Expression of the P-glycoprotein gene in multidrug-resistant Chinese hamster ovary cells.

A series of multidrug-resistant (MDR) Chinese hamster ovary (CHO) cells was established to survive in stepwise increasing concentrations of vincristine. These MDR cells contained amplified P-glycoprotein (mdr) genes. Using monoclonal antibody against the P-glycoprotein, we present here results showing that the levels of P-glycoprotein and its phosphorylation and glycosylation did not correlate with those of drug resistance. Our results suggest that overproduction of the P-glycoprotein cannot account for the differences in drug resistance in these MDR cells, and that multiple modalities are involved in multiple drug resistance in mammalian cells.