The cholesterol turnover, synthesis, and absorption in two sisters with familial hypercholesterolemia (type IIa)

Plasma Non-cholesterol Sterols as Markers of Cholesterol Synthesis and Intestinal Absorption: A Critical Review

Plasma concentrations of phytosterols and non-cholesterol sterol precursors of cholesterol synthesis have been used as markers of intestinal cholesterol absorption and synthesis in inherited and secondary dyslipidemias and in population-based investigations to evaluate the risk for cardiovascular disease, respectively. The method aims at replacing initial research procedures such as the use of stable isotopes associated with fecal steroid balance, which are limited by the high cost and tedious procedures. However, we show in this review that numerous results obtained with serum sterol measurements are contradictory. In this regard, the following points are discussed: 1) how phytosterols relate to atherosclerosis considering that defects in biliary output or in the transport of phytosterols from the intestinal mucosa back into the intestinal lumen provide increased content of phytosterols and other sterols in plasma and tissues, thus not allowing to conclude that their presence in arteries and atheromas represents the etiology of atherosclerosis; 2) serum non-cholesterol sterols as markers of cholesterol synthesis and absorption, such as cholestanol, present discrepant results, rendering them often inadequate to identify cases of coronary artery disease as well as alterations in the whole body cholesterol metabolism; 3) such methods of measurement of cholesterol metabolism are confounded by factors like diabetes mellitus, body weight and other pathologies including considerable hereditary hyperlipidemias biological variabilities that influence the efficiency of synthesis and intestinal absorption of cholesterol.

An Interesting Case of Familial Homozygous Hypercholesterolemia-A Brief Review. Indian J Clin Biochem. 2012;27:309-313. [PMID: 26405394 DOI: 10.1007/s12291-011-0165-8]

Familial hypercholesterolemia (FH) is a form of primary hyperlipoproteinemia, is an autosomal co-dominant disorder, characterized by an increase in serum LDL cholesterol concentrations, presence of xanthomas and premature atherosclerosis. Homozygous familial hypercholesterolemia is of rare occurence in which approximately 1 in 1 million persons in the general population are affected. Here we report an interesting case of familial homozygous hypercholesterolemia for its classical presentation and rarity.

The Farnesoid X receptor: a molecular link between bile acid and lipid and glucose metabolism

Bile acids are the end products of cholesterol metabolism. They are synthesized in the liver and secreted via bile into the intestine, where they aid in the absorption of fat-soluble vitamins and dietary fat. Subsequently, bile acids return to the liver to complete their enterohepatic circulation. The Farnesoid X receptor (FXR) is a member of the nuclear receptor superfamily and has emerged as a key player in the control of multiple metabolic pathways. On its activation by bile acids, FXR regulates bile acid synthesis, conjugation, and transport, as well as various aspects of lipid and glucose metabolism. This review summarizes recent advances in deciphering the role of FXR in the context of hepatic lipid and glucose homeostasis and discusses the potential of FXR as a pharmacological target for therapeutic applications.

Intestinal absorption of cholesterol by patients with Smith-Lemli-Opitz syndrome

The Smith-Lemli-Opitz syndrome (SLOS) is a disorder of impaired cholesterol biosynthesis because of a deficiency of the enzyme 7-dehydrocholesterol-Delta(7)-reductase, in the last step in cholesterol biosynthesis. Dietary cholesterol has been proposed as a potential therapy for SLOS and is being tested currently. Because there is no information on cholesterol absorption in SLOS, we recruited 12 SLOS patients into the General Clinical Research Center for 1-wk periods for administration of test meals and for blood and stool collections. A test breakfast that contained tracer cholesterol-4-C(14) with egg yolk or with crystalline cholesterol in suspension was given subsequently. Twenty-four and 48-h blood and 1-wk stool samples then were collected. The radioactivities in these samples were analyzed to determine the absorption of cholesterol by these patients. In 11 patients who were given egg yolk cholesterol, cholesterol absorption was 27.3 +/- 6.7%. The absorption was slightly less at 20.5 +/- 10.3% but not significantly different for the six patients who were given crystalline cholesterol. There was a positive correlation between the absorption of isotopic cholesterol as measured by determination of radioactive cholesterol in stool and the amount of isotopic cholesterol in the plasma at 24 and 48 h after the meal. Our data indicated that SLOS patients absorb cholesterol from the diet. However, the percentage of absorption is lower than reported values for normal adults and for hypercholesterolemic children. The absorption of crystalline cholesterol in suspension was slightly lower than the absorption of cholesterol in egg yolk cholesterol by these patients. The absorption of cholesterol may ameliorate some of the biochemical and developmental deficits in SLOS patients.

ApoB metabolism in familial hypercholesterolemia. Inconsistencies with the LDL receptor paradigm

The biology of the low-density lipoprotein (LDL) receptor has been examined in detail, and a paradigm for LDL metabolism has evolved from comparative studies of cholesterol metabolism in a variety of cells cultured from normal individuals and subjects with familial hypercholesterolemia (FH). Cultured cells from patients with homozygous FH lack a functional LDL receptor and show diminished LDL clearance, induction of the enzyme hydroxymethylglutaryl coenzyme A (HMG-CoA) reductase, increased cholesterol synthesis, decreased cholesterol ester production, and depleted cholesterol ester stores. The observed decrease in the fractional catabolic rate (FCR) of LDL is attributed to the mutated LDL receptor gene. However, in the experimental animal model of this disease, the Watanabe heritable hyperlipidemic (WHHL) rabbit, cholesterol ester stores are increased, while hepatic cholesterol synthesis is decreased. Furthermore, in humans HMG-CoA reductase is suppressed, and the LDL apolipoprotein (apo) B production rate is increased in patients with FH. These findings raise questions about the adequacy of the paradigm in understanding hepatic cholesterol metabolism in vivo. In humans, apoB metabolism is believed to be principally determined by the liver, where apoB is both synthesized and catabolized. Assuming the neutral lipid content of the liver is the major determinant of apoB metabolism, we postulated that the changes in apoB metabolism in FH are predictable when based on the assumption of an increase in hepatic cholesterol and cholesterol ester content, as observed both in the WHHL rabbit and in humans. We examined this hypothesis in vivo in patients with heterozygous FH by using tracer kinetic methodology and have used similar data from normal and hypertriglyceridemic (HTG) subjects as controls. Whereas normal and HTG subjects secrete apoB primarily as large, triglyceride-enriched very-low-density lipoprotein (VLDL), heterozygous FH patients have an absolute decrease in apoB production and secrete almost 40% of apoB as smaller intermediate-density lipoprotein (IDL)/LDL. In normal humans, about half of secreted apoB is catabolized rather than being converted to LDL. In HTG subjects two thirds of apoB follows this same route, by which VLDL remnants remaining after triglyceride hydrolysis are largely returned to the liver. In contrast, in FH subjects secreted apoB is fully converted to LDL. Thus, although total apoB secretion is reduced in FH subjects, total LDL production is greater than in either normal or HTG subjects. Under basal conditions the elevated LDL in heterozygous FH is due to both decreased LDL receptor-mediated catabolism and increased LDL production. However, the number of LDL receptors actually expressed is suppressed below the number of potentially functional receptors.(ABSTRACT TRUNCATED AT 400 WORDS)

Reduction in plasma low-density lipoprotein cholesterol and urinary mevalonic acid by lovastatin in patients with heterozygous familial hypercholesterolemia

The effects of lovastatin, an inhibitor of 3-hydroxy-3-methyl glutaryl coenzyme A reductase (HMG CoA reductase), on 24-hour urinary excretion rates of mevalonic acid (an intermediate in cholesterol biosynthesis) and plasma low-density lipoprotein (LDL) cholesterol concentrations were evaluated in patients with heterozygous familial hypercholesterolemia (FH). The mean rates of urinary mevalonate excretion of 28 FH patients were initially higher (2.95 +/- 0.29 (+/- SEM) mumols/d) than in 17 control subjects (1.82 +/- 0.12 mumols/d). Patients with FH were treated with sequentially increasing doses of lovastatin (10, 20, 40, and 80 mg daily, taken as a twice daily dosage) for a period of 6 weeks on each dose. When compared to baseline, LDL cholesterol levels fell by 22%, 26%, 30%, and 35% respectively, on these different doses. The mean daily urinary mevalonate excretion decreased from baseline by 19% after 4 weeks on 10 mg daily of lovastatin, 35% on 20 mg, and 31% on 40 mg and 80 mg daily. Similar decreases in urinary mevalonate excretions were observed when patients with FH were treated directly with 40 mg (20 mg twice daily) or 80 mg (40 mg twice daily) mg of lovastatin daily. The magnitude of decrease in LDL cholesterol did not show any significant correlation with the changes in urinary excretion of mevalonic acid. Lovastatin therapy decreases rates of urinary mevalonate excretion (which has previously been shown to reflect rates of cholesterol synthesis) by up to 35% at doses of 20 to 80 mg/d; such a decrease seems unlikely to compromise other important cellular requirements for mevalonate.

Bile acid metabolism in heterozygous familial hypercholesterolaemia: a study comparing affected and unaffected siblings of four kindreds

Previous studies have indicated that quantitative as well as qualitative abnormalities of bile acid metabolism frequently occur in hypercholesterolaemia. In order to determine if this is a feature of familial hypercholesterolaemia, bile acid kinetics and biliary lipid composition were determined in 15 affected (heterozygous) and six unaffected siblings of four kindreds with familial hypercholesterolaemia. Furthermore, serum levels of cholic acid, chenodeoxycholic acid and deoxycholic acid were measured with a mass fragmentographic technique in 15 members of two of the kindreds, and secretion rates of biliary lipids were measured in six members of two kindreds. No differences with regard to these parameters between affected and unaffected siblings could be detected. There was a close resemblance between relatives of a given kindred concerning bile acid pool size and serum bile acid levels. No evidence for a defective bile acid metabolism in familial hypercholesterolaemia could be gained from the present study. It is concluded that the deficient receptor-mediated elimination of low density lipoprotein cholesterol in this disorder does not influence the maintenance of normal bile acid metabolism.

Long-term changes in cholesterol biosynthesis and the effect of plasmapheresis therapy in a hypercholesterolemia homozygote

Synthesis of cholesterol was measured in a familial hypercholesterolemia homozygote on four occasions from age 1.1 to 9.9 years by the sterol balance technique. Both the fecal neutral steroid and fecal bile acid components of sterol balance were elevated initially. Over the decade of study, neutral steroid excretion/kg declined 61% whereas bile acid excretion/kg was unchanged. Chronic plasmapheresis therapy every two weeks for 3.4 years reduced plasma low-density lipoprotein cholesterol 54% but had little effect on the rate of cholesterol biosynthesis.

Effects of high cholesterol high fat diet on plasma lipoproteins in familial hypercholesterolemia

Heterozygous individuals with familial hypercholesterolemia possess about half of the normal numbers of functioning receptors on their cells. This is thought to be responsible for their hypercholesterolemia. In normals, dietary cholesterol increases LDL production and decreases LDL receptor-related LDL clearance, resulting in elevations in LDL cholesterol levels of approximately 30 mg/dL. To assess the effects of high fat and high cholesterol diets on the lipoproteins of individuals with diminished LDL receptors, three kinds of diets, including ones high in cholesterol, were fed to four patients with familial hypercholesterolemia, in the expectation that diet effects on apoB- or apoE-containing lipoproteins would be exaggerated. The basal diet consisted of 15% protein, 30% fat, 55% carbohydrate, 300 mg/d cholesterol, P/S ratio 0.4; the high fat diet was identical except that fat calories were 55% and carbohydrate 30%; the high fat-high cholesterol diet was identical with the high fat diet except approximately 750 or approximately 1,500 mg/d of cholesterol were added. Each diet was eaten for five weeks at home and for the sixth week at the general Clinical Research Center. Fasting (12-14 hours) plasmas were collected every two weeks for lipoprotein-lipid and apoprotein quantitation. At the end of each period, fasting and 4-hour postprandial samples were analyzed also by zonal ultracentrifugation and gel permeation chromatography.(ABSTRACT TRUNCATED AT 250 WORDS)

Abnormalities of high density lipoproteins in homozygous familial hypercholesterolemia

The possibility that low high density lipoprotein (HDL) levels may add to the risk of occlusive atherosclerosis in familial hypercholesterolemia (FH) is supported by the frequency and severity of this finding in these patients, particularly homozygotes. To investigate this abnormality in greater detail, we measured HDL lipid and apoprotein values in nine homozygotes with FH. Compared to their unaffected relatives, they had markedly reduced HDL cholesterol and apo A-I and A-II levels. The values found in heterozygote relatives were between those of homozygotes and those of controls. Density gradient ultracentrifugation of serum or isolated HDL from homozygotes demonstrated little or no HDL2; there was also a shift of homozygotes' HDL to an above normal peak density (d = 1.15 g/ml). Heterozygotes had milder abnormalities. The reduced HDL in FH seems to be related to elevated low density lipoprotein levels although the pathophysiology is unknown. These findings add weight to the concept that HDL abnormalities contribute to the relentless course of atherosclerosis in familial hypercholesterolemia.

Hypercholesterolemia associated with alpha-1 antitrypsin deficiency and hepatitis: lipoprotein and apoprotein determinations, sterol balance and treatment

Lipid metabolism was investigated in a 4-year-old boy with alpha-1 antitrypsin deficiency (ZZ phenotype) and liver disease. Plasma cholesterol and triglyceride levels were 604 mg/dl and 336 mg/dl respectively. Both parents had normal plasma lipids. Lipoprotein X was present at a concentration of 855 mg/dl and levels of apoproteins A-I, A-II, B and C-III were elevated. The plasma free fatty acid pattern was normal. Plasma cholesterol esterification was greatly depressed. Cholesterol absorption on two occasions was reduced about 13% compared with adult controls. Neutral and total steroid excretion was normal with increased excretion of bile acids. A low-cholesterol, low-fat diet reduced plasma cholesterol to 374 mg/dl and triglyceride to 236 mg/dl in two months. Cholesterol and lipoprotein X concentrations were elevated far out of proportion to the severity of the liver disease (total bilirubin 3.7 mg/dl, SGOT 280 IU/L). This suggests that lipoprotein metabolism in patients with this disorder is unusual and may differ from the derangements seen in other forms of liver disease.

Homozygous familial hypercholesterolemia occurring with apoprotein E3 deficiency. Report of two cases

This is the first report of homozygous familial hypercholesterolemia (FH) occurring together with dysbetalipoproteinemia. The former was demonstrated by deficiency of specific receptors for apoprotein B of low density lipoproteins and the latter by isoelectric focusing of the E isoapoproteins and the presence of a broad-beta band on electrophoresis. Two young boys of Lebanese extraction had extensive tuberous and tendinous xanthomata, serum cholesterol concentrations of 29.9 and 28.4 mmol/liter, respectively, and mildly raised serum triglycerides due to an accumulation of lipoprotein remnant particles. Homozygosity for FH was demonstrated in both boys by the deficiency of specific binding of low density lipoprotein to cultured skin fibroblasts (less than 15% and less than 10% of normal, respectively). The E apoprotein phenotypes showed E3/E2 in one boy and E2/E2 in the other. The treatment of both boys with cholestyramine and probucol reduced the serum cholesterol concentration to between 15 and 18 mmol/liter and dramatically lessened the severity of xanthomatosis.

The dietary treatment of hyperlipidemia. Rationale, technique and efficacy

Working from the foundation of the role of diet in the pathogenesis of hyperlipidemia, the authors present a rational, detailed therapy for treating the disorder. A three-phase approach is offered so that the patient can gradually incorporate new eating behavior into his or her lifestyle.

Therapeutic effects of ML-236B in primary hypercholesterolemia

ML-236B, a competitive inhibitor of 3-hydroxy-3-methylglutaryl CoA reductase, was administered to 11 patients with primary hypercholesterolemia. After 4--8 weeks of drug treatment at doses of 50--150 mg/day, serum cholesterol levels were reduced by 11--37% (27% on average) in cases of heterozygous familial hypercholesterolemia and combined hyperlipidemia. A marked reduction in tuberous xanthomas was noticed in a homozygous case of familial hypercholesterolemia, but here the drug was less effective in reducing the serum cholesterol level and a higher dose was required for treatment. Softening of Achilles tendon xantomas was observed in a case of combined hyperlipidemia.

Sterol balance in abetalipoproteinemia: studies in a patient with homozygous familial hypobetalipoproteinemia

A new case of homozygous familial hypobetalipoproteinemia is reported in a 16-yr-old girl. Apoprotein B was absent from plasma and the patient had acanthocytes and steatorrhea, but minimal neurologic dysfunction. Total body cholesterol synthesis was assessed intermittently over a 30-mo period by sterol balance techniques. The rate of synthesis of cholesterol was higher (15.0 +/- 2.9 mg/kg/day) in the patient (8.3 +/- 0.4 mg/kg/day than in 3 control children, p less than 0.005). Bile acid synthesis was similar (4.6 +/- 1.8 versus 4.0 +/- 1.7 mg/kg/day) in the patient and controls, but total body sterol synthesis was significantly higher (19.6 +/- 3.0 versus 12.2 +/- 2.0, p less than 0.005). The absorption of orally administered [1,2,(3)H] cholesterol in the patient was low and less than 0.5% of the label appeared in the total plasma volume at all times up to 48 hr. Estimates of the extent that malabsorption of biliary cholesterol contributes to the enhanced excretion of neutral sterols in this case indicate that all of the increase can be explained on this basis. Thus, although the mechanisms for the increased sterol synthesis in this case may relate to the absence of chylomicrons and low density lipoproteins in plasma, the magnitude of the increase can be fully explained on the basis of a compensatory mechanism to maintain cholesterol homeostasis in the face of enhanced fecal losses.