肝细胞中活化转录因子ATF6抑制SREBP1的转录活性

内质网膜定位的活化转录因子ATF6和SREBP1均是经过蛋白酶切水解激活,激活后的ATF6(N)和SREBP1(N)进入细胞核内,分别指导内质网膜未折叠蛋白聚集反应相关基因和脂肪酸合成相关基因的表达.研究发现,肝细胞内葡萄糖饥饿激活ATF6并抑制SREBP1的转录活性及其靶基因的表达.过表达ATF6(N)能够抑制SREBP1介导的转录及其下游基因的表达.免疫共沉淀实验显示,ATF6(N)在细胞核内结合SREBP1(N),这种结合在无糖状况下增强.不同功能区缺失分析表明,ATF6和SREBP1通过亮氨酸拉链(leucinezipper)功能区相互作用.在葡萄糖饥饿状况下,ATF6对SREBP1转录活性的抑制保证了细胞基本生命活动所需要的能量.     

Sterol-responsive element-binding protein (SREBP) 2 down-regulates ATP-binding cassette transporter A1 in vascular endothelial cells: a novel role of SREBP in regulating cholesterol metabolism

ATP-binding cassette transporter A1 (ABCA1) is a pivotal regulator of cholesterol efflux from cells to apolipoproteins, whereas sterol-responsive element-binding protein 2 (SREBP2) is the key protein regulating cholesterol synthesis and uptake. We investigated the regulation of ABCA1 by SREBP2 in vascular endothelial cells (ECs). Our results showed that sterol depletion activated SREBP2 and increased its target, low density lipoprotein receptor mRNA, with a concurrent decrease in the ABCA1 mRNA. Transient transfection analysis revealed that sterol depletion decreased the ABCA1 promoter activity by 50%, but low density lipoprotein receptor promoter- and the sterol-responsive element-driven luciferase activities were increased. Overexpression of the N terminus of SREBP2 (SREBP2(N)), an active form of SREBP2, also inhibited the ABCA1 promoter activity. Functionally adenovirus-mediated SREBP2(N) expression increased cholesterol accumulation and decreased apoA-I-mediated cholesterol efflux. The conserved E-box motif was responsible for the SREBP2(N)-mediated inhibition since mutation of the E-box increased the basal activity of the ABCA1 promoter and abolished the inhibitory effect of SREBP2(N). Furthermore sterol depletion and SREBP2(N) overexpression induced the binding of SREBP2(N) to both consensus and ABCA1-specific E-box. Chromatin immunoprecipitation assay demonstrated that serum starvation enhanced the association of SREBP2 and the ABCA1 promoter in ECs. To correlate this mechanism pathophysiologically, we found that oscillatory flow caused the activation of SREBP2 and therefore attenuated ABCA1 promoter activity in ECs. Thus, this SREBP-regulated mechanism may control the efflux of cholesterol, which is a newly defined function of SREBP2 in ECs in addition to its role in cholesterol uptake and biosynthesis.