A 190-KILODALTON PROTEIN OVEREXPRESSED IN NON-P-GLYCOPROTEIN-CONTAINING MULTIDRUG-RESISTANT CELLS AND ITS RELATIONSHIP TO THE MRP GENE

Background: A 190k (190-kilodalton) membrane protein has been identifi ed in several multidrug-resistant (MDR) cell lines that show decreased drug accumulation without expression of P-glycoprotein. It is not cle ar whether this 190k protein is involved directly in drug efflux. Rece ntly, a gene for a putative transporter protein, MRP (multidrug resist ance-associated protein) has been sequenced and localized to chromosom e 16. The protein encoded by this gene contains a 7-amino-acid sequenc e present in the synthetic peptide used to generate the antiserum reco gnizing the 190k protein. Purpose: The study was undertaken to clarify the relationship of the 190k protein to MRP gene expression in non-P- glycoprotein-containing MDR cells of the large-cell and adenocarcinoma lung cancer lines, COR-L23 and MOR. Methods: Expression of the 190k p rotein was determined by Western blot analysis and that of the MRP gen e by polymerase chain reaction amplification of complementary DNA reve rse transcribed from RNA. Abnormalities of chromosome 16 were investig ated in chromosome spreads by fluorescence in situ hybridization. Resu lts: The amount of detectable 190k protein is closely associated with degree of drug resistance. Cell lines surviving in higher drug concent rations have greater amounts of protein, and revertant lines grown wit hout drug for up to 28 weeks show reduced expression of the protein to gether with enhanced drug sensitivity. The 190k protein appears to be one of the major proteins differentially expressed in membranes of dru g-resistant cells. The amount of MRP messenger RNA correlates closely with that of the 190k protein. The MDR cells contain amplified chromos ome 16 material with many double minutes in the large-cell lung tumor lines and an enlarged chromosome 16 in the adenocarcinoma lines. Concl usion: The 190k protein detected immunologically is likely to be the p rotein, encoded by the MRP gene, which becomes overexpressed in these cells as a consequence of chromosomal amplification and fragmentation. Implication: Though associated with drug resistance, enhanced drug ef flux, and decreased drug accumulation in cell lines, the role of this protein in clinical resistance has yet to be determined.