Deficient ileal 3-hydroxy-3-methylglutaryl coenzyme A reductase activity in sitosterolemia: sitosterol is not a feedback inhibitor of intestinal cholesterol biosynthesis

Molecular actions of hypocholesterolaemic compounds from edible mushrooms

Edible mushrooms contain bioactive compounds able to modulate the expression of genes related to absorption, biosynthesis and transport of cholesterol and regulation of its homeostasis.

Sitosterolemia Presenting as Pseudohomozygous Familial Hypercholesterolemia

A young girl, age 8.5 years, presented with profound hypercholesterolemia and early xanthomatosis, suggesting homozygous familial (or type II) hypercholesterolemia. The patient's low density lipoprotein (LDL) receptor function and parental lipoprotein profiles were determined to be normal, prompting revision of the initial diagnosis to pseudohomozygous familial hypercholesterolemia. When she subsequently presented with giant platelets, the case was presented to colleagues on an electronic mailing list. It was recommended that plasma and sterol analysis be performed, which led to a diagnosis of sitosterolemia. The presentation of profound hypercholesterolomia in childhood that ultimately is not attributed as due to homozygous or compound heterozygous defects in the LDL receptor gene has been termed pseudohomozygous familial (or type II) hypercholesterolemia (PHT2HC). Patients diagnosed with PHT2HC subsequently confirmed to have sitosterolemia have been previously reported only rarely. The challenge of achieving accurate specific diagnosis and appropriate workup for these conditions in children is discussed in the context of this rare case and review of the historical literature concerning these conditions.

Dietary xenosterols lead to infertility and loss of abdominal adipose tissue in sterolin-deficient mice

The investigation of the human disease sitosterolemia (MIM 210250) has shed light not only on the pathways by which dietary sterols may traffic but also on how the mammalian body rids itself of cholesterol and defends against xenosterols. Two genes, ABCG5 and ABCG8, located at the sitosterolemia locus, each encodes a membrane-bound ABC half-transporter and constitutes a functional unit whose activity has now been shown to account for biliary and intestinal sterol excretion. Knockout mice deficient in Abcg5 or Abcg8 recapitulate many of the phenotypic features of sitosterolemia. During the course of our studies to characterize these knockout mice, we noted that these mice, raised on normal rodent chow, exhibited infertility as well as loss of abdominal fat. We show that, although sitosterolemia does not lead to any structural defects or to any overt endocrine defects, fertility could be restored if xenosterols are specifically blocked from entry and that the loss of fat is also reversed by a variety of maneuvers that limit xenosterol accumulation. These studies show that xenosterols may have a significant biological impact on normal mammalian physiology and that the Abcg5 or Abcg8 knockout mouse model may prove useful in investigating the role of xenosterols on mammalian physiology.

New insights into the molecular actions of plant sterols and stanols in cholesterol metabolism

Plant sterols and stanols (phytosterols/phytostanols) are known to reduce serum low-density lipoprotein (LDL)-cholesterol level, and food products containing these plant compounds are widely used as a therapeutic dietary option to reduce plasma cholesterol and atherosclerotic risk. The cholesterol-lowering action of phytosterols/phytostanols is thought to occur, at least in part, through competition with dietary and biliary cholesterol for intestinal absorption in mixed micelles. However, recent evidence suggests that phytosterols/phytostanols may regulate proteins implicated in cholesterol metabolism both in enterocytes and hepatocytes. Important advances in the understanding of intestinal sterol absorption have provided potential molecular targets of phytosterols. An increased activity of ATP-binding cassette transporter A1 (ABCA1) and ABCG5/G8 heterodimer has been proposed as a mechanism underlying the hypocholesterolaemic effect of phytosterols. Conclusive studies using ABCA1 and ABCG5/G8-deficient mice have demonstrated that the phytosterol-mediated inhibition of intestinal cholesterol absorption is independent of these ATP-binding cassette (ABC) transporters. Other reports have proposed a phytosterol/phytostanol action on cholesterol esterification and lipoprotein assembly, cholesterol synthesis and apolipoprotein (apo) B100-containing lipoprotein removal. The accumulation of phytosterols in ABCG5/G8-deficient mice, which develop features of human sitosterolaemia, disrupts cholesterol homeostasis by affecting sterol regulatory element-binding protein (SREBP)-2 processing and liver X receptor (LXR) regulatory pathways. This article reviews the progress to date in studying these effects of phytosterols/phytostanols and the molecular mechanisms involved.

Phytosterols and vascular disease

PURPOSE OF REVIEW

Phytosterols and stanols are plant derivatives that compete with cholesterol for intestinal absorption and thereby lower serum cholesterol concentrations. They have been developed as food additives to help lower serum cholesterol but there is concern that these additives could inadvertently increase cardiovascular risk. This concern arises from the observation that patients with the rare genetic condition phytosterolemia overabsorb phytosterols and develop premature atherosclerosis. This review evaluates the relationship between phytosterol and stanol supplementation and cardiovascular risk.

RECENT FINDINGS

Plant sterol supplementation produces minimal increases in blood phytosterol concentrations in humans. Recent animal studies suggest that phytosterols reduce atherosclerosis in the Apo-E deficient mouse model. The evidence from human studies is mixed and does not prove or disprove an increase in atherosclerotic risk from serum phytosterol levels. An increase in risk seems unlikely, but additional studies should address this possibility.

SUMMARY

Phytosterols are effective in lowering low-density lipoprotein-cholesterol levels, and do not appear to increase atherosclerotic risk, but additional research on this topic is necessary.

Efficacy and safety of sitosterol in the management of blood cholesterol levels

Elevated levels of plasma LDL cholesterol (LDL-C) represent a major risk factor for cardiovascular disease. Treatments aimed at reducing levels of circulating LDL are regarded, therefore, as cardioprotective. The cholesterol lowering properties of plant sterols have been known for some time and many clinical studies have confirmed the efficacy of sitosterol in lowering plasma LDL-C concentrations. Animal studies have also shown reductions in LDL by sitosterol. The use of animal models has been useful in facilitating the elucidation of specific mechanisms by which this sterol exerts its hypocholesterolemic action. It is well known that plant sterols compete with cholesterol for space within bile salt micelles in the intestinal lumen thereby reducing cholesterol absorption. The understanding of the function of plant sterols in impeding cholesterol absorption has been clarified with the discovery of the adenosine binding cassette transporters, ABCG5/8, involved in the regulation of sterol absorption and secretion into the enterocyte and hepatocyte. Compared to cholesterol and other sterols, sitosterol is preferentially pumped out to the intestinal lumen by the ABCG5/8 transporters. This selective binding of sitosterol to the transporters ultimately results in significant lowering of plasma cholesterol. However, some findings support the hypothesis that plant sterols might be an additional risk factor for coronary heart disease. From the review of these studies, it is apparent that sitosterol is a useful dietary supplement for the lowering of plasma cholesterol. Nevertheless, this plant sterol should be used with caution in certain individuals who have a higher absorption rate of sitosterol.

Sitosterolemia: a gateway to new knowledge about cholesterol metabolism

Sitosterolemia (OMIM 210250) is a rare, autosomal recessive lipid disorder initially described almost 30 years ago. The disease is characterized by elevated plasma levels of plant sterols due to increased intestinal absorption and reduced biliary secretion of neutral sterols. Patients with sitosterolemia are frequently hypercholesterolemic, and develop xanthomas and premature coronary heart disease (CHD). Hemolysis, arthralgias and arthritis are also frequently associated with the disorder. Recently, sitosterolemia has been revealed to be due to mutations in either of the two ATP-binding cassette (ABC) half-transporters. ABCG5 or ABCG8. These two genes are expressed almost exclusively in the liver and intestine, and are co-regulated by the nuclear hormone receptor, liver X receptor (LXR). Genetically modified mice, which express either high levels or no ABCG5 and ABCG8 have been developed. Analyses of these mice confirm that these two transporters play key roles in regulating the absorption of dietary and biliary sterols, and in mediating the excretion of neutral sterols from the liver to the bile. The elucidation of the gene defects responsible for sitosterolemia provides potential therapeutic targets for the treatment of hyperlipidemias in the general population.

Inhibition of cholesterol biosynthesis by Delta22-unsaturated phytosterols via competitive inhibition of sterol Delta24-reductase in mammalian cells

Dietary phytosterols are cholesterol-lowering agents that interfere with the intestinal absorption of cholesterol. In the present study, we have studied their effects on cholesterol biosynthesis in human cells, particularly in the sterol-conversion pathway. For this, both Caco-2 (intestinal mucosa) and HL-60 (promyelocytic) human cell lines were incubated with [(14)C]acetate, and the incorporation of radioactivity into sterols was determined using HPLC and radioactivity detection online. Sterols containing a double bond at C-22 in the side chain (stigmasterol, brassicasterol and ergosterol) dramatically inhibited the activity of sterol Delta(24)-reductase, as indicated by the decrease in radioactivity incorporation into cholesterol and the accumulation of its precursors (mainly desmosterol). Phytosterols with the saturated side chain (beta-sitosterol and campesterol) were inactive in this regard. The inhibition of sterol (24)-reductase was confirmed in rat liver microsomes by using (14)C-labelled desmosterol as the substrate. The (22)-unsaturated phytosterols acted as competitive inhibitors of sterol (24)-reductase, with K(i) values (41.1, 42.7 and 36.8 microM for stigmasterol, brassicasterol and ergosterol respectively) similar to the estimated K(m) for desmosterol (26.3 microM). The sterol 5,22-cholestedien-3beta-ol, an unusual desmosterol isomer that lacks the alkyl groups characteristic of phytosterols, acted as a much stronger inhibitor of (24)-reductase (K(i)=3.34 microM). The usually low intracellular concentrations of the physiological substrates of (24)-reductase explains the strong inhibition of cholesterol biosynthesis that these compounds exert in cells. Given that inhibition of sterol (24)-reductase was achieved at physiologically relevant concentrations, it may represent an additional mechanism for the cholesterol-lowering action of phytosterols, and opens up the possibility of using certain (22)-unsaturated sterols as effective hypocholesterolaemic agents.

3-Hydroxy-3-methylglutaryl-coenzyme A reductase activity is inhibited by cholesterol and up-regulated by sitosterol in sitosterolemic fibroblasts

Sitosterolemia is an inherited recessive disease characterized by abnormally increased plasma and tissue plant sterol concentrations. Patients hyperabsorb sitosterol. In addition, hepatic, ileal, and mononuclear leukocyte 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase, the rate-controlling enzyme in the cholesterol biosynthetic pathway, is markedly suppressed in this disease. It is still controversial whether the down-regulation is due to accumulated sitosterol, but the effect of sitosterol on HMG-CoA reductase activity has not been studied in sitosterolemic tissues. To investigate whether sitosterol inhibits HMG-CoA reductase activity in sitosterolemia, we measured the enzyme activities in liver and cultured skin flbroblasts from patients. Hepatic HMG-CoA reductase activities in patients were decreased 76% (P < .05) as compared with results in control subjects. In contrast, HMG-CoA reductase activities in sitosterolemic fibroblasts were not decreased as compared with results in control fibroblasts, and the activities in all cells were up-regulated similarly when they were exposed to delipidated medium. Because the cultured sitosterolemic fibroblasts contained only trace amounts of plant sterols, we added 20 microg/mL sitosterol directly to the cell medium. Raising the intracellular sitosterol concentration to 7% of cellular cholesterol level increased HMG-CoA reductase activity 23% (P < .05), while the addition of the same amount of cholesterol to the cells reduced the activity 46% (P < .05). Thus, when sitosterolemic skin fibroblasts were used, it was possible to distinguish between the effects of cholesterol and those of sitosterol on the activity of HMG-CoA reductase. These results suggest that reduced HMG-CoA reductase activity in this disease is caused by secondary effects of unknown regulator(s) other than sitosterol.

Effects of dietary phytosterols on cholesterol metabolism and atherosclerosis: clinical and experimental evidence

Although plant sterols (phytosterols) and cholesterol have similar chemical structures, they differ markedly in their synthesis, intestinal absorption, and metabolic fate. Phytosterols inhibit intestinal cholesterol absorption, thereby lowering plasma total and low-density lipoprotein (LDL) cholesterol levels. In 16 recently published human studies that used phytosterols to reduce plasma cholesterol levels in a total of 590 subjects, phytosterol therapy was accompanied by an average 10% reduction in total cholesterol and 13% reduction in LDL cholesterol levels. Phytosterols may also affect other aspects of cholesterol metabolism that contribute to their antiatherogenic properties, and may interfere with steroid hormone synthesis. The clinical and biochemical features of hereditary sitosterolemia, as well as its treatment, are reviewed, and the effects of cholestyramine treatment in 12 sitosterolemic subjects are summarized. Finally, new ideas for future research into the role of phytosterols in health and disease are discussed.

Mapping a gene involved in regulating dietary cholesterol absorption. The sitosterolemia locus is found at chromosome 2p21

The molecular mechanisms regulating the amount of dietary cholesterol retained in the body as well as the body's ability to selectively exclude other dietary sterols are poorly understood. Studies of the rare autosomal recessively inherited disease sitosterolemia (OMIM 210250) may shed some light on these processes. Patients suffering from this disease appear to hyperabsorb both cholesterol and plant sterols from the intestine. Additionally, there is failure of the liver's ability to preferentially and rapidly excrete these non-cholesterol sterols into bile. Consequently, people who suffer from this disease have very elevated plasma plant sterol levels and develop tendon and tuberous xanthomas, accelerated atherosclerosis, and premature coronary artery disease. Identification of this gene defect may therefore throw light on regulation of net dietary cholesterol absorption and lead to an advancement in the management of this important cardiovascular risk factor. By studying 10 well-characterized families with this disorder, we have localized the genetic defect to chromosome 2p21, between microsatellite markers D2S1788 and D2S1352 (maximum lodscore 4.49, theta = 0.0).

Phytosterolaemia: diagnosis, characterization and therapeutical approaches

Phytosterolaemia (sitosterolaemia) is a very rare inherited sterol storage disease characterized by tendon and tuberous xanthomas and by a strong predisposition to premature coronary atherosclerosis. In addition to increased or normal serum cholesterol, patients are found to have markedly elevated concentrations of the phytosterols sitosterol and campesterol. These sterols accumulate in all tissues, except the brain. Increased intestinal absorption of plant sterols, impaired biliary excretion, and decreased cholesterol synthesis are suggested as causes for this disease. However, the primary defect has not yet been identified. As well as dietary restrictions of cholesterol and plant sterols, therapeutic approaches based on interruption of the enterohepatic circulation of bile acids by administration of bile acid-binding resins or ileal bypass surgery have been recommended as therapeutic approaches to reduce all serum sterols. Administration of sitostanol, a nonabsorbable saturated plant sterol, showed a significant reduction of serum plant sterols and cholesterol in two patients with phytosterolaemia, presumably by competitive inhibition of sterol absorption.

Enhanced efficacy of sitostanol-containing versus sitostanol-free phytosterol mixtures in altering lipoprotein cholesterol levels and synthesis in rats

To investigate the action and mechanism of a dietary phytosterol mixture naturally containing sitostanol, derived from tall-oil, on circulating cholesterol and lipoprotein levels, five groups of rats were fed a control elemental diet (group 1), a control elemental diet with 1% cholesterol alone (group 2) or with sitostanol mixtures or a sitostanol-free mixture supplemented at 0.2% (group 3), 0.5% (group 4) or 1% (group 5) of dietary levels. One per cent supplementation of sitostanol (21%) compared with sitostanol-free mixtures decreased (P < 0.02) total serum cholesterol. Dietary sitostanol (16% or 21%) mixture at 1% dietary levels decreased (P < 0.05) low density lipoprotein (LDL) cholesterol and increased (P < 0.05) high density lipoprotein (HDL) cholesterol levels. The decrease of LDL and increase of HDL cholesterol were correlated (P < 0.01) with the level of sitostanol mixture in the diet. Consumption of the sitostanol-containing mixture (1% dietary levels) caused a compensatory increase in cholesterol synthesis as indicated by elevated (P < 0.05) lathosterol/ cholesterol ratios in plasma and hepatic cholesterol fractional synthesis rate (FSR) (P < 0.02). Both sitostanol and sitostanol-free mixtures at 0.5% or 1% dietary intake levels increased plasma campesterol and beta-sitosterol levels, while plasma sitostanol levels were negligible. The absence of sitostanol in plasma and the increase in cholesterol synthesis induced by dietary sitostanol mixtures in addition to elevation of plasma campesterol and beta-sitosterol by sitostanol or sitostanol-free mixtures suggest that sitostanol mixtures effectively modify circulating lipoprotein cholesterol concentrations at the level of the intestine, rather than internally at the level of cholesterogenesis.