Macrophage-specific Up-regulation of Apolipoprotein E Gene Expression by STAT1 Is Achieved via Long Range Genomic Interactions

In atherogenesis, macrophage-derived apolipoprotein E (apoE) has an athero-protective role by a mechanism that is not fully understood. We investigated the regulatory mechanisms involved in the modulation of apoE expression in macrophages. The experiments showed that the promoters of all genes of the apoE/apoCI/apoCIV/apoCII gene cluster are enhanced by multienhancer 2 (ME. 2), a regulatory region that is located 15.9 kb downstream of the apoE gene. ME. 2 interacts with the apoE promoter in a macrophage-specific manner. Transient transfections in RAW 264.7 macrophages showed that the activity of ME. 2 was strongly decreased by deletion of either 87 bp from the 5' end or 131 bp from the 3' end. We determined that the minimal fragment of this promoter that can be activated by ME. 2 is the proximal -100/+73 region. The analysis of the deletion mutants of ME. 2 revealed the importance of the 5' end of ME.2 in apoE promoter transactivation. Chromatin conformational capture assays demonstrated that both ME. 2 and ME. 1 physically interacted with the apoE promoter in macrophages. Our data showed that phorbol 12-myristate 13-acetate-induced differentiation of macrophages is accompanied by a robust induction of apoE and STAT1 expression. In macrophages (but not in hepatocytes), STAT1 up-regulated apoE gene expression via ME.2. The STAT1 binding site was located in the 174-182 region of ME.2. In conclusion, the specificity of the interactions between the two multienhancers (ME.1 and ME.2) and the apoE promoter indicates that these distal regulatory elements play an important role in the modulation of apoE gene expression in a cell-specific manner.