Background: Although surgery, chemotherapy, and radiotherapy eliminate clinically apparent ovarian tumor, the 5-year survival rate is no more than 45%. Cancer stem cells (CSCs) have been identified for precaution of tumor metastasis and recurrence in many kinds of cancers including ovarian cancer. 
Aim: This study aims to explore the function of OCT4, a CSC marker, in ovarian cancer progression and to investigate its underlying mechanism. 
Materials and methods: By Hoechst side population (SP) technique, CSC-like SP cells from human ovarian cancer SKOV3 and A2780 cells were isolated and used for this study. shRNA and lentivirus targeting human OCT4  gene were used to knock down OCT4 in SP cells and upregulate OCT4 in non-SP (NSP) cells stably. Peficitinib was used to inhibit JAK/STAT signaling. Cell counting kit-8, flow cytometry, and in vivo xenograft model were used to evaluate the effects of OCT4/JAK/STAT on the viability, drug resistance, apoptosis, cycle, and tumorigenesis of the SP cells. Immunofluorescence staining was used to detect the location of STAT6. 
Results: Results showed that OCT4 was upregulated in the SP of SKOV3 and A2780 cells when compared with the NSP cells. Downregulation of OCT4 inhibited SP cell viability, tumorigenesis, and reduced cell drug resistance and induced a G2/M phase arrest, while upregulation of OCT4 conferred NSP cell malignant features. Besides, OCT4 upregulation in NSP cells increased the phosphorylated levels of proteins in JAK and STAT families, especially in JAK1 and STAT6. Furthermore, the roles of apoptosis inhibition and viability, invasion, and tumorigenesis promotions induced by OCT4 in NSP cells were all abolished when adding peficitinib. 
Conclusion: Our study demonstrated that OCT4 accelerated ovarian cancer progression through activating JAK/STAT signaling pathway.
Keywords: OCT4, JAK/STAT signaling, ovarian cancer, side population