In this report we characterized the transcriptional regulation of the rat mdrlb gene by xenobiotics. The expression of this gene was increased in primary rat hepatocytes and in the H4-ll-E hepatoma cell line by exposure t o carcinogens such as aflatoxin B1, N-acetoxy-2-acetylaminofluorene, and methyl methanesulfonate.
Nuclear run-on experiments indicated that the higher steady-state levels o f mdrlb mRNA were due to an increase in transcription. The 5'-flanking region of the rndrlb gene was isolated, sequenced, and functionally characterized in transient and stable transfection assays. A single transcription start site was identified for this gene; no alternate start sites were used after induction with aftatoxin B1. Deletion analysis of this promoter demonstrated that the sequence between nt -214 and -178 was critical for basal promoter activity. This region did not contain any consensus-binding sites for previously identified transcription factors. A negative regulatory region was also identified between nt -940 and -250. No specific carcinogen-responsive element was identified; the xenobiotic response required a large part o f the promoter. These data suggest that the carcinogen induction of mdrlb expression is mediated through sequences that overlap or that are identical t o the basal promoter element.