Reverse cholesterol transport in man: promotion of fecal steroid excretion by infusion of reconstituted HDL

Reverse cholesterol transport is a complex process, which transfers cholesterol from peripheral cells to the liver for subsequent elimination as bile acids and neutral steroids. Although apo A-I in high density lipoproteins (HDL) is believed to have a crucial role in this process, clinical conditions with very low HDL cholesterol levels appear to maintain normal cholesterol excretion. On the other hand, infusion of 'artificial HDL' in the form of recombinant proapo A-I (4 g) liposome complexes results in increased fecal steroid excretion, corresponding to a removal of approximately 0.5 g cholesterol daily for up to 9 days. This occurs without evidence of increased cholesterol synthesis, and could not be reproduced by infusion of liposomes only. These data indicate that stimulation of reverse cholesterol transport may be induced by infusion of 'artificial HDL' in humans, and that a more detailed knowledge of this process may be useful in the treatment of atherosclerosis.

Regulation of hepatic low-density lipoprotein receptor, 3-hydroxy-3-methylglutaryl coenzyme A reductase, and cholesterol 7alpha-hydroxylase mRNAs in human liver

To characterize the coordinate regulation of cholesterol metabolism in human liver, we simultaneously quantified mRNA levels of cholesterol 7alpha-hydroxylase (CYP7A1), 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMGR), and low- density lipoprotein receptors (LDLRs) in liver biopsies from 76 patients undergoing cholecystectomy. The three transcript levels were not different between untreated gallstone and gallstone-free patients and not significantly altered by 10-d exclusion of dietary cholesterol. Treatment with chenodeoxycholic acid suppressed CYP7A1 and to a lesser extent HMGR mRNA levels. Cholestyramine treatment increased CYP7A1, but also HMGR and LDLR mRNA, and statins only increased HMGR mRNA. Resin + statin treatment increased all mRNA species. In untreated patients, the mRNA levels of HMGR and LDLR were more strongly correlated (r = +0.60) than those of CYP7A1 and HMGR (r = +0.49) or CYP7A1 and LDLR (r = +0.21). In the treated patients, in whom bile acid synthesis was suppressed or stimulated, mRNA levels of CYP7A1 and HMGR (r = +0.84) as well as CYP7A1 and LDLR (r = +0.62) were more strongly correlated than those of HMGR and LDLR (r = +0.59). The coordinate control of HMGR and LDLR mRNA levels reflects their common regulation by shared transcriptional activation. In contrast, following changes in bile acid flux through the liver, CYP7A1 gene expression becomes a strong modulator of hepatic cholesterol metabolism.

Accumulation of foam cells in liver X receptor-deficient mice

BACKGROUND

The nature of some of the target genes for liver X receptors (LXRs)-alpha and -beta, such as sterol regulatory element binding protein-1 and ATP-binding cassette transporter proteins, suggests a pivotal role of these nuclear receptors in the regulation of fatty acid and cholesterol homeostasis. The present study aimed to elucidate the physiological relevance of both LXRs with regard to lipid metabolism and macrophage cholesterol efflux.

METHODS AND RESULTS

Mice depleted for LXRalpha, LXRbeta, or both were fed low-fat rodent chow for 18 months before investigations. The combined deficiency of LXRalpha and LXRbeta was linked to impaired triglyceride metabolism, increased LDL and reduced HDL cholesterol levels, and cholesterol accumulation in macrophages (foam cells) of the spleen, lung, and arterial wall.

CONCLUSIONS

Our data demonstrate the physiological importance of both LXRs in lipid metabolism and strongly indicate that both LXRs have a protective role against the development of atherosclerosis.

Requirement for thyroid hormone receptor beta in T3 regulation of cholesterol metabolism in mice

T3 potently influences cholesterol metabolism through the nuclear thyroid hormone receptor beta (TRbeta), the most abundant TR isoform in rodent liver. Here, we have tested if TRalpha1, when expressed at increased levels from its normal locus, can replace TRbeta in regulation of cholesterol metabolism. By the use of TRalpha2-/-beta-/- animals that overexpress hepatic TRalpha1 6-fold, a near normalization of the total amount of T3 binding receptors was achieved. These mice are similar to TRbeta-/- and TRalpha1-/-beta-/- mice in that they fail to regulate cholesterol 7alpha-hydroxylase expression properly, and that their serum cholesterol levels are unaffected by T3. Thus, hepatic overexpression of TRalpha1 cannot substitute for absence of TRbeta, suggesting that the TRbeta gene has a unique role in T3 regulation of cholesterol metabolism in mice. However, examination of T3 regulation of hepatic target genes revealed that dependence on TRbeta is not general: T3 regulation of type I iodothyronine deiodinase and the low density lipoprotein receptor were partially rescued by TRalpha1 overexpression. These in vivo data show that TRbeta is necessary for the effects of T3 on cholesterol metabolism. That TRalpha1 only in some instances can substitute for TRbeta indicates that T3 regulation of physiological and molecular processes in the liver occurs in an isoform-specific fashion.

Genetic characterization of Swedish patients with familial hypercholesterolemia: a heterogeneous pattern of mutations in the LDL receptor gene

Familial hypercholesterolemia (FH) is an autosomal codominant disease, caused by mutations in the LDL receptor gene. To characterize the distribution of genetic aberrations in Swedish FH-patients fulfilling the clinical criteria of FH, we have investigated 150 unrelated Swedish patients for mutations in the LDL receptor gene and for the most common mutation causing familial ligand defective apo B-100 (FDB). Of the patients, 77 were recruited from Huddinge University Hospital in Stockholm and 73 from Sahlgren's University Hospital in Göteborg. Screening was carried out using SSCP and Southern blotting techniques, combined with DNA sequence analysis. In total, mutations regarded as cause for disease were identified in 55 patients (37%), representing 32 different types of mutations. In the LDL receptor gene we detected four nonsense mutations, 13 missense mutations, seven splice junction mutations, and four major rearrangements. In addition, two small deletions were identified and one base exchange in the promoter region. The most common mutation (apo B3500) causing FDB was found in three patients. The most frequent mutation was FH-Helsinki, reflecting the admixture of Finnish immigrants. We further identified 15 point mutations which were not considered to affect the function of the gene, and thus were regarded as polymorphic changes. This multitude of mutations reflects a heterogeneous genetic background in our series of Swedish FH-patients and differs from the situation in the other Scandinavian countries. Future studies should aim at characterizing the importance of other genes for the development of the FH phenotype.

Differences in the regulation of the classical and the alternative pathway for bile acid synthesis in human liver. No coordinate regulation of CYP7A1 and CYP27A1

It has been reported that there is a coordinate regulation of sterol 27-hydroxylase (CYP27A1) and cholesterol 7alpha-hydroxylase (CYP7A1) in rats. Thus, the levels of the mRNA corresponding to these two enzymes were found to change in the same direction in rat liver and in isolated rat hepatocytes. In contrast, other groups have not seen such regulation of CYP27A1 in rabbit liver or in rat liver when using an activity assay. In the present work, the effect of bile acid treatment on human CYP27A1/luciferase reporter activity was studied in a transient transfection assay in human liver-derived HepG2 cells. Neither the endogenous 27-hydroxylase activity nor the CYP27A1/luciferase reporter activity were down-regulated by treatment of HepG2 cells with chenodeoxycholic acid or taurochenodeoxycholic acid. We also measured CYP27A1 mRNA and CYP7A1 mRNA in liver of humans subjected to treatment with chenodeoxycholic acid, ursodeoxycholic acid, hydroxymethylglutaryl (HMG)-CoA reductase inhibitor and a combination of HMG-CoA reductase inhibitor and cholestyramine. There was a 60-fold variation in the levels of CYP7A1 mRNA but only a 5-fold variation in the levels of CYP27A1 mRNA. There was no correlation between the two mRNA species. It is concluded that, in humans, there is little or no coordinate regulation of CYP7A1 and CYP27A1 at the transcriptional level, and that CYP27A1 is not subject to a negative feedback control by bile acids. The results underline that marked species differences may exist in mechanisms for control of synthesis of bile acids and cholesterol homeostasis.

Prolonged stimulation of the adrenals by corticotropin suppresses hepatic low-density lipoprotein and high-density lipoprotein receptors and increases plasma cholesterol

Pituitary ACTH has been shown to strongly stimulate adrenal receptors for low-density lipoprotein (LDL) and high-density lipoprotein (HDL) scavenger receptor class B type 1(SR-BI) to provide precursor cholesterol for glucocorticoid synthesis. The present study aimed to determine the effects of ACTH on hepatic cholesterol metabolism and plasma lipoproteins. Treatment of Sprague Dawley rats or normal C57BL/6J mice with ACTH for 3.5 d reduced hepatic SR-BI and LDL receptors. Simultaneously, cholesterol in plasma LDL and HDL was increased. None of these effects could be reproduced using glucocorticoids instead of ACTH, and they were abolished in adrenalectomized rats, indicating an obligate role of the adrenals for the effects of ACTH observed in the liver. When ACTH was given to LDL receptor-deficient mice, plasma LDL did not increase and the increase in HDL cholesterol remained, as did the suppression of hepatic SR-BI. Our data show that prolonged ACTH treatment suppresses hepatic SR-BI and LDL receptors in vivo in rodents, resulting in elevated plasma HDL and LDL. The adrenals are obligate for these effects, suggesting that ACTH releases some factor(s) that suppresses hepatic LDL and SR-BI receptors. Hypothetically, this novel mechanism would further promote channeling of cholesterol to the adrenals in situations of prolonged stress.

Dexamethasone stimulates very low density lipoprotein (VLDL) receptor gene expression in differentiating 3T3-L1 cells

To characterize endocrine mechanisms of very low density lipoprotein (VLDL) receptor regulation we studied mouse adipocytic 3T3-L1 cells. Lipid filled adipocyte-like cells are formed during a 5-7 day time course in the presence of insulin, dexamethasone and isobutylmethylxanthine (IBMX). The VLDL receptor protein, in the form of its approximately 120 and approximately 100 kDa type I and type II isoforms, as well as binding of (125)I-beta-VLDL, was induced several-fold during differentiation. Among the three different constituents added to the culture medium only dexamethasone (1 microM), but not insulin or IBMX, induced a time- and dose-dependent increase of VLDL receptor expression. Inclusion of RU-486 (10 microM) blocked the stimulatory effect of dexamethasone on VLDL receptor mRNA and protein levels. 3.6 kb of the 5'-untranslated region representing the VLDL receptor promoter were cloned and sequenced, and the transcriptional start site was determined by primer extension to be located 574 bases upstream from the initiating methionine. To investigate the functionality of the promoter, luciferase reporter gene constructs for the region -181 to -3726 bases were assembled and transfected into 3T3-L1 cells. An increased reporter gene activity was recorded when comparing preconfluent cells to fully differentiated cells. Between day 0 and day 2 (48 h after transfection) reporter gene activity was induced by dexamethasone, but not by insulin or IBMX. RU-486 inhibited this stimulatory effect for all constructs tested. No classical glucocorticoid receptor (GR) response element was found in the sequenced region of the VLDL receptor promoter. Thus, an indirect stimulatory effect mediated via GR on VLDL receptor gene transcription is the most likely mechanism of VLDL receptor gene activation in differentiating 3T3-L1 cells.

Analysis of the ileal bile acid transporter gene, SLC10A2, in subjects with familial hypertriglyceridemia

Familial hypertriglyceridemia (FHTG), a disease characterized by elevated plasma very low density lipoprotein triglyceride levels, has been associated with impaired intestinal absorption of bile acids. The aim of this study was to test the hypothesis that defects in the active ileal absorption of bile acids are a primary cause of FHTG. Single-stranded conformation polymorphism analysis was used to screen the ileal Na(+)/bile acid cotransporter gene (SLC10A2) for FHTG-associated mutations. Analysis of 20 hypertriglyceridemic patients with abnormal bile acid metabolism revealed 3 missense mutations (V98I, V159I, and A171S), a frame-shift mutation (646insG) at codon 216, and 4 polymorphisms in the 5' flanking sequence of SLC10A2. The SLC10A2 missense mutations and 5' flanking sequence polymorphisms were not correlated with bile acid production or turnover in the hypertriglyceridemic patients and were equally prevalent in the unaffected control subjects. In transfected COS cells, the V98I, V159I, and A171S isoforms all transported bile acids similar to the wild-type SLC10A2. The 646insG frame-shift mutation abolished bile acid transport activity in transfected COS cells but was found in only a single FHTG patient. These findings indicate that the decreased intestinal bile acid absorption in FHTG patients is not commonly associated with inherited defects in SLC10A2.

[Blood lipid derangement common in peripheral artery disease. Lipid-lowering agents may alleviate symptoms and slow the atherosclerotic process]

In the new guidelines from the Swedish Medical Products Agency, an aggressive approach is recommended for the treatment of hyperlipidemia in all patients with manifest atherosclerotic disease. Patients with intermittent claudication should therefore receive lipid-lowering treatment on the same indications as patients with coronary artery disease. The present article reviews our knowledge of hyperlipidemia as a risk factor for the development of peripheral artery disease. Hyperlipidemia is frequently found in these patients and the most common lipid derangements are low levels of HDL-cholesterol and hypertriglyceridemia. Hard end-point data concerning morbidity and mortality during lipid-lowering treatment in this specific population is largely lacking, although previous studies indicate that lipid-lowering treatment slows the atherosclerotic process and induces pain relief.

Growth hormone reduces plasma cholesterol in LDL receptor-deficient mice

Growth hormone (GH) has pleiotropic effects on cholesterol and lipoprotein metabolism. Pituitary GH is important for the normal regulation of hepatic LDL receptors (LDLR), for the enzymatic activity of bile acid regulatory cholesterol 7alpha-hydroxylase (C7alphaOH), and for the maintenance of resistance to dietary cholesterol. The present study aimed to determine whether GH has beneficial effects on plasma lipids and hepatic cholesterol metabolism in mice devoid of LDLR. Compared with wild-type controls, LDLR-deficient mice had approximately 250% elevated plasma total cholesterol and approximately 50% increased hepatic cholesterol levels; hepatic HMG CoA reductase activity was reduced by 70%, whereas C7alphaOH activity was increased by 40%. In LDLR mice, GH infusion reduced plasma cholesterol and triglycerides up to 40%, whereas HMG CoA reductase and C7alphaOH activities were stimulated by approximately 50% and 110% respectively. GH also stimulated HMG CoA reductase and C7alphaOH activities in control mice, whereas hepatic LDLR and plasma lipoproteins were unchanged. The effects of cholestyramine and atorvastatin on C7alphaOH in LDLR-deficient mice were potentiated by GH, and this was associated with a further reduction in plasma cholesterol. GH treatment reduces plasma cholesterol and triglycerides and stimulates C7alphaOH activity in mice devoid of LDLR, particularly in combination with resin or statin treatment. The potential of GH therapy in patients with homozygous familial hypercholesterolemia should be evaluated.

Thyroid hormone receptor beta-deficient mice show complete loss of the normal cholesterol 7alpha-hydroxylase (CYP7A) response to thyroid hormone but display enhanced resistance to dietary cholesterol

Thyroid hormone (T3) influences hepatic cholesterol metabolism, and previous studies have established an important role of this hormone in the regulation of cholesterol 7alpha-hydroxylase (CYP7A), the rate-limiting enzyme in the synthesis of bile acids. To evaluate the respective contribution of thyroid hormone receptors (TR) alpha1 and beta in this regulation, the responses to 2% dietary cholesterol and T3 were studied in TRalpha1 and TRbeta knockout mice under hypo- and hyperthyroid conditions. Our experiments show that the normal stimulation in CYP7A activity and mRNA level by T3 is lost in TRbeta-/- but not in TRalpha1-/-mice, identifying TRbeta as the mediator of T3 action on CYP7A and, consequently, as a major regulator of cholesterol metabolism in vivo. Somewhat unexpectedly, T3-deficient TRbeta-/- mice showed an augmented CYP7A response after challenge with dietary cholesterol, and these animals did not develop hypercholesterolemia to the extent as did wild-type (wt) controls. The latter results lend strong support to the concept that TRs may exert regulatory effects in vivo independent of T3.

Hepatic cholesterol metabolism and resistance to dietary cholesterol in LXRbeta-deficient mice

The nuclear oxysterol-receptor paralogues LXRalpha and LXRbeta share a high degree of amino acid identity and bind endogenous oxysterol ligands with similar affinities. While LXRalpha has been established as an important regulator of cholesterol catabolism in cholesterol-fed mice, little is known about the function of LXRbeta in vivo. We have generated mouse lines with targeted disruptions of each of these LXR receptors and have compared their responses to dietary cholesterol. Serum and hepatic cholesterol levels and lipoprotein profiles of cholesterol-fed animals revealed no significant differences between LXRbeta(-/-) and wild-type mice. Steady-state mRNA levels of 3-hydroxy-3-methylglutaryl coenzyme A reductase, farnesyl diphosphate synthase, and squalene synthase were increased in LXRbeta(-/-) mice compared with LXRbeta(+/+) mice, when fed standard chow. The mRNA levels for cholesterol 7alpha-hydroxylase, oxysterol 7alpha-hydroxylase, sterol 12alpha-hydroxylase, and sterol 27-hydroxylase, respectively, were comparable in these strains, both on standard and 2% cholesterol chow. Our results indicate that LXRbeta(-/-) mice - in contrast to LXRalpha(-/-) mice - maintain their resistance to dietary cholesterol, despite subtle effects on the expression of genes coding for enzymes involved in lipid metabolism. Thus, our data indicate that LXRbeta has no complete overlapping function compared with LXRalpha in the liver.

Obesity and disturbed lipoprotein profile in estrogen receptor-alpha-deficient male mice

Clinical case reports have documented disturbances of carbohydrate and lipid metabolism in aromatase deficient and estrogen resistant males. The aim of the present study was to explore the metabolic functions of estrogens in male mice and to dissect the estrogen receptor (ER) specificity of such effects. Total body fat content and serum levels of leptin were followed in ERalpha knockout (ERKO), ERbeta knockout (BERKO), and ERalpha/beta double knockout (DERKO) mice. Neither the total body fat nor serum leptin levels were altered in any group before or during sexual maturation. However, after sexual maturation ERKO and DERKO, but not BERKO, demonstrated a clear increase in total body fat and enhanced serum leptin levels. Serum cholesterol was increased and a qualitative change in the lipoprotein profile, including smaller LDL particles, was observed in ERKO and DERKO mice. In conclusion, ERalpha but not ERbeta-inactivated male mice develop obesity after sexual maturation.

Thyroid hormone receptor beta-deficient mice show complete loss of the normal cholesterol 7alpha-hydroxylase (CYP7A) response to thyroid hormone but display enhanced resistance to dietary cholesterol

Thyroid hormone (T3) influences hepatic cholesterol metabolism, and previous studies have established an important role of this hormone in the regulation of cholesterol 7alpha-hydroxylase (CYP7A), the rate-limiting enzyme in the synthesis of bile acids. To evaluate the respective contribution of thyroid hormone receptors (TR) alpha1 and beta in this regulation, the responses to 2% dietary cholesterol and T3 were studied in TRalpha1 and TRbeta knockout mice under hypo- and hyperthyroid conditions. Our experiments show that the normal stimulation in CYP7A activity and mRNA level by T3 is lost in TRbeta-/- but not in TRalpha1-/-mice, identifying TRbeta as the mediator of T3 action on CYP7A and, consequently, as a major regulator of cholesterol metabolism in vivo. Somewhat unexpectedly, T3-deficient TRbeta-/- mice showed an augmented CYP7A response after challenge with dietary cholesterol, and these animals did not develop hypercholesterolemia to the extent as did wild-type (wt) controls. The latter results lend strong support to the concept that TRs may exert regulatory effects in vivo independent of T3.

LDL-apheresis in patients with nephrotic syndrome: effects on serum albumin and urinary albumin excretion

BACKGROUND

Hyperlipidemia is a common feature of the nephrotic syndrome (NS). From retrospective studies, it has been suggested that aggressive lipid-lowering with low-density lipoprotein apheresis (LDL-A) may not only improve dyslipidemia but also decrease urinary albumin excretion and increase serum levels of albumin in patients with focal segmental sclerosis.

METHODS

Seven patients (6 males) aged 44 +/-7 years (SEM) with NS (duration 29+/-11 months) of diverse etiologies were investigated in a prospective study. A fixed protocol of LDL-A was designed for treatment twice-a-week for 3 weeks and then once a week for 7 weeks. The effects of LDL-A on lipid parameters (cholesterol, triglycerides, HDL, Lp(a), apo A-I, apo B) and renal parameters (iohexol clearance, serum albumin and 24-h urinary albumin excretion) were evaluated.

RESULTS

Following treatment by LDL-A a remission in the severity of the NS was observed in two patients whereas a clear improvement was observed in four of the patients. A small, but significant (P<0.05), increase in serum albumin levels from 20+/-2 to 24+/-2 g L(-1) was noted after LDL-A. As expected, serum lipid parameters improved during LDL-A, and significant decreases in serum cholesterol, apo B and plasma Lp(a) were observed at different time-points of LDL-A. Conversely, no significant changes in either triglyceride, HDL or apo A-I levels were observed during LDL-A.

CONCLUSIONS

The present uncontrolled prospective study shows that LDL-A causes a rapid 30-40% decrease in serum cholesterol and plasma Lp(a) levels in patients with NS. The present prospective study also suggests that short-term LDL-A treatment may increase serum albumin levels in nephrotic patients.

Biphasic effects of the natural estrogen 17beta-estradiol on hepatic cholesterol metabolism in intact female rats

The protective influence of estrogens in cardiovascular disease is believed to be partly due to beneficial effects on cholesterol metabolism. Much of the experimental data are based on models in which synthetic estrogens have been used in pharmacological doses, and therefore, the physiological role of estrogens in cholesterol metabolism is uncertain. To evaluate this important issue, we performed experiments in intact female rats with use of the natural estrogen 17beta-estradiol (E2) administered either subcutaneously or orally. After physiological doses of E2 (< or =0.04 mg. kg(-1). d(-1)) were administered, plasma levels of high density lipoprotein (HDL) cholesterol and apolipoprotein (apo) A-I were increased. In the liver, 3-hydroxy-3-methylglutaryl coenzyme A reductase and cholesterol 7alpha-hydroxylase activities were increased, as well as cholesterol 7alpha-hydroxylase mRNA levels. These effects were abolished during treatment with higher doses of E2, whereas apo A-I mRNA increased in a dose-dependent way. After treatment with pharmacological doses of E2 (> or =0.2 mg. kg(-1). d(-1)), the number of hepatic low density lipoprotein receptors increased and plasma cholesterol was reduced. These effects were similar after both oral and subcutaneous administration of E2. Our results show that the responses to E2 are biphasic: plasma HDL, apo A-I, and hepatic enzyme activities governing bile acid and cholesterol synthesis increased only at physiological doses of E2. At pharmacological doses of E2, hepatic low density lipoprotein receptors are stimulated and plasma cholesterol is reduced. Therefore, under physiological conditions, E2 exerts its major effects on hepatic cholesterol metabolism through mechanisms other than stimulation of low density lipoprotein receptor expression.