OVEREXPRESSION OF P-GLYCOPROTEIN BUT NOT ITS MESSENGER-RNA IN MULTIDRUG-RESISTANT CELLS SELECTED WITH HYDROXYRUBICIN

Previous studies have revealed that cultured cells treated with lipoph ilic natural products containing aromatic rings and basic amino group usually yielded multidrug resistant (MDR) variants. These MDR cells ov erexpress P-glycoprotein (P-gp), most often due to gene amplification or transcriptional activation of mdr/P-gp genes. Doxorubicin (Dox) is an anthracycline that belongs to this group of compounds. To explore t he possible resistance mechanism(s) to anthracyclines that do nor invo lve P-gp, we use a Dox analog, hydroxyrubicin (HyR) or WP159, which co ntains a C3' hydroxy group in replacement of the amino group in the su gar moiety of Dox thereby reducing basicity and eliminating positive c harge in the parental compound to establish HyR-resistant cell lines. These resistant cells displayed the MDR phenotype and overexpressed P- gp as analyzed by Western blot analyses and immunohistochemical staini ng using two different anti-P-gp antibodies. Strikingly, the levels of P-gp mRNA in the majority of these MDR cells remained comparable to t hose in the drug-sensitive counterparts by slot blot hybridization. Th ese results implicate that the basic center of the selecting agent is a critical determinant for generating diverse MDR variants, and that H yR may have a posttranscriptional effect on P-gp biosynthesis. This is the first report suggesting that cultured cells exposed to a particul ar selecting agent may give rise to particular subtype of MDR variants .