Plasma forms of somatomedin and the binding protein phenomenon

Endocrine and cellular physiology and pathology of the insulin-like growth factor acid-labile subunit

The acid-labile subunit (ALS) of the insulin-like growth factor (IGF) binding protein (IGFBP) complex, encoded in humans by IGFALS, has a vital role in regulating the endocrine transport and bioavailability of IGF-1 and IGF-2. Accordingly, ALS has a considerable influence on postnatal growth and metabolism. ALS is a leucine-rich glycoprotein that forms high-affinity ternary complexes with IGFBP-3 or IGFBP-5 when they are occupied by either IGF-1 or IGF-2. These complexes constitute a stable reservoir of circulating IGFs, blocking the potentially hypoglycaemic activity of unbound IGFs. ALS is primarily synthesized by hepatocytes and its expression is lower in non-hepatic tissues. ALS synthesis is strongly induced by growth hormone and suppressed by IL-1β, thus potentially serving as a marker of growth hormone secretion and/or activity and of inflammation. IGFALS mutations in humans and Igfals deletion in mice cause modest growth retardation and pubertal delay, accompanied by decreased osteogenesis and enhanced adipogenesis. In hepatocellular carcinoma, IGFALS is described as a tumour suppressor; however, its contribution to other cancers is not well delineated. This Review addresses the endocrine physiology and pathology of ALS, discusses the latest cell and proteomic studies that suggest emerging cellular roles for ALS and outlines its involvement in other disease states.

Are insulin-like growth factor and its binding proteins 1 and 3 clinically useful as markers of malnutrition, sarcopenia and inflammation in end-stage renal disease?

INTRODUCTION

Malnutrition is common in end-stage renal disease (ESRD) and affects both morbidity and mortality. The growth hormone-dependent insulin-like growth factor (IGF)-I may be a good marker of malnutrition because of its short half-life. In the present study, we investigate the influence of decreasing residual renal function as well as of chronic inflammation on the IGF system to assess its usefulness in this patient group.

PATIENTS AND METHODS

Cross-sectional analysis of 220 ESRD patients (140 males) with a mean age of 52+/-1 years. Biochemical analyses of insulin, IGF-I, IGFBP-1, IGFBP-3, IL-6, high sensitivity (hs)-CRP and other routine markers. Malnutrition status was recorded using subjective global assessment (SGA), body mass index, estimated protein intake from nitrogen appearance (nPNA), handgrip strength (HGS) and insulin resistance (HOMA-IR). Dual energy X-ray absorptiometry was used to assess body composition.

RESULTS

Both IGF-I and IGFBP-1 showed significant and opposite correlations with most markers of nutritional status, including SGA (rho=-0.29 and 0.27; P<0.001), nPNA (rho=0.18 and -0.22; P<0.05), S-creatinine (rho=0.19 and -0.19; P<0.01) and HGS (rho=0.21 and -0.25; P<0.01). IFG-I was strongly correlated with IGFBP-3 (rho=0.62; P<0.001) and inversely correlated with IGFBP-1 (rho=0.44; P<0.001). Both IGF-I and IGFBP-3, but not IGFBP-1, were significantly correlated with age (rho=-0.25 for IGF-I and -0.35 for IGFBP-3; P<0.001) and hsCRP (rho=-0.21 and -0.32; P<0.01). In multivariate analysis, SGA and s-albumin were independent predictors of both IGF-I and IGFBP-1.

CONCLUSION

Both IGF-I and IGFBP-1 appear to correlate well with markers of protein-energy malnutrition and sarcopenia. However, IGF-I is also influenced by age, whereas IGFBP-1 is influenced by glucose metabolism. IGFBP-3 does not correlate with nutritional status in ESRD, perhaps because of a strong association with inflammation.

Insulin-like growth factor-I as a possible hormonal mediator of nutritional regulation of reproduction in cattle

The current review aims to establish insulin-like growth factor-1 (IGF-I) as the factor that signals nutritional status to the reproductive axis, and show that assessment of IGF-I in blood early postpartum during the negative energy balance (NEB) period could be used to predict both nutritional and reproductive status in dairy cattle. The review also explores the effect of nutritional status on circulating IGF-I concentrations and the endocrine role of IGF-I on the reproductive axis. IGF-I plays an important role in gonadotropin-induced folliculogenesis, ovarian steroidogenesis and corpus luteum (CL) function. It also modulates pituitary and hypothalamus function. IGF-I clearly has an endocrine role on the reproductive axis. Severe under nutrition significantly reduces plasma IGF-I concentrations. During the critical period of NEB in high yielding dairy cattle early postpartum, IGF-I concentrations are low in blood and its levels are positively correlated to energy status and reproductive function during this period. Changes in circulating IGF-I immediately postpartum may help predict both nutritional and reproductive status in dairy cattle. IGF-I is therefore one of the long sought factors that signal nutritional status to the reproductive axis.

Influence of gonadotropins on insulin- and insulin-like growth factor-I (IGF-I)-induced steroid production by bovine granulosa cells

To determine the effect of gonadotropins on insulin- and insulin-like growth factor (IGF-I)-induced bovine granulosa cell functions, granulosa cells from bovine ovarian follicles were cultured for 2 days in the presence of 10% fetal calf serum (FCS), and then cultured for an additional 2 days in serum-free medium with added hormones. In the presence of 0 or 1 ng/mL of insulin or IGF-I, FSH had little or no effect (P > 0.05) on estradiol production by granulosa cells from both small (1-5mm) and large (> or = 8mm) follicles. However, in the presence of > or = 3 ng/mL of insulin, FSH increased (P < 0.05) estradiol production by granulosa cells from small and large follicles such that the estimated dose (ED(50)) of insulin necessary to stimulate 50% of the maximum estradiol production was decreased by 2- to 3-fold from 22 to 28 ng/mL in the absence of FSH to 7-14 ng/mL in the presence of FSH. Similarly, in the presence of > or = 3 ng/mL of IGF-I, FSH increased (P< 0.05) estradiol production by granulosa cells from small and large follicles such that the ED(50) of IGF-I for estradiol production was decreased by 4- to 5-fold from 25 to 36 ng/mL in the absence of FSH to 5-6 ng/mL in the presence of FSH. In the presence of FSH, the maximal effect of insulin on estradiol production was much greater than that of IGF-I (137- versus 12-fold increase) and were not additive; when combined, 100 ng/mL of IGF-I completely blocked the stimulatory effect of 100 ng/mL of insulin. In the absence of FSH, the maximal effect of insulin and IGF-I on estradiol production was similar. Concomitant treatment with 30 ng/mL of LH reduced (P<0.05) insulin-stimulated estradiol production by 52% on day 1 and 19% on day 2 of treatment. Insulin, IGF-I and FSH also increased (P<0.05) granulosa cell numbers and progesterone production but their maximal effects were less (i.e., < 4-fold increase) than their effects on estradiol production. In conclusion, insulin and IGF-I synergize with FSH to directly regulate ovarian follicular function in cattle, particularly granulosa cell aromatase activity.

Insulin-like growth factors in central nervous system tumors

Insulin-like growth factors (IGFs) appear to play a role in the development of tumors in general and brain tumors in particular. Specific receptors for IGFs have been identified in normal human and rat brain, and evidence suggests that components of the IGF signal transduction system may play a role in the transformation process. Secretion of IGFs by a variety of human brain tumors has been confirmed, and these growth factors appear to have an autocrine stimulatory effect on these tumors. IGFs circulate in the blood stream bound to at least six distinct binding proteins which may modulate the effects of these growth factors on target tissues. Sex steroids may also regulate the behavior of certain brain tumors such as meningiomas at least in part through their effects on the expression of IGFs and their binding proteins. Recently, antisense gene technology against certain IGFs or their receptors have resulted in potent antitumor effects in the case of several gliomas, although the mechanism for this remains unclear.

Insulin-like growth factors in poultry

A large amount of research, primarily in mammals, has defined to a great extent the pleiotropic effects of the IGF system on growth, development, and intermediary metabolism. Similar elucidations in poultry were hindered to some extent by the absence of native peptides (IGF-I and IGF-II) until their purification, followed by the production of recombinant chicken IGFs. In many ways IGF physiology in birds is similar to that in other species, including but not limited to the fact that IGF-I synthesis is both GH- and GH-independent, and that autocrine-paracrine IGF action is evident. However, it is clear that several unique differences in IGF physiology exist between birds and mammals. For example, more IGF is present in the free form in chickens, and the biological responses to the IGFs is different in several metabolic pathways in birds compared to mammals. To date, no unique IGF-II receptor has been identified in birds. Despite an increasing understanding of the IGFs in aves, several important questions remain to be answered. What is the role of IGF-II in embryo development and posthatch growth? Does an IGF-II receptor entity exist in nonmammalian species? How does nutrition affect IGF-I and IGF-II gene expression, and can this information be used to enhance poultry production? What is the biochemical composition of the IGFBPs, and what are their roles in birds? Can the genetic variation present in poultry be used to positively modify IGF gene expression and physiology? How do the IGFs regulate intermediary metabolism? What is the role of the IGFs in the etiology of several disease states associated with rapid growth in poultry, including tibial dyschondroplasia, obesity, ascites, and spiking mortality syndrome? Answers to these questions are relevant to our understanding of the basic mechanisms of IGF physiology as well as possibly assisting in the amelioration of problems found in modern poultry production.

Serum levels of insulin-like growth factor-binding protein-3 in normal pregnancy

The aim of this study was to investigate circulating levels of insulin-like growth factor-binding protein-3 (IGFBP-3) in women with normal pregnancy. A total of 468 pregnant women at various gestational ages and 30 non-pregnant females were recruited in this study. Serum concentration of IGFBP-3 was determined by radioimmunoassay. Maternal serum levels of IGFBP-3 increased as pregnancy progressed. The concentration of IGFBP-3 in pregnancy serum was positively associated with the gestational age. The mean values of circulating IGFBP-3 in the second and third trimesters of pregnancy were higher than those in the first trimester of pregnancy and in non-pregnant females. In addition, there was a highly significant correlation between IGFBP-3 and IGF-I in pregnancy serum. We conclude that, during pregnancy, IGFBP-3 appears to be the principal binding protein for circulating IGF-I and may be involved in the regulation of fetal growth.

Evidence that human bone cells in culture secrete insulin-like growth factor (IGF)-II and IGF binding protein-3 but not acid-labile subunit both under basal and regulated conditions

Insulin-like growth factors (IGFs) are found in human circulation predominantly as part of a growth hormone (GH)-dependent complex of 125-150 kD, which is composed of three subunits: IGF-I or IGF-II, an acid stable IGF binding protein (IGFBP)-3, and an acid labile subunit (ALS). Although recent studies demonstrate that a number of cell types in culture secrete IGFs and IGFBP-3, very little is known with regard to the origin of circulating ALS. To test the hypothesis that human bone cells (HBCs), which produce abundant amounts of IGF-II and IGFBP-3, also produce ALS, we measured the IGF-I, IGF-II, IGFBP-3, and ALS levels using specific radioimmunoassays (RIAs) in the conditioned medium (CM) of untransformed normal HBCs and SaOS-2 osteosarcoma cells treated with various effectors (IGF-II, osteogenic protein-1 [OP-1, bone morphogenetic protein-7] and human GH) for 48 h. No detectable levels (< 3 ng/ml) of ALS were found in the CM of various HBC types under basal conditions. In contrast, CM collected from liver explants in culture contained significant amount of ALS prepared and assayed under identical conditions. The IGF-I level was also undetectable in the CM of various HBC types. In the IGF-II (3, 30 ng/ml)-treated HBC CM, the IGFBP-3 level was increased in a dose-dependent manner but neither IGF-I nor ALS could be detected. In the SaOS-2 cell culture, OP-1 (1, 100 ng/ml) increased both IGF-II and IGFBP-3 secretion but neither ALS nor IGF-I secretion. Treatment of HBCs with GH (1, 10, 100 ng/ml) had no significant effect on the secretion of either IGF-I, IGF-II, IGFBP-3, or ALS.(ABSTRACT TRUNCATED AT 250 WORDS)

The annals of IGF binding proteins: into the next millennium

From a small beginning 21 years ago, the investigation of the insulin-like growth factor binding proteins (IGFBPs) has grown to occupy a major part of the research on the insulin-like growth factors (IGFs). A single abstract at the Endocrine Society in 1974 has grown into a torrent of more than 1400 papers published in the past two decades, and more than 200 in the past year alone. Instead of one possible binding protein for somatomedin we now have seven members of a homologous protein family that was completely uncharacterized only a decade ago. The Third International Symposium on IGF Binding Proteins clearly marked a milestone in the development of this field of research. Concentrated on the three days of this meeting was a rich collection of 50 oral presentations and 89 posters. The quality and breadth of the presentations proved that not only is research on the IGFBPs vigorous and healthy, but it is clearly dealing with important scientific issues. The publication of this collection of papers from the Third International Symposium on IGF Binding Proteins serves as a roadmap for where this field of research is now, and the likely direction for developments as we approach the third millennium.

Protease-catalyzed conversion of insulin-like growth factor-1 and interleukin-6 into high-molecular-mass species through the sequential action of hematopoietic surface-associated cathepsin G and gamma-glutamyl transpeptidase-related activities

Interleukin-6 (IL-6) and insulin-like growth-factor-1 (IGF-1) are cytokines produced by a variety of cells that act on a wide range of tissues, influencing cell growth and differentiation. Purified plasma membranes from human U937 monoblastic cells produced in vitro dimeric species of IL-6- and IGF-1-derived peptides through the sequential actions of surface-associated enzymes cathepsin G and transpeptidase activities. Cathepsin G degraded native unglycosylated IL-6 and IGF-1 molecules into 8-kDa and 7-kDa peptides respectively. Subsequent dimerisation of these intermediate forms into 16-kDa IL-6- and 14-kDa IGF-1-derived peptides was inhibited by acivicin and glutathione which are specific inhibitors of the standard cell-surface gamma-glutamyl transpeptidase (gamma-GT). However U937 plasma membranes, cleared of gamma-GT activity by immunoprecipitation with anti-gamma-GT and adsorption on protein-G-Sepharose, were still able to convert the intermediate forms of IL-6 and IGF-1 into dimers. Together, these observations indicate that the transpeptidase involved in the formation of the dimeric species of IL-6 and IGF-1 was related to, but distinct from, standard cell-surface gamma-GT. Cells of all hematopoietic lineages expressed gamma-GT-related activity. In contrast to the 16-kDa IL-6-derived peptide that did not retain growth-stimulating activity, the 14-kDa IGF-1 peptide was at least equipotent with native IGF-1 in the BALB/c 3T3 fibroblast DNA synthesis response. The N/O-glycosylated IL-6 was clearly as sensitive to cathepsin-G- and gamma-GT-related activities as the unglycosylated IL-6 from Escherichia coli, thus indicating that the sugar chains did not protect the cleavage sites of the two proteases on the IL-6 molecule. Our in vitro findings raise the possibility that similar proteases participate in the regulation of the catabolism of IL-6 and IGF-1 in vivo.

Insulin like growth factor-1 and -2 and their role in the re-epithelialisation of wounds; interactions with insulin like growth factor binding protein type 1

Insulin like growth factor (IGF) 1 and 2 which are present and actively synthesised in the wound fluid stimulate several cell types involved in the process of wound healing. To investigate the role of IGF-1 and 2 and in addition, the association between IGF and their carrier proteins, IGF binding proteins (IGFBP), we have used a newly established model for human wound healing in fresh biopsy material. Histological examination shows that IGF-1 stimulates efficient reepithelialisation of the wounds both alone and in the presence of recombinant IGFBP-1. In contrast, IGF-2 stimulates healing only when used in combination with IGFBP-1. These findings suggest that the two IGFs and their carrier proteins may function during different phases of wound healing and that both IGF-1 and 2 act as potent inducers of wound healing; this may have direct clinical implications.

Role of the insulin-like growth factors in the endocrine control of glucose homeostasis

There is a growing body of evidence that the insulin-like growth factors (IGF-I and IGF-II) are dynamically involved in the regulation of glucose homeostasis, with one of their binding proteins, IGFBP-1, playing a counterregulatory role. The IGFs are structurally and functionally related to insulin and in the circulation they represent a huge hypoglycemic potential which is buffered by their association with the IGFBPs. The predominant IGFBP in serum, IGFBP-3, is able to form a high molecular weight complex with the IGFs and this complex is retained in the circulation and appears to act as a reservoir of IGFs. The IGFs and IGFBP-3 are regulated in the long term by changes in nutritional status. In contrast, IGFBP-1 is acutely regulated in a manner similar to glucose counterregulatory hormones. IGFBP-1 is able to block the insulin-like actions of the circulating IGFs and when administered alone as a bolus infusion causes an increase in blood glucose levels. There is recent evidence that more IGFs are available for an endocrine glucoregulatory role than indicated by estimates of steady-state 'free' IGF levels. The IGF/IGFBP system may thus play a complementary role to insulin and the classical counterregulatory hormones in the control of blood glucose.

Fibroblast stimulation of breast cancer cell growth in a serum-free system

Conditioned media from 14 short term fibroblast cell lines were mitogenic for human breast cancer cells with different steroid receptor profiles in serum-free culture. Fibroblast-conditioned medium stimulated tritiated thymidine incorporation in short term culture and growth in a longer proliferation study as measured by the MTT colorimetric assay. Conditioned media from benign and malignant epithelial cells were non-stimulatory for breast cancer cells but that derived from endothelial cells showed similar stimulation to fibroblasts. Partial purification of fibroblast-conditioned medium identified a peptide with a molecular weight of approximately 8 kDa that showed no affinity for heparin and was mitogenic for MCF-7 breast cancer cells.

Hypoglycemic and insulin response to a continuous intravenous infusion of CGP-35126 recombinant human insulin-like growth factor-I (rhIGF-I) in healthy males

Recombinant human insulin-like growth factor-I (rhIGF-I) produced by expression in a yeast vector was evaluated in seven normal men to determine effects on plasma glucose and insulin levels. Each subject received an initial intravenous infusion of normal saline and on the following day, rhIGF-I at a rate of 21.4 micrograms/kg/hour. Each infusion lasted for 7 hours. The subjects' fasting baseline glucose and insulin levels were not statistically different (P > .05) from their pre-dose levels. Compared with the saline (control) infusion, serum glucose levels were statistically lower (P < .05) 2 hours into the rhIGF-I infusion. These lower glucose levels were maintained until the subjects consumed a standard lunch (4 hours into the infusion). Insulin levels demonstrated a similar response to rhIGF-I, but decreases in insulin levels occurred after the rhIGF-I hypoglycemic effect. This observation suggests that suppression of insulin levels may be due to secondary hypoglycemia rather than to a direct rhIGF-I effect. This study demonstrated the desired rhIGF-I effect of lowering subjects' glucose levels without clinically significant hypoglycemia. This finding suggests that rhIGF-I may have potential clinical utility in hyperglycemic states.

Monkey retinal pigment epithelial cells in vitro synthesize, secrete, and degrade insulin-like growth factor binding proteins

Cultured monkey retinal pigment epithelial (RPE) cells rapidly secrete large amounts of insulin-like growth factor binding proteins (IGF-BPs). IGF-II tracer binding activity in conditioned media is two to three times greater than that of IGF-I. Under reducing SDS-PAGE conditions, 125I-IGF affinity-crosslinked binding protein (BP) is visualized as a broad band between 36 +/- 2.9 and 49 +/- 3.3 kDa. Because the electrophoretic mobility of the crosslinked BP is increased under non-reducing conditions (33-45 kDa), intramolecular sulfhydryl bonding may be present. Frequently, the radiographic band representing affinity-crosslinked binding protein exhibits a complex pattern of non-uniform densities that suggests structural or functional IGF-BP micro-heterogeneity. IGF-BPs synthesized by RPE also exhibit heterogeneity with respect to the absence or presence of oligosaccharide side chains. In particular, the larger, but not the mid-sized or smaller IGF-BPs exhibit side chains linked to the core protein with N-glycosidic linkage. None of the crosslinked IGF-BPs exhibit O-linked side chains. Long-term (12, 24, 48 hr) conditioning studies revealed that IGF-BP fails to accumulate in culture media beyond 12 hr, but that replacement of conditioned media with fresh media allows a second period of binding protein accumulation. Other short-term (12 hr) experiments indicate that, in fresh medium, the levels of IGF-BP increase during the first 6-8 hr and then remain stable. To examine the processes contributing to these steady state levels of IGF-BP, aliquots of 8-hr conditioned medium were removed from the cells and either frozen on dry ice or incubated at 37 degrees C for 16 hr. Importantly, it was found that incubation at 37 degrees C resulted in a near total loss of binding activity. This is the first report of IGF-BP degrading activity in a cell culture system. These findings indicate that 1) primate RPE cells rapidly secrete a complex mixture of N-glycosylated and non-glycosylated IGF-BPs, and 2) the steady state levels of secreted IGF-BP are tightly regulated at least in part through a concomitant IGF-BP inactivating activity. Cultured RPE cells may be of utility in examining the mechanisms of IGF-BP synthesis, secretion, and degradation at the cellular level.

Function of the growth hormone-insulin-like growth factor I axis in the profoundly growth-retarded diabetic child: evidence for defective target organ responsiveness in the Mauriac syndrome

Mauriac syndrome (MS) consists of a triad of poorly controlled diabetes, profound growth retardation, and hepatomegaly. The mechanisms involved in the growth retardation of those patients are not well understood. In an attempt to determine whether the growth retardation was secondary to somatroph secretory failure, abnormal pulsatile secretion, deletion of the growth hormone (GH) receptor, inadequate insulin-like growth factor I (IGF-I) generation, or abnormal IGF-I binding proteins (IGFBPs) two patients with MS were studied and their results compared with those of age-matched diabetic boys of similar glycemic control who were growing well. Overnight GH profiles in the MS and normally growing diabetics were analyzed by the CLUSTER program. The mean 12-hour GH concentrations, pulse amplitude, and pulse frequency were not different in either group of patients and did not change during acute normalization of the serum glucose overnight in the MS patients. The GH-binding proteins (GHBPs) relative binding were found to be the same in both groups of patients and did not differ from normal nondiabetic sera (62% +/- 8.0% relative specific binding in MS patients, v 53% +/- 4.3% in diabetic controls). The IGF-I concentrations were normal and comparable in both groups of patients (1.1 +/- 0.1 U/mL MS, v 1.1 +/- 0.3 diabetic controls). The IGFBPs were comparable in both groups of patients as well. One of the patients with MS had no meaningful increase in his growth velocity after 1 year on GH therapy despite good compliance. In conclusion, our data show normal hypothalamic-pituitary function, normal GHBP, IGF-I generation, and IGFBPs in two patients with MS when compared with normally growing diabetic children. These data, and the lack of linear growth in response to exogenous GH therapy in one patient, suggest a GH-resistant state, either secondary to impaired bioactivity of IGF-I, or a defect at or distal to the IGF-I receptor.

Radioimmunoassay of insulin-like growth factors in cyst fluid of central nervous system tumors

Tumor cells are characterized by abnormalities in growth and metabolism, including the autocrine secretion of certain growth factors. The authors have previously shown the presence of insulin-like growth factor receptors in tumors of the central nervous system (CNS) and in this study examine whether CNS tumors are capable of autocrine secretion of insulin-like growth factors in situ. To investigate the production of insulin-like growth factors I and II by CNS tumors, the authors have developed specific radioimmunoassays for these growth factors. In situ production of insulin-like growth factors was studied by immunoassay of CNS tumor cyst fluid aspirated at the time of surgery from 12 cystic tumors: seven primary brain tumors, four metastatic tumors, and one spinal schwannoma. For immunoassay, cyst fluid was treated overnight with acetic acid, then insulin-like growth factors were separated from binding proteins by a refined solid-phase technique, then dried and reconstituted in immunoassay buffer. Normal human serum and cerebrospinal fluid served as controls. Insulin-like growth factor I was detected in all 12 tumors studied. In contrast, insulin-like growth factor II was detected only in three low-grade astrocytomas, the spinal schwannoma (which had the highest insulin-like growth factor II level of all tumors studied), and three metastatic lung cancers. These results suggest that CNS tumors may be capable of autocrine production of insulin-like growth factors in situ. Furthermore, there appears to be a difference in the type of insulin-like growth factors produced by different types of CNS tumors. Preferential production of insulin-like growth factors may be an important marker of tumor differentiation and useful as a diagnostic tool.

Differential regulation of insulin-like growth factor binding protein secretion from human decidual cells by IGF-I, insulin, and relaxin

Several growth hormone-independent 25-31,000 kD insulin-like growth factor binding proteins (IGF-BPs) have been identified in plasma, extravascular fluids, and various cell-conditioned media. Cultured human decidual cells release three IGF-BPs with 24,000, 30,000, and 34,000 Mr. Using ligand blot analysis and an RIA for the 30,000-Mr form (IGF-BP-1), we examined the effects of IGF-I (10-1,000 ng/ml), insulin (10-10,000 ng/ml), and relaxin (10-250 ng/ml) on decidual cell IGF-BP release after 120 h of hormone exposure. IGF-I inhibited release of both IGF-BP-1 and the 24,000 Mr form. Inhibition of IGF-BP-1 release was noted after 48 h of treatment and was progressive throughout the subsequent 120 h. Insulin stimulated a fourfold increase in release of the 24,000-Mr protein while inhibiting IGF-BP-1 release comparable to IGF-I, alpha-IR3, a monoclonal antibody to the IGF-I receptor, blocked approximately 33% of the IGF-I response but had no effect on insulin-mediated IGF-BP-1 inhibition. Relaxin stimulated a 2.4-fold increase in release of the 24,000-Mr form and a 16-fold increase in the 30,000-Mr protein after 120 h. Stimulation of the 30,000-Mr protein was inhibited by the addition of cycloheximide (50 micrograms/ml). Both IGF-I and insulin also blocked the relaxin-mediated increase in IGF-BP-1. These studies suggest that three structurally related proteins differentially regulate IGF-BP secretion possibly via activation of distinct receptor subtypes.

Effects of human growth hormone in men over 60 years old

BACKGROUND

The declining activity of the growth hormone--insulin-like growth factor I (IGF-I) axis with advancing age may contribute to the decrease in lean body mass and the increase in mass of adipose tissue that occur with aging.

METHODS

To test this hypothesis, we studied 21 healthy men from 61 to 81 years old who had plasma IGF-I concentrations of less than 350 U per liter during a six-month base-line period and a six-month treatment period that followed. During the treatment period, 12 men (group 1) received approximately 0.03 mg of biosynthetic human growth hormone per kilogram of body weight subcutaneously three times a week, and 9 men (group 2) received no treatment. Plasma IGF-I levels were measured monthly. At the end of each period we measured lean body mass, the mass of adipose tissue, skin thickness (epidermis plus dermis), and bone density at nine skeletal sites.

RESULTS

In group 1, the mean plasma IGF-I level rose into the youthful range of 500 to 1500 U per liter during treatment, whereas in group 2 it remained below 350 U per liter. The administration of human growth hormone for six months in group 1 was accompanied by an 8.8 percent increase in lean body mass, a 14.4 percent decrease in adipose-tissue mass, and a 1.6 percent increase in average lumbar vertebral bone density (P less than 0.05 in each instance). Skin thickness increased 7.1 percent (P = 0.07). There was no significant change in the bone density of the radius or proximal femur. In group 2 there was no significant change in lean body mass, the mass of adipose tissue, skin thickness, or bone density during treatment.

CONCLUSIONS

Diminished secretion of growth hormone is responsible in part for the decrease of lean body mass, the expansion of adipose-tissue mass, and the thinning of the skin that occur in old age.

Interaction of insulin-like growth factors (IGF) with isolated sheep hepatocytes. II. Binding, internalization and degradation of IGF-II

This study examines the binding and degradation of IGF-II by the ovine liver. Binding and degradation of 125I-IGF-II to isolated hepatocytes was time, temperature and cell number dependent. Ovine and human IGF-II were 2-5 times more effective in inhibiting 125I-hIGF-II binding than were the IGF-I preparations. Insulin did not affect binding. Autoradiographs of 125I-hIGF-II affinity cross-linked to hepatocytes showed a major band of molecular weight 271,000 under reduced conditions. This band was eliminated by 100 nM hIGF-II or oIGF-II but not by excess hIGF-I, oIGF-I or insulin. The internalization of IGF-II was examined by treating the cells with trypsin or sodium acetate to remove surface-bound IGF-II. Both treatments showed that 20-25% of 125I-hIGF-II was internalized. Mannose-6-phosphate at 1, 2 and 4 mM enhanced the binding of 125I-hIGF-II to hepatocytes 3.5, 12.8 and 16.4%, respectively. The lysosomal inhibitors ammonium chloride, chloroquine and leupeptin had no effect on 125I-hIGF-II degradation or cell-associated radioactivity indicating a nonlysosomal pathway of degradation for 125I-hIGF-II in the ovine hepatocyte. The low molecular weight sheep serum IGF binding protein inhibited binding of 125I-hIGF-II in a dose-dependent manner but had no effect on degradation, which suggests that degradation of 125I-hIGF-II is independent of receptor interaction. These studies demonstrate that IGF-II binds to specific high affinity sites in sheep hepatocytes which display the characteristics of type II IGF receptors. A significant fraction of the receptor bound IGF-II is internalized but not degraded by these cells, which suggests that the biological actions of IGF-II may be exerted by an intracellular pathway in sheep hepatocytes.

Insulin-like growth factor-I and insulin-like growth factor-binding protein in the toad, Bufo woodhousei

Molecular weight characteristics and plasma concentrations of insulin-like growth factor-I (IGF-I) and its binding protein (IGF-BP) were investigated in the toad, Bufo woodhousei. IGF-I and IGF-BP were measured by radioimmunoassay (RIA, Kd = 0.37 +/- 0.04 ng/ml) and charcoal-separated ligand binding assay, respectively, in male toad plasma and adult male human donor plasma using a synthetic human IGF-I standard. Prior to the IGF-I RIA, samples were acid-ethanol extracted. Molecular weight characteristics were determined using size exclusion chromatography. At neutral pH (pH = 7.4), IGF-I immunoreactivity and IGF-BP eluted at molecular weight greater than 66 kDa in both toad and human plasma. Acid chromatography (pH approximately 3) resulted in the separation of IGF-I from its binding protein and consequently a shift of IGF-I immunoreactivity to the low molecular weight fractions (approximately 8 kDa) for both toad and human. IGF-BP activity shifted to molecular weight approximately 50 kDa. Toad plasma IGF-I and IGF-BP activity exhibited differences according to season: IGF-I levels were low in the spring (March = 0.48 +/- 0.11 ng eq/ml), increased progressively to reach a peak in July (5.84 +/- 2.5 ng eq/ml), and decreased to low levels again in the fall (October = 0.60 +/- 0.08, November = 0.45 +/- 0.09 ng eq/ml). Plasma IGF-BP activity demonstrated a similar pattern (March = 17.4 +/- 2.5, July = 35.0 +/- 2.4, November = 12.6 +/- 3.2% specific binding). IGF-I was produced for at least 72 hr when toad liver explants were cultured in serum-free medium, indicating that the liver is a source of IGF-I in anurans.

Recognition of insulin-like-growth-factor-binding proteins in serum and amniotic fluid by an antiserum against a low-molecular-mass insulin-like-growth-factor-inhibitor/binding protein

An antiserum (R8) raised against a purified specific low-Mr (16,000-18,000) insulin-like-growth-factor (IGF)-inhibitor/binding protein, which is immunologically related to the native growth hormone (GH)-dependent Mr-150,000 IGF-binding protein in serum, has been used to probe a possible additional relationship to the predominant non-GH-dependent IGF-binding protein (Mr approximately 30,000) of human amniotic fluid. Amniotic-fluid fractions and an IGF-inhibitory fraction of serum were analysed by covalent cross-linking, ligand-blotting and immunoblotting techniques. Western blotting of the serum fraction using the R8 antiserum gave five immunoreactive (ir) bands, of which at least three (Mr 38,000, 34,000 and 23,000) possessed IGF-binding activity, as indicated by ligand (125I-IGF-I) blotting. Western blotting of two differently prepared amniotic-fluid fractions, which both showed potent IGF-inhibitory bioactivity, gave several (Mr range 14,500-73,000) ir bands, of which two (Mr 28,000 and 17,000) were most prominent. Ligand-blotting analysis gave a single intensely labelled band at Mr 28,000, consistent with the major presence of the Mr-28,000-30,000 amniotic-fluid IGF-binding protein. Covalent cross-linking of the amniotic-fluid fractions to 125I-IGF-I gave three specifically cross-linked complexes (Mr 36,000, 32,000 and 23,000), which, assuming a 1:1 binding stoichiometry with IGF (Mr 7500), represent binding proteins of Mr 28,500, 24,500 and 15,500 respectively. The Mr-15,500 binding protein, very similar to the Mr-17,000 ir band, in all likelihood represents the IGF-inhibitor protein. Our results indicate that inhibitor-sized binding proteins and IGF-inhibitor bioactivity are present in human amniotic fluid, and that the IGF-inhibitor protein (Mr 16,000) and amniotic-fluid IGF-binding protein (Mr 28,000) are immunologically related. Since the IGF-inhibitor protein is also immunologically and structurally related to the native, GH-dependent, Mr-150,000 binding protein in serum, our data suggest a heretofore-unrecognized immunological similarity between the Mr-150,000 binding protein and the amniotic-fluid binding protein and its serum analogue, the Mr approximately 30,000, non-GH-dependent, binding protein.

The binding of endogenous and exogenous substance-P in human plasma

High levels of substance-P are present in the plasma of patients with carcinoid tumours and some thyrotoxic conditions. The majority of the substance-P in the blood plasma was shown, by immunoassay, to be associated with high molecular-weight material in a complex that could be dissociated by repeated gel-filtration. Smaller amounts of an intermediate molecular-weight (about 65,000 Da) complex were also detected. Chemical crosslinking with glutaraldehyde was used to show that the radioactively-labelled derivative [125I]Tyr-8-substance-P was able to bind to the high-Mr fraction of human plasma and also to human serum albumin. Binding to serum albumin was also demonstrated by equilibrium gel-filtration. Substance-P added to human plasma from a thyrotoxic subject, which contained high endogenous levels of the tachykinin (980 pg/mL), was rapidly degraded during incubation at 37 degrees, whereas the endogenous substance-P was considerably more stable. These results suggest that the binding of substance-P to blood plasma components may play an important role in protecting it against degradation. Furthermore, immunoassay techniques involving prior extraction, which fail to detect the bound substance-P, will give inaccurate measurements of the levels of this peptide in plasma.

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

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

Recombinant human insulin-like growth factor I induces its own specific carrier protein in hypophysectomized and diabetic rats

The physiology of the specific serum binding proteins which constitute the main storage pool for insulin-like growth factors (IGFs) in mammals is still incompletely understood. We have, therefore, investigated the regulation of these proteins in (i) hypophysectomized (hypox) rats infused with recombinant human growth hormone (rhGH) or recombinant human IGF 1 (rhIGF I) and (ii) streptozotocin-diabetic rats infused with insulin or rhIGF I. The main carrier protein, a GH-dependent complex of apparent molecular mass 200 kDa, contains N-glycosylated IGF-binding subunits (42, 45, and 49 kDa) that differ in their glycosyl but not in their protein moiety. These subunits are lacking in hypox and diabetic rats. They are induced by GH and insulin, respectively, and appear in the 200-kDa complex. Infusion of rhIGF I induces the subunits in both states; however, only in diabetic, not in hypox, rats do they form the 200-kDa complex. Glycosylated carrier protein subunits do not appear before 8 hr of rhIGF I infusion. During that period, hypox rats may become severely hypoglycemic. After 16 hr, glycosylated subunits are clearly induced, and blood sugar values are normal. We conclude: (i) The N-glycosylated subunits of the 200-kDa complex reflect the IGF I status. (ii) IGF I may mediate the induction of these subunits by GH. (iii) Significant association to the 200-kDa complex occurs only in the presence of GH. It is likely that GH, but not IGF I, induces a component, which itself does not bind IGF, but associates with the glycosylated IGF-binding subunits. (iv) The glycosylated subunits protect against IGF-induced hypoglycemia and may be involved in tissue-specific targeting of IGFs.

Serum levels of insulin-like growth factor (IGF) I, II and IGF binding protein in diabetic adolescents treated with continuous subcutaneous insulin infusion

IGF-I and IGF-II as well as the low molecular type of IGF binding protein (IGFPB) were determined in serum from 11 adolescents with insulin-dependent diabetes mellitus (IDDM) during a cross-over study with conventional and continuous subcutaneous insulin infusion (CIT and CSII) therapy. At the onset of the study the mean IGF-I level, 127 +/- 15 ng ml-1, was significantly decreased (P less than 0.001) in comparison with age-matched controls, whereas the mean IGF-II level, 1024 +/- 48 ng ml-1, was increased. A significant correlation (r = 0.70, P less than 0.05) was found between IGF-II and HbA1c levels. The mean morning level of IGFBP, 75 +/- 17 ng ml-1, at the onset of the study, was increased threefold above that in age-matched controls (P less than 0.01). There was a significant correlation between IGFBP and blood glucose values (r = 0.66, P less than 0.05). During CSII therapy a significant decrease (P less than 0.05) of the IGFBP levels was seen in subjects with a decrease in glucose levels, whereas no change was observed in IGF levels. The findings of elevated IGF-II and IGFBP levels and correlations between IGFBP and blood glucose concentration as well as IGF-II and HbA1c levels in adolescents with IDDM indicate that both IGF-II and IGFBP reflect a deranged metabolism caused by inadequate insulin administration.

Characterization of insulin-like growth factor binding proteins of cultured rat astroglial and neuronal cells

Insulin-like growth factor binding proteins produced by cultured rat neurons, astrocytes, and rat cell lines BRL-3A and B104 were compared to binding proteins found in rat serum, using affinity labeling, deglycosylation, and Western ligand blotting studies. Each source elaborated an unique pattern of heterogeneous binding proteins. Some of the binding proteins from different sources behaved similarly in each experimental system suggesting that subsets of these binding proteins may be structurally related. In particular, our data suggest that cultured astrocytes and neurons make the major binding protein produced by BRL-3A cells.

Insulin-like growth factor binding proteins from cultured human fibroblasts. Characterization and hormonal regulation

Specific, high affinity insulin-like growth factor (IGF) binding proteins are secreted by human fibroblasts in culture. By multiple criteria, the species of IGF binding proteins produced by human fibroblasts are distinct from the HepG2/amniotic fluid IGF binding protein, but share many characteristics with the growth hormone-dependent IGF binding protein forms predominant in normal adult human plasma. Treatment of cultured human fibroblasts with growth hormone produced an increase in IGF binding protein activity in the medium, while addition of glucocorticoids markedly diminished IGF binding activity. Insulin, epidermal growth factor, platelet-derived growth factor, and progesterone had no effect on IGF binding activity in fibroblast media. In comparison, HepG2 IGF binding activity was enhanced by progesterone, decreased by insulin, and unaffected by growth hormone or glucocorticoid treatment. Five molecular forms of IGF binding proteins were identified by Western ligand blots in human fibroblast conditioned medium, with Mr = 41,500, 37,000, 32,000, 28,000, and 23,000. In human fibroblast conditioned medium, the Mr = 41,500 and 37,000 IGF binding protein species were abundant, as in normal human plasma, with a major Mr = 23,000 form which was a minor component in plasma.

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

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

Insulin-like growth factor (IGF)-I and IGF-binding activity in normal and fast-growing chickens

Insulin-like growth factor(IGF)-binding activity were characterized in sera normal, fast growing, and very fast growing chicken strains. In contrast to a previous report, specific IGF-binding activity was observed in this nonmammalian species. Age-related IGF-binding activity levels did not differ between strains. IGF-I levels were significantly higher for the normal as compared to the 2 fast growing strains. Chromatographic studies show that IGF-I is associated with acid-dissociable high MW complexes in chicken serum, and the MW patterns are similar to human serum. Circulating levels of IGF-I or IGF-binding activity do not account for accelerated somatic growth in inbred large-bodied chicken strains.

Isolation and NH2-terminal amino acid sequences of rat serum carrier proteins for insulin-like growth factors

Three N-glycosylated carrier proteins (CP) for insulin-like growth factors (apparent molecular weights 30-32, 42 and 45 kDa) were isolated from adult rat serum. They share the same amino terminus (up to amino acid 31) and are constituents of the growth hormone-dependent native 150-200 kDa IGF carrier complex. Residues 12-31 display 60 and 50% sequence homology, respectively, to residues 2-21 of fetal rat and to residues 4-22 of a human amniotic fluid IGF carrier protein. No homology exists with the type I or II IGF receptors. Adult rat serum also contains a fourth IGF CP (24 kDa) whose 9 NH2-terminal amino acids are identical to those of the fetal form. Our findings suggest that the three N-glycosylated components originate from the same IGF carrier protein (adult form) and that the 24 kDa protein is a separate (fetal) species.

High molecular weight forms of insulin-like growth factor II and its binding protein identified by protein immunoblotting

Insulin-like growth factor (IGF)II is a mitogenic polypeptide which circulates in association with a binding protein(s). Immunoblotting studies were performed in human serum and indicate that:(1)a approximately 200 kDa covalently-linked IGF-II/binding protein complex is antigenically related to the 30 kDa binding protein, (2)IGF-II prohormone is associated with this complex, and (3)a major portion of the IGF-II prohormone immunoreactivity in human serum is present in fractions which would not be detected by standard radioimmunoassay methods. Our data provide insight regarding the inter-relationships of IGF-II and its binding protein, and direct evidence for the presence of IGF-II prohormone in human serum.

Heterogeneity of insulin-like growth factor binding proteins and relationships between structure and affinity. 1. Circulating forms in man

Circulating insulin-like growth factors (IGFs) are bound to specific, high-affinity binding proteins (BPs), and form complexes with relative molecular masses of about 150,000 ('large' complex) and 40,000 ('small' complex). The large complex appears to be under growth-hormone control and its proportions vary with those of the IGFs. Molecular heterogeneity among the binding proteins was revealed by polyacrylamide gel electrophoresis (SDS-PAGE) of serum in which they were cross-linked to 125I-labelled IGF I or II. Out of the six specific bands observed, of 150,000, 120,000, 49,000, 40,000 and 37,000 Mr, the last three appeared in both complexes, whereas the first three were visible only in the large complex. Some or all of the 49,000-37,000-Mr species may constitute the subunits of 150,000-Mr and/or 120,000-Mr IGF-BP complexes. With electrophoresis followed by transfer onto nitrocellulose and incubation with either 125I-labelled IGF I or II (western blot), the different binding proteins were identified per se. There were five molecular forms with Mr of 41,500, 38,500, 34,000, 30,000 and 24,000. In normal serum the 41,500 and 38,500-Mr forms were the major binding proteins. They appeared in both complexes, but were predominant in the large complex where they constitute the elementary binding units. These two proteins therefore bind to IGFs to form both 'monomeric' IGF-BP and 'oligomeric' (IGF-BP)n complexes. The 34,000, 30,000 and 24,000-Mr forms, by contrast, were visible only in the small complex. Different mechanisms appear to regulate the different binding proteins: in acromegalic serum the 41,500 and 38,500-Mr forms were augmented and the 34,000-Mr form diminished, whereas in hypopituitary serum the reverse was true and, in addition, the 30,000-Mr form was augmented. With chromatofocusing, the 34,000, 30,000 and 24,000-Mr forms eluted in three peaks between pH 6.0 and 4.0, whereas the 41,500 and 38,500-Mr forms eluted throughout the gradient, principally at pH 7.5 and 7.0. Competitive binding studies, done on binding proteins separated either by chromatofocusing or by SDS-PAGE and transfer onto nitrocellulose, revealed different affinities for the IGFs among the different molecular forms. The 41,500 and 24,000-Mr binding proteins preferentially bound IGF I and the 38,500, 34,000 and 30,000-Mr proteins preferentially bound IGF II. Our findings demonstrate the molecular heterogeneity of the binding proteins and the existence of a relationship between their structure and their affinities for the IGFs.(ABSTRACT TRUNCATED AT 400 WORDS)

Type II insulin-like growth factor receptor is present in rat serum

We previously identified in fetal rat serum a component capable of specifically binding radiolabeled insulin-like growth factor type II (IGF-II) that is considerably larger than both the fetal (40 kDa) and the adult (150 kDa) carrier proteins. We now present immunologic and affinity crosslinking data to show that this binding species is the type II IGF receptor. Rat serum was gel-filtered on a Sephadex G-200 column (0.05 M NH4HCO3, pH 8), and 125I-labeled IGF-II (125I-IGF-II) binding was measured in individual column fractions. 125I-IGF-II binding activity was found in the void volume of the column in addition to the carrier protein regions. Competitive binding studies using 125I-IGF-II and binding activity from the Sephadex G-200 void volume showed the characteristics of the type II receptor: IGF-II was more potent than IGF-I, and insulin did not compete. Moreover, a specific anti-type II IGF receptor antibody (no. 3637) completely blocked 125I-IGF-II binding. 125I-IGF-I did not bind to the void volume pool, demonstrating the absence of the type I IGF receptor in rat serum. Affinity crosslinking of 125I-IGF-II to the Sephadex G-200 void volume material demonstrated a specific band at 210 kDa without reduction and at 240 kDa after reduction of disulfide bonds. The serum type II IGF receptor size was confirmed by immunoblotting the void volume material with antiserum 3637, which revealed a band slightly smaller (approximately 10 kDa) than the type II IGF receptor from rat placental membranes. Immunoquantitation by immunoblotting using pure type II IGF receptor from rat placental membranes as standard showed a developmental pattern. In fetal rat serum (19-days gestation) and in sera from 3-, 10-, and 20-day-old rats, the concentrations of receptor protein were similar (1-5 micrograms/ml). The level of the type II IGF receptor in serum declined dramatically between age 20 and 40 days, but the receptor was still measurable at age 12 mo. We conclude that the type II IGF receptor is present in rat serum and is developmentally regulated.

Binding proteins for insulin-like growth factors in adult rat serum. Comparison with other human and rat binding proteins

Insulin-like growth factor (IGF) binding protein has been purified from adult rat serum by affinity chromatography on agarose-IGF-II and high performance reverse-phase chromatography. The final preparation contains two components, of apparent molecular mass 50 and 56 kDa nonreduced, or 44 and 48 kDa reduced, both of which specifically bind IGF-I and IGF-II. Competitive binding data indicate association constants of 5-10 X 10(10) l/mol for both IGFs, with a slightly higher affinity for IGF-II than IGF-I. Amino-terminal sequence analysis yields a unique sequence, identical in 11 of the first 15 amino acids with that of a human plasma IGF binding protein (Martin, J. L., and Baxter, R. C. (1986) J. Biol Chem. 261, 8754-8760), and with slight homology to other human and rat IGF binding proteins characterized to date. By analogy with the binding protein from human plasma, it is likely that the rat protein is part of the growth-hormone dependent complex which appears to carry most or all of the circulating IGFs.

Short-term metabolic effects of recombinant human insulin-like growth factor I in healthy adults

Insulin-like growth factor I (IGF I) is structurally similar to insulin and shares many of its biologic properties. We compared the short-term metabolic effects of recombinant IGF I (100 micrograms [13.3 nmol] per kilogram of body weight) and insulin (0.15 IU [1 nmol] per kilogram) in eight healthy volunteers (four men and four women). The hypoglycemic responses to both hormones were nearly identical in the doses used. The lowest blood glucose levels were reached after 30 minutes: 1.98 +/- 0.44 mmol per liter after IGF I and 1.78 +/- 0.29 after insulin. On a molar basis, IGF I was only 6 percent as potent as insulin in the production of hypoglycemia. Insulin also inhibited lipolysis more effectively than IGF I. Levels of epinephrine, norepinephrine, growth hormone, glucagon, and cortisol responded similarly to both agents. The hypoglycemia produced by IGF I is probably due to the supraphysiologic concentrations of the free peptide that result from its rapid intravenous injection. Fifteen minutes after injection, the serum level of IGF I increased from 144 +/- 38 ng per milliliter at base line to 424 +/- 56, of which 80 percent was free in the plasma (not bound to IGF carrier proteins). The determination of whether any of the short-term metabolic effects of IGF I have any clinical application will require further investigation.

Absence of serum growth hormone binding protein in patients with growth hormone receptor deficiency (Laron dwarfism)

It has recently been recognized that human serum contains a protein that specifically binds human growth hormone (hGH). This protein has the same restricted specificity for hGH as the membrane-bound GH receptor. To determine whether the GH-binding protein is a derivative of, or otherwise related to, the GH receptor, we have examined the serum of three patients with Laron-type dwarfism, a condition in which GH refractoriness has been attributed to a defect in the GH receptor. The binding of 125I-labeled hGH incubated with serum has been measured after gel filtration of the serum through an Ultrogel AcA 44 minicolumn. Nonspecific binding was determined when 125I-hGH was incubated with serum in the presence of an excess of GH. Results are expressed as percent of specifically bound 125I-hGH and as specific binding relative to that of a reference serum after correction is made for endogenous GH. The mean +/- SEM of specific binding of sera from eight normal adults (26-46 years of age) was 21.6 +/- 0.45%, and the relative specific binding was 101.1 +/- 8.6%. Sera from 11 normal children had lower specific binding of 12.5 +/- 1.95% and relative specific binding of 56.6 +/- 9.1%. Sera from three children with Laron-type dwarfism lacked any demonstrable GH binding, whereas sera from 10 other children with other types of nonpituitary short stature had normal relative specific binding. We suggest that the serum GH-binding protein is a soluble derivative of the GH receptor. Measurement of the serum GH-binding protein may permit recognition of other abnormalities of the GH receptor.

Risk of hypoglycaemia with alternate-day growth hormone injections

Fasting hypoglycaemia developed in three growth hormone deficient children after the start of treatment with synthetic growth hormone. The effects, which occurred 36-60 h after each injection, may have been due to insulin-like effects of endogenous somatomedins after waning of insulin antagonism induced by growth hormone. Daily growth hormone injections may be necessary to maintain normal plasma glucose levels in young children.

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.

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.

Paracrine functions of somatomedins

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

Regulation of lipoprotein lipase in primary cultures of isolated human adipocytes

To study the regulation of adipose tissue lipoprotein lipase (LPL) in human adipocytes, omental adipose tissue was obtained from healthy subjects and digested in collagenase. The isolated adipocytes thus obtained were suspended in Medium 199 and cultured at 37 degrees C. Cell viability was demonstrated in adipocytes cultured for up to 72 h by constancy of cell number, cell size, trypan-blue exclusion, and specific 125I-insulin binding. In addition, chloroquine induced an increase in cell-associated 125I-insulin at 24, 48, and 72 h after preparation. Thus, isolated adipocytes retained their ability to bind, internalize, and degrade insulin. LPL was measured as activity secreted into the culture medium (CM), released from cells by heparin (HR), and extracted from cell digests. A broad range of heparin concentrations produced a prompt release of LPL from a rapidly replenishable pool of cellular activity. When cells were cultured in medium containing 10% fetal bovine serum, there was a marked stimulation of CM and HR. The secretory response to serum (CM) correlated strongly with HR 24 h after preparation (rs = 0.731, P less than 0.001). In addition, HR was found to correlate logarithmically and inversely with body mass index (r = -0.731, P less than 0.001). Insulin, at 400 ng/ml only, increased HR by 36 +/- 10%, an effect simulated by lower concentrations of insulin-like growth factor-1 (IGF1). Thus, LPL is produced and regulated in isolated human adipocytes. The degree of adiposity and serum are important regulators of HR activity, whereas insulin is stimulatory only at a pharmacologic concentration. This effect of insulin may be mediated through the IGF1 receptor. Isolated human adipocytes represent a novel and useful system for the study of LPL and lipid metabolism as well as for other aspects of adipocyte biology.