MET exon 14 skipping mutation is a hepatocyte growth factor (HGF)-dependent oncogenic driver in vitro and in humanised HGF knock-in mice

Exon skipping mutations of the MET receptor tyrosine kinase (METex14), increasingly reported in cancers, occur in 3-4% of non-small-cell lung cancer (NSCLC). Only 50% of patients have a beneficial response to treatment with MET-tyrosine kinase inhibitors (TKIs), underlying the need to understand the mechanism of METex14 oncogenicity and sensitivity to TKIs. Whether METex14 is a driver mutation and whether it requires hepatocyte growth factor (HGF) for its oncogenicity in a range of in vitro functions and in vivo has not been fully elucidated from previous preclinical models. Using CRISPR/Cas9, we developed a METex14/WT isogenic model in nontransformed human lung cells and report that the METex14 single alteration was sufficient to drive MET-dependent in vitro anchorage-independent survival and motility and in vivo tumorigenesis, sensitising tumours to MET-TKIs. However, we also show that human HGF (hHGF) is required, as demonstrated in vivo using a humanised HGF knock-in strain of mice and further detected in tumour cells of METex14 NSCLC patient samples. Our results also suggest that METex14 oncogenicity is not a consequence of an escape from degradation in our cell model. Thus, we developed a valuable model for preclinical studies and present results that have potential clinical implication.

Combined regulation of pro-inflammatory cytokines production by STAT3 and STAT5 in a model of B. pertussis infection of alveolar macrophages

Bordetella pertussis is a highly contagious respiratory pathogen responsible for whooping-cough or pertussis. Despite high vaccination coverage worldwide, this gram-negative bacterium continues to spread among the population. B. pertussis is transmitted by aerosol droplets from an infected individual to a new host and will colonize its upper respiratory tract. Alveolar macrophages (AMs) are effector cells of the innate immune system that phagocytose B. pertussis and secrete both pro-inflammatory and antimicrobial mediators in the lungs. However, understanding their role in B. pertussis pathogenesis at the molecular level is hampered by the limited number of primary AMs that can be collected in vivo. In order to decipher the regulation of innate response induced by B. pertussis infection, we used for the first time self-renewing, non-transformed cells, called Max Planck Institute (MPI) cells, which are phenotypically and functionally very close to pulmonary AMs. Using optimized infection conditions, we characterized the entry and the clearance of B. pertussis within MPI macrophages. We showed that under these conditions, MPI cells exhibit a pro-inflammatory phenotype with the production of TNF, IL-1β, IL-6 and MIP-2α, similarly to primary AMs purified from broncho-alveolar fluids of mice. In addition, we explored the yet uncharacterized role of the signal transduction activator of transcription (STAT) proteins family in the innate immune response to B. pertussis infection and showed for the first time the parallel regulation of pro-inflammatory cytokines by STAT3 and STAT5 in MPI macrophages infected by B. pertussis. Altogether, this work highlights the interest of using MPI cells for experiments optimization and preliminary data acquisition to understand B. pertussis interaction with AMs, and thus significantly reduce the number of animals to be sacrificed.

Transforming properties of MET receptor exon 14 skipping can be recapitulated by loss of the CBL ubiquitin ligase binding site

MET is a receptor tyrosine kinase that is activated in many cancers through various mechanisms. MET exon 14 (Ex14) skipping occurs in 3% of nonsmall cell lung tumors. However, the contribution of the regulatory sites lost upon this skipping, which include a phosphorylated serine (S985) and a binding site for the E3 ubiquitin ligase CBL (Y1003), remains elusive. Sequencing of 2808 lung tumors revealed 71 mutations leading to MET exon 14 skipping and three mutations affecting Y1003 or S985. In addition, MET exon 14 skipping and MET Y1003F induced similar transcriptional programs, increased the activation of downstream signaling pathways, and increased cell mobility. Therefore, the MET Y1003F mutation is able to fully recapitulate responses induced by MET exon 14 skipping, suggesting that loss of the CBL binding site is the main contributor of cell transformation induced by MET Ex14 mutations.

MET kinase mutations in lung cancer patients

e21014

Background: Although mutations in the kinase domain of MET are well described in papillary renal cell carcinoma, they are rare and poorly studied in lung cancer, but could constitute a new therapeutic target given the availability of potent MET inhibitors. The characteristics of patients with MET kinase mutation in lung cancer are still unknown. Our objective was to describe the demographic, clinical and biological characteristics of these patients, and analyze their survival. Methods: We conducted a multicenter retrospective study including patients with lung cancer harboring a MET kinase mutation. Patients who had already received treatment with a MET TKI before the MET kinase mutation was found were excluded. Results: We identified 37 patients with a MET kinase mutation. Among them, 2 had already received a MET inhibitor and in 8 cases, the data were not accessible. A total of 27 patients were included in the final analysis, including 17 males (63%) and only 2 (8%) never smokers. The median age was 64 (range 43-86). Most patients had adenocarcinoma (n = 25, 93%). 19 patients were diagnosed at an advanced stage. The main metastatic sites were brain (n = 8, 42%), bones (n = 7, 37%) and lungs (n = 5, 19%). PDL1 expression level was available for 21 patients, and was < 1%, 1-49% and ≥50% in 4 (19%), 5 (24%) and 12 (57%) patients respectively. MET kinase mutations involved exon 15, 16, 17, 18 or 19 in 6 (22%), 7 (26%), 7 (26%), 3 (11%) and 4 (15%) patients respectively. The 2 most common mutations were H1112Y (4 patients) and H1097R (3 patients). 18 patients (67%) had a concurrent alteration by NGS including TP53 mutations (n = 11, 41%), KRAS mutations (n = 6, 22%) and NRAS mutations (n = 3, 11%). Overall, co-alterations involving known driver oncogenes were detected in 13 patients (48%). 19 patients received a first-line systemic treatment for advanced disease. Median progression-free survival with chemotherapy and immunotherapy was 10.5 and 6.4 months, respectively (p = 0.55). Median overall survival was 12.2 months. No patient received a MET inhibitor. Conclusions: MET kinase mutations are not associated with specific clinical characteristics and frequently occur together with other oncogene mutations. These results suggest that these mutations may not be sufficient to drive carcinogenesis by themselves.

Abstract 3683: MET exon 14 skipping mutations in lung cancer: Screening, functional and clinical impact

Mutations affecting exon 14 splice sites of the gene encoding the MET receptor have been recently revealed in non-small cell lung cancer (NSCLC). These mutations induce MET exon 14 skipping (METex14), leading to receptor activation through deletion of a regulatory domain. Importantly, these mutations represent a promising therapeutic opportunity since MET tyrosine kinase inhibitors (TKI) are available. Nevertheless, these mutations raise several scientific and clinical questions. (i) Their functional consequences are still poorly understood,(ii) these mutations are highly heterogeneous which makes them difficult to detect, and (iii) efficacy of MET-TKI seems limited by largely unknown resistances. To address these issues, we first created by genome editing a panel of pulmonary cells expressing either METex14 or MET receptor mutated in each known active site of the regulatory domain. Comparison of signalling pathways, transcriptional landscapes and cellular responses revealed that METex14 activation is recapitulated by mutation of the CBL binding site involved in MET internalization, but provide also an unexpected resistance to apoptosis through abrogating its caspase cleavage. Second, in order to detect METex14 mutations in clinical routine practice, we developed an optimized targeted next generation sequencing panel covering the METex14 in addition to the usual targets. This panel revealed METex14 alterations in 2.2% NSCLC patients and presence of various concurrent alterations. Third, by further characterization of the concurrent alterations, we found high rate of PI3K pathway alterations in METex14 patients. In addition, MET-TKI treatment in 3 patients harboring these alterations had shown progressive disease, suggesting their involvement in resistance. Using a patient-derived cell line with primary resistance and cell lines in which MET or PI3K alterations were inserted, we confirmed involvement of PI3K activation in the resistance process, which was overcome with PI3K inhibitor. Overall, our integrated study reveals that METex14 mutations induce an original activation involving cooperation between regulatory mechanisms, but offering sensitivity to MET-TKI. Therefore, these mutations, now detectable in routine practice, are druggable by MET-TKI providing a novel therapeutic line for NSCLC, but have to face to innate resistance including PI3K alterations.

Citation Format: Marie Fernandes, Philippe Jamme, Sonia Paget, Angela Morabito, Frédéric Leprêtre, Martin Figeac, Clotilde Descarpentries, Fabienne Escande, Simon Baldacci, Anne Chotteau-Lelièvre, Luca Grumolato, Marie-Christine Copin, Zoulika Kherrouche, Alexis B. Cortot, David Tulasne. MET exon 14 skipping mutations in lung cancer: Screening, functional and clinical impact [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 3683.

High MET Overexpression Does Not Predict the presence of MET exon 14 Splice Mutations in NSCLC: Results From the IFCT PREDICT.amm study

INTRODUCTION

MET proto-oncogene (MET) exon 14 splice site (METex14) mutations were recently described in NSCLC and has been reported to correlate with efficacy of MET tyrosine kinase inhibitors. High diversity of these alterations makes them hard to detect by DNA sequencing in clinical practice. Because METex14 mutations induce increased stabilization of the MET receptor, it is anticipated that these mutations are associated with MET overexpression. We aim to determine whether NSCLC with high MET overexpression could define a subset of patients with a high rate of METex14 mutations.

METHODS

From The French Cooperative Thoracic Intergroup PREDICT.amm cohort of 843 consecutive patients with a treatment-naive advanced NSCLC who were eligible for a first-line therapy, 108 NSCLC samples with high MET overexpression defined by an immunochemistry score 3+ were tested for METex14 mutations using fragment length analysis combined with optimized targeted next-generation sequencing. MET copy number analysis was also derived from the sequencing data.

RESULTS

METex14 mutations were detected in two patients (2.2%) who also displayed a TP53 mutation and a PIK3CA mutation, respectively. An MET gene copy number increase was observed in seven additional patients (7.7%). Next-generation sequencing analysis revealed inactivating mutations in TP53 (52.7%) and PTEN (1.1%), and oncogenic mutations in KRAS (28.6%), EGFR (7.7%), PIK3CA (4.4%), BRAF (4.4%), NRAS (2.2%), GNAS (1.1%), and IDH1 (1.1%).

CONCLUSIONS

The rate of METex14 mutations in NSCLC with high MET overexpression was similar to that found in unselected NSCLC. Moreover, we observed a high frequency of driver alterations in other oncogenes. Consequently these findings do not support the use of MET immunohistochemistry as a surrogate marker for METex14 mutations.

Relevance of Detection of Mechanisms of Resistance to ALK Inhibitors in ALK-Rearranged NSCLC in Routine Practice

BACKGROUND

Anaplastic lymphoma kinase (ALK) tyrosine kinase inhibitors (TKIs) have shown efficacy in the treatment of ALK-rearranged non-small-cell lung cancer (NSCLC), but the disease eventually progresses in all patients. In many cases, resistance to ALK TKIs arises through ALK mutations. Although clinical and biological data suggest variations in TKI efficacy according to the mechanism of resistance, ALK mutations are still rarely investigated in routine practice.

MATERIALS AND METHODS

We performed a retrospective multicentric study with an aim to determine the frequency and clinical relevance of ALK alterations detected using targeted next-generation sequencing in patients with advanced ALK-rearranged NSCLC after progression during an ALK TKI treatment. Data on clinical, pathological, and molecular characteristics and patient outcomes were collected.

RESULTS

We identified 23 patients with advanced ALK-rearranged NSCLC who, between January 2012 and May 2017, had undergone at least 1 repeat biopsy at progression during an ALK TKI treatment. A resistance mechanism was identified in 9 of the 23 patients (39%). The anomalies involved included 9 ALK mutations in 8 patients and one ALK amplification. The ALK mutation rate was 15% after failure of a first ALK TKI and 33% after failure of 2 ALK TKI treatments. Five of 7 patients who received a different ALK TKI after detection of an ALK mutation achieved an objective response. All of the patients who received a TKI presumed to act on the detected ALK mutant achieved disease control.

CONCLUSION

Targeted next-generation sequencing is suitable for detecting ALK resistance mutations in ALK-rearranged NSCLC patients in routine practice. It might help select the best treatment at the time of disease progression during treatment with an ALK TKI.

MET amplification increases the metastatic spread of EGFR-mutated NSCLC

BACKGROUND

Five to 20% of metastatic EGFR-mutated non-small cell lung cancers (NSCLC) develop acquired resistance to EGFR tyrosine kinase inhibitors (EGFR-TKI) through MET amplification. The effects of MET amplification on tumor and patient phenotype remain unknown.

METHODS

We investigated,in vitro and in vivo, the impact of MET amplification on the biological properties of the HCC827 cell line, derived from an EGFR-mutated NSCLC. We further evaluated the time to new metastases after EGFR-TKI progression in EGFR-mutated NSCLC, exhibiting MET amplification or high MET overexpression.

RESULTS

MET amplification significantly enhanced proliferation, anchorage independent growth, anoikis resistance, migration, and induced an epithelial to mesenchymal transition. In vivo, MET amplification significantly increased the tumor growth and metastatic spread. Treatment with a MET-TKI reversed this aggressive phenotype. We found that EGFR-mutated NSCLC patients exhibiting MET amplification on a re-biopsy, performed after EGFR-TKI progression, displayed a shorter time to new metastases after EGFR-TKI progression than patients with high MET overexpression but no MET amplification.

CONCLUSION

MET amplification increases metastatic spread even in the context of an already pre-existing strong driver mutation such as EGFR mutation. These results prompt development of therapeutic strategies aiming at preventing emergence of MET amplification.

[MET exon 14 splicing sites mutations: A new therapeutic opportunity in lung cancer]

The mutations leading to MET exon 14 skipping represent a new class of molecular alterations described in various cancers. These alterations are observed in 2 to 3 % of cases of non-small cell lung cancer (NSCLC). Several cases of NSCLC carrying such alterations and achieving objective response to MET tyrosine kinase inhibitorshave recently been published. This review summarizes the molecular mechanisms responsible for MET exon 14 skipping as well as the consequences of the loss of this exon on receptor activity. We also describe the clinical characteristics of patients with METΔ14 mutations. Finally, we address the issues related to the detection of these mutations in lung cancer, and the need to anticipate resistance to MET inhibitors.

Outcome of EGFR-mutated NSCLC patients with MET-driven resistance to EGFR tyrosine kinase inhibitors

Background

Several mechanisms of acquired resistance to EGFR tyrosine kinase inhibitors (TKIs) in EGFR-mutated NSCLC have been described including the T790M mutation and MET amplification. Whereas T790M mutation confers prolonged survival and sensitivity to 3rd generation TKIs, data are lacking on clinical features and outcome of MET-driven resistant EGFR-mutated NSCLC patients.

Methods

Patients with metastatic EGFR-mutated NSCLC displaying high MET overexpression or MET amplification, detected on a biopsy performed after progression on EGFR TKI, were identified in 15 centers. Clinical and molecular data were retrospectively collected.

Results

Forty two patients were included. The median overall survival (OS), and the median post EGFR TKI progression overall survival (PPOS) were 36.2 months [95%CI 27.3-66.5] and 18.5 months [95%CI 10.6-27.4] respectively. Nineteen out of 36 tumors tested for MET FISH had MET amplification. A T790M mutation was found in 11/41 (26.8%) patients. T790M-positive patients had a better OS than T790M-negative patients (p=0.0224). Nineteen patients received a MET TKI. Objective response was reported in 1 out of 12 evaluable patients treated with a MET inhibitor as a single agent and in 1 of 2 patients treated with a combination of MET and EGFR TKIs.

Conclusion

MET-driven resistance to EGFR TKI defines a specific pattern of resistance characterized by low objective response rate to MET inhibitors given alone and overlapping with T790M mutations. Further studies are warranted to define adequate therapeutic strategies for MET-driven resistance to EGFR TKI.

PEA3 transcription factors are downstream effectors of Met signaling involved in migration and invasiveness of Met-addicted tumor cells

Various solid tumors including lung or gastric carcinomas display aberrant activation of the Met receptor which correlates with aggressive phenotypes and poor prognosis. Although downstream signaling of Met is well described, its integration at the transcriptional level is poorly understood. We demonstrate here that in cancer cells harboring met gene amplification, inhibition of Met activity with tyrosine kinase inhibitors or specific siRNA drastically decreased expression of ETV1, ETV4 and ETV5, three transcription factors constituting the PEA3 subgroup of the ETS family, while expression of the other members of the family were less or not affected. Similar link between Met activity and PEA3 factors expression was found in lung cancer cells displaying resistance to EGFR targeted therapy involving met gene amplification. Using silencing experiments, we demonstrate that the PEA3 factors are required for efficient migration and invasion mediated by Met, while other biological responses such as proliferation or unanchored growth remain unaffected. PEA3 overexpression or silencing revealed that they participated in the regulation of the MMP2 target gene involved in extracellular matrix remodeling. Our results demonstrated that PEA3-subgroup transcription factors are key players of the Met signaling integration involved in regulation of migration and invasiveness.

[Thirty years of Met receptor research: from the discovery of an oncogene to the development of targeted therapies]

In 1984, the Met receptor and its ligand, the HGF/SF, were discovered thanks to their ability to induce cell transformation and proliferation. Thirty years of research highlighted their crucial role in the development and homeostasis of various structures, including many epithelial organs. This period also allowed unraveling the structural basis of their interaction and their complex signaling network. In parallel, Met was shown to be deregulated and associated with a poor prognosis in many cancers. Met involvement in resistance to current therapies is also being deciphered. Based on these data, pharmaceutical companies developed a variety of Met inhibitors, some of which are evaluated in phase III clinical trials. In this review, we trace the exemplary track record of research on Met receptor, which allowed moving from bench to bedside through the development of therapies targeting its activity. Many questions still remain unanswered such as the involvement of Met in several processes of development, the mechanisms involving Met in resistance to current therapies or the likely emergence of resistances to Met-targeted therapies.

Caspase-generated fragment of the Met receptor favors apoptosis via the intrinsic pathway independently of its tyrosine kinase activity

The receptor tyrosine kinase Met and its ligand, the hepatocyte growth factor, are essential to embryonic development, whereas the deregulation of Met signaling is associated with tumorigenesis. While ligand-activated Met promotes survival, caspase-dependent generation of the p40 Met fragment leads to apoptosis induction - hallmark of the dependence receptor. Although the survival signaling pathways induced by Met are well described, the pro-apoptotic signaling pathways are unknown. We show that, although p40 Met contains the entire kinase domain, it accelerates apoptosis independently of kinase activity. In cell cultures undergoing apoptosis, the fragment shows a mitochondrial localization, required for p40 Met-induced cell death. Fulminant hepatic failure induced in mice leads to the generation of p40 Met localized also in the mitochondria, demonstrating caspase cleavage of Met in vivo. According to its localization, the fragment induces mitochondrial permeabilization, which is inhibited by Bak silencing and Bcl-xL overexpression. Moreover, Met silencing delays mitochondrial permeabilization induced by an apoptotic treatment. Thus, the Met-dependence receptor in addition to its well-known role in survival signaling mediated by its kinase activity, also participates in the intrinsic apoptosis pathway through the generation of p40 Met - a caspase-dependent fragment of Met implicated in the mitochondrial permeabilization process.

Loss of a negative feedback loop involving pea3 and cyclin d2 is required for pea3-induced migration in transformed mammary epithelial cells

The Ets family transcription factor Pea3 (ETV4) is involved in tumorigenesis especially during the metastatic process. Pea3 is known to induce migration and invasion in mammary epithelial cell model systems. However, the molecular pathways regulated by Pea3 are still misunderstood. In the current study, using in vivo and in vitro assays, Pea3 increased the morphogenetic and tumorigenic capacity of mammary epithelial cells by modulating their cell morphology, proliferation, and migration potential. In addition, Pea3 overexpression favored an epithelial-mesenchymal transition (EMT) triggered by TGF-β1. During investigation for molecular events downstream of Pea3, Cyclin D2 (CCND2) was identified as a new Pea3 target gene involved in the control of cellular proliferation and migration, a finding that highlights a new negative regulatory loop between Pea3 and Cyclin D2. Furthermore, Cyclin D2 expression was lost during TGF-β1-induced EMT and Pea3-induced tumorigenesis. Finally, restored Cyclin D2 expression in Pea3-dependent mammary tumorigenic cells decreased cell migration in an opposite manner to Pea3. As such, these data demonstrate that loss of the negative feedback loop between Cyclin D2 and Pea3 contributes to Pea3-induced tumorigenesis.

IMPLICATIONS

This study reveals molecular insight into how the Ets family transcription factor Pea3 favors EMT and contributes to tumorigenesis via a negative regulatory loop with Cyclin D2, a new Pea3 target gene.

Shedding-generated Met receptor fragments can be routed to either the proteasomal or the lysosomal degradation pathway

The receptor tyrosine kinase Met and its ligand, the hepatocyte growth factor/scatter factor, are essential for embryonic development, whereas deregulation of Met signaling pathways is associated with tumorigenesis and metastasis. The presenilin-regulated intramembrane proteolysis (PS-RIP) is involved in ligand-independent downregulation of Met. This proteolytic process involves shedding of the Met extracellular domain followed by γ-secretase cleavage, generating labile intracellular fragments degraded by the proteasome. We demonstrate here that upon shedding both generated Met N- and C-terminal fragments are degraded directly in the lysosome, with C-terminal fragments escaping γ-secretase cleavage. PS-RIP and lysosomal degradation are complementary, because their simultaneous inhibition induces synergistic accumulation of fragments. Met N-terminal fragments associate with the high-affinity domain of HGF/SF, confirming its decoy activity which could be reduced through their routing to the lysosome at the expense of extracellular release. Finally, the DN30 monoclonal antibody inducing Met shedding promotes receptor degradation through induction of both PS-RIP and the lysosomal pathway. Thus, we demonstrate that Met shedding initiates a novel lysosomal degradation which participates to ligand-independent downregulation of the receptor.

Abstract 2559: Preclinical evaluation of dual PI3K-mTOR inhibitors and histone deacetylase inhibitors in head and neck squamous cell carcinoma

In this study we have examined the potential value of a series of clinically relevant PI3K-mTOR inhibitors alone, or in combination with histone deacetylase inhibitors, in a model of head and neck squamous cell carcinoma (HNSCC). We present data relating to in vitro and in vivo pharmacological effects and efficacy for both these classes of drugs.

HNSCC cell lines, human keratinocyte and HNSCC xenograft models were treated with histone deacetylase inhibitors (HDACIs) and new generation PI3K and dual PI3K-mTOR inhibitors either alone or in combination. Cell and tumour tissue viability and proliferation were determined in vitro and in vivo following treatments. Effect upon activation status of signalling pathways was also investigated.

We report that PI3K, AKT, and dual PI3K-mTOR inhibitors (LY294002, Wortmannin, AKT VIII, BEZ235, BKM120 and BGT226) caused a marked in vitro enhancement of cytotoxicity induced by HDACIs (vorinostat, depsipeptide, valproic acid and LBH589) in HNSCC cancer cells (SCC25, Cal27, Detroit 562). This effect correlates with intense and duration of AKT inhibition, reactive oxygen species (ROS) accumulation, and is attenuated by expression of constitutively active AKT. LBH589, BEZ235, BGT226 and BKM120 inhibited tumour growth alone and in combination in a xenograft model of HNSCC. However, we saw no evidence of improved efficacy with an HDACI / PI3KI combination. Importantly, we observed intratumoural HDAC inhibition and PI3K inhibition as assessed by histone H3 acetylation status and phospho-AKT staining respectively. Finally, further studies indicated that the PI3KI/mTOR inhibitor, BGT226, could significantly enhance cisplatin or taxane-induced cytotoxicity in HNSCC cell lines in vitro.This study indicates, for the first time, that new generation clinically relevant PI3K and dual PI3K-mTOR inhibitors possess antitumour effect against SCC in vivo. These findings provide a rational basis for pursuing clinical trials of PI3KIs in HNSCC.

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 2559. doi:10.1158/1538-7445.AM2011-2559

Concurrent fast and slow cycling of a transcriptional activator at an endogenous promoter

For gene regulation, some transcriptional activators bind periodically to promoters with either a fast (approximately 1 minute) or a slow (approximately 15 to 90 minutes) cycle. It is uncertain whether the fast cycle occurs on natural promoters, and the function of either cycle in transcription remains unclear. We report that fast and slow cycling can occur simultaneously on an endogenous yeast promoter and that slow cycling in this system reflects an oscillation in the fraction of accessible promoters rather than the recruitment and release of stably bound transcriptional activators. This observation, combined with single-cell measurements of messenger RNA (mRNA) production, argues that fast cycling initiates transcription and that slow cycling regulates the quantity of mRNA produced. These findings counter the prevailing view that slow cycling initiates transcription.

ASK-1 (apoptosis signal-regulating kinase 1) is a direct E2F target gene

In the present study, we show that E2Fs (E2 promoter-binding factors) regulate the expression of ASK-1 (apoptosis signal-regulating kinase 1), which encodes a mitogen-activated protein kinase kinase kinase, also known as MAP3K5. Its mRNA expression is cell-cycle-regulated in human T98G cells released from serum starvation. Moreover, overexpression and RNA interference experiments support the requirement of endogenous E2F/DP (E2F dimerization partner) activity for ASK-1 expression. Characterization of the human ASK-1 promoter demonstrates that the -95/+11 region is critical for E2F-mediated up-regulation. Chromatin immunoprecipitation assays show that E2F1-E2F4 are bound in vivo to the ASK-1 promoter in cycling cells, probably through a non-consensus E2F-binding site located 12 bp upstream of the transcription start site. Mutation of this site completely abolishes the ASK-1 promoter response to E2Fs as well as the E2F1 binding in electrophoretic mobility-shift experiments. Our results indicate that E2Fs modulate the expression of ASK-1 and suggest that some of the cellular functions of ASK-1 may be under the control of E2F transcription factors. Moreover, the up-regulation of ASK-1 may also favour the p53-independent E2F1 apoptotic activity.

The NRF-1/alpha-PAL transcription factor regulates human E2F6 promoter activity

E2F6 is widely expressed in human tissues and cell lines. Recent studies have demonstrated its involvement in developmental patterning and in the regulation of various genes implicated in chromatin remodelling. Despite a growing number of studies, nothing is really known concerning the E2F6 expression regulation. To understand how cells control E2F6 expression, we analysed the activity of the previously cloned promoter region of the human E2F6 gene. DNase I footprinting, gel electrophoreticmobility shift, transient transfection and site-directed mutagenesis experiments allowed the identification of two functional NRF-1/alpha-PAL (nuclear respiratory factor-1/alpha-palindrome-binding protein)-binding sites within the human E2F6 core promoter region, which are conserved in the mouse and rat E2F6 promoter region. Moreover, ChIP (chromatin immunoprecipitation) analysis demonstrated that overexpressed NRF-1/alpha-PAL is associated in vivo with the E2F6 promoter. Furthermore, overexpression of full-length NRF-1/alpha-PAL enhanced E2F6 promoter activity, whereas expression of its dominant-negative form reduced the promoter activity. Our results indicate that NRF-1/alpha-PAL is implicated in the regulation of basal E2F6 gene expression.

Human E2F6 is alternatively spliced to generate multiple protein isoforms

E2F6 protein belongs to the family of the E2F transcription factors. Here, we showed that the human E2F6 gene contains nine exons distributed along 20.4kbp of genomic DNA on chromosome 2 leading to the transcription of six alternatively spliced E2F6 mRNAs that encode four different E2F6 proteins. Moreover, we identified an E2F6 pseudogene localized on chromosome 22 completely spliced and devoid of exons 2, 3, and 4, and part of exons 1 and 5. Definition of the transcriptional initiation site and sequence analysis show that the gene contains a TATA less, CAAT less, GC-rich promoter with multiple transcription start sites. Regulatory elements necessary for basal transcription reside within a 134bp fragment as determined by transient transfection experiments.

Roles of stem cell factor/c-Kit and effects of Glivec/STI571 in human uveal melanoma cell tumorigenesis

The B-Raf(V599E)-mediated constitutive activation of ERK1/2 is involved in establishing the transformed phenotype of some uveal melanoma cells (Calipel, A., Lefevre, G., Pouponnot, C., Mouriaux, F., Eychene, A., and Mascarelli, F. (2003) J. Biol. Chem. 278, 42409-42418). We have shown that stem cell factor (SCF) is involved in the proliferation of normal uveal melanocytes and that c-Kit is expressed in 75% of primary uveal melanomas. This suggests that the acquisition of autonomous growth during melanoma progression may involve the SCF/c-Kit axis. We used six human uveal melanoma tumor-derived cell lines and normal uveal melanocytes to characterize the SCF/c-Kit system and to assess its specific role in transformation. We investigated the possible roles of activating mutations in c-KIT, the overexpression of this gene, and ligand-dependent c-Kit overactivation in uveal melanoma cell tumorigenesis. Four cell lines (92.1, SP6.5, Mel270, and TP31) expressed both SCF and c-Kit, and none harbored the c-KIT mutations in exons 9, 11, 13, and 17 that have been shown to induce SCF-independent c-Kit activation. Melanoma cell proliferation was strongly inhibited by small interfering RNA-mediated depletion of c-Kit in these cells, despite the presence of (V599E)B-Raf in SP6.5 and TP31 cells. We characterized the signaling pathways involved in SCF/c-Kit-mediated cell growth and survival in normal and tumoral melanocytes and found that constitutive ERK1/2 activation played a key role in both the SCF/c-Kit autocrine loop and the gain of function of (V599E)B-Raf for melanoma cell proliferation and transformation. We also provide the first evidence that Glivec/STI571, a c-Kit tyrosine kinase inhibitor, could be used to treat uveal melanomas.

Microphthalmia transcription factor analysis in posterior uveal melanomas

PURPOSE

The protein encoded by the Microphthalmia gene (MITF) is a transcription factor essential for the development and survival of melanocytes. It serves as a master regulator in modulating extracellular signals. Because of its central role in melanocytes survival and to assess its potential use as a histopathological marker for melanoma, MITF expression was examined in human choroidal melanomas.

METHODS

Fifty-seven paraffin-embedded sections of choroidal melanoma specimens and 1 choroidal melanoma cell line were analyzed using immunochemistry and RT-PCR. Normal choroids and normal choroidal melanocyte cells were used as control.

RESULTS

Sixty-five percent of the tumoral specimens stained positively for MITF with a predominant nuclear pattern of reactivity. MITF-M and MITF-A isoforms were detected by RT-PCR in all specimens examined. Using a chimeric protein resulting from the fusion of each Mitf protein with the GFP, Mitf-M exhibited an exclusive nuclear staining whereas Mitf-A exhibited a mixed nuclear and cytoplasmic staining. No correlation between MITF-positivity and parameters such as cell type, largest tumor diameter, sclera invasion, mitotic figures was observed. In contrast, a significant negative association was found between MITF staining and the pigmentation (p=0.02) and a positive correlation between MITF staining and the proliferative marker Ki67 was found (p=0.02).

CONCLUSION

MITF may be implicated in choroidal melanoma pigmentation and proliferation. Further analysis should provide new insights into the mechanisms underlying the molecular and cellular changes of choroidal melanomas.

Expression of the c-kit receptor in choroidal melanomas

The KIT gene encodes c-kit, a transmembrane receptor that has tyrosine kinase activity and plays a role in haematopoiesis, gametogenesis and melanogenesis. The c-kit protein is found in normal cutaneous and choroidal melanocytes, and there is evidence that expression is lost in melanoma. Expression of c-kit was analysed in 57 paraffin-embedded sections of choroidal melanoma specimens and three choroidal melanoma cell lines using immunochemistry and Western blotting. Of the tumour specimens, 75% stained positively for c-kit with a membrane pattern of reactivity. Of the six patients who underwent proton beam therapy before enucleation, five tumours exhibited no c-kit immunoreactivity and the other tumour demonstrated weak staining. Of the three melanoma cell lines used, c-kit expression was observed in only one. No correlations between c-kit positivity and parameters such as cell type, largest macroscopic tumour dimension, scleral invasion or pigmentation were observed. In contrast, a significant positive association was found between c-kit staining and mitotic activity (P = 0.02). However, c-kit expression did not significantly influence survival when evaluated by univariate analysis. In conclusion, c-kit is expressed in most choroidal melanoma tumours. Further analysis should provide new insights into the mechanisms underlying the molecular and cellular changes in choroidal melanomas.

Hepatocyte growth factor/scatter factor activates the ETS1 transcription factor by a RAS-RAF-MEK-ERK signaling pathway

Hepatocyte growth factor/scatter factor (HGF/SF) induces scattering and morphogenesis of epithelial cells through the activation of the MET tyrosine kinase receptor. Although the activated MET receptor recruits a number of signaling proteins, little is known of the downstream signaling pathways activated by HGF/SF. In this study, we wished to examine the signaling pathway leading to activation of the ETS1 transcription factor. Using in vitro and in vivo kinase assays, we found that HGF/SF activates the ERK1 MAP kinase, leading to the phosphorylation of the threonine 38 residue of ETS1 within a putative MAP kinase phosphorylation site (PLLT38P). This threonine residue was neither phosphorylated by JNK1, nor by p38 MAP kinases and was required for the induction of transcriptional activity of ETS1 by HGF/SF. Using kinase and transcription assays, we further demonstrated that phosphorylation and activation of ETS1 occurs downstream of a RAS-RAF-MEK-ERK pathway. The functional involvement of this pathway in HGF/SF action was demonstrated using U0126, a pharmacological inhibitor of MEK, which blocked phosphorylation and activation of ETS1, RAS-dependent transcriptional responses, cell scattering and morphogenesis. These data demonstrated that ETS1 is a downstream target of HGF/SF acting through a RAS-RAF-MEK-ERK pathway and provides a signaling pathway leading to the regulation of gene expression by HGF/SF.

Molecular cloning and characterization of the mouse E2F6 gene

E2F6 is the most recently identified member of the E2F family. In this study, the murine E2F6 gene was cloned and found to consist of eight exons. Analysis of its 5' flanking region revealed two transcription start sites. The proximal promoter region contained no TATA or CAAT box. We also identified a novel E2F6 mRNA containing the alternative exon 2. The E2F6 mRNAs are highly expressed during mouse embryogenesis and are present in all adult tissues examined. Moreover, E2F6 shows a unique expression pattern in synchronized mouse embryonic fibroblasts. E2F6 expression rapidly increases during the G0-G1 transition, reaching its higher level in mid-G1, and remains relatively constant thereafter. These findings suggest that E2F6 may contribute to the regulation of events throughout the cell cycle. Isolation of the murine E2F6 gene is a step toward generation of genetically modified mouse models that will help to understand the functions of E2F6.

Atypical nucleoprotein complexes mediate CRE-dependent regulation of the early promoter of minute virus of mice

The P4 promoter of the parvovirus minute virus of mice (MVMp) directs transcription of the genes encoding non-structural proteins. We have previously shown that functional upstream CRE elements contribute to both the ras oncogene-dependent activation of promoter P4 and its down-modulation by known activators of cyclic AMP-dependent protein kinase A (PKA). In the present work, the nucleoprotein complexes formed with the P4 CRE elements were characterized with regard to their polypeptide constituents and the nucleotides taking part in the interaction. Atypical interactions, both at the protein-protein and protein-DNA level, were observed, which may be a reflection of the divergence of the parvoviral CREs from the usual consensus. The CRE-mediated regulation of promoter P4 by PKA and Ras is discussed in light of these findings.

Isolation and characterization of a chicken homologue of the Spi-1/PU.1 transcription factor

Spi-1/PU.1 is a member of the Ets family of transcription factors important in regulation of hematopoiesis. We have isolated a chicken cDNA homologuous to the mammalian Spi-1/PU.1 gene with an open reading frame of 250 amino acids (aa). The chicken Spi-1/PU.1 protein is 14 aa and 16 aa shorter than its human and mouse counterparts but is extremely well conserved with 78.8% and 75.2% identity respectively. The carboxy terminal DNA binding region, or ETS binding domain, is 100% identical to that of human and mouse. Some differences with the mammalian homologues are seen in the N-terminal part of the protein and in the PEST connecting domain. However, the differences are mainly conservative and all the features underlying functional aspects seem preserved. The major discrepancy lies in a 12 aa deletion in an already poorly conserved part of the PEST sequence. Spi-1/PU.1 transcripts were detected at high levels in spleen and Fabricius bursa of chick embryos by Northern blot and in situ hybridization. Our results show that the chicken Spi-1/PU.1 protein behaves like a bonafide Spi-1/PU.1 transcription factor in its DNA binding and transactivating properties.

Upstream CREs participate in the basal activity of minute virus of mice promoter P4 and in its stimulation in ras-transformed cells

The activity of the P4 promoter of the parvovirus minute virus of mice (prototype strain MVMp) is stimulated in ras-transformed FREJ4 cells compared with the parental FR3T3 line. This activation may participate in the oncolytic effect of parvoviruses, given that P4 drives a transcriptional unit encoding cytotoxic nonstructural proteins. Our results suggest that the higher transcriptional activity of promoter P4 in FREJ4 cells is mediated at least in part by upstream CRE elements. Accordingly, mutations in the CRE motifs impair P4 function more strongly in the FREJ4 derivative than in its FR3T3 parent. Further evidence that these elements contribute to hyperactivity of the P4 promoter in the ras transformant is the fact that they form distinct complexes with proteins from FREJ4 and FR3T3 cell extracts. This difference can be abolished by treating the FREJ4 cell extracts with cyclic AMP-dependent protein kinase (PKA) or treating original cultures with a PKA activator. These findings can be linked with two previously reported features of ras-transformed cells: the activation of a PKA-inhibited protein kinase cascade and the reduction of PKA-induced protein phosphorylation. In keeping with these facts, P4-directed gene expression can be up- or downmodulated in vivo by exposing cells to known inhibitors or activators of PKA, respectively.

Rab2 nucleotide coding sequence in gallus gallus and it phylogenetic position

The nucleic acid sequence of the chicken rab2 mRNA was determined by sequencing a full length cDNA. The phylogeny of rab2 sequences was established.