LONG-TERM TREATMENT OF SWISS 3T3 FIBROBLASTS WITH DEXAMETHASONE ATTENUATES MAP KINASE ACTIVATION-INDUCED BY INSULIN-LIKE GROWTH-FACTOR-I (IGF-I)

Bone formation is reduced in hyperglucocorticoid states, e.g. Cushing' s syndrome or long-term treatment with synthetic glucocorticoids durin g rheumatic diseases. possibly related to decreased sensitivity of the target to insulin-like growth factor-I (IGF-I). In this study, we hav e sought to identify postreceptor-mechanisms for glucocorticoid-induce d resistance to insulin-like peptides in a model system. Treatment of Swiss 3T3 fibroblasts with 100nM dexamethasone for 48h reduced IGF-I-i nduced activation of mitogen-activated protein kinase (MAP kinase). Th e level of insulin receptor substrate-1 (IRS-1) was reduced in dexamet hasone-treated cells, as measured by Western blot; however, the patter n of tyrosine-phosphorylated protein subsequent to stimulation with IG F-I (1 min) was not altered. No inhibitory effect of dexamethasone was observed on the level of phosphotyrosine in IRS-1 in extracts from IG F-I-treated cells. The amount of IGF-I-induced association of insulin receptor substrate-1 and phosphatidylinositol 3-kinase was increased i n steroid treated cells. Addition of IGF-I increased the synthesis of lipid, glycogen and protein, and the reduction of a tetrazolium dye, M TS, in untreated cells. The response to IGF-I in terms of glycogen syn thesis was blunted, whereas the effect of IGF-I was unaffected for the other three parameters in cells pretreated with dexamethasone. These findings indicate that the activation of MAP kinase may be dissociated from IGF-I-induced anabolic pathways and tyrosine phosphorylationof I RS-1. The results agree with the previously proposed role for the acti vation of MAP kinase in the regulation of glycogen synthesis. Furtherm ore, they suggest that dexamethasone-induced reduction of IRS-1 expres sion may be important for the impaired activation of MAP kinase by ins ulin-like peptides in steroid-treated cells.