INDUCTION OF CYCLOOXYGENASE-2 BY SECRETORY PHOSPHOLIPASES A(2) IN NERVE GROWTH FACTOR-STIMULATED RAT SEROSAL MAST-CELLS IS FACILITATED BY INTERACTION WITH FIBROBLASTS AND MEDIATED BY A MECHANISM INDEPENDENT OFTHEIR ENZYMATIC FUNCTIONS

Mast cells exhibit a biphasic (immediate and delayed) eicosanoid-biosy nthetic response after stimulation with particular cytokines or Fc eps ilon RI (high affinity receptor for IgE) cross-linking. Treatment of r at serosal connective tissue mast cells (CTMC) with nerve growth facto r (NGF) induced only the delayed phase of PGD(2) generation that depen ded on inducible cyclooxygenase-2 (COX-2), but not constitutive COX-1, even though the subcellular distributions of these isoforms were simi lar. Experiments using several phospholipase A(2) (PLA(2)) isozyme-spe cific probes and inhibitors suggested that both constitutive cytosolic PLA(2) and inducible type IIA secretory PLA(2) (sPLA(2)) are involved in NGF-initiated, COX-a-dependent, delayed PGD(2) generation in rat C TMC, A type IIA sPLA(2) inhibitor, but neither cytosolic PLA(2) nor CO X inhibitors, reduced, while adding exogenous type IIA sPLA(2) augment ed, NGF-induced COX-2 expression and its attendant PGD(2) generation, indicating that the sPLA(2)-mediated increase in delayed PGD(2) genera tion was attributable mainly to enhanced COX-2 expression. Type IIA sP LA(2) and its close relative type V sPLA(2) associated with fibroblast ic cell surfaces increased NGF-induced COX-2 expression more efficient ly than the soluble enzymes, revealing a particular juxtacrine sPLA(2) presentation route. Surprisingly, catalytically inactive type IIA sPL A(2) mutants, which were incapable of promoting arachidonic acid relea se from cytokine-primed cells, retained the ability to enhance COX-2 e xpression in CTMC, indicating that the COX-2-inducing activities of sP LA(2) are independent of their catalytic functions.