Transcriptional induction of cyclooxygenase-2 in osteoblasts is involved in interleukin-6-induced osteoclast formation

Interleukin-6 (IL-6) induces osteoclast-like cell (osteoclast) formation in a dose-dependent fashion in cocuItures of mouse bone marrow cells and osteoblastic cells when soluble IL-6 receptor (sIL-6R) is present. Simultaneous treatment with submaximal doses of IL-1 alpha and IL-6 with sIL-6R caused marked induction of osteoclast formation and PGE(2) synthesis. These effects were suppressed by adding neutralizing antibodies against IL-1 alpha or IL-6R and were totally abolished by adding nonsteroidal antiinflammatory drugs, such as indomethacin and a selective cyclooxygenase-a (COX-2) inhibitor (NS398). In mouse osteoblastic cells, both IL-ln and IL-6 with sIL-6R markedly induced messenger RNA expression of COX-2, but not COX-1, as determined by Northern blot analysis, and luciferase activity in cells stably transfected with a COX-2 promoter-luciferase fusion construct. IL-6 and sIL-6R, when added separately, did not stimulate COX-2 messenger RNA expression. Simultaneous addition of IL-1 alpha and IL-6 with sIL-6R to osteoblast cultures cooperatively induced transcription of COX-2, which was associated with a marked increase in COX activity measured by the conversion of arachidonic acid into PGE(2). The increased PGE, synthesis by osteoblasts may play an important role in osteoclastagenesis induced by submaximal doses of IL-1 and IL-6.