Activation of Trk neurotrophin receptors in the absence of neurotrophins

Neurotrophins regulate neuronal cell survival and synaptic plasticity through activation of Trk receptor tyrosine kinases. Binding of neurotrophins to Trk receptors results in receptor autophosphorylation and downstream phosphorylation cascades. Here, we describe an approach to use small molecule agonists to transactivate Trk neurotrophin receptors. Activation of TrkA receptors in PC12 cells and TrkB in hippocampal neurons was observed after treatment with adenosine, a neuromodulator that acts through G protein-coupled receptors. These effects were reproduced by using the adenosine agonist CGS 21680 and were counteracted with the antagonist ZM 241385, indicating that this transactivation event by adenosine involves adenosine 2A receptors. The increase in Trk activity could be inhibited by the use of the Src family-specific inhibitor, PP1, or K252a, an inhibitor of Trk receptors. In contrast to other G protein-coupled receptor transactivation events, adenosine used Trk receptor signaling with a longer time course. Moreover, adenosine activated phosphatidylinositol 3-kinase/Akt through a Trk-dependent mechanism that resulted in increased cell survival after nerve growth factor or brain-derived neurotrophic factor withdrawal. Therefore, adenosine acting through the A(2A) receptors exerts a trophic effect through the engagement of Trk receptors. These results provide an explanation for neuroprotective actions of adenosine through a unique signaling mechanism and raise the possibility that small molecules may be used to elicit neurotrophic effects for the treatment of neurodegenerative diseases.     

Activation of Trk neurotrophin receptors by glucocorticoids provides a neuroprotective effect

Glucocorticoids (GCs) display both protective and destructive effects in the nervous system. In excess, GCs produce neuronal damage after stress or brain injury; however, the neuroprotective effects of adrenal steroids also have been reported. The mechanisms that account for the positive actions are not well understood. Here we report that GCs can selectively activate Trk receptor tyrosine kinases after in vivo administration in the brain and in cultures of hippocampal and cortical neurons. Trk receptors are normally activated by neurotrophins, such as NGF and brain-derived neurotrophic factor, but the activation of Trk receptors by GCs does not depend on increased production of neurotrophins. Other tyrosine kinase receptors, such as EGF and FGF receptors, were not activated by GCs. The ability of GCs to increase Trk receptor activity resulted in the neuroprotection of neurons deprived of trophic support and could be modulated by steroid-converting enzymes. Pharmacological and shRNA experiments indicate that Trk receptor activation by GCs depends on a genomic action of the GC receptor. The ability of GCs to promote Trk receptor activity represents a molecular mechanism that integrates the actions of GCs and neurotrophins.     

神经营养因子受体Trk小分子激动剂研究进展

神经营养因子作用于Trk受体酪氨酸激酶后能激活细胞内磷酸肌醇3-激酶、细胞外信号调节激酶等信号通路,促进神经元的存活和分化。开发非肽类小分子Trk激动剂以及神经营养因子模拟物,能够在激动Trk受体信号转导的同时避免神经营养因子的诸多缺点,为治疗神经系统疾病提供了新的治疗思路。     

Epidermal growth factor-induced activation of the insulin-like growth factor I receptor in rat hepatocytes

Insulin-like growth factor I (IGF-I) plays a critical role in the induction of cell cycle progression and survival in many cell types. However, there is minimal IGF-I binding to hepatocytes, and a role for IGF-I in hepatocyte signaling has not been elucidated. The dynamics of IGF-I receptor (IGF-IR) activation were examined in freshly isolated rat hepatocytes. IGF-I did not activate the IGF-IR. However, des(1-3)IGF-I, which weakly binds IGF binding protein-3 (IGFBP-3), induced IGF-IR phosphorylation. IGFBP-3 surface coating was identified by confocal immunofluorescence microscopy. In contrast with the inactivity of IGF-I, epidermal growth factor (EGF) induced the tyrosine phosphorylation of the IGF-IR in parallel with EGF receptor phosphorylation. Transactivation of the IGF-IR by EGF was inhibited by tyrphostin I-Ome-AG538, a tyrosine kinase inhibitor with high specificity for the IGF-IR. Src kinase inhibitors pyrazolopyrimidine PP-1 and PP-2 inhibited transactivation of the IGF-IR by EGF. EGF stimulated the tyrosine phosphorylation of Src, and induced its association with the IGF-IR. EGF-induced phosphorylations of insulin-related substrate (IRS)-1, IRS-2, Akt, and p42/44 mitogen-activated protein kinases (MAPKs) were inhibited variably by I-Ome-AG538. In conclusion, the data show an EGF- and Src-mediated transactivation pathway for IGF-IR activation in hepatocytes, and indicate a role for the IGF-IR in hepatocyte intracellular signaling. The findings also show a role for IGFBP-3 in the inhibition of IGF-I signaling in hepatocytes.     

Sphingosine 1-phosphate transactivates the platelet-derived growth factor beta receptor and epidermal growth factor receptor in vascular smooth muscle cells

Sphingosine 1-phosphate (S1P) is a bioactive lipid generated during vascular injury that regulates cell growth, differentiation, survival, and motility via endothelial differentiation gene (EDG) family G protein-coupled receptors. Although several G protein-coupled receptor ligands transactivate receptor tyrosine kinases, such as the epidermal growth factor receptor (EGFR), S1P-stimulated receptor tyrosine kinase transactivation has not been well studied. We show that platelet-derived growth factor beta receptor (PDGFbetaR) and EGFR are tyrosine phosphorylated in response to S1P in rat aortic vascular smooth muscle cells (VSMCs). S1P-stimulated transactivation of PDGFbetaR and EGFR was mediated via Gi-coupled EDG receptors. S1P-stimulated transactivation of EGFR and PDGFbetaR was dependent on Src, reactive oxygen species, and cholesterol-rich membranes. A phosphoinositide 3-kinase-Akt pathway was activated by S1P and blocked by AG1296 and AG1478. Activation of extracellular signal-regulated kinase (ERK) 1 and ERK2 pathway by S1P was blocked only by AG1478. In Chinese hamster ovary cells that expressed exogenous EDG-1, activation of Akt and ERK1/2 in response to S1P was observed and was enhanced by coexpression of PDGFbetaR or EGFR. S1P-mediated VSMC proliferation was shown to be secondary to transactivation, because it was suppressed by AG1296 and AG1478. These data establish S1P as an important stimulus for EGFR and PDGFbetaR activation in VSMCs that may have important implications for the vessel response to injury.     

Transactivation of fetal liver kinase-1/kinase-insert domain-containing receptor by lysophosphatidylcholine induces vascular endothelial cell proliferation

Lysophosphatidylcholine (LPC), a major lipid component of oxidized low-density lipoprotein, is a bioactive lipid molecule involved in numerous biological processes including the progression of atherosclerosis. Recently orphan G protein-coupled receptors were identified as high-affinity receptors for LPC. Although several G protein-coupled receptor ligands transactivate receptor tyrosine kinases, LPC-stimulated transactivation of receptor tyrosine kinase has not yet been reported. Here we observed for the first time that LPC treatment of human umbilical vein endothelial cells (HUVECs) induces tyrosyl phosphorylation of vascular endothelial growth factor receptor 2 [fetal liver kinase-1/kinase-insert domain-containing receptor, Flk-1/KDR)]. Flk-1/KDR transactivation by LPC was inhibited by vascular endothelial growth factor receptor tyrosine kinase inhibitors, SU1498 and 4-[(4'-chloro-2'-fluoro) phenylamino]6,7-dimethoxyquinazoline (VTKi) in immunoprecipitation. Furthermore, we examined the effects of the Src family kinases inhibitors, herbimycin A and 4-amino-5-(4-chlorophenyl)-7-(t-butyl) pyrazolo[3,4-d] pyrimidine (PP2), on LPC-induced Flk-1/KDR transactivation. Results from Western blots, c-Src is involved in LPC-induced Flk-1/KDR transactivation because herbimycin A and PP2 inhibited this transactivation. Kinase-inactive (KI) Src transfection also inhibited LPC-induced Flk-1/KDR transactivation. In addition, results from Western blots, ERK1/2 and Akt, which are downstream effectors of Flk-1/KDR, were also activated by LPC, and this was inhibited by SU1498, VTKi, herbimycin A, PP2, and KI Src transfection in HUVECs. LPC-induced stimulation of HUVEC proliferation was shown to be secondary to transactivation because it was suppressed by SU1498, VTKi, herbimycin A, PP2, and KI Src transfection in dimethylthiazoldiphenyltetra-zoliumbromide assay. These findings suggest that LPC-induced Flk-1/KDR transactivation via c-Src may have important implications for the progression of atherosclerosis.     

Trk-signaling endosomes are generated by Rac-dependent macroendocytosis

Why neurotrophins and their Trk receptors promote neuronal differentiation and survival whereas receptor tyrosine kinases for other growth factors, such as EGF, do not, has been a long-standing question in neurobiology. We provide evidence that one difference lies in the selective ability of Trk to generate long-lived signaling endosomes. We show that Trk endocytosis is distinguished from the classical clathrin-based endocytosis of EGF receptor (EGFR). Although Trk and EGFR each stimulate membrane ruffling, only Trk undergoes both selective and specific macroendocytosis at ruffles, which uniquely requires the Rho-GTPase, Rac, and the trafficking protein, Pincher. This process leads to Trk-signaling endosomes, which are immature multivesicular bodies that retain Rab5. In contrast, EGFR endosomes rapidly exchange Rab5 for Rab7, thereby transiting into late-endosomes/lysosomes for degradation. Sustained endosomal signaling by Trk does not reflect intrinsic differences between Trk and EGFR, because each elicits long-term Erk-kinase activation from the cell surface. Thus, a population of stable Trk endosomes, formed by specialized macroendocytosis in neurons, provides a privileged endosome-based system for propagation of signals to the nucleus.     

Muscle-specific trk-related receptor with a kringle domain defines a distinct class of receptor tyrosine kinases.

Little is known about the signaling pathways by which motoneurons induce synapses on muscle fibers, and no receptors for synapse-inducing signals have yet been identified. Because several other inductive events in development are mediated by receptor tyrosine kinases (RTKs), and because phosphotyrosine staining within muscle fibers is concentrated at synaptic sites, one possibility is that synapse-inducing signals are transduced by a RTK within the muscle fiber. We have used PCR to search for tyrosine kinases within the electric organ of the electric ray Torpedo californica, since this tissue is homologous to muscle but is much more densely innervated and is therefore a rich source of synaptic molecules. We have isolated a RTK that is specifically expressed in electric organ and skeletal muscle. The kinase domain of this receptor is related to the trk family of neurotrophin receptors, but unlike any previously described receptor, the extracellular region of this Torpedo RTK contains a kringle domain close to the transmembrane domain.     

Ganglioside GM1 binds to the Trk protein and regulates receptor function.

Several lines of evidence have suggested that ganglioside GM1 stimulates neuronal sprouting and enhances the action of nerve growth factor (NGF), but its precise mechanism is yet to be elucidated. We report here that GM1 directly and tightly associates with Trk, the high-affinity tyrosine kinase-type receptor for NGF, and strongly enhances neurite outgrowth and neurofilament expression in rat PC12 cells elicited by a low dose of NGF that alone is insufficient to induce neuronal differentiation. The potentiation of NGF activity by GM1 appears to involve tyrosine-autophosphorylation of Trk, which contains intrinsic tyrosine kinase activity that has been localized to the cytoplasmic domain. In the presence of GM1 in culture medium, there is a > 3-fold increase in NGF-induced autophosphorylation of Trk as compared with NGF alone. We also found that GM1 could directly enhance NGF-activated autophosphorylation of immunoprecipitated Trk in vitro. Monosialoganglioside GM1, but not polysialogangliosides, is tightly associated with immunoprecipitated Trk. Furthermore, such tight association of GM1 with Trk appears to be specific, since a similar association was not observed with other growth factor receptors, such as low-affinity NGF receptor (p75NGR) and epidermal growth factor receptor (EGFR). Thus, these results strongly suggest that GM1 functions as a specific endogenous activator of NGF receptor function, and these enhanced effects appear to be due, at least in part, to tight association of GM1 with Trk.     

Growth hormone receptor: structure function relationships

BACKGROUND: The growth hormone receptor (GHR) was the first class 1 cytokine receptor to be cloned, and it has been studied intensively. The crystal structures of the bound and unbound forms have been solved and the energetic contributions of residues involved in the binding interaction have been quantified. Two receptor subunits bind to opposite sides of the hormone through site 1 and site 2, and a third interaction occurs between receptors in the lower β-sandwich module at site 3. All three interactions are required for receptor activation, which was thought to be a consequence of hormone-induced receptor dimerization. However, substantial data support the existence of a constitutive receptor dimer that interacts via the transmembrane domain (TMD), with receptor activation triggered by a hormone-induced conformational change. Mutagenesis studies and crystal structure data indicate that receptor activation involves a relative rotation and scissor movement of subunits to activate the associated tyrosine kinase, Janus kinase 2 (JAK2). We have recently reported that a second tyrosine kinase, an Src-family kinase, also associates constitutively with the receptor and activates the Ras-extracellular-signal-regulated kinase pathway. Activation of this kinase requires a conformational change in a loop of the lower sandwich module, and a different orientation of the TMDs than needed for JAK2 activation. CONCLUSIONS: This structural difference could allow differential activation of these two kinases by GH analogues such as I179M human GH.     

Progression of prostate cancer by synergy of AKT with genotropic and nongenotropic actions of the androgen receptor

Classic work by Huggins and Hodges demonstrated that human prostate cancer regresses dramatically during antihormonal therapy but recurs frequently with androgen independence. Perturbations in the androgen receptor (AR) and PTEN-AKT signaling axes are significantly correlated with the progression of prostate cancer. Genetic alterations of the AR cause receptor hypersensitivity, promiscuity, and androgen-independent receptor transactivation. Prostate cancers maintain an elevated AKT activity through the loss of PTEN function or the establishment of autocrine signaling by growth factors and cytokines. We used an in vivo prostate regeneration system to investigate the biological potency of the potential crosstalk between these two signal transduction pathways. We demonstrate a direct synergy between AKT and AR signaling that is sufficient to initiate and progress na?ve adult murine prostatic epithelium to frank carcinoma and override the effect of androgen ablation. Both genotropic and nongenotropic signals mediated by AR are essential for this synergistic effect. However, phosphorylation of AR by AKT at Ser-213 and Ser-791 is not critical for this synergy. These results suggest that more efficient therapeutics for advanced prostate cancer may need to target simultaneously AR signaling and AKT or the growth factor receptor tyrosine kinases that activate AKT.     

Herpes simplex virus 1 infected cell protein 0 forms a complex with CIN85 and Cbl and mediates the degradation of EGF receptor from cell surfaces

Infected cell protein 0 (ICP0) is a 775-residue multifunctional herpes simplex virus protein associated with numerous functions related to transactivation of gene expression and repression of host defenses to infection. We report that an uncharted domain of ICP0 located between residues 245 and 510 contains multiple SH3 domain binding motifs similar to those required for binding to CIN85, the M(r) 85,000 protein that interacts with Cbl. CIN85 and Cbl are involved in endocytosis and negative regulation of numerous receptor tyrosine kinases. We report that ICP0 binds CIN85 in a reciprocal manner and that the complexes pulled down by ICP0 also contain Cbl. We tested the role of ICP0 in the down-regulation of receptor tyrosine kinases by using epidermal growth factor receptor (EGFR) as a prototypic receptor. In transfection assays, ICP0, in the absence of other viral genes, down-regulated EGF-dependent expression of a reporter gene (luciferase). ICP0 also down-regulated both total and cell surface levels of EGFR in EGF-independent manner. In wild-type virus-infected cells, the surface levels of EGFR were also decreased in the absence of EGF stimulation. Stimulation by EGF enhanced the decrease in surface EGFR. We conclude that ICP0 encodes SH3 domain binding sites that function to down-regulate signaling pathways associated with receptor tyrosine kinases. The results suggest that ICP0 precludes signaling to the infected cells through the receptor tyrosine kinases.     

Targeting receptor tyrosine kinases in gastric cancer

Molecularly targeted therapeutic agents are constantly being developed and have been shown to be effective in various clinical trials.One group of representative targeted oncogenic kinases,the receptor tyrosine kinases(RTKs),has been associated with gastric cancer development.Trastuzumab,an inhibitor of ERBB2,has been approved for the treatment of gastric cancer,although other receptor tyrosine kinases,such as epidermal growth factor receptor,vascular endothelial growth factor,platelet-derived growth factor receptor,c-Met,IGF-1R and fibroblast growth factor receptor 2,are also activated in gastric cancer.The promising results of the trastuzumab clinical trial for gastric cancer resulted in the approval of trastuzumab-based therapy as a first-line treatment for human epidermal growth factor receptor 2-positive patients.On the other hand,the trial examining bevacizumab in combination with conventional chemotherapy did not meet its primary goal of increasing the overall survival time of gastric cancer patients;however,a significantly higher response rate and a longer progression-free survival were observed in the bevacizumab arm of the trial.Other clinical trials,especially phaseⅢtrials that have tested drugs targeting RTKs,such as cetuximab,panitumumab,gefitinib,erlotinib,figitumumab,sorafenib,sunitinib and lapatinib,have shown that these drugs have modest effects against gastric cancer.This review summarizes the recent results from the clinical trials of molecularly targeted drugs and suggests that further improvements in the treatment of advanced gastric cancer can be achieved through the combination of conventional drugs with the new molecularly targeted therapies.     

ErbB receptors, their ligands, and the consequences of their activation and inhibition in the myocardium

The epidermal growth factor (EGF) receptor (or ErbB1) and the related ErbB4 are transmembrane receptor protein tyrosine kinases which bind extracellular ligands of the EGF family. ErbB2 and ErbB3 are “co-receptors” structurally related to ErbB1/ErbB4, but ErbB2 is an “orphan” receptor and ErbB3 lacks tyrosine kinase activity. However, both are important in transmembrane signalling. All ErbB receptors/ligands are intimately involved in the regulation of cell growth, differentiation and survival, and their dysregulation contributes to some human malignancies. After extracellular ligand binding, receptor dimerisation and transautophosphorylation of intracellular C-terminal tyrosine residues, they bind signalling proteins which recognise specific tyrosine-phosphorylated motifs. This leads to activation of multiple signalling pathways, notably the extracellular signal-regulated kinase 1/2 (ERK1/2) cascade and the phosphoinositide 3-kinase (PI3K)/protein kinase B [PKB/(Akt)] pathway. In heart, targeted deletion of ErbB2, ErbB3, ErbB4 and some ErbB receptor extracellular ligands leads to embryonic lethality resulting from cardiovascular defects. ErbB receptor ligands improve cardiac myocyte viability and are hypertrophic, partly because of activation of ERK1/2 and/or PI3K/PKB(Akt). Furthermore, ErbB transactivation by Gq protein-coupled receptor (GqPCR) signalling may mediate the hypertrophic effects of GqPCR agonists. The utility of anthracyclines in cancer chemotherapy can be limited by their cardiotoxic side effects and these may be counteracted by ErbB receptor ligands. ErbB2 is the target of anti-cancer monoclonal antibody trastuzumab (Herceptin), and its myocardial downregulation may account for the occasional cardiotoxicity of this therapy. Here, we review the basic biochemistry of ErbB receptors/ligands, and emphasise their particular roles in the myocardium.     

Engagement of β-arrestin by transactivated insulin-like growth factor receptor is needed for V2 vasopressin receptor-stimulated ERK1/2 activation

G protein-coupled receptors (GPCRs) have been shown to activate the mitogen-activated protein kinases, ERK1/2, through both G protein-dependent and -independent mechanisms. Here, we describe a G protein-independent mechanism that unravels an unanticipated role for β-arrestins. Stimulation of the V2 vasopressin receptor (V2R) in cultured cells or in vivo in rat kidney medullar collecting ducts led to the activation of ERK1/2 through the metalloproteinase-mediated shedding of a factor activating the insulin-like growth factor receptor (IGFR). This process was found to be both Src- and β-arrestin-dependent. Whereas Src was found to act upstream of the metalloproteinase activation and be required for the release of the IGFR-activating factor, β-arrestins were found to act downstream of the IGFR transactivation. Unexpectedly, the engagement of β-arrestins by the IGFR but not by the V2R was needed to promote the vasopressin-stimulated ERK1/2 activation, indicating that a pool of β-arrestins distinct from those β-arrestins recruited to the V2R acts downstream of the receptor tyrosine kinase to activate ERK1/2. Such a dual site of action for β-arrestins helps explain the pleiotropic actions of this scaffolding protein. Given the role that V2R-stimulated ERK1/2 plays in kidney cell proliferation, this transactivation mechanism may have important implications for renal pathophysiology. Still, the role of β-arrestins downstream of a transactivation event is not limited to the V2R, because we observed a similar involvement for an unrelated GPCR (the platelet-activating factor receptor), indicating that it may be a general mechanism shared among GPCRs.     

Determination of the exact molecular requirements for type 1 angiotensin receptor epidermal growth factor receptor transactivation and cardiomyocyte hypertrophy

Major interest surrounds how angiotensin II triggers cardiac hypertrophy via epidermal growth factor receptor transactivation. G protein-mediated transduction, angiotensin type 1 receptor phosphorylation at tyrosine 319, and β-arrestin-dependent scaffolding have been suggested, yet the mechanism remains controversial. We examined these pathways in the most reductionist model of cardiomyocyte growth, neonatal ventricular cardiomyocytes. Analysis with [(32)P]-labeled cardiomyocytes, wild-type and [Y319A] angiotensin type 1 receptor immunoprecipitation and phosphorimaging, phosphopeptide analysis, and antiphosphotyrosine blotting provided no evidence for tyrosine phosphorylation at Y319 or indeed of the receptor, and mutation of Y319 (to A/F) did not prevent either epidermal growth factor receptor transactivation in COS-7 cells or cardiomyocyte hypertrophy. Instead, we demonstrate that transactivation and cardiomyocyte hypertrophy are completely abrogated by loss of G-protein coupling, whereas a constitutively active angiotensin type 1 receptor mutant was sufficient to trigger transactivation and growth in the absence of ligand. These results were supported by the failure of the β-arrestin-biased ligand SII angiotensin II to transactivate epidermal growth factor receptor or promote hypertrophy, whereas a β-arrestin-uncoupled receptor retained these properties. We also found angiotensin II-mediated cardiomyocyte hypertrophy to be attenuated by a disintegrin and metalloprotease inhibition. Thus, G-protein coupling, and not Y319 phosphorylation or β-arrestin scaffolding, is required for epidermal growth factor receptor transactivation and cardiomyocyte hypertrophy via the angiotensin type 1 receptor.     

Gambogic amide, a selective agonist for TrkA receptor that possesses robust neurotrophic activity, prevents neuronal cell death

Nerve growth factor (NGF) binds to TrkA receptor and triggers activation of numerous signaling cascades, which play critical roles in neuronal plasticity, survival, and neurite outgrowth. To mimic NGF functions pharmacologically, we developed a high-throughput screening assay to identify small-molecule agonists for TrkA receptor. The most potent compound, gambogic amide, selectively binds to TrkA, but not TrkB or TrkC, and robustly induces its tyrosine phosphorylation and downstream signaling activation, including Akt and MAPKs. Further, it strongly prevents glutamate-induced neuronal cell death and provokes prominent neurite outgrowth in PC12 cells. Gambogic amide specifically interacts with the cytoplasmic juxtamembrane domain of TrkA receptor and triggers its dimerization. Administration of this molecule in mice substantially diminishes kainic acid-triggered neuronal cell death and decreases infarct volume in the transient middle cerebral artery occlusion model of stroke. Thus, gambogic amide might not only establish a powerful platform for dissection of the physiological roles of NGF and TrkA receptor but also provide effective treatments for neurodegenerative diseases and stroke.     

After chronic myelogenous leukemia: tyrosine kinase inhibitors in other hematologic malignancies

Tyrosine kinases phosphorylate proteins on tyrosine residues, producing a biologic signal that influences many aspects of cellular function including cell growth, proliferation, differentiation, and death. Constitutive or unregulated activity through mutation or overexpression of these enzymes is a common pathologic feature in many acute and chronic leukemias. Inhibition of tyrosine kinases represents a strategy to disrupt signaling pathways that promote neoplastic growth and survival in hematologic malignancies and likely in other neoplasias as well. This review focuses on tyrosine kinases that have been implicated in the pathogenesis of hematologic diseases other than chronic myelogenous leukemia and discusses the evidence for the use of small molecules to target these kinases.