Induction of autophagy by the MG-132 proteasome inhibitor is associated with endoplasmic reticulum stress in MCF-7 cells

The aim of the present study was to investigate whether endoplasmic reticulum (ER) stress is involved in MG-132-induced autophagy, and to determine the effects of the inhibition of autophagy and ER stress on cell viability following MG-132 treatment. The proteasome inhibitor, MG-132, was used to induce autophagy in MCF-7 cells, and 3-methyladenine (3-MA) and salubrinal were used to inhibit autophagy and ER stress, respectively. An MTT assay was used to analyze cell viability. Apoptosis and the cell cycle were analyzed using flow cytometry. The expression levels of apoptosis- and ER stress-associated genes were investigated using western blot and reverse transcription-quantitative polymerase chain reaction analyses. MG-132 inhibited cell proliferation, and induced apoptosis and cell cycle arrest at the G(2) phase of the cell cycle. Notably, MG-132 increased the autophagy-associated conversion of microtubule-associated protein 1 light chain 3 (LC3)-I to LC3-II, which was partially attenuated by the ER stress inhibitor, salubrinal. In addition, MG-132 inhibited the protein expression of the anti-apoptotic protein, B-cell lymphoma (Bcl)-2, whereas the expression levels of Bcl-2-associated X protein and caspase-3 were upregulated. These effects were enhanced by co-treatment with either 3-MA or salubrinal. Furthermore, the mRNA and protein levels of the ER stress-associated genes, glucose-regulated protein 78, growth arrest and DNA damage induced gene-153, and caspase-12, were upregulated by MG132, and these levels were significantly inhibited by co-treatment of the cells with salubrinal. Taken together, the results of the present study indicated that the induction of autophagy by the proteasome inhibitor was associated with ER stress in the MCF-7 cells, and that the inhibition of autophagy or ER stress enhanced MG-132-induced apoptosis. These findings suggest the potential application of inhibitors of ER stress and autophagy, in combination with proteasomal inhibitors, for the development of combinatorial targeted cancer therapy.