Characterizing oncogenic effects of NADPH oxidase 5 in head and neck squamous cell carcinomas

Abstract

Head and neck squamous cell carcinoma (HNSCC) has always been among the top ten most common cancers worldwide, with increased incidence and mortality in recent years. About two-thirds of HNSCC patients present at an advanced stage when first diagnosed, and the prognosis of these patients remains unfavorable. Surgery, radiotherapy and chemotherapy are major therapeutic options for HNSCC patients. However, combinations of these conventional treatments offer minimal improvements in survival rates. Despite the rapid development of targeted therapies for cancers over the years, only two types of targeted therapy agents have been approved for HNSCC to date. Therefore, exploring novel targets for treating HNSCC is in high demand, especially for advanced stage patients. A failed clinical trial of antioxidants has brought new insights to find novel targets in managing HNSCC. Reactive oxygen species (ROS) are frequently involved in HNSCC carcinogenesis with an increased level in tumor tissues. However, a clinical trial using antioxidants to eliminate ROS resulted in no benefits in either improving the survival rate or preventing the second primary tumors of HNSCC patients. One possible explanation is that ROS were not fully eliminated by those antioxidants, because the major cellular generator of ROS - the NADPH oxidase (NOX) family - might replenish them, which contributes to the failure of antioxidant therapy. Therefore, targeting NOX might be a promising strategy to treat HNSCC. Unfortunately, studies of the NOX family in HNSCC remain extremely limited. This study characterized the expression, function and clinical significance of the potential NOX in HNSCC with an integrated employment of bioinformatics, clinical samples, animal models and cell lines. Specifically, NOX5 was found as the most potential target among the NOX family through online database analysis. Next, highly expressed NOX5 was identified in Hong Kong advanced stage and metastatic HNSCC patients, with correlation to poor clinical outcome. Functional studies showed that knockdown of NOX5 inhibited HNSCC cell growth, and this reduction was more pronounced under hypoxia. Mechanism studies demonstrated that NOX5 regulated HIF-2α, and this regulation together with MAPK/ERK1/2 activation were mediated by NOX5-derived superoxide. Further studies demonstrated that NOX5 promoted angiogenesis by upregulating VEGF and facilitated metastasis via inducing the epithelial-mesenchymal transition (EMT) process. The characterization of the oncogenic effects of NOX5 in HNSCC provides a promising target for treating HNSCC patients. Although unspecific inhibitors of the NOX family exist already, this study underlines that the specific inhibitor of NOX5 deserves further clinical development. Moreover, the identification of NOX5-related pathways has provided the possible combination options of blocking several signals at the same time – such as the combinations of HIF-2α inhibitor, VEGF inhibitor, MAPK/ERK1/2 inhibitor or superoxide scavenger – to achieve better efficacy when applying the targeted therapies. In summary, this study pointed out the unfavorable situation of targeted therapy development in HNSCC. By integrating the bioinformatic data, clinical tissues of patients, animal models and cell experiments, the current study provided evidence that NOX5 promotes HNSCC progression and metastasis by activating HIF-2α, VEGF, MAPK/ERK1/2 and EMT, demonstrating that NOX5 is a potential target for HNSCC targeted therapy.