BH3 mimetics selectively eliminate chemotherapy-induced senescent cells and improve response in TP53 wild-type breast cancer.

TP53 wild-type breast tumors rarely undergo a complete pathological response after chemotherapy treatment. These patients have an extremely poor survival rate and studies show these tumors preferentially undergo senescence instead of apoptosis. These senescent cells persist after chemotherapy and secrete cytokines and chemokines comprising the senescence associated secretory phenotype, which promotes survival, proliferation, and metastasis. We hypothesized that eliminating senescent tumor cells would improve chemotherapy response and extend survival. Previous studies have shown "senolytic" agents selectively kill senescent normal cells, but their efficacy in killing chemotherapy-induced senescent cancer cells is unknown. We show that ABT-263, a BH3 mimetic that targets antiapoptotic proteins BCL2/BCL-XL/BCL-W, had no effect on proliferating cells, but rapidly and selectively induced apoptosis in a subset of chemotherapy-treated cancer cells, though sensitivity required days to develop. Low NOXA expression conferred resistance to ABT-263 in some cells, necessitating additional MCL1 inhibition. Gene editing confirmed breast cancer cells relied on BCL-XL or BCL-XL/MCL1 for survival in senescence. In a mouse model of breast cancer, ABT-263 treatment following chemotherapy led to apoptosis, greater tumor regression, and longer survival. Our results reveal cancer cells that have survived chemotherapy by entering senescence can be eliminated using BH3 mimetic drugs that target BCL-XL or BCL-XL/MCL1. These drugs could help minimize residual disease and extend survival in breast cancer patients that otherwise have a poor prognosis and are most in need of improved therapies.

Chemotherapy-induced senescent cancer cells engulf other cells to enhance their survival.

In chemotherapy-treated breast cancer, wild-type p53 preferentially induces senescence over apoptosis, resulting in a persisting cell population constituting residual disease that drives relapse and poor patient survival via the senescence-associated secretory phenotype. Understanding the properties of tumor cells that allow survival after chemotherapy treatment is paramount. Using time-lapse and confocal microscopy to observe interactions of cells in treated tumors, we show here that chemotherapy-induced senescent cells frequently engulf both neighboring senescent or nonsenescent tumor cells at a remarkable frequency. Engulfed cells are processed through the lysosome and broken down, and cells that have engulfed others obtain a survival advantage. Gene expression analysis showed a marked up-regulation of conserved macrophage-like program of engulfment in chemotherapy-induced senescent cell lines and tumors. Our data suggest compelling explanations for how senescent cells persist in dormancy, how they manage the metabolically expensive process of cytokine production that drives relapse in those tumors that respond the worst, and a function for their expanded lysosomal compartment.