FOXO3a orchestrates glioma cell responses to starvation conditions and promotes hypoxia-induced cell death

Abstract

Forkhead box O (FOXO) transcription factors are homeostatic regulators adjusting diverse cellular processes crucial for metabolism and survival. In gliomas, FOXOs have been shown to modulate cell death, proliferation and differentiation. Here, we investigated the role of FOXO3a in human malignant gliomas with special regard to starvation conditions. Expression of FOXO3a increased with WHO grade and was accentuated in the perinecrotic niche, colocalizing with hypoxiainducible factor 1 (HIF1) expression. FOXO3a was upregulated in hypoxia and translocation of FOXO3a from the cytoplasm to the nucleus was induced by serum starvation, pharmacological inhibition of protein kinase B (PKB) and hypoxia. Overexpression of FOXO3a induced tumor necrosis factorrelated apoptosisinducing ligand (TRAIL) expression and resulted in spontaneous cell death. Knockdown of FOXO3a (shFOXO3a), on the one hand, enhanced the sensitivity of glioma cells towards H2O2 under normoxia. On the other hand, it decreased consumption of glucose and oxygen, resulting in improved survival during glucose and oxygen deprivation. Mechanistically, in shFOXO3a cells, hypoxia-response element reporter activity, as well as the expression of common HIF1 target genes, was increased, suggesting disinhibited HIF1 signaling. However, glucose transporter 1 (GLUT1) expression was inversely regulated, and this may have been caused by an upregulation of TP53 in shFOXO3a cells. These data reveal a novel role of FOXO3a-dependent gene regulation in the complex adaptive responses of gliomas towards starvation signals. Strategies that target FOXO3a function may directly or indirectly alter glioma cell behavior and viability in the hypoxic niche.