Submitted on August 16, 2007
Accepted on October 15, 2007
Intestinal alkaline phosphatase: Selective endocytosis from the enterocyte brush border during fat absorption.
Gert H. Hansen1, Lise-Lotte Niels-Christiansen2, Lissi Immerdal2, Birthe T. Nystrom3, and E. Michael Danielsen1*
1 Dept. of Cellular and Molecular Medicine, The Panum Institute, University of Copenhagen, Copenhagen, Denmark
2 Dept. of Cellular and Molecular Medicine, The Panum Institute, University of Copenhagen, Copenhagen, Denmark; Copenhagen, Denmark
3 Dept. of Biomedicine, The Panum Institute, University of Copenhagen, Copenhagen, Denmark
* To whom correspondence should be addressed. E-mail: midan{at}imbg.ku.dk.
Absorption of dietary fat in the small intestine is accompanied by a rise in the serum of intestinal alkaline phosphatase (IAP) as well as in the secretion of IAP-containing surfactant-like particles (SLP's) from the enterocytes. In the present work fat absorption was studied in organ cultured mouse intestinal explants. By immunofluorescence microscopy, fat absorption caused a translocation of IAP from the enterocyte brush border to the interior of the cell whereas other brush border enzymes were unaffected. By electron microscopy, the translocation occurred by a rapid (5 min) induction of endocytosis via clathrin-coated pits. By 60 min, IAP was seen in subapical endosomes and along membranes surrounding fat droplets. IAP is a well-known lipid raft-associated protein, and fat absorption was accompanied by a marked change in the density and morphology of the detergent resistant membranes (DRM's) harbouring IAP. A lipid analysis revealed that fat absorption caused a marked increase in the microvillar membrane contents of free fatty acids. In conclusion, fat absorption rapidly induces a transient clathrin-dependent endocytosis via coated pits from the enterocyte brush border. The process selectively internalizes IAP and may contribute to the appearance of the enzyme in serum and SLP's.
