Hepatocellular triglyceride synthesis and transfer to lipid droplets and nascent very low density lipoproteins.

Abstract

The transfer of triglyceride from sites of synthesis in the endoplasmic reticulum to cytoplasmic lipid droplets and nascent VLDL (very low density lipoproteins) in rat liver in vivo has been examined with [3H]glycerol, cell fractionation, and electron microscopy. Rates of mass transfer of newly synthesized triglyceride were estimated from the specific radioactivity of triglyceride present in microsomal membranes and the radioactivity observed in recipient triglyceride pools. Fasting decreased the transfer of triglyceride to nascent VLDL without affecting transfer to lipid droplets. Stimulation of triglyceride synthesis with 2-tetradecylglycidic acid (TDGA) increased transfer of triglyceride to nascent VLDL 5-fold, and to lipid droplets 14-fold, 1 hr after TDGA administration. Triglyceride transfer to nascent VLDL was increased 6-fold, and to lipid droplets 37-fold, above control rates 6 hr following TDGA treatment, indicative of saturation of triglyceride assembly into nascent VLDL and storage of excess triglyceride in lipid droplet reservoirs. These liver triglyceride pools were concurrently expanded and electron microscopy demonstrated more abundant VLDL particles in the endoplasmic reticulum together with a proliferation of lipid droplets in hepatocytes. TDGA progressively decreased hepatic sn-glycerol-3-phosphate in fasting rats while triglyceride synthesis increased, indicating that sn-glycerol-3-phosphate does not limit the rate of triglyceride synthesis in this metabolic state. Results implicate triglyceride transfer from endoplasmic reticulum membranes to nascent VLDL as a regulated determinant of hepatic VLDL assembly and VLDL triglyceride secretion in vivo.