Electron microscopic studies of the assembly, intracellular transport, and secretion of chylomicrons by rat intestine

Synthesis and release of lipids and lipoproteins by isolated rat jejunal enterocytes in the presence of sodium taurocholate

Isolated rat jejunal villus and crypt cells prepared by differential scraping and hyaluronidase dispersion were used in the presence of 8 mM sodium taurocholate to study the incorporation of sn-[3H]glycerol-2-monooleate, [1-14C]palmitate, [1-14C]acetate, L-[4,5(n)-3H]leucine and D-[1-14C]glucosamine into cellular and medium lipids and proteins, respectively. The villus cells were capable of an apparently normal biosynthesis of triacylglycerols and phospholipids, as well as of proteins and glycoproteins despite an altered dye permeability and increased release of cytosolic and membrane enzymes. About 20-30% of the newly formed triacylglycerols and about 35% of the newly formed phospholipids were secreted into the medium and were recovered as triacylglycerol-rich particles. Labelled proteins and glycoproteins were also recovered from this fraction. The crypt cells synthesized about one-half as much triacylglycerol and phospholipid as did the villus cells, but secreted little or no labelled lipid into the postincubation medium. The release into the medium of triacylglycerols synthesized by the villus cells was blocked by a pretreatment of the isolated cells with the microtubule disruptors, nocodazole, colchicine and colcemid; by the amino sugar, D-galactosamine; by the inhibitors of protein synthesis, puromycin and cycloheximide, and by the inhibitor of the biosynthesis of phosphatidylcholine, chlorocholine. These results indicate that the secretion of labelled lipids, proteins and glycoproteins by the upper villus enterocytes in the presence of sodium taurocholate is not entirely due to cell breakage and spillage of contents. It is concluded that incubations of isolated villus cells of rat jejunum with mixed micellar solutions containing 8 mM taurocholate are compatible with an apparently normal biosynthesis and secretion of triacylglycerol-rich particles.

Lipid absorption and intestinal lipoprotein formation

Lipid absorption is a complex process which involves coordinated gastric, intestinal, biliary and pancreatic function. Emulsification of dietary lipid occurs in the stomach and upper intestine where a series of enzymic events also occur. Phospholipids are digested by phospholipases. Colipase anchors lipase to the emulsion surface overcoming the interfering effect of bile salts. The products of lipolysis, monoglycerides and fatty acids, are removed from the emulsion surface by bile salts in the form of mixed micelles which transport lipid digestive products across the unstirred water layer to the epithelial cell. Within the intestinal epithelial cell a series of synthetic events occur resulting in the formation of chylomicrons and very low density lipoprotein (VLDL). Chylomicrons consists of an oily core of triglyceride surrounded by a membrane of phospholipids, free cholesterol and apoproteins which maintain the solubility of the particle in plasma. Chylomicrons from both experimental animals and man have specific apoproteins associated with them. These proteins include apoA-I, the major protein of plasma high density lipoproteins. During chylomicron metabolism, apoA-I and phospholipid from the chylomicron surface contribute to plasma high density lipoproteins. Other chylomicron apoproteins include apoB and apoA-IV, which are synthesized in the intestine, and apoC and apoE which are absorbed onto the chylomicron surface from other lipoproteins. The intestines also synthesizes, very low density sized particles (VLDL) while fasting and during lipid absorption. There is evidence that the intestine also synthesizes high density lipoproteins. The intestine has recently been recognized as a major site of synthesis of plasma lipoproteins and apoproteins, especially apoA-I for plasma high density lipoproteins.

Milk lipid absorption and chylomicron production in the suckling rat

Structural correlates of milk lipid absorption and chylomicron production were studied in 10-day-old suckled rats. The gastric and duodenal contents and duodenal mucosae were examined with the light and electron microscopes. In the gastric lumen the milk lipid globule cores were smooth, circular and uniformly electron opaque. Many membranes and lamellar structures with a trilaminar and multilamellar appearance were adherent to the peripheries of the cores. In the central duodenal lumen the milk lipid globule cores were also smooth, circular and uniformly electron opaque. Very few milk lipid globules in the duodenal lumen showed adherent membranes or lamellae. Membrane fragments and lamellae were present in the lumen separate from the milk lipid globules. In the duodenal lumen between villi the milk lipid globules had multiple electron lucent indentations of the core. It is believed that the irregular peripheries of the milk lipid globule cores are the result of lipolysis within the duodenal lumen acting at the milk lipid globule surface. This lipolysis of triacylglycerol would produce amphiphilic lipids which may result in the electron lucent spaces at the milk lipid globule periphery. The absorptive epithelial cells along the length of the duodenal villus varied in structure relative to their position at the tip, middle, or base of the villus. Typical mid-villus epithelial cells contained lipid droplets averaging 0.3-micrometer diameter in the smooth and rough endoplasmic reticulum and in Golgi complexes in the apical cytoplasm. Villus tip and villus base cells contained large lipid droplets between 7-16 micrometers. Only a few 0.3-micrometer lipid droplets were present within these cells. These large lipid droplets appeared to be accumulations of triacylglycerol present in the apical cytoplasm associated with lamellar and membranous structures. Numerous chylomicrons were present between epithelial cells located in the middle region of the villus while significantly fewer chylomicrons were seen between epithelial cells at the tip and base of the villus. These observations suggest that the cells at the middle of the duodenal villus of suckling rats were more efficient in the production of chylomicron triacylglycerol derived from incoming milk triacylglycerol than cells at the tip and base of the villus.

Influence of cholesterol/fat feeding on cholesterol esterification and morphological structures in intestinal mucosa from guinea pigs

Guinea pigs were fed a semisynthetic diet containing 10% (by weight) cottonseed oil and 1% cholesterol. In response to cholesterol/fat feeding there was an increase in both the unesterified cholesterol (UC) and cholesteryl ester (CE) of the intestinal mucosal cell. Along with the increased cholesterol levels there was a 4-fold increase in the microsomal acylCoA:cholesterol acyltransferase (ACAT) activity after only two days of cholesterol/fat feeding. After 6 days on the experimental diet the ACAT activity was up to 8-fold the activity of the control, and then remained at this level for up to 20 days. The increased ACAT activity was probably not due to increased substrate concentration alone, since the fractional esterification of cholesterol also increased when the cholesterol/fat containing diet was given. There was also an increase in the triglyceride content of the intestinal mucosal cells from guinea pigs on the experimental diet. The mucosal cells of the cholesterol/fat fed animals accumulated varying amounts of lipid droplets, which were without an enveloping membrane, suggesting that the uptake of lipids from the intestinal lumen was higher than the capacity to synthesize and/or secrete lipoproteins. Simultaneously the size and amount of secondary lysosomes increased. A considerable increase in lipid droplets, lipolysosomes, and residual bodies was observed in the lamina propria macrophages while no crystalline clefts were seen.

Apolipoprotein A-IV: a protein occurring in human mesenteric lymph chylomicrons and free in plasma. Isolation and quantification

1. Human mesenteric lymph chylomicrons were isolated from chylous ascites fluid by ultra-centrifugation and agarose/gel chromatography and their apoprotein composition was analysed by dodecylsulfate/polyacrylamide gel electrophoresis, analytical isoelectric focusing and immuno-chemically. Major components of mesenteric lymph chylomicrons were apoprotein A-I, proteins of Mr less than 15 000 including the C-group apoproteins and a protein of Mr 46 000. Minor components were apoprotein E and a protein of Mr approximately equal to 200 000 (B-like protein). This apoprotein composition was qualitatively identical with that of chylomicrons from intestinal lymph of the rat, but was distinctly different from plasma chylomicrons of humans with fasting chylomicronaemia. 2. The protein of Mr approximately equal to 46 000 has been isolated by preparative dodecylsulfate/polyacrylamide gel electrophoresis from human and rat lymph chylomicrons and was compared to a protein of identical Mr present in rat high-density lipoproteins (apoplipoprotein A-IV) and in the rho less than 1.006 g/ml serum lipoprotein fraction of individual humans with alimentary hypertriglyceridaemia. In both species the 46 000-Mr proteins isolated from lymph and serum were identical according to amino acid composition and isoelectric point in 6 M urea. The human proteins from both sources were also immunologically identical. The similarities in the molecular properties of the human apolipoprotein and rat apolipoprotein A-IV indicate that these proteins are homologous. 3. Plasma levels of human apolipoprotein A-IV determined by electroimmunodiffusion were 14.15 +/- 3.66 mg/100 ml (n = 59), but greater than 90% of the protein was unassociated with the major lipoprotein fractions. It is concluded, that apolipoprotein A-IV is a main protein component of human lymph chylomicrons, that is removed from the particles in the plasma compartment.