Elevated MDR gene expression is determined by multiple mechanisms in drug resistant rat hepatoma cell lines

Resistance to cytotoxic drugs is a sever hindrance in the effective use of chemotherapy. There is therefore a great interest in determining the molecular mechanisms resulting in elevated MDR activity of drug resistant cells. We studied the molecular cause of drug resistance of rat hepatoma cell lines established by cholhicine selection. In rodents two related genes encode two major drug transporters. In accord with the increased drug efflux activity, the mRNA levels corresponding to both Abcb1 genes were increased in the drug resistant cells. In the copy numbers of Abcb1 genes we found only small increase in the drug resistant lines, and similarly, only a small increase in half life of one of the Abcb1b message was detectable in the drug resistant cells. To study the role of epigenic regulation in altered Abcb1 expression we treated the cells with histone deacetylase inhibitor. Western blots indicated a general increase of acetylated histoneH4 level in TSA treated cells and results of ChIP experiment verified the elevated level of H3K9ac in the regulatory regions of both Abcb1genes. Despite that, the mRNA levels corresponding to Abcb1a and Abcb1b changed in opposing direction; while the former decreased the latter increased upon TSA treatment. Our data indicate marked differences in the regulation of the related Abcb1 genes and opposing response in their expression to a change in histone acetylation level.