Localization of the lipid receptors CD36 and CLA-1/SR-BI in the human gastrointestinal tract: Towards the identification of receptors mediating the intestinal absorption of dietary lipids

The scavenger receptors CLA-1/SR-BI and CD36 interact with native and modif ied lipoproteins and with some anionic phospholipids. In addition, CD36 bin ds/transports long-chain free fatty acids. Recent biochemical evidences ind icates that the rabbit CLA-1/SR-BI receptor can be detected in enterocytes, and previous studies showed the presence of mRNA for both CLA-1/SR-BI and CD36 in some segments of the intestinal tract. These findings prompted us t o study their respective localization and distribution from the human stoma ch to the colorectal segments, using immunohistochemical methods. Their exp ression in the colorectal carcinoma-derived cell line Caco-2 was analyzed b y Northern blotting. In the human intestinal tract, CLA-1/SR-BI was found i n the brush-border membrane of enterocytes from the duodenum to the rectum, However, CD36 was found only in the duodenal and jejunal epithelium, where as enterocytes from other intestinal segments were not stained. In the duod enum and jejunum, CD36 co-localized with CLA-1/SR-BI in the apical membrane of enterocytes. The gastric epithelium was immunonegative for both glycopr oteins. We also found that CLA-1/SR-BI mRNA was expressed in Caco-2 cells a nd that its expression levels increased concomitantly with their differenti ation. In contrast, the CD36 transcript was not found in this colon cell li ne, in agreement with the absence of this protein in colon epithelium. The specific localization of CLA-1/SR-BI and CD36 along the human gastrointesti nal tract and their ability to interact with a large variety of lipids stro ngly support a physiological role for them in absorption of dietary lipids.