Distinct arachidonate-releasing functions of mammalian secreted phospholipase A(2)s in human embryonic kidney 293 and rat mastocytoma RBL-2H3 cells through heparan sulfate shuttling and external plasma membrane mechanisms

We analyzed the ability of a diverse set of mammalian secreted phospholipas e A, (sPLA(2)) to release arachidonate for lipid mediator generation in two transfected cell lines. In human embryonic kidney 293 cells, the heparin-b inding enzymes sPLA(2)-IIA, -IID, and -V promote stimulus-dependent arachid onic acid release and prostaglandin E, production in a manner dependent on the heparan sulfate proteoglycan glypican, In contrast, sPLA(2)-IB, -IIC, a nd -IIE, which bind weakly or not at all to heparanoids, fail to elicit ara chidonate release, and addition of a heparin binding site to sPLA(2)-IIC al lows it to release arachidonate, Heparin nonbinding sPLA(2)-X liberates ara chidonic acid most likely from the phosphatidylcholine-rich outer plasma me mbrane in a glypican-independent manner. In rat mastocytoma RBL-2H3 cells t hat lack glypican, sPLA(2)-V and -X, which are unique among sPLA(2)s in bei ng able to hydrolyze phosphatidylcholine-rich membranes, act most likely on the extracellular face of the plasma membrane to markedly augment IgE-depe ndent immediate production of leukotriene C, and platelet-activating factor . sPLA(2)-IB, -IIA, -IIC, -IID, and -IIE exert minimal effects in RBL-2H3 c ells. These results are also supported by studies with sPLA(2), mutants and immunocytostaining and reveal that sPLA(2)-dependent lipid mediator genera tion occur by distinct (heparanoid-dependent and -independent) mechanisms i n HEK293 and RBL-2H3 cells.