E.R. Froesch

Preferential enhancement of myoblast differentiation by insulin-like growth factors (IGF I and IGF II) in primary cultures of chicken embryonic cells

Cells prepared from the body walls of chicken embryos were plated in the absence of serum. Insulin‐like growth factors (IGFs) barely stimulated cell replication, but preferentially enhanced the differentiation of muscle cells. Myoblast fusion was favoured in the presence of IGF (or insulin). Concomitantly, acetylcholinesterase activity increased. IGF I and IGF II were equipotent and active in low physiological concentrations, in contrast to insulin, which was known for a long time to exert such effects at pharmacological concentrations.

Intact but not truncated insulin-like growth factor binding protein-3 (IGFBP-3) blocks IGF I-induced stimulation of osteoblasts : control of IGF signalling to bone cells by IGFBP-3-specific proteolysis ?

IGFBP-3 is the predominant IGFBP in serum and the major IGFBP secreted by osteoblasts. Native and recombinant IGFBP-3 and a truncated form lacking the carboxyterminal third were tested for their effects on 2 osteoblastic cell lines. Intact but not truncated IGFBP-3 blocked IGF I-stimulated DNA and glycogen synthesis. Inhibition was dose-dependent and found whenever the concentration of intact IGFBP-3 exceeded the concentration of IGF I. Truncated IGFBP-3 appears to result from proteolytic cleavage and does occur in vivo. The loss of inhibition by IGFBP-3 may be regulated at the site of IGF target cells and thus be essential for IGF I-induced osteoblast growth.

Insulin-like growth factor I supports differentiation of cultured osteoblast-like cells

Rat calvaria cells grown in culture for one week had properties of osteoblasts: a high content in alkaline phosphatase and a marked cyclic AMP response to parathyroid hormone (PTH). In short‐term experiments, insulin‐like growth factor I (IGFI) stimulated the incorporation of (14C]glucose into glycogen. When IGF I was present in the medium during 6 days the cell number increased slightly and there was a substantial, disproportionate rise in alkaline phosphatase activity of the cultures. Thus, IGFI stimulates growth, and in addition, and in contrast to other growth factors, mainly enhances differentiation of osteoblasts.

Insulin-like growth factors stimulate synthesis of nucleic acids and glycogen in cultured calvaria cells

SummaryA (sub)population of cells obtained from newborn rat calvaria by (sequential) collagenase digestion is grown to confluence in serum-containing medium. These cells are osteoblast-like with respect to high alkaline phosphatase activity and marked responsiveness (cAMP) to parathormone.Insulin-like growth factors (IGFs) enhance net incorporation of the labeled precursors thymidine, uridine, and glucose into the respective macromolecules DNA, RNA, and glycogen. Human IGF I is five times as potent as IGF II in evoking these anabolic responses in cultured rat calvaria cells. In contrast to insulin, the factors are effective in concentrations in which they are present in serum.

Differential regulation of insulin-like growth factor binding protein (IGFBP)-2 mRNA in liver and bone cells by insulin and retinoic acid in vitro

Isolated cells produce insulin‐like growth factors (IGFs) and their binding proteins (IGFBPs). Two distinct cell types were studied with regard to IGFBP‐2 expression: (i) rat hepatocytes, which produce IGF I at a high rate and thus regulate its plasma concentration; and (ii) rat osteoblasts, which are targets of IGF I action. IGFBP‐2 expression is low in hepatocytes prepared from normal adult rats and high in calvaria cells from newborn rats. Retinoic acid stimulates IGFBP‐2 production by liver cells. Insulin suppresses both basal and retinoic acid‐induced IGFBP‐2 mRNA expression in hepatocytes and has no such effect on osteoblasts. Retinoic acid and insulin regulate IGFBP‐2 expression in a tissue‐specific manner.

Production of carrier proteins for insulin-like growth factors (IGFs) by rat osteoblastic cells Regulation by IGF I and cortisol

A bone‐derived rat cell line, PyMS, releases IGF I and IGF carrier proteins which are similar to those found in rat serum. Western blot analysis of culture media conditioned by hormone‐treated cells shows that growth hormone and IGF I stimulate and cortisol inhibits production of IGF carrier proteins in vitro. A glycosylated carrier protein species of 49‐42 kDa is closely related to the subunits of the growth hormone‐dependent carrier protein complex found in rat serum. In addition, rhIGF I rapidly induces a 32 kDa, non‐glycosylated IGF‐binding protein whose accumulation is markedly increased by cortisol.

Effects of hormones on cyclic AMP release from rat adipose tissue in vitro

Lipolysis is regulated, at least in part, by the intracellular level of cyclic AMP [ 1,2] . The effects of several hormones and of lipolytic and antilipolytic agents on CAMP accumulation in adipose tissue and adipocytes have been extensively studied [3-61. Most experiments were carried out with isolated fat cells [4-81. Little attention has been paid to CAMP release from intact adipose tissue [3]. During liver perfusion CAMP is released into the perfusion medium in response to glucagon and to epinephrine [9, lo] . Therefore, we examined to what extent CAMP was released from adipose tissue in response to these lipolytic hormones and whether lipolysis was correlated with CAMP release, which may reflect metabolically available ‘free’ CAMP in the cell [9] . Furthermore, we were interested to find out whether partially purified nonsuppressible insulin-like activity (NSILA-S) which has been shown to exert the same metabolic effects on adipose tissue as insulin (for review see [ 111) affects epinephrine-induced CAMP release. The results of this study show that: 1) Considerable amounts of CAMP are released from adipose tissue only when theophylline is present in addition to lipolytic hormones. After 30 min CAMP, which has accumulated in the medium, rapidly disappears possibly due to enhanced degradation. 2) CAMP release is inhibited by insulin and NSILA-S. 3) Concomitant inhibition by insulin of epinephrine-induced lipolysis and of CAMP release is observed only at low epinephrine concentrations, whereas at higher epinephrine concentrations lipolysis

Binding of nonsuppressible insulin-like activity (NSILA) to isolated fat cells: Evidence for two separate membrane acceptor sites

The biological effects of insulin and NSILA, a polypeptide purified from human serum, on adipose tissue and isolated fat cells have been shown to be identical [l-5 ] . Among these, carrier-mediated stimulation of glucose uptake across the fat cell membrane [2] and inhibition of epinephrine-induced cyclic AMP-release from adipose tissue [6] suggest that the mechanism by which both hormones act at the cell membrane level is also the same. Therefore, we expected that the primary step of action of insulin and NSILA on adipose tissue involves binding to the same membrane acceptor site. To test this hypothesis binding studies with 1251-labelled insulin and NSILA were performed with isolated rat fat cells. The results are compatible with the presence of two membrane binding sites for NSILA on adipocytes, one which appears to be identical with the insulin acceptor, but with a lower affinity for NSILA, and a second one which displays specificity and high affinity for NSILA, but not for insulin.

Expression, effects, and fate of IGFBP-5 are different in normal and malignant osteoblastic cells

Normal osteoblasts from newborn rat calvaria and human osteosarcoma (Saos-2) cells express IGFBP-5 mRNA. IGF I increases IGFBP-5 mRNA levels in both cell types, whereas retinoic acid stimulates IGFBP-5 mRNA expression in calvaria but suppresses it in Saos-2 cells. IGFBP-5 mRNA expression is stimulated in normal bone cells by parathyroid hormone. 30 nM IGFBP-5 stimulates 3H-thymidine incorporation in calvaria (which produce IGF I contributing to basal proliferation in serum-free medium) but not in Saos-2 cells which produce little IGF I and IGF II. Among the 5 rhIGFBPs tested (IGFBP-2 to -6), only IGFBP-5 stimulates DNA synthesis in calvaria cells, and only IGFBP-6 in Saos-2 cells. RhIGFBP-5 displays a short half-life (approximately 30 min) in serum-free medium of calvaria cells and a long half-life (approximately 15 h) in the medium of Saos-2 cells. Fragments of 20 and 14 kDa accumulate in the media of both cell types. Intact (31 kDa) IGFBP-5 is associated and remains with the extracellular matrix of mature calvaria osteoblasts but not of Saos-2 cells. Among the IGFBPs produced IGFBP-5 is unique with regard to its marked affinity to matrix of normal bone cells, its short half-life when released, and its stimulatory effects on DNA synthesis.

Effects of Insulin‐Like Growth Factor I in Man

ABSTRACT. A 6‐day period of subcutaneous infusion with recombinant human insulin‐like growth factor I in three healthy male volunteers resulted in an increase in the ratio of insulin to C‐peptide levels and significant decreases in triglyceride levels and the ratio of total to high density lipoprotein—cholesterol in serum. Increased renal plasma flow and glomerular filtration rates were also observed.