Cancer stem cell-associated pathways regulating chemoresistance in ovarian cancer

Abstract

Epithelial ovarian cancer (EOC) is the most lethal of the gynaecologic malignancies, with an overall 5-year survival rate of only <30%. This poor prognosis is due to the advanced-stage disease at initial diagnosis in approximately 70% of women. Despite good initial response to chemotherapy treatment, almost 80% of these patients will relapse and ultimately die within 5 years due to the development of chemoresistant recurrent disease. Emerging evidence suggests that a subpopulation of cancer stem cells (CSCs) that possess stem cell-like self-renewal and pluripotency properties are responsible for drug resistance and disease recurrence. The results from this thesis demonstrated that the expression of JAK2, STAT3, Src and EGFR activation was significantly higher in the ascites-derived tumour cells of recurrent serous EOC patients previously treated with chemotherapy compared to that of untreated chemo-naïve patients. STAT3 is a major regulator in stem cell regulation, and is predominantly activated by upstream JAK2 tyrosine kinase.

Hence, this thesis aimed to explore whether targeting the JAK2/STAT3 pathway is sufficient in suppressing the development of ovarian CSCs which consequently would overcome chemoresistance-associated recurrence, and improve the disease-free survival period of EOC patients.

To achieve the aim, in the first instance, this thesis investigated the in vitro suppression of paclitaxel-induced JAK2/STAT3 pathway activation with Momelotinib (a potent, small molecular inhibitor of JAK2) and paclitaxel-induced Src/STAT3 activation with dasatinib (a potent, small molecular inhibitor of Src family kinases) on the development of CSC-like phenotypes and tumour cell viability in HEY high-grade serous carcinoma and TOV21G clear cell carcinoma cell lines. The in vitro results demonstrated that suppression of JAK2/STAT3 with Momelotinib and paclitaxel was relatively more effective at reducing CSC-like markers and cell viability than suppression of Src/STAT3 with dasatinib and paclitaxel.

Next, an in vivo mouse xenograft model was included to determine the effect of combination treatment with paclitaxel and Momelotinib on the sustained suppression of CSC-like emergence, tumour burden and disease-free survival of mice. Intraperitoneally injected HEY cells in Balb/c nude mice that received a combination of weekly paclitaxel and daily Momelotinib treatment survived the longest and produced the smallest tumour burden that exhibited significant reduction in paclitaxel-induced JAK2/STAT3 activation and CSC-like (Oct4 and c-Kit) tumour staining, but enhanced Src activation. However, termination of combination treatment resulted in the re-activation of JAK2/STAT3 and an increased in CSC-like tumour staining, but reduced Src activation.

This thesis is the first to show that maintenance therapy with Momelotinib in a group of mice pre-treated with paclitaxel and Momelotinib further reduced the overall tumour burden and extended the overall disease-free survival period.

Collectively, the results presented in this thesis supports the role of JAK2/STAT3 activation, in CSC-mediated chemoresistance and recurrence in EOC, while the activation of Src pathway compensated for the effects of Momelotinib. As we are now entering an era of personalized therapy, the findings in this thesis have increased our understanding of the resistance mechanisms and the complexity underlying successful clinical implementation, which have profound impact in developing a more effective treatment paradigm for EOC patients.