Project Summary/Abstract Breast cancer is the second most common type of cancer diagnosed worldwide and is also the second leading cause of cancer-related deaths among women overall. While much progress has been made in the diagnosis and treatment of breast cancer, traditional chemotherapeutic treatments can lead to the development of chemotherapeutic resistance and remains a major challenge in overall patient outcomes. The activation of the Pregnane X Receptor (PXR) by numerous endo- and xeno-biotics, including several chemotherapeutic drugs, is known to regulate many of the metabolic pathways associated with drug metabolism and resistance. The long-term goal is to understand the transcriptional and metabolic targets of PXR in human breast cancers, and to apply this knowledge in their treatment in order to improve patient outcomes. The objective of this application is to elucidate the effects of PXR expression and activation on the transcriptome and phenotypic changes in human breast cancer cell lines. The central hypothesis, which is formulated on PXRs known roles in drug metabolism in healthy and pathogenic tissues as well as preliminary data, is that PXR activation increases xenobiotic metabolism via the induction of Phase I and II enzymes, as well as Phase III transporters, in addition to alterations in the cell cycle and apoptosis in breast cancer. The rationale for the proposed research is that once the transcriptional and phenotypic alterations in response to PXR activation are identified, pharmacological agents or regimes that interfere with these pathways could be developed to decrease the chemoresistance of breast cancer. The objective of this project will be accomplished by two specific aims: (1) Identify global gene expressions changes in human breast cancers associated with pregnane x receptor expression and/or transcription factor activation. An RNA-seq based approach will be used to identify changes to the transcriptome in response to endogenous and exogenous PXR expression and activation. (2) Examine the effects of PXR expression and activation in models of breast cancer on cell survival and chemoresistance. In- vitro based assays examining the effects of PXR on various aspects of the cell cycle, survival, apoptosis, and invasiveness will be utilized. This study is innovative because it shifts the focus of PXRs roles from normal liver and intestinal drug metabolism in order to fill a large void in our understanding of PXRs influence on breast cancer, and potentially in many other types of cancer as well. The proposed project is significant because it will uncover genes and metabolic pathways that are critical to the development of chemotherapeutic resistance in breast cancer, identifying new potential therapeutic targets.