From shape-shifting embryonic cells to oncology: The fascinating history of epithelial mesenchymal transition.

Epithelial-to-mesenchymal transition or transformation (EMT) is a cell shape-changing process that is utilized repeatedly throughout embryogenesis and is critical to the attainment of a precise body plan. In the adult, EMT is observed under both normal and pathological conditions, such as during normal wounding healing, during development of certain fibrotic states and vascular anomalies, as well as in some cancers when malignant cells progress to become more aggressive, invasive, and metastatic. Epithelia derived from any of the three embryonic germ layers can undergo EMT, including those derived from mesoderm, such as endothelial cells (sometimes termed Endo-MT) and those derived from endoderm such as fetal liver stroma. At the cellular level, EMT is defined as the transformation of epithelial cells towards a mesenchymal phenotype and is marked by attenuation of expression of epithelial markers and de novo expression of mesenchymal markers. This process is induced by extracellular factors and can be reversible, resulting in mesenchymal-to-epithelial transformation (MET). It is now clear that a cell can simultaneously express properties of both epithelia and mesenchyme, and that such transitional cell-types drive tumor cell heterogeneity, an important aspect of cancer progression, development of a stem-like cell state, and drug resistance. Here we review some of the earliest studies demonstrating the existence of EMT during embryogenesis and discuss the discovery of the extracellular factors and intracellular signaling pathways that contribute to this process, with components of the TGFß signaling superfamily playing a prominent role. We mention early controversies surrounding in vivo EMT during embryonic development and in adult diseased states, and the maturation of the field to a stage wherein targeting EMT to control disease states is an aspirational goal.

Gene networks reveal stem-cell state convergence during preneoplasia and progression to malignancy in multistage skin carcinogenesis.

Adult mammalian stem cells play critical roles in normal tissue homeostasis, as well as in tumor development, by contributing to cell heterogeneity, plasticity, and development of drug resistance. The relationship between different types of normal and cancer stem cells is highly controversial and poorly understood. Here, we carried out gene expression network analysis of normal and tumor samples from genetically heterogeneous mice to create network metagenes for visualization of stem-cell networks, rather than individual stem-cell markers, at the single-cell level during multistage carcinogenesis. We combined this approach with lineage tracing and single-cell RNASeq of stem cells and their progeny, identifying a previously unrecognized hierarchy in which Lgr6+ stem cells from tumors generate progeny that express a range of other stem-cell markers including Sox2, Pitx1, Foxa1, Klf5, and Cd44. Our data identify a convergence of multiple stem-cell and tumor-suppressor pathways in benign tumor cells expressing markers of lineage plasticity and oxidative stress. This same single-cell population expresses network metagenes corresponding to markers of cancer drug resistance in human tumors of the skin, lung and prostate. Treatment of mouse squamous carcinomas in vivo with the chemotherapeutic cis-platin resulted in elevated expression of the genes that mark this cell population. Our data have allowed us to create a simplified model of multistage carcinogenesis that identifies distinct stem-cell states at different stages of tumor progression, thereby identifying networks involved in lineage plasticity, drug resistance, and immune surveillance, providing a rich source of potential targets for cancer therapy.

TGFß: Signaling Blockade for Cancer Immunotherapy.

Discovered over four decades ago, transforming growth factor ß (TGFß) is a potent pleiotropic cytokine that has context-dependent effects on most cell types. It acts as a tumor suppressor in some cancers and/or supports tumor progression and metastasis through its effects on the tumor stroma and immune microenvironment. In TGFß-responsive tumors it can promote invasion and metastasis through epithelial-mesenchymal transformation, the appearance of cancer stem cell features, and resistance to many drug classes, including checkpoint blockade immunotherapies. Here we consider the biological activities of TGFß action on different cells of relevance toward improving immunotherapy outcomes for patients, with a focus on the adaptive immune system. We discuss recent advances in the development of drugs that target the TGFß signaling pathway in a tumor-specific or cell type-specific manner to improve the therapeutic window between response rates and adverse effects.

Integrin αvß8 on T cells suppresses anti-tumor immunity in multiple models and is a promising target for tumor immunotherapy.

αvß8 integrin, a key activator of transforming growth factor ß (TGF-ß), inhibits anti-tumor immunity. We show that a potent blocking monoclonal antibody against αvß8 (ADWA-11) causes growth suppression or complete regression in syngeneic models of squamous cell carcinoma, mammary cancer, colon cancer, and prostate cancer, especially when combined with other immunomodulators or radiotherapy. αvß8 is expressed at the highest levels in CD4+CD25+ T cells in tumors, and specific deletion of ß8 from T cells is as effective as ADWA-11 in suppressing tumor growth. ADWA-11 increases expression of a suite of genes in tumor-infiltrating CD8+ T cells normally inhibited by TGF-ß and involved in tumor cell killing, including granzyme B and interferon-γ. The in vitro cytotoxic effect of tumor CD8 T cells is inhibited by CD4+CD25+ cells, and this suppressive effect is blocked by ADWA-11. These findings solidify αvß8 integrin as a promising target for cancer immunotherapy.

TGFß biology in cancer progression and immunotherapy.

TGFß signalling has key roles in cancer progression: most carcinoma cells have inactivated their epithelial antiproliferative response and benefit from increased TGFß expression and autocrine TGFß signalling through effects on gene expression, release of immunosuppressive cytokines and epithelial plasticity. As a result, TGFß enables cancer cell invasion and dissemination, stem cell properties and therapeutic resistance. TGFß released by cancer cells, stromal fibroblasts and other cells in the tumour microenvironment further promotes cancer progression by shaping the architecture of the tumour and by suppressing the antitumour activities of immune cells, thus generating an immunosuppressive environment that prevents or attenuates the efficacy of anticancer immunotherapies. The repression of TGFß signalling is therefore considered a prerequisite and major avenue to enhance the efficacy of current and forthcoming immunotherapies, including in tumours comprising cancer cells that are not TGFß responsive. Herein, we introduce the mechanisms underlying TGFß signalling in tumours and their microenvironment and discuss approaches to inhibit these signalling mechanisms as well as the use of these approaches in cancer immunotherapies and their potential adverse effects.

Chronic TGF-ß exposure drives stabilized EMT, tumor stemness, and cancer drug resistance with vulnerability to bitopic mTOR inhibition.

Tumors comprise cancer stem cells (CSCs) and their heterogeneous progeny within a stromal microenvironment. In response to transforming growth factor-ß (TGF-ß), epithelial and carcinoma cells undergo a partial or complete epithelial-mesenchymal transition (EMT), which contributes to cancer progression. This process is seen as reversible because cells revert to an epithelial phenotype upon TGF-ß removal. However, we found that prolonged TGF-ß exposure, mimicking the state of in vivo carcinomas, promotes stable EMT in mammary epithelial and carcinoma cells, in contrast to the reversible EMT induced by a shorter exposure. The stabilized EMT was accompanied by stably enhanced stem cell generation and anticancer drug resistance. Furthermore, prolonged TGF-ß exposure enhanced mammalian target of rapamycin (mTOR) signaling. A bitopic mTOR inhibitor repressed CSC generation, anchorage independence, cell survival, and chemoresistance and efficiently inhibited tumorigenesis in mice. These results reveal a role for mTOR in the stabilization of stemness and drug resistance of breast cancer cells and position mTOR inhibition as a treatment strategy to target CSCs.

Enhanced TGF-ß Signaling Contributes to the Insulin-Induced Angiogenic Responses of Endothelial Cells.

Angiogenesis, the development of new blood vessels, is a key process in disease. We reported that insulin promotes translocation of transforming growth factor ß (TGF-ß) receptors to the plasma membrane of epithelial and fibroblast cells, thus enhancing TGF-ß responsiveness. Since insulin promotes angiogenesis, we addressed whether increased autocrine TGF-ß signaling participates in endothelial cell responses to insulin. We show that insulin enhances TGF-ß responsiveness and autocrine TGF-ß signaling in primary human endothelial cells, by inducing a rapid increase in cell surface TGF-ß receptor levels. Autocrine TGF-ß/Smad signaling contributed substantially to insulin-induced gene expression associated with angiogenesis, including TGF-ß target genes encoding angiogenic mediators; was essential for endothelial cell migration; and participated in endothelial cell invasion and network formation. Blocking TGF-ß signaling impaired insulin-induced microvessel outgrowth from neonatal aortic rings and modified insulin-stimulated blood vessel formation in zebrafish. We conclude that enhanced autocrine TGF-ß signaling is integral to endothelial cell and angiogenic responses to insulin.

Smad3-mediated recruitment of the methyltransferase SETDB1/ESET controls Snail1 expression and epithelial-mesenchymal transition.

During epithelial-mesenchymal transition (EMT), reprogramming of gene expression is accompanied by histone modifications. Whether EMT-promoting signaling directs functional changes in histone methylation has not been established. We show here that the histone lysine methyltransferase SETDB1 represses EMT and that, during TGF-ß-induced EMT, cells attenuate SETDB1 expression to relieve this inhibition. SETDB1 also controls stem cell generation, cancer cell motility, invasion, metastatic dissemination, as well as sensitivity to certain cancer drugs. These functions may explain the correlation of breast cancer patient survival with SETDB1 expression. At the molecular level, TGF-ß induces SETDB1 recruitment by Smad3, to repress Smad3/4-activated transcription of SNAI1, encoding the EMT "master" transcription factor SNAIL1. Suppression of SNAIL1-mediated gene reprogramming by SETDB1 occurs through H3K9 methylation at the SNAI1 gene that represses its H3K9 acetylation imposed by activated Smad3/4 complexes. SETDB1 therefore defines a TGF-ß-regulated balance between histone methylation and acetylation that controls EMT.

Targeting TGF-ß Signaling for Therapeutic Gain.

Transforming growth factor ßs (TGF-ßs) are closely related ligands that have pleiotropic activity on most cell types of the body. They act through common heterotetrameric TGF-ß type II and type I transmembrane dual specificity kinase receptor complexes, and the outcome of signaling is context-dependent. In normal tissue, they serve a role in maintaining homeostasis. In many diseased states, particularly fibrosis and cancer, TGF-ß ligands are overexpressed and the outcome of signaling is diverted toward disease progression. There has therefore been a concerted effort to develop drugs that block TGF-ß signaling for therapeutic benefit. This review will cover the basics of TGF-ß signaling and its biological activities relevant to oncology, present a summary of pharmacological TGF-ß blockade strategies, and give an update on preclinical and clinical trials for TGF-ß blockade in a variety of solid tumor types.

ShcA Protects against Epithelial-Mesenchymal Transition through Compartmentalized Inhibition of TGF-ß-Induced Smad Activation.

Epithelial-mesenchymal transition (EMT) is a normal cell differentiation event during development and contributes pathologically to carcinoma and fibrosis progression. EMT often associates with increased transforming growth factor-ß (TGF-ß) signaling, and TGF-ß drives EMT, in part through Smad-mediated reprogramming of gene expression. TGF-ß also activates the Erk MAPK pathway through recruitment and Tyr phosphorylation of the adaptor protein ShcA by the activated TGF-ß type I receptor. We found that ShcA protects the epithelial integrity of nontransformed cells against EMT by repressing TGF-ß-induced, Smad-mediated gene expression. p52ShcA competed with Smad3 for TGF-ß receptor binding, and down-regulation of ShcA expression enhanced autocrine TGF-ß/Smad signaling and target gene expression, whereas increased p52ShcA expression resulted in decreased Smad3 binding to the TGF-ß receptor, decreased Smad3 activation, and increased Erk MAPK and Akt signaling. Furthermore, p52ShcA sequestered TGF-ß receptor complexes to caveolin-associated membrane compartments, and reducing ShcA expression enhanced the receptor localization in clathrin-associated membrane compartments that enable Smad activation. Consequently, silencing ShcA expression induced EMT, with increased cell migration, invasion, and dissemination, and increased stem cell generation and mammosphere formation, dependent upon autocrine TGF-ß signaling. These findings position ShcA as a determinant of the epithelial phenotype by repressing TGF-ß-induced Smad activation through differential partitioning of receptor complexes at the cell surface.

Matters of context guide future research in TGFß superfamily signaling.

The highly conserved wiring of the SMAD-dependent transforming growth factor ß (TGFß) superfamily signaling pathway has been mapped over the last 20 years after molecular discovery of its component parts. Numerous alternative TGFß-activated signaling pathways that elicit SMAD-independent biological responses also exist. However, the molecular mechanisms responsible for the renowned context dependency of TGFß signaling output remains an active and often confounding area of research, providing a prototype relevant to regulation of other signaling pathways. Highlighting discoveries presented at the 9th FASEB meeting, The TGFß Superfamily: Signaling in Development and Disease (July 12-17th 2015 in Snowmass, Colorado), this Review outlines research into the rich contextual nature of TGFß signaling output and offers clues for therapeutic advances.

Genetic variation in the functional ENG allele inherited from the non-affected parent associates with presence of pulmonary arteriovenous malformation in hereditary hemorrhagic telangiectasia 1 (HHT1) and may influence expression of PTPN14.

HHT shows clinical variability within and between families. Organ site and prevalence of arteriovenous malformations (AVMs) depend on the HHT causative gene and on environmental and genetic modifiers. We tested whether variation in the functional ENG allele, inherited from the unaffected parent, alters risk for pulmonary AVM in HHT1 mutation carriers who are ENG haploinsufficient. Genetic association was found between rs10987746 of the wild type ENG allele and presence of pulmonary AVM [relative risk = 1.3 (1.0018-1.7424)]. The rs10987746-C at-risk allele associated with lower expression of ENG RNA in a panel of human lymphoblastoid cell lines (P = 0.004). Moreover, in angiogenically active human lung adenocarcinoma tissue, but not in uninvolved quiescent lung, rs10987746-C was correlated with expression of PTPN14 (P = 0.004), another modifier of HHT. Quantitative TAQMAN expression analysis in a panel of normal lung tissues from 69 genetically heterogeneous inter-specific backcross mice, demonstrated strong correlation between expression levels of Eng, Acvrl1, and Ptpn14 (r2 = 0.75-0.9, P < 1 × 10(-12)), further suggesting a direct or indirect interaction between these three genes in lung in vivo. Our data indicate that genetic variation within the single functional ENG gene influences quantitative and/or qualitative differences in ENG expression that contribute to risk of pulmonary AVM in HHT1, and provide correlative support for PTPN14 involvement in endoglin/ALK1 lung biology in vivo. PTPN14 has been shown to be a negative regulator of Yap/Taz signaling, which is implicated in mechanotransduction, providing a possible molecular link between endoglin/ALK1 signaling and mechanical stress. EMILIN2, which showed suggestive genetic association with pulmonary AVM, is also reported to interact with Taz in angiogenesis. Elucidation of the molecular mechanisms regulating these interactions in endothelial cells may ultimately provide more rational choices for HHT therapy.

Genetic variants of Adam17 differentially regulate TGFß signaling to modify vascular pathology in mice and humans.

Outcome of TGFß1 signaling is context dependent and differs between individuals due to germ-line genetic variation. To explore innate genetic variants that determine differential outcome of reduced TGFß1 signaling, we dissected the modifier locus Tgfbm3, on mouse chromosome 12. On a NIH/OlaHsd genetic background, the Tgfbm3b(C57) haplotype suppresses prenatal lethality of Tgfb1(-/-) embryos and enhances nuclear accumulation of mothers against decapentaplegic homolog 2 (Smad2) in embryonic cells. Amino acid polymorphisms within a disintegrin and metalloprotease 17 (Adam17) can account, at least in part, for this Tgfbm3b effect. ADAM17 is known to down-regulate Smad2 signaling by shedding the extracellular domain of TGFßRI, and we show that the C57 variant is hypomorphic for down-regulation of Smad2/3-driven transcription. Genetic variation at Tgfbm3 or pharmacological inhibition of ADAM17, modulates postnatal circulating endothelial progenitor cell (CEPC) numbers via effects on TGFßRI activity. Because CEPC numbers correlate with angiogenic potential, this suggests that variant Adam17 is an innate modifier of adult angiogenesis, acting through TGFßR1. To determine whether human ADAM17 is also polymorphic and interacts with TGFß signaling in human vascular disease, we investigated hereditary hemorrhagic telangiectasia (HHT), which is caused by mutations in TGFß/bone morphogenetic protein receptor genes, ENG, encoding endoglin (HHT1), or ACVRL1 encoding ALK1 (HHT2), and considered a disease of excessive abnormal angiogenesis. HHT manifests highly variable incidence and severity of clinical features, ranging from small mucocutaneous telangiectases to life-threatening visceral and cerebral arteriovenous malformations (AVMs). We show that ADAM17 SNPs associate with the presence of pulmonary AVM in HHT1 but not HHT2, indicating genetic variation in ADAM17 can potentiate a TGFß-regulated vascular disease.

Epistatic interactions between Tgfb1 and genetic loci, Tgfbm2 and Tgfbm3, determine susceptibility to an asthmatic stimulus.

TGFß activation and signaling have been extensively studied in experimental models of allergen-induced asthma as potential therapeutic targets during chronic or acute phases of the disease. Outcomes of experimental manipulation of TGFß activity have been variable, in part due to use of different model systems. Using an ovalbumin (OVA)-induced mouse model of asthma, we here show that innate variation within TGFß1 genetic modifier loci, Tgfbm2 and Tgfbm3, alters disease susceptibility. Specifically, Tgfbm2(129) and Tgfbm3(C57) synergize to reverse accentuated airway hyperresponsiveness (AHR) caused by low TGFß1 levels in Tgfb1(+/-) mice of the NIH/OlaHsd strain. Moreover, epistatic interaction between Tgfbm2(129) and Tgfbm3(C57) uncouples the inflammatory response to ovalbumin from those of airway remodeling and airway hyperresponsiveness, illustrating independent genetic control of these responses. We conclude that differential inheritance of genetic variants of Tgfbm genes alters biological responses to reduced TGFß1 signaling in an experimental asthma model. TGFß antagonists for treatment of lung diseases might therefore give diverse outcomes, dependent on genetic variation.

Targeting the TGFß signalling pathway in disease.

Many drugs that target transforming growth factor-ß (TGFß) signalling have been developed, some of which have reached Phase III clinical trials for a number of disease applications. Preclinical and clinical studies indicate the utility of these agents in fibrosis and oncology, particularly in augmentation of existing cancer therapies, such as radiation and chemotherapy, as well as in tumour vaccines. There are also reports of specialized applications, such as the reduction of vascular symptoms of Marfan syndrome. Here, we consider why the TGFß signalling pathway is a drug target, the potential clinical applications of TGFß inhibition, the issues arising with anti-TGFß therapy and how these might be tackled using personalized approaches to dosing, monitoring of biomarkers as well as brief and/or localized drug-dosing regimens.

Complexities of TGF-ß targeted cancer therapy.

Many advanced tumors produce excessive amounts of Transforming Growth Factor-ß (TGF-ß) which, in normal epithelial cells, is a potent growth inhibitor. However, in oncogenically activated cells, the homeostatic action of TGF-ß is often diverted along alternative pathways. Hence, TGF-ß signaling elicits protective or tumor suppressive effects during the early growth-sensitive stages of tumorigenesis. However, later in tumor development when carcinoma cells become refractory to TGF-ß-mediated growth inhibition, the tumor cell responds by stimulating pathways with tumor progressing effects. At late stages of malignancy, tumor progression is driven by TGF-ß overload. The tumor microenvironment is a target of TGF-ß action that stimulates tumor progression via pro-tumorigenic effects on vascular, immune, and fibroblastic cells. Bone is one of the richest sources of TGF-ß in the body and a common site for dissemination of breast cancer metastases. Osteoclastic degradation of bone matrix, which accompanies establishment and growth of metastases, triggers further release of bone-derived TGF-ß. This leads to a vicious positive feedback of tumor progression, driven by ever increasing levels of TGF-ß released from both the tumor and bone matrix. It is for this reason, that pharmaceutical companies have developed therapeutic agents that block TGF-ß signaling. Nonetheless, the choice of drug design and dosing strategy can affect the efficacy of TGF-ß therapeutics. This review will describe pre-clinical and clinical data of four major classes of TGF-ß inhibitor, namely i) ligand traps, ii) antisense oligonucleotides, iii) receptor kinase inhibitors and iv) peptide aptamers. Long term dosing strategies with TGF-ß inhibitors may be ill-advised, since this class of drug has potentially highly pleiotropic activity, and development of drug resistance might potentiate tumor progression. Current paradigms for the use of TGF-ß inhibitors in oncology have therefore moved towards the use of combinatorial therapies and short term dosing, with considerable promise for the clinic.