Purification from transformed mouse fibroblast of a cell growth inhibitor which is an IGF-binding protein

The pro-forms of insulin-like growth factor I (IGF-I) are predominant in skeletal muscle and alter IGF-I receptor activation

IGF-I is a key regulator of muscle development and growth. The pre-pro-peptide produced by the Igf1gene undergoes several posttranslational processing steps to result in a secreted mature protein, which is thought to be the obligate ligand for the IGF-I receptor (IGF-IR). The goals of this study were to determine what forms of IGF-I exist in skeletal muscle, and whether the mature IGF-I protein was the only form able to activate the IGF-IR. We measured the proportion of IGF-I species in murine skeletal muscle and found that the predominant forms were nonglycosylated pro-IGF-I and glycosylated pro-IGF-I, which retained the C-terminal E peptide extension, instead of mature IGF-I. These forms were validated using samples subjected to viral expression of IGF-I combined with furin and glycosidase digestion. To determine whether the larger molecular weight IGF-I forms were also ligands for the IGF-IR, we generated each specific form through transient transfection of 3T3 cells and used the enriched media to perform kinase receptor activation assays. Compared with mature IGF-I, nonglycosylated pro-IGF-I had similar ability to activate the IGF-IR, whereas glycosylation of pro-IGF-I significantly reduced receptor activation. Thus, it is important to understand not only the quantity, but also the proportion of IGF-I forms produced, to evaluate the true biological activity of this growth factor.

Insulin-like growth factor binding protein-1 inhibits arterial smooth muscle cell proliferation in vitro but does not reduce the neointimal response to balloon catheter injury

The biological effects of the insulin-like growth factors (IGFs) are modulated by circulating binding proteins (BPs), including IGFBP-1. We have investigated the effects of recombinant IGFBP-1 on smooth muscle cell (SMC) proliferation in vitro using cultured rat aortic SMCs and in vivo using the ballooned rat carotid artery model. IGFBP-1 inhibited IGF-1 induced and spontaneous SMC proliferation dose-dependently. In vivo, the effective half-life of IGFBP-1 was approximately 5 h when administered by intraperitoneal injection. High peri-operative plasma levels of IGFBP-1 (mean 1780 ng/ml) were attained by giving and intravenous dose immediately prior to balloon injury in 9 rats. Animals injected with human serum albumin or saline were used as controls. In vivo cell proliferation was assessed by BrdU pulse labeling each animal prior to the termination of the experiment, 6 days after balloon injury. Absolute intimal thickness, intima-media ratio and cell proliferation indices were measured for each animal. Although IGFBP-1 inhibited SMC proliferation in vitro, high plasma concentrations of IGFBP-1 did not reduce neointimal size or cell proliferation. IGFBP-1 administration was, however, associated with a significantly greater loss of body weight (P < 0.05), indicating that the peptide had a profound metabolic effect. Our data suggest that IGF-1 does not have a major role in inducing SMC proliferation in the early phases following angioplasty.

Diminished expression of insulin-like growth factor (IGF) binding protein-5 and activation of IGF-I-mediated autocrine growth in simian virus 40-transformed human fibroblasts

The reduced growth factor requirements of murine fibroblasts transformed by simian virus 40 (SV 40) have been attributed to insulin-like growth factor (IGF)-I induction by T antigen and consequent activation of IGF-I receptor signaling. The present study shows that the autonomous growth of SV 40-transformed human fibroblasts also requires type-I IGF-I receptor activation but that this is not due to de novo induction of IGF-I gene expression since untransformed human fibroblasts, which fail to proliferate in the absence of serum, also showed IGF-I gene expression under serum-free conditions. DNA synthesis assays confirmed that untransformed cells were responsive to exogenous IGF and indicated that transformed cells were already maximally stimulated. In untransformed fibroblasts, IGF binding was principally to abundant membrane-associated IG-FBP-5, whereas in transformed fibroblasts this protein was minimally expressed, and IGF binding was to IGF receptors. Loss of detectable membrane-associated IG-FBP-5 in transformed cells was associated with diminished IGFBP-5 gene expression and with loss of IGF-II gene expression. Exogenous IGFBP-5 associated with the membranes of transformed cells and inhibited the autocrine growth of these cells. These findings suggest that loss of IGFBP-5 in SV 40-transformed fibroblasts facilitates interaction of endogenously produced IGF-I with the IGF-I receptor and increases their sensitivity to autocrine stimulation.

Production of autostimulatory growth factors by the human carcinoma line, RPMI 2650

The human carcinoma line RPMI 2650 produces autocrine factors; they are detected by the ability of RPMI 2650 conditioned medium (CM) to stimulate growth in soft agar of RPMI 2650 cells plated at low density. The autocrine activity in crude CM can be fractionated by ultrafiltration into a lower molecular weight (MW) fraction (R1-30), which concentrates molecules in the 1000-30,000 Da range; and a higher MW fraction (R30) with molecules greater than 30,000 Da in a more concentrated form. R1-30 is labile to acid, base, and heat treatment, whereas R30 is stable to (and sometimes activated by) these treatments. Boiling of R30, however, renders it labile to acid, base, and trypsin treatments. CM can be separated into a weakly heparin-binding fraction (with stability properties similar, but not identical, to R1-30), and a non-heparin binding fraction (with stability properties similar to R30). RPMI 2650 cells secrete transforming growth factor (TGF)alpha- and TGF beta-like molecules, but the R1-30 fraction can be distinguished from these TGFs, and from most other known growth factors, by its unusual combination of acid lability and weak affinity for heparin. Since the R30/non-heparin binding fraction is rendered labile by boiling or acid treatment, it may represent a bound or conformationally stable form of a growth factor.