Role of the Endoplasmic Reticulum Unfolded Protein Response in Glomerular Epithelial Cell Injury

Andrey V Cybulsky,Tomoko Takano,Joan Papillon,Krikor Bijian

doi:10.1074/jbc.m500729200

Andrey V Cybulsky, Tomoko Takano + Show 2 more

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https://doi.org/10.1074/jbc.m500729200

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Abstract

C5b-9-induced glomerular epithelial cell (GEC) injury in vivo (in passive Heymann nephritis) and in culture is associated with damage to the endoplasmic reticulum (ER) and increased expression of ER stress proteins. Induction of ER stress proteins is enhanced via cytosolic phospholipase A(2) (cPLA(2)) and limits complement-dependent cytotoxicity. The present study addresses another aspect of the ER unfolded protein response, i.e. activation of protein kinase R-like ER kinase (PERK or pancreatic ER kinase), which phosphorylates eukaryotic translation initiation factor 2-alpha (eIF2alpha), thereby generally suppressing translation and decreasing the protein load on a damaged ER. Phosphorylation of eIF2alpha was enhanced significantly in glomeruli of proteinuric rats with passive Heymann nephritis, compared with control. In cultured GECs, complement induced phosphorylation of eIF2alpha and reduced protein synthesis, and complement-stimulated phosphorylation of eIF2alpha was enhanced by overexpression of cPLA(2). Ischemia-reperfusion in vitro (deoxyglucose plus antimycin A followed by glucose re-exposure) also stimulated eIF2alpha phosphorylation and reduced protein synthesis. Complement and ischemia-reperfusion induced phosphorylation of PERK (which correlates with activation), and fibroblasts from PERK knock-out mice were more susceptible to complement- and ischemia-reperfusion-mediated cytotoxicity, as compared with wild type fibroblasts. The GEC protein, nephrin, plays a key role in maintaining glomerular permselectivity. In contrast to a general reduction in protein synthesis, translation regulated by the 5'-end of mouse nephrin mRNA during ER stress was paradoxically maintained, probably due to the presence of short open reading frames in this mRNA segment. Thus, phosphorylation of eIF2alpha and consequent general reduction in protein synthesis may be a novel mechanism for limiting complement- or ischemia-reperfusion-dependent GEC injury.

Highlights

The unfolded protein response (UPR)1 is one of several cell stress responses, which in part consists of up-regulating the
Phosphorylation of eIF2␣ and consequent general reduction in protein synthesis may be a novel mechanism for limiting complement- or ischemiareperfusion-dependent glomerular epithelial cell (GEC) injury
The present study focuses on another aspect of the endoplasmic reticulum (ER) unfolded protein response, i.e. activation of PERK, which leads to phosphorylation of eIF2␣, and suppression of global translation initiation and protein synthesis

Summary

Introduction

The unfolded protein response (UPR) is one of several cell stress responses, which in part consists of up-regulating the. Upon activation of the complement cascade near a cell surface, there is assembly of terminal components, exposure of hydrophobic domains, and insertion of the C5b-9 membrane attack complex into the lipid bilayer of the plasma membrane [6, 7]. An example of sublytic C5b-9-mediated cell injury in vivo is passive Heymann nephritis (PHN) in the rat, a widely accepted model of human membranous nephropathy [11, 12]. Assembly of C5b-9 in GEC in culture and in vivo increased expression of the ER stress proteins, bip and grp, in a cPLA2-dependent manner, and induction of these proteins restricted complement-mediated GEC injury. The aim of the present study was to determine if induction of ER stress in GECs activates the PERK pathway and suppresses protein synthesis. Translation regulated by the 5Ј-end of nephrin mRNA was, maintained following ischemiareperfusion injury

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