Targeting YAP in malignant pleural mesothelioma

Hippo pathway in non-small cell lung cancer: mechanisms, potential targets, and biomarkers

Lung cancer is the primary contributor to cancer-related deaths globally, and non-small cell lung cancer (NSCLC) constitutes around 85% of all lung cancer cases. Recently, the emergence of targeted therapy and immunotherapy revolutionized the treatment of NSCLC and greatly improved patients' survival. However, drug resistance is inevitable, and extensive research has demonstrated that the Hippo pathway plays a crucial role in the development of drug resistance in NSCLC. The Hippo pathway is a highly conserved signaling pathway that is essential for various biological processes, including organ development, maintenance of epithelial balance, tissue regeneration, wound healing, and immune regulation. This pathway exerts its effects through two key transcription factors, namely Yes-associated protein (YAP) and transcriptional co-activator PDZ-binding motif (TAZ). They regulate gene expression by interacting with the transcriptional-enhanced associate domain (TEAD) family. In recent years, this pathway has been extensively studied in NSCLC. The review summarizes a comprehensive overview of the involvement of this pathway in NSCLC, and discusses the mechanisms of drug resistance, potential targets, and biomarkers associated with this pathway in NSCLC.

Cancer cell response to extrinsic and intrinsic mechanical cue: opportunities for tumor apoptosis strategies

Increasing studies have revealed the importance of mechanical cues in tumor progression, invasiveness and drug resistance. During malignant transformation, changes manifest in either the mechanical properties of the tissue or the cellular ability to sense and respond to mechanical signals. The major focus of the review is the subtle correlation between mechanical cues and apoptosis in tumor cells from a mechanobiology perspective. To begin, we focus on the intracellular force, examining the mechanical properties of the cell interior, and outlining the role that the cytoskeleton and intracellular organelle-mediated intracellular forces play in tumor cell apoptosis. This article also elucidates the mechanisms by which extracellular forces guide tumor cell mechanosensing, ultimately triggering the activation of the mechanotransduction pathway and impacting tumor cell apoptosis. Finally, a comprehensive examination of the present status of the design and development of anti-cancer materials targeting mechanotransduction is presented, emphasizing the underlying design principles. Furthermore, the article underscores the need to address several unresolved inquiries to enhance our comprehension of cancer therapeutics that target mechanotransduction.

A Novel Irreversible TEAD Inhibitor, SWTX-143, Blocks Hippo Pathway Transcriptional Output and Causes Tumor Regression in Preclinical Mesothelioma Models

The Hippo pathway and its downstream effectors, the YAP and TAZ transcriptional coactivators, are deregulated in multiple different types of human cancer and are required for cancer cell phenotypes in vitro and in vivo, while largely dispensable for tissue homeostasis in adult mice. YAP/TAZ and their main partner transcription factors, the TEAD1-4 factors, are therefore promising anticancer targets. Because of frequent YAP/TAZ hyperactivation caused by mutations in the Hippo pathway components NF2 and LATS2, mesothelioma is one of the prime cancer types predicted to be responsive to YAP/TAZ-TEAD inhibitor treatment. Mesothelioma is a devastating disease for which currently no effective treatment options exist. Here, we describe a novel covalent YAP/TAZ-TEAD inhibitor, SWTX-143, that binds to the palmitoylation pocket of all four TEAD isoforms. SWTX-143 caused irreversible and specific inhibition of the transcriptional activity of YAP/TAZ-TEAD in Hippo-mutant tumor cell lines. More importantly, YAP/TAZ-TEAD inhibitor treatment caused strong mesothelioma regression in subcutaneous xenograft models with human cells and in an orthotopic mesothelioma mouse model. Finally, SWTX-143 also selectively impaired the growth of NF2-mutant kidney cancer cell lines, suggesting that the sensitivity of mesothelioma models to these YAP/TAZ-TEAD inhibitors can be extended to other tumor types with aberrations in Hippo signaling. In brief, we describe a novel and specific YAP/TAZ-TEAD inhibitor that has potential to treat multiple Hippo-mutant solid tumor types.

The past, present, and future of targeted therapeutic approaches in patients with diffuse pleural mesotheliomas

Despite our growing understanding of the genomic landscape of diffuse pleural mesotheliomas (DPM), there has been limited success in targeted therapeutic strategies for the disease. This review summarizes attempts to develop targeted therapies in DPM, focusing on the following targets being clinically explored in recent and ongoing clinical trials: vascular endothelial growth factor, mesothelin, BRCA1-associated protein 1, Wilms tumor 1 protein, NF2/YAP/TAZ, CDKN2, methylthioadenosine phosphorylase, v-domain Ig suppressor T-cell activation, and argininosuccinate synthetase 1. Although preclinical data for these targets are promising, few have efficaciously translated to benefit our patients. Future efforts should seek to expand the availability of preclinical models that faithfully recapitulate DPM biology, develop clinically relevant biomarkers, and refine patient selection criteria for clinical trials.

The genetic landscape and possible therapeutics of neurofibromatosis type 2

Neurofibromatosis type 2 (NF2) is a genetic condition marked by the development of multiple benign tumors in the nervous system. The most common tumors associated with NF2 are bilateral vestibular schwannoma, meningioma, and ependymoma. The clinical manifestations of NF2 depend on the site of involvement. Vestibular schwannoma can present with hearing loss, dizziness, and tinnitus, while spinal tumor leads to debilitating pain, muscle weakness, or paresthesias. Clinical diagnosis of NF2 is based on the Manchester criteria, which have been updated in the last decade. NF2 is caused by loss-of-function mutations in the NF2 gene on chromosome 22, leading the merlin protein to malfunction. Over half of NF2 patients have de novo mutations, and half of this group are mosaic. NF2 can be managed by surgery, stereotactic radiosurgery, monoclonal antibody bevacizumab, and close observation. However, the nature of multiple tumors and the necessity of multiple surgeries over the lifetime, inoperable tumors like meningiomatosis with infiltration of the sinus or in the area of the lower cranial nerves, the complications caused by the operation, the malignancies induced by radiotherapy, and inefficiency of cytotoxic chemotherapy due to the benign nature of NF-related tumors have led a march toward exploring targeted therapies. Recent advances in genetics and molecular biology have allowed identifying and targeting of underlying pathways in the pathogenesis of NF2. In this review, we explain the clinicopathological characteristics of NF2, its genetic and molecular background, and the current knowledge and challenges of implementing genetics to develop efficient therapies.

Mesothelioma cancer cells are glutamine addicted and glutamine restriction reduces YAP1 signaling to attenuate tumor formation

Glutamine addiction is an important phenotype displayed in some types of cancer. In these cells, glutamine depletion results in a marked reduction in the aggressive cancer phenotype. Mesothelioma is an extremely aggressive disease that lacks effective therapy. In this study, we show that mesothelioma tumors are glutamine addicted suggesting that glutamine depletion may be a potential therapeutic strategy. We show that glutamine restriction, by removing glutamine from the medium or treatment with inhibitors that attenuate glutamine uptake (V-9302) or conversion to glutamate (CB-839), markedly reduces mesothelioma cell proliferation, spheroid formation, invasion, and migration. Inhibition of the SLC1A5 glutamine importer, by knockout or treatment with V-9302, an SLC1A5 inhibitor, also markedly reduces mesothelioma cell tumor growth. A relationship between glutamine utilization and YAP1/TEAD signaling has been demonstrated in other tumor types, and the YAP1/TEAD signaling cascade is active in mesothelioma cells and drives cell survival and proliferation. We therefore assessed the impact of glutamine depletion on YAP1/TEAD signaling. We show that glutamine restriction, SLC1A5 knockdown/knockout, or treatment with V-9302 or CB-839, reduces YAP1 level, YAP1/TEAD-dependent transcription, and YAP1/TEAD target protein (e.g., CTGF, cyclin D1, COL1A2, COL3A1, etc.) levels. These changes are observed in both cells and tumors. These findings indicate that mesothelioma is a glutamine addicted cancer, show that glutamine depletion attenuates YAP1/TEAD signaling and tumor growth, and suggest that glutamine restriction may be useful as a mesothelioma treatment strategy.

Small RNA-Seq Transcriptome Profiling of Mesothelial and Mesothelioma Cell Lines Revealed microRNA Dysregulation after Exposure to Asbestos-like Fibers

Environmental exposure to fibers of respirable size has been identified as a risk for public health. Experimental evidence has revealed that a variety of fibers, including fluoro-edenite, can develop chronic respiratory diseases and elicit carcinogenic effects in humans. Fluoro-edenite (FE) is a silicate mineral first found in Biancavilla (Sicily, Italy) in 1997. Environmental exposure to its fibers has been correlated with a cluster of malignant pleural mesotheliomas. This neoplasm represents a public health problem due to its long latency and to its aggression not alerted by specific symptoms. Having several biomarkers providing us with data on the health state of those exposed to FE fibers or allowing an early diagnosis on malignant pleural mesothelioma, still asymptomatic patients, would be a remarkable goal. To these purposes, we reported the miRNA transcriptome in human normal mesothelial cell line (MeT-5A) and in the human malignant mesothelioma cell line (JU77) exposed and not exposed to FE fibers. The results showed a difference in the number of deregulated miRNAs between tumor and nontumor samples both exposed and not exposed to FE fibers. As a matter of fact, the effect of exposure to FE fibers is more evident in the expression of miRNA in the tumor samples than in the nontumor samples. In the present paper, several pathways involved in the pathogenesis of malignant pleural mesothelioma have been analyzed. We especially noticed the involvement of pathways that have important functions in inflammatory processes, angiogenesis, apoptosis, and necrosis. Besides this amount of data, further studies will be designed for the selection of the most significant miRNAs to test and validate their diagnostic potential, alone or in combination with other protein biomarkers, in high-risk individuals' liquid biopsy to have a noninvasive tool of diagnosis for this neoplasm.

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

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

The Rocky Road from Preclinical Findings to Successful Targeted Therapy in Pleural Mesothelioma

Pleural mesothelioma (PM) is a rare and aggressive disease that arises from the mesothelial cells lining the pleural cavity. Approximately 80% of PM patients have a history of asbestos exposure. The long latency period of 20-40 years from the time of asbestos exposure to diagnosis, suggests that multiple somatic genetic alterations are required for the tumorigenesis of PM. The genomic landscape of PM has been characterized by inter- and intratumor heterogeneity associated with the impairment of tumor suppressor genes such as CDKN2A, NF2, and BAP1. Current systemic therapies have shown only limited efficacy, and none is approved for patients with relapsed PM. Advances in understanding of the molecular landscape of PM has facilitated several biomarker-driven clinical trials but so far, no predictive biomarkers for targeted therapies are in clinical use. Recent advances in the PM genetics have provided optimism for successful molecular strategies in the future. Here, we summarize the molecular mechanism underlying PM pathogenesis and review potential therapeutic targets.

The Evolving Therapeutic Landscape for Malignant Pleural Mesothelioma

PURPOSE OF REVIEW

For patients with malignant pleural mesothelioma, prognosis is poor with extremely low 5-year survival rates and limited therapeutic options. Here, we review the current treatment landscape for mesothelioma and highlight promising future therapeutic directions.

RECENT FINDINGS

Evolving frontline therapeutic options for mesothelioma include VEGF inhibition in combination with chemotherapy and dual immune checkpoint inhibition, with synergisms between the therapies and response prediction via biomarkers also being explored. Evolving experimental treatments for mesothelioma include PARP and ALK inhibitors, dendritic and CAR T-cell therapies, anti-mesothelin vaccines, and oncolytic viral therapies, representing timely advances in the field. The therapeutic landscape for malignant pleural mesothelioma is evolving and preferred treatment in the frontline and later settings will likely evolve with it. However, this does not preclude the evidence for including multi-modal therapies spanning angiogenesis and immune checkpoint inhibitors, and biomarker utilization, in current clinical trials and management.

Extracellular Vesicles Isolated from Malignant Mesothelioma Cancer-Associated Fibroblasts Induce Pro-Oncogenic Changes in Healthy Mesothelial Cells

Malignant mesothelioma is an aggressive tumour of the pleura (MPM) or peritoneum with a clinical presentation at an advanced stage of the disease. Current therapies only marginally improve survival and there is an urgent need to identify new treatments. Carcinoma-associated fibroblasts (CAFs) represent the main component of a vast stroma within MPM and play an important role in the tumour microenvironment. The influence of CAFs on cancer progression, aggressiveness and metastasis is well understood; however, the role of CAF-derived extracellular vesicles (CAF-EVs) in the promotion of tumour development and invasiveness is underexplored. We purified CAF-EVs from MPM-associated cells and healthy dermal human fibroblasts and examined their effect on cell proliferation and motility. The data show that exposure of healthy mesothelial cells to EVs derived from CAFs, but not from normal dermal human fibroblasts (NDHF) resulted in activating pro-oncogenic signalling pathways and increased proliferation and motility. Consistent with its role in suppressing Yes-Associated Protein (YAP) activation (which in MPM is a result of Hippo pathway inactivation), treatment with Simvastatin ameliorated the pro-oncogenic effects instigated by CAF-EVs by mechanisms involving both a reduction in EV number and changes in EV cargo. Collectively, these data determine the significance of CAF-derived EVs in mesothelioma development and progression and suggest new targets in cancer therapy.

Molecular Alterations in Malignant Pleural Mesothelioma: A Hope for Effective Treatment by Targeting YAP

Malignant pleural mesothelioma is a rare and aggressive neoplasm, which has primarily been attributed to the exposure to asbestos fibers (83% of cases); yet, despite a ban of using asbestos in many countries, the incidence of malignant pleural mesothelioma failed to decline worldwide. While little progress has been made in malignant pleural mesothelioma diagnosis, bevacizumab at first, then followed by double immunotherapy (nivolumab plus ipilumumab), were all shown to improve survival in large phase III randomized trials. The morphological analysis of the histological subtyping remains the primary indicator for therapeutic decision making at an advanced disease stage, while a platinum-based chemotherapy regimen combined with pemetrexed, either with or without bevacizumab, is still the main treatment option. Consequently, malignant pleural mesothelioma still represents a significant health concern owing to poor median survival (12-18 months). Given this context, both diagnosis and therapy improvements require better knowledge of the molecular mechanisms underlying malignant pleural mesothelioma's carcinogenesis and progression. Hence, the Hippo pathway in malignant pleural mesothelioma initiation and progression has recently received increasing attention, as the aberrant expression of its core components may be closely related to patient prognosis. The purpose of this review was to provide a critical analysis of our current knowledge on these topics, the main focus being on the available evidence concerning the role of each Hippo pathway's member as a promising biomarker, enabling detection of the disease at earlier stages and thus improving prognosis.

Dysregulation of microRNAs and tRNA-derived ncRNAs in mesothelial and mesothelioma cell lines after asbestiform fiber exposure

Experimental evidence demonstrated that fluoro-edenite (FE) can develop chronic respiratory diseases and elicit carcinogenic effects. Environmental exposure to FE fibers is correlated with malignant pleural mesothelioma (MPM). An early diagnosis of MPM, and a comprehensive health monitoring of the patients exposed to FE fibers are two clinical issues that may be solved by the identification of specific biomarkers. We reported the microRNA (miRNA) and transfer RNA-derived non coding RNA (tRNA-derived ncRNA) transcriptome in human normal mesothelial and malignant mesothelioma cell lines exposed or not exposed to several concentration FE fibers. Furthermore, an interactive mesothelioma-based network was derived by using NetME tool. In untreated condition, the expression of miRNAs and tRNA-derived ncRNAs in tumor cells was significantly different with respect to non-tumor samples. Moreover, interesting and significant changes were found after the exposure of both cells lines to FE fibers. The network-based pathway analysis showed several signaling and metabolic pathways potentially involved in the pathogenesis of MPM. From papers analyzed by NetME, it is clear that many miRNAs can positively or negatively influence various pathways involved in MPM. For the first time, the analysis of tRNA-derived ncRNAs molecules in the context of mesothelioma has been made by using in vitro systems. Further studies will be designed to test and validate their diagnostic potential in high-risk individuals' liquid biopsies.

ATG7 immunohistochemical expression in malignant pleural mesothelioma. A preliminary report

Literature evidence has demonstrated a high incidence of asbestos-related malignant pleural mesothelioma (MPM) in a Sicilian town (Biancavilla, Italy), where fluoro-edenite (FE) fibers were discovered some decades ago. As ATG7 immunohistochemical analysis has been ascribed as a prognostic tool of improved survival, we decided to investigate, in MPM patients, exposed and not exposed to FE fibers, the immunohistochemical expression of this autophagy-related protein named ATG7. We analyzed the correlation between ATG7 immunohistochemical level and clinicopathological parameters. Twenty MPM tissue samples, from patients with available clinical and follow-up data, were included in paraffin and processed for immunohistochemistry. The immunohistochemical results confirmed activation of the autophagic process in MPM. Densitometric and morphometric expressions of ATG7 were significantly increased in MPMs when compared to the control tissues. A significant association of a high level of ATG7 with increased survival was demonstrated, with a mean overall survival (OS) of 12.5 months for patients with high expression vs. a mean OS of 4.5 months for patients with low ATG7 expression. In addition, a significant correlation between ATG7 expression and the survival time of MPM patients was observed. This study represents a starting point to hypothesize the prognostic role of ATG7 which could be a reliable prognostic indicator in MPM.

Extracellular Vesicle Enriched miR-625-3p Is Associated with Survival of Malignant Mesothelioma Patients

Malignant mesothelioma (MM) is characterized by poor prognosis and short survival. Extracellular vesicles (EVs) are membrane-bound particles released from cells into various body fluids, and their molecular composition reflects the characteristics of the origin cell. Blood EVs or their miRNA cargo might serve as new minimally invasive biomarkers that would enable earlier detection of MM or treatment outcome prediction. Our aim was to evaluate miRNAs enriched in serum EVs as potential prognostic biomarkers in MM patients in a pilot longitudinal study. EVs were isolated from serum samples obtained before and after treatment using ultracentrifugation on 20% sucrose cushion. Serum EV-enriched miR-103-3p, miR-126-3p and miR-625-3p were quantified using qPCR. After treatment, expression of miR-625-3p and miR-126-3p significantly increased in MM patients with poor treatment outcome (p = 0.012 and p = 0.036, respectively). A relative increase in miR-625-3p expression after treatment for more than 3.2% was associated with shorter progression-free survival (7.5 vs. 19.4 months, HR = 3.92, 95% CI = 1.20-12.80, p = 0.024) and overall survival (12.5 vs. 49.1 months, HR = 5.45, 95% CI = 1.06-28.11, p = 0.043) of MM patients. Bioinformatic analysis showed enrichment of 33 miR-625-3p targets in eight biological pathways. Serum EV-enriched miR-625-3p could therefore serve as a prognostic biomarker in MM and could contribute to a more personalized treatment.

Biological basis for novel mesothelioma therapies

Mesothelioma is an aggressive cancer that is associated with exposure to asbestos. Although asbestos is banned in several countries, including the UK, an epidemic of mesothelioma is predicted to affect middle-income countries during this century owing to their heavy consumption of asbestos. The prognosis for patients with mesothelioma is poor, reflecting a failure of conventional chemotherapy that has ultimately resulted from an inadequate understanding of its biology. However, recent work has revolutionised the study of mesothelioma, identifying genetic and pathophysiological vulnerabilities, including the loss of tumour suppressors, epigenetic dysregulation and susceptibility to nutrient stress. We discuss how this knowledge, combined with advances in immunotherapy, is enabling the development of novel targeted therapies.

Combined Analysis of RNA Sequence and Microarray Data Reveals a Competing Endogenous RNA Network as Novel Prognostic Markers in Malignant Pleural Mesothelioma

Malignant pleural mesothelioma (MPM) is a highly aggressive cancer with short survival time. Unbalanced competing endogenous RNAs (ceRNAs) have been shown to participate in the tumor pathogenesis and served as biomarkers for the clinical prognosis. However, the comprehensive analyses of the ceRNA network in the prognosis of MPM are still rarely reported. In this study, we obtained the transcriptome data of the MPM and the normal samples from TCGA, EGA, and GEO databases and identified the differentially expressed (DE) mRNAs, lncRNAs, and miRNAs. The functions of the prognostic genes and the overlapped DEmRNAs were further annotated by the multiple enrichment analyses. Then, the targeting relationships among lncRNA–miRNA and miRNA–mRNA were predicted and calculated, and a prognostic ceRNA regulatory network was established. We included the prognostic 73 mRNAs and 13 miRNAs and 26 lncRNAs into the ceRNA network. Moreover, 33 mRNAs, three miRNAs, and seven lncRNAs were finally associated with prognosis, and a model including seven mRNAs, two lincRNAs, and some clinical factors was finally established and validated by two independent cohorts, where CDK6 and SGMS1-AS1 were significant to be independent prognostic factors. In addition, the identified co-expressed modules associated with the prognosis were overrepresented in the ceRNA network. Multiple enrichment analyses showed the important roles of the extracellular matrix components and cell division dysfunction in the invasion of MPM potentially. In summary, the prognostic ceRNA network of MPM was established and analyzed for the first time and these findings shed light on the function of ceRNAs and revealed the potential prognostic and therapeutic biomarkers of MPM.

RGS12 is a novel tumor suppressor in osteosarcoma that inhibits YAP-TEAD1-Ezrin signaling

Osteosarcoma (OS) is the most common primary malignancy of the bone that predominantly affects children and adolescents. Hippo pathway is a crucial regulator of organ size and tumorigenesis. However, how Hippo pathway regulates the occurrence of osteosarcoma is largely unknown. Here, we reported the regulator of G protein signaling protein 12 (RGS12) is a novel Hippo pathway regulator and tumor suppressor of osteosarcoma. Depletion of Rgs12 promotes osteosarcoma progression and lung metastasis in an orthotopic xenograft mouse model. Our data showed that the knockdown of RGS12 upregulates Ezrin expression through promoting the GNA12/13-RhoA-YAP pathway. Moreover, RGS12 negatively regulates the transcriptional activity of YAP/TEAD1 complex through its PDZ domain function to inhibit the expression and function of the osteosarcoma marker Ezrin. PDZ domain peptides of RGS12 can inhibit the development of intratibial tumor and lung metastases. Collectively, this study identifies that the RGS12 is a novel tumor suppressor in osteosarcoma through inhibiting YAP-TEAD1-Ezrin signaling pathway and provides a proof of principle that targeting RGS12 may be a therapeutic strategy for osteosarcoma.

NF2 and Canonical Hippo-YAP Pathway Define Distinct Tumor Subsets Characterized by Different Immune Deficiency and Treatment Implications in Human Pleural Mesothelioma

Simple Summary

It is a long-held notion that loss-of-function mutations in negative regulators of the Hippo-YAP pathway, such as NF2, LATS1/2, have a similar potential to promote nuclear YAP activity, which is thought to play an essential role in the pathogenesis of MPM. Whether loss-of-function in these individual regulators uniformly affects the Hippo-YAP activity and contributes to a similar disease phenotype has not yet been revealed in MPM. Surprisingly and interestingly, we found in this study that loss-of-function in the upstream regulator NF2 of the Hippo pathway is linked to the aberrant activation of Hippo-YAP-independent signaling. More importantly, our work showed NF2 loss-of-function and dysregulated Hippo-YAP pathway define distinct MPM subsets that differ in molecular features, therapeutic implications, patients’ prognosis, and in particular, infiltrative immune signatures. Our findings in this study may be instrumental for the precise management of immunotherapy and/or targeted therapy for MPM patients.

Abstract

(1) Inactivation of the tumor suppressor NF2 is believed to play a major role in the pathogenesis of malignant pleural mesothelioma (MPM) by deregulating the Hippo-YAP signaling pathway. However, NF2 has functions beyond regulation of the Hippo pathway, raising the possibility that NF2 contributes to MPM via Hippo-independent mechanisms. (2) We performed weighted gene co-expression analysis (WGCNA) in transcriptomic and proteomic datasets obtained from The Cancer Gene Atlas (TCGA) MPM cohort to identify clusters of co-expressed genes highly correlated with NF2 and phospho (p)-YAP protein, surrogate markers of active Hippo signaling and YAP inactivation. The potential targets are experimentally validated using a cell viability assay. (3) MPM tumors with NF2 loss-of-function are not associated with changes in p-YAP level nor YAP/TAZ activity score, but are characterized by a deficient B-cell receptor (BCR) signaling pathway. Conversely, MPM tumors with YAP activation display exhausted CD8 T-cell-mediated immunity together with significantly upregulated PD-L1, which is validated in an independent MPM cohort, suggesting a potential benefit of immune-checkpoint inhibitors (ICI) in this patient subset. In support of this, mutations in core Hippo signaling components including LATS2, but not NF2, are independently associated with better overall survival in response to ICI in patients. Additionally, based on cancer cell line models, we show that MPM cells with a high Hippo-YAP activity are particularly sensitive to inhibitors of BCR-ABL/SRC, stratifying a unique MPM patient subset that may benefit from BCR-ABL/SRC therapies. Furthermore, we observe that NF2 physically interacts with a considerable number of proteins that are not involved in the canonical Hippo-YAP pathway, providing a possible explanation for its Hippo-independent role in MPM. Finally, survival analyses show that YAP/TAZ scores together with p-YAP protein level, but not NF2, predict the prognosis of MPM patients. (4) NF2 loss-of-function and dysregulated Hippo-YAP pathway define distinct MPM subsets that differ in their molecular features and prognosis, which has important clinical implications for precision oncology in MPM patients.

Biomarker-guided targeted and immunotherapies in malignant pleural mesothelioma

Malignant pleural mesothelioma (MPM) is a lethal thoracic malignancy whose incidence is still increasing worldwide. MPM is characterized by frequent inactivation of tumor-suppressor genes (TSGs), e.g., the homozygous deletion of CDKN2A/2B and various genetic alterations that inactivate BAP1, NF2, LATS1/2, and TP53. The leading cause for the poor prognosis of patients with MPM is the lack of effective treatment options, with conventional chemotherapy being the standard of care in the clinic, which has remained unchanged for almost 20 years. Precision oncology, a burgeoning effort to provide precise cancer treatment tailored to unique molecular changes in individual patients, has made tremendous progress in the last decade in several cancers, but not in MPM. Recent studies indicate a high degree of tumor heterogeneity in MPM and the importance to optimize histological and molecular classifications for improved treatment. In this review, we provide an up-to-date overview of recent advances in MPM by focusing on new stratifications of tumor subgroups, specific vulnerabilities associated with functional loss of TSGs and other biomarkers, and potential clinical implications. The molecularly based subdivisions not only deepen our understanding of MPM pathobiology, but more importantly, they may raise unprecedented new hopes for personalized treatment of MPM patients with biomarker-guided targeted and immunotherapies.

The Role of Photoactivated and Non-Photoactivated Verteporfin on Tumor

Verteporfin (VP) has long been clinically used to treat age-related macular degeneration (AMD) through photodynamic therapy (PDT). Recent studies have reported a significant anti-tumor effect of VP as well. Yes-associated protein (YAP) is a pro-tumorigenic factor that is aberrantly expressed in various cancers and is a central effector of the Hippo signaling pathway that regulates organ size and tumorigenesis. VP can inhibit YAP without photoactivation, along with suppressing autophagy, and downregulating germinal center kinase-like kinase (GLK) and STE20/SPS1-related proline/alanine-rich kinase (SPAK). In addition, VP can induce mitochondrial damage and increase the production of reactive oxygen species (ROS) upon photoactivation, and is an effective photosensitizer (PS) in anti-tumor PDT. We have reviewed the direct and adjuvant therapeutic action of VP as a PS, and its YAP/TEA domain (TEAD)-dependent and independent pharmacological effects in the absence of light activation against cancer cells and solid tumors. Based on the present evidence, VP may be repositioned as a promising anti-cancer chemotherapeutic and adjuvant drug.

Therapeutic Landscape of Malignant Pleural Mesothelioma: Collateral Vulnerabilities and Evolutionary Dependencies in the Spotlight

Malignant pleural mesothelioma (MPM) is the epitome of a recalcitrant cancer driven by pharmacologically intractable tumor suppressor proteins. A significant but largely unmet challenge in the field is the translation of genetic information on alterations in tumor suppressor genes (TSGs) into effective cancer-specific therapies. The notion that abnormal tumor genome subverts physiological cellular processes, which creates collateral vulnerabilities contextually related to specific genetic alterations, offers a promising strategy to target TSG-driven MPM. Moreover, emerging evidence has increasingly appreciated the therapeutic potential of genetic and pharmacological dependencies acquired en route to cancer development and drug resistance. Here, we review the most recent progress on vulnerabilities co-selected by functional loss of major TSGs and dependencies evolving out of cancer development and resistance to cisplatin based chemotherapy, the only first-line regimen approved by the US Food and Drug Administration (FDA). Finally, we highlight CRISPR-based functional genomics that has emerged as a powerful platform for cancer drug discovery in MPM. The repertoire of MPM-specific “Achilles heel” rises on the horizon, which holds the promise to elucidate therapeutic landscape and may promote precision oncology for MPM.

Genomics and Functional Genomics of Malignant Pleural Mesothelioma

Malignant pleural mesothelioma (MPM) is a rare, aggressive cancer of the mesothelial cells lining the pleural surface of the chest wall and lung. The etiology of MPM is strongly associated with prior exposure to asbestos fibers, and the median survival rate of the diagnosed patients is approximately one year. Despite the latest advancements in surgical techniques and systemic therapies, currently available treatment modalities of MPM fail to provide long-term survival. The increasing incidence of MPM highlights the need for finding effective treatments. Targeted therapies offer personalized treatments in many cancers. However, targeted therapy in MPM is not recommended by clinical guidelines mainly because of poor target definition. A better understanding of the molecular and cellular mechanisms and the predictors of poor clinical outcomes of MPM is required to identify novel targets and develop precise and effective treatments. Recent advances in the genomics and functional genomics fields have provided groundbreaking insights into the genomic and molecular profiles of MPM and enabled the functional characterization of the genetic alterations. This review provides a comprehensive overview of the relevant literature and highlights the potential of state-of-the-art genomics and functional genomics research to facilitate the development of novel diagnostics and therapeutic modalities in MPM.

Identification of a Five-Gene Signature for Predicting Survival in Malignant Pleural Mesothelioma Patients

Malignant pleural mesothelioma (MPM), predominantly caused by asbestos exposure, is a highly aggressive cancer with poor prognosis. The staging systems currently used in clinics is inadequate in evaluating the prognosis of MPM. In this study, a five-gene signature was developed and enrolled into a prognostic risk score model by LASSO Cox regression analysis based on two expression profiling datasets (GSE2549 and GSE51024) from Gene Expression Omnibus (GEO). The five-gene signature was further validated using the Cancer Genome Atlas (TCGA) MPM dataset. Univariate and multivariate Cox analyses proved that the five-gene signature was an independent prognostic factor for MPM. The signature remained statistically significant upon stratification by Brigham stage, AJCC stage, gender, tumor size, and lymph node status. Time-dependent receiver operating characteristic (ROC) curve indicated good performance of our model in predicting 1- and 2-years overall survival in MPM patients. The C-index was 0.784 for GSE2549 and 0.753 for the TCGA dataset showing moderate predictive accuracy of our model. Furthermore, Gene Set Enrichment Analysis suggested that the five-gene signature was related to pathways resulting in MPM tumor progression. Together, we have established a five-gene signature significantly associated with prognosis in MPM patients. Hence, the five-genes signature may serve as a potentially useful prognostic tool for MPM patients.

Hippo signaling-a central player in cystic kidney disease?

Cystic transformation of kidney tissue is a key feature of various disorders including autosomal dominant polycystic kidney disease (ADPKD), autosomal recessive polycystic kidney disease (ARPKD), and disorders of the nephronophthisis spectrum (NPH). While ARPKD and NPH typically affect children and adolescents, pediatric onset of ADPKD is less frequently found. While both ADPKD and ARPKD are characterized by formation of hundreds of cysts accompanied by hyperproliferation of tubular epithelia with massive renal enlargement, NPH patients usually show kidneys of normal or reduced size with cysts limited to the corticomedullary border. Recent results suggest the hippo pathway to be a central regulator at the crossroads of the renal phenotype in both diseases. Hippo signaling regulates organ size and proliferation by keeping the oncogenic transcriptional co-activators Yes associated protein 1 (YAP) and WW domain containing transcription regulator 1 (TAZ) in check. Once this inhibition is released, nuclear YAP/TAZ interacts with TEAD family transcription factors and the consecutive transcriptional activation of TEA domain family members (TEAD) target genes mediates an increase in proliferation. Here, we review the current knowledge on the impact of NPHP and ADPKD mutations on Hippo signaling networks. Furthermore, we provide an outlook towards potential future therapeutic strategies targeting Hippo signaling to alleviate cystic kidney disease.

The Crosstalk between Src and Hippo/YAP Signaling Pathways in Non-Small Cell Lung Cancer (NSCLC)

The advancement of new therapies, including targeted therapies and immunotherapies, has improved the survival of non-small-cell lung cancer (NSCLC) patients in the last decade. Some NSCLC patients still do not benefit from therapies or encounter progressive disease during the course of treatment because they have intrinsic resistance, acquired resistance, or lack a targetable driver mutation. More investigations on the molecular biology of NSCLC are needed to find useful biomarkers for current therapies and to develop novel therapeutic strategies. Src is a non-receptor tyrosine kinase protein that interacts with cell surface growth factor receptors and the intracellular signaling pathway to maintain cell survival tumorigenesis in NSCLC. The Yes-associated protein (YAP) is one of the main effectors of the Hippo pathway and has been identified as a promoter of drug resistance, cancer progression, and metastasis in NSCLC. Here, we review studies that have investigated the activation of YAP as mediated by Src kinases and demonstrate that Src regulates YAP through three main mechanisms: (1) direct phosphorylation; (2) the activation of pathways repressing Hippo kinases; and (3) Hippo-independent mechanisms. Further work should focus on the efficacy of Src inhibitors in inhibiting YAP activity in NSCLC. In addition, future efforts toward developing potentially reasonable combinations of therapy targeting the Src–YAP axis using other therapies, including targeted therapies and/or immunotherapies, are warranted.

PRMT5 silencing selectively affects MTAP-deleted mesothelioma: In vitro evidence of a novel promising approach

Malignant mesothelioma (MM) is an aggressive asbestos‐related cancer of the serous membranes. Despite intensive treatment regimens, MM is still a fatal disease, mainly due to the intrinsic resistance to current therapies and the lack of predictive markers and new valuable molecular targets. Protein arginine methyltransferase 5 (PRMT5) inhibition has recently emerged as a potential therapy against methylthioadenosine phosphorylase (MTAP)‐deficient cancers, in which the accumulation of the substrate 5'‐methylthioadenosine (MTA) inhibits PRMT5 activity, thus sensitizing the cells to further PRMT5 inhibition. Considering that the MTAP gene is frequently codeleted with the adjacent cyclin‐dependent kinase inhibitor 2A (CDKN2A) locus in MM, we assessed whether PRMT5 could represent a therapeutic target also for this cancer type. We evaluated PRMT5 expression, the MTAP status and MTA content in normal mesothelial and MM cell lines. We found that both administration of exogenous MTA and stable PRMT5 knock‐down, by short hairpin RNAs (shRNAs), selectively reduced the growth of MTAP‐deleted MM cells. We also observed that PRMT5 knock‐down in MTAP‐deficient MM cells reduced the expression of E2F1 target genes involved in cell cycle progression and of factors implicated in epithelial‐to‐mesenchymal transition. Therefore, PRMT5 targeting could represent a promising new therapeutic strategy against MTAP‐deleted MMs.

The YAP1 Signaling Inhibitors, Verteporfin and CA3, Suppress the Mesothelioma Cancer Stem Cell Phenotype

Mesothelioma is an aggressive cancer that has a poor prognosis. Tumors develop in the mesothelial lining of the pleural and peritoneal cavities in response to asbestos exposure. Surgical debulking followed by chemotherapy is initially effective, but this treatment ultimately selects for resistant cells that form aggressive and therapy-resistant recurrent tumors. Mesothelioma cancer stem cells (MCS) are a highly aggressive subpopulation present in these tumors that are responsible for tumor maintenance and drug resistance. In this article, we examine the impact of targeting YAP1/TAZ/TEAD signaling in MCS cells. YAP1, TAZ, and TEADs are transcriptional mediators of the Hippo signaling cascade that activate gene expression to drive tumor formation. We show that two YAP1 signaling inhibitors, verteporfin and CA3, attenuate the MCS cell phenotype. Verteporfin or CA3 treatment reduces YAP1/TEAD level/activity to suppress MCS cell spheroid formation, Matrigel invasion, migration, and tumor formation. These agents also increase MCS cell apoptosis. Moreover, constitutively active YAP1 expression antagonizes inhibitor action, suggesting that loss of YAP1/TAZ/TEAD signaling is required for response to verteporfin and CA3. These agents are active against mesothelioma cells derived from peritoneal (epithelioid) and patient-derived pleural (sarcomatoid) mesothelioma, suggesting that targeting YAP1/TEAD signaling may be a useful treatment strategy. IMPLICATIONS: These studies suggest that inhibition of YAP1 signaling may be a viable approach to treating mesothelioma.

Long Non-coding RNA MIR4435-2HG Promotes Colorectal Cancer Proliferation and Metastasis Through miR-206/YAP1 Axis

Objective: Long non-coding RNAs (lncRNAs) are critical to colorectal cancer (CRC) progression. In the current study, the objective was the exploration of the role played by lncRNA MIR4435-2HG in CRC proliferation and metastasis.

Methods: lncRNA MIR4435-2HG expression and its association with CRC were analyzed using database and clinical specimens. The influences exerted by MIR4435-2HG on cell proliferating process, invading process, and migrating process of CRC were identified after MIR4435-2HG knockdown. The influences exerted by MIR4435-2HG on tumor growth and metastasis were assessed in vivo. The underlying mechanistic associations between MIR4435-2HG, microRNA miR-206, and the transcription factor Yes-associated protein 1 (YAP1) were assessed using bioinformatics and a luciferase reporter gene assay.

Results: MIR4435-2HG was highly expressed in CRC tissue in contrast with that in regular tissues and displayed relations to poor prognosis. MIR4435-2HG knockdown could suppress CRC cell proliferation, invasion, and migration. Moreover, MIR4435-2HG knockdown inhibited CRC growth and liver metastasis in vitro. We found MIR4435-2HG knockdown reduced YAP1, CTGF, AREG, vimentin, Snail, Slug, and Twist expression but enhanced E-cadherin expression. Functionally, MIR4435-2HG acted as a competing endogenous RNA (ceRNA) to upregulate YAP1 by sponging miR-206.

Conclusions: MIR4435-2HG promoted CRC growth and metastasis through miR-206/YAP1 axis and is likely to play prognostic marker roles and be therapeutically targeted in CRC.

Inhibition of cyclin-dependent kinase 7 down-regulates yes-associated protein expression in mesothelioma cells

Cyclin-dependent kinase 7 (CDK7) is a protein kinase that plays a major role in transcription initiation. Yes-associated protein (YAP) is a main effector of the Hippo/YAP signalling pathway. Here, we investigated the role of CDK7 on YAP regulation in human malignant pleural mesothelioma (MPM). We found that in microarray samples of human MPM tissue, immunohistochemistry staining showed correlation between the expression level of CDK7 and YAP (n = 70, r = .513). In MPM cells, CDK7 expression level was significantly correlated with GTIIC reporter activity (r = .886, P = .019). Inhibition of CDK7 by siRNA decreased the YAP protein level and the GTIIC reporter activity in the MPM cell lines 211H, H290 and H2052. Degradation of the YAP protein was accelerated after CDK7 knockdown in 211H, H290 and H2052 cells. Inhibition of CDK7 reduced tumour cell migration and invasion, as well as tumorsphere formation ability. Restoration of the CDK7 gene rescued the YAP protein level and GTIIC reporter activity after siRNA knockdown in 211H and H2052 cells. Finally, we performed a co-immunoprecipitation analysis using an anti-YAP antibody and captured the CDK7 protein in 211H cells. Our results suggest that CDK7 inhibition reduces the YAP protein level by promoting its degradation and suppresses the migration and invasion of MPM cells. Cyclin-dependent kinase 7 may be a promising therapeutic target for MPM.

Epidermal Growth Factor Receptor (EGFR) Pathway, Yes-Associated Protein (YAP) and the Regulation of Programmed Death-Ligand 1 (PD-L1) in Non-Small Cell Lung Cancer (NSCLC)

The epidermal growth factor receptor (EGFR) pathway is a well-studied oncogenic pathway in human non-small cell lung cancer (NSCLC). A subset of advanced NSCLC patients (15–55%) have EGFR-driven mutations and benefit from treatment with EGFR-tyrosine kinase inhibitors (TKIs). Immune checkpoint inhibitors (ICIs) targeting the PD-1/PDL-1 axis are a new anti-cancer therapy for metastatic NSCLC. The anti-PD-1/PDL-1 ICIs showed promising efficacy (~30% response rate) and improved the survival of patients with metastatic NSCLC, but the role of anti-PD-1/PDL-1 ICIs for EGFR mutant NSCLC is not clear. YAP (yes-associated protein) is the main mediator of the Hippo pathway and has been identified as promoting cancer progression, drug resistance, and metastasis in NSCLC. Here, we review recent studies that examined the correlation between the EGFR, YAP pathways, and PD-L1 and demonstrate the mechanism by which EGFR and YAP regulate PD-L1 expression in human NSCLC. About 50% of EGFR mutant NSCLC patients acquire resistance to EGFR-TKIs without known targetable secondary mutations. Targeting YAP therapy is suggested as a potential treatment for NSCLC with acquired resistance to EGFR-TKIs. Future work should focus on the efficacy of YAP inhibitors in combination with immune checkpoint PD-L1/PD-1 blockade in EGFR mutant NSCLC without targetable resistant mutations.

Dissecting heterogeneity in malignant pleural mesothelioma through histo-molecular gradients for clinical applications

Malignant pleural mesothelioma (MPM) is recognized as heterogeneous based both on histology and molecular profiling. Histology addresses inter-tumor and intra-tumor heterogeneity in MPM and describes three major types: epithelioid, sarcomatoid and biphasic, a combination of the former two types. Molecular profiling studies have not addressed intra-tumor heterogeneity in MPM to date. Here, we use a deconvolution approach and show that molecular gradients shed new light on the intra-tumor heterogeneity of MPM, leading to a reconsideration of MPM molecular classifications. We show that each tumor can be decomposed as a combination of epithelioid-like and sarcomatoid-like components whose proportions are highly associated with the prognosis. Moreover, we show that this more subtle way of characterizing MPM heterogeneity provides a better understanding of the underlying oncogenic pathways and the related epigenetic regulation and immune and stromal contexts. We discuss the implications of these findings for guiding therapeutic strategies, particularly immunotherapies and targeted therapies.

A functional interaction between Hippo-YAP signalling and SREBPs mediates hepatic steatosis in diabetic mice

The Hippo pathway is an evolutionarily conserved regulator of organ size and tumorigenesis that negatively regulates cell growth and survival. Whether the Hippo pathway regulates cell metabolism is unknown. Here, we report that in the nucleus of hepatocytes, Yes‐associated protein(YAP)—the terminal effector of the Hippo pathway—directly interacts with sterol regulatory element binding proteins (SREBP‐1c and SREBP‐2) on the promoters of the fatty acid synthase (FAS) and 30‐hydroxylmethyl glutaryl coenzyme A reductase (HMGCR), thereby stimulating their transcription and promoting hepatocyte lipogenesis and cholesterol synthesis. In diet‐induced diabetic mice, either Lats1 overexpression or YAP knockdown protects against hepatic steatosis and hyperlipidaemia through suppression of the interaction between YAP and SREBP‐1c/SREBP‐2. These results suggest that YAP is a nuclear co‐factor of SREBPs and that the Hippo pathway negatively affects hepatocyte lipogenesis by inhibiting the function of YAP‐SREBP complexes.

WW Domain-Containing Proteins YAP and TAZ in the Hippo Pathway as Key Regulators in Stemness Maintenance, Tissue Homeostasis, and Tumorigenesis

The Hippo pathway is a conserved signaling pathway originally defined in Drosophila melanogaster two decades ago. Deregulation of the Hippo pathway leads to significant overgrowth in phenotypes and ultimately initiation of tumorigenesis in various tissues. The major WW domain proteins in the Hippo pathway are YAP and TAZ, which regulate embryonic development, organ growth, tissue regeneration, stem cell pluripotency, and tumorigenesis. Recent reports reveal the novel roles of YAP/TAZ in establishing the precise balance of stem cell niches, promoting the production of induced pluripotent stem cells (iPSCs), and provoking signals for regeneration and cancer initiation. Activation of YAP/TAZ, for example, results in the expansion of progenitor cells, which promotes regeneration after tissue damage. YAP is highly expressed in self-renewing pluripotent stem cells. Overexpression of YAP halts stem cell differentiation and yet maintains the inherent stem cell properties. A success in reprograming iPSCs by the transfection of cells with Oct3/4, Sox2, and Yap expression constructs has recently been shown. In this review, we update the current knowledge and the latest progress in the WW domain proteins of the Hippo pathway in relevance to stem cell biology, and provide a thorough understanding in the tissue homeostasis and identification of potential targets to block tumor development. We also provide the regulatory role of tumor suppressor WWOX in the upstream of TGF-β, Hyal-2, and Wnt signaling that cross talks with the Hippo pathway.

MST1/Hippo promoter gene methylation predicts poor survival in patients with malignant pleural mesothelioma in the IFCT-GFPC-0701 MAPS Phase 3 trial

BACKGROUND

The Mesothelioma Avastin Cisplatin Pemetrexed Study (MAPS/NCT00651456) phase 3 trial demonstrated the superiority of bevacizumab plus pemetrexed-cisplatin triplet over chemotherapy alone in 448 malignant pleural mesothelioma (MPM) patients. Here, we evaluated the prognostic role of Hippo pathway gene promoter methylation.

METHODS

Promoter methylations were assayed using methylation-specific polymerase chain reaction in samples from 223 MAPS patients, evaluating their prognostic value for overall survival (OS) and disease-free survival in univariate and multivariate analyses. MST1 inactivation effects on invasion, soft agar growth, apoptosis, proliferation, and YAP/TAZ activation were investigated in human mesothelial cell lines.

RESULTS

STK4 (MST1) gene promoter methylation was detected in 19/223 patients tested (8.5%), predicting poorer OS in univariate and multivariate analyses (adjusted HR: 1.78, 95% CI (1.09-2.93), p = 0.022). Internal validation by bootstrap resampling supported this prognostic impact. MST1 inactivation reduced cellular basal apoptotic activity while increasing proliferation, invasion, and soft agar or in suspension growth, resulting in nuclear YAP accumulation, yet TAZ cytoplasmic retention in mesothelial cell lines. YAP silencing decreased invasion of MST1-depleted mesothelial cell lines.

CONCLUSIONS

MST1/hippo kinase expression loss is predictive of poor prognosis in MPM patients, leading to nuclear YAP accumulation and electing YAP as a putative target for therapeutic intervention in human MPM.

The Role of Yes-Associated Protein (YAP) in Regulating Programmed Death-Ligand 1 (PD-L1) in Thoracic Cancer

The programmed death-ligand 1(PD-L1)/PD-1 pathway is an immunological checkpoint in cancer cells. The binding of PD-L1 and PD-1 promotes T-cell tolerance and helps tumor cells escape from host immunity. Immunotherapy targeting the PD-L1/PD-1 axis has been developed as an anti-cancer therapy and used in treating advanced human non-small cell lung cancer (NSCLC) and malignant pleural mesothelioma (MPM). Yes-associated protein (YAP) is a key mediator of the Hippo/YAP signaling pathway, and plays important roles in promoting cancer development, drug resistance and metastasis in human NSCLC and MPM. YAP has been suggested as a new therapeutic target in NSCLC and MPM. The role of YAP in regulating tumor immunity such as PD-L1 expression has just begun to be explored, and the correlation between YAP-induced tumorigenesis and host anti-tumor immune responses is not well known. Here, we review recent studies investigating the correlation between YAP and PD-L1 and demonstrating the mechanism by which YAP regulates PD-L1 expression in human NSCLC and MPM. Future work should focus on the interactions between Hippo/YAP signaling pathways and the immune checkpoint PD-L1/PD-1 pathway. The development of new synergistic drugs for immune checkpoint PD-L1/PD-1 blockade in NSCLC and MPM is warranted.

Assessment of signaling pathway inhibitors and identification of predictive biomarkers in malignant pleural mesothelioma

OBJECTIVES

Malignant pleural mesothelioma (MPM) is an aggressive tumor with limited therapeutic options, requiring the development of efficient targeted therapies based on molecular phenotype of the tumor and to identify predictive biomarkers of the response.

MATERIALS AND METHODS

The effect of inhibitors was investigated by cell viability assessment on primary MPM cell lines established in our laboratory from patient tumors, well characterized at the molecular level. Effects on apoptosis, cell proliferation and viability on MPM growing in multicellular spheroid were also assessed for verteporfin. Gene and protein expression, and gene knockdown by RNA interference were used to define mechanism of inhibition and specific predictive biomarkers.

RESULTS

Anti-tumor effect of eight major signaling pathways inhibitors involved in mesothelial carcinogenesis was investigated. Three inhibitors were more efficient than cisplatin, the drug used as first-line chemotherapy in patients with MPM: verteporfin, a putative YAP inhibitor, defactinib, a FAK inhibitor and NSC668394, an Ezrin inhibitor. Verteporfin, the most efficient inhibitor, induced cell proliferation arrest and cell death, and is effective on 3D spheroid multicellular model. Verteporfin sensitivity was YAP-independent and related to molecular classification of the tumors. Biomarkers based on gene expression were identified to predict accurately sensitivity to these three inhibitors.

CONCLUSION

Our study shows that drug screening on well-characterized MPM cells allows for the identification of novel potential therapeutic strategies and defining specific biomarkers predictive of the drug response.

Hippo signaling dysfunction induces cancer cell addiction to YAP

Over the past decades, the Hippo has been established as a crucial pathway involved in organ size control and cancer suppression. Dysregulation of Hippo signaling and hyperactivation of its downstream effector YAP are frequently associated with various human cancers. However, the underlying significance of such YAP activation in cancer development and therapy has not been fully characterized. In this study, we reported that the Hippo signaling deficiency can lead to a YAP-dependent oncogene addiction for cancer cells. Through a clinical compound library screen, we identified histone deacetylase (HDAC) inhibitors as putative inhibitors to suppress YAP expression. Importantly, HDAC inhibitors specifically targeted the viability and xenograft tumor growth for the cancer cells in which YAP is constitutively active. Taken together, our results not only establish an active YAP-induced oncogene addiction in cancer cells, but also lay the foundation to develop targeted therapies for the cancers with Hippo dysfunction and YAP activation.

Epithelioid Hemangioendothelioma as a Model of YAP/TAZ-Driven Cancer: Insights from a Rare Fusion Sarcoma

Epithelioid hemangioendothelioma (EHE) is a rare soft-tissue sarcoma involving cells with histologic markers that suggest an endothelial origin. Around 90% of EHEs are caused by the fusion of Transcriptional Co-activator with a PDZ-motif (TAZ) with Calmodulin Binding Transcription Activator 1 (CAMTA1), a central nervous system-specific transcription activator. The 10% of EHEs that lack the TAZ–CAMTA1 fusion instead have a fusion of Yes-associated Protein (YAP) and Transcription Factor E3 (TFE3) genes (YAP-TFE3). YAP and TAZ are well-defined downstream effectors in the Hippo pathway that promote cell growth when translocated to the nucleus. The TAZ–CAMTA1 fusion transcript is insensitive to the Hippo inhibitory signals that normally prevent this process and thus constitutively activates the TAZ transcriptome. In EHE, this causes tumors to form in a variety of organs and tissue types, most commonly the liver, lung, and bone. Its clinical course is unpredictable and highly variable. TAZ activation is known to contribute to key aspects of the cancer phenotype, including metastasis and fibrosis, and increased expression of TAZ is thought to be causally related to the progression of many cancers, including breast, lung, and liver. Therefore, understanding TAZ biology and the molecular mechanisms by which it promotes unregulated cell proliferation will yield insights and possibly improved treatments for both EHE as well as much more common cancers.

Critical role of the Ror-family of receptor tyrosine kinases in invasion and proliferation of malignant pleural mesothelioma cells

Malignant pleural mesothelioma (MPM) is a highly aggressive tumor with poor prognosis and closely related to exposure to asbestos. MPM is a heterogeneous tumor with three main histological subtypes, epithelioid, sarcomatoid, and biphasic types, among which sarcomatoid type shows the poorest prognosis. The Ror-family of receptor tyrosine kinases, Ror1 and Ror2, is expressed in various types of tumor cells at higher levels and affects their aggressiveness. However, it is currently unknown whether they are expressed in and involved in aggressiveness of MPM. Here, we show that Ror1 and Ror2 are expressed in clinical specimens and cell lines of MPM with different histological features. Studies using MPM cell lines indicate that expression of Ror2 is associated tightly with high invasiveness of MPM cells, whereas Ror1 can contribute to their invasion in the absence of Ror2. However, both Ror1 and Ror2 promote proliferation of MPM cells. We also show that promoted invasion and proliferation of MPM cells by Ror signaling can be mediated by the Rho-family of small GTPases, Rac1, and Cdc42. These findings elucidate the critical role of Ror signaling in promoting invasion and proliferation of MPM cells.

YAP/TAZ Activation as a Target for Treating Metastatic Cancer

Yes-Associated Protein (YAP) and Transcriptional Co-activator with PDZ-binding Motif (TAZ) have both emerged as important drivers of cancer progression and metastasis. YAP and TAZ are often upregulated or nuclear localized in aggressive human cancers. There is abundant experimental evidence demonstrating that YAP or TAZ activation promotes cancer formation, tumor progression, and metastasis. In this review we summarize the evidence linking YAP/TAZ activation to metastasis, and discuss the roles of YAP and TAZ during each step of the metastatic cascade. Collectively, this evidence strongly suggests that inappropriate YAP or TAZ activity plays a causal role in cancer, and that targeting aberrant YAP/TAZ activation is a promising strategy for the treatment of metastatic disease. To this end, we also discuss several potential strategies for inhibiting YAP/TAZ activation in cancer and the challenges each strategy poses.

NF2/Merlin Inactivation and Potential Therapeutic Targets in Mesothelioma

The neurofibromatosis type 2 (NF2) gene encodes merlin, a tumor suppressor protein frequently inactivated in schwannoma, meningioma, and malignant mesothelioma (MM). The sequence of merlin is similar to that of ezrin/radixin/moesin (ERM) proteins which crosslink actin with the plasma membrane, suggesting that merlin plays a role in transducing extracellular signals to the actin cytoskeleton. Merlin adopts a distinct closed conformation defined by specific intramolecular interactions and regulates diverse cellular events such as transcription, translation, ubiquitination, and miRNA biosynthesis, many of which are mediated through Hippo and mTOR signaling, which are known to be closely involved in cancer development. MM is a very aggressive tumor associated with asbestos exposure, and genetic alterations in NF2 that abrogate merlin’s functional activity are found in about 40% of MMs, indicating the importance of NF2 inactivation in MM development and progression. In this review, we summarize the current knowledge of molecular events triggered by NF2/merlin inactivation, which lead to the development of mesothelioma and other cancers, and discuss potential therapeutic targets in merlin-deficient mesotheliomas.

Inhibition of yes-associated protein suppresses brain metastasis of human lung adenocarcinoma in a murine model

Yes‐associated protein (YAP) is a main mediator of the Hippo pathway and promotes cancer development and progression in human lung cancer. We sought to determine whether inhibition of YAP suppresses metastasis of human lung adenocarcinoma in a murine model. We found that metastatic NSCLC cell lines H2030‐BrM3(K‐ras^G12C mutation) and PC9‐BrM3 (EGFR^Δexon19 mutation) had a significantly decreased p‐YAP(S127)/YAP ratio compared to parental H2030 (K‐ras^G12C mutation) and PC9 (EGFR^Δexon19 mutation) cells (P < .05). H2030‐BrM3 cells had significantly increased YAP mRNA and expression of Hippo downstream genes CTGF and CYR61 compared to parental H2030 cells (P < .05). Inhibition of YAP by short hairpin RNA (shRNA) and small interfering RNA (siRNA) significantly decreased mRNA expression in downstream genes CTGF and CYR61 in H2030‐BrM3 cells (P < .05). In addition, inhibiting YAP by YAP shRNA significantly decreased migration and invasion abilities of H2030‐BrM3 cells (P < .05). We are first to show that mice inoculated with YAP shRNA‐transfected H2030‐BrM3 cells had significantly decreased metastatic tumour burden and survived longer than control mice (P < .05). Collectively, our results suggest that YAP plays an important role in promoting lung adenocarcinoma brain metastasis and that direct inhibition of YAP by shRNA suppresses H2030‐BrM3 cell brain metastasis in a murine model.

Inhibition of yes-associated protein down-regulates PD-L1 (CD274) expression in human malignant pleural mesothelioma

Although tumour PD‐L1 (CD274) expression had been used as a predictive biomarker in checkpoint immunotherapy targeting the PD1/PD‐L1 axis in various cancers, the regulation of PD‐L1 (CD274) expression is unclear. Yes‐associated protein (YAP), an important oncogenic protein in Hippo signalling pathway, reportedly promotes cancer development. We investigated whether inhibition of YAP down‐regulates PD‐L1 (CD274) in human malignant pleural mesothelioma (MPM). Western blotting showed that 2 human MPM cell lines (H2052 and 211H) had increased PD‐L1 protein expression compared to H290, MS‐1 and H28 cells. In H2052 and 211H cells, PD‐L1 mRNA expression was significantly increased compared to other MPM cell lines; YAP knockdown by small interfering RNA decreased PD‐L1 protein and mRNA expression. Forced overexpression of the YAP gene increased PD‐L1 protein expression in H2452 cells. Chromatin immunoprecipitation (ChIP) assay showed the precipitation of PD‐L1 enhancer region encompassing 2 putative YAP‐TEAD‐binding sites in H2052 cells. We found that, in human MPM tissue microarray samples, YAP and PD‐L1 concurrently expressed in immunohistochemistry stain (n = 70, P < .05, chi‐square). We conclude that PD‐L1 is correlated with YAP expression, and inhibition of YAP down‐regulates PD‐L1 expression in human MPM. Further study of how YAP regulates PD‐L1 in MPM is warranted.

Targeting the Hippo Pathway Is a New Potential Therapeutic Modality for Malignant Mesothelioma

Malignant mesothelioma (MM) constitutes a very aggressive tumor that arises from the pleural or peritoneal cavities and is highly refractory to conventional therapies. Several key genetic alterations are associated with the development and progression of MM including mutations of the CDKN2A/ARF, NF2, and BAP1 tumor-suppressor genes. Notably, activating oncogene mutations are very rare; thus, it is difficult to develop effective inhibitors to treat MM. The NF2 gene encodes merlin, a protein that regulates multiple cell-signaling cascades including the Hippo pathway. MMs also exhibit inactivation of Hippo pathway components including LATS1/2, strongly suggesting that merlin-Hippo pathway dysregulation plays a key role in the development and progression of MM. Furthermore, Hippo pathway inactivation has been shown to result in constitutive activation of the YAP1/TAZ transcriptional coactivators, thereby conferring malignant phenotypes to mesothelial cells. Critical YAP1/TAZ target genes, including prooncogenic CCDN1 and CTGF, have also been shown to enhance the malignant phenotypes of MM cells. Together, these data indicate the Hippo pathway as a therapeutic target for the treatment of MM, and support the development of new strategies to effectively target the activation status of YAP1/TAZ as a promising therapeutic modality for this formidable disease.

Drug development against the hippo pathway in mesothelioma

Advances in the treatments for malignant pleural mesothelioma (MPM) have been disappointing until recently. Conventional cytotoxic drugs fail in MPM in part because they do not address the cancer stem cell population or stem cell pathways that drive tumor resistance and resurgence following treatment. The Hippo stem cell pathway regulates cell contact inhibition with tumor suppressor genes such as NF2 (Neurofibromatosis 2) upstream controlling YAP (Yes-associated protein 1) oncogenes. NF2 is mutated in 40-50% of all MPM and downstream YAP is constitutively active in greater than 70% of MPM, making the downstream YAP/TEAD (transcriptional enhancer associate domain) complex the ultimate target. Novel small molecule YAP inhibitors are showing promising results in preclinical studies and may prove to be effective chemotherapy drugs in MPM.