TrkC binds to the type II TGF-beta receptor to suppress TGF-beta signaling

TrkC, a novel prognostic marker, induces and maintains cell survival and metastatic dissemination of Ewing sarcoma by inhibiting EWSR1-FLI1 degradation

Upregulation of EWSR1-FLI1 expression has been associated with invasiveness, induced cell survival, metastatic dissemination, and acquisition of self-renewal traits in Ewing sarcoma (ES). Although existing evidence implies that TrkC expression is linked to the pathogenesis of other cancer types, its role and the mechanism behind its correlation with EWSR1-FLI1 in the pathogenesis of ES remain unclear. In this study, we uncovered a novel physiological role of TrkC as a key regulator of EWSR1-FLI1 involved in the survival and metastatic dissemination of ES. TrkC was observed to be frequently overexpressed in human metastatic ES cells in vitro and in vivo, facilitating enhanced survival, tumorigenicity, and metastasis of ES cells. TrkC-mediated metastasis of ES cells was induced by the inhibition of the proteasomal degradation of EWSR1-FLI1 via the TrkC/EWSR1-FLI1 complex, which subsequently enabled the induction of the target proteins, EGR2 and NKX2.2. Moreover, TrkC significantly inhibited tumor suppressor activity of TGF-β through reduction of the mRNA expression of one of its receptors, TGFBR2 via TrkC-induced stabilization of EWSR1-FLI1. Furthermore, loss of TrkC expression inhibited tumor growth and metastasis in experimental mouse models. This study is the first to report the involvement and functional role of TrkC in the pathogenesis of ES, suggesting important implications for understanding the alterations of TrkC in Ewing tumors.

Dependence receptors: new targets for cancer therapy

Dependence receptors are known to promote survival and positive signaling such as proliferation, migration, and differentiation when activated, but to actively trigger apoptosis when unbound to their ligand. Their abnormal regulation was shown to be an important feature of tumorigenesis, allowing cancer cells to escape apoptosis triggered by these receptors while promoting in parallel major aspects of tumorigenesis such as proliferation, angiogenesis, invasiveness, and chemoresistance. This involvement in multiple cancer hallmarks has raised interest in dependence receptors as targets for cancer therapy. Although additional studies remain necessary to fully understand the complexity of signaling pathways activated by these receptors and to target them efficiently, it is now clear that dependence receptors represent very exciting targets for future cancer treatment. This manuscript reviews current knowledge on the contribution of dependence receptors to cancer and highlights the potential for therapies that activate pro-apoptotic functions of these proteins.

TrkB-Induced Inhibition of R-SMAD/SMAD4 Activation is Essential for TGF-β-Mediated Tumor Suppressor Activity

TrkB-mediated activation of the IL6/JAK2/STAT3 signaling pathway is associated with the induction of the epithelial–mesenchymal transition (EMT) program and the acquisition of metastatic potential by tumors. Conversely, the transforming of growth factor-β (TGF-β) is implicated in tumor suppression through the canonical SMAD-dependent signaling pathway. Hence, TrkB could play a role in disrupting the potent TGF-β-mediated growth inhibition, a concept that has not been fully explored. Here, we identified TrkB to be a crucial regulator of the TGF-β signaling pathway as it inhibits the TGF-β-mediated tumor suppression and the activation of TrkB kinase. We further show that the interactions between TrkB and SMADs inhibit TGF-β-mediated R-SMAD/SMAD4 complex formation and suppress TGF-β-induced nuclear translocation and target gene expression. Additionally, the knockdown of TrkB restored the tumor inhibitory activity of TGF-β signaling. These observations suggest that interactions between TrkB and SMADs are critical for the inhibition of TGF-β tumor suppressor activity in cancer cells.

TGF-β Signaling

Transforming growth factor-β (TGF-β) represents an evolutionarily conserved family of secreted polypeptide factors that regulate many aspects of physiological embryogenesis and adult tissue homeostasis. The TGF-β family members are also involved in pathophysiological mechanisms that underlie many diseases. Although the family comprises many factors, which exhibit cell type-specific and developmental stage-dependent biological actions, they all signal via conserved signaling pathways. The signaling mechanisms of the TGF-β family are controlled at the extracellular level, where ligand secretion, deposition to the extracellular matrix and activation prior to signaling play important roles. At the plasma membrane level, TGF-βs associate with receptor kinases that mediate phosphorylation-dependent signaling to downstream mediators, mainly the SMAD proteins, and mediate oligomerization-dependent signaling to ubiquitin ligases and intracellular protein kinases. The interplay between SMADs and other signaling proteins mediate regulatory signals that control expression of target genes, RNA processing at multiple levels, mRNA translation and nuclear or cytoplasmic protein regulation. This article emphasizes signaling mechanisms and the importance of biochemical control in executing biological functions by the prototype member of the family, TGF-β.

Roles of TrkC Signaling in the Regulation of Tumorigenicity and Metastasis of Cancer

Tropomyosin receptor kinase (Trk) C contributes to the clinicopathology of a variety of human cancers, and new chimeric oncoproteins containing the tyrosine kinase domain of TrkC occur after fusion to the partner genes. Overexpression of TrkC and TrkC fusion proteins was observed in patients with a variety of cancers, including mesenchymal, hematopoietic, and those of epithelial cell lineage. Both microRNAs (miRNAs) and long non-coding RNAs (lncRNAs) were involved in the regulation of TrkC expression through transcriptional and posttranscriptional alteration. Aberrant activation of TrkC and TrkC fusion proteins markedly induces the epithelial-mesenchymal transition (EMT) program, growth rate, tumorigenic capacity via constitutive activation of Ras-MAP kinase (MAPK), PI3K-AKT, and the JAK2-STAT3 pathway. The clinical trial of TrkC or TrkC fusion-positive cancers with newly developed Trk inhibitors demonstrated that Trk inhibitors were highly effective in inducing tumor regression in patients who do not harbor mutations in the kinase domain. Recently, there has been a progressive accumulation of mutations in TrkC or the TrkC fusion protein detected in the clinic and its related cancer cell lines caused by high-throughput DNA sequencing. Despite given the high overall response rate against Trk or Trk fusion proteins-positive solid tumors, acquired drug resistance was observed in patients with various cancers caused by mutations in the Trk kinase domain. To overcome acquired resistance caused by kinase domain mutation, next-generation Trk inhibitors have been developed, and these inhibitors are currently under investigation in clinical trials.

MEST induces Twist-1-mediated EMT through STAT3 activation in breast cancers

The loss of imprinting of MEST has been linked to certain types of cancer by promoter switching. However, MEST-mediated regulation of tumorigenicity and metastasis are yet to be understood. Herein, we reported that MEST is a key regulator of IL-6/JAK/STAT3/Twist-1 signal pathway-mediated tumor metastasis. Enhanced MEST expression is significantly associated with pathogenesis of breast cancer patients. Also, MEST induces metastatic potential of breast cancer through induction of the EMT-TFs-mediated EMT program. Moreover, MEST leads to Twist-1 induction by STAT3 activation and subsequently enables the induction of activation of the EMT program via the induction of STAT3 nuclear translocation. Furthermore, the c-terminal region of MEST was essential for STAT3 activation via the induction of JAK2/STAT3 complex formation. Finally, MEST is required for metastasis in an experimental metastasis model. These observations suggest that MEST is a promising target for intervention to prevent tumor metastasis.

TGF-β family co-receptor function and signaling

Transforming growth factor-β (TGF-β) family members, which include TGF-βs, activins and bone morphogenetic proteins, are pleiotropic cytokines that elicit cell type-specific effects in a highly context-dependent manner in many different tissues. These secreted protein ligands signal via single-transmembrane Type I and Type II serine/threonine kinase receptors and intracellular SMAD transcription factors. Deregulation in signaling has been implicated in a broad array of diseases, and implicate the need for intricate fine tuning in cellular signaling responses. One important emerging mechanism by which TGF-β family receptor signaling intensity, duration, specificity and diversity are regulated and/or mediated is through cell surface co-receptors. Here, we provide an overview of the co-receptors that have been identified for TGF-β family members. While some appear to be specific to TGF-β family members, others are shared with other pathways and provide possible ways for signal integration. This review focuses on novel functions of TGF-β family co-receptors, which continue to be discovered.

An agent for optical imaging of TrkC-expressing, breast cancer

Tropomyosin receptor kinases receptor C is expressed at high levels on the surface of tumors from metastatic breast cancer, metastatic melanoma, glioblastoma, and neuroblastoma. Previous studies have shown synthetic TrkC ligands bearing agents for photodynamic therapy could be used to completely ablate 4T1 metastatic breast tumors and suppress metastatic spread in vivo. Modification of these probes (A in the text) to make them suitable for near infrared optical imaging in vivo would require a substantial increase in molecular mass (and hence increased vulnerability to undesirable absorption, metabolism and immunogenicity effects), or significant changes to the probe design which might compromise binding to TrkC in histochemical studies and on live cells. The research featured here was undertaken to investigate if the second strategy could be achieved without compromising binding to TrkC-expressing tissues. Specifically, an "aza-BODIPY" probe was synthesized to replace a spacer fragment in the original probe A. In the event, this new probe design (1a in the text) binds TrkC⁺ breast cancer in live cell cultures, in histochemical studies and in an in vivo murine model. Probe 1a binds TrkC⁺ tissues with good contrast with respect to healthy tissues, and much more strongly than an isomeric, non-TrkC binding, probe (1b) prepared as a negative control.

Tropomyosin Receptor Kinase C Targeted Delivery of a Peptidomimetic Ligand-Photosensitizer Conjugate Induces Antitumor Immune Responses Following Photodynamic Therapy

Tropomyosin receptor kinase C (TrkC) targeted ligand-photosensitizer construct, IYIY-diiodo-boron-dipyrromethene (IYIY-I₂-BODIPY) and its scrambled counterpart YIYI-I₂-BODIPY have been prepared. IYIY-I₂-BODIPY binds TrkC similar to neurotrophin-3 (NT-3), and NT-3 has been reported to modulate immune responses. Moreover, it could be shown that photodynamic therapy (PDT) elevates antitumor immune responses. This prompted us to investigate the immunological impacts mediated by IYIY-I₂-BODIPY in pre- and post-PDT conditions. We demonstrated that IYIY-I₂-BODIPY (strong response) and YIYI-I₂-BODIPY (weak response) at 10 mg/kg, but not I₂-BODIPY control, increased the levels of IL-2, IL-4, IL-6 and IL-17, but decreased the levels of systemic immunoregulatory mediators TGF-β, myeloid-derived suppressor cells and regulatory T-cells. Only IYIY-I₂-BODIPY enhanced the IFN-γ⁺ and IL-17⁺ T-lymphocytes, and delayed tumor growth (~20% smaller size) in mice when administrated daily for 5 days. All those effects were observed without irradiation; when irradiated (520 nm, 100 J/cm², 160 mW/cm²) to produce PDT effects (drug-light interval 1 h), IYIY-I₂-BODIPY induced stronger responses. Moreover, photoirradiated IYIY-I₂-BODIPY treated mice had high levels of effector T-cells compared to controls. Adoptive transfer of immune cells from IYIY-I₂-BODIPY-treated survivor mice that were photoirradiated gave significantly delayed tumor growth (~40–50% smaller size) in recipient mice. IYIY-I₂-BODIPY alone and in combination with PDT modulates the immune response in such a way that tumor growth is suppressed. Unlike immunosuppressive conventional chemotherapy, IYIY-I₂-BODIPY can act as an immune-stimulatory chemotherapeutic agent with potential applications in clinical cancer treatment.

Dysregulated JAK2 expression by TrkC promotes metastasis potential, and EMT program of metastatic breast cancer

Metastatic breast cancers are aggressive tumors associated with high levels of epithelial-mesenchymal transition (EMT) markers, activation of IL6/JAK2/STAT3 and PI3K/AKT pathways for cell growth, mobility, invasion, metastasis, and CSC status. We identified a new molecular and functional network present in metastasis that regulates and coordinates with TrkC. Inhibition of SOCS3-mediated JAK2 degradation by TrkC increases total JAK2/STAT3 expression, and then leads to upregulation of Twist-1 through activation of JAK2/STAT3 cascade. Also, TrkC increases secretion and expression of IL-6, suggesting that this autocrine loop generated by TrkC maintains the mesenchymal state by continued activation of the JAK2/STAT3 cascade and upregulation of Twist expression. Moreover, TrkC interacts with the c-Src/Jak2 complex, which increases Twist-1 and Twist-2 levels via regulation of JAK2/STAT3 activation and JAK2/STAT3 expression. Furthermore, TrkC enhances metastatic potential of breast cancer via induction of EMT by upregulating Twist-1 and Twist-2. Additionally, TrkC significantly enhances the ability of breast cancer cells to form pulmonary metastases and primary tumor formation. Unexpectedly, we found that TrkC expression and clinical breast tumor pathological phenotypes show significant correlation. These findings suggest that TrkC plays a central role in tumorigenicity, metastasis, and self-renewal traits of metastatic breast cancer.

Signaling Receptors for TGF-β Family Members

Transforming growth factor β (TGF-β) family members signal via heterotetrameric complexes of type I and type II dual specificity kinase receptors. The activation and stability of the receptors are controlled by posttranslational modifications, such as phosphorylation, ubiquitylation, sumoylation, and neddylation, as well as by interaction with other proteins at the cell surface and in the cytoplasm. Activation of TGF-β receptors induces signaling via formation of Smad complexes that are translocated to the nucleus where they act as transcription factors, as well as via non-Smad pathways, including the Erk1/2, JNK and p38 MAP kinase pathways, and the Src tyrosine kinase, phosphatidylinositol 3'-kinase, and Rho GTPases.

Regulation of TGF-β Receptors

In cells responding to extracellular polypeptide ligands, regulatory mechanisms at the level of cell surface receptors are increasingly seen to define the nature of the ligand-induced signaling responses. Processes that govern the levels of receptors at the plasma membrane, including posttranslational modifications, are crucial to ensure receptor function and specify the downstream signals. Indeed, extracellular posttranslational modifications of the receptors help define stability and ligand binding, while intracellular modifications mediate interactions with signaling mediators and accessory proteins that help define the nature of the signaling response. The use of various molecular biology and biochemistry techniques, based on chemical crosslinking, e.g., biotin or radioactive labeling, immunofluorescence to label membrane receptors and flow cytometry, allows for quantification of changes of cell surface receptor presentation. Here, we discuss recent progress in our understanding of the regulation of TGF-β receptors, i.e., the type I (TβRI) and type II (TβRII) TGF-β receptors, and describe basic methods to identify and quantify TGF-β cell surface receptors.

SPSB1, a Novel Negative Regulator of the Transforming Growth Factor-β Signaling Pathway Targeting the Type II Receptor

Appropriate cellular signaling is essential to control cell proliferation, differentiation, and cell death. Aberrant signaling can have devastating consequences and lead to disease states, including cancer. The transforming growth factor-β (TGF-β) signaling pathway is a prominent signaling pathway that has been tightly regulated in normal cells, whereas its deregulation strongly correlates with the progression of human cancers. The regulation of the TGF-β signaling pathway involves a variety of physiological regulators. Many of these molecules act to alter the activity of Smad proteins. In contrast, the number of molecules known to affect the TGF-β signaling pathway at the receptor level is relatively low, and there are no known direct modulators for the TGF-β type II receptor (TβRII). Here we identify SPSB1 (a Spry domain-containing Socs box protein) as a novel regulator of the TGF-β signaling pathway. SPSB1 negatively regulates the TGF-β signaling pathway through its interaction with both endogenous and overexpressed TβRII (and not TβRI) via its Spry domain. As such, TβRII and SPSB1 co-localize on the cell membrane. SPSB1 maintains TβRII at a low level by enhancing the ubiquitination levels and degradation rates of TβRII through its Socs box. More importantly, silencing SPSB1 by siRNA results in enhanced TGF-β signaling and migration and invasion of tumor cells.

TSPYL2 is an essential component of the REST/NRSF transcriptional complex for TGFβ signaling activation

REST/NRSF is a transcriptional repressor of neuronal genes that has been implicated in development and cancer. In epithelial tissues, REST acts as a tumor suppressor and in breast cancer, loss of REST is associated with disease recurrence and poor prognosis. Here, we identify TSPYL2 (also known as CDA1 and DENTT) as a novel component of the REST protein complex. We show that REST and TSPYL2 are regulators of TGFβ signaling and that cell-cycle arrest induced by TGFβ requires both REST and TSPYL2. Importantly, knockdown of REST or TSPYL2 resulted in transformation of human mammary epithelial cells. Mechanistically, we demonstrate that the TSPYL2/REST complex promotes TGFβ signaling by repressing the expression of genes, such as the proto-oncogene neurotrophic tyrosine kinase receptor C (TrkC). These data provide insight into the role of REST as a tumor suppressor in epithelial tissues through the regulation of the TGFβ pathway.

Tropomyosin receptor kinases B and C are tumor progressive and metastatic marker in colorectal carcinoma

Members of the tropomyosin receptor kinase (Trk) family have a high affinity for neurotrophins and regulate neuronal survival. The role of Trks in cancer is still controversial. The expression and role of TrkB and TrkC were examined in colorectal cancer (CRC). Immunohistochemical analysis of TrkB and TrkC was performed in 133 patients with CRC. Using human CRC cell lines, expression of vascular endothelial growth factor (VEGF) and transforming growth factor β, cell growth, invasion, and apoptosis were examined by knockdown methods. Immunohistochemistry showed positive results of TrkB and TrkC (23.3% and 12.8%, respectively). TrkB expression was associated with local progression (P = .0284), clinical stage (P = .0026), nodal metastasis (P = .0068), and peritoneal metastasis (P = .0026). TrkC expression was only related to liver metastasis (P = .0001). Coexpression of TrkB or TrkC and their ligands was found in 80.6% and 82.4% of cases, respectively. In vitro analysis using human CRC cells showed that TrkB positively regulated gene expression of VEGF-A (P < .05) and VEGF-C (P < .05), whereas TrkC suppressed transforming growth factor β expression (P < .05). TrkB and TrkC induced cell growth (P < .05) and invasion (P < .05), respectively. Both TrkB and TrkC showed antiapoptotic effect (P < .05). These results suggest that TrkB and TrkC have a tumor progressive function and may be a useful diagnostic and therapeutic target in CRC.

Role of TGF-β1 and its receptors in malignant progression of hepatocellular carcinoma

Transforming growth factor-beta 1 (TGF-β1) is a cytokine which exerts a wide range of biological activities by initiating downstream signaling. As transmembrane receptors, TGFBRⅠ, TGFBRⅡ and TGFBRⅢ have been shown to play an important role in mediating TGF-β1 signal transduction. Owing to roles in promoting cell growth and development, alterations in TGF-β1 and its receptors may result in many diseases, especially tumors. Hepatocellular carcinoma (HCC) is a common malignant solid tumor with high risk of metastasis and recurrence and is associated with a high fatality. Recent studies point to a close relationship between hepatocellular carcinoma and the dysregulation of TGF-β1 signaling pathway. The changes in the levels of TGF-β1 and its receptors in tumor microenvironment may facilitate the invasive potential of HCC. In this article we mainly summarize the structure and function of TGF-β1 and its receptors and assess their role in metastasis and recurrence of hepatocellular carcinoma.

Mediators of receptor tyrosine kinase activation in infantile fibrosarcoma: a Children's Oncology Group study

Infantile fibrosarcoma (IFS; also known as cellular congenital mesoblastic nephroma, CMN, when in the kidney) is a rare, undifferentiated tumour often characterized by the ETV6-NTRK3 fusion transcript. Our goal was to identify downstream pathways, diagnostic markers and potential therapeutic targets for IFS/CMN. Global gene expression, reverse-phase protein array and ETV6-NTRK3 fusion analyses were performed on 14 IFS/CMN and compared with 41 other paediatric renal tumours. These analyses confirm significant receptor tyrosine kinase (RTK) activation, with evidence of PI3-Akt, MAPK and SRC activation. In particular, GAB2 docking protein, STAT5-pTyr-694, STAT3-pSer-729 and YAP-pSer-127 were elevated, and TAZ-pSer-89 was decreased. This provides mRNA and proteomic evidence that GAB2, STAT activation and phosphorylation of the Hippo pathway transcription co-activators YAP and TAZ contribute to the RTK signal transduction in IFS/CMN. All IFS/CMN tumours displayed a distinctive gene expression pattern that may be diagnostically useful. Unexpectedly, abundant ETV6-NTRK3 transcript copies were present in only 7/14 IFS, with very low copy number in 3/14. An additional 4/14 were negative by RT-PCR and absence of ETV6-NTRK3 was confirmed by FISH for both ETV6 and NTRK3. Therefore, molecular mechanisms other than ETV6-NTRK3 fusion are responsible for the development of some IFS/CMNs and the absence of ETV6-NTRK3 fusion products should not exclude IFS/CMN as a diagnosis.

Trks are novel oncogenes involved in the induction of neovascularization, tumor progression, and nodal metastasis in oral squamous cell carcinoma

The function of tropomyosin receptor kinase (Trk) family including TrkA, TrkB, and TrkC in cancer remains unknown. The role of Trks in oral squamous cell carcinoma (OSCC) was examined. Knockdown of Trks provided inhibition of growth or invasion and decrease of apoptosis in OSCC cells, which expressed Trks at high levels. VEGF expression was associated with TrkA and TrkB expression; a decrease of VEGF-C and VEGF-D was observed in OSCC cells with TrkB knockdown. TrkC did not affect the expression of VEGF family. An immunohistochemical analysis of 102 OSCCs showed that TrkB expression was related to microvessel density (MVD), lymph vessel density (LVD), and poor prognosis. TrkC expression was correlated with clinical stage, lymph node metastasis, MVD, LVD, and poor prognosis. TrkA expression was associated with VEGF expression, whereas TrkB expression was associated with the expressions of VEGF, VEGF-C and VEGF-D. No significant association was found between the expression of TrkC and genes of the VEGF family. Expression of Trks was not associated with RUNX3 silencing by methylation in OSCC cells. Trks expression was inversely correlated with RUNX3 expression in the OSCC cases. These results suggested that Trks enhances progression of OSCC through angiogenesis and lymphangiogenesis.

Post-translational regulation of TGF-β receptor and Smad signaling

TGF-β family signaling through Smads is conceptually a simple and linear signaling pathway, driven by sequential phosphorylation, with type II receptors activating type I receptors, which in turn activate R-Smads. Nevertheless, TGF-β family proteins induce highly complex programs of gene expression responses that are extensively regulated, and depend on the physiological context of the cells. Regulation of TGF-β signaling occurs at multiple levels, including TGF-β activation, formation, activation and destruction of functional TGF-β receptor complexes, activation and degradation of Smads, and formation of Smad transcription complexes at regulatory gene sequences that cooperate with a diverse set of DNA binding transcription factors and coregulators. Here we discuss recent insights into the roles of post-translational modifications and molecular interaction networks in the functions of receptors and Smads in TGF-β signal responses. These layers of regulation demonstrate how a simple signaling system can be coopted to exert exquisitely regulated, complex responses.

p130Cas is required for mammary tumor growth and transforming growth factor-beta-mediated metastasis through regulation of Smad2/3 activity

During breast cancer progression, transforming growth factor-beta (TGF-beta) switches from a tumor suppressor to a pro-metastatic molecule. Several recent studies suggest that this conversion in TGF-beta function depends upon fundamental changes in the TGF-beta signaling system. We show here that these changes in TGF-beta signaling are concomitant with aberrant expression of the focal adhesion protein, p130Cas. Indeed, elevating expression of either the full-length (FL) or just the carboxyl terminus (CT) of p130Cas in mammary epithelial cells (MECs) diminished the ability of TGF-beta1 to activate Smad2/3, but increased its coupling to p38 MAPK. This shift in TGF-beta signaling evoked (i) resistance to TGF-beta-induced growth arrest, and (ii) acinar filling upon three-dimensional organotypic cultures of p130Cas-FL or -CT expressing MECs. Furthermore, rendering metastatic MECs deficient in p130Cas enhanced TGF-beta-stimulated Smad2/3 activity, which restored TGF-beta-induced growth inhibition both in vitro and in mammary tumors produced in mice. Additionally, whereas elevating TbetaR-II expression in metastatic MECs had no affect on their phosphorylation of Smad2/3, this event markedly enhanced their activation of p38 MAPK, leading to increased MEC invasion and metastasis. Importantly, depleting p130Cas expression in TbetaR-II-expressing metastatic MECs significantly increased their activation of Smad2/3, which (i) reestablished the physiologic balance between canonical and noncanonical TGF-beta signaling, and (ii) reversed cellular invasion and early mammary tumor cell dissemination stimulated by TGF-beta. Collectively, our findings identify p130Cas as a molecular rheostat that regulates the delicate balance between canonical and noncanonical TGF-beta signaling, a balance that is critical to maintaining the tumor suppressor function of TGF-beta during breast cancer progression.

CDYL bridges REST and histone methyltransferases for gene repression and suppression of cellular transformation

The neuronal gene repressor REST/NRSF recruits corepressors, including CoREST, to modify histones and repress transcription. REST also functions as a tumor suppressor, but the mechanism remains unclear. We identified chromodomain on Y-like (CDYL) as a REST corepressor that physically bridges REST and the histone methylase G9a to repress transcription. Importantly, RNAi knockdown of REST, CDYL, and G9a, but not CoREST, induced oncogenic transformation of immortalized primary human cells and derepression of the proto-oncogene TrkC. Significantly, transgenic expression of TrkC also induced transformation. This implicates CDYL-G9a, but not CoREST, in REST suppression of transformation, possibly by oncogene repression. CDYL knockdown also augments transformation in a cell culture model of cervical cancer, where loss of heterozygosity of the CDYL locus occurs. These findings demonstrate molecular strategies by which REST carries out distinct biological functions via different corepressors and provide critical insights into the role of histone-modifying complexes in regulating cellular transformation.

High-affinity neurotrophin receptors and ligands promote leukemogenesis

Neurotrophins (NTs) and their receptors play a key role in neurogenesis and survival. The TRK (tropomyosin-related kinase) receptor protein tyrosine kinases (TRKA, TRKB, TRKC) are high-affinity NT receptors that are expressed in a variety of human tissues. Their role in normal and malignant hematopoiesis is poorly understood. In a prospective study involving 94 adult patients we demonstrate for the first time cell-surface expression of the 3 TRKs and constitutive activation in blasts from patients with de novo or secondary acute leukemia. At least one TRK was expressed in 55% of the analyzed cases. We establish a clear correlation between the TRK expression pattern and FAB classification. Although only few point mutations were found in TRK sequences by reverse-transcriptase-polymerase chain reaction (RT-PCR), we observed coexpression of BDNF (ligand for TRKB) in more than 50% of TRKB(+) cases (16/30). Activation of TRKA or TRKB by NGF and BDNF, respectively, efficiently rescued murine myeloid cells from irradiation-induced apoptosis. Coexpression of TRKB/BDNF or TRKA/NGF in murine hematopoietic cells induced leukemia. Moreover, activation of TRKs was important for survival of both human and murine leukemic cells. Our findings suggest that TRKs play an important role in leukemogenesis and may serve as a new drug target.