VOLUME-SENSITIVE CHLORIDE CURRENTS IN 4 EPITHELIAL-CELL LINES ARE NOTDIRECTLY CORRELATED TO THE EXPRESSION OF THE MDR-1 GENE

It has been shown recently that heterologous expression of human MDR-1 gene, which is responsible for multidrug resistance during cancer the rapy, causes appearance of volume-sensitive Cl- currents, thus suggest ing that the product of the MDR-1 gene (the P-glycoprotein) has a Cl- channel activity (Valverde, M. A., Diaz, M., Sepulveda, M. A., Gill, D . R., Hyde, S. C., and Higgins, C, F. (1992) Nature 355, 830-833). In the present work, we have tested four epithelial cell lines both for t he expression of MDR-1 gene and for the presence of volume-sensitive C l- currents. LoVo/H and LoVo/Dx cells derive from a human colon adenoc arcinoma, the latter cell line being resistant to high concentrations of the antitumoral drug doxorubicin. 9HTEo- cells were obtained by tra nsformation of human tracheal epithelium. The 9HTEo-/Dx cell line was established from these cells by selection in doxorubicin. As expected, higher levels of P-glycoprotein expression were detected in LoVo/Dx a nd 9HTEo-/Dx by means of reverse transcriptase polymerase chain reacti on technique, indirect immunofluorescence, and Western immunoblot assa ys. In contrast with these data, the size of swelling-induced Cl- curr ent was the same in the sensitive cell line and in its drug-resistant counterpart. Actually, the Cl- conductance of 9HTEo- and 9HTEo-/Dx was 4-fold higher than that of either LoVo/H or LoVo/Dx cells. This indic ates that the amplitude of this conductance is not directly related to the expression of the MDR-1 gene.