High density lipoprotein metabolism in low density lipoprotein receptor-deficient mice

Reverse Cholesterol Transport Dysfunction Is a Feature of Familial Hypercholesterolemia

PURPOSE OF REVIEW

We seek to establish whether high-density lipoprotein HDL metabolism and reverse cholesterol transport (RCT) impairment is an intrinsic feature of familial hypercholesterolemia (FH).

RECENT FINDINGS

RCT from macrophages (m-RCT), a vascular cell type of major influence on atherosclerosis, is impaired in FH due to defective low-density lipoprotein receptor (LDLR) function via both the HDL- and LDL-mediated pathways. Potential mechanisms include impaired HDL metabolism, which is linked to increased LDL levels, as well as the increased transport of cellular unesterified cholesterol to LDL, which presents a defective catabolism. RCT dysfunction is consistently associated with mutation-positive FH linked to decreased HDL levels as well as impaired HDL remodeling and LDLR function. It remains to be explored whether these alterations are also present in less well-characterized forms of FH, such as cases with no identified mutations, and whether they are fully corrected by current standard treatments.

The c.*52 A/G and c.*773 A/G Genetic Variants in the UTR'3 of the LDLR Gene Are Associated with the Risk of Acute Coronary Syndrome and Lower Plasma HDL-Cholesterol Concentration

Dyslipidemia has a substantial role in the development of acute coronary syndrome (ACS). Low-density lipoprotein receptor (LDLR) plays a critical role in plasma lipoprotein hemostasis, which is involved in the formation of atherosclerotic plaque. This study aimed to evaluate whether LDLR gene polymorphisms are significantly associated with ACS and the plasma lipids profile. Three LDLR gene polymorphisms located in the UTR′3 region (c.*52 A/G, c.*504 A/G, and c.* 773 A/G) were determined using TaqMan genotyping assays in a group of 618 ACS patients and 666 healthy controls. Plasma lipids profile concentrations were determined by enzymatic/colorimetric assays. Under co-dominant and recessive models, the c.*52 A allele of the c.*52 A/G polymorphism was associated with a higher risk of ACS (OR = 2.02, pC_Co-dom = 0.033, and OR = 2.00, pC_Res = 0.009, respectively). In the same way, under co-dominant and recessive models, the c.*773 G allele of the c.*773 A/G polymorphism was associated with a high risk of ACS (OR = 2.04, pC_Co-dom = 0.027, and OR = 2.01, pC_Res = 0.007, respectively). The “AAG” haplotype was associated with a high risk of ACS (OR = 1.22, pC = 0.016). The c.*52 AA genotype showed a lower HDL-C concentration than individuals with the GG genotype. In addition, carriers of c.*773 GG genotype carriers had a lower concentration of the high-density lipoprotein-cholesterol (HDL-C) than subjects with the AA genotype. Our data suggest the association of the LDLRc.*773 A/G and LDLR c.*52 A/G polymorphisms with both the risk of developing ACS and with a lower concentration of HDL-C in the study population.

NR4A1 Deletion in Marginal Zone B Cells Exacerbates Atherosclerosis in Mice-Brief Report

Supplemental Digital Content is available in the text.

NR4A orphan receptors have been well studied in vascular and myeloid cells where they play important roles in the regulation of inflammation in atherosclerosis. NR4A1 (nerve growth factor IB) is among the most highly induced transcription factors in B cells following BCR (B-cell receptor) stimulation. Given that B cells substantially contribute to the development of atherosclerosis, we examined whether NR4A1 regulates B-cell function during atherogenesis.

LDL Receptor Regulates the Reverse Transport of Macrophage-Derived Unesterified Cholesterol via Concerted Action of the HDL-LDL Axis: Insight From Mouse Models

RATIONALE

The HDL (high-density lipoprotein)-mediated stimulation of cellular cholesterol efflux initiates macrophage-specific reverse cholesterol transport (m-RCT), which ends in the fecal excretion of macrophage-derived unesterified cholesterol (UC). Early studies established that LDL (low-density lipoprotein) particles could act as efficient intermediate acceptors of cellular-derived UC, thereby preventing the saturation of HDL particles and facilitating their cholesterol efflux capacity. However, the capacity of LDL to act as a plasma cholesterol reservoir and its potential impact in supporting the m-RCT pathway in vivo both remain unknown.

OBJECTIVE

We investigated LDL contributions to the m-RCT pathway in hypercholesterolemic mice.

METHODS AND RESULTS

Macrophage cholesterol efflux induced in vitro by LDL added to the culture media either alone or together with HDL or ex vivo by plasma derived from subjects with familial hypercholesterolemia was assessed. In vivo, m-RCT was evaluated in mouse models of hypercholesterolemia that were naturally deficient in CETP (cholesteryl ester transfer protein) and fed a Western-type diet. LDL induced the efflux of radiolabeled UC from cultured macrophages, and, in the simultaneous presence of HDL, a rapid transfer of the radiolabeled UC from HDL to LDL occurred. However, LDL did not exert a synergistic effect on HDL cholesterol efflux capacity in the familial hypercholesterolemia plasma. The m-RCT rates of the LDLr (LDL receptor)-KO (knockout), LDLr-KO/APOB100, and PCSK9 (proprotein convertase subtilisin/kexin type 9)-overexpressing mice were all significantly reduced relative to the wild-type mice. In contrast, m-RCT remained unchanged in HAPOB100 Tg (human APOB100 transgenic) mice with fully functional LDLr, despite increased levels of plasma APO (apolipoprotein)-B-containing lipoproteins.

CONCLUSIONS

Hepatic LDLr plays a critical role in the flow of macrophage-derived UC to feces, while the plasma increase of APOB-containing lipoproteins is unable to stimulate m-RCT. The results indicate that, besides the major HDL-dependent m-RCT pathway via SR-BI (scavenger receptor class B type 1) to the liver, a CETP-independent m-RCT path exists, in which LDL mediates the transfer of cholesterol from macrophages to feces. Graphical Abstract: A graphical abstract is available for this article.

Targeted Deletion of Hepatocyte Abca1 Increases Plasma HDL (High-Density Lipoprotein) Reverse Cholesterol Transport via the LDL (Low-Density Lipoprotein) Receptor

OBJECTIVE

The role of hepatocyte Abca1 (ATP binding cassette transporter A1) in trafficking hepatic free cholesterol (FC) into plasma versus bile for reverse cholesterol transport (RCT) is poorly understood. We hypothesized that hepatocyte Abca1 recycles plasma HDL-C (high-density lipoprotein cholesterol) taken up by the liver back into plasma, maintaining the plasma HDL-C pool, and decreasing HDL-mediated RCT into feces. Approach and Results: Chow-fed hepatocyte-specific Abca1 knockout (HSKO) and control mice were injected with human HDL radiolabeled with ¹²⁵I-tyramine cellobiose (¹²⁵I-TC; protein) and ³H-cholesteryl oleate (³H-CO). ¹²⁵I-TC and ³H-CO plasma decay, plasma HDL ³H-CO selective clearance (ie, ³H-¹²⁵I fractional catabolic rate), liver radiolabel uptake, and fecal ³H-sterol were significantly greater in HSKO versus control mice, supporting increased plasma HDL RCT. Twenty-four hours after ³H-CO-HDL injection, HSKO mice had reduced total hepatic ³H-FC (ie, ³H-CO hydrolyzed to ³H-FC in liver) resecretion into plasma, demonstrating Abca1 recycled HDL-derived hepatic ³H-FC back into plasma. Despite similar liver LDLr (low-density lipoprotein receptor) expression between genotypes, HSKO mice treated with LDLr-targeting versus control antisense oligonucleotide had slower plasma ³H-CO-HDL decay, reduced selective ³H-CO clearance, and decreased fecal ³H-sterol excretion that was indistinguishable from control mice. Increased RCT in HSKO mice was selective for ³H-CO-HDL, since macrophage RCT was similar between genotypes.

CONCLUSIONS

Hepatocyte Abca1 deletion unmasks a novel and selective FC trafficking pathway that requires LDLr expression, accelerating plasma HDL-selective CE uptake by the liver and promoting HDL RCT into feces, consequently reducing HDL-derived hepatic FC recycling into plasma.

Endocytosis of lipoproteins

During their metabolism, all lipoproteins undergo endocytosis, either to be degraded intracellularly, for example in hepatocytes or macrophages, or to be re-secreted, for example in the course of transcytosis by endothelial cells. Moreover, there are several examples of internalized lipoproteins sequestered intracellularly, possibly to exert intracellular functions, for example the cytolysis of trypanosoma. Endocytosis and the subsequent intracellular itinerary of lipoproteins hence are key areas for understanding the regulation of plasma lipid levels as well as the biological functions of lipoproteins. Indeed, the identification of the low-density lipoprotein (LDL)-receptor and the unraveling of its transcriptional regulation led to the elucidation of familial hypercholesterolemia as well as to the development of statins, the most successful therapeutics for lowering of cholesterol levels and risk of atherosclerotic cardiovascular diseases. Novel limiting factors of intracellular trafficking of LDL and the LDL receptor continue to be discovered and to provide drug targets such as PCSK9. Surprisingly, the receptors mediating endocytosis of high-density lipoproteins or lipoprotein(a) are still a matter of controversy or even new discovery. Finally, the receptors and mechanisms, which mediate the uptake of lipoproteins into non-degrading intracellular itineraries for re-secretion (transcytosis, retroendocytosis), storage, or execution of intracellular functions, are largely unknown.

FoxO transcription factors are required for hepatic HDL cholesterol clearance

Insulin resistance and type 2 diabetes are associated with low levels of high-density lipoprotein cholesterol (HDL-C). The insulin-repressible FoxO transcription factors are potential mediators of the effect of insulin on HDL-C. FoxOs mediate a substantial portion of insulin-regulated transcription, and poor FoxO repression is thought to contribute to the excessive glucose production in diabetes. In this work, we show that mice with liver-specific triple FoxO knockout (L-FoxO1,3,4), which are known to have reduced hepatic glucose production, also have increased HDL-C. This was associated with decreased expression of the HDL-C clearance factors scavenger receptor class B type I (SR-BI) and hepatic lipase and defective selective uptake of HDL cholesteryl ester by the liver. The phenotype could be rescued by re-expression of SR-BI. These findings demonstrate that hepatic FoxOs are required for cholesterol homeostasis and HDL-mediated reverse cholesterol transport to the liver.

High-density lipoproteins and adrenal steroidogenesis: A population-based study

BACKGROUND

Cholesterol trafficked within plasma lipoproteins, in particular high-density lipoproteins (HDL), may represent an important source of cholesterol that is required for adrenal steroidogenesis. Based on a urinary gas chromatography method, compromised adrenal function has been suggested in men but not in women with (genetically determined) low plasma HDL-cholesterol (HDL-C).

OBJECTIVE

The objective of the article was to examine the extent to which glucocorticoid production relates to HDL-C in a population-based cohort.

METHODS

A total of 240 subjects (120 men and 120 women, aged 20-79 years) without relevant comorbidities were recruited from the general population. Glucocorticoid metabolites were measured by gas chromatography with tandem mass spectrometric detection in 24-hour urine collections to estimate total glucocorticoid production (TGP). Fasting plasma (apo)lipoproteins were assayed by routine methods.

RESULTS

TGP was not decreased but tended to be increased in subjects with low HDL-C (NCEP-ATPIII criteria; P = .094). In univariate analysis, TPG was correlated inversely with HDL-C (r = -0.353, P < .001) and apoA-I (r = -0.263, P = .01). Multivariable linear regression analysis demonstrated that TGP was still inversely related to HDL-C (β = -0.145, P = .019) or alternatively to low HDL-C (β = -0.129, P = .013) taking age, sex, current smoking, and other metabolic syndrome components into account.

CONCLUSION

In this population-based study, urinary glucocorticoid metabolite excretion was inversely associated with HDL-C. We found no evidence for an attenuated adrenal function in men and women with low HDL-C.

Advances in the Study of the Antiatherogenic Function and Novel Therapies for HDL

The hypothesis that raising high-density lipoprotein cholesterol (HDL-C) levels could improve the risk for cardiovascular disease (CVD) is facing challenges. There is multitudinous clear clinical evidence that the latest failures of HDL-C-raising drugs show no clear association with risks for CVD. At the genetic level, recent research indicates that steady-state HDL-C concentrations may provide limited information regarding the potential antiatherogenic functions of HDL. It is evident that the newer strategies may replace therapeutic approaches to simply raise plasma HDL-C levels. There is an urgent need to identify an efficient biomarker that accurately predicts the increased risk of atherosclerosis (AS) in patients and that may be used for exploring newer therapeutic targets. Studies from recent decades show that the composition, structure and function of circulating HDL are closely associated with high cardiovascular risk. A vast amount of data demonstrates that the most important mechanism through which HDL antagonizes AS involves the reverse cholesterol transport (RCT) process. Clinical trials of drugs that specifically target HDL have so far proven disappointing, so it is necessary to carry out review on the HDL therapeutics.

Genetic alterations affecting cholesterol metabolism and human fertility

Single nucleotide polymorphisms (SNPs) represent genetic variations among individuals in a population. In medicine, these small variations in the DNA sequence may significantly impact an individual's response to certain drugs or influence the risk of developing certain diseases. In the field of reproductive medicine, a significant amount of research has been devoted to identifying polymorphisms which may impact steroidogenesis and fertility. This review discusses current understanding of the effects of genetic variations in cholesterol metabolic pathways on human fertility that bridge novel linkages between cholesterol metabolism and reproductive health. For example, the role of the low-density lipoprotein receptor (LDLR) in cellular metabolism and human reproduction has been well studied, whereas there is now an emerging body of research on the role of the high-density lipoprotein (HDL) receptor scavenger receptor class B type I (SR-BI) in human lipid metabolism and female reproduction. Identifying and understanding how polymorphisms in the SCARB1 gene or other genes related to lipid metabolism impact human physiology is essential and will play a major role in the development of personalized medicine for improved diagnosis and treatment of infertility.