Different effects on mitogenesis and transformation of a mutation at tyrosine 1251 of the insulin-like growth factor I receptor

Insulin Receptor Isoforms and Insulin Growth Factor-like Receptors: Implications in Cell Signaling, Carcinogenesis, and Chemoresistance

This comprehensive review thoroughly explores the intricate involvement of insulin receptor (IR) isoforms and insulin-like growth factor receptors (IGFRs) in the context of the insulin and insulin-like growth factor (IGF) signaling (IIS) pathway. This elaborate system encompasses ligands, receptors, and binding proteins, giving rise to a wide array of functions, including aspects such as carcinogenesis and chemoresistance. Detailed genetic analysis of IR and IGFR structures highlights their distinct isoforms, which arise from alternative splicing and exhibit diverse affinities for ligands. Notably, the overexpression of the IR-A isoform is linked to cancer stemness, tumor development, and resistance to targeted therapies. Similarly, elevated IGFR expression accelerates tumor progression and fosters chemoresistance. The review underscores the intricate interplay between IRs and IGFRs, contributing to resistance against anti-IGFR drugs. Consequently, the dual targeting of both receptors could present a more effective strategy for surmounting chemoresistance. To conclude, this review brings to light the pivotal roles played by IRs and IGFRs in cellular signaling, carcinogenesis, and therapy resistance. By precisely modulating these receptors and their complex signaling pathways, the potential emerges for developing enhanced anti-cancer interventions, ultimately leading to improved patient outcomes.

Non-glycosylated IGF2 prohormones are more mitogenic than native IGF2

Insulin-like Growth Factor-2 (IGF2) is important for the regulation of human embryonic growth and development, and for adults' physiology. Incorrect processing of the IGF2 precursor, pro-IGF2(156), leads to the formation of two IGF2 proforms, big-IGF2(87) and big-IGF2(104). Unprocessed and mainly non-glycosylated IGF2 proforms are found at abnormally high levels in certain diseases, but their mode of action is still unclear. Here, we found that pro-IGF2(156) has the lowest ability to form its inactivating complexes with IGF-Binding Proteins and has higher proliferative properties in cells than IGF2 and other IGF prohormones. We also showed that big-IGF2(104) has a seven-fold higher binding affinity for the IGF2 receptor than IGF2, and that pro-IGF2(87) binds and activates specific receptors and stimulates cell growth similarly to the mature IGF2. The properties of these pro-IGF2 forms, especially of pro-IGF2(156) and big-IGF2(104), indicate them as hormones that may be associated with human diseases related to the accumulation of IGF-2 proforms in the circulation.

Insulin-like growth factors: Ligands, binding proteins, and receptors

BACKGROUND

The insulin-like growth factor family of ligands (IGF-I, IGF-II, and insulin), receptors (IGF-IR, M6P/IGF-IIR, and insulin receptor [IR]), and IGF-binding proteins (IGFBP-1-6) play critical roles in normal human physiology and disease states.

SCOPE OF REVIEW

Insulin and insulin receptors are the focus of other chapters in this series and will therefore not be discussed further. Here we review the basic components of the IGF system, their role in normal physiology and in critical pathology's. While this review concentrates on the role of IGFs in human physiology, animal models have been essential in providing understanding of the IGF system, and its regulation, and are briefly described.

MAJOR CONCLUSIONS

IGF-I has effects via the circulation and locally within tissues to regulate cellular growth, differentiation, and survival, thereby controlling overall body growth. IGF-II levels are highest prenatally when it has important effects on growth. In adults, IGF-II plays important tissue-specific roles, including the maintenance of stem cell populations. Although the IGF-IR is closely related to the IR it has distinct physiological roles both on the cell surface and in the nucleus. The M6P/IGF-IIR, in contrast, is distinct and acts as a scavenger by mediating internalization and degradation of IGF-II. The IGFBPs bind IGF-I and IGF-II in the circulation to prolong their half-lives and modulate tissue access, thereby controlling IGF function. IGFBPs also have IGF ligand-independent cell effects.

Novel Regulators of the IGF System in Cancer

The insulin-like growth factor (IGF) system is a dynamic network of proteins, which includes cognate ligands, membrane receptors, ligand binding proteins and functional downstream effectors. It plays a critical role in regulating several important physiological processes including cell growth, metabolism and differentiation. Importantly, alterations in expression levels or activation of components of the IGF network are implicated in many pathological conditions including diabetes, obesity and cancer initiation and progression. In this review we will initially cover some general aspects of IGF action and regulation in cancer and then focus in particular on the role of transcriptional regulators and novel interacting proteins, which functionally contribute in fine tuning IGF1R signaling in several cancer models. A deeper understanding of the biological relevance of this network of IGF1R modulators might provide novel therapeutic opportunities to block this system in neoplasia.

Characterization of viral insulins reveals white adipose tissue-specific effects in mice

Objective

Members of the insulin/insulin-like growth factor (IGF) superfamily are well conserved across the evolutionary tree. We recently showed that four viruses in the Iridoviridae family possess genes that encode proteins highly homologous to human insulin/IGF-1. Using chemically synthesized single-chain (sc), i.e., IGF-1-like, forms of the viral insulin/IGF-1-like peptides (VILPs), we previously showed that they can stimulate human receptors. Because these peptides possess potential cleavage sites to form double chain (dc), i.e., more insulin-like, VILPs, in this study, we have characterized dc forms of VILPs for Grouper iridovirus (GIV), Singapore grouper iridovirus (SGIV) and Lymphocystis disease virus-1 (LCDV-1) for the first time.

Methods

The dcVILPs were chemically synthesized. Using murine fibroblast cell lines overexpressing insulin receptor (IR-A or IR-B) or IGF1R, we first determined the binding affinity of dcVILPs to the receptors and characterized post-receptor signaling. Further, we used C57BL/6J mice to study the effect of dcVILPs on lowering blood glucose. We designed a 3-h dcVILP in vivo infusion experiment to determine the glucose uptake in different tissues.

Results

GIV and SGIV dcVILPs bind to both isoforms of human insulin receptor (IR-A and IR-B) and to the IGF1R, and for the latter, show higher affinity than human insulin. These dcVILPs stimulate IR and IGF1R phosphorylation and post-receptor signaling in vitro and in vivo. Both GIV and SGIV dcVILPs stimulate glucose uptake in mice. In vivo infusion experiments revealed that while insulin (0.015 nmol/kg/min) and GIV dcVILP (0.75 nmol/kg/min) stimulated a comparable glucose uptake in heart and skeletal muscle and brown adipose tissue, GIV dcVILP stimulated 2-fold higher glucose uptake in white adipose tissue (WAT) compared to insulin. This was associated with increased Akt phosphorylation and glucose transporter type 4 (GLUT4) gene expression compared to insulin in WAT.

Conclusions

Our results show that GIV and SGIV dcVILPs are active members of the insulin superfamily with unique characteristics. Elucidating the mechanism of tissue specificity for GIV dcVILP will help us to better understand insulin action, design new analogs that specifically target the tissues and provide new insights into their potential role in disease.

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Viral insulin/IGF1-like peptides (VILPs) are microbial members of the insulin superfamily.

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VILPs bind to human IR and IGF1R and stimulate post-receptor signaling.

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Grouper iridovirus (GIV) VILP has white adipose tissue (WAT)-specific characteristics.

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GIV VILP stimulates increased glucose uptake in WAT via increased GLUT4 expression.

IGF-1 receptor activity in the Golgi of migratory cancer cells depends on adhesion-dependent phosphorylation of Tyr1250 and Tyr1251

Although insulin-like growth factor 1 (IGF-1) signaling promotes tumor growth and cancer progression, therapies that target the IGF-1 receptor (IGF-1R) have shown poor clinical efficacy. To address IGF-1R activity in cancer cells and how it differs from that of the closely related insulin receptor (IR), we focused on two tyrosines in the IGF-1R C-terminal tail that are not present in the IR and are essential for IGF-1-mediated cancer cell survival, migration, and tumorigenic growth. We found that Tyr¹²⁵⁰ and Tyr¹²⁵¹ (Tyr^1250/1251) were autophosphorylated in a cell adhesion-dependent manner. To investigate the consequences of this phosphorylation, we generated phosphomimetic Y1250E/Y1251E (EE) and nonphosphorylatable Y1250F/Y1251F (FF) mutant forms of IGF-1R. Although fully competent in kinase activity and signaling, the EE mutant was more rapidly internalized and degraded than either the wild-type or FF receptor. IGF-1 promoted the accumulation of wild-type and EE IGF-1R within the Golgi apparatus, whereas the FF mutant remained at the plasma membrane. Golgi-associated IGF-1R signaling was a feature of migratory cancer cells, and Golgi disruption impaired IGF-1-induced signaling and cell migration. Upon the formation of new cell adhesions, IGF-1R transiently relocalized to the plasma membrane from the Golgi. Thus, phosphorylation at Tyr^1250/1251 promoted IGF-1R translocation to and signaling from the Golgi to support an aggressive cancer phenotype. This process distinguishes IGF-1R from IR signaling and could contribute to the poor clinical efficacy of antibodies that target IGF-1R on the cell surface.

Modeling cell line-specific recruitment of signaling proteins to the insulin-like growth factor 1 receptor

Receptor tyrosine kinases (RTKs) typically contain multiple autophosphorylation sites in their cytoplasmic domains. Once activated, these autophosphorylation sites can recruit downstream signaling proteins containing Src homology 2 (SH2) and phosphotyrosine-binding (PTB) domains, which recognize phosphotyrosine-containing short linear motifs (SLiMs). These domains and SLiMs have polyspecific or promiscuous binding activities. Thus, multiple signaling proteins may compete for binding to a common SLiM and vice versa. To investigate the effects of competition on RTK signaling, we used a rule-based modeling approach to develop and analyze models for ligand-induced recruitment of SH2/PTB domain-containing proteins to autophosphorylation sites in the insulin-like growth factor 1 (IGF1) receptor (IGF1R). Models were parameterized using published datasets reporting protein copy numbers and site-specific binding affinities. Simulations were facilitated by a novel application of model restructuration, to reduce redundancy in rule-derived equations. We compare predictions obtained via numerical simulation of the model to those obtained through simple prediction methods, such as through an analytical approximation, or ranking by copy number and/or K_D value, and find that the simple methods are unable to recapitulate the predictions of numerical simulations. We created 45 cell line-specific models that demonstrate how early events in IGF1R signaling depend on the protein abundance profile of a cell. Simulations, facilitated by model restructuration, identified pairs of IGF1R binding partners that are recruited in anti-correlated and correlated fashions, despite no inclusion of cooperativity in our models. This work shows that the outcome of competition depends on the physicochemical parameters that characterize pairwise interactions, as well as network properties, including network connectivity and the relative abundances of competitors.

Cells rely on networks of interacting biomolecules to sense and respond to environmental perturbations and signals. However, it is unclear how information is processed to generate appropriate and specific responses to signals, especially given that these networks tend to share many components. For example, receptors that detect distinct ligands and regulate distinct cellular activities commonly interact with overlapping sets of downstream signaling proteins. Here, to investigate the downstream signaling of a well-studied receptor tyrosine kinase (RTK), the insulin-like growth factor 1 (IGF1) receptor (IGF1R), we formulated and analyzed 45 cell line-specific mathematical models, which account for recruitment of 18 different binding partners to six sites of receptor autophosphorylation in IGF1R. The models were parameterized using available protein copy number and site-specific affinity measurements, and restructured to allow for network generation. We find that recruitment is influenced by the protein abundance profile of a cell, with different patterns of recruitment in different cell lines. Furthermore, in a given cell line, we find that pairs of IGF1R binding partners may be recruited in a correlated or anti-correlated fashion. We demonstrate that the simulations of the model have greater predictive power than protein copy number and/or binding affinity data, and that even a simple analytical model cannot reproduce the predicted recruitment ranking obtained via simulations. These findings represent testable predictions and indicate that the outputs of IGF1R signaling depend on cell line-specific properties in addition to the properties that are intrinsic to the biomolecules involved.

IGF1 receptor signaling pathways

Insulin-like growth factors (IGFs) bind specifically to the IGF1 receptor on the cell surface of targeted tissues. Ligand binding to the α subunit of the receptor leads to a conformational change in the β subunit, resulting in the activation of receptor tyrosine kinase activity. Activated receptor phosphorylates several substrates, including insulin receptor substrates (IRSs) and Src homology collagen (SHC). Phosphotyrosine residues in these substrates are recognized by certain Src homology 2 (SH2) domain-containing signaling molecules. These include, for example, an 85 kDa regulatory subunit (p85) of phosphatidylinositol 3-kinase (PI 3-kinase), growth factor receptor-bound 2 (GRB2) and SH2-containing protein tyrosine phosphatase 2 (SHP2/Syp). These bindings lead to the activation of downstream signaling pathways, PI 3-kinase pathway and Ras-mitogen-activated protein kinase (MAP kinase) pathway. Activation of these signaling pathways is known to be required for the induction of various bioactivities of IGFs, including cell proliferation, cell differentiation and cell survival. In this review, the well-established IGF1 receptor signaling pathways required for the induction of various bioactivities of IGFs are introduced. In addition, we will discuss how IGF signals are modulated by the other extracellular stimuli or by themselves based on our studies.

Type I insulin-like growth factor receptor signaling in hematological malignancies

The insulin-like growth factor (IGF) signaling system plays key roles in the establishment and progression of different types of cancer. In agreement with this idea, substantial evidence has shown that the type I IGF receptor (IGF-IR) and its primary ligand IGF-I are important for maintaining the survival of malignant cells of hematopoietic origin. In this review, we discuss current understanding of the role of IGF-IR signaling in cancer with a focus on the hematological neoplasms. We also address the emergence of IGF-IR as a potential therapeutic target for the treatment of different types of cancer including plasma cell myeloma, leukemia, and lymphoma.

Insulin Receptor Isoforms in Physiology and Disease: An Updated View

The insulin receptor (IR) gene undergoes differential splicing that generates two IR isoforms, IR-A and IR-B. The physiological roles of IR isoforms are incompletely understood and appear to be determined by their different binding affinities for insulin-like growth factors (IGFs), particularly for IGF-2. Predominant roles of IR-A in prenatal growth and development and of IR-B in metabolic regulation are well established. However, emerging evidence indicates that the differential expression of IR isoforms may also help explain the diversification of insulin and IGF signaling and actions in various organs and tissues by involving not only different ligand-binding affinities but also different membrane partitioning and trafficking and possibly different abilities to interact with a variety of molecular partners. Of note, dysregulation of the IR-A/IR-B ratio is associated with insulin resistance, aging, and increased proliferative activity of normal and neoplastic tissues and appears to sustain detrimental effects. This review discusses novel information that has generated remarkable progress in our understanding of the physiology of IR isoforms and their role in disease. We also focus on novel IR ligands and modulators that should now be considered as an important strategy for better and safer treatment of diabetes and cancer and possibly other IR-related diseases.

Human insulin analogues modified at the B26 site reveal a hormone conformation that is undetected in the receptor complex

The structural characterization of the insulin-insulin receptor (IR) interaction still lacks the conformation of the crucial B21-B30 insulin region, which must be different from that in its storage forms to ensure effective receptor binding. Here, it is shown that insulin analogues modified by natural amino acids at the TyrB26 site can represent an active form of this hormone. In particular, [AsnB26]-insulin and [GlyB26]-insulin attain a B26-turn-like conformation that differs from that in all known structures of the native hormone. It also matches the receptor interface, avoiding substantial steric clashes. This indicates that insulin may attain a B26-turn-like conformation upon IR binding. Moreover, there is an unexpected, but significant, binding specificity of the AsnB26 mutant for predominantly the metabolic B isoform of the receptor. As it is correlated with the B26 bend of the B-chain of the hormone, the structures of AsnB26 analogues may provide the first structural insight into the structural origins of differential insulin signalling through insulin receptor A and B isoforms.

IGF1R inhibition in mammary epithelia promotes canonical Wnt signaling and Wnt1-driven tumors

Triple-negative breast cancer (TNBC) is an aggressive disease subtype that, unlike other subtypes, lacks an effective targeted therapy. Inhibitors of the insulin-like growth factor receptor (IGF1R) have been considered for use in treating TNBC. Here, we provide genetic evidence that IGF1R inhibition promotes development of Wnt1-mediated murine mammary tumors that offer a model of TNBC. We found that in a double transgenic mouse model carrying activated Wnt1 and mutant Igf1r, a reduction in IGF1R signaling reduced tumor latency and promoted more aggressive phenotypes. These tumors displayed a squamous phenotype with increased expression of keratins 5/6 and β-catenin. Notably, cell lineage analyses revealed an increase in basal (CD29(hi)/CD24(+)) and luminal (CD24(+)/CD61+/CD29(lo)) progenitor cell populations, along with increased Nanog expression and decreased Elf5 expression. In these doubly transgenic mice, lung metastases developed with characteristics of the primary tumors, unlike MMTV-Wnt1 mice. Mechanistic investigations showed that pharmacologic inhibition of the IGF1R in vitro was sufficient to increase the tumorsphere-forming efficiency ofMMTV-Wnt1 tumor cells. Tumors from doubly transgenic mice also exhibited an increase in the expression ratio of the IGF-II-sensitive, A isoform of the insulin receptor versus the IR-B isoform, which when stimulated in vitro resulted in enhanced expression of β-catenin. Overall, our results revealed that in Wnt-driven tumors, an attenuation of IGF1R signaling accelerates tumorigenesis and promotes more aggressive phenotypes with potential implications for understanding TNBC pathobiology and treatment.

Down-regulation of cyclin G2 by insulin, IGF-I (insulin-like growth factor 1) and X10 (AspB10 insulin): role in mitogenesis

The mechanisms whereby insulin analogues may cause enhanced mitogenicity through activation of either the IR (insulin receptor) or the IGF-IR (insulin-like growth factor 1 receptor) are incompletely understood. We demonstrate that in L6 myoblasts expressing only IGF-IRs as well as in the same cells overexpressing the IR, IGF-I (insulin-like growth factor 1), insulin and X10 (AspB10 insulin) down-regulate the mRNA expression level of the cell cycle inhibitor cyclin G2, as measured by qRT-PCR (quantitative reverse transcription-PCR), and induce cell growth measured by [6-(3)H]thymidine incorporation into DNA. Western blotting showed a marked down-regulation of cyclin G2 at the protein level in both cell lines. Overexpression of cyclin G2 in the two cell lines diminished the mitogenic effect of all three ligands. The use of specific inhibitors indicated that both the MAPK (mitogen-activated protein kinase) and the PI3K (phosphoinositide 3-kinase) pathways mediate the down-regulation of Ccng2. The down-regulation of CCNG2 by the three ligands was also observed in other cell lines: MCF-7, HMEC, Saos-2, R(-)/IR and INS-1. These results indicate that regulation of cyclin G2 is a key mechanism whereby insulin, insulin analogues and IGF-I stimulate cell proliferation.

Decorin differentially modulates the activity of insulin receptor isoform A ligands

The proteoglycan decorin, a key component of the tumor stroma, regulates the action of several tyrosine-kinase receptors, including the EGFR, Met and the IGF-IR. Notably, the action of decorin in regulating the IGF-I system differs between normal and transformed cells. In normal cells, decorin binds with high affinity to both the natural ligand IGF-I and the IGF-I receptor (IGF-IR) and positively regulates IGF-IR activation and downstream signaling. In contrast, in transformed cells, decorin negatively regulates ligand-induced IGF-IR activation, downstream signaling and IGF-IR-dependent biological responses. Whether decorin may bind another member of the IGF-I system, the insulin receptor A isoform (IR-A) and its cognate ligands, insulin, IGF-II and proinsulin, have not been established. Here we show that decorin bound with high affinity insulin and IGF-II and, to a lesser extent, proinsulin and IR-A. We utilized as a cell model system mouse embryonic fibroblasts homozygous for a targeted disruption of the Igf1r gene (designated R(-) cells) which were stably transfected with a human construct harboring the IR-A isoform of the receptor. Using these R(-)/IR-A cells, we demonstrate that decorin did not affect ligand-induced phosphorylation of the IR-A but enhanced IR-A downregulation after prolonged IGF-II stimulation without affecting insulin and proinsulin-dependent effects on IR-A stability. In addition, decorin significantly inhibited IGF-II-mediated activation of the Akt pathways, without affecting insulin and proinsulin-dependent signaling. Notably, decorin significantly inhibited IGF-II-mediated cell proliferation of R(-)/IR-A cells but affected neither insulin- nor proinsulin-dependent mitogenesis. Collectively, these results suggest that decorin differentially regulates the action of IR-A ligands. Decorin preferentially inhibits IGF-II-mediated biological responses but does not affect insulin- or proinsulin-dependent signaling. Thus, decorin loss may contribute to tumor initiation and progression in malignant neoplasms which depend on an IGF-II/IR-A autocrine loop.

Ligand-mediated endocytosis and trafficking of the insulin-like growth factor receptor I and insulin receptor modulate receptor function

The insulin-like growth factor system and its two major receptors, the IGF receptor I (IGF-IR) and IR, plays a central role in a variety of physiological cellular processes including growth, differentiation, motility, and glucose homeostasis. The IGF-IR is also essential for tumorigenesis through its capacity to protect cancer cells from apoptosis. The IR is expressed in two isoforms: the IR isoform A (IR-A) and isoform B (IR-B). While the role of the IR-B in the regulation of metabolic effects has been known for several years, more recent evidence suggests that the IR, and in particular the IR-A, may be involved in the pathogenesis of cancer. Ligand-mediated endocytosis of tyrosine-kinases receptors plays a critical role in modulating the duration and intensity of receptors action but while the signaling pathways induced by the IGF-IR and IR are quite characterized, very little is still known about the mechanisms and proteins that regulate ligand-induced IGF-IR and IR endocytosis and trafficking. In addition, how these processes affect receptor downstream signaling has not been fully characterized. Here, we discuss the current understanding of the mechanisms and proteins regulating IGF-IR and IR endocytosis and sorting and their implications in modulating ligand-induced biological responses.

Something old, something new and something borrowed: emerging paradigm of insulin-like growth factor type 1 receptor (IGF-1R) signaling regulation

The insulin-like growth factor type 1 receptor (IGF-1R) plays a key role in the development and progression of cancer; however, therapeutics targeting it have had disappointing results in the clinic. As a receptor tyrosine kinase (RTK), IGF-1R is traditionally described as an ON/OFF system, with ligand stabilizing the ON state and exclusive kinase-dependent signaling activation. Newly added to the traditional model, ubiquitin-mediated receptor downregulation and degradation was originally described as a response to ligand/receptor interaction and thus inseparable from kinase signaling activation. Yet, the classical model has proven over-simplified and insufficient to explain experimental evidence accumulated over the last decade, including kinase-independent signaling, unbalanced signaling, or dissociation between signaling and receptor downregulation. Based on the recent findings that IGF-1R “borrows” components of G-protein coupled receptor (GPCR) signaling, including β-arrestins and G-protein-related kinases, we discuss the emerging paradigm for the IGF-1R as a functional RTK/GPCR hybrid, which integrates the kinase signaling with the IGF-1R canonical GPCR characteristics. The contradictions to the classical IGF-1R signaling concept as well as the design of anti-IGF-1R therapeutics treatment are considered in the light of this paradigm shift and we advocate recognition of IGF-1R as a valid target for cancer treatment.

Molecular basis of signaling specificity of insulin and IGF receptors: neglected corners and recent advances

Insulin and insulin-like growth factor (IGF) receptors utilize common phosphoinositide 3-kinase/Akt and Ras/extracellular signal-regulated kinase signaling pathways to mediate a broad spectrum of "metabolic" and "mitogenic" responses. Specificity of insulin and IGF action in vivo must in part reflect expression of receptors and responsive pathways in different tissues but it is widely assumed that it is also determined by the ligand binding and signaling mechanisms of the receptors. This review focuses on receptor-proximal events in insulin/IGF signaling and examines their contribution to specificity of downstream responses. Insulin and IGF receptors may differ subtly in the efficiency with which they recruit their major substrates (IRS-1 and IRS-2 and Shc) and this could influence effectiveness of signaling to "metabolic" and "mitogenic" responses. Other substrates (Grb2-associated binder, downstream of kinases, SH2Bs, Crk), scaffolds (RACK1, β-arrestins, cytohesins), and pathways (non-receptor tyrosine kinases, phosphoinositide kinases, reactive oxygen species) have been less widely studied. Some of these components appear to be specifically involved in "metabolic" or "mitogenic" signaling but it has not been shown that this reflects receptor-preferential interaction. Very few receptor-specific interactions have been characterized, and their roles in signaling are unclear. Signaling specificity might also be imparted by differences in intracellular trafficking or feedback regulation of receptors, but few studies have directly addressed this possibility. Although published data are not wholly conclusive, no evidence has yet emerged for signaling mechanisms that are specifically engaged by insulin receptors but not IGF receptors or vice versa, and there is only limited evidence for differential activation of signaling mechanisms that are common to both receptors. Cellular context, rather than intrinsic receptor activity, therefore appears to be the major determinant of whether responses to insulin and IGFs are perceived as "metabolic" or "mitogenic."

Insulin and insulin-like growth factor II differentially regulate endocytic sorting and stability of insulin receptor isoform A

The insulin receptor isoform A (IR-A) binds both insulin and insulin-like growth factor (IGF)-II, although the affinity for IGF-II is 3-10-fold lower than insulin depending on a cell and tissue context. Notably, in mouse embryonic fibroblasts lacking the IGF-IR and expressing solely the IR-A (R-/IR-A), IGF-II is a more potent mitogen than insulin. As receptor endocytosis and degradation provide spatial and temporal regulation of signaling events, we hypothesized that insulin and IGF-II could affect IR-A biological responses by differentially regulating IR-A trafficking. Using R-/IR-A cells, we discovered that insulin evoked significant IR-A internalization, a process modestly affected by IGF-II. However, the differential internalization was not due to IR-A ubiquitination. Notably, prolonged stimulation of R-/IR-A cells with insulin, but not with IGF-II, targeted the receptor to a degradative pathway. Similarly, the docking protein insulin receptor substrate 1 (IRS-1) was down-regulated after prolonged insulin but not IGF-II exposure. Similar results were also obtained in experiments using [NMeTyr(B26)]-insulin, an insulin analog with IR-A binding affinity similar to IGF-II. Finally, we discovered that IR-A was internalized through clathrin-dependent and -independent pathways, which differentially regulated the activation of downstream effectors. Collectively, our results suggest that a lower affinity of IGF-II for the IR-A promotes lower IR-A phosphorylation and activation of early downstream effectors vis à vis insulin but may protect IR-A and IRS-1 from down-regulation thereby evoking sustained and robust mitogenic stimuli.

Plasma and tissue insulin-like growth factor-I receptor (IGF-IR) as a prognostic marker for prostate cancer and anti-IGF-IR agents as novel therapeutic strategy for refractory cases: a review

Cancer database analysis indicates that prostate cancer is one of the most seen cancers in men meanwhile composing the leading cause of morbidity and mortality among developed countries. Current available therapies are surgery, radiotherapy and androgene ablation for prostate carcinoma. The response rate is as high nearly 90% however, most of these recur or become refractory and androgene independent (AI). Therefore recent studies intensified on molecular factors playing role on development of prostate carcinoma and novel treatment strategies targetting these factors and their receptors. Insulin-like growth factor-I (IGF-I) and its primary receptor insulin-like growth factor receptor-I (IGF-IR) are among these factors. Biologic functions and role in malign progression are primarily achieved via IGF-IR which is a type 2 tyrosine kinase receptor. IGF-IR plays an important role in mitogenesis, angiogenesis, transformation, apoptosis and cell motility. It also generates intensive proliferative signals leading to carcinogenesis in prostate tissue. So IGF-IR and its associated signalling system have provoked considerable interest over recent years as a novel therapeutic target in cancer. In this paper it is aimed to sum up the lately published literature searching the relation of IGF-IR and prostate cancer in terms of incidence, pathologic features, and prognosis. This is followed by a discussion of the different possible targets within the IGF-1R system, and drugs developed to interact at each target. A systems-based approach is then used to review the in vitro and in vivo data in the published literature of the following compounds targeting IGF-1R components using specific examples: growth hormone releasing hormone antagonists (e.g. JV-1-38), growth hormone receptor antagonists (e.g. pegvisomant), IGF-1R antibodies (e.g. CP-751,871, AVE1642/EM164, IMC-A12, SCH-717454, BIIB022, AMG 479, MK-0646/h7C10), and IGF-1R tyrosine kinase inhibitors (e.g. BMS-536942, BMS-554417, NVP-AEW541, NVP-ADW742, AG1024, potent quinolinyl-derived imidazo (1,5-a)pyrazine PQIP, picropodophyllin PPP, nordihydroguaiaretic acid Insm-18/NDGA). And the other end point is to yield an overview on the recent progress about usage of this receptor as a novel anticancer agent of targeted therapies in treatment of prostate carcinoma.

The insulin receptor: a new target for cancer therapy

A large body of evidences have shown that both the IGF-I receptor (IGF-IR) and the insulin receptor (IR) play a role in cancer development and progression. In particular, IR overactivation by IGF-II is common in cancer cells, especially in dedifferentiated/stem-like cells. In spite of these findings, until very recently, only IGF-IR but not IR has been considered a target in cancer therapy. Although several preclinical studies have showed a good anti-cancer activity of selective anti-IGF-IR drugs, the results of the clinical first trials have been disappointing. In fact, only a small subset of malignant tumors has shown an objective response to these therapies. Development of resistance to anti-IGF-IR drugs may include upregulation of IR isoform A (IR-A) in cancer cells and its overactivation by increased secretion of autocrine IGF-II. These findings have led to the concept that co-targeting IR together with IGF-IR may increase therapy efficacy and prevent adaptive resistance to selective anti-IGF-IR drugs. IR blockade should be especially considered in tumors with high IR-A:IGF-IR ratio and high levels of autocrine IGF-II. Conversely, insulin sensitizers, which ameliorate insulin resistance associated with metabolic disorders and cancer treatments, may have important implications for cancer prevention and management. Only few drugs co-targeting the IR and IGF-IR are currently available. Ideally, future IR targeting strategies should be able to selectively inhibit the tumor promoting effects of IR without impairing its metabolic effects.

Effect of single-chain antibody targeting of the ligand-binding domain in the anaplastic lymphoma kinase receptor

The tyrosine kinase receptor anaplastic lymphoma kinase (ALK) and its ligand, the growth factor pleiotrophin (PTN), are highly expressed during the development of the nervous system and have been implicated in the malignant progression of different tumor types. Here, we describe human single-chain variable fragment (scFv) antibodies that target the ligand-binding domain (LBD) in ALK and show the effect in vitro and in vivo. The ALK LBD was used as a bait in a yeast two-hybdrid system to select human scFv from a library with randomized complementarity-determining region 3 domains. Surface plasmon resonance showed high-affinity binding of the selected scFv. The anti-ALK scFv competed for binding of PTN to ALK in intact cells and inhibited PTN-dependent signal transduction through endogenous ALK. Invasion of an intact endothelial cell monolayer by U87MG human glioblastoma cells was inhibited by the anti-ALK scFv. In addition, the growth of established tumor xenografts in mice was reversed after the induction of the conditional expression of the anti-ALK scFv. In archival malignant brain tumors expression levels of ALK and PTN were found elevated and appear correlated with poor patient survival. This suggests a rate-limiting function of the PTN/ALK interaction that may be exploited therapeutically.

c-Abl and insulin receptor signalling

Insulin Receptor (IR) and IGF-I receptor (IGF-IR) are homolog but display distinct functions: IR is mainly metabolic, while IGF-IR is mitogenic. However, in some conditions like foetal growth, cancer and diabetes, IR may display some non-metabolic effects like proliferation and migration. The molecular mechanisms underlying this 'functional switch of IR' have been attributed to several factors including overexpression of ligands and receptors, predominant IR isoform expression, preferential recruitment of intracellular substrates. Here, we report that c-Abl, a cytoplasmic tyrosine kinase regulating several signal transduction pathways, is involved in this functional switch of IR. Indeed, c-Abl tyrosine kinase is involved in IR signalling as it shares with IR some substrates like Tub and SORBS1 and is activated upon insulin stimulation. Inhibition of c-Abl tyrosine kinase by STI571 attenuates the effect of insulin on Akt/GSK-3beta phosphorylation and glycogen synthesis, and at the same time, it enhances the effect of insulin on ERK activation, cell proliferation and migration. This effect of STI571 is specific to c-Abl inhibition, because it does not occur in Abl-null cells and is restored in c-Abl-reconstituted cells. Numerous evidences suggest that focal adhesion kinase (FAK) is involved in mediating this c-Abl effect. First, c-Abl tyrosine kinase activation is concomitant with FAK dephosphorylation in response to insulin, whereas c-Abl inhibition is accompanied by FAK phosphorylation in response to insulin, a response similar to that observed with IGF-I. Second, the c-Abl effects on insulin signalling are not observed in cells devoid of FAK (FAK(-/-) cells). Taken together these results suggest that c-Abl activation by insulin, via a modification of FAK response, may play an important role in directing mitogenic versus metabolic insulin receptor signalling.

Implications of insulin-like growth factor-I for prostate cancer therapies

In the last decade, abundant evidence has suggested that the insulin-like growth factor (IGF) family comprises a multi-component network of molecules involved in the regulation of both physiological and pathological growth processes in the prostate. The IGF axis plays an important role in the tumorigenesis and neoplastic growth of prostate cancer. Epidemiological observations indicate that circulating IGF-I levels are positively associated with increased risk of prostate cancer. Activation of IGF-I receptor (IGF-IR) by IGF-I has mitogenic and anti-apoptotic effects on normal and malignant prostate cells. Therapeutic alternatives in men with progressive prostate cancer after androgen ablation are very limited and more effective therapies are needed for such patients. Inactivation of the IGF-I axis represents a potential target to treat androgen-independent prostate cancer. This review addresses epidemiological studies of IGF-I and therapeutic strategies including reduction of IGF-I levels, inhibition of IGF-IR and the signaling mechanisms involved.

IGF and insulin receptor signaling in breast cancer

Major molecular abnormalities in breast cancer include the deregulation of several components of the IGF system. It is well recognized that the epithelial breast cancer cells commonly overexpress the IGF-I receptor while IGF-II is expressed by the tumor stroma. In view to the fact that the IGF-IR has mitogenic, pro-invasive and anti-apoptotic effects and mediates resistance to a variety of anti-cancer therapies, breast cancer is expected to be a candidate to therapeutic approaches aimed to inhibit the IGF-IR. However, there is increasing awareness that IGF system in cancer undergoes signal diversification by various mechanisms. One of these mechanisms is the aberrant expression of insulin receptor (IR) isoform A (IR-A), which is a high affinity receptor for both insulin and IGF-II, in breast cancer cells. Moreover, overexpression of both IGF-IR and IR-A in breast cancer cells, leads to overexpression of hybrid IR/IGF-IR receptors (HRs) as well. Upon binding to IGF-II, both IR-A and HRs may activate unique signaling patterns, which predominantly mediate proliferative effects. A better understanding of IGF system signal diversification in breast cancer has important implications for cancer prevention measures, which should include control of insulin resistance and associated hyperinsulinemia. Moreover, in addition to the IGF-IR, both IR-A and HRs should be also considered as molecular targets for anti-cancer therapies.

A novel role for IGF-1R in p53-mediated apoptosis through translational modulation of the p53-Mdm2 feedback loop

Insulin-like growth factor 1 receptor (IGF-1R) is important in cancer cell growth and survival and has been implicated in cancer pathophysiology and treatment. Here we report a novel function for IGF-1R in p53-dependent apoptotic response. We show that inhibition or loss of IGF-1R activity reduces translational synthesis of p53 and Mdm2 protein. Notably, IGF-1R inhibition increases p53 protein stability by reducing p53 ubiquitination and maintains p53 at low levels by decreasing p53 synthesis, thus rendering p53 insensitive to stabilization after DNA damage. The accumulation and apoptosis of DNA-damage-induced p53 is therefore reduced in Igf-1r(-/-) mouse embryonic fibroblasts or tumor cells treated with the IGF-1R inhibitor. Furthermore, we find that inhibition of IGF-1R reduces p53 and Mdm2 translation through a gene-specific mechanism mediated by the respective 5' untranslated region of p53 and mdm2 messenger RNA. The eukaryotic translation initiation factor 4F complex is also involved in this translational inhibition. These results demonstrate an unexpected role for translational control by IGF-1R in p53-mediated apoptosis.

Nampt/PBEF/Visfatin regulates insulin secretion in beta cells as a systemic NAD biosynthetic enzyme

Intracellular nicotinamide phosphoribosyltransferase (iNampt) is an essential enzyme in the NAD biosynthetic pathway. An extracellular form of this protein (eNampt) has been reported to act as a cytokine named PBEF or an insulin-mimetic hormone named visfatin, but its physiological relevance remains controversial. Here we show that eNampt does not exert insulin-mimetic effects in vitro or in vivo but rather exhibits robust NAD biosynthetic activity. Haplodeficiency and chemical inhibition of Nampt cause defects in NAD biosynthesis and glucose-stimulated insulin secretion in pancreatic islets in vivo and in vitro. These defects are corrected by administration of nicotinamide mononucleotide (NMN), a product of the Nampt reaction. A high concentration of NMN is present in mouse plasma, and plasma eNampt and NMN levels are reduced in Nampt heterozygous females. Our results demonstrate that Nampt-mediated systemic NAD biosynthesis is critical for beta cell function, suggesting a vital framework for the regulation of glucose homeostasis.

Modifying IGF1 activity: an approach to treat endocrine disorders, atherosclerosis and cancer

Insulin-like growth factor 1 (IGF1) is a polypeptide hormone that has a high degree of structural similarity to human proinsulin. Owing to its ubiquitous nature and its role in promoting cell growth, strategies to inhibit IGF1 actions are being pursued as potential adjunctive measures for treating diseases such as short stature, atherosclerosis and diabetes. In addition, most tumour cell types possess IGF1 receptors and conditions in the tumour microenvironment, such as hypoxia, can lead to enhanced responsiveness to IGF1. Therefore, inhibiting IGF1 action has been proposed as a specific mechanism for potentiating the effects of existing anticancer therapies or for directly inhibiting tumour cell growth.

Transformation of late passage insulin-like growth factor-I receptor null mouse embryo fibroblasts by SV40 T antigen

There is evidence that the insulin-like growth factor-I (IGF-I) receptor is required for transformation by a variety of viral and cellular oncogenes in a mouse embryo fibroblast model. To further investigate the IGF-I receptor signaling pathways that are required for the permissive effect of the receptor on transformation by SV40 T antigen, we established three independent fibroblast cell lines each from wild-type and IGF-I receptor null embryos (R-). We transfected the wild-type and R- cell lines with an SV40 T antigen plasmid and selected three clones from each cell line that expressed T antigen. As in previous reports, none of the cloned R- cell lines expressing T antigen were transformed as measured by the ability to form large colonies in soft agar. However, with further passage, all three T antigen-expressing clones from one of the R- cell lines (R(-)3) formed large colonies in soft agar and the transformation of these T antigen-expressing clones was confirmed by tumorigenesis experiments in immunodeficient mice. DNA microarray analysis comparing gene expression between early passage and late passage R(-)3/T antigen clones showed, among other changes, an increase in the expression of ErbB-3 mRNA in the late passage clones. Also, the expression of ErbB-3 protein was dramatically increased in the late passage R(-)3/T antigen clones. We conclude that late passage IGF-I receptor null mouse embryo fibroblasts can be transformed by SV40 T antigen, and that ErbB-3 may play a role in permitting transformation by T antigen.

The insulin-like growth factor system and its pleiotropic functions in brain

In recent years, much interest has been devoted to defining the role of the IGF system in the nervous system. The ubiquitous IGFs, their cell membrane receptors, and their carrier binding proteins, the IGFBPs, are expressed early in the development of the nervous system and are therefore considered to play a key role in these processes. In vitro studies have demonstrated that the IGF system promotes differentiation and proliferation and sustains survival, preventing apoptosis of neuronal and brain derived cells. Furthermore, studies of transgenic mice overexpressing components of the IGF system or mice with disruptions of the same genes have clearly shown that the IGF system plays a key role in vivo.

Interaction of 14-3-3 proteins with the insulin-like growth factor I receptor (IGFIR): evidence for a role of 14-3-3 proteins in IGFIR signaling

We have extended our previous yeast two-hybrid findings to show that 14-3-3beta also interacts with the insulin-like growth factor I receptor (IGFIR) in mammalian cells overexpressing both proteins and that the interaction involves serine 1283 and is dependent on receptor activation. Treatment of cells with the phorbol ester PMA stimulates the interaction of 14-3-3beta with the IGFIR in the absence of receptor tyrosine phosphorylation, suggesting that receptor activation leads to activation of an endogenous protein kinase that catalyzes the phosphorylation of serine 1283. To investigate the role of 14-3-3 proteins in IGF signal transduction, IGFIR structure-function studies were performed. Mutation of serine 1283 alone (S1283A) (a mutation that decreases but does not abolish the interaction of the IGFIR with 14-3-3) did not affect anchorage-independent growth of NIH 3T3 fibroblasts overexpressing the mutant receptor. However, the simultaneous mutation of this residue and the truncation of the C-terminal 27 residues of the receptor (Delta1310/S1283A) abolished the interaction of the receptor with 14-3-3 and reversed the enhanced colony formation observed with the IGFIR truncation mutation alone (Delta1310). The difference between the Delta1310 and Delta1310/S1283A transfectants in the soft agar assay was confirmed by tumorigenesis experiments. These findings suggest that 14-3-3 proteins interact with the IGFIR in vivo and that this interaction may play a role in a transformation pathway signaled by the IGFIR.

Impaired Shc, Ras, and MAPK activation but normal Akt activation in FL5.12 cells expressing an insulin-like growth factor I receptor mutated at tyrosines 1250 and 1251

The Y1250F/Y1251F mutant of the insulin-like growth factor I receptor (IGF-IR) has tyrosines 1250 and 1251 mutated to phenylalanines and is deficient in IGF-I-mediated suppression of apoptosis in FL5.12 lymphocytic cells. To address the mechanism of loss of function in this mutant we investigated signaling responses in FL5.12 cells overexpressing either a wild-type (WT) or Y1250F/Y1251F (mutant) IGF-IR. Cells expressing the mutant receptor were deficient in IGF-I-induced phosphorylation of the JNK pathway and had decreased ERK and p38 phosphorylation. IGF-I induced phosphorylation of Akt was comparable in WT and mutant expressing cells. The decreased activation of the mitogen-activated protein kinase (MAPK) pathways was accompanied by greatly decreased Ras activation in response to IGF-I. Although phosphorylation of Gab2 was similar in WT and mutant cell lines, phosphorylation of Shc on Tyr(313) in response to IGF-I was decreased in cells expressing the mutant receptor, as was recruitment of Grb2 and Ship to Shc. However, phosphorylation of Shc on Tyr(239), the Src phosphorylation site, was normal. A role for JNK in the survival of FL5.12 cells was supported by the observation that the JNK inhibitor SP600125 suppressed IGF-I-mediated protection from apoptosis. Altogether these data demonstrate that phosphorylation of Shc, and assembly of the Shc complex necessary for activation of Ras and the MAPK pathways are deficient in cells expressing the Y1250F/Y1251F mutant IGF-IR. This would explain the loss of IGF-I-mediated survival in FL5.12 cells expressing this mutant and may also explain why this mutant IGF-IR is deficient in functions associated with cellular transformation and cell migration in fibroblasts and epithelial tumor cells.

Roles of the insulin-like growth factor I receptor C-terminus in cellular radioresistance

Available evidence suggests that insulin-like growth factor I receptor (IGF-IR) expression leads to increased cellular radioresistance. The most direct explanation of these findings predicts that IGF-IR is the source of survival signals in resistant cells. Mutational analysis revealed that protein truncated at amino acid 1245 in the C-terminus retained the ability of IGF-IR to confer radioresistance whereas point mutations at both Tyr-1250 and Tyr-1251 abrogated this effect using IGF-IR-deficient mouse embryo fibroblasts (R-) as a recipient. In cells expressing the latter mutant receptors, both phosphatidylinositol-3(') kinase (PI3-K) and mitogen-activated protein kinase (MAPK) signaling pathways remained intact, and addition of exogenous IGF-I could not change the radiosensitivity of these cells. Further analysis indicated that the abrogation of radioresistance required the presence of His-1293 and Lys-1294. These results suggest a novel regulatory role of the C-terminus of IGF-IR in mediating cellular radioresistance that may be independent of survival signals transmitted through this receptor.

Redundancy of radioresistant signaling pathways originating from insulin-like growth factor I receptor

The insulin-like growth factor I receptor (IGF-IR) has the ability to confer clonogenic radioresistance following ionizing irradiation. We attempted to determine the downstream pathways involved in IGF-IR-mediated radioresistance and used mouse embryo fibroblasts deficient in endogenous IGF-IR (R-) as recipients for a number of mutant IGF-IRs. Mutational analysis revealed that the tyrosine at residue 950 (Tyr-950) of IGF-IR, as well as the C-terminal domain, are required for radioresistance and that both domains must be mutated to abrogate the phenotype. Furthermore, the contribution of downstream pathways was analyzed by combining the use of wild-type or Tyr-950 and C-terminal mutants with specific inhibitors of phosphatidylinositol 3'-kinase (PI3-K) or mitogen-activated protein extracellular signal-regulated kinase (ERK) kinase (MEK). Radioresistance could be induced by IGF-IR as long as the ability of the receptor to stimulate the MEK/ERK pathway was retained. This was confirmed by the expression of constitutively active MEK in R- cells. The ability to stimulate the PI3-K pathway alone was not sufficient, but PI3-K activation coupled with MEK/ERK pathway-independent signals from the C terminus was able to induce radioresistance. Taken together, these results indicate that the IGF-IR-mediated radioresistant signaling mechanism progresses through redundant downstream pathways.

Reduced growth of human sarcoma xenografts in hosts homozygous for the lit mutation

BACKGROUND AND OBJECTIVES

Prior studies have shown that sarcoma growth can be stimulated by insulin-like growth factor-I (IGF-I). To extend this line of research, we carried out in vivo growth studies of primary human sarcoma in immunosuppressed control and IGF-I-deficient mice.

METHODS

Human sarcoma specimens (one osteosarcoma and seven soft tissue sarcomas) were harvested in the operating room and implanted in immunosuppressed mice. Second-generation sarcomas were transplanted to control (GH replete lit/+ mice) and to experimental (GH/IGF-I-deficient lit/lit) animals. When tumors reached 1,000 mm(3) in one group, average tumor size was compared in the two groups. IGF-I receptor expression was measured by RT-PCR and IGF-I receptor binding sites were assayed by radiolabeled IGF-I.

RESULTS

Five of eight sarcomas demonstrated reduced growth in the GH/IGF-I-deficient lit/lit animals. In four of the five sarcomas that demonstrated growth inhibition, IGF-R was elevated relative to placenta or a positive control cell line (MCF-7, which is known to be responsive to IGF-I in vitro and in vivo). In three of the five sarcomas that demonstrated growth suppression, IGF-R was elevated twofold after implantation in the experimental IGF-I-deficient animals.

CONCLUSIONS

The GH-IGF axis may be an important stimulator of tumor growth in sarcomas. These experiments suggest that IGF suppression may inhibit sarcoma growth in vivo.

Insulin-like growth factor I receptor is expressed at normal levels in Nijmegen breakage syndrome cells

Nijmegen breakage syndrome (NBS) is an autosomal recessive disorder sharing a pleiotropic phenotype with ataxia-telangiectasia (A-T), including increased radiosensitivity and cancer disposition. Insulin-like growth factor I receptor (IGF-IR) expression is reportedly decreased in A-T cells, which is thought to contribute to its increased radiosensitivity. In this study, we investigated whether the same mechanism underlies the radiosensitivity of NBS cells. GM7166VA7 cells lacking NBS1 protein displayed a phenotype of increased radiosensitivity, while the introduction of NBS1 cDNA conferred radioresistance comparable to normal cells. IGF-IR expression levels were essentially the same among normal, NBS, and NBS1-complemented NBS cells. There was no significant difference between NBS and NBS1-complemented cells in activation of major downstream pathways of IGF-IR upon IGF-I stimulation, including phosphatidylinositol-3(') kinase (PI3-K) and mitogen-activated protein kinase (MAPK). Collectively, IGF-IR-related events are unlikely to be disrupted in NBS cells, and therefore, defects in IGF-IR signaling do not explain the increased radiosensitivity of NBS cells.

Regulation of Id1 protein expression in mouse embryo fibroblasts by the type 1 insulin-like growth factor receptor

The activated type 1 insulin-like growth factor (IGF-IR) increases the expression of Id1 proteins in mouse embryo fibroblasts (MEF). Up-regulation depends on a functional receptor and on multiple pathways originating from different domains of the receptor. In MEF, Id1 protein expression is also up-regulated by serum and certain oncogenes. Signaling through Stat3 plays an important, but not exclusive, role in the up-regulation of Id1 protein levels. In all instances, the increase in Id1 protein expression is paralleled by a corresponding increase in Id1 promoter activity, as measured with a reporter gene.

Zebrafish insulin-like growth factor-I receptor: molecular cloning and developmental expression

The biological actions of the insulin-like growth factors (IGFs) are mediated primarily by the IGF-I receptor (IGF-IR), and the IGF family has been highly conserved throughout vertebrate evolution. In this study we report the isolation of a 3 kb cDNA clone for the zebrafish IGF-IR that includes the complete 3' untranslated region and polyA tail and mapping of the receptor gene to zebrafish linkage group 7. The open reading frame deduced from the cDNA sequence encompasses the juxtamembrane and protein tyrosine kinase portions of the receptor, and is 70 and 67% identical to the corresponding regions of the IGF-IRs of the turbot and Xenopus, respectively. By RT-PCR, zebrafish IGF-IR expression was detected from early blastula to early larval stages of development. Using whole mount in situ hybridization, IGF-IR expression was detected after gastrulation. Expression was evident in most tissues but was particularly evident in the tail, in eye and ear primordia and in the brain.

Cooperative regulation of the invasive and metastatic phenotypes by different domains of the type I insulin-like growth factor receptor beta subunit

The receptor for the type 1 insulin-like growth factor (IGF-I) regulates multiple cellular functions impacting on the metastatic phenotype of tumor cells, including cellular proliferation, anchorage-independent growth, survival, migration, synthesis of the 72-kDa type IV collagenase and invasion. We have used site-directed mutagenesis to generate domain-specific mutants of the receptor beta subunit to analyze the role of specific tyrosines in the regulation of the invasive/metastatic phenotype. Poorly invasive M-27 carcinoma cells expressing low receptor numbers were transfected with a plasmid vector expressing IGF-I receptor cDNA in which single or multiple tyrosine codons in the kinase domain, namely Tyr-1131, Tyr-1135, and Tyr-1136 or the C-terminal tyrosines 1250 and 1251 were substituted with phenylalanine. Changes in the invasive and metastatic properties were analyzed relative to M-27 cells expressing the wild type receptor. We found that cells expressing the Y1131F,Y1135F,Y1136F or Y1135F receptor mutants lost all IGF-IR-dependent functions and their phenotypes were indistinguishable from, or suppressed relative to, the parent line. The Y1250F,Y1251F substitution abolished anchorage-independent growth, cell spreading, and the anti-apoptotic effect of IGF-I whereas all other IGF-IR-dependent phenotypes were either unperturbed (i.e. mitogenicity) or only partially reduced (migration and invasion). The results identify three types of receptor-dependent functions in this model: those dependent only on an intact kinase domain (DNA synthesis), those dependent equally on kinase domain and Tyr-1250/1251 signaling (e.g. apoptosis, soft agar cloning) and those dependent on kinase domain and enhanced through Tyr-1250/1251 signaling (migration, invasion). They suggest that signals derived from both regions of the receptor cooperate to enhance tumor metastasis.

A PDZ domain protein interacts with the C-terminal tail of the insulin-like growth factor-1 receptor but not with the insulin receptor

In this study, we report on the isolation of a PDZ domain protein, here designated as IIP-1, insulin-like growth factor-1 (IGF-1) receptor-interacting protein-1, which binds to the IGF-1 receptor, but not to the related insulin receptor, and which is involved in the regulation of cell motility. The interaction between the IGF-1 receptor and IIP-1 as well as a splice variant IIP-1/p26 was demonstrated in the yeast two-hybrid system. Using co-precipitation experiments, we confirmed the interaction in transfected cells as well as in vitro. Analysis of deletion mutants indicates that the PDZ domain of IIP-1 mediates interaction with the C-terminal tail of the IGF-1 receptor (serine-threonine-cysteine). This finding demonstrates that the C terminus of the IGF-1 receptor acts as novel PDZ domain binding site. Immunofluorescence analysis revealed an overlapping localization of IIP-1 and the IGF-1 receptor in the breast cancer cell line MCF-7. A functional connection between IIP-1 and the IGF-1 receptor is further supported by the finding that the level of expression of IIP-1 and the IGF-1 receptor strongly correlates in different normal and cancer cells. Furthermore, overexpression of IIP-1 resulted in an attenuation of migration of MCF-7 cells, which is one of the biological activities mediated by the IGF-1 signaling system.

Anchorage-independent growth of fibroblasts that express a truncated IGF-I receptor

The purpose of this investigation was to study signaling by an insulin-like growth factor I receptor (IGF-I R) that lacks the extracellular portion of the receptor. We transfected IGF-I R-negative mouse embryo fibroblasts with a truncated IGF-I R consisting of only the transmembrane and cytoplasmic part of the beta subunit. Proliferation as assessed by counting cells was the same for vector only transfectants and the truncated receptor transfectants in defined medium containing EGF and PDGF. In contrast, anchorage-independent growth as measured by colony formation in soft agar was markedly increased for the truncated IGF-I R transfectants compared to the vector transfectants. MAP-kinase activity in the truncated IGF-I R transfectants was not higher than in the vector transfectants; however, PI 3-kinase activity was significantly higher in the IGF-I R transfectants. These results provide evidence that an IGF-I receptor consisting of only the transmembrane and cytoplasmic domain of the beta subunit can signal pathways leading to anchorage-independent growth.

Characterization of insulin-like growth factor I (IGF-I) receptor mutants for their effects on IGF-I- and interleukin 4-mediated DNA synthesis of 32D cells

Recently we demonstrated that overexpression of the wild type insulin-like growth factor I receptor (IGF-IRWT) in 32D myeloid progenitor cells led to cell proliferation in response to interleukin 4 (IL-4) as well as insulin-like growth factor I (IGF-I) in the absence of insulin receptor substrate expression (Soon, L., Flechner, L., Gutkind, J. S., Wang, L. H., Baserga, R., Pierce, J. H., and Li, W. (1999) Mol. Cell. Biol. 19, 3816-3828). To understand the structural importance of insulin-like growth factor I receptor (IGF-IR) in mediating IL-4- and IGF-I-induced DNA synthesis, we transfected various mutants of IGF-IR to 32D cells. Our results show that most mutants, including Y1250F, Y1251F, Y1250F/Y1251F, S1280A/S1281A/S1282A/S1283A, Y1316F, and 1245d, still retained mitogenic response toward IGF-I or IL-4. However, the Y950F, Y1131F, and Y1135F mutants were not able to respond to either ligand. The H1293F/K1294R and 1293d mutants reduced response toward IGF-I but not to IL-4. Phosphorylation of Shc was greatly reduced in those three mutants that lost mitogenic response. The MAPK activity was much lower in Y1131F and Y1135F mutants, indicating the importance of the Shc/MAPK pathway in IGF-I-induced mitogenesis. Importantly, the synergistic effect of these two factors on DNA synthesis was not affected in cells expressing most of the mutants, even in those three that had lower mitogenic response toward a single ligand. These results suggest that an unidentified pathway(s) may be induced upon co-addition of IGF-I and IL-4 that sustains the intact mitogenesis.

The contradictions of the insulin-like growth factor 1 receptor

In recent years, the type 1 insulin-like growth factor receptor (IGF-IR) has emerged as a receptor that plays a very important role in the growth of cells, both in vivo and in vitro. The ability of the IGF-IR to induce mitogenesis and to promote survival of cells against a variety of apoptotic agents is well documented. Somewhat less known are other functions of the IGF-IR, like its ability to induce differentiation, to regulate cell size and to affect the organization of the cytoskeleton of cells. This review will focus on these lesser known functions of the IGF-IR. At the same time, we will emphasize how the IGF-IR can send contradictory signals, which depend on different domains of the receptor and the availability of downstream transducing molecules.

Inhibition of the type I insulin-like growth factor receptor expression and signaling: novel strategies for antimetastatic therapy

The receptor for the type 1 insulin-like growth factor (IGF-1R) plays a critical role in the acquisition of the malignant phenotype. Using a highly metastatic murine lung carcinoma model, it was demonstrated that this receptor regulates several cellular functions that can impact on the metastatic potential of the cells, including cellular proliferation, anchorage-independent growth, cell migration, and invasion. The tumor model was used to develop several strategies for altering receptor expression and function as means of abrogating the metastatic potential of the cells. They include stable expression in the tumor cells of IGF-1R antisense RNA and dominant negative receptor mutants in which tyrosines in the kinase domain were substituted with phenylalanine. In addition, a novel strategy was used based on altering post ligand-binding receptor turnover. This led to inhibition of receptor re-expression and signaling and resulted in increased tumor cell apoptosis. When combined with the development of viral vectors designed to deliver genetic information with high efficiency, these strategies could form the basis for development of highly specific, antimetastatic therapy in tumors with known IGF-IR involvement.

IGF-I receptor activation and BCL-2 overexpression prevent early apoptotic events in human neuroblastoma

The type I insulin-like growth factor receptor (IGF-IR) is important for mitogenesis, transformation, and survival of tumor cells. The current study examines the effect of IGF-IR expression and activation on apoptosis in SHEP human neuroblastoma cells. SHEP cells undergo apoptosis which is prevented by IGF-I addition or overexpression of the IGF-IR (SHEP/IGF-IR cells). High mannitol treatment activates caspase-3 by 1 h in SHEP cells while caspase-3 activation is delayed by 3 h in SHEP/IGF-IR cells. Transfection with Bcl-2 (SHEP/Bcl-2 cells) prevents serum withdrawal and mannitol induced apoptosis and caspase-3 activation. Mannitol induces mitochondrial membrane depolarization in both SHEP and SHEP/IGF-IR cells. IGF-IR activation or overexpression of Bcl-2 in SHEP cells prevents mitochondrial membrane depolarization. Collectively, these results suggest that IGF-IR or Bcl-2 overexpression in neuroblastoma cells promotes cell survival by preventing mitochondrial membrane depolarization and caspase-3 activation, ultimately leading to increased tumor growth.

Domains of the insulin-like growth factor I receptor required for the activation of extracellular signal-regulated kinases

The type 1 insulin-like growth factor receptor (IGF-IR) activates the extracellular signal-regulated kinases (ERK1 and -2). The two major substrates of the IGF-IR, insulin receptor substrate-1 (IRS-1) and the Shc proteins, are known to contribute to this activation. We investigated the domains of the IGF-IR required for the activation of the ERK proteins. To facilitate this study, we used a cell line (32D cells) that lacks IRS-1. In the absence of IRS-1, ERK activation is inhibited if the IGF-IR is mutated at two domains: tyrosine Y950 and a serine quartet at 1280-1283. Expression of IRS-1 in 32D cells expressing the double mutant IGF-IR restores ERK activation. The importance of the C-terminus of the IGF-IR in ERK activation (in the absence of IRS-1) is confirmed by the failure of the insulin receptor to give a sustained activation of ERK. In this model system, there is a good, but not exact, correlation between ERK activation and cell survival after withdrawal of growth factors.