Platelet agonists induce Ca2+ transients in tumor cells by opening distinct receptor-operated channels: An effect unrelated to the presence of classical multi-drug resistance phenotype

Background - Modulation of the cytoplasmic calcium concentration is a mechanism of signal transduction regulating several biological phenomena and may also play a role in the stimulation of cell proliferation. In the present study we have investigated the effect of different platelet agonists on cytoplasmic Ca²⁺ levels in tumor cells with or without the multi-drug resistance (MDR) phenotype and the effects of verapamil on agonist induced Ca²⁺ transients and on in-vitro tumor cell growth. Methods - LoVo cells and doxorubicin-resistant LoVoDx cells, derived from a human colon adenocarcinoma, were cultured in vitro using standard methods. Cytoplasmic Ca²⁺ levels in aequorin-loaded tumor cells were determined in a Platelet Ionized Calcium Aggregometer. Results - ADP, GRGDS, PAF, collagen and thrombin were able to induce Ca²⁺ transients in both cell lines, while U46619, a thromboxane A₂ mimetic agent, PDGF and carbachol were not. Tumor cells of both cell lines became refractory to thrombin after the first addition, but remained sensitive to the other inducers. Furthermore, the calcium channel blocker verapamil significantly inhibited thrombin-induced Ca²⁺ fluxes in both LoVo cells and LoVoDx cells and had no significant effect on Ca²⁺ movements induced by the other agonists. Finally, the drug inhibited the in-vitro growth of both cell lines in a dose-dependent manner, with an effect more evident in resistant cells. Conclusions. - These data may help to explain the ability of verapamil to reverse the MDR phenotype and may contribute to identifying new mechanisms for the two-way interaction of tumors with the hemostatic system.