Shaikh Saame Raza, Rockett Benjamin Drew, Salameh Muhammad, Carraway Kristen
Department of Biochemistry and Molecular Biology, Brody School of Medicine, East Carolina University, Greenville, NC 27834, USA.
J Nutr. 2009 Sep;139(9):1632-9. doi: 10.3945/jn.109.108720. Epub 2009 Jul 29.
An emerging molecular mechanism by which docosahexaenoic acid (DHA) exerts its effects is modification of lipid raft organization. The biophysical model, based on studies with liposomes, shows that DHA avoids lipid rafts because of steric incompatibility between DHA and cholesterol. The model predicts that DHA does not directly modify rafts; rather, it incorporates into nonrafts to modify the lateral organization and/or conformation of membrane proteins, such as the major histocompatibility complex (MHC) class I. Here, we tested predictions of the model at a cellular level by incorporating oleic acid, eicosapentaenoic acid (EPA), and DHA, compared with a bovine serum albumin (BSA) control, into the membranes of EL4 cells. Quantitative microscopy showed that DHA, but not EPA, treatment, relative to the BSA control diminished lipid raft clustering and increased their size. Approximately 30% of DHA was incorporated directly into rafts without changing the distribution of cholesterol between rafts and nonrafts. Quantification of fluorescence colocalization images showed that DHA selectively altered MHC class I lateral organization by increasing the fraction of the nonraft protein into rafts compared with BSA. Both DHA and EPA treatments increased antibody binding to MHC class I compared with BSA. Antibody titration showed that DHA and EPA did not change MHC I conformation but increased total surface levels relative to BSA. Taken together, our findings are not in agreement with the biophysical model. Therefore, we propose a model that reconciles contradictory viewpoints from biophysical and cellular studies to explain how DHA modifies lipid rafts on several length scales. Our study supports the notion that rafts are an important target of DHA's mode of action.
二十二碳六烯酸（DHA）发挥其作用的一种新出现的分子机制是对脂筏组织的修饰。基于脂质体研究的生物物理模型表明，由于DHA与胆固醇之间存在空间不相容性，DHA会避开脂筏。该模型预测，DHA不会直接修饰脂筏；相反，它会整合到非脂筏中，以修饰膜蛋白的侧向组织和/或构象，例如主要组织相容性复合体（MHC）I类。在此，我们通过将油酸、二十碳五烯酸（EPA）和DHA（与牛血清白蛋白（BSA）对照相比）整合到EL4细胞的膜中，在细胞水平上测试了该模型的预测。定量显微镜显示，相对于BSA对照，DHA处理而非EPA处理减少了脂筏聚集并增加了其大小。约30%的DHA直接整合到脂筏中，而没有改变胆固醇在脂筏和非脂筏之间的分布。荧光共定位图像的定量显示，与BSA相比，DHA通过增加非脂筏蛋白进入脂筏的比例，选择性地改变了MHC I类的侧向组织。与BSA相比，DHA和EPA处理均增加了抗体与MHC I类的结合。抗体滴定显示，DHA和EPA没有改变MHC I的构象，但相对于BSA增加了总表面水平。综上所述，我们的发现与生物物理模型不一致。因此，我们提出了一个模型，该模型调和了生物物理和细胞研究中的矛盾观点，以解释DHA如何在几个长度尺度上修饰脂筏。我们的研究支持脂筏是DHA作用模式的重要靶点这一观点。
