Inactivation of p21-Activated Kinase 2 (Pak2) Inhibits the Development of Nf2-Deficient Tumors by Restricting Downstream Hedgehog and Wnt Signaling

Because loss of the NF2 tumor suppressor gene results in p21-activated kinase (Pak) activation, PAK inhibitors hold promise for the treatment of NF2-deficient tumors. To test this possibility, we asked if loss of Pak2, a highly expressed group I PAK member, affects the development of malignant mesothelioma in Nf2;Cdkn2a-deficient (NC) mice and the growth properties of NC mesothelioma cells in culture. In vivo, deletion of Pak2 resulted in a markedly decreased incidence and delayed onset of both pleural and peritoneal malignant mesotheliomas in NC mice. In vitro, Pak2 deletion decreased malignant mesothelioma cell viability, migration, clonogenicity, and spheroid formation. RNA-sequencing analysis demonstrated downregulated expression of Hedgehog and Wnt pathway genes in NC;Pak2-/- mesothelioma cells versus NC;Pak2+/+ mesothelioma cells. Targeting of the Hedgehog signaling component Gli1 or its target gene Myc inhibited cell viability and spheroid formation in NC;P+/+ mesothelioma cells. Kinome profiling uncovered kinase changes indicative of EMT in NC;Pak2-/- mesothelioma cells, suggesting that Pak2-deficient malignant mesotheliomas can adapt by reprogramming their kinome in the absence of Pak activity. The identification of such compensatory pathways offers opportunities for rational combination therapies to circumvent resistance to anti-PAK drugs.

IMPLICATIONS

We provide evidence supporting a role for PAK inhibitors in treating NF2-deficient tumors. NF2-deficient tumors lacking Pak2 eventually adapt by kinome reprogramming, presenting opportunities for combination therapies to bypass anti-PAK drug resistance.

Somatic Epigenetic Silencing of RIPK3 Inactivates Necroptosis and Contributes to Chemoresistance in Malignant Mesothelioma

PURPOSE

Receptor-interacting protein kinase 3 (RIPK3) phosphorylates effector molecule MLKL to trigger necroptosis. Although RIPK3 loss is seen in several human cancers, its role in malignant mesothelioma is unknown. This study aimed to determine whether RIPK3 functions as a potential tumor suppressor to limit development of malignant mesothelioma.

EXPERIMENTAL DESIGN

RIPK3 expression was examined in 66 malignant mesothelioma tumors and cell lines. Promoter methylation and DNMT1 siRNA studies were performed to assess the mode of RIPK3 silencing in RIPK3-deficient malignant mesothelioma cells. Restoration of RIPK3 expression in RIPK3-negative malignant mesothelioma cells, either by treatment with 5-aza-2'-deoxycytidine or lentiviral expression of cDNA, was performed to assess effects on cell viability, necrosis, and chemosensitization.

RESULTS

Loss of RIPK3 expression was observed in 42/66 (63%) primary malignant mesotheliomas and malignant mesothelioma cell lines, and RT-PCR analysis demonstrated that downregulation occurs at the transcriptional level, consistent with epigenetic silencing. RIPK3-negative malignant mesothelioma cells treated with 5-aza-2'-deoxycytidine resulted in reexpression of RIPK3 and chemosensitization. Ectopic expression of RIPK3 also resulted in chemosensitization and led to necroptosis, the latter demonstrated by phosphorylation of downstream target MLKL and confirmed by rescue experiments. Mining of RIPK3 expression and survival outcomes among patients with malignant mesothelioma available from The Cancer Genome Atlas repository revealed that promoter methylation of RIPK3 is associated with reduced RIPK3 expression and poor prognosis.

CONCLUSIONS

These data suggest that RIPK3 acts as a tumor suppressor in malignant mesothelioma by triggering necroptosis and that epigenetic silencing of RIPK3 by DNA methylation impairs necroptosis and contributes to chemoresistance and poor survival in this incurable disease.

Challenging Global Waste Management - Bioremediation to Detoxify Asbestos

As the 21st century uncovers ever-increasing volumes of asbestos and asbestos-contaminated waste, we need a new way to stop 'grandfather's problem' from becoming that of our future generations. The production of inexpensive, mechanically strong, heat resistant building materials containing asbestos has inevitably led to its use in many public and residential buildings globally. It is therefore not surprising that since the asbestos boom in the 1970s, some 30 years later, the true extent of this hidden danger was exposed. Yet, this severely toxic material continues to be produced and used in some countries, and in others the disposal options for historic uses - generally landfill - are at best unwieldy and at worst insecure. We illustrate the global scale of the asbestos problem via three case studies which describe various removal and/or end disposal issues. These case studies from both industrialised and island nations demonstrate the potential for the generation of massive amounts of asbestos contaminated soil. In each case, the final outcome of the project was influenced by factors such as cost and land availability, both increasing issues, worldwide. The reduction in the generation of asbestos containing materials will not absolve us from the necessity of handling and disposal of contaminated land. Waste treatment which relies on physico-chemical processes is expensive and does not contribute to a circular model economy ideal. Although asbestos is a mineral substance, there are naturally occurring biological-mediated processes capable of degradation (such as bioweathering). Therefore, low energy options, such as bioremediation, for the treatment for asbestos contaminated soils are worth exploring. We outline evidence pointing to the ability of microbe and plant communities to remove from asbestos the iron that contributes to its carcinogenicity. Finally, we describe the potential for a novel concept of creating ecosystems over asbestos landfills ('activated landfills') that utilize nature's chelating ability to degrade this toxic product effectively.

Inactivation of Bap1 Cooperates with Losses of Nf2 and Cdkn2a to Drive the Development of Pleural Malignant Mesothelioma in Conditional Mouse Models

Pleural malignant mesothelioma is a therapy-resistant cancer affecting the serosal lining of the thoracic cavity. Mutations/deletions of BAP1, CDKN2A, and NF2 are the most frequent genetic lesions in human malignant mesothelioma. We introduced various combinations of these deletions in the pleura of conditional knockout (CKO) mice, focusing on the contribution of Bap1 loss. While homozygous CKO of Bap1, Cdkn2a, or Nf2 alone gave rise to few or no malignant mesotheliomas, inactivation of Bap1 cooperated with loss of either Nf2 or Cdkn2a to drive development of malignant mesothelioma in approximately 20% of double-CKO mice, and a high incidence (22/26, 85%) of malignant mesotheliomas was observed in Bap1;Nf2;Cdkn2a (triple)-CKO mice. Malignant mesothelioma onset was rapid in triple-CKO mice, with a median survival of only 12 weeks, and malignant mesotheliomas from these mice were consistently high-grade and invasive. Adenoviral-Cre treatment of normal mesothelial cells from Bap1;Nf2;Cdkn2a CKO mice, but not from mice with knockout of one or any two of these genes, resulted in robust spheroid formation in vitro, suggesting that mesothelial cells from Bap1;Nf2;Cdkn2a mice have stem cell-like potential. RNA-seq analysis of malignant mesotheliomas from triple-CKO mice revealed enrichment of genes transcriptionally regulated by the polycomb repressive complex 2 (PRC2) and others previously implicated in known Bap1-related cellular processes. These data demonstrate that somatic inactivation of Bap1, Nf2, and Cdkn2a results in rapid, aggressive malignant mesotheliomas, and that deletion of Bap1 contributes to tumor development, in part, by loss of PRC2-mediated repression of tumorigenic target genes and by acquisition of stem cell potential, suggesting a potential avenue for therapeutic intervention. SIGNIFICANCE: Combinatorial deletions of Bap1, Nf2, and Cdkn2a result in aggressive mesotheliomas, with Bap1 loss contributing to tumorigenesis by circumventing PRC2-mediated repression of oncogenic target genes.

Inactivation of Tp53 and Pten drives rapid development of pleural and peritoneal malignant mesotheliomas

Malignant mesothelioma (MM) is a therapy-resistant cancer arising primarily from the lining of the pleural and peritoneal cavities. The most frequently altered genes in human MM are cyclin-dependent kinase inhibitor 2A (CDKN2A), which encodes components of the p53 (p14ARF) and RB (p16INK4A) pathways, BRCA1-associated protein 1 (BAP1), and neurofibromatosis 2 (NF2). Furthermore, the p53 gene (TP53) itself is mutated in ~15% of MMs. In many MMs, the PI3K-PTEN-AKT-mTOR signaling node is hyperactivated, which contributes to tumor cell survival and therapeutic resistance. Here, we demonstrate that the inactivation of both Tp53 and Pten in the mouse mesothelium is sufficient to rapidly drive aggressive MMs. Pten^L/L ;Tp53^L/L mice injected intraperitoneally or intrapleurally with adenovirus-expressing Cre recombinase developed high rates of peritoneal and pleural MMs (92% of mice with a median latency of 9.4 weeks and 56% of mice with a median latency of 19.3 weeks, respectively). MM cells from these mice showed consistent activation of Akt-mTor signaling, chromosome breakage or aneuploidy, and upregulation of Myc; occasional downregulation of Bap1 was also observed. Collectively, these findings suggest that when Pten and Tp53 are lost in combination in mesothelial cells, DNA damage is not adequately repaired and genomic instability is widespread, whereas the activation of Akt due to Pten loss protects genomically damaged cells from apoptosis, thereby increasing the likelihood of tumor formation. Additionally, the mining of an online dataset (The Cancer Genome Atlas) revealed codeletions of PTEN and TP53 and/or CDKN2A/p14ARF in ~25% of human MMs, indicating that cooperative losses of these genes contribute to the development of a significant proportion of these aggressive neoplasms and suggesting key target pathways for therapeutic intervention.

Haploinsufficiency in tumor predisposition syndromes: altered genomic transcription in morphologically normal cells heterozygous for VHL or TSC mutation

Tumor suppressor genes and their effector pathways have been identified for many dominantly heritable cancers, enabling efforts to intervene early in the course of disease. Our approach on the subject of early intervention was to investigate gene expression patterns of morphologically normal one-hit cells before they become hemizygous or homozygous for the inherited mutant gene which is usually required for tumor formation. Here, we studied histologically non-transformed renal epithelial cells from patients with inherited disorders that predispose to renal tumors, including von Hippel-Lindau (VHL) disease and Tuberous Sclerosis (TSC). As controls, we studied histologically normal cells from non-cancerous renal epithelium of patients with sporadic clear cell renal cell carcinoma (ccRCC). Gene expression analyses of VHLmut/wt or TSC1/2mut/wt versus wild-type (WT) cells revealed transcriptomic alterations previously implicated in the transition to precancerous renal lesions. For example, the gene expression changes in VHLmut/wt cells were consistent with activation of the hypoxia response, associated, in part, with the Warburg effect. Knockdown of any remaining VHL mRNA using shRNA induced secondary expression changes, such as activation of NF?B and interferon pathways, that are fundamentally important in the development of RCC. We posit that this is a general pattern of hereditary cancer predisposition, wherein haploinsufficiency for VHL or TSC1/2, or potentially other tumor susceptibility genes, is sufficient to promote development of early lesions, while cancer results from inactivation of the remaining normal allele. The gene expression changes identified here are related to the metabolic basis of renal cancer and may constitute suitable targets for early intervention.

Abstract 842: The NALP3 inflammasome and IL-1â signaling link asbestos-induced inflammation with the development of malignant mesothelioma

Exposure to asbestos is causally associated with the development of malignant mesothelioma (MM), a cancer of cells lining the internal body cavity. MM is an aggressive cancer that is resistant to all current therapies. Once inhaled or ingested, asbestos causes inflammation in and around tissues that come in contact with these carcinogenic fibers. Recent studies suggest that inflammation is a major contributing factor in the development of many types of cancer, including MM. The NALP3 inflammasome, including the component ASC, is an important mediator of inflammation that senses extracellular insults, such as infection or asbestos, and activates a signaling cascade resulting in release of mature IL-1â and recruitment of inflammatory cells. To determine if inflammasome-mediated inflammation contributes to asbestos-induced MM, we chronically exposed Asc-deficient mice to asbestos and evaluated tumor incidence, latency and overall survival. Asc-deficient mice showed a significant delay in tumor onset compared to wild-type animals, suggesting that the NALP3 inflammasome plays a role in MM carcinogenesis. We also tested whether inflammation-related release of IL-1â promotes tumor development in an accelerated mouse model of asbestos-induced MM. Nf2+/-;Cdkn2a+/- mice exposed to asbestos in the presence of the IL-1â antagonist, anakinra, showed a 50% longer disease-free survival than in similarly exposed mice given vehicle control, consistent with IL-1â signaling contributing significantly to MM development. Collectively, these studies provide evidence for a link between asbestos-associated inflammation/IL-1â signaling and the development of MM; furthermore, these findings provide rationale for chemoprevention strategies targeting inflammation-related IL-1â signaling in high risk, asbestos-exposed populations.

Citation Format: Yuwaraj Kadariya, Craig W. Menges, Jacqueline Talarchek, Qi Cai, Andres J. Klein-Szanto, Ralph A. Pietrofesa, Melpo Christofidou-Solomidou, Brooke T. Mossman, Arti Shukla, Joseph R. Testa. The NALP3 inflammasome and IL-1â signaling link asbestos-induced inflammation with the development of malignant mesothelioma. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 842.

Inflammation-Related IL1β/IL1R Signaling Promotes the Development of Asbestos-Induced Malignant Mesothelioma

Exposure to asbestos is causally associated with the development of malignant mesothelioma, a cancer of cells lining the internal body cavities. Malignant mesothelioma is an aggressive cancer resistant to all current therapies. Once inhaled or ingested, asbestos causes inflammation in and around tissues that come in contact with these carcinogenic fibers. Recent studies suggest that inflammation is a major contributing factor in the development of many types of cancer, including malignant mesothelioma. The NALP3/NLRP3 inflammasome, including the component ASC, is thought to be an important mediator of inflammation in cells that sense extracellular insults, such as asbestos, and activate a signaling cascade resulting in release of mature IL1β and recruitment of inflammatory cells. To determine if inflammasome-mediated inflammation contributes to asbestos-induced malignant mesothelioma, we chronically exposed Asc-deficient mice and wild-type littermates to asbestos and evaluated differences in tumor incidence and latency. The Asc-deficient mice showed significantly delayed tumor onset and reduced malignant mesothelioma incidence compared with wild-type animals. We also tested whether inflammation-related release of IL1β contributes to tumor development in an accelerated mouse model of asbestos-induced malignant mesothelioma. Nf2(+/-);Cdkn2a(+/-) mice exposed to asbestos in the presence of anakinra, an IL1 receptor (IL1R) antagonist, showed a marked delay in the median time of malignant mesothelioma onset compared with similarly exposed mice given vehicle control (33.1 weeks vs. 22.6 weeks, respectively). Collectively, these studies provide evidence for a link between inflammation-related IL1β/IL1R signaling and the development of asbestos-induced malignant mesothelioma. Furthermore, these findings provide rationale for chemoprevention strategies targeting IL1β/IL1R signaling in high-risk, asbestos-exposed populations. Cancer Prev Res; 9(5); 406-14. ©2016 AACR.

Bap1 Is a Bona Fide Tumor Suppressor: Genetic Evidence from Mouse Models Carrying Heterozygous Germline Bap1 Mutations

Individuals harboring inherited heterozygous germline mutations in BAP1 are predisposed to a range of benign and malignant tumor types, including malignant mesothelioma, melanoma, and kidney carcinoma. However, evidence to support a tumor-suppressive role for BAP1 in cancer remains contradictory. To test experimentally whether BAP1 behaves as a tumor suppressor, we monitored spontaneous tumor development in three different mouse models with germline heterozygous mutations in Bap1, including two models in which the knock-in mutations are identical to those reported in human BAP1 cancer syndrome families. We observed spontaneous malignant tumors in 54 of 93 Bap1-mutant mice (58%) versus 4 of 43 (9%) wild-type littermates. All three Bap1-mutant models exhibited a high incidence and similar spectrum of neoplasms, including ovarian sex cord stromal tumors, lung and mammary carcinomas, and spindle cell tumors. Notably, we also observed malignant mesotheliomas in two Bap1-mutant mice, but not in any wild-type animals. We further confirmed that the remaining wild-type Bap1 allele was lost in both spontaneous ovarian tumors and mesotheliomas, resulting in the loss of Bap1 expression. Additional studies revealed that asbestos exposure induced a highly significant increase in the incidence of aggressive mesotheliomas in the two mouse models carrying clinically relevant Bap1 mutations compared with asbestos-exposed wild-type littermates. Collectively, these findings provide genetic evidence that Bap1 is a bona fide tumor suppressor gene and offer key insights into the contribution of carcinogen exposure to enhanced cancer susceptibility. Cancer Res; 76(9); 2836-44. ©2016 AACR.

Flaxseed lignans enriched in secoisolariciresinol diglucoside prevent acute asbestos-induced peritoneal inflammation in mice

Following acute exposure to crocidolite asbestos fibers, flaxseed lignans, enriched in secoisolariciresinol diglucoside (SDG), significantly reduced peritoneal inflammation, proinflammatory/profibrogenic cytokine release and oxidative/nitrosative stress in mice. Our findings support the potential role of SDG, which is safe and well-tolerated, in the chemoprevention of malignant mesothelioma.

Malignant mesothelioma (MM), linked to asbestos exposure, is a highly lethal form of thoracic cancer with a long latency period, high mortality and poor treatment options. Chronic inflammation and oxidative tissue damage caused by asbestos fibers are linked to MM development. Flaxseed lignans, enriched in secoisolariciresinol diglucoside (SDG), have antioxidant, anti-inflammatory and cancer chemopreventive properties. As a prelude to chronic chemoprevention studies for MM development, we tested the ability of flaxseed lignan component (FLC) to prevent acute asbestos-induced inflammation in MM-prone Nf2^+/mu mice. Mice (n = 16–17 per group) were placed on control (CTL) or FLC-supplemented diets initiated 7 days prior to a single intraperitoneal bolus of 400 µg of crocidolite asbestos. Three days post asbestos exposure, mice were evaluated for abdominal inflammation, proinflammatory/profibrogenic cytokine release, WBC gene expression changes and oxidative and nitrosative stress in peritoneal lavage fluid (PLF). Asbestos-exposed mice fed CTL diet developed acute inflammation, with significant (P < 0.0001) elevations in WBCs and proinflammatory/profibrogenic cytokines (IL-1ß, IL-6, TNFα, HMGB1 and active TGFß1) relative to baseline (BL) levels. Alternatively, asbestos-exposed FLC-fed mice had a significant (P < 0.0001) decrease in PLF WBCs and proinflammatory/profibrogenic cytokine levels relative to CTL-fed mice. Importantly, PLF WBC gene expression of cytokines (IL-1ß, IL-6, TNFα, HMGB1 and TGFß1) and cytokine receptors (TNFαR1 and TGFßR1) were also downregulated by FLC. FLC also significantly (P < 0.0001) blunted asbestos-induced nitrosative and oxidative stress. FLC reduces acute asbestos-induced peritoneal inflammation, nitrosative and oxidative stress and may thus prove to be a promising agent in the chemoprevention of MM.

Abstract 5542: Flaxseed and its lignan component protect from asbestos-induced inflammation in mice

Introduction/Background: Malignant mesothelioma (MM) is a highly lethal form of thoracic cancer with high mortality and poor treatment options. Development of MM has been linked directly to exposure to asbestos fibers, but with a long latency period. Recent studies have indicated that the pathogenesis of asbestos-induced cancers is due, in part, to chronic inflammation and oxidative tissue damage caused by persistent asbestos fibers. Whole grain flaxseed (FS) has known antioxidant, anti-inflammatory and cancer chemopreventive properties. Rationale: As a prelude to future chemoprevention studies, we tested the ability of oral FS and its lignan component, FLC which is enriched in the lignan secoisolariciresinol diglucoside (SDG), to prevent acute asbestos-induced inflammation and inflammatory cytokine release in MM-prone Nf2+/mut;Cdkn2a+/mut mice. Methods: Mice were given a single ip bolus of 400 mM of crocidolite asbestos. They were then placed on 10% FS or 10% FLC supplemented diets 1 day prior (-1) to or after (+1) asbestos instillation. All mice were evaluated 3 days after injection of asbestos for abdominal inflammation and proinflammatory cytokine release. The Nf2+/mut;Cdkn2a+/mut model was selected as it develops an accelerated form of MM when exposed to asbestos. Results: Using liquid chromatography and tandem mass spectrometry (LC/MS/MS), we showed that systemic levels (plasma) of flaxseed lignan metabolites (such as the mammalian lignans Enterolactone (EL) and Enterodiol (ED)) were comparable to those in other mouse models where FS was shown to be an effective chemopreventive agent. The numbers of macrophages (MF) and neutrophils (PMN) in peritoneal lavage fluid indicated that both FS and FLC blunted acute abdominal inflammation induced by asbestos. In addition, the levels of pro-inflammatory cytokines TNF alpha and IL-1 beta in lavage fluid were also decreased by the dietary agents. Conclusions: Our findings suggest that the known chemopreventive properties of FS and its lignan component appear to reduce short-term asbestos-induced inflammation and may thus prove to be a promising dietary agent in the chemoprevention of MM.

Funding: This study was supported in part by Penn's SRP Center Grant (P42 ES027320), “Asbestos fate, exposure, remediation and adverse health effects” from the National Institute of Environmental Health (NIEHS) and 1R03CA180548-02 (MCS).

Citation Format: Steven M. Albelda, Ralph A. Pietrofesa, Craig W. Menges, Joseph R. Testa, Melpo Christofidou-Solomidou. Flaxseed and its lignan component protect from asbestos-induced inflammation in mice. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 5542. doi:10.1158/1538-7445.AM2015-5542

Germline mutation of Bap1 accelerates development of asbestos-induced malignant mesothelioma

Malignant mesotheliomas are highly aggressive tumors usually caused by exposure to asbestos. Germline-inactivating mutations of BAP1 predispose to mesothelioma and certain other cancers. However, why mesothelioma is the predominate malignancy in some BAP1 families and not others, and whether exposure to asbestos is required for development of mesothelioma in BAP1 mutation carriers are not known. To address these questions experimentally, we generated a Bap1(+/-) knockout mouse model to assess its susceptibility to mesothelioma upon chronic exposure to asbestos. Bap1(+/-) mice exhibited a significantly higher incidence of asbestos-induced mesothelioma than wild-type (WT) littermates (73% vs. 32%, respectively). Furthermore, mesotheliomas arose at an accelerated rate in Bap1(+/-) mice than in WT animals (median survival, 43 weeks vs. 55 weeks after initial exposure, respectively) and showed increased invasiveness and proliferation. No spontaneous mesotheliomas were seen in unexposed Bap1(+/-) mice followed for up to 87 weeks of age. Mesothelioma cells from Bap1(+/-) mice showed biallelic inactivation of Bap1, consistent with its proposed role as a recessive cancer susceptibility gene. Unlike in WT mice, mesotheliomas from Bap1(+/-) mice did not require homozygous loss of Cdkn2a. However, normal mesothelial cells and mesothelioma cells from Bap1(+/-) mice showed downregulation of Rb through a p16(Ink4a)-independent mechanism, suggesting that predisposition of Bap1(+/-) mice to mesothelioma may be facilitated, in part, by cooperation between Bap1 and Rb. Drawing parallels to human disease, these unbiased genetic findings indicate that BAP1 mutation carriers are predisposed to the tumorigenic effects of asbestos and suggest that high penetrance of mesothelioma requires such environmental exposure.

Tumor suppressor alterations cooperate to drive aggressive mesotheliomas with enriched cancer stem cells via a p53-miR-34a-c-Met axis

Malignant mesothelioma is a highly aggressive, asbestos-related cancer frequently marked by mutations of both NF2 and CDKN2A. We demonstrate that germline knockout of one allele of each of these genes causes accelerated onset and progression of asbestos-induced malignant mesothelioma compared with asbestos-exposed Nf2(+/-) or wild-type mice. Ascites from some Nf2(+/-);Cdkn2a(+/-) mice exhibited large tumor spheroids, and tail vein injections of malignant mesothelioma cells established from these mice, but not from Nf2(+/-) or wild-type mice, produced numerous tumors in the lung, suggesting increased metastatic potential of tumor cells from Nf2(+/-);Cdkn2a(+/-) mice. Intraperitoneal injections of malignant mesothelioma cells derived from Nf2(+/-);Cdkn2a(+/-) mice into severe combined immunodeficient mice produced tumors that penetrated the diaphragm and pleural cavity and harbored increased cancer stem cells (CSC). Malignant mesothelioma cells from Nf2(+/-);Cdkn2a(+/-) mice stained positively for CSC markers and formed CSC spheroids in vitro more efficiently than counterparts from wild-type mice. Moreover, tumor cells from Nf2(+/-);Cdkn2a(+/-) mice showed elevated c-Met expression/activation, which was partly dependent on p53-mediated regulation of miR-34a and required for tumor migration/invasiveness and maintenance of the CSC population. Collectively, these studies demonstrate in vivo that inactivation of Nf2 and Cdkn2a cooperate to drive the development of highly aggressive malignant mesotheliomas characterized by enhanced tumor spreading capability and the presence of a CSC population associated with p53/miR-34a-dependent activation of c-Met. These findings suggest that cooperativity between losses of Nf2 and Cdkn2a plays a fundamental role in driving the highly aggressive tumorigenic phenotype considered to be a hallmark of malignant mesothelioma.

Abstract 924: FAK inhibition preferentially attenuates growth of Merlin-negative malignant mesotheliomas: role of cancer stem cells

Malignant pleural mesothelioma (MPM) is an aggressive tumor in the pleural lining of the lung often caused by asbestos exposure. MPM patients are usually diagnosed at an advanced stage of the disease and the prognosis is poor. Median survival after diagnosis is 9 to 12 months and standard-of-care agents such as pemetrexed are relatively ineffective in increasing median survival time for MPM patients. New therapeutic modalities are urgently needed for MPM patients.

Neurofibromatosis 2 (NF2) is a tumor suppressor gene that encodes the protein Merlin. Biallelic inactivation of NF2 by mutation and/or deletion occurs in ∼40% of MPMs leading to inactive or absent Merlin. Merlin has been demonstrated to play roles in cell adhesion, invasion and cell motility in tumor cell lines partially through regulation of focal adhesion kinase (FAK) which in turn mediates signal transduction by integrins and growth factor receptors. Increased activation of FAK has been demonstrated in NF2-mutated mesothelioma cells, indicating that FAK may represent an important therapeutic target for MPM.

A potent and selective FAK inhibitor VS-4718 was evaluated in a panel of MPM cell lines with wild-type or mutated NF2. Mutant NF2 MPM cells were found to be especially sensitive to the FAK inhibitor VS-4718 in a 3D Matrigel assay with EC50 values below 100 nM, in contrast to wild type NF2 MPM cell lines which were less sensitive with EC50 values above 1 μM. Decrease in cell growth in response to FAK inhibitor treatment was found to result from reduction of cell proliferation and induction of apoptosis. Ectopic expression of a non-phosphorylatable mutant form of NF2 (NF2-S518A) in NF2 mutant MPM cells abolished the enhanced sensitivity to VS-4718 in both proliferation and apoptosis assays, confirming the hypothesis that Merlin loss confers hypersensitivity to FAK inhibition. In addition, VS-4718 inhibited tumor cell growth in an orthotopic Merlin-negative mesothelioma xenograft model in a dose-dependent manner with corresponding inhibition of tumor FAK autophosphorylation. It has been reported that cancer stem cells (CSCs) are enriched in pemetrexed-resistant mesothelioma. Using Aldefluor activity as a measure of CSCs, we have found that the FAK inhibitor VS-4718 preferentially reduced the percentage of CSCs in Merlin-negative MPM in contrast to the standard-of-care agent pemetrexed which increased the percentage of CSCs. In summary, our results indicate that the FAK inhibitor VS-4718 is especially potent in Merlin-negative MPM tumor cells, and that NF2 status may be a valuable stratification marker for response to FAK inhibition. Furthermore, cancer stem cells in Merlin-negative mesotheliomas appear to be particularly resistant to pemetrexed, but sensitive to VS-4718. These data support the clinical development of a selective FAK inhibitor for treatment of Merlin-negative malignant mesothelioma.

Citation Format: Irina M. Shapiro, Vihren N. Kolev, Christian M. Vidal, Jennifer E. Ring, Mitchell Keegan, Qunli Xu, Craig W. Menges, Joseph R. Testa, Jonathan A. Pachter. FAK inhibition preferentially attenuates growth of Merlin-negative malignant mesotheliomas: role of cancer stem cells. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 924. doi:10.1158/1538-7445.AM2013-924

Group I p21-activated kinases (PAKs) promote tumor cell proliferation and survival through the AKT1 and Raf-MAPK pathways

Group I p21-activated kinases (PAK) are important effectors of the small GTPases Rac and Cdc42, which regulate cell motility/migration, survival, proliferation, and gene transcription. Hyperactivation of these kinases have been reported in many tumor types, making PAKs attractive targets for therapeutic intervention. PAKs are activated by growth factor-mediated signaling and are negatively regulated by the tumor suppressor neurofibromatosis type 2 (NF2)/Merlin. Thus, tumors characterized by NF2 inactivation would be expected to show hyperactivated PAK signaling. On the basis of this rationale, we evaluated the status of PAK signaling in malignant mesothelioma, an aggressive neoplasm that is resistant to current therapies and shows frequent inactivation of NF2. We show that group I PAKs are activated in most mesotheliomas and mesothelioma cell lines and that genetic or pharmacologic inhibition of PAKs is sufficient to inhibit mesothelioma cell proliferation and survival. We also identify downstream effectors and signaling pathways that may contribute mechanistically to PAK-related tumorigenesis. Specifically, we show that inhibition of PAK results in attenuation of AKT and Raf-MAPK signaling and decreased tumor cell viability. Collectively, these data suggest that pharmacologic inhibition of group I PAKs may have therapeutic efficacy in tumors characterized by PAK activation.

Abstract 4312: Loss of Ink4a and Arf each contributes to increased susceptibility to asbestos-induced malignant mesothelioma development

The CDKN2A/ARF locus encompasses overlapping tumor suppressor genes p16(INK4A) and p14(ARF), which are frequently co-deleted in human malignant mesothelioma (MM). The importance of p16(INK4A) loss in human cancer is well established, but the relative significance of p14(ARF) loss has been debated. The tumor predisposition of mice singly deficient for either Ink4a or Arf, due to targeting of exons 1α or 1β, respectively, supports the idea that both play significant and nonredundant roles in suppressing spontaneous tumors. To further test this notion, we exposed Ink4a(+/-) and Arf(+/-) mice to asbestos, the major cause of MM. Asbestos-treated Ink4a(+/-) and Arf(+/-) mice showed increased incidence and shorter latency of MM relative to wild-type littermates. MMs from Ink4a(+/-) mice exhibited biallelic inactivation of Ink4a, loss of Arf or p53 expression and frequent loss of p15(Ink4b). In contrast, MMs from Arf (+/-) mice exhibited loss of Arf expression, but did not require loss of Ink4a or Ink4b. Mice doubly deficient for Ink4a and Arf, due to deletion of Cdkn2a/Arf exon 2, showed accelerated asbestos-induced MM formation relative to mice deficient for Ink4a or Arf alone, and MMs exhibited biallelic loss of both tumor suppressor genes. The tumor suppressor function of Arf in MM was p53-independent, since MMs with loss of Arf retained functional p53. Collectively, these investigations provide in vivo evidence indicating that both CDKN2A/ARF gene products suppress asbestos carcinogenicity. Furthermore, while inactivation of ARF appears to be crucial for MM pathogenesis, the inactivation of both p16(INK4A) and p19(ARF) cooperate to accelerate asbestos-induced tumorigenesis.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 4312. doi:10.1158/1538-7445.AM2011-4312

Upregulation of DLX5 promotes ovarian cancer cell proliferation by enhancing IRS-2-AKT signaling

The distal-less homeobox gene (dlx) 5 encodes a transcription factor that controls jaw formation and appendage differentiation during embryonic development. We had previously found that Dlx5 is overexpressed in an Akt2 transgenic model of T-cell lymphoma. To investigate if DLX5 is involved in human cancer, we screened its expression in the NCI 60 cancer cell line panel. DLX5 was frequently upregulated in cell lines derived from several tumor types, including ovarian cancer. We next validated its upregulation in primary ovarian cancer specimens. Stable knockdown of DLX5 by lentivirus-mediated transduction of short hairpin RNA (shRNA) resulted in reduced proliferation of ovarian cancer cells due to inhibition of cell cycle progression in connection with the downregulation of cyclins A, B1, D1, D2, and E, and decreased phosphorylation of AKT. Cell proliferation resumed following introduction of a DLX5 cDNA harboring wobbled mutations at the shRNA-targeting sites. Cell proliferation was also rescued by transduction of a constitutively active form of AKT. Intriguingly, downregulation of IRS-2 and MET contributed to the suppression of AKT signaling. Moreover, DLX5 was found to directly bind to the IRS-2 promoter and augmented its transcription. Knockdown of DLX5 in xenografts of human ovarian cancer cells resulted in markedly diminished tumor size. In addition, DLX5 was found to cooperate with HRAS in the transformation of human ovarian surface epithelial cells. Together, these data suggest that DLX5 plays a significant role in the pathogenesis of some ovarian cancers.

A Phosphotyrosine Proteomic Screen Identifies Multiple Tyrosine Kinase Signaling Pathways Aberrantly Activated in Malignant Mesothelioma

Malignant mesothelioma (MM) is a highly aggressive cancer that is refractory to all current chemotherapeutic regimens. Therefore, uncovering new rational therapeutic targets is imperative in the field. Tyrosine kinase signaling pathways are aberrantly activated in many human cancers and are currently being targeted for chemotherapeutic intervention. Thus, we sought to identify tyrosine kinases hyperactivated in MM. An unbiased phosphotyrosine proteomic screen was employed to identify tyrosine kinases activated in human MM cell lines. From this screen, we have identified novel signaling molecules, such as JAK1, STAT1, cortactin (CTTN), FER, p130Cas (BCAR1), SRC and FYN as tyrosine phosphorylated in human MM cell lines. Additionally, STAT1 and SRC family kinases (SFK) were confirmed to be active in primary MM specimens. We also confirmed that known signal transduction pathways previously implicated in MM, such as EGFR and MET signaling axes, are co-activated in the majority of human MM specimens and cell lines tested. EGFR, MET, and SFK appear to be co-activated in a significant proportion of MM cell lines, and dual inhibition of these kinases was demonstrated to be more efficacious for inhibiting MM cell viability and downstream effector signaling than inhibition of a single tyrosine kinase. Consequently, these data suggest that TKI mono-therapy may not represent an efficacious strategy for the treatment of MM, due to multiple tyrosine kinases potentially signaling redundantly to cellular pathways involved in tumor cell survival and proliferation.

FAS-associated factor 1 (FAF1): diverse functions and implications for oncogenesis

FAS-associated factor 1, FAF1, is an evolutionarily conserved protein that has several protein interaction domains. Although FAF1 was initially identified as a member of the FAS death-inducing signaling complex, subsequent work has revealed that FAF1 functions in diverse biological processes. FAF1 has been shown to play an important role in normal development and neuronal cell survival, whereas FAF1 downregulation may contribute to multiple aspects of tumorigenesis. In particular, there is compelling evidence implicating FAF1 as a tumor suppressor involved in the regulation of apoptosis and NFkappaB activity, as well as in ubiquitination and proteasomal degradation. Here, we highlight FAF1's role in NFkappaB signaling and postulate that this pathway has critical connotations for the pathogenesis and treatment of human cancers.

Constitutive activation of the Raf-MAPK pathway causes negative feedback inhibition of Ras-PI3K-AKT and cellular arrest through the EphA2 receptor

The Raf-mitogen-activated protein kinase (MAPK) and phosphatidylinositide 3-kinase (PI3K)-AKT pathways are two downstream effectors of the small GTPase Ras. Although both pathways are positively regulated by Ras, the Raf-MAPK and PI3K-AKT pathways have been shown to control opposing functions within the cell, suggesting a need for cross-talk regulation. The PI3K-AKT pathway can inhibit the Raf-MAPK pathway directly during processes such as muscle differentiation. Here we describe the ability of the Raf-MAPK pathway to negatively regulate the PI3K-AKT pathway during cellular arrest. Constitutive activation of Raf or methyl ethyl ketone 1 (MEK1) leads to inhibition of AKT and cellular arrest. Furthermore, we show that activation of Raf-MEK1 signaling causes negative feedback inhibition of Ras through the ephrin receptor EphA2. EphA2-mediated negative feedback inhibition is required for Raf-induced AKT inhibition and cell cycle arrest, therefore establishing the inhibition of the Ras-PI3K-AKT pathway as a necessary event for the Raf-MEK1-regulated cellular arrest.

Human papillomavirus type 16 E7 up-regulates AKT activity through the retinoblastoma protein

Human papillomaviruses (HPV) are small DNA tumor viruses causally associated with cervical cancer. The early gene product E7 from high-risk HPV is considered the major transforming protein expressed by the virus. Although many functions have been described for E7 in disrupting normal cellular processes, we describe in this study a new cellular target in primary human foreskin keratinocytes (HFK), the serine/threonine kinase AKT. Expression of HPV type 16 E7 in HFK caused inhibition of differentiation, hyperproliferation, and up-regulation of AKT activity in organotypic raft cultures. The ability of E7 to up-regulate AKT activity is dependent on its ability to bind to and inactivate the retinoblastoma (Rb) gene product family of proteins. Furthermore, we show that knocking down Rb alone, with short hairpin RNAs, was sufficient to up-regulate AKT activity in differentiated keratinocytes. Up-regulation of AKT activity and loss of Rb was also observed in HPV-positive cervical high-grade squamous intraepithelial lesions when compared with normal cervical tissue. Together, these data provide evidence linking inactivation of Rb by E7 in the up-regulation of AKT activity during cervical cancer progression.

AKT1 provides an essential survival signal required for differentiation and stratification of primary human keratinocytes

Keratinocyte differentiation and stratification are complex processes involving multiple signaling pathways, which convert a basal proliferative cell into an inviable rigid squame. Loss of attachment to the basement membrane triggers keratinocyte differentiation, while in other epithelial cells, detachment from the extracellular matrix leads to rapid programmed cell death or anoikis. The potential role of AKT in providing a survival signal necessary for stratification and differentiation of primary human keratinocytes was investigated. AKT activity increased during keratinocyte differentiation and was attributed to the specific activation of AKT1 and AKT2. Targeted reduction of AKT1 expression, but not AKT2, by RNA interference resulted in an abnormal epidermis in organotypic skin cultures with a thin parabasal region and a pronounced but disorganized cornified layer. This abnormal stratification was due to significant cell death in the suprabasal layers and was alleviated by caspase inhibition. Normal expression patterns of both early and late markers of keratinocyte differentiation were also disrupted, producing a poorly developed stratum corneum.