Oestrogenic Compounds Modulate Cytokine-induced Nitric Oxide Production in Mouse Osteoblast-like Cells

Nitric oxide (NO) is a mediator of bone metabolism with effects on both bone resorption and formation. Its production by both the constitutive and inducible isoforms of nitric oxide synthase (NOS) is affected by oestrogen in several types of cell and in tissues other than bone cells. Recently, oestrogens were found to increase basal NO production by osteoblasts via enhanced activity or expression, or both, of NOS‐3. Inflammatory cytokines, however, increase NO by increasing the expression of NOS‐2. In this study we have examined whether cytokine‐induced NO production by osteoblastic cells was affected by oestrogenic compounds by studying the effect of 17β‐oestradiol and the anti‐oestrogens ICI164,384 and 4‐hydroxytamoxifen on cytokine‐induced NO production in oestrogen receptor positive MC3T3‐E1 osteoblast‐like cells. Combinations of the inflammatory cytokines interleukin‐1β, tumour necrosis factor‐α, and interferon‐γ with lipopolysaccharide stimulated NO production up to 11‐fold. This cytokine‐induced NO production was further increased dose‐dependently by the anti‐oestrogens ICI164,384 and 4‐hydroxytamoxifen (133.3±3.2% and 146.0±13.2%, respectively). 17β‐Oestradiol either had no effect on or slightly inhibited cytokine‐induced NO production. It did, however, dose‐dependently counteract the stimulatory effect of the anti‐oestrogens. Concentrations of 17β‐oestradiol needed to prevent the stimulatory effect of 4‐hydroxytamoxifen were ca tenfold that of ICI164,384. These findings show that, in addition to the stimulatory effect of oestrogen on basal NO production by NOS‐3, cytokine‐induced NO production is also affected by oestrogenic compounds in osteoblasts.