Lipoprotein synthesis by the colonic adenocarcinoma cell line Caco-2 was investigated to assess the utility of this cell line as a model for the in vitro study of human intestinal lipid metabolism. Electron micrographic analysis of conditioned medium revealed that under basal conditions of culture post-confluent Caco-2 cells synthesize and secrete lipoprotein particles. Lipoproteins of density (d) less than 1.063 g/ml consist of a heterogeneous population of particles (diameter from 10 to 90 nm). This fraction consists of very low density lipoproteins (d less than 1.006 g/ml) and low density lipoproteins (d = 1.019-1.063 g/ml). Analysis by sodium dodecyl sulfate-polyacrylamide gel electrophoresis of [35S]methionine-labeled Caco-2 lipoproteins revealed that very low density lipoproteins contain apolipoprotein E (apoE) and C apolipoproteins, while low density lipoproteins contained apoB-100, apoE, apoA-I, and C apolipoproteins. The 1.063-1.21 g/ml density fraction contained two morphological entities, discoidal (diameter 15.6 +/- 3.9 nm) and round high density lipoprotein particles (diameter 10.2 +/- 2.3 nm). The high density lipoproteins contained apoA-I, apoB-100, apoB-48, apoE, and the C apolipoproteins. Using isoelectric focusing polyacrylamide gel electrophoresis newly secreted apoA-I was identified as pro-apoA-I. ApoE and apoC-III released by Caco-2 cells were highly sialylated. mRNA species for apoA-I, apoC-III, and apoE, but not apoA-IV were identified by Northern blot analysis. ApoA-I, apoB, and apoE were visualized in Caco-2 cells by immunolocalization analysis. This intestinal cell line may be useful for in vitro studies of nutritional and hormonal regulation of lipoprotein synthesis.