Functional Analysis of Nuclear Receptors

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Members of the superfamily of nuclear receptors are evolutionarily-related DNA-binding transcription factors. The focus of the current research proposal is on the constitutive androstane receptor (CAR; gene designation NR1I3), which regulates the expression of specific genes involved not only in glucose homeostasis, lipid metabolism, and bone homeostasis, but also transport, bioactivation, and detoxification of xenobiotics (also referred to as foreign chemicals; e.g. drugs, herbal medicines, and environmental pollutants) and endogenous substances (e.g. hormones, vitamins, and fatty acids). The longer-term objective of my research program is to enhance our understanding of the various molecular and cellular factors that regulate the function and expression of nuclear receptors (e.g. CAR) involved in controlling the expression of genes that play an important role in the transport, bioactivation, and detoxification of xenobiotics and endogenous substances. The specific short-term aims are to: 1) identify microRNAs in regulating the expression and function of the wild-type isoform of human CAR (hCAR-WT); and 2) delineate the function of specific hCAR splice variants (e.g. hCAR-SV22 and hCAR-SV26) and determine whether the function of hCAR isoforms is subject to regulation by other nuclear receptors. Under Aim #1, the experimental issues to be addressed will include: 1) the effect of microRNA on expression and function of hCAR-WT and the consequence on hCAR-WT target gene expression; 2) the consequences of microRNA regulation of retinoic X receptor-alpha (RXR-alpha) on hCAR transcriptional activity; and 3) the effect of ligands of hCAR-WT on expression of microRNAs, including the ones that regulate the expression and function of hCAR-WT. Under Aim #2, the experimental issues are to: 1) to characterize the functionality of specific hCAR splice variants (e.g. hCAR-SV22 and hCAR-SV26); and 2) determine whether there is functional interaction (i.e. cross-talk) between the various hCAR isoforms (e.g. hCAR-WT, hCAR-SV23, hCAR-SV24) and other nuclear receptors, such as small heterodimer partner (SHP) and liver X receptor-alpha (LXR-alpha). Our proposed research program will enhance our fundamental understanding of the influence of epigenetics on the expression and function of hCAR, the functionality of the various splice variants of human CAR, and whether they are subject to regulation by other nuclear receptors. The information generated will help us to predict and explain inter-individual differences in how we respond to endogenous substances and xenobiotics, including drugs, and increase our understanding of the various factors that regulate hCAR expression and function.