The Hippo pathway effector TAZ induces TEAD-dependent liver inflammation and tumors

Role of Hippo pathway dysregulation from gastrointestinal premalignant lesions to cancer

BACKGROUND

First identified in Drosophila melanogaster, the Hippo pathway is considered a major regulatory cascade controlling tissue homeostasis and organ development. Hippo signaling components include kinases whose activity regulates YAP and TAZ final effectors. In response to upstream stimuli, YAP and TAZ control transcriptional programs involved in cell proliferation, cytoskeletal reorganization and stemness.

MAIN TEXT

While fine tuning of Hippo cascade components is essential for maintaining the balance between proliferative and non-proliferative signals, pathway signaling is frequently dysregulated in gastrointestinal cancers. Also, YAP/TAZ aberrant activation has been described in conditions characterized by chronic inflammation that precede cancer development, suggesting a role of Hippo effectors in triggering carcinogenesis. In this review, we summarize the architecture of the Hippo pathway and discuss the involvement of signaling cascade unbalances in premalignant lesions of the gastrointestinal tract, providing a focus on the underlying molecular mechanisms.

CONCLUSIONS

The biology of premalignant Hippo signaling dysregulation needs further investigation in order to elucidate the evolutionary trajectories triggering cancer inititation and develop effective early therapeutic strategies targeting the Hippo/YAP pathway.

Molecular mechanisms in MASLD/MASH-related HCC

Liver cancer is the third leading cause of cancer-related deaths and ranks as the sixth most prevalent cancer type globally. NAFLD or metabolic dysfunction-associated steatotic liver disease, and its more severe manifestation, NASH or metabolic dysfunction-associated steatohepatitis (MASH), pose a significant global health concern, affecting approximately 20%-25% of the population. The increased prevalence of metabolic dysfunction-associated steatotic liver disease and MASH is parallel to the increasing rates of obesity-associated metabolic diseases, including type 2 diabetes, insulin resistance, and fatty liver diseases. MASH can progress to MASH-related HCC (MASH-HCC) in about 2% of cases each year, influenced by various factors such as genetic mutations, carcinogen exposure, immune microenvironment, and microbiome. MASH-HCC exhibits distinct molecular and immune characteristics compared to other causes of HCC and affects both men and women equally. The management of early to intermediate-stage MASH-HCC typically involves surgery and locoregional therapies, while advanced HCC is treated with systemic therapies, including anti-angiogenic therapies and immune checkpoint inhibitors. In this comprehensive review, we consolidate previous research findings while also providing the most current insights into the intricate molecular processes underlying MASH-HCC development. We delve into MASH-HCC-associated genetic variations and somatic mutations, disease progression and research models, multiomics analysis, immunological and microenvironmental impacts, and discuss targeted/combined therapies to overcome immune evasion and the biomarkers to recognize treatment responders. By furthering our comprehension of the molecular mechanisms underlying MASH-HCC, our goal is to catalyze the advancement of more potent treatment strategies, ultimately leading to enhanced patient outcomes.

Molecules in the hippo pathway that regulate Th17 differentiation reveal the severity of ankylosing spondylitis

AIM

Ankylosing spondylitis (AS) is a chronic, progressive, and inflammatory autoimmune disease of unknown origin that affects the axial skeleton and sacroiliac joints, resulting in pain and loss of function. AS is characterized by the overdifferentiation of T helper 17 (Th17) cells, which contribute to the development of the disease. The Hippo signaling pathway is an important regulator of Th17 differentiation, but its role in patients with AS is unclear. We aimed to investigate the role of key molecules of the Hippo signaling pathway in inflammatory Th17 differentiation in patients with AS and to examine their correlation with disease stages.

METHODS

We examined the activity of the Hippo pathway in patients with AS and the regulation of Th17 differentiation during AS-mediated inflammation. Blood samples were collected from 60 patients with AS at various stages and 30 healthy controls. Peripheral blood mononuclear cells (PBMCs) were isolated from peripheral blood by density gradient centrifugation. The Serum Interleukin-17 (IL-17) levels in patients with AS and healthy controls were quantified by ELISA. The key molecules of Hippo pathway were assessed by real-time PCR for their mRNA expression, and protein levels were determined by Western blot analysis.

RESULTS

Elevated serum interleukin-17 (IL-17) levels were observed in patients with AS compared with healthy controls. The protein and mRNA levels of retinoic acid receptor-related orphan receptor γt (RORγt), transcriptional coactivator with a PDZ-binding motif (TAZ), and key upstream transcription factors in the Hippo signaling pathway were measured. The expression of RORγt and TAZ was increased in the blood of patients with AS, whereas the expression of other Hippo pathway proteins, such as MST1/2 and NDR1/2, was significantly decreased. Increased levels of IL-17 and TAZ were significantly associated with disease activity. In addition, MST1, MST2, and NDR1 levels were negatively correlated with TAZ, RORγt, and IL-17 levels.

CONCLUSION

Our findings suggest that the Hippo pathway plays a significant role in the regulation of Th17 differentiation and disease activity in patients with AS. The upregulation of TAZ and downregulation of key Hippo pathway proteins, such as MST1/2 and NDR1/2, may contribute to AS pathogenesis. These proteins may serve as biomarkers and may lead to the development of novel therapeutic strategies for AS.

Control of stem cell renewal and fate by YAP and TAZ

Complex physiological processes control whether stem cells self-renew, differentiate or remain quiescent. Two decades of research have placed the Hippo pathway, a highly conserved kinase signalling cascade, and its downstream molecular effectors YAP and TAZ at the nexus of this decision. YAP and TAZ translate complex biological cues acting on stem cells - from mechanical forces to cellular metabolism - into genome-wide effects to mediate stem cell functions. While aberrant YAP/TAZ activity drives stem cell dysfunction in ageing, tumorigenesis and disease, therapeutic targeting of Hippo signalling and YAP/TAZ can boost stem cell activity to enhance regeneration. In this Review, we discuss how YAP/TAZ control the self-renewal, fate and plasticity of stem cells in different contexts, how dysregulation of YAP/TAZ in stem cells leads to disease, and how therapeutic modalities targeting YAP/TAZ may benefit regenerative medicine and cancer therapy.

TAZ facilitates breast tumor growth by promoting an immune-suppressive tumor microenvironment

The core Hippo pathway module consists of a tumour-suppressive kinase cascade that inhibits the transcriptional coactivators Yes-associated protein (YAP) and WW domain-containing transcription regulator protein 1 (WWTR1; also known as TAZ). When the Hippo pathway is downregulated, as often occurs in breast cancer, YAP/TAZ activity is induced. To elaborate the roles of TAZ in triple-negative breast cancer (TNBC), we depleted Taz in murine TNBC 4T1 cells, using either CRISPR/Cas9 or small hairpin RNA (shRNA). TAZ-depleted cells and their controls, harbouring wild-type levels of TAZ, were orthotopically injected into the mammary fat pads of syngeneic BALB/c female mice, and mice were monitored for tumour growth. TAZ depletion resulted in smaller tumours compared to the tumours generated by control cells, in line with the notion that TAZ functions as an oncogene in breast cancer. Tumours, as well as their corresponding in vitro cultured cells, were then subjected to gene expression profiling by RNA sequencing (RNA-seq). Interestingly, pathway analysis of the RNA-seq data indicated a TAZ-dependent enrichment of 'Inflammatory Response', a pathway correlated with TAZ expression levels also in human breast cancer tumours. Specifically, the RNA-seq analysis predicted a significant depletion of regulatory T cells (Tregs) in TAZ-deficient tumours, which was experimentally validated by the staining of tumour sections and by quantitative cytometry by time of flight (CyTOF). Strikingly, the differences in tumour size were completely abolished in immune-deficient mice, demonstrating that the immune-modulatory capacity of TAZ is critical for its oncogenic activity in this setting. Cytokine array analysis of conditioned medium from cultured cells revealed that TAZ increased the abundance of a small group of cytokines, including plasminogen activator inhibitor 1 (Serpin E1; also known as PAI-1), CCN family member 4 (CCN4; also known as WISP-1) and interleukin-23 (IL-23), suggesting a potential mechanistic explanation for its in vivo immunomodulatory effect. Together, our results imply that TAZ functions in a non-cell-autonomous manner to modify the tumour immune microenvironment and dampen the anti-tumour immune response, thereby facilitating tumour growth.

TGF-β/SMAD canonical pathway induces the expression of transcriptional cofactor TAZ in liver cancer cells

The TGF-β and Hippo pathways are critical for liver size control, regeneration, and cancer progression. The transcriptional cofactor TAZ, also named WWTR1, is a downstream effector of Hippo pathway and plays a key role in the maintenance of liver physiological functions. However, the up-regulation of TAZ expression has been associated with liver cancer progression. Recent evidence shows crosstalk of TGF-β and Hippo pathways, since TGF-β modulates TAZ expression through different mechanisms in a cellular context-dependent manner but supposedly independent of SMADs. Here, we evaluate the molecular interplay between TGF-β pathway and TAZ expression and observe that TGF-β induces TAZ expression through SMAD canonical pathway in liver cancer HepG2 cells. Therefore, TAZ cofactor is a primary target of TGF-β/SMAD-signaling, one of the pathways altered in liver cancer.

Anti-apoptotic effect of adrenomedullin gene delivery on Leydig cells by suppressing TGF-β1 via the Hippo signaling pathway

This study aims to establish whether adrenomedullin (ADM) is capable to restore the steroidogenic functions of Leydig cells by suppressing transforming growth factor-β1 (TGF-β1) through Hippo signaling. Primary Leydig cells were treated with lipopolysaccharide (LPS), an adeno-associated virus vector that expressed ADM (Ad-ADM) or sh-RNA of TGF-β1 (Ad-sh-TGF-β1). The cell viability and medium concentrations of testosterone were detected. Gene expression and protein levels were determined for steroidogenic enzymes, TGF-β1, RhoA, YAP, TAZ and TEAD1. The role of Ad-ADM in the regulation of TGF-β1 promoter was confirmed by ChIP and Co-IP. Similar to Ad-sh-TGF-β1, Ad-ADM mitigated the decline in the number of Leydig cells and plasma concentrations of testosterone by restoring the gene and protein levels of SF-1, LRH1, NUR77, StAR, P450scc, 3β-HSD, CYP17 and 17β-HSD. Similar to Ad-sh-TGF-β1, Ad-ADM not only inhibited the LPS-induced cytotoxicity and cell apoptosis but also restored the gene and protein levels of SF-1, LRH1, NUR77, StAR, P450scc, 3β-HSD, CYP17 and 17β-HSD, along with the medium concentrations of testosterone in LPS-induced Leydig cells. Like Ad-sh-TGF-β1, Ad-ADM improved LPS-induced TGF-β1 expression. In addition, Ad-ADM suppressed RhoA activation, enhanced the phosphorylation of YAP and TAZ, reduced the expression of TEAD1 which interacted with HDAC5 and then bound to TGF-β1 gene promoter in LPS-exposed Leydig cells. It is thus suspected that ADM can exert anti-apoptotic effect to restore the steroidogenic functions of Leydig cells by suppressing TGF-β1 through Hippo signaling.

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.

Inhibition of YAP1 activity ameliorates acute lung injury through promotion of M2 macrophage polarization

The balance of M1/M2 macrophage polarization plays an important role in regulating inflammation during acute lung injury (ALI). Yes-associated protein (YAP1) is a key protein in the Hippo-YAP1 signaling pathway and is involved in macrophage polarization. We aimed to determine the role of YAP1 in pulmonary inflammation following ALI and regulation of M1/M2 polarization. Pulmonary inflammation and injury with upregulation of YAP1 were observed in lipopolysaccharide (LPS)-induced ALI. The YAP1 inhibitor, verteporfin, attenuated pulmonary inflammation and improved lung function in ALI mice. Moreover, verteporfin promoted M2 polarization and inhibited M1 polarization in the lung tissues of ALI mice and LPS-treated bone marrow-derived macrophages (BMMs). Additionally, siRNA knockdown confirmed that silencing Yap1 decreased chemokine ligand 2 (CCL2) expression and promoted M2 polarization, whereas silencing large tumor suppressor 1 (Lats1) increased CCL2 expression and induced M1 polarization in LPS-treated BMMs. To investigate the role of inflammatory macrophages in ALI mice, we performed single-cell RNA sequencing of macrophages isolated from the lungs. Thus, verteporfin could activate the immune-inflammatory response, promote the potential of M2 macrophages, and alleviate LPS-induced ALI. Our results reveal a novel mechanism where YAP1-mediated M2 polarization alleviates ALI. Therefore, inhibition of YAP1 may be a target for the treatment of ALI.

An allosteric pan-TEAD inhibitor blocks oncogenic YAP/TAZ signaling and overcomes KRAS G12C inhibitor resistance

The Hippo pathway is a key growth control pathway that is conserved across species. The downstream effectors of the Hippo pathway, YAP (Yes-associated protein) and TAZ (transcriptional coactivator with PDZ-binding motif), are frequently activated in cancers to drive proliferation and survival. Based on the premise that sustained interactions between YAP/TAZ and TEADs (transcriptional enhanced associate domain) are central to their transcriptional activities, we discovered a potent small-molecule inhibitor (SMI), GNE-7883, that allosterically blocks the interactions between YAP/TAZ and all human TEAD paralogs through binding to the TEAD lipid pocket. GNE-7883 effectively reduces chromatin accessibility specifically at TEAD motifs, suppresses cell proliferation in a variety of cell line models and achieves strong antitumor efficacy in vivo. Furthermore, we uncovered that GNE-7883 effectively overcomes both intrinsic and acquired resistance to KRAS (Kirsten rat sarcoma viral oncogene homolog) G12C inhibitors in diverse preclinical models through the inhibition of YAP/TAZ activation. Taken together, this work demonstrates the activities of TEAD SMIs in YAP/TAZ-dependent cancers and highlights their potential broad applications in precision oncology and therapy resistance.

Mechanosensation to inflammation: Roles for YAP/TAZ in innate immune cells

Innate immune cells are responsible for eliminating foreign infectious agents and cellular debris, and their ability to perceive, respond to, and integrate biochemical and mechanical cues from their microenvironment eventually determines their behavior. In response to tissue injury, pathogen invasion, or a biomaterial implant, immune cells activate many pathways to initiate inflammation in the tissue. In addition to common inflammatory pathways, studies have demonstrated the role of the mechanosensitive proteins and transcriptional coactivators YAP and TAZ (YAP/TAZ) in inflammation and immunity. We review our knowledge of YAP/TAZ in controlling inflammation and immunity in innate immune cells. Furthermore, we discuss the roles of YAP/TAZ in inflammatory diseases, wound healing, and tissue regeneration and how they integrate mechanical cues with biochemical signaling during disease progression. Last, we comment on possible approaches that can be exploited to harness the therapeutic potential of YAP/TAZ in inflammatory diseases.

ACSL4 promotes microglia-mediated neuroinflammation by regulating lipid metabolism and VGLL4 expression

Acyl-CoA synthetase long-chain family member 4 (ACSL4) is an important isozyme in polyunsaturated fatty acid (PUFA) metabolism. The role of ACSL4 in the lipopolysaccharide (LPS)-induced inflammation of microglia, and the effects of ACSL4-mediated inflammation on the progression of Parkinson's disease (PD) are unknown. In this study, we found that ACSL4 expression was increased after LPS stimulation. Knocking down ACSL4 in microglia decreased proinflammatory cytokine production. Mechanistically, ACSL4 reduced vestigial-like family member 4(VGLL4) expression to promote NF-κB signal transduction; and ACSL4 regulated lipid composition after LPS stimulation. In addition, knocking down ACSL4 alleviated neuroinflammation in a systemic LPS model and acute l-methyl-4-phenyl-l,2,3,6-tetrahydropyridine (MPTP) model. These data revealed ACSL4 to be a novel regulator that promotes microglia-mediated neuroinflammation by regulating VGLL4 expression and lipid metabolism.

Hepatocyte-specific miR-33 deletion attenuates NAFLD-NASH-HCC progression

The complexity of the multiple mechanisms underlying non-alcoholic fatty liver disease (NAFLD) progression remains a significant challenge for the development of effective therapeutics. miRNAs have shown great promise as regulators of biological processes and as therapeutic targets for complex diseases. Here, we study the role of hepatic miR-33, an important regulator of lipid metabolism, during the progression of NAFLD. We report that miR-33 is overexpressed in hepatocytes isolated from mice with NAFLD and demonstrate that its specific suppression in hepatocytes (miR-33 HKO ) improves multiple aspects of the disease, including insulin resistance, steatosis, and inflammation and limits the progression to non-alcoholic steatohepatitis (NASH), fibrosis and hepatocellular carcinoma (HCC). Mechanistically, we find that hepatic miR-33 deficiency reduces lipid biosynthesis and promotes mitochondrial fatty acid oxidation to reduce lipid burden in hepatocytes. Additionally, miR-33 deficiency improves mitochondrial function, reducing oxidative stress. In miR-33 deficient hepatocytes, we found an increase in AMPKα activation, which regulates several pathways resulting in the attenuation of liver disease. The reduction in lipid accumulation and liver injury resulted in decreased transcriptional activity of the YAP/TAZ pathway, which may be involved in the reduced progression to HCC in the HKO livers. Together, these results suggest suppressing hepatic miR-33 may be an effective therapeutic approach at different stages of NAFLD/NASH/HCC disease progression.

Recent insight into the role and therapeutic potential of YAP/TAZ in gastrointestinal cancers

With the rapid development of cancer treatment, gastrointestinal (GI) cancers are still the most prevalent malignancies with high morbidity and mortality worldwide. Dysregulation of the Hippo signaling pathway has been recognized to play a critical role during cancer development and adopted for monitoring disease progression and therapy response. Despite the well-documented tumor proliferation and metastasis, recent efforts in two core Hippo components, Yes-associated protein (YAP) and transcriptional coactivator with PDZ-binding motif (TAZ), have identified as the driving forces behind cancer metabolism, stemness, tumor immunity, and therapy resistance. Understanding the molecular mechanisms by which YAP/TAZ facilitates the tumorigenesis and progression of GI cancer, and identifying novel therapeutic strategies for targeting YAP/TAZ are crucial to GI cancer treatment and prevention. In this study, we summarize the latest findings on the function and regulatory mechanisms of YAP/TAZ in GI cancers, and highlight the translational significance of targeting YAP/TAZ for cancer therapies.

Roles of Yes-associated protein and transcriptional coactivator with PDZ-binding motif in non-neoplastic liver diseases

The prevalence of liver disease has been increasing worldwide. Moreover, the burden of end-stage liver disease, including cirrhosis and liver cancer, is high because of high mortality and suboptimal treatment. The pathological process of liver disease includes steatosis, hepatocyte death, and fibrosis, which ultimately lead to cirrhosis and liver cancer. Clinical and preclinical evidence indicates that non-neoplastic liver diseases, particularly cirrhosis, are major risk factors for liver cancer, although the mechanism underlying this association remains unclear. Yes-associated protein (YAP) and transcriptional coactivator with PDZ-binding motif (TAZ) are transcriptional activators that regulate organ size and cancer development. YAP and TAZ play important roles in liver development, regeneration, and homeostasis. Abnormal YAP and TAZ levels have also been implicated in non-neoplastic liver diseases (e.g., non-alcoholic fatty liver disease, alcoholic liver disease, liver injury, and liver fibrosis). Here, we review recent findings on the roles of YAP and TAZ in non-neoplastic liver diseases and discuss directions for future research. This review provides a basis for the study of non-neoplastic liver diseases.

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.

Loss of Yap/Taz in cardiac fibroblasts attenuates adverse remodelling and improves cardiac function

AIMS

Fibrosis is associated with all forms of adult cardiac diseases including myocardial infarction (MI). In response to MI, the heart undergoes ventricular remodelling that leads to fibrotic scar due to excessive deposition of extracellular matrix mostly produced by myofibroblasts. The structural and mechanical properties of the fibrotic scar are critical determinants of heart function. Yes-associated protein (Yap) and transcriptional coactivator with PDZ-binding motif (Taz) are the key effectors of the Hippo signalling pathway and are crucial for cardiomyocyte proliferation during cardiac development and regeneration. However, their role in cardiac fibroblasts, regulating post-MI fibrotic and fibroinflammatory response, is not well established.

METHODS AND RESULTS

Using mouse model, we demonstrate that Yap/Taz are activated in cardiac fibroblasts after MI and fibroblasts-specific deletion of Yap/Taz using Col1a2Cre(ER)T mice reduces post-MI fibrotic and fibroinflammatory response and improves cardiac function. Consistently, Yap overexpression elevated post-MI fibrotic response. Gene expression profiling shows significant downregulation of several cytokines involved in post-MI cardiac remodelling. Furthermore, Yap/Taz directly regulate the promoter activity of pro-fibrotic cytokine interleukin-33 (IL33) in cardiac fibroblasts. Blocking of IL33 receptor ST2 using the neutralizing antibody abrogates the Yap-induced pro-fibrotic response in cardiac fibroblasts. We demonstrate that the altered fibroinflammatory programme not only affects the nature of cardiac fibroblasts but also the polarization as well as infiltration of macrophages in the infarcted hearts. Furthermore, we demonstrate that Yap/Taz act downstream of both Wnt and TGFβ signalling pathways in regulating cardiac fibroblasts activation and fibroinflammatory response.

CONCLUSION

We demonstrate that Yap/Taz play an important role in controlling MI-induced cardiac fibrosis by modulating fibroblasts proliferation, transdifferentiation into myofibroblasts, and fibroinflammatory programme.

Target Therapy for Hepatocellular Carcinoma: Beyond Receptor Tyrosine Kinase Inhibitors and Immune Checkpoint Inhibitors

Simple Summary

Hepatocellular carcinoma (HCC) is the most common type of primary liver cancer and its incidence is steadily increasing. The development of HCC is a complex, multi-step process that is accompanied by alterations in multiple signaling cascades. Recent years have seen advancement in understanding molecular signaling pathways that play central roles in hepatocarcinogenesis. Aberrant activation of YAP/TAZ, Hedgehog, or Wnt/β-catenin signaling is frequently found in a subset of HCC patients. Targeting the signaling pathway via small molecule inhibitors could be a promising therapeutic option for the subset of patients. In this review, we will introduce the signaling pathways, discuss their roles in the development of HCC, and propose a therapeutic approach targeting the signaling pathways in the context of HCC.

Abstract

Hepatocellular carcinoma (HCC) is a major health concern worldwide, and its incidence is increasing steadily. To date, receptor tyrosine kinases (RTKs) are the most favored molecular targets for the treatment of HCC, followed by immune checkpoint regulators such as PD-1, PD-L1, and CTLA-4. With less than desirable clinical outcomes from RTK inhibitors as well as immune checkpoint inhibitors (ICI) so far, novel molecular target therapies have been proposed for HCC. In this review, we will introduce diverse molecular signaling pathways that are aberrantly activated in HCC, focusing on YAP/TAZ, Hedgehog, and Wnt/β-catenin signaling pathways, and discuss potential therapeutic strategies targeting the signaling pathways in HCC.

TAZ-induced Cybb contributes to liver tumor formation in non-alcoholic steatohepatitis

BACKGROUND & AIMS

Non-alcoholic steatohepatitis (NASH) is a leading cause of hepatocellular carcinoma (HCC), but mechanisms linking NASH to eventual tumor formation remain poorly understood. Herein, we investigate the role of TAZ/WWTR1, which is induced in hepatocytes in NASH, in the progression of NASH to HCC.

METHODS

The roles of hepatocyte TAZ and its downstream targets were investigated in diet-induced and genetic models of NASH-HCC using gene-targeting, adeno-associated virus 8 (AAV8)-H1-mediated gene silencing, or AAV8-TBG-mediated gene expression. The biochemical signature of the newly elucidated pathway was probed in liver specimens from humans with NASH-HCC.

RESULTS

When hepatocyte-TAZ was silenced in mice with pre-tumor NASH using AAV8-H1-shTaz (short-hairpin Taz), subsequent HCC tumor development was suppressed. In this setting, the tumor-suppressing effect of shTaz was not dependent of TAZ silencing in the tumors themselves and could be dissociated from the NASH-suppressing effects of shTaz. The mechanism linking pre-tumor hepatocyte-TAZ to eventual tumor formation involved TAZ-mediated induction of the NOX2-encoding gene Cybb, which led to NADPH-mediated oxidative DNA damage. As evidence, DNA damage and tumor formation could be suppressed by treatment of pre-tumor NASH mice with AAV8-H1-shCybb; AAV8-TBG-OGG1, encoding the oxidative DNA-repair enzyme 8-oxoguanine glycosylase; or AAV8-TBG-NHEJ1, encoding the dsDNA repair enzyme non-homologous end-joining factor 1. In surrounding non-tumor tissue from human NASH-HCC livers, there were strong correlations between TAZ, NOX2, and oxidative DNA damage.

CONCLUSIONS

TAZ in pre-tumor NASH-hepatocytes, via induction of Cybb and NOX2-mediated DNA damage, contributes to subsequent HCC tumor development. These findings illustrate how NASH provides a unique window into the early molecular events that can lead to tumor formation and suggest that NASH therapies targeting TAZ might also prevent NASH-HCC.

LAY SUMMARY

Non-alcoholic steatohepatitis (NASH) is emerging as the leading cause of a type of liver cancer called hepatocellular carcinoma (HCC), but molecular events in pre-tumor NASH hepatocytes leading to HCC remain largely unknown. Our study shows that a protein called TAZ in pre-tumor NASH-hepatocytes promotes damage to the DNA of hepatocytes and thereby contributes to eventual HCC. This study reveals a very early event in HCC that is induced in pre-tumor NASH, and the findings suggest that NASH therapies targeting TAZ might also prevent NASH-HCC.

SIRPγ-expressing cancer stem-like cells promote immune escape of lung cancer via Hippo signaling

Cancer stem-like cells (CSLCs) acquire enhanced immune checkpoint responses to evade immune cell killing and promote tumor progression. Here we showed that signal regulatory protein γ (SIRPγ) determined CSLC properties and immune evasiveness in a small population of lung adenocarcinoma (LUAD) cancer cells. A SIRPγ^hi population displayed CSLC properties and transmitted the immune escape signal through sustaining CD47 expression in both SIRPγ^hi and SIRPγ^lo/– tumor cells. SIRPγ bridged MST1 and PP2A to facilitate MST1 dephosphorylation, resulting in Hippo/YAP activation and leading to cytokine release by CSLCs, which stimulated CD47 expression in LUAD cells and consequently inhibited tumor cell phagocytosis. SIRPγ promoted tumor growth and metastasis in vivo through YAP signaling. Notably, SIRPγ targeting with genetic SIRPγ knockdown or a SIRPγ-neutralizing antibody inhibited CSLC phenotypes and elicited phagocytosis that suppressed tumor growth in vivo. SIRPG was upregulated in human LUAD and its overexpression predicted poor survival outcome. Thus, SIRPγ^hi cells serve as CSLCs and tumor immune checkpoint–initiating cells, propagating the immune escape signal to the entire cancer cell population. Our study identifies Hippo/YAP signaling as the first mechanism by which SIRPγ is engaged and reveals that targeting SIRPγ represents an immune- and CSLC-targeting strategy for lung cancer therapy.

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.

Engineering Modular 3D Liver Culture Microenvironments In Vitro to Parse the Interplay between Biophysical and Biochemical Microenvironment Cues on Hepatic Phenotypes

In vitro models of human liver functions are used across a diverse range of applications in preclinical drug development and disease modeling, with particular increasing interest in models that capture facets of liver inflammatory status. This study investigates how the interplay between biophysical and biochemical microenvironment cues influence phenotypic responses, including inflammation signatures, of primary human hepatocytes (PHH) cultured in a commercially available perfused bioreactor. A 3D printing-based alginate microwell system was designed to form thousands of hepatic spheroids in a scalable manner as a comparator 3D culture modality to the bioreactor. Soft, synthetic extracellular matrix (ECM) hydrogel scaffolds with biophysical properties mimicking features of liver were engineered to replace polystyrene scaffolds, and the biochemical microenvironment was modulated with a defined set of growth factors and signaling modulators. The supplemented media significantly increased tissue density, albumin secretion, and CYP3A4 activity but also upregulated inflammatory markers. Basal inflammatory markers were lower for cells maintained in ECM hydrogel scaffolds or spheroid formats than polystyrene scaffolds, while hydrogel scaffolds exhibited the most sensitive response to inflammation as assessed by multiplexed cytokine and RNA-seq analyses. Together, these engineered 3D liver microenvironments provide insights for probing human liver functions and inflammatory response in vitro.

TAZ is indispensable for c-MYC-induced hepatocarcinogenesis

BACKGROUND & AIMS

Mounting evidence implicates the Hippo downstream effectors Yes-associated protein (YAP) and transcriptional co-activator with PDZ-binding motif (TAZ) in hepatocellular carcinoma (HCC). We investigated the functional contribution of YAP and/or TAZ to c-MYC-induced liver tumor development.

METHODS

The requirement for YAP and/or TAZ in c-Myc-driven hepatocarcinogenesis was analyzed using conditional Yap, Taz, and Yap;Taz knockout (KO) mice. An hepatocyte-specific inducible TTR-CreER^T2 KO system was applied to evaluate the role of YAP and TAZ during tumor progression. Expression patterns of YAP, TAZ, c-MYC, and BCL2L12 were analyzed in human HCC samples.

RESULTS

We found that the Hippo cascade is inactivated in c-Myc-induced mouse HCC. Intriguingly, TAZ mRNA levels and activation status correlated with c-MYC activity in human and mouse HCC, but YAP mRNA levels did not. We demonstrated that TAZ is a direct transcriptional target of c-MYC. In c-Myc induced murine HCCs, ablation of Taz, but not Yap, completely prevented tumor development. Mechanistically, TAZ was required to avoid c-Myc-induced hepatocyte apoptosis during tumor initiation. The anti-apoptotic BCL2L12 gene was identified as a novel target regulated specifically by YAP/TAZ, whose silencing strongly suppressed c-Myc-driven murine hepatocarcinogenesis. In c-Myc murine HCC lesions, conditional knockout of Taz, but not Yap, led to tumor regression, supporting the requirement of TAZ for c-Myc-driven HCC progression.

CONCLUSIONS

TAZ is a pivotal player at the crossroad between the c-MYC and Hippo pathways in HCC. Targeting TAZ might be beneficial for the treatment of patients with HCC and c-MYC activation.

LAY SUMMARY

The identification of novel treatment targets and approaches for patients with hepatocellular carcinoma is crucial to improve survival outcomes. We identified TAZ as a transcriptional target of c-MYC which plays a critical role in c-MYC-dependent hepatocarcinogenesis. TAZ could potentially be targeted for the treatment of patients with c-MYC-driven hepatocellular carcinoma.

The YAP/TAZ Signaling Pathway in the Tumor Microenvironment and Carcinogenesis: Current Knowledge and Therapeutic Promises

The yes-associated protein (YAP) and the transcriptional coactivator with PDZ-binding motif (TAZ) are transcriptional coactivators, members of the Hippo signaling pathway, which play a critical role in cell growth regulation, embryonic development, regeneration, proliferation, and cancer origin and progression. The mechanism involves the nuclear binding of the un-phosphorylated YAP/TAZ complex to release the transcriptional enhanced associate domain (TEAD) from its repressors. The active ternary complex is responsible for the aforementioned biological effects. Overexpression of YAP/TAZ has been reported in cancer stem cells and tumor resistance. The resistance involves chemotherapy, targeted therapy, and immunotherapy. This review provides an overview of YAP/TAZ pathways’ role in carcinogenesis and tumor microenvironment. Potential therapeutic alternatives are also discussed.

Genome-wide identification of microRNA targets reveals positive regulation of the Hippo pathway by miR-122 during liver development

Liver development is a highly complex process that is regulated by the orchestrated interplay of epigenetic regulators, transcription factors, and microRNAs (miRNAs). Owing to the lack of global in vivo targets of all miRNAs during liver development, the mechanisms underlying the dynamic control of hepatocyte differentiation by miRNAs remain elusive. Here, using Argonaute (Ago) high-throughput sequencing of RNA isolated by crosslinking immunoprecipitation (HITS-CLIP) in the mouse liver at different developmental stages, we characterized massive Ago-binding RNAs and obtained a genome-wide map of liver miRNA-mRNA interactions. The dynamic changes of five clusters of miRNAs and their potential targets were identified to be differentially involved at specific stages, a dozen of high abundant miRNAs and their epigenetic regulation by super-enhancer were found during liver development. Remarkably, miR-122, a liver-specific and most abundant miRNA in newborn and adult livers, was found by its targetome and pathway reporter analyses to regulate the Hippo pathway, which is crucial for liver size control and homeostasis. Mechanistically, we further demonstrated that miR-122 negatively regulates the outcomes of the Hippo pathway transcription factor TEAD by directly targeting a number of hippo pathway regulators, including the coactivator TAZ and a key factor of the phosphatase complex PPP1CC, which contributes to the dephosphorylation of YAP, another coactivator downstream of the Hippo pathway. This study identifies for the first time the genome-wide miRNA targetomes during mouse liver development and demonstrates a novel mechanism of terminal differentiation of hepatocytes regulated by the miR-122/Hippo pathway in a coordinated manner. As the Hippo pathway plays important roles in cell proliferation and liver pathological processes like inflammation, fibrosis, and hepatocellular carcinoma (HCC), our study could also provide a new insight into the function of miR-122 in liver pathology.

Mechano-Signaling Aspects of Hepatocellular Carcinoma

HCC is one of the leading causes of cancer related death worldwide and comprises about 90% of the cases of primary liver cancer. It is generally accompanied by chronic liver fibrosis characterised by deposition of collagen fibres, which, in turn, causes enhanced stiffness of the liver tissue. Changes of tissue stiffness give rise to alterations of signalling pathways that are associated to mechanical properties of the cells and the extracellular matrix, and that can be subsumed as "mechano-signaling pathways", like, e.g., the YAP/TAZ pathway, or the SRF pathway. Stiffness of the liver tissue modulates mechanical regulation of many genes involved in HCC progression. However, mechano-signaling is still rather underrepresented in our concepts of cancer in comparison to "classical" biochemical signalling pathways. This review aims to give an overview of various stiffness induced mechano-biological aspects of HCC.

25(OH)-Vitamin D alleviates neonatal infectious pneumonia via regulating TGFβ-mediated nuclear translocation mechanism of YAP/TAZ

Neonatal infectious pneumonia (NIP) is a common infectious disease that develops in the neonatal period. The purpose of our study was to explore the potential roles of 25(OH)-Vitamin D (25-OH-VD) and its anti-inflammatory mechanism in NIP. The results showed that serum 25-OH-VD level was negatively correlated with the severity of NIP, whereas Spearman’s correlation analysis showed a significant positive correlation between the severity of NIP and the levels of pneumonia markers procalcitonin (PCT) and interleukin-6 (IL-6). The expression of vitamin D receptor (VDR) was down-regulated, while the transforming growth factor β (TGFβ), nuclear YAP, and TAZ were up-regulated in the peripheral blood mononuclear cells (PBMCs) of neonates with severe pneumonia. Neonates with 25-OH-VD deficiency were associated with an increased risk of NIP. In BEAS-2B cells, down-regulation of nuclear YAP and TAZ was found in the lipopolysaccharide (LPS) + VD group relative to the LPS-induced group. Additionally, positive rate of nuclear YAP, as detected by immunocytochemistry (ICC), and the nuclear translocation of nuclear YAP/TAZ by IFA in the LPS+VD group showed an intermediate level between that of the control and LPS-induced groups. Furthermore, the expressions of VDR and CYP27B1 were significantly increased in the LPS+VD group as compared to those in the LPS-induced group. The anti-inflammatory mechanism in NIP was achieved due to the 25-OH-VD mediating TGFβ/YAP/TAZ pathway, which suggested that using 25-OH-VD might be a potential strategy for NIP treatment.

Hippo-YAP/MCP-1 mediated tubular maladaptive repair promote inflammation in renal failed recovery after ischemic AKI

Acute kidney injury (AKI) is associated with significant morbidity and its chronic inflammation contributes to subsequent chronic kidney disease (CKD) development. Yes-associated protein (YAP), the major transcriptional coactivator of the Hippo pathway, has been shown associated with chronic inflammation, but its role and mechanism in AKI-CKD transition remain unclear. Here we aimed to investigate the role of YAP in AKI-induced chronic inflammation. Renal ischemia/reperfusion (I/R) was used to induce a mouse model of AKI-CKD transition. We used verteporfin (VP), a pharmacological inhibitor of YAP, to treat post-IRI mice for a period, and evaluated the influence of YAP inhibition on long-term outcomes of AKI. In our results, severe IRI led to maladaptive tubular repair, macrophages infiltration, and progressive fibrosis. Following AKI, the Hippo pathway was found significantly altered with YAP persistent activation. Besides, tubular YAP activation was associated with the maladaptive repair, also correlated with interstitial macrophage infiltration. Monocyte chemoattractant protein 1 (MCP-1) was found notably upregulated with YAP activation. Of note, pharmacological inhibition of YAP in vivo attenuated renal inflammation, including macrophage infiltration and MCP-1 overexpression. Consistently, in vitro oxygen-glucose deprivation and reoxygenation (OGD/R) induced YAP activation and MCP-1 overproduction whereas these could be inhibited by VP. In addition, we modulated YAP activity by RNA interference, which further confirmed YAP activation enhances MCP-1 expression. Together, we concluded tubular YAP activation with maladaptive repair exacerbates renal inflammation probably via promoting MCP-1 production, which contributes to AKI-CKD transition.

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.

TEAD family transcription factors in development and disease

The balance between stem cell potency and lineage specification entails the integration of both extrinsic and intrinsic cues, which ultimately influence gene expression through the activity of transcription factors. One example of this is provided by the Hippo signalling pathway, which plays a central role in regulating organ size during development. Hippo pathway activity is mediated by the transcriptional co-factors Yes-associated protein (YAP) and transcriptional co-activator with PDZ-binding motif (TAZ), which interact with TEA domain (TEAD) proteins to regulate gene expression. Although the roles of YAP and TAZ have been intensively studied, the roles played by TEAD proteins are less well understood. Recent studies have begun to address this, revealing that TEADs regulate the balance between progenitor self-renewal and differentiation throughout various stages of development. Furthermore, it is becoming apparent that TEAD proteins interact with other co-factors that influence stem cell biology. This Primer provides an overview of the role of TEAD proteins during development, focusing on their role in Hippo signalling as well as within other developmental, homeostatic and disease contexts.

AAV-mediated YAP expression in cardiac fibroblasts promotes inflammation and increases fibrosis

Fibrosis is a hallmark of heart disease independent of etiology and is thought to contribute to impaired cardiac dysfunction and development of heart failure. However, the underlying mechanisms that regulate the differentiation of fibroblasts to myofibroblasts and fibrotic responses remain incompletely defined. As a result, effective treatments to mitigate excessive fibrosis are lacking. We recently demonstrated that the Hippo pathway effector Yes-associated protein (YAP) is an important mediator of myofibroblast differentiation and fibrosis in the infarcted heart. Yet, whether YAP activation in cardiac fibroblasts is sufficient to drive fibrosis, and how fibroblast YAP affects myocardial inflammation, a significant component of adverse cardiac remodeling, are largely unknown. In this study, we leveraged adeno-associated virus (AAV) to target cardiac fibroblasts and demonstrate that chronic YAP expression upregulated indices of fibrosis and inflammation in the absence of additional stress. YAP occupied the Ccl2 gene and promoted Ccl2 expression, which was associated with increased macrophage infiltration, pro-inflammatory cytokine expression, collagen deposition, and cardiac dysfunction in mice with cardiac fibroblast-targeted YAP overexpression. These results are consistent with other recent reports and extend our understanding of YAP function in modulating fibrotic and inflammatory responses in the heart.

Small Molecule Dysregulation of TEAD Lipidation Induces a Dominant-Negative Inhibition of Hippo Pathway Signaling

The transcriptional enhanced associate domain (TEAD) family of transcription factors serves as the receptors for the downstream effectors of the Hippo pathway, YAP and TAZ, to upregulate the expression of multiple genes involved in cellular proliferation and survival. Recent work identified TEAD S-palmitoylation as critical for protein stability and activity as the lipid tail extends into a hydrophobic core of the protein. Here, we report the identification and characterization of a potent small molecule that binds the TEAD lipid pocket (LP) and disrupts TEAD S-palmitoylation. Using a variety of biochemical, structural, and cellular methods, we uncover that TEAD S-palmitoylation functions as a TEAD homeostatic protein level checkpoint and that dysregulation of this lipidation affects TEAD transcriptional activity in a dominant-negative manner. Furthermore, we demonstrate that targeting the TEAD LP is a promising therapeutic strategy for modulating the Hippo pathway, showing tumor stasis in a mouse xenograft model.

Maladaptive regeneration - the reawakening of developmental pathways in NASH and fibrosis

With the rapid expansion of the obesity epidemic, nonalcoholic fatty liver disease is now the most common chronic liver disease, with almost 25% global prevalence. Nonalcoholic fatty liver disease ranges in severity from simple steatosis, a benign 'pre-disease' state, to the liver injury and inflammation that characterize nonalcoholic steatohepatitis (NASH), which in turn predisposes individuals to liver fibrosis. Fibrosis is the major determinant of clinical outcomes in patients with NASH and is associated with increased risks of cirrhosis and hepatocellular carcinoma. NASH has no approved therapies, and liver fibrosis shows poor response to existing pharmacotherapy, in part due to an incomplete understanding of the underlying pathophysiology. Patient and mouse data have shown that NASH is associated with the activation of developmental pathways: Notch, Hedgehog and Hippo-YAP-TAZ. Although these evolutionarily conserved fundamental signals are known to determine liver morphogenesis during development, new data have shown a coordinated and causal role for these pathways in the liver injury response, which becomes maladaptive during obesity-associated chronic liver disease. In this Review, we discuss the aetiology of this reactivation of developmental pathways and review the cell-autonomous and cell-non-autonomous mechanisms by which developmental pathways influence disease progression. Finally, we discuss the potential prognostic and therapeutic implications of these data for NASH and liver fibrosis.

The role of the Hippo pathway in the pathogenesis of inflammatory bowel disease

Inflammatory bowel disease (IBD) is a chronic and recurrent inflammatory disorder that primarily comprises Crohn's disease (CD) and ulcerative colitis (UC). Owing to its increasing prevalence in Eastern countries and the intractable challenges faced during IBD treatment, extensive research on IBD has been carried out over the last few years. Although the precise aetiology of IBD is undefined, the currently accepted hypothesis for IBD pathogenesis considers it to be a combination of environment, genetic predisposition, gut microbiota, and abnormal immunity. A recently emerged signalling pathway, the Hippo pathway, acts as a key regulator of cell growth, tissue homoeostasis, organ size, and has been implicated in several human cancers. In the past few years, studies have revealed the importance of the Hippo pathway in gastrointestinal tract physiology and gastrointestinal diseases, such as colorectal cancer and IBD. However, the role of the Hippo pathway and its exact impact in IBD remains to be elucidated. This review summarises the latest scientific literature on the involvement of this pathway in IBD from the following perspectives that account for the IBD pathogenesis: intestinal epithelial cell regeneration, immune regulation, gut microbiota, and angiogenesis. A comprehensive understanding of the specific role of the Hippo pathway in IBD will provide novel insights into future research directions and clinical implications of the Hippo pathway.

YAP/TAZ deficiency reprograms macrophage phenotype and improves infarct healing and cardiac function after myocardial infarction

Adverse cardiac remodeling after myocardial infarction (MI) causes structural and functional changes in the heart leading to heart failure. The initial post-MI pro-inflammatory response followed by reparative or anti-inflammatory response is essential for minimizing the myocardial damage, healing, and scar formation. Bone marrow–derived macrophages (BMDMs) are recruited to the injured myocardium and are essential for cardiac repair as they can adopt both pro-inflammatory or reparative phenotypes to modulate inflammatory and reparative responses, respectively. Yes-associated protein (YAP) and transcriptional coactivator with PDZ-binding motif (TAZ) are the key mediators of the Hippo signaling pathway and are essential for cardiac regeneration and repair. However, their functions in macrophage polarization and post-MI inflammation, remodeling, and healing are not well established. Here, we demonstrate that expression of YAP and TAZ is increased in macrophages undergoing pro-inflammatory or reparative phenotype changes. Genetic deletion of YAP/TAZ leads to impaired pro-inflammatory and enhanced reparative response. Consistently, YAP activation enhanced pro-inflammatory and impaired reparative response. We show that YAP/TAZ promote pro-inflammatory response by increasing interleukin 6 (IL6) expression and impede reparative response by decreasing Arginase-I (Arg1) expression through interaction with the histone deacetylase 3 (HDAC3)-nuclear receptor corepressor 1 (NCoR1) repressor complex. These changes in macrophages polarization due to YAP/TAZ deletion results in reduced fibrosis, hypertrophy, and increased angiogenesis, leading to improved cardiac function after MI. Also, YAP activation augmented MI-induced cardiac fibrosis and remodeling. In summary, we identify YAP/TAZ as important regulators of macrophage-mediated pro-inflammatory or reparative responses post-MI.

Adverse cardiac remodeling after myocardial infarction causes structural and functional changes in the heart, leading to heart failure. This study shows that the Hippo pathway influences post-injury cardiac inflammation by modulating macrophage polarization.

The Hippo Pathway in Liver Homeostasis and Pathophysiology

Studies of the regenerative capacity of the liver have converged on the Hippo pathway, a serine/threonine kinase cascade discovered in Drosophila and conserved from unicellular organisms to mammals. Genetic studies of mouse and rat livers have revealed that the Hippo pathway is a key regulator of liver size, regeneration, development, metabolism, and homeostasis and that perturbations in the Hippo pathway can lead to the development of common liver diseases, such as fatty liver disease and liver cancer. In turn, pharmacological targeting of the Hippo pathway may be utilized to boost regeneration and to prevent the development and progression of liver diseases. We review current insights provided by the Hippo pathway into liver pathophysiology. Furthermore, we present a path forward for future studies to understand how newly identified components of the Hippo pathway may control liver physiology and how the Hippo pathway is regulated in the liver.

Distinct and Overlapping Roles of Hippo Effectors YAP and TAZ During Human and Mouse Hepatocarcinogenesis

BACKGROUND & AIMS

Yes-associated protein (YAP) and its paralog transcriptional co-activator with post synaptic density protein, drosophila disc large tumor suppressor and zonula occludens-1-binding motif (TAZ) are 2 co-activators downstream of Hippo tumor-suppressor cascade. Both have been implicated in the development of hepatocellular carcinoma (HCC). However, whether YAP and TAZ have distinct or overlapping functions during hepatocarcinogenesis remains unknown.

METHODS

Expression patterns of YAP and TAZ were analyzed in human HCC samples. The requirement of Yap and/or Taz in protein kinase B (Akt)/ neuroblastoma RAS viral oncogene homolog (NRas) -driven liver tumorigenesis was analyzed using conditional Yap, Taz, and Yap;Taz knockout mice. Transcriptional programs regulated by YAP and/or TAZ were identified via RNA sequencing.

RESULTS

We found that in human HCC samples, an almost ubiquitous activation of YAP or TAZ occurs, underlying their role in this tumor type. Intriguingly, 70% of HCC samples showed only nuclear YAP or TAZ immunoreactivity. In the Akt/NRas liver tumor model, where nuclear Yap and Taz can be detected readily, deletion of Yap or Taz alone only mildly delayed liver tumor development, whereas their concomitant ablation strongly inhibited tumor cell proliferation and significantly suppressed Akt/NRas-driven hepatocarcinogenesis. In HCC cell lines, silencing of either YAP or TAZ led to decreased expression of both overlapping and distinct sets of genes, with the most prominent gene signatures related to cell-cycle progression and DNA replication.

CONCLUSIONS

YAP and TAZ have overlapping and distinct roles in hepatocarcinogenesis. HCCs may display unique activation of YAP or TAZ, thus relying on either YAP or TAZ for their growth.

Machine-Learning and Chemicogenomics Approach Defines and Predicts Cross-Talk of Hippo and MAPK Pathways

Hippo pathway dysregulation occurs in multiple cancers through genetic and nongenetic alterations, resulting in translocation of YAP to the nucleus and activation of the TEAD family of transcription factors. Unlike other oncogenic pathways such as RAS, defining tumors that are Hippo pathway-dependent is far more complex due to the lack of hotspot genetic alterations. Here, we developed a machine-learning framework to identify a robust, cancer type-agnostic gene expression signature to quantitate Hippo pathway activity and cross-talk as well as predict YAP/TEAD dependency across cancers. Further, through chemical genetic interaction screens and multiomics analyses, we discover a direct interaction between MAPK signaling and TEAD stability such that knockdown of YAP combined with MEK inhibition results in robust inhibition of tumor cell growth in Hippo dysregulated tumors. This multifaceted approach underscores how computational models combined with experimental studies can inform precision medicine approaches including predictive diagnostics and combination strategies. SIGNIFICANCE: An integrated chemicogenomics strategy was developed to identify a lineage-independent signature for the Hippo pathway in cancers. Evaluating transcriptional profiles using a machine-learning method led to identification of a relationship between YAP/TAZ dependency and MAPK pathway activity. The results help to nominate potential combination therapies with Hippo pathway inhibition.This article is highlighted in the In This Issue feature, p. 521.

Inhibition of Yes-Associated Protein-1 (YAP1) Enhances the Response of Invasive Breast Cancer Cells to the Standard Therapy

Purpose

The deregulation of the Hippo pathway results in translocation ofYes-associated protein-1 (YAP1) to the nucleus to exert an oncogenic effect. This effect has been demonstrated in several malignancies, yet, in breast cancer (BC), it remains controversial. The present study aimed to investigate the significance of YAP1 expression in BC, its relation to cancer stem cells (CSCs), and the effect of its inhibition on tumor cell survival.

Patients and Methods

We evaluated the expression of YAP1 protein and gene using immunohistochemistry (IHC) and RT-qPCR in FFPE tissue from normal and breast cancer cases. We also studied its association with CSC expression (OCT4, NANOG, and SOX2) and with different clinicopathologic characteristics. Two BC cell lines (MCF7 and MDA-MB-231) were exposed to different concentrations of YAP1 inhibitor “verteporfin” and cell viability was subsequently assessed.

Results

YAP1 mRNA was higher in BC compared to the normal breast tissue (p-value=0.040) and was higher in luminal tumors compared to triple-negative breast cancer (TNBC) (p-value= 0.017). Its expression in tumors was significantly associated with the expression of pluripotency markers (OCT4 and NANOG) (p-value= 0.030 and 0.035, respectively) and its inhibition resulted in a significant reduction of CSC expression in both MCF-7 and MDA-MB-231 cells. YAP1 nuclear expression by IHC, which signifies its activation, was more evident in invasive carcinomas compared to normal breast tissue and in-situ foci where the expression was limited to the cytoplasm. The pretreatment of BC cells (MCF7 and MDA-MB-231) with YAP1 inhibitor “verteporfin” resulted in their sensitization to the effect of tamoxifen and doxorubicin, respectively, and significantly decreased tumor cell proliferation and survival.

Conclusion

Our results imply that YAP1 is highly expressed and activated in BC and its inhibition could represent a possible novel therapeutic strategy that should be further explored and investigated to improve the outcome of breast cancer patients.

Hepatic Hippo signaling inhibits development of hepatocellular carcinoma

Primary liver cancer is one of the most common cancer worldwide. Hepatocellular carcinoma (HCC) in particular, is the second leading cause of cancer deaths in the world. The Hippo signaling pathway has emerged as a major oncosuppressive pathway that plays critical roles inhibiting hepatocyte proliferation, survival, and HCC formation. A key component of the Hippo pathway is the inhibition of yes-associated protein (YAP)/transcriptional co-activator with PDZ-binding motif (TAZ) transcription factors by the Hippo kinase cascade. Aberrant activation of YAP or TAZ has been found in several human cancers including HCC. It is also well established that YAP/TAZ activation in hepatocytes causes HCC in mouse models, indicating that YAP/TAZ are potential therapeutic targets for human liver cancer. In this review, we summarize the recent findings regarding the multifarious roles of Hippo/YAP/TAZ in HCC development, and focus on their cell autonomous roles in controlling hepatocyte proliferation, differentiation, survival and metabolism as well as their non-cell autonomous in shaping the tumor microenvironment.

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) Disrupts Control of Cell Proliferation and Apoptosis in a Human Model of Adult Liver Progenitors

The aryl hydrocarbon receptor (AhR) activation has been shown to alter proliferation, apoptosis, or differentiation of adult rat liver progenitors. Here, we investigated the impact of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-mediated AhR activation on a human model of bipotent liver progenitors, undifferentiated HepaRG cells. We used both intact undifferentiated HepaRG cells, and the cells with silenced Hippo pathway effectors, yes-associated protein 1 (YAP) and transcriptional coactivator with PDZ-binding motif (TAZ), which play key role(s) in tissue-specific progenitor cell self-renewal and expansion, such as in liver, cardiac, or respiratory progenitors. TCDD induced cell proliferation in confluent undifferentiated HepaRG cells; however, following YAP, and, in particular, double YAP/TAZ knockdown, TCDD promoted induction of apoptosis. These results suggested that, unlike in mature hepatocytes, or hepatocyte-like cells, activation of the AhR may sensitize undifferentiated HepaRG cells to apoptotic stimuli. Induction of apoptosis in cells with silenced YAP/TAZ was associated with upregulation of death ligand TRAIL, and seemed to involve both extrinsic and mitochondrial apoptosis pathways. Global gene expression analysis further suggested that TCDD significantly altered expression of constituents and/or transcriptional targets of signaling pathways participating in control of expansion or differentiation of liver progenitors, including EGFR, Wnt/β-catenin, or tumor growth factor-β signaling pathways. TCDD significantly upregulated cytosolic proapoptotic protein BMF (Bcl-2 modifying factor) in HepaRG cells, which could be linked with an enhanced sensitivity of TCDD-treated cells to apoptosis. Our results suggest that, in addition to promotion of cell proliferation and alteration of signaling pathways controlling expansion of human adult liver progenitors, AhR ligands may also sensitize human liver progenitor cells to apoptosis.

The Hippo Pathway in Innate Anti-microbial Immunity and Anti-tumor Immunity

The Hippo pathway responds to diverse environmental cues and plays key roles in cell fate determination, tissue homeostasis, and organ regeneration. Aberrant Hippo signaling, on the other hand, has frequently been implicated in diversified pathologies such as cancer and immune dysfunction. Here, we summarize the recent but rapid progress in understanding the involvement of the Hippo pathway in innate immunity, with a special focus on the intrinsic mechanisms and mutual interactions between Hippo-YAP signaling and the innate immune response and its physiological impacts on anti-microbial immunity and anti-tumor immunity. Moving forward, we believe that systematic investigations under the physiological setting are needed to draw a clearer picture of the actions of Hippo in innate immunity.

La plasticidad del hepatocito y su relevancia en la fisiología y la patología hepática

El hígado es uno de los principales órganos encargados de mantener la homeostasis en vertebrados,  además de poseer una gran capacidad regenerativa. El hígado está constituido por diversos tipos celulares que de forma coordinada contribuyen para que el órgano funcione eficientemente. Los hepatocitos representan el tipo celular principal de este órgano y llevan a cabo la mayoría de sus actividades; además, constituyen una población heterogénea de células epiteliales con funciones especializadas en el metabolismo. El fenotipo de los hepatocitos está controlado por diferentes vías de señalización, como la vía del TGFβ/Smads, la ruta Hippo/YAP-TAZ y la vía Wnt/β-catenina, entre otras. Los hepatocitos son células que se encuentran normalmente en un estado quiescente, aunque cuentan con una plasticidad intrínseca que se manifiesta en respuesta a diversos daños en el hígado; así, estas células reactivan su capacidad proliferativa o cambian su fenotipo a través de procesos celulares como la transdiferenciación o la transformación, para contribuir a mantener la homeostasis del órgano en condiciones saludables o desarrollar diversas  patologías.

Hyaluronan Degradation Promotes Cancer via Hippo-YAP Signaling: An Intervention Point for Cancer Therapy

High-molecular-weight hyaluronan acts as a ligand of the tumor-suppressive Hippo signal, whereas degradation of hyaluronan from a high-molecular-weight form to a low-molecular-weight forms by hyaluronidase 2 inhibits Hippo signal activation and thereby activates the pro-oncogenic transcriptional coactivator yes-associated protein (YAP), which creates a cancer-predisposing microenvironment and drives neoplastic transformation of cells through both cell-autonomous and non-cell-autonomous mechanisms. In fact, accumulation of low-molecular-weight hyaluronan in tissue stroma is observed in many types of cancers. Since inhibition of YAP activity suppresses tumor growth in vivo, pharmacological intervention of the Hippo-YAP signal is an attractive approach for future drug development. In this review, pharmacological intervention of excessive hyaluronan degradation as a novel approach for inhibition of the Hippo-YAP signal is also discussed. Development of hyaluronidase inhibitors may provide novel therapeutic strategies for human malignant tumors.

Cholesterol Stabilizes TAZ in Hepatocytes to Promote Experimental Non-alcoholic Steatohepatitis

Incomplete understanding of how hepatosteatosis transitions to fibrotic non-alcoholic steatohepatitis (NASH) has limited therapeutic options. Two molecules that are elevated in hepatocytes in human NASH liver are cholesterol, whose mechanistic link to NASH remains incompletely understood, and TAZ, a transcriptional regulator that promotes fibrosis but whose mechanism of increase in NASH is unknown. We now show that increased hepatocyte cholesterol upregulates TAZ and promotes fibrotic NASH. ASTER-B/C-mediated internalization of plasma membrane cholesterol activates soluble adenylyl cyclase (sAC; ADCY10), triggering a calcium-RhoA-mediated pathway that suppresses β-TrCP/proteasome-mediated TAZ degradation. In mice fed with a cholesterol-rich NASH-inducing diet, hepatocyte-specific silencing of ASTER-B/C, sAC, or RhoA decreased TAZ and ameliorated fibrotic NASH. The cholesterol-TAZ pathway is present in primary human hepatocytes, and associations among liver cholesterol, TAZ, and RhoA in human NASH liver are consistent with the pathway. Thus, hepatocyte cholesterol contributes to fibrotic NASH by increasing TAZ, suggesting new targets for therapeutic intervention.

Hippo kinases MST1 and MST2 control the differentiation of the epididymal initial segment via the MEK-ERK pathway

Although the roles of the Hippo pathway in organogenesis and tumorigenesis have been well studied in multiple organs, its role in sperm maturation and male fertility has not been investigated. The initial segment (IS) of the epididymis plays a critical role in sperm maturation. IS differentiation is governed by ERK1/2, but the mechanisms of ERK1/2 activation in IS are not fully understood. Here we show that double knockout (dKO) of mammalian sterile 20-like kinases 1 and 2 (Mst1 and Mst2), homologs of Hippo in Drosophila, in the epididymal epithelium led to male infertility in mice. Sperm in the cauda epididymides of mutant mice were immotile with flagellar angulation and severely disorganized structures. Loss of Mst1/2 activated YAP and increased proliferation and cell death in all the segments of epididymis. The mutant mice showed substantially suppressed MEK/ERK signaling in the IS and failed IS differentiation. Deletion of Yap restored the reduced MEK/ERK signaling, and partially rescued the defective IS differentiation and fertility in Mst1/2 dKO mice. Our results demonstrate that YAP inhibits the MEK/ERK pathway in IS epithelial cells, and MST1/2 control IS differentiation and fertility at least partially by repressing YAP. Taken together, the Hippo pathway is essential for sperm maturation and male fertility.

Tubule-Specific Mst1/2 Deficiency Induces CKD via YAP and Non-YAP Mechanisms

BACKGROUND

The serine/threonine kinases MST1 and MST2 are core components of the Hippo pathway, which has been found to be critically involved in embryonic kidney development. Yes-associated protein (YAP) and transcriptional coactivator with PDZ-binding motif (TAZ) are the pathway's main effectors. However, the biologic functions of the Hippo/YAP pathway in adult kidneys are not well understood, and the functional role of MST1 and MST2 in the kidney has not been studied.

METHODS

We used immunohistochemistry to examine expression in mouse kidneys of MST1 and MST2, homologs of Hippo in Drosophila. We generated mice with tubule-specific double knockout of Mst1 and Mst2 or triple knockout of Mst1, Mst2, and Yap. PCR array and mouse inner medullary collecting duct cells were used to identify the primary target of Mst1/Mst2 deficiency.

RESULTS

MST1 and MST2 were predominantly expressed in the tubular epithelial cells of adult kidneys. Deletion of Mst1/Mst2 in renal tubules increased activity of YAP but not TAZ. The kidneys of mutant mice showed progressive inflammation, tubular and glomerular damage, fibrosis, and functional impairment; these phenotypes were largely rescued by deletion of Yap in renal tubules. TNF-α expression was induced via both YAP-dependent and YAP-independent mechanisms, and TNF-α and YAP amplified the signaling activities of each other in the tubules of kidneys with double knockout of Mst1/Mst2.

CONCLUSIONS

Our findings show that tubular Mst1/Mst2 deficiency leads to CKD through both the YAP and non-YAP pathways and that tubular YAP activation induces renal fibrosis. The pathogenesis seems to involve the reciprocal stimulation of TNF-α and YAP signaling activities.

Hippo-YAP1 Is a Prognosis Marker and Potentially Targetable Pathway in Advanced Gallbladder Cancer

Gallbladder cancer is an aggressive disease with late diagnosis and no efficacious treatment. The Hippo-Yes-associated protein 1 (YAP1) signaling pathway has emerged as a target for the development of new therapeutic interventions in cancers. However, the role of the Hippo-targeted therapy has not been addressed in advanced gallbladder cancer (GBC). This study aimed to evaluate the expression of the major Hippo pathway components mammalian Ste20-like protein kinase 1 (MST1), YAP1 and transcriptional coactivator with PDZ-binding motif (TAZ) and examined the effects of Verteporfin (VP), a small molecular inhibitor of YAP1-TEA domain transcription factor (TEAD) protein interaction, in metastatic GBC cell lines and patient-derived organoids (PDOs). Immunohistochemical analysis revealed that advanced GBC patients had high nuclear expression of YAP1. High nuclear expression of YAP1 was associated with poor survival in GBC patients with subserosal invasion (pT2). Additionally, advanced GBC cases showed reduced expression of MST1 compared to chronic cholecystitis. Both VP treatment and YAP1 siRNA inhibited the migration ability in GBC cell lines. Interestingly, gemcitabine resistant PDOs with high nuclear expression of YAP1 were sensitive to VP treatment. Taken together, our results suggest that key components of the Hippo-YAP1 signaling pathway are dysregulated in advanced gallbladder cancer and reveal that the inhibition YAP1 may be a candidate for targeted therapy.

Phase separation of TAZ compartmentalizes the transcription machinery to promote gene expression

TAZ promotes growth, development and tumorigenesis by regulating the expression of target genes. However, the manner in which TAZ orchestrates the transcriptional responses is poorly defined. Here we demonstrate that TAZ forms nuclear condensates through liquid-liquid phase separation to compartmentalize its DNA-binding cofactor TEAD4, coactivators BRD4 and MED1, and the transcription elongation factor CDK9 for transcription. TAZ forms phase-separated droplets in vitro and liquid-like nuclear condensates in vivo, and this ability is negatively regulated by Hippo signalling through LATS-mediated phosphorylation and is mediated by the coiled-coil (CC) domain. Deletion of the TAZ CC domain or substitution with the YAP CC domain prevents the phase separation of TAZ and its ability to induce the expression of TAZ-specific target genes. Thus, we identify a mechanism of transcriptional activation by TAZ and demonstrate that pathway-specific transcription factors also engage the phase-separation mechanism for efficient and specific transcriptional activation.

The Crosstalk Between Hippo-YAP Pathway and Innate Immunity

Recognition of pathogen-associated molecular patterns (PAMPs) triggers expression of antiviral interferons and proinflammatory cytokines, which functions as the frontier of host defense against microbial pathogen invasion. Hippo-YAP pathway regulates cell proliferation, survival, differentiation and is involved in diverse life processes, including tissue homeostasis and tumor suppression. Emerging discoveries elucidated that the components of Hippo-YAP pathway, such as MST1/2, NDR1/2, and YAP/TAZ played crucial regulatory roles in innate immunity. Meanwhile the innate immune signaling also exhibited regulatory effect on Hippo-YAP pathway. As for the importance of these two pathways, it would be interesting to figure out the deeper biological implications of their interplays. This review focuses on the regulation between Hippo-YAP pathway and innate immune signaling. We also propose the possible contribution of these interplays to tumor development.