Background: The underlying mechanism involved in ovarian cancer stemness and chemoresistance remains largely unknown. Here, we explored whether the regulation of c-Kit and plasma membrane prohibitin (PHB) affects ovarian cancer stemness and chemotherapy resistance.Methods: Mass spectrum analysis and an in vitro kinase assay were conducted to examine the phosphorylation of PHB at tyrosine 259 by c-Kit. The in vitro effects of c-Kit on membrane raft-PHB in ovarian cancer were determined using tissue microarray (TMA)-based immunofluorescence, western blotting, immunoprecipitation, colony and spheroid formation, cell migration and cell viability assays. In vivo tumor initiation and carboplatin treatment were conducted in nude mice.Results: We found that c-Kit and PHB colocalized in the raft domain and were positively correlated in human ovarian serous carcinoma. c-Kit interacted with PHB and facilitated the phosphorylation of PHB at tyrosine 259 (phospho-PHB Y259 ) in the membrane raft to enhance ovarian cancer cell motility. The generation of SKOV3GL-G4, a metastatic phenotype of SKOV3 green fluorescent protein and luciferase (GL) ovarian cancer cells, in xenograft murine ascites showed a correlation between metastatic potential and stem cell characteristics, as indicated by the expression of c-Kit, Notch3, Oct4, Nanog and SOX2. Further study revealed that after activation by c-Kit, raft-phospho-PHB Y259 interacted with Notch3 to stabilize Notch3 and increase the downstream target PBX1. Downregulation of raft-phospho-PHB Y259 increased the protein degradation of Notch3 through a lysosomal pathway and inhibited the β-catenin-ABCG2 signaling pathway. Moreover, raft-phospho-PHB Y259 played an important role in ovarian cancer stemness and tumorigenicity as well as resistance to platinum drug treatment in vitro and in vivo.Conclusions: These findings thus reveal a hitherto unreported interrelationship between c-Kit and PHB as well as the effects of raft-phospho-PHB Y259 on ovarian cancer stemness and tumorigenicity mediated by the Notch3 and β-catenin signaling pathways. Targeting the c-Kit/raft-phospho-PHB Y259 axis may provide a new therapeutic strategy for treating patients with ovarian cancer.
Phosphorylation of prohibitin (PHB) at threonine 258 (phospho-PHBT258) and tyrosine 259 (phospho-PHBY259) in human cancer-cell membranes is associated with an increase in the invasiveness and metastasis of cancer cells. Phospho-PHBY259 can regulate phospho-PHBT258 but not vice versa. Here, we found that stably overexpressing c-Kit in SKOV3 ovarian cancer cells (SKOV3_c-Kit) increased phospho-PHBY259, phospho-PHBT258 and PHB levels in the lipid raft domain of the plasma membrane. On the other hand, inhibition of c-Kit expression decreased PHB expression in SKOV3_c-Kit and Kuramochi ovarian cancer cells. Immunoprecipitation analysis showed that c-Kit interacts with PHB. SKOV3_c-Kit cells or SKOV3 cells overexpressing PHB in the lipid raft had higher levels of Notch3, NICD3 (Notch3 intracellular domain) and ABCG2 protein expression as compared to wild-type SKOV3 cells. Moreover, generation of SKOV3-G4 cells through serial passage of SKOV3 cells into nude mice ovary 4 times, revealed that the levels of c-Kit, Notch3, NICD3, phospho-PHBY259, phospho-PHBT258 and PHB in the lipid raft domain of SKOV3-G4 were significantly higher than wild-type SKOV3 cells. The SKOV3-G4 cells formed spheroids with stem cell-like properties such as higher expression of stem cell markers (Oct4, Nanog, SOX2). In addition, SKOV3-G4 cells exhibited more clonogenic potential and migration/invasion ability than wild-type SKOV3. These results suggest that c-Kit interacts with PHB in the lipid raft domain to phosphorylate PHB to increase invasiveness and stemness in ovarian cancer. Citation Format: Shu-Mei Y. Liang, Chia-Hsun Fang, Pu-Yuan Chang, Chi-Ming Liang. c-Kit enhances ovarian cancer stemness via upregulating phospho-PHB in the lipid raft domain of the plasma membrane [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr LB-048.
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