Abstract
Inflammation elevates intracellular Ca(2+) ([Ca(2+)]i) concentrations in airway smooth muscle (ASM). Store-operated Ca(2+) entry (SOCE) is an important source of [Ca(2+)]i mediated by stromal interaction molecule-1 (STIM1), a sarcoplasmic reticulum (SR) protein. In transducing SR Ca(2+) depletion, STIM1 aggregates to form puncta, thereby activating SOCE via interactions with a Ca(2+) release-activated Ca(2+) channel protein (Orai1) in the plasma membrane. We hypothesized that STIM1 aggregation is enhanced by inflammatory cytokines, thereby augmenting SOCE in human ASM cells. We used real-time fluorescence microscopic imaging to assess the dynamics of STIM1 aggregation and SOCE after exposure to TNF-α or IL-13 in ASM cells overexpressing yellow fluorescent protein-tagged wild-type STIM1 (WT-STIM1) and STIM1 mutants lacking the Ca(2+)-sensing EF-hand (STIM1-D76A), or lacking the cytoplasmic membrane binding site (STIM1ΔK). STIM1 aggregation was analyzed by monitoring puncta size during the SR Ca(2+) depletion induced by cyclopiazonic acid (CPA). We found that puncta size was increased in cells expressing WT-STIM1 after CPA. However, STIM1-D76A constitutively formed puncta, whereas STIM1ΔK failed to form puncta. Furthermore, cytokines increased basal WT-STIM1 puncta size, and the SOCE triggered by SR Ca(2+) depletion was increased in cells expressing WT-STIM1 or STIM1-D76A. Meanwhile, SOCE in cells expressing STIM1ΔK and STIM1 short, interfering RNA (siRNA) was decreased. Similarly, in cells overexpressing STIM1, the siRNA knockdown of Orai1 blunted SOCE. However, exposure to cytokines increased SOCE in all cells, increased basal [Ca(2+)]i, and decreased SR Ca(2+) content. These data suggest that cytokines induce a constitutive increase in STIM1 aggregation that contributes to enhanced SOCE in human ASM after inflammation. Such effects of inflammation on STIM1 aggregations may contribute to airway hyperresponsiveness.