Protein disulfide isomerase (PDI) is a redox responsive enzyme mainly involved in the signalling of the endoplasmic reticulum stress and Unfolded Protein Response (UPR). Accumulating evidence indicates that PDI is implicated in the regulation of several biological processes expanding from the fine tuning of the cellular redox state and the balance between pro-survival and pro-death pathways to the communication between endoplasmic reticulum and mitochondria, the control of the process of antigen presentation and the coordination of the immune responses. In this study, the role of several estrogen-like compounds including tamoxifen (TMX), raloxifene (RLX), anastrazole (ANA), B-estradiol-17 (E2), and dexamethasone (DEX) and DNA-damaging agents including deferoxamine (DFO), Interferon-y (IFN-y), etoposide (ETOP) on modulating oxidative stress and their consequences on PDI level and activity have been investigated in breast cancer cell lines. PDI was silenced in MCF-7 and MDA-MB-231 under diverse oxidative stress to assess the role of PDI on regulating different physiological functions in which PDI is known to participate in such as reactive oxygen species (ROS), mitochondrial membrane disruption, glutathione homeostasis, classical and non-classical MHC-I antigen presentation, ATP production, CD44 levels, and Ki-67 levels. The association between PDI and these physiological functions were followed in MCF-7 and MDA-MB-231 cells to investigate the role of the interaction between PDI and ERa in regulating these physiological functions. The involvement of PDI as a transcriptional cofactor in the regulation of gene expression of genes targeted by ERa, p53 and NF-kB led to the hypothesis that silencing of PDI gene expression would unravel the pathways that are modulated by these transcription factors through the participation of PDI. To test this hypothesis RNA-sequencing (RNA-seq) was carried out in PDI-silenced MCF-7 and MDA-MB-231 breast cancer cells treated with IFN-y or ETOP to create diverse oxidative stress and the differentially expressed genes under these conditions were investigated. The results provide evidence to support the view that PDI plays a pro-apoptotic role in MCF-7 cells and a pro-survival role in MDA-MB-231 cells. In addition, ATP production is inhibited in the presence of PDI in MCF-7 cells and it is induced in MDA-MB-231 cells. HLA-G surface level is upregulated in both MCF-7 and MDA-MB-231 cells in the presence of PDI. RNA-seq results suggested that PDI is involved in regulating the transcription of p53 and estrogen receptor signalling pathways in MCF-7 cells as genes involved in these pathways is upregulated in PDI-silenced MCF-7 cells. Finally, RNA-seq results indicated that PDI regulate MHC-I classical at the transcriptional level, in which PDI downregulated the transcription of genes that are involved in the classical MHC-I antigen presentation in MCF-7 and MDA-MB-231 cells.