Effects of TGF-β, TNF-α, IL-β and IL-6 alone or in combination, and tyrosine kinase inhibitor on cyclooxygenase expression, prostaglandin E₂ production and bone resorption in mouse calvarial bone cells

Cyclooxygenase-2 (COX-2) and tyrosine kinase, which are involved in the biosynthesis of prostaglandin E₂ (PGE₂) in mouse calvarial osteoblasts, are stimulated by cytokine interleukin-1β (IL-1β), tumor necrosis factor-α (TNF-α) and/or interleukin-6 (IL-6). IL-1β and IL-6 and, to a lesser extent, TNF-α, enhances COX-2 mRNA levels in calvarial osteoblasts. Simultaneous treatment with IL-6 and IL-1β and TNF-α resulted in enhanced COX-2 mRNA levels accompanied by the cooperative stimulation of PGE₂ biosynthesis compared to cells treated with IL-1β or TNF-α or IL-6 alone. In contrast, the presence of TGF-β reduced COX-2 mRNA level, PGE₂ biosynthesis and bone resorption induced by IL-1β, TNF-α, IL-6 or a combination thereof. However, neither IL-1β, TNF-α, IL-6 nor a combination of IL-1β, TNF-α, IL-6 enhanced COX-1 mRNA levels in calvarial osteoblasts. A novel Src tyrosine kinase inhibitor, Herbimycin A (HERB), reduced COX-2 mRNA levels as well as PGE₂ production induced by IL-1β, TNF-α and IL-6 or a combination of IL-1β, TNF-α, IL-6, whereas COX-1 mRNA levels remained unaffected. Finally, HERB was found to inhibit in vitro bone resorption. These results indicate that the cooperative effects of IL-β, TNF-α, IL-6 on PGE₂ production are due to the enhanced expression of the COX-2 gene and that tyrosine kinase(s) are involved in COX-2 signal transduction in mouse calvarial osteoblasts. Thus, the Src family of kinase inhibitors may be useful in treating diseases associated with elevated bone loss.