Kerstin Hall

Evidence for the presence of specific receptors for insulin-like growth factors 1 (IGF-1) and 2 (IGF-2) and insulin throughout the adult human brain☆

The present study examines adult human brain tissue for the presence of receptors for insulin-like growth factors (IGFs) 1 and 2 and insulin. Binding sites for all hormones were widely distributed throughout the brain removed at autopsy, with the highest specific binding being found in the various cortical regions. Cross-reaction studies performed on frontal lobe biopsy material indicated specific binding sites for IGF-1 and insulin whereas iodinated IGF-2 was equally displaced by IGF-1 and IGF-2, suggesting the presence of an additional IGF-2-like binding site in the adult brain.

The somatomedin-binding protein isolated from a human hepatoma cell line is identical to the human amniotic fluid somatomedin-binding protein.

Serum-free medium conditioned by the human hepatoma cell line HEP G2 was shown to contain a somatomedin-binding protein with a relative molecular mass of about 35,000. This binding protein was purified to homogeneity by the use of immunoaffinity chromatography and subsequent size exclusion chromatography. Antibodies for the immunoaffinity step were raised in rabbits against a previously isolated human amniotic fluid somatomedin-binding protein. The total composition and N-terminal amino acid sequence showed the protein to be identical to the binding protein from human amniotic fluid. Both have the N-terminal structure Ala-Pro-Trp-Gln-. The HEP G2 cell line offers a useful model to study the regulation of the synthesis and secretion of human somatomedin-binding proteins.

A comparison of the biological activity of the recombinant intact and truncated insulin-like growth factor 1 (IGF-1)

A truncated form of insulin-like growth factor 1 (IGF-1), which lacked the aminoterminal tripeptide Gly-Pro-Glu has been isolated from human fetal and adult brain. This truncated IGF-1 displayed more potent cross-reactivity and biological action on brain cells than IGF-1 isolated from human serum. We now present data on a recombinant DNA-derived truncated IGF-1 lacking the aminoterminal tripeptide. Recombinant truncated IGF-1 was 1.4-5-times more potent than recombinant and natural IGF-1 in displacing [125 I]IGF-1 from human fetal and adult brain and placenta membranes. These differences were slightly enhanced when truncated IGF-1 was used as radioligand. The relative potencies compared to insulin-like growth factor 2 (IGF-2) in displacing [125I]IGF-2 from rat liver membranes were recombinant truncated IGF-1, 0.3% and recombinant IGF-1, 0.2%. Recombinant truncated IGF-1 displayed 100-fold reduced affinity for the low molecular weight binding protein (IGF-BP) isolated from human amniotic fluid when compared to recombinant IGF-1. Likewise, the IGF-BP was 100-fold less potent in inhibiting the receptor binding of recombinant truncated IGF-1 than that of recombinant IGF-1. Recombinant truncated IGF-1 was 4-times more potent than recombinant and natural IGF-1 in stimulating DNA synthesis in fetal rat brain cells. This biological activity of recombinant truncated IGF-1 was not affected by the IGF-BP at concentrations which abolished the biological activity of recombinant IGF-1. The hypothesis that IGF-BP bound intact IGF-1 represents the endocrine form of IGF-1, whereas truncated IGF-1 represents the paracrine or autocrine form of IGF-1, is proposed.

IGF-2 stimulated growth mediated by the somatomedin type 2 receptor

Human Insulinlike Growth Factor 2 (IGF-2) can promote cell proliferation via the type 2 receptor in K562 cells, a human erythroleukemia cell line with IGF-2/type 2 receptors and insulin receptors but lacking IGF-1/type 1 receptors. Cells are grown in semi-solid agar in the absence and presence of increasing amounts of insulin, IGF-1 and IGF-2. Two strains of K562 cells have been studied, with different concentrations of insulin and IGF-2 receptors. The effect of IGF-2 is proportional to the IGF-2 receptor concentration.

Insulin-like growth factor-I and insulin-like growth factor binding protein-1 in a representative population of type 2 diabetic patients in Sweden

OBJECTIVE To study the influence of type 2 diabetes on the insulin-like growth factor-I (IGF-I) and insulin-like growth factor binding protein-1 (IGFBP-1) serum levels in an area-based population of type 2 patients previously described. RESULTS The patients (n = 151) were elderly (70.6 +/- 0.7 years of age) and moderately overweight (BMI 27.0 +/- 0.4 kg/m2). Most patients (83%) were treated with either diet alone or diet in combination with sulphonylurea. Metabolic control measured as HbAlc deteriorated with duration (p < 0.001) and between groups treated with diet (HbA1c 5.8 +/- 0.6), sulphonylurea (7.5 +/- 0.2) and insulin (7.7 +/- 0.4). Mean levels of IGF-I were within reported normal range, but were lower in the insulin-treated as compared to the non-insulin-treated patients. Levels of IGF-I decreased with diabetes duration and with increased blood glucose. There was a positive correlation between IGF-I and insulin levels and also an inverse correlation between IGF-I and IGFBP-1 levels. The IGFBP-1 levels were twofold higher than reported in non-diabetic individuals. In multiple stepwise correlation analysis, 37% of the variability in IGFBP-1 could be explained by BMI, IGF-I SD score, age, IGF-I, and fasting blood glucose. CONCLUSION Our study indicates that influence of type 2 diabetes on IGF-I bioavailability in individual patients is modulated by insulin, body weight (presumably reflecting insulin sensitivity) and metabolic control. Furthermore, increased levels of IGFBP-1 are strongly associated with decreased b-cell function in type 2 diabetes mellitus.

Relations between sex hormones, sex hormone binding globulin, insulin-like growth factor-I and insulin-like growth factor binding protein-1 in post-menopausal breast cancer patients.

OBJECTIVE Oestrogens, androgens and anti‐endocrine drugs such as tamoxifen and aminoglutethimide influence plasma Insulin‐like growth factor‐I (IGF‐I). IGF‐I, in turn, has been found to stimulate the peripheral aromatase in vitro. The aim of this study was to examine relations between sex hormones, IGF‐I and insulin‐like growth factor binding protein‐1 (IGFBP‐1) In post‐menopausal women with breast cancer.

Insulin-Like Growth Factor Binding Protein 1 as a Novel Specific Marker of Hepatic Insulin Sensitivity

BACKGROUND AND AIMS The liver is the main source and insulin the main regulator of IGF binding protein 1 (IGFBP-1) in humans. Here we examined how serum IGFBP-1 concentrations are related to directly measured hepatic insulin sensitivity and liver fat content in humans. METHODS We measured fasting serum (fS) IGFBP-1 concentrations and liver fat content by proton magnetic resonance spectroscopy in 113 nondiabetic subjects. In addition, hepatic insulin sensitivity was measured using the euglycemic hyperinsulinemic clamp (insulin 0.3 mU/kg.min) technique in combination with the infusion of [3-(3)H]glucose in 78 subjects. RESULTS fS-IGFBP-1 concentrations were inversely related to liver fat content (r = -0.38, P < 0.0001). Of circulating parameters, fS-IGFBP-1 was better correlated to hepatic insulin sensitivity (r = 0.48, P < 0.0001) than fS-insulin (r = -0.42, P = 0.0001), fS-C-peptide (r = -0.41, P = 0.0002), fS-triglyceride (r = -0.33, P = 0.003), or fS-high-density lipoprotein cholesterol (r = 0.30, P = 0.007). In multiple linear regression analyses, body mass index (P < 0.0001) and fS-IGFBP-1 (P = 0.008), but neither age nor gender, were independently associated with hepatic insulin sensitivity (P < 0.0001 for ANOVA). Neither fS-insulin nor fS-C-peptide were independent determinants of hepatic insulin sensitivity after adjusting for age, gender, and body mass index. CONCLUSIONS fS-IGFBP-1 is inversely correlated with liver fat and is an obesity-independent and liver-specific circulating marker of hepatic insulin sensitivity.

Influence of treatment with tamoxifen and change in tumor burden on the IGF‐system in breast cancer patients

Plasma levels of IGF‐I IGFBP‐I and IGFBP‐3 were measured before and during treatment with tamoxifen up to 19 + months in 34 post‐menopausal patients with advanced breast cancer. In 28 patients, pro‐IGF‐IIE (IGF‐IIE) levels were determined and IGFBP‐3 was evaluated by immunoblot in 27 patients. Tamoxifen suppressed plasma levels of IGF‐I by a mean value of 25.5%–37.7% at different times. This effect was fully developed after 1ndash;2 months of treatment. IGF‐IIE was decreased by a mean value of 7.7–23.2% at different time intervals during treatment with tamoxifen, but this effect was significant after long‐term treatment (19 months +) only. Plasma IGFBP‐I increased by a mean value varying between 48.6% and 190.1%. Tamoxifen had no significant effect on total IGFBP‐3 levels. However, patients responding to treatment had a 28% reduction in fragmentation of IGFBP‐3, while patients with progressive disease had a 36% increase in fragmentation. The difference between responders and non‐responders was highly significant. These findings confirm and extend previous observations regarding the effects of treatment with tamoxifen on IGF‐I and IGFBP‐I. The finding that patients responding to tamoxifen achieve a reduction in the ratio of fragmented to intact IGFBP‐3 while patients progressing on therapy experience an increase in the IGFBP‐3 fragmentation ratio, suggest that the tumor burden influences IGFBP‐3 protease activity in breast‐cancer patients.

Specific binding of human growth hormone but not insulin-like growth factors by human adipocytes.

Binding of human GH (hGH) and insulin‐like growth factors I and II (IGFI and II) to isolated human adipocytes from adult subjects was studied. Binding equilibrium for hGH at 24°C was reached at 120 min and half‐maximal specific binding at 6–8 . Apparent K a was 2.1 × 109 M−1 and B max 7.3 × 10−11 M/106 cells. The human fat cellgrowth hormone receptor recognized neither bovine, ovine or rat GH nor human prolactin or placental lactogen. No specific receptors for human IGFII could be demonstrated. Thus, human adipocytes do not possess IGF receptors but have specific GH receptors which recognize hGH but not GH from lower species.

SERUM LEVELS OF INSULIN-LIKE GROWTH FACTOR I AND INSULIN-LIKE GROWTH FACTOR-BINDING PROTEIN 1 CORRELATE WITH SERUM FREE TESTOSTERONE AND SEX HORMONE BINDING GLOBULIN LEVELS IN HEALTHY YOUNG AND MIDDLE-AGED MEN

OBJECTIVE Administration of testosterone has been reported to increase serum levels of IGF‐I in men with isolated hypogonadotrophic hypogonadism. An inverse relation between serum IGF‐I and sex hormone binding globulin (SHBG) is seen in GH deficient children. The biological action of IGF‐I is thought to be influenced by binding proteins, one of which is insulin‐like growth factor‐binding protein‐1 (IGFBP‐1), which is not only a carrier protein but also actively regulates the cellular actions of IGF‐I. These observations suggest associations between IGF‐I, IGFBP‐1, testosterone and SHBG in serum. The aim of the present study was to investigate these associations in normal healthy men.