Bile acids target proteolipid nano-assemblies of EGFR and phosphatidic acid in the plasma membrane for stimulation of MAPK signaling

Bile acids are critical biological detergents in the gastrointestinal tract and also act as messengers to regulate a multitude of intracellular signaling events, including mitogenic signaling, lipid metabolism and endo/exocytosis. In particular, bile acids stimulate many receptors and ion channels on the cell surface, the mechanisms of which are still poorly understood. Membrane-associating proteins depend on the local spatial distribution of lipids in the plasma membrane (PM) for their function. Here, we report that the highly amphipathic secondary bile acid deoxycholic acid (DCA), a major constituent in the human bile, at doses <1μM enhances the nanoclustering and the PM localization of phosphatidic acid (PA) but disrupts the local segregation of phosphatidylserine in the basolateral PM of the human colorectal adenocarcinoma Caco-2 cells. PA is a key structural component of the signaling nano-domains of epidermal growth factor receptor (EGFR) on the cell surface. We show that DCA promotes the co-localization between PA and EGFR, the PA-driven EGFR dimerization/oligomerization and EGFR signaling. Depletion of PA abolishes the stimulatory effects of DCA on the EGFR oligomerization and signaling. This effect occurs in the cultured Caco-2 cells and the ex vivo human intestinal enteroids. We propose a novel mechanism, where the amphiphilic DCA monomers alter the nano-assemblies of anionic phospholipids and in turn change the dynamic structural integrity of the lipid-driven oligomerization of cell surface receptors and their signal transduction.

胆汁酸是胃肠道中关键的生物去污剂，同时可作为信号分子调控诸多细胞内信号转导事件，包括有丝分裂原信号通路、脂质代谢以及胞吞/胞吐过程。尤为特别的是，胆汁酸可激活细胞表面的多种受体与离子通道，但其背后的分子机制仍未被充分阐明。膜结合蛋白的功能依赖于质膜（plasma membrane, PM）内脂质的局部空间分布。本研究发现，作为人体胆汁主要成分的两亲性次级胆汁酸——脱氧胆酸（deoxycholic acid, DCA），在低于1μM的浓度下可增强磷脂酸（phosphatidic acid, PA）的纳米簇集与质膜定位，但会破坏人大肠腺癌Caco-2细胞基底外侧质膜内磷脂酰丝氨酸的局部分异分布。磷脂酸（PA）是细胞表面表皮生长因子受体（epidermal growth factor receptor, EGFR）信号纳米域的关键结构组分。本研究证实，DCA可促进PA与EGFR的共定位，以及PA介导的EGFR二聚化/寡聚化与EGFR信号转导。PA的耗竭可消除DCA对EGFR寡聚化与信号转导的激活效应。该效应在培养的Caco-2细胞以及离体人肠道类器官中均得到验证。本研究提出了一种全新的机制：两亲性DCA单体可改变阴离子磷脂的纳米组装结构，进而改变脂质介导的细胞表面受体寡聚化的动态结构完整性及其信号转导过程。