Human GRB-IRbeta/GRB10. Splice variants of an insulin and growth factor receptor-binding protein with PH and SH2 domains

Syringic Acid in Canarium odontophyllum for Diabetes and Obesity - A Review

Syringic acid (SA) is a phenolic compound with a significant role in the treatment of diabetes and obesity. Syringic acid possesses anti-obesity and anti-diabetic properties; however, the potential of syringic acid derived from the native Bornean fruit Canarium odontophyllum (C. odontophyllum) for managing diabetes and obesity remains undocumented. This brief discussion explores the possible mechanisms associated with syringic acid's structure and its potential therapeutic effects in managing diabetes and obesity. The relevant information is gathered from previous reports on syringic acid, related to molecular docking studies involving syringic acid-associated enzymes and protein residues. The potential mechanism of syringic acid derived from C. odontophyllum with chemical structure characterized by a benzene ring with hydrogen bonds and its high affinity for enzymes and protein residues targeting diabetes and obesity, including hexokinase 2 (HK2), glycogen synthase kinase (GSK), 2BEL, protein kinase D (PKD), insulin receptor substrate-1 (IRS-1), and insulin receptor beta (IR-β). This review paper provides alternative insights into syringic acid derived from the seasonal fruit of native Bornean fruit associated with molecular docking, structural advantages and mechanism of action in diabetes treatment.

Genetic variation near GRB10 associated with bone growth and osteosarcoma risk in canine and human populations

BACKGROUND

Canine and human osteosarcoma are similar in clinical presentation and tumor genomics. Giant breed dogs experience elevated osteosarcoma incidence, and taller stature remains a consistent risk factor for human osteosarcoma. Whether evolutionarily conserved genes contribute to both human and canine osteosarcoma predisposition merits evaluation.

METHODS

A multi-center sample of childhood osteosarcoma patients and controls underwent genome-wide genotyping and imputation. Ancestry-adjusted SNP associations were calculated within each dataset using logistic regression, then meta-analyzed across the three datasets, totaling 1091 patients and 3026 controls. Ten regions previously associated with canine osteosarcoma risk were mapped to the human genome, spanning ∼6 Mb. We prioritized association testing of 5985 human SNPs mapping to candidate osteosarcoma risk regions detected in Irish wolfhounds, the largest dog breed studied. Secondary analyses explored 6289 additional human SNPs mapping to candidate osteosarcoma risk regions identified in Rottweilers and greyhounds.

RESULTS

Fourteen SNPs were associated with human osteosarcoma risk after adjustment for multiple comparisons, all within a 42 kb region of human Chromosome 7p12.1. The lead variant was rs17454681 (OR=1.25, 95 %CI: 1.12-1.39; P=4.1×10-5), and independent risk variants were not observed in conditional analyses. While the associated region spanned 2.1 Mb and contained eight genes in Irish wolfhounds, associations were localized to a 50-fold smaller region of the human genome and strongly implicate GRB10 (growth factor receptor-bound protein 10) in canine and human osteosarcoma predisposition. PheWAS analysis in UK Biobank data identified noteworthy associations of the rs17454681 risk allele with varied measures of height and pubertal timing.

CONCLUSIONS

Our comparative oncology analysis identified a novel human osteosarcoma risk allele near GRB10, a growth inhibitor that suppresses activated receptor tyrosine kinases including IGF1R, PDGFRB, and EGFR. Epidemiologists may benefit from leveraging cross-species comparisons to identify haplotypes in highly susceptible but genetically homogenous populations of domesticated animals, then fine-mapping these associations in diverse human populations.

Grb7, Grb10 and Grb14, encoding the growth factor receptor-bound 7 family of signalling adaptor proteins have overlapping functions in the regulation of fetal growth and post-natal glucose metabolism

BACKGROUND

The growth factor receptor bound protein 7 (Grb7) family of signalling adaptor proteins comprises Grb7, Grb10 and Grb14. Each can interact with the insulin receptor and other receptor tyrosine kinases, where Grb10 and Grb14 inhibit insulin receptor activity. In cell culture studies they mediate functions including cell survival, proliferation, and migration. Mouse knockout (KO) studies have revealed physiological roles for Grb10 and Grb14 in glucose-regulated energy homeostasis. Both Grb10 KO and Grb14 KO mice exhibit increased insulin signalling in peripheral tissues, with increased glucose and insulin sensitivity and a modestly increased ability to clear a glucose load. In addition, Grb10 strongly inhibits fetal growth such that at birth Grb10 KO mice are 30% larger by weight than wild type littermates.

RESULTS

Here, we generate a Grb7 KO mouse model. We show that during fetal development the expression patterns of Grb7 and Grb14 each overlap with that of Grb10. Despite this, Grb7 and Grb14 did not have a major role in influencing fetal growth, either alone or in combination with Grb10. At birth, in most respects both Grb7 KO and Grb14 KO single mutants were indistinguishable from wild type, while Grb7:Grb10 double knockout (DKO) were near identical to Grb10 KO single mutants and Grb10:Grb14 DKO mutants were slightly smaller than Grb10 KO single mutants. In the developing kidney Grb7 had a subtle positive influence on growth. An initial characterisation of Grb7 KO adult mice revealed sexually dimorphic effects on energy homeostasis, with females having a significantly smaller renal white adipose tissue depot and an enhanced ability to clear glucose from the circulation, compared to wild type littermates. Males had elevated fasted glucose levels with a trend towards smaller white adipose depots, without improved glucose clearance.

CONCLUSIONS

Grb7 and Grb14 do not have significant roles as inhibitors of fetal growth, unlike Grb10, and instead Grb7 may promote growth of the developing kidney. In adulthood, Grb7 contributes subtly to glucose mediated energy homeostasis, raising the possibility of redundancy between all three adaptors in physiological regulation of insulin signalling and glucose handling.

Grb7 knockout mice develop normally but litters born to knockout females fail to thrive

BACKGROUND

Growth factor receptor-bound 7 (Grb7) is an adaptor protein involved in signal transduction downstream of multiple receptor tyrosine kinases, including ERBB, FGFR, and PDGFR pathways. Experimental studies have implicated Grb7 in regulating cell proliferation, survival, migration, and invasion through its large repertoire of protein-protein interactions.

RESULTS

Here, we describe the generation and characterization of a Grb7 knockout mouse. These mice are viable and fertile. A lacZ knock-in reporter was used to visualize Grb7 promoter activity patterns in adult tissues, indicating widespread Grb7 expression in glandular epithelium, the central nervous system, and other tissues. The sole defect observed in these animals was a failure of Grb7 knockout females to successfully raise pups to weaning age, a phenotype that was independent of both paternal and pup genotypes.

CONCLUSIONS

These data suggest a regulatory role for Grb7 in mammary lactational physiology.

Whole-genome resequencing reveals recent signatures of selection in five populations of largemouth bass ( Micropterus salmoides)

Largemouth bass ( Micropterus salmoides) is an economically important fish species in North America, Europe, and China. Various genetic improvement programs and domestication processes have modified its genome sequence through selective pressure, leaving nucleotide signals that can be detected at the genomic level. In this study, we sequenced 149 largemouth bass fish, including protospecies (imported from the US) and improved breeds (four domestic breeding populations from China). We detected genomic regions harboring certain genes associated with improved traits, which may be useful molecular markers for practical domestication, breeding, and selection. Subsequent analyses of genetic diversity and population structure revealed that the improved breeds have undergone more rigorous genetic changes. Through selective signal analysis, we identified hundreds of putative selective sweep regions in each largemouth bass line. Interestingly, we predicted 103 putative candidate genes potentially subjected to selection, including several associated with growth (p sst1 and grb10), early development ( klf9, sp4, and sp8), and immune traits ( pkn2, sept2, bcl6, and ripk2). These candidate genes represent potential genomic landmarks that could be used to improve important traits of biological and commercial interest. In summary, this study provides a genome-wide map of genetic variations and selection footprints in largemouth bass, which may benefit genetic studies and accelerate genetic improvement of this economically important fish.

Real role of growth factor receptor-binding protein 10: Linking lipid metabolism to diabetes cardiovascular complications

Cardiovascular complications of patients with type 2 diabetes mellitus (T2DM) threaten the health and life of numerous individuals. Recently, growth factor receptor-binding protein 10 (GRB10) was found to play a pivotal role in vascular complications of T2DM, which participates in the regulation of lipid metabolism of T2DM patients. The genetic variation of GRB10 rs1800504 is closely related to the risk of coronary heart disease in patients with T2DM. The development of GRB10 as a key mediator in the association of lipid metabolism with cardiovascular complications in T2DM is detailed in and may provide new potential concerns for the study of cardiovascular complications in T2DM patients.

La proteína 7 unida al receptor del factor de crecimiento (GRB7) en cáncer de mama

El cáncer de mama debe considerarse como un problema de salud pública ya que es la causa principal de muerte en mujeres en el mundo. Se conoce que es multifactorial y heterogéneo de manera que cada tumor tiene características genéticas y moleculares propias, lo cual se refleja en el comportamiento clínico, respuesta al tratamiento y pronóstico. La proteína 7 unida al receptor del factor de crecimiento (GRB7) hace parte de un grupo de proteínas GRB que median la interacción entre receptores tirosina cinasa y proteínas efectoras en algunas vías de señalización involucradas en transducción de señales, migración celular y angiogénesis. Esta proteína es codificada por el gen GRB7 localizado en el cromosoma 17 en el locus 17q11–21, cerca del gen ERBB2, lo que sugiere coamplificación y coexpresión de estos dos genes en el desarrollo del cáncer. Se ha visto que la proteína GRB7 por sí sola está presente en la biología molecular implícita del cáncer de mama, interviniendo en la proliferación y migración celular facilitando así la invasión y posibles metástasis. Se considera como un factor de mal pronóstico en esta enfermedad.

Feedback, Crosstalk and Competition: Ingredients for Emergent Non-Linear Behaviour in the PI3K/mTOR Signalling Network

The PI3K/mTOR signalling pathway plays a central role in the governing of cell growth, survival and metabolism. As such, it must integrate and decode information from both external and internal sources to guide efficient decision-making by the cell. To facilitate this, the pathway has evolved an intricate web of complex regulatory mechanisms and elaborate crosstalk with neighbouring signalling pathways, making it a highly non-linear system. Here, we describe the mechanistic biological details that underpin these regulatory mechanisms, covering a multitude of negative and positive feedback loops, feed-forward loops, competing protein interactions, and crosstalk with major signalling pathways. Further, we highlight the non-linear and dynamic network behaviours that arise from these regulations, uncovered through computational and experimental studies. Given the pivotal role of the PI3K/mTOR network in cellular homeostasis and its frequent dysregulation in pathologies including cancer and diabetes, a coherent and systems-level understanding of the complex regulation and consequential dynamic signalling behaviours within this network is imperative for advancing biology and development of new therapeutic approaches.

Role of Grb10 in mTORC1-dependent regulation of insulin signaling and action in human skeletal muscle cells

Growth factor receptor-bound protein 10 (Grb10) is an adaptor protein that binds to the insulin receptor, upon which insulin signaling and action are thought to be inhibited. Grb10 is also a substrate for the mechanistic target of rapamycin complex 1 (mTORC1) that mediates its feedback inhibition on phosphatidylinositide 3-kinase (PI3K)/Akt signaling. To characterize the function of Grb10 and its regulation by mTORC1 in human muscle, primary skeletal muscle cells were isolated from healthy lean young men and then induced to differentiate into myotubes. Knockdown of Grb10 enhanced insulin-induced PI3K/Akt signaling and glucose uptake in myotubes, reinforcing the notion underlying its function as a negative regulator of insulin action in human muscle. The increased insulin responsiveness in Grb10-silenced myotubes was associated with a higher abundance of the insulin receptor. Furthermore, insulin and amino acids independently and additively stimulated phosphorylation of Grb10 at Ser476. However, acute inhibition of mTORC1 with rapamycin blocked Grb10 Ser476 phosphorylation and repressed a negative-feedback loop on PI3K/Akt signaling that increased myotube responsiveness to insulin. Chronic rapamycin treatment reduced Grb10 protein abundance in conjunction with increased insulin receptor protein levels. Based on these findings, we propose that mTORC1 controls PI3K/Akt signaling through modulation of insulin receptor abundance by Grb10. These findings have potential implications for obesity-linked insulin resistance, as well as clinical use of mTORC1 inhibitors.

Grb7, a Critical Mediator of EGFR/ErbB Signaling, in Cancer Development and as a Potential Therapeutic Target

The partner of activated epidermal growth factor receptor (EGFR), growth factor receptor bound protein-7 (Grb7), a functionally multidomain adaptor protein, has been demonstrated to be a pivotal regulator for varied physiological and pathological processes by interacting with phospho-tyrosine-related signaling molecules to affect the transmission through a number of signaling pathways. In particular, critical roles of Grb7 in erythroblastic leukemia viral oncogene homolog (ERBB) family-mediated cancer development and malignancy have been intensively evaluated. The overexpression of Grb7 or the coamplification/cooverexpression of Grb7 and members of the ERBB family play essential roles in advanced human cancers and are associated with decreased survival and recurrence of cancers, emphasizing Grb7's value as a prognostic marker and a therapeutic target. Peptide inhibitors of Grb7 are being tested in preclinical trials for their possible therapeutic effects. Here, we review the molecular, functional, and clinical aspects of Grb7 in ERBB family-mediated cancer development and malignancy with the aim to reveal alternative and effective therapeutic strategies.

De-silencing Grb10 contributes to acute ER stress-induced steatosis in mouse liver

The growth factor receptor bound protein GRB10 is an imprinted gene product and a key negative regulator of the insulin, IGF1 and mTORC1 signaling pathways. GRB10 is highly expressed in mouse fetal liver but almost completely silenced in adult mice, suggesting a potential detrimental role of this protein in adult liver function. Here we show that the Grb10 gene could be reactivated in adult mouse liver by acute endoplasmic reticulum stress (ER stress) such as tunicamycin or a short-term high-fat diet (HFD) challenge, concurrently with increased unfolded protein response (UPR) and hepatosteatosis. Lipogenic gene expression and acute ER stress-induced hepatosteatosis were significantly suppressed in the liver of the liver-specific GRB10 knockout mice, uncovering a key role of Grb10 reactivation in acute ER stress-induced hepatic lipid dysregulation. Mechanically, acute ER stress induces Grb10 reactivation via an ATF4-mediated increase in Grb10 gene transcription. Our study demonstrates for the first time that the silenced Grb10 gene can be reactivated by acute ER stress and its reactivation plays an important role in the early development of hepatic steatosis.

The adaptors Grb10 and Grb14 are calmodulin-binding proteins

We identified the Grb7 family members, Grb10 and Grb14, as Ca²⁺ -dependent CaM-binding proteins using Ca²⁺ -dependent CaM-affinity chromatography as we previously did with Grb7. The potential CaM-binding sites were identified and experimentally tested using fluorescent-labeled peptides corresponding to these sites. The apparent affinity constant of these peptides for CaM, and the minimum number of calcium ions bound to CaM that are required for effective binding to these peptides were also determined. We prepared deletion mutants of the three adaptor proteins lacking the identified sites and determined that they lost or strongly diminished their CaM-binding capacity following the sequence Grb7 > > Grb14 > Grb10. More than one CaM-binding site and/or accessory CaM-binding sites appear to exist in Grb10 and Grb14, as compared to a single one present in Grb7.

Grb7 and Hax1 may colocalize partially to mitochondria in EGF-treated SKBR3 cells and their interaction can affect Caspase3 cleavage of Hax1

Growth factor receptor bound protein 7 (Grb7) is a signal-transducing adaptor protein that mediates specific protein-protein interactions in multiple signaling pathways. Grb7, with Grb10 and Grb14, is members of the Grb7 protein family. The topology of the Grb7 family members contains several protein-binding domains that facilitate the formation of protein complexes, and high signal transduction efficiency. Grb7 has been found overexpressed in several types of cancers and cancer cell lines and is presumed involved in cancer progression through promotion of cell proliferation and migration via interactions with the erythroblastosis oncogene B 2 (human epidermal growth factor receptor 2) receptor, focal adhesion kinase, Ras-GTPases, and other signaling partners. We previously reported Grb7 binds to Hax1 (HS1 associated protein X1) isoform 1, an anti-apoptotic protein also involved in cell proliferation and calcium homeostasis. In this study, we confirm that the in vitro Grb7/Hax1 interaction is exclusive to these two proteins and their interaction does not depend on Grb7 dimerization state. In addition, we report Grb7 and Hax1 isoform 1 may colocalize partially to mitochondria in epidermal growth factor-treated SKBR3 cells and growth conditions can affect this colocalization. Moreover, Grb7 can affect Caspase3 cleavage of Hax1 isoform 1 in vitro, and Grb7 expression may slow Caspase3 cleavage of Hax1 isoform 1 in apoptotic HeLa cells. Finally, Grb7 is shown to increase cell viability in apoptotic HeLa cells in a time-dependent manner. Taken together, these discoveries provide clues for the role of a Grb7/Hax1 protein interaction in apoptosis pathways involving Hax1. Copyright © 2016 John Wiley & Sons, Ltd.

Growth receptor binding protein 10 inhibits glucose-stimulated insulin release from pancreatic β-cells associated with suppression of the insulin/insulin-like growth factor-1 signalling pathway

Growth receptor binding protein 10 (Grb10) is an adaptor protein that interacts with the insulin receptor and insulin-like growth factor (IGF)-1 receptor. Overexpression of Grb10 in muscle cells and adipocytes inhibits insulin signalling, and transgenic mice overexpressing Grb10 exhibit impaired glucose tolerance. However, the roles of Grb10 in β-cells remain unknown. The aim of the present study was to explore the effect of Grb10 on β-cell function. The effects of Grb10 on glucose-stimulated insulin secretion (GSIS) and the insulin/IGF-1 signalling pathway were investigated in rat islets and/or dispersed islet cells with Grb10 overexpresion by adenovirus transfection. Protein expression was detected by western blot analysis. We found that Grb10 was expressed in both human and rat pancreas. Expression of Grb10 was increased in islets isolated from rats fed a high-fat plus high-sugar diet compared with islets isolated from rats fed normal chow diet, as well as in INS 832/13 cells exposed to high levels of glucose (20 mmol/L), palmitate (1 mmol/L) and interleukin-1β (50 U/mL). Overexpression of Grb10 in INS 832/13 cells or rat islets impaired GSIS compared with the respective control (all P < 0.05). Moreover, inhibition of GSIS by Grb10 overexpression was associated with a decrease in insulin- and IGF-1-induced Akt and extracellular signal-regulated kinase 1/2 phosphorylation. The results of the present study demonstrate that Grb10 is an important negative regulator of insulin/IGF-1 signalling in pancreatic β-cells and a potential target to improve β-cell function.

Grb10 is a dual regulator of receptor tyrosine kinase signaling

The adaptor protein Grb10 is a close homolog of Grb7 and Grb14. These proteins are characterized by an N-terminal proline-rich region, a Ras-GTPase binding domain, a PH domain, an SH2 domain and a BPS domain in between the PH and SH2 domains. Human Grb10 gene encodes three splice variants. These variants show differences in functionality. Grb10 associates with multiple proteins including tyrosine kinases in a tyrosine phosphorylation dependent or independent manner. Association with multiple proteins allows Grb10 to regulate different signaling pathways resulting in different biological consequences.

Regulation of insulin and type 1 insulin-like growth factor signaling and action by the Grb10/14 and SH2B1/B2 adaptor proteins

The effects of insulin and type 1 insulin-like growth factor (IGF-1) on metabolism, growth and survival are mediated by their association with specific receptor tyrosine kinases, which results in both receptor and substrate phosphorylation. Phosphotyrosine residues on receptors and substrates provide docking sites for signaling proteins containing SH2 (Src homology 2) domains, including molecular adaptors. This review focuses on the regulation of insulin/IGF-1 signaling and action by two adaptor families with a similar domain organization: the growth factor receptor-bound proteins Grb7/10/14 and the SH2B proteins. Both Grb10/14 and SH2B1/B2 associate with the activation loop of insulin/IGF-1 receptors through their SH2 domains, but association of Grb10/14 also involves their unique BPS domain. Consistent with Grb14 binding as a pseudosubstrate to the kinase active site, insulin/IGF-induced activation of receptors and downstream signaling pathways in cultured cells is inhibited by Grb10/14 adaptors, but is potentiated by SH2B1/B2 adaptors. Accordingly, Grb10 and Grb14 knockout mice show improved insulin/IGF sensitivity in vivo, and, for Grb10, overgrowth and increased skeketal muscle and pancreatic β-cell mass. Conversely, SH2B1-depleted mice display insulin and IGF-1 resistance, with peripheral depletion leading to reduced adiposity and neuronal depletion leading to obesity through associated leptin resistance. Grb10/14 and SH2B1 adaptors also modulate insulin/IGF-1 action by interacting with signaling components downstream of receptors and exert several tissue-specific effects. The identification of Grb10/14 and SH2B1 as physiological regulators of insulin signaling and action, together with observations that variants at their gene loci are associated with obesity and/or insulin resistance, highlight them as potential therapeutic targets for these conditions.

Disruption of growth factor receptor-binding protein 10 in the pancreas enhances β-cell proliferation and protects mice from streptozotocin-induced β-cell apoptosis

Defects in insulin secretion and reduction in β-cell mass are associated with type 2 diabetes in humans, and understanding the basis for these dysfunctions may reveal strategies for diabetes therapy. In this study, we show that pancreas-specific knockout of growth factor receptor-binding protein 10 (Grb10), which is highly expressed in pancreas and islets, leads to elevated insulin/IGF-1 signaling in islets, enhanced β-cell mass and insulin content, and increased insulin secretion in mice. Pancreas-specific disruption of Grb10 expression also improved glucose tolerance in mice fed with a high-fat diet and protected mice from streptozotocin-induced β-cell apoptosis and body weight loss. Our study has identified Grb10 as an important regulator of β-cell proliferation and demonstrated that reducing the expression level of Grb10 could provide a novel means to increase β-cell mass and reduce β-cell apoptosis. This is critical for effective therapeutic treatment of both type 1 and 2 diabetes.

FLT3 signals via the adapter protein Grb10 and overexpression of Grb10 leads to aberrant cell proliferation in acute myeloid leukemia

The adaptor protein Grb10 plays important roles in mitogenic signaling. However, its roles in acute myeloid leukemia (AML) are predominantly unknown. Here we describe the role of Grb10 in FLT3-ITD-mediated AML. We observed that Grb10 physically associates with FLT3 in response to FLT3-ligand (FL) stimulation through FLT3 phospho-tyrosine 572 and 793 residues and constitutively associates with oncogenic FLT3-ITD. Furthermore endogenous Grb10-FLT3 association was observed in OCI-AML-5 cells. Grb10 expression did not alter FLT3 receptor activation or stability in Ba/F3-FLT3 cells. However, expression of Grb10 enhanced FL-induced Akt phosphorylation without affecting Erk or p38 phosphorylation in Ba/F3-FLT3-WT and Ba/F3-FLT3-ITD. Selective Grb10 depletion reduced Akt phosphorylation in Ba/F3-FLT3-WT and OCI-AML-5 cells. Grb10 transduces signal from FLT3 by direct interaction with p85 and Ba/F3-FLT3-ITD cells expressing Grb10 exhibits higher STAT5 activation. Grb10 regulates the cell cycle by increasing cell population in S-phase. Expression of Grb10 furthermore resulted in an increased proliferation and survival of Ba/F3-FLT3-ITD cells as well as increased colony formation in semisolid culture. Grb10 expression was significantly increased in AML patients compared to healthy controls and was also elevated in patients carrying FLT3-ITD mutants. The elevated Grb10 expression partially correlated to relapse as well as to poor prognosis. These results suggest that Grb10 binds to both normal and oncogenic FLT3 and induces PI3K-Akt and STAT5 signaling pathways resulting in an enhanced proliferation, survival and colony formation of hematopoietic cells.

Lentivirus shRNA Grb10 targeting the pancreas induces apoptosis and improved glucose tolerance due to decreased plasma glucagon levels

AIMS/HYPOTHESIS

The physiological significance of growth factor receptor-bound protein-10 (GRB10) in the pancreas is unclear. We hypothesised that GRB10 is involved in pancreatic apoptosis, as GRB10 binds with a family of cell-survival-related proteins implicated in apoptosis.

METHODS

Lentiviral vector small hairpin RNA (shRNA) targeting Grb10 was injected in vivo via an intraductal pancreatic route to target pancreatic tissues in adult mice, which were studied 2 weeks post-injection.

RESULTS

Using the TUNEL assay, we demonstrated for the first time that in vivo injection of lentivirus shRNA Grb10 directly into the adult mouse pancreas induced apoptosis in both exocrine and endocrine (alpha and beta) cells. This effect was more pronounced in alpha cells. Levels of the pro-apoptotic protein BCL2-interacting mediator of cell death (BIM) in islets was higher in lentivirus shRNA Grb10 than in lentivirus shRNA scramble mice. In the apoptotic pathway, BIM initiates apoptosis signalling, leading to activation of the caspase cascade. We propose that, when complexed with GRB10, BIM is inactive. On activation by stress signalling or, in the present study, following injection of lentivirus shRNA Grb10 into pancreas, BIM becomes unbound from GRB10 and activates the caspase cascade. Indeed, caspase-3 activity in islets was higher in the experimental than in the control group. Apoptosis induced by shRNA Grb10 resulted in a 34% decrease in fasting plasma glucagon. Mice injected with shRNA Grb10 had improved glucose tolerance despite reduced insulin secretion compared with shRNA scramble control mice.

CONCLUSIONS/INTERPRETATION

GRB10 is critically involved in alpha cell survival and, as a result, plays an important role in regulating basal glucagon secretion and glucose tolerance in adult mice.

Genomic organization and control of the grb7 gene family

Grb7 and their related family members Grb10 and Grb14 are adaptor proteins, which participate in the functionality of multiple signal transduction pathways under the control of a variety of activated tyrosine kinase receptors and other tyrosine-phosphorylated proteins. They are involved in the modulation of important cellular and organismal functions such as cell migration, cell proliferation, apoptosis, gene expression, protein degradation, protein phosphorylation, angiogenesis, embryonic development and metabolic control. In this short review we shall describe the organization of the genes encoding the Grb7 protein family, their transcriptional products and the regulatory mechanisms implicated in the control of their expression. Finally, the alterations found in these genes and the mechanisms affecting their expression under pathological conditions such as cancer, diabetes and some congenital disorders will be highlighted.

Tyrosine phosphorylation of Grb14 by Tie2

Background

Growth factor receptor bound (Grb) proteins 7, 10 and 14 are a family of structurally related multi-domain adaptor proteins involved in a variety of biological processes. Grb7, 10 and 14 are known to become serine and/or threonine phosphorylated in response to growth factor (GF) stimulation. Grb7 and 10 have also been shown to become tyrosine phosphorylated under certain conditions. Under experimental conditions Grb7 is tyrosine phosphorylated by the Tie2/Tie-2/Tek angiogenic receptor tyrosine kinase (RTK). Furthermore, Grb14 has also been shown to interact with Tie2, however tyrosine phosphorylation of this Grb family member has yet to be reported.

Results

Here we report for the first time tyrosine phosphorylation of Grb14. This phosphorylation requires a kinase competent Tie2 as well as intact tyrosines 1100 and 1106 (Y1100 and Y1106) on the receptor. Furthermore, a complete SH2 domain on Grb14 is required for Grb14 tyrosine phosphorylation by Tie2. Grb14 was also able to become tyrosine phosphorylated in primary endothelial cells when treated with a soluble and potent variant of the Tie2 ligand, cartilage oligomeric matrix protein (COMP) Ang1.

Conclusion

Our results show that Grb14, like its family members Grb7 and Grb10, is able to be tyrosine phosphorylated. Furthermore, our data indicate a role for Grb14 in endothelial signaling downstream of the Tie2 receptor.

Growth factor receptor-bound protein-7 (Grb7) as a prognostic marker and therapeutic target in breast cancer

BACKGROUND

Growth factor receptor-bound protein-7 (Grb7) is an adapter-type signaling protein recruited to various tyrosine kinases, including HER2/neu. Grb7-specific inhibitors are in early development. As with other targeted therapies, response to therapy might be associated with target expression.

MATERIALS AND METHODS

Tissue microarrays containing 638 primary breast cancer specimens with 15-year patient follow-up were employed to assess Grb7 expression using our Automated QUantitative Analysis method; cytokeratin defines pixels as breast cancer (tumor mask) within the histospot, and Grb7 expression within the mask is measured with Cy5-conjugated antibodies.

RESULTS

High Grb7 expression was strongly associated with decreased survival in the entire cohort and in the node-positive subset (P = 0.0034 and P = 0.0019, respectively). On multivariable analysis, it remained an independent prognostic marker (P = 0.01). High Grb7 was strongly associated with high HER2/neu, and coexpression of these molecules was associated with worse prognosis than HER2/neu overexpression alone.

CONCLUSIONS

High Grb7 defines a subset of breast cancer patients with decreased survival, indicating that Grb7 might be a valuable prognostic marker and drug target. Coexpression with HER2/neu indicates that cotargeting these molecules might be an effective approach for treating HER2/neu-positive breast cancers. Future studies using Grb7-targeting agents should include assessment of Grb7 levels.

Differential modulation of cardiac potassium channels by Grb adaptor proteins

Scaffolding growth factor receptor-bound (Grb) adaptor proteins are components of macromolecular signaling complexes at the plasma membrane and thus are putative regulators of ion channel activity. The present study aimed to define the impact of Grb adaptor proteins on the function of cardiac K(+) channels. To this end channel proteins were coinjected with the adaptor proteins in Xenopus oocytes and channel activity analyzed with two-electrode voltage-clamp. It is shown that coexpression of Grb adaptor proteins can reduce current amplitudes of coexpressed channels. Grb7 and 10 significantly inhibited functional currents generated by hERG, Kv1.5 and Kv4.3 channels. Only Grb10 significantly inhibited KCNQ1/KCNE1 K(+) channels, and only Grb7 reduced Kir2.3 activity, whereas neither Grb protein significantly affected the closely related Kir2.1 and Kir2.2 channels. The present observations for the first time provide evidence for a selective and modulatory role of Grb adaptor proteins in the functional expression of cardiac K(+) channels.

Paternal deletion of Meg1/Grb10 DMR causes maternalization of the Meg1/Grb10 cluster in mouse proximal Chromosome 11 leading to severe pre- and postnatal growth retardation

Mice with maternal duplication of proximal Chromosome 11 (MatDp(prox11)), where Meg1/Grb10 is located, exhibit pre- and postnatal growth retardation. To elucidate the responsible imprinted gene for the growth abnormality, we examined the precise structure and regulatory mechanism of this imprinted region and generated novel model mice mimicking the pattern of imprinted gene expression observed in the MatDp(prox11) by deleting differentially methylated region of Meg1/Grb10 (Meg1-DMR). It was found that Cobl and Ddc, the neighboring genes of Meg1/Grb10, also comprise the imprinted region. We also found that the mouse-specific repeat sequence consisting of several CTCF-binding motifs in the Meg1-DMR functions as a silencer, suggesting that the Meg1/Grb10 imprinted region adopted a different regulatory mechanism from the H19/Igf2 region. Paternal deletion of the Meg1-DMR (+/DeltaDMR) caused both upregulation of the maternally expressed Meg1/Grb10 Type I in the whole body and Cobl in the yolk sac and loss of paternally expressed Meg1/Grb10 Type II and Ddc in the neonatal brain and heart, respectively, demonstrating maternalization of the entire Meg1/Grb10 imprinted region. We confirmed that the +/DeltaDMR mice exhibited the same growth abnormalities as the MatDp(prox11) mice. Fetal and neonatal growth was very sensitive to the expression level of Meg1/Grb10 Type I, indicating that the 2-fold increment of the Meg1/Grb10 Type I is one of the major causes of the growth retardation observed in the MatDp(prox11) and +/DeltaDMR mice. This suggests that the corresponding human GRB10 Type I plays an important role in the etiology of Silver-Russell syndrome caused by partial trisomy of 7p11-p13.

Brain-derived neurotrophic factor modulation of Kv1.3 channel is disregulated by adaptor proteins Grb10 and nShc

BACKGROUND

Neurotrophins are important regulators of growth and regeneration, and acutely, they can modulate the activity of voltage-gated ion channels. Previously we have shown that acute brain-derived neurotrophic factor (BDNF) activation of neurotrophin receptor tyrosine kinase B (TrkB) suppresses the Shaker voltage-gated potassium channel (Kv1.3) via phosphorylation of multiple tyrosine residues in the N and C terminal aspects of the channel protein. It is not known how adaptor proteins, which lack catalytic activity, but interact with members of the neurotrophic signaling pathway, might scaffold with ion channels or modulate channel activity.

RESULTS

We report the co-localization of two adaptor proteins, neuronal Src homology and collagen (nShc) and growth factor receptor-binding protein 10 (Grb10), with Kv1.3 channel as demonstrated through immunocytochemical approaches in the olfactory bulb (OB) neural lamina. To further explore the specificity and functional ramification of adaptor/channel co-localization, we performed immunoprecipitation and Western analysis of channel, kinase, and adaptor transfected human embryonic kidney 293 cells (HEK 293). nShc formed a direct protein-protein interaction with Kv1.3 that was independent of BDNF-induced phosphorylation of Kv1.3, whereas Grb10 did not complex with Kv1.3 in HEK 293 cells. Both adaptors, however, co-immunoprecipitated with Kv1.3 in native OB. Grb10 was interestingly able to decrease the total expression of Kv1.3, particularly at the membrane surface, and subsequently eliminated the BDNF-induced phosphorylation of Kv1.3. To examine the possibility that the Src homology 2 (SH2) domains of Grb10 were directly binding to basally phosphorylated tyrosines in Kv1.3, we utilized point mutations to substitute multiple tyrosine residues with phenylalanine. Removal of the tyrosines 111-113 and 449 prevented Grb10 from decreasing Kv1.3 expression. In the absence of either adaptor protein, channel co-expression reciprocally down-regulated expression and tyrosine phosphorylation of TrkB kinase and related insulin receptor kinase. Finally, through patch-clamp electrophysiology, we found that the BDNF-induced current suppression of the channel was prevented by both nShc and Grb10.

CONCLUSION

We report that adaptor protein alteration of kinase-induced Kv1.3 channel modulation is related to the degree of direct protein-protein association and that the channel itself can reciprocally modulate receptor-linked tyrosine kinase expression and activity.

Grb10/Nedd4-mediated multiubiquitination of the insulin-like growth factor receptor regulates receptor internalization

The adaptor protein Grb10 is an interacting partner of the IGF-I receptor (IGF-IR) and the insulin receptor (IR). Previous work from our laboratory has established the role of Grb10 as a negative regulator of IGF-IR-dependent cell proliferation. We have shown that Grb10 binds the E3 ubiquitin ligase Nedd4 and promotes IGF-I-stimulated ubiquitination, internalization, and degradation of the IGF-IR, thereby giving rise to long-term attenuation of signaling. Recent biochemical evidence suggests that tyrosine-kinase receptors (RTK) may not be polyubiquitinated but monoubiquitinated at multiple sites (multiubiquitinated). However, the type of ubiquitination of the IGF-IR is still not defined. Here we show that the Grb10/Nedd4 complex upon ligand stimulation mediates multiubiquitination of the IGF-IR, which is required for receptor internalization. Moreover, Nedd4 by promoting IGF-IR ubiquitination and internalization contributes with Grb10 to negatively regulate IGF-IR-dependent cell proliferation. We also demonstrate that the IGF-IR is internalized through clathrin-dependent and-independent pathways. Grb10 and Nedd4 remain associated with the IGF-IR in early endosomes and caveosomes, where they may participate in sorting internalized receptors. Grb10 and Nedd4, unlike the IGF-IR, which is targeted for lysosomal degradation are not degraded and likely directed into recycling endosomes. These results indicate that Grb10 and Nedd4 play a critical role in mediating IGF-IR down-regulation by promoting ligand-dependent multiubiquitination of the IGF-IR, which is required for receptor internalization and regulates mitogenesis.

Grb7 SH2 domain structure and interactions with a cyclic peptide inhibitor of cancer cell migration and proliferation

Background

Human growth factor receptor bound protein 7 (Grb7) is an adapter protein that mediates the coupling of tyrosine kinases with their downstream signaling pathways. Grb7 is frequently overexpressed in invasive and metastatic human cancers and is implicated in cancer progression via its interaction with the ErbB2 receptor and focal adhesion kinase (FAK) that play critical roles in cell proliferation and migration. It is thus a prime target for the development of novel anti-cancer therapies. Recently, an inhibitory peptide (G7-18NATE) has been developed which binds specifically to the Grb7 SH2 domain and is able to attenuate cancer cell proliferation and migration in various cancer cell lines.

Results

As a first step towards understanding how Grb7 may be inhibited by G7-18NATE, we solved the crystal structure of the Grb7 SH2 domain to 2.1 Å resolution. We describe the details of the peptide binding site underlying target specificity, as well as the dimer interface of Grb 7 SH2. Dimer formation of Grb7 was determined to be in the μM range using analytical ultracentrifugation for both full-length Grb7 and the SH2 domain alone, suggesting the SH2 domain forms the basis of a physiological dimer. ITC measurements of the interaction of the G7-18NATE peptide with the Grb7 SH2 domain revealed that it binds with a binding affinity of K_d = ~35.7 μM and NMR spectroscopy titration experiments revealed that peptide binding causes perturbations to both the ligand binding surface of the Grb7 SH2 domain as well as to the dimer interface, suggesting that dimerisation of Grb7 is impacted on by peptide binding.

Conclusion

Together the data allow us to propose a model of the Grb7 SH2 domain/G7-18NATE interaction and to rationalize the basis for the observed binding specificity and affinity. We propose that the current study will assist with the development of second generation Grb7 SH2 domain inhibitors, potentially leading to novel inhibitors of cancer cell migration and invasion.

Interaction between the insulin receptor and Grb14: A dynamic study in living cells using BRET

Grb14 is a molecular adaptor that binds to the activated insulin receptor (IR) and negatively regulates insulin signaling. We have studied the dynamics of interaction of the IR with Grb14, in real time, in living HEK cells, using bioluminescence resonance energy transfer (BRET). Insulin rapidly and dose-dependently stimulated this interaction. Removing insulin from the incubation medium only resulted in a modest decrease in BRET signal, indicating that the interaction between the IR and Grb14 can remain long after insulin stimulus has disappeared. BRET saturation experiments indicated that insulin markedly increases the affinity between IR and Grb14, resulting in recruitment of the adaptor to the activated IR. In addition, using both BRET and co-immunoprecipitation experiments, we demonstrated that insulin induced the dimerization of Grb14, most likely as a result of simultaneous binding of two Grb14 molecules on the activated IR. We also investigated the relationships between IR, Grb14 and the protein tyrosine phosphatase PTP1B. We observed that insulin-induced BRET between the IR and PTP1B was markedly reduced by Grb14, suggesting that Grb14 regulated this interaction in living cells. Using site-specific antibodies against phosphorylated tyrosines of the insulin receptor, we showed that Grb14 protected the three tyrosines of the kinase loop from dephosphorylation by PTP1B, while favouring dephosphorylation of tyrosine 972. This resulted in decreased IRS-1 binding to the IR and decreased activation of the ERK pathway. Our work suggests that Grb14 may regulate signalling through the insulin receptor by controlling its tyrosine-dephosphorylation in a site-specific manner.

Interaction with Grb14 results in site-specific regulation of tyrosine phosphorylation of the insulin receptor

The dynamics of interaction of the insulin receptor (IR) with Grb14 was monitored, in real time, in living human embryonic kidney cells, using bioluminescence resonance energy transfer (BRET). We observed that insulin rapidly and dose-dependently stimulated this interaction. We also observed that insulin-induced BRET between the IR and protein tyrosine phosphatase 1B (PTP1B) was markedly reduced by Grb14, suggesting that Grb14 regulated this interaction in living cells. Using site-specific antibodies against phosphorylated tyrosines of the IR, we showed that Grb14 protected the three tyrosines of the kinase loop from dephosphorylation by PTP1B, while favouring dephosphorylation of tyrosine 972. This resulted in decreased IRS-1 binding to the IR and decreased activation of the extracellular signal-regulated kinase pathway. Increased Grb14 expression in human liver-derived HuH7 cells also seemed to specifically decrease the phosphorylation of Y972. Our work therefore suggests that Grb14 may regulate signalling through the IR by controlling its tyrosine dephosphorylation in a site-specific manner.

Grb10 and Grb14: enigmatic regulators of insulin action--and more?

The Grb proteins (growth factor receptor-bound proteins) Grb7, Grb10 and Grb14 constitute a family of structurally related multidomain adapters with diverse cellular functions. Grb10 and Grb14, in particular, have been implicated in the regulation of insulin receptor signalling, whereas Grb7 appears predominantly to be involved in focal adhesion kinase-mediated cell migration. However, at least in vitro, these adapters can bind to a variety of growth factor receptors. The highest identity within the Grb7/10/14 family occurs in the C-terminal SH2 (Src homology 2) domain, which mediates binding to activated receptors. A second well-conserved binding domain, BPS [between the PH (pleckstrin homology) and SH2 domains], can act to enhance binding to the IR (insulin receptor). Consistent with a putative adapter function, some non-receptor-binding partners, including protein kinases, have also been identified. Grb10 and Grb14 are widely, but not uniformly, expressed in mammalian tissues, and there are various isoforms of Grb10. Binding of Grb10 or Grb14 to autophosphorylated IR in vitro inhibits tyrosine kinase activity towards other substrates, but studies on cultured cell lines have been conflicting as to whether Grb10 plays a positive or negative role in insulin signalling. Recent gene knockouts in mice have established that Grb10 and Grb14 act as inhibitors of intracellular signalling pathways regulating growth and metabolism, although the phenotypes of the two knockouts are distinct. Ablation of Grb14 enhances insulin action in liver and skeletal muscle and improves whole-body tolerance, with little effect on embryonic growth. Ablation of Grb10 results in disproportionate overgrowth of the embryo and placenta involving unidentified pathways, and also impacts on hepatic glycogen synthesis, and probably on glucose homoeostasis. This review discusses the extent to which previous studies in vitro can account for the observed phenotype of knockout animals, and considers evidence that aberrant function of Grb10 or Grb14 may contribute to disorders of growth and metabolism in humans.

Meg1/Grb10 overexpression causes postnatal growth retardation and insulin resistance via negative modulation of the IGF1R and IR cascades

The Meg1/Grb10 protein has been implicated as an adapter protein in the signaling pathways from insulin receptor (IR) and insulin-like growth factor 1 receptor (IGF1R) in vitro. To elucidate its in vivo function, four independent Meg1/Grb10 transgenic mouse lines were established, and the effects of excess Meg1/Grb10 on both postnatal growth and glucose metabolism were examined. All of the Meg1/Grb10 transgenic mice showed growth retardation after weaning (3-4 weeks), which indicates that ectopic overexpression of Meg1/Grb10 inhibits postnatal growth that is mediated by IGF1 via IGF1R. In addition, the mice became hyperinsulinemic owing to high levels of insulin resistance, which demonstrates that Meg1/Grb10 also modulates the insulin receptor cascade negatively in vivo. Type II diabetes arose frequently in the two transgenic lines, which also showed impaired glucose tolerance. In these mice, severe atrophy of the pancreatic acinus cells was associated with high-level production of Meg1/Grb10 in the pancreas. These results suggest that Meg1/Grb10 inhibits the function of both insulin and IGF1 receptors in these cells, since a similar phenotype has been reported for Ir and Igf1r double knockout mice. Taken together, these results indicate that Meg1/Grb10 interacts with both insulin and IGF1 receptors in vivo, and negatively regulates the IGF growth pathways via these receptors.

Hormonal regulation of the Grb14 signal modulator and its role in cell cycle progression of MCF-7 human breast cancer cells

Growth factor receptor bound (Grb)14 is a member of the Grb7 family of src homology (SH)2 domain-containing proteins. These proteins perform both adaptor and modulatory roles in receptor tyrosine kinase (RTK) signaling, although their regulation is poorly understood. In this study, a positive correlation between Grb14 protein expression and ER alpha status in breast cancer cell lines led us to investigate regulation of Grb14 by estradiol and insulin, which synergize in the regulation of breast cancer cell proliferation. In MCF-7 cells maintained in charcoal-stripped serum, Grb14 expression was downregulated by estradiol and increased by the pure anti-estrogen ICI 182780. Under serum-free conditions, insulin enhanced Grb14 expression but this effect was repressed by estradiol when both hormones were used in combination. Using a system in which c-Myc induction drives cell cycle progression independently of estradiol, we demonstrated that Grb14 regulation was specific to estradiol treatment. Finally, we demonstrated a novel functional role for Grb14 whereby its overexpression inhibited not only insulin- but also estrogen-induced cell cycle progression. This was associated with decreased extracellular signal-regulated kinase (Erk)1/2 activation in insulin-stimulated Grb14-overexpressing cells. These data represent the first demonstration of regulation of Grb14 expression levels in response to hormonal stimuli, and are consistent with its role as a repressor of insulin signaling where it is induced as a negative feedback mechanism. A role for Grb14 is also shown in estrogen/insulin crosstalk since estradiol blocks the insulin-induced induction of this protein.

Distinct Grb10 domain requirements for effects on glucose uptake and insulin signaling

The adapter protein Grb10 binds to phosphotyrosine residues in insulin receptors via its C-terminal region and regulates insulin signaling. This study investigated Grb10 regulation of glucose uptake and the importance of the Grb10 N-terminal region using 3T3-L1 adipocytes overexpressing full-length (FL-Grb10) or N-terminally truncated Grb10 (BPS-SH2). Overexpression of FL-Grb10 inhibited insulin-stimulated receptor autophosphorylation and glucose uptake. In contrast, the BPS-SH2 fragment of Grb10 had no effect on receptor phosphorylation or glucose uptake. In spite of these differences, both FL-Grb10 and the BPS-SH2 fragment inhibited insulin-stimulated phosphorylation of IRS1, IRS2, Akt/PKB, Shc, ERK1/2, APS, and c-Cbl to a similar extent. Co-precipitation studies demonstrated more sustained binding of the BPS-SH2 fragment than FL-Grb10 to insulin receptors. Although receptor binding domains of Grb10 are sufficient to inhibit insulin effects on proximal post-receptor signaling responses, N-terminal domains of Grb10 are essential for the effects of this adapter protein on receptor phosphorylation and glucose uptake.

Grb10 prevents Nedd4-mediated vascular endothelial growth factor receptor-2 degradation

One of the cellular mechanisms used to prevent continuous and enhanced activation in response to growth factors is the internalization and degradation of their receptors. Little is known about the molecular mechanisms involved in vascular endothelial growth factor receptor-2 (VEGF-R2) degradation. In a previous work, we have shown that the adaptor protein Grb10 is a positive regulator of the VEGF signaling pathway. Indeed, VEGF stimulates Grb10 expression, and Grb10 overexpression induces an increase in the amount and the tyrosine phosphorylation of VEGF-R2. In the present manuscript, we demonstrate that Grb10 stimulates VEGF-R2 expression by inhibiting the Nedd4-mediated VEGF-R2 degradation. First, we show that proteasome inhibition by MG132 induces an increase in VEGF-R2 amount, and that VEGF-R2 is ubiquitinated in response to VEGF. Expression of Nedd4, a HECT domain-containing ubiquitin ligase, induces the disappearance of VEGF-R2 in cells, suggesting that Nedd4 is involved in VEGF-R2 degradation. To determine whether Nedd4 directly ubiquitinates VEGF-R2, we expressed a ubiquitin ligase-deficient mutant Nedd4C854S. In the presence of Nedd4C854S, VEGF-R2 is expressed and ubiquitinated. These results suggest that VEGF-R2 is ubiquitinated but that Nedd4 is not involved in this process. Finally, we show that Grb10 constitutively associates with Nedd4. Co-expression of Nedd4 and Grb10 restores the expression of VEGF-R2, suggesting that Grb10 inhibits the Nedd4-mediated degradation of VEGF-R2. In this study, we show that Grb10 acts as a positive regulator in VEGF-R2 signaling and protects VEGF-R2 from degradation by interacting with Nedd4, a component of the endocytic machinery.

Gene expression pattern in hepatic stem/progenitor cells during rat fetal development using complementary DNA microarrays

To identify new and differentially expressed genes in rat fetal liver epithelial stem/progenitor cells during their proliferation, lineage commitment, and differentiation, we used a high throughput method-mouse complementary DNA (cDNA) microarrays-for analysis of gene expression. The gene expression pattern of rat hepatic cells was studied during their differentiation in vivo: from embryonic day (ED) 13 until adulthood. The differentially regulated genes were grouped into two clusters: a cluster of up-regulated genes comprised of 281 clones and a cluster of down-regulated genes comprised of 230 members. The expression of the latter increased abruptly between ED 16 and ED 17. Many of the overexpressed genes from the first cluster fall into distinct, differentially expressed functional groups: genes related to development, morphogenesis, and differentiation; calcium- and phospholipid-binding proteins and signal transducers; and cell adhesion, migration, and matrix proteins. Several other functional groups of genes that are initially down-regulated, then increase during development, also emerged: genes related to inflammation, blood coagulation, detoxification, serum proteins, amino acids, lipids, and carbohydrate metabolism. Twenty-eight genes overexpressed in fetal liver that were not detected in adult liver are suggested as potential markers for identification of liver progenitor cells. In conclusion, our data show that the gene expression program of fetal hepatoblasts differs profoundly from that of adult hepatocytes and that it is regulated in a specific manner with a major switch at ED 16 to 17, marking a dramatic change in the gene expression program during the transition of fetal liver progenitor cells from an undifferentiated to a differentiated state. Supplementary material for this article can be found on the HEPATOLOGY website (http://interscience.wiley.com/jpages/0270-9139/suppmat/index.html).

The PIR domain of Grb14 is an intrinsically unstructured protein: implication in insulin signaling

Grb14 belongs to the Grb7 family of adapter proteins and was identified as a negative regulator of insulin signal transduction. Its inhibitory effect on the insulin receptor kinase activity is controlled by a newly discovered domain called PIR. To investigate the biochemical and biophysical characteristics of this new domain, we cloned and purified recombinant PIR-SH2, PIR, and SH2 domains. The isolated PIR and PIR-SH2 domains were physiologically active and inhibited insulin-induced reinitiation of meiosis in the Xenopus oocytes system. However, NMR experiments on (15)N-labelled PIR revealed that it did not present secondary structure. These results suggest that the PIR domain belongs to the growing family of intrinsically unstructured proteins.

Negative regulation of insulin-stimulated mitogen-activated protein kinase signaling by Grb10

Grb10 is a Pleckstrin homology and Src homology 2 (SH2) domain-containing protein that binds to the tyrosine-phosphorylated insulin receptor in response to insulin stimulation. Loss of Grb10 function in mice results in fetal and placental overgrowth; however, the molecular mechanism remains unknown. In the present study, we show that overexpression of Grb10 in Chinese hamster ovary cells expressing the insulin receptor or in 3T3-L1 adipocytes reduced insulin-stimulated phosphorylation of MAPK. Overexpression of Grb10 in rat primary adipocytes also inhibited insulin-stimulated phosphorylation of the MAPK downstream substrate Elk1. To determine the mechanism by which Grb10 inhibited insulin-stimulated MAPK signaling, we examined whether Grb10 affects the phosphorylation of MAPK upstream signaling components. We found that overexpression of Grb10 inhibited the insulin-stimulated phosphorylation of Shc, a positive regulator of the MAPK signaling pathway. The inhibitory effect was diminished when the SH2 domain of Grb10 was deleted. The negative role of Grb10 in insulin signaling was established by suppression of endogenous Grb10 by RNA interference in HeLa cells overexpressing the insulin receptor, which enhanced insulin-stimulated phosphorylation of MAPK, Shc, and Akt. Taken together, our findings suggest that Grb10 functions as a negative regulator in the insulin-stimulated MAPK signaling pathway. In addition, the inhibitory effect of Grb10 on the MAPK pathway is most likely due to a direct block of insulin-stimulated Shc tyrosine phosphorylation.

Grb10 adapter protein as regulator of insulin-like growth factor receptor signaling

Grb10 is a member of a superfamily of adapter proteins that includes Grb10, 7, 14, and a protein of Caenorhabditis elegans called Mig10. Grb10 proteins are binding partners for several trans-membrane tyrosine-kinase receptors, including the insulin-like growth factor receptor (IGF-IR) and the insulin receptor (IR). Many recent reports have suggested a very important role of Grb10 in regulating IGF-IR signaling. In this review, we will focus on the role of Grb10 in IGF-I-induced mitogenesis and we will discuss the recent findings that show the involvement of Grb10 in the regulation of ligand-induced ubiquitination, internalization, and stability of the IGF-IR.

Growth factor receptor-binding protein 10 (Grb10) as a partner of phosphatidylinositol 3-kinase in metabolic insulin action

The regulation of the metabolic insulin response by mouse growth factor receptor-binding protein 10 (Grb10) has been addressed in this report. We find mouse Grb10 to be a critical component of the insulin receptor (IR) signaling complex that provides a functional link between IR and p85 phosphatidylinositol (PI) 3-kinase and regulates PI 3-kinase activity. This regulatory mechanism parallels the established link between IR and p85 via insulin receptor substrate (IRS) proteins. A direct association was demonstrated between Grb10 and p85 but was not observed between Grb10 and IRS proteins. In addition, no effect of mouse Grb10 was observed on the association between IRS-1 and p85, on IRS-1-associated PI 3-kinase activity, or on insulin-mediated activation of IR or IRS proteins. A critical role of mouse Grb10 was observed in the regulation of PI 3-kinase activity and the resulting metabolic insulin response. Dominant-negative Grb10 domains, in particular the SH2 domain, eliminated the metabolic response to insulin in differentiated 3T3-L1 adipocytes. This was consistently observed for glycogen synthesis, glucose and amino acid transport, and lipogenesis. In parallel, the same metabolic responses were substantially elevated by increased levels of Grb10. A similar role of Grb10 was confirmed in mouse L6 cells. In addition to the SH2 domain, the Pro-rich amino-terminal region of Grb10 was implicated in the regulation of PI 3-kinase catalytic activity. These regulatory roles of Grb10 were extended to specific insulin mediators downstream of PI 3-kinase including PKB/Akt, glycogen synthase kinase, and glycogen synthase. In contrast, a regulatory role of Grb10 in parallel insulin response pathways including p70 S6 kinase, ubiquitin ligase Cbl, or mitogen-activated protein kinase p38 was not observed. The dissection of the interaction of mouse Grb10 with p85 and the resulting regulation of PI 3-kinase activity should help elucidate the complexity of the IR signaling mechanism.

Systematic identification of regulatory proteins critical for T-cell activation

BACKGROUND

The activation of T cells, mediated by the T-cell receptor (TCR), activates a battery of specific membrane-associated, cytosolic and nuclear proteins. Identifying the signaling proteins downstream of TCR activation will help us to understand the regulation of immune responses and will contribute to developing therapeutic agents that target immune regulation.

RESULTS

In an effort to identify novel signaling molecules specific for T-cell activation we undertook a large-scale dominant effector genetic screen using retroviral technology. We cloned and characterized 33 distinct genes from over 2,800 clones obtained in a screen of 7 x 108 Jurkat T cells on the basis of a reduction in TCR-activation-induced CD69 expression after expressing retrovirally derived cDNA libraries. We identified known signaling molecules such as Lck, ZAP70, Syk, PLC gamma 1 and SHP-1 (PTP1C) as truncation mutants with dominant-negative or constitutively active functions. We also discovered molecules not previously known to have functions in this pathway, including a novel protein with a RING domain (found in a class of ubiquitin ligases; we call this protein TRAC-1), transmembrane molecules (EDG1, IL-10R alpha and integrin alpha2), cytoplasmic enzymes and adaptors (PAK2, A-Raf-1, TCPTP, Grb7, SH2-B and GG2-1), and cytoskeletal molecules (moesin and vimentin). Furthermore, using truncated Lck, PLC gamma 1, EDG1 and PAK2 mutants as examples, we showed that these dominant immune-regulatory molecules interfere with IL-2 production in human primary lymphocytes.

CONCLUSIONS

This study identified important signal regulators in T-cell activation. It also demonstrated a highly efficient strategy for discovering many components of signal transduction pathways and validating them in physiological settings.

Two novel proteins that are linked to insulin-like growth factor (IGF-I) receptors by the Grb10 adapter and modulate IGF-I signaling

Grb10 is a protein that binds to the intracellular domains of activated tyrosine kinase receptors, including insulin-like growth factor (IGF-I) and insulin receptors. This occurs through the interaction of two C-terminal Grb10 motifs (BPS and Src homology domains) with receptor phosphotyrosine residues. Published data from transfection/overexpression studies support both positive and negative regulatory effects of Grb10, thus leaving its physiological role unclear. Because Grb10 has the structure of an adapter protein, the objective of this study was to determine whether Grb10 links other proteins to IGF-I receptors and thus modulates IGF-I signaling. Using yeast two-hybrid screening, the N terminus of Grb10 was shown to interact with two novel proteins, designated GIGYF1 (Grb10 interacting GYF protein 1) and GIGYF2. Mutation analysis indicates that a 17-amino acid sequence in GIGYF1 and GIGYF2, homologous to the GYF domain described previously, binds to tandem proline-rich regions in the N terminus of Grb10. In IGF-I receptor-expressing R+ fibroblasts, there is detectable binding of a Myc-tagged fragment of GIGYF1 to Grb10 in the basal state. Stimulation with IGF-I results in increased binding of GIGYF1 to Grb10 and transient binding of both Grb10 and GIGYF1 to IGF-I receptors, presumably via the adapter function of Grb10. At later time points, GIGYF1 dissociates, but Grb10 remains linked to IGF-I receptors. Overexpression of the Grb10 binding fragment of GIGYF1 in R+ cells results in a significant increase in IGF-I-stimulated receptor tyrosine phosphorylation. In conclusion, we have identified two members of a novel protein family, which become transiently linked to IGF-I receptors by the Grb10 adapter protein following IGF-I stimulation. Grb10 and GIGYFs may act cooperatively to regulate receptor signaling.

NIK is a component of the EGF/heregulin receptor signaling complexes

Nuclear factor kappaB-inducing kinase (NIK) is a member of the MAP kinase kinase kinase family that was first identified as a component of the TNF-R1-induced NF-kappaB activation pathway (TNF, tumor necrosis factor; nuclear factor kappaB, NF-kappaB). Gene knockout study, however, suggests that NIK is dispensable for TNF-R1- but required for lymphotoxin-beta receptor-induced NF-kappaB activation. A NIK kinase inactive mutant is a potent inhibitor of NF-kappaB activation triggered by various stimuli, suggesting that NIK is involved in a broad range of NF-kappaB activation pathways. To unambiguously identify signaling pathways that NIK participates in, we screened antibody arrays for proteins that are associated with NIK. This effort identified ErbB4, one of the EGF/heregulin receptors, and Grb7, an adapter protein associated with ErbB4 (ErbB, epidermal growth factor receptor family protein; EGF, epidermal growth factor; Grb, growth factor receptor bound). Coimmunoprecipitation experiments demonstrated that NIK interacted with Grb7, as well as Grb10 and Grb14, but not Grb2. Domain mapping experiments indicated that the central GM domain of Grb7 was sufficient for its interaction with NIK. Coimmunoprecipitation experiments also indicated that Grb7 and NIK could be simultaneously recruited into signaling complexes of all known EGF/heregulin receptors, including EGFR, ErbB2, ErbB3, and ErbB4. In reporter gene assays, NIK could potentiate Grb7, ErbB2/ErbB4, and EGF-induced NF-kappaB activation. A NIK kinase inactive mutant could block ErbB2/ErbB4 and EGF-induced NF-kappaB activation. Moreover, EGF/heregulin receptors activated NF-kappaB in wild-type, but not NIK-/- embryonic fibroblasts. Our findings suggest that NIK is a component of the EGF/heregulin receptor signaling complexes and involved in NF-kappaB activation triggered by these receptors.

The Grb10/Nedd4 complex regulates ligand-induced ubiquitination and stability of the insulin-like growth factor I receptor

The adapter protein Grb10 belongs to a superfamily of related proteins, including Grb7, -10, and -14 and Caenorhabditis elegans Mig10. Grb10 is an interacting partner of the insulin-like growth factor I receptor (IGF-IR) and the insulin receptor (IR). Previous work showed an inhibitory effect of mouse Grb10 (mGrb10alpha) on IGF-I-mediated mitogenesis (A. Morrione et al., J. Biol. Chem. 272:26382-26387, 1997). With mGrb10alpha as bait in a yeast two-hybrid screen, mouse Nedd4 (mNedd4-1), a ubiquitin protein ligase, was previously isolated as an interacting protein of Grb10 (A. Morrione et al., J. Biol. Chem. 274:24094-24099, 1999). However, Grb10 is not ubiquitinated by Nedd4 in cells. Here we show that in mouse embryo fibroblasts overexpressing Grb10 and the IGF-IR (p6/Grb10), there is a strong ligand-dependent increase in ubiquitination of the IGF-IR compared with that in parental cells (p6). This increased ubiquitination is associated with a shorter half-life and increased internalization of the IGF-IR. The IGF-IR is stabilized following treatment with both MG132 and chloroquine, indicating that both the proteasome and lysosomal pathways mediate degradation of the receptor. Ubiquitination of the IGF-IR likely occurs at the plasma membrane, prior to the formation of endocytic vesicles, as it is insensitive to dansylcadaverine, an inhibitor of early endosome formation in IGF-IR endocytosis. Grb10 coimmunoprecipitates with the IGF-IR and endogenous Nedd4 in p6/Grb10 cells, suggesting the presence of a Grb10/Nedd4/IGF-IR complex. Ubiquitination of the IGF-IR in p6/Grb10 cells is severely impaired by overexpression of a catalytically inactive Nedd4 mutant (Nedd4-CS), which also stabilizes the receptor. Likewise, overexpression of a Grb10 mutant lacking the Src homology 2 (SH2) domain impaired ubiquitination of the IGF-IR in parental p6 and p6/Grb10 cells, indicating that Grb10 binding to Nedd4 is critical for ubiquitination of the receptor. These results suggest a role for the Grb10/Nedd4 complex in regulating ubiquitination and stability of the IGF-IR, and they suggest that Grb10 serves as an adapter to form a bridge between Nedd4 and the IGF-IR. This is the first demonstration of regulation of stability of a tyrosine kinase receptor by the Nedd4 (HECT) family of E3 ligases.

Structural basis for dimerization of the Grb10 Src homology 2 domain. Implications for ligand specificity

Grb7, Grb10, and Grb14 are members of a distinct family of adapter proteins that interact with various receptor tyrosine kinases upon receptor activation. Proteins in this family contain several modular signaling domains including a pleckstrin homology (PH) domain, a BPS (between PH and SH2) domain, and a C-terminal Src homology 2 (SH2) domain. Although SH2 domains are typically monomeric, we show that the Grb10 SH2 domain and also full-length Grb10 gamma are dimeric in solution under physiologic conditions. The crystal structure of the Grb10 SH2 domain at 1.65-A resolution reveals a non-covalent dimer whose interface comprises residues within and flanking the C-terminal alpha helix, which are conserved in the Grb7/Grb10/Grb14 family but not in other SH2 domains. Val-522 in the BG loop (BG3) and Asp-500 in the EF loop (EF1) are positioned to interfere with the binding of the P+3 residue of a phosphopeptide ligand. These structural features of the Grb10 SH2 domain will favor binding of dimeric, turn-containing phosphotyrosine sequences, such as the phosphorylated activation loops in the two beta subunits of the insulin and insulin-like growth factor-1 receptors. Moreover, the structure suggests the mechanism by which the Grb7 SH2 domain binds selectively to pTyr-1139 (pYVNQ) in Her2, which along with Grb7 is co-amplified in human breast cancers.

Grb10 inhibits insulin-stimulated insulin receptor substrate (IRS)-phosphatidylinositol 3-kinase/Akt signaling pathway by disrupting the association of IRS-1/IRS-2 with the insulin receptor

Grb10 has been proposed to inhibit or activate insulin signaling, depending on cellular context. We have investigated the mechanism by which full-length hGrb10gamma inhibits signaling through the insulin receptor substrate (IRS) proteins. Overexpression of hGrb10gamma in CHO/IR cells and in differentiated adipocytes significantly reduced insulin-stimulated tyrosine phosphorylation of IRS-1 and IRS-2. Inhibition occurred rapidly and was sustained for 60 min during insulin stimulation. In agreement with inhibited signaling through the IRS/PI 3-kinase pathway, we found hGrb10gamma to both delay and reduce phosphorylation of Akt at Thr(308) and Ser(473) in response to insulin stimulation. Decreased phosphorylation of IRS-1/2 may arise from impaired catalytic activity of the receptor, since hGrb10gamma directly associates with the IR kinase regulatory loop. However, yeast tri-hybrid studies indicated that full-length Grb10 blocks association between IRS proteins and IR, and that this requires the SH2 domain of Grb10. In cells, hGrb10gamma inhibited insulin-stimulated IRS-1 tyrosine phosphorylation in a dose-dependent manner, but did not affect IR catalytic activity toward Tyr(972) in the juxtamembrane region and Tyr(1158/1162/1163) in the regulatory domain. We conclude that binding of hGrb10gamma to IR decreases signaling through the IRS/PI 3-kinase/AKT pathway by physically blocking IRS access to IR.

The adapter protein ZIP binds Grb14 and regulates its inhibitory action on insulin signaling by recruiting protein kinase Czeta

Grb14 is a member of the Grb7 family of adapters and acts as a negative regulator of insulin-mediated signaling. Here we found that the protein kinase Czeta (PKCzeta) interacting protein, ZIP, interacted with Grb14. Coimmunoprecipitation experiments demonstrated that ZIP bound to both Grb14 and PKCzeta, thereby acting as a link in the assembly of a PKCzeta-ZIP-Grb14 heterotrimeric complex. Mapping studies indicated that ZIP interacted through its ZZ zinc finger domain with the phosphorylated insulin receptor interacting region (PIR) of Grb14. PKCzeta phosphorylated Grb14 under in vitro conditions and in CHO-IR cells as demonstrated by in vivo labeling experiments. Furthermore, Grb14 phosphorylation was increased under insulin stimulation, suggesting that the PKCzeta-ZIP-Grb14 complex is involved in insulin signaling. The PIR of Grb14, which also interacts with the catalytic domain of the insulin receptor (IR) and inhibits its activity, was preferentially phosphorylated by PKCzeta. Interestingly, the phosphorylation of Grb14 by PKCzeta increased its inhibitory effect on IR tyrosine kinase activity in vitro. The role of ZIP and Grb14 in insulin signaling was further investigated in vivo in Xenopus laevis oocytes. In this model, ZIP potentiated the inhibitory action of Grb14 on insulin-induced oocyte maturation. Importantly, this effect required the recruitment of PKCzeta and the phosphorylation of Grb14, providing in vivo evidences for a regulation of Grb14-inhibitory action by ZIP and PKCzeta. Together, these results suggest that Grb14, ZIP, and PKCzeta participate in a new feedback pathway of insulin signaling.

Two adaptor proteins differentially modulate the phosphorylation and biophysics of Kv1.3 ion channel by SRC kinase

The Shaker family K(+) channel protein, Kv1.3, is tyrosine phosphorylated by v-Src kinase at Tyr(137) and Tyr(449) to modulate current magnitude and kinetic properties. Despite two proline rich sequences and these phosphotyrosines contained in the carboxyl and amino terminals of the channel, v-Src kinase fails to co-immunoprecipitate with Kv1.3 as expressed in HEK 293 cells, indicating a lack of direct Src homology 3- or Src homology 2-mediated protein-protein interaction between the channel and the kinase. We show that the adaptor proteins, n-Shc and Grb10, are expressed in the olfactory bulb, a region of the brain where Kv1.3 is highly expressed. In HEK 293 cells, co-expression of Kv1.3 plus v-Src with Grb10 causes a decrease in v-Src-induced Kv1.3 tyrosine phosphorylation and a reversal of v-Src-induced Kv1.3 current suppression, increase in inactivation time constant (tau(inact)), and disruption of cumulative inactivation properties. Co-expression of Kv1.3 plus v-Src with n-Shc did not significantly alter v-Src-induced Kv1.3 current suppression but reversed v-Src induced increased tau(inact) and restored the right-shifted voltage at half-activation (V(1/2)) induced by v-Src. The v-Src-induced shift in V(1/2) and increased tau(inact) was retained when Tyr(220), Tyr(221), and Tyr(304) in the CH domain of n-Shc were mutated to Phe (triple Shc mutant) but was reversed back to control values when either wild-type Shc or the family member Sck, which is not a substrate for Src kinase, was substituted for the triple Shc mutant. Thus the portion of the CH domain that includes Tyr(220), Tyr(221), and Tyr(304) may regulate a shift in Kv1.3 voltage dependence and inactivation kinetics produced by n-Shc in the presence of v-Src. Collectively these data indicate that Grb10 and n-Shc adaptor molecules differentially modulate the degree of Kv1.3 tyrosine phosphorylation, the channel's biophysical properties, and the physical complexes associated with Kv1.3 in the presence of Src kinase.