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.