Verapamil inhibits to different extents agonist-induced Ca2+ transients in human tumor cells and in vitro tumor cell growth

Platelet agonists are known to contribute to the regulation of cytoplasmic Ca2+ levels in tumor cells and this property could be relevant in the stimulation of cell proliferation. In the present study we investigated the ability of ADP, collagen and thrombin to increase cytoplasmic Ca2+ levels in different human tumor cell lines (mesothelioma, DND-1A melanoma, HeLa uterine carcinoma) and we analyzed the effect of the calcium channel blocker verapamil on Ca2+ fluxes and on in vitro tumor cell growth. ADP was able to induce a transient increase in the cytoplasmic Ca2+ concentration in tumor cells from all lines; collagen showed this effect in mesothelioma cells and in HeLa cells, and thrombin was effective only in mesothelioma cells. Verapamil inhibited Ca2+ fluxes induced by the effective agonists in a dose-dependent manner. Values of IC50 for inhibition of ADP-induced Ca2+ transients were 63.5 microM in mesothelioma cells, 97.3 microM in DND-1A cells and 93.5 microM in HeLa cells, while those for inhibition of collagen-induced Ca2+ movements were slightly higher (170.2 microM in mesothelioma cells and 112.3 microM in HeLa cells) and the value of IC50 for inhibition of thrombin-induced Ca2+ fluxes (evaluated only in mesothelioma cells) was lower (22.5 microM). The drug dose-dependently also inhibited the in vitro growth of tumor cells; values of IC50 for growth inhibition were 21.8 microM in mesothelioma cells, 9.1 microM in DND-1A cells and 6.4 microM in HeLa cells, suggesting that the antiproliferative activity of verapamil was partly Ca(2+)-independent. These data may be of interest to elucidate the mechanisms of the two-way interactions of tumors with the hemostatic system and may help to identify new pharmacologic strategies for their control.