Hypoxia-Induced Metabolic and Functional Changes in Oral CSCs: Implications for Stemness and Viability Modulation Through BNIP3-Driven Mitophagy

Abstract

Oral squamous cell carcinomas (OSCCs), like several solid tumours, contain heterogeneous subpopulations of a small subset of cancer cells, termed cancer stem cells (CSCs), that are highly relevant to cancer metastasis and invasive properties. CSCs have also shown a high capacity to survive against various stressful environments, such as hypoxia. However, the molecular underpinnings behind the high potential of CSCs to survive under this stress remain unclear. The current study aimed to investigate the significance of autophagy systems in oral CSC maintenance and survival under stress conditions. Human OSCC cell lines OECM-1 and OECM-1 CSCs were cultured in different hypoxic time periods for proliferation and cytotoxicity analyses. The stemness property of CSCs is evaluated by sphere formation, transwell and wound healing assays protein expression of stemness, and epithelial-to-mesenchymal transition markers. Mitochondrial functions, including mitochondrial ROS generation, mitochondria dynamics, mitophagy, and mitochondrial metabolism (glycolysis and oxidative phosphorylation [OXPHOS]) were examined by western blotting, immunohistochemistry, and XF-seahorse assays, respectively. Under hypoxia, oral CSCs showed a higher proliferation rate with increased invasion/migration/EMT properties than OECM-1 cells. Further, hypoxia-induced BNIP3-driven mitophagy was activated in OECM-1 CSCs than in OECM-1 cells, which also triggered a metabolic shift towards OXPHOS, and BNIP3/-L silencing by siRNA significantly attenuated OECM-1 CSCs stemness features. TCGA data analyses also revealed a higher BNIP3 expression in head and neck squamous carcinoma patients' tumour samples associated with lower patient survival. Collectively, our results revealed a BNIP3/-L-driven autophagy contributes to the OECM-1 CSCs stemness features under hypoxia, suggesting a novel therapeutic strategy involving BNIP3 and autophagy inhibition in oral CSCs.