ABSTRACT
Functions and regulation of selected human UDP-glucuronosyltransferases (UGT1A1, UGT1A4, UGT1A6, UGT1A9, UGT2B7, UGT2B15) are summarized. Evidence for at least two PAH-inducible UGTs (UGT1A6 and UGT1A9) is presented, which, however, are also constitutively expressed in a tissue- and cell-specific manner. These isoforms have recently been characterized to conjugate planar and bulky phenols, respectively. Using a selective RT-PCR method, UGT1A6 expression was detected in a variety of tissues (liver, kidney, lung, intestine, and pharyngeal mucosa). PAH-inducible UGTs may cooperate in the metabolism of phenolic metabolites of benzo(a)pyrene. Studies with stably expressed isoforms suggest that UGT1A9 is responsible for the formation of benzo(a)pyrene-3.6-diphenol diglucuronide, the major biliary metabolite of benzo(a)pyrene.
Subject(s)
Gene Expression Regulation, Enzymologic , Glucuronosyltransferase/genetics , Polycyclic Aromatic Hydrocarbons/pharmacology , Enzyme Induction , Glucuronosyltransferase/physiology , Humans , Polycyclic Aromatic Hydrocarbons/metabolism , Transcription, Genetic/drug effectsABSTRACT
Human colon carcinoma Caco-2 cells were used to study the induction of UDP glucuronosyltransferase (UGT) isoforms UGT1A6, UGT1A9, and UGT2B7 by aryl hydrocarbon receptor agonists and by antioxidant-type inducers with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and t-butylhydroquinone (TBHQ), respectively. Early- (PF11) and late-passage clones (TC7) of Caco-2 cells, which show low and high constitutive UGT1A6 expression, respectively, were selected. The following results were obtained: 1) In Caco-2 cells UGT activity (4-methylumbelliferone as substrate) was significantly enhanced by 10 nM TCDD or 40 to 80 microM TBHQ and 2) duplex reverse-transcription-polymerase chain reaction analysis showed for the first time that the expression of human UGT1A6, UGT1A9, and UGT2B7 was enhanced by 40 to 80 microM TBHQ; both UGT1A6 and UGT1A9 were induced by 10 nM TCDD, whereas UGT2B7 was not induced by TCDD. The results suggest that at least two human UGTs (UGT1A6 and UGT1A9) are inducible by aryl hydrocarbon receptor agonists and even more isoforms (UGT1A6, UGT1A9, and UGT2B7) are inducible by antioxidant-type inducers in Caco-2 cells.
Subject(s)
Antioxidants/pharmacology , Caco-2 Cells/enzymology , Environmental Pollutants/pharmacology , Glucuronosyltransferase/biosynthesis , Hydroquinones/pharmacology , Polychlorinated Dibenzodioxins/pharmacology , Caco-2 Cells/drug effects , Enzyme Induction , Humans , Isoenzymes/biosynthesisABSTRACT
Transcriptional regulation and function of rat and human PAH-inducible UDP-glucuronosyltransferase (UGT) isoforms have been studied. 1. At least two PAH-inducible UGT isoforms are expressed in a variety of tissues, the rat isoforms UGT1A6 and UGT1A7, and the human isoforms UGT1A6 and UGT1A9. 2. For the rat and human UGT1A6 isoforms two modes of tissue- and cell-specific regulation were found, PAH-inducible and constitutive expression. 3. Transient transfection studies, using human UGT1A6/CAT fusion constructs and colon carcinoma Caco-2 cells, revealed that PAH induction of human UGT1A6 is mediated by the Ah receptor. 4. Cell-expressed UGT isoforms were used to study their function in PAH metabolism. Rat UGT1A7 and human UGT1A9 appear to be more efficient than the corresponding UGT1A6 isoforms in catalyzing glucuronide formation of PAH phenols and diphenols. Several isoforms may act together in the formation of benzo(a)pyrene-3.6-diol diglucuronide, the major glucuronide found in rat bile. The results suggest complex modes of transcriptional regulation of PAH-inducible UGTs. They also suggest a major role of these UGT isoforms in detoxication of PAHs.
