The original observation that insulin-like growth factors (IGFs) are present in native serum as high-molecular-weight forms (1–3) had not attracted particular attention for a surprisingly long period of time. All efforts had then concentrated on the biological and chemical characterization of a low-molecular-weight component with a non-suppressible insulin-like activity (NSILA-s; s stands for the material that was soluble in acid–ethanol during one of the first steps of purification). Even the finding, several years later, that NSILA-s bound with high specificity and affinity to high-molecular-weight serum proteins, suggesting the presence of specific IGF binding proteins (IGFBPs) (4, 5) received relatively little attention. The main interest, rather, centered around the newly identified constituents of NSILA-s, i.e. IGF-I and IGF-II (6), whose complete amino acid sequences had been published in 1978 (7, 8). Thus, it took nearly another 10 years until the first two human IGFBPs had been isolated and characterized by their
Knud W. Kastrup and Jürgen Zapf
Severe growth retardation is found in patients with high levels of growth hormone and low sulphation factor activity or somatomedin. Also nonsuppressible insulin-like activity (NSILA-s) has been found to be very low in a patient with this condition as measured by bioassay, protein binding assay and radioimmunoassay and to be below activities found in hypopituitary patients. Partially purified NSILA-s restored the ability of serum to increase sulphation activity although full restitution may still depend on other factors. These findings support the hypothesis that NSILA-s belongs to the family of somatomedin and thus is involved in promoting growth, and that low activity of these growth factors is a primary cause of the growth retardation found in these patients.
Ines Zangger, Jürgen Zapf and E. Rudolf Froesch
Abstract. Insulin-like growth factor I and II (IGF I and II) were determined by five different assays in human serum, in the sera of ten mammalian species and in chicken, turtle, and frog serum. Sera of all tested mammals contain two different IGFs corresponding to human immunoreactive IGF I and receptor reactive IGF II. Receptor reactive IGF II of most animal species does not show significant cross-reactivity in the RIA for human IGF II. IGF activity was also detected in sera of non-mammals, such as chicken and turtles, but not in frog serum. The IGF values obtained with the different assay system corresponded rather well: there is a good correlation between the values obtained in the protein binding and the fat cell assay, and between the results of the latter assays and the sum of immunoreactive IGF I and receptor reactive IGF II. The results suggest that those regions in the IGF I and II molecules which are responsible for reactivity with the type I IGF and the insulin receptor have not essentially changed during evolution. Similarly, the C-region, which mainly determines the immunological properties of IGFs, appears to have remained relatively constant in the IGF I, but not in the IGF II molecule.
Katharina Binz, Jürgen Zapf and E. Rudolf Froesch
Insulin-deficient, streptozotocin-diabetic rats show severe metabolic disturbances and stop growing. Besides insulin, these animals also lack growth hormone and insulin-like growth factor-I. We examined whether or not growth parameters correlate with IGF-I serum levels in young rats with streptozotocin-diabetes of different severity. In the diabetic rats, blood glucose varied between 18.4 and 38.6 mmol/1 (healthy controls between 6.1 and 9.3), IGF-I serum levels between 2.6 and 15.6 nmol/1 (controls between 19.6 and 26.5), and serum insulin levels between 0.05 and 0.14 nmol/1 (controls between 0.36 and 0.55). We found a highly significant linear correlation between IGF-I serum levels and the two investigated growth parameters, tibial epiphyseal width and longitudinal tibial bone growth. The finding that these indices of growth are strongly correlated with IGF-I serum levels in young rats with diabetes of different severity, suggests that IGF-I is a major determinant of growth. This is in keeping with our earlier demonstration that exogenously infused IGF-I promotes growth in diabetic rats.
Susanne Keller, Christoph Schmid, Jürgen Zapf and E. Rudolf Froesch
IGF-I infused at pharmacological doses in healthy men markedly decreases C-peptide levels, whereas insulin levels remain within the normal range. One possible explanation is decreased insulin removal. As the liver is the major site of insulin degradation, we studied insulin degradation by HepG2 cells in the presence of IGF. We found that IGF-I at a concentration of 130 nmol/l inhibits insulin degradation by HepG2 cells when the initial insulin concentration is 0.34 nmol/l. The effect of IGF-I on insulin degradation is dose-dependent and the rate of insulin degradation is dependent on the insulin concentration. IGF-II is 6 to 10 times more potent than IGF-I in inhibiting 125I-insulin binding to HepG2 cells and in protecting insulin from being degraded. Thus, IGF-I and IGF-II inhibit insulin degradation most likely by competing for binding at insulin binding sites of liver cells.
Armin Kurtz, Robert Matter, Kai-Uwe Eckardt and Jürgen Zapf
In this study we have examined the correlation between activity of erythropoiesis and serum concentrations of erythropoietin and insulin-like growth factor I in male and female rats during accelerated growth (day 30-90). We found that fractional incorporation of iron into newly formed red blood cells was linearily correlated with body weight gain. Total iron incorporation into newly formed red blood cells reflecting total daily red cell formation increased almost linearily between day 25 and 80 after birth in both sexes. While serum erythropoietin concentrations decreased in the time interval investigated (25–120 days), serum IGF-I levels increased in both sexes between day 25 and 55. In this period, individual values of total iron incorporation into red blood cells and serum IGF-I concentrations were linearily correlated. Our observations support the concept that IGF-I rather than erythropoietin modulates erythropoiesis during accelerated growth and thus manages a proportional increase in body mass and oxygen transport capacity.
Hans-Peter Guler, Jürgen Zapf, Christoph Schmid and E. Rudolf Froesch
IGF-I and -II share specific serum carrier proteins which elute on neutral Sephadex G-200 gel permeation chromatography at apparent molecular masses of 50 and 200 kD. The half-lives of free and carrier protein-bound 125I-IGF-I and -II were determined after bolus injections of the tracers into two normal adults. Labelled IGF-I and -II migrated first with the 50-kD and later with the 200-kD complex. In these complexes their apparent half-lives were 20–30 min and 12–15 h, respectively. The apparent half-life of free 125I-IGF-I and -II was 10–12 min. In a second set of experiments, recombinant human insulin-like growth factor I was infused during 6 days in two healthy adults at a dose of 20 μg · kg−1 · h−1 (corresponding to around 30 mg/day). Serum obtained before and during the infusion was subjected to neutral Sephadex G-200 gel permeation chromatography and fractions were pooled according to the apparent molecular masses at which the carrier protein complexes elute. IGF-I and -II in these pools were determined by RIA. Before the IGF-I infusion, 92 and 272 μg/l of IGF-I and -II were found in the 200-kD complex, 45 and 91 μg/l in the 50-kD complex, and 15 and 5 μg/l were present in the free form. Corresponding figures during the IGF-I infusion were 389 and 18 μg/l for the 200-Kd complex, 201 and 54 μg/l for the 50-kD complex, and 80 and < 1 μg/l for free IGF-I and -II. Using the half-lives of the tracer studies and the levels of the different molecular weight forms of IGF in serum, the production rates for IGF-I and -II were calculated to be 10 mg and 13 mg per day.
Peter D. Zenobi, Hans-Peter Guler, Jürgen Zapf and E. Rudolf Froesch
Abstract. IGF I was determined by a radioimmunoassay and IGF II by a radioreceptorassay in 20 Göttinger miniature (mini)-pigs and 13 domestic pigs of different weight and age. Immunoreactive IGF I serum levels of mini-pigs were similar to those of domestic pigs in corresponding age-classes (150–250 and 100–270 μg/l, respectively). No differences were detectable between receptor-reactive IGF II serum levels in mini-pigs (150–200 μg/l) and domestic pigs (110–270 μg/l) nor did the biological insulin-like activites (measured in the rat fat cell assay) differ in mini- and domestic pigs (81–100 and 71–98 mU insulin/l, respectively). IGF I and IGF II decreased drastically after hypophysectomy in one of the mini-pigs. Intravenous bolus injections of 30 μg/kg of recombinant human IGF I in 4 mini-pigs caused a similar degree of hypoglycemia (nadir of blood glucose 1.33 ± 0.61 mmol/l) as 0.15 IU insulin/kg, followed by a sharp growth hormone peak. We conclude that the marked difference between mini- and domestic pigs regarding body size is unrelated to serum levels of IGF I and II, a lack of response of tissues to IGF I or a reduced growth hormone secretory capacity in the mini-pig.
Hans-Peter Guler, Katharina Binz, Eugen Eigenmann, Silvia Jäggi, Daniel Zimmermann, Jürgen Zapf and E. Rudolf Froesch
The short stature of mini-poodles is associated with low serum levels of IGF-I. Standard poodles are taller and have considerably higher serum levels of IGF-I. Low IGF-I serum levels may be a symptom or the cause of small stature. We, therefore, undertook a study in which serum IGF-I levels of mini-poodles were elevated over a prolonged period of time by a constant infusion of rhIGF-I and the growth rate of the mini-poodles was followed. We infused four mini-poodles from day 91 to day 221 of age with 6 mg/day of recombinant human insulin-like growth factor I (rhIGF-I). Serum levels of IGF-I rose from about 160 to about 500 μg/l. Blood glucose remained within normal limits. Stimulation tests with clonidine and with GHRH revealed suppression of endogenous GH secretion during the IGF-I infusion. Serum levels of IGF-II and of creatinine were lower in the IGF-I-infused animals. Radial length and body weight did not increase to a greater extent in the IGF-I infused dogs than in controls. However, 'adapted body mass index' (aBMI = gram body weight/(mm radial length)2) decreased in each of the IGF-I infused animals, whereas it increased in each of the control dogs (p <0.05). We conclude that long-term infusion of IGF-I does not stimulate growth in young minipoodles, but may change body composition.
Hans-Peter Guler, Kai-U. Eckardt, Jürgen Zapf, Christian Bauer and E. Rudolf Froesch
Recombinant IGF-I was infused sc at a dose of 20 μg · kg−1 · h−1 to 2 healthy subjects during a total of 79 h. Serum levels of IGF-I rose from 93 and 177 to 502 and 616 μg/l, respectively. Fasting blood glucose remained normal. During the infusion, glomerular filtration rate increased by 31% in subject No. 1 and by 32% in subject No.2. Concomitantly, renal plasma flow increased by 26% and 22%, respectively. Proximal and distal tubular reabsorption of fluid and sodium as determined by lithium clearance was elevated to a similar extent. When determined again one week after the end of the IGF-I infusion, all parameters of renal function had returned to baseline. Sodium excretion, body weight and blood pressure did not change. We conclude that IGF-I infused at pharmacological doses has marked effects on kidney function. Future studies will be necessary to define the clinical potential of recombinant IGF-I in the treatment of diseases characterized by impaired renal perfusion and filtration.