Improved Autophagic Flux in Escapers from Doxorubicin-Induced Senescence/Polyploidy of Breast Cancer Cells.

The induction of senescence/polyploidization and their role in cancer recurrence is still a poorly explored issue. We showed that MDA-MB-231 and MCF-7 breast cancer cells underwent reversible senescence/polyploidization upon pulse treatment with doxorubicin (dox). Subsequently, senescent/polyploid cells produced progeny (escapers) that possessed the same amount of DNA as parental cells. In a dox-induced senescence/polyploidization state, the accumulation of autophagy protein markers, such as LC3B II and p62/SQSTM1, was observed. However, the senescent cells were characterized by a very low rate of new autophagosome formation and degradation, estimated by autophagic index. In contrast to senescent cells, escapers had a substantially increased autophagic index and transcription factor EB activation, but a decreased level of an autophagy inhibitor, Rubicon, and autophagic vesicles with non-degraded cargo. These results strongly suggested that autophagy in escapers was improved, especially in MDA-MB-231 cells. The escapers of both cell lines were also susceptible to dox-induced senescence. However, MDA-MB-231 cells which escaped from senescence were characterized by a lower number of γH2AX foci and a different pattern of interleukin synthesis than senescent cells. Thus, our studies showed that breast cancer cells can undergo senescence uncoupled from autophagy status, but autophagic flux resumption may be indispensable in cancer cell escape from senescence/polyploidy.