Tyrosine phosphorylation of p62(Dok) induced by cell adhesion and insulin: possible role in cell migration

Dok-1 regulates mast cell degranulation negatively through inhibiting calcium-dependent F-actin disassembly

In food allergies, antigen-induced aggregation of FcεRI on mast cells initiates highly ordered and sequential signaling events. Dok-1(downstream of tyrosine kinase 1), undergoes intense tyrosine phosphorylation upon FcεRI stimulation, which negatively regulates Ras/Erk signaling and the subsequent cytokine release, but it remains unclear whether Dok-1 regulates Fc-mediated degranulation. In this study, we investigated the role of Dok-1 in FcεRI-mediated degranulation. Dok-1 overexpressing RBL-2H3 cells were established. Degranulation, immunoprecipitation, co-immunoprecipitation, immunoblotting and flow cytometry assay were performed to explore the effects of Dok-1 and its underlying mechanisms. We found that, following FcεRI activation, Dok-1 was recruited to the plasma membrane, leading to tyrosine phosphorylation. Phosphorylated Dok-1 inhibits FcεRI-operated calcium influx, and negatively regulated degranulation by inhibiting calcium-dependent disassembly of actin filaments. Our data revealed that Dok-1 is a negative regulator of FcεRI-mediated mast cell degranulation. These findings contribute to the identification of therapeutic targets for food allergies.

Structure, Function, and Regulation of the SRMS Tyrosine Kinase

Src-related kinase lacking C-terminal regulatory tyrosine and N-terminal myristoylation sites (SRMS) is a tyrosine kinase that was discovered in 1994. It is a member of a family of nonreceptor tyrosine kinases that also includes Brk (PTK6) and Frk. Compared with other tyrosine kinases, there is relatively little information about the structure, function, and regulation of SRMS. In this review, we summarize the current state of knowledge regarding SRMS, including recent results aimed at identifying downstream signaling partners. We also present a structural model for the enzyme and discuss the potential involvement of SRMS in cancer cell signaling.

Nck adapter proteins promote podosome biogenesis facilitating extracellular matrix degradation and cancer invasion

Background

Podosomes are membrane‐bound adhesive structures formed by actin remodeling. They are capable of extracellular matrix (ECM) degradation, which is a prerequisite for cancer cell invasion and metastasis. The signaling mechanism of podosome formation is still unknown in cancer. We previously reported that Nck adaptors regulate directional cell migration and endothelial lumen formation by actin remodeling, while deficiency of Nck reduces cancer metastasis. This study evaluated the role of Nck adaptors in podosome biogenesis and cancer invasion.

Methods

This study was conducted in vitro using both healthy cells (Human Umbilical Vein Endothelial Cell, 3T3 fibroblasts) and cancer cells (prostate cancer cell line; PC3, breast cancer cell line; MDA‐MB‐231). Confocal and TIRF imaging of cells expressing Green Fluorescence Protein (GFP)  mutant under altered levels of Nck or downstream of kinase 1 (Dok1) was used to evaluate the podosome formation and fluorescent gelatin matrix degradation. Levels of Nck in human breast carcinoma tissue sections were detected by immune histochemistry using Nck polyclonal antibody. Biochemical interaction of Nck/Dok1 was detected in podosome forming cells using immune precipitation and far‐western blotting.

Results

This study demonstrates that ectopic expression of Nck1 and Nck2 can induce the endothelial podosome formation in vitro. Nck silencing by short‐hairpin RNA blocked podosome biogenesis and ECM degradation in cSrc‐Y530F transformed endothelial cells in this study. Immunohistochemical analysis revealed the Nck overexpression in human breast carcinoma tissue sections. Immunoprecipitation and far‐western blotting revealed the biochemical interaction of Nck/p62Dok in podosome forming cells.

Conclusions

Nck adaptors in interaction with Dok1 induce podosome biogenesis and ECM degradation facilitating cancer cell invasion, and therefore a bona fide target of cancer therapy.

A crucial role for DOK1 in PDGF-BB-stimulated glioma cell invasion through p130Cas and Rap1 signalling

DOK1 regulates platelet-derived growth factor (PDGF)-BB-stimulated glioma cell motility. Mechanisms regulating tumour cell motility are essential for invasion and metastasis. We report here that PDGF-BB-mediated glioma cell invasion and migration are dependent on the adaptor protein downstream of kinase 1 (DOK1). DOK1 is expressed in several glioma cell lines and in tumour biopsies from high-grade gliomas. DOK1 becomes tyrosine phosphorylated upon PDGF-BB stimulation of human glioma cells. Knockdown of DOK1 or expression of a DOK1 mutant (DOK1FF) containing Phe in place of Tyr at residues 362 and 398, resulted in inhibition of both the PDGF-BB-induced tyrosine phosphorylation of p130Cas (also known as BCAR1) and the activation of Rap1. DOK1 colocalises with tyrosine phosphorylated p130Cas at the cell membrane of PDGF-BB-treated cells. Expression of a non-tyrosine-phosphorylatable substrate domain mutant of p130Cas (p130Cas15F) inhibited PDGF-BB-mediated Rap1 activation. Knockdown of DOK1 and Rap1 inhibited PDGF-BB-induced chemotactic cell migration, and knockdown of DOK1 and Rap1 and expression of DOK1FF inhibited PDGF-mediated three-dimensional (3D) spheroid invasion. These data show a crucial role for DOK1 in the regulation of PDGF-BB-mediated tumour cell motility through a p130Cas-Rap1 signalling pathway. [Corrected]

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.

Functional dissociation between PIKfyve-synthesized PtdIns5P and PtdIns(3,5)P2 by means of the PIKfyve inhibitor YM201636

PIKfyve is an essential mammalian lipid kinase with pleiotropic cellular functions whose genetic knockout in mice leads to preimplantation lethality. Despite several reports for PIKfyve-catalyzed synthesis of phosphatidylinositol 5-phosphate (PtdIns5P) along with phosphatidylinositol-3,5-biphosphate [PtdIns(3,5)P(2)] in vitro and in vivo, the role of the PIKfyve pathway in intracellular PtdIns5P production remains underappreciated and the function of the PIKfyve-synthesized PtdIns5P pool poorly characterized. Hence, the recently discovered potent PIKfyve-selective inhibitor, the YM201636 compound, has been solely tested for inhibiting PtdIns(3,5)P(2) synthesis. Here, we have compared the in vitro and in vivo inhibitory potency of YM201636 toward PtdIns5P and PtdIns(3,5)P(2). Unexpectedly, we observed that at low doses (10-25 nM), YM201636 inhibited preferentially PtdIns5P rather than PtdIns(3,5)P(2) production in vitro, whereas at higher doses, the two products were similarly inhibited. In cellular contexts, YM201636 at 160 nM inhibited PtdIns5P synthesis twice more effectively compared with PtdIns(3,5)P(2) synthesis. In 3T3L1 adipocytes, human embryonic kidney 293 and Chinese hamster ovary (CHO-T) cells, levels of PtdIns5P dropped by 62-71% of the corresponding untreated controls, whereas those of PtdIns(3,5)P(2) fell by only 28-46%. The preferential inhibition of PtdIns5P versus PtdIns(3,5)P(2) at low doses of YM201636 was explored to probe contributions of the PIKfyve-catalyzed PtdIns5P pool to insulin-induced actin stress fiber disassembly in CHO-T cells, GLUT4 translocation in 3T3L1 adipocytes, and induction of aberrant cellular vacuolation in these or other cell types. The results provide the first experimental evidence that the principal pathway for PtdIns5P intracellular production is through PIKfyve and that insulin effect on actin stress fiber disassembly is mediated entirely by the PIKfyve-produced PtdIns5P pool.

Molecular basis of signaling specificity of insulin and IGF receptors: neglected corners and recent advances

Insulin and insulin-like growth factor (IGF) receptors utilize common phosphoinositide 3-kinase/Akt and Ras/extracellular signal-regulated kinase signaling pathways to mediate a broad spectrum of "metabolic" and "mitogenic" responses. Specificity of insulin and IGF action in vivo must in part reflect expression of receptors and responsive pathways in different tissues but it is widely assumed that it is also determined by the ligand binding and signaling mechanisms of the receptors. This review focuses on receptor-proximal events in insulin/IGF signaling and examines their contribution to specificity of downstream responses. Insulin and IGF receptors may differ subtly in the efficiency with which they recruit their major substrates (IRS-1 and IRS-2 and Shc) and this could influence effectiveness of signaling to "metabolic" and "mitogenic" responses. Other substrates (Grb2-associated binder, downstream of kinases, SH2Bs, Crk), scaffolds (RACK1, β-arrestins, cytohesins), and pathways (non-receptor tyrosine kinases, phosphoinositide kinases, reactive oxygen species) have been less widely studied. Some of these components appear to be specifically involved in "metabolic" or "mitogenic" signaling but it has not been shown that this reflects receptor-preferential interaction. Very few receptor-specific interactions have been characterized, and their roles in signaling are unclear. Signaling specificity might also be imparted by differences in intracellular trafficking or feedback regulation of receptors, but few studies have directly addressed this possibility. Although published data are not wholly conclusive, no evidence has yet emerged for signaling mechanisms that are specifically engaged by insulin receptors but not IGF receptors or vice versa, and there is only limited evidence for differential activation of signaling mechanisms that are common to both receptors. Cellular context, rather than intrinsic receptor activity, therefore appears to be the major determinant of whether responses to insulin and IGFs are perceived as "metabolic" or "mitogenic."

The SH2 domain-containing proteins in 21 species establish the provenance and scope of phosphotyrosine signaling in eukaryotes

The Src homology 2 (SH2) domains are participants in metazoan signal transduction, acting as primary mediators for regulated protein-protein interactions with tyrosine-phosphorylated substrates. Here, we describe the origin and evolution of SH2 domain proteins by means of sequence analysis from 21 eukaryotic organisms from the basal unicellular eukaryotes, where SH2 domains first appeared, through the multicellular animals and increasingly complex metazoans. On the basis of our results, SH2 domains and phosphotyrosine signaling emerged in the early Unikonta, and the numbers of SH2 domains expanded in the choanoflagellate and metazoan lineages with the development of tyrosine kinases, leading to rapid elaboration of phosphotyrosine signaling in early multicellular animals. Our results also indicated that SH2 domains coevolved and the number of the domains expanded alongside protein tyrosine kinases and tyrosine phosphatases, thereby coupling phosphotyrosine signaling to downstream signaling networks. Gene duplication combined with domain gain or loss produced novel SH2-containing proteins that function within phosphotyrosine signaling, which likely have contributed to diversity and complexity in metazoans. We found that intra- and intermolecular interactions within and between SH2 domain proteins increased in prevalence along with organismal complexity and may function to generate more highly connected and robust phosphotyrosine signaling networks.

Characterization of DOK1, a candidate tumor suppressor gene, in epithelial ovarian cancer

In attempt to discover novel aberrantly hypermethylated genes with putative tumor suppressor function in epithelial ovarian cancer (EOC), we applied expression profiling following pharmacologic inhibition of DNA methylation in EOC cell lines. Among the genes identified, one of particular interest was DOK1, or downstream of tyrosine kinase 1, previously recognized as a candidate tumor suppressor gene (TSG) for leukemia and other human malignancies. Using bisulfite sequencing, we determined that a 5'-non-coding DNA region (located at nt -1158 to -850, upstream of the DOK1 translation start codon) was extensively hypermethylated in primary serous EOC tumors compared with normal ovarian specimens; however, this hypermethylation was not associated with DOK1 suppression. On the contrary, DOK1 was found to be strongly overexpressed in serous EOC tumors as compared to normal tissue and importantly, DOK1 overexpression significantly correlated with improved progression-free survival (PFS) values of serous EOC patients. Ectopic modulation of DOK1 expression in EOC cells and consecutive functional analyses pointed toward association of DOK1 expression with increased EOC cell migration and proliferation, and better sensitivity to cisplatin treatment. Gene expression profiling and consecutive network and pathway analyses were also confirmative for DOK1 association with EOC cell migration and proliferation. These analyses were also indicative for DOK1 protective role in EOC tumorigenesis, linked to DOK1-mediated induction of some tumor suppressor factors and its suppression of pro-metastasis genes. Taken together, our findings are suggestive for a possible tumor suppressor role of DOK1 in EOC; however its implication in enhanced EOC cell migration and proliferation restrain us to conclude that DOK1 represents a true TSG in EOC. Further studies are needed to more completely elucidate the functional implications of DOK1 and other members of the DOK gene family in ovarian tumorigenesis.

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.

Downstream of tyrosine kinase 1 and 2 play opposing roles in CD200 receptor signaling

The CD200 receptor (CD200R) negatively regulates myeloid cells by interacting with its widely expressed ligand CD200. CD200R signals through a unique inhibitory pathway involving a direct interaction with the adaptor protein downstream of tyrosine kinase 2 (Dok2) and the subsequent recruitment and activation of Ras GTPase-activating protein (RasGAP). Ligand engagement of CD200R also results in tyrosine phosphorylation of Dok1, but this protein is not essential for inhibitory CD200R signaling in human myeloid cells. In this paper, we show that CD200R-induced phosphorylation of Dok2 precedes phosphorylation of Dok1, and that Dok2 and Dok1 recruit different downstream proteins. Compared with Dok2, Dok1 recruits substantially less RasGAP. In addition to binding RasGAP, Dok2 recruits the adaptor molecule Nck in response to ligand engagement of CD200R. CD200R-induced phosphorylation of Dok1 results in the recruitment of CT10 sarcoma oncogene cellular homologue-like (CrkL), whereas the closely related CT10 sarcoma oncogene cellular homologue interacts constitutively with Dok1. Knockdown of Dok1 or CrkL expression in U937 cells resulted in increased Dok2 phosphorylation and RasGAP recruitment to Dok2. These data are consistent with a model in which Dok1 negatively regulates Dok2-mediated CD200R signaling through the recruitment of CrkL.

Docking proteins

Docking proteins comprise a distinct category of intracellular, noncatalytic signalling protein, that function downstream of a variety of receptor and receptor-associated tyrosine kinases and regulate diverse physiological and pathological processes. The growth factor receptor bound 2-associated binder/Daughter of Sevenless, insulin receptor substrate, fibroblast growth factor receptor substrate 2 and downstream of tyrosine kinases protein families fall into this category. This minireview focuses on the structure, function and regulation of these proteins.

Adapters in the organization of mast cell signaling

Mast cells are pivotal in innate immunity and play an important role in amplifying adaptive immunity. Nonetheless, they have long been known to be central to the initiation of allergic disorders. This results from the dysregulation of the immune response whereby normally innocuous substances are recognized as non-self, resulting in the production of IgE antibodies to these 'allergens'. Preformed and newly synthesized inflammatory (allergic) mediators are released from the mast cell following allergen-mediated aggregation of allergen-specific IgE bound to the high-affinity receptors for IgE (FcepsilonRI). Thus, the process by which the mast cell is able to interpret the engagement of FcepsilonRI into the molecular events necessary for release of their allergic mediators is of considerable therapeutic interest. Unraveling these molecular events has led to the discovery of a functional class of proteins that are essential in organizing activated signaling molecules and in coordinating and compartmentalizing their activity. These so-called 'adapters' bind multiple signaling proteins and localize them to specific cellular compartments, such as the plasma membrane. This organization is essential for normal mast cell responses. Here, we summarize the role of adapter proteins in mast cells focusing on the most recent advances toward understanding how these molecules work upon FcepsilonRI engagement.

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.

Analysis of DOK-6 function in downstream signaling of RET in human neuroblastoma cells

Point mutations and structural alterations of the RET tyrosine kinase gene cause multiple endocrine neoplasia type 2 (MEN 2) and papillary thyroid carcinoma, respectively. RET activation by glial cell line-derived neurotrophic factor (GDNF) is essential for the development of the enteric nervous system and the kidney. The signal through RET tyrosine kinase requires several adaptor proteins including the DOK (downstream of kinase) family of proteins. Of the seven members of the DOK protein family, DOK-1, -4, -5, and -6 have been reported to play roles in the GDNF-RET signaling pathway. Although DOK-6 has been shown to bind to RET and promote GDNF-induced neurite outgrowth in mouse Neuro2A cells, DOK-6 function in human cells remains unclear. In the present study, we investigated the role of DOK-6 in GDNF-RET signaling in human cells including neuroblastoma cells. DOK-6 was constitutively localized to the plasma membrane via its pleckstrin homology (PH) domain, and was phosphorylated following RET activation via a MEN2A mutation or GDNF stimulation. However, DOK-6 could not significantly affect downstream signaling and neurite outgrowth in human neuroblastoma cells. The binding affinity of the DOK-6 phosphotyrosine-binding (PTB) domain to RET was much lower than that of the DOK-1, DOK-4, and SHC PTB domains to RET. These findings indicate that DOK-6 is involved in RET signaling with less influence when compared with DOK-1, DOK-4, and SHC.

The Irs1 branch of the insulin signaling cascade plays a dominant role in hepatic nutrient homeostasis

We used a Cre-loxP approach to generate mice with varied expression of hepatic Irs1 and Irs2 to establish the contribution of each protein to hepatic nutrient homeostasis. While nutrient-sensitive transcripts were expressed nearly normally in liver lacking Irs2 (LKO2 mice), these transcripts were significantly dysregulated in liver lacking Irs1 (LKO1 mice) or Irs1 and Irs2 together (DKO mice). Similarly, a set of key gluconeogenic and lipogenic genes was regulated nearly normally by feeding in liver retaining a single Irs1 allele without Irs2 (DKO/1 mice) but was poorly regulated in liver retaining one Irs2 allele without Irs1 (DKO/2 mice). DKO/2 mice, but not DKO/1 mice, also showed impaired glucose tolerance and insulin sensitivity-though both Irs1 and Irs2 were required to suppress hepatic glucose production during hyperinsulinemic-euglycemic clamp. In contrast, either hepatic Irs1 or Irs2 mediated suppression of HGP by intracerebroventricular insulin infusion. After 12 weeks on a high-fat diet, postprandial tyrosine phosphorylation of Irs1 increased in livers of control and LKO2 mice, whereas tyrosine phosphorylation of Irs2 decreased in control and LKO1 mice. Moreover, LKO1 mice -- but not LKO2 mice -- that were fed a high-fat diet developed postprandial hyperglycemia. We conclude that Irs1 is the principal mediator of hepatic insulin action that maintains glucose homeostasis.

CD45 recruits adapter protein DOK-1 and negatively regulates JAK-STAT signaling in hematopoietic cells

It has been extensively documented that CD45 positively regulates T cell receptor-mediated signaling through the activation of Src-family kinases. The mechanism whereby CD45 negatively regulates the JAK/STAT pathway, however, has not been fully elucidated. Here we describe the mechanism by which CD45 negatively regulates the JAK/STAT pathway through the recruitment of the inhibitory molecule Downstream of Kinase 1 (DOK-1) in hematopoietic cells. We present evidences that CD45 recruits DOK-1 to associate with tyrosine-phosphorylated DOK-1, and that the DOK-1-Y296F mutant completely abrogates its interaction with CD45. Moreover, CD45 expression is required for DOK-1 targeting to the plasma membrane in response to anti-CD3 stimulation. Functional studies further showed that stable expression of DOK-1 in K562 cells markedly decreased both JAK-2 and STAT-3/5 phosphorylation following IL-3 and IFN-alpha stimulation. Likewise, stable expression of DOK-1 in Jurkat cells significantly decreased JAK-2 phosphorylation. Similarly, both IL-3 and IFN-alpha-induced JAK-2 phosphorylations were significantly increased in CD45 deficient Jurkat cells. Consistently, silencing of the DOK-1 gene resulted in rescue of MAP kinases and JAKs activities in CD45 positive Jurkat cells. Accordingly, CD45 recruits adaptor DOK-1 to the proximal plasma membrane to serve as a downstream effector, resulting in negative regulation of the JAK/STAT signaling pathway.

Nck adapter proteins: functional versatility in T cells

Nck is a ubiquitously expressed adapter protein that is almost exclusively built of one SH2 domain and three SH3 domains. The two isoproteins of Nck are functionally redundant in many aspects and differ in only few amino acids that are mostly located in the linker regions between the interaction modules. Nck proteins connect receptor and non-receptor tyrosine kinases to the machinery of actin reorganisation. Thereby, Nck regulates activation-dependent processes during cell polarisation and migration and plays a crucial role in the signal transduction of a variety of receptors including for instance PDGF-, HGF-, VEGF- and Ephrin receptors. In most cases, the SH2 domain mediates binding to the phosphorylated receptor or associated phosphoproteins, while SH3 domain interactions lead to the formation of larger protein complexes. In T lymphocytes, Nck plays a pivotal role in the T cell receptor (TCR)-induced reorganisation of the actin cytoskeleton and the formation of the immunological synapse. However, in this context, two different mechanisms and adapter complexes are discussed. In the first scenario, dependent on an activation-induced conformational change in the CD3epsilon subunits, a direct binding of Nck to components of the TCR/CD3 complex was shown. In the second scenario, Nck is recruited to the TCR complex via phosphorylated Slp76, another central constituent of the membrane proximal activation complex. Over the past years, a large number of putative Nck interactors have been identified in different cellular systems that point to diverse additional functions of the adapter protein, e.g. in the control of gene expression and proliferation.

Insulin resistance: a proinflammatory state mediated by lipid-induced signaling dysfunction and involved in atherosclerotic plaque instability

The dysregulation of the insulin-glucose axis represents the crucial event in insulin resistance syndrome. Insulin resistance increases atherogenesis and atherosclerotic plaque instability by inducing proinflammatory activities on vascular and immune cells. This condition characterizes several diseases, such as type 2 diabetes, impaired glucose tolerance (IGT), impaired fasting glucose (IFG), obesity, hypertension, dyslipidemia, and other endocrinopathies, but also cancer. Recent studies suggest that the pathophysiology of insulin resistance is closely related to interferences with insulin-mediated intracellular signaling on skeletal muscle cells, hepatocytes, and adipocytes. Strong evidence supports the role of free fatty acids (FFAs) in promoting insulin resistance. The FFA-induced activation of protein kinase C (PKC) delta, inhibitor kappaB kinase (IKK), or c-Jun N-terminal kinase (JNK) modulates insulin-triggered intracellular pathway (classically known as PI3-K-dependent). Therefore, reduction of FFA levels represents a selective target for modulating insulin resistance.

Global impact of oncogenic Src on a phosphotyrosine proteome

Elevated activity of Src, the first characterized protein-tyrosine kinase, is associated with progression of many human cancers, and Src has attracted interest as a therapeutic target. Src is known to act in various receptor signaling systems to impact cell behavior, yet it remains likely that the spectrum of Src protein substrates relevant to cancer is incompletely understood. To better understand the cellular impact of deregulated Src kinase activity, we extensively applied a mass spectrometry shotgun phosphotyrosine (pTyr) proteomics strategy to obtain global pTyr profiles of Src-transformed mouse fibroblasts as well as their nontransformed counterparts. A total of 867 peptides representing 563 distinct pTyr sites on 374 different proteins were identified from the Src-transformed cells, while 514 peptides representing 275 pTyr sites on 167 proteins were identified from nontransformed cells. Distinct characteristics of the two profiles were revealed by spectral counting, indicative of pTyr site relative abundance, and by complementary quantitative analysis using stable isotope labeling with amino acids in cell culture (SILAC). While both pTyr profiles are replete with sites on signaling and adhesion/cytoskeletal regulatory proteins, the Src-transformed profile is more diverse with enrichment in sites on metabolic enzymes and RNA and protein synthesis and processing machinery. Forty-three pTyr sites (32 proteins) are predicted as major biologically relevant Src targets on the basis of frequent identification in both cell populations. This select group, of particular interest as diagnostic biomarkers, includes well-established Src sites on signaling/adhesion/cytoskeletal proteins, but also uncharacterized sites of potential relevance to the transformed cell phenotype.

Constitutive plasma membrane targeting and microdomain localization of Dok5 studied by single-molecule microscopy

In the present study, single-molecule fluorescence microscopy was used to examine the characteristics of plasma membrane targeting and microdomain localization of enhanced yellow fluorescent protein (eYFP)-tagged wild-type Dok5 and its variants in living Chinese hamster ovary (CHO) cells. We found that Dok5 can target constitutively to the plasma membrane, and the PH domain is essential for this process. Furthermore, single-molecule trajectories analysis revealed that Dok5 can constitutively partition into microdomain on the plasma membrane. Finally, the potential mechanism of microdomain localization of Dok5 was discussed. This study provided insights into the characteristics of plasma membrane targeting and microdomain localization of Dok5 in living CHO cells.

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.

An integrin phosphorylation switch: the effect of beta3 integrin tail phosphorylation on Dok1 and talin binding

Integrins play a fundamental role in cell migration and adhesion; knowledge of how they are regulated and controlled is vital for understanding these processes. Recent work showed that Dok1 negatively regulates integrin activation, presumably by competition with talin. To understand how this occurs, we used NMR spectroscopy and x-ray crystallography to investigate the molecular details of interactions with integrins. The binding affinities of beta3 integrin tails for the Dok1 and talin phosphotyrosine binding domains were quantified using 15N-1H hetero-nuclear single quantum correlation titrations, revealing that the unphosphorylated integrin tail binds more strongly to talin than Dok1. Chemical shift mapping showed that unlike talin, Dok1 exclusively interacts with the canonical NPXY motif of the beta3 integrin tail. Upon phosphorylation of Tyr 747 in the beta3 integrin tail, however, Dok1 then binds much more strongly than talin. Thus, we show that phosphorylation of Tyr 747 provides a switch for integrin ligand binding. This switch may represent an in vivo mechanism for control of integrin receptor activation. These results have implications for the control of integrin signaling by proteins containing phosphotyrosine binding domains.

Effects of SHIP-1 on MMP2 Secretion and Invasion of SR3Y1 Cells

SHIP-1 is an SH2 domain containing inositol-5-phosphatase that appears to be a negative regulator of hematopoiesis. To the potential effects of SHIP-1 on MMP2 secretion and migration of cancer cells, three murine SHIP-1 mutants were made: DeltaSH2-SHIP-1, DeltaPtase-SHIP-1, DeltaCter-SHIP-1. These mutant forms were subcloned as well as the wild type (WT) of murine SHIP-1 cDNA were subcloned into pcDNA3 expression vector, then transfected into and overexpressed SHIP-1 and its mutants in a Src-transformed 3Y1 cell line (SR3Y1). The results showed that overexpression of wild type of SHIP-1 does not affect the MMP2 secretion in both SR3Y1 and 3Y1 cells, but can induce MMP9 secretion, while either WT SHIP-1, the SH2 domain, phosphatase domain, or C terminus deletion mutants could significantly block the MMP2 and MMP9 secretion in SR3Y1 cells and suppress cell invasion ability. The results confirmed SHIP-1 as a negative regulator for cell migration and invasion in transformed cells, and implied that it may function through each of its three domains.

Regulating insulin signaling and beta-cell function through IRS proteins

Diabetes mellitus is a complex disorder that arises from various causes, including dysregulated glucose sensing and impaired insulin secretion (maturity onset diabetes of youth, MODY), autoimmune-mediated beta-cell destruction (type 1), or insufficient compensation for peripheral insulin resistance (type 2). Type 2 diabetes is the most prevalent form that usually occurs at middle age; it afflicts more than 30 million people over the age of 65, but is appearing with greater frequency in children and adolescents. Dysregulated insulin signaling exacerbated by chronic hyperglycemia promotes a cohort of systemic disorders--including dyslipidemia, hypertension, cardiovascular disease, and female infertility. Understanding the molecular basis of insulin resistance can prevent these disorders and their inevitable progression to type 2 diabetes.

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.

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.

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.

Proteomic analysis reveals novel molecules involved in insulin signaling pathway

The binding of insulin to its receptor triggers a signaling cascade regulated by protein complexes via tyrosine phosphorylation events on a multitude of associated proteins. To search novel phosphotyrosine proteins or associated proteins involved in insulin signaling pathway, we employed a method in which Rat1 cells stably expressing the human insulin receptor were stimulated with or without insulin and sub-fractionated prior to enrichment of phosphotyrosine proteins by immunoprecipitation and analysis by LC-MS/MS. Bioinformatic analysis and manual confirmation of peptide phosphorylation site assignments led to identification of 35 phosphotyrosine sites derived from 31 protein groups. Over 50% of these proteins were reported for the first time as tyrosine phosphorylated, including gigaxonin, XIAP and CDK10. In addition, we also found that calcium/calmodulin-dependent protein serine kinase (CASK), a key protein in protein-targeting and vesicle transport in neurons, forms a complex with two unidentified phosphotyrosine proteins pp100 and pp95 in response to insulin-stimulation, though CASK is not itself tyrosine phosphorylated. Furthermore, insulin was able to decrease CASK nuclear location, as well as down-regulate the expression of CASK targeted genes. Our results imply CASK as a novel joint knot connecting CASK-mediated pathways with the insulin signaling. Our data provide a wealth of information potentially paving the way to identify new components in the insulin signaling network.

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.

PTEN and SHIP2 phosphoinositide phosphatases as negative regulators of insulin signalling

Insulin resistance in peripheral tissues is the primary cause responsible for onset of type II diabetes mellitus. Recently, the genetic and biochemical dissection of intracellular signalling pathways transducing the metabolic and mitogenic effects of insulin has contributed to the understanding of the molecular causes of this insulin resistance. In particular, important efforts have been developed to comprehend the role of negative regulators of insulin signalling, since they might represent future therapeutical targets to reduce insulin resistance in peripheral tissues. Herein, we will briefly review major intracellular signalling pathways activated by insulin and how they are negatively regulated by distinct mechanisms. In particular, the role of PTEN and SHIP2, two phosphoinositide phosphatases recently implicated as negative modulators of insulin signalling, is in focus. Current knowledge on the role of PTEN and SHIP2 in insulin resistance, type II diabetes and related disorders will also be discussed.

Drosophila Dok is required for embryonic dorsal closure

Embryonic dorsal closure (DC) in Drosophila is a series of morphogenetic movements involving the bilateral dorsal movement of the epidermis (cell stretching) and dorsal suturing of the leading edge (LE) cells to enclose the viscera. The Syk family tyrosine kinase Shark plays a crucial role in this Jun amino-terminal kinase (JNK)-dependent process, where it acts upstream of JNK in LE cells. Using a yeast two-hybrid screen, the unique Drosophila homolog of the downstream of kinase (Dok) family, Ddok,was identified by its ability to bind Shark SH2 domains in a tyrosine phosphorylation-dependent fashion. In cultured S2 embryonic cells, Ddok tyrosine phosphorylation is Src dependent; Shark associates with Ddok and Ddok localizes at the cell cortex, together with a portion of the Shark protein. The embryonic expression pattern of Ddok resembles the expression pattern of Shark. Ddok loss-of-function mutant (DdokPG155)germ-line clones possess DC defects, including the loss of JNK-dependent expression of dpp mRNA in LE cells, and decreased epidermal F-actin staining and LE actin cable formation. Epistatic analysis indicates that Ddok functions upstream of shark to activate JNK signaling during DC. Consistent with these observations, Ddok mutant embryos exhibit decreased levels of tyrosine phosphorylated Shark at the cell periphery of LE and epidermal cells. As there are six mammalian Dok family members that exhibit some functional redundancy, analysis of the regulation of DC by Ddok is expected to provide novel insights into the function of the Dok adapter proteins.

Irs1 and Irs2 signaling is essential for hepatic glucose homeostasis and systemic growth

Insulin receptor substrates, including Irs1 and Irs2, integrate insulin and IGF receptor signals with heterologous pathways to coordinate growth and metabolism. Since Irs2 is thought to be especially important in hepatic nutrient homeostasis, we deleted Irs2 [corrected] from hepatocytes of WT mice (called LKO) or genetically insulin-resistant Irs1-/- mice (called LKO::Irs1-/-). Viable LKO::Irs1-/- mice were 70% smaller than WT or LKO mice, and 40% smaller than Irs1-/- mice. Hepatic insulin receptors were functional in all the mice, but insulin signaling via the Akt-FoxO1 pathway was reduced in Irs1-/- and LKO liver, and undetected in LKO::Irs1-/- liver; however, Gsk3beta phosphorylation (Ser9) and hepatic glycogen stores were nearly normal in all of the mice. LKO and Irs1-/- mice developed insulin resistance and glucose intolerance that never progressed to diabetes, whereas LKO::Irs1-/- mice developed hyperglycemia and hyperinsulinemia immediately after birth. Regardless, few hepatic genes changed expression significantly in Irs1-/- or LKO mice, whereas hundreds of genes changed in LKO::Irs1-/- mice--including elevated levels of Pck1, G6pc, Ppargc1, Pparg, and Igfbp1. Thus, signals delivered by Irs1 or Irs2 regulate hepatic gene expression that coordinates glucose homeostasis and systemic growth.

Dok1 expression and mutation in Burkitt's lymphoma cell lines

Dok1 is an adaptor tyrosine kinase substrate with tumor-suppressive activity. The gene encoding Dok1 maps to human chromosome 2p13, which is frequently rearranged in human tumors. We have previously reported a frameshift mutation of this gene and the down-regulation of its expression in chronic lymphocytic leukemia. In this study, we have determined the expression levels of Dok1 in Burkitt's lymphoma (BL) cell lines, lymphoblastoid cell lines from patients with X-linked lymphoproliferative (XLP-LCL), or from control healthy donors. We have also screened for Dok1 gene mutations by heteroduplex analysis and direct sequencing. Dok1 expression was down-regulated in all BL and XLP-LCL cell lines in comparison to the control cells. No Dok1 mutation or polymorphism was found in the coding region of Dok1 in the three types of cells. However, DNA sequence analysis revealed the presence of four nucleotide changes in Dok1 gene, T(90172)C (intron 1), C(89487)T and (89433)InsCTCT (intron 2), and A(87714)G (3' UTR). T(90172)C and (89433)InsCTCT that were detected in about 7% of BL, 9% of XLP-LCL and 4% of normal samples may represent a common polymorphism. C(89487)T and A(87714)G changes were detected in 9 and 6% of analyzed BL lines, respectively, but never in the control and XLP-LCL cells, indicating that these nucleotide substitution occurred during tumor development. Interestingly, the C(89487)T variant is associated with a significantly lower level of Dok1 expression compared to the control samples. A positive association was also found between the presence of EBV in BL and the Dok1 genetic variation. Our data show that Dok1 expression and structure are affected in a subset of Burkitt's lymphoma samples, suggesting its possible role in this type of cancer.

Signaling through Disabled 1 requires phosphoinositide binding

The Reelin signaling pathway plays a critical role in the correct positioning of neurons within the developing brain. Within this pathway, Disabled 1 (Dab1) serves as an intracellular adaptor that is tyrosine phosphorylated when Reelin, a secreted glycoprotein, binds to the lipoprotein receptors VLDLR and ApoER2 on the surface of neurons. The phosphotyrosine-binding (PTB) domain within its amino terminus enables Dab1 to recognize and bind to a conserved sequence motif within the cytoplasmic tails of the receptors. In addition, the PTB contains a Pleckstrin Homology-like subdomain that binds to phosphoinositides. Here, we show that the phosphoinositide-binding region within Dab1 PTB domain is required for membrane localization and basal tyrosine phosphorylation of Dab1 independently of VLDLR and ApoER2. Furthermore, receptor-independent membrane targeting of Dab1 is required for its interaction with Src and Crk, and disruption of phosphoinositide binding also blocks subsequent Reelin-induced tyrosine phosphorylation of Dab1.

Mitochondrial Dok-4 Recruits Src Kinase and Regulates NF-κB Activation in Endothelial Cells

The downstream of kinase (Dok) family of adapter proteins consists of at least five members structurally characterized by an NH2-terminal tandem of conserved pleckstrin homology and phosphotyrosine binding domains linked to a unique COOH-terminal region. To determine the role of the novel adapter protein Dok-4 in endothelial cells, we first investigated the cell localization of Dok-4. Most surprisingly, immunofluorescence microscopy, cell fractionation studies, and studies with enhanced green fluorescent protein chimeras showed that wild type Dok-4 (Dok-4-WT) specifically localized in mitochondria. An NH2-terminal deletion mutant of Dok-4 (Dok-4-(deltaN11-29)), which lacks the mitochondrial targeting sequence, could not accumulate in mitochondria. Co-immunoprecipitation revealed an interaction of c-Src with Dok-4-WT in endothelial cells. Most interestingly, overexpression of Dok-4-WT, but not Dok-4-(deltaN1-99), increased mitochondrial c-Src expression, whereas knock-down of endogenous Dok-4 with a small interfering RNA vector greatly inhibited mitochondrial localization of c-Src, suggesting a unique function for Dok-4 as an anchoring protein for c-Src in mitochondria. Dok-4-WT significantly decreased 39-kDa subunit complex I expression. PP2, a specific Src kinase inhibitor, prevented the Dok-4-mediated complex I decrease, suggesting the involvement of Src kinase in regulation of complex I expression. Dok-4-WT enhanced tumor necrosis factor-alpha (TNF-alpha)-mediated reactive oxygen species (ROS) production, supporting the functional relevance of a Dok-4-Src-complex I/ROS signaling pathway in mitochondria. Finally, Dok-4 enhanced TNF-alpha-mediated NF-kappaB activation, whereas this was inhibited by transfection with Dok-4 small interfering RNA. In addition, Dok-4-induced NF-kappaB activation was also inhibited by transfection of a dominant negative form of c-Src. These data suggest a role for mitochondrial Dok-4 as an anchoring molecule for the tyrosine kinase c-Src, and in turn as a regulator of TNF-alpha-mediated ROS production and NF-kappaB activation.

Molecular mechanisms of insulin resistance

Currently, we observe an epidemic expansion of diabetes mellitus. In subjects with Type 2 diabetes the resistance of fat, muscle and liver to insulin is the central pathophysiological event in the development of this disease. Genetic and environmental factors play a major role in this process, although the precise pathogenesis of insulin resistance and Type 2 diabetes is still largely unknown. However, recent studies have contributed to a deeper understanding of the molecular mechanisms underlying this process. In this review we therefore summarize the current developments in understanding the pathophysiological process of insulin resistance and Type 2 diabetes. Among the many molecules involved in the intracellular processing of the signal provided by insulin, insulin receptor substrate (IRS)-2, the protein kinase B (PKB)-beta isoform and the forkhead transcription factor Foxo1a (FKHR) are of particular interest in this context as recent data have provided strong evidence that dysfunction of these proteins results in insulin resistance in-vivo. Furthermore, we have now increasing evidence that the adipose tissue not only produces free fatty acids that contribute to insulin resistance, but also acts as a relevant endocrine organ producing mediators (adipokines) that can modulate insulin signalling. The identification of the molecular pathophysiological mechanisms of insulin resistance and Type 2 diabetes is essential for the development of novel and more effective therapies to better treat our patients with insulin resistance and Type 2 diabetes.

DOK1 Mediates SHP-2 Binding to the αVβ3 Integrin and Thereby Regulates Insulin-like Growth Factor I Signaling in Cultured Vascular Smooth Muscle Cells

Recruitment of the Src homology 2 domain tyrosine phosphatase (SHP-2) to the phosphorylated beta3 subunit of the alphaVbeta3 integrin is required for insulin-like growth factor I (IGF-I)-stimulated cell migration and proliferation in vascular smooth muscle cells. Because SHP-2 does not bind directly to beta3, we attempted to identify a linker protein that could mediate SHP-2/beta3 association. DOK1 is a member of insulin receptor substrate protein family that binds beta3 and contains YXXL/I motifs that are potential binding sites for SHP-2. Our results show that IGF-I induces DOK1 binding to beta3 and to SHP-2. Preincubation of cells with synthetic peptides that blocked either DOK1/beta3 or DOK1/SHP-2 association inhibited SHP-2 recruitment to beta3. Expression of a DOK1 mutant that does not bind to beta3 also disrupts SHP-2/beta3 association. As a result of SHP-2/beta3 disruption, IGF-I dependent phosphorylation of Akt and p44/p42 mitogen-activated protein kinase and its ability to stimulate cell migration and proliferation were significantly impaired. These results demonstrate that DOK1 mediates SHP-2/beta3 association in response to IGF-I thereby mediating the effect of integrin ligand occupancy on IGF-IR-linked signaling in smooth muscle cells.

Dok-1 tyrosine residues at 336 and 340 are essential for the negative regulation of Ras-Erk signalling, but dispensable for rasGAP-binding

Dok-1 is a common substrate of many protein tyrosine kinases (PTKs). It recruits rasGAP and other SH2-containing proteins and negatively regulates Ras-Erk signalling downstream of PTKs. However, the mechanisms of its inhibitory effect are yet unclear. Here, a series of C-terminal deletion mutants of Dok-1 delineated the core domain for the inhibition of Erk from 334 to 346 amino acid, which contains two SH2-binding motifs having Tyr-336 or Tyr-340. The Dok-1 mutants having tyrosine-to-phenylalanine (YF) substitution(s) at Tyr-336 and/or Tyr-340 lost their inhibitory effect on Ras and Erk downstream of Src-like PTK, Lyn or Fyn, whereas the rasGAP-binding of each mutant remained intact. However, the Dok-1 mutant having YF substitutions at the rasGAP-binding sites (Tyr-295 and Tyr-361) also showed incapability of Ras and Erk inhibition. Moreover, the Dok-1 mutant having YF substitutions at Tyr-336 and Tyr-340 showed an impaired inhibitory effect on v-Abl-induced transformation of NIH-3T3 cells. These results demonstrate that Tyr-336 and Tyr-340 of Dok-1 are dispensable for rasGAP-binding but essential for inhibition of Ras-Erk signalling and cellular transformation downstream of PTKs. Thus, Dok-1 probably recruits as yet unidentified molecule(s), which, in concert with rasGAP, negatively regulate Ras-Erk signalling.

IkappaB kinase beta phosphorylates Dok1 serines in response to TNF, IL-1, or gamma radiation

Dok1 is an abundant Ras-GTPase-activating protein-associated tyrosine kinase substrate that negatively regulates cell growth and promotes migration. We now find that IkappaB kinase beta (IKKbeta) associated with and phosphorylated Dok1 in human epithelial cells and B lymphocytes. IKKbeta phosphorylation of Dok1 depended on Dok1 S(439), S(443), S(446), and S(450). Recombinant IKKbeta also phosphorylated Dok1 or Dok1 amino acids 430-481 in vitro. TNF-alpha, IL-1, gamma radiation, or IKKbeta overexpression phosphorylated Dok1 S(443), S(446), and S(450) in vivo, as detected with Dok1 phospho-S site-specific antisera. Moreover, Dok1 with S(439), S(443), S(446), and S(450) mutated to A was not phosphorylated by IKKbeta in vivo. Surprisingly, mutant Dok1 A(439), A(443), A(446), and A(450) differed from wild-type Dok1 in not inhibiting platelet-derived growth factor-induced extracellular signal-regulated kinase 1/2 phosphorylation or cell growth. Mutant Dok1 A(439), A(443), A(446), and A(450) also did not promote cell motility, whereas wild-type Dok1 promoted cell motility, and Dok1 E(439), E(443), E(446), and E(450) further enhanced cell motility. These data indicate that IKKbeta phosphorylates Dok1 S(439)S(443) and S(446)S(450) after TNF-alpha, IL-1, or gamma-radiation and implicate the critical Dok1 serines in Dok1 effects after tyrosine kinase activation.

Insulin receptor substrate proteins and diabetes

The discovery of insulin receptor substrate (IRS) proteins and their role to link cell surface receptors to the intracellular signaling cascades is a key step to understanding insulin and insulin-like growth factor (IGF) action. Moreover, IRS-proteins coordinate signals from the insulin and IGF receptor tyrosine kinases with those generated by proinflammatory cytokines and nutrients. The IRS2-branch of the insulin/IGF signaling cascade has an important role in both peripheral insulin response and pancreatic beta-cell growth and function. Dysregulation of IRS2 signaling in mice causes the failure of compensatory hyperinsulinemia during peripheral insulin resistance. IRS protein signaling is down regulated by serine phosphorylation or proteasome-mediated degradation, which might be an important mechanism of insulin resistance during acute injury and infection, or chronic stress associated with aging or obesity. Understanding the regulation and signaling by IRS1 and IRS2 in cell growth, metabolism and survival will reveal new strategies to prevent or cure diabetes and other metabolic diseases.

Dok-6, a Novel p62 Dok family member, promotes Ret-mediated neurite outgrowth

Activation of Ret, the receptor-tyrosine kinase for the glial cell line-derived neurotrophic factor (GDNF) family ligands (GFLs), results in the recruitment and assembly of adaptor protein complexes that function to transduce signals downstream of the receptor. Here we identify Dok-6, a novel member of the Dok-4/5 subclass of the p62 Dok family of intracellular adaptor molecules, and characterize its interaction with Ret. Expression analysis reveals that Dok-6 is highly expressed in the developing central nervous system and is co-expressed with Ret in several locations, including sympathetic, sensory, and parasympathetic ganglia, as well as in the ureteric buds of the developing kidneys. Pull-down assays using the Dok-6 phosphotyrosine binding (PTB) domain and GDNF-activated Ret indicate that Dok-6 binds to the phosphorylated Ret Tyr(1062) residue. Moreover, ligand activation of Ret resulted in phosphorylation of tyrosine residue(s) located within the unique C terminus of Dok-6 predominantly through a Src-dependent mechanism, indicating that Dok-6 is a substrate of the Ret-Src signaling pathway. Interestingly, expression of Dok-6 potentiated GDNF-induced neurite outgrowth in GDNF family receptor alpha1 (GFRalpha1)-expressing Neuro2A cells that was dependent upon the C-terminal residues of Dok-6. Taken together, these data identify Dok-6 as a novel Dok-4/5-related adaptor molecule that may function in vivo to transduce signals that regulate Ret-mediated processes such as axonal projection.

Insulin signaling in microdomains of the plasma membrane

Although the effects of insulin on glucose and lipid metabolism are well documented, gaps remain in our understanding of the precise molecular mechanisms of signal transduction. Recent evidence suggests that compartmentalization of signaling molecules and metabolic enzymes may explain the unique cellular effects of the hormone. Signal initiation from the insulin receptor is restricted in part to caveolae microdomains of the plasma membrane. A fraction of the insulin receptor directly interacts with caveolin, thus directing the protein to caveolae. Following its activation by insulin, the receptor recruits a series of adapter proteins, resulting in the activation of the G protein TC10, which also resides in caveolae. TC10 can influence a number of cellular processes, including changes in the actin cytoskeleton, recruitment of effector including the adapter protein CIP4, and assembly of the exocyst complex. These events play crucial roles in the trafficking, docking and fusion of vesicles containing the insulin-responsive glucose transporter Glut4 at the plasma membrane.

c-Abl phosphorylates Dok1 to promote filopodia during cell spreading

Filopodia are dynamic F-actin structures that cells use to explore their environment. c-Abl tyrosine kinase promotes filopodia during cell spreading through an unknown mechanism that does not require Cdc42 activity. Using an unbiased approach, we identified Dok1 as a specific c-Abl substrate in spreading fibroblasts. When activated by cell adhesion, c-Abl phosphorylates Y361 of Dok1, promoting its association with the Src homology 2 domain (SH2)/SH3 adaptor protein Nck. Each signaling component was critical for filopodia formation during cell spreading, as evidenced by the finding that mouse fibroblasts lacking c-Abl, Dok1, or Nck had fewer filopodia than cells reexpressing the product of the disrupted gene. Dok1 and c-Abl stimulated filopodia in a mutually interdependent manner, indicating that they function in the same signaling pathway. Dok1 and c-Abl were both detected in filopodia of spreading cells, and therefore may act locally to modulate actin. Our data suggest a novel pathway by which c-Abl transduces signals to the actin cytoskeleton through phosphorylating Dok1 Y361 and recruiting Nck.

Specialised adaptors in immune cells

Adaptors are modular proteins implicated in the orchestration of intracellular signalling pathways. Studies of adaptors specifically expressed in immune cells have provided clear examples of the importance of adaptor molecules in normal mammalian biology. Moreover, they have led to the identification of naturally occurring mutations in adaptors that can be linked to human diseases. Lastly, they have highlighted the plasticity of protein-protein interaction modules, and have shed light onto the mechanisms dictating the specificity of adaptor-mediated signals.

Pleckstrin Homology and Phosphotyrosine-binding Domain-dependent Membrane Association and Tyrosine Phosphorylation of Dok-4, an Inhibitory Adapter Molecule Expressed in Epithelial Cells

Dok-like adapter molecules represent an expanding family of pleckstrin homology (PH) and phosphotyrosine-binding (PTB) domain-containing tyrosine kinase substrates with negative regulatory functions in hematopoietic cell signaling. In a search for nonhematopoietic counterparts to Dok molecules, we identified and characterized Dok-4, a recently cloned member of the family. dok-4 mRNA was strongly expressed in nonhematopoietic organs, particularly the intestine, kidney, and lung, whereas both mRNA and protein were expressed at high levels in cells of epithelial origin. In Caco-2 human colon cancer cells, endogenous Dok-4 underwent tyrosine phosphorylation in response to pervanadate stimulation. In transfected COS cells, Dok-4 was a substrate for the cytosolic tyrosine kinases Src and Fyn as well as for Jak2. Dok-4 could also be phosphorylated by the receptor tyrosine kinase Ret but not by platelet-derived growth factor receptor-beta or IGF-IR. In both mammalian cells and yeast, Dok-4 was constitutively localized at the membrane in a manner that required both its PH and PTB domains. The PH and PTB domains of Dok-4 were also required for tyrosine phosphorylation of Dok-4 by Fyn and Ret. Finally, wild type Dok-4 strongly inhibited activation of Elk-1 induced by either Ret or Fyn. The attenuation of this inhibitory effect by deletion of the PH domain and its restoration by the addition of a myristoylation signal suggested an important role for constitutive membrane localization of Dok-4. In summary, Dok-4 is a constitutively membrane-localized adapter molecule that may function as an inhibitor of tyrosine kinase signaling in epithelial cells.