| The biological effects of xenobiotics are greatly influenced by their rate of elimination. Xenobiotics often undergo oxidative and/or conjugative biotransformation, processes that may accelerate or delay elimination. The cytochrome P450 (CYP), UDP-glucuronosyltransferase (UGT), and sulfotransferase (SULT) families of “drug-metabolizing enzymes” (DMEs) are important contributors to xenobiotic biotransformation. In rodents, exposure to certain compounds, including drugs, simultaneously increases (induction) or decreases (suppression) the expression of multiple DMEs. While clinical evidence suggests that such coordinate regulation may occur in humans, corroborative experimental proof is limited. The main objective of this dissertation was to determine whether xenobiotic exposure elicits a coordinated, or “pleiotypic” response in human DME expression.; The bDNA Signal Amplification Assay was used to ascertain the pleiotypic DME response in primary cultures of rat hepatocytes. CYP and UGT mRNA levels were co-induced following exposure to β-napthoflavone (CYP1A1, CYP1A2, UGT1A6 and UGT1A7), phenobarbital (CYP2B1/2, UGT1A6 and UGT2B1), and pregnenolone 16α-carbonitrile (CYP3A1/2 and UGT1A2).; The bDNA assay was also used to evaluate mRNA expression in donated human tissue. CYP, UGT and SULT genes were found to be differentially expressed in liver, small intestine, kidney and lung. Common patterns in expression were evident, suggesting shared mechanisms of transcriptional regulation. It was also determined that hepatic CYP mRNA levels are rarely a good predictor of corresponding enzymatic activity.; Lastly, the coordinate induction of human DMEs was studied in vitro using primary cultures of human hepatocytes exposed to microsomal enzyme inducers. Inter-culture variability in gene expression precluded unambiguous interpretation of results. Nonetheless, cluster analysis of relative changes in DME mRNA levels indicated simultaneous induction of CYP, UGT and SULT genes expression following exposure to certain xenobiotics, including the drugs omeprazole, phenobarbital and rifampin.; To summarize, patterns in human DME expression in vivo and in vitro suggest coordinate gene regulation. Multiple CYP, UGT and SULT enzymes were co-induced in cultured human hepatocytes exposed to microsomal enzyme inducers. Additional work is needed to confirm that this experimental system accurately predicts human DME regulation in vivo . Such coordinate induction may profoundly affect the clearance of certain drugs and underlie clinically important drug-drug interactions. |
