Metformin radiosensitizes OSCC in 2D and 3D models: possible involvement of CAF-1

Objective. This study investigated metformin as a sensitizer for radiotherapy in oral squamous cell carcinoma (OSCC) to reduce the radiation intensity. It evaluated the drug’s effect on Chromatin Assembly Factor-1 (CAF-1) expression, whose high levels correlate with worse prognosis of this cancer. 

Methods. The effects of metformin, alone and with radiotherapy, were evaluated on CAL27 (HPV-) and SCC154 (HPV+) OSCC cells. The analyses were performed on cell monolayers by colony-forming assay, motility, and confocal microscopy. In spheroid 3D models, the sensitizing effect of metformin was assessed by measuring areas. CAF-1 expression affected by metformin was evaluated via Western blot, and its role was investigated by siRNAs. 

Results. Metformin reduced the cells’ ability to form colonies, migrate and invade, and promoted the acquisition of a less aggressive phenotype by increased E-cadherin and decreased N-cadherin expressions. Moreover, metformin lowered the IC50 of radiotherapy and showed strong effects on spheroid growth. Metformin downmodulated the expression of the major subunits of CAF-1, and the knockdown of this protein by siRNAs elicited a metformin-like effect on cell aggressiveness. 

Conclusions. Metformin emerged as a promising adjuvant drug in OSCC because of its effects on cell aggressiveness and radiosensitizing action. These activities could be CAF-1-mediated.