Regulation of triglyceride-rich lipoprotein secretion by fatty acids in CaCo-2 cells.

The effect of fatty acids on secretion of triglyceride-rich lipoprotein (d less than 1.006 g/ml) by CaCo-2 cells was studied. Of the fatty acids studied, oleic acid (18:1) was the most potent stimulator of newly synthesized triglyceride secretion in triglyceride-rich lipoproteins followed in descending order by 18:2, 18:3, and 16:0 = 14:0. All the fatty acids increased intracellular triglyceride synthesis. Fatty acids 14:0, 16:0, 18:2, and 18.3 caused similar increases; however, 18:1 caused the highest rates of triglyceride synthesis. Oleic acid (18:1) was used to further study the secretion of lipoproteins of density less than 1.006 g/ml by CaCo-2 cells. There was a step-wise increase in cellular triglyceride synthesis with increasing oleic acid concentration. Above 250 microM of the fatty acid, however, newly synthesized triglyceride secretion in triglyceride-rich lipoproteins plateaued, suggesting saturation of the secretory pathway. After stimulating triglyceride synthesis by oleic acid, radiolabeled triglyceride secreted in triglyceride-rich lipoproteins was initially delayed resulting in a sigmoid-shaped curve for secretion. This was most pronounced in control cells, which were not incubated with the fatty acid. Over 6 hr, cells incubated with oleic acid secreted more newly synthesized triglyceride in triglyceride-rich lipoproteins compared to control cells. The secretion of lipoproteins of density less than 1.006 g/ml was dependent upon protein synthesis and normal microtubular function in as much as cycloheximide and colchicine significantly decreased triglyceride transport without changing cellular triglyceride synthesis. Triglyceride and unesterified cholesterol mass in lipoproteins of density less than 1.006 g/ml were increased 57 and 244%, respectively, in medium from cells incubated with oleic acid compared to control cells. By 24 hr, 0.17% of lipoproteins of density less than 1.006 g/ml were taken up and degraded. Over the same period, approximately 50% of the lipoprotein triglyceride was hydrolyzed. Under conditions whereby lipoprotein secretion was stimulated fourfold by oleic acid, the activities of HMG-CoA reductase and ACAT were unchanged from activities in control cells. The data suggest that CaCo-2 cells secrete triglyceride-rich lipoproteins of density less than 1.006 g/ml in response to fatty acids in the medium. Triglyceride-rich lipoprotein secretion is a saturable process and dependent on protein synthesis and normal microtubular function. An increase in triglyceride-rich lipoprotein secretion is accompanied by an increase in triglyceride mass in lipoproteins of density less than 1.006 g/ml. By 24 hr, significant postsecretory