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L L Rudel

Publications and source records attributed to L L Rudel.

At least 73 records · Page 4Linked to original sources
PubMed1990

Lecithin-cholesterol acyltransferase (LCAT) catalyzes transacylation of intact cholesteryl esters. Evidence for the partial reversal of the forward LCAT reaction.

Lecithin-cholesterol acyltransferase (LCAT) catalyzes the intravascular synthesis of lipoprotein cholesteryl esters by converting cholesterol and lecithin to cholesteryl ester and lysolecithin. LCAT is unique in that it catalyzes sequential reactions within a single polypeptide sequence, a phospholipase A2 reaction followed by a transacylation reaction. In this report we find that LCAT mediates a partial reverse reaction, the transacylation of lipoprotein cholesteryl oleate, in whole plasma and in a purified, reconstituted system. As a result of the reverse transacylation reaction, a linear accumulation of [3H]cholesterol occurred during incubations of plasma containing high density lipoprotein labeled with [3H]cholesteryl oleate. When high density lipoprotein labeled with cholesteryl [14C]oleate was also included in the incubation the labeled fatty acyl moiety remained in the cholesteryl [14C]oleate pool showing that the formation of labeled cholesterol did not result from hydrolysis of the doubly labeled cholesteryl esters. The rate of release of [3H]cholesterol was only about 10% of the forward rate of esterification of cholesterol using partially purified human LCAT and was approximately 7% in whole monkey plasma. Therefore, net production of cholesterol via the reverse LCAT reaction would not occur. [3H]Cholesterol production from [3H]cholesteryl oleate was almost completely inhibited by a final concentration of 1.4 mM 5,5'-dithiobis(nitrobenzoic acid) during incubation with either purified LCAT or whole plasma. Addition of excess lysolecithin to the incubation system did not result in the formation of [14C]oleate-labeled lecithin, showing that the reverse reaction found here for LCAT was limited to the last step of the reaction. To explain these results we hypothesize that LCAT forms a [14C]oleate enzyme thioester intermediate after its attack on the cholesteryl oleate molecule. Formation of this intermediate allows [3H]cholesterol to be liberated from the enzyme by exchange with unlabeled cholesterol of plasma lipoproteins. The liberated [3H]cholesterol thereby becomes available for reesterification by LCAT as indicated by its appearance as newly synthesized cholesteryl linoleate.

Animals↗
PubMed1990

Effect of fish oil on atherosclerosis and lipoprotein metabolism.

This review primarily covers work on the effects of dietary n-3 fatty acids on lipoprotein metabolism and atherosclerosis that has been done in a nonhuman primate model, the African green monkey, and puts it in context with work of others using humans and other experimental animals. Detection of effects of n-3 fatty acids in the monkey model was facilitated by a considerable enrichment of dietary fat with fish oil (about 20% of dietary calories came from menhaden oil in the fish oil group or about 5 g/1000 cal of n-3 fatty acids). This group was compared with a group fed lard isocalorically substituted for fish oil, such that the percentage of saturated fatty acids was essentially equivalent in the 2 dietary groups. Cholesterol concentrations in whole plasma, LDL and HDL were about 1/3 lower in the fish oil group, as was apo A-I concentration, but apo B concentration was not different. The fish oil group had plasma LDL particles that were smaller, contained fewer cholesteryl ester molecules and had lower cholesteryl ester transition temperatures due to a relative enrichment of n-3 fatty acids in the CE fraction. In addition, hepatic cholesterol and cholesteryl ester concentrations were significantly lower in the animals fed fish oil. Liver perfusion was used to show that hepatic secretion of cholesterol and triglyceride was lower in the fish oil group, although the number of cholesterol and triglyceride enriched apo B-containing particles secreted was not different. We also demonstrated a lower plasma LCAT reactivity for the plasma phospholipids of the animals fed fish oil. Taken together, these findings clearly demonstrate important effects of n-3 fatty acids on cholesterol metabolism in a primate model that have not been previously recognized. In addition, the monkeys fed fish oil had less atherosclerosis in the coronary arteries and in the aorta. Thus, these findings indicate that, in addition to the many other effects of fish oil in eicosanoid production, fish oil effects on cholesterol metabolism, per se, can have an important role in limiting atherosclerosis.

Animals↗
PubMed1990

Hepatic apo B-100 lipoproteins and plasma LDL heterogeneity in African green monkeys.

The contribution of hepatic apolipoprotein (apo) B-100 lipoproteins to plasma low-density lipoprotein (LDL) metabolic heterogeneity was examined in African green monkeys. Hepatic 3H-labeled very low-density lipoproteins (VLDL) (d less than 1.006, where d is density in g/ml) or hepatic 131I-labeled LDL (1.030 less than d less than 1.063) were isolated from perfused livers and injected simultaneously with autologous plasma 125I-LDL into African green monkeys. Serial blood samples were taken, and the distribution of radioactivity among various subfractions of apo B-100 lipoproteins was determined using density-gradient ultracentrifugation. Compartmental models were developed to describe simultaneously the kinetics of hepatic lipoproteins and plasma LDL. In five of seven studies, the metabolic behavior of LDL derived from radiolabeled hepatic lipoprotein precursors differed from the metabolic behavior of radiolabeled autologous plasma LDL. These differences could be described by different models supporting two hypotheses with different physiological interpretations: 1) lipoproteins of donor and recipient animals are kinetically distinct, and/or 2) plasma LDL derived from various potential sources are kinetically distinct. Compartmental modeling was used to test these hypotheses, which were not accessible to testing by conventional experimental methodologies. The kinetic analyses of these studies suggest that plasma LDL may be derived from a variety of precursors, including hepatic VLDL and hepatic LDL, with each source giving rise to metabolically distinct plasma LDL.

Animals↗
PubMed1990

Effects on plasma lipoproteins of monounsaturated, saturated, and polyunsaturated fatty acids in the diet of African green monkeys.

Work by other investigators has shown that an increase in dietary content of monounsaturated fatty acids can result in a decreased plasma low density lipoprotein (LDL) cholesterol concentration. This observation, combined with the epidemiologic evidence that monounsaturated fat-rich diets are associated with decreased rates of death from coronary heart disease, suggests that inclusion of increased amounts of mono-unsaturated fat in the diet may be beneficial. The present study was carried out in a primate model, the African green monkey, to evaluate the effects of dietary monounsaturated fat on plasma lipoprotein cholesterol endpoints. Two study periods were carried out in which the fatty acid compositions of the experimental diets were varied. All diets contained 35% of calories as fat. In the first experimental period, a mixture of fats was used to set the dietary fatty acid composition to be approximately 50-60% of the desired fatty acid, either saturated, monounsaturated, or polyunsaturated (n-6). In the second experimental period, pure fats were used (palm oil, oleic acid-rich safflower oil, and linoleic acid-rich safflower oil) to maximize the difference in fatty acid composition. The effects of the more exaggerated dietary fatty acid differences of period 2 were similar to those that have been reported in humans. For the group fed the diet enriched in monounsaturated fat compared to saturated fat, whole plasma and LDL cholesterol concentrations were significantly lower while high density lipoprotein (HDL) cholesterol concentrations were not affected. For the group fed the diet enriched in polyunsaturated fat compared to saturated fat, both LDL and HDL cholesterol concentrations were significantly lower than in the group fed saturated fat. LDL cholesterol concentrations were comparable in the monounsaturated and polyunsaturated fat groups and the percentage of cholesterol in LDL was lowest in the monounsaturated fat fed group. Trends were similar for the mixed fat diets, although no statistically significant differences in plasma lipoprotein endpoints could be attributed to monounsaturated fatty acids in this dietary comparison. Since effects on plasma lipoproteins similar to those seen in humans were identified in this primate model, relevant mechanisms for the effects of dietary fatty acids on lipoprotein endpoints related to coronary artery atherosclerosis, per se, can subsequently be examined.

Analysis of Variance↗
PubMed1990

Effect of fish oil diet on hepatic lipid metabolism in nonhuman primates: lowering of secretion of hepatic triglyceride but not apoB.

African green monkeys were fed diets containing either 11% (by weight) fish oil or lard for 2.5 yr. To test the hypothesis that fish oil decreases hepatic secretion of triglyceride (TG) and apoB, livers from these animals were perfused with a fatty acid mixture [85% (w/w) oleate containing [14C]oleate and 15% n-3 containing [3H]eicosapentaenoic acid (EPA)] at a rate of 0.1 mumol fatty acid/min per g liver. Liver perfusate was sampled every 30 min during 4 h of recirculating perfusion. The concentration of triglyceride was similar for livers of animals of both groups and there was no difference between groups in the extent of incorporation of [3H]EPA or [14C]oleate into hepatic TG. While the secretion rate for the mass of TG was less in the fish oil-fed group (8.3 +/- 2.5 vs 18.3 +/- 4.4 mg/h per 100 g liver, P less than 0.05), the apoB secretion rate was similar (0.92 +/- 0.15 vs 1.01 +/- 0.13 mg/h per 100 g liver). Significantly less [3H]EPA was incorporated into secreted TG in the fish oil group (0.4 +/- 0.1 vs 1.0 +/- 0.1% infused dose/h; P less than 0.01). The rate of secretion of [14C]TG was similar for both groups (1.3 +/- 0.3 vs 1.4 +/- 0.1% infused dose/h for fish oil and lard groups, respectively). No significant diet-related differences in [3H]TG or [14C]TG fatty acid specific activity were observed for perfusate TG or hepatic TG. After perfusion, livers from fish oil-fed monkeys contained significantly more [3H]EPA in hepatic phospholipid than livers from lard-fed monkeys (19.5 +/- 1.8 vs 11.4 +/- 1.7% infused dose; P less than 0.01) although hepatic phospholipid mass concentrations were similar. The liver phospholipids of the fish oil group were enriched in n-3 fatty acid mass and were relatively depleted of oleate and linoleate. We conclude that although apoB secretion was unaffected, dietary fish oil significantly decreased hepatic TG secretion through relatively poor utilization of EPA for the synthesis of TG destined for secretion in VLDL; at the same time, increased incorporation of [3H]EPA into hepatic phospholipid accompanied the decreased incorporation into secreted TG and these events may be coupled.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1990

Nascent VLDL from liver perfusions of cynomolgus monkeys are preferentially enriched in RRR- compared with SRR-alpha-tocopherol: studies using deuterated tocopherols.

The transport and secretion of vitamin E in lipoproteins have been studied in cynomolgus monkeys fed tocopherols labeled with different amounts of deuterium. The animals were fed a single dose of vitamin E containing 60 mumol of each 2R,4'R,8'R-alpha-(5,7-(C2H3)2)tocopheryl acetate (d6-RRR-alpha-tocopheryl acetate; alpha-tocopherol with natural stereochemistry), 2S,4'R,8'R-alpha-5-(C2H3)tocopheryl acetate (d3-SRR-alpha-tocopheryl acetate; alpha-tocopherol with unnatural stereochemistry), and 2R,4'R,8'R-gamma-(3,4-2H)tocopherol (d2-RRR-gamma-tocopherol; gamma-tocopherol with natural stereochemistry). Chylomicrons, as well as the other plasma lipoproteins, contained equal concentrations of all three tocopherols at the earliest time points after feeding suggesting that all three tocopherols were absorbed equally. At later times plasma lipoproteins became preferentially enriched in d6-RRR-alpha-tocopherol. This is likely to be due to hepatic secretion of VLDL (very low density lipoproteins) and other lipoproteins, which were enriched in d6-RRR-alpha-tocopherol, as demonstrated in the lipoproteins isolated from perfused livers that had been obtained 24 h following the administration of the deuterated tocopherols. Taken together these data demonstrate that the liver, not the intestine, is the likely site of discrimination between tocopherol isomers and that the liver secretes nascent lipoproteins preferentially enriched in d6-RRR-alpha-tocopherol.

Animals↗
PubMed1989

Studies on the expression of genes encoding apolipoproteins B100 and B48 and the low density lipoprotein receptor in nonhuman primates. Comparison of dietary fat and cholesterol.

African green monkeys were fed diets containing low and moderate cholesterol concentrations with either polyunsaturated or unsaturated fat as 40% of calories. Plasma total cholesterol, low density lipoprotein (LDL) cholesterol, and apoB concentrations generally were higher in animals fed (a) the higher dietary cholesterol concentration and (b) saturated fat. At necropsy, liver and intestine were removed, and measurement of mRNAs for LDL receptors (liver) and for apolipoprotein B (liver and intestine) was done. Monkey small intestine mucosa made exclusively apoB48 while the liver made only apoB100, although apoB mRNA in both tissues was the same size (14 kilobases). No dietary cholesterol or fat effects were found for apoB mRNA abundance in the liver, while the animals fed the higher dietary cholesterol level had 50% lower levels of hepatic LDL receptor mRNA. In a separate group of animals, livers were perfused and the rate of apoB secretion was measured. No dietary fat effect on apoB secretion rate was found, and no relationship between plasma LDL cholesterol concentration and the rate of hepatic apoB production existed. These findings support the idea that the dietary factors that increase LDL concentrations act by reducing clearance of apoB-containing particles rather than by increasing production of these lipoproteins. Hepatic LDL receptor mRNA was similar in abundance in polyunsaturated fat and saturated fat-fed animals, suggesting that the difference in plasma cholesterol concentration between these groups is not mediated via effects on LDL receptor mRNA abundance. The level of intestinal apoB mRNA was about 30% higher in animals fed the moderate dietary cholesterol concentration. Earlier studies have shown that more cholesterol is transported in chylomicrons from the intestine when dietary cholesterol levels are higher, and the increased intestinal apoB mRNA abundance may reflect increased intestinal cholesterol transport and chylomicron apoB48 production.

Animals↗
PubMed1989

The reactivity of plasma phospholipids with lecithin:cholesterol acyltransferase is decreased in fish oil-fed monkeys.

The size of low density lipoproteins (LDL) is strongly correlated with LDL cholesteryl ester (CE) content and coronary artery atherosclerosis in monkeys fed cholesterol and saturated fat. African green monkeys fed 11% (weight) fish oil diets have smaller LDL and less CE per LDL particle than lard-fed animals. We hypothesized that this might be due to a lower plasma lecithin:cholesterol acyltransferase (LCAT) activity in fish oil-fed animals. Using recombinant particles made of egg yolk lecithin-[14C]cholesterol-apoA-I as exogenous substrate, we found no difference in plasma LCAT activity (27 versus 28 nmol CE formed per h/ml) of fish oil- versus lard-fed animals, respectively; furthermore, no diet-induced difference in immunodetectable LCAT was found. However, plasma phospholipids from fish oil-fed animals were over 4-fold enriched in n-3 fatty acids in the sn-2 position compared to those of lard-fed animals. Additionally, the proportion of n-3 fatty acid-containing CE products formed by LCAT, relative to the available n-3 fatty acid in the sn-2 position of phospholipids, was less than one-tenth of that for linoleic acid. The overall rate of LCAT-catalyzed CE formation with phospholipid substrates from fish oil-fed animals was lower (5-50%) than with phospholipid substrates from lard-fed animals. These data show that n-3 fatty acids in phospholipids are not readily utilized by LCAT for formation of CE; rather, LCAT preferentially utilizes linoleic acid for CE formation. The amount of linoleic acid in the sn-2 position of plasma phospholipids is reduced and replaced with n-3 fatty acids in fish oil-fed animals. As a result, LCAT-catalyzed plasma CE formation in vivo is likely reduced in fish oil-fed animals contributing to the decreased cholesteryl ester content and smaller size of LDL particles in the animals of this diet group.

Animals↗
PubMed1989

Delivery of high-density lipoprotein free and esterified cholesterol to bile by the perfused monkey liver.

The movement of cholesterol from high-density lipoproteins (HDL) into bile has been studied using perfused livers from cholesterol-fed African Green monkeys. Mass amounts of HDL were isolated from the plasma of African Green monkeys and were doubly labeled with either 125I-apolipoprotein and [3H]cholesteryl ester or with [3H]cholesteryl ester and [14C]cholesterol. For 3 h of perfusion HDL-free cholesterol was cleared from perfusate at a faster rate than HDL ester cholesterol which, in turn, was cleared at a faster rate than HDL protein. [14C]cholesterol from HDL appeared in biliary bile acids and cholesterol at a higher rate than [3H]esterified cholesterol from HDL. The specific activities of biliary [14C]cholesterol and HDL-free [14C]cholesterol had equilibrated by 60 min of perfusion, although the specific activity of whole liver free [14C]cholesterol was still only 46% of that in bile at 180 min of perfusion. In contrast, the specific activity of total liver free [3H]cholesterol was equal to that of biliary [3H]cholesterol by 180 min of perfusion. We conclude that, in this primate model, HDL-free cholesterol enters into a hepatic compartment that communicates with biliary cholesterol and bile acid precursor pools more efficiently than with other liver pools of cholesterol, whereas HDL-esterified cholesterol appears to mix with all liver pools with equal efficiency. Overall, these data support the concept of compartmentalization of cholesterol in the liver.

Animals↗
PubMed1989

Differential effects of dietary fat on the tissue-specific expression of the apolipoprotein A-I gene: relationship to plasma concentration of high density lipoproteins.

Isocaloric substitution of polyunsaturated fat for saturated fat reduces concentrations of total plasma cholesterol and high density lipoproteins (HDL) in nonhuman primates. The biochemical mechanisms through which polyunsaturated fat lowers plasma HDL concentrations are not well understood but must involve changes in HDL production or HDL clearance from plasma, or both. To determine whether dietary polyunsaturated fat (P/S = 2.2) alters apolipoprotein (apo) A-I production, African green monkeys (Cercopithecus aethiops) were fed diets containing polyunsaturated fat or saturated fat (P/S = 0.3) each in combination with high (0.8 mg/kcal) and low (0.03 mg/kcal) amounts of dietary cholesterol. Animals fed polyunsaturated fat at either cholesterol level had lower plasma concentrations of total cholesterol and HDL cholesterol. Plasma apoA-I concentration was reduced by 16% by polyunsaturated fat in the high cholesterol group. The rate of hepatic apoA-I secretion, as estimated by the accumulation of perfusate apoA-I during recirculating liver perfusion, was reduced by 19% in animals consuming the high cholesterol, polyunsaturated fat diet. Hepatic apoA-I mRNA concentrations, as measured by DNA-excess solution hybridization, also were reduced by 22% in the high cholesterol, polyunsaturated fat-fed animals. In contrast, intestinal apoA-I mRNA concentrations were not altered by the type of dietary fat. Plasma apoA-II and hepatic apoA-II mRNA concentrations also were not altered by the type of dietary fat. These data indicate that dietary polyunsaturated fat can selectively alter the expression of the apoA-I gene in a tissue-specific manner.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1989

Fish oil decreases hepatic cholesteryl ester secretion but not apoB secretion in African green monkeys.

Two groups of African green monkeys were fed diets containing 40% of calories as fat with half of the fat calories as either fish oil or lard. The fish oil-fed animals had lower cholesterol concentrations in blood plasma (33%) and low density lipoproteins (LDL) (34%) than did animals fed lard. Size and cholesteryl ester (CE) content of LDL, strong predictors of coronary artery atherosclerosis in monkeys, were significantly less for the fish oil-fed animals although the apoB and LDL particle concentrations in plasma were similar for both diet groups. We hypothesized that decreased hepatic CE secretion led to the smaller size and reduced CE content of LDL in the fish oil-fed animals. Hepatic CE secretion was studied using recirculating perfusion of monkey livers that were infused during perfusion with fatty acids (85% 18:1 and 15% n-3) at a rate of 0.1 mumol/min per g liver. The rate of cholesterol secretion was less (P = 0.055) for the livers of fish oil versus lard-fed animals (3.3 +/- 0.5 vs. 6.0 +/- 1.2 mg/h per 100 g, mean +/- SEM) but the rate of apoB secretion was similar for both groups (0.92 +/- 0.15 vs. 1.01 +/- 0.13 mg/h per 100 g, respectively). The hepatic triglyceride secretion rate was also less (P less than 0.05) for the fish oil-fed animals (8.3 +/- 2.5 vs. 18.3 +/- 4.4 mg/h per 100 g). Liver CE content was lower (P less than 0.006) in fish oil-fed animals (4.1 +/- 0.8 vs. 7.4 +/- 0.7 mg/g) and this was reflected in a lower (P less than 0.04) esterified to total cholesterol ratio of perfusate VLDL (0.21 +/- 0.045 vs. 0.41 +/- 0.06). The hepatic VLDL of animals fed fish oil had 40-50% lower ratios of triglyceride to protein and total cholesterol to protein. From these data we conclude that livers from monkeys fed fish oil secreted similar numbers of VLDL particles as those of lard-fed animals although the hepatic VLDL of fish oil-fed animals were smaller in size and relatively enriched in surface material and depleted of core constituents. Positive correlations between plasma LDL size and both hepatic CE content (r = 0.87) and hepatic VLDL cholesterol secretion rate (r = 0.84) were also found.(ABSTRACT TRUNCATED AT 400 WORDS)

Analysis of Variance↗
PubMed1989

Metabolic behavior of hepatic VLDL and plasma LDL apoB-100 in African green monkeys.

Recently, evidence has accumulated suggesting that significant amounts of plasma low density lipoproteins (LDL) may be derived by direct production. These plasma very low density lipoprotein (VLDL)-independent sources include the production and secretion of LDL-like particles directly by the liver, and/or a small pool of nascent precursor particles that are converted rapidly to LDL. The current studies were designed to test the hypothesis that hepatic VLDL represent a rapidly turning over precursor pool to plasma LDL in African green monkeys. Livers from African green monkeys were perfused with serum-free medium containing [3H]leucine or 3H-labeled amino acids for 4-6 hr. Hepatic [3H]VLDL and autologous plasma 125I-labeled LDL were injected simultaneously into recipient animals and density gradient ultracentrifugation and gel filtration were used to characterize the distribution of 3H and 125I radioactivity at selected times after injection. These studies show that 4 to 66% of the injected dose of hepatic VLDL [3H]apoB-100 was metabolized extremely rapidly into particles that resembled the recipient's plasma LDL by size and density. Based on the kinetic model developed to describe the metabolic behavior of hepatic VLDL [3H]apoB-100, the estimated maximal pool size of hepatic VLDL apoB-100 in these animals was very small (0.042 and 0.112 mg) and represented, at best, approximately 10% of the average plasma VLDL apoB-100 mass found in cholesterol-fed African green monkeys. In addition, the radiolabeled hepatic LDL appear to be metabolized similarly to plasma LDL. That is, the rapid conversion of hepatic VLDL as well as the direct production of hepatic particles within the LDL density range appear to contribute to plasma LDL. Metabolic heterogeneity was also seen within the LDL class. The more buoyant subfraction (LDL1) had a higher turnover rate than the more dense subfraction (LDL2) and hepatic VLDL-derived [3H]LDL1 had a slower final rate of plasma disappearance than the plasma-derived 125I-labeled LDL1 in most animals. The results from these studies suggest that a small pool of hepatic VLDL can be converted very rapidly to plasma LDL and may contribute significantly to the large plasma pool of LDL seen in cholesterol-fed African green monkeys. This pathway may be analogous to the pathway in some human subjects in which a portion of human plasma VLDL is converted rapidly into LDL without passing through a delipidation cascade, often referred to as direct LDL production.

Animals↗
PubMed1988

Neutral lipid transfer and lipolysis convert high molecular weight LDL from cholesterol-fed nonhuman primates towards normal: a molecular analysis.

In cynomolgus monkeys (Macaca fascicularis) fed an atherogenic diet, large, cholesterol ester-rich LDL (Mr greater than 3.5.10(6] are found at the same time that the plasma triacylglycerol levels are low. We studied whether the presence of higher concentrations of triacylglycerol-rich lipoproteins (VLDL) during in vitro incubations would allows depletion from LDL of cholesterol ester and a decreased LDL molecular weight. Three high Mr LDL (Mr = (3.7-4.8).10(6)), rich in cholesterol ester (50 +/- 1.4% by weight), were isolated from three animals by zonal ultracentrifugation, and were then incubated with human VLDL at 37 degrees C for 18 h in lipoprotein-deficient human plasma containing neutral lipid transfer activity. After incubation, modified LDL (M-LDL) was isolated by zonal ultracentrifugation. M-LDL was triacylglycerol-rich (36 +/- 5% by weight) and cholesterol ester-poor (20 +/- 3%), and cholesterol ester had transferred into VLDL. Purified lipoprotein lipase was added to the M-LDL, and triacylglycerol was hydrolyzed. The size of the post-lipolysis M-LDL (Mp-LDL) particles became smaller (mean diameters of 253 A and 228 A for two native LDLs and 215 A and 193 A for Mp-LDL, respectively). Both analytical and zonal ultracentrifugation showed Mp-LDL to be more dense than native LDL. Estimated molecular weights for Mp-LDL were 40%-50% less than that of the original LDL, and fell within the molecular weight range for normal human and monkey LDL. Lipid exchanges, but not apoprotein transfers, were responsible for LDL remodelling, as supported by three separate methods of analysis. Cholesterol ester losses accounted for about two-thirds of the molecular weight decrease. These in vitro results suggest that cholesterol ester enrichment of apoprotein B lipoprotein particles can be reversed by providing adequate levels of VLDL in the presence of neutral lipid transfer processes and lipolytic activity.

Animals↗
PubMed1988

Apolipoprotein (apo) A-I production and mRNA abundance explain plasma apoA-I and high density lipoprotein differences between two nonhuman primate species with high and low susceptibilities to diet-induced hypercholesterolemia.

Earlier studies have shown that African green monkeys develop a more modest hypercholesterolemia, higher high density lipoprotein (HDL) concentrations, and less atherosclerosis than cynomolgus monkeys fed diets with the same cholesterol content. In the present study, cynomolgus monkeys were fed less cholesterol than was fed to African green monkeys to induce equivalent hypercholesterolemia in both species. African green monkeys still had 2-fold higher plasma HDL cholesterol concentrations and 2.7-fold higher plasma apolipoprotein (apo) A-I concentrations. Therefore, the higher HDL concentration in African green monkeys appears to result from factors that act independently of dietary cholesterol intake or total plasma cholesterol concentration. Two aspects of HDL production were examined to determine the metabolic basis of the species difference in HDL concentration. The rate of hepatic apoA-I secretion, as estimated by the accumulation of apoA-I in the medium during recirculating liver perfusion, was 5-fold higher in livers of African green monkeys. In addition, the concentration of apoA-I mRNA was 2-fold higher in the liver and 3.7-fold higher in the intestine of African green monkeys. Taken together, these findings indicate that differences in apoA-I production in the liver and small intestine are large enough to be responsible for the differences in the plasma concentrations of HDL and apoA-I between these species. Factors which regulate apoA-I secretion, including modulation of tissue apoA-I mRNA concentrations, are important determinants of plasma HDL concentrations and may contribute to the relative resistance of African green monkeys to dietary cholesterol-induced hypercholesterolemia and atherosclerosis. ApoA-I mRNA was also detected at low levels in the kidney and testis of African green and cynomolgus monkeys but not in the adrenal or brain. The tissue distribution and abundance of apoA-I mRNA in peripheral tissues was very different than that seen for apoE mRNA. Kidney and testis apoA-I mRNAs were the same size as liver apoA-I mRNA when examined by Northern blot analysis. Testis apoA-I mRNA appeared to be functionally active as judged by its presence in cytoplasmic polyribosomes. The low levels of apoA-I expression in kidney and testis are unlikely to contribute significantly to the plasma apoA-I pool but might function in some aspect of local lipid metabolism within these tissues.

Animals↗
PubMed1987

Lipoproteins and atherosclerosis.

The plasma lipoproteins are the primary means of transport of cholesterol among tissues. In particular, the apo B-containing lipoproteins (VLDL, IDL and LDL) are important for the delivery of cholesterol from the liver to peripheral tissues, while HDL appear to mediate the reverse process of movement of cholesterol from tissues back to the liver. Both of these transport processes are necessary for efficient whole body cholesterol homeostasis, because the liver is the major site of both the production and excretion of cholesterol. However, deviations from a proper balance of transport of cholesterol, either increases in LDL levels or decreases in HDL cholesterol flux, may result in accumulation of cholesterol in extrahepatic tissues. Increased risk of atherosclerosis and CHD may be associated with elevation in the number of LDL particles, increase or decrease in LDL particle size, or changes in the composition of plasma LDL. These modifications of plasma LDL may be brought about following perturbation of one of several aspects of LDL metabolism. These include decreased LDL receptor activity, increased VLDL production and cholesterol enrichment of the liver-derived VLDL. The events in the arterial wall that make some LDL particles apparently atherogenic are not well understood. In the case of nonhuman primates, large-size LDL are associated with an increased risk of CHD. One characteristic of these LDL is that their core lipids are rich in saturated cholesteryl esters and their transition temperatures are frequently above body temperature. The liquid crystalline cholesteryl ester cores of such LDL may modulate the conformation of apo B on the surface and thereby affect the interaction of these LDL with cellular receptors or connective tissue matrix proteoglycans. It is likely, though, that changes in LDL particle number, LDL particle size and LDL particle composition may each contribute to progression of atherosclerosis. The presumed metabolic events that make HDL protective against atherosclerosis have been termed reverse cholesterol transport, and suggest that small HDL that are deficient in free cholesterol acquire this lipid from cell membranes. The HDL cholesterol is esterified by LCAT in the circulation, forming large HDL that can then deliver the cholesteryl ester to the liver by both direct and indirect means. In most circumstances, it is assumed that an increase in plasma HDL cholesterol concentration reflects an increase in the rate at which HDL is removing cholesterol from tissues and, consequently, a decrease in atherosclerosis.(ABSTRACT TRUNCATED AT 400 WORDS)

Apolipoproteins B↗
PubMed1987

Nascent lipoproteins from recirculating and nonrecirculating liver perfusions and from the hepatic Golgi apparatus of African green monkeys.

Perfusate apoB-100-containing lipoproteins from the isolated, perfused livers of African green monkeys consist of significant amounts of d greater than 1.006 g/ml particles in addition to very low density lipoproteins (VLDL). Distinguishing characteristics of these perfusate lipoproteins are the relative abundance of surface lipids and deficiency of core lipids. The present studies were performed to determine the likelihood that the d greater than 1.006 g/ml perfusate lipoproteins are secretion products instead of products of post-secretory modification (e.g., lipolysis) of secreted VLDL. [14C]Leucine from the perfusate became incorporated into the apoB of each of the perfusate lipoprotein classes to a similar extent in both recirculating and nonrecirculating perfusions. When endogenously radiolabeled perfusate VLDL from one liver was recirculated through a second liver, only about 15% of the radiolabeled protein appeared in the d greater than 1.006 g/ml fraction. The particle morphology and the cholesterol and apoB distribution between VLDL and d greater than 1.006 g/ml fractions were similar in recirculating and nonrecirculating perfusions. A Golgi apparatus-rich fraction was isolated from the homogenates of fresh liver samples and the isolated Golgi VLDL and d greater than 1.006 g/ml lipoproteins exhibited morphologic evidence of extra surface material analogous to that seen in perfusate. Taken together, these data support the possibility that significant amounts of d greater than 1.006 g/ml lipoproteins, many with surface-rich properties, are nascent, secretory products of the primate liver. The low level of lecithin:cholesterol acyltransferase (LCAT) in this perfusion system appears to permit detection of these secretion products and it is significant to note that the perfusate lipoprotein profile, which is unlike that of normal plasma, is similar to that of LCAT-deficient patients.

Animals↗
PubMed1987

Intravascular metabolism of lipoprotein cholesteryl esters in African green monkeys: differential fate of doubly labeled cholesteryl oleate.

High density lipoproteins (HDL), doubly labeled with [3H]cholesteryl oleate and cholesteryl [14C]oleate, were reinjected to study HDL cholesteryl ester metabolism in African green monkeys. The transfer of labeled HDL cholesteryl ester to low density lipoprotein (LDL) was rapid and equilibration of the [3H]cholesteryl oleate and cholesteryl [14C]oleate specific activities in LDL and HDL occurred within 90 min after reinjection. The apparent rates of disappearance from the circulation of the two moieties of the cholesteryl ester were different. In the same four animals, the residence time for the turnover of plasma [3H]cholesterol averaged 6.1 days while the residence time for the removal of cholesteryl [14C]oleate from plasma was approximately 2.1 days. These results suggest that for some lipoprotein cholesteryl esters removed from plasma, the cholesterol moiety subsequently reappeared in plasma. The difference between the rate of decay of the 14C-labeled fatty acid moiety, which represents all of the cholesteryl ester removed from plasma (0.48 pools/day) and the decay of the 3H-labeled cholesterol moiety, which represents the sum of cholesteryl ester removal and cholesterol reappearance (0.16 pools/day), is the fraction of the cholesteryl ester pool recycled per day (0.32 pools/day or 22.5 mg/kg per day). In other words, approximately 68% of the cholesterol moiety that was removed from plasma as cholesteryl oleate reappeared in the plasma cholesterol pool. These studies support the concept that an efficient reutilization cycle for plasma cholesterol occurs, i.e., the cholesteryl ester molecule can exit and the cholesterol moiety can re-enter plasma without effective equilibration of the cholesterol moiety with extravascular cholesterol pools.

Animals↗