Shc isoform-specific tyrosine phosphorylation by the insulin and epidermal growth factor receptors

Differences in the effects of Kenyan, Tanzanian, and Ethiopian coffee intake on interstitial glucose levels measured by FreeStyle Libre: A pilot case study

Background

Although generally considered part of a healthy diet, coffee consumption has been suspected to be associated with elevated epinephrine levels and increasing insulin resistance.

Objectives

We studied the effects of the intake of 3 different types of coffee (Tanzanian, Ethiopian, and Kenyan) on postprandial interstitial glucose levels.

Method

Interstitial glucose levels were measured every 15 minutes using the FreeStyle Libre glucose monitoring system (Abbott Diabetes Care Ltd, Witney, United Kingdom) in each individual after drinking coffee compared with when not consuming coffee.

Results

Unlike Tanzanian and Ethiopian coffees, Kenyan coffee suppressed the increase of postprandial interstitial glucose levels. Kenyan coffee beans contain less anhydrous caffeine and more chlorogenic acid than Tanzanian and Ethiopian coffee beans. These findings may explain the different effects of these coffee types on postprandial interstitial glucose levels. Furthermore, Kenyan coffee beans inhibited α-glucosidase activity, which may partially explain why Kenyan coffee reduces postprandial interstitial glucose levels.

Conclusions

Coffee is widely consumed as a beverage worldwide, and our findings suggest that patients with diabetes mellitus may benefit from drinking Kenyan coffee because of its ability to reduce postprandial interstitial glucose levels. (Curr Ther Res Clin Exp. 2020; 81:XXX-XXX).

FAM83G Is a Novel Inducer of Apoptosis

The family with sequence similarity 83 (FAM83) protein family G (FAM83G) possesses a predicted consensus phosphorylation motif for serine/threonine-protein kinase D1/protein kinase C mu (PKD1/PKCμ) at serine residue 356 (S356). In this study, overexpressed wild-type FAM83G coimmunoprecipitated with PKD1/PKCμ in Chinese hamster ovary (CHO) cells inhibited heat shock protein 27 (HSP27) phosphorylation at S82 and reduced the living cell number. The expression of a FAM83G phosphorylation-resistant mutant (S356A-FAM83G) had no effect on the living cell number or the induction of spontaneous apoptosis. By contrast, the introduction of a synthetic peptide encompassing FAM83G S356 into HCT116 and HepG2 cells decreased HSP27 S15 and S82 phosphorylation and induced spontaneous apoptosis. On the other hand, the introduction of FAM83G phosphorylation-resistant mutant synthesized peptides (S356A-AF-956 and S356A-AG-066) did not reduce the living cell number or induce spontaneous apoptosis. The endogenous expression of HSP27 and FAM83G was apparently greater in HCT116 and HepG2 cells compared with in CHO cells. In various types of lung cancer cell lines, the FAM83G messenger RNA (mRNA) level in non-small lung cancer cells was at a similar level to that in non-cancerous cells. However, the FAM83G mRNA level in the small cell lung cancer cell lines was variable, and the HSP27 mRNA level in FAM83G mRNA-rich types was greater than that in FAM83G mRNA-normal range types. Taken together, these data demonstrate that FAM83G S356 phosphorylation modulates HSP27 phosphorylation and apoptosis regulation and that HSP27 is a counterpart of FAM83G.

Cathepsin Inhibition Prevents Autophagic Protein Turnover and Downregulates Insulin Growth Factor-1 Receptor-Mediated Signaling in Neuroblastoma

Inhibition of the major lysosomal proteases, cathepsins B, D, and L, impairs growth of several cell types but leads to apoptosis in neuroblastoma. The goal of this study was to examine the mechanisms by which enzyme inhibition could cause cell death. Cathepsin inhibition caused cellular accumulation of fragments of the insulin growth factor 1 (IGF-1) receptor. The fragments were located in dense organelles that were characterized as autophagosomes. This novel discovery provides the first clear link between lysosomal function, autophagy, and IGF-1- mediated cell proliferation. A more in-depth analysis of the IGF1 signaling pathway revealed that the mitogen-activated protein kinase (MAPK) cell-proliferation pathway was impaired in inhibitor treated cells, whereas the Akt cell survival pathway remained functional. Shc, an adapter protein that transmits IGF-1 signaling through the MAPK pathway, was sequestered in autophagosomes; whereas IRS-2, an adapter protein that transmits IGF-1 signaling through the Akt pathway, was unaffected by cathepsin inhibition. Furthermore, Shc was sequestered in autophagosomes as its active form, indicating that autophagy is a key mechanism for downregulating IGF-1-induced cell proliferation. Cathepsin inhibition had a greater effect on autophagic sequestration of the neuronal specific adapter protein, Shc-C, than ubiquitously expressed Shc-A, providing mechanistic support for the enhanced sensitivity of neuronally derived tumor cells. We also observed impaired activation of MAPK by epidermal growth factor treatment in inhibitor-treated cells. The Shc adapter proteins are central to transducing proliferation signaling by a range of receptor tyrosine kinases; consequently, cathepsin inhibition may become an important therapeutic approach for treating neuroblastoma and other tumors of neuronal origin.

Nucleobindin-2 is a positive modulator of EGF-dependent signals leading to enhancement of cell growth and suppression of adipocyte differentiation

Nucleobindin-2 is a 420-amino-acid EF-hand calcium-binding protein that undergoes proteolytic processing to generate an 82-amino-acid amino-terminal peptide termed nesfatin-1. To determine whether nucleobindin-2 has any biological function, nucleobindin-2 was either overexpressed or knocked down by short hairpin RNA in cultured CHO cells expressing the human insulin and epidermal growth factor (EGF) receptors (CHO/IE) and in 3T3-L1 cells. Reduction in nucleobindin-2 expression inhibited EGF-stimulated MAPK kinase (S217/S221) and Erk phosphorylation (T202/Y204). In contrast, there was no significant effect on EGF-stimulated EGF receptor phosphorylation, EGF receptor internalization, or 52-kDa Shc and c-Raf phosphorylation. Although kinase suppressor of Ras-1 and protein phosphatase 2A expression was not changed, intracellular calcium concentrations and PP2A activity was significantly increased in nucleobindin-2 knocked-down cells. Concomitant with these alterations in EGF-stimulated signaling, cell proliferation was significantly reduced in nucleobindin-2 knocked-down cells. Moreover, reduced nucleobindin-2 expression in 3T3-L1 preadipocytes resulted in a greater extent of 3T3-L1 cell adipocyte differentiation. Taken together, these data indicate that nucleobindin-2 regulates EGF-stimulated MAPK kinase/Erk signaling, cell proliferation, and adipocyte differentiation.

Mutant p53 disrupts role of ShcA protein in balancing Smad protein-dependent and -independent signaling activity of transforming growth factor-β (TGF-β)

Biomarkers are lacking for identifying the switch of transforming growth factor-β (TGF-β) from tumor-suppressing to tumor-promoting. Mutated p53 (mp53) has been suggested to switch TGF-β to a tumor promoter. However, we found that mp53 does not always promote the oncogenic role of TGF-β. Here, we show that endogenous mp53 knockdown enhanced cell migration and phosphorylation of ERK in DU145 prostate cancer cells. Furthermore, ectopic expression of mp53 in p53-null PC-3 prostate cancer cells enhanced Smad-dependent signaling but inhibited TGF-β-induced cell migration by down-regulating activated ERK. Reactivation of ERK by the expression of its activator, MEK-1, restored TGF-β-induced cell migration. Because TGF-β is known to activate the MAPK/ERK pathway through direct phosphorylation of the adaptor protein ShcA and MAPK/ERK signaling is pivotal to tumor progression, we investigated whether ShcA contributed to mp53-induced ERK inhibition and the conversion of the role of TGF-β during carcinogenesis. We found that mp53 expression led to a decrease of phosphorylated p52ShcA/ERK levels and an increase of phosphorylated Smad levels in a panel of mp53-expressing cancer cell lines and in mammary glands and tumors from mp53 knock-in mice. By manipulating ShcA levels to regulate ERK and Smad signaling in human untransformed and cancer cell lines, we showed that the role of TGF-β in regulating anchorage-dependent and -independent growth and migration can be shifted between growth suppression and migration promotion. Thus, our results for the first time suggest that mp53 disrupts the role of ShcA in balancing the Smad-dependent and -independent signaling activity of TGF-β and that ShcA/ERK signaling is a major pathway regulating the tumor-promoting activity of TGF-β.

Impaired TrkB-mediated ERK1/2 activation in huntington disease knock-in striatal cells involves reduced p52/p46 Shc expression

Altered neurotrophic support as a result of reduced brain-derived neurotrophic factor (BDNF) expression and trafficking has been revealed as a key factor in Huntington disease (HD) pathology. BDNF binds to and activates the tyrosine kinase receptor TrkB, leading to activation of intracellular signaling pathways to promote differentiation and cell survival. In order to design new neuroprotective therapies based on BDNF delivery, it is important to define whether BDNF-mediated TrkB signaling is affected in HD. Here, we demonstrate reduced TrkB-mediated Ras/MAPK/ERK1/2 signaling but unchanged phosphatidylinositol 3-kinase/Akt and phospholipase Cgamma activation in knock-in HD striatal cells. Altered BDNF-mediated ERK1/2 activation in mutant huntingtin cells is associated with reduced expression of p52/p46 Shc docking proteins. Notably, reduced BDNF-induced ERK1/2 activation increases the sensitivity of mutant huntingtin striatal cells to oxidative damage. Accordingly, pharmacological activation of the MAPK pathway with PMA prevents cell death induced by oxidative stress. Taken together, our results suggest that in addition to reduced BDNF, diminished Ras/MAPK/ERK1/2 activation is involved in neurotrophic deficits associated with HD pathology. Therefore, pharmacological approaches aimed to directly modulate the MAPK/ERK1/2 pathway may represent a valuable therapeutic strategy in HD.

p66shc negatively regulates insulin-like growth factor I signal transduction via inhibition of p52shc binding to Src homology 2 domain-containing protein tyrosine phosphatase substrate-1 leading to impaired growth factor receptor-bound protein-2 membrane recruitment

Our previous studies have indicated an essential role of p52shc in mediating IGF-I activation of MAPK in smooth muscle cells (SMC). However, the role of the p66 isoform of shc in IGF-I signal transduction is unclear. In the current study, two approaches were employed to investigate the role of p66shc in mediating IGF-I signaling. Knockdown p66shc by small interfering RNA enhanced IGF-I-stimulated p52shc tyrosine phosphorylation and growth factor receptor-bound protein-2 (Grb2) association, resulting in increased IGF-I-dependent MAPK activation. This was associated with enhanced IGF-I-stimulated cell proliferation. In contrast, knockdown of p66shc did not affect IGF-I-stimulated IGF-I receptor tyrosine phosphorylation. Overexpression of p66shc impaired IGF-I-stimulated p52shc tyrosine phosphorylation and p52shc-Grb2 association. In addition, IGF-I-dependent MAPK activation was also impaired, and SMC proliferation in response to IGF-I was inhibited. IGF-I-dependent cell migration was enhanced by p66shc knockdown and attenuated by p66shc overexpression. Mechanistic studies indicated that p66shc inhibited IGF-I signal transduction via competitively inhibiting the binding of Src homology 2 domain-containing protein tyrosine phosphatase-2 (SHP-2) to SHP substrate-1 (SHPS-1), leading to the disruption of SHPS-1/SHP-2/Src/p52shc complex formation, an event that has been shown previously to be essential for p52shc phosphorylation and Grb2 recruitment. These findings indicate that p66shc functions to negatively regulate the formation of a signaling complex that is required for p52shc activation in response to IGF-I, thus leading to attenuation of IGF-I-stimulated cell proliferation and migration.

Simplification of stochastic chemical reaction models with fast and slow dynamics

Biological systems often involve chemical reactions occurring in low-molecule-number regimes, where fluctuations are not negligible and thus stochastic models are required to capture the system behaviour. The resulting models are generally quite large and complex, involving many reactions and species. For clarity and computational tractability, it is important to be able to simplify these systems to equivalent ones involving fewer elements. While many model simplification approaches have been developed for deterministic systems, there has been limited work on applying these approaches to stochastic modelling. Here, we describe a method that reduces the complexity of stochastic biochemical network models, and apply this method to the reduction of a mammalian signalling cascade and a detailed model of the process of bacterial gene expression. Our results indicate that the simplified model gives an accurate representation for not only the average numbers of all species, but also for the associated fluctuations and statistical parameters.

Expression of the ErbB4 receptor causes reversal regulation of PP2A in the Shc signal transduction pathway in human cancer cells

Expression of ErbB4 receptor is correlated with the incidence of non-metastatic types of human cancers, whereas the overexpression of other ErbB receptor families (ErbB1/EGFR, ErbB2 and ErbB3) is correlated to the formation of metastatic tumors. However, the molecular mechanism underlying this phenomenon has been unclear. Earlier, we demonstrated that okadaic acid (OA), an inhibitor of a serine/threonine phosphatase PP2A, stimulated the growth hormone-induced ERK phosphorylation in the wild type Chinese hamster ovary (CHO) cells and the cells expressing ErbB1 receptor, but suppressed ERK activation in CHO cells that express ErbB4 receptor. PP2A had been understood as a negative regulator of the growth hormone-stimulated signal transduction pathways, however, this observation suggested that expression of ErbB4 receptor reversed the regulation of PP2A in the ErbB4 signalling pathway. In this study, we found that OA suppressed phosphorylation of Shc at Tyr317, therefore it down-regulated ERK phosphorylation in the ErbB4 expressing CHO cells. Accordingly, basal PP2A contributed to the phosphorylation of Shc Tyr317 in ErbB4 expressing CHO cells, nevertheless it had been reported that PP2A negatively regulates Shc tyrosine phosphorylation in the EGF- or IGF-I-induced signalling pathways. By testing OA for human cancer cell lines that express different types of ErbB receptors, we found that ErbB4 receptor expression was accompanied with positive regulation of PP2A for phosphorylation of Shc Tyr317 and its downstream ERK phosphorylation in MCF-7 and SK-OV-3 cell lines, but not in LNCaP and PC-3 cells. Thus, PP2A regulates the ERK activity in a cell-specific manner, and it is speculated that distinct regulation of PP2A in the ErbB4 receptor signalling pathway may cause a difference in progression of cancer phenotypes.

Regulation of endogenous human NPFF2 receptor by neuropeptide FF in SK-N-MC neuroblastoma cell line

Neuropeptide FF has many functions both in the CNS and periphery. Two G protein-coupled receptors (NPFF1 and NPFF2 receptors) have been identified for neuropeptide FF. The expression analysis of the peptide and receptors, together with pharmacological and physiological data, imply that NPFF2 receptor would be the primary receptor for neuropeptide FF. Here, we report for the first time a cell line endogenously expressing hNPFF2 receptor. These SK-N-MC neuroblastoma cells also express neuropeptide FF. We used the cells to investigate the hNPFF2 receptor function. The pertussis toxin-sensitive inhibition of adenylate cyclase activity upon receptor activation indicated coupling to Gi/o proteins. Upon agonist exposure, the receptors were internalized and the mitogen-activated protein kinase cascade was activated. Upon neuropeptide FF treatment, the actin cytoskeleton was reorganized in the cells. The expression of hNPFF2 receptor mRNA was up-regulated by neuropeptide FF. Concomitant with the receptor mRNA, the receptor protein expression was increased. The homologous regulation of hNPFF2 receptor correlates with our previous results in vivo showing that during inflammation, the up-regulation of neuropeptide FF mRNA precedes that of NPFF2 receptor. The regulation of hNPFF2 receptor by NPFF could also be important in the periphery where neuropeptide FF has been suggested to function as a hormone.

Sustained MAPK activation is dependent on continual NGF receptor regeneration

It still remains intriguing how signal specificity is achieved when different signals are relayed by the common intracellular signal transduction pathways. A well documented example for signal specificity determination is found in rat phaeochromocytoma PC12 cells where epidermal growth factor (EGF) stimulation produces a transient mitogen-activated protein kinase (MAPK) activation and leads to cell proliferation while nerve growth factor (NGF) initiates a sustained MAPK activation and induces cell differentiation. In this simulation, we demonstrated that NGF-induced sustained MAPK activation may mainly depend on continual regeneration of NGF receptors and that the presence of a small pool of surface receptors is enough to maintain a sustained MAPK activation. On the other hand, MAPK activation is not significantly sensitive to the half-life of internalized receptors and the levels of NGF-specific MAPK phosphatase MAP kinase phosphatase-3 (MKP-3), though cytoplasmic persistence of internalized NGF-bound receptors and the MKP-3 dependent feedback control also contribute to the sustaining of MAPK activation. These results are consistent with the recent experimental evidence that persistent tyrosine receptor kinase A (TrkA) activity is necessary to maintain transcription in the differentiating PC12 cells (Chang et al. 2003) and a sustained Src kinase activity is detected in response to NGF stimulation (Gatti 2003). It is suggested that sustained or transient MAPK activation induced by different growth factor and neurotrophins, which is crucial to their signaling specificity, could be satisfactorily accounted for by their specific receptor turnover kinetics rather than by the activation of specific downstream signaling cascades.

Differential activation of epidermal growth factor (EGF) receptor downstream signaling pathways by betacellulin and EGF

To determine the downstream signaling pathways regulated by betacellulin (BTC) in comparison with epidermal growth factor (EGF), we used Chinese hamster ovary cells overexpressing the human EGF receptor (ErbB1/EGFR). The overall time-dependent activation of EGFR autophosphorylation was identical in cells treated with 1 nm BTC or 1.5 nm EGF. Analysis of site-specific EGFR phosphorylation demonstrated that the BTC and EGF tyrosine phosphorylation of Y1086 was not significantly different. In contrast, the autophosphorylation of Y1173 was markedly reduced in BTC-stimulated cells, compared with EGF stimulation that directly correlated with a reduced BTC stimulation of Shc tyrosine phosphorylation, Ras, and Raf-1 activation. On the other hand, Y1068 phosphorylation was significantly increased after BTC stimulation, compared with EGF in parallel with a greater extent of Erk phosphorylation. Expression of a dominant interfering MEK kinase 1 (MEKK1) and Y1068F EGFR more efficiently blocked the enhanced Erk activation by BTC, compared with EGF. Interestingly BTC had a greater inhibitory effect on apoptosis, compared with EGF, and expression of Y1068F EGFR abolished this enhanced inhibitory effect. Together, these data indicated that although BTC and EGF share overlapping signaling properties, the ability of BTC to enhance Erk activation occurs independent of Ras. The increased BTC activation results from a greater extent of Y1068 EGFR tyrosine phosphorylation and subsequent increased recruitment of the Grb2-MEKK1 complex to the plasma membrane, compared with EGF stimulation. The increased Erk activation by BTC associated with antiapoptotic function.

A Cryptic Targeting Signal Induces Isoform-specific Localization of p46Shc to Mitochondria

The human Src homology and collagen (Shc) gene encodes three protein isoforms of 46, 52, and 66 kDa that belong to a family of molecular adapters involved in several signal transduction pathways. Recently, the 66-kDa isoform has been shown to play a central role in controlling reactive oxygen species metabolism and life span in mammals. Despite the large amount of information available on the biology and biochemistry of Shc proteins, very little is known regarding the regulation of their subcellular localization. Here we demonstrate the specific and selective localization of p46Shc to the mitochondrial matrix. Through deletion mapping experiments, we show that targeting of p46Shc to mitochondria is mediated by its first 32 amino acids, which behave as a bona fide mitochondrial targeting sequence. We further demonstrate that the N-terminal location of the signal peptide is critical for its function. This accounts for the observation that p52Shc and p66Shc, containing the same sequence but more internally located, display a remarkably different subcellular localization. These findings indicate that p46Shc may exert a non-redundant biological function in signal transduction pathways involving mitochondria.

Shc-dependent pathway is redundant but dominant in MAPK cascade activation by EGF receptors: a modeling inference

In cell signaling cascades, one stimulus often leads to various physiological functions by multiple pathways. Perturbation of one pathway by blocking or overexpressing one of its components will result in changes in multiple pathways and multiple cell functions. Thus, it is important to reveal the relative contribution of each pathway to each function in order to assess the consequence of perturbations (e.g. drug delivery). By exploring an established mathematical model, the Shc-dependent pathway is found to be both redundant and dominant during activation of the mitogen-activated protein kinase cascade by epidermal growth factor receptor (EGFR). Its dominance results from the majority consumption of the common precursor ((EGF-EGFR*)2-GAP) by this pathway. The key steps for the dominance are the binding and phosphorylation of Shc. In conclusion, cells may prefer the long Shc-dependent pathway to the short Shc-independent pathway.

Adaptor ShcA protein binds tyrosine kinase Tie2 receptor and regulates migration and sprouting but not survival of endothelial cells

Angiopoietin-1 can promote migration, sprouting, and survival of endothelial cells through activation of different signaling pathways triggered by the Tie2 tyrosine kinase receptor. ShcA adapter proteins are targets of activated tyrosine kinases and are implicated in the transmission of activation signals to the Ras/mitogen-activated protein kinase pathway. Here we report the identification of an interaction between the adapter protein ShcA and the cytoplasmic domain of Tie2 through in vitro co-immunoprecipitation analysis. Stimulation of endogenous Tie2 in endothelial cells with its ligand angiopoietin-1 increased its association with ShcA and phosphorylation of the adapter protein. The interaction requires the SH2 domain of ShcA and the tyrosine phosphorylation of Tie2 as shown by pull-down experiments. Furthermore, Tyr-1101 of Tie2 was identified as the primary binding site for the SH2 domain of ShcA. Overexpression of a dominant-negative form of ShcA affects angiopoietin-1-induced chemotaxis and sprouting, although it has no effect on survival of endothelial cells. Furthermore, this mutant partially reduces the tyrosine phosphorylation of the regulatory p85 subunit of phosphatidylinositol 3-kinase. Together, our results identified a novel interaction between Tie2 with the adapter molecule ShcA and suggested that this interaction may play a role in the regulation of migration and three-dimensional organization of endothelial cells induced by angiopoietin-1.

Coupling of folding and binding in the PTB domain of the signaling protein Shc

The notion that certain proteins lack intrinsic globular structure under physiological conditions and that the attainment of fully folded structure only occurs upon the binding of target molecules has been recently gaining popularity. We report here the solution structure of the PTB domain of the signaling protein Shc in the free form. Comparison of this structure with that of the complex form, obtained previously with a phosphopeptide ligand, reveals that the Shc PTB domain is structurally disordered in the free form, particularly around the regions constituting the peptide binding pocket. The binding of the ligand appears to reorganize this pocket through local folding events triggering a conformational switch between the free and the complex forms.

Identification of MEK- and phosphoinositide 3-kinase-dependent signalling as essential events during Chlamydia pneumoniae invasion of HEp2 cells

The ability of Chlamydia pneumoniae to survive and cause disease is predicated on efficient invasion of cellular hosts. While it is recognized that chlamydial determinants are important for mediating attachment and uptake into non-phagocytic cells, little is known about the bacterial ligands and cellular receptors that facilitate invasion or host cell signal transduction pathways implicated in this process. We used transmission and scanning electron microscopy to demonstrate that attachment of bacteria to host cells induced the appearance of microvilli on host cell membranes. Invasion occurred 30-120 min after cell contact with the subsequent loss of membrane microvilli. Using an epithelial cell infection model, C. pneumoniae invasion caused a rapid and sustained increase in MEK-dependent phosphorylation and activation of ERK1/2, followed by PI 3-kinase-dependent phosphorylation and activation of Akt. Tyrosine phosphorylation of focal adhesion kinase (FAK) preceded its appearance in a complex with the p85 subunit of PI 3-kinase during chlamydial invasion and isoform-specific tyrosine phosphorylation of the docking protein Shc also occurred at the time of attachment and entry of bacteria. Chlamydia entry but not attachment could be abrogated with specific inhibitors of MEK, PI 3-kinase and actin polymerization, demonstrating the importance of these signalling pathways and an intact actin cytoskeleton for C. pneumoniae invasion. These results suggest that activation of specific cell signalling pathways is an essential strategy used by C. pneumoniae to invade epithelial cells.

Identification and genetic analysis of human and mouse activated Cdc42 interacting protein-4 isoforms

By yeast two-hybrid screening with the Src kinase Lyn as bait, we identified a novel gene product with features of a scaffolding protein. Reported as Felic ( es-related, with homology to Ezrin, Lyn interactor with Cdc42), it is related to the CIP4 (Cdc42 Interacting Protein-4) gene. Southern blotting for CIP4/Felic of genomic DNA shows a single band, suggesting no gene duplication. Felic differs from CIP4 because of a 29 nucleotide sequence derived from the end of intron 13. Consequently, there is an out-of-frame translation that destroys an SH3 domain. Analysis of various tissues shows that the original CIP4 is the predominant transcript. Therefore, we propose to call that, CIP4a and Felic, CIP4b. During screening of the colorectal CaCo2 cell line, clones corresponding to a third CIP4-related transcript (CIP4c) were identified. CIP4c encodes a premature stop codon, resulting in the loss of the SH3 domain. A fourth, relatively abundant transcript (CIP4h) was isolated from heart, lung, and trachea tissue. CIP4h retains the SH3 domain. CIP4 levels are modified by all-trans-retinoic acid. The presence of alternative splice transcripts, with or without SH3 domains, suggests that CIP4 regulates cytoskeletal organization through structural-functional differences in a tissue-specific manner.

Signaling via Shc family adapter proteins

The adapter protein Shc was initially identified as an SH2 containing proto-oncogene involved in growth factor signaling. Since then a number of studies in multiple systems have implicated a role for Shc in signaling via many different types of receptors, such as growth factor receptors, antigen receptors, cytokine receptors, G-protein coupled receptors, hormone receptors and integrins. In addition to the ubiquitous ShcA, two other shc gene products, ShcB and ShcC, which are predominantly expressed in neuronal cells, have also been identified. ShcA knockout mice are embryonic lethal and have clearly suggested an important role for ShcA in vivo. Based on dominant negative studies and mouse embryos deficient in ShcA, a clear role for Shc in leading to mitogen activated protein kinase (MAPK) activation has been established. However MAPK activation may not be the sole function of Shc proteins. Although Shc has also been linked to other signaling events such as c-Myc activation and cell survival, the mechanistic understanding of these signaling events remains poorly characterized. Given the apparently central role that Shc plays signaling via many receptors, delineating the precise mechanism(s) of Shc-mediated signaling may be critical to our understanding of the effects mediated through these receptors.

Autoregulation of cell-specific MAP kinase control of the tryptophan hydroxylase promoter

The neurotransmitter serotonin controls a wide range of biological systems, including its own synthesis and release. As the rate-limiting enzyme in serotonin biosynthesis, tryptophan hydroxylase (TPH) is a potential target for this autoregulation. Using the serotonergic neuron-like CA77 cell line, we have demonstrated that treatment with a 5-hydroxytryptamine autoreceptor agonist, CGS 12066A, can lower TPH mRNA levels and promoter activity. We reasoned that this repression might involve inhibition of MAP kinases, since 5-HT1 receptors can increase mitogen-activated protein (MAP) kinase phosphatase levels. To test this hypothesis, we first showed that the TPH promoter can be activated 20-fold by mitogen-activated extracellular-signal regulated kinase kinase kinase (MEKK), an activator of MAP kinases. This activation was then blocked by CGS 12066A. The maximal MAP kinase and CGS repression regulatory region was mapped to between -149 and -45 base pairs upstream of the transcription start site. The activation by MEKK appears to be cell-specific, because MEKK did not activate the TPH promoter in nonneuronal cell lines. At least part, but not all, of the MAP kinase responsiveness was mapped to an inverted CCAAT box that binds the transcription factor NF-Y. These data suggest a model for the autoregulation of serotonin biosynthesis by repression of MAP kinase stimulation of the TPH promoter.

Role of insulin and insulin receptor in learning and memory

As one of the most extensively studied protein hormones, insulin and its receptor have been known to play key roles in a variety of important biological functions. Until recent years, the functions of insulin and insulin receptor (IR) in the central nervous system (CNS) have largely remained unclear. IR is abundantly expressed in several specific brain regions that govern fundamental behaviors such as food intake, reproduction and high cognition. The IR from the periphery and CNS exhibit differences in both structure and function. In addition to that from the peripheral system, locally synthesized insulin in the brain has also been identified. Accumulated evidence has demonstrated that insulin/IR plays important roles in associative learning, as suggested by results from both interventive and correlative studies. Interruption of insulin production and IR activity causes deficits in learning and memory formation. Abnormal insulin/IR levels and activities are seen in Alzheimer's dementia, whereas administration of insulin significantly improves the cognitive performance of these patients. The synaptic bases for the action of insulin/IR include modifying neurotransmitter release processes at various types of presynaptic terminals and modulating the activities of both excitatory and inhibitory postsynaptic receptors such as NMDA and GABA receptors, respectively. At the molecular level, insulin/IR participates in regulation of learning and memory via activation of specific signaling pathways, one of which is shown to be associated with the formation of long-term memory and is composed of intracellular molecules including the shc, Grb-r/SOS, Ras/Raf, and MEK/MAP kinases. Cross-talk with another IR pathway involving IRS1, PI3 kinase, and protein kinase C, as well as with the non-receptor tyrosine kinase pp60c-src, may also be associated with memory processing.

Differential feedback regulation of the MAPK cascade underlies the quantitative differences in EGF and NGF signalling in PC12 cells

Although epidermal growth factor (EGF) induces transient activation of Ras and the mitogen-activated protein kinase (MAPK) cascade in PC12 cells, whereas nerve growth factor (NGF) stimulates sustained activation, the basis for these contrasting responses is not known. We have developed a computer simulation of EGF-induced MAPK cascade activation, which provides quantitative evidence that feedback inhibition of the MAPK cascade is the most important factor in determining the duration of cascade activation. Hence, we propose that the observed quantitative differences in EGF and NGF signalling can be accounted for by differential feedback regulation of the MAPK cascade.

Gab-1-mediated IGF-1 signaling in IRS-1-deficient 3T3 fibroblasts

The insulin receptor substrate (IRS) family of proteins mediate a variety of intracellular signaling events by serving as signaling platforms downstream of several receptor tyrosine kinases including the insulin and insulin-like growth factor-1 (IGF-1) receptors. Recently, several new members of this family have been identified including IRS-3, IRS-4, and growth factor receptor-binding protein 2-associated binder-1 (Gab-1). 3T3 cell lines derived from IRS-1-deficient embryos exhibit a 70-80% reduction in IGF-1-stimulated S-phase entry and a parallel decrease in the induction of the immediate-early genes c-fos and egr-1 but unaltered activation of the mitogen-activated protein kinases extracellular signal-regulated kinase-1 and extracellular signal-regulated kinase-2. Reconstitution of IRS-1 expression in IRS-1-deficient fibroblasts by retroviral mediated gene transduction is capable of restoring these defects. Overexpression of Gab-1 in IRS-1-deficient fibroblasts also results in the restoration of egr-1 induction to levels similar to those achieved by IRS-1 reconstitution and markedly increases IGF-1-stimulated S-phase progression. Gab-1 is capable of regulating these biological end points despite the absence of IGF-1 stimulated tyrosine phosphorylation. These data provide evidence that Gab-1 may serve as a unique signaling intermediate in insulin/IGF-1 signaling for induction of early gene expression and stimulation of mitogenesis without direct tyrosine phosphorylation.

Chemokine receptors CXCR-1/2 activate mitogen-activated protein kinase via the epidermal growth factor receptor in ovarian cancer cells

Ovarian cancer typically disseminates widely in the abdomen, a characteristic that limits curative therapy. The mechanisms that promote ovarian cancer cell migration are incompletely understood. We studied model SK-OV-3 ovarian cancer cells and observed robust expression of the alpha chemokine receptors CXCR-1 and CXCR-2. Interleukin-8 (IL-8) treatment caused shape changes in the cells, with membrane ruffling and formation/retraction of thin actin-like projections, as detected by time-lapse microscopy. Stimulation of the CXCR-1/2 receptors by human interleukin 8 (IL-8) rapidly activated the p44/42 mitogen-activated protein (extracellular signal-regulated kinase (Erk1/2)) kinase pathway. Treatment of SK-OV-3 cells with the inhibitors genestein and herbimycin A indicated that tyrosine kinases were involved in the IL-8 activation of Erk1 and Erk2. Of note, IL-8 induced transient phosphorylation of the epidermal growth factor (EGF) receptor and its association with the adaptor molecules Shc and Grb2. This transactivation of the EGF receptor was dependent on intracellular Ca(2+) mobilization. Furthermore AG1478, a specific inhibitor of the EGF receptor kinase, blocked Erk1 and Erk2 activation. c-Src kinase was not involved in the IL-8-mediated phosphorylation of the EGF receptor, but was critical for Shc phosphorylation and downstream Erk1/2 kinase activation. These results suggest important "cross-talk" between chemokine and growth factor pathways that may link signals of cell migration and proliferation in ovarian cancer.

In vitro endosome-lysosome transfer of dephosphorylated EGF receptor and Shc in rat liver

We have studied the endosome-lysosome transfer of internalized epidermal growth factor receptor (EGFR) complexes in a cell-free system from rat liver. Analytical subfractionation of a postmitochondrial supernatant fraction showed that a pulse of internalized [(125)I]EGF was largely associated with a light endosomal fraction devoid of lysosomal markers. After an additional 30 min incubation in vitro in the presence of an ATP-regenerating system, the amount of [(125)I]EGF in this compartment decreased by 39%, with an increase in [(125)I]EGF in lysosomes. No transfer of [(125)I]EGF to the cytosol was detected. To assess the fate of the internalized EGFR protein over the time course of the endo-lysosomal transfer of the ligand, the effect of a saturating dose of native EGF on subsequent lysosomal targeting of the EGFR was evaluated by immunoblotting. A massive translocation of the EGFR to the endosomal compartment was observed in response to ligand injection coincident with its tyrosine phosphorylation and receptor recruitment of the tyrosine-phosphorylated adaptor protein Shc. During cell-free endosome-lysosome fusion, a time-dependent increase in the content of the EGFR and the two 55- and 46-kDa Shc isoforms was observed in lysosomal fractions with a time course superimposable with the lysosomal transfer of the ligand; no transfer of the 66-kDa Shc isoform was detected. The relationship between EGFR tyrosine kinase activity and EGFR sorting in endosomes investigated by immunoblot studies with anti-phosphotyrosine antibodies revealed that endosomal dephosphorylation of EGFR and Shc preceded lysosomal transfer. These results support the view that a lysosomal targeting machinery distinct from the endosomal receptor kinase activity, such as the recruitment of the signaling molecule Shc, may regulate this sorting event in the endosome.

Quantification of short term signaling by the epidermal growth factor receptor

During the past decade, our knowledge of molecular mechanisms involved in growth factor signaling has proliferated almost explosively. However, the kinetics and control of information transfer through signaling networks remain poorly understood. This paper combines experimental kinetic analysis and computational modeling of the short term pattern of cellular responses to epidermal growth factor (EGF) in isolated hepatocytes. The experimental data show transient tyrosine phosphorylation of the EGF receptor (EGFR) and transient or sustained response patterns in multiple signaling proteins targeted by EGFR. Transient responses exhibit pronounced maxima, reached within 15-30 s of EGF stimulation and followed by a decline to relatively low (quasi-steady-state) levels. In contrast to earlier suggestions, we demonstrate that the experimentally observed transients can be accounted for without requiring receptor-mediated activation of specific tyrosine phosphatases, following EGF stimulation. The kinetic model predicts how the cellular response is controlled by the relative levels and activity states of signaling proteins and under what conditions activation patterns are transient or sustained. EGFR signaling patterns appear to be robust with respect to variations in many elemental rate constants within the range of experimentally measured values. On the other hand, we specify which changes in the kinetic scheme, rate constants, and total amounts of molecular factors involved are incompatible with the experimentally observed kinetics of signal transfer. Quantitation of signaling network responses to growth factors allows us to assess how cells process information controlling their growth and differentiation.

Insulin induces tyrosine phosphorylation of the insulin receptor and SHC, and SHC/GRB2 association in cerebellum but not in forebrain cortex of rats

A growth-related branch of the insulin-signaling pathway was studied in the forebrain cortex and cerebellum of Wistar rats. Anesthetized rats received a bolus injection of saline or insulin through the cava vein after which fragments of cerebellum and forebrain cortex were excised and immediately homogenized. Insulin receptor and p46SHCA phosphorylation, and p46SHCA/GRB2 association were detected by immunoprecipitation and blotting with specific antibodies. Insulin stimulated the rapid phosphorylation of its receptor in cerebellum, followed by p46SHCA phosphorylation and GRB2 recruitment. The optimal insulin dose for the induction of p46SHCA/GRB2 binding was 60 microg, and time-course experiments showed that maximum phosphorylation/binding occurred 2-3 min after stimulation. Although insulin receptors and SHC were present in forebrain cortex, there was no increase in their phosphorylation, nor was there any recruitment of GRB2 following stimulation with insulin. Thus, although elements involved in the early intracellular response to insulin are present in the central nervous system, differences in their activation/regulation may account for the functional roles of insulin in these tissues.

The dynamic expression of the epidermal growth factor receptor and epidermal growth factor ligand family in a differentiating intestinal epithelial cell line

The Caco-2 intestinal epithelial cell line differentiates when cultured on plastic or permeable filters, and offers a valuable system to study events associated with enterocytic differentiation in vitro. Little is known as to whether the expression of the epidermal growth factor receptor (EGFR) and its ligands changes as intestinal epithelial cells differentiate. We found that total cellular EGFR protein and mRNA transcript levels were relatively unchanged during Caco-2 cell differentiation, but the expression of surface EGFR and patterns of steady state epidermal growth factor (EGF)-family ligand expression changed significantly. EGFR affinity, surface EGFR expression levels, and the repertoire of expressed EGF-family ligands, were different between Caco-2 cells cultured on plastic and filters. Functionally, EGFR-mediated cell proliferation and tyrosine phosphorylation of the signal transduction protein SHC could be inhibited in Caco-2 cells cultured on filters, but not on plastic. Thus, the substrate on which the cells were grown and the degree of cell differentiation strongly modulate EGFR affinity, EGFR surface expression, the steady state expression of EGF-family ligands, as well as, EGFR-mediated cellular responses. Our results suggest that the EGFR system is regulated during intestinal epithelial cell differentiation primarily at the level of ligand expression.

Insulin receptor substrate 2 and Shc play different roles in insulin-like growth factor I signaling

The major substrates for the type I insulin-like growth factor (IGF-I) receptor are Shc and insulin receptor substrate (IRS) proteins. In the current study, we report that IGF-I induces a sustained tyrosine phosphorylation of Shc and its association with Grb2 in SH-SY5Y human neuroblastoma cells. The time course of Shc tyrosine phosphorylation parallels the time course of IGF-I-stimulated activation of extracellular signal-regulated kinase (ERK). Transfection of SH-SY5Y cells with a p52 Shc mutant decreases Shc tyrosine phosphorylation and Shc-Grb2 association. This results in the inhibition of IGF-I-mediated ERK tyrosine phosphorylation and neurite outgrowth. In contrast, IGF-I induces a transient tyrosine phosphorylation of IRS-2 and an association of IRS-2 with Grb2. The time course of IRS-2 tyrosine phosphorylation and IRS-2-Grb2 and IRS-2-p85 association closely resembles the time course of IGF-I-mediated membrane ruffling. Treating cells with the phosphatidylinositol 3'-kinase inhibitors wortmannin and LY294002 blocks IGF-I-induced membrane ruffling. The ERK kinase inhibitor PD98059, as well as transfection with the p52 Shc mutant, has no effect on IGF-I-mediated membrane ruffling. Immunolocalization studies show IRS-2 and Grb2, but not Shc, concentrated at the tip of the extending growth cone where membrane ruffling is most active. Collectively, these results suggest that the association of Shc with Grb2 is essential for IGF-I-mediated neurite outgrowth, whereas the IRS-2-Grb2-phosphatidylinositol 3'-kinase complex may regulate growth cone extension and membrane ruffling.

Adenovirus-mediated transfer of a modified human proinsulin gene reverses hyperglycemia in diabetic mice

The human proinsulin cDNA was introduced into a replication-defective adenovirus and was found to confer proinsulin expression to a hepatocyte (H4-II-E) cell line upon infection. A second virus was constructed in which the dibasic prohormone convertase recognition sequence was mutated to a tetrabasic furin cleavage site. Cells infected with this virus synthesized both proinsulin and mature insulin. Gel filtration chromatography, competition of insulin binding, and activation of the insulin receptor kinase activity demonstrated that this mature insulin was functionally identical to that of authentic processed insulin. Injection of these viral constructs into the external jugular vein of mice resulted in insulin gene expression in the liver. Expression from the mutated proinsulin virus dramatically improved the glycemic state of diabetic mice. However, the effects of the viral infection were transient, being maximal at approximately 5-7 days and returning to steady-state levels by 14-21 days. These data demonstrate that somatic cell insulin gene delivery by the use of recombinant adenovirus can be used to transiently reverse the diabetic state in mice.

Effect of nutritional state on the formation of a complex involving insulin receptor IRS-1, the 52 kDa Src homology/collagen protein (Shc) isoform and phosphatidylinositol 3'-kinase activity

The Src homology and collagen protein (Shc) is tyrosine phosphorylated in response to insulin; however, evidence for its interaction with insulin receptor (IR) in normal tissues is missing. Interactions between IR, Shc and regulatory subunits of the phosphatidylinositol 3'-kinase (PI 3'-kinase) were characterized in the present study in liver and muscles of chickens submitted to various nutritional states. A chicken liver Shc cDNA fragment encoding a 198 amino acid long fragment, including the phosphotyrosine binding domain was sequenced. It shows 89% homology with the corresponding human homologue. The amounts of the three Shc isoforms (66, 52 and 46 kDa) and Shc messenger were not altered by the nutritional state. Shc tyrosine phosphorylation was decreased by fasting in both liver and muscle. Importantly, Shc was immunoprecipitated by IR antibody (mostly the 52 kDa isoform) or by alphaIRS-1(mostly the 46 kDa isoform). IR-Shc association was decreased by fasting and restored by refeeding. In liver, alphaShc immunoprecipitated the three forms of regulatory subunits of PI 3'-kinase and a PI 3'-kinase activity which was decreased by fasting. In muscle, alphaShc immunoprecipitated only the p85 isoform; the associated PI 3'-kinase activity was not altered by the nutritional state. Conversely, in both tissues anti-p85 antibody precipitated only the 52 kDa Shc isoform. In liver, antibodies to insulin receptor substrate-1 (alphaIRS-1), Shc or IR immunoprecipitated the three regulatory subunits of PI 3'-kinase and an equal PI 3'-kinase activity, without any residual activity left in the supernatants, suggesting the presence of a large complex involving IR, IRS-1, Shc (mainly the 52 kDa isoform) and PI 3'-kinase activity. The presence of another complex containing IRS-1 and the 46 kDa Shc isoform, but no PI 3'-kinase activity, is suggested.

Inhibition of clathrin-mediated endocytosis selectively attenuates specific insulin receptor signal transduction pathways

To examine the role of clathrin-dependent insulin receptor internalization in insulin-stimulated signal transduction events, we expressed a dominant-interfering mutant of dynamin (K44A/dynamin) by using a recombinant adenovirus in the H4IIE hepatoma and 3T3L1 adipocyte cell lines. Expression of K44A/dynamin inhibited endocytosis of the insulin receptor as determined by both cell surface radioligand binding and trypsin protection analysis. The inhibition of the insulin receptor endocytosis had no effect on either the extent of insulin receptor autophosphorylation or insulin receptor substrate 1 (IRS1) tyrosine phosphorylation. In contrast, expression of K44A/dynamin partially inhibited insulin-stimulated Shc tyrosine phosphorylation and activation of the mitogen-activated protein kinases ERK1 and -2. Although there was an approximately 50% decrease in the insulin-stimulated activation of the phosphatidylinositol 3-kinase associated with IRS1, insulin-stimulated Akt kinase phosphorylation and activation were unaffected. The expression of K44A/dynamin increased the basal rate of amino acid transport, which was additive with the effect of insulin but had no effect on the basal or insulin-stimulated DNA synthesis. In 3T3L1 adipocytes, expression of K44A/dynamin increased the basal rate of glucose uptake, glycogen synthesis, and lipogenesis without any significant effect on insulin stimulation. Together, these data demonstrate that the acute actions of insulin are largely independent of insulin receptor endocytosis and are initiated by activation of the plasma membrane-localized insulin receptor.

Modulation of insulin-like growth factor I mitogenic signaling in 3T3-L1 preadipocyte differentiation

Insulin-like growth factor I (IGF-I) stimulates mitogenesis in proliferating 3T3-L1 preadipocytes. However, IGF-I functions to stimulate differentiation once growth arrest occurs at confluence. Epidermal growth factor (EGF) is also a potent mitogen in these cells, but inhibits differentiation of preadipocytes. We compared mitogenic signaling via the mitogen-activated protein kinase (MAPK) pathway in response to IGF-I or EGF in proliferating, growth-arrested, and differentiating 3T3-L1 cells. IGF-I stimulation of MAPK was rapid and maximal in proliferating 3T3-L1 preadipocytes, but decreased greatly in differentiating cells. EGF was more potent than IGF-I in stimulating MAPK activity in 3T3-L1 cells, and activation of MAPK was maintained in differentiating cells. These results suggest an uncoupling of MAPK activation specific to IGF-I-mediated 3T3-L1 preadipocyte differentiation. Studies of proximal signaling revealed Shc phosphorylation and Shc/Grb2 complex formation in IGF-I-treated proliferating, but not differentiating, cells. Insulin receptor substrate-1 phosphorylation after IGF-I treatment was present in proliferating, quiescent, and differentiating preadipocytes. Shc phosphorylation and Grb2 association after EGF treatment were present throughout all phases of growth. The change in IGF-I signaling via Shc phosphorylation and MAPK activity during 3T3-L1 differentiation indicates that loss of IGF-I mitogenic signaling via the MAPK pathway is part of the differentiation process.

A novel, multifuntional c-Cbl binding protein in insulin receptor signaling in 3T3-L1 adipocytes

The protein product of the c-Cbl proto-oncogene is prominently tyrosine phosphorylated in response to insulin in 3T3-L1 adipocytes and not in 3T3-L1 fibroblasts. After insulin-dependent tyrosine phosphorylation, c-Cbl specifically associates with endogenous c-Crk and Fyn. These results suggest a role for tyrosine-phosphorylated c-Cbl in 3T3-L1 adipocyte activation by insulin. A yeast two-hybrid cDNA library prepared from fully differentiated 3T3-L1 adipocytes was screened with full-length c-Cbl as the target protein in an attempt to identify adipose-specific signaling proteins that interact with c-Cbl and potentially are involved in its tyrosine phosphorylation in 3T3-L1 adipocytes. Here we describe the isolation and the characterization of a novel protein that we termed CAP for c-Cbl-associated protein. CAP contains a unique structure with three adjacent Src homology 3 (SH3) domains in the C terminus and a region showing significant sequence similarity with the peptide hormone sorbin. Both CAP mRNA and proteins are expressed predominately in 3T3-L1 adipocytes and not in 3T3-L1 fibroblasts. CAP associates with c-Cbl in 3T3-L1 adipocytes independently of insulin stimulation in vivo and in vitro in an SH3-domain-mediated manner. Furthermore, we detected the association of CAP with the insulin receptor. Insulin stimulation resulted in the dissociation of CAP from the insulin receptor. Taken together, these data suggest that CAP represents a novel c-Cbl binding protein in 3T3-L1 adipocytes likely to participate in insulin signaling.

Interleukin 5 signals through Shc and Grb2 in human eosinophils

Eosinophils are potent effector cells contributing to allergic inflammation and asthma. The differentiation, recruitment, and effector functions of eosinophils are greatly affected by interleukin (IL)-5. In the eosinophil, signal transduction pathways including Jak-STAT and Ras-Raf-MAP kinase are stimulated by IL-5 and enzymatic activation of tyrosine kinases Jak-2 and Lyn has been demonstrated. The participation of adapter proteins in the responses of the Ras-Raf-MAP kinase pathway has been documented in many cytokine family receptors but the expression and activation of these proteins have not been demonstrated in eosinophils. In these studies, we have found three isoforms of the adapter protein, Shc, to be expressed in eosinophils. One of these isoforms, p52 Shc, was tyrosine phosphorylated following IL-5 treatment of eosinophils. A second adapter protein, Grb2, coimmunoprecipitated with Shc following IL-5 stimulation of eosinophils. Furthermore, p52 Shc was increasingly associated with a cell fraction resistant to detergent solubilization, following IL-5 administration. This cell fraction of limited detergent solubility is a complex mixture of proteins and the adapter protein Grb2, the tyrosine kinases Jak-2 and Lyn, the nucleotide exchange factor Vav, and the serine-threonine kinases p45 MAP kinase, Raf-1, and PKCbeta, were distributed either wholly or partially in the same fraction, as were the cytoskeletal proteins actin and vimentin. Only p52 Shc, however, demonstrated discernibly increased association with this fraction following IL-5 stimulation of eosinophils. These data suggest that IL-5 activates a signal transduction pathway utilizing the adapter proteins Shc and Grb2 in the human eosinophil.

The 66-kDa Shc isoform is a negative regulator of the epidermal growth factor-stimulated mitogen-activated protein kinase pathway

In addition to tyrosine phosphorylation of the 66-, 52-, and 46-kDa Shc isoforms, epidermal growth factor (EGF) treatment of Chinese hamster ovary cells expressing the human EGF receptor also resulted in the serine/threonine phosphorylation of approximately 50% of the 66-kDa Shc proteins. The serine/threonine phosphorylation occurred subsequent to tyrosine phosphorylation and was prevented by pretreatment of the cells with the MEK-specific inhibitor PD98059. Surprisingly, only the gel-shifted 66-kDa Shc isoform (serine/threonine phosphorylated) was tyrosine phosphorylated and associated with Grb2. In contrast, only the non-serine/threonine-phosphorylated fraction of 66-kDa Shc was associated with the EGF receptor. To assess the relationship between the three Shc isoforms in EGF-stimulated signaling, the cDNA encoding the 66-kDa Shc species was cloned from a 16-day-old mouse embryo library. Sequence alignment confirmed that the 66-kDa Shc cDNA resulted from alternative splicing of the primary Shc transcript generating a 110-amino acid extension at the amino terminus. Co-immunoprecipitation of Shc and Grb2 from cells overexpressing the 52/46-kDa Shc isoforms versus the 66-kDa Shc species directly demonstrated a competition of binding for a limited pool of Grb2 proteins. Furthermore, expression of the 66-kDa Shc isoform markedly accelerated the inactivation of ERK following EGF stimulation. Together, these data indicate that the serine/threonine phosphorylation of 66-kDa Shc impairs its ability to associate with the tyrosine-phosphorylated EGF receptor and can function in a dominant-interfering manner by inhibiting EGF receptor downstream signaling pathways.

Identification of ErbB3-stimulated genes using modified representational difference analysis

The epidermal growth factor receptor (EGFR) family of tyrosine kinases is involved in the growth of normal and tumour cells. The specific contribution of each of the four family members to these processes remains unclear. In the present study we have used a PCR-based subtractive approach to identify differences in messages induced in response to activation of ErbB3 and EGFR. The approach described is a modification of the representational difference analysis technique adapted for analysis of cDNA, which we have modified to permit identification of differential gene expression using as little as 20 microg of total RNA as the starting material. The mRNA obtained from EGF-stimulated NIH-3T3 cells expressing chimaeric EGFR-ErbB3 receptors provided the tester amplicons (small PCR-amplified fragments) which were subtracted against driver amplicons derived from unstimulated NIH-3T3 cells expressing the EGFR-ErbB3 chimaera or EGF-stimulated NIH-3T3 cells overexpressing the EGFR. A total of 22 different clones were isolated, 90% of which showed increased expression in the tester amplicons. Six of these, corresponding to known DNA sequences, were selected for further Northern blot analysis against total RNA prepared from the starting cell lines. Of these, the gene encoding the protein dlk (or a closely related protein, Pref-1) was identified as being regulated by ErbB3 but not by the EGFR. Other genes appeared to be elevated by both ErbB3 and EGFR, including those encoding c-jun, Ret finger protein (RFP), neuroleukin and amyloid protein precursor. One gene product, TIS11, was identified as being regulated by EGFR but not by ErbB3.

Differential signaling by insulin receptor substrate 1 (IRS-1) and IRS-2 in IRS-1-deficient cells

Mice made insulin receptor substrate 1 (IRS-1) deficient by targeted gene knockout exhibit growth retardation and abnormal glucose metabolism due to resistance to the actions of insulin-like growth factor 1 (IGF-1) and insulin (E. Araki et al., Nature 372:186-190, 1994; H. Tamemoto et al., Nature 372:182-186, 1994). Embryonic fibroblasts and 3T3 cell lines derived from IRS-1-deficient embryos exhibit no IGF-1-stimulated IRS-1 phosphorylation or IRS-1-associated phosphatidylinositol 3-kinase (PI 3-kinase) activity but exhibit normal phosphorylation of IRS-2 and Shc and normal IRS-2-associated PI 3-kinase activity. IRS-1 deficiency results in a 70 to 80% reduction in IGF-1-stimulated cell growth and parallel decreases in IGF-1-stimulated S-phase entry, PI 3-kinase activity, and induction of the immediate-early genes c-fos and egr-1 but unaltered activation of the mitogen-activated protein kinases ERK 1 and ERK 2. Expression of IRS-1 in IRS-1-deficient cells by retroviral gene transduction restores IGF-1-stimulated mitogenesis, PI 3-kinase activation, and c-fos and egr-1 induction in proportion to the level of reconstitution. Increasing the level of IRS-2 in these cells by using a retrovirus reconstitutes IGF-1 activation of PI 3-kinase and immediate-early gene expression to the same degree as expression of IRS-1; however, IRS-2 overexpression has only a minor effect on IGF-1 stimulation of cell cycle progression. These results indicate that IRS-1 is not necessary for activation of ERK 1 and ERK 2 and that activation of ERK 1 and ERK 2 is not sufficient for IGF-1-stimulated activation of c-fos and egr-1. These data also provide evidence that IRS-1 and IRS-2 are not functionally interchangeable signaling intermediates for stimulation of mitogenesis despite their highly conserved structure and many common functions such as activating PI 3-kinase and early gene expression.

Roles of insulin receptor substrate-1 and Shc on insulin-like growth factor I receptor signaling in early passages of cultured human fibroblasts

Insulin-like growth factor-I (IGF-I) improves glucose metabolism and growth in patients with leprechaunism. We investigated signal transduction through IGF-I receptor in comparison with epidermal growth factor (EGF) receptor in early passages of cultured skin fibroblasts from a normal subject and a patient with leprechaunism whose insulin receptor tyrosine kinase was almost nonexistent. Insulin receptor substrate-1 (IRS-1) became tyrosine-phosphorylated and bound growth factor receptor-bound protein 2 (GRB2) quickly by IGF-I. The association of Shc with GRB2 by IGF-I was detected by immunoblot with anti-Shc antibody but was hardly visible with antiphosphotyrosine antibody, which was in marked contrast to efficient tyrosine phosphorylation of Shc by EGF. However, the potency of IGF-I for DNA synthesis was far stronger than EGF, which was not parallel with the potency of these growth factors to activate Shc or MAP kinase. Rather, phosphatidylinositol (PI) 3-kinase activity, which was activated by IGF-I about 5- to 10-fold more strongly than EGF, appeared to correlate with mitogenesis. Signal transduction pathways following IGF-I receptor or EGF receptor activation were indistinguishable between the normal subject and the patient. Our results strongly suggest that in human skin fibroblasts, which represent a more physiological cell culture: 1) IRS-1, rather than Shc, is the major tyrosine-phosphorylated protein binding GRB2 in initial phase of IGF-I signaling; 2) mitogenic potency of receptor tyrosine kinases such as IGF-I receptor and EGF receptor may not be determined solely by the amount of Shc-GRB2 complex or the activity of MAP kinase; and 3) in contrast to previous reports, IGF-I and EGF receptor signalings are not defective in leprechaunism.

Characterization of the phosphotyrosine-binding domain of the Drosophila Shc protein

The phosphotyrosine-binding (PTB) domain of Drosophila Shc (dShc) binds in vitro to phosphopeptides containing the sequence motif NPXpY, and physically associates with the activated Drosophila epidermal growth factor receptor homologue (DER) in vivo. The structural elements, specificity and binding kinetics of the dShc PTB domain have now been characterized. The dShc PTB domain appeared similar to the insulin-like receptor substrate-1 PTB domain in secondary structure as suggested by Fourier transform infrared spectroscopy. Surface plasmon resonance measurements indicated that the dShc PTB domain bound with high affinity to phosphopeptides (Der) derived from the Tyr1228 site of the DER receptor. The kinetics of the dShc PTB domain-Der phosphopeptide interaction differed from those of a typical SH2 domain-ligand interaction, in that the PTB domain displayed slower on/off rates. Competition binding assays using truncated versions of the Der peptides revealed that high affinity binding to the dShc PTB domain requires, in addition to the NPXpY motif, the presence of hydrophobic residues at both positions -5 and -7 relative to phosphotyrosine. The dShc PTB domain showed a similar binding specificity to the human Shc (hShc) PTB domain, but subtle differences were noted; such that the hShc PTB domain bound preferentially to a phosphopeptide from the mammalian nerve growth factor receptor, whereas the dShc PTB domain bound preferentially to phosphopeptides from the Drosophila DER receptor. The invertebrate dShc PTB domain therefore possesses a binding specificity for tyrosine-phosphorylated peptides that is optimally suited for recognition of the activated DER receptor.

Recombinant human alpha 2-HS glycoprotein inhibits insulin-stimulated mitogenic pathway without affecting metabolic signalling in Chinese hamster ovary cells overexpressing the human insulin receptor

Insulin acts on its target tissues by specific interaction with the cell surface insulin receptor (IR). The IR possesses an intrinsic tyrosine kinase (TK) activity which is stimulated by insulin binding. This TK activity is required for many aspects of insulin signalling. We had earlier reported that human plasma alpha 2-HS glycoprotein (alpha 2-HSG) inhibits insulin-stimulated mitogenesis at the level of IR-TK (Mol Endo 7: 1445-1455, 1993). In the present study, using recombinant alpha 2-HSG, which possesses 50-100 times the specific activity of plasma alpha 2-HSG, we have further investigated the molecular basis of this effect. We examined the insulin-stimulated Ras signalling pathway in Chinese Hamster Ovary cells overexpressing the human IR. alpha 2-HSG inhibits insulin-induced tyrosine phosphorylation of IRS-1 and the subsequent association of GRB2, as well as Sos, with IRS-1. This inhibition results in reduced guanine nucleotide exchange in p21ras. alpha 2-HSG also inhibits the stimulation of Raf phosphorylation, in response to insulin, leading to inhibition of MEK activity. In a parallel pathway, alpha 2-HSG also inhibits insulin-induced tyrosine phosphorylation of Shc. However, alpha 2-HSG does not affect any of the metabolic actions of insulin rested in these cells. These results suggest that, while insulin's mitogenic effects can be abolished by inhibition of insulin-induced IR-TK, propagation of signals for metabolic activities might utilize alternate of rescue mechanisms.

Different pathways of postreceptor desensitization following chronic insulin treatment and in cells overexpressing constitutively active insulin receptors

We have reported previously that substitution of the transmembrane domain of the insulin receptor with that of the erbB-2 oncogene (IRerbV-->E) results in constitutive activation of the insulin receptor kinase. Compared to NIH3T3 cells overexpressing wild-type insulin receptors (IRwt), cells overexpressing IRerbV-->E displayed a decrease in IRS-1 protein content by 55%, but basal tyrosine phosphorylation of IRS-1 was increased. This resulted in an increased association of IRS-1 with the p85 subunit of phosphatidylinositol 3-kinase, increased phosphatidylinositol 3-kinase activity in anti-IRS-1 immunoprecipitates, constitutive activation of p70 S6 protein kinase, and an increased association of Grb2 with Shc in the absence of ligand. However, Grb2 association with IRS-1 could not be detected in the basal or insulin-stimulated states, and mitogen-activated protein kinase (MAPK) activity could not be stimulated by insulin, epidermal growth factor, or platelet-derived growth factor. In contrast to IRerbV-->E, the insulin receptor content and its tyrosine phosphorylation were significantly decreased in IRwt cells chronically stimulated (>24 h) with insulin. With decreased IRS-1 content, tyrosine phosphorylation of IRS-1 was decreased by over 75%, leading to decreased IRS-1-associated PI 3-kinase and Grb2. In addition, Grb2 association with Shc and activation of MAPK and the p70 S6 kinase were insensitive to insulin stimulation. By contrast, association of Grb2 with Shc and activation of MAPK, but not the p70 S6 kinase, could be stimulated by epidermal growth factor or platelet-derived growth factor. These data suggest that there are multiple levels of postreceptor desensitization to insulin action. These are used somewhat differently in these two different models, probably due in part to the difference in receptor down-regulation.

Functional roles of the Shc phosphotyrosine binding and Src homology 2 domains in insulin and epidermal growth factor signaling

Shc is involved in the activation of Ras in response to many growth factors. Shc contains two phosphotyrosine binding domains, an Src homology 2 (SH2) domain in the carboxyl terminus of the protein and a phosphotyrosine binding (PTB) domain in the amino terminus. Since functional roles for these two domains have not been established, we microinjected glutathione S-transferase fusion proteins of either the Shc PTB or SH2 domains into fibroblasts expressing insulin and epidermal growth factor receptors and measured their effects on DNA synthesis. We found that the Shc PTB was necessary for insulin-induced mitogenic signaling, whereas the SH2 domain was not. In contrast, for epidermal growth factor signaling, the Shc SH2 was functionally more important. These differential modes of signal transduction may be an important factor in determining the specificity of the response of a cell to external stimuli.

Interactions between Src homology (SH) 2/SH3 adapter proteins and the guanylnucleotide exchange factor SOS are differentially regulated by insulin and epidermal growth factor

Co-immunoprecipitation of whole cell extracts demonstrated that the guanylnucleotide exchange factor SOS was associated with the small adapter proteins Grb2, CrkII, and Nck. In vitro binding indicated a similar binding affinity of SOS for all three adapter proteins but with a slightly lower Kd for Grb2 (approximately 2.5-fold) compared with Nck and CrkII. Insulin stimulation resulted in co-immunoprecipitation of tyrosine-phosphorylated IRS1 with Grb2 and to a lesser extent CrkII. Although Grb2 also associated with tyrosine-phosphorylated Shc, there was no detectable interaction of CrkII with Shc. In contrast, EGF stimulation resulted in the predominant co-immunoprecipitation of Grb2 with the EGF receptor, whereas CrkII primarily associated with an unidentified 120-130-kDa protein. Similar to the ability of insulin to induce the dissociation of the Grb2-SOS complex, there was a concomitant time-dependent dissociation of the CrkII-SOS and Nck-SOS complexes. However, EGF stimulation had no effect on the association state of the Grb2-SOS or the Nck-SOS complexes but did result in a time-dependent dissociation of the CrkII from SOS. Together, these data demonstrate that different cellular pools of SOS associate with different adapter proteins forming various signaling complexes, each undergoing distinct patterns of assembly/disassembly following growth factor stimulation.

Insulin induces tyrosine phosphorylation of JAK2 in insulin-sensitive tissues of the intact rat

The Janus kinase family of protein tyrosine kinases constitutes a novel type of signal transduction pathway activated in response to a wide variety of polypeptide ligands and has four known members: JAK1, JAK2, JAK3, and Tyk2. In this study, we examined the ability of insulin to stimulate JAK2 tyrosine phosphorylation in insulin-sensitive tissues of the intact rat using immunoprecipitation and immunoblotting. The results demonstrate that after an infusion of insulin, JAK2 is rapidly tyrosine phosphorylated (and the kinase is activated) in the liver, adipose tissue, skeletal muscle, heart, and isolated adipocytes. The presence of phosphorylated JAK2 was detectable after an infusion of insulin that increased serum insulin to physiological postprandial levels (40-70 microunits/ml). Co-immunoprecipitation with anti-insulin receptor antibody, anti-JAK2 antibody, and anti-IRS-1 antibody showed that JAK2 interacts with the insulin receptor and IRS-1 to form stable complexes following stimulation by insulin. In two animal models of insulin resistance the regulation of JAK2 tyrosine phosphorylation after insulin infusion paralleled the phosphorylation of the insulin receptor and of IRS-1. In conclusion, our data indicate that after physiological stimulation by insulin in the intact animal, JAK2 associates with the insulin receptor and is tyrosine phosphorylated in insulin-sensitive tissues in a time- and dose-dependent fashion.

Role of c-Src tyrosine kinase in G protein-coupled receptor- and Gbetagamma subunit-mediated activation of mitogen-activated protein kinases

Several G protein-coupled receptors that interact with pertussis toxin-sensitive heterotrimeric G proteins mediate Ras-dependent activation of mitogen-activated protein (MAP) kinases. The mechanism involves Gbetagamma subunit-mediated increases in tyrosine phosphorylation of the Shc adapter protein, Shc*Grb2 complex formation, and recruitment of Ras guanine nucleotide exchange factor activity. We have investigated the role of the ubiquitous nonreceptor tyrosine kinase c-Src in activation of the MAP kinase pathway via endogenous G protein-coupled lysophosphatidic acid (LPA) receptors or by transient expression of Gbetagamma subunits in COS-7 cells. In vitro kinase assays of Shc immunoprecipitates following LPA stimulation demonstrated rapid, transient recruitment of tyrosine kinase activity into Shc immune complexes. Recruitment of tyrosine kinase activity was pertussis toxin-sensitive and mimicked by cellular expression of Gbetagamma subunits. Immunoblots for coprecipitated proteins in Shc immunoprecipitates revealed a transient association of Shc and c-Src following LPA stimulation, which coincided with increases in Shc-associated tyrosine kinase activity and Shc tyrosine phosphorylation. LPA stimulation or expression of Gbetagamma subunits resulted in c-Src activation, as assessed by increased c-Src autophosphorylation. Overexpression of wild-type or constitutively active mutant c-Src, but not kinase inactive mutant c-Src, lead to increased tyrosine kinase activity in Shc immunoprecipitates, increased Shc tyrosine phosphorylation, and Shc.Grb2 complex formation. MAP kinase activation resulting from LPA receptor stimulation, expression of Gbetagamma subunits, or expression of c-Src was sensitive to dominant negatives of mSos, Ras, and Raf. Coexpression of Csk, which inactivates Src family kinases by phosphorylating the regulatory C-terminal tyrosine residue, inhibited LPA stimulation of Shc tyrosine phosphorylation, Shc.Grb2 complex formation, and MAP kinase activation. These data suggest that Gbetagamma subunit-mediated formation of Shc.c-Src complexes and c-Src kinase activation are early events in Ras-dependent activation of MAP kinase via pertussis toxin-sensitive G protein-coupled receptors.