Basis for the loss of aryl hydrocarbon receptor gene expression in clones of a mouse hepatoma cell line.

Rare benzo[a]pyrene-resistant clones were previously isolated from the mouse hepatoma cell line, Hepa-1 (Hepa1c1c7), and shown to be deficient in induction of CYP1A1 mRNA by ligands for the aryl hydrocarbon receptor (AHR). Clones belonging to complementation group B were shown to have reduced levels of ligand binding to AHR. It is shown here that all 15 independently derived B clones analyzed had much reduced levels of AHR mRNA, but in each case, the mRNA was normal in size. Infection of B clones with a retroviral expression vector for AHR restores CYP1A1 inducibility (although viral AHR expression is progressively silenced and CYP1A1 expression progressively diminishes as the cells are maintained in culture). Treatment of the B clones with the histone deacetylase inhibitors sodium butyrate or trichostatin A restores AHR expression and also restores CYP1A1 inducibility to nearly 100% of the cells in the treated cultures. Fusion of a representative B clone with a rat hepatoma cell line restores expression to the mouse AHR gene encoded by the B clone's genome. These results demonstrate that the loss of CYP1A1 inducibility in B clones is probably totally ascribable to their reduced levels of AHR and that the clones are most probably not mutated in the AHR gene but are deficient in its expression. The evidence suggests that the reduction in expression of mRNA encoded by the endogenous AHR gene in the B clones is not due to an epigenetic alteration in chromatin structure but that the clones are probably defective either in a transcription factor for the AHR gene or in a protein required for generating an open chromatin configuration over the gene.