Expression of Sar1b enhances chylomicron assembly and key components of the coat protein complex II system driving vesicle budding.

OBJECTIVE: SAR1b plays a significant role in the assembly, organization, and function of the coat protein complex II, a critical complex for the transport of proteins from the endoplasmic reticulum to the Golgi. Recently, mutations in SARA2 have been associated with lipid absorption disorders. However, functional studies on Sar1b-mediated lipid synthesis pathways and lipoprotein packaging have not been performed. METHODS AND RESULTS: Sar1b was overexpressed in Caco-2/15 cells and resulted in significantly augmented triacylglycerol, cholesteryl ester, and phospholipid esterification and secretion and markedly enhanced chylomicron production. It also stimulated monoacylglycerol acyltransferase/diacylglycerol acyltransferase activity and enhanced apolipoprotein B-48 protein synthesis, as well as elevated microsomal triglyceride transfer protein activity. Along with the enhanced chylomicrons, microsomes were characterized by abundant Sec12, the guanine exchange factor that promotes the localization of Sar1b in the endoplasmic reticulum. Furthermore, coimmunoprecipitation experiments revealed high levels of the complex components Sec23/Sec24 and p125, the Sec23-interacting protein. Finally, a pronounced interaction of Sec23/Sec24 with sterol regulatory element binding protein (SREBP) cleavage-activating protein and SREBP-1c was noted, thereby permitting the transfer of the transcription factor SREBP-1c to the nucleus for the activation of genes involved in lipid metabolism. CONCLUSION: Our data suggest that Sar1b expression may promote intestinal lipid transport with the involvement of the coat protein complex II network and the processing of SREBP-1c.

AMPK in the small intestine in normal and pathophysiological conditions.

The role of AMPK in regulating energy storage and depletion remains unexplored in the intestine. This study will to define its status, composition, regulation and lipid function, as well as to examine the impact of insulin resistance and type 2 diabetes on intestinal AMPK activation, insulin sensitivity, and lipid metabolism. Caco-2/15 cells and Psammomys obesus (P. obesus) animal models were experimented. We showed the predominance of AMPKα1 and the prevalence of α1/ß2/γ1 heterotrimer in Caco-2/15 cells. The activation of AMPK by 5-aminoimidazole-4-carboxamide ribonucleoside and metformin resulted in increased phospho(p)-ACC. However, the down-regulation of p-AMPK by compound C and high glucose lowered p-ACC without affecting 3-hydroxy-3-methylglutaryl-coenzyme A reductase. Administration of metformin to P. obesus with insulin resistance and type 2 diabetes led to 1) an up-regulation of intestinal AMPK signaling pathway typified by ascending p-AMPKα(-Thr172); 2) a reduction in ACC activity; 3) an elevation of carnitine palmitoyltransferase 1; 4) a trend of increase in insulin sensitivity portrayed by augmentation of p-Akt and phospho-glycogen synthetase kinase 3ß; 5) a reduced phosphorylation of p38-MAPK and ERK1/2; and 6) a decrease in diabetic dyslipidemia following lowering of intracellular events that govern lipoprotein assembly. These data suggest that AMPK fulfills key functions in metabolic processes in the small intestine.