Scavenger receptor class B type I (SR-BI) in pig enterocytes: trafficking from the brush border to lipid droplets during fat absorption

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Abstract
Background: Scavenger receptor class B type I (SR-BI) is known to mediate cellular uptake of cholesterol from high density lipoprotein particles and is particularly abundant in liver and steroidogenic tissues. In addition, SR-BI expression in the enterocyte brush border has also been reported but its role in the small intestine remains unclear. Aim and methods: To gain insight into the possible function of pig SR-BI during uptake of dietary fat, its localisation in enterocytes was studied in the fasting state and during fat absorption by immunogold electron microscopy and subcellular fractionation. Results: In the fasting state, SR-BI was mainly localised in the microvillar membrane and in apical invaginations/pits between adjacent microvilli. In addition, a subapical compartment and small cytoplasmic lipid droplets were distinctly labelled. During lipid absorption, the receptor was found in clathrin positive apical coated pits and vesicles. In addition, cytoplasmic lipid droplets that greatly increased in size and number were strongly labelled by the SR-BI antibody whereas apolipoprotein A-1 positive chylomicrons were largely devoid of the receptor. Conclusion: During absorption of dietary fat, SR-BI is endocytosed from the enterocyte brush border and accumulates in cytoplasmic lipid droplets. Internalisation of the receptor occurs mainly by clathrin coated pits rather than by a caveolae/lipid raft based mechanism.