Constitutive phosphorylation of the receptor for insulinlike growth factor I in cells transformed by the src oncogene

Short-Term Biomarker Modulation Study of Dasatinib for Estrogen Receptor-Negative Breast Cancer Chemoprevention

Objective

Risk-reducing therapy with selective estrogen receptor (ER) modulators and aromatase inhibitors reduce breast cancer risk. However, the effects are limited to ER-positive breast cancer. Therefore, new agents with improved toxicity profiles that reduce the risk in ER-negative breast cancers are urgently needed. The aim of this prospective, short-term, prevention study was to evaluate the effect of dasatinib, an inhibitor of the tyrosine kinase Src, on biomarkers in normal (but increased risk) breast tissue and serum of women at high risk for a second, contralateral primary breast cancer.

Materials and Methods

Women with a history of unilateral stage I, II, or III ER-negative breast cancer, having no active disease, and who completed all adjuvant therapies were eligible. Patients underwent baseline fine-needle aspiration (FNA) of the contralateral breast and serum collection for biomarker analysis and were randomized to receive either no treatment (control) or dasatinib at 40 or 80 mg/day for three months. After three months, serum collection and breast FNA were repeated. Planned biomarker analysis consisted of changes in cytology and Ki-67 on breast FNA, and changes in serum levels of insulin-like growth factor 1 (IGF-1), IGF-binding protein 1, and IGF-binding protein 3. The primary objective was to evaluate changes in Ki-67 and secondary objective included changes in cytology in breast tissue and IGF-related serum biomarkers. Toxicity was also evaluated.

Results

Twenty-three patients started their assigned treatments. Compliance during the study was high, with 86.9% (20/23) of patients completing their assigned doses. Dasatinib was well tolerated and no drug-related grade 3 and 4 adverse events were observed. Since only one patient met the adequacy criteria for the paired FNA sample, we could not evaluate Ki-67 level or cytological changes. No significant change in serum biomarkers was observed among the three groups.

Conclusion

Dasatinib was well tolerated but did not induce any significant changes in serum biomarkers. The study could not fulfill its primary objective due to an inadequate number of paired FNA samples. Further, larger studies are needed to evaluate the effectiveness of Src inhibitors in breast cancer prevention.

Epidermal growth factor receptor and K-Ras mutations and resistance of lung cancer to insulin-like growth factor 1 receptor tyrosine kinase inhibitors

BACKGROUND

Most patients with nonsmall cell lung cancer (NSCLC) have responded poorly to epidermal growth factor receptor (EGFR) tyrosine kinase inhibitors (TKIs). The authors investigated the involvement of insulinlike growth factor 1 receptor (IGF-1R) signaling in primary resistance to EGFR TKIs and the molecular determinants of resistance to IGF-1R TKIs.

METHODS

Phosphorylated IGF-1R/insulin receptor (pIGF-1R/IR) was immunohistochemically evaluated in an NSCLC tissue microarray. The authors analyzed the antitumor effects of an IGF-1R TKI (PQIP or OSI-906), either alone or in combination with a small-molecular inhibitor (PD98059 or U0126) or with siRNA targeting K-Ras or mitogen-activated protein kinase/extracellular signal-regulated kinase kinase (MEK), in vitro and in vivo in NSCLC cells with variable histologic features and EGFR or K-Ras mutations.

RESULTS

pIGF-1R/IR expression in NSCLC specimens was associated with a history of tobacco smoking, squamous cell carcinoma histology, mutant K-Ras, and wild-type (WT) EGFR, all of which have been strongly associated with poor response to EGFR TKIs. IGF-1R TKIs exhibited significant antitumor activity in NSCLC cells with WT EGFR and WT K-Ras but not in those with mutations in these genes. Introduction of mutant K-Ras attenuated the effects of IGF-1R TKIs on NSCLC cells expressing WT K-Ras. Conversely, inactivation of MEK restored sensitivity to IGF-TKIs in cells carrying mutant K-Ras.

CONCLUSIONS

The mutation status of both EGFR and K-Ras could be a predictive marker of response to IGF-1R TKIs. Also, MEK antagonism can abrogate primary resistance of NSCLC cells to IGF-1R TKIs.

Control of apoptosis in human multiple myeloma by insulin-like growth factor I (IGF-I)

Human multiple myeloma (MM) is characterized by the expansion of neoplastic plasmablasts/plasma cells with complex genetic aberrations and high dependence for survival and growth on cytokines produced in the bone marrow microenvironment. As tools in the study of MM about 80 authentic MM cell lines and a few relevant in vivo mouse models are available. The dependence on insulin-like growth factor receptor (IGF-IR) signaling in the development and maintenance of the malignant phenotype in a variety of cancers is a rationale for attempts to improve tumor treatment by selectively inhibiting the IGF-IR in malignant cells by neutralizing antibodies, dominant negative IGF-IR, and IGF-IR siRNA. Testing the hypothesis that abrogating IGF-IR-mediated signaling of survival should make MM cells more susceptible to apoptosis, our studies have so far provided proof-of-principle by the demonstration that inhibition of a signaling pathway stimulating survival renders cells susceptible to drug-induced apoptosis when the drug (dexamethasone) and inhibitor (rapamycin) converge on the same target, that is p70(S6K). The recent publication of the three-dimensional structure of the IGF-IR kinase domain has facilitated the development of IGF-IR inhibitors of the cyclolignan family, that is picropodophyllin, with capacity to distinguish also in vivo between the IGF-IR and the insulin receptor. Studies in vitro and in vivo with picropodophyllin show promising effects, that is apoptosis induction and growth arrest, and have made it possible to evaluate the biological and therapeutic effects of inhibition of the IGF-IR signaling in MM.

IGF-I-induced oligodendrocyte progenitor proliferation requires PI3K/Akt, MEK/ERK, and Src-like tyrosine kinases

Insulin-like growth factor-I (IGF-I) is required for the growth of oligodendrocytes, although the underlying mechanisms are not fully understood. Our aim was to investigate the role of phosphatidylinositol 3-kinase (PI3K), mitogen-activated protein kinase kinase (MEK1), and Src family tyrosine kinases in IGF-I-stimulated proliferation of oligodendrocyte progenitors. IGF-I treatment increased the proliferation of cultured oligodendrocyte progenitors as determined by measuring incorporation of [(3)H]-thymidine and bromodeoxy-uridine (BrdU). IGF-I stimulated a transient phosphorylation of 3-phosphoinositide-dependent kinase-1 (PDK1) and extracellular signal-regulated kinases (ERK1/2) (targets of MEK1), as well as a rapid and sustained activation of Akt (a target of PI3K). Furthermore, inhibitors of PI3K (LY294002 and Wortmannin), MEK1 (PD98059 and U0126), and Src family tyrosine kinases (PP2) decreased IGF-I-induced proliferation, and blocked ERK1/2 activation. LY294002, Wortmannin and PP2 also blocked Akt activation. To further determine whether Akt is required for IGF-I stimulated oligodendrocyte progenitor proliferation, cultures were infected with adenovirus vectors expressing dominant-negative mutants of Akt or treated with pharmacological inhibitors of Akt. All treatments reduced IGF-I-induced oligodendrocyte progenitor proliferation. Our data indicate that stimulation of oligodendrocyte progenitor proliferation by IGF-I requires Src-like tyrosine kinases as well as the PI3K/Akt and MEK1/ERK signaling pathways.

Targeting insulin-like growth factor pathways

Some cancer cells depend on the function of specific molecules for their growth, survival, and metastatic potential. Targeting of these critical molecules has arguably been the best therapy for cancer as demonstrated by the success of tamoxifen and trastuzumab in breast cancer. This review will evaluate the type I IGF receptor (IGF-IR) as a potential target for cancer therapy. As new drugs come forward targeting this receptor system, several issues will need to be addressed in the early clinical trials using these agents.

Activation of insulin-like growth factor II signaling by mutant type p53: physiological implications for potentiation of IGF-II signaling by p53 mutant 249

The aim of the present study was to investigate the intracellular mediators of the third base mutant of codon 249 in p53 gene (p53mt249) mutation that potentiate IGF-II dependent IGF-I receptor (IGF-IR) signaling. p53mt249 enhanced IGF-II dependent IGF-IR signaling in p53 negative Hep3B hepatoma cells which were specifically prevented by IGF-IR antibody, alpha IR3 and lovastin. p53mt249 increased the number of IGF-II binding sites with no change in the affinity of IGF-IR. Enhanced levels of IGF-IR expression and transcription were identified in p53mt249 transfected Hep3B cells. Pre-transfection of cultured hepatoma cells with p53mt249 resulted in a three to fourfold increase in IGF-IR phosphorylation and downstream mediator IRS-I phosphorylation but, enhanced more than 15-fold after IGF-II treatment, which coincides well with the cell growth and thymidine uptake results. Our results showed that p53mt249 modulate IGF-II dependent IGF-IR signaling by upregulating IGF-IR and potentiating IGF-IRs where IGF-IRs became more sensitive on treatment with IGF-II. We concluded that p53mt249 stimulates IGF-II dependent IGF-IR signaling by upregulating the expression of both ligand (IGF-II) and receptor (IGF-IR) through an autocrine and/or paracrine loop and we outline the physiological significance of potentiation of IGF-IR by p53 mutation in the development of hepatocellular carcinoma (HCC).

Potentiation of neuronal L calcium channels by IGF-1 requires phosphorylation of the alpha1 subunit on a specific tyrosine residue

Insulin-like growth factor 1 (IGF-1) rapidly potentiates N and L calcium channel currents in cerebellar granule neurons by an unknown mechanism. Here, we show that the L channel alpha1C subunit is tyrosine phosphorylated in response to IGF-1. Moreover, expression of kinase-dead c-Src in neurons or acute block of Src family kinases with a cell-permeable inhibitor specifically blocks L channel potentiation. Purified Src kinase phosphorylates tyrosine residue Y2122 of the C terminus of neuronal alpha1C in vitro, and c- and v-Src directly bind the C terminus. When expressed in neuroblastoma cells, point mutation of Y2122 prevents both tyrosine phosphorylation of alpha1C and IGF-1 potentiation. Our data provide a biochemical mechanism whereby phosphorylation of a single specific tyrosine residue rapidly modifies ion channel physiology.

C-terminal Src kinase associates with ligand-stimulated insulin-like growth factor-I receptor

Increased expression of the insulin-like growth factor-I receptor (IGF-IR) protein-tyrosine kinase occurs in several kinds of cancer and induces neoplastic transformation in fibroblast cell lines. The transformed phenotype can be reversed by interfering with the function of the IGF-IR. The IGF-IR is required for transformation by a number of viral and cellular oncoproteins, including SV40 large T antigen, Ras, Raf, and Src. The IGF-IR is a substrate for Src in vitro and is phosphorylated in v-Src-transformed cells. We observed that the IGF-IR and IR associated with the C-terminal Src kinase (CSK) following ligand stimulation. We found that the SH2 domain of CSK binds to the tyrosine-phosphorylated form of IGF-IR and IR. We determined the tyrosine residues in the IGF-IR and in the IR responsible for this interaction. We also observed that fibroblasts stimulated with IGF-I or insulin showed a rapid and transient decrease in c-Src tyrosine kinase activity. The results suggest that c-Src and CSK are involved in IGF-IR and IR signaling and that the interaction of CSK with the IGF-IR may play a role in the decrease in c-Src activity following IGF-I stimulation.

2,3,7,8-Tetrachlorodibenzo-p-dioxin suppresses apoptosis and leads to hyperphosphorylation of p53 in rat hepatocytes

Inhibition of apoptosis of preneoplastic cells is thought to represent a major mechanism of action of tumor promoters. 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), the most potent promoter of liver carcinogenesis in rodents, suppressed apoptosis in rat hepatocytes pretreated in vitro with an apoptogenic dose of UV light. This effect, which was also observable in DNA fragmentation analysis, coincided with a pronounced inhibition of the p53 increase usually seen after UV irradiation of rat hepatocytes. Interestingly, TCDD also led to a very minor but consistent enhancement of DNA fragmentation and to a slight increase in p53. Furthermore, TCDD resulted in a dose-dependent increase in p53 phosphorylation in intact cells. The concentration-response curves of the effects of TCDD on p53 phosphorylation and aromatic hydrocarbon receptor (AhR)-dependent induction of cytochrome P450 1A1 activity were almost superimposable, suggesting that TCDD induces p53 phosphorylation via an AhR-linked kinase activity. In an extract prepared from rat liver homogenate, 1 nM TCDD also stimulated p53 phosphorylation. Since the tyrosine kinase c-src was previously shown by others to be activated upon binding of TCDD to the AhR, extracts were pretreated with anti-src-antibodies. This treatment almost completely abrogated the effect of TCDD on p53 phosphorylation suggesting a key role for AhR-associated c-src. This mode of action may result in the observed suppression of the p53 response to apoptogenic UV irradiation, and may contribute to the inhibition of apoptosis.

Dysregulation of the type 1 IGF receptor as a paradigm in tumor progression

The Type 1 IGF receptor plays a critical role in cell progression. During normal ontogeny it is expressed by every proliferating cell, where it functions as a potent cell survival agent. Disruption of the Type 1 IGF receptor gene by homologous recombination results in severely growth retarded animals which invariably die at birth. Most importantly, fibroblasts derived from mice embryos lacking the receptor cannot be transformed by any of a number of oncogenes, indicating that the Type 1 IGF receptor has a crucial role in the transformation process. Consistently, the receptor displays potent mitogenic and antiapoptotic activities [corrected]. A number of transcription factors have been identified that control the expression of the IGF receptor promoter, thus stimulating cellular proliferation. On the other hand, certain tumour suppressors including p53 and WT1 were shown to repress the activity of the IGF receptor promoter. Mutant forms of these and other tumour suppressors are potentially impaired in their ability to suppress expression of the IGF receptor gene, thus helping to expand neoplastic populations.

Cellular functions regulated by Src family kinases

Src family protein tyrosine kinases are activated following engagement of many different classes of cellular receptors and participate in signaling pathways that control a diverse spectrum of receptor-induced biological activities. While several of these kinases have evolved to play distinct roles in specific receptor pathways, there is considerable redundancy in the functions of these kinases, both with respect to the receptor pathways that activate these kinases and the downstream effectors that mediate their biological activities. This chapter reviews the evidence implicating Src family kinases in specific receptor pathways and describes the mechanisms leading to their activation, the targets that interact with these kinases, and the biological events that they regulate.

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) causes an Ah receptor-dependent and ARNT-independent increase in membrane levels and activity of p60(Src)

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is known to affect various cellular activities including growth factor signal transduction, hormone responses, and cell differentiation. The purpose of this study was to examine more closely the very early effects of TCDD on protein tyrosine kinase activity, specifically p60(Src). We found that TCDD causes rapid changes in the plasma-microsomal membrane levels and activity of p60(Src) in Hepa 1c1c7, Hepa c4 cells, and SR3Y1 cells, a p60(v-Src) overexpressing cell line. Such cellular changes occur within 30 minutes after 10 nM TCDD treatment, as measured by Western blot analysis. TCDD's ability to increase p60(Src) levels was found to be: (1) dose-dependent, with an estimated EC(50) between 10(-10) and 10(-11) M TCDD; (2) Ah receptor (AhR)-dependent, since TCDD's effect was blocked by co-administration with 1 μM α-naphthoflavone, an AhR antagonist; and interestingly (3) ARNT-independent, since TCDD's effect was observed in Hepa c4 cells, an ARNT(-) mutant cell line. Since ARNT is a heterodimerization partner of the AhR required for binding of the ligand-activated AhR to dioxin-responsive elements on DNA in the nucleus to transactivate genes controlled by the AhR, an alternative mechanism for TCDD's action is discussed which does not require ARNT. Along with increased membrane levels of p60(Src), we observed a corresponding increase in the activity of a 60 kDa protein tyrosine kinase using two different kinase detection assays. This effect of TCDD was also found to be AhR-dependent, ARNT-independent, and independent of de novo protein synthesis since cycloheximide was unable to completely abolish TCDD's effect. The present findings provide a potentially important mechanism by which TCDD can alter cell growth and differentiation.

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

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

Role of insulin-like growth factors in autocrine growth of human retinoblastoma Y79 cells

In this study, we have demonstrated that human retinoblastoma Y79 cells produce insulin-like growth factors (IGFs) type I and type II and release them into the medium. We have also ascertained, by means of competitive studies and cross-linking procedure, that Y79 cells contain the type-I IGF receptor (IGF-IR). Furthermore, surface-bound IGF-I is internalised by the receptor, then degraded to amino acids. Insulin, IGF-I and IGF-II caused down-regulation of IGF-IR; the effect is concentration and time dependent. Scatchard analysis demonstrated that incubation with insulin markedly decreased the binding capacity measured for IGF-I while the apparent Kd value calculated for IGF-I binding was not significantly modified. IGF-I, IGF-II and insulin induced tyrosine phosphorylation of IGF-IR. Tyrosine phosphorylation of this receptor with, however, a less strong signal, was detectable even in cells cultured in serum-free medium without the addition of any exogenous growth factor. Similar results have been found concerning the tyrosine phosphorylation of insulin receptor substrate-1 (IRS 1). Tyrosine phosphorylation of both IGF-IR and IRS 1, either under basal conditions or after stimulation with growth factors, was strongly inhibited when alpha-IR3, a monoclonal antibody to IGF-IR, was added to the culture. IGF-I was capable of inducing Y79 cell proliferation and its effect was entirely inhibited by the addition of alpha-IR3. This antibody also markedly reduced the proliferation of Y79 cells cultured in serum-free medium not supplemented with stimulatory factors. Our results indicate that IGF-I and IGF-IR mediate an autocrine growth mechanism in Y79 cells.

Oncogenes, Protein Tyrosine Kinases, and Signal Transduction

Many oncogenes encode protein tyrosine kinases (PTKs). Oncogenic mutations of these genes invariably result in constitutive activation of these PTKs. Autophosphorylation of the PTKs and tyrosine phosphorylation of their cellular substrates are essential events for transmission of the mitogenic signal into cells. The recent discovery of the characteristic amino acid sequences, of the src homology domains 2 and 3 (SH2 and SH3), and extensive studies on proteins containing the SH2 and SH3 domains have revealed that protein tyrosine-phosphorylation of PTKs provides phosphotyrosine sites for SH2 binding and allows extracellular signals to be relayed into the nucleus through a chain of protein-protein interactions mediated by the SH2 and SH3 domains. Studies on oncogenes, PTKs and SH2/SH3-containing proteins have made a tremendous contribution to our understanding of the mechanisms for the control of cell growth, oncogenesis, and signal transduction. This review is intended to provide an outline of the most recent progress in the study of signal transduction by PTKs. Copyright 1994 S. Karger AG, Basel

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

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

Over expression of insulin-like growth factor receptor type-I in T-cell lines infected with human T-lymphotropic virus types-I and -II

To determine if aberrant expression of tyrosine kinase growth factor receptors may be related to the cell transformation capabilities of human T-lymphotropic viruses (HTLVs), we examined the expression of the epidermal growth factor receptor (EGF-R), insulin receptor (INS-R), and insulin-like growth factor receptor type-I (IGFR-I) in cell lines infected with HTLV type I (MT-2, HuT-102) and HTLV type II (Mo-T). Levels of mRNA transcripts for IGFR-I were significantly higher in both MT-2, HuT-102 (HTLV-I) and Mo-T (HTLV-II) cell lines than in uninfected cell lines (HuT-78, Jurkat); no detectable levels of EGF-R or INS-R mRNA transcript were observed in HTLV-infected or uninfected cell lines. Southern blot analysis demonstrated that no amplification or rearrangement of the IGFR-I gene occurred in either the MT-2 or Mo-T cell line. Flow cytometry analysis demonstrated that while IGFR-I protein was constitutively expressed on the cell surface in both MT-2 and Mo-T cell lines, neither EGF-R nor INS-R proteins could be detected. Ligand binding studies with MT-2 and Mo-T cell lines demonstrating binding of 125I insulin-like growth factor type-1 (IGF-I) in a dose-dependent manner and this response could be inhibited by increasing concentrations of cold IGF-I. These data demonstrate that deregulated expression of functional IGFR-I, the regular component of the growth control machinery of normal cells, may contribute to cellular proliferation and eventual transformation in HTLV-I- and HTLV-II-infected cell lines.

Modulating effects of the extracellular sequence of the human insulinlike growth factor I receptor on its transforming and tumorigenic potential

We reported previously that an N-terminally truncated insulinlike growth factor I receptor (IGFR) fused to avian sarcoma virus UR2 gag p19 had a greater transforming potential than did the native IGFR, but it failed to cause tumors in vivo. To investigate whether the 36 amino acids (aa) of the IGFR extracellular (EC) sequence in the gag-IGFR fusion protein encoded by the retrovirus UIGFR have a modulatory effect on the biological and biochemical properties of the protein, four mutants, NM1, NM2, NM3, and NM4 of the EC sequence were constructed. NM1 lacks the entire 36 aa residues; NM2 lacks the N-terminal 16 aa residues (aa 870 to 885), including two potential N-linked glycosylation sites of the EC sequence; NM3 contains a deletion of the C-terminal 20 aa residues (aa 886 to 905) of the EC sequence; and NM4 contains N-to-Q substitutions at both N-linked glycosylation sites. NM1 was the strongest of the four mutants in promoting anchorage-independent growth of transfected chicken embryo fibroblasts, while NM2 and NM4 had weaker transforming potential than did the original UIGFR virus. Only NM1 and NM3 were able to induce sarcomas in chickens. The four NM mutant-transformed cells expressed the expected proteins with comparable steady-state levels. The in vitro tyrosine kinase activity of P53NM1 was about fourfold higher than that of the parental P57-75UIGFR, whereas NM2 and NM4 proteins exhibited four- to fivefold-lower kinase activities. Despite lacking the IGFR EC sequence, P53NM1 formed covalent dimers similar to those formed by the parental P57-75UIGFR. Increased phosphatidylinositol (PI) 3-kinase activity was found to be associated with the mutant IGFR proteins. Among NM4 proteins. Elevated tyrosine phosphorylation of cellular proteins of 35, 120, 140, 160, and 170 kDa was detected in all mutant IGFR-transformed cells. We conclude that the EC 36-aa sequence of IGFR in the gag-IGFR fusion protein exerts intricate modulatory effects on the protein's transforming and tumorigenic potential. The 20 aa residues immediately upstream of the transmembrane domain have an inhibitory effect on the tumorigenic potential of gag-IGFR, whereas N-linked glycosylation within the EC sequence appears to have a positive effect on the transforming potential of UIGFR. Increased in vitro kinase activity and, to a lesser extent, in vivo tyrosine phosphorylation as well as the elevated association of PI 3-kinase activity with IGFR proteins seem to be correlated with the transforming potential of IGFR mutant proteins.

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

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