Hypoxia Accelerates Aggressiveness of Hepatocellular Carcinoma Cells Involving Oxidative Stress, Epithelial-Mesenchymal Transition and Non-Canonical Hedgehog Signaling

Abstract

Background/Aims: Hypoxic microenvironment, a common feature of hepatocellular carcinoma (HCC), can induce HIF-1α expression and promote the epithelial-mesenchymal transition (EMT) and invasion of cancer cells. However, the underlying molecular mechanisms have not fully elucidated. Methods: HCC cells were cultured under controlled hypoxia conditions or normoxic conditions. Transwell assays were used to examine the migration and invasion capacity. HIF-1α siRNA, cyclopamine (a SMO antagonist) and GLI1 siRNA were used to inhibit HIF-1α transcription or Hh signaling activation. Results: In present study, we first observed a strongly positive correlation between HIF-1α and GLI1 expression in HCC tissues. Then, we showed that hypoxia significantly promoted EMT process and invasion of HCC cells, associated with activating the non-canonical Hh pathway without affecting SHH and PTCH1 expression. HIF-1α knockdown mitigated hypoxia-induced SMO and GLI1 expression, EMT invasion of HCC cells. Moreover, the SMO inhibitor or GLI1 siRNA also reversed the hypoxia-driven EMT and invasion of HCC cells under hypoxia condition. Here, we show that non-canonical Hh signaling is required as an important role to switch on hypoxia-induced EMT and invasion in HCC cells. In addition, we found that hypoxia increased ROS production and that ROS inhibitors (NAC) blocked GLI1-dependent EMT process and invasion under hypoxic conditions. To determine a major route of ROS production, we tested whether nicotinamide adenine dinucleotide phosphate (NADPH) oxidase 4 (NOX4) is involved in hypoxia-induced ROS production. NOX4 expression was found to be increased at both mRNA and protein levels in hypoxic HCC cells. Furthermore, siRNA-mediated knockdown of NOX4 expression abolished hypoxia induced ROS generation and GLI1-dependent activation and invasion of HCC cells. Conclusion: Our findings indicate that hypoxia triggers ROS-mediated GLI1-dependent EMT progress and invasion of HCC cells through induction of NOX4 expression. Thus, hypoxia-driven ROS mediated non-canonical Hh signaling may play an important role in the initiation of EMT and provides a potential marker for cancer prevention and treatment.