Permeabilization of enterocytes induced by absorption of dietary fat

Absorption of dietary fat in the small intestine involves epithelial exposure to potentially harmful molecules such as bile salts and free fatty acids. We used organ culture of porcine jejunal explants incubated with a pre-digested mixture of fat (plant oil), bile and pancreatin to mimick the physiological process of dietary fat absorption, and short exposures to the fat mixture caused fat droplet accumulation within villus enterocytes. Lucifer yellow (LY), a fluorescent membrane-impermeable polar tracer was included to monitor epithelial integrity. Both in controls and during fat absorption LY penetrated the epithelium and accumulated in the basal lamina and the lamina propria. LY was also seen in the paracellular space, whereas villus enterocytes were generally only weakly labeled except for small amounts taken up by apical endocytosis. In the crypts, however, fat absorption induced cell permeabilization with LY accumulating in the cytosol and nucleus. Morphologically, both apical and basolateral membranes appeared intact, indicating that the leakiness was caused by minor lesions in the membrane. Albeit to a lesser extent, bile alone was capable of permeabilizing crypt cells, implying that the surfactant properties of bile salts are involved in the process. In addition to LY, crypt enterocytes also became permeable for albumin, ovalbumin and insulin. In conclusion, during fat absorption the permeability of the gut epithelium is increased mainly in the crypts. A possible explanation is that cell membranes of immature crypt cells, lacking detergent-resistant lipid raft microdomains, are less resistant to the deleterious effects of bile salts and free fatty acids.