Xu X X, Tabas I
Department of Medicine, Columbia University College of Physicians and Surgeons, New York, New York 10032.
J Biol Chem. 1991 Sep 15;266(26):17040-8.
Activation of acyl-CoA:cholesterol actyltransferase (ACAT) in macrophages by lipoproteins is a key event in atheroma foam cell formation. To help elucidate the mechanisms whereby lipoproteins stimulate ACAT, the early cellular events of lipoprotein-induced ACAT stimulation were studied in mouse peritoneal macrophages. As a function of increasing lipoprotein-cholesterol influx to the cell during the first few hours of incubation, ACAT activity was markedly stimulated by beta-very low density lipoprotein (beta-VLDL) and acetyl-low density lipoprotein (acetyl-LDL) only after lipoprotein-cholesterol influx reached a threshold level of approximately 25% above the basal cell cholesterol content. In contrast, LDL stimulated ACAT only minimally at this level of lipoprotein-cholesterol influx. In further experiments, the source of ACAT cholesterol substrate during the initial stimulation of ACAT was shown to be a mixture of cellular (approximately 75%) and lipoprotein-cholesterol (approximately 25%) in proportions that approximated the proportions of originally cellular and lipoprotein-cholesterol in the cell. Thus, lipoprotein-cholesterol rapidly mixed with most or all of cellular cholesterol before ACAT esterification. Additional studies showed that LDL caused significant efflux of cellular cholesterol, thus providing at least a partial explanation for the relatively weak ACAT stimulatory potential of LDL. To support this idea, LDL that was modified to decrease its ability to induce net cellular cholesterol efflux stimulated ACAT 2-fold greater than control LDL when matched for lysosomal LDL-cholesterol influx. Moreover, when the effective efflux potentials of beta-VLDL and acetyl-LDL were increased, ACAT stimulation was markedly decreased despite unchanged lipoprotein-cholesterol influx. Thus, macrophage ACAT is stimulated not directly by the influx of newly hydrolyzed lipoprotein-cholesterol but rather by net expansion of cellular cholesterol pools to a particular threshold level. This scheme has potentially important implications regarding the cellular and molecular mechanisms of foam cell formation.
脂蛋白激活巨噬细胞中的酰基辅酶A：胆固醇酰基转移酶（ACAT）是动脉粥样硬化泡沫细胞形成的关键事件。为了帮助阐明脂蛋白刺激ACAT的机制，在小鼠腹腔巨噬细胞中研究了脂蛋白诱导的ACAT刺激的早期细胞事件。在孵育的最初几个小时内，随着进入细胞的脂蛋白胆固醇流入量增加，仅在脂蛋白胆固醇流入量达到比基础细胞胆固醇含量高约25%的阈值水平后，β-极低密度脂蛋白（β-VLDL）和乙酰低密度脂蛋白（乙酰-LDL）才会显著刺激ACAT活性。相比之下，在这个脂蛋白胆固醇流入水平下，低密度脂蛋白（LDL）对ACAT的刺激作用微乎其微。在进一步的实验中，ACAT最初受到刺激时，其胆固醇底物的来源显示为细胞胆固醇（约75%）和脂蛋白胆固醇（约25%）的混合物，其比例与细胞中原始细胞胆固醇和脂蛋白胆固醇的比例相近。因此，在ACAT酯化之前，脂蛋白胆固醇迅速与大部分或全部细胞胆固醇混合。额外的研究表明，LDL会导致细胞胆固醇的显著流出，从而至少部分解释了LDL相对较弱的ACAT刺激潜力。为支持这一观点，经修饰以降低其诱导细胞胆固醇净流出能力的LDL，在溶酶体LDL胆固醇流入量相匹配时，对ACAT的刺激作用比对照LDL大2倍。此外，当β-VLDL和乙酰-LDL的有效流出潜力增加时，尽管脂蛋白胆固醇流入量不变，但ACAT刺激作用明显降低。因此，巨噬细胞ACAT不是直接由新水解的脂蛋白胆固醇的流入刺激，而是由细胞胆固醇池净扩张到特定阈值水平刺激。该方案对泡沫细胞形成的细胞和分子机制具有潜在的重要意义。
