Complement-mediated Activation of Calcium-independent Phospholipase A2γ ROLE OF PROTEIN KINASES AND PHOSPHORYLATION

In experimental membranous nephropathy, complement C5b-9-induces glomerular epithelial cell (GEC) injury and proteinuria. The effects of C5b-9 are mediated via signaling pathways, including calcium-independent phospholipase A(2)gamma (iPLA(2)gamma), and mitogen-activated protein kinases (MAPKs) such as extracellular signal-regulated kinase (ERK), c-Jun N-terminal kinase (JNK), and p38. The iPLA(2)gamma pathway is cytoprotective. This study addresses the mechanisms of iPLA(2)gamma activation. iPLA(2)gamma activity was monitored by quantifying prostaglandin E-2 (PGE(2)) production. In GECs, iPLA(2)gamma localized at the endoplasmic reticulum and mitochondria. Complement-mediated production of PGE2 was amplified in GECs that overexpress iPLA(2)gamma, compared with control cells, and was blocked by the iPLA(2)gamma inhibitor bromoenol lactone in both iPLA(2)gamma-overexpressing and control GECs. In GECs that overexpress iPLA(2)gamma, complement-mediated PGE2 production was reduced by inhibitors of MAP/ERK kinase 1 (MEK1) and p38 but not JNK. In COS-1 cells that overexpress iPLA(2)gamma and cyclooxygenase-1, PGE2 production was induced by co-expression of constitutively active MEK1 or MAPK-interacting kinase 1 (MNK1) as well as by stimulation with epidermal growth factor (EGF) + ionomycin. Complement-and EGF + ionomycin-stimulated iPLA(2)gamma activity was attenuated by the S511A/S515A double mutation. Moreover, complement and EGF + ionomycin enhanced phosphorylation of Ser-511. Thus, complement-mediated activation of iPLA(2)gamma is mediated via ERK and p38 pathways, and phosphorylation of Ser-511 and/or Ser-515 plays a key role in the catalytic activity and signaling of iPLA(2)gamma. Defining the mechanisms by which complement activates iPLA(2)gamma provides opportunities for development of novel therapeutic approaches to GEC injury and proteinuria.