Cytokine-induced prostaglandin E-2 synthesis and cyclooxygenase-2 activityare regulated both by a nitric oxide-dependent and -independent mechanism in rat osteoblasts in vitro

Osteoblasts respond to stimulation with interleukin-1 (IL-1), tumor necrosi s factor-alpha (TNF-alpha), and interferon-gamma (IFN-gamma) by production of nitric oxide and prostaglandins (PGs), In this study the relationship be tween nitric oxide and PG synthesis was investigated after cytokine stimula tion of cultured rat osteoblasts. IL-1, TNF-alpha, IFN-gamma, and exogenous sodium nitroprusside, a nitric oxide donor, all stimulated PGE(2) producti on in a dose-dependent manner. PGE(2) production was blocked-by L-nitro-arg inine methyl ester, an inhibitor of nitric oxide production, after IFN-gamm a stimulation and was partially blocked after TNF-alpha stimulation. Howeve r, IL-1-induced PGE(2) was unaffected. Similarly, expression of the cycloox ygenase-2 protein was stimulated by cytokines, and IFN-gamma-induced expres sion was again blocked by L-nitro-arginine methyl ester, In contrast, all c ytokines induced the cyclooxygenase-2 mRNA expression independently of nitr ic oxide production, although exogenous sodium nitroprusside was able to in duce the cyclooxygenase-a mRNA in the absence of cytokines, The results sho w that nitric oxide can induce PG synthesis and cyclooxygenase 2 expression and may regulate cyclooxygenase-a expression at both transcriptional and p ost-transcriptional levels. In addition, the data show the existence of bot h nitric oxide-dependent and -independent pathways of PG synthesis after cy tokine stimulation of osteoblasts. The results suggest that nitric oxide ma y be an important mediator of PG production in inflammatory bone diseases.