Association of Sos Ras exchange protein with Grb2 is implicated in tyrosine kinase signal transduction and transformation

Microarray analysis of gene expression in adult retinal ganglion cells

Retinal ganglion cells (RGCs) transfer visual information to the brain and are known to be susceptible to selective degeneration in various neuropathies such as glaucoma. This selective vulnerability suggests that these highly specialized neurons possess a distinct gene expression profile that becomes altered by neuropathy-associated stresses, which lead to the RGC death. In this study, to identify genes expressed predominantly in adult RGCs, a global transcriptional profile of purified primary RGCs has been compared to that of the whole retina. To avoid alterations of the original gene expression profile by cell culture conditions, we isolated RNA directly from adult RGCs purified by immunopanning without prior sub-cultivation. Genes expressed predominantly in RGCs included: Nrg1, Rgn, 14-3-3 family (Ywhah, Ywhaz, Ywhab), Nrn1, Gap43, Vsnl1, Rgs4. Some of these genes may serve as novel markers for these neurons. Our analysis revealed enrichment in genes controlling the pro-survival pathways in RGCs as compared to other retinal cells.

Assembly of b/HLH/z proteins c-Myc, Max, and Mad1 with cognate DNA: importance of protein-protein and protein-DNA interactions

Among the best characterized of the transcription factors are the b/HLH/z proteins: USF, Max, Myc, and Mad. These proteins bind to the DNA E-box, a six base pair sequence, CACGTG. Max and Myc form a heterodimer that has strong oncogenic potential but can also repress transcription, while Mad and Max form a heterodimer that acts as a transcription repressor. We have used fluorescence anisotropy to measure protein-protein and protein-DNA affinity. The specific binding between MLP DNA and Max (K = 2.2 +/- 0.5 nM) is about 10-fold higher affinity than LCR DNA and about 100-fold higher than for a nonspecific DNA. USF has a similar binding affinity as Max to MLP DNA (K = 15 +/- 10 nM), but Max binds more tightly to LCR and nonspecific DNA. A series of oligonucleotides designated E-box, half-E-box, and non-E-box were constructed to examine the effects of DNA sequence. The binding results indicate that for Max protein most of the binding energy can be attributed to individual elements with little cooperativity among the two halves of the E-box. Further studies measured the equilibria for the entire thermodynamic cycle of monomer-dimer-DNA interactions. Surprisingly, the affinity of the Max monomer-DNA for the second monomer was greatly reduced (K for the first monomer in the nanomolar range and for the second monomer in the micromolar range). Looked at from the perspective of the Max protein, the binding of DNA to Max significantly reduces the affinity of the Max protein for the second monomer, whether the second monomer is Myc, Mad, or Max. These data suggest the importance of protein-protein interactions in assembly of a transcription initiation complex.

T-type calcium channel trigger p21ras signaling pathway to ERK in Cav3.1-expressed HEK293 cells

We constructed a new cell line which stably expressed Cav3.1 and Kir2.1 subunits in HEK293 cells (HEK293/Cav3.1/Kir2.1) in order to investigate the unknown cellular signaling pathways of T-type voltage-dependent calcium channels. The new cell line has a stable resting membrane potential and can activate T-type Ca(2+) channels by KCl-mediated depolarization. We showed that Cav3.1 activation resulted in the level of p21(ras)-GTP in the cells being rapidly decreased during the first 2 min, and then recovering between 2 min and 15 min. The kinetics of ERK activation following Cav3.1 stimulation was also investigated. ERK activation was decreased from 2 min to 5 min after KCl stimulation, which means that Cav3.1 activation reduced ERK activity in the very early stages of activation. In addition, similar results for Cav3.1 activation were also shown in the case of Sos1, Grb2, and Shc, which means that Cav3.1 activation triggers p21(ras) and that this signal is transferred to ERK by Sos1, Grb2, and Shc.

A network model of early events in epidermal growth factor receptor signaling that accounts for combinatorial complexity

We consider a model of early events in signaling by the epidermal growth factor (EGF) receptor (EGFR). The model includes EGF, EGFR, the adapter proteins Grb2 and Shc, and the guanine nucleotide exchange factor Sos, which is activated through EGF-induced formation of EGFR-Grb2-Sos and EGFR-Shc-Grb2-Sos assemblies at the plasma membrane. The protein interactions involved in signaling can potentially generate a diversity of protein complexes and phosphoforms; however, this diversity has been largely ignored in models of EGFR signaling. Here, we develop a model that accounts more fully for potential molecular diversity by specifying rules for protein interactions and then using these rules to generate a reaction network that includes all chemical species and reactions implied by the protein interactions. We obtain a model that predicts the dynamics of 356 molecular species, which are connected through 3749 unidirectional reactions. This network model is compared with a previously developed model that includes only 18 chemical species but incorporates the same scope of protein interactions. The predictions of this model are reproduced by the network model, which also yields new predictions. For example, the network model predicts distinct temporal patterns of autophosphorylation for different tyrosine residues of EGFR. A comparison of the two models suggests experiments that could lead to mechanistic insights about competition among adapter proteins for EGFR binding sites and the role of EGFR monomers in signal transduction.

Grb2 negatively regulates epidermal growth factor-induced phospholipase C-γ1 activity through the direct interaction with tyrosine-phosphorylated phospholipase C-γ1

Phospholipase C-gamma1 (PLC-gamma1) plays pivotal roles in cellular growth and proliferation. Upon the stimulation of growth factors and hormones, PLC-gamma1 is rapidly phosphorylated at three known sites; Tyr771, Tyr783 and Tyr1254 and its enzymatic activity is up-regulated. In this study, we demonstrate for the first time that Grb2, an adaptor protein, specifically interacts with tyrosine-phosphorylated PLC-gamma1 at Tyr783. The association of Grb2 with PLC-gamma1 was induced by the treatment with epidermal growth factor (EGF). Replacement of Tyr783 with Phe completely blocked EGF-induced interaction of PLC-gamma1 with Grb2, indicating that tyrosine phosphorylation of PLC-gamma1 at Tyr783 is essential for the interaction with Grb2. Interestingly, the depletion of Grb2 from HEK-293 cells by RNA interference significantly enhanced increased EGF-induced PLC-gamma1 enzymatic activity and mobilization of the intracellular Ca2+, while it did not affect EGF-induced tyrosine phosphorylation of PLC-gamma1. Furthermore, overexpression of Grb2 inhibited PLC-gamma1 enzymatic activity. Taken together, these results suggest Grb2, in addition to its key function in signaling through Ras, may have a negatively regulatory role on EGF-induced PLC-gamma1 activation.

Dynamic characteristics of transient responses

Transient responses of signaling molecules are seen in a wide variety of cellular processes that are mediated by distinct molecular mechanisms. Although transient responses might intuitively be thought to depend on the absolute concentration of growth factors or the intensity of stimulation, we here introduce that some transient responses are prompted by temporal rate of increase of stimulation, rather than intensity of stimulation, by three independent mechanisms. These include the Ras system with fast activation and slow inactivation, the ERK-dependent negative feedback loop system, and the receptor degradation system, all of which can be commonly seen in various signaling networks. In addition, we show the distinct transient and steady state characteristics of these systems.

Toward the Elucidation of the Structural Determinants Responsible for the Molecular Recognition between Mad1 and Max

Mad1 is a member of the Mad family. This family is part of the larger Myc/Max/Mad b-HLH-LZ eukaryotic transcription-factor network. Mad1 forms a specific heterodimer with Max and acts as a transcriptional repressor when bound to an E-box sequence (CACGTG) found in the promoter of c-Myc target genes. Mad1 cannot form a complex with DNA by itself under physiological conditions. A global model for the molecular recognition has emerged in which the Mad1 b-HLH-LZ homodimer is destabilized and the Mad/Max b-HLH-LZ heterodimer is favored. The detailed structural determinants responsible for the molecular recognition remain largely unknown. In this study, we focus on the elucidation of the structural determinants responsible for the destabilization of the Mad1 b-HLH-LZ homodimer. Conserved acidic residues at the dimerization interface (position a) of the LZ of all Max-interacting proteins have been hypothesized to be involved in the destabilization of the homodimeric states. In Mad1, this position corresponds to residue Asp 112. As reported for the complete gene product of Mad1, we show that wild-type b-HLH-LZ does not homodimerize or bind DNA under physiological conditions. On the other hand, the single mutation of Asp 112 to an Asn enables the b-HLH-LZ to dimerize and bind DNA. Our results suggest that Asp 112 is implicated in the destabilization of Mad1 b-HLH-LZ homodimer. Interestingly, this side chain is observed to form a salt bridge at the interface of the LZ domain in the crystal structure of Mad1/Max heterodimeric b-HLH-LZ bound to DNA [Nair, S. K., and Burley, S. K. (2003) Cell 112, 193-205]. This clearly suggests that Asp 112 plays a crucial role in the molecular recognition between Max and Mad1.

Differentiation of neuroblastoma cell line N1E-115 involves several signaling cascades

No systematic searches for differential expression of signaling proteins (SP) in undifferentiated vs. differentiated cell lineages were published and herein we used protein profiling for this purpose. The NIE-115 cell line was cultivated and an aliquot was differentiated with dimethylsulfoxide (DMSO), that is known to lead to a neuronal phenotype. Cell lysates were prepared, run on two-dimensional gel electrophoresis followed by MALDI-TOF-TOF identification of proteins and maps of identified SPs were generated. Seven SPs were comparable, 27 SPs: GTP-binding/Ras-related proteins, kinases, growth factors, calcium binding proteins, phosphatase-related proteins were observed in differentiated NIE-115 cells and eight SPs of the groups mentioned above were observed in undifferentiated cells only. Switching-on/off of several individual SPs from different signaling cascades during the differentiation process is a key to understand mechanisms involved. The findings reported herein are challenging in vitro and in vivo studies to confirm a functional role for deranged SPs.

Disruption of the Shc/Grb2 complex during abelson virus transformation affects proliferation, but not apoptosis

The v-Abl protein tyrosine kinase encoded by Abelson murine leukemia virus (Ab-MLV) induces pre-B-cell transformation. Signals emanating from the SH2 domain of the protein are required for transformation, and several proteins bind this region of v-Abl. One such protein is the adaptor molecule Shc, a protein that complexes with Grb2/Sos and facilitates Ras activation, an event associated with Ab-MLV transformation. To test the role this interaction plays in growth and survival of infected pre-B cells, dominant-negative (DN) Shc proteins were coexpressed with v-Abl and transformation was examined. Expression of DN Shc reduced Ab-MLV pre-B-cell transformation and decreased the ability of v-Abl to stimulate Ras activation and Erk phosphorylation in a Raf-dependent but Rac-independent fashion. Further analysis revealed that Shc is required for v-Abl-mediated Raf tyrosine 340 and 341 phosphorylation, an event associated with Erk phosphorylation. In contrast to effects on proliferation, survival of the cells and activation of Akt were not affected by expression of DN Shc. Together, these data reveal that v-Abl-Shc interactions are a critical part of the growth stimulatory signals delivered during transformation but that they do not affect antiapoptotic pathways. Furthermore, these data highlight a novel role for Shc in signaling from v-Abl to Raf.

Genetic Deletion of the p66 Shc Adaptor Protein Protects From Angiotensin II–Induced Myocardial Damage

Angiotensin II (Ang II), acting through its G protein-coupled AT1 receptor (AT1), contributes to the precocious heart senescence typical of patients with hypertension, atherosclerosis, and diabetes. AT1 was suggested to transactivate an intracellular signaling controlled by growth factors and their tyrosin-kinase receptors. In cultured vascular smooth muscle cells, this downstream mechanism comprises the p66Shc adaptor protein, previously recognized to play a role in vascular cell senescence and death. The aim of the present study was 2-fold: (1) to characterize the cardiovascular phenotype of p66Shc knockout mice (p66Shc(-/-)), and (2) to test the novel hypothesis that disrupting the p66Shc might protect the heart from the damaging action of elevated Ang II levels. Compared with wild-type littermates (p66Shc(+/+)), p66Shc(-/-) showed similar blood pressure, heart rate, and left ventricular wall thickness. However, cardiomyocyte number was increased in mutant animals, indicating a condition of myocardial hyperplasia. In p66Shc(+/+), infusion of a sub-pressor dose of Ang II (300 nmol/kg body weight [BW] daily for 28 days) caused left ventricular hypertrophy and apoptotic death of cardiomyocytes and endothelial cells. In contrast, p66Shc(-/-) were resistant to the proapoptotic/hypertrophic action of Ang II. Consistently, in vitro experiments showed that Ang II causes apoptotic death of cardiomyocytes isolated from p66Shc(+/+) hearts to a greater extent as compared with p66Shc(-/-) cardiomyocytes. Our results indicate a fundamental role of p66Shc in Ang II-mediated myocardial remodeling. In perspective, p66Shc inhibition may be envisioned as a novel way to prevent the deleterious effects of Ang II on the heart.

Identification of Grb2 As a Novel Binding Partner of the Signaling Lymphocytic Activation Molecule-Associated Protein Binding Receptor CD229

Ag recognition by the TCR determines the subsequent fate of the T cell and is regulated by the involvement of other cell surface molecules, termed coreceptors. CD229 is a lymphocyte cell surface molecule that belongs to the CD150 family of receptors. Upon tyrosine phosphorylation, CD229 recruits various signaling molecules to the membrane. One of these molecules is the signaling lymphocytic activation molecule-associated protein, of which a deficiency leads to the X-linked lymphoproliferative syndrome. We report that CD229 interacts in a phosphorylation-dependent manner with Grb2. We mapped this interaction showing that the Src homology 2 domain of Grb2 and the tyrosine residue Y606 in CD229 are required for CD229-Grb2 complex formation. The Grb2 motif in the cytoplasmic tail of CD229 is distinct and independent from the two tyrosines required for efficient signaling lymphocytic activation molecule-associated protein recruitment. CD229, but not other members of the CD150 family, directly bound Grb2. We also demonstrate that CD229 precipitates with Grb2 in T lymphocytes after pervanadate treatment, as well as CD229 or TCR ligation. Interestingly, the CD229 mutant lacking the Grb2 binding site is not internalized after CD229 engagement with specific Abs. Moreover, a dominant negative form of Grb2 (containing only Src homology 2 domain) impaired CD229 endocytosis. Unexpectedly, Erk phosphorylation was partially inhibited after activation of CD229 plus CD3. Consistent with this, CD229 ligation partially inhibited TCR signaling in peripheral blood cells and CD229-Jurkat cells transfected with the 3XNFAT-luciferase reporter construct. Altogether, the data suggest a model whereby CD229 ligation attenuates TCR signaling and Grb2 recruitment to CD229 controls its rate of internalization.

Regulation of growth factor receptor bound 2 by electroconvulsive seizure

Electroconvulsive seizure (ECS) is a well-established non-chemical antidepressant that is effective in the treatment of severe depression and also in subjects resistant to chemical antidepressant treatment. Although the molecular mechanism governing the antidepressant efficacy of ECS is unknown, recent work suggests that an amplification of growth/neurotrophic signaling might play a role in mediating the therapeutic effects. In this context, we examined the regulation of growth factor receptor bound 2 (Grb2), an important adaptor molecule in several growth factor signaling cascades. In situ hybridization analysis revealed a more than 2-fold induction of Grb2 mRNA in the hippocampal dentate gyrus as well as superficial and deep layers of the cortex with both acute and chronic ECS. Grb2 also exhibited a time-dependent induction 4 and 8 h after acute ECS, returning to basal levels at 24 h. These results provide further evidence of increased growth factor signaling in response to ECS.

Phosphorylation of p66Shc and forkhead proteins mediates Abeta toxicity

Excessive accumulation of amyloid β-peptide (Aβ) plays an early and critical role in synapse and neuronal loss in Alzheimer's Disease (AD). Increased oxidative stress is one of the mechanisms whereby Aβ induces neuronal death. Given the lessened susceptibility to oxidative stress exhibited by mice lacking p66Shc, we investigated the role of p66Shc in Aβ toxicity. Treatment of cells and primary neuronal cultures with Aβ caused apoptotic death and induced p66Shc phosphorylation at Ser36. Ectopic expression of a dominant-negative SEK1 mutant or chemical JNK inhibition reduced Aβ-induced JNK activation and p66Shc phosphorylation (Ser36), suggesting that JNK phosphorylates p66Shc. Aβ induced the phosphorylation and hence inactivation of forkhead transcription factors in a p66Shc-dependent manner. Ectopic expression of p66ShcS36A or antioxidant treatment protected cells against Aβ-induced death and reduced forkhead phosphorylation, suggesting that p66Shc phosphorylation critically influences the redox regulation of forkhead proteins and underlies Aβ toxicity. These findings underscore the potential usefulness of JNK, p66Shc, and forkhead proteins as therapeutic targets for AD.

Human Bone Derived Cell (HBDC) Behaviour of Sol-Gel Derived Carbonate Hydroxyapatite Coatings on Titanium Alloy Substrates

Poor cell adhesion to orthopaedic and dental implants results in implant failure. Establishing and maintaining mature bone at the bone/device interface is critical to the long-term success of the prostheses. Considerable effort has been devoted to alter the surface characteristics of these biomaterials in order to improve the initial interlocking of device and skeleton in the noncemented joint prosthesis. Previously we demonstrated that surface chemistry modification of bioceramics induced osteogenesis. In the present work, we investigate the effect of surface chemistry modification of titanium alloy (Ti-6Al-4V) with alkoxide-derived carbonate hydroxyapatite (CHAp) using sol-gel coating methods on human bone derived cell (HBDC)behaviour. Western blotting demonstrated that sol gel coating of Ti-6Al-4V with CHAp upregulated the expression of key signalling protein Shc isoforms (p46, p52, p66) and phosphorylated Erk1/2. CHAp-modification of Ti-6Al-4V is associated with signal transduction pathways involving the key signalling protein Shc and ERK1/2 which may lead to enhanced gene expression of extracellular matrix proteins at the skeletal tissue/device interface.

Prediction and validation of the distinct dynamics of transient and sustained ERK activation

To elucidate the hidden dynamics of extracellular-signal-regulated kinase (ERK) signalling networks, we developed a simulation model of ERK signalling networks by constraining in silico dynamics based on in vivo dynamics in PC12 cells. We predicted and validated that transient ERK activation depends on rapid increases of epidermal growth factor and nerve growth factor (NGF) but not on their final concentrations, whereas sustained ERK activation depends on the final concentration of NGF but not on the temporal rate of increase. These ERK dynamics depend on Ras and Rap1 dynamics, the inactivation processes of which are growth-factor-dependent and -independent, respectively. Therefore, the Ras and Rap1 systems capture the temporal rate and concentration of growth factors, and encode these distinct physical properties into transient and sustained ERK activation, respectively.

The SH2 domain: versatile signaling module and pharmaceutical target

The Src homology 2 (SH2) domain is the most prevalent protein binding module that recognizes phosphotyrosine. This approximately 100-amino-acid domain is highly conserved structurally despite being found in a wide variety proteins. Depending on the nature of neighboring protein module(s), such as catalytic domains and other protein binding domains, SH2-containing proteins play many different roles in cellular protein tyrosine kinase (PTK) signaling pathways. Accumulating evidence indicates SH2 domains are highly versatile and exhibit considerable flexibility in how they bind to their ligands. To illustrate this functional versatility, we present three specific examples: the SAP, Cbl and SOCS families of SH2-containing proteins, which play key roles in immune responses, termination of PTK signaling, and cytokine responses. In addition, we highlight current progress in the development of SH2 domain inhibitors designed to antagonize or modulate PTK signaling in human disease. Inhibitors of the Grb2 and Src SH2 domains have been extensively studied, with the aim of targeting the Ras pathway and osteoclastic bone resorption, respectively. Despite formidable difficulties in drug design due to the lability and poor cell permeability of negatively charged phosphorylated SH2 ligands, a variety of structure-based strategies have been used to reduce the size, charge and peptide character of such ligands, leading to the development of high-affinity lead compounds with potent cellular activities. These studies have also led to new insights into molecular recognition by the SH2 domain.

Structural analysis of autoinhibition in the Ras activator Son of sevenless

The classical model for the activation of the nucleotide exchange factor Son of sevenless (SOS) involves its recruitment to the membrane, where it engages Ras. The recent discovery that Ras*GTP is an allosteric activator of SOS indicated that the regulation of SOS is more complex than originally envisaged. We now present crystallographic and biochemical analyses of a construct of SOS that contains the Dbl homology-pleckstrin homology (DH-PH) and catalytic domains and show that the DH-PH unit blocks the allosteric binding site for Ras and suppresses the activity of SOS. SOS is dependent on Ras binding to the allosteric site for both a lower level of activity, which is a result of Ras*GDP binding, and maximal activity, which requires Ras*GTP. The action of the DH-PH unit gates a reciprocal interaction between Ras and SOS, in which Ras converts SOS from low to high activity forms as Ras*GDP is converted to Ras*GTP by SOS.

Malignant peripheral nerve sheath tumors

The rarity of MPNSTs and the lack of any singular diagnostic radiologic or pathologic signature lead to several management challenges. These tumors are best managed as part ofa multidisciplinary team so as to optimize patient care and facilitate research. Suspicion of an MPNST based on clinical or radiologic alteration of a soft tissue mass in proximity to a peripheral nerve, especially in the context of NF I, should lead to referral to such a tertiary center. Early diagnosis followed by oncologic surgery to obtain tumor-free margins provides the best chance for long-term cure. Psychologic support and occupational rehabilitation are vital components of the overall care of these relatively young patients faced with often disabling surgery. Current adjuvant therapy with radiation and chemotherapy is suboptimal. There have been major inroads toward the molecular biologic understanding of MPNSTs,with several biologic targets that are of potential therapeutic interest. Proper evaluation of these novel and promising management strategies requires a concerted effort to refer these patients to the tertiary centers through which multi-institutional clinical trials can be undertaken.

Rac1 contributes to maximal activation of STAT1 and STAT3 in IFN-gamma-stimulated rat astrocytes

Rac1 GTPase is implicated as a signaling mediator in various cellular events. In this study, we show that Rac1 contributes to IFN-gamma-induced inflammatory responses in rat astrocytes. We revealed that IFN-gamma rapidly stimulated activation of Rac1 in C6 astroglioma cells by investigating GST-PAK-PBD-binding ability. We also found that Rac1 deficiency led to attenuation of IFN-gamma-responsive transcriptional responses. Compared with levels in control cells, IFN-gamma-induced IFN-gamma-activated sequence promoter activity was markedly reduced in both C6 astroglioma cells and primary astrocytes expressing RacN17, a well-characterized Rac1-negative mutant. The expression of several IFN-gamma-responsive genes, such as MCP-1 and ICAM-1, was also reduced in cells expressing RacN17. Consistent with these observations, IFN-gamma-induced phosphorylation of STAT1 and STAT3 was lower in C6 cells expressing RacN17 (referred to as C6-RacN17) than in control cells. However, there was no difference in expression level of IFN-gammaRalpha subunit and IFN-gamma-induced phosphorylation of JAK1 between C6 control and C6-RacN17 cells. Interestingly, Rac1 appeared to associate with IFN-gammaRalpha and augment the interaction of IFN-gammaR with either STAT1 or STAT3 in response to IFN-gamma. Taken together, we suggest that Rac1 may serve as an auxiliary mediator of IFN-gamma-signaling, at least at the level of STAT activation, thus contributing to maximal activation of IFN-gamma-responsive inflammatory signaling in rat astrocytes.

GTP hydrolysis mechanism of Ras-like GTPases

The Ras-like GTPases regulate diverse cellular functions via the chemical cycle of binding and hydrolyzing GTP molecules. They alternate between GTP- and GDP-bound conformations. The GTP-bound conformation is biologically active and promotes a cellular function, such as signal transduction, cytoskeleton organization, protein synthesis/translocation, or a membrane budding/fusion event. GTP hydrolysis turns off the GTPase switch by converting it to the inactive GDP-bound conformation. The fundamental GTP hydrolysis mechanism by these GTPases has generated considerable interest over the last two decades but remained to be firmly established. This review provides an update on the catalytic mechanism with discussions on recent developments from kinetic, structural, and model studies in the context of the various GTP hydrolysis models proposed over the years.

The complexity of complexes in signal transduction

Many activities of cells are controlled by cell-surface receptors, which in response to ligands, trigger intracellular signaling reactions that elicit cellular responses. A hallmark of these signaling reactions is the reversible nucleation of multicomponent complexes, which typically begin to assemble when ligand-receptor binding allows an enzyme, often a kinase, to create docking sites for signaling molecules through chemical modifications, such as tyrosine phosphorylation. One function of such docking sites is the co-localization of enzymes with their substrates, which can enhance both enzyme activity and specificity. The directed assembly of complexes can also influence the sensitivity of cellular responses to ligand-receptor binding kinetics and determine whether a cellular response is up- or downregulated in response to a ligand stimulus. The full functional implications of ligand-stimulated complex formation are difficult to discern intuitively. Complex formation is governed by conditional interactions among multivalent signaling molecules and influenced by quantitative properties of both the components in a system and the system itself. Even a simple list of the complexes that can potentially form in response to a ligand stimulus is problematic because of the number of ways signaling molecules can be modified and combined. Here, we review the role of multicomponent complexes in signal transduction and advocate the use of mathematical models that incorporate detail at the level of molecular domains to study this important aspect of cellular signaling.

Tandem histone folds in the structure of the N-terminal segment of the ras activator Son of Sevenless

The Ras activator Son of Sevenless (Sos) contains a Cdc25 homology domain, responsible for nucleotide exchange, as well as Dbl/Pleckstrin homology (DH/PH) domains. We have determined the crystal structure of the N-terminal segment of human Sos1 (residues 1-191) and show that it contains two tandem histone folds. While the N-terminal domain is monomeric in solution, its structure is surprisingly similar to that of histone dimers, with both subunits of the histone "dimer" being part of the same peptide chain. One histone fold corresponds to the region of Sos that is clearly similar in sequence to histones (residues 91-191), whereas the other is formed by residues in Sos (1-90) that are unrelated in sequence to histones. Residues that form a contiguous patch on the surface of the histone domain of Sos are conserved from C. elegans to humans, suggesting a potential role for this domain in protein-protein interactions.

Role of the p66Shc isoform in insulin-like growth factor I receptor signaling through MEK/Erk and regulation of actin cytoskeleton in rat myoblasts

To investigate the role of Shc in IGF action and signaling in skeletal muscle cells, Shc protein levels were reduced in rat L6 myoblasts by stably overexpressing a Shc cDNA fragment in antisense orientation (L6/Shcas). L6/Shcas myoblasts showed marked reduction of the p66Shc protein isoform and no change in p52Shc or p46Shc proteins compared with control myoblasts transfected with the empty vector (L6/Neo). When compared with control, L6/Shcas myoblasts demonstrated 3-fold increase in Erk-1/2 phosphorylation under basal conditions and blunted Erk-1/2 stimulation by insulin-like growth factor I (IGF-I), in the absence of changes in total Erk-1/2 protein levels. Increased basal Erk-1/2 activation was paralleled by a greater proportion of phosphorylated Erk-1/2 in the nucleus of L6/Shcas myoblasts in the absence of IGF-I stimulation. The reduction of p66Shc in L6/Shcas myoblasts resulted in marked phenotypic abnormalities, such as rounded cell shape and clustering in islets or finger-like structures, and was associated with impaired DNA synthesis in response to IGF-I and lack of terminal differentiation into myotubes. In addition, L6/Shcas myoblasts were characterized by complete disruption of actin filaments and cell cytoskeleton. Treatment of L6/Shcas myoblasts with the MEK inhibitor PD98059 reduced the abnormal increase in Erk-1/2 activation to control levels and restored the actin cytoskeleton, re-establishing the normal cell morphology. Thus, the p66Shc isoform exerts an inhibitory effect on the mitogen-activated protein kinase signaling pathway in rodent myoblasts, which is necessary for maintenance of IGF responsiveness of the MEK/Erk pathway and normal cell phenotype.

The Cytoplasmic Tyrosine Kinase Pyk2 as a Novel Effector of Fibroblast Growth Factor Receptor 3 Activation

Activating mutations within fibroblast growth factor receptor 3 (FGFR3), a receptor tyrosine kinase, are responsible for human skeletal dysplasias including achondroplasia and the neonatal lethal syndromes thanatophoric dysplasia types I and II. Several of these same FGFR3 mutations have also been identified somatically in human cancers, including multiple myeloma, bladder carcinoma, and cervical cancer. The molecular pathways exploited by FGFR3 to stimulate abnormal proliferation during neoplasia are unclear. The nonreceptor protein-tyrosine kinase Pyk2 (proline-rich tyrosine kinase 2) has been shown previously to regulate apoptosis in multiple myeloma cells. Here we describe a novel interaction between FGFR3 and Pyk2, mediated by the juxtamembrane domain of FGFR3 and the kinase domain of Pyk2. Within the FGFR family, Pyk2 also interacted significantly with FGFR2. Overexpression of Pyk2 alone led to its spontaneous activation and tyrosine phosphorylation, resulting in activation of Stat5B, indicated by the reporter GFP-Stat5B. These effects were completely dependent upon Tyr(402), the autophosphorylation site of Pyk2, which allows recruitment of Src family members for further activating phosphorylations at other sites on Pyk2. In the presence of activated FGFR3, the activation of Pyk2 itself became independent of Tyr(402), indicating that FGFR3 activation circumvents the requirement for c-Src recruitment at Tyr(402) of Pyk2. We also examined the role of the tyrosine phosphatase Shp2 in antagonizing Pyk2 activation. Taken together, these results suggest that signaling pathways regulated by FGFR3 may converge with Pyk2-dependent pathways to provide maximal activation of Stat5B.

A potential role of 15-deoxy-delta(12,14)-prostaglandin J2 for induction of human articular chondrocyte apoptosis in arthritis

The cyclopentenone prostaglandin (PG) J2 is formed within the cyclopentenone ring of the endogenous prostaglandin PG D2 by a nonenzymatic reaction. The PG J family is involved in mediating various biological effects including the regulation of cell cycle progression and inflammatory responses. Here we demonstrate the potential role of 15-deoxy-Delta(12,14)-prostaglandin J2 (15d-PG J2) in human articular chondrocyte apoptosis. 15d-PG J2 was released by human articular chondrocytes and found in joint synovial fluids taken from osteoarthritis or rheumatoid arthritis patients. Proinflammatory cytokines such as interleukin-1beta (IL-1beta) and tumor necrosis factor-alpha (TNF-alpha) up-regulated chondrocyte release of 15d-PG J2. PG D2 synthase mRNA expression was up-regulated by IL-1beta, TNF-alpha, or nitric oxide. 15d-PG J2 induced apoptosis of chondrocytes from osteoarthritis or rheumatoid arthritis patients as well as control nonarthritic subjects in a time- and dose-dependent manner and in a peroxisome proliferator-activated receptor gamma-dependent manner. Peroxisome proliferator-activated receptor gamma expression was up-regulated by IL-1beta and TNF-alpha. Inhibition of NF-kappaB, and the activation of p38 MAPK were also found to be involved in 15d-PG J2-induced chondrocyte apoptosis. Such signal pathways led to the activation of the downstream pro-apoptotic molecule p53 and caspase cascades. Together, these results suggest that 15d-PGJ2 may play an important role in the pathogenesis of arthritic joint destruction via a regulation of chondrocyte apoptosis.

Grb2 SH2 domain-binding peptide analogs as potential anticancer agents

The growth factor receptor-bound protein 2 (Grb2) plays an important role in the Ras signaling pathway. Several proteins were found to be overexpressed by oncogenes in the Ras signaling pathway, rendering Grb2 a potential target for the design of antitumor agents. Blocking the interaction between the phosphotyrosine-containing activated receptor and the Src-homology 2 (SH2) domain of Grb2 thus constitutes an important strategy for the development of potential anticancer agents. X-ray, NMR structural investigations, and molecular modeling studies have provided the target structure of Grb2 SH2 domain-alone or complexed with a phosphotyrosine-containing peptide-which is useful for the structure-based design of peptides or peptidomimetics with high affinity for the Grb2 SH2 domain. We review here the variety of approaches to Grb2 SH2 pepide inhibitors developed with the aim of interrupting Grb2 recognition. Inhibitory effects of peptide analogs on the Grb2 SH2 domain and their binding affinities for Grb2 SH2 were determined by ELISA, cell-based assays, or Surface Plasman Resonance (SPR) technology. Results of theses studies provide important information for further modifications of lead peptides, and should lead to the discovery of potent peptides as anticancer agents.

Induction of mitogenic signalling in the 1LN prostate cell line on exposure to submicromolar concentrations of cadmium+

Cadmium exposure increases the risk of prostate cancer. We now describe the effects of Cd2+ on signalling and proliferation in 1LN prostate cells. Cd2+ increased [3H]thymidine uptake and cell number twofold. Cd2+ elevated intracellular IP3, cytosolic-free Ca2+, phosphorylated MEK1/2, ERK1/2, p38 MAPK and JNK two- to threefold. Increased PDK1 and phosphorylation of the 85-kDa regulatory subunit of PI 3-kinase, Akt and p70s6k were also observed. Cd2+ treatment increased transcription factors NFkappaB and CREB, and the expression of c-fos and c-myc. Cd2+-induced increased uptake of [3H]thymidine was abolished by translational and transcriptional inhibitors, and Ca2+ channel blockers. Inhibition of phospholipase C and of Ca2+ binding to IP3 receptors inhibited Cd2+-induced DNA synthesis as did inhibition of tyrosine kinases, protein kinase C, PI 3-kinase, farnesyl transferase, MEK1/2, ERK1/2 and p38MAPK. Thus signalling events, which are triggered on exposure of 1LN cells to submicromolar concentrations of Cd2+, induce increased proliferation of these cells.

Molecular mechanisms of renal tissue repair in survival from acute renal tubule necrosis: role of ERK1/2 pathway

Our earlier studies with S-(1,2-dichlorovinyl)-L-cysteine (DCVC) showed that prior administration of a low priming dose of 15 mg/kg, i.p. to mice, given 72 hours before administration of a normally lethal dose of DCVC (75 mg/kg, i.p.) led to renal tubule necrosis, however sustained renal tubule regeneration was observed and these mice recovered from renal failure and survived. The objective of the present study was to investigate the role of extracellular signal-regulated kinase (ERK) pathway in this autoprotection model. Following the priming dose of DCVC, IL-6 protein and mRNA increased markedly as early as 1 hour after dosing, peaking at 3 hours with a 1.5-fold increase in plasma. Immunocytochemistry on kidney sections using specific antibodies against TGF-alpha, HB-EGF, EGFr, IGF-1Rbeta, Grb-2, and phospho-p44/42 MAP kinase (ERK1/2) revealed a significantly higher staining of these molecules 3 to 72 hours after dosing, indicating up regulation of the ERK pathway. Following a lethal dose of DCVC (75 mg/kg) the early increase in these signaling molecules was not sustained, being markedly reduced 24 and 36 hours after dosing, leading to inhibition of S-phase DNA synthesis, cell division and renal tubule repair. In contrast, prior treatment with a low dose of DCVC, followed by a high dose led to a sustained stimulation of the renal ERK pathway, renal tubule regeneration and recovery from acute renal failure. These results suggest that a sustained activation of the ERK1/2 pathway may be a key factor in enabling a continued renal tubule repair and hence protection from the progressive phase of DCVC-induced acute renal tubular necrosis in the mouse.

Dystroglycan, a scaffold for the ERK-MAP kinase cascade

Dystroglycan is an important cell adhesion receptor linking the actin cytoskeleton, via utrophin and dystrophin, to laminin in the extracellular matrix. To identify adhesion-related signalling molecules associated with dystroglycan, we conducted a yeast two-hybrid screen and identified mitogen-activated protein (MAP) kinase kinase 2 (MEK2) as a beta-dystroglycan interactor. Pull-down experiments and localization studies substantiated a physiological link between beta-dystroglycan and MEK and localized MEK with dystroglycan in membrane ruffles. Moreover, we also identified active extracellular signal-regulated kinase (ERK), the downstream kinase from MEK, as another interacting partner for beta-dystroglycan and localized both active ERK and dystroglycan to focal adhesions in fibroblast cells. These studies suggest a role for dystroglycan as a multifunctional adaptor or scaffold capable of interacting with components of the ERK-MAP kinase cascade including MEK and ERK. These findings have important implications for our understanding of the role of dystroglycan in normal cellular processes and in disease states such as muscular dystrophy.

Estrogens in the Nervous System: Mechanisms and Nonreproductive Functions

The past decade has witnessed a growing interest in estrogens and their activity in the central nervous system, which was originally believed to be restricted to the control of reproduction. It is now well accepted that estrogens modulate the activity of all types of neural cells through a multiplicity of mechanisms. Estrogens, by binding to two cognate receptors ERalpha and ERbeta, may interact with selected promoters to initiate the synthesis of target proteins. Alternatively, the hormone receptor complex may interfere with intracellular signaling at both cytoplasmic and nuclear levels. The generation of cellular and animal models, combined with clinical and epidemiological studies, has allowed us to appreciate the neurotrophic and neuroprotective effects of estrogens. These findings are of major interest because estradiol might become an important therapeutic agent to maintain neural functions during aging and in selected neural diseases.

A novel MET-interacting protein shares high sequence similarity with RanBPM, but fails to stimulate MET-induced Ras/Erk signaling

MET is a receptor protein tyrosine kinase for hepatocyte growth factor, a multifunctional cytokine controlling cell growth, morphogenesis, and motility. In our previous study, RanBPM/RanBP9, whose name originated from its ability to interact with Ran, was identified as a MET-interacting protein. RanBPM/RanBP9 activates the Ras/Erk signaling pathway by serving as an adaptor protein of MET to recruit Sos. In this study, we identify a protein sharing a high amino acid sequence identity with RanBPM/RanBP9, especially in its SPRY domain, the region responsible for MET binding. This protein lacks the N-terminal poly-proline and poly-glutamine (Poly-PQ) stretch present in RanBPM/RanBP9 and has less homology with RanBPM/RanBP9 in its mid-region. We subsequently named this protein RanBP10 after demonstrating its interaction with Ran. We show that, like RanBPM/RanBP9, RanBP10 interacts with the tyrosine kinase domain of MET via its SPRY domain and these two proteins can compete with each other to bind to MET. Interestingly, unlike RanBPM/RanBP9, overexpression of RanBP10 cannot induce Erk1/2 phosphorylation and serum response element-luciferase (SRE-LUC) reporter gene expression. More importantly, co-transfection of RanBPM/RanBP9 and RanBP10 significantly represses SRE-LUC reporter gene expression induced by overexpression of RanBPM/RanBP9. Additional binding assays demonstrate that RanBP10 fails to interact with Sos, which explains its inability to activate the Ras/Erk pathway. Furthermore, we show that the N-terminus of RanBPM/RanBP9 with the Poly-PQ stretch is required for recruiting Sos and a truncated RanBPM/RanBP9 lacking this region fails to recruit Sos, indicating that the functional difference between RanBP10 and RanBPM/RanBP9 lies in their sequence difference in their N-termini.

Insulin signalling: the role of insulin receptor substrate 1

The insulin receptor is a ligand-activated tyrosine kinase that phosphorylates its major substrate protein, insulin receptor substrate 1 (IRS1), at multiple sites. Tyrosine-phosphorylated IRS1 then serves as a docking/effector protein for at least four Src homology 2 (SH2)-domain proteins involved in signal transduction. This initial step in signalling distinguishes the insulin receptor from other receptor tyrosine kinases, which directly bind several SH2-domain proteins, and establishes IRS1 as a founding member of a group of proteins whose function is to link activated tyrosine kinases to SH2-domain proteins.

Requirement for Ras guanine nucleotide releasing protein 3 in coupling phospholipase C-gamma2 to Ras in B cell receptor signaling

Two important Ras guanine nucleotide exchange factors, Son of sevenless (Sos) and Ras guanine nucleotide releasing protein (RasGRP), have been implicated in controlling Ras activation when cell surface receptors are stimulated. To address the specificity or redundancy of these exchange factors, we have generated Sos1/Sos2 double- or RasGRP3-deficient B cell lines and determined their ability to mediate Ras activation upon B cell receptor (BCR) stimulation. The BCR requires RasGRP3; in contrast, epidermal growth factor receptor is dependent on Sos1 and Sos2. Furthermore, we show that BCR-induced recruitment of RasGRP3 to the membrane and the subsequent Ras activation are significantly attenuated in phospholipase C-gamma2-deficient B cells. This defective Ras activation is suppressed by the expression of RasGRP3 as a membrane-attached form, suggesting that phospholipase C-gamma2 regulates RasGRP3 localization and thereby Ras activation.

Cytoskeletal reorganization dependence of signaling by the gonadotropin-releasing hormone receptor

Activation of classical G protein-coupled receptors (GPCRs) like the mammalian gonadotropin-releasing hormone receptor (GnRHR) typically stimulates heterotrimeric G protein molecules that subsequently activate downstream effectors. Receptor activation of heterotrimeric G protein pathways primarily controls intermediary cell metabolism by elevation or diminution of soluble cytoplasmic second messenger molecules. We have demonstrated here that stimulation of the GnRHR also results in a dramatic change in both cell adhesion and superstructural morphology. Gonadotropin-releasing hormone (GnRH) receptor activation rapidly increases the capacity of HEK293 cells expressing the GnRHR to remain matrix-adherent in the face of fluid insults. Coinciding with this profound elevation in matrix adherence, we demonstrated a GnRH-induced alteration in both cell morphology and the de novo generation of polymerized actin structures. GnRH induction of cytoskeletal remodeling was correlated with significant increases in the tyrosine phosphorylation status of a series of cytoskeletal associated proteins, e.g. focal adhesion kinase (FAK), c-Src, and microtubule-associated protein kinase (MAPK or ERK1/2). The activation of the distal downstream effector ERK1/2 was demonstrated to be sensitive to the disrupters of cytoskeletal rearrangement, cytochalasin D and latrunculin B. In addition to the sensitivity of ERKs to cytoskeletal integrity, GnRH-induced FAK and c-Src kinase activation were sensitive to these agents and the fibronectin-integrin antagonistic RGDS peptide. Activation of ERK was dependent on its protein-protein assembly with FAK and c-Src at focal adhesion complexes. Induction of the cell remodeling event leading to this signaling complex assembly occurred primarily via GnRHR activation of the monomeric G protein Rac but not RhoA. These findings demonstrated a clear divergence of GnRHR signaling via the Rac monomeric G protein focal adhesion signaling complex assembly and cytoskeletal remodeling independent of the classical heterotrimeric G protein-controlled phospholipase C-beta pathway.

A novel SHP-1/Grb2-dependent mechanism of negative regulation of cytokine-receptor signaling: contribution of SHP-1 C-terminal tyrosines in cytokine signaling

SHP-1, an src homology 2 (SH2) domain containing protein tyrosine phosphatase, functions as a negative regulator of signaling downstream of cytokine receptors, receptor tyrosine kinases and receptor complexes of the immune system. Dephosphorylation of receptors and/or receptor-associated kinases has been described as the mechanism for the function of SHP-1. Here we demonstrate a novel mechanism by which SHP-1 down-regulates the Janus kinase-2 (Jak2)/signal transducer and activator of transcription-5 (Stat5) pathway downstream of the prolactin receptor (PRLR) and the erythropoietin receptor (EPOR) in a catalytic activity-independent manner. Structural/functional analysis of SHP-1 defined the C-terminal tyrosine residues (Y278, Y303, Y538, Y566) within growth factor receptor-bound protein 2 (Grb-2) binding motif to be responsible for delivering the inhibitory effects. Our results further indicate that these tyrosine residues, via recruitment of the adaptor protein Grb-2, are required for targeting the inhibitory protein suppressor of cytokine signaling-1 (SOCS-1) to Jak2 kinase. Finally, loss of SOCS-1 expression in SOCS-1(-/-) mouse embryonic fibroblast (MEF) cells led to attenuation in SHP-1 function to down-regulate PRL-induced Stat5 activation. All together, our results indicate that SHP-1 inhibits PRLR and EPOR signaling by recruitment and targeting of SOCS-1 to Jak2, highlighting a new mechanism of SHP-1 regulation of cytokine-receptor signaling.

Rac regulates cardiovascular superoxide through diverse molecular interactions: more than a binary GTP switch

The small G protein Rac has been implicated in multiple cardiovascular processes. Rac has two major functions: 1) it regulates the organization of the actin cytoskeleton, and 2) it controls the activity of the key enzyme complex NADPH oxidase to control superoxide production in both phagocytes and nonphagocytic cells. In phagocytes, superoxide derived from NADPH has a bactericidal function, whereas Rac-derived superoxide in the cardiovascular system has a diverse array of functions that have recently been a subject of intense interest. Rac is differentially activated by cellular receptors coupled to distinct Rac-activating adapter molecules, with each leading to pathway-specific arrays of downstream effects. Thus it may be important to investigate not just whether Rac is activated but also where, how, and for what effector. An understanding of the biochemical functions of Rac and its effectors lays the groundwork for a dissection of the exact array of effects produced by Rac in common cardiovascular processes, including cardiac and vascular hypertrophy, hypertension, leukocyte migration, platelet biology, and atherosclerosis. In addition, investigation of the spatiotemporal regulation of both Rac activation and consequent superoxide generation may produce new insights into the development of targeted antioxidant therapies for cardiovascular disease and enhance our understanding of important cardiovascular drugs, including angiotensin II antagonists and statins, that may depend on Rac modulation for their effect.

The four distal tyrosines are required for LAT-dependent signaling in FcepsilonRI-mediated mast cell activation

The linker for activation of T cells (LAT) is an adaptor protein critical for Fc epsilon RI-mediated mast cell activation. LAT is a substrate of the tyrosine kinases activated after TCR and Fc epsilon RI engagement. After phosphorylation of the cytosolic domain of LAT, multiple signaling molecules such as phospholipase C-gamma1, Grb2, and Gads associate with phosphorylated LAT via their SH2 domains. The essential role of the four distal tyrosines in TCR-mediated signaling and T cell development has been demonstrated by experiments using LAT-deficient cell lines and genetically modified mice. To investigate the role of these four tyrosines of LAT in Fc epsilon RI-mediated mast cell activation, bone marrow-derived mast cells from LAT-deficient mice were infected with retroviral vectors designed to express wild-type or mutant LAT. Examination of bone marrow-derived mast cells expressing various tyrosine to phenylalanine mutants in LAT demonstrates a differential requirement for these different binding sites. In these studies, assays of biochemical pathways, degranulation, and cytokine and chemokine release were performed. Finally, the role of these tyrosines was also evaluated in vivo using genetically modified animals. Deletion of all four distal tyrosines, and in particular, loss of the primary phospholipase C-gamma-binding tyrosine had a significant effect on antigen-induced histamine release.

Jun N-terminal kinase pathway enhances signaling of monocytic differentiation of human leukemia cells induced by 1,25-dihydroxyvitamin D3

Recent studies revealed that the MEK/ERK module of the mitogen-activated protein kinase (MAPK) signaling cascades is up-regulated in the early stages of 1alpha,25-dihydroxyvitamin D(3) (1,25D(3))-induced monocytic differentiation of human leukemia cells HL60. In the present study, we investigated whether another MAPK module, the JNK pathway, also participates in this form of differentiation. We found that the dependence on the concentration of the inducer, the vitamin-hormone 1,25D(3), in two types of human leukemia cells, HL60 and U937, and the kinetics of monocytic differentiation in HL60 cells, parallel the degree of the activation of the JNK pathway. A blockade of JNK signaling by a stable expression of dominant negative (dn) JNK1 mutant in U937 cells resulted in reduced c-jun phosphorylation, and the differentiation of these cells was markedly decreased. Similarly, inhibition of JNK1 and JNK2 activities by the selective inhibitor SP600125 led to both dose-dependent reduction of c-jun and ATF-2 phosphorylation, and of the differentiation of HL60 cells. In addition, we found that JNK activity is essential for the AP-1 DNA binding induced by 1,25D(3) in HL60 and U937 cells. The results indicate that in cultured human leukemia cells, the JNK pathway participates in the induction of monocytic differentiation by 1,25D(3), probably by activating the AP-1 transcription factor.

Allosteric inhibition of protein--DNA complexes by polyamide--intercalator conjugates

The sequence-specific inhibition of essential protein-DNA contacts in the promoter of a gene is a central issue for the regulation of gene expression by chemical methods. Hairpin polyamides have been shown to inhibit protein-DNA complexes in some but not all cases. For example, polyamides co-occupy the same DNA sequence in the minor groove in the presence of major-groove binding bZip proteins. Four hairpin polyamide-acridine conjugates were synthesized and shown to bind the minor groove of DNA with high affinity in a sequence-specific manner. The polyamide-acridine conjugates were shown to unwind DNA (phi = 14-15 degrees), evidence for intercalation by the acridine moiety. Importantly, the polyamide-intercalator conjugates, which combine sequence-specific groove binding with proximal local unwinding, inhibit major-groove DNA binding by the GCN4 bZip protein. This class of DNA binding molecules creates a sequence-specific allosteric change in DNA structure and has the potential to be a general inhibitor of transcription factor binding independent of the specific protein-DNA structure.

SH2 and PTB domains in tyrosine kinase signaling

Intracellular signaling pathways that involve protein tyrosine kinases (PTKs) are critical for the control of most cellular processes. Dysfunctions in PTKs, or in the signaling pathways that they regulate, result in a variety of diseases such as cancer, diabetes, immune deficiency, and many others. SH2 (Src homology region 2) and PTB (phosphotyrosine-binding) domains are small protein modules that mediate protein-protein interactions involved in many signal transduction pathways. Both domains were initially identified as modules that recognize phosphorylated tyrosines in receptor tyrosine kinases and other signaling proteins. Subsequent studies have shown that, while binding of SH2 domains to their target proteins is strictly regulated by tyrosine phosphorylation, most PTB domains actually bind to their (nonphosphorylated) targets constitutively. The functions of SH2 and PTB domains include targeting of their host proteins to different cellular compartments, assembly of key components of signaling pathways in response to extracellular signals, and the control of autoinhibition, activation and dimerization of their host proteins. The information flow from the cell surface to different cellular compartments to regulate the cell cycle, cell shape and movement, cell proliferation, differentiation and cell survival are all controlled in part by SH2 and PTB domains that can recognize phosphotyrosine or particular amino acid sequence motifs in a wide variety of target molecules.

Kos1, a nonreceptor tyrosine kinase that suppresses Ras signaling

Kinase of embryonic stem cells (Kos1), a nonreceptor protein tyrosine kinase (NRPTK), was identified and cloned from differentiating murine embryonic stem cells. Kos1 is localized on mouse chromosome 11 that corresponds to human chromosome 17p13.1 and is homologous to Tnk1, Ack1 and Ack2, making it a new member of the Ack family of NRPTKs. Kos1 is a ubiquitously expressed 47-kDa protein with autotyrosine kinase activity that is developmentally regulated during embryogenesis. Kos1 is also upregulated following IL3 withdrawal from factor-dependent murine NSF/N1.H7 cells that undergo apoptosis, suggesting a role in growth inhibition. Stable overexpression of Kos1 inhibits growth of NIH 3T3 cells, while the kinase-dead Kos1(CN) promotes cell growth in both liquid culture and soft agar. In addition, forced expression of Kos1 inhibits Ras activity in an indirect mechanism that results in the downregulation of the Ras-Raf1-MAPK growth pathway. Furthermore, overexpression of Kos1 in NCI-H82 lung cancer cells that express oncogenic Ha-Ras(G12V) inhibits cell growth under reduced serum (0.5%) conditions in close association with the upregulation of the Ras inhibitor, Rap1A. Collectively, these data support a negative regulatory role for Kos1 in regulating the Ras-Raf1-MAPK growth pathway by a mechanism that requires its autotyrosine kinase activity.

p27Kip1 inhibition of GRB2-SOS formation can regulate Ras activation

p27(Kip1) (p27) is often inappropriately downregulated in aggressive human cancers. Although p27 can inhibit cyclin-dependent kinases (CDKs), low p27 does not always correlate with increased CDK activity. Furthermore, cells derived from p27(-/-) mice respond to antimitogens, maintain restriction point control, and do not deregulate CDKs. Thus, disruption of a p27 function other than CDK inhibition may contribute to the disease state. A yeast two-hybrid screen identified growth factor receptor-bound protein 2 (GRB2) as a p27 binding partner. We now demonstrate that p27 can inhibit GRB2 function by blocking its association with the guanine nucleotide exchange factor SOS. Endogenous p27 is rapidly exported from the nucleus to the cytoplasm in response to mitogen stimulation, where it binds GRB2 concomitant with a decrease in GRB2-associated SOS. As predicted, mitogen-stimulated p27(-/-) cells maintained their GRB2-SOS complexes for significantly longer. The Ras/mitogen-activated protein kinase pathway does not appear to be deregulated in cells lacking p27 despite excess GRB2-SOS, suggesting that additional control mechanisms are present. A transient-transfection approach was employed to show that p27 can inhibit Ras activation by targeting GRB2 and further revealed that the CDK and GRB2 inhibitory functions of p27 are separable and distinct. Thus, p27 downregulation may compromise control of Ras, one of the most common oncogenic events in human cancer.

Four-dimensional organization of protein kinase signaling cascades: the roles of diffusion, endocytosis and molecular motors

Extracellular signals received by membrane receptors are processed, encoded and transferred to the nucleus via phosphorylation and spatial relocation of protein members of multiple component pathways, such as mitogen activated protein kinase (MAPK) cascades. The receptor-induced membrane recruitment of the cytoplasmic protein SOS results in the activation of the Ras/MAPK cascade. It has been suggested that the membrane recruitment of signaling proteins causes an increase in the diffusion-limited rates. We have recently shown that this increase is too small to be responsible for enhanced signal transduction. Instead we demonstrate that the function of membrane localization is to increase the number (or average lifetime) of complexes between signaling partners. A hallmark of signaling pathways is the spatial separation of activation and deactivation mechanisms; e.g. a protein can be phosphorylated at the cell surface by a membrane-bound kinase and dephosphorylated in the cytosol by a cytosolic phosphatase. Given the measured values of protein diffusion coefficients and of phosphatase and kinase activities, the spatial separation is shown to result in precipitous phospho-protein gradients. When information transfer is hampered by slow protein diffusion and rapid dephosphorylation, phospho-protein trafficking within endocytic vesicles may be an efficient way to deliver messages to physiologically relevant locations. The proposed mechanism explains recent observations that various inhibitors of endocytosis can inhibit MAPK activation. Additional mechanisms facilitating the relay of signals from cell-surface receptors to the nucleus can involve the assembly of protein kinases on a scaffolding protein and active transport of signaling complexes by molecular motors. We also discuss long-range signaling within a cell, such as survival signaling in neurons. We hypothesize that ligand-independent waves of receptor activation or/and traveling waves of phosphorylated kinases emerge to spread the signals over long distances.

Ligand-induced changes in dynamics in the RT loop of the C-terminal SH3 domain of Sem-5 indicate cooperative conformational coupling

We report the effects of peptide binding on the (15)N relaxation rates and chemical shifts of the C-SH3 of Sem-5. (15)N spin-lattice relaxation time (T(1)), spin-spin relaxation time (T(2)), and ((1)H)-(15)N NOE were obtained from heteronuclear 2D NMR experiments. These parameters were then analyzed using the Lipari-Szabo model free formalism to obtain parameters that describe the internal motions of the protein. High-order parameters (S(2) > 0.8) are found in elements of regular secondary structure, whereas some residues in the loop regions show relatively low-order parameters, notably the RT loop. Peptide binding is characterized by a significant decrease in the (15)N relaxation in the RT loop. Concomitant with the change in dynamics is a cooperative change in chemical shifts. The agreement between the binding constants calculated from chemical shift differences and that obtained from ITC indicates that the binding of Sem-5 C-SH3 to its putative peptide ligand is coupled to a cooperative conformational change in which a portion of the binding site undergoes a significant reduction in conformational heterogeneity.

Phosphorylation of the Ras-GRF1 exchange factor at Ser916/898 reveals activation of Ras signaling in the cerebral cortex

The Ras-GRF1 exchange factor, which is regulated by increases in intracellular calcium and the release of G beta gamma subunits from heterotrimeric G proteins, plays a critical role in the activation of neuronal Ras. Activation of G protein-coupled receptors stimulates an increase in the phosphorylation of Ras-GRF1 at certain serine residues. The first of these sites to be identified, Ser(916) in the mouse sequence (equivalent to Ser(898) in the rat sequence), is required for full activation of the Ras exchange factor activity of Ras-GRF1 by muscarinic receptors. We demonstrate here that Ras-GRF1 is highly expressed in rat brain compared with the Sos exchange factor and that there is an increase in incorporation of (32)P into Ser(898) of brain Ras-GRF1 following activation of protein kinase A. Phosphorylation of Ras-GRF1 at Ser(916) is also required for maximal induction of Ras-dependent neurite outgrowth in PC12 cells. A novel antibody (termed 2152) that selectively recognizes Ras-GRF1 when it is phosphorylated at Ser(916/898) confirmed the regulated phosphorylation of Ras-GRF1 by Western blotting in both model systems of transfected COS-7 and PC12 cells and also of the endogenous protein in rat forebrain slices. Indirect confocal immunofluorescence of transfected PC12 cells using antibody 2152 demonstrated reactivity only under conditions in which Ras-GRF1 was phosphorylated at Ser(916/898). Confocal immunofluorescence of cortical slices of rat brain revealed widespread and selective phosphorylation of Ras-GRF1 at Ser(898). In the prefrontal cortex, there was striking phosphorylation of Ras-GRF1 in the dendritic tree, supporting a role for Ras activation and signal transduction in neurotransmission in this area.

p21-activated kinase 1 (PAK1) interacts with the Grb2 adapter protein to couple to growth factor signaling

A variety of intracellular signaling pathways are linked to cell surface receptor signaling through their recruitment by Src homology 2 (SH2)/SH3-containing adapter molecules. p21-activated kinase 1 (PAK1) is an effector of Rac/Cdc42 GTPases that has been implicated in the regulation of cytoskeletal dynamics, proliferation, and cell survival signaling. In this study, we describe the specific interaction of PAK1 with the Grb2 adapter protein both in vitro and in vivo. We identify the site of this interaction as the second proline-rich SH3 binding domain of PAK1. Stimulation of the epidermal growth factor receptor (EGFR) in HaCaT cells enhances the level of EGFR-associated PAK1 and Grb2, although the PAK1-Grb2 association is itself independent of this stimulation. A cell-permeant TAT-tagged peptide encompassing the second proline-rich SH3 binding domain of PAK1 simultaneously blocked Grb2 and activated EGFR association with PAK1, in vitro and in vivo, indicating that Grb2 mediates the interaction of PAK1 with the activated EGFR. Blockade of this interaction decreased the epidermal growth factor-induced extension of membrane lamellae. Thus Grb2 may serve as an important mechanism for linking downstream PAK signaling to various upstream pathways.