This inhibitor (10 μM) prevented completely the increase of [Ca++ i caused by OUA (Figure 2c), while the L-type Ca++ channel blocker nifedipine (Nif) (10 μM) was ineffective (Figure 2c). These results were obtained with ouabain either 500 nM or 100 μM, suggesting that also at low concentration OUA impairs NCX, with the result of Ca++ entry in the cells. NCX promotes cell survival Cell death was evaluated by detection of trypan blue-excluding cells and of subG1 events in U937 cells pretreated

with KBR (10 μM) and then with OUA for 24 h. In particular, NCX learn more inhibition by KBR of U937 cells exposed to OUA 100 nM caused a pronounced increase of cell death (66±7% of subG1 events and 20±15% of trypan blue-excluding cells) in comparison with cells treated only with OUA (20±3% of subG1 events and 80±5% of trypan blue-excluding cells) (Figure 3a,b). Nifedipine (10 μM) did not modify these parameters in comparison with OUA treated cells.

Under the same conditions, neither the inhibitors nor DMSO affected cell viability (Figure 3a,b). Monensin (Mon) is a Na+ ionophore which causes the entry of Ca++ through NCX (L.D.R. unpublished results) [32]. We selected the concentration 5 μM of this drug because it activates a survival pathway in U937 cells resulting in 20±3% of subG1 events and 78±3% of trypan blue-excluding cells (L.D.R. unpublished results). Also in this case the inhibition of NCX by KBR brought upon a pronounced Crizotinib increase of U937 cell death (63±8% of subG1 events and 22±5% of trypan blue-excluding cells) (Figure 3c,d). Tunicamycin (TN) is an ER stressor, which does not impair NCX. At the concentration 1 μM it activates a survival pathway in U937 cells [33], Adenosine triphosphate which

was not affected by KBR (Figure 3c,d). Figure 3 Survival of U937 cells treated with OUA depends on the activity of NCX. U937 cells were exposed or not to KBR (10 μM) or to Nifedipine (10 μM) or to DMSO for 30 min and then to OUA 100 nM or again to DMSO for 24 h. (a) Cells were fixed and stained with propidium iodide; subG1 events in the cell cycle were evaluated under cytofluorimetry. (b) a portion of unfixed cells cells were counted in a hemocytometer as excluding and not excluding trypan blue. Viability was obtained by calculating live (trypan blue-excluding) cells as a percentage of all counted cells. The reported values represent the means and the error bars the S.D. of the percentage of live cells (trypan blue-excluding) or subG1 events of four independent experiments. Assessment of cell survival was investigated and statistically significant differences (P<0.01) were found between the data obtained in OUA and in (KBR + OUA) treated cells. (c, d) U937 cells were pretreated with KBR (10 μM) for 30 min and then exposed to Monensin (3 μM) or Tunicamycin (1 μM) for 24 h. The reported values represent the means and the error bars the SD of the percentage of live cells (trypan blue-excluding) or of subG1 events of four independent experiments.