Different Effects of RNAi-Mediated Downregulation or Chemical Inhibition of NAMPT in an Isogenic IDH Mutant and Wild-Type Glioma Cell Model

The IDH1(R132H) mutation in glioma results in the neoenzymatic function of IDH1, leading to the production of the oncometabolite 2-hydroxyglutarate (2-HG), alterations in energy metabolism and changes in the cellular redox household. Although shifts in the redox ratio NADPH/NADP(+) were described, the consequences for the NAD(+) synthesis pathways and potential therapeutic interventions were largely unexplored. Here, we describe the effects of heterozygous IDH1(R132H) on the redox system in a CRISPR/Cas edited glioblastoma model and compare them with IDH1 wild-type (IDH1(wt)) cells. Besides an increase in 2-HG and decrease in NADPH, we observed an increase in NAD(+) in IDH1(R132H) glioblastoma cells. RT-qPCR analysis revealed the upregulation of the expression of the NAD(+) synthesis enzyme nicotinamide phosphoribosyltransferase (NAMPT). Knockdown of NAMPT resulted in significantly reduced viability in IDH1(R132H) glioblastoma cells. Given this dependence of IDH1(R132H) cells on NAMPT expression, we explored the effects of the NAMPT inhibitors FK866, GMX1778 and GNE-617. Surprisingly, these agents were equally cytotoxic to IDH1(R132H) and IDH1(wt) cells. Altogether, our results indicate that targeting the NAD(+) synthesis pathway is a promising therapeutic strategy in IDH mutant gliomas; however, the agent should be carefully considered since three small-molecule inhibitors of NAMPT tested in this study were not suitable for this purpose.