There is significant human exposure to polycyclic aromatic hydrocarbons (PAHs), many of which are bioactivated by the cytochrome P450 (P450) 1A family of enzymes to metabolites that are capable of covalently binding to DNA, a critical step in the initation of carcinogenesis. We reported earlier that exposure of rats to 3-methylcholanthrene (MC) causes sustained induction of hepatic cytochrome P4501A expression for up to 45 days. Here, we tested the hypothesis that MC elicits persistent induction of other genes that are regulated by the Ah receptor (AHR). Female Sprague-Dawley rats were treated with MC (100 μmol/kg) ip once daily for 4 days, and gene expression patterns were investigated using total liver RNA isolated from animals at 1, 15, and 28 days after MC withdrawal. Gene expression was studied by cDNA microarrray analyses using 4608 unique clones from liver-derived expressed sequence tag (EST) libraries fortified with clones of known liver genes representing ∼4000 genes. Several phase I (P4501A1, -1A2) and phase II [e.g., glutathione-S- transferase (GST)-M1, UDP-glucuronosyl transferases (UGT)] genes were persistently induced (3-10-fold) by MC for 15-28 days. The persistent induction of P4501A1 gene expression was confirmed by real time reverse transcriptase polymerase chain reaction (RT-PCR) experiments. MC also elicited a 5-fold persistent augmentation of acute phase genes such as orosomucoid 1 and α-1-acid glycoprotein (AGP), and this was accompanied by sustained liver damage and inflammation in the MC-exposed rats. In conclusion, our results strongly suggest that sustained induction of P4501A1 by MC is accompanied by persistent expression of other genes belonging to the Ah gene battery, as well as certain other genes involved in toxic responses. Elucidating the mechanisms of persistent induction of P4501A1 and other genes by MC might lead to a better understanding of the mechanisms of toxicity mediated by PAHs.
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