Interleukin-1 alpha (IL-1 alpha) and IL-1 beta are cytokines produced by cells of the immune system and nonimmune cells, such as osteoblasts. IL-1 alpha and IL-1 beta have potent stimulatory effects on bone resorption and also have mixed effects on bone formation, suggesting that they play an important role in local regulation of bone turnover. The present study examined paracrine mechanisms underlying effects of IL-1 on bone formation. Bone formation is regulated at the local level by polypeptide growth factors such as insulin-like growth factor-I (IGF-I). IGF-I is a relatively abundant growth factor in bone matrix, is produced by osteoblasts, and stimulates osteoblastic cell proliferation and bone matrix synthesis. Recent evidence suggests that IGF-I also stimulates osteoclast formation and bone resorption. To determine whether increased or decreased IGF-I production in bone might mediate some of the effects of IL-1 on bone turnover, the effects of IL-1 alpha and IL-1 beta on IGF-I release from neonatal mouse calvaria in organ culture were examined. IL-1 alpha and IL-1 beta stimulated resorption and increased the release of IGF-I into the medium during 6 days of culture. Maximal stimulation of resorption occurred at lower concentrations of IL-1 alpha and IL-1 beta (1 pM) than were required for maximum stimulation of IGF-I release (10 pM). IL-1 beta also increased steady state levels of 7.5- and 0.9-kilobase IGF-I mRNA transcripts in total RNA extracted from cultured calvaria at 24 h. The cyclooxygenase inhibitor indomethacin (1 microM) inhibited IL-1 beta-induced prostaglandin E2 (PGE2) production and inhibited resorption and release of IGF-I from calvaria. Indomethacin had a smaller effect on PTH-induced IGF-I release and had no significant effect on PTH-induced resorption. These results suggest that the effects of IL-1 on resorption and release of IGF-I may be mediated in part by a prostaglandin-dependent mechanism. Consistent with this hypothesis, PGE1 and PGE2 stimulated resorption and release of IGF-I. The dose responses for PGE2 stimulation of resorption and stimulation of IGF-I release were equivalent. The results are consistent with the hypotheses that 1) IL-1 stimulates IGF-I production by bone cells, in part by PG-dependent mechanisms; 2) the effects of IL-1 and PGs on bone formation and resorption may be mediated by locally induced secretion of IGF-I and other growth factors in bone.