TAZ and YAP are frequently activated oncoproteins in sarcomas

Soft Tissue Sarcoma Cancer Stem Cells: An Overview

Soft tissue sarcomas (STSs) are an uncommon group of solid tumors that can arise throughout the human lifespan. Despite their commonality as non-bony cancers that develop from mesenchymal cell precursors, they are heterogeneous in their genetic profiles, histology, and clinical features. This has made it difficult to identify a single target or therapy specific to STSs. And while there is no one cell of origin ascribed to all STSs, the cancer stem cell (CSC) principle—that a subpopulation of tumor cells possesses stem cell-like properties underlying tumor initiation, therapeutic resistance, disease recurrence, and metastasis—predicts that ultimately it should be possible to identify a feature common to all STSs that could function as a therapeutic Achilles' heel. Here we review the published evidence for CSCs in each of the most common STSs, then focus on the methods used to study CSCs, the developmental signaling pathways usurped by CSCs, and the epigenetic alterations critical for CSC identity that may be useful for further study of STS biology. We conclude with discussion of some challenges to the field and future directions.

Injury promotes sarcoma development in a genetically and temporally restricted manner

Cancer results from the accumulation of genetic mutations in a susceptible cell of origin. We and others have also shown that injury promotes sarcoma development, but how injury cooperates with genetic mutations at the earliest stages of tumor formation is not known. Here, we utilized dual recombinase technology to dissect the complex interplay of the timing of KrasG12D activation, p53 deletion, and muscle injury in sarcomagenesis using a primary mouse model of soft tissue sarcoma. When mutations in oncogenic Kras and p53 are separated by 3 weeks, few sarcomas develop without injury. However, the transformation potential of these tumor-initiating cells can be unmasked by muscle injury. In the absence of Kras mutations, injury of the muscle with global deletion of p53 results in sarcomas with amplification of chromosomal regions encompassing the Met or Yap1 gene. These findings demonstrate a complex interplay between the timing of genetic mutations and perturbations in the tumor microenvironment, which provides insight into the earliest stages of sarcoma development.

A comprehensive evaluation of Hippo pathway silencing in sarcomas

TAZ and YAP are transcriptional coactivators negatively regulated by the Hippo pathway that have emerged as key oncoproteins in several cancers including sarcomas. We hypothesized that loss of expression of the Hippo kinases might be a mechanism of activating TAZ and YAP. By immunohistochemistry, TAZ/YAP activated clinical sarcoma samples demonstrated loss of MST1 (47%), MST2 (26%), LATS1 (19%), and LATS2 (27%). Western blot similarly demonstrated loss of MST1 (58%), MST2 (25%), and LATS2 (17%). Treatment with MG132 demonstrated an accumulation of MST2 in 25% of sarcoma cell lines, indicating that proteosomal degradation regulates MST2 expression. qRT-PCR in sarcoma cell lines demonstrated loss of expression of the Hippo kinases at the RNA level, most pronounced in MST1 (42%) and MST2 (25%). 5-azacytidine treatment in sarcoma cell lines modestly reversed expression of predominantly MST1 (8%) and MST2 (17%), indicating CpG island hypermethylation can silence expression of MST1 and MST2. Trichostatin A treatment reversed expression of MST1 (58%) and MST2 (67%), indicating histone deacetylation also plays a role in silencing expression of MST1 and MST2. Loss of expression of the Hippo kinases is frequent in sarcomas and is due to a variety of mechanisms including regulation at the post-translational level and epigenetic silencing.

Potential Value of YAP Staining in Rhabdomyosarcoma

Rhabdomyosarcoma (RMS) is a common malignancy of soft tissue, subclassified as alveolar (ARMS), pleomorphic (PRMS), spindle cell/sclerosing (SRMS), and embryonal (ERMS) types. The Yes-associated protein (YAP) is a member of the Hippo pathway and a transcriptional regulator that controls cell proliferation. We have studied the immunohistochemical expression of YAP in different RMSs, arranged in tissue microarray (TMA) and whole slide formats. Pertinent clinical data including patient age, gender, tumor location, and clinical stage were collected. Out of 96 TMA cases, 30 cases (31%) were pleomorphic, 27 (28%) were embryonal, 24 (25%) alveolar, and 15 (16%) spindle cell. Positive nuclear YAP staining was seen in the PRMS (17/30, 56.7%), SRMS (7/15, 46.7%), ERMS (19/27 or 70%), and less in ARMS (37.5%). YAP nuclear staining was significantly more prevalent in ERMS than ARMS ( p=0.02). Of the 41 whole slide cases, nuclear staining was detected in all ARMS but was restricted in distribution to <30% of the cells, in contrast to ERMS and SRMS, which had diffuse or >30% staining. These results highlight the role of YAP in RMS tumorigenesis, a fact that can be useful in engineering targeted therapy. Restricted nuclear YAP staining (<30% of cells) may be of value in the diagnosis of ARMS.

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.

YAP promotes tumorigenesis and cisplatin resistance in neuroblastoma

The transcriptional co-activator Yes-associated protein (YAP) is essential for Hippo pathway-driven tumorigenesis in various cancers. However, the expression and function of YAP in neuroblastoma remains elusive. Here, we show that YAP was highly expressed in Neuroblastoma (NB) and expression levels correlated with advanced tumor staging. Knockdown of YAP significantly impaired neuroblastoma proliferation, tumorigenesis, and invasion in vitro. Injection of the YAP inhibitor, Peptide 17, dramatically prevented neuroblastoma subcutaneous tumor growth by efficiently downregulating YAP expression in tumors. Additionally, less proliferative and more apoptotic cells were found in the Peptide 17 treatment group. Furthermore, YAP inhibition significantly inhibited cisplatin-resistant neuroblastoma proliferation, tumorigenesis, and invasion in vitro. The combination of Peptide 17 with low-dose cisplatin efficiently impaired cisplatin-resistant NB subcutaneous tumor growth, being as effective as high-dose cisplatin. Notably, the combination therapy caused lesser liver toxicity in mice compared to the high-dose cisplatin treatment group. Collectively, this work identifies YAP as a novel regulator of neuroblastoma proliferation, tumorigenesis, and invasion and indicates that YAP is a potential therapeutic target for cisplatin-resistant neuroblastoma.

The Transcriptional Coactivator TAZ Is a Potent Mediator of Alveolar Rhabdomyosarcoma Tumorigenesis

Purpose: Alveolar rhabdomyosarcoma (aRMS) is a childhood soft tissue sarcoma driven by the signature PAX3-FOXO1 (P3F) fusion gene. Five-year survival for aRMS is <50%, with no improvement in over 4 decades. Although the transcriptional coactivator TAZ is oncogenic in carcinomas, the role of TAZ in sarcomas is poorly understood. The aim of this study was to investigate the role of TAZ in P3F-aRMS tumorigenesis.Experimental Design: After determining from publicly available datasets that TAZ is upregulated in human aRMS transcriptomes, we evaluated whether TAZ is also upregulated in our myoblast-based model of P3F-initiated tumorigenesis, and performed IHC staining of 63 human aRMS samples from tissue microarrays. Using constitutive and inducible RNAi, we examined the impact of TAZ loss of function on aRMS oncogenic phenotypes in vitro and tumorigenesis in vivo Finally, we performed pharmacologic studies in aRMS cell lines using porphyrin compounds, which interfere with TAZ-TEAD transcriptional activity.Results: TAZ is upregulated in our P3F-initiated aRMS model, and aRMS cells and tumors have high nuclear TAZ expression. In vitro, TAZ suppression inhibits aRMS cell proliferation, induces apoptosis, supports myogenic differentiation, and reduces aRMS cell stemness. TAZ-deficient aRMS cells are enriched in G₂-M phase of the cell cycle. In vivo, TAZ suppression attenuates aRMS xenograft tumor growth. Preclinical studies show decreased aRMS xenograft tumor growth with porphyrin compounds alone and in combination with vincristine.Conclusions: TAZ is oncogenic in aRMS sarcomagenesis. While P3F is currently not therapeutically tractable, targeting TAZ could be a promising novel approach in aRMS. Clin Cancer Res; 24(11); 2616-30. ©2018 AACR.

Prognostic value of the Hippo pathway transcriptional coactivators YAP/TAZ and β1-integrin in conventional osteosarcoma

Introduction

Currently, very few studies are available concerning the mammalian Hippo pathway in bone sarcomas. YAP/TAZ transcription co-activators are key downstream effectors of this pathway and may also have oncogenic properties. Additionally, recent in-vitro experiments showed that expression of β1-integrin promoted metastasis in osteosarcomas. This study investigated the expression of YAP/TAZ and β1-integrin in human osteosarcomas.

Materials and methods

We performed automated immunohistochemistry on tissue-microarrays (TMA) in which 69 conventional osteosarcomas biopsies performed prior to chemotherapy were embedded. Cellular localization and semi-quantitative analysis of each immunostain was performed using Immunoreactive Score (IRS) and correlated to clinico-pathological data.

Results

Cytoplasmic expression of β1-integrin was noted in 54/59 osteosarcomas (92%), with 33/59 cases (56%) displaying membranous staining. YAP/TAZ was expressed in 27/45 osteosarcomas (60%), with 14 cases (31%) showing cytoplasmic expression while 13 other cases (28%) displayed nuclear expression. No link was found between YAP/TAZ or β1-integrin expression and response to chemotherapy. In univariate analysis, YAP/TAZ immunoreactive score was pejoratively correlated with overall survival (p = 0.01). Expression of β1-integrin on cell membrane was also pejorative for OS (p = 0.045). In multivariate analysis, YAP/TAZ nuclear expression was an independent prognostic factor for PFS (p = 0.035).

Conclusion

this study indicates that β1-integrin and YAP/TAZ proteins are linked to prognosis and therefore could be therapeutic targets in conventional osteosarcomas.

Overexpression of FZD7 promotes glioma cell proliferation by upregulating TAZ

Gliomas are the most prevalent type of primary brain tumors in adults, accounting for more than 40% of neoplasm in the central nervous system. Frizzled-7 (FZD7) is a seven-pass trans-membrane Wnt receptor that plays a critical role in the development of various tumors. In this study, we detected high-level FZD7 expression in glioma and its overexpression was associated with advanced tumor stage. In vitro functional assays showed that forced overexpression of FZD7 promoted proliferation of gliomas cells, whereas knockdown of endogenous FZD7 significantly suppressed proliferation ability of these cells. In a xenograft assay, FZD7 was also found to promote the growth of glioma cells. We further found that FZD7 could activate transcriptional coactivator with PDZ-binding motif (TAZ), and TAZ was required for FZD7 to promote cell proliferation in glioma. Furthermore, the univariate analysis of survival shows that glioma patients with high FZD7 expression have a shorter survival. In conclusion, our findings demonstrate that FZD7 may promote glioma cell proliferation via upregulation of TAZ.

CD31 Expression Determines Redox Status and Chemoresistance in Human Angiosarcomas

Purpose: Angiosarcomas are soft tissue sarcomas with endothelial differentiation and vasoformative capacity. Most angiosarcomas show strong constitutive expression of the endothelial adhesion receptor CD31/PECAM-1 pointing to an important role of this molecule. However, the biological function of CD31 in angiosarcomas is unknown.Experimental Design: The expression levels of CD31 in angiosarcoma cells and its effects on cell viability, colony formation, and chemoresistance were evaluated in human angiosarcoma clinical samples and in cell lines through isolation of CD31^high and CD31^low cell subsets. The redox-regulatory CD31 function linked to YAP signaling was determined using a CD31-blocking antibody and siRNA approach and was further validated in CD31-knockout endothelial cells.Results: We found that most angiosarcomas contain a small CD31^low cell population. CD31^low cells had lost part of their endothelial properties and were more tumorigenic and chemoresistant than CD31^high cells due to more efficient reactive oxygen species (ROS) detoxification. Active downregulation of CD31 resulted in loss of endothelial tube formation, nuclear accumulation of YAP, and YAP-dependent induction of antioxidative enzymes. Addition of pazopanib, a known enhancer of proteasomal YAP degradation resensitized CD31^low cells for doxorubicin resulting in growth suppression and induction of apoptosis.Conclusions: Human angiosarcomas contain a small aggressive CD31^low population that have lost part of their endothelial differentiation programs and are more resistant against oxidative stress and DNA damage due to intensified YAP signaling. Our finding that the addition of YAP inhibitors can resensitize CD31^low cells toward doxorubicin may aid in the rational development of novel combination therapies to treat angiosarcomas. Clin Cancer Res; 24(2); 460-73. ©2017 AACR.

USP21 regulates Hippo pathway activity by mediating MARK protein turnover

The Hippo pathway, which acts to repress the activity of YAP and TAZ trancriptional co-activators, serve as a barrier for oncogenic transformation. Unlike other oncoproteins, YAP and TAZ are rarely activated by mutations or amplified in cancer. However, elevated YAP/TAZ activity is frequently observed in cancer and often correlates with worse survival. The activity and stability of Hippo pathway components, including YAP/TAZ, AMOT and LATS1/2, are regulated by ubiquitin-mediated protein degradation. Aberrant expression of ubiquitin ligase complexes that regulate the turnover of Hippo components and deubiquitylating enzymes that counteract these ubiquitin ligases have been implicated in human cancer. Here we identify the USP21 deubiquitylating enzyme as a novel regulator of Hippo pathway activity. We provide evidence that USP21 regulates YAP/TAZ activity by controlling the stability of MARK kinases, which promote Hippo signaling. Low expression of USP21 in early stage renal clear cell carcinoma suggests that USP21 may be a useful biomarker.

YAP and the Hippo pathway in pediatric cancer

The Hippo pathway is an important signaling pathway that controls cell proliferation and apoptosis. It is evolutionarily conserved in mammals and is stimulated by cell-cell contact, inhibiting cell proliferation in response to increased cell density. During early embryonic development, the Hippo signaling pathway regulates organ development and size, and its functions result in the coordinated balance between proliferation, apoptosis, and differentiation. Its principal effectors, YAP and TAZ, regulate signaling by the embryonic stem cells and determine cell fate and histogenesis. Dysfunction of this pathway contributes to cancer development in adults and children. Emerging studies have shed light on the upregulation of Hippo pathway members in several pediatric cancers and may offer prognostic information on rhabdomyosarcoma, osteosarcoma, Wilms tumor, neuroblastoma, medulloblastoma, and other brain gliomas. We review the results of such published studies and highlight the potential clinical application of this pathway in pediatric oncologic and pathologic studies. These studies support targeting this pathway as a novel treatment strategy.