Translational and Experimental Medicine, Warwick Medical School, Coventry CV4 7AL, United Kingdom.
Department of Chemistry, University of Warwick, Coventry CV4 7AL, United Kingdom.
J Lipid Res. 2019 Jan;60(1):111-120. doi: 10.1194/jlr.M089300. Epub 2018 Nov 5.
We investigated whether, in view of its activity being expressed on both aspects of the endoplasmic reticulum (ER; dual membrane topology), diacylglycerol acyltransferase 1 (DGAT1) plays a distinctive role in determining the triglyceride (TAG) content of VLDL particles secreted by the liver. Mice in which the DGAT1 gene was specifically ablated in hepatocytes (DGAT1-LKO mice) had the same number of VLDL particles (apoB concentration) in the plasma 1 h after Triton 1339 treatment, but these particles were approximately half the size of VLDL particles secreted by control mice and had a proportionately decreased content of TAG, with normal cholesterol and cholesteryl ester contents. Analyses of purified microsomal fractions prepared from 16 h fasted control and DAGT1-LKO mice showed that the TAG/protein ratio in the ER was significantly lower in the latter. Electron micrographs of these livers showed that those from DGAT1-LKO mice did not show the increased lipid content of the smooth ER shown by control livers. The effects of DGAT1- and DGAT2-specific inhibitors on apoB secretion by HepG2 cells showed that DGAT1 is not indispensable for apoB secretion and demonstrated redundancy in the ability of the two enzymes to support apoB secretion. Therefore, our findings show that DGAT1 is essential for the complete lipidation and maturation of VLDL particles within the lumen of the ER, consistent with its dual topology within the ER membrane. In the mouse, DGAT2 can support apoB secretion (particle number) even when TAG availability for full VLDL lipidation is restricted in the absence of DGAT1.
我们研究了二酰基甘油酰基转移酶 1（DGAT1）是否因其在内质网（ER；双膜拓扑结构）的两个方面的活性表达而在确定肝脏分泌的极低密度脂蛋白（VLDL）颗粒的甘油三酯（TAG）含量方面发挥独特作用。在肝细胞中特异性缺失 DGAT1 基因的小鼠（DGAT1-LKO 小鼠）在 Triton 1339 处理后 1 小时血浆中的 VLDL 颗粒（apoB 浓度）相同，但这些颗粒的大小约为对照组小鼠分泌的 VLDL 颗粒的一半，并且 TAG 含量相应减少，胆固醇和胆固醇酯含量正常。对禁食 16 小时的对照和 DAGT1-LKO 小鼠的纯化微粒体部分进行的分析表明，后者 ER 中的 TAG/蛋白比显着降低。这些肝脏的电子显微镜照片显示，DGAT1-LKO 小鼠的肝脏没有显示出对照肝脏中光滑 ER 增加的脂质含量。DGAT1 和 DGAT2 特异性抑制剂对 HepG2 细胞 apoB 分泌的影响表明，DGAT1 对于 apoB 分泌不是必不可少的，并且两种酶支持 apoB 分泌的能力具有冗余性。因此，我们的发现表明，DGAT1 对于 ER 腔中 VLDL 颗粒的完全脂质化和成熟是必不可少的，这与其在 ER 膜内的双拓扑结构一致。在小鼠中，即使在缺乏 DGAT1 时限制 TAG 可用性以完全 VLDL 脂质化的情况下，DGAT2 也可以支持 apoB 分泌（颗粒数）。
