Expression of human hepatic lipase in the rabbit model preferentially enhances the clearance of triglyceride-enriched versus native high-density lipoprotein apolipoprotein A-I

White Adipose Tissue Dysfunction: Pathophysiology and Emergent Measurements

White adipose tissue (AT) dysfunction plays an important role in the development of cardiometabolic alterations associated with obesity. AT dysfunction is characterized by the loss of the expansion capacity of the AT, an increment in adipocyte hypertrophy, and changes in the secretion profile of adipose cells, associated with accumulation of macrophages and inflammation. Since not all people with an excess of adiposity develop comorbidities, it is necessary to find simple tools that can evidence AT dysfunction and allow the detection of those people with the potential to develop metabolic alterations. This review focuses on the current pathophysiological mechanisms of white AT dysfunction and emerging measurements to assess its functionality.

HDL functionality in type 1 and type 2 diabetes: new insights

PURPOSE OF REVIEW

To critically appraise new insights into HDL structure and function in type 1 diabetes (T1DM) and type 2 diabetes (T2DM).

RECENT FINDINGS

In young T1DM patients with early renal impairment and a high inflammatory score, both HDL antioxidative activity and endothelial vasodilatory function were impaired, revealing a critical link between HDL dysfunction, subclinical vascular damage, systemic inflammation and end organ damage. HDL may inhibit development of T2DM by attenuating endoplasmic reticulum (ER) stress and apoptotic loss of pancreatic β-cells, an effect due in part to ABC transporter-mediated efflux of specific oxysterols with downstream activation of the hedghehog signalling receptor, Smoothened. The apoM-sphingosine-1-phosphate complex is critical to HDL antidiabetic activity, encompassing protection against insulin resistance, promotion of insulin secretion, enhanced β-cell survival and inhibition of hepatic glucose production. Structure-function studies of HDL in hyperglycemic, dyslipidemic T2DM patients revealed both gain and loss of lipidomic and proteomic components. Such changes attenuated both the optimal protective effects of HDL on mitochondrial function and its capacity to inhibit endothelial cell apoptosis. Distinct structural components associated with individual HDL functions.

SUMMARY

Extensive evidence indicates that both the proteome and lipidome of HDL are altered in T1DM and T2DM, with impairment of multiple functions.

Obesity-Related Changes in High-Density Lipoprotein Metabolism and Function

In obese individuals, atherogenic dyslipidemia is a very common and important factor in the increased risk of cardiovascular disease. Adiposity-associated dyslipidemia is characterized by low high-density lipoprotein cholesterol (HDL-C) levels and an increase in triglyceride-rich lipoproteins. Several factors and mechanisms are involved in lowering HDL-C levels in the obese state and HDL quantity and quality is closely related to adiponectin levels and the bioactive lipid sphingosine-1-phosphate. Recent studies have shown that obesity profoundly alters HDL metabolism, resulting in altered HDL subclass distribution, composition, and function. Importantly, weight loss through gastric bypass surgery and Mediterranean diet, especially when enriched with virgin olive oil, is associated with increased HDL-C levels and significantly improved metrics of HDL function. A thorough understanding of the underlying mechanisms is crucial for a better understanding of the impact of obesity on lipoprotein metabolism and for the development of appropriate therapeutic approaches. The objective of this review article was to summarize the newly identified changes in the metabolism, composition, and function of HDL in obesity and to discuss possible pathophysiological consequences.

Transcriptomic analysis of the liver of cholesterol-fed rabbits reveals altered hepatic lipid metabolism and inflammatory response

Rabbits are a suitable animal model for atherosclerosis due to their sensitivity to dietary cholesterol. Moreover, rabbits have lipoprotein profiles that are more similar to humans than those of other laboratory animals. However, little is known about the transcriptomic information related to atherosclerosis in rabbits. We aimed to determine the changes in the livers of rabbits fed a normal chow diet (control) or high cholesterol diet (HCD) by histological examinations and RNA sequencing analysis. Compared with the control group, the lipid levels and small LDL subfractions in plasma were increased, and aortic atherosclerotic plaques were formed in the HCD group. Most importantly, HCD resulted in lipid accumulation and inflammation in the livers. Transcriptomic analysis of the liver showed that HCD induces 1183 differentially expressed genes (DEGs) that mainly participate in the regulation of inflammation and lipid metabolism. Furthermore, the signaling pathways involved in inflammation and lipid metabolism were enriched by KEGG pathway analysis. In addition, hepatic DEGs of the HCD group were further validated by real-time PCR. These results suggest that HCD causes liver lipid accumulation and inflammatory response. Although the relationships between these hepatic changes and atherogenesis need further investigation, these findings provide a fundamental framework for future research on human atherosclerosis using rabbit models.

Bone marrow-derived HL mitigates bone marrow-derived CETP-mediated decreases in HDL in mice globally deficient in HL and the LDLr

The objective of this study was to determine the combined effects of HL and cholesteryl ester transfer protein (CETP), derived exclusively from bone marrow (BM), on plasma lipids and atherosclerosis in high-fat-fed, atherosclerosis-prone mice. We transferred BM expressing these proteins into male and female double-knockout HL-deficient, LDL receptor-deficient mice (HL(-/-)LDLr(-/-)). Four BM chimeras were generated, where BM-derived cells expressed 1) HL but not CETP, 2) CETP and HL, 3) CETP but not HL, or 4) neither CETP nor HL. After high-fat feeding, plasma HDL-cholesterol (HDL-C) was decreased in mice with BM expressing CETP but not HL (17 ± 4 and 19 ± 3 mg/dl, female and male mice, respectively) compared with mice with BM expressing neither CETP nor HL (87 ± 3 and 95 ± 4 mg/dl, female and male mice, respectively, P < 0.001 for both sexes). In female mice, the presence of BM-derived HL mitigated this CETP-mediated decrease in HDL-C. BM-derived CETP decreased the cholesterol component of HDL particles and increased plasma cholesterol. BM-derived HL mitigated these effects of CETP. Atherosclerosis was not significantly different between BM chimeras. These results suggest that BM-derived HL mitigates the HDL-lowering, HDL-modulating, and cholesterol-raising effects of BM-derived CETP and warrant further studies to characterize the functional properties of these protein interactions.

Elevated cholesteryl ester transfer protein (CETP) activity, a major determinant of the atherogenic dyslipidemia, and atherosclerotic cardiovascular disease in South Asians

AIMS

Why South Asians are at increased risk of premature atherosclerotic cardiovascular diseases compared with other ethnic groups is not fully understood. Atherogenic dyslipoproteinemia - hypertriglyceridemia, elevated numbers of low-density lipoprotein (LDL) particles and low high-density lipoprotein cholesterol (HDL-C) - is more common in South Asians but the mechanisms responsible have not been explicated. Here we examined whether the circulating lipid transfer protein, cholesteryl ester transfer protein (CETP), plays a role in the pathogenesis of the atherogenic dyslipoproteinemia among South Asians.

METHODS AND RESULTS

CETP activity was determined by exogenous substrate assay in the serum of healthy, metabolically well-characterized individuals of South Asian and European descent (N = 244 and 238, respectively). Serum and lipoprotein lipids and apolipoproteins were measured and lipoprotein particle number and size were quantified via nuclear magnetic resonance spectroscopy. All the elements of the atherogenic dyslipoproteinemia were more severe in South Asians and CETP activity was significantly greater by 30% in South Asians compared with Europeans, adjusted for age, sex, body mass index and waist circumference (p < 0.0001). CETP activity was directly associated with serum triglycerides and inversely with HDL-C in the whole population. CETP activity was also directly related to apoB and LDL particle number. Finally, increased CETP activity was associated with pro-atherogenic reductions in HDL and LDL particle size.

CONCLUSIONS

We identified novel associations between elevated CETP activity and the triad of quantitative and qualitative lipoprotein abnormalities in the atherogenic dyslipidemia in South Asians, a major contributor of increased atherosclerotic cardiovascular diseases in South Asians.

Oxidized high-density lipoprotein is associated with increased plasma glucose in non-diabetic dyslipidemic subjects

BACKGROUND

Oxidized high-density lipoprotein (oxHDL) has reduced capacity for cholesterol efflux and some of other anti-atherogenic properties of HDL, but the role of oxHDL in the pathogenesis of cardiometabolic disease has not been fully demonstrated. This study investigated the association of oxHDL with plasma glucose (PG) and the other atherosclerotic risk variables in non-diabetic dyslipidemic subjects.

METHODS

Conventional atherosclerotic markers and LDL particle size (LDL-PS), as determined by gel electrophoresis, were measured in 155 non-diabetic subjects (mean age of 57 years) with dyslipidemia. Serum oxHDL levels were quantified using an antibody against oxidized human apoA-I in a sandwich ELISA format.

RESULTS

Multiple regression analysis adjusted for possible confounders revealed that HDL-cholesterol was independently, significantly and positively correlated with LDL-PS and oxHDL. By multiple regression analysis, oxHDL was independently, significantly and positively correlated with fasting PG (β=0.19, P=0.01). Subjects in the highest PG tertile group had approximately 30% higher oxHDL levels than the lowest PG tertile group.

CONCLUSIONS

These results suggest that high PG levels may contribute to the HDL oxidation, irrespective of HDL-cholesterol levels, even in non-diabetic subjects with dyslipidemia, and that the measurement of oxHDL may be a useful marker of dysfunctional HDL.

Liver triacylglycerol lipases

The hallmark of obesity and one of the key contributing factors to insulin resistance, type 2 diabetes and cardiovascular disease is excess triacylglycerol (TG) storage. In hepatocytes, excessive accumulation of TG is the common denominator of a wide range of clinicopathological entities known as nonalcoholic fatty liver disease, which can eventually progress to cirrhosis and associated complications including hepatic failure, hepatocellular carcinoma and death. A tight regulation between TG synthesis, hydrolysis, secretion and fatty acid oxidation is required to prevent lipid accumulation as well as lipid depletion from hepatocytes. Therefore, understanding the pathways that regulate hepatic TG metabolism is crucial for development of therapies to ameliorate pathophysiological conditions associated with excessive hepatic TG accumulation, including dyslipidemias, viral infection and atherosclerosis. This review highlights the physiological roles of liver lipases that degrade TG in cytosolic lipid droplets, endoplasmic reticulum, late endosomes/lysosomes and along the secretory route. This article is part of a Special Issue entitled Triglyceride Metabolism and Disease.

Impact of serum amyloid A on high density lipoprotein composition and levels

Serum amyloid A (SAA) is an acute-phase protein mainly associated with HDL. To study the role of SAA in mediating changes in HDL composition and metabolism during inflammation, we generated mice in which the two major acute-phase SAA isoforms, SAA1.1 and SAA2.1, were deleted [SAA knockout (SAAKO) mice], and induced an acute phase to compare lipid and apolipoprotein parameters between wild-type (WT) and SAAKO mice. Our data indicate that SAA does not affect apolipoprotein A-I (apoA-I) levels or clearance under steady-state conditions. HDL and plasma triglyceride levels following lipopolysaccharide administration, as well as the decline in liver expression of apoA-I and apoA-II, did not differ between both groups of mice. The expected size increase of WT acute-phase HDL was surprisingly also seen in SAAKO acute-phase HDL despite the absence of SAA. HDLs from both mice showed increased phospholipid and unesterified cholesterol content during the acute phase. We therefore conclude that in the mouse, SAA does not impact HDL levels, apoA-I clearance, or HDL size during the acute phase and that the increased size of acute-phase HDL in mice is associated with an increased content of surface lipids, particularly phospholipids, and not surface proteins. These data need to be transferred to humans with caution due to differences in apoA-I structure and remodeling functions.

CETP does not affect triglyceride production or clearance in APOE*3-Leiden mice

The cholesteryl ester transfer protein (CETP) facilitates the bidirectional transfer of cholesteryl esters and triglycerides (TG) between HDL and (V)LDL. By shifting cholesterol in plasma from HDL to (V)LDL in exchange for VLDL-TG, CETP aggravates atherosclerosis in hyperlipidemic APOE*3-Leiden (E3L) mice. The aim of this study was to investigate the role of CETP in TG metabolism and high-fat diet-induced obesity by using E3L mice with and without the expression of the human CETP gene. On chow, plasma lipid levels were comparable between both male and female E3L and E3L.CETP mice. Further mechanistic studies were performed using male mice. CETP expression increased the level of TG in HDL. CETP did not affect the postprandial plasma TG response or the hepatic VLDL-TG and VLDL-apolipoprotein B production rate. Moreover, CETP did not affect the plasma TG clearance rate or organ-specific TG uptake after infusion of VLDL-like emulsion particles. In line with the absence of an effect of CETP on tissue-specific TG uptake, CETP also did not affect weight gain in response to a high-fat diet. In conclusion, the CETP-induced increase of TG in the HDL fraction of E3L mice is not associated with changes in the production of TG or with tissue-specific clearance of TG from the plasma.

Leukocyte-derived hepatic lipase increases HDL and decreases en face aortic atherosclerosis in LDLr-/- mice expressing CETP

In addition to hepatic expression, cholesteryl ester transfer protein (CETP) and hepatic lipase (HL) are expressed by human macrophages. The combined actions of these proteins have profound effects on HDL structure and function. It is not known how these HDL changes influence atherosclerosis. To elucidate the role of leukocyte-derived HL on atherosclerosis in a background of CETP expression, we studied low density lipoprotein receptor-deficient mice expressing human CETP (CETPtgLDLr -/-) with a leukocyte-derived HL deficiency (HL -/- BM). HL(-/-) bone marrow (BM), CETPtgLDLr(-/-) mice were generated via bone marrow transplantation. Wild-type bone marrow was transplanted into CETPtgLDLr(-/-) mice to generate HL +/+ BM, CETPtgLDLr(-/-) controls. The chimeras were fed a high-fat, high-cholesterol diet for 14 weeks to promote atherosclerosis. In female HL(-/-) BM, CETPtgLDLr(-/-) mice plasma HDL-cholesterol concentration during high-fat feeding was decreased 27% when compared with HL +/+ BM, CETPtgLDLr(-/-) mice (P < 0.05), and this was associated with a 96% increase in en face aortic atherosclerosis (P < 0.05). In male CETPtgLDLr(-/-) mice, leukocyte-derived HL deficiency was associated with a 16% decrease in plasma HDL-cholesterol concentration and a 25% increase in aortic atherosclerosis. Thus, leukocyte-derived HL in CETPtgLDLr(-/-) mice has an atheroprotective role that may involve increased HDL levels.

Enhanced cellular uptake of remnant high-density lipoprotein particles: a mechanism for high-density lipoprotein lowering in insulin resistance and hypertriglyceridemia

A low level of high-density lipoprotein (HDL) cholesterol is characteristic of insulin resistance and hypertriglyceridemia and likely contributes to the increased risk of cardiovascular disease associated with these conditions. One pathway involves enhanced clearance of lipolytically modified HDL particles, but the underlying mechanisms remain poorly understood. Here, we examine the effect of triglyceride enrichment and hepatic lipase hydrolysis on HDL binding, internalization, and degradation in cultured liver and kidney cells. Maximal binding of remnant HDL (HDL enriched with triglycerides followed by hepatic lipase hydrolysis), but not binding affinity, was markedly higher than native and triglyceride-rich HDL in both HepG2 cells and HEK293 cells. Compared with native and triglyceride-rich HDL, remnant HDL was internalized to a greater extent in both cell types and was more readily degraded in HEK293 cells. The increased binding of remnant HDL was not mediated by the low-density lipoprotein receptor or scavenger receptor class B type I (SR-BI), because enhanced remnant HDL binding was observed in low-density lipoprotein receptor-deficient cells with or without SR-BI overexpression. Disruption of cell surface heparan sulfate proteoglycans or blockage of apolipoprotein E-mediated lipoprotein binding also did not abolish the enhanced remnant HDL binding. Our observations indicate that remodeling of triglyceride-enriched HDL by hepatic lipase may result in enhanced binding, internalization, and degradation in tissues involved in HDL catabolism, contributing to rapid clearance and overall lowering of plasma HDL cholesterol in insulin resistance and hypertriglyceridemia.

Obesity and atherogenic dyslipidemia

Obesity is associated with an increased risk of coronary heart disease, in part due to its strong association with atherogenic dyslipidemia, characterized by high triglycerides and low high-density lipoprotein (HDL) cholesterol. There has been substantial research effort focused on the mechanisms of the link between obesity and atherogenic dyslipidemia, both in the absence and presence of insulin resistance. After a brief overview of the epidemiology of atherogenic dyslipidemia, this article details the known molecular mechanisms of adipocyte function and its relationship to apoB-containing lipoprotein assembly and metabolism, both in the healthy as well as in the obese states. We also discuss the pathophysiology of low HDL cholesterol in obesity and the implications for cardiovascular disease risk.

Genetic etiology of isolated low HDL syndrome: incidence and heterogeneity of efflux defects

OBJECTIVE

We have used a multitiered approach to identify genetic and cellular contributors to high-density lipoprotein (HDL) deficiency in 124 human subjects.

METHODS AND RESULTS

We resequenced 4 candidate genes for HDL regulation and identified several functional nonsynonymous mutations including 2 in apolipoprotein A-I (APOA1), 4 in lecithin:cholesterol acyltransferase (LCAT), 1 in phospholipid transfer protein (PLTP), and 7 in the ATP-binding cassette transporter ABCA1, leaving 88% (110/124) of HDL deficient subjects without a genetic diagnosis. Cholesterol efflux assays performed using cholesterol-loaded monocyte-derived macrophages from the 124 low HDL subjects and 48 control subjects revealed that 33% (41/124) of low HDL subjects had low efflux, despite the fact that the majority of these subjects (34/41) were not carriers of dysfunctional ABCA1 alleles. In contrast, only 2% of control subjects presented with low efflux (1/48). In 3 families without ABCA1 mutations, efflux defects were found to cosegregate with low HDL.

CONCLUSIONS

Efflux defects are frequent in low HDL syndromes, but the majority of HDL deficient subjects with cellular cholesterol efflux defects do not harbor ABCA1 mutations, suggesting that novel pathways contribute to this phenotype.

Determinants of plasma HDL concentrations and reverse cholesterol transport

PURPOSE OF REVIEW

One of the major mechanisms whereby HDL particles are felt to protect against atherosclerosis is that of reverse cholesterol transport from atherosclerotic lesion macrophages to the liver, with subsequent excretion of cholesterol in the bile. This review focuses on recent progress in our understanding of reverse cholesterol transport and the factors that determine plasma HDL cholesterol concentrations.

RECENT FINDINGS

The liver and intestine are the major sites of apolipoprotein A-I synthesis and nascent HDL particle secretion. The liver has recently been shown to be a major contributor to the plasma HDL-cholesterol concentration, but the precise site or mechanism whereby hepatically-synthesized HDL acquire the bulk of their lipid content remains to be determined. Contrastingly, macrophages contribute little to the plasma HDL cholesterol pool, whereas the quantitatively small macrophage-specific reverse cholesterol transport contributes disproportionately to protection against atherosclerosis. Studies have highlighted the coordinate action of cell surface lipid transporters, cholesterol esterification enzymes and lipid transfer factors in the early steps of reverse cholesterol transport and the recycling of pre-beta HDL particles to create a ready supply of cholesterol acceptor HDL particles. Most of the variation in plasma HDL-cholesterol levels in human populations is accounted for by variations in HDL clearance rather than production.

SUMMARY

Our understanding of the in-vivo metabolism of HDL particles and their role in reverse cholesterol transport is rapidly evolving, with long-standing concepts being constantly challenged by emerging evidence. An in-depth understanding of HDL metabolism will guide the rational design of novel pharmacological therapies that effectively protect against atherosclerosis.

Different VLDL apo B, and HDL apo AI and apo AII metabolism in two heterozygous carriers of unrelated mutations in the lipoprotein lipase gene

BACKGROUND

Lipoprotein lipase (LPL) deficiency has been suggested as a cause of low HDL-cholesterol (HDL-C) plasma levels, by a mechanism that involves an enhanced catabolism of HDL apolipoprotein (apo) AI. To verify the role of 2 different LPL gene mutations on HDL metabolism, we studied the in vivo turnover of the apo AI and apo AII in heterozygous carriers of LPL deficiency.

METHODS

Apo AI and AII kinetics were studied by a 10-h primed constant infusion of 5,5,5-2H3-leucine approach in 2 carriers, 1 man (patient 1) and 1 woman (patient 2), and 5 control subjects. The rates of HDL apolipoproteins production (PR) and catabolism (FCR) were estimated using a one-compartment model-based analysis.

RESULTS

Both carriers had low HDL-C plasma levels and only patient 1 was hypertriglyceridemic. VLDL apo B was 4-times slower in patient 1 as compared to patient 2. The FCRs of apo AI in both carriers was within the range of the controls (0.200, 0.221 and 0.211+/-0.051 day(-1), respectively). Apo AII FCR in patient 1 was about 20% lower than the mean of the control group whereas being normal in patient 2. Apo AI PR in patient 1 (9.20 mg kg(-1) day(-1)) was below the lowest value in controls (range, 10.52-13.24 mg kg(-1) day(-1)) whereas in patient 2 it was normal. Apo AII PR in both patients was similar to controls.

CONCLUSION

The heterozygous carriers of 2 different mutations in the LPL gene had different VLDL apo B FCR, and from normal to slightly low HDL apolipoprotein FCR and PR. These results disagree with the putative enhanced apo AI FCR in LPL deficient patients and suggest the need to reconsider the effects of LPL activity on HDL metabolism.

Thematic review series: patient-oriented research. What have we learned about HDL metabolism from kinetics studies in humans?

Plasma measurements of lipids, lipoproteins, and apolipoproteins provide information on the static levels of these fractions without providing key information on the dynamic fluxes of lipoproteins in the circulation. Kinetics studies, in contrast, provide additional information on the production and clearance rates of lipoproteins and the flow of lipids and apolipoproteins through lipoprotein fractions. This information is crucial in accurately delineating the metabolism of HDL in plasma, because plasma concentrations of HDL are the net result of the de novo production and catabolism of HDL as well as the recycling of HDL particles and the contribution to HDL from components of other lipoproteins. Studies aimed at measuring the metabolism of HDL particles have shown that HDL metabolism in vivo is complex and consists of multiple components. Kinetics studies provide a window into the metabolism of HDL, allowing us to better understand the mechanisms of HDL decrease in human conditions and the functionality of HDL particles. Here, we review the progress in our understanding of HDL metabolism derived from in vivo kinetics studies, focusing primarily on studies in humans but also reviewing key studies in animal models.

Common genetic and environmental effects on lipid phenotypes: the HERITAGE family study

OBJECTIVE

Despite the well known genetic component influencing plasma lipid-lipoprotein levels and the observed correlations among these traits, little is known about pleiotropic heritable determinants among them. Our aim is to investigate pair-wise polygenic and environmental correlations among lipid-lipoprotein levels at baseline and in response to regular exercise in Whites and Blacks.

METHODS

Common pair-wise genetic and environmental correlations among levels of total cholesterol (TC), LDL-C, ApoB, HDL-C (also HDL2-C and HDL3-C), triglycerides (TG, HDL-TG and LDL-TG) and ApoA-1 were investigated at baseline and again after a 20-week endurance exercise program using a variance-components-decomposition.

RESULTS

With a few exceptions, all lipid phenotypes were heritable at baseline and for training responses in Blacks and Whites. Strong to high genetic and environmental correlations (0.4 < rho(g) < 0.7) were observed for the majority of the baseline pair-wise traits. For training responses, many of the same patterns were noted, although fewer genetic correlations were significant as compared to the baseline results.

CONCLUSIONS

Results suggest that the observed phenotypic correlations among many of these traits may be due to in part to pleiotropic genes, in particular between LDL-C and ApoB and between TG and HDL-C. This shared genetic architecture should be considered in follow-up gene finding studies.

New Insights Into the Regulation of HDL Metabolism and Reverse Cholesterol Transport

The metabolism of high-density lipoproteins (HDL), which are inversely related to risk of atherosclerotic cardiovascular disease, involves a complex interplay of factors regulating HDL synthesis, intravascular remodeling, and catabolism. The individual lipid and apolipoprotein components of HDL are mostly assembled after secretion, are frequently exchanged with or transferred to other lipoproteins, are actively remodeled within the plasma compartment, and are often cleared separately from one another. HDL is believed to play a key role in the process of reverse cholesterol transport (RCT), in which it promotes the efflux of excess cholesterol from peripheral tissues and returns it to the liver for biliary excretion. This review will emphasize 3 major evolving themes regarding HDL metabolism and RCT. The first theme is that HDL is a universal plasma acceptor lipoprotein for cholesterol efflux from not only peripheral tissues but also hepatocytes, which are a major source of cholesterol efflux to HDL. Furthermore, although efflux of cholesterol from macrophages represents only a tiny fraction of overall cellular cholesterol efflux, it is the most important with regard to atherosclerosis, suggesting that it be specifically termed macrophage RCT. The second theme is the critical role that intravascular remodeling of HDL by lipid transfer factors, lipases, cell surface receptors, and non-HDL lipoproteins play in determining the ultimate metabolic fate of HDL and plasma HDL-c concentrations. The third theme is the growing appreciation that insulin resistance underlies the majority of cases of low HDL-c in humans and the mechanisms by which insulin resistance influences HDL metabolism. Progress in our understanding of HDL metabolism and macrophage reverse cholesterol transport will increase the likelihood of developing novel therapies to raise plasma HDL concentrations and promote macrophage RCT and in proving that these new therapeutic interventions prevent or cause regression of atherosclerosis in humans.

Evidence of QTLs on chromosomes 13q and 14q for triglycerides before and after 20 weeks of exercise training: the HERITAGE Family Study

Fasting triglyceride (TG) levels in total plasma and in lipoprotein subfractions were assessed both at baseline and after a 20-week exercise training intervention in 99 White and 101 Black families. A genome-wide multipoint variance component linkage analysis was performed separately by race, using 509 markers. The strongest evidence of linkage was for the TG subfractions of low-density lipoprotein (LDL-TG) and high-density lipoprotein (HDL-TG) rather than for overall levels of TG. For baseline levels, the maximum LOD score was 3.8 on 13q12-q14 with HDL-TG in Whites. Additional linkage evidence was found on 14q31 (LOD=3.2) and 10p14 (LOD=2.9) for baseline LDL-TG in Whites. Suggestive linkage signal at baseline in Whites was detected for HDL-TG (LOD=2.6) on 12q24 and for LDL-TG on 19p13. For training response in Whites, suggestive signal (LOD=2.2) was observed on 13q12-q14 with LDL-TG and for HDL-TG on 10q23. For Blacks, weak signals (LODs<2.0) were found either for baseline and responses to training, perhaps due to small sample sizes that reduced the power of the linkage analysis. These results represent the first report of linkage for the lipoprotein subfractions and for the lipid and lipoprotein responses to exercise training. It is interesting that the strongest signals were found for the LDL-TG and HDL-TG subfractions, given their particular relationships with the atherogenic lipid profile including dense LDL and HDL particles.

Genome-wide linkage analysis replicates susceptibility locus for fasting plasma triglycerides: NHLBI Family Heart Study

Recent reports implicate chromosomal regions linked to inter-individual variation in plasma triglycerides. We conducted genome-wide scans to replicate these linkages and/or identify other loci influencing plasma triglycerides in the NHLBI Family Heart Study (FHS). Data were obtained for 501 three-generational families. Genotyping was done by the Utah Molecular Genetics Laboratory and NHLBI Mammalian Genotyping Service; markers from both were placed on one genetic map. Analysis was done using multipoint variance components linkage. Fasting plasma triglycerides were log-transformed and age-, sex-, and field center-adjusted; suggestive linkage evidence was found on chromosome 8 (LOD=2.80 at 89 cM, marker D8S1141). Further adjustment for waist girth, BMI, diabetes, hypertension, and lipid-lowering drugs suggested linkage regions on chromosomes 6 (LOD=2.29 at 79 cM, marker D6S295) and 15 (LOD=1.85 at 43 cM, marker D15S659). Since HDL is correlated with triglycerides and because it was linked to this region on chromosome 15 in FHS, we created a composite triglyceride-HDL phenotype. The combined phenotype LOD score was 3.0 at the same marker on chromosome 15. Chromosome 15 likely harbors a susceptibility locus with an influence on triglycerides and HDL. Regions on chromosomes 6 and 8 may also contain loci contributing to inter-individual variation in plasma triglycerides.

Lipoprotein lipase and its role in regulation of plasma lipoproteins and cardiac risk

For over 50 years, biologists and clinicians have studied lipoprotein lipase (LPL) and learned about its structure, function, cellular production, physiology, and human genetics. LPL is the principal enzyme that removes triglyceride from the bloodstream. It also determines plasma levels of high-density lipoprotein. Surprisingly, within the past several years, a number of new and unexpected proteins have been discovered that regulate the actions of LPL. These include the very low-density lipoprotein receptor, angiopoetin-like protein 3, and apolipoprotein A-V. In addition, mouse genetic studies have confirmed tissue culture findings of nonenzymatic roles of LPL both in lipid metabolism and atherogenesis. These basic observations are now being related to new information on human genetic polymorphism in this gene that is likely to affect clinical evaluation of lipoprotein disorders and cardiac risk.

Evidence that hepatic lipase deficiency in humans is not associated with proatherogenic changes in HDL composition and metabolism

The aim of the present study was to characterize the composition and metabolism of HDL in subjects with complete hepatic lipase (HL) deficiency. Analyses were carried out in three complete and three partial HL-deficient subjects as well as in eight normotriglyceridemic (NTG) and two hypertriglyceridemic controls. Complete HL deficiency was associated with hypertriglyceridemia and with a 3.5-fold increase in HDL-triglyceride (TG) levels. The in vivo kinetics of apolipoprotein A-I (apoA-I) and apoA-II (d < 1.25 g/l) were studied in the fasted state using a primed-constant infusion of l-(5,5,5-D3)leucine for 12 h. Complete HL deficiency was associated with a reduced fractional catabolic rate of apoA-I in the HL-deficient female proband (-47%) and in her two brothers (-21%) compared with gender- and TG-matched controls. Total plasma and HDL from complete HL-deficient patients were able to mediate normal cholesterol efflux from human skin fibroblasts labeled with [3H]cholesterol. Complete HL deficiency was also associated with normal levels of prebeta-migrating apoA-I-containing HDL separated by two-dimensional gel electrophoresis and with an accumulation of large HDL particles compared with NTG controls. These results suggest that HL activity is important for adequate HDL metabolism, although its presence may not be necessary for normal HDL-mediated reverse cholesterol transport.

Formation and Metabolism of Prebeta-Migrating, Lipid-Poor Apolipoprotein A-I

The preferred extracellular acceptor of cell phospholipids and unesterified cholesterol in the process mediated by the ATP-binding cassette A1 (ABCA1) transporter is a monomolecular, prebeta-migrating, lipid-poor or lipid-free form of apolipoprotein (apo) A-I. This monomolecular form of apoA-I is quite distinct from the prebeta-migrating, discoidal high-density lipoprotein (HDL) that contains two or three molecules of apoA-I per particle and which are present as minor components of the HDL fraction in human plasma. The mechanism of the ABCA1-mediated efflux of phospholipid and cholesterol from cells has been studied extensively. In contrast, much less attention has been given to the origin and subsequent metabolism of the acceptor lipid-free/lipid-poor apoA-I. There is a substantial body of evidence from studies conducted in vitro that a monomolecular, lipid-free/lipid-poor form of apoA-I dissociates from HDL during the remodeling of HDLs by plasma factors such as cholesteryl ester transfer protein, hepatic lipase, and phospholipid transfer protein. The rate at which apoA-I dissociates from HDL is influenced by the phospholipid composition of the particles and by the presence of apoA-II. This review describes current knowledge regarding the formation, metabolism, and regulation of monomolecular, lipid-free/lipid-poor apoA-I in plasma.

Mechanisms of HDL lowering in insulin resistant, hypertriglyceridemic states: the combined effect of HDL triglyceride enrichment and elevated hepatic lipase activity

Hypertriglyceridemia, low plasma concentrations of high density lipoproteins (HDL) and qualitative changes in low density lipoproteins (LDL) comprise the typical dyslipidemia of insulin resistant states and type 2 diabetes. Although isolated low plasma HDL-cholesterol (HDL-c) and apolipoprotein A-I (apo A-I, the major apolipoprotein component of HDL) can occur in the absence of hypertriglyceridemia or any other features of insulin resistance, the majority of cases in which HDL-c is low are closely linked with other clinical features of insulin resistance and hypertriglyceridemia. We and others have postulated that triglyceride enrichment of HDL particles secondary to enhanced CETP-mediated exchange of triglycerides and cholesteryl ester between HDL and triglyceride-rich lipoproteins, combined with the lipolytic action of hepatic lipase (HL), are driving forces in the reduction of plasma HDL-c and apoA-I plasma concentrations. The present review focuses on these metabolic alterations in insulin resistant states and their important contributions to the reduction of HDL-c and HDL-apoA-I plasma concentrations.