The insulin-like growth factor-I receptor. Structure, ligand-binding mechanism and signal transduction

The role of zinc in growth and cell proliferation

The inhibition of growth is a cardinal symptom of zinc deficiency. In animals fed a zinc-inadequate diet, both food intake and growth are reduced within 4-5 d. Despite the concomitant reduction in food intake and growth, reduced energy intake is not the limiting factor in growth, because force-feeding a zinc-inadequate diet to animals fails to maintain growth. Hence, food intake and growth appear to be regulated by zinc through independent, although well coordinated, mechanisms. Despite the long-term study of zinc metabolism, the first limiting role of zinc in cell proliferation remains undefined. Zinc participates in the regulation of cell proliferation in several ways; it is essential to enzyme systems that influence cell division and proliferation. Removing zinc from the extracellular milieu results in decreased activity of deoxythymidine kinase and reduced levels of adenosine(5')tetraphosphate(5')-adenosine. Hence, zinc may directly regulate DNA synthesis through these systems. Zinc also influences hormonal regulation of cell division. Specifically, the pituitary growth hormone (GH)-insulin-like growth factor-I (IGF-I) axis is responsive to zinc status. Both increased and decreased circulating concentrations of GH have been observed in zinc deficiency, although circulating IGF-I concentrations are consistently decreased. However, growth failure is not reversed by maintaining either GH or IGF-I levels through exogenous administration, which suggests the defect occurs in hormone signaling. Zinc appears to be essential for IGF-I induction of cell proliferation; the site of regulation is postreceptor binding. Overall, the evidence suggests that reduced zinc availability affects membrane signaling systems and intracellular second messengers that coordinate cell proliferation in response to IGF-I.

Transgenic mouse models for studying the functions of insulin-like growth factor-binding proteins

The insulin-like growth factor-binding proteins (IGFBPs) comprise a family of six related peptides that interact with high affinity with IGFs. IGFBPs compete with IGF receptors for IGF binding, and as a consequence of this competition they can affect cell growth. In addition, IGF-independent regulatory mechanisms of IGFBPs have been described. Despite their common property to interact with IGFs every IGFBP is expressed in a tightly regulated time- and tissue-specific manner suggesting that each protein may have its own distinct functions. Several transgenic mouse models overexpressing IGFBP-1, -2, -3, or -4 were developed in the past few years. Brain abnormalities were a common feature of IGFBP-1 transgenic models. Individual strains showed alterations in glucose homeostasis, reproductive performance, and a reduction of somatic growth as the most prominent phenotypes. The latter was also the main effect observed in IGFBP-2 transgenic mice. The overexpression of IGFBP-3 under the control of an ubiquitous promoter resulted in selective organomegaly, whereas mammary gland-targeted expression of this protein caused an altered involution after pregnancy in this organ. Tissue-specific overexpression of IGFBP-4 resulted in hypoplasia and reduced weight of smooth muscle-rich tissues such as bladder, aorta, and stomach. This review summarizes the current knowledge about the actions of IGFBPs in vivo based on the presently established transgenic mice.

Transgenic mice overexpressing insulin-like growth factor-II in beta cells develop type 2 diabetes

During embryonic development, insulin-like growth factor-II (IGF-II) participates in the regulation of islet growth and differentiation. We generated transgenic mice (C57BL6/SJL) expressing IGF-II in beta cells under control of the rat Insulin I promoter in order to study the role of islet hyperplasia and hyperinsulinemia in the development of type 2 diabetes. In contrast to islets from control mice, islets from transgenic mice displayed high levels of IGF-II mRNA and protein. Pancreases from transgenic mice showed an increase in beta-cell mass (about 3-fold) and in insulin mRNA levels. However, the organization of cells within transgenic islets was disrupted, with glucagon-producing cells randomly distributed throughout the core. We also observed enhanced glucose-stimulated insulin secretion and glucose utilization in islets from transgenic mice. These mice displayed hyperinsulinemia, mild hyperglycemia, and altered glucose and insulin tolerance tests, and about 30% of these animals developed overt diabetes when fed a high-fat diet. Furthermore, transgenic mice obtained from the N1 backcross to C57KsJ mice showed high islet hyperplasia and insulin resistance, but they also developed fatty liver and obesity. These results indicate that local overexpression of IGF-II in islets might lead to type 2 diabetes and that islet hyperplasia and hypersecretion of insulin might occur early in the pathogenesis of this disease.

Developmental expression of insulin receptor substrate-2 during dimethylsulfoxide-induced differentiation of human HL-60 cells

Insulin receptor substrate-2 (IRS-2) is phosphorylated on tyrosine by a number of cytokine receptors and is implicated in the activation of phosphatidylinositol 3'-kinase (PI3-kinase). Here, we demonstrate that induction of granulocytic differentiation of human promyeloid HL-60 cells leads to an increase in the amount of IRS-2 that is phosphorylated in response to insulin-like growth factor (IGF)-I. Although PI3-kinase is often activated following interaction with IRS-1, we could not detect IRS-1 protein, IRS-1 mRNA, or IRS-1-precipitable PI3-kinase enzymatic activity. However, PI3-kinase activity that was coimmunoprecipitated with either anti-phosphotyrosine or anti-IRS-2 following IGF-I stimulation was increased 100-fold. Heightened tyrosine phosphorylation of IRS-2 during granulocytic differentiation was not caused by an increase in expression of the tyrosine kinase IGF-I receptor, as measured by the amount of both the alpha- and beta-subunits. Instead, immunoblotting experiments with an Ab to IRS-2 revealed that induction of granulocytic differentiation caused a large increase in IRS-2, and this occurred in the absence of detectable IRS-1 protein. These IRS-2-positive cells could not differentiate into more mature myeloid cells in serum-free medium unless IGF-I was added. These data are consistent with a model of granulocytic differentiation that requires at least two signals, the first of which leads to an increase in the cytoplasmic pool of IRS-2 protein and a second molecule that acts to tyrosine phosphorylate IRS-2 and enhance granulocytic differentiation.

Insulin-like growth factor-I prevents caspase-mediated apoptosis in Schwann cells

Both neurons and glia succumb to programmed cell death (PCD) when deprived of growth factors at critical periods in development or following injury. Insulin-like growth factor-I (IGF-I) prevents apoptosis in neurons in vitro. To investigate whether IGF-I can protect Schwann cells (SC) from apoptosis, SC were harvested from postnatal day 3 rats and maintained in serum-containing media until confluency. When cells were switched to serum-free defined media (DM) for 12-72 h, they underwent PCD. Addition of insulin or IGF-I prevented apoptosis. Bisbenzamide staining revealed nuclear condensation and formation of apoptotic bodies in SC grown in DM alone, but SC grown in DM plus IGF-I had normal nuclear morphology. The phosphatidylinositol 3-kinase (PI 3-K) inhibitor LY294002 blocked IGF-I-mediated protection. Caspase-3 activity was rapidly activated upon serum withdrawal in SC, and the caspase inhibitor BAF blocked apoptosis. These results suggest that IGF-I rescues SC from apoptosis via PI 3-K signaling which is upstream from caspase activation.

Insulin-like growth factor-I (IGF-I)-dependent activation of pp42/44 mitogen-activated protein kinase occurs independently of IGF-I receptor kinase activation and IRS-1 tyrosine phosphorylation

The proliferation and metabolism of H4IIE hepatoma cells is apparently mediated through the insulin receptor. These cells, however, also have high-affinity binding sites for insulin-like growth factor-I (IGF-I). Addition of insulin to H4IIE cells increased RNA synthesis, DNA synthesis and cell number. IGF-I, on the other hand, was ineffective at concentrations equivalent to the lowest effective insulin dose, although stimulation was observed with concentrations 100-fold higher. Similar results were obtained when glucose uptake was measured. Western blot analysis demonstrated that tyrosine phosphorylation patterns produced by insulin and IGF-I differed. In particular, phosphorylation of insulin receptor substrate-1 (IRS-1) was evident after treatment with insulin, but not after treatment with IGF-I. Correspondingly, insulin, but not IGF-I, stimulated receptor tyrosine kinase activity. In contrast with these results, both insulin and IGF-I induced mitogen-activated protein (MAP) kinase phosphorylation and activity at a concentration of 10 nM. The correlation between insulin-dependent and IGF-I-dependent MAP kinase activation was confirmed by Western blot analysis of phosphorylated MAP kinase kinase (MEK). These results suggest that phosphorylation of IRS-1 is essential for both cell proliferation and glucose metabolism, but is uncoupled from the MAP kinase cascade. Furthermore, stimulation of MEK and MAP kinase is independent of receptor tyrosine kinase activity.

Igf1 promotes longitudinal bone growth by insulin-like actions augmenting chondrocyte hypertrophy

Longitudinal bone growth, and hence stature, are functions of growth plate chondrocyte proliferation and hypertrophy. Insulin-like growth factor 1 (Igf1) is reputed to augment longitudinal bone growth by stimulating growth plate chondrocyte proliferation. In this study, however, we demonstrate that chondrocyte numbers and proliferation are normal in Igf1 null mice despite a 35% reduction in the rate of long bone growth. Igf1 null hypertrophic chondrocytes differentiate normally in terms of expressing specialized proteins such as collagen X and alkaline phosphatase, but are smaller than wild-type at all levels of the hypertrophic zone. The terminal hypertrophic chondrocytes, which form the scaffold on which long bone growth extends, are reduced in linear dimension by 30% in Igf1 null mice, accounting for most of their decreased longitudinal growth. The expression of the insulin-sensitive glucose transporter, GLUT4, is significantly decreased and the insulin-regulated enzyme glycogen synthase kinase 3beta (GSK3) is hypo-phosphorylated in Igf1 null chondrocytes. Glycogen levels were significantly decreased and ribosomal RNA levels were reduced by almost 75% in Igf1 null chondrocytes. These data suggest that Igf1 promotes longitudinal bone growth by 'insulin-like' anabolic actions which augment chondrocyte hypertrophy.

Activation of protein kinase C pathway by phorbol ester results in a proteoglycan synthesis increase in peritubular cells from immature rat testis

In cultured peritubular cells (PT) from rat testis, protein kinase C (PKC) was activated by phorbol 12-myristate 13-acetate (PMA). PMA enhanced the synthesis of proteoglycans (PG) and to a lesser extent their catabolism; the stimulation of the synthesis appeared to be due to an increase in PG protein moiety production and, at the same time, to an increase in the glycanation process as revealed by the use of an exogenous acceptor, p-nitrophenyl-beta-d-xyloside. In the presence of PMA, the molecular weight of neosynthesized PG and the length of their constitutive glycosaminoglycan chains were not modified. Moreover, the distribution of proteochondroitin sulfate and proteoheparan sulfate in medium and in cell layer remained unchanged. However, PMA reduced the sulfation level of chondroitin sulfate and heparan sulfate chains, suggesting that PKC activation resulted in an independent modulation of the sugar chain formation and of the sulfate residue transfer. PMA effect on the synthesis of hyaluronan was also determined: PMA dramatically enhanced its production by PT cells.

Expression and regulation of the insulin-like growth factor-1 receptor by growing and quiescent H4IIE hepatoma

Recent evidence that insulin-like growth factor-1 (IGF-1) influences certain properties of H4IIE hepatoma cells independent of insulin led us to examine whether H4IIE cells express IGF-1 receptors. Competitive binding experiments demonstrated IGF-1, but not insulin or IGF-II, could compete with [125I]IGF-1. Chemical crosslinking detected a protein with an apparent mass of 175 kDa and its identity as the IGF-1 receptor alpha-subunit was confirmed by Western blotting. The apparent molecular mass of this protein decreased to 135 kDa following deglycosylation. Immunofluorescence microscopy verified that both insulin and IGF-1 receptors were present, although measurement of IGF-1 receptor quantity revealed they were less abundant than insulin receptors. Binding of IGF-1 was low in growing cells and higher in a quiescent cell population. Scatchard analysis confirmed that receptor density was increased in non-growing H4IIE cells while there was no apparent difference in receptor affinity. Western blot analysis and RT-PCR revealed that both protein and mRNA levels were elevated as cell growth ceased. Interestingly, addition of insulin to quiescent H4IIE cells, which stimulates cell proliferation, further increased IGF-1 receptor protein levels with a peak at 12-24 h. Distinct modes of regulating IGF-1 receptor expression are indicated.

Genetic engineering in mice: impact on insulin signalling and action

The expression of a number of genes encoding key players in insulin signalling and action, including insulin, insulin receptor (IR), downstream signalling molecules such as insulin receptor substrate-1 (IRS-1) and IRS-2, glucose transporters (GLUT4, GLUT2) and important metabolic enzymes such as glucokinase, has now been altered in transgenic or knockout mice. Such mice presented with phenotypes ranging from mild defects, revealing complementarity between key molecules or pathways, to severe diabetes with ketoacidosis and early postnatal death. Insulin action could also be improved by overproduction of proteins acting at regulatory steps. The development of diabetes by combining mutations, which alone do not lead to major metabolic alterations, validated the 'diabetogenes' concept of non-insulin-dependent diabetes mellitus. Genes encoding insulin-like growth factors (IGF-I and IGF-II) and their type I receptor (IGF-IR) have also been disrupted. It appears that although IR and IGF-IR are both capable of metabolic and mitogenic signalling, they are not fully redundant. However, IR could replace IGF-IR if efficiently activated by IGF-II. Studies with cell lines lacking IR or IGF-IR lend support to such conclusions. Concerning the issues of specificity and redundancy, studies with cell lines derived from IRS-1-deficient mice showed that IRS-1 and IRS-2 are also not completely interchangeable.

Multivalent cations and ligand affinity of the type 1 insulin-like growth factor receptor on P2A2-LISN muscle cells

Mouse P2A2-LISN myoblasts are transfected cells that overexpress the human type 1 insulin-like growth factor (IGF) receptor. Because the type 1 IGF receptor is the major binding site for both IGF-I and IGF-II, this cell line is an excellent model to determine the effect of multivalent cations on ligand binding specifically to this type of receptor. Competitive binding assays were performed to characterize IGF binding and Scatchard analysis to quantify affinity (Ka). 125I-IGF-I, 125I-IGF-II, and 125I-R3-IGF-I bind only to the type 1 IGF receptor on these cells. Zn2+ increased binding of the three ligands to the type 1 IGF receptor by 17 to 35%. Cd2+ significantly increased binding of 125I-IGF-I, although by only 8%. La3+ and Cr3+ did not effect binding. Au3+ decreased IGF binding by approximately 56%. Scatchard analysis produced nonlinear concave-down plots yielding binding constants for high and low affinity sites. Zn2+ increased the strength of only the high affinity sites. Au3+ decreased the affinity of both high and low affinity sites. Zn2+ increased binding with a half-maximal effect between 40 microM and 60 microM. Half-maximal dose of Au3+ was >130 microM. Zinc, gold, and cadmium bind to similar regions within proteins (a zinc-binding motif) and only these cations were found to affect receptor binding indicating similar mechanisms of action. Thus, multivalent cations may alter IGF binding to cell surface receptors ultimately controlling growth. Physiologically this may be especially important for the growth promoting effects of Zn2+.

Identification of insulin-like growth factor I in bovine seminal plasma and its receptor on spermatozoa: influence on sperm motility

Insulin-like growth factor I (IGF-I) has been identified in human seminal plasma. This study was conducted to determine whether IGF-I is present in bovine seminal plasma, whether sperm cells express the IGF-I receptor (IGF-IR), and whether IGF-I affects sperm motility. Semen samples were collected from bulls by electroejaculation and maintained at 37 degrees C, and motility of sperm was assessed. After centrifugation to separate sperm cells from seminal plasma, the seminal plasma was submitted to a validated heterologous RIA for IGF-I. Significant concentrations of IGF-I (116.29 +/- 40.83 ng/ml expressed as mean +/- SD) were measured in bovine seminal plasma. Sperm cells were washed with buffer and subjected to either radioreceptor assay (RRA) or immunocytochemistry (IC). RRA revealed a single high affinity for the IGF-IR with a Kd of 0.83 nM as determined by the computer program LIGAND. IC, using three monoclonal antibodies, localized the IGF-IR to the acrosomal region of the sperm. Computer-assisted sperm-motion analysis was used to determine the effects of IGF-I and IGF-II on bovine sperm motility parameters. Both IGF-I and IGF-II increased sperm motility and straight-line velocity (p < 0.05) relative to the control. The presence of IGF-IR on sperm, the presence of IGF-I in semen, and the ability of IGF-I to stimulate sperm motility provide evidence that the IGF system may be involved in the fertilization process in the bovine species.

The Hyperresponsiveness of Cells Expressing Truncated Erythropoietin Receptors Is Contingent on Insulin-Like Growth Factor-1 in Fetal Calf Serum

We demonstrate herein that the well documented hyperresponsiveness to erythropoietin (Epo) of Ba/F3 cells expressing C-terminal truncated erythropoietin receptors (EpoRs) is contingent on these cells being in fetal calf serum (FCS). In the absence of FCS, their Epo-induced proliferation is far poorer than Ba/F3 cells expressing wild-type (WT) EpoRs. This hyporesponsiveness in the absence of serum is also seen in DA-3 cells expressing these truncated EpoRs. In fact, long-term proliferation studies performed in the absence of serum show that even at saturating concentrations of Epo, Ba/F3 cells expressing these truncated receptors die via apoptosis, while cells bearing WT EpoRs do not, and this programmed cell death correlates with an inability of Epo-stimulated Ba/F3 cells expressing truncated EpoRs to induce the tyrosine phosphorylation of MAPK and the activation of p70S6K. Using neutralizing antibodies to insulin-like growth factor (IGF)-1, we show that a major non-Epo factor in FCS that contributes to the hyperresponsive phenotype of Ba/F3 cells expressing truncated EpoRs is IGF-1. Our results suggest that the Epo-hypersensitivity of truncated EpoR expressing Ba/F3 cells is due to the combined effects of these EpoRs not possessing a binding site for the negative regulator, SHP-1, and the triggering of proliferation-inducing/apoptosis-inhibiting cascades, lost through EpoR truncation, by IGF-1.

The Hyperresponsiveness of Cells Expressing Truncated Erythropoietin Receptors Is Contingent on Insulin-Like Growth Factor-1 in Fetal Calf Serum

We demonstrate herein that the well documented hyperresponsiveness to erythropoietin (Epo) of Ba/F3 cells expressing C-terminal truncated erythropoietin receptors (EpoRs) is contingent on these cells being in fetal calf serum (FCS). In the absence of FCS, their Epo-induced proliferation is far poorer than Ba/F3 cells expressing wild-type (WT) EpoRs. This hyporesponsiveness in the absence of serum is also seen in DA-3 cells expressing these truncated EpoRs. In fact, long-term proliferation studies performed in the absence of serum show that even at saturating concentrations of Epo, Ba/F3 cells expressing these truncated receptors die via apoptosis, while cells bearing WT EpoRs do not, and this programmed cell death correlates with an inability of Epo-stimulated Ba/F3 cells expressing truncated EpoRs to induce the tyrosine phosphorylation of MAPK and the activation of p70S6K. Using neutralizing antibodies to insulin-like growth factor (IGF)-1, we show that a major non-Epo factor in FCS that contributes to the hyperresponsive phenotype of Ba/F3 cells expressing truncated EpoRs is IGF-1. Our results suggest that the Epo-hypersensitivity of truncated EpoR expressing Ba/F3 cells is due to the combined effects of these EpoRs not possessing a binding site for the negative regulator, SHP-1, and the triggering of proliferation-inducing/apoptosis-inhibiting cascades, lost through EpoR truncation, by IGF-1.

Bidirectional regulation of p38 kinase and c-Jun N-terminal protein kinase by insulin-like growth factor-I

We have previously shown that insulin-like growth factor I (IGF-I) activation of the IGF-I receptor rescues SH-SY5Y human neuroblastoma cells from high glucose-mediated programmed cell death (PCD). In the current study, we further explored the potential points in the cell death cascade where IGF-I receptor activation may afford neuroprotection. As an initial step, we examined the effects of the PCD stimulus, high glucose, on stress-activated protein kinases, specifically the two mitogen-activated protein kinases p38 kinase and c-Jun N-terminal kinase (JNK). High glucose treatment activated the tyrosine phosphorylation of both p38 kinase and JNK in a dose- and time-dependent fashion. We next examined the effects of IGF-I on JNK and p38 kinase under normoglycemic and hyperglycemic conditions. IGF-I activated p38 kinase alone and had additive effects on glucose-induced p38 kinase phosphorylation. In contrast, IGF-I inhibited glucose activation of JNK phosphorylation and JNK activity. IGF-I also inhibited the glucose-induced nuclear translocation of JNK, but did not effect glucose-induced translocation of p38 kinase. Finally, IGF-I inhibition of JNK phosphorylation was blocked by the mitogen-activated protein kinase/extracellular signal-regulated kinase inhibitor, PD98059. Collectively, these data imply cross-talk between the mitogen-activated protein kinase pathway and JNK and suggest that IGF-I activation of mitogen-activated protein kinases interferes with JNK activation and protects cells from PCD.

Effects of human plasma proteins on endothelin binding and function

We have previously shown that the endothelin (ET) isoforms ET-1 and ET-3 and ET receptor antagonists exhibit a high degree (> 98%) of binding to plasma proteins, especially serum albumin. This study examines the effects of human plasma proteins on ET-1 binding and ET-1 stimulated biologic responses. When ET-1 binding to rat pituitary MMQ cells was examined, human serum albumin (HSA) inhibited ET-1 binding in a concentration-dependent manner, with 92% inhibition observed at 5% (w/v) HSA. A similar observation was made when human plasma was tested. Surprisingly, addition of human plasma (0-100%) or HSA (0-5%) did not have a significant effect on ET-1-stimulated phosphatidylinositol (PI) hydrolysis in these cells. In human pericardial smooth-muscle cells, HSA again exhibited profound inhibitory effects on ET-1 binding; 5% HSA inhibited ET-1 binding by 84%. However, ET-1-stimulated arachidonic acid release in these cells was not significantly affected by HSA (0-5%). Addition of increasing concentrations of human plasma incrementally decreased the potency of PD-156707, and ETA-selective antagonist, on inhibiting ET-1-stimulated PI hydrolysis in the MMQ cells. These results suggest that, although ET-1 binding to the receptor is inhibited by human plasma proteins, especially HSA, ET-1-induced signal transduction is not significantly affected. Furthermore, plasma proteins decrease the potency of an ET receptor antagonist in inhibiting ET-1-stimulated biologic responses. This study may help to explain the discrepancy between in vitro and in vivo potencies of ET receptor antagonists.

Insulin and insulin-like growth factor-I receptor mediated differentiation of 3T3-F442A cells into adipocytes: effect of PI 3-kinase inhibition

The ability of insulin and insulin-like growth factors (IGF-I and IGF-II) to induce differentiation of 3T3-F442A cells into adipocytes was examined at various hormone concentrations. Both insulin and the IGFs promoted differentiation at concentrations compatible with binding to their cognate receptors, suggesting that both insulin and IGF-I receptors are capable of promoting this differentiation. Adipocyte conversion of 3T3-F442A cells was completely blocked in the presence of LY294002, a specific inhibitor of PI 3-kinase, indicating that PI 3-kinase activity plays a crucial role in the initial signalling events that trigger this differentiation process.

Endocrine responses to weaning and changes in post-weaning diet in the young pig

The effects of weaning and changing post-weaning diet composition on growth patterns and growth-related hormonal profiles were evaluated in neonatal pigs. Forty-eight crossbred piglets were assigned to two groups (n = 24/group) based on weaning at 2 or 3 wk of age (2W and 3W groups. respectively). At weaning, piglets were removed from the sow and placed on a commercial starter ration for the first 11 d post-weaning (Phase I diet). At Day 12 post-weaning, pigs were placed on a growing ration for the remainder of the study (Phase II diet). Body weights and blood samples were collected twice weekly from birth until 42 d of age. Serum insulin-like growth factor (IGF)-1, IGF-2, and average daily gain (ADG) were reduced (P < 0.05) in both groups as a result of weaning, whereas serum growth hormone (GH) was elevated (P < 0.05). Earlier weaning resulted in a greater reduction in growth rate and serum IGF-2 values (P < 0.05). Mild reductions in ADG occurred after the Phase I to II dietary change in both weaning groups (P < 0.05), but serum IGF-1 decreased only in the 2W group (P < 0.05). Growth hormone concentrations tended to increase after the change in post-weaning diets (P = 0.07 and 0.16 in 2W and 3W, respectively). Serum thyroxine (T4) and triiodothyronine (T3) levels were unaltered by weaning but declined in both groups after the change in starter diets (P < 0.05). Changes in cortisol concentrations were not associated with weaning or the change in post-weaning diets. With the exception of serum IGF-1 concentrations, which were elevated in the 2W group, growth and endocrine endpoints were equivalent between experimental groups at the end of the study (42 d of age). These results indicate that earlier weaning and changing solid diets can more severely affect patterns of early growth and related hormone secretion, but effective compensatory mechanisms restore normal physiological and physical development.

Insulin receptor-deficient cells as a new tool for dissecting complex interplay in insulin and insulin-like growth factors

Cell systems derived from knockout mice for the insulin receptor (IR) or the IGF-1 receptor (IGF-1R) represent unique tools for dissecting complex interplay in the actions of insulin and insulin-like growth factors through their cognate versus non-cognate receptor. In this study, we used a fibroblast cell line derived from IR-deficient mice to investigate metabolic and mitogenic effects of IGF-1 and insulin. IGF-1 was able to stimulate glucose uptake, glucose incorporation into glycogen and thymidine incorporation in such cells. Phosphatidylinositol 3-kinase and mitogen-activated protein kinase, two enzymes of major metabolic-mitogenic signaling pathways, were activated upon stimulating these cells with IGF-1. All these effects were also achieved when IR-deficient cells were stimulated with insulin. Thus, IGF-1R can represent an alternative receptor through which insulin might exert some of its effects.

Role for cyclic adenosine monophosphate in modulating insulin-like growth factor binding protein secretion by muscle cells

The modulation of insulin-like growth factor-binding protein (IGFBP) secretion is an important variable affecting muscle cell metabolism, proliferation, and differentiation. We have previously shown that secretion of IGFBP-4 and IGFBP-5 by L6 and BC3H-1 muscle cells was stimulated by treatment with either insulin, IGF-I, or IGF-II. Herein, these cells were used to further identify mechanisms involved in controlling IGFBP secretion. Agents that elevate intracellular cAMP concentrations (dcAMP, forskolin, isoproterenol, and prostaglandin [PGE1]) increase secretion of IGFBP-4 and IGFBP-5 from L6 cells. Similar increases in IGFBP secretion were found by treatment with either insulin, IGF-I, or dcAMP. The effects of dcAMP and either insulin or IGF-I were additive, but the effects of insulin and IGF-I were not additive. These results suggest that insulin/IGF-I and dcAMP are acting via distinct mechanisms to stimulate IGFBP secretion. Indomethacin, which blocks endogenous prostaglandin synthesis, and progesterone, which decreases intracellular cAMP levels, decreased IGFBP-4 and IGFBP-5 secretion. IGFBP-5 secretion by BC3H-1 cells was increased by either insulin or IGF-I. Agents which elevate intracellular cAMP concentrations did not increase IGFBP-5 secretion. Additionally, these agents were not synergistic with either insulin or IGF-I. However, indomethacin and progesterone depressed IGFBP-5 secretion by BC3H-1 cells. In summary, there appear to be at least two intracellular signaling mechanisms controlling IGFBP-4 and IGFBP-5 secretion by L6 and BC3H-1 muscle cells. IGFBP secretion by L6 cells is stimulated by both insulin/IGF-I and cAMP-dependent pathways, whereas IGFBP-5 secretion by BC3H-1 cells is stimulated only by the insulin/IGF pathway. IGFBP secretion by both cell lines can be decreased by agents which depress cAMP levels. Our results suggest that two divergent but synergistic pathways modulate IGFBP production and these mechanisms can potentially modulate IGF activity during muscle cell proliferation and differentiation.

Insulin-like growth factors regulate neuronal differentiation and survival

Insulin-like growth factor I (IGF-I) and IGF-II are potent trophic factors for motor and sensory neurons and glial cells. The actions of IGF-I and IGF-II are mediated via the IGF-I receptor (IGF-IR). IGF:IGF-IR binding activates distinct signaling cascades, which in turn mediate the trophic effects of the IGFs. We discuss three main IGF coupled events: growth cone motility, long-term neurite outgrowth, and neuroprotection. Our data suggest that IGF-I enhances growth cone motility by promoting reorganization of actin and activation of focal adhesion proteins via the phosphatidylinositol-3 kinase (Pl-3K) pathway. Long-term treatment with IGF-I activates the mitogen-activated protein (MAP) kinase cascade and promotes neurite outgrowth. A separable, but likely linked, action of the IGFs via Pl-3K is protection of neurons from apoptosis. These pleotrophic effects of IGFs suggest that this family of growth factors may have potential clinical utility in the treatment of neurological disorders.

Binding affinities of insulin-like growth factor-I (IGF-I) fusion proteins to IGF binding protein 1 and IGF-I receptor are not correlated with mitogenic activity

In this report, comparisons between molecular affinities and cellular proliferation activities have been made for insulin-like growth factor-I (IGF-I) and two IGF-I fusion proteins in order to evaluate fusion proteins as tools for receptor binding studies. Binding affinities and growth promoting effects of the N-terminal fusion Z-IGF-I and the C-terminal fusion IGF-I-Z, and native recombinant human IGF-I, were analyzed. Binding kinetic properties of the three IGF-I variants were analyzed using BIAcore kinetic interaction analysis testing for binding to both human IGF binding protein 1 (IGFBP-1) and a soluble form of the human IGF type I receptor extracellular domains (sIGF-IR). The growth promoting effects on SaOS-2 human osteosarcoma cells of the different fusion proteins were analyzed. A comparison of receptor binding affinities and growth promoting effects shows that the fusion protein receptor affinity does not correlate with proliferative potential. The IGF-I-Z fusion, with the lowest receptor affinity, shows similar proliferative potential to native IGF-I. However, the Z-IGF-I fusion protein, with twice the receptor affinity of IGF-I-Z, displays only about 70% of the IGF-I-Z growth promoting activity. Both IGF-I fusion proteins possess similar affinity to IGFBP-1. These results indicate that determinants other than the receptor affinity could be involved in the regulation of IGF-I proliferative action. This study demonstrates that ligand fusion proteins may be useful to study mechanisms of ligand induced receptor activation.

Colocalisation of insulin and IGF-1 receptors in cultured rat sensory and sympathetic ganglion cells

Peripheral sensory and autonomic neurons are known to possess insulin receptors. These have been considered to be of the peripheral type, i.e. similar to those of hepatic and fat cells rather than of the brain type which show dual specificity for both insulin and insulin-like growth factor (IGF-1). We have examined the localisation of insulin and IGF-1 receptors in cultured sensory and sympathetic ganglion cells using confocal microscopy and indirect labelling with FITC (fluorescein isothiocyanate) and TRITC (tetramethyl rhodamine isothiocyanate) respectively. We have shown that in cultured U266B1 multiple myeloma cells these receptors display separate localisation, whereas they are colocalised in IM-9 lymphocytes which are known to possess hybrid receptors. We have confirmed the sequestration of insulin and IGF-1 receptors in the cytoplasm of sensory and sympathetic neurons, consistent with a brain-type receptor. The colocalisation of insulin and IGF-1 receptors in sensory and sympathetic ganglion cells is consistent with the view that they are hybrid receptors, similar to those present in the CNS. The function of these receptors, as suggested for the CNS, may be related to trophic support for neurons.

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.

IGF-1 stimulates synthesis of undersulfated proteoglycans and of hyaluronic acid by peritubular cells from immature rat testis

The exposure of confluent peritubular (PT) cells from immature rat testis to insulin-like growth factor-1 (IGF-1) induced a time and dose-dependent increase of [35S]-sulfate and [3H]-D-glucosamine incorporations in newly synthesized proteoglycans (PG). This increased content of PG was the result of an enhancement of PG synthesis rather than a decreased rate of degradation. IGF-1 had no effect on the molecular weight of synthesized PG nor on the nature and distribution of the constitutive glycosaminoglycan chains, both in medium and in cell layer. The stimulation of PG synthesis by IGF-1 appeared to be due, at least partially, to an increase of glycosylation processes. IGF-1 effect was mediated by the classical tyrosine kinase signalling process, since IGF-1 action on PG synthesis was abolished by genistein and tyrphostin A9, two well known tyrosine kinase inhibitors. The increase of PG synthesis was accompanied with an undersulfation of constitutive glycosaminoglycan (GAG) chains (chondroitin sulfate and heparan sulfate chains) since the [35S]/[3H] ratio was reduced by about 20-25% in presence of IGF-1. Although the mechanism of hyaluronic acid synthesis was completely different from those of other GAG, IGF-1 also dramatically enhanced its production by PT cells.

Logical analysis of timing-dependent receptor signalling specificity: application to the insulin receptor metabolic and mitogenic signalling pathways

We present a method for logical analysis of signal-transduction networks, focusing on metabolic and mitogenic signalling by the insulin receptor, with specific emphasis on dependence of the signalling properties on the timing of binding events. We discuss a basic model which demonstrates this dependence (hormone binding leads to activation of the receptor which can lead to a commitment to mitogenic signalling), and show how residence time of the hormone on the receptor can determine the specificity of signalling between the alternative metabolic or mitogenic pathways. The method gives conditions for the selection of specific branches in the signalling pathway expressed in terms of inequalities among the characteristic activation or deactivation times of components of that pathway. In this way, the conditions for mitogenic signalling can be given in terms of a required range of values of the hormone residence time on the receptor, which is directly related to the kinetic dissociation rate.

Growth-promoting interaction of IGF-II with the insulin receptor during mouse embryonic development

Genetic analyses of dwarfing phenotypes resulting from targeted mutagenesis of the genes encoding the insulin-like growth factors (IGF-I and IGF-II) and their cognate type 1 IGF receptor (IGF1R) have demonstrated that this signaling system is a major determinant of mouse embryonic growth. Of the two IGF ligands, IGF-I interacts exclusively with IGF1R, whereas IGF-II recognizes an additional receptor (XR), because the growth retardation of embryos lacking both IGR1R and IGF-II (30% of normal birthweight) is more severe than that manifested in either class of single Igf1r or Igf2 null mutants (45 and 60% of normal, respectively). To determine whether XR is the insulin receptor (IR), we examined embryos nullizygous for both Igf1r and Insr. While the growth of embryos lacking solely IR is affected very mildly and only at the end of gestation, concomitant absence of IGF1R results in a severe growth-deficiency phenotype (30% of normal size at birth) that is first detected at Embryonic Day 13.5 and is also characterized by transient edema, curly tail, generalized organ hypoplasia, including the muscles, developmental delays in ossification, and thin epidermis. The Igf1r/Insr double nullizygotes are phenotypically indistinguishable from double mutants lacking IGF1R and IGF-II and from other double and triple mutants in which all of the IGF ligand/receptor interactions have been eliminated. Therefore, these results provide genetic evidence that the growth-promoting function of IGF-II during mouse embryogenesis is mediated in part by signaling through the insulin receptor.

Insulin and insulin-like growth factor-1 regulate tau phosphorylation in cultured human neurons

Hyperphosphorylated tau is the major component of paired helical filaments in neurofibrillary lesions associated with Alzheimer's disease. Hyperphosphorylation reduces the affinity of tau for microtubules and is thought to be a critical event in the pathogenesis of this disease. Recently, glycogen-synthase kinase-3 has been shown to phosphorylate tau in vitro and in non-neuronal cells transfected with tau. The activity of glycogen-synthase kinase-3 can be down-regulated in response to insulin or insulin-like growth factor-1 through the activation of the phosphatidylinositol 3-kinase pathway. We therefore hypothesize that insulin or insulin-like growth factor-1 may affect tau phosphorylation through the inhibition of glycogen-synthase kinase-3 in neurons. Using cultured human neuronal NT2N cells, we demonstrate that glycogen-synthase kinase-3 phosphorylates tau and reduces its affinity for microtubules and that insulin and insulin-like growth factor-1 stimulation reduces tau phosphorylation and promotes tau binding to microtubules. We further demonstrate that these effects of insulin and insulin-like growth factor-1 are mediated through the inhibition of glycogen-synthase kinase-3 via the phosphatidylinositol 3-kinase/protein kinase B signaling pathway.

Angiotensin II stimulates tyrosine phosphorylation and activation of insulin receptor substrate 1 and protein-tyrosine phosphatase 1D in vascular smooth muscle cells

Angiotensin II (Ang II) and insulin-like growth factor I (IGF I) stimulate intracellular signaling events through binding to their respective G-protein-coupled and growth factor receptors. In rat aortic vascular smooth muscle cells, IGF I (20 ng/ml) induced a sustained (>30 min) increase in the tyrosine phosphorylation of both Src-homology 2 domain-docking insulin receptor substrate 1 (IRS-1) and Src-homology 2-binding tyrosine phosphatase 1D (PTP-1D). In addition, IGF I stimulated PTP-1D phosphatase activity. Ang II (10(-7) M) also increased the tyrosine phosphorylation of IRS-1 (4-fold), PTP-1D (5-fold), and PTP-1D activity (3-4-fold), but with a more transient time course. Ang II also induced PTP-1D.IRS-1 complex formation. These Ang II-induced events were not affected by preincubation with an anti-IGF I antibody, suggesting that Ang II's actions were not mediated via the autocrine secretion of IGF I. Anti-PTP-1D antibody electroporation attenuated Ang II-induced PTP-1D.IRS-1 complex formation and PTP-1D tyrosine phosphorylation and activation. Our findings show that the tyrosine phosphorylation of IRS-1 and PTP-1D represents a convergent intracellular signaling cascade stimulated by both growth factor (i.e. IGF I) and G-protein-coupled (i.e. AT1) receptors.

Insulin-like growth factor II stimulates cell proliferation through the insulin receptor

R- cells are 3T3-like fibroblasts generated from mouse embryos nullizygous for a targeted disruption of the genes encoding the type 1 insulin-like growth factor (IGF) receptor (IGF1R). These cells fail to proliferate in serum-free medium supplemented with purified growth factors, in contrast to their wild-type counterparts. However, when R- cells overexpress the insulin receptor from a stably integrated plasmid, R-/IR cells, they become capable of growing in serum-free medium supplemented solely with insulin or IGF-II, but not with IGF-I. Moreover, the introduction into R-/IR cells of an additional plasmid expressing IGF-II causes these cells to proliferate in serum-free medium without growth factor supplementation. From these results, we conclude that IGF-II can stimulate cell proliferation not only through its cognate IGF1R but also through the insulin receptor.

Antiapoptotic signalling by the insulin-like growth factor I receptor, phosphatidylinositol 3-kinase, and Akt

We have found that insulin-like growth factor I (IGF-I) can protect fibroblasts from apoptosis induced by UV-B light. Antiapoptotic signalling by the IGF-I receptor depended on receptor kinase activity, as cells overexpressing kinase-defective receptor mutants could not be protected by IGF-I. Overexpression of a kinase-defective receptor which contained a mutation in the ATP binding loop functioned as a dominant negative and sensitized cells to apoptosis. The antiapoptotic capacity of the IGF-I receptor was not shared by other growth factors tested, including epidermal growth factor (EGF) and thrombin, although the cells expressed functional receptors for all the agonists. However, EGF was antiapoptotic for cells overexpressing the EGF receptor, and expression of activated pp60v-src also was protective. There was no correlation between protection from apoptosis and activation of mitogen-activated protein kinase, p38/HOG1, or p70S6 kinase. On the other hand, protection by any of the tyrosine kinases against UV-induced apoptosis was blocked by wortmannin, implying a role for phosphatidylinositol 3-kinase (PI3 kinase). To test this, we transiently expressed constitutively active or kinase-dead PI3 kinase and found that overexpression of activated phosphatidylinositol 3-kinase (PI3 kinase) was sufficient to provide protection against apoptosis. Because Akt/PKB is believed to be a downstream effector for PI3 kinase, we also examined the role of this serine/threonine protein kinase in antiapoptotic signalling. We found that membrane-targeted Akt was sufficient to protect against apoptosis but that kinase-dead Akt was not. We conclude that the endogenous IGF-I receptor has a specific antiapoptotic signalling capacity, that overexpression of other tyrosine kinases can allow them also to be antiapoptotic, and that activation of PI3 kinase and Akt is sufficient for antiapoptotic signalling.

Suppression of somatotroph function induced by growth hormone treatment in neonatal pigs

The effect of recombinant porcine growth hormone (pGH) treatment on pituitary function was evaluated in young pigs. Piglets received intraperitoneal recombinant pGH implants (0.5 mg/d sustained release) or vehicle implants beginning at 3 d of age. Ten piglets were sacrificed at 4 and 6 wk of age (five piglets/treatment group) for the collection of pituitary glands, blood, and liver tissue. Blood samples also were drawn at 3 and 12 d of age. Serum concentrations of GH, prolactin (PRL), thyroid-stimulating hormone (TSH), insulin-like growth factor-1 (IGF-1) and IGF-2 were evaluated. Levels of IGF-1 and IGF-2 mRNA were determined in liver samples. Treatment with GH increased circulating levels of GH and IGF-1 (P < 0.01), but not PRL, TSH, or IGF-2. Hepatic IGF-1, but not IGF-2, mRNA levels were increased by pGH (P < 0.001). Cultured pituitary cells from each animal were challenged with 0.1, 1, and 10 nM GH-releasing hormone (GHRH); 2 nM 8-Br-cAMP; or 100 nM phorbol myristate acetate. The release of GH from cultured pituitary cells was stimulated by all secretagogues (P < 0.001). The secretion of GH, but not PRL or TSH, in culture was inhibited by previous in vivo GH treatment (P < 0.001). Similarly, cellular GH, but not PRL or TSH, content was lower in the GH-implant group (P = 0.005). Cell cultures from 6-wk-old piglets secreted more GH, but not PRL or TSH, than cultures from 4-wk-old piglets (P < 0.05). Likewise, cellular GH, but not PRL or TSH, content was greatest in cultures from 6-wk-old animals (P = 0.002). Piglet growth was not affected by exogenous GH treatment (P = 0.67). These results demonstrate that exogenous pGH treatment selectively down-regulates somatotroph function in young pigs.

Spatial polarization of insulin-like growth factor receptors on the human syncytiotrophoblast

The expression of IGF receptors on the maternal-facing, microvillous membrane (MVM) surface and the fetal-facing, basal membrane (BM) surface of the syncytiotrophoblast was studied using standard ligand binding assays, covalent cross-linking techniques, and immunoblot analysis. Scatchard analysis of [125I]IGF-I and -II binding revealed the presence of both high and low affinity binding sites associated with each membrane preparation that did not clearly distinguish between the two membrane preparations. Cross-linking analysis, however, demonstrated type I and type II IGF receptors associated primarily with MVM, suggesting that nonreceptor binding sites may contribute to total membrane binding. Ligand blot analysis revealed that BM are uniquely associated with 29- and 24-kD IGF binding proteins (IGFBPs). [125I]QAYL-IGF-I, having reduced affinity for IGFBPs, was therefore used to study receptor-specific binding. Approximately 5-fold more type I IGF receptors were shown to be associated with MVM than BM by Scatchard and cross-linking analyses. This was confirmed by immunoblot analysis. By contrast, immunoblot analysis revealed approximately 50-100% more type II IGF receptor protein associated with BM, whereas cross-linking to [125I]IGF-II revealed a MVM predominance. In the presence of 5 mM mannose 6-phosphate, however, a substantial increase in [125I]IGF-II cross-linked to the type II IGF receptor was observed in BM but not MVM consistent with immunoblot analysis. These data demonstrate that type 1 and unoccupied type II IGF receptors are expressed primarily on the maternal-facing. MVM surface of the syncytiotrophoblast.

Type I insulin-like growth factor receptor activation regulates apoptotic proteins

Activation of the type I insulin-like growth factor receptor (IGF-IR) blocks osmotic mediated programmed cell death (PCD) in neurons. We speculated that IGF-IR activation could afford neuroprotection either by effecting the negative regulators of the death pathway, Bcl-2 and Bcl-xL, or by altering activity of the ced-3/ICE-like proteases. Here we report that osmotic stress decreases total neuronal Bcl-2 by 4-fold and that hyperosmotic PCD correlates with proteolytic processing of neuronal ced-3/ICE-like proteases. IGF-IR activation maintains normal Bcl-2 levels, and signaling via the IGF-IR:phosphatidylinositol 3-kinase pathway prevents ICE/LAP-3 and Yama/CPP32 processing. Finally, increased neuronal IGF-IR expression enhances the negative death regulator Bcl-xL. We suggest that IGF-IR signaling exerts its short-term inhibitory effects upon PCD "upstream" of both Bcl proteins and ced-3/ICE-like proteases, while chronic increased IGF-IR expression may modulate susceptibility to death signals by mediating the negative death regulator, Bcl-xL.

Src phosphorylates the insulin-like growth factor type I receptor on the autophosphorylation sites. Requirement for transformation by src

The insulin-like growth factor type I (IGF-I) receptor can become tyrosine phosphorylated and enzymatically activated either in response to ligand or because of the activity of the Src tyrosine kinase (Peterson, J. E., Jelinek, T., Kaleko, M., Siddle, K., and Weber, M. J. (1994) J. Biol. Chem. 269, 27315-27321). The goal of the present study was to analyze the mechanistic basis and functional significance of the Src-induced phosphorylation and activation of the IGF-I receptor. 1) We mapped the sites of IGF-I receptor autophosphorylation to peptides representing three different receptor domains: tyrosines 943 and 950 in the juxtamembrane region; tyrosines 1131, 1135, and 1136 within the kinase domain; and tyrosine 1316 in the carboxyl-terminal domain. The juxtamembrane and kinase-domain peptides were phosphorylated both in vivo and in vitro. The carboxyl-terminal site, although phosphorylated in vitro and in src-transformed cells, was not a major site of ligand-induced phosphorylation in vivo. 2) We determined that the sites of Src-induced phosphorylation of the IGF-I receptor are the same as the ligand-induced autophosphorylation sites and that the Src kinase can catalyze these phosphorylations directly. 3) We showed that cells cultured from mice in which the IGF-I receptor has been knocked out by homologous recombination are defective for morphological transformation by src. Thus, the Src kinase can substitute for the receptor kinase in phosphorylating and activating the IGF-I receptor, and this receptor phosphorylation and activation are essential for transformation by src.

Reexamining interaction of the SH2 domains of SYP and GAP with insulin and IGF-1 receptors in the two-hybrid system

Direct interaction of effector proteins such as the p85 regulatory subunit of phosphatidylinositol 3-kinase (PI 3-kinase), SYP (SH2-domain-containing tyrosine phosphatase) and GAP (Ras-GTPase activating protein) with the insulin receptor (IR) and insulin-like growth factor-1 (IGF-1) type 1 receptor (IGF-1R) has been reported in some studies. Interaction of SYP and GAP with IR and IGF-1R was re-investigated here in the two-hybrid system by assessing his3/lacZ activation in S. cerevisiae. The experiments were performed with the cytoplasmic beta domain of IR and IGF-1R and various SH2-subdomains of SYP and GAP. None of the subdomains of SYP and GAP tested were able to activate his3/lacZ, whereas these reporter genes were strongly activated when p85 was used as we have recently shown. Thus, interaction of SYP and GAP with IR and IGF-1R, if any, would be weak and/or transient as compared to that of p85.

The avian IGF type 1 receptor: cDNA analysis and in situ hybridization reveal conserved sequence elements and expression patterns relevant for the development of the nervous system

Insulin-like growth factor type 1 receptor (IGF-1R) is a tyrosine kinase with a key role in development. The primary structure of IGF-1R is known for mammalian species, but not for birds. The avian embryo, however, provides an ideal system for the experimental study of neurogenesis. We therefore cloned the complete coding sequence of the chicken IGF-1R from a cDNA library and analyzed its embryonic expression by Northern blot and in situ hybridization. The deduced chicken IGF-1R precursor of 1363 amino acids was 85% identical to human IGF-1R and did not show deletions or insertions in critical positions, when compared to its mammalian homologues. Notably, all cysteine residues in the extracellular domains, and 15 of the 17 N-linked glycosylation sites found in human IGF-1R were also present in the chicken receptor. An 11 kb transcript was abundant in developing nervous tissues, kidney, pancreas and the gastrointestinal tract. The early in situ expression patterns in 20-somite embryos revealed high levels of IGF-1R mRNA in the neuroepithelia, notochord and somites. At embryonic day 4 (E4), high concentrations of IGF-1R transcripts were found again primarily in the neuroepithelia and, to a lesser degree, in the sensory ganglia and diverse mesenchymal derivatives. During the second half of embryonic development, IGF-1R expression in the CNS was particularly abundant in telencephalic regions, including the olfactory bulb, hippocampus, striatum and piriform cortex, and also in the optic tectum and cerebellum. By the use of cDNA cloning and in situ hybridization this study reveals conserved amino acid sequence elements between birds and mammals, and developmental expression patterns that are compatible with an important role of this receptor in growth, differentiation and maturation of the avian CNS.

Thyrotropin via cyclic AMP induces insulin receptor expression and insulin Co-stimulation of growth and amplifies insulin and insulin-like growth factor signaling pathways in dog thyroid epithelial cells

Despite the similarity of their receptors and signal transduction pathways, insulin is regarded as a regulator of glucose, protein, and lipid metabolism, whereas insulin-like growth factors (IGF-I and IGF-II) mainly act as mitogenic hormones. In the dog thyroid primary culture model, the triggering of DNA synthesis by thyrotropin (TSH) through cAMP, or by cAMP-independent factors including epidermal growth factor, hepatocyte growth factor and phorbol esters, requires insulin or IGFs as comitogenic factors. In the present study, in TSH-treated cells, IGF-I receptors and insulin receptors were paradoxically equivalent in their capacity to elicit the comitogenic pathway, which, however, was mediated only by IGF-I receptors in dog thyroid cells stimulated by cAMP-independent mitogens. Moreover, prior cell exposure to TSH or forskolin increased their responsiveness to insulin, IGF-I, and IGF-II, as seen on DNA synthesis and activation of a common insulin/IGF signaling pathway. To understand these observations, binding characteristics and expression of insulin and IGF-I receptors were examined. To analyze IGF-I receptor characteristics, the unexpected interference of a huge presence of IGF-binding proteins at the cell membrane was avoided using labeled Long R3 IGF-I instead of IGF-I. Strikingly, TSH, through cAMP, time-dependently induced insulin binding and insulin receptor mRNA and protein accumulation without any effect on IGF-I receptors. These findings constitute a first example of an induction of insulin receptor gene expression by a cAMP-mediated hormone. In dog thyroid cells, this allows low physiological insulin concentrations to act as a comitogenic factor and might explain in part the enhanced responsiveness to IGFs in response to TSH. This raises the possibility that TSH-insulin interactions may play a role in the regulation of thyroid growth and function in vivo.

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.

Insulin-like growth factor receptors and binding proteins

The insulin-like growth factor receptors are integral membrane proteins and demonstrate separate, but important effects on the regulation of cellular processes. The IGF-I receptor signals multiple cascades via its inherent tyrosine kinase activity. The IGF-II/M-6-P receptor on the other hand is primarily involved in targeting of enzymes to various subcellular compartments. In contrast, the insulin-like binding proteins are secreted by the cells and accumulate in the extracellular matrix or on the external surface of the cell. They are also involved in regulating cellular processes more indirectly. They modulate the interactions of the IGFs with their receptors, and in addition, may have some IGF-independent effects probably by direct interaction with integrin and other cell membrane receptor proteins. The recent studies, as outlined in this review, strongly suggest an important, if not essential role for the IGF system in normal physiology and disease states. The challenge now is to define the mechanisms involved in these effects. More studies are required to fully understand the post-receptor mechanism involved in IGF-I receptor signal transduction and the mechanisms whereby the IGFBPs exert their interesting effects. Understanding these mechanisms will enable investigators to create new therapeutic modalities for diseases that are affected by the IGF system.

Interaction of p85 subunit of PI 3-kinase with insulin and IGF-1 receptors analysed by using the two-hybrid system

Interaction of the p85 subunit of PI 3-kinase with the insulin receptor (IR) and the IGF-1 receptor (IGF-1R) was investigated using the two-hybrid system by assessing for his3 and lacZ activation in S. cerevisiae. The experiments were performed with the cytoplasmic beta domain (wild type or mutated) of IR and IGF-1R and p85 or its subdomains (N + C-SH2, N-SH2, C-SH2, SH3 + N-SH2). The results of his3 activation indicated that p85, N + C-SH2 and C-SH2 interact with both IR beta and IGF-1R beta, whereas N-SH2 and SH3 + N-SH2 interact only with IR beta. Interaction of p85 and N+C-SH2 with IR beta (delta C-43) or IGF-1R beta(delta C-43) in which the C-terminal 43 amino acids (including the YXXM motif) were deleted, persisted. The internal binding site thus revealed was not altered by further mutating Y960/F for IR or Y950/F for IGF-1R. Activation of lacZ upon interaction of p85 with IR beta(delta C-43) was 4-fold less as compared to IR beta. This activation with p85 and IGF-1R beta was 4-fold less as compared to IR beta and was somewhat increased (2-fold) for IGF-1R beta (delta C-43). Thus, the C-terminal domain in IGF-1R appears to exert a negative control on binding of p85 thereby providing a possible regulatory mechanism for direct activation of the PI 3-kinase pathway.