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

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Abstract
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.

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