Abstract
Pyruvate carboxylation is essential for the proliferation of SDH-deficient cells.
Major finding: Pyruvate carboxylation is essential for the proliferation of SDH-deficient cells.
Mechanism: Pyruvate carboxylase supports aspartate synthesis to maintain growth of SDH-deficient cells.
Impact: The dependence of SDH-deficient cells on pyruvate carboxylase may be exploited therapeutically.
Inactivating mutations in the genes that encode for the subunits of succinate dehydrogenase (SDH), an enzyme involved in the tricarboxylic acid (TCA) cycle, are associated with increased tumor susceptibility and result in a pseudohypoxic and hypermethylator phenotype. However, the effects of SDH loss on tumor cell metabolism and how these effects promote cell growth have not been fully elucidated. To unmask tumor cell metabolic rewiring induced by SDH loss, Cardaci and colleagues generated Sdhb-ablated immortalized mouse kidney cells, which were devoid of SDH activity due to the total absence of SDHB, one of the subunits of SDH. Isotope-tracing experiments showed that loss of SDH activity in Sdhb-ablated cells curtailed the TCA cycle, disabled mitochondrial respiration, and caused a dependence on glycolysis to meet their bioenergetic needs. To maintain maximal glycolytic flux and proliferation, Sdhb-ablated cells required the uptake of extracellular pyruvate. Metabolomic profiling revealed that the increased glycolytic flux driven by loss of SDH activity resulted in decreased aspartate biosynthesis, which induced the dependence of Sdhb-ablated cells on extracellular pyruvate. Consistent with this finding, additional isotope-labeling studies demonstrated that glucose-derived carbons were a major source of the aspartate pool in Sdhb-null cells. Analysis of the expression of pyruvate carboxylase (PC), which catabolizes pyruvate to generate the TCA intermediate and aspartate precursor oxaloacetate, revealed that PC is overexpressed in Sdhb-ablated cells as well as human neuroendocrine tumors and renal cell carcinomas with mutations in SDH. Knockdown of PC reduced the proliferation and tumorigenic potential of Sdhb-ablated cells and mutant RAS–transformed Sdhb-ablated cells, respectively. Proliferation of PC-silenced Sdhb-ablated cells was rescued by the addition of excess aspartate, but not palmitate, a fatty acid that provides energy via β-oxidation. Taken together, these findings show that SDH-deficient cells rely upon pyruvate carboxylation to sustain Warburg-like bioenergetic features, and suggest PC as a potential therapeutic vulnerability in SDH-associated tumors.
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