Pitas R E, Boyles J K, Lee S H, Foss D, Mahley R W
Biochim Biophys Acta. 1987 Jan 13;917(1):148-61. doi: 10.1016/0005-2760(87)90295-5.
We have previously demonstrated that astrocytes synthesize and secrete apolipoprotein E in situ. In the present work, primary cultures of rat brain astrocytes were used to study apolipoprotein E synthesis, secretion, and metabolism in vitro. The astrocytes in culture contained immunoreactive apolipoprotein E in the area of the Golgi apparatus. Incubation of the astrocytes with [35S]methionine resulted in the secretion of labeled immunoprecipitable apolipoprotein E, which constituted 1-3% of the total secreted proteins. The apolipoprotein E secreted in culture and the apolipoprotein E in rat brain extracts differed from serum apolipoprotein E in two respects: both had a slightly higher apparent molecular weight (approx. 36,000) and more acidic isoforms than serum apolipoprotein E. Sialylation of the newly secreted apolipoprotein accounted for the difference in both the apparent molecular weight and isoelectric focusing pattern of newly secreted apolipoprotein E and plasma apolipoprotein E. The astrocytes possessed apolipoprotein B,E(LDL) receptors capable of binding and internalizing apolipoprotein E-containing lipoproteins. The uptake of lipoproteins by the cells led to a reduction in the number of cell surface receptors and to the intracellular accumulation of cholesteryl esters. Since apolipoprotein E is present within the brain, and since brain cells can express apolipoprotein B,E(LDL) receptors, apolipoprotein E-containing lipoproteins may function to redistribute lipid and regulate cholesterol homeostasis within the brain.
我们之前已经证明，星形胶质细胞在原位合成并分泌载脂蛋白E。在本研究中，我们使用大鼠脑星形胶质细胞的原代培养物来研究载脂蛋白E在体外的合成、分泌和代谢。培养的星形胶质细胞在高尔基体区域含有免疫反应性载脂蛋白E。用[35S]甲硫氨酸孵育星形胶质细胞会导致分泌标记的可免疫沉淀的载脂蛋白E，其占总分泌蛋白的1%-3%。培养物中分泌的载脂蛋白E和大鼠脑提取物中的载脂蛋白E在两个方面与血清载脂蛋白E不同：两者的表观分子量略高（约36,000），并且酸性异构体比血清载脂蛋白E更多。新分泌的载脂蛋白的唾液酸化解释了新分泌的载脂蛋白E和血浆载脂蛋白E在表观分子量和等电聚焦模式上的差异。星形胶质细胞拥有载脂蛋白B、E（低密度脂蛋白）受体，能够结合并内化含载脂蛋白E的脂蛋白。细胞对脂蛋白的摄取导致细胞表面受体数量减少，并导致胆固醇酯在细胞内积累。由于载脂蛋白E存在于脑内，并且由于脑细胞可以表达载脂蛋白B、E（低密度脂蛋白）受体，含载脂蛋白E的脂蛋白可能起到重新分配脂质和调节脑内胆固醇稳态的作用。
