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IMPACT OF SAR1B OVEREXPRESSION ON INTESTINAL LIPIDS ASSEMBLY AND EXPORT
A Sané1, A BenDjoudi Ouadda1, M Elchebly2, E Harmel1, M Laville3, M Dominguez1, S Spahis1, E Delvin2, E Levy1 1Departements of Nutrition; 2Departements of Biochemistry, Université de Montréal, Montreal, Quebec; 3Centre de recherche Rhône-Alpes en nutrition humaine, Hôpital Edouard-Herriot, Faculté de Médicine, Université de Lyon-1, Lyon, France
AIMS: Sar1b, encoded by SARA2 gene, 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: Sar1b was overexpressed in Caco-2 cells.
RESULTS: Sar1B enhanced triacylglycerol esterification and chylomicron overproduction. Increased monoacylglycerol/diacylglycerol acyltransferase activity and apolipoprotein B-48 protein expression and synthesis, as well as microsomal triglyceride transfer protein activity represented the main mechanisms for the stimulation of chylomicron packaging in cells overexpressing Sar1b. Furthermore, Sar1b overexpression caused a significant augmentation in the phosphorylation of the catalytic subunit alpha of AMPK, a crucial regulator of lipid metabolism. The activation of AMPK led to the raised phosphorylation level and hence inactivation, of acetyl-coenzyme A carboxylase, the rate-limiting enzyme in fatty acid synthesis. Given the relationship of p38 MAPK with lipid metabolism, we examined its absolute protein contents and phosphorylation level. Inspection of Caco-2 cells with Sar1b overexpression revealed a rise in the phosphorylation degree (Thr180/Tyr182) even if the p38-MAP kinase protein remained unchanged. These findings were not accompanied with changes in the protein expression and phosphorylation of protein kinase B, the regulatory protein c-Jun NH2-terminal kinase 1 and insulin receptor, which are implicated in insulin resistance.
CONCLUSIONS: Taken together, these results suggest that Sar1b overexpression enhances intestinal lipid transport while regulating lipogenesis and modulating the specific p38 MAPK signalisation pathway.