Dominant Role of Orai1 with STIM1 on the Cytosolic Entry and Cytotoxicity of Lead Ions

Pb2+ ions cause severe damages to living cells. In particular, our previous study showed that the Orai-STIM1 (stromal interacting protein 1)-formed store-operated Ca2+ channels (SOCs) allow Pb2+ entry. In relation to this, the present study investigates the molecular gating mechanism of Pb2+ entry by Orai1 with STIM1, as well as the resulting cytotoxicity on human embryonic kidney HEK293 cells. The store-operated Ca2+ entry (SOCE, activity of SOCs) and Pb2+ entry were measured using the fura-2 imaging method and indo-1 quenching strategy, as well as through an atomic absorption spectrophotometer. The results of RT-PCR, Western blot, fast confocal, and fluorescent lifetime imaging microscopy indicated the endogenous expression of Orai1 and STIM1 in HEK cells and the functional interaction between these two proteins during SOCE. Both SOCE and Pb2+ entry largely increased when Orai1 and STIM1 were overexpressed (3- and 1.64-folds, respectively) compared with nonfluorescent cells, and they were significantly attenuated when the E106Q mutation of Orail with STIM1 was cotransfected (6- and 2.25-folds decrease, respectively) compared with Orai1-STIM1 coexpressed cells. The ion gating for Pb2+ could be governed by the E106 region of Orai1. After sorting and subsequent cultures, the Orai1-STIM1 positive expressed cells behaved more sensitively to Pb2+ than the Orai1-STIM1 negative cells. In summary, the data suggest that Orai1, together with STIM1, plays a critical role in Pb2+ entry and the toxicity of Pb2+.