Use of insulin-like growth factor-binding protein-2 (IGFBP-2), IGFBP-3, and IGF-I for assessing growth hormone status in short children

Insulin-like growth factor-binding protein-2 and -3 (IGFBP-2 and -3) are members of a family of proteins that are present in extracellular fluids and bind IGF-I and -II. IGFBP-2 is regulated differently from IGF-I and IGFBP-3, because its serum concentrations are elevated in some adults with GH deficiency (GHD), whereas IGF-I and IGFBP-3 concentrations are usually decreased. The purposes of this study were to define the normal range of IGFBP-2 concentrations in children, to determine its efficacy in the diagnosis of GHD, and to compare the diagnostic value of measurements of the serum GH response to provocative testing with results of measurements of IGFBP-2, IGFBP-3, and IGF-I. Mean serum IGFBP-2 concentrations ranged from 263 +/- 101 ng/mL (mean +/- SD) during infancy to 136 +/- 38 ng/mL in normal 15- to 18-yr-olds (P < 0.001), whereas IGFBP-3 increased from 1211 +/- 384 to 2781 +/- 382 ng/mL in the same age groups. Thirty-nine of 49 children with GHD and low IGF-I values (serum GH response, < or = 1 ng/mL after 2 provocative tests) had serum IGFBP-2 concentrations that were greater than 2 SD above their corresponding age-adjusted means. In contrast all 49 of these children had IGFBP-3 values that were below normal for age. Because serum IGFBP-2 concentrations are regulated by GH directly and not through IGF-I, the IGFBP-2 to IGF-I ratio was used to determine whether it improved diagnostic accuracy. Fifty of 57 GH-deficient children had IGFBP-2/IGF-I ratios that were greater than 2 SD above the mean. This included 48 of 49 children with low IGF-I and 2 of 8 children with normal IGF-I. Fifty-three of the 57 children with GHD had decreased IGFBP-3 values. Among 23 children with idiopathic short stature (ISS) who had normal responses to GH stimulation testing (serum GH, > 10 ng/mL), 7 had low IGF-I values. Of the 7, all had an increased IGFBP-2/IGF-I ratio and a low IGFBP-3 level. Of the remaining 16 children with normal IGF-I, 13 had a normal IGFBP-2/IGF-I ratio and normal IGFBP-3 values. Three had low IGFBP-3 and an increased IGFBP-2/IGF-I ratio. In 76% of the 80 short-statured patients studied, there was concordance among serum GH responses to provocative tests, IGF-I, IGFBP-2/IGF-I ratio, and IGFBP-3.(ABSTRACT TRUNCATED AT 400 WORDS)

A highly conserved insulin-like growth factor-binding protein (IGFBP-5) is expressed during myoblast differentiation

Insulin-like growth factor-binding proteins (IGFBPs) are a family of secreted proteins that bind insulin-like growth factors I and II (IGFs I and II) and are capable of modulating IGF actions on target cells. We have shown previously that C2 myoblasts secrete a single approximately 29-kDa IGFBP during their terminal differentiation (Tollefsen, S. E., Lajara, R., McCusker, R. H., Clemmons, D. R., and Rotwein, P. (1989) J. Biol. Chem. 264, 13810-13817). In this study, we have purified the protein from C2 cell-conditioned media by conventional and IGF-affinity chromatography, cloned its cDNA by PCR-based and traditional library screening, and identified it as mouse IGFBP-5. The resultant 5561 nucleotide cDNA encodes a 252-amino acid mature protein (predicted M(r) approximately 28,400) that is 97% identical to rat and human IGFBP-5. In differentiating C2 myoblasts and in F3 azamyoblasts the > 6-kilobase IGFBP-5 mRNA accumulates concomitantly with induction of myogen mRNA, an early marker of muscle differentiation. Ligand blot analysis shows that IGFBP-5 protein is secreted within 12 h of the onset of differentiation in these cells and that it is the only IGFBP produced in several fusing skeletal muscle cell lines. In vivo, IGFBP-5 transcripts are expressed in a variety of mouse tissues including striated muscle, but, unlike other IGFBPs, it is barely detectable in liver. IGFBP-5 is more conserved than other IGFBPs in mammals; its conserved structure and sequence also extends to non-mammalian vertebrates. Hybridization of a mouse BP5 coding region probe to RNA from several chicken and Xenopus tissues demonstrated similarly sized transcripts in these species. A partial Xenopus cDNA is identical in 38/45 deduced amino acids to the mammalian proteins. Identification of an IGF-binding protein that is produced during myoblast differentiation provides a model system in which to study the potential modulatory role of IGFBPs in development.

Extracellular matrix contains insulin-like growth factor binding protein-5: potentiation of the effects of IGF-I

Insulin-like growth factor binding proteins (IGFBPs) have been shown to serve as carrier proteins for the insulin-like growth factors (IGFs) and to modulate their biologic effects. Since extracellular matrix (ECM) has been shown to be a reservoir for IGF-I and IGF-II, we examined the ECM of cultured human fetal fibroblasts and found that IGFBP-5 was incorporated intact into ECM, while mostly inert proteolytic fragments were found in the medium. In contrast, two other forms of IGFBP that are secreted by these cells were either present in ECM in minimal amounts (IGFBP-3) or not detected (IGFBP-4). Likewise, when purified IGFBPs were incubated with ECM, IGFBP-5 bound preferentially. IGFBP-5 was found to bind to types III and IV collagen, laminin, and fibronectin. Increasing salt concentrations inhibited the binding of IGFBP-5 to ECM and accelerated the release of IGFBP-5 from ECM, suggesting an ionic basis for this interaction. ECM-associated IGFBP-5 had a sevenfold decrease in affinity for IGF-I compared to IGFBP-5 in solution. Furthermore, when IGFBP-5 was present in cell culture substrata, it potentiated the growth stimulatory effects of IGF-I on fibroblasts. When IGFBP-5 was present only in the medium, it was degraded to a 22-kD fragment and had no effect on IGF-I-stimulated growth. We conclude that IGFBP-5 is present in fibroblast ECM, where it is protected from degradation and can potentiate the biologic actions of IGF-I. These findings provide a molecular explanation for the association of the IGF's with the extracellular matrix, and suggest that the binding of the IGF's to matrix, via IGFBP-5, may be important in mediating the cellular growth response to these growth factors.

Human IGFBP-1 is phosphorylated on 3 serine residues: effects of site-directed mutagenesis of the major phosphoserine

Human IGFBP-1 is phosphorylated by cells in culture and is present in both phosphorylated and nonphosphorylated forms in human fetal serum and amniotic fluid. We have found immunoprecipitable [32P]IGFBP-1 in the conditioned media of both Chinese hamster ovary (CHO) cells (stabley transfected and secreting human IGFBP-1) and human hepatoma (HepG2) cells metabolically labelled with [32P]orthophosphate. Phosphoamino acid analysis of this [32P]IGFBP-1 demonstrates that only serine residues are phosphorylated. Four phosphorylated isoforms of IGFBP-1 can be separated from one nonphosphorylated form by nondenaturing gel electrophoresis. Since we have shown that the nonphosphorylated form of IGFBP-1 has a lower affinity for IGF-I compared to phosphorylated forms and a greater potentiating effect of IGF-I actions, we determined which serine residues in human IGFBP-1 are phosphorylated. After metabolically labelling IGFBP-1 with 32P, the purified phosphoprotein was digested first with trypsin and then with endoproteinase Glu-C. By radiosequencing the resulting 32P-labelled phosphopeptides, we found 3 serine residues to be phosphorylated. Approximately 70% of incorporated 32P was attributed to Ser101, while Ser169 accounted for approximately 25% and Ser119 for 5%. To investigate the physiologic importance of Ser101, this residue (and the nonphosphorylated Ser98) were changed to alanine by site directed mutagenesis of a human IGFBP-1 expression vector, followed by transfection into CHO cells. The [Ala98,101]IGFBP-1 purified from the conditioned media of these cells had the following characteristics: 1) when labelled with [32P]orthophosphate, it contained 63% less radioactivity than wild type IGFBP-1; 2) when analyzed by nondenaturing gel electrophoresis, it contained none of the most rapidly migrating and most rapidly migrating and most highly phosphorylated isoform, more of the nonphosphorylated isoform, and more of the most slowly migrating phosphorylated isoform; and 3) its affinity for IGF-I was reduced 2.5-fold and was midway between wild type IGFBP-1 from transfected CHO cells and dephosphorylated IGFBP-1. We conclude that Ser101 represents the major site of phosphorylation of IGFBP-1 and that while phosphorylation of Ser101 increases affinity of IGFBP-1 for IGF-I, phosphorylation of Ser169 and/or Ser119 also contributes to the high affinity of fully phosphorylated IGFBP-1.

Identification of the sites of phosphorylation in insulin-like growth factor binding protein-1. Regulation of its affinity by phosphorylation of serine 101

Serine phosphorylation of insulin-like growth factor binding protein-1 (IGFBP-1) has been shown to alter its affinity for the insulin-like growth factors (IGF-I and IGF-II) and to modify its capacity to modulate cellular responses to the IGFs. Because of this, we determined the sites of serine phosphorylation. Purification of 32P-labeled IGFBP-1 was followed by digestion with trypsin and endoproteinase Glu-C and radiosequencing of labeled peptides. Three serines were found to be phosphorylated, with Ser101, Ser119, and Ser169 containing 70%, 5%, and 25% of the incorporated 32P, respectively. A mutated IGFBP-1, substituting alanine for serine at positions 98 and 101, was expressed in CHO cells. On nondenaturing gels, the wild type protein migrated as five isoforms (one non-phosphorylated and four phosphorylated). However, in the mutated protein, the most rapidly migrating band (a phosphorylated form) was not present. The cells containing the mutated cDNA incorporated 60% less 32P into immunoprecipitable IGFBP-1. The mutated protein had a 3-fold reduction in affinity for IGF-I compared to the wild type protein. We conclude that Ser101 represents the major site of phosphorylation containing 63% of the total 32P incorporated and that phosphorylation of Ser101 is important for maintenance of high affinity binding for this growth factor.

Competition for binding to insulin-like growth factor (IGF) binding protein-2, 3, 4, and 5 by the IGFs and IGF analogs

The insulin-like growth factors (IGF) I and II bind to IGF binding proteins (BP) with high affinity. The affinity of each of the IGFs for individual BPs and the regions of the IGF-I molecule that are required for this high affinity binding have been defined only for IGFBP-1 and IGFBP-3. The present studies have determined the affinity of several IGF analogs (prepared using in vitro mutagenesis) for pure IGFBP-2, 3, 4, and 5. The results show IGFBP-2 binds these analogs in a manner similar to IGFBP-1. For example, a mutation in the A chain region (positions 49, 50, 51) or B chain (positions 3, 4) results in greater than 20-fold reduction in affinity for either IGFBP-1 or 2. In contrast, mutations in the A chain region have minimal effect on binding to IGFBP-3, whereas substitutions at the 3, 4, 15, 16 positions of the B chain reduce IGF-I affinity by at least 50-fold. At pH 7.4, binding of the analogs to IGFBP-4 is less affected by substitutions at the B chain 3, 4 positions compared to IGFBP-1, 2, and 3, but IGFBP-4 affinity for analogs containing the A chain substitutions is greatly reduced similarly to IGFBP-1 and 2. Binding to IGFBP-5 is greatly reduced by either A or B chain substitutions and most of the mutations result in greater than 100-fold reduction in affinity. Acidic pH 6.0 was associated with increased affinity of IGFBP-4 for the A chain containing mutants. The results indicate that only IGFBP-1 and 2 have nearly identical affinity for each of these analogs, whereas IGFBP-3, 4, and 5 have similarities and significant differences. The findings suggest that different binding proteins have differential structural requirements for optimal IGF-I binding.

Identification of the forms of insulin-like growth factor-binding proteins produced by human fibroblasts and the mechanisms that regulate their secretion

Human fibroblasts secrete insulin-like growth factor-binding proteins (IGFBPs) that can modify insulin-like growth factor (IGF) I action. We have determined the molecular identities of three forms of IGFBPs that are secreted by human fibroblasts in vitro. Ligand blot analysis of fibroblast conditioned media revealed that the M(r) 43,000 and 39,000 forms were the most abundant, but that M(r) 31,000 and 24,000 forms were also present. An antiserum that was specific for IGFBP-5 reacted with the M(r) 31,000 form, and an IGFBP-4-specific antiserum recognized only the M(r) 24,000 form. The M(r) 39,000 and 43,000 forms were detected by IGFBP-3 antiserum. Further proof that fibroblasts synthesized these forms of IGFBPs was obtained by Northern blotting. A cDNA probe for IGFBP-3 hybridized with a 2.4-kilobase (kb) transcript, whereas a cDNA probe for IGFBP-5 recognized a single 6.0-kb transcript, and an IGFBP-4 cDNA probe recognized 2.2- and 2.0-kb transcripts. IGF-I and -II caused a minimal (less than 43%) increase in IGFBP-5 mRNA abundance and had no effect on IGFBP-4 mRNA abundance. IGF-I and -II (100 ng/ml) stimulated 6-8-fold increases in IGFBP-5 levels, whereas IGFBP-4 was inhibited. Insulin failed to elicit any change in IGFBP-5, suggesting that binding of the IGFs to IGFBPs was required to detect the increase. Immunoblotting for IGFBP-5 revealed an M(r) 23,000 (non-IGF-I-binding) fragment. To determine if the IGFs were influencing proteolytic degradation of IGFBP-5, pure IGFBP-5 was added to fibroblast cultures and incubated for 4 h at 37 degrees C. The amount of fragment formation was attenuated by the presence of IGF-I and -II, but not insulin, suggesting that this is a mechanism by which the IGFs act to modulate IGFBP-5 concentration. In contrast to the IGFs, forskolin, which increased IGFBP-4 and -5 mRNA abundance and secretion, had no effect on fragment formation. The results show that human fibroblasts synthesize and secrete IGFBP-3, -4, and -5 and that changes in intracellular cAMP regulate synthesis, whereas the IGFs regulate IGFBP-4 and -5 levels by post-transcriptional mechanisms.

Cloning and sequence determination of bovine insulin-like growth factor binding protein-2 (IGFBP-2): comparison of its structural and functional properties with IGFBP-1

Insulin-like growth factor binding proteins (IGFBPs) are secreted by several cell types and can modify IGF actions. Mandin-Darby Bovine Kidney (MDBK) cells have been shown to secrete a 34,000 Da form of IGF binding protein whose N-terminal sequence is similar to a form of IGFBP purified from rat BRL-3A cells that has recently been named IGFBP-2. These studies report the complete amino acid sequence of bovine IGFBP-2 and compare its functional properties with human IGFBP-1. The protein is 81% identical to rat IGFBP-2. When compared with both rat IGFBP-2 and human IGFBP-1, the positions of all 18 cysteine residues are conserved. Similarly an RGD sequence is present near the carboxyl terminus in both proteins. IGFBP-2 has a higher affinity for IGF-II than for IGF-I and its affinity for both forms of IGF is greater than for human IGFBP-1. Like IGFBP-1 the protein can enhance the DNA synthesis response of porcine aortic smooth muscle cells to IGF-I; however, IGFBP-2 was much less potent. The maximum potentiation of the IGF-mediated mitogenic response that could be achieved was approximately 42% that of IGFBP-1. This potentiation is dependent upon a factor contained in platelet poor plasma and if this factor is omitted from the incubation medium, IGFBP-2 inhibits DNA synthesis. The purification of IGFBP-2 will allow more detailed comparisons to be made between it and other forms of IGFBPs in physiologic test systems.

Evaluation of the developmental and nutritional changes in porcine insulin-like growth factor-binding protein-1 and -2 serum levels by immunoassay

Porcine serum contains five insulin-like growth factor-binding proteins (IGFBPs), whose regulation has been studied by ligand blotting. To more accurately quantify changes in two specific forms of IGFBP a heterologous RIA for porcine (p) IGFBP-2 was developed, and IGFBP-1 levels were analyzed by immunoblotting. By RIA, postnatal hypophysectomy caused a 7-fold increase in serum pIGFBP-2 levels compared to controls (2,622 +/- 378 vs. 382 +/- 10 ng/ml, respectively). Fetal pIGFBP-2 levels were higher at 110 vs. 45 days gestation (1,074 +/- 214 vs. 418 +/- 30 ng/ml, respectively), rose to 1,905 +/- 167 ng/ml within 12 h after birth, then decreased to 1,010 +/- 10 ng/ml at 48 h. By immunoblot analysis, bovine IGFBP-2 antiserum reacted with a 34,000 mol wt (Mr) IGFBP and did not react with other forms of IGFBP detected by ligand blotting. Serum levels of the 34,000 Mr IGFBP, as detected by ligand blot analysis, are decreased when neonatal pigs are fasted for 48 h. In contrast, by RIA, pIGFBP-2 concentrations increased 4-fold. Immunoblots of these sera showed two lower Mr (22,000 and 14,000 Mr) bands that did not bind either [125I]IGF-I or [125I]IGF-II and were distinct from a smaller (20,000 Mr) IGFBP which bound only [125I]IGF-II. These two bands were increased in serum of 48-h fasted compared to fed piglets, suggesting that they are proteolytic fragments of pIGFBP-2. In vitro incubation of 48-h fasted pig serum with intact IGFBP-2 failed to reveal proteolytic fragments, indicating that the IGFBP-2 fragments were not generated by a protease that was released into the serum. Analysis performed with human IGFBP-1 antiserum revealed a 29,000 Mr immunoreactive band whose abundance was increased by either postnatal hypophysectomy or fasting. No fragments of IGFBP-1 were found in any serum tested. We conclude that heterologous antibodies can be used to identify and quantify IGFBP-1 and IGFBP-2 in porcine serum. Changes in pIGFBP-2 levels measured during fasting are due to the combination of changes in intact 34,000 Mr IGFBP-2 and smaller non-IGF-binding fragments. Changes in levels of specific forms of IGFBP as well as the presence of fragments have the potential to modulate the transport of IGF-I and -II out of the vasculature.

Variables controlling the secretion of insulin-like growth factor binding protein-2 in normal human subjects

Insulin-like growth factor binding protein-2 (IGFBP-2) is one of a family of IGFBPs that are present in extracellular fluids, and binds both IGF-II and IGF-I with high affinity. These studies were conducted to determine the nutritional and hormonal variables that regulate plasma IGFBP-2 concentrations in humans. The mean plasma IGFBP-2 concentration for 38 normal adult subjects was 150 +/- 61 micrograms/L and was 4.7-fold greater than their mean fasting IGFBP-1 value. Mean IGFBP-2 values in cord sera of 26 normal term infants was 3.8-fold greater than the normal adult mean value. Likewise, the mean value for 44 hypopituitary adults was increased 2-fold compared to normal. There was no suppression of IGFBP-2 values in acromegaly. Normal adult subjects showed minimal fluctuations (less than 2-fold changes) in plasma IGFBP-2 concentrations during a 48-h sampling period. These changes were significantly less than the changes that occurred in plasma IGFBP-1 during the same interval. Plasma IGFBP-2 did not change significantly post prandially or after a glucose infusion. Extreme insulin deficiency, after 9 days of fasting, was associated with a 1.7-fold increase in plasma IGFBP-2. Administration of GH, which is known to cause a major decrease in plasma IGFBP-1 and in IGFBP-2 in hypophysectomized animals, did not result in a change in calorically restricted normal adult subjects, suggesting that a normal caloric intake is required for GH to suppress IGFBP-2. In summary, these results show that plasma IGFBP-2 is regulated differently than IGFBP-1. Acute stimulation of insulin secretion does not suppress IGFBP-2, and there is much less daily fluctuation compared to IGFBP-1. These findings suggest that plasma IGFBP-2 levels are more stable than IGFBP-1, and therefore IGFBP-2 may serve as a larger reservior that is available for IGF transport.

Insulin-like growth factor (IGF) binding to cell monolayers is directly modulated by the addition of IGF-binding proteins

Insulin-like growth factor-I (IGF-I) binds to specific receptors and IGF-binding proteins (IGFBPs) that are present on cell surfaces. The analysis of [125I]IGF-I binding to human fibroblasts is complicated by IGFBPs on the cell surface and their release into the medium during the binding assay. This release alters the distribution of [125I]IGF-I between type I IGF receptors and both soluble as well as cell surface-associated IGFBPs. In the present study we have determined the effects of three different forms of IGFBPs on [125I]IGF-I binding to cell surface binding sites of human fetal fibroblasts (GM10 cells) and porcine smooth muscle cells. Human 29,000 mol wt (Mr; IGFBP-1), bovine 34,000 Mr (IGFBP-2), and bovine 46,000 Mr (IGFBP-3) forms of IGFBP were compared. Each of the three IGFBPs inhibited [125I]IGF-I binding to the cell surface of both cell types. This effect was due to increased binding of [125I]IGF-I by the IGFBPs in the assay buffer. At equimolar concentrations, IGFBP-3 was more effective than either IGFBP-1 or IGFBP-2 in blocking cell surface binding. The addition of increasing concentrations of unlabeled IGF-I in the presence of each IGFBP showed that either IGFBP-1 or IGFBP-3, but not IGFBP-2, resulted in a paradoxical increase in [125I]IGF-I binding to the cell surface. The paradoxical increase occurred in the presence of excess insulin, indicating that unsaturated type I IGF receptors are not required to demonstrate this phenomenon. In a physiological salt solution, the order of affinity of the IGFBPs for IGF-I was IGFBP-3 greater than IGFBP-1 greater than IGFBP-2. These differences in affinity appear to account for the differences in IGF-I competition for binding that are seen when each of the three proteins is added. Thus, IGFBPs have the potential to alter the partitioning of IGF-I between cell surface-associated IGFBPs, membrane receptors, and the IGFBPs in extracellular fluids. The various forms of IGFBP affect IGF cell surface binding differently, and therefore, each may have distinct effects on IGF target cell actions.

The direct in vitro effect of insulin-like growth factors (IGFs) on normal bovine mammary cell proliferation and production of IGF binding proteins

Mammary epithelial cells isolated from pregnant, nonlactating heifers were grown in vitro using collagen substrates. Using these systems, the truncated form of insulin growth factor-1 (IGF-1) (des-3-IGF-1), IGF-1, and IGF-2 all stimulated a significant (0.5 to 1 fold) increase in cell proliferation (des-3-IGF-1 greater than IGF-1 greater than IGF-2). When grown in media containing serum plus IGF-1, normal bovine mammary cells also produced and secreted at least four species of IGF-binding protein (IGFBP) ranging from 21K to 48K (as demonstrated by ligand blot analysis). However, cells grown in serum free media secreted detectable quantities of only 2 major forms of IGFBP of 34K and 48K. Using immunoblot analysis, these proteins were identified as IGFBP-2 and IGFBP-3, respectively. Both proteins were inducible by the addition of IGF to the serum free media (relative potency; IGF-1 greater than des-3-IGF-1 greater than IGF-2). Using RIA analysis, bovine mammary cells cultured in the presence of IGF-1 produced 20-25 ng/ml IGFBP-2 compared to control cultures which secrete approximately 1.0 ng/ml. Cells exposed to des-3-IGF-1 produced 40-60% less IGFBP-2 whereas insulin and IGF-2 did not stimulate significant IGFBP-2 production. These data indicate that normal bovine mammary cells secret IGFBP-2 and IGFBP-3. This secretion is stimulated by IGF-1 and des-3-IGF-1 suggesting a mechanism for regulating local IGF activity.

Transcapillary permeability and subendothelial distribution of endothelial and amniotic fluid insulin-like growth factor binding proteins in the rat heart

Insulin-like growth factor (IGF) binding proteins (IGFBP) were purified from conditioned media of cultured bovine endothelial cells (ECBP) and from human amniotic fluid (IGFBP-1), and then labeled by radioiodination. 125I-ECBP and 125I-IGFBP-1 were perfused through isolated beating rat hearts for 1 and 5 min, and the hearts fixed and analyzed for 125I-BP content and distribution. One to 4% of the perfused 125I-ECBP and 125I-IGFBP-1 crossed the capillary boundary. The ECBPs predominantly localized as intact 125I-BP in connective tissue elements of the heart with less 125I-BP in cardiac muscle. The ratio of 125I-ECBP in connective tissue: muscle (normalized to percent vol of these compartments) was greater than or equal to 10:1. In contrast, the IGFBP-1 had a greater affinity for cardiac muscle with ratios of 125I-IGFBP-1 in connective tissue:muscle of approximately 1:2. When 125I-IGF-I, in the absence of any BPs, was perfused through the hearts approximately 3-5% left the microcirculation and was found in subendothelial tissues. 125I-IGF-I localized primarily to cardiac muscle with a distribution of connective tissue:cardiac muscle of about 1:3. The findings in the isolated perfused heart were confirmed in intact animals. After 125I-IGFBP-1 was injected into anesthetized rats and allowed to circulate for 5 min, substantial radioactivity was associated with the heart. As in the isolated heart, the IGFBP-1 preferentially localized to cardiac muscle with a connective tissue:cardiac muscle ratio of 1:3. We conclude that IGFBPs produced by endothelial cells and the IGFBP-1 contained in amniotic fluid can cross the capillary boundaries of the rat heart, and that the ECBPs preferentially localize in connective tissue elements of the myocardium, whereas IGFBP-1 predominantly localizes in cardiac muscle.

Differential regulation of insulin-like growth factor binding protein secretion from human decidual cells by IGF-I, insulin, and relaxin

Several growth hormone-independent 25-31,000 kD insulin-like growth factor binding proteins (IGF-BPs) have been identified in plasma, extravascular fluids, and various cell-conditioned media. Cultured human decidual cells release three IGF-BPs with 24,000, 30,000, and 34,000 Mr. Using ligand blot analysis and an RIA for the 30,000-Mr form (IGF-BP-1), we examined the effects of IGF-I (10-1,000 ng/ml), insulin (10-10,000 ng/ml), and relaxin (10-250 ng/ml) on decidual cell IGF-BP release after 120 h of hormone exposure. IGF-I inhibited release of both IGF-BP-1 and the 24,000 Mr form. Inhibition of IGF-BP-1 release was noted after 48 h of treatment and was progressive throughout the subsequent 120 h. Insulin stimulated a fourfold increase in release of the 24,000-Mr protein while inhibiting IGF-BP-1 release comparable to IGF-I, alpha-IR3, a monoclonal antibody to the IGF-I receptor, blocked approximately 33% of the IGF-I response but had no effect on insulin-mediated IGF-BP-1 inhibition. Relaxin stimulated a 2.4-fold increase in release of the 24,000-Mr form and a 16-fold increase in the 30,000-Mr protein after 120 h. Stimulation of the 30,000-Mr protein was inhibited by the addition of cycloheximide (50 micrograms/ml). Both IGF-I and insulin also blocked the relaxin-mediated increase in IGF-BP-1. These studies suggest that three structurally related proteins differentially regulate IGF-BP secretion possibly via activation of distinct receptor subtypes.

Three distinct forms of insulin-like growth factor binding proteins are released by decidual cells in culture

Recent studies have shown that human decidual explants synthesize a 25,272 dalton form of insulin-like growth factor binding protein (IGFBP-1) that is identical to the most abundant form of IGFBP detected in human amniotic fluid. To determine whether decidual cells also secrete other structurally distinct forms of IGFBP, conditioned medium obtained from human decidual cells was analyzed by ligand blotting and immunoblotting using antisera that were specific for IGFBP-1 or 2. IGFBP-2 is a form of binding protein that is structurally distinct from IGFBP-1. Ligand blotting analysis (which detects all forms of IGFBPs) showed that 3 forms of IGFBP with Mr estimates of 34,000, 30,000 and 24,000 were present in basal, unstimulated, conditioned media. Immunoblotting showed that the 30,000 Mr form reacted with an antibody that is specific for IGFBP-1, whereas the 34,000 Mr form reacted with an antibody that is specific for IGFBP-2. The 24,000 Mr band did not react with antisera to either IGFBP-1 or 2. Basal IGFBP-1 release from human decidual cultures, as measured by RIA, showed a significant decrease during the 5 day period from 34 +/- 1.6 on day 1 to 12 +/- 1.9 ng/ml on day 5. Exposure of the cells to (Bu)2cAMP (1.0 mM) for 5 days had no significant effect on IGFBP-1 release during the first day of exposure, but caused a progressive stimulation of release during the subsequent 4 days. By day 5, exposure to (Bu)2cAMP induced a 5- to 6-fold increase in IGFBP-1 release that was completely inhibited by exposure to cycloheximide. Exposure of the cells to cholera toxin (0.1 micrograms/ml) for 4 days caused a 3.7-fold increase in IGFBP-1 release. The stimulation of IGFBP-1 release by (Bu)2cAMP was completely inhibited by simultaneous exposure to insulin (100 ng/ml) and/or IGF-I (100 ng/ml) (day 5 control = 35 ng/ml, (Bu)2cAMP alone = 248 ng/ml, insulin + (Bu)2cAMP = 36 ng/ml, or IGF-I + (Bu)2cAMP = 10.1 ng/ml). Hydrocortisone and the phorbol ester (phorbol myristate acetate) caused no significant changes in basal IGFBP-1 release but prevented the decrease in the basal rate of release that occurred between day 1 and 5. Ligand blotting studies showed that the 24,000 Mr form of binding protein was also stimulated by (Bu)2cAMP (3.3-fold increase); however, cholera toxin and phorbol myristate acetate were without effect. In contrast, the release of the 34,000 Mr form was unaffected by exposure to any of these agents, suggesting that regulation of this form of IGFBP is distinct from the other 2 forms.(ABSTRACT TRUNCATED AT 400 WORDS)

Insulin differentially alters transcapillary movement of intravascular IGFBP-1, IGFBP-2 and endothelial cell IGF-binding proteins in the rat heart

Insulin-like growth factor binding-proteins 1 and 2 (IGFBP-1, IGFBP-2) and endothelial cell IGF binding proteins (ECBP) were individually perfused through isolated beating rat hearts in the absence and presence of insulin. Insulin caused an increased movement of IGFBP-1 from the vascular space to tissues of the heart. Subendothelial content of IGFBP-1 was 110%, 126% (p less than .01) and 132% (p less than 0.05) of control hearts when perfused with 1, 10 and 100 ng/ml insulin, respectively. . In contrast, insulin treatment was associated with a decrease in ECBP content in cardiac tissue, being 83%, 62% (p less than 0.005) and 73% (p less than 0.05) of control when perfused with 1, 10 and 100 ng/ml insulin. The efflux of IGFBP-2 from the intravascular space was unaffected by insulin. The subendothelial tissue distribution of the transported binding proteins was not changed by insulin perfusion, with IGFBP-1 and IGFBP-2 localizing predominantly in cardiac muscle and ECBP having greater affinity for connective tissue elements. We conclude that in the perfused rat heart, insulin can differentially alter transcapillary movement of IGFBP-1, IGFBP-2 and endothelial cell IGF-binding proteins. Such insulin-facilitated changes could potentially mediate nutrient-dependent transport of IGF-I and IGF-II to peripheral tissues.

Purified preparations of the amniotic fluid-derived insulin-like growth factor-binding protein contain multimeric forms that are biologically active

The insulin-like growth factors (IGFs) appear to be secreted into interstitial fluids by many cell types, along with specific, high affinity binding proteins (IGF-BPs). These proteins, therefore, have the potential to bind IGF-I and -II and modify their ability to interact with specific cell surface receptors In these studies we report the detection of high mol wt, multimeric forms of one form of IGF-BP that has been purified from human amniotic fluid. The multimeric forms, which are either not or barely detectable in native amniotic fluid, are the result of intermolecular disulfide bond formation and can be reduced to a monomeric form by exposure to dithiothreitol. After reduction, the multimers are reduced to either monomeric or dimeric forms, as detected by Western blotting. The multimers can be separated from monomeric and dimeric forms by gel filtration chromatography. The purified multimers were fully biologically active in potentiating the effect of IGF-I on porcine aortic smooth muscle cell DNA synthesis. The monomeric form was also bioactive. No significant differences in the affinity of the monomeric and multimeric forms for IGF-I or -II could be detected. In summary, multimeric forms of this form of IGF-BP are detected during purification. The formation of these multimers is through intermolecular disulfide bonds and does not disrupt IGF binding or potentiation of the cellular growth response to IGF-I. These findings indicate that these higher mol wt forms may be fully active in biological test systems.

Radioimmunoassay of a 26,000-dalton plasma insulin-like growth factor-binding protein: control by nutritional variables

Insulin-like growth factor I (IGF-I) is a peptide growth factor that circulates bound to carrier proteins. One form of carrier protein (mol wt, approximately 26K) is not believed to be GH dependent, is relatively unsaturated, and modulates the cellular response to IGF-I. This study was undertaken to determine the variables that control the plasma concentration of this protein, which was measured using a specific RIA. The mean plasma 26K IGF-binding protein (IGF-BP) concentration in 15 normal fasting subjects at 0800 h was 9.4 +/- 4.4 (+/- SD) micrograms/L. The mean value in GH-deficient patients was increased to 19.5 +/- 10.1 micrograms/L (n = 60; P less than 0.05), and it was 7.3 +/- 4.3 micrograms/L in patients with acromegaly (n = 31). The GH dependency of these changes is further supported by the observation that subjects who received GH injections had a 51% reduction in their fasting values. Nutritional intake appeared to be a more important controlling variable than GH. During an overnight fast plasma 26K IGF-BP values increased approximately 4-fold in 6 normal subjects. After 2 days of fasting, the mean value in 7 obese subjects rose progressively from 6.5 +/- 2.3 to 11.7 +/- 5.4 micrograms/L (P less than 0.001), and it increased further to 19.2 +/- 5.9 micrograms/L by day 4 of fasting; after 2 days of refeeding it returned to the prefasting level of 6.8 +/- 1.9 micrograms/L. Likewise, ingestion of a standard test meal resulted in a significant decrease in mean plasma 26K IGF-BP from a fasting value of 8.4 +/- 2.9 to 5.6 +/- 2.8 micrograms/L 4 h postprandially (P less than 0.05). In summary, the plasma concentrations of the 26K IGF-I-BP fluctuate widely in response to dietary manipulation, whereas GH status appears to be a secondary controlling variable.

Purification of a 31,000-dalton insulin-like growth factor binding protein from human amniotic fluid. Isolation of two forms with different biologic actions

Human amniotic fluid has been shown to contain a protein that binds insulin-like growth factor I and II (IGF-I and IGF-II). Partially purified preparations of this protein have been reported to inhibit the biologic actions of the IGFs. In these studies our laboratory has used a modified purification procedure to obtain a homogeneous preparation of this protein as determined by polyacrylamide gel electrophoresis and amino acid sequence analysis. During purification the ion exchange chromatography step resulted in two peaks of material with IGF binding activity termed peaks B and C. Each peak was purified separately to homogeneity. Both peaks were estimated to be 31,000 daltons by polyacrylamide gel electrophoresis and their amino acid compositions were nearly identical. Amino acid sequence analysis showed that both peaks had identical N-terminal sequences through the first 28 residues. Neither protein had detectable carbohydrate side chains and each had a similar affinity for radiolabeled IGF-I (1.7-2.2 x 10(10) liters/mol). In contrast, these two forms had marked differences in bioactivity. Concentrations of peak C material between 2 and 20 ng/ml inhibited IGF-I stimulation of [3H]thymidine incorporation into smooth muscle cell DNA. In contrast, when peak B (100 ng/ml) was incubated with IGF-I there was a 4.4-fold enhancement of stimulation of DNA synthesis. Additionally, pure peak B was shown to adhere to cell surfaces, whereas peak C was not adherent. The non-adherent peak C inhibited IGF-I binding to its receptor and to adherent peak B. We conclude that human amniotic fluid contains two forms of IGF binding protein that have very similar physiochemical characteristics but markedly different biologic actions. Since both have similar if not identical amino acid compositions, N-terminal sequences, and do not contain carbohydrate, we conclude that they differ in some other as yet undefined post-translational modification.

Cloning, characterization, and expression of a human insulin-like growth factor binding protein

Insulin-like growth factors (IGFs) bind to specific proteins present in extracellular fluids. One of these binding proteins (IGF-BP) was purified from human amniotic fluid and was shown to potentiate the effects of IGF-I in vitro (10). In these studies, a polyclonal antibody to this protein was used to isolate a cDNA clone from a human decidua library. This clone encodes a polypeptide of 25,832 daltons that includes the sequences of 9 tryptic peptides that had been prepared from the purified IGF-BP. The protein has 15 cysteines that are clustered at the amino and carboxy ends of the molecule. The protein has an RGD sequence near its C-terminus, which may account for its ability to attach to cells and to potentiate the biological actions of IGF-I.