Aberrant epithelial morphology and persistent epidermal growth factor receptor signaling in a mouse model of renal carcinoma

Renal Cell Carcinoma of Variant Histology: Biology and Therapies

The term variant histology renal cell carcinomas (vhRCCs), also known as non-clear cell RCCs, refers to a diverse group of malignancies with distinct biologic and therapeutic considerations. The management of vhRCC subtypes is often based on extrapolating results from the more common clear cell RCC studies or basket trials that are not specific to each histology. The unique management of each vhRCC subtype necessitates accurate pathologic diagnosis and dedicated research efforts. Herein, we discuss tailored recommendations for each vhRCC histology informed by ongoing research and clinical experience.

MFN2 suppresses clear cell renal cell carcinoma progression by modulating mitochondria-dependent dephosphorylation of EGFR

BACKGROUND

Clear cell renal cell carcinoma (ccRCC) is the most lethal renal cancer. An overwhelming increase of patients experience tumor progression and unfavorable prognosis. However, the molecular events underlying ccRCC tumorigenesis and metastasis remain unclear. Therefore, uncovering the underlying mechanisms will pave the way for developing novel therapeutic targets for ccRCC. In this study, we sought to investigate the role of mitofusin-2 (MFN2) in supressing ccRCC tumorigenesis and metastasis.

METHODS

The expression pattern and clinical significance of MFN2 in ccRCC were analyzed by using the Cancer Genome Atlas datasets and samples from our independent ccRCC cohort. Both in vitro and in vivo experiments, including cell proliferation, xenograft mouse models and transgenic mouse model, were used to determine the role of MFN2 in regulating the malignant behaviors of ccRCC. RNA-sequencing, mass spectrum analysis, co-immunoprecipitation, bio-layer interferometry and immunofluorescence were employed to elucidate the molecular mechanisms for the tumor-supressing role of MFN2.

RESULTS

we reported a tumor-suppressing pathway in ccRCC, characterized by mitochondria-dependent inactivation of epidermal growth factor receptor (EGFR) signaling. This process was mediated by the outer mitochondrial membrane (OMM) protein MFN2. MFN2 was down-regulated in ccRCC and associated with favorable prognosis of ccRCC patients. in vivo and in vitro assays demonstrated that MFN2 inhibited ccRCC tumor growth and metastasis by suppressing the EGFR signaling pathway. In a kidney-specific knockout mouse model, loss of MFN2 led to EGFR pathway activation and malignant lesions in kidney. Mechanistically, MFN2 preferably binded small GTPase Rab21 in its GTP-loading form, which was colocalized with endocytosed EGFR in ccRCC cells. Through this EGFR-Rab21-MFN2 interaction, endocytosed EGFR was docked to mitochondria and subsequently dephosphorylated by the OMM-residing tyrosine-protein phosphatase receptor type J (PTPRJ).

CONCLUSIONS

Our findings uncover an important non-canonical mitochondria-dependent pathway regulating EGFR signaling by the Rab21-MFN2-PTPRJ axis, which contributes to the development of novel therapeutic strategies for ccRCC.

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.

Renal Deletion of LRRC8/VRAC Channels Induces Proximal Tubulopathy

BACKGROUND

Volume-regulated anion channels (VRACs) are heterohexamers of LRRC8A with LRRC8B, -C, -D, or -E in various combinations. Depending on the subunit composition, these swelling-activated channels conduct chloride, amino acids, organic osmolytes, and drugs. Despite VRACs' role in cell volume regulation, and large osmolarity changes in the kidney, neither the localization nor the function of VRACs in the kidney is known.

METHODS

Mice expressing epitope-tagged LRRC8 subunits were used to determine the renal localization of all VRAC subunits. Mice carrying constitutive deletions of Lrrc8b-e, or with inducible or cell-specific ablation of Lrrc8a, were analyzed to assess renal functions of VRACs. Analysis included histology, urine and serum parameters in different diuresis states, and metabolomics.

RESULTS

The kidney expresses all five VRAC subunits with strikingly distinct localization. Whereas LRRC8C is exclusively found in vascular endothelium, all other subunits are found in the nephron. LRRC8E is specific for intercalated cells, whereas LRRC8A, LRRC8B, and LRRC8D are prominent in basolateral membranes of proximal tubules. Conditional deletion of LRRC8A in proximal but not distal tubules and constitutive deletion of LRRC8D cause proximal tubular injury, increased diuresis, and mild Fanconi-like symptoms.

CONCLUSIONS

VRAC/LRRC8 channels are crucial for the function and integrity of proximal tubules, but not for more distal nephron segments despite their larger need for volume regulation. LRRC8A/D channels may be required for the basolateral exit of many organic compounds, including cellular metabolites, in proximal tubules. Proximal tubular injury likely results from combined accumulation of several transported molecules in the absence of VRAC channels.

Systemic Therapies for the Management of Non-Clear Cell Renal Cell Carcinoma: What Works, What Doesn't, and What the Future Holds

Non-clear cell renal cell carcinoma (nccRCC) is a broad term that refers to a diverse group of tumors, each with its own distinct biologic and therapeutic profile. The management of nccRCCs is often based on extrapolating data from clinical trials in the more common clear cell renal cell carcinoma, but our emerging prospective and retrospective clinical experience in nccRCC allows us to make more precise recommendations tailored to each histology. The systemic therapy options for metastatic nccRCC include targeted therapies such as tyrosine kinase inhibitors, immune checkpoint inhibitors, and, for specific rare subtypes, cytotoxic chemotherapy. Each nccRCC histology may respond differently to these regimens, which makes accurate pathologic diagnosis imperative. In the present review, we discuss the available clinical and biological data that can help guide systemic therapy recommendations for specific nccRCC subtypes.

Regulation of Hippo signaling pathway in cancer: A MicroRNA perspective

Recent studies have suggested that Hippo signaling is not only involved in controlling organ size in Drosophila but can also regulate cell proliferation, tissue homeostasis, differentiation, apoptosis and regeneration. Any dysregulation of Hippo signaling, especially the hyper activation of its downstream effectors YAP/TAZ, can lead to uncontrolled cell proliferation and malignant transformation. In majority of cancers, expression of YAP/TAZ is extremely high and this increased expression of YAP/TAZ has been shown to be an independent predictor of prognosis and indicator of increased cell proliferation, metastasis and poor survival. In this review, we have summarized the most recent findings about the cross talk of Hippo signaling pathway with other signaling pathways and its regulation by different miRNAs in various cancer types. Recent evidence has suggested that Hippo pathway is also involved in mediating the resistance of different cancer cells to chemotherapeutic drugs and in a few cancer types, this is brought about by regulating miRNAs. Therefore, the delineation of the underlying mechanisms regulating the chemotherapeutic resistance might help in developing better treatment options. This review has attempted to provide an overview of different drugs/options which can be utilized to target oncogenic YAP/TAZ proteins for therapeutic interventions.

Merlin regulates signaling events at the nexus of development and cancer

Background

In this review, we describe how the cytoskeletal protein Merlin, encoded by the Neurofibromin 2 (NF2) gene, orchestrates developmental signaling to ensure normal ontogeny, and we discuss how Merlin deficiency leads to aberrant activation of developmental pathways that enable tumor development and malignant progression.

Main body

Parallels between embryonic development and cancer have underscored the activation of developmental signaling pathways. Hippo, WNT/β-catenin, TGF-β, receptor tyrosine kinase (RTK), Notch, and Hedgehog pathways are key players in normal developmental biology. Unrestrained activity or loss of activity of these pathways causes adverse effects in developing tissues manifesting as developmental syndromes. Interestingly, these detrimental events also impact differentiated and functional tissues. By promoting cell proliferation, migration, and stem-cell like phenotypes, deregulated activity of these pathways promotes carcinogenesis and cancer progression. The NF2 gene product, Merlin, is a tumor suppressor classically known for its ability to induce contact-dependent growth inhibition. Merlin plays a role in different stages of an organism development, ranging from embryonic to mature states. While homozygous deletion of Nf2 in murine embryos causes embryonic lethality, Merlin loss in adult tissue is implicated in Neurofibromatosis type 2 disorder and cancer. These manifestations, cumulatively, are reminiscent of dysregulated developmental signaling.

Conclusion

Understanding the molecular and cellular repercussions of Merlin loss provides fundamental insights into the etiology of developmental disorders and cancer and has the potential, in the long term, to identify new therapeutic strategies.

Analysis of the role of the Hippo pathway in cancer

Cancer is a serious health issue in the world due to a large body of cancer-related human deaths, and there is no current treatment available to efficiently treat the disease as the tumor is often diagnosed at a serious stage. Moreover, Cancer cells are often resistant to chemotherapy, radiotherapy, and molecular-targeted therapy. Upon further knowledge of mechanisms of tumorigenesis, aggressiveness, metastasis, and resistance to treatments, it is necessary to detect the disease at an earlier stage and for a better response to therapy. The hippo pathway possesses the unique capacity to lead to tumorigenesis. Mutations and altered expression of its core components (MST1/2, LATS1/2, YAP and TAZ) promote the migration, invasion, malignancy of cancer cells. The biological significance and deregulation of it have received a large body of interests in the past few years. Further understanding of hippo pathway will be responsible for cancer treatment. In this review, we try to discover the function of hippo pathway in different diversity of cancers, and discuss how Hippo pathway contributes to other cellular signaling pathways. Also, we try to describe how microRNAs, circRNAs, and ZNFs regulate hippo pathway in the process of cancer. It is necessary to find new therapy strategies for cancer.

Identification of EGFR as a Novel Key Gene in Clear Cell Renal Cell Carcinoma (ccRCC) through Bioinformatics Analysis and Meta-Analysis

Clear cell renal cell carcinoma (ccRCC) was the most aggressive histological type of renal cell carcinoma (RCC) and accounted for 70-80% of cases of all RCC. The aim of this study was to identify the potential biomarker in ccRCC and explore their underlying mechanisms. Four profile datasets were downloaded from the GEO database to identify DEGs. GO and KEGG analysis of DEGs were performed by DAVID. A protein-protein interaction (PPI) network was constructed to predict hub genes. The hub gene expression within ccRCC across multiple datasets and the overall survival analysis were investigated utilizing the Oncomine Platform and UALCAN dataset, separately. A meta-analysis was performed to explore the relationship between the hub genes: EGFR and ccRCC. 127 DEGs (55 upregulated genes and 72 downregulated genes) were identified from four profile datasets. Integrating the result from PPI network, Oncomine Platform, and survival analysis, EGFR, FLT1, and EDN1 were screened as key factors in the prognosis of ccRCC. GO and KEGG analysis revealed that 127 DEGs were mainly enriched in 21 terms and 4 pathways. The meta-analysis showed that there was a significant difference of EGFR expression between ccRCC tissues and normal tissues, and the expression of EGFR in patients with metastasis was higher. This study identified 3 importance genes (EGFR, FLT1, and EDN1) in ccRCC, and EGFR may be a potential prognostic biomarker and novel therapeutic target for ccRCC, especially patients with metastasis.

Deficiency of tumor suppressor Merlin facilitates metabolic adaptation by co-operative engagement of SMAD-Hippo signaling in breast cancer

The NF2 gene encodes the tumor and metastasis suppressor protein Merlin. Merlin exerts its tumor suppressive role by inhibiting proliferation and inducing contact-growth inhibition and apoptosis. In the current investigation, we determined that loss of Merlin in breast cancer tissues is concordant with the loss of the inhibitory SMAD, SMAD7, of the TGF-β pathway. This was reflected as dysregulated activation of TGF-β signaling that co-operatively engaged with effectors of the Hippo pathway (YAP/TAZ/TEAD). As a consequence, the loss of Merlin in breast cancer resulted in a significant metabolic and bioenergetic adaptation of cells characterized by increased aerobic glycolysis and decreased oxygen consumption. Mechanistically, we determined that the co-operative activity of the Hippo and TGF-β transcription effectors caused upregulation of the long non-coding RNA Urothelial Cancer-Associated 1 (UCA1) that disengaged Merlin's check on STAT3 activity. The consequent upregulation of Hexokinase 2 (HK2) enabled a metabolic shift towards aerobic glycolysis. In fact, Merlin deficiency engendered cellular dependence on this metabolic adaptation, endorsing a critical role for Merlin in regulating cellular metabolism. This is the first report of Merlin functioning as a molecular restraint on cellular metabolism. Thus, breast cancer patients whose tumors demonstrate concordant loss of Merlin and SMAD7 may benefit from an approach of incorporating STAT3 inhibitors.

G3BP1 promotes tumor progression and metastasis through IL-6/G3BP1/STAT3 signaling axis in renal cell carcinomas

The chronic inflammatory microenvironment within or surrounding the primary renal cell carcinoma (RCC) site promotes oncogenic transformation as well as contributes to the development of metastasis. G3BP stress granule assembly factor 1 (G3BP1) was found to be involved in the regulation of multiple cellular functions. However, its functions in RCC have not been previously explored. Here, we first showed that the expression of G3BP1 is elevated in human RCC and correlates with RCC progression. In cultured RCC cells, knockdown of G3BP1 results in inhibition of tumor cell proliferation, migration, and invasion, consistently with the alteration of epithelial–mesenchymal transition (EMT) and cell proliferative markers, including Cadherins, Vimentin, Snail, Slug, c-Myc, and cyclin D1. Remarkably, knockdown of G3BP1 dramatically impaired the signaling connection of pro-inflammatory cytokine IL-6 stimulation and downstream STAT3 activation in RCC, thus eventually contributing to the disruption of IL-6-elicited RCC migration and metastasis. In addition, in vivo orthotopic tumor xenografts results confirmed that knockdown of G3BP1 suppressed RCC tumor growth and metastasis in mice. Collectively, our findings support the notion that G3BP1 promotes tumor progression and metastasis through IL-6/G3BP1/STAT3 signaling axis in RCC.

Neurofibromatosis type 2 tumor suppressor protein is expressed in oligodendrocytes and regulates cell proliferation and process formation

The neurofibromatosis type 2 (NF2) tumor suppressor protein Merlin functions as a negative regulator of cell growth and actin dynamics in different cell types amongst which Schwann cells have been extensively studied. In contrast, the presence and the role of Merlin in oligodendrocytes, the myelin forming cells within the CNS, have not been elucidated. In this work, we demonstrate that Merlin immunoreactivity was broadly distributed in the white matter throughout the central nervous system. Following Merlin expression during development in the cerebellum, Merlin could be detected in the cerebellar white matter tract at early postnatal stages as shown by its co-localization with Olig2-positive cells as well as in adult brain sections where it was aligned with myelin basic protein containing fibers. This suggests that Merlin is expressed in immature and mature oligodendrocytes. Expression levels of Merlin were low in oligodendrocytes as compared to astrocytes and neurons throughout development. Expression of Merlin in oligodendroglia was further supported by its identification in either immortalized cell lines of oligodendroglial origin or in primary oligodendrocyte cultures. In these cultures, the two main splice variants of Nf2 could be detected. Merlin was localized in clusters within the nuclei and in the cytoplasm. Overexpressing Merlin in oligodendrocyte cell lines strengthened reduced impedance in XCELLigence measurements and Ki67 stainings in cultures over time. In addition, the initiation and elongation of cellular projections were reduced by Merlin overexpression. Consistently, cell migration was retarded in scratch assays done on Nf2-transfected oligodendrocyte cell lines. These data suggest that Merlin actively modulates process outgrowth and migration in oligodendrocytes.

p120-Catenin is an obligate haploinsufficient tumor suppressor in intestinal neoplasia

p120-Catenin (p120) functions as a tumor suppressor in intestinal cancer, but the mechanism is unclear. Here, using conditional p120 knockout in Apc-sensitized mouse models of intestinal cancer, we have identified p120 as an "obligatory" haploinsufficient tumor suppressor. Whereas monoallelic loss of p120 was associated with a significant increase in tumor multiplicity, loss of both alleles was never observed in tumors from these mice. Moreover, forced ablation of the second allele did not further enhance tumorigenesis, but instead induced synthetic lethality in combination with Apc loss of heterozygosity. In tumor-derived organoid cultures, elimination of both p120 alleles resulted in caspase-3-dependent apoptosis that was blocked by inhibition of Rho kinase (ROCK). With ROCK inhibition, however, p120-ablated organoids exhibited a branching phenotype and a substantial increase in cell proliferation. Access to data from Sleeping Beauty mutagenesis screens afforded an opportunity to directly assess the tumorigenic impact of p120 haploinsufficiency relative to other candidate drivers. Remarkably, p120 ranked third among the 919 drivers identified. Cofactors α-catenin and epithelial cadherin (E-cadherin) were also among the highest scoring candidates, indicating a mechanism at the level of the intact complex that may play an important role at very early stages of of intestinal tumorigenesis while simultaneously restricting outright loss via synthetic lethality.

Hippo vs. Crab: tissue-specific functions of the mammalian Hippo pathway

The Hippo signaling pathway is a vital suppressor of tumorigenesis that is often inactivated in human cancers. In normal cells, the Hippo pathway is triggered by external forces such as cell crowding, or changes to the extracellular matrix or cell polarity. Once activated, Hippo signaling down-regulates transcription supported by the paralogous cofactors YAP1 and TAZ. The Hippo pathway's functions in normal and cancer biology have been dissected by studies of mutant mice with null or conditional tissue-specific mutations of Hippo signaling elements. In this review, we attempt to systematically summarize results that have been gleaned from detailed in vivo characterizations of these mutants. Our goal is to describe the physiological roles of Hippo signaling in several normal organ systems, as well as to emphasize how disruption of the Hippo pathway, and particularly hyperactivation of YAP1/TAZ, can be oncogenic.

Hippo transducer TAZ promotes epithelial mesenchymal transition and supports pancreatic cancer progression

Transcriptional co-activator with PDZ binding motif (TAZ) is a transducer of the Hippo pathway and promotes cancer development and progression. In the present study, we sought to determine the roles and underlying mechanisms of elevated expression and activation of TAZ in pancreatic cancer development and progression. The mechanistic role of TAZ and Hippo signaling in promotion of pancreatic cancer development and progression was examined using cell culture, molecular biology, and mouse models. The relevance of our experimental and mechanistic findings was validated using human pancreatic tumor specimens. We found that TAZ expression was markedly higher in pancreatic tumors than in normal pancreatic tissue. Further analysis of the correlation of TAZ expression with tissue microarray clinicopathologic parameters revealed that this expression was positively associated with tumor differentiation. Also, TAZ expression was higher in pancreatic cancer cell lines than in pancreatic ductal epithelial cells. TAZ activation in pancreatic cancer cells promoted their proliferation, migration, invasion, and epithelial-mesenchymal transition. Further mechanistic studies demonstrated that aberrant expression and activation of TAZ in pancreatic cancer cells resulted from suppression of the expression of Merlin, a positive regulator upstream of the Hippo pathway, and that the oncogenic function of TAZ in pancreatic cancer cells was mediated by TEA/ATTS domain transcription factors. Therefore, TAZ functioned as an oncogene and promoted pancreatic cancer epithelial-mesenchymal transition and progression. TAZ thus may be a target for effective therapeutic strategies for pancreatic cancer.

Molecular analysis of aggressive renal cell carcinoma with unclassified histology reveals distinct subsets

Renal cell carcinomas with unclassified histology (uRCC) constitute a significant portion of aggressive non-clear cell renal cell carcinomas that have no standard therapy. The oncogenic drivers in these tumours are unknown. Here we perform a molecular analysis of 62 high-grade primary uRCC, incorporating targeted cancer gene sequencing, RNA sequencing, single-nucleotide polymorphism array, fluorescence in situ hybridization, immunohistochemistry and cell-based assays. We identify recurrent somatic mutations in 29 genes, including NF2 (18%), SETD2 (18%), BAP1 (13%), KMT2C (10%) and MTOR (8%). Integrated analysis reveals a subset of 26% uRCC characterized by NF2 loss, dysregulated Hippo–YAP pathway and worse survival, whereas 21% uRCC with mutations of MTOR, TSC1, TSC2 or PTEN and hyperactive mTORC1 signalling are associated with better clinical outcome. FH deficiency (6%), chromatin/DNA damage regulator mutations (21%) and ALK translocation (2%) distinguish additional cases. Altogether, this study reveals distinct molecular subsets for 76% of our uRCC cohort, which could have diagnostic and therapeutic implications.

A subset of renal cell carcinomas have uncertain histology and are aggressive in nature. Here, the authors examine this group of unclassified renal cancers using genomics techniques and identify further subclasses of the tumours that have differing prognoses.

Hippo signaling in the kidney: the good and the bad

The Hippo signaling pathway is an evolutionarily conserved kinase cascade, playing multiple roles in embryonic development that controls organ size, cell proliferation, and apoptosis. At the center of this network lie the Hippo kinase target and downstream pathway effector Yes-associated protein (YAP) and its paralog TAZ. In its phosphorylated form, cytoplasmic YAP is sequestered in an inactive state. When it is dephosphorylated, YAP, a potent oncogene, is activated and relocates to the nucleus to interact with a number of transcription factors and signaling regulators that promote cell growth, differentiation, and survival. The identification of YAP activation in human cancers has made it an attractive target for chemotherapeutic drug development. Little is known to date about the function of the Hippo pathway in the kidney, but that is rapidly changing. Recent studies have shed light on the role of Hippo-YAP signaling in glomerular and lower urinary tract embryonic development, maintenance of podocyte homeostasis, the integrity of the glomerular filtration barrier, regulation of renal tubular cyst growth, renal epithelial injury in diabetes, and renal fibrogenesis. This review summarizes the current knowledge of the Hippo-YAP signaling axis in the kidney under normal and disease conditions.

New agents and new targets for renal cell carcinoma

The vascular endothelial growth factor (VEGF) pathway blockers and mammalian target of rapamycin (mTOR) inhibitors have dramatically improved the treatment options and outcome for patients with advanced renal cell carcinoma (RCC). However, because the vast majority of patients will still succumb to their disease, novel treatment approaches are still necessary. Efforts to identify novel therapeutic target treatments are focused on better understanding unique aspects of tumor cell biology guided the Cancer Genome Atlas analyses and the interaction of the tumor with its microenvironment. Areas of promising investigation include a) the identification of mechanisms of acquired resistance to VEGF pathway inhibition and developing agents targeting these in combination with VEGF receptor (VEGFR) pathway blockade; b) the identification of novel therapeutic targets, particularly for patients with VEGF pathway blocker refractory disease; and c) the development of novel immunotherapies, particularly those involving checkpoint inhibitors used alone or in combination with other immunotherapies of VEGF pathway blockers. Specific targets or agents of interest include angiopoietins (trebaninib), c-Met (cabozantinib), activin receptor-like kinase-1 (ALK-1; dalantercept), interleukin (IL)-8, and HDM2 for acquired resistance to VEGF pathway inhibition; hypoxia inducible factor-2 alpha (HIF-2 alpha), TORC1/2, and the Hippo pathway for novel targets, and PD1 and PDL1 antibodies given either alone or in combination with other checkpoint inhibitors, other immunotherapies, or VEGF pathway blockers for novel immunotherapies. In addition, the application of genetic, immunologic, or other biomarkers developed in the context of this research has the potential to select patients with specific tumor types for therapy targeted to specific vulnerabilities within the tumor or tumor microenvironment. Together, these developments should enable the transition to a new era of rational and more effective therapy for patients with advanced RCC.

NF2/Merlin mediates contact-dependent inhibition of EGFR mobility and internalization via cortical actomyosin

Merlin and Ezrin are central to a mechanism whereby mechanical forces transduced across the apical actomyosin cytoskeleton from cell junctions control the mobility and internalization of EGFR, providing novel insight into how cells inhibit mitogenic signaling in response to cell contact.

The proliferation of normal cells is inhibited at confluence, but the molecular basis of this phenomenon, known as contact-dependent inhibition of proliferation, is unclear. We previously identified the neurofibromatosis type 2 (NF2) tumor suppressor Merlin as a critical mediator of contact-dependent inhibition of proliferation and specifically found that Merlin inhibits the internalization of, and signaling from, the epidermal growth factor receptor (EGFR) in response to cell contact. Merlin is closely related to the membrane–cytoskeleton linking proteins Ezrin, Radixin, and Moesin, and localization of Merlin to the cortical cytoskeleton is required for contact-dependent regulation of EGFR. We show that Merlin and Ezrin are essential components of a mechanism whereby mechanical forces associated with the establishment of cell–cell junctions are transduced across the cell cortex via the cortical actomyosin cytoskeleton to control the lateral mobility and activity of EGFR, providing novel insight into how cells inhibit mitogenic signaling in response to cell contact.

Merlin/NF2 Suppresses Pancreatic Tumor Growth and Metastasis by Attenuating the FOXM1-Mediated Wnt/β-Catenin Signaling

Merlin, the protein encoded by the NF2 gene, is a member of the band 4.1 family of cytoskeleton-associated proteins and functions as a tumor suppressor for many types of cancer. However, the roles and mechanism of Merlin expression in pancreatic cancer have remained unclear. In this study, we sought to determine the impact of Merlin expression on pancreatic cancer development and progression using human tissue specimens, cell lines, and animal models. Decreased expression of Merlin was pronounced in human pancreatic tumors and cancer cell lines. Functional analysis revealed that restored expression of Merlin inhibited pancreatic tumor growth and metastasis in vitro and in vivo. Furthermore, Merlin suppressed the expression of Wnt/β-catenin signaling downstream genes and the nuclear expression of β-catenin protein, and overexpression of Forkhead box M1 (FOXM1) attenuated the suppressive effect of Merlin on Wnt/β-catenin signaling. Mechanistically, Merlin decreased the stability of FOXM1 protein, which plays critical roles in nuclear translocation of β-catenin. Collectively, these findings demonstrated that Merlin critically regulated pancreatic cancer pathogenesis by suppressing FOXM1/β-catenin signaling, suggesting that targeting novel Merlin/FOXM1/β-catenin signaling is an effective therapeutic strategy for pancreatic cancer.

MYC oncogene overexpression drives renal cell carcinoma in a mouse model through glutamine metabolism

The MYC oncogene is frequently mutated and overexpressed in human renal cell carcinoma (RCC). However, there have been no studies on the causative role of MYC or any other oncogene in the initiation or maintenance of kidney tumorigenesis. Here, we show through a conditional transgenic mouse model that the MYC oncogene, but not the RAS oncogene, initiates and maintains RCC. Desorption electrospray ionization-mass-spectrometric imaging was used to obtain chemical maps of metabolites and lipids in the mouse RCC samples. Gene expression analysis revealed that the mouse tumors mimicked human RCC. The data suggested that MYC-induced RCC up-regulated the glutaminolytic pathway instead of the glycolytic pathway. The pharmacologic inhibition of glutamine metabolism with bis-2-(5-phenylacetamido-1,2,4-thiadiazol-2-yl)ethyl sulfide impeded MYC-mediated RCC tumor progression. Our studies demonstrate that MYC overexpression causes RCC and points to the inhibition of glutamine metabolism as a potential therapeutic approach for the treatment of this disease.

Hippo signaling in stress response and homeostasis maintenance

Co-ordination of cell proliferation, differentiation, and apoptosis maintains tissue development and homeostasis under normal or stress conditions. Recently, the highly conserved Hippo signaling pathway, discovered in Drosophila melanogaster and mammalian system, has been implicated as a key regulator of organ size control. Importantly, emerging evidence suggests that Hippo pathway is involved in the responses to cellular stresses, including mechanic stress, DNA damage, and oxidative stress, to maintain homeostasis at the cellular and organic levels. The mutation or deregulation of the key components in the pathway will result in degenerative disorder, developmental defects, or tumorigenesis. The purpose of this review is to summarize the recent findings and discuss how Hippo pathway responds to cellular stress and regulates early development events, tissue homeostasis as well as tumorigenesis.

The biology of YAP/TAZ: hippo signaling and beyond

The transcriptional regulators YAP and TAZ are the focus of intense interest given their remarkable biological properties in development, tissue homeostasis and cancer. YAP and TAZ activity is key for the growth of whole organs, for amplification of tissue-specific progenitor cells during tissue renewal and regeneration, and for cell proliferation. In tumors, YAP/TAZ can reprogram cancer cells into cancer stem cells and incite tumor initiation, progression and metastasis. As such, YAP/TAZ are appealing therapeutic targets in cancer and regenerative medicine. Just like the function of YAP/TAZ offers a molecular entry point into the mysteries of tissue biology, their regulation by upstream cues is equally captivating. YAP/TAZ are well known for being the effectors of the Hippo signaling cascade, and mouse mutants in Hippo pathway components display remarkable phenotypes of organ overgrowth, enhanced stem cell content and reduced cellular differentiation. YAP/TAZ are primary sensors of the cell's physical nature, as defined by cell structure, shape and polarity. YAP/TAZ activation also reflects the cell "social" behavior, including cell adhesion and the mechanical signals that the cell receives from tissue architecture and surrounding extracellular matrix (ECM). At the same time, YAP/TAZ entertain relationships with morphogenetic signals, such as Wnt growth factors, and are also regulated by Rho, GPCRs and mevalonate metabolism. YAP/TAZ thus appear at the centerpiece of a signaling nexus by which cells take control of their behavior according to their own shape, spatial location and growth factor context.

Nf2/Merlin controls spinal cord neural progenitor function in a Rac1/ErbB2-dependent manner

Objective

Individuals with the neurofibromatosis type 2 (NF2) cancer predisposition syndrome develop spinal cord glial tumors (ependymomas) that likely originate from neural progenitor cells. Whereas many spinal ependymomas exhibit indolent behavior, the only treatment option for clinically symptomatic tumors is surgery. In this regard, medical therapies are unfortunately lacking due to an incomplete understanding of the critical growth control pathways that govern the function of spinal cord (SC) neural progenitor cells (NPCs).

Methods

To identify potential therapeutic targets for these tumors, we leveraged primary mouse Nf2-deficient spinal cord neural progenitor cells.

Results

We demonstrate that the Nf2 protein, merlin, negatively regulates spinal neural progenitor cell survival and glial differentiation in an ErbB2-dependent manner, and that NF2-associated spinal ependymomas exhibit increased ErbB2 activation. Moreover, we show that Nf2-deficient SC NPC ErbB2 activation results from Rac1-mediated ErbB2 retention at the plasma membrane.

Significance

Collectively, these findings establish ErbB2 as a potential rational therapeutic target for NF2-associated spinal ependymoma.

Hippo signaling mediates proliferation, invasiveness, and metastatic potential of clear cell renal cell carcinoma

Recent work has identified dysfunctional Hippo signaling to be involved in maintenance and progression of various human cancers, although data on clear cell renal cell carcinoma (ccRCC) have been limited. Here, we provide evidence implicating aberrant Hippo signaling in ccRCC proliferation, invasiveness, and metastatic potential. Nuclear overexpression of the Hippo target Yes-associated protein (YAP) was found in a subset of patients with ccRCC. Immunostaining was particularly prominent at the tumor margins and highlighted neoplastic cells invading the tumor-adjacent stroma. Short hairpin RNA-mediated knockdown of YAP significantly inhibited proliferation, migration, and anchorage-independent growth of ccRCC cells in soft agar and led to significantly reduced murine xenograft growth. Microarray analysis of YAP knockdown versus mock-transduced ccRCC cells revealed down-regulation of endothelin 1, endothelin 2, cysteine-rich, angiogenic inducer, 61 (CYR61), and c-Myc in ccRCC cells as well as up-regulation of the cell adhesion molecule cadherin 6. Signaling pathway impact analysis revealed activation of the p53 signaling and cell cycle pathways as well as inhibition of mitogen-activated protein kinase signaling on YAP down-regulation. Our data suggest CYR61 and c-Myc as well as signaling through the endothelin axis as bona fide downstream effectors of YAP and establish aberrant Hippo signaling as a potential therapeutic target in ccRCC.

Loss of NF2/Merlin expression in advanced sporadic colorectal cancer

PURPOSE

NF2/Merlin was first identified through its association with neurofibromatosis type 2 (NF2). However, accumulating evidence suggests a more general involvement in tumorigenesis and, in particular, a broader role in tumor suppression. The aim of this study was to examine NF2/Merlin involvement in sporadic colorectal cancer.

METHODS

This study is the first to examine the role of NF2/Merlin in sporadic colorectal cancer through LOH analysis at the NF2 locus and mRNA expression analysis via quantitative RT-PCR of total NF2, NF2 isoform I and II. In addition, Merlin protein expression was assessed by immunohistochemistry and Western blotting.

RESULTS

NF2 LOH was detected in 20.0 % of heterozygous cases and was found to be more frequent in tumors larger than 5 cm in diameter (p = 0.041) and in tumors with a less differentiated phenotype (p = 0.027). No differences were observed in total NF2 and NF2 isoform I/isoform II mRNA expression between the tumors and their corresponding normal mucous tissues. NF2 isoform II was the most predominant isoform in all samples analyzed. mRNA expression levels of total NF2 and isoforms I and II were significantly lower in poorly differentiated tumors (p = 0.033, p = 0.036 and p = 0.044, respectively). Weak Merlin immunostaining was more frequent in poorly differentiated tumors (p = 0.034) and tumors classified as Dukes' C (p = 0.023). A distinct pattern of Merin phosphorylation was observed in tumors compared to normal mucous tissues.

CONCLUSION

Our data indicate that NF2/Merlin may serve as a potential target in the management of colorectal cancer.

Tumor suppressor NF2/Merlin is a microtubule stabilizer

Cancer-associated mutations in oncogene products and tumor suppressors contributing to tumor progression manifest themselves, at least in part, by deregulating microtubule-dependent cellular processes that play important roles in many cell biological pathways, including intracellular transport, cell architecture, and primary cilium and mitotic spindle organization. An essential characteristic of microtubules in the performance of these varied cell processes is their ability to continuously remodel, a phenomenon known as dynamic instability. It is therefore conceivable that part of the normal function of certain cancer-causing genes is to regulate microtubule dynamic instability. Here, we report the results of a high-resolution live-cell image-based RNA interference screen targeting a collection of 70 human tumor suppressor genes to uncover cancer genes affecting microtubule dynamic instability. Extraction and computational analysis of microtubule dynamics from EB3-GFP time-lapse image sequences identified the products of the tumor suppressor genes NF1 and NF2 as potent microtubule-stabilizing proteins. Further in-depth characterization of NF2 revealed that it binds to and stabilizes microtubules through attenuation of tubulin turnover by lowering both rates of microtubule polymerization and depolymerization as well as by reducing the frequency of microtubule catastrophes. The latter function appears to be mediated, in part, by inhibition of hydrolysis of tubulin-bound GTP on the growing microtubule plus end.

Candidate tumor suppressor and pVHL partner Jade-1 binds and inhibits AKT in renal cell carcinoma

The von Hippel-Lindau (VHL) tumor suppressor pVHL is lost in the majority of clear-cell renal cell carcinomas (RCC). Activation of the PI3K/AKT/mTOR pathway is also common in RCC, with PTEN loss occurring in approximately 30% of the cases, but other mechanisms responsible for activating AKT at a wider level in this setting are undefined. Plant homeodomain protein Jade-1 (PHF17) is a candidate renal tumor suppressor stabilized by pVHL. Here, using kinase arrays, we identified phospho-AKT1 as an important target of Jade-1. Overexpressing or silencing Jade-1 in RCC cells increased or decreased levels of endogenous phospho-AKT/AKT1. Furthermore, reintroducing pVHL into RCC cells increased endogenous Jade-1 and suppressed endogenous levels of phospho-AKT, which colocalized with and bound to Jade-1. The N-terminus of Jade-1 bound both the catalytic domain and the C-terminal regulatory tail of AKT, suggesting a mechanism through which Jade-1 inhibited AKT kinase activity. Intriguingly, RCC precursor cells where Jade-1 was silenced exhibited an increased capacity for AKT-dependent anchorage-independent growth, in support of a tumor suppressor function for Jade-1 in RCC. In support of this concept, an in silico expression analysis suggested that reduced Jade-1 expression is a poor prognostic factor in clear-cell RCC that is associated with activation of an AKT1 target gene signature. Taken together, our results identify 2 mechanisms for Jade-1 fine control of AKT/AKT1 in RCC, through loss of pVHL, which decreases Jade-1 protein, or through attenuation in Jade-1 expression. These findings help explain the pathologic cooperativity in clear-cell RCC between PTEN inactivation and pVHL loss, which leads to decreased Jade-1 levels that superactivate AKT. In addition, they prompt further investigation of Jade-1 as a candidate biomarker and tumor suppressor in clear-cell RCC.

The Hippo pathway and human cancer

The Hippo pathway controls organ size in diverse species, whereas pathway deregulation can induce tumours in model organisms and occurs in a broad range of human carcinomas, including lung, colorectal, ovarian and liver cancer. Despite this, somatic or germline mutations in Hippo pathway genes are uncommon, with only the upstream pathway gene neurofibromin 2 (NF2) recognized as a bona fide tumour suppressor gene. In this Review, we appraise the evidence for the Hippo pathway as a cancer signalling network, and discuss cancer-relevant biological functions, potential mechanisms by which Hippo pathway activity is altered in cancer and emerging therapeutic strategies.

Merlin/ERM proteins establish cortical asymmetry and centrosome position

The ability to generate asymmetry at the cell cortex underlies cell polarization and asymmetric cell division. Here we demonstrate a novel role for the tumor suppressor Merlin and closely related ERM proteins (Ezrin, Radixin, and Moesin) in generating cortical asymmetry in the absence of external cues. Our data reveal that Merlin functions to restrict the cortical distribution of the actin regulator Ezrin, which in turn positions the interphase centrosome in single epithelial cells and three-dimensional organotypic cultures. In the absence of Merlin, ectopic cortical Ezrin yields mispositioned centrosomes, misoriented spindles, and aberrant epithelial architecture. Furthermore, in tumor cells with centrosome amplification, the failure to restrict cortical Ezrin abolishes centrosome clustering, yielding multipolar mitoses. These data uncover fundamental roles for Merlin/ERM proteins in spatiotemporally organizing the cell cortex and suggest that Merlin's role in restricting cortical Ezrin may contribute to tumorigenesis by disrupting cell polarity, spindle orientation, and, potentially, genome stability.

Spatial regulation of receptor tyrosine kinases in development and cancer

During development and tissue homeostasis, patterns of cellular organization, proliferation and movement are highly choreographed. Receptor tyrosine kinases (RTKs) have a crucial role in establishing these patterns. Individual cells and tissues exhibit tight spatial control of the RTKs that they express, enabling tissue morphogenesis and function, while preventing unwarranted cell division and migration that can contribute to tumorigenesis. Indeed, RTKs are deregulated in most human cancers and are a major focus of targeted therapeutics. A growing appreciation of the essential role of spatial RTK regulation during development prompts the realization that spatial deregulation of RTKs is likely to contribute broadly to cancer development and may affect the sensitivity and resistance of cancer to pharmacological RTK inhibitors.

Thyroid-specific knockout of the tumor suppressor mitogen-inducible gene 6 activates epidermal growth factor receptor signaling pathways and suppresses nuclear factor-κB activity

BACKGROUND

Mitogen-inducible gene 6 (Mig-6) is a putative tumor suppressor gene and prognostic biomarker in papillary thyroid cancer. We hypothesized that Mig-6 knockout would activate pro-oncogenic signaling in mouse thyrocytes.

METHODS

We performed a thyroid-specific knockout using the Cre/loxP recombinase system.

RESULTS

Four knockout and 4 control mouse thyroids were harvested at 2 months of age. Immunoblotting confirmed Mig-6 ablation in knockout mice thyrocytes. Epidermal growth factor receptor (EGFR) and extracellular signal-regulated kinase (ERK) phosphorylation levels were increased in Mig-6 knockout compared to wild-type mice. Total EGFR levels were similar in knockout and wild-type mice. However, EGFR was absent in the caveolae-containing membrane fraction of knockout mice, indicating that Mig-6 depletion is associated with a change in the membrane distribution of EGFR. Although p65 localized to the nucleus in wild-type mice, it was distributed in both cytoplasm and nucleus in knockouts, suggesting that Mig-6 loss decreases p65 activity.

CONCLUSION

Our results confirm the feasibility of targeted, thyroid-specific gene knockout as a strategy for studying the relevance of specific genes in thyroid oncogenesis. We suggest that the loss of Mig-6 alters the membrane distribution of EGFR, which may limit receptor degradation and activate this oncogenic signaling pathway.

NPHP4, a cilia-associated protein, negatively regulates the Hippo pathway

The cilia-associated protein NPHP4 is a negative regulator of Hippo signaling that modulates cell proliferation in mammals.

The conserved Hippo signaling pathway regulates organ size in Drosophila melanogaster and mammals and has an essential role in tumor suppression and the control of cell proliferation. Recent studies identified activators of Hippo signaling, but antagonists of the pathway have remained largely elusive. In this paper, we show that NPHP4, a known cilia-associated protein that is mutated in the severe degenerative renal disease nephronophthisis, acts as a potent negative regulator of mammalian Hippo signaling. NPHP4 directly interacted with the kinase Lats1 and inhibited Lats1-mediated phosphorylation of the Yes-associated protein (YAP) and TAZ (transcriptional coactivator with PDZ-binding domain), leading to derepression of these protooncogenic transcriptional regulators. Moreover, NPHP4 induced release from 14-3-3 binding and nuclear translocation of YAP and TAZ, promoting TEA domain (TEAD)/TAZ/YAP-dependent transcriptional activity. Consistent with these data, knockdown of NPHP4 negatively affected cellular proliferation and TEAD/TAZ activity, essentially phenocopying loss of TAZ function. These data identify NPHP4 as a negative regulator of the Hippo pathway and suggest that NPHP4 regulates cell proliferation through its effects on Hippo signaling.

Neurofibromatosis type 2/Merlin: sharpening the myth of prometheus

The molecular signals that control the maintenance and activation of liver stem/progenitor cells are poorly understood, and the role of liver progenitor cells in hepatic tumorigenesis is unclear. We report here that liver-specific deletion of the neurofibromatosis type 2 (NF2) tumor suppressor gene in the developing or adult mouse specifically yields a dramatic, progressive expansion of progenitor cells throughout the liver without affecting differentiated hepatocytes. All surviving mice eventually developed both cholangiocellular and hepatocellular carcinoma,suggesting that Nf2-/-progenitors can be a cell of origin for these tumors. Despite the suggested link between NF2 and the Hpo/Wts/Yki signaling pathway in Drosophila, and recent studies linking the corresponding Mst/Lats/Yap pathway to mammalian liver tumorigenesis, our molecular studies suggest that Merlin is not a major regulator of YAP in liver progenitors,and that the overproliferation of Nf2-/-liver progenitors is instead driven by aberrant epidermal growth factor receptor (EGFR) activity. Indeed, pharmacologic inhibition of EGFR blocks the proliferation of Nf2-/-liver progenitors in vitro and in vivo, consistent with recent studies indicating that the NF2-encoded protein Merlin can control the abundance and signaling of membrane receptors such as EGFR. Together,our findings uncover a critical role for NF2/Merlin in controlling homeostasis of the liver stem cell niche.

A tight junction-associated Merlin-angiomotin complex mediates Merlin's regulation of mitogenic signaling and tumor suppressive functions

The Merlin/NF2 tumor suppressor restrains cell growth and tumorigenesis by controlling contact-dependent inhibition of proliferation. We have identified a tight-junction-associated protein complex comprising Merlin, Angiomotin, Patj, and Pals1. We demonstrate that Angiomotin functions downstream of Merlin and upstream of Rich1, a small GTPase Activating Protein, as a positive regulator of Rac1. Merlin, through competitive binding to Angiomotin, releases Rich1 from the Angiomotin-inhibitory complex, allowing Rich1 to inactivate Rac1, ultimately leading to attenuation of Rac1 and Ras-MAPK pathways. Patient-derived Merlin mutants show diminished binding capacities to Angiomotin and are unable to dissociate Rich1 from Angiomotin or inhibit MAPK signaling. Depletion of Angiomotin in Nf2(-/-) Schwann cells attenuates the Ras-MAPK signaling pathway, impedes cellular proliferation in vitro and tumorigenesis in vivo.

Development and characterization of clinically relevant tumor models from patients with renal cell carcinoma

BACKGROUND

Animal models are instrumental in understanding disease pathophysiology and mechanisms of therapy action and resistance in vivo.

OBJECTIVE

To establish and characterize a panel of mouse models of renal cell carcinoma (RCC) derived from patients undergoing radical nephrectomy.

DESIGN, SETTING, AND PARTICIPANTS

In vivo and in vitro animal experiments.

MEASUREMENTS

Tumor tissues obtained during surgery were implanted into the subcutaneous space of female BALB/c nude mice and serially passaged into new mice. Tumors were characterized by histology, short tandem repeat (STR) fingerprinting, von Hippel-Lindau (VHL) gene sequencing, and single nucleotide polymorphism (SNP) analysis. Tumor-bearing mice were treated with sunitinib or everolimus. Primary cell cultures were derived from patient tumors and transfected with a lentivirus carrying the luciferase gene. Four subcutaneous xenograft mouse models were developed, representing papillary type 1, papillary type 2, clear cell, and clear cell with sarcomatoid features RCC.

RESULTS AND LIMITATIONS

RCC mouse models were established from four patients with distinct histologies of RCC. Tumor growth was dependent on histologic type, the size of the implanted tumor chip, and the passage number. Mouse tumors accurately represented their respective original patient tumors, as STR fingerprints were matching, histology was comparable, and SNP profiles and VHL mutation status were conserved with multiple passages. Bioluminescence imaging results were commensurate with subcutaneous xenograft growth patterns. Mice treated with sunitinib and everolimus exhibited an initial response, followed by a later stage of resistance to these agents, which mimics the clinical observations in patients with RCC.

CONCLUSIONS

We developed four mouse xenograft models of RCC with clear-cell and papillary histologies, with stable histologic and molecular characteristics. These models can be used to understand the basic biology of RCC as well as response and resistance to therapy.

Merlin in organ size control and tumorigenesis: Hippo versus EGFR?

The role of the NF2 gene as a tumor suppressor has been well established. In this issue of Genes & Development, Benhamouche and colleagues (pp. 1718-1730) demonstrate that NF2 is also involved in the regulation of organ size control in mammals. Conditional knockout of Nf2 in the mouse liver results in massive organ enlargement and eventual tumor development, which is attributed to the specific expansion of oval cells. Here we discuss these findings and the proposed molecular mechanisms involved within the context of our current understanding of the pathways regulated by NF2.

Nf2/Merlin controls progenitor homeostasis and tumorigenesis in the liver

The molecular signals that control the maintenance and activation of liver stem/progenitor cells are poorly understood, and the role of liver progenitor cells in hepatic tumorigenesis is unclear. We report here that liver-specific deletion of the neurofibromatosis type 2 (Nf2) tumor suppressor gene in the developing or adult mouse specifically yields a dramatic, progressive expansion of progenitor cells throughout the liver without affecting differentiated hepatocytes. All surviving mice eventually developed both cholangiocellular and hepatocellular carcinoma, suggesting that Nf2(-/-) progenitors can be a cell of origin for these tumors. Despite the suggested link between Nf2 and the Hpo/Wts/Yki signaling pathway in Drosophila, and recent studies linking the corresponding Mst/Lats/Yap pathway to mammalian liver tumorigenesis, our molecular studies suggest that Merlin is not a major regulator of YAP in liver progenitors, and that the overproliferation of Nf2(-/-) liver progenitors is instead driven by aberrant epidermal growth factor receptor (EGFR) activity. Indeed, pharmacologic inhibition of EGFR blocks the proliferation of Nf2(-/-) liver progenitors in vitro and in vivo, consistent with recent studies indicating that the Nf2-encoded protein Merlin can control the abundance and signaling of membrane receptors such as EGFR. Together, our findings uncover a critical role for Nf2/Merlin in controlling homeostasis of the liver stem cell niche.

What a fish can learn from a mouse: principles and strategies for modeling human cancer in mice

This review highlights the current techniques used to generate transgenic mouse models of cancer, with an emphasis on recent advances in the use of ubiquitous promoters, models that use Cre-loxP and Flip-FRT recombinase technology, inducible systems, RNAi to target genes, and transposon mutagenesis. A concluding section discusses new imaging systems that visualize tumor progression and the microenvironment in vivo. In this review, these techniques and strategies used in mouse models of cancer are highlighted, as they are pertinent and relevant to the development of zebrafish models of cancer.