The goal of this study was to compare changes in gene expression induced by exposure to different carcinogens and to anchor these changes to the induced levels of toxicity and mutagenesis. The human TK6 lymphoblastoid cell line was used as an in vitro model system, and reactive metabolites of two human carcinogens, benzo[a]pyrene and 4-aminobiphenyl, were used as model compounds. We first determined the toxicity of the model compounds N-hydroxy-4- acetylaminobiphenyl (N-OH-AABP) and benzo[a]pyrene diol epoxide (BPDE) in TK6 cells. BPDE was about 1000-fold more toxic and mutagenic than N-OH-AABP in TK6 cells on a molar basis. We next treated cells with three doses of each compound that resulted in low, medium, and high toxicities (5, 15, and 40%) and harvested cells at different times after exposure. Using comparable levels of toxicity as the phenotypic anchor, we compared the patterns of gene expression induced by each reactive metabolite using printed cDNA microarrays comprising ∼18 000 human gene/EST sequences. The microarray data from the N-OH-AABP and BPDE treatment groups were compared using self-organizing map clustering algorithms, as well as a statistical regression modeling approach. While subsets of genes indicative of a generalized stress response [Hsp 40 homologue (DNAJ), Hsp70, Hsp105, and Hsp 125] were detected after exposure to both compounds at all concentrations, there were also many differentially regulated genes, including phase I xenobiotic metabolism [e.g., glutathione transferase ω (GSTTLp28) and antioxidant enzymes (Apxl)]. Other differentially regulated genes included those encoding proteins involved in all major DNA repair pathways, including excision repair (e.g., ERCC5), mismatch repair (e.g., MLH3), damage specific DNA binding protein (e.g., DDB2), and cisplatin resistance-associated overexpressed protein (LUC7A, CRA). Differences in the transcriptional response of TK6 cells to N-OH-AABP or BPDE exposure may explain the dramatic differences in the toxicity and mutagenicity of these human carcinogens.
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