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Cellular senescence forms a barrier to tumorigenesis, by inducing cell cycle arrest in damaged and mutated cells. However, once formed, senescent cells often emit paracrine signals that can either promote or suppress tumorigenesis. There is evidence that, in addition to cancer cells, subsets of tumor stromal cells, including fibroblasts, endothelial cells, and immune cells, undergo senescence. Such senescent stromal cells can influence cancer development and progression and represent potential targets for therapy. However, understanding of their characteristics and roles is limited and few studies have dissected their functions in vivo. Here, we discuss current knowledge and pertinent questions regarding the presence of senescent stromal cells in cancers, the triggers that elicit their formation, and their potential roles within the tumor microenvironment.

IntroductionClassically, cancer therapy approaches lead to tumor cell death or an alternative fate such as senescence. Senescent cells remain viable but without proliferation markers. Senescent cells secrete factors with diverse actions that influence surrounding cells, the extracellular matrix (ECM) and the immune system, a phenomenon termed the senescence-associated secretory phenotype (SASP). Cyclin-dependent kinase 4/6 inhibitors (CDKi) are game-changers in the therapy of metastatic hormone receptor-positive, HER2-negative breast cancer. A potential mechanism of the action of these agents is induction of senescence in breast tumor cells, going beyond cell cycle arrest. However, senescence can be studied mainly using invasive biopsies, and therefore the prevalence and importance of senescence in patients are largely unknown. Moreover, only few circulating biomarkers exist to predict activity or measure CDKi effects. In order to develop a correlative liquid biopsy for CDKi activity, we have analyzed epigenetic (methylation) changes following therapy-induced senescence in vitro. We sought to identify (un)methylated loci that will be affected by different approaches and later can be discovered in circulating cell-free DNA. MethodsWe have treated luminal breast cancer MCF-7 cells with Doxorubicin, CDKi (Palbociclib) or by irradiation. This protocol resulted in up to 80% of cells with senescence-related beta-galactosidase activity. DNA methylation was profiled using Illumina Infinium MethylationEPIC 850K BeadChip. Differentially methylated loci (mDNA) were identified using GenomeStudio and Minfi. Analyses were done using GREAT, CSGene, and Reactome databases. Methylation age was analyzed using the Horvath Methylation Calculator. Statistical significance was defined as p ResultsSignificantly differentially methylated sites (in comparison to untreated cells) were revealed as following: 9111 sites in Doxorubicin treated cells, 3828 sites in Palbociclib treated cells, and 694 sites in irradiated cells. These loci comprise 1%, 0.45%, and 0.08% of analyzed methylation sites, respectively. 324 loci were similarly changed following the three treatment options ('common sites'). We found that the 'geographic' distribution of intragenic and intergenic methylation sites (5' UTR, gene body etc.) in all treated cells was similar. Gene set over-representation analysis revealed that sites associated with genes of the 'collagen metabolic process' set were significantly altered in drug-treated cells. Pathway investigation of the 324 'common sites' revealed that ECM-related 'focal adhesion assembly' is the most significant pathway involved. Specific analysis of senescence-related gene sets showed that 15.1% (76/503) of senescence genes changed after Doxorubicin treatment, 6.2% (31/503) after Palbociclib and 1% (5/503) after irradiation. Pathway analysis of these genes showed that drugs affected the 'oxidative stress-induced senescence' pathway, while irradiated cells had SASP-related genes affected.Despite the above mentioned methylation changes, age calculation based on methylation clock showed that all samples had similar age (6), regardless of manipulation. ConclusionSingle treatment of MCF-7 cells with known senescence inducers results in changes in methylation patterns. A significant number of common loci changed following all types of treatments, suggesting them as potential surrogate loci of senescence. Also, we have reproduced the well-known interplay between collagen and senescence/SASP.This preliminary data sheds light on epigenetic changes following treatment-induced senescence. Further studies are needed to validate whether these methylation changes can be found in vivo in tumors and in patients’ cell-free DNA following therapy with CDKi. Citation Format: Albert Grinshpun, Anatoli Kustanovich, Joshua Moss, Yael Gabai, Yuval Dor, Ittai Ben-Porath. Senescence-related methylation changes following therapy as potential biomarker for CDK4/6 inhibitor activity [abstract]. In: Proceedings of the 2020 San Antonio Breast Cancer Virtual Symposium; 2020 Dec 8-11; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2021;81(4 Suppl):Abstract nr PS5-43.

Summary: Differentiation events contribute to phenotypic cellular heterogeneity within tumors and influence disease progression and response to therapy. Here, we dissect mechanisms controlling intratumoral heterogeneity within triple-negative basal-like breast cancers. Tumor cells expressing the cytokeratin K14 possess a differentiation state that is associated with that of normal luminal progenitors, and K14-negative cells are in a state closer to that of mature luminal cells. We show that cells can transition between these states through asymmetric divisions, which produce one K14+ and one K14− daughter cell, and that these asymmetric divisions contribute to the generation of cellular heterogeneity. We identified several regulators that control the proportion of K14+ cells in the population. EZH2 and Notch increase the numbers of K14+ cells and their rates of symmetric divisions, and FOXA1 has an opposing effect. Our findings demonstrate that asymmetric divisions generate differentiation transitions and heterogeneity, and identify pathways that control breast cancer cellular composition. : Granit et al. study the sources of phenotypic cellular heterogeneity in triple-negative breast cancers. They find that cancer cells can undergo asymmetric divisions that produce K14+ and K14− daughters and thereby generate heterogeneity. K14+ cells possess a progenitor-associated, tumorigenic phenotype, and the authors identify regulators that control their relative numbers. Keywords: triple-negative breast cancer, basal-like breast cancer, tumor heterogeneity, asymmetric divisions, mammary progenitor cells, EZH2, Notch, FOXA1, NFIB, KLF5

Cellular senescence is thought to contribute to age-associated deterioration of tissue physiology. The senescence effector p16(Ink4a) is expressed in pancreatic beta cells during aging and limits their proliferative potential; however, its effects on beta cell function are poorly characterized. We found that beta cell-specific activation of p16(Ink4a) in transgenic mice enhances glucose-stimulated insulin secretion (GSIS). In mice with diabetes, this leads to improved glucose homeostasis, providing an unexpected functional benefit. Expression of p16(Ink4a) in beta cells induces hallmarks of senescence--including cell enlargement, and greater glucose uptake and mitochondrial activity--which promote increased insulin secretion. GSIS increases during the normal aging of mice and is driven by elevated p16(Ink4a) activity. We found that islets from human adults contain p16(Ink4a)-expressing senescent beta cells and that senescence induced by p16(Ink4a) in a human beta cell line increases insulin secretion in a manner dependent, in part, on the activity of the mechanistic target of rapamycin (mTOR) and the peroxisome proliferator-activated receptor (PPAR)-γ proteins. Our findings reveal a novel role for p16(Ink4a) and cellular senescence in promoting insulin secretion by beta cells and in regulating normal functional tissue maturation with age.

The mechanisms regulating breast cancer differentiation state are poorly understood. Of particular interest are molecular regulators controlling the highly aggressive and poorly differentiated traits of basal-like breast carcinomas. Here we show that the Polycomb factor EZH2 maintains the differentiation state of basal-like breast cancer cells, and promotes the expression of progenitor associated and basal-lineage genes. Specifically, EZH2 regulates the composition of basal-like breast cancer cell populations by promoting a ‘bi-lineage’ differentiation state, in which cells co-express basal- and luminal-lineage markers. We show that human basal-like breast cancers contain a subpopulation of bi-lineage cells, and that EZH2-deficient cells give rise to tumors with a decreased proportion of such cells. Bi-lineage cells express genes that are active in normal luminal progenitors, and possess increased colony-formation capacity, consistent with a primitive differentiation state. We found that GATA3, a driver of luminal differentiation, performs a function opposite to EZH2, acting to suppress bi-lineage identity and luminal-progenitor gene expression. GATA3 levels increase upon EZH2 silencing, mediating a decrease in bi-lineage cell numbers. Our findings reveal a novel role for EZH2 in controlling basal-like breast cancer differentiation state and intra-tumoral cell composition.