FUNCTIONAL SIGNIFICANCE OF LIPID DROPLETS AS A NEW MEMBRANE DOMAIN

脂滴被简单地视为特殊分化细胞(如白色脂肪细胞)中的脂类储存库。但事实上，LDs几乎存在于任何类型的细胞中，但它们的功能一直不明确。在本研究中，我们试图主要研究LDs在非脂肪细胞中的生理功能，得到以下结果：1)Caveolin，尤其是Caveolin-2β优先定位于LDs表面。小窝蛋白-2β的中央疏水部分是定位所必需的。当灯盏花素A抑制内质网到高尔基体的转运时，其他小窝蛋白，包括小窝蛋白-1和小窝蛋白-2α也定位于LD。2)LD表面是含有游离胆固醇的磷脂单分子层。LD磷脂的脂肪酸组成是独特的，与内质网和RAFT的脂肪酸组成不同。3)adrp(或adiophlin)是一种在非脂肪细胞中特异定位于LDS的蛋白质，它通过使用来自小窝蛋白的不同序列线索被靶向于LDS。4)通过对纯化的LDS进行蛋白质组学分析，Rab18被鉴定为LD特异的蛋白质，过表达的Rab18消除了LD表面的ADRP，并诱导了LD-ER的形成。当用RNA干扰下调ADRP的表达或用布洛菲尔丁A处理细胞时，也形成了相同的结合。这些结果表明，LDS不仅存储过多的脂肪，而且可能参与了包括细胞内脂质运输和代谢在内的多种重要功能。

Lipid droplets(LDs) have been regarded simply as a depot of lipids in specially differentiated cells, e.g., white adipose cells. But in fact LDs are found virtually in any cell type, but their function has been left ambiguous. In the present study, we attempted to examine the physiological function of LDs mainly in non-adipose cells, and obtained the following results.1)Caveolins, especially caveolin-2β, was preferentially localized in the surface of LDs. The central hydrophobic portion of caveolin-2βwas necessary for the localization. Other caveolins, including caveolin-1 and caveolin-2α were also localized to LDs when the ER-to-Golgi transport was inhibited by brefeldin A.2)The LD surface was shown to be a phospholipid monolayer containing free cholesterol. The fatty acid composition of the LD phospholipids was unique and different from that of the ER and rafts.3)ADRP (or adipophilin), a protein specifically localized to LDs in non-adipose cells, was targeted to LDs by using different sequence cues from caveolins. Two redundant signals were found in the N and C termini of the ADRP molecule.4)ADRP was shown to bind to ARF1,and was dissociated from LDs when cells were treated with brefeldin A or GDP-bound ARF1.5)By proteomic analysis of purified LDs, Rab18 was identified as an LD-specific protein.Overexpression of Rab18 eliminated ADRP from the LD surface, and induced formation of the LD-ER apposition. The same apposition was formed when the expression of ADRP was knocked down by RNA interference or when cells were treated by brefeldin A.These results indicate that LDs do not simply store excessive lipids, but may be involved in various important functions including intracellular lipid trafficking and metabolism.