Abstract 12293: Retinoic Acid Signaling Regulates Smooth Muscle Cell Phenotypic Switching in Atherosclerosis Through Epigenetic Modulation of Gene Expression

Abstract

Background: Smooth muscle cells (SMCs) substantially contribute to atherosclerosis through “phenotypic switching.” Our previous work identified an intermediate SMC-derived cell type, termed “SEM” cells, which was multipotent and activated in inflammatory response. Activation of retinoic acid (RA) signaling by all-trans retinoic acid (ATRA) attenuated atherosclerosis in mice coincident with dramatic suppression of SEM cell formation from SMCs. However, the regulatory mechanisms by which RA signaling modulates SMC transition to SEM cells are largely unknown. Methods: We employed molecular and cell biology techniques, SMC-linage tracing and atheroprone mouse models, and next-generation sequencing (e.g., RNA-seq, ChIP-seq) to reveal how RA signaling modulates SMC transition to SEM cells. Results: Activation of RA signaling with ATRA significantly reduced SEM cells in established atherosclerosis, as well as downregulated the expression of SEM cell marker genes (e.g., Ly6a , Ly6c1 ) and repressed inflammatory response in ex vivo SEM cells, whereas inhibition of the signaling with antagonist, BMS49334, showed opposite results. RARα occupied the promoter regions of SEM cell marker genes, and ATRA treatment significantly increased the enrichment of NCOR1 at promoters of these genes. These findings suggest that RA signaling suppresses SMC transition to SEM cells via directly repressing the expression of SEM cell marker genes. Furthermore, we found EZH2, one of the subunits of PRC2, physically interacted with RARα in SMCs and occupied the promoters of SEM cell marker genes, and its methyltransferase activity at the promoter regions was responsible for the repression of SEM cell maker genes. Moreover, activation of RA signaling inhibited SEM cell inflammatory response through LXR-mediated suppression of a series of inflammatory genes. Finally, multiple epigenetic signatures (e.g., H3K27me3, H3K27ac, H3K4me3, etc.) at TSS of SEM cell marker genes and inflammatory genes were extensively altered in response to the activation of RA signaling. Conclusions: Our findings indicate that RA signaling modulates maintenance of SEM cell identity and inflammatory function in atherosclerosis by epigenetic regulation of gene expression.