Targeting Cancer Stem Cells in Mucoepidermoid Carcinoma.

Abstract

Salivary gland mucoepidermoid carcinoma is rare but causes significant morbidity in patients who are diagnosed. Slow, persistent growth as well as resistance to chemotherapy treatment has greatly hindered the response of these tumors to conventional therapies. Emerging research has identified a population of highly tumorigenic cells, termed cancer stem cells, which possess stem cell-like properties of multipotency, self-renewal, and unique tumorigenic potential compared to non-cancer stem cells. Importantly, cancer stem cells have been shown to be resistant to chemotherapy and radiation therapies. Selective targeting of stem cell associated pathways could prevent therapy resistance. Here, we investigated the presence of cancer stem cell markers ALDH, CD44, CD24, and CD10 and the tumorigenic potential of human mucoepidermoid carcinoma (HMC) cells sorted for these stem cell markers in in-vitro salisphere formation and in-vivo xenograft growth models. FACS-sorted ALDHhighCD44high cells preferentially formed salispheres in primary and secondary ultra-low attachment, serum-free culture compared to ALDHlowCD44low cells suggesting that ALDHhighCD44high cells are capable of self-renewal. Importantly, ALDHhighCD44high sorted cells consistently formed primary and secondary tumors compared to ALDHlowCD44low cells when implanted in-vivo, each tumor replicating the original ALDH/CD44 sub-populations suggesting that ALDHhighCD44high cells are capable of tumorigenicity and multipotency. Together, these results suggest that ALDHhighCD44high cells identify a population of uniquely tumorigenic cancer stem cells. In an effort to identify a therapeutic to target the ALDHhighCD44high cells, we used the MDM2/p53 small molecule inhibitor, MI-773, and treated human mucoepidermoid carcinoma cells using low doses of the drug. We found a significant reduction of ALDHhighCD44high cells in-vitro as well as in-vivo. Importantly, we also noted an increase in p21 expression and decrease in Bmi-1 expression suggesting that accumulation of p53 in the cell by inhibiting MDM2 greatly affects self-renewal and differentiation associated protein. Treatment of human mucoepidermoid carcinoma cells with MI-773 also induced apoptosis and cell-cycle arrest both in-vitro and in-vivo. Together, these results demonstrate the presence of highly tumorigenic cancer stem cells in salivary gland mucoepidermoid carcinoma. Further, we propose the use of MDM2 inhibitors in the selective ablation of cancer stem cells as a therapeutic target for the treatment of salivary gland mucoepidermoid carcinomas.