TGF-beta signaling engages an ATM-CHK2-p53-independent RAS-induced senescence and prevents malignant transformation in human mammary epithelial cells

Regulation of cell function and identity by cellular senescence

During aging and in some contexts, like embryonic development, wound healing, and diseases such as cancer, senescent cells accumulate and play a key role in different pathophysiological functions. A long-held belief was that cellular senescence decreased normal cell functions, given the loss of proliferation of senescent cells. This view radically changed following the discovery of the senescence-associated secretory phenotype (SASP), factors released by senescent cells into their microenvironment. There is now accumulating evidence that cellular senescence also promotes gain-of-function effects by establishing, reinforcing, or changing cell identity, which can have a beneficial or deleterious impact on pathophysiology. These effects may involve both proliferation arrest and autocrine SASP production, although they largely remain to be defined. Here, we provide a historical overview of the first studies on senescence and an insight into emerging trends regarding the effects of senescence on cell identity.

Multi-omics and single cell characterization of cancer immunosenescence landscape

Cellular senescence (CS) is closely related to tumor progression. However, the studies about CS genes across human cancers have not explored the relationship between cancer senescence signature and telomere length. Additionally, single-cell analyses have not revealed the evolutionary trends of malignant cells and immune cells at the CS level. We defined a CS-associated signature, called "senescence signature", and found that patients with higher senescence signature had worse prognosis. Higher senescence signature was related to older age, higher genomic instability, longer telomeres, increased lymphocytic infiltration, higher pro-tumor immune infiltrates (Treg cells and MDSCs), and could predict responses to immune checkpoint inhibitor therapy. Single-cell analysis further reveals malignant cells and immune cells share a consistent evolutionary trend at the CS level. MAPK signaling pathway and apoptotic processes may play a key role in CS, and senescence signature may effectively predict sensitivity of MEK1/2 inhibitors, ERK1/2 inhibitors and BCL-2 family inhibitors. We also developed a new CS prediction model of cancer survival and established a portal website to apply this model ( https://bio-pub.shinyapps.io/cs_nomo/ ).

Plasma membrane damage is a new trigger of cellular senescence

By blocking proliferation and inducing a secretory phenotype, cellular senescence has beneficial and deleterious effects, the latter being linked to aging. Suda et al. recently reported that plasma membrane (PM) damage (PMD) triggers senescence, suggesting that PMD inducers promote senescence and that the PMD repair machinery can regulate it.

RSK3 switches cell fate: from stress-induced senescence to malignant progression

BACKGROUND

TGFβ induces several cell phenotypes including senescence, a stable cell cycle arrest accompanied by a secretory program, and epithelial-mesenchymal transition (EMT) in normal epithelial cells. During carcinogenesis cells lose the ability to undergo senescence in response to TGFβ but they maintain an EMT, which can contribute to tumor progression. Our aim was to identify mechanisms promoting TGFβ-induced senescence escape.

METHODS

In vitro experiments were performed with primary human mammary epithelial cells (HMEC) immortalized by hTert. For kinase library screen and modulation of gene expression retroviral transduction was used. To characterize gene expression, RNA microarray with GSEA analysis and RT-qPCR were used. For protein level and localization, Western blot and immunofluorescence were performed. For senescence characterization crystal violet assay, Senescence Associated-β-Galactosidase activity, EdU staining were conducted. To determine RSK3 partners FLAG-baited immunoprecipitation and mass spectrometry-based proteomic analyses were performed. Proteosome activity and proteasome enrichment assays were performed. To validate the role of RSK3 in human breast cancer, analysis of METABRIC database was performed. Murine intraductal xenografts using MCF10DCIS.com cells were carried out, with histological and immunofluorescence analysis of mouse tissue sections.

RESULTS

A screen with active kinases in HMECs upon TGFβ treatment identified that the serine threonine kinase RSK3, or RPS6KA2, a kinase mainly known to regulate cancer cell death including in breast cancer, reverted TGFβ-induced senescence. Interestingly, RSK3 expression decreased in response to TGFβ in a SMAD3-dependent manner, and its constitutive expression rescued SMAD3-induced senescence, indicating that a decrease in RSK3 itself contributes to TGFβ-induced senescence. Using transcriptomic analyses and affinity purification coupled to mass spectrometry-based proteomics, we unveiled that RSK3 regulates senescence by inhibiting the NF-κΒ pathway through the decrease in proteasome-mediated IκBα degradation. Strikingly, senescent TGFβ-treated HMECs display features of epithelial to mesenchymal transition (EMT) and during RSK3-induced senescence escaped HMECs conserve EMT features. Importantly, RSK3 expression is correlated with EMT and invasion, and inversely correlated with senescence and NF-κΒ in human claudin-low breast tumors and its expression enhances the formation of breast invasive tumors in the mouse mammary gland.

CONCLUSIONS

We conclude that RSK3 switches cell fate from senescence to malignancy in response to TGFβ signaling.

Oncogene-Induced Senescence Is a Crucial Antitumor Defense Mechanism of Human Endometrial Stromal Cells

Being the major cellular component of highly dynamic tissue, endometrial stromal cells (EnSCs) are exposed to cycles of proliferation upon hormonal stimulation, which might pose risks for the accumulation of mutations and malignization. However, endometrial stromal tumors are rare and uncommon. The present study uncovered defense mechanisms that might underlie the resistance of EnSCs against oncogenic transformation. All experiments were performed in vitro using the following methods: FACS, WB, RT-PCR, IF, molecular cloning, lentiviral transduction, and CRISPR/Cas9 genome editing. We revealed that the expression of the mutant HRASG12V leads to EnSC senescence. We experimentally confirmed the inability of HRASG12V-expressing EnSCs to bypass senescence and resume proliferation, even upon estrogen stimulation. At the molecular level, the induction of oncogene-induced senescence (OIS) was accompanied by activation of the MEK/ERK, PI3K/AKT, p53/p21WAF/CIP/Rb, and p38/p16INK4a/Rb pathways; however, inhibiting either pathway did not prevent cell cycle arrest. PTEN loss was established as an additional feature of HRASG12V-induced senescence in EnSCs. Using CRISPR-Cas9-mediated PTEN knockout, we identified PTEN loss-induced senescence as a reserve molecular mechanism to prevent the transformation of HRASG12V-expressing EnSCs. The present study highlights oncogene-induced senescence as an antitumor defense mechanism of EnSCs controlled by multiple backup molecular pathways.

Inflammation and DNA damage: cause, effect or both

Inflammation is a biological response involving immune cells, blood vessels and mediators induced by endogenous and exogenous stimuli, such as pathogens, damaged cells or chemicals. Unresolved (chronic) inflammation is characterized by the secretion of cytokines that maintain inflammation and redox stress. Mitochondrial or nuclear redox imbalance induces DNA damage, which triggers the DNA damage response (DDR) that is orchestrated by ATM and ATR kinases, which modify gene expression and metabolism and, eventually, establish the senescent phenotype. DDR-mediated senescence is induced by the signalling proteins p53, p16 and p21, which arrest the cell cycle in G1 or G2 and promote cytokine secretion, producing the senescence-associated secretory phenotype. Senescence and inflammation phenotypes are intimately associated, but highly heterogeneous because they vary according to the cell type that is involved. The vicious cycle of inflammation, DNA damage and DDR-mediated senescence, along with the constitutive activation of the immune system, is the core of an evolutionarily conserved circuitry, which arrests the cell cycle to reduce the accumulation of mutations generated by DNA replication during redox stress caused by infection or inflammation. Evidence suggests that specific organ dysfunctions in apparently unrelated diseases of autoimmune, rheumatic, degenerative and vascular origins are caused by inflammation resulting from DNA damage-induced senescence.

Recombinant human chorionic gonadotropin induces signaling pathways towards cancer prevention in the breast of BRCA1/2 mutation carriers

BACKGROUND

Strategies for breast cancer prevention in women with germline BRCA1/2 mutations are limited. We previously showed that recombinant human chorionic gonadotropin (r-hCG) induces mammary gland differentiation and inhibits mammary tumorigenesis in rats. The present study investigated hCG-induced signaling pathways in the breast of young nulliparous women carrying germline BRCA1/2 mutations.

METHODS

We performed RNA-sequencing on breast tissues from 25 BRCA1/2 mutation carriers who received r-hCG treatment for 3 months in a phase II clinical trial, we analyzed the biological processes, reactome pathways, canonical pathways, and upstream regulators associated with genes differentially expressed after r-hCG treatment, and validated genes of interest.

RESULTS

We observed that r-hCG induces remarkable transcriptomic changes in the breast of BRCA1/2 carriers, especially in genes related to cell development, cell differentiation, cell cycle, apoptosis, DNA repair, chromatin remodeling, and G protein-coupled receptor signaling. We revealed that r-hCG inhibits Wnt/β-catenin signaling, MYC, HMGA1 , and HOTAIR , whereas activates TGFB/TGFBR-SMAD2/3/4, BRCA1, TP53, and upregulates BRCA1 protein.

CONCLUSION

Our data suggest that the use of r-hCG at young age may reduce the risk of breast cancer in BRCA1/2 carriers by inhibiting pathways associated with stem/progenitor cell maintenance and neoplastic transformation, whereas activating genes crucial for breast epithelial differentiation and lineage commitment, and DNA repair.

Low-Dose Radiation Therapy (LDRT) against Cancer and Inflammatory or Degenerative Diseases: Three Parallel Stories with a Common Molecular Mechanism Involving the Nucleoshuttling of the ATM Protein?

Very early after their discovery, X-rays were used in multiple medical applications, such as treatments against cancer, inflammation and pain. Because of technological constraints, such applications involved X-ray doses lower than 1 Gy per session. Progressively, notably in oncology, the dose per session increased. However, the approach of delivering less than 1 Gy per session, now called low-dose radiation therapy (LDRT), was preserved and is still applied in very specific cases. More recently, LDRT has also been applied in some trials to protect against lung inflammation after COVID-19 infection or to treat degenerative syndromes such as Alzheimer's disease. LDRT illustrates well the discontinuity of the dose-response curve and the counterintuitive observation that a low dose may produce a biological effect higher than a certain higher dose. Even if further investigations are needed to document and optimize LDRT, the apparent paradox of some radiobiological effects specific to low dose may be explained by the same mechanistic model based on the radiation-induced nucleoshuttling of the ATM kinase, a protein involved in various stress response pathways.

Expression of the Calcium-Binding Protein CALB1 Is Induced and Controls Intracellular Ca2+ Levels in Senescent Cells

In response to many stresses, such as oncogene activation or DNA damage, cells can enter cellular senescence, a state of proliferation arrest accompanied by a senescence-associated secretory phenotype (SASP). Cellular senescence plays a key role in many physiopathological contexts, including cancer, aging and aging-associated diseases, therefore, it is critical to understand how senescence is regulated. Calcium ions (Ca²⁺) recently emerged as pivotal regulators of cellular senescence. However, how Ca²⁺ levels are controlled during this process is barely known. Here, we report that intracellular Ca²⁺ contents increase in response to many senescence inducers in immortalized human mammary epithelial cells (HMECs) and that expression of calbindin 1 (CALB1), a Ca²⁺-binding protein, is upregulated in this context, through the Ca²⁺-dependent calcineurin/NFAT pathway. We further show that overexpression of CALB1 buffers the rise in intracellular Ca²⁺ levels observed in senescent cells. Finally, we suggest that increased expression of Ca²⁺-binding proteins calbindins is a frequent mark of senescent cells. This work thus supports that, together with Ca²⁺channels, Ca²⁺-binding proteins modulate Ca²⁺ levels and flux during cellular senescence. This opens potential avenues of research to better understand the role of Ca²⁺ and of Ca²⁺-binding proteins in regulating cellular senescence.

Preclinical studies for improving radiosensitivity of non-small cell lung cancer cell lines by combining glutaminase inhibition and senolysis

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Abnormal glutaminolysis is common in cancer cells with mutations.

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The glutaminase inhibitor CB839 enhanced radiosensitivity in A549 and H460 cells.

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Glutaminolysis inhibition led to an increase in cell senescence.

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The Bcl-2 family inhibitor ABT-263 induced transition from senescence to apoptosis.

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Combined glutaminolysis and senolysis may improve radiosensitivity in cancer cells.

Glutamine metabolism, known as glutaminolysis, is abnormally activated in many cancer cells with KRAS or BRAF mutations or active c-MYC. Glutaminolysis plays an important role in the proliferation of cancer cells with oncogenic mutations. In this study, we characterized radiation-induced cell death, which was enhanced by glutaminolysis inhibition in non-small cell lung cancer A549 and H460 cell lines with KRAS mutation. A clonogenic survival assay revealed that treatment with a glutaminase inhibitor, CB839, enhanced radiosensitivity. X-irradiation increased glutamate production, mitochondrial oxygen consumption, and ATP production, whereas CB839 treatment suppressed these effects. The data suggest that the enhancement of glutaminolysis-dependent energy metabolism for ATP production is important for survival after X-irradiation. Evaluation of the cell death phenotype revealed that glutaminolysis inhibitory treatment with CB839 or a low-glutamine medium significantly promoted the proliferation of β-galactosidase-positive and IL-6/IL-8 secretory cells among X-irradiated tumor cells, corresponding to an increase in the senescent cell population. Furthermore, treatment with ABT263, a Bcl-2 family inhibitor, transformed senescent cells into apoptotic cells. The findings suggest that combination treatment with a glutaminolysis inhibitor and a senolytic drug is useful for efficient radiotherapy.

Mechanisms and Regulation of Cellular Senescence

Cellular senescence entails a state of an essentially irreversible proliferative arrest in which cells remain metabolically active and secrete a range of pro-inflammatory and proteolytic factors as part of the senescence-associated secretory phenotype. There are different types of senescent cells, and senescence can be induced in response to many DNA damage signals. Senescent cells accumulate in different tissues and organs where they have distinct physiological and pathological functions. Despite this diversity, all senescent cells must be able to survive in a nondividing state while protecting themselves from positive feedback loops linked to the constant activation of the DNA damage response. This capacity requires changes in core cellular programs. Understanding how different cell types can undergo extensive changes in their transcriptional programs, metabolism, heterochromatin patterns, and cellular structures to induce a common cellular state is crucial to preventing cancer development/progression and to improving health during aging. In this review, we discuss how senescent cells continuously evolve after their initial proliferative arrest and highlight the unifying features that define the senescent state.

FGF-2 induces a failure of cell cycle progression in cells harboring amplified K-Ras, revealing new insights into oncogene-induced senescence

Paradoxically, oncogenes that drive cell cycle progression may also trigger pathways leading to senescence, thereby inhibiting the growth of tumorigenic cells. Knowledge of how these pathways operate, and how tumor cells may evade these pathways, is important for understanding tumorigenesis. The Y1 cell line, which harbors an amplification of the proto-oncogene Ras, rapidly senesces in response to the mitogen fibroblast growth factor-2 (FGF-2). To gain a more complete picture of how FGF-2 promotes senescence, we employed a multi-omics approach to analyze histone modifications, mRNA and protein expression, and protein phosphorylation in Y1 cells treated with FGF-2. Compared to control cells treated with serum alone, FGF-2 caused a delayed accumulation of acetylation on histone H4 and higher levels of H3K27me3. Sequencing analysis revealed decreased expression of cell cycle-related genes with concomitant loss of H3K27ac. At the same time, FGF-2 promoted the expression of p21, various cytokines, and MAPK-related genes. Nuclear envelope proteins, particularly lamin B1, displayed increased phosphorylation in response to FGF-2. Proteome analysis suggested alterations in cellular metabolism, as evident by modulated expression of enzymes involved in purine biosynthesis, tRNA aminoacylation, and the TCA cycle. We propose that Y1 cells senesce due to an inability to progress through the cell cycle, which may stem from DNA damage or TGFb signaling. Altogether, the phenotype of Y1 cells is consistent with rapidly established oncogene-induced senescence, demonstrating the synergy between growth factors and oncogenes in driving senescence and bringing additional insight into this tumor suppressor mechanism.

Induction and transmission of oncogene-induced senescence

Senescence is a cellular stress response triggered by diverse stressors, including oncogene activation, where it serves as a bona-fide tumour suppressor mechanism. Senescence can be transmitted to neighbouring cells, known as paracrine secondary senescence. Secondary senescence was initially described as a paracrine mechanism, but recent evidence suggests a more complex scenario involving juxtacrine communication between cells. In addition, single-cell studies described differences between primary and secondary senescent end-points, which have thus far not been considered functionally distinct. Here we discuss emerging concepts in senescence transmission and heterogeneity in primary and secondary senescence on a cellular and organ level.

The Small GTPase RAC1B: A Potent Negative Regulator of-and Useful Tool to Study-TGFβ Signaling

RAC1 and its alternatively spliced isoform, RAC1B, are members of the Rho family of GTPases. Both isoforms are involved in the regulation of actin cytoskeleton remodeling, cell motility, cell proliferation, and epithelial-mesenchymal transition (EMT). Compared to RAC1, RAC1B exhibits a number of distinctive features with respect to tissue distribution, downstream signaling and a role in disease conditions like inflammation and cancer. The subcellular locations and interaction partners of RAC1 and RAC1B vary depending on their activation state, which makes RAC1 and RAC1B ideal candidates to establish cross-talk with cancer-associated signaling pathways-for instance, interactions with signaling by transforming growth factor β (TGFβ), a known tumor promoter. Although RAC1 has been found to promote TGFβ-driven tumor progression, recent observations in pancreatic carcinoma cells surprisingly revealed that RAC1B confers anti-oncogenic properties, i.e., through inhibiting TGFβ-induced EMT. Since then, an unexpected array of mechanisms through which RAC1B cross-talks with TGFβ signaling has been demonstrated. However, rather than being uniformly inhibitory, RAC1B interacts with TGFβ signaling in a way that results in the selective blockade of tumor-promoting pathways, while concomitantly allowing tumor-suppressive pathways to proceed. In this review article, we are going to discuss the specific interactions between RAC1B and TGFβ signaling, which occur at multiple levels and include various components such as ligands, receptors, cytosolic mediators, transcription factors, and extracellular inhibitors of TGFβ ligands.

The Histone Code of Senescence

Senescence is the end point of a complex cellular response that proceeds through a set of highly regulated steps. Initially, the permanent cell-cycle arrest that characterizes senescence is a pro-survival response to irreparable DNA damage. The maintenance of this prolonged condition requires the adaptation of the cells to an unfavorable, demanding and stressful microenvironment. This adaptation is orchestrated through a deep epigenetic resetting. A first wave of epigenetic changes builds a dam on irreparable DNA damage and sustains the pro-survival response and the cell-cycle arrest. Later on, a second wave of epigenetic modifications allows the genomic reorganization to sustain the transcription of pro-inflammatory genes. The balanced epigenetic dynamism of senescent cells influences physiological processes, such as differentiation, embryogenesis and aging, while its alteration leads to cancer, neurodegeneration and premature aging. Here we provide an overview of the most relevant histone modifications, which characterize senescence, aging and the activation of a prolonged DNA damage response.

Inactivation of PP2A by a recurrent mutation drives resistance to MEK inhibitors

The serine/threonine Protein Phosphatase 2A (PP2A) functions as a tumor suppressor by negatively regulating multiple oncogenic signaling pathways. The canonical PP2A holoenzyme comprises a scaffolding subunit (PP2A Aα/β), which serves as the platform for binding of both the catalytic C subunit and one regulatory B subunit. Somatic heterozygous missense mutations in PPP2R1A, the gene encoding the PP2A Aα scaffolding subunit, have been identified across multiple cancer types, but the effects of the most commonly mutated residue, Arg-183, on PP2A function have yet to be fully elucidated. In this study, we used a series of cellular and in vivo models and discovered that the most frequent Aα R183W mutation formed alternative holoenzymes by binding of different PP2A regulatory subunits compared with wild-type Aα, suggesting a rededication of PP2A functions. Unlike wild-type Aα, which suppressed tumorigenesis, the R183W mutant failed to suppress tumor growth in vivo through activation of the MAPK pathway in RAS-mutant transformed cells. Furthermore, cells expressing R183W were less sensitive to MEK inhibitors. Taken together, our results demonstrate that the R183W mutation in PP2A Aα scaffold abrogates the tumor suppressive actions of PP2A, thereby potentiating oncogenic signaling and reducing drug sensitivity of RAS-mutant cells.

A FAM83A Positive Feed-back Loop Drives Survival and Tumorigenicity of Pancreatic Ductal Adenocarcinomas

Pancreatic ductal adenocarcinomas (PDAC) are deadly on account of the delay in diagnosis and dearth of effective treatment options for advanced disease. The insurmountable hurdle of targeting oncogene KRAS, the most prevalent genetic mutation in PDAC, has delayed the availability of targeted therapy for PDAC patients. An alternate approach is to target other tumour-exclusive effector proteins important in RAS signalling. The Family with Sequence Similarity 83 (FAM83) proteins are oncogenic, tumour-exclusive and function similarly to RAS, by driving the activation of PI3K and MAPK signalling. In this study we show that FAM83A expression is significantly elevated in human and murine pancreatic cancers and is essential for the growth and tumorigenesis of pancreatic cancer cells. Elevated FAM83A expression maintains essential MEK/ERK survival signalling, preventing cell death in pancreatic cancer cells. Moreover, we identified a positive feed-forward loop mediated by the MEK/ERK-activated AP-1 transcription factors, JUNB and FOSB, which is responsible for the elevated expression of oncogenic FAM83A. Our data indicates that targeting the MEK/ERK-FAM83A feed-forward loop opens up additional avenues for clinical therapy that bypass targeting of oncogenic KRAS in aggressive pancreatic cancers.

Expansion and maintenance of primary corneal epithelial stem/progenitor cells by inhibition of TGFβ receptor I-mediated signaling

Transforming growth factor β (TGFβ) signaling is one of the most important signaling pathways regulating cell behavior in ocular tissues. Its functions are mainly linked to tissue fibrosis and inflammatory responses in ophthalmology. In epithelial cells, however, the growth inhibitory activity of TGFβ was reported in both non-ocular and ocular tissues. Since TGFβ is a bifunctional regulator that either inhibits or stimulates cell proliferation according to the specific context, we examined the effect of inhibition of TGFβ receptor (TβR) I-mediated signaling on primary corneal epithelial cells (CECs) in serum- and feeder-free conditions. The mouse CECs were isolated from the eyeballs of 6-8 weeks old female C57BL/6 mice using dispase and trypsin separately, cultivated in defined Keratinocyte serum-free medium (KSFM) with supplements (the complete medium) without feeder layer. Cells were divided into three groups, those cultured in complete medium additionally supplemented with 10 μM SB-431542, a specific inhibitor of TβR-I, were SB-CECs; those cultured in complete medium additionally supplemented with 10 ng/ml SRI-011381, a TGF-beta signaling agonist, were SRI-CECs; those cultured in complete medium without SB-431542 or SRI-011381 were control CECs. The growth rate and morphology were analyzed by light microscopy. The identity and stemness of cells was investigated through marker staining of p63, inhibitor of differentiation 1 (ID1), cytokeratin 12 (K12), cytokeratin 14 (K14), PAX6, pSmad3, alpha smooth muscle Actin (αSMA) and E-cadherin (E-cad); Real-time quantitative (RT-PCR) analysis of p63; Western blot analysis of ID1; as well as colony forming assay, sphere forming assay, healing wound in vitro assay and air-lifting interface assay. The results showed SB-CECs subcultured steadily, achieved sustained expansion, and expanded almost thrice faster than control CECs. Expanded SB-CECs exhibited smaller and more compact morphology, up-regulated p63 and ID1, as well as better performed colony-forming capacity, sphere-forming capacity, in vitro wound healing capacity, and the capacity to stratify and differentiate on air-lifting interface. Preliminary tests on human limbal epithelial cells (HLECs) showed the same results as mouse CECs. Interestingly, the ID1 expression pattern was almost identical to p63, the typical marker for corneal epithelial stem/progenitor cell (CESC/CEPC), in cultured CECs and normal corneal sections. Since ID1 has been proven to be regulated negatively by TGFβ signaling in epithelial cells and plays a role in blocking cell differentiation, its derepression by TβR-I inhibitor could be, at least in part, the underlying cause of CESC/CEPC expansion and the synchronously up-regulated expression of p63 in SB-CECs. In conclusion, inhibition of TβR-I-mediated signaling, CESCs/CEPCs achieved efficient long-term expansion in a feeder- and serum-free condition in vitro. And derepression of ID1 could be the underlying cause. Meanwhile, ID1 could serve as a marker for CESC/CEPC. These results may advance the basic and clinical CESC/CEPC research.

A prospect of cell immortalization combined with matrix microenvironmental optimization strategy for tissue engineering and regeneration

Cellular senescence is a major hurdle for primary cell-based tissue engineering and regenerative medicine. Telomere erosion, oxidative stress, the expression of oncogenes and the loss of tumor suppressor genes all may account for the cellular senescence process with the involvement of various signaling pathways. To establish immortalized cell lines for research and clinical use, strategies have been applied including internal genomic or external matrix microenvironment modification. Considering the potential risks of malignant transformation and tumorigenesis of genetic manipulation, environmental modification methods, especially the decellularized cell-deposited extracellular matrix (dECM)-based preconditioning strategy, appear to be promising for tissue engineering-aimed cell immortalization. Due to few review articles focusing on this topic, this review provides a summary of cell senescence and immortalization and discusses advantages and limitations of tissue engineering and regeneration with the use of immortalized cells as well as a potential rejuvenation strategy through combination with the dECM approach.

RAC1B: A Rho GTPase with Versatile Functions in Malignant Transformation and Tumor Progression

RAC1B is an alternatively spliced isoform of the monomeric GTPase RAC1. It differs from RAC1 by a 19 amino acid in frame insertion, termed exon 3b, resulting in an accelerated GDP/GTP-exchange and an impaired GTP-hydrolysis. Although RAC1B has been ascribed several protumorigenic functions such as cell cycle progression and apoptosis resistance, its role in malignant transformation, and other functions driving tumor progression like epithelial-mesenchymal transition, migration/invasion and metastasis are less clear. Insertion of exon 3b endows RAC1B with specific biochemical properties that, when compared to RAC1, encompass both loss-of-functions and gain-of-functions with respect to the type of upstream activators, downstream targets, and binding partners. In its extreme, this may result in RAC1B and RAC1 acting in an antagonistic fashion in regulating a specific cellular response with RAC1B behaving as an endogenous inhibitor of RAC1. In this review, we strive to provide the reader with a comprehensive overview, rather than critical discussions, on various aspects of RAC1B biology in eukaryotic cells.

Targeting 17q23 amplicon to overcome the resistance to anti-HER2 therapy in HER2+ breast cancer

Chromosome 17q23 amplification occurs in ~11% of human breast cancers. Enriched in HER2+ breast cancers, the 17q23 amplification is significantly correlated with poor clinical outcomes. In addition to the previously identified oncogene WIP1, we uncover an oncogenic microRNA gene, MIR21, in a majority of the WIP1-containing 17q23 amplicons. The 17q23 amplification results in aberrant expression of WIP1 and miR-21, which not only promotes breast tumorigenesis, but also leads to resistance to anti-HER2 therapies. Inhibiting WIP1 and miR-21 selectively inhibits the proliferation, survival and tumorigenic potential of the HER2+ breast cancer cells harboring 17q23 amplification. To overcome the resistance of trastuzumab-based therapies in vivo, we develop pH-sensitive nanoparticles for specific co-delivery of the WIP1 and miR-21 inhibitors into HER2+ breast tumors, leading to a profound reduction of tumor growth. These results demonstrate the great potential of the combined treatment of WIP1 and miR-21 inhibitors for the trastuzumab-resistant HER2+ breast cancers.

The 17q23 amplicon containing the WIP1 oncogene is frequently amplified in HER2+ breast cancer. Here they find MIR21 to be present in WIP1-containing amplicons, and report nanoparticle based co-delivery of WIP1 and miR-21 inhibitors to be effective in trastuzumab-resistant HER2+ breast cancer.

The SCN9A channel and plasma membrane depolarization promote cellular senescence through Rb pathway

Oncogenic signals lead to premature senescence in normal human cells causing a proliferation arrest and the elimination of these defective cells by immune cells. Oncogene‐induced senescence (OIS) prevents aberrant cell division and tumor initiation. In order to identify new regulators of OIS, we performed a loss‐of‐function genetic screen and identified that the loss of SCN9A allowed cells to escape from OIS. The expression of this sodium channel increased in senescent cells during OIS. This upregulation was mediated by NF‐κB transcription factors, which are well‐known regulators of senescence. Importantly, the induction of SCN9A by an oncogenic signal or by p53 activation led to plasma membrane depolarization, which in turn, was able to induce premature senescence. Computational and experimental analyses revealed that SCN9A and plasma membrane depolarization mediated the repression of mitotic genes through a calcium/Rb/E2F pathway to promote senescence. Taken together, our work delineates a new pathway, which involves the NF‐κB transcription factor, SCN9A expression, plasma membrane depolarization, increased calcium, the Rb/E2F pathway and mitotic gene repression in the regulation of senescence. This work thus provides new insight into the involvement of ion channels and plasma membrane potential in the control of senescence.

Selective elimination of senescent cells by mitochondrial targeting is regulated by ANT2

Cellular senescence is a form of cell cycle arrest that limits the proliferative potential of cells, including tumour cells. However, inability of immune cells to subsequently eliminate senescent cells from the organism may lead to tissue damage, inflammation, enhanced carcinogenesis and development of age-related diseases. We found that the anticancer agent mitochondria-targeted tamoxifen (MitoTam), unlike conventional anticancer agents, kills cancer cells without inducing senescence in vitro and in vivo. Surprisingly, it also selectively eliminates both malignant and non-cancerous senescent cells. In naturally aged mice treated with MitoTam for 4 weeks, we observed a significant decrease of senescence markers in all tested organs compared to non-treated animals. Mechanistically, we found that the susceptibility of senescent cells to MitoTam is linked to a very low expression level of adenine nucleotide translocase-2 (ANT2), inherent to the senescent phenotype. Restoration of ANT2 in senescent cells resulted in resistance to MitoTam, while its downregulation in non-senescent cells promoted their MitoTam-triggered elimination. Our study documents a novel, translationally intriguing role for an anticancer agent targeting mitochondria, that may result in a new strategy for the treatment of age-related diseases and senescence-associated pathologies.

Human cytomegalovirus glycoprotein B inhibits migration of breast cancer MDA-MB-231 cells and impairs TGF-β/Smad2/3 expression

Breast cancer is a leading cause of cancer-associated mortality in females worldwide and evidence suggests that human cytomegalovirus (HCMV) infection may be implicated in the progress of breast cancer. HCMV glycoprotein B (gB) is the most abundant envelope protein and serves an important role in host cell entry. The present study aimed to clarify the role of HCMV gB in breast cancer cells. A HCMV gB construct (UL55) was generated and stable vUL55 gene lentivirus-transfected MDA-MB-231 cells were established. Subsequently, the effect of HCMV gB on the apoptosis and proliferation of MDA-MB-231 cells was measured by flow cytometry and Cell Counting Kit-8 assay. Furthermore, whether HCMV gB may modulate MDA-MB-231 cell migration was examined using Transwell and cell scratch assays. In addition, alterations in HCMV gB-modulated protein levels of transforming growth factor-β (TGF-β) and Mothers against decapentaplegic homologs 2/3 (Smad2/3) were detected using western blot analysis. The results indicated that UL55 cDNA was stably transfected into MDA-MB-231 cells, and that HCMV gB protein was stably expressed. No significant differences in cell apoptosis and proliferation between transfected (231-GB-OE) and negative control (231-NC) cells were observed, while the rate of cell migration was significantly decreased in the 231-GB-OE cells compared with the 231-NC cells. Additionally, the expression level of TGF-β and phosphorylation level of Smad2/3 were also decreased in 231-GB-OE cells compared with the 231-NC cells. Although certain previous studies indicated that HCMV infection was associated with breast carcinogenesis, the results of the present study indicate that the envelope protein HCMV gB exhibits no effect on cell apoptosis and proliferation, but inhibits breast cancer cell migration. This may be due to downregulated TGF-β/Smad signaling. Taken together, these studies may assist in developing anti-TGF-β agents that contribute to tumor suppression.

Oxygen concentration modulates cellular senescence and autophagy in human trophoblast cells

PROBLEM

We investigated the effect of oxygen concentrations on cellular senescence and autophagy and examined the role of autophagy in human trophoblast cells.

METHOD OF STUDY

Human first-trimester trophoblast cells (Sw.71) were incubated under 21%, 5%, or 1% O₂ concentrations for 24 hours. We examined the extent of senescence caused using senescence-associated β-galactosidase (SA-β-Gal) and senescence-associated secretory phenotype (SASP) as markers. Moreover, we examined the role of autophagy in causing cellular senescence using an autophagy inhibitor (3-methyladenine, 3MA).

RESULTS

Physiological normoxia (5% O₂ ) decreased SA-β-Gal-positive cells and SASP including interleukin-6 (IL-6) and IL-8 compared with cultured cells in 21% O₂ . Pathophysiological hypoxia (1% O₂ ) caused cytotoxicity, including extracellular release of ATP and lactate dehydrogenase, and decreased senescence phenotypes. 3MA-treated trophoblast cells significantly suppressed senescence markers (SA-β-Gal-positive cells and SASP secretion) in O₂ -independent manner.

CONCLUSION

We conclude that O₂ concentration modulates cellular senescence phenotypes regulating autophagy in the human trophoblast cells. Moreover, inhibiting autophagy suppresses cellular senescence, suggesting that autophagy contributes to oxygen stress-induced cellular senescence.

The Immortal Senescence

Activation of oncogenic signaling paradoxically results in the permanent withdrawal from cell cycle and induction of senescence (oncogene-induced senescence (OIS)). OIS is a fail-safe mechanism used by the cells to prevent uncontrolled tumor growth, and, as such, it is considered as the first barrier against cancer. In order to progress, tumor cells thus need to first overcome the senescent phenotype. Despite the increasing attention gained by OIS in the past 20 years, this field is still rather young due to continuous emergence of novel pathways and processes involved in OIS. Among the many factors contributing to incomplete understanding of OIS are the lack of unequivocal markers for senescence and the complexity of the phenotypes revealed by senescent cells in vivo and in vitro. OIS has been shown to play major roles at both the cellular and organismal levels in biological processes ranging from embryonic development to barrier to cancer progression. Here we will briefly outline major advances in methodologies that are being utilized for induction, identification, and characterization of molecular processes in cells undergoing oncogene-induced senescence. The full description of such methodologies is provided in the corresponding chapters of the book.

[Cellular senescence and colorectal cancer]

Apart from proliferation and cell death, cellular senescence is an important response to numerous stress-associated stimuli. Originally described as an in vitro phenomenon, it is involved in various physiological and pathophysiological processes. For example, during the development of solid and generalized tumors, senescence induction poses an important barrier against disease progression. This could be demonstrated for malignant lymphomas, melanomas and various carcinomas using sophisticated animal models. However, senescent cells remain highly secretorically active and have a profound effect on neighboring cells as well as the entire tissue network. This article tries to provide insight into the current literature and discusses clinical implications and future applications.

TGF-β-Dependent Growth Arrest and Cell Migration in Benign and Malignant Breast Epithelial Cells Are Antagonistically Controlled by Rac1 and Rac1b

Despite improvements in diagnosis and treatment, breast cancer is still the most common cancer type among non-smoking females. TGF-β can inhibit breast cancer development by inducing cell cycle arrest in both, cancer cells and, as part of a senescence program in normal human mammary epithelial cells (HMEC). Moreover, TGF-β also drives cell migration and invasion, in part through the small GTPases Rac1 and Rac1b. Depletion of Rac1b or Rac1 and Rac1b in MDA-MB-231 or MDA-MB-435s breast cancer cells by RNA interference enhanced or suppressed, respectively, TGF-β1-induced migration/invasion. Rac1b depletion in MDA-MB-231 cells also increased TGF-β-induced p21^WAF1 expression and ERK1/2 phosphorylation. Senescent HMEC (P15/P16), when compared to their non-senescent counterparts (P11/P12), presented with dramatically increased migratory activity. These effects were paralleled by elevated expression of genes associated with TGF-β signaling and metastasis, downregulated Rac1b, and upregulated Rac1. Our data suggest that acquisition of a motile phenotype in HMEC resulted from enhanced autocrine TGF-β signaling, invasion/metastasis-associated gene expression, and a shift in the ratio of antimigratory Rac1b to promigratory Rac1. We conclude that although enhanced TGF-β signaling is considered antioncogenic in HMEC by suppressing oncogene-induced transformation, this occurs at the expense of a higher migration and invasion potential.

Haplodeficiency of Ataxia Telangiectasia Mutated Accelerates Heart Failure After Myocardial Infarction

Background

Cell senescence is involved in the process of organ damage and repair; however, the underlying molecular mechanism needs to be further explored.

Methods and Results

Senescence‐related genes (ie, p21, p53, and ataxia telangiectasia mutated [ATM]) were shown to be elevated after myocardial infarction (MI) in both mouse and human hearts. Ten‐ to 12‐week‐old male wild‐type littermates (ATM^+/+) and ATM heterozygous mice (ATM^+/−) were subjected to MI. Cardiac echography showed that ATM haplodeficiency did not affect the survival rate but aggravated heart failure at day 28 post MI. Histologic analysis showed increased fibrosis in the noninfarct area of ATM^+/− mice compared with that in ATM^+/+ mice. Senescence‐associated β‐galactosidase staining showed that the number of senescent fibroblasts was decreased when ATM was haplodeficient both in vivo and in vitro. Costaining of α‐smooth muscle actin with p53 or p19 showed fewer senescent myofibroblasts in ATM^+/− mouse hearts. Moreover, angiogenesis was also examined using the endothelial markers CD31 both at early (day 7) and late stages (day 28) after MI, and ATM haplodeficiency reduced angiogenesis after MI. Finally, cardiac fibroblasts were isolated from infarcted mouse heart and the medium were tested for its capacity of endothelial tubing formation, revealing that ATM haplodeficiency led to lower vascular endothelial growth factor production from cardiac fibroblast and reduced capacity of endothelial tube formation in vitro.

Conclusions

The present study shows that ATM haplodeficiency decreases fibroblast senescence and vascular endothelial growth factor production and impaired angiogenesis in response to MI, leading to accelerated heart failure.

Rapamycin enhances the antiproliferative effect of transforming growth factor-β on MCF-7 human breast cancer cells

Transforming growth factor-β (TGF-β), a well-known cytokine with pleiotropic biological functions, has an important role in the regulation of cellular proliferation. Rapamycin has specific antagonistic activity on the function of the mammalian target of the rapamycin signaling pathway. The cooperation of TGF-β and rapamycin on the proliferation of Michigan Cancer Foundation (MCF)-7 human breast cancer cells is unclear. The present study demonstrated that TGF-β had a growth-arresting effect on MCF-7 cancer cells. TGF-β stimulation resulted in the upregulation of several cyclin-dependent kinase inhibitors, including p14^ARF, p15^INK4b, p16I^NK4a and p21^WAF1/CIP1. The present study also demonstrated that rapamycin enhances the antiproliferative effect of TGF-β. The combination of rapamycin and TGF-β induced apoptosis of MCF-7 tumor cells. These findings advance the current understanding of the biological effects of TGF-β and rapamycin.

TGF-β Family Signaling in the Control of Cell Proliferation and Survival

The transforming growth factor β (TGF-β) family controls many fundamental aspects of cellular behavior. With advances in the molecular details of the TGF-β signaling cascade and its cross talk with other signaling pathways, we now have a more coherent understanding of the cytostatic program induced by TGF-β. However, the molecular mechanisms are still largely elusive for other cellular processes that are regulated by TGF-β and determine a cell's proliferation and survival, apoptosis, dormancy, autophagy, and senescence. The difficulty in defining TGF-β's roles partly stems from the context-dependent nature of TGF-β signaling. Here, we review our current understanding and recent progress on the biological effects of TGF-β at the cellular level, with the hope of providing a framework for understanding how cells respond to TGF-β signals in specific contexts, and why disruption of such mechanisms may result in different human diseases including cancer.

Oncostatin M promotes cancer cell plasticity through cooperative STAT3-SMAD3 signaling

Increasing evidence supports the idea that cancer cell plasticity promotes metastasis and tumor recurrence, resulting in patient mortality. While it is clear that the tumor microenvironment (TME) contributes to cancer cell plasticity, the specific TME factors most actively controlling plasticity remain largely unknown. Here, we performed a screen to identify TME cytokines and growth factors that promote epithelial–mesenchymal plasticity, and acquisition of cancer stem cell (CSC) properties. Of 28 TME cytokines and growth factors tested, we identified Oncostatin M (OSM) as the most potent inducer of mesenchymal/CSC properties. OSM-induced plasticity was Signal Transducer and Activator of Transcription 3 (STAT3)-dependent, and also required a novel intersection with transforming growth factor-β (TGF-β)/SMAD signaling. OSM/STAT3 activation promoted SMAD3 nuclear accumulation, DNA binding and induced SMAD3-dependent transcriptional activity. Suppression of TGF-β receptor activity or ablation of SMAD3 or SMAD4, but not SMAD2, strongly suppressed OSM/STAT3-mediated plasticity. Moreover, removal of OSM or inhibition of STAT3 or SMAD3 resulted in a marked reversion to a non-invasive, epithelial phenotype. We propose that targeted blockade of the STAT3/SMAD3 axis in tumor cells may represent a novel therapeutic approach to prevent the plasticity required for metastatic progression and tumor recurrence.

STAT3-mediated SMAD3 activation underlies Oncostatin M-induced Senescence

Cytokines in the developing tumor microenvironment (TME) can drive transformation and subsequent progression toward metastasis. Elevated levels of the Interleukin-6 (IL-6) family cytokine Oncostatin M (OSM) in the breast TME correlate with aggressive, metastatic cancers, increased tumor recurrence, and poor patient prognosis. Paradoxically, OSM engages a tumor-suppressive, Signal Transducer and Activator of Transcription 3 (STAT3)-dependent senescence response in normal and non-transformed human mammary epithelial cells (HMEC). Here, we identify a novel link between OSM-activated STAT3 signaling and the Transforming Growth Factor-β (TGF-β) signaling pathway that engages senescence in HMEC. Inhibition of functional TGF-β/SMAD signaling by expressing a dominant-negative TGF-β receptor, treating with a TGF-β receptor inhibitor, or suppressing SMAD3 expression using a SMAD3-shRNA prevented OSM-induced senescence. OSM promoted a protein complex involving activated-STAT3 and SMAD3, induced the nuclear localization of SMAD3, and enhanced SMAD3-mediated transcription responsible for senescence. In contrast, expression of MYC (c-MYC) from a constitutive promoter abrogated senescence and strikingly, cooperated with OSM to promote a transformed phenotype, epithelial-mesenchymal transition (EMT), and invasiveness. Our findings suggest that a novel STAT3/SMAD3-signaling axis is required for OSM-mediated senescence that is coopted during the transformation process to confer aggressive cancer cell properties. Understanding how developing cancer cells bypass OSM/STAT3/SMAD3-mediated senescence may help identify novel targets for future "pro-senescence" therapies aiming to reengage this hidden tumor-suppressive response.

Suppression of Transforming Growth Factor-β Signaling Delays Cellular Senescence and Preserves the Function of Endothelial Cells Derived from Human Pluripotent Stem Cells

Transplantation of vascular cells derived from human pluripotent stem cells (hPSCs) offers an attractive noninvasive method for repairing the ischemic tissues and for preventing the progression of vascular diseases. Here, we found that in a serum‐free condition, the proliferation rate of hPSC‐derived endothelial cells is quickly decreased, accompanied with an increased cellular senescence, resulting in impaired gene expression of endothelial nitric oxide synthase (eNOS) and impaired vessel forming capability in vitro and in vivo. To overcome the limited expansion of hPSC‐derived endothelial cells, we screened small molecules for specific signaling pathways and found that inhibition of transforming growth factor‐β (TGF‐β) signaling significantly retarded cellular senescence and increased a proliferative index of hPSC‐derived endothelial cells. Inhibition of TGF‐β signaling extended the life span of hPSC‐derived endothelial and improved endothelial functions, including vascular network formation on Matrigel, acetylated low‐density lipoprotein uptake, and eNOS expression. Exogenous transforming growth factor‐β1 increased the gene expression of cyclin‐dependent kinase inhibitors, p15^Ink4b, p16^Ink4a, and p21^CIP1, in endothelial cells. Conversely, inhibition of TGF‐β reduced the gene expression of p15^Ink4b, p16^Ink4a, and p21^CIP1. Our findings demonstrate that the senescence of newly generated endothelial cells from hPSCs is mediated by TGF‐β signaling, and manipulation of TGF‐β signaling offers a potential target to prevent vascular aging. Stem Cells Translational Medicine2017;6:589–600

Cellular senescence in aging and age-related disease: from mechanisms to therapy

Cellular senescence, a process that imposes permanent proliferative arrest on cells in response to various stressors, has emerged as a potentially important contributor to aging and age-related disease, and it is an attractive target for therapeutic exploitation. A wealth of information about senescence in cultured cells has been acquired over the past half century; however, senescence in living organisms is poorly understood, largely because of technical limitations relating to the identification and characterization of senescent cells in tissues and organs. Furthermore, newly recognized beneficial signaling functions of senescence suggest that indiscriminately targeting senescent cells or modulating their secretome for anti-aging therapy may have negative consequences. Here we discuss current progress and challenges in understanding the stressors that induce senescence in vivo, the cell types that are prone to senesce, and the autocrine and paracrine properties of senescent cells in the contexts of aging and age-related diseases as well as disease therapy.

HBP1-mediated Regulation of p21 Protein through the Mdm2/p53 and TCF4/EZH2 Pathways and Its Impact on Cell Senescence and Tumorigenesis

The activity of the CDK inhibitor p21 is associated with diverse biological activities, including cell proliferation, senescence, and tumorigenesis. However, the mechanisms governing transcription of p21 need to be extensively studied. In this study, we demonstrate that the high-mobility group box-containing protein 1 (HBP1) transcription factor is a novel activator of p21 that works as part of a complex mechanism during senescence and tumorigenesis. We found that HBP1 activates the p21 gene through enhancing p53 stability by inhibiting Mdm2-mediated ubiquitination of p53, a well known positive regulator of p21. HBP1 was also found to enhance p21 transcription by inhibiting Wnt/β-catenin signaling. We identified histone methyltransferase EZH2, the catalytic subunit of polycomb repressive complex 2, as a target of Wnt/β-catenin signaling. HBP1-mediated repression of EZH2 through Wnt/β-catenin signaling decreased the level of trimethylation of histone H3 at lysine 27 of overall and specific histone on the p21 promoter, resulting in p21 transactivation. Although intricate, the reciprocal partnership of HBP1 and p21 has exceptional importance. HBP1-mediated elevation of p21 through the Mdm2/p53 and TCF4/EZH2 pathways contributes to both cellular senescence and tumor inhibition. Together, our results suggest that the HBP1 transcription factor orchestrates a complex regulation of key genes during cellular senescence and tumorigenesis with an impact on protein ubiquitination and overall histone methylation state.

SIP1 is a downstream effector of GADD45G in senescence induction and growth inhibition of liver tumor cells

Cellular senescence evasion caused by the inactivation of tumor suppressive programs is implicated in tumor initiation and therapeutic resistance. Our previous study has shown that the downregulation of growth arrest and DNA damage 45G (GADD45G) contributes to senescence bypass in hepatocellular carcinoma (HCC). Here, we report that the Smad-interacting protein-1 (SIP1) is transcriptionally activated and functions critically in the GADD45G-induced tumor cell senescence. Knockdown of SIP1 significantly abrogates the suppressive effects of GADD45G on the growth of xenografted liver tumor in vivo. The essential role of SIP1 in GADD45G activities is further validated in the model of the proteasome inhibitor MG132-induced cell senescence. We further show that JNK but not p38 MAPK activation is involved in the GADD45G-mediated SIP1 upregulation, and that JNK inhibition counteracts the GADD45G-induced cellular senescence. More importantly, we show that GADD45G and SIP1 expression are coincidently downregulated in primary human HCC tissues. Together, our results establish that the dowregulation of GADD45G-SIP1 axis may contribute to cellular senescence evasion and HCC development.

Regulation of p53 during senescence in normal human keratinocytes

p53, the guardian of the genome, is a tumor suppressor protein and critical for the genomic integrity of the cells. Many studies have shown that intracellular level of p53 is enhanced during replicative senescence in normal fibroblasts, and the enhanced level of p53 is viewed as the cause of senescence. Here, we report that, unlike in normal fibroblasts, the level of intracellular p53 reduces during replicative senescence and oncogene-induced senescence (OIS) in normal human keratinocytes (NHKs). We found that the intracellular p53 level was also decreased in age-dependent manner in normal human epithelial tissues. Senescent NHKs exhibited an enhanced level of p16^INK4A, induced G₂ cell cycle arrest, and lowered the p53 expression and transactivation activity. We found that low level of p53 in senescent NHKs was due to reduced transcription of p53. The methylation status at the p53 promoter was not altered during senescence, but senescent NHKs exhibited notably lower level of acetylated histone 3 (H3) at the p53 promoter in comparison with rapidly proliferating cells. Moreover, p53 knockdown in rapidly proliferating NHKs resulted in the disruption of fidelity in repaired DNA. Taken together, our study demonstrates that p53 level is diminished during replicative senescence and OIS and that such diminution is associated with H3 deacetylation at the p53 promoter. The reduced intracellular p53 level in keratinocytes of the elderly could be a contributing factor for more frequent development of epithelial cancer in the elderly because of the loss of genomic integrity of cells.

Immortalization of normal human mammary epithelial cells in two steps by direct targeting of senescence barriers does not require gross genomic alterations

Telomerase reactivation and immortalization are critical for human carcinoma progression. However, little is known about the mechanisms controlling this crucial step, due in part to the paucity of experimentally tractable model systems that can examine human epithelial cell immortalization as it might occur in vivo. We achieved efficient non-clonal immortalization of normal human mammary epithelial cells (HMEC) by directly targeting the 2 main senescence barriers encountered by cultured HMEC. The stress-associated stasis barrier was bypassed using shRNA to p16^INK4; replicative senescence due to critically shortened telomeres was bypassed in post-stasis HMEC by c-MYC transduction. Thus, 2 pathologically relevant oncogenic agents are sufficient to immortally transform normal HMEC. The resultant non-clonal immortalized lines exhibited normal karyotypes. Most human carcinomas contain genomically unstable cells, with widespread instability first observed in vivo in pre-malignant stages; in vitro, instability is seen as finite cells with critically shortened telomeres approach replicative senescence. Our results support our hypotheses that: (1) telomere-dysfunction induced genomic instability in pre-malignant finite cells may generate the errors required for telomerase reactivation and immortalization, as well as many additional “passenger” errors carried forward into resulting carcinomas; (2) genomic instability during cancer progression is needed to generate errors that overcome tumor suppressive barriers, but not required per se; bypassing the senescence barriers by direct targeting eliminated a need for genomic errors to generate immortalization. Achieving efficient HMEC immortalization, in the absence of “passenger” genomic errors, should facilitate examination of telomerase regulation during human carcinoma progression, and exploration of agents that could prevent immortalization.

Extracellular matrix as a contextual determinant of transforming growth factor-β signaling in epithelial-mesenchymal transition and in cancer

Extracellular matrix (ECM) provides both structural support and contextual information to cells within tissues and organs. The combination of biochemical and biomechanical signals from the ECM modulates responses to extracellular signals toward differentiation, proliferation, or apoptosis; alterations in the ECM are necessary for development and remodeling processes, but aberrations in the composition and organization of ECM are associated with disease pathology and can predispose to development of cancer. The primary cell surface sensors of the ECM are the integrins, which provide the physical connection between the ECM and the cytoskeleton and also convey biochemical information about the composition of the ECM. Transforming growth factor-β (TGF-β) is an extracellular signaling molecule that is a powerful controller of a variety of cellular functions, and that has been found to induce very different outcomes according to cell type and cellular context. It is becoming clear that ECM-mediated signaling through integrins is reciprocally influenced by TGF-β: integrin expression, activation, and responses are affected by cellular exposure to TGF-β, and TGF-β activation and cellular responses are in turn controlled by signaling from the ECM through integrins. Epithelial-mesenchymal transition (EMT), a physiological process that is activated by TGF-β in normal development and in cancer, is also affected by the composition and structure of the ECM. Here, we will outline how signaling from the ECM controls the contextual response to TGF-β, and how this response is selectively modulated during disease, with an emphasis on recent findings, current challenges, and future opportunities.

Multidrug resistance protein 3 loss promotes tumor formation by inducing senescence escape

Oncogenic-stress-induced senescence (OIS) is a stress response allowing normal cells, when receiving oncogenic signals, to stably arrest their proliferation. OIS thus acts to prevent aberrant cell proliferation and tumor formation. To identify novel tumor suppressive pathways, we have recently completed a loss-of-function genetic screen to identify novel genes promoting escape from OIS and thus, potentially, tumor formation when their functions are lost. Using this approach, we unexpectedly found that loss of function of the multidrug resistance protein 3 (MRP3 or ABCC3) promotes escape from OIS in human epithelial cells. Importantly, ABCC3 expression is reduced in human skin tumors, and ABCC3-knockout mice display increased sensitivity to RAS-induced skin carcinogenesis, concomitantly with decreased OIS. ABCC3 participates in resistance to chemotherapy via its transporter activity. Our data show that this transporter activity is involved in ABCC3-induced senescence, demonstrating that this protein has a complex role in cancer, since its loss of function may promote escape from OIS and tumor formation whereas its gain of function promotes resistance to chemotherapy.

Breast cancer anti-estrogen resistance 3 inhibits transforming growth factor β/Smad signaling and associates with favorable breast cancer disease outcomes

INTRODUCTION

This study helps to define the implications of breast cancer anti-estrogen resistance 3 (BCAR3) in breast cancer and extends the current understanding of its molecular mechanism of action. BCAR3 has been shown to promote cell proliferation, migration and attachment to extracellular matrix components. However, in a cohort of metastatic breast cancer patients who received tamoxifen treatment, high BCAR3 mRNA levels were associated with favorable progression-free survival outcome. These results suggest that, besides its established roles, BCAR3 may have additional mechanisms of action that regulate breast cancer aggressive phenotype. In this study, we investigated whether BCAR3 is a novel antagonist of the canonical transforming growth factor β (TGFβ) pathway, which induces potent migration and invasion responses in breast cancer cells.

METHODS

We surveyed functional genomics databases for correlations between BCAR3 expression and disease outcomes of breast cancer patients. We also studied how BCAR3 could regulate the TGFβ/Smad signaling axis using Western blot analysis, coimmunoprecipitation and luciferase assays. In addition, we examined whether BCAR3 could modulate TGFβ-induced cell migration and invasion by using an automated imaging system and a confocal microscopy imaging-based matrix degradation assay, respectively.

RESULTS

Relatively low levels of BCAR3 expression in primary breast tumors correlate with poor distant metastasis-free survival and relapse-free survival outcomes. We also found a strong correlation between the loss of heterozygosity at BCAR3 gene alleles and lymph node invasion in human breast cancer, further suggesting a role for BCAR3 in preventing disease progression. In addition, we found BCAR3 to inhibit Smad activation, Smad-mediated gene transcription, Smad-dependent cell migration and matrix digestion in breast cancer cells. Furthermore, we found BCAR3 to be downregulated by TGFβ through proteasome degradation, thus defining a novel positive feedback loop mechanism downstream of the TGFβ/Smad signaling pathway.

CONCLUSION

BCAR3 is considered to be associated with aggressive breast cancer phenotypes. However, our results indicate that BCAR3 acts as a putative suppressor of breast cancer progression by inhibiting the prometastatic TGFβ/Smad signaling pathway in invasive breast tumors. These data provide new insights into BCAR3's molecular mechanism of action and highlight BCAR3 as a novel TGFβ/Smad antagonist in breast cancer.

Exome-wide mutation profile in benzo[a]pyrene-derived post-stasis and immortal human mammary epithelial cells

Genetic mutations are known to drive cancer progression and certain tumors have mutation signatures that reflect exposures to environmental carcinogens. Benzo[a]pyrene (BaP) has a known mutation signature and has proven capable of inducing changes to DNA sequence that drives normal pre-stasis human mammary epithelial cells (HMEC) past a first tumor suppressor barrier (stasis) and toward immortality. We analyzed normal, pre-stasis HMEC, three independent BaP-derived post-stasis HMEC strains (184Aa, 184Be, 184Ce) and two of their immortal derivatives(184A1 and 184BE1) by whole exome sequencing. The independent post-stasis strains exhibited between 93 and 233 BaP-induced mutations in exons. Seventy percent of the mutations were C:G>A:T transversions, consistent with the known mutation spectrum of BaP. Mutations predicted to impact protein function occurred in several known and putative cancer drivers including p16, PLCG1, MED12, TAF1 in 184Aa; PIK3CG, HSP90AB1, WHSC1L1, LCP1 in 184Be and FANCA, LPP in 184Ce. Biological processes that typically harbor cancer driver mutations such as cell cycle, regulation of cell death and proliferation, RNA processing, chromatin modification and DNA repair were found to have mutations predicted to impact function in each of the post-stasis strains. Spontaneously immortalized HMEC lines derived from two of the BaP-derived post-stasis strains shared greater than 95% of their BaP-induced mutations with their precursor cells. These immortal HMEC had 10 or fewer additional point mutations relative to their post-stasis precursors, but acquired chromosomal anomalies during immortalization that arose independent of BaP. The results of this study indicate that acute exposures of HMEC to high dose BaP recapitulate mutation patterns of human tumors and can induce mutations in a number of cancer driver genes.

Are there roles for brain cell senescence in aging and neurodegenerative disorders?

The term cellular senescence was introduced more than five decades ago to describe the state of growth arrest observed in aging cells. Since this initial discovery, the phenotypes associated with cellular senescence have expanded beyond growth arrest to include alterations in cellular metabolism, secreted cytokines, epigenetic regulation and protein expression. Recently, senescence has been shown to play an important role in vivo not only in relation to aging, but also during embryonic development. Thus, cellular senescence serves different purposes and comprises a wide range of distinct phenotypes across multiple cell types. Whether all cell types, including post-mitotic neurons, are capable of entering into a senescent state remains unclear. In this review we examine recent data that suggest that cellular senescence plays a role in brain aging and, notably, may not be limited to glia but also neurons. We suggest that there is a high level of similarity between some of the pathological changes that occur in the brain in Alzheimer's and Parkinson's diseases and those phenotypes observed in cellular senescence, leading us to propose that neurons and glia can exhibit hallmarks of senescence previously documented in peripheral tissues.

MYC synergizes with activated BRAFV600E in mouse lung tumor development by suppressing senescence

The activated RAS/RAF cascade plays a crucial role in lung cancer, but is also known to induce cellular senescence, a major barrier imposed on tumor cells early in tumorigenesis. MYC is a key factor in suppression of RAS/BRAF(V600E)-induced senescence in vitro. However, it is still unclear whether MYC has the same role during tumor development in vivo. Using a conditional, compound knock-in model of Cre-activated BRAF(V600E) and tamoxifen-regulatable MycER, we show that tamoxifen-induced activation of MYC accelerated the onset and increased the number and size of BRAF(V600E)-driven adenomas in a dose-dependent manner, resulting in reduced survival. Furthermore, MYC activation leads to reduced expression of the senescence markers p16(INK4A), p21(CIP1), and H3K9me3-containing heterochromatin foci, and an increased percentage of Ki67(+) tumor cells. This suggests that MYC already early during tumor formation suppresses a BRAF(V600E)-induced senescence-like state. Initial activation of MYC followed by tamoxifen withdrawal still resulted in an increased number of tumors and reduced survival. However, these tumors were of smaller size, showed increased expression of p16(INK4A) and p21(CIP1), and reduced number of Ki67(+) cells, indicating that MYC inactivation restores BRAF(V600E)-induced senescence. Surprisingly, MYC activation did not promote adenoma to carcinoma progression. This suggests that senescence suppression by MYC is a discrete step in tumor development important for sustained tumor growth but preceding malignant transformation and that additional oncogenic events are required for carcinoma development and metastasis. These findings contribute to our understanding of the neoplastic transformation process, with implications for future treatment strategies.

Protein O-glucosyltransferase 1 overexpression downregulates p16 in BT474 human breast cancer cells

Protein O-glucosyltransferase 1 (POGLUT1) is a novel gene that was initially isolated and identified from the bone marrow cells of patients with myelodysplastic syndrome/acute myeloid leukemia. Previous findings have suggested that POGLUT1 promotes the proliferation of U937 human tissue lymphoma cells. Furthermore, POGLUT1 has been identified in other tissues, including the mammary glands, lymph nodes, intestine, liver and spleen. In the present study, in order to investigate the function and target of POGLUT1 in BT474 breast cancer cells, the effect of POGLUT1 on cell proliferation, differentiation, apoptosis and key proteins in the transforming growth factor (TGF)-β1 signaling pathway was investigated in BT474 cells. The overexpression of POGLUT1 in the presence of TGF-β1 was found to significantly enhance cell viability. Flow cytometric and quantitative polymerase chain reaction analyses revealed that POGLUT1 had an effect on the cell cycle and inhibited the TGF-β1-induced transcriptional upregulation of p16, a major cyclin-dependent kinase inhibitor (CDKI). Furthermore, phosphorylated (p)-Smad3, which has a key role in mediating the TGF-β antiproliferative response, was greatly inhibited by exogenous POGLUT1, suggesting a role for POGLUT1 in the TGF-β1-mediated signaling pathway in the BT474 cell cycle. However, no significant changes were observed in the expression of other CDKIs or in cell apoptosis. The findings of the present study show that the increase in BT474 cell viabilty induced by POGLUT1 is associated with POGLUT1-induced inhibition of the transcriptional upregulation of p16 by TGF-β1, which may be a result of the inhibition of p-Smad3.

Tumor microenvironmental signaling elicits epithelial-mesenchymal plasticity through cooperation with transforming genetic events

Epithelial-to-mesenchymal transition (EMT) facilitates the escape of epithelial cancer cells from the primary tumor site, which is a key event early in metastasis. Here, we explore how extrinsic, tumor microenvironmental cytokines cooperate with intrinsic, genetic changes to promote EMT in human mammary epithelial cells (HMECs). Viral transduction of transforming genetic events into HMECs routinely generated two distinct cell populations. One population retained epithelial characteristics, while an emergent population spontaneously acquired a mesenchymal morphology and properties associated with cancer stem cells (CSCs). Interestingly, the spontaneous mesenchymal/CSCs were unable to differentiate and lacked epithelial-mesenchymal plasticity. In contrast, exposure of the transformed HMECs retaining epithelial characteristics to exogenous transforming growth factor-β (TGF-β) generated a mesenchymal/CSC population with remarkable plasticity. The TGF-β-induced mesenchymal/CSC population was dependent on the continued presence of TGF-β. Removal of TGF-β or pharmacologic or genetic inhibition of TGF-β/SMAD signaling led to the reversion of mesenchymal/CSC to epithelial/non-CSC. Our results demonstrate that targeting exogenous cytokine signaling disrupts epithelial-mesenchymal plasticity and may be an effective strategy to inhibit the emergence of circulating tumor cells. The model of epithelial-mesenchymal plasticity we describe here can be used to identify novel tumor microenvironmental factors and downstream signaling that cooperate with intrinsic genetic changes to drive metastasis. Understanding the interaction between extrinsic and intrinsic factors that regulate epithelial-mesenchymal plasticity will allow the development of new therapies that target tumor microenvironmental signals to reduce metastasis.