HPV-E6-induced Nurr1 promoted cancer aggressiveness, self-renewal and radioresistance via MEK/ERK and PI3K/Akt/mTOR pathways in cervical cancer

Abstract

Human papillomavirus (HPV) is regarded as the etiological agent for cervical cancer affecting half a million individuals annually. Despite the generally accepted association of high-risk HPV with cervical cancer, the detailed mechanisms by which viral oncoproteins promote carcinogenesis remain poorly understood. Concurrent chemoradiation is the standard treatment against cervical cancer. Though it is able to initially shrink the tumor mass, it fails to completely eradicate the lesions due to the residing chemo- and radio-resistant cervical cancer stem-like cells, resulting in tumor relapse. Studies reporting the cervical stem cells-specific HPV infection and HPV-induced changes of stem cell dynamics prompted us to explore the association between HPV oncoprotein and cancer stemness. Besides, stem-like populations are frequently associated with more aggressive and metastatic phenotypes. Therefore, it is imperative to identify HPV-induced targets that promote tumorigenesis and to investigate whether targeting these cancer stem-like populations can be a promising therapeutic strategy for cervical cancer treatment. In this study, I demonstrated for the first time that HPV-E6 oncoprotein induced Nurr1 expression in E6-overexpressing HPV-negative C33A cells, HPV16- and HPV18-positive cervical cancer cell lines. Up-regulation of Nurr1 was observed in cervospheres-derived from primary cervical cancer tissues in our previous study, justifying its association with stem-like properties. Immunohistochemistry confirmed a higher expression of Nurr1 in malignant cervical tissues. Further, stronger expression of Nurr1 was detected in the cytosol as tumor grades propagated. Functional studies demonstrated that Nurr1 promoted cancer aggressiveness by showing higher proliferative output, greater metastatic capability, better anchorage-independent growth and stronger tumorigenicity in vivo. Moreover, Nurr1 augmented self-renewal ability as demonstrated by greater spheroid-forming ability and ability to form second-generation xenografts in serial transplantation. In addition, Nurr1 was found to promoted radioresistance of cervical cancer cell lines. Regarding the Nurr1-regulated molecular pathways, I am the first to report that Nurr1-augmented malignant phenotypes were mediated via PI3K/Akt/mTOR and MEK/ERK signaling cascades. Treating Nurr1-overexpressing cells with an inhibitor specific to MEK1/2 or a dual inhibitor specific to PI3K and mTOR abrogated Nurr1-induced cancer aggressiveness and self-renewal ability via up-regulation of p21 and p27 and suppression of MMP9 and KLF4. In term of clinical significance, PI3K/mTOR dual inhibitor abolished Nurr1-mediated radioresistance and sensitized the cervical cancer-stem like cells to irradiation, suggesting PI3K/mTOR inhibitor as a potential candidate to target radioresistant populations in concurrent chemoradiotherapy. In summary, our finding identified Nurr1 as a novel HPV-E6-induced target, which promoted tumorigenesis and self-renewal in cervical cancer-stem like cells. I also reported a novel activation mechanism of MEK/ERK and PI3K/Akt/mTOR cascades by Nurr1, which subsequently inhibited expression of p21 and p27 expression and up-regulated MMP9 and KLF4 expression. Besides, targeting PI3K/Akt/mTOR pathways was demonstrated as an effective approach to abrogate radioresistance in cervical cancer stem-like cells. The identification of HPV-E6-induced Nurr1 provided new insight in how HPV oncoprotein promoted cervical carcinogenesis and created new avenues to the development of more effective cancer treatments.