Abstract 1294: XBP1 is a Key Protein Inducing Endothelial Cell Death in vivo and in vitro
Background: X-box binding protein 1 (XBP1) is a key signal transducer in endoplasmic reticulum stress response, and its potential role in the atherosclerosis development is unknown. This study aims to explore the impact of XBP1 in maintaining endothelial integrity related to atherosclerosis and to delineate the underlying mechanism
Methods and Results: En face staining using anti-XBP1 antibody indicated that XBP1 was highly expressed in endothelium of the bifurcation and lesion area in the artery wall in ApoE−/− mice. RT-PCR and Western blot analysis further revealed that the XBP1 levels, especially that of the spliced one, was related to the severity of lesion development. Since blood flow is considered to be an important factor for the development of lesions, we performed experiments with different flow system in vitro. It was found that disturbed flow sustainedly activated XBP1 expression. Both disturbed flow and cholesterol overloading activated XBP1 splicing through the translocation of grp78 from ER membrane to cell plasma membrane. Secondly, over-expression of spliced XBP1 caused vacuolization in HUVECs, maybe due to the resconstitution of membrane system, followed by cell apoptosis. ChIP assay showed that XBP1 bound to VE-cadherin promoter region, leading to the down-regulation of transcription. In addition, MMP inhibitors partially suppressed spliced XBP1s-induced VE-cadherin degradation, while lysosome inhibitors suppressed XBP1s-induced eNOS degradation, suggesting sustained activation of XBP1 splicing activates MMP and lysosome proteases. Caspase 2 and 3 were also found to be activated by spliced XBP1. Over-expression of spliced XBP1 caused endothelial cell loss from artery ex vivo, which could be partially rescued by MMP inhibitors and casepase inhibitors. Arterial endothelial cells infected by Ad-XBP1 induced atherosclerotic lesion development in ApoE−/− mice.
Conclusions: We provide the first evidence that XBP1 plays an essential role in mediating endothelial death, in which sustained activation of XBP1 splicing can induce endothelial apoptosis via down-regulation of VE-cadherin and eNOS, resulting in endothelial cell loss from the intact arterial wall, leading to the development of atherosclerotic lesions.