CDK4/6 inhibitor-mediated cell overgrowth triggers osmotic and replication stress to promote senescence

Pharmacological modulation of cellular senescence: Implications for breast cancer progression and therapeutic strategies

Senescence, defined by the cessation of cell proliferation, plays a critical and multifaceted role in breast cancer progression and treatment. Senescent cells produce senescence-associated secretory phenotypes (SASP) comprising inflammatory cytokines, chemokines, and small molecules, which actively shape the tumor microenvironment, influencing cancer development, progression, and metastasis. This review provides a comprehensive analysis of the types and origins of senescent cells in breast cancer, alongside their markers and detection methods. Special focus is placed on pharmacological strategies targeting senescence, including drugs that induce or inhibit senescence, their molecular mechanisms, and their roles in therapeutic outcomes when combined with chemotherapy and radiotherapy. By exploring these pharmacological interventions and their impact on breast cancer treatment, this review underscores the potential of senescence-targeting therapies to revolutionize breast cancer management.

p21-Dependent Senescence Induction by BMP4 Renders Glioblastoma Cells Vulnerable to Senolytics

Glioblastoma (GBM) is a highly malignant brain tumor with extensive cellular heterogeneity and plasticity. Bone morphogenetic protein 4 (BMP4) has shown potential as a therapeutic agent by promoting differentiation, but its effects are complex and context dependent. While BMP4's role in differentiation is well established, its impact on senescence remains unclear. This study investigates BMP4's ability to induce senescence in GBM cells. Primary GBM cultures were treated with BMP4 and analyzed for senescence markers, including cell enlargement, p21 expression, senescence-related gene enrichment, and senescence-associated-β-galactosidase activity. A p21 knockout model was used to determine its role in BMP4-induced senescence, and sensitivity to the senolytic agent navitoclax was evaluated. BMP4 induced senescence in the GBM cultures, particularly in mesenchymal (MES)-like GBM cells with high baseline p21 levels. The knockout of p21 nearly abolished BMP4-induced senescence, maintaining cell size and proliferation. Furthermore, navitoclax effectively eliminated BMP4-induced senescent cells through apoptosis, while sparing cells with normal p21 expression. Our findings highlight BMP4 as an inducer of p21-dependent senescence in GBM, particularly in MES-like cells. This study clarifies BMP4's dual roles in differentiation and senescence, emphasizing their context dependence. Given the strong link between MES-like cells and therapy resistance, their heightened susceptibility to senescence may aid in developing targeted therapies for GBM and potentially other cancers with similar cellular dynamics.

Tumor Suppressors Condition Differential Responses to the Selective CDK2 Inhibitor BLU-222

Cyclin-dependent kinase 2 (CDK2) inhibitors have recently been developed and have entered clinical trials. Combination approaches can help broaden the use of therapeutic agents and establish more effective treatments. Here, we evaluated the selective CDK2 inhibitor BLU-222 for mechanisms of response in the context of ovarian and breast cancer models. Sensors of cellular CDK activity indicated that sensitivity to either CDK4/6 or CDK2 inhibition was related to the differential dependence on a single CDK for G1-S transition. Unlike CDK4/6 inhibitors, BLU-222 was able to robustly inhibit proliferation through cell-cycle inhibition in both G1 and G2 phases. However, it remained possible for cells to reenter the cell cycle upon drug withdrawal. The antiproliferative strength and impact on G1-S versus G2-M accumulation was mediated by the RB tumor suppressor. To broaden the sensitivity to CDK2 inhibition, combinatorial drug screens were performed that identified both synergistic (e.g., CDK4/6 inhibitors) and antagonistic (e.g., WEE1 inhibitors) relationships. Models that were exceptionally sensitive to CDK2 inhibition displayed coordinate expression of cyclin E1 and P16INK4A, an endogenous CDK4/6 inhibitor. Functional studies demonstrated that P16INK4A and CDK4/6 activity were key mediators of sensitivity to BLU-222. Clinical gene and protein expression analyses revealed a positive correlation between cyclin E1 and P16INK4A and identified that ∼25% of ovarian cancers exhibited coordinate expression of cyclin E, P16INK4A, and RB, indicative of strong sensitivity to CDK2 inhibition. Together, this work advances a precision strategy for the use of CDK2 inhibitors in the context of ovarian and breast cancers. Significance: The CDK2-specific inhibitor BLU-222 shows preclinical efficacy in breast and ovarian cancer with select determinants of response and holds promise in combinatorial strategies. See related article by House and colleagues, p. 1297.

Curcumin Alleviates DON-Induced Intestinal Epithelial Barrier Disruption by Improving Ribotoxic Stress-Associated p38 Pathway-Mediated TJ Injury, Apoptosis, and Cell Cycle Arrest

Deoxynivalenol (DON) is a pervasive ribotoxic stressor that induces intestinal epithelial barrier disruption by impairing tight junctions (TJs) and causing cellular damage. Curcumin (CUR), known for its enteroprotective properties and low toxicity, has been shown to attenuate DON-induced intestinal epithelial barrier injury. However, the underlying mechanisms are still unclear. In this study, we established in vivo and in vitro models using 30 male Kunming mice and IPEC-J2 cells to investigate the mechanisms by which CUR alleviates DON-induced intestinal epithelial barrier injury. The results showed that CUR markedly reduced DON-induced increases in intestinal permeability by restoring TJ protein expression (Claudin-4 and occludin) and preventing fiber-shaped actin (F-actin) contraction. CUR also attenuated DON-induced apoptosis by downregulating p53 and caspase activation and alleviated the G1 cell cycle arrest by reducing p21 expression. Mechanistically, CUR inhibited the activation of the ribosomal stress response (RSR)-associated p38 pathway, evidenced by decreased phosphorylation of p38, GSK3β, and ATF-2. The p38 activator dehydrocorydaline reversed CUR's protective effects. In conclusion, CUR alleviates DON-induced intestinal epithelial barrier disruption by improving RSR-associated p38 pathway-mediated TJ injury, apoptosis, and cell cycle arrest. These findings highlight the potential of CUR as a therapeutic agent for mitigating mycotoxin-induced intestinal dysfunction and suggest new avenues for drug target discovery.

Temporal myc dynamics permit mitotic bypass, promoting polyploid phenotypes

High Myc phenotypes are extensively documented in the hyperproliferative cell cycle of cancer cells, as well as non-proliferative endoreplication cycles engaged during normal development and stress response. Notably, endoreplication in cancer produces chemotherapy resistant polyploid cells, necessitating a clearer understanding of altered cell cycle regulation that uncouples DNA replication and mitotic cell division. The c-Myc oncogene is a well-established transcriptional regulator of cell cycle progression and has been extensively published as an essential driver of the G1/S transition. Beyond S phase, Myc transcriptionally activates the proteins that drive mitotic entry. Sustained activation of Myc through the cell cycle transcriptionally couples DNA replication and mitotic cell division. Based on the literature in this field, we propose a new model of temporal regulation of Myc activity that serves to either couple or uncouple these two processes, determining cell cycle fate - proliferation or polyploidy. The mitotic cell cycle requires two pulses of Myc activity - the first driving the G1/S transition and the second driving the G2/M transition. During mitosis, Myc activity must be silenced to achieve high-fidelity division. Absence of the second activity pulse during G2 results in the downregulation of the proteins essential for mitotic entry and permits premature activation of APC/C, inducing mitotic bypass. A subsequent rise of Myc activity following mitotic bypass permits genome re-replication, driving polyploid phenotypes. This model serves to provide a new level of understanding to the global regulation of S phase-mitosis coupling, as well as a new lens to view low Myc phenotypes.

The Regulation of Cellular Senescence in Cancer

Cellular senescence is a stable state of cell cycle arrest caused by telomere shortening or various stresses. After senescence, cells cease dividing and exhibit many age-related characteristics. Unlike the halted proliferation of senescence cells, cancer cells are considered to have unlimited growth potential. When cells display senescence-related features, such as telomere loss or stem cell failure, they can inhibit tumor development. Therefore, inducing cells to enter a senescence state can serve as a barrier to tumor cell development. However, many recent studies have found that sustained senescence of tumor cells or normal cells under certain circumstances can exert environment-dependent effects of tumor promotion and inhibition by producing various cytokines. In this review, we first introduce the causes and characteristics of induced cellular senescence, analyze the senescence process of immune cells and cancer cells, and then discuss the dual regulatory role of cell senescence on tumor growth and senescence-induced therapies targeting cancer cells. Finally, we discuss the role of senescence in tumor progression and treatment opportunities, and propose further studies on cellular senescence and cancer therapy.

A meta-analysis and systematic review of different cyclin-dependent kinase 4/6 inhibitors in breast cancer

Objective

The objective of this study was to assess the effectiveness and safety of CDK4/6 inhibitors in the treatment of hormone receptor-positive (HR+) breast cancer by using meta-analysis.

Methods

To gather comprehensive and reliable data for our analysis, we systematically searched multiple databases for relevant studies. We utilized RevMan5.3 software to perform the meta-analysis.

Results

Following a rigorous screening and evaluation process, we ultimately included a total of 13 studies in our analysis. Our findings showed that compared to endocrine therapy alone, the combination of CDK4/6 inhibitors with endocrine therapy significantly increased both PFS [HR 0.54 (95%CI: 0.50, 0.58), P<0.00001], OS [HR 0.77 (95%CI: 0.50, 0.58), P<0.00001] and ORR [RR 1.39 (95% CI: 1.21, 1.60), P<0.00001). However, it was also found that CDK4/6 inhibitors caused adverse drug reactions related to the blood system and digestive system (P<0.0001).

Conclusions

Our meta-analysis demonstrates that the addition of CDK4/6 inhibitors to endocrine therapy can result in improved PFS and OS for HR+ breast cancer patients. Meanwhile, we recommend close monitoring and management of these potential side effects when utilizing these inhibitors in breast cancer treatment.

Systematic Review Registration

https://www.crd.york.ac.uk/PROSPERO, identifier CRD42023490499.

The efferocytosis process in aging: Supporting evidence, mechanisms, and therapeutic prospects for age-related diseases

BACKGROUND

Aging is characterized by an ongoing struggle between the buildup of damage caused by a combination of external and internal factors. Aging has different effects on phagocytes, including impaired efferocytosis. A deficiency in efferocytosis can cause chronic inflammation, aging, and several other clinical disorders.

AIM OF REVIEW

Our review underscores the possible feasibility and extensive scope of employing dual targets in various age-related diseases to reduce the occurrence and progression of age-related diseases, ultimately fostering healthy aging and increasing lifespan. Key scientific concepts of review Hence, the concurrent implementation of strategies aimed at augmenting efferocytic mechanisms and anti-aging treatments has the potential to serve as a potent intervention for extending the duration of a healthy lifespan. In this review, we comprehensively discuss the concept and physiological effects of efferocytosis. Subsequently, we investigated the association between efferocytosis and the hallmarks of aging. Finally, we discuss growing evidence regarding therapeutic interventions for age-related disorders, focusing on the physiological processes of aging and efferocytosis.

Pro-survival roles for p21(Cip1/Waf1) in non-small cell lung cancer

BACKGROUND

Quiescence is reversible proliferative arrest. Multiple mechanisms regulate quiescence that are not fully understood. High expression of the CDK inhibitor p21Cip1/Waf1 correlates with a poor prognosis in non-small cell lung cancer (NSCLC) and, in non-transformed cells, p21 promotes quiescence after replication stress. We tested whether NSCLC cells enter p21-dependent quiescence and if this is advantageous to NSCLC cells.

METHODS

Through analysis of patient data and quantitative, single-cell, timelapse imaging of genetically-engineered NSCLC reporter cell lines we investigated the role of p21 in NSCLC during normal proliferation and after chemotherapy.

RESULTS

High p21 expression correlates with a poor prognosis in TP53 wild-type, but not TP53 mutant, NSCLC patients and TP53 wild-type NSCLC cells can enter p21-dependent quiescence, downstream of replication stress. Without p21, unrepaired DNA damage propagates into S-phase and cells display increased genomic instability. p21 expression confers survival advantages to TP53 wild-type NSCLC cells, during proliferation and after chemotherapy. p21 can promote tumour relapse by allowing recovery from both G1 and G2 arrests after chemotherapy.

CONCLUSIONS

p21-dependent quiescence exists in TP53 wild-type NSCLC cells and provides survival advantages to these cells. Targeting p21 function in TP53 wild-type tumours could lead to better outcomes for chemotherapy treatment in NSCLC patients.

Cell enlargement modulated by GATA4 and YAP instructs the senescence-associated secretory phenotype

Dynamic changes in cell size are associated with development and pathological conditions, including aging. Although cell enlargement is a prominent morphological feature of cellular senescence, its functional implications are unknown; moreover, how senescent cells maintain their enlargement state is less understood. Here we show that an extensive remodeling of actin cytoskeleton is necessary for establishing senescence-associated cell enlargement and pro-inflammatory senescence-associated secretory phenotype (SASP). This remodeling is attributed to a balancing act between the SASP regulator GATA4 and the mechanosensor YAP on the expression of the Rho family of GTPase RHOU. Genetic or pharmacological interventions that reduce cell enlargement attenuate SASP with minimal effect on senescence growth arrest. Mechanistically, actin cytoskeleton remodeling couples cell enlargement to the nuclear localization of GATA4 and NF-κB via the Linker of Nucleoskeleton and Cytoskeleton (LINC) complex. RhoU protein accumulates in mouse adipose tissue under senescence-inducing conditions. Furthermore, RHOU expression correlates with SASP expression in adipose tissue during human aging. Thus, our study highlights an unexpected instructive role of cell enlargement in modulating the SASP and reveals a mechanical branch in the senescence regulatory network.

The PRC2.1 subcomplex opposes G1 progression through regulation of CCND1 and CCND2

Progression through the G1 phase of the cell cycle is the most highly regulated step in cellular division. We employed a chemogenetic approach to discover novel cellular networks that regulate cell cycle progression. This approach uncovered functional clusters of genes that altered sensitivity of cells to inhibitors of the G1/S transition. Mutation of components of the Polycomb Repressor Complex 2 rescued proliferation inhibition caused by the CDK4/6 inhibitor palbociclib, but not to inhibitors of S phase or mitosis. In addition to its core catalytic subunits, mutation of the PRC2.1 accessory protein MTF2, but not the PRC2.2 protein JARID2, rendered cells resistant to palbociclib treatment. We found that PRC2.1 (MTF2), but not PRC2.2 (JARID2), was critical for promoting H3K27me3 deposition at CpG islands genome-wide and in promoters. This included the CpG islands in the promoter of the CDK4/6 cyclins CCND1 and CCND2, and loss of MTF2 lead to upregulation of both CCND1 and CCND2. Our results demonstrate a role for PRC2.1, but not PRC2.2, in antagonizing G1 progression in a diversity of cell linages, including chronic myeloid leukemia (CML), breast cancer, and immortalized cell lines.

Genome concentration limits cell growth and modulates proteome composition in Escherichia coli

Defining the cellular factors that drive growth rate and proteome composition is essential for understanding and manipulating cellular systems. In bacteria, ribosome concentration is known to be a constraining factor of cell growth rate, while gene concentration is usually assumed not to be limiting. Here, using single-molecule tracking, quantitative single-cell microscopy, and modeling, we show that genome dilution in Escherichia coli cells arrested for DNA replication limits total RNA polymerase activity within physiological cell sizes across tested nutrient conditions. This rapid-onset limitation on bulk transcription results in sub-linear scaling of total active ribosomes with cell size and sub-exponential growth. Such downstream effects on bulk translation and cell growth are near-immediately detectable in a nutrient-rich medium, but delayed in nutrient-poor conditions, presumably due to cellular buffering activities. RNA sequencing and tandem-mass-tag mass spectrometry experiments further reveal that genome dilution remodels the relative abundance of mRNAs and proteins with cell size at a global level. Altogether, our findings indicate that chromosome concentration is a limiting factor of transcription and a global modulator of the transcriptome and proteome composition in E. coli. Experiments in Caulobacter crescentus and comparison with eukaryotic cell studies identify broadly conserved DNA concentration-dependent scaling principles of gene expression.

A hybrid epithelial-mesenchymal transition program enables basal epithelial cells to bypass stress-induced stasis and contributes to a metaplastic breast cancer progenitor state

BACKGROUND

Human mammary epithelial cell (HMEC) cultures encounter a stress-associated barrier termed stasis, during which most cells adopt a senescence-like phenotype. From these cultures, rare variants emerge from the basal epithelial population, re-initiating growth. Variants exhibit pre-malignant properties, including an aberrant epigenetic program that enables continued proliferation and acquisition of genetic changes. Following oncogenic transformation, variants produce tumors that recapitulate the histopathological characteristics of metaplastic breast cancer (MBC), a rare and aggressive subtype marked by the differentiation of neoplastic epithelium into squamous and mesenchymal elements.

METHODS

Using a serum-free HMEC culture system, we probed the capacity for phenotypic plasticity inherent to basal epithelial cell populations from human breast tissue as they navigated stasis and emerged as variant populations.

RESULTS

We observed robust activation of a TGF-β-dependent epithelial-mesenchymal transition (EMT) program in basal epithelial cells during stasis, followed by subsequent attenuation of this program in emerging variants. Inhibition of the TGF-β pathway or depleting the EMT regulators Snail or Slug allowed basal epithelial cells to collectively bypass stasis, demonstrating that cellular dysfunction and arrest resulting from TGF-β and EMT activation are central to this in vitro barrier. The spontaneous emergence of variants from stasis cultures was associated with a restricted EMT trajectory, characterized by the stabilization of hybrid EMT states associated with greater proliferative capacity, rather than progressing to a complete mesenchymal state characterized by irreversible growth arrest. Epigenetic mechanisms, which contributed to the dysregulated growth control characteristic of the variant phenotype, also contributed to the stability of the hybrid EMT program in variants. By overcoming the cellular dysfunction and growth arrest resulting from TGF-β and complete EMT, variants exhibited a higher oncogenic transformation efficiency compared to pre-stasis basal epithelial cells. Inhibiting the TGF-β pathway prior to stasis significantly reduced EMT in the basal epithelial population, alleviated selective pressure driving variant emergence, and also enhanced oncogenic transformation efficiency, resulting in tumors with markedly diminished metaplastic differentiation.

CONCLUSIONS

This study reveals how an epigenetic program governs basal epithelial cell fate decisions and contributes to the development of MBC progenitors by restricting access to terminal mesenchymal states that induce growth arrest and, instead, favoring hybrid EMT states with enhanced tumorigenic potential.

mTOR potentiates senescent phenotypes and primary cilia formation after cisplatin-induced G2 arrest in retinal pigment epithelial cells

Cisplatin, a platinum-based anticancer drug, is used to treat several types of cancer. Despite its effectiveness, cisplatin-induced side effects have often been reported. Although cisplatin-induced toxicities, such as apoptosis and/or necrosis, have been well studied, the fate of cells after exposure to sublethal doses of cisplatin needs further elucidation. Treatment with a sublethal dose of cisplatin induced cell cycle arrest at the G2 phase in retinal pigment epithelial cells. Following cisplatin withdrawal, the cells irreversibly exited the cell cycle and became senescent. Notably, the progression from the G2 to the G1 phase occurred without mitotic entry, a phenomenon referred to as mitotic bypass, resulting in the accumulation of cells containing 4N DNA content. Cisplatin-exposed cells exhibited morphological changes associated with senescence, including an enlarged size of cell and nucleus and increased granularity. In addition, the senescent cells possessed primary cilia and persistent DNA lesions. Senescence induced by transient exposure to cisplatin involves mTOR activation. Although transient co-exposure with an mTORC1 inhibitor rapamycin did not prevent mitotic bypass and entry into senescence, it delayed the progression of senescence and attenuated senescent phenotypes, resulting in shorter primary cilia formation. Conclusively, cisplatin induces senescence in retinal pigment epithelial cells by promoting mTOR activation.

Genome dilution by cell growth drives starvation-like proteome remodeling in mammalian and yeast cells

Cell size is tightly controlled in healthy tissues and single-celled organisms, but it remains unclear how cell size influences physiology. Increasing cell size was recently shown to remodel the proteomes of cultured human cells, demonstrating that large and small cells of the same type can be compositionally different. In the present study, we utilize the natural heterogeneity of hepatocyte ploidy and yeast genetics to establish that the ploidy-to-cell size ratio is a highly conserved determinant of proteome composition. In both mammalian and yeast cells, genome dilution by cell growth elicits a starvation-like phenotype, suggesting that growth in large cells is restricted by genome concentration in a manner that mimics a limiting nutrient. Moreover, genome dilution explains some proteomic changes ascribed to yeast aging. Overall, our data indicate that genome concentration drives changes in cell composition independently of external environmental cues.

Uncoupling of mTORC1 from E2F activity maintains DNA damage and senescence

DNA damage is a primary trigger for cellular senescence, which in turn causes organismal aging and is a promising target of anti-aging therapies. Most DNA damage occurs when DNA is fragile during DNA replication in S phase, but senescent cells maintain DNA damage long-after DNA replication has stopped. How senescent cells induce DNA damage and why senescent cells fail to repair damaged DNA remain open questions. Here, we combine reversible expression of the senescence-inducing CDK4/6 inhibitory protein p16INK4 (p16) with live single-cell analysis and show that sustained mTORC1 signaling triggers senescence in non-proliferating cells by increasing transcriptional DNA damage and inflammation signaling that persists after p16 is degraded. Strikingly, we show that activation of E2F transcriptional program, which is regulated by CDK4/6 activity and promotes expression of DNA repair proteins, repairs transcriptionally damaged DNA without requiring DNA replication. Together, our study suggests that senescence can be maintained by ongoing mTORC1-induced transcriptional DNA damage that cannot be sufficiently repaired without induction of protective E2F target genes.

The PRC2.1 Subcomplex Opposes G1 Progression through Regulation of CCND1 and CCND2

Progression through the G1 phase of the cell cycle is the most highly regulated step in cellular division. We employed a chemogenetic approach to discover novel cellular networks that regulate cell cycle progression. This approach uncovered functional clusters of genes that altered sensitivity of cells to inhibitors of the G1/S transition. Mutation of components of the Polycomb Repressor Complex 2 rescued proliferation inhibition caused by the CDK4/6 inhibitor palbociclib, but not to inhibitors of S phase or mitosis. In addition to its core catalytic subunits, mutation of the PRC2.1 accessory protein MTF2, but not the PRC2.2 protein JARID2, rendered cells resistant to palbociclib treatment. We found that PRC2.1 (MTF2), but not PRC2.2 (JARID2), was critical for promoting H3K27me3 deposition at CpG islands genome-wide and in promoters. This included the CpG islands in the promoter of the CDK4/6 cyclins CCND1 and CCND2, and loss of MTF2 lead to upregulation of both CCND1 and CCND2. Our results demonstrate a role for PRC2.1, but not PRC2.2, in antagonizing G1 progression in a diversity of cell linages, including CML, breast cancer and immortalized cell lines.

Eukaryotic cell size regulation and its implications for cellular function and dysfunction

Depending on cell type, environmental inputs, and disease, the cells in the human body can have widely different sizes. In recent years, it has become clear that cell size is a major regulator of cell function. However, we are only beginning to understand how the optimization of cell function determines a given cell's optimal size. Here, we review currently known size control strategies of eukaryotic cells and the intricate link of cell size to intracellular biomolecular scaling, organelle homeostasis, and cell cycle progression. We detail the cell size-dependent regulation of early development and the impact of cell size on cell differentiation. Given the importance of cell size for normal cellular physiology, cell size control must account for changing environmental conditions. We describe how cells sense environmental stimuli, such as nutrient availability, and accordingly adapt their size by regulating cell growth and cell cycle progression. Moreover, we discuss the correlation of pathological states with misregulation of cell size and how for a long time this was considered a downstream consequence of cellular dysfunction. We review newer studies that reveal a reversed causality, with misregulated cell size leading to pathophysiological phenotypes such as senescence and aging. In summary, we highlight the important roles of cell size in cellular function and dysfunction, which could have major implications for both diagnostics and treatment in the clinic.

An adaptive Epithelial-Mesenchymal Transition Program Enables Basal Epithelial Cells to Bypass Stress-Induced Stasis and Contributes to Metaplastic Breast Cancer Progenitor State

Background

Human mammary epithelial cell (HMEC) cultures encounter a stress-associated barrier termed stasis, during which most cells adopt a senescence-like phenotype. From these cultures, rare variants emerge from the basal epithelial population, re-initiating growth. Variants exhibit pre-malignant properties, including an aberrant epigenetic program that enables continued proliferation and acquisition of genetic changes. Following oncogenic transformation, variants produce tumors that recapitulate the histopathological characteristics of metaplastic breast cancer (MBC), a rare subtype characterized by squamous and mesenchymal differentiation.

Methods

Using the conventional serum-free HMEC culture system, we probed the capacity for phenotypic plasticity inherent to basal epithelial cell populations from human breast tissue as they navigated stasis and emerged as variant populations.

Results

We observed robust activation of a TGF-β-dependent epithelial-mesenchymal transition (EMT) program in basal epithelial cells during stasis, followed by subsequent attenuation of this program in emerging variants. Inhibiting the TGF-β pathway or depleting the EMT regulators Snail or Slug allowed basal epithelial cells to collectively bypass stasis, demonstrating that cellular dysfunction and arrest resulting from TGF-β and EMT activation are central to this in vitro barrier. The spontaneous emergence of variants from stasis cultures was associated with a restricted EMT trajectory, which diverted cells away from a complete mesenchymal state characterized by irreversible growth arrest, and instead limited variants to epithelial and intermediate EMT states associated with greater proliferative capacity and stemness. Epigenetic mechanisms, which contributed to the dysregulated growth control characteristic of the variant phenotype, also contributed to the constrained EMT program in variants. By overcoming the cellular dysfunction and growth arrest resulting from TGF-β and EMT activation, variants exhibited increased oncogenic transformation efficiency compared to pre-stasis basal epithelial cells. Inhibiting the TGF-β pathway prior to stasis significantly reduced EMT in the basal epithelial population, alleviated selective pressure driving variant emergence, and enhanced oncogenic transformation efficiency, resulting in tumors with markedly diminished metaplastic differentiation.

Conclusions

This study reveals how adaptive EMT reprogramming governs basal epithelial cell fate decisions and contributes to the development of MBC progenitors by restricting access to terminal mesenchymal states that induce growth arrest and, instead, favoring intermediate states with enhanced tumorigenic potential.

Mechanisms of sensitivity and resistance to CDK4/CDK6 inhibitors in hormone receptor-positive breast cancer treatment

Cell cycle dysregulation is a hallmark of cancer that promotes eccessive cell division. Cyclin-dependent kinase 4 (CDK4) and cyclin-dependent kinase 6 (CDK6) are key molecules in the G1-to-S phase cell cycle transition and are crucial for the onset, survival, and progression of breast cancer (BC). Small-molecule CDK4/CDK6 inhibitors (CDK4/6i) block phosphorylation of tumor suppressor Rb and thus restrain susceptible BC cells in G1 phase. Three CDK4/6i are approved for the first-line treatment of patients with advanced/metastatic hormone receptor-positive (HR+)/human epidermal growth factor receptor 2-negative (HER2-) BC in combination with endocrine therapy (ET). Though this has improved the clinical outcomes for survival of BC patients, there is no established standard next-line treatment to tackle drug resistance. Recent studies suggest that CDK4/6i can modulate other distinct effects in both BC and breast stromal compartments, which may provide new insights into aspects of their clinical activity. This review describes the biochemistry of the CDK4/6-Rb-E2F pathway in HR+ BC, then discusses how CDK4/6i can trigger other effects in BC/breast stromal compartments, and finally outlines the mechanisms of CDK4/6i resistance that have emerged in recent preclinical studies and clinical cohorts, emphasizing the impact of these findings on novel therapeutic opportunities in BC.

An unscheduled switch to endocycles induces a reversible senescent arrest that impairs growth of the Drosophila wing disc

A programmed developmental switch to G / S endocycles results in tissue growth through an increase in cell size. Unscheduled, induced endocycling cells (iECs) promote wound healing but also contribute to cancer. Much remains unknown, however, about how these iECs affect tissue growth. Using the D. melanogaster wing disc as model, we find that populations of iECs initially increase in size but then subsequently undergo a heterogenous arrest that causes severe tissue undergrowth. iECs acquired DNA damage and activated a Jun N-terminal kinase (JNK) pathway, but, unlike other stressed cells, were apoptosis-resistant and not eliminated from the epithelium. Instead, iECs entered a JNK-dependent and reversible senescent-like arrest. Senescent iECs promoted division of diploid neighbors, but this compensatory proliferation did not rescue tissue growth. Our study has uncovered unique attributes of iECs and their effects on tissue growth that have important implications for understanding their roles in wound healing and cancer.

Targeting senescence-associated secretory phenotypes to remodel the tumour microenvironment and modulate tumour outcomes

Tumour cell senescence can be induced by various factors, including DNA damage, inflammatory signals, genetic toxins, ionising radiation and nutrient metabolism. The senescence-associated secretory phenotype (SASP), secreted by senescent tumour cells, possesses the capacity to modulate various immune cells, including macrophages, T cells, natural killer cells and myeloid-derived suppressor cells, as well as vascular endothelial cells and fibroblasts within the tumour microenvironment (TME), and this modulation can result in either the promotion or suppression of tumorigenesis and progression. Exploring the impact of SASP on the TME could identify potential therapeutic targets, yet limited studies have dissected its functions. In this review, we delve into the causes and mechanisms of tumour cell senescence. We then concentrate on the influence of SASP on the tumour immune microenvironment, angiogenesis, extracellular matrix and the reprogramming of cancer stem cells, along with their associated tumour outcomes. Last, we present a comprehensive overview of the diverse array of senotherapeutics, highlighting their prospective advantages and challenge for the treatment of cancer patients. KEY POINTS: Senescence-associated secretory phenotype (SASP) secretion from senescent tumour cells significantly impacts cancer progression and biology. SASP is involved in regulating the remodelling of the tumour microenvironment, including immune microenvironment, vascular, extracellular matrix and cancer stem cells. Senotherapeutics, such as senolytic, senomorphic, nanotherapy and senolytic vaccines, hold promise for enhancing cancer treatment efficacy.

Metabolic remodeling in cancer and senescence and its therapeutic implications

Cellular metabolism is a flexible and plastic network that often dictates physiological and pathological states of the cell, including differentiation, cancer, and aging. Recent advances in cancer metabolism represent a tremendous opportunity to treat cancer by targeting its altered metabolism. Interestingly, despite their stable growth arrest, senescent cells - a critical component of the aging process - undergo metabolic changes similar to cancer metabolism. A deeper understanding of the similarities and differences between these disparate pathological conditions will help identify which metabolic reprogramming is most relevant to the therapeutic liabilities of senescence. Here, we compare and contrast cancer and senescence metabolism and discuss how metabolic therapies can be established as a new modality of senotherapy for healthy aging.

APC/C prevents a noncanonical order of cyclin/CDK activity to maintain CDK4/6 inhibitor-induced arrest

Regulated cell cycle progression ensures homeostasis and prevents cancer. In proliferating cells, premature S phase entry is avoided by the E3 ubiquitin ligase anaphasepromoting complex/cyclosome (APC/C), although the APC/C substrates whose degradation restrains G1-S progression are not fully known. The APC/C is also active in arrested cells that exited the cell cycle, but it is not clear whether APC/C maintains all types of arrest. Here, by expressing the APC/C inhibitor, EMI1, we show that APC/C activity is essential to prevent S phase entry in cells arrested by pharmacological cyclin-dependent kinases 4 and 6 (CDK4/6) inhibition (Palbociclib). Thus, active protein degradation is required for arrest alongside repressed cell cycle gene expression. The mechanism of rapid and robust arrest bypass from inhibiting APC/C involves CDKs acting in an atypical order to inactivate retinoblastoma-mediated E2F repression. Inactivating APC/C first causes mitotic cyclin B accumulation which then promotes cyclin A expression. We propose that cyclin A is the key substrate for maintaining arrest because APC/C-resistant cyclin A, but not cyclin B, is sufficient to induce S phase entry. Cells bypassing arrest from CDK4/6 inhibition initiate DNA replication with severely reduced origin licensing. The simultaneous accumulation of S phase licensing inhibitors, such as cyclin A and geminin, with G1 licensing activators disrupts the normal order of G1-S progression. As a result, DNA synthesis and cell proliferation are profoundly impaired. Our findings predict that cancers with elevated EMI1 expression will tend to escape CDK4/6 inhibition into a premature, underlicensed S phase and suffer enhanced genome instability.

Improving the power of drug toxicity measurements by quantitative nuclei imaging

Imaging-based anticancer drug screens are becoming more prevalent due to development of automated fluorescent microscopes and imaging stations, as well as rapid advancements in image processing software. Automated cell imaging provides many benefits such as their ability to provide high-content data, modularity, dynamics recording and the fact that imaging is the most direct way to access cell viability and cell proliferation. However, currently most publicly available large-scale anticancer drugs screens, such as GDSC, CTRP and NCI-60, provide cell viability data measured by assays based on colorimetric or luminometric measurements of NADH or ATP levels. Although such datasets provide valuable data, it is unclear how well drug toxicity measurements can be integrated with imaging data. Here we explored the relations between drug toxicity data obtained by XTT assay, two quantitative nuclei imaging methods and trypan blue dye exclusion assay using a set of four cancer cell lines with different morphologies and 30 drugs with different mechanisms of action. We show that imaging-based approaches provide high accuracy and the differences between results obtained by different methods highly depend on drug mechanism of action. Selecting AUC metrics over IC50 or comparing data where significantly drugs reduced cell numbers noticeably improves consistency between methods. Using automated cell segmentation protocols we analyzed mitochondria activity in more than 11 thousand drug-treated cells and showed that XTT assay produces unreliable data for CDK4/6, Aurora A, VEGFR and PARP inhibitors due induced cell size growth and increase in individual mitochondria activity. We also explored several benefits of image-based analysis such as ability to monitor cell number dynamics, dissect changes in total and individual mitochondria activity from cell proliferation, and ability to identify chromatin remodeling drugs. Finally, we provide a web tool that allows comparing results obtained by different methods.

An unscheduled switch to endocycles induces a reversible senescent arrest that impairs growth of the Drosophila wing disc

A programmed developmental switch to G / S endocycles results in tissue growth through an increase in cell size. Unscheduled, induced endocycling cells (iECs) promote wound healing but also contribute to cancer. Much remains unknown, however, about how these iECs affect tissue growth. Using the D. melanogasterwing disc as model, we find that populations of iECs initially increase in size but then subsequently undergo a heterogenous arrest that causes severe tissue undergrowth. iECs acquired DNA damage and activated a Jun N-terminal kinase (JNK) pathway, but, unlike other stressed cells, were apoptosis-resistant and not eliminated from the epithelium. Instead, iECs entered a JNK-dependent and reversible senescent-like arrest. Senescent iECs promoted division of diploid neighbors, but this compensatory proliferation did not rescue tissue growth. Our study has uncovered unique attributes of iECs and their effects on tissue growth that have important implications for understanding their roles in wound healing and cancer.

The search for CDK4/6 inhibitor biomarkers has been hampered by inappropriate proliferation assays

CDK4/6 inhibitors are effective at treating advanced HR+ /HER2- breast cancer, however biomarkers that can predict response are urgently needed. We demonstrate here that previous large-scale screens designed to identify which tumour types or genotypes are most sensitive to CDK4/6 inhibitors have misrepresented the responsive cell lines because of a reliance on metabolic proliferation assays. CDK4/6-inhibited cells arrest in G1 but continue to grow in size, thereby producing more mitochondria. We show that this growth obscures the arrest using ATP-based proliferation assays but not if DNA-based assays are used instead. Furthermore, lymphoma lines, previously identified as the most sensitive, simply appear to respond the best using ATP-based assays because they fail to overgrow during the G1 arrest. Similarly, the CDK4/6 inhibitor abemaciclib appears to inhibit proliferation better than palbociclib because it also restricts cellular overgrowth through off-target effects. DepMap analysis of screening data using reliable assay types, demonstrates that palbociclib-sensitive cell types are also sensitive to Cyclin D1, CDK4 and CDK6 knockout/knockdown, whereas the palbociclib-resistant lines are sensitive to Cyclin E1, CDK2 and SKP2 knockout/knockdown. Potential biomarkers of palbociclib-sensitive cells are increased expression of CCND1 and RB1, and reduced expression of CCNE1 and CDKN2A. Probing DepMap with similar data from metabolic assays fails to reveal these associations. Together, this demonstrates why CDK4/6 inhibitors, and any other anti-cancer drugs that arrest the cell cycle but permit continued cell growth, must now be re-screened against a wide-range of cell types using an appropriate proliferation assay. This would help to better inform clinical trials and to identify much needed biomarkers of response.

Cellular senescence and kidney aging

Life expectancy is increasing worldwide, and by 2050 the proportion of the world's population over 65 years of age is estimated to surpass 1.5 billion. Kidney aging is associated with molecular and physiological changes that cause a loss of renal function and of regenerative potential. As the aging population grows, it is crucial to understand the mechanisms underlying these changes, as they increase the susceptibility to developing acute kidney injury (AKI) and chronic kidney disease (CKD). Various cellular processes and molecular pathways take part in the complex process of kidney aging. In this review, we will focus on the phenomenon of cellular senescence as one of the involved mechanisms at the crossroad of kidney aging, age-related disease, and CKD. We will highlight experimental and clinical findings about the role of cellular senescence in kidney aging and CKD. In addition, we will review challenges in senescence research and emerging therapeutic aspects. We will highlight the great potential of senolytic strategies for the elimination of harmful senescent cells to promote healthy kidney aging and to avoid age-related disease and CKD. This review aims to give insight into recent discoveries and future developments, providing a comprehensive overview of current knowledge on cellular senescence and anti-senescent therapies in the kidney field.

Active growth signaling promotes senescence and cancer cell sensitivity to CDK7 inhibition

Tumor growth is driven by continued cellular growth and proliferation. Cyclin-dependent kinase 7's (CDK7) role in activating mitotic CDKs and global gene expression makes it therefore an attractive target for cancer therapies. However, what makes cancer cells particularly sensitive to CDK7 inhibition (CDK7i) remains unclear. Here, we address this question. We show that CDK7i, by samuraciclib, induces a permanent cell-cycle exit, known as senescence, without promoting DNA damage signaling or cell death. A chemogenetic genome-wide CRISPR knockout screen identified that active mTOR (mammalian target of rapamycin) signaling promotes samuraciclib-induced senescence. mTOR inhibition decreases samuraciclib sensitivity, and increased mTOR-dependent growth signaling correlates with sensitivity in cancer cell lines. Reverting a growth-promoting mutation in PIK3CA to wild type decreases sensitivity to CDK7i. Our work establishes that enhanced growth alone promotes CDK7i sensitivity, providing an explanation for why some cancers are more sensitive to CDK inhibition than normally growing cells.

Genome homeostasis defects drive enlarged cells into senescence

Cellular senescence refers to an irreversible state of cell-cycle arrest and plays important roles in aging and cancer biology. Because senescence is associated with increased cell size, we used reversible cell-cycle arrests combined with growth rate modulation to study how excessive growth affects proliferation. We find that enlarged cells upregulate p21, which limits cell-cycle progression. Cells that re-enter the cell cycle encounter replication stress that is well tolerated in physiologically sized cells but causes severe DNA damage in enlarged cells, ultimately resulting in mitotic failure and permanent cell-cycle withdrawal. We demonstrate that enlarged cells fail to recruit 53BP1 and other non-homologous end joining (NHEJ) machinery to DNA damage sites and fail to robustly initiate DNA damage-dependent p53 signaling, rendering them highly sensitive to genotoxic stress. We propose that an impaired DNA damage response primes enlarged cells for persistent replication-acquired damage, ultimately leading to cell division failure and permanent cell-cycle exit.

Too big not to fail: Different paths lead to senescence of enlarged cells

In this issue of Molecular Cell, Crozier et al.,1 Foy et al.,2 Manohar et al.,3 and Wilson et al.4 show how excessive cell growth caused by a temporary G1 arrest leads to permanent cell cycle exit at different stages of the cell cycle.

Oncogenic signals prime cancer cells for toxic cell overgrowth during a G1 cell cycle arrest

CDK4/6 inhibitors are remarkable anti-cancer drugs that can arrest tumor cells in G1 and induce their senescence while causing only relatively mild toxicities in healthy tissues. How they achieve this mechanistically is unclear. We show here that tumor cells are specifically vulnerable to CDK4/6 inhibition because during the G1 arrest, oncogenic signals drive toxic cell overgrowth. This overgrowth causes permanent cell cycle withdrawal by either preventing progression from G1 or inducing genotoxic damage during the subsequent S-phase and mitosis. Inhibiting or reverting oncogenic signals that converge onto mTOR can rescue this excessive growth, DNA damage, and cell cycle exit in cancer cells. Conversely, inducing oncogenic signals in non-transformed cells can drive these toxic phenotypes and sensitize the cells to CDK4/6 inhibition. Together, this demonstrates that cell cycle arrest and oncogenic cell growth is a synthetic lethal combination that is exploited by CDK4/6 inhibitors to induce tumor-specific toxicity.

Many Ways to the Cell Cycle Exit after Inhibition of CDK4/6

Cyclin-dependent kinases (CDKs) are master regulators of proliferation, and therefore they represent attractive targets for cancer therapy. Deve-lopment of selective CDK4/6 inhibitors including palbociclib revolutionized the treatment of advanced HR+/HER2- breast cancer. Inhibition of CDK4/6 leads to cell cycle arrest in G0/G1 phase and eventually to a permanent cell cycle exit called senescence. One of the main features of the senescence is an increased cell size. For many years, it was believed that the non-dividing cells simply continue to grow and as a result, they become excessively large. There is now emerging evidence that the increased cell size is a cause rather than consequence of the cell cycle arrest. This review aims to summarize recent advances in our understanding of senescence induction, in particular that resulting from treatment with CDK4/6 inhibitors.