Interleukin-1 (IL-1) is an important factor in bone metabolism, and its actions may be mediated in part via prostaglandins. Prostaglandin G/H synthase (PGHS), a critical enzyme in the synthesis of prostaglandins, has two isoforms, PGHS-1, which is generally constitutively expressed, and PGHS-2, which is inducible. This study examines the effects of IL-1 on PGHS-2 mRNA expression in human osteosarcoma MG-63 cells, the human osteoblast-like initial transfectant (HOBIT) cell line, and primary human osteoblastic (HOB) cells. IL-1 induced PGHS-2 mRNA expression in MG-63 cells within 1 h, and expression was maintained for 24 h. There was a dose-related increase in PGHS-2 mRNA levels with 1-100 ng/ml of IL-1. Induction of PGHS-2 protein and media prostaglandin E2 (PGE2) paralleled induction of PGHS-2 mRNA levels. IL-1 similarly induced PGHS-2 mRNA expression and PGE2 production in HOBIT and HOB cells. Among other potential agonists, phorbol myristate acetate (PMA) was a potent inducer of PGHS-2 expression, while forskolin (FSK), serum, and prostaglandins had little effect. Cycloheximide enhanced effects of both IL-1 and PMA, suggesting that de novo protein synthesis is not required for induction of PGHS-2. Twenty-four hours of PMA pretreatment blocked the induction of PGHS-2 by PMA but not by IL-1, suggesting that IL-1 induction of PGHS-2 mRNA is not dependent on the protein kinase C pathway. Although FSK alone had little effect, it enhanced induction of PGHS-2 mRNA by IL-1. PGHS-1 was constitutively expressed and showed little change with treatment. In summary, we show that IL-1 is a potent inducer of PGHS-2 expression and PGE2 production in human osteosarcoma cells as well as in osteoblastic cells derived from normal human bone.