Functional complexes between YAP2 and ZO-2 are PDZ domain-dependent, and regulate YAP2 nuclear localization and signalling. Biochem J. 2010;432:461-472. [PMID: 20868367 DOI: 10.1042/bj20100870]

RhoGDI1 regulates cell-cell junctions in polarized epithelial cells

Cell-cell contact formation of polarized epithelial cells is a multi-step process that involves the co-ordinated activities of Rho family small GTPases. Consistent with the central role of Rho GTPases, a number of Rho guanine nucleotide exchange factors (GEFs) and Rho GTPase-activating proteins (GAPs) have been identified at cell-cell junctions at various stages of junction maturation. As opposed to RhoGEFs and RhoGAPs, the role of Rho GDP dissociation inhibitors (GDIs) during cell-cell contact formation is poorly understood. Here, we have analyzed the role of RhoGDI1/ARHGDIA, a member of the RhoGDI family, during cell-cell contact formation of polarized epithelial cells. Depletion of RhoGDI1 delays the development of linear cell-cell junctions and the formation of barrier-forming tight junctions. In addition, RhoGDI1 depletion impairs the ability of cells to stop migration in response to cell collision and increases the migration velocity of collectively migrating cells. We also find that the cell adhesion receptor JAM-A promotes the recruitment of RhoGDI1 to cell-cell contacts. Our findings implicate RhoGDI1 in various processes involving the dynamic reorganization of cell-cell junctions.

Tight Junction Proteins as Therapeutic Targets to Treat Liver Fibrosis and Hepatocellular Carcinoma

In the last decade tight junction proteins exposed at the surface of liver or cancer cells have been uncovered as mediators of liver disease biology: Claudin-1 and Occludin are host factors for hepatitis C virus entry and Claudin-1 has been identified as a driver for liver fibrosis and hepatocellular carcinoma (HCC). Moreover, Claudins have emerged as therapeutic targets for liver disease and HCC. CLDN1 expression is upregulated in liver fibrosis and HCC. Monoclonal antibodies (mAbs) targeting Claudin-1 have completed preclinical proof-of-concept studies for treatment of liver fibrosis and HCC and are currently in clinical development for advanced liver fibrosis. Claudin-6 overexpression is associated with an HCC aggressive phenotype and treatment resistance. Claudin-6 mAbs or chimeric antigen receptor-T cells therapies are currently being clinically investigated for Claudin-6 overexpressing tumors. In conclusion, targeting Claudin proteins offers a novel clinical opportunity for the treatment of patients with advanced liver fibrosis and HCC.

SRY-Box transcription factor 9 triggers YAP nuclear entry via direct interaction in tumors

The translocation of YAP from the cytoplasm to the nucleus is critical for its activation and plays a key role in tumor progression. However, the precise molecular mechanisms governing the nuclear import of YAP are not fully understood. In this study, we have uncovered a crucial role of SOX9 in the activation of YAP. SOX9 promotes the nuclear translocation of YAP by direct interaction. Importantly, we have identified that the binding between Asp-125 of SOX9 and Arg-124 of YAP is essential for SOX9-YAP interaction and subsequent nuclear entry of YAP. Additionally, we have discovered a novel asymmetrical dimethylation of YAP at Arg-124 (YAP-R124me2a) catalyzed by PRMT1. YAP-R124me2a enhances the interaction between YAP and SOX9 and is associated with poor prognosis in multiple cancers. Furthermore, we disrupted the interaction between SOX9 and YAP using a competitive peptide, S-A1, which mimics an α-helix of SOX9 containing Asp-125. S-A1 significantly inhibits YAP nuclear translocation and effectively suppresses tumor growth. This study provides the first evidence of SOX9 as a pivotal regulator driving YAP nuclear translocation and presents a potential therapeutic strategy for YAP-driven human cancers by targeting SOX9-YAP interaction.

ZO-1 Regulates Hippo-Independent YAP Activity and Cell Proliferation via a GEF-H1- and TBK1-Regulated Signalling Network

Tight junctions are a barrier-forming cell-cell adhesion complex and have been proposed to regulate cell proliferation. However, the underlying mechanisms are not well understood. Here, we used cells deficient in the junction scaffold ZO-1 alone or together with its paralog ZO-2, which disrupts the junctional barrier. We found that ZO-1 knockout increased cell proliferation, induced loss of cell density-dependent proliferation control, and promoted apoptosis and necrosis. These phenotypes were enhanced by double ZO-1/ZO-2 knockout. Increased proliferation was dependent on two transcriptional regulators: YAP and ZONAB. ZO-1 knockout stimulated YAP nuclear translocation and activity without changes in Hippo-dependent phosphorylation. Knockout promoted TANK-binding kinase 1 (TBK1) activation and increased expression of the RhoA activator GEF-H1. Knockdown of ZO-3, another paralog interacting with ZO1, was sufficient to induce GEF-H1 expression and YAP activity. GEF-H1, TBK1, and mechanotransduction at focal adhesions were found to cooperate to activate YAP/TEAD in ZO-1-deficient cells. Thus, ZO-1 controled cell proliferation and Hippo-independent YAP activity by activating a GEF-H1- and TBK1-regulated mechanosensitive signalling network.

A retinoid analogue, TTNPB, promotes clonal expansion of human pluripotent stem cells by upregulating CLDN2 and HoxA1

Enzymatic dissociation of human pluripotent stem cells (hPSCs) into single cells during routine passage leads to massive cell death. Although the Rho-associated protein kinase inhibitor, Y-27632 can enhance hPSC survival and proliferation at high seeding density, dissociated single cells undergo apoptosis at clonal density. This presents a major hurdle when deriving genetically modified hPSC lines since transfection and genome editing efficiencies are not satisfactory. As a result, colonies tend to contain heterogeneous mixtures of both modified and unmodified cells, making it difficult to isolate the desired clone buried within the colony. In this study, we report improved clonal expansion of hPSCs using a retinoic acid analogue, TTNPB. When combined with Y-27632, TTNPB synergistically increased hPSC cloning efficiency by more than 2 orders of magnitude (0.2% to 20%), whereas TTNPB itself increased more than double cell number expansion compared to Y-27632. Furthermore, TTNPB-treated cells showed two times higher aggregate formation and cell proliferation compared to Y-27632 in suspension culture. TTNPB-treated cells displayed a normal karyotype, pluripotency and were able to stochastically differentiate into all three germ layers both in vitro and in vivo. TTNBP acts, in part, by promoting cellular adhesion and self-renewal through the upregulation of Claudin 2 and HoxA1. By promoting clonal expansion, TTNPB provides a new approach for isolating and expanding pure hPSCs for future cell therapy applications.

To squeeze or not: Regulation of cell size by mechanical forces in development and human diseases

Physical constraints, such as compression, shear stress, stretching and tension play major roles during development and tissue homeostasis. Mechanics directly impact physiology, and their alteration is also recognized as having an active role in driving human diseases. Recently, growing evidence has accumulated on how mechanical forces are translated into a wide panel of biological responses, including metabolism and changes in cell morphology. The aim of this review is to summarize and discuss our knowledge on the impact of mechanical forces on cell size regulation. Other biological consequences of mechanical forces will not be covered by this review. Moreover, wherever possible, we also discuss mechanosensors and molecular and cellular signaling pathways upstream of cell size regulation. We finally highlight the relevance of mechanical forces acting on cell size in physiology and human diseases.

Uveitic glaucoma-like features in Yap conditional knockout mice

Glaucoma is a multifactorial neurodegenerative disease characterized by the progressive and irreversible degeneration of the optic nerve and retinal ganglion cells. Despite medical advances aiming at slowing degeneration, around 40% of treated glaucomatous patients will undergo vision loss. It is thus of utmost importance to have a better understanding of the disease and to investigate more deeply its early causes. The transcriptional coactivator YAP, an important regulator of eye homeostasis, has recently drawn attention in the glaucoma research field. Here we show that Yap conditional knockout mice (Yap cKO), in which the deletion of Yap is induced in both Müller glia (i.e. the only retinal YAP-expressing cells) and the non-pigmented epithelial cells of the ciliary body, exhibit a breakdown of the aqueous-blood barrier, accompanied by a progressive collapse of the ciliary body. A similar phenotype is observed in human samples that we obtained from patients presenting with uveitis. In addition, aged Yap cKO mice harbor glaucoma-like features, including deregulation of key homeostatic Müller-derived proteins, retinal vascular defects, optic nerve degeneration and retinal ganglion cell death. Finally, transcriptomic analysis of Yap cKO retinas pointed to early-deregulated genes involved in extracellular matrix organization potentially underlying the onset and/or progression of the observed phenotype. Together, our findings reveal the essential role of YAP in preserving the integrity of the ciliary body and retinal ganglion cells, thereby preventing the onset of uveitic glaucoma-like features.

p120 RasGAP and ZO-2 are essential for Hippo signaling and tumor-suppressor function mediated by p190A RhoGAP

ARHGAP35, which encodes p190A RhoGAP (p190A), is a major cancer gene. p190A is a tumor suppressor that activates the Hippo pathway. p190A was originally cloned via direct binding to p120 RasGAP (RasGAP). Here, we determine that interaction of p190A with the tight-junction-associated protein ZO-2 is dependent on RasGAP. We establish that both RasGAP and ZO-2 are necessary for p190A to activate large tumor-suppressor (LATS) kinases, elicit mesenchymal-to-epithelial transition, promote contact inhibition of cell proliferation, and suppress tumorigenesis. Moreover, RasGAP and ZO-2 are required for transcriptional modulation by p190A. Finally, we demonstrate that low ARHGAP35 expression is associated with shorter survival in patients with high, but not low, transcript levels of TJP2 encoding ZO-2. Hence, we define a tumor-suppressor interactome of p190A that includes ZO-2, an established constituent of the Hippo pathway, and RasGAP, which, despite strong association with Ras signaling, is essential for p190A to activate LATS kinases.

YAP-mediated trophoblast dysfunction: the common pathway underlying pregnancy complications

Yes-associated protein (YAP) is a pivotal regulator in cellular proliferation, survival, differentiation, and migration, with significant roles in embryonic development, tissue repair, and tumorigenesis. At the maternal-fetal interface, emerging evidence underscores the importance of precisely regulated YAP activity in ensuring successful pregnancy initiation and progression. However, despite the established association between YAP dysregulation and adverse pregnancy outcomes, insights into the impact of aberrant YAP levels in fetal-derived, particularly trophoblast cells, and the ensuing dysfunction at the maternal-fetal interface remain limited. This review comprehensively examines YAP expression and its regulatory mechanisms in trophoblast cells throughout pregnancy. We emphasize its integral role in placental development and maternal-fetal interactions and delve into the correlations between YAP dysregulation and pregnancy complications. A nuanced understanding of YAP's functions during pregnancy could illuminate intricate molecular mechanisms and pave the way for innovative prevention and treatment strategies for pregnancy complications. Video Abstract.

Insertion of the CXXC domain of KMT2A into YAP1: An unusual mechanism behind the formation of a chimeric oncogenic protein

Most neoplasia-associated gene fusions are formed through the fusion of the 5'-part of one gene with the 3'-part of another. We here describe a unique mechanism, by which a part of the KMT2A gene through an insertion replaces part of the YAP1 gene. The resulting YAP1::KMT2A::YAP1 (YKY) fusion was verified by RT-PCR in three cases of sarcoma morphologically resembling sclerosing epithelioid fibrosarcoma (SEF-like sarcoma). In all cases, a portion (exons 4/5-6) encoding the CXXC domain of KMT2A was inserted between exon 4/5 and exon 8/9 of YAP1. The inserted sequence from KMT2A thus replaced exons 5/6-8 of YAP1, which encode an important regulatory sequence of YAP1. To evaluate the cellular impact of the YKY fusion, global gene expression profiles from fresh frozen and formalin-fixed YKY-expressing sarcomas were compared with control tumors. The effects of the YKY fusion, as well as YAP1::KMT2A and KMT2A::YAP1 fusion constructs, were further studied in immortalized fibroblasts. Analysis of differentially upregulated genes revealed significant overlap between tumors and cell lines expressing YKY, as well as with previously reported YAP1 fusions. Pathway analysis of upregulated genes in cells and tumors expressing YKY revealed an enrichment of genes included in key oncogenic signaling pathways, such as Wnt and Hedgehog. As these pathways are known to interact with YAP1, it seems likely that the pathogenesis of sarcomas with the YKY fusion is linked to distorted YAP1 signaling.

SPTAN1/NUMB axis senses cell density to restrain cell growth and oncogenesis through Hippo signaling

The loss of contact inhibition is a key step during carcinogenesis. The Hippo-Yes-associated protein (Hippo/YAP) pathway is an important regulator of cell growth in a cell density-dependent manner. However, how Hippo signaling senses cell density in this context remains elusive. Here, we report that high cell density induced the phosphorylation of spectrin α chain, nonerythrocytic 1 (SPTAN1), a plasma membrane-stabilizing protein, to recruit NUMB endocytic adaptor protein isoforms 1 and 2 (NUMB1/2), which further sequestered microtubule affinity-regulating kinases (MARKs) in the plasma membrane and rendered them inaccessible for phosphorylation and inhibition of the Hippo kinases sterile 20-like kinases MST1 and MST2 (MST1/2). WW45 interaction with MST1/2 was thereby enhanced, resulting in the activation of Hippo signaling to block YAP activity for cell contact inhibition. Importantly, low cell density led to SPTAN1 dephosphorylation and NUMB cytoplasmic location, along with MST1/2 inhibition and, consequently, YAP activation. Moreover, double KO of NUMB and WW45 in the liver led to appreciable organ enlargement and rapid tumorigenesis. Interestingly, NUMB isoforms 3 and 4, which have a truncated phosphotyrosine-binding (PTB) domain and are thus unable to interact with phosphorylated SPTAN1 and activate MST1/2, were selectively upregulated in liver cancer, which correlated with YAP activation. We have thus revealed a SPTAN1/NUMB1/2 axis that acts as a cell density sensor to restrain cell growth and oncogenesis by coupling external cell-cell contact signals to intracellular Hippo signaling.

The impact of interaction between verteporfin and yes-associated protein 1/transcriptional coactivator with PDZ-binding motif-TEA domain pathway on the progression of isocitrate dehydrogenase wild-type glioblastoma

Verteporfin and 5-ALA are used for visualizing malignant tissue components in different body tumors and as photodynamic therapy in treating isocitrate dehydrogenase (IDH) wild-type glioblastoma (GBM). Additionally, verteporfin interferes with Yes-associated protein 1 (YAP)/Transcriptional coactivator with PDZ-binding motif - TEA domain (TAZ-TEAD) pathway, thus inhibiting the downstream effect of these oncogenes and reducing the malignant properties of GBM. Animal studies have shown verteporfin to be successful in increasing survival rates, which have led to the conduction of phase 1 and 2 clinical trials to further investigate its efficacy in treating GBM. In this article, we aimed to review the novel mechanism of verteporfin's action, the impact of its interaction with YAP/TAZ-TEAD, its effect on glioblastoma stem cells, and its role in inducing ferroptosis.

p120 RasGAP and ZO-2 are essential for Hippo signaling and tumor suppressor function mediated by p190A RhoGAP

ARHGAP35 , which encodes p190A RhoGAP (p190A), is a major cancer gene. p190A is a tumor suppressor that activates the Hippo pathway. p190A was originally cloned via direct binding to p120 RasGAP (RasGAP). Here, we determine that a novel interaction of p190A with the tight junction-associated protein ZO-2 is dependent on RasGAP. We establish that both RasGAP and ZO-2 are necessary for p190A to activate LATS kinases, elicit mesenchymal-to-epithelial transition, promote contact inhibition of cell proliferation and suppress tumorigenesis. Moreover, RasGAP and ZO-2 are required for transcriptional modulation by p190A. Finally, we demonstrate that low ARHGAP35 expression is associated with shorter survival in patients with high, but not low, transcript levels of TJP2 encoding ZO-2. Hence, we define a tumor suppressor interactome of p190A that includes ZO-2, an established constituent of the Hippo pathway, and RasGAP, which despite strong association with Ras signaling, is essential for p190A to activate LATS kinases.

Optogenetic control of YAP can enhance the rate of wound healing

BACKGROUND

Tissues need to regenerate to restore function after injury. Yet, this regenerative capacity varies significantly between organs and between species. For example, in the heart, some species retain full regenerative capacity throughout their lifespan but human cardiac cells display a limited ability to repair the injury. After a myocardial infarction, the function of cardiomyocytes is impaired and reduces the ability of the heart to pump, causing heart failure. Therefore, there is a need to restore the function of an injured heart post myocardial infarction. We investigate in cell culture the role of the Yes-associated protein (YAP), a transcriptional co-regulator with a pivotal role in growth, in driving repair after injury.

METHODS

We express optogenetic YAP (optoYAP) in three different cell lines. We characterised the behaviour and function of optoYAP using fluorescence imaging and quantitative real-time PCR of downstream YAP target genes. Mutant constructs were generated using site-directed mutagenesis. Nuclear localised optoYAP was functionally tested using wound healing assay.

RESULTS

Utilising optoYAP, which enables precise control of pathway activation, we show that YAP induces the expression of downstream genes involved in proliferation and migration. optoYAP can increase the speed of wound healing in H9c2 cardiomyoblasts. Interestingly, this is not driven by an increase in proliferation, but by collective cell migration. We subsequently dissect specific phosphorylation sites in YAP to identify the molecular driver of accelerated healing.

CONCLUSIONS

This study shows that optogenetic YAP is functional in H9c2 cardiomyoblasts and its controlled activation can potentially enhance wound healing in a range of conditions.

ZNF582 overexpression restrains the progression of clear cell renal cell carcinoma by enhancing the binding of TJP2 and ERK2 and inhibiting ERK2 phosphorylation

Recent evidences have suggested that Zinc finger protein 582 (ZNF582) plays different important roles in various tumors, but its clinical role, biological function and regulatory mechanism in clear cell renal cell carcinoma (ccRCC) are still vague. Through analyzing GEO and TCGA-KIRC data and validation with local samples, we identified the low expression pattern of ZNF582 in ccRCC. Decreased ZNF582 expression is correlated with higher tumor stage and grade, distant metastasis and poor prognosis. By analyzing the DNA methylation data of ccRCC in TCGA-KIRC and using Massarray DNA methylation and demethylation analysis, we confirmed the hypermethylation status of ZNF582 in ccRCC and its negative regulation on ZNF582 expression. Using cell phenotype experiments and orthotopic kidney tumor growth models, we determined the inhibitory effect of ZNF582 overexpression on ccRCC growth and metastasis in vivo and in vitro. Mechanistically, using TMT (Tandem mass tags) quantitative proteomics test, Co-IP (Co-immunoprecipitation) and Western Blot experiments, we clarified that ZNF582 binds to TJP2 and up-regulates TJP2 protein expression. Increased TJP2 protein combines with ERK2 to promote ERK2 protein expression and suppresses the phosphorylation of ERK2, thereby inhibiting the growth and metastasis of ccRCC. In general, our findings provide the first solid theoretical rationale for targeting ZNF582/TJP2/ERK2 axis to improve ccRCC treatment.

Mucosal healing and inflammatory bowel disease: Therapeutic implications and new targets

Mucosal healing (MH) is vital in maintaining homeostasis within the gut and protecting against injury and infections. Multiple factors and signaling pathways contribute in a dynamic and coordinated manner to maintain intestinal homeostasis and mucosal regeneration/repair. However, when intestinal homeostasis becomes chronically disturbed and an inflammatory immune response is constitutively active due to impairment of the intestinal epithelial barrier autoimmune disease results, particularly inflammatory bowel disease (IBD). Many proteins and signaling pathways become dysregulated or impaired during these pathological conditions, with the mechanisms of regulation just beginning to be understood. Consequently, there remains a relative lack of broadly effective therapeutics that can restore MH due to the complexity of both the disease and healing processes, so tissue damage in the gastrointestinal tract of patients, even those in clinical remission, persists. With increased understanding of the molecular mechanisms of IBD and MH, tissue damage from autoimmune disease may in the future be ameliorated by developing therapeutics that enhance the body’s own healing response. In this review, we introduce the concept of mucosal healing and its relevance in IBD as well as discuss the mechanisms of IBD and potential strategies for altering these processes and inducing MH.

Targeting YAP1/TAZ in nonsmall-cell lung carcinoma: From molecular mechanisms to precision medicine

Accumulating evidence has underscored the importance of the Hippo-YAP1 signaling in lung tissue homeostasis, whereas its deregulation induces tumorigenesis. YAP1 and its paralog TAZ are the key downstream effectors tightly controlled by the Hippo pathway. YAP1/TAZ exerts oncogenic activities by transcriptional regulation via physical interaction with TEAD transcription factors. In solid tumors, Hippo-YAP1 crosstalks with other signaling pathways such as Wnt/β-catenin, receptor tyrosine kinase cascade, Notch and TGF-β to synergistically drive tumorigenesis. As YAP1/TAZ expression is significantly correlated with unfavorable outcomes for the patients, small molecules have been developed for targeting YAP1/TAZ to get a therapeutic effect. In this review, we summarize the recent findings on the deregulation of Hippo-YAP1 pathway in nonsmall cell lung carcinoma, discuss the molecular mechanisms of its dysregulation in leading to tumorigenesis, explore the therapeutic strategies for targeting YAP1/TAZ, and provide the research directions for deep investigation. We believe that detailed delineation of Hippo-YAP1 regulation in tumorigenesis provides novel insight for accurate therapeutic intervention.

Hippo pathway dysregulation in gastric cancer: from Helicobacter pylori infection to tumor promotion and progression

The Hippo pathway plays a critical role for balancing proliferation and differentiation, thus regulating tissue homeostasis. The pathway acts through a kinase cascade whose final effectors are the Yes-associated protein (YAP) and its paralog transcriptional co‑activator with PDZ‑binding motif (TAZ). In response to a variety of upstream signals, YAP and TAZ activate a transcriptional program that modulates cellular proliferation, tissue repair after injury, stem cell fate decision, and cytoskeletal reorganization. Hippo pathway signaling is often dysregulated in gastric cancer and in Helicobacter pylori-induced infection, suggesting a putative role of its deregulation since the early stages of the disease. In this review, we summarize the architecture and regulation of the Hippo pathway and discuss how its dysregulation fuels the onset and progression of gastric cancer. In this setting, we also focus on the crosstalk between Hippo and other established oncogenic signaling pathways. Lastly, we provide insights into the therapeutic approaches targeting aberrant YAP/TAZ activation and discuss the related clinical perspectives and challenges.

ZO-1 Guides Tight Junction Assembly and Epithelial Morphogenesis via Cytoskeletal Tension-Dependent and -Independent Functions

Formation and maintenance of tissue barriers require the coordination of cell mechanics and cell-cell junction assembly. Here, we combined methods to modulate ECM stiffness and to measure mechanical forces on adhesion complexes to investigate how tight junctions regulate cell mechanics and epithelial morphogenesis. We found that depletion of the tight junction adaptor ZO-1 disrupted junction assembly and morphogenesis in an ECM stiffness-dependent manner and led to a stiffness-dependant reorganisation of active myosin. Both junction formation and morphogenesis were rescued by inhibition of actomyosin contractility. ZO-1 depletion also impacted mechanical tension at cell-matrix and E-cadherin-based cell-cell adhesions. The effect on E-cadherin also depended on ECM stiffness and correlated with effects of ECM stiffness on actin cytoskeleton organisation. However, ZO-1 knockout also revealed tension-independent functions of ZO-1. ZO-1-deficient cells could assemble functional barriers at low tension, but their tight junctions remained corrupted with strongly reduced and discontinuous recruitment of junctional components. Our results thus reveal that reciprocal regulation between ZO-1 and cell mechanics controls tight junction assembly and epithelial morphogenesis, and that, in a second, tension-independent step, ZO-1 is required to assemble morphologically and structurally fully assembled and functionally normal tight junctions.

The Regulation of the Hippo Pathway by Intercellular Junction Proteins

The Hippo pathway is an evolutionarily conserved pathway that serves to promote cell death and differentiation while inhibiting cellular proliferation across species. The downstream effectors of this pathway, yes-associated protein (YAP) and transcriptional co-activator with PDZ-binding motif (TAZ), are considered vital in promoting the output of the Hippo pathway, with activation of upstream kinases negatively regulating YAP/TAZ activity. The upstream regulation of the Hippo pathway is not entirely understood on a molecular level. However, several studies have shown that numerous cellular and non-cellular mechanisms such as cell polarity, contact inhibition, soluble factors, mechanical forces, and metabolism can convey external stimuli to the intracellular kinase cascade, promoting the activation of key components of the Hippo pathway and therefore regulating the subcellular localisation and protein activity of YAP/TAZ. This review will summarise what we have learnt about the role of intercellular junction-associated proteins in the activation of this pathway, including adherens junctions and tight junctions, and in particular our latest findings about the desmosomal components, including desmoglein-3 (DSG3), in the regulation of YAP signalling, phosphorylation, and subcellular translocation.

ZO-2/Tjp2 suppresses Yap and Wwtr1/Taz-mediated hepatocyte to cholangiocyte transdifferentiation in the mouse liver

TJP2/ZO-2-inactivating mutations in humans cause progressive cholestatic liver disease. Liver-specific deletion of Tjp2 in the mouse (Tjp2 cKO mice) leads to mild progressive cholestasis without an overt degradation of the bile-blood barrier (BBB). These mice are more susceptible to cholic acid (CA) induced liver injury. Interestingly, while initially also more susceptible, Tjp2 cKO mice develop tolerance to a DDC-supplemented diet. The DDC diet induces an exacerbated ductular reaction in Tjp2 cKO mice, which arises from the transdifferentiation of hepatocytes to cholangiocytes. Consequently, this transdifferentiation is only observed if Tjp2 is inactivated in hepatocytes, but not if deleted in cholangiocytes. The DDC-diet-induced hepatocyte transdifferentiation in Tjp2 cKO mice requires Yap and Wwtr1/Taz, whose protein expression is upregulated in hepatocytes lacking Tjp2, but is independent of Notch2. Although inactivating Tjp2 is sufficient for the upregulation of Yap and Wwtr1/Taz protein, efficient transdifferentiation requires the DDC-diet insult. Thus, Tjp2 negatively regulates Yap/Taz-mediated transdifferentiation of hepatocytes to cholangiocytes in response to DDC-diet-induced liver injury. Furthermore, transdifferentiation is regulated at multiple levels and the type of injury inflicted on the Tjp2 deficient liver plays an important role in the resulting pathophysiology.

Hippo in Gastric Cancer: From Signalling to Therapy

The Hippo pathway is one of the most important ones in mammals. Its key functions in cell proliferation, tissue growth, repair, and homeostasis make it the most crucial one to be controlled. Many means have been deployed for its regulation, since this pathway is not only composed of core regulatory components, but it also communicates with and regulates various other pathways, making this signalisation even more complex. Its role in cancer has been studied more and more over the past few years, and it presents YAP/TAZ as the major oncogenic actors. In this review, we relate how vital this pathway is for different organs, and how regulatory mechanisms have been bypassed to lead to cancerous states. Most studies present an upregulation status of YAP/TAZ, and urge the need to target them. A focus is made here on gastric carcinogenesis, its main dysregulations, and the major strategies adopted and tested to counteract Hippo pathway disbalance in this disease. Hippo pathway targeting can be achieved by various means, which are described in this review. Many studies have tested different potential molecules, which are detailed hereby. Though not all tested in gastric cancer, they could represent a real interest.

Role of zonula occludens in gastrointestinal and liver cancers

A growing body of evidence suggests that tight junction (TJ) proteins play a crucial role in the pathogenesis of various diseases, including gastrointestinal (GI) cancer and hepatocellular carcinoma (HCC). TJ proteins primarily maintain the epithelial and endothelial cells intact together through integral proteins however, recent reports suggest that they also regulate gene expression necessary for cell proliferation, angiogenesis, and metastasis through adapter proteins such as zonula occludens (ZO). ZO proteins are membrane-associated cytosolic scaffolding proteins that modulate cell proliferation by interacting with several transcription factors. Reduced ZO proteins in GI cancer and HCC are correlated with tumor development and poor prognosis. Pubmed has searched for using the keyword ZO and gastric cancer, ZO and cancer, and ZO and HCC for the last ten years to date. This review summarized the role of ZO proteins in cell proliferation and their expression in GI cancer and HCC. Furthermore, therapeutic interventions targeting ZO in GI and liver cancers are reviewed.

Mechanical control of nuclear import by Importin-7 is regulated by its dominant cargo YAP

Mechanical forces regulate multiple essential pathways in the cell. The nuclear translocation of mechanoresponsive transcriptional regulators is an essential step for mechanotransduction. However, how mechanical forces regulate the nuclear import process is not understood. Here, we identify a highly mechanoresponsive nuclear transport receptor (NTR), Importin-7 (Imp7), that drives the nuclear import of YAP, a key regulator of mechanotransduction pathways. Unexpectedly, YAP governs the mechanoresponse of Imp7 by forming a YAP/Imp7 complex that responds to mechanical cues through the Hippo kinases MST1/2. Furthermore, YAP behaves as a dominant cargo of Imp7, restricting the Imp7 binding and the nuclear translocation of other Imp7 cargoes such as Smad3 and Erk2. Thus, the nuclear import process is an additional regulatory layer indirectly regulated by mechanical cues, which activate a preferential Imp7 cargo, YAP, which competes out other cargoes, resulting in signaling crosstalk.

The translation of mechanical cues into gene expression changes is dependent on the nuclear import of mechanoresponsive transcriptional regulators. Here the authors identify that Importin-7 drives the nuclear import of one such regulator YAP while YAP then controls Importin-7 response to mechanical cues and restricts Importin-7 binding to other cargoes.

The Hippo pathway in cancer: YAP/TAZ and TEAD as therapeutic targets in cancer

Tumorigenesis is a highly complex process, involving many interrelated and cross-acting signalling pathways. One such pathway that has garnered much attention in the field of cancer research over the last decade is the Hippo signalling pathway. Consisting of two antagonistic modules, the pathway plays an integral role in both tumour suppressive and oncogenic processes, generally via regulation of a diverse set of genes involved in a range of biological functions. This review discusses the history of the pathway within the context of cancer and explores some of the most recent discoveries as to how this critical transducer of cellular signalling can influence cancer progression. A special focus is on the various recent efforts to therapeutically target the key effectors of the pathway in both preclinical and clinical settings.

The LINC Complex Assists the Nuclear Import of Mechanosensitive Transcriptional Regulators

Mechanical forces play pivotal roles in directing cell functions and fate. To elicit gene expression, either intrinsic or extrinsic mechanical information are transmitted into the nucleus beyond the nuclear envelope via at least two distinct pathways, possibly more. The first and well-known pathway utilizes the canonical nuclear transport of mechanoresponsive transcriptional regulators through the nuclear pore complex, which is an exclusive route for macromolecular trafficking between the cytoplasm and nucleoplasm. The second pathway depends on the linker of the nucleoskeleton and cytoskeleton (LINC) complex, which is a molecular bridge traversing the nuclear envelope between the cytoskeleton and nucleoskeleton. This protein complex is a central component in mechanotransduction at the nuclear envelope that transmits mechanical information from the cytoskeleton into the nucleus to influence the nuclear structure, nuclear stiffness, chromatin organization, and gene expression. Besides the mechanical force transducing function, recent increasing evidence shows that the LINC complex plays a role in controlling nucleocytoplasmic transport of mechanoresponsive transcriptional regulators. Here we discuss recent findings regarding the contribution of the LINC complex to the regulation of intracellular localization of the most-notable mechanosensitive transcriptional regulators, β-catenin, YAP, and TAZ.

Evidence for a nuclear role for Drosophila Dlg as a regulator of the NURF complex

Scribble (Scrib), Discs-large (Dlg), and Lethal giant larvae (Lgl) are basolateral regulators of epithelial polarity and tumor suppressors whose molecular mechanisms of action remain unclear. We used proximity biotinylation to identify proteins localized near Dlg in the Drosophila wing imaginal disc epithelium. In addition to expected membrane- and cytoskeleton-associated protein classes, nuclear proteins were prevalent in the resulting mass spectrometry dataset, including all four members of the nucleosome remodeling factor (NURF) chromatin remodeling complex. Subcellular fractionation demonstrated a nuclear pool of Dlg and proximity ligation confirmed its position near the NURF complex. Genetic analysis showed that NURF activity is also required for the overgrowth of dlg tumors, and this growth suppression correlated with a reduction in Hippo pathway gene expression. Together, these data suggest a nuclear role for Dlg in regulating chromatin and transcription through a more direct mechanism than previously thought.

Identification, subcellular localization, and functional comparison of novel Yap splicing isoforms in mouse embryonic stem cells

Hippo signaling pathway is involved in many biological processes including the fate decision of embryonic stem cells (ESCs). Yes-associated protein (Yap) function as a key effector of Hippo pathway, but its role in ESCs is still controversial. So far, only two isoforms of Yap have been identified and they have both overlapping and distinct functions. Here, we identify six novel isoforms of mouse Yap, bringing the total number of isoforms to eight. According to the differences in the first exon, they are divided into two subtypes (a and b). Isoform-a and isoform-b exhibit different subcellular localizations. Moreover, isoform-a can fully reverse the impaired self-renewal phenotype induced by Yap knockout (KO). Upon overexpression, isoform-a moderately promotes mESCs self-renewal and markedly delays differentiation. On the contrary, no significant pro-self-renewal phenotype is observed when isoform-b overexpressed in wildtype (WT) mESCs or re-expressed in Yap KO cell lines. These finding not only help to clarify the role of Yap in mESCs, but also lay the foundation for advancing functional researches of Yap in other processes.

Yes-Associated Protein Is Required for ZO-1-Mediated Tight-Junction Integrity and Cell Migration in E-Cadherin-Restored AGS Gastric Cancer Cells

Yes-associated protein (YAP) regulates numerous cellular homeostasis processes and malignant transformation. We found that YAP influences ZO-1-mediated cell migration using E-cadherin-restored EC96 cells derived from gastric malignant AGS cells. Ectopic expression of E-cadherin enhanced straightforward migration of cells, in comparison to the meandering movement of parental AGS cells. In EC96 cells, YAP and ZO-1 expression increased but nuclear YAP levels and activity were reduced. Nuclear factor-κB (NF-κB) mediated the increase in ZO-1 expression, possibly stabilizing cytoplasmic YAP post-translationally. Downregulation of YAP expression using siYAP RNA or stable knock-down inhibited straightforward cell migration by fragmenting ZO-1 containing tight junctions (TJs) but not adherens junctions, implying involvement of YAP in ZO-1-mediated cell migration. The association of YAP with ZO-1 was mediated by angiomotin (AMOT) because downregulation of AMOT dissociated YAP from ZO-1 and reduced cell migration. E-cadherin restoration in malignant cancer cells induced NF-κB signaling to enhance ZO-1 expression and subsequently stabilize YAP. At high expression levels, YAP associates with ZO-1 via AMOT at TJs, influencing ZO-1-mediated cell migration and maintaining TJ integrity.

Strong as a Hippo's Heart: Biomechanical Hippo Signaling During Zebrafish Cardiac Development

The heart is comprised of multiple tissues that contribute to its physiological functions. During development, the growth of myocardium and endocardium is coupled and morphogenetic processes within these separate tissue layers are integrated. Here, we discuss the roles of mechanosensitive Hippo signaling in growth and morphogenesis of the zebrafish heart. Hippo signaling is involved in defining numbers of cardiac progenitor cells derived from the secondary heart field, in restricting the growth of the epicardium, and in guiding trabeculation and outflow tract formation. Recent work also shows that myocardial chamber dimensions serve as a blueprint for Hippo signaling-dependent growth of the endocardium. Evidently, Hippo pathway components act at the crossroads of various signaling pathways involved in embryonic zebrafish heart development. Elucidating how biomechanical Hippo signaling guides heart morphogenesis has direct implications for our understanding of cardiac physiology and pathophysiology.

Role of the Hippo pathway and mechanisms for controlling cellular localization of YAP/TAZ

The Hippo pathway is a crucial signaling mechanism that inhibits the growth of cells and organs during development and in disease. When the Hippo pathway is activated, YAP/TAZ transcriptional coactivators are phosphorylated by upstream kinases, preventing nuclear localization of YAP/TAZ. However, when the Hippo pathway is inhibited, YAP/TAZ localize mainly in the nucleus and induce the expression of target genes related to cell proliferation. Abnormal proliferation of cells is one of the hallmarks of cancer initiation, and activation of Hippo pathway dampens such cell proliferation. Various types of diseases including cancer can occur due to the dysregulation of the Hippo pathway. Therefore, a better understanding of the Hippo pathway signaling mechanisms, and in particular how YAP/TAZ exist in the nucleus, may lead to the identification of new therapeutic targets for treating cancer and other diseases. In this review, we summarize the overall Hippo pathway and discuss mechanisms related to nuclear localization of YAP/TAZ.

From the Matrix to the Nucleus and Back: Mechanobiology in the Light of Health, Pathologies, and Regeneration of Oral Periodontal Tissues

Among oral tissues, the periodontium is permanently subjected to mechanical forces resulting from chewing, mastication, or orthodontic appliances. Molecularly, these movements induce a series of subsequent signaling processes, which are embedded in the biological concept of cellular mechanotransduction (MT). Cell and tissue structures, ranging from the extracellular matrix (ECM) to the plasma membrane, the cytosol and the nucleus, are involved in MT. Dysregulation of the diverse, fine-tuned interaction of molecular players responsible for transmitting biophysical environmental information into the cell's inner milieu can lead to and promote serious diseases, such as periodontitis or oral squamous cell carcinoma (OSCC). Therefore, periodontal integrity and regeneration is highly dependent on the proper integration and regulation of mechanobiological signals in the context of cell behavior. Recent experimental findings have increased the understanding of classical cellular mechanosensing mechanisms by both integrating exogenic factors such as bacterial gingipain proteases and newly discovered cell-inherent functions of mechanoresponsive co-transcriptional regulators such as the Yes-associated protein 1 (YAP1) or the nuclear cytoskeleton. Regarding periodontal MT research, this review offers insights into the current trends and open aspects. Concerning oral regenerative medicine or weakening of periodontal tissue diseases, perspectives on future applications of mechanobiological principles are discussed.

RAB11A-mediated YAP localization to adherens and tight junctions is essential for colonic epithelial integrity

Within the intestinal epithelium, regulation of intracellular protein and vesicular trafficking is of utmost importance for barrier maintenance, immune responses, and tissue polarity. RAB11A is a small GTPase that mediates the anterograde transport of protein cargos to the plasma membrane. Loss of RAB11A-dependent trafficking in mature intestinal epithelial cells results in increased epithelial proliferation and nuclear accumulation of Yes-associated protein (YAP), a key Hippo-signaling transducer that senses cell–cell contacts and regulates tissue growth. However, it is unclear how RAB11A regulates YAP intracellular localizations. In this report, we examined the relationship of RAB11A to epithelial junctional complexes, YAP, and the associated consequences on colonic epithelial tissue repair. We found that RAB11A controls the biochemical associations of YAP with multiple components of adherens and tight junctions, including α-catenin, β-catenin, and Merlin, a tumor suppressor. In the absence of RAB11A and Merlin, we observed enhanced YAP–β-catenin complex formation and nuclear translocation. Upon chemical injury to the intestine, mice deficient in RAB11A were found to have reduced epithelial integrity, decreased YAP localization to adherens and tight junctions, and increased nuclear YAP accumulation in the colon epithelium. Thus, RAB11A-regulated trafficking regulates the Hippo–YAP signaling pathway for rapid reparative response after tissue injury.

Tight junction protein ZO-2 modulates the nuclear accumulation of transcription factor TEAD

The presence of tight junction protein zonula occludens 2 (ZO-2) at the nucleus inhibits the transcription of genes regulated by TEAD transcription factor. Here, we analyzed whether the movement of ZO-2 into the nucleus modulates the nuclear concentration of TEAD. In sparse cultures of ZO-2 knockdown Madin–Darby canine kidney cells, nuclear TEAD was diminished, as in parental cells transfected with a ZO-2 construct without nuclear localization signals, indicating that ZO-2 facilitates the entry of TEAD into the nucleus. Inhibition of nPKCδ in parental cells triggers the interaction between ZO-2 and TEAD at the cytoplasm and facilitates TEAD/ZO-2 complex nuclear importation. Using proximity ligation, immunoprecipitation, and pull-down assays, TEAD/ZO-2 interaction was confirmed. Nuclear TEAD is phosphorylated, and its exit in parental cells is enhanced by activation of a ZO-2 nuclear exportation signal by nPKCε, while the nuclear accumulation of ZO-2 triggered by the mutation of ZO-2 nuclear export signals induces no change in TEAD nuclear concentration. In summary, our results indicate that the movements of ZO-2 in and out of the nucleus modulate the intracellular traffic of TEAD through a process regulated by nPKCδ and ε and provide a novel role of ZO-2 as a nuclear translocator of TEAD.

Endothelial connexin-integrin crosstalk in vascular inflammation

Cardiovascular diseases including blood vessel disorders represent a major cause of death globally. The essential roles played by local and systemic vascular inflammation in the pathogenesis of cardiovascular diseases have been increasingly recognized. Vascular inflammation triggers the aberrant activation of endothelial cells, which leads to the functional and structural abnormalities in vascular vessels. In addition to humoral mediators such as pro-inflammatory cytokines and prostaglandins, the alteration of physical and mechanical microenvironment - including vascular stiffness and shear stress - modify the gene expression profiles and metabolic profiles of endothelial cells via mechano-transduction pathways, thereby contributing to the pathogenesis of vessel disorders. Notably, connexins and integrins crosstalk each other in response to the mechanical stress, and, thereby, play an important role in regulating the mechano-transduction of endothelial cells. Here, we provide an overview on how the inter-play between connexins and integrins in endothelial cells unfold during the mechano-transduction in vascular inflammation.

YAP1 and its fusion proteins in cancer initiation, progression and therapeutic resistance

YAP1 is a transcriptional co-activator whose activity is controlled by the Hippo signaling pathway. In addition to important functions in normal tissue homeostasis and regeneration, YAP1 has also prominent functions in cancer initiation, aggressiveness, metastasis, and therapy resistance. In this review we are discussing the molecular functions of YAP1 and its roles in cancer, with a focus on the different mechanisms of de-regulation of YAP1 activity in human cancers, including inactivation of upstream Hippo pathway tumor suppressors, regulation by intersecting pathways, miRNAs, and viral oncogenes. We are also discussing new findings on the function and biology of the recently identified family of YAP1 gene fusions, that constitute a new type of activating mutation of YAP1 and that are the likely oncogenic drivers in several subtypes of human cancers. Lastly, we also discuss different strategies of therapeutic inhibition of YAP1 functions.

The Hippo-YAP Signaling as Guardian in the Pool of Intestinal Stem Cells

Despite endogenous insults such as mechanical stress and danger signals derived from the microbiome, the intestine can maintain its homeostatic condition through continuous self-renewal of the crypt–villus axis. This extraordinarily rapid turnover of intestinal epithelium, known to be 3 to 5 days, can be achieved by dynamic regulation of intestinal stem cells (ISCs). The crypt base-located leucine-rich repeat-containing G-protein-coupled receptor 5-positive (Lgr5⁺) ISCs maintain intestinal integrity in the steady state. Under severe damage leading to the loss of conventional ISCs, quiescent stem cells and even differentiated cells can be reactivated into stem-cell-like cells with multi-potency and contribute to the reconstruction of the intestinal epithelium. This process requires fine-tuning of the various signaling pathways, including the Hippo–YAP system. In this review, we summarize recent advances in understanding the correlation between Hippo–YAP signaling and intestinal homeostasis, repair, and tumorigenesis, focusing specifically on ISC regulation.

Role of Clostridium perfringens Enterotoxin on YAP Activation in Colonic Sessile Serrated Adenoma/ Polyps with Dysplasia

Sessile serrated adenoma/polyp with dysplasia (SSA/P-D) is an SSA/P with cellular dysplasia and has a higher risk of progressing to colon carcinogenesis. Previously, we reported that tight junction impairment by Clostridiumperfringens enterotoxin (CPE) leads to activation of the transcriptional co-activator yes-associated protein (YAP) in oral squamous cell carcinoma. Here, we investigated whether CPE activates YAP to promote the malignant progression of SSA/P. E-cadherin expression was lower in the 12 cases with SSA/P-D examined than that in normal mucosa, SSA/P, or tubular adenoma (TA). Furthermore, intracellular translocation of claudin-4 (CLDN4) and nuclear translocation of YAP were observed. The CPE gene was detected in DNA extracted from SSA/P-D lesions, but not in SSA/P or TA. Treatment of the rat intestinal epithelial cell line IEC6 with low-dose CPE resulted in intracellular translocation of CLDN4 to the cytoplasmic membrane. Cytoplasmic CLDN4 showed co-precipitation with transcriptional co-activator with PDZ-binding motif, zonula occludens (ZO)-1, large tumor suppressor, and mammalian Ste20-like. Additionally, YAP co-precipitated with ZO-2 under CPE treatment led to decreased YAP phosphorylation and nuclear translocation. YAP activation promoted increase in nuclear TEA domain family member level, expression of cyclin D1, snail, vimentin, CD44, NS and decrease in E-cadherin levels, thereby inducing stemness and epithelial-mesenchymal-transition (EMT). The Hippo complex with the incorporation of CLDN4 increased stability. Upon low-dose CPE treatment, HT29 cells with BRAF^V600E gene mutation showed increased growth, enhanced invasive potential, stemness, and induced EMT phenotype, whereas HCT116 cells, which carry KRAS^G13D gene mutation, did not show such changes. In an examination of 10 colorectal cancers, an increase in EMT and stemness was observed in CPE (+) and BRAF mutation (+) cases. These findings suggest that C.perfringens might enhance the malignant transformation of SSA/P-D via YAP activation. Our findings further highlight the importance of controlling intestinal flora using probiotics or antibiotics.

Non-cell autonomous promotion of astrogenesis at late embryonic stages by constitutive YAP activation

Although astrocytes have gained increased recognition as an important regulator in normal brain function and pathology, the mechanisms underlying their genesis are not well understood. In this study, we show that constitutive YAP activation by in utero introduction of a non-degradable form of the YAP gene (YAP 5SA) causes productive GFAP⁺ cell generation at late embryonic periods, and this activity is nuclear localization- and TEAD transcription factor-dependent. Moreover, we found that the GFAP⁺ cells were not YAP 5SA-expressing cells themselves but cells in the vicinity in vivo. Conditioned medium prepared from YAP 5SA-expressing cells induced GFAP⁺ cell production in vitro, suggesting that a soluble factor(s) was mediating the astrogenic activity of YAP 5SA. Indeed, YAP 5SA expression greatly increased CNTF and BMP4 transcription in neural progenitor cells, and a neutralizing antibody against CNTF reduced the astrogenic effects of YAP 5SA-conditioned medium. Furthermore, the YAP 5SA-expressing cells were identified as FN1⁺ mesenchymal cells which are responsible for the precocious astrogenesis. These results suggest a novel molecular mechanism by which YAP activation can induce astrogenesis in a non-cell autonomous manner.

Relationship between apical junction proteins, gene expression and cancer

The apical junctional complex (AJC) is a cell-cell adhesion system present at the upper portion of the lateral membrane of epithelial cells integrated by the tight junction (TJ) and the adherens junction (AJ). This complex is crucial to initiate and stabilize cell-cell adhesion, to regulate the paracellular transit of ions and molecules and to maintain cell polarity. Moreover, we now consider the AJC as a hub of signal transduction that regulates cell-cell adhesion, gene transcription and cell proliferation and differentiation. The molecular components of the AJC are multiple and diverse and depending on the cellular context some of the proteins in this complex act as tumor suppressors or as promoters of cell transformation, migration and metastasis outgrowth. Here, we describe these new roles played by TJ and AJ proteins and their potential use in cancer diagnostics and as targets for therapeutic intervention.

Bile canaliculi remodeling activates YAP via the actin cytoskeleton during liver regeneration

The mechanisms of organ size control remain poorly understood. A key question is how cells collectively sense the overall status of a tissue. We addressed this problem focusing on mouse liver regeneration. Using digital tissue reconstruction and quantitative image analysis, we found that the apical surface of hepatocytes forming the bile canalicular network expands concomitant with an increase in F-actin and phospho-myosin, to compensate an overload of bile acids. These changes are sensed by the Hippo transcriptional co-activator YAP, which localizes to apical F-actin-rich regions and translocates to the nucleus in dependence of the integrity of the actin cytoskeleton. This mechanism tolerates moderate bile acid fluctuations under tissue homeostasis, but activates YAP in response to sustained bile acid overload. Using an integrated biophysical-biochemical model of bile pressure and Hippo signaling, we explained this behavior by the existence of a mechano-sensory mechanism that activates YAP in a switch-like manner. We propose that the apical surface of hepatocytes acts as a self-regulatory mechano-sensory system that responds to critical levels of bile acids as readout of tissue status.

Clostridium perfringens enterotoxin induces claudin-4 to activate YAP in oral squamous cell carcinomas

Claudin (CLDN)-4 expression has been associated with malignancy in various cancers. When CLDN4 expression was examined in oral squamous cell carcinoma (OSCC), 22 out of 57 (39%) cases showed immunoreactivity in the nucleus. Nuclear CLDN4-positive cases showed a stronger correlation with cancer progression than the negative cases. Intratumoral anaerobic bacterial DNA examination revealed nuclear CLDN4 expression in 81% of Clostridium perfringens-positive cases. Treatment of human oral squamous cell carcinoma cell lines HSC3 and HSC4 with Clostridium perfringens enterotoxin (CPE), induced CLDN4 nuclear translocation to enhance epithelial-mesenchymal transition (EMT), stemness, cell proliferation and invasive ability. In addition, CPE treatment suppressed phosphorylation of yes-associated protein-1 (YAP1) and promoted YAP1 nuclear translocation, resulting in increased expression of YAP1 target genes; cyclin D1 and connective tissue growth factor. Moreover, it was revealed that the complex of YAP1, CLDN4 and zona occludens-2 (ZO-2) was formed by CPE treatment, further suppressing YAP1 phosphorylation by LATS1 and activating it. Thus YAP activation in OSCC was regarded important in promoting malignant phenotypes. Our research suggested that the control of oral anaerobic bacteria may suppress YAP activation and in turn tumor progression.

The Hippo Pathway and Viral Infections

The Hippo signaling pathway is a novel tumor suppressor pathway, initially found in Drosophila. Recent studies have discovered that the Hippo signaling pathway plays a critical role in a wide range of biological processes, including organ size control, cell proliferation, cancer development, and virus-induced diseases. In this review, we summarize the current understanding of the biological feature and pathological role of the Hippo pathway, focusing particularly on current findings in the function of the Hippo pathway in virus infection and pathogenesis.

Claudin-2 suppresses GEF-H1, RHOA, and MRTF, thereby impacting proliferation and profibrotic phenotype of tubular cells

The tight junctional pore-forming protein claudin-2 (CLDN-2) mediates paracellular Na⁺ and water transport in leaky epithelia and alters cancer cell proliferation. Previously, we reported that tumor necrosis factor-α time-dependently alters CLDN-2 expression in tubular epithelial cells. Here, we found a similar expression pattern in a mouse kidney injury model (unilateral ureteral obstruction), consisting of an initial increase followed by a drop in CLDN-2 protein expression. CLDN-2 silencing in LLC-PK¹ tubular cells induced activation and phosphorylation of guanine nucleotide exchange factor H1 (GEF-H1), leading to Ras homolog family member A (RHOA) activation. Silencing of other claudins had no such effects, and re-expression of an siRNA-resistant CLDN-2 prevented RHOA activation, indicating specific effects of CLDN-2 on RHOA. Moreover, kidneys from CLDN-2 knockout mice had elevated levels of active RHOA. Of note, CLDN-2 silencing reduced LLC-PK1 cell proliferation and elevated expression of cyclin-dependent kinase inhibitor P27 (P27KIP1) in a GEF-H1/RHOA-dependent manner. P27KIP1 silencing abrogated the effects of CLDN-2 depletion on proliferation. CLDN-2 loss also activated myocardin-related transcription factor (MRTF), a fibrogenic RHOA effector, and elevated expression of connective tissue growth factor and smooth muscle actin. Finally, CLDN-2 down-regulation contributed to RHOA activation and smooth muscle actin expression induced by prolonged tumor necrosis factor-α treatment, because they were mitigated by re-expression of CLDN-2. Our results indicate that CLDN-2 suppresses GEF-H1/RHOA. CLDN-2 down-regulation, for example, by inflammation, can reduce proliferation and promote MRTF activation through RHOA. These findings suggest that the initial CLDN-2 elevation might aid epithelial regeneration, and CLDN-2 loss could contribute to fibrotic reprogramming.

ZO-2 Is a Master Regulator of Gene Expression, Cell Proliferation, Cytoarchitecture, and Cell Size

ZO-2 is a cytoplasmic protein of tight junctions (TJs). Here, we describe ZO-2 involvement in the formation of the apical junctional complex during early development and in TJ biogenesis in epithelial cultured cells. ZO-2 acts as a scaffold for the polymerization of claudins at TJs and plays a unique role in the blood–testis barrier, as well as at TJs of the human liver and the inner ear. ZO-2 movement between the cytoplasm and nucleus is regulated by nuclear localization and exportation signals and post-translation modifications, while ZO-2 arrival at the cell border is triggered by activation of calcium sensing receptors and corresponding downstream signaling. Depending on its location, ZO-2 associates with junctional proteins and the actomyosin cytoskeleton or a variety of nuclear proteins, playing a role as a transcriptional repressor that leads to inhibition of cell proliferation and transformation. ZO-2 regulates cell architecture through modulation of Rho proteins and its absence induces hypertrophy due to inactivation of the Hippo pathway and activation of mTOR and S6K. The interaction of ZO-2 with viral oncoproteins and kinases and its silencing in diverse carcinomas reinforce the view of ZO-2 as a tumor regulator protein.

The Hippo Signaling Pathway in Development and Disease

The Hippo signaling pathway regulates diverse physiological processes, and its dysfunction has been implicated in an increasing number of human diseases, including cancer. Here, we provide an updated review of the Hippo pathway; discuss its roles in development, homeostasis, regeneration, and diseases; and highlight outstanding questions for future investigation and opportunities for Hippo-targeted therapies.