Yes-associated protein regulates the hepatic response after bile duct ligation

[Zheng Gan Decoction inhibits diethylnitrosamine-induced hepatocellular carcinoma in rats by activating the Hippo/YAP signaling pathway]

OBJECTIVES

To investigate the inhibitory effect of Zheng GanDecoction (ZGF) on tumor progression in a rat model of diethylnitrosamine (DEN)-induced hepatocellular carcinoma (HCC) and explore the possible mechanism.

METHODS

Seventy SD rats were subjected to regular intraperitoneal injections of DEN (50 mg/kg) for 12 weeks to induce HCC tumorigenesis, with another 10 rats receiving saline injections as the normal control. After successful modeling, the rats were randomized into 5 groups (n=10) for daily treatment with distilled water ( model group), Huaier Granules (4 g/kg; positive control group), or ZGF at low, medium, and high doses (2, 4, and 8 g/kg, respectively) via gavage for 17 weeks. Body weight changes of the rats were monitored, and after completion of the treatments, the rats were euthanized for measurement of liver, spleen and thymus indices and morphological and histopathological examinations of the liver tissues using HE staining. The expressions of YAP, p-YAP, MST1, LATS1 and p-LATS1 in the liver tissues were detected using immunohistochemistry and Western blotting.

RESULTS

Compared with the normal control rats, the rat models with DEN-induced HCC exhibited much poorer general condition with a significantly reduced survival rate, increased body weight and liver and spleen indices, and a lowered thymus index. ZGF treatment obviously reduced liver and spleen indices, increased the thymus index, and improved pathologies of the liver tissues of the rat models. Immunohistochemistry and Western blotting showed a dose-dependent reduction of YAP expression and an increment of p-YAP expression in ZGF-treated rats, which also exhibited significantly upregulated hepatic expressions of MST1, LATS1 and p-LATS1.

CONCLUSIONS

ZGF inhibits DEN-induced HCC in rats by activating the Hippo/YAP pathway via upregulating MST1 and LATS1 expression, which promotes YAP phosphorylation and degradation to suppress proliferation and induce apoptosis of the tumor cells.

Microenvironmental control of the ductular reaction: balancing repair and disease progression

The ductular reaction (DR) is a dynamic adaptive cellular response within the liver, triggered by various hepatic insults and characterized by an expansion of dysmorphic biliary epithelial cells and liver progenitors. This complex response presents a dual role, playing a pivotal function in liver regeneration but, paradoxically, contributing to the progression of liver diseases, depending upon specific contextual factors and signaling pathways involved. This comprehensive review aims to offer a holistic perspective on the DR, focusing into its intricate cellular and molecular mechanisms, highlighting its pathological significance, and exploring its potential therapeutic implications. An up-to-date understanding of the DR in the context of different liver injuries is provided, analyzing its contributions to liver regeneration, inflammation, fibrosis, and ultimately carcinogenesis. Moreover, the review highlights the role of multiple microenvironmental factors, including the influence of extracellular matrix, tissue mechanics and the interplay with the intricate hepatic cell ecosystem in shaping the DR's regulation. Finally, in vitro and in vivo experimental models of the DR will be discussed, providing insights into how researchers can study and manipulate this critical cellular response. By comprehensively addressing the multifaceted nature of the DR, this review contributes to a more profound understanding of its pathophysiological role in liver diseases, thus offering potential therapeutic avenues for hepatic disorders and improving patient outcomes.

Selective Degradation of TEADs by a PROTAC Molecule Exhibited Robust Anticancer Efficacy In Vitro and In Vivo

Genetic mutations in components of the Hippo pathway frequently lead to the aberrant activation of TEADs, which is often associated with cancer. Consequently, TEADs have been actively pursued as therapeutic targets for diseases driven by TEAD overactivation. In this study, we report two series of TEAD PROTACs based on CRBN binders and VHL binders. Both series yielded potent TEAD degraders, including 19 and 40 (H122), which induced TEAD1 degradation with DC50 < 10 nM. Mechanistic studies demonstrated that the degradation of TEAD1 induced by 40 relied on CRBN binding, TEAD1 binding, E3 ligase activity, and a functional proteasome. RNA-seq analyses indicated that 40 significantly downregulated the expression of Myc target genes, as highlighted by GSEA analysis. More importantly, 40 exhibited robust antitumor efficacy in the MSTO-211H mouse xenograft model. Collectively, our results suggest that TEAD PROTACs have therapeutic potential for the treatment of cancers associated with TEAD overactivation.

The Hippo pathway: Organ size control and beyond

The Hippo signaling pathway is a highly conserved signaling network for controlling organ size, tissue homeostasis, and regeneration. It integrates a wide range of intracellular and extracellular signals, such as cellular energy status, cell density, hormonal signals, and mechanical cues, to modulate the activity of YAP/TAZ transcriptional coactivators. A key aspect of Hippo pathway regulation involves its spatial organization at the plasma membrane, where upstream regulators localize to specific membrane subdomains to regulate the assembly and activation of the pathway components. This spatial organization is critical for the precise control of Hippo signaling, as it dictates the dynamic interactions between pathway components and their regulators. Recent studies have also uncovered the role of biomolecular condensation in regulating Hippo signaling, adding complexity to its control mechanisms. Dysregulation of the Hippo pathway is implicated in various pathological conditions, particularly cancer, where alterations in YAP/TAZ activity contribute to tumorigenesis and drug resistance. Therapeutic strategies targeting the Hippo pathway have shown promise in both cancer treatment, by inhibiting YAP/TAZ signaling, and regenerative medicine, by enhancing YAP/TAZ activity to promote tissue repair. The development of small molecule inhibitors targeting the YAP-TEAD interaction and other upstream regulators offers new avenues for therapeutic intervention. SIGNIFICANCE STATEMENT: The Hippo signaling pathway is a key regulator of organ size, tissue homeostasis, and regeneration, with its dysregulation linked to diseases such as cancer. Understanding this pathway opens new possibilities for therapeutic approaches in regenerative medicine and oncology, with the potential to translate basic research into improved clinical outcomes.

Mechanosignaling via Integrins: Pivotal Players in Liver Fibrosis Progression and Therapy

Liver fibrosis, a consequence of chronic liver injury, represents a major global health burden and is the leading cause of liver failure, morbidity, and mortality. The pathological hallmark of this condition is excessive extracellular matrix deposition, driven primarily by integrin-mediated mechanotransduction. Integrins, transmembrane heterodimeric proteins that serve as primary ECM receptors, orchestrate complex mechanosignaling networks that regulate the activation, differentiation, and proliferation of hepatic stellate cells and other ECM-secreting myofibroblasts. These mechanical signals create self-reinforcing feedback loops that perpetuate the fibrotic response. Recent advances have provided insight into the roles of specific integrin subtypes in liver fibrosis and revealed their regulation of key downstream effectors-including transforming growth factor beta, focal adhesion kinase, RhoA/Rho-associated, coiled-coil containing protein kinase, and the mechanosensitive Hippo pathway. Understanding these mechanotransduction networks has opened new therapeutic possibilities through pharmacological manipulation of integrin-dependent signaling.

TWEAK is an activator of Hippo-YAP signaling protecting against hepatic Ischemia/ reperfusion injury

Hepatic ischemia-reperfusion injury (IRI) represents a formidable complication commonly linked with hemorrhagic shock, liver resection, and transplantation. This study aims to elucidate the role of Tumor Necrosis Factor-like Weak Inducer of Apoptosis (TWEAK) in the pathogenesis of hepatic I/R injury and to delineate the underlying mechanisms involved. Utilizing a hypoxia-reoxygenation model in human liver organoids (HLOs) alongside a murine model of warm ischemia-reperfusion injury, we systematically investigated the interplay between TWEAK, its receptor Fn14, and the HIPPO signaling pathway. Our findings indicate that TWEAK pretreatment significantly mitigates IRI in murine livers as well as hypoxia/reoxygenation injury in HLOs. Notably, administration of adeno-associated virus (AAV) to knock down Fn14 abrogated the protective effects of TWEAK in the murine model. Transcriptome sequencing analysis revealed that the interaction between TWEAK and Fn14 enhances cellular resistance to IRI by activating the HIPPO signaling pathway. Overall, TWEAK emerges as a promising therapeutic target for mitigating hepatic I/R injury, potentially improving outcomes in liver transplantation.

MCC950 as a promising candidate for blocking NLRP3 inflammasome activation: A review of preclinical research and future directions

The NOD-like receptor thermal protein domain associated protein 3 (NLRP3) inflammasome is a key component of the innate immune system that triggers inflammation and pyroptosis and contributes to the development of several diseases. Therefore, blocking the activation of the NLRP3 inflammasome has therapeutic potential for the treatment of these diseases. MCC950, a selective small molecule inhibitor, has emerged as a promising candidate for blocking NLRP3 inflammasome activation. Ongoing research is focused on elucidating the specific targets of MCC950 as well as assessfing its metabolism and safety profile. This review discusses the diseases that have been studied in relation to MCC950, with a focus on stroke, Alzheimer's disease, liver injury, atherosclerosis, diabetes mellitus, and sepsis, using bibliometric analysis. It then summarizes the potential pharmacological targets of MCC950 and discusses its toxicity. Furthermore, it traces the progression from preclinical to clinical research for the treatment of these diseases. Overall, this review provides a solid foundation for the clinical therapeutic potential of MCC950 and offers insights for future research and therapeutic approaches.

HIF-1α induced FGF18 alleviates renal ischemia/reperfusion injury via YAP

Acute kidney injury (AKI) is a devastating clinical condition characterized by an abrupt loss of renal function. The pathophysiology of AKI involves diverse processes and elements, of which survival and regeneration have been established to be significant hallmarks. And early studies have confirmed the fundamental role of FGFs in the regulation of AKI pathology, although the association between FGF18 and AKI still remains elusive. Our study demonstrates a substantial up-regulation of FGF18 in the renal tubules of mice subjected to ischemia. Notably, targeted overexpression of FGF18 effectively mitigates the impairment of kidney function induced by AKI. Mechanistically, FGF18 facilitates cell proliferation and anti-apoptosis in RTECs by enhancing the expression of YAP and facilitating its translocation to the nucleus. Aside from that, we also discovered that the substantial expression of FGF18 under ischemic conditions is HIF-1α dependent. This study aims to uncover the inherent mechanism behind the beneficial effects of FGF18 in attenuating AKI. By doing so, it aims to offer novel insights into the development of therapeutic strategies for AKI.

Microglia-mediated pericytes migration and fibroblast transition via S1P/S1P3/YAP signaling pathway after spinal cord injury

Platelet-derived growth factor receptor β positive (PDGFRβ+) pericytes detach from the microvascular wall and migrate into the injury center following spinal cord injury (SCI), which has been widely regarded as the main source of fibrotic scar, but the mechanism of migration and fibroblast transition remains elusive. Here we show the associated spatiotemporal distribution between microglia and pericytes at three and seven days post-injury (dpi). The increased expression of Sphingosine kinase-1 (SPHK1) in microglia significantly raised the concentration of Sphingosine-1-phosphate (S1P) in the spinal cord, which promotes migration and fibroblast transition of pericyte. In vitro experiments, we found the elevated Sphingosine 1-phosphate receptor 3 (S1P3), the S1P/S1PR3 axis inhibited the phosphorylation of YAP and promoted its nuclear translocation, which contributed to the formation of alpha-smooth muscle actin (α-SMA) and collagen type I (COL1) protein, This process can be blocked by an S1P3 specific inhibitor TY52156 in vitro. The S1P/S1P3/YAP pathway might be a potential target for treatment in SCI.

Cell competition in primary and metastatic colorectal cancer

Adult tissues set the scene for a continuous battle between cells, where a comparison of cellular fitness results in the elimination of weaker "loser" cells. This phenomenon, named cell competition, is beneficial for tissue integrity and homeostasis. In fact, cell competition plays a crucial role in tumor suppression, through elimination of early malignant cells, as part of Epithelial Defense Against Cancer. However, it is increasingly apparent that cell competition doubles as a tumor-promoting mechanism. The comparative nature of cell competition means that mutational background, proliferation rate and polarity all factor in to determine the outcome of these processes. In this review, we explore the intricate and context-dependent involvement of cell competition in homeostasis and regeneration, as well as during initiation and progression of primary and metastasized colorectal cancer. We provide a comprehensive overview of molecular and cellular mechanisms governing cell competition and its parallels with regeneration.

Sortilin in Biliary Epithelial Cells Promotes Ductular Reaction and Fibrosis during Cholestatic Injury

Cholestatic injuries are accompanied by ductular reaction, initiated by proliferation and activation of biliary epithelial cells (BECs), leading to fibrosis. Sortilin (encoded by Sort1) facilitates IL-6 secretion and leukemia inhibitory factor (LIF) signaling. This study investigated the interplay between sortilin and IL-6 and LIF in cholestatic injury-induced ductular reaction, morphogenesis of new ducts, and fibrosis. Cholestatic injury was induced by bile duct ligation (BDL) in wild-type and Sort1-/- mice, with or without augmentation of IL-6 or LIF. Mice with BEC sortilin deficiency (hGFAPcre.Sort1fl/fl) and control mice were subjected to BDL and 3,5-diethoxycarbonyl-1,4-dihydrocollidine diet (DDC) induced cholestatic injury. Sort1-/- mice displayed reduced BEC proliferation and expression of BEC-reactive markers. Administration of LIF or IL-6 restored BEC proliferation in Sort1-/- mice, without affecting BEC-reactive or inflammatory markers. Sort1-/- mice also displayed impaired morphogenesis, which was corrected by LIF treatment. Similarly, hGFAPcre.Sort1fl/fl mice exhibited reduced BEC proliferation, but similar reactive and inflammatory marker expression. Serum IL-6 and LIF were comparable, yet liver pSTAT3 was reduced, indicating that sortilin is essential for co-activation of LIF receptor/gp130 signaling in BECs, but not for IL-6 secretion. hGFAPcre.Sortfl/fl mice displayed impaired morphogenesis and diminished fibrosis after BDL and DDC. In conclusion, sortilin-mediated engagement of LIF signaling in BECs promoted ductular reaction and morphogenesis during cholestatic injury. This study indicates that BEC sortilin is pivotal for the development of fibrosis.

The Hippo signaling pathway in development and regeneration

The Hippo signaling pathway is a central growth control mechanism in multicellular organisms. By integrating diverse mechanical, biochemical, and stress cues, the Hippo pathway orchestrates proliferation, survival, differentiation, and mechanics of cells, which in turn regulate organ development, homeostasis, and regeneration. A deep understanding of the regulation and function of the Hippo pathway therefore holds great promise for developing novel therapeutics in regenerative medicine. Here, we provide updates on the molecular organization of the mammalian Hippo signaling network, review the regulatory signals and functional outputs of the pathway, and discuss the roles of Hippo signaling in development and regeneration.

Dynamic YAP expression in the non-parenchymal liver cell compartment controls heterologous cell communication

INTRODUCTION

The Hippo pathway and its transcriptional effectors yes-associated protein (YAP) and transcriptional coactivator with PDZ-binding motif (TAZ) are targets for cancer therapy. It is important to determine if the activation of one factor compensates for the inhibition of the other. Moreover, it is unknown if YAP/TAZ-directed perturbation affects cell-cell communication of non-malignant liver cells.

MATERIALS AND METHODS

To investigate liver-specific phenotypes caused by YAP and TAZ inactivation, we generated mice with hepatocyte (HC) and biliary epithelial cell (BEC)-specific deletions for both factors (YAPKO, TAZKO and double knock-out (DKO)). Immunohistochemistry, single-cell sequencing, and proteomics were used to analyze liver tissues and serum.

RESULTS

The loss of BECs, liver fibrosis, and necrosis characterized livers from YAPKO and DKO mice. This phenotype was weakened in DKO tissues compared to specimens from YAPKO animals. After depletion of YAP in HCs and BECs, YAP expression was induced in non-parenchymal cells (NPCs) in a cholestasis-independent manner. YAP positivity was detected in subgroups of Kupffer cells (KCs) and endothelial cells (ECs). The secretion of pro-inflammatory chemokines and cytokines such as C-X-C motif chemokine ligand 11 (CXCL11), fms-related receptor tyrosine kinase 3 ligand (FLT3L), and soluble intercellular adhesion molecule-1 (ICAM1) was increased in the serum of YAPKO animals. YAP activation in NPCs could contribute to inflammation via TEA domain transcription factor (TEAD)-dependent transcriptional regulation of secreted factors.

CONCLUSION

YAP inactivation in HCs and BECs causes liver damage, and concomitant TAZ deletion does not enhance but reduces this phenotype. Additionally, we present a new mechanism by which YAP contributes to cell-cell communication originating from NPCs.

The inhibition of YAP Signaling Prevents Chronic Biliary Fibrosis in the Abcb4-/- Model by Modulation of Hepatic Stellate Cell and Bile Duct Epithelium Cell Pathophysiology

Primary sclerosing cholangitis (PSC) represents a chronic liver disease characterized by poor prognosis and lacking causal treatment options. Yes-associated protein (YAP) functions as a critical mediator of fibrogenesis; however, its therapeutic potential in chronic biliary diseases such as PSC remains unestablished. The objective of this study is to elucidate the possible significance of YAP inhibition in biliary fibrosis by examining the pathophysiology of hepatic stellate cells (HSC) and biliary epithelial cells (BEC). Human liver tissue samples from PSC patients were analyzed to assess the expression of YAP/connective tissue growth factor (CTGF) relative to non-fibrotic control samples. The pathophysiological relevance of YAP/CTGF in HSC and BEC was investigated in primary human HSC (phHSC), LX-2, H69, and TFK-1 cell lines through siRNA or pharmacological inhibition utilizing verteporfin (VP) and metformin (MF). The Abcb4-/- mouse model was employed to evaluate the protective effects of pharmacological YAP inhibition. Hanging droplet and 3D matrigel culture techniques were utilized to investigate YAP expression and activation status of phHSC under various physical conditions. YAP/CTGF upregulation was observed in PSC patients. Silencing YAP/CTGF led to inhibition of phHSC activation and reduced contractility of LX-2 cells, as well as suppression of epithelial-mesenchymal transition (EMT) in H69 cells and proliferation of TFK-1 cells. Pharmacological inhibition of YAP mitigated chronic liver fibrosis in vivo and diminished ductular reaction and EMT. YAP expression in phHSC was effectively modulated by altering extracellular stiffness, highlighting YAP's role as a mechanotransducer. In conclusion, YAP regulates the activation of HSC and EMT in BEC, thereby functioning as a checkpoint of fibrogenesis in chronic cholestasis. Both VP and MF demonstrate effectiveness as YAP inhibitors, capable of inhibiting biliary fibrosis. These findings suggest that VP and MF warrant further investigation as potential therapeutic options for the treatment of PSC.

Comparative genomic analysis of Fischer F344 rat livers exposed for 90 days to 3-methylfuran or its parental compound furan

Furan is a naturally forming compound found in heat-processed foods such as coffee, canned meats, and jarred baby food. It is concurrently found with analogues including 2-methylfuran (2-MF) and 3-methylfuran (3-MF), and toxicity studies demonstrate all are potent liver toxins. Toxicity studies found 3-MF is more toxic than either furan, or 2-MF. The present analysis assesses the transcriptional response in liver samples taken from male Fischer (F344) rats exposed to furan or 3-MF from 0 to 2.0 and 0-1.0 mg/kg bw/day, respectively, for 90 days. Transcriptional analyses found decreased liver function and fatty acid metabolism are common responses to both furan and 3-MF exposure. Furan liver injury promotes a ductular reaction through Hippo and TGFB signalling, which combined with increased immune response results in ameliorating perturbed bile acid homeostasis in treated rats. Failure to activate these pathways in 3-MF exposed rats and decreased p53 activity leads to cholestasis, and increased toxicity. Finally, BMD analysis indicate many of the most sensitive pathways affected by furan and 3-MF exposure relate to metabolism - malate dehydrogenase and glucose metabolism with BMDLs of 0.03 and 0.01 mg/kg bw/day for furan and 3-MF exposure, respectively, which agrees with BMDLs previously reported for apical and microarray data.

YAP targetome reveals activation of SPEM in gastric pre-neoplastic progression and regeneration

Peptic ulcer disease caused by environmental factors increases the risk of developing gastric cancer (GC), one of the most common and deadly cancers in the world. However, the mechanisms underlying this association remain unclear. A major type of GC uniquely undergoes spasmolytic polypeptide-expressing metaplasia (SPEM) followed by intestinal metaplasia. Notably, intestinal-type GC patients with high levels of YAP signaling exhibit a lower survival rate and poor prognosis. YAP overexpression in gastric cells induces atrophy, metaplasia, and hyperproliferation, while its deletion in a Notch-activated gastric adenoma model suppresses them. By defining the YAP targetome genome-wide, we demonstrate that YAP binds to active chromatin elements of SPEM-related genes, which correlates with the activation of their expression in both metaplasia and ulcers. Single-cell analysis combined with our YAP signature reveals that YAP signaling is activated during SPEM, demonstrating YAP as a central regulator of SPEM in gastric neoplasia and regeneration.

Biallelic ANKS6 null variants cause notable extrarenal phenotypes in a nephronophthisis patient and lead to hepatobiliary abnormalities by YAP1 deficiency

The ankyrin repeat and sterile alpha motif domain containing 6 (ANKS6) gene, encoding an inversin compartment protein of the primary cilium, was recently reported as a pathogenic gene of nephronophthisis (MIM PS256100). Extrarenal manifestations are frequently observed in this disease, however, potential genotype-phenotype correlations and the underlying mechanisms remain poorly understood. Here we described an infant with kidney failure, hepatobiliary abnormalities, and heart disease, in whom whole exome sequencing identified compound heterozygous variants in ANKS6, including a novel nonsense variant p.Trp458* and a recurrent splicing variant c.2394+1G > A. mRNA expression studies showed that the splicing variant caused aberrant mRNA splicing with exon 13 skipping and the biallelic variants were predicted to cause loss of ANKS6 function. We systematically characterized the clinical and genetic spectra of the disease and revealed that biallelic null variants in ANKS6 cause more severe kidney disease and more extrarenal manifestations, thus establishing a clear genotype-phenotype correlation for the disease. Further evaluations showed that ANKS6 deficiency reduced YAP1 expression in the patient's bile duct epithelium and ANKS6 promotes YAP1 transcriptional activity in a dose-dependent manner, indicating that loss of ANKS6 function causes hepatobiliary abnormalities through YAP1 deficiency during biliary morphogenesis and development, which may offer new therapeutic targets.

The Hippo Pathway Effectors YAP/TAZ-TEAD Oncoproteins as Emerging Therapeutic Targets in the Tumor Microenvironment

Various cancer cell-associated intrinsic and extrinsic inputs act on YAP/TAZ proteins to mediate the hyperactivation of the TEAD transcription factor-based transcriptome. This YAP/TAZ-TEAD activity can override the growth-limiting Hippo tumor-suppressor pathway that maintains normal tissue homeostasis. Herein, we provide an integrated summary of the contrasting roles of YAP/TAZ during normal tissue homeostasis versus tumor initiation and progression. In addition to upstream factors that regulate YAP/TAZ in the TME, critical insights on the emerging functions of YAP/TAZ in immune suppression and abnormal vasculature development during tumorigenesis are illustrated. Lastly, we discuss the current methods that intervene with the YAP/TAZ-TEAD oncogenic signaling pathway and the emerging applications of combination therapies, gut microbiota, and epigenetic plasticity that could potentiate the efficacy of chemo/immunotherapy as improved cancer therapeutic strategies.

P4HA2 induces hepatic ductular reaction and biliary fibrosis in chronic cholestatic liver diseases

BACKGROUNDS

Prolyl-4-hydroxylases (P4Hs) are key enzymes in collagen synthesis. The P4HA subunit (P4HA1, P4HA2, and P4HA3) contains a substrate binding and catalyzation domain. We postulated that P4HA2 would play a key role in the cholangiocyte pathology of cholestatic liver diseases.

METHODS

We studied humans with primary biliary cholangitis (PBC) and Primary sclerosing cholangitis (PSC), P4HA2 -/- mice injured by DDC, and P4HA2 -/- /MDR2 -/- double knockout mice. A parallel study was performed in patients with PBC, PSC, and controls using immunohistochemistry and immunofluorescence. In the murine model, the level of ductular reaction and biliary fibrosis were monitored by histology, qPCR, immunohistochemistry, and Western blotting. Expression of Yes1 Associated Transcriptional Regulator (YAP) phosphorylation was measured in isolated mouse cholangiocytes. The mechanism of P4HA2 was explored in RBE and 293T cell lines by using qPCR, Western blot, immunofluorescence, and co-immunoprecipitation.

RESULTS

The hepatic expression level of P4HA2 was highly elevated in patients with PBC or PSC. Ductular reactive cholangiocytes predominantly expressed P4HA2. Cholestatic patients with more severe liver injury correlated with levels of P4HA2 in the liver. In P4HA2 -/- mice, there was a significantly reduced level of ductular reaction and fibrosis compared with controls in the DDC-induced chronic cholestasis. Decreased liver fibrosis and ductular reaction were observed in P4HA2 -/- /MDR2 -/- mice compared with MDR2 -/- mice. Cholangiocytes isolated from P4HA2 -/- /MDR2 -/- mice displayed a higher level of YAP phosphorylation, resulting in cholangiocytes proliferation inhibition. In vitro studies showed that P4HA2 promotes RBE cell proliferation by inducing SAV1 degradation, eventually resulting in the activation of YAP.

CONCLUSIONS

P4HA2 promotes hepatic ductular reaction and biliary fibrosis by regulating the SAV1-mediated Hippo signaling pathway. P4HA2 is a potential therapeutic target for PBC and PSC.

Downregulation of Yap1 during limb regeneration results in defective bone formation in axolotl

The Hippo pathway plays an imperative role in cellular processes such as differentiation, regeneration, cell migration, organ growth, apoptosis, and cell cycle. Transcription coregulator component of Hippo pathway, YAP1, promotes transcription of genes involved in cell proliferation, migration, differentiation, and suppressing apoptosis. However, its role in epimorphic regeneration has not been fully explored. The axolotl is a well-established model organism for developmental biology and regeneration studies. By exploiting its remarkable regenerative capacity, we investigated the role of Yap1 in the early blastema stage of limb regeneration. Depleting Yap1 using gene-specific morpholinos attenuated the competence of axolotl limb regeneration evident in bone formation defects. To explore the affected downstream pathways from Yap1 down-regulation, the gene expression profile was examined by employing LC-MS/MS technology. Based on the generated data, we provided a new layer of evidence on the putative roles of increased protease inhibition and immune system activities and altered ECM composition in diminished bone formation capacity during axolotl limb regeneration upon Yap1 deficiency. We believe that new insights into the roles of the Hippo pathway in complex structure regeneration were granted in this study.

An involvement of Hippo-yes-associated protein pathway in biliary epithelial senescence in primary biliary cholangitis

BACKGROUND & AIMS

Accumulating evidence suggest that Hippo-yes-associated protein (YAP) pathway plays important roles in development and repair after injuries in biliary system. We disclosed that senescent biliary epithelial cells (BECs) participate in the pathogenesis of primary biliary cholangitis (PBC). We hypothesized that dysregulation of Hippo-YAP pathway may be associated with biliary epithelial senescence in pathogenesis of PBC.

APPROACH & RESULTS

Cellular senescence was induced in cultured BECs by treatment with serum depletion or glycochenodeoxycholic acid. The expression and activity of YAP1 were significantly decreased in senescent BECs (p<0.01). Cellular senescence and apoptosis were significantly increased (p<0.01) and a proliferation activity and a 3D-cyst formation activity were significantly decreased (p<0.01) by a knockdown of YAP1 in BECs. The expression of YAP1 were immunohistochemically determined in livers taken from the patients with PBC (n = 79) and 79 control diseased and normal livers and its association with senescent markers p16^INK4a and p21^WAF1/Cip1 was analyzed. The nuclear expression of YAP1, which indicates activation of YAP1, was significantly decreased in BECs in small bile ducts involved in cholangitis and ductular reactions in PBC, compared to control livers (p<0.01). The decreased expression of YAP1 was seen in senescent BECs showing expression of p16^INK4a and p21^WAF1/Cip1 in bile duct lesions.

CONCLUSION

Dysregulation of Hippo-YAP1 pathway may be involved in the pathogenesis of PBC in association with biliary epithelial senescence.

YAP/TAZ as master regulators in cancer: modulation, function and therapeutic approaches

Our understanding of the function of the transcriptional regulators YAP and TAZ (YAP/TAZ) in cancer is advancing. In this Review, we provide an update on recent progress in YAP/TAZ biology, their regulation by Hippo signaling and mechanotransduction and highlight open questions. YAP/TAZ signaling is an addiction shared by multiple tumor types and their microenvironments, providing many malignant attributes. As such, it represents an important vulnerability that may offer a broad window of therapeutic efficacy, and here we give an overview of the current treatment strategies and pioneering clinical trials.

Role of the Hippo pathway in liver regeneration and repair: recent advances

Although the signaling pathways involved in normal liver regeneration have been well characterized, less has been done for livers affected by chronic tissue damage. These "abnormal livers" have an impaired regenerative response that leads to liver repair and fibrosis. The tumor suppressor Hippo pathway plays a key role in liver regeneration and repair. On this basis, this review discusses recent studies focusing on the involvement of the Hippo signaling pathway during "normal healthy liver regeneration" (i.e., in a normal liver after 2/3 partial hepatectomy) and "abnormal liver regeneration" (i.e., in a liver damaged by chronic disease). This could be an important question to address with respect to new therapies aimed at improving impaired liver regenerative responses. The studies reported here have shown that activation of the Hippo coactivators YAP/TAZ during normal liver regeneration promotes the formation of a new bile duct network through direct BEC proliferation or/and hepatocyte dedifferentiation to HPCs which can trans-differentiate to BECs. Moreover, YAP/TAZ signaling interaction with other signaling pathways mediates the recruitment and activation of Kupffer cells, which release mitogenic cytokines for parenchymal and/or non-parenchymal cells and engage in phagocytosis of cellular debris. In addition, YAP-mediated activation of stellate cells (HSCs) promotes liver regeneration through the synthesis of extracellular matrix. However, in chronically diseased livers, where the predetermined threshold for proper liver regeneration is exceeded, YAP/TAZ activation results in a reparative process characterized by liver fibrosis. In this condition, YAP/TAZ activation in parenchymal and non-parenchymal cells results in (i) differentiation of quiescent HSCs into myofibroblastic HSCs; (ii) recruitment of macrophages releasing inflammatory cytokines; (iii) polarization of macrophages toward the M2 phenotype. Since accumulation of damaged hepatocytes in chronic liver injury represent a significant risk factor for the development of hepatocarcinoma, this review also discussed the involvement of the Hippo pathway in the clearance of damaged cells.

Yap signalling regulates ductular reactions in mice with CRISPR/Cas9-induced glycogen storage disease type Ia

Glycogen storage disease type Ia (GSD-Ia) is caused by a deficiency in the glucose-6-phosphatase (G6Pase, G6pc) enzyme, which catalyses the final step of gluconeogenesis and glycogenolysis. Accumulation of G6pc can lead to an increase in glycogen and development of fatty liver. Ductular reactions refer to the proliferation of cholangiocytes and hepatic progenitors, which worsen fatty liver progress. To date, however, ductular reactions in GSD-Ia remain poorly understood. Here, we studied the development and potential underlying mechanism of ductular reactions in GSD-Ia in mice. We first generated GSD-Ia mice using CRISPR/Cas9 to target the exon 3 region of the G6pc gene. The typical GSD-Ia phenotype in G6pc ^-/- mice was then analysed using biochemical and histological assays. Ductular reactions in G6pc ^-/- mice were tested based on the expression of cholangiocytic markers cytokeratin 19 (CK19) and epithelial cell adhesion molecule (EpCAM). Yes-associated protein 1 (Yap) signalling activity was measured using western blot (WB) analysis and quantitative real-time polymerase chain reaction (qRT-PCR). Verteporfin was administered to the G6pc ^-/- mice to inhibit Yap signalling. The CRISPR/Cas9 system efficiently generated G6pc ^-/- mice, which exhibited typical GSD-Ia characteristics, including retarded growth, hypoglycaemia, and fatty liver disease. In addition, CK19- and EpCAM-positive cells as well as Yap signalling activity were increased in the livers of G6pc ^-/- mice. However, verteporfin treatment ameliorated ductular reactions and decreased Yap signalling activity. This study not only improves our understanding of GSD-Ia pathophysiology, but also highlights the potential of novel therapeutic approaches for GSD-Ia such as drug targeting of ductular reactions.

YAP-VGLL4 antagonism defines the major physiological function of the Hippo signaling effector YAP

The Hippo-YAP signaling pathway plays a critical role in development, homeostasis, regeneration, and tumorigenesis by converging on YAP, a coactivator for the TEAD family DNA-binding transcription factors, to regulate downstream transcription programs. Given its pivotal role as the nuclear effector of the Hippo pathway, YAP is indispensable in multiple developmental and tissue contexts. Here we report that the essentiality of YAP in liver and lung development can be genetically bypassed by simultaneous inactivation of the TEAD corepressor VGLL4. This striking antagonistic epistasis suggests that the major physiological function of YAP is to antagonize VGLL4. We further show that the YAP-VGLL4 antagonism plays a widespread role in regulating Hippo pathway output beyond normal development, as inactivation of Vgll4 dramatically enhanced intrahepatic cholangiocarcinoma formation in Nf2-deficient livers and ameliorated CCl4-induced damage in normal livers. Interestingly, Vgll4 expression is temporally regulated in development and regeneration and, in certain contexts, provides a better indication of overall Hippo pathway output than YAP phosphorylation. Together, these findings highlight the central importance of VGLL4-mediated transcriptional repression in Hippo pathway regulation and inform potential strategies to modulate Hippo signaling in cancer and regenerative medicine.

Loss of Mst1/2 activity promotes non-mitotic hair cell generation in the neonatal organ of Corti

Mammalian sensory hair cells (HCs) have limited capacity for regeneration, which leads to permanent hearing loss after HC death. Here, we used in vitro RNA-sequencing to show that the Hippo signaling pathway is involved in HC damage and self-repair processes. Turning off Hippo signaling through Mst1/2 inhibition or Yap overexpression induces YAP nuclear accumulation, especially in supporting cells, which induces supernumerary HC production and HC regeneration after injury. Mechanistically, these effects of Hippo signaling work synergistically with the Notch pathway. Importantly, the supernumerary HCs not only express HC markers, but also have cilia structures that are able to form neural connections to auditory regions in vivo. Taken together, regulating Hippo suggests new strategies for promoting cochlear supporting cell proliferation, HC regeneration, and reconnection with neurons in mammals.

[Changes of YAP activity at the early stage of nonalcoholic steatohepatitis and its spatiotemporal relationship with ductular reaction in mice]

OBJECTIVE

To explore the changes in Yes-associated protein (YAP) activity at the early stage of nonalcoholic steatohepatitis (NASH) and the spatiotemporal relationship between YAP and ductular reaction (DR).

METHODS

Male C57BL/6J mouse models of NASH were established by feeding with a methionine- and choline-deficient (MCD) diet or a thioacetamide (TAA) diet for 12 weeks. At different time points during the feeding, liver histology of the mice was observed with HE and Masson trichrome staining. The mRNA expressions of YAP and its target genes (Ctgf, Cyr61, Acta2) were determined by qPCR, and the total protein expression level of YAP was measured with immunoblotting. The expression and distribution of YAP and the markers of DR (K19 and Sox9) were observed with immunohistochemical staining.

RESULTS

At the early stage of NASH induced by MCD diet (1 to 4 weeks), the mRNA expression of YAP and its target genes and the total protein expression of YAP increased significantly (P < 0.01). The number of YAP-positive hepatocytes reached the peak level of 90.8 (cells per ×400 field of view) at week 2 and then decreased to 30.8 at week 4 (P < 0.001); YAP-positive ductular cells appeared near the portal area, where DR began to occur. From 8 to 12 weeks, numerous K19/Sox9-positive DR cells were observed in the hepatic lobules around the central vein (P < 0.01), while only a few YAP-positive hepatocytes were present in the liver tissue (P > 0.05), and the number of YAP-positive ductular cells gradually increased with time (P < 0.001). At the early stage of NASH induced by TAA diet (3 days to 2 weeks), the mRNA expression of YAP and its target genes and the total protein expression of YAP increased significantly (P < 0.05), and the number of YAP-positive hepatocytes reached the peak of 69.2 at week 2 and then decreased to 55.2 at week 4 (P < 0.001); YAP-positive ductular cells first appeared at the initial location of DR near the central vein. From 6 to 12 weeks, numerous K19/Sox9-positive DR cells occurred in the hepatic lobules around the central vein (P < 0.01). While the number of YAP-positive hepatocytes decreased (P < 0.001), the number of YAP-positive ductular cells continued to increase (P < 0.001).

CONCLUSION

During the development of NASH, YAP activation occurs earlier than DR but they are spatiotemporally correlated. YAP activation in hepatocytes may participate in DR by promoting hepatocyte dedifferentiation.

Biliary NIK promotes ductular reaction and liver injury and fibrosis in mice

Excessive cholangiocyte expansion (ductular reaction) promotes liver disease progression, but the underlying mechanism is poorly understood. Here we identify biliary NF-κB-inducing kinase (NIK) as a pivotal regulator of ductular reaction. NIK is known to activate the noncanonical IKKα/NF-κB2 pathway and regulate lymphoid tissue development. We find that cholangiocyte NIK is upregulated in mice with cholestasis induced by bile duct ligation (BDL), 5-diethoxycarbonyl-1,4-dihydrocollidine (DDC), or α-naphtyl-isothiocyanate (ANIT). DDC, ANIT, or BDL induces ductular reaction, liver injury, inflammation, and fibrosis in mice. Cholangiocyte-specific deletion of NIK, but not IKKα, blunts these pathological alterations. NIK inhibitor treatment similarly ameliorates DDC-induced ductular reaction, liver injury, and fibrosis. Biliary NIK directly increases cholangiocyte proliferation while suppressing cholangiocyte death, and it also promotes secretion of cholangiokines from cholangiocytes. Cholangiokines stimulate liver macrophages and hepatic stellate cells, augmenting liver inflammation and fibrosis. These results unveil a NIK/ductular reaction axis and a NIK/cholangiokine axis that promote liver disease progression.

Excessive expansion of cholangiocytes in the liver leads to ductular reaction and liver disease. Here, the authors show that genetic ablation, or pharmacological inhibition, of biliary NIK blocks ductular reaction, liver inflammation, and liver fibrosis in mice by modulating secretion of cholangiokines that mediate liver inflammation and fibrosis.

Cellular crosstalk during liver regeneration: unity in diversity

The liver is unique in its ability to regenerate from a wide range of injuries and diseases. Liver regeneration centers around hepatocyte proliferation and requires the coordinated actions of nonparenchymal cells, including biliary epithelial cells, liver sinusoidal endothelial cells, hepatic stellate cells and kupffer cells. Interactions among various hepatocyte and nonparenchymal cells populations constitute a sophisticated regulatory network that restores liver mass and function. In addition, there are two different ways of liver regeneration, self-replication of liver epithelial cells and transdifferentiation between liver epithelial cells. The interactions among cell populations and regenerative microenvironment in the two modes are distinct. Herein, we first review recent advances in the interactions between hepatocytes and surrounding cells and among nonparenchymal cells in the context of liver epithelial cell self-replication. Next, we discuss the crosstalk of several cell types in the context of liver epithelial transdifferentiation, which is also crucial for liver regeneration.

BMP4 and Gremlin 1 regulate hepatic cell senescence during clinical progression of NAFLD/NASH

The role of hepatic cell senescence in human non-alcoholic fatty liver disease (NAFLD) and non-alcoholic steatohepatitis (NASH) is not well understood. To examine this, we performed liver biopsies and extensive characterization of 58 individuals with or without NAFLD/NASH. Here, we show that hepatic cell senescence is strongly related to NAFLD/NASH severity, and machine learning analysis identified senescence markers, the BMP4 inhibitor Gremlin 1 in liver and visceral fat, and the amount of visceral adipose tissue as strong predictors. Studies in liver cell spheroids made from human stellate and hepatocyte cells show BMP4 to be anti-senescent, anti-steatotic, anti-inflammatory and anti-fibrotic, whereas Gremlin 1, which is particularly highly expressed in visceral fat in humans, is pro-senescent and antagonistic to BMP4. Both senescence and anti-senescence factors target the YAP/TAZ pathway, making this a likely regulator of senescence and its effects. We conclude that senescence is an important driver of human NAFLD/NASH and that BMP4 and Gremlin 1 are novel therapeutic targets.

Decoding YAP dependent transcription in the liver

The transcriptional coactivator YAP is emerging as a master regulator of cell growth. In the liver, YAP activity is linked to hepatomegaly, regeneration, dedifferentiation, and aggressive tumor growth. Here we present genomic studies to address how YAP may elicit such profound biological changes in murine models. YAP bound the genome in a TEAD-dependent manner, either at loci constitutively occupied by TEAD or by pioneering enhancers, which comprised a fraction of HNF4a/FOXA-bound embryonic enhancers active during embryonic development but silent in the adult. YAP triggered transcription on promoters by recruiting BRD4, enhancing H3K122 acetylation, and promoting RNApol2 loading and pause-release. YAP also repressed HNF4a target genes by binding to their promoters and enhancers, thus preventing RNApol2 pause-release. YAP activation led to the induction of hepatocyte proliferation, accompanied by tissue remodeling, characterized by polarized macrophages, exhausted T-lymphocytes and dedifferentiation of endothelial cells into proliferative progenitors. Overall, these analyses suggest that YAP is a master regulator of liver function that reshapes the enhancer landscape to control transcription of genes involved in metabolism, proliferation, and inflammation, subverts lineage specification programs by antagonizing HNF4a and modulating the immune infiltrate and the vascular architecture of the liver.

Emerging roles of the Hippo signaling pathway in modulating immune response and inflammation-driven tissue repair and remodeling

Inflammation is an evolutionarily conserved process and part of the body's defense mechanism. Inflammation leads to the activation of immune and non-immune cells that protect the host tissue/organs from injury or intruding pathogens. The Hippo pathway is an evolutionarily conserved kinase cascade with an established role in regulating cell proliferation, survival, and differentiation. It is involved in diverse biological processes, including organ size control and tissue homeostasis. Recent clinical and pre-clinical studies have shown that the Hippo signaling pathway is also associated with injury- and pathogen-induced tissue inflammation and associated immunopathology. In this review, we have summarized the recent findings related to the involvement of the Hippo signaling pathway in modulating the immune response in different acute and chronic inflammatory diseases and its impact on tissue repair and remodeling.

New Insights into Hippo/YAP Signaling in Fibrotic Diseases

Fibrosis results from defective wound healing processes often seen after chronic injury and/or inflammation in a range of organs. Progressive fibrotic events may lead to permanent organ damage/failure. The hallmark of fibrosis is the excessive accumulation of extracellular matrix (ECM), mostly produced by pathological myofibroblasts and myofibroblast-like cells. The Hippo signaling pathway is an evolutionarily conserved kinase cascade, which has been described well for its crucial role in cell proliferation, apoptosis, cell fate decisions, and stem cell self-renewal during development, homeostasis, and tissue regeneration. Recent investigations in clinical and pre-clinical models has shown that the Hippo signaling pathway is linked to the pathophysiology of fibrotic diseases in many organs including the lung, heart, liver, kidney, and skin. In this review, we have summarized recent evidences related to the contribution of the Hippo signaling pathway in the development of organ fibrosis. A better understanding of this pathway will guide us to dissect the pathophysiology of fibrotic disorders and develop effective tissue repair therapies.

Hippo signalling in the liver: role in development, regeneration and disease

The Hippo signalling pathway has emerged as a major player in many aspects of liver biology, such as development, cell fate determination, homeostatic function and regeneration from injury. The regulation of Hippo signalling is complex, with activation of the pathway by diverse upstream inputs including signals from cellular adhesion, mechanotransduction and crosstalk with other signalling pathways. Pathological activation of the downstream transcriptional co-activators yes-associated protein 1 (YAP) and transcriptional co-activator with PDZ-binding motif (TAZ, encoded by WWTR1), which are negatively regulated by Hippo signalling, has been implicated in multiple aspects of chronic liver disease, such as the development of liver fibrosis and tumorigenesis. Thus, development of pharmacological inhibitors of YAP-TAZ signalling has been an area of great interest. In this Review, we summarize the diverse roles of Hippo signalling in liver biology and highlight areas where outstanding questions remain to be investigated. Greater understanding of the mechanisms of Hippo signalling in liver function should help facilitate the development of novel therapies for the treatment of liver disease.

Liver regeneration and ethanol detoxification: A new link in YAP regulation of ALDH1A1 during alcohol-related hepatocyte damage

Yes-associated protein (YAP), a central effector in the Hippo pathway, is involved in the regulation of organ size, stem cell self-renewal, and tissue regeneration. In this study, we observed YAP activation in patients with alcoholic steatosis, hepatitis, and cirrhosis. Accumulation of this protein in the nucleus was also observed in murine livers that were damaged after chronic-plus-single binge or moderate ethanol ingestion combined with carbon tetrachloride intoxication (ethanol/CCl4 ). To understand the role of this transcriptional coactivator in alcohol-related liver injury, we knocked out the Yap1 gene in hepatocytes of floxed homozygotes through adeno-associated virus (AAV8)-mediated deletion utilizing Cre recombinase. Yap1 hepatocyte-specific knockouts (KO) exhibited hemorrhage, massive hepatic necrosis, enhanced oxidative stress, elevated hypoxia, and extensive infiltration of CD11b+ inflammatory cells into hepatic microenvironments rich for connective tissue growth factor (Ctgf) during ethanol/CCl4 -induced liver damage. Analysis of whole-genome transcriptomics indicated upregulation of genes involved in hypoxia and extracellular matrix (ECM) remodeling, whereas genes related to hepatocyte proliferation, progenitor cell activation, and ethanol detoxification were downregulated in the damaged livers of Yap1 KO. Acetaldehyde dehydrogenase (Aldh)1a1, a gene that encodes a detoxification enzyme for aldehyde substrates, was identified as a potential YAP target because this gene could be transcriptionally activated by a hyperactive YAP mutant. The ectopic expression of the human ALDH1A1 gene caused increase in hepatocyte proliferation and decrease in hepatic necrosis, oxidative stress, ECM remodeling, and inflammation during ethanol/CCl4 -induced liver damage. Taken together, these observations indicated that YAP was crucial for liver repair during alcohol-associated injury. Its regulation of ALDH1A1 represents a new link in liver regeneration and detoxification.

YAP1 activation and Hippo pathway signaling in the pathogenesis and treatment of intrahepatic cholangiocarcinoma

Intrahepatic cholangiocarcinoma (iCCA), the second most common primary liver cancer, is a highly lethal epithelial cell malignancy exhibiting features of cholangiocyte differentiation. iCCAs can potentially develop from multiple cell types of origin within liver, including immature or mature cholangiocytes, hepatic stem cells/progenitor cells, and from transdifferentiation of hepatocytes. Understanding the molecular mechanisms and genetic drivers that diversely drive specific cell lineage pathways leading to iCCA has important biological and clinical implications. In this context, activation of the YAP1-TEAD dependent transcription, driven by Hippo-dependent or -independent diverse mechanisms that lead to the stabilization of YAP1 is crucially important to biliary fate commitment in hepatobiliary cancer. In preclinical models, YAP1 activation in hepatocytes or cholangiocytes is sufficient to drive their malignant transformation into iCCA. Moreover, nuclear YAP1/TAZ is highly prevalent in human iCCA irrespective of the varied etiology, and significantly correlates with poor prognosis in iCCA patients. Based on the ubiquitous expression and diverse physiologic roles for YAP1/TAZ in the liver, recent studies have further revealed distinct functions of active YAP1/TAZ in regulating tumor metabolism, as well as the tumor immune microenvironment. In the current review, we discuss our current understanding of the various roles of the Hippo-YAP1 signaling in iCCA pathogenesis, with a specific focus on the roles played by the Hippo-YAP1 pathway in modulating biliary commitment and oncogenicity, iCCA metabolism, and immune microenvironment. We also discuss the therapeutic potential of targeting the YAP1/TAZ-TEAD transcriptional machinery in iCCA, its current limitations, and what future studies are needed to facilitate clinical translation.

ADAM Metalloproteinase Domain 17 Regulates Cholestasis-Associated Liver Injury and Sickness Behavior Development in Mice

Disintegrin and metalloproteinase domain-containing protein 17 (ADAM17) is a ubiquitously expressed membrane-bound enzyme that mediates shedding of a wide variety of important regulators in inflammation including cytokines and adhesion molecules. Hepatic expression of numerous cytokines and adhesion molecules are increased in cholestatic liver diseases including primary biliary cholangitis (PBC) and primary sclerosing cholangitis (PSC), however, the pathophysiological role of ADAM17 in regulating these conditions remains unknown. Therefore, we evaluated the role of ADAM17 in a mouse model of cholestatic liver injury due to bile duct ligation (BDL). We found that BDL enhanced hepatic ADAM17 protein expression, paralleled by increased ADAM17 bioactivity. Moreover, inhibition of ADAM17 bioactivity with the specific inhibitor DPC 333 significantly improved both biochemical and histological evidence of liver damage in BDL mice. Patients with cholestatic liver disease commonly experience adverse behavioral symptoms, termed sickness behaviors. Similarly, BDL in mice induces reproducible sickness behavior development, driven by the upregulated expression of cytokines and adhesion molecules that are in turn regulated by ADAM17 activity. Indeed, inhibition of ADAM17 activity significantly ameliorated BDL-associated sickness behavior development. In translational studies, we evaluated changes in ADAM17 protein expression in liver biopsies obtained from patients with PBC and PSC, compared to normal control livers. PSC and PBC patients demonstrated increased hepatic ADAM17 expression in hepatocytes, cholangiocytes and in association with liver-infiltrating immune cells compared to normal controls. In summary, cholestatic liver injury in mice and humans is associated with increased hepatic ADAM17 expression. Furthermore, inhibition of ADAM17 activity improves both cholestatic liver injury and associated sickness behavior development, suggesting that ADAM17 inhibition may represent a novel therapeutic approach for treating patients with PBC/PSC.

Role of YAP1 Signaling in Biliary Development, Repair, and Disease

Yes-associated protein 1 (YAP1) is a transcriptional coactivator that activates transcriptional enhanced associate domain transcription factors upon inactivation of the Hippo signaling pathway, to regulate biological processes like proliferation, survival, and differentiation. YAP1 is most prominently expressed in biliary epithelial cells (BECs) in normal adult livers and during development. In the current review, we will discuss the multiple roles of YAP1 in the development and morphogenesis of bile ducts inside and outside the liver, as well as in orchestrating the cholangiocyte repair response to biliary injury. We will review how biliary repair can occur through the process of hepatocyte-to-BEC transdifferentiation and how YAP1 is pertinent to this process. We will also discuss the liver's capacity for metabolic reprogramming as an adaptive mechanism in extreme cholestasis, such as when intrahepatic bile ducts are absent due to YAP1 loss from hepatic progenitors. Finally, we will discuss the roles of YAP1 in the context of pediatric pathologies afflicting bile ducts, such as Alagille syndrome and biliary atresia. In conclusion, we will comprehensively discuss the spatiotemporal roles of YAP1 in biliary development and repair after biliary injury while describing key interactions with other well-known developmental pathways.

Hippo-Yap/Taz signalling in zebrafish regeneration

The extent of tissue regeneration varies widely between species. Mammals have a limited regenerative capacity whilst lower vertebrates such as the zebrafish (Danio rerio), a freshwater teleost, can robustly regenerate a range of tissues, including the spinal cord, heart, and fin. The molecular and cellular basis of this altered response is one of intense investigation. In this review, we summarise the current understanding of the association between zebrafish regeneration and Hippo pathway function, a phosphorylation cascade that regulates cell proliferation, mechanotransduction, stem cell fate, and tumorigenesis, amongst others. We also compare this function to Hippo pathway activity in the regenerative response of other species. We find that the Hippo pathway effectors Yap/Taz facilitate zebrafish regeneration and that this appears to be latent in mammals, suggesting that therapeutically promoting precise and temporal YAP/TAZ signalling in humans may enhance regeneration and hence reduce morbidity.

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.

Novel Therapeutic Targets in Liver Fibrosis

Liver fibrosis is end-stage liver disease that can be rescued. If irritation continues due to viral infection, schistosomiasis and alcoholism, liver fibrosis can progress to liver cirrhosis and even cancer. The US Food and Drug Administration has not approved any drugs that act directly against liver fibrosis. The only treatments currently available are drugs that eliminate pathogenic factors, which show poor efficacy; and liver transplantation, which is expensive. This highlights the importance of clarifying the mechanism of liver fibrosis and searching for new treatments against it. This review summarizes how parenchymal, nonparenchymal cells, inflammatory cells and various processes (liver fibrosis, hepatic stellate cell activation, cell death and proliferation, deposition of extracellular matrix, cell metabolism, inflammation and epigenetics) contribute to liver fibrosis. We highlight discoveries of novel therapeutic targets, which may provide new insights into potential treatments for liver fibrosis.

Pantoprazole ameliorates liver fibrosis and suppresses hepatic stellate cell activation in bile duct ligation rats by promoting YAP degradation

Liver fibrosis is one of the most severe pathologic consequences of chronic liver diseases, and effective therapeutic strategies are urgently needed. Proton pump inhibitors (PPIs) are H⁺/K⁺-ATPase inhibitors and currently used to treat acid-related diseases such as gastric ulcers, which have shown other therapeutic effects in addition to inhibiting acid secretion. However, few studies have focused on PPIs from the perspective of inhibiting hepatic fibrosis. In the present study, we investigated the effects of pantoprazole (PPZ), a PPI, against liver fibrosis in a bile duct ligation (BDL) rat model, human hepatic stellate cell (HSC) line LX-2 and mouse primary HSCs (pHSCs), and explored the potential mechanisms underlying the effects of PPZ in vitro and in vivo. In BDL rats, administration of PPZ (150 mg· kg^-1· d^-1, i.p. for 14 d) significantly attenuated liver histopathological injury, collagen accumulation, and inflammatory responses, and suppressed fibrogenesis-associated gene expression including Col1a1, Acta2, Tgfβ1, and Mmp-2. In LX-2 cells and mouse pHSCs, PPZ (100-300 μM) dose-dependently suppressed the levels of fibrogenic markers. We conducted transcriptome analysis and subsequent validation in PPZ-treated LX-2 cells, and revealed that PPZ inhibited the expression of Yes-associated protein (YAP) and its downstream targets such as CTGF, ID1, survivin, CYR61, and GLI2. Using YAP overexpression and silencing, we demonstrated that PPZ downregulated hepatic fibrogenic gene expression via YAP. Furthermore, we showed that PPZ promoted the proteasome-dependent degradation and ubiquitination of YAP, thus inhibiting HSC activation. Additionally, we showed that PPZ destabilized YAP by disrupting the interaction between a deubiquitinating enzyme OTUB2 and YAP, and subsequently blocked the progression of hepatic fibrosis.

TNFAIP3 Interacting Protein 3 Is an Activator of Hippo-YAP Signaling Protecting Against Hepatic Ischemia/Reperfusion Injury

BACKGROUND AND AIMS

Hepatic ischemia/reperfusion (I/R) injury, a common clinical problem that occurs during liver surgical procedures, causes a large proportion of early graft failure and organ rejection cases. The identification of key regulators of hepatic I/R injury may provide potential strategies to clinically improve the prognosis of liver surgery. Here, we aimed to identify the role of tumor necrosis factor alpha-induced protein 3-interacting protein 3 (TNIP3) in hepatic I/R injury and further reveal its immanent mechanisms.

APPROACH AND RESULTS

In the present study, we found that hepatocyte TNIP3 was markedly up-regulated in livers of both persons and mice subjected to I/R surgery. Hepatocyte-specific Tnip3 overexpression effectively attenuated I/R-induced liver necrosis and inflammation, but improved cell proliferation in mice, whereas TNIP3 ablation largely aggravated liver injury. This inhibitory effect of TNIP3 on hepatic I/R injury was found to be dependent on significant activation of the Hippo-YAP signaling pathway. Mechanistically, TNIP3 was found to directly interact with large tumor suppressor 2 (LATS2) and promote neuronal precursor cell-expressed developmentally down-regulated 4-mediated LATS2 ubiquitination, leading to decreased Yes-associated protein (YAP) phosphorylation at serine 112 and the activated transcription of factors downstream of YAP. Notably, adeno-associated virus delivered TNIP3 expression in the liver substantially blocked I/R injury in mice.

CONCLUSIONS

TNIP3 is a regulator of hepatic I/R injury that alleviates cell death and inflammation by assisting ubiquitination and degradation of LATS2 and the resultant YAP activation.TNIP3 represents a promising therapeutic target for hepatic I/R injury to improve the prognosis of liver surgery.

Autophagy-mediated negative feedback attenuates the oncogenic activity of YAP in pancreatic cancer

Pancreatic ductal adenocarcinoma (PDAC) is the most lethal malignancy in humans, and new therapeutic targets are urgently needed. Yes-associated protein (YAP) plays a significant role in cancer progression. Autophagy is also closely associated with various human cancers. However, the interplay between YAP and autophagy in PDAC remains poorly understood. In this study, we found that YAP was upregulated and activated in PDAC. Further analysis revealed that there is a YAP-autophagy feedback loop in pancreatic cancer. Mechanistically, YAP activates autophagy by promoting Atg5 transcription via TEAD1-mediated binding, while autophagy negatively regulates YAP through autophagic degradation. The hyperactivation of YAP in PDAC unbalances the YAP-autophagy circuit and promotes cancer progression. Inhibition of autophagy enhances the oncogenic activity of YAP in PDAC. The autophagy activator rapamycin promotes the antitumor effect of verteporfin, a YAP inhibitor. Therefore, our study elucidated the interaction between YAP and autophagy in PDAC and our results suggest that targeting the YAP-autophagy circuit may be a new therapeutic strategy for pancreatic cancer.

The hippo pathway: A master regulator of liver metabolism, regeneration, and disease

The liver is the only visceral organ in the body with a tremendous capacity to regenerate in response to insults that induce inflammation, cell death, and injury. Liver regeneration is a complicated process involving a well-orchestrated activation of non-parenchymal cells in the injured area and proliferation of undamaged hepatocytes. Furthermore, the liver has a Hepatostat, defined as adjustment of its volume to that required for homeostasis. Understanding the mechanisms that control different steps of liver regeneration is critical to informing therapies for liver repair, to help patients with liver disease. The Hippo signaling pathway is well known for playing an essential role in the control and regulation of liver size, regeneration, stem cell self-renewal, and liver cancer. Thus, the Hippo pathway regulates dynamic cell fates in liver, and in absence of its downstream effectors YAP and TAZ, liver regeneration is severely impaired, and the proliferative expansion of liver cells blocked. We will mainly review upstream mechanisms activating the Hippo signaling pathway following partial hepatectomy in mouse model and patients, its roles during different steps of liver regeneration, metabolism, and cancer. We will also discuss how targeting the Hippo signaling cascade might improve liver regeneration and suppress liver tumorigenesis.

Yes-associated protein regulates the hepatoprotective effect of vitamin D receptor activation through promoting adaptive bile duct remodeling in cholestatic mice

Mounting clinical evidence has revealed that the vitamin D receptor (VDR) is associated with cholestatic liver injury, although the functions of VDR in this condition remain largely unexplored. Here, we investigated the effects of VDR activation on bile duct ligation (BDL) mice, and the underlying mechanisms were further investigated. A low-calcemic VDR agonist, paricalcitol (PAL, 200 ng/kg), was intraperitoneally injected into BDL mice every other day for 5 days or 28 days. Liver histology, liver function indicators, cholangiocyte proliferation, fibrosis scores, and inflammation were evaluated. Mice treated with PAL were rescued from the decreased survival rate induced by BDL and liver damage was reduced. Mechanistically, PAL promoted cholangiocyte proliferation, which was likely conducive to proliferating bile duct maturation and increased branching of bile ducts. PAL treatment also increased the expression of Yes-associated protein (YAP) and its target protein epithelial cell adhesion molecule (EpCam) and decreased the level of inactive cytoplasmic phosphorylated YAP. YAP knockdown abrogated PAL-induced primary bile duct epithelial cell proliferation, confirmed with YAP inhibitor administration. In addition, BDL-induced liver fibrosis and inflammatory cell infiltration were reduced by PAL treatment at both day 5 and day 28 post-BDL. In conclusion, VDR activation mitigates cholestatic liver injury by promoting adaptive bile duct remodeling through cholangiocytic YAP upregulation. Because PAL is an approved clinical drug, it may be useful for treatment of cholestatic liver disease. © 2021 The Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.

Mst1/2 Kinases Inhibitor, XMU-MP-1, Attenuates Angiotensin II-Induced Ascending Aortic Expansion in Hypercholesterolemic Mice

Background: Ascending and abdominal aortic aneurysms (AAs) are asymptomatic, permanent dilations of the aorta with surgical intervention as the currently available therapy. Hippo-Yap signaling cascade plays a critical role in stem cell self-renewal, tissue regeneration and organ size control. By using XMU-MP-1, a pharmacological inhibitor of the key component of Hippo-Yap signaling, MST1/2, we examined the functional contribution of Hippo-Yap in the development of AAs in Angiotensin II (AngII)-infused hypercholesterolemic mice.

Methods and Results: MST, p-MST, p-YAP, p-MOB and TAZ proteins in AngII-infused ascending and abdominal aortas were assessed by immunohistochemical and western blot analyses. To examine the effect of MST1/2 inhibition on AAs, western diet-fed low density lipoprotein (LDL) receptor −/− mice infused with AngII were administered with either vehicle or XMU-MP-1 for 5 weeks. Hippo-YAP signaling proteins were significantly elevated in AngII infused ascending and abdominal aortas. XMU-MP-1 administration resulted in the attenuation of AngII-induced ascending AAs without influencing abdominal AAs and aortic atherosclerosis. Inhibition of Hippo-YAP signaling also resulted in the suppression of AngII-induced matrix metalloproteinase 2 (MMP2) activity, macrophage accumulation, aortic medial hypertrophy and elastin breaks in the ascending aorta.

Conclusions: The present study demonstrates a pivotal role for the Hippo-YAP signaling pathway in AngII-induced ascending AA development.

Regeneration Defects in Yap and Taz Mutant Mouse Livers Are Caused by Bile Duct Disruption and Cholestasis

BACKGROUND AND AIMS

The Hippo pathway and its downstream effectors YAP and TAZ (YAP/TAZ) are heralded as important regulators of organ growth and regeneration. However, different studies provided contradictory conclusions about their role during regeneration of different organs, ranging from promoting proliferation to inhibiting it. Here we resolve the function of YAP/TAZ during regeneration of the liver, where Hippo's role in growth control has been studied most intensely.

METHODS

We evaluated liver regeneration after carbon tetrachloride toxic liver injury in mice with conditional deletion of Yap/Taz in hepatocytes and/or biliary epithelial cells, and measured the behavior of different cell types during regeneration by histology, RNA sequencing, and flow cytometry.

RESULTS

We found that YAP/TAZ were activated in hepatocytes in response to carbon tetrachloride toxic injury. However, their targeted deletion in adult hepatocytes did not noticeably impair liver regeneration. In contrast, Yap/Taz deletion in adult bile ducts caused severe defects and delay in liver regeneration. Mechanistically, we showed that Yap/Taz mutant bile ducts degenerated, causing cholestasis, which stalled the recruitment of phagocytic macrophages and the removal of cellular corpses from injury sites. Elevated bile acids activated pregnane X receptor, which was sufficient to recapitulate the phenotype observed in mutant mice.

CONCLUSIONS

Our data show that YAP/TAZ are practically dispensable in hepatocytes for liver development and regeneration. Rather, YAP/TAZ play an indirect role in liver regeneration by preserving bile duct integrity and securing immune cell recruitment and function.

Shh-Yap signaling controls hepatic ductular reactions in CCl4 -induced liver injury

Carbon tetrachloride (CCl₄ ) exposure can induce hepatic ductular reactions. To date, however, the related mechanism remains largely unknown. Sonic hedgehog (Shh) and Yes-associated protein (Yap) signaling are correlated with liver injury and regeneration. Herein, we investigated the role of Shh and Yap signaling in the fate of ductular reaction cells in CCl₄ -treated livers and the possible mechanisms. Wild-type and Shh-EGFP-Cre male mice were exposed to CCl₄ (2 mL/kg), and then treated with or without the Shh signaling inhibitor Gant61. The level of liver injury, proliferation of ductular reaction cells, and expression levels of mRNA and protein related to the Shh and Yap signaling components were assessed. Results showed that CCl₄ treatment induced liver injury and promoted activation and proliferation of ductular reaction cells. In addition, CCl₄ induced the expression of Shh ligands in hepatocytes, accompanied by activation of Shh and Yap1 signaling in the liver. Furthermore, administration of Gant61 ameliorated liver regeneration, inhibited hepatic ductular reactions, and decreased Shh and Yap1 signaling activity. Thus, Shh-Yap1 signaling appears to play an integral role in the proliferation of ductular reaction cells in CCl₄ -induced liver injury. This study should improve our understanding of the mechanism of CCl₄ -induced liver injury and ductular reactions and provide support for future investigations on liver disease therapy.