Augmented prostaglandin E2 generation resulting from increased activities of cytosolic and secretory phospholipase A2 and induction of cyclooxygenase-2 in interleukin-1β-stimulated rat calvarial cells during the mineralizing phase

Abstract.

Objective and Design: To assess prostaglandin (PG) E₂ production by osteoblasts during the mineralizing phase after interleukin (IL)-1β stimulation, using an in vitro system of rat calvarial cells cultured for 21 days.¶Methods: The cells, which reached confluence after 3 days, were designated day 0 cells. Culture was continued for a further 21 days after confluence. The cells on the 21st day of the culture were designated day 21 cells.¶Results: The PGE₂ concentration in the medium of the day 21 cells was increased 72 h after IL-1β treatment, and reached a peak level approximately 1,400 times that of the day 0 cells 6 h after IL-1β treatment. We examined the effects of IL-1β on PGE₂ production and changes in the relevant enzyme activities, and found that the activities of cytosolic phospholipase A₂ (cPLA₂), type II secretory PLA₂ (sPLA₂) and cyclooxygenase (COX)-2 in the day 21 cells were increased. Both selective COX-2 inhibitor and cPLA₂ inhibitor abolished PGE₂ generation, whereas an sPLA₂ inhibitor partially inhibited it. Taken together, these results indicate that COX-2 and cPLA₂ play pivotal roles and sPLA₂ is involved in IL-1β-stimulated PGE₂ production by these cells. Furthermore, we found that IL-1β treatment induced PGE synthase activity and this correlated well with PGE2 production.¶Conclusion: Augmented PGE₂ production by mineralizing osteoblasts after IL-1β treatment, and the involvement of IL-1β-induced cPLA₂, sPLA₂, COX-2 and PGE synthase activities in this phenomenon were demonstrated.