Stable transfection of the p-glycoprotein promoter reproduces the endogenous overexpression phenotype: The role of MED-1

Cellular resistance to multiple chemotherapeutic agents is most often due to the overexpression of P-glycoprotein (Pgp). The mechanism(s) underlying Pgp overexpression had not been determined, due, in part, to a failure to reproduce the overexpression in transient transfection assays. We now report that stable transfection of a Pgp (pgp1) promoter/luciferase construct in the drug-sensitive Chinese hamster cell line DC-3F and its drug-resistant sublines reproduced the overexpression phenotype, with up to 18-fold higher activity observed in the resistant cell lines compared with DC-3F. Moreover, mutation of a pgp1 promoter element, multiple start site element downstream (MED-1), decreased transcription in drug-resistant cells without affecting activity in drug-sensitive cells. This is the first report of a Pgp promoter element differentially regulated in drug-resistant cells. Moreover, these data suggest that the regulation of Pgp transcription is modulated by chromatin structure, and that stable transfections may be more suitable for identifying promoter elements important for overexpression in drug-resistant cells.