The Role of Mitochondrial NADPH-Dependent Isocitrate Dehydrogenase in Cancer Cells

Abstract

Isocitrate dehydrogenase 2 (IDH2) is located in the mitochondrial matrix. IDH2 acts in the forward Krebs cycle as an NADP(+)-consuming enzyme, providing NADPH for maintenance of the reduced glutathione and peroxiredoxin systems and for self-maintenance by reactivation of cystine-inactivated IDH2 by glutaredoxin 2. In highly respiring cells, the resulting NAD(+) accumulation then induces sirtuin-3-mediated activating IDH2 deacetylation, thus increasing its protective function. Reductive carboxylation of 2-oxoglutarate by IDH2 (in the reverse Krebs cycle direction), which consumes NADPH, may follow glutaminolysis of glutamine to 2-oxoglutarate in cancer cells. When the reverse aconitase reaction and citrate efflux are added, this overall "anoxic" glutaminolysis mode may help highly malignant tumors survive aglycemia during hypoxia. Intermittent glycolysis would hypothetically be required to provide ATP. When oxidative phosphorylation is dormant, this mode causes substantial oxidative stress. Arg172 mutants of human IDH2-frequently found with similar mutants of cytosolic IDH1 in grade 2 and 3 gliomas, secondary glioblastomas, and acute myeloid leukemia-catalyze reductive carboxylation of 2-oxoglutarate and reduction to D-2-hydroxyglutarate, which strengthens the neoplastic phenotype by competitive inhibition of histone demethylation and 5-methylcytosine hydroxylation, leading to genome-wide histone and DNA methylation alternations. D-2-hydroxyglutarate also interferes with proline hydroxylation and thus may stabilize hypoxia-induced factor α.

Figure 1

Main domains of the mitochondrial isocitrate dehydrogenase, IDH2.The homodimeric IDH2 of two 413-amino acid subunits; each 47 kDa subunit is schematically illustrated according the solved crystal structure of porcine IDH2 [50]. Residues 142–187 make up two stacked four-stranded antiparallel β-sheets (middle white marble-shaded domain). Mn²⁺ and isocitrate in the active site are depicted as a blue circle and rod, respectively. Six Arg residues, which provide hydrogen bonds with isocitrate oxygens [50], are not depicted. Arg 83 (R83) interacts by hydrogen bonding with the 3′-OH of the nicotinamide ribose and thus enhances NADP⁺ affinity [53]. An Arg residue corresponding to human Arg172, which is often mutated in gliomas and AML, is indicated by an asterisk (∗). For activity, the glutathionylation of Cys269 [46] (GS–S) must be removed (X) by reactivation aided by glutaredoxin-2 in the presence of reduced glutathionine, and the protein must be deacetylated by SIRT3 [44]. We hypothesize that Lys212, Lys374, and Lys260 (in porcine notation) are the prime candidates for acetylation causing IDH2 inactivation.

Figure 2

Consequences of IDH2 functions for redox homeostasis in cancer cells and nonmalignant cells.