Oxidation of ER resident proteins upon oxidative stress: Effects of altering cellular redox/antioxidant status and implications for protein maturation

Previous work showed that from all cellular proteins, the endoplasmic reticulum (ER) resident proteins are most sensitive to oxidative stress [hydrogen peroxide (H2O2)], as determined using the oxidation-sensitive, membrane-permeable, acetylTyrFluo probe. Because of the importance of these proteins in proper cellular functioning, we studied (a) whether modifying the cellular redox state/antioxidant status alters the susceptibility of those proteins toward H2O2 oxidative stress and (b) whether H2O2 affects ER function with regard to protein folding. The cellular redox and/or antioxidative capacity was modified in several ways. Lowering the capacity increased H2O 2-induced protein oxidation, and increasing the capacity lowered H2O2-induced protein oxidation. The effect of H 2O2 on ER-related protein maturation was investigated, using the maturation of the low-density lipoprotein receptor as a model. Its maturation was not affected at low concentrations of H2O2 (≤400 μM), which do result in oxidation of ER resident proteins. Maturation was slowed down or reversibly inhibited at higher concentrations of H2O2 (1.5-2.0 mM). These results might be caused by several events, including oxidation of the low-density lipoprotein receptor itself or ER resident proteins resulting in decreased folding (capacity). Alternatively, oxidation of cytosolic proteins involved in ER Golgi transport might attenuate transport and maturation. Clearly, the mechanism(s) responsible for the impairment of maturation need further investigation.