Evidence for separate pathways of chylomicron and very low-density lipoprotein assembly and transport by rat small intestine

Abstract

Previously, we demonstrated that the hydrophobic surfactant Pluronic L-81 (L-81) inhibits the intestinal formation and transport of chylomicrons (CM) but not of very low-density lipoprotein-sized (VLDL) particles. The present study was undertaken to determine whether infusion of egg lecithin results mainly in secretion of VLDL by the small intestine and whether L-81 has any effect on their formation and secretion. Intestinal fistula rats were infused intraduodenally at a rate of 3 ml/h with a lipid emulsion containing 20 mM egg lecithin and 19 mM sodium taurocholate for 8 h. This was then followed by another 8 h of infusion of a similar lipid emulsion but with 0.5 mg/h of L-81 added. Lymphatic lipid output was measured, and lymph lipoproteins were sized by use of electron microscopy. Whether L-81 was present or not, no significant difference was detected in the lymphatic triglyceride, phospholipid, or cholesterol outputs. Based on agarose gel electrophoresis, sizing of intestinal lymph lipoproteins, and also the determination of lipid in the intestinal lymph CM and VLDL as separated by ultracentrifugation, VLDL were the major lipoproteins present in lymph during the infusion of egg lecithin. Thus, intraduodenal infusion of egg lecithin in the rat results mainly in the transport of VLDL and is not affected by the administration of L-81. The results suggest that CM and VLDL are assembled separately by the enterocytes and indicate the usefulness of L-81 in further investigating the pathways and regulation of intestinal lipoprotein synthesis, assembly, and secretion.

Intestinal alkaline phosphatase release is not associated with chylomicron formation

“…2) indicates that the amount of Pluronic L-81 used in our experiment was clearly sufficient to block the formation of chylomicrons and thus prevent the increase in lymphatic triacylglycerol output during fat absorption. These data support numerous other reports of the effects of Pluronic L-81 on chylomicron formation (21,(24)(25)(26)(27).…”

“…Chylomicron formation was inhibited by the compound Pluronic L-81 (21,(24)(25)(26)(27), and yet, it is evident from Figs. 3 and 4 that Pluronic L-81 did not inhibit increase in lymphatic alkaline phosphatase secretion induced by active lipid absorption.…”

“…Once inside the intestinal cells, triacylglycerols are synthesized from the fatty acids and 2-monoacylglycerols and transported to the lymph via chylomicrons. Because Pluronic L-81 blocks chylomicron formation and secretion, but does not interfere with the uptake and the reesterification of monoacylglycerols and fatty acids to form triacylglycerols (21,(24)(25)(26)(27), we determined whether the stimulation of lymphatic alkaline phosphatase secretion by lipid absorption is mediated by the presence of intracellular lipid or by the formation and secretion of chylomicrons. As shown in Fig.…”

“…Olestra, a fat substitute comprising sucrose esterified with 6-8 fatty acids (16)(17)(18), is currently commercialized in food products. In addition, we used the hydrophobic surfactant Pluronic L-81, because it blocks the formation of chylomicrons by blocking trafficking of prechylomicrons from endoplasmic reticulum to Golgi apparatus without altering reesterification of the digestion products (21,(24)(25)(26)(27).…”

Cholesterol: from feeding to gene regulation

“…Caco-2 cells) have been used extensively to study the formation and the secretion of lipoproteins [49,54,55,74,113] and the genetic expression and posttranslational modification of apolipoproteins [75]. During fasting, very low-density lipoproteins (VLDLs) are the only lipoproteins produced by the small intestine [87,88,114]. Following a meal, the small intestine produces predominantly CMs [114,135].…”