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.
p53 is a transcription factor that regulates expression of genes involved in cell cycle arrest, senescence, and apoptosis. TP53 harbors mutations that inactivate its transcriptional activity in roughly 30% of breast cancers, and these tumors are much more likely to undergo a pathological complete response to chemotherapy. Thus, the gene expression program activated by wild-type p53 contributes to a poor response. We used an in vivo genetic model system to comprehensively define the p53- and p21-dependent genes and pathways modulated in tumors following doxorubicin treatment. We identified genes differentially expressed in spontaneous mammary tumors harvested from treated MMTV-Wnt1 mice that respond poorly (Trp53+/+) or favorably (Trp53-null) and those that lack the critical senescence/arrest p53 target gene Cdkn1a. Trp53 wild-type tumors differentially expressed nearly 10-fold more genes than Trp53-null tumors after treatment. Pathway analyses showed that genes involved in cell cycle, senescence, and inflammation were enriched in treated Trp53 wild-type tumors; however, no genes/pathways were identified that adequately explain the superior cell death/tumor regression observed in Trp53-null tumors. Cdkn1a-null tumors that retained arrest capacity (responded poorly) and those that proliferated (responded well) after treatment had remarkably different gene regulation. For instance, Cdkn1a-null tumors that arrested upregulated Cdkn2a (p16), suggesting an alternative, p21-independent route to arrest. Live animal imaging of longitudinal gene expression of a senescence/inflammation gene reporter in Trp53+/+ tumors showed induction during and after chemotherapy treatment, while tumors were arrested, but expression rapidly diminished immediately upon relapse.
Transformed cells have properties that allow them to survive and proliferate inappropriately. These characteristics often arise as a result of mutations caused by DNA damage. p53 suppresses transformation by removing the proliferative or survival capacity of cells with DNA damage or inappropriate cell-cycle progression. Cellular senescence, marked by morphological and gene expression changes, is a critical component of p53-mediated tumor suppression. In response to stress, p53 can facilitate an arrest and senescence program in cells exposed to stresses such as DNA damage and oncogene activation, preventing transformation. Senescent cells are evident in precancerous adenoma-type lesions, whereas proliferating, malignant tumors have bypassed senescence, either by p53 mutation or inactivation of the p53 pathway by other means. Tumors that have retained wild-type p53 often show a p53-mediated senescence response to chemotherapy. This response is actually detrimental in some tumor types, as senescent cells can drive relapse by persisting and producing cytokines and chemokines through an acquired secretory phenotype.
Phenotypic and functional cancer cell heterogeneity limits the efficacy of targeted and immuno-therapies. The transcription factor MITF is known to regulate melanoma cell plasticity and, consequently, response to drugs. However, the underlying mechanisms of this phenomenon remain incompletely understood. Here, we show that MITF levels control functional melanoma cell heterogeneity by fine-tuning the ability to contract the extracellular matrix, the maturation of focal adhesions and ROCK-mediated melanoma cell contractility.Modulation of MITF expression alters extracellular matrix organization, melanoma cell morphology and solid stress in three-dimensional melanoma spheroids, thereby accounting for spatial differences in cell cycle dynamics. Together, our data identify MITF as a master regulator of the melanoma micro-architecture and point towards novel targeting strategies for cancer cell heterogeneity.Significance. Development of drug resistance is a major cause of melanoma therapy failure.The role of MITF in melanoma response to therapy has been discussed controversially, which can be explained, at least in part, through the rheostat model linking MITF activity to cell proliferation. Heterogeneity is widely associated with therapy resistance, however, whether cell phenotype switching, mediated by MITF, is responsible for treatment resistance is not known. Our findings provide an in-depth mechanistic understanding of the MITF-mediated regulation of cell cycle behavior and physical regulation of the tumor architecture. As MITF is not amenable to direct drug targeting, the identification of mediators of MITF-triggered functional heterogeneity reveals novel targets that can be deployed to control this phenomenon.
The breast tumors that are most difficult to eradicate with chemotherapy have wild-type TP53 and preferentially enter senescence after treatment. One factor contributing to the persistence of senescent cells in residual disease: acquisition of a novel phenotype that allows cannibalism of entire cells and engulfment of other substrates.
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