The arylamide 2-acetylaminofluorene (AAF) is a powerful carcinogen displaying a marked promoting activity. also known to regulate expression of liver detoxifying proteins. In this study we identified CYP3A23. a major inducible cytochrome P-450 (CYP) isoform, as an AAF target in hepatocytes. Indeed. exposure to AAF of primary rat hepatocytes resulted in a marked up-regulation of CYP3A23 expression at both mRNA and protein levels. Using CYP3A23 reporter gene constructs. we further demonstrated that AAF activated the CYP3A23 Direct Repeat 3 (DR3) promoter element interacting with the nuclear pregnane X receptor (PXR). Moreover, the PXR antagonist ecteinascidin-743 fully suppressed AAF-related CYP3A23 induction. Low doses of AAF inhibiting DNA synthesis in hepatocytes however failed to trigger PXR-related CYP3A23 induction and PXR-negative epithelial liver cells remained sensitive to the mito-inhibitory effects of AAF. Such data indicate that AAF up-regulates CYP3A23 through PXR activation but does not require PXR for exerting its carcinogenic promoting properties based on inhibition of cell growth. (C) 2002 Elsevier Science (USA). All rights reserved.