An autophagy inhibitor enhances the inhibition of cell proliferation induced by a proteasome inhibitor in MCF-7 cells.

The ubiquitin-proteasome system and the autophagy-lysosome pathway are the two main routes for eukaryotic intracellular protein clearance. Inhibition of proteasome activity leads to cell death. Due to the dual roles of autophagy in tumor cell survival and death, the effect of suppression of autophagy on breast cancer cells remains to be elucidated. We investigated whether inhibition of the proteasome is capable of inducing autophagy, and we assessed the effect of combined inhibition of the proteasome and autophagy on human breast cancer MCF-7 cells. The proteasome inhibitor bortezomib was used to induce autophagy in MCF-7 cells, and the effect of autophagy on the proliferation of MCF-7 cells was investigated using the autophagy inhibitor 3-MA. Cell viability was measured by MTT assay. The expression of autophagy‑related proteins was determined by Western blot analysis and the GFP-LC3 redistribution was detected using a fluorescence microscope after MCF-7 cells were infected with a GFP-LC3-expressing adenovirus. MCF-7 cell proliferation was inhibited and autophagy was activated in the same dose‑dependent manner. Bortezomib induced a dose‑dependent increase in LC3-II. However, when MCF-7 cells were co-treated with bortezomib and 3-MA, 3-MA blocked the increase in LC3-II protein expression and led to a significant inhibition of cell proliferation. Inhibition of the proteasome may induce autophagy in human breast cancer MCF-7 cells and 3-MA could inhibit autophagy induced by the proteasome inhibitor. A combination of 3-MA and bortezomib increases cell death. These findings indicate that suppression of the two intracellular degradation systems may shed new light on breast cancer control.