Regulation of ACAT, an enzyme involved in cholesterol esterification and foam cell formation in vascular walls

酰基辅酶A胆固醇酰基转移酶（ACAT）在细胞内催化游离胆固醇转化为胆固醇酯，并在动脉粥样硬化病变中巨噬细胞衍生泡沫细胞的形成中起关键作用。ACAT有两个同工异构体，ACAT-1和ACAT-2。利用一种针对人ACAT-1的特异性抗体，我们通过免疫组织化学方法证明了ACAT-1在人动脉粥样硬化病变中，特别是巨噬细胞衍生的泡沫细胞中高度表达。培养的人单核-巨噬细胞的Western blotting显示，在单核细胞向巨噬细胞分化的过程中，ACAT-1蛋白表达增加。为了确定单核细胞-巨噬细胞分化过程中参与ACAT-1诱导的因素，我们用THP-1细胞作为人类单核细胞模型测试了一些候选细胞。当thp -1细胞被生理浓度的1,25-二羟维生素D3（一种已知的刺激单核细胞分化的因子）处理时，ACAT-1被大量诱导，这表明1,25-二羟维生素D3在单核细胞分化过程中参与了ACAT-1的诱导。人体组织免疫组化分析显示，ACAT-1在皮肤巨噬细胞及其相关细胞如肺泡巨噬细胞、库普弗细胞、朗格汉斯细胞中高表达。类固醇激素产生细胞如肾上腺皮质细胞、颗粒细胞和间质细胞也大量表达ACAT-1。免疫电镜观察显示，ACAT-1位于人巨噬细胞粗面内质网。然而，当细胞与乙酰化LDL形成泡沫细胞时，ACAT-1更广泛地分布在细胞内形成小泡。这些囊泡可能在ACAT-1介导的胆固醇酯化过程中起一定作用。

Acyl-coenzyme A cholesterol acyltransferase (ACAT) catalyzes conversion of free cholesterol to cholesteryl ester inside the cells and plays a key role in formation of macrophage-derived foam cells in atherosclerotic lesions. There are two isoforms of ACAT, ACAT-1 and ACAT-2. Using a specific antibody to human ACAT-1, we immunohistochemically demonstrated that ACAT-1 is highly expressed in human atherosclerotic lesions, particularly in macrophage-derived foam cells. Western blotting of cultured human monocyte-macrophages showed that ACAT-1 protein increases during differentiation of monocytes into macrophages. To identify the factors involved in ACAT-1 induction during monocyte-macrophage differentiation, we tested some candidates with THP-1 cells as a model of human monocytes. WhenTHP-1 cells were treated with a physiological concentration of 1,25-dihydroxyvitamin D3, a factor known to stimulate monocytic differentiation, ACAT-1 was substantially induced suggesting that 1,25-dihydroxyvitamin D3 is involved in ACAT-1 induction during monocytic differentiation.Immunohistochemical analysis of human tissues showed that ACAT-1 is highly expressed in macrophages and their related cells such as alveolar macrophages, Kupffer cells and Langerhans cells in the skin. Steroid hormone-producing cells such as adrenocortical cells, granulosa cells and Leydig cells also showed abundant expression of ACAT-1. Immunoelectronmicroscopic observation revealed that ACAT-1 is located in rough endoplasmic reticulum in human macrophages. However, when cells were convened to foam cells with acetylated LDL, ACAT-1 is more diffusely distributed inside the cells forming small vesicles. These vesicles may play some roles in cholesterol esterification by ACAT-1.