Insulin-like growth factor binding proteins from cultured human fibroblasts. Characterization and hormonal regulation

Direct regulation of IGF-binding protein 1 promoter by interleukin-1β via an insulin- and FoxO-1-independent mechanism

The level of insulin-like growth factor-binding protein 1 (IGFBP1), a liver-produced serum protein that regulates insulin-like growth factor-I bioactivity, glucose homeostasis, and tissue regeneration, increases during inflammation. This manuscript describes a novel pathway for the regulation of hepatic IGFBP1 mRNA and protein levels by interleukin (IL)-1β. Experiments with the luciferase reporter system show that IL-1β stimulates transcriptional activity from the 1-kb promoter region of IGFBP1. Although IL-1β stimulation suppresses the insulin activation of protein kinase B, the major upstream regulator of IGFBP1 mRNA transcription, the induction of IGFBP1 by IL-1β did not require an intact insulin response element. Furthermore, neither overexpression nor silencing of FoxO-1 had any effect on the IL-1β-induced increase in IGFBP1 mRNA levels and promoter activity. However, inhibition of the ERK MAP kinases effectively prevented the IL-1β effects. Inhibition of neutral sphingomyelinase, a key player in the IL-1β signaling cascade that acts upstream of ERK, also suppressed the IL-1β effects, while increasing the ceramide, through the addition of C2-ceramide or via treatment with exogenous sphingomyelinase, was sufficient to induce IGFBP1 promoter-driven luciferase activity. Studies in primary rat hepatocytes where the levels of neutral sphingomyelinase were either elevated or suppressed using adenoviral constructs affirmed the key role of neutral sphingomyelinase and ceramide (exerted likely through ERK activation) in the IL-1β-induced IGFBP1 production. Finally, the IL-1β effects on IGFBP1 mRNA production and protein secretion could be abolished by the addition of insulin, either at very late time points or at very high doses.

Quantitative analysis of the concentrations of IGFs and several IGF-binding proteins in a large fibrous abdominal tumor and the circulation of a patient with hypoglycemia

The syndrome of nonislet cell tumor induced hypoglycemia (NICTH) represent extreme cases of excessive expression and production of incompletely processed high-molecular-mass pro-IGF-II forms (big IGF-II) by an often large tumor. Tumor-derived big IGF-II is responsible for enhanced insulin-like effects in the body through complicated mechanisms, leading to hypoglycemia. Case studies on NICTH usually focus on measurements of diagnostic parameters in the circulation of patients. Some studies have also reported on qualitative immunohistochemical analysis of tumor tissue, in particular with respect to the expression of IGF-II at the mRNA or protein level. However, quantitative data on the concentrations of IGFs and IGFBPs in tumor specimen causing NICTH, in relation to their corresponding plasma levels are lacking. Such an analysis would provide an estimate of the total potential of (big) IGF-II retained by the tumor and more insight in the relative levels of different IGFBPs and their origin in the circulation, that is, systemically induced by tumor related factors or directly tumor-derived. Here we investigated quantitatively the levels of IGFs and IGFBPs in a large, 1.76 kg weighing, solitary fibrous tumor from a typical case of NICTH using highly specific immunometric assays. Besides a high level of big IGF-II, patient's plasma also contained increased levels of both IGFBP-2 and -6 which declined after removal of the tumor. These IGFBPs have a higher affinity for (pro-) IGF-II than IGF-I and exhibit intrinsic IGF-independent bioactivities. Tumor tissue contained high amounts of big IGF-II and IGFBP-6, exceeding that in patient's circulation many-fold. A relatively low tumor content of IGFBP-2 was found suggesting that the preoperative high levels in plasma were attributable to systemic mechanisms. The background literature and possible implications of these findings are briefly discussed. Based on the present results we postulate that tumor tissue is not the source of the elevated levels of IGFBP-2 often seen in NICTH patients. Large tumors that cause NICTH can produce IGFBP-6 leading to enhanced levels of this IGFBP in the circulation. Hence, the measurement of IGFBP-6 in plasma may serve as an additional marker of this disease pattern.

Differential regulation of insulin-like growth factor binding protein-1 and -2 by insulin in the baboon (Papio anubis) endometrium

BACKGROUND

The purpose of this study was to examine the effect of insulin on expression and synthesis of IGFBP-1 and IGFBP-2 in the baboon endometrium in vitro.

METHODS

Baboon endometrial explants collected from cycling, ovariectomized, steroid-treated, simulated-pregnant and pregnant animals were cultured for 48 h in the presence or absence of insulin, with or without estradiol, progesterone and hCG.

RESULTS

Insulin clearly inhibited IGFBP-1 production and mRNA expression in a time- and dose-dependent manner, whereas IGFBP-2 synthesis was not significantly affected. The inhibitory effects of insulin on IGFBP-1 were more evident in explants of non-pregnant tissue or tissue away from the implantation site. In the absence of insulin, synthesis of IGFBP-1 was induced in explants with low levels of de novo synthesis whereas IGFBP-2 synthesis was inhibited. This effect was potentiated by steroids and hCG in the explant cultures.

CONCLUSION

Insulin differentially regulates endometrial IGFBP-1 and IGFBP-2 secretion in the baboon.

Expression and regulation of the insulin-like growth factor axis components in rat liver myofibroblasts

Apart from hepatic stellate cells (HSC), liver myofibroblasts (MF) represent a second mesenchymal cell population involved in hepatic fibrogenesis. The IGF system including the insulin-like growth factors I and II (IGF-I, -II), their receptors (IGF-I receptor, IGF-IR; IGF-II/mannose 6-phosphate receptor, IGF-II/M6-PR), and six high affinity IGF binding proteins (IGFBPs) participate in the regulation of growth and differentiation of cells of the fibroblast lineage, possibly contributing to the fibrogenic process. The aim of this work was to study the expression and regulation of the IGF axis components in rat liver MF.

METHODS

Cultures of MF from passages 1 to 4 (P1-4) were studied. IGFBP secretion was analyzed by [(125)I]-IGF-I ligand and immunoblotting. IGF-I, IGF-IR, IGF-II/M6-PR, and IGFBP messenger RNA (mRNA) expression was assessed by Northern blot hybridization. DNA synthesis was evaluated by 5-bromo-2'-deoxyuridine (BrdU) incorporation assay.

RESULTS

MF from P1 to 4 constitutively expressed mRNA transcripts specific for IGF-I, IGF-IR, and IGF-II/M6-PR. In MF, biosynthesis of IGFBP-3 and -2 was observed that was stimulated by IGF-I, insulin, and transforming growth factor beta (TGF-beta), whereas platelet-derived growth factor (PDGF-BB) revealed inhibitory effects. IGF-I and to a lesser extent insulin increased DNA synthesis of MF. Simultaneous addition of recombinant human IGFBP-2 or -3 with IGF-I diminished the mitogenic effect of IGF-I on MF whereas preincubation of MF with IGFBP-2 or -3 further potentiated the IGF-I stimulated DNA synthesis. In conclusion, the present study demonstrates that the IGF axis may play a role in the regulation of MF proliferation in vitro which might be relevant in vivo for the process of fibrogenesis during acute and chronic liver injury.

Specific amino acid substitutions determine the differential contribution of the N- and C-terminal domains of insulin-like growth factor (IGF)-binding protein-5 in binding IGF-I

We have previously reported that two highly conserved amino acids in the C-terminal domain of rat insulin-like growth factor-binding protein (IGFBP)-5, Gly(203) and Gln(209), are involved in binding to insulin-like growth factor (IGF)-1. Here we report that mutagenesis of both amino acids simultaneously (C-Term mutant) results in a cumulative effect and an even greater reduction in IGF-I binding: 30-fold measured by solution phase IGF binding assay and 10-fold by biosensor analysis. We compared these reductions in ligand binding to the effects of specific mutations of five amino acids in the N-terminal domain (N-Term mutant), which had previously been shown by others to cause a very large reduction in IGF-I binding (). Our results confirm this as the major IGF-binding site. To prove that the mutations in either N- or C-Term were specific for IGF-I binding, we carried out CD spectroscopy and showed that these alterations did not lead to gross conformational changes in protein structure for either mutant. Combining these mutations in both domains (N+C-Term mutant) has a cumulative effect and leads to a 126-fold reduction in IGF-I binding as measured by biosensor. Furthermore, the equivalent mutations in the C terminus of rat IGFBP-2 (C-Term 2) also results in a significant reduction in IGF-I binding, suggesting that the highly conserved Gly and Gln residues have a conserved IGF-I binding function in all six IGFBPs. Finally, although these residues lie within a major heparin-binding site in IGFBP-5 and -3, we also show that the mutations in C-Term have no effect on heparin binding.

Expression and release of insulin-like growth factor binding proteins in isolated epiphyseal growth plate chondrocytes from the ovine fetus

Insulin-like growth factor-II (IGF-II) is an autocrine modulator of epiphyseal chondrogenesis in the fetus. The cellular availability of IGFs are influenced by the IGF-binding proteins (IGFBPs). In this study, we investigated the control of expression and release of IGFBPs from isolated epiphyseal growth plate chondrocytes from the ovine fetus by hormones and growth factors implicated in the chondrogenic process. Chondrocytes were isolated from the proliferative zone of the fetal ovine proximal tibial growth plate and maintained in monolayer culture at early passage number. Culture media conditioned by chondrocytes under basal conditions released IGFBPs of 24, 34, and 29 kDa, and a less abundant species of 39-43 kDa that were identified immunologically as IGFBP-4, IGFBP-2, IGFBP-5, and IGFBP-3, respectively. Messenger RNAs encoding each species were identified by Northern blot analysis within chondrocytes, as was mRNA encoding IGFBP-6. Exposure to IGF-I or IGF-II (13 or 26 nM) caused an increase in expression and release of IGFBP-3. The release of IGFBP-2 and IGFBP-5 were also potentiated without changes to steady state mRNA, and for IGFBP-5 this was due in part to a release from the cell membrane in the presence of IGF-II. Insulin (16.7 or 167 nM) selectively increased mRNA and the release of IGFBP-3, while cortisol (1 or 5 microM) inhibited both mRNA and release of IGFBP-2 and IGFBP-5. Transforming growth factor-beta1 (TGF-beta1) (0.1 or 0.2 nM) increased the expression and release of IGFBP-3, and caused an increase in mRNAs encoding IGFBP-2 and IGFBP-5. Neither growth hormone (GH), fibroblast growth factor-2, nor thyroxine (T(4)) had any effect on IGFBP expression or release. The results suggest that IGFBP expression and release within the developing growth plate can be modulated by IGF-II and other trophic factors, thus controlling IGF availability and action.

Post-transcriptional and post-translational regulation of insulin-like growth factor binding protein-3 and -4 by insulin-like growth factor-I in uterine myometrial cells

Involution of the uterus induced by oestrogen depletion is associated with a decrease in uterine insulin-like growth factor (IGF)-I and an increase in IGF binding protein (IGFBP) gene expression. We examined the effects of IGF-I on primary uterine myometrial cell proliferation, and on IGFBP-3 and IGFBP-4 gene expression. IGF-I enhanced DNA synthesis in these cells. In conditioned media, IGF-I increased IGFBP-3 accumulation by release of cell associated IGFBP-3. A low dose of IGF-I increased IGFBP-4 accumulation, and a high dose caused IGFBP-4 to disappear. In cell-free conditioned media IGF-I protected IGFBP-3 and enhanced IGFBP-4 proteolysis. Co-incubation of [(125)I]-IGFBP-4 with cell-free conditioned media cleaved IGFBP-4 into 18 and 12 kDa fragments. Northern blot analysis indicated that IGF-I increased IGFBP-4 mRNA accumulation by stabilizing the mRNA while IGFBP-3 gene expression was slightly decreased. The results demonstrate that IGF-I regulates IGFBP-4 post-trancriptionally and post-translationally, whereas IGFBP-3 is only affected post-translationally. By enhancing IGFBP-4 proteolysis, increasing cell-associated IGFBP-3 and stabilizing IGFBP-3, IGF-I may initiate a mitogenic response.

Modification of plasma insulin-like growth factors and binding proteins during oral contraceptive use and the normal menstrual cycle

OBJECTIVE

Sex steroid regulation of the insulin-like growth factor axis is a subject of contention. We examined the effect of combined oral contraceptives and investigated the cyclic variations in the insulin-like growth factor axis.

STUDY DESIGN

Fasting blood samples were taken from 9 women receiving oral contraceptives, 10 women receiving no medication, and 10 male subjects.

RESULTS

In women receiving oral contraceptives, insulin-like growth factor binding protein 1 remained highly phosphorylated and levels were acutely increased by sex steroid treatment (305 +/- 110 microg/L on day 14 of the cycle [medication phase] vs 118 +/- 70 microg/L during the medication-free period, P <.03). In women receiving no medication, insulin-like growth factor binding protein 1 levels were significantly lower (69 +/- 50 microg/L on day 14 of the menstrual cycle, P <.001) and varied cyclically, with a rise in the late-secretory phase that coincided with the appearance of nonphosphorylated and less phosphorylated insulin-like growth factor binding protein 1 isoforms. Compared with those in untreated women and in men, insulin-like growth factor I levels were decreased in women receiving oral contraceptives (405 +/- 104 ng/mL in untreated women and 330 +/- 28 ng/mL in men vs 287 +/- 73 ng/mL in women receiving oral contraceptives, P <.004). Oral contraceptive use had no effect on insulin-like growth factor II levels, and neither insulin-like growth factor I nor insulin-like growth factor II showed cyclic variation.

CONCLUSION

The bioavailability of insulin-like growth factor I is reduced in users of oral contraceptives. This may contribute to the metabolic changes observed in such subjects.

Human renal fibroblasts modulate proximal tubule cell growth and transport via the IGF-I axis

To determine the paracrine interactions involved in the tubulointerstitial response to progressive renal disease, the role of insulin-like growth factor-I (IGF-I) and its binding proteins (IGFBPs) in in vitro interactions between human proximal tubule cells (PTC) and renal cortical fibroblasts (CF) were studied in primary cell culture. PTC growth and transport were increased in the presence of CF-conditioned media (CF-CM), as shown by increased thymidine incorporation, cellular protein content and sodium-hydrogen exchange (NHE) activity, to 185 +/- 31% (P < 0.01), 150 +/- 18% (P < 0.05) and 195 +/- 27% (P < 0.01) of the control values, respectively. IGF-I was produced by cultured CF at a rate of 64.6 +/- 7.5 ng/mg protein/day. Exogenous IGF-I applied to PTC provoked similar enhancement of growth and NHE activity as CF-CM and the stimulatory effect of CF-CM was blocked by specific immunoneutralization of IGF-I receptors. These receptors were threefold more abundant on PTC basolateral versus apical membranes. IGF binding proteins (IGFBP)-2 and IGFBP-3 were secreted by CF at rates of 694 +/- 88 and 1769 +/- 45 ng/mg/day, with the release of IGFBP-3 being enhanced in the presence of PTC-CM (120.0 +/- 9.7% of control, P < 0.01). Moreover, the addition of CF-CM to PTC increased cell-associated IGFBP-3 on PTC surfaces, without changes in IGF-I receptor numbers or affinity and without changes in PTC mRNA for IGFBP-3. Des(1-3)IGF-I, an analog that binds to the IGF-I receptor but not to IGFBPs, provided a less potent stimulus for PTC growth compared with IGF-I, indicating that cell-associated IGFBP-3 facilitates the action of IGF-I on PTC. The results support important paracrine roles for both IGF-I and IGFBPs in the interstitial regulation of proximal tubule growth and transport.

Effect of insulin-like growth factor binding proteins on the response of proximal tubular cells to insulin-like growth factor-I

The insulin-like growth factor binding proteins (IGFBP) are major modulators of insulin-like growth factor-I (IGF-I) action, but relatively little is known about their production by kidney tubular cells or about their modulating effects on the action of IGF-I on these cells. In this study we demonstrated that rabbit proximal tubular cells express the genes for IGFBP-2, -4 and -5 and secrete 24 and 32 kDa size binding proteins. The rate of IGFBP production by these cells was regulated by several growth factors including hydrocortisone, which was potently stimulatory, and EGF, which was inhibitory. The overall effect of these kidney cell-secreted IGFBPs was to inhibit the mitogenic activity of IGF-I. Similarly, recombinant IGFBP-3, the major circulating IGFBP that in kidney is produced close to the proximal tubules, also inhibited IGF-I stimulated DNA synthesis in cultured rabbit proximal tubular cells and in cultured opossum kidney (OK) cells. IGFBP-3 also inhibited basal DNA synthesis in OK cells in the absence of added IGF-I, suggesting that this IGFBP may have an IGF-I independent action. These findings highlight the important effect that IGFBPs have on the action of IGF-I on kidney cells and support the notion that the changes in IGFBPs observed in various renal diseases may contribute to the pathophysiology of these diseases.

Bioactivity of a 29-kilodalton insulin-like growth factor binding protein-3 fragment present in excess in chronic renal failure serum

Children with chronic renal failure (CRF) have normal or high serum levels of GH, IGF-I, and IGF-II. Despite this, the serum of CRF patients has low IGF bioactivity, which may contribute to CRF growth failure. Recent studies suggest that excess IGF binding proteins (IGFBPs) in the approximately 35-kD fractions of CRF serum contribute to this low IGF bioactivity. This report characterizes a 29-kD form of IGFBP-3, IGFBP-3(29), which accumulates in the approximately 35-kD fractions of CRF serum and peritoneal dialysate. Deglycosylation and [125I]IGF ligand blot studies show that IGFBP-3(29) is a glycosylated IGFBP-3 fragment with low affinity for IGF peptides. Using an IGFBP-3 antibody column, IGFBP-3(29) was purified to homogeneity from the approximately 35-kD fractions of peritoneal dialysate from children with CRF. Compared with native IGFBP-3, pure IGFBP-3(29) has a 4-10-fold lower affinity for IGF-II and a 200-fold lower affinity for IGF-I. Consistent with the binding data, IGFBP-3(29) inhibited IGF-II-stimulated thymidine incorporation in chondrosarcoma cells, but was a less potent inhibitor than native IGFBP-3; also, native IGFBP-3 clearly inhibited IGF-I-stimulated thymidine incorporation in chondrosarcoma cells and potentiated IGF-I-stimulated aminoisobutyric acid uptake in bovine fibroblasts, but higher concentrations of IGFBP-3(29) had no effect on these IGF-I actions. Thus, the 29-kD IGFBP-3 form that accumulates in CRF serum and extravascular spaces is an IGFBP-3 fragment that may modulate IGF-II, but not IGF-I, effects on target tissues. Whether IGFBP-3(29) plays any role in the growth failure of children with CRF remains to be determined.

Evidence for insulin-like growth factor (IGF)-independent transcriptional regulation of IGF binding protein-3 by growth hormone in SKHEP-1 human hepatocarcinoma cells

Insulin-like growth factor (IGF)-binding protein-3 (IGFBP-3) is a polypeptide that forms a ternary complex with IGFs and an acid-labile subunit. The hormonal regulation of the components of this complex is highly controversial, and both IGF-I and GH have been shown to mediate the expression/synthesis of IGFBP-3. This study investigates the regulation of IGFBP-3 protein, measured by RIA and Western ligand blot, and messenger RNA (mRNA) expression, measured by Northern analysis and reverse transcriptase-PCR, in SKHEP-1 human hepatocarcinoma cells. SKHEP-1 cells significantly increased the IGFBP-3 concentrations in conditioned medium (CM) when treated with GH (0.1-10 ng/ml), IGF-I (1-100 ng/ml), or Des(1-3)-IGF-I (1-100 ng/ml) in a dose-dependent manner (>3-fold). The increase in IGFBP-3 protein concentrations in CM was accompanied by a corresponding increase in IGFBP-3 mRNA levels. Interestingly, time-course studies showed that the GH-induced increase in IGFBP-3 mRNA preceded the IGF-I-induced increase (6 h for GH-induced IGFBP-3 mRNA; 12 h for IGF-I-induced IGFBP-3 mRNA). The half-life of IGFBP-3 mRNA was evaluated after transcriptional arrest by treatment with a RNA polymerase II inhibitor (5,6-dichloro-1beta-D-ribofuranosylbenzimidazole), and was found to be 14-18 h and unaltered by GH or IGF-I treatment. The induction of IGFBP-3 by GH was not due to the indirect action of locally synthesized IGF-I, because 1) no immunoreactive IGF-I was detected in the CM of control or GH-treated cells; 2) Northern blots revealed no IGF-I mRNA expression in SKHEP-1 cells; 3) reverse transcriptase-PCR did not detect any expression of the IGF-I gene; and 4) time-course studies showed an earlier increase in IGFBP-3 mRNA after GH treatment than after IGF-I treatment. Thus, the results obtained in this study are consistent with an IGF-I-independent regulation of IGFBP-3 gene expression by GH.

Growth hormone stimulates the secretion of insulin-like growth factor binding protein-2 (IGFBP-2) by monolayer cultures of sheep costal growth plate chondrocytes

Using monolayer cultures of costal chondrocytes established from four week old Clun Forest lambs, we have demonstrated that, under serum free conditions the cells release three IGFBPs (32, 29 and 21 kDa) into the medium. The most abundant of these--the 32 kDa BP--was shown to be IGFBP-2 by Western blotting. Furthermore we demonstrate that the levels of IGFBP-2 in conditioned medium are acutely increased (6, 12 and 24 h time points) following treatment of cells with bovine GH (1-100 ng/ml). In a parallel set of experiments, using ovine fibroblasts (derived from dermis) we show that IGFBPs of Mr 32, 29 and 21 kDa are also secreted by this cell type. However the relative abundance of these BPs differed from that seen in the chondrocyte cultures, with the 21 kDa species now the most abundant. In addition, prolonged exposure of autoradiographs indicated that fibroblasts secreted a higher Mr IGFBP (most probably ovine BP-3) that was not detected in any of our chondrocyte cultures. Most significant however was the demonstration that bGH did not dramatically affect the levels of IGFBPs in fibroblast cell cultures. We conclude that GH stimulates BP-2 production from chondrocytes and this is a cell-type specific effect in as much as it is not replicated in cultures of dermal fibroblasts.

Measurement of insulin-like growth factor (IGF)-II binding to purified IGF binding proteins 1-6: comparison of charcoal adsorption and high performance size exclusion chromatography

Insulin-like growth factor (IGF) binding to IGF binding proteins is commonly assessed by adsorbing free IGF to albumin-coated charcoal and quantitating bound IGF in the supernatant, but the validity of this technique has been questioned and many variations have been described. We compared the measurement of binding affinity and capacity of purified IGFBPs 1-6 for IGF-II using charcoal adsorption and Superdex G75 high performance size exclusion chromatography (HPSEC) to separate free and bound 125I-IGF-II. Optimal HPSEC recovery and resolution was obtained for IGFBP-1 and IGFBP-6 with low salt buffer (0.1 M sodium phosphate, pH 7.4), whereas phosphate buffer supplemented with 0.5 M NaCl was optimal for IGFBPs 2-5. Measurement of binding of 125I-IGF-II to IGFBPs 3-5 using the charcoal assay was also increased by the use of high salt buffer. Under optimal conditions, charcoal measurements of 125I-IGF-II binding to IGFBPs 1-5 were consistently lower than HPSEC measurements. By competitive binding using unlabeled IGF-II, the binding affinity of each of the IGFBPs for IGF-II was the same using both methods. Similarly, binding affinities as measured by charcoal assay were not affected by buffer composition. Differences in total binding obtained using the two methods and under different conditions were therefore due to differences in binding capacity. Charcoal adsorbs 15% of cross-linked 125I-IGF-II:IGFBP complexes which may partially explain the lower binding capacity for IGFBPs 1-5 determined by charcoal adsorption. Charcoal adsorption and HPSEC, therefore, are both valid methods for the measurement of binding affinities of IGFBPs for IGF-II, but assay conditions must be validated prior to measurement of binding capacity.

Synthesis of insulin-like growth factor binding proteins and of the acid-labile subunit of the insulin-like growth factor ternary binding protein complex in primary cultures of human hepatocytes

BACKGROUND/AIMS

The liver is the main source of circulating insulin-like growth factor binding proteins. In man, the cellular origin of insulin-like growth factor binding proteins has remained obscure.

METHODS

Human hepatocytes isolated from surgical specimens were purified and cultured using a collagen gel immobilization technique. Gene expression of individual insulin-like growth factor binding proteins and of the acid-labile subunit of the insulin-like growth factor binding proteins by Western ligand blotting and immunoblot analysis. Neutral size chromatography of medium samples was used to detect insulin-like growth factors binding protein complexes.

RESULTS

In cultured hepatocytes transcripts for insulin-like growth factor binding protein-1, -2, -3, -4 and for acid labile subunit could be demonstrated. Ligand blotting revealed the secretion of insulin-like growth factor binding proteins of molecular weights of 24 kD, 30 kD, 34 kD, 43 kD and 46 kD, respectively. Using polyclonal antisera, these proteins were identified as insulin-like growth factor binding protein-1, -2 and the insulin-like growth factor binding protein-3 doublet. Neural size chromatography of culture supernatants showed the presence of an insulin-like growth factor binding protein complex of approximately 40 kD, but absence of the high molecular weight ternary complex of 150 kD.

CONCLUSIONS

It is concluded that in man parenchymal liver cells have to be regarded as a source of acid-labile subunit and of circulating insulin-like growth factor binding proteins including insulin-like growth factor binding protein-3.

Differential effects of glucocorticoids on insulin-like growth factor I action in cultured human fibroblasts

Glucocorticoids act synergistically with insulin-like growth factor I (IGF-I) to stimulate DNA synthesis and replication of cultured human fibroblasts. In the present study, we further define glucocorticoid and IGF-I interactive effects on human fibroblast metabolism and growth. IGF-I stimulated dose-dependent increases in early metabolic events. Half-maximal effectiveness was seen at 5-8 ng/ml IGF-I, with mean maximal responses of 1.5-, 2-, and 6-fold for [3H]2-deoxyglucose uptake, [14C]glucose incorporation, and [14C]aminoisobutyric acid (AIB) uptake, respectively. A 48-hour preincubation with 10(-7) M dexamethasone markedly enhanced both the sensitivity and maximal effectiveness of IGF-I stimulation of AIB uptake. In contrast, dexamethasone had no effect on IGF-I-stimulated glucose uptake and utilization. Maximum specific binding of [125I]IGF-I to fibroblast monolayers was identical in ethanol control and glucocorticoid-treated cells, with 50% displacement at approximately 5 ng/ml IGF-I. In addition to its synergism with IGF-I, preincubation with dexamethasone augmented insulin and epidermal growth factor (EGF) stimulation of [3H]thymidine incorporation; dexamethasone had no effect on platelet-derived growth factor or fibroblast growth factor action. Two-dimensional gel electrophoresis identified two specific glucocorticoid-induced proteins in human fibroblast cell extracts with molecular weights of 45K and 53K and pIs of 6.8 and 6.3, respectively. These data indicate that IGF-I receptor-mediated actions in human fibroblasts are differentially modulated by glucocorticoids. Glucocorticoids are synergistic with IGF-I in stimulating mitogenesis and amino acid uptake, without having any apparent effect on IGF-I-stimulating glucose metabolism. Glucocorticoid enhancement of growth factor bioactivity may involve modulation of a regulatory event in the mitogenic signaling pathway subsequent to cell surface receptor activation.

Glucocorticoids regulate the secretion of a 21kDa-IGF-binding protein by sheep adipose tissue explants

Using a solution phase assay we have demonstrated that sheep adipose tissue explants secrete insulin-like growth factor binding proteins (IGFBPs) when cultured in serum-free medium over a 24 h period. Further, we demonstrate that secretion of IGFBP(s) is inhibited (up to 50%) by incubation of the cultures in the presence of 10(-8) M dexamethasone. This inhibitory effect is overcome when insulin (10 ng/ml) and ovine growth hormone (100 ng/ml) are incubated together (but not separately) with glucocorticoid. Further characterisation of this IGF binding activity by high performance size exclusion chromatography and Western ligand blot analysis indicated that under our culture conditions sheep adipose tissue explants secrete one predominant 21 kDa IGFBP and it is this BP which is hormonally regulated as described above. We discuss our results in the context of endocrine/paracrine/autocrine control of adipose tissue metabolism and differentiation.

Cleavage analysis of insulin-like growth factor (IGF)-dependent IGF-binding protein-4 proteolysis and expression of protease-resistant IGF-binding protein-4 mutants

Cultured human fibroblasts and osteoblast-like cells secrete an insulin-like growth factor (IGF)-dependent protease that cleaves IGF-binding protein-4 (IGFBP-4) into two fragments of approximately 18 and 14 kDa. Edman degradation of the isolated proteins established the amino termini of the reaction products. Sequence analysis of the 14-kDa carboxyl-terminal half of IGFBP-4 suggested cleavage after methionine at position 135 of the mature protein. Four variant IGFBP-4 molecules with single amino acid substitutions around this cleavage site were constructed and expressed. Wild-type and mutant IG-FBPs-4 bound IGF-I and IGF-II with equivalent affinities and, in the intact state, were equally effective inhibitors of IGF-I action. However, the IGFBP-4 mutants were relatively resistant to IGF-dependent proteolysis. A 5-6-h incubation in human fibroblast conditioned medium in the presence of IGF-II was sufficient for near total hydrolysis of wild-type IGFBP-4, whereas the mutant IGFBPs-4 were only minimally affected at this time. After a 24-h incubation with IGF-II, all mutant IGFBPs-4 showed extensive proteolysis, generating 18- and 14-kDa fragments. Pre-exposure of human fibroblasts in serum-free conditioned medium to IGF-II for 5 h potentiated subsequent IGF-I stimulation of DNA synthesis. When added with IGF-II, the protease-resistant mutant IG-FBPs-4, but not wild-type IGFBP-4, suppressed IGF-II enhancement of IGF-I-stimulated DNA synthesis. These biological studies suggest that the IGFBP-4/IGFBP-4 protease system may play a role modulating local cellular response to IGF-I.

Diminished expression of insulin-like growth factor (IGF) binding protein-5 and activation of IGF-I-mediated autocrine growth in simian virus 40-transformed human fibroblasts

The reduced growth factor requirements of murine fibroblasts transformed by simian virus 40 (SV 40) have been attributed to insulin-like growth factor (IGF)-I induction by T antigen and consequent activation of IGF-I receptor signaling. The present study shows that the autonomous growth of SV 40-transformed human fibroblasts also requires type-I IGF-I receptor activation but that this is not due to de novo induction of IGF-I gene expression since untransformed human fibroblasts, which fail to proliferate in the absence of serum, also showed IGF-I gene expression under serum-free conditions. DNA synthesis assays confirmed that untransformed cells were responsive to exogenous IGF and indicated that transformed cells were already maximally stimulated. In untransformed fibroblasts, IGF binding was principally to abundant membrane-associated IG-FBP-5, whereas in transformed fibroblasts this protein was minimally expressed, and IGF binding was to IGF receptors. Loss of detectable membrane-associated IG-FBP-5 in transformed cells was associated with diminished IGFBP-5 gene expression and with loss of IGF-II gene expression. Exogenous IGFBP-5 associated with the membranes of transformed cells and inhibited the autocrine growth of these cells. These findings suggest that loss of IGFBP-5 in SV 40-transformed fibroblasts facilitates interaction of endogenously produced IGF-I with the IGF-I receptor and increases their sensitivity to autocrine stimulation.

Regulation of insulin-like growth factor 1 binding protein 3 levels by epidermal growth factor and retinoic acid in cervical epithelial cells

Insulin-like growth factors (IGFs) are important regulators of epithelial cell growth. The mitogenic activity of these factors is influenced by the levels of extracellular IGF binding proteins, including insulin-like growth factor binding protein 3 (IGFBP-3). In the present report we study the effects of epidermal growth factor (EGF) and all-trans-retinoic acid (RA) on IGFBP-3 RNA and protein levels in human papillomavirus-immortalized cervical epithelial cells. Treatment of ECE16-1 cells with 3-20 ng/ml EGF causes a marked reduction in IGFBP-3 levels. In contrast, 1 microM RA increases IGFBP-3 mRNA and protein levels in the presence or absence of 20 ng/ml EGF. The response is concentration dependent with a half-maximal increase observed at 1 nM RA. RA is able to reverse the EGF suppression when added simultaneously or 3 days after initiation of EGF treatment. Conversely, when cells are treated with RA, IGFBP-3 levels increase within 24 h and subsequent addition of EGF is without effect. Thus, the RA-dependent increase in IGFBP-3 levels is dominant over the EGF suppression. The increased IGFBP-3 levels are correlated with RA suppression of proliferation. Similar RA effects on IGFBP-3 mRNA levels were observed in other cervical epithelial cell lines (i.e., ECE16-D1, ECE16-D2, and CaSki). These results suggest that RA may act to inhibit cervical cell growth by increasing IGFBP-3 levels and reducing the extracellular concentration of free insulin-like growth factor I (IGFI) and/or alternatively, IGFBP-3 may inhibit cell growth by direct effects on the cell, independent of IGFI.

Expression of insulin-like growth factor binding proteins by T-47D human breast cancer cells: regulation by progestins and antiestrogens

We have used ligand blotting and Northern blotting techniques to examine the effects of progestins and antiestrogens on expression of insulin-like growth factor binding proteins (IGFBPs) by T-47D human breast cancer cells under conditions where these agents are growth inhibitory. Under basal conditions, conditioned medium from T-47D cells was found to contain IGFBPs of 39, 33, and 27 kDa. Northern blot and/or Western blot analysis have identified these as IGFBP 2, 5, and 4, respectively. Medroxyprogesterone acetate (MPA) treatment resulted in a time- and dose-dependent decrease in IGFBP 4 and 5 mRNA abundance and secretion of these proteins, while little if any effect was observed on IGFBP 2 expression. A decrease in the steady state mRNA levels for IGFBP 4 and 5 was observed with as little as 0.1 nM MPA. Using 10 nM MPA a maximum decrease in IGFBP 4 and 5 mRNA levels was observed between 12 and 24 hours. While RU 486 alone had little or no effect on IGFBP 4 expression, it inhibited the effect of MPA. However, in the same samples, IGFBP 5 expression was inhibited by RU 486, and RU 486 was unable to reverse the effects of progestins on the expression of IGFBP 5. Furthermore, another synthetic progestin, Org 2058, but not dexamethasone, inhibited IGFBP 4 and IGFBP 5 expression. The antiestrogen ICI 164384 also transiently decreased the steady state mRNA levels of both IGFBP 4 and IGFBP 5. Regulation of expression of the IGFBPs by these agents suggests a potential role for the IGFBPs in the growth response of T-47D cells to these agents.

Regulation of insulin-like growth factor binding protein 4 by a specific insulin-like growth factor binding protein 4 proteinase in normal human osteoblast-like cells: implications in bone cell physiology

Insulin-like growth factor binding protein 4 (IGFBP-4) is secreted by normal human osteoblast-like cells (hOB) and is a potent inhibitor of insulin-like growth factor (IGF) action in vitro. In previous studies, IGF treatment of hOB in culture led to markedly reduced medium levels of IGFBP-4 as detected by western ligand blotting. In the present study, incubation of hOB-conditioned medium (hOB-CM) with IGF under cell-free conditions resulted in a similar loss of IGFBP-4. Both IGF-I and IGF-II were capable of inducing a decrease in IGFBP-4; however, IGF-II was more effective. When the six characterized IGFBP were added to hOB-CM, only IGFBP-4 disappeared in response to IGF-II addition. This IGF-regulated loss of IGFBP-4 was inhibited by metalloproteinase inhibitors and appeared to be due to a proteinase that cleaved IGFBP-4 in 18 and 14 kD fragments identified by western immunoblotting. Conditioned media from eight of eight different donor hOB lines tested exhibited IGFBP-4 proteinase activity. To assess the biologic consequences of IGF-II-induced IGFBP-4 proteolysis, we treated hOB with IGF-II for 5 h, which decreased medium IGF-BP-4 by 70%, and then measured IGF-I and insulin stimulation of [3H]thymidine incorporation. IGF-II itself was not mitogenic and had no effect on insulin-stimulated [3H]thymidine incorporation. However, pretreatment of cultured hOB with IGF-II enhanced IGF-I-stimulated [3H]thymidine incorporation threefold.(ABSTRACT TRUNCATED AT 250 WORDS)

Insulin-like growth factor binding protein production and regulation in fetal rat lung cells

Insulin-like growth factor binding proteins (IGFBPs) are expressed in lung from early in gestation and may modulate IGF-stimulated fetal lung cell proliferation and/or differentiation. To begin to define IGFBP production and regulation in lung cells during development, we prepared primary cultures of 19 day gestation fetal rat lung fibroblasts and epithelial cells and identified IGFBPs secreted into medium. Ligand blot analysis of conditioned media (CM) from both cell types demonstrated IGFBP bands of approximately 39,000-45,000, 32,000, 24,000, and 22,000 M(r). These migration characteristics allowed the identification of the 39,000-45,000 M(r) bands as IGFBP-3 and the 24,000 M(r) band as IGFBP-4, while Western immunoblot analyses localized IGFBP-2 to the 32,000 M(r) band and IGFBP-5 to the 22,000 M(r) band. Polymerase chain reaction amplification of cDNAs generated by reverse transcription of fibroblast and epithelial cell RNA using specific oligodeoxynucleotide primers for IGFBPs 1 through 6, demonstrated the presence of amplified products for IGFBP-2, -3, -4, -5, and -6. In both cell types, IGFBP-2 and -3 production was sustained during 48 h of incubation in serum-free medium, whereas IGFBP-4 abundance increased only during the first 6 to 12 h of incubation. CM from fibroblasts and epithelial cells plated at low densities contained a high abundance of IGFBP-2 per microgram cellular DNA compared with cells at higher densities. In contrast, IGFBP-3 and -4 abundance normalized to cell DNA did not change with differing cell densities.(ABSTRACT TRUNCATED AT 250 WORDS)

Posttranslational regulation of insulin-like growth factor binding protein-4 in normal and transformed human fibroblasts. Insulin-like growth factor dependence and biological studies

Insulin-like growth factor binding protein-4 (IGFBP-4) is a 24-26-kD protein expressed by a variety of cell types in vivo and in vitro. Treatment of normal adult human fibroblasts with 10 nM insulin-like growth factor II (IGF-II) for 24 h resulted in an 85% decrease in endogenous IGFBP-4, as assessed by Western ligand blot analysis of the conditioned medium. Incubation of human fibroblast-conditioned medium (HFCM) with IGF-II under cell-free conditions led to a similar loss of IGFBP-4. This posttranslationally regulated decrease in IGFBP-4 appeared to be due to a protease in HFCM: (a) It could be prevented with specific protease inhibitors or incubation at 4 degrees C; (b) proteolysis of recombinant human (rh) IGFBP-4 required HFCM; (c) immunoblotting and radiolabeling confirmed cleavage of IGFBP-4 into 18- and 14-kD IGFBP-4 fragments. The protease was specific for IGFBP-4, and was strictly dependent on IGFs for activation. IGF-II was the most effective of the natural and mutant IGFs tested, inducing complete hydrolysis of rhIGFBP-4 at a molar ratio of 0.25:1 (IGF/IGFBP-4). Simian virus 40-transformed adult human fibroblasts also expressed IGFBP-4 and IGFBP-4 protease, as well as an inhibitor of IGFBP-4 proteolysis. In biological studies, intact rhIGFBP-4 potently inhibited IGF-I-stimulated [3H]aminoisobutyric acid uptake, whereas proteolyzed rhIGFBP-4 had no inhibitory effect. In conclusion, these data provide evidence for a novel IGF-dependent IGFBP-4-specific protease that modifies IGFBP-4 structure and function, and indicate a preferential role for IGF-II in its activation. Posttranslational regulation of IGFBP-4 may provide a means for cooperative control of local cell growth by IGF-I and IGF-II.

The effect of insulin-like growth factor-I and human growth hormone on periodontal ligament fibroblast morphology, growth pattern, DNA synthesis, and receptor binding

Repopulation of the detached root surface by cells from the periodontal ligament (PDL) is a prerequisite for new attachment formation. Stimulation of PDL-cell growth may therefore serve as an essential method to enhance formation of new attachment. Studies have demonstrated that insulin-like growth factor-I (IGF-I) has a mitogenic effect on fibroblasts originating from various connective tissues and cell-lines. Further, human growth hormone (hGH) is known to regulate the plasma concentration of IGF-I and to mediate cellular biological effects. In the present study we examined the effect of IGF-I and hGH on morphology, growth pattern, and DNA synthesis. The expression of IGF-I and hGH receptors on the surface of cultured PDL fibroblasts is also described. A primary fibroblastic cell line was established from rat PDL tissue, and blind, photographic recordings of morphology and growth pattern, as well as incorporation of [3H]thymidine in cellular DNA, was carried out in the presence and absence of IGF-I and hGH. The presence of specific membrane receptors was investigated by binding of [125I]IGF-I and [125I]hGH. The analysis of photographs showed that IGF-I and hGH had no effect on morphology and growth pattern. Incorporation of 3H-thymidine, however, was increased in a dose-dependent manner by IGF-I, whereas hGH alone or in combination with IGF-I produced no dose-dependent response. Maximum effect (% of control) on DNA synthesis was 176% for IGF-I and 91% for hGH.(ABSTRACT TRUNCATED AT 250 WORDS)

Obstructive nephropathy in the pig. Possible roles for insulin-like growth factor I

Kidney growth was investigated in 30-kg pigs after 72 h of unilateral ureteral obstruction. The data were compared to control kidneys from normal non-operated pigs at same weight. Kidney wet weight was determined. Cortex and medulla were separated, and from both regions RNA, DNA, protein and kidney tissue insulin-like growth factor I was determined. Unilateral obstruction caused a doubling of the wet hydronephrotic kidney weight and an ipsilateral 76% increase in total kidney protein content. RNA increased by 45% in the cortex and 76% in the medulla. Kidney protein in the contralateral cortex increased by 23% and RNA by 42%. In the hydronephrotic kidney DNA was reduced by 13% in the cortex and by 21% in the medulla. Contralaterally, DNA was the same as in the controls. Mean kidney insulin-like growth factor I increased sevenfold in the ipsilateral medulla but in the cortex it was the same as in the controls. Serum insulin-like growth factor I concentration was 1.7 +/- 1.1 micrograms/l in the hydronephrotic animals and 1.2 +/- 0.8 micrograms/l in controls. At this stage of obstruction, our data demonstrate (1) hydronephrotic growth that is most probably hyperplastic in the medulla, associated with an increase in medullary insulin-like growth factor I, (2) hyperplastic growth in the cortex, and (3) contralateral kidney growth that is mainly hypertrophic after 72 h of contralateral ureteral obstruction.

Insulin and insulin-like growth factors in central nervous system tumors. Part V: Production of insulin-like growth factors I and II in vitro

The authors have previously reported the presence of insulin-like growth factor (IGF) receptors in central nervous system (CNS) tumors and the production of IGF's and their binding proteins by CNS tumors in situ. This study was designed to investigate whether CNS tumor cells are capable of autocrine secretion of IGF-I and IGF-II in vitro. Production of IGF's was studied by specific radioimmunoassay of tumor-cell-conditioned serum-free media from 34 CNS tumors: 12 gliomas, 12 meningiomas, and 10 miscellaneous tumors. Normal human serum and cerebrospinal fluid served as controls. Insulin-like growth factor I was detected in five of 12 meningiomas but in none of the gliomas studied. In contrast, IGF-II was detected in four of 12 gliomas and in six of 11 meningiomas studied. Four miscellaneous tumors produced IGF-I and/or IGF-II. These results suggest that CNS tumors differentially produce IGF-I and IGF-II in vitro. Preferential production of IGF's may be an important marker of the tumor-cell differentiation or malignancy and may be useful as a clinical diagnostic tool. These results add further support to the concept that IGF's may play a role in the regulation of the behavior of CNS tumors.

Platelet-derived growth factor isoforms decrease insulin-like growth factor I gene expression in rat vascular smooth muscle cells and selectively stimulate the biosynthesis of insulin-like growth factor binding protein 4

Platelet-derived growth factor (PDGF) is believed to be a critical mediator of vascular smooth muscle cell (SMC) proliferation. Because insulin-like growth factor (IGF) I (IGF-I) functions as a progression factor for the mitogenic effects of PDGF, we hypothesized that IGF-I gene expression and the production of IGF binding proteins (IGFBPs) by cultured rat aortic SMCs might be regulated by one or more of the three isoforms of PDGF: PDGF-AA, -BB, and -AB. IGF-I gene expression was highly dependent on cell density: IGF-I mRNA transcripts decreased markedly as a function of cell confluence. IGF-I mRNA content was inhibited to a similar degree by PDGF-AA, -BB, and -AB through a mechanism requiring protein synthesis. The inhibition was readily apparent at 4 hours, reaching approximately 25% of control levels after 24 hours. Radioimmunoassayable IGF-I was only barely detectable in SMC-conditioned serum-free medium and not significantly modulated by PDGF. Western ligand blot revealed that vascular SMCs release 30-kd and 24-kd IGFBP into serum-free conditioned medium. PDGF isoforms did not significantly alter release of the 30-kd IGFBP but evoked a fivefold to sixfold increase in the 24-kd IGFBP. The 24-kd IGFBP was found to comigrate with IGFBP-4, a recently identified binding protein that inhibits IGF action. The 30-kd protein was not merely a glycosylated form of IGFBP-4, because it was not sensitive to N-glycanase digestion. PDGF-AA, -BB, and -AB markedly induced expression of IGFBP-4 mRNA in a time- and concentration-dependent fashion. Vascular SMCs also express IGFBP-2 mRNA, but its abundance was not induced by PDGF. In conclusion, PDGF evokes a complex pattern of regulation of genes in the IGF/IGFBP system. By inhibiting IGF-I production and specifically inducing biosynthesis of the inhibitory binding protein IGFBP-4, PDGF may set in motion mechanisms to limit the final magnitude of the mitogenic response.

Insulin-like growth factor I and II binding in human myometrium and leiomyomas

OBJECTIVES

The purpose of this study is to determine if the human myometrium has receptors for insulin-like growth factors I and II and whether the concentration of these receptors is increased in leiomyomas.

STUDY DESIGN

Specific binding of iodine 125-labeled insulin-like growth factor I and II was examined in the membrane preparations of myometrium and leiomyomas obtained from 10 women with uterine leiomyomas.

RESULTS

Binding studies indicate presence of specific binding sites for both insulin-like growth factors I and II in the myometrium and leiomyoma. The concentration of binding sites for insulin-like growth factor I, but not for insulin-like growth factor II, was significantly (p less than 0.01) higher in leiomyomas than in the myometrium. The dissociation constants for insulin-like growth factors I and II receptors in both myometrium and leiomyoma were similar.

CONCLUSION

insulin-like growth factor I, but not insulin-like growth factor II, receptors are increased in leiomyomas compared with those in myometrium, indicating that insulin-like growth factor I may play a role in the generation and/or growth of this tumor.

Insulin-like growth factor binding protein 3 accumulates to high levels in culture medium of senescent and quiescent human fibroblasts

Insulin-like growth factor binding protein 3 (IGFBP-3) mRNA levels were consistently higher in both senescent normal human diploid fibroblasts (HDFs) at late passage (old cells) and prematurely senescent HDFs from a subject with Werner syndrome (WS) during serum depletion and repletion of growth medium and during proliferation from sparse to high-density inhibited cultures, compared to normal early-passage (young) HDFs. However, IGFBP-3 protein accumulated to higher levels in conditioned medium of old cells than in medium of WS and young cells, in that order, under the same conditions. Insulin-like growth factor I (IGF-I) was not detected in naive medium or in any of the media conditioned by these three cell types, whereas IGF-II was detectable in serum-repleted medium and remained relatively constant. Thus, molar ratios of IGFBP-3/IGF-II were consistently higher in old and WS cells and increased substantially as all three cell types became quiescent, due to either serum depletion or high cell density. These data are consistent with either an adaptive or a causal role for IGFBP-3 protein in the senescent and quiescent growth arrest of HDFs.

A unique receptor-independent mechanism by which insulinlike growth factor I regulates the availability of insulinlike growth factor binding proteins in normal and transformed human fibroblasts

Insulin-like growth factor I and II (IGF-I and IGF-II) associate with specific IGF binding proteins (IGFBPs) present in plasma and extracellular fluids that can modulate the anabolic effects of these peptides. IGF-I has been shown to increase IGFBP concentrations in vivo and in vitro, but the mechanism and significance of this action are unknown. We examined these issues using normal and simian virus 40-transformed adult human fibroblasts (SV40-HF) in culture. Treatment with IGF-I markedly stimulated the appearance of IGFBP-3 (42/38 kD doublet), a 36 kD IGFBP, and 28-32 kD IGFBPs in the medium of these cells, as assessed by Western ligand blotting; IGF-I decreased levels of 24 kD IGFBP in normal HF cultures. The IGF-I-induced change in IGFBP levels was not a type I IGF receptor-mediated effect on IGFBP synthesis because (a) high concentrations of insulin did not mimic IGF-I's effect; (b) IGF-II and IGF-I analogues having reduced affinity for the IGF-I receptor were equipotent with IGF-I in increasing medium IGFBPs; (c) [QAYL]IGF-I, and IGF-I analogue having normal receptor affinity and decreased affinity for IGFBPs, had no effect; and (d) alpha IR-3, a monoclonal antibody specific for the type I IGF receptor, did not block IGF-I-stimulated increases in IGFBPs. In physiological studies, preincubation with 1 nM IGF-I had no effect on type I IGF receptor binding in normal HF and SV40-HF. In contrast, preincubation of cells with an equivalent concentration of [QAYL]IGF-I downregulated the receptors 40-50%. Changes in cell surface receptor number were reflected in cell responsiveness to IGF-I-stimulated [3H]thymidine incorporation and [3H]aminoisobutyric acid uptake. In conclusion, IGF-I regulates the availability of specific IGFBPs in cultured human fibroblasts by a novel receptor-independent mechanism. Rapid changes in levels of soluble IGFBPs as a direct response to extracellular IGF-I, in turn, modulate IGF-I peptide and receptor interaction, and may constitute an important level of control in IGF cellular physiology.

Function of the growth hormone-insulin-like growth factor I axis in the profoundly growth-retarded diabetic child: evidence for defective target organ responsiveness in the Mauriac syndrome

Mauriac syndrome (MS) consists of a triad of poorly controlled diabetes, profound growth retardation, and hepatomegaly. The mechanisms involved in the growth retardation of those patients are not well understood. In an attempt to determine whether the growth retardation was secondary to somatroph secretory failure, abnormal pulsatile secretion, deletion of the growth hormone (GH) receptor, inadequate insulin-like growth factor I (IGF-I) generation, or abnormal IGF-I binding proteins (IGFBPs) two patients with MS were studied and their results compared with those of age-matched diabetic boys of similar glycemic control who were growing well. Overnight GH profiles in the MS and normally growing diabetics were analyzed by the CLUSTER program. The mean 12-hour GH concentrations, pulse amplitude, and pulse frequency were not different in either group of patients and did not change during acute normalization of the serum glucose overnight in the MS patients. The GH-binding proteins (GHBPs) relative binding were found to be the same in both groups of patients and did not differ from normal nondiabetic sera (62% +/- 8.0% relative specific binding in MS patients, v 53% +/- 4.3% in diabetic controls). The IGF-I concentrations were normal and comparable in both groups of patients (1.1 +/- 0.1 U/mL MS, v 1.1 +/- 0.3 diabetic controls). The IGFBPs were comparable in both groups of patients as well. One of the patients with MS had no meaningful increase in his growth velocity after 1 year on GH therapy despite good compliance. In conclusion, our data show normal hypothalamic-pituitary function, normal GHBP, IGF-I generation, and IGFBPs in two patients with MS when compared with normally growing diabetic children. These data, and the lack of linear growth in response to exogenous GH therapy in one patient, suggest a GH-resistant state, either secondary to impaired bioactivity of IGF-I, or a defect at or distal to the IGF-I receptor.

Altered affinity of insulin-like growth factor II (IGF-II) for receptors and IGF-binding proteins, resulting from limited modifications of the IGF-II molecule

The binding affinities of seven analogues of recombinant human insulin-like growth factor II (hIGF-II) were characterized for the IGF type-I and type-II receptors and insulin receptors, as well as for IGF-binding protein (IGFBP)-1, IGFBP-2, IGFPB-3 and human serum IGFBPs. A switch of two of the three cysteine bridges in hIGF-II, 9-47 and 46-51 to 9-46 and 47-51, severely impaired the binding of this analogue to all receptors and to the IGFBPs. The affinities for the IGF type-I receptor and the IGFBPs were decreased over 100-fold, while the binding to the insulin receptor and the IGF type-II receptor was less affected, with a 6-10-fold decrease in affinity. Slight modifications of the N-terminus had only minor effects upon the binding of hIGF-II to the IGFBPs or to the receptors. Deletion of both the N-terminal amino acid and the two C-terminal amino acids resulted in moderate decreases in affinity, with a 60% decrease in affinity for IGFBP-1 and the IGF type-I receptor. Acetylation of the N-terminus of Ala1 and the epsilon-nitrogen of Lys65 decreased the affinity, by 60-90%, of hIGF-II for all of the IGFBPs and receptors. The experiments involving acetylation of IGF-II or switching of its cysteine bridges indicated that these modifications (no substitution, deletion or addition of any of the 67 amino acids of hIGF-II) may lead to a severe impairment of the binding affinity of IGF-II for both the IGFBPs and the receptors. Acetylation of the epsilon-nitrogen of Lys65, which causes a charge change, or alteration of the three-dimensional structure, as shown by the cysteine bridge switch, lead to a severe impairment of the binding affinity for the binding proteins and for the receptors. In general, care should be taken with the synthesis of analogues and the interpretation of resulting binding data, since affinity alterations ascribed to amino acid changes may instead be caused by alterations of the charge or the three-dimensional structure of the protein.

Secretion of insulinlike growth factor I and insulinlike growth factor-binding proteins by murine bone marrow stromal cells

Insulin-like growth factor I (IGF-I) stimulates hematopoiesis. We examined whether bone marrow stromal cells synthesize IGF-I. Secretion of IGF-I immunoreactivity by cells from TC-1 murine bone marrow stromal cells was time-dependent and inhibited by cycloheximide. Gel filtration chromatography under denaturing conditions of TC-1 conditioned medium demonstrated two major peaks of apparent IGF-I immunoreactivity with molecular weights of approximately 7.5-8.0 kD, the size of native IGF-I, and greater than 25 kD. Expression of IGF-I mRNA was identified by both RNase protection assay and reverse transcription/polymerase chain reaction. To determine whether the greater than 25-kD species identified by RIA possessed IGF-binding activity, a potential cause of artifactual IGF-I immunoreactivity, charcoal adsorption assay of these gel filtration fractions was performed. The peak of IGF-binding activity coeluted with apparent IGF-I immunoreactivity suggesting that TC-1 cells secrete IGF-binding protein(s). Unfractionated conditioned medium exhibited linear dose-dependent increase in specific binding of [125I]-IGF-I with a pattern of displacement (IGF-I and IGF-II much greater than insulin) characteristic of IGF-binding proteins. Western ligand analysis of conditioned medium showed three IGF-I binding species of approximately 31, 38, and 40 kD. These data indicate that TC-1 bone marrow stromal cells synthesize and secrete IGF-I and IGF-binding proteins and constitute a useful model system to study their regulation and role in hematopoiesis.

Insulin-like growth factor-II (IGF-II) and IGF binding proteins in human endometrium

The insulin-like growth factor autocrine/paracrine system is believed to play a role in steroid-mediated endometrial differentiation. It is constituted of the mitogenic peptides (IGF-I and IGF-II), membrane receptors, and a family of high-affinity binding proteins (IGFBPs) that regulate the actions of the IGFs at their target cells. We have investigated expression of the mRNAs encoding the three major IGFBPs (IGFBP-1, IGFBP-2, and IGFBP-3) in human endometrium and have found, by Northern analysis, differential expression of all three mRNAs in secretory compared to proliferative endometrium, different steroidal milieux. IGF-II mRNAs were also detected in secretory endometrium. Finally, we found that human endometrial stromal cells in culture synthesize and secrete IGFBP-2 and IGFBP-3, and that the synthesis of IGFBP-2 is regulated by steroid hormones.

Cytochalasin B stimulates insulin-like growth factor-binding protein-1 production by Hep G2 cells

Hep G2 cells were used to study the early sequence of events regulating production of insulin-like growth factor-binding protein-1 (IGFBP-1). Cytochalasin B (100 microM) specifically inhibited 2-deoxyglucose uptake by Hep G2 cells and stimulated IGFBP-1 production 2-fold. Insulin (300 nM) did not stimulate hexose uptake but inhibited IGFBP-1 production more than 50%. A change in IGFBP-1 secretion was observed as early as 2 h after a 15-min or 2-h pulse exposure to either effector. In contrast to IGFBP-1, albumin production was diminished in the presence of cytochalasin B and increased by insulin. From these results we conclude that IGFBP-1 synthesis is (i) stimulated by transient inhibition of cellular glucose uptake and further stimulated by long-term glucose deprivation, and (ii) inhibited by transient exposure to insulin with further inhibition on long-term exposure. These effects are consistent with the dynamic regulation of IGFBP-1 by nutritional status.

Sequence analysis, expression and chromosomal localization of a gene, isolated from a subtracted human retina cDNA library, that encodes an insulin-like growth factor binding protein (IGFBP2)

The metabolic functions of insulin-like growth factors (IGFs) I and II are modulated by a family of binding proteins which are present in biological fluids and are synthesized by a variety of cell types. A cDNA clone, isolated at random from a subtracted human retina library, has been identified to code for a novel IGF-binding protein (IGFBP2) by its sequence homology to the peptide sequence of IGF binding proteins purified from bovine MDBK and rat BRL-3A cells. The complete nucleotide sequence of the IGFBP2 cDNA is 1406 bp long, contains 66% G-Cs and an open reading frame of 328 amino acids with a putative signal or pro-peptide of 39 residues. The mature polypeptide of 289 amino acids has 18 cysteines, a putative ATP-binding site and an RGD tripeptide. The 1.4 kb IGFBP2 transcript is expressed in several human tissues including fetal eye and fetal brain, but not in the human lymphoblastoid cell line against which the retinal cDNA library was subtracted. In situ hybridization to sections of mouse retina localized the mRNA for IGFBP2 primarily in the outer nuclear layer of photoreceptors. Southern blot analysis of DNA from human x rodent and mouse x rodent somatic cell hybrids assigned the gene for IGFBP2 to human chromosome 2q33-qter and mouse chromosome 1 in a known conserved syntenic region.

Endogenous insulin-like growth factor (IGF) binding proteins cause IGF-1 resistance in cultured fibroblasts from a patient with short stature

The ED50 of insulin-like growth factor (IGF)-I-stimulated alpha-aminoisobutyric acid (AIB) uptake (mean +/- SD) in cultured fibroblasts from a child with short stature that we have reported (1.40 +/- 0.24 nM), is significantly higher than the ED50 of IGF-I-stimulated AIB uptake in fibroblasts from 11 normal subjects (0.42 +/- 0.12 nM) and from 127 short children (0.35 +/- 0.11 nM). Similarly, the ED50 of IGF-I-stimulated thymidine incorporation in fibroblasts from this child is 2.8 times higher than that in fibroblasts from four normal subjects. To minimize potential modulation of IGF-I action by endogenous IGF binding proteins in these assays, fibroblast responsiveness to [Q3,A4,Y15,L16]IGF-I, an IGF-I variant that has a 600-fold reduced affinity for serum IGF binding proteins, has been examined. The biological activity of this variant is comparable in the patient's and normal fibroblasts, suggesting that the resistance to IGF-I action cannot be attributed to a defective IGF-I receptor. To investigate directly the possibility that IGF-I sensitivity in the patient's fibroblasts is reduced by endogenous IGF binding proteins (IGFBP), binding proteins that are secreted into AIB assay buffer during a 3-h collection and that are cell-associated at the end of the collection have been analyzed. Ligand blot analysis of conditioned AIB assay buffer demonstrates that fibroblasts from the patient secrete 1.3-2.2 times more of Mr 46,400/42,900, 32,000, and 26,800 binding proteins than normal fibroblasts. The major difference between fibroblasts from the patient and from normal subjects is a striking 10-fold increase in the amount of a cell surface Mr 32,000 binding protein in the patient's fibroblasts. The Mr 32,000 binding protein is similar in size to IGFB-1 and different from IGFBP-2 and IGFBP-3, but it does not cross-react with an antibody against IGFBP-1. We conclude that the resistance to IGF-I action in the patient's fibroblasts is caused by an abnormal production and/or cell association of IGF binding proteins.

Molecular mechanisms of tubulointerstitial hypertrophy and hyperplasia

Adult kidneys, which are principally composed of tubulointerstitium, do not normally regenerate or expand their working pool of functional cells at a very high rate. Loss of kidney tissue, however, can lead to some compensatory renal enlargement. The catalytic forces initiating such exchanges have not been fully articulated by current experimental endeavors. Increasing evidence, nevertheless, does suggest that factors other than simple changes in renal hemodynamics may be involved in this process. Different cellular elements in the tubulointerstitial microenvironment probably modulate changes in tubular enlargement or size through a complex cytokine network. Autocrine and paracrine stimulation of enlargement by different local growth factors also seem to play a pivotal role. After binding to cellular receptors, these factors activate signal transduction pathways resulting in expression of immediate early genes, which by themselves can synchronize the expression of subsequent genes through the medium of transacting factors. The renal enlargement response can also be modified by endocrine hormones that can activate such genes directly and/or stimulate other adjunctive processes, like receptor expression for the regional binding of growth factors. Furthermore, renal enlargement is under negative feedback of inhibitory factors like TGF beta. It is possible, for example, that special genes exist which are only expressed to arrest enlargement. It has been further suggested that activation of the Na+/H+ antiporter is a common denominator in renal enlargement. Recent findings, however, indicate that the activation of this antiporter is not always necessary, and might rather be a parallel event rather than a key phenomena in tubular enlargement. G0/G1 transition of tubular cells seems to involve similar factors in tubular hypertrophy and hyperplasia. The factors which are responsible for the final determination of the enlargement pattern (hypertrophy vs. proliferation) are unknown. The separation between hypertrophy and hyperplasia, although suggested by striking differences in cellular regulation, may be somewhat artificial, since responses leading to tubular enlargement also exist in circumstances where hyperplasia and hypertrophy are combined events. Recently it has been proposed that growth factors stimulate gluconeogenesis in proximal tubular cells producing hyperplasia, whereas factors inhibiting gluconeogenesis might induce hypertrophy. Whether the common pathway message of this intriguing hypothesis is correct still requires further validation.(ABSTRACT TRUNCATED AT 400 WORDS)

Growth factors in the human prostate

Recent studies have focused on the potential role of local polypeptide growth-regulating factors in the etiology of benign prostatic hyperplasia (BPH) and prostatic carcinoma. In our studies we confirmed the presence of specific receptors for epidermal growth factor (EGF) in prostatic tissues from patients affected by BPH. In addition, we demonstrated that specific receptors for insulin-like growth factor type I (IGF-I) are present in BPH tissues. In order to identify a possible interaction between androgens and these growth-regulating factors, we investigated the effect of testicular suppression-induced androgen withdrawal on both EGF and IGF-I receptor concentrations in prostatic tissue from patients affected by BPH treated with a long-acting luteinizing hormone-releasing hormone analog. Both EGF and IGF-I binding capacities were significantly increased after treatment. This finding suggests that in vivo IGF-I and EGF receptor levels may be under negative androgenic regulation, indicating a potential role for these growth-regulating factors in the mechanism of response to the castration-induced regression of androgen-dependent prostatic tissue. Moreover, preliminary studies indicate that in human BPH prostatic tissue multiple IGF-binding proteins (IGF-BP) are present. This finding suggests a possible role of IGF-BP in modulating IGFs biological activities at the prostate level.

Insulin-like growth factors are mitogenic for human keratinocytes and a squamous cell carcinoma

Normal adult human keratinocytes in monolayer culture and SCL-1, a skin-derived squamous-cell carcinoma cell line, were investigated for the expression of receptors for insulin-like growth factors (IGF) and insulin. As demonstrated by affinity crosslinking, radiolabeled IGF-1, IGF-2, and insulin bound specifically to both cell types. Each cell expressed type I IGF receptors, with affinity for IGF-1 greater than IGF-2 much greater than insulin. Insulin receptors, with highest affinity for insulin, were also present on both cells. However, keratinocytes and SCL-1 cells differed in 125I-IGF-2 binding. 125I-IGF-2-bound to both type I and type II IGF receptors in normal keratinocytes, but bound predominantly to membrane-associated IGF binding proteins in SCL-1. IGF-1 was slightly more potent than IGF-2 in stimulating growth of both keratinocytes and SCL-1 cells. In keratinocytes, concentrations of IGF-1 ranging from 5-100 ng/ml, and of IGF-2 from 50-100 ng/ml, resulted in a significant increase in cell number. At the maximum dose of 100 ng/ml, either IGF-1 or IGF-2 caused a 2.3-times increase in cell number. In SCL-1 cells, IGF-1 was more potent than IGF-2 or insulin at lower concentrations, but either IGF-1 or IGF-2 at the maximal concentration of 333 ng/ml stimulated a 4.7-times increase in thymidine incorporation. The stimulatory effect of insulin in SCL-1 was 10-50 times less potent than that of the IGF. The effect of either IGF on SCL-1 was completely inhibited by the type I IGF receptor antibody alpha IR-3, suggesting that both IGFs are mitogenic through the type I IGF receptor. Insulin action was partially blocked by alpha IR-3, suggesting that insulin can act through both the insulin and type I IGF receptors. It thus appears that IGF-1 and IGF-2 are mitogens for normal and transformed human keratinocytes and that their actions are primarily mediated through the type I IGF receptor, whereas insulin is a mitogen through both the IGF-1 receptor and the insulin receptor.

Insulin-like growth factor-binding protein-1 inhibits binding of IGF-I on fetal skin fibroblasts but stimulates their DNA synthesis

Insulin-like growth factor-binding protein-1 (IGFBP-1) was purified from human midtrimester amniotic fluid using monoclonal anti-IGFBP-1 affinity column. Two peaks were obtained in anion exchange chromatography. Both had the same molecular mass of 30 kDa. In monolayer cultures of fetal skin fibroblasts both forms of IGFBP-1 inhibited binding of [125I]IGF-I onto the cells, but amplified the IGF-I-stimulated [3H]thymidine incorporation into the same cells. Radiolabeled IGFBP-1 did not bind to the cells. No detectable IGFBP-1 was released into conditioned medium from the cells, and they contained no specific IGFBP-1 mRNA. Recently we found that the same IGFBP-1 preparation inhibits IGF-I-stimulated [3H]thymidine incorporation into human hyperstimulated granulosa cells. These results show that, depending on target cells, the same protein is capable of either stimulating or inhibiting DNA synthesis.

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.