The functional role of Dihydroorotate Dehydrogenase in oral cancer

Abstract

Oral cancer is one of the ten most common cancers in the world, of which squamous cell carcinoma is the major histological form. Regional invasion and lymph node metastasis are closely associated with high mortality and poor prognosis. Hence, understanding and identifying the critical factor involved in oral cancer invasion and metastasis have a profound impact on treatment and prognosis. The hypoxic tumor microenvironment is a significant contributing factor to oral cancer invasion and metastasis. At low oxygen tension, oral cancers modified their gene expression signature to adapt to hypoxic stress. The hypoxic responses are mediated via a key transcription factor known as hypoxia-inducible factor 1 (HIF-1). High HIF-1 expression is a poor prognostic marker in oral cancer patients. However, the molecular signaling mechanisms governing HIF-1 upregulation in oral cancer remain poorly understood. To date, effective HIF-1 inhibitor remains unavailable for clinical use. Here, we attempt to identify the upstream HIF-1 regulator and explore the potential use as a therapeutic target in oral cancer treatment.

Dihydroorotate Dehydrogenase (DHODH) is a mitochondrial protein located at the outer surface of the inner mitochondrial membrane. DHODH is first identified as an enzyme which catalyzes the de novo pyrimidine biosynthesis process. DHODH could function as a reactive oxygen species (ROS) generating enzymes. As ROS can function as a secondary messenger in signal transduction, we speculated that DHODH might be involved in the molecular signaling pathway in oral cancers. This project aims at identifying the molecular regulator governing HIF-1 upregulation in oral cancer. We hypothesize that DHODH can stimulate HIF-1 expression by the ROS-generating function in oral cancers. Also, we tested whether DHODH is a potential target for oral cancer treatment. Analysis of the data in head and neck cancers database suggested that head and neck cancer tissues had higher DHODH expression as compared with the normal counterpart. The DHODH transcript level in cancer tissue increased in parallel with tumor stage development, suggesting that DHODH is possibly involved in cancer progression. Silencing DHODH resulted in a significant decrease in cancer cell proliferation, migration, and invasion. For DHODH-overexpressing oral cancer cell lines, the ability to form metastatic foci was remarkably increased. In addition, we noted that DHODH expression is associated with the transcription upregulation of HIF1A (Hypoxia inducible factor 1 subunit alpha) in oral cancer cells. DHODH is a ROS-generating enzyme which could involve in mediating HIF1A transcription. We attempted to test whether targeting DHODH is a feasible approach to suppress HIF1A. Our results suggest that targeting DHODH using clinically available DHODH inhibitors can suppress the hypoxia-induced HIF1A protein accumulation in oral cancer cells. In conclusion, our results suggest that DHODH is functionally involved in oral cancer and is associated with the development of aggressive phenotype. DHODH expression is correlated with HIF1A expression in oral cancers, suggesting that mitochondria DHODH activity play a role in regulating the hypoxic responses of oral cancers. DHODH inhibitors have HIF1A inhibitory function. Thus, targeting DHODH might be a potentially useful strategy for the treatment of the hypoxic oral cancers.