Isocitrate dehydrogenase mutations rescue lactate dehydrogenase-deficient cells from cell death under hypoxia

Mutations in isocitrate dehydrogenase (mtIDH) are crucial in gliomagenesis, causing cellular metabolic dysfunction and heterogeneity. Yet, it remains unclear how cells with mtIDH mutations are selected and favored during pre-tumor progression. Lactate, produced by lactate dehydrogenase (LDH) from pyruvate, supports cell survival in hypoxia, notably in the Central Nervous System. In mtIDH gliomas, lacking LDH expression implies IDH mutations enable hypoxic survival. This mechanism involves Nocturnin (NOCT), converting NADP+ to NAD+, vital for ATP production during hypoxia. Our goal is to understand how mtIDH mutations contributes to glioma emergence and the advantages they offer cells in the pre-malignant stage. We conducted western blotting and immunohistochemistry on biopsies from 12 patients to confirm LDH and mtIDH expression, leveraging bioinformatic analysis of TCGA data. LDH expression was suppressed in murine astrocytes via siRNA or CRISPR, and cells were cultured under normoxia or hypoxia. Then, mtIDH expression was induced in LDH-deficient murine astrocytes to assess the mutation's impact on cell survival. Cell death was assessed using MTT assays and cell counting with Trypan blue. NOCT activity was inhibited using Calcium Chloride (CaCl2). Our results, derived from both patient biopsies and TCGA analysis, confirmed decreased LDH expression in mtIDH gliomas, revealing an inverse correlation between mtIDH and LDH levels. LDH knockdown in murine astrocytes led to hypoxic cell death, but ectopic mtIDH expression rescued LDH-deficient cells, suggesting a compensatory role. The inhibition of NOCT activity induced cell death in mtIDH and LDH-deleted cells under hypoxia. Thus, mtIDH gliomas may lack or down-regulate LDH expression, with mtIDH expression rescuing LDH-deficient cells from hypoxic death. NOCT may contribute to the survival of LDH-deficient and mtIDH gliomas by converting NADP+ to NAD+, ensuring ATP production in hypoxia, as a compensatory effect.