Chief, Oxidative Stress and Cancer, CEA-Saclay
Disturbance of glutathione metabolism is a hallmark of numerous diseases, yet glutathione functions are still poorly understood. One key to address this question is to consider its functional compartmentation. In the endoplasmic reticulum (ER), protein folding requires disulfide bond formation catalyzed by the thiol oxidase Ero1 and protein disulfide isomerase (PDI). In the ER, glutathione is thought to counterbalance ER oxidation by the Ero1-PDI redox relay, which explain its relative more oxidized redox state (EGSH -242 mV), relative to the cytoplasm (EGSH = -295 mV). To access the function of GSH in this compartment, we first asked about the mechanism that maintains the ER EGSH homeostatic value. As GSH is exclusively synthesized in the cytoplasm and there is no GSH reductase in the ER to recycle the GSSG produced by the activity of the ER Ero1-PDI oxidative pathway, maintenance of ER EGSH likely depends on an ER import of GSH and export of GSSG. We found that GSH enters the ER by facilitated diffusion through the protein-conducting channel Sec61. We also found that an oxidized form of the chaperone Bip (Kar2 in yeast) inhibits this transport. We show that increased ER transport of GSH triggers Ero1 activation by reduction of its negative regulatory disulfides, which in turn leads to Bip oxidation by the H2O2 by-product of Ero1 activity. This regulated transport is strongly activated during ER stress by the UPR induction of Ero1 and an increase in GSH biosynthesis. Thus, the ER poise is tuned by a reciprocal control of GSH import into the ER and Ero1 activation. Such a reciprocal control is aimed at preventing a “short circuit” between the ER oxidative and reductive pathways, which would lead to Ero1 chronic cycling, cellular GSH consumption and cell death.