Previous studies have indicated that lipopolysaccharide(LPS)from Gram-negative bacteria inplaque induces the release of prostaglandin E_2(PGE_2),which promotes alveolar bone resorption in periodontitis,and that tobacco smoking might be an important risk factor for the development and severity of periodontitis.We determined the effect of nicotine and LPS on alkaline phosphatase(ALPase)activity,PGE_2 production,and the expression of cyclooxygenase(COX-1,COX-2),PGE_2 receptors Ep1-4,and macrophage colonystimulating factor(M-CSF)in human osteoblastic Saos-2 cells.The cells were cultured with 10~(-3)M nicotinein the presence of 0,1,or 10μg/ml LPS,or with LPS alone.ALPase activity decreased in cells cultured withnicotine or LPS alone,and decreased further in those cultured with both nicotine and LPS,whereas PGE_2production significantly increased in the former and increased further in the latter.By itself,nicotine did notaffect expression of COX-1,COX-2,any of the PGE_2 receptors,or M-CSF,but when both nicotine and LPSwere present,expression of COX-2,Ep3,Ep4,and M-CSF increased significantly.Simultaneous addition of10~(-4)M indomethacin eliminated the effects of nicotine and LPS on ALPase activity,PGE_2 production,and M-CSF expression.Phosphorylation of protein kinase A was high in cells cultured with nicotine and LPS.Theseresults suggest that LPS enhances the production of nicotine-induced PGE_2 by an increase in COX-2 expres-sion in osteoblasts,that nicotine-LPS-induced PGE_2 interacts with the osteoblast Ep4 receptor primarily inautocrine or paracrine mode,and that the nicotine-LPS-induced PGE_2 then decreases ALPase activity andincreases M-CSF expression. Previous studies have indicated that lipopolysaccharide (LPS) from Gram-negative bacteria in plaque induces the release of prostaglandin E_2 (PGE_2), which promotes alveolar bone resorption in periodontitis, and that tobacco smoking might be an important risk factor for the development and severity of periodontitis . We determined the effect of nicotine and LPS on alkaline phosphatase (ALPase) activity, PGE_2 production, and the expression of cyclooxygenase (COX-1, COX-2), PGE_2 receptors Ep1-4, and macrophage colonystimulating factor (M-CSF) in human osteoblastic Saos-2 cells. The cells were cultured with 10-3 M nicotinein the presence of 0,1, or 10 μg / ml LPS, or with LPS alone. ALPase activity decreased in cells cultured withnicotine or LPS alone, and decreased further in those cultured with both nicotine and LPS, and PGE 2 production significantly increased in the former and increased further in the latter. BY itself, nicotine did not affect expression of COX-1, COX-2, any of the PGE 2 receptors, or M -CSF, b ut when both both nicotine and LPSwere present, expression of COX-2, Ep3, Ep4, and M-CSF increased significantly. Simultaneous addition of10-4Mindomethacin eliminated the effects of nicotine and LPS on ALPase activity, PGE2 production, and M-CSF expression. Phosphorylation of protein kinase A was high in cells cultured with nicotine and LPS. These results suggest suggest that LPS enhances the production of nicotine-induced PGE_2 by an increase in COX-2 expres-sion in osteoblasts, that nicotine-LPS- induced PGE_2 interacts with the osteoblast Ep4 remodeling in inocrine or paracrine mode, and that the nicotine-LPS-induced PGE_2 then decreases ALPase activity and in creases M-CSF expression.