分析PRAP1在脂肪運輸的功能

Abstract

PRAP1在許多真核生物內具有保守性。生化分析顯示PRAP1可與MTP結合,並促進其脂肪轉運的能力,進而促成ApoB的分泌。E85V, 一個PRAP1 的點突變蛋白,則沒有辦法促進此現象。在這篇研究裡,我們主要探究PRAP1在脂肪運輸的功能。之前的研究發現缺乏PRAP1時,在油脂灌食後大部分的油脂會累積在小腸,而延遲的送到血液循環裡。我們發現PRAP1主要會在高基氏體裡,而部分位於內質網中-MTP主要作用的地方。PRAP1在肝臟細胞株中能夠協助ApoB的脂化,進而形成類高密度脂蛋白的顆粒。缺乏PRAP1老鼠所生成的乳糜蛋白油脂量與ApoB的釋出量也較野生種少。有趣的是,即使WT蛋白與E85V突變都可以跟脂肪直接連結，過量表現PRAP1在HeLa細胞中可以減少油脂在胞內累積的現象,而E85V突變不行，代表這其中可能牽涉複雜的脂肪運輸機致。同時，PRAP1也能夠跟高密度脂蛋白中的主要成分ApoA1有相互作用,並可促進肝臟細胞攝取膽固醇及巨噬細胞排出膽固醇的能力。綜上所述,我們的研究顯示認為PRAP1是個新的脂蛋白,去調節脂肪運輸的能力及其他脂蛋白的釋出或功能，並可能作為人類脂肪代謝相關疾病的一種解決方針。

Proline-rich acidic protein 1 (PRAP1) is highly conserved among eukaryotic organisms. Our earlier studies indicated that PRAP1 forms a complex with microsomal triglyceride transfer protein (MTP) and promotes MTP-mediated lipid transfer and facilitates ApoB secretion. The E85V mutant, a point mutation form of PRAP1, however, fails to promote the MTP activity in vitro. In this study, we investigated the functions of PRAP1 in lipid transport, since PRAP1-deficient mice manifest lipid accumulation in the small intestine and delayed secretion of lipids into bloodstream following oral gavage of lipids. Although PRAP1 is mainly localized to the Golgi, it is indeed partially colocalized with MTP, which mainly resides on the ER. PRAP1 promotes ApoB secretion to form HDL-like particles in hepatoma cell lines. Further analysis revealed that PRAP1 promotes the assembly and secretion of ApoB in vivo. Both wt and the E85V mutant of PRAP1 directly bind to lipids. However, overexpression of wt but not the E85V results in less lipid accumulation in HeLa cells, suggesting a possible mechanistic link to its complex functions in lipid transport. Last, we demonstrate that PRAP1 interacts with ApoA1 and facilitates cholesterol uptake into hepatoma cell lines and promotes cholesterol efflux from the macrophage cell line. Taken together, our results suggest that PRAP1 plays a role in lipid transport and lipoprotein metabolism, and may serve as a novel therapeutic target for human diseases.