PP1A-mediated dephosphorylation positively regulates YAP2 activity

ROS-induced cytosolic release of mitochondrial PGAM5 promotes colorectal cancer progression by interacting with MST3

Aberrant release of mitochondrial reactive oxygen species (mtROS) in response to cellular stress is well known for promoting cancer progression. However, precise molecular mechanism by which mtROS contribute to epithelial cancer progression remains only partially understood. Here, using colorectal cancer (CRC) models, we show that upon sensing excessive mtROS, phosphatase PGAM5, which normally localizes to the mitochondria, undergoes aberrant cleavage by presenilin-associated rhomboid-like protein (PARL), becoming released into the cytoplasm. Cytosolic PGAM5 then directly binds to and dephosphorylates MST3 kinase. This, in turn, prevents STK25-mediated LATS1/2 phosphorylation, leading to YAP activation and CRC progression. Importantly, depletion of MST3 reciprocally promotes accumulation of cytosolic PGAM5 by inducing mitochondrial damage. Taken together, these findings demonstrate how mtROS promotes CRC progression by activating YAP via a post-transcriptional positive feedback loop between PGAM5 and MST3, both of which can serve as potential targets for developing next-generation anti-colon cancer therapeutics.

The Pivotal Role of LACTB in the Process of Cancer Development

The mitochondrial serine β-lactamase-like protein LACTB has emerged as a critical regulator in cancer biology, distinguished by its unique structural and functional attributes. Defined by its conserved penicillin-binding proteins and β-lactamases (PBP-βLs) domain and SXXK catalytic motif, LACTB demonstrates properties distinct from its prokaryotic homologs, including the ability to polymerize into filaments. These structural characteristics enable LACTB to modulate mitochondrial organization and enzymatic activity, influencing lipid metabolism and indirectly affecting cellular proliferation. Importantly, the expression and functional roles of LACTB exhibit cancer-type-specific variation, underscoring its dual function as both a tumor suppressor and an oncogene. Decreased LACTB expression is associated with poor clinical outcomes in cancers such as breast cancer, lung cancer, and colorectal cancer, while specific mutations and regulatory mechanisms have been linked to its oncogenic activity in osteosarcoma and pancreatic adenocarcinoma. Mechanistically, LACTB regulates key processes in cancer progression, including mitochondrial dynamics, epithelial-mesenchymal transition (EMT), and cell death pathways. This duality highlights LACTB as a promising therapeutic target and underscores its relevance in advancing precision oncology strategies. This review provides a comprehensive analysis of expression level, structure-function relationships, and the diverse roles of LACTB in oncogenesis, underscoring its promise as a focal point for precision cancer therapies.

New Horizons in Cancer Progression and Metastasis: Hippo Signaling Pathway

The Hippo pathway is highly evolved to maintain tissue homeostasis in diverse species by regulating cell proliferation, differentiation, and apoptosis. In tumor biology, the Hippo pathway is a prime example of signaling molecules involved in cancer progression and metastasis. Hippo core elements LATS1, LATS2, MST1, YAP, and TAZ have critical roles in the maintenance of traditional tissue architecture and cell homeostasis. However, in cancer development, dysregulation of Hippo signaling results in tumor progression and the formation secondary cancers. Hippo components not only transmit biochemical signals but also act as mediators of mechanotransduction pathways during malignant neoplasm development and metastatic disease. This review confers knowledge of Hippo pathway core components and their role in cancer progression and metastasis and highlights the clinical role of Hippo pathway in cancer treatment. The Hippo signaling pathway and its unresolved mechanisms hold great promise as potential therapeutic targets in the emerging field of metastatic cancer research.

Sirtuin 5-mediated deacetylation of TAZ at K54 promotes melanoma development

PURPOSE

Nuclear accumulation of YAP/TAZ promotes tumorigenesis in several cancers, including melanoma. Although the mechanisms underlying the nuclear retention of YAP are known, those underlying the retention of TAZ remain unclear. Our study investigates a novel acetylation/deacetylation switch in TAZ, governing its subcellular localization in melanoma tumorigenesis.

METHODS

Immunoprecipitation/Western blot assessed TAZ protein interactions and acetylation. SIRT5 activity was quantified with enzyme-linked immunosorbent assay. Immunofluorescence indicated TAZ nuclear localization. TEAD transcriptional activity was measured through luciferase reporter assays. ChIP detected TAZ binding to the CTGF promoter. Transwell and wound healing assays quantified melanoma cell invasiveness and migration. Metastasis was evaluated using a mouse model via tail vein injections. Clinical relevance was explored via immunohistochemical staining of patient tumors.

RESULTS

CBP facilitated TAZ acetylation at K54 in response to epidermal growth factor stimulation, while SIRT5 mediated deacetylation. Acetylation correlated with phosphorylation, regulating TAZ's binding with LATS2 or TEAD. TAZ K54 acetylation enhanced its S89 phosphorylation, promoting cytosolic retention via LATS2 interaction. SIRT5-mediated deacetylation enhanced TAZ-TEAD interaction and nuclear retention. Chromatin IP showed SIRT5-deacetylated TAZ recruited to CTGF promoter, boosting transcriptional activity. In a mouse model, SIRT5 overexpression induced melanoma metastasis to lung tissue following the injection of B16F10 melanocytes via the tail vein, and this effect was prevented by verteporfin treatment.

CONCLUSIONS

Our study revealed a novel mechanism of TAZ nuclear retention regulated by SIRT5-mediated K54 deacetylation and demonstrated the significance of TAZ deacetylation in CTGF expression. This study highlights the potential implications of the SIRT5/TAZ axis for treating metastatic melanoma.

Deregulated circRNAs in Epithelial Ovarian Cancer With Activity in Preclinical In Vivo Models: Identification of Targets and New Modalities for Therapeutic Intervention

Epithelial ovarian cancer (EOC) is associated with a dismal prognosis due to development of resistance to chemotherapy and metastasis in the peritoneal cavity and distant organs. In order to identify new targets and treatment modalities we searched the literature for up- and and down-regulated circRNAs with efficacy in preclinical EOC-related in vivo systems. Our search yielded circRNAs falling into the following categories: cisplatin and paclitaxel resistance, transmembrane receptors, secreted factors, transcription factors, RNA splicing and processing factors, RAS pathway-related components, proteolysis and cell-cycle regulation, signaling-related proteins, and circRNAs regulating proteins in additional categories. These findings can be potentially translated by validation and manipulation of the corresponding targets, inhibition of circRNAs with antisense oligonucleotides (ASO), small interfering RNAs (siRNA) or small hairpin RNA (shRNA) or by reconstituting their activity.

TRIP6 promotes neural stem cell maintenance through YAP-mediated Sonic Hedgehog activation

In the adult mammalian brain, new neurons are continuously generated from neural stem cells (NSCs) in the subventricular zone (SVZ)-olfactory bulb (OB) pathway. YAP, a transcriptional co-activator of the Hippo pathway, promotes cell proliferation and inhibits differentiation in embryonic neural progenitors. However, the role of YAP in postnatal NSCs remains unclear. Here, we showed that YAP was present in NSCs of the postnatal mouse SVZ. Forced expression of Yap promoted NSC maintenance and inhibited differentiation, whereas depletion of Yap by RNA interference or conditional knockout led to the decline of NSC maintenance, premature neuronal differentiation, and collapse of neurogenesis. For the molecular mechanism, thyroid hormone receptor-interacting protein 6 (TRIP6) recruited protein phosphatase PP1A to dephosphorylate LATS1/2, therefore inducing YAP nuclear localization and activation. Moreover, TRIP6 promoted NSC maintenance, cell proliferation, and inhibited differentiation through YAP. In addition, YAP regulated the expression of the Sonic Hedgehog (SHH) pathway effector Gli2 and Gli1/2 mediated the effect of YAP on NSC maintenance. Together, our findings demonstrate a novel TRIP6-YAP-SHH axis, which is critical for regulating postnatal neurogenesis in the SVZ-OB pathway.

Evaluation of Quantification and Normalization Strategies for Phosphoprotein Phosphatase Affinity Proteomics: Application to Breast Cancer Signaling

Accurate quantification of proteomics data is essential for revealing and understanding biological signaling processes. We have recently developed a chemical proteomic strategy termed phosphatase inhibitor beads and mass spectrometry (PIB-MS) to investigate endogenous phosphoprotein phosphatase (PPP) dephosphorylation signaling. Here, we compare the robustness and reproducibility of status quo quantification methods for optimal performance and ease of implementation. We then apply PIB-MS to an array of breast cancer cell lines to determine differences in PPP signaling between subtypes. Breast cancer, a leading cause of cancer death in women, consists of three main subtypes: estrogen receptor-positive (ER+), human epidermal growth factor receptor two positive (HER2+), and triple-negative (TNBC). Although there are effective treatment strategies for ER+ and HER2+ subtypes, tumors become resistant and progress. Furthermore, TNBC has few targeted therapies. Therefore, there is a need to identify new approaches for treating breast cancers. Using PIB-MS, we distinguished TNBC from non-TNBC based on subtype-specific PPP holoenzyme composition. In addition, we identified an increase in PPP interactions with Hippo pathway proteins in TNBC. These interactions suggest that phosphatases in TNBC play an inhibitory role on the Hippo pathway and correlate with increased expression of YAP/TAZ target genes both in TNBC cell lines and in TNBC patients.

The Impact of the Hippo Pathway and Cell Metabolism on Pathological Complete Response in Locally Advanced Her2+ Breast Cancer: The TRISKELE Multicenter Prospective Study

The Hippo pathway and its two key effectors, Yes-associated protein (YAP) and transcriptional coactivator with PDZ-binding motif (TAZ), are consistently altered in breast cancer. Pivotal regulators of cell metabolism such as the AMP-activated protein kinase (AMPK), Stearoyl-CoA-desaturase 1 (SCD1), and HMG-CoA reductase (HMGCR) are relevant modulators of TAZ/YAP activity. In this prospective study, we measured the tumor expression of TAZ, YAP, AMPK, SCD1, and HMGCR by immunohistochemistry in 65 Her2+ breast cancer patients who underwent trastuzumab-based neoadjuvant treatment. The aim of the study was to assess the impact of the immunohistochemical expression of the Hippo pathway transducers and cell metabolism regulators on pathological complete response. Low expression of cytoplasmic TAZ, both alone and in the context of a composite signature identified by machine learning including also low nuclear levels of YAP and HMGCR and high cytoplasmic levels of SCD1, was a predictor of residual disease in the univariate logistic regression. This finding was not confirmed in the multivariate model including estrogen receptor > 70% and body mass index > 20. However, our findings were concordant with overall survival data from the TCGA cohort. Our results, possibly affected by the relatively small sample size of this study population, deserve further investigation in adequately sized, ad hoc prospective studies.

A non-transcriptional function of Yap regulates the DNA replication program

In multicellular eukaryotic organisms, the initiation of DNA replication occurs asynchronously throughout S-phase according to a regulated replication timing program. Here, using Xenopus egg extracts, we showed that Yap (Yes-associated protein 1), a downstream effector of the Hippo signalling pathway, is required for the control of DNA replication dynamics. We found that Yap is recruited to chromatin at the start of DNA replication and identified Rif1, a major regulator of the DNA replication timing program, as a novel Yap binding protein. Furthermore, we show that either Yap or Rif1 depletion accelerates DNA replication dynamics by increasing the number of activated replication origins. In Xenopus embryos, using a Trim-Away approach during cleavage stages devoid of transcription, we found that either Yap or Rif1 depletion triggers an acceleration of cell divisions, suggesting a shorter S-phase by alterations of the replication program. Finally, our data show that Rif1 knockdown leads to defects in the partitioning of early versus late replication foci in retinal stem cells, as we previously showed for Yap. Altogether, our findings unveil a non-transcriptional role for Yap in regulating replication dynamics. We propose that Yap and Rif1 function as brakes to control the DNA replication program in early embryos and post-embryonic stem cells.

Insights Into the Properties, Biological Functions, and Regulation of USP21

Deubiquitylating enzymes (DUBs) antagonize ubiquitination by removing ubiquitin from their substrates. The role of DUBs in controlling various physiological and pathological processes has been extensively studied, and some members of DUBs have been identified as potential therapeutic targets in diseases ranging from tumors to neurodegeneration. Ubiquitin-specific protease 21 (USP21) is a member of the ubiquitin-specific protease family, the largest subfamily of DUBs. Although USP21 was discovered late and early research progress was slow, numerous studies in the last decade have gradually revealed the importance of USP21 in a wide variety of biological processes. In particular, the pro-carcinogenic effect of USP21 has been well elucidated in the last 2 years. In the present review, we provide a comprehensive overview of the current knowledge on USP21, including its properties, biological functions, pathophysiological roles, and cellular regulation. Limited pharmacological interventions for USP21 have also been introduced, highlighting the importance of developing novel and specific inhibitors targeting USP21.

Hippo/YAP signaling pathway protects against neomycin-induced hair cell damage in the mouse cochlea

The Hippo/Yes-associated protein (YAP) signaling pathway has been shown to be able to maintain organ size and homeostasis by regulating cell proliferation, differentiation, and apoptosis. The abuse of aminoglycosides is one of the main causes of sensorineural hearing loss (SSNHL). However, the role of the Hippo/YAP signaling pathway in cochlear hair cell (HC) damage protection in the auditory field is still unclear. In this study, we used the YAP agonist XMU-MP-1 (XMU) and the inhibitor Verteporfin (VP) to regulate the Hippo/YAP signaling pathway in vitro. We showed that YAP overexpression reduced neomycin-induced HC loss, while downregulated YAP expression increased HC vulnerability after neomycin exposure in vitro. We next found that activation of YAP expression inhibited C-Abl-mediated cell apoptosis, which led to reduced HC loss. Many previous studies have reported that the level of reactive oxygen species (ROS) is significantly increased in cochlear HCs after neomycin exposure. In our study, we also found that YAP overexpression significantly decreased ROS accumulation, while downregulation of YAP expression increased ROS accumulation. In summary, our results demonstrate that the Hippo/YAP signaling pathway plays an important role in reducing HC injury and maintaining auditory function after aminoglycoside exposure. YAP overexpression could protect against neomycin-induced HC loss by inhibiting C-Abl-mediated cell apoptosis and decreasing ROS accumulation, suggesting that YAP could be a novel therapeutic target for aminoglycosides-induced sensorineural hearing loss in the clinic.

Adaptations in Hippo-Yap signaling and myofibroblast fate underlie scar-free ear appendage wound healing in spiny mice

Spiny mice (Acomys cahirinus) are terrestrial mammals that evolved unique scar-free regenerative wound-healing properties. Myofibroblasts (MFs) are the major scar-forming cell type in skin. We found that following traumatic injury to ear pinnae, MFs appeared rapidly in both Acomys and mouse yet persisted only in mouse. The timing of MF loss in Acomys correlated with wound closure, blastema differentiation, and nuclear localization of the Hippo pathway target protein Yap. Experiments in vitro revealed an accelerated PP2A-dependent dephosphorylation activity that maintained nuclear Yap in Acomys dermal fibroblasts (DFs) and was not detected in mouse or human DFs. Treatment of Acomys in vivo with the nuclear Yap-TEAD inhibitor verteporfin prolonged MF persistence and converted tissue regeneration to fibrosis. Forced Yap activity prevented and rescued TGF-β1-induced human MF formation in vitro. These results suggest that Acomys evolved modifications of Yap activity and MF fate important for scar-free regenerative wound healing in vivo.

Modulation of serine/threonine-protein phosphatase 1 (PP1) complexes: A promising approach in cancer treatment

Cancer is the second leading cause of death worldwide. Despite the availability of numerous therapeutic options, tumor heterogeneity and chemoresistance have limited the success of these treatments, and the development of effective anticancer therapies remains a major focus in oncology research. The serine/threonine-protein phosphatase 1 (PP1) and its complexes have been recognized as potential drug targets. Research on the modulation of PP1 complexes is currently at an early stage, but has immense potential. Chemically diverse compounds have been developed to disrupt or stabilize different PP1 complexes in various cancer types, with the objective of inhibiting disease progression. Beneficial results obtained in vitro now require further pre-clinical and clinical validation. In conclusion, the modulation of PP1 complexes seems to be a promising, albeit challenging, therapeutic strategy for cancer.

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.

Protein Phosphatase 2A Mediates YAP Activation in Endothelial Cells Upon VEGF Stimulation and Matrix Stiffness

Angiogenesis is an essential process during development. Abnormal angiogenesis also contributes to many disease conditions such as tumor and retinal diseases. Previous studies have established the Hippo signaling pathway effector Yes-associated protein (YAP) as a crucial regulator of angiogenesis. In ECs, activated YAP promotes endothelial cell proliferation, migration and sprouting. YAP activity is regulated by vascular endothelial growth factor (VEGF) and mechanical cues such as extracellular matrix (ECM) stiffness. However, it is unclear how VEGF or ECM stiffness signal to YAP, especially how dephosphorylation of YAP occurs in response to VEGF stimulus or ECM stiffening. Here, we show that protein phosphatase 2A (PP2A) is required for this process. Blocking PP2A activity abolishes VEGF or ECM stiffening mediated YAP activation. Systemic administration of a PP2A inhibitor suppresses YAP activity in blood vessels in developmental and pathological angiogenesis mouse models. Consistently, PP2A inhibitor also inhibits sprouting angiogenesis. Mechanistically, PP2A directly interacts with YAP, and this interaction requires proper cytoskeleton dynamics. These findings identify PP2A as a crucial mediator of YAP activation in ECs and hence as an important regulator of angiogenesis.

Regulation and mechanism of YAP/TAZ in the mechanical microenvironment of stem cells (Review)

Stem cells receive cues from their physical and mechanical microenvironment via mechanosensing and mechanotransduction. These cues affect proliferation, self‑renewal and differentiation into specific cell fates. A growing body of evidence suggests that yes‑associated protein (YAP) and transcriptional coactivator with PDZ‑binding motif (TAZ) mechanotransduction is key for driving stem cell behavior and regeneration via the Hippo and other signaling pathways. YAP/TAZ receive a range of physical cues, including extracellular matrix stiffness, cell geometry, flow shear stress and mechanical forces in the cytoskeleton, and translate them into cell‑specific transcriptional programs. However, the mechanism by which mechanical signals regulate YAP/TAZ activity in stem cells is not fully understand. The present review summarizes the current knowledge of the mechanisms involved in YAP/TAZ regulation on the physical and mechanical microenvironment, as well as its potential effects on stem cell differentiation.

YAP and TAZ Mediators at the Crossroad between Metabolic and Cellular Reprogramming

Cell reprogramming can either refer to a direct conversion of a specialized cell into another or to a reversal of a somatic cell into an induced pluripotent stem cell (iPSC). It implies a peculiar modification of the epigenetic asset and gene regulatory networks needed for a new cell, to better fit the new phenotype of the incoming cell type. Cellular reprogramming also implies a metabolic rearrangement, similar to that observed upon tumorigenesis, with a transition from oxidative phosphorylation to aerobic glycolysis. The induction of a reprogramming process requires a nexus of signaling pathways, mixing a range of local and systemic information, and accumulating evidence points to the crucial role exerted by the Hippo pathway components Yes-Associated Protein (YAP) and Transcriptional Co-activator with PDZ-binding Motif (TAZ). In this review, we will first provide a synopsis of the Hippo pathway and its function during reprogramming and tissue regeneration, then we introduce the latest knowledge on the interplay between YAP/TAZ and metabolism and, finally, we discuss the possible role of YAP/TAZ in the orchestration of the metabolic switch upon cellular reprogramming.

Small extracellular vesicles with LncRNA H19 "overload": YAP Regulation as a Tendon Repair Therapeutic Tactic

Functional healing of tendon injuries remains a great challenge. Small extracellular vesicles (sEVs) have received attention as pro-regenerative agents. H19 overexpression could bring tendon regenerative ability, but the mechanism is still not fully elucidated, and reliable method for delivery of long non-coding RNAs (LncRNAs) was demanded. We identified the downstream mechanism of H19, the activation of yes-associated protein (YAP) via the H19-PP1-YAP axis. We established tendon stem/progenitor cells (TSPCs) stably overexpressing H19 with CRISPR-dCas9-based hnRNP A2/B1 activation (H19-CP-TSPCs). H19-OL-sEVs (H19 “overloading” sEVs) could be produced effectively from H19-CP-TSPCs. Only H19-OL-sEVs were able to significantly load large amounts of H19 rather than other competitors, and the potential of H19-OL-sEVs to promote tendon healing was far better than that of other competitors. Our study established a relatively reliable method for enrichment of LncRNAs into sEVs, providing new hints for modularized sEV-based therapies, and modularized sEVs represented a potential strategy for tendon regeneration.

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H19 overexpression enhances tendon regeneration

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H19 dephosphorylates and activates YAP

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hnRNP A2/B1 assists the enrichment of H19 into sEVs

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H19-OL-sEVs promote tendon regeneration

Plasma miR-218a-5p as a biomarker for acute cholestatic liver injury in rats and investigation of its pathophysiological roles

MicroRNAs (miRNA) have received considerable attention as potential biomarkers for drug-induced liver injury. We recently reported that the plasma levels of miR-143-3p and miR-218a-5p increased in severe cholestasis in rats. This study aimed to investigate whether these miRNAs increase in a severity-dependent manner and to elucidate their pathophysiological roles in cholestasis. Male Sprague-Dawley rats were orally administered different doses of α-naphthylisothiocyanate or 4,4-methylenedianiline to induce acute cholestasis. They were also orally administered acetaminophen or thioacetamide to induce hepatocellular injury. We found that plasma miR-143-3p and miR-218a-5p levels increased in a dose-dependent manner in cholestatic rats but not in hepatocellular injury. Bioinformatic analysis provided putative target genes of hsa-miR-218-5p, rno-miR-218a-5p, and mmu-miR-218-5p, among which GNAI2, PPP1CB, and PPP2R5A were experimentally validated as their direct target genes in human cholangiocyte line MMNK-1. Proliferation of MMNK-1 cells was significantly suppressed after overexpression of miR-218-5p and transduction of siRNAs for GNAI2, PPP1CB, and PPP2R5A. In the cholestatic livers of rats, Ppp1cb and Ppp2r5a expression levels decreased, whereas Gnai2 expression levels increased compared with those in vehicle-treated rats, suggesting that Ppp1cb and Ppp2r5a may be under the control of miR-218a-5p in vivo. In conclusion, our data suggest that miR-218(a)-5p is involved in the suppression of cholangiocyte proliferation by inhibiting the expression of PPP1CB and PPP2R5A, thereby contributing to the pathogenesis of cholestasis; and miR-218a-5p leaks into the plasma probably from damaged cholangiocytes in a severity-dependent manner in rats. Therefore, miR-218a-5p overexpression could be one of the underlying mechanisms of acute cholestatic liver injury in rats.

Signaling pathways in hepatocellular carcinoma

Despite the recent introduction of new effective systemic agents, the survival of patients with hepatocellular carcinoma (HCC) at advanced stages remains dismal. This underscores the need for new therapies, which has spurred extensive research on the identification of the main drivers of pathway de-regulation as a source of novel therapeutic targets. Frequently altered pathways in HCC involve growth factor receptors (e.g., VEGFR, FGFR, TGFA, EGFR, IGFR) and/or its cytoplasmic intermediates (e.g., PI3K-AKT-mTOR, RAF/ERK/MAPK) as well as key pathways in cell differentiation (e.g., Wnt/β-catenin, JAK/STAT, Hippo, Hedgehog, Notch). Somatic mutations, chromosomal aberrations and epigenetic changes are common mechanisms for pathway deregulation in HCC. Aberrant pathway activation has also been explored as a biomarker to predict response to specific therapies, but currently, these strategies are not implemented when deciding systemic therapies in HCC patients. Beyond the well-established molecular cascades, there are numerous emerging signaling pathways also deregulated in HCC (e.g., tumor microenvironment, non-coding RNA, intestinal microbiota), which have opened new avenues for therapeutic exploration.

Concerted localization-resets precede YAP-dependent transcription

Yes-associated protein 1 (YAP) is a transcriptional regulator with critical roles in mechanotransduction, organ size control, and regeneration. Here, using advanced tools for real-time visualization of native YAP and target gene transcription dynamics, we show that a cycle of fast exodus of nuclear YAP to the cytoplasm followed by fast reentry to the nucleus ("localization-resets") activates YAP target genes. These "resets" are induced by calcium signaling, modulation of actomyosin contractility, or mitosis. Using nascent-transcription reporter knock-ins of YAP target genes, we show a strict association between these resets and downstream transcription. Oncogenically-transformed cell lines lack localization-resets and instead show dramatically elevated rates of nucleocytoplasmic shuttling of YAP, suggesting an escape from compartmentalization-based control. The single-cell localization and transcription traces suggest that YAP activity is not a simple linear function of nuclear enrichment and point to a model of transcriptional activation based on nucleocytoplasmic exchange properties of YAP.

Interaction of the Hippo Pathway and Phosphatases in Tumorigenesis

The Hippo pathway is an emerging tumor suppressor signaling pathway involved in a wide range of cellular processes. Dysregulation of different components of the Hippo signaling pathway is associated with a number of diseases including cancer. Therefore, identification of the Hippo pathway regulators and the underlying mechanism of its regulation may be useful to uncover new therapeutics for cancer therapy. The Hippo signaling pathway includes a set of kinases that phosphorylate different proteins in order to phosphorylate and inactivate its main downstream effectors, YAP and TAZ. Thus, modulating phosphorylation and dephosphorylation of the Hippo components by kinases and phosphatases play critical roles in the regulation of the signaling pathway. While information regarding kinase regulation of the Hippo pathway is abundant, the role of phosphatases in regulating this pathway is just beginning to be understood. In this review, we summarize the most recent reports on the interaction of phosphatases and the Hippo pathway in tumorigenesis. We have also introduced challenges in clarifying the role of phosphatases in the Hippo pathway and future direction of crosstalk between phosphatases and the Hippo pathway.

Androgen attenuates the inactivating phospho-Ser-127 modification of yes-associated protein 1 (YAP1) and promotes YAP1 nuclear abundance and activity

The transcriptional coactivator YAP1 (yes-associated protein 1) regulates cell proliferation, cell-cell interactions, organ size, and tumorigenesis. Post-transcriptional modifications and nuclear translocation of YAP1 are crucial for its nuclear activity. The objective of this study was to elucidate the mechanism by which the steroid hormone androgen regulates YAP1 nuclear entry and functions in several human prostate cancer cell lines. We demonstrate that androgen exposure suppresses the inactivating post-translational modification phospho-Ser-127 in YAP1, coinciding with increased YAP1 nuclear accumulation and activity. Pharmacological and genetic experiments revealed that intact androgen receptor signaling is necessary for androgen's inactivating effect on phospho-Ser-127 levels and increased YAP1 nuclear entry. We also found that androgen exposure antagonizes Ser/Thr kinase 4 (STK4/MST1) signaling, stimulates the activity of protein phosphatase 2A, and thereby attenuates the phospho-Ser-127 modification and promotes YAP1 nuclear localization. Results from quantitative RT-PCR and CRISPR/Cas9-aided gene knockout experiments indicated that androgen differentially regulates YAP1-dependent gene expression. Furthermore, an unbiased computational analysis of the prostate cancer data from The Cancer Genome Atlas revealed that YAP1 and androgen receptor transcript levels correlate with each other in prostate cancer tissues. These findings indicate that androgen regulates YAP1 nuclear localization and its transcriptional activity through the androgen receptor-STK4/MST1-protein phosphatase 2A axis, which may have important implications for human diseases such as prostate cancer.

Molecular Mechanism of Hippo-YAP1/TAZ Pathway in Heart Development, Disease, and Regeneration

The Hippo-YAP1/TAZ pathway is a highly conserved central mechanism that controls organ size through the regulation of cell proliferation and other physical attributes of cells. The transcriptional factors Yes-associated protein 1 (YAP1) and PDZ-binding motif (TAZ) act as downstream effectors of the Hippo pathway, and their subcellular location and transcriptional activities are affected by multiple post-translational modifications (PTMs). Studies have conclusively demonstrated a pivotal role of the Hippo-YAP1/TAZ pathway in cardiac development, disease, and regeneration. Targeted therapeutics for the YAP1/TAZ could be an effective treatment option for cardiac regeneration and disease. This review article provides an overview of the Hippo-YAP1/TAZ pathway and the increasing impact of PTMs in fine-tuning YAP1/TAZ activation; in addition, we discuss the potential contributions of the Hippo-YAP1/TAZ pathway in cardiac development, disease, and regeneration.

A combat with the YAP/TAZ-TEAD oncoproteins for cancer therapy

The transcriptional co-regulators YAP and TAZ pair primarily with the TEAD family of transcription factors to elicit a gene expression signature that plays a prominent role in cancer development, progression and metastasis. YAP and TAZ endow cells with various oncogenic traits such that they sustain proliferation, inhibit apoptosis, maintain stemness, respond to mechanical stimuli, engineer metabolism, promote angiogenesis, suppress immune response and develop resistance to therapies. Therefore, inhibiting YAP/TAZ- TEAD is an attractive and viable option for novel cancer therapy. It is exciting to know that many drugs already in the clinic restrict YAP/TAZ activities and several novel YAP/TAZ inhibitors are currently under development. We have classified YAP/TAZ-inhibiting drugs into three groups. Group I drugs act on the upstream regulators that are stimulators of YAP/TAZ activities. Many of the Group I drugs have the potential to be repurposed as YAP/TAZ indirect inhibitors to treat various solid cancers. Group II modalities act directly on YAP/TAZ or TEADs and disrupt their interaction; targeting TEADs has emerged as a novel option to inhibit YAP/TAZ, as TEADs are major mediators of their oncogenic programs. TEADs can also be leveraged on using small molecules to activate YAP/TAZ-dependent gene expression for use in regenerative medicine. Group III drugs focus on targeting one of the oncogenic downstream YAP/TAZ transcriptional target genes. With the right strategy and impetus, it is not far-fetched to expect a repurposed group I drug or a novel group II drug to combat YAP and TAZ in cancers in the near future.

Metformin attenuates PD-L1 expression through activating Hippo signaling pathway in colorectal cancer cells

Programmed cell death ligand 1 (PD-L1) is a key suppressor of the cytotoxic immune response. In colorectal carcinoma (CRC), PD-L1 expression results in immune escape and poor prognosis. Extensive researches suggested that metformin had a potential efficacy of enhancing anti-tumor immune response in different types of cancer. However, the detail mechanisms underlying the efficacy in CRC are unclear. Here, we showed that metformin decreases PD-L1 and YAP1 expression in vitro and in vivo. After silencing or inhibiting YAP1 expression by Verteporfin (VP), the inhibitor of YAP1, the expression of PD-L1 were decreased in protein level in CRC cells. Furthermore, metformin directly phosphorylated YAP1 and restricted YAP1 to entry in the nucleus, so that PD-L1 was reduced via western blot and immunofluorescence assays in SW480 and HCT116 cells. Finally, subcutaneous xenotransplanted tumor models of HCT116 cells were established in BALB/c nude mice. Compared with the control group, PD-L1 and YAP1 expressions in tumor tissues, detected by immunohistochemistry, were reduced in the group of metformin treatment. These findings illuminate a new regulatory mechanism, metformin activates Hippo signaling pathway to regulate PD-L1 expression and suggests that metformin has the possibility to increase the efficacy of immunotherapy in human CRC.

POPX2 is a novel LATS phosphatase that regulates the Hippo pathway

The Hippo pathway regulates cell proliferation, survival, apoptosis and differentiation. During carcinogenesis, members of the Hippo pathway are mutated to avoid anoikis and promote anchorage independent growth. Although many regulators of the Hippo pathway have been reported, negative regulators of the hippo kinases are not well studied. Through an interactome screen, we found that POPX2 phosphatase interacts with several of the Hippo pathway core kinases, including LATS1 which is the direct kinase regulating the transcription co-activators, YAP and TAZ. Phosphorylated YAP/TAZ are retained in the cytoplasm and prevented from translocation into the nucleus to activate transcription of target genes. We found that POPX2 could dephosphorylate LATS1 on Threonine-1079, leading to inactivation of LATS1 kinase. As a result, YAP/TAZ are not phosphorylated and are able to translocate into the nucleus to activate target genes involved in cell proliferation. Furthermore, POPX2 knock-out using CRISPR in the highly metastatic MDA-MB-231 breast cancer cells results in decreased cell proliferation and impairment of anchorage independent growth. We propose that POPX2 act as a suppressor of the Hippo pathway through LATS1 dephosphorylation and inactivation.

YAPping about and not forgetting TAZ

The Hippo pathway has emerged as a major eukaryotic signalling pathway and is increasingly the subject of intense interest, as are the key effectors of canonical Hippo signalling, YES-associated protein (YAP) and TAZ. The Hippo pathway has key roles in diverse biological processes, including network signalling regulation, development, organ growth, tissue repair and regeneration, cancer, stem cell regulation and mechanotransduction. YAP and TAZ are multidomain proteins and function as transcriptional coactivators of key genes to evoke their biological effects. YAP and TAZ interact with numerous partners and their activities are controlled by a complex set of processes. This review provides an overview of Hippo signalling and its role in growth. In particular, the functional domains of YAP and TAZ and the complex mechanisms that regulate their protein stability and activity are discussed. Notably, the similarities and key differences are highlighted between the two paralogues including which partner proteins interact with which functional domains to regulate their activity.

Protease-activated receptor 2 signaling modulates susceptibility of colonic epithelium to injury through stabilization of YAP in vivo

Hippo signaling plays critical roles in intestinal regeneration. However, the mechanisms which regulate its activity in vivo are largely unknown. We hypothesize that protease-activated receptor 2 (PAR2) signaling, which could be activated by trypsin, might affect YAP activity in the setting of tissue damage and regeneration. It is found that knockout of PAR2 severely aggravates the mucosal damage induced by dextran sodium sulfate (DSS) in mouse, which correlated with notable repression of YAP protein in colonic epithelial cells. Although the cytokine expression is reduced, the damage of colonic crypt is more severe after DSS-induced colitis in PAR2-/- mouse. In vitro, PAR2 activation causes the accumulation of YAP, while knockdown of PAR2 with shRNA dramatically represses the expression of YAP protein in different intestinal epithelial cell lines. Moreover, forced expression of YAP significantly reduces the production of reactive oxygen species (ROS) and the sensitivity to nitric oxide-induced apoptosis in PAR2-deficient condition. Further studies show that PAR2 signaling stabilizes YAP protein but independent of Lats. Nevertheless PAR2 activation increased the binding of YAP with protein phosphatase PP1. Inhibition of PP1 with specific siRNA blocked PAR2-induced dephosphorylation of YAP. Taken together, PAR2 signaling might modulate susceptibility of colonic epithelium to injury through stabilization of YAP.

Interplay between YAP/TAZ and Metabolism

Yes-associated protein (YAP) and transcriptional coactivator with PDZ-binding motif (TAZ) are two homologous transcriptional coactivators that promote cell proliferation, stem cell maintenance, and tissue homeostasis. Under favorable conditions, YAP and TAZ are active to promote cell growth through a transcriptional program mediated by the TEAD family transcription factors. Given the indispensability of cellular energy and metabolites for survival and growth, YAP and TAZ are inhibited when energy level is low. Indeed, glucose, fatty acids, hormones, and other metabolic factors have been recently revealed to regulate YAP and TAZ. Conversely, YAP and TAZ are also involved in metabolism regulation, such as to promote glycolysis, lipogenesis, and glutaminolysis, suggesting YAP and TAZ as emerging nodes in coordinating nutrient availability with cell growth and tissue homeostasis. In this Review, we summarize recent findings and provide a current overview of YAP and TAZ in metabolism by focusing on the role of YAP and TAZ as integrators for metabolic cues and cell growth.

Multifaceted regulation and functions of YAP/TAZ in tumors (Review)

The Hippo pathway, initially identified through screenings for mutant tumor suppressors in Drosophila, is an evolutionarily conserved signaling pathway that controls organ size by regulating cell proliferation and apoptosis. Abnormal regulation of the Hippo pathway may lead to cancer in mammals. As the major downstream effectors of the Hippo pathway, unphosphorylated Yes-associated protein (YAP) and its homolog transcriptional co-activator TAZ (also called WWTR1) (hereafter called YAP/TAZ) are translocated into the nucleus. In the nucleus, in order to induce target gene expression, YAP/TAZ bind to the TEA domain (TEAD) proteins, and this binding subsequently promotes cell proliferation and inhibits apoptosis. In contrast, as key regulators of tumorigenesis and development, YAP/TAZ are phosphorylated and regulated by multiple molecules and pathways including Lats1/2 of Hippo, Wnt and G-protein-coupled receptor (GPCR) signaling, with a regulatory role in cell physiology, tumor cell development and pathological abnormalities simultaneously. In particular, the crucial role of YAP/TAZ in tumors ensures their potential as targets in designing anticancer drugs. To date, mounting research has elucidated the suppression of YAP/TAZ via effective inhibitors, which significantly highlights their application in cancer treatment. In the present review, we focus on the functions of YAP/TAZ in cancer, discuss their potential as new therapeutic target for tumor treatment, and provide valuable suggestions for further study in this field.

Regulation of Yki/Yap subcellular localization and Hpo signaling by a nuclear kinase PRP4K

Hippo (Hpo) signaling pathway controls tissue growth by regulating the subcellular localization of Yorkie (Yki)/Yap via a cytoplasmic kinase cassette containing an upstream kinase Hpo/MST1/2 and a downstream kinase Warts (Wts)/Lats1/2. Here we show that PRP4K, a kinase involved in mRNA splicing, phosphorylates Yki/Yap in the nucleus to prevent its nuclear accumulation and restrict Hpo pathway target gene expression. PRP4K inactivation accelerates whereas excessive PRP4K inhibits Yki-driven tissue overgrowth. PRP4K phosphorylates a subset of Wts/Lats1/2 sites on Yki/Yap to inhibit the binding of Yki/Yap to the Scalloped (Sd)/TEAD transcription factor and exclude Yki/Yap nuclear localization depending on nuclear export. Furthermore, PRP4K inhibits proliferation and invasiveness of cultured breast cancer cells and its high expression correlates with good prognosis in breast cancer patients. Our study unravels an unanticipated layer of Hpo pathway regulation and suggests that PRP4K-mediated Yki/Yap phosphorylation in the nucleus provides a fail-safe mechanism to restrict aberrant pathway activation.

The Hippo signaling pathway controls tissue growth by regulating the subcellular localization of Yorkie /Yap. Here the authors show that PRP4K, a kinase involved in mRNA splicing, phosphorylates Yki/Yap in the nucleus, which prevents its nuclear accumulation and inhibits Hippo signaling.

Metformin mediates resensitivity to 5-fluorouracil in hepatocellular carcinoma via the suppression of YAP

Metformin plays an anti-proliferative role in tumor cells in many types of cancer. However, the correlation between metformin and sensitivity to chemotherapeutic agents in hepatocellular carcinoma (HCC) and the relevant mechanism are unclear. The present study showed that HCC patients with type 2 diabetes mellitus benefited from metformin administration, with a longer overall survival. Metformin resensitized Bel-7402/5-fluorouracil (Bel/Fu) cells to 5-fluorouracil (5-Fu) in vitro and in vivo, and the combination of metformin and 5-Fu inhibited cell proliferation, promoted cell apoptosis and induced G0/G1 cell cycle arrest in the Bel/Fu cells. Moreover, metformin repressed YAP by both decreasing the total protein expression and accelerating the phosphorylation of YAP. The inhibition of YAP subsequently promoted the expression of PTEN, and suppressed the Akt pathway. Therefore, the expression of P-gp and MRP1 was downregulated. Taken together, our findings suggested that metformin may increase sensitivity to chemotherapeutic agents by suppressing YAP in hepatocellular carcinoma.

Platelets reduce anoikis and promote metastasis by activating YAP1 signaling

Thrombocytosis is present in more than 30% of patients with solid malignancies and correlates with worsened patient survival. Tumor cell interaction with various cellular components of the tumor microenvironment including platelets is crucial for tumor growth and metastasis. Although it is known that platelets can infiltrate into tumor tissue, secrete pro-angiogenic and pro-tumorigenic factors and thereby increase tumor growth, the precise molecular interactions between platelets and metastatic cancer cells are not well understood. Here we demonstrate that platelets induce resistance to anoikis in vitro and are critical for metastasis in vivo. We further show that platelets activate RhoA-MYPT1-PP1-mediated YAP1 dephosphorylation and promote its nuclear translocation which induces a pro-survival gene expression signature and inhibits apoptosis. Reduction of YAP1 in cancer cells in vivo protects against thrombocytosis-induced increase in metastasis. Collectively, our results indicate that cancer cells depend on platelets to avoid anoikis and succeed in the metastatic process.

Platelets have been associated with increased tumor growth and metastasis but the mechanistic details of this interaction are still unclear. Here the authors show that platelets improve anoikis resistance of cancer cells and increase metastasis by activating Yap through a RhoA/MYPT-PP1 pathway.

Dual function of partitioning-defective 3 in the regulation of YAP phosphorylation and activation

Partitioning-defective 3 (Par3), a key component of the evolutionarily conserved polarity PAR complex (Par3/Par6/aPKC), controls cell polarity and contributes to cell migration, proliferation and tumor development. Emerging evidence indicates that cell polarity proteins function as upstream modulators that regulate the Hippo pathway. However, little is known about Par3’s involvement in the Hippo pathway. Here, we find Par3 and YAP dynamically co-localize in different subcellular compartments; that is, the membrane, cytoplasm and nucleus, in a cell-density-dependent manner. Interestingly, Par3 knockdown promotes YAP phosphorylation, leading to a significant impairment of YAP nuclear translocation at low cell density, but not at high density, in MDCK cells. Furthermore, via its third PDZ domain, Par3 directly binds to the PDZ-binding motif of YAP. The interaction is required for regulating YAP phosphorylation and nuclear localization. Mechanistically, Par3, as a scaffold protein, associates with LATS1 and protein phosphatase 1, α subunit (PP1A) in the cytoplasm and nucleus. Par3 promotes the dephosphorylation of LATS1 and YAP, thus enhancing YAP activation and cell proliferation. Strikingly, we also find that under the condition of PP1A knockdown, Par3 expression promotes YAP hyperphosphorylation, leading to the suppression of YAP activity and its downstream targets. Par3 expression results in differential effects on YAP phosphorylation and activation in different tumor cell lines. These findings indicate that Par3 may have a dual role in regulating the activation of the Hippo pathway, in a manner possibly dependent on cellular context or cell type in response to cell–cell contact and cell polarity signals.

Control of YAP/TAZ Activity by Metabolic and Nutrient-Sensing Pathways

Metabolism is a fundamental cellular function that can be reprogrammed by signaling pathways and oncogenes to meet cellular requirements. An emerging paradigm is that signaling and transcriptional networks can be in turn regulated by metabolism, allowing cells to coordinate their metabolism and behavior in an integrated manner. The activity of the YAP/TAZ transcriptional coactivators, downstream transducers of the Hippo cascade and powerful pro-oncogenic factors, was recently found to be regulated by metabolic pathways, such as aerobic glycolysis and mevalonate synthesis, and by the nutrient-sensing LKB1-AMPK and TSC-mTOR pathways. We discuss here current data linking YAP/TAZ to metabolism and suggest how this coupling might coordinate nutrient availability with genetic programs that sustain tissue growth, neoplastic cell proliferation, and tumor malignancy.

YAP and TAZ Take Center Stage in Cancer

The Hippo pathway was originally identified and named through screening for mutations in Drosophila, and the core components of the Hippo pathway are highly conserved in mammals. In the Hippo pathway, MST1/2 and LATS1/2 regulate downstream transcription coactivators YAP and TAZ, which mainly interact with TEAD family transcription factors to promote tissue proliferation, self-renewal of normal and cancer stem cells, migration, and carcinogenesis. The Hippo pathway was initially thought to be quite straightforward; however, recent studies have revealed that YAP/TAZ is an integral part and a nexus of a network composed of multiple signaling pathways. Therefore, in this review, we will summarize the latest findings on events upstream and downstream of YAP/TAZ and the ways of regulation of YAP/TAZ. In addition, we also focus on the crosstalk between the Hippo pathway and other tumor-related pathways and discuss their potential as therapeutic targets.

Hippo circuitry and the redox modulation of hippo components in cancer cell fate decisions

Meticulous and precise control of organ size is undoubtedly one of the most pivotal processes in mammalian development and regeneration along with cell differentiation, morphogenesis and programmed cell death. These processes are strictly regulated by complex and highly coordinated mechanisms to maintain a steady growth state. There are a number of extrinsic and intrinsic factors that dictate the total number and/or size of cells by influencing growth, proliferation, differentiation and cell death. Multiple pathways, such as those involved in promoting organ size and others that restrict disproportionate tissue growth act simultaneously to maintain cellular and tissue homeostasis. Aberrations at any level in these organ size-regulating processes can lead to various pathological states with cancers being the most formidable one (Yin and Zhang, 2011). Extensive research in the realm of growth control has led to the identification of the Hippo-signaling pathway as a critical network in modulating tissue growth via its effect on multiple signaling pathways and through intricate crosstalk with proteins that regulate cell polarity, adhesion and cell-cell interactions (Zhao et al., 2011b). The Hippo pathway controls cell number and organ size by transducing signals from the plasma membrane to the nucleus to regulate the expression of genes involved in cell fate determination (Shi et al., 2015). In this review, we summarize the recent discoveries concerning Hippo pathway, its diversiform regulation in mammals as well as its implications in cancers, and highlight the possible role of oxidative stress in Hippo pathway regulation.

Control of Proliferation and Cancer Growth by the Hippo Signaling Pathway

The control of cell division is essential for normal development and the maintenance of cellular homeostasis. Abnormal cell proliferation is associated with multiple pathological states, including cancer. Although the Hippo/YAP signaling pathway was initially thought to control organ size and growth, increasing evidence indicates that this pathway also plays a major role in the control of proliferation independent of organ size control. In particular, accumulating evidence indicates that the Hippo/YAP signaling pathway functionally interacts with multiple other cellular pathways and serves as a central node in the regulation of cell division, especially in cancer cells. Here, recent observations are highlighted that connect Hippo/YAP signaling to transcription, the basic cell-cycle machinery, and the control of cell division. Furthermore, the oncogenic and tumor-suppressive attributes of YAP/TAZ are reviewed, which emphasizes the relevance of the Hippo pathway in cancer.

YAP1 and AR interactions contribute to the switch from androgen-dependent to castration-resistant growth in prostate cancer

The transcriptional co-activator Yes-associated protein 1 (YAP1), a key nuclear effector of the Hippo pathway, is a potent oncogene, and yet, the interaction between YAP1 and androgen receptor (AR) remains unexplored. Here we identify YAP1 as a physiological binding partner and positive regulator of AR in prostate cancer. YAP1 and AR co-localize and interact with each other predominantly within cell nuclei by an androgen-dependent mechanism in a hormone naive and an androgen-independent mechanism in castration-resistant prostate cancer cells. The growth suppressor MST1 kinase modulates androgen-dependent and -independent nuclear YAP1-AR interactions through directly regulating YAP1 nuclear accumulation. Disruption of YAP1 signalling by genetic (RNAi) and pharmacological (Verteporfin) approaches suppresses AR-dependent gene expression and prostate cancer cell growth. These findings indicate that the YAP1-AR axis may have a critical role in prostate cancer progression and serves as a viable drug target.

Emerging role of Hpo signaling and YAP in hepatocellular carcinoma

Hepatocellular carcinoma (HCC) is the sixth most common cancer and the third most common cause of cancer-related mortality worldwide. Due to the poor prognosis and limited therapeutic options, there is great interest in further understanding better the molecular underpinnings and potential molecular targets associated with HCC. The Hippo (Hpo) signaling pathway and YAP, its principal downstream effector, represent an innovative area of research in HCC. Pioneered in Drosophila melanogaster, the Hpo cascade controls tissue homeostasis including organ size, cell proliferation, apoptosis, as well as cell-cycle regulation and differentiation. This conserved kinase cascade in mammals depends on central control by the tumor suppressor mammalian sterile 20-like kinase 1/2 (Mst1/2). The Mst1/2 commences the downstream kinase cascade, ultimately activating the oncoprotein YAP and allowing its physical association with downstream targets to enhance the gene expression signatures that are involved in proliferation and survival. Alterations in YAP expression and defective regulation of other key Hpo pathway members, such as Mst1/2, Salvador, neurofibromatosis and Mer (Nf2/mer), large tumor suppressor homolog 1/2 (Lats1/2), and Mps one binder kinase activator-like 1A and 1B (Mob1) drive carcinogenesis in animal models. The dysregulation of the Hpo pathway – resulting in an unchecked activation of YAP – culminates in the development of a broad range of human tumor types, including HCC. The abrogation of Mst1/2-mediated YAP phosphorylation permits YAP entry into the nucleus in murine models and functions similarly in human HCCs. Chemoresistance mechanisms displayed by HCC tumors occur in a YAP-dependent manner. The HCC specimens exhibit YAP overexpression, and YAP serves as an independent prognostic marker for disease-free survival and overall survival in patients with HCC. Recently, the small molecule inhibitor, verteporfin has been shown to attenuate YAP activity in murine models, perhaps offering a novel therapeutic approach for patients with advanced HCC.

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

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

IMPORTANCE

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

Netrin-1 exerts oncogenic activities through enhancing Yes-associated protein stability

Yes-associated protein (YAP), a transcription coactivator, is the major downstream effector of the Hippo pathway, which plays a critical role in organ size control and cancer development. However, how YAP is regulated by extracellular stimuli in tumorigenesis remains incompletely understood. Netrin-1, a laminin-related secreted protein, displays proto-oncogenic activity in cancers. Nonetheless, the downstream signaling mediating its oncogenic effects is not well defined. Here we show that netrin-1 via its transmembrane receptors, deleted in colorectal cancer and uncoordinated-5 homolog, up-regulates YAP expression, escalating YAP levels in the nucleus and promoting cancer cell proliferation and migration. Inactivating netrin-1, deleted in colorectal cancer, or uncoordinated-5 homolog B (UNC5B) decreases YAP protein levels, abrogating cancer cell progression by netrin-1, whereas knockdown of mammalian STE20-like protein kinase 1/2 (MST1/2) or large tumor suppressor kinase 1/2 (Lats1/2), two sets of upstream core kinases of the Hippo pathway, has no effect in blocking netrin-1-induced up-regulation of YAP. Netrin-1 stimulates phosphatase 1A to dephosphorylate YAP, which leads to decreased ubiquitination and degradation, enhancing YAP accumulation and signaling. Hence, our findings support that netrin-1 exerts oncogenic activity through YAP signaling, providing a mechanism coupling extracellular signals to the nuclear YAP oncogene.

Roles of Hippo signaling pathway in size control of organ regeneration

Animals have an intrinsic regeneration ability for injured tissues and organs. Species that have high regeneration ability such as newts can regenerate an organ with exactly the same size and shape as those of the original one. It has been unclear how a regenerating organ grows and ceases growth at an appropriate size. Organ size control in regeneration is seen in various organs of various species that have high regeneration ability. In animal species that do not have sufficient regeneration ability, a wound heals (the injury is closed, but lost parts are not regenerated), but an organ cannot be restored to its original size. On the other hand, perturbation of regeneration sometimes results in oversized or extra structures. In this sense, organ size control plays essential roles in proper regeneration. In this article, we introduce the concept of size control in organ regeneration regulated by the Hippo signaling pathway. We focused on the transcriptional regulator Yap, which shuttles between the nuclei and cytoplasm to exert a regulatory function in a context-dependent manner. The Yap-mediated Hippo pathway is thought to sense cell density, extracellular matrix (ECM) contact and cell position and to regulate gene expression for control of organ size. This mechanism can reasonably explain size control of organ regeneration.

Casein kinase II controls TBK1/IRF3 activation in IFN response against viral infection

By sensing viral nucleic acids, host innate receptors elicit signaling pathways converging on TBK1-IFN regulatory factor (IRF)3 axis in mediating IFN-αβ induction and defense mechanisms. In contrast, viruses have evolved with diverse immune evasion/interference mechanisms to undermine innate receptor signaling and IFN response. In this regard, approaches enabling host to overcome such immune evasion/interference mechanisms are urgently needed to combat infections by epidemic/pandemic viruses. In this study, we report that protein kinase CK2 serves as a key component controlling TBK1 and IRF3 activation in IFN-inducing TLR, RIG-I-like receptors, and cGAS/STING signaling pathways. Accordingly, knocking down of CK2 expression or genetic ablation of its kinase activity resulted in elevated IFN-αβ response in response to infection by DNA and RNA viruses. Moreover, PP2A was identified as one of the intermediate phosphatases responsible for CK2-regulated IFN response, suggesting that CK2 may regulate TBK1 and IRF3 activation indirectly. Importantly, blockade of CK2 activity by small molecule inhibitor was able to activate TBK1, whereby eliciting effective host defense mechanisms against hepatitis C virus infection. Taken together, our results identify CK2 as a novel regulator of TBK1 and IRF3 and suggest that targeting CK2 by small molecular inhibitor may be a viable approach to prevent and treat viral infections.

HDAC2 selectively regulates FOXO3a-mediated gene transcription during oxidative stress-induced neuronal cell death

All neurodegenerative diseases are associated with oxidative stress-induced neuronal death. Forkhead box O3a (FOXO3a) is a key transcription factor involved in neuronal apoptosis. However, how FOXO3a forms complexes and functions in oxidative stress processing remains largely unknown. In the present study, we show that histone deacetylase 2 (HDAC2) forms a physical complex with FOXO3a, which plays an important role in FOXO3a-dependent gene transcription and oxidative stress-induced mouse cerebellar granule neuron (CGN) apoptosis. Interestingly, we also found that HDAC2 became selectively enriched in the promoter region of the p21 gene, but not those of other target genes, and inhibited FOXO3a-mediated p21 transcription. Furthermore, we found that oxidative stress reduced the interaction between FOXO3a and HDAC2, leading to an increased histone H4K16 acetylation level in the p21 promoter region and upregulated p21 expression in a manner independent of p53 or E2F1. Phosphorylation of HDAC2 at Ser 394 is important for the HDAC2-FOXO3a interaction, and we found that cerebral ischemia/reperfusion reduced phosphorylation of HDAC2 at Ser 394 and mitigated the HDAC2-FOXO3a interaction in mouse brain tissue. Our study reveals the novel regulation of FOXO3a-mediated selective gene transcription via epigenetic modification in the process of oxidative stress-induced cell death, which could be exploited therapeutically.

Integration of Hippo signalling and the unfolded protein response to restrain liver overgrowth and tumorigenesis

The role of the unfolded protein response (UPR) in tissue homeostasis remains largely unknown. Here we find that loss of Mst1/2, the mammalian Hippo orthologues, or their regulator WW45, leads to a remarkably enlarged endoplasmic reticulum (ER) size-associated UPR. Intriguingly, attenuation of the UPR by tauroursodeoxycholic acid (TUDCA) diminishes Mst1/2 mutant-driven liver overgrowth and tumorigenesis by promoting nuclear exit and degradation of Hippo downstream effector Yap. Yap is required for UPR activity and ER expansion to alleviate ER stress. During the adaptive stage of the UPR, PERK kinase-eIF2α axis activates Yap, while prolonged ER stress-induced Hippo signalling triggers assembly of the GADD34/PP1 complex in a negative feedback loop to inhibit Yap and promote apoptosis. Significantly, the deregulation of UPR signals associated with Yap activation is found in a substantial fraction of human hepatocellular carcinoma (HCC). Thus, we conclude Yap integrates Hippo and UPR signalling to control liver size and tumorigenesis.

LATS2-mediated YAP1 phosphorylation is involved in HCC tumorigenesis

YAP (yes-associated protein) is a transcriptional co-activator that acts downstream of the Hippo signaling pathway and regulates multiple cellular processes, including proliferation and apoptosis. Although YAP plays an important role in various tumors, the underlying mechanism in hepatocellular carcinoma (HCC) tumorigenesis remains unclear. In this study, we observed that the LATS2 was highly expressed in Bel-7402 and HepG2 cell lines, and LATS2 protein level was negatively correlated with YAP1 in HCC cells. And then, we inhibited LATS2 expression by transfecting with siRNA. Western blot and Immunofluorescent staining analysis demonstrated that LATS2 inhibition decreased the dephosphorylation of YAP1 protein and promoted YAP1 nuclear accumulation in HCC cells. Moreover, Immunoprecipitation assay results also indicated that Yap binds directly to TEAD2 and LATS2 inhibition-mediated dephosphorylation increased the YAP1/TEAD2 association, leading to YAP1/TEAD2 transcriptional activation, which in turn upregulated cell invasion in HCC cells. Taken together, our current data indicated a new regulatory mechanism of YAP1 by the LATS2-mediated phosphorylation that was involved in HCC tumorigenesis.

The regulation and function of YAP transcription co-activator

The Hippo pathway was initially identified in Drosophila by genetic mosaic screens for tumor suppressor genes. Researches indicated that the Hippo pathway is a key regulator of organ size and is conserved during evolution. Furthermore, studies of mouse models and clinical samples demonstrated the importance of Hippo pathway dysregulation in human cancer development. In addition, the Hippo pathway contributes to progenitor cell and stem cell self-renewal and is thus involved in tissue regeneration. In the Hippo pathway, MST1/2 kinases together with the adaptor protein SAV phosphorylate LATS1/2 kinases. Interaction with an adaptor protein MOB is also important for LATS1/2 activation. Activated LATS1/2 in turn phosphorylate and inhibit Yes-associated protein (YAP). YAP is a key downstream effector of the Hippo pathway, and is a transcriptional co-activator that mainly interacts with TEAD family transcription factors to promote gene expression. Alteration of gene expression by YAP leads to cell proliferation, apoptosis evasion, and also stem cell amplification. In this review, we mainly focus on YAP, discussing its regulation and mechanisms of action in the context of organ size control, tissue regeneration and tumorigenesis.

The biology of YAP/TAZ: hippo signaling and beyond

The transcriptional regulators YAP and TAZ are the focus of intense interest given their remarkable biological properties in development, tissue homeostasis and cancer. YAP and TAZ activity is key for the growth of whole organs, for amplification of tissue-specific progenitor cells during tissue renewal and regeneration, and for cell proliferation. In tumors, YAP/TAZ can reprogram cancer cells into cancer stem cells and incite tumor initiation, progression and metastasis. As such, YAP/TAZ are appealing therapeutic targets in cancer and regenerative medicine. Just like the function of YAP/TAZ offers a molecular entry point into the mysteries of tissue biology, their regulation by upstream cues is equally captivating. YAP/TAZ are well known for being the effectors of the Hippo signaling cascade, and mouse mutants in Hippo pathway components display remarkable phenotypes of organ overgrowth, enhanced stem cell content and reduced cellular differentiation. YAP/TAZ are primary sensors of the cell's physical nature, as defined by cell structure, shape and polarity. YAP/TAZ activation also reflects the cell "social" behavior, including cell adhesion and the mechanical signals that the cell receives from tissue architecture and surrounding extracellular matrix (ECM). At the same time, YAP/TAZ entertain relationships with morphogenetic signals, such as Wnt growth factors, and are also regulated by Rho, GPCRs and mevalonate metabolism. YAP/TAZ thus appear at the centerpiece of a signaling nexus by which cells take control of their behavior according to their own shape, spatial location and growth factor context.