Bmi-1 inhibition sensitizes head and neck cancer stem cells to cytotoxic chemotherapy

Cancer stem cells (CSC) drive therapeutic resistance and recurrence in head and neck squamous cell carcinoma (HNSCC). We and others have shown that treatment with cytotoxic chemotherapy agents (e.g. Cisplatin, Carboplatin) induce Bmi-1 expression and increase the fraction of highly tumorigenic CSC in HNSCC. Notably, Bmi-1 is a master regulator of stem cell self-renewal and DNA repair. The purpose of this work was to test whether therapeutic inhibition of Bmi-1 sensitizes HNSCC cancer stem cells to chemotherapy. HNSCC cells (UM-SCC-1,-22A,-22B) were treated with Cisplatin or Carboplatin and subjected to stemness analyses to evaluate the impact of Bmi-1 on chemoresistance. We observed that both, shRNA-mediated Bmi-1 silencing or pharmacologic inhibition of Bmi-1 with the small molecule inhibitor PTC596, blocked chemotherapy-induced cancer stemness (i.e. increase in the fraction of ALDH^highCD44^high cells), CSC self-renewal (i.e. orosphere formation) and inhibited protective DNA damage responses in HNSCC. Further, it is known that high IL-6 serum levels correlate with poor HNSCC patient survival, and that platinum-based therapies induce IL-6 signaling. Here, we observed that Bmi-1 silencing (or PTC596 treatment) inhibited the IL-6R/STAT3 signaling pathway even in presence of platinum-based cytotoxic agents (i.e. Cisplatin, Carboplatin). In vivo, Bmi-1 inhibition with PTC596 suppressed Cisplatin-mediated increase in the fraction of ALDH^highCD44^high cells (cancer stemness). Collectively, these preclinical results demonstrate that Bmi-1 is a key mediator of head and neck cancer stemness and suggest that HNSCC patients might benefit from treatment with a Bmi-1 inhibitor combined with a conventional chemotherapeutic agent.