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⁺.