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

Control of very low-density lipoprotein secretion by N-ethylmaleimide-sensitive factor and miR-33

RATIONALE

Several reports suggest that antisense oligonucleotides against miR-33 might reduce cardiovascular risk in patients by accelerating the reverse cholesterol transport pathway. However, conflicting reports exist about the impact of anti-miR-33 therapy on the levels of very low-density lipoprotein-triglycerides (VLDL-TAG).

OBJECTIVE

We test the hypothesis that miR-33 controls hepatic VLDL-TAG secretion.

METHODS AND RESULTS

Using therapeutic silencing of miR-33 and adenoviral overexpression of miR-33, we show that miR-33 limits hepatic secretion of VLDL-TAG by targeting N-ethylmaleimide-sensitive factor (NSF), both in vivo and in primary hepatocytes. We identify conserved sequences in the 3'UTR of NSF as miR-33 responsive elements and show that Nsf is specifically recruited to the RNA-induced silencing complex following induction of miR-33. In pulse-chase experiments, either miR-33 overexpression or knock-down of Nsf lead to decreased secretion of apolipoproteins and TAG in primary hepatocytes, compared with control cells. Importantly, Nsf rescues miR-33-dependent reduced secretion. Finally, we show that overexpression of Nsf in vivo increases global hepatic secretion and raises plasma VLDL-TAG.

CONCLUSIONS

Together, our data reveal key roles for the miR-33-NSF axis during hepatic secretion and suggest that caution should be taken with anti-miR-33-based therapies because they might raise proatherogenic VLDL-TAG levels.

Dietary and genetic probes of atherogenic dyslipidemia

A goal of dietary management of cardiovascular disease risk in patients with obesity and metabolic syndrome is improvement in the atherogenic dyslipidemia comprising elevated triglyceride, reduced high-density lipoprotein (HDL) cholesterol, and increased numbers of small, dense low-density lipoprotein (LDL) particles. Individuals with a genetically influenced trait characterized by a high proportion of small, dense LDL (phenotype B) respond to a low-fat, high-carbohydrate diet with greater reduction of LDL cholesterol, apoprotein B, and mid-sized LDL2 particles than unaffected subjects (phenotype A). In contrast, in phenotype A subjects there is a reciprocal shift from large LDL1 to small LDL3 such that a high proportion convert to phenotype B. There is evidence for heritable effects on these diet-induced subclass changes and for the involvement of specific genes. For example, a haplotype of the APOA5 gene associated with increased plasma triglyceride and small, dense LDL predicts greater diet-induced reduction of LDL2, a haplotype-specific effect that is strongly correlated with both increased VLDL precursors and LDL4 products. Understanding of such diet-genotype interactions may help to elucidate mechanisms that are responsible for phenotype B and for its differential dietary responsiveness. This information may also ultimately help in identifying those individuals who are most likely to achieve cardiovascular risk benefit from specific dietary interventions.

Metabolic origins and clinical significance of LDL heterogeneity

LDLs in humans comprise multiple distinct subspecies that differ in their metabolic behavior and pathologic roles. Metabolic turnover studies suggest that this heterogeneity results from multiple pathways, including catabolism of different VLDL and IDL precursors, metabolic remodeling, and direct production. A common lipoprotein profile designated atherogenic lipoprotein phenotype is characterized by a predominance of small dense LDL particles. Multiple features of this phenotype, including increased levels of triglyceride rich lipoprotein remnants and IDLs, reduced levels of HDL and an association with insulin resistance, contribute to increased risk for coronary heart disease compared with individuals with a predominance of larger LDL. Increased atherogenic potential of small dense LDL is suggested by greater propensity for transport into the subendothelial space, increased binding to arterial proteoglycans, and susceptibility to oxidative modification. Large LDL particles also can be associated with increased coronary disease risk, particularly in the setting of normal or low triglyceride levels. Like small LDL, large LDL exhibits reduced LDL receptor affinity compared with intermediate sized LDL. Future delineation of the determinants of heterogeneity of LDL and other apoB-containing lipoproteins may contribute to improved identification and management of patients at high risk for atherosclerotic disease.

Apolipoprotein B metabolism: tracer kinetics, models, and metabolic studies

The study of apolipoprotein (apo) B metabolism is central to our understanding of lipoprotein metabolism. However, the assembly and secretion of apoB-containing lipoproteins is a complex process. Specialized techniques, developed and applied to in vitro and in vivo studies of apoB metabolism, have provided insights into the mechanisms involved in the regulation of this process. Moreover, these studies have important implications for understanding both the pathophysiology as well as the therapeutic options for the dyslipidemias. The purpose of this review is to examine the role of apoB in lipoprotein metabolism and to explore the applications of kinetic analysis and multicompartmental modeling to the study of apoB metabolism. New developments and significant advances over the last decade are discussed.

Soluble fiber and soybean protein reduce atherosclerotic lesions in guinea pigs. Sex and hormonal status determine lesion extension

These studies were undertaken to assess guinea pigs as potential models for early atherosclerosis development. For that purpose, male, female, and ovariectomized (to mimic menopause) guinea pigs were fed a control or a TEST diet for 12 wk. Differences between diets were the type of protein (60% casein/40% soybean vs. 100% soybean) and the type of fiber (12.5% cellulose vs. 2.5% cellulose/5% pectin/5% psyllium) for control and TEST diets, respectively. Diet had no effect on plasma cholesterol or triacylglycerol (TAG) concentrations; however, there were significant effects related to sex/hormonal status. Ovariectomized guinea pigs had higher plasma cholesterol and TAG concentrations than males or females (P < 0.01). In contrast to effects on plasma lipids, hepatic cholesterol and TAG were 50% lower in the TEST groups (P < 0.01) compared to controls. Low density lipoproteins (LDL) from guinea pigs fed the TEST diet had a lower number of cholesteryl ester (CE) molecules and a smaller diameter than LDL from controls. Atherosclerotic lesions were modulated by both diet (P < 0.0001) and sex (P < 0.0001). Guinea pigs fed the TEST diet had 25% less lesion extension whereas males had 20% larger occlusion of the arteries compared to both female and ovariectomized guinea pigs. Significant positive correlations were found between LDL CE and atherosclerotic lesions (r = 0.495, P < 0.05) and LDL size and fatty streak area (r = 0.56, P < 0.01). In addition, females fed the TEST diet had the lowest plasma and hepatic cholesterol concentrations, the smallest LDL particles, and the least atherosclerosis involvement compared to the other groups. These data indicate that dietary factors and sex/hormonal status play a role in determining plasma lipids and atherosclerosis in guinea pigs.

Effects of simvastatin on apoB metabolism and LDL subfraction distribution

Seven moderately hypercholesterolemic subjects were studied before and after 10 weeks of simvastatin therapy (20 mg/day). Therapy reduced low density lipoprotein (LDL) cholesterol by 39% (p < 0.001), whereas high density lipoprotein and very low density lipoprotein (VLDL) cholesterol were unchanged. Apolipoprotein (apo) B-containing lipoproteins were divided into VLDL1 (Sf 60-400), VLDL2 (Sf 20-60), intermediate density lipoprotein (IDL) (Sf 12-20), and LDL (Sf 0-12), and metabolic changes were sought in dual-tracer VLDL1 and VLDL2 turnover studies. VLDL1 apoB pool size was unaltered by therapy, as were its rates of synthesis, catabolism, and delipidation to VLDL2. Similarly, the VLDL2 apoB pool size was unchanged, but its metabolic fate was altered. The IDL pool size fell significantly (27%, p < 0.01) due entirely to an increased fractional catabolism of the lipoprotein. In our subjects, the circulating mass of LDL apoB decreased (49%, p < 0.01) primarily due to a reduction in its synthesis. Before therapy, 30% of the apoB entering the delipidation cascade in these hyperlipidemic subjects was converted to LDL. On therapy the input remained the same, but direct catabolism from VLDL2 and IDL was increased (p < 0.05), and as a result only 16% eventually appeared in LDL. These kinetic changes were associated with a fall in particle cholesteryl ester content throughout the delipidation cascade. We also observed a link between LDL kinetics and its subfraction distribution. Simvastatin influences the metabolism of LDL, IDL, and VLDL2 but not VLDL1.

George Lyman Duff Memorial Lecture. Multifactorial etiology of hypercholesterolemia. Implications for prevention of coronary heart disease

This review underlines the concept that multiple factors are responsible for hypercholesterolemia in the American public. Dietary factors (cholesterol, saturated fatty acids, and obesity) clearly raise the cholesterol level, and they are important causes of borderline-high cholesterol. Still, the unexplained decline of LDL receptor activity with aging contributes importantly to borderline-high levels and cannot be ignored. The loss of estrogen-stimulated LDL receptor synthesis after menopause is an important contributor to elevated cholesterol in postmenopausal women. In addition, several genetic defects inherited singly appear to be responsible for moderate hypercholesterolemia. Some of these defects may represent genetic hypersensitivity to diet, and dietary therapy alone may provide adequate cholesterol lowering. Other defects impart resistance to dietary control, and use of a single cholesterol-lowering drug may be required. With the exception of heterozygous FH, most cases of severe hypercholesterolemia appear to be the result of the coexistence of at least two defects in LDL metabolism, and as a rule, they can be treated successfully only by using cholesterol-lowering drugs in combination.

Dietary fat saturation in rhesus monkeys affects LDL concentrations by modulating the independent production of LDL apolipoprotein B

In a recent study from this laboratory, rhesus monkeys fed a 90% palm oil/10% soybean oil-containing diet (PS), rich in 16:0 and 18:1 fatty acids, had decreased total and LDL cholesterol concentrations compared to monkeys fed a 90% coconut oil/10% soybean oil-containing diet (CS), rich in 12:0 and 14:0 fatty acids. To investigate the metabolic basis of these changes, homologous 125I-VLDL and 131I-LDL were injected simultaneously into eight monkeys (four per dietary group). Analysis of apo B specific activity curves revealed that PS monkeys had an increased pool size of VLDL apo B (P less than 0.02), a 3-fold increase in the total VLDL apo B transport rate (P less than 0.001), a decreased pool size of LDL apo B (P less than 0.01) and a 2-fold decrease in the total transport rate of LDL apo B (P less than 0.001), while the irreversible FCR for VLDL apo B and LDL apo B was similar between dietary groups. PS monkeys derived a greater percentage of LDL apo B from VLDL catabolism resulting in a greater transport rate of LDL apo B from VLDL catabolism (P less than 0.055), in comparison to CS monkeys. For CS monkeys the proportion as well as the amount of LDL apo B derived from VLDL-independent catabolism (i.e., LDL apo B derived from sources other than VLDL catabolism) was higher (P less than 0.001) than the values obtained in PS monkeys. In both dietary groups the proportion of VLDL apo B converted to LDL apo B was similar, although the absolute amount was higher for the PS monkeys (P less than 0.06). The proportion of VLDL apo B directly removed from the circulation was similar for both dietary groups, with the absolute amount being higher for the PS monkeys (P less than 0.001). Consistent with the lower pool size of LDL apo B and the higher pool size of VLDL apo B observed in PS monkeys, plasma and LDL cholesterol concentrations tended to be lower, whereas plasma triacylglycerol and VLDL cholesterol concentrations tended to be higher, but these changes were not statistically significant. Although total apo B and VLDL apo B transport rates were increased 2-3-fold in PS monkeys, LDL apo B concentration was reduced by 40% (P less than 0.02) attributed to a significant reduction in the mass and proportion of LDL apo B derived independent of VLDL catabolism.(ABSTRACT TRUNCATED AT 400 WORDS)

Effect of dietary fish oil on coronary artery and aortic atherosclerosis in African green monkeys

Studies were carried out for 2.5 to 3 years in adult male African green monkeys (grivet subspecies) fed diets containing 22% of calories as lard or fish oil with 40% of calories as fat and 0.75 mg cholesterol/Kcal to determine if isocaloric substitution of menhaden fish oil for lard affects coronary artery atherosclerosis. The average total plasma cholesterol concentrations during the experimental period were significantly lower for the fish-oil group (231 +/- 37 mg/dl) compared to the lard group (360 +/- 44 mg/dl), but this difference did not become apparent until after 5 months of experimental diet consumption. High density lipoprotein cholesterol concentrations were 30% lower (p less than 0.01) for the fish-oil group also (57 +/- 5 vs. 82 +/- 6 mg/dl). Plasma triglyceride concentrations were low for both groups, but after about 5 months of diet consumption, they were higher for the animals fed fish oil (25 +/- 2 mg/dl) compared to their lard-fed counterparts (15 +/- 1 mg/dl). Coronary artery intimal area (in this case a measure of early atherosclerotic lesion size) was low in all animals but was significantly less (p less than 0.03) for the fish oil vs. lard groups (0.01 +/- 0.002 vs. 0.03 +/- 0.009 mm2). More atherosclerosis was found in other arteries, and a trend was seen of less atherosclerosis in the thoracic aorta and common carotid arteries of the fish-oil group. The size of lesions in the abdominal aorta was similar between diet groups, but microscopic examination of arteries of the lard group revealed relatively more cholesterol monohydrate crystals compared to the arteries of the fish-oil group. Chemical analysis showed that there was less esterified cholesterol (1.46 +/- 0.71 vs. 3.43 +/- 0.74 mg/g, p = 0.04) and free cholesterol (3.7 +/- 2.15 vs. 7.05 +/- 1.68 mg/g, p = 0.08) in the abdominal aortas taken from the animals fed fish oil. There was a significant correlation between low density lipoprotein (LDL) cholesteryl ester (CE) fatty acid ratio (i.e., saturated + monounsaturated/polyunsaturated species) and the amount of esterified (r = 0.59) and free (r = 0.63) cholesterol in the abdominal aortas. Compared to the lard group, animals fed fish oil had significantly lower LDL CE melting temperatures (26 +/- 1 vs. 38 +/- 1 degree C) and significantly smaller LDL particles (2.68 +/- 0.10 vs. 3.25 +/- 0.38 g/mumol). Therefore, the potentially antiatherogenic effects of dietary fish oil include its ability to decrease the concentration, size, CE content, and CE melting temperature of plasma LDL.

Pravastatin therapy in primary moderate hypercholesterolaemia: changes in metabolism of apolipoprotein B-containing lipoproteins

This study examined the actions of pravastatin on the metabolism of apolipoprotein B (apo B) in very low-, intermediate-, and low-density lipoproteins (VLDL, IDL, and LDL) in 10 patients with primary moderate hypercholesterolaemia. 131I-VLDL apo B was used as a tracer, and appearance of label was followed into IDL apo B and LDL apo B. Compared to placebo, pravastatin therapy reduced levels of cholesterol in total plasma. LDL, VLDL, and IDL cholesterol by 25%, 29%, 31%, and 47%, respectively. Pravastatin treatment also significantly decreased concentrations of apo B in LDL, IDL, and VLDL. The drug significantly reduced the mean production rate for VLDL apo B by 40%, and decreased production rates for LDL apo B in eight of 10 patients. In contrast, fractional catabolic rates (FCRs) were not altered significantly in any of the three lipoprotein fractions on pravastatin therapy. Further, pravastatin produced no consistent changes in LDL particle size, composition, or LDL subclass pattern. Thus pravastatin seemingly reduced input rates for all apo B-containing lipoproteins. Consistent with previous studies, this response was most likely the result of enhanced removal of nascent lipoproteins by increased activity of LDL receptors, although decreased synthesis of apo B in the liver is a possible second action.

Effect of estrogen and progesterone on metabolism of apoprotein B in baboons

To determine the metabolic mechanisms by which estrogen and progesterone alter levels of apoprotein B (apo B)-containing lipoproteins, 12 ovariectomized and hysterectomized baboons, maintained on a high-cholesterol (1.7 mg/kcal) and a high-fat (40% from lard) diet and divided into four groups, were treated with estrogen, progesterone, estrogen plus progesterone, and a placebo. After 12 wk, plasma cholesterol was unchanged in the control and progesterone groups but was reduced in the estrogen- and estrogen plus progesterone-treated groups. The reduction was primarily because of decreased low-density lipoprotein (LDL) cholesterol. LDL apo B levels decreased parallel to the LDL cholesterol. Very low-density lipoprotein (VLDL) and LDL apo B metabolism were studied using a double-label turnover study. Multicompartmental modeling suggested that LDL apo B was kinetically heterogeneous and that there exists an extravascular pool, perhaps consisting of hepatic remnants, that contributes significantly to LDL apo B transport. The model was used to estimate apo B production rates and residence times. VLDL apo B production was not affected by estrogen but was increased by progesterone. LDL apo B production was increased by both estrogen and progesterone. The residence time of LDL apo B was decreased by estrogen and estrogen plus progesterone but not by progesterone. Thus estrogen and progesterone have independent effects on apo B metabolism in baboons.