The multidrug-resistance modifiers verapamil, cyclosporine A and tamoxifen induce an intracellular acidification in colon carcinoma cell lines in vitro.
In this study we have investigated the effects of the multidrug-resistance (MDR) modifiers verapamil (VPM), cyclosporin A (CsA) and tamoxifen (TMX) on the intracellular pH(pHi) of four colon carcinoma-derived cell lines with low P-glycoprotein expression (CaCo-2, HT-29, SW 620 and SW 480). Addition of VPM (1 mu M), CsA (1 microgram/ml) or TMX (2 microM) in HEPES- or bicarbonate/CO2-buffered Ringer's solution was followed by dose-dependent and reversible decreases of the pHi (0.1-0.3 units) of all cell lines, as measured ratiometrically by the changes in the pH-dependent fluorescence of bis(carboxyethyl)carboxyfluorescein (BCECF). Testing the effects of the resistance modifiers on the Na+/H+ antiporter and bicarbonate trans-porters under appropriate buffer conditions and addition of inhibitors (amiloride, DIDS) revealed that the chemomodulator-induced acidification does not interfere with the function of these major pHi-regulating acid-base transporters. The induction of changes in pHi shows no correlation with MDR-reversing activity of the drugs and our data do not support the P-gp-inhibition-mediated accumulation of acidic substrates as underlying mechanism. In addition to the P-gp-directed MDR-reversal, chemomodulator-induced intracellular acidification may enhance the chemosensitivity of the cells especially under alkaline extracellular conditions, and contribute to the decreased efficacy of MDR-modifiers in acidic extracellular environments and to the chemosensitising effect of VPM in P-gp-negative cell lines.
Hamilton
G
G
Department of Surgery, AKH, University School of Medicine, Vienna, Austria.
Cosentini
E P
EP
Teleky
B
B
Koperna
T
T
Zacheri
J
J
Riegler
M
M
Feil
W
W
Schiessel
R
R
Wenzi
E
E
D020168:ATP Binding Cassette Transporter, Subfamily B, Member 1; D000230:Adenocarcinoma; D000584:Amiloride / Q000494:pharmacology; D002352:Carrier Proteins / Q000096:biosynthesis*; D002455:Cell Division / Q000187:drug effects; D002460:Cell Line; D002470:Cell Survival / Q000187:drug effects; D003110:Colonic Neoplasms; D016572:Cyclosporine / Q000494:pharmacology*; D004317:Doxorubicin / Q000633:toxicity; D004351:Drug Resistance / Q000502:physiology*; D006801:Humans; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D008562:Membrane Glycoproteins / Q000096:biosynthesis*; D013629:Tamoxifen / Q000494:pharmacology*; D014407:Tumor Cells, Cultured; D014700:Verapamil / Q000494:pharmacology*
D020168:ATP Binding Cassette Transporter, Subfamily B, Member 1; D002352:Carrier Proteins; D008562:Membrane Glycoproteins; D013629:Tamoxifen; D000584:Amiloride; D004317:Doxorubicin; D016572:Cyclosporine; D014700:Verapamil