Insulin-like growth factor I rapidly stimulates tyrosine phosphorylation of a Mr 185,000 protein in intact cells

Phosphorylation Codes in IRS-1 and IRS-2 Are Associated with the Activation/Inhibition of Insulin Canonical Signaling Pathways

Insulin receptor substrates 1 and 2 (IRS-1 and IRS-2) are signaling adaptor proteins that participate in canonical pathways, where insulin cascade activation occurs, as well as in non-canonical pathways, in which phosphorylation of substrates is carried out by a diverse array of receptors including integrins, cytokines, steroid hormones, and others. IRS proteins are subject to a spectrum of post-translational modifications essential for their activation, encompassing phosphorylation events in distinct tyrosine, serine, and threonine residues. Tyrosine residue phosphorylation is intricately linked to the activation of the insulin receptor cascade and its interaction with SH2 domains within a spectrum of proteins, including PI3K. Conversely, serine residue phosphorylation assumes a different function, serving to attenuate the effects of insulin. In this review, we have identified over 50 serine residues within IRS-1 that have been reported to undergo phosphorylation orchestrated by a spectrum of kinases, thereby engendering the activation or inhibition of different signaling pathways. Furthermore, we delineate the phosphorylation of over 10 distinct tyrosine residues at IRS-1 or IRS-2 in response to insulin, a process essential for signal transduction and the subsequent activation of PI3K.

Imbalanced Insulin Actions in Obesity and Type 2 Diabetes: Key Mouse Models of Insulin Signaling Pathway

Since the discovery of the tyrosine kinase activity of the insulin receptor (IR), researchers have been engaged in intensive efforts to resolve physiological functions of IR and its major downstream targets, insulin receptor substrate 1 (Irs1) and Irs2. Studies conducted using systemic and tissue-specific gene-knockout mice of IR, Irs1, and Irs2 have revealed the physiological roles of these molecules in each tissue and interactions among multiple tissues. In obesity and type 2 diabetes, selective downregulation of Irs2 and its downstream actions to cause reduced insulin actions was associated with increased insulin actions through Irs1 in variety tissues. Thus, we propose the novel concept of "organ- and pathway-specific imbalanced insulin action" in obesity and type 2 diabetes, which includes and extends "selective insulin resistance." This Review focuses on recent progress in understanding insulin signaling and insulin resistance using key mouse models for elucidating pathophysiology of human obesity and type 2 diabetes.

Insulin-like growth factors in the gastrointestinal tract and liver

The liver is a major source of insulin-like growth factors (IGFs) and IGF binding proteins (IGFBPs) that are present in the circulation and have important endocrine activities relating to energy metabolism, body size, carcinogenesis, and various organ-specific functions. Although IGFs have only minor effects on the normal liver itself, production of IGFs and IGFBPs in a tissue-specific manner in the gastrointestinal tract exert important regulatory effects on cellular proliferation, survival, and apoptosis. IGFs and IGFBPs play important regulatory roles in the response of both the liver and the gastrointestinal tract to inflammation and in the development of neoplasia.

Intracellular events mediating insulin-like growth factor I-induced oligodendrocyte development: modulation by cyclic AMP

Insulin-like growth factor I (IGF-I) is a potent inducer of oligodendrocyte development and myelination. Although IGF-I intracellular signaling has been well described in several cell types, intracellular mechanisms for IGF-I-induced oligodendrocyte development have not been defined. By using specific inhibitors of intracellular signaling pathways, we report here that the MAPK and phosphatidylinositol 3-kinase signaling pathways are required for the full effect of IGF-I on oligodendrocyte development in primary mixed rat cerebrocortical cell cultures. The MAPK activation, but not the phosphatidylinositol 3-kinase activation, leads to phosphorylation of the cAMP response element-binding protein, which is necessary for IGF-I to induce oligodendrocyte development. cAMP, although it does not show any effect on oligodendrocyte development, has an inhibitory effect on IGF-I-induced oligodendrocyte development that is mediated by the cAMP-dependent protein kinase. Furthermore, cAMP also has an inhibitory effect on IGF-I-dependent MAPK activation. This is a cAMP-dependent protein kinase-independent effect and probably contributes to the cAMP action on IGF-I-induced oligodendrocyte development.

Relative expression and localization of the insulin-like growth factor system components in the fetal, child and adult intestine

BACKGROUND

The insulin-like growth factors (IGFs) are important in the development and maintenance of the gastrointestinal tract.

OBJECTIVES

To compare the expression of IGFs and their receptors in the stomach and duodenum of the fetus, the child and the adult. To identify the cells mainly responsible for the production of the members of the IGF system.

METHODS

Tissue was obtained from fetus after abortion and from children and adults during diagnostic endoscopy and biopsy. The expression of the IGFs and their receptors was estimated by an RNAse protection assay and sections were stained with antisera to the components of IGF system.

RESULTS

The tissues from the stomach and the duodenum expressed the two IGFs and their receptors at all stages of life. The fetal IGF receptors I and II, were approximately ten times higher than in the child and IGF-II was five times higher. Immunohistochemical staining showed the components of the IGF system to be localized to the gastric glands and to the basotlateral border of the gastric epithelial cells. In the duodenum, they were concentrated at the apical portion of the epithelial tissue. They could also be identified in ganglion cells and nerves.

CONCLUSIONS

The IGFs and their receptors in the stomach and duodenum are expressed in all age groups and mostly are highest in the fetus. The IGF system proteins were located in the gastric glands and epithelium and in the apical portion of the villous epithelium of the duodenum.

Role of IGF system of mitogens in the induction of fibroblast proliferation by keloid-derived keratinocytes in vitro

Keloids are proliferative dermal growths representing a pathological wound-healing response. We report high proliferation rates in normal (NF) and keloid-derived fibroblasts (KF) cocultured with keloid-derived keratinocytes (KK). IGF binding protein (IGFBP)-3 mRNA and secreted IGFBP-3 in conditioned media were increased in NF cocultured with KK compared with NF but markedly reduced in KF cocultured with KK or normal keratinocytes (NK). IGFBP-2 and IGFBP-4 mRNA levels were elevated, whereas IGFBP-5 mRNA was decreased in KF cocultured with KK or NK. Significant increases in IGFBP-2 and -4 mRNA in KF cocultured with KK did not correlate with protein secretion. Downstream IGF signaling cascade components, phospho-Raf, phospho-MEK1/2, phospho-MAPK, PI-3 kinase, phospho-Akt, and phospho-Elk-1, were elevated in KF cocultured with KK. Addition of recombinant human IGFBP-3 or antibodies against IGF-I or IGF-IR significantly inhibited proliferation of KF. The bioavailability of IGF-I may be related to the levels of IGFBP-3 produced, which in turn influences KF proliferation, suggesting that modulation of IGF-I, IGF-IR, and IGFBP-3, individually or in combination, may represent novel approaches to the treatment of keloids.

Cell adhesion properties and effects on receptor-mediated insulin endocytosis are independent properties of pp120, a substrate of the insulin receptor tyrosine kinase

pp120 undergoes phosphorylation by the tyrosine kinase of the insulin, not the insulin-like growth factor 1 (IGF-1), receptor. Moreover, pp120 stimulates receptor-mediated insulin, but not IGF-1, endocytosis, suggesting that pp120 phosphorylation underlies its effect on insulin endocytosis. pp120 phosphorylation also underlies its bile acid transport and tumor suppression functions. In addition to depending on the intracellular tail, the cell adhesion property of pp120 depends on Arg98 in the N-terminal IgV-like ectoplasmic domain. To investigate whether this domain mediates the effect of pp120 on insulin endocytosis, we mutated Arg98 to Ala and examined whether this mutation altered pp120 phosphorylation and its effect on ligand endocytosis in transfected NIH 3T3 cells. This mutation did not modify either pp120 phosphorylation or its effect on receptor-mediated ligand endocytosis. These findings support the hypothesis that stimulation of insulin endocytosis by pp120 is not mediated by Arg98 in the N-terminal IgV-like ectoplasmic domain of pp120.

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.

Inhibition of PI 3-kinase and RAS blocks IGF-I and insulin-induced uncoupling protein 1 gene expression in brown adipocytes

Fetal brown adipocytes expressed uncoupling protein 1 (UCP1) mRNA, this expression being blunted throughout culture for 24 h in a serum-free medium. At physiological doses, either insulin-like growth factor I (IGF-I) or insulin turned out to be as potent as dibutyryl cAMP (dbcAMP) in increasing UCP1 gene transcription rate (1 h) and also UCP1 mRNA accumulation (3 h), their maximal effect (15-fold increase) reached upon treatment for 24 h. Upon treatment with either IGF-I or insulin for 48 h, a 7-fold increase in the UCP1 protein content relative to levels in the control cells was found, this induction being abolished in the presence of cycloheximide. Moreover, either IGF-I or insulin transactivates the UCP1-chloramphenicol acetyl transferase (CAT) fusion gene after transient transfection of primary brown adipocytes, these effects being tissue-specific. Transient transfection of dominant-negative form of phosphatidylinositol (PI) 3-kinase completely blocked the transactivation of the fusion gene UCP1-CAT induced by either IGF-I or insulin, although inhibition of p70S6kinase with rapamycin does not preclude transactivation of the UCP1 promoter by insulin. Furthermore, transient transfection of dominant-negative form of p21-ras or treatment of cells with a mitogen-activated protein kinase kinase (MEK-1) inhibitor (PD098059) completely abolished insulin-induced UCP1-CAT transactivation. Cotransfection with dominant-negative p85 or with dominant-negative Ras also produced down-regulation of the insulin or IGF-I-induced 12-O-tetradecanoylphorbol-13-acetate response element (TRE)-CAT (five AP-1, activating protein-1, binding sites arranged in tandem) transactivation. In addition, insulin induced AP-1 DNA binding activity, this effect being totally prevented in the presence of MEK-1 inhibitor. These results strongly suggest that either IGF-I or insulin induced thermogenic-differentiation through AP-1 activity in a PI 3-kinase and Ras/MAPK dependent manner in brown adipocytes.

Insulin-like growth factor-I and diabetes. A review

Although diabetes is a heterogeneous condition, IGF-I has been shown to improve glycaemic control and reduce insulin requirements in both IDDM and NIDDM. In IDDM, the therapeutic rationale for IGF-I is as a replacement therapy "topping up" low circulating IGF-I levels. There is now convincing evidence that this is associated with a reduction in GH secretion resulting in an improvement in insulin sensitivity and glycaemic control. The mechanism may simply be reduced GH-secretion, but pre- and post-receptor effects on insulin sensitivity are also likely. It is not clear what effect IGF-I treatment has on IGF binding proteins, but with the restoration of a more normal GH/IGF-I axis they are likely to be restored to normal concentrations which may in turn have a direct effect on glucose metabolism. In NIDDM, the mechanism of action of IGF-I remains unclear. At high doses, IGF-I may mimic insulin, but at levels resulting in unacceptable "acromegalic" IGF-I levels and side-effects. The most exciting data concerning IGF-I is with a low dose where IGF-I improves insulin sensitivity by an unknown mechanism. This may be mediated via the IGF-I receptor, by cross-reactivity with the insulin receptor, or by activation of hybrid receptors. The exact mechanism and interaction remains to be elucidated. In severe insulin-resistant states, IGF-I-treatment appears to be effective, and may be the only realistic therapeutic measure in the near future, and warrants further investigation. Detailed genetic characterization of these syndromes following treatment with IGF-I may also help to characterize the mechanism of action of IGF-I and its interactions with the insulin receptor. Thus, IGF-I appears to have a future as a therapeutic agent in treating diabetes, but long-term studies addressing safety and short-term studies addressing mechanisms are essential. With only a few pharmaceutical companies having the capability to produce IGF-I for scientific and therapeutic investigation, it is important that short-term marketing strategy does not prevent the proper exploration of this exciting peptide hormone as a therapeutic agent for all types of diabetes.

Mitogenic signaling of insulin-like growth factor I in MCF-7 human breast cancer cells requires phosphatidylinositol 3-kinase and is independent of mitogen-activated protein kinase

Addition of insulin-like growth factor I (IGF-I) to quiescent breast tumor-derived MCF-7 cells causes stimulation of cyclin D1 synthesis, hyperphosphorylation of the retinoblastoma protein pRb, DNA synthesis, and cell division. All of these effects are independent of the mitogen-activated protein kinase (MAPK) pathway since none of them is blocked by PD098059, the specific inhibitor of the MAPK activating kinase MEK1. This observation is consistent with the finding that the phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA), a strong inducer of MAPK activity in MCF-7 cells, effectively inhibits proliferation. The anti-proliferative effect of TPA in these cells may be accounted for, at least in part, by the MAPK-dependent stimulation of the synthesis of p21(WAF1/CIP1), an inhibitor of cyclin/cyclin-dependent kinase complexes. In contrast, all of the observed stimulatory effects of IGF-I on cell cycle progression, cyclin D1 synthesis, and pRb hyperphosphorylation were blocked by the specific phosphatidylinositol 3-kinase inhibitor LY294002, suggesting that phosphatidylinositol 3-kinase activity but not MAPK activity is required for transduction of the mitogenic IGF-I signal in MCF-7 cells.

Neutralizing IGF-1 monoclonal antibody with cross-species reactivity

We report the generation of a murine IGF-1 monoclonal antibody designated 35I17, which exhibits unique cross-species reactivity. The antibody recognizes recombinant human and rat IGF-1 in ELISA, Western blots, and in an 125I-recombinant human IGF-1 Scintillation Proximity Assay. In addition, 35I17 blocks cell proliferation induced by recombinant human and rat IGF-1, and inhibits cell proliferation induced by sera from human, rat, calf, dog, goat, or mouse. The antibody inhibits rat IGF-1 binding to IGF-1 receptors, and prevents IGF-1-stimulated receptor and IRS-1 phosphorylation in LISN C4 cells, an IGF-1 receptor-transfected cell line. The cross-species and neutralizing properties of 35I17 may be useful in in vitro and in vivo animal studies for elucidating the role of IGF-1 in cancer, rheumatoid arthritis, and other diseases.

Differential phosphorylation of pp120 by insulin and insulin-like growth factor-1 receptors: role for the C-terminal domain of the beta-subunit

pp 120, a plasma membrane glycoprotein expressed by hepatocytes, is a substrate of the insulin receptor tyrosine kinase. Since insulin-like growth factor-1 (IGF-1) and insulin receptors are structurally homologous, we investigated whether pp120 is also a substrate of the IGF-1 receptor tyrosine kinase. IGF-1 receptor failed to phosphorylate pp120 in response to IGF-1 in stably transfected NIH 3T3 fibroblasts. However, replacement of the C-terminal domain of the beta-subunit of the IGF-1 receptor with the corresponding fragment in the insulin receptor restored ligand-stimulated pp120 phosphorylation, suggesting that this domain plays a regulatory role in pp120 phosphorylation. Since pp120 is the first identified substrate specific for the insulin vis-à-vis the IGF-1 receptor tyrosine kinase, the pp120 signaling pathway may constitute a novel mechanism for the distinct cellular effects of insulin and IGF-1, the former being principally involved in metabolism, and the latter in mitogenesis.

Phosphatidylinositol 3-kinase is a requirement for insulin-like growth factor I-induced differentiation, but not for mitogenesis, in fetal brown adipocytes

In the present study we have examined the role of phosphatidylinositol 3-kinase (PI 3-kinase) in the insulin-like growth factor I (IGF-I)-signaling pathways involved in differentiation and in mitogenesis in fetal rat brown adipocytes. Activation of PI 3-kinase in response to IGF-I was markedly inhibited by two PI 3-kinase inhibitors (wortmannin and LY294002) in a dose-dependent manner. IGF-I-stimulated glucose uptake was also inhibited by both compounds. The expression of adipogenic-related genes such as fatty acid synthase, malic enzyme, glycerol 3-phosphate dehydrogenase, and acetylcoenzyme A carboxylase induced by IGF-I was totally prevented in the presence of IGF-I and any of those inhibitors, resulting in a marked decrease of the cytoplasmic lipid content. Moreover, the expression of the thermogenic marker uncoupling protein induced by IGF-I was also down-regulated in the presence of wortmannin/LY294002. IGF-I-induced adipogenic- and thermogenic-related gene expression was only partly inhibited by the p70S6k inhibitor rapamycin. In addition, pretreatment of brown adipocytes with either wortmannin or LY294002, but not with rapamycin, blocked protein kinase C zeta activation by IGF-I. In contrast, IGF-I-induced fetal brown adipocyte proliferation was PI 3-kinase-independent. Our results show for the first time an essential requirement of PI 3-kinase in the IGF-I-signaling pathways leading to fetal brown adipocyte differentiation, but not leading to mitogenesis. In addition, protein kinase C zeta seems to be a signaling molecule also involved in the IGF-I differentiation pathways downstream from PI 3-kinase.

IGF-I: a mitogen also involved in differentiation processes in mammalian cells

The main source of insulin-like growth factor I (IGF-I) postnatally is the liver, under growth hormone stimulation, although IGF-I is already present in embryonic tissues and in fetal serum, when its expression is independent of growth hormone. The extracellular alpha-subunit of the IGF-I receptor (IGF-IR) contains an IGF-I binding domain, and the beta-subunit possesses tyrosine kinase activity, which is greatly enhanced when IGF-I binds to the alpha-subunit and leads to its autophosphorylation. Insulin receptor substrate 1 (IRS-1) is the most well characterized cellular substrate for IGF-I, containing at least 20 potential tyrosine phosphorylation sites. The tyrosine phosphorylated form of IRS-1 acts as a docking protein by associating SH2-containing proteins including the p85 regulatory subunit of phosphatidylinositol-3-kinase (P13-kinase), the protein tyrosine phosphatase SH-PTP2, the SH2- and SH3-containing adaptor protein Nck and the growth factor receptor-bound protein-2 (Grb2/Sem5) protein. Grb2 is found associated with mSOS, a GTP/GDP exchange factor involved in converting the inactive Ras-GDP to the active Ras-GTP. The p85 regulatory subunit of PI3-kinase can be also a direct in vitro substrate of the IGF-IR. Although IRS-1 is the major substrate of the IGF-IR, there is another early phosphotyrosine substrate termed SHC, which also activates Ras via Grb2-mSos complex. Activation of p21-Ras induces a serine/threonine kinase cascade leading to the activation of MAP-kinases. The importance of IGF-I as a mitogen throughout development has been clearly demonstrated in IGF-I and IGF-IR knockout mouse studies and also in transgenic mice over-expressing IGF-I. IGF-I is a mitogen in many cell types in culture such as T lymphocytes, chondrocytes or osteoblasts and it is considered to be a progression factor in mouse fibroblasts. IGF-I is also involved in muscle, neurons and adipogenic differentiation of mesenchymal cells. However, IGF-I induces proliferation and differentiation in fetal brown adipocytes, suggesting that both cellular processes are not necessarily mutually exclusive in fetal cells.

Identification of functional insulin-like growth factor-II/mannose-6-phosphate receptors in isolated bone cells

The role of the IGF-II/cation independent mannose-6-phosphate (IGF-II/M6P) receptor in the transduction of cellular effects evoked by IGF-II has been extensively debated in the literature. Many reports suggest that IGF-II transduces its effects through the IGF-I receptor, while others show that IGF-II utilizes the type II receptor to affect cellular activity. This study 1) verifies the presence of the IGF-II/M6P receptor in rat calvarial osteoblasts, and 2) evaluates the ability of the receptor to initiate intracellular signals. Using conventional receptor binding assays, it was found that osteoblasts bind IGF-II with high affinity. Scatchard analyses indicated that there are 5.08 x 10(4) IGF-II/M6P receptors per osteoblast with a Kd near 2.0 nM). The receptor protein was further identified by cross-linking with 125I-IGF-II. Northern analysis was used to identify an mRNA transcript for the IGF-II/M6P receptor protein. To examine if the IGF-II/M6P receptor can initiate second messenger signals, the ability of IGF-II to evoke Ca2+ transients was evaluated. Osteoblasts pretreated with IGF-I did not lose their ability to respond to IGF-II. Further, a polyclonal antibody against the rat IGF-II/M6P receptor (R-II-PAB1) 1) was able to evoke its own Ca2+ response, and 2) was able to block the generation of Ca2+ transients caused by IGF-II. The data in this report show that the osteoblastic Ca2+ response to IGF-II appears to be caused by an intracellular release of Ca2+ which is mediated by the IGF-II/M6P receptor making it possible to envision how the receptor may be an important modulator of osteoblast mediated osteogenesis.

Localization of the insulin-like growth factor I receptor binding sites for the SH2 domain proteins p85, Syp, and GTPase activating protein

Potential signaling substrates for the insulin-like growth factor I (IGF-I) receptor are SH2 domain proteins including the p85 subunit of phosphatidylinositol 3-kinase, the tyrosine phosphatase Syp, GTPase activating protein (GAP), and phospholipase C-gamma (PLC-gamma). In this study, we demonstrate an association between the IGF-I receptor and p85, Syp, and GAP, but not with PLC-gamma in lysates of cells overexpressing the human IGF-I receptor. We further investigated these interactions using glutathione S-transferase (GST) fusion proteins containing the amino-terminal SH2 domains of p85 or GAP, or both SH2 domains of Syp or PLC-gamma to precipitate the IGF-I receptor from purified receptor preparations and from whole cell lysates. p85-, Syp-, and GAP-GSTs precipitated the IGF-I receptor, whereas the PLC-gamma-GST did not. Using phosphopeptides corresponding to IGF-I receptor phosphorylation sites, we determined that the p85- and Syp-GST association with the IGF-I receptor could be inhibited by a carboxyl-terminal peptide containing pY1316 and that the GAP-GST association could be inhibited by a NPXY domain peptide. The GAP-GST binding site was confirmed by showing that a mutant IGF-I receptor with a deletion of the NPXY domain including tyrosine 950 was poorly precipitated by the GAP-GST. We conclude that p85 and Syp may bind directly to the IGF-I receptor at tyrosine 1316, and that GAP may bind to the IGF-I receptor at and PLC-gamma was not evident. p85, Syp, and GAP are potential modulators of IGF-I receptor signal transduction.

Growth hormone stimulates the tyrosine phosphorylation of the insulin receptor substrate-1 and its association with phosphatidylinositol 3-kinase in primary adipocytes

Insulin receptor substrate-1 (IRS-1) is tyrosine-phosphorylated in response to insulin resulting in association with and activation of phosphatidylinositol 3-kinase (PI 3-kinase), thereby initiating some of the effects of insulin. We have recently shown that the insulin-like effects of growth hormone (GH) in adipocytes can be inhibited by the selective PI 3-kinase inhibitor wortmannin (Ridderstråle, M., and Tornqvist, H. (1994) Biochem. Biophys. Res. Commun. 203, 306-310), suggesting a similar role for PI 3-kinase in GH action. Here we show that IRS-1 is tyrosine-phosphorylated in a time- and dose-dependent manner in response to GH in primary rat adipocytes. This phosphorylation coincided with the extent of interaction between IRS-1 and the 85-kDa subunit of PI 3-kinase as evidenced by coimmunoprecipitation. Stimulation with 23 nM GH increased the PI 3-kinase activity associated with IRS1 4-fold. Our data suggest that GH-induced tyrosine phosphorylation of IRS-1 and the subsequent docking of PI 3-kinase are important postreceptor events in GH action. The mechanism for the phosphorylation of IRS-1 induced by GH is unknown, but involvement of JAK2, the only known GH receptor-associated tyrosine kinase, seems possible.

Activation of mitogen-activated protein kinase and phosphatidylinositol 3'-kinase is not sufficient for the hormonal stimulation of glucose uptake, lipogenesis, or glycogen synthesis in 3T3-L1 adipocytes

The precise mechanism by which insulin regulates glucose metabolism is not fully understood. However, it is known that insulin activates two enzymes, phosphatidylinositol 3'-kinase (PI 3'-K) and mitogen-activated protein kinase (MAPK), which may be involved in stimulating the metabolic effects of insulin. The role of these enzymes in glucose metabolism was examined by comparing the effects of insulin, platelet-derived growth factor (PDGF) and epidermal growth factor (EGF) in 3T3-L1 adipocytes. Treatment of the cells with PDGF or EGF for 5 min increased the MAPK activity 3-5-fold, while insulin treatment produced a 2.5-fold increase. The MAPK activity remained elevated for 1 h after either PDGF or insulin treatment. PDGF and insulin, but not EGF, caused a transient increase in the amount PI 3'-K activity coprecipitated with tyrosine phosphorylated proteins. Although PDGF and insulin caused a similar increase in the activities of these two enzymes, only insulin caused substantial increases in glucose utilization. Insulin increased the transport of glucose and the synthesis of lipid 4- and 17-fold, respectively, while PDGF did not affect these processes significantly. Glycogen synthesis was increased 15-fold in response to insulin and only 3-fold in response to PDGF. Thus, the activation of MAPK and PI 3'-K are not sufficient for the complete stimulation of glucose transport, lipid synthesis, or glycogen synthesis by hormones in 3T3-L1 adipocytes, suggesting a requirement for other signaling mechanisms that may be uniquely responsive to insulin.

Involvement of Src-homology/collagen (SHC) proteins in signaling through the insulin receptor and the insulin-like-growth-factor-I-receptor

Src homology/collagen (SHC) proteins are thought to participate in signaling through both receptor tyrosine kinases, such as the insulin receptor and the EGF (epidermal growth factor) receptor, and cytoplasmic tyrosine kinases, such as v-src and v-fps. Here we approached the insulin-induced and the insulin-like-growth-factor-I-induced (IGF-I-induced) phosphorylation of SHC proteins, and the possible role of these proteins in insulin and IGF-I signaling. First, we showed that SHC proteins are phosphorylated on tyrosine residues upon insulin and IGF-I treatment of fibroblasts transfected with a SHC cDNA construct. More important, ligand-activated insulin and IGF-I receptors phosphorylate SHC proteins in vitro, indicating that SHC proteins could be direct substrates for insulin and IGF-I receptors. Further, insulin or IGF-I treatment of SHC-transfected fibroblasts leads to immunoprecipitation of SHC proteins with insulin-receptor substrate 1 (IRS-1). We next looked at the possible effect of SHC proteins on biological responses in SHC-transfected fibroblasts. We found that the expression of exogenous SHC proteins results in an increased basal MEK (MAPK/ERK-activating kinase) activity. Further, neither the basal nor the insulin-induced or IGF-I-induced PtdIns-3-kinase activity were modified by expression of exogenous SHC proteins. These results illustrate that SHC proteins are implicated in the MAP (mitogen-activated protein)-kinase pathway, but not in that of PtdIns-3-kinase. Finally, we show that SHC-transfected cells, unlike control cells, are able to advance into the early phases of the cell cycle, and are more sensitive to the growth-promoting effect of insulin. In conclusion, SHC proteins are substrates for insulin and IGF-I receptors, and would appear to function as early post-receptor signaling components.

Recombinant insulin-like growth factor binding protein-1 inhibits IGF-I, serum, and estrogen-dependent growth of MCF-7 human breast cancer cells

The insulin-like growth factors (IGFs) are potent mitogens for breast cancer cells and their activity is modulated by high affinity binding proteins (IGFBPs). We have recently shown that IGFBP-1 purified from human amniotic fluid neutralizes IGF-I-dependent growth of MCF-7 cells. In this study we examined the effects of recombinant IGFBP-1 (rBP-1) on IGF-I, estradiol (E2), and serum-induced monolayer and anchorage independent growth (AIG) of MCF-7 cells. Under serum-free conditions, rBP-1 had no effect on MCF-7 basal monolayer growth. However, 40 nM rBP-1 completely blocked the mitogenic action of both IGF-I and 5% charcoal stripped serum (CSS). This concentration of rBP-1 partially inhibited E2-induced growth, while 80 nM rBP-1 completely abolished E2 mitogenicity. The addition of either excess IGF-I or 5 nM [Arg3]IGF-I, a species that does not bind IGFBPs, neutralized rBP-1 inhibitory effects. In AIG assays, 80 nM rBP-1 reduced colony number by at least 70% and decreased colony size in all treatment groups compared to control. We examined rBP-1 effects on both IGF-I binding to MCF-7 membranes and activation of type I IGF receptor (IGFR1) and found that 80 nM rBP-1 reduced IGF-I receptor binding to levels of nonspecific binding and completely abolished ligand-dependent IGFR1 phosphorylation. However, neither treatment with 5% CSS nor exposure to E2 resulted in IGFR1 phosphorylation suggesting that different mechanism(s) are responsible for rBP-1 inhibitory action under this condition. Our data suggest rBP-1 may serve as an antagonist of human breast cancer growth by interfering with growth factor-mediated cell proliferation.

Evidence for two insulin-like growth factor I receptors with distinct primary structure that are differentially expressed during development

We have previously presented evidence for two IGF I receptor species in rat skeletal muscle. One form of IGF I receptor is selectively expressed in fetal and early postnatal life, and the second is present in both fetal and adult animals. These two IGF I receptors were shown to have similar tryptic phosphopeptide maps but to differ in beta subunit molecular weight (105,000 for the fetal vs. 95,000 for the adult type receptor). In this study, we have used specific antibodies to investigate the structural relationships between the two IGF I receptors. Anti-IGF I receptor beta subunit antibodies were generated against synthetic peptides corresponding to residues 1284-1293 and 1308-1318 of the cloned human IGF I receptor, and the capacity of these antibodies to interact with the two IGF I receptors was investigated. Both anti-peptide antibodies selectively immunoprecipitated the higher molecular weight fetal receptor and not the adult receptor from rat muscle. Human placenta and muscle were shown to contain two receptors similar to those observed in rat muscle. In human muscle, the anti-peptide antibodies and the human-specific monoclonal alpha subunit antibody alpha-IR3 also selectively immunoprecipitated the fetal type receptor. The presence of a 95,000 M(r) IGF I receptor beta subunit distinct from the insulin receptor beta subunit in human muscle was confirmed by the demonstration of an IGF I sensitive receptor with a beta subunit of this size after insulin receptor immunodepletion. These data strongly support the conclusion that the fetal and adult type IGF I receptors differ in primary structure. The fetal receptor corresponds to the cloned and sequenced IGF I receptor, and the primary structure of the adult type receptor has not yet been established.

Insulin-like growth factor-I (IGF-I) dependent phosphorylation of the IGF-I receptor in MG-63 cells

Insulin-like growth factor I (IGF-I) stimulates multiplication of the human osteosarcoma cell line, MG-63. by acting through the IGF-I receptor. We have characterized IGF-I stimulated phosphorylation of the IGF-I receptor in this cell line. Serum starved MG-63 cells were metabolically labeled with [32P]orthophosphoric acid and the cells were treated with IGF-I. Phosphotyrosine containing proteins were immunoprecipitated from the cell lysates with antiphosphotyrosine-Agarose and eluted with phenyl phosphate. Further immunoprecipitation with IGF-I receptor monoclonal antibodies (alpha IR-3, 18E9) and analysis by sodium dodecylsulfate polyacrylamide gel electrophoresis and autoradiography demonstrated IGF-I dependent autophosphorylation of the IGF-I receptor. Phosphoamino acid analysis of the IGF-I receptor beta subunit and the observation that antiphosphotyrosine-Agarose did not immunoprecipitate [35S]methionine-labeled receptor from unstimulated cells, demonstrated that in the absence of IGF-I, the receptor was not phosphorylated on tyrosine residues. Western blotting of cell lysates with a monoclonal phosphotyrosine antibody did not identify the IGF-I receptor or pp185 but demonstrated IGF-I dependent phosphorylation on tyrosine residues in three other proteins, p110, p70 and p40.

Specific phosphorylation of membrane proteins of Mr 44,000 and Mr 32,000 by the autophosphorylated insulin receptor from the hepatopancreas of the shrimp Penaeus monodon (Crustacea: Decapoda)

The insulin receptor, purified from the hepatopancreas of the shrimp Penaeus monodon, is a hydrophobic heterodimer of subunits with relative masses (Mr) of 70,000 and 58,000, as estimated by FPLC on Superose 12 and SDS-PAGE. Only the subunit of Mr 70,000 was autophosphorylated after the addition of insulin. The autophosphorylation occurred specifically at Tyr residues, as demonstrated by the specific subsequent dephosphorylation by the phosphotyrosyl protein phosphatase from the hepatopancreas of the shrimp Penaeus monodon. Proteins of Mr 44,000 and Mr 32,000 on the plasma membrane from the hepatopancreas of the shrimp Panaeus monodon were phosphorylated by the autophosphorylated insulin receptor from the shrimp hepatopancreas, but not by that from the human placenta. The detergent, Triton X-100, caused noticeable enhancement of the autophosphorylation of both shrimp and human insulin receptors.

Paradoxical biological effects of overexpressed insulin-like growth factor-1 receptors in Chinese hamster ovary cells

One major approach to the study of growth factor receptor action has been to overexpress wild-type or mutant receptors in cultured cells and to evaluate biological responses to exogenous ligand. Studies of this type with insulin and insulin-like growth factor-I (IGF-I) receptors often use Chinese hamster ovary (CHO) cells. We have compared the effect of receptor overexpression in CHO cells and in NIH-3T3 fibroblasts in order to assess the suitability of CHO cells for studies of this nature and the contribution of cell type-specific factors to those responses generally assayed. Overexpression of IGF-I receptors in NIH-3T3 cells resulted in increased sensitivity and maximal responsiveness of thymidine incorporation, 2-deoxyglucose uptake, and phosphatidylinositol-3 (PI3) kinase activation to IGF-I stimulation. In CHO cells, on the other hand, overexpression of either IGF-I or insulin receptors increased the sensitivity of thymidine incorporation to ligand, but maximal responsiveness was unchanged or decreased. Overexpression of the insulin receptor increased sensitivity of glucose uptake and the maximal response of PI3 kinase activation to insulin. Overexpression of the IGF-I receptor did not affect sensitivity or maximal responsiveness of glucose uptake or PI3 kinase activation to IGF-I. These data suggest that IGF-I and insulin signal pathways may differ in CHO cells, and that there may even be divergent IGF-I signaling pathways for short vs. long-term effects. Whether this is a result of differences in the number of endogenous receptors, hybrid receptor formation, or defects in post-receptor signaling, the use of CHO cells to assess receptor function must be approached with caution.

The role of insulin-like growth factors and IGF-binding proteins in the physiological and pathological processes of the kidney

Insulin-like growth factors (IGFs) and their binding proteins are implicated in the growth regulation of the kidney during embryogenesis and differentiation. Recent evidence also suggests that IGFs play a role in kidney physiology (glomerular filtration rate, renal plasma flow) and pathology (diabetic renal hypertrophy, nephritis, glomerulosclerosis, kidney tumours, chronic renal failure). This review focuses on the biology of IGFs at the molecular, protein and receptor levels and considers their importance in renal physiology and pathology. The current data demonstrate a central role for the IGFs in the mediation of a wide variety of effects on renal growth, function and malignancy.

Quantitative dissociation between EGF effects on c-myc and c-fos gene expression, DNA synthesis, and epidermal growth factor receptor tyrosine kinase activity

The exact relationship between EGF-stimulated tyrosine phosphorylation, induction of the cellular proto-oncogenes c-myc and c-fos, and DNA synthesis remains uncertain. Madin-Darby Canine Kidney (MDCK) cells possess EGF receptor sites with high binding capacity, and in contrast to A431 cells, respond to EGF by increasing DNA synthesis. Following EGF stimulation of intact MDCK cells, there was a rapid and marked increase in the autophosphorylation of the EGF receptor. This was associated with an increase in the tyrosine phosphorylation of a 120 kDa phosphoprotein believed to be an endogenous substrate of this receptor kinase. The ED50 for stimulation of phosphorylation of pp120 was approximately 0.05 nM versus 1.0 nM for receptor autophosphorylation, consistent with amplification of signalling at this step in EGF action. Stimulation of DNA synthesis occurred after 12 to 24 hours and revealed even further amplification with an ED50 of about 0.1 nM. Intermediate between these events was a time-dependent activation of c-fos and c-myc gene expression. However, the ED50 for these processes was approximately 10 nM, indicating a relatively lower sensitivity of EGF for stimulation of proto-oncogene expression. Tyrphostin (RG 50864), a compound reported to inhibit specifically the EGF receptor kinase, completely blocked EGF stimulation of proto-oncogene induction. Interestingly, under the same experimental conditions, EGF receptor autophosphorylation was decreased only 60%. These data, along with the dose-response studies, indicate that proto-oncogene induction requires near maximal stimulation of EGF receptor autophosphorylation. They also suggest that, in MDCK cells, the EGF dependent induction of the c-fos and c-myc genes is not strictly correlated to the extent of EGF receptor autophosphorylation or EGF-stimulated DNA synthesis, and that EGF stimulation of DNA synthesis likely involves additional rate-limiting intermediate steps.

The insulin receptor and type I IGF receptor: comparison of structure and function

The insulin receptor and type I IGF receptor are closely related in structure and function. The receptors are heterotetrameric glycoproteins, of structure alpha beta beta alpha, which are widely distributed in mammalian tissues. A third member of this receptor family has been described, the insulin receptor-related receptor for which a ligand has still to be identified. It has also been demonstrated that the insulin receptor and IGF receptor form alpha beta beta alpha hybrids in cells expressing both receptors. The key elements in the function of any receptor are recognition of ligand and transmission of an intracellular signal. In the insulin and IGF receptors, determinants of binding specificity are contained within amino-terminal and cysteine-rich domains of the extracellular alpha-subunit. Intracellular signalling is dependent on ligand activated tyrosine kinase activity in the transmembrane beta-subunit, which phosphorylates both the receptor itself and the specific substrate insulin receptor substrate-1 (IRS-1). Phosphorylated IRS-1 binds the enzyme phosphatidylinositol 3-kinase and may act as a multivalent docking site for SH2 domains of other proteins involved in signalling. The possibility that some signalling molecules interact directly with the receptors has not been ruled out. The specificity of action of insulin and IGFs in vivo depends on differences between the respective receptors in tissue distribution, ligand binding specificity and intrinsic signalling capacity. However, the detailed aspects of gene and receptor structure which underly these functional differences are still poorly understood. Moreover, the issue of specificity is complicated by the existence of hybrid and atypical receptors, which in principle could bind and respond to both insulin and IGF-I, although the physiological significance of these receptor subtypes is at present unclear.

The molecular and cellular biology of insulin-like growth factor II

Insulin-like growth factor II (IGF-II) is a 67 amino acid polypeptide that belongs to the family of insulin-like peptides. The IGF-II gene is coupled to the insulin gene and paternally imprinted. Multiple IGF-II mRNAs with identical coding regions and 3' untranslated regions (UTRs) but different 5' UTRs are generated from 3 promoters. The transcripts are translationally discriminated and inactivated by a specific endonucleolytic cleavage in their 3' UTR. These features may be important in the control of IGF-II production. IGF-II functions in an auto- and paracrine manner and binds to two types of receptors. The IGF-I receptor that is a tyrosine kinase and closely related with the insulin receptor and the IGF-II/mannose 6-phosphate (IGF-II/Man 6-P) receptor that is identical with the cation-independent mannose 6-phosphate receptor. The mitogenic and metabolic actions of IGF-II are propagated by the IGF-I receptor. In contrast, the IGF-II/Man 6-P receptor, that target lysosomal enzymes from the Golgi apparatus or the plasma membrane to the lysosomes, mediates the rapid internalization and degradation of IGF-II. IGF-II is expressed at high levels during foetal life and it is a major growth factor for the foetus in rodents. The developmental profiles and tissue distribution of the IGF-I and the maternally imprinted IGF-II/Man 6-P receptors both parallel that of IGF-II. In this scenario IGF-II promotes the growth of the embryo through the IGF-I receptor, whereas the IGF-II/Man 6-P receptor balance the activity by controlling the extracellular level of IGF-II.

The insulin receptor: signalling mechanism and contribution to the pathogenesis of insulin resistance

The insulin receptor is a heterotetrameric structure consisting of two alpha-subunits of Mr 135 kilodalton on the outside of the plasma membrane connected by disulphide bonds to beta-subunits of Mr 95 kilodalton which are transmembrane proteins. Insulin binding to the alpha-subunit induces conformational changes which are transduced to the beta-subunit. This leads to the activation of a tyrosine kinase activity which is intrinsic to the cytoplasmatic domains of the beta-subunit. Activation of the tyrosine kinase activity of the insulin receptor represents an essential step in the transduction of an insulin signal across the plasma membrane of target cells. Signal transduction on the post-kinase level is not yet understood in detail, possible mechanisms involve phosphorylation of substrate proteins at tyrosine residues, activation of serine kinases, the interaction with G-proteins, phospholipases and phosphatidylinositol kinases. Studies in multiple insulin-resistant cell models have demonstrated that an impaired response of the tyrosine kinase to insulin stimulation is one potential mechanism causing insulin resistance. An impairment of the insulin effect on tyrosine kinase activation in all major target tissues of insulin, in particular the skeletal muscle was demonstrated in Type 2 (non-insulin-dependent) diabetic patients. There is no evidence that the impaired tyrosine kinase response in the skeletal muscle is a primary defect, however, it is likely that this abnormality of insulin signal transduction contributes significantly to the pathogenesis of the insulin-resistant state in Type 2 diabetes.

Kidney epithelial cells release growth factors in response to extracellular signals

The growth of nontransformed monkey kidney epithelial cells in culture appears to be regulated by the interplay of positive and negative autocrine growth factors. Reduction of the potassium or sodium concentration of the medium induces rapid release of novel growth-promoting activities, whereas addition of the mitogen adenosine diphosphate stimulates the appearance of a platelet-derived growth factor-like protein which could function in a paracrine manner. These observations suggest that autocrine and paracrine growth factors could play an important role in physiological and pathological states in the kidney.