Imiquimod simultaneously induces autophagy and apoptosis in human basal cell carcinoma cells

Background Imiquimod shows antitumour activity through the stimulation of cell-mediated immunity in vivo. Recent studies have shown that imiquimod promotes apoptosis in melanoma cells and induces autophagy in macrophage cell lines. Objectives To evaluate the imiquimod-induced apoptosis, autophagy and their relationship in a basal cell carcinoma (BCC) cell line. Methods Cell viability was determined by XTT test. Apoptosis was evaluated by DNA content assay, annexin V/propidium iodide staining assay and terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate nick-end labelling assay. Autophagy was determined by LC3 immunoblotting, EGFP-LC3 puncta formation and quantification of acidic vesicular organelles with acridine orange staining. The temporal and spatial differences of imiquimod-induced apoptosis and autophagy were examined by immunoblotting and simultaneously monitored by staining the EGFP-LC3 transfected cells with caspase 3 fluorogenic substrate. We inhibited the apoptosis and autophagy by pancaspase inhibitor and siRNA for Beclin 1 or Atg5, respectively, to evaluate the interplay between imiquimod-induced apoptosis and autophagy. Results We found that imiquimod induces autophagy and apoptosis in BCC cells in a time- and dose-dependent manner. Imiquimod not only induced EGFP-LC3 puncta formation for autophagy, but also simultaneously activated an apoptotic caspase cascade in the same cells. Both apoptosis and autophagy induced by imiquimod cooperate to cause BCC cell death. However, inhibition of imiquimod-induced apoptosis increased the strength of autophagy, and inhibition of imiquimod-induced autophagy further promoted cell apoptosis. Conclusions This study not only demonstrates that imiquimod can directly induce autophagy and apoptosis in BCC cells, but also shows the cooperation and coordination between these two processes to induce cell death.