Unstable aryl hydrocarbon hydroxylase-deficient variants of a rat hepatoma line

The H-4-II-E-C3 line has high aryl hydrocarbon hydroxylase (AHH) activity. It was also found to be very sensitive to benzo(a)pyrene (BP) toxicity, and a single-step selection procedure for isolating BP-resistant (BPr) clones was designed. All the BPr variants tested had reduced AHH activities under both inducing and noninducing conditions. As H-4-II-E-C3 was propagated, it gradually declined in AHH activity and increased in cloning efficiency in BP. The spontaneous rate of origin of the BPr variants was approximately 3 x 10(-5) events per cell generation. However, the variants gradually increased in AHH activity and decreased in cloning efficiency in BP as they were propagated. Partially resistant variants also generated, at an appreciable rate, cells with even greater degrees of resistance to BP. The instabilities of the wild-type and variant cultures suggest that the variants arose by an epigenetic mechanism or by changes in gene dosage, rather than by point mutations.