Binding properties of chimeric insulin receptors containing the cysteine-rich domain of either the insulin-like growth factor I receptor or the insulin receptor related receptor.

We constructed and expressed chimeric receptor cDNAs with insulin receptor exon 3 (residues 191-297 of the cysteine-rich region) replaced with either the comparable region of the insulin-like growth factor I receptor (IGF-IR) or the insulin receptor related receptor (IRR). Both chimeric receptors still could bind insulin with as high affinity as the wild-type receptor. In addition, chimeric receptors containing exon 3 of the IGF-IR could also bind with high affinity both IGF-I and IGF-II. In contrast, chimeric receptors containing exon 3 of IRR did not bind either IGF-I, IGF-II, or relaxin. These results indicate that (1) the high affinity of binding of insulin to its receptor can occur in the absence of insulin receptor specific residues encoded by exon 3, the cysteine-rich region; (2) the cysteine-rich region of the IGF-I receptor can confer high-affinity binding to both IGF-I and IGF-II; and 3) the IRR is unlikely to be a receptor for either IGF-I, IGF-II, or relaxin.     

Insulin-like growth factors I and II in fetal and adult bovine serum. Purification, primary structures, and immunological cross-reactivities

Insulin-like growth factors I and II (IGF-I and IGF-II) were prepared from fetal calf serum and adult bovine serum by gel filtration, immunoaffinity chromatography, chromatofocusing, and reverse-phase high pressure liquid chromatography, and their complete amino acid sequences determined. IGF-I and -II are found in both adult and fetal serum. The sequence of bovine IGF-I is found to be identical to that of human IGF-I, whereas 3 out of 67 amino acid residues are found to be different between bovine and human IGF-II. The differences are located in the C-peptide region of the molecule. Bovine IGF-II shows less than 10% immunological cross-reactivity with antisera against human and rat IGF-II, but is equipotent to human IGF-II in displacing human 125I-labeled IGF-II from human placental receptor. Bovine IGF-I was equipotent to human IGF-I in both radioimmunoassays and radioreceptor assays within the limits of the assay.     

Hybrid receptors formed by insulin receptor (IR) and insulin-like growth factor I receptor (IGF-IR) have low insulin and high IGF-1 affinity irrespective of the IR splice variant

Insulin receptor (IR) and insulin-like growth factor I receptor (IGF-IR) are both from the same subgroup of receptor tyrosine kinases that exist as covalently bound receptor dimers at the cell surface. For both IR and IGF-IR, the most described forms are homodimer receptors. However, hybrid receptors consisting of one-half IR and one-half IGF-IR are also present at the cell surface. Two splice variants of IR are expressed that enable formation of two isoforms of the IGF-IR/IR hybrid receptor. In this study, these two splice variants of hybrid receptors were studied with respect to binding affinities of insulin, insulin-like growth factor I (IGF-I), and insulin-like growth factor II (IGF-II). Unlike previously published data, in which semipurified receptors have been studied, we found that the two hybrid receptor splice variants had similar binding characteristics with respect to insulin, IGF-I, and IGF-II binding. We studied both semipurified and purified hybrid receptors. In all cases we found that IGF-I had at least 50-fold higher affinity than insulin, irrespective of the splice variant. The binding characteristics of insulin and IGF-I to both splice variants of the hybrid receptors were similar to classical homodimer IGF-IR.     

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.     

Insulin/insulin-like growth factor I hybrid receptors have different biological characteristics depending on the insulin receptor isoform involved

The insulin receptor (IR) and the insulin-like growth factor I receptor (IGF-IR) have a highly homologous structure, but different biological effects. Insulin and IGF-I half-receptors can heterodimerize, leading to the formation of insulin/IGF-I hybrid receptors (Hybrid-Rs) that bind IGF-I with high affinity. As the IR exists in two isoforms (IR-A and IR-B), we evaluated whether the assembly of the IGF-IR with either IR-A or IR-B moieties may differently affect Hybrid-R signaling and biological role. Three different models were studied: (a) 3T3-like mouse fibroblasts with a disrupted IGF-IR gene (R(-) cells) cotransfected with the human IGF-IR and with either the IR-A or IR-B cDNA; (b) a panel of human cell lines variably expressing the two IR isoforms; and (c) HepG2 human hepatoblastoma cells predominantly expressing either IR-A or IR-B, depending on their differentiation state. We found that Hybrid-Rs containing IR-A (Hybrid-Rs(A)) bound to and were activated by IGF-I, IGF-II, and insulin. By binding to Hybrid-Rs(A), insulin activated the IGF-I half-receptor beta-subunit and the IGF-IR-specific substrate CrkII. In contrast, Hybrid-Rs(B) bound to and were activated with high affinity by IGF-I, with low affinity by IGF-II, and insignificantly by insulin. As a consequence, cell proliferation and migration in response to both insulin and IGFs were more effectively stimulated in Hybrid-R(A)-containing cells than in Hybrid-R(B)-containing cells. The relative abundance of IR isoforms therefore affects IGF system activation through Hybrid-Rs, with important consequences for tissue-specific responses to both insulin and IGFs.     

Differential activation of insulin receptor substrates 1 and 2 by insulin-like growth factor-activated insulin receptors

The insulin-like growth factors (insulin-like growth factor I [IGF-I] and IGF-II) exert important effects on growth, development, and differentiation through the IGF-I receptor (IGF-IR) transmembrane tyrosine kinase. The insulin receptor (IR) is structurally related to the IGF-IR, and at high concentrations, the IGFs can also activate the IR, in spite of their generally low affinity for the latter. Two mechanisms that facilitate cross talk between the IGF ligands and the IR at physiological concentrations have been described. The first of these is the existence of an alternatively spliced IR variant that exhibits high affinity for IGF-II as well as for insulin. A second phenomenon is the ability of hybrid receptors comprised of IGF-IR and IR hemireceptors to bind IGFs, but not insulin. To date, however, direct activation of an IR holoreceptor by IGF-I at physiological levels has not been demonstrated. We have now found that IGF-I can function through both splice variants of the IR, in spite of low affinity, to specifically activate IRS-2 to levels similar to those seen with equivalent concentrations of insulin or IGF-II. The specific activation of IRS-2 by IGF-I through the IR does not result in activation of the extracellular signal-regulated kinase pathway but does induce delayed low-level activation of the phosphatidylinositol 3-kinase pathway and biological effects such as enhanced cell viability and protection from apoptosis. These findings suggest that IGF-I can function directly through the IR and that the observed effects of IGF-I on insulin sensitivity may be the result of direct facilitation of insulin action by IGF-I costimulation of the IR in insulin target tissues.     

Insulin-like growth factor binding protein (IGFBP)-3-bound IGF-I and IGFBP-3-bound IGF-II in growth hormone deficiency

In blood, circulating IGFs are bound to six high-affinity IGFBPs, which modulate IGF delivery to target cells. Serum IGFs and IGFBP-3, the main carrier of IGFs, are upregulated by GH. The functional role of serum IGFBP-3-bound IGFs is not well understood, but they constitute the main reservoir of IGFs in the circulation. We have used an equation derived from the law of mass action to estimate serum IGFBP-3-bound IGF-I and IGFBP-3-bound IGF-II, as well as serum free IGF-I and free IGF-II, in 129 control children and adolescents (48 girls and 81 boys) and in 13 patients with GHD. Levels of serum total IGF-I, total IGF-II, IGFBP-1, IGFBP-2 and IGFBP-3 were determined experimentally, while those of IGFBP-4, IGFBP-5 and IGFPB-6, as well as the 12 affinity constants of association of the two IGFs with the six IGFBPs, were taken from published values. A correction for in vivo proteolysis of serum IGFBP-3 was also considered. In controls, serum total IGF-I, total IGF-II, IGFBP-3, IGFBP-3-bound IGF-I, IGFBP-3-bound IGF-II and free IGF-I increased linearly with age, from less than 1 to 15 years, in the two sexes. The concentrations of serum free IGF-I and free IGF-II were approximately two orders of magnitude below published values, as well as below the affinity constant of association of IGF-I with the type-1 IGF receptor. Therefore, it is unlikely that these levels can interact with the receptor. In the 13 patients with GHD, mean (+/- SD) SDS of serum IGFBP-3-bound IGF-I was -2.89 +/- 0.97. It was significantly lower than serum total IGF-I, free IGF-I or IGFBP-3 SDSs (-2.35 +/- 0.83, -1.12 +/- 0.78 and -2.55 +/- 1.07, respectively, p = 0.0001). The mean SDS of serum total IGF-II, IGFBP-3-bound IGF-II and free IGF-II were -1.25 +/- 0.68, -2.03 +/- 0.87 and 0.59 +/- 1.10, respectively, in GHD. In control subjects, 89.8 +/- 4.47% of serum total IGF-I and 77.3 +/- 9.4% of serum total IGF-II were bound to serum IGFBP-3. In patients with GHD, the mean serum IGFBP-3-bound IGF-I and IGFBP-3-bound IGF-II were 8.63 +/- 8. 53 and 19.1 +/- 14.7% below the respective means of control subjects (p < 0.02). In conclusion, in GHD there was a relative change in the distribution of serum IGFs among IGFBPs, due to the combined effects of the decrease in both total IGF-I and IGFBP-3. As a result, serum IGFBP-3-bound IGF-I and IGFBP-3 bound IGF-II, the main reservoirs of serum IGFs, were severely affected. This suggests that the decrease in serum IGFPB-3-bound IGF-I and IGFBP-3-bound IGF-II might have a negative effect for growth promotion and other biological effects of IGF-I and IGF-II. Finally, the estimation of serum IGFBP-3-bound IGF-I, or the percentage of total IGF-I and IGF-II bound to IGFBP-3, might be useful markers in the diagnosis of GHD.     

Characterization of a chimeric monoclonal antibody against the insulin-like growth factor-I receptor

The insulin-like growth factors (IGFs) signaling system has been shown to play important roles in neoplasia. The IGF receptor type 1 (IGF-IR) is overexpressed in many types of solid and hematopoietic malignancies, and there is substantial experimental and clinical evidence that targeting IGF-IR is a promising therapeutic strategy against cancer. It has been previously reported that a mouse monoclonal antibody (mAb), 4G11, blocked IGF-I binding to IGF-IR and downregulated the IGF-IR in MCF-7 cells. We cloned this antibody, constructed a human-mouse chimeric antibody, designated m590, and characterized it. The chimeric IgG1 m590 bound to cell-associated IGF-IR on NWT c43 stably transfected cells and MCF-7 breast cancer cells as efficiently as the parental murine antibody. Using purified IGF-IR extracellular domains, we found that both the chimeric m590 and the parental 4G11 antibodies bind to conformational epitopes on IGF-IR. Neither of these antibodies bound to the insulin receptor (IR) ectodomain. Furthermore, IgG1 m590 blocked the binding of IGF-I and IGF-II to IGF-IR, and inhibited both IGF-I and IGF-II induced phosphorylation of IGF-IR in MCF-7 cells. These results suggest that m590 could be an useful antibody in diagnosis and treatment of cancer, as well as a research tool.     

Characterization of a chimeric monoclonal antibody against the insulin-like growth factor-I receptor

The insulin-like growth factors (IGFs) signaling system has been shown to play important roles in neoplasia. The IGF receptor type 1 (IGF-IR) is overexpressed in many types of solid and hematopoietic malignancies, and there is substantial experimental and clinical evidence that targeting IGF-IR is a promising therapeutic strategy against cancer. It has been previously reported that a mouse monoclonal antibody (mAb), 4G11, blocked IGF-I binding to IGF-IR and downregulated the IGF-IR in MCF-7 cells. We cloned this antibody, constructed a human-mouse chimeric antibody, designated m590, and characterized it. The chimeric IgG1 m590 bound to cell-associated IGF-IR on NWT c43 stably transfected cells and MCF-7 breast cancer cells as efficiently as the parental murine antibody. Using purified IGF-IR extracellular domains, we found that both the chimeric m590 and the parental 4G11 antibodies bind to conformational epitopes on IGF-IR. Neither of these antibodies bound to the insulin receptor (IR) ectodomain. Furthermore, IgG1 m590 blocked the binding of IGF-I and IGF-II to IGF-IR, and inhibited both IGF-I and IGF-II induced phosphorylation of IGF-IR in MCF-7 cells. These results suggest that m590 could be an useful antibody in diagnosis and treatment of cancer, as well as a research tool.     

Expression and function of insulin-like growth factor receptors on anti-CD3-activated human T lymphocytes.

The pattern of expression of receptors for insulin-like growth factors (IGF-I and IGF-II) and insulin was studied on monocyte-depleted human peripheral blood T cells activated via anti-CD3. Binding assays demonstrated the sequential appearance of receptors for IGF-I, IGF-II, and insulin on activated T cells. IGF-IR appeared early, their expression reaching maximum levels at or before the peak of cellular proliferation. IGF-IIR expression generally followed that of the IGF-IR and was more transient, with increases and decreases in expression paralleling the rise and decline of cellular proliferation. Insulin receptor expression remained low throughout the activation time course. The identity of the IGFR on anti-CD3-activated T cells was confirmed in affinity cross-linking experiments. These data demonstrated a 135,000 Mr peptide that specifically binds radiolabeled IGF-I and corresponds to the alpha subunit of the type I IGF-IR, and a 260,000 Mr peptide that specifically binds radiolabeled IGF-II and corresponds to the type II IGFR. We have additionally found that IGF-I and IGF-II (in nanomolar concentrations) produce as much as a threefold enhancement of T cell proliferation early in the activation process, correlating with the early appearance of IGF-IR. The effect of both IGF appeared to be mediated through the type I receptor, since an antibody (alpha IR3), which blocks binding to the alpha subunit of this receptor, inhibited enhancement by up to 83%. Furthermore, we have found expression of IGF-IR on T cells after activation to be associated with both CD4+ and CD8+ T cell subpopulations. These observations provide a foundation for investigating the contribution of IGF in regulating T cell proliferation, differentiation, and effector function.     

Hypertrophy of cultured adult rat ventricular cardiomyocytes induced by antibodies against the insulin-like growth factor (IGF)-I or the IGF-I receptor is IGF-II-dependent

Antibodies against the insulin-like growth factor-I (IGF-I) or the IGF-I receptor (IGF-IR) directly initiate a rapid (within 6 h) hypertrophy of isolated adult rat ventricular cardiomyocytes cultured in the absence of serum. Further, cardiomyocytes treated with either of these agonistic antibodies upregulate the expression of their genes for insulin-like growth factor-II (IGF-II) and the IGF-II receptor (IGF-IIR). Genistein, an inhibitor of the tyrosine kinase IGF-IR, also induces the cardiomyocytes to hypertrophy. Anti-IGF-II antibody inhibits the cardiomyocyte hypertrophy induced by anti-IGF-I and anti-IGF-IR antibodies or by genistein. Results are consistent with a model in which local production of IGF-II is upregulated when the IGF-IR signaling pathway is blocked and in which an IGF-II-mediated pathway, likely involving the IGF-IIR, then stimulates hypertrophy of the cardiomyocytes.     

Loss of insulin-like growth factor II receptor expression promotes growth in cancer by increasing intracellular signaling from both IGF-I and insulin receptors

The insulin-like growth factor-II receptor (IGF-IIR) is frequently mutated or deleted in some malignant human tumors, suggesting that the IGF-IIR is a tumor suppressor. However, the exact mechanism by which IGF-IIR suppresses growth in tumors has not been definitively established. We demonstrate that IGF-IIR-deficient murine L cells (D9) have higher growth rates than IGF-IIR-positive L cells (Cc2) in response to IGF-II. IGF-II levels are higher in growth-conditioned medium from D9 versus Cc2 cells. Receptor neutralization studies and measurements of insulin receptor substrate 1 phosphorylation confirm that the enhanced growth of D9 cells is due to increased stimulation of the IGF-I and insulin receptors by IGF-II. In contrast, the levels of secreted latent and active transforming growth factor beta (TGF-beta) are similar for both D9 and Cc2 cells, indicating that the slower growth of Cc2 cells is not due to activation of latent TGF-beta by IGF-IIR and growth inhibition. The results directly demonstrate that down regulation of the IGF-IIR promotes the growth of transformed D9 cells by sustaining IGF-II, which binds to and activates IGF-IR and insulin receptor to increase intracellular growth signals.     

Primary structure of a putative receptor for a ligand of the insulin family

Nucleotide sequence analysis of human and guinea pig genomic DNA encoding a new member of the insulin receptor (IR) family revealed that the predicted primary structure of this IR-related protein is as similar to the IR and insulin-like growth factor (IGF) I receptor as the IR and IGF-IR are to each other. The conservation of this IR-related sequence among mammals and with the IR and IGF-IR suggests that this IR-related protein is a novel receptor for insulin, IGF-I, IGF-II, or an as yet unidentified peptide hormone or growth factor belonging to the insulin family.     

Potency of Full- Length MGF to Induce Maximal Activation of the IGF-I R Is Similar to Recombinant Human IGF-I at High Equimolar Concentrations

Aims

To compare full-length mechano growth factor (full-length MGF) with human recombinant insulin-like growth factor-I (IGF-I) and human recombinant insulin (HI) in their ability to activate the human IGF-I receptor (IGF-IR), the human insulin receptor (IR-A) and the human insulin receptor-B (IR-B), respectively. In addition, we tested the stimulatory activity of human MGF and its stabilized analog Goldspink-MGF on the IGF-IR.

Methods

The effects of full-length MGF, IGF-I, human mechano growth factor (MGF), Goldspink-MGF and HI were compared using kinase specific receptor activation (KIRA) bioassays specific for IGF-I, IR-A or IR-B, respectively. These assays quantify activity by measuring auto-phosphorylation of the receptor upon ligand binding.

Results

IGF-IR: At high equimolar concentrations maximal IGF-IR stimulating effects generated by full-length MGF were similar to that of IGF-I (89-fold vs. 77-fold, respectively). However, EC50 values of IGF-I and full-length MGF for the IGF-I receptor were 0.86 nmol/L (95% CI 0.69–1.07) and 7.83 nmol/L (95% CI: 4.87–12.58), respectively. No IGF-IR activation was observed by human MGF and Goldspink-MGF, respectively. IR-A/IR-B: At high equimolar concentrations similar maximal IR-A stimulating effects were observed for full -length MGF and HI, but maximal IR-B stimulation achieved by full -length MGF was stronger than that by HI (292-fold vs. 98-fold). EC50 values of HI and full-length MGF for the IR-A were 1.13 nmol/L (95% CI 0.69–1.84) and 73.11 nmol/L (42.87–124.69), respectively; for IR-B these values were 1.28 nmol/L (95% CI 0.64–2.57) and 35.10 nmol/L (95% 17.52–70.33), respectively.

Conclusions

Full-length MGF directly stimulates the IGF-IR. Despite a higher EC50 concentration, at high equimolar concentrations full-length MGF showed a similar maximal potency to activate the IGF-IR as compared to IGF-I. Further research is needed to understand the actions of full-length MGF in vivo and to define the physiological relevance of our in vitro findings.     

Involvement of ligand occupancy in Insulin-like growth factor-I (IGF-I) induced cell growth in osteoblast like MC3T3-E1 cells

Growth factors and matrix proteins regulate the proliferation and differentiation of osteoblasts. The insulin-like growth factor (IGF) system comprises IGF-I, IGF-II, and six high-affinity IGF-binding proteins (IGFBPs). IGFs stimulate cell growth in many types of tissue; IGF-binding proteins regulate cellular actions and can affect cell growth. IGF-I is involved in differentiation, proliferation, and matrix formation in osteoblasts; IGFBP-5 is associated with the extracellular matrix (ECM) and can potentiate the actions of IGF-I. We investigated the effect of ECM proteins on the responses of MC3T3-E1 osteoblast cells to IGF-I and IGFBP-5. In addition, because extracellular signal-regulated kinases 1 and 2 (Erk 1/2) affect cell growth, we evaluated the effects of IGFBP-5 on Erk 1/2 phosphorylation in MC3T3-E1 cells. IGF-I caused an increase in IGFBP-5 expression in cultured MC3T3-E1 cells, and IGF-I plus IGFBP-5 significantly increased cell growth. Likewise, the addition of IGF-I and IGFBP-5 to cultured MC3T3-E1 cells increased the synthesis of the ECM proteins osteopontin (OPN) and thrombospondin-1 (TSP-1), which can bind to alphaVbeta3 integrin receptors on the cell surface. By contrast, the addition of an antibody against ECM proteins inhibited the effects of OPN and TSP-1 on IGFBP-5 expression. The stimulatory effect of IGFBP-5 was mediated via Erk 1/2 activation. These data suggest that IGFBP-5 regulates Erk 1/2 phosphorylation in cultured MC3T3-E1 cells via ECM proteins that may ultimately stimulate the growth of osteoblasts. We determined whether occupation of the alphaVbeta3 integrin receptor affects IGF-I receptor (IGF-IR)-mediated signaling and function in MC3T3-E1 osteoblast cells. Occupation of the alphaVbeta3 integrin receptor with ECM proteins induced IGF-I-stimulated IGF-IR phosphorylation. Conversely, in the presence of the alphaVbeta3-specific disintegrin echistatin, IGF-I-stimulated IGF-IR activation was inhibited. IGF-I-stimulated IGF-IR phosphorylation was accompanied by IRS-1 phosphorylation and MAPK activation. However, these effects were attenuated by echistatin. Thus, occupancy of the alphaVbeta3 disintegrin receptor modulates IGF-I-induced IGF-IR activation and IGF-IR-mediated function in MC 3T3-E1 osteoblasts.     

Insulin-like growth factor family and combined antisense approach in therapy of lung carcinoma

BACKGROUND: Perturbation in a level of any peptide from insulin-like growth factor (IGF) family (ligands, receptors, and binding proteins) seems to be implicated in lung cancer formation; IGF ligands and IGF-I receptor through their mitogenic and anti-apoptotic action, and the mannose 6-phosphate/insulin-like growth factor II receptor (M6-P/IGF-IIR) possibly as a tumor suppressor. MATERIALS AND METHODS: To determine the identity, role, and mutual relationship of IGFs in lung cancer growth and maintenance, we examined IGF's gene (by RT-PCR) and protein (by immunohistochemistry) expression in 69 human lung carcinoma tissues. We also examined IGF-I receptor numbers (Scatchard analysis) and IGF-II production and release (by Western blot) in IGF-II/IGF-IR mRNA positive and negative lung carcinomas. Finally, the potential role of IGF-IR and IGF-II as growth promoting factors in lung cancer was studied using antisense oligodeoxynucleotides that specifically inhibit IGF-IR and IGF-II mRNA. RESULTS: Thirty-two tumors were positive for IGF-I, 39 for IGF-II, 48 for IGF-IR, and 35 for IGFBP-4 mRNA. Seventeen tumors were concomitantly positive for all four IGFs, whereas 34 were positive for IGF-II, IGF-IR, and IGFBP-4 mRNA. An elevated amount of IGF-II peptide was secreted into the growth medium of cell cultures established from five different IGF-II/IGF-IR mRNA positive lung cancer tissues. The cells also expressed elevated numbers of IGF-IR. Nine IGF-II-negative and 19 IGF-II-positive lung cancers of different stages were selected, and M6-P/ IGF-II receptor was determined immunohistochemically. Most of the IGF-II-negative tumors were strongly positive for M6-P/IGF-IIR. IGF-II-positive tumors were mostly negative for M6-P/IGF-II receptors. Antisense oligodeoxynucleotides to IGF-II significantly inhibited, by 25-60%, the in vitro growth of all six lung cancer cell lines. However, the best results (growth inhibition of up to 80%) were achieved with concomitant antisense treatment (to IGF-IR and IGF-II). CONCLUSION: Our data suggest that lung cancer cells produce IGF-IR and IGF-II, which in turn stimulates their proliferation by autocrine mechanism. Cancer cell proliferation can be abrogated or alleviated by blocking the mRNA activity of these genes indicating that an antisense approach may represent an effective and practical cancer gene therapy strategy.     

A possible role for insulin-like growth factor-binding protein-3 autocrine/paracrine loops in controlling hepatocellular carcinoma cell proliferation.

Hepatocellular carcinoma (HCC) is a common malignancy, but treatment outcomes have generally remained poor. Specific factors important for the pathogenesis of HCC are incompletely understood. Insulin-like growth factors (IGFs) are potent autocrine and paracrine mitogens for liver cancer cell proliferation, and their bioactivity is reduced by IGF-binding protein 3 (IGFBP-3). In the present study, we report that IGFBP-3 protein levels were either undetectable (28.5%) or low (71.5%) in human HCC samples examined compared with matched non-neoplastic liver tissue by Western blotting. IGFBP-3 was localized to nontumor liver cells by immunohistochemistry with greater immunointensity than neoplastic liver cells. Levels of type I receptor (IGF-IR) were found to be low in approximately 39% of human HCC samples examined compared with matched nontumor tissues. IGF-II was overexpressed in 32%, whereas IGF-I expression was decreased in 100% of HCC samples. In vitro studies revealed that IGF-I and IGF-II induced HepG2 cell proliferation in a dose-dependent manner. Treatment of HepG2 cells with either human recombinant IGFBP-3 (hrIGFBP-3) or IGF-II antibody led to a significant reduction in cell proliferation. Cotreating these cells with hrIGFBP-3 significantly attenuated the mitogenic activity of IGF-I. IGF-I-induced phosphorylation of IGF-IR beta subunit, IRS-1, mitogen-activated protein kinase, Elk-1, and Akt-1 as well as phosphatidylinositol 3'-kinase activity was significantly attenuated when hepG2 cells were pretreated with hrIGFBP-3. Our data indicate that loss of autocrine/paracrine IGFBP-3 loops may lead to HCC tumor growth and suggest that modulating production of the IGFs, IGFBP-3, and IGF-IR may represent a novel approach in the treatment of HCC.     

Does IGF-I stimulate pancreatic islet cell growth?

Both IGF-I and its receptor (IGF-IR) are specifically expressed in various cell types of the endocrine pancreas. IGF-I has long been considered a growth factor for islet cells as it induces DNA synthesis in a glucose-dependent manner, prevents Fas-mediated autoimmune β-cell destruction and delays onset of diabetes in non-obese diabetic (NOD) mice. Islet-specific IGF-I overexpression promotes islet cell regeneration in diabetic mice. However, in the last few years, results from most gene-targeted mice have challenged this view. For instance, combined inactivation of insulin receptor and IGF-IR or IGF-I and IGF-II genes in early embryos results in no defect on islet cell development; islet β-cell-specific inactivation of IGF-IR gene causes no change in β-cell mass; liver- and pancreatic-specific IGF-I gene deficiency (LID and PID mice) suggests that IGF-I exerts an inhibitory effect on islet cell growth albeit indirectly through controlling growth hormone release or expression of Reg family genes. These results need to be evaluated with potential gene redundancy, model limitations, indirect effects and ligand-receptor cross-activations within the insulin/IGF family. Although IGF-I causes islet β-cell proliferation and neogenesis directly, what occur in normal physiology, pathophysiology or during development of an organism might be different. Locally produced and systemic IGF-I does not seem to play a positive role in islet cell growth. Rather, it is probably a negative regulator through controlling growth hormone and insulin release, hyperglycemia, or Reg gene expression. These results complicate the perspective of an IGF-I therapy for β-cell loss.     

Insulin-like growth factor-binding protein-3 activates a phosphotyrosine phosphatase. Effects on the insulin-like growth factor signaling pathway

The proliferative action of insulin-like growth factors (IGF-I and -II) is mediated via the type I IGF receptor (IGF-IR) and is modulated by their association with high affinity binding proteins, IGFBP-1 to -6. We recently found that, in addition to its ability to bind IGFs, IGFBP-3 also inhibits IGF-IR activation independently of IGF binding and without interacting directly with IGF-IR. Here, we show that IGFBP-3 is capable of blocking the signal triggered by IGFs. Breast carcinoma-derived cells (MCF-7) were stimulated by des(1-3)IGF-I or [Gln(3),Ala(4),Tyr(15),Leu(16)]IGF-I, two IGF analogues with intact affinity for IGF-IR, but with weak or virtually no affinity for IGFBPs, then incubated with IGFBP-3. The activated IGF-IR was desensitized through reversal of its autophosphorylation, following which both phosphatidylinositol 3-kinase and p42(MAPK) activities were depressed. Direct measurement of phosphotyrosine phosphatase activity and reconstitution experiments using tyrosine-phosphorylated insulin receptor substrate-1 (IRS-1) indicated that IGFBP-3 activated a phosphotyrosine phosphatase (PTPase). This action appeared to be peculiar to IGFBP-3 among the IGFBPs, since neither IGFBP-1 nor IGFBP-5 (structurally the closest to IGFBP-3), had any such effect. Several cell lines derived from normal or tumor cells responsive to IGF-I were used to show that IGFBP-3-stimulated PTPase is cell type-specific. Although the precise nature of the phosphatase remains to be determined, the results of this study demonstrate that IGFBP-3 stimulates a phosphotyrosine phosphatase activity that down-regulates the IGF-I signaling pathway, suggesting a major role for IGFBP-3 in regulating cell proliferation.