MRP GENE OVEREXPRESSION IN A HUMAN DOXORUBICIN-RESISTANT SCLC CELL-LINE - ALTERATIONS IN CELLULAR PHARMACOKINETICS AND IN PATTERN OF CROSS-RESISTANCE

The development of non-P-glycoprotein-mediated multidrug resistance is a frequent event among rung-cancer cell lines. In an attempt to under stand the underlying mechanisms of this phenotype, we have selected a multi-drug-resistant subline (POGB/DX) in vitro for doxorubicin resist ance. The original cell line (POGB) was established in vitro from a no n-treated patient with a small-cell lung cancer. POGB/DX cells were cr oss-resistant to other drugs, associated with MDR phenotype. In contra st, they were not resistant to taxol, camptothecin or melphalan, but w ere instead hypersensitive to 5-fluorouracil. Although expression of t he mdr-l gene was not detected in POGB/DX cells, cellular pharmacokine tics showed a reduced drug accumulation and altered intracellular loca lization in the POGB/DX cell line. This defect in drug accumulation wa s associated with overexpression and amplification of the MRP gene. in terestingly, verapamil, a known modulator of P-glycoprotein function, was able to reverse drug resistance and to increase drug accumulation. In Northern-blot analysis no differences in expression of topoisomera se I and II (alpha and beta), DNA polymerase beta, or HSP70 and HSP60 genes were observed between POGB and POGB/DX. Coupled to lack of chang es in expression of known resistance factors, overexpression of MRP an d modulation by verapamil strongly support a role for this gene produc t in the development of drug resistance in this SCLC cell system. This study provides evidence that (a) altered cellular pharmacokinetics is related to MRP expression; (b) MRP-mediated phenotype is characterize d by a specific pattern of cross-resistance, which does not involve ta xol; and (c) verapamil may be effective in modulating the function of the MRP gene product. (C) 1995 Wiley-Liss, Inc.