Stromal Interacting Molecule 1 Signaling and Lung Vascular Barrier Function

Stromal interacting molecule 1 (STIM1) regulates store-operated Ca2+ entry (SOCE) in endothelial cells (ECs). Here, we show that STIM1 expression in ECs is increased during sepsis and thus contributes to hyper-permeability. LPS induced STIM1 mRNA and protein expression in human and mouse lung ECs. The induced STIM1 expression was associated with augmented SOCE as well as permeability increase in both in vitro and in vivo models. Since activation of both NF-κB and p38 MAPK signaling pathways, downstream of TLR4, amplify vascular inflammation, we studied the influence of these two pathways on LPS-induced STIM1 expression. Inhibition of either NF-κB or p38 MAPK activation by pharmacological agents prevented LPS-induced STIM1 expression. Silencing of the NF-κB proteins (p65/RelA or p50/NF-κB1) or the p38 MAPK isoform p38α prevented LPS-induced STIM1 expression and increased SOCE in ECs. In support of these findings, we found NF-κB and AP1 binding sites in the 5’-regulatory region of human and mouse STIM1 genes. Further, we demonstrated that LPS induced time-dependent binding of the transcription factors NF-κB (p65/RelA) and AP1 (c-Fos/c-Jun) to the STIM1 promoter. Interestingly, silencing of c-Fos, but not c-Jun markedly reduced LPS-induced STIM1 expression in ECs. Also, we observed that silencing of p38α prevented c-Fos expression in response to LPS in ECs, suggesting that p38α signaling mediates expression of c-Fos. These results support the notion that cooperative signaling of both NF-κB and AP1 (via p38α) amplifies STIM1 expression in ECs and thereby contributes to the lung vascular hyper-permeability response during sepsis. To further validate our findings in an in vivo model, endothelial-restricted STIM1 [STIM1EC-/-] mice was generated. Loss of STIM1 expression was observed in freshly isolated lung endothelial cells (LECs) from STIM1EC-/- mice which was associated with abrogated PAR-1-mediated SOCE in LECs. We further demonstrate that STIM1EC-/- mice is completely protected from LPS potentiated PAR-1-mediated endothelial barrier dysfunction. These observations confirm the critical role for endothelial cell-expressed STIM1 in mediating microvascular leak during sepsis.