Increased response to cholesterol feeding in apolipoprotein C1-deficient mice

Genetic deletion of Abcc6 disturbs cholesterol homeostasis in mice

Genetic studies link adenosine triphosphate-binding cassette transporter C6 (ABCC6) mutations to pseudoxanthoma elasticum (PXE). ABCC6 sequence variations are correlated with altered HDL cholesterol levels and an elevated risk of coronary artery diseases. However, the role of ABCC6 in cholesterol homeostasis is not widely known. Here, we report reduced serum cholesterol and phytosterol levels in Abcc6-deficient mice, indicating an impaired sterol absorption. Ratios of cholesterol precursors to cholesterol were increased, confirmed by upregulation of hepatic 3-hydroxy-3-methylglutaryl coenzyme A reductase (Hmgcr) expression, suggesting activation of cholesterol biosynthesis in Abcc6-/- mice. We found that cholesterol depletion was accompanied by a substantial decrease in HDL cholesterol mediated by lowered ApoA-I and ApoA-II protein levels and not by inhibited lecithin-cholesterol transferase activity. Additionally, higher proprotein convertase subtilisin/kexin type 9 (Pcsk9) serum levels in Abcc6-/- mice and PXE patients and elevated ApoB level in knockout mice were observed, suggesting a potentially altered very low-density lipoprotein synthesis. Our results underline the role of Abcc6 in cholesterol homeostasis and indicate impaired cholesterol metabolism as an important pathomechanism involved in PXE manifestation.

Human apolipoprotein C1 transgenesis reduces atherogenesis in hypercholesterolemic rabbits

BACKGROUND AND AIMS

Apolipoprotein (apo) C1 is a 6.6 kDa protein associated with HDL and VLDL. ApoC1 alters triglyceride clearance, and it also favors cholesterol accumulation in HDL, especially by inhibiting CETP in human plasma. Apart from studies in mice, which lack CETP, the impact of apoC1 on atherosclerosis in animal models expressing CETP, like in humans, is not known. This study aimed at determining the net effect of human apoC1 on atherosclerosis in rabbits, a species with naturally high CETP activity but with endogenous apoC1 without CETP inhibitory potential.

METHODS

Rabbits expressing a human apoC1 transgene (HuApoC1Tg) were generated and displayed significant amounts of human apoC1 in plasma.

RESULTS

After cholesterol feeding, atherosclerosis lesions were significantly less extensive (-22%, p < 0.05) and HDL displayed a reduced ability to serve as CETP substrates (-25%, p < 0.05) in HuApoC1Tg rabbits than in WT littermates. It was associated with rises in plasma HDL cholesterol level and PON-1 activity, and a decrease in the plasma level of the lipid oxidation markers 12(S)-HODE and 8(S)HETE. In chow-fed animals, the level of HDL-cholesterol was also significantly higher in HuApoC1Tg than in WT animals (0.83 ± 0.11 versus 0.73 ± 0.11 mmol/L, respectively, p < 0.05), and it was associated with significantly lower CETP activity (cholesteryl ester transfer rate, -10%, p < 0.05; specific CETP activity, -14%, p < 0.05).

CONCLUSIONS

Constitutive expression of fully functional human apoC1 in transgenic rabbit attenuates atherosclerosis. It was found to relate, at least in part, to the inhibition of plasma CETP activity and to alterations in plasma HDL.

Mining the genome for lipid genes

Mining of the genome for lipid genes has since the early 1970s helped to shape our understanding of how triglycerides are packaged (in chylomicrons), repackaged (in very low density lipoproteins; VLDL), and hydrolyzed, and also how remnant and low-density lipoproteins (LDL) are cleared from the circulation. Gene discoveries have also provided insights into high-density lipoprotein (HDL) biogenesis and remodeling. Interestingly, at least half of these key molecular genetic studies were initiated with the benefit of prior knowledge of relevant proteins. In addition, multiple important findings originated from studies in mouse, and from other types of non-genetic approaches. Although it appears by now that the main lipid pathways have been uncovered, and that only modulators or adaptor proteins such as those encoded by LDLRAP1, APOA5, ANGPLT3/4, and PCSK9 are currently being discovered, genome wide association studies (GWAS) in particular have implicated many new loci based on statistical analyses; these may prove to have equally large impacts on lipoprotein traits as gene products that are already known. On the other hand, since 2004 - and particularly since 2010 when massively parallel sequencing has become de rigeur - no major new insights into genes governing lipid metabolism have been reported. This is probably because the etiologies of true Mendelian lipid disorders with overt clinical complications have been largely resolved. In the meantime, it has become clear that proving the importance of new candidate genes is challenging. This could be due to very low frequencies of large impact variants in the population. It must further be emphasized that functional genetic studies, while necessary, are often difficult to accomplish, making it hazardous to upgrade a variant that is simply associated to being definitively causative. Also, it is clear that applying a monogenic approach to dissect complex lipid traits that are mostly of polygenic origin is the wrong way to proceed. The hope is that large-scale data acquisition combined with sophisticated computerized analyses will help to prioritize and select the most promising candidate genes for future research. We suggest that at this point in time, investment in sequence technology driven candidate gene discovery could be recalibrated by refocusing efforts on direct functional analysis of the genes that have already been discovered. This article is part of a Special Issue entitled: From Genome to Function.

Transcriptional profiling and pathway analysis of CSF-1 and IL-34 effects on human monocyte differentiation

CSF-1 is the well-known ligand for CSF-1R, which plays a vital role in monocyte-macrophage generation, survival, and function. IL-34 is a newly discovered cytokine that also signals through CSF-1R. Although there are limited data for downstream signaling and pathway activation for CSF-1, none are published, to date, for expression profiles of IL-34. The objective of this study was to characterize and compare the signaling pathways downstream of the CSF-1R receptor, based on these two ligands. This was accomplished through transcriptional profiling and pathway analysis of CD14(+) human monocytes differentiated with each ligand. Additionally, cells were treated with a CSF-1R inhibitor GW2580 to establish that observations associated with each ligand were CSF-1R mediated. Gene expression profiles were generated for each condition using Agilent 4x44K Whole Human Genome Microarrays. Overall profiles generated by each cytokine were similar (~75% of genes) with a dampened effect noted on some pathways (~25% of genes) with IL-34. One key difference observed, between the two cytokines was in the repression of CCR2 message. A similar divergence in protein level was established by FACS analysis. The differential effect on CCR2 expression has major implications for monocyte/macrophage biology including homeostasis and function. Further study of IL-34 effects on monocyte/macrophage biology will shed light on the specific role each ligand plays and the context in which these roles are important. To our knowledge, this study is the first to illustrate downstream transcriptional profiles and pathways of IL-34 in comparison with CSF-1 and identify notable differences in CCR2 expression.

Apolipoprotein CI inhibits scavenger receptor BI and increases plasma HDL levels in vivo

Apolipoprotein CI (apoCI) has been suggested to influence HDL metabolism by activation of LCAT and inhibition of HL and CETP. However, the effect of apoCI on scavenger receptor BI (SR-BI)-mediated uptake of HDL-cholesteryl esters (CE), as well as the net effect of apoCI on HDL metabolism in vivo is unknown. Therefore, we evaluated the effect of apoCI on the SR-BI-mediated uptake of HDL-CE in vitro and determined the net effect of apoCI on HDL metabolism in mice. Enrichment of HDL with apoCI dose-dependently decreased the SR-BI-dependent association of [(3)H]CE-labeled HDL with primary murine hepatocytes, similar to the established SR-BI-inhibitors apoCIII and oxLDL. ApoCI deficiency in mice gene dose-dependently decreased HDL-cholesterol levels. Adenovirus-mediated expression of human apoCI in mice increased HDL levels at a low dose and increased the HDL particle size at higher doses. We conclude that apoCI is a novel inhibitor of SR-BI in vitro and increases HDL levels in vivo.

Hepatic denervation impairs the assembly and secretion of VLDL-TAG

VLDL secretion is a regulated process that depends on the availability of lipids, apoB and MTP. Our aim was to investigate the effect of liver denervation upon the secretion of VLDL and the expression of proteins involved in this process. Denervation was achieved by applying a 85% phenol solution onto the portal tract, while control animals were treated with 9% NaCl. VLDL secretion was evaluated by the Tyloxapol method. The hepatic concentration of TAG and cholesterol, and the plasma concentration of TAG, cholesterol, VLDL-TAG, VLDL-cholesterol and HDL-cholesterol were measured, as well as mRNA expression of proteins involved in the process of VLDL assembly. Hepatic acinar distribution of MTP and apoB was evaluated by immunohistochemistry. Denervation increased plasma concentration of cholesterol (125.3 +/- 10.1 vs. 67.1 +/- 4.9 mg dL(-1)) and VLDL-cholesterol (61.6 +/- 5.6 vs. 29.4 +/- 3.3 mg dL(-1)), but HDL-cholesterol was unchanged (45.5 +/- 6.1 vs. 36.9 +/- 3.9 mg dL(-1)). Secretion of VLDL-TAG (47.5 +/- 23.8 vs. 148.5 +/- 27.4 mg dL h(-1)) and mRNA expression of CPT I and apoB were reduced (p < 0.01) in the denervated animals. MTP and apoB acinar distribution was not altered in the denervated animals, but the intensity of the reaction was reduced in relation to controls.

Apolipoprotein C-I binds free fatty acids and reduces their intracellular esterification

Mice that overexpress human apolipoprotein C-I (apoC-I) homozygously (APOC1(+/+) mice) are protected against obesity and show cutaneous abnormalities. Although these effects can result from our previous observation that apoC-I inhibits FFA generation by LPL, we have also found that apoC-I impairs the uptake of a FFA analog in adipose tissue. In this study, we tested the hypothesis that apoC-I interferes with cellular FFA uptake independent of LPL activity. The cutaneous abnormalities of APOC1(+/+) mice were not affected after transplantation to wild-type mice, indicating that locally produced apoC-I prevents lipid entry into the skin. Subsequent in vitro studies with apoC-I-deficient versus wild-type macrophages revealed that apoC-I reduced the cell association and subsequent esterification of [(3)H]oleic acid by approximately 35% (P < 0.05). We speculated that apoC-I binds FFA extracellularly, thereby preventing cell association of FFA. We showed that apoC-I was indeed able to mediate the binding of oleic acid to otherwise protein-free VLDL-like emulsion particles involving electrostatic interaction. We conclude that apoC-I binds FFA in the circulation, thereby reducing the availability of FFA for uptake by cells. This mechanism can serve as an additional mechanism behind the resistance to obesity and the cutaneous abnormalities of APOC1(+/+) mice.

Apolipoprotein CI aggravates atherosclerosis development in ApoE-knockout mice despite mediating cholesterol efflux from macrophages

OBJECTIVE

Apolipoprotein CI (apoCI) is expressed in the liver and in macrophages, and has several roles in lipid metabolism. Since macrophage apoCI expression might affect macrophage lipid homeostasis and atherosclerotic lesion development locally in the arterial wall, we investigated the effect of both systemic and macrophage apoCI on atherosclerotic lesion development.

METHODS AND RESULTS

To investigate whether physiological expression levels of apoCI affect atherosclerosis development, we first assessed the effect of systemic endogenous apoCI expression on atherosclerosis in apoe-/- apoc1+/+ as compared to apoe-/- apoc1-/- mice at 26 weeks of age. ApoCI expression increased plasma levels of triglycerides (TG) (+70%; P<0.01) and cholesterol (+30%; P<0.05), and increased the atherosclerotic lesion area in the aortic root (+87%; P<0.05). Paradoxically, incubation of apoc1+/+ and apoc1-/- murine peritoneal macrophages with AcLDL (50 microg/mL; 48 h) revealed that macrophage apoCI decreased the accumulation of cellular cholesteryl esters (CE) relatively to free cholesterol (-22%; P<0.05). Accordingly, exogenous human apoCI increased cholesterol efflux from AcLDL-laden wild-type macrophages, and to a similar extent as apoAI and apoE. To evaluate whether atherosclerosis development would be affected by macrophage apoCI expression in vivo, we assessed atherosclerotic lesion development at 16 weeks after transplantation of bone marrow from apoe-/- apoc1-/- or apoe-/- apoc1+/+ mice to apoe-/- apoc1+/+ mice. However, in the situation wherein the liver and adipose tissue still produce apoCI, macrophage apoCI expression did not affect plasma lipid levels or the atherosclerotic lesion area.

CONCLUSIONS

Systemic apoCI increases atherosclerosis, probably by inducing hyperlipidemia. Despite decreasing macrophage lipid accumulation in vitro, apoCI production by macrophages locally in the arterial wall does not affect atherosclerosis development in vivo.

High-density lipoprotein and transport of cholesterol and triglyceride in blood

High-density lipoproteins (HDL) contain approximately 25% of the cholesterol and <5% of the triglyceride in the plasma of human blood. However, the dynamic exchange of lipids and lipid-binding proteins is not revealed by simply considering the mass of material at any point in time. HDL are the most complex of lipoprotein species with multiple protein constituents, which facilitate cholesterol secretion from cells, cholesterol esterification in plasma, and transfer of cholesterol to other lipoproteins and to the liver for excretion. They also play a major role in triglyceride transport by providing for activation of lipoprotein lipase, exchange of triglyceride among the lipoproteins, and removal of triglyceride rich remnants of chylomicrons and very-low-density lipoproteins after lipase action. In addition, antioxidative enzymes and phospholipid transfer proteins are important components of HDL. Many of the proteins of HDL are exchangeable with other lipoproteins, including chylomicrons and very-low-density lipoproteins. The constantly changing content of lipids and apolipoproteins in HDL particles generate a series of structures that can be analyzed by using separation techniques that depend on size or charge of the particles. Interaction of these various structures can be very different with cell surfaces depending on the size or apolipoprotein content. A series of different transport proteins preferentially exchange lipids with specific structures among the HDL but interact poorly or not at all with others. The role of these differing forms of HDL and their interactions with cells and other lipoprotein species in plasma is the subject of intense study stimulated by the potential for reducing atherogenesis. The strength of this is only partially indicated by the correlation of higher total levels of the HDL particles with reduced incidence of vascular disease in various clinical trials and epidemiological studies.

Hepatic lipid accumulation in apolipoprotein C-I-deficient mice is potentiated by cholesteryl ester transfer protein

The impact of apolipoprotein C-I (apoC-I) deficiency on hepatic lipid metabolism was addressed in mice in the presence or the absence of cholesteryl ester transfer protein (CETP). In addition to the expected moderate reduction in plasma cholesterol levels, apoCIKO mice showed significant increases in the hepatic content of cholesteryl esters (+58%) and triglycerides (+118%) and in biliary cholesterol concentration (+35%) as compared with wild-type mice. In the presence of CETP, hepatic alterations resulting from apoC-I deficiency were enforced, with up to 58% and 302% increases in hepatic levels of cholesteryl esters and triglycerides in CETPTg/apoCIKO mice versus CETPTg mice, respectively. Biliary levels of cholesterol, phospholipids, and bile acids were increased by 88, 77, and 20%, respectively, whereas total cholesterol, HDL cholesterol, and triglyceride concentrations in plasma were further reduced in CETPTg/apoCIKO mice versus CETPTg mice. Finally, apoC-I deficiency was not associated with altered VLDL production rate. In line with the previously recognized inhibition of lipoprotein clearance by apoC-I, apoC-I deficiency led to decreased plasma lipid concentration, hepatic lipid accumulation, and increased biliary excretion of cholesterol. The effect was even greater when the alternate reverse cholesterol transport pathway via VLDL/LDL was boosted in the presence of CETP.

Apolipoprotein CI stimulates the response to lipopolysaccharide and reduces mortality in Gram‐negative sepsis

Gram-negative sepsis is a major death cause in intensive care units. Accumulating evidence indicates the protective role of plasma lipoproteins such as high-density lipoprotein (HDL) in sepsis. It has recently been shown that septic HDL is almost depleted from apolipoprotein CI (apoCI), suggesting that apoCI may be a protective factor in sepsis. Sequence analysis revealed that apoCI possesses a highly conserved consensus KVKEKLK binding motif for lipopolysaccharide (LPS), an outer-membrane component of gram-negative bacteria. Through avid binding to LPS involving this motif, apoCI improved the presentation of LPS to macrophages in vitro and in mice, thereby stimulating the inflammatory response to LPS. Moreover, apoCI dose-dependently increased the early inflammatory response to Klebsiella pneumoniae-induced pneumonia, reduced the number of circulating bacteria, and protected mice against fatal sepsis. Our data support the hypothesis that apoCI is a physiological protector against infection by enhancing the early inflammatory response to LPS and suggest that timely increase of apoCI levels could be used to efficiently prevent and treat early sepsis.

Severe hypertriglyceridemia in human APOC1 transgenic mice is caused by apoC-I-induced inhibition of LPL

Studies in humans and mice have shown that increased expression of apolipoprotein C-I (apoC-I) results in combined hyperlipidemia with a more pronounced effect on triglycerides (TGs) compared with total cholesterol (TC). The aim of this study was to elucidate the main reason for this effect using human apoC-I-expressing (APOC1) mice. Moderate plasma human apoC-I levels (i.e., 4-fold higher than human levels) caused a 12-fold increase in TG, along with a 2-fold increase in TC, mainly confined to VLDL. Cross-breeding of APOC1 mice on an apoE-deficient background resulted in a marked 55-fold increase in TG, confirming that the apoC-I-induced hyperlipidemia cannot merely be attributed to blockade of apoE-recognizing hepatic lipoprotein receptors. The plasma half-life of [3H]TG-VLDL-mimicking particles was 2-fold increased in APOC1 mice, suggesting that apoC-I reduces the lipolytic conversion of VLDL. Although total postheparin plasma LPL activity was not lower in APOC1 mice compared with controls, apoC-I was able to dose-dependently inhibit the LPL-mediated lipolysis of [3H]TG-VLDL-mimicking particles in vitro with a 60% efficiency compared with the main endogenous LPL inhibitor apoC-III. Finally, purified apoC-I impaired the clearance of [3H]TG-VLDL-mimicking particles independent of apoE-mediated hepatic uptake in lactoferrin-treated mice. Therefore, we conclude that apoC-I is a potent inhibitor of LPL-mediated TG-lipolysis.

Apolipoprotein C-I induces apoptosis in human aortic smooth muscle cells via recruiting neutral sphingomyelinase

OBJECTIVE

Apolipoprotein C-I (apoC-I) influences lipoprotein metabolism, but little is known about its cellular effects in aortic smooth muscle cells (ASMC).

METHODS AND RESULTS

In cultured human ASMC, apoC-I and immunoaffinity purified apoC-I-enriched high-density lipoproteins (HDL) markedly induced apoptosis (5- to 25-fold), compared with control cells, apoC-I-poor HDL, and apolipoprotein C-III (apoC-III) as determined by 4', 6-diamidino-2-phenylindole dihydrochloride staining and DNA ladder assay. Preincubation of cells with GW4869, an inhibitor of neutral sphingomyelinase (N-SMase), blocked apoC-I-induced apoptosis, an effect that was bypassed by C-2 ceramide. The activity of N-SMase was increased 2- to 3-fold in ASMC by apoC-I, apoC-I-enriched HDL, and tumor necrosis factor alpha (TNF-alpha) (positive control) after 10 minutes and then decreased over 60 minutes, which is a kinetic pattern not seen with controls, apoC-III, and apoC-I-poor HDL. ApoC-I and apoC-I-enriched HDL stimulated the generation of ceramide, the release of cytochrome c from mitochondria, and activation of caspase-3 greater than that found in controls, apoC-III, and apoC-I-poor HDL. GW4869 inhibited apoC-I-induced production of ceramide and cytochrome c release.

CONCLUSIONS

ApoC-I and apoC-I-enriched HDL activate the N-SMase-ceramide signaling pathway, leading to apoptosis in human ASMC, which is an effect that may promote plaque rupture in vivo.

Overexpression of apoC-I in apoE-null mice: severe hypertriglyceridemia due to inhibition of hepatic lipase

Apolipoprotein C-I (apoC-I) has been proposed to act primarily via interference with apoE-mediated lipoprotein uptake. To define actions of apoC-I that are independent of apoE, we crossed a moderately overexpressing human apoC-I transgenic, which possesses a minimal phenotype in the WT background, with the apoE-null mouse. Surprisingly, apoE-null/C-I mice showed much more severe hyperlipidemia than apoE-null littermates in both the fasting and non-fasting states, with an almost doubling of cholesterol, primarily in IDL+LDL, and a marked increase in triglycerides; 3-fold in females to 260 +/- 80 mg/dl and 14-fold in males to 1409 +/- 594 mg/dl. HDL lipids were not significantly altered but HDL were apoC-I-enriched and apoA-II-depleted. Production rates of VLDL triglyceride were unchanged as was the clearance of post-lipolysis remnant particles. Plasma post-heparin hepatic lipase and lipoprotein lipase levels were undiminished as was the in vitro hydrolysis of apoC-I transgenic VLDL. However, HDL from apoC-I transgenic mice had a marked inhibitory effect on hepatic lipase activity, as did purified apoC-I. LPL activity was minimally affected. Atherosclerosis assay revealed significantly increased atherosclerosis in apoE-null/C-I mice assessed via the en face assay. Inhibition of hepatic lipase may be an important mechanism of the decrease in lipoprotein clearance mediated by apoC-I.

Plasma kinetics of VLDL and HDL apoC-I in normolipidemic and hypertriglyceridemic subjects

ApoC-I has several different lipid-regulating functions including, inhibition of receptor-mediated uptake of plasma triglyceride-rich lipoproteins, inhibition of cholesteryl ester transfer activity, and mediation of tissue fatty acid uptake. Since little is known about the rate of production and catabolism of plasma apoC-I in humans, the present study was undertaken to determine the plasma kinetics of VLDL and HDL apoC-I using a primed constant (12 h) intravenous infusion of deuterium-labeled leucine. Data were obtained for 14 subjects: normolipidemics (NL, n = 4), hypertriglyceridemics (HTG, n = 4) and combined hyperlipidemics (CHL, n = 6). Plasma VLDL triglyceride (TG) levels were 0.59 +/- 0.03, 4.32 +/- 0.77 (P < 0.01 vs. NL), and 2.20 +/- 0.39 mmol/l (P < 0.01 vs. NL), and plasma LDL cholesterol (LDL-C) levels were 2.34 +/- 0.22, 2.48 +/- 0.26, and 5.35 +/- 0.48 mmol/l (P < 0.01 vs. NL), respectively. HTG and CHL had significantly (P < 0.05) increased levels of total plasma apoC-I (12.5 +/- 1.2 and 12.4 +/- 1.3 mg/dl, respectively) versus NL (7.9 +/- 0.6 mg/dl), due to significantly (P < 0.01) elevated levels of VLDL apoC-I (5.8 +/- 0.8 and 4.5 +/- 0.8 vs. 0.3 +/- 0.1 mg/dl). HTG and CHL also had increased rates of VLDL apoC-I transport (i.e., production) versus NL: 2.29 +/- 0.34 and 3.04 +/- 0.53 versus 0.24 +/- 0.11 mg/kg.day (P < 0.01), with no significant change in VLDL apoC-I residence times (RT): 1.16 +/- 0.12 versus 0.69 +/- 0.06 versus 0.74 +/- 0.17. Although HDL apoC-I concentrations were not significantly lower in HTG and CHL versus NL, HDL apoC-I rates of transport were inversely related to plasma and VLDL-TG levels (r = -0.63 and -0.62, respectively, P < 0.05). Our results demonstrate that increased levels of plasma and VLDL apoC-I in hypertriglyceridemic subjects (with or without elevated LDL-C levels) are associated with increased levels of plasma VLDL apoC-I production.

Apolipoprotein CI deficiency markedly augments plasma lipoprotein changes mediated by human cholesteryl ester transfer protein (CETP) in CETP transgenic/ApoCI-knocked out mice

Transgenic mice expressing human cholesteryl ester transfer protein (HuCETPTg mice) were crossed with apolipoprotein CI-knocked out (apoCI-KO) mice. Although total cholesterol levels tended to be reduced as the result of CETP expression in HuCETPTg heterozygotes compared with C57BL6 control mice (-13%, not significant), a more pronounced decrease (-28%, p < 0.05) was observed when human CETP was expressed in an apoCI-deficient background (HuCETPTg/apoCI-KO mice). Gel permeation chromatography analysis revealed a significant, 6.1-fold rise (p < 0.05) in the cholesteryl ester content of very low density lipoproteins in HuCETPTg/apoCI-KO mice compared with control mice, whereas the 2.7-fold increase in HuCETPTg mice did not reach the significance level in these experiments. Approximately 50% decreases in the cholesteryl ester content and cholesteryl ester to triglyceride ratio of high density lipoproteins (HDL) were observed in HuCETPTg/apoCI-KO mice compared with controls (p < 0.05 in both cases), with intermediate -20% changes in HuCETPTg mice. The cholesteryl ester depletion of HDL was accompanied with a significant reduction in their mean apparent diameter (8.68 +/- 0.04 nm in HuCETPTg/apoCI-KO mice versus 8.83 +/- 0.02 nm in control mice; p < 0.05), again with intermediate values in HuCETPTg mice (8.77 +/- 0.04 nm). In vitro purified apoCI was able to inhibit cholesteryl ester exchange when added to either total plasma or reconstituted HDL-free mixtures, and coincidently, the specific activity of CETP was significantly increased in the apoCI-deficient state (173 +/- 75 pmol/microg/h in HuCETPTg/apoCI-KO mice versus 72 +/- 19 pmol/microg/h in HuCETPTg, p < 0.05). Finally, HDL from apoCI-KO mice were shown to interact more readily with purified CETP than control HDL that differ only by their apoCI content. Overall, the present observations provide direct support for a potent specific inhibition of CETP by plasma apoCI in vivo.

Gene dose effect of the APOE-epsilon4 allele on plasma HDL cholesterol level in patients with Alzheimer's disease

It has been reported that decreased serum high density lipoprotein (HDL) cholesterol level is related with severity of Alzheimer's disease (AD). Here, 82 patients with AD and 40 non-demented individuals were examined to determine how the APOE-epsilon4 allele modifies plasma cholesterol fractions. In adjusting for age, sex and plasma albumin level, plasma HDL cholesterol level was inversely correlated with the APOE-epsilon4 dose only in the patients (P = 0.0048), while plasma low density lipoprotein (LDL) cholesterol level tended to be correlated with the APOE-epsilon4 dose in both groups, although this was not significant. The ratio of LDL to HDL cholesterol in the patients showed a similar correlation with the APOE-epsilon4 dose to that in the controls, and this correlation was evident (P = 0.0069) after putting all subjects into one group. However, neither HDL nor LDL cholesterol levels showed significant differences between the groups. These results indicated that the APOE-epsilon4 dose affects the composition of plasma cholesterol, and suggested that the genetic effect on plasma lipid metabolism could be distinctive in patients with AD.

The interaction of human apolipoprotein C-I with sub-micellar phospholipid

Mature human apolipoprotein C-I (apoC-I), comprising 57 amino acids, is the smallest member of the plasma apolipoprotein family. Amphipathic helical regions within apoC-I, common to this class of proteins, are mediators of lipid binding, a process that underlies the functional properties of apoC-I, including the capacity to activate the plasma enzyme LCAT, to disrupt apoE mediated receptor interactions and to inhibit cholesterol ester transfer protein. To examine apoC-I/phospholipid interactions, we have developed an expression system in Escherichia coli to obtain purified apoC-I with yields of approximately 4-5 mg per L of culture. The purified product has properties similar to plasma-derived apoC-I including self-association in the lipid-free state and induced alpha-helical content in the presence of egg-yolk phosphatidylcholine and dimyristoylglycerophosphocholine vesicles. We chose the short-chain phospholipid, dihexanoylglycerophosphocholine (Hex2Gro-PCho), to examine the interaction of apoC-I with submicellar phospholipid. Circular dichroism spectroscopy and cross-linking experiments show that apoC-I acquires helical content and remains self-associated at submicellar concentrations of Hex2Gro-PCho (4 mM). Sedimentation equilibrium studies of apoC-I at submicellar levels of Hex2Gro-PCho and analysis of the effects of apoC-I on the 1H NMR spectrum of Hex2Gro-PCho indicate micelle induction by apoC-I, and establish the capacity of apoC-I to assemble individual phospholipid molecules.

Apolipoproteins C-I and C-III as important modulators of lipoprotein metabolism

Apolipoprotein (apo)C-I and apoC-III are constituents of HDL and of triglyceride-rich lipoproteins that slow the clearance of triglyceride-rich lipoproteins by a variety of mechanisms. ApoC-I is an inhibitor of lipoprotein binding to the LDL receptor, LDL receptor-related protein, and VLDL receptor. It also is the major plasma inhibitor of cholesteryl ester transfer protein, and appears to interfere directly with fatty acid uptake. ApoC-III also interferes with lipoprotein particle clearance, but its principal role is as an inhibitor of lipolysis, both through the biochemical inhibition of lipoprotein lipase and by interfering with lipoprotein binding to the cell-surface glycosaminoglycan matrix where lipolytic enzymes and lipoprotein receptors reside. Variation in the expression of apoC-III has been credibly documented to have an important role in hypertriglyceridemia. Variation in the expression of apoC-I may also be important for hypertriglyceridemia under certain circumstances.

Adipophilin is a sensitive marker for lipid loading in human blood monocytes

Adipophilin, a marker of lipid accumulation initially described in adipocytes, was recently shown to be induced in macrophage foam cells. We found that even freshly isolated blood monocytes express adipophilin and that the amount of adipophilin protein is variable in monocytes from different healthy individuals. However, the physiological expression of adipophilin does not correlate with the levels of free fatty acids, cholesterylesters or free cholesterol. Enzymatically modified low-density lipoprotein (E-LDL) induces rapid foam cell formation in monocytes and upregulates adipophilin mRNA and protein within 2 h of incubation. This rapid induction of adipophilin is accompanied by a significant increase of free fatty acids in monocytes incubated with E-LDL. Adipophilin facilitates the uptake of free fatty acids, and here we demonstrate that free fatty acids increase is related to the early upregulation of adipophilin expression in blood monocytes. Fatty acids are ligands for peroxisome proliferator-activated receptor-gamma (PPARgamma), and the upregulation of adipophilin mRNA by PPARgamma agonists like 15d-PGJ(2) and ciglitazone indicates that PPARgamma may mediate the induction of adipophilin expression in human blood monocytes.

Genetic determinants of plasma triglycerides: impact of rare and common mutations

Raised plasma triglyceride (TG) levels are an independent risk factor for coronary artery disease (CAD), and thus understanding the genetic and environmental determinants of TG levels are of major importance. TG metabolism is a process for delivering free fatty acids for energy storage or b-oxidation, and involves a number of different hydrolytic enzymes and apolipoproteins (apo). The genes encoding these proteins are, therefore, candidates for determining plasma TGs. Although rare mutations in lipoprotein lipase (LPL), the major TG-hydrolyzing enzyme, and apo CII (APOC2), its essential activator, result in extremely high plasma TG levels, their low frequency means they have little impact upon TG levels in the general population. Common mutations in LPL, apo CIII (APOC3), and apo E (APOE) have the strongest effect on plasma TG levels at the population level. In addition, environmental factors such as diet, obesity, and smoking interact with genetic determinants of TG to produce a modulating high-risk environment.

Human apolipoprotein C-I accounts for the ability of plasma high density lipoproteins to inhibit the cholesteryl ester transfer protein activity

The aim of the present study was to identify the protein that accounts for the cholesteryl ester transfer protein (CETP)-inhibitory activity that is specifically associated with human plasma high density lipoproteins (HDL). To this end, human HDL apolipoproteins were fractionated by preparative polyacrylamide gradient gel electrophoresis, and 30 distinct protein fractions with molecular masses ranging from 80 down to 2 kDa were tested for their ability to inhibit CETP activity. One single apolipoprotein fraction was able to completely inhibit CETP activity. The N-terminal sequence of the 6-kDa protein inhibitor matched the N-terminal sequence of human apoC-I, the inhibition was completely blocked by specific anti-apolipoprotein C-I antibodies, and mass spectrometry analysis confirmed the identity of the isolated inhibitor with full-length human apoC-I. Pure apoC-I was able to abolish CETP activity in a concentration-dependent manner and with a high efficiency (IC(50) = 100 nmol/liter). The inhibitory potency of total delipidated HDL apolipoproteins completely disappeared after a treatment with anti-apolipoprotein C-I antibodies, and the apoC-I deprivation of native plasma HDL by immunoaffinity chromatography produced a mean 43% rise in cholesteryl ester transfer rates. The main localization of apoC-I in HDL and not in low density lipoprotein in normolipidemic plasma provides further support for the specific property of HDL in inhibiting CETP activity.

Role of the enterohepatic circulation of bile salts in lipoprotein metabolism

The enterohepatic circulation of bile salts and cholesterol plays a central role in maintaining whole body cholesterol homeostasis. Hepatic lipoprotein metabolism is reviewed and the role of disturbances in bile salt metabolism in the pathogenesis of dyslipidemias is discussed. Further, the manipulation of bile salt metabolism to treat dyslipidemia is reviewed.

Receptor-mediated mechanisms of lipoprotein remnant catabolism

Chylomicron and VLDL are triglyceride-rich lipoprotein particles assembled by the intestine and liver respectively. These particles are not metabolized by the liver in their native form. However, upon entry into the plasma, their triglyceride component is rapidly hydrolyzed by lipoprotein lipase and they are converted to cholesterol-rich remnant particles. The remnant particles are recognized by the liver and rapidly cleared from the plasma. This process is believed to occur in two steps. (i) An initial sequestration of remnant particles on hepatic cell surface proteoglycans, and (ii) receptor-mediated endocytosis of remnants by hepatic parenchymal cells. The initial binding to proteoglycans may be facilitated by lipoprotein lipase and hepatic lipase which possess both lipid- and heparin-binding domains. The subsequent endocytic process may be mediated by LDL receptors and/or LRP. Both receptors have a high affinity for apoE, a major apolipoprotein component of remnant particles. The lipases may also serve as ligands for these receptors. An impairment of any component of this complex process may result in an accumulation of remnant particles in the plasma leading to atherosclerosis and coronary heart disease.

Lipoproteins and atherogenesis

Like many complex disease processes, atherogenesis represents the interaction of an array of genetic and environmental factors. From nonhuman animal models to the investigation of epidemiologic factors in man, no single, overriding cause for the development of this indolent vascular disease has been identified. However, the cholesterol-enriched lipoprotein particles are closely tied to the development of the disease. The genetic and environmental influences on the concentrations of specific lipoprotein subspecies provide a context for identifying patients at risk as well as for developing effective therapeutic strategies to influence and prevent the sequelae of atherogenesis.

Hyperlipidemia and cutaneous abnormalities in transgenic mice overexpressing human apolipoprotein C1

Transgenic mice were generated with different levels of human apolipoprotein C1 (APOC1) expression in liver and skin. At 2 mo of age, serum levels of cholesterol, triglycerides (TG), and FFA were strongly elevated in APOC1 transgenic mice compared with wild-type mice. These elevated levels of serum cholesterol and TG were due mainly to an accumulation of VLDL particles in the circulation. In addition to hyperlipidemia, APOC1 transgenic mice developed dry and scaly skin with loss of hair, dependent on the amount of APOC1 expression in the skin. Since these skin abnormalities appeared in two independent founder lines, a mutation related to the specific insertion site of the human APOC1 gene as the cause for the phenotype can be excluded. Histopathological analysis of high expressor APOC1 transgenic mice revealed a disorder of the skin consisting of epidermal hyperplasia and hyperkeratosis, and atrophic sebaceous glands lacking sebum. In line with these results, epidermal lipid analysis showed that the relative amounts of the sebum components TG and wax diesters in the epidermis of high expressor APOC1 transgenic mice were reduced by 60 and 45%, respectively. In addition to atrophic sebaceous glands, the meibomian glands were also found to be severely atrophic in APOC1 transgenic mice. High expressor APOC1 transgenic mice also exhibited diminished abdominal adipose tissue stores (a 60% decrease compared with wild-type mice) and a complete deficiency of subcutaneous fat. These results indicate that, in addition to the previously reported inhibitory role of apoC1 on hepatic remnant uptake, overexpression of apoC1 affects lipid synthesis in the sebaceous gland and/or epidermis as well as adipose tissue formation. These APOC1 transgenic mice may serve as an interesting in vivo model for the investigation of lipid homeostasis in the skin.

Use of transgenic mice in lipoprotein metabolism and atherosclerosis research

In APOE*3-Leiden transgenic mice the atherosclerotic lesion size is correlated with plasma cholesterol. In these mice the plasma lipid levels are positively correlated with the relative amount of APOE 3-Leiden protein on the VLDL particle. The plasma cholesterol levels are influenced by diet, age and gender, mainly due to an effect of these factors on VLDL production rate. Excess of APOC1 protein does inhibit the hepatic clearance of VLDL remnant particles, whereas excess of apoE leads to a hampered extra-hepatic lipolysis of VLDL triglyceride.

Reduced very-low-density lipoprotein fractional catabolic rate in apolipoprotein C1-deficient mice

The function of apolipoprotein (apo) C1 in vivo is not clearly defined. Because transgenic mice overexpressing human apoC1 show elevated triacylglycerol (TG) levels [Simonet, Bucay, Pitas, Lauer and Taylor (1991) J. Biol. Chem. 266, 8651-8654], an as yet unknown role for apoC1 in TG metabolism has been suggested. Here we investigated directly the effect of the complete absence of apoC1 on very-low-density lipoprotein (VLDL)-TG lipolysis, clearance and production, by performing studies with the previously generated apoC1-deficient mice. On a sucrose-rich, low fat/low cholesterol (LFC) diet, apoC1-deficient mice accumulate in their circulation VLDL particles, which contain relatively lower amounts of lipids when compared with VLDL isolated from control mice. Lipolysis assays in vitro on VLDL from apoC1-deficient and control mice showed no differences in apparent K(m) and Vmax values (0.27 +/- 0.06 versus 0.24 +/- 0.03 mmol of TG/litre and 0.40 +/- 0.03 versus 0.36 +/- 0.03 mmol of non-esterified fatty acid (NEFA)/min per litre respectively). To correct for potential differences in the size of the VLDL particles, the resulting K(m) values were also expressed relative to apoB concentration. Under these conditions apoC1-deficient VLDL displayed a lower, but not significant, K(m) value when compared with control VLDL (3.44 +/- 0.71 versus 4.44 +/- 0.52 mmol of TG2/g apoB per litre). VLDL turnover studies with autologous injections of [3H]TG-VLDL in vivo showed that the VLDL fractional catabolic rate (FCR) was decreased by up to 50% in the apoC1-deficient mice when compared with control mice (10.5 +/- 3.4 versus 21.0 +/- 1.2/h of pool TG). No significant differences between apoC1-deficient and control mice were observed in the hepatic VLDL production estimated by Triton WR139 injections (0.19 +/- 0.02 versus 0.21 +/- 0.05 mmol/h of TG per kg) and in the extra-hepatic lipolysis of VLDL-TG (4.99 +/- 1.62 versus 3.46 +/- 1.52/h of pool TG) in vivo. Furthermore, [125I]VLDL-apoB turnover experiments in vivo also showed a 50% decrease in the FCR of VLDL in apoC1-deficient mice when compared with control mice on the LFC diet (1.1 +/- 0.3 versus 2.1 +/- 0.1/h of pool apoB). When mice were fed a very high fat/high cholesterol (HFC) diet, the VLDL-apoB FCR was further decreased in apoC1-deficient mice (0.4 +/- 0.1 versus 1.4 +/- 0.4/h of pool apoB). We conclude that, in apoC1-deficient mice, the FCR of VLDL is reduced because of impaired uptake of VLDL remnants by hepatic receptors, whereas the production and lipolysis of VLDL-TG is not affected.

In the absence of the low density lipoprotein receptor, human apolipoprotein C1 overexpression in transgenic mice inhibits the hepatic uptake of very low density lipoproteins via a receptor-associated protein-sensitive pathway

To study the role of apoC1 in lipoprotein metabolism, we have generated transgenic mice expressing the human APOC1 gene. On a sucrose-rich diet, male transgenic mice with high APOC1 expression in the liver showed elevated levels of serum cholesterol and triglyceride compared with control mice (5.7+/-0.7 and 3.3+/-2.1 vs. 2.7+/-0.1 and 0.4+/-0.1 mmol/liter, respectively). These elevated levels were mainly confined to the VLDL fraction. Female APOC1 transgenic mice showed less pronounced elevated serum lipid levels. In vivo VLDL turnover studies revealed that, in hyperlipidemic APOC1 transgenic mice, VLDL particles are cleared less efficiently from the circulation as compared with control mice. No differences were observed in the hepatic production and extrahepatic lipolysis of VLDL-triglyceride. Also, VLDL isolated from control and APOC1 transgenic mice were found to be equally good substrates for bovine lipoprotein lipase in vitro. These data indicate that the hyperlipidemia in APOC1 transgenic mice results primarily from impaired hepatic VLDL particle clearance, rather than a defect in the hydrolysis of VLDL-triglyceride. To investigate which hepatic receptor is involved in the apoC1-mediated inhibition of VLDL clearance, APOC1 transgenic mice were bred with an LDL receptor-deficient (LDLR(-/-)) background. In addition, control, LDLR(-/-), and LDLR(-/-)/APOC1 mice were transfected with adenovirus carrying the gene for the receptor-associated protein (Ad-RAP). Both serum cholesterol and triglyceride levels were strongly elevated in LDLR(-/-)/APOC1 mice compared with LDLR(-/-) mice (52+/-19 and 36+/-19 vs. 8.4+/-0.9 and 0.5+/-0.2 mmol/liter, respectively), indicating that apoC1 inhibits the alternative VLDL clearance pathway via the remnant receptor. Transfection of LDLR(-/-) mice with Ad-RAP strongly increased serum cholesterol and triglyceride levels, but to a lesser extent than those found in LDLR(-/-)/APOC1 mice (39+/-8 and 17+/-8 vs. 52+/-19 and 36+/-19 mmol/liter, respectively). However, in LDLR(-/-)/APOC1 mice the transfection with Ad-RAP did not further increase serum cholesterol and triglyceride levels (52+/-19 and 36+/-19 vs. 60+/-10 and 38+/-7 mmol/liter, respectively). From these studies we conclude that, in the absence of the LDLR, apoC1 inhibits the hepatic uptake of VLDL via a RAP-sensitive pathway.

Both lipolysis and hepatic uptake of VLDL are impaired in transgenic mice coexpressing human apolipoprotein E*3Leiden and human apolipoprotein C1

Transgenic mice overexpressing human APOE*3Leiden are highly susceptible to diet-induced hyperlipoproteinemia and atherosclerosis due to a defect in hepatic uptake of remnant lipoproteins. In addition to the human APOE*3Leiden gene, these mice carry the human APOC1 gene (APOE*3Leiden-C1). To investigate the possible effect of simultaneous expression of the human APOC1 gene, we examined the phenotypic expression in these APOE*3Leiden-C1 mice in relation to transgenic mice expressing the APOE*3Leiden gene without the APOC1 gene (APOE*3Leiden-HCR). APOE*3Leiden-C1 and APOE*3Leiden-HCR mice had comparable liver expression for the APOE*3Leiden transgene and high total cholesterol levels on a sucrose-based diet compared with control mice (4.3 and 4.3 versus 2.1 mmol/L). In addition, on this diet APOE*3Leiden-C1 mice displayed significantly higher serum triglyceride levels than APOE*3Leiden-HCR mice and control mice (4.4 versus 0.6 and 0.2 mmol/L). Elevated triglyceride and cholesterol levels were mainly in the VLDL-sized lipoproteins. In vivo turnover studies with endogenously triglyceride-labeled VLDL showed a reduced VLDL triglyceride fractional catabolic rate for APOE*3Leiden-C1 and APOE*3Leiden-HCR mice compared with control mice (3.5 and 11.0 versus 20.4 pools per hour). To study whether the difference in fractional catabolic rates between the two transgenic strains was due to an inhibiting effect of apoC1 on the extrahepatic lipolysis or hepatic-mediated uptake of VLDL, turnover experiments were performed in functionally hepatectomized mice. Strikingly, both APOE*3Leiden-C1 and APOE*3Leiden-HCR mice showed a decreased lipolytic rate of VLDL triglyceride in the extrahepatic circulation compared with control mice (1.5 and 1.8 versus 6.3 pools per hour). We conclude that next to an impaired hepatic uptake, overexpression of the APOE*3Leiden gene influences the extrahepatic lipolysis of VLDL triglycerides, whereas simultaneous overexpression of the APOC1 gene leads to a further decrease in hepatic clearance of VLDL.

Combined hyperlipidemia in transgenic mice overexpressing human apolipoprotein Cl

We have generated transgenic mice over-expressing human apolipoprotein CI (apo CI) using the native gene joined to the downstream 154-bp liver-specific enhancer that we defined for apo E. Human apo CI (HuCI)-transgenic mice showed elevation of plasma triglycerides (mg/dl) compared to controls in both the fasted (211 +/- 81 vs 123 +/- 52, P = 0.0001) and fed (265 +/- 105 vs 146 +/- 68, P < 0.0001) states. Unlike the human apo CII (HuCII)- and apo CIII (HuCIII)-transgenic mouse models of hypertriglyceridemia, plasma cholesterol was disproportionately elevated (95 +/- 23 vs 73 +/- 23, P = 0.002, fasted and 90 +/- 24 vs 61 +/- 14, P < 0.0001, fed). Lipoprotein fractionation showed increased VLDL and IDL + LDL with an increased cholesterol/triglyceride ratio (0.114 vs 0.065, P = 0.02, in VLDL). The VLDL apo E/apo B ratio was decreased 3.4-fold (P = 0.05) and apo CII and apo CIII decreased in proportion to apo E. Triglyceride and apo B production rates were normal, but clearance rates of VLDL triglycerides and postlipolysis lipoprotein "remnants" were significantly slowed. Plasma apo B was significantly elevated. Unlike HuCII- and HuCIII-transgenic mice, VLDL from HuCI transgenic mice bound heparin-Sepharose, a model for cell-surface glycosaminoglycans, normally. In summary, apo CI overexpression is associated with decreased particulate uptake of apo B-containing lipoproteins, leading to increased levels of several potentially atherogenic species, including cholesterol-enriched VLDL, IDL, and LDL.