Reactive oxygen species produced by the knockdown of manganese-superoxide dismutase up-regulate hypoxia-inducible factor-1α expression in oral squamous cell carcinoma cells

Hypoxia-inducible factor-1α (HIF-1α) is a central regulator that controls the hypoxic response of mammalian cells through the induction of various target genes, and its expression contributes to the development and malignant progression of many tumors. We previously reported that some chemotherapeutic drugs and γ-rays induce HIF-1α expression through increased production of reactive oxygen species (ROS) in oral squamous cell carcinoma (OSCC) cells. However, the mechanism by which intracellular ROS activate HIF-1α expression is poorly understood. In this study, we investigated the influence of ROS on HIF-1α signaling in OSCC cells by the transfection of manganese-superoxide dismutase (Mn-SOD)-specific small interfering RNA (siRNA). The levels of HIF-1α protein and mRNA were increased by siRNA under both normoxic and hypoxic conditions in parallel with the increase in intracellular ROS levels. The accumulation of HIF-1α protein was enhanced through inhibition of the recruitment of von Hippel–Lindau protein and HIF-1α ubiquitination without a change in prolyl hydroxylase mRNA and protein levels. Furthermore, the transactivation of HIF-1α was enhanced via cap-dependent and internal ribosome entry site-mediated mechanisms. These results suggest that intracellular ROS produced by the knockdown of Mn-SOD enhance HIF-1α expression in OSCC cells through transcriptional, translational, and posttranslational regulation.