The regulation of apolipoprotein B (apo B) metabolism by eicosapentaenoic acid was investigated in CaCo-2 cells. Cells cultured on semipermeable membranes that separated an upper from a lower well were incubated for 48 hours with albumin alone or 1 mM eicosapentaenoic acid or oleic acid attached to albumin (4:1, mol/mol). Compared with cells incubated with oleic acid, cells incubated with eicosapentaenoic acid synthesized and secreted less [3H]glycerol-labeled triglycerides. Although both fatty acids increased cellular triglyceride mass compared with control cells, less triglycerides accumulated in cells incubated with the n-3 fatty acid. The secretion of triglyceride and apo B mass by cells incubated with eicosapentaenoic acid was less than that observed by cells incubated with oleate. The amount of apo B mass within cells, however, was not altered by either of the fatty acids and was similar to amounts found in control cells. Apo B mRNA abundance was decreased fourfold in cells exposed for 48 hours to eicosapentaenoic acid. In contrast, in cells incubated with oleic acid, apo B mRNA levels were not significantly altered. Pulse-chase experiments were performed to investigate the regulation of apo B synthesis and degradation by the fatty acids. In cells incubated with eicosapentaenoic acid, the synthesis and basolateral secretion of newly synthesized apo B-100 and apo B-48 were significantly less compared with control cells or cells incubated with oleic acid. In contrast, the synthesis and secretion of newly synthesized apo B in cells exposed to oleic acid were similar to control cells. Rates of apo A-I synthesis were similar in cells incubated with either of the fatty acids. Compared with control cells and cells incubated with eicosapentaenoic acid, the residence time of labeled apo B in cells incubated with oleic acid was prolonged. The percentage of newly synthesized apo B that was degraded was less in cells incubated with oleic acid. In contrast, residence times and the percentages of apo A-I and apo B-48 degraded were similar in control cells and cells incubated with the fatty acids. Thus, in CaCo-2 cells, compared with the effects of oleic acid, eicosapentaenoic acid impairs triglyceride transport in part by inhibiting apo B synthesis and secretion. The inhibition of apo B synthesis by eicosapentaenoic acid may be related to a decrease in gene transcription or a decrease in mRNA stability, as apo B mRNA levels were significantly decreased in cells incubated with this fatty acid.(ABSTRACT TRUNCATED AT 400 WORDS)