Abstract
Apoptosis and autophagy have been shown to be negatively regulated by prosurvival Bcl-2 proteins. We determined whether the anticancer agent celecoxib, alone or combined with a small molecule Bcl-2/Bcl-x(L) antagonist (ABT-737), can induce autophagy in colon cancer cells. Furthermore, we determined whether inhibition of autophagy can drive colon cancer cells into apoptosis. Celecoxib was shown to induce apoptosis that was attenuated by ectopic Bcl-2 or Bax knockout. ABT-737 synergistically enhanced celecoxib-induced cytotoxicity that was primarily due to apoptosis as shown by caspase cleavage and Annexin V labeling that was attenuated by a pan caspase inhibitor (z-VAD-fmk). Celecoxib triggered conversion of the autophagosome-associated protein light chain 3 (LC3) from a cytosolic (LC3I) to a membrane-bound (LC3II) form, as shown by immunoblotting and a punctate fluorescence pattern of an ectopic GFP-LC3 protein. Celecoxib-induced conversion of LC3 was due to autophagy induction, as supported using the lysosome inhibitor, bafilomycin A1, which produced an accumulation of LC3II. ABT-737 enhanced celecoxib-induced LC3 conversion and p62/SQSTM1 degradation. Inhibition of autophagy was then studied in an effort to drive cells into apoptosis. 3-methyladenine (3-MA) blocked LC3 conversion, and 3-MA and wortmannin significantly enhanced apoptotic signaling in cells treated with celecoxib plus ABT-737. Furthermore, knockdown of Atg8/LC3B or Vps34 using siRNA attenuated p62 degradation and enhanced apoptotic signaling; Vps34 siRNA potentiated annexin V(+), PI(-) labeled cells induced by celecoxib + ABT-737. In conclusion, celecoxib induces apoptosis and autophagy that can both be potentiated by ABT-737. Inhibition of autophagy was shown to enhance apoptosis, suggesting a novel therapeutic strategy against colon cancer.