Effects of AOMA on cholesterol metabolism in man

Ezetimibe markedly attenuates hepatic cholesterol accumulation and improves liver function in the lysosomal acid lipase-deficient mouse, a model for cholesteryl ester storage disease

Lysosomal acid lipase (LAL) plays a critical role in the intracellular handling of lipids by hydrolyzing cholesteryl esters (CE) and triacylglycerols (TAG) contained in newly internalized lipoproteins. In humans, mutations in the LAL gene result in cholesteryl ester storage disease (CESD), or in Wolman disease (WD) when the mutations cause complete loss of LAL activity. A rat model for WD and a mouse model for CESD have been described. In these studies we used LAL-deficient mice to investigate how modulating the amount of intestinally-derived cholesterol reaching the liver might impact its mass, cholesterol content, and function in this model. The main experiment tested if ezetimibe, a potent cholesterol absorption inhibitor, had any effect on CE accumulation in mice lacking LAL. In male Lal(-/-) mice given ezetimibe in their diet (20 mg/day/kg bw) for 4 weeks starting at 21 days of age, both liver mass and hepatic cholesterol concentration (mg/g) were reduced to the extent that whole-liver cholesterol content (mg/organ) in the treated mice (74.3±3.4) was only 56% of that in those not given ezetimibe (133.5±6.7). There was also a marked improvement in plasma alanine aminotransferase (ALT) activity. Thus, minimizing cholesterol absorption has a favorable impact on the liver in CESD.

Inhibition of cholesterol absorption: targeting the intestine

Atherosclerosis, the gradual formation of a lipid-rich plaque in the arterial wall is the primary cause of Coronary Artery Disease (CAD), the leading cause of mortality worldwide. Hypercholesterolemia, elevated circulating cholesterol, was identified as a key risk factor for CAD in epidemiological studies. Since the approval of Mevacor in 1987, the primary therapeutic intervention for hypercholesterolemia has been statins, drugs that inhibit the biosynthesis of cholesterol. With improved understanding of the risks associated with elevated cholesterol levels, health agencies are recommending reductions in cholesterol that are not achievable in every patient with statins alone, underlying the need for improved combination therapies. The whole body cholesterol pool is derived from two sources, biosynthesis and diet. Although statins are effective at reducing the biosynthesis of cholesterol, they do not inhibit the absorption of cholesterol, making this an attractive target for adjunct therapies. This report summarizes the efforts to target the gastrointestinal absorption of cholesterol, with emphasis on specifically targeting the gastrointestinal tract to avoid the off-target effects sometimes associated with systemic exposure.

Inhibiting intestinal NPC1L1 activity prevents diet-induced increase in biliary cholesterol in Golden Syrian hamsters

Niemann-Pick C1-like 1 (NPC1L1) facilitates the uptake of sterols into the enterocyte and is the target of the novel cholesterol absorption inhibitor, ezetimibe. These studies used the Golden Syrian hamster as a model to delineate the changes in the relative mRNA expression of NPC1L1 and other proteins that regulate sterol homeostasis in the enterocyte during and following cessation of ezetimibe treatment and also to address the clinically important question of whether the marked inhibition of cholesterol absorption alters biliary lipid composition. In hamsters fed a low-cholesterol, low-fat basal diet, the abundance of mRNA for NPC1L1 in the small intestine far exceeded that in other regions of the gastrointestinal tract, liver, and gallbladder. In the first study, female hamsters were fed the basal diet containing ezetimibe at doses up to 2.0 mg.day(-1).kg body wt(-1). At this dose, cholesterol absorption fell by 82%, fecal neutral sterol excretion increased by 5.3-fold, and hepatic and intestinal cholesterol synthesis increased more than twofold, but there were no significant changes in either fecal bile acid excretion or biliary lipid composition. The ezetimibe-induced changes in intestinal cholesterol handling were reversed when treatment was withdrawn. In a second study, male hamsters were given a diet enriched in cholesterol and safflower oil without or with ezetimibe. The lipid-rich diet raised the absolute and relative cholesterol levels in bile more than fourfold. This increase was largely prevented by ezetimibe. These data are consistent with the recent finding that ezetimibe treatment significantly reduced biliary cholesterol saturation in patients with gallstones.

Delineation of molecular changes in intrahepatic cholesterol metabolism resulting from diminished cholesterol absorption

The absorption of cholesterol by the small intestine is a major route for the net entry of cholesterol into the body and can therefore affect the plasma low density lipoprotein-cholesterol (LDL-C) concentration. These studies used ezetimibe, a potent inhibitor of cholesterol absorption, to delineate the biochemical and molecular changes in intrahepatic metabolism and biliary lipid secretion when there is a major reduction in chylomicron cholesterol delivery to the liver. In female LDL receptor (LDLR)-deficient (LDLR-/-) mice fed a basal diet containing ezetimibe (0-10 mg/day/kg body weight), cholesterol absorption was reduced up to 91%, fecal neutral sterol excretion was increased up to 4.7-fold, and plasma total cholesterol concentrations decreased by up to 18%. Blocking cholesterol absorption prevented the accumulation of very low density lipoproteins and LDL in the circulation of LDLR-/- mice fed a lipid-rich diet. In female LDLR+/+ mice fed the lipid-rich diet with ezetimibe, the relative mRNA level for the LDLR in the liver was 2-fold greater than in matching mice given the lipid-rich diet alone. We conclude that in the mouse the reduction in plasma LDL-C levels induced by blocking cholesterol absorption reflects both a diminished rate of LDL-C production and a modest increase in hepatic LDLR expression.

Cholesterol metabolism and therapeutic targets: rationale for targeting multiple metabolic pathways

The liver is the major regulator of the plasma low density lipoprotein cholesterol (LDL-C) concentration because it is not only the site of formation of very low density lipoproteins (VLDL), the precursors of most LDL in the circulation, but it is also the organ where the bulk of receptor-mediated clearance of LDL takes place. The liver also initially clears all the cholesterol that is absorbed from the small intestine. The absorption of excess cholesterol can potentially increase the amount of cholesterol stored in the liver. This, in turn, can result in increased VLDL secretion, and hence LDL formation, and also downregulation of hepatic LDL receptor activity. Such events will potentially increase plasma LDL-C levels. The converse situation occurs when cholesterol absorption is inhibited. Cholesterol enters the lumen of the small intestine principally from bile and diet. The major steps involved in the absorption process have been characterized. On average, about half of all cholesterol entering the intestine is absorbed, but the fractional absorption rate varies greatly among individuals. While the basis for this variability is not understood, it may partly explain why some patients respond poorly or not at all to statins and other classes of lipid-lowering drugs. There are few data relating to racial differences in cholesterol absorption. One study reported a significantly higher rate in African Americans compared with non-African Americans. Multiple lipid-lowering drugs that target pathways involving the absorption, synthesis, transport, storage, catabolism, and excretion of cholesterol are available. Ezetimibe selectively blocks cholesterol absorption and lowers plasma LDL-C levels by an average of 18%. When ezetimibe is coadministered with lower doses of statins, there is an additive reduction in LDL-C level, which equals the reduction achieved with maximal doses of statins alone. Dual inhibition of cholesterol synthesis and absorption is an effective new strategy for treating hypercholesterolemia.

Inhibition of cholesterol absorption by SCH 58053 in the mouse is not mediated via changes in the expression of mRNA for ABCA1, ABCG5, or ABCG8 in the enterocyte

Intestinal cholesterol absorption is a major determinant of plasma low density lipoprotein-cholesterol (LDL-C) concentrations. Ezetimibe (SCH 58235) and its analogs SCH 48461 and SCH 58053 are novel potent inhibitors of cholesterol absorption whose mechanism of action is unknown. These studies investigated the effect of SCH 58053 on cholesterol metabolism in female 129/Sv mice. In mice fed a low cholesterol rodent diet containing SCH 58053, cholesterol absorption was reduced by 46% and fecal neutral sterol excretion was increased 67%, but biliary lipid composition and bile acid synthesis, pool size, and pool composition were unchanged. When the dietary cholesterol content was increased either 10- or 50-fold, those animals given SCH 58053 manifested lower hepatic and biliary cholesterol concentrations than did their untreated controls. Cholesterol feeding increased the relative mRNA level for adenosine triphosphate-binding cassette transporter A1 (ABCA1), ABC transporter G5 (ABCG5), and ABC transporter G8 (ABCG8) in the jejunum, and of ABCG5 and ABCG8 in the liver, but the magnitude of this increase was generally less if the mice were given SCH 58053. We conclude that the inhibition of cholesterol absorption effected by this new class of agents is not mediated via changes in either the size or composition of the intestinal bile acid pool, or the level of mRNA expression of proteins that facilitate cholesterol efflux from the enterocyte, but rather may involve disruption of the uptake of luminal sterol across the microvillus membrane.

Effect of feeding psyllium and cholestyramine in combination on low density lipoprotein metabolism and fecal bile acid excretion in hamsters with dietary-induced hypercholesterolemia

We wished to determine the effectiveness of submaximal doses of cholestyramine and psyllium given in combination in reversing dietary-induced hypercholesterolemia in Golden Syrian hamsters, and to investigate the mechanism or mechanisms of action through which these agents together decrease plasma low density lipoprotein cholesterol (LDL-C) levels in this model. For 30 days, male hamsters were fed a cholesterol-rich cereal-based diet containing either a submaximal dose of cholestyramine (1% wt/wt) alone or in combination with psyllium (either 2 or 4%), or a high dose of cholestyramine (3%) alone. Although the greatest cholesterol-reducing action was achieved with 3% resin alone, in the animals fed one third as much cholestyramine combined with psyllium (4%) LDL-C production decreased from 288 +/- 15 to 187 +/- 17 micrograms/h per 100 g body weight, the suppression of LDL-receptor activity was almost fully reversed, plasma LDL-C levels were reduced from 90 +/- 8 to 41 +/- 5 mg/dl, and hepatic cholesterol content decreased from 17.1 +/- 1.9 to 2.4 +/- 0.1 mg/g. In the group that received 1% resin alone, the plasma LDL-C and hepatic cholesterol levels were 60 +/- 3 mg/dl and 7.2 +/- 0.6 mg/g, respectively. As compared with animals that received 1% resin alone, those fed both agents manifested higher rates of fecal bile acid excretion and lower levels of intestinal cholesterol absorption. A significant cholesterol-lowering benefit can be derived from using these nonsystemic agents in combination at lower, more tolerable doses.

Mechanisms of LDL-cholesterol lowering action of psyllium hydrophillic mucilloid in the hamster

Psyllium hydrophillic mucilloid (psyllium) is a soluble fiber that significantly lowers plasma low-density lipoprotein (LDL)-cholesterol levels in humans and experimental animals. These studies were designed to determine whether this action is the result of a reduction in LDL-cholesterol production, an increase in receptor-mediated LDL clearance by the tissues, or a combination of these mechanisms. Adult male Golden Syrian hamsters were fed ad libitum for 30 days a cereal-based diet containing added cholesterol (0.1%) and hydrogenated coconut oil (10%), as well as either microcrystalline cellulose (Avicel) (7.5%) or psyllium (7.5%). In contrast to their Avicel-fed controls, the hamsters given psyllium had markedly lower plasma total (122.1 +/- 4.1 vs. 399.4 +/- 39.4 mg/dl) and LDL-cholesterol (46.0 +/- 2.2 vs. 143.5 +/- 12.0 mg/dl) levels. Psyllium feeding also prevented both the dramatic increase in hepatic total cholesterol levels (2.6 +/- 0.1 vs. 16.6 +/- 1.1 mg/g), and the suppression of hepatic cholesterol synthesis (165.1 +/- 27.1 vs. 26.1 +/- 1.2 nmol/h per g) that occurred in the animals given Avicel. Compared to their controls, the psyllium-fed animals also manifested a 44% lower rate of LDL-cholesterol production (167.6 +/- 8.1 vs. 300.2 +/- 16.0 micrograms/h per 100 g bw), and a 2.2-fold higher rate of hepatic LDL clearance (50.1 +/- 2.3 vs. 22.6 +/- 2.1 microliters/h per g). When expressed as a percentage of corresponding values obtained for hamsters fed the basal diet without any additions, the relative rate of LDL-cholesterol production was 175 +/- 10% and 99 +/- 4% for the Avicel- and psyllium-fed groups, respectively. It was similarly determined that the level of whole animal relative LDL receptor activity was marginally higher in the hamsters given psyllium (55.9 +/- 1.4%) than in those fed Avicel (47.5 +/- 3.3%). Thus, it was concluded that while the LDL-cholesterol lowering action of psyllium in the hamster is mediated through two mechanisms, the major effect is exerted at the level of LDL-cholesterol production.

Regulation of hepatic 7 alpha-hydroxylase expression and response to dietary cholesterol in the rat and hamster

Although dietary cholesterol raises plasma total and low density lipoprotein (LDL) cholesterol concentrations, the response to a given intake of cholesterol varies enormously among different species and even among individuals of the same species. The mechanisms responsible for differing sensitivity to dietary cholesterol were examined by comparing the rat, which is able to adapt to large fluctuations in sterol intake or loss with little change in plasma LDL levels, with the hamster, where changes in sterol balance strongly influence plasma LDL concentrations. When fed the same cholesterol-free diet, hepatic 7 alpha-hydroxylase activity was 16-fold higher in the rat than in the hamster. As a consequence, rates of hepatic cholesterol synthesis were 20-fold higher in the rat than in the hamster. In both species, hepatic cholesterol synthesis was suppressed > 90% in response to increasing loads of dietary cholesterol. However, the quantitative importance of this adaptive mechanism was much greater in the rat since the absolute reduction in hepatic cholesterol synthesis in the rat (2,110 nmol/h/g) was much larger than in the hamster (103 nmol/h/g). In the rat, the high basal level of 7 alpha-hydroxylase expression was further induced by substrate (cholesterol) allowing these animals to convert excess dietary cholesterol to bile acids efficiently. In contrast, the low basal level of enzyme expression in the hamster was not induced by dietary cholesterol. Thus, the low basal rates of bile acid and cholesterol synthesis coupled with a lack of 7 alpha-hydroxylase induction by cholesterol render the hamster much more sensitive than the rat to the cholesterolemic effects of dietary cholesterol.

Psyllium augments the cholesterol-lowering action of cholestyramine in hamsters by enhancing sterol loss from the liver

BACKGROUND/AIMS

Psyllium hydrophilic mucilloid is a nonabsorbable soluble fiber that lowers plasma cholesterol levels in several species, including humans. However, its mechanism of action has not been fully elucidated. Therefore, using a hamster model, experiments were performed to determine whether psyllium given alone or in combination with a submaximal dose of cholestyramine blocks intestinal cholesterol absorption.

METHODS

The efficiency of cholesterol absorption and concentrations of plasma and hepatic total cholesterol were measured in male hamsters fed a cholesterol-enriched chow diet (0.1%) that contained either avicel (cellulose) (7.5%), surfomer (3%), cholestyramine (1% or 3%), or psyllium (7.5%) as single agents or a fixed level of cholestyramine (1%) combined with variable levels of psyllium (2%, 4%, 6%, or 8%).

RESULTS

Psyllium, cholestyramine, and surfomer, when given alone, markedly lowered plasma and hepatic cholesterol concentrations. Surfomer, and cholestyramine at the higher dose (3%), blocked cholesterol absorption by 54% and 75%, respectively, whereas psyllium had no effect. Combining psyllium with a submaximal dose of cholestyramine augmented the cholesterol-lowering action of the resin without effecting any marked change in the level of cholesterol absorption, except at the highest dose used.

CONCLUSIONS

Psyllium, given either as a single agent or as an adjunct to treatment with cholestyramine, exerts a significant hypocholesterolemic effect by enhancing net negative sterol balance across the liver.

Antihypercholesterolemic effect of dipyridamole in chickens

The administration of a 4 mg/kg dose of dipyridamole daily in chickens fed a diet supplemented with 2% cholesterol reversed the hypercholesterolemic effects of the diet. In particular, it reduced the plasma cholesterol concentration in approximately 18%; the levels of very-low-density lipoproteins and intermediate-density lipoproteins and the liver cholesterol content. Although the mechanism was not fully elucidated, the increased excretion of cholesterol seemed to be responsible for the lipid lowering effect. When dipyridamole was administered in chickens fed the same diet without cholesterol no significant changes were observed. Inasmuch as the chicken lipoprotein metabolism differs in several aspects to human, the extrapolation of the hypocholesterolemic effect of dipyridamole to man must be made with care.

Cholesterol-lowering action of psyllium mucilloid in the hamster: sites and possible mechanisms of action

These studies were undertaken to examine and compare the metabolic effects of psyllium mucilloid and two other nonabsorbable polymers (cholestyramine and surfomer) on sterol metabolism in the hamster. These three agents all significantly lowered the plasma total cholesterol concentration and the level of cholesterol carried in low-density lipoproteins (LDL). Rates of cholesterol synthesis were markedly increased in the livers of the psyllium-fed animals, but not in other tissues. In contrast, cholestyramine and surfomer feeding increased both hepatic and intestinal sterol synthesis. When cholesterol and saturated triacylglycerols were added to the diet, psyllium feeding essentially completely blocked the increase in the plasma cholesterol concentration and hepatic cholesterol content and the suppression of cholesterol synthesis. The pool of bile acid in the small intestine was increased from the control value (17.9 mumol/animal) by both psyllium (23.0 mumol) and cholestyramine (21.9 mumol) feeding. However, this pool was readily absorbed and secreted into the bile in the psyllium-fed animals (27.9 mumol/4 h), but not in the cholestyramine-treated hamsters (13.0 mumol/4 h). This was consistent with the further observation that there was no binding of bile acid by psyllium under in vitro conditions. Thus, these findings indicate that all three polymers lower plasma cholesterol concentrations by inducing a net negative cholesterol balance across the liver. With psyllium, this effect is presumably articulated through a reduction in cholesterol absorption, as well as an increase in the rate of degradation of cholesterol to bile acids.

Mechanisms of low density lipoprotein lowering by hypolipidemic agents

Most hypolipidemic agents have been developed to reduce to normal high plasma concentrations of low density lipoprotein (LDL), the most atherogenic substance in human plasma. Various agents work with many different mechanisms, some of which are not understood. Mechanistic studies are helpful--e.g., for understanding of the various effects of treatment, elucidating regulatory disturbances in different hyperlipidemias and selecting treatment for patients' specific problems. This paper assesses some of the agents that reduce the concentration of LDL focusing on various mechanistic pathways.

Lipid metabolism in bile acid malabsorption

Malabsorption of bile acid increases cholesterol synthesis and activates hepatic LDL receptors which leads to enhanced elimination of cholesterol from the body. Interruption of enterohepatic circulation of bile acids may lead to a smaller bile acid pool, which, in turn, impairs cholesterol and fat absorption by reduced micellar solubilization. Together with reduced cholesterol absorption, the increased cholesterol loss as bile acids also reduces plasma cholesterol concentrations and the biliary cholesterol excretion, too. Diminished biliary cholesterol in bile acid malabsorption may contribute to the increased incidence of gallstones associated with ileal dysfunction. Malabsorption of bile acid leads to a fall in LDL-cholesterol concentration, and an increase of HDL-cholesterol concentration has been reported. VLDL-triglyceride concentrations are almost invariably raised. Enhanced cholesterol and bile acid synthesis in ileal dysfunction is reflected by raised concentrations of plasma cholesterol precursors, especially lathosterols, which can be used as an indicator of increased bile acid loss to faeces. Cholesterol absorption, in turn, correlates positively with plasma plant sterol concentrations levels and the ratio of lathosterols to campesterols can be used as a screening measurement for ileal dysfunction. Plasma fatty acid composition is also altered as a response to fat malabsorption associated with ileal dysfunction. The proportion of essential fatty acids is inversely correlated with faecal fat excretion and endogenous fatty acid synthesis is activated.

Cholesterol absorption regulates cholesterol metabolism and plasma lipoprotein levels in patients with gut exclusions

The occurrence of cholesterol malabsorption and its role in the regulation of cholesterol metabolism were studied in 30 patients with an earlier gut resection and 9 patients with a jejunoileal bypass for treatment of obesity. Fractional cholesterol absorption varied from 0.1% to 70%, and was lowest in jejunoileal bypass (8.3%) associated with severe fat and moderate bile acid malabsorptions and in 15 patients with a long small-intestinal resection (20.4%) associated with severe bile acid and moderate fat malabsorption. Seven resected patients with normal fecal fat and bile acids and 8 resected patients with malabsorption of only bile acid had normal cholesterol absorption. Low fractional cholesterol absorption was associated with a short length of the remaining proximal small intestine, high dietary intake of plant sterol, and high fecal fat and neutral sterol excretions, but not with bile acid malabsorption. In the whole study population, plasma levels of total, low-density lipoprotein, and high-density lipoprotein cholesterol were positively correlated with fractional cholesterol absorption and the amount of total, dietary, and biliary absorbed cholesterol and were negatively correlated with fecal cholesterol elimination as neutral sterols (less so as bile acids) and cholesterol synthesis. The results emphasize that, in patients with ileal exclusion, plasma levels of low-density lipoprotein and high-density lipoprotein cholesterol are regulated more effectively by cholesterol than by bile acid malabsorption. Moreover, although the fecal loss of bile acids is the main determinant in cholesterol elimination and stimulation of cholesterol synthesis in patients with intestinal exclusions, intestinal cholesterol absorption also contributes noticeably to the regulation of cholesterol synthesis.

Heterogeneity of cholesterol homeostasis in man. Response to changes in dietary fat quality and cholesterol quantity

Studies were carried out to examine the effects of dietary fat and cholesterol on cholesterol homeostasis in man. 75 12-wk studies were carried out during intake of 35% of calories as either saturated or polyunsaturated fat, first low and then high in dietary cholesterol. Dietary fat and cholesterol intakes, plasma lipid and lipoprotein levels, cholesterol absorption and sterol synthesis in isolated blood mononuclear leukocytes were measured during each diet period. In 69% of the studies the subjects compensated for the increased cholesterol intake by decreasing cholesterol fractional absorption and/or endogenous cholesterol synthesis. When an increase in plasma cholesterol levels was observed there was a failure to suppress endogenous cholesterol synthesis. Plasma cholesterol levels were more sensitive to dietary fat quality than to cholesterol quantity. The results demonstrate that the responses to dietary cholesterol and fat are highly individualized and that most individuals have effective feedback control mechanisms.

Esterases in inbred strains of mice with differential cholesterolemic responses to a high-cholesterol diet

Specific esterase isoenzyme patterns in plasma may be associated with responsiveness of serum cholesterol to dietary cholesterol. In rabbits and rats the presence and absence of a high-mobility, anodal esterase band on electrophoresis have been shown to be associated with hypo- and hyperresponsiveness, respectively. We fed for 28 days male mice of 7 inbred strains either a low-cholesterol, commercial diet or a diet containing 2% (w/w) cholesterol, 0.5% cholic acid and 5% olive oil. Feeding the high-cholesterol diet revealed marked inter-strain differences in the responses of plasma and liver cholesterol; the increases ranged from 21 to 129% and from 10 to 80-fold, respectively. There was no association between esterase isoenzyme patterns in plasma and the sensitivity to the high-cholesterol diet. The mean baseline plasma total esterase activity tended to be positively associated with the absolute response of plasma cholesterol to the high-cholesterol diet (r = 0.56; n = 7), but the positive relationship between the baseline concentration of the ES-1 component in plasma and the cholesterolemic response was stronger (r = 0.84; n = 7; P less than 0.05). The high-cholesterol diet caused a significant increase in plasma total esterase activities in 6 out of the 7 strains. Evidence is presented that the increase in plasma total esterase activity, which was associated with an increase in the activity and concentration of the so-called ES-2 isoenzyme, is the result of an enhanced release of esterases from the intestine, rather than from the liver. A significant, positive correlation was found between the baseline intestinal esterase activity and the cholesterolemic response after cholesterol feeding (r = 0.83; n = 7; P less than 0.05).

Hypo- and hyperresponders: individual differences in the response of serum cholesterol concentration to changes in diet

The feeding of cholesterol-rich diets to random-bred animals results in marked interindividual differences in the response of serum cholesterol. Certain animals show only small responses (hyporesponders), whereas others develop high degrees of hypercholesterolemia (hyperresponders). Inbred strains of rabbits, rats, and mice differing in their sensitivity to dietary cholesterol are available. In these animals, and also in monkeys, the responsiveness to high-cholesterol diets has a strong genetic basis. The existence of hyper- and hyporesponders also holds in humans, though not as pronounced as in laboratory animals. Repeated trials with the same subjects have shown that persons exist with a consistently low or high response to increased intakes of cholesterol. However, "spontaneous," diet-independent within-person variations in the level of serum cholesterol markedly inflate the between-person variation in the response of serum cholesterol; both variations are of the same order of magnitude. Hypo- and hyperresponsiveness to dietary cholesterol extends to other hypercholesterolemic components of the diet. In humans and rabbits hyperresponsiveness to dietary cholesterol is associated with responsiveness to dietary saturated fatty acids. The mechanisms underlying hypo- and hyperresponsiveness to dietary cholesterol have not yet been unraveled. On the basis of available data, we propose that in hyperresponders, compared with hyporesponders, there is a higher hepatic efflux of cholesterol in low-density lipoproteins (LDL), or its precursors, after cholesterol consumption. This may be caused by insufficient inhibition of cholesterol biosynthesis and/or the high capacity of cholesterol absorption in the hyperresponders. The stimulation of LDL production accounts for the increase in LDL cholesterol in serum. The number of hepatic LDL receptors, which may be already decreased in hyperresponders, will decrease further through down-regulation. The receptor-mediated LDL clearance decreases, but the absolute amount of LDL cholesterol taken up by the cells via the receptor and by the receptor-independent pathway increases because of the increased level of LDL cholesterol. In this way a new equilibrium is reached in which LDL production equals LDL catabolism. The phenomenon of hypo- and hyperresponsiveness may have implications for counseling subjects who attempt to lower their serum cholesterol by diet. However, identification of true hyper- and hyporesponders is greatly hampered by within-person fluctuations of the level of serum cholesterol. No simple test is available to discriminate hypo- from hyperresponders.(ABSTRACT TRUNCATED AT 400 WORDS)

Turnover of low density lipoproteins during inhibition of cholesterol absorption by neomycin

The mechanisms for the hypocholesterolemic actin of neomycin were examined in six patients with various levels of plasma total cholesterol and triglycerides. All patients were studied on a metabolic ward. The first period of 6 weeks was for control. Thereafter, neomycin (1.5 g/day) was started, and the patients were readmitted for another 6-week period after 2 to 3 months of treatment with the drug. Cholesterol balance studies showed that neomycin increased fecal excretion of neutral steroids by an average of 45%; the drug also inhibited absorption of exogenous cholesterol by an average of 44%. During treatment with neomycin, the plasma total cholesterol fell by an average of 20%, low density lipoproteins (LDL) fell by 25%, and high density lipoproteins, by 16%. Neomycin did not change plasma triglyceride levels. Turnover of the apoprotein of LDL (apoLDL) was measured following injection of 125I-apoLDL. Neomycin decreased synthesis of apoLDL by 28%. The decrease in plasma apoLDL level was correlated positively with the decrease in apoLDL synthetic rate. The effect of the drug on clearance of LDL was less constant; four of six patients had an increase in fractional clearance rates of apoLDL, but the change for the whole group was not statistically significant. These data suggest that a decrease in production of LDL is a major factor in the lowering of LDL following inhibition of cholesterol absorption; however, an increase in clearance rates may occur in some patients.