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Mutations in isocitrate dehydrogenase 1 (IDH1) characterize most adult low-grade gliomas. Mutant IDH1 catalyzes production of the oncometabolite 2-hydroxyglutarate (2-HG). We recently discovered that the IDH1 mutation also reprograms pyruvate metabolism in a 2-HG-dependent manner, and that reprogramming of pyruvate metabolism is essential for cell proliferation in glioma cells with mutant IDH1.
Metabolic reprogramming is increasingly viewed as a hallmark of cancer, with several common metabolic alterations such as increased glycolysis reported in a range of tumor types;1 however, the metabolic alterations associated with recently discovered oncogenic mutations in isocitrate dehydrogenase 1 (IDH1) remain to be fully elucidated.
Mutations in IDH1 characterize over 80% of adult low-grade gliomas. Cytosolic IDH1 normally catalyzes the production of α-ketoglutarate (α-KG) from isocitrate and plays an important role in the regulation of redox status, lipogenesis, and glucose and glutamine metabolism. Mutations in IDH1, most commonly at the R132 residue in the active site of the enzyme, instead lead to the conversion of α-KG to 2-hydroxyglutarate (2-HG). By inhibiting the activity of a variety of cellular α-KG-dependent enzymes, 2-HG induces epigenetic changes that block cellular differentiation and induce tumorigenesis.2 We and others have shown that, in addition to an altered epigenetic profile and elevated 2-HG levels, IDH1 mutant cells also undergo broader metabolic reprogramming compared to their wild-type IDH1 counterparts.3,4 Most notably, we observed a significant reduction in 1H magnetic resonance spectroscopy (MRS)-detectable steady-state levels of lactate, phosphocholine, and glutamate in 2 genetically engineered cell models expressing mutant IDH1—a U87 glioblastoma-based model and a normal human astrocyte (NHA) model.4 In a separate study we discovered that pyruvate dehydrogenase (PDH) activity was reduced in IDH1 mutant NHA cells.5 Given that PDH is an important regulatory point for glucose oxidation via the tricarboxylic acid (TCA) cycle and, as a result, for glutamate production, we questioned the role of PDH in IDH1 mutant glioma cells.6
In a recently published study6 we confirmed a significant reduction in PDH activity in both our U87 and NHA mutant IDH1 cells compared to wild-type.6 13C MRS probing of the fate of 1-13C-glucose to 4-13C-glutamate, and hyperpolarized 13C MRS probing of the fate of 2-13C-pyruvate to 5–-3C-glutamate, showed that reduced PDH activity also resulted in a reduction in glucose flux to glutamate in IDH1 mutant cells relative to wild-type. This was consistent with, and mostly explained, the decrease in steady-state glutamate levels. We further found that IDH1 mutant cells showed increased expression, at both mRNA and protein levels, of pyruvate dehydrogenase kinase 3 (PDK3), a well-known regulator of PDH activity. The increase in PDK3 expression correlated with increased inhibitory phosphorylation of PDH, thereby explaining the reduction in PDH activity in IDH1 mutant cells. This effect was associated with increased levels of hypoxia inducible factor-1α (HIF-1α) in IDH1 mutant cells, consistent with previous observations and the stabilization of HIF-1α via inhibition of α-KG-dependent-prolyl hydroxylases.7,8 Finally, we also showed that treatment of IDH1 wild-type cells with 2-HG recapitulated the effects of the IDH1 mutation, with increased levels of PDK3 and HIF-1α and decreased PDH activity in 2-HG treated cells. Collectively, our results therefore suggested that IDH1 mutation results in increased PDK3 expression thereby reducing PDH activity and explaining the reduction in glutamate levels. Fig. 1 summarizes our findings and the mechanism by which mutant IDH1 leads to inhibition of PDH activity. Our finding was important because it indicated that the reprogramming of pyruvate metabolism is not linked to 2-HG-induced hypermethylation, but occurs on a timescale that is faster and possibly more amenable to intervention.
To test the value of PDH as a therapeutic target, we went on to address the functional consequences of reduced PDH activity in IDH1 mutant cells. Our study showed, to our knowledge for the first time, that reversing the metabolic reprogramming of PDH in mutant IDH1 cells was detrimental to the proliferation and clonogenic potential of these cells. Specifically, treatment with dichloroacetate (DCA), a PDK inhibitor,9 not only increased PDH activity and glutamate production in IDH1 mutant cells, but also completely abrogated the increased clonogenicity observed in cells expressing mutant IDH1. Moreover, DCA also inhibited proliferation of patient-derived mutant IDH1 neurosphere cultures, thereby also validating our findings in clinically relevant models. DCA treatment also reversed the metabolic alterations detected by 1H and 13C MRS.
Our results thus suggest that the reduction in PDH activity induced by the IDH1 mutation is essential for proliferation and clonogenicity in IDH1 mutant glioma cells, and identify PDH as a possible therapeutic target for the treatment of mutant IDH1 cells. Our findings also highlight the value of MRS in elucidating the mechanisms of metabolic reprogramming in IDH1 mutant glioma cells. Importantly, 1H MRS has been used as a non-invasive method of evaluating brain tumors in human patients, and, more recently, the value of hyperpolarized 13C MRS was also demonstrated in patients.10
In summary, our recent study6 identifies a potential therapeutic target for mutant IDH1 low-grade gliomas, as well as an associated companion MRS biomarker for agents that would modulate that target.
No potential conflicts of interest were disclosed.
This work was supported by NIH R01CA172845, NIH R01CA154915, NIH R21CA161545 and the Terry Fox Research Institute and Foundation.