Kyriakos E. Kypreos

HDL biogenesis and functions: Role of HDL quality and quantity in atherosclerosis

Coronary heart disease (CHD) is a leading cause of death in western societies. In the last few decades, a number of epidemiological studies have shown that a disproportion between atheroprotective and atherogenic lipoproteins in plasma is one of the most important contributors towards atherosclerosis and CHD. Thus, based on the classical view, reduced HDL cholesterol levels independently predict ones risk factor for developing cardiovascular disease, while elevated HDL levels protect from atherosclerosis. However, more recent studies have suggested that the relationship between HDL and cardiovascular risk is more complex and extends beyond the levels of HDL in plasma. These studies challenge the existing view on HDL and cardiovascular risk and trigger a discussion as to whether low HDL is a causal effect for the development of heart disease. In this article we provide a review of the current literature on the biogenesis of HDL and its proposed functions in atheroprotection. In addition, we discuss the significance of both HDL quality and quantity in assessing cardiovascular risk.

Pathway of biogenesis of apolipoprotein E-containing HDL in vivo with the participation of ABCA1 and LCAT

We have investigated the ability of apoE (apolipoprotein E) to participate in the biogenesis of HDL (high-density lipoprotein) particles in vivo using adenovirus-mediated gene transfer in apoA-I-/- (apolipoprotein A-I) or ABCA1-/- (ATP-binding cassette A1) mice. Infection of apoA-I-/- mice with 2x10(9) pfu (plaque-forming units) of an apoE4-expressing adenovirus increased both HDL and the triacylglycerol-rich VLDL (very-low-density lipoprotein)/IDL (intermediate-density lipoprotein)/LDL (low-density lipoprotein) fraction and generated discoidal HDL particles. ABCA1-/- mice treated similarly failed to form HDL particles, suggesting that ABCA1 is essential for the generation of apoE-containing HDL. Combined infection of apoA-I-/- mice with a mixture of adenoviruses expressing both apoE4 (2x10(9) pfu) and human LCAT (lecithin:cholesterol acyltransferase) (5x10(8) pfu) cleared the triacylglycerol-rich lipoproteins, increased HDL and converted the discoidal HDL into spherical HDL. Similarly, co-infection of apoE-/- mice with apoE4 and human LCAT corrected the hypercholesterolaemia and generated spherical particles, suggesting that LCAT is essential for the maturation of apoE-containing HDL. Overall, the findings indicate that apoE has a dual functionality. In addition to its documented functions in the clearance of triacylglycerol-rich lipoproteins, it participates in the biogenesis of HDL-sized apoE-containing particles. HDL particles generated by this pathway may account at least for some of the atheroprotective functions of apoE.

Androgen deficiency and atherosclerosis: The lipid link.

The relationship between androgen deficiency and atherosclerosis is complex, poorly understood, and remains controversial. The aim of this review is to evaluate the data in the literature to determine if androgen deficiency modulates lipid profiles and contributes to atherosclerosis development or progression. Studies in animals and humans suggest that androgen deficiency is associated with increased triglycerides (TGs), total cholesterol (TC), and low-density lipoprotein cholesterol (LDL-C). Although the effects of androgen deficiency on high-density lipoprotein cholesterol (HDL-C) remains controversial, recent data suggest that androgen therapy is associated with increased levels of HDL-C and may improve reverse cholesterol transport. Animal studies suggested that androgen deprivation adversely affect lipid profiles and this was reversed by androgen treatment. Furthermore, androgen treatment of hypogonadal men significantly improved lipid profiles. Emerging data indicate that androgens play an important role in lipid metabolism. Therefore androgens are critical in the prevention and progression of atherosclerosis. Androgen deficiency contributes to increased TGs, TC, LDL-C and reduced HDL-C while androgen treatment results in a favorable lipid profile, suggesting that androgens may provide a protective effect against the development and/or progression of atherosclerosis.

Apolipoprotein E predisposes to obesity and related metabolic dysfunctions in mice

Obesity is a central feature of the metabolic syndrome and is associated with increased risk for insulin resistance and type II diabetes. Here, we investigated the contribution of human apoliprotein E3 and mouse apoliprotein E to the development of diet‐induced obesity in response to western‐type diet. Our data show that apolipoprotein E contributes to the development of obesity and other related metabolic disorders, and that human apolipoprotein E3 is more potent than mouse apolipoprotein E in promoting obesity in response to western‐type diet. Specifically, we found that apolipoprotein E3 knock‐in mice fed western‐type diet for 24 weeks became obese and developed hyperglycemia, hyperinsulinemia, hyperleptinemia, glucose intolerance and insulin resistance that were more severe than in C57BL/6 mice. In contrast, apolipoprotein E‐deficient mice fed western‐type diet for the same period were resistant to diet‐induced obesity, had normal plasma glucose, leptin and insulin levels, and exhibited normal responses to glucose tolerance and insulin resistance tests. Furthermore, low‐density lipoprotein receptor‐deficient mice were more sensitive to the development of diet‐induced obesity and insulin resistance than apolipoprotein E‐deficient mice, but were still more resistant than C57BL/6 mice, raising the possibility that low‐density lipoprotein receptor mediates, at least in part, the effects of apolipoprotein E on obesity. Taken together, our findings suggest that, in addition to other previously identified mechanisms of obesity, apolipoprotein E and possibly the chylomicron pathway are also important contributors to the development of obesity and related metabolic dysfunctions in mice.

Testosterone and cardiovascular disease: An old idea with modern clinical implications

The role of sex steroid hormones in modulating vascular function in men is of great importance, given that androgen deficiency is strongly associated with common medical conditions including metabolic syndrome, obesity, diabetes, hypertension and atherosclerosis. Testosterone deficiency afflicts approximately 30% of men ages 40-79 years. Testosterone replacement in deficient men with such co-morbidities ameliorates or partially reverses their progression. Studies in animal and humans suggest that androgen deficiency is associated with increased triglycerides (TGs), total cholesterol (TC), and low-density lipoprotein cholesterol (LDL-C). Emerging evidence indicates that androgens may provide a protective effect against the development and/or progression of atherosclerosis in men.

Probing the pathways of chylomicron and HDL metabolism using adenovirus-mediated gene transfer

Purpose of the review This review clarifies the functions of key proteins of the chylomicron and the HDL pathways. Recent findings Adenovirus-mediated gene transfer of several apolipoprotein (apo)E forms in mice showed that the amino-terminal 1-185 domain of apoE can direct receptor-mediated lipoprotein clearance in vivo. Clearance is mediated mainly by the LDL receptor. The carboxyl-terminal 261-299 domain of apoE induces hypertriglyceridemia, because of increased VLDL secretion, diminished lipolysis and inefficient VLDL clearance. Truncated apoE forms, including apoE2-202, have a dominant effect in remnant clearance and may have future therapeutic applications for the correction of remnant removal disorders. Permanent expression of apoE and apoA-I following adenoviral gene transfer protected mice from atherosclerosis. Functional assays, protein cross-linking, and adenovirus-mediated gene transfer of apoA-I mutants in apoA-I deficient mice showed that residues 220-231, as well as the central helices of apoA-I, participate in ATP-binding cassette transporter A1-mediated lipid efflux and HDL biogenesis. Following apoA-I gene transfer, an amino-terminal deletion mutant formed spherical α-HDL, a double amino- and carboxyl-terminal deletion mutant formed discoidal HDL, and a carboxyl-terminal deletion mutant formed only pre-β-HDL. The findings support a model of cholesterol efflux that requires direct physical interactions between apoA-I and ATP-binding cassette transporter A1, and can explain Tangier disease and other HDL deficiencies. Summary New insights are provided into the role of apoE in cholesterol and triglyceride homeostasis, and of apoA-I in the biogenesis of HDL. Clearance of the lipoprotein remnants and increase in HDL synthesis are obvious targets for therapeutic interventions.

Domains of apolipoprotein E contributing to triglyceride and cholesterol homeostasis in vivo. Carboxyl-terminal region 203-299 promotes hepatic very low density lipoprotein-triglyceride secretion.

Apolipoprotein (apo) E has been implicated in cholesterol and triglyceride homeostasis in humans. At physiological concentration apoE promotes efficient clearance of apoE-containing lipoprotein remnants. However, high apoE plasma levels correlate with high plasma triglyceride levels. We have used adenovirus-mediated gene transfer in apoE-deficient mice (E− /−) to define the domains of apoE required for cholesterol and triglyceride homeostasis in vivo. A dose of 2 × 109 plaque-forming units of apoE4-expressing adenovirus reduced slightly the cholesterol levels of E− /− mice and resulted in severe hypertriglyceridemia, due to accumulation of cholesterol and triglyceride-rich very low density lipoprotein particles in plasma. In contrast, the truncated form apoE4–202 resulted in a 90% reduction in the plasma cholesterol levels but did not alter plasma triglyceride levels in the E− /− mice. ApoE secretion by cell cultures, as well as the steady-state hepatic mRNA levels in individual mice expressing apoE4 or apoE4–202, were similar. In contrast, very low density lipoprotein-triglyceride secretion in mice expressing apoE4, but not apoE4–202, was increased 10-fold, as compared with mice infected with a control adenovirus. The findings suggest that the amino-terminal 1–202 region of apoE4 contains the domains required for the in vivo clearance of lipoprotein remnants. Furthermore, the carboxyl-terminal 203–299 residues of apoE promote hepatic very low density lipoprotein-triglyceride secretion and contribute to apoE-induced hypertriglyceridemia.

Increased CUG triplet repeat-binding protein-1 predisposes to impaired adipogenesis with aging

Preadipocyte differentiation capacity declines between middle and old age. Expression of the adipogenic transcription factors, CCAAT/enhancer-binding protein (C/EBP) α and peroxisome proliferator-activated receptor γ (PPARγ), is lower in differentiating preadipocytes from old than young animals, although no age-related changes occur in C/EBPβ mRNA, which is upstream of C/EBPα and PPARγ. C/EBPβ-liver-enriched inhibitory protein (C/EBPβ-LIP), a truncated C/EBPβ isoform that is a dominant inhibitor of differentiation, increases with aging in rat fat tissue and preadipocytes. CUG triplet repeat-binding protein-1 (CUGBP1) binds to C/EBPβ mRNA, increasing C/EBPβ-LIP translation. Abundance and nucleotide binding activity of CUGBP1 increased with aging in preadipocytes. CUGBP1 overexpression in preadipocytes from young animals increased C/EBPβ-LIP and impaired adipogenesis. Decreasing CUGBP1 in preadipocytes from old rats by RNA interference reduced C/EBPβ-LIP abundance and promoted adipogenesis. Tumor necrosis factor-α, levels of which are elevated in fat tissue with aging, increased CUGBP1 protein, CUGBP1 binding activity, and C/EBPβ-LIP in preadipocytes from young rats. Thus, CUGBP1 contributes to regulation of adipogenesis in primary preadipocytes and is responsive to tumor necrosis factor-α. With aging, preadipocyte CUGBP1 abundance and activity increases, resulting in enhanced translation of the C/EBPβ-LIP isoform, thereby blocking effects of adipogenic transcription factors, predisposing preadipocytes from old animals to resist adipogenesis. Altered translational processing, possibly related to changes in cytokine milieu and activation of stress responses, may contribute to changes in progenitor differentiation and tissue function with aging.

Mechanisms of obesity and related pathologies: Role of apolipoprotein E in the development of obesity

Apolipoprotein E is a polymorphic glycoprotein in humans with a molecular mass of 34.5 kDa. It is a component of chylomicron remnants, very low density lipoprotein, low density lipoprotein and high density lipoprotein, and is primarily responsible for maintaining plasma lipid homeostasis. In addition to these well‐documented functions, recent studies in experimental mouse models, as well as population studies, show that apolipoprotein E also plays an important role in the development of obesity and insulin resistance. It is widely accepted that disruption in homeostasis between food intake and energy expenditure, and the subsequent deposition of excess fatty acids into fat cells in the form of triglycerides, leads to the development of obesity. Despite the pivotal role of obesity and dyslipidemia in the development of the metabolic syndrome and heart disease, the functional interactions between adipose tissue and components of the lipoprotein transport system have not yet been investigated thoroughly. In this minireview, we focus on the current literature pertinent to the involvement of apolipoprotein E in the development of pathologies associated with the metabolic syndrome.

Perturbations in the HDL metabolic pathway predispose to the development of osteoarthritis in mice following long-term exposure to western-type diet

OBJECTIVE Recent data suggest that obesity and related metabolic aberrations are associated with osteoarthritis (OA) development, a phenomenon that is attributed at least in part to the consumption of lipid-rich diets. To date, the molecular mechanisms that govern the lipid-OA connection remain largely unknown. Given the important role of high-density lipoprotein (HDL) in plasma and tissue lipid metabolism, the main purpose of the present study was to investigate the role of HDL metabolism in the pathobiology of OA. METHODS We used apolipoprotein A-I (apoA-I)(-/-) mice that lack classical apoA-I containing HDL, LCAT(-/-) mice that have only immature HDL and relatively reduced HDL-cholesterol levels and control C57BL/6 mice. Mice were placed on chow or western-type (WTD) and monitored for 24 weeks. Knee joints were removed and articular cartilage was isolated for further analyses. RESULTS The LCAT(-/-) mice were significantly more sensitive to the development of diet-induced obesity compared to the C57BL/6 and apoA-I(-/-) mice. Morphological, biochemical and molecular analyses revealed that the LCAT(-/-) obese mice developed OA, while the C57BL/6 mice that were fed WTD did not. Notably, apoA-I(-/-) mice that received WTD also developed OA although their body-weight gain was similar to their wild-type counterparts. Interestingly, bone marrow from LCAT(-/-) and apoA-I(-/-) mice contained significantly increased number of adipocytes, compared to the other groups. CONCLUSIONS Our findings suggest that perturbations in HDL metabolism predispose to OA following chronic insult with WTD and raise the challenging possibility that HDL has a causative relation to OA in patients with metabolic syndrome.