A preferential binding site for insulin-like growth factor II in human and rat placental membranes

IGF-II and MMP9 as surgical repair indicators of ventricular septal defects

OBJECTIVE

The insulin-like growth factor-I (IGF-I), insulin-like growth factor binding protein-3 (IGFBP-3) and human growth hormone (h-GH) have been recognized as therapeutic targets for the heart disease therapy. The bioavailability and actions of insulin-like growth factors-II (IGF-II) and matrix metalloproteinase-9 (MMP9) are important for embryonic development and cardiomyocyte differentiation as well. However, the clinical manifestations following the change in the serum IGF-II and MMP9 in infants with isolated ventricular septal defect (VSD) undergoing surgical repair have not been clearly defined.

STUDY DESIGN

Serum samples were collected from 72 infants: Twenty normal infants (group I) and 51 consecutive infants with echocardiography established isolated VSD (aged from 3 months to 1 year) were investigated. Among the 51 infants with VSD, 28 with shunt fraction, Qp/Qs < or = 1.5 were free of congestive heart failure symptoms (group II); 23 with shunt fraction, Qp/Qs > or = 2.0 were in congestive heart failure (group IIIa); and 23 of these 23 infants had undergone VSD repair 6 months before their second study (group IIIb). All insulin-like growth factors-II (IGF-II) and human growth hormone (h-GH), insulin like growth factor binding protein-3 (IGFBP-3) and its specific serum protease-MMP9 concentration were analyzed using ELISA and zymography, respectively.

RESULTS

Serum IGF-II and MMP9 exhibited significant decreasing trends among the three groups and significantly lower concentrations of IGF-II, IGF-II/IGFBP-3 ratio and MMP9, were found only in the severe group whereas h-GH/IGF-II ratio became significantly higher in this group. Moreover, there were no significant differences in these parameters between the infants after surgical correction and the normal ones.

CONCLUSIONS

The improvement in IGF-II and MMP9 serum concentration was identified in infants with VSD after surgical repair. These findings also indicate a significant relationship between IGF-II, MMP9 and VSD which might be used as diagnosis and prognosis indicators for this defect. Slight reductions in IGF-II/IGFBP3 ratio and slight increase in the h-GH/IGF-II ratio indicate mild VSD. The reductions in the MMP9, IGF-II, and IGF-II/IGFBP3 ratio plus high increase in the h-GH/IGF-II ratio indicate severe VSD.

Design and characterisation of long-R3-insulin-like growth factor-I muteins which show resistance to pepsin digestion

Site-directed mutagenesis was used to construct pepsin-resistant, single-point mutations of the N-terminal extended IGF-I analogue, long-R3-IGF-I. In order to identify the most susceptible sites, the kinetics of long-R3-IGF-I digestion by purified porcine pepsin were determined. Pepsin initially cleaved the Leu10-Phe11 bond in the N-terminal extension peptide to generate FVN-R3-IGF-I, followed in rapid succession by cleavage at Gln15-Phe16, Tyr24-Phe25, Leu10-Val11 and Met59-Tyr60 in the IGF-I moiety. Single-point mutations at these sites were designed on the basis of the preferred cleavage bonds for pepsin, as well as amino acid substitutions less likely to disturb protein structure. These included Leu10Val, Phe16Ala, Phe25Leu, Asp53Glu and Met59Gln. All five muteins retained growth-promoting activity equivalent to or higher than that of IGF-I. In terms of pepsin susceptibility, Leu10Val and Asp53Glu were degraded as rapidly as the parent long-R3-IGF-I, Met59Gln and Phe25Leu were partially stabilised, and Phe16Ala showed a marked improvement in stability over a wide range of pepsin:substrate ratios. Accordingly, the Phe16Ala mutein, long-R3A16-IGF-I, has potential for oral applications to enhance gastric growth and repair.

Growth promoting peptides in diabetic and non-diabetic pregnancy: interactions with trophoblastic receptors and serum carrier proteins

Infantile macrosomia in diabetic pregnancy (DP) is commonly attributed to fetal hyperinsulinism. However, insulin-like growth factors in the mother and the fetus, their binding proteins and their placental receptors may also play roles in the process of fetal overgrowth. We measured levels of maternal and cord serum IGF-I, IGF-II, C-peptide, IGFBP-1, IGFBP-2 and IGFBP-3 in 8 White Class B insulin dependent DP and 8 non-diabetic pregnancies (NP). These results were correlated with the concentration and affinity of placental trophoblastic membrane receptors (TR) for insulin (IN), IGF-I and IGF-II as well as with infant and placenta weights and maternal body mass indices. Significant respective differences between the diabetic and non-diabetic groups were found in mean infant weight, 4248 +/- 114 vs 3555 +/- 119 g (p < 0.001), placental weight 765 +/- 51 vs 575 +/- 24 g (p < 0.01), maternal body mass index 32.8 +/- 3.8 vs 21.3 +/- 1.2 (p < 0.02), cord serum IGF-I 136.8 +/- 6.6 vs 85.9 +/- 5.7 ng/ml (p < 0.01), cord serum C-peptide 18.7 +/- 3.5 vs 9.0 +/- 1.7 ng/ml (p < 0.025), cord serum IGFBP-1 21.9 +/- 4.7 vs 133.2 +/- 43.2 ng/ml (p < 0.025), cord serum IGFBP-2 672.0 +/- 76 vs 1206 +/- 220 ng/ml (p < 0.05) and cord serum IGFBP-3 11.5 +/- 1.0 vs 5.6 +/- 0.6 ng/ml (p < 0.001). No significant differences were found between DP and NP with respect to cord serum IGF-II, maternal serum IGF-I, IGF-II, C-peptide, IGFBP-1, IGFBP-2 and IGFBP-3, and the concentration and affinity of TR for IN, IGF-I and IGF-II. Analysis of variance revealed an interaction between infant weight and the weight of the placenta (p < 0.01), cord IGF-I (p < 0.02), cord C-peptide (p < 0.01) and cord IGFBP-3 (p < 0.01). Regression analysis revealed significant correlations of cord IGF-I with cord values of IGFBP-2 (r = -0.52, p = 0.04) and IGFBP-3 (r = 0.66, p < 0.005). Maternal serum IGF-I significantly correlated only with maternal IGFBP-3 (r = 0.65, p < 0.01). These results suggest that increased fetal production of insulin and IGF-I may contribute to the development of infantile macrosomia in DP. Concomitant changes in fetal production of IGFBPs, particularly IGFBP-2 and IGFBP-3, may modulate the action of insulin and IGFs. The lack of change in number or binding affinity of placental trophoblastic receptors for insulin, IGF-I and IGF-II tends to exclude a significant regulatory role of these receptors in the production of fetal macrosomia.

Functional alterations of type I insulin-like growth factor receptor in placenta of diabetic rats

The presence of type I insulin-like growth factor (IGF-I) receptors on placental membranes led to the hypothesis that these receptors might play a critical role in the rapid growth of this organ. Diabetes induces feto-placental overgrowth, but it is not known whether it modifies IGF-I receptor activity in fetal and/or placental tissues. To answer this question, we have partially purified and characterized placental receptors from normal and streptozotocin-induced diabetic rats. In normal rats, binding of 125I-IGF-I to a 140 kDa protein corresponding to the alpha subunit of the receptor was observed in cross-linking experiments performed under reducing conditions. Stimulation by IGF-I induces the autophosphorylation of a 105 kDa phosphoprotein representing the beta subunit of the receptor. In rats made hyperglycaemic and insulinopenic by streptozotocin injection on day 1 of pregnancy, placental IGF-I receptor-binding parameters were not different from controls on day 20 of pregnancy. In contrast, the autophosphorylation and kinase activity of IGF-I receptors of diabetic rats were increased 2-3-fold in the basal state and after IGF-I stimulation. The present study indicates that the rat placental IGF-I receptor possesses structural characteristics similar to that reported for fetal-rat muscle, and suggests that the high-molecular-mass beta subunit could represent a type of receptor specifically expressed during prenatal development. In addition, it clearly demonstrates that diabetes induces functional alterations in IGF-I receptor kinase activity that may play a major role in the placental overgrowth in diabetic pregnancy.

The insulin-like growth factor II/mannose-6-phosphate receptor : IGF-II/Man-6-P receptor

Recent evidence from molecular cloning, biochemical and immunological experiments has established that the cation-independent mannose-6-phosphate (Man-6-P) receptor and insulin-like growth factor-II (IGF-II) receptor are the same protein. Although the role of the IGF-II/Man-6-P receptor as a transporter of hydrolytic enzymes in the biogenesis of lysosomes is certain, elucidation of the receptor's structure has not yet provided major insights into the function of IGF-II binding. Mutually exclusive binding of IGF-II and naturally occurring phosphomannosyl ligands to distinct but proximal sites on the receptor suggests that the IGF-II/Man-6-P receptor cannot simultaneously fulfill the functional requirements of both IGF-II and lysosomal enzymes. Does the receptor transduce on intracellular signal in order to mediate the biological effects of IGF-II? If so, then the receptor must interact with an effector molecule, perhaps a G protein, in the mechanism of IGF-II action. Further information from ligand binding and especially mutagenesis experiments will be needed to elucidate the potentially multiple functions of the IGF-II/Man-6-P receptor.

Functional receptors for insulin-like growth factors I and II in rat thymocytes and mouse thymoma cells

Functional receptors for insulin-like growth factors (IGF) I and II have been identified in rat thymocytes and mouse thymoma cell lines R1.1 and S49.1. IGF-I receptor alpha-subunit (MW 130,000) bind IGF-I and IGF-II with equal affinity (Kd approximately 4-7 nM), and insulin with approximately 100 times lower affinity. Tyrosine kinase activity and autophosphorylation of the IGF-I receptor beta-subunit (MW 95,000) are stimulated by IGF-I and IGF-II with equal potency (ED50 approximately 0.5 nM). IGF-II receptors (MW 250,000) bind IGF-II with Kd approximately 0.3 nM and IGF-I with 30 times lower affinity, but not insulin. IGF-I and IGF-II do not cross-react with the insulin receptor to which insulin binds with an apparent Kd approximately 1 nM, and stimulates its tyrosine kinase activity with ED50 approximately 3 nM. In thymocytes, alpha-aminoisobutyric acid transport is stimulated 2-fold by IGF-I and IGF-II with identical potency (ED50 approximately 2 nM), and by insulin with ED50 approximately 10 nM. Activation of thymocytes by concanavalin A increased the number of IGF-II receptors 2-fold, whereas IGF-I receptor binding and IGF-stimulated amino acid transport were unaltered. We conclude that the effect of IGF-I and IGF-II in thymocytes is mediated via binding to the IGF-I receptor and stimulation of its tyrosine kinase. The presence of functional IGF receptors on thymocytes and thymoma cells suggests that IGF-I and IGF-II play a role in the regulation of thymic functions.

Delineation of atypical insulin receptors from classical insulin and type I insulin-like growth factor receptors in human placenta

Insulin-like growth factor (IGF)-binding sites copurifying with human placental insulin receptors during insulin-affinity chromatography consist of two immunologically distinct populations. One reacts with monoclonal antibody alpha IR-3, but not with antibodies to the insulin receptor, and represents Type I IGF receptors; the other reacts only with antibodies to the insulin receptor and is precipitated with a polyclonal receptor antibody (B-10) after labelling with 125I-multiplication-stimulating activity (MSA, rat IGF-II). The latter is a unique sub-population of atypical insulin receptors which differ from classical insulin receptors by their unusually high affinity for MSA (Ka = 2 x 10(9) M-1 compared with 5 x 10(7) M-1) and relative potencies for insulin, MSA and IGF-I (40:5:1 compared with 150:4:1). They represent 10-20% of the total insulin receptor population and account for 25-50% of the 125I-MSA binding activity in Triton-solubilized placental membranes. Although atypical and classical insulin receptors are distinct, their immunological properties are very similar, as are their binding properties in response to dithiothreitol, storage at -20 degrees C and neuraminidase digestion. We conclude that atypical insulin receptors with moderately high affinity for IGFs co-exist with classical insulin receptors and Type I IGF receptors in human placenta. They provide an explanation for the unusual IGF-II binding properties of human placental membranes and may have a specific role in placental growth and/or function.

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

Previous investigations have demonstrated specific receptors and associated mitogenic actions for insulin and insulinlike growth factors I and II (IGF-I and II) in postnatal bovine aortic smooth muscle. Using fetal tissue we have observed different patterns of binding and action for these peptides. Smooth muscle cells isolated from near-term fetal bovine aortae were studied in early passage. Specific receptors for both IGF-I and IGF-II were identified. Specific binding averaged 5.7%/2.5 X 10(5) cells for IGF-I, and 16.2% for IGF-II, and 0.3% for insulin. High affinity Kd for both IGF receptors were nanomolar. IGF-II was fivefold less potent than IGF-I in displacing IGF-I binding. IGF-I showed no affinity for the IGF-II receptor. Insulin, at physiologic concentrations, was incapable of displacing either IGF-I or IGF-II binding. Cellular incorporation of [methyl-3H]thymidine was stimulated at the lowest dose of IGF-I tested, 0.5 ng/ml. IGF-II showed no effect up to 100 ng/ml, after which a sharp increase in incorporation was noted. Insulin had a similar effect only at concentrations greater than 0.5 micrograms/ml, with a maximal response noted at 5 to 10 micrograms/ml. Our results indicate that fetal bovine aortic smooth muscle cells have an abundance of IGF receptors but lack specific insulin receptors. In addition, IGF-II binding levels are three times higher than for IGF-I. These results are consistent with observations in other species, in which a predominance of IGF over insulin receptors has been demonstrated in fetal tissue, and provide further evidence for a role for the IGFs in embryonic cellular metabolism.

Parallel effects of insulin-like growth factor-II and insulin on glucose metabolism of developing mouse embryonic limb buds in culture

The present study was undertaken to evaluate the functional significance of the previously observed increase in the level of IGF-II receptors despite the loss of growth promoting activity of IGF-II in differentiated limb buds in organ culture. In the present study IGF-II and insulin at similar concentration (0.1-1 microgram/ml) stimulated the glucose uptake by the tissue and incorporation into glycogen (approximately 2 and 4 times control) and stimulated the activity of glycogen synthase (approximately 2 times control) of the limb buds irrespective of the differentiation stage of the tissues. IGF-I had little or no effect. None of the hormones (IGF-I, II or insulin) had any effect on the CO2 production by the limb buds. These results suggest a regulatory role of IGF-II, complementary or overlapping with insulin, in glucose metabolism of the developing limb buds. The absence of the effects of both IGF-II and insulin on CO2 production by the tissue, however, indicate that the IGF-II and insulin regulation of glucose metabolism of the limb buds is predominantly on glycogen synthesis.

Specificity of insulin-like growth factor binding to type-II IGF receptors in rabbit mammary gland and hypophysectomized rat liver

We have reevaluated IGF binding specificity to membrane receptors in rabbit mammary gland (RMG) and hypophysectomized rat liver (HRL) using recombinant DNA-derived and synthetic analogues of human IGF-I and highly purified IGF-II. SDS-PAGE demonstrated that [125I]IGF-I bound to type-I IGF receptors in RMG; this binding was inhibited in a similar fashion by the IGF-I analogues (IC50 = 10 ng/ml) and to a lesser extent by IGF-II (IC50 = 60 ng/ml). [125I]IGF-II bound to type-II IGF receptors in both RMG and HRL. The IC50 for IGF-II was 9 and 3 ng/ml with RMG and HRL, respectively. At a dose as high as 1 microgram/ml, IGF-I analogues inhibited less than 20% of [125I]IGF-II binding. These results suggest that IGF-I has little or no affinity for type-II IGF receptors.

Ontogeny of insulin receptors in the rat hemochorial placenta

Binding of 125I-insulin to rat placental membranes was time and protein concentration dependent, reversible, and specific. Unlabeled porcine insulin competed for 125I-insulin binding with an IC50 of 65 nM, while IGF-I was much less potent with an IC50 of 2.12 mM. Specific binding of 125I-insulin decreased during the second half of gestation from Days 11 to 19. Scatchard analysis of the binding data for membranes prepared from Gestation Days 11 and 19 yielded typical curvilinear plots which showed a marked decrease in the number of binding sites in late gestation placenta. Beginning on Day 14, insulin binding was characterized with isolated labyrinth and basal zone portions of the hemochorial placenta. There was no evidence for differences in Kd values or the number of binding sites in these two functionally distinct portions of the rat placenta. Crosslinking of 125I-insulin followed by SDS-PAGE showed a single protein with a molecular weight of 130,000 from placental tissues on Gestation Days 11 and 19 and confirmed a gestational decrease in the number of insulin receptors. In solubilized, lectin-purified preparations from placenta and liver membranes, insulin stimulated the phosphorylation of a Mr 95,000 protein. 32P-incorporation into this 95,000 protein was stimulated fivefold by insulin in Day 11 placenta receptor, whereas no detectable 32P-incorporation was found in Day 19 placenta. Thus, while the alpha- and beta-subunits of insulin receptors in mid and late gestation placenta have molecular weights which are similar to receptors in maternal liver, data indicate the presence of a functional difference in insulin-stimulated kinase activities.

Heterogeneity of insulin-like growth factor-I affinity for the insulin-like growth factor-II receptor: comparison of natural, synthetic and recombinant DNA-derived insulin-like growth factor-I

Although insulin-like growth factors (IGF) I and II bind with high affinity to structurally discrete receptors, they bind with a lesser affinity to each other's receptor. We have evaluated the affinity of five different IGF-I preparations (three natural IGF-I preparations, one synthetic preparation, and one recombinant DNA-derived) for the IGF-II receptor in rat placental membranes, 18-54,SF cells and BRL-3A cells. In all tissues tested, the natural IGF-I preparations demonstrated an affinity for the IGF-II receptor which was 10-20% that of IGF-II. However, the recombinant and synthetic IGF-I preparations exhibited substantially lower affinities than natural IGF-I for this receptor, with only 10-25% reduction in (125-I)iodo IGF-II binding at peptide concentrations up to 400 ng/ml. Radioimmunoassay of the natural IGF-I preparations with an antibody directed against the unique C-peptide region of IGF-II demonstrated that contamination of IGF-I preparations with immunoreactive IGF-II could not exceed 5%. These results demonstrate that IGF-I purified from human plasma has a different affinity for the IGF-II receptor than does synthetic or recombinant IGF-I. Furthermore, these data are consistent with the hypothesis that IGF-I, itself, may be heterogeneous, and that subforms may vary in their affinities for the IGF receptors. Alternatively, IGF-I preparations which have been considered to be pure may be contaminated with small amounts of IGF-II, resulting in overestimation of the affinity of IGF-I for the type II IGF receptor.

Physiology of the somatotropic axis with particular reference to the ruminant

The physiology of growth hormone and the insulin-like growth factors are reviewed with particular reference to the dairy industry. Growth hormone secretion in the ruminant is pulsatile in nature and nutritional factors have a major impact on its secretion. Isolation of growth hormone-releasing factor has allowed further progress in understanding the mechanisms underlying growth hormone release. The receptors appear to be under active endocrine and metabolic control, and nutritional influences on the somatotropic axis are in large part mediated through changes in somatotropic receptors. The mode of action of growth hormone to induce acute metabolic affects and lipolysis remains to be resolved, but there is increasing evidence that its anabolic actions are mediated by the insulin-like growth factors. Recent studies of measurement of insulin-like growth factor-1 and -2 in the ruminant and the role of growth hormone, nutrition, insulin, and sex steroids in their regulation are reviewed. The relative role of the two factors and the multiple forms of their receptors remain to be resolved. It is well-documented that growth hormone is galactopoietic. The evidence that this effect is largely due to enhanced nutrient supply to the mammary gland is not convincing. Effects of growth hormone are indirect and may be mediated by the insulin-like growth factors. The potential is considerable for manipulating the growth hormone insulin-like growth factor axis to enhance lactation.

Receptors for insulin-like growth factors I and II in developing embryonic mouse limb bud

Insulin-like growth factors (IGF) or somatomedins (SM) have been classically defined as promoting the actions of growth hormone in skeletal growth. IGF is divided into two groups, IGF-I and II, and are presumed to act via IGF type I (higher affinity for IGF-I and II and very low affinity for insulin) and II (higher affinity for IGF-II than I and no affinity for insulin) receptors, respectively. Recently, a switchover role of IGF-II to I during fetal to adult growth has been suggested. We have investigated the possible transitional role of IGF-II to I in a developing mouse embryonic limb bud organ culture model. In this in vitro system, limb bud develops from the blastoma stage to a well-differentiated cartilage tissue. Both IGF type I and II receptors were found to be present in limb buds at all stages of differentiation. Type I receptor decreased with differentiation while Type II receptor increased. The effect of IGF-I on [3H]thymidine and [35S]sulfate uptake by the tissue increased with differentiation while the effect of IGF-II on [3H]thymidine uptake of the undifferentiated tissue was abolished with differentiation of the tissue. The increase of the IGF-I response with decreased type I receptor may reflect an altered receptor sensitivity (occupancy) during differentiation. The decrease of the IGF-II response with increased type II receptor with differentiation may on the other hand suggest that IGF-II in differentiated tissue no longer acts as a classical growth factor. These results tend to support the hypothesis of the switchover role of IGF-I and II during fetal and adult growth, however, confirmation of the precise role of IGF-I and II in biological growth may have to wait until further studies clarifying the significance of the increased IGF type II receptor in differentiated tissue are made.

Insulin-like growth factors (IGF) I and II in diabetic pregnancy: suppression of normal pregnancy-induced rise of IGF-I

The concentrations of somatomedins/insulin-like growth factors were measured by a specific radioimmunoassay for insulin-like growth factor-I and a specific radioreceptor assay for insulin-like growth factor-II in sera of term normal and Type 1 (insulin-dependent) diabetic pregnant women and in various cord sera of their newborn infants. Serum insulin-like growth factor-I levels in normal (non-diabetic) maternal serum were higher than in non-pregnant women (486 +/- 26 versus 215 +/- 26 ng/ml). The normal pregnancy-induced increment of insulin-like growth factor-I was markedly reduced in diabetic pregnancy. It was not different in patients with good or poor glycaemic control, as judged by normal or elevated blood levels of haemoglobin A1c content. Insulin-like growth factor-I levels in cord serum of infants of diabetic women with good glycaemic control (86 +/- 11 ng/ml) and poor glycaemic control (91 +/- 19 ng/ml) were significantly higher (p less than 0.01) than in infants of non-diabetic women (43 +/- 42 ng/ml). The fetal birth weight ratios were not significantly correlated with insulin-like growth factor-I levels in cord serum. Serum insulin-like growth factor-II levels in maternal and cord serum in diabetic and normal pregnancy were not different from each other or from normal non-pregnant women. The increment in insulin-like growth factor-I levels in maternal serum in pregnancy may influence placental structure and function. Lack of this increment in maternal diabetes may have direct implication in placental abnormalities in diabetes and indirect implications on fetal development and metabolism.(ABSTRACT TRUNCATED AT 250 WORDS)

Purification of the insulin-like growth factor II (IGF-II) receptor from an IGF-II-producing cell line, and generation of an antibody which both immunoprecipitates and blocks the type 2 IGF receptor

18,54-SF cells, which secrete rat insulin-like growth factor II (rIGF-II), have abundant type 2 IGF receptors. We have purified the type 2 receptor from these cells by solubilization of crude membranes in Triton X-100, followed by chromatography on agarose-immobilized rIGF-II. A partially purified receptor preparation, obtained by chromatography of solubilized membranes over wheat germ agglutinin, was used to immunize a rabbit. The antibody generated both immunoprecipitates the type 2 receptor, and specifically inhibits IGF-II binding to a variety of rat tissues, including 18,54-SF cells, BRL-3A cells and placenta. The presence of abundant type 2 receptors on an rIGF-II-secreting cell line is consistent with an autocrine role for IGF-II in select cells.

Identification of receptors for insulin-like growth factor II in two insulin-like growth factor II producing cell lines

Specific high affinity membrane receptor(s) for insulin-like growth factor II have been characterized in two cell lines which produce this hormone and have the ability to proliferate in serum-free media. These receptor(s) have no affinity for either insulin or biosynthetic insulin-like growth factor I. Affinity cross-linking and sodium dodecyl sulfate-polyacrylamide gel electrophoresis revealed an apparent Mr of 250K which does not change with disulfide bond reduction. Our findings are consistent with an autocrine function for insulin-like growth factor II and indicate that these continuous cell lines may provide unique systems for further investigations of this hormone and its receptor.

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.

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.

Fetal growth control: the role of insulin and related peptides

Both insulin and the related peptides, the insulin-like growth factors/somatomedins, may function as anabolic factors in the regulation of fetal body size. Infants born to women suffering from diabetes mellitus may show increased deposition of subcutaneous fat and enhanced lean body mass, findings reproduced in experimental animal fetuses with induced hyperinsulinaemia. Fetal adiposity may be associated with a life-time tendency to obesity and its associated diseases. Insulin-like growth factors I (IGFI) and II are present in the circulation of the newborn infant and animal fetus and correlate positively with birth size. The fetal tissues are biologically responsive to IGFs in vitro and are rich in specific cell membrane receptors, those predominantly recognizing IGFI being structurally and functionally similar to the insulin receptor. Insulin could theoretically influence fetal tissues by an interaction with either the insulin or IGF receptor. IGF release is a property of multiple fetal tissues in vitro, but, in contrast to postnatal life, is not dependent on growth hormone. Fetal IGF production may be influenced by placental lactogen, especially IGFII which rapidly declines in the circulation following parturition in the rat and sheep. A positive association also exists between circulating levels of insulin and IGFs when the former is experimentally manipulated in the animal fetus. Similarly the infant born with transient diabetes mellitus has low cord blood levels of insulin and IGFI. Insulin has a dual role in prenatal life. In the last trimester insulin functions as a glucoregulatory hormone, but from much earlier in gestation creates an anabolic environment in the fetus supplied with optimal nutrients. This latter mechanism of action is unclear and probably heterogeneous, but in overview is permissive rather than obligatory. In contrast the growth-promoting role of the IGFs is direct but their interaction with fetal tissues, and thus the overall emphasis of fetal growth, may be paracrine.

Basic somatomedin (B-SM) receptors in human term placenta explants

Binding of basic somatomedin (B-SM) and effect on alpha-aminoisobutyrate (AIB) uptake was studied in explants of term human placenta. B-SM demonstrated specific binding to the cultured explants. The binding was weakly inhibited by insulin. No significant degradation of the hormone was observed during incubation. The affinity cross-linking of [125I]B-SM binding to the explant membrane revealed a binding subunit of Mr 140 K. Addition of B-SM to the serum free medium stimulated the placental explant AIB uptake in a dose dependent manner. The incubated media containing FBS had less SM activity than the pre-incubated medium. The serum free incubated media did not contain measurable SM activity. The above findings that placental explants bind and have biological response to B-SM but do not produce B-SM suggest that SM has an endocrine rather than a paracrine action on human placenta.

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.

IGF-II binding on human lymphoid cells: demonstration of a common high affinity receptor for insulin like peptides

We have studied the binding of 125I-GF-II to the IM-9 human lymphoid cell line, and to human placental membranes. All of IGF-II radioligand binding to IM-9 cells, and half of the binding to human placental membranes is to a previously unrecognized common (Type-III) high affinity receptor site for insulin-like peptides, in which IGF-I and IGF-II are equipotent and insulin only slightly less potent. This common receptor represents another mechanism by which insulin, and the somatomedins can exert biological action.

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.

Structure of the insulin-like growth factor receptor in chicken embryo fibroblasts

The insulin-like growth factors (IGFs) and insulin stimulate DNA synthesis and cell multiplication in chicken embryo fibroblasts in culture. This response appears to be mediated by interaction with a single type of IGF receptor. The present study examines the subunit structure of this receptor by covalently crosslinking two 125I-labeled IGFs, IGF-I and multiplication-stimulating activity (MSA), to chicken embryo fibroblasts by using disuccinimidyl suberate. After solubilization, NaDodSO4/polyacrylamide gel electrophoresis, and autoradiography, IGF receptor complexes of appropriate specificity were identified; they had Mr approximately 130,000 (major band) and approximately 260,000 (minor band) under reducing conditions and Mr greater than 300,000 without disulfide reduction. The proportion of the Mr 260,000 component increased with increasing concentration of crosslinking agent, suggesting that it was formed from smaller proteins during the crosslinking procedure. The IGF receptor in chicken embryo fibroblasts resembles the insulin receptor in size and structure but can be distinguished by a higher affinity for IGF-I and MSA than for insulin. Although IGF receptors with different structure and specificity have been recognized in other tissues, the function of these binding sites is unknown. The present study demonstrates that the IGF receptor of chicken embryo fibroblasts that appears to mediate the growth-promoting effects of the IGFs contains a Mr approximately 130,000 binding subunit and exists as a native receptor complex of Mr greater than 300,000.