The Hippo pathway effectors TAZ and YAP in development, homeostasis and disease

Cell density regulates cancer metastasis via the Hippo pathway

Metastatic spread of cancer cells from the primary tumor site to distant organs is the major cause of death in cancer patients. To disseminate, cancer cells detach from the primary tumor, enter the blood stream and extravasate at distant organ sites such as the liver, lung, bone or brain. While cancer cells are known to evade contact inhibition during growth in culture, we found that cell density is still sensed and can signal through the Hippo pathway effectors LATS1 and YAP. These effectors control cancer cell invasive behavior into stromal tissues, expression of cytokines that recruit inflammatory cells and progression toward metastatic spread. In this perspective, we discuss the drivers and the significance of pathways controlled by cell growth density.

Thyroid development in zebrafish lacking Taz

Taz is a signal-responsive transcriptional coregulator implicated in several biological functions, from chondrogenesis to regulation of organ size. Less well studied, however, is its role in thyroid formation. Here, we explored the in vivo effects on thyroid development of morpholino (MO)-mediated knockdown of wwtr1, the gene encoding zebrafish Taz. The wwtr1 gene is expressed in the thyroid primordium and pharyngeal tissue of developing zebrafish. Compared to mammalian cells, in which Taz promotes expression of thyroid transcription factors and thyroid differentiation genes, wwtr1 MO injection in zebrafish had little or no effect on the expression of thyroid transcription factors, and differentially altered the expression of thyroid differentiation genes. Analysis of wwtr1 morphants at later stages of development revealed that the number and the lumen of thyroid follicles, and the number of thyroid follicle cells, were significantly smaller. In addition, Taz-depleted larvae displayed patterning defects in ventral cranial vessels that correlate with lateral displacement of thyroid follicles. These findings indicate that the zebrafish Taz protein is needed for the normal differentiation of the thyroid and are the first to suggest that Taz confers growth advantage to the endocrine gland.

Histone Deacetylase Inhibitors Target the Leukemic Microenvironment by Enhancing a Nherf1-Protein Phosphatase 1α-TAZ Signaling Pathway in Osteoblasts

Disrupting the protective signals provided by the bone marrow microenvironment will be critical for more effective combination drug therapies for acute myeloid leukemia (AML). Cells of the osteoblast lineage that reside in the endosteal niche have been implicated in promoting survival of AML cells. Here, we investigated how to prevent this protective interaction. We previously showed that SDF-1, a chemokine abundant in the bone marrow, induces apoptosis of AML cells, unless the leukemic cells receive protective signals provided by differentiating osteoblasts (8, 10). We now identify a novel signaling pathway in differentiating osteoblasts that can be manipulated to disrupt the osteoblast-mediated protection of AML cells. Treating differentiating osteoblasts with histone deacetylase inhibitors (HDACi) abrogated their ability to protect co-cultured AML cells from SDF-1-induced apoptosis. HDACi prominently up-regulated expression of the Nherf1 scaffold protein, which played a major role in preventing osteoblast-mediated protection of AML cells. Protein phosphatase-1α (PP1α) was identified as a novel Nherf1 interacting protein that acts as the downstream mediator of this response by promoting nuclear localization of the TAZ transcriptional modulator. Moreover, independent activation of either PP1α or TAZ was sufficient to prevent osteoblast-mediated protection of AML cells even in the absence of HDACi. Together, these results indicate that HDACi target the AML microenvironment by enhancing activation of the Nherf1-PP1α-TAZ pathway in osteoblasts. Selective drug targeting of this osteoblast signaling pathway may improve treatments of AML by rendering leukemic cells in the bone marrow more susceptible to apoptosis.

Connecting physical cues and tissue patterning

Several signaling pathways work together, via a protein called Amotl2a, to establish the size and shape of a zebrafish sense organ primordium.

MAP4K family kinases act in parallel to MST1/2 to activate LATS1/2 in the Hippo pathway

The Hippo pathway plays a central role in tissue homoeostasis, and its dysregulation contributes to tumorigenesis. Core components of the Hippo pathway include a kinase cascade of MST1/2 and LATS1/2 and the transcription co-activators YAP/TAZ. In response to stimulation, LATS1/2 phosphorylate and inhibit YAP/TAZ, the main effectors of the Hippo pathway. Accumulating evidence suggests that MST1/2 are not required for the regulation of YAP/TAZ. Here we show that deletion of LATS1/2 but not MST1/2 abolishes YAP/TAZ phosphorylation. We have identified MAP4K family members—Drosophila Happyhour homologues MAP4K1/2/3 and Misshapen homologues MAP4K4/6/7—as direct LATS1/2-activating kinases. Combined deletion of MAP4Ks and MST1/2, but neither alone, suppresses phosphorylation of LATS1/2 and YAP/TAZ in response to a wide range of signals. Our results demonstrate that MAP4Ks act in parallel to and are partially redundant with MST1/2 in the regulation of LATS1/2 and YAP/TAZ, and establish MAP4Ks as components of the expanded Hippo pathway.

A variety of signals have been reported to either activate or inhibit the Hippo kinase cascade. Here, Meng et al. show that mitogen activated protein kinase kinase kinase kinase (MAP4K) family members function in parallel to and are partially redundant with MST1/2 in regulating LATS in response to upstream signals.

Differential control of Yorkie activity by LKB1/AMPK and the Hippo/Warts cascade in the central nervous system

The Hippo (Hpo) pathway is a highly conserved tumor suppressor network that restricts developmental tissue growth and regulates stem cell proliferation and differentiation. At the heart of the Hpo pathway is the progrowth transcriptional coactivator Yorkie [Yki-Yes-activated protein (YAP)/transcriptional coactivator with PDZ-binding motif (TAZ) in mammals]. Yki activity is restricted through phosphorylation by the Hpo/Warts core kinase cascade, but increasing evidence indicates that core kinase-independent modes of regulation also play an important role. Here, we examine Yki regulation in the Drosophila larval central nervous system and uncover a Hpo/Warts-independent function for the tumor suppressor kinase liver kinase B1 (LKB1) and its downstream effector, the energy sensor AMP-activated protein kinase (AMPK), in repressing Yki activity in the central brain/ventral nerve cord. Although the Hpo/Warts core cascade restrains Yki in the optic lobe, it is dispensable for Yki target gene repression in the late larval central brain/ventral nerve cord. Thus, we demonstrate a dramatically different wiring of Hpo signaling in neighboring cell populations of distinct developmental origins in the central nervous system.

A YAP/TAZ-induced feedback mechanism regulates Hippo pathway homeostasis

In this study, Moroishi et al. show that YAP and TAZ activation induces the negative regulators LATS1 kinase and LATS2 kinase to constitute a negative feedback mechanism to control YAP/TAZ activation duration and cellular response. This study provides new insights into the dynamic regulation and maintenance of Hippo pathway homeostasis.

YAP (Yes-associated protein) and TAZ (transcriptional coactivator with PDZ-binding motif) are major downstream effectors of the Hippo pathway that influences tissue homeostasis, organ size, and cancer development. Aberrant hyperactivation of YAP/TAZ causes tissue overgrowth and tumorigenesis, whereas their inactivation impairs tissue development and regeneration. Dynamic and precise control of YAP/TAZ activity is thus important to ensure proper physiological regulation and homeostasis of the cells. Here, we show that YAP/TAZ activation results in activation of their negative regulators, LATS1/2 (large tumor suppressor 1/2) kinases, to constitute a negative feedback loop of the Hippo pathway in both cultured cells and mouse tissues. YAP/TAZ in complex with the transcription factor TEAD (TEA domain family member) directly induce LATS2 expression. Furthermore, YAP/TAZ also stimulate the kinase activity of LATS1/2 through inducing NF2 (neurofibromin 2). This feedback regulation is responsible for the transient activation of YAP upon lysophosphatidic acid (LPA) stimulation and the inhibition of YAP-induced cell migration. Thus, this LATS-mediated feedback loop provides an efficient mechanism to establish the robustness and homeostasis of YAP/TAZ regulation.

Yap and Taz regulate retinal pigment epithelial cell fate

The optic vesicle comprises a pool of bi-potential progenitor cells from which the retinal pigment epithelium (RPE) and neural retina fates segregate during ocular morphogenesis. Several transcription factors and signaling pathways have been shown to be important for RPE maintenance and differentiation, but an understanding of the initial fate specification and determination of this ocular cell type is lacking. We show that Yap/Taz-Tead activity is necessary and sufficient for optic vesicle progenitors to adopt RPE identity in zebrafish. A Tead-responsive transgene is expressed within the domain of the optic cup from which RPE arises, and Yap immunoreactivity localizes to the nuclei of prospective RPE cells. yap (yap1) mutants lack a subset of RPE cells and/or exhibit coloboma. Loss of RPE in yap mutants is exacerbated in combination with taz (wwtr1) mutant alleles such that, when Yap and Taz are both absent, optic vesicle progenitor cells completely lose their ability to form RPE. The mechanism of Yap-dependent RPE cell type determination is reliant on both nuclear localization of Yap and interaction with a Tead co-factor. In contrast to loss of Yap and Taz, overexpression of either protein within optic vesicle progenitors leads to ectopic pigmentation in a dosage-dependent manner. Overall, this study identifies Yap and Taz as key early regulators of RPE genesis and provides a mechanistic framework for understanding the congenital ocular defects of Sveinsson's chorioretinal atrophy and congenital retinal coloboma.

Hippo and TGF-β interplay in the lung field

The Hippo pathway is comprised of a kinase cascade that involves mammalian Ste20-like serine/threonine kinases (MST1/2) and large tumor suppressor kinases (LATS1/2) and leads to inactivation of transcriptional coactivator with PDZ-binding motif (TAZ) and yes-associated protein (YAP). Protein stability and subcellular localization of TAZ/YAP determine its ability to regulate a diverse array of biological processes, including proliferation, apoptosis, differentiation, stem/progenitor cell properties, organ size control, and tumorigenesis. These actions are enabled by interactions with various transcription factors or through cross talk with other signaling pathways. Interestingly, mechanical stress has been shown to be an upstream regulator of TAZ/YAP activity, and this finding provides a novel clue for understanding how mechanical forces influence a broad spectrum of biological processes, which involve cytoskeletal structure, cell adhesion, and extracellular matrix (ECM) organization. Transforming growth factor-β (TGF-β) pathway is a critical component of lung development and the progression of lung diseases including emphysema, fibrosis, and cancer. In addition, TGF-β is a key regulator of ECM remodeling and cell differentiation processes such as epithelial-mesenchymal transition. In this review, we summarize the current knowledge of the Hippo pathway regarding lung development and diseases, with an emphasis on its interplay with TGF-β signaling.

Hippo signaling is required for Notch-dependent smooth muscle differentiation of neural crest

Notch signaling has well-defined roles in the assembly of arterial walls and in the development of the endothelium and smooth muscle of the vasculature. Hippo signaling regulates cellular growth in many tissues, and contributes to regulation of organ size, in addition to other functions. Here, we show that the Notch and Hippo pathways converge to regulate smooth muscle differentiation of the neural crest, which is crucial for normal development of the aortic arch arteries and cranial vasculature during embryonic development. Neural crest-specific deletion of the Hippo effectors Yap and Taz produces neural crest precursors that migrate normally, but fail to produce vascular smooth muscle, and Notch target genes such as Jagged1 fail to activate normally. We show that Yap is normally recruited to a tissue-specific Jagged1 enhancer by directly interacting with the Notch intracellular domain (NICD). The Yap-NICD complex is recruited to chromatin by the DNA-binding protein Rbp-J in a Tead-independent fashion. Thus, Hippo signaling can modulate Notch signaling outputs, and components of the Hippo and Notch pathways physically interact. Convergence of Hippo and Notch pathways by the mechanisms described here might be relevant for the function of these signaling cascades in many tissues and in diseases such as cancer.

Crumbs3-Mediated Polarity Directs Airway Epithelial Cell Fate through the Hippo Pathway Effector Yap

Epithelial cells undergo dynamic polarity changes as organs pattern, but the relationship between epithelial polarity and cell fate is poorly understood. Using the developing lung as a model, we found that distinct alterations in apical-basal polarity dictate airway epithelial differentiation. We demonstrate that Crb3, a Crumbs isoform that determines epithelial apical domain identity, is required for airway differentiation by controlling the localization of the transcriptional regulator Yap. We show that Crb3 promotes the interaction between Yap and the Hippo pathway kinases Lats1/2 at apical cell junctions to induce Yap phosphorylation and cytoplasmic retention, which drive cell differentiation. Loss of Crb3 in developing mouse airways or isolated adult airway progenitors results in unrestricted nuclear Yap activity and consequent cell differentiation defects. Our findings demonstrate that polarity-dependent cues control airway cell differentiation, offering important molecular insights into organ patterning.

A conserved MST1/2-YAP axis mediates Hippo signaling during lung growth

Hippo signaling is a critical player in controlling the growth of several tissues and organs in diverse species. The current model of Hippo signaling postulates a cascade of kinase activity initiated by the MST1/2 kinases in response to external stimuli. This leads to inactivation of the transcriptional coactivators, YAP/TAZ, due to their cytoplasmic retention and degradation that is correlated with YAP/TAZ phosphorylation. In most tissues examined, YAP plays a more dominant role than TAZ. Whether a conserved Hippo pathway is utilized during lung growth and development is unclear. In particular, the regulatory relationship between MST1/2 and YAP/TAZ in the lung remains controversial. By employing the Shh-Cre mouse line to efficiently inactivate genes in the lung epithelium, we show that loss of MST1/2 kinases in the epithelium can lead to neonatal lethality caused by lung defects. This is manifested by perturbation of lung epithelial cell proliferation and differentiation. These phenotypes are more severe than those produced by Nkx2.1-Cre, highlighting the effects of differential Cre activity on phenotypic outcomes. Importantly, expression of YAP targets is upregulated and the ratio of phospho-YAP to total YAP protein levels is reduced in Mst1/2-deficient lungs, all of which are consistent with a negative role of MST1/2 in controlling YAP function. This model gains further support from both in vivo and in vitro studies. Genetic removal of one allele of Yap or one copy of both Yap and Taz rescues neonatal lethality and lung phenotypes due to loss of Mst1/2. Moreover, knockdown of Yap in lung epithelial cell lines restores diminished alveolar marker expression caused by Mst1/2 inactivation. These results demonstrate that MST1/2 inhibit YAP/TAZ activity and establish a conserved MST1/2-YAP axis in coordinating lung growth during development.

The Human Adenovirus Type 5 E4orf4 Protein Targets Two Phosphatase Regulators of the Hippo Signaling Pathway

When expressed alone at high levels, the human adenovirus E4orf4 protein exhibits tumor cell-specific p53-independent toxicity. A major E4orf4 target is the B55 class of PP2A regulatory subunits, and we have shown recently that binding of E4orf4 inhibits PP2A(B55) phosphatase activity in a dose-dependent fashion by preventing access of substrates (M. Z. Mui et al., PLoS Pathog 9:e1003742, 2013, http://dx.doi.org/10.1371/journal.ppat.1003742). While interaction with B55 subunits is essential for toxicity, E4orf4 mutants exist that, despite binding B55 at high levels, are defective in cell killing, suggesting that other essential targets exist. In an attempt to identify additional targets, we undertook a proteomics approach to characterize E4orf4-interacting proteins. Our findings indicated that, in addition to PP2A(B55) subunits, ASPP-PP1 complex subunits were found among the major E4orf4-binding species. Both the PP2A and ASPP-PP1 phosphatases are known to positively regulate effectors of the Hippo signaling pathway, which controls the expression of cell growth/survival genes by dephosphorylating the YAP transcriptional coactivator. We find here that expression of E4orf4 results in hyperphosphorylation of YAP, suggesting that Hippo signaling is affected by E4orf4 interactions with PP2A(B55) and/or ASPP-PP1 phosphatases. Furthermore, knockdown of YAP1 expression was seen to enhance E4orf4 killing, again consistent with a link between E4orf4 toxicity and inhibition of the Hippo pathway. This effect may in fact contribute to the cancer cell specificity of E4orf4 toxicity, as many human cancer cells rely heavily on the Hippo pathway for their enhanced proliferation.

IMPORTANCE

The human adenovirus E4orf4 protein has been known for some time to induce tumor cell-specific death when expressed at high levels; thus, knowledge of its mode of action could be of importance for development of new cancer therapies. Although the B55 form of the phosphatase PP2A has long been known as an essential E4orf4 target, genetic analyses indicated that others must exist. To identify additional E4orf4 targets, we performed, for the first time, a large-scale affinity purification/mass spectrometry analysis of E4orf4 binding partners. Several additional candidates were detected, including key regulators of the Hippo signaling pathway, which enhances cell viability in many cancers, and results of preliminary studies suggested a link between inhibition of Hippo signaling and E4orf4 toxicity.

Yes-associated protein (Yap) is necessary for ciliogenesis and morphogenesis during pronephros development in zebrafish (Danio Rerio)

The Hippo signaling pathway and its transcriptional co-activator Yap are known as essential regulators for cell proliferation and organ size. However, little is known about their roles in kidney development and ciliogenesis. We examined expression of Yap during zebrafish embryogenesis, and its transcripts were detected in pronephric duct, while Yap protein was found to be localized in the cytoplasm and apical membrane in kidney epithelium cells. By morpholino (MO) knockdown of yap expression in zebrafish, the injected larve exhibits pronephic cysts and many aspects of ciliopathy, which can be rescued by full-length yap mRNA, but not yap^S127A mRNA. With transgenic Tg(Na⁺/K⁺ ATPase:EGFP), we found that lacking Yap led to expansion and discontinuities of pronephric duct, as well as disorganization of cloaca during pronephros morphogenesis. Mis-located Na⁺/K⁺ ATPase and ciliary abnormalities are also detected in pronephric duct of yap morphants. In addition, genetic analysis suggests that yap interacts with ift20, ift88 and arl13b in pronephric cyst formation. Taken together, our data reveals that Yap is required for pronephric duct integrity, maintenance of baso-lateral cell polarity, and ciliogenesis during zebrafish kidney development.

Lats1 Deletion Causes Increased Germ Cell Apoptosis and Follicular Cysts in Mouse Ovaries

The Hippo signaling pathway is essential for regulating proliferation and apoptosis in mammalian cells. The LATS1 kinase is a core member of the Hippo signaling pathway that phosphorylates and inactivates the transcriptional co-activators YAP1 and WWTR1. Deletion of Lats1 results in low neonate survival and ovarian stromal tumors in surviving adults, but the effects of Lats1 on early follicular development are not understood. Here, the expression of Hippo pathway components including Wwtr1, Stk4, Stk3, Lats2, and Yap1 transcripts were decreased by 50% in mouse ovaries between 2 and 8 days of age while expression was maintained from 8 days to 21 days and after priming with eCG. LATS1, LATS2, and MOB1B were localized to both germ and somatic cells of primordial to antral follicles. Interestingly, YAP1 was predominantly cytoplasmic, whereas WWTR1 was nuclear in oocytes and somatic cells. Deletion of Lats1 caused an increase in germ cell apoptosis from 1.7% in control ovaries to 3.6% in Lats1 mutant ovaries and a 58% and 32% decrease in primordial and activated follicle numbers in cultured mutant ovaries. Surprisingly, there was an increase in Bmp15 but not Gdf9, Figla, Nobox transcripts or the somatic-specific transcripts Amh and Wnt4 in cultured Lats1 mutant ovaries. Last, Lats1 mutant ovaries developed ovarian cysts at a higher frequency (43%) than heterozygous (24%) and control ovaries (8%). Results showed that the Hippo pathway is active in ovarian follicles and that LATS1 is required to maintain the pool of germ cells and primordial follicles.

YAP and TAZ: a nexus for Hippo signaling and beyond

The Hippo pathway is a potent regulator of cellular proliferation, differentiation, and tissue homeostasis. Here we review the regulatory mechanisms of the Hippo pathway and discuss the function of Yes-associated protein (YAP)/transcriptional coactivator with a PDZ-binding domain (TAZ), the prime mediators of the Hippo pathway, in stem cell biology and tissue regeneration. We highlight their activities in both the nucleus and the cytoplasm and discuss their role as a signaling nexus and integrator of several other prominent signaling pathways such as the Wnt, G protein-coupled receptor (GPCR), epidermal growth factor (EGF), bone morphogenetic protein (BMP)/transforming growth factor beta (TGFβ), and Notch pathways.

TAZ promotes temozolomide resistance by upregulating MCL-1 in human glioma cells

Temozolomide is a novel cytotoxic agent currently used as first-line chemotherapy for glioblastoma multiforme (GBM). However, intrinsic or acquired chemoresistance to temozolomide remains the greatest obstacle to the successful treatment of human GBM. The principal mechanism responsible for this resistance is largely unknown. In the present study, we showed that expression of transcriptional co-activator with PDZ-binding motif (TAZ) in glioma cells correlated with temozolomide chemoresistance in human glioma cells. Overexpression of TAZ promoted temozolomide resistance in U-87MG cells, whereas knockdown of TAZ expression sensitized temozolomide-resistant U-251MG cells to temozolomide. Further, TAZ inhibits temozolomide induced apoptosis via upregulation of MCL-1 (myeloid cell leukemia 1) and high expression of TAZ predicts a poor prognosis for GBM patients. In conclusion, our results suggest that TAZ had a critical role in the resistance to temozolomide in glioma cells, and it may provide a promising target for improving the therapeutic outcome of temozolomide-resistant gliomas.

Physiological mechanisms and therapeutic potential of bone mechanosensing

Skeletal loading is an important physiological regulator of bone mass. Theoretically, mechanical forces or administration of drugs that activate bone mechanosensors would be a novel treatment for osteoporotic disorders, particularly age-related osteoporosis and other bone loss caused by skeletal unloading. Uncertainty regarding the identity of the molecular targets that sense and transduce mechanical forces in bone, however, has limited the therapeutic exploitation of mechanosesning pathways to control bone mass. Recently, two evolutionally conserved mechanosensing pathways have been shown to function as "physical environment" sensors in cells of the osteoblasts lineage. Indeed, polycystin-1 (Pkd1, or PC1) and polycystin-2 (Pkd2, or PC2' or TRPP2), which form a flow sensing receptor channel complex, and TAZ (transcriptional coactivator with PDZ-binding motif, or WWTR1), which responds to the extracellular matrix microenvironment act in concert to reciprocally regulate osteoblastogenesis and adipogenesis through co-activating Runx2 and a co-repressing PPARγ activities. Interactions of polycystins and TAZ with other putative mechanosensing mechanism, such as primary cilia, integrins and hemichannels, may create multifaceted mechanosensing networks in bone. Moreover, modulation of polycystins and TAZ interactions identify novel molecular targets to develop small molecules that mimic the effects of mechanical loading on bone.

Tendon mechanobiology: Current knowledge and future research opportunities

Tendons mainly function as load-bearing tissues in the muscloskeletal system; transmitting loads from muscle to bone. Tendons are dynamic structures that respond to the magnitude, direction, frequency, and duration of physiologic as well as pathologic mechanical loads via complex interactions between cellular pathways and the highly specialized extracellular matrix. This paper reviews the evolution and current knowledge of mechanobiology in tendon development, homeostasis, disease, and repair. In addition, we review several novel mechanotransduction pathways that have been identified recently in other tissues and cell types, providing potential research opportunities in the field of tendon mechanobiology. We also highlight current methods, models, and technologies being used in a wide variety of mechanobiology research that could be investigated in the context of their potential applicability for answering some of the fundamental unanswered questions in this field. The article concludes with a review of the major questions and future goals discussed during the recent ORS/ISMMS New Frontiers in Tendon Research Conference held on September 10 and 11, 2014 in New York City.

Expression of α-Smooth Muscle Actin Determines the Fate of Mesenchymal Stromal Cells

Pro-fibrotic microenvironments of scars and tumors characterized by increased stiffness stimulate mesenchymal stromal cells (MSCs) to express α-smooth muscle actin (α-SMA). We investigated whether incorporation of α-SMA into contractile stress fibers regulates human MSC fate. Sorted α-SMA-positive MSCs exhibited high contractile activity, low clonogenicity, and differentiation potential limited to osteogenesis. Knockdown of α-SMA was sufficient to restore clonogenicity and adipogenesis in MSCs. Conversely, α-SMA overexpression induced YAP translocation to the nucleus and reduced the high clonogenicity and adipogenic potential of α-SMA-negative MSCs. Inhibition of YAP rescued the decreased adipogenic differentiation potential induced by α-SMA, establishing a mechanistic link between matrix stiffness, α-SMA, YAP, and MSC differentiation. Consistent with in vitro findings, nuclear localization of YAP was positively correlated in α-SMA expressing stromal cells of adiposarcoma and osteosarcoma. We propose that α-SMA mediated contraction plays a critical role in mechanically regulating MSC fate by controlling YAP/TAZ activation.

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The α-SMA-positive myofibroblast fraction of human MSCs exhibits low clonogenicity

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Formation of α-SMA stress fibers enhances nuclear translocation of YAP/TAZ in MSCs

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α-SMA knockdown favors adipogenesis, while overexpression promotes osteogenesis

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α-SMA-mediated lineage choice of MSCs is YAP dependent

Hippo Component TAZ Functions as a Co-repressor and Negatively Regulates ΔNp63 Transcription through TEA Domain (TEAD) Transcription Factor

Transcriptional co-activator with a PDZ binding domain (TAZ) is a WW domain-containing transcriptional co-activator and a core component of an emerging Hippo signaling pathway that regulates organ size, tumorigenesis, metastasis, and drug resistance. TAZ regulates these biological functions by up-regulating downstream cellular genes through transactivation of transcription factors such as TEAD and TTF1. To understand the molecular mechanisms underlying TAZ-induced tumorigenesis, we have recently performed a gene expression profile analysis by overexpressing TAZ in mammary cells. In addition to the TAZ-up-regulated genes that were confirmed in our previous studies, we identified a large number of cellular genes that were down-regulated by TAZ. In this study, we have confirmed these down-regulated genes (including cytokines, chemokines, and p53 gene family members) as bona fide downstream transcriptional targets of TAZ. By using human breast and lung epithelial cells, we have further characterized ΔNp63, a p53 gene family member, and shown that TAZ suppresses ΔNp63 mRNA, protein expression, and promoter activity through interaction with the transcription factor TEAD. We also show that TEAD can inhibit ΔNp63 promoter activity and that TAZ can directly interact with ΔNp63 promoter-containing TEAD binding sites. Finally, we provide functional evidence that down-regulation of ΔNp63 by TAZ may play a role in regulating cell migration. Altogether, this study provides novel evidence that the Hippo component TAZ can function as a co-repressor and regulate biological functions by negatively regulating downstream cellular genes.

Cell signaling and transcription factors regulating cell fate during formation of the mouse blastocyst

The first cell fate decisions during mammalian development establish tissues essential for healthy pregnancy. The mouse has served as a valuable model for discovering pathways regulating the first cell fate decisions because of the ease with which early embryos can be recovered and the availability of an arsenal of classical and emerging methods for manipulating gene expression. We summarize the major pathways that govern the first cell fate decisions in mouse development. This knowledge serves as a paradigm for exploring how emergent properties of a self-organizing system can dynamically regulate gene expression and cell fate plasticity. Moreover, it brings to light the processes that establish healthy pregnancy and ES cells. We also describe unsolved mysteries and new technologies that could help to overcome experimental challenges in the field.

Crosstalk between Hippo and TGFβ: Subcellular Localization of YAP/TAZ/Smad Complexes

The Hippo pathway plays a crucial role in growth control, proliferation and tumor suppression. Activity of the signaling pathway is associated with cell density sensing and tissue organization. Furthermore, the Hippo pathway helps to coordinate cellular processes through crosstalk with growth-factor-mediated signaling pathways such as TGFβ. Here we have examined the localization of interactions between proteins of the Hippo pathway (YAP/TAZ) and TGFβ (Smad2/3) signaling pathway by using in situ proximity ligation assays. We investigated the formation of protein complexes between YAP/TAZ and Smad2/3 and examined how these interactions were affected by TGFβ stimulation and cell density in HaCaT keratinocytes and in Smad4-deficient HT29 colon cancer cells. We demonstrate that TGFβ induces formation of YAP/TAZ-Smad2/3 complexes in HaCaT cells. Under sparse cell conditions, the complexes were detected to a higher degree and were predominantly located in the nucleus, while under dense culture conditions, the complexes were fewer and mainly located in the cytoplasm. Surprisingly, we could not detect any YAP/TAZ-Smad2/3 complexes in HT29 cells. To examine if Smad4 deficiency was responsible for the absence of interactions, we treated HaCaT cells with siRNA targeting Smad4. However, we could still observe complex formation in the siRNA-treated cells, suggesting that Smad4 is not essential for the YAP-Smad2/3 interaction. In conclusion, this study shows localized, density-dependent formation of YAP/TAZ-Smad2/3 complexes in HaCaT cells and provides evidence supporting a crosstalk between the Hippo and the TGFβ signaling pathways.

SMADs and YAP compete to control elongation of β-catenin:LEF-1-recruited RNAPII during hESC differentiation

The Wnt3a/β-catenin and Activin/SMAD2,3 signaling pathways synergize to induce endodermal differentiation of human embryonic stem cells; however, the underlying mechanism is not well understood. Using ChIP-seq and GRO-seq analyses, we show here that Wnt3a-induced β-catenin:LEF-1 enhancers recruit cohesin to direct enhancer-promoter looping and activate mesendodermal (ME) lineage genes. Moreover, we find that LEF-1 and other hESC enhancers recruit RNAPII complexes (eRNAPII) that are highly phosphorylated at Ser5, but not Ser7. Wnt3a signaling further increases Ser5P-RNAPII at LEF-1 sites and ME gene promoters, indicating that elongation remains limiting. However, subsequent Activin/SMAD2,3 signaling selectively increases transcription elongation, P-TEFb occupancy, and Ser7P-RNAPII levels at these genes. Finally, we show that the Hippo regulator, YAP, functions with TEAD to regulate binding of the NELF negative elongation factor and block SMAD2,3 induction of ME genes. Thus, the Wnt3a/β-catenin and Activin/SMAD2,3 pathways act in concert to counteract YAP repression and upregulate ME genes during early hESC differentiation.

The Hippo effector YAP regulates the response of cancer cells to MAPK pathway inhibitors

RAF- and MEK-targeted therapies are approved for patients with BRAF(V600E) melanoma and under investigation in a several other tumor types, but resistance remains a major challenge. We uncovered yes-associated protein 1 (YAP1) as a mechanism of resistance to RAF-MEK inhibition in BRAF- and RAS-mutant cancers, providing a rationale for co-targeting YAP and RAF-MEK to enhance patient outcomes.

A YAP/TAZ-Regulated Molecular Signature Is Associated with Oral Squamous Cell Carcinoma

Oral squamous cell carcinoma (OSCC) is a prevalent form of cancer that develops from the epithelium of the oral cavity. OSCC is on the rise worldwide, and death rates associated with the disease are particularly high. Despite progress in understanding the mutational and expression landscape associated with OSCC, advances in deciphering these alterations for the development of therapeutic strategies have been limited. Further insight into the molecular cues that contribute to OSCC is therefore required. Here, we show that the transcriptional regulators YAP (YAP1) and TAZ (WWTR1), which are key effectors of the Hippo pathway, drive protumorigenic signals in OSCC. Regions of premalignant oral tissues exhibit aberrant nuclear YAP accumulation, suggesting that dysregulated YAP activity contributes to the onset of OSCC. Supporting this premise, we determined that nuclear YAP and TAZ activity drives OSCC cell proliferation, survival, and migration in vitro, and is required for OSCC tumor growth and metastasis in vivo. Global gene expression profiles associated with YAP and TAZ knockdown revealed changes in the control of gene expression implicated in protumorigenic signaling, including those required for cell cycle progression and survival. Notably, the transcriptional signature regulated by YAP and TAZ significantly correlates with gene expression changes occurring in human OSCCs identified by The Cancer Genome Atlas (TCGA), emphasizing a central role for YAP and TAZ in OSCC biology.

IMPLICATIONS

This study defines a YAP/TAZ-regulated transcriptional program in OSCC and reveals novel roles for nuclear YAP/TAZ activity in the onset and progression of this cancer.

Cell growth density modulates cancer cell vascular invasion via Hippo pathway activity and CXCR2 signaling

Metastasis of cancer cells involves multiple steps, including their dissociation from the primary tumor and invasion through the endothelial cell barrier to enter the circulation and finding their way to distant organ sites where they extravasate and establish metastatic lesions. Deficient contact inhibition is a hallmark of invasive cancer cells, yet surprisingly the vascular invasiveness of commonly studied cancer cell lines is regulated by the density at which cells are propagated in culture. Cells grown at high density were less effective at invading an endothelial monolayer than cells grown at low density. This phenotypic difference was also observed in a zebrafish model of vascular invasion of cancer cells after injection into the yolk sac and extravasation of cancer cells into tissues from the vasculature. The vascular invasive phenotypes were reversible. A kinome-wide RNAi screen was used to identify drivers of vascular invasion by panning shRNA library transduced non-invasive cancer cell populations on endothelial monolayers. The selection of invasive subpopulations showed enrichment of shRNAs targeting the LATS1 (large tumor suppressor 1) kinase that inhibits the activity of the transcriptional coactivator YAP in the Hippo pathway. Depletion of LATS1 from non-invasive cancer cells restored the invasive phenotype. Complementary to this, inhibition or depletion of YAP inhibited invasion in vitro and in vivo. The vascular invasive phenotype was associated with a YAP-dependent up-regulation of the cytokines IL6, IL8, and CXCL1, 2, and 3. Antibody blockade of cytokine receptors inhibited invasion and confirmed that they are rate-limiting drivers that promote cancer cell vascular invasiveness and could provide therapeutic targets.

A laminin 511 matrix is regulated by TAZ and functions as the ligand for the α6Bβ1 integrin to sustain breast cancer stem cells

Chang et al. found that breast cancer stem cells (CSCs) produce a laminin (LM) 511 matrix that promotes self-renewal and tumor initiation by engaging the α6Bβ1 integrin and activating the Hippo transducer TAZ. TAZ regulates the transcription of the α5 subunit of LM511 and the formation of a LM511 matrix.

Understanding how the extracellular matrix impacts the function of cancer stem cells (CSCs) is a significant but poorly understood problem. We report that breast CSCs produce a laminin (LM) 511 matrix that promotes self-renewal and tumor initiation by engaging the α6Bβ1 integrin and activating the Hippo transducer TAZ. Although TAZ is important for the function of breast CSCs, the mechanism is unknown. We observed that TAZ regulates the transcription of the α5 subunit of LM511 and the formation of a LM511 matrix. These data establish a positive feedback loop involving TAZ and LM511 that contributes to stemness in breast cancer.

Roles of the Hippo pathway in lung development and tumorigenesis

Lung cancer is the most commonly diagnosed cancer and accounts for one fifth of all cancer deaths worldwide. Although significant progress has been made toward our understanding of the causes of lung cancer, the 5-year survival is still lower than 15%. Therefore, there is an urgent need for novel lung cancer biomarkers and drug targets. The Hippo signaling pathway is an emerging signaling pathway that regulates various biological processes. Recently, increasing evidence suggests that the Hippo pathway may play important roles in not only lung development but also lung tumorigenesis. In this review article, we will summarize the most recent advances and predict future directions on this new cancer research field.

YAP1-TFE3 epithelioid hemangioendothelioma: a case without vasoformation and a new transcript variant

Epithelioid hemangioendothelioma is a rare vascular tumor of borderline malignancy characterized by recurrent WWTR1-CAMTA1 gene fusions in approximately 90 % of cases. In addition, a recurrent YAP1-TFE3 gene fusion has been identified in WWTR1-CAMTA1 negative epithelioid hemangioendotheliomas. This subset has been reported as having a distinct morphology with more obvious vasoformation, voluminous eosinophilic cytoplasm, and TFE3 positivity on immunohistochemistry. We report a case of a YAP1-TFE3 translocated epithelioid hemangioendothelioma arising in a groin lymph node in a 29-year-old male. Plump spindle cell morphology and absence of vasoformation made correct diagnosis particularly difficult. Immunohistochemistry showed nuclear positivity for both ERG and TFE3, fluorescence in situ hybridization showed break apart for TFE3 and RT-PCR identified a YAP1 exon1 to TFE3 exon 6 transcript, a previously unreported fusion variant. Awareness of this solid morphology and variant fusion will aid in identification of future cases of this rare vascular tumor.

The emerging roles of YAP and TAZ in cancer

Yes-associated protein (YAP) and transcriptional co-activator with PDZ-binding motif (TAZ) are the major downstream effectors of the Hippo pathway, which regulates tissue homeostasis, organ size, regeneration and tumorigenesis. In this Progress article, we summarize the current understanding of the biological functions of YAP and TAZ, and how the regulation of these two proteins can be disrupted in cancer. We also highlight recent findings on their expanding role in cancer progression and describe the potential of these targets for therapeutic intervention.

Cancer stem cells: the promise and the potential

Despite the advancement of treatment modalities, many cancer patients experience tumor recurrence and metastasis at regional or distant sites. Evolving understanding of tumor biology has led to the hypothesis that tumors may possess a stem cell-like subpopulation known as cancer stem cells (CSCs) that may be involved in driving tumor propagation and pathogenesis. Like normal stem cells (NSCs), CSCs can be identified by markers such as CD133, CD44, and ALDH. CSCs have the ability to self-renew and differentiate into different tumor components through stemness pathways, such as Wnt, TGF-β, STAT, and Hippo-YAP/TAZ, among others. In NSCs, stemness pathways are strictly regulated and control many important biologic processes, including embryogenesis and intestinal crypt cellular regulation. In contrast, stemness pathways in CSCs are significantly dysregulated. Combining current drugs with the targeting of these stemness pathways may significantly improve patient prognosis. The aim of this supplement is to update clinicians on the accumulated evidence characterizing the role of CSCs in tumor initiation, heterogeneity, therapy resistance, and recurrence and metastasis, and the potential for effectively treating patients.

Oncoprotein YAP regulates the spindle checkpoint activation in a mitotic phosphorylation-dependent manner through up-regulation of BubR1

The transcriptional co-activator YAP (Yes-associated protein) functions as an oncogene; however, it is largely unclear how YAP exerts its oncogenic role. In this study, we further explored the functional significance of YAP and its mitotic phosphorylation in the spindle checkpoint. We found that the dynamic mitotic phosphorylation of YAP was CDC14-dependent. We also showed that YAP was required for the spindle checkpoint activation induced by spindle poisons. Mitotic phosphorylation of YAP was required for activation of the spindle checkpoint. Furthermore, enhanced expression of active YAP hyperactivated the spindle checkpoint and induced mitotic defects in a mitotic phosphorylation-dependent manner. Mechanistically, we documented that mitotic phosphorylation of YAP controlled transcription of genes associated with the spindle checkpoint. YAP constitutively associated with BubR1 (BUB1-related protein kinase), and knockdown of BubR1 relieved YAP-driven hyperactivation of the spindle checkpoint. Finally, we demonstrated that YAP promoted epithelial cell invasion via both mitotic phosphorylation and BubR1-dependent mechanisms. Together, our results reveal a novel link between YAP and the spindle checkpoint and indicate a potential mechanism underlying the oncogenic function of YAP through dysregulation of the spindle checkpoint.

β-Spectrin regulates the hippo signaling pathway and modulates the basal actin network

Emerging evidence suggests functional regulation of the Hippo pathway by the actin cytoskeleton, although the detailed molecular mechanism remains incomplete. In a genetic screen, we identified a requirement for β-Spectrin in the posterior follicle cells for the oocyte repolarization process during Drosophila mid-oogenesis. β-spectrin mutations lead to loss of Hippo signaling activity in the follicle cells. A similar reduction of Hippo signaling activity was observed after β-Spectrin knockdown in mammalian cells. We further demonstrated that β-spectrin mutations disrupt the basal actin network in follicle cells. The abnormal stress fiber-like actin structure on the basal side of follicle cells provides a likely link between the β-spectrin mutations and the loss of the Hippo signaling activity phenotype.

YAP immunoreactivity is directly related to pilomatrixoma size and proliferation rate

Organ size regulation is a highly coordinated process involving complex mechanisms. Yes-associated protein (YAP) and transcriptional co-activator with PDZ-binding motif, also known as WWTR1 (TAZ) have recently been linked to organ size determination and cell proliferation. Pilomatrixoma (PM) is a benign tumor of the adnexal appendages with a certain degree of differentiation toward the matrix of the hair follicle. PM presents as a dermal nodule that usually ranges from 0.5 to 2 cm, rarely exceeding 3 cm. We recently observed a case of unusual "giant" (6.5 cm) PM. Our hypothesis was that YAP and TAZ could be related to PM growth. We analyzed YAP and TAZ immunohistochemical expression in the giant and in ten usual size PMs in relation with tumor size and proliferation rate. YAP nuclear expression was remarkably higher in the giant PM in comparison with usual size PMs and statistically correlated, in a direct manner, with size and proliferation rate of PMs. Contrariwise, TAZ nuclear expression seemed stochastic. Our findings suggest that YAP could play a role in PM growth.

Hippo/Yap signaling controls epithelial progenitor cell proliferation and differentiation in the embryonic and adult lung

The Hippo/Yap pathway is a well-conserved signaling cascade that regulates cell proliferation and differentiation to control organ size and stem/progenitor cell behavior. Following airway injury, Yap was dynamically regulated in regenerating airway epithelial cells. To determine the role of Hippo signaling in the lung, the mammalian Hippo kinases, Mst1 and Mst2, were deleted in epithelial cells of the embryonic and mature mouse lung. Mst1/2 deletion in the fetal lung enhanced proliferation and inhibited sacculation and epithelial cell differentiation. The transcriptional inhibition of cell proliferation and activation of differentiation during normal perinatal lung maturation were inversely regulated following embryonic Mst1/2 deletion. Ablation of Mst1/2 from bronchiolar epithelial cells in the adult lung caused airway hyperplasia and altered differentiation. Inhibitory Yap phosphorylation was decreased and Yap nuclear localization and transcriptional targets were increased after Mst1/2 deletion, consistent with canonical Hippo/Yap signaling. YAP potentiated cell proliferation and inhibited differentiation of human bronchial epithelial cells in vitro. Loss of Mst1/2 and expression of YAP regulated transcriptional targets controlling cell proliferation and differentiation, including Ajuba LIM protein. Ajuba was required for the effects of YAP on cell proliferation in vitro. Hippo/Yap signaling regulates Ajuba and controls proliferation and differentiation of lung epithelial progenitor cells.

Signaling pathway cooperation in TGF-β-induced epithelial-mesenchymal transition

Transdifferentiation of epithelial cells into cells with mesenchymal properties and appearance, that is, epithelial-mesenchymal transition (EMT), is essential during development, and occurs in pathological contexts, such as in fibrosis and cancer progression. Although EMT can be induced by many extracellular ligands, TGF-β and TGF-β-related proteins have emerged as major inducers of this transdifferentiation process in development and cancer. Additionally, it is increasingly apparent that signaling pathways cooperate in the execution of EMT. This update summarizes the current knowledge of the coordination of TGF-β-induced Smad and non-Smad signaling pathways in EMT, and the remarkable ability of Smads to cooperate with other transcription-directed signaling pathways in the control of gene reprogramming during EMT.

Polyomavirus small T antigen interacts with yes-associated protein to regulate cell survival and differentiation

Murine polyomavirus small t antigen (PyST) regulates cell cycle, cell survival, apoptosis, and differentiation and cooperates with middle T antigen (MT) to transform primary cells in vitro and in vivo. Like all polyomavirus T antigens, PyST functions largely via its interactions with host cell proteins. Here, we show that PyST binds both Yes-associated protein 1 (YAP1) and YAP2, integral parts of the Hippo signaling pathway, which is a subject of increasing interest in human cancer. The transcription factor TEAD, which is a known target of YAP, is also found in PyST complexes. PyST enhanced YAP association with protein phosphatase 2A (PP2A), leading to decreased YAP phosphorylation. PyST increased YAP levels by decreasing its degradation. This effect was mediated by a reduction in YAP association with β-transducin repeat protein (βTRCP), which is known to regulate YAP turnover in a phosphorylation-dependent manner. Genetic analysis has identified PyST mutants defective in YAP binding. These mutants demonstrated that YAP binding is important for PyST to block myoblast differentiation and to synergize with the phosphodiesterase inhibitor isobutylmethylxanthine (IBMX) to promote cell death in 3T3-L1 preadipocytes placed under differentiation conditions. In addition to YAP binding, both of these phenotypes require PyST binding to PP2A. Importance: The Hippo/YAP pathway is a highly conserved cascade important for tissue development and homeostasis. Defects in this pathway are increasingly being associated with cancer. Polyomavirus small t antigen is a viral oncogene that cooperates with middle T antigen in transformation. On its own, small t antigen controls cell survival and differentiation. By binding YAP, small t antigen brings it together with protein phosphatase 2A. This work shows how this association of small t antigen with YAP is important for its effects on cell phenotype. It also suggests that PyST can be used to characterize cellular processes that are regulated by YAP.

Timing of developmental events in the early mouse embryo

The timing of developmental events during early mouse development has been investigated in embryos that have been subject to experimental manipulation of cell number and tissue mass. These phenomenological studies revealed that the timing of preimplantation events, such as compaction, formation of blastocyst cavity and lineage allocation is correlated with the rounds of cleavage division or DNA replication of the blastomeres. Timing of postimplantation processes, such as formation of proamniotic cavity and onset of gastrulation is sensitive to cell number and probably the tissue mass, which may be measured by a mechanosensory signaling mechanism. Developmental changes in these two physical attributes are correlated with the cell proliferative activity and the growth trajectory of the whole embryo prior to the transit to organogenesis. During organogenesis, timing of morphogenesis appears to be regulated by individual devices that could be uncoupled during compensatory growth. Insights of the timing mechanism may be gleaned from the analysis of genomic activity associated with the transition through developmental milestones.