Antiproliferative effect of Ca2+ channel blockers on human epidermoid carcinoma A431 cells

The effects of Ca(2+) channel blockers on the proliferation of human epidermoid carcinoma A431 cells were investigated by microtiter tetrazolium (MTT) proliferation assay and bromodeoxyuridine (BrdU) incorporation assay. Dihydropyridine derivatives, such as amlodipine, nicardipine, and nimodipine inhibited A431 cell growth and the incorporation of BrdU into cells with IC(50) values of 20-30 microM, while verapamil, diltiazem and dihydropyridine nifedipine inhibited neither the cell growth nor BrdU incorporation at the same concentration. Though extracellular Ca(2+) is indispensable to the cell growth, an L-type Ca(2+) channel agonist, 1,4-dihydro-2,6-dimethyl-5-nitro-4-[2-(trifluoromethyl) phenyl]pyridine-3-carboxylic acid methyl ester (200 nM), did not affect the antiproliferative action of amlodipine. Thapsigargin, an inhibitor of Ca(2+)-ATPase of the endoplasmic reticulum, inhibited itself the growth of A431 cells and also showed a synergistic effect with the antiproliferative action of amlodipine. In the fluorimetric measurement of intracellular free Ca(2+) concentration in fura-2 or fluo-3 loaded A431 cells, amlodipine blunted the thapsigargin- or cyclopiazonic acid-induced Ca(2+) release from endoplasmic reticulum and the ensuing Ca(2+) influx through Ca(2+)-permeable channels. The effect on the thapsigargin-induced Ca(2+) responses could be reproduced by nicardipine and nimodipine but not by nifedipine or verapamil, lacking antiproliferative potency. These findings suggest that the intracellular Ca(2+) control system responsible for thapsigargin- and cyclopiazonic acid-sensitive endoplasmic reticulum, but not L-type Ca(2+) channels, may be modulated by amlodipine, which results in the inhibition of A431 cell growth.