Curtiss L K, Edgington T S
J Clin Invest. 1978 May;61(5):1298-308. doi: 10.1172/JCI109047.
The present study demonstrates the existence on human peripheral blood lymphocytes of a saturable cell surface receptor for low density lipoprotein inhibitor (LDL-In), a subset of normal human serum low density lipoprotein (LDL) that has been previously demonstrated to suppress selected lymphocyte functions in vivo and in vitro. The binding of radioiodinated LDL-In of demonstrable biological activity occurs rapidly and is quantitatively augmented by prior cultivation of the lymphocytes in lipoprotein-depleted serum, suggesting regulation of receptor density by lipoproteins in vivo. Binding is temperature dependent, facilitated by calcium ions, saturable at 4 degrees C within 40-60 min, and blocked by prior exposure to unlabeled LDL-In. The lymphocyte receptor is trypsin sensitive and regenerates in vitro with a t1/2 of 3.6 h. LDL-In receptors are calculated to have a maximum density of 4,860 +/- 460 per cell if uniformly distributed on all lymphocyte subsets. These receptors have an estimated average association constant of 1.47 X 10(7) liters/mol. When considered in context of the estimated concentration of LDL-In in blood, the receptors should be partially occupied in vivo by endogenous plasma LDL-In. Prior site occupancy inhibition experiments designed to analyze the specificity of LDL-In binding demonstrate that (a) LDL-In is 13.7-fold more effective than whole LDL in blocking the subsequent binding of 125I-LDL-In to cells; and that (b) LDL is 11-fold more effective than LDL-In in blocking the binding of 125I-LKL. This is consistent with the degree of contamination of each lipoprotein with the other lipoprotein. An independent identity of the LDL-In receptor is also supported by observations that in contrast to the previously described LDL receptor, synthesis and expression of the LDL-In receptor on lymphocytes are not suppressed by cultivation of the cells in the presence of 25-hydroxycholesterol and cholesterol. These findings suggest the existence of a previously undescribed and discrete receptor on lymphocytes for LDL-In, and that the modulation of lymphocyte function by LDL-In may be mediated by a specific cell surface receptor pathway.
本研究证明，人类外周血淋巴细胞存在低密度脂蛋白抑制剂（LDL-In）的可饱和细胞表面受体，LDL-In是正常人血清低密度脂蛋白（LDL）的一个亚组，先前已证明其在体内和体外均可抑制特定淋巴细胞功能。具有可证明生物活性的放射性碘化LDL-In的结合迅速发生，并且通过在脂蛋白缺乏的血清中预先培养淋巴细胞而在数量上增加，这表明体内脂蛋白对受体密度有调节作用。结合是温度依赖性的，受钙离子促进，在4℃下40 - 60分钟内可饱和，并且可被预先暴露于未标记的LDL-In所阻断。淋巴细胞受体对胰蛋白酶敏感，在体外以3.6小时的半衰期再生。如果LDL-In受体均匀分布在所有淋巴细胞亚组上，则计算得出每个细胞的最大密度为4860±460。这些受体的估计平均缔合常数为1.47×10⁷升/摩尔。考虑到血液中LDL-In的估计浓度，这些受体在体内应被内源性血浆LDL-In部分占据。旨在分析LDL-In结合特异性的先前位点占据抑制实验表明：（a）LDL-In在阻断¹²⁵I-LDL-In与细胞的后续结合方面比完整LDL有效13.7倍；并且（b）LDL在阻断¹²⁵I-LKL的结合方面比LDL-In有效11倍。这与每种脂蛋白被另一种脂蛋白污染的程度一致。与先前描述的LDL受体相反，在25-羟基胆固醇和胆固醇存在的情况下培养细胞，淋巴细胞上LDL-In受体的合成和表达并未受到抑制，这一观察结果也支持了LDL-In受体具有独立的特性。这些发现表明淋巴细胞上存在一种先前未描述的离散的LDL-In受体，并且LDL-In对淋巴细胞功能的调节可能由特定的细胞表面受体途径介导。
