Epidemiologic evidence for high-density lipoprotein cholesterol as a risk factor for coronary artery disease

Plasma high-density lipoprotein cholesterol (HDL-C) is inversely related to coronary artery disease incidence in both women and men, with a proportionately increasing effect of HDL-C throughout the average (or slightly above average)-to-low concentration range. A substantial body of evidence from major epidemiologic studies has established that this coronary artery disease risk associated with HDL-C is entirely independent of plasma low-density lipoprotein cholesterol, other lipid parameters (triglycerides, total cholesterol), and other nonlipid risk factors. The coronary artery disease relative risk for patients with low levels of HDL-C in conjunction with low total cholesterol is also markedly higher than in patients with a profile of high levels of these 2 lipid parameters. Similarly, the risk associated with a low HDL-C level is of particular significance in patients with established coronary artery disease or with type 2 diabetes, and it is a more pertinent factor to consider when managing dyslipidemia.

Limited proteolysis of a disulfide-linked apoA-I dimer in reconstituted HDL

The apolipoprotein A-I(Milano) (apoA-I(M)) is a molecular variant of apoA-I characterized by the Arg(173)-->Cys substitution, leading to the formation of homodimers A-I(M)/A-I(M). Upon interaction with palmitoyloleoylphosphatidylcholine, A-I(M)/A-I(M) forms only two species of reconstituted HDL (rHDL) particles, with diameters of 7.8 and 12.5 nm. We used limited proteolysis to analyze the conformation of A-I(M)/A-I(M) in the two rHDL particles, in comparison with that of apoA-I in rHDL of similar size. ApoA-I in the small, 7.8-nm rHDL is degraded to a greater extent (50% after 6 h) than in the large rHDL (<10% degraded after 6 h). The protease susceptibility of A-I(M)/A-I(M) in small and large rHDL is instead remarkably the same, with A-I(M)/A-I(M) being much more sensitive to proteolytic digestion (50% degraded after 10 min) than apoA-I. The identification of the proteolytic fragments by immunoblotting, N-terminal sequencing, and molecular mass determination, shows that the N-terminus of both proteins is resistant to proteolysis, with six cleavage sites located in the central and carboxy-terminal portions of the molecules. Cleavage in the middle of apoA-I occurs at distinct sites in 7.8-nm (Lys(118)) and 12.7-nm (Arg(123)) rHDL, indicating a different conformation in small and large rHDL particles. The A-I(M)/A-I(M) instead adopts a unique and identical conformation in small and large rHDL, with the carboxy-terminal portion of the molecule being remarkably more accessible to the proteases than in apoA-I. This suggests the presence of a novel carboxy-terminal domain in A-I(M)/A-I(M), not organized in a compact structure and not shared by wild-type apoA-I, which may account for the unique functional properties of A-I(M)/A-I(M).

Development of a lipid-rich, soft plaque in rabbits, monitored by histology and intravascular ultrasound

Lipid rich, soft plaques in the clinic are a common forerunner to occlusive thrombus formation, even with modest arterial stenosis. Animal models of atherosclerosis, obtained by various methods, do not generally allow direct in vivo evaluation of the lesion and, furthermore, cannot be examined more than once. The aim of the study was the generation of a rabbit model of atherosclerosis, with morphological characteristics similar to human lipid-rich, soft atheromatous plaques, and the evaluation of the reliability of intravascular ultrasound (IVUS) technology in the study of the development of atherosclerotic lesions in this model. Briefly, New Zealand white rabbits undergo perivascular electrical injury at both common carotid arteries, together with a 1.5% cholesterol diet for up to 90 days. The lesioned arterial segments show progressive changes, from diffuse cellular mortality, to macrophage infiltration in the media, up to the final migration of macrophages to the neointima, resulting in bulky, eccentric, macrophage and lipid-rich lesions. At IVUS, the produced lesions clearly resemble those described as 'soft plaques' in the clinical setting, with minimal calcification and reduced echo-reflectivity versus the adventitial layer. Quantitative and morphometric analysis of plaques shows a significant correlation between histological and IVUS measurements at each time point. In conclusion, vascular injury in the common carotids of rabbits generates atherosclerotic lipid-rich, soft plaques, that can be properly assessed by the IVUS methodology. The easy accessibility of the arterial lesion allows serial IVUS investigations and the direct evaluation of a number of locally or generally delivered therapeutic agents.

Cardiovascular status of carriers of the apolipoprotein A-I(Milano) mutant: the Limone sul Garda study

BACKGROUND

Carriers of the apolipoprotein A-I(Milano) (apoA-I(M)) mutant present with very low plasma HDL cholesterol and moderate hypertriglyceridemia, apparently not leading to premature coronary heart disease. The objective of this study was to establish whether this high-risk lipid/lipoprotein profile is associated with structural changes in the carotid arteries and heart, indicative of preclinical atherosclerosis.

METHODS AND RESULTS

Twenty-one A-I(M) carriers were compared with age- and sex-matched control subjects from the same kindred and with 2 series of matched subjects with primary hypoalphalipoproteinemia (HA). Structural changes in the carotid arteries were defined as the intima-media thickness (IMT) measured by B-mode ultrasound. HA subjects, both recruited among patients attending our Lipid Clinic and blood donors, showed significant thickening of the carotids (average IMT, 0.86+/-0.25 and 0.88+/-0.29 mm, respectively) compared with control subjects (average IMT, 0.64+/-0.12 mm); the apoA-I(M) carriers instead showed normal arterial thickness (average IMT, 0.63+/-0.10 mm). Moreover, a significantly higher prevalence of atherosclerotic plaques was found in patients and blood donors with HA (both 57%) compared with apoA-I(M) carriers (33%) and control subjects (21%). Echocardiographic findings and maximal treadmill ECG did not differ significantly between apoA-I(M) carriers and control subjects, apart from a slight increase in left ventricular end-diastolic dimension in the carriers.

CONCLUSIONS

Despite severe HA, carriers of the apoA-I(M) mutant do not show structural changes in the arteries and heart, in contrast to HA subjects, who are characterized by a marked increase in carotid IMT and increased prevalence of atherosclerotic plaques.

A point mutation in ABC1 gene in a patient with severe premature coronary heart disease and mild clinical phenotype of Tangier disease

The proband is a 50 year-old woman born from a consanguineous marriage. She has been suffering from angina pectoris since the age of 38 and underwent coronary bypass surgery for three-vessel disease at 48. The presence of low plasma levels of total cholesterol and high density lipoprotein (HDL) cholesterol (2.4 and 0.1 mmol/l) and apo AI (<15 mg/dl), associated with corneal lesions and a mild splenomegaly suggested the diagnosis of Tangier disease. However, none of the other features of Tangier disease, including hepatomegaly, anemia and peripheral neuropathy, were present. The analysis of the dinucleotide microsatellites located in chromosome 9q31 region demonstrated that the proband was homozygous for the alleles of D9S53, D9S1784 and D9S1832. The mother and son of the proband, both with low levels of HDL cholesterol, shared one of the proband's haplotypes, whereas neither of these haplotypes was present in the normolipidemic proband's sister. The sequence of ATP-binding cassette transporter 1 (ABC1-1) cDNA obtained by reverse transcription-PCR (RT-PCR) of total RNA isolated from cultured fibroblasts showed that the proband was homozygous for a C>T transition in exon 13, which caused a tryptophane for arginine substitution (R527W). This mutation was confirmed by direct sequencing of exon 13 amplified from genomic DNA. It can be easily screened, as the nucleotide change introduces a restriction site for the enzyme Afl III. R527W substitution occurs in a highly conserved region of the NH2 cytoplasmic domain of ABC1 protein. R527W co-segregates with the low HDL phenotype in the family and was not found in 200 chromosomes from normolipidemic individuals.

Metabolism of apolipoproteins AI and AII in subjects carrying similar apoAI mutations, apoAI Milano and apoAI Paris

ApoAI Milano (AI(M)) and apoAI Paris (AI(P)) are mutant forms of apoAI in which cysteine is substituted for arginine at residues 173 and 151 respectively leading to the formation of homodimers and heterodimers with apoAII. Heterozygous subjects with these mutants are characterized by low levels of plasma HDL cholesterol and apoAI. The present study analyzed the metabolism of the different complexes of apoAI in three subjects, two AI(M) and one AI(P), using a primed-constant infusion of trideuterated leucine. In AI(M) carriers, the mutant form was almost equally distributed in AI(M) dimer, AI(M):AII heterodimer and the monomer, whereas, in the AI(P) subject, the mutant apoAI was essentially in the apoAI(P):AII complex. Normal apoAI was low in the AI(M) subjects (20 and 16 mg/dl) but very low in the AI(P) subject (0.3 mg/dl). In the AI(M) subjects, the low levels of apoAI were due to a rapid catabolism with a normal synthetic rate. However, the apoAI kinetics were heterogeneous with a rapid catabolism of the AI(M):AII complex (FCR of 0.430 and 0.401 day(-1)) and the AI(M) monomer (FCR of 0.570 and 0.406 day(-1)) whereas the AI(M) dimer was catabolized slowly (FCR of 0.114 and 0. 118 day(-1)). In contrast, AI(P) was catabolized relatively slowly with a FCR of 0.263, 0.182 and 0.258 day(-1) for AI(P) homodimer, apoAI(P):AII heterodimer and AI(P) monomer. In the three subjects, normal apoAI was catabolized quickly, with an FCR of 0.805 and 0.601 day(-1) in AI(M) carriers and 0.526 day(-1) in the AI(P) carrier. Therefore, the low level of apoAI in the AI(P) carrier is caused by a low production rate of apoAI, particularly of normal apoAI. In conclusion, apoAI is kinetically heterogeneous in AI(M) and in AI(P) subjects. Moreover, the two mutations lead to significant differences in the kinetic behavior of mutant apoAI depending on its inclusion in its complexes.

Omacor in familial combined hyperlipidemia: effects on lipids and low density lipoprotein subclasses

Elevations of plasma cholesterol and/or triglycerides, and the prevalence of small, dense LDL particles remarkably increase coronary risk in patients with familial combined hyperlipidemia (FCHL). A total of 14 FCHL patients were studied, to investigate the ability of Omacor, a drug containing the n-3 fatty acids eicosapentaenoic and docosahexaenoic acid (EPA and DHA), to favorably correct plasma lipid/lipoprotein levels and LDL particle distribution. The patients received four capsules daily of Omacor (providing 3.4 g EPA+DHA per day) or placebo for 8 weeks in a randomized, double-blind, cross-over study. Omacor significantly lowered plasma triglycerides and VLDL-cholesterol levels, by 27 and 18%, respectively. Total cholesterol did not change but LDL-cholesterol and apolipoprotein B (apoB) concentrations increased by 21 and 6%. As expected, LDL particles were small (diameter=24.9+/-0.3 nm) and apoB-rich (LDL-cholesterol/apoB ratio=1.27+/-0.26) in the selected subjects. After Omacor treatment LDL became enriched in cholesterol (LDL-cholesterol/apoB ratio=1.40+/-0.17), mainly cholesteryl esters, indicating accumulation in plasma of more buoyant and core enriched LDL particles. Indeed, the separation of LDL subclasses by rate zonal ultracentrifugation showed an increase of the plasma concentration of IDL and of the more buoyant, fast floating LDL-1 and LDL-2 subclasses after Omacor, with a parallel decrease in the concentration of the denser, slow floating LDL-3 subclass. However, the average LDL size did not change after Omacor (25.0+/-0.3 nm). The resistance of the small LDL pattern to drug-induced modifications implies that a maximal lipid-lowering effect must be achieved to reduce coronary risk in FCHL patients.

Cell cholesterol efflux to reconstituted high-density lipoproteins containing the apolipoprotein A-IMilano dimer

The apolipoprotein A-IMilano (apoA-IM) is a molecular variant of apoA-I characterized by the Arg(173)-->Cys substitution, resulting in the formation of homodimers A-IM/A-IM. The introduction of the interchain disulfide bridge in the A-IM dimer limits the apolipoprotein conformational flexibility and restricts HDL particle size heterogeneity, thus possibly affecting HDL function in lipid metabolism and atherosclerosis protection. To investigate whether the structural changes in A-IM/A-IM affect apoA-I capacity for cell cholesterol uptake, we tested the ability of four reconstituted HDL (rHDL), that contained either apoA-I or A-IM/A-IM, to remove cholesterol from Fu5AH hepatoma cells and cholesterol-loaded murine primary macrophages (MPM). As the HDL particle size is known to affect the rHDL capacity for cell cholesterol uptake, the reconstitution conditions were carefully selected to produce two sets of rHDL particles of small and large size (7.8 and 12.5 nm in diameter). The small A-IM/A-IM rHDL were more efficient than the corresponding apoA-I particles as acceptors of membrane cholesterol from Fu5AH cells and MPM, and as inhibitors of cholesterol esterification in MPM. The large rHDL and the lipid-free apolipoproteins displayed instead similar capacities for cell cholesterol efflux. These results suggest that cell cholesterol efflux to rHDL particles of different size occurs through different mechanisms. Large HDL accommodate and retain the cholesterol molecules that have desorbed from the cell membrane into the extracellular fluid, in a process that is less sensitive to protein conformation. Small HDL accelerate the desorption of cholesterol from the cell membrane, in a process that is influenced by the conformation of the proteins on the surface of the acceptor particle. The enhanced efficiency of small A-IM/A-IM rHDL seems related to the peculiar structure of the protein on the rHDL surface, with a hydrophobic C-terminal domain extending out of the rHDL particle, available for anchoring the acceptor to the plasma membrane.

Control of chaotic spatiotemporal spiking by time-delay autosynchronization

A global time-delayed feedback control is applied to a globally coupled reaction-diffusion system describing charge transport in a bistable semiconductor. We demonstrate that a variety of spatiotemporal unstable periodic orbits (UPOs) embedded in a chaotic attractor of the spatially extended system can be stabilized using an extended time-delay autosynchronization algorithm. These UPOs correspond to spiking current filaments. We critically evaluate analytical approximations for the limits of control originally developed for low-dimensional temporal chaos and show that the delay time can be extrapolated with high accuracy, while the theoretical limit for the control of UPOs in terms of the product of the period and the largest Lyapunov exponent is not reached. If the global feedback is modified by a spatial filter, we achieve stabilization of different spatial patterns.

Increased cholesterol efflux potential of sera from ApoA-IMilano carriers and transgenic mice

The ability of HDL to remove cholesterol from peripheral cells and drive it to the liver for excretion is believed to explain most of the strong inverse correlation between plasma HDL cholesterol levels and coronary heart disease. Carriers of the ApoA-IMilano (A-IM) mutant have a severe hypoalphalipoproteinemia but are not at increased risk for premature of coronary heart disease. To explain this apparent paradox, we compared the capacity of serum from A-IM and control subjects to extract cholesterol from Fu5AH cells. Because the A-IM carriers are all heterozygotes for the mutation, we also compared the cholesterol efflux capacity of serum from transgenic mice expressing A-IM or wild-type ApoA-I (A-IWT), in the absence of murine ApoA-I. In the whole series of human or mouse sera, cholesterol efflux was significantly correlated with several HDL-related parameters; after adjustment for concomitant variables, the only parameter that remained significantly correlated with cholesterol efflux was the serum ApoA-I concentration (r2=0.85 in humans and 0.84 in mice). The same was true when samples from control subjects, A-IM carriers, A-IWT or A-IM mice were analyzed separately. Cholesterol efflux to sera from the A-IM carriers was only reduced slightly compared with control sera (25.0+/-4.2% versus 30.4+/-3.3%), although there was a large reduction (-45%) in the serum ApoA-I concentration in the former. Cholesterol efflux was also lower to sera from A-IM than A-IWT mice (15.6+/-3.8% versus 30. 1+/-7.1%), but less than expected from the 70% reduction in serum ApoA-I concentration. A relative efflux potential of serum was calculated in each group as the slope of the regression line fitting cholesterol efflux to ApoA-I concentrations. Therefore, the relative efflux potential reflects the relative efficiency of ApoA-I in determining cell cholesterol efflux. The relative efflux potential of mouse and human sera was in the following order: A-IM mice>A-IM carriers>A-IWT mice=control subjects, suggesting a gene-dosage effect of the A-IM mutation on the efficiency of serum to extract cholesterol from cells. The high efficiency of A-IM-containing HDL for cell cholesterol uptake would result in an improved reverse cholesterol transport in the A-IM carriers, possibly explaining the low susceptibility to atherosclerosis development.

[Axillary lymphadenectomy]

Axillary lymphadenectomy represents one of the historical hallmarks in the surgical strategy for breast cancer treatment. In recent years the role of axillary dissection is becoming matter for a renewed debate: up to date better therapeutic results together with new biologic acquisitions and progresses in early diagnosis have been influencing current indications and extension of axillary lymphadenectomy; also, cosmetic and financial considerations play a relevant role. This is the background of a large number of studies investigating the possibility of a more limited applications of axillary dissection provided that oncologic results remain unchanged. In this context sentinel node biopsy is one of the most promising lines of research. It can be stated that, as far as early stages are concerned, a trend towards more conservative resection techniques that started with the first quadrantectomies two decades ago, is now involving axillary dissection too. Possibly, in the next future, an elective axillary lymphadenectomy will not be performed any more if the presence of positive nodes will not be preliminarily ascertained. Nevertheless such conservative strategy can not be recommended to date, until the technique of sentinel node biopsy will be optimized and its results confirmed by randomized trials.

Recombinant apolipoproteins for the treatment of vascular diseases

The protein components of human lipoproteins, apolipoproteins, allow the redistribution of cholesterol from the arterial wall to other tissues and exert beneficial effects on systems involved in the development of arterial lesions, like inflammation and hemostasis. Because of these properties, the antiatherogenic apolipoproteins, particularly apo A-I and apo E, may provide an innovative approach to the management of vascular diseases. The recent availability of extractive or biosynthetic molecules is allowing a detailed overview of their therapeutic potential in a number of animal models of arterial disease. Infusions of apo E, or more dramatically, of apo A-I, both recombinant or extractive, cause a direct reduction of the atherosclerotic burden in experimental animals. Naturally, as the apo A-I(Milano) (apo A-I(M)) dimer, or engineered recombinant apolipoproteins with prolonged permanence in plasma and improved function may offer an even better approach to the therapeutic handling of arterial disease. This progress will go on in parallel with innovations in the technologies for direct, non invasive assessments of human atherosclerosis, thus allowing closer monitoring of this potential new approach to therapy.

Human apolipoproteins A-I and A-II in cell cholesterol efflux: studies with transgenic mice

The first step in reverse cholesterol transport is the movement of cholesterol out of cells onto lipoprotein acceptors in the interstitial fluid. The contribution of specific lipoprotein components to this process remains to be established. In this study, the role of human apolipoproteins (apo) A-I and A-II in the efflux of cellular cholesterol was investigated in transgenic mouse models in which the expression of murine apoA-I was abolished due to gene targeting (A-IKO). Serum from A-IKO mice and from mice expressing human apoA-I and/or human apoA-II was incubated with [3H]cholesterol-labeled Fu5AH rat hepatoma cells for 4 hours at 37 degrees C. The cholesterol efflux to the serum of A-IKO mice was markedly lower than that to the serum of mice transgenic for human apoA-I (5.0 +/- 1.5% versus 25.0 +/- 4.0%). Expression of human apoA-II alone did not modify the cholesterol efflux capacity of A-IKO mouse serum. Cholesterol efflux to serum of mice expressing human apoA-II together with human apoA-I was significantly lower than that to human apoA-I mouse serum (20.0 +/- 2.3% versus 25.0 +/- 4.0%). Regression analysis of cholesterol efflux versus the lipid/apolipoprotein concentrations of mouse serum suggested that 3 independent factors contribute to determine the cholesterol efflux potential of serum: the apolipoprotein composition of HDL, the serum concentration of HDL phospholipids, and the presence of a small fraction of particles containing apoA-I.

Modulated serum activities and concentrations of paraoxonase in high density lipoprotein deficiency states

Paraoxonase is a high density lipoprotein (HDL) associated enzyme with a hypothesised role in the protection of low density lipoproteins (LDL) from oxidative stress. The present study examined paraoxonase in several genetically distinct HDL deficiency states. Despite reduction or even absence of detectable HDL, enzyme activity was present in sera from A-I-Pisa, A-I-Helsinki, A-I-Milano and Tangier patients. Both enzyme activities and peptide concentrations were modulated (reduced) but specific activities were broadly similar to controls, suggesting an impact on peptide concentration rather than an inhibition of enzyme activity. Despite the absence of HDL in A-I-Pisa and Tangier subjects, there was no association of paraoxonase with very low density lipoproteins or LDL. Paraoxonase function is maintained in HDL deficient states. It implies that certain HDL-associated anti-atherogenic processes may not be entirely compromised by HDL deficiency. This has important implications for the cardiovascular risk associated with modulated HDL concentrations.