Prostaglandin E-2 amplifies cytosolic phospholipase A(2)- 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 that is genetically type IIA secretory phospholipase A(2) (sPLA(2))-deficient, to reveal the type IIA sPLA(2)-independent route of the prostanglandin (PG) biosynthetic pathway, Kinetic and pharmacological studies 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 cyclooxygenase (COX)-2. Expression of these two enzymes was reduced by cPLA(2) or COX-2 inhibitors and restored by adding exogenous arachidonic acid or PGE(2), indicating that PGE(2) produced by these cells acted as an autocrine amplifier of delayed PGE(2) generation through enhanced cPLA(2) and COX-2 expression, Exogenous addition or enforced expression of type IIA sPLA(2) significantly increased IL-1 beta/TNF alpha-initiated PGE(2) generation, which was accompanied by increased expression of both cPLA(2) and COX-2 and suppressed by inhibitors of these enzymes. Thus, our results revealed a particular cross-talk between the two PLA(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, enhances PGE(2) generation by increasing cPLA(2) and COX-2 expression via endogenous PGE(2).