A panel of monoclonal antibodies for the type I insulin-like growth factor receptor. Epitope mapping, effects on ligand binding, and biological activity.

We obtained 20 mouse monoclonal antibodies specific for human type I insulin-like growth factor (IGF) receptors, using transfected cells expressing high levels of receptors (IGF-1R/3T3 cells) as immunogen. The antibodies immunoprecipitated receptor.125I-IGF-I complexes and biosynthetically labeled receptors from IGF-1R/3T3 cells but did not react with human insulin receptors or rat type I IGF receptors. Several antibodies stimulated DNA synthesis in IGF-1R/3T3 cells, but the maximum stimulation was only 25% of that produced by IGF-I. The antibodies fell into seven groups recognizing distinct epitopes and with different effects on receptor function. All the antibodies reacted with the extracellular portion of the receptor, and epitopes were localized to specific domains by investigating their reaction with a series of chimeric IGF/insulin receptor constructs. Binding of IGF-I was inhibited up to 90% by antibody 24-60 reacting in the region 184-283, and by antibody 24-57 reacting in the region 440-586. IGF-I binding was stimulated up to 2.5-fold by antibodies 4-52 and 16-13 reacting in the region 62-184, and by antibody 26-3 reacting downstream of 283. The latter two groups of antibodies also dramatically stimulated insulin binding to intact IGF-1R/3T3 cells (by up to 50-fold), and potentiated insulin stimulation of DNA synthesis. Scatchard analysis indicated that in the presence of these antibodies, the affinity of the type I IGF receptor for insulin was comparable with that of the insulin receptor. These data indicate that regions both within and outside the cysteine-rich domain of the receptor alpha-subunit are important in determining the affinity and specificity of ligand binding. These antibodies promise to be valuable tools in resolving issues of IGF-I receptor heterogeneity and in studying the structure and function of classical type I receptors and insulin/IGF receptor hybrids.     

Nuclear targeting of IGF-1 receptor in orbital fibroblasts from Graves' disease: apparent role of ADAM17

Insulin-like growth factor-1 receptor (IGF-1R) comprises two subunits, including a ligand binding domain on extra- cellular IGF-1Rα and a tyrosine phosphorylation site located on IGF-1Rβ. IGF-1R is over-expressed by orbital fibroblasts in the autoimmune syndrome, Graves' disease (GD). When activated by IGF-1 or GD-derived IgG (GD-IgG), these fibroblasts produce RANTES and IL-16, while those from healthy donors do not. We now report that IGF-1 and GD-IgG provoke IGF-1R accumulation in the cell nucleus of GD fibroblasts where it co-localizes with chromatin. Nuclear IGF-1R is detected with anti-IGF-1Rα-specific mAb and migrates to approximately 110 kDa, consistent with its identity as an IGF-1R fragment. Nuclear IGF-1R migrating as a 200 kDa protein and consistent with an intact receptor was undetectable when probed with either anti-IGF-1Rα or anti-IGF-1Rβ mAbs. Nuclear redistribution of IGF-1R is absent in control orbital fibroblasts. In GD fibroblasts, it can be abolished by an IGF-1R-blocking mAb, 1H7 and by physiological concentrations of glucocorticoids. When cell-surface IGF-1R is cross-linked with (125)I IGF-1, (125)I-IGF-1/IGF-1R complexes accumulate in the nuclei of GD fibroblasts. This requires active ADAM17, a membrane associated metalloproteinase, and the phosphorylation of IGF-1R. In contrast, virally encoded IGF-1Rα/GFP fusion protein localizes equivalently in nuclei in both control and GD fibroblasts. This result suggests that generation of IGF-1R fragments may limit the accumulation of nuclear IGF-1R. We thus identify a heretofore-unrecognized behavior of IGF-1R that appears limited to GD-derived fibroblasts. Nuclear IGF-1R may play a role in disease pathogenesis.     

Crosstalk between the extracellular domain of the ErbB2 receptor and IGF-1 receptor signaling

Insulin-like growth factor 1 receptor (IGF-1R) plays an important role in cell growth and malignant transformation. To investigate IGF-1R-dependent signaling events and its effects on apoptosis induction and cellular proliferation, we generated a constitutively active, ligand-independent IGF-1R variant. We fused the cytoplasmic domain of the IGF-1R to the extracellular and transmembrane domains of the oncogenic ErbB2 receptor (ErbB2^V→E/IGF-1). A fusion protein in which the wild-type sequence of the ErbB2 receptor was used, served as a control (ErbB2^V/IGF-1R). ErbB2^V/IGF-1R, ErbB2^V→E/IGF-1R and IGF-1R were stably transfected into interleukin 3 (IL-3)-dependent BaF/3 cells. ErbB2^V→E/IGF-1R expressing cells exhibited ligand-independent, constitutive tyrosine phosphorylation of the receptor fusion protein. Constitutively, activated ErbB2^V→E/IGF-1R conferred IL-3 independence for growth and survival to the transfected BaF/3 cells. Constitutive activation of the IGF-1R results in cellular growth and protection against apoptosis upon IL-3 withdrawal in BaF/3 cells.     

MiR-223 Regulates Human Embryonic Stem Cell Differentiation by Targeting the IGF-1R/Akt Signaling Pathway

Currently, there are difficulties associated with the culturing of pluripotent human embryonic stem cells (hESCs), and knowledge regarding their regulatory mechanisms is limited. MicroRNAs (miRNAs) regulate gene expression and have critical functions in stem cell self-renewal and differentiation. Moreover, fibroblast growth factor (FGF) and the insulin-like growth factor receptor (IGF-1R) are key activators of signaling in hESCs. Based on the identification of complementary binding sites in miR-223 and IGF-1R mRNA, it is proposed that miR-223 acts as a local regulator of IGF-1R. Therefore, levels of miR-223 were detected in differentiated versus undifferentiated hESCs. In addition, proliferation, apoptosis, and differentiation were assayed in these two hESC populations and were compared in the presence of exogenous miR-223 and miR-223 inhibitor. Inhibition of miR-223 was found to maintain the undifferentiated state of hESCs, while addition of miR-223 induced differentiation. Furthermore, these effects were found to be likely dependent on IGF-1R/Akt signaling.     

Estrogen receptor alpha rapidly activates the IGF-1 receptor pathway

Estrogen and insulin-like-growth factor 1 (IGF-1) are potent mitogenic stimuli that share important properties in the control of cellular proliferation. However, the coupling between the signaling cascades of estrogen receptors alpha and beta and the IGF-1 receptor (IGF-1R) is poorly understood. Therefore, we selectively transfected estrogen receptor alpha or beta in COS7 and HEK293 cells, which contain IGF-1R. In presence of estrogen receptor alpha but not beta, 17beta-estradiol (E2) rapidly induces phosphorylation of the IGF-1R and the extracellular signal-regulated kinases 1/2. Furthermore, upon stimulation with E2, estrogen receptor alpha but not beta bound rapidly to the IGF-1R in COS7 as well as L6 cells, which express all investigated receptors endogenously. Control experiments in the IGF-1R-deficient fibroblast cell line R(-) showed that after stimulation with E2 only estrogen receptor alpha bound to the transfected IGF-1R. Overexpression of dominant negative mitogen-activated protein kinases kinase inhibited this effect. Finally, estrogen receptor alpha but not beta is required to induce the activation of the estrogen receptor-responsive reporter ERE-LUC in IGF-1-stimulated cells. Taken together, these data demonstrate that ligand bound estrogen receptor alpha is required for rapid activation of the IGF-1R signaling cascade.     

IGF-1R Regulates the Extracellular Level of Active MMP-2, Pathological Neovascularization,and Functionality in Retinas of OIR Mouse Model

In ischemic proliferative diseases such as retinopathies, persistent hypoxia leads to the release of numerous neovascular factors that participate in the formation of abnormal vessels and eventually cause blindness. The upregulation and activation of metalloproteinases (MMP-2 and MMP-9) represent a final common pathway in this process. Although many regulators of the neovascular process have been identified, the complete role of the insulin-like growth factor 1 (IGF-1) and its receptor (IGF-1R) appears to be significantly more complex. In this study, we used an oxygen-induced retinopathy (OIR) mouse model as well as an in vitro model of hypoxia to study the role of MMP-2 derived from Müller glial cells (MGCs) and its relation with the IGF-1/IGF-1R system. We demonstrated that MMP-2 protein expression increased in P17 OIR mice, which coincided with the active phase of the neovascular process. Also, glutamine synthetase (GS)-positive cells were also positive for MMP-2, whereas IGF-1R was expressed by GFAP-positive cells, indicating that both proteins were expressed in MGCs. In addition, in the OIR model a single intravitreal injection of the IGF-1R blocking antibody (αIR3) administered at P12 effectively prevented pathologic neovascularization, accelerated physiological revascularization, and improved retinal functionality at P17. Finally, in MGC supernatants, the blocking antibody abolished the IGF-1 effect on active MMP-2 under normoxic and hypoxic conditions without affecting the extracellular levels of pro-MMP-2. These results demonstrate, for the first time, that the IGF-1/IGF-1R system regulates active MMP-2 levels in MGCs, thus contributing to MEC remodeling during the retinal neovascular process.     

Differential signalling potential of insulin- and IGF-1-receptor cytoplasmic domains.

The human receptors for insulin-like growth factor 1 (IGF-1) and insulin, and two chimeric receptors consisting of ligand-binding, extracellular insulin receptor and intracellular IGF-1 receptor structures, have been expressed in NIH-3T3 fibroblasts. All four receptor types were synthesized, processed and transported to the cell surface to form high-affinity binding sites. All normal and chimeric receptors had an active tyrosine kinase which was regulated by homologous or heterologous ligands respectively. In addition, cell surface receptors were internalized efficiently and subjected to accelerated degradation in the presence of ligand. While all four types of receptor stimulated glucose transport with similar efficiency, they displayed significant differences in their mitogenic signalling potentials. Receptors with an IGF-1 receptor cytoplasmic domain were 10 times more active in stimulating DNA synthesis than the insulin receptor. In NIH-3T3 cells overexpressing wild-type and chimeric receptors, maximal growth responses obtained with IGF-1 or insulin alone were equivalent to those obtained with 10% fetal calf serum. We conclude that in the cell system employed the receptors for IGF-1 and insulin mediate short-term responses similarly, but display distinct characteristics in their long-term mitogenic signalling potentials.     

Targeting of the protein interaction site between FAK and IGF-1R

The interaction of focal adhesion kinase (FAK) and insulin-like growth factor-1 receptor (IGF-1R) plays an important role in cancer cell survival. Targeting this interaction with small molecule drugs could be a novel strategy in cancer therapy. By a series of pull-down assays using GST-tagged FAK fragments and His-tagged IGF-1R intracellular fragments, we showed that the FAK-NT2 (a.a. 127-243) domain directly interacts with the N-terminal part of the IGF-1R intracellular domain. Overexpressed FAK-NT2 domain was also shown to co-localize with IGF-1R in pancreatic cells. Computational modeling was used to predict the binding configuration of these two domains and to screen for small molecules binding to the interaction site. This strategy successfully identified a lead compound that disrupts FAK/IGF-1R interaction.     

Identification of a heregulin binding site in HER3 extracellular domain

HER3 (also known as c-Erb-b3) is a type I receptor tyrosine kinase similar in sequence to the epidermal growth factor (EGF) receptor. The extracellular segment of this transmembrane receptor contains four domains. Domains I and II are similar in sequence to domains III and IV, respectively, and domains II and IV are cysteine-rich. We show that the EGF-like domain of heregulin (hrg) binds to domains I and II of HER3, in contrast to the EGF receptor, for which prior studies have shown that a construct consisting of domains III and portions of domain IV binds EGF. Next, we identified a putative hrg binding site by limited proteolysis of the recombinant extracellular domains of HER3 (HER3-ECD(I-IV)) in both the presence and absence of hrg. In the absence of hrg, HER3-ECD(I-IV) is cleaved after position Tyr(50), near the beginning of domain I. Binding of hrg to HER3-ECD(I-IV) fully protects position Tyr(50) from proteolysis. To confirm that domain I contains a hrg binding site, we expressed domains I and II (HER3-ECD(I-II)) and find that it binds hrg with 68 nm affinity. These data suggest that domains I and II of HER3-ECD(I-IV) act as a functional unit in folding and binding of hrg. Thus, our biochemical findings reinforce the structural hypothesis of others that HER3-ECD(I-IV) is similar to the insulin-like growth factor-1 receptor (IGF-1R), as follows: 1) The protected cleavage site in HER3-ECD(I-IV) corresponds to a binding footprint in domain I of IGF-1R; 2) HER3-ECD(I-II) binds hrg with a 68 nm dissociation constant, supporting the hypothesis that domain I is involved in ligand binding; and 3) the large accessible surface area (1749 A) of domain L1 of IGF-1R that is buried by domain S1, as well as the presence of conserved contacts in this interface of type 1 RTKs, suggests that domains L1 and S1 of IGF-1R function as a unit as observed for HER3-ECD(I-II). Our results are consistent with the proposal that HER3 has a structure similar to IGF-1R and binds ligand at a site in corresponding domains.     

Insulin-like growth factor type-1 receptor transactivation in vasoactive peptide and oxidant-induced signaling pathways in vascular smooth muscle cells

Transactivation of epidermal growth factor receptor (EGFR) is a well-documented mechanism by which vasoactive peptides and H2O2 elicit their cellular responses. However, a role for the insulin-like growth factor type-1 receptor (IGF-1R) transactivation in mediating the effects of angiotensin II (Ang II) and H2O2 in vascular smooth muscle cells from different artery types have also been recently recognized. By using a series of pharmacological inhibitors of various growth factor receptor tyrosine kinases and a direct analysis of the phosphorylation status of the beta-subunit of IGF-1R, a requirement of this growth factor receptor in Ang II and H2O2 response has been demonstrated. This review discusses some of the studies that highlight the importance of IGF-1R transactivation in mediating Ang II- and H2O2-induced mitogen-activated protein kinase and protein kinase B signaling pathways.     

A Highly Selective Dual Insulin Receptor (IR)/Insulin-like Growth Factor 1 Receptor (IGF-1R) Inhibitor Derived from an Extracellular Signal-regulated Kinase (ERK) Inhibitor

Dual inhibitors of the closely related receptor tyrosine kinases insulin-like growth factor 1 receptor (IGF-1R) and insulin receptor (IR) are promising therapeutic agents in cancer. Here, we report an unusually selective class of dual inhibitors of IGF-1R and IR identified in a parallel screen of known kinase inhibitors against a panel of 300 human protein kinases. Biochemical and structural studies indicate that this class achieves its high selectivity by binding to the ATP-binding pocket of inactive, unphosphorylated IGF-1R/IR and stabilizing the activation loop in a native-like inactive conformation. One member of this compound family was originally reported as an inhibitor of the serine/threonine kinase ERK, a kinase that is distinct in the structure of its unphosphorylated/inactive form from IR/IGF-1R. Remarkably, this compound binds to the ATP-binding pocket of ERK in an entirely different conformation to that of IGF-1R/IR, explaining the potency against these two structurally distinct kinase families. These findings suggest a novel approach to polypharmacology in which two or more unrelated kinases are inhibited by a single compound that targets different conformations of each target kinase.     

The receptor binding domain of apolipoprotein E, linked to a model class A amphipathic helix, enhances internalization and degradation of LDL by fibroblasts

Human apolipoprotein E (apo E) consists of two distinct domains, the lipid-associating domain (residues 192-299) and the globular domain (residues 1-191) which contains the LDL receptor (LDLR) binding site (residues 129-169). To test the hypothesis that an arginine-rich apo E receptor binding domain (residues 141-150) is sufficient to enhance low-density lipoprotein (LDL) uptake and clearance when covalently linked to a class A amphipathic helix, a peptide in which the receptor binding domain of human apo E, LRKLRKRLLR (hApoE[141-150]), is linked to 18A, a well-characterized high-affinity lipid-associating peptide (DWLKAFYDKVAEKLKEAF), we synthesized the peptide hApoE[141-150]-18A (hE18A) and its end-protected analogue, Ac-hE18A-NH(2). The importance of positively charged residues and the role of the hydrophobic residues in the receptor binding domain were also studied using four analogues. Ac-LRRLRRRLLR-18A-NH(2) [Ac-hE(R)18A-NH(2)] and Ac-LRKMRKRLMR-18A-NH(2) (Ac-mE18A-NH(2)) contained an extended hydrophobic face, including the receptor binding region. Control peptides, Ac-LRLLRKLKRR-18A-NH(2) [Ac-hE(Sc)18A-NH(2)], had the amino acid residues of the apo E receptor binding domain scrambled to disrupt the extended hydrophobic face, and Ac-RRRRRRRRRR-18A-NH(2) (Ac-R(10)18A-NH(2)) had only positively charged Arg residues as the receptor binding domain. The effect of the dual-domain peptides on the uptake and degradation of human LDL by fibroblasts was determined in murine embryonic fibroblasts (MEF1). LDL internalization was enhanced 3-, 5-, and 7-fold by Ac-mE18A-NH(2), Ac-hE18A-NH(2), and Ac-hE(R)18A-NH(2), respectively, whereas the control peptides had no significant biological activity. All three active peptides increased the level of degradation of LDL by 100%. The LDL binding and internalization to MEF1 cells in the presence of these peptides was not saturable over the LDL concentration range that was studied (1-10 microgram/mL). Furthermore, a similar enhancement of LDL internalization was observed independent of the presence of the LDL receptor-related protein (LRP), LDLR, or both. Pretreatment of cells with heparinase and heparitinase abolished more than 80% of the enhanced peptide-mediated LDL uptake and degradation by cells. We conclude that the dual-domain peptides enhanced LDL uptake and degradation by fibroblasts via a heparan sulfate proteoglycan (HSPG)-mediated pathway.     

Specific binding of insulin-like growth factors 1 and 2 to the type 1 and type 2 receptors respectively.

1. Competitive binding and receptor cross-linking experiments have been used to examine the receptor-ligand interactions between three bovine insulin-like growth factors (IGF) and monolayer cultures of myoblasts and fibroblasts. 2. Labelled IGF-2 bound predominantly to the type 2 receptor with negligible label cross-linked to the type 1 receptor, notwithstanding the ability of IGF-2 to compete effectively for the binding of IGF-1 to the type 1 receptor. Approx. 100-fold higher concentrations of IGF-1 or the N-terminal truncated (des-Gly-Pro-Glu) IGF-1 (-3N:IGF-1) were required to produce competition equivalent to IGF-2. 3. All IGF peptides, but especially IGF-1, enhanced the binding of labelled IGF-2 to the type 2 receptor of lung fibroblasts. This unusual effect was probably a consequence of the displacement of labelled IGF-2 otherwise bound to a medium protein, a conclusion supported by the demonstration of a 38 kDa membrane protein cross-linked to labelled IGF-2. 4. Both IGF-1 and -3N:IGF-1 bound only to the type 1 IGF receptor in L6 myoblasts, rat vascular smooth-muscle cells and human lung fibroblasts. The peptides competed for labelled IGF-1 binding with potencies in the order -3N:IGF-1 greater than IGF-1 greater than IGF-2 much greater than insulin. Since the IGF peptides were equipotent in skin fibroblasts, it was proposed that the apparently higher affinity of -3N:IGF-1 for receptors in the other cell types was instead a consequence of a low affinity of this peptide for the competing 38 kDa binding protein.     

Casein Kinase 2 Dependent Phosphorylation of Neprilysin Regulates Receptor Tyrosine Kinase Signaling to Akt

Neprilysin (NEP) is a type II membrane metalloproteinase that cleaves physiologically active peptides at the cell surface thus regulating the local concentration of these peptides available for receptor binding and signal transduction. In addition, the cytoplasmic N-terminal domain of NEP interacts with the phosphatase and tensin homologue deleted on chromosome 10 (PTEN) thereby regulating intracellular signaling via Akt. Thus, NEP serves dual functions in extracellular and intracellular signal transduction. Here, we show that NEP undergoes phosphorylation at serine residue 6 within the N-terminal cytoplasmic domain. In vitro and cell culture experiments demonstrate that Ser 6 is efficiently phosphorylated by protein kinase CK2. The phosphorylation of the cytoplasmic domain of NEP inhibits its interaction with PTEN. Interestingly, expression of a pseudophosphorylated NEP variant (Ser6Asp) abrogates the inhibitory effect of NEP on insulin/insulin-like growth factor-1 (IGF-1) stimulated activation of Akt. Thus, our data demonstrate a regulatory role of CK2 in the interaction of NEP with PTEN and insulin/IGF-1 signaling.     

Precise mapping of an IGF-I-binding site on the IGF-1R

The IGF-1R [type 1 IGF (insulin-like growth factor) receptor] is activated upon binding to IGF-I and IGF-II leading to cell growth, survival and migration of both normal and cancerous cells. We have characterized the binding interaction between the IGF-1R and its ligands using two high-affinity mouse anti-IGF-1R mAbs (monoclonal antibodies), 7C2 and 9E11. These mAbs both block IGF-I binding to the IGF-1R but have no effect on IGF-II binding. Epitope mapping using chimaeras of the IGF-1R and insulin receptor revealed that the mAbs bind to the CR (cysteine-rich) domain of IGF-1R. The epitope was finely mapped using single point mutations in the IGF-1R. Mutation of Phe241, Phe251 or Phe266 completely abolished 7C2 and 9E11 binding. The three-dimensional structure showed that these residues cluster on the surface of the CR-domain. BIAcore analyses revealed that IGF-I and a chimaeric IGF-II with the IGF-I C-domain competed for the binding of both mAbs with the IGF-1R, whereas neither IGF-II nor a chimaeric IGF-I with the IGF-II C-domain affected antibody binding. We therefore conclude the IGF-I C-domain interacts with the CR (cysteine-rich) domain of the receptor at the cluster of residues Phe241, Phe251 and Phe266. These results allow precise orientation of IGF-I within the IGF-I-IGF-1R complex involving the IGF-I C-domain binding to the IGF-1R CR domain. In addition, mAbs 7C2 and 9E11 inhibited both IGF-I- and IGF-II-induced cancer cell proliferation, migration and IGF-1R down-regulation, demonstrating that targeting the IGF-1R is an effective strategy for inhibition of cancer cell growth.     

An investigation of the ligand binding properties and negative cooperativity of soluble insulin-like growth factor receptors

To investigate the interaction of the insulin-like growth factor (IGF) ligands with the insulin-like growth factor type 1 receptor (IGF-1R), we have generated two soluble variants of the IGF-1R. We have recombinantly expressed the ectodomain of IGF-1R or fused this domain to the constant domain from the Fc fragment of mouse immunoglobulin. The ligand binding properties of these soluble IGF-1Rs for IGF-I and IGF-II were investigated using conventional ligand competition assays and BIAcore biosensor technology. In ligand competition assays, the soluble IGF-1Rs both bound IGF-I with similar affinities and a 5-fold lower affinity than that seen for the wild type receptor. In addition, both soluble receptors bound IGF-II with similar affinities to the wild type receptor. BIAcore analyses showed that both soluble IGF-1Rs exhibited similar ligand-specific association and dissociation rates for IGF-I and for IGF-II. The soluble IGF-1R proteins both exhibited negative cooperativity for IGF-I, IGF-II, and the 24-60 antibody, which binds to the IGF-1R cysteine-rich domain. We conclude that the addition of the self-associating Fc domain to the IGF-1R ectodomain does not affect ligand binding affinity, which is in contrast to the soluble ectodomain of the IR. This study highlights some significant differences in ligand binding modes between the IGF-1R and the insulin receptor, which may ultimately contribute to the different biological activities conferred by the two receptors.     

Expression of the insulin-like growth factor 1 receptor (IGF-1R) in breast cancer cells: evidence for a regulatory role of dolichyl phosphate in the transition from an intracellular to an extracellular IGF-1 pathway.

In this study we provide evidence that the low expression of IGF-1R at the cell surface of estrogen-independent breast cancer cells is due to a low rate of de novo synthesis of dolichyl phosphate. The analyses were performed on the estrogen receptor-negative breast cancer cell line MDA231 and, in comparison, the melanoma cell line SK-MEL-2, which expresses a high number of plasma membrane-bound IGF-1R. Whereas the MDA231 cells had little or no surface expression of IGF-1R, they expressed functional (i.e., ligand-binding) intracellular receptors. By measuring the incorporation of [3H]mevalonate into dolichyl phosphate, we could demonstrate that the rate of dolichyl phosphate synthesis was considerably lower in MDA231 cells than in SK-MEL-2 cells. Furthermore, N-linked glycosylation of the alpha-subunit of IGF-1R was 8-fold higher in the melanoma cells. Following addition of dolichyl phosphate to MDA231 cells, N-linked glycosylation of IGF-1R was drastically increased, which in turn was correlated to a substantial translocation of IGF-1R to the plasma membrane, as assayed by IGF-1 binding analysis and by Western blotting of plasma membrane proteins. The dolichyl phosphate-stimulated receptors were proven to be biochemically active since they exhibited autophosphorylation. Under normal conditions MDA231 cells, expressing very few IGF-1R at the cell surface, were not growth-arrested by an antibody (alphaIR-3) blocking the binding of IGF-1 to IGF-1R. However, after treatment with dolichyl phosphate, leading to a high cell surface expression of IGF-1R, alphaIR-3 efficiently blocked MDA231 cell growth. Taken together with the fact that the breast cancer cells produce IGF-1 and exhibit intracellular binding, our data suggest that the level of de novo -synthesized dolichyl phosphate may be critical for whether the cells will use an intracellular or an extracellular autocrine IGF-1 pathway.     

Doxorubicin Impairs the Insulin-Like Growth Factor-1 System and Causes Insulin-Like Growth Factor-1 Resistance in Cardiomyocytes

BackgroundInsulin-like growth factor-1 (IGF-1) promotes the survival of cardiomyocytes by activating type 1 IGF receptor (IGF-1R). Within the myocardium, IGF-1 action is modulated by IGF binding protein-3 (IGFBP-3), which sequesters IGF-1 away from IGF-1R. Since cardiomyocyte apoptosis is implicated in anthracycline cardiotoxicity, we investigated the effects of the anthracycline, doxorubicin, on the IGF-1 system in H9c2 cardiomyocytes.ConclusionsDoxorubicin down-regulates IGF-1R and up-regulates IGFBP-3 via p53 and oxidative stress in H9c2 cells. This leads to resistance to IGF-1 that may contribute to doxorubicin-initiated apoptosis. Further studies are needed to confirm these findings in human cardiomyocytes and explore the possibility of manipulating the IGF-1 axis to protect against anthracycline cardiotoxicity.     

{beta}-Arrestin is crucial for ubiquitination and down-regulation of the insulin-like growth factor-1 receptor by acting as adaptor for the MDM2 E3 ligase

The insulin-like growth factor-1 receptor (IGF-1R) plays important roles in physiological growth and aging as well as promoting several crucial functions in cancer cells. However, the molecular mechanisms involved in expression and down-regulation of IGF-1R are still poorly understood. Here we provide evidence that beta-arrestin, otherwise known to be involved in the regulation of G protein-coupled receptors, serves as an adaptor to bring the oncoprotein E3 ubiquitin ligase MDM2 to the IGF-1R. In this way, beta-arrestin acts as a crucial component in the ubiquitination and down-regulation of the receptor. Both MDM2 and beta-arrestin co-immunoprecipitated with the IGF-1R. The beta-arrestin isoform 1 appeared to be more strongly associated with the receptor than isoform 2, and in a molecular context it was 4-fold more efficient in inducing polyubiquitination of IGF-1R, a reaction that required the presence of beta-arrestin and MDM2. Ligand stimulation accelerated IGF-1R ubiquitination. In mouse P6 cells (overexpressing human IGF-1R) absence of beta-arrestin 1, but not of beta-arrestin 2, blocked ubiquitination of IGF-1R. Conversely, in the two studied human melanoma cell lines both beta-arrestin isoforms seemed to be involved in IGF-1R ubiquitination. However, because depletion of beta-arrestin 1 almost completely eliminated degradation, and IGF-1 induced down-regulation of the receptor in these cells, whereas beta-arrestin 2 only had a partial effect, beta-arrestin 1 seems to the more important isoform in affecting the expression of IGF-1R. To our knowledge this is the first study demonstrating a defined molecular role of beta-arrestin with direct relevance to cell growth and cancer.     

IGF-1 receptor contributes to the malignant phenotype in human and canine osteosarcoma

To further define the role of insulin-like growth factor-1 (IGF-1) and its receptor (IGF-1R) in osteosarcoma (OS), human OS cell lines with low (SAOS-2) and high (SAOS-LM2) metastatic potential and three canine OS-derived cell lines were studied. Cell lines were evaluated for: IGF-1R expression; expression of IGF binding proteins (IGFBPs); effect of IGF-1 on tumor cell growth, invasion, expression of urokinase plasminogen activator (uPA), and soluble uPA receptor (suPAR), and; ectopic and orthotopic tumorigenicity of the canine OS cells in athymic mice. All cell lines exhibited steady-state mRNA expression of IGF-1R. The SAOS-2 and SAOS-LM2 cells expressed 9,138 and 10,234 cell-associated binding sites, respectively. Canine OS cells expressed from 1,728 to 3,883 binding sites. Two IGF-1-treated cell lines displayed enhanced proliferation. Two cell lines formed colonies in semisolid media, and IGF-1 increased colony number. Matrigel invasion was enhanced in one cell line following IGF-1 treatment. uPA and suPAR were unchanged in SAOS-2 and SAOS-LM2 cells following IGF-1 treatment, but the highly metastatic OS line SAOS-LM2 expressed five times more suPAR and displayed enhanced invasion compared to the parental, low metastatic SAOS-2. IGFBP-5 was detected in four of five cell lines, and IGFBP-3 was detected in two canine OS cell lines. Two canine OS lines were tumorigenic, and one metastasized spontaneously. In conclusion, OS cells express IGF-1R, which can contribute to their growth and invasion. There is suggestive evidence that increasing receptor number may contribute to in vivo tumorigenesis. Additional studies are needed to determine how IGF-1/IGF-1R interactions contribute to the malignant phenotype of OS.