COMMON CHARACTERISTICS FOR NA+-DEPENDENT SUGAR-TRANSPORT IN CACO-2 CELLS AND HUMAN-FETAL COLON

The recent demonstration that the human colon adenocarcinoma cell line Caco-2 was susceptible to spontaneous enterocytic differentiation led us to consider the question as to whether Caco-2 cells would exhibit sodium-coupled transport of sugars. This problem was investigated using isotopic tracer flux measurements of the nonmetabolizable sugar analog .alpha.-methyl-glucoside (AMG). AMG accumulation in confluent monolayers was inhibited the same extent by sodium replacement, 200 .mu.M phlorizin, 1 mM phloretin, and 25 mM D-glucose, but was not inhibited further in the presence of both phlorizin and phloretin. Kinetic studies were compatible with the presence of both a simple diffusive process and a single, Na+-dependent, phlorizin- and phloretin-sensitive AMG transport system. These results also ruled out any interaction between AMG and a Na+-independent, phloretin-sensitive, facilitated diffusion pathway. The brush-border membrane localization of the Na+-dependent system was inferred from the observations that its functional differentiation was synchronous with the development of brush-border membrane enzyme activities and that phlorizin and phloretin addition 1 hr after initiating sugar transport produced immediate inhibition of AMG uptake as compared to ouabain. Finally, it was shown that brush-border membrane vesicles isolated from the human fetal colonic mucosa do possess a Na+-dependent transport pathway(s) for D-glucose which was inhibited by AMG and both phlorizin and phloretin. Caco-2 cells thus appear as a valuable cell culture model to study the mechanisms involved in the differentiation and regulation of intestinal transport functions.