Multidrug resistance (MDR) genes, which are ATP-binding cassette family genes, encode the cell surface glycoprotein, P-glycoprotein, which functions as an energy-dependent drug efflux pump. Two relevant human genes, PGY1 and PGY3, are located on human chromosome 7, and three relevant mouse genes, mdrla, mdrlb, and mdr2, are located on mouse chromosome 5. An LMD1 cell line was established after the transfer of a 580-kb yeast artificial chromosome (YAC) clone carrying the human MDR locus into mouse L cells; the cell line was shown to have stably integrated YAC DNA in an apparent intact form. Using LMD1 cells as the parental cell line, five vincristine-resistant sublines, designated LMD1-V50, LMD1-V100, LMD1-V200, LMD1-V500, and LMD1-V1000, were isolated by exposure to increasing concentrations of the drug. LMD1-V50, LMD1-V100, LMD1-V200, LMD1-V500, and LMD1-V1000 showed 3-, 7-, 13-, 45-, and 110-fold higher resistance to the cytotoxic effects of vincristine, respectively, than their parental counterpart, LMD1. Immunofluorescence, Western blot, and Northern blot analyses revealed that the human PGY1 gene or its product was overexpressed, accompanied by gene amplification. The human PGY3 gene was also overexpressed in the LMD1-V20, LMD1-V100, and LMD1-V1000 cell lines. Southern blot and fluorescence in situ hybridization (FISH) analyses demonstrated that although essentially the entire YAC DNA was integrated in mouse genome and amplified, the endogenous mouse mdr genes were not amplified in these drug-resistant cell lines. Similar results were obtained by the analyses of vincristine-resistant cell lines isolated from four independent subclones of LMD1 cells. Thus, in contrast to their mouse counterparts, the integrated human MDR genes retained susceptibility to both gene activation and amplification, during the selection of drug-resistant mouse cell lines. The possibility that transferred YACs may retain regulatory properties observed in the cells of origin, and may have a chromatin structure that favors augmented expression, is discussed.
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