Somatomedin carrier proteins

Role and Importance of IGF-1 in Traumatic Brain Injuries

It is increasingly affirmed that most of the long-term consequences of TBI are due to molecular and cellular changes occurring during the acute phase of the injury and which may, afterwards, persist or progress. Understanding how to prevent secondary damage and improve outcome in trauma patients, has been always a target of scientific interest. Plans of studies focused their attention on the posttraumatic neuroendocrine dysfunction in order to achieve a correlation between hormone blood level and TBI outcomes. The somatotropic axis (GH and IGF-1) seems to be the most affected, with different alterations between the acute and late phases. IGF-1 plays an important role in brain growth and development, and it is related to repair responses to damage for both the central and peripheral nervous system. The IGF-1 blood levels result prone to decrease during both the early and late phases after TBI. Despite this, experimental studies on animals have shown that the CNS responds to the injury upregulating the expression of IGF-1; thus it appears to be related to the secondary mechanisms of response to posttraumatic damage. We review the mechanisms involving IGF-1 in TBI, analyzing how its expression and metabolism may affect prognosis and outcome in head trauma patients.

Neuroendocrine dysfunction in the acute phase of traumatic brain injury

BACKGROUND

Pituitary hormone abnormalities have been reported in up to 50% of survivors of traumatic brain injury (TBI) who were investigated several months or longer following the event. The frequency of pituitary dysfunction in the early post-TBI period is unknown.

AIM

To evaluate the prevalence of anterior and posterior pituitary dysfunction in the early phase following TBI.

SUBJECTS

Fifty consecutive patients admitted to the neurosurgical unit with severe or moderate TBI [initial Glasgow Coma Scale (GCS) score 3-13], and 31 matched healthy control volunteers were studied.

METHODS

The glucagon stimulation test (GST) was performed at a median of 12 days (range 7-20) following TBI. Baseline thyroid function, PRL, IGF-1, gonadotrophins, testosterone or oestradiol, plasma sodium, plasma and urine osmolalities or the standard observed water deprivation test were performed. The control subjects underwent the GST for GH and cortisol responses; other parameters were compared to locally derived reference ranges.

RESULTS

Control data indicated that peak serum GH of > 5 ng/ml and cortisol > 450 nmol/l following glucagon stimulation should be taken as normal. Nine TBI patients (18%) had GH response < 5 ng/ml (12 mU/l). Eight patients (16%) had peak cortisol responses < 450 nmol/l. Compared to controls, basal cortisol values were significantly lower in patients with subnormal cortisol responses to glucagon and significantly higher in patients with normal cortisol responses (P < 0.05). GH and cortisol deficiencies were unrelated to patient age, BMI, initial GCS or IGF-1 values (P > 0.05). Forty patients (80%) had gonadotrophin deficiency, with low sex steroid concentrations, which was unrelated to the presence of hyperprolactinaemia. In males there was a positive correlation between serum testosterone concentration and GCS (r = 0.32, P = 0.04). One patient had TSH deficiency. Hyperprolactinaemia was present in 26 patients (52%) and serum PRL levels correlated negatively with the GCS score (r =-0.36, P = 0.011). Thirteen patients (26%) had cranial diabetes insipidus (DI) and seven (14%) had syndrome of inappropriate ADH secretion.

CONCLUSION

Our data show that post-traumatic neuroendocrine abnormalities occur early and with high frequency, which may have significant implications for recovery and rehabilitation of TBI patients.

Primary culture of canine tracheal smooth muscle cells in serum-free medium: effects of insulin-like growth factor I and insulin

The effects of growth factors on cell growth and muscarinic receptor (mAChR) expression of canine tracheal smooth muscle cells (TSMCs) were observed under serum-free medium supplemented with 0.1% BSA. In the presence of 0.1% BSA, TSMCs withdraw from cell cycle as compared with 10% FBS and allow to determine the effects of growth factors on mAChR expression. The individual components of growth factors (IGF-I, insulin, and aFGF) at the concentration used are not sufficient to stimulate growth of TSMCs in the primary culture with 0.1% BSA. IGF-I (10 ng/ml) and insulin (1 microgram/ml), alone or in combination, could stimulate the expression of mAChRs of cultured TSMCs. Heparin could inhibit these stimulatory effects of mAChR expression. The stimulatory effects of IGF-I and insulin on mAChR expression were mediated through their own receptors since these effects were reversed by pretreatment of TSMCs with antibodies of the respective growth factor receptors. The pharmacological response of functional mAChRs, determined as accumulation of inositol phosphates induced by carbachol, is greater in the medium containing IGF-I and insulin than that cultured in 0.1% BSA. These results firmly establish that IGF-I and insulin could stimulate the expression of mAChRs in TSMCs under serum-free culture condition.

Insulin-like growth factor-I levels and gene expression in ovine hereditary chondrodysplasia (spider lamb syndrome)

Hereditary Chondrodysplasia or Spider Lamb Syndrome (SLS) is an inherited, semi-lethal, musculo-skeletal disease affecting lambs primarily of Suffolk or Hampshire breeding. Deformities of the limbs and spinal column along with multiple sites of ossification at the anconeal process are diagnostic for the disease. Muscle atrophy is also predominant. We have investigated the relationship between SLS and circulating levels of IGF-I and the IGF-BPs in older (50-80 d of age) animals. Serum IGF-I levels were lower (P less than 0.01) in SLS affected lambs (117 ng/ml) than in phenotypically normal lambs (188 ng/ml) while serum levels of the 32 kDa BP increased (P less than 0.01) 77% in SLS affected lambs as compared to contemporary controls. All other IGF-BPs appeared to be unaffected in this group. Gene expression of IGF-I and -II in the liver and muscle of younger (16-22 d of age) lambs was also measured. There were no differences in IGF-II expression in either muscle or liver between SLS affected and phenotypically normal control lambs. Muscle IGF-I expression also did not differ. However, liver IGF-I expression in SLS affected lambs was nearly double that of control lambs (P less than 0.01). These data suggest that the regulation of IGF-I and the IGF-BPs may be involved in the physical manifestations of this disorder.

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.

Production of immunoreactive insulin-like growth factor-I (IGF-I) and IGF-I binding proteins by human lung tumours

The production of insulin-like growth factor I (IGF-I) and IGF-I binding proteins (BPs) by human lung tumour cell lines in vitro has been examined and the levels of these substances in the serum of lung cancer patients investigated. While small cell lung cancer (SCLC) cell lines secreted both IGF-I and BPs, non-small cell lung cancer (NSCLC) cell lines secreted BPs only. No evidence of increased serum IGF-I levels was obtained in a cohort of 52 lung cancer patients having SCLC and NSCLC histologies. In contrast, serum levels of low molecular weight BPs were markedly elevated in the majority of lung cancer patients.

Exercise patterns and trabecular bone density in college women

To assess the effects of physical activity patterns on trabecular bone density in college women, we studied three groups of nonsmoking eumenorrheic women with different (but chronic) exercise regimens. There were nine sedentary (SED) women exercising less than 1 h/week, nine women who performed aerobic (AER) exercise greater than 2.5 h/week, and nine women who supplemented aerobics with muscle-building activities (MB) for more than 1 h/week. Resting energy expenditure, calorie, protein, and calcium intake, total body weight, and body mass index were not statistically different among the three groups. AER and SED women had similar lumbar bone mineral density (BMD). MB women had significantly greater spinal bone density (p less than 0.007 versus SED, AER). IGF-1 (insulin-like growth factor) concentrations were greatest in MB (p less than 0.01), and hours muscle building per week correlated with IGF-1 (r = 0.86, p less than 0.03). For all 27 women (mean age 24.5 years), body mass index was the single best predictor of lumbar BMD (r = 0.42, p less than 0.03); hours in muscle-building exercise per week conferred an additive effect on lumbar BMD. This cross-sectional study of young women suggests chronic muscle-building exercises may augment lumbar bone mass. The additive effect of anaerobic exercise on bone density may be mediated by both local weight-bearing changes and possible systemic factors.

Characterization of the latent transforming growth factor beta complex in bone

Transforming growth factor beta (TGF-beta) is a 25 kD multifunctional polypeptide with pronounced effects on the proliferation and differentiation of a variety of cells in vitro. TGF-beta is a potent regulator of the activity of cells with the osteoblast phenotype and of isolated osteoclasts. It is released in increased amounts by bone cultures stimulated to resorb. Organ cultures of neonatal mouse calvaria produce TGF-beta as an inert large-molecular-weight complex that must be dissociated to release biologically active TGF-beta (5-8 ng/ml). We have shown recently that stimulated isolated avian osteoclasts release active TGF-beta from this bone-derived biologically latent form. In this report we have characterized this bone latent form of TGF-beta. Only small amounts of active TGF-beta (less than 0.5 ng/ml) and no free binding protein are detectable in conditioned medium from bone cultures. Active TGF-beta can be detected in acid-treated calvarial conditioned media in which none or only minute amounts could previously be detected. Following incubation at 37 degrees C, this activated TGF-beta gradually loses activity. Cross-linking studies using 125I-labeled TGF-beta show that this loss of activity is due to TGF-beta binding to a protein of approximately 300 kD. The TGF-beta latent complex accumulates in a linear manner and is stable in the presence of serum and the protease trypsin. Increases in temperature and pH extremes dissociate the complex to release active TGF-beta. Decreases in pH result in an exponential increase in TGF-beta activity. Significant activation of the latent TGF-beta was detectable at pH values as high as 4 and 5. Since the osteoclastic microenvironment is acidic during bone resorption, these data suggest that this acidic microenvironment may regulate TGF-beta activity by releasing active TGF-beta from its latent complex.

Cloning and expression of human insulin-like growth factor binding protein 3

Insulin-like growth factors (IGFs) found in plasma and other body fluids circulate in association with specific binding proteins. We report here the cloning and the nucleotide sequence of cDNAs for the growth-hormone-dependent acid-stable IGF binding protein, hIGF-BP3. The derived protein begins with a putative 27-amino acid signal peptide followed by 264 residues of the mature polypeptide. The predicted sequence contains three potential N-linked glycosylation sites and shares two region of homology with the low-molecular-weight non-growth-hormone-dependent binding proteins BP-1 and BP-2. The protein contains 18 cysteine residues clustered in the amino and carboxy termini. Chinese Hamster ovary cells transfected with this cDNA secrete a 43-45 kD protein doublet, which bound IGF. The expressed IGF-binding protein is indistinguishable from the native BP-3 found in human plasma.

Production of insulin-like growth factor binding proteins by small-cell lung cancer cell lines

Conditioned serum-free media (CM) from small-cell lung cancer (SCLC) cell lines were examined for the presence of insulin-like growth-factor-binding proteins (IGF-BP). 6/9 SCLC cell lines secreted binding proteins with high affinity for IGFs. When [125I]IGF-I or [125I]IGF-II was incubated with the CMs, complexes of tracer with proteins could be demonstrated by gel filtration, by precipitation with polyethylenglycol, and after adsorption of unbound tracer with activated charcoal. Analysis of binding data according to the method of Scatchard resulted in linear plots for IGF-I and IGF-II. The dissociation constants were determined to be 0.106 nM for IGF-I and 0.209 nM for IGF-II binding. Cross-linking of [125I]IGF-I or [125I]IGF-II to the CMs followed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) under nonreducing conditions revealed the presence of IGF-BPs with molecular masses in the range 24-32 kDa. The binding was competitively inhibited by addition of cold IGF-I and IGF-II but not by insulin. Northern blot hybridization with an IGF-BP cDNA probe encoding a low-molecular-weight IGF-BP from a human placenta cDNA library and Western blot analysis with a corresponding polyclonal antibody showed no expression of this gene. These data demonstrate that SCLC cell lines release IGF-BPs in culture supernatants, which differ from IGF-BPs detected in liver and placenta. These IGF-BPs might be important mediators in the autocrine/paracrine growth regulation of IGFs in SCLC.

Circulating insulin-like growth factor I in type 1 (insulin-dependent) diabetic patients with retinopathy

The relationship between insulin-like growth factor I (IGF I) and diabetic retinopathy was investigated. This somatomedin circulates bound to at least two large carrier proteins with molecular weights of approximately 150,000 and 35,000. Total and protein binding profiles of insulin-like growth factor I were determined in the serum of 18 patients who had had Type 1 (insulin-dependent) diabetes for 15-20 years, but had no signs of nephropathy and a similar degree of mild subclinical neuropathy. Nine had preproliferative or proliferative retinopathy and 9 had little or no background retinopathy but there was no difference in diabetes duration, insulin doses or glycaemic control between the two groups. In the latter group, the amounts of the somatomedin I and the serum profiles were similar to those in 9 healthy control subjects. In patients with advanced retinopathy, however, binding of insulin-like growth factor I to the carrier proteins was significantly altered. Binding to the low molecular weight protein increased to 140% whereas binding to the large molecular weight protein decreased to 70% of the normal level. In the latter Type 1 diabetic patients total serum insulin-like growth factor I was decreased to 60% of the normal level (p less than 0.02). When the alteration in serum profile was adjusted for, the level of somatomedin associated with the small carrier complex was normal whereas that associated with the large carrier complex was reduced by almost 60% in Type 1 diabetic patients with retinopathy. It is proposed that the total circulating somatomedin level is low in advanced diabetic retinopathy.(ABSTRACT TRUNCATED AT 250 WORDS)

Heterogeneity of insulin-like growth factor binding proteins in swine

Proteins in porcine amniotic fluid and sera (both fetal and adult) were separated electrophoretically in sodium dodecyl sulfate-containing polyacrylamide gels and transferred to nitrocellulose sheets. Western blots were analysed for proteins that would bind (a) radioiodinated insulin-like growth factor-I (IGF-I) and (b) antibodies to a rat insulin-like growth factor binding protein. Multiple insulin-like growth factor binding proteins were identified in sera. The binding proteins ranged in size from Mr 192,000 to 26,000. One immunologically cross-reactive protein (Mr 36,000) was detected. No binding proteins were detected routinely in amniotic fluids. Sera from adult swine were fractionated by preparative isoelectric focusing. Two binding proteins (Mr 192,000, 46,000) were located in acidic fractions which also contained IGF-I and IGF-II. Two binding proteins (Mr 36,000, 26,000) were located in neutral to basic fractions which contained primarily IGF-II. Immunoglobulin-sized material from adult sera fractionated over Sephadex G-200 contained two binding proteins (Mr 46,000, 42,000) whereas albumin-sized material contained one (Mr 36,000). Porcine insulin-like growth factor binding proteins are as heterogeneous as those from humans.

Binding proteins for the insulin-like growth factors: structure, regulation and function

Binding proteins for the insulin-like growth factors (IGF-I and IGF-II) are increasingly being recognized as modulators of IGF actions in both inhibitory and stimulatory ways. At least three distinct classes of binding protein are thought to exist, differing in their primary structures and binding characteristics, although all are able to bind both IGF-I and IGF-II. This review outlines the purification and characterization of the binding proteins that have been identified to date, and describes the regulation of their production and of their levels in the circulation. Current views on their potential biological roles are also discussed.

Purification of the serum acid-stable insulin-like growth factor binding protein from the pig (Sus scrofa)

1. An acid-stable IGF binding protein was isolated and purified from porcine serum. 2. The protein comprised two major species with Mrs of 45 and 41 kDa determined using SDS-PAGE under reducing conditions. 3. The IGFBP preparation specifically bound both IGF-I and II. 4. Four distinct protein bands (Mrs of 23, 45, 50 and 75 kDa) in the porcine IGFBP preparation specifically bound radiolabelled IGF-I. 5. The porcine IGFBP exhibited sequence homology with IGFBPs from human plasma and rat serum. 6. This is the first report of the purification and characterization of the acid-stable IGFBP from porcine serum.

Organization of the gene encoding the insulin-like growth factor binding protein IBP-1

Human genomic clones encompassing the tissue specific expressed gene IBP-1, an insulin-like growth factor binding protein were isolated and characterized. The gene is organized in four exons and spans 5.9 kb. S1 nuclease analysis determined a single transcription start site. The first exon and 5' flanking region are highly GC rich and located in a CpG island. The CpG island enclose the CAAT box, the TATA box, the transcription start site and a potential SP1 transcription factor binding site. The presumptive promoter region is characteristic for genes expressed in a tissue specific fashion. All signals required for cleavage/polyadenylation are located within exon IV, predicting a mRNA of 1.5 kb which is consistent with the size seen on RNA blots.

Isolation of a biologically active fragment from the carboxy terminus of the fetal rat binding protein for insulin-like growth factors

We have purified a 14 kDa fragment of the 30 kDa binding protein for insulin-like growth factors (IGFs) from BRL-3A cell conditioned medium. The fragment binds IGF-I and IGF-II with similar specificity to the 30 kDa binding protein, but with lower affinity. It corresponds to the carboxy terminus of the native binding protein (residues 148-270), and is thought to arise by proteolysis. We infer that this region of the native binding protein contains, at least in part, the IGF binding domain.

Human recombinant insulin-like growth factor I. I. Development of a serum-free medium for clonal density assay of growth factors using BALB/c 3T3 mouse embryo fibroblasts

A serum-free clonal density growth assay was developed for the quantification of the biological activity of human recombinant insulin-like growth factor I (IGF-I). The assay measures IGF-I stimulated growth of Balb/c 3T3 cells cultured over 4 d on poly-D-lysine-coated plastic surfaces in a serum-free medium formulation composed of a 1:1 (vol/vol) mixture of Ham's F12 and Dulbecco's modified Eagle's media, supplemented with 3.0 ng/ml bovine basic fibroblast growth factor (bFGF), 10 micrograms/ml human transferrin, 100 micrograms/ml ovalbumin, and 1.0 microM dexamethasone. Low-temperature trypsinization of serum-supplemented stock cultures combined with the use of poly-D-lysine-coated plates made it unnecessary to use serum or fibronectin to promote cell attachment and survival. Serum-free growth conditions were optimized with respect to the concentrations of the supplements. Addition of IGF-I resulted in 3.5-fold more cells than control cultures without IGF-I after 4 d. Deletion of BFGF resulted in no IGF-I stimulation of growth. The concentrations of various preparations of IGF-I required to achieve one-half maximal stimulation of cell number (ED50) ranged between 1.25 and 4.7 ng/ml. In parallel assays, IGF-I was 6.6 times more potent than human recombinant insulin-like growth factor II and 32 times more potent than insulin. When cells were seeded into medium containing IGF-I, transferrin, ovalbumin, and dexamethasone but no bFGF, growth was minimal. Dose-response addition of bFGF showed an ED50 of 0.9 ng/ml. The methods reported are useful to monitor the biological potency of recombinant and natural-source growth factors as well as providing a new means of studying the multiple growth factor requirements of Balb/c 3T3 cells in culture.

A new serum-free method of measuring growth factor activities for human breast cancer cells in culture

Growth of the MCF-7, T47D, and ZR-75-1 human breast cancer cells was established in a serum-free defined medium (MOM-1) composed of a 1:1 (vol/vol) mixture of Ham's F12 medium and Dulbecco's modified Eagle's medium containing 15 mM HEPES (pH 7.2), 2 mM 1-glutamine, 20 micrograms/ml glutathione, 10 micrograms/ml insulin, 10 micrograms/ml transferrin (Tf), 10 ng/ml selenous acid, 0.3 nM triiodothyronine, 50 micrograms/ml ethanolamine, 20 ng/ml epidermal growth factor, 2.0 nM 17 beta-estradiol, and 1.0 mg/ml bovine serum albumin (BSA). Proliferation in MOM-1 was 50 to 70% of the serum stimulated rate. Deletion of components from MOM-1 gave a medium (Tf-BSA) containing only HEPES, 10 micrograms/ml Tf, and 200 micrograms/ml BSA, which sustained MCF-7 and T47D cells in a slowly dividing and mitogen responsive state; ZR-75-1 cells required Tf plus 1.0 mg/ml BSA. In Tf-BSA, insulin and insulin-like growth factor I (IGF-I) were mitogenic with ED50 values of 2 to 3 ng/ml and 30 to 150 pg/ml, respectively, with MCF-7 cells. The T47D cells were responsive to these factors in Tf-BSA but required 10-fold higher concentrations for ED50. At saturating concentrations, insulin and IGF-I promoted 1.5 to 3.5 cell population doublings over controls in 8 d. At less than or equal to ng/ml concentrations, epidermal growth factor, insulin-like growth factor II, and basic fibroblast growth factor were mitogenic for human breast cancer cells in Tf-BSA. Mitogen activities in uterus and pituitary extracts were assayed readily in Tf-BSA. This new method offers a convenient means of comparing the potencies of growth-promoting factors on human breast cancer cells without interfering activities known to be present in serum.

Demonstration of insulin-like growth factor (IGF-I and -II) receptors and binding protein in human breast cancer cell lines

The insulin like growth factors (IGFs), potent mitogens for a variety of normal and transformed cells, have been reported to be secreted by several human breast cancer cell lines (BC). We have investigated the binding characteristics of IGF-I and -II in four human BC: MCF-7, T-47D, MDA 231 and Hs578T. Binding studies in microsomal membrane preparations detected high specific binding for both IGF in all four BC studied. Cross-linking with 125I-IGF-I, followed by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) under reduced conditions, revealed the presence of an alpha subunit of apparent Mr = 130,000 in MCF-7, T-47D and MDA 213 cells. When 125I-IGF-II was cross-linked, a major band of apparent Mr = 260,000 was seen in all BC. This band was inhibited by IGF-II, but not by insulin. Cross-linking of 125I-IGF-I to conditioned media from BC demonstrated the presence of three binding proteins of apparent Mr = 45,000, 36,000 and 29,000 in all BC but T-47D, in which the 36,000 band was not seen. These data demonstrate that BC possess classical receptors for both IGF-I and -II and, furthermore, that BC produce specific binding proteins for these growth factors.

Human blood-brain barrier insulin-like growth factor receptor

Insulin-like growth factor (IGF)-1 and IGF-2, may be important regulatory molecules in the CNS. Possible origins of IGFs in brain include either de novo synthesis or transport of circulating IGFs from blood into brain via receptor mediated transcytosis mechanisms at the brain capillary endothelial wall, ie, the blood-brain barrier (BBB). In the present studies, isolated human brain capillaries are used as an in vitro model system of the human BBB and the characteristics of IGF-1 or IGF-2 binding to this preparation were assessed. The total binding of IGF-2 at 37 degrees C exceeded 130% per mg protein and was threefold greater than the total binding for IGF-1. However, at 37 degrees C nonsaturable binding equaled total binding, suggesting that endocytosis is rate limiting at physiologic temperatures. Binding studies performed at 4 degrees C slowed endocytosis to a greater extent than membrane binding, and specific binding of either IGF-1 or IGF-2 was detectable. Scatchard plots for either peptide were linear and the molar dissociation constant of IGF-1 and IGF-2 binding was 2.1 +/- 0.4 and 1.1 +/- 0.1 nmol/L, respectively. Superphysiologic concentrations of porcine insulin inhibited the binding of both IGF-1 (ED50 = 2 micrograms/mL) and IGF-2 (ED50 = 0.5 microgram/mL). Affinity cross linking of 125I-IGF-1, 125I-IGF-2, and 125I-insulin to isolated human brain capillaries was performed using disuccinimidylsuberate (DSS). These studies revealed a 141 kd binding site for both IGF-1 and IGF-2, and a 133 kd binding site for insulin.(ABSTRACT TRUNCATED AT 250 WORDS)

The insulin-like growth factors and their binding proteins

1. This review provides a brief overview of the structure of the insulin-like growth factors (IGFs or somatomedins), their mRNA and genes; the regulation and sites of production of these peptides; their binding and actions in target tissues; and the structure and biological role of their binding proteins. 2. Molecular cloning techniques have allowed the prediction of precursor forms of IGF-I and IGF-II, have provided tools to study the regulation of the synthesis and translation of IGF mRNAs, and have recently yielded the primary sequence of the IGF-I receptor, supplementing other rapidly-accumulating structural data. 3. Several of the IGF binding proteins have also been purified, and initial structural studies performed. 4. The increased knowledge of the structures of the IGFs, their receptors and binding proteins should now permit rapid progress in understanding the physiology and functions of these proteins.

The molecular basis of growth hormone deficiency

A well-known law states that 'if a thing can go wrong it will go wrong'. This clearly applies to the hypothalamic-pituitary-somatic axis as to many other physiological and biochemical systems. Defects of this axis, giving rise to stunted growth, can occur at several different points, as has been discussed in detail in this review. Defects at the level of the brain can lead to inadequate production or secretion of the factors that control growth hormone secretion. Defects at the level of the pituitary can lead to failure to produce or secrete adequate quantities of growth hormone, or to production of inactive hormone. Defects at the level of target organs can lead to inability to respond to growth hormone or somatomedins. The axis involved in the production and effects of growth hormone is a complex one, and defects have been identified at most of the points that 'could go wrong', although in many cases the molecular details are far from fully understood. Increased understanding of the biochemistry and physiology of the hormonal control of growth, and of the impairments to which it is subject, should provide an improved basis for treatment of growth defects. Nevertheless, there remain many points at which our knowledge is very incomplete. The field is a rapidly moving one and further developments in both basic understanding and clinical treatment are to be expected during the next few years.

Preferential association of the insulin-like growth factors I and II with metabolically inactive and active carrier-bound complexes in serum

Ion-exchange chromatography of serum on DEAE-Sephadex A-50 using a stepwise NaCl gradient showed that complexes enriched with insulin-like growth factors I and II (IGF-I and IGF-II) could be preferentially eluted. A fraction eluted with 0.075 M-NaCl preferentially contained immunoreactive IGF-I with peak levels appearing in fractions of Mr approx. 110,000. The IGF-I-binding protein complex itself had low bioactivity as measured in a non-suppressible insulin-like (NSILA) bioassay. On conversion to free IGF-I by gel-permeation chromatography on Sephadex G-75 in 1% formic acid, however, the IGF-I did express its intrinsic NSILA bioactivity. In contrast, an IGF-II-enriched complex was eluted from the DEAE-Sephadex with 0.15 M-NaCl. Practically all of the recovered NSILA of the original serum was present in this fraction, in the Mr range 70,000-300,000 with a peak of 150,000. Chromatography on Sephadex G-75 in 1% formic acid separated this high-Mr NSILA into low-Mr (less than 15000) IGF-II and high-Mr acid-stable NSILA-P. The high-Mr IGF-II complex bound to concanavalin A-Sepharose, suggesting that it was a glycoprotein. The results confirm previous reports that a large portion of the NSILA of whole serum can be accounted for by a biologically active acid-dissociable complex. These data show for the first time that this active complex consists of an IGF-II-preferring binding protein. In direct contrast, the IGF-I-preferring complex does not express NSILA bioactivity until the IGF-I is liberated through acidification. The presence of a metabolically active IGF-II complex in serum raises questions as to its possible biological role in the adult.

Stimulation of proteoglycan biosynthesis by serum and insulin-like growth factor-I in cultured bovine articular cartilage

The addition of foetal calf serum to explant cultures of adult bovine articular cartilage is known to stimulate proteoglycan synthesis in a dose-dependent manner. We have now shown the activity in serum responsible for this effect to be heat- and acid-stable, to be associated with a high-Mr complex in normal serum but converted to a low-Mr form under acid conditions. The activity has an apparent Mr approximately 10,000 and isoelectric points similar to those reported for insulin-like growth factors (IGFs). Addition of a monoclonal antibody against insulin-like growth factor-I (IGF-I) prevented foetal calf serum from stimulating proteoglycan synthesis. Physiological concentrations of recombinant IGF-I or pharmacological levels of insulin when added to cartilage cultures mimicked the proteoglycan-stimulatory activity of serum. IGF-I appeared to act by increasing the rate of proteoglycan synthesis and did not change the nature of the proteoglycan synthesized nor the rate of proteoglycan catabolism by the tissue, suggesting that IGF-I may be important in the regulation of proteoglycan metabolism in adult articular cartilage. Furthermore, IGF-I can replace foetal calf serum in the culture medium, thereby allowing the use of a fully-defined medium which will maintain the synthesis and tissue levels of proteoglycan in adult articular cartilage explants for up to 5 days.

Identification and characterization of specific binding proteins for growth hormone in normal human sera

The well-recognized "big" forms (45,000-100,000 mol wt) of immunoreactive human growth hormone (hGH) in human serum have been reported to be random aggregates or formal polymers. However, we have now investigated the possibility that they are protein-bound forms. After incubation of monomeric 125I-hGH with normal serum, gel chromatography indicated a peak of bound 125I-hGH (at approximately 120,000 mol wt), which was completely displaced by excess unlabeled hGH. When serum alone was chromatographed two peaks of specific binding were subsequently detected, the major peak, eluting between 74,000 and 85,000 mol wt corresponded to the 125I-hGH-binding protein complex observed at approximately 120,000 mol wt. Using a mini-gel filtration system for separating bound from free hormone, binding of 125I-hGH by normal human serum was dependent on time (equilibrium was reached in 2 h at 21 degrees C), temperature (21 degrees C greater than 37 degrees C), Ca2+ and serum concentrations. Binding was reversible and highly specific for hGH, not being displayed by GH or prolactins from several species. Scatchard analysis revealed linear plots with an affinity (KA) of 0.32 +/- 0.06 X 10(9) M-1 (n = 7). Human serum with low endogenous hGH levels, when added to rabbit liver membranes, decreased the binding of 125I-hGH in this tissue in a dose-dependent manner. These data indicate that human sera contain a specific, high affinity binding protein for hGH and that this may account, at least in part, for the known size heterogeneity of GH in serum. Its effect on GH binding to target tissues may indicate a role for the binding protein in the regulation of GH action.

Characterization of Multiplication-Stimulating Activity (MSA) carrier protein

A cloned rat liver cell line (BRL-3A) synthesizes and secretes the somatomedin, Multiplication-Stimulating Activity (MSA), in association with its specific carrier protein (MCP). Affinity-purified MCP is a single-chain polypeptide with a molecular weight of 31,500 under non-reducing conditions and 34,000 when fully reduced. The formation of a Mr 42,000 complex following chemical crosslinking of purified MSA (Mr 8700) and MCP (Mr 34,000) suggests that these components bind in a 1:1 molar ratio on the basis of the sum of their combined molecular weights. The amino acid composition and the N-terminal amino acid sequence of MCP were also determined. Polyclonal MCP-antibody preparations were used to determine if MCP could be detected in normal rat sera. MCP could not be detected in adult rat serum, but was present at high concentrations in fetal rat serum. These results suggest that MCP is a fetal somatomedin carrier protein and that MSA-MCP complexes may be important during fetal development. The availability of antibodies directed against a purified somatomedin carrier protein will provide the opportunity to investigate further the role of carrier proteins in the biological activity of the somatomedins.

Analysis of serum insulin-like growth factor binding proteins using western blotting: use of the method for titration of the binding proteins and competitive binding studies

A nitrocellulose gel transfer technique has been adapted to study the insulin-like growth factor (IGF) binding proteins of human serum. Normal and hypopituitary sera were subjected to sodium dodecyl sulfate-polyacrylamide gel electrophoresis, followed by electroblotting to nitrocellulose or nylon membrane. Nonidet-P40 (3%) and Tween 20 (0.1%) were required for quenching and to allow detection of the IGF binding proteins by autoradiography after overlay with either 125I-labeled IGF I or IGF II. Several forms of IGF binding protein have been identified with molecular weights of 41,500, 38,500, 34,000, 30,000, and 24,000. Titration and competitive binding studies with IGF were performed on the transferred IGF binding proteins, indicating that binding proteins isolated by this technique can be characterized.

Paracrine functions of somatomedins

Evidence is growing that the somatomedins act by a paracrine and/or autocrine mechanism. The importance of these mechanisms relative to the traditional endocrine actions is not clear, and it is possible that these growth factors act through all three mechanisms. Supporting the possible paracrine/autocrine mechanisms are reports that production of somatomedins or somatomedin-like peptides is widespread throughout the body. Additionally, the somatomedins have biological actions on remarkably diverse cell types, and these responsive cells are found in close proximity to cells known to produce somatomedin. Finally, factors that alter the growth rate of cultured cells produce parallel changes in somatomedin secretion, suggesting that these phenomena are closely linked.

Two insulin-like growth factor I messenger RNAs are expressed in human liver

Through use of a synthetic oligonucleotide probe, human insulin-like growth factor I (IGF-I) cDNA clones were isolated from a liver library. Two types of cDNAs were defined by restriction enzyme analysis and DNA sequencing. Both encode IGF-I precursors of either 195 or 153 amino acids. The two predicted protein precursors are identical from their amino terminus to a lysine residue 16 codons beyond the IGF-I sequence, and then they diverge. Both cDNAs predict additional unique carboxyl-terminal extension peptides. Since there is only one IGF-I gene in the human genome, the finding of two different cDNAs suggests that alternative RNA processing plays a role in IGF-I gene expression. The functions of the different extension peptides remain to be elucidated.

Evidence for the specific binding of growth hormone to a receptor-like protein in rabbit serum

The binding in vitro of 125I-human or bovine growth hormone (GH) to normal female rabbit serum has been studied using gel filtration to separate bound and free hormone. On Ultrogel AcA34 columns, a substantial peak of specific 125I-GH binding was observed at a MW approximately 120 000. This peak was not precipitable by 12.5% polyethylene glycol, a method used widely for solubilized hormone receptors. Assuming a 1:1 binding stoichiometry between GH (MW 21 000) and the binding protein, the MW of the binding protein would be approximately 100 000. Gel filtration of serum alone, followed by assessment of 125I-hGH binding in column fractions, indicated the binding protein had a similar MW (83 000-107 000). Specific binding of 125I-hGH was dependent on incubation time (equilibrium being reached in 2 h at 21 degrees C), Ca2+ concentration (0.5-2.0 mM) and serum concentration (a 1:5 dilution of serum giving 45.2 +/- 1.7% specific binding; mean +/- SE, n = 10). Binding was completely reversible (t 1/2 approximately 1.5 h) and specific for somatotrophic but not lactogenic hormones. Scatchard analysis revealed linear plots with a Ka 1.59 +/- 0.11 X 10(9) M-1 and capacity 3700 fmol/ml serum. The presence in rabbit serum of a high affinity, GH-specific, binding protein raises important questions regarding its identity and possible physiological role in modulating the delivery and/or activity of GH in vivo.