Low Density Lipoprotein: A METABOLIC PATHWAY FOR RETURN OF CHOLESTEROL TO THE SPLANCHNIC BED

The mechanism(s) by which cholesterol returns to the splanchnic bed from peripheral tissues are not well understood. To study this phenomenon in fasting man, lipoproteins were isolated from plasma obtained from hepatic vein and aorta. Cholesterol content of each lipoprotein class was determined and arteriovenous (AV) differences could be calculated for each patient. The results in the first 24 patients indicated splanchnic secretion of very low density lipoprotein cholesterol (mean AV difference - 3 mg/100 ml, P < 0.01), but not significant AV difference for total cholesterol, high density lipoprotein cholesterol, or low density lipoprotein (LDL) B protein. In contrast, for LDL (d 1.006-1.063 g/ml), there was significant uptake of cholesterol across the AV bed +8 mg/100 ml, P < 0.0002). In a further 15 patients, similar samples were obtained and intermediate density lipoprotein isolated at d 1.006-1.019 g/ml and LDL at 1.019-1.063 g/ml. The AV difference previously noted could now be localized to the 1.019-1.063 cholesterol ester moiety (+8 mg/100 ml, P < 0.0005). In the final 14 patients, the LDL cholesterol AV difference was again confirmed and shown to be unrelated to heparin. As well, there was secretion of triglyceride in the hepatic vein LDL. These quantitative data obtained in man raise the possibility that LDL rather than high density lipoprotein transports cholesterol ester to the splanchnic bed.     

Effects of chronic ethanol intake on mobilization and excretion of cholesterol in baboons.

To investigate the effects of chronic ethanol administration on the mobilization and excretion of cholesterol, turnover and balance studies were carried out in baboons pair-fed cholesterol-free diets containing 50% of energy either as ethanol or as additional carbohydrate for several years. Ethanol feeding increased free cholesterol in all plasma lipoprotein fractions, and esterified cholesterol in very low density lipoprotein, intermediate density lipoprotein, and high density lipoprotein (HDL). The major increase occurred in HDL, mainly as esterified cholesterol. The latter was associated with decreased transfer of esterified cholesterol from HDL to low density lipoprotein. By contrast, the smaller increase in HDL-free cholesterol was associated with increased turnover in the plasma, increased splanchnic uptake, and increased fecal excretion of plasma cholesterol, mainly as neutral steroids. Cholesterol extraction predominated over release in the splanchnic vascular bed, suggesting that the excess of cholesterol excreted in the feces originated in extrasplanchnic tissues. Thus, these findings indicate that alcohol consumption favors mobilization of tissue free cholesterol for hepatic removal and excretion. By contrast the increase in HDL-cholesterol (mainly esterified) appears to be a poor indicator of cholesterol mobilization.     

Effect of cyclosporine on HMG-CoA reductase, cholesterol 7alpha-hydroxylase, LDL receptor, HDL receptor, VLDL receptor, and lipoprotein lipase expressions

Long-term administration of cyclosporine (CsA) has been shown to cause hypercholesteremia, hypertriglyceridemia, and elevations of plasma low-density and very low-density lipoprotein (LDL and VLDL) levels in humans. This study was undertaken to explore the effects of CsA on expressions of the key lipid regulatory enzymes and receptors. Thus, hepatic expressions of cholesterol 7alpha-hydroxylase (the rate-limiting step in cholesterol conversion to bile acids), LDL receptor, and high-density lipoprotein (HDL) receptor proteins, as well as 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase activity were determined in rats treated with CsA (18 mg/kg/day) or placebo for 3 weeks. In addition, skeletal muscle and adipose tissue expressions of lipoprotein lipase and VLDL receptor were measured. Western blot analysis was used for all protein measurements using appropriate antibodies against the respective proteins. CsA-treated animals showed mild but significant elevations of plasma cholesterol and triglyceride concentrations. This was associated with a marked down-regulation of cholesterol 7alpha-hydroxylase in the liver and a severe reduction of lipoprotein lipase abundance in skeletal muscle and adipose tissue. However, hepatic LDL receptor and HDL receptor expressions and HMG-CoA reductase activity were not altered by CsA therapy. Likewise, skeletal muscle and adipose tissue VLDL receptor protein expressions were unaffected by CsA administration under the given condition. In conclusion, CsA administration for 3 weeks resulted in a significant reduction of hepatic cholesterol 7alpha-hydroxylase and marked down-regulation of skeletal muscle and adipose tissue lipoprotein lipase abundance in rats. The former abnormality can contribute to hypercholesterolemia by limiting cholesterol catabolism, whereas the latter may contribute to hypertriglyceridemia and VLDL accumulation by limiting triglyceride-rich lipoprotein clearance in CsA-treated animals.     

High density lipoproteins, but not other lipoproteins, provide a vehicle for sterol transport to bile.

Unesterified cholesterol (UC) that is taken up by the liver from lipoproteins is rapidly mixed by exchange with liver UC. Thus, it is not possible to quantitate the transport of UC from different lipoproteins into bile using radiolabeled UC. However, plant sterols do not exchange with UC and are secreted in bile with the same kinetics as UC. To compare the contribution to bile of sterols from different lipoproteins, we perfused isolated rat livers with VLDL, LDL, and HDL that were obtained from patients with hereditary phytosterolemia and were rich in plant sterols. After 30-min recirculating perfusions, hepatic concentrations of plant sterols were not different after different lipoproteins were perfused. However, biliary plant sterol secretion was markedly different: with the perfusion of either VLDL or LDL there was no increase in plant sterols in bile, but with perfusion of HDL, the secretion of plant sterols was increased two- to threefold (P = 0.0005). The increase in biliary plant sterols was detected 5-10 min after HDL was added to perfusates and was similarly large for each of three individual plant sterols that was tracked. Results show that when sterol transport from lipoproteins into bile can be determined, only HDL provides a vehicle for UC elimination in bile that is consistent with its putative function in reverse cholesterol transport.     

Dyslipidemia in insulin resistance and its improvement by troglitazone

Dyslipideia in insulin resistant state are characterized by 3 major components; increased triglyceride levels, decreased high-density lipoprotein (HDL) cholesterol, and change in the composition of low-density lipoprotein (LDL) cholesterol particle which results in small-dense LDL. Insulin resistance is thought to lead to overproduction of very low-density lipoprotein (VLDL) cholesterol through the decreased peripheral lipoprotein lipase (LPL) activity, increased production of apolipoprotein B-100 and decreased clearance of remnant particles. Troglitazone, an insulin action enhancer, decreases the triglyceride level, increases HDL cholesterol level and decreases th proportion of small-dense LDL through the direct effect on lipoprotein metabolism. However, activation of PPAR-gamma is considered to possese potential atherogeneity and more closer examination is needed.     

Metabolic and health complications of obesity

Overnutrition manifested by obesity has emerged as a major health problem in affluent countries. In spite of increased interest in fitness, obesity is on the increase in the United States. This is particularly so among children and adolescents. Although obesity is associated with many risk factors for diseases, the mechanisms whereby it enhances disease risk are not fully understood. Such an understanding is needed to develop strategies for management of these conditions. In this report we suggest that overnutrition produces clinical diseases only in individuals who already possess a metabolic weakness or "defect" in a given system. In the absence of such underlying defects, overnutrition, or obesity, is well tolerated. One of the most common consequences of obesity is dyslipidemia, that is, elevations of very low-density lipoprotein (VLDL) triglycerides and low-density lipoprotein (LDL) cholesterol and low concentrations of high-density lipoprotein (HDL) cholesterol. The major effect of overnutrition on lipoprotein metabolism is to stimulate the production of VLDL. For patients who have an underlying defect in lypolysis of VLDL triglycerides, hypertriglyceridemia will develop in the obese state. For those who have defective clearance of LDL, obesity will accentuate hypercholesterolemia. Both of these effects can be explained by overproduction of VLDL, due to obesity, combined with a genetic defect in clearance of VLDL or LDL. The mechanism whereby obesity causes a lowering of HDL cholesterol is uncertain, although it could enhance removal of HDL by an excess of adipose tissue. Another disease associated with obesity is cholesterol gallstones. The presence of obesity more than doubles the risk for gallstones. Two underlying factors increase the danger for gallstones: a deficiency of hepatic secretion of bile acids and a tendency for formation of cholesterol crystals in bile. Overnutrition promotes the synthesis of whole-body cholesterol, and the only route for excretion of this excess cholesterol is through the biliary tree.(ABSTRACT TRUNCATED AT 400 WORDS)     

Resolving hyperlipidemia after liver transplantation in a patient with primary sclerosing cholangitis

A 64-year-old man with primary sclerosing cholangitis (PSC) and resultant liver failure presented to our hospital with severe dyslipidemia (total cholesterol, 525 mg/dL; low-density lipoprotein (LDL) cholesterol, 489 mg/dL; high-density lipoprotein (HDL) cholesterol, 13 mg/dL; triglycerides, 114 mg/d) and coronary artery disease. The abnormal lipid profile of patients with cholestatic liver disease, such as PSC, includes an abnormal atherogenic LDL called lipoproteinX. The patient's dyslipidemia persisted despite treatment with a statin. Lipids normalized only after liver transplantation (total cholesterol, 135 mg/dL; LDL cholesterol, 60 mg/dL; high-density lipoprotein cholesterol, 48 mg/dL; triglycerides, 130 mg/dL). To the best of our knowledge, the dramatic improvement in the lipid profile after liver transplantation represents the first such published report for PSC. The recognition of dyslipidemia and atherosclerosis in those with cholestatic liver disease and the normalization of lipid profile after liver transplantation warrant further study. We present a review of dyslipidemia in cholestatic liver disease, its relationship to atherosclerosis, and its treatment.     

On the mechanism of plasma cholesterol reduction in the rat given probucol

1. The effects of the cholesterol-lowering drug probucol on lipoprotein metabolism and on the key enzymes that regulate hepatic cholesterol metabolism in the rat were studied. 2. Probucol given for 2 weeks was accompanied by a significant reduction in plasma concentrations of low-density and high-density lipoproteins (LDL,HDL). The fractional catabolic rates of the apolipoproteins of HDL and LDL (apoHDL, apoLDL) were not affected by probucol, although the absolute rates of catabolism of both the apolipoproteins were significantly reduced. 3. The activities of 3-hydroxy-3-methylglutaryl-coenzyme A (CoA) reductase and cholesterol 7 alpha-mono-oxygenase, as well as the rate of hepatic sterol synthesis, were unchanged during the first 2 weeks of probucol. More prolonged probucol led to inhibition of the activity of these enzymes and reduction in sterol synthesis, although the liver cellular content of cholesterol significantly increased. 4. It is postulated that a principal mode of action of the drug is to reduce the rate of lipoprotein synthesis.     

Hepatobiliary cholesterol transport is not impaired in Abca1-null mice lacking HDL

The ABC transporter ABCA1 regulates HDL levels and is considered to control the first step of reverse cholesterol transport from the periphery to the liver. To test this concept, we studied the effect of ABCA1 deficiency on hepatic metabolism and hepatobiliary flux of cholesterol in mice. Hepatic lipid contents and biliary secretion rates were determined in Abca1(-/-), Abca1(+/-), and Abca1(+/+) mice with a DBA background that were fed either standard chow or a high-fat, high-cholesterol diet. Hepatic cholesterol and phospholipid contents in Abca1(-/-) mice were indistinguishable from those in Abca1(+/-) and Abca1(+/+) mice on both diets. In spite of the absence of HDL, biliary secretion rates of cholesterol, bile salts, and phospholipid were unimpaired in Abca1(-/-) mice. Neither the hepatic expression levels of genes controlling key steps in cholesterol metabolism nor the contribution of de novo synthesis to biliary cholesterol and bile salts were affected by Abca genotype. Finally, fecal excretion of neutral and acidic sterols was similar in all groups. We conclude that plasma HDL levels and ABCA1 activity do not control net cholesterol transport from the periphery via the liver into the bile, indicating that the importance of HDL in reverse cholesterol transport requires re-evaluation.     

2型糖尿病合并非酒精性脂肪肝42例临床分析

目的探讨2型糖尿病患者非酒精性脂肪肝的发生是否与胰岛素抵抗及血脂代谢紊乱有关。方法收集本院96例2型糖尿病患者，全部病例肝脏B超扫描证实，将其分为2型糖尿病合并脂肪肝组42例与2型糖尿病无脂肪肝组54例，同时测定两组病人的体重、TG、TC、HDL、LDL、空腹血糖（FBs）、空腹血胰岛素水平（FINS），并计算胰岛素抵抗指数（IRI）等。结果2型糖尿病合并脂肪肝组与无脂肪肝组相比较，血TG、LDL、FINS、IRI增高（P≤0．05）；HDL下降（P≤0．05）：TC、FBS无明显差异（P〉0．05）。结论2型糖尿病患者合并非酒精性脂肪肝与血脂代谢紊乱、胰岛素抵抗及肥胖等有关。     

Lipoprotein physiology in nondiabetic and diabetic states. Relationship to atherogenesis

Abnormalities of plasma lipid and lipoprotein concentrations are common in both insulin-dependent (IDDM) and non-insulin-dependent (NIDDM) diabetes mellitus. In general, individuals with IDDM who are untreated or inadequately treated have elevations in both postprandial and fasting triglyceride levels in association with reduced activity of lipoprotein lipase. Low-density lipoprotein (LDL) cholesterol levels can rise when insulin deficiency impacts on LDL-receptor function. When patients with IDDM are treated and plasma glucose levels well controlled, plasma very-low-density lipoprotein (VLDL) triglyceride and LDL cholesterol levels are usually normal. In addition, plasma high-density lipoprotein (HDL) cholesterol levels are normal or elevated in well-controlled IDDM subjects. In NIDDM, increased VLDL triglyceride and reduced HDL cholesterol concentrations are common and are only partially related to glycemic control. Overproduction of VLDL leads to hypertriglyceridemia, which can be exacerbated if lipoprotein lipase activity is also reduced. The regulation of LDL levels is complex; catabolism can be reduced if significant insulin deficiency exists or increased if significant hypertriglyceridemia is present. The reduced levels of HDL cholesterol in NIDDM appear to be related to increased exchange of HDL cholesteryl esters for VLDL triglycerides, although other mechanisms may exist. The roles of insulin resistance, obesity, and independently inherited abnormalities of lipoprotein metabolism in the etiology of dyslipidemia of NIDDM are complex and require further investigation. Finally, the effects of diabetes on glycosylation of apoproteins; on other lipid enzymes, particularly hepatic triglyceride lipase; on lipoprotein surface lipids; and on hepatic uptake of remnants have only just begun to be defined. In view of the marked increase in atherosclerotic cardiovascular disease in individuals with diabetes mellitus, prompt attention to and aggressive therapy for dyslipidemia should be a central component of care for these patients.     

Lipoprotein particle analysis by nuclear magnetic resonance spectroscopy

Laboratory measurements of plasma lipids (principally cholesterol and triglycerides) and lipoprotein lipids (principally low-density lipoprotein [LDL] and low-density lipoprotein [HDL] cholesterol) are the cornerstone of the clinical assessment and management of atherosclerotic cardiovascular disease (CVD) risk. LDL particles, and to a lesser extent very-low-density lipoprotein [VLDL] particles, cause atherosclerosis, whereas HDL particles prevent or reverse this process through reverse cholesterol transport. The overall risk for CVD depends on the balance between the "bad" LDL (and VLDL) and "good" HDL particles. Direct assessment of lipoprotein particle numbers us now possible through nuclear magnetic resonance spectroscopic analysis.     

The effects of triple vs. dual and monotherapy with rosiglitazone, glimepiride, and atorvastatin on lipid profile and glycemic control in type 2 diabetes mellitus rats

The present study was undertaken to investigate the effects of triple oral therapy and different combination of rosiglitazone, atorvastatin, and glimepiride on streptozotocin (STZ)-induced diabetic rats. The various biochemical parameters studied included glycosylated hemoglobin (A1c), fasting plasma sugar levels, triglycerides, low-density lipoprotein (LDL) cholesterol, high-density lipoprotein (HDL) cholesterol, and very low-density lipoprotein (VLDL) cholesterol in diabetic and normal rats. The present study demonstrates that atorvastatin could increase the effect of rosiglitazone and glimepiride and lipid-lowering effect of combination of rosiglitazone and glimepiride (GLIM). According to our finding, similar results for rosiglitazone plus atorvastatin were obtained in terms of correcting lipid parameters, whereas the suppressive action of triple oral therapy of rosiglitazone and glimepiride, and atorvastatin on blood glucose, total cholesterol, LDL, VLDL, HDL cholesterol, and triglyceride was more beneficial than that of dual therapy of different combinations and monotherapy.     

Improved method for simultaneous determination of cholesterol in high- and low-density lipoproteins.

A previously described procedure for simultaneous determination of cholesterol in high- and low-density lipoproteins (HDL and LDL) [Clin. Chem. 24, 1504--1508 (1978)] has been improved by using centrifugation instead of ultrafiltration. Addition of NICl2 (2.0 mmol/L) to the reagent produced a good separation of HDL from the complex of low- and very-low-density lipoprotein-heparin-Ca2+ on centrifugation at 3000 rpm for 15 min. Replicate analyses for high-density lipoprotein cholesterol by the present method demonstrated the following intra-assay precision: mean = 389 mg/L, SD = 11 mg/L, CV = 2.8%. The present (y) and original (x) methods gave results that agreed reasonably well (n = 50, r = 0.960, y = 1.0x + 0.9). The enzymic method for HDL- or (HDL + LDL)-cholesterol after the separation of their fractions gave erroneous results, in particular in the cases of hyperbilirubinemic sera and in (HDL + LDL) fractions.     

Basal and post-methionine serum homocysteine and lipoprotein abnormalities in patients with chronic liver disease.

BACKGROUND: Lipoprotein abnormalities are commonly found in chronic liver diseases (CLDs), particularly hypercholesterolemia in primary biliary cirrhosis (PBC). However, affected patients may not be at increased risk of coronary heart disease. Cirrhotic patients display impaired methionine clearance, and an increased level of homocysteine, a methionine metabolite, is an independent risk factor for coronary heart disease. Thus, we hypothesized that the low risk of coronary heart disease in patients with CLD may be related to low serum levels of homocysteine. The aim of this study was to test this hypothesis after methionine load and to describe the serum lipoprotein profile in patients with PBC and in patients with hepatocellular liver disease. METHODS: Fifteen female patients (mean age, 58.2 +/- 11.7 years) with PBC, 15 female patients (mean age, 54.5 +/- 9.6 years) with other causes of CLD, and 15 healthy sex- and age-matched controls were given L-methionine (50 mg/kg of ideal body weight). Basal fasting serum homocysteine level and 2, 4, and 6 hours of post-methionine load were determined using high-performance liquid chromatography with a fluorometric detector. Levels of fasting serum cholesterol, triglycerides, high-density lipoprotein (HDL), low-density lipoprotein (LDL), lipoprotein (a) (Lp(a)), and apoprotein B were also determined. RESULTS: Results showed that mean basal and post-methionine load (6 hours) serum homocysteine levels were statistically significantly higher in the patients with PBC and with CLD than in the control group (P=0.04) and that levels of serum cholesterol, LDL, HDL, and apoprotein B were significantly higher in the PBC patients than in the other two groups (P < or = 0.05). There was no correlation between any of these parameters and the severity of liver disease. Serum HDL was significantly lower in the CLD group (P < or = 0.05) and correlated with severity of liver disease. There was no significant difference in serum cholesterol, LDL, or apoprotein B between the CLD group and the controls. Serum triglyceride and Lp(a) levels were similar for all three groups. CONCLUSIONS: In contrast to previous reports, the site of the methionine metabolic impairment was found to be below the homocysteine synthesis level. For most patients with CLD, factors other than serum homocysteine or Lp(a) are responsible for the reduction in the risk of coronary heart disease. Further studies with larger samples are needed.     

Cholesterol guidelines, lipoprotein cholesterol levels, and triglyceride levels: potential for misclassification of coronary heart disease risk

By using National Cholesterol Education Program guidelines for serum cholesterol (less than 200 mg/dl is designated "desirable," and 200 to 239 mg/dl is designated "borderline-high," and greater than or equal to 240 mg/dl is designated "high"), low-density and high-density lipoprotein (LDL, HDL) cholesterol levels and triglyceride levels were quantitated in 897 self-referred fasting subjects to assess the potential for coronary risk misclassification. With cholesterol less than 200 mg/dl, misclassification was arbitrarily identified by an LDL level greater than or equal to the 75th percentile, a triglyceride level greater than or equal to the 90th percentile, or an HDL level less than or equal to the 10th percentile. With the cholesterol level in the 200 to 239 mg/dl range, misclassification was identified by an LDL level greater than or equal to the 75th percentile, a triglyceride level greater than or equal to the 90th percentile, and an HDL level less than or equal to the 10th percentile or greater than or equal to the 90th percentile (or both). With a cholesterol level greater than or equal to 240 mg/dl, misclassification was identified by an HDL level less than or equal to the 10th percentile, or greater than or equal to the 90th percentile. With the cholesterol level less than 200 mg/dl, misclassification is rare, occurring in 14.5% of the subjects. With the cholesterol level in the 200 to 239 mg/dl range, and greater than or equal to 240 mg/dl, misclassification occurred in 46.7% and 17.6% of the subjects, respectively. The importance of routine lipoprotein analysis when the cholesterol level is greater than or equal to 240 mg/dl is emphasized by the finding that 65% of the subjects in this category had top quartile LDL levels, 8% had bottom decile HDL levels, and 30% had top decile triglyceride levels. To avoid misclassification, fasting HDL, LDL, and triglyceride levels should probably be measured in all subjects with screening cholesterol levels greater than or equal to 200. There is remarkably little misclassification with top quartile LDL or bottom decile HDL levels (or both) when the cholesterol level is less than 200 mg/dl.     

Increased levels of high-density lipoprotein cholesterol are ineffective in inhibiting the development of immune responses to oxidized low-density lipoprotein and atherosclerosis in transgenic rabbits expressing human apolipoprotein (apo) A-I with severe hypercholesterolaemia

High levels of high-density lipoprotein (HDL) cholesterol have been reported to protect against the development of atherosclerosis in humans by increasing reverse cholesterol transport and inhibiting the oxidation of low-density lipoprotein (LDL) due to the paraoxonase content of HDL. The purpose of the present study was to assess if there are any relationships between in vivo increases in serum levels of immunological LDL oxidation markers [autoantibodies against oxidized LDL, autoantibodies against malondialdehyde-modified LDL, LDL immune complexes and anti-cardiolipin autoantibodies], paraoxonase activity and the development of atherosclerosis in control rabbits and in transgenic rabbits expressing human apolipoprotein (apo) A-I. A total of 13 apo A-I transgenic rabbits and 18 non-transgenic littermates were fed on a cholesterol-rich diet (0.4%, w/w) for 14 weeks, and were monitored at weeks 0, 2, 6, 10 and 14. Aortic atherosclerotic lesions were measured at the end of this period. Human apo A-I transgenic rabbits with high HDL cholesterol levels were not protected against the development of atherosclerosis when they were fed on a cholesterol-rich diet which induced dramatic hypercholesterolaemia. Immunological markers of LDL oxidation increased and serum paraoxonase activity decreased similarly in control and transgenic rabbits. In conclusion, the present study demonstrates that high HDL cholesterol levels are ineffective in inhibiting increases in immunological markers of LDL oxidation and the development of atherosclerosis in a mammal with severe hypercholesterolaemia.     

Lipoprotein transport in the metabolic syndrome: pathophysiological and interventional studies employing stable isotopy and modelling methods

The accompanying review in this issue of Clinical Science [Chan, Barrett and Watts (2004) Clin. Sci. 107, 221-232] presented an overview of lipoprotein physiology and the methodologies for stable isotope kinetic studies. The present review focuses on our understanding of the dysregulation and therapeutic regulation of lipoprotein transport in the metabolic syndrome based on the application of stable isotope and modelling methods. Dysregulation of lipoprotein metabolism in metabolic syndrome may be due to a combination of overproduction of VLDL [very-LDL (low-density lipoprotein)]-apo (apolipoprotein) B-100, decreased catabolism of apoB-containing particles and increased catabolism of HDL (high-density lipoprotein)-apoA-I particles. These abnormalities may be consequent on a global metabolic effect of insulin resistance, partly mediated by depressed plasma adiponectin levels, that collectively increases the flux of fatty acids from adipose tissue to the liver, the accumulation of fat in the liver and skeletal muscle, the hepatic secretion of VLDL-triacylglycerols and the remodelling of both LDL (low-density lipoprotein) and HDL particles in the circulation. These lipoprotein defects are also related to perturbations in both lipolytic enzymes and lipid transfer proteins. Our knowledge of the pathophysiology of lipoprotein metabolism in the metabolic syndrome is well complemented by extensive cell biological data. Nutritional modifications may favourably alter lipoprotein transport in the metabolic syndrome by collectively decreasing the hepatic secretion of VLDL-apoB and the catabolism of HDL-apoA-I, as well as by potentially increasing the clearance of LDL-apoB. Several pharmacological treatments, such as statins, fibrates or fish oils, can also correct the dyslipidaemia by diverse kinetic mechanisms of action, including decreased secretion and increased catabolism of apoB, as well as increased secretion and decreased catabolism of apoA-I. The complementary mechanisms of action of lifestyle and drug therapies support the use of combination regimens in treating dyslipoproteinaemia in subjects with the metabolic syndrome.     

Alterations of serum lipids in breast cancer: effects of disease activity, treatment, and hormonal factors.

Fasting venous blood collected from 83 patients with breast cancer was analyzed for triglycerides; total, high-density lipoprotein (HDL), and low-density lipoprotein (LDL) cholesterol; tumor necrosis factor (TNF alpha); glucose; creatinine; insulin; glucagon; growth hormone; cortisol; and thyrotropin. Patients with stage IV disease had significantly higher (P less than 0.05) triglyceride concentrations and significantly lower (P less than 0.05) concentrations of total and HDL cholesterol than did patients with less advanced disease or age-matched controls. Furthermore, LDL cholesterol concentrations in patients with boney metastases were significantly lower (P less than 0.05) than concentrations in patients with liver or liver plus boney metastases or in controls. These results could not be attributed to smoking habits, alcohol consumption, or treatment. We observed no correlations between serum concentrations of lipid and concentrations of TNF alpha, insulin, glucose, creatinine, cortisol, growth hormone, or thyrotropin. However, there was a significant (P less than 0.05) negative correlation between total cholesterol and glucagon and between LDL cholesterol and glucagon for patients with stage II, III, and IV disease, suggesting that glucagon may reduce LDL cholesterol concentrations by an as-yet-unidentified mechanism.     

Regulation of biliary cholesterol secretion. Functional relationship between the canalicular and sinusoidal cholesterol secretory pathways in the rat.

The functional interrelationship between biliary cholesterol secretion, sinusoidal lipoprotein cholesterol secretion and bile salt synthesis was studied in the rat. Diosgenin, fructose, and colestipol in the diet were used to, respectively, influence biliary cholesterol output, VLDL production and bile salt synthesis. In the acute bile fistula rat, biliary cholesterol output was 700% increased by diosgenin and 50% decreased by fructose. In the rats fed both diosgenin and fructose, biliary cholesterol secretion was increased only by approximately 200%, whereas biliary bile salts and phospholipid outputs were unchanged. In the isolated perfused liver, VLDL-cholesterol output was 50% reduced by diosgenin alone, but was unchanged following feeding of diosgenin plus fructose. However, the livers of rats fed diosgenin plus fructose exhibited a 700% increase in VLDL-triglyceride production and a 200% increase in VLDL-cholesterol output. A significant reciprocal relationship between VLDL-cholesterol secretion and the coupling ratio of cholesterol to bile salts in bile was observed. Colestipol added to the diet maintained both sinusoidal and biliary cholesterol outputs within the normal range. In the chronic bile fistula rat, colestipol increased bile salt synthesis by 100% while diosgenin and fructose diets had no effect. Similarly, the addition of fructose to the colestipol diet did not decrease bile salt synthesis. These data suggest a reciprocal relationship between biliary cholesterol secretion and hepatic secretion of cholesterol as VLDL particles. The free cholesterol pool used for bile salt synthesis seems functionally unrelated to the pool from which VLDL-cholesterol and biliary cholesterol originate. These findings support the idea that metabolic compartmentalization of hepatic cholesterol is a major determinant of the quantity of cholesterol available for recruitment by the bile salt-dependent biliary cholesterol secretory mechanism.