A Pilot Study to Develop Paraneoplastic Cerebellar Degeneration Mouse Model.

Modeling paraneoplastic neurological diseases to understand the immune mechanisms leading to neuronal death is a major challenge given the rarity and terminal access of patients' autopsies. Here, we present a pilot study aiming at modeling paraneoplastic cerebellar degeneration with Yo autoantibodies (Yo-PCD). Female mice were implanted with an ovarian carcinoma cell line expressing CDR2 and CDR2L, the known antigens recognized by anti-Yo antibodies. To boost the immune response, we also immunized the mice by injecting antigens with diverse adjuvants and immune checkpoint inhibitors. Ataxia and gait instability were assessed in treated mice as well as autoantibody levels, Purkinje cell density, and immune infiltration in the cerebellum. We observed the production of anti-Yo antibodies in the CSF and serum of all immunized mice. Brain immunoreaction varied depending on the site of implantation of the tumor, with subcutaneous administration leading to a massive infiltration of immune cells in the meningeal spaces, choroid plexus, and cerebellar parenchyma. However, we did not observe massive Purkinje cell death nor any motor impairments in any of the experimental groups. Self-sustained neuro-inflammation might require a longer time to build up in our model. Unusual tumor antigen presentation and/or intrinsic, species-specific factors required for pro-inflammatory engagement in the brain may also constitute strong limitations to achieve massive recruitment of antigen-specific T-cells and killing of antigen-expressing neurons in this mouse model.

Oral cavity squamous cell carcinomas in patients with no identified risk factors: Feeling like an outsider.

OBJECTIVES: Head and neck squamous cell carcinomas (HNSCCs) mainly affect smokers and drinkers. However, oral cavity squamous cell cancers (OCSCCs) are increasingly affecting patients with no identified risk factors (NIRFs). This study aimed at characterising their experience of the disease. METHODS: Qualitative study based on semi-structured interviews of 20 survivors of OCSCC with NIRF. Methods used in grounded theory approach were applied. RESULTS: Patients with NIRF had a similar experience to that of patients with risk factors regarding treatments. The absence of identified causes led to stigma and led the patients to distance themselves, both from the stereotypic HNSCC cancer patient and the identity as a cancer patient. Furthermore, having no identified risk factors seemed to reinforce the fear of recurrence. CONCLUSION: This study is the first to address key gaps in knowledge regarding patients with NIRF having survived OCSCC. Their experience is similar to that of patients with orphan diseases. Owing to confrontation with other patients and the repeated questions of caregivers about tobacco/alcohol consumption, these patients felt stigmatised during their treatment. OCSCC patients with NIRF may benefit from guidance and support on how to engage in prevention.

p58(IPK)-mediated attenuation of the proapoptotic PERK-CHOP pathway allows malignant progression upon low glucose.

As solid tumors expand, oxygen and nutrients become limiting owing to inadequate vascularization and diffusion. How malignant cells cope with this potentially lethal metabolic stress remains poorly understood. We found that glucose shortage associated with malignant progression triggers apoptosis through the endoplasmic reticulum (ER) unfolded protein response (UPR). ER stress is in part caused by reduced glucose flux through the hexosamine pathway. Deletion of the proapoptotic UPR effector CHOP in a mouse model of K-ras(G12V)-induced lung cancer increases tumor incidence, strongly supporting the notion that ER stress serves as a barrier to malignancy. Overcoming this barrier requires the selective attenuation of the PERK-CHOP arm of the UPR by the molecular chaperone p58(IPK). Furthermore, p58(IPK)-mediated adaptive response enables cells to benefit from the protective features of chronic UPR. Altogether, these results show that ER stress activation and p58(IPK) expression control the fate of malignant cells facing glucose shortage.

Toll-like receptor 3 downregulation is an escape mechanism from apoptosis during hepatocarcinogenesis.

BACKGROUND & AIMS: Low levels of toll-like receptor 3 (TLR3) in patients with hepatocellular carcinoma (HCC) are associated with poor prognosis, primarily owing to the loss of inflammatory signaling and subsequent lack of immune cell recruitment to the liver. Herein, we explore the role of TLR3-triggered apoptosis in HCC cells. METHODS: Quantitative reverse transcription PCR, western blotting, immunohistochemistry and comparative genomic hybridization were used to analyze human and mouse HCC cell lines, as well as surgically resected primary human HCCs, and to study the impact of TLR3 expression on patient outcomes. Functional analyses were performed in HCC cells, following the restoration of TLR3 by lentiviral transduction. The role of TLR3-triggered apoptosis in HCC was analyzed in vivo in a transgenic mouse model of HCC. RESULTS: Lower expression of TLR3 in tumor compared to non-tumor matched tissue was observed at both mRNA and protein levels in primary HCC, and was predictive of shorter recurrence-free survival after surgical resection in both univariate (hazard ratio [HR] 1.79; 95% CI 1.04-3.06; p = 0.03) and multivariate analyses (HR 1.73; CI 1.01-2.97; p = 0.04). Immunohistochemistry confirmed frequent downregulation of TLR3 in human and mouse primary HCC cells. None of the 6 human HCC cell lines analyzed expressed TLR3, and ectopic expression of TLR3 following lentiviral transduction not only restored the inflammatory response but also sensitized cells to TLR3-triggered apoptosis. Lastly, in the transgenic mouse model of HCC, absence of TLR3 expression was accompanied by a lower rate of preneoplastic hepatocyte apoptosis and accelerated hepatocarcinogenesis without altering the tumor immune infiltrate. CONCLUSION: Downregulation of TLR3 protects transforming hepatocytes from direct TLR3-triggered apoptosis, thereby contributing to hepatocarcinogenesis and poor patient outcome. LAY SUMMARY: Hepatocellular carcinoma (HCC) is a heterogeneous disease associated with a poor prognosis. In patients with HCC, TLR3 downregulation is associated with reduced survival. Herein, we show that the absence of TLR3 is associated with a lower rate of apoptosis, and subsequently more rapid hepatocarcinogenesis, without any change to the immune infiltrate in the liver. Therefore, the poor prognosis associated with low TLR3 expression in HCC is likely linked to tumors ability to escape apoptosis. TLR3 may become a promising therapeutic target in TLR3-positive HCC.

Cleaved/associated TLR3 represents the primary form of the signaling receptor.

TLR3 belongs to the family of intracellular TLRs that recognize nucleic acids. Endolysosomal localization and cleavage of intracellular TLRs play pivotal roles in signaling and represent fail-safe mechanisms to prevent self-nucleic acid recognition. Indeed, cleavage by cathepsins is required for native TLR3 to signal in response to dsRNA. Using novel Abs generated against TLR3, we show that the conserved loop exposed in LRR12 is the single cleavage site that lies between the two dsRNA binding sites required for TLR3 dimerization and signaling. Accordingly, we found that the cleavage does not dissociate the C- and N-terminal fragments, but it generates a very stable "cleaved/associated" TLR3 present in endolysosomes that recognizes dsRNA and signals. Moreover, comparison of wild-type, noncleavable, and C-terminal-only mutants of TLR3 demonstrates that efficient signaling requires cleavage of the LRR12 loop but not dissociation of the fragments. Thus, the proteolytic cleavage of TLR3 appears to fulfill function(s) other than separating the two fragments to generate a functional receptor.

Dual function of MyD88 in inflammation and oncogenesis: implications for therapeutic intervention.

PURPOSE OF REVIEW: Inflammation is emerging as a new hallmark of cancer, and the toll-like receptor and interleukin-1 receptor adaptor molecule MyD88 has been linked to tumorigenesis. The purpose of this review is to give a brief overview of the latest advances in understanding the complexity of MyD88 implication in tumorigenesis. RECENT FINDINGS: MyD88 is shown to play a protumorigenic role through two mechanisms. First, it activates the nuclear factor kappa-light-chain-enhancer of activated B cells signaling pathway in the hematopoietic compartment and in tumor cells, inducing an inflammatory environment favorable to carcinogenesis. Second, it plays a cell-autonomous role in Ras signaling and transformation, independently of its role in inflammatory signaling. MyD88 mediates the optimal activation of the Ras/extracellular signal-regulated kinase (ERK) pathway by binding to ERK and protecting it from dephosphorylation. This optimal activation of the Ras pathway is essential for the expression of important DNA repair enzymes, allowing cancer cells to efficiently repair damaged DNA. MyD88 is also shown in certain cases to play an antitumoral role through modulation of the immune response SUMMARY: These findings present a new dual function model for MyD88 implication in carcinogenesis making it a potential therapeutic target in cancer.

Soft extracellular matrix drives endoplasmic reticulum stress-dependent S quiescence underlying molecular traits of pulmonary basal cells.

Cell culture on soft matrix, either in 2D and 3D, preserves the characteristics of progenitors. However, the mechanism by which the mechanical microenvironment determines progenitor phenotype, and its relevance to human biology, remains poorly described. Here we designed multi-well hydrogel plates with a high degree of physico-chemical uniformity to reliably address the molecular mechanism underlying cell state modification driven by physiological stiffness. Cell cycle, differentiation and metabolic activity could be studied in parallel assays, showing that the soft environment promotes an atypical S-phase quiescence and prevents cell drift, while preserving the differentiation capacities of human bronchoepithelial cells. These softness-sensitive responses are associated with calcium leakage from the endoplasmic reticulum (ER) and defects in proteostasis and enhanced basal ER stress. The analysis of available single cell data of the human lung also showed that this non-conventional state coming from the soft extracellular environment is indeed consistent with molecular feature of pulmonary basal cells. Overall, this study demonstrates that mechanical mimicry in 2D culture supports allows to maintain progenitor cells in a state of high physiological relevance for characterizing the molecular events that govern progenitor biology in human tissues. STATEMENT OF SIGNIFICANCE: This study focuses on the molecular mechanism behind the progenitor state induced by a soft environment. Using innovative hydrogel supports mimicking normal human lung stiffness, the data presented demonstrate that lung mechanics prevent drift while preserving the differentiation capabilities of lung epithelial cells. Furthermore, we show that the cells are positioned in a quiescent state in the atypical S phase. Mechanistically, we demonstrate that this quiescence: i) is driven by calcium leakage from the endoplasmic reticulum (ER) and basal activation of the PERK branch of ER stress signalling, and ii) protects cells from lethal ER stress caused by metabolic stress. Finally, we validate using human single-cell data that these molecular features identified on the soft matrix are found in basal lung cells. Our results reveal original and relevant molecular mechanisms orchestrating cell fate in a soft environment and resistance to exogenous stresses, thus providing new fundamental and clinical insights into basal cell biology.

Cancer selective cell death induction by a bivalent CK2 inhibitor targeting the ATP site and the allosteric αD pocket.

Although the involvement of protein kinase CK2 in cancer is well-documented, there is a need for selective CK2 inhibitors suitable for investigating CK2 specific roles in cancer-related biological pathways and further exploring its therapeutic potential. Here, we report the discovery of AB668, an outstanding selective inhibitor that binds CK2 through a bivalent mode, interacting both at the ATP site and an allosteric αD pocket unique to CK2. Using caspase activation assay, live-cell imaging, and transcriptomic analysis, we have compared the effects of this bivalent inhibitor to representative ATP-competitive inhibitors, CX-4945, and SGC-CK2-1. Our results show that in contrast to CX-4945 or SGC-CK2-1, AB668, by targeting the CK2 αD pocket, has a distinct mechanism of action regarding its anti-cancer activity, inducing apoptotic cell death in several cancer cell lines and stimulating distinct biological pathways in renal cell carcinoma.

Molecular landscapes of oral cancers of unknown etiology.

The incidence of the mobile tongue cancer in young patients has been rising. This oral cancer (OC) type has no identified risk factors (NIRF), no established molecular markers and is not yet recognized as a distinct clinical entity. To understand this emerging malignancy, we innovatively analyzed the public head and neck cancer multi-omics data. We identified mutational signatures that successfully stratified 307 OC and 109 laryngeal cancer cases according to their clinico-pathological characteristics. The NIRF OCs exhibited significantly increased activities of endogenous clock-like and APOBEC-associated mutagenesis, alongside specific cancer driver gene mutations, distinct methylome patterns and prominent antimicrobial transcriptomic responses. Furthermore, we show that mutational signature SBS16 in OCs reflects the combined effects of alcohol drinking and tobacco smoking. Our study characterizes the unique disease histories and molecular programs of the NIRF OCs revealing that this emerging cancer subtype is likely driven by increased endogenous mutagenesis correlated with responses to microbial insults.

The stress sensor GCN2 differentially controls ribosome biogenesis in colon cancer according to the nutritional context.

Nutrient availability is a key determinant of tumor cell behavior. While nutrient-rich conditions favor proliferation and tumor growth, scarcity, and particularly glutamine starvation, promotes cell dedifferentiation and chemoresistance. Here, linking ribosome biogenesis plasticity with tumor cell fate, we uncover that the amino acid sensor general control non-derepressible 2 (GCN2; also known as eIF-2-alpha kinase 4) represses the expression of the precursor of ribosomal RNA (rRNA), 47S, under metabolic stress. We show that blockade of GCN2 triggers cell death by an irremediable nucleolar stress and subsequent TP53-mediated apoptosis in patient-derived models of colon adenocarcinoma (COAD). In nutrient-rich conditions, a cell-autonomous GCN2 activity supports cell proliferation by stimulating 47S rRNA transcription, independently of the canonical integrated stress response (ISR) axis. Impairment of GCN2 activity prevents nuclear translocation of methionyl-tRNA synthetase (MetRS), resulting in nucleolar stress, mTORC1 inhibition and, ultimately, autophagy induction. Inhibition of the GCN2-MetRS axis drastically improves the cytotoxicity of RNA polymerase I (RNA pol I) inhibitors, including the first-line chemotherapy oxaliplatin, on patient-derived COAD tumoroids. Our data thus reveal that GCN2 differentially controls ribosome biogenesis according to the nutritional context. Furthermore, pharmacological co-inhibition of the two GCN2 branches and RNA pol I activity may represent a valuable strategy for elimination of proliferative and metabolically stressed COAD cells.

The Kynurenine Pathway and Cancer: Why Keep It Simple When You Can Make It Complicated.

The kynurenine pathway has been highlighted as a gatekeeper of immune-privileged sites through its ability to generate from tryptophan a set of immunosuppressive metabolic intermediates. It additionally constitutes an important source of cellular NAD+ for the organism. Hijacking of its immunosuppressive functions, as recurrently observed in multiple cancers, facilitates immune evasion and promotes tumor development. Based on these observations, researchers have focused on characterizing indoleamine 2,3-dioxygenase (IDO1), the main enzyme catalyzing the first and limiting step of the pathway, and on developing therapies targeting it. Unfortunately, clinical trials studying IDO1 inhibitors have thus far not met expectations, highlighting the need to unravel this complex signaling pathway further. Recent advances demonstrate that these metabolites additionally promote tumor growth, metastatic dissemination and chemoresistance by a combination of paracrine and autocrine effects. Production of NAD+ also contributes to cancer progression by providing cancer cells with enhanced plasticity, invasive properties and chemoresistance. A comprehensive survey of this complexity is challenging but necessary to achieve medical success.

The hexosamine pathway and coat complex II promote malignant adaptation to nutrient scarcity.

The glucose-requiring hexosamine biosynthetic pathway (HBP), which produces UDP-N-acetylglucosamine for glycosylation reactions, promotes lung adenocarcinoma (LUAD) progression. However, lung tumor cells often reside in low-nutrient microenvironments, and whether the HBP is involved in the adaptation of LUAD to nutrient stress is unknown. Here, we show that the HBP and the coat complex II (COPII) play a key role in cell survival during glucose shortage. HBP up-regulation withstood low glucose-induced production of proteins bearing truncated N-glycans, in the endoplasmic reticulum. This function for the HBP, alongside COPII up-regulation, rescued cell surface expression of a subset of glycoproteins. Those included the epidermal growth factor receptor (EGFR), allowing an EGFR-dependent cell survival under low glucose in anchorage-independent growth. Accordingly, high expression of the HBP rate-limiting enzyme GFAT1 was associated with wild-type EGFR activation in LUAD patient samples. Notably, HBP and COPII up-regulation distinguished LUAD from the lung squamous-cell carcinoma subtype, thus uncovering adaptive mechanisms of LUAD to their harsh microenvironment.

Pemetrexed Hinders Translation Inhibition upon Low Glucose in Non-Small Cell Lung Cancer Cells.

Genetic alterations in non-small cell lung cancers (NSCLC) stimulate the generation of energy and biomass to promote tumor development. However, the efficacy of the translation process is finely regulated by stress sensors, themselves often controlled by nutrient availability and chemotoxic agents. Yet, the crosstalk between therapeutic treatment and glucose availability on cell mass generation remains understudied. Herein, we investigated the impact of pemetrexed (PEM) treatment, a first-line agent for NSCLC, on protein synthesis, depending on high or low glucose availability. PEM treatment drastically repressed cell mass and translation when glucose was abundant. Surprisingly, inhibition of protein synthesis caused by low glucose levels was partially dampened upon co-treatment with PEM. Moreover, PEM counteracted the elevation of the endoplasmic reticulum stress (ERS) signal produced upon low glucose availability, providing a molecular explanation for the differential impact of the drug on translation according to glucose levels. Collectively, these data indicate that the ERS constitutes a molecular crosstalk between microenvironmental stressors, contributing to translation reprogramming and proteostasis plasticity.

A Gender-Dependent Molecular Switch of Inflammation via MyD88/Estrogen Receptor-Alpha Interaction.

INTRODUCTION: Most Toll-like receptors and IL-1/IL-18 receptors activate a signaling cascade via the adaptor molecule MyD88, resulting in NF-κB activation and inflammatory cytokine and chemokine production. Females are less susceptible than males to inflammatory conditions, presumably due to protection by estrogen. The exact mechanism underlying this protection is unknown. METHODS: MCF7 cells expressing wild-type or mutated LXXLL motif were used to determine MyD88/estrogen receptor (ER)-a interaction by immunoprecipitation and cell activation by ELISA and luciferase reporter assay. IL-1b and/or E2 were used to activate MCF7 cells expressing normal or knocked down levels of PRMT1. Finally, in situ proximity ligation assay with anti-MyD88 and anti-methylated ER-a (methER-a) antibodies was used to evaluate MyD88/methylated ER-a interaction in THP1 cells and histological sections. RESULTS: We show that MyD88 interacts with a methylated, cytoplasmic form of estrogen receptor-alpha (methER-α). This interaction is required for NF-κB transcriptional activity and pro-inflammatory cytokine production, and is dissociated by estrogen. Importantly, we show a strong gender segregation in gametogenic reproductive organs, with MyD88/methER-α interactions found in testicular tissues and in ovarian tissues from menopausal women, but not in ovaries from women age 49 and less - suggesting a role for estrogen in disrupting this complex in situ. DISCUSSION: Collectively, our results indicate that the formation of MyD88/methER-α complexes during inflammatory signaling and their disruption by estrogen may represent a mechanism that contributes to gender bias in inflammatory responses.

ATF4-Dependent NRF2 Transcriptional Regulation Promotes Antioxidant Protection during Endoplasmic Reticulum Stress.

Endoplasmic reticulum (ER) stress generates reactive oxygen species (ROS) that induce apoptosis if left unabated. To limit oxidative insults, the ER stress PKR-like endoplasmic reticulum Kinase (PERK) has been reported to phosphorylate and activate nuclear factor erythroid 2-related factor 2 (NRF2). Here, we uncover an alternative mechanism for PERK-mediated NRF2 regulation in human cells that does not require direct phosphorylation. We show that the activation of the PERK pathway rapidly stimulates the expression of NRF2 through activating transcription factor 4 (ATF4). In addition, NRF2 activation is late and largely driven by reactive oxygen species (ROS) generated during late protein synthesis recovery, contributing to protecting against cell death. Thus, PERK-mediated NRF2 activation encompasses a PERK-ATF4-dependent control of NRF2 expression that contributes to the NRF2 protective response engaged during ER stress-induced ROS production.

Cisplatin unleashes Toll-like receptor 3-mediated apoptosis through the downregulation of c-FLIP in malignant mesothelioma.

Toll-like receptor 3 (TLR3) is an immune receptor that behaves like a death receptor in tumor cells, thereby providing an original target for cancer therapy. The therapeutic potential of TLR3 targeting in malignant mesothelioma, an aggressive and incurable neoplasia of the pleura and peritoneum, has so far not been addressed. We investigated TLR3 expression and sensitivity of human mesothelioma cell lines to the synthetic dsRNA Poly(I:C), alone or in combination with cisplatin, the gold standard chemotherapy in mesothelioma. Activation of TLR3 by Poly(I:C) induced apoptosis of 4/8 TLR3-positive cell lines but not of TLR3-negative cell lines. The combined cisplatin/Poly(I:C) treatment enhanced apoptosis of 3/4 Poly(I:C)-sensitive cell lines and overcame resistance to Poly(I:C) or cisplatin alone in 2/4 cell lines. Efficacy of the combined treatment relied on cisplatin-induced downregulation of c-FLIP, the main regulator of the extrinsic apoptotic pathway, leading to an enhanced caspase-8-mediated pathway. Of note, 6/6 primary cell samples isolated from patients with peritoneal mesothelioma expressed TLR3. Patient-derived cells were sensitive to Poly(I:C) alone while the combined cisplatin/Poly(I:C) treatment induced dramatic cell death. Our findings demonstrate that TLR3 targeting in combination with cisplatin presents an innovative therapeutic strategy in mesothelioma.

Disruption of the langerin/CD207 gene abolishes Birbeck granules without a marked loss of Langerhans cell function.

Langerin is a C-type lectin expressed by a subset of dendritic leukocytes, the Langerhans cells (LC). Langerin is a cell surface receptor that induces the formation of an LC-specific organelle, the Birbeck granule (BG). We generated a langerin(-/-) mouse on a C57BL/6 background which did not display any macroscopic aberrant development. In the absence of langerin, LC were detected in normal numbers in the epidermis but the cells lacked BG. LC of langerin(-/-) mice did not present other phenotypic alterations compared to wild-type littermates. Functionally, the langerin(-/-) LC were able to capture antigen, to migrate towards skin draining lymph nodes, and to undergo phenotypic maturation. In addition, langerin(-/-) mice were not impaired in their capacity to process native OVA protein for I-A(b)-restricted presentation to CD4(+) T lymphocytes or for H-2K(b)-restricted cross-presentation to CD8(+) T lymphocytes. langerin(-/-) mice inoculated with mannosylated or skin-tropic microorganisms did not display an altered pathogen susceptibility. Finally, chemical mutagenesis resulted in a similar rate of skin tumor development in langerin(-/-) and wild-type mice. Overall, our data indicate that langerin and BG are dispensable for a number of LC functions. The langerin(-/-) C57BL/6 mouse should be a valuable model for further functional exploration of langerin and the role of BG.

Release of c-FLIP brake selectively sensitizes human cancer cells to TLR3-mediated apoptosis.

Toll-like receptor 3 (TLR3) mediates innate immune responses by sensing viral dsRNA, but also induces apoptosis selectively in cancer cells. Our analysis by immunohistochemistry revealed that TLR3 is frequently overexpressed in 130 non-small cell lung cancer (NSCLC) patients' samples compared with normal bronchial epithelium (P < 0.0001, Mann-Whitney test), supporting the therapeutic potential of TLR3 ligand for this type of cancer. However, a proportion of TLR3-expressing cancer cell lines, including NSCLC, remain resistant to TLR3-mediated apoptosis, and the underlying mechanism of resistance remains unclear. We here investigated the molecular basis conferring resistance to non-transformed vs. transformed cells against TLR3-mediated cell death. In non-transformed epithelial cells cellular FLICE-like inhibitory protein (c-FLIP) and cellular Inhibitor of APoptosis (cIAPs) ubiquitin ligases exerted an efficient double brake on apoptosis signaling. In contrast, releasing only one of these two brakes was sufficient to overcome the resistance of 8/8 cancer cell lines tested. Remarkably, the release of the c-FLIP, but not cIAPs, brake only results in the sensitization of all human cancer cells to TLR3-mediated apoptosis. Taking advantage of the difference between transformed and non-transformed cells, we developed a rational strategy by combining the chemotherapeutic agent paclitaxel, which decreases c-FLIP expression, with TLR3 ligand. This combination was highly synergistic for triggering apoptosis in cancer cells but not in non-transformed cells. In vivo, the combination of paclitaxel with dsRNA delayed tumor growth and prolonged survival in a mouse xenograft lung tumor model. In conclusion, combining the release of the c-FLIP brake with TLR3 ligand synergizes to selectively kill cancer cells, and could represent an efficient and safe therapy against TLR3-expressing cancers such as NSCLC.

ERK1/2/MAPK pathway-dependent regulation of the telomeric factor TRF2.

Telomere stability is a hallmark of immortalized cells, including cancer cells. While the telomere length is maintained in most cases by the telomerase, the activity of a protein complex called Shelterin is required to protect telomeres against unsuitable activation of the DNA damage response pathway. Within this complex, telomeric repeat binding factor 2 (TRF2) plays an essential role by blocking the ataxia telangiectasia-mutated protein (ATM) signaling pathway at telomeres and preventing chromosome end fusion. We showed that TRF2 was phosphorylated in vitro and in vivo on serine 323 by extracellular signal-regulated kinase (ERK1/2) in both normal and cancer cells. Moreover, TRF2 and activated ERK1/2 unexpectedly interacted in the cytoplasm of tumor cells and human tumor tissues. The expression of non-phosphorylatable forms of TRF2 in melanoma cells induced the DNA damage response, leading to growth arrest and tumor reversion. These findings revealed that the telomere stability is under direct control of one of the major pro-oncogenic signaling pathways (RAS/RAF/MEK/ERK) via TRF2 phosphorylation.