White A L
Department of Virology and Immunology, Southwest Foundation for Biomedical Research, San Antonio, TX, USA.
Clin Genet. 1997 Nov;52(5):326-37. doi: 10.1111/j.1399-0004.1997.tb04350.x.
Lipoprotein(a) [Lp(a)] biogenesis was examined in primary cultures of hepatocytes isolated from mice transgenic for both human apolipoprotein(a) [apo(a)] and human apoB. Steady-state and pulse-chase labeling experiments demonstrated that newly synthesized human apo(a) had a prolonged residence time (approximately 60 min) in the endoplasmic reticulum (ER) before maturation and secretion. Apo(a) was inefficiently secreted by the hepatocytes and a large portion of the protein was retained and degraded intracellularly. Apo(a) exhibited a prolonged and complex folding pathway in the ER, which included incorporation of apo(a) into high molecular weight, disulfide-linked aggregates. These folding characteristics could account for long ER residence time and inefficient secretion of apo(a). Mature apo(a) bound via its kringle domains to the hepatocyte cell surface before appearing in the culture medium. Apo(a) could be released from the cell surface by apoB-containing lipoproteins. These studies are consistent with a model in which the efficiency of post-translational processing of apo(a) strongly influences human plasma Lp(a) levels, and suggest that cell surface assembly may be one pathway of human Lp(a) production in vivo. Transgenic mouse hepatocytes thus provide a valuable model system with which to study factors regulating human Lp(a) biogenesis.
在从同时转染了人载脂蛋白(a)[apo(a)]和人载脂蛋白B的小鼠分离的原代肝细胞培养物中研究了脂蛋白(a)[Lp(a)]的生物合成。稳态和脉冲追踪标记实验表明，新合成的人apo(a)在成熟和分泌之前在内质网(ER)中的停留时间延长(约60分钟)。apo(a)由肝细胞分泌效率低下，大部分蛋白质被保留并在细胞内降解。apo(a)在内质网中表现出延长且复杂的折叠途径，包括将apo(a)掺入高分子量、二硫键连接的聚集体中。这些折叠特性可以解释apo(a)在内质网中的长时间停留和分泌效率低下。成熟的apo(a)在出现在培养基中之前通过其kringle结构域与肝细胞表面结合。apo(a)可以被含apoB的脂蛋白从细胞表面释放。这些研究与一种模型一致，即apo(a)的翻译后加工效率强烈影响人血浆Lp(a)水平，并表明细胞表面组装可能是体内人Lp(a)产生的一条途径。因此，转基因小鼠肝细胞提供了一个有价值的模型系统，用于研究调节人Lp(a)生物合成的因素。
