Abstract 460: Reversibility of chemotherapy-induced senescence in breast tumor cells in culture and in vivo

Despite the ability of chemotherapy (and/or radiation) to eliminate the majority of tumor cells, some residual surviving cells may be able to escape cell death by entering into a prolonged growth arrest from which they can ultimately re-emerge and proliferative . In this study, we evaluated two mechanisms, autophagy and senescence, that in theory could allow tumor cells to escape apoptotic cell death, and which therefore might be responsible for cancer recurrence. 4T1 murine breast cancer cells exposed to the chemotherapeutic agent, Adriamycin (doxorubicin), underwent both autophagy and senescence. Senescence induced by anticancer drugs or radiation is a state of prolonged growth arrest that is thought to be irreversible and is considered an alternative (but nevertheless desirable) response to apoptosis. Our studies, however, suggest that Therapy Induced Senescence (TIS) is reversible and serves as a mechanism of evading cell death. When subjected to acute treatment with Adriamycin, some of the 4T1 cells underwent apoptosis; however, a large population entered into a state of prolonged growth arrest that lasted for several days, after which time the cells recovered proliferative capacity and formed colonies. At the time points when cells were arrested, prominent Senescence Associated β-Galactosidase staining and polyploidy were detected, markers indicative of senescence. Inhibition of autophagy did not prevent the tumor cells from undergoing senescence and later escaping growth arrest. To further establish the reversibility of senescence, senescent cells that were isolated by Flow Cytometry (based on their enlarged morphology and detection of a fluorescent β-Gal substrate (C12FDG)), plated and monitored over time; these cells were also shown to recover, as was the case from with the non-sorted senescent cells. Sorted and isolated senescent cells implanted into either NSG (immunodeficient) or syngeneic (immunocompetent) mice also recovered and developed into tumors; recovery was accelerated in the NSG mice, suggesting that the immune system may (at least transiently) recognize and eliminate the tumor cells either at senescence or during recovery from senescence. Senescent cells are characterized by a specific (though often heterogeneous) secretory profile (Senescence Associated Secretory Phenotype or SASP), some components of which have been shown to promote tumorigenesis and may assist in neoplastic escape after therapy. Furthermore, senescence and escape from senescence may represent a form of tumor dormancy and disease recurrence, respectively. Therefore, it is important to treat Therapy Induced Senescence as a deleterious outcome of treatment and identify strategies that might prevent senescent cells from neoplastic escape and induce cell death to completely eliminate residual tumors. Citation Format: Liliya Tyutyunyk, Joseph Landry, Tareq Saleh, David Gewirtz. Reversibility of chemotherapy-induced senescence in breast tumor cells in culture and in vivo [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 460.