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1 Department of Medicine, Division of Gastroenterology, Washington University School of Medicine, St Louis, MO, USA
* To whom correspondence should be addressed. E-mail: dalpers{at}im.wustl.edu.
Surfactant-like particles (SLP) are unilamellar secreted membranes associated with the process of lipid absorption, and isolated previously only from the apical surface of enterocytes. In this paper the intracellular membrane has been isolated from corn oil fed animals, identified by its content of the marker protein, intestinal alkaline phosphatase (IAP). Another brush border protein, cubilin, and its anchoring protein, megalin, have been identified as components of extracellular SLP, but only cubilin is present to any extent in intracellular SLP. During fat absorption IAP is modestly enriched in intracellular SLP, but full-length cubilin (migrating at 210 kDa in fat fed mucosal fractions) falls by half, although fragments of cubilin are abundant in the intracellular SLP. Both IAP and cubilin co-localize to the same cells during corn oil absorption, and co-localize around lipid droplets. This localization is more intense during feeding of corn oil with Pluronic L-81, a detergent that allows uptake of fatty acids and monoglycerides from the lumen, but blocks chylomicron secretion. Confocal microscopy confirms the co-localization of IAP and the ligand for cubilin, intrinsic factor. Possible roles for cubilin in intracellular SLP include facilitating movement of the lipid droplet through the cell and binding to the basolateral membrane prior to reverse endocytosis.
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