Differential overexpression of three mdr gene family members in multidrug-resistant J774.2 mouse cells. Evidence that distinct P-glycoprotein precursors are encoded by unique mdr genes

A hallmark of the multidrug-resistant phenotype is the overproduction of a family of 130-180-kDa integral membrane phosphoglycoproteins collectively called P-glycoprotein. Gene-specific hybridization probes were derived from three classes of mouse P-glycoprotein cDNAs. These probes revealed the differential amplification and/or transcriptional activation of three distinct but closely related mdr genes (mdr1a, mdr1b, and mdr2) in independently selected multidrug-resistant J774.2 mouse cell lines. Overexpression of mdr1a and mdr1b was found to correlate, in general, with the differential overproduction of either a 120- or 125-kDa P-glycoprotein precursor, respectively. This same correlation was observed in a single cell line during the course of stepwise selection for resistance to vinblastine in which a switch in gene expression from mdr1b to mdr1a resulted in a switch from the 125- to 120-kDa P-glycoprotein precursor. These findings suggest that differential overexpression of distinct mdr genes which encode unique P-glycoprotein isoforms is a possible mechanism for generating diversity in the multidrug-resistant phenotype.