p62(dok), a negative regulator of Ras and mitogen-activated protein kinase (MAPK) activity, opposes leukemogenesis by p210(bcr-abl)

GL-V9 induce apoptosis of CML cells via MAPK signaling pathway

GL-V9, a derivative of wogonin, has shown potent antitumor effects in various cancers, yet its impact on chronic myeloid leukemia (CML) remains unexplored. In this study, we found that GL-V9 significantly decreased the viability of CML cells. Annexin V/PI staining demonstrated that GL-V9 induced apoptosis in a concentration-dependent manner. The JC-1 assay indicated a significant reduction in mitochondrial membrane potential (ΔΨm) in cells treated with GL-V9. Additionally, GL-V9 altered reactive oxygen species (ROS) levels in CML cells. Through transcriptomic sequencing and Western blot analysis, we further revealed that GL-V9 activated the MAPK pathway. These results suggest that GL-V9 is a promising therapeutic candidate for CML.

Diverse p120RasGAP interactions with doubly phosphorylated partners EphB4, p190RhoGAP, and Dok1

RasGAP (p120RasGAP), the founding member of the GTPase-activating protein (GAP) family, is one of only nine human proteins to contain two SH2 domains and is essential for proper vascular development. Despite its importance, its interactions with key binding partners remains unclear. In this study we provide a detailed viewpoint of RasGAP recruitment to various binding partners and assess their impact on RasGAP activity. We reveal the RasGAP SH2 domains generate distinct binding interactions with three well-known doubly phosphorylated binding partners: p190RhoGAP, Dok1, and EphB4. Affinity measurements demonstrate a 100-fold weakened affinity for RasGAP-EphB4 binding compared to RasGAP-p190RhoGAP or RasGAP-Dok1 binding, possibly driven by single versus dual SH2 domain engagement with a dominant N-terminal SH2 interaction. Small-angle X-ray scattering reveals conformational differences between RasGAP-EphB4 binding and RasGAP-p190RhoGAP binding. Importantly, these interactions do not impact catalytic activity, implying RasGAP utilizes its SH2 domains to achieve diverse spatial-temporal regulation of Ras signaling in a previously unrecognized fashion.

DOK7 Inhibits Cell Proliferation, Migration, and Invasion of Breast Cancer via the PI3K/PTEN/AKT Pathway

Recently, increasing attention has been paid to the correlation between the expression of downstream of kinase 7 (DOK7) and the occurrence and development of various tumors. In this study, we clarified the effects of DOK7 in breast cancer. First, we showed that DOK7 expression was obviously reduced in the breast cancer tissues and lower levels of DOK7 linked to more aggressive behaviors and worse prognosis of patients. Furthermore, DOK7 expression of various breast cancer cell lines was lower than that of human noncancerous MCF-10A cells. Overexpression of DOK7 inhibited proliferation, migration, and invasion, while silencing DOK7 expression promoted the malignancy of breast cancer. In addition, overexpression of DOK7 suppressed tumor proliferation and lung metastasis in animal models. Finally, to investigate the possible signaling mechanism, we first found that the level of p-AKT protein was extremely downregulated and the level of PTEN protein was remarkably upregulated after overexpressing DOK7 in breast cancer cells. Repression of PTEN expression using PTEN siRNA or SF1670 (PTEN inhibitor) rescued the tumor-inhibiting effect induced by DOK7 overexpression, suggesting that DOK7 inhibits proliferation, migration, and invasion of breast cancer cells though the PI3K/PTEN/AKT pathway. These results suggest that the downregulation of DOK7 may become a novel breast cancer therapeutic target.

New insights into the novel anti-inflammatory mode of action of glucocorticoids

Inflammation is a physiological intrinsic host response to injury meant for removal of noxious stimuli and maintenance of homeostasis. It is a defensive body mechanism that involves immune cells, blood vessels and molecular mediators of inflammation. Glucocorticoids (GCs) are steroidal hormones responsible for regulation of homeostatic and metabolic functions of body. Synthetic GCs are the most useful anti-inflammatory drugs used for the treatment of chronic inflammatory diseases such as asthma, chronic obstructive pulmonary disease (COPD), allergies, multiple sclerosis, tendinitis, lupus, atopic dermatitis, ulcerative colitis, rheumatoid arthritis and osteoarthritis whereas, the long term use of GCs are associated with many side effects. The anti-inflammatory and immunosuppressive (desired) effects of GCs are usually mediated by transrepression mechanism whereas; the metabolic and toxic (undesired) effects are usually manifested by transactivation mechanism. Though GCs are most potent anti-inflammatory and immunosuppressive drugs, the common problem associated with their use is GC resistance. Several research studies are rising to comprehend these mechanisms, which would be helpful in improving the GC resistance in asthma and COPD patients. This review aims to focus on identification of new drug targets in inflammation which will be helpful in the resolution of inflammation. The ample understanding of GC mechanisms of action helps in the development of novel anti-inflammatory drugs for the treatment of inflammatory and autoimmune disease with reduced side effects and minimal toxicity.

Analysis of the DOK1 gene in breast cancer

Breast cancer, which is the most common type of cancer among women, is a heterogenous disease. It results from progressive accumulation of genetic and epigenetic alterations in different genes. The Dok1 protein has been identified as the major substrate of protein tyrosine kinases in hematopoietic cells. It is considered as a tumor suppressor due to the reports which describe its inhibitory effect on major oncogenic signaling pathways such as Mek/Erk/PI3k/Akt and Wnt/β-catenin. In this study, we investigated the mutation frequency of the DOK1 gene in 118 breast tumors using Sanger sequencing and DOK1 mRNA expression level in 63 breast cancer samples using qRT-PCR methods. Although the mutation frequency was low DOK1 mRNA expression levels were significantly reduced (63.5%) in the tumors compared to adjacent non-cancerous tissue. We also correlated expression changes with clinicopathological characteristics. Low mRNA levels correlated with age (p = 0.01) and c-erbB-2 (p = 0.05). In most of the previous reports, down-regulation of DOK1 mRNA expression has been associated with promoter methylation. We identified four different coding sequence alterations in 5.1% (6/118) of the tumor samples. However, all of these alterations were located in the functional domains of the protein. Therefore, these mutations may affect the function and/or cellular localization of the protein and contribute to cancer progression by this way. In conclusion our data indicate that DOK1 acts as a tumor suppressor in breast cancer and association of Dok1 with the c-erbB-2 mediated mechanism of action in breast cancer needs to be investigated.

DOK6 promoter methylation serves as a potential biomarker affecting prognosis in de novo acute myeloid leukemia

BACKGROUND

Downstream of tyrosine kinase 6 (DOK6), which is specifically expressed in the nervous system, was previously recognized as an adapter only in neurite outgrowth. Recent studies also demonstrated the potential role of DOK6 in solid tumors such as gastric cancer and breast cancer. However, previous studies of DOK6 have not dealt with its roles in myeloid malignancies. Herein, we verified the promoter methylation status of DOK6 and further explored its clinical implication in de novo acute myeloid leukemia (AML).

METHODS

A total of 100 newly diagnosed adult AML patients were involved in the current study. DOK6 expression and methylation were detected by real-time qPCR and methylation-specific PCR (MSP), respectively. Bisulfite sequencing PCR (BSP) was performed to assess the methylation density of the DOK6 promoter.

RESULTS

Downstream of tyrosine kinase 6 promoter methylation was significantly increased in AML patients compared to controls (P = .037), whereas DOK6 expression significantly decreased in AML patients (P < .001). The expression of DOK6 was markedly up-regulated after treated by 5-aza-2'-deoxycytidine (5-aza-dC) in THP-1 cell lines. The methylation status of the DOK6 promoter was associated with French-American-British classifications (P = .037). There was no significant correlation existed between DOK6 expression and its promoter methylation (R = .077, P = .635). Interestingly, of whole-AML and non-APL AML patients, both have a tendency pertaining to the DOK6 methylation group and a significantly longer overall survival (OS) than the DOK6 unmethylation group (P = .042 and .036, respectively).

CONCLUSION

Our study suggested that DOK6 promoter hypermethylation was a common molecular event in de novo AML patients. Remarkably, DOK6 promoter methylation could serve as an independent and integrated prognostic biomarker not only in non-APL AML patients but also in AML patients who are less than 60 years old.

DNA methyltransferases in virus-associated cancers

Human tumor viruses are either casually linked or contribute in the development of human cancers. Viruses can stimulate oncogenesis through affecting diverse biological pathways in human cells. Growing data have demonstrated frequent involvement of one of the most characteristic parts of cellular epigenetic machinery, DNA methylation, in the oncogenesis. DNA methylation of cellular genes is catalyzed by DNA methyltransferases (DNMTs) as a key effector enzyme in this process. Dysregulation of DNMTs can cause aberrant gene methylation in promoter of cancer-related genes including tumor suppressor genes, resulting in gene silencing. In this regard, the role of tumor viruses is remarkable. Here, in this review, we used published information to elucidate whether tumor viruses are able to manipulate DNMT regulation, and if so, what are its consequences in the process of oncogenesis. This essay also aims to shed light on which cellular pathways have been engaged by viruses to induce DNMTs.

Rasal2 deficiency reduces adipogenesis and occurrence of obesity-related disorders

Objective

Identification of additional regulatory factors involved in the onset of obesity is important to understand the mechanisms underlying this prevailing disease and its associated metabolic disorders and to develop therapeutic strategies. Through isolation and analysis of a mutant, we aimed to uncover the function of a Ras-GAP gene, Rasal2 (Ras protein activator like 2), in the development of obesity and related metabolic disorders and to obtain valuable insights regarding the mechanism underlying the function.

Methods

An obesity-based genetic screen was performed to identify an insertional mutation that disrupts the expression of Rasal2 (Rasal2^PB/PB mice). Important metabolic parameters, such as fat mass and glucose tolerance, were measured in Rasal2^PB/PB mice. The impact of Rasal2 on adipogenesis was evaluated in the mutant mice and in 3T3-L1 preadipocytes treated with Rasal2 siRNA. Ras and ERK activities were then evaluated in Rasal2-deficient preadipocytes or mice, and their functional relationships with Rasal2 on adipogenesis were investigated by employing Ras and MEK inhibitors.

Results

Rasal2^PB/PB mice showed drastic decrease in Rasal2 expression and a lean phenotype. The mutant mice displayed decreased adiposity and resistance to high-fat diet induced metabolic disorders. Further analysis indicated that Rasal2 deficiency leads to impaired adipogenesis in vivo and in vitro. Moreover, while Rasal2 deficiency resulted in increased activity of both Ras and ERK in preadipocytes, reducing Ras, but not ERK, suppressed the impaired adipogenesis.

Conclusions

Rasal2 promotes adipogenesis, which may critically contribute to its role in the development of obesity and related metabolic disorders and may do so by repressing Ras activity in an ERK-independent manner.

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Rasal2-deficient mice show decreased adiposity fed on either high-fat or normal-chow diet.

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Rasal2-deficient mice are resistant to high-fat diet-induced obesity and related metabolic disorders.

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Rasal2 deficiency causes a decrease in adipogenesis in vivo and in vitro.

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Rasal2 likely regulates adipogenesis by repressing Ras activity through an ERK-independent mechanism.

In Vitro Apoptotic Effects of Farnesyltransferase blockade in Acute Myeloid Leukemia Cells

Farnesyltransferase inhibitors (FTIs) are a class of oral anti-cancer drugs currently tested in phase I-II clinical trials for treatment of hematological malignancies. The in vitro effects of various FTIs (alpha-hydroxyfarnesylphosphonic acid, manumycin-A and SCH66336) were tested on CD34⁺ KG1a cell line and in primary acute myeloid leukemia (AML) cells from 64 patients. By cell viability and clonogeneic methylcellulose assays, FTIs showed a significant inhibitory activity in CD34⁺ KG1a and primary bone marrow (BM) leukemic cells from 56% of AML patients. FTIs also induced activation of caspase-3 and Fas-independent apoptosis, confirmed by the finding that inhibition of caspase-8 was not associated with the rescue of FTI-treated cells. We concluded that other cellular events induced by FTIs may trigger activation of caspase-3 and subsequent apoptosis, but the expression of proapoptotic molecules, as Bcl-2 and Bcl-XL, and antiapoptotic, as Bcl-X(s), were not modified by FTIs. By contrast, expression of inducible nitric oxide synthase (iNOS) was increased in FTI-treated AML cells. Our results suggest a very complex mechanism of action of FTIs that require more studies for a better clinical use of the drugs alone or in combination in the treatment of hematological malignancies.

The non-genomic loss of function of tumor suppressors: an essential role in the pathogenesis of chronic myeloid leukemia chronic phase

BACKGROUND

Chronic Myeloid Leukemia was always referred as a unique cancer due to the apparent independence from tumor suppressors' deletions/mutations in the early stages of the disease. However, it is now well documented that even genetically wild-type tumor suppressors can be involved in tumorigenesis, when functionally inactivated. In particular, tumor suppressors' functions can be impaired by subtle variations of protein levels, changes in cellular compartmentalization and post-transcriptional/post-translational modifications, such as phosphorylation, acetylation, ubiquitination and sumoylation. Notably, tumor suppressors inactivation offers challenging therapeutic opportunities. The reactivation of an inactive and genetically wild-type tumor suppressor could indeed promote selective apoptosis of cancer cells without affecting normal cells.

MAIN BODY

Chronic Myeloid Leukemia (CML) could be considered as the paradigm for non-genomic loss of function of tumor suppressors due to the ability of BCR-ABL to directly promote functionally inactivation of several tumor suppressors.

SHORT CONCLUSION

In this review we will describe new insights on the role of FoxO, PP2A, p27, BLK, PTEN and other tumor suppressors in CML pathogenesis. Finally, we will describe strategies to promote tumor suppressors reactivation in CML.

Dok-1 negatively regulates platelet integrin αIIbβ3 outside-in signalling and inhibits thrombosis in mice

Adaptor proteins play a critical role in the assembly of signalling complexes after engagement of platelet receptors by agonists such as collagen, ADP and thrombin. Recently, using proteomics, the Dok (downstream of tyrosine kinase) adapter proteins were identified in human and mouse platelets. In vitro studies suggest that Dok-1 binds to platelet integrin β3, but the underlying effects of Dok-1 on αIIbβ3 signalling, platelet activation and thrombosis remain to be elucidated. In the present study, using Dok-1-deficient (Dok-1-/-) mice, we determined the phenotypic role of Dok-1 in αIIbβ3 signalling. We found that platelets from Dok-1-/- mice displayed normal aggregation, activation of αIIbβ3 (assessed by binding of JON/A), P-selectin surface expression (assessed by anti-CD62P), and soluble fibrinogen binding. These findings indicate that Dok-1 does not affect "inside-out" platelet signalling. Compared with platelets from wild-type (WT) mice, platelets from Dok-1-/- mice exhibited increased clot retraction (p < 0.05 vs WT), increased PLCγ2 phosphorylation, and enhanced spreading on fibrinogen after thrombin stimulation (p < 0.01 vs WT), demonstrating that Dok-1 negatively regulates αIIbβ3 "outside-in" signalling. Finally, we found that Dok-1-/- mice exhibited significantly shortened bleeding times and accelerated carotid artery thrombosis in response to photochemical injury (p < 0.05 vs WT mice). We conclude that Dok-1 modulates thrombosis and haemostasis by negatively regulating αIIbβ3 outside-in signalling.

Dok proteins are recruited to the phagosome and degraded in a GP63-dependent manner during Leishmania major infection

Three adaptor molecules of the Dok family, Dok-1, Dok-2 and Dok-3 are expressed in macrophages and are involved in the negative regulation of signaling in response to lipopolysaccharide and various cytokines and growth factors. We investigated the role and the fate of these proteins following infection with Leishmania major promastigotes in macrophages. The protozoan parasite L. major causes cutaneous leishmaniasis and is known for its capacity to alter host-cell signaling and function. Dok-1/Dok-2(-/-) bone marrow-derived macrophages displayed normal uptake of L. major promastigotes. Following Leishmania infection, Dok-1 was barely detectable by confocal microscopy. By contrast, phagocytosis of latex beads or zymosan led to the recruitment of Dok-1 to phagosomes. In the absence of the Leishmania pathogenesis-associated metalloprotease GP63, Dok-1 was also, partially, recruited to phagosomes containing L. major promastigotes. Further biochemical analyses revealed that similar to Dok-1, Dok-2 and Dok-3 were targets of GP63. Moreover, we showed that upon infection with wild-type or Δgp63 L. major promastigotes, production of nitric oxide and tumor necrosis factor by interferon-γ-primed Dok-1/Dok-2(-/-) macrophages was reduced compared to WT macrophages. These results suggest that Dok proteins may be important regulators of macrophage responses to Leishmania infection.

mRNA expression of DOK1-6 in human breast cancer

AIM: To examine the expression of downstream of tyrosine kinase (DOK)1-6 genes in normal and breast cancer tissue and correlated this with several clinico-pathological and prognostic factors.

METHODS: DOK1-6 mRNA extraction and reverse transcription were performed on fresh frozen breast cancer tissue samples (n = 112) and normal background breast tissue (n = 31). Tissues were collected between 1991 and 1996 at two centres and all patients underwent mastectomy and ipsilateral axillary node dissection. All tissues were randomly numbered and the details were only made known after all analyses were completed. Transcript levels of expression were determined using real-time polymerase chain reaction and analyzed against TNM stage, tumour grade and clinical outcome over a 10-year follow-up period.

RESULTS: DOK-2 and DOK-6 expression decreased with increasing TNM stage. DOK-6 expression decreased with increasing Nottingham Prognostic Index (NPI) [NPI-1 vs NPI-3 (mean copy number 15.4 vs 0.22, 95%CI: 2.7-27.6, P = 0.018) and NPI-2 vs NPI-3 (mean copy number 7.6 vs 0.22, 95%CI: 0.1-14.6, P = 0.048)]. After a median follow up period of 10 years, higher levels of DOK-2 expression were found among patients who remained disease-free compared to those who developed local or distant recurrence (mean copy number 3.94 vs 0.0000096, 95%CI: 1.0-6.85, P = 0.0091), and distant recurrence (mean copy number 3.94 vs 0.0025, 95%CI: 1.0-6.84, P = 0.0092). Patients who remained disease-free had higher levels of DOK-6 expression compared to those who died from breast cancer.

CONCLUSION: Decreasing expression levels of DOK-2 and DOK-6 with increased breast tumour progression supports the notion that DOK-2 and DOK-6 behave as tumour suppressors in human breast cancer.

Epstein-Barr virus down-regulates tumor suppressor DOK1 expression

The DOK1 tumor suppressor gene encodes an adapter protein that acts as a negative regulator of several signaling pathways. We have previously reported that DOK1 expression is up-regulated upon cellular stress, via the transcription factor E2F1, and down-regulated in a variety of human malignancies due to aberrant hypermethylation of its promoter. Here we show that Epstein Barr virus (EBV) infection of primary human B-cells leads to the down-regulation of DOK1 gene expression via the viral oncoprotein LMP1. LMP1 alone induces recruitment to the DOK1 promoter of at least two independent inhibitory complexes, one containing E2F1/pRB/DNMT1 and another containing at least EZH2. These events result in tri-methylation of histone H3 at lysine 27 (H3K27me3) of the DOK1 promoter and gene expression silencing. We also present evidence that the presence of additional EBV proteins leads to further repression of DOK1 expression with an additional mechanism. Indeed, EBV infection of B-cells induces DNA methylation at the DOK1 promoter region including the E2F1 responsive elements that, in turn, lose the ability to interact with E2F complexes. Treatment of EBV-infected B-cell-lines with the methyl-transferase inhibitor 5-aza-2′-deoxycytidine rescues DOK1 expression. In summary, our data show the deregulation of DOK1 gene expression by EBV and provide novel insights into the regulation of the DOK1 tumor suppressor in viral-related carcinogenesis.

Many oncogenic viruses exhibit cellular transforming properties, often involving oncogenes activation and tumor suppressor genes inactivation. The DOK1 gene is a newly identified tumor suppressor gene with altered expression via hypermethylation of its promoter in a variety of human cancers, including head and neck, lung, gastric and others. In addition, a correlation has been reported between DOK1 aberrant hypermethylation and the presence of oncogenic viruses such as hepatitis B virus (HBV) in hepatocellular carcinoma (HCC) and Epstein-Barr virus (EBV) in Burkitt's lymphoma-derived cell lines. Here we demonstrate for the first time that EBV is directly involved in the inhibition of DOK1 expression in B-cells. We show that EBV leads to epigenetic repression of DOK1 through increased DNA methylation of its promoter and H3K27 tri-methylation. The LMP1 oncoprotein plays a key role in the repression of DOK1 expression. It promotes the formation and the recruitment to the DOK1 promoter of transcriptionally inhibitory complexes composed of E2F1/pRB/DNMT1 and of EZH2 which is part of the polycomb repressive complex 2. Interestingly, one or more additional EBV protein(s) cooperate(s) with LMP1 in inducing massive DNA methylation at the DOK1 promoter, leading to the loss of E2F1 complexes recruitment and even stronger repression of DOK1 expression.

BRK targets Dok1 for ubiquitin-mediated proteasomal degradation to promote cell proliferation and migration

Breast tumor kinase (BRK), also known as protein tyrosine kinase 6 (PTK6), is a non-receptor tyrosine kinase overexpressed in more that 60% of human breast carcinomas. The overexpression of BRK has been shown to sensitize mammary epithelial cells to mitogenic signaling and to promote cell proliferation and tumor formation. The molecular mechanisms of BRK have been unveiled by the identification and characterization of BRK target proteins. Downstream of tyrosine kinases 1 or Dok1 is a scaffolding protein and a substrate of several tyrosine kinases. Herein we show that BRK interacts with and phosphorylates Dok1 specifically on Y362. We demonstrate that this phosphorylation by BRK significantly downregulates Dok1 in a ubiquitin-proteasome-mediated mechanism. Together, these results suggest a novel mechanism of action of BRK in the promotion of tumor formation, which involves the targeting of tumor suppressor Dok1 for degradation through the ubiquitin proteasomal pathway.

The unique N-terminal region of SRMS regulates enzymatic activity and phosphorylation of its novel substrate docking protein 1

SRMS (Src-related tyrosine kinase lacking C-terminal regulatory tyrosine and N-terminal myristoylation sites) belongs to a family of nonreceptor tyrosine kinases, which also includes breast tumour kinase and Fyn-related kinase. SRMS, similar to breast tumour kinase and Fyn-related kinase, harbours a Src homology 3 and Src homology 2, as well as a protein kinase domain. However, unlike breast tumour kinase and Fyn-related kinase, SRMS lacks a C-terminal regulatory tail but distinctively possesses an extended N-terminal region. Both breast tumour kinase and Fyn-related kinase play opposing roles in cell proliferation and signalling. SRMS, however, is an understudied member of this family. Although cloned in 1994, information on the biochemical, cellular and physiological roles of SRMS remains unreported. The present study is the first to explore the expression pattern of SRMS in breast cancers, its enzymatic activity and autoregulatory elements, and the characterization of docking protein 1 as its first bonafide substrate. We found that, similar to breast tumour kinase, SRMS is highly expressed in most breast cancers compared to normal mammary cell lines and tissues. We generated a series of SRMS point and deletion mutants and assessed enzymatic activity, subcellular localization and substrate recognition. We report for the first time that ectopically-expressed SRMS is constitutively active and that its N-terminal region regulates the enzymatic activity of the protein. Finally, we present evidence indicating that docking protein 1 is a direct substrate of SRMS. Our data demonstrate that, unlike members of the Src family, the enzymatic activity of SRMS is regulated by the intramolecular interactions involving the N-terminus of the enzyme and that docking protein 1 is a bona fide substrate of SRMS.

Modulation of the transcriptional activity of peroxisome proliferator-activated receptor gamma by protein-protein interactions and post-translational modifications

Peroxisome proliferator-activated receptor gamma (PPARγ) belongs to a nuclear receptor superfamily; members of which play key roles in the control of body metabolism principally by acting on adipose tissue. Ligands of PPARγ, such as thiazolidinediones, are widely used in the treatment of metabolic syndromes and type 2 diabetes mellitus (T2DM). Although these drugs have potential benefits in the treatment of T2DM, they also cause unwanted side effects. Thus, understanding the molecular mechanisms governing the transcriptional activity of PPARγ is of prime importance in the development of new selective drugs or drugs with fewer side effects. Recent advancements in molecular biology have made it possible to obtain a deeper understanding of the role of PPARγ in body homeostasis. The transcriptional activity of PPARγ is subject to regulation either by interacting proteins or by modification of the protein itself. New interacting partners of PPARγ with new functions are being unveiled. In addition, post-translational modification by various cellular signals contributes to fine-tuning of the transcriptional activities of PPARγ. In this review, we will summarize recent advancements in our understanding of the post-translational modifications of, and proteins interacting with, PPARγ, both of which affect its transcriptional activities in relation to adipogenesis.

Stem Cell Factor Receptor/c-Kit: From Basic Science to Clinical Implications

Stem cell factor (SCF) is a dimeric molecule that exerts its biological functions by binding to and activating the receptor tyrosine kinase c-Kit. Activation of c-Kit leads to its autophosphorylation and initiation of signal transduction. Signaling proteins are recruited to activated c-Kit by certain interaction domains (e.g., SH2 and PTB) that specifically bind to phosphorylated tyrosine residues in the intracellular region of c-Kit. Activation of c-Kit signaling has been found to mediate cell survival, migration, and proliferation depending on the cell type. Signaling from c-Kit is crucial for normal hematopoiesis, pigmentation, fertility, gut movement, and some aspects of the nervous system. Deregulated c-Kit kinase activity has been found in a number of pathological conditions, including cancer and allergy. The observation that gain-of-function mutations in c-Kit can promote tumor formation and progression has stimulated the development of therapeutics agents targeting this receptor, e.g., the clinically used inhibitor imatinib mesylate. Also other clinically used multiselective kinase inhibitors, for instance, sorafenib and sunitinib, have c-Kit included in their range of targets. Furthermore, loss-of-function mutations in c-Kit have been observed and shown to give rise to a condition called piebaldism. This review provides a summary of our current knowledge regarding structural and functional aspects of c-Kit signaling both under normal and pathological conditions, as well as advances in the development of low-molecular-weight molecules inhibiting c-Kit function.

Transcriptional regulation of the human tumor suppressor DOK1 by E2F1

The expression of the tumor suppressor DOK1 is repressed in a variety of human tumors as a result of hypermethylation of its promoter region. However, the molecular mechanisms by which DOK1 expression is regulated have been poorly investigated. Here, we show that the expression of DOK1 is regulated mainly by the transcription factor E2F1. We identified three putative E2F1 response elements (EREs) in the DOK1 promoter region. E2F1 had a relatively higher binding affinity for the ERE located between bp -498 and -486 compared with the other two EREs. E2F1 gene silencing strongly inhibited DOK1 expression. E2F1-driven DOK1 transcription occurred in the presence of cellular stresses, such as accumulation of DNA damage induced by etoposide. DOK1 silencing promoted cell proliferation and protected against etoposide-induced apoptosis, indicating that DOK1 acts as a key mediator of cellular stress-induced cell death. Most importantly, we observed that DNA methylation of the DOK1 core promoter region found in head and neck cancer cell lines hampered the recruitment of E2F1 to the DOK1 promoter and compromised DOK1 expression. In summary, our data show that E2F1 is a key factor in DOK1 expression and provide novel insights into the regulation of these events in cancer cells.

The inositol 5-phosphatase SHIP-1 and adaptors Dok-1 and 2 play central roles in CD4-mediated inhibitory signaling

CD4 functions to enhance the sensitivity of T cells to antigenic peptide/MHC class II. However, if aggregated in isolation, e.g. in the absence of T cell receptor (TCR), CD4 can transduce yet undefined signals that lead to T cell unresponsiveness to antigen and apoptosis. In Human Immunodeficiency Virus-1 (HIV-1) disease, CD4(+) T cell loss can result from gp120-induced CD4 signaling in uninfected cells. We show here that CD4 aggregation leads to Lck-dependent phosphorylation of the RasGAP adaptors Downstream of kinase-1/2 (Dok-1/2) and the inositol 5-phosphatase-1 (SHIP-1) and association of the two molecules. Studies using SHIP-1 shRNA, knockout mice and decoy inhibitors further indicate that CD4-mediated inhibition of TCR-mediated T cell activation is SHIP-1 and Dok-1/2 dependent, and involves SHIP-1 hydrolysis of Phosphatidylinositol 3,4,5-trisphosophate (PI(3,4,5)P3) needed for TCR signaling. Our studies provide evidence for a novel mechanism by which ill-timed CD4-mediated signals activated by ligands such as HIV-1 gp120 lead to disarmament of the immune system.

Feedback circuits monitor and adjust basal Lck-dependent events in T cell receptor signaling

The Src family kinase Lck is crucial for the initiation of TCR signaling. The activity of Lck is tightly controlled to prevent erroneous immune activation, yet it enables rapid cellular responses over a range of sensitivities to antigens. Here, in experiments with an analog-sensitive variant of the tyrosine kinase Csk, we report that Lck in T cells is dynamically controlled by an equilibrium between Csk and the tyrosine phosphatase CD45. By rapidly inhibiting Csk, we showed that changes in this equilibrium were sufficient to activate canonical TCR signaling pathways independently of ligand binding to the TCR. The activated signaling pathways showed sustained and enhanced phosphorylation compared to that in TCR-stimulated cells, revealing a feedback circuit that was sensitive to the basal signaling machinery. We identified the inhibitory adaptor molecule Dok-1 (downstream of kinase 1) as a candidate that may respond to alterations in basal signaling activity. Our results also suggest a role for Csk in the termination or dampening of TCR signals.

Inactivation of the putative suppressor gene DOK1 by promoter hypermethylation in primary human cancers

The DOK1 gene is a putative tumour suppressor gene located on the human chromosome 2p13 which is frequently rearranged in leukaemia and other human tumours. We previously reported that the DOK1 gene can be mutated and its expression down-regulated in human malignancies. However, the mechanism underlying DOK1 silencing remains largely unknown. We show here that unscheduled silencing of DOK1 expression through aberrant hypermethylation is a frequent event in a variety of human malignancies. DOK1 was found to be silenced in nine head and neck cancer (HNC) cell lines studied and DOK1 CpG hypermethylation correlated with loss of gene expression in these cells. DOK1 expression could be restored via demethylating treatment using 5-aza-2'deoxycytidine. In addition, transduction of cancer cell lines with DOK1 impaired their proliferation, consistent with the critical role of epigenetic silencing of DOK1 in the development and maintenance of malignant cells. We further observed that DOK1 hypermethylation occurs frequently in a variety of primary human neoplasm including solid tumours (93% in HNC, 81% in lung cancer) and haematopoietic malignancy (64% in Burkitt's lymphoma). Control blood samples and exfoliated mouth epithelial cells from healthy individuals showed a low level of DOK1 methylation, suggesting that DOK1 hypermethylation is a tumour specific event. Finally, an inverse correlation was observed between the level of DOK1 gene methylation and its expression in tumour and adjacent non tumour tissues. Thus, hypermethylation of DOK1 is a potentially critical event in human carcinogenesis, and may be a potential cancer biomarker and an attractive target for epigenetic-based therapy.

An RNAi-based system for loss-of-function analysis identifies Raf1 as a crucial mediator of BCR-ABL-driven leukemogenesis

Genetic loss-of-function studies in murine tumor models have been essential in the analysis of downstream mediators of oncogenic transformation. Unfortunately, these studies are frequently limited by the availability of genetically modified mouse strains. Here we describe a versatile method allowing the efficient expression of an oncogene and simultaneous knockdown of targets of interest (TOI) from a single retroviral vector. Both oncogene and TOI-specific miR30-based shRNA are under the control of the strong viral long terminal repeat promoter, resulting in a single shared RNA transcript. Using this vector in a murine syngeneic BM transplantation model for BCR-ABL-induced chronic myeloid leukemia, we find that oncogene expression and target knockdown in primary hematopoietic cells with this vector is efficient both in vitro and in vivo, and demonstrate that Raf1, but not BRAF, modulates BCR-ABL-dependent ERK activation and transformation of hematopoietic cells. This expression system could facilitate genetic loss-of-function studies and allow the rapid validation of potential drug targets in a broad range of oncogene-driven murine tumor models.

Oncogenic tyrosine kinases target Dok-1 for ubiquitin-mediated proteasomal degradation to promote cell transformation

Cellular transformation induced by oncogenic tyrosine kinases is a multistep process involving activation of growth-promoting signaling pathways and inactivation of suppressor molecules. Dok-1 is an adaptor protein that acts as a negative regulator of tyrosine kinase-initiated signaling and opposes oncogenic tyrosine kinase-mediated cell transformation. Findings that its loss facilitates transformation induced by oncogenic tyrosine kinases suggest that Dok-1 inactivation could constitute an intermediate step in oncogenesis driven by these oncoproteins. However, whether Dok-1 is subject to regulation by oncogenic tyrosine kinases remained unknown. In this study, we show that oncogenic tyrosine kinases, including p210(bcr-abl) and oncogenic forms of Src, downregulate Dok-1 by targeting it for degradation through the ubiquitin-proteasome pathway. This process is dependent on the tyrosine kinase activity of the oncoproteins and is mediated primarily by lysine-dependent polyubiquitination of Dok-1. Importantly, restoration of Dok-1 levels strongly suppresses transformation of cells expressing oncogenic tyrosine kinases, and this suppression is more pronounced in the context of a Dok-1 mutant that is largely refractory to oncogenic tyrosine kinase-induced degradation. Our findings suggest that proteasome-mediated downregulation of Dok-1 is a key mechanism by which oncogenic tyrosine kinases overcome the inhibitory effect of Dok-1 on cellular transformation and tumor progression.

Dasatinib reduces FAK phosphorylation increasing the effects of RPI-1 inhibition in a RET/PTC1-expressing cell line

Background

TPC-1 is a papillary thyroid carcinoma (PTC)-derived cell line that spontaneously expresses the oncogene RET/PTC1. TPC-1 treated with the RET/PTC1 inhibitor RPI-1 displayed a cytostatic and reversible inhibition of cell proliferation and a strong activation of focal adhesion kinase (FAK). As dasatinib inhibition of Src results in reduction of FAK activation, we evaluated the effects of TPC-1 treatment with dasatinib in combination with RPI-1.

Results

Dasatinib (100 nM) strongly reduced TPC-1 proliferation and induced marked changes in TPC-1 morphology. Cells appeared smaller and more contracted, with decreased cell spreading, due to the inhibition of phosphorylation of important cytoskeletal proteins (p130^CAS, Crk, and paxillin) by dasatinib. The combination of RPI-1 with dasatinib demonstrated enhanced effects on cell proliferation (more than 80% reduction) and on the phosphotyrosine protein profile. In particular, RPI-1 reduced the phosphorylation of RET, MET, DCDB2, CTND1, and PLCγ, while dasatinib acted on the phosphorylation of EGFR, EPHA2, and DOK1. Moreover, dasatinib completely abrogated the phosphorylation of FAK at all tyrosine sites (Y576, Y577, Y861, Y925) with the exception of the autoactivation site (Y397). Notably, the pharmacological treatments induced an overexpression of integrin β1 (ITB1) that was correlated with a mild enhancement in phosphorylation of ERK1/2 and STAT3, known for their roles in prevention of apoptosis and in increase of proliferation and survival. A reduction in Akt, p38 and JNK1/2 activation was observed.

Conclusions

All data demonstrate that the combination of the two drugs effectively reduced cell proliferation (by more than 80%), significantly decreased Tyr phosphorylation of almost all phosphorylable proteins, and altered the morphology of the cells, supporting high cytostatic effects. Following the combined treatment, cell survival pathways appeared to be mediated by STAT3 and ERK activities resulting from integrin clustering and FAK autophosphorylation. EphA2 may also contribute, at least in part, to integrin and FAK activation. In conclusion, these data implicate ITB1 and EphA2 as promising therapeutic targets in PTC.

The proximal signaling network of the BCR-ABL1 oncogene shows a modular organization

BCR-ABL1 is a fusion tyrosine kinase, which causes multiple types of leukemia. We used an integrated proteomic approach that includes label-free quantitative protein complex and phosphorylation profiling by mass spectrometry to systematically characterize the proximal signaling network of this oncogenic kinase. The proximal BCR-ABL1 signaling network shows a modular and layered organization with an inner core of three leukemia transformation-relevant adaptor protein complexes (Grb2/Gab2/Shc1 complex, CrkI complex and Dok1/Dok2 complex). We introduced an 'interaction directionality' analysis, which annotates static protein networks with information on the directionality of phosphorylation-dependent interactions. In this analysis, the observed network structure was consistent with a step-wise phosphorylation-dependent assembly of the Grb2/Gab2/Shc1 and the Dok1/Dok2 complexes on the BCR-ABL1 core. The CrkI complex demonstrated a different directionality, which supports a candidate assembly on the Nedd9 (Hef1, CasL) scaffold. As adaptor protein family members can compensate for each other in leukemic transformation, we compared members of the Dok and Crk protein families and found both overlapping and differential binding patterns. We identified an additional level of regulation for the CrkII protein via binding to 14-3-3 proteins, which was independent from its inhibitory phosphorylation. We also identified novel components of the inner core complexes, including the kinases Pragmin (Sgk223) and Lrrk1 (Lrrk2 paralog). Pragmin was found as a component of the CrkI complex and is a potential link between BCR-ABL1/CrkI and RhoA signaling. Lrrk1 is an unusual kinase with a GTPase domain. We detected Lrrk1 as a component of the Grb2/Gab2/Shc1 complex and found that it functionally interacts with the regulator of small GTPases Arap1 (Centd2) and possibly participates in the mitogen-activated protein kinase response to cellular stresses. This modular and phosphorylation-driven interaction network provides a framework for the integration of pleiotropic signaling effects of BCR-ABL1 toward leukemic transformation.

The roles of Dok family adapters in immunoreceptor signaling

The mammalian Dok protein family has seven members (Dok-1-Dok-7). The Dok proteins share structural similarities characterized by the NH2-terminal pleckstrin homology and phosphotyrosine-binding domains followed by SH2 target motifs in the COOH-terminal moiety, indicating an adapter function. Indeed, Dok-1 was originally identified as a 62 kDa protein that binds with p120 rasGAP, a potent inhibitor of Ras, upon tyrosine phosphorylation by a variety of protein tyrosine kinases. Among the Dok family, only Dok-1, Dok-2, and Dok-3 are preferentially expressed in hematopoietic/immune cells. Dok-1 and its closest relative Dok-2 act as negative regulators of the Ras-Erk pathway downstream of many immunoreceptor-mediated signaling systems, and it is believed that recruitment of p120 rasGAP by Dok-1 and Dok-2 is critical to their negative regulation. By contrast, Dok-3 does not bind with p120 rasGAP. However, accumulating evidence has demonstrated that Dok-3 is a negative regulator of the activation of JNK and mobilization of Ca2+ in B-cell receptor-mediated signaling, where the interaction of Dok-3 with SHIP-1 and Grb2 appears to be important. Here, we review the physiological roles and underlying mechanisms of Dok family proteins.

Downstream of tyrosine kinase/docking protein 6, as a novel substrate of tropomyosin-related kinase C receptor, is involved in neurotrophin 3-mediated neurite outgrowth in mouse cortex neurons

BACKGROUND

The downstream of tyrosine kinase/docking protein (Dok) adaptor protein family has seven members, Dok1 to Dok7, that act as substrates of multiple receptor tyrosine kinase and non-receptor tyrosine kinase. The tropomyosin-related kinase (Trk) receptor family, which has three members (TrkA, TrkB and TrkC), are receptor tyrosine kinases that play pivotal roles in many stages of nervous system development, such as differentiation, migration, axon and dendrite projection and neuron patterning. Upon related neurotrophin growth factor stimulation, dimerisation and autophosphorylation of Trk receptors can occur, recruiting adaptor proteins to mediate signal transduction.

RESULTS

In this report, by using yeast two-hybrid assays, glutathione S-transferase (GST) precipitation assays and coimmunoprecipitation (Co-IP) experiments, we demonstrate that Dok6 selectively binds to the NPQY motif of TrkC through its phosphotyrosine-binding (PTB) domain in a kinase activity-dependent manner. We further confirmed their interaction by coimmunoprecipitation and colocalisation in E18.5 mouse cortex neurons, which provided more in vivo evidence. Next, we demonstrated that Dok6 is involved in neurite outgrowth in mouse cortex neurons via the RNAi method. Knockdown of Dok6 decreased neurite outgrowth in cortical neurons upon neurotrophin 3 (NT-3) stimulation.

CONCLUSIONS

We conclude that Dok6 interacts with the NPQY motif of the TrkC receptor through its PTB domain in a kinase activity-dependent manner, and works as a novel substrate of the TrkC receptor involved in NT-3-mediated neurite outgrowth in mouse cortex neurons.

Identification of DOK genes as lung tumor suppressors

Genome-wide analyses of human lung adenocarcinoma have identified regions of consistent copy-number gain or loss, but in many cases the oncogenes and tumor suppressors presumed to reside in these loci remain to be determined. Here we identify the downstream of tyrosine kinase (Dok) family members Dok1, Dok2 and Dok3 as lung tumor suppressors. Single, double or triple compound loss of these genes in mice results in lung cancer, with penetrance and latency dependent on the number of lost Dok alleles. Cancer development is preceded by an aberrant expansion and signaling profile of alveolar type II cells and bronchioalveolar stem cells. In human lung adenocarcinoma, we identify DOK2 as a target of copy-number loss and mRNA downregulation and find that DOK2 suppresses lung cancer cell proliferation in vitro and in vivo. Given the genomic localization of DOK2, we propose it as an 8p21.3 haploinsufficient human lung tumor suppressor.

Phosphorylation of ARD1 by IKKbeta contributes to its destabilization and degradation

IkappaB kinase beta (IKKbeta), a major kinase downstream of various proinflammatory signals, mediates multiple cellular functions through phosphorylation and regulation of its substrates. On the basis of protein sequence analysis, we identified arrest-defective protein 1 (ARD1), a protein involved in apoptosis and cell proliferation processes in many human cancer cells, as a new IKKbeta substrate. We provided evidence showing that ARD1 is indeed a bona fide substrate of IKKbeta. IKKbeta physically associated with ARD1 and phosphorylated it at Ser209. Phosphorylation by IKKbeta destabilized ARD1 and induced its proteasome-mediated degradation. Impaired growth suppression was observed in ARD1 phosphorylation-mimic mutant (S209E)-transfected cells as compared with ARD1 non-phosphorylatable mutant (S209A)-transfected cells. Our findings of molecular interactions between ARD1 and IKKbeta may enable further understanding of the upstream regulation mechanisms of ARD1 and of the diverse functions of IKKbeta.

Overexpression ofCyclin D1in Accelerated-Phase Chronic Myeloid Leukemia

Chronic myeloid leukemia (CML) is a bi- or triphasic disease. Molecular markers distinct for the phase evolution would be clinically helpful. For signaling transformation and proliferation activities in CML, Bcr-Abl is the pivotal protein. As downstream signals of Bcr-Abl , RAS, PI3-K, and Stat 5 may lead to cell cycle progression mediated by increased expression of cyclin Ds. We analyzed copy numbers of bcr-abl and cyclin D1 transcripts by reverse transcription (RT) and competitive PCR titration in bone marrow cells of 20 patients with CML, 10 in chronic phase (CP) and the other 10 in accelerated phase (AP). The level of bcr-abl transcripts in the AP was not significantly higher than that in the CP; in contrast, the level of cyclin D1 transcripts in the AP was significantly higher than that in the CP (p <0.001). Cyclin D1 RNA expression in the CP of CML was also found to have clinical relevance to time to AP transformation. The median time to AP transformation for the CP patients with cyclin D1 transcripts of 1.50 x 10(4)/microg RNA was significantly shorter than that for those with cyclin D1 transcripts < 1.50 x 10(4)/microg RNA (15 vs. 67 months, p=0.0354) although confirmation to conduct in a larger patient group is required. These results suggest that the expression level of cyclin D1 RNA in bone marrow cells is predictive of the phase evolution in CML and may be helpful in treatment decision-making.

Advances in targeting IKK and IKK-related kinases for cancer therapy

IkappaB kinases (IKK) and IKK-related kinases play critical roles in regulating the immune response through nuclear factor-kappaB and IFN regulatory factor-dependent signaling transduction cascades. Recently, these kinases have been implicated in the pathogenesis of many human diseases, including cancer. In fact, dysregulation of IKK activities promotes tumor survival, proliferation, migration, metastasis, and angiogenesis-common characteristics of many types of human cancers. Because of their oncogenic effects in human cancer development, targeting IKK and IKK-related kinases is becoming an increasingly popular avenue for the development of novel therapeutic interventions for cancer. This review will briefly cover the recent discovery of the downstream substrates of IKK and IKK-related kinases, and outline the strategies used for targeting IKK as a therapeutic intervention for cancer.

p21 Ras/impedes mitogenic signal propagation regulates cytokine production and migration in CD4 T cells

The propensity of T cells to generate coordinated cytokine responses is critical for the host to develop resistance to pathogens while maintaining the state of immunotolerance to self-antigens. The exact mechanisms responsible for preventing the overproduction of proinflammatory cytokines including interferon (IFN)-gamma are not fully understood, however. In this study, we examined the role of a recently described Ras GTPase effector and repressor of the Raf/MEK/ERK cascade called impedes mitogenic signal propagation (Imp) in limiting the induction of T-cell cytokines. We found that stimulation of the T cell receptor complex leads to the rapid development of a physical association between Ras and Imp. Consistent with the hypothesis that Imp inhibits signal transduction, we also found that disengagement of this molecule by the Ras(V12G37) effector loop mutant or RNA interference markedly enhances the activation of the NFAT transcription factor and IFN-gamma secretion. A strong output of IFN-gamma is responsible for the distinct lymphocyte traffic pattern observed in vivo because the transgenic or retroviral expression of Ras(V12G37) caused T cells to accumulate preferentially in the lymph nodes and delayed their escape from the lymphoid tissue, respectively. Together, our results describe a hitherto unrecognized negative regulatory role for Imp in the production of IFN-gamma in T cells and point to Ras-Imp binding as an attractive target for therapeutic interventions in conditions involving the production of this inflammatory cytokine.

PML targeting eradicates quiescent leukaemia-initiating cells

The existence of a small population of 'cancer-initiating cells' responsible for tumour maintenance has been firmly demonstrated in leukaemia. This concept is currently being tested in solid tumours. Leukaemia-initiating cells, particularly those that are in a quiescent state, are thought to be resistant to chemotherapy and targeted therapies, resulting in disease relapse. Chronic myeloid leukaemia is a paradigmatic haematopoietic stem cell disease in which the leukaemia-initiating-cell pool is not eradicated by current therapy, leading to disease relapse on drug discontinuation. Here we define the critical role of the promyelocytic leukaemia protein (PML) tumour suppressor in haematopoietic stem cell maintenance, and present a new therapeutic approach for targeting quiescent leukaemia-initiating cells and possibly cancer-initiating cells by pharmacological inhibition of PML.

MEK inhibitor enhances the inhibitory effect of imatinib on pancreatic cancer cell growth

Imatinib mesylate (imatinib) inhibits the c-Kit-dependent tyrosine kinase activities and highly effective in the treatment of CML and GIST patients. Although pancreatic cancer is reported to express c-Kit, imatinib does not effectively inhibit pancreatic cancer cell growth at physiological concentrations. Therefore, we investigated the mechanism of resistance of pancreatic cancer to imatinib treatment. Imatinib inhibited growth of pancreatic cancer cell lines in concentration and time-dependent fashion regardless of c-Kit expression. However, 5 microM imatinib, which is almost a mean maximal plasma concentration in clinical setting, failed to suppress pancreatic cancer cell growth. Western blot analysis demonstrated that 5 microM imatinib treatment for 1h activated the MEK-MAPK pathway and the activation was independent of Ras activation. Administration of 5 microM imatinib and 1 microM U0126 (MEK inhibitor) significantly suppressed pancreatic cell growth. Our results indicate that a combination therapy of imatinib and MEK inhibitor can be a new therapeutic strategy to suppress the progression of pancreatic cancer.

Dok1 mediates high-fat diet-induced adipocyte hypertrophy and obesity through modulation of PPAR-gamma phosphorylation

Insulin receptor substrate (IRS)-1 and IRS-2 have dominant roles in the action of insulin, but other substrates of the insulin receptor kinase, such as Gab1, c-Cbl, SH2-B and APS, are also of physiological relevance. Although the protein downstream of tyrosine kinases-1 (Dok1) is known to function as a multisite adapter molecule in insulin signaling, its role in energy homeostasis has remained unclear. Here we show that Dok1 regulates adiposity. Expression of Dok1 in white adipose tissue was markedly increased in mice fed a high-fat diet, whereas adipocytes lacking this adapter were smaller and showed a reduced hypertrophic response to this dietary manipulation. Dok1-deficient mice were leaner and showed improved glucose tolerance and insulin sensitivity compared with wild-type mice. Embryonic fibroblasts from Dok1-deficient mice were impaired in adipogenic differentiation, and this defect was accompanied by an increased activity of the protein kinase ERK and a consequent increase in the phosphorylation of peroxisome proliferator-activated receptor (PPAR)-gamma on Ser112. Mutation of this negative regulatory site for the transactivation activity of PPAR-gamma blocked development of the lean phenotype caused by Dok1 ablation. These results indicate that Dok1 promotes adipocyte hypertrophy by counteracting the inhibitory effect of ERK on PPAR-gamma and may thus confer predisposition to diet-induced obesity.

Anti-inflammatory functions of glucocorticoid-induced genes

There is a broad consensus that glucocorticoids (GCs) exert anti-inflammatory effects largely by inhibiting the function of nuclear factor kappaB (NFkappaB) and consequently the transcription of pro-inflammatory genes. In contrast, side effects are thought to be largely dependent on GC-induced gene expression. Biochemical and genetic evidence suggests that the positive and negative effects of GCs on transcription can be uncoupled from one another. Hence, novel GC-related drugs that mediate inhibition of NFkappaB but do not activate gene expression are predicted to retain therapeutic effects but cause fewer or less severe side effects. Here, we critically re-examine the evidence in favor of the consensus, binary model of GC action and discuss conflicting evidence, which suggests that anti-inflammatory actions of GCs depend on the induction of anti-inflammatory mediators. We propose an alternative model, in which GCs exert anti-inflammatory effects at both transcriptional and post-transcriptional levels, both by activating and inhibiting expression of target genes. The implications of such a model in the search for safer anti-inflammatory drugs are discussed.

Dok-3 plays a nonredundant role in negative regulation of B-cell activation

p62(dok) and Dok-3 are members of the Dok family of adaptors found in B cells, with the former cloned as a substrate of the p210(bcr/abl) oncoprotein in Ph + chronic myelogenous leukemia. A role for p62(dok) in FcgammaRIIB-mediated negative regulation of B-cell proliferation had been established previously. Here, we generated Dok-3(-/-) mice to assess the function of Dok-3 in B cells. Mice lacking Dok-3 have normal B-cell development but possess higher level of IgM antibodies in their sera. In comparison to wild-type mice, Dok-3(-/-) mice mounted significantly enhanced humoral immune responses to T cell-independent type I and II antigens. Dok-3-deficient B cells hyperproliferated, exhibited elevated level of calcium signaling as well as enhanced activation of NF-kappaB, JNK, and p38MAPK in response to B-cell receptor (BCR) engagement. In the absence of Dok-3, the localization of the inhibitory phosphatase SHIP-1 to the plasma membrane is intact while its phosphorylation is compromised, suggesting that Dok-3 could function to facilitate or sustain the activation of SHIP-1. The phenotype and responses of Dok-3(-/-) mice and B cells could be differentiated from those of the Dok-1(-/-) counterparts. Hence, we propose that Dok-3 plays a distinct and nonredundant role in the negative regulation of BCR signaling.

Identification of Dok-4b, a Dok-4 splice variant with enhanced inhibitory properties

Dok adapter proteins have been primarily implicated in negative regulation of tyrosine kinase signaling, but Dok-4 has been reported to exert both inhibitory and stimulatory effects. We have identified a splice variant of Dok-4, Dok-4b, which contains a 39 aa insert within the its C-terminal region. The approximately 45kDa Dok-4b protein was detected in several human epithelial cell lines. Based on genomic sequences, Dok-4b was also predicted to exist in primates and possibly bovines, but not in rodents or other species. Compared to Dok-4, Dok-4b inhibited the tyrosine kinase-induced activation of both Erk and Elk-1 more strongly. Truncation of the C-terminal region of Dok-4 (Dok-4 DeltaCT) also enhanced the inhibitory activity of Dok-4, whereas expression of the isolated C-terminal domain enhanced Elk-1 activation, suggesting that the N-terminus and C-terminus of Dok-4 possess opposing inhibitory and stimulatory properties, respectively, the balance of which is altered by alternative splicing of Dok-4 to Dok-b.

A chemical approach to the identification of tensin-binding proteins

Many protein-protein interactions are mediated by small modular domains, which recognize short peptide motifs in their partner proteins. However, for the great majority of these domains, the identity of their partner proteins remains unknown. In this work, a chemical/bioinformatics approach has been developed to identify phosphotyrosyl (pY) proteins that bind to tensin, a protein involved in the formation of actin cytoskeleton and signal transduction. A pY peptide library was chemically synthesized and screened against the Src homology 2 (SH2) domain of tensin to identify the peptide motifs that bind to the SH2 domain. Next, protein databases were searched for proteins containing the SH2 domain-binding peptide motifs. Finally, the potential tensin-binding proteins were confirmed (or disproved) by in vitro pull-down and coimmunoprecipitation assays. This procedure identified phosphoinositide-dependent kinase-1 and downstream of tyrosine kinase 2 as novel tensin-binding proteins. In addition, a cell-permeable pY peptide was designed as tensin SH2 domain inhibitor, which caused the disruption of actin filaments in NIH 3T3 cells. This method should be generally applicable to other modular domains that recognize small peptide motifs.

Differential regulation of adapter proteins Dok2 and Dok1 in platelets, leading to an association of Dok2 with integrin alphaIIbbeta3

BACKGROUND

We previously demonstrated that Dok2 is rapidly phosphorylated on tyrosine residues in platelets in response to thrombin, the immunoreceptor tyrosine-based activation motif-coupled collagen receptor glycoprotein (GP) VI, and by integrin alphaIIbbeta3.

OBJECTIVES AND METHODS

In this study we further delineate the regulation of phosphorylation of Dok2 and compare this to the related adapter Dok1.

RESULTS

We demonstrate expression of Dok1 in platelets and the unexpected observation that the adapter protein undergoes tyrosine phosphorylation in response to thrombin but not to GPVI or integrin alphaIIbbeta3. Furthermore, Dok1 phosphorylation is transient, peaking at 30 s and returning to basal by 5 min, whereas Dok2 phosphorylation is delayed but sustained. Dok2 phosphorylation, but not that of Dok1, is inhibited by Src kinase inhibitors and by chelation of intracellular calcium. Further, phosphorylation of Dok2 by thrombin and integrin alphaIIbbeta3 in mouse platelets is independent of Syk and phospholipase Cgamma2. Additionally, Dok2 coimmunoprecipitates with integrin alphaIIbbeta3 downstream of Src kinases.

CONCLUSIONS

These results demonstrate differential modes of regulation of Dok1 and Dok2 in platelets. Further, they raise the interesting possibility that Dok2 plays an important role in integrin outside-in signaling through a physical and functional interaction with integrin alphaIIbbeta3.

T cell receptor for antigen induces linker for activation of T cell-dependent activation of a negative signaling complex involving Dok-2, SHIP-1, and Grb-2

Adaptor proteins positively or negatively regulate the T cell receptor for antigen (TCR) signaling cascade. We report that after TCR stimulation, the inhibitory adaptor downstream of kinase (Dok)-2 and its homologue Dok-1 are involved in a multimolecular complex including the lipid phosphatase Src homology 2 domain–containing inositol polyphosphate 5′-phosphatase (SHIP)-1 and Grb-2 which interacts with the membrane signaling scaffold linker for activation of T cells (LAT). Knockdown of LAT and SHIP-1 expression indicated that SHIP-1 favored recruitment of Dok-2 to LAT. Knockdown of Dok-2 and Dok-1 revealed their negative control on Akt and, unexpectedly, on Zap-70 activation. Our findings support the view that Dok-1 and -2 are critical elements of a LAT-dependent negative feedback loop that attenuates early TCR signal. Dok-1 and -2 may therefore exert a critical role in shaping the immune response and as gatekeepers for T cell tolerance.

All-trans retinoic acid induces p62DOK1 and p56DOK2 expression which enhances induced differentiation and G0 arrest of HL-60 leukemia cells

p62(DOK1) (DOK1) and p56(DOK2) (DOK2) are sequence homologs that act as docking proteins downstream of receptor or nonreceptor tyrosine kinases. Originally identified in chronic myelogenous leukemia cells as a highly phosphorylated substrate for the chimeric p210(bcr-abl) protein, DOK1 was suspected to play a role in leukemogenesis. However, p62(DOK1-/-) fibroblast knockout cells were found to have enhanced MAPK signaling and proliferation due to growth factors, suggesting negative regulatory capabilities for DOK1. The role of DOK1 and DOK2 in leukemogeneis thus is enigmatic. The data in this report show that both the DOK1 and the DOK2 adaptor proteins are constitutively expressed in the myelomonoblastic leukemia cell line, HL-60, and that expression of both proteins is induced by the chemotherapeutic differentiation causing agents, all-trans retinoic acid (atRA) and 1,25-dihydroxyvitamin D3 (VD3). Ectopic expression of either protein enhances atRA- or VD3-induced growth arrest, differentiation, and G(0)/G(1) cell cycle arrest and results in increased ERK1/2 phosphorylation. DOK1 and DOK2 are similarly effective in these capabilities. The data provide evidence that DOK1 and DOK2 proteins have a similar role in regulating cell proliferation and differentiation and are positive regulators of the MAPK signaling pathway in this context.

A nuclear export signal and phosphorylation regulate Dok1 subcellular localization and functions

Dok1 is believed to be a mainly cytoplasmic adaptor protein which down-regulates mitogen-activated protein kinase activation, inhibits cell proliferation and transformation, and promotes cell spreading and cell migration. Here we show that Dok1 shuttles between the nucleus and cytoplasm. Treatment of cells with leptomycin B (LMB), a specific inhibitor of the nuclear export signal (NES)-dependent receptor CRM1, causes nuclear accumulation of Dok1. We have identified a functional NES (348LLKAKLTDPKED359) that plays a major role in the cytoplasmic localization of Dok1. Src-induced tyrosine phosphorylation prevented the LMB-mediated nuclear accumulation of Dok1. Dok1 cytoplasmic localization is also dependent on IKKbeta. Serum starvation or maintaining cells in suspension favor Dok1 nuclear localization, while serum stimulation, exposure to growth factor, or cell adhesion to a substrate induce cytoplasmic localization. Functionally, nuclear NES-mutant Dok1 had impaired ability to inhibit cell proliferation and to promote cell spreading and cell motility. Taken together, our results provide the first evidence that Dok1 transits through the nucleus and is actively exported into the cytoplasm by the CRM1 nuclear export system. Nuclear export modulated by external stimuli and phosphorylation may be a mechanism by which Dok1 is maintained in the cytoplasm and membrane, thus regulating its signaling functions.

Distinctive role of the cKit receptor tyrosine kinase signaling in mammalian melanocytes

The cKit receptor plays a critical role in melanocyte physiology, influencing melanogenesis, proliferation, migration, and survival of the pigment-producing cells. However, pathways of cKit-mediated intracellular signaling and molecular mechanisms, which regulate specific cellular responses to the activation of the receptor in melanocytes, remain incompletely understood. Here, by using the genetically altered mouse melanocytes expressing an endogenous, constitutively active mutant (D814Y) cKit receptor, we investigated physiological cellular responses to the ligand-independent activation of the receptor tyrosine kinase. It was anticipated that such activation would either trigger uncontrolled proliferation of the melanocytes or stimulate melanin biosynthesis. In contrast to the expectation, we found that constitutive signaling from the cKit receptor did not stimulate melanogenesis and proliferation, but significantly promoted migration of the melanocytes both in vitro and in vivo. We also showed that such signaling is not associated with tumorigenic transformation of the pigment-producing cells. Taken together, our observations suggest that, in mammalian melanocytes, activation of the cKit receptor tyrosine kinase is primarily responsible for transmission of pro-migration signals, which may antagonize proliferation and melanogenesis. Our data also provide an additional explanation as to why malignant melanocytes lose cKit expression during melanoma progression.

Dok5 is substrate of TrkB and TrkC receptors and involved in neurotrophin induced MAPK activation

Tropomyosin-related kinase (Trk) family receptors are a group of high affinity receptors for neurotrophin growth factors, which have pivotal functions in many physiological processes of nervous system. Trk receptors can dimerize and autophosphorylate upon neurotrophin stimulation, then recruit multiple adaptor proteins to transduct signal. In this report, we identified Dok5, a member of Dok family, as a new substrate of TrkB/C receptors. In yeast two-hybrid assay, Dok5 can interact with intracellular domain of TrkB and TrkC receptor through its PTB domain, but not with that of TrkA receptor. The interaction was then confirmed by GST pull-down assay and Co-IP experiment. Dok5 co-localized with TrkB and TrkC in differentiated PC12 cells, providing another evidence for their interaction. By using mutational analysis, we characterized that Dok5 PTB domain bound to Trk receptor NPQY motif in a kinase-activity-dependent manner. Furthermore, competition experiment indicated that Dok5 competed with N-shc for binding to the receptors at the same site. Finally, we showed that Dok5 was involved in the activation of MAPK pathway induced by neurotrophin stimulation. Taken together, these results suggest that Dok5 acts as substrate of TrkB/C receptors and is involved in neurotrophin induced MAPK signal pathway activation.

Dok-1 independently attenuates Ras/mitogen-activated protein kinase and Src/c-myc pathways to inhibit platelet-derived growth factor-induced mitogenesis

The Dok adaptor proteins play key regulatory roles in receptor and non-receptor kinase-initiated signaling pathways. Dok-1, the prototype member of this family, negatively regulates cell proliferation elicited by numerous growth factors, including platelet-derived growth factor (PDGF). However, how Dok-1 exerts its negative effect on mitogenesis has remained elusive. Using Dok-1 knockout cells and Dok-1 mutants deficient in binding to specific Dok-1-interacting proteins, we show that Dok-1 interferes with PDGF-stimulated c-myc induction and Ras/mitogen-activated protein kinase (MAPK) activation by tethering different signaling components to the cell membrane. Specifically, Dok-1 attenuates PDGF-elicited c-myc induction by recruiting Csk to active Src kinases, whereupon their activities and consequent c-myc induction are diminished. On the other hand, Dok-1 negatively regulates PDGF-induced MAPK activation by acting on Ras-GAP and at least one other Dok-1-interacting protein. Importantly, we demonstrate that Dok-1's actions on both of these signaling pathways contribute to its inhibitory effect on mitogenesis. Our data suggest a mechanistic basis for the inhibitory effect of Dok-1 on growth factor-induced mitogenesis and its role as a tumor suppressor.