PROSTAGLANDIN E-2 AMPLIFIES CYTOSOLIC PHOSPHOLIPASE A(2)-DEPENDENT AND CYCLOOXYGENASE-2-DEPENDENT DELAYED PROSTAGLANDIN E-2 GENERATION IN MOUSE OSTEOBLASTIC CELLS - ENHANCEMENT BY SECRETORY PHOSPHOLIPASE A(2)

We used the MC3T3-E1 cell line, which originates from C57BL/6J mouse t hat is genetically type IIA secretory phospholipase A(2) (sPLA(2))-def icient, to reveal the type IIA sPLA(2)-independent route of the prosta nglandin (PG) biosynthetic pathway, Kinetic and pharmacological studie s showed that delayed PGE(2) generation by this cell line in response to interleukin (IL)-1 beta and tumor necrosis factor alpha (TNF alpha) was dependent upon cytosolic phospholipase A(2) (cPLA(2)) and cycloox ygenase (COX)-2. Expression of these two enzymes was reduced by cPLA(2 ) or COX-2 inhibitors and restored by adding exogenous arachidonic aci d or PGE(2), indicating that PGE(2) produced by these cells acted as a n autocrine amplifier of delayed PGE(2) generation through enhanced cP LA(2) and COX-2 expression, Exogenous addition or enforced expression of type IIA sPLA(2) significantly increased IL-1 beta/TNF alpha-initia ted PGE(2) generation, which was accompanied by increased expression o f both cPLA(2) and COX-2 and suppressed by inhibitors of these enzymes . Thus, our results revealed a particular cross-talk between the two P LA(2) enzymes and COX-2 for delayed PGE(2) biosynthesis by a type IIA sPLA(2)-deficient cell line, cPLA(2) is responsible for initiating COX -2-dependent delayed PGE(2) generation, and sPLA(2), if introduced, en hances PGE(2) generation by increasing cPLA(2) and COX-2 expression vi a endogenous PGE(2).