Functional coupling between secretory and cytosolic phospholipase A2 modulates tumor necrosis factor-α- and interleukin-1β-induced NF-κB activation

Tumor necrosis factor (TNF)-alpha and interleukin (IL)-1 beta are potent activators of the transcription factor NF-kappaB, induced during inflammatory conditions. We have previously shown that both secretory and cytosolic phospholipase A(2) (PLA(2)) are involved in TNF-alpha- and IL-1 beta -induced NF-kappaB activation. In this study, we have addressed the mechanism of PLA, involvement with respect to downstream arachidonic acid (AA) metabolites and the functional coupling between PLA(2)s mediating NF-kappaB activation. We show that in addition to inhibitors of secretory and cytosolic PLA2s, 5-lipoxygenase inhibitors attenuate TNF-alpha- and IL-1 beta -stimulated NF-KB activation. Exogenous addition of leukotriene B-4 (LTB4) restored NF-kappaB activation reduced by 5-lipoxygenase inhibitors or an LTB4 receptor antagonist, thus identifying LTB4 as a mediator in signaling to NF-kappaB. TNF-alpha- and IL-1 beta -induced AA release from cellular membranes was accompanied by phosphorylation of cytosolic PLA(2). Inhibitors of secretory PLA(2) and of 5-lipoxygenase/LTB4 functionality markedly reduced AA release and nearly completely abolished cytosolic PLA2 phosphorylation. This demonstrates that secretory PLA(2) through 5-lipoxygenase metabolites, is an essential upstream regulator of cytosolic PLA2 and AA release. Our results therefore suggest the existence of a functional link between secretory and cytosolic PLA2 in cytokine-activated keratinocytes, providing a molecular explanation for the participation of both secretory and cytosolic PLA(2) in arachidonic acid signaling and NF-KB activation in response to proinflammatory cytokines.