Everolimusinduces G1 cell cycle arrest through autophagy-mediated protein degradation of cyclin D1 in breast cancer cells.

PurposeEverolimusinhibit mammalian target of rapamycin (mTORC1) and are known to cause induction of autophagy and G1 cell cycle arrest. However, it remains unknown whether everolimus-induced autophagy plays a critical role in its regulation of cell cycle. We, for the first time, suggested that everolimus could stimulate autophagy-mediated Cyclin D1 degradation in breast cancer cells. MethodsEverolimus-induced Cyclin D1 degradation through autophagy pathway was investigated in MCF-10DCIS.COM and MCF-7 cell lines upon autophagy inhibitor treatment using western blot assay. Everolimus stimulated autophagy and decreased Cyclin D1 were also tested in explant human breast tissue.Results Inhibiting mTORC1 with everolimus rapidly increased Cyclin D1 degradation, while 3-Methyladenine, chloroquine and bafilomycin A1, the classic autophagy inhibitors could attenuate everolimus-induced Cyclin D1 degradation. Similarly, knockdown of autophagy related gene 7 (Atg-7) also repressed everolimus-triggered Cyclin D1 degradation. In addition, everolimus-induced autophagy occurred earlier than its induction of G1 arrest and blockade of autophagy attenuated everolimus-induced G1 arrest. We also found that everolimus stimulated autophagy and decreased Cyclin D1 levels in explant human breast tissue. ConclusionsThese data support the conclusion that the autophagy induced by everolimusin human mammary epithelial cells appears to cause Cyclin D1 degradation resulting in G1 cell cycle arrest. Our findings contribute to our knowledge of the interplay between autophagy and cell cycle regulation mediated by mTORC1 signaling and Cyclin D1 regulation.