Suresh S, Yan Z, Patel R C, Patel Y C, Patel S C
Neurobiology Research Laboratory, VA Connecticut Healthcare System, Newington 06111, USA.
J Neurochem. 1998 Jan;70(1):242-51. doi: 10.1046/j.1471-4159.1998.70010242.x.
Apolipoprotein D (apoD), a member of the lipocalin superfamily of ligand transporters, has been implicated in the transport of several small hydrophobic molecules including sterols and steroid hormones. We have previously established that apoD is a secreted protein from cultured mouse astrocytes and that treatment with the oxysterol 25-hydroxycholesterol markedly stimulates apoD release. Here, we have investigated expression and cellular processing of apoD in the Niemann-Pick type C (NPC) mouse, an animal model of human NPC, which is a genetic disorder affecting cellular cholesterol transport. NPC is phenotypically characterized by symptoms of chronic progressive neurodegeneration. ApoD gene expression was up-regulated in cultured NPC astrocytes and in NPC brain. ApoD protein levels were also increased in NPC brain with up to 30-fold higher apoD content in the NPC cerebellum compared with control mice. Subcellular fractionation of NPC brain homogenates revealed that most of the apoD was associated with the myelin fraction. ApoD was found to be a secreted protein from cultured normal astrocytes and treatment with the oxysterol, 25-hydroxycholesterol, markedly stimulated apoD release (by five- to 10-fold). By contrast, secretion of apoD from NPC astrocytes was markedly reduced and could not be stimulated by oxysterol treatment. Secretion of apoE, another apolipoprotein normally produced by astrocytes, was similar in NPC and control cells. Furthermore, apoE secretion was not potentiated by oxysterol treatment in either cell type. Plasma levels of apoD were sixfold higher in NPC, whereas hepatic levels were substantially reduced compared with controls, possibly reflecting reduced hepatic clearance of the circulating protein. These results reveal hitherto unrecognized defects in apoD metabolism in NPC that appear to be linked to the known defects in cholesterol homeostasis in this disorder.
载脂蛋白D（apoD）是脂质运载蛋白超家族中配体转运蛋白的一员，与包括固醇和类固醇激素在内的多种小疏水分子的转运有关。我们之前已经确定apoD是培养的小鼠星形胶质细胞分泌的一种蛋白质，并且用氧化固醇25-羟基胆固醇处理可显著刺激apoD的释放。在此，我们研究了人尼曼-匹克C型（NPC）病动物模型NPC小鼠中apoD的表达和细胞加工过程，NPC病是一种影响细胞胆固醇转运的遗传性疾病。NPC在表型上的特征是慢性进行性神经退行性变的症状。在培养的NPC星形胶质细胞和NPC脑中，apoD基因表达上调。NPC脑中的apoD蛋白水平也有所增加，与对照小鼠相比，NPC小脑的apoD含量高出多达30倍。NPC脑匀浆的亚细胞分级分离显示，大部分apoD与髓磷脂部分相关。发现apoD是培养的正常星形胶质细胞分泌的一种蛋白质，用氧化固醇25-羟基胆固醇处理可显著刺激apoD的释放（增加5至10倍）。相比之下，NPC星形胶质细胞中apoD的分泌显著减少，并且不能被氧化固醇处理所刺激。另一种通常由星形胶质细胞产生的载脂蛋白apoE的分泌在NPC细胞和对照细胞中相似。此外，在两种细胞类型中，氧化固醇处理均未增强apoE的分泌。NPC患者血浆中apoD水平比对照高6倍，而肝脏中apoD水平与对照相比显著降低，这可能反映了循环蛋白肝脏清除率的降低。这些结果揭示了NPC中apoD代谢迄今未被认识到的缺陷，这些缺陷似乎与该疾病中已知的胆固醇稳态缺陷有关。
