Phospholipase A2 inhibitors and leukotriene synthesis inhibitors block TNF-induced NF-kappaB activation.

Tumour necrosis factor (TNF) is a key regulator of inflammation and immunity. The cellular effects exerted by TNF depend, apart from NF-kappaB-directed gene transcription, largely on its ability to activate phospholipase A2(PLA2), yielding the release of arachidonic acid (AA) and its metabolites. AA metabolites, especially the leukotrienes, act as second messengers in TNF receptor signalling, as different inhibitors of AA metabolism impair a variety of TNF-induced biochemical events. The role, however, of AA and its metabolites in TNF-induced NF-kappaB activation is still obscure. Here we report that 4-bromophenacyl bromide (4-BPB; an inhibitor of PLA2), nordihydroguaretic acid (NDGA; a 5-lipoxygenase inhibitor), as well as MK-886 [an inhibitor of 5-lipoxygenase-activating protein (FLAP)] interfere with TNF-induced NF-kappaB-mediated transactivation. However, only 4-BPB inhibited the DNA-binding activity of NF-kappaB, whereas NDGA and MK-886 did not. Thus, different inhibitors interfere at different points in TNF-induced signalling leading to NF-kappaB-dependent transcription. Artificial induction of AA metabolism induced neither DNA-binding activity of NF-kappaB nor NF-kappaB-dependent transactivation. It was concluded that although TNF-induced signalling to NF-kappaB-dependent transcription is sensitive to inhibitors of AA metabolism at multiple points during this signalling, AA release is essential but not sufficient for NF-kappaB-activation.