Radioimmunological determination of insulinlike growth factors I and II in normal subjects and in patients with growth disorders and extrapancreatic tumor hypoglycemia

Radioimmunoassay for the measurement of insulin-like growth factor I in patients with pituitary disease in comparison with commercially available somatomedin-C radioimmunoassays

A highly sensitive and specific radioimmunoassay for the measurement of insulin-like growth factor I (IGF I) has been developed. The IGF I concentrations were measured in sera of normal subjects, patients with acromegaly, hypophysectomized patients and patients with hyperprolactinaemia. The results were compared with the results obtained after measurement of IGF I in the sera of the same patients with two commercially available radioimmunoassays for somatomedin-C. IGF I was separated from its carrier protein using Sep-Pak C18 cartridges. The total recovery of IGF I by this method was about 100%. The in-house assay shows a high specificity for IGF I and a high sensitivity. As little as 0.1 microgram/l of IGF I can be detected. Only a small amount of serum (25 microliter) is necessary for the IGF I determination and a great number of serum samples (more than 100 per day) can be processed. The mean immunoreactive IGF I concentration was 379 +/- 159 micrograms/l in normal adult subjects, 3340 +/- 1094 micrograms/l in acromegalic patients, and 52 +/- 11 micrograms/l in growth-hormone deficient patients. Patients with hyperprolactinaemia had an IGF I concentration of 433 +/- 112 micrograms/l.

Acute metabolic effects and half-lives of intravenously administered insulinlike growth factors I and II in normal and hypophysectomized rats

Insulinlike growth factors (IGF) act qualitatively like insulin on insulin target tissues in vitro. In the circulation in vivo they are bound to specific carrier proteins. In this form or when continuously infused into hypophysectomized (hypox) rats they do not exert acute insulinlike effects on glucose homeostasis. This study definitively shows that intravenous bolus injections of pure IGF I or II act acutely on glucose homeostasis: they lower the blood sugar, enhance the disappearance of U-[14C]glucose from serum and increase its incorporation into diaphragm glycogen in normal and hypox rats in the presence of antiinsulin serum. The same effects were obtained with recombinant human IGF I injected intravenously either with or without antiinsulin serum into normal rats. Free fatty acid levels decreased transiently only in normal animals. Lipid synthesis from glucose in adipose tissue was not stimulated in hypox and barely stimulated in normal rats. The half-life of injected IGF I or II in normal rats (approximately 4 h) is strikingly different from that in hypophysectomized rats (20-30 min) and appears to depend on the growth hormone-induced 150,000-200,000-mol wt IGF carrier protein that is lacking in hypophysectomized rats. 15 min after the bolus serum IGF I and II concentrations were similar to steady state levels during long-term infusion in hypox rats. Free IGF was barely detectable, however, in the infused animals, whereas 40-100% was found free 15 min after the bolus. These observations for the first time confirm the hypothesis that only free IGF, but not the IGF carrier protein complex, is bioavailable to insulin target tissues.

Affinity chromatography with amniotic fluid somatomedin binding protein in the purification of insulin-like growth factor I

A simplified procedure has been developed for the isolation of insulin-like growth factor I from human plasma by use of affinity chromatography with the somatomedin binding protein. After acidification of human plasma and separation of insulin-like growth factor I and endogenous binding protein by cation exchange chromatography on SP-Sephadex the material was passed through a column packed with pure human amniotic fluid binding protein covalently coupled to Sepharose. The bound insulin-like growth factors I and II were eluted by 1M acetic acid and separated on a Mono S cation exchange column by use of a salt gradient. The 30 micrograms insulin-like growth factor I and 18 micrograms insulin-like growth factor II recovered from 1 liter plasma gave an overall recovery of 30% for insulin-like growth factor I but only 2.5% for insulin-like growth factor II.

Insulin-like growth factors in patients with nonislet cell tumors and hypoglycemia

Insulin-like growth factors (IGF) in serum from 3 patients with nonislet cell tumors and hypoglycemia were measured by radioimmunoassay and by means of a rat liver membrane assay (specific for IGF II). The concentration of IGF II by receptor assay was greater than normal when the samples were initially assayed, but subsequently decreased 48% to 80% over the next 8 to 15 weeks. In the same serum samples, concentrations of both IGF I and IGF II by radioimmunoassay were consistently less than normal. In all 3 patients growth hormone (GH) responses to intravenous arginine were depressed and in 2 of the 3 patients, a GH-dependent 150 K serum protein carrier of IGF was absent. None of these abnormalities were seen in 7 patients with insulinomas and chronic hypoglycemia. The data suggest that some patients with nonislet cell tumors and hypoglycemia produce a receptor-active, nonimmunoreactive IGF-II-like material. This material appears (a) more labile than normal IGF II and (b) capable of inhibiting GH secretion. The latter effect may decrease immunoreactive IGF I and II, as well as decrease the GH-dependent 150 K protein carrier of these factors. The extreme lability of the IGF-II-like material produced by tumors probably explains the previous contradictory reports on this topic.

Measurement of insulin-like growth factor-II by radioreceptor assay using ovine placental membranes

A new radioreceptor assay for insulin-like growth factor-II (IGF-II), using receptors on ovine placental membranes, is described. Half-maximal displacement of specifically bound radioiodinated human IGF-II tracer was seen at 1.0 ng/tube of unlabelled IGF-II. The cross-reactivity of IGF-I was 1%, and insulin was entirely without effect. Measured on serum samples from 100 healthy adults, the mean IGF-II concentration (+/- SD) was 576 +/- 160 ng/ml. Identical mean values were seen for all adult age groups up to 65 years. The mean value for 10 acromegalic adults was 583 +/- 155 ng/ml, and for 9 GH-deficient subjects, 161 +/- 26 ng/ml (P less than 0.001 compared to normals). Of eight patients with chronic renal failure, none had an IGF-II level less than 2SD above the normal mean. No significant effect of renal dialysis was seen. In groups of patients with gastric, breast, lung, testicular, oat cell, ovarian, colonic and prostatic carcinoma, Hodgkin's lymphoma, non-Hodgkin's lymphoma, sarcoma and teratoma (5-12 patients per group), mean IGF-II levels were in the lower part of the normal range. Thus this study does not provide evidence supporting a role for excessive IGF-II production in the growth of any of these tumour types.

Receptors for insulin-like growth factors and insulin on murine fetal cortical brain cells

Fetal murine neuronal cells bear somatomedin receptors which can be classified according to their affinities for IGF-I, IGF-II and insulin. Binding of 125I-IGF-I is half-maximally displaced by 7 ng/ml IGF-I while 15- and 700-fold higher concentrations are required for, respectively, IGF-II and insulin. Linear Scatchard plots of competitive-binding data with IGF-I suggest one single class of type I IGF receptors (Ka = 2.6 X 10(9) M-1; Ro = 4500 sites per cell). The occurrence of IGF-II receptors appears from the specific binding of 125I-IGF-II and competition by unlabeled IGF-II; the IGF-II binding sites display a low affinity for IGF-II and no affinity for insulin. IGF-II also interacts with insulin receptors although 50- to 100-fold less potent than insulin in competing for 125I-insulin binding. The presence of distinct receptors for IGF-I, IGF-II and insulin on fetal neuronal cells is consistent with a role of these peptides in neuronal development, although our data also indicate that IGF-I receptors could mediate the growth promoting effects of insulin.

Circulating insulin-like growth factor I levels in newborn premature and full-term infants followed longitudinally

Longitudinal circulating levels of insulin-like growth factor I (IGF-I) were measured by radioimmunoassay after acid/ethanol extraction of serum or plasma in 44 appropriate-for-gestational age (AGA) premature infants, 7 small-for-gestational age (SGA) premature infants and 9 AGA full-term infants. The subjects were divided into cohorts with gestational age at birth 26-29 weeks, 30-33 weeks, 34-37 weeks and 38-42 weeks (full-term). The premature infants in this study exhibited diminished growth as compared with normal intrauterine growth. In all but the earliest premature infant cohort there was an immediate fall from the mean fetal IGF-I level, as reflected by the cord value, to a basal postnatal circulating level of IGF-I. The basal level of circulating IGF-I in premature infants was related only to gestational age. It increased slowly from 25 weeks gestation until four weeks after full-term equivalent and was independent of time of birth. Full-term infants were distinguished from early premature infants by the occurrence of a prominent postnatal surge in circulating IGF-I levels that was characterised by a significant (P less than 0.02) increase between day 1 and days 10-15. The SGA and AGA infants in the 34-37 week cohort showed similar profiles of circulating IGF-I with no significant difference in cord values between the two groups.

Effect of bromocriptine treatment on circulating somatomedins in tall adolescents

Somatomedin and bromocriptine levels were measured before and after 3 months of bromocriptine treatment (5 mg/day) in the sera of 16 tall adolescents with excessive adult height prediction. Somatomedins were measured by RIA for somatomedin C and IgFII and by measuring thymidine incorporation into human lectin-activated lymphocytes. Mean +/- SEM levels of bromocriptine after three months of treatment were 0.61 +/- 0.08 ng/ml. No changes in radioimmunoassayable somatomedin C were observed after bromocriptine intake respectively 1.34 +/- 0.17 U/ml before and 1.4 +/- 0.01 U/ml during treatment. On the opposite a significant decrease of thymidine activity (p less than 0.002) from 1.45 +/- 0.17 U/ml to 1.12 +/- 0.19 U/ml was observed. No changes of IgFII levels were observed in the sera of the 8 patients where they were measured. In order to test a possible direct effect of bromocriptine on peripheral tissues bromocriptine mesylate was added in somatomedin bioassays. Inhibitory effect of bromocriptine in vitro is seen at higher levels (1 microM) compared to the circulating one (1 nM). This study demonstrates that the major effect of bromocriptine which is the acceleration of bone maturation is not related to changes in somatomedin C.

Comparative binding of bovine, human and rat insulin-like growth factors to membrane receptors and to antibodies against human insulin-like growth factor-1

The immunological properties of human, bovine and rat insulin-like growth factors (IGF) and insulin were compared in competitive binding studies with Tr10 and NPA polyclonal antisera raised in rabbits against human IGF-1. Bovine IGF-1 was 11-19% as effective as human IGF-1 in competing for binding with 125I-labelled human IGF-1, whereas IGF-2 reacted poorly and insulin did not compete. Similar competitive binding curves were obtained with the mouse monoclonal anti-(human IGF-1) antibody 3D1, except that bovine IGF-1 showed a severalfold greater affinity for the monoclonal antibody than for either polyclonal antiserum. Membranes isolated from human placenta, sheep placenta and foetal-human liver were used as sources of cellular receptors. In human placental membranes, most of the binding of IGF-1 tracers could be attributed to a type-1 receptor, because insulin inhibited up to 65% of tracer binding. The other two tissues apparently contain only type-2 receptors, as evidenced by the very low potency of bovine or human IGF-1 in competing for binding with IGF-2 tracers and the absence of any competition by insulin. In competition for binding with labelled bovine or human IGF-1 to human placental membranes, bovine IGF-1 had a similar potency to human IGF-1, whereas bovine IGF-1 was more potent in binding studies with tissues rich in type-2 receptors. Rat IGF-2 was considerably less effective than human IGF-2 in competition for receptors on any of the membrane preparations.

Activity in fetal bovine serum that stimulates erythroid colony formation in fetal mouse livers is insulinlike growth factor I

In the present study, the erythropoietic activity of fetal serum was characterized. Using fetal bovine serum (FBS) as a source of the erythropoietic activity and serum-free cultures of fetal mouse livers (FMLC assay) as a detection system, we found that FBS stimulated colony formation from late erythroid progenitor cells (CFU-E) in a dose-dependent fashion. The slope of the dose-response curve, however, was significantly different from that for erythropoietin (Ep), the best-characterized erythropoietic activity so far. The erythropoietic activity of FBS was found in the 120-160- and 40-70-kD range at neutral pH. In the presence of 1 M acetic acid, however, the erythropoietic activity had an apparent molecular mass between 3 and 13 kD. From ion exchange experiments with DEAE-cellulose, the isoionic point of the activity was estimated to about pH 5. Furthermore, the erythropoietic activity of FBS was found to be co-eluted on Sephadex G-150 with the binding proteins of insulinlike growth factors (IGF). The IGF I concentration determined by radioimmunoassay was 70 ng IGF I/ml. The Ep activity of FBS was less than 5 mU/ml when determined with the posthypoxic polycythemic mouse assay for Ep. These results suggest that the erythropoietic activity of FBS is related to IGF and not to Ep. The erythropoietic activity of FBS was abolished by an antiserum against IGF I. Furthermore, IGF I was a factor of approximately 40 more potent than IGF II in stimulating erythroid colony formation. All of these findings suggest that the erythropoietic activity of FBS is IGF I.

Interactions of insulin, insulinlike growth factor II, and platelet-derived growth factor in erythropoietic culture

To examine the influence of insulin and insulinlike growth factor (IGF) on erythropoiesis, we tested their effects in human bone marrow cultures prepared with biochemically defined medium or a platelet-poor plasma-derived serum (PDS) that was depleted of hormones by adsorption to activated charcoal. Erythroid colony formation was enhanced two- to threefold by 10 ng/ml of electrophoretically pure IGF-II and 100 ng/ml of highly purified insulin (P less than 0.05). Dose-response curves for IGF-II were parallel to and shifted by one to two orders of magnitude to the left relative to those for insulin. When added together to culture, IGF-II and insulin expressed additive activities. In contrast, their activities were synergistic with those of erythropoietin and burst-promoting activity. The erythropoietic actions of IGF-II and insulin were similar in PDS and whole blood serum (WBS) containing cultures. Furthermore, when added to cultures with electrophoretically pure platelet-derived growth factor, their respective activities were synergistic. We conclude that insulin and IGF-II potentiate human marrow erythropoiesis in vitro. Their activities appear to be mediated by a similar receptor or postreceptor system.

Insulin-like growth factors and insulin: comparative aspects

IGF I and IGF II are two insulin-like growth factors resembling insulin in many respects. They stem from a common precursor, act through receptors similar to the insulin receptor with which they cross-react. When administered in large amounts they produce hypoglycemia. Their major effects, however, are on replication and differentiation of cells of mesodermal origin. IGF I is the major growth promoting factor in vivo. The synthesis and secretion of IGF I by the liver depend on the growth hormone status, insulin and nutrition. In contrast to insulin, the IGFs circulate in blood bound to the carrier proteins. Their half-life in man is in the order of 16 h. IGF I deficiency results in dwarfism (pygmy, Laron dwarf, toy poodle) despite normal or elevated growth hormone secretion. The anabolic actions of insulin and of the IGFs appear to complement each other as shown in Figure 7.

Insulin-like growth factors I and II of nonmammalian sera

We have compared the insulin-like growth factors (IGFs) of a number of mammalian and nonmammalian vertebrate sera. Sera were subjected to acid gel filtration through G-75 Sephadex and separate binding protein and free IGF fractions were pooled. The IGF pools were subjected to flatbed isoelectric focusing and the concentration of IGF I in the fractions was detected with an immunoassay using 125I-hIGF I and the somatomedin C antibody of Underwood and Van Wyk. IGF II in the fractions was detected with either the 125I-IGF rat placental membrane RRA or the 125I-IGF II serum binding protein assay. One or more basic peaks of IGF I immunoactivity were detected in all the mammalian sera studied (human, guinea pig, rat, bovine, kangaroo, and opossum) and all the nonmammalian species studied (chicken, turtle, toad, and trout). Neutral and slightly acidic peaks of IGF II receptor reactivity were observed in human, cow, and guinea pig serum. No distinct peaks of IGF II were observed in adult rat, opossum, or kangaroo serum. Distinct peaks of IGF II activity in the neutral to slightly acidic range were clearly recognized in chicken and turtle serum, no such peaks were recognized in toad and trout serum. All mammalian sera studied contained acid stable, binding proteins with high affinity, saturable binding of 125I-IGF II. None of the nonmammalian sera studied contained demonstrable specific 125I-IGF II binding. These observations document the presence of separate IGF I and IGF II peptides in chickens and turtles but only IGF I in amphibians and fish. These observations indicate that the gene duplication giving rise to two separate IGFs arose before reptilian evolution. The acquisition of a specific IGF binding protein is limited to mammals.

Somatomedin-C in accelerated growth of children with precocious puberty

To assess the role of somatomedin-C as a possible mediator of the growth spurt in children with central precocious puberty, we compared Sm-C levels in 40 children with central precocious puberty, 87 age-matched normal children, and 110 normal pubertal controls. Somatomedin C levels were significantly elevated for age in the children with precocious puberty (P less than 0.01), and were similar to the levels observed during normal puberty. The patients with precocious puberty were given the luteinizing hormone releasing hormone analogue D-Trp6-Pro9-NEt-LHRH (LHRHa) for 6 months. Treatment caused a significant decrease in secondary sexual characteristics, growth rate, plasma gonadotropins, sex steroids (estradiol in the girls and testosterone in the boys), and Sm-C levels. Growth during LHRHa treatment returned to the age-appropriate rate, whereas plasma Sm-C levels, although lower than pretreatment levels, remained significantly elevated for age (P less than 0.002). In addition, growth rates before and during treatment did not correlate with the plasma somatomedin C levels, nor did the decreases in growth rate during LHRHa therapy correlate with the decreases in somatomedin C levels. Growth rates did correlate significantly, however, with plasma estradiol levels in the girls (P less than 0.0005) and with plasma testosterone levels in the boys (P less than 0.025). We conclude that the growth spurt in children with precocious puberty cannot be explained by the plasma level of somatomedin C.

Amino acid sequence of a variant pro-form of insulin-like growth factor II

Human serum contains, in addition to the "classical" 7.5-kDa insulin-like growth factors (IGFs) I and II, small amounts of larger IGF-II. A 10-kDa IGF-II was isolated by gel filtration, immunoaffinity chromatography, and reversed-phase HPLC. Upon amino acid sequence determination, a substitution of Cys-Gly-Asp for Ser-33 was found as well as a COOH-terminal extension of 21 residues (E peptide). These sequence differences suggest that 10-kDa IGF-II is a precursor of a variant IGF-II. Since the substitution is not located at a known intron/exon hinge region, the finding of this variant IGF-II is evidence for the presence of more than one gene for IGF-II.

Insulin-like growth factor II (IGF II) in human brain: regional distribution of IGF II and of higher molecular mass forms

Twenty-four distinct areas of human brain were analyzed for the presence of insulin-like growth factor (IGF). As reported for cerebrospinal fluid, only IGF II-like immunoreactivity, but no significant amounts of IGF I-like immunoreactivity, could be found. Upon gel permeation chromatography, two to five distinct size classes were separated on the basis of their immunoreactivity. The smallest component had an apparent molecular mass of 7.5 kDa, identical to the one of purified IGF II from human serum. Radioimmunoassays and a bioassay also gave results indistinguishable from those of serum IGF II. The highest amounts of IGF II-like immunoreactivity occur in the anterior pituitary--namely, 20-25 pmol equivalents/g of wet weight. This is up to 100 times more than in most other brain regions analyzed. The higher molecular mass immunoreactive species were partially characterized. After immunoaffinity purification, the 38- and 26-kDa species are active in a bioassay. Specific IGF-binding protein activity could be shown after purification of the 38- and 26-kDa species on an IGF-affinity column. The 13-kDa species released significant amounts of 7.5-kDa material. The results are interpreted as evidence for the presence of IGF II synthesized locally in human brain. The structure of the larger forms of IGF II-like immunoreactive material as well as the function of IGF II in brain are not yet known.

Comparative effects of somatomedin C and insulin on the metabolism and growth of cultured human fibroblasts

At concentrations of 25 ng/ml in serum-free medium, somatomedin C (SM-C) and insulin stimulated 3H-thymidine incorporation in adult human fibroblasts 4- and 1.5-fold, respectively. The presence of 0.25% human hypopituitary serum (HHS), which by itself had little effect, enhanced the mitogenicity of both SM-C and insulin. Furthermore, 10(-7)M dexamethasone dramatically potentiated SM-C stimulation (70-fold) and insulin stimulation (28-fold) of 3H-thymidine incorporation. With dexamethasone and 0.25% HHS, significant stimulation of DNA synthesis was seen at 2.5 ng/ml for both SM-C and insulin. The effects of SM-C and insulin on 3H-thymidine incorporation were additive. These 3H-thymidine incorporation results were clearly supported by cell replication studies. On the other hand, SM-C and insulin had equivalent, nonadditive effects on RNA and protein synthesis and protein degradation. Half-maximal effects were seen for both peptides on all three metabolic processes at 2-5 ng/ml. In contrast to their synergism with SM-C in the stimulation of DNA synthesis and cell replication, HHS and dexamethasone did not enhance SM-C stimulation of RNA or protein synthesis or protein degradation. These data indicate that SM-C and insulin stimulate DNA, RNA, and protein synthesis, protein degradation, and cell replication in adult human fibroblasts at nanomolar concentrations, suggesting that each peptide is capable of acting through its own receptor. Both SM-C and insulin are also capable of synergism with low concentrations of serum and dexamethasone in the stimulation of DNA synthesis and cell replication. It is proposed that SM-C and insulin both participate in the regulation of cell growth and metabolism in vivo.

Isolation of a cDNA clone encoding rat insulin-like growth factor-II precursor

Insulin-like growth factor-I (IGF-I) and IGF-II are mitogenic polypeptides of relative molecular mass (Mr) approximately 7,500 isolated from human plasma each containing four peptide domains in a single chain and identical at more than 60% of their amino acid loci. The B- and A-domains of the IGFs are approximately 40% identical to the B- and A-chains of human insulin. IGF-I and IGF-II have similar in vitro biological activities and receptor reactivity, but are immunologically distinct. IGF-I appears to mediate the effects of growth hormone on cartilage to promote skeletal growth whereas IGF-II may have a special role in fetal development and in the central nervous system. To investigate the in vivo role of IGF-II, we have studied IGF-II biosynthesis in the BRL-3A rat liver cell line. BRL-3A cells synthesize and secrete a 7,484 Mr protein 93% identical to human IGF-II and representing rat IGF-II (rIGF-II). Rat IGF-II is synthesized as a approximately 22,000 Mr prepro-rIGF-II (ref. 12) from 12 S poly(A)+mRNA. In addition, approximately 20,000 Mr pro-rIGF-II has been identified in lysates of biosynthetically labelled intact BRL-3A cells. We report here the isolation of an almost complete cDNA clone for rIGF-II. Our results indicate that pro-rIGF-II is synthesized as a 156 amino acid peptide precursor (17,619 Mr) containing mature rIGF-II 1-67 at its amino-terminus and an 89-residue carboxy-terminal peptide extension.

Dual regulation of glycogen metabolism by insulin and insulin-like growth factors in human hepatoma cells (HEP-G2). Analysis with an anti-receptor monoclonal antibody

Insulin and the insulinlike growth factors (IGF-I and IGF-II) are members of a family of hormones that regulate the metabolism and growth of many tissues. Cultured HEP-G2 cells (a minimal deviation human hepatoma) have insulin receptors and respond to insulin by increasing their glycogen metabolism. In the present study with HEP-G2 cells, we used 125I-labeled insulin, IGF-I, and IGF-II to identify distinct receptors for each hormone by competition-inhibition studies. Unlabeled insulin was able to inhibit 125I-IGF-I binding but not 125I-IGF-II binding. A mouse monoclonal antibody to the human insulin receptor that inhibits insulin binding and blocks insulin action inhibited 75% of 125I-insulin binding, but inhibited neither 125I-IGF-I nor 125I-IGF-II binding. When glycogen metabolism was studied, insulin stimulated [3H]glucose incorporation into glycogen in a biphasic manner; one phase that was 20-30% of the maximal response occurred over 1-100 pM, and the other phase occurred over 100 pM-100 nM. The anti-receptor monoclonal antibody inhibited the first phase of insulin stimulation but not the second. Both IGF-I and IGF-II stimulated [3H]glucose incorporation over the range of 10 pM-10 nM; IGF-I was three to fivefold more potent. The monoclonal antibody, however, was without effect on IGF regulation of glycogen metabolism. Therefore, these studies indicate that insulin as well as the IGFs at physiological concentrations regulate glycogen metabolism in HEP-G2 cells. Moreover, this regulation of glycogen metabolism is mediated by both the insulin receptor and the IGF receptors.

Proteolytic conversion of insulin-like growth factors to an acidic form(s)

The relative amounts of the various forms of bioassayable insulin-like growth factors (IGF) isolated from human serum or serum fraction Cohn IV-1 depend on the purification procedure. With acid gel filtration or acid/ethanol extraction as the initial step, IGF-II (pI approximately 6.5) was the most abundant (40-70%) followed by somatomedin A (pI approximately 7.4; 15-23%), an acidic form of insulin-like activity (ILA pI 4.8) (13-21%) and IGF-I (pI approximately 8.5; 5-27%). If, however, pH 5.5 ion-exchange chromatography on SP-Sephadex was used prior to acid gel filtration, the acidic pI 4.8 form was the major (greater than 90%) species recovered and was accompanied by a quantitative loss of the other IGF species. This suggested a possible conversion of IGF-I, somatomedin A and/or IGF-II to the acidic ILA pI 4.8 form(s) during the SP-Sephadex procedure. Further experiments indicated that differences in the yields of ILA pI 4.8 were not due simply to differences in the initial pH conditions of the various methods (i.e. acid versus neutral), although exposure to pH 9.7 (a pH experienced during elution of IGF activity from the SP-Sephadex) did appear to play a role. The involvement of the carrier protein in the conversion process was tested by subjecting carrier-free IGF-I and IGF-II to the SP-Sephadex procedure. No conversion of the free forms to ILA pI 4.8 occurred. To examine the possible role of proteinase in the conversion of IGFs to ILA pI 4.8, SP-Sephadex chromatography was performed in the presence of a broad spectrum proteinase inhibitor. The IGF distribution pattern obtained closely resembled the 'normal' pattern seen with acid gel filtration, indicating that proteinase inactivation had prevented conversion to ILA pI 4.8. These data suggest that proteolytic conversion of IGF-I, somatomedin A and IGF-II to more acidic ILA pI 4.8 form(s) (i) occurs during SP-Sephadex chromatography, (ii) is not prevented simply by prior acid exposure, and (iii) takes place only when IGF-I and -II are in their high-Mr carrier-bound forms. Since IGF-I and IGF-II, although homologous, have unique amino acid sequences, the conversion of both IGFs implies that at least two acidic ILA forms exist. Nevertheless, because ILA pI 4.8 retains the full spectrum of IGF bioactivities in vitro, and significant quantities are present in normal human serum (21%), it would suggest that proteolytic conversion of IGF-I, somatomedin A and IGF-II to ILA pI 4.8 in vivo may be a physiologically significant event.

Insulin-like growth factor characteristics of an acidic non-suppressible insulin-like activity

The biological activities of an acidic form of non-suppressible insulin-like activity (ILA pI 4.8) have been studied. ILA pI 4.8 was isolated from Cohn fraction IV-1 of human serum by pH 5.5 ion-exchange chromatography on SP-Sephadex. Carrier-bound ILA was eluted at pH 9.7 and then sequentially gel chromatographed in 1% formic acid on Sephadex G-75 and Bio-Gel P-30. The low-Mr (7000) active material was subjected to flat bed isoelectric focusing. Overall recovery was 87 munit of insulin equivalents/100 g of Cohn fraction IV-1, with a specific activity in the range 4-10 munit/mg of protein, representing a purity of 1-6%. This material has been tested in a variety of insulin-like growth factor (IGF)/somatomedin assay systems. It stimulated, in a dose-related manner, [14C]glucose conversion into lipid by isolated rat adipocytes, 35SO4(2-) incorporation into weanling rat costal cartilage and [3H]thymidine incorporation into DNA of cultured human fibroblasts. Like IGF-I and -II, ILA pI 4.8 was able to inhibit degradation of 125I-insulin by crude homogenates of rat liver. In addition, the biological activity of ILA pI 4.8 was completely suppressible by a recently described inhibitor of IGF-I and IGF-II. ILA pI 4.8 was able to compete, in a parallel manner, with 125I-IGF-I and 125I-IGF-II and, at higher doses, with 125I-insulin in a placental radioreceptor assay. No cross-reactivity was seen in a radioimmunoassay for IGF-I and -II C-peptides, but at higher concentrations parallel displacement was observed in a somatomedin C/IGF-I radioimmunoassay using two different antisera. These data indicate that ILA pI 4.8 does possess many of the biological activities previously reported for the IGFs. Since ILA pI 4.8 does occur naturally in serum, it would appear reasonable to tentatively include it as one of the IGF/somatomedin family.

Localization of insulin-like growth factor genes to human chromosomes 11 and 12

The insulin-like growth factors IGF-I and IGF-II are required for growth and development. Both are single-chain proteins (of 70 and 67 amino acids respectively) derived from precursors by proteolytic processing. IGF-I may be particularly important in promoting normal stature and IGF-II may be a fetal growth hormone. The IGF proteins are probably synthesized by many normal tissues and by some tumours. The secretion of growth factors by tumours and tumour-derived cell lines suggests that they may act as autocrine regulators of cell proliferation. Because of the possible role of these proteins in growth disorders and cancer, and their sequence homology with insulin, we have determined their chromosomal localization. Using somatic cell hybrids and cloned cDNA probes for these proteins, we have assigned the genes for IGF-I and IGF-II to human chromosomes 12 and 11, respectively. We present evidence that the IGF-II gene is located on the short arm of chromosome 11 with a ras proto-oncogene and the insulin structural gene, and also suggest the existence of a fragment length polymorphism using the IGF-I probe.

The characterization of somatomedin A, isolated by microcomputer-controlled chromatography, reveals an apparent identity to insulin-like growth factor 1

The polypeptide termed somatomedin A (SMA) was isolated from outdated human plasma by a new purification procedure, not using acid ethanol extraction. Fractions containing SMA were monitored by a placenta radioreceptorassay and a radioimmunoassay for SMA. The purification method utilized a microcomputer-controlled chromatography system, yielding both SMA (identified as insulin-like growth factor 1 (IGF-1) or a deamidated derivative) and insulin-like growth factor 2 (IGF-2). The first step of CM-Affigel blue adsorbed at neutral pH the majority of somatomedins detectable by the radioreceptorassay for SMA. Exclusion chromatography on Sephadex G-50 in 0.1 M acetic acid separated this active material from albumin and NaCl. Separation between SMA and IGF-2 was achieved on two different cation-exchange columns, but not in the final high-performance liquid chromatography step. The isoelectric points, determined by chromatofocusing, were 8.0 for SMA and 6.2 for IGF-2. The amino acid compositions of the two isolated peptides were indistinguishable from the known compositions of IGF-1 and IGF-2. Sequence analysis up to position 39 of the peptide with a pI of 6.2 also proved identity with IGF-2 for all positions examined. The peptide with a pI of 8.0, corresponding to SMA, was degraded directly as well as after CNBr cleavage. The results show that it is identical to IGF-1, with the possible exception of acid/amide assignment, which could correspond to a deamidation. If occurring in the native preparation before analysis, it could explain the chromatographic properties and isoelectric point of SMA versus IGF-1 isolated by other techniques.

Somatomedin/insulin-like growth factor tissue receptors

There are two types of Sm/IGF receptors based on results of competitive binding experiments and investigations of receptor structure. The type I receptor preferentially interacts with IGF I rather than IGF II and interacts weakly with insulin. This receptor has a binding subunit of Mr = 130 000 which is disulphide bonded to form larger structures of Mr greater than 300 000. The type II receptor prefers IGF II to IGF I and does not interact with insulin. Its binding subunit is not linked by disulphide bonds to other membrane components (Mr = 260 000 with reduction, 220 000 without reduction). Subunit organization of the type I receptor appears to be similar to that of the insulin receptor but it is unlikely that the insulin and Sm/IGF binding sites are on a common alpha subunit. The type I receptor is down-regulated by IGFs and insulin. A rapid increase in ligand binding to the type II receptor by insulin has been described in intact rat adipocytes. The original idea that an IGF receptor mediates the growth-promoting action of both IGFs and insulin while acute metabolic effects of insulin and IGFs are mediated by the insulin receptor is an oversimplification . There now are clear examples of insulin stimulating growth by acting through the insulin receptor and, conversely, instances of IGF stimulating glucose transport by acting through an IGF receptor. Radioreceptor assays which measure IGF I in preference to IGF II (human placental membrane) and which measure IGF II in preference to IGF I (rat liver and rat placental membranes) have been utilized for clinical measurements of Sm/IGF levels, but are less specific than the respective radioimmunoassays. With the demonstration of Sm/IGF receptors on circulating human mononuclear cells and human skin fibroblasts, it is expected that these systems will be useful for investigations of patients with possible end-organ resistance to Sm/IGF.

Insulin and insulin-like growth factors/somatomedins in fetal and neonatal development

In this presentation we have attempted to review the status of knowledge of the hormonal regulation of fetal and neonatal growth, and have placed particular emphasis on the roles of insulin and somatomedin in fetal growth. The inaccessibility of the fetus and the physical and ethical constraints on study of its growth have resulted in slow progress. Furthermore, the opportunity for the fetus to benefit from homologous hormones from a variety of sources has made it difficult to assess the effect of withdrawal of individual hormones. Many of the advances made have resulted from attempts to determine hormone concentrations in fetal blood, identify hormone binding by fetal tissues, and examine the effects of natural or experimentally induced deviations in hormone availability. In the future, attention should be focused on developing study models which better isolate the fetus from the influence of multiple, homologous hormones. Progress should also come as the result of more detailed study of the influence of individual growth factors on in vitro growth of fetal cells and tissues, assessment of control mechanisms for growth factors in the fetus, and experiments directed at recognizing the complex interactions between individual growth factors and between growth factors and hormones.

Prospective clinical trial of human growth hormone in short children without growth hormone deficiency

Ten unselected, apparently healthy short children who were capable of normal growth hormone secretion were given human growth hormone (0.1 U/kg 1M thrice weekly) for 6 months to determine whether such treatment might lead to an increase in growth velocity. During treatment, all patients increased their growth rate (from 4.3 +/- 0.3 cm/yr to 7.4 +/- 0.5 cm/yr P less than 0.001). No adverse effects were detected. During the four-day IGF generation test, IGF I and IGF II levels rose significantly from 0.32 +/- 0.04 U/ml to 0.62 +/- 0.13 U/ml and from 279 +/- 36 ng/ml to 434 +/- 49 ng/ml, respectively. However, the growth response was not predicted by either the acute rise in IGF I or that in IGF II. Human growth hormone in standard doses may be capable of inducing accelerated growth in some short children without growth hormone deficiency. Measurements of IGF I and II cannot be used to predict which children will respond.

Tissue concentrations of somatomedin C: further evidence for multiple sites of synthesis and paracrine or autocrine mechanisms of action

We have validated a method for extracting and measuring the tissue content of somatomedin C (Sm-C)/insulin-like growth factor I (IGF-I), a growth-hormone-dependent, growth-promoting peptide. The Sm-C content of tissue extracts was strongly growth-hormone dependent because most of the tissues studied from hypophysectomized rats contained significantly less Sm-C than normal tissues. The intraperitoneal administration of ovine growth hormone (oGH) to hypophysectomized rats caused tissue extractable Sm-C to increase in kidney, liver, lung, heart, and testes. Tissue Sm-C responses to oGH were maximal after 12 hr, 6 hr before the maximal increment in serum. In liver and lung, the tissue Sm-C response to various doses of oGH fit linear regression models, and the doses of oGH needed to increase the Sm-C are in the range of those required to increase protein synthesis. Although these results do not exclude the possibility that the somatomedins act by hormone-like endocrine mechanisms, they add support to the concept that these peptides act through autocrine or paracrine mechanisms, being produced at multiple sites and acting at or near their sites of production.

The presence of normal levels of serum immunoreactive insulin-like growth factor 2 (IGF-2) in patients with Down's syndrome

The present study was performed to further investigate the levels of somatomedins in Down's syndrome (DS). The results show that the serum levels of immunoreactive insulin-like growth factor 2 (RIA-IGF-2) in patients with DS is within the normal adult range. No age variation among the patients was observed. Since we earlier reported a deficiency of immunoreactive insulin-like growth factor 1 (IGF-1) in DS patients, present study showing normal RIA-IGF-2 levels suggests a selective deficiency of IGF-1 in DS.

Insulin-like growth factors I and II, prolactin, and insulin in 19 growth hormone-deficient children with excessive, normal, or decreased longitudinal growth after operation for craniopharyngioma

We studied insulin-like growth factors (IGF) I and II, prolactin, and the insulin response to arginine in 19 children with craniopharyngioma and documented growth hormone deficiency. Patients were divided into three groups according to their growth rate during the first postoperative year. Seven patients with excessive growth (Group A) had hyperinsulinism, normal IGF values, elevated basal prolactin levels, and a delayed thyrotropin response to thyrotropin-releasing hormone, which was compatible with hypothalamic lesions. In the six patients with normal growth (Group B), the insulin level was low; all other hormone values were similar to those of Group A. In the six patients with decreased growth (Group C), levels of IGF I, insulin, prolactin, and thyrotropin were low, indicating the presence of severe pituitary damage and explaining the failure to grow. Patients in all groups had low or undetectable basal levels of growth hormone. We conclude that in Group B, normal IGF permitted normal growth, and prolactin hypersecretion may have been responsible for normal IGF I values. Excessive growth in Group A may have been caused by hyperinsulinism associated with hyperphagia and obesity of hypothalamic origin.

Insulin-like growth factors. Studies in diabetics with and without retinopathy

To determine whether two insulin-like growth factors (IGF I and IGF II) influence the course of diabetic retinopathy, we measured the concentrations of these factors in 80 adult patients with diabetes and in 62 control subjects. In seven patients with Type I diabetes and rapidly deteriorating vision as a result of proliferative and exudative retinopathy, the serum concentration of IGF I was 722 +/- 41 ng per milliliter (mean +/- S.E.M.), as compared with 381 +/- 48 ng per milliliter in 26 patients who had Type I diabetes without retinopathy or with less severe forms of it, and 302 +/- 15 ng per milliliter in the controls (P less than 0.001 for both comparisons). Serum concentrations of IGF II were normal in subjects with Type I diabetes but were somewhat depressed in those with Type II disease. Whether elevated serum concentrations of IGF I cause the accelerated development of retinopathy in some patients remains to be determined. Such levels do appear to identify patients at high risk for rapid deterioration of vision, and hence may be useful in selecting patients for more intensive or alternative forms of therapy.

Insulin-like growth factor I/somatomedin-C (IGF-I/SM-C) and glucocorticoids synergistically regulate mitosis in competent human fibroblasts

In serum-free medium, insulin-like growth factor-I/somatomedin-C (IGF-I/SM-C) was weakly mitogenic for adult human fibroblasts in culture. However, in the presence of 0.5% human hypopituitary serum (HHS), which by itself had little effect, there was a marked dose-dependent response to IGF-I/SM-C with a 10- to 20-fold increase in [3H]thymidine incorporation at 25 ng/ml IFG-I/SM-C. With the further addition of dexamethasone or hydrocortisone to the combination of IGF-I/SM-C + 0.5% HHS, there was a dramatic synergistic effect resulting in a 60- to 70-fold increase in [3H]thymidine incorporation. This stimulation was two times greater than that seen with 20% FCS. In contrast, glucocorticoids had no effect in serum-free medium or with HHS alone. These [3H]thymidine incorporation results were clearly supported by cell replication studies. Dose-response curves for 125I IGF-I/SM-C binding and IGF-I/SM-C stimulation of [3H]thymidine incorporation were similar with 1/2 maximal effects for both at 5 ng/ml. However, the striking synergism seen with glucocorticoids occurred in the absence of any glucocorticoid-induced change in IGF-I/SM-C binding, indicating that the interaction of IGF-I/SM-C and glucocorticoids occurs at a postreceptor level. These data demonstrate that in the presence of a low concentration of HHS, IGF-I/SM-C and glucocorticoids stimulate complete cell cycle traverse and replication of human fibroblasts.

Resistance to subcutaneous and intramuscular insulin associated with deficiency of insulin-like growth factor (IGF) 2

A diabetic patient is described whose serum was deficient in IGF 2. The patient responded appropriately to intravenous insulin but was resistant to subcutaneous and intramuscular insulin. His serum degraded insulin in vitro. This degradation was inhibited by IGF 2 and to a lesser extent by IGF 1 and insulin. We propose that this patient inactivated insulin at the injection site because of an insulin protease in his tissues that would normally be inhibited by serum IGF 2.

Recent advances in the biochemistry and physiology of the insulin-like growth factor/somatomedin family

The insulin-like growth factors (IGF) or somatomedins (Sm) are a family of low molecular weight circulating growth factors which have a major, but not absolute, dependence on GH, and have been shown to stimulate body growth and skeletal metabolism in vivo. They are thus considered to mediate the effects of GH on skeletal growth. In humans, the family consists of two well-characterized forms--IGF-I or SmC (a basic peptide) and IGF-II (a "neutral" peptide)--as well as perhaps two less well characterized forms--SmA (a neutral peptide) and an acidic insulin-like activity (ILA pI 4.8). Similar IGF/Sm species have been found and well-characterized in rat serum. Some higher mol wt forms also exist in tissues and body fluids and may represent possible precursor forms. On the basis of in vitro, clinical and in vivo evidence it has been postulated that IGF-I is the primary growth factor in the adult, whilst IGF-II is probably a major foetal growth factor. In vitro the IGF/Sms have a variety of effects including (1) acute insulin-like metabolic actions, which are observed primarily in insulin target tissues and are initiated largely at insulin receptors, and (2) longer term effects, associated with cell growth and skeletal tissue metabolism, and which occur in traditionally non-insulin target tissues, primarily via IGF/Sm receptors. These observations, together with the circumstantial clinical evidence favouring a close association between IGF levels and growth status, clearly indicate a role for IGF/Sms in growth regulation. This concept is now fully supported by the demonstration that IGF-I infused into hypophysectomized (GH-deficient) rats results in increased growth and skeletal metabolism. The physiological regulation of the expression of net IGF activity in vivo is complex and is controlled by the following three determinants: the levels of IGFs, the levels of the specific carrier-proteins and the levels of IGF inhibitors. Both IGFs and their carrier-proteins are influenced by the GH status of the animal as well as by other hormones, nutritional status and chronic illness. Little is known yet about the control of the various IGF inhibitors that have been described. Of importance, however, is the general concept that normal growth is dependent on an adequate balance between all three determinants and that some regard must be had for the contribution of each in determining the overall potential for growth under given circumstances.

A new candidate for the regulation of erythropoiesis. Insulin-like growth factor I

The effect of pure human insulin-like growth factor I (IGF I) on the colony formation of late stage erythroid precursor cells (CFU-e) from fetal mouse liver and adult bone marrow was studied in a serum-free culture system. We found that IGF I in physiological concentrations stimulated erythroid colony formation. The combined effect of IGF I and erythropoietin was smaller than the sum of their single effects. The number of colonies induced by IGF I was linearly dependent on the number of plated cells. Our results indicate that IGF I is the first clearly defined mitogen that stimulates the late stages of erythroid differentiation independently of erythropoietin.