Increase of CYP1A1 mRNA and AHH activity by inhibitors of either protein or RNA synthesis in mouse hepatocytes in primary culture.

Regulation of CYP1A1 gene expression in mouse hepatocytes from C57/BL6 strain in primary culture was investigated with respect to aryl hydrocarbon hydroxylase (AHH) activity and mRNA levels. Small amounts of the CYP1A1 gene transcripts were detected without the presence of any known AHH inducers but after medium change. Maximal level of expression was approximately 6-9 h after the procedure, followed by a decrease to an undetectable level 24 h later. After temporary treatment of hepatocytes for 10 h with cycloheximide, an inhibitor of protein synthesis, AHH activity measured 14 h later was at a normal low level, although medium-change-associated CYP1A1 mRNA were increased by cycloheximide treatment. However, if cells were treated with either actinomycin D, alpha-amanitin or cordycepin, which are inhibitors of RNA synthesis, after exposure to cycloheximide, prominent induction of AHH activity was observed, the levels being almost equal to those for hepatocytes treated with benz[a]anthracene, a potent AHH inducer. With the same experimental protocol benz[a]anthracene-induced AHH activity was enhanced approximately 4-fold. After washing out cycloheximide and/or benz[a]anthracene, the decrease in the mRNA amounts was delayed in the presence of actinomycin D and half amounts were found even 12 h later; while without actinomycin D they reduced with a half-life of 3-4 h. The observations indicate that CYP1A1 gene expression might be regulated at the post-transcriptional level with compensatory mRNA stabilization under conditions of blocked further production, resulting in elevation of AHH activity.