DEVELOPMENTAL REGULATION OF THE DRUG-PROCESSING GENOME IN MOUSE LIVER

Abstract

Despite the recent progress in understanding the expression patterns and regulatory mechanisms of drug-processing genes, namely phase-I and -II drug metabolizing enzymes and transporters in adults, very little is known of the alterations of these genes during liver development. Therefore, newborns and children are potentially at a higher risk of adverse drug reactions. The purpose of my dissertation is to characterize the expression and regulatory mechanisms of the drug-processing genes during postnatal liver maturation. The present study integrated various research technologies, including genetically-engineered mice, messenger RNA and protein assays, ChIP-on-chip, ChIP-Seq, transcription-factor binding assays, LC-MS/MS, and bioinformatics analysis. Cluster analysis demonstrated that the ontogenic expression of 82 drug-processing genes separate into 4 distinct patterns: perinatal enriched, early-adolescent enriched, late-adolescent enriched, and adult enriched. Critical nuclear receptors, including the xenobiotic sensor pregnane X receptor (PXR, N1I2), and the bile-acid sensor farnesoid X receptor (FXR, NR1H4), are crucial in regulating the expression of drug-processing genes during liver development. Initiation of bile-acid signaling, mediated largely via FXR, is a hallmark of the neonatal induction of major liver transporters involved in the enterohepatic circulation of bile acids, whereas PXR is more important for the induction of xenobiotic-processing genes in adolescent and adult period. Because the accessibility of transcription factors to the target genes is determined by chromatin epigenetic mechanisms, I have also determined the correlations between the expression of drug-processing genes and distinct chromatin epigenetic marks, and identified that histone H3 lysine 4 di-methylation (H3K4Me2) appeared to be the choice of nature to induce numerous drug-processing genes during postnatal liver development. In conclusion, in the present dissertation, I have performed a systemic characterization of critical drug-processing genes and transcription factors during postnatal liver maturation. I have demonstrated that the developmental regulation of drug metabolism and transport is a sequential event associated with changes of chromatin epigenetic signatures, which set a permissive environment for ligand-activated nuclear receptors to gain access to the target genes prior to transcription initiation. The current work has generated basic knowledge that will serve as a foundation for further understanding of pediatric pharmacology and toxicology in humans.