Induction of AP-1 by YAP/TAZ contributes to cell proliferation and organ growth

Emerging Principles in the Transcriptional Control by YAP and TAZ

Simple Summary

YAP and TAZ are transcriptional cofactors that integrate several upstream signals to generate context-dependent transcriptional responses. This requires extensive integration with epigenetic regulators and other transcription factors. The molecular and genomic characterization of YAP and TAZ nuclear function has broad implications both in physiological and pathological settings.

Abstract

Yes-associated protein (YAP) and TAZ are transcriptional cofactors that sit at the crossroad of several signaling pathways involved in cell growth and differentiation. As such, they play essential functions during embryonic development, regeneration, and, once deregulated, in cancer progression. In this review, we will revise the current literature and provide an overview of how YAP/TAZ control transcription. We will focus on data concerning the modulation of the basal transcriptional machinery, their ability to epigenetically remodel the enhancer–promoter landscape, and the mechanisms used to integrate transcriptional cues from multiple pathways. This reveals how YAP/TAZ activation in cancer cells leads to extensive transcriptional control that spans several hallmarks of cancer. The definition of the molecular mechanism of transcriptional control and the identification of the pathways regulated by YAP/TAZ may provide therapeutic opportunities for the effective treatment of YAP/TAZ-driven tumors.

Cellular feedback dynamics and multilevel regulation driven by the hippo pathway

The Hippo pathway is a dynamic cellular signalling nexus that regulates differentiation and controls cell proliferation and death. If the Hippo pathway is not precisely regulated, the functionality of the upstream kinase module is impaired, which increases nuclear localisation and activity of the central effectors, the transcriptional co-regulators YAP and TAZ. Pathological YAP and TAZ hyperactivity consequently cause cancer, fibrosis and developmental defects. The Hippo pathway controls an array of fundamental cellular processes, including adhesion, migration, mitosis, polarity and secretion of a range of biologically active components. Recent studies highlight that spatio-temporal regulation of Hippo pathway components are central to precisely controlling its context-dependent dynamic activity. Several levels of feedback are integrated into the Hippo pathway, which is further synergized with interactors outside of the pathway that directly regulate specific Hippo pathway components. Likewise, Hippo core kinases also ‘moonlight’ by phosphorylating multiple substrates beyond the Hippo pathway and thereby integrates further flexibility and robustness in the cellular decision-making process. This topic is still in its infancy but promises to reveal new fundamental insights into the cellular regulation of this therapeutically important pathway. We here highlight recent advances emphasising feedback dynamics and multilevel regulation of the Hippo pathway with a focus on mitosis and cell migration, as well as discuss potential productive future research avenues that might reveal novel insights into the overall dynamics of the pathway.

Sox2 controls neural stem cell self-renewal through a Fos-centered gene regulatory network

The Sox2 transcription factor is necessary for the long-term self-renewal of neural stem cells (NSCs). Its mechanism of action is still poorly defined. To identify molecules regulated by Sox2, and acting in mouse NSC maintenance, we transduced, into Sox2-deleted NSC, genes whose expression is strongly downregulated following Sox2 loss (Fos, Jun, Egr2), individually or in combination. Fos alone rescued long-term proliferation, as shown by in vitro cell growth and clonal analysis. Furthermore, pharmacological inhibition by T-5224 of FOS/JUN AP1 complex binding to its targets decreased cell proliferation and expression of the putative target Suppressor of cytokine signaling 3 (Socs3). Additionally, Fos requirement for efficient long-term proliferation was demonstrated by the reduction of NSC clones capable of long-term expansion following CRISPR/Cas9-mediated Fos inactivation. Previous work showed that the Socs3 gene is strongly downregulated following Sox2 deletion, and its re-expression by lentiviral transduction rescues long-term NSC proliferation. Fos appears to be an upstream regulator of Socs3, possibly together with Jun and Egr2; indeed, Sox2 re-expression in Sox2-deleted NSC progressively activates both Fos and Socs3 expression; in turn, Fos transduction activates Socs3 expression. Based on available SOX2 ChIPseq and ChIA-PET data, we propose a model whereby Sox2 is a direct activator of both Socs3 and Fos, as well as possibly Jun and Egr2; furthermore, we provide direct evidence for FOS and JUN binding on Socs3 promoter, suggesting direct transcriptional regulation. These results provide the basis for developing a model of a network of interactions, regulating critical effectors of NSC proliferation and long-term maintenance.

Hippo signaling effectors YAP and TAZ induce Epstein-Barr Virus (EBV) lytic reactivation through TEADs in epithelial cells

The Epstein-Barr virus (EBV) human herpesvirus is associated with B-cell and epithelial-cell malignancies, and both the latent and lytic forms of viral infection contribute to the development of EBV-associated tumors. Here we show that the Hippo signaling effectors, YAP and TAZ, promote lytic EBV reactivation in epithelial cells. The transcriptional co-activators YAP/TAZ (which are inhibited by Hippo signaling) interact with DNA-binding proteins, particularly TEADs, to induce transcription. We demonstrate that depletion of either YAP or TAZ inhibits the ability of phorbol ester (TPA) treatment, cellular differentiation or the EBV BRLF1 immediate-early (IE) protein to induce lytic EBV reactivation in oral keratinocytes, and show that over-expression of constitutively active forms of YAP and TAZ reactivate lytic EBV infection in conjunction with TEAD family members. Mechanistically, we find that YAP and TAZ interact with, and activate, the EBV BZLF1 immediate-early promoter. Furthermore, we demonstrate that YAP, TAZ, and TEAD family members are expressed at much higher levels in epithelial cell lines in comparison to B-cell lines, and find that EBV infection of oral keratinocytes increases the level of activated (dephosphorylated) YAP and TAZ. Finally, we have discovered that lysophosphatidic acid (LPA), a known YAP/TAZ activator that plays an important role in inflammation, induces EBV lytic reactivation in epithelial cells through a YAP/TAZ dependent mechanism. Together these results establish that YAP/TAZ are powerful inducers of the lytic form of EBV infection and suggest that the ability of EBV to enter latency in B cells at least partially reflects the extremely low levels of YAP/TAZ and TEADs in this cell type.

Molecular control of cell density-mediated exit to quiescence

Contact inhibition of cell proliferation regulates tissue size and prevents uncontrolled cell expansion. When cell density increases, contact inhibition can force proliferating cells into quiescence. Here we show that the variable memory of local cell density experienced by a mother cell controls the levels of the cyclin-dependent kinase (CDK) activator cyclin D1 and inhibitor p27 in newborn daughters, which direct cells to proliferation or quiescence. Much of this regulation can be explained by rapid suppression of ERK activity by high cell density in mothers, which leads to lower cyclin D1 and higher p27 levels in daughters. Strikingly, cell density and mitogen signals compete by shifting the ratio of cyclin D1/p27 levels below or above a single sharp threshold that controls the proliferation decision. Thus, the history of competing cell density and mitogen signals experienced by mothers is funneled into a precise activator-inhibitor balance that decides the fate of daughter cells.

Using live single-cell microscopy, Fan and Meyer show that the decision of newborn daughter cells to proliferate or become quiescent is controlled by the memory of local cell density inherited from mother cells. This memory is mediated by an ultrasensitive activator-inhibitor balance between cyclin D1 and p27.

Molecular and Metabolic Reprogramming: Pulling the Strings Toward Tumor Metastasis

Metastasis is a major hurdle to the efficient treatment of cancer, accounting for the great majority of cancer-related deaths. Although several studies have disclosed the detailed mechanisms underlying primary tumor formation, the emergence of metastatic disease remains poorly understood. This multistep process encompasses the dissemination of cancer cells to distant organs, followed by their adaptation to foreign microenvironments and establishment in secondary tumors. During the last decades, it was discovered that these events may be favored by particular metabolic patterns, which are dependent on reprogrammed signaling pathways in cancer cells while they acquire metastatic traits. In this review, we present current knowledge of molecular mechanisms that coordinate the crosstalk between metastatic signaling and cellular metabolism. The recent findings involving the contribution of crucial metabolic pathways involved in the bioenergetics and biosynthesis control in metastatic cells are summarized. Finally, we highlight new promising metabolism-based therapeutic strategies as a putative way of impairing metastasis.

Extracellular signal-regulated kinase signaling pathway and silicosis

Silicosis is a scarring lung disease caused by inhaling fine particles of crystalline silica in the workplace of many industries. Due to the lack of effective treatment and management, the continued high incidence of silicosis remains a major public health concern worldwide, especially in the developing countries. Till now, related molecular mechanisms underlying silicosis are still not completely understood. Multiple pathways have been reported to be participated in the pathological process of silicosis, and more complex signaling pathways are receiving attention. The activated extracellular signal-regulated kinase (ERK) signaling pathway has been recognized to control some functions in the cell. Recent studies have identified that the ERK signaling pathway contributes to the formation and development of silicosis through regulating the processes of oxidative stress, inflammatory response, proliferation and activation of fibroblasts, epithelial-mesenchymal transformation, autophagy, and apoptosis of cells. In this review article, we summarize the latest findings on the role of ERK signaling pathway in silica-induced experimental models of silicosis, as well as clinical perspectives.

Cell Shape and Matrix Stiffness Impact Schwann Cell Plasticity via YAP/TAZ and Rho GTPases

Schwann cells (SCs) are a highly plastic cell type capable of undergoing phenotypic changes following injury or disease. SCs are able to upregulate genes associated with nerve regeneration and ultimately achieve functional recovery. During the regeneration process, the extracellular matrix (ECM) and cell morphology play a cooperative, critical role in regulating SCs, and therefore highly impact nerve regeneration outcomes. However, the roles of the ECM and mechanotransduction relating to SC phenotype are largely unknown. Here, we describe the role that matrix stiffness and cell morphology play in SC phenotype specification via known mechanotransducers YAP/TAZ and RhoA. Using engineered microenvironments to precisely control ECM stiffness, cell shape, and cell spreading, we show that ECM stiffness and SC spreading downregulated SC regenerative associated proteins by the activation of RhoA and YAP/TAZ. Additionally, cell elongation promoted a distinct SC regenerative capacity by the upregulation of Rac1/MKK7/JNK, both necessary for the ECM and morphology changes found during nerve regeneration. These results confirm the role of ECM signaling in peripheral nerve regeneration as well as provide insight to the design of future biomaterials and cellular therapies for peripheral nerve regeneration.

LACTB suppresses melanoma progression by attenuating PP1A and YAP interaction

Very limited progress has been made in the management of advanced melanoma, especially melanoma of uveal origin. Lactamase β (LACTB) is a novel tumor suppressor; however, its biological function in melanoma remains unknown. Herein we demonstrated markedly lower LACTB expression levels in melanoma tissues and cell lines. Overexpression of LACTB suppressed the proliferation, migration and invasion of melanoma cells in vitro. Mechanistically, LACTB inhibited the activity of yes-associated protein (YAP). We showed that the level of phospho-YAP (Serine 127) was increased upon LACTB overexpression, which prevented the translocation of YAP to the nucleus. Further, LACTB could directly bind to PP1A and attenuate the interaction between PP1A and YAP, resulting in decreased YAP dephosphorylation and inactivation in a LATS1-independent manner. Additionally, transfection of phosphorylation-defective YAP mutants reversed LACTB-induced tumor suppression. Upstream, we demonstrated that SOX10 binds to the LACTB promoter and negatively regulates its transcription. Overexpression of LACTB also suppressed the tumorigenicity and lung metastasis of MUM2B uveal melanoma cells in vivo. Taken together, our findings indicate a novel SOX10/LACTB/PP1A signaling cascade that renders YAP inactive and modulates melanoma progression, offering a new therapeutic target for melanoma treatment.

DDR2 upregulation confers ferroptosis susceptibility of recurrent breast tumors through the Hippo pathway

Recurrent breast cancer presents significant challenges with aggressive phenotypes and treatment resistance. Therefore, novel therapeutics are urgently needed. Here, we report that murine recurrent breast tumor cells, when compared with primary tumor cells, are highly sensitive to ferroptosis. Discoidin Domain Receptor Tyrosine Kinase 2 (DDR2), the receptor for collagen I, is highly expressed in ferroptosis-sensitive recurrent tumor cells and human mesenchymal breast cancer cells. EMT regulators, TWIST and SNAIL, significantly induce DDR2 expression and sensitize ferroptosis in a DDR2-dependent manner. Erastin treatment induces DDR2 upregulation and phosphorylation, independent of collagen I. Furthermore, DDR2 knockdown in recurrent tumor cells reduces clonogenic proliferation. Importantly, both the ferroptosis protection and reduced clonogenic growth may be compatible with the compromised YAP/TAZ upon DDR2 inhibition. Collectively, these findings identify the important role of EMT-driven DDR2 upregulation in recurrent tumors in maintaining growth advantage but activating YAP/TAZ-mediated ferroptosis susceptibility, providing potential strategies to eradicate recurrent breast cancer cells with mesenchymal features.

Endothelial YAP/TAZ Signaling in Angiogenesis and Tumor Vasculature

Solid tumors are dependent on vascularization for their growth. The hypoxic, stiff, and pro-angiogenic tumor microenvironment induces angiogenesis, giving rise to an immature, proliferative, and permeable vasculature. The tumor vessels promote tumor metastasis and complicate delivery of anti-cancer therapies. In many types of tumors, YAP/TAZ activation is correlated with increased levels of angiogenesis. In addition, endothelial YAP/TAZ activation is important for the formation of new blood and lymphatic vessels during development. Oncogenic activation of YAP/TAZ in tumor cell growth and invasion has been studied in great detail, however the role of YAP/TAZ within the tumor endothelium remains insufficiently understood, which complicates therapeutic strategies aimed at targeting YAP/TAZ in cancer. Here, we overview the upstream signals from the tumor microenvironment that control endothelial YAP/TAZ activation and explore the role of their downstream targets in driving tumor angiogenesis. We further discuss the potential for anti-cancer treatments and vascular normalization strategies to improve tumor therapies.

Single-cell analyses reveal YAP/TAZ as regulators of stemness and cell plasticity in Glioblastoma

Glioblastoma (GBM) is a devastating human malignancy. GBM stem-like cells (GSCs) drive tumor initiation and progression. Yet, the molecular determinants defining GSCs in their native state in patients remain poorly understood. Here we used single cell datasets and identified GSCs at the apex of the differentiation hierarchy of GBM. By reconstructing the GSCs' regulatory network, we identified the YAP/TAZ coactivators as master regulators of this cell state, irrespectively of GBM subtypes. YAP/TAZ are required to install GSC properties in primary cells downstream of multiple oncogenic lesions, and required for tumor initiation and maintenance in vivo in different mouse and human GBM models. YAP/TAZ act as main roadblock of GSC differentiation and their inhibition irreversibly lock differentiated GBM cells into a non-tumorigenic state, preventing plasticity and regeneration of GSC-like cells. Thus, GSC identity is linked to a key molecular hub integrating genetics and microenvironmental inputs within the multifaceted biology of GBM.

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

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

Unraveling the Complex Interplay Between Transcription Factors and Signaling Molecules in Thyroid Differentiation and Function, From Embryos to Adults

Thyroid differentiation of progenitor cells occurs during embryonic development and in the adult thyroid gland, and the molecular bases of these complex and finely regulated processes are becoming ever more clear. In this Review, we describe the most recent advances in the study of transcription factors, signaling molecules and regulatory pathways controlling thyroid differentiation and development in the mammalian embryo. We also discuss the maintenance of the adult differentiated phenotype to ensure the biosynthesis of thyroid hormones. We will focus on endoderm-derived thyroid epithelial cells, which are responsible for the formation of the thyroid follicle, the functional unit of the thyroid gland. The use of animal models and pluripotent stem cells has greatly aided in providing clues to the complicated puzzle of thyroid development and function in adults. The so-called thyroid transcription factors - Nkx2-1, Foxe1, Pax8 and Hhex - were the first pieces of the puzzle identified in mice. Other transcription factors, either acting upstream of or directly with the thyroid transcription factors, were subsequently identified to, almost, complete the puzzle. Among them, the transcription factors Glis3, Sox9 and the cofactor of the Hippo pathway Taz, have emerged as important players in thyroid differentiation and development. The involvement of signaling molecules increases the complexity of the puzzle. In this context, the importance of Bmps, Fgfs and Shh signaling at the onset of development, and of TSH, IGF1 and TGFβ both at the end of terminal differentiation in embryos and in the adult thyroid, are well recognized. All of these aspects are covered herein. Thus, readers will be able to visualize the puzzle of thyroid differentiation with most - if not all - of the pieces in place.

CBX2 depletion inhibits the proliferation, invasion and migration of gastric cancer cells by inactivating the YAP/β-catenin pathway

Gastric cancer (GC) is the most common and fast-growing malignancy of the digestive system, which has a high mortality. Chromobox homolog 2 (CBX2) has been reported to be highly expressed in cancer tissues compared with adjacent normal tissues. It has also been established that CBX2 is upregulated in GC cell lines by searching the Cancer Cell Line Encyclopedia. The aim of the present study was to investigate the biomolecular role and underlying mechanism of CBX2 in the proliferation, invasion and migration of GC cells. Short hairpin RNA-CBX2 and yes-associated protein (YAP) overexpression plasmids were constructed to regulate CBX2 and YAP expression, respectively. Additionally, the expression of certain mRNAs and proteins involved in the YAP/β-catenin pathway and those associated with cell invasion were assessed by western blotting and reverse transcription-quantitative PCR, respectively. The cellular behaviors of MFC cells were analyzed using Cell Counting Kit-8, colony formation wound-healing and Transwell assays. The results of the present study revealed that increased CBX2 expression was observed in GC cell lines compared with normal gastric cells. In addition, CBX2 knockdown inhibited the nuclear cytoplasm translocation of YAP, inducing its phosphorylation, and suppressing the activation of the β-catenin signaling pathway. The results also demonstrated that CBX2 depletion inhibited the proliferation, migration and invasion of GC cells by inactivating the YAP/β-catenin pathway. It was determined that CBX2 promoted the proliferation, invasion and migration of GC cells by activating the YAP/β-catenin pathway, suggesting that CBX2 is involved in the pathogenesis of GC and may represent a novel target for the clinical treatment of GC.

YAP and AP-1 Cooperate to Initiate Pancreatic Cancer Development from Ductal Cells in Mice

The development of pancreatic cancer is heavily dependent upon the aberrant activation of KRAS signaling. Among the downstream targets of KRAS, the effectors of the Hippo pathway YAP and TAZ (YAP/TAZ) are crucial during cancer initiation and progression. However, little is known about the cell type-specific effects of YAP/TAZ on the development of pancreatic cancer. Here we clarify the unique consequences of YAP/TAZ activation in the ductal cell population of the pancreas by generating mice with pancreatic duct cell-specific, inducible knockouts of Lats1 and Lats2, the main kinases upstream of YAP/TAZ. Oncogenic activation of YAP by deletion of Lats1/2 in ductal cells led to the rapid transformation of the pancreas, which was accompanied by a robust increase in the expression of YAP and AP-1 target genes. Pharmacologic inhibition of AP-1 activity induced death in Lats1/2 knockout organoids and attenuated YAP-dependent transformation of the pancreas in vivo. Both YAP and AP-1 were activated during the development of KRAS-dependent cancer in mice and human patients with pancreatic ductal adenocarcinoma, suggesting that this signaling hub represents an important mediator of pancreatic cancer development and progression. Collectively, these data define a YAP-dependent mechanism of pancreatic cancer cell development and suggest that inhibition of AP-1 can suppress this development. SIGNIFICANCE: A pancreatic ductal cell-specific knockout mouse model featuring constitutively active YAP allows for the study of YAP-dependent transformation of the pancreas and for screening pharmacologically active inhibitors.

Yap/Taz promote the scavenging of extracellular nutrients through macropinocytosis

The uptake of macromolecules and cellular debris through macropinocytosis has emerged as an important nutrient acquisition strategy of cancer cells. Genetic alterations commonly found in human cancers (e.g. mutations in KRAS or loss of PTEN) have been shown to increase macropinocytosis. To identify additional effectors that enable cell growth dependent on the uptake of extracellular proteins, pancreatic ductal adenocarcinoma (PDA) cells were selected for growth in medium where extracellular albumin was the obligate source of the essential amino acid leucine. Analysis of global changes in chromatin availability and gene expression revealed that PDA cells selected under these conditions exhibited elevated activity of the transcriptional activators Yap/Taz. Knockout of Yap/Taz prevented growth of PDA cells in leucine-deficient medium, but not in complete medium. Furthermore, constitutively active forms of Yap or Taz were sufficient to stimulate macropinocytosis of extracellular protein. In addition to promoting the uptake of plasma proteins, Yap/Taz also promoted the scavenging of apoptotic cell bodies and necrotic debris by PDA cells. The Yap/Taz transcriptional target Axl was found to be essential for cell growth dependent on the uptake of dead cells and cell debris. Together, these studies suggest that the Hippo pathway effectors Yap and Taz are important transcriptional regulators of endocytic nutrient uptake.

MicroRNA-21-Enriched Exosomes as Epigenetic Regulators in Melanomagenesis and Melanoma Progression: The Impact of Western Lifestyle Factors

DNA mutation-induced activation of RAS-BRAF-MEK-ERK signaling associated with intermittent or chronic ultraviolet (UV) irradiation cannot exclusively explain the excessive increase of malignant melanoma (MM) incidence since the 1950s. Malignant conversion of a melanocyte to an MM cell and metastatic MM is associated with a steady increase in microRNA-21 (miR-21). At the epigenetic level, miR-21 inhibits key tumor suppressors of the RAS-BRAF signaling pathway enhancing proliferation and MM progression. Increased MM cell levels of miR-21 either result from endogenous upregulation of melanocytic miR-21 expression or by uptake of miR-21-enriched exogenous exosomes. Based on epidemiological data and translational evidence, this review provides deeper insights into environmentally and metabolically induced exosomal miR-21 trafficking beyond UV-irradiation in melanomagenesis and MM progression. Sources of miR-21-enriched exosomes include UV-irradiated keratinocytes, adipocyte-derived exosomes in obesity, airway epithelium-derived exosomes generated by smoking and pollution, diet-related exosomes and inflammation-induced exosomes, which may synergistically increase the exosomal miR-21 burden of the melanocyte, the transformed MM cell and its tumor environment. Several therapeutic agents that suppress MM cell growth and proliferation attenuate miR-21 expression. These include miR-21 antagonists, metformin, kinase inhibitors, beta-blockers, vitamin D, and plant-derived bioactive compounds, which may represent new options for the prevention and treatment of MM.

The bZIP Proteins of Oncogenic Viruses

Basic leucine zipper (bZIP) transcription factors (TFs) govern diverse cellular processes and cell fate decisions. The hallmark of the leucine zipper domain is the heptad repeat, with leucine residues at every seventh position in the domain. These leucine residues enable homo- and heterodimerization between ZIP domain α-helices, generating coiled-coil structures that stabilize interactions between adjacent DNA-binding domains and target DNA substrates. Several cancer-causing viruses encode viral bZIP TFs, including human T-cell leukemia virus (HTLV), hepatitis C virus (HCV) and the herpesviruses Marek’s disease virus (MDV), Epstein–Barr virus (EBV) and Kaposi’s sarcoma-associated herpesvirus (KSHV). Here, we provide a comprehensive review of these viral bZIP TFs and their impact on viral replication, host cell responses and cell fate.