Abstract P132: Heterogeneously Expressed Bcl6 Controls Endothelial Inflammatory Response

Abstract

We recently reported that BCL6 , a DNA binding protein important in germinal center responses and lymphoncogenesis, was heterogeneously expressed across human endothelial cells (EC). Its expression correlated with more intense inflammatory responses. Here, we asked what role BCL6 might play in endothelial inflammation. We examined public expression and epigenomic data and tested BCL6 function in vitro . BCL6 was significantly enriched in cardiac microvascular and endocardial EC, associated with cardiovascular disease in GWAS studies, and was elevated in rejecting cardiac transplant biopsies. The chromatin landscape around BCL6 reflected that of a superenhancer in EC, with high H3K27ac and H3K4me3 signal indicative of active/poised transcription. EC treated with TNFα, IFNγ or IL-1β upregulated BCL6 within 2-3hr, maintained through 24hr. The BCL6 gene had putative NFκB/REL transcription factor binding sequences, and p65/RelA binding to the BCL6 gene increased more than 8-fold under TNFα or IL-1β stimulation. BCL6 DNA binding sites were found in EC adhesion molecule and chemokine genes, including VCAM1 , CXCL2 , CX3CL1 , SELE , CCL5 ; as well as NFKB genes themselves. Perturbation of BCL6 complexes with inhibitors (FX1, BI-3812) led to dramatic changes in the TNFα-induced transcriptome and at the protein level in endothelial cells. For example, FX1 and BI-3812 significantly blocked TNFα-induction of VCAM1 and CXCL2 /GROβ, but upregulated CCL5 /RANTES and CXCL10 /IP-10. Interestingly, TNFα-induced expression of miR155, a pro-inflammatory microRNA important in atherosclerosis, was also suppressed. We hypothesize that BCL6 is a novel NFκB-inducible regulator of vascular inflammation. Surprisingly, BCL6 corepressor antagonism both enhanced and suppressed TNFα mediated EC activation. Therefore BCL6 performs transcriptional modification that is context and cell-type dependent. Its role in vascular inflammation merits further investigation.