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

by Selleckchem | Aug 29 2023

Objectives: Adenoid Cystic Carcinomas (ACC) typically show modest responseto cytotoxic therapy. Cancer stem cells (CSC) have been implicated in chemoresistance and tumor relapse. However, their role in ACC remains unknown. The purpose of this work was to evaluate the impact of targeting ACC CSCs with Bmi-1 inhibitors on resistance to cytotoxic therapy and tumor relapse.

Materials and methods: Therapeutic efficacy of a small molecule inhibitor of Bmi-1 (PTC596; Unesbulin) and/or Cisplatin on ACC stemness was evaluated in immunodeficient mice harboring PDX ACC tumors (UM-PDX-HACC-5) and in human ACC cell-lines (UM-HACC-2A,-14) or low passage primary human ACC cells (UM-HACC-6). The effect of therapy on stemness was examined by salisphere assays, flow cytometry for ALDH activity and CD44 expression, and Western blots for Bmi-1 (self-renewal marker) and Oct4 (embryonic stem cell marker) expression.

Results: Platinum-based agents (Cisplatin, Carboplatin) induced Bmi-1 and Oct4 expression, increased salisphere formation and the CSC fraction in vitro and in vivo. In contrast, PTC596 inhibited expression of Bmi-1, Oct4 and pro-survival proteins Mcl-1 and Claspin; decreased the number of salispheres, and the fraction of ACC CSCs in vitro. Silencing Claspin decreased salisphere formation and CSC fraction. Both, single agent PTC596 and PTC596/Cisplatin combination decreased the CSC fraction in PDX ACC tumors. Notably, short-term combination therapy (2 weeks) with PTC596/Cisplatin prevented tumor relapse for 150 days in a preclinical trial in mice.

Conclusion: Therapeutic inhibition of Bmi-1 ablates chemoresistant CSCs and prevents ACC tumor relapse. Collectively, these results suggest that ACC patients might benefit from Bmi-1-targeted therapies.

Comments:

The study described aimed to investigate the impact of targeting Adenoid Cystic Carcinoma (ACC) Cancer Stem Cells (CSCs) with Bmi-1 inhibitors on resistance to cytotoxic therapy and tumor relapse. The researchers utilized various materials and methods to evaluate the therapeutic efficacy of a small molecule inhibitor of Bmi-1 called PTC596 (Unesbulin) in combination with or without Cisplatin, a commonly used platinum-based chemotherapeutic agent.

The evaluation was conducted using immunodeficient mice harboring patient-derived xenograft (PDX) ACC tumors (UM-PDX-HACC-5), as well as human ACC cell lines (UM-HACC-2A, -14) and low passage primary human ACC cells (UM-HACC-6). The impact of therapy on stemness, a characteristic of CSCs, was assessed through salisphere assays, flow cytometry analysis of ALDH activity and CD44 expression, and Western blot analysis of Bmi-1 and Oct4 expression. Bmi-1 is a marker associated with self-renewal, while Oct4 is an embryonic stem cell marker.

The results of the study demonstrated that platinum-based agents such as Cisplatin and Carboplatin induced the expression of Bmi-1 and Oct4, increased salisphere formation (a characteristic of CSCs), and expanded the CSC fraction both in vitro and in vivo. In contrast, treatment with PTC596 inhibited the expression of Bmi-1, Oct4, and pro-survival proteins Mcl-1 and Claspin. PTC596 also reduced the number of salispheres and decreased the fraction of ACC CSCs in vitro. Silencing Claspin, one of the pro-survival proteins, also decreased salisphere formation and the CSC fraction.

Furthermore, both single-agent PTC596 and the combination of PTC596 with Cisplatin decreased the CSC fraction in PDX ACC tumors. Notably, when the combination therapy of PTC596 and Cisplatin was administered for a short duration (2 weeks), it prevented tumor relapse for a significant period of 150 days in a preclinical trial using mice.

In conclusion, the study suggests that therapeutic inhibition of Bmi-1 effectively eliminates chemoresistant CSCs and prevents tumor relapse in ACC. These findings indicate that ACC patients may potentially benefit from Bmi-1-targeted therapies.