Complement C5b-9 Membrane Attack Complex Increases Expression of Endoplasmic Reticulum Stress Proteins in Glomerular Epithelial Cells*

In the passive Heymann nephritis (PHN) model of membranous nephropathy, complement C5b-9 induces glomerular epithelial cell (GEC) injury, proteinuria, and activation of cytosolic phospholipase A₂ (cPLA₂). This study addresses the role of endoplasmic reticulum (ER) stress proteins (bip, grp94) in GEC injury. GEC that overexpress cPLA₂ (produced by transfection) and “neo” GEC (which expresses cPLA₂at a lower level) were incubated with complement (40 min), and leakage of constitutively expressed bip and grp94 from ER into cytosol was measured to monitor ER injury. Greater leakage of bip and grp94 occurred in complement-treated GEC that overexpress cPLA₂, as compared with neo, implying that cPLA₂ activation perturbed ER membrane integrity. After chronic incubation (4–24 h), C5b-9 increased bip and grp94 mRNAs and proteins, and the increases were dependent on cPLA₂. Expression of bip-antisense mRNA reduced stimulated bip protein expression and enhanced complement-dependent GEC injury. Glomerular bip and grp94 proteins were up-regulated in proteinuric rats with PHN, as compared with normal control. Pretreatment of rats with tunicamycin or adriamycin, which increase ER stress protein expression, reduced proteinuria in PHN. Thus, C5b-9 injures the ER and enhances ER stress protein expression, in part, via activation of cPLA₂. ER stress protein induction is a novel mechanism of protection from complement attack.