This dissertation, "A Study of Biological Role of Reactive Oxygen Species in Cellular Response in Stress" by Dennis, Lam, 林勁行, was obtained from The University of Hong Kong (Pokfulam, Hong Kong) and is being sold pursuant to Creative Commons: Attribution 3.0 Hong Kong License. The content of this dissertation has not been altered in any way. We have altered the formatting in order to facilitate the ease of printing and reading of the dissertation. All rights not granted by the above license are retained by the author. Abstract: When proteins are unable to fold properly in the endoplasmic reticulum (ER), the resultant formation of misfolded proteins causes stress of the ER. Cells with ER stress often have a higher abundance of reactive oxygen species (ROS). Previous studies suggest that ROS could aggravate ER stress by further disrupting the ER protein folding process. More recent studies suggest that the unfolded protein response signaling pathways activated by ER stress could lead to the production of ROS. Such studies lead to the hypothesis that ER stress could be promoted by ROS, and vice versa. The aim of the present study is to test the above hypothesis by studying how ROS could be generated in ER-stressed cells. This is followed by investigating if ROS could increase or decrease the level of ER stress in cells. Finally, the extent of ER stress induced cell death in the presence and absence of ROS is assessed. The treatment of HeLa cells with tunicamycin (Tm), a common ER-stress inducing agent, resulted in the elevation of intracellular ROS that could be detected with the ROS-reactive probe dichlorodihydrofluorescein (DCF), but not dihydroethidium which is relatively specific towards superoxide anion. The Tm-induced elevation of ROS could be prevented by co-incubation of cells with thiol reductants such as dithiothreitol and N-acetylcysteine but not with the free radical scavenger ascorbate. The tunicamycin-induced elevation of ROS level could also be prevented by the over-expression of catalase in HeLa. These data is consistent with the idea that hydrogen peroxide is a major form of ROS produced in Tm-treated cells. In addition to elevation of ROS level, HeLa cells treated with tunicamycin also resulted in the phosphorylation of PERK and eIF2α, and the splicing of XBP-1. In the presence of cycloheximide to inhibit protein synthesis so as to deplete protein substrates for folding in the ER, tunicamycin-induced ER stress was greatly minimized as was evident by the absence of both the phosphorylation of PERK and splicing of XBP-1. However, the phosphorylation of eIF2α and elevation of DCF-detectable ROS remained unaffected. The cycloheximde-resistant phosphorylation of eIF2α could be prevented when cells were co-treated with thiol reductants, or upon the over-expression of catalase. These data suggest that the production of ROS in Tm-treated cells does not require the presence of ER stress as a prerequisite. Furthermore, the ROS so produced could induce phosphorylation of eIF2α without the need to cause ER stress in the first place. The quenching of ROS through the use of thiol reductants, or the over-expression of catalase, had no effect on inhibition of protein synthesis in cells treated with tunicamycin. However, the extent of cell death was significantly increased. The data obtained in this study is not consistent with the idea that ROS is a downstream product of ER stress, capable of inducing more ER-stress by a feedback mechanism. Therefore, a mutually enhancing effect between ER stress and ROS may not exist. The ROS found in stressed cells may serve to extend cellular survival under the condition of continuous stress.