Generation of lipid polarity in intestinal epithelial (Caco-2) cells: Sphingolipid synthesis in the Golgi complex and sorting before vesicular traffic to the plasma membrane

Generation of intestinal epithelial lipid polarity was studied in Caco-2 cells. Confluent monolayers on filters incorporated the exchangeable lipid N-6-NBD-aminocaproyl-sphingosine (C6-NBD-ceramide) from liposomes. The fluorescent ceramide was converted equally to C6-NBD-glycosylceramide and C6-NBD-sphingomyelin, analogues of lipids enriched on the apical and basolateral surface, respectively, of intestinal cells in vivo. Below 16°C, where vesicular traffic is essentially blocked, each fluorescent product accumulated in the Golgi area. At 37°C, 50% had been transported to the cell surface within 0.5 h, as measured by selective extraction of the fluorescent lipids onto BSA in the medium ('back-exchange') at 10°C. Transport to the two surfaces could be assayed separately, as a diffusion barrier existed for both NBD-lipids and BSA. C6-NBD-glucosylceramide was enriched twofold apically, whereas C6-NBD-sphingomyelin was equally distributed over both domains. Polarities did not decrease when 37°C incubations were carried out in the presence of increasing BSA concentrations to trap the fluorescent lipids immediately after their arrival at the cel surface. Within 10 min from the start of synthesis, both products displayed their typical surface polarity. Lipid transcytosis displayed a half time of hours. In conclusion, newly synthesized sphingolipids in Caco-2 cells are sorted before reaching the cell surface. Transcytosis is not required for generating the in vivo lipid polarity.