Extracellular glutamate and IDH1 inhibitor promote glioma growth by boosting redox potential.

Purpose: Somatic mutations of the isocitrate dehydrogenase 1 (IDH1) gene, mostly substituting Arg132 with histidine, are associated with better patient survival, but glioma recurrence and progression are nearly inevitable, resulting in disproportionate morbidity and mortality. Our previous studies demonstrated that in contrast to hemizygous IDH1 (loss of wild-type allele), heterozygous IDH1 is intrinsically glioma suppressive but its suppression of three-dimensional (3D) growth is negated by extracellular glutamate and reducing equivalent. This study sought to understand the importance of 3D culture in IDH1 biology and the underlying mechanism of the glutamate effect.

Methods: RNA sequencing data of IDH1-heterozygous and IDH1-hemizygous glioma cells cultured under two-dimensional (2D) and 3D conditions were subjected to unsupervised hierarchal clustering and gene set enrichment analysis. IDH1-heterozygous and IDH1-hemizygous tumor growth were compared in subcutaneous and intracranial transplantations. Short-hairpin RNA against glutamate dehydrogenase 2 gene (GLUD2) expression was employed to determine the effects of glutamate and the mutant IDH1 inhibitor AGI-5198 on redox potential in IDH1-heterozygous cells.

Results: In contrast to IDH1-heterozygous cells, 3D-cultured but not 2D-cultured IDH1-hemizygous cells were clustered with more malignant gliomas, possessed the glioblastoma mesenchymal signature, and exhibited aggressive tumor growth. Although both extracellular glutamate and AGI-5198 stimulated redox potential for 3D growth of IDH1-heterozygous cells, GLUD2 expression was required for glutamate, but not AGI-5198, stimulation.

Conclusion: 3D culture is more relevant to IDH1 glioma biology. The importance of redox homeostasis in IDH1 glioma suggests that metabolic pathway(s) can be explored for therapeutic targeting, whereas IDH1 inhibitors may have counterproductive consequences in patient treatment.