The Hippo pathway in intestinal regeneration and disease

Expression of the Hippo transducer TAZ in association with WNT pathway mutations impacts survival outcomes in advanced gastric cancer patients treated with first-line chemotherapy

Background

An extensive crosstalk co-regulates the Hippo and Wnt pathway. Preclinical studies revealed that the Hippo transducers YAP/TAZ mediate a number of oncogenic functions in gastric cancer (GC). Moreover, comprehensive characterization of GC demonstrated that the Wnt pathway is targeted by oncogenic mutations. On this ground, we hypothesized that YAP/TAZ- and Wnt-related biomarkers may predict clinical outcomes in GC patients treated with chemotherapy.

Methods

In the present study, we included 86 patients with advanced GC treated with first-line chemotherapy in prospective phase II trials or in routine clinical practice. Tissue samples were immunostained to evaluate the expression of YAP/TAZ. Mutational status of key Wnt pathway genes (CTNNB1, APC and FBXW7) was assessed by targeted DNA next-generation sequencing (NGS). Survival curves were estimated and compared by the Kaplan–Meier product-limit method and the log-rank test, respectively. Variables potentially affecting progression-free survival (PFS) were verified in univariate Cox proportional hazard models. The final multivariate Cox models were obtained with variables testing significant at the univariate analysis, and by adjusting for all plausible predictors of the outcome of interest (PFS).

Results

We observed a significant association between TAZ expression and Wnt mutations (Chi-squared p = 0.008). Combined TAZ expression and Wnt mutations (TAZ^pos/WNT^mut) was more frequently observed in patients with the shortest progression-free survival (negative outliers) (Fisher p = 0.021). Uni-and multivariate Cox regression analyses revealed that patients whose tumors harbored the TAZ^pos/WNT^mut signature had an increased risk of disease progression (univariate Cox: HR 2.27, 95% CI 1.27–4.05, p = 0.006; multivariate Cox: HR 2.73, 95% CI 1.41–5.29, p = 0.003). Finally, the TAZ^pos/WNT^mut signature negatively impacted overall survival.

Conclusions

Collectively, our findings indicate that the oncogenic YAP/TAZ–Wnt crosstalk may be active in GC, conferring chemoresistant traits that translate into adverse survival outcomes.

Electronic supplementary material

The online version of this article (10.1186/s12967-018-1385-y) contains supplementary material, which is available to authorized users.

Down-regulation of transcription factor OVOL2 contributes to epithelial-mesenchymal transition in a noninvasive type of trophoblast implantation to the maternal endometrium

Embryo implantation into the uterine endometrium is required for pregnancy establishment in most mammals. By using global expression analysis, we investigated the molecules that are related to epithelial-mesenchymal transition (EMT) in noninvasive bovine trophoblasts and found that the transcription factor, ovo-like zinc finger 2 ( OVOL2), which is essential for mesenchymal-epithelial transition in various cancers, was down-regulated after trophoblast attachment to the endometrial epithelium in utero. In cultured bovine trophoblast cells, OVOL2 down-regulation occurred only when cells were allowed to attach to bovine endometrial epithelial cells via the TEAD3/YAP signaling pathway. This resulted in the up-regulation of the EMT-associated transcription factors, ZEB1 and SNAI2, and the mesenchymal cell markers, N-cadherin ( CDH2) and vimentin ( VIM), whereas epithelial cell marker, E-cadherin ( CDH1), was down-regulated. In contrast, OVOL2 overexpression in bovine trophoblast cells exhibited a decrease in ZEB1 transcripts and an increase in E-cadherin. These observations revealed that ovo-like protein (OVOL)2 down-regulation occurred concurrently with conceptus implantation into the uterine endometrium via the YAP/TEAD3 signaling pathway, and suggest that the down-regulation of OVOL2 expression contributes to the up-regulation of EMT-related transcription factor expression, which enables EMT progression in the noninvasive bovine trophectoderm postimplantation.-Bai, R., Kusama, K., Nakamura, K., Sakurai, T., Kimura, K., Ideta, A., Aoyagi, Y., Imakawa, K. Down-regulation of transcription factor OVOL2 contributes to epithelial-mesenchymal transition in a noninvasive type of trophoblast implantation to the maternal endometrium.

The Hippo Signaling Pathway in Pancreatic β-Cells: Functions and Regulations

Hippo signaling is an evolutionarily conserved pathway that critically regulates development and homeostasis of various tissues in response to a wide range of extracellular and intracellular signals. As an emerging important player in many diseases, the Hippo pathway is also involved in the pathophysiology of diabetes on the level of the pancreatic islets. Multiple lines of evidence uncover the importance of Hippo signaling in pancreas development as well as in the regulation of β-cell survival, proliferation, and regeneration. Hippo therefore represents a potential target for therapeutic agents designed to improve β-cell function and survival in diabetes. In this review, we summarize recent data on the regulation of the Hippo signaling pathway in the pancreas/in pancreatic islets, its functions on β-cell homeostasis in physiology and pathophysiology, and its contribution toward diabetes progression. The current knowledge related to general mechanisms of action and the possibility of exploiting the Hippo pathway for therapeutic approaches to block β-cell failure in diabetes is highlighted.

A role for Hippo/YAP-signaling in FGF-induced lens epithelial cell proliferation and fibre differentiation

Recent studies indicate an important role for the transcriptional co-activator Yes-associated protein (YAP), and its regulatory pathway Hippo, in controlling cell growth and fate during lens development; however, the exogenous factors that promote this pathway are yet to be identified. Given that fibroblast growth factor (FGF)-signaling is an established regulator of lens cell behavior, the current study investigates the relationship between this pathway and Hippo/YAP-signaling during lens cell proliferation and fibre differentiation. Rat lens epithelial explants were cultured with FGF2 to induce epithelial cell proliferation or fibre differentiation. Immunolabeling methods were used to detect the expression of Hippo-signaling components, Total and Phosphorylated YAP, as well as fibre cell markers, Prox-1 and β-crystallin. FGF-induced lens cell proliferation was associated with a strong nuclear localisation of Total-YAP and low-level immuno-staining for phosphorylated-YAP. FGF-induced lens fibre differentiation was associated with a significant increase in cytoplasmic phosphorylated YAP (inactive state) and enhanced expression of core Hippo-signaling components. Inhibition of YAP with Verteporfin suppressed FGF-induced lens cell proliferation and ablated cell elongation during lens fibre differentiation. Inhibition of either FGFR- or MEK/ERK-signaling suppressed FGF-promoted YAP nuclear translocation. Here we propose that FGF promotes Hippo/YAP-signaling during lens cell proliferation and differentiation, with FGF-induced nuclear-YAP expression playing an essential role in promoting the proliferation of lens epithelial cells. An FGF-induced switch from proliferation to differentiation, hence regulation of lens growth, may play a key role in mediating Hippo suppression of YAP transcriptional activity.

Targeting IRF3 as a YAP agonist therapy against gastric cancer

Jiao et al. show that the key player of antiviral immunity IRF3 binds to and promotes the transactivation of the YAP–TEAD4 complex to coregulate transcription of Hippo pathway target genes and that therapeutic targeting of IRF3 suppresses YAP-driven gastric cancer.

The Hippo pathway plays a vital role in tissue homeostasis and tumorigenesis. The transcription factor IRF3 is essential for innate antiviral immunity. In this study, we discovered IRF3 as an agonist of Yes-associated protein (YAP). The expression of IRF3 is positively correlated with that of YAP and its target genes in gastric cancer; the expression of both IRF3 and YAP is up-regulated and prognosticates patient survival. IRF3 interacts with both YAP and TEAD4 in the nucleus to enhance their interaction, promoting nuclear translocation and activation of YAP. IRF3 and YAP–TEAD4 are associated genome-wide to cobind and coregulate many target genes of the Hippo pathway. Overexpression of active IRF3 increased, but depletion of IRF3 reduced, the occupancy of YAP on the target genes. Knockdown or pharmacological targeting of IRF3 by Amlexanox, a drug used clinically for antiinflammatory treatment, inhibits gastric tumor growth in a YAP-dependent manner. Collectively, our study identifies IRF3 as a positive regulator for YAP, highlighting a new therapeutic target against YAP-driven cancers.

The Hippo pathway in organ development, homeostasis, and regeneration

The Hippo pathway is a universal governor of organ size, tissue homeostasis, and regeneration. A growing body of work has advanced our understanding of Hippo pathway regulation of cell proliferation, differentiation, and spatial patterning not only in organ development but also upon injury-induced regeneration. The pathway's central role in stem cell biology thus implicates its potential for therapeutic manipulation in mammalian organ regeneration. In this review, we survey recent literature linking the Hippo pathway to the development, homeostasis, and regeneration of various organs, including Hippo-independent roles for YAP, defined here as YAP functions that are not regulated by the Hippo pathway kinases LATS1/2.

Critical research gaps and recommendations to inform research prioritisation for more effective prevention and improved outcomes in colorectal cancer

OBJECTIVE

Colorectal cancer (CRC) leads to significant morbidity/mortality worldwide. Defining critical research gaps (RG), their prioritisation and resolution, could improve patient outcomes.

DESIGN

RG analysis was conducted by a multidisciplinary panel of patients, clinicians and researchers (n=71). Eight working groups (WG) were constituted: discovery science; risk; prevention; early diagnosis and screening; pathology; curative treatment; stage IV disease; and living with and beyond CRC. A series of discussions led to development of draft papers by each WG, which were evaluated by a 20-strong patient panel. A final list of RGs and research recommendations (RR) was endorsed by all participants.

RESULTS

Fifteen critical RGs are summarised below: RG1: Lack of realistic models that recapitulate tumour/tumour micro/macroenvironment; RG2: Insufficient evidence on precise contributions of genetic/environmental/lifestyle factors to CRC risk; RG3: Pressing need for prevention trials; RG4: Lack of integration of different prevention approaches; RG5: Lack of optimal strategies for CRC screening; RG6: Lack of effective triage systems for invasive investigations; RG7: Imprecise pathological assessment of CRC; RG8: Lack of qualified personnel in genomics, data sciences and digital pathology; RG9: Inadequate assessment/communication of risk, benefit and uncertainty of treatment choices; RG10: Need for novel technologies/interventions to improve curative outcomes; RG11: Lack of approaches that recognise molecular interplay between metastasising tumours and their microenvironment; RG12: Lack of reliable biomarkers to guide stage IV treatment; RG13: Need to increase understanding of health related quality of life (HRQOL) and promote residual symptom resolution; RG14: Lack of coordination of CRC research/funding; RG15: Lack of effective communication between relevant stakeholders.

CONCLUSION

Prioritising research activity and funding could have a significant impact on reducing CRC disease burden over the next 5 years.

Sophisticated lessons from simple organisms: appreciating the value of curiosity-driven research

For hundreds of years, biologists have studied accessible organisms such as garden peas, sea urchins collected at low tide, newt eggs, and flies circling rotten fruit. These organisms help us to understand the world around us, attracting and inspiring each new generation of biologists with the promise of mystery and discovery. Time and time again, what we learn from such simple organisms has emphasized our common biological origins by proving to be applicable to more complex organisms, including humans. Yet, biologists are increasingly being tasked with developing applications from the known, rather than being allowed to follow a path to discovery of the as yet unknown. Here, we provide examples of important lessons learned from research using selected non-vertebrate organisms. We argue that, for the purpose of understanding human disease, simple organisms cannot and should not be replaced solely by human cell-based culture systems. Rather, these organisms serve as powerful discovery tools for new knowledge that could subsequently be tested for conservation in human cell-based culture systems. In this way, curiosity-driven biological research in simple organisms has and will continue to pay huge dividends in both the short and long run for improving the human condition.

RHOA GTPase Controls YAP-Mediated EREG Signaling in Small Intestinal Stem Cell Maintenance

RHOA, a founding member of the Rho GTPase family, is critical for actomyosin dynamics, polarity, and morphogenesis in response to developmental cues, mechanical stress, and inflammation. In murine small intestinal epithelium, inducible RHOA deletion causes a loss of epithelial polarity, with disrupted villi and crypt organization. In the intestinal crypts, RHOA deficiency results in reduced cell proliferation, increased apoptosis, and a loss of intestinal stem cells (ISCs) that mimic effects of radiation damage. Mechanistically, RHOA loss reduces YAP signaling of the Hippo pathway and affects YAP effector epiregulin (EREG) expression in the crypts. Expression of an active YAP (S112A) mutant rescues ISC marker expression, ISC regeneration, and ISC-associated Wnt signaling, but not defective epithelial polarity, in RhoA knockout mice, implicating YAP in RHOA-regulated ISC function. EREG treatment or active β-catenin Catnb^lox(ex3) mutant expression rescues the RhoA KO ISC phenotypes. Thus, RHOA controls YAP-EREG signaling to regulate intestinal homeostasis and ISC regeneration.

Cloning and expression analysis of BmYki gene in silkworm, Bombyx mori

The transcriptional coactivator Yorkie(Yki), is a critical downstream effector of the Hippo(Hpo) signaling pathway that controls organ size through the regulation of cell proliferation and apoptosis. During the past ten years the biological function of Yki has been studied extensively in Drosophila and a few other insects, however, little is known about it in the silkworm, Bombyx mori, a major research model of lepidopteran insect. Here, we describe the isolation, characterization and expression of the B. mori Yki ortholog, BmYki. The coding sequence of the BmYki was 1314 bp in length, encoding a protein of 437 amino acids containing two conserved WW domains. BmYki transcripts were ubiquitous but not abundant in all detected tissues and developmental stages. Comparatively, it was expressed at pretty high level in silk glands and at the stage of fifth-instar day-3 larvae. Overexpression of BmYki in cultured B. mori embryonic cells significantly promoted transcription of genes associated with cell proliferation and apoptosis, indicating that BmYki functions in the regulation of organ growth-related biological processes. Interestingly, transcription of silk protein-coding genes and transcription factors regulating the synthesis of silk proteins was downregulated remarkably, suggesting that BmYki was involved in the regulation of silk protein synthesis. This study provides new insights into the role of BmYki in Hpo pathway regulation in silkworm.

Palmitic acid dysregulates the Hippo-YAP pathway and inhibits angiogenesis by inducing mitochondrial damage and activating the cytosolic DNA sensor cGAS-STING-IRF3 signaling mechanism

Impaired angiogenesis and wound healing carry significant morbidity and mortality in diabetic patients. Metabolic stress from hyperglycemia and elevated free fatty acids have been shown to inhibit endothelial angiogenesis. However, the underlying mechanisms remain poorly understood. In this study, we show that dysregulation of the Hippo-Yes-associated protein (YAP) pathway, an important signaling mechanism in regulating tissue repair and regeneration, underlies palmitic acid (PA)-induced inhibition of endothelial angiogenesis. PA inhibited endothelial cell proliferation, migration, and tube formation, which were associated with increased expression of mammalian Ste20-like kinases 1 (MST1), YAP phosphorylation/inactivation, and nuclear exclusion. Overexpression of YAP or knockdown of MST1 prevented PA-induced inhibition of angiogenesis. When searching upstream signaling mechanisms, we found that PA dysregulated the Hippo-YAP pathway by inducing mitochondrial damage. PA treatment induced mitochondrial DNA (mtDNA) release to cytosol, and activated cytosolic DNA sensor cGAS-STING-IRF3 signaling. Activated IRF3 bound to the MST1 gene promoter and induced MST1 expression, leading to MST1 up-regulation, YAP inactivation, and angiogenesis inhibition. Thus, mitochondrial damage and cytosolic DNA sensor cGAS-STING-IRF3 signaling are critically involved in PA-induced Hippo-YAP dysregulation and angiogenesis suppression. This mechanism may have implication in impairment of angiogenesis and wound healing in diabetes.

SAV1 represses the development of human colorectal cancer by regulating the Akt-mTOR pathway in a YAP-dependent manner

OBJECTIVES

SAV1 is a human homologue of Salvador that contains two protein-protein interaction modules known as WW domains and acts as a scaffolding protein. SAV1 participates in the development of diverse types of cancer. We aimed to investigate the role of SAV1 in human colorectal cancer.

MATERIALS AND METHODS

Human colorectal cancer samples were used to study the expression of SAV1 and YAP. Loss-of-function and gain-of-function strategies were used to study the effects of SAV1 on colorectal cancer cell growth. Rapamycin was used to treat cells and mice to investigate the effect of mTOR signalling.

RESULTS

SAV1 represses the development of colorectal cancer by inhibiting the Akt-mTOR signalling in a YAP-dependent manner. The mRNA and protein levels of SAV1 are down-regulated in human colorectal cancer tissues compared with adjacent non-cancer tissues. SAV1 knockdown promotes the growth of colorectal cancer cells in vitro and in vivo, whereas SAV1 overexpression leads to opposing results. SAV1 represses the activation of the Akt-mTOR signalling, and rapamycin treatment blunts the effects of SAV1 on in vitro and in vivo growth of colorectal cancer cells. Finally, we show that SAV1 promotes the phosphorylation and inactivation of YAP, which contributes to the effect of SAV1 on Akt-mTOR signalling pathway.

CONCLUSIONS

SAV1 is a repressor during the development of human colorectal cancer by inhibiting the YAP-Akt-mTOR signalling pathway.

Recent advances in understanding contextual TGFβ signaling

The appearance of the first animal species on earth coincides with the emergence of transforming growth factor β (TGFβ) pathways. The evolution of these animals into more complex organisms coincides with a progressively increased TGFβ repertoire through gene duplications and divergence, making secreted TGFβ molecules the largest family of morphogenetic proteins in humans. It is therefore not surprising that TGFβ pathways govern numerous aspects of human biology from early embryonic development to regeneration, hematopoiesis, neurogenesis, and immunity. Such heavy reliance on these pathways is reflected in the susceptibility to minor perturbations in pathway components that can lead to dysregulated signaling and a diverse range of human pathologies such as cancer, fibrosis, and developmental disorders. Attempts to comprehensively resolve these signaling cascades are complicated by the long-recognized paradoxical role the pathway plays in cell biology. Recently, several groups have probed examples of the disparate aspects of TGFβ biology in a variety of animal models and uncovered novel context-dependent regulatory mechanisms. Here, we briefly review recent advancements and discuss their overall impact in directing future TGFβ research.

TAZ overexpression is associated with epithelial-mesenchymal transition in cisplatin-resistant gastric cancer cells

Gastric cancer is one of the common malignant diseases. The poor treatment outcome is mainly due to chemotherapeutic resistance. Therefore, it is important to determine the molecular mechanism of drug resistance in gastric cancer. To explore the mechanisms of cisplatin resistance in gastric cancer cells, several approaches were performed including MTT assay, real-time RT-PCR, western blot analysis, migration and invasion assays, wound healing assay, and transfection. We found that cisplatin-resistant (CR) gastric cancer cells acquired epithelial-mesenchymal transition (EMT) phenotype. The CR cells with EMT features obtained higher migratory and invasive activities. Moreover, we observed that TAZ was highly expressed in CR cells. Consistently, depletion of TAZ caused partial reversal of EMT to MET in CR cells. Our results suggest that TAZ plays a pivotal role in CR-induced EMT. Targeting TAZ could be a potential therapeutic strategy for gastric cancer.

ETS (E26 transformation-specific) up-regulation of the transcriptional co-activator TAZ promotes cell migration and metastasis in prostate cancer

Prostate cancer is a very common malignant disease and a leading cause of death for men in the Western world. Tumorigenesis and progression of prostate cancer involves multiple signaling pathways, including the Hippo pathway. Yes-associated protein (YAP) is the downstream transcriptional co-activator of the Hippo pathway, is overexpressed in prostate cancer, and plays a vital role in the tumorigenesis and progression of prostate cancer. However, the role of the YAP paralog and another downstream effector of the Hippo pathway, transcriptional co-activator with PDZ-binding motif (TAZ), in prostate cancer has not been fully elucidated. Here, we show that TAZ is a basal cell marker for the prostate epithelium. We found that overexpression of TAZ promotes the epithelial-mesenchymal transition (EMT), cell migration, and anchorage-independent growth in the RWPE1 prostate epithelial cells. Of note, knock down of TAZ in the DU145 prostate cancer cells inhibited cell migration and metastasis. We also found that SH3 domain binding protein 1 (SH3BP1), a RhoGAP protein that drives cell motility, is a direct target gene of TAZ in the prostate cancer cells, mediating TAZ function in enhancing cell migration. Moreover, the prostate cancer-related oncogenic E26 transformation-specific (ETS) transcription factors, ETV1, ETV4, and ETV5, were required for TAZ gene transcription in PC3 prostate cancer cells. MAPK inhibitor U0126 treatment decreased TAZ expression in RWPE1 cells, and ETV4 overexpression rescued TAZ expression in RWPE1 cells with U0126 treatment. Our results show a regulatory mechanism of TAZ transcription and suggest a significant role for TAZ in the progression of prostate cancer.

YAP and WWTR1: New targets for skin cancer treatment

The core components of the Hippo signaling pathway are a cascade of kinases that govern the phosphorylation of downstream transcriptional co-activators, namely, YES-associated protein (YAP) and WW domain-containing transcription regulator protein 1 (WWTR1, also known as TAZ). The Hippo signaling pathway is considered an important tumor-suppressor pathway, and its dysregulation has been noted in a variety of human cancers, in which YAP/WWTR1 enable cancerous cells to overcome contact inhibition, and to grow and spread uncontrollably. Interestingly, however, recent studies have told a somewhat different but perhaps more intriguing YAP/WWTR1 story, as these studies found that YAP/WWTR1 function as a central hub that integrates signals from multiple upstream signaling pathways, cell-cell interactions and mechanical forces and then bind to and activate different downstream transcriptional factors to direct cell social behavior and cell-cell interactions. In this review, we present the latest findings on the role of YAP/WWTR1 in skin physiology, pathology and tumorigenesis and discuss the statuses of newly developed therapeutic interventions that target YAP/WWTR1 in human cancers, as well as their prospects for use as skin cancer treatments.

The TAZ-miR-224-SMAD4 axis promotes tumorigenesis in osteosarcoma

Transcriptional co-activator with PDZ-binding motif (TAZ) is a downstream effector of the Hippo signaling pathway that participates in tumorigenesis. The aim of this study was to identify the miRNA counterpart for TAZ and elucidate the mechanism underlying the tumorigenic effect of TAZ. We demonstrated that TAZ is upregulated in osteosarcoma (OS) tissues and cell lines, and that TAZ overexpression can induce cell migration, invasion and proliferation. Moreover, miRNA-224 (miR-224), a TAZ phenocopy that functions downstream of TAZ, was found to be upregulated with TAZ overexpression. Further, a mechanistic study revealed that miR-224 functions by inhibiting the tumor suppressor, SMAD4, to support the proliferation and migration of OS cells. Our findings indicate that targeting TAZ and miR-224 could be a promising approach for the treatment of OS.

Targeting the Hippo Signaling Pathway for Tissue Regeneration and Cancer Therapy

The Hippo signaling pathway is a highly-conserved developmental pathway that plays an essential role in organ size control, tumor suppression, tissue regeneration and stem cell self-renewal. The YES-associated protein (YAP) and the transcriptional co-activator with PDZ-binding motif (TAZ) are two important transcriptional co-activators that are negatively regulated by the Hippo signaling pathway. By binding to transcription factors, especially the TEA domain transcription factors (TEADs), YAP and TAZ induce the expression of growth-promoting genes, which can promote organ regeneration after injury. Therefore, controlled activation of YAP and TAZ can be useful for regenerative medicine. However, aberrant activation of YAP and TAZ due to deregulation of the Hippo pathway or overexpression of YAP/TAZ and TEADs can promote cancer development. Hence, pharmacological inhibition of YAP and TAZ may be a useful approach to treat tumors with high YAP and/or TAZ activity. In this review, we present the mechanisms regulating the Hippo pathway, the role of the Hippo pathway in tissue repair and cancer, as well as a detailed analysis of the different strategies to target the Hippo signaling pathway and the genes regulated by YAP and TAZ for regenerative medicine and cancer therapy.

Biphasic Regulation of Yes-associated Protein (YAP) Cellular Localization, Phosphorylation, and Activity by G Protein-coupled Receptor Agonists in Intestinal Epithelial Cells: A NOVEL ROLE FOR PROTEIN KINASE D (PKD)

We examined the regulation of Yes-associated protein (YAP) localization, phosphorylation, and transcriptional activity in intestinal epithelial cells. Our results show that stimulation of intestinal epithelial IEC-18 cells with the G protein-coupled receptor (GPCR) agonist angiotensin II, a potent mitogen for these cells, induced rapid translocation of YAP from the nucleus to the cytoplasm (within 15 min) and a concomitant increase in YAP phosphorylation at Ser(127) and Ser(397) Angiotensin II elicited YAP phosphorylation and cytoplasmic accumulation in a dose-dependent manner (ED50 = 0.3 nm). Similar YAP responses were provoked by stimulation with vasopressin or serum. Treatment of the cells with the protein kinase D (PKD) family inhibitors CRT0066101 and kb NB 142-70 prevented the increase in YAP phosphorylation on Ser(127) and Ser(397) via Lats2, YAP cytoplasmic accumulation, and increase in the mRNA levels of YAP/TEAD-regulated genes (Ctgf and Areg). Furthermore, siRNA-mediated knockdown of PKD1, PKD2, and PKD3 markedly attenuated YAP nuclear-cytoplasmic shuttling, phosphorylation at Ser(127), and induction of Ctgf and Areg expression in response to GPCR activation. These results identify a novel role for the PKD family in the control of biphasic localization, phosphorylation, and transcriptional activity of YAP in intestinal epithelial cells. In turn, YAP and TAZ are necessary for the stimulation of the proliferative response of intestinal epithelial cells to GPCR agonists that act via PKD. The discovery of interaction between YAP and PKD pathways identifies a novel cross-talk in signal transduction and demonstrates, for the first time, that the PKDs feed into the YAP pathway.

The MST/Hippo Pathway and Cell Death: A Non-Canonical Affair

The MST/Hippo signalling pathway was first described over a decade ago in Drosophila melanogaster and the core of the pathway is evolutionary conserved in mammals. The mammalian MST/Hippo pathway regulates organ size, cell proliferation and cell death. In addition, it has been shown to play a central role in the regulation of cellular homeostasis and it is commonly deregulated in human tumours. The delineation of the canonical pathway resembles the behaviour of the Hippo pathway in the fly where the activation of the core kinases of the pathway prevents the proliferative signal mediated by the key effector of the pathway YAP. Nevertheless, several lines of evidence support the idea that the mammalian MST/Hippo pathway has acquired new features during evolution, including different regulators and effectors, crosstalk with other essential signalling pathways involved in cellular homeostasis and the ability to actively trigger cell death. Here we describe the current knowledge of the mechanisms that mediate MST/Hippo dependent cell death, especially apoptosis. We include evidence for the existence of complex signalling networks where the core proteins of the pathway play a central role in controlling the balance between survival and cell death. Finally, we discuss the possible involvement of these signalling networks in several human diseases such as cancer, diabetes and neurodegenerative disorders.