Sensitization of human fibroblasts to insulin-like growth factor I by serum deprivation and dexamethasone pretreatment

Differential effects of glucocorticoids on insulin-like growth factor I action in cultured human fibroblasts

Glucocorticoids act synergistically with insulin-like growth factor I (IGF-I) to stimulate DNA synthesis and replication of cultured human fibroblasts. In the present study, we further define glucocorticoid and IGF-I interactive effects on human fibroblast metabolism and growth. IGF-I stimulated dose-dependent increases in early metabolic events. Half-maximal effectiveness was seen at 5-8 ng/ml IGF-I, with mean maximal responses of 1.5-, 2-, and 6-fold for [3H]2-deoxyglucose uptake, [14C]glucose incorporation, and [14C]aminoisobutyric acid (AIB) uptake, respectively. A 48-hour preincubation with 10(-7) M dexamethasone markedly enhanced both the sensitivity and maximal effectiveness of IGF-I stimulation of AIB uptake. In contrast, dexamethasone had no effect on IGF-I-stimulated glucose uptake and utilization. Maximum specific binding of [125I]IGF-I to fibroblast monolayers was identical in ethanol control and glucocorticoid-treated cells, with 50% displacement at approximately 5 ng/ml IGF-I. In addition to its synergism with IGF-I, preincubation with dexamethasone augmented insulin and epidermal growth factor (EGF) stimulation of [3H]thymidine incorporation; dexamethasone had no effect on platelet-derived growth factor or fibroblast growth factor action. Two-dimensional gel electrophoresis identified two specific glucocorticoid-induced proteins in human fibroblast cell extracts with molecular weights of 45K and 53K and pIs of 6.8 and 6.3, respectively. These data indicate that IGF-I receptor-mediated actions in human fibroblasts are differentially modulated by glucocorticoids. Glucocorticoids are synergistic with IGF-I in stimulating mitogenesis and amino acid uptake, without having any apparent effect on IGF-I-stimulating glucose metabolism. Glucocorticoid enhancement of growth factor bioactivity may involve modulation of a regulatory event in the mitogenic signaling pathway subsequent to cell surface receptor activation.

Glucocorticoids regulate the secretion of a 21kDa-IGF-binding protein by sheep adipose tissue explants

Using a solution phase assay we have demonstrated that sheep adipose tissue explants secrete insulin-like growth factor binding proteins (IGFBPs) when cultured in serum-free medium over a 24 h period. Further, we demonstrate that secretion of IGFBP(s) is inhibited (up to 50%) by incubation of the cultures in the presence of 10(-8) M dexamethasone. This inhibitory effect is overcome when insulin (10 ng/ml) and ovine growth hormone (100 ng/ml) are incubated together (but not separately) with glucocorticoid. Further characterisation of this IGF binding activity by high performance size exclusion chromatography and Western ligand blot analysis indicated that under our culture conditions sheep adipose tissue explants secrete one predominant 21 kDa IGFBP and it is this BP which is hormonally regulated as described above. We discuss our results in the context of endocrine/paracrine/autocrine control of adipose tissue metabolism and differentiation.

Endogenous insulin-like growth factor (IGF) binding proteins cause IGF-1 resistance in cultured fibroblasts from a patient with short stature

The ED50 of insulin-like growth factor (IGF)-I-stimulated alpha-aminoisobutyric acid (AIB) uptake (mean +/- SD) in cultured fibroblasts from a child with short stature that we have reported (1.40 +/- 0.24 nM), is significantly higher than the ED50 of IGF-I-stimulated AIB uptake in fibroblasts from 11 normal subjects (0.42 +/- 0.12 nM) and from 127 short children (0.35 +/- 0.11 nM). Similarly, the ED50 of IGF-I-stimulated thymidine incorporation in fibroblasts from this child is 2.8 times higher than that in fibroblasts from four normal subjects. To minimize potential modulation of IGF-I action by endogenous IGF binding proteins in these assays, fibroblast responsiveness to [Q3,A4,Y15,L16]IGF-I, an IGF-I variant that has a 600-fold reduced affinity for serum IGF binding proteins, has been examined. The biological activity of this variant is comparable in the patient's and normal fibroblasts, suggesting that the resistance to IGF-I action cannot be attributed to a defective IGF-I receptor. To investigate directly the possibility that IGF-I sensitivity in the patient's fibroblasts is reduced by endogenous IGF binding proteins (IGFBP), binding proteins that are secreted into AIB assay buffer during a 3-h collection and that are cell-associated at the end of the collection have been analyzed. Ligand blot analysis of conditioned AIB assay buffer demonstrates that fibroblasts from the patient secrete 1.3-2.2 times more of Mr 46,400/42,900, 32,000, and 26,800 binding proteins than normal fibroblasts. The major difference between fibroblasts from the patient and from normal subjects is a striking 10-fold increase in the amount of a cell surface Mr 32,000 binding protein in the patient's fibroblasts. The Mr 32,000 binding protein is similar in size to IGFB-1 and different from IGFBP-2 and IGFBP-3, but it does not cross-react with an antibody against IGFBP-1. We conclude that the resistance to IGF-I action in the patient's fibroblasts is caused by an abnormal production and/or cell association of IGF binding proteins.