Ionic currents in multidrug resistant K562 human leukemic cells.

In this study, the expression and functional characterization of currents through the CFTR (cystic fibrosis transmembrane regulator) and ORCC (outwardly rectifying chloride channels) were determined in wild-type K562 chronic human leukemia cells (K562-WT) and in its resistant counterpart, the vincristine resistant cell line (K562-Vinc). Expression of the CFTR and MDR1 (multidrug resistant) gene products was determined by a semi-quantitative RT-PCR protocol. The amplified products in K562-WT and K562-Vinc showed two bands corresponding to CFTR and MDR1. MDR1 mRNA increased by 20-fold in K562-Vinc whereas no change in CFTR mRNA levels was observed. CFTR and ORCC channel activity were measured with a whole cell configuration of the patch clamp technique. Forskolin (40 microM n activator of adenylate cyclase, added to the extracellular side increased the current in both cell lines. A fraction of the activated whole cell currents was inhibited by 500 microM 4,4-diisothiocyanatostilbene-2,2-disulfonic acid (DIDS) and subsequent addition of 500 microM diphenylamine-2-carboxylate (DPC plus DIDS) further inhibited the remaining currents. The levels of forskolin-activated currents and subsequent blockade were similar in both cell lines. The effect of forskolin was prevented in cells previously exposed to 500 microM DPC. The effects of DIDS and DPC on the forskolin-activated whole cell currents support the idea that both CFTR and ORCC are generating a significant fraction of these currents with DIDS inhibiting ORCC currents and DPC inhibiting CFTR currents when the blockers are added one after another to the extracellular side. Finally, we show that exposure of K562 cells to vincristine which results in the over expression of MDR1 is not accompanied by a significant down regulation of CFTR as in other cells.