Taiwan cobra phospholipase A₂ suppresses ERK-mediated ADAM17 maturation, thus reducing secreted TNF-α production in human leukemia U937 cells

The goal of this study was to explore the signaling pathway regulating the processing of proADAM17 into ADAM17 in Taiwan cobra phospholipase A₂ (PLA₂)-treated human leukemia U937 cells. PLA₂ induced reactive oxygen species (ROS)-elicited p38 MAPK activation and ERK inactivation in U937 cells. Catalytically inactive bromophenacylated PLA₂ (BPB-PLA₂) and PLA₂ mutants evoked Ca²⁺- mediated p38 MAPK activation, and the level of phosphorylated ERK remained unchanged. PLA₂ treatment reduced mature ADAM17 expression and secreted TNF-α (sTNF-α) production. Co-treatment of SB202190 (p38 MAPK inhibitor) and catalytically inactive PLA₂ increased ERK phosphorylation, ADAM17 maturation and sTNF-α production. Nevertheless, mRNA levels of ADAM17 and TNF-α were insignificantly altered after PLA₂ and SB202190/BPB-PLA₂ treatment. ADAM17 activity assay and knock-down of ADAM17 revealed that ADAM17 was involved in sTNF-α production. Restoration of ERK activation increased the processing of proADAM17 into ADAM17 in PLA₂-treated cells, while inactivation of ERK reduced ADAM17 maturation in untreated and SB202190/BPB-PLA₂-treated cells. Removal of cell surface heparan sulfate abrogated PLA₂ and SB202190/BPB-PLA₂ effect on ADAM17 maturation. Taken together, the present data reveal that PLA₂ suppresses ERK-mediated ADAM17 maturation, thus reducing sTNF-α production in U937 cells. Moreover, the binding with heparan sulfate is crucial for the PLA₂ effect.