An electron microscopic study of endogenous very low density lipoprotein production in the intestine of rat and man

Direct evidence for a two-step assembly of ApoB48-containing lipoproteins in the lumen of the smooth endoplasmic reticulum of rabbit enterocytes

The aim of this study was to investigate the types and characteristics of chylomicron precursors in the lumen of the secretory compartment of rabbit enterocytes. Luminal contents were separated into density subfractions in two continuous self-generating gradients of different density profiles. In enterocytes from rabbits fed a low fat diet, newly synthesized and immunodetectable apoB48 was only in the subfraction of density similar to high density lipoprotein (dense particles); the luminal triacylglycerol (TAG) content was low and only in the subfraction of density similar to that of chylomicrons/very low density lipoproteins (light particles). After feeding fat, newly synthesized, and immunodetectable apoB48 was in both dense (phospholipid-rich) and light (TAG-rich) particles. Luminal TAG mass and synthesis increased after fat feeding and was only in light particles. Pulse-chase experiments showed that the luminal-radiolabeled apoB48 lost from the dense particles was recovered in the light particles and the secreted chylomicrons. All of the light particle lipids (mass and newly synthesized) co-immunoprecipitated with apoB48. However, in the dense particles, there was a preferential co-precipitation of the preexisting rather than newly synthesized phospholipid. Assembly of apoB48-containing TAG-enriched lipoproteins is therefore a two-step process. The first step produces dense apoB48 phospholipid-rich particles, which accumulate in the smooth endoplasmic reticulum lumen. In the second step, these dense particles rapidly acquire the bulk of the TAG and additional phospholipid in a single and rapid step.

Synthesis, secretion and immunoelectron microscopic demonstration of apolipoprotein B-containing lipoprotein particles in the visceral rat yolk sac

Electron microscopic investigations on the involvement of the fetal membranes of the rat (visceral yolk sac) in the lipid metabolism revealed the occurrence of lipoprotein-sized particles located in cisternal Golgi stacks, Golgi vesicles and secretory vesicles of the cells of the visceral yolk sac epithelium as well as in distended areas of the intercellular space between adjacent epithelial cells. Application of the protein A-gold technique with specific anti-apoB antiserum resulted in a specific location of immunogold both over the different compartments of the lipoprotein pathway (RER, Golgi complex, secretory vesicles) as well as over the distended intercellular spaces, thus confirming these particles to be lipoproteins in nature. Isolated visceral epithelial cells prepared by a tryptic digestion method exhibited some ultrastructural alterations, such as a loss of apical brush border, a change from columnar to spherical cell shape, a decrease in phagolysosomes, but an increase in autophagosomal structures after 6 h incubation at a vitality rate of at least 85%. Within this period the epithelial cells secreted measurable amounts of apoB-containing lipoproteins into the medium floating in the density classes d less than 1.006 g/ml, d = 1.006-1.020 g/ml and d = 1.020-1.064 g/ml. The production of the lipoproteins was partly inhibited by cycloheximide indicating the secretion of particles with performed as well as newly synthesized apoB. Negative staining of the particles revealed an average diameter of 34 nm of VLDL, 31 nm of IDL and 24 nm of LDL. In summary, our studies demonstrate that in the feto-placental unit of the rat the fetal membranes are capable of synthesizing and secreting lipoproteins. The cells of the visceral yolk sac epithelium were shown to be the producers of apoB-containing particles.

The ontogeny and site of intestinal lipid and lipoprotein synthesis

The developmental aspects of characteristic intestinal lipoprotein synthesis, chlomicrons (CM), very low density lipoproteins (VLDL) and high density lipoproteins (HDL), are unknown. Our objective was to determine the ontogeny of intestinal lipid and lipoprotein synthesis in both the jejunum and the ileum. Explants of the jejunum and the ileum from fetal (F) (18-19 days of gestation), suckling (S) (5 days old) and weaning (W) (23 days old) rats were cultured in the presence of [14C]-oleic acid to examine lipid synthesis. The results indicate the following. (1) The incorporation of oleic acid is higher for the fetal explants. However, the efficiency of esterification of free fatty acids (FFA) into triglycerides (TG) in the jejunum increases with age (33% F, 37% S, 48% W) (P less than 0.05, by ANOVA). (2) The same profile is found at the ideal site for the incorporation of oleic acid. However, the capacity for the synthesis of TG is more intense at the suckling period (34% F, 54% S, 42% W) (P less than 0.05, by ANOVA). (3) The relative content of TG in CM changes with age: F, 90-93%; S, 80-84%; W, 33-40%. (4) A low percentage of TG content is found in CM at the weaning period while high levels are detectable in VLDL (40-42%). (5) A most significant difference is noted between the jejunum and the ileum in TG synthesis at the suckling period. The ileum synthesizes 53% more TG than the jejunum (P less than 0.025). (6) HDL particles contain substantial amounts of FFA. Nevertheless, they were also found to be able to transport TG mainly in the suckling rat. Thus, this study demonstrates that with growth the rat is able to synthesize CM, VLDL and HDL. Our findings indicate changes in the synthesis of intestinal lipids and lipoproteins, depending on both the development and the site, which suggests an ontogeny. These modifications can be attributed to dietary and hormonal influences present during the period of development.

Immunocytochemical localization of hepatic fatty acid binding protein in the rat intestine: effect of fasting

Localization of hepatic fatty acid binding protein (h-FABP) in the small and large intestines of rats was studied by light and electron microscopic histochemistry using the peroxidase-antiperoxidase (PAP) method. In the small intestine of rats fed ad libitum, an intense FABP immunoreactivity was confined to the absorptive epithelial cells of the villi, but not of the crypts. The lowest margin of the immunoreactive cell sheet was closer to the crypts in the proximal than in the distal portions of the small intestine. In the large intestine, FABP immunoreactivity was present in the surface epithelial cells, with higher intensity in the proximal than in the distal portions of the intestine. After fasting rats for two days, many crypt cells exhibited intense immunoreactivity for h-FABP, resulting in an extension of the lowest margin of the immunoreactive cell sheet deep into the crypts. Such expansion of the immunoreactive cell population was reversed by refeeding the animals. With regard to the intracellular localization of immunoreactivity in the jejunum, the basolateral portion of the cytoplasm exhibited a more intense immunoreaction than the apical portion in the majority of immunoreactive cells lining the villi, whether the animals were fed or fasted. The immunoreactive products appeared in the cytoplasmic matrix without association with any subcellular structures.(ABSTRACT TRUNCATED AT 250 WORDS)

A post-embedding immunoelectron-microscopic demonstration of apolipoprotein-B-containing lipoprotein particles in hepatocytes of fetal rats

We used the protein-A gold technique to demonstrate the presence of apolipoprotein-B in ultrathin sections of fetal rat liver tissue. It was possible to show for the first time that the electron-dense, osmiophilic particles with diameters of 20-40 nm located within the RER cisternae and Golgi complexes of fetal rat hepatocytes contain apolipoprotein-B components and therefore are lipoproteins. After specific labelling an accumulation of gold label was observed on the RER cisternae, Golgi cisternae and the Golgi-associated secretory vesicles of hepatocytes. The specificity of this labelling pattern was assessed by comparison with cytochemical controls. Our qualitative findings were confirmed by a quantitative analysis of the mean labelling intensity (mean number of gold particles per square micron of the surface area of a particular cellular compartment) on the RER, Golgi complexes, mitochondria, nuclei and the remaining cytoplasm of hepatocytes. It is concluded that the hepatocytes of fetal rats are capable of forming apolipoprotein-B-containing lipoprotein particles. With respect to the size-distribution pattern of the observed intra-hepatic lipoprotein particles, we suggest that the hepatocytes of fetal rats produce lipoproteins of the low- and very low-density-lipoprotein type.

Comparative electron microscopic features of normal intermediate and metaplastic pyloric epithelium

The ultrastructural appearance of surface epithelial cells of normal and abnormal pyloric epithelium is presented. In addition to areas of complete intestinal metaplasia (IM) an incomplete metaplasia is described which contains poorly differentiated mucous cells, goblet cells, an occasional immature absorptive cell as well as the mature mucus-secreting surface epithelial cells of normal pyloric epithelium. This type of epithelium frequently appeared hyperplastic and showed alterations in the surface microvilli, terminal webs, plasma membranes and mucin granules. Apical vesicles, small electron-dense bodies and larger electron-dense multivesicular bodies were also found in the intermediate cells of incomplete IM and in the mature absorptive cells of complete IM. The intermediate cells described here may be synonymous with the sulphomucin-secreting cells found in adenocarcinomas of the stomach and oesophagus.

Lipoprotein particles in the jejunal mucosa of postnatal developing rats

The jejunal mucosa of neonatal rats contains lipid particles of the same size, electron density and intracellular and extracellular distribution as particles identified by others in adult jejunum as lipoprotein particles. As in fetal jejunum obtained during the last three days of gestation, the jejunal mucosa of unsuckled newborn rats contains exclusively lipoprotein particles the size of very low density lipoproteins (VLDL). Within one day after initiation of suckling, there is in the mucosa a spectrum of lipoprotein particles ranging widely in size from those of VLDL particles to those of chylomicrons. These particles are seen in the endoplasmic reticulum and Golgi material of absorptive cells and within interepithelial cell spaces, the extracellular spaces of the lamina propria and lymphatic lacteals. VLDL-sized and chylomicron-sized particles are also seen, although in decreasing number, in the jejunal mucosa of 18-day-old suckling rats. However, in rats of comparable age, fasted for 48 or 72 hours, only VLDL-sized particles are seen in the jejunal mucosa. Ligation and transection of bile duct followed by fasting in rats of this age results in a marked decrease in the number of lipoprotein particles in absorptive cells. The results indicate that endogenous lipid contributes to the formation of VLDL particles whereas dietary triglycerides are needed for formation of chylomicrons.

Metabolism of triglyceride in rabbit plasma low and high density lipoproteins: studies in vivo and in vitro

The metabolism of triglyceride in the plasma low density lipoproteins (LDL) and high density lipoproteins (HDL) was studied in the normal-fed rabbit. Preparations of LDL and HDL la-eled with 3H in the triglyceride moiety were obtained from donor rabbits that had previously been injected with 3H-palmitic acid. These labeled lipoproteins were subsequently either reinjected into other rabbits or incubated in vitro with unlabeled lipoprotein fractions. In vivo, there were bidirectional transfers of 3H-triglyceride between the LDL and HDL, the rate of which considerably exceeded that of any irreversible removal from the plasma compartment. To a much lesser extent, there was also a transfer of 3H-triglyceride from both LDL and HDL to very low density lipoproteins (VLDL). In vitro, so long as the dialysed 1.21 g/ml infranate of serum was present there was a significant transfer of 3H-triglyceride from both LDL and HDL into each of the other lipoprotein fractions that in no case could be accounted for by a corresponding net transfer of triglyceride mass. It was concluded that the pools of triglyceride in LDL and HDL, and to a lesser extent in VLDL, comprise parts of a larger, progressively equilibrating pool.

Increased production of triglyceride-rich lipoproteins after partial biliary diversion in the rhesus monkey

An interrelationship between bile acid and triglyceride metabolism is suggested by observations in patients with endogenous hypertriglyceridemia and in subjects treated with bile acid-binding resins or chenodeoxycholic acid. We investigated this possible interrelationship in the rhesus monkey. The effect of 14 days of partial bile diversion on plasma concentration and secretion rates of triglyceride-rich lipoproteins was studied by means of a stream-splitting device that permitted controlled interruption of the enterohepatic circulation. Diversion of 25% of the enterohepatic circulation resulted in increased bile acid synthesis and decreased bile acid secretion and pool size, and was associated with a 75% increase in fasting serum triglyceride concentration and a twofold increase in the rate of secretion of triglyceride-rich lipoproteins into plasma. Intraduodenal replacemtn of cholic acid during the period of bile diversion completely prevented the increase in lipoprotein secretion and partially prevented the increase in fasting plasma triglycerides, suggesting that these changes resulted from loss of bile acid rather than other components of bile. These studies show that partial removal of bile acids from the enterohepatic circulation augments the secretion of triglyceride-rich lipoprotein into plasma in the rhesus monkey. Elucidation of the mechanism of this effect may provide important new information concerning the control of hepatic lipoprotein secretion.

Intestinal contribution to secretion of very low density lipoproteins into plasma

To resolve the question of the magnitude of the intestine's contribution to circulating very low density lipoproteins (VLDL), measurements of intestinal, hepatic, and total VLDL--triglyceride were made on the same animals or on animals studied under comparable conditions. Animals were examined in the fasted state and during infusion of a fat-free meal. Intestinal VLDL secretion was determined through timed collections of lymph from the mesenteric lymph duct; hepatic and total VLDL secretion rates were estimated by the accumulation of plasma VLDL after injections of Triton WR 1339. Results indicate that the intestine contributes only a minor portion (11%) of the amount of triglyceride entering into the plasma compartment in the fasted state. Although intestinal triglyceride production is increased by 50% (p less than 0.01) in fed rats, the overall contribution of the intestine is not significantly altered in fed rats and represents only 14--17% of total body VLDL secretion. Thus, although intestinal VLDL secretion can be modified experimentally, its total impact on endogenous triglyceride production in normotriglyceridemic rats is small.

Localization of apolipoprotein B in intestinal epithelial cells

Indirect immunofluorescence techniques were employed to determine the distribution within intestinal epithelial cells of apolipoprotein B, a protein essential for the normal transport of fat. Isolated intestinal cells were prepared from rats either during active lipid absorption or after biliary diversion. Specific immunofluorescence from an antiserum to apolipoprotein B was detected in the apical portion of epithelial cells from bile-diverted animals, demonstrating that a pool of apolipoprotein B is present in the nonabsorptive epithelial cell and may be a component of intestinal cell membranes. During lipid absorption in normal rats, an early and sustained increase in immunofluorescence was demonstrated, consistent with an increase synthesis of apolipoprotein B during lipid absorption. This study demonstrates the presence of apolipoprotein B within intestinal epithelium and provides evidence for the participation of this apoprotein in intestinal lipid transport.

Contrasting effects on plasma lipoproteins of intravenous versus oral administration of a triglyceride-phospholipid emulsion

The effect of fat on plasma lipoproteins was compared when administered by the oral and intravenous routes to healthy control subjects on a low fat diet for 5-6 weeks. During this time each subject underwent two 5 day periods of fat supplementation with a soya bean triglyceride-egg yolk phospholipid emulsion (Intralipid), once via intragastric tube, once intravenously. Changes in plasma lipoproteins were assessed by measurement of their lipid and protein content. Intragastric fat administration significantly decreased the level of very low density lipoprotein (VLDL), whereas intravenous fat administration caused a significant rise in low density lipoprotein (LDL). Morphological evidence of persistence of exogenous phospholipid in plasma following intravenous fat administration was confirmed by alterations in the fatty acid composition of lecithin in the d 1.006-1.063 fraction of plasma. In vitro studies showed that exchange readily occurred between Intralipid and LDL lecithin, causing the latter to assume a more saturated pattern. It is concluded that the occurrence of similar changes in vivo could explain the rise in low density lipoprotein following intravenous fat, possibly by influencing the catabolic rate of the apoprotein. Hydrolysis of ingested phospholipids during absorption presumably explains why none of these changes occurred after oral fat supplementation, and would seem to be an important regulatory function of the small intestine in relation to plasma lipoprotein metabolism.

Fatty acid composition of plasma lipoproteins in control subjects and in patients with malabsorption

The fatty acid composition of cholesterol (cholesteryl) ester, triglyceride, and lecithin has been investigated in whole plasma and individual lipoproteins of healthy control subjects, of patients with malabsorption, and also of patients without malabsorption. The results show a decreased proportion of the essential fatty acid linoleic acid in all three lipid classes in both groups of patients as compared with the healthy controls. This abnormality was more marked in the malabsorbers, especially those with steatorrhoea secondary to intestinal resection. Unequivocal biochemical evidence of essential fatty acid deficiency, as indicated by the appearance of 5, 8, 11 eicosatrienoic acid in plasma lecithin, was observed in two patients, both of whom had undergone major intestinal resections. The results suggest that intestinal resection predisposes to the development of essential fatty acid deficiency.