IDENTIFICATION OF POTASSIUM FLUX PATHWAYS AND THEIR ROLE IN THE CYTOTOXICITY OF ESTRAMUSTINE IN HUMAN-MALIGNANT GLIOMA, PROSTATIC-CARCINOMA AND PULMONARY-CARCINOMA CELL-LINES

Clinically-used drugs such as furosemide, bumetanide and cardiac glycosides, are modulators of transmembrane fluxes of cations. Recently, it has been suggested that the regulation of intracellular cation concentrations could be a primary target for anti-neoplastic drugs, and that the cytotoxic activity may be altered by inhibitors of cation fluxes at the level of the plasma membrane. Therefore, we investigated the mechanisms by which cations are translocated across the plasma membrane of malignant glioma (U251 MG), prostatic carcinoma (PC3) and pulmonary carcinoma (P31) cell lines. The interactions between cation flux inhibitors and the cytotoxicity of estramustine were also evaluated. Ouabain, the classical inhibitor of Na+, K(+)ATPase, markedly reduced Rb-86 (K+) influx in all three lines, indicating that this ion transport system is present in the cells. Furosemide and especially bumetanide inhibited the Rb-86 influx, indicating the presence of the Na+, K+, Cl- co-transport system. The potassium channel blocker, tetraethylammonium, but not apamin reduced the influx of Rb-86 showing that high conductance K+ channels are present, but that channels of low conductance probably do not exist in these cell lines. The Na+, K+, Cl- co-transport inhibitors furosemide and bumetanide significantly reduced cytotoxicity of estramustine in P31 sells, whereas no interaction between other K+ flux inhibitors and the anti-neoplastic drugs were detected in any of the cell lines investigated. Thus, the data show that Na+, K+, ATPase and Na+, K+, Cl- co-transport systems and K+ channels of high conductance are present in malignant glioma (U251 MG), prostatic carcinoma (PC3) and pulmonary carcinoma (P31) cell Lines, and that inhibition of the Na+, K+, Cl- co-transport system in P31 is associated with reduced cytotoxicity of estramustine. The results justify further studies evaluating the role of these cation flux pathways in terms of targets for anti-neoplastic therapy.