Production of β-Lipoprotein by Intestine in the Rat

Altered Metabolic Profile of Triglyceride-Rich Lipoproteins in Gut-Lymph of Rodent Models of Sepsis and Gut Ischemia-Reperfusion Injury

BACKGROUND

Triglyceride-rich lipoproteins are important in dietary lipid absorption and subsequent energy distribution in the body. Their importance in the gut-lymph may have been overlooked in sepsis, the most common cause of critical illness, and in gut ischemia-reperfusion injury, a common feature of many critical illnesses.

AIMS

We aimed to undertake an exploratory study of triglyceride-rich lipoprotein fractions in gut-lymph using untargeted metabolic profiling to identify altered metabolites in sepsis or gut ischemia-reperfusion.

METHODS

The gut-lymph was collected from rodent sham, sepsis, and gut ischemia-reperfusion models. The triglyceride-rich lipoprotein-enriched fractions isolated from the gut-lymph were subjected to a dual metabolomics analysis approach: non-polar metabolite analysis by ultra-high performance liquid chromatography-mass spectrometry and polar metabolite analysis by gas chromatography-mass spectrometry.

RESULTS

The metabolite analysis of gut-lymph triglyceride-rich lipoprotein fractions revealed a significant increase (FDR-adjusted P value < 0.05) in myo-inositol in the sepsis group and monoacylglycerols [(18:1) and (18:2)] in gut ischemia-reperfusion. There were no significantly increased specific metabolites in the lipoprotein-enriched fractions of both sepsis and gut ischemia-reperfusion. In contrast, there was a widespread decrease in multiple lipid species in sepsis (35 out of 190; adjusted P < 0.05), but not in the gut ischemia-reperfusion.

CONCLUSIONS

Increased levels of myo-inositol and monoacylglycerols, and decreased multiple lipid species in the gut-lymph triglyceride-rich lipoprotein fraction could be candidates for new biomarkers and/or involved in the progression of sepsis and gut ischemia-reperfusion pathobiology.

Fasting and postprandial overproduction of intestinally derived lipoproteins in an animal model of insulin resistance. Evidence that chronic fructose feeding in the hamster is accompanied by enhanced intestinal de novo lipogenesis and ApoB48-containing lipoprotein overproduction

Insulin-resistant states are characterized by hypertriglyceridemia, predominantly because of overproduction of hepatic very low density lipoprotein particles. The additional contribution of intestinal lipoprotein overproduction to the dyslipidemia of insulin-resistant states has not been previously appreciated. Here, we have investigated intestinal lipoprotein production in a fructose-fed hamster model of insulin resistance previously documented to have whole body and hepatic insulin resistance, and hepatic very low density lipoprotein overproduction. Chronic fructose feeding for 3 weeks induced significant oversecretion of apolipoprotein B48 (apoB48)-containing lipoproteins in the fasting state and during steady state fat feeding, based on (a) in vivo Triton WR1339 studies of apoB48 production as well as (b) ex vivo pulse-chase labeling of intestinal enterocytes from fasted and fed hamsters. ApoB48 particle overproduction was accompanied by increased intracellular apoB48 stability, enhanced lipid synthesis, higher abundance of microsomal triglyceride transfer protein mass, and a significant shift toward the secretion of larger chylomicron-like particles. ApoB48 particle overproduction was not observed with short-term fructose feeding or in vitro incubation of enterocytes with fructose. Secretion of intestinal apoB48 and triglyceride was closely linked to intestinal enterocyte de novo lipogenesis, which was up-regulated in fructose-fed hamsters. Inhibition of fatty acid synthesis by cerulenin, a fatty acid synthase inhibitor, resulted in a dose-dependent decrease in intestinal apoB48 secretion. Overall, these findings further suggest that intestinal overproduction of apoB48 lipoproteins should also be considered as a major contributor to the fasting and postprandial dyslipidemia observed in response to chronic fructose feeding and development of an insulin-resistant state.

Modulation of intestinal and liver fatty acid-binding proteins in Caco-2 cells by lipids, hormones and cytokines

Intestinal and liver fatty acid binding proteins (I- and L-FABP) are thought to play a role in enterocyte fatty acid (FA) trafficking. Their modulation by cell differentiation and various potential effectors was investigated in the human Caco-2 cell line. With the acquisition of enterocytic features, Caco-2 cells seeded on plastic progressively increased L-FABP quantities, whereas I-FABP was not detectable even very late in the maturation process. On permeable filters that improved differentiation markers (sucrase, alkaline phosphatase, transepithelial resistance), Caco-2 cells furthered their L-FABP content and expressed I-FABP. Western blot analysis showed a significant increase in I- and L-FABP expression following an 8-hour incubation period with butyric acid, oleic acid, and phosphatidylcholine. However, in all cases, I-FABP levels were higher than L-FABP concentrations regardless of the lipid substrates added. Similarly, hydrocortisone and insulin enhanced the cellular content of I- and L-FABP whereas leptin triggered I-FABP expression only after an 8-hour incubation. Finally, tumor necrosis factor-alpha was more effective in increasing the cytosolic amount of I-FABP levels. In conclusion, our data demonstrate that I-FABP expression is limited to fully differentiated Caco-2 cells and can be more easily regulated than L-FABP by lipids, hormones, and cytokines.

Inhibition of hydroxymethylglutaryl coenzyme A reductase activity does not affect the secretion rate of apolipoproteins B and AI by CaCo-2 cells

It is believed that the major mechanisms by which hydroxymethylglutaryl coenzyme A reductase inhibitors lower plasma cholesterol levels are by inducing hepatic low-density lipoprotein (LDL) receptor activity and by decreasing apolipoprotein B (apoB) secretion by the liver. However, the intestine is also an important cholesterogenic organ and the possibility that this class of drugs may alter lipoprotein secretion by the intestine has not been fully studied. The purpose of the present study was to examine the possible role of cholesterol in regulating apoB secretion by the intestine by testing if the suppression of cholesterol synthesis by the reductase inhibitor lovastatin affected the secretion of apoB by CaCo-2 human intestinal cells. Differentiated post-confluent CaCo-2 cells were incubated for 24-72 h in serum-free medium in the presence or absence of 5 microM lovastatin, and the secretion rate of lipids, as well as apoB and apolipoprotein AI (apoAI) into the medium, was measured. Lovastatin markedly inhibited the incorporation of [1-14C]acetate into cholesterol for at least 48 h, lowered the content of esterified cholesterol in cells, and reduced their rate of cholesterol secretion. However, under basal conditions lovastatin had no effect on the secretion rate of apoB. After stimulation of apoB secretion by addition of 0.8 mM oleic acid to the medium, lovastatin did not alter apoB secretion in the first 2 days of incubation, but reduced the content of apoB in media from the 3rd day by 30%. This could not be explained by an increase in the rate of LDL degradation. Furthermore, supplementation with mevalonic acid only reversed about one-half of the effect of lovastatin, suggesting that this effect was at least parly nonspecific or unrelated to inhibition of cholesterol biosynthesis. There was also no specific effect of lovastatin on apoAI secretion. When cells were cultured with [1-14C]acetate for 24 or 72 h, the specific activity of cholesterol in medium at the end of the incubation was the same as in cells, suggesting that cholesterol used for lipoprotein secretion was in equilibrium with bulk cellular cholesterol and was not from a segregated compartment derived from newly synthesized cholesterol. This may explain why apoB secretion by CaCo-2 cells was unaffected by inhibition of cholesterol synthesis with lovastatin.

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.

Regulation of lipid metabolism by dietary sorbose in laying hens

Single Comb White Leghorn laying hens received ad libitum diets (10 birds per diet) containing varying concentrations of sorbose (0, 100, and 200 g/kg diet) for 4 wk. Body weight gain and feed intake decreased with increasing dietary sorbose. Serum triglyceride, cholesterol, low density lipoprotein, very low density lipoprotein, and chylomicron levels were significantly reduced, in dose-dependent fashions, as dietary sorbose increased, whereas serum glucose level remained unchanged by the dietary regimens. Absolute and relative weights of abdominal fat were also decreased by increasing dietary sorbose. Liver color improved; it became less white and less yellow with the supplement of dietary sorbose. Hen-day egg production rate was not affected with dietary sorbose, and hence, it has been shown that dietary sorbose can be used as a potential regulator of lipid metabolism in the laying hen.

Intestinal very-low-density lipoprotein secretion in the genetically obese Zucker rat

The objectives of this study were to measure intestinal very-low-density lipoprotein (VLDL) production in obese Zucker rats and to assess an eventual effect of a high-fat diet. VLDL secretion was specifically inhibited by orotic acid, and intestinal VLDL output was measured following the Triton WR-1339 method. After a control diet, total VLDL secretion (without orotic acid) was 4.8 +/- 0.3 and 1.4 +/- 0.1 mg triacylglycerol/ml in obese and lean rats, respectively, decreasing by 30% in obese rats after fat-feeding. Intestinal VLDL production was similar in obese and lean rats fed the control diet (0.32 +/- 0.05 and 0.27 +/- 0.05 mg triacylglycerol/ml, respectively), increasing 2.5-fold after fat-feeding in both genotypes. Thus, intestine contributed 21 and 60% of total VLDL in lean but only 7 and 24% in obese rats with the control and high-fat diets, respectively. These results show that the intestine of obese Zucker rats does not contribute to their hypertriglyceridemia, suggesting that it originates solely from liver. Moreover, their intestinal VLDL production was stimulated by fat-feeding to the same extent as in lean animals.

Effects of Triton WR 1339 and orotic acid on lipid metabolism in rats

In order to investigate the effect of hepatic cholesterol flux on biliary bile acids, Triton WR 1339 and orotic acid were administered to rats, and the biliary cholesterol, phospholipids and bile acids were analyzed together with serum lipoproteins and hepatic lipids. Triton, which raised serum very low density lipoprotein and lipid levels and decreased serum high density lipoprotein liver lipid levels, increase the biliary cholic acid group/chenodeoxycholic acid group ratio (CA/CDCA) in the bile without affecting the total amount of bile acids and the other biliary lipids. Orotic acid, which decreased serum lipid and lipoprotein concentrations and increased liver lipid levels, increased the biliary excretion of cholesterol and phospholipids, but produced no significant change in the total amount of bile acids and in the CA/CDCA ratio in bile.

Studies on the in vivo transfer of retinoids from parenchymal to stellate cells in rat liver

Studies were conducted to examine the in vivo transfer of chylomicron (dietary) retinoid from rat liver parenchymal to stellate cells. We specifically addressed the question of whether chylomicron retinyl ester is transferred directly from hepatic parenchymal to stellate cells without first undergoing hydrolysis. [14C]Retinyl palmitate and its non-hydrolyzable ether analog, retinyl [3H]hexadecyl ether, were utilized to answer this question. Chylomicrons labeled with these retinoids were injected intravenously into rats. Liver cell fractions, highly enriched in parenchymal or in stellate cells, were isolated 0.5 h, 4.5 h and 24 h after chylomicron injection. The ratio of 3H: 14C found in parenchymal cell preparations 4.5 h after injection was 1.8 times the ratio for the injected chylomicrons, and 24 h postinjection the ratio had increased to 2.5 times that of the chylomicrons. In the stellate-cell-enriched preparations the 3H: 14C ratio was found to be 0.39, 0.29, and 0.23 times the ratio found in the injected labeled chylomicrons at 0.5 h, 4.5 h and 24 h after injection respectively. From the levels of 14C observed in the isolated stellate cells, it is estimated that 0.5 h postinjection the stellate cells contained approximately 34% of the 14C (i.e. the retinol injected as chylomicron retinyl ester) present in the liver. By 4.5 h the 14C present in isolated stellate cells had risen to approximately 41% of that present in the total liver, and 24 h after injection approximately 55% of hepatic total 14C was found in the stellate cells. These findings suggest that chylomicron retinyl ester is not transferred directly from the parenchymal to stellate cells without first undergoing hydrolysis to retinol.

Ethanol and lipids

The interaction of ethanol with lipid metabolism is complex. When ethanol is present, it becomes a preferred fuel for the liver and displaces fat as a source of energy. This favors fat accumulation. In addition, the altered redox state secondary to the oxidation of ethanol promotes lipogenesis, for instance, through enhanced formation of acylglycerols. The depressed oxidative capacity of the mitochondria injured by chronic alcohol feeding also contributes to the development of the fatty liver. Accumulation of fat acts as a stimulus for the secretion of lipoproteins and the development of hyperlipemia. Hyperlipemia may also be facilitated by the proliferation of the endoplasmic reticulum after chronic ethanol consumption and the associated increase of enzymes involved in the production of triglycerides and lipoproteins. The propensity to enhance lipoprotein secretion is offset, at least in part, by a decrease in microtubules and an impairment of the secretory capacity of the liver. The level of blood lipids depends on the balance between these two opposite changes: At the early stage of alcohol abuse, when liver damage is still small, hyperlipemia will prevail, whereas the opposite occurs with severe liver injury. When hyperlipemia occurs, it involves all lipoprotein classes, including high density lipoprotein (HDL). The latter have been suggested to be responsible for the lower incidence of coronary complications of moderate drinkers compared to teetotalers, but in fact, the subtype of HDL involved (HDL3) differs from the HDL2 subtype associated with protection.

Changes in lipid and apolipoprotein composition of pig lipoproteins facilitated by rabbit lipid transfer protein

A lipid transfer protein has been purified from the lipoprotein-free fraction of rabbit plasma. Rabbit lipid transfer protein, which was purified 600-700-fold with a 4% recovery, has an apparent Mr of 68 000 and facilitates the transfer of isotopically labelled cholesteryl ester, triacylglycerol and phosphatidylcholine between low-density lipoprotein (LDL) and high-density lipoprotein (HDL). Rabbit lipid transfer protein, which appears to be very similar to the cholesteryl ester exchange protein previously purified from human plasma, was incubated with pig plasma at 37 degrees C for up to 6 h. Analysis of very-low-density lipoprotein (VLDL, d less than 1.006 g/ml), LDL (d 1.019-1.063 g/ml) and HDL (d 1.090-1.21 g/ml) after incubation showed that lipid transfer protein had a marked effect on the composition of the lipoprotein classes. The VLDL became enriched with cholesteryl ester and depleted of triacylglycerol. The LDL and HDL became enriched with triacylglycerol. In addition to these changes in lipid composition there were also changes in apolipoprotein composition. The most prominent change in apolipoprotein distribution was a marked increase in the apolipoprotein E content of LDL which was observed only after incubation in the presence of lipid transfer protein.

Intestinal very low density lipoprotein secretion in rats fed various amounts of fat

1. The effect of a high-fat diet (30% fat by wt.) on intestinal very low density lipoprotein (VLDL) secretion was studied in male rats after specific inhibition of hepatic VLDL secretion by dietary orotic acid. Total VLDL secretion (from liver and intestine) was measured in animals not receiving orotic acid. 2. Fat-feeding resulted in a 32% decreased post-Triton secretion of total serum VLDL triacylglycerols as compared to a control (low fat) diet. Concomitantly, a large stimulation of post-Triton intestinal VLDL triacylglycerols secretion was measured in fat-fed rats. Thus, the major part (64%) of circulating triacylglycerols transported as VLDL originated from the intestine in these animals, leading presumably to an increased secretion of intestinal apolipoproteins. 3. Intestinal VLDL and chylomicron secretion rates increased with the amount of fat in the diet (7, 13, 20 or 30% fat by wt.). Whereas the chylomicron secretion was linearly related to the dietary fat content, the relationship between intestinal VLDL secretion and fat content of the diet was sigmoidal. The highest stimulation of intestinal VLDL formation was observed within a narrow range of dietary fat content (between 10 and 20%).

Metabolism of orotic acid: lack of orotate phosphoribosyltransferase in rat intestinal mucosa

The main enzymes involved in orotic acid metabolism, orotate phosphoribosyltransferase and orotidine 5'-phosphate decarboxylase, are associated as a multienzyme complex (complex U) which is present in the liver of most vertebrate species. Orotic-acid-enriched diets produce increased pyrimidine synthesis which competes with purine synthesis for 5-phosphoribosyl diphosphate, resulting in decreased adenylate levels in liver cells. Inhibition of secretion of very low density lipoproteins and hepatic steatosis is then observed. In contrast, lipoproteins secretion by the intestine is not impaired and fat does not accumulate in enterocytes. The aim of this work was to investigate whether orotate is differently metabolized in gut and in liver thus explaining the lack of effect on the intestinal lipoproteins secretion. Complex U was found in appreciable amounts in rat, mouse and rabbit livers; the intestinal mucosa of the two last species contains a much lower level of multienzyme complex whereas in rat intestine its activity cannot be detected. Indeed, radioactive aspartate and orotate were not incorporated into intestinal cells RNA. The absence of orotate metabolisation by lack of orotate phosphoribosyltransferase and orotidine 5'-phosphate decarboxylase activity in rat intestine would explain why this organ, in contrast to the liver, is protected against disturbances of nucleotide metabolism and lipoproteins secretion induced by orotic-acid-supplemented diets.

Integrated regulation of very low density lipoprotein triglyceride and apolipoprotein-B kinetics in man: normolipemic subjects, familial hypertriglyceridemia and familial combined hyperlipidemia

Turnover kinetics of triglycerides (TG) and apolipoprotein-B (apo-B) of plasma very low density lipoprotein (VLDL) and their relationship to plasma VLDL composition and VLDL apo-B conversion to low density lipoprotein (LDL) were determined in age and weight-matched groups of normolipemic (NL) healthy subjects, patients with familial combined hyperlipidemia (FCHL) and patients with familial hypertriglyceridemia (FHTG). In NL subjects, a significant correlation as observed between VLDL TG or VLDL apo-B turnover rate and its circulating mass, suggesting that the plasma level of VLDL was determined by the secretion rate of VLDL TG and apo-B. The positive significant correlation between VLDL TG and apo-B also suggests that the production of these moieties was integrated at the synthetic and/or secretory sites to maintain the ratio of TG to apo-B in plasma VLDL. In moderately obese NL subjects, proportionate increases in VLDL TG and apo-B turnover rates resulted in enhanced secretion of VLDL particles. Both groups with genetic hypertriglyceridemia had increased VLDL TG and VLDL apo-B turnover rates. This increase accounted for the increase in circulating VLDL TG and apo-B mass. In patients with FCHL, turnover rates of VLDL TG and apo-B were equally increased, hence, the ratios between major VLDL constituents were within normal limits. On the other hand, the increase in VLDL TG turnover in patients with FHTG was disproportionately greater than that of apo-B resulting in a higher ratio of TG to other VLDL components. In NL subjects, approximately 72% of VLDL apo-B released into plasma was converted to LDL. This conversion correlated positively with VLDL apo-B turnover rate and inversely with VLDL TG turnover rate. Formation of LDL from VLDL was significantly greater in the obese individuals. In FCHL, conversion of VLDL to LDL represented the major pathway for VLDL apo-B catabolism. The increased VLDL apo-B load was predominantly catabolized to LDL. The greater increase in VLDL TG turnover relative to apo-B in FHTG, on the other hand, resulted in a smaller fraction of VLDL apo-B recovered in LDL, most of the VLDL apo-B being removed via a pathway that did not involve this conversion. We conclude that the composition and metabolic fate of plasma VLDL may be greatly influenced by the secretion rates of VLDL TG and apo-B. If VLDL conversion to LDL and the subsequent catabolism of the latter provides a major route for delivery of cholesterol ester to peripheral tissues, then the increased LDL production in FCHL compared to FHTG may account for a higher cardiovascular risk.

Effect of a high-fat diet on rat very low density lipoprotein secretion

1. Very low density lipoprotein (VLDL) secretion rates were studied on rats adapted to a high-fat diet (71% calories as lard) for 3-4 weeks, compared to control (starch-fed) rats. 2. Experiments were performed at 14.00 h, at which time all animals had the same circulating free fatty acids. Fat-fed rats presented an apparent liver stealosis, a high post-Triton chylomicron secretion, but a 40% decreased VLDL secretion. 3. Injection of [1-14C]palmitic acid showed that the tracer was incorporated less in liver triacylglycerols of the fat-fed rats, presumably because of an enhanced ketogenesis. Secretion of labelled VLDL-triacylglycerols in 1 h was diminished 5-fold, even after a correction for the lower hepatic esterification. 4. Two complementary experiments were carried out, with the following results: at 08.00 h, when serum free fatty acid concentrations were comparable in both groups of rats [5,10], post-Triton VLDL secretion was diminished by 45% in the fat-fed rats; at 20.00 h, the fat-fed rats had significantly elevated plasma free fatty acids [5,10], but their VLDL secretion was the same as in control rats. 5. So it appears that in fat-fed rats circulating free fatty acids do not stimulate VLDL secretion as expected. It is suggested that the decreased VLDL secretion with the high-fat diet may result from inhibition of hepatic lipogenesis.

Gastrointestinal Function

This chapter discusses the functions of gastrointestinal tract. The principal functions of the gastrointestinal tract are assimilation of nutrients and excretion of the waste products of digestion. Within the gastrointestinal tract, these substances are solubilized and degraded enzymatically to simple molecules, sufficiently small in size and in a form that permits absorption across the mucosal epithelium. The distribution of the different types of secretory cells in the salivary glands varies among species. The mandibular and sublingual glands are mixed salivary glands containing both mucous and serous types of cells, and produce a viscous secretion that contains large amounts of mucus. The cytoplasm of the secretory cells contains numerous zymogen granules that vary in size and number depending on the activity of the gland. These granules contain the precursors of the hydrolytic enzymes responsible for digestion of the major dietary components. The cells of the terminal ducts probably secrete the bicarbonate ion responsible for neutralizing hydrochloric acid that enters the duodenum from the stomach.

Serum and liver lipid composition and lecithin: cholesterol acyltransferase in horses, Equus caballus

1. The lipid composition of serum and liver and some properties of serum lecithin: cholesterol acyltransferase of the horse were investigated. 2. Phospholipids and cholesterol were the major components of serum lipids and the concentration of triglyceride was considerably low. The concentration of liver lipids was comparable with that of other mammals. 3. Fatty acid composition of serum cholesterol ester resembled that of the 2-position of lecithin, except palmitic acid. 4. The activity of serum cholesterol esterifying enzyme was found to be 0.03-0.09 mumol/hr per ml. There was an equimolar decrease in free cholesterol and lecithin during incubation, and changes in unsaturated fatty acids in these two components were in good agreement. 5. Cholesterol esterification was reversibly inhibited by 5,5'-dithiobis-(2-nitrobenzoic acid). The acyl-transferase had a specificity for linoleic acid.

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.

Transport kinetics of plasma free fatty acid, very low density lipoprotein triglycerides and apoprotein in patients with endogenous hypertriglyceridaemia: effects of 2,2-dimethyl, 5(2, 5-xylyoxy) valeric acid therapy

The effects of C1-719 on plasma lipid and lipoprotein concentrations have been examined in four patients with endogenous hypertriglyceridaemia maintained on an isocaloric diet for a period of 6 months. During therapy (400 mg/day) the mean plasma triglyceride and cholesterol concentrations were reduced by 35% and 15% respectively, while the administration of 800 mg/day reduced these by 49% and 31%. This hypolipidaemic effect was due to a reduction in the circulating level of very low density lipoproteins (VLDL) without a change in their composition. Before treatment the plasma VLDL triglyceride turnover, and FFA flux, were higher than that of normal subjects maintained on a similar diet. The plasma VLDL B-apoprotein turnover was similarly higher than in the controls. Administration of C1-719 decreased the plasma VLDL triglyceride turnover, FFA flux and VLDL B-approtein turnover. The drug reduced the insulin response following a glucose load with some decrease in glucose levels. The results suggest that the increase in plasma triglyceride concentration in patients with endogenous hypertriglyceridaemia is due to increased production of plasma VLDL triglyceride and its apoptein associated with an enhanced supply of FFA for hepatic triglyceride synthesis. C1-719 exerts a hypolipidaemic effect through a reduction of VLDL production, consequent upon inhibition of lipolysis as well as decreased synthesis of the apoprotein carrier. These effects could in part be explained by an improvement in peripheral tissue responsiveness to insulin and decreased exposure of the liver to high levels of insulin. However, a direct effect of the drug on adipose tissue and liver metabolism has to be considered.

Delayed clearance of chylomicron remnants following vitamin-A-containing oral fat loads in broad-beta disease (type III hyperlipoproteinemia)

Chylomicron "remnants" are formed by the selective removal of triglyceride catalyzed by lipoprotein lipase. To investigate a possible defect in the clearance of these remnants in the pathophysiology of broad-beta disease (type III hyperlipoproteinemia), subjects with this disorder and comparison subjects with endogenous hypertriglyceridemia (and type IV lipoprotein patterns) ingested an oral fat load (corn oil: cocoa butter, 1:1, 50 g/sq M) containing retinyl ester, 100 mg, with or without 15 muCi 15-(14) C-retinol (43.7 mCi/mg). The content of triglyceride and vitamin A was sequentially determined in chylomicrons (Sf more than 400) and very low density lipoproteins (VLDS, Sf20-400) over the ensuing 24-72 hr. Vitamin A was chosen as a marker for exogenous sterol assimilation since, like cholesterol, it is absorbed in the small intestine and cosecreted in esterified form with triglyceride in the chylomicron core; however, unlike cholesterol, once having been removed by the liver, it cannot be recycled inot VLDL, but subsequently circulates only as a complex with the high density retinol binding protein. Thus measurements of the vitamin A/triglyceride ratio in Sf greater than 20 lipoproteins reflected the relative efficiency of vitamin A versus triglyceride removal within these lipoproteins. These studies confirmed the intital concentration of exogenous vitamin A in chylomicrons but invariably disclosed an increasing proportion of the remaining Sf greater than 20 vitamin A in VLDL 24 hr after its ingestion. The vitamin A/triglyceride ratio also invariably increased between 6 and 24 hr in the Sf20-30 subfraction, reflecting the formation of vitamin A-rich "remnants" as intermediate species in the catabolism of chylomicrons and VLDL. Among those with mild to moderate endogenous hypertriglyceridemia the Sf greater than 400 vitamin A/triglyceride ratio declined between 6 and 24 hr, reflecting the efficient passage of the vitamin A through this fraction and/or continued secretion of Sf greater than 400 particles rich in triglyceride. Among those with severe endogenous hypertriglyceridemia, both the peak and decline in the Sf greater than 400 vitamin A/triglyceride ratio were delayed. However, among those with broad-beta disease, an increasing vitamin A/triglyceride ratio between 6 and 24 hr was frequent within all VLDL subfractions and invariable among lipoproteins of Sf greater than 400 regardless of the degree of antecedent hypertriglyceridemia. Although additional experiments disclosed a similar delay in both vitamin A and triglyceride assimilation when basal triglyceride levels were high in these subjects, marked reduction of triglyceride levels did not correct the rise in the Sf greater than 400 vitamin A/triglyceride ratio between 6 and 24 hr. Experiments employing preparative electrophoresis confirmed the identity of VLDL containing a high vitamin A/triglyceride ratio with the beta-VLDL which accumulate in broad-beta disease...

Effects of sex on formation and properties of plasma very low density lipoprotein in vivo

The concentration and composition of the very low density lipoprotein (VLDL) lipids and the behavior of the VLDL in a density gradient in the zonal ultracentrifuge were examined in plasma obtained from normal fed male and female rats before and after intravenous injection of Triton WR-1339. Concentration of lipids in plasma VLDL of female rats was about half that of male animals. Following injection with Triton WR-1339, the concentration of VLDL lipids was higher in female rats (triacylglycerol) or similar (phospholipid, cholesterol, and cholesteryl esters) in both sexes. Female rats secreted much more VLDL traicylglycerol into the plasma compartment than did the male animals under the same experimental conditions. No differences were observed in lipid composition of the VLDL or in the position of the VLDL in the zonal rotor after ultracentrifugation in a density gradient of the lipoprotein from plasma of normal male and female rats before treatment with the detergent. However, after treatment with Triton, a higher proportion of the VLDL particles isolated from plasma of female rats displayed a more rapid rate-zonal flotation in the ultracentrifuge than did the VLDL produced by the male. The VLDL secreted by female rats contained fewer moles of phospholipid and free sterol per mol triacylglycerol than did the VLDL secreted by male animals under identical experimental conditions. The molar ratio of free cholesterol:cholesteryl ester in the VLDL secreted after treatment with Trition increased in both male and female rats. Simultaneously, the content of arachidonic acid in phospholipid of VLDL increased with a concomitant decrease in cholesteryl ester. These changes in fatty acid composition suggest that the inhibitory effect of Triton on lecithin-cholesterol acyl transferase activity affects the exchange of lipids between VLDL and high density lipoprotein. It can be concluded from the data reported here that sex influences the concentration of plasma lipids in vivo and the output and properties of the VLDL.

Metabolism of lipoproteins in nonhuman primates. Reduced secretion of very low density lipoproteins in squirrel monkeys with diet-induced hypercholesterolemia

We have studied the effects of diet-induced hypercholesterolemia on the rates of secretion of triglycerides into the plasma of fasted squirrel monkeys. Two groups of monkeys were studied: control animals which were fed a semipurified diet not associated with hyperlipemia (plasma cholesterol 127 +/- 8 mg/100 ml), and animals made hypercholesterolemic (plasma cholesterol 307 +/- 31 mg/100 ml) by being fed a diet containing 25% butter and 0.5% cholesterol. After intravenous infusion of Triton WR 1339 (300 mg/kg body wt), plasma triglycerides increased almost linearly for 9-12 hours. Analysis of individual lipoproteins separated by ultracentrifugation showed that newly secreted triglycerides were present almost exclusively in the very low density lipoprotein fraction. The rates of triglyceride secretion in the hypercholesterolemic group of monkeys (5.15 +/- 0.86 mg/kg/hr) were less than half those of the control animals (10.96 +/- 2.15 mg/kg/hr). We suggest that in monkeys with diet-induced hypercholesterolemia high concentrations of plasma low density lipoproteins may inhibit the synthesis and/or secretion of their parent very low density lipoprotein molecules into the circulation.

Lipd profiles of plasma lipoproteins of fasted and fed normal and choline-deficient rats

Three major density classes of lipoproteins and a residual protein (d greater than 1.21) were isolated by ultracentrifugation from plasma of fasted, fed normal, and choline-deficient rats. Lipid extracts were obtained from total plasma and the various density classes of lipoproteins, and each extract was examined in detail by thin layer and gas chromatographies. The results indicated essentially identical compositions of molecular species of phosphatidyl choline, which suggested their rapid equilibration among the different plasma lipoprotein classes. In contrast, the molecular species of the triacylglycerols and cholesteryl esters showed significant differences among the chylomicrons, very low and low, and high density lipoproteins, which excluded the possibility of their ready equilibration in vivo. Omission of choline from diet resulted in a sharp and statistically significant decrease in all lipid components of the very low and low density lipoproteins with 2 days. After 10 days of choline deficiency, the lipid levels of chylomicrons and very low and low density lipoproteins were ca. one-half the levels found in the choline supplemented animals, and there were discernible distortions in their lipid composition. Reintroduction of choline led to a prompt return to normal levels and lipid composition of both chylomicron and very low and low density lipoprotein fractions. The lack of equilibration of the triacylglycerols among the lipoprotein classes under normal conditions and in choline deficiency demonstrates an as yet unrecgnized source of compartmentation of plasma lipids.

Paradoxical increase in rate of catabolism of low-density lipoproteins after hepatectomy

It has been suggested that the liver may be a major site for irreversible degradation of low-density lipoprotein (LDL). The disappearance of autologous (125)I-labeled LDL from plasma was compared in intact and in hepatectomized swine. Contrary to expectations, the rate of irreversible removal of LDL from plasma was increased rather then decreased by hepatectomy. These studies suggest that the liver is not a major site for LDL removal. We propose further that the liver (or some function requiring an intact liver) may affect the metabolism of LDL in a manner that prolongs its lifetime in the plasma compartment.