Induction of MDR1 gene expression by anthracycline analogues in a human drug resistant leukaemia cell line

The effects of 4-demethoxydaunorubicin (idarubicin, IDA) and MX2, a new mor pholino-anthracycline, on up-regulation of the MDR1 gene in the low-level m ultidrug resistant (MDR) cell line CEM/A7R were compared at similar concent rations (IC10, IC50 and IC90) over a short time exposure (4 and 24 h). The chemosensitivity of each drug was determined by a 2-day cell growth inhibit ion assay. Compared with epirubicin (EPI), IDA and MX2 were 17- and eightfo ld more effective in the CEM/A7R line respectively. No cross-resistance to 5-FU was seen in the CEM/A7R line. Verapamil (5 mu M) and PSC 833 (1 mu M), which dramatically reversed resistance to EPI in the CEM/A7R line, had no sensitizing effect on the resistance of this line to MX2, but slightly decr eased resistance to IDA. The sensitivity to 5-FU was unchanged by these mod ulators. The induction of MDR1 mRNA expression by IDA, MX2 and 5-FU was ana lysed by Northern blotting and semiquantitatively assessed by scanning Nort hern blots on a phosphorimager. The relative level of MDR1 expression was e xpressed as a ratio of MDR1 mRNA to the internal RNA control glyceraldehyde -3-phospbate dehydrogenase (GAPDH), IDA, MX2 and 5-FU differentially up-reg ulated MDR1 mRNA in the CEM/A7R line in a dose-dependent manner. Both IDA a nd MX2 induced MDR1 expression within 4 h, 5-FU up-regulated MDR1 expressio n only when drug exposure was prolonged to 24 h. Based on MRK 16 binding, f low cytometric analysis of P-glycoprotein (Pgp) expression paralleled the i ncrease in MDR1 mRNA revels. For the three anthracyclines, the increase in MDR1 expression was stable in cells grown in the absence of drug for more t han 3 weeks after drug treatment. The induction of MDR1 expression by 5-FU was transient, associated with a rapid decrease in the increased Pgp levels which returned to baseline 72 h after the removal of 5-FU, This study demo nstrates that MDR1 expression can be induced by analogues of anthracyclies not pumped by Pgp, and that this induction appears to be stable despite a 3 -week drug-free period.