SCAP/SREBP2/LDLr通路在LPS诱导人血管平滑肌细胞泡沫化中的作用

目的：研究炎症介质——脂多糖（LPS）是否通过扰乱固醇调节元件结合蛋白裂解激活蛋白-固醇调节元件结合蛋白2（SCAP-SREBP2）复合物介导的低密度脂蛋白受体（LDLr）负反馈调控,增加人血管平滑肌细胞对天然低密度脂蛋白胆固醇（LDL）摄取,导致泡沫细胞形成。方法：人血管平滑肌细胞在无血清培养基中培养24 h后,分为对照组（继续无血清培养）;高脂组（加入LDL负荷,终浓度为25μg/ml）;高脂加LPS组（加入LDL负荷,终浓度为25μg/ml,同时加入LPS,终浓度400 ng/ml）;高脂加LPS加肝素组（加入LDL负荷,终浓度为25μg/ml,加入LPS,终浓度400 ng/ml,加入肝素,终浓度5 mg/ml）,以上各组细胞培养24 h后收获。酶化学法检测细胞内胆固醇浓度,油红O染色法检测细胞内脂质水平,Real time PCR法检测LDLr、SREBP2表达水平,细胞免疫化学法检测SCAP蛋白表达,激光共聚焦检测SCAP在内质网与高尔基体间的转位情况。结果：细胞内胆固醇测定及油红O染色发现,LPS通过增加人血管平滑肌细胞对非修饰LDL摄取,导致人血管平滑肌细胞转变为泡沫细胞。在LPS不存在时,25μg/ml LDL减少LDLr mRNA水平（P〈0.05）。然而,LPS增加LDLr mRNA水平,逆转25μg/ml LDL对LDLr的抑制效应,不恰当的增加了血管平滑肌细胞对LDL摄取（P〈0.05）,LPS刺激也导致了SCAP蛋白表达增加和SREBP2的mRNA水平升高（P〈0.05）,同时促进了SCAP从内质网转移到高尔基体。这些结果提示LPS通过增加SCAP/SREBP2复合物从内质网到高尔基体转位,干扰了胆固醇介导的LDLr负反馈调控,使非修饰LDL在血管平滑肌细胞内堆积,导致泡沫细胞形成。结论：本研究提示,在LPS刺激条件下,SCAP/SREBP2/LDLr通路可能是血管平滑肌细胞泡沫化的另一条通路。

Effect of SCAP/SREBP2/LDLr pathway on foam-cell formation induced by LPS in human vascular smooth muscle cells

Objective： To investigate if LPS increases the uptake of LDL in human vascular smooth muscle cells, causing foam- cell formation and its mechanism. Methods： Human vascular smooth muscle cells were divided into control group, LDL group, and LDL plus LPS group, incubated in serum free medium without LDL with LDL and with LDL plus lipopolysaccharide（LPS） respectively. Intracellular cholesterol concentration, SCAP protein expression and translocation from endoplasmic reticulum（ER） to Golgi and mRNA level of LDLr and SREBP2 in the treated cells were assessed by cholesterol enzymatic assay, oil red O staining, confocol microscopy,real time quantitative polymerase chain reaction（PCR） and immunocytochemistry, respectively. Results： LPS enhanced human vascular smooth muscle cells transforming into foam cells by increasing the uptake of unmodified LDL. In the absence of LPS, 25 μg/ml LDL decreased LDLr mRNA level（P〈 0.05）. However, In the presence of LPS, LPS enhanced LDLr mRNA level, overcoming the suppression of LDLr induced by LDL and abnormally increasing LDL uptake（P〈 0.05）. Exposure to LPS also caused over-expression of SCAP protein expression and SREBP2 mRNA level（P〈 0.05）, resulting in an escape of SCAP-SREBP2 complex from ER to Golgi in thepresence of high concentration of intracellular cholesterol. Conclusion： These results indicate that LPS disrupts cholesterol-mediated LDLr feedback regulation via enhancing the escape of SCAP-SREBP2 complex from ER to Golgi, permitting intracellular accumulation of unmodified LDL and causing foam cell formation. The implication of these findings is that SACP/SREBP2/LDLr may be another important pathway for foam cell formation in human vascular smooth muscle cells under stimulation of LPS.