AMPLIFICATION AND OVEREXPRESSION OF THE MOUSE MDR 1A GENE IN 9 INDEPENDENTLY DERIVED MULTIDRUG-RESISTANT SEWA MURINE CELL-LINES

Many different drugs may be used in selecting cells for multidrug resi stance (MDR). Enhanced expression and/or gene amplification is known t o cause over-production of membrane-bound 170,000 P-glycoproteins, res ponsible for the MDR. In rodents, the P-glycoproteins are encoded by a small gene family: mdr 1a, mdr 1b, and mdr2 To evaluate the relations hip between the pattern of MDR and the selecting drug, nine MDR sublin es were independently selected from a sensitive mouse tumor cell line, SEWATC13K, using three different drugs. Each MDR subline displayed am plification of one or more of the three mdr genes, but only one, mdr 1 a, was consistently overexpressed. Thus, our results indicate that the pattern of mdr gene amplification and overexpression is independent o f the selective agent. Furthermore, in four of the MDR sublines, where all three mdr genes had been originally amplified, pulsed field gel e lectrophoresis (PFGE) revealed that amplification of mdr 1a, only, was a second step of gene amplification. In addition, the gene for the ca lcium-binding protein, sorcin, was coamplified in eight of the nine MD R sublines. The sorcin gene was overexpressed in seven of these eight sublines. Finally, hybridizations with a probe homologous with a putat ive region of RFLP (restriction fragment length polymorphism), indicat ed that the amplified sequences originate from one or the other of the two homologous chromosomes with no preference.