Oncogenic mutations in isocitrate dehydrogenase 1 (IDH1) or 2 (IDH2) compromise their normal activity and induce NADPH-dependent (D)2-hydroxyglutarate (2HG) production within the cytosol or mitochondria. Given the critical functions of these enzymes in tricarboxylic acid (TCA) metabolism and cellular redox homeostasis, such mutations are likely to cause metabolic reprogramming in tumors. To identify metabolic abnormalities in cells harboring IDH mutations we applied 13C metabolic flux analysis (MFA) to isogenic cells with heterozygous IDH1 mutations. MFA results indicate that IDH1 mutant cells exhibit increased oxidative TCA metabolism and oxygen consumption under hypoxia relative to parental cells. This metabolic phenotype occurs independently of 2HG accumulation and renders IDH1 mutant cells more sensitive to electron transport chain (ETC) inhibition compared to IDH wild-type and IDH2 mutant cells. These data suggest that targeting mitochondrial metabolism may be synthetically lethal in tumors expressing mutant IDH1 and can be combined with strategies that selectively inhibit its neomorphic activity.
In addition, we have exploited the NADPH-dependent 2HG production by mutant IDH1 and IDH2 to characterize the function of compartmentalized cofactor cycles that occur in cancer cells. Importantly, pathway segregation in distinct organelles has prevented traditional MFA approaches from accurately measuring fluxes within specific compartments. By applying 2H isotope tracers that specifically label NADPH and inducibly expressing IDH1-R132H or IDH2-R172K in the cytosol or mitochondria, respectively, we can reliably quantify NADPH metabolism in each compartment. Using this approach we have elucidated the directionality of metabolic cycles coordinated between the cytosol and mitochondria that are critical for tumor growth.