss of MPC in the heart increases glycolytic metabolism and ultimately glycogen accumulation and heart failure, while a ketogenic diet can reverse both the glycogen accumulation and heart failure. We conclude that maintaining mitochondrial pyruvate import and metabolism is critical for the heart, unless cardiac pyruvate metabolism is dramatically reduced by consumption of a ketogenic diet.

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Direct mitochondrial import of lactate supports resilient carbohydrate oxidation

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