Aberrant CYP1A1 Induction: Discrepancy of CYP1A1 mRNA and Aryl Hydrocarbon Hydroxylase Activity in Mutant Cells of Mouse Hepatoma Line, Hepa‐1

We have isolated new benzo[α]pyrene‐resistant clones, cl‐21 and cl‐32, of the mouse hepatoma line, Hepa‐1. CYP1A1‐dependent aryl hydrocarbon hydroxylase activity is not inducible by 2,3,7,8‐tetrachlorodibenzo‐p‐dioxin or 3‐methylcholanthrene in these two cell lines. However, mRNA of CYP1A1 is inducible in cl‐21 and cl‐32 cells, as in the wild‐type cells, in spite of an undetectable level of cytosolic Ah receptor. The cl‐21 cDNA of Cypla‐1 was found to have a single mutation leading to an amino acid substitution from Leu (118) to Arg (118). However, the CYP1A1 protein band was not detected on Western immunoblots. The cDNA of cl‐32 was found to have a single mutation leading to an amino acid change from Arg (359) to Trp (359). The presence of the mature protein in cl‐32 was confirmed by Western blot analysis. Somatic cell hybridization experiments demonstrated that the phenotype of cl‐21 and cl‐32 is recessive and that these clones belong to the same complementation group. These data suggest that there may be a non‐Ah receptor‐mediated mechanism of CYP1A1 induction.