Distinct regulation of cytosolic phospholipase A2 phosphorylation, translocation, proteolysis and activation by tumour necrosis factor-receptor subtypes

The hormonally regulated Ca²⁺-dependent enzyme, cytosolic phospholipase A₂ (cPLA₂) is activated by a range of inflammatory stimuli. Tumour necrosis factor-α (TNF) is one of the first known stimuli for cPLA₂ but it is not known whether both TNF receptor subtypes are involved in activating the lipase. In the present study, we show for the first time that both type I 55 kDa TNFR (TNFR1) and type II 75 kDa TNFR (TNFR2) stimulate cPLA₂ enzyme, but with distinct signalling mechanisms. TNFR1 activates mitogen-activated protein kinase (MAPK) and p38MAPK. TNFR1 then phosphorylates and activates cPLA₂ in a MAPK-dependent fashion. Furthermore, TNFR1 causes the translocation and caspase-dependent proteolysis of cPLA₂ as part of its activation profile. TNFR2, on the other hand, does not cause the phosphorylation of cPLA₂ as it does not activate MAPK or p38MAPK, but instead activates cPLA₂ by causing its translocation to plasma membrane and perinuclear subcellular regions. TNFR2 activation causes a delayed, slight increase in [Ca²⁺]_i of <50 nM that may contribute towards the translocation and activation of cPLA₂. Therefore both TNF receptor subtypes play a role in cPLA₂ activation, but by means of separate signal-transduction pathways.