Apoprotein B in fasting and postprandial human jejunal mucosa

Steatorrhea and disorders of chylomicron synthesis and secretion

Knowledge concerning the absorptive phase of fat remains relatively scanty as compared to the wealth of information available on the digestive phase. However, the past years have seen important developments in our understanding of chylomicron formation and secretion. This has come about thanks to clinical studies of rare congenital disorders of chylomicron synthesis and exocytosis and to the creation of experimental models.

Epitopes of apolipoprotein B-100 and B-48 in both liver and intestine. Expression and evidence for local synthesis in recessive abetalipoproteinemia

The presence of apolipoprotein (apo) B in liver and intestine from a patient with abetalipoproteinemia was evaluated by immunohistochemistry with a polyclonal and six monoclonal antibodies to different apo B-48 and B-100 epitopes. In normal liver, apo B was present inside and outside hepatocytes. The patients liver exhibited staining in the cytoplasm with the polyclonal and three monoclonal antibodies. By immunoelectron-microscopy, apo B was found to be present in the smooth endoplasmatic reticulum and the Golgi complex. Normal intestinal epithelium was labeled with polyclonal and all monoclonal antibodies, including those specific for apo B-100. The patients epithelium stained with polyclonal and six monoclonals, and apo B was present in the Golgi complex. Thus, normal intestinal mucosa expressed apo B-48 and B-100 epitopes, which indicates apo B-100 synthesis in the gut. The synthesis of the apo B molecule in the patient seems to be retained in both liver and gut, which suggests a posttranslational defect.

The effects of fat feeding on apolipoprotein AI secretion from rat small intestinal epithelium

The small intestine is known to be an important synthetic site for certain apolipoproteins, which are subsequently secreted from the enterocyte into the mesenteric lymph. We have studied apolipoprotein AI and CIII content of the enterocyte during the course of fat feeding in order to determine their relative synthetic and secretory rates. Rat intestinal enterocytes were isolated from the entire jejunal villus after fat feeding in vivo. The apo AI content fell 50% as determined by RIA one and two hours after fat feeding. By four hours, the intracellular cellular levels had returned to prefeeding levels. These changes in apolipoprotein AI levels were not seen in the terminal ileum. Apolipoprotein CIII levels remained unchanged afer fat feeding. To determine the effect of free fatty acids on apolipoprotein AI secretion, organ culture explants were incubated for four hours in the presence and absence of 360 microM oleic acid bound to albumin. Apolipoprotein AI detected in the incubation media reflected release from the lamina propria (which was not colchicine sensitive), and secretion from the enterocyte (which was inhibited by colchicine). In the absence of oleic acid, enterocyte secretion of apolipoprotein AI accounted for about half of the apo AI recovered in the medium. In the presence of oleic acid, the total apolipoprotein AI content of the tissue increased by 50 percent. A similar increase in colchicine sensitive secretion was observed. The secretion of apolipoprotein AI from explants was more rapid in the presence of oleic acid and began without the half hour lag noted when oleic acid was absent. The mid intestine was most active in the secretion of apolipoprotein AI. These data are consistent with the hypothesis that in the first few hours after feeding the rate of secretion of apolipoprotein AI exceeds the synthetic capacity of the small intestinal epithelium.

The independent synthesis of intermediate density lipoproteins in type III hyperlipoproteinemia

The kinetics of VLDL (Sf 60-400) and IDL (Sf 12-60) B apoprotein were examined in five type III hyperlipoproteinemic subjects. The production rates for B apoprotein were significantly greater in the IDL fraction than in VLDL. Precursor-product relationships between VLDL and IDL B apoprotein illustrated that a significant proportion of IDL B apoprotein was derived from some source other than VLDL catabolism. These observations indicated that, in the five individuals studied, an 'unusual' B apoprotein synthetic pathway operated whereby B apoprotein was directly entering the IDL fraction. Furthermore, this second pathway resulted in an overproduction of IDL B apoprotein and possibly was the major defect leading to the development of the type III hyperlipoproteinemia lipid profile. In two subjects who were restudied following hormonal treatment (estrogen or thyroxine replacement) and in whom the type III hyperlipoproteinemia lipid profile no longer existed, it was found that the pathway for direct synthesis of IDL B apoprotein had been abolished. From these studies we have concluded that a pathway for the direct synthesis of IDL B apoprotein operates in type III hyperlipoproteinemics and it is a major causative factor in the development of the IDL accumulation characteristic of this metabolic disorder.

Detection of apolipoprotein C in human and rat enterocytes

Apoproteins have important physiologic functions in lipoprotein metabolism. Several apoproteins are produced in the intestine including ApoA-I, ApoA-IV, and ApoB. Each appears to participate in intestinal lipid transport. The liver also produces several apoproteins, including ApoC-II and ApoC-III, but the data demonstrating the ability of the intestine to produce ApoC is incomplete. Our aim was to ascertain whether ApoC-II and ApoC-III were present in human and rat jejunum, and if so, whether their presence was altered by fat feeding. The technique of immunolocalization and a newly developed double antibody radioimmunoassay for rat ApoC-III3 were used. ApoC-III3 was found in the supranuclear regions of enterocytes along the entire lengths of villi in the jejuna of 12-h-fasted rats. 1 hour after the gastric ingestion of corn oil. ApoC-III3 was found primarily in between cells and in the lamina propria. Similar results were obtained in human jejunal biopsies with ApoC-II and ApoC-III. ApoC-III3 was also detected by radioimmunoassay in enterocytes isolated from jejuna of neonatal and adult rats. Thus, ApoC-II and ApoC-III are clearly present in the intestine as well as in the liver. In addition, because their localization is altered after fat feeding, they are also likely to be produced in the enterocyte.

Lipoprotein synthesis and secretion by cultured human intestinal mucosa

We tested whether cultured human jejunal mucosa incorporates fatty acid into esterified lipids and whether organ culture can serve as a model system to study lipoprotein secretion by the human gut. Jejunal biopsies obtained from seven subjects were cultured for 24 h in medium containing 2 microCi of [3H]palmitic acid. More than 95% of the radioactivity incorporated by the tissue was found to be associated with esterified lipids: triglycerides, cholesterol esters and phospholipids, 7 . 8 +/- 1 . 2, 0 . 5 +/- 0 . 1 and 14 . 0 +/- 1 . 5 nmol/10 mg weight respectively. During the culture labelled triglycerides, phospholipids and cholesterol esters were secreted to the medium. Most of the newly synthesized esterified lipids in the medium were found in the d < 1 . 019 g/ml and d = 1 . 019--1 . 063 g/ml fractions. The majority of the newly synthesized triglycerides were found in the d < 1 . 019 g/ml fraction. Labelled cholesterol esters were enriched in the d = 1 . 063--1 . 21 g/ml fraction. Boiled biopsies adsorbed negligible amounts of radioactive palmitic acid and did not synthesize esterified lipids. The addition of puromycin to the culture medium and preincubation with colchicine resulted in decreased uptake of the labelled fatty acid and decreased secretion of esterified lipids to the medium. These experiments indicate that cultured human intestinal mucosa is a suitable model to study lipid synthesis and lipoprotein secretion by the human intestine.

Human apolipoprotein A-IV. Intestinal origin and distribution in plasma

The role of the human intestine has been explored as a site of synthesis of apoA-IV, a major apoprotein of human intestinal triglyceride-rich lipoproteins. Intestinal biopsies were performed on normal volunteers while fasting and after lipid ingestion. Indirect immunofluorescence demonstrated a marked increase in immunofluorescence for apoA-IV during lipid absorption consistent with an increased intracellular content. ApoA-IV comprised 10-13% of chylomicron apoprotein and 24-30% of intestinal very low density lipoprotein (VLDL) as assessed by densitometry of sodium dodecyl sulfate gels of lipoproteins from chylous urine (mesenteric lymphatic-urinary fistula) and thoracic duct lymph (postoperative fistula). After one subject with chyluria ingested 40 g of corn oil, triglyceride excretion in urine was accompanied by an increased excretion of apoA-IV. 11.5 g of triglyceride and 81 mg of apoA-IV were recovered in the urine. In chylous urine 56% of apoA-IV was in the triglyceride-rich lipoproteins (chylomicrons and intestinal VLDL) and 44% in the d > 1.006-g/ml fraction. Normal plasma apoA-IV was 15.7+/-0.9 mg/dl (n = 14) whereas four subjects with abetalipoproteinemia had reduced levels 1.2, 7.6, 9.6, and 8.3 mg/dl, respectively. Lipid feeding in normal volunteers resulted in a rise in plasma apoA-IV (16.1+/-0.7 mg/dl to 18.5+/-0.7 mg/dl, n = 5, P < 0.01). In fasting plasma, 98% of apoA-IV was in the d > 1.21-g/ml fraction. In lipemic plasma, 10% of apoA-IV was associated with triglyceride-rich lipoproteins and 90% with the d > 1.21-g/ml fraction. Agarose column chromatography of fasting plasma confirmed that the bulk of plasma apoA-IV is free, unassociated with lipoproteins. These results demonstrate that apoA-IV is present in human intestinal epithelial cells and is secreted as a chylomicron and VLDL apoprotein. Within fasting plasma most of the apoA-IV is found free, unassociated with lipoproteins. After lipid ingestion apoA-IV is also found in plasma chylomicrons indicating that some apoA-IV remains associated with chylomicrons in plasma during chylomicron metabolism, although some may be transferred from the chylomicron surface.

Fat malabsorption--advances in our understanding

The intestinal absorption of triglyceride constitutes a multistep process with active involvement of the pancreatobiliary system, the intestine and lymphatics. It is only through the integrated function of these organs that dietary triglyceride can be efficiently absorbed and delivered to the peripheral blood for subsequent metabolism. In this review we discuss each aspect of triglyceride absorption and chylomicron formation and illustrate how various diseases may interfere with the process resulting in fat malabsorption. In addition, the role of the intestine as a major synthetic source of lipoprotein constituents for circulating lipoproteins is discussed.