Association of apolipoproteins C-I and C-II truncations with coronary heart disease and progression of coronary artery calcium: Multi-Ethnic Study of Atherosclerosis

BACKGROUND AND AIMS

Higher truncated-to-native proteoform ratios of apolipoproteins (apo) C-I (C-I'/C-I) and C-II (C-II'/C-II) are associated with less atherogenic lipid profiles. We examined prospective relationships of C-I'/C-II and C-II'/C-II with coronary heart disease (CHD) and coronary artery calcium (CAC).

METHODS

ApoC-I and apoC-II proteoforms were measured by mass spectrometry immunoassay in 5790 MESA baseline plasma samples. CHD events (myocardial infarction, resuscitated cardiac arrest, fatal CHD, n = 434) were evaluated for up to 17 years. CAC was measured 1-4 times over 10 years for incident CAC (if baseline CAC = 0), and changes (follow-up adjusted for baseline) in CAC score and density (if baseline CAC>0).

RESULTS

C-II'/C-II was inversely associated with CHD (n = 434 events) after adjusting for non-lipid cardiovascular risk factors (Hazard ratio: 0.89 [95% CI: 0.81-0.98] per SD), however, the association was attenuated after further adjustment for HDL levels (0.93 [0.83-1.03]). There was no association between C-I'/C-I and CHD (0.98 [0.88-1.08]). C-II'/C-II was positively associated with changes in CAC score (3.4% [95%CI: 0.6, 6.3]) and density (6.3% [0.3, 4.2]), while C-I'/C-I was inversely associated with incident CAC (Risk ratio: 0.89 [95% CI: 0.81, 0.98]) in fully adjusted models that included plasma lipids. Total apoC-I and apoC-II concentrations were not associated with CHD, incident CAC or change in CAC score.

CONCLUSIONS

Increased apoC-II truncation was associated with reduced CHD, possibly explained by differences in lipid metabolism. Increased apoC-I and apoC-II truncations were also associated with less CAC progression and/or development of denser coronary plaques.

Is Apo-CIII the new cardiovascular target? An analysis of its current clinical and dietetic therapies

AIMS

Recently, Apolipoprotein CIII (Apo-CIII) has gained remarkable attention since its overexpression has been strongly correlated to cardiovascular disease (CVD) occurrence. The aim of this review was to summarize the latest findings of Apo-CIII as a CVDs and diabetes risk factor, as well as the plausible mechanisms involved in the development of these pathologies, with particular emphasis on current clinical and dietetic therapies.

DATA SYNTHESIS

Apo-CIII is a small protein (∼8.8 kDa) that, among other functions, inhibits lipoprotein lipase, a key enzyme in lipid metabolism. Apo-CIII plays a fundamental role in the physiopathology of atherosclerosis, type-1, and type-2 diabetes. Apo-CIII has become a potential clinical target to tackle these multifactorial diseases. Dietetic (omega-3 fatty acids, stanols, polyphenols, lycopene) and non-dietetic (fibrates, statins, and antisense oligonucleotides) therapies have shown promising results to regulate Apo-CIII and triglyceride levels. However, more information from clinical trials is required to validate it as a new target for atherosclerosis and diabetes types 1 and 2.

CONCLUSIONS

There are still several pathways involving Apo-CIII regulation that might be affected by bioactive compounds that need further research. The mechanisms that trigger metabolic responses following bioactive compounds consumption are mainly related to higher LPL expression and PPARα activation, although the complete pathways are yet to be elucidated.

Altered apolipoprotein C expression in association with cognition impairments and hippocampus volume in schizophrenia and bipolar disorder

Proteomic analyses facilitate the interpretation of molecular biomarker probes which are very helpful in diagnosing schizophrenia (SZ). In the current study, we attempt to test whether potential differences in plasma protein expressions in SZ and bipolar disorder (BD) are associated with cognitive deficits and their underlying brain structures. Forty-two plasma proteins of 29 SZ patients, 25 BD patients and 93 non-clinical controls were quantified and analysed using multiple reaction monitoring-based triple quadrupole mass spectrometry approach. We also computed group comparisons of protein expressions between patients and controls, and between SZ and BD patients, as well. Potential associations of protein levels with cognitive functioning (psychomotor speed, executive functioning, crystallised intelligence) as well as underlying brain volume in the hippocampus were explored, using bivariate correlation analyses. The main finding of this study was that apolipoprotein expression differed between patients and controls and that these alterations in both disease groups were putatively related to cognitive impairments as well as to hippocampus volumes. However, none of the protein level differences were related to clinical symptom severity. In summary, altered apolipoprotein expression in BD and SZ was linked to cognitive decline and underlying morphological changes in both disorders. Our results suggest that the detection of molecular patterns in association with cognitive performance and its underlying brain morphology is of great importance for understanding of the pathological mechanisms of SZ and BD, as well as for supporting the diagnosis and treatment of both disorders.

[Correlation of metabolic syndrome clinical signs and genetic determinants at children with obese]

The aim of the work was to study the clinical and genetic factors at children with obese that predispose to the development of MS, and the development of algorithm for generating risk of MS.

MATERIALS AND METHODS

Two comparable age and sex groups of children--148 children with obesity and 46--with normal body weight. We assessed anthropometric indices, blood pressure (BP), lipid profile, carbohydrate metabolism, the level of uric acid. 83 children with obesity were genotyped for polymorphisms: I/D gene ACE, G-75A ApoA1, S19W ApoA5, Sstl ApoC3, E2/E3/E4 ApoE and W/R ADRB3.

RESULTS

98,0% of children had abdominal obesity. In 35,8% was identified high blood pressure. In 47,4% was diagnosed hypo-alpha cholesterolemia and/or hypertriglyceridemia (HTG). In 21,0% of children was identified hyperglycemia. 25,7%were suffered from hyperuricemia. Among the genotyped children 57,0% of homo-and heterozygous carriers of D allele ACE gene had high blood pressure. More than half of the holders of 19W-allele ApoA5 (68,5%),--75A-allele of ApoA1 (56,0%), 52-allele of the gene ApoC3 (53,0%), E4-ApoE gene (85,7%), in the heterozygous state had metabolic TG and/or HDL. In 60,3% of the carriers W/W genotype of ADRB3 gene revealed a combination of hyperglycemia with hyperinsulinemia and/or TG.

CONCLUSION

As a result of, aiming aimed at early detection of the major manifestations of MS clinical and genetic study was revealed stable combination of constitutional, metabolic and molecular-genetic factors. Based on these data was developed algorithm for forming groups at risk of MS and individual tactics to prevent and/or therapy.

Expression of apolipoprotein C-IV is regulated by Ku antigen/peroxisome proliferator-activated receptor gamma complex and correlates with liver steatosis

BACKGROUND/AIMS

We previously reported that hepatitis C virus (HCV) core protein up regulated transcription of apolipoprotein C-IV (ApoC-IV, 10.7-fold increase), a member of the apolipoprotein family implicated in liver steatosis. Here, we identified host transcription factors regulating the ApoC-IV gene expression.

METHODS

Transcriptional regulators were identified by DNA affinity purification and steatosis was detected by oil red O staining and triglyceride assay.

RESULTS

We defined a 163-bp ApoC-IV promoter as a core protein responsive element, and identified Ku antigen complex (Ku70 and Ku80) as well as nuclear receptors PPARgamma/RXRalpha as key regulators of ApoC-IV gene expression. Both Ku70 overexpression and PPARgamma agonist significantly increased ApoC-IV promoter activity; conversely, Ku70 silencing or mutation of PPARgamma binding site diminished the ApoC-IV promoter activity. Interestingly, transient transfection of ApoC-IV cDNA into a human hepatoma cell line was able to trigger moderate lipid accumulation. In agreement with this in vitro study, ApoC-IV transcript level was increased in HCV infected livers which correlated with triglyceride accumulation.

CONCLUSIONS

ApoC-IV overexpression may perturb lipid metabolism leading to lipid accumulation. HCV core protein may modulate ApoC-IV expression through Ku antigen and PPARgamma/RXRalpha complex.

Antisense oligonucleotides as therapeutics for hyperlipidaemias

Hyperlipidaemia, due to elevations of low-density lipoprotein cholesterol (LDL-C) or triglycerides (TGs), is recognised as a significant risk factor contributing to the development of coronary heart disease (CHD), the leading cause of morbidity and mortality in the Western world. Even though a variety of established antihyperlipidaemic agents are available, the majority of high-risk patients do not reach their lipid goals, indicating the need for new and more effective therapeutics to be used alone or as combination agents with existing drugs. Antisense oligonucleotides (ASOs), designed to specifically and selectively inhibit novel targets involved in cholesterol/TG homeostasis, represent a new class of agents that may prove beneficial for the treatment of hyperlipidaemias resulting from various genetic, metabolic or behavioural factors. This article describes the antisense technology platform, highlights the advantages of these novel drugs for the treatment of hyperlipidaemia and reviews the current research in this area.

Inhibition of hepatocyte nuclear factor 4 transcriptional activity by the nuclear factor kappaB pathway

HNF-4 (hepatocyte nuclear factor 4) is a key regulator of liver-specific gene expression in mammals. We have shown previously that the activity of the human APOC3 (apolipoprotein C-III) promoter is positively regulated by the anti-inflammatory cytokine TGFbeta (transforming growth factor beta) and its effectors Smad3 (similar to mothers against decapentaplegic 3) and Smad4 proteins via physical and functional interactions between Smads and HNF-4. We now show that the pro-inflammatory cytokine TNFalpha (tumour necrosis factor alpha) antagonizes TGFbeta for the regulation of APOC3 gene expression in hepatocytes. TNFalpha was a strong inhibitor of the activity of apolipoprotein promoters that harbour HNF-4 binding sites and this inhibition required HNF-4. Using specific inhibitors of TNFalpha-induced signalling pathways, it was shown that inhibition of the APOC3 promoter by TNFalpha involved NF-kappaB (nuclear factor kappaB). Latent membrane protein 1 of the Epstein-Barr virus, which is an established potent activator of NF-kappaB as well as wild-type forms of various NF-kappaB signalling mediators, also inhibited strongly the APOC3 promoter and the transactivation function of HNF-4. TNFalpha had no effect on the stability or the nuclear localization of HNF-4 in HepG2 cells, but inhibited the binding of HNF-4 to the proximal APOC3 HRE (hormone response element). Using the yeast-transactivator-GAL4 system, we showed that both AF-1 and AF-2 (activation functions 1 and 2) of HNF-4 are inhibited by TNFalpha and that this inhibition was abolished by overexpression of different HNF-4 co-activators, including PGC-1 (peroxisome-proliferator-activated-receptor-gamma co-activator 1), CBP [CREB (cAMP-response-element-binding protein) binding protein] and SRC3 (steroid receptor co-activator 3). In summary, our findings indicate that TNFalpha, or other factors that trigger an NF-kappaB response in hepatic cells, inhibit the transcriptional activity of the APOC3 and other HNF-4-dependent promoters and that this inhibition could be accounted for by a decrease in DNA binding and the down-regulation of the transactivation potential of the AF-1 and AF-2 domains of HNF-4.

Apolipoprotein C-III isoforms: kinetics and relative implication in lipid metabolism

Apolipoprotein C-III (apoC-III) production rate (PR) is strongly correlated with plasma triglyceride (TG) levels. ApoC-III exists in three different isoforms, according to the sialylation degree of the protein. We investigated the kinetics and respective role of each apoC-III isoform in modulating intravascular lipid/lipoprotein metabolism. ApoC-III kinetics were measured in a sample of 18 healthy men [mean age (+/-SD) 42.1 +/- 9.5 years, body mass index 29.8 +/- 4.6 kg/m2] using a primed-constant infusion of l-(5,5,5-D3) leucine for 12 h. Mono-sialylated and di-sialylated apoC-III (apo-CIII1 and apoC-III2) exhibited similar PRs (means +/- SD, 1.22 +/- 0.49 mg/kg/day vs. 1.15 +/- 0.59 mg/kg/day, respectively) and similar fractional catabolic rates (FCRs) (0.51 +/- 0.13 pool/day vs. 0.61 +/- 0.24 pool/day, respectively). Nonsialylated apoC-III (apoC-III0) had an 80% lower PR (0.25 +/- 0.12 mg/kg/day) and a 60% lower FCR (0.21 +/- 0.07 pool/day) (P < 0.0001 for comparison with CIII1 and CIII2 isoforms). The PRs of apoC-III1 and apoC-III2 were more strongly correlated with plasma TG levels (r > 0.8, P < 0.0001) than was apoC-III0 PR (r = 0.54, P < 0.05). Finally, the PR of apoC-III2 was strongly correlated with the proportion of LDL <255 A (r = 0.72, P = 0.002). These results suggest that all apoC-III isoforms, especially the predominant CIII1 and CIII2 isoforms, contribute to hypertriglyceridemia and that apoC-III2 may play a significant role in the expression of the small, dense LDL phenotype.

Use of Intralipid for kinetic analysis of HDL apoC-III: evidence for a homogeneous kinetic pool of apoC-III in plasma

Apolipoprotein C-III (apoC-III) is an important regulator of lipoprotein metabolism. Radioisotope and stable isotope kinetic studies show differing results in relation to the kinetics of apoC-III in HDL. Kinetic analysis of HDL apoC-III may be difficult because of its low concentration, as well as the presence of other apoproteins at higher concentration, in the HDL fraction. We used Intralipid(R) (IL), known to preferentially extract apoC proteins from plasma, as a means of extracting apoC-III from HDL before apoprotein separation by isoelectric focusing gel electrophoresis for the measurement of tracer enrichment. Protein purity was assessed by an isoleucine-to-leucine (Ile/Leu) ratio, as apoC-III contains no isoleucine. We compared apoC-III kinetics in 14 men using a bolus infusion of deuterated leucine. The Ile/Leu ratio for IL-extracted HDL (IL-HDL) apoC-III (3.0 +/- 0.7%) was not different from that of VLDL apoC-III (2.6 +/- 0.6%) but was significantly lower than that of untreated HDL apoC-III (9.0 +/- 2.9%) (P < 0.001). The isotopic enrichment curves and fractional catabolic rates (FCRs) for IL-HDL apoC-III were not different from those of VLDL apoC-III. In contrast, HDL apoC-III had significantly lower isotopic enrichments and FCRs than IL-HDL apoC-III (P < 0.001). In conclusion, this simple IL method can be used to isolate apoC-III from HDL with minimal interference from other HDL apoproteins, and it demonstrates that the kinetics of apoC-III in VLDL and HDL are similar, supporting the concept of a single kinetically homogeneous pool of apoC-III in plasma.

Partnership of PGC-1α and HNF4α in the Regulation of Lipoprotein Metabolism

Peroxisome proliferator-activated receptor gamma coactivator-1alpha (PGC-1alpha) is a transcriptional coactivator involved in several aspects of energy metabolism. It is induced or activated under different stimuli in a highly tissue-specific manner and subsequently partners with certain transcription factors in those tissues to execute various biological programs. In the fasted liver, PGC-1alpha is induced and interacts with hepatocyte nuclear factor 4alpha (HNF4alpha) and other transcription factors to activate gluconeogenesis and increase hepatic glucose output. Given the broad spectrum of liver genes responsive to HNF4alpha, we sought to determine those that were specifically targeted by the combination of PGC-1alpha and HNF4alpha. Coexpression of these two molecules in murine stem cells reveals a high induction of mRNA for apolipoproteins A-IV and C-II. Forced expression of PGC-1alpha in mouse and human hepatoma cells increases the mRNA of a subset of apolipoproteins implicated in very low density lipoprotein and triglyceride metabolism, including apolipoproteins A-IV, C-II, and C-III. Coactivation of the apoC-III/A-IV promoter region by PGC-1alpha occurs through a highly conserved HNF4alpha response element, the loss of which completely abolishes activation by PGC-1alpha and HNF4alpha. Adenoviral infusion of PGC-1alpha into live mice increases hepatic expression of apolipoproteins A-IV, C-II, and C-III and increases serum and very low density lipoprotein triglyceride levels. Conversely, knock down of PGC-1alpha in vivo causes a decrease in both apolipoprotein expression and serum triglyceride levels. These data point to a crucial role for the PGC-1alpha/HNF4alpha partnership in hepatic lipoprotein metabolism.

In-vivo metabolism of VLDL-apolipoprotein-B, -CIII and -E in normolipidemic subjects

BACKGROUND AND AIM

ApoE and apoC-III are important components of lipoprotein metabolism. While the function of both apoproteins is relatively well understood, little is known about the in vivo metabolism of these proteins, partly because of the lack of a standardized method to isolate these apoproteins in large sample numbers.

METHODS AND RESULTS

We developed a new reverse phase HPLC method (acetonitril/phosphate gradient; Aquapore RP-300, 7 microm, 220 x 4.6 mm) to isolate a number of different apoproteins, including apoC-III and apoE from VLDL. This method was then used in a study which aimed at determining VLDL-apoE-3 and VLDL-apoC-III metabolism. In addition VLDL-apoB and LDL-apoB metabolism was determined. Endogenous labeling with d(3)-leucine, mass spectrometry and multicompartmental modeling was used in 6 normolipidemic healthy male subjects. Tracer/tracee ratios of free plasma leucine, VLDL-apoE, -apoC-III, -apoB, and LDL-apoB leucine were determined over 60 h following a bolus of d(3)-leucine (5 mg kg(-1)). In all subjects sufficient apoC-III could be isolated by reverse phase HPLC to derive metabolic parameters, while apoE metabolic parameters could only be determined if apoE plasma concentration was 0.75 mg dl(-1) or higher. Compared to VLDL-apoB (FCR 10.4 +/- 3.3 d(-1), production 17.8 +/- 4.5 mg kg(-1) d(-1)), VLDL-apoE-3 (FCR 1.03 +/- 0.11 d(-1), production 0.50 +/- 0.29 mg kg(-1) d(-1)) and VLDL-apoC-III (FCR 1.67 +/- 1.22 d(-1), production 0.44 +/- 0.24 mg kg(-1) d(-1)) parameters were much lower. This indicates that apoE-3 and apoC-III recirculate in plasma and that only a small fraction of apoE and apoC-III on VLDL is newly synthesized.

CONCLUSIONS

We conclude that HPLC methodology can be used to isolate VLDL-apoC-III and apoE for metabolic studies and that the metabolic fate of apoC-III and apoE is different from that of apoB because both apoproteins recycle through the VLDL fraction.

Human apoA-I expression in CETP transgenic rats leads to lower levels of apoC-I in HDL and to magnification of CETP-mediated lipoprotein changes

Plasma cholesteryl ester transfer protein (CETP) has a profound effect on neutral lipid transfers between HDLs and apolipoprotein B (apoB)-containing lipoproteins when it is expressed in combination with human apoA-I in HuAI/CETP transgenic (Tg) rodents. In the present study, human apoA-I-mediated lipoprotein changes in HuAI/CETPTg rats are characterized by 3- to 5-fold increments in the apoB-containing lipoprotein-to-HDL cholesterol ratio, and in the cholesteryl ester-to-triglyceride ratio in apoB-containing lipoproteins. These changes occur despite no change in plasma CETP concentration in HuAI/CETPTg rats, as compared with CETPTg rats. A number of HDL apolipoproteins, including rat apoA-I and rat apoC-I are removed from the HDL surface as a result of human apoA-I overexpression. Rat apoC-I, which is known to constitute a potent inhibitor of CETP, accounts for approximately two-thirds of CETP inhibitory activity in HDL from wild-type rats, and the remainder is carried by other HDL-bound apolipoprotein inhibitors. It is concluded that human apoA-I overexpression modifies HDL particles in a way that suppresses their ability to inhibit CETP. An apoC-I decrease in HDL of HuAI/CETPTg rats contributes chiefly to the loss of the CETP-inhibitory potential that is normally associated with wild-type HDL.

Apolipoprotein CI causes hypertriglyceridemia independent of the very-low-density lipoprotein receptor and apolipoprotein CIII in mice

We have recently shown that the predominant hypertriglyceridemia in human apolipoprotein C1 (APOC1) transgenic mice is mainly explained by apoCI-mediated inhibition of the lipoprotein lipase (LPL)-dependent triglyceride (TG)-hydrolysis pathway. Since the very-low-density lipoprotein receptor (VLDLr) and apoCIII are potent modifiers of LPL activity, our current aim was to study whether the lipolysis-inhibiting action of apoCI would be dependent on the presence of the VLDLr and apoCIII in vivo. Hereto, we employed liver-specific expression of human apoCI by using a novel recombinant adenovirus (AdAPOC1). In wild-type mice, moderate apoCI expression leading to plasma human apoCI levels of 12-33 mg/dl dose-dependently and specifically increased plasma TG (up to 6.6-fold, P < 0.001), yielding the same hypertriglyceridemic phenotype as observed in human APOC1 transgenic mice. AdAPOC1 still increased plasma TG in vldlr(-/-) mice (4.1-fold, P < 0.001) and in apoc3(-/-) mice (6.8-fold, P < 0.001) that were also deficient for the low-density lipoprotein receptor (LDLr) and LDLr-related protein (LRP) or apoE, respectively. Thus, irrespective of receptor-mediated remnant clearance by the liver, liver-specific expression of human apoCI causes hypertriglyceridemia in the absence of the VLDLr and apoCIII. We conclude that apoCI is a powerful and direct inhibitor of LPL activity independent of the VLDLr and apoCIII.

High density lipoproteins bind Abeta and apolipoprotein C-II amyloid fibrils

Disease-associated amyloid deposits contain both fibrillar and nonfibrillar components. The majority of these amyloid components originate or coexist in the bloodstream. To understand the nature of the interaction between the nonfibrillar and fibrillar components, we have developed a centrifugation method to isolate fibril binding proteins from human serum. Amyloid fibrils composed of either Abeta peptide or apolipoprotein C-II (apoC-II) cosedimented with specific serum proteins. Gel electrophoresis, mass spectrometry peptide fingerprinting, and Western analysis identified the major binding species as proteins found in HDL particles, including apoA-I, apoA-II, apoE, clusterin, and serum amyloid A. Sedimentation analysis showed that purified human HDL and recombinant apoA-I lipid particles bound directly to Abeta and apoC-II amyloid fibrils. These studies reveal a novel function of HDL that may contribute to the well-established protective effect of this lipoprotein class in heart disease.

Visceral Adiposity and Apolipoprotein C-III in Apolipoprotein B-Containing Lipoproteins Are Independent Predictors in Determining Atherogenic Lipid Profiles

BACKGROUND/AIMS

Little is known about how visceral adiposity and apolipoprotein CIII in apoB-containing lipoproteins influence atherogenic lipids profiles. The purpose of this study was to determine the relationships of visceral adiposity and apoC-III in apoB-containing lipoproteins (LpB:C-III) with lipoprotein lipids in circulating plasma.

METHODS

A subgroup of 46 men (n = 20, aged 29.1-33.4 years) and women (n = 26, aged 29.1-33.8 years) were recruited from an ongoing population study at our institution. Anthropometric variables including weight, height, and waist circumference were measured using standard procedures, and body mass index was calculated (kg/m(2)). Visceral adipose tissue (VAT) was measured with magnetic resonance imaging. Plasma apolipoproteins, lipids, glucose, and insulin were measured after an overnight fasting.

RESULTS

The men had a significantly higher waist circumference, glucose, and TC/HDL-C ratio, while the women had a significantly higher HDL-C and apoA-I. In particular, this is the first study to report that VAT and LpB:C-III were independent predictors in determining plasma triglyceride concentrations.

CONCLUSION

The relations of plasma triglyceride concentration with VAT and LpB:C-III suggest that information on VAT and apoC-III in apoB-containing lipoproteins may provide additional information on the atherogenic lipid profiles.

Ginsenoside Rf, a component of ginseng, regulates lipoprotein metabolism through peroxisome proliferator-activated receptor α

We investigated whether ginseng regulates lipoprotein metabolism by altering peroxisome proliferator-activated receptor alpha (PPARalpha)-mediated pathways, using a PPARalpha-null mouse model. Administration of ginseng extract, ginsenosides, and ginsenoside Rf (Rf) to wild-type mice not only significantly increased basal levels of hepatic apolipoprotein (apo) A-I and C-III mRNA compared with wild-type controls, but also substantially reversed the reductions in mRNA levels of apo A-I and C-III expected following treatment with the potent PPARalpha ligand Wy14,643. In contrast, no effect was detected in the PPARalpha-null mice. Testing of eight main ginsenosides on PPARalpha reporter gene expression indicated that Rf was responsible for the effects of ginseng on lipoprotein metabolism. Furthermore, the inhibition of PPARalpha-dependent transactivation by Rf seems to occur at the level of DNA binding. These results demonstrate that ginseng component Rf regulates apo A-I and C-III mRNA and the actions of Rf on lipoprotein metabolism are mediated via interactions with PPARalpha.

Apolipoprotein C-II is a novel substrate for matrix metalloproteinases

We previously reported an efficient proteomic approach to identify matrix metalloproteinase (MMP) substrates from complex protein mixture. Using the proteomic approach, apolipoprotein C-II (apoC-II), which is a cofactor of lipoprotein lipase (LPL) and a component of very-low density lipoprotein and chylomicron, has been identified as a putative MMP-14 substrate. Cleavage of apoC-II, with various MMPs, demonstrated that apoC-II is cleaved most efficiently by MMP-14, and also by MMP-7, among the tested MMPs. The 79-amino acid residue apoC-II was cleaved between Asn35 and Leu36 by MMP-14, and between Phe14 and Leu15 and between Asn35 and Leu36 by MMP-7. Cleavage of apoC-II by MMP-14 markedly decreased LPL activity and would thus impair hydrolysis of triglycerides in plasma and transfer of fatty acids to tissues. Our result suggests that cleavage of apoC-II by MMPs would be important for development of pathophysiological situations of apoC-II deficiency such as atherosclerosis.

Molecular Mechanism of the Blockade of Plasma Cholesteryl Ester Transfer Protein by Its Physiological Inhibitor Apolipoprotein CI

Genetically engineered mice demonstrated that apolipoprotein (apo) CI is a potent, physiological inhibitor of plasma cholesteryl ester transfer protein (CETP) activity. The goal of this study was to determine the molecular mechanism of the apoCI-mediated blockade of CETP activity. Kinetic analyses revealed that the inhibitory property of apoCI is independent of the amount of active CETP, but it is tightly dependent on the amount of high density lipoproteins (HDL) in the incubation mixtures. The electrostatic charge of HDL, i.e. the main carrier of apoCI in human plasma, is gradually modified with increasing amounts of apoCI, and the neutralization of apoCI lysine residues by acetylation produces a marked reduction in its inhibitory potential. The inhibitory property of full-length apoCI is shared by its C-terminal alpha-helix with significant electrostratic properties, whereas its N-terminal alpha-helix with no CETP inhibitory property has no effect on HDL electronegativity. Finally, binding experiments demonstrated that apoCI and to a lower extent its C-terminal alpha-helix are able to disrupt CETP-lipoprotein complexes in a concentration-dependent manner. It was concluded that the inhibition of CETP activity by apoCI is in direct link with its specific electrostatic properties, and the apoCI-mediated reduction in the binding properties of lipoproteins results in weaker CETP-HDL interactions and fewer cholesteryl ester transfers.

Apolipoproteins C-II and C-III inhibit selective uptake of low- and high-density lipoprotein cholesteryl esters in HepG2 cells

Plasma low- and high-density lipoproteins (LDL and HDL) are cleared from the circulation by specific receptors and are either totally degraded or their cholesteryl esters (CE) are selectively delivered to cells by receptors such as the scavenger receptor class B type I (SR-BI). The aim of the present study was to define the effect of apoC-II and apoC-III on the uptake of LDL and HDL by HepG2 cells. Stable transformants were obtained with sense or antisense strategies that secrete 47-294% the normal level of apoC-II or 60-200% that of apoC-III. Different levels of secreted apoC-II or apoC-III had little effect on LDL and HDL protein degradation by HepG2 cells. However, compared to controls, cells under-expressing apoC-II showed a 160% higher capacity to selectively take up HDL-CE, while cells under-expressing apoC-III demonstrated 70 and 160% higher capacity to take up CE from LDL and HDL, respectively. In experiments conducted with exogenously added apoC-II or apoC-III, no significant effect was observed on lipoprotein-protein association/degradation; however, LDL-CE and HDL-CE selective uptake was significantly reduced in a dose-dependent manner. These results indicate that apoC-II and apoC-III inhibit CE-selective uptake.

Apolipoprotein CI overexpression is not a relevant strategy to block cholesteryl ester transfer protein (CETP) activity in CETP transgenic mice

ApoCI (apolipoprotein CI) is a potent inhibitor of plasma CETP [CE (cholesteryl ester) transfer protein]. The relevance of apoCI overexpression as a method for CETP blockade in vivo was addressed in the present study in CETPTg/apoCITg mice (mice expressing both human CETP and apoCI). Despite a significant reduction in specific CETP activity in CETPTg/apoCITg mice compared with CETPTg mice [transgenic mouse to human CETP; 46.8+/-11.1 versus 101.8+/-25.7 pmol x h(-1).(mug of plasma CETP)(-1) respectively; P<0.05], apoCI overexpression increased both the CETP mass concentration (3-fold increase; P<0.05) and the hepatic CETP mRNA level (4-fold increase, P<0.005), leading to an increase in total plasma CE transfer activity (by 39%, P<0.05). The ratio of apoB-containing lipoprotein to HDL (high-density lipoprotein) CE was 10-fold higher in CETPTg/apoCITg mice than in apoCITg mice (P<0.0005). It is proposed that the increased CETP expression in CETPTg/apoCITg mice is a direct consequence of liver X receptor activation in response to the accumulation of cholesterol-rich apoB-containing lipoproteins. In support of the latter view, hepatic mRNA levels of other liver X receptor-responsive genes [ABCG5 (ATP-binding cassette transporter GS) and SREBP-1c (sterol-regulatory-binding protein-1c)] were higher in CETPTg/apoCITg mice compared with CETPTg mice. In conclusion, overexpression of apoCI, while producing a significant inhibitory effect on specific CETP activity, does not represent a suitable method for decreasing total CE transfer activity in CETPTg/apoCITg mice, owing to an hyperlipidaemia-mediated effect on CETP gene expression.

A combined defect in the biosynthesis of N- and O-glycans in patients with cutis laxa and neurological involvement: the biochemical characteristics

Based on our preliminary observation of abnormal glycosylation in a cutis laxa patient, nine cutis laxa patients were analyzed for congenital defects of glycosylation (CDG). Isoelectric focusing of plasma transferrin and apolipoproteinC-III showed that three out of nine patients had a defect in the biosynthesis of N-glycans and core 1 mucin type O-glycans, respectively. Mass spectrometric N-glycan analyses revealed a relative increase of glycans lacking sialic acid and glycans lacking sialic acid and galactose residues. Mutation analysis of the fibulin-5 gene (FBLN5), which has been reported in cases of autosomal recessive cutis laxa, revealed no mutations in the patients' DNA. Evidence is presented that extracellular matrix (ECM) proteins of skin are likely to be highly glycosylated with N- and/or mucin type O-glycans by using algorithms for predicting glycosylation. The conclusions in this study were that the clinical phenotype of autosomal recessive cutis laxa seen in three patients is not caused by mutations in the FBLN5 gene. Our findings define a novel form of CDG with cutis laxa and neurological involvement due to a defect in the sialylation and/or galactosylation of N- and O-glycans. Improper glycosylation of ECM proteins of skin may form the pathophysiological basis for the cutis laxa phenotype.

Apolipoprotein A-V-heparin interactions: implications for plasma lipoprotein metabolism

Transgenic and gene disruption experiments in mice have revealed that apolipoprotein (apo) A-V is a potent regulator of plasma triglyceride (TG) levels. To investigate the molecular basis of apoA-V function, the ability of isolated recombinant apoA-V to modulate lipoprotein lipase (LPL) activity was examined in vitro. With three distinct lipid substrate particles, including very low-density lipoprotein (VLDL), a TG/phospholipid emulsion, or dimyristoylphosphatidylcholine liposomes, apoA-V had little effect on LPL activity. In the absence or presence apolipoprotein C-II, apoA-V marginally inhibited LPL activity. On the other hand, apoA-V-dimyristoylphosphatidylcholine disc particles bound to heparin-Sepharose and were specifically eluted upon application of a linear gradient of NaCl. The interaction of apoA-V with sulfated glycosaminoglycans was further studied by surface plasmon resonance spectroscopy. ApoA-V showed strong binding to heparin-coated chips, and binding was competed by free heparin. ApoA-V enrichment enhanced binding of apoC-II-deficient chylomicrons and VLDL to heparin-coated chips. When LPL was first bound to the heparin-coated chip, apoA-V-enriched chylomicrons showed binding. Finally, human pre- and post-heparin plasma samples were subjected to immunoblot analysis with anti-apoA-V IgG. No differences in the amount of apoA-V present were detected. Taken together, the results show that apoA-V lipid complexes bind heparin and, when present on TG-rich lipoprotein particles, may promote their association with cell surface heparan sulfate proteoglycans. Through such interactions, apoA-V may indirectly affect LPL activity, possibly explaining its inverse correlation with plasma TG levels.

Genetic determinants of low high-density lipoprotein cholesterol

PURPOSE OF REVIEW

High-density lipoprotein cholesterol (HDL-C) has been well established as an inverse predictor of coronary heart disease (CHD), and in recent years, investigations have focused on the genetic regulation of high-density lipoprotein. Although numerous candidate genes contribute to the low HDL-C phenotype, their impact on CHD is heterogeneous, reflecting diverse gene-gene interactions and gene-environmental relationships. This review summarizes recent data involving HDL regulatory genes and their role in atherothrombosis.

RECENT FINDINGS

The primary genetic determinants associated with relative HDL-C deficiency states are the ATP binding cassette protein, ABCA1; apolipoprotein (APO) A1; and lecithin cholesteryl acyl transferase. Other potentially important candidates invoked in low HDL-C syndromes in humans include APOC3, lipoprotein lipase, sphingomyelin phosphodiesterase 1, and glucocerebrosidase. Molecular variation in ABCAI and APOAI and, in selected cases, lecithin cholesteryl acyl transferase deficiency have been associated with increased CHD, whereas two notable variants, APOAIMilano and APOAIParis, are associated with reduced risk.

SUMMARY

Low HDL-C syndromes have generally been correlated with an increased risk of CHD. However, single-gene abnormalities responsible for HDL-C deficiency states may have variable effects on atherothrombotic risk.

ApoC-III bound to apoB-containing lipoproteins increase with insulin resistance in Cherokee Indian youth

Because Native Americans are predisposed to obesity and type 2 diabetes associated with coronary artery disease, we assessed whether apoC-III bound to apoB-containing (LpB:C-III) and apoA-containing (LpA:C-III) lipoproteins, total apoC-III, apoB, and plasma lipids are associated with insulin resistance, body mass index (BMI), and waist circumference in Cherokee children and adolescents aged 5 to 19 years (n = 975). A cross-sectional analysis was done to determine associations of the lipoproteins with the homeostasis index (HOMA-IR) and BMI. When the data were grouped by quartiles for HOMA-IR and separated by three 5-year age groups (5-9, 10-14, and 15-19 years), the trend for LpB:C-III, triglyceride, and BMI z score to increase was significant for all age groups and both genders (P < .001). The trend to increase LpB:C-III with age was greater in boys (P < .0001) than in girls (P < .05) who tended to plateau after the age of 10 years. In contrast, the ratio of LpA:C-III to LpB:C-III decreased and the decrease was greater in boys (P < .0001) than girls (P < .01). Body mass index z score and waist circumference were correlated with LpB:C-III, triglyceride, apoB, and non-high-density lipoprotein cholesterol within each gender (P < .001). In multiple regression models, LpB:C-III, the dependent variable, was associated with HOMA-IR for both genders. We conclude that increases in LpB:C-III in childhood and adolescence are associated with insulin resistance and obesity supporting the need for prevention programs.

Post-transcriptional regulation of apoC-I synthesis and secretion in human HepG2 cells

ApoC-I plays an important role in controlling plasma lipid metabolism, however little is known about factors regulating the hepatic synthesis and secretion of this apolipoprotein. In the present study, we have carried out experiments with human hepatoma (HepG2) cells, in order to determine the effect of different tissue culture conditions on cellular lipid levels and on the production of apoC-I (and apoE) at the protein and mRNA level. Cells incubated for 48 h with 10% human serum had significantly higher cellular triglyceride (22%, P<0.05) and cholesterol levels (19%, P<0.01), higher medium apoC-I and apoE levels (2.6- and 2.9-fold, respectively), but similar levels of apoC-I and apoE mRNA, compared to cells incubated with 10% human lipoprotein-deficient serum (LPDS). Serum containing only HDL, or containing HDL with LDL, also increased cellular lipids and increased secreted apoC-I and apoE levels without altering apoC-I and apoE mRNA levels. Incubation of cells with Intralipid triglyceride (625 microM), increased cellular triglyceride (2.8-fold, P<0.001), decreased cellular cholesterol (32%, P<0.01), decreased cellular and medium apoC-I (24 and 26%, P<0.01) and had no effect on apoC-I mRNA levels. Additional experiments in which cells were loaded with cholesterol (incubation with 10 microg/ml cholesterol plus 1 microg/ml 25-hydroxycholesterol) or depleted of cholesterol (statin treatment) confirmed that secretion of apoC-I by HepG2 cells was dependent on cellular cholesterol levels and independent of changes in apoC-I mRNA levels. These results demonstrate that cellular cholesterol rather than triglyceride levels play a role in controlling apoC-I production by HepG2 cells and that this regulation occurs at a post-transcriptional level.

Defective protein glycosylation in patients with cutis laxa syndrome

Congenital cutis laxa is a genetically heterogeneous condition presenting with loose and redundant skin folds, decreased elasticity of the skin, connective tissue involvement and a highly variable spectrum of associated features. The most common forms are inherited in an autosomal recessive or dominant fashion. Fibulin 5 and elastin mutations were detected in a limited number of patients, but in most cases the etiology is not known. Based on a previous observation of an abnormal transferrin isoelectric focusing pattern in a patient with cutis laxa indicating an N-glycosylation defect, we performed a screening for disorders of protein glycosylation in unrelated children with cutis laxa syndrome, including a recently developed test for defective O-glycosylation. Here, we describe five patients from consanguineous marriages with a cutis laxa syndrome with skeletal and joint involvement, developmental delay and neurological findings. Three of these five children have an inborn error of glycan biosynthesis affecting the synthesis of both N- and O-linked glycans. Two patients had normal glycosylation patterns. All known causes of secondary glycosylation disorders were excluded in the children. No mutations were found in the FBLN5 gene. In conclusion, we have identified a new combined glycosylation defect with a distinct clinical phenotype. Our results suggest that a combined defect of glycosylation might be a causative factor in congenital cutis laxa. This is the first report where abnormal N- and O-linked glycosylation is implicated in the etiology of cutis laxa syndrome.

Greater enrichment of triacylglycerol-rich lipoproteins with apolipoproteins E and C-III after meals rich in saturated fatty acids than after meals rich in unsaturated fatty acids

BACKGROUND

Although there is considerable interest in the postprandial events involved in the absorption of dietary fats and the subsequent metabolism of diet-derived triacylglycerol-rich lipoproteins, little is known about the effects of meal fatty acids on the composition of these particles.

OBJECTIVE

We examined the effect of meal fatty acids on the lipid and apolipoprotein contents of triacylglycerol-rich lipoproteins.

DESIGN

Ten normolipidemic men received in random order a mixed meal containing 50 g of a mixture of palm oil and cocoa butter [rich in saturated fatty acids (SFAs)], safflower oil [n-6 polyunsaturated fatty acids (PUFAs)], or olive oil [monounsaturated fatty acids (MUFAs)] on 3 occasions. Fasting and postprandial apolipoproteins B-48, B-100, E, C-II, and C-III and lipids (triacylglycerol and cholesterol) were measured in plasma fractions with Svedberg flotation rates (S(f)) >400, S(f) 60-400, and S(f) 20-60.

RESULTS

Calculation of the composition of the triacylglycerol-rich lipoproteins (expressed per mole of apolipoprotein B) showed notable differences in the lipid and apolipoprotein contents of the SFA-enriched particles in the S(f) > 400 and S(f) 60-400 fractions. After the SFA meal, triacylglycerol-rich lipoproteins in these fractions showed significantly greater amounts of triacylglycerol and of apolipoproteins C-II (S(f) 60-400 fraction only), C-III, and E than were found after the MUFA meal (P < 0.02) and more cholesterol, apolipoprotein C-III (S(f) > 400 fraction only), and apolipoprotein E than after the PUFA meal (P < 0.02).

CONCLUSIONS

Differences in the composition of S(f) > 400 and S(f) 60-400 triacylglycerol-rich lipoproteins formed after saturated compared with unsaturated fatty acid-rich meals may explain differences in the metabolic handling of dietary fats.

Clinical and biochemical presentation of siblings with COG-7 deficiency, a lethal multiple O- and N-glycosylation disorder

Congenital disorders of glycosylation (CDG) represent a group of inherited multiorgan diseases caused by defects in the biosynthesis of glycoproteins. We report on two dysmorphic siblings with severe liver disease who died at the age of a few weeks. Increased activities of lysosomal enzymes in plasma were found, though total sialic acid in plasma was strongly decreased. Isoelectric focusing of serum sialotransferrins showed a type 2-like CDG pattern. Some of the known CDG subtypes were excluded. O-Glycosylation was investigated by isoelectric focusing of apolipoprotein C-III, which showed increased fractions of hyposialylated isoforms. In a consecutive study a defect in the conserved oligomeric Golgi complex was established at the level of subunit COG-7, leading to disruption of multiple glycosylation functions of the Golgi. This report on patients with a new variant of CDG, due to a multiple Golgi defect, emphasizes in addition to sialotransferrins the importance of analysis of a serum O-linked glycoprotein, e.g. apolipoprotein C-III, in unclassified CDG-X cases.

Foxo1 mediates insulin action on apoC-III and triglyceride metabolism

The apolipoprotein apoC-III plays an important role in plasma triglyceride metabolism. It is predominantly produced in liver, and its hepatic expression is inhibited by insulin. To elucidate the inhibitory mechanism of insulin in apoC-III expression, we delivered forkhead box O1 (Foxo1) cDNA to hepatocytes by adenovirus-mediated gene transfer. Foxo1 stimulated hepatic apoC-III expression and correlated with the ability of Foxo1 to bind to its consensus site in the apoC-III promoter. Deletion or mutation of the Foxo1 binding site abolished insulin response and Foxo1-mediated stimulation. Likewise, Foxo1 also mediated insulin action on intestinal apoC-III expression in enterocytes. Furthermore, elevated Foxo1 production in liver augmented hepatic apoC-III expression, resulting in increased plasma triglyceride levels and impaired fat tolerance in mice. Transgenic mice expressing a constitutively active Foxo1 allele exhibited hypertriglyceridemia. Moreover, we show that hepatic Foxo1 expression becomes deregulated as a result of insulin deficiency or insulin resistance, culminating in significantly elevated Foxo1 production, along with its skewed nuclear distribution, in livers of diabetic NOD or db/db mice. While loss of insulin response is associated with unrestrained apoC-III production and impaired triglyceride metabolism, these data suggest that Foxo1 provides a molecular link between insulin deficiency or resistance and aberrant apoC-III production in the pathogenesis of diabetic hypertriglyceridemia.

Synthesis and secretion of apoC-I and apoE during maturation of human SW872 liposarcoma cells

Little is known about the regulation of apolipoprotein (apo) C-I production by human adipocytes. The aim of the present study, therefore, was to investigate the effect of different tissue culture conditions on the synthesis and secretion of apoC-I and apoE in human SW872 liposarcoma cells. After 3-4 d in culture (0.5 x 10(6) cells/well, DMEM/F-12 medium with 10% fetal calf serum), cells reached confluence and became growth arrested. The molar ratio of apoE:apoC-I in the cell was 8.9 +/- 0.6 and in the medium was 6.6 +/- 0.5. After 17 d in culture, SW872 cells contained significantly more cholesterol (100%) and triglyceride (3-fold) and secreted more apoC-I [4 vs. 17 d: 0.11 +/- 0.01 vs. 0.23 +/- 0.01 pmol/(10(6) cells . 24 h), P < 0.001] and apoE [0.7 +/- 0.1 vs. 3.1 +/- 0.3 pmol/(10(6) cells . 24 h), P < 0.001]. Cellular apoC-I increased 7-fold and apoE increased 16-fold. Cell maturation was associated with significantly higher levels of apoE mRNA but not apoC-I mRNA. Increases in cell lipids, apoC-I, and apoE were not dependent on the presence of extracellular lipids because similar changes occurred in cells incubated with lipoprotein-deficient serum or in cells incubated without serum. Treatment (7 d) of cells during maturation with insulin (10 or 1000 nmol/L) significantly reduced the secretion of apoC-I and apoE. These results demonstrate that in maturing SW872 cells, cholesterol and triglyceride accumulation in the presence or absence of extracellular lipids, is associated with increased apoC-I and apoE production. Furthermore, apoC-I and apoE production are differentially regulated at the transcriptional level, and long-term treatment with insulin has an inhibitory rather than stimulatory effect on apoC-I and apoE production.

Impaired lipid metabolism in aged mice as revealed by fasting-induced expression of apolipoprotein mRNAs in the liver and changes in serum lipids

BACKGROUND

Changes in apolipoprotein (Apo) metabolism can cause an increased incidence of diseases such as cardiovascular disorders and diabetes with advancing age. Limited reports are available on this topic, however.

OBJECTIVE

To investigate age-related changes in mobilization of stored lipid, we studied the effects of fasting on the gene expression of Apos in the liver as well as serum triglyceride (TG) and cholesterol levels in the serum.

METHODS

Using young (6- to 8-month-old) and old (24- to 28-month-old) fasted and re-fed mice, Northern blots of hepatic mRNAs for Apos A-I, A-IV, C-II, C-III, and liver-type fatty acid-binding protein and HPLC analyses of serum lipids were conducted.

RESULTS

Fasting induced 4- and 20-fold increases in the mRNA of Apo C-II and A-IV, respectively, in young mice while only 1.1- and 7-fold increases, respectively, were detected in old mice. In contrast, the Apo C-III gene expression was significantly reduced by fasting in the young mice but the reduction was small in the old. In view of the stimulating effect of Apo C-II and A-IV and the inhibiting effect of C-III on lipoprotein lipase (LPL), these findings suggest that the fasting-induced activation of LPL may be considerably decreased in old mice. The amount of TG in very low-density lipoprotein (VLDL), a major form of the transport of TG to peripheral tissues, was significantly greater in the young than in the old mice. Despite possible activation of LPL by fasting, the amount of TG in VLDL, a major form of the transport of TG to peripheral tissues, was significantly greater in the young mice than in the old. It is indicated that the synthesis of VLDL in the liver is high in the young but low in the old mice, which also may be true for the rate of transport of TG.

CONCLUSION

The present findings suggest that mobilization of lipids is impaired in old animals due to decreased gene expression of Apos, possibly leading in the long run to excessive lipid accumulation in tissues such as the liver, adipose tissues and blood vessels even in normal feeding, and resulting in an increased incidence of age-related diseases.

Insulin in UW solution exacerbates hepatic ischemia / reperfusion injury by energy depletion through the IRS-2 / SREBP-1c pathway

Ischemia / reperfusion (I / R) injury is related to tissue graft energy status. Insulin, which is currently used in the University of Wisconsin (UW) preservation solution with insulin (UWI), is an anabolic hormone and was shown to exacerbate the hepatic I / R injury in our previous study. In this study, the energy status and regulation of metabolism genes by insulin were investigated in liver grafts preserved by UW solution. Insulin could significantly decrease adenosine triphosphate (ATP) level after 3 hours of preservation, as well as total adenine nucleotides (TANs) and energy charge (EC) levels. Energy regeneration deteriorated in the grafts preserved by insulin in terms of ATP and EC levels at 24 hours after transplantation. The insulin signal was transduced through the insulin receptor substrate-2 (IRS-2) pathway and the activity of IRS-2 was decreased gradually at the messenger ribonucleic acid (mRNA) level during cold preservation. Downstream targeting genes such as sterol regulatory element-binding protein-1c (SREBP-1c), glucokinase (GKC), and fatty acid synthase (FAS) genes, as well as phospho-glycogen synthase kinase-3beta (GSK-3beta) were activated and they showed the similar expression profiles during cold preservation. Lipoprotein metabolism was accelerated by insulin through upregulation of the activity of apolipoprotein C-III (Apo C-III) during cold preservation. The insulin-like growth factor-binding protein-1 pathway was inhibited during cold preservation. In conclusion, insulin in UW solution exacerbates hepatic I / R injury by energy depletion as the graft maintains its anabolic activity. The key enzyme activities of the energy-consuming process of glycogen and fatty acid synthesis as well as lipoprotein metabolism were accelerated by insulin through the IRS-2 / SREBP-1c pathway.

Apolipoprotein C-III SstI genotypes modulate exercise-induced hypotriglyceridemia

PURPOSE

To investigate whether the efficacy of exercise as an antitriglyceridemic treatment was dependent on the apolipoprotein (apo) C-III SstI gene polymorphism.

METHODS

A subgroup of 100 Korean men, who were prescreened by genotypes (S1S1 = 40, S1S2 = 40, S2S2 = 20), participated in a 6-d moderate-exercise intervention using either a motor-driven treadmill or a cycle ergometer. Height and weight for body mass index and waist circumference were measured. Fasting total cholesterol, triglycerides (TG), high-density lipoprotein cholesterol, low-density lipoprotein cholesterol, glucose, and insulin were measured. Univariate analysis of variance and ANCOVA followed by Bonferroni post hoc tests were used to compare the preintervention values and the pre- to postexercise change (delta) scores in the measured variables across the genotypes, respectively. Multivariate linear regressions were carried out with age, smoking, waist circumference, body mass index, and genotype as independent variables and lipids, glucose, and insulin as dependent variables.

RESULTS

Subjects with the S2S2 genotype had a significantly higher preintervention TG concentration than subjects with the S1S1 genotype (P = 0.022). Waist circumference and genotype were the only significant predictors for the preintervention TG (P < 0.001). With respect to the delta scores, the subjects with the S2S2 genotype had a greater reduction in TG than the subjects with the S1S1 genotype, adjusted for the preintervention difference (P = 0.047).

CONCLUSION

This study suggests that the apo C-III SstI gene polymorphism might be useful for determining who is most susceptible to intervention by exercise in lowering fasting TG concentration.

Isotretinoin and fenofibrate induce adiposity with distinct effect on metabolic profile in a rat model of the insulin resistance syndrome

OBJECTIVE

To investigate the effect of transcription-modulating drugs, fenofibrate and isotretinoin, on metabolic profile, insulin sensitivity of adipose and muscle tissues and gene expression in a genetic model of insulin resistance syndrome, polydactylous rat strain (PD/Cub).

DESIGN

Administration of fenofibrate (100 mg/kg/day), isotretinoin (30 mg/kg/day) or vehicle to adult male PD/Cub rats fed standard laboratory chow for 15 days.

MEASUREMENTS

Parameters of lipid and carbohydrate metabolism-oral glucose tolerance test, serum concentrations of insulin, triglycerides (TG), free fatty acids (FFA), glycerol, total cholesterol (CH); morphometric variables, in vitro insulin sensitivity of adipose and muscle tissues, catecholamine-stimulated lipolysis and the expression of ApoC-III and Hnf-4 genes in liver.

RESULTS

Both experimental groups displayed an increase in adiposity with contrasting effects on TG (lowered by fenofibrate and increased by isotretinoin) and gene expression (no change in fibrate-treated rats and increased expression of ApoC-III and Hnf-4 in isotretinoin-treated group). Fenofibrate-treated rats also showed decreased concentrations of FFA and CH with concomitant decrease of catecholamine-induced lipolysis in adipocytes, but also hyperinsulinemia and the highest insulin/glucose ratio. Isotretinoin increased glycerol concentrations and decreased the insulin sensitivity of peripheral tissues.

CONCLUSION

The PD/Cub rat showed a distinct pharmacogenetic reaction to fenofibrate and isotretinoin administration. Several lines of evidence now implicate specific variant(s) of ApoC-III and/or ApoA-V alleles as responsible for the dyslipidemia observed in this genetic model. The PD/Cub strain may also serve as a pharmacogenetic model for dissection of the retinoid-induced hypertriglyceridemia.

Hepatic CCAAT/enhancer binding protein alpha mediates induction of lipogenesis and regulation of glucose homeostasis in leptin-deficient mice

CCAAT/enhancer binding protein alpha (C/EBP alpha) is a critical factor in glucose metabolism in the neonate as revealed by conventional C/EBP alpha-null mice that do not survive beyond the first day after birth because of severe hypoglycemia and a deficiency in hepatic glycogen accumulation. To elucidate the function of C/EBP alpha in leptin-deficient mouse (ob/ob) liver, a C/EBP alpha-liver null mouse on an ob/ob background (ob/ob-C/EBP alpha/Cre(+)) was produced using a floxed C/EBP alpha allele and Cre recombinase under control of the albumin promoter (AlbCre). The C/EBP alpha-deficient liver in ob/ob mice had significantly decreased triglyceride content compared with equivalent mice lacking the AlbCre transgene (ob/ob-C/EBP alpha/Cre(-)). Expression of genes involved in lipogenesis including fatty acid synthase, acetyl-coenzyme A carboxylase, stearoyl-coenzyme A desaturase 1 and ATP-citrate lyase dramatically decreased in ob/ob-C/EBP alpha/Cre(+) mouse liver. Induction of these lipogenic genes by a high-carbohydrate diet caused an exacerbation in the development of fatty liver and an increase in liver size, hepatic triglyceride, and cholesterol contents in ob/ob-C/EBP alpha/Cre(-) mice but not in ob/ob-C/EBP alpha/Cre(+) mice. Deficiency in hepatic C/EBP alpha expression caused an exacerbation of hyperglycemia because of decreased insulin secretion. Taken together, these results indicate that hepatic C/EBP alpha plays a critical role in the acceleration of lipogenesis in ob/ob mice and in glucose homeostasis by the indirect regulation of insulin secretion.

The structure and interactions of human apolipoprotein C-II in dodecyl phosphocholine

The structure of human apolipoprotein C-II (apoC-II) in the presence of dodecyl phosphocholine (DPC) micelles has been investigated by NMR spectroscopy. The resulting structural information is compared to that available for apoC-II in the presence of sodium dodecyl sulfate, revealing a high level of overall similarity but several significant differences. These findings further our understandings of the structural basis for apoC-II function. The interactions of the protein with the detergent micelle are probed using intermolecular nuclear Overhauser effects (NOEs) and paramagnetic agents. These interactions are seen across almost the full length of apoC-II and show the periodicity expected for an amphipathic helix interacting with the amphipathic surface of the DPC micelle. Furthermore, we observe specific contacts between lysine residues of apoC-II and protons near the phosphate group of DPC, consistent with the predictions of the so-called "snorkel hypothesis" of the structural basis for the apolipoprotein/lipid interaction (Segrest, J. P., Jackson, R. L., Morrisett, J. D., and Gotto, A. M., Jr. (1974) A molecular theory of lipid-protein interactions in the plasma lipoproteins, FEBS Lett 38, 247-258.). These findings offer the most detailed structural information available for the interaction between an apolipoprotein and the phospholipids of the lipoprotein surface and provide the first direct structural support for the snorkel hypothesis.

Mechanism of triglyceride lowering in mice expressing human apolipoprotein A5

Overexpression of human APOA5 in mice results in dramatically decreased plasma triglyceride levels. In this study we explored the mechanism underlying this hypotriglyceridemic effect. Initially we found that triglyceride turnover was faster in hAPOA5 transgenic mice compared to controls, and this strongly correlated with increased LPL activity in postheparin plasma. Furthermore, we show that in vitro recombinant apoAV interacts physically with lipoprotein lipase and significantly increased its activity. We show that both apoB and apoCIII are decreased in hAPOA5 transgenic mice indicating a decrease in VLDL number. To further investigate the mechanism of hAPOA5 in a hyperlipidemic background, we inter-crossed hAPOA5 and hAPOC3 transgenic mice. We found a marked decrease in VLDL triglyceride and cholesterol, as well as apolipoprotein B and CIII levels. These data indicated that apoAV induces a decrease in VLDL size by activating lipolysis and an increase of VLDL clearance. In a postprandial state, the normal triglyceride response found in wild-type mice was significantly reduced in hAPOA5 transgenics. In addition, we demonstrated that in response to this fat load in hAPOA5xhAPOC3 mice, apoAV, but not apoCIII, was redistributed from primarily HDL to VLDL. This shift of apoAV in VLDL appears to limit the increase of triglyceride by activating the lipoprotein lipase.

Contributions of domain structure and lipid interaction to the functionality of exchangeable human apolipoproteins

Exchangeable apolipoproteins function in lipid transport as structural components of lipoprotein particles, cofactors for enzymes and ligands for cell-surface receptors. Recent findings with apoA-I and apoE suggest that the tertiary structures of these two members of the human exchangeable apolipoprotein gene family are related. Characteristically, these proteins contain a series of proline-punctuated, 11- or 22-amino acid, amphipathic alpha-helical repeats that can adopt a helix bundle conformation in the lipid-free state. The amino- and carboxyl-terminal regions form separate domains with the latter being primarily responsible for lipid binding. Interaction with lipid induces changes in the conformation of the amino-terminal domain leading to alterations in function; for example, opening of the amino-terminal four-helix bundle in apolipoprotein E upon lipid binding is associated with enhanced receptor-binding activity. The concept of a two-domain structure for the larger exchangeable apolipoproteins is providing new molecular insights into how these apolipoproteins interact with lipids and other proteins, such as receptors. The ways in which structural changes induced by lipid interaction modulate the functionality of these apolipoproteins are reviewed.

The role and mode of action of apolipoproteins CIII and AV: synergistic actors in triglyceride metabolism?

PURPOSE OF REVIEW

Apolipoprotein (apo)CIII and apoAV play an important role in triglyceride metabolism as evidenced by the unambiguous and opposing phenotypes of transgenic and knockout mouse models. In this review we discuss studies on the genetics, protein structure, and regulation of apoCIII and apoAV and compare their potential molecular mechanisms of action in triglyceride metabolism. We examine the hypothesis that apoCIII and apoAV synergistically affect triglyceride metabolism.

RECENT FINDINGS

It has now been firmly established that variation in plasma triglyceride levels in a wide range of human populations is strongly associated with genetic variation at the chromosomal locus encoding both the APOC3 and APOA5 genes, the APOA1/C3/A4/A5 gene cluster. The close physical linkage of these genes and the frequent concurrence of genetic variants, however, complicate the assignment of specific metabolic defects to specific polymorphisms. Recent insight into the regulation of APOC3 and APOA5 gene expression and structural modeling studies on the apoAV protein have provided novel clues for the potential molecular mechanisms responsible for the effects of apoCIII and apoAV on triglyceride metabolism.

SUMMARY

Hypertriglyceridemia is a major independent risk factor in the development of cardiovascular disease. Moreover, triglyceride-derived fatty acids are thought to play a key role in the development and progression of the metabolic syndrome. As modulators of triglyceride metabolism, apoCIII and apoAV are key players and potential therapeutic targets. However, little is known of their molecular mechanism and potential cooperativity. Rational therapeutic application will require the filling of this hiatus in our knowledge.

Sexually dimorphic P450 gene expression in liver-specific hepatocyte nuclear factor 4alpha-deficient mice

Hepatocyte nuclear factor (HNF) 4alpha is a liver-enriched nuclear receptor that plays a critical role in regulating the expression of numerous hepatic genes, including members of the cytochrome P450 (CYP) superfamily, several of which are expressed in a sex-dependent manner. Presently, we use a liver-specific Hnf4alpha-deficient mouse model to investigate the role of HNF4alpha in regulating liver-enriched transcription factors and sexually dimorphic Cyps in liver in vivo. Real-time PCR analysis of RNA isolated from livers of wild-type and Hnf4alpha-deficient mice revealed the following: 1) HNF4alpha exerts both positive regulation (Hnfalpha, C/ebpalpha, and C/ebpbeta) and negative regulation (Hnf3alpha and the HNF4alpha coactivator Pgc-1alpha) on liver transcription factor expression; 2) a strong dependence on HNF4alpha characterizes several male-predominant Cyps (2d9 and 8b1), female-predominant Cyps (2b10, 2b13, 3a41, and 3a44) and Cyps, whose expression is sex independent (3a11, 3a25); 3) HNF4alpha confers a unique, positive regulation of two male-expressed genes (Cyp4a12 and GSTpi) and a negative regulation of several female-predominant genes (Cyp2a4, Cyp2b9, Hnf3beta, and Hnf6), both of which are manifest in male but not female mouse liver. These trends were confirmed at the protein level by Western blot analysis using antibodies raised to Cyp2a, Cyp2b, and Cyp3a family members. Thus, HNF4alpha is an essential player in the complex regulatory network of liver-enriched transcription factors and the sexually dimorphic mouse Cyp genes that they regulate. HNF4alpha is proposed to contribute to the sex specificity of liver gene expression by positively regulating a subset of male-specific Cyp genes while concomitantly inhibiting the expression of certain female-specific Cyps and liver transcription factors, by mechanisms that are operative in male, but not female, mouse liver.

The structure and refinement of apocrustacyanin C2 to 1.3 A resolution and the search for differences between this protein and the homologous apoproteins A1 and C1

The blue carotenoprotein alpha-crustacyanin of Homarus gammarus lobster carapace is comprised chemically of five 20 kDa subunits. Only two genes for the proteins have been isolated (J. B. C. Findlay, personal communication) and the five apoproteins fall into two sets of homologous proteins based on their chemical properties (CRTC, consisting of apoproteins C(1), C(2) and A(1), and CRTA, consisting of apoproteins A(2) and A(3)). The diffraction quality of apo C(2) has been improved from 2.2 to 1.3 A and its structure solved. The structure is compared with the A(1) and C(1) proteins determined at 1.4 A [Cianci et al. (2001), Acta Cryst. D57, 1219-1229] and 1.15 A, respectively [Gordon et al. (2001), Acta Cryst. D57, 1230-1237] and found to be very similar. Normalized B-factor difference plots per residue of different types were used to try to find chemically modified residues; none were found at these resolutions. It remains possible that the differences between the CRTC proteins result from differences in amidation. By comparison of a crystal grown with glycerol (studied at 1.6 A) and one grown without glycerol (studied at 1.3 A) it was seen that glycerol bound at the astaxanthin site.

Dietary restriction in aged mice can partially restore impaired metabolism of apolipoprotein A-IV and C-III

Dietary restriction (DR) is only one well-established non-genetic experimental means to retard aging in various species of animals. Here we demonstrated that DR in mice for 3 months initiated late in life, at the age of 22 months, partially restores age-related decline of serum apolipoprotein A-IV (apo A-IV) level and the gene expression found in the liver of ad libitum fed young animals as revealed by fasting. In contrast, increase in gene expression of apo C-III by fasting was higher in the aged than that in the young, but it was reduced to the level of young animals in DR counterparts of the aged. In view of the implication that apo A-IV and C-III are involved in the activation and inhibition of lipoprotein lipase, respectively, the adult onset DR can conceivably upregulate the activity of this enzyme that is likely reduced in aged animals and thus improve the lipid metabolism. The present findings suggest that DR initiated even relatively late in life may reduce risk of age-related diseases associated with impaired lipid metabolism.

Plasma turnover of HDL apoC-I, apoC-III, and apoE in humans: in vivo evidence for a link between HDL apoC-III and apoA-I metabolism

Numerous factors are known to affect the plasma metabolism of HDL, including lipoprotein receptors, lipid transfer protein, lipolytic enzymes and HDL apolipoproteins. In order to better define the role of HDL apolipoproteins in determining plasma HDL concentrations, the aims of the present study were: a) to compare the in vivo rate of plasma turnover of HDL apolipoproteins [i.e., apolipoprotein A-I (apoA-I), apoC-I, apoC-III, and apoE], and b) to investigate to what extent these metabolic parameters are related to plasma HDL levels. We thus studied 16 individuals with HDL cholesterol levels ranging from 0.56-1.66 mmol/l and HDL apoA-I levels ranging from 89-149 mg/dl. Plasma kinetics of HDL apolipoproteins were investigated using a primed constant (12 h) infusion of deuterated leucine. Plasma HDL apolipoprotein levels were 41.8 +/- 1.5, 9.7 +/- 0.5, 4.9 +/- 0.5, and 0.7 +/- 0.1 micromol/l for apoA-I, apoC-I, apoC-III and apoE. Plasma transport rates (TRs) were 388.6 +/- 24.7, 131.5 +/- 12.5, 66.5 +/- 9.1, and 31.4 +/- 3.3 nmol.kg-1.day-1; and residence times (RTs) were 5.1 +/- 0.4, 3.7 +/- 0.3, 3.6 +/- 0.3, and 1.1 +/- 0.1 days, respectively. HDL cholesterol and apoA-I levels were significantly correlated with HDL apoA-I RT (r = 0.69 and r = 0.56), and were not significantly correlated with HDL apoA-I TR. In contrast, HDL apoC-I, apoC-III, and apoB levels were all positively related to their TRs and not their RTs. HDL apoC-III TR was positively correlated with levels of HDL apoC-III (r = 0.73, P < 0.01), and with those of HDL cholesterol and apoA-I (r = 0.54 and r = 0.53, P < 0.05, respectively). HDL apoC-III TR was in turn related to HDL apoA-I RT (r = 0.51, P < 0.05). Together, these results provide in vivo evidence for a link between the metabolism of HDL apoC-III and apoA-I, and suggest a role for apoC-III in the regulation of plasma HDL levels.

Overexpression of APOC1 in obob mice leads to hepatic steatosis and severe hepatic insulin resistance

Obese obob mice with strong overexpression of the human apolipoprotein C1 (APOC1) exhibit excessive free fatty acid (FFA) and triglyceride (TG) levels and severely reduced body weight (due to the absence of subcutaneous adipose tissue) and skin abnormalities. To evaluate the effects of APOC1 overexpression on hepatic and peripheral insulin sensitivity in a less-extreme model, we generated obob mice with mild overexpression of APOC1 (obob/APOC1(+/-)) and performed hyperinsulinemic clamp analysis. Compared with obob littermates, obob/APOC1(+/-) mice showed reduced body weight (-25%) and increased plasma levels of TG (+632%), total cholesterol (+134%), FFA (+65%), glucose (+73%), and insulin (+49%). Hyperinsulinemic clamp analysis revealed severe whole-body and hepatic insulin resistance in obob/APOC1(+/-) mice and, in addition, increased hepatic uptake of FFA and hepatic TG content. Treatment of obob/APOC1(+/-) mice with rosiglitazone strongly improved whole-body insulin sensitivity as well as hepatic insulin sensitivity, despite a further increase of hepatic fatty acid (FA) uptake and a panlobular increase of hepatic TG accumulation. We conclude that overexpression of APOC1 prevents rosiglitazone-induced peripheral FA uptake leading to severe hepatic steatosis. Interestingly, despite rosiglitazone-induced hepatic steatosis, hepatic insulin sensitivity improves dramatically. We hypothesize that the different hepatic fat accumulation and/or decrease in FA intermediates has a major effect on the insulin sensitivity of the liver.

1,1'-bis(anilino)-4-,4'-bis(naphtalene)-8,8'-disulfonate acts as an inhibitor of lipoprotein lipase and competes for binding with apolipoprotein CII

Lipoprotein lipase (LPL) is dependent on apolipoprotein CII (apoCII), a component of plasma lipoproteins, for function in vivo. The hydrophobic fluorescent probe 1,1'-bis(anilino)-4,4'-bis(naphthalene)-8,8'-disulfonate (bis-ANS) was found to be a potent inhibitor of LPL. ApoCII prevented the inhibition by bis-ANS, and was also able to restore the activity of inhibited LPL in a competitive manner, but only with triacylglycerols with acyl chains longer than three carbons. Studies of fluorescence and surface plasmon resonance indicated that LPL has an exposed hydrophobic site for binding of bis-ANS. The high affinity interaction was characterized by an equilibrium constant Kd of 0.10-0.26 microm and by a relatively high on rate constant kass = 2.0 x 10(4) m(-1) s(-1) and a slow off-rate with a dissociation rate constant kdiss = 1.2 x 10(-4) s(-1). The high affinity binding of bis-ANS did not influence interaction of LPL with heparin or with lipid/water interfaces and did not dissociate the active LPL dimer into monomers. Analysis of fragments of LPL after photoincorporation of bis-ANS indicated that the high affinity binding site was located in the middle part of the N-terminal folding domain. We propose that bis-ANS binds to an exposed hydrophobic area that is located close to the active site. This area may be the binding site for individual substrate molecules and also for apoCII.

Effect of atorvastatin on ApoE and ApoC-I synthesis and secretion by THP-1 macrophages

Apolipoprotein (apo) E and C-I are plasma apolipoproteins that have been implicated in the etiology of atherosclerosis and obesity, respectively. Both proteins are synthesized and secreted by macrophages, though pharmacological regulation of their production is poorly understood. The authors compared the effect of 2 HMG-CoA reductase inhibitors, atorvastatin and cerivastatin, on the synthesis and secretion of apoE and apoC-I by THP-1 macrophages. Atorvastatin reduced medium apoE and cellular apoE mRNA of PMA-activated THP-1 cells in a dose-dependent manner (-24% and -22%, respectively, at 1-micromol/L, P < 0.01). ApoC-I in the medium was also reduced by atorvastatin in a dose-dependent manner, though to a lesser extent (-15% at 1-micromol/L, P < 0.05). Cerivastatin similarly reduced medium apoE (-20% at 1-micromol/L, P < 0.05) and cellular apoE mRNA (-31% at 1-micromol/L, P < 0.05), and significantly lowered cellular apoC-I mRNA (-15%, P < 0.05), but not apoC-I in the medium. In experiments with THP-1 macrophages loaded with cholesterol (ie, 24-hour incubation with acetyl-LDL), atorvastatin and cerivastatin (1-micromol/L) significantly (P < 0.05) reduced both medium apoE (-30% and -25%, respectively) and cellular apoE mRNA (-25% and -17%, respectively). A lower and less consistent effect was observed on medium apoC-I (-6% and -18%, respectively) and cellular apoC-I mRNA (-13% and -19%, respectively). These data demonstrate that statins have the capacity to reduce the synthesis and secretion of both apoE and apoC-I in THP-1 macrophages loaded or unloaded with cholesterol.

Farnesoid X receptor agonists suppress hepatic apolipoprotein CIII expression

BACKGROUND & AIMS

Increased serum triglyceride levels constitute a risk factor for coronary heart disease. Apolipoprotein CIII (Apo CIII) is a determinant of serum triglyceride metabolism. In this study, we investigated whether activators of the nuclear farnesoid X receptor (FXR) modulate Apo CIII gene expression.

METHODS

The influence of bile acids and synthetic FXR activators on Apo CIII and triglyceride metabolism was studied in vivo by using FXR wild-type and FXR-deficient mice and in vitro by using human primary hepatocytes and HepG2 cells.

RESULTS

In mice, treatment with the FXR agonist taurocholic acid strongly decreased serum triglyceride levels, an effect associated with reduced Apo CIII serum and liver messenger RNA levels. By contrast, no change was observed in FXR-deficient mice. Incubation of human primary hepatocytes and HepG2 cells with bile acids or the nonsteroidal synthetic FXR agonist GW4064 resulted in a dose-dependent down-regulation of Apo CIII gene expression. Promoter transfection experiments and mutation analysis showed that bile acid-activated FXR decrease human Apo CIII promoter activity via a negative FXR response element located in the I(4) footprint between nucleotides -739 and -704. Chromatin immunoprecipitation experiments showed that bile acid treatment led to binding of FXR/retinoid X receptor heterodimers to and displacement of HNF4alpha from this site. Bile acid treatment still repressed liver Apo CIII gene expression in hepatic HNF4alpha-deficient mice, suggesting an active rather than a competitive mechanism of Apo CIII repression by the FXR.

CONCLUSIONS

We identified bile acid and synthetic activators of the nuclear FXR as negative regulators of Apo CIII expression, an effect that may contribute to the triglyceride-decreasing action of FXR agonists.

Identification of a lipoprotein lipase cofactor-binding site by chemical cross-linking and transfer of apolipoprotein C-II-responsive lipolysis from lipoprotein lipase to hepatic lipase

To localize the regions of lipoprotein lipase (LPL) that are responsive to activation by apoC-II, an apoC-II peptide fragment was cross-linked to bovine LPL. Following chemical hydrolysis and peptide separation, a specific fragment of LPL (residues 65-86) was identified to interact with apoC-II. The fragment contains regions of amino acid sequence dissimilarity compared with hepatic lipase (HL), a member of the same gene family that is not responsive to apoC-II. Using site-directed mutagenesis, two sets of chimeras were created in which the two regions of human LPL (residues 65-68 and 73-79) were exchanged with the corresponding human HL sequences. The chimeras consisted of an HL backbone with the suspected LPL regions replacing the corresponding HL sequences either individually (HLLPL-(65-68) and HLLPL-(73-79)) or together (HLLPLD). Similarly, LPL chimeras were created in which the candidate regions were replaced with the corresponding HL sequences (LPLHL-(77-80), LPLHL-(85-91), and LPLHLD). Using a synthetic triolein substrate, the lipase activity of the purified enzymes was measured in the presence and absence of apoC-II. Addition of apoC-II to HLLPL-(65-68) and HLLPL-(73-79) did not significantly alter their enzyme activity. However, the activity of HLLPLD increased approximately 5-fold in the presence of apoC-II compared with an increase in native LPL activity of approximately 11-fold. Addition of apoC-II to LPLHL-(77-80) resulted in approximately 10-fold activation, whereas only approximately 6- and approximately 4-fold activation of enzyme activity was observed in LPLHL-(85-91) and LPLHLD, respectively. In summary, our results have identified 11 amino acid residues in the N-terminal domain of LPL (residues 65-68 and 73-79) that appear to act cooperatively to enable substantial activation of human LPL by apoC-II.