LIPID, APOLIPOPROTEIN, AND LIPOPROTEIN SYNTHESIS AND SECRETION DURINGCELLULAR-DIFFERENTIATION IN CACO-2 CELLS

Although Caco-2 cells are frequently employed for the study of enteroc yte lipid metabolism, variable results have been reported regarding th eir ability to synthesize and secrete lipids and apolipoproteins. The major goal of this investigation is to examine the capacity of Caco-2 cells to elaborate and secrete lipids, lipoproteins, and apolipoprotei ns at different degrees of morphological and functional differentiatio n. Cells were cultured in medium with 5% fetal bovine serum (FBS), on permeable polycarbonate filters from 2 to 30 d in the presence of C-14 -oleate or S-35-methionine. Cellular differentiation, as assessed by m orphology (light and electron microscopy), transepithelial resistance, free fatty acid flux, and sucrase activity. progressed steadily up to 20 d of culture. Caco-2 cells esterified oleic acid mainly into phosp holipids, triglycerides (TG), and smaller amounts of cholesterol ester s. Lipid synthesis began as early as 2 d, and TG secretion was enhance d with increased duration of culture. However, very low efficiency of lipid export was observed at all levels of differentiation, reaching a maximum of only 6% of intracellular lipids. VLDL and LDL were the dom inant Lipoproteins secreted, with HDL comprising < 20% of the total. V LDL secretion increased, while LDL decreased, whereas the Lipid compos ition of lipoproteins varied little with increasing duration of cultur e. Apoprotein B and A-I synthesis and secretion increased markedly fro m 11 to 20 d of culture, The ratio of apo B-100/B-48 decreased between II and 30 d, consistent with enhanced apo B editing of more mature en terocytes. Taken together, our data suggest that from 20 d of culture, Caco-2 cells are morphologically and functionally mature, capable of lipid esterification, and lipoprotein and apolipoprotein synthesis. Ho wever, despite their functional and morphological similarities to matu re enterocytes, Caco-2 cells have a very limited lipid export capacity .