| Summary: | Background - Sustained activation of spliced x-box binding protein-1 (XBP1), an endoplasmic reticulum stress response transcription factor, results in the development of atherosclerosis in apoE -/- mice. Histone deacetylases (HDACs) play a crucial role in transcriptional regulation through modulation of chromatin structure. In particular, HDAC3 is involved in maintaining endothelial integrity. HDAC3 and XBP1 are similarly expressed in the bifurcation regions of aorta. Unspliced XBP1 (XBPlu) is expressed in a similar manner in endothelial cells. In the present study we investigated the role of XBPlu and XBPls in the maintenance of endothelial integrity and endothelial cell migration. Furthermore, the crosstalk between HDACS and XBP1 signalling pathways was examined. Methods and Results - Our study demonstrated that disturbed flow upregulated HDACS and XBPlu protein production through the VEGF receptor 2 / PI-3-kinase pathway. Knockdown of XBP1 by shRNA lentiviral transfection ablated disturbed flow-induced HDAC3 upregulation. Similarly to HDACS, overexpression of XBPlu by adenoviral gene transfer increased Akt phosphorylation at serine 473 and haem oxygenase 1 gene transcription, which showed a protective role in hydrogen peroxide-induced apoptosis of endothelial cells. Co-immunoprecipitation assays demonstrated that HDACS physically associates with XBPlu and Akt. The use of truncated HDACS constructs demonstrated that XBP1 binds to the central section of HDACS, which is predicted to contain a nuclear export signal. Furthermore, we identified a role for XBP1 in mediating endothelial cell migration. Overexpression of both XBPlu and XBPls enhanced the ability of endothelial cells to migrate. Ablation of XBP1 expression and XBP1 splicing, through knockdown of IRE la expression, reduced endothelial cell migration; however this was without a corresponding decrease in eNOS expression and NO production. Conclusions - These results suggest that XBPlu protects endothelial cells from oxidative stress, including that produced by disturbed flow. The interaction with HDAC3 could be crucial for this effect. The ratio between XBPlu and XBPls is also crucial for correct endothelial cell migration. Modulation of this balance and the interaction with HDAC3 may provide novel therapeutic strategies in vascular disease whilst maintaining endothelial integrity.
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