Distinctive effects of the carboxyl-terminal sequence of the insulin-like growth factor I receptor on its signaling functions

The tale of a tail: The secret behind IGF-1R's oncogenic power

The C-terminal tail of insulin-like growth factor 1 receptor (IGF-1R) has long been appreciated to drive much of this receptor's oncogenic power. In this issue of Science Signaling, Rieger et al. have shown that Tyr¹²⁵⁰ and Tyr¹²⁵¹ of IGF-1R are autophosphorylated in a cell adhesion-dependent manner, uncovering a previously unknown plasma membrane-Golgi trafficking route for IGF-1R in migratory cells, an integral part of the malignant phenotype.

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.

Novel agents for advanced pancreatic cancer

Pancreatic cancer is relatively insensitive to conventional chemotherapy. Therefore, novel agents targeting dysregulated pathways (MAPK/ERK, EGFR, TGF-β, HEDGEHOG, NOTCH, IGF, PARP, PI3K/AKT, RAS, and Src) are being explored in clinical trials as monotherapy or in combination with cytotoxic chemotherapy. This review summarizes the most recent advances with the targeted therapies in the treatment of patients with advanced pancreatic cancer.

Targeting insulin-like growth factor axis in hepatocellular carcinoma

The insulin-like growth factor (IGF) axis contains ligands, receptors, substrates, and ligand binding proteins. The essential role of IGF axis in hepatocellular carcinoma (HCC) has been illustrated in HCC cell lines and in animal xenograft models. Preclinical evidence provides ample indication that all four components of IGF axis are crucial in the carcinogenic and metastatic potential of HCC. Several strategies targeting this system including monoclonal antibodies against the IGF 1 receptor (IGF-1R) and small molecule inhibitors of the tyrosine kinase function of IGF-1R are under active investigation. This review describes the most up-to-date understanding of this complex axis in HCC, and provides relevant information on clinical trials targeting the IGF axis in HCC with a focus on anti-IGF-1R approach. IGF axis is increasingly recognized as one of the most relevant pathways in HCC and agents targeting this axis can potentially play an important role in the treatment of HCC.

Targeting insulin-like growth factor axis in hepatocellular carcinoma

The insulin-like growth factor (IGF) axis contains ligands, receptors, substrates, and ligand binding proteins. The essential role of IGF axis in hepatocellular carcinoma (HCC) has been illustrated in HCC cell lines and in animal xenograft models. Preclinical evidence provides ample indication that all four components of IGF axis are crucial in the carcinogenic and metastatic potential of HCC. Several strategies targeting this system including monoclonal antibodies against the IGF 1 receptor (IGF-1R) and small molecule inhibitors of the tyrosine kinase function of IGF-1R are under active investigation. This review describes the most up-to-date understanding of this complex axis in HCC, and provides relevant information on clinical trials targeting the IGF axis in HCC with a focus on anti-IGF-1R approach. IGF axis is increasingly recognized as one of the most relevant pathways in HCC and agents targeting this axis can potentially play an important role in the treatment of HCC.

Aldosterone stimulates elastogenesis in cardiac fibroblasts via mineralocorticoid receptor-independent action involving the consecutive activation of Galpha13, c-Src, the insulin-like growth factor-I receptor, and phosphatidylinositol 3-kinase/Akt

We previously demonstrated that aldosterone, which stimulates collagen production through the mineralocorticoid receptor (MR)-dependent pathway, also induces elastogenesis via a parallel MR-independent mechanism involving insulin-like growth factor-I receptor (IGF-IR) signaling. The present study provides a more detailed explanation of this signaling pathway. Our data demonstrate that small interfering RNA-driven elimination of MR in cardiac fibroblasts does not inhibit aldosterone-induced IGF-IR phosphorylation and subsequent increase in elastin production. These results exclude the involvement of the MR in aldosterone-induced increases in elastin production. Results of further experiments aimed at identifying the upstream signaling component(s) that might be activated by aldosterone also eliminate the putative involvement of pertussis toxin-sensitive Galphai proteins, which have previously been shown to be responsible for some MR-independent effects of aldosterone. Instead, we found that small interfering RNA-dependent elimination of another heterotrimeric G protein, Galpha13, eliminates aldosterone-induced elastogenesis. We further demonstrate that aldosterone first engages Galpha13 and then promotes its transient interaction with c-Src, which constitutes a prerequisite step for aldosterone-dependent activation of the IGF-IR and propagation of consecutive downstream elastogenic signaling involving phosphatidylinositol 3-kinase/Akt. In summary, the data we present reveal new details of an MR-independent cellular signaling pathway through which aldosterone stimulates elastogenesis in human cardiac fibroblasts.

RACK1 recruits STAT3 specifically to insulin and insulin-like growth factor 1 receptors for activation, which is important for regulating anchorage-independent growth

Current understanding of the activation of STATs is through binding between the SH2 domain of STATs and phosphotyrosine of tyrosine kinases. Here we demonstrate a novel role of RACK1 as an adaptor for insulin and insulin-like growth factor 1 receptor (IGF-1R)-mediated STAT3 activation specifically. Intracellular association of RACK1 via its N-terminal WD domains 1 to 4 (WD1-4) with insulin receptor (IR)/IGF-1R is augmented upon respective ligand stimulation, whereas association with STAT3 is constitutive. Purified RACK1 or RACK1 WD1-4 associates directly with purified IR, IGF-1R, and STAT3 in vitro. Insulin induces multiprotein complex formation of RACK1, IR, and STAT3. Overexpression or downregulation of RACK1 greatly enhances or decreases, respectively, IR/IGF-1R-mediated activation of STAT3 and its target gene expression. Site-specific mutants of IR and IGF-1R impaired in RACK1 binding are ineffective in mediating recruitment and activation of STAT3 as well as in insulin- or IGF-1-induced protection of cells from anoikis. RACK1-mediated STAT3 activation is important for insulin and IGF-1-induced anchorage-independent growth in certain ovarian cancer cells. We conclude that RACK1 mediates recruitment of STAT3 to IR and IGF-1R specifically for activation, suggesting a general paradigm for the need of an adaptor in mediating activation of STATs by receptor protein tyrosine kinases.

The insulin-like growth factor-I receptor as a target for cancer therapy

This review examines the rationale for targeting the insulin-like growth factor (IGF)-I receptor in the therapy of human tumours and their metastases. The rationale is based on two crucial findings: 1) in experimental animals, normal cells are only partially affected by the deletion of the IGF-I receptor, whereas tumour cells undergo apoptosis when the IGF-I receptor is downregulated; and 2) cells with a deleted IGF-I receptor are refractory to transformation by viral and cellular oncogenes. This review focuses on the mechanisms underlying the experimental findings, and discusses the possibility of extrapolating the results obtained in animals to the cure of human tumours.

RACK1, an insulin-like growth factor I (IGF-I) receptor-interacting protein, modulates IGF-I-dependent integrin signaling and promotes cell spreading and contact with extracellular matrix

The insulin-like growth factor I (IGF-I) receptor (IGF-IR) is known to regulate a variety of cellular processes including cell proliferation, cell survival, cell differentiation, and cell transformation. IRS-1 and Shc, substrates of the IGF-IR, are known to mediate IGF-IR signaling pathways such as those of mitogen-activated protein kinase (MAPK) and phosphatidylinositol 3-kinase (PI3K), which are believed to play important roles in some of the IGF-IR-dependent biological functions. We used the cytoplasmic domain of IGF-IR in a yeast two-hybrid interaction trap to identify IGF-IR-interacting molecules that may potentially mediate IGF-IR-regulated functions. We identified RACK1, a WD repeat family member and a Gbeta homologue, and demonstrated that RACK1 interacts with the IGF-IR but not with the closely related insulin receptor (IR). In several types of mammalian cells, RACK1 interacted with IGF-IR, protein kinase C, and beta1 integrin in response to IGF-I and phorbol 12-myristate 13-acetate stimulation. Whereas most of RACK1 resides in the cytoskeletal compartment of the cytoplasm, transformation of fibroblasts and epithelial cells by v-Src, oncogenic IR or oncogenic IGF-IR, but not by Ros or Ras, resulted in a significantly increased association of RACK1 with the membrane. We examined the role of RACK1 in IGF-IR-mediated functions by stably overexpressing RACK1 in NIH 3T3 cells that expressed an elevated level of IGF-IR. RACK1 overexpression resulted in reduced IGF-I-induced cell growth in both anchorage-dependent and anchorage-independent conditions. Overexpression of RACK1 also led to enhanced cell spreading, increased stress fibers, and increased focal adhesions, which were accompanied by increased tyrosine phosphorylation of focal adhesion kinase and paxillin. While IGF-I-induced activation of IRS-1, Shc, PI3K, and MAPK pathways was unaffected, IGF-I-inducible beta1 integrin-associated kinase activity and association of Crk with p130(CAS) were significantly inhibited by RACK1 overexpression. In RACK1-overexpressing cells, delayed cell cycle progression in G(1) or G(1)/S was correlated with retinoblastoma protein hypophophorylation, increased levels of p21(Cip1/WAF1) and p27(Kip1), and reduced IGF-I-inducible Cdk2 activity. Reduction of RACK1 protein expression by antisense oligonucleotides prevented cell spreading and suppressed IGF-I-dependent monolayer growth. Our data suggest that RACK1 is a novel IGF-IR signaling molecule that functions as a positive mediator of cell spreading and contact with extracellular matrix, possibly through a novel IGF-IR signaling pathway involving integrin and focal adhesion signaling molecules.

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.

IGF-I receptor signalling in transformation and differentiation

The type 1 insulin-like growth factor receptor (IGF-IR) sends several signals, some of which are contradictory. When the concentrations of insulin receptor substrate 1 (IRS-1), a major substrate of the IGF-IR, are high, the signal is mitogenic, anti-apoptotic, and can even cause malignant transformation. However, in the absence of IRS-1, the IGF-IR sends a differentiation signal, which leads to granulocytic differentiation in haemopoietic cells. The mitogenic signal of the IGF-IR/IRS-1 combination depends largely, but not exclusively, on the activation of the phosphatidylinositol-3 kinase (PI3K).

Differential requirements of the MAP kinase and PI3 kinase signaling pathways in Src- versus insulin and IGF-1 receptors-induced growth and transformation of rat intestinal epithelial cells

There have been few studies on the specific signaling pathways involved in the transformation of epithelial cells by oncogenic protein tyrosine kinases. Here we investigate the requirement of MAP (MAPK) and phosphatidylinositol 3- (PI3K) kinases in the transformation of rat intestinal epithelial (RIE) cells by oncogenic forms of insulin receptor (gag-IR), insulin-like growth factor-1 receptor (gag-IGFR), and v-Src. MAPK is not significantly activated in cells transformed by gag-IR and gag-IGFR but is activated in v-Src transformed cells. Treatment with PD98059, a MEK inhibitor, at concentrations where MAPK activity was reduced below the basal level showed that MAPK is partially required for the monolayer growth of parental and transformed RIE cells. However, MAPK is not essential for the focus forming ability of the three oncogene-transformed cells. It is also not necessary for the colony forming ability of gag-IR- and gag-IGFR-, but is partially required for v-Src-transformed cells. PI3K is significantly activated in all three oncogene transformed RIE cells. LY294002, a PI3K inhibitor, potently inhibited monolayer growth of all three oncogene-transformed cells. However, at concentrations of LY294002 where activated forms of Akt, a downstream component of the PI3K pathway, were undetectable, colony and focus forming abilities of the v-Src-RIE cells were only slightly affected whereas those of gag-IR/IGFR-RIE cells were greatly inhibited. These results were confirmed using a different pharmacological inhibitor, wortmannin, and a dominant negative form of PI3K, Ap85. Similarly, rapamycin, known to inhibit p70S6 kinase, a downstream component of the PI3K-Akt pathway, also inhibited gag-IR/IGFR-induced, but not v-Src-induced, focus and colony formation. We conclude that the MAPK and PI3K signaling pathways are differentially required for transformation of RIE cells by oncogenic IR and IGFR versus Src and the pattern of requirements is different from that of fibroblast transformation.

Mechanism of action in thalidomide teratogenesis

In this commentary, we describe a model to explain the mechanism of the embryopathy of thalidomide. We propose that thalidomide affects the following pathway during development: insulin-like growth factor I (IGF-I) and fibroblast growth factor 2 (FGF-2) stimulation of the transcription of alphav and beta3 integrin subunit genes. The resulting alphavbeta3 integrin dimer stimulates angiogenesis in the developing limb bud, which promotes outgrowth of the bud. The promoters of the IGF-I and FGF-2 genes, the genes for their binding proteins and receptors, as well as the alphav and beta3 genes, lack typical TATA boxes, but instead contain multiple GC boxes (GGGCGG). Thalidomide, or a breakdown product of thalidomide, specifically binds to these GC promoter sites, decreasing transcription efficiency of the associated genes. A cumulative decrease interferes with normal angiogenesis, which results in truncation of the limb. Intercalation into G-rich promoter regions of DNA may explain why certain thalidomide analogs are not teratogenic while retaining their anti-tumor necrosis factor-alpha (TNF-alpha) activity, and suggests that we look elsewhere to explain the action of thalidomide on TNF-alpha. On the other hand, the anti-cancer action of thalidomide may be based on its antiangiogenic action, resulting from specific DNA intercalation. The tissue specificity of thalidomide and its effect against only certain neoplasias may be explained by the fact that various developing tissues and neoplasias depend on different angiogenesis or vasculogenesis pathways, only some of which are thalidomide-sensitive.

Loss of IGF-II imprinting in endometrial tumors: overexpression in carcinosarcoma

The genomic imprinting of the maternal allele defines the monoallelic expression of the IGF-II gene in most human tissues. The loss of imprinting (LOI) leading to biallelic overexpression of IGF-II has been reported in several human malignancies, including uterine leiomyosarcoma. To ascertain if LOI occurs in endometrial malignancies, the allelic expression of the IGF-II gene was examined in samples of normal human endometrium (n=22) and endometrial tumors (n=12) by assessing the ApaI polymorphism in cDNA segments amplified by RT-PCR. The biallelic overexpression of IGF-II mRNA, involving activation of all four (P1-P4) promoters, was detected in one normal endometrium and in one endometrial carcinosarcoma. Low level biallelic expression of IGF-II was also detected in two samples of hormone-unresponsive/Type II endometrial carcinomas. The level of IGF-I mRNA in these four samples was low. The IGF-IR mRNA was overexpressed in all endometrial cancers including the carcinosarcoma sample, but not in normal endometrium. These data suggest that LOI associated with overexpression of IGF-II and concomitant overexpression of IGF-IR may play a role in the rare carcinosarcoma of the endometrium.

Dissociation between resistance to apoptosis and the transformed phenotype in IGF-I receptor signaling

Programmed Cell Death (PCD) is known to play an important role in both the development and the growth rate of human tumors. It has in fact been suggested that suppression of the apoptotic pathway is a requirement for the establishment of the transformed phenotype. In order to elucidate the relationship between resistance to apoptosis and transformation, we have asked in this investigation whether or not the two processes can be directly correlated. For this purpose, we have used mouse embryo fibroblasts (MEF) expressing either the wild-type or several mutants of the type 1 insulin-like growth factor receptor (IGF-IR). The wild-type IGF-IR has both transforming and anti-apoptotic activities, and we have asked whether these two activities can be or not separated in mutant receptors. Using this well-defined system, our results show that certain mutants of the IGF-IR that have strong anti-apoptotic and mitogenic activities, are incapable of transforming MEF (colony formation in soft agar). We have, instead, a good correlation between mitogenic and anti-apoptotic activities, suggesting the possibility that the two processes may share similar signaling pathways from the IGF-IR. On the other hand, our results indicate that transformation requires an additional signal, above and beyond the mitogenic and survival signals. Our conclusion is that, at least in this system, the establishment of the malignant phenotype and resistance to apoptosis can be dissociated, implying the possibility of separate targeting.

DBI-1, a novel gene related to the notch family, modulates mitogenic response to insulin-like growth factor 1

The insulin-like growth factor 1 (IGF-1) receptor has been found to transform fibroblast cells when overexpressed. The removal of 108 aa from the C-terminus of the IGF-1 receptor abolishes the transforming ability of the receptor without affecting its ability to induce cell growth. The availability of this mutant receptor provides a means to examine the changes in gene expression which take place during transformation, solely in response to an increased number of IGF-1 receptors. Using differential display, we have examined differences in gene expression between cells expressing a wild-type, transforming IGF-1 receptor and cells expressing a C-terminally truncated, nontransforming IGF-1 receptor. We have cloned a novel 6. 3-kb cDNA transcript (DBI-1) which is expressed at much lower levels in cells containing the wild-type IGF-1 receptor. The predicted protein sequence of DBI-1 contains seven EGF-like repeats, which bear >90% sequence identity to the rat Notch 2 protein. The cDNA also contains a potential DEAD box in the C-terminal region. The DBI-1 message is detected at relatively high levels in cardiac tissue and at lower levels in lung, liver, and kidney. Antibodies generated to a unique region of the DBI-1 protein recognize a protein of 88 kDa, which is localized in the nucleus. Overexpression of DBI-1 in cells which contain the wild-type IGF-1 receptor diminishes the mitogenic response to IGF-1.

Structure and function of the insulin-like growth factor I receptor

Insulin-like growth factors I and II (IGF-I, IGF-II) were originally identified as potent mitogens and as the mediators of growth hormone action. Besides being mitogenic, however, these polypeptide growth factors play a crucial role in cell survival, and contribute to transformation and to maintenance of the malignant phenotype. Here we will discuss signaling by the IGFs, focusing specifically on the structure and function of the IGF-I receptor and the domains of this receptor responsible for distinct IGF functions: mitogenesis, transformation, and protection from apoptosis. We will also compare the structural domains of the related but functionally distinct receptor for insulin.

14-3-3 proteins interact with the insulin-like growth factor receptor but not the insulin receptor

We have used a yeast two-hybrid system to identify proteins which bind to the cytosolic portion of the type 1 insulin-like growth factor (IGF) receptor (IGFIR) but not the insulin receptor (IR). This analysis identified 14-3-3beta and zeta proteins. 14-3-3beta also binds to the IGFIR but not the IR in vitro and 14-3-3-IGFIR complexes are present in insect cells overexpressing the IGFIR cytoplasmic domain. 14-3-3 proteins are substrates of the IGFIR in the yeast system and in vitro. The interaction of 14-3-3 with the IGFIR requires receptor-kinase activity and maps to the C-terminus of the receptor, but does not depend on tyrosine residues in this or the juxtamembrane regions. Instead, the binding maps to serine residue 1283 and requires phosphorylation of this residue. 14-3-3 proteins are phosphoserine-binding proteins which have been shown to interact directly with components of the mitogenic and apoptotic signalling pathways, suggesting that they participate in growth regulation. Our findings suggest that 14-3-3 proteins may play a role in IGFIR signal transduction and may contribute to the differences in IGF and IR signalling capabilities.

Tyrosine residues in the C-terminal domain of the insulin-like growth factor-I receptor mediate mitogenic and tumorigenic signals

We investigated cellular proliferation, the transforming activity, and activation of known signal transduction pathways in NIH-3T3 cells stably expressing insulin-like growth factor-I receptors (IGF-IRs) with amino acid substitutions in the carboxy(C)-terminal domain. The mutant receptors contained substitutions of both tyrosines 1250 and 1251 with phenylalanine and histidine (amino acids present in the analogous positions in the insulin receptor), as well as phenylalanine 1310 replaced by tyrosine (IsY clones) to resemble the placement of tyrosine residues in the C-terminal domain of the insulin receptor. As a control for the IsY clones, a second mutant receptor was expressed with a substitution of phenylalanine 1310 with tyrosine only (DBY clones). Clones expressing IGF-IRs with the IsY substitutions had a significantly slower rate of growth compared with cells expressing an equivalent number of wild-type IGF-IRs (NWT). In contrast, the DBY clones showed relatively normal growth rates. Cells with wild-type IGF-IR demonstrated a transformed phenotype in soft agar assays. The IsY clones lost the transforming ability of the wild type IGF-IR, whereas DBY clones formed colonies. IGF-I-stimulated autophosphorylation of the IGF-IR and tyrosine phosphorylation of IRS-1 and SHC, known substrates in the IGF-IR signal transduction pathway, were studied. Mutated IGF-IRs (IsY and DBY) did not alter the IGF-I-induced tyrosine phosphorylation of these proteins. Furthermore, the mutated IGF-IRs did not alter Grb2 association with phosphorylated IRS-1 and SHC. IGF-I stimulation of Crk-II phosphorylation, a novel substrate of the IGF-IR, was similar in cells expressing mutated and wild-type IGF-IRs. IGF-I-induced activation of phosphatidylinositol (PI) 3'-kinase was equivalent in cells expressing either mutant or wild-type IGF-IRs. These data suggest that the IGF-IR mediates, at least in part, cellular proliferation and increased transforming ability through its C-terminal domain. The exact postreceptor signaling pathway(s) involved have yet to be fully elucidated.

Insulin-like growth factor I receptor signaling in transformation by src oncogenes

R- cells, a line of mouse embryo fibroblasts with a targeted disruption of the insulin-like growth factor I (IGF-I) receptor genes, are refractory to transformation by several viral and cellular oncogenes. Using colony formation in soft agar as a measure of full transformation, we report here that R- cells can be transformed by v-src, although they still cannot be transformed by the activated c-src527 (mutation at tyrosine 527 to phenylalanine), which readily transforms mouse embryo cells with a wild-type number of IGF-I receptors (W cells). Although v-src is a more potent inducer of tyrosine phosphorylation than c-src527, the extent of phosphorylation of either insulin receptor substrate 1 or Shc, two of the major substrates of the IGF-I receptor, does not seem sufficiently different to explain the qualitative difference in soft agar growth. v-src, however, is considerably more efficient than c-src527 in its ability to tyrosyl phosphorylate, in R- cells, the focal adhesion kinase, Stat1, and p130cas. These results indicate that v-src, but not c-src527, can bypass the requirement for a functional IGF-I receptor in the full transformation of mouse embryo fibroblasts and suggest that qualitative and quantitative differences between the two oncogenes can be used to identify some of the signals relevant to the mechanism(s) of transformation.

Identification of signalling components in tyrosine kinase cascades using phosphopeptide affinity chromatography

Various methods are now available to identify the molecular partners of the component of a signal transduction pathway. Some interactions, however, can be technically difficult to detect because they depend upon transient tyrosine phosphorylation. Here, we present a simple affinity chromatography approach based on synthetic phosphopeptides to purify potential partners of phosphotyrosine-containing proteins. With this approach, we confirm the previously characterized interaction between Grb2 and the EGF receptor, and we identify novel partners of the IGF-1 receptor and of the JAK proteins. Methenyltetrahydrofolate synthetase (MTHFS) was identified as a potential mediator of IGF-1R dependent transformation. P85alpha, the regulatory subunit of PI3 kinase, was identified as one of four proteins recruited by a phosphopeptide mimicking a motif conserved in all JAK family members.

The IGF-I receptor in mitogenesis and transformation of mouse embryo cells: role of receptor number

The type 1 receptor for insulin-like growth factors (IGF-IR) plays an important role in the growth and transformation of several types of cells. We have investigated the role of IGF-IR number in IGF-I-mediated mitogenesis and transformation of mouse embryo fibroblasts. We have used R- cells (3T3-like cells originating from mouse embryos with a targeted disruption of the IGF-IR genes) transfected with a plasmid expressing the human IGF-IR cDNA to generate clones with receptor numbers ranging from zero to 10(6) receptors per cell. In this model, between 15,000 and 22,000 receptors per cell are sufficient to render mouse embryo cells competent to grow in serum-free medium supplemented solely with IGF-I. For growth in soft agar, 30,000 receptors per cell seem to be the minimum requirement. These experiments indicate that a small increment in the number of receptors per cell, well within the physiological range, can modulate the mitogenic and transforming activities of the IGF-IR in 3T3-like cells.

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.

Effect of mutations at serines 1280-1283 on the mitogenic and transforming activities of the insulin-like growth factor I receptor

The insulin-like growth factor I receptor (IGF-IR) controls the extent of cell proliferation in a variety of cell types by at least 3 different ways: it is mitogenic, it causes transformation, and it protects cells from apoptosis. Previous reports indicated that certain domains in the C terminus of the IGF-IR transmitted a transforming signal that is additional to and separate from the mitogenic signal. We have now mutated the four serine residues at 1280-1283 of the IGF-IR, and transfected the mutant receptor into R- cells. Cells expressing the mutant receptor are fully responsive to IGF-I mediated mitogenesis, but are not transformed (no colony formation in soft agar). Several downstream signal transducers are not affected by the mutation, again suggesting a separate pathway for transformation. The mutant receptor can act as a dominant negative for growth, but cannot induce apoptosis in cells with endogenous wild-type receptors.

Effect of tyrosine mutations on the kinase activity and transforming potential of an oncogenic human insulin-like growth factor I receptor

The tyrosines in the cytoplasmic domain of an oncogenic human insulin-like growth factor I receptor (gag-IGFR) were systematically mutated to phenylalanines to investigate the role of those tyrosines in the enzymatic and biological function of the gag-IGFR. Our results indicate that tyrosines 1131, 1135, 1136, and 1221 are important for the receptor protein-tyrosine kinase (PTK) activity. However, mutation of Tyr-1136 only slightly affects the kinase activity but dramatically reduces the transforming ability and overall substrate phosphorylation, in particular, annexin II, which is strongly phosphorylated by the gag-IGFR but not by the Phe-1136 mutant. Single mutation of either Tyr-943 or Tyr-950 resulted in significantly reduced phosphorylation of the receptor but not on its PTK activity or transforming ability. Tyr-950 together with its surrounding sequence is involved in mediating the interaction between the gag-IGFR and insulin receptor substrate 1. Our data also suggest that Tyr-1316 is involved in phosphorylation of phospholipase C-gamma, which is, however, not important for cell transforming activity. Overall, our study has identified several tyrosine residues of IGFR important for its PTK activity and substrate interaction. The transforming potential of the gag-IGFR correlates well with its ability to phosphorylate overall cellular substrates and to activate phosphatidylinositol 3-kinase via insulin receptor substrate 1.

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

The wild type insulin-like growth factor I (IGF-I) receptor has both mitogenic and transforming activities. We have examined the effect of point mutations at tyrosine residues 1250 and 1251 on these two properties of the receptor. For this purpose, we stably transfected plasmids expressing mutant and wild type receptors into R- cells, which are 3T3-like cells, derived from mouse embryos with a targeted disruption of the IGF-I receptor genes, and therefore devoid of endogenous IGF-I receptors. A tyrosine to phenylalanine mutation of either the 1250 or 1251 residue, or both, has no effect on the ability of the receptor to transmit a mitogenic signal. However, the tyrosine 1251 mutant receptor and the double mutant have lost the ability to transform R- cells (colony formation in soft agar), even when the receptors are expressed at very high levels, while the Y1250F mutant is fully transforming. These experiments show that the 1251 tyrosine residue is required for the transforming activity of the IGF-I receptor.

Failure of the bovine papillomavirus to transform mouse embryo fibroblasts with a targeted disruption of the insulin-like growth factor I receptor genes

Mouse embryo cells with a targeted disruption of the insulin-like growth factor I receptor (IGF-IR) genes (R- cells) are refractory to transformation by the simian virus 40 large T antigen and/or an activated and overexpressed Ras, both of which readily transform cells from wild-type littermate embryos and other 3T3-like cells. R- cells are also refractory to transformation induced by overexpressed epidermal growth factor receptor and platelet-derived growth factor receptor beta. Since the platelet-derived growth factor receptor beta is required for transformation by bovine papillomavirus, we inquired whether the IGF-IR was also required for transformation by bovine papillomavirus E5 oncoprotein. We show here that R- cells are refractory to transformation by E5; reintroduction into R- cells of a human IGF-IR restores the susceptibility to transformation.

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