Non-esterified fatty acids (NEFAs) and insulin are important factors in the control of lipoprotein secretion, but the mechanism of action is unclear. The present study was undertaken to determine whether insulin and NEFAs modulated hepatic secretion of triacylglycerol and apolipoprotein B (apo-B) by regulation of hepatic intracellular apo-B content. The experiments were performed with the human hepatoblastoma cell line Hep G2, for periods of up to 72 h in the presence and absence of NEFAs and insulin. Higher concentrations of eicosapentanoate (EPA) sustained for 72 h decreased cellular protein content (at 250 microM) or caused cell death (at 750 microM), and this effect was not observed with the other NEFAs studied, whereas 75 microM-EPA did not affect cell viability. Compared with the absence of NEFA, 75 microM-EPA did not alter the intracellular triacylglycerol content, but decreased the intracellular content of apo-B by 47% (P < 0.01) and decreased secreted triacylglycerol and secreted apo-B by 13% (P < 0.05) and 21% (P < 0.01) respectively, after 72 h. However 250 microM-oleate increased the intracellular triacylglycerol by 36% (P < 0.01), intracellular apo-B by 22% (P < 0.05) and secreted triacylglycerol and apo-B by 20-30% (P < 0.05-0.01). Insulin decreased secreted triacylglycerol and apo-B in the presence of each NEFA studied by 20-30%. There was no correlation between the changes in intracellular triacylglycerol and the rate of secretion. However, when the secreted triacylglycerol or apo-B was plotted against intracellular apo-B content a significant correlation was observed (r = 0.89, P < 0.001 for both analyses). Apo-B mRNA levels did not change after 72 h incubation with oleate or EPA. These results demonstrate that EPA can be toxic to hepatocytes and that NEFAs and insulin control secretion of triacylglycerol and apo-B by regulation of the intracellular apo-B concentration, thus controlling assembly of apo-B with triacylglycerol to form lipoproteins.