Differential roles of STIM1, STIM2 and Orai1 in the control of cell proliferation and SOCE amplitude in HEK293 cells.

Orai1, together with STIM1 and STIM2, constitutes the molecular basis for store-operated calcium entry (SOCE) and we have investigated their role in cell proliferation and cell cycle progression in HEK293 cells. 48-h serum deprival, and a 24-h treatment with 1 mM hydroxyurea or with 10 microM RO-3306--a cyclin-dependent kinase 1 inhibitor--induced cell cycle block in G1, S and G2/M, respectively. SOCE amplitude, monitored in whole-cell voltage clamped cells, was markedly reduced (60-70%) in all conditions, with full reversibility within 4h. Silencing of Orai and STIM1 using siRNA resulted in a large inhibition of SOCE (70-80%) whereas siSTIM2 had a smaller but significant effect (30%). However, the cell population doubling time was not affected in siSTIM1 cells (18 h, the same as in control cells) but was increased in both siOrai1 cells (29 h) and in siSTIM2 (23 h) even when combined with siSTIM1. This suggests that STIM1 plays no role in cell proliferation in HEK293 cells while STIM2 is involved in both SOCE and cell proliferation in these cells. Finally, the cell cycle block induced SOCE inhibition was associated with reduced Orai1 expression with full recovery within 4h, whereas the expression of STIM1 and STIM2 remained unaltered. These observations reveal a tight relation between cell proliferation, calcium entry and Orai1 expression in HEK293 cells.