Beclin 1 Enhances Proteasome Inhibition-Mediated Cytotoxicity of Thyroid Cancer Cells in Macroautophagy-Independent Manner

Context: The ubiquitin–proteasome system and macroautophagy are two major pathways for intracellular protein degradation. Emerging lines of evidence have shown that blockade of ubiquitin–proteasome system by proteasome inhibitors activates macroautophagy. Objective: The purpose of this study was to determine the involvement of autophagy essential gene Beclin 1 in cytotoxicity of thyroid cancer cells mediated by proteasome inhibitors. Design: Autophagy was measured by acidic-trophic dye staining and EGF-LC3 distribution using fluorescence microscopy, as well as LC3-II transition using Western blot. To ascertain the effect of Beclin 1, cells were transfected with Beclin 1 plasmid or shRNA against Beclin 1. Cell viability and apoptotic cells were measured using MTT assay and flow cytometry, respectively. Results: Proteasome inhibitors decreased Beclin 1 expression. In addition, treatment with PI3K inhibitors 3-MA or wortmannin, as well as knockdown of Beclin 1 expression, was unable to affect autophagic responses mediated by proteasome inhibitors. Overexpression of Beclin 1 enhanced proteasome inhibitor–mediated cytotoxicity of thyroid cancer cells via suppression of survivin. Conclusions: Proteasome inhibitors cause Beclin 1–independent macroautophagic responses of thyroid cancer cells in a Beclin 1–independent manner. Beclin 1 possesses autophagy-independent antitumoral effects upon exposure of thyroid cancer cells to proteasome inhibitors.