Relationship of the parameters of body cholesterol metabolism with plasma levels of HDL cholesterol and the major HDL apoproteins

Atherosclerosis Regression and Cholesterol Efflux in Hypertriglyceridemic Mice

[Figure: see text].

HDL flux is higher in patients with nonalcoholic fatty liver disease

Altered lipid metabolism and inflammation are involved in the pathogenesis of both nonalcoholic fatty liver disease (NAFLD) and cardiovascular disease (CVD). Even though high-density lipoprotein (HDL), a CVD protective marker, is decreased, whether HDL metabolism and function are perturbed in NAFLD are currently unknown. We examined the effect of NAFLD and disease severity on HDL metabolism and function in patients with biopsy-proven simple steatosis (SS), nonalcoholic steatohepatitis (NASH), and healthy controls. HDL turnover and HDL protein dynamics in SS (n = 7), NASH (n = 8), and healthy controls (n = 9) were studied in vivo. HDL maturation and remodeling, antioxidant, cholesterol efflux properties, and activities of lecithin-cholesterol ester acyltransferase and cholesterol ester transfer protein (CETP) were quantified using in vitro assays. All patients with NAFLD had increased turnover of both HDL cholesterol (HDLc; 0.16 ± 0.09 vs. 0.34 ± 0.18 days, P < 0.05) and apolipoprotein A1 (ApoAI) (0.26 ± 0.04 vs. 0.34 ± 0.06 days, P < 0.005) compared with healthy controls. The fractional catabolic rates of other HDL proteins, including ApoAII (and ApoAIV) were higher (P < 0.05) in patients with NAFLD who also had higher CETP activity, ApoAI/HDLc ratio (P < 0.05). NAFLD-induced alterations were associated with lower antioxidant (114.2 ± 46.6 vs. 220.5 ± 48.2 nmol·mL-1·min-1) but higher total efflux properties of HDL (23.4 ± 1.3% vs. 25.5 ± 2.3%) (both P < 0.05), which was more pronounced in individuals with NASH. However, no differences were observed in either HDL turnover, antioxidant, and cholesterol efflux functions of HDL or HDL proteins' turnover between subjects with SS and subjects with NASH. Thus, HDL metabolism and function are altered in NAFLD without any significant differences between SS and NASH.

Glycation Reduces the Stability of ApoAI and Increases HDL Dysfunction in Diet-Controlled Type 2 Diabetes

CONTEXT

Hyperglycemia plays a key role in the pathogenesis of cardiovascular complications of diabetes. Type 2 diabetes mellitus (T2DM) is associated with high-density lipoprotein (HDL) dysfunction and increased degradation of apolipoprotein I (ApoAI). The mechanism(s) of these changes is largely unknown.

OBJECTIVE

To study the role of hyperglycemia-induced glycation on ApoAI kinetics and stability in patients with diet-controlled T2DM.

DESIGN

2H2O-metabolic labeling approach was used to study ApoAI turnover in patients with diet-controlled T2DM [n = 9 (5 F); 59.3 ± 8.5 years] and matched healthy controls [n = 8 (4 F); 50.7 ± 11.6 years]. The effect of Amadori glycation on in vivo ApoAI stability and the antioxidant and cholesterol efflux properties of HDL were assessed using a proteomics approach and in vitro assays.

RESULTS

Patients with T2DM had increased turnover of ApoAI and impaired cholesterol efflux and antioxidant properties of HDL. Glycated hemoglobin was negatively correlated with the half-life of ApoAI and cholesterol efflux function of HDL. Proteomics analysis identified several nonenzymatic early (Amadori) glycations of ApoAI at lysine sites. The kinetics analysis of glycated and native ApoAI peptides in patients with T2DM revealed that glycation resulted in a threefold shorter ApoAI half-life.

CONCLUSIONS

The 2H2O method allowed the detection of early in vivo impairments in HDL metabolism and function that were related to hyperglycemia-induced glycation of ApoAI in T2DM.

2H2O-based high-density lipoprotein turnover method for the assessment of dynamic high-density lipoprotein function in mice

OBJECTIVE

High-density lipoprotein (HDL) promotes reverse cholesterol transport from peripheral tissues to the liver for clearance. Reduced HDL-cholesterol (HDLc) is associated with atherosclerosis; however, as a predictor of cardiovascular disease, HDLc has limitations because it is not a direct marker of HDL functionality. Our objective was to develop a mass spectrometry-based method for the simultaneous measurement of HDLc and ApoAI kinetics in mice, using a single (2)H2O tracer, and use it to examine genetic and drug perturbations on HDL turnover in vivo.

APPROACH AND RESULTS

Mice were given (2)H2O in the drinking water, and serial blood samples were collected at different time points. HDLc and ApoAI gradually incorporated (2)H, allowing experimental measurement of fractional catabolic rates and production rates for HDLc and ApoAI. ApoE(-/-) mice displayed increased fractional catabolic rates (P<0.01) and reduced production rates of both HDLc and ApoAI (P<0.05) compared with controls. In human ApoAI transgenic mice, levels and production rates of HDLc and human ApoAI were strikingly higher than in wild-type mice. Myriocin, an inhibitor of sphingolipid synthesis, significantly increased both HDL flux and macrophage-to-feces reverse cholesterol transport, indicating compatibility of this HDL turnover method with the macrophage-specific reverse cholesterol transport assay.

CONCLUSIONS

(2)H2O-labeling can be used to measure HDLc and ApoAI flux in vivo, and to assess the role of genetic and pharmacological interventions on HDL turnover in mice. Safety, simplicity, and low cost of the (2)H2O-based HDL turnover approach suggest that this assay can be scaled for human use to study effects of HDL targeted therapies on dynamic HDL function.

Beyond high-density lipoprotein cholesterol levels evaluating high-density lipoprotein function as influenced by novel therapeutic approaches

A number of therapeutic strategies targeting high-density lipoprotein (HDL) cholesterol and reverse cholesterol transport are being developed to halt the progression of atherosclerosis or even induce regression. However, circulating HDL cholesterol levels alone represent an inadequate measure of therapeutic efficacy. Evaluation of the potential effects of HDL-targeted interventions on atherosclerosis requires reliable assays of HDL function and surrogate markers of efficacy. Promotion of macrophage cholesterol efflux and reverse cholesterol transport is thought to be one of the most important mechanisms by which HDL protects against atherosclerosis, and methods to assess this pathway in vivo are being developed. Indexes of monocyte chemotaxis, endothelial inflammation, oxidation, nitric oxide production, and thrombosis reveal other dimensions of HDL functionality. Robust, reproducible assays that can be performed widely are needed to move this field forward and permit effective assessment of the therapeutic potential of HDL-targeted therapies.

LDL-cholesterol lowering or HDL-cholesterol raising for cardiovascular prevention

A number of reports have indicated that both lowering low density lipoprotein (LDL)-cholesterol and raising high density lipoprotein (HDL)-cholesterol can result in significant cardiovascular benefit, both in terms of reduction of events and also, to a variable extent, of atheromatous lesions. LDL and HDL have opposite roles in body cholesterol regulation and, in theory, both reduced deposition (LDL reduction) and increased removal (raised HDL) can improve vascular disease. A number of reports over the last 30 years have attempted to quantitate with cholesterol balance/turnover studies, the correlations between LDL and HDL levels and body cholesterol pool sizes. More recently, these studies have evaluated the effects of LDL or HDL changes on cholesterol elimination. Data have, at times, been fully consistent with theoretical expectations, whereas at others they have not. Evaluation of these, at times, historical data provides, however, an important clue to the understanding of current results with different medications for the management of lipoprotein disorders.

Quantitative trait loci that determine lipoprotein cholesterol levels in DBA/2J and CAST/Ei inbred mice

To investigate genetic contributions to individual variations of lipoprotein cholesterol concentrations, we performed quantitative trait locus/loci (QTL) analyses of an intercross of CAST/Ei and DBA/2J inbred mouse strains after feeding a high-cholesterol cholic acid diet for 10 weeks. In total, we identified four QTL for HDL cholesterol. Three of these were novel and were named Hdlq10 [20 centimorgans (cM), chromosome 4], Hdlq11 (48 cM, chromosome 6), and Hdlq12 (68 cM, chromosome 6). The fourth QTL, Hdl1 (48 cM, chromosome 2), confirmed a locus discovered previously using a breeding cross that employed different inbred mouse strains. In addition, we identified one novel QTL for total and non-HDL cholesterol (8 cM, chromosome 9) that we named Chol6. Hdlq10, colocalized with a mutagenesis-induced point mutation (Lch), also affecting HDL. We provide molecular evidence for Abca1 as the gene underlying Hdlq10 and Ldlr as the gene underlying Chol6 that, coupled with evidence generated by other researchers using knockout and transgenic models, causes us to postulate that polymorphisms of these genes, different from the mutations leading to Tangier's disease and familial hypercholesterolemia, respectively, are likely primary genetic determinants of quantitative variation of lipoprotein levels in mice and, by orthology, in the human population.

Human serum Paraoxonase/Arylesterase's retained hydrophobic N-terminal leader sequence associates with HDLs by binding phospholipids : apolipoprotein A-I stabilizes activity

In serum, human paraoxonase/arylesterase (PON1) is found exclusively associated with high density lipoprotein (HDL) and contributes to its antiatherogenic properties by inhibiting low density lipoprotein (LDL) oxidation. Difficulties in purifying PON1 from apolipoprotein A-I (apoA-I) suggested that PON1's association with HDL may occur through a direct binding between these 2 proteins. An unusual property of PON1 is that the mature protein retains its hydrophobic N-terminal signal sequence. By expressing in vitro a mutant PON1 with a cleavable N-terminus, we demonstrate that PON1 associates with lipoproteins through its N-terminus by binding phospholipids directly rather than binding apoA-I. Nonetheless, apoA-I stabilized arylesterase activity more than did phospholipid alone, apoA-II, or apoE. Consequently, we studied the role of apoA-I in PON1 expression and HDL association in mice genetically deficient in apoA-I. Though present in HDL fractions at decreased levels, PON1 arylesterase activity was less stable than in control mice. Furthermore, PON1 could be competitively removed from HDL by phospholipids, suggesting that PON1's retained N-terminal peptide allows transfer of the enzyme between phospholipid surfaces. Thus, our data suggest that PON1 is stabilized by apoA-I, and its binding to HDL and physiological distribution are dependent on the direct binding of the retained hydrophobic N-terminus to phospholipids optimally presented in association with apoA-I.

Is reverse cholesterol transport a misnomer for suggesting its role in the prevention of atheroma formation?

Reverse cholesterol transport from peripheral tissues, including the arterial wall, involves high density lipoprotein (HDL) uptake of unesterified cell cholesterol, its esterification by lecithin-cholesterol-acyl-transferase (LCAT), direct HDL-cholesteryl ester uptake by the liver and the indirect pathway consisting of the cholesteryl ester transfer protein (CETP)-mediated transfer of HDL-cholesteryl ester to apolipoprotein (apo) B-containing lipoproteins (very low density lipoprotein (VLDL) and LDL). Although the first route should be regarded as anti-atherogenic, ambiguous interpretations are drawn from the indirect pathway since it is potentially atherogenic to the extent that it may raise the plasma cholesteryl ester concentration in lipoproteins that are taken up by arterial wall macrophages. In addition, controversial roles are played in reverse cholesterol transport by LCAT and liver uptake of HDL-cholesteryl ester mediated by hepatic lipase (HL). HDL may exert several antiatherogenic effects unrelated to its role in cell cholesterol removal.

Effects of exercise and fat ingestion on high density lipoprotein production by peripheral tissues

The peripheral production of high density lipoprotein (HDL) cholesterol and of the subclasses HDL2 and HDL3 was assessed by measurement of the arteriovenous fluxes across the human forearm, at rest and after 20 min isometric exercise in the forearm. Eight subjects were studied twice--fasting and after a high-fat meal--and one other subject was studied only after fat loading. In the fasted state the net fluxes of HDL2 and HDL3 cholesterol were slightly negative in the resting forearm, but they became positive during exercise, indicating greater production during short-term muscular activity. The effect of exercise, particularly that on HDL3 cholesterol, was greatly increased by a high-fat meal; the difference in HDL3 cholesterol arteriovenous flux between rest and exercise was significant (-0.06 [SEM 0.05] vs 0.51 [0.17] mumol/100 ml forearm/min). By contrast, there was no peripheral production of HDL2 or HDL3 cholesterol during exercise in two patients with lipoprotein lipase deficiency. These findings suggest that formation of HDL3 during lipolysis by lipoprotein lipase in the muscle capillary bed is influenced by the supply of chylomicrons and other lipoprotein substrates for this enzyme. Muscle blood flow may therefore be an important determinant of HDL production by this mechanism. The effect of exercise in raising HDL cholesterol, and the inverse relation between exercise and coronary heart disease, may be partly the result of this process.

On the associations of body cholesterol pool size with age, HDL cholesterol and plasma total cholesterol concentration in humans

Data from 17 subjects, in whom cholesterol kinetics had been measured by two-pool analysis of medium-term plasma cholesterol specific activity-time curves, were examined by multiple linear regression to explore the determinants of the size of the slowly exchanging cholesterol pool (MBmin) in humans. Pool size was independently and positively related to body weight (regression coefficient, 0.94 g per kg; P = 0.05) and age (1.77 g per year; P = 0.02). After allowance for these effects, MBmin retained a significant negative association with the plasma high density lipoprotein (HDL) cholesterol concentration (-0.56 g per mg/dl; P = 0.03), but was unrelated to plasma total cholesterol. This result is consistent with published data on the composition of those human tissues whose cholesterol is known to be largely a component of the slowly exchanging pool. It differs, however, from that of a recent study of cholesterol turnover [Blum et al, J. Lipid Res., 1985; 26: 1079-1088] in which pool size, measured by three-pool analysis of long-term decay curves, was unrelated to HDL and directly related to plasma total cholesterol. On the basis of other published data, it is considered that this discrepancy is unlikely to be a consequence of the difference between our respective studies in the duration and method of analysis of the specific activity decay curves. Differences in the variances of HDL cholesterol and plasma total cholesterol concentration that were examined, and in the biochemical-genetic factors underlying these variances, provide a more likely explanation. The overall weight of evidence favours the view that the pool of slowly exchangeable cholesterol in many human tissues expands during ageing at a rate which is increased in the presence of severe hypercholesterolemia, and which under some, but not all, circumstances also varies inversely with HDL cholesterol. The critical components of HDL metabolism which affect this process remain to be identified.

Plasma lipid concentrations: the concept of "normality" and its implications for detection of high cardiovascular risk

The relation between serum cholesterol concentrations and the incidence of coronary heart disease is continuous and curvilinear; there is neither epidemiological nor biological evidence to support the existence of a threshold value. There is a clinical need, however, for an acceptable definition of action limits and desirable ranges, based on the evidence that raised cholesterol concentrations are causally related to atherosclerotic heart disease. The European Atherosclerosis Society has proposed a set of cut off points, which, together with age and the presence of other risk factors, direct the clinician to an appropriate level of treatment. Because the changes of serum cholesterol during adult life appear unphysiological, these action limits do not require adjustment for age. The distribution of serum cholesterol in the United Kingdom population is such that a case finding strategy is required to identify the many persons at very high risk of coronary disease. Measurements of triglyceride, high density lipoprotein, apolipoproteins, and the investigation of hyperlipoproteinemia are informative but less mandatory.

High-density lipoprotein: a major risk factor for coronary atherosclerosis

The high-density lipoproteins (HDL) are a polydisperse family of lipid--protein complexes whose principal functions in lipid transport are: (1) to act as a reservoir of C apoproteins required for triglyceride transport; (2) to act as a 'scavenger' of surplus cholesterol and phospholipid liberated from lipolysed triglyceride-rich lipoproteins; and (3) to transport surplus cholesterol from peripheral tissues to the liver for excretion and catabolism (reverse cholesterol transport), both directly and indirectly via other lipoproteins and the lipid transfer protein. The concentration of HDL cholesterol (mostly cholesteryl ester) has been found to be a strong risk factor for coronary atherosclerosis, and its clinical complications in most industrialized communities have been studied. The association with disease risk is independent of other lipoproteins and risk factors, has been found in both sexes, and persists following reduction of plasma lipids by diet and certain drugs. It is not yet clear whether or not certain HDL subclasses and/or apoproteins are better predictors of risk than HDL cholesterol. Indirect evidence from clinical studies and data from animal experiments suggests that certain pharmacologically induced increases in HDL cholesterol concentration are associated with a reduction of atherogenesis. However, the mechanism of the link between HDL and atherogenesis is not yet clear: although the original suggestion that it reflects the function of HDL in reverse cholesterol transport remains plausible, alternative mechanisms are possible. These include effects of HDL on platelet function and prostacyclin synthesis. Alternatively, the association might be indirect, reflecting an atherogenic effect of triglyceride-rich lipoproteins and/or their remnants, the plasma concentrations of which are correlated with HDL cholesterol.