Reaven E, Shi X Y, Azhar S
Department of Medicine, Stanford University School of Medicine, California.
J Biol Chem. 1990 Nov 5;265(31):19100-11.
Plasma membranes of ovarian luteal and adrenal cortical cells from "microvillar channels," a unique extracellular compartment formed by the close apposition of flattened microvillar surfaces. Microvillar channels have unusual affinity for cholesterol-rich lipoproteins, and, in vivo, may provide an increased surface area for these particles. In this research, we have isolated a plasma membrane-enriched fraction from rat luteinized ovaries, in which closely apposed membrane (i.e. microvillar channels) comprise about 30% of the preparation. Following in vitro incubations (approximately 1 h) of this plasma membrane fraction with different plasma lipoproteins, the closely apposed plasma membrane surfaces widen and become filled with lipoprotein particles (up to about 30 nm), whereas other membranes of the fraction show little binding. Competition experiments show that rat high density lipoproteins have the highest affinity for binding to the plasma membrane fraction. Radiolabeled plasma lipoprotein and the tissue-specific hormone, human chorionic gonadotropin, showed specific and saturable binding to the plasma membrane fraction, whereas other macromolecules used as controls did not. Radioautographic analyses of 125I-labeled lipoproteins and human chorionic gonadotropin indicate that binding occurs predominantly to the closely apposed plasma membranes (i.e. microvillar channels of the fraction). These studies show that microvillar channels of steroid-secreting cells entrap large numbers of plasma lipoproteins, particularly high density lipoproteins particles, presumably functioning in the delivery of cholesterol to these cells.
卵巢黄体细胞和肾上腺皮质细胞的质膜来自“微绒毛通道”，这是一种由扁平微绒毛表面紧密贴合形成的独特细胞外区室。微绒毛通道对富含胆固醇的脂蛋白具有异常亲和力，在体内可能为这些颗粒提供增加的表面积。在本研究中，我们从大鼠黄体化卵巢中分离出富含质膜的部分，其中紧密贴合的膜（即微绒毛通道）约占制剂的30%。用不同的血浆脂蛋白对该质膜部分进行体外孵育（约1小时）后，紧密贴合的质膜表面变宽并充满脂蛋白颗粒（最大约30纳米），而该部分的其他膜显示几乎没有结合。竞争实验表明，大鼠高密度脂蛋白对结合到质膜部分具有最高亲和力。放射性标记的血浆脂蛋白和组织特异性激素人绒毛膜促性腺激素显示与质膜部分有特异性和可饱和结合，而用作对照的其他大分子则没有。对125I标记的脂蛋白和人绒毛膜促性腺激素的放射自显影分析表明，结合主要发生在紧密贴合的质膜上（即该部分的微绒毛通道）。这些研究表明，类固醇分泌细胞的微绒毛通道捕获大量血浆脂蛋白，特别是高密度脂蛋白颗粒，推测其在向这些细胞输送胆固醇中起作用。
