Moderately decreased cholesterol absorption rates are associated with a large atheroprotective effect

Oxy- and Phytosterols as Biomarkers: Current Status and Future Perspectives

Oxysterols and phytosterols are sterol compounds present at markedly low levels in tissues and serum of healthy individuals. A wealth of evidence suggests that they could be employed as biomarkers for human diseases or for cholesterol absorption.An increasing number of reports suggest circulating or tissue oxysterols as putative biomarkers for cardiovascular and neurodegenerative diseases or cancers. Thus far most of the studies have been carried out on small study populations. To achieve routine biomarker use, large prospective cohort studies are absolutely required. This, again, would necessitate thorough standardization of the oxysterol analytical methodology across the different laboratories, which now employ different technologies resulting in inconsistencies in the measured oxysterol levels. Routine use of oxysterol biomarkers would also necessitate the development of a new targeted analytical methodology suitable for high-throughput platforms.The most important use of phytosterols as biomarkers involves their use as markers for cholesterol absorption. For this to be achieved, (1) their quantitative analyses should be available in routine lipid laboratories, (2) it should be generally acknowledgment that the profile of cholesterol metabolism can reveal the risk of the development of atherosclerotic cardiovascular diseases (ASCVD), and (3) screening of the profile of cholesterol metabolism should be included in the ASCVD risk surveys. This should be done e.g. in families with a history of early onset or frequent ASCVD and in young adults aged 18-20 years, to exclude the presence of high cholesterol absorption. Individuals in high cholesterol absorption families need preventive measures from young adulthood to inhibit the possible development and progression of atherosclerosis.

Lycopene Reduces Cholesterol Absorption and Prevents Atherosclerosis in ApoE-/- Mice by Downregulating HNF-1α and NPC1L1 Expression

Our previous study showed that lycopene reduced the absorption of cholesterol in Caco-2 cells through inhibiting Niemann-Pick C1-Like 1 (NPC1L1) expression. Herein, we aimed to explore whether lycopene supplementation can decrease cholesterol absorption in the intestine and prevent atherosclerosis progression in high-fat diet (HFD)-fed apolipoprotein E knockout (ApoE^-/-) mice. Male ApoE^-/- mice were fed a high-fat diet with or without lycopene for 19 weeks. Supplementation of lycopene markedly lowered serum total cholesterol and low-density lipoprotein cholesterol (LDL-C) levels. Additionally, serum high-density lipoprotein cholesterol (HDL-C) levels were increased after lycopene administration. Lycopene also downregulated the expression of NPC1L1 and hepatocyte nuclear factor-1α (HNF-1α) in the small intestine. Furthermore, the Oil Red O staining of the aorta and aortic sinus showed that lycopene supplementation remarkably reduced atherosclerotic lesions. These results indicated that lycopene inhibited intestinal cholesterol absorption and protected against HFD-induced atherosclerosis through inhibiting HNF-1α and NPC1L1 expression. Lycopene exhibits a potential antiatherosclerotic effect through suppressing intestinal cholesterol absorption.

Supplementation with curcumin inhibits intestinal cholesterol absorption and prevents atherosclerosis in high-fat diet-fed apolipoprotein E knockout mice

Atherosclerosis is a major cause of cardiovascular disease caused by high cholesterol. Reduced intestinal cholesterol absorption has been shown to exert strong cholesterol-lowering and antiatherogenic effects. Previously, we reported that curcumin reduced cholesterol absorption in high-fat diet-fed hamster by downregulating the intestinal expression of Niemann-Pick C1-like 1. Here, we tested the hypothesis that supplementation with curcumin can also reduce intestinal cholesterol absorption in high-fat diet-fed apolipoprotein E knockout (ApoE^-/-) mice and prevent atherosclerosis development. ApoE^-/- mice were fed a high-fat diet supplemented with or without curcumin (0.1% w/w) for 16 weeks. Aortic sinus sections revealed that curcumin supplementation reduced the extent of atherosclerotic lesions by 45%. Curcumin treatment also reduced cholesterol accumulation in the aortas by 56% and lowered plasma total cholesterol and low-density lipoprotein cholesterol levels. Moreover, the antiatherogenic and cholesterol-lowering effects of curcumin coincided with a significant decrease in intestinal cholesterol absorption. It was reduced by nearly 51%, and the decreased cholesterol absorption was modulated by inhibiting the intestinal expression of Niemann-Pick C1-like 1, predominantly in the duodenal and jejunal segments of the small intestine. These findings support the hypothesis that curcumin supplementation reduces intestinal cholesterol absorption and prevents atherosclerosis in high-fat diet-fed ApoE^-/- mice. Curcumin affords a potent antiatherogenic action by inhibiting intestinal cholesterol absorption in the mouse.

Liver-specific induction of Abcg5 and Abcg8 stimulates reverse cholesterol transport in response to ezetimibe treatment

OBJECTIVE

Previous studies have shown ezetimibe treatment results in a 2-6-fold increase in reverse cholesterol transport (RCT). However, recent sterol balance studies question the role of biliary sterol secretion in RCT, and challenge the hypothesis that ezetimibe increases RCT through decreased absorption of biliary cholesterol in the intestine. We set out to determine whether ezetimibe may increase RCT by mechanisms that are independent of its well-established inhibition of intestinal cholesterol absorption.

METHODS

C57BL/6J, Npc1l1-KO, and/or Abcg8-KO mice were fed a chow diet with or without ezetimibe and fecal [(14)C]-neutral and [(14)C]-acidic sterols were measured to examine macrophage-to-feces RCT. We measured the expression of RCT related genes in the liver and jejunum in these mice. To confirm our significant gene expression findings, we utilized primary human hepatocytes cultured with or without a glucuronated metabolite of ezetimibe.

RESULTS

Our studies revealed that treatment with ezetimibe was associated with increased expression of hepatic Abcg5 and Abcg8. Ezetimibe did not directly affect expression in the liver, but this expression was due to the inhibition of intestinal cholesterol absorption. This conclusion was supported by the absence of an ABCG5/ABCG8 expression response to treatment with an ezetimibe metabolite in primary human hepatocytes. Finally, we found that the ezetimibe-dependent stimulation of RCT was attenuated in the absence of Abcg8.

CONCLUSIONS

Our study is the first to demonstrate ezetimibe treatment cooperatively stimulated macrophage-to-feces RCT by indirectly increasing liver Abcg5/Abcg8 expression in addition to its known suppression of intestinal cholesterol absorption.

Lactobacillus acidophilus ATCC 4356 prevents atherosclerosis via inhibition of intestinal cholesterol absorption in apolipoprotein E-knockout mice

The objective of this study was to investigate the effect of Lactobacillus acidophilus ATCC 4356 on the development of atherosclerosis in apolipoprotein E-knockout (ApoE(-/-)) mice. Eight-week-old ApoE(-/-) mice were fed a Western diet with or without L. acidophilus ATCC 4356 daily for 16 weeks. L. acidophilus ATCC 4356 protected ApoE(-/-) mice from atherosclerosis by reducing their plasma cholesterol levels from 923 ± 44 to 581 ± 18 mg/dl, likely via a marked decrease in cholesterol absorption caused by modulation of Niemann-Pick C1-like 1 (NPC1L1). In addition, suppression of cholesterol absorption induced reverse cholesterol transport (RCT) in macrophages through the peroxisome proliferator-activated receptor/liver X receptor (PPAR/LXR) pathway. Fecal lactobacillus and bifidobacterium counts were significantly (P < 0.05) higher in the L. acidophilus ATCC 4356 treatment groups than in the control groups. Furthermore, L. acidophilus ATCC 4356 was detected in the rat small intestine, colon, and feces during the feeding trial. The bacterial levels remained high even after the administration of lactic acid bacteria had been stopped for 2 weeks. These results suggest that administration of L. acidophilus ATCC 4356 can protect against atherosclerosis through the inhibition of intestinal cholesterol absorption. Therefore, L. acidophilus ATCC 4356 may be a potential therapeutic material for preventing the progression of atherosclerosis.

Role of the apical and basolateral domains of the enterocyte in the regulation of cholesterol transport by a high glucose concentration

We have recently shown that a high glucose (HG) concentration raised intestinal cholesterol (CHOL) transport and metabolism in intestinal epithelial cells. The objective of the present work is to determine whether the stimulus for increased CHOL absorption by glucose originates from the apical site (corresponding to the intestinal lumen) or from the basolateral site (related to blood circulation). We tackled this issue by using differentiated Caco-2/15 cells. Only basolateral medium, supplemented with 25 mmol/L glucose, stimulated [(14)C]-CHOL uptake via the up-regulation of the critical CHOL transporter NPC1L1 protein, as confirmed by its specific ezetimibe inhibitor that abolished the rise in glucose-mediated CHOL capture. No significant changes were noted in SR-BI and CD36. Elevated CHOL uptake was associated with an increase in the transcription factors SREBP-2, LXR-β, and ChREBP, along with a fall in RXR-α. Interestingly, although the HG concentration in the apical medium caused modest changes in CHOL processing, its impact was synergetic with that of the basolateral medium. Our results suggest that HG concentration influences positively intestinal CHOL uptake when present in the basolateral medium. In addition, excessive consumption of diets containing high levels of carbohydrates may strengthen intestinal CHOL uptake in metabolic syndrome, thereby contributing to elevated levels of circulating CHOL and, consequently, the risk of developing type 2 diabetes and cardiovascular disease.

Assessing the functional properties of high-density lipoproteins: an emerging concept in cardiovascular research

Although plasma concentrations of high-density lipoprotein (HDL) cholesterol correlate inversely with the incidence of atherosclerotic cardiovascular disease, results from recent epidemiological, genetic and pharmacological intervention studies resulted in a shift of concept. Rather than HDL cholesterol mass levels, the functionality of HDL particles is increasingly regarded as potentially clinically important. This review provides an overview of four key functional properties of HDL, namely cholesterol efflux and reverse cholesterol transport; antioxidative activities; anti-inflammatory activities; and the ability of HDL to increase vascular nitric oxide production resulting in vasorelaxation. Currently available assays are put into context with different HDL isolation procedures yielding compositional heterogeneity of the particle. Gathered knowledge on the impact of different disease states on HDL function is discussed together with potential underlying causative factors modulating HDL functionalities. In addition, a perspective is provided regarding how a better understanding of the determinants of (dys)functional HDL might impact clinical practice and the future design of rational and specific therapeutic approaches targeting atherosclerotic cardiovascular disease.

The role of the gut in reverse cholesterol transport--focus on the enterocyte

In the arterial intima, macrophages become cholesterol-enriched foam cells and atherosclerotic lesions are generated. This atherogenic process can be attenuated, prevented, or even reversed by HDL particles capable of initiating a multistep pathway known as the macrophage-specific reverse cholesterol transport. The macrophage-derived cholesterol released to HDL is taken up by the liver, secreted into the bile, and ultimately excreted in the feces. Importantly, the absorptive epithelial cells lining the lumen of the small intestine, the enterocytes, express several membrane-associated proteins which mediate the influx of luminal cholesterol and its subsequent efflux at their apical and basolateral sides. Moreover, generation of intestinal HDL and systemic effects of the gut microbiota recently revealed a direct link between the gut and the cholesterol cargo of peripheral macrophages. This review summarizes experimental evidence establishing that the reverse cholesterol transport pathway which initiates in macrophages is susceptible to modulation in the small intestine. We also describe four paths which govern cholesterol passage across the enterocyte and define a role for the gut in the regulation of reverse cholesterol transport. Understanding the concerted function of these paths may be useful when designing therapeutic strategies aimed at removing cholesterol from the foam cells which occupy atherosclerotic lesions.

Atherosclerosis: lessons from LXR and the intestine

Modulation of the cholesterol-sensing liver X receptors (LXRs) and their downstream targets has emerged as promising therapeutic avenues in atherosclerosis. The intestine is important for its unique capabilities to act as a gatekeeper for cholesterol absorption and to participate in the process of cholesterol elimination in the feces and reverse cholesterol transport (RCT). Pharmacological and genetic intestine-specific LXR activation have been shown to protect against atherosclerosis. In this review we discuss the LXR-targeted molecular players in the enterocytes as well as the intestine-driven pathways contributing to cholesterol homeostasis with therapeutic potential as targets in the prevention and treatment of atherosclerosis..

Dietary sphingomyelin lowers hepatic lipid levels and inhibits intestinal cholesterol absorption in high-fat-fed mice

Controlling intestinal lipid absorption is an important strategy for maintaining lipid homeostasis. Accumulation of lipids in the liver is a major risk factor for metabolic syndrome and nonalcoholic fatty liver disease. It is well-known that sphingomyelin (SM) can inhibit intestinal cholesterol absorption. It is, however, unclear if dietary SM also lowers liver lipid levels. In the present study (i) the effect of pure dietary egg SM on hepatic lipid metabolism and intestinal cholesterol absorption was measured with [¹⁴C]cholesterol and [³H]sitostanol in male C57BL/6 mice fed a high-fat (HF) diet with or without 0.6% wt/wt SM for 18 days; and (ii) hepatic lipid levels and gene expression were determined in mice given a HF diet with or without egg SM (0.3, 0.6 or 1.2% wt/wt) for 4 weeks. Mice supplemented with SM (0.6% wt/wt) had significantly increased fecal lipid and cholesterol output and reduced hepatic [¹⁴C]cholesterol levels after 18 days. Relative to HF-fed mice, SM-supplemented HF-fed mice had significantly lower intestinal cholesterol absorption (−30%). Liver weight was significantly lower in the 1.2% wt/wt SM-supplemented mice (−18%). Total liver lipid (mg/organ) was significantly reduced in the SM-supplemented mice (−33% and −40% in 0.6% wt/wt and 1.2% wt/wt SM, respectively), as were triglyceride and cholesterol levels. The reduction in liver triglycerides was due to inactivation of the LXR-SREBP-1c pathway. In conclusion, dietary egg SM has pronounced hepatic lipid-lowering properties in mice maintained on an obesogenic diet.

LXRβ activation increases intestinal cholesterol absorption, leading to an atherogenic lipoprotein profile

OBJECTIVES

Liver X receptors (LXRs) are essential for the regulation of intestinal cholesterol absorption. Because two isoforms exist, LXRα and LXRβ, with overlapping but not identical functions, we investigated whether LXRα and LXRβ exert different effects on intestinal cholesterol absorption.

DESIGN

Wild-type (WT), LXRα(-/-) and LXRβ(-/-) mice were fed control diet, 0.2% cholesterol-enriched diet or 0.2% cholesterol-enriched diet plus the LXR agonist GW3965.

RESULTS

When fed a control diet, all three genotypes showed similar levels of cholesterol absorption. Of interest, a significant increase in cholesterol absorption was found in the LXRα(-/-) mice, but not in the WT or LXRβ(-/-) animals, when fed a diet enriched with 0.2% cholesterol or 0.2% cholesterol + GW3965. Reduced faecal neutral sterol excretion and a hydrophobic bile acid profile were also observed in LXRα(-/-) mice. Greater increases in the apolipoprotein (apo)B-containing lipoproteins in serum were seen in the LXRα(-/-) mice. A 0.2% cholesterol +GW3965 diet suppressed intestinal Npc1l1 protein expression to the same extent for all genotypes, while Abca1 and Abcg5 were elevated to the same degree.

CONCLUSIONS

In the intestine, LXRα and LXRβ seem to exert similar effects on expression of cholesterol-transporting proteins such as Npc1l1. Selective activation of LXRβ may generate effects such as increased cholesterol absorption and elevated serum levels of apoB-containing lipoproteins, which seem to be counteracted by LXRα. Therefore, an intestinal LXRβ-specific pathway might exist in terms of cholesterol transportation in addition to the main pathway.

Plant sterols and cardiovascular disease: a systematic review and meta-analysis

The impact of increased serum concentrations of plant sterols on cardiovascular risk is unclear. We conducted a systematic review and meta-analysis aimed to investigate whether there is an association between serum concentrations of two common plant sterols (sitosterol, campesterol) and cardiovascular disease (CVD). We systematically searched the databases MEDLINE, EMBASE, and COCHRANE for studies published between January 1950 and April 2010 that reported either risk ratios (RR) of CVD in relation to serum sterol concentrations (either absolute or expressed as ratios relative to total cholesterol) or serum sterol concentrations in CVD cases and controls separately. We conducted two meta-analyses, one based on RR of CVD contrasting the upper vs. the lower third of the sterol distribution, and another based on standardized mean differences between CVD cases and controls. Summary estimates were derived by fixed and random effects meta-analysis techniques. We identified 17 studies using different designs (four case–control, five nested case–control, three cohort, five cross-sectional) involving 11 182 participants. Eight studies reported RR of CVD and 15 studies reported serum concentrations in CVD cases and controls. Funnel plots showed evidence for publication bias indicating small unpublished studies with non-significant findings. Neither of our meta-analyses suggested any relationship between serum concentrations of sitosterol and campesterol (both absolute concentrations and ratios to cholesterol) and risk of CVD. Our systematic review and meta-analysis did not reveal any evidence of an association between serum concentrations of plant sterols and risk of CVD.

Regulation of reverse cholesterol transport - a comprehensive appraisal of available animal studies

Plasma levels of high density lipoprotein (HDL) cholesterol are strongly inversely correlated to the risk of atherosclerotic cardiovascular disease. A major recognized functional property of HDL particles is to elicit cholesterol efflux and consequently mediate reverse cholesterol transport (RCT). The recent introduction of a surrogate method aiming at determining specifically RCT from the macrophage compartment has facilitated research on the different components and pathways relevant for RCT. The current review provides a comprehensive overview of studies carried out on macrophage-specific RCT including a quick reference guide of available data. Knowledge and insights gained on the regulation of the RCT pathway are summarized. A discussion of methodological issues as well as of the respective relevance of specific pathways for RCT is also included.

Therapies targeting exogenous cholesterol uptake: new insights and controversies

Exogenous cholesterol uptake involves a complex process in the intestines for the absorption of cholesterol and bile acids. This process is regulated by intestinal nuclear transcription factors such as LXR that affect sterol transporters NPC1L1, ABCG5/G8, and ABCG1, and enzymes such as ACAT-2. Plant sterol/stanols, ezetimibe, and bile acid sequestrants have a variety of effects on these various transporters, and new insights into their mechanism(s) of action have provided a plethora of exciting targets for metabolic diseases, dyslipidemia, and atherosclerosis.

Intestinal specific LXR activation stimulates reverse cholesterol transport and protects from atherosclerosis

Several steps of the HDL-mediated reverse cholesterol transport (RCT) are transcriptionally regulated by the nuclear receptors LXRs in the macrophages, liver, and intestine. Systemic LXR activation via synthetic ligands induces RCT but also causes increased hepatic fatty acid synthesis and steatosis, limiting the potential therapeutic use of LXR agonists. During the last few years, the participation of the intestine in the control of RCT has appeared more evident. Here we show that while hepatic-specific LXR activation does not contribute to RCT, intestinal-specific LXR activation leads to decreased intestinal cholesterol absorption, improved lipoprotein profile, and increased RCT in vivo in the absence of hepatic steatosis. These events protect against atherosclerosis in the background of the LDLR-deficient mice. Our study fully characterizes the molecular and metabolic scenario that elects the intestine as a key player in the LXR-driven protective environment against cardiovascular disease.

Abolished synthesis of cholic acid reduces atherosclerotic development in apolipoprotein E knockout mice

To investigate the effects of abolished cholic acid (CA) synthesis in the ApoE knockout model [apolipoprotein E (apoE) KO],a double-knockout (DKO) mouse model was created by crossbreeding Cyp8b1 knockout mice (Cyp8b1 KO), unable to synthesize the primary bile acid CA, with apoE KO mice. After 5 months of cholesterol feeding, the development of atherosclerotic plaques in the proximal aorta was 50% less in the DKO mice compared with the apoE KO mice. This effect was associated with reduced intestinal cholesterol absorption, decreased levels of apoB-containing lipoproteins in the plasma, enhanced bile acid synthesis, reduced hepatic cholesteryl esters, and decreased hepatic activity of ACAT2. The upregulation of Cyp7a1 in DKO mice seemed primarily caused by reduced expression of the intestinal peptide FGF15. Treatment of DKO mice with the farnesoid X receptor (FXR) agonist GW4064 did not alter the intestinal cholesterol absorption, suggesting that the action of CA in this process is confined mainly to formation of intraluminal micelles and less to its ability to activate the nuclear receptor FXR. Inhibition of CA synthesis may offer a therapeutic strategy for the treatment of hyperlipidemic conditions that lead to atherosclerosis.

The associations of cholesterol metabolism and plasma plant sterols with all-cause and cardiovascular mortality

Moderately elevated levels of plasma plant sterols have been suspected to be causally involved in atherosclerosis. The aim of this study was to investigate whether plant sterols and other markers of sterol metabolism predicted all-cause and cardiovascular mortality in participants of the Ludwigshafen Risk and Cardiovascular health (LURIC) study. A total of 1,257 individuals who did not use statins and at baseline had a mean (+/- SD) age of 62.8 (+/- 11.0) years were included in the present analysis. Lathosterol, cholestanol, campesterol, and sitosterol were measured to estimate cholesterol synthesis and absorption. The mean (+/- SD) time of the follow-up for all-cause and cardiovascular mortality was 7.32 (+/- 2.3) years. All-cause (P = 0.001) and cardiovascular (P = 0.006) mortality were decreased in the highest versus the lowest lathosterol to cholesterol tertile. In contrast, subjects in the third cholestanol to cholesterol tertile had increased all-cause (P < 0.001) and cardiovascular mortality (P = 0.010) compared with individuals in the first tertile. The third campesterol to cholesterol tertile was associated with increased all-cause mortality (P = 0.025). Sitosterol to cholesterol tertiles were not significantly related to all-cause or cardiovascular mortality. The data suggest that high absorption and low synthesis of cholesterol predict increased all-cause and cardiovascular mortality in LURIC participants.

Pleiotropic effects of ezetimibe: do they really exist?

Ezetimibe represents a new lipid lowering agent which inhibits cholesterol absorption. It effectively reduces low-density lipoprotein cholesterol when administered either alone or in combination with statins. However, its effect on cardiovascular mortality remains under question since it failed to demonstrate any significant changes in the primary endpoints of the recently published ENHANCE and SEAS studies. A possible explanation for this unsuccessful outcome is that ezetimibe lacks pleiotropic effects. This article aims to review the potential pleiotropic effects of the drug mainly on inflammation markers, lipoprotein subfractions and endothelial function.