THE FUNCTIONS OF 5 DISTINCT MAMMALIAN PHOSPHOLIPASE A(2)S IN REGULATING ARACHIDONIC-ACID RELEASE - TYPE IIA AND TYPE-V SECRETORY PHOSPHOLIPASE A(2)S ARE FUNCTIONALLY REDUNDANT AND ACT IN CONCERT WITH CYTOSOLICPHOSPHOLIPASE A(2)

We examined the relative contributions of five distinct mammalian phos pholipase A(2) (PLA(2)) enzymes (cytosolic PLA(2) (cPLA(2); type IV), secretory PLA(2)s (sPLA(2)s; types IIA, V, and IIC), and Ca2+-independ ent PLA(2) (iPLA(2); type VI)) to arachidonic acid (AA) metabolism by overexpressing them in human embryonic kidney 293 fibroblasts and Chin ese hamster ovary cells. Analyses using these transfectants revealed t hat cPLA(2) was a prerequisite for both the calcium ionophore-stimulat ed immediate and the interleukin (IL)-1- and serum-induced delayed pha ses of AA release, Type IIA sPLA, (sPLA(2)-IIA) mediated delayed AA re lease and, when expressed in larger amounts, also participated in imme diate AA release. sPLA(2)-V, but not sPLA(2)-IIC, behaved in a manner similar to sPLA(2)-IIA. Both sPLA(2)s-IIA and -V, but not sPLA(2)-IIC, were heparin-binding PLA(2)s that exhibited significant affinity for cell-surface proteoglycans, and site-directed mutations in residues re sponsible for their membrane association or catalytic activity markedl y reduced their ability to release AA from activated cells. Pharmacolo gical studies using selective inhibitors as well as co-expression expe riments supported the proposal that cPLA(2) is crucial for these sPLA( 2)s to act properly. The AA-releasing effects of these sPLA(2)s were i ndependent of the expression of the M-type sPLA(2) receptor. Both cPLA (2) sPLA(2)s-IIA, and -V were able to supply AA to downstream cyclooxy genase-2 for IL-1-induced prostaglandin E-2 biosynthesis, iPLA(2) incr eased the spontaneous release of fatty acids, and this was further aug mented by serum but not by IL-1, Finally, iPLA(2)-derived AA was not m etabolized to prostaglandin E-2. These observations provide evidence f or the functional cross-talk or segregation of distinct PLA(2)s in mam malian cells in regulating AA metabolism and phospholipid turnover.