Isolation from human cerebrospinal fluid of a new insulin-like growth factor-binding protein with a selective affinity for IGF-II

In biological fluids IGF-I and IGF-II are bound to specific, high-affinity binding protein (BPs). Two human BPs have been isolated, one from serum, which is GH-dependent, the other from amniotic fluid (AF BP), and their cDNAs have recently been cloned. We report here the isolation of another, new species from cerebrospinal fluid (CSF) where this BP predominates. The protein was purified to homogeneity by a four-step procedure: gel filtration, chromatofocusing, hydrophobic-interaction chromatography and reverse-phase chromatography. Thereafter, SDS-polyacrylamide gel electrophoresis gave an Mr of 34,000 (non-reduced), chromatofocusing gave an isoelectric point of 5.0m and its affinity for IGF-II (3 x 10(10) M-1) was 10 times that for IGF-I. The N-terminal amino acid sequence of the first 15 residues determined in a BP preparation from the CSF of children was Leu-Ala-Pro-Gly-(/)-Gly-Gln-Gly-Val-Gln-Ala-Gly-Ala-Pro-Gly. A similar sequence was found for adult CSF, apart from residues 12 and 13 (-Leu-Leu-). These are highly analogous with the sequences starting from residue 69 of the GH-dependent BP, and from residue 61 of the AF BP. The new BP isolated is therefore related to, but distinct from, the other human BPs.     

Characterization of insulin-like growth factor binding proteins from human breast cancer cells

The insulin-like growth factor binding proteins (IGF-BPs) are structurally and immunologically distinct from the IGF type 1 or type 2 receptors and are characterized by two major forms: a large, GH-dependent BP found in human plasma (Mr = 150 k) and a small GH-independent BP (Mr = 28-42 k) present in human plasma, amniotic fluid, and HEP G2 cells. Using affinity cross-linking techniques, we have identified several binding proteins secreted by human breast cancer cell lines (Hs578T, MDA-231, T-47D, and MCF-7). Under nonreducing conditions these proteins migrated at an apparent Mr = 35, 28, 27, and 24 k, while reducing conditions revealed bands of apparent Mr = 35, 32, 27, and 24 k. Competitive binding studies in T-47D-conditioned media demonstrated that these BPs bound more IGF-II than IGF-I, and that IGF-II potently inhibited binding of either IGF-I or -II. Immunological studies using a polyclonal antibody against the HEP G2 small BP revealed no immunoreactive BP in conditioned media from MCF-7 and T-47D and only slight immunoreactivity in conditioned media from Hs578T and MDA 231. Analysis by Northern blot, using a probe from the cDNA sequence of the HEP G2 BP, demonstrated that Hs578T and MDA-231 cell lines contained small amounts of the 1.65 kilobase mRNA characteristic of the HEP G2 BP, while MCF-7 and T-47D tested negative.(ABSTRACT TRUNCATED AT 250 WORDS)     

A Unique Bivalent Binding and Inhibition Mechanism by the Yatapoxvirus Interleukin 18 Binding Protein

Interleukin 18 (IL18) is a cytokine that plays an important role in inflammation as well as host defense against microbes. Mammals encode a soluble inhibitor of IL18 termed IL18 binding protein (IL18BP) that modulates IL18 activity through a negative feedback mechanism. Many poxviruses encode homologous IL18BPs, which contribute to virulence. Previous structural and functional studies on IL18 and IL18BPs revealed an essential binding hot spot involving a lysine on IL18 and two aromatic residues on IL18BPs. The aromatic residues are conserved among the very diverse mammalian and poxviruses IL18BPs with the notable exception of yatapoxvirus IL18BPs, which lack a critical phenylalanine residue. To understand the mechanism by which yatapoxvirus IL18BPs neutralize IL18, we solved the crystal structure of the Yaba-Like Disease Virus (YLDV) IL18BP and IL18 complex at 1.75 Å resolution. YLDV-IL18BP forms a disulfide bonded homo-dimer engaging IL18 in a 2∶2 stoichiometry, in contrast to the 1∶1 complex of ectromelia virus (ECTV) IL18BP and IL18. Disruption of the dimer interface resulted in a functional monomer, however with a 3-fold decrease in binding affinity. The overall architecture of the YLDV-IL18BP:IL18 complex is similar to that observed in the ECTV-IL18BP:IL18 complex, despite lacking the critical lysine-phenylalanine interaction. Through structural and mutagenesis studies, contact residues that are unique to the YLDV-IL18BP:IL18 binding interface were identified, including Q67, P116 of YLDV-IL18BP and Y1, S105 and D110 of IL18. Overall, our studies show that YLDV-IL18BP is unique among the diverse family of mammalian and poxvirus IL-18BPs in that it uses a bivalent binding mode and a unique set of interacting residues for binding IL18. However, despite this extensive divergence, YLDV-IL18BP binds to the same surface of IL18 used by other IL18BPs, suggesting that all IL18BPs use a conserved inhibitory mechanism by blocking a putative receptor-binding site on IL18.     

Receptor Binding, Endocytosis, and Mitogenesis of Insulin-Like Growth Factors I and II in Fetal Rat Brain Neurons

Cell surface binding, internalization, and biological effects of insulin-like growth factors (IGFs) I and II have been studied in primary neuronal cultures from developing rat brain (embryonic day 15). Two types of IGF binding sites are present on the cell surface. The IGF-I receptor alpha-subunit (Mr 125,000) binds IGF-I with a KD of 1 nM and IGF-II with 10 times lower affinity. The mannose-6-phosphate (Man-6-P)/IGF-II receptor (Mr 250,000) binds IGF-II with a KD of 0.5 nM and IGF-I with 100 times lower affinity. Surface-bound IGF-I and IGF-II are internalized by their respective receptors binding and internalization of IGF-II but not those of IGF-I. Neuronal synthesis of RNA and DNA is increased twofold by IGF-I with 10 times higher potency than IGF-II. Antibody 3637, which blocks receptor binding of IGF-II, has no effect on the DNA response to IGF-I or IGF-II. Double immunocytochemical staining with antibodies to bromodeoxyuridine and neurofilament shows that greater than 80% of the bromodeoxyuridine-positive cells become neurofilament positive. It is concluded that IGF-I and IGF-II bind to two receptors on the surface of neuronal precursor cells that mediate endocytosis and degradation of IGF-I and IGF-II. Proliferation of neuronal precursor cells is stimulated by IGF-I and IGF-II via activation of the IGF-I receptor.     

Insulin-like growth factor-binding protein from human plasma. Purification and characterization

Insulin-like growth factor (IGF)-binding protein (BP) has been purified from Cohn fraction IV of human plasma by acidification, ion exchange to remove endogenous ligands, and affinity chromatography on agarose-IGF-II. The pure protein appeared as a single peak by high performance reverse-phase and gel permeation chromatography (molecular mass, 45-50 kDa), but on sodium dodecyl sulfate-polyacrylamide gel electrophoresis gave a major band at 53 kDa and a minor band at 47 kDa, unreduced, or 43 and 40 kDa, respectively, reduced. The two bands stained for both protein and carbohydrate. After storage at 2 degrees C for 5 months at pH 3, two additional bands, at 26 and 22 kDa on unreduced gels, were also present. Autoradiography after affinity labeling with IGF-I or IGF-II tracer revealed a single labeled band of 61 kDa. BP, quantitated using a specific radioimmunoassay, was retained by agarose-immobilized IGF-I, IGF-II, concanavalin A, and wheat germ lectin, but not Helix pomatia lectin. Competitive binding curves using pure BP and human IGF-I and IGF-II as both labeled and unlabeled ligands indicated association constants of 2-3 X 10(10) liters/mol for both peptides, with a slightly higher affinity for IGF-II than IGF-I, and 0.9 binding sites for either peptide per 53-kDa protein. The exact relationship of this acid-stable IGF BP to the 150-kDa complex from which it is derived remains to be determined.     

Regulation of breast cancer growth by insulin-like growth factors

The IGFs may be important autocrine, paracrine or endocrine growth factors for human breast cancer. IGF-I and II stimulate growth of cultured human breast cancer cells. IGF-I is slightly more potent, paralleling its higher affinity for the IGF-I receptor. Antibody blockade of the IGF-I receptor inhibits growth stimulation induced by both IGFs, suggesting that this receptor mediates the growth effects of both peptides. However, IGF-I receptor blockade does not inhibit estrogen (E₂)-induced growth suggesting that secreted IGFs are not the major mediators of E₂ action. Several breast cancer cell lines express IGF-II mRNA by both Northern analysis and RNase protection assay. IGF-II activity is found in conditioned medium by radioimmuno and radioreceptor assay, after removal of somatomedin binding proteins (BP) which are secreted in abundance. IGF-I is undetectable. BPs of 25 and 40 K predominate in ER-negative cell lines while BPs of 36 K predominate in ER-positive cells. Blockade of the IGF-I receptor inhibits anchorage-independent and monolayer growth in serum of a panel of breast cancer cell lines. Growth of one line (MDA-231) was also inhibited in vivo by receptor antibody treatment of nude mice. The antibody had no effect on growth of MCF-7 tumors. These data suggest the IGFs are important regulators of breast cancer cell proliferation and that antagonism of this pathway may offer a new treatment strategy.     

Effects of bisphosphonates on keratinocytes and fibroblasts having a role in the development of osteonecrosis of the jaw

Bisphosphonates (BPs) have became the treatment of choice for the prevention of skeletal complications in cancer patients with bone metastases as well as in patients suffering from osteoporosis, Paget's disease and rheumatoid arthritis. Osteonecrosis of the jaw (ONJ) is a recently described complication associated with the use of BPs in which the key finding is exposed necrotic bone in the oral cavity. Often, the precipitating event appears to be a dental invasive procedure. We recently provided evidence that ONJ is associated with dental extractions and use of dentures. It has been reported that the primary lesion lies in the bone and it is related to over-suppression of bone turnover, but it is unclear why such a lesion should present with loss of the soft tissue covering the jawbone. We propose that BP could be impairing molecular signalling not only of osteoblasts and osteoclasts but also of fibroblasts and keratinocytes, via cell to cell endocrine and paracrine interactions in a double manner. Such an impairment would result to fibroblast and keratinocyte impaired multiplication, proliferation and migration thereby leading to defective mucosal wound healing. This provides an open entry point for the oral flora to reach the underlying jawbone which is considered to have poor metabolic and immune properties when under BP treatment. We demonstrate that ONJ is associated with mucosal damage, which could be mediated via BP induced soft tissue toxicity. BPs have been reported to promote keratinocyte and fibroblast apoptosis and to impair various cellular activities like apoptosis, RANK, RANK-L and OPG signalling, bone morphogenetic protein signalling, growth factor signalling, immune homeostasis and wound healing. We discuss potential consequences of the above hypothesis for practitioners and investigators.     

Molecular characterization of the interaction between porins of Neisseria gonorrhoeae and C4b-binding protein

Neisseria gonorrhoeae, the causative agent of gonorrhea, is a natural infection only in humans. The resistance of N. gonorrhoeae to normal human serum killing correlates with porin (Por)-mediated binding to the complement inhibitor, C4b-binding protein (C4BP). The entire binding site for both porin molecules resides within complement control protein domain 1 (CCP1) of C4BP. Only human and chimpanzee C4BPs bind to Por1B-bearing gonococci, whereas only human C4BP binds to Por1A strains. We have now used these species-specific differences in C4BP binding to gonococci to map the porin binding sites on CCP1 of C4BP. A comparison between human and chimpanzee or rhesus C4BP CCP1 revealed differences at 4 and 12 amino acid positions, respectively. These amino acids were targeted in the construction of 13 recombinant human mutant C4BPs. Overall, amino acids T43, T45, and K24 individually and A12, M14, R22, and L34 together were important for binding to Por1A strains. Altering D15 (found in man) to N15 (found in rhesus) introduced a glycosylation site that blocked binding to Por1A gonococci. C4BP binding to Por1B strains required K24 and was partially shielded by additional glycosylation in the D15N mutant. Only those recombinant mutant C4BPs that bound to bacteria rescued them from 100% killing by rhesus serum, thereby providing a functional correlate for the binding studies and highlighting C4BP function in gonococcal serum resistance.     

Comparative analysis of two biliproteins, BP1 and BP2, from haemolymph of cabbage white butterfly, Pieris rapae

Two blue-pigment binding proteins, BP1 and BP2, are present in larval and pupal haemolymph of cabbage white butterfly, Pieris rapae, and fluctuate in expression during development. Both BP1 and BP2 are found in pupal haemolymph in varying proportions as well as in adult haemolymph, while only small amounts of BP2 are found in larval haemolymph. BPs are separated by 75% ammonium sulfate, and then purified effectively by ion exchange column chromatography and preparative gel electrophoresis. It was shown that BP1 and BP2 have molecular masses of 20,244 and 19,878 Da, and isoelectric points of 7.0 and 6.8, respectively. Considering their amino acid compositions and N-terminal amino acid sequences, the two proteins are almost identical except the first N-terminal amino acid. The first amino acid of BP1 is asparagine, whereas the initial residue of BP2 is aspartic acid. Anti-BP1 cross-reacts with BP2, indicating that they have immunological homogeneity. Western blotting analyses revealed that only BP1 was present in the larval tissues such as fat body, integument, muscle, and hindgut. However, BP1 was not found in midgut, Malphigian tubules, and silk gland. BP1 was also present in the protein bodies, and both cuticle and hemocoel sides of larval epidermis cells by the transmission electron microscopic observation. The information in this report will facilitate studies on the molecular biology and biological significance of insect BPs.     

Identification of a type 1 insulin-like growth factor binding protein (IGF BP) in serum from rats with diabetes mellitus

Circulating insulin-like growth factor binding protein (IGF BP) activity is increased in animals with streptozotocin-induced diabetes. Separation of BPs by SDS/PAGE for ligand and immunoblot analysis revealed that a 32,000 molecular weight BP is present and increased in diabetic serum. This BP is immunologically distinct from the low molecular weight fetal rat BP (rBP2) and is related to the human amniotic fluid BP (hBP1) that is increased in patients with insulin dependent diabetes mellitus.     

Computational model for the IGF-II/IGF2r complex that is predictive of mutational and surface plasmon resonance data

Insulin-like growth factors (IGFs) are key regulators of cell proliferation, differentiation, and transformation, and are thus pivotal in cancer, especially breast, prostate, and colon neoplasm. Their potent mitogenic and anti-apoptotic actions depend primarily on their availability to bind to the signaling IGF cell surface receptors. One mechanism by which IGF-II availability is thought to be modulated is by binding to the nonsignaling IGF-II receptor (IGF2R). This binding is essentially mediated by domain 11 in the multidomain IGF2R extracellular region. The crystal structure of domain 11 of the human IGF-II receptor (IGF2R-d11) has identified a putative IGF-II binding site, and a nuclear magnetic resonance (NMR) solution structure for the IGF-II ligand has also been characterized. These structures have now been used to model in silico the protein-protein interaction between IGF-II and IGF2R-d11 using the program 3D-Dock. Because the IGF-II data comprise an ensemble of 20 structures, all of which satisfy the NMR constraints, the docking procedure was applied to each member of the ensemble. Only those models in which residue Ile1572 of IGF2R-d11, known to be essential for the binding of IGF-II, was at the interface were considered further. These plausible complexes were then critically assessed using an array of analysis techniques including consideration of additional mutagenesis data. One model was strongly supported by these analyses and is discussed here in detail. Furthermore, we demonstrate in vitro experimental support for this model by studying the binding of chimeras of IGF-I and IGF-II to IGF2R fragments.     

Characteristics of binding of insulin-like growth factor (IGF)-I and IGF-II analogues to the type 1 IGF receptor determined by BIAcore analysis.

Insulin-like growth factor (IGF) binding to the type 1 IGF receptor (IGF1R) elicits mitogenic effects, promotion of differentiation and protection from apoptosis. This study has systematically measured IGF1R binding affinities of IGF-I, IGF-II and 14 IGF analogues to a recombinant high-affinity form of the IGF1R using BIAcore technology. The analogues assessed could be divided into two groups: (a) those designed to investigate binding of IGF-binding protein, which exhibited IGF1R-binding affinities similar to those of IGF-I or IGF-II; (b) those generated to probe IGF1R interactions with greatly reduced IGF1R-binding affinities. The relative binding affinities of IGF-I analogues and IGF-I for the IGF1R determined by BIAcore analysis agreed closely with existing data from receptor-binding assays using cells or tissue membranes, demonstrating that BIAcore technology is a powerful tool for measuring affinities of IGFs for IGF1R. In parallel studies, IGF1R-binding affinities were related to ability to protect against serum withdrawal-induced apoptosis in three different assays including Hoechst 33258 staining, cell survival, and DNA fragmentation assays using the rat pheochromocytoma cell line, PC12. In this model system, IGF-I and IGF-II at low nanomolar concentrations are able to prevent apoptosis completely. We conclude that ability to protect against apoptosis is directly related to ability to bind the IGF1R.     

Secretion of noncomplexed 'Big' (10-18 kD) forms of insulin-like growth factor-II by 12 soft tissue sarcoma cell lines

The paraneoplastic production of pro-insulin-like growth factor-II (IGF-II) forms causes tumour hypoglycaemias and presumably also has an effect on tumour cell growth. We investigated the molecular weights of IGF-II forms and their ability to form complexes with IGF binding proteins (IGFBPs) in conditioned culture media (CM) from 12 paediatric soft tissue sarcoma (STS) cell lines and from two healthy fibroblast lines. Untreated CM were separated by size exclusion chromatography using biocompatible HPLC. Subsequently, IGF-II, IGFBP-2 and IGFBP-3 were determined in the HPLC fractions by specific RIAs. In the CM, IGF-II concentrations between 0.5 and 8.6 ng/10(6) cells were measured but no IGF-I was detectable. Parallel to this investigation, a high IGF-II mRNA level averaging 44.4 +/- 29.7% was measured by semi-quantitative RT-PCR. The STS cell lines secreted a higher proportion of big-IGF-II forms reaching 10-18 kD (10-33% of the total IGF-II secreted) compared to the healthy fibroblasts (2.5-5%). At the same time, the proportion of IGF-II bound with IGFBP in complexes of 35- 70 kD and 150 kD was reduced by up to 85% in CM from tumour cells. The tumour cell lines apparently secrete a different spectrum of IGF-II forms than healthy fibroblasts. The reduced ability to form complexes with IGFBP and the higher molecular weight of the IGF-II forms produced by the tumour cells indicate that these forms could in fact be the known tumour-associated pro-IGF-II forms. Due to these characteristics, the big-IGF-II forms probably have an altered biological effect on the tumour cells when compared to IGF-II.     

Genome-wide association between branch point properties and alternative splicing

The branch point (BP) is one of the three obligatory signals required for pre-mRNA splicing. In mammals, the degeneracy of the motif combined with the lack of a large set of experimentally verified BPs complicates the task of modeling it in silico, and therefore of predicting the location of natural BPs. Consequently, BPs have been disregarded in a considerable fraction of the genome-wide studies on the regulation of splicing in mammals. We present a new computational approach for mammalian BP prediction. Using sequence conservation and positional bias we obtained a set of motifs with good agreement with U2 snRNA binding stability. Using a Support Vector Machine algorithm, we created a model complemented with polypyrimidine tract features, which considerably improves the prediction accuracy over previously published methods. Applying our algorithm to human introns, we show that BP position is highly dependent on the presence of AG dinucleotides in the 3' end of introns, with distance to the 3' splice site and BP strength strongly correlating with alternative splicing. Furthermore, experimental BP mapping for five exons preceded by long AG-dinucleotide exclusion zones revealed that, for a given intron, more than one BP can be chosen throughout the course of splicing. Finally, the comparison between exons of different evolutionary ages and pseudo exons suggests a key role of the BP in the pathway of exon creation in human. Our computational and experimental analyses suggest that BP recognition is more flexible than previously assumed, and it appears highly dependent on the presence of downstream polypyrimidine tracts. The reported association between BP features and the splicing outcome suggests that this, so far disregarded but yet crucial, element buries information that can complement current acceptor site models.     

Endocytosis of insulin-like growth factor II by a mini-receptor based on repeat 11 of the mannose 6-phosphate/insulin-like growth factor II receptor

The mannose 6-phosphate/insulin-like growth factor II receptor (M6P/IGF-II receptor) plays an important role in controlling the extracellular level of the insulin-like growth factor II (IGF-II) by mediating its binding at the cell surface and delivery to lysosomes. Loss of the receptor is associated with an accumulation of IGF-II, which can cause perinatal lethality if it is systemic, or local proliferation and tumorgenesis if it is spatially restricted. The extracytoplasmic domain of the receptor consists of 15 homologous repeats, of which repeat 11 carries the IGF-II-binding site of the multifunctional receptor. To investigate whether repeat 11 is sufficient to mediate binding and internalization of IGF-II, a construct consisting of repeat 11 fused to the transmembrane and cytoplasmic domain of the M6P/IGF-II receptor was transfected into mouse embryonic fibroblasts. The construct was expressed as a stable membrane protein which binds IGF-II with a 10-fold lower affinity as observed for the M6P/IGF-II receptor and is found at the cell surface and in endosomes. It mediates the internalization of IGF-II and its delivery to lysosomes, suggesting that it can function as a IGF-II mini-receptor controlling the extracellular IGF-II level.     

Binding of nitrogen‐containing bisphosphonates (N‐BPs) to the Trypanosoma cruzi farnesyl diphosphate synthase homodimer

Bisphosphonates (BPs) are a class of compounds that have been used extensively in the treatment of osteoporosis and malignancy‐related hypercalcemia. Some of these compounds act through inhibition of farnesyl diphosphate synthase (FPPS), a key enzyme in the synthesis of isoprenoids. Recently, nitrogen‐containing bisphosphonates (N‐BPs) used in bone resorption therapy have been shown to be active against Trypanosoma cruzi, the parasite that causes American trypanosomiasis (Chagas disease), suggesting that they may be used as anti‐trypanosomal agents. The crystal structures of TcFPPS in complex with substrate (isopentenyl diphosphate, IPP) and five N‐BP inhibitors show that the C‐1 hydroxyl and the nitrogen‐containing groups of the inhibitors alter the binding of IPP and the conformation of two TcFPPS residues, Tyr94 and Gln167. Isothermal titration calorimetry experiments suggest that binding of the first N‐BPs to the homodimeric TcFPPS changes the binding properties of the second site. This mechanism of binding of N‐BPs to TcFPPS is different to that reported for the binding of the same compounds to human FPPS. Proteins 2010. © 2009 Wiley‐Liss, Inc.     

Structurally different bisphosphonates exert opposing effects on alkaline phosphatase and mineralization in marrow osteoprogenitors

Bisphosphonates (BPs) are inhibitors of bone resorption and soft tissue calcification. The biological effects of the BPs in calcium-related disorders are attributed mainly to their incorporation in bone, enabling direct interaction with osteoclasts and/or osteoblasts through a variety of biochemical pathways. Structural differences account for the considerable differences in the pharmacological activity of BPs. We compared the effects of two structurally different compounds, alendronate and 2-(3′-dimethylaminopyrazinio)ethylidene-1,1-bisphosphonic acid betaine (VS-6), in an osteoprogenitor differentiation system. The BPs were examined in a bone marrow stromal-cell culture system, which normally results in osteoprogenitor differentiation. The drugs were present in the cultures from days 2 to 11 of osteogenic stimulation, a period estimated as being comparable to the end of proliferation and the matrix-maturation stages. We found that the two different BPs have opposing effects on specific alkaline phosphatase (ALP) activity, on stromal-cell proliferation, and on cell-mediated mineralization. These BPs differentially interact with cell-associated phosphohydrolysis, particularly at a concentration of 10⁻² of ALP K_m, in which alendronate inhibits whereas VS-6 did not inhibit phosphatase activity. VS-6 treatment resulted in similar and significantly increased mineralization at 10 and 1 μM drug concentrations, respectively. In contrast, mineralization was similar to control, and significantly decreased at 10 and 1 μM drug concentrations, respectively, under alendronate treatment. J. Cell. Biochem. 68:186–194, 1998. © 1998 Wiley-Liss, Inc.     

Interactions of cathepsin-D and insulin-like growth factor-II (IGF-II) on the IGF-II/mannose-6-phosphate receptor in human breast cancer cells and possible consequences on mitogenic activity of IGF-II

The lysosomal enzyme cathepsin-D (cath-D) and insulin-like growth factor-II (IGF-II), which share a common IGF-II/mannose-6-phosphate (M6P) transmembrane receptor, are both synthesized and secreted by breast cancer cells, upon which they might exert an intracrine/autocrine control on proliferation. We have evaluated the binding of 125I-immunopurified human cath-D in different breast cell membrane preparations. The concentration of high affinity M6P reversible binding sites (mean Kd, 0.85 nM) varied among the different breast cancer cells (0-0.82 pmol/mg membrane protein), but there was no correlation between the presence of steroid receptor and M6P-dependent binding. Cross-linking experiments with [125I]cath-D and [125I]IGF-II showed the formation of complexes with the 270,000 mol wt IGF-II/M6P receptor molecule which migrated, respectively, at 330,000 and 270,000 mol wt in 3-10% gradient sodium dodecyl sulfate-polyacrylamide gels. [125I]IGF-II cross-linking was increased by M6P (20% above control), whereas cath-D strongly inhibited IGF-II interaction by 80%. Conversely, IGF-II reduced [125I]cath-D cross-linking by 55%. Direct ligand binding on receptors transferred onto nitrocellulose sheets by Western blotting confirmed the interaction of both ligands on the same receptor molecule. By studying IGF-II's growth-promoting activity in these cells in a wide range of concentrations, we show that IGF-II triggers its mitogenic response via IGF-II/M6P receptor at low concentrations, whereas it is mainly acting via IGF-I receptor at high concentrations. Three lines of evidences lead us to that conclusion.(ABSTRACT TRUNCATED AT 250 WORDS)     

Presence of insulinlike growth factor receptors and lack of insulin receptors on fetal bovine smooth muscle cells

Summary Previous investigations have demonstrated specific receptors and associated mitogenic actions for insulin and insulinlike growth factors I and II (IGF-I and II) in postnatal bovine aortic smooth muscle. Using fetal tissue we have observed different patterns of binding and action for these peptides. Smooth muscle cells isolated from near-term fetal bovine aortae were studied in early passage. Specific receptors for both IGF-I and IGF-II were identified. Specific binding averaged 5.7%/2.5×10⁵ cells for IGF-I, and 16.2% for IGF-II, and 0.3% for insulin. High affinity K _d for both IGF receptors were nanomolar. IGF-II was fivefold less potent than IGF-I in displacing IGF-I binding. IGF-I showed no affinity for the IGF-II receptor. Insulin, at physiologic concentrations, was incapable of displacing either IGF-I or IGF-II binding. Cellular incorporation of [methyl-³H]thymidine was stimulated at the lowest dose of IGF-I tested, 0.5 ng/ml. IGF-II showed no effect up to 100 ng/ml, after which a sharp increase in incorporation was noted. Insulin had a similar effect only at concentrations >0.5 μg/ml, with a maximal response noted at 5 to 10 μg/ml. Our results indicate that fetal bovine aortic smooth muscle cells have an abundance of IGF receptors but lack specific insulin receptors. In addition, IGF-II binding levels are three times higher than for IGF-I. These results are consistent with observations in other species, in which a predominance of IGF over insulin receptors has been demonstrated in fetal tissue, and provide further evidence for a role for the IGFs in embryonic cellular metabolism. This project was supported by grants AM22190 (R. L. H.), AM28229 (R. G. R.) from the National Institutes of Health, Bethesda, MD, and Research Career Development Award AM01275 from the NIH (R. G. R.). Dr. Lee was the recipient of a fellowship award from the Juvenile Diabetes Foundation International and is currently supported by funds from the American Diabetes Association. Dr. Benitz is the recipient of a Clinician-Scientist Award from the American Heart Association, with funds contributed in part by the California Affiliate.