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 f ibroblasts. 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. H alf-maximal effectiveness was seen at 5-8 ng/ml IGF-I, with mean maxim al responses of 1.5-, 2-, and 6-fold for [H-3]2-deoxyglucose uptake, [ C-14]glucose incorporation, and [C-14]aminoisobutyric acid (AIB) uptak e, respectively. A 48-hour preincubation with 10(-7) M dexamethasone m arkedly 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 [I-125]IGF-I to fibroblast monolayers was identical in etha nol control and glucocorticoid-treated cells, with 50% displacement at similar to 5 ng/ml IGF-I. In addition to its synergism with IGF-I, pr eincubation with dexamethasone augmented insulin and epidermal growth factor (EGF) stimulation of [H-3]thymidine incorporation; dexamethason e had no effect on platelet-derived growth factor or fibroblast growth factor action. Two-dimensional gel electrophoresis identified two spe cific glucocorticoid-induced proteins in human fibroblast cell extract s with molecular weights of 45K and 53K and pls of 6.8 and 6.3, respec tively. These data indicate that IGF-I receptor-mediated actions in hu man fibroblasts are differentially modulated by glucocorticoids. Gluco corticoids are synergistic with IGF-I in stimulating mitogenesis and a mino acid uptake, without having any apparent effect on IGF-I-stimulat ed glucose metabolism. Glucocorticoid enhancement of growth factor bio activity may involve modulation of a regulatory event in the mitogenic signaling pathway subsequent to cell surface receptor activation. (C) 1995 Wiley-Liss, Inc.