Therapeutic inhibition of Bmi-1 ablates chemoresistant cancer stem cells in adenoid cystic carcinoma

Objectives: Adenoid Cystic Carcinomas (ACC) typically exhibit limited responsiveness to cytotoxic therapies. Cancer stem cells (CSCs) are believed to play a significant role in chemoresistance and tumor recurrence, but their specific involvement in ACC remains unclear. This study aims to assess the effects of targeting ACC CSCs with Bmi-1 inhibitors on overcoming chemoresistance and preventing tumor relapse.

Materials and Methods: The therapeutic effects of a small molecule Bmi-1 inhibitor (PTC596; Unesbulin) alone or in combination with Cisplatin were evaluated in immunodeficient mice carrying PDX ACC tumors (UM-PDX-HACC-5) as well as in human ACC cell lines (UM-HACC-2A, -14) and low-passage primary human ACC cells (UM-HACC-6). The impact of therapy on stemness was analyzed using salisphere assays, flow cytometry for ALDH activity and CD44 expression, and Western blot analysis for Bmi-1 (a self-renewal marker) and Oct4 (an embryonic stem cell marker).

Results: Platinum-based agents (Cisplatin, Carboplatin) upregulated Bmi-1 and Oct4 expression, promoted salisphere formation, and increased the CSC fraction both in vitro and in vivo. In contrast, PTC596 inhibited the expression of Bmi-1, Oct4, and pro-survival proteins Mcl-1 and Claspin; reduced salisphere formation; and decreased the CSC fraction in vitro. Silencing Claspin also led to a reduction in salisphere formation and CSC fraction. Both PTC596 alone and the combination of PTC596 and Cisplatin reduced the CSC fraction in PDX ACC tumors. Remarkably, a short-term combination therapy (2 weeks) of PTC596 and Cisplatin prevented tumor relapse for 150 days in a preclinical mouse model.

Conclusion: Targeting Bmi-1 therapeutically eliminates chemoresistant CSCs and prevents relapse of ACC tumors. These findings suggest that ACC patients could potentially benefit from Bmi-1-targeted treatments.