Insulin receptor substrate 1 mediates insulin and insulin-like growth factor I-stimulated maturation of Xenopus oocytes

The brominated flame retardant, TBCO, impairs oocyte maturation in zebrafish (Danio rerio)

The brominated flame retardant, 1,2,5,6-tetrabromocyclooctane (TBCO), has been shown to decrease fecundity in Japanese medaka (Oryzias latipes) and there is indirect evidence from analysis of the transcriptome and proteome that this effect might be due to impaired oogenesis. An assay for disruption of oocyte maturation by chemical stressors has not been developed in Japanese medaka. Thus, using zebrafish (Danio rerio) as a model, objectives of the present study were to determine whether exposure to TBCO has effects on maturation of oocytes and to investigate potential mechanisms. Sexually mature female zebrafish were given a diet of 35.3 or 628.8 μg TBCO / g food for 14 days after which, stage IV oocytes were isolated to assess maturation in response to maturation inducing hormone. To explore potential molecular mechanisms, abundances of mRNAs of a suite of genes that regulate oocyte maturation were quantified by use of quantitative real-time PCR, and abundances of microRNAs were determined by use of miRNAseq. Ex vivo maturation of oocytes from fish exposed to TBCO was significantly less than maturation of oocytes from control fish. The percentage of oocytes which matured from control fish and those exposed to low and high TBCO were 89, 71, and 67%, respectively. Among the suite of genes known to regulate oocyte maturation, mRNA abundance of insulin like growth factor-3 was decreased by 1.64- and 3.44-fold in stage IV oocytes from females given the low and high concentrations of TBCO, respectively, compared to the control group. Abundances of microRNAs regulating the expression of proteins that regulate oocyte maturation, including processes related to insulin-like growth factor, were significantly different in stage IV oocytes from fish exposed to TBCO. Overall, results of this study indicated that impaired oocyte maturation might be a mechanism of reduced reproductive performance in TBCO-exposed fish. Results also suggested that effects of TBCO on oocyte maturation might be due to molecular perturbations on insulin-like growth factor signaling and expression of microRNAs.

Steroid-induced oocyte maturation in Indian shad Tenualosa ilisha (Hamilton, 1822) is dependent on phosphatidylinositol 3 kinase but not MAP kinase activation

Fully grown fish and amphibian oocytes exposed to a maturation-inducing steroid (MIS) activates multiple signal transduction pathways, leading to formation and activation of maturation-promoting factor (MPF) and induction of germinal vesicle breakdown (GVBD). The present study was to investigate if phosphatidylinositol 3 kinase (PI3 kinase) and mitogen-activated protein kinase (MAP kinase) activation are required for naturally occurring MIS, 17α,20β-dihydroxy-4-pregnen-3-one (17,20β-P)-induced cdc2 activation and oocyte maturation (OM) in Tenualosa ilisha. We observed that 17,20β-P-induced OM was significantly inhibited by PI3 kinase inhibitors Wortmannin and LY29400. 17,20 β-P was shown to activate PI3 kinase maximally at 90 min and cdc2 kinase at 16 h of treatment. Relative involvement of PI3 kinase, MAP kinase and cdc2 kinase in 17,20β-P-induced OM was examined. MAP kinase was rapidly phosphorylated and activated (60-120 min) after MIS treatment and this response preceded the activation of cdc2 kinase by several hours. A selective inhibitor of MAP kinase (MEK), PD98059, sufficiently blocked the phosphorylation and activation of MAP kinase. Inhibition of MAP kinase activity using PD98059 however, had no effect on MIS-induced cdc2 kinase activation and GVBD. These results demonstrate that activation of the PI3 kinase is required for 17,20β-P-induced cdc2 kinase activation and OM in T. ilisha. MAP kinase although was activated in response to 17,20β-P and PI3 kinase activation, it is not necessary for cdc2 activation and OM in this species.

IGF1R signaling in Ewing sarcoma is shaped by clathrin-/caveolin-dependent endocytosis

Receptor endocytosis is critical for cell signaling. IGF1R mediates an autocrine loop that is de-regulated in Ewing Sarcoma (ES) cells. Here we study the impact of IGF1R internalization, mediated by clathrin and caveolin-1 (CAV1), in ES signaling. We used clathrin and CAV1-siRNA to interfere in clathrin- and caveolin-dependent endocytosis. Chlorpromazine (CPMZ) and methyl-beta-cyclo-dextrin (MCD) were also used in order to inhibit clathrin- and caveolin-dependent endocytosis, respectively. We analyzed IGF1R internalization and co-localization with clathrin and CAV1 upon ligand binding, as well as the status of the IGF1R pathway, cellular proliferation, and the apoptosis of interfered and inhibited ES cells. We performed a high-throughput tyrosine kinase phosphorylation assay to analyze the effects of combining the IGF1R tyrosine kinase inhibitor AEW541 (AEW) with CPMZ or MCD on the intracellular phospho-proteome. We observed that IGF1R is internalized upon ligand binding in ES cells and that this process is dependent on clathrin or CAV1. The blockage of receptor internalization inhibited AKT and MAPK phosphorylation, reducing the proliferative rate of ES cells and increasing the levels of apoptosis. Combination of AEW with CPMZ or MCD largely enhanced these effects. CAV1 and clathrin endocytosis controls IGF1R internalization and signaling and has a profound impact on ES IGF1R-promoted survival signaling. We propose the combination of tyrosine-kinase inhibitors with endocytosis inhibitors as a new therapeutic approach to achieve a stronger degree of receptor inhibition in this, or other neoplasms dependent on IGF1R signaling.

Serine-385 phosphorylation of inwardly rectifying K+ channel subunit (Kir6.2) by AMP-dependent protein kinase plays a key role in rosiglitazone-induced closure of the K(ATP) channel and insulin secretion in rats

AIMS/HYPOTHESIS

Rosiglitazone, an insulin sensitiser, not only improves insulin sensitivity but also enhances insulin secretory capacity by ameliorating gluco- and lipotoxicity in beta cells. Rosiglitazone can stimulate insulin secretion at basal and high glucose levels via a phosphatidylinositol 3-kinase (PI3K)-dependent pathway. We hypothesised that regulation of phosphorylation of the ATP-sensitive potassium (K(ATP)) channel might serve as a key step in the regulation of insulin secretion.

METHODS

Insulin secretory responses were studied in an isolated pancreas perfusion system, cultured rat islets and MIN6 and RINm5F beta cells. Signal transduction pathways downstream of PI3K were explored to link rosiglitazone to K(ATP) channel conductance with patch clamp techniques and insulin secretion measured by ELISA.

RESULTS

Rosiglitazone stimulated AMP-activated protein kinase (AMPK) activity and induced inhibition of the K(ATP) channel conductance in islet beta cells; both effects were blocked by the PI3K inhibitor LY294002. Following stimulation of AMPK by 5-aminoimidazole-4-carboxamide ribonucleoside (AICAR), a pharmacological activator, both AICAR-stimulated insulin secretion and inhibition of K(ATP) channel conductance were unaffected by LY294002, indicating that AMPK activation occurs at a site downstream of PI3K activity. The serine residue at amino acid position 385 of Kir6.2 was found to be the substrate phosphorylation site of AMPK when activated by rosiglitazone or AICAR.

CONCLUSIONS/INTERPRETATION

Our data indicate that PI3K-dependent activation of AMPK is required for rosiglitazone-stimulated insulin secretion in pancreatic beta cells. Phosphorylation of the Ser(385) residue of the Kir6.2 subunit of the K(ATP) channel by AMPK may play a role in insulin secretion.

A novel, rapid, inhibitory effect of insulin on alpha1beta2gamma2s gamma-aminobutyric acid type A receptors

In the CNS, GABA and insulin seem to contribute to similar processes, including neuronal survival; learning and reward; and energy balance and food intake. It is likely then that insulin and GABA may interact, perhaps at the GABA(A) receptor. One such interaction has already been described [Q. Wan, Z.G. Xiong, H.Y. Man, C.A. Ackerley, J. Braunton, W.Y. Lu, L.E. Becker, J.F. MacDonald, Y.T. Wang, Recruitment of functional GABA(A) receptors to postsynaptic domains by insulin, Nature 388 (1997) 686-690]; in it a micromolar concentration of insulin causes the insertion of GABA(A) receptors into the cell membrane, increasing GABA current. I have discovered another effect of insulin on GABA(A) currents. Using a receptor isoform, alpha(1)beta(2)gamma(2s) that is the likely main neuronal GABA(A) isoform expressed recombinantly in Xenopus oocytes, insulin inhibits GABA-induced current when applied simultaneously with low concentrations of GABA. Insulin will significantly inhibit currents induced by EC(30-50) concentrations of GABA by about 38%. Insulin is potent in this effect; IC(50) of insulin was found to be about 4.3 x 10(-10) M. The insulin effect on the GABA dose responses looked like that of an antagonist similar to bicuculline or beta-carbolines. However, an effect of phosphorylation on the GABA(A) receptor from the insulin receptor signal transduction pathway cannot yet be dismissed.

Growth hormone, insulin-like growth factors, and the skeleton

GH and IGF-I are important regulators of bone homeostasis and are central to the achievement of normal longitudinal bone growth and bone mass. Although GH may act directly on skeletal cells, most of its effects are mediated by IGF-I, which is present in the systemic circulation and is synthesized by peripheral tissues. The availability of IGF-I is regulated by IGF binding proteins. IGF-I enhances the differentiated function of the osteoblast and bone formation. Adult GH deficiency causes low bone turnover osteoporosis with high risk of vertebral and nonvertebral fractures, and the low bone mass can be partially reversed by GH replacement. Acromegaly is characterized by high bone turnover, which can lead to bone loss and vertebral fractures, particularly in patients with coexistent hypogonadism. GH and IGF-I secretion are decreased in aging individuals, and abnormalities in the GH/IGF-I axis play a role in the pathogenesis of the osteoporosis of anorexia nervosa and after glucocorticoid exposure.

Activation of insulin and IGF-1 signaling pathways by melatonin through MT1 receptor in isolated rat pancreatic islets

Melatonin diminishes insulin release through the activation of MT1 receptors and a reduction in cAMP production in isolated pancreatic islets of neonate and adult rats and in INS-1 cells (an insulin-secreting cell line). The pancreas of pinealectomized rats exhibits degenerative pathological changes with low islet density, indicating that melatonin plays a role to ensure the functioning of pancreatic beta cells. By using immunoprecipitation and immunoblotting analysis we demonstrated, in isolated rat pancreatic islets, that melatonin induces insulin growth factor receptor (IGF-R) and insulin receptor (IR) tyrosine phosphorylation and mediates the activities of the PI3K/AKT and MEK/ERKs pathways, which are involved in cell survival and growth, respectively. Thus, the effects of melatonin on pancreatic islets do not involve a reduction in cAMP levels only. This indoleamine may regulate growth and differentiation of pancreatic islets by activating IGF-I and insulin receptor signaling pathways.

Oncogenic Met receptor induces cell-cycle progression in Xenopus oocytes independent of direct Grb2 and Shc binding or Mos synthesis, but requires phosphatidylinositol 3-kinase and Raf signaling

Biological responses of hepatocyte growth factor (HGF) are mediated by the Met receptor tyrosine kinase. Although HGF is a potent mitogen for a variety of cells, the signals required for cell-cycle progression by the Met/HGF receptor are poorly defined. In this study, we have used the Xenopus oocyte system to define the role of various Met proximal-binding partners and downstream signaling pathways in cell-cycle regulation. We show that cell-cycle progression and activation of MAPK and JNK mediated by the oncogenic Met receptor, Tpr-Met, are dependent on its kinase activity and the presence of the twin phosphotyrosine (Y482 & Y489) residues in its C-terminus, but that the recruitment of Grb2 and Shc adaptor proteins is dispensable, implicating other signaling molecules. However, using Met receptor oncoproteins engineered to recruit specific signaling proteins, we demonstrate that recruitment of Grb2 or Shc adaptor proteins is sufficient to induce cell-cycle progression and activation of MAPK and JNK, while the binding of phospholipase-Cgamma or phosphatidylinositol 3-kinase alone fails to elicit these responses. Using various means to block phosphatidylinositol 3-kinase, phospholipase-Cgamma, MEK, JNK, Mos, and Raf1 activity, we show that unlike the fibroblast growth factor receptor, MEK-dependent and independent signaling contribute to Met receptor-mediated cell-cycle progression, but phospholipase-Cgamma or JNK activity and Mos synthesis are not critical. Notably, we demonstrate that Raf1 and phosphatidylinositol 3-kinase signaling are required for cell-cycle progression initiated by the Met receptor, a protein frequently deregulated in human tumors.

Metformin (Glucophage) inhibits tyrosine phosphatase activity to stimulate the insulin receptor tyrosine kinase

Metformin is a commonly used anti-diabetic but whether its mechanism involves action on the insulin receptor or on downstream events is still controversial. With a time course that was slow compared with insulin action, metformin increased tyrosine phosphorylation of the regulatory domain of the insulin receptor (specifically, tyrosine residues 1150 and 1151). In a direct action, therapeutic levels of metformin stimulated the tyrosine kinase activity of the soluble intracellular portion of the beta subunit of the human insulin receptor toward a substrate derived from the insulin receptor regulatory domain. However, metformin did not alter the order of substrate phosphorylation by the insulin receptor kinase. Using a Xenopus oocyte preparation, we simultaneously recorded tyrosine kinase and phosphatase activities that regulate the insulin receptor by measuring the tyrosine phosphorylation and dephosphorylation of peptides derived from the regulatory domain of the human insulin receptor. In an indirect stimulation of the insulin receptor, metformin inhibited endogenous tyrosine phosphatases and purified human protein tyrosine phosphatase 1B that dephosphorylate and inhibit the insulin receptor kinase. Thus, there was evidence that metformin acted directly upon the insulin receptor and indirectly through inhibition of tyrosine phosphatases.

Progesterone and insulin stimulation of CPEB-dependent polyadenylation is regulated by Aurora A and glycogen synthase kinase-3

Progesterone stimulation of Xenopus oocyte maturation requires the cytoplasmic polyadenylation-induced translation of mos and cyclin B mRNAs. One cis element that drives polyadenylation is the CPE, which is bound by the protein CPEB. Polyadenylation is stimulated by Aurora A (Eg2)-catalyzed CPEB serine 174 phosphorylation, which occurs soon after oocytes are exposed to progesterone. Here, we show that insulin also stimulates Aurora A-catalyzed CPEB S174 phosphorylation, cytoplasmic polyadenylation, translation, and oocyte maturation. However, these insulin-induced events are uniquely controlled by PI3 kinase and PKC-zeta, which act upstream of Aurora A. The intersection of the progesterone and insulin signaling pathways occurs at glycogen synthase kinase 3 (GSK-3), which regulates the activity of Aurora A. GSK-3 and Aurora A interact in vivo, and overexpressed GSK-3 inhibits Aurora A-catalyzed CPEB phosphorylation. In vitro, GSK-3 phosphorylates Aurora A on S290/291, the result of which is an autophosphorylation of serine 349. GSK-3 phosphorylated Aurora A, or Aurora A proteins with S290/291D or S349D mutations, have reduced or no capacity to phosphorylate CPEB. Conversely, Aurora A proteins with S290/291A or S349A mutations are constitutively active. These results suggest that the progesterone and insulin stimulate maturation by inhibiting GSK-3, which allows Aurora A activation and CPEB-mediated translation.

Insulin-like growth factors and their binding proteins: Potential relevance to reproductive physiology

Cyclic ovarian follicular development is a complex process that involves proliferation, differentiation, and death of follicle cells. Gonadotropins produced by the pituitary gland have a central role in the regulation of these processes. In addition, a wide range of paracrine and autocrine factors produced in the reproductive organs have been proposed as regulators of reproductive functions. Components of the insulin-like growth factors (IGF) system are widely expressed in the female reproductive tract. The IGFs and their binding proteins play a significant role in several processes of reproductive physiology, including ovarian follicular development, oogenesis and oocyte maturation, ovulation, luteal function, follicular atresia, and testicular function. The majority of these physiological actions of the IGFs are believed to occur via activation of the IGF-I receptor, although the IGF-I effects are modulated by IGF binding proteins (IGFBPs). As much of the data obtained to date have been in the rodent reproductive organs, it may not be possible to directly extrapolate the results to the primate organs. There is a distinct species-difference in the gene expression and functional roles of the IGF-IGFBP system in reproductive organs. However, the disturbance of the IGF-IGFBP system in human reproductive physiology may lead to anovulation, disorders of androgen excess, infertility associated with implantation failure, and male infertility. Further research is needed in domestic animals to determine if manipulation of the IGF-IGFBP system may result in improved reproductive efficiency. As our understanding of the IGF-IGFBP system increases, the uses of human recombinant IGF peptides and IGFBPs as clinical therapy for disease states is becoming a reality. (Reprod Med Biol 2003; 2: 1-24).

Cloning of the mouse insulin receptor substrate-3 (mIRS-3) promoter, and its regulation by p53

The insulin receptor susbtrate-3 (IRS-3) is a member of a family of intermediate adapter proteins that function as major intracellular targets for phosphorylation by the activated insulin and IGF-I receptors. Among the four IRS proteins identified so far, IRS-3 exhibits a rather peculiar expression pattern during both the embryonic development and adult life, suggesting a different mechanism of regulation of its expression. In this study, we cloned the 5' flanking region of the mIRS-3 gene and analyzed its promoter activity. The mIRS-3 promoter is inhibited by wild-type p53, and this effect is completely abolished by cotransfection of a dominant negative p53. Tumor-derived p53 mutants show variable, but lower suppressing capability than wt p53. In addition, treatment with doxorubicin inhibits endogenous expression of mIRS-3 mRNA in C2C12 and 3T3-L1 cells. The DNA region spanning from nucleotides -287 and -178 in the mIRS-3 promoter is responsible for a 32.2% reduction of the mouse double minute 2 (MDM2) promoter activity, suggesting its involvement in the p53-mediated inhibitory effect. In conclusion, our study demonstrates that the mIRS-3 promoter is regulated by p53 at the transcriptional level. The inhibition of mIRS-3 promoter by wild-type p53, and its de-repression by tumor-derived p53 mutants, appears to be similar to that previously reported for the IGF-I receptor promoter, suggesting a common role of these two genes in p53-mediated cell growth and differentiation.

Regulated cationic channel function in Xenopus oocytes expressing Drosophila big brain

Big brain (bib) is a neurogenic gene that when mutated causes defects in cell fate determination during Drosophila neurogenesis through an unknown mechanism. The protein Big Brain (BIB) has sequence identity with the major intrinsic protein family that includes the water- and ion-conducting aquaporin channels. We show here that BIB expressed heterologously in Xenopus oocytes provides a voltage-insensitive, nonselective cation channel function with permeability to K+ > Na+ >> tetraethylammonium. The conductance, activated in response to endogenous signaling pathways in BIB-expressing oocytes, is decreased after treatment with 20 microm insulin and is enhanced with 10 microm lavendustin A, a tyrosine kinase inhibitor. Western blot analysis confirms that BIB is tyrosine-phosphorylated. Both tyrosine phosphorylation and the potentiating effect of lavendustin A are removed by partial deletion of the C terminus (amino acids 317-700). Current activation is not observed in control oocytes or in oocytes expressing a nonfunctional mutant (BIB E71N) that appears to be expressed on the plasma membrane by confocal microscopy and Western blotting. These results indicate that BIB can participate in tyrosine kinase-regulated transmembrane signaling and may suggest a role for membrane depolarization in the neurogenic function of BIB in early development.

Tyrosine phosphorylation-dependent and -independent role of Shc in the regulation of IGF-1-induced mitogenesis and glycogen synthesis

To examine the functional role of Shc tyrosine phosphorylation in IGF-1 signaling, wild-type (WT)-Shc and Y239,240,317F (3F)-Shc were transiently transfected into L6 myoblasts. IGF-1 signaling was compared among the transfected cells. IGF-1-induced tyrosine phosphorylation of Shc and its subsequent association with Grb2 were increased in WT-Shc cells, whereas they were decreased in 3F-Shc cells compared with those in parental L6 cells. Consistent with their changes, IGF-1-induced MAPK activation and thymidine incorporation were enhanced in WT-Shc cells, whereas they were again decreased in 3F-Shc cells. It is possible that Shc and insulin receptor substrate (IRS)-1 can interact competitively, via their phosphotyrosine binding (PTB) domains, with the activated IGF-1 receptor. In this regard, IGF-1-induced tyrosine phosphorylation of IRS-1 was decreased by overexpressing both WT-Shc and 3F-Shc cells. Consistent with the decrease, IGF-1-induced IRS-1 association with the p85 subunit of PI3K and activation of PI3K and Akt were reduced in both WT-Shc and 3F-Shc cells. As a result, IGF-1-induced glycogen synthesis was also decreased in both cells. Furthermore, expression of Shc PTB domain alone inhibited IGF-1 stimulation of Akt and glycogen synthesis. These results indicate that tyrosine phosphorylation of Shc is important for IGF-1 stimulation of MAPK leading to mitogenesis and that Shc, via its PTB domain, negatively regulates IGF-1-induced glycogen synthesis by competing with IRS-1, which is not relevant to Shc tyrosine phosphorylation.

Insulin-secreting pituitary GH3 cells: a potential beta-cell surrogate for diabetes cell therapy

In a companion article, we describe the engineering and characterization of pituitary GH3 cell clones stably transfected with a furin-cleavable human insulin cDNA (InsGH3 cells). This article describes the performance of InsGH3 (clones 1 and 7) cell grafts into streptozotocin (STZ)-induced diabetic nude mice. Subcutaneous implantation of 2 x 10(6) InsGH3 cells resulted in the progressive reversal of hyperglycemia and diabetic symptoms, even though the progressive growth of the transplanted cells (clone 7) eventually led to glycemic levels below the normal mouse range. Proinsulin transgene expression was maintained in harvested InsGH3 grafts that, conversely, lose the expression of the prolactin (PRL) gene. Elevated concentrations of circulating mature human insulin were detected in graft recipients, demonstrating that proinsulin processing by InsGH3 cells did occur in vivo. Histologic analysis showed that transplanted InsGH3 grew in forms of encapsulated tumors composed of cells with small cytoplasms weakly stained for the presence of insulin. Conversely, intense insulin immunoreactivity was detected in graft-draining venules. Compared to pancreatic betaTC3 cells, InsGH3 cells showed in vitro a higher rate of replication, an elevate resistance to apoptosis induced by serum deprivation and proinflammatory cytokines, and significantly higher antiapoptotic Bcl-2 protein levels. Moreover, InsGH3 cells were resistant to the streptozotocin toxicity that, in contrast, reduced betaTC3 cell viability to 50-60% of controls. In conclusion, proinsulin gene expression and mature insulin secretion persisted in transplanted InsGH3 cells that reversed hyperglycemia in vivo. InsGH3 cells might represent a potential beta-cell surrogate because they are more resistant than pancreatic beta cells to different apoptotic insults and might therefore be particularly suitable for encapsulation.

Increased insulin receptor substrate-1 and enhanced skeletal muscle insulin sensitivity in mice lacking CCAAT/enhancer-binding protein beta

CCAAT/enhancer-binding protein beta (C/EBPbeta) controls gene transcription and metabolic processes in a variety of insulin-sensitive tissues; however, its role in regulating insulin responsiveness in vivo has not been investigated. We performed hyperinsulinemic-euglycemic clamps in awake, non-stressed, chronically catheterized adult mice homozygous for a deletion in the gene for C/EBPbeta (C/EBPbeta(-/-)). Fasting plasma insulin, glucose, and free fatty acid (FFA) levels were significantly lower in C/EBPbeta(-/-) mice compared with wild-type (WT) controls. Acute hyperinsulinemia (4 h) suppressed hepatic glucose production, phosphoenolpyruvate carboxykinase mRNA, and plasma FFA to a similar extent in WT and C/EBPbeta(-/-) mice, suggesting that C/EBPbeta deletion does not alter the metabolic and gene regulatory response to insulin in liver and adipose tissue. In contrast, using submaximal (5 milliunits/kg/min) and maximal (20 milliunits/kg/min) insulin infusions, whole-body glucose disposal was 77% (p < 0.01) and 33% (p < 0.05) higher in C/EBPbeta(-/-) mice, respectively, compared with WT mice. Maximal insulin-stimulated 3-O-methylglucose uptake in isolated soleus muscle was 54% greater in C/EBPbeta(-/-) mice (p < 0.05). Furthermore, insulin-stimulated insulin receptor and Akt Ser(473) phosphorylation and phosphatidylinositol 3-kinase activity were 1.6-2.5-fold greater in skeletal muscle from C/EBPbeta(-/-) mice compared with WT mice. The level of insulin receptor substrate-1 protein was increased 2-fold in skeletal muscle from C/EBPbeta(-/-) mice. These results demonstrate that C/EBPbeta deletion decreases plasma FFA levels and increases insulin signal transduction specifically in skeletal muscle, and both contribute to increased whole-body insulin sensitivity.

Association of insulin receptor substrate proteins with Bcl-2 and their effects on its phosphorylation and antiapoptotic function

Insulin receptor substrate (IRS) proteins are docking proteins that couple growth factor receptors to various effector molecules, including phosphoinositide-3 kinase, Grb-2, Syp, and Nck. Here we show that IRS-1 associates with the loop domain of Bcl-2 and synergistically up-regulates antiapoptotic function of Bcl-2. IRS-2 but not IRS-3 binds to Bcl-2, and IRS-1 associates with Bcl-XL but not with Bax or Bik. Overexpression of IRS-1 suppresses phosphorylation of Bcl-2 induced by stimulation with insulin, and the hypophosphorylation may lead to its enhanced antiapoptotic activity. The binding site for Bcl-2 is located on the carboxyl half-domain of IRS-1. IRS-3, which lacks the corresponding region, dominant-negatively abrogates the survival effects of IRS-1 and Bcl-2. For the antiapoptotic activity of IRS-1, binding to Bcl-2 is more critical than activating phosphoinositide-3 kinase. Our results indicate that IRS proteins transmit signals from the insulin receptor to Bcl-2, thus regulating cell survival probably through regulating phosphorylation of Bcl-2.

Insulin-like growth factors and ovarian physiology

OBJECTIVE

To review the available information regarding the roles of insulin-like growth factor (IGF)-IGF binding protein (IGFBP) system in ovarian physiology.

DESIGN

Studies that specifically relate to the roles of ovarian folliculogenesis, oocyte maturation, and ovulation were identified through the literature and Medline searches.

RESULTS

Numerous actions of the IGFs have been demonstrated in the ovary, including an enhancement of cell proliferation, aromatase activity, and progesterone biosynthesis. The ovarian IGF system, comprised of IGF-I and IGF-II peptides, IGFBPs and IGF receptors, plays a significant role in the process of follicular development. In addition, IGF-I stimulates the meiotic maturation of follicle-enclosed oocytes in vitro via the IGF-I receptors. IGFBP-3 significantly inhibit gonadotropin-induced ovulation and oocyte maturation by neutralizing endogenously produced IGF-I. Thus, the intraovarian IGF-IGFBP system play a significant role in the processes of follicular development, oocyte maturation, and ovulation.

CONCLUSION

IGF-IGFBP systems have autocrine/paracrine regulatory actions in ovarian physiology. The disturbance of the IGF-IGFBP system in human ovaries may lead to an ovulation, disorders of androgen excess, and infertility.

A particle-receptor model for the insulin-induced closure of connexin43 channels

Connexin43(Cx43) channels can be regulated by a variety of factors, including low pHi. Structure/function studies from this laboratory have demonstrated that pH gating follows a particle-receptor mechanism, similar to the "ball-and-chain" model of voltage-dependent inactivation of ion channels. The question whether the particle-receptor model is applicable only to pH gating or to other forms of Cx43 regulation as well remains. To address this question, we looked at the uncoupling effects of insulin and of insulin-like growth factor-1 (IGF) on Cx43 channels expressed in Xenopus oocytes. These agonists do not induce changes in pHi. Junctional conductance (Gj) was measured by the dual 2-electrode voltage-clamp technique. Control studies showed that relative Gj did not change spontaneously as a function of time. Continuous exposure of Cx43-expressing oocytes to insulin (10 micro/L) led to a decrease in Gj. After 80 minutes, Gj was 54+/-5% from control (n= 12). Exposure of oocytes to IGF (10 nmol/L) caused an even more pronounced change in Gj (37+/-4% of control, n=6). The time course of the IGF-induced uncoupling was similar to that observed after insulin exposure. The effect of insulin was abolished by truncation of the carboxyl-terminal domain of Cx43 at amino acid 257 (M257). Interestingly, as in the case of pH gating, coexpression of the carboxyl-terminal domain (amino acids 258 to 282) together with M257 rescued the ability of insulin to reduce coupling (Gj, 39+/-12% from control; n=6). Structure/function experiments using various deletion mutants of the carboxyl-terminal domain showed that insulin treatment does not modify Gj if amino acids 261 to 280 are missing from the Cx43 sequence. Our results suggest that a particle-receptor (or ball-and-chain) mechanism, similar to that described for pH gating, also applies to chemical regulation of Cx43 by other factors.

Progesterone release of lipid second messengers at the amphibian oocyte plasma membrane: role of ceramide in initiating the G2/M transition

Treatment with either sphingomyelinase (SMase), or soluble forms of ceramide, has been reported to induce meiosis in oocytes from Xenopus laevis, a species which can breed throughout most of the year. In this paper the sphingomyelin-derived second messenger, ceramide, is compared with progesterone for its ability to induce meiosis in oocytes from the seasonal breeder, Rana pipiens. Serum gonadotropin levels normally rise as Rana females emerge from hibernation in the spring, stimulating follicular synthesis of progesterone and subsequent ovulation. Injection of gonadotropins can induce earlier meiosis and ovulation, effective from the previous October through the following spring. During the same period, defolliculated oocytes respond to exogenous progesterone by meiosis, as indicated by nuclear breakdown. We find that in the spring, treatment of defolliculated Rana oocytes with exogenous C2- or C8-ceramide or SMase did induce meiosis, but not during the fall or winter. A 50% response was seen by late April and a 100% response by early May. Exposure of [3H]palmitate-labeled Rana oocytes to either exogenous progesterone or to SMase produced a rapid and comparable release of intracellular [3H]ceramide within 1-2 min in fall, winter or spring. Our results from this and from previous experiments indicate that increased ceramide is not the initiating event in meiotic induction in Rana, but is associated with a subsequent pathway which depends upon a threshold level of progesterone.

Grb10 interacts differentially with the insulin receptor, insulin-like growth factor I receptor, and epidermal growth factor receptor via the Grb10 Src homology 2 (SH2) domain and a second novel domain located between the pleckstrin homology and SH2 domains

The Grb10 protein appears to be an adapter protein of unknown function that has been implicated in insulin receptor (IR) signaling. The interaction of this protein with the IR has been shown to be mediated in part by the Src homology 2 (SH2) domain of Grb10. Here we demonstrate the existence of a second novel domain within Grb10 that interacts with the IR and insulin-like growth factor receptor in a kinase-dependent manner. This domain was localized to a region of approximately 50 amino acids, and we term it the BPS domain to denote its location between the PH and SH2 domains. The BPS domain does not bear any obvious resemblance to other known protein interaction domains but is highly conserved among the Grb10-related proteins Grb7 and Grb14. We show that the BPS domain interaction is dependent upon receptor tyrosine kinase activity. Furthermore, interaction of the BPS domain requires the kinase domain of the IR, since mutation of the paired tyrosine residues (Y1150F/Y1151F) within the IR activation loop dramatically reduced the interaction. Last, our data suggest that the presence of two distinct protein interaction domains may help to determine the specificity by which Grb10 interacts with different receptors. Specifically, the IR, which appears to interact most strongly with Grb10, interacts well with both the SH2 and BPS domains. Conversely, the insulin-like growth factor receptor and EGFR, which interact less avidly with Grb10, interact well only with the BPS domain or the SH2 domain, respectively. In summary, our findings demonstrate the existence of a previously unidentified tyrosine kinase activity-dependent binding domain located between the Pleckstrin homology and SH2 domains of Grb10.

Molecular cloning and characterization of Xenopus insulin-like growth factor-1 receptor: its role in mediating insulin-induced Xenopus oocyte maturation and expression during embryogenesis

We have cloned a complementary DNA encoding the putative Xenopus insulin-like growth factor-1 (xIGF-1) receptor. Injection of messenger RNA derived from the cloned complementary DNA into Xenopus oocytes resulted in the expression and correct processing of the receptor's alpha- and beta-subunits. Using antibodies generated against protein expressed against the cloned sequence, we demonstrated that the endogenous xIGF-1 receptor in Xenopus oocytes was activated by nanomolar concentrations of mammalian IGF-1 and by insulin approximately 100-fold higher in concentration. This receptor activation profile correlated with hormone-induced Xenopus oocyte maturation. Furthermore, injection of a neutralizing antiinsulin receptor antibody into Xenopus oocytes inhibited hormone-induced xIGF-1 receptor activation. These results provide molecular and biochemical evidence supporting a role for xIGF-1 receptor in mediating insulin/IGF-1-induced Xenopus oocyte maturation. We also report here that embryonic transcription of xIGF-1 receptor is activated during the formation of the central nervous system in early Xenopus embryos.

Insulin-like growth factor-I receptor internalization regulates signaling via the Shc/mitogen-activated protein kinase pathway, but not the insulin receptor substrate-1 pathway

Insulin-like growth factor-I (IGF-I) receptors activate divergent signaling pathways by phosphorylating multiple cellular proteins, including insulin receptor substrate-1 (IRS-1) and the Shc proteins. Following hormone binding, IGF-I receptors cluster into clathrin-coated pits and are internalized via an endocytotic mechanism. This study investigates the relationship between IGF-I receptor internalization and signaling via IRS-1 and Shc. A mutation in the C terminus of the IGF-I receptor decreased both the rate of receptor internalization and IGF-I-stimulated Shc phosphorylation by more than 50%, but did not affect IRS-1 phosphorylation. Low temperature (15 degrees C) decreased IGF-I receptor internalization and completely inhibited Shc phosphorylation. Although receptor and IRS-1 phosphorylation were decreased in accordance with delayed binding kinetics at 15 degrees C, the ratio of IRS-1 to receptor phosphorylation was increased more than 2-fold. Dansylcadaverine decreased receptor internalization and Shc phosphorylation, but did not change receptor or IRS-1 phosphorylation. Consistent with these findings, dansylcadaverine inhibited IGF-I-stimulated Shc-Grb2 association, mitogen-activated protein kinase phosphorylation, and p90 ribosomal S6 kinase activation, but did not affect the association of phosphatidylinositide 3-kinase with IRS-1 or activation of p70 S6 kinase. These data support the concept that Shc/mitogen-activated protein kinase pathway activation requires IGF-I receptor internalization, whereas the IRS-1 pathway is activated by both cell surface and endosomal receptors.

Differential signaling to glycogen synthesis by the intracellular domain of the insulin versus the insulin-like growth factor-1 receptor. Evidence from studies of TrkC-chimeras

Insulin and insulin-like growth factor-1 (IGF-1) have similar cell-surface receptors yet subserve different physiological functions. To examine whether these differences relate to intrinsic signaling properties of the intracellular domains of their respective receptors, chimeric receptors were constructed using the extracellular domain of the neurotrophin-3 (NT-3) receptor, TrkC, and the intracellular domain of either the insulin receptor or the IGF-1 receptor. TrkC-IR (TIR) and TrkC-IGF-1R (TIGR) were stably expressed in 3T3-L1 cells. While TIR and TIGR cell lines expressing similar numbers of chimeric receptors showed a similar dose-response relationship in NT-3 stimulated DNA synthesis, NT-3 stimulated glycogen synthesis was greater in TIR than in TIGR cells (maximum TIGR response was 35% of maximum TIR response). Additionally, the concentration of NT-3 at which significant stimulation of glycogen synthesis was seen was 0.1 ng/ml in TIR and 1 ng/ml in TIGR cells. Basal levels of thymidine incorporation but not glycogen synthesis were consistently higher in TIR than in TIGR expressing cells. No detectable basal autophosphorylation of chimeric receptors was seen in any of the cell lines. However, exposure of cell lines to the phosphatase inhibitor bisperoxovanadate resulted in greater basal autophosphorylation of the TIR and endogenous murine IR than the TIGR and endogenous murine IGF-1R. Thus, in this system, the intracellular domain of the IR appears to couple more effectively to glycogen synthesis than that of the IGF-1R, whereas the intracellular domains of both receptors have a similar capacity to stimulate DNA synthesis.

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.

The adapter protein Grb10 associates preferentially with the insulin receptor as compared with the IGF-I receptor in mouse fibroblasts

To identify receptor-associated proteins that may contribute to the specificity of insulin and IGF-I signaling responses, a mouse embryo library was screened using the yeast two-hybrid system. Multiple receptor-interactive clones encoding the SH2 domain of the adapter protein Grb10 were isolated. Subsequent cloning of the full-length Grb10 sequence from a mouse fat cDNA library defined a previously unknown Grb10 variant, that appears to be the predominant isoform in mouse tissues. Receptor-deficient R- cells (fibroblasts from mice with homologous disruption of the IGF-I receptor gene) and transfected R- cells expressing either insulin receptors (R-IR cells) or IGF-I receptors (R+ cells) were used to investigate the specificity of Grb10 interaction with the two related receptors. Hormone-activated insulin receptors in R-IR cells coprecipitated with three species, all recognized as Grb10 isoforms by specific Grb10 antibody. Under the same conditions, Grb10 was essentially undetectable in IGF-I receptor immunoprecipitates from stimulated R+ cells. Grb10 association with insulin receptors was maximal at 10 nM insulin stimulation and sustained from 5-10 min after hormone stimulation in R-IR cells. In conclusion, Grb10 interacts preferentially with insulin vs. IGF-I receptors in intact cells and, thus, may have a role in mediating insulin receptor-specific cellular responses.

A casein kinase I activity is constitutively associated with Nck

Nck is a 47-kDa cytosolic protein devoid of intrinsic catalytic activity and consisting of Src homology 2 and 3 (SH2 and SH3) domains organized as follows: SH3-SH3-SH3-SH2. Nck is believed to act as an adaptor protein mediating signal transduction initiated by receptor tyrosine kinases (RTKs). Through its SH2 domain, Nck recognizes a specific phosphotyrosine residue on RTKs or on protein substrates of RTKs like insulin receptor substrate-1, the major substrate of the insulin receptor, and through its SH3 domains it interacts with poorly characterized effector molecules. To identify novel proteins that might interact with Nck, we have used the amino-terminal segment of Nck encompassing its three SH3 domains in the yeast two-hybrid system. Among the polypeptides that associate with Nck, we have identified the gamma2 isoform of the serine/threonine casein kinase I (CKI-gamma2). In transformed rat hepatocytes overexpressing the insulin receptor (HTC-IR cells), serine/threonine protein kinase activity coimmunoprecipitates with Nck, an interaction mediated mainly by the third SH3 domain of Nck. This kinase activity is not apparently modulated by insulin, nor is it sensitive to staurosporine or heparin, and it does not use GTP as a phosphate donor. However the kinase activity coimmunoprecipitated with Nck is completely abolished by N-(2-aminoethyl)-5-chloroisoquinoline-8-sulfonamide, a specific inhibitor of casein kinase I. In an in vitro renaturation gel kinase assay, a protein kinase of 70-75 kDa was detected associated with the SH3 domains of Nck. Far Western analysis demonstrated that the SH3 domains of Nck bound directly to a cytosolic protein of 70-75 kDa. A rabbit polyclonal antibody raised against the C-terminal region of CKI-gamma2 protein kinase immunoprecipitated a single specific protein of 70-75 kDa from HTC-IR cell lysates and detected CKI-gamma2 among the proteins coimmunoprecipitated with Nck. These results support an in vivo interaction between Nck and CKI-gamma2 and suggest that CKI-gamma2 could be involved in signaling pathways downstream of RTKs.

Mutant of insulin receptor substrate-1 incapable of activating phosphatidylinositol 3-kinase did not mediate insulin-stimulated maturation of Xenopus laevis oocytes

Insulin receptor substrate-1 (IRS-1) is rapidly phosphorylated on multiple tyrosine residues in response to insulin and binds several Src homology 2 domain-containing proteins, thereby initiating downstream signaling. To assess the tyrosine phosphorylation sites that mediate relevant downstream signaling and biological effects, we created site-directed mutants of IRS-1 and overexpressed them in the Xenopus laevis oocyte. In oocytes overexpressing IRS-1 or IRS-1-895F (Tyr-895 replaced with phenylalanine), insulin activated phosphatidylinositol (PI) 3-kinase, p70 S6 kinase, and mitogen-activated protein kinase and induced oocyte maturation. In contrast, in oocytes overexpressing IRS-1-4F (Tyr-460, Tyr-608, Tyr-939, and Tyr-987 of IRS-1 replaced with phenylalanine), insulin did not activate PI 3-kinase, p70 S6 kinase, and mitogen-activated protein kinase and failed to induce oocyte maturation. These observations indicate that in X. laevis oocytes overexpressing IRS-1, the association of PI 3-kinase rather than Grb2 (growth factor-bound protein 2) with IRS-1 plays a major role in insulin-induced oocyte maturation. Activation of PI 3-kinase may lie upstream of mitogen-activated protein kinase activation and p70 S6 kinase activation in response to insulin.

Growth and survival signals transmitted via two distinct NPXY motifs within leukocyte tyrosine kinase, an insulin receptor-related tyrosine kinase

Leukocyte tyrosine kinase (LTK) is a receptor tyrosine kinase, which belongs to the insulin receptor family and is mainly expressed in pre-B cells and brain. In this study, we show that LTK utilizes insulin receptor substrate-1 (IRS-1) and Shc as major two substrates and possesses two NPXY motifs for them separately, tyrosine 485 of one NPXY motif at the juxtamembrane domain for IRS-1 and tyrosine 862 of another NPXY motif at the carboxyl-terminal domain for Shc. By using Ba/F3 cells expressing epidermal growth factor receptor-LTK chimeric receptors containing a mutation at each NPXY site, we showed that while both NPXY motifs equally contribute to activation of the Ras pathway and generation of mitogenic signals, only tyrosine 485 of LTK transmits cell survival signals. These data suggest that IRS-1 possesses anti-apoptotic function at least in LTK signaling. Moreover, our data indicate that the survival signaling pathway of LTK is distinct from the Ras pathway and the p70(S6) kinase pathway. Our results provide a useful insight in understanding the distinctive roles of Shc and IRS-1 in the signal transduction system of the insulin receptor family, and this anti-apoptotic function of IRS-1 may explain the survival effects of insulin, IGF-1, and interleukin 4.

Overexpression of Rad inhibits glucose uptake in cultured muscle and fat cells

Rad is a Ras-like GTPase that was isolated by subtraction cloning of human muscle and shown to have increased expression in some individuals with Type II diabetes. To ascertain the potential role of Rad in insulin-mediated signaling, we have overexpressed Rad in myocyte and adipocyte cell lines. Expression of Rad resulted in a 50-90% reduction in insulin-stimulated 2-deoxyglucose glucose uptake in C2C12 murine myotubes, L6 rat myotubes, and 3T3-L1 adipocytes and a 25% reduction in 3-O-methylglucose uptake in 3T3-L1 adipocytes. This occurred despite unaltered levels of glucose transporter expression, with no detectable change in Glut4 translocation and with no alteration in insulin receptor or substrate phosphorylation or phosphatidylinositol 3-kinase activity. These data indicate that Rad is a negative regulator of glucose uptake and that this effect may be due to a decrease in the intrinsic activity of the transporter molecules, rather than an effect on the translocation of Glut4.

Interaction of a GRB-IR splice variant (a human GRB10 homolog) with the insulin and insulin-like growth factor I receptors. Evidence for a role in mitogenic signaling

We have utilized the yeast two-hybrid system to identify proteins that interact with the cytoplasmic domain of the insulin receptor. We identified a human cDNA that is a splice variant of the human GRB10 homolog GRB-IR, which we term GRB10/IR-SV1 (for GRB10/GRB-IR splice variant 1). The protein encoded by the GRB10/IR-SV1 cDNA contains an SH2 domain and a pleckstrin homology domain. Cloning of a full-length human cDNA revealed a predicted coding sequence that was similar to the mouse GRB10 protein, although GRB10/IR-SV1 contained an 80-amino acid deletion. The GRB10/IR-SV1 cDNA is a splice variant of the GRB-IR cDNA such that GRB10/IR-SV1 contains an intact pleckstrin homology domain and a distinct amino terminus. The interaction of GRB10/IR-SV1 with the insulin receptor and the insulin-like growth factor I (IGF-I) receptor is mediated by the SH2 domain, and we show that glutathione S-transferase-SH2 domain fusion proteins interact specifically in vitro with the insulin receptor derived from mammalian cells. The GRB10/IR-SV1 SH2 domain also interacted with an approximately 135-kDa phosphoprotein from unstimulated cell lysates, an interaction that decreased after insulin stimulation. We present evidence that the GRB10/IR-SV1 protein plays a functional role in insulin and IGF-I signaling by showing that microinjection of an SH2 domain fusion protein inhibited insulin- and IGF-I-stimulated mitogenesis in fibroblasts, yet had no effect on mitogenesis induced by epidermal growth factor. Our findings suggest that GRB10/IR-SV1 may serve to positively link the insulin and IGF-I receptors to an uncharacterized mitogenic signaling pathway.

Functional importance of amino-terminal domain of Shc for interaction with insulin and epidermal growth factor receptors in phosphorylation-independent manner

Shc has two distinct domains, amino-terminal and SH2 domain, which can interact with activated growth factor receptors. Shc interacts with insulin receptor via Shc-amino-terminal (N) domain, whereas Shc associates with epidermal growth factor (EGF) receptor through both Shc-N and -SH2 domains. In accordance with the different functional roles between insulin and EGF receptors, EGF stimulated tyrosine phosphorylation of Shc faster than insulin. To clarify the functional importance of three distinct Shc domains on insulin and EGF signaling, we microinjected glutathione S-transferase (GST) fusion proteins containing the amino terminus plus collagen homology domain (NCH), collagen homology domain (CH), and Src homology 2 domain (SH2) into Rat1 fibroblasts expressing insulin receptors (HIRc). Bromodeoxyuridine (BrdUrd) incorporation into newly synthesized DNA was subsequently studied to assess the importance of the three distinct domains of Shc. Microinjection of the NCH-GST fusion protein inhibited BrdUrd incorporation induced by both EGF and insulin, whereas microinjection of the SH2-GST fusion protein inhibited EGF, but not insulin stimulation of DNA synthesis. Neither EGF- nor insulin-induced BrdUrd incorporation was inhibited by the CH-GST fusion protein. Following EGF or insulin stimulation, Shc is phosphorylated on single Tyr-317 residue serving as a docking site for Grb2. Microinjection of Shc-N+CH GST fusion protein with Tyr-317 --> Phe replacement (Y317F) also inhibited insulin stimulation of DNA synthesis. Next, we stably overexpressed wild-type Shc or Y317F mutant Shc into HIRc cells. Insulin-induced tyrosine phosphorylation of IRS-1 was compared among the transfected cell lines, since IRS-1 and Shc could competitively interact with insulin receptor. Insulin-stimulated tyrosine phosphorylation of IRS-1 was decreased in both WT-Shc and Y317F-Shc cells compared with that in HIRc cells. Furthermore, overexpression of the Shc-SH2 domain or Shc-N+CH domain with Y317F mutation interfered with EGF-stimulated endogenous Shc phosphorylation. These results suggest that the amino terminus domain of Shc is functionally important in insulin- and EGF-induced cell cycle progression and that the phosphorylation of Shc Tyr-317 residue is independent of Shc interaction with these receptors.

Interaction of insulin receptor substrate-2 (IRS-2) with the insulin and insulin-like growth factor I receptors. Evidence for two distinct phosphotyrosine-dependent interaction domains within IRS-2

Insulin receptor substrate 2 (IRS-2) has recently been shown to be a substrate of the insulin receptor (IR). In this study we utilize the yeast two-hybrid system and assays of in vitro interaction to demonstrate that IRS-2 interacts directly with the IR and the insulin-like growth factor I receptor. We show that, like IRS-1, the region of IRS-2 that contains the putative phosphotyrosine binding and SAIN elements (188-591) is sufficient for receptor interaction and that this interaction is dependent upon the NPX(p)Y (where (p)Y is phosphotyrosine) motifs within the juxtamembrane domains of the receptors. In addition to this amino-terminal NPX(p)Y-binding domain, an additional domain of strong interaction was identified in the central region of IRS-2 and was localized between amino acids 591 and 733. This interaction was found to be dependent upon receptor phosphorylation but was NPX(p)Y-independent. This region does not appear to have either an SH2 or a phosphotyrosine binding domain. Both of the interactions could also be demonstrated in vitro using IRS-2 glutathione S-transferase fusion proteins. We conclude that IRS-2, unlike IRS-1, can interact with tyrosine-phosphorylated receptors such as the IR and insulin-like growth factor I receptor via multiple independent binding motifs. Our findings suggest the existence of a previously unidentified phosphotyrosine-dependent binding domain within the central region of IRS-2.

Phosphorylation of insulin receptor substrate-1 on multiple serine residues, 612, 632, 662, and 731, modulates insulin action

Okadaic acid has been described previously as being a negative regulator of insulin signaling, as it inhibits insulin stimulation of glucose transport. In addition, this drug induces on insulin receptor substrate-1 (IRS-1) a decrease in tyrosine phosphorylation, concomitantly with an increase in serine/threonine phosphorylation. The present work was aimed at the identification of the serine/threonine residues that, upon phosphorylation, might be involved in modulating insulin signaling. To this end, we studied double-point mutants of IRS-1, in which serines 612/632 and 662/731 were replaced with alanine. These are four plausible sites of phosphorylation by mitogen-activated protein kinases and are in the immediate proximity of tyrosine residues, which are potential sites of interaction with phosphatidylinositol 3-kinase Src homology 2 domains. Using transient expression in 293 EBNA cells, we demonstrate that serines 612, 632, 662, and 731 and mitogen-activated protein kinases are not involved in the okadaic acid effect on IRS-1. Rather, these serines appear to play a role in modulating basal and insulin-stimulated IRS-1 tyrosine phosphorylation, association of IRS-1, with p85, and phosphatidylinositol 3-kinase activity in the IRS-1.p85 immune complex, since mutation of these sites enhances these events. Our findings suggest the existence of an IRS-1 desensitization mechanism resulting from serine/threonine phosphorylation, occurring at least on serines 612, 632, 662, and 731.

4PS/insulin receptor substrate (IRS)-2 is the alternative substrate of the insulin receptor in IRS-1-deficient mice

Insulin receptor substrate-1 (IRS-1) is the major cytoplasmic substrate of the insulin and insulin-like growth factor (IGF)-1 receptors. Transgenic mice lacking IRS-1 are resistant to insulin and IGF-1, but exhibit significant residual insulin action which corresponds to the presence of an alternative high molecular weight substrate in liver and muscle. Recently, Sun et al. (Sun, X.-J., Wang, L.-M., Zhang, Y., Yenush, L. P., Myers, M. G., Jr., Glasheen, E., Lane, W.S., Pierce, J. H., and White, M. F. (1995) Nature 377, 173-177) purified and cloned 4PS, the major substrate of the IL-4 receptor-associated tyrosine kinase in myeloid cells, which has significant structural similarity to IRS-1. To determine if 4PS is the alternative substrate of the insulin receptor in IRS-1-deficient mice, we performed immunoprecipitation, immunoblotting, and phosphatidylinositol (PI) 3-kinase assays using specific antibodies to 4PS. Following insulin stimulation, 4PS is rapidly phosphorylated in liver and muscle, binds to the p85 subunit of PI 3-kinase, and activates the enzyme. Insulin stimulation also results in the association of 4PS with Grb 2 in both liver and muscle. In IRS-1-deficient mice, both the phosphorylation of 4PS and associated PI 3-kinase activity are enhanced, without an increase in protein expression. Immunodepletion of 4PS from liver and muscle homogenates removes most of the phosphotyrosine-associated PI 3-kinase activity in IRS-1-deficient mice. Thus, 4PS is the primary alternative substrate, i.e. IRS-2, which plays a major role in physiologic insulin signal transduction via both PI 3-kinase activation and Grb 2/Sos association. In IRS-1-deficient mice, 4PS/IRS-2 provides signal transduction to these two major pathways of insulin signaling.

Insulin and insulin-like growth factor I (IGF-I) stimulate GLUT4 glucose transporter translocation in Xenopus oocytes

1. The heterologous expression of glucose transporters GLUT4 and GLUT1 in Xenopus oocytes has been shown to cause a differential targeting of these glucose-carrier isoforms to cellular membranes and a distinct induction of glucose transport activity. In this study we have evaluated the effect of insulin and insulin-like growth factor I (IGF-I) on glucose uptake and glucose transporter distribution in Xenopus oocytes expressing mammalian GLUT4 and GLUT1 glucose carriers. 2. Insulin and IGF-I stimulated 2-deoxyglucose uptake in GLUT4-expressing oocytes, but not in GLUT1-expressing oocytes or in water-injected oocytes. The stimulatory effect of insulin and IGF-I on 2-deoxyglucose uptake in GLUT4-expressing oocytes occurred via activation of the IGF-I receptor. 3. Subcellular-fractionation studies indicated that insulin and IGF-I stimulated translocation of GLUT4 to the cell surface of the oocyte. 4. Incubation of intact oocytes with insulin stimulated phosphatidylinositol 3-kinase activity, an effect that was blocked by the additional presence of wortmannin. Furthermore, wortmannin totally abolished the insulin-induced stimulation of 2-deoxyglucose uptake in GLUT4-expressing oocytes. 5. In this study, both the insulin-induced GLUT4 carrier translocation and GLUT4-dependent insulin-stimulated glucose transport have been reconstituted in the Xenopus oocyte. These observations, together with the fact that wortmannin, as found in adipocytes, inhibits insulin-stimulated glucose transport in oocytes, suggest that the heterologous expression of GLUT4 in oocytes is a useful experimental model by which to study the cell biology of insulin-induced GLUT4 translocation.

The structure and function of p55PIK reveal a new regulatory subunit for phosphatidylinositol 3-kinase

Phosphatidylinositol 3-kinase (PI-3 kinase) is implicated in the regulation of diverse cellular processes, including insulin-stimulated glucose transport. PI-3 kinase is composed of a 110-kDa catalytic subunit and an 85-kDa regulatory subunit. Here, we describe p55PIK, a new regulatory subunit that was isolated by screening expression libraries with tyrosine-phosphorylated insulin receptor substrate 1 (IRS-1). p55PIK is composed of a unique 30-residue NH2 terminus followed by a proline-rich motif and two Src homology 2 (SH2) domains with significant sequence identify to those in p85. p55PIK mRNA is expressed early during development, remains abundant in adult mouse brain and testis tissue, and is detectable in adult adipocytes and heart and kidney tissues. p55PIK forms a stable complex with p110, and it associates with IRS-1 during insulin stimulation. Moreover, the activated insulin receptor phosphorylates p55PIK in Sf9 cells, and insulin stimulates p55PIK phosphorylation in CHOIR/p55PIK cells. The unique features of p55PIK suggest that it is important in receptor signaling.

Molecular cloning of an amphibian insulin receptor substrate 1-like cDNA and involvement of phosphatidylinositol 3-kinase in insulin-induced Xenopus oocyte maturation

An insulin receptor substrate 1 (IRS-1)-like cDNA was isolated from a Xenopus ovary cDNA library by low-stringency hybridization using rat IRS-1 cDNA as a probe. The deduced amino acid sequence encoded by this cDNA (termed XIRS-L) is 67% identical (77% similar) to that of rat IRS-1. Significantly, all the insulin-induced tyrosine phosphorylation sites identified in rat IRS-1, including those responsible for binding to the Src homology domains of phosphatidylinositol (PI) 3-kinase, Syp and Grb2, are conserved in XIRS-L. Both mRNA and protein corresponding to the cloned XIRS-L can be detected in immature Xenopus oocytes. Recombinant XIRS-L protein produced in insect cells or a bacterial glutathione S-transferase fusion protein containing the putative PI 3-kinase binding site can be phosphorylated in vitro by purified insulin receptor kinase (IRK) domain, and the IRK-catalyzed phosphorylation renders both proteins capable of binding PI 3-kinase in Xenopus oocyte lysates. Another glutathione S-transferase fusion protein containing the C terminus of XIRS-L and including several putative tyrosine phosphorylation sites is also phosphorylated by IRK in vitro, but it failed to bind PI 3-kinase. Insulin stimulation of immature Xenopus oocytes activates PI 3-kinase in vivo [as indicated by an elevation of PI(3,4)P2 and PI(3,4,5)P3] as well as oocyte maturation (as indicated by germinal vesicle breakdown). Pretreatment of these oocytes with wortmannin inhibited insulin-induced activation of PI 3-kinase in vivo. The same treatment also abolished insulin-induced, but not progesterone-induced, germinal vesicle breakdown. These results (i) identify an IRS-1-like molecule in immature Xenopus oocytes, suggesting that the use of IRS-1-like Scr homology 2 domain-docking proteins in signal transduction is conserved in vertebrates, and (ii) strongly implicate PI 3-kinase as an essential effector of insulin-induced oocyte maturation.

Non-SH2 domains within insulin receptor substrate-1 and SHC mediate their phosphotyrosine-dependent interaction with the NPEY motif of the insulin-like growth factor I receptor

Insulin receptor substrate-1 (IRS-1) and SHC become rapidly phosphorylated upon tyrosines after insulin-like growth factor I receptor (IGFIR) activation. In this study we demonstrate that IRS-1, SHC, and the p85 subunit of phosphatidylinositol 3-kinase interact directly and specifically with the IGFIR. The interaction of all three proteins is dependent upon IGFIR kinase activity and, furthermore, substitution of Tyr-950 with Phe within the NPEY motif of the IGFIR eliminated interaction with both SHC and IRS-1 but had no effect upon p85 interaction. We show that residues 160-516 of IRS-1 and 1-238 of SHC are sufficient and necessary for receptor interaction in the yeast two-hybrid assay. We also demonstrate a direct in vitro interaction between the IGFIR and a fusion protein containing SHC amino acids 1-238. No interaction was observed with a SHC protein containing only the SH2 domain. We conclude that SHC and IRS-1 interact with the tyrosine-phosphorylated NPEY motif of the IGFIR, and that both proteins interact via related motifs located in their amino termini. We conclude that the interactions of SHC and IRS-1 with the IGFIR are similar to those which we have previously defined with the insulin receptor.

Growth hormone, interferon-gamma, and leukemia inhibitory factor promoted tyrosyl phosphorylation of insulin receptor substrate-1

The identification of JAK2 as a growth hormone (GH) receptor-associated, GH-activated tyrosine kinase has established tyrosyl phosphorylation as a signaling mechanism for GH. In the present study, GH is shown to stimulate tyrosyl phosphorylation of insulin receptor substrate 1 (IRS-1), the principle substrate of the insulin receptor. Tyrosyl phosphorylation of IRS-1 is a critical step in insulin signaling and provides binding sites for proteins with the appropriate Src homology 2 domains, including the 85-kDa regulatory subunit of phosphatidylinositol (PI) 3'-kinase. In 3T3-F442A fibroblasts, GH-dependent tyrosyl phosphorylation of IRS-1 was detected by 1 min and at GH concentrations as low as 5 ng/ml (0.23 nM). Tyrosyl phosphorylation of IRS-1 was transient, with maximal stimulation detected at 30 min and diminished signal detected at 60 min. The ability of GH receptor (GHR) to transduce the signal for IRS-1 tyrosyl phosphorylation is mediated by the intracellular region of GHR between amino acids 295 and 380 by a mechanism not involving the two tyrosines in this region. This region of GHR is required for GH-dependent JAK2 association and activation (VanderKuur, J. A., Wang, X., Zhang, L., Campbell, G. S., Allevato, G., Billestrup, N., Norstedt, G., and Carter-Su, C. (1994) J. Biol. Chem. 269, 21709-21717). When other cytokines that activate JAK2 were tested for the ability to stimulate the tyrosyl phosphorylation of IRS-1, stimulation was detected with interferon-gamma and leukemia inhibitory factor. The correlation between JAK2 tyrosyl phosphorylation and IRS-1 tyrosyl phosphorylation in response to GH, interferon-gamma, and leukemia inhibitory factor and in cells expressing different GHR mutants, provides evidence that IRS-1 may interact with JAK2 or an auxiliary molecule that binds to JAK2. GH is also shown to stimulate binding of IRS-1 to the 85-kDa regulatory subunit of PI 3'-kinase. The ability of GH to stimulate tyrosyl phosphorylation of IRS-1 and its association with PI 3'-kinase provides a biochemical basis for responses shared by insulin and GH including the well characterized insulin-like metabolic effects of GH observed in a variety of cell types.

Insulin signaling in chick embryos exposed to alcohol

Although insulin is known to be an important generator of regulatory signals during fetal growth and development, neither the immediate nor long-term effects of alcohol (ethanol) on insulin action are well understood. In the rat, fetal exposure to alcohol has been shown to be correlated with a subsequent abnormal response to a glucose load in the neonate and adult. Further, fetal hypoplasia secondary to maternal alcohol consumption is correlated with decreased placental glucose transport and with a lowering of the glucose levels in fetal tissues. However, the fetal effects of alcohol cannot be completely overcome by glucose/caloric supplementation, suggesting that factors other than glucose transport are involved. Using an embryonic chick model that negates the factors of maternal/placental metabolism and transport, the current study found that fetal alcohol exposure markedly increased insulin binding in developing tissue, but had little effect on the binding of the insulin-like growth factors. Competitive binding experiments revealed a marked increase in insulin receptor numbers, but no change in binding affinity as a result of the alcohol exposure. Basal uptake of 2-deoxyglucose by fetal tissue was lowered by alcohol exposure, but incubation with exogenous porcine insulin (1 x 10(-7) M) resulted in a significant increase in glucose uptake by the alcohol-exposed embryos. The increases in insulin binding and in insulin-dependent glucose uptake notwithstanding, exogenous insulin could not induce normal levels of ornithine decarboxylase activity in embryonic cells previously exposed to alcohol.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of Rad, a new member of Ras/GTPase superfamily, and its regulation by a unique GTPase-activating protein (GAP)-like activity

We have recently identified a new member of the Ras/GTPase superfamily termed Rad which has unique sequence features and is overexpressed in the skeletal muscle of humans with type II diabetes (Reynet, C., and Kahn, C. R. (1993) Science, 262, 1441-1444). When expressed in bacteria as a glutathione S-transferase fusion protein, Rad bound [alpha-32P]GTP quickly and saturably. Binding was specific for guanine nucleotides and displayed unique magnesium dependence such that both GTP and GDP binding were optimal at relatively high Mg2+ concentrations (1-10 mM). Rad had low intrinsic GTPase activity which was greatly enhanced by a GTPase-activating protein (GAP) activity present in various tissues and cell lines. Several known GAPs had no stimulatory effect toward Rad. Conversion of Ser to Asn at position 66 in Rad (equivalent to position 12 in Ras) resulted in a total loss of GTP binding. Mutation of Pro61 (equivalent to Gly12 in Ras) or Gln109 (equivalent to Gln61 in Ras) had no effect on Rad GTPase activity, whereas creation of a double mutation at these positions resulted in exceptionally high intrinsic GTPase activity. In vitro, Rad was phosphorylated by the catalytic subunit of cAMP-dependent protein kinase (PK). Phosphopeptide mapping indicated two PKA phosphorylation sites near the COOH terminus. Rad also co-precipitated a serine/threonine kinase activity from extracts of various tissues and cell lines which catalyzed phosphorylation on Rad but was not inhibited by PKA inhibitor. Thus, Rad is a GTP-binding protein and a GTPase which has some structure/function similarities to Ras, but displays unique features. Rad may also be phosphorylated on serine/threonine residues by PKA and other kinases, as well as regulated by its own GAP which is present in many tissues and cell types.

Enhancement or inhibition of insulin signaling by insulin receptor substrate 1 is cell context dependent

Insulin treatment of Chinese hamster ovary (CHO) cells expressing high levels of the insulin receptor (CHO/IR cells) activates both c-fos serum response element and activator protein 1 (AP-1) reporter genes approximately 10-fold. In contrast, parental CHO cells display only two- to threefold insulin stimulation of reporter gene activity. Transient transfection of parental CHO cells with an insulin receptor substrate 1 (IRS1) expression plasmid enhanced insulin downstream signaling in a biphasic manner, whereas IRS1 transfection of CHO/IR cells inhibited insulin signaling in a dose-dependent fashion. Further, expression of Grb2 in parental CHO cells had no effect on insulin signaling, whereas Grb2 increased insulin activation of reporter gene expression in CHO/IR cells. These data suggest that the expression levels of various effector molecules can either enhance or inhibit insulin downstream signaling events. To assess the relative effects of various insulin receptor, IRS1, and Grb2 levels on insulin signaling, parental CHO cells were transiently transfected with various combinations of expression plasmids encoding these proteins. Although expression of IRS1 resulted in a biphasic increase of insulin signaling in parental CHO cells, coexpression of IRS1 with the insulin receptor resulted in inhibition of signaling. This inhibition of insulin signaling directly correlated with an increased association of Grb2 with IRS1 and a concomitant sequestration of Grb2 away from Shc. Consistent with the Shc-Grb2 pathway as the major route for insulin-stimulated c-Fos and AP-1 transcriptional activation, the IRS1-mediated inhibition was reversed by transfection with an expression plasmid for Grb2. These data demonstrate that the extent of insulin-stimulated downstream signaling was dependent not only on the levels of individual signaling molecules but also on the formation of multiprotein complexes with specific stoichiometries.