Circulating intestinal fibroblast growth factor 19 has a pronounced diurnal variation and modulates hepatic bile acid synthesis in man

Bile acids (BAs) traversing the enterohepatic circulation exert several important metabolic effects. Their hepatic synthesis, controlled by the enzyme cholesterol 7alpha-hydroxylase (CYP7A1), has a unique diurnal variation in man. Here we provide evidence that the transintestinal flux of BAs regulates serum levels of intestinal fibroblast growth factor 19 (FGF19) that in turn modulate BA production in human liver. Basal FGF19 levels varied by 10-fold in normal subjects, and were reduced following treatment with a BA-binding resin and increased upon feeding the BA chenodeoxycholic acid. Serum FGF19 levels exhibited a pronounced diurnal rhythm with peaks occurring 90-120 min after the postprandial rise in serum BAs. The FGF19 peaks in turn preceded the declining phase of BA synthesis. The diurnal rhythm of serum FGF19 was abolished upon fasting. We conclude that, in humans, circulating FGF19 has a diurnal rhythm controlled by the transintestinal BA flux, and that FGF19 modulates hepatic BA synthesis. Through its systemic effects, circulating FGF19 may also mediate other known BA-dependent effects on lipid and carbohydrate metabolism.

Lipoprotein profiles in plasma and interstitial fluid analyzed with an automated gel-filtration system

BACKGROUND

High-quality methods for lipoprotein characterization are warranted in studies on various metabolic diseases.

MATERIALS AND METHODS

An automated system for size-exclusion chromatography (SEC) of lipoproteins using commercially available components is described. Cholesterol or triglyceride content in separated lipoproteins from plasma and interstitial fluid (IF) was continuously determined on-line using microlitre sample volumes.

RESULTS

The lipoprotein assay showed a good concordance with the classic ultra-centrifugation/precipitation technique using fresh or frozen samples. Determination of lipoproteins in IF obtained from vacuum-induced skin blisters from 18 healthy subjects revealed that very low density lipoprotein (VLDL), low density lipoprotein (LDL) and high density lipoprotein (HDL) cholesterol levels were 18%, 19% and 25%, respectively, of concomitant plasma concentrations. The size-exclusion chromatography (SEC) system also allows for triglyceride determination on-line and it could be shown that the system is advantageous for an accurate determination of triglycerides in conditions when there are high levels of glycerol, e.g. in mice and in patients with hyperglycerolaemia (pseudo-hypertriglyceridaemia).

CONCLUSIONS

The described system should be of value in studies where detailed lipoprotein analysis is warranted and particularly when significant sample series with small volumes are available. Our data also suggest that there is a 4-5.5-fold concentration gradient between plasma and IF for the three major plasma lipoproteins.

Bile acid synthesis in humans has a rapid diurnal variation that is asynchronous with cholesterol synthesis

BACKGROUND & AIMS

The conversion of cholesterol to bile acids by the liver is an important regulator of body cholesterol homeostasis. In rodents, both cholesterol and bile acid synthesis have marked diurnal rhythms that peak synchronously at midnight. The aim of this study was to establish whether such diurnal rhythms are also present in healthy humans.

METHODS

Serum levels of the markers 7alpha-hydroxy-4-cholesten-3-one (C4) monitoring bile acid biosynthesis and lathosterol reflecting cholesterol synthesis were determined at 90-minute intervals in 8 human volunteers during standardized dietary conditions.

RESULTS

Serum C4 showed 2 distinct peaks (2- to 4-fold above baseline) during a 24-hour period, the first at 1:00 pm and the second at 9:00 pm. During the night, C4 levels declined, and they returned to baseline levels the next morning. In contrast, serum lathosterol levels peaked at night, between midnight and 4:00 am. The diurnal changes of C4 were not synchronous with serum lipid changes or with the postprandial increase in serum bile acids and were maintained in cholecystectomized subjects.

CONCLUSIONS

Bile acid synthesis in humans has a diurnal rhythm, with 2 peaks during the daytime, that is opposite from the circadian rhythm of cholesterol synthesis. This is completely different from the pattern in rodents and indicates the presence of an important species variation in the regulation of cholesterol homeostasis.

Pituitary control of cholesterol metabolism in normal and LDL receptor knock-out mice: Effects of hypophysectomy and growth hormone treatment

The pituitary is important in the control of lipid metabolism and studies of hypophysectomized (Hx) rats have shown strong effects of growth hormone (GH) on bile acid synthesis, hepatic LDL receptor (LDLR) expression and on the sensitivity to dietary cholesterol. It is unclear if mice may be used in such studies. The aim of the current study was to evaluate if Hx mice may be used to further explore how GH modulates cholesterol and bile acid metabolism, and to define the importance of the LDLR in this regulation by studying LDLR-deficient mice (LDLRko). Experiments on three mouse strains showed that, following Hx, HDL were reduced and LDL increased. Cholesterol/fat feeding of Hx mice increased serum cholesterol levels 2- to 3-fold. Serum triglycerides were reduced 50% in Hx mice; a further 30% reduction was seen after dietary cholesterol/fat. A serum marker for CYP7A1-mediated bile acid synthesis (C4) increased 2-fold in intact mice on cholesterol/fat diet. In Hx mice C4 levels were reduced by 50% as compared to intact controls, but were unexpectedly increased to levels seen in normal mice upon cholesterol/fat feeding. Hx of LDLRko mice moderately increased LDL-cholesterol and reduced triglycerides and GH treatment attenuated these effects; serum C4 levels were increased by GH treatment in all groups. In conclusion, mice can be used to explore the role of the pituitary in lipid metabolism. CYP7A1 is generally reduced in Hx mice but has a normal stimulatory response following dietary cholesterol suggesting that faulty regulation of CYP7A1 is not important for the reduced resistance to dietary cholesterol in Hx mice. Further, the LDLR is only to a minor part involved in the pituitary regulation of serum cholesterol in mice.

Telling the liver (not) to make bile acids: a new voice from the gut?

The maintenance of adequate amounts of bile acids in the liver, biliary tract, and intestine requires a finely tuned control of their synthesis. A paper in this issue of Cell Metabolism by Inagaki et al. (2005) indicates that sensing of the levels of bile acids in the intestine may trigger the secretion of a hormone which regulates bile acid production in the liver.

Common polymorphisms in the CYP7A1 gene do not contribute to variation in rates of bile acid synthesis and plasma LDL cholesterol concentration

Transcriptional regulation of the cholesterol 7alpha-hydroxylase (CYP7AI) gene is of critical importance for bile acid and cholesterol metabolism. We evaluated the physiological significance of two common polymorphisms (-203C/A and -469T/C) in the promoter region of the CYP7AI gene. No evidence was found for physiological differences between either the -203C and -203A alleles or the -469T and -469C alleles in transient transfection studies using native 834bp promoter constructs. Moreover, no association was observed between the CYP7AI promoter polymorphisms and the hepatic cholesterol 7alpha-hydroxylase activity and parameters of bile acid synthesis rates, as analyzed in subjects with gallstone disease. In addition, no relationships were found between the promoter polymorphisms and plasma LDL cholesterol concentration in association studies conducted in three different groups of middle-aged Swedish men. Finally, near complete allelic association was found between the two promoter polymorphisms and the IVS6+363G/A polymorphism at the 3' end of the CYP7AI gene (|D'|=0.98), indicating strong linkage disequilibrium across the whole CYP7AI gene. It is concluded that common polymorphisms of the CYP7A1 gene do not contribute to variation in cholesterol 7alpha-hydroxylase activity, rates of bile acid synthesis and plasma LDL cholesterol concentration in humans.

Selective thyroid receptor modulation by GC-1 reduces serum lipids and stimulates steps of reverse cholesterol transport in euthyroid mice

Thyroid hormones [predominantly 3,5,3'-triiodo-L-thyronine (T3)] regulate cholesterol and lipoprotein metabolism, but cardiac effects restrict their use as hypolipidemic drugs. T3 binds to thyroid hormone receptors (TRs) alpha and beta. TRbeta is the predominant isoform in liver, whereas T3 effects on heart rate are mediated mostly by TRalpha. Drugs that target TRbeta or exhibit tissue-selective uptake may improve plasma lipid levels while sparing the heart. Here, we asked how the TRbeta- and liver uptake-selective agonist GC-1 influences cholesterol and triglyceride metabolism in euthyroid mice. GC-1 treatment reduced serum cholesterol levels by 25% and serum triglycerides by 75% in chow-fed mice and also attenuated diet-induced hypercholesterolemia. GC-1 reduced plasma high-density lipoprotein cholesterol levels; increased expression of the hepatic high-density lipoprotein receptor, SR-BI; stimulated activity of cholesterol 7alpha-hydroxylase; and increased fecal excretion of bile acids. Collectively, these results suggest that GC-1 stimulates important steps in reverse cholesterol transport. Use of TRbeta and uptake selective agonists such as GC-1 should be further explored as a strategy to improve lipid metabolism in dyslipoproteinemia.

Effects of cholesteryl ester transfer protein inhibition on high-density lipoprotein subspecies, apolipoprotein A-I metabolism, and fecal sterol excretion

OBJECTIVE

Pharmacological inhibition of the cholesteryl ester transfer protein (CETP) in humans increases high-density lipoprotein (HDL) cholesterol (HDL-C) levels; however, its effects on apolipoprotein A-I (apoA-I) containing HDL subspecies, apoA-I turnover, and markers of reverse cholesterol transport are unknown. The present study was designed to address these issues.

METHODS AND RESULTS

Nineteen subjects, 9 of whom were taking 20 mg of atorvastatin for hypercholesterolemia, received placebo for 4 weeks, followed by the CETP inhibitor torcetrapib (120 mg QD) for 4 weeks. In 6 subjects from the nonatorvastatin cohort, the everyday regimen was followed by a 4-week period of torcetrapib (120 mg BID). At the end of each phase, subjects underwent a primed-constant infusion of (5,5,5-2H3)-L-leucine to determine the kinetics of HDL apoA-I. The lipid data in this study have been reported previously. Relative to placebo, 120 mg daily torcetrapib increased the amount of apoA-I in alpha1-migrating HDL in the atorvastatin (136%; P<0.001) and nonatorvastatin (153%; P<0.01) cohorts, whereas an increase of 382% (P<0.01) was observed in the 120 mg twice daily group. HDL apoA-I pool size increased by 8+/-15% in the atorvastatin cohort (P=0.16) and by 16+/-7% (P<0.0001) and 34+/-8% (P<0.0001) in the nonatorvastatin 120 mg QD and BID cohorts, respectively. These changes were attributable to reductions in HDL apoA-I fractional catabolic rate (FCR), with torcetrapib reducing HDL apoA-I FCR by 7% (P=0.10) in the atorvastatin cohort, by 8% (P<0.001) in the nonatorvastatin 120 mg QD cohort, and by 21% (P<0.01) in the nonatorvastatin 120 mg BID cohort. Torcetrapib did not affect HDL apoA-I production rate. In addition, torcetrapib did not significantly change serum markers of cholesterol or bile acid synthesis or fecal sterol excretion.

CONCLUSIONS

These data indicate that partial inhibition of CETP via torcetrapib in patients with low HDL-C: (1) normalizes apoA-I levels within alpha1-migrating HDL, (2) increases plasma concentrations of HDL apoA-I by delaying apoA-I catabolism, and (3) does not significantly influence fecal sterol excretion.

Prenatal exposure to interleukin-6 results in hypertension and increased hypothalamic-pituitary-adrenal axis activity in adult rats

During pregnancy, systemic inflammatory responses induce cytokines that may stress the fetus and contribute to cardiovascular and neuroendocrine dysfunction in adulthood. We evaluated the effects of early and late prenatal exposure to IL-6 on mean systolic arterial pressure (MSAP) and hypothalamic-pituitary-adrenal (HPA) axis regulation in male and female rats at 5-24 wk of age. MSAP and ACTH and corticosterone levels were measured basally and in response to a novel environment, immobilization stress, and stimulation with corticotropin-releasing factor (CRF) and ACTH. In addition, mRNA expression and protein levels of glucocorticoid receptor, mineralocorticoid receptor, CRF receptor type 1, and CRF were estimated in brain areas thought to mediate central effects of corticosteroids on the HPA axis and on central neuroendocrine regulation of MSAP. Both early and late prenatal IL-6 exposure led to hypertension, which was evident in females at 5 wk of age. In adult rats, basal ACTH and corticosterone levels were elevated, the responses to stress and stimulation tests were of extended duration, and circadian rhythm during the light period was flattened and reversed. Mineralocorticoid receptor and glucocorticoid receptor mRNA expression was reduced in the hippocampus, the CRF level was increased in the hypothalamus, and CRF receptor type 1 mRNA expression was increased in the pituitary. These findings suggest that fetal stress induced by prenatal exposure to IL-6 leads to hypertension and dysregulation of HPA axis activity during adulthood.

Autosomal recessive hypercholesterolaemia: normalization of plasma LDL cholesterol by ezetimibe in combination with statin treatment

BACKGROUND

Severe hereditary hypercholesterolaemia is most frequently due to familial hypercholesterolaemia (FH), caused by mutations in the LDL receptor (LDLR) gene. However, a phenotype very similar to FH may also be caused by defects in other genes like the genes for apolipoprotein (apo) B-100 or autosomal recessive hypercholesterolaemia (ARH).

SUBJECT

An 8-year-old male of Lebanese origin was diagnosed with severe hypercholesterolaemia and extensive cutaneous and tendon xanthomas. Plasma LDL cholesterol before treatment was 17 mmol L(-1), whilst parents and both siblings had normal levels.

DIAGNOSIS

Degradation of (125)I-labelled LDL in blood lymphocytes was reduced, but not abolished. Sequencing analysis of the LDLR and apoB-100 genes were negative, whilst a splice acceptor mutation in intron 1 (IVS 1 -1G>C) was detected in the ARH gene. The patient was homozygous for the mutation, whilst the parents were heterozygous. These findings were in agreement with a diagnosis of ARH.

TREATMENT AND CLINICAL COURSE

Monthly LDL apheresis and atorvastatin 120 mg daily reduced LDL cholesterol preapheresis level to 4.8 mmol L(-1). When ezetimibe was given 10 mg day(-1) in combination with rosuvastatin 80 mg day(-1), LDL cholesterol was further lowered to 1.6 mmol L(-1), which made apheresis unnecessary. Cutaneous and tendon xanthomas disappeared completely and the intima-media thickness of the common carotid arteries decreased. At age 23 he developed a small myocardial infarction.

CONCLUSION

ARH should be considered in cases of severe hypercholesterolaemia with a pattern of recessive inheritance. Combination therapy with high-dose statin and ezetimibe seems to be the treatment of choice in ARH and may reduce or eliminate the need for LDL apheresis treatment.

ACAT2 is localized to hepatocytes and is the major cholesterol-esterifying enzyme in human liver

BACKGROUND

Two acyl-coenzyme A:cholesterol acyltransferase (ACAT) genes, ACAT1 and ACAT2, have been identified that encode 2 proteins responsible for intracellular cholesterol esterification.

METHODS AND RESULTS

In this study, immunohistology was used to establish their cellular localization in human liver biopsies. ACAT2 protein expression was confined to hepatocytes, whereas ACAT1 protein was found in Kupffer cells only. Studies with a highly specific ACAT2 inhibitor, pyripyropene A, in microsomal activity assays demonstrated that ACAT2 activity was highly variable among individual human liver samples, whereas ACAT1 activity was more similar in all specimens. ACAT2 provided the major cholesterol-esterifying activity in 3 of 4 human liver samples examined.

CONCLUSIONS

The data suggest that in diseases in which dysregulation of cholesterol metabolism occurs, such as hypercholesterolemia and atherosclerosis, ACAT2 should be considered a target for prevention and treatment.

Pituitary control of lipoprotein and bile acid metabolism in male rats: growth hormone effects are not mediated by prolactin

Previous studies have established that growth hormone (GH) has many important effects on the regulation of cholesterol and lipoprotein metabolism. However, human GH (hGH) can also bind to prolactin receptors, eliciting prolactin receptor-mediated effects. In this study, we evaluated whether hGH can exert such responses in currently used animal models and whether prolactin affects lipoprotein and/or hepatic cholesterol metabolism. Normal and hypophysectomized (Hx) male rats were given either hGH or bovine GH, the latter unable to bind to the prolactin receptor. The hormones were continuously infused by use of subcutaneous osmotic mini-pumps for 7 days; blood and livers were collected after euthanasia. Both hormones stimulated hepatic LDL receptor expression and bile acid synthesis to a similar extent and normalized the altered plasma lipoprotein pattern in Hx rats. Prolactin, injected twice daily to Hx male rats, did not exert any effects on the plasma lipoprotein pattern or on cholesterol metabolism. We conclude that previously established effects of hGH on cholesterol metabolism are not mediated by prolactin in male rats.

Angiogenesis inhibitor, TNP-470, prevents diet-induced and genetic obesity in mice

Adipose tissue growth has been proposed to involve recruitment of new blood vessels. Here, we test the hypothesis that delivery of an angiogenesis inhibitor in mice may prevent diet-induced obesity, the most common type of obesity in humans. We show that systemic administration of a selective angiogenesis inhibitor, TNP-470 (AGM-1470), prevents obesity in high caloric diet-fed wt mice as well as in genetically leptin-deficient ob/ob mice. Inhibition of obesity in mice by TNP-470 involves a reduction of vascularity in the adipose tissue. This therapeutic strategy appears to selectively affect the growth of adipose tissue as measured by the ratio between total fat and lean body mass. Interestingly, the treatment with TNP-470 results in decreased serum levels of low-density lipoprotein cholesterol. Furthermore, insulin levels are reduced, which indicates increased insulin sensitivity, suggesting that angiogenesis inhibitors may prevent the development of type II diabetes. Our findings suggest that similarly to growth and organogenesis in other tissues, adipose tissue growth is dependent on angiogenesis. Our observations may have conceptual implications for the prevention of obesity and related disorders.

Growth Hormone Induces Low-Density Lipoprotein Clearance but not Bile Acid Synthesis in Humans

OBJECTIVE

Growth hormone (GH) induces hepatic low-density lipoprotein (LDL) receptors and lowers plasma cholesterol. We characterized the influence of GH treatment on plasma LDL clearance in normal humans and investigated the relative role of LDL receptor (LDLR) activity and stimulation of bile acid synthesis in subjects with different LDLR expression.

METHODS AND RESULTS

Plasma clearance of autologous 125I-LDL was measured before and during 3 weeks of treatment with GH (0.1 IU/kg per day) in 9 healthy young males. Plasma LDL cholesterol was reduced by 13% and the fractional catabolic rate of LDL increased by 27%. More marked changes were seen in a patient with hypopituitarism substituted with GH (0.07 IU/kg per day) for 3 months. In a second study, GH dose-dependently reduced LDL cholesterol and increased Lp(a) levels in 3 groups of males: younger and elderly healthy subjects and heterozygous familial hypercholesterolemia (FH). No effect on bile acid synthesis measured by the plasma marker 7alpha-hydroxy-4-cholesten-3-one was observed. In an LDLR-deficient FH homozygote, LDL cholesterol was not affected by GH.

CONCLUSIONS

GH treatment reduces plasma LDL cholesterol by inducing LDL clearance. In humans, LDLR expression is a prerequisite for this effect, whereas it is not related to stimulation of bile acid synthesis.

Leptin induces the hepatic high density lipoprotein receptor scavenger receptor B type I (SR-BI) but not cholesterol 7alpha-hydroxylase (Cyp7a1) in leptin-deficient (ob/ob) mice

Cholesterol elimination from the body involves reverse cholesterol transport from peripheral tissues in which the elimination of high density lipoprotein (HDL) and low density lipoprotein (LDL) cholesterol by the liver and subsequent biliary excretion as free cholesterol and bile acids are important. In situations of peripheral fat and cholesterol accumulation, such as obesity, these pathways may be overloaded, contributing to increased cholesterol deposition. Leptin has an important role in obesity, suppressing food intake and increasing energy expenditure. This hormone, which is absent in genetically obese ob/ob mice, is also thought to be involved in the coordination of lipid excretion pathways, although available data are somewhat inconsistent. We therefore studied the expression of the hepatic HDL receptor, scavenger receptor class B type I (SR-BI), and the LDL receptor as well as the rate-limiting enzyme in bile acid synthesis, cholesterol 7alpha-hydroxylase (Cyp7a1), in leptin-deficient ob/ob mice and their wild-type controls. In ob/ob mice, protein levels of both LDL receptor and SR-BI were reduced, whereas LDL receptor mRNA levels were increased and those of SR-BI were reduced, regardless of challenge with a 2% cholesterol diet. In ob/ob mice, the enzymatic activity and mRNA for Cyp7a1 were reduced, and the increase in response to dietary cholesterol was blunted. Upon short-term (2 days) treatment with leptin, a dose-dependent increase was seen in the SR-BI protein and mRNA, whereas the Cyp7a1 protein and mRNA were reduced. Our findings indicate that leptin is an important regulator of hepatic SR-BI expression and, thus, HDL cholesterol levels, whereas it does not stimulate Cyp7a1 and bile acid synthesis.

Mutations in a Sar1 GTPase of COPII vesicles are associated with lipid absorption disorders

Dietary fat is an important source of nutrition. Here we identify eight mutations in SARA2 that are associated with three severe disorders of fat malabsorption. The Sar1 family of proteins initiates the intracellular transport of proteins in COPII (coat protein)-coated vesicles. Our data suggest that chylomicrons, which vastly exceed the size of typical COPII vesicles, are selectively recruited by the COPII machinery for transport through the secretory pathways of the cell.

Monitoring hepatic cholesterol 7alpha-hydroxylase activity by assay of the stable bile acid intermediate 7alpha-hydroxy-4-cholesten-3-one in peripheral blood

We describe an accurate method for monitoring the enzymatic activity of hepatic cholesterol 7alpha-hydroxylase (C7alphaOH; CYP7A1), the rate-limiting and major regulatory enzyme in the synthesis of bile acids. Assay of 7alpha-hydroxy-4-cholesten-3-one (C4), an intermediate in bile acid synthesis, revealed that the level of C4 in peripheral blood serum or plasma showed a strong correlation to the enzymatic activity of hepatic C7alphaOH, both at steady-state conditions (r = 0.929) as well as during the rapid changes that occur during the diurnal phases. This assay should be of value in clarifying the regulation of bile acid synthesis in vivo in laboratory animals and humans since it allows for the monitoring of hepatic C7alphaOH activity using peripheral blood samples.

Pharmacological interference with intestinal bile acid transport reduces plasma cholesterol in LDL receptor/apoE deficiency

Reduction of plasma cholesterol by statins is fundamental to prevent coronary heart disease. Such therapy is often sub-optimal, however, particularly in patients with reduced LDL receptors (familial hypercholesterolemia), and novel or adjuvant therapies are therefore warranted. Cholesterol elimination is profoundly influenced by the rate of its conversion to bile acids (BA), regulated by the enzyme Cyp7a1. Induced fecal loss of BA by resin treatment reduces plasma cholesterol, presumably through induction of hepatic LDL receptors (LDLR). We here describe the effect of PR835, a drug belonging to a new class of lipid-lowering agents that inhibit the Slc10a2 protein, the intestinal transporter responsible for active uptake of BA. Treatment reduced plasma cholesterol by 40% in mice devoid of both the LDLR and its ligand, apoE, while triglycerides and HDL cholesterol were unchanged. Cyp7a1 enzyme activity and mRNA were induced several-fold, and hepatic HMG CoA reductase mRNA increased, mirroring an induced synthesis of BA and cholesterol. The addition of a statin potentiated the effect, leading to reductions of plasma total and LDL cholesterol by 64% and 70%, respectively. These effects could not be attributed to induction of other known hepatic lipoprotein receptors and indicate the presence of new points of targeting in lipid-lowering therapy.