Dual roles for lipolysis and oxidation in peroxisome proliferation-activator receptor responses to electronegative low density lipoprotein

Lipids and lipoproteins may play a role in the neuropathology of Alzheimer's disease

Alzheimer's disease (AD) and other classes of dementia are important public health problems with overwhelming social, physical, and financial effects for patients, society, and their families and caregivers. The pathophysiology of AD is poorly understood despite the extensive number of clinical and experimental studies. The brain's lipid-rich composition is linked to disturbances in lipid homeostasis, often associated with glucose and lipid abnormalities in various neurodegenerative diseases, including AD. Moreover, elevated low-density lipoprotein (LDL) cholesterol levels may be related to a higher probability of AD. Here, we hypothesize that lipids, and electronegative LDL (L5) in particular, may be involved in the pathophysiology of AD. Although changes in cholesterol, triglyceride, LDL, and glucose levels are seen in AD, the cause remains unknown. We believe that L5-the most electronegative subfraction of LDL-may be a crucial factor in understanding the involvement of lipids in AD pathology. LDL and L5 are internalized by cells through different receptors and mechanisms that trigger separate intracellular pathways. One of the receptors involved in L5 internalization, LOX-1, triggers apoptotic pathways. Aging is associated with dysregulation of lipid homeostasis, and it is believed that alterations in lipid metabolism contribute to the pathogenesis of AD. Proposed mechanisms of lipid dysregulation in AD include mitochondrial dysfunction, blood-brain barrier disease, neuronal signaling, inflammation, and oxidative stress, all of which lead ultimately to memory loss through deficiency of synaptic integration. Several lipid species and their receptors have essential functions in AD pathogenesis and may be potential biomarkers.

Can Electronegative LDL Act as a Multienzymatic Complex?

Electronegative LDL (LDL(-)) is a minor form of LDL present in blood for which proportions are increased in pathologies with increased cardiovascular risk. In vitro studies have shown that LDL(-) presents pro-atherogenic properties, including a high susceptibility to aggregation, the ability to induce inflammation and apoptosis, and increased binding to arterial proteoglycans; however, it also shows some anti-atherogenic properties, which suggest a role in controlling the atherosclerotic process. One of the distinctive features of LDL(-) is that it has enzymatic activities with the ability to degrade different lipids. For example, LDL(-) transports platelet-activating factor acetylhydrolase (PAF-AH), which degrades oxidized phospholipids. In addition, two other enzymatic activities are exhibited by LDL(-). The first is type C phospholipase activity, which degrades both lysophosphatidylcholine (LysoPLC-like activity) and sphingomyelin (SMase-like activity). The second is ceramidase activity (CDase-like). Based on the complementarity of the products and substrates of these different activities, this review speculates on the possibility that LDL(-) may act as a sort of multienzymatic complex in which these enzymatic activities exert a concerted action. We hypothesize that LysoPLC/SMase and CDase activities could be generated by conformational changes in apoB-100 and that both activities occur in proximity to PAF-AH, making it feasible to discern a coordinated action among them.

Modified Lipoproteins Induce Arterial Wall Inflammation During Atherogenesis

Circulating apolipoprotein B-containing lipoproteins, notably the low-density lipoproteins, enter the inner layer of the arterial wall, the intima, where a fraction of them is retained and modified by proteases, lipases, and oxidizing agents and enzymes. The modified lipoproteins and various modification products, such as fatty acids, ceramides, lysophospholipids, and oxidized lipids induce inflammatory reactions in the macrophages and the covering endothelial cells, initiating an increased leukocyte diapedesis. Lipolysis of the lipoproteins also induces the formation of cholesterol crystals with strong proinflammatory properties. Modified and aggregated lipoproteins, cholesterol crystals, and lipoproteins isolated from human atherosclerotic lesions, all can activate macrophages and thereby induce the secretion of proinflammatory cytokines, chemokines, and enzymes. The extent of lipoprotein retention, modification, and aggregation have been shown to depend largely on differences in the composition of the circulating lipoprotein particles. These properties can be modified by pharmacological means, and thereby provide opportunities for clinical interventions regarding the prevention and treatment of atherosclerotic vascular diseases.

Whole-body insulin resistance and energy expenditure indices, serum lipids, and skeletal muscle metabolome in a state of lipoprotein lipase overexpression

INTRODUCTION

Overexpression of lipoprotein lipase (LPL) protects against high-fat-diet (HFD)-induced obesity and insulin resistance in transgenic rabbits; however, the molecular mechanisms remain unclear. Skeletal muscle is a major organ responsible for insulin-stimulated glucose uptake and energy expenditure.

OBJECTIVES

The main purpose of the current study was to examine the effects of the overexpression of LPL on the skeletal muscle metabolomic profiles to test our hypothesis that the mitochondrial oxidative metabolism would be activated in the skeletal muscle of LPL transgenic rabbits and that the higher mitochondrial oxidative metabolism activity would confer better phenotypic metabolic outcomes.

METHODS

Under a HFD, insulin resistance index was measured using the intravenous glucose tolerance test, and total energy expenditure (TEE) was measured by doubly-labeled water in control and LPL transgenic rabbits (n = 12, each group). Serum lipids, such as triglycerides and free fatty acid, were also measured. The skeletal muscle metabolite profile was analyzed using capillary electrophoresis time-of flight mass spectrometry in the two groups (n = 9, each group). A metabolite set enrichment analysis (MSEA) with muscle metabolites and a false discovery rate q < 0.2 was performed to identify significantly different metabolic pathways between the 2 groups.

RESULTS

The triglycerides and free fatty acid levels and insulin resistance index were lower, whereas the TEE was higher in the LPL transgenic rabbits than in the control rabbits. Among 165 metabolites detected, the levels of 37 muscle metabolites were significantly different between the 2 groups after false discovery rate correction (q < 0.2). The MSEA revealed that the TCA cycle and proteinogenic amino acid metabolism pathways were significantly different between the 2 groups (P < 0.05). In the MSEA, all four selected metabolites for the TCA cycle (2-oxoglutaric acid, citric acid, malic acid, fumaric acid), as well as eight selected metabolites for proteinogenic amino acid metabolism (asparagine, proline, methionine, phenylalanine, histidine, arginine, leucine, isoleucine) were consistently increased in the transgenic rabbits compared with control rabbits, suggesting that these two metabolic pathways were activated in the transgenic rabbits. Some of the selected metabolites, such as citric acid and methionine, were significantly associated with serum lipids and insulin resistance (P < 0.05).

CONCLUSION

The current results suggest that the overexpression of LPL may lead to increased activities of TCA cycle and proteinogenic amino acid metabolism pathways in the skeletal muscle, and these enhancements may play an important role in the biological mechanisms underlying the anti-obesity/anti-diabetes features of LPL overexpression.

Molecular and Cellular Mechanisms of Electronegative Lipoproteins in Cardiovascular Diseases

Dysregulation of glucose and lipid metabolism increases plasma levels of lipoproteins and triglycerides, resulting in vascular endothelial damage. Remarkably, the oxidation of lipid and lipoprotein particles generates electronegative lipoproteins that mediate cellular deterioration of atherosclerosis. In this review, we examined the core of atherosclerotic plaque, which is enriched by byproducts of lipid metabolism and lipoproteins, such as oxidized low-density lipoproteins (oxLDL) and electronegative subfraction of LDL (LDL(−)). We also summarized the chemical properties, receptors, and molecular mechanisms of LDL(−). In combination with other well-known markers of inflammation, namely metabolic diseases, we concluded that LDL(−) can be used as a novel prognostic tool for these lipid disorders. In addition, through understanding the underlying pathophysiological molecular routes for endothelial dysfunction and inflammation, we may reassess current therapeutics and might gain a new direction to treat atherosclerotic cardiovascular diseases, mainly targeting LDL(−) clearance.

Electronegative LDL: An Active Player in Atherogenesis or a By- Product of Atherosclerosis?

Low-density lipoproteins (LDLs) are the major plasma carriers of cholesterol. However, LDL particles must undergo various molecular modifications to promote the development of atherosclerotic lesions. Modified LDL can be generated by different mechanisms, but as a common trait, show an increased electronegative charge of the LDL particle. A subfraction of LDL with increased electronegative charge (LDL(-)), which can be isolated from blood, exhibits several pro-atherogenic characteristics. LDL(-) is heterogeneous, due to its multiple origins but is strongly related to the development of atherosclerosis. Nevertheless, the implication of LDL(-) in a broad array of pathologic conditions is complex and in some cases anti-atherogenic LDL(-) properties have been reported. In fact, several molecular modifications generating LDL(-) have been widely studied, but it remains unknown as to whether these different mechanisms are specific or common to different pathological disorders. In this review, we attempt to address these issues examining the most recent findings on the biology of LDL(-) and discussing the relationship between this LDL subfraction and the development of different diseases with increased cardiovascular risk. Finally, the review highlights the importance of minor apolipoproteins associated with LDL(-) which would play a crucial role in the different properties displayed by these modified LDL particles.

Efficacy of functional foods mixture in improving hypercholesterolemia, inflammatory and endothelial dysfunction biomarkers-induced by high cholesterol diet

Background

Hypercholesterolemia associated with cardiovascular diseases is a global health issue that could be alleviated by functional foods. This study aimed to explore the effects of a high-cholesterol diet on lipid profile, cardiac, inflammatory, and endothelial dysfunction biomarkers, and the possible improvement by functional foods mixture.

Methods

Male albino rats weighing 100–150 g were randomly divided into four equal groups: 1st control, giving a normal diet; the 2nd received high-cholesterol diet for 8 weeks, the 3rd received the high-cholesterol diet + functional foods mixture, and the 4th administered high-cholesterol diet +atorvastatin (20 mg) orally.

Results

The results showed a significant increase in lipid profile and cardiac biomarkers levels (lactate dehydrogenase, creatine kinase and homocystein), also inflammatory markers, as, tumor necrotic factor alpha and chronic reactive proteins were elevated, moreover, vascular adhesion molecule-1 and nitric oxide synthase were disturbed in high-cholesterol diet compared with normal group. While administration of atorvastatin and functional foods mixture ameliorated these alterations.

Conclusions

Administration of functional foods mixture and atorvastatin were effective in treating hypercholesterolemia, reduce the risk of inflammation and cardiovascular biomarkers with a high safety margin. These efficiencies may be due to its active ingredient that improve the imbalance in the measured biomarkers.

Low-density lipoprotein electronegativity is a novel cardiometabolic risk factor

BACKGROUND

Low-density lipoprotein (LDL) plays a central role in cardiovascular disease (CVD) development. In LDL chromatographically resolved according to charge, the most electronegative subfraction-L5-is the only subfraction that induces atherogenic responses in cultured vascular cells. Furthermore, increasing evidence has shown that plasma L5 levels are elevated in individuals with high cardiovascular risk. We hypothesized that LDL electronegativity is a novel index for predicting CVD.

METHODS

In 30 asymptomatic individuals with metabolic syndrome (MetS) and 27 healthy control subjects, we examined correlations between plasma L5 levels and the number of MetS criteria fulfilled, CVD risk factors, and CVD risk according to the Framingham risk score.

RESULTS

L5 levels were significantly higher in MetS subjects than in control subjects (21.9±18.7 mg/dL vs. 11.2±10.7 mg/dL, P:0.01). The Jonckheere trend test revealed that the percent L5 of total LDL (L5%) and L5 concentration increased with the number of MetS criteria (P<0.001). L5% correlated with classic CVD risk factors, including waist circumference, body mass index, waist-to-height ratio, smoking status, blood pressure, and levels of fasting plasma glucose, triglyceride, and high-density lipoprotein. Stepwise regression analysis revealed that fasting plasma glucose level and body mass index contributed to 28% of L5% variance. The L5 concentration was associated with CVD risk and contributed to 11% of 30-year general CVD risk variance when controlling the variance of waist circumference.

CONCLUSION

Our findings show that LDL electronegativity was associated with multiple CVD risk factors and CVD risk, suggesting that the LDL electronegativity index may have the potential to be a novel index for predicting CVD. Large-scale clinical trials are warranted to test the reliability of this hypothesis and the clinical importance of the LDL electronegativity index.

Electronegative LDL: a circulating modified LDL with a role in inflammation

Electronegative low density lipoprotein (LDL(−)) is a minor modified fraction of LDL found in blood. It comprises a heterogeneous population of LDL particles modified by various mechanisms sharing as a common feature increased electronegativity. Modification by oxidation is one of these mechanisms. LDL(−) has inflammatory properties similar to those of oxidized LDL (oxLDL), such as inflammatory cytokine release in leukocytes and endothelial cells. However, in contrast with oxLDL, LDL(−) also has some anti-inflammatory effects on cultured cells. The inflammatory and anti-inflammatory properties ascribed to LDL(−) suggest that it could have a dual biological effect.

Electronegative low-density lipoprotein. A link between apolipoprotein B misfolding, lipoprotein aggregation and proteoglycan binding

PURPOSE OF REVIEW

Subendothelial retention of lipoproteins is considered the first step in the development of atherosclerosis, but the molecular mechanisms involved are poorly understood. Recent findings on the atherogenic properties of a minor electronegative fraction of LDL (LDL(-)) could contribute to a better understanding of this process.

RECENT FINDINGS

Circular dichroism, Trp-fluorescence and two-dimensional nuclear magnetic resonance have shown that apolipoprotein B (apoB) in LDL(-) has an abnormal, misfolded conformation. Immunochemical analysis revealed a different conformation, mainly in the N-terminal and C-terminal extremes. These alterations contribute to the high susceptibility to aggregation of LDL(-). Moreover, LDL(-) can seed the aggregation of native LDL, suggesting an amyloidogenic character that has been attributed to the amphipathic helix cluster in the α2-domain. A phospholipase C (PLC)-like activity associated to LDL(-) seems to play a major role in the LDL(-)-induced aggregation. The aggregation of LDL(-) increases its binding to proteoglycans because of the abnormal conformation of the N-terminal extreme of apoB.

SUMMARY

LDL(-) could play a relevant role in atherogenesis by acting as a priming factor that stimulates lipoprotein aggregation. This process, which appears to be mediated by a PLC-like activity intrinsic to LDL(-), increases the binding of LDL to proteoglycans and could promote subendothelial retention of these lipoproteins.

Chemical composition-oriented receptor selectivity of L5, a naturally occurring atherogenic low-density lipoprotein

Anion-exchange chromatography resolves human plasma low-density lipoprotein (LDL) into 5 subfractions, with increasing negative surface charge in the direction of L1 to L5. Unlike the harmless L1 to L4, the exclusively atherogenic L5 is rejected by the normal LDL receptor (LDLR) but endocytosed into vascular endothelial cells through the lectin-like oxidized LDL receptor-1 (LOX-1). Analysis with SDS-PAGE and 2-dimensional electrophoresis showed that the protein framework of L1 was composed mainly of apolipoprotein (apo) B100, with an isoelectric point (pI) of 6.620. There was a progressively increased association of additional proteins, including apoE (pI 5.5), apoAI (pI 5.4), apoCIII (pI 5.1), and apo(a) (pI 5.5), from L1 to L5. LC/MSE was used to quantify protein distribution in all subfractions. On the basis of weight percentages, L1 contained 99% apoB-100 and trace amounts of other proteins. In contrast, L5 contained 60% apoB100 and substantially increased amounts of apo(a), apoE, apoAI, and apoCIII. The compositional characteristics contribute to L5's electronegativity, rendering it unrecognizable by LDLR. LOX-1, which has a high affinity for negatively charged ligands, is known to mediate the signaling of proinflammatory cytokines. Thus, the chemical composition-oriented receptor selectivity hinders normal metabolism of L5, enhancing its atherogenicity through abnormal receptors, such as LOX-1.

Poloxamer 407 as a general lipase inhibitor: its implications in lipid metabolism and atheroma formation in C57BL/6 mice

OBJECTIVES

The aims of this investigation were to determine if the activity of endothelial lipase was inhibited in vitro by the polymeric surfactant, poloxamer 407 (P-407), and to review the action of P-407 on the biological activity of several other critical lipases involved in lipid metabolism. The overall goal was to suggest a possible explanation for the observed dyslipidaemia and formation of aortic atherosclerotic lesions when present and previous findings, along with key findings proposed by others, were combined and reviewed in the context of the P-407-induced mouse model of atherogenesis.

KEY FINDINGS

Endothelial lipase was expressed using a recombinant adenovirus; subconfluent COS cells were exposed to recombinant adenoviruses and triglyceride lipase activity was determined using standard assay methods. It was demonstrated that P-407 inactivates endothelial lipase in vitro. Endothelial lipase was inhibited in vitro by P-407, with an IC50 of approximately 11.3 µm. It is suggested that one possible explanation for the eventual formation of aortic atherosclerotic lesions in the P-407-induced mouse model of atherogenesis may be related to the capacity of P-407 to inhibit the activity of several critical lipases involved in lipid metabolism.

CONCLUSIONS

The following physiological and biochemical processes are all observed in the P-407-induced mouse model of dyslipidaemia and atherosclerosis: inactivation of endothelial lipase (both in vitro and in vivo), hepatic lipase and lipoprotein lipase by P-407; reduced plasma levels of high-density lipoprotein cholesterol; no activation of either human or mouse peroxisome proliferator-activated receptor α (as well as peroxisome proliferator-activated receptor γ) in vitro or in vivo; increased plasma concentrations of soluble vascular cell adhesion molecule 1, soluble intercellular adhesion molecule 1 and soluble E-selectin; and aortic atherosclerotic lesion formation following 12-16 weeks of P-407 administration (0.5 g/kg administered every 3 days by intraperitoneal injection). This biochemical-based pathway, or series of events, may contribute, in part, to the dyslipidaemia and eventual formation of aortic atherosclerotic lesions observed in the P-407-induced mouse model of atherogenesis.

The vascular contribution to Alzheimer's disease

AD (Alzheimer's disease) is a progressive neurodegenerative disease of unknown origin. Despite questions as to the underlying cause(s) of this disease, shared risk factors for both AD and atherosclerotic cardiovascular disease indicate that vascular mechanisms may critically contribute to the development and progression of both AD and atherosclerosis. An increased risk of developing AD is linked to the presence of the apoE4 (apolipoprotein E4) allele, which is also strongly associated with increased risk of developing atherosclerotic cardiovascular disease. Recent studies also indicate that cardiovascular risk factors, including elevated blood cholesterol and triacylglycerol (triglyceride), increase the likelihood of AD and vascular dementia. Lipids and lipoproteins in the circulation interact intimately with the cerebrovasculature, and may have important effects on its constituent brain microvascular endothelial cells and the adjoining astrocytes, which are components of the neurovascular unit. The present review will examine the potential mechanisms for understanding the contributions of vascular factors, including lipids, lipoproteins and cerebrovascular Abeta (amyloid beta), to AD, and suggest therapeutic strategies for the attenuation of this devastating disease process. Specifically, we will focus on the actions of apoE, TGRLs (triacylglycerol-rich lipoproteins) and TGRL lipolysis products on injury of the neurovascular unit and increases in blood-brain barrier permeability.

Validation of a novel ELISA for measurement of electronegative low-density lipoprotein

BACKGROUND

Oxidative modification of low-density lipoprotein (LDL) plays a key role in the pathogenesis of atherosclerosis. LDL(-) is present in blood plasma of healthy subjects and at higher concentrations in diseases with high cardiovascular risk, such as familial hypercholesterolemia or diabetes.

METHODS

We developed and validated a sandwich ELISA for LDL(-) in human plasma using two monoclonal antibodies against LDL(-) that do not bind to native LDL, extensively copper-oxidized LDL or malondialdehyde-modified LDL. The characteristics of assay performance, such as limits of detection and quantification, accuracy, inter- and intra-assay precision were evaluated. The linearity, interferences and stability tests were also performed.

RESULTS

The calibration range of the assay is 0.625-20.0 mU/L at 1:2000 sample dilution. ELISA validation showed intra- and inter-assay precision and recovery within the required limits for immunoassays. The limits of detection and quantification were 0.423 mU/L and 0.517 mU/L LDL(-), respectively. The intra- and inter-assay coefficient of variation ranged from 9.5% to 11.5% and from 11.3% to 18.9%, respectively. Recovery of LDL(-) ranged from 92.8% to 105.1%.

CONCLUSIONS

This ELISA represents a very practical tool for measuring LDL(-) in human blood for widespread research and clinical sample use.

Mediation of electronegative low-density lipoprotein signaling by LOX-1: a possible mechanism of endothelial apoptosis

The lectin-like oxidized LDL receptor LOX-1 mediates endothelial cell (EC) uptake of experimentally prepared copper-oxidized LDL (oxLDL). To confirm the atherogenic role of this receptor cloned against copper-oxLDL, we examined whether it mediates EC uptake of L5, an electronegative LDL abundant in dyslipidemic but not normolipidemic human plasma. Hypercholesterolemic (LDL-cholesterol, >160 mg/dL) human LDL was fractionated into L1-L5, increasingly electronegative, by ion-exchange chromatography. In cultured bovine aortic ECs (BAECs), L5 upregulated LOX-1 and induced apoptosis. Transfection of BAECs with LOX-1-specific small interfering RNAs (siLOX-1) minimized baseline LOX-1 production and restrained L5-induced LOX-1 upregulation. Internalization of labeled L1-L5 was monitored in BAECs and human umbilical venous ECs by fluorescence microscopy. LOX-1 knockdown with siLOX-1 impeded the endocytosis of L5 but not L1-L4. In contrast, blocking LDL receptor with RAP (LDL receptor-associated protein) stopped the internalization of L1-L4 but not L5. Although chemically different, L5 and oxLDL competed for EC entry through LOX-1. Via LOX-1, L5 signaling hampered Akt phosphorylation and suppressed EC expression of fibroblast growth factor-2 and Bcl-2. L5 also selectively inhibited Bcl-xL expression and endothelial nitric oxide synthase phosphorylation but increased synthesis of Bax, Bad, and tumor necrosis factor-alpha. Blocking Akt phosphorylation with wortmannin increased LOX-1 expression, suggesting a modulatory role of Akt in LOX-1 synthesis; L5 upregulated LOX-1 by dephosphorylating Akt. Because endothelial nitric oxide synthase and Bcl-2 activities are Akt-dependent, L5 impairs Akt-mediated growth and survival signals in vascular ECs by way of LOX-1. Thus, the L5/LOX-1 complex may play a critical role in atherogenesis and illuminate important targets for disease intervention.

PPARgamma in the endothelium regulates metabolic responses to high-fat diet in mice

Although endothelial dysfunction, defined as abnormal vasoreactivity, is a common early finding in individuals with type 2 diabetes, the endothelium has not been known to regulate metabolism. As PPARgamma, a transcriptional regulator of energy balance, is expressed in endothelial cells, we set out to investigate the role of endothelial cell PPARgamma in metabolism using mice that lack PPARgamma in the endothelium and BM (gammaEC/BM-KO). When gammaEC/BM-KO mice were fed a high-fat diet, they had decreased adiposity and increased insulin sensitivity compared with control mice, despite increased serum FFA and triglyceride (TG) levels. After fasting or olive oil gavage, gammaEC/BM-KO mice exhibited significant dyslipidemia and failed to respond to the FFA and TG lowering effects of the PPARgamma agonist rosiglitazone. BM transplantation studies, which reconstituted hematopoietic PPARgamma, established that these metabolic phenotypes were due to endothelial PPARgamma deficiency. We further found that the impairment in TG-rich lipoprotein metabolism in gammaEC/BM-KO mice was associated with fatty acid-mediated lipoprotein lipase inhibition and changes in a PPARgamma-regulated endothelial cell transcriptional program. Despite their metabolic improvements, high-fat diet-fed gammaEC/BM-KO mice had impaired vasoreactivity. Taken together, these data suggest that PPARgamma in the endothelium integrates metabolic and vascular responses and may contribute to the effects of PPARgamma agonists, thus expanding what endothelial function and dysfunction may entail.

Conjugated linoleic acid modulation of risk factors associated with atherosclerosis

Conjugated linoleic acid (CLA) has been the subject of extensive investigation regarding its possible benefits on a variety of human diseases. In some animal studies, CLA has been shown to have a beneficial effect on sclerotic lesions associated with atherosclerosis, be a possible anti-carcinogen, increase feed efficiency, and act as a lean body mass supplement. However, the results have been inconsistent, and the effects of CLA on atherogenesis appear to be dose-, isomer-, tissue-, and species-specific. Similarly, CLA trials in humans have resulted in conflicting findings. Both the human and animal study results may be attributed to contrasting doses of CLA, isomers, the coexistence of other dietary fatty acids, length of study, and inter-and/or intra-species diversities. Recent research advances have suggested the importance of CLA isomers in modulating gene expression involved in oxidative damage, fatty acid metabolism, immune/inflammatory responses, and ultimately atherosclerosis. Although the possible mechanisms of action of CLA have been suggested, they have yet to be determined.

Conjugated linoleic acid modulation of risk factors associated with atherosclerosis

Conjugated linoleic acid (CLA) has been the subject of extensive investigation regarding its possible benefits on a variety of human diseases. In some animal studies, CLA has been shown to have a beneficial effect on sclerotic lesions associated with atherosclerosis, be a possible anti-carcinogen, increase feed efficiency, and act as a lean body mass supplement. However, the results have been inconsistent, and the effects of CLA on atherogenesis appear to be dose-, isomer-, tissue-, and species-specific. Similarly, CLA trials in humans have resulted in conflicting findings. Both the human and animal study results may be attributed to contrasting doses of CLA, isomers, the coexistence of other dietary fatty acids, length of study, and inter-and/or intra-species diversities. Recent research advances have suggested the importance of CLA isomers in modulating gene expression involved in oxidative damage, fatty acid metabolism, immune/inflammatory responses, and ultimately atherosclerosis. Although the possible mechanisms of action of CLA have been suggested, they have yet to be determined.

Novel phospholipolytic activities associated with electronegative low-density lipoprotein are involved in increased self-aggregation

Electronegative low-density lipoprotein (LDL(-)) is a minor LDL subfraction present in plasma with increased platelet-activating factor acetylhydrolase (PAF-AH) activity. This activity could be involved in the proinflammatory effects of LDL(-). Our aim was to study the presence of additional phospholipolytic activities in LDL(-). Total LDL was fractionated into electropositive (LDL(+)) and LDL(-) by anion-exchange chromatography, and phospholipolytic activities were measured by fluorometric methods. Phospholipolytic activity was absent in LDL(+) whereas LDL(-) presented activity against lysophosphatidylcholine (LPC, 82.4 +/- 34.9 milliunits/mg of apoB), sphingomyelin (SM, 53.3 +/- 22.5 milliunits/mg of apoB), and phosphatidylcholine (PC, 25.7 +/- 4.3 milliunits/mg of apoB). LDL(-), but not LDL(+), presented spontaneous self-aggregation at 37 degrees C in parallel to phospholipid degradation. This was observed in the absence of lipid peroxidation and suggests the involvement of phospholipolytic activity in self-aggregation of LDL(-). Phospholipolytic activity was not due to PAF-AH, apoE, or apoC-III and was not increased in LDL(+) modified by Cu (2+) oxidation, acetylation, or secretory phospholipase A 2 (PLA 2). However, LDL(-) efficiently degraded phospholipids of lipoproteins enriched in LPC, such as oxidized LDL or PLA 2-LDL, but not native or acetylated LDL. This finding supports that LPC is the best substrate for LDL(-)-associated phospholipolytic activity. These results reveal novel properties of LDL(-) that could play a significant role in its atherogenic properties.

Electronegative LDL impairs vascular endothelial cell integrity in diabetes by disrupting fibroblast growth factor 2 (FGF2) autoregulation

OBJECTIVE

L5, a circulating electronegative LDL identified in patients with hypercholesterolemia or type 2 diabetes, induces endothelial cell (EC) apoptosis by suppressing fibroblast growth factor (FGF)2 expression. FGF2 plays a pivotal role in endothelial regeneration and compensatory arteriogenesis. It is likely that vasculopathy and poor collateralization in diabetes is a result of FGF2 dysregulation.

RESEARCH DESIGN AND METHODS

To investigate this mechanism, we isolated L5 from type 2 diabetic patients. In cultured bovine aortic ECs (BAECs), L5 inhibited FGF2 transcription and induced apoptosis. Because FGF2 stimulates the phosphatidylinositol 3-kinase (PI3K)-Akt pathway, we examined whether FGF2 transcription is regulated by Akt through a feedback mechanism.

RESULTS

Diabetic L5 reduced FGF2 release to the medium but enhanced caspase-3 activity, with resultant apoptosis. Inhibition of PI3K with wortmannin or suppression of Akt activation with dominant-negative Akt inhibited FGF2 expression. Transfection of BAECs with FGF2 antisense cDNA depleted endogenous FGF2 protein. In these cells, not only was Akt phosphorylation inhibited, but FGF2 transcription was also critically impaired. In contrast, transfecting BAECs with FGF2 sense cDNA augmented Akt phosphorylation. Treatment with constitutively active Akt enhanced FGF2 expression. Augmentation of either FGF2 transcription or Akt phosphorylation rendered BAECs resistant to L5.

CONCLUSIONS

These findings suggest that FGF2 is the primary initiator of its own expression, which is autoregulated through a novel FGF2-PI3K-Akt loop. Thus, by disrupting FGF2 autoregulation in vascular ECs, L5 may impair reendothelialization and collateralization in diabetes.

Zinc deficiency alters lipid metabolism in LDL receptor deficient mice treated with rosiglitazone

Zinc is a structural and functional component of PPAR and zinc deficiency may be associated with an increased risk for cardiovascular diseases. We tested the hypothesis that zinc deficiency compromises lipid metabolism in rosiglitazone (RSG)-treated mice lacking the LDL-receptor (LDL-R) gene. LDL-R-deficient mice were maintained for 3 wk on low-fat (7 g/100 g) diets that were either zinc deficient or zinc adequate. Subsequently, diets were adjusted to a high-fat (HF) (15 g/100 g) regimen for 1 wk to produce a biological environment of mild oxidative and inflammatory stress. One-half of the mice within each zinc group was gavaged daily with the PPARgamma agonist RSG starting 2 d prior to the HF feeding. Selected lipid parameters were studied. Zinc deficiency increased plasma total cholesterol, which was also elevated by RSG. Zinc deficiency also caused an increased lipoprotein-cholesterol distribution toward the non-HDL fraction (VLDL, intermediate density lipoprotein, LDL). Plasma total fatty acids tended to increase during zinc deficiency and RSG treatment resulted in similar changes in the fatty acid profile in zinc-deficient mice. Fatty acid translocase (FAT/CD36) expression in abdominal aorta was upregulated by RSG only in zinc-deficient mice. In contrast, RSG treatment markedly increased lipoprotein lipase (LPL) expression only in zinc-adequate mice. In vitro studies confirmed that adequate zinc is required for RSG-induced PPARgamma activity to transactivate target genes. These data suggest that in this atherogenic mouse model treated with RSG, lipid metabolism can be compromised during zinc deficiency and that adequate dietary zinc may be considered during therapy with the antidiabetic medicine RSG.

Heparin, lipoproteins, and oxygenated fatty acids in blood: a cautionary note

We measured 16 nonesterified oxygenated fatty acid derivatives (oxylipids) in plasmas from seven human subjects. Two arterial samples from each subject were analyzed, drawn approximately 2h apart. We observed a marked increase in levels of most oxylipids in the second sample, as high as 470-fold. Between the first and second samples, subjects received approximately 800-1000 IU of heparin to prevent clotting in intravascular catheters. We postulate that heparin activated lipoprotein lipases, which, in turn, released oxylipids from triglycerides and phospholipids in plasma lipoproteins. Some of that lipolysis may have occurred during sample storage. Measurements of nonesterified lipids in human plasma may be distorted if heparin is administered to subjects before blood is drawn and if lipase inhibitors are omitted from stored samples.

Coffee drinking induces incorporation of phenolic acids into LDL and increases the resistance of LDL to ex vivo oxidation in humans

BACKGROUND

Epidemiologic and intervention studies indicate that both diet as a whole and single dietary components are involved in the risk of atherosclerosis. The resistance of LDL to oxidative modification is an ex vivo indicator of risk, which is modulated by dietary components. Coffee contains phenolic compounds with antioxidant activity. These molecules are found in plasma after the consumption of coffee, and it has been shown that, in vitro, they are able to decrease the susceptibility of LDL to oxidation.

OBJECTIVE

The aim of this study was to evaluate the effect of coffee consumption on the redox status of LDL as modulated by the possible incorporation of phenolic acids into LDL.

DESIGN

Ten healthy volunteers, after an overnight fast, drank 200 mL filtered coffee. Blood was drawn before and 30 and 60 min after drinking. Changes in LDL redox status were evaluated by the measure of LDL resistance to oxidative modification and the concentration of LDL(-), a mildly modified, electronegative LDL subfraction. Chlorogenic and phenolic acids concentration in LDL were measured by electrochemical HPLC.

RESULTS

The resistance of LDL to oxidative modification increased significantly after coffee drinking, but the LDL(-) concentration did not increase. The concentration into LDL of conjugated forms of caffeic, p-coumaric, and ferulic acids increased significantly after coffee drinking.

CONCLUSION

Drinking 200 mL (1 cup) coffee induces an increase in the resistance of LDL to oxidative modification, probably as a result of the incorporation of coffee's phenolic acids into LDL.

Proteinuria increases oxylipid concentrations in VLDL and HDL but not LDL particles in the rat

We previously established that proteinuria alters the apolipoprotein content of lipoproteins. This study was conducted to establish whether proteinuria also alters the concentrations of oxidized lipids within lipoprotein density fractions. To this end, we induced passive Heymann nephritis in Sprague Dawley rats and measured an array of alkaline-stable oxylipids in VLDL, LDL, and HDL particles. Proteinuria increased the total oxylipid amounts in the HDL and VLDL fractions. More importantly, these levels were increased when expressed per unit lipoprotein protein, indicating that the oxidized lipid load per particle was increased. Epoxides and diols increased approximately 2-fold in HDL and approximately 5-fold in VLDL, whereas LDL showed approximately 2-fold decreases. The hydroxyeicosatetraenoic acids and hydroxyoctadecadienoic acids (HODEs) increased >4-fold in HDL and >20-fold in VLDL, whereas LDL showed approximately 2-fold decreases in the HODEs. Therefore, nephrotic syndrome alters the lipoprotein oxylipid composition independently of an increase in total lipoprotein levels. These proteinuria-induced changes may be associated with the cardiovascular risk of lipoprotein oxidation.

Peroxisome proliferator-activated receptors as transcriptional nodal points and therapeutic targets

Peroxisome proliferator-activated receptors (PPARs) are ligand-activated transcription factors involved in the transcriptional regulation of key metabolic pathways such as lipid metabolism, adipogenesis, and insulin sensitivity. More recent work implicates all 3 PPAR isotypes (alpha, gamma, and delta, also known as beta or beta/delta) in inflammatory and atherosclerotic pathways. Because these nuclear receptors are activated by extracellular signals and control multiple gene targets, PPARs can be seen as nodes that control multiple inputs and outputs involved in energy balance, providing insight into how metabolism and the vasculature may be integrated. The ongoing clinical use of fibrates, which activate PPARalpha, and thiazolidinediones, which activate PPARgamma, establishes these receptors as viable drug targets, whereas considerable in vitro animal model and human surrogate marker studies suggest that PPAR activation may limit inflammation and atherosclerosis. Together, these various observations have stimulated intense interest in PPARs as therapeutic targets and led to large-scale cardiovascular end-point trials with PPAR agonists. The first of these studies has generated mixed results that require careful review, especially in anticipation of additional clinical trial data and ongoing attempts to develop novel PPAR modulators. Such analysis of the existing PPAR data, the appropriate use of currently approved PPAR agonists, and continued progress in PPAR therapeutics will be predicated on a better understanding of PPAR biology.

Inflammation in diabetes mellitus: role of peroxisome proliferator-activated receptor-alpha and peroxisome proliferator-activated receptor-gamma agonists

Patients with type 2 diabetes mellitus and/or the metabolic syndrome have considerable cardiovascular risk. Treatment with 3-hydroxy-3-methylglutaryl coenzyme A reductase inhibitors (statins) and with antihypertensive and some antihyperglycemic agents reduces this risk, but residual macrovascular morbidity and mortality persist, even in patients assigned to intensive multifactorial intervention programs. Therapeutic strategies that target inflammation and lipid abnormalities not well addressed by statins may offer additional opportunities for improving the prognosis of these patients. Inflammation, a key mechanism of atherogenesis, appears to have particular relevance to diabetic vascular complications, as well as in the development of diabetes itself. Oxidative stress and hyperglycemia also figure among the pathogenic factors that promote cardiovascular complications in patients with the metabolic syndrome and/or diabetes and may augment the ongoing inflammation. Peroxisome proliferator-activated receptor (PPAR)-alpha and PPAR-gamma, members of the nuclear receptor family, form ligand-activated transcription factors that regulate key important metabolic pathways. PPARs have become therapeutic targets through the use of the fibrate class of antidyslipidemic drugs (PPAR-alpha) and the insulin-sensitizing thiazolidinediones (PPAR-gamma). The activation of these PPARs may also suppress inflammation and atherosclerosis. Recent clinical trials (Fenofibrate Intervention and Event Lowering in Diabetes [FIELD], Prospective Pioglitazone Clinical Trial in Macrovascular Events [PROactive]) have considered the impact of these PPAR agonists on cardiovascular disease, with mixed effects that require careful analysis, especially given ongoing trials and additional PPAR agonists in development.

Atherogenic and inflammatory profile of human arterial endothelial cells (HUAEC) in response to LDL subfractions

BACKGROUND

Electronegative LDL (LDL(-)) is a minor modified LDL fraction present in plasma that has been demonstrated to be inflammatory in endothelial cells isolated from human umbilical vein (HUVEC).

METHODS

A protein array able to measure 42 cytokines, chemokines and related compounds involved in atherogenesis was used to determine their release into the culture medium of human arterial endothelial cells (HUAEC) activated or not by two low-density lipoprotein (LDL) fractions isolated from human plasma by anion-exchange chromatography.

RESULTS

The results of the protein array (confirmed using specific ELISAs for each induced factor) revealed that HUAEC in the absence of stimuli released small amounts of interleukin 8 (IL-8), monocyte chemotactic protein 1 (MCP-1) and growth-related oncogene (GRO). The major native LDL fraction (named LDL(+)) increased the release of these molecules and also those of interleukin 6 (IL-6) and GROalpha. Compared to LDL(+), the minor modified fraction, named electronegative LDL (LDL(-)), increased all these factors to a greater degree and also induced the release of granulocyte-monocyte colony-stimulating factor (GM-CSF) and platelet-derived growth factor B (PDGF-B). These results were confirmed by ELISA.

CONCLUSIONS

All these results indicate that, compared to LDL(+), LDL(-) fraction promotes not only the release of proinflammatory factors but also those of atherogenic factors in endothelial cells of arterial origin, thereby suggesting a new role for LDL(-) in atherogenesis.

Electronegative low-density lipoprotein subfraction from type 2 diabetic subjects is proatherogenic and unrelated to glycemic control

BACKGROUND

The physicochemical and biological characteristics of electronegative low-density lipoprotein (LDL) (LDL(-)) from type 2 diabetic patients (DM2), before and after insulin therapy, were studied.

METHODS

Total LDL was subfractionated in LDL(+) (native LDL) and LDL(-) by anion-exchange chromatography.

RESULTS

The proportion of LDL(-) was increased in plasma from DM2 patients compared to control subjects (13.8 +/- 4.6% versus 6.1 +/- 2.5, P < 0.05) and was not modified after glycemic optimization (14.0 +/- 5.9%). LDL(-) from DM2 patients presented similar differential characteristics versus LDL(+) than LDL(-) from controls; that is, decreased apoB and oxidizability, and increased triglyceride, nonesterified fatty acids (NEFA), apoE, apoC-III, platelet-activating factor (PAF) acetylhydrolase activity and aggregability. No difference in particle size, antioxidants, malondialdehyde (MDA), fructosamine or glycated low-density lipoprotein (gLDL) was observed between LDL subfractions. Concerning differences between LDL subfractions isolated from DM2 and from control subjects, the former showed increased MDA, fructosamine and gLDL proportion and decreased LDL size and antioxidant content. The only effect of glycemic optimization was a decrease in fructosamine and gLDL in LDL(+) from DM2 subjects. LDL(-) from DM2 patients presented low binding affinity to the low-density lipoprotein receptor (LDLr) in cultured fibroblasts compared to LDL(+) and two- to threefold increased ability to release interleukin-8 (IL-8) and monocyte chemotactic protein 1 (MCP-1) in endothelial cells.

CONCLUSION

These results suggest that, although nonenzymatic glycosylation and oxidation are increased in type 2 diabetes, these features would not be directly involved in the generation of LDL(-). Moreover, LDL(-) properties suggest that the high proportion observed in plasma could promote accelerated atherosclerosis in DM2 patients through increased residence time in plasma and induction of inflammatory responses in artery wall cells.

Nuclear receptors as drug targets in metabolic diseases: new approaches to therapy

Nuclear receptors represent novel targets for the development of therapeutic agents for the treatment of numerous diseases, including type 2 diabetes, obesity dyslipidemia, atherosclerosis and the metabolic syndrome. There have been many recent advances in the development of new therapeutic agents for a subset of these receptors, including the peroxisome proliferator-activated receptors, the liver X receptors and the farnesoid X receptor. To date, the synthesis of selective modulators that regulate the activity of these receptors has been empirical. However, a detailed understanding of the molecular basis for selective modulation, as well as new insights into the biology of these receptors, might open the door to the rational design of a new generation of therapeutic agents with improved safety and efficacy.

Reciprocal and coordinate regulation of serum amyloid A versus apolipoprotein A-I and paraoxonase-1 by inflammation in murine hepatocytes

OBJECTIVE

During inflammation, the serum amyloid A (SAA) content of HDL increases, whereas apolipoprotein A-I (apoA-I) and paraoxonase-1 (PON-1) decrease. It remains unclear whether SAA physically displaces apoA-I or if these changes derive from coordinated but inverse transcriptional regulation of the HDL apolipoprotein genes. Because cytokines stimulate the hepatic expression of inflammatory markers, we investigated their role in regulating SAA, apoA-I, and PON-1 expression.

METHODS AND RESULTS

A cytokine mixture (tumor necrosis factor [TNF]-alpha, interleukin [IL]-1beta, and IL-6) simultaneously induced SAA and repressed apoA-I and PON-1 expression levels. These effects were partially inhibited in cells pretreated with either nuclear factor kappaB (NF-kappaB) inhibitors (pyrrolidine dithiocarbamate, SN50, and overexpression of super-repressor inhibitor kappaB) or after exposure to the peroxisome proliferator-activated receptor-alpha (PPARalpha) ligands (WY-14643 and fenofibrate). Consistent with these findings, the basal level of SAA was increased, whereas apoA-I and PON-1 decreased in primary hepatocytes from PPARalpha-deficient mice as compared with wild-type mice. Moreover, neither WY-14643 nor fenofibrate had any effect on SAA, apoA-I, or PON-1 expression in the absence of PPARalpha.

CONCLUSIONS

These results suggest that cytokines increase the expression of SAA through NF-kappaB transactivation, while simultaneously decreasing the expression of apoA-I and PON-1 by inhibiting PPARalpha activation. Inflammation may convert HDL de novo into a more proatherogenic form by coordinate but inverse transcriptional regulation in the liver, rather than by physical displacement of apoA-I by SAA.

Deregulated expression of fat and muscle genes in B-cell chronic lymphocytic leukemia with high lipoprotein lipase expression

Lipoprotein lipase (LPL) is a prognostic marker in B-cell chronic lymphocytic leukemia (B-CLL) related to immunoglobulin V(H) gene (IgV(H))mutational status. We determined gene expression profiles using Affymetrix U133A GeneChips in two groups of B-CLLs selected for either high ('LPL+', n=10) or low ('LPL-', n=10) LPL mRNA expression. Selected genes were verified by real-time PCR in an extended patient cohort (n=42). A total of 111 genes discriminated LPL+ from LPL- B-CLLs. Of these, the top three genes associated with time to first treatment were Septin10, DMD and Gravin (P</=0.01). The relationship of LPL+ and LPL- B-CLL gene expression signatures to 52 tissues was statistically analyzed. The LPL+ B-CLL expression signature, represented by 64 genes was significantly related to fat, muscle and PB dendritic cells (P<0.001). Exploration of microarray data to define functional alterations related to the biology of LPL+ CLL identified two functional modules, fatty acid degradation and MTA3 signaling, as being altered with higher statistical significance. Our data show that LPL+ B-CLL cells have not only acquired gene expression changes in fat and muscle-associated genes but also in functional pathways related to fatty acid degradation and signaling which may ultimately influence CLL biology and clinical outcome.

Wide proinflammatory effect of electronegative low-density lipoprotein on human endothelial cells assayed by a protein array

Electronegative low-density lipoprotein (LDL(-)) is a modified subfraction of LDL present in plasma able to induce the release of interleukin 8 (IL-8) and monocyte chemotactic protein 1 (MCP-1) by human umbilical vein endothelial cells (HUVEC). To ascertain whether further inflammation mediator release could be induced by LDL(-), a protein array system was used to measure 42 cytokines and related compounds. Native LDL and LDL(-) isolated from normolipemic subjects were incubated for 24 h with HUVEC and culture supernatants were used to measure inflammation mediator release. The protein array revealed that IL-6, granulocyte/monocyte colony-stimulating factor (GM-CSF) and growth-related oncogene (GRO) release were increased by cultured HUVEC in response to LDL(-). LDL(-) enhanced production of IL-6 (4-fold vs. LDL(+)), GM-CSF (4-fold), GRObeta (2-fold) and GROgamma (7-fold) was confirmed by ELISA. Time-course experiments revealed that IL-6 was released earlier than the other inflammation mediators, suggesting a first-wave cytokine action. However, the addition of IL-6 alone did not stimulate the production of IL-8, MCP-1 or GM-CSF. Moreover, IL-8, MCP-1 or GM-CSF alone did not promote the release of the other inflammatory molecules. Modification of LDL(+) by phospholipase A(2)-mediated lipolysis or by loading with non-esterified fatty acids (NEFA) reproduced the action of LDL(-), thereby suggesting the involvement of NEFA and/or lysophosphatidylcholine in the release of these molecules. Our results indicate that LDL(-) promotes a proinflammatory phenotype in endothelial cells through the production of cytokines, chemokines and growth factors.

High-density lipoprotein hydrolysis by endothelial lipase activates PPARalpha: a candidate mechanism for high-density lipoprotein-mediated repression of leukocyte adhesion

Although high-density lipoprotein (HDL) is known to inhibit endothelial adhesion molecule expression, the mechanism for this anti-inflammatory effect remains obscure. Surprisingly, we observed that HDL no longer decreased adhesion of U937 monocytoid cells to tumor necrosis factor (TNF)alpha-stimulated human endothelial cells (EC) in the presence of the general lipase inhibitor tetrahydrolipstatin. In considering endothelial mechanisms responsible for this effect, we found that endothelial lipase (EL) overexpression in both EC and non-EL-expressing NIH/3T3 mouse embryonic fibroblasts cells significantly decreased TNFalpha-induced VCAM1 expression and promoter activity in a manner dependent on HDL concentration and intact EL activity. Given recent evidence for lipolytic activation of peroxisome proliferator-activated receptors (PPARs)-nuclear receptors implicated in metabolism, atherosclerosis, and inflammation-we hypothesized HDL hydrolysis by EL is an endogenous endothelial mechanism for PPAR activation. In both EL-transfected NIH cells and bovine EC, HDL significantly increased PPAR ligand binding domain activation in the order PPAR-alpha> >-gamma>-delta. Moreover, HDL stimulation induced expression of the canonical PPARalpha-target gene acyl-CoA-oxidase (ACO) in a PPARalpha-dependent manner in ECs. Conditioned media from EL-adenovirus transfected cells but not control media exposed to HDL also activated PPARalpha. PPARalpha activation by EL was most potent with HDL as a substrate, with lesser effects on LDL and VLDL. Finally, HDL inhibited leukocyte adhesion to TNFalpha-stimulated ECs isolated from wild-type but not PPARalpha-deficient mice. This data establishes HDL hydrolysis by EL as a novel, distinct natural pathway for PPARalpha activation and identifies a potential mechanism for HDL-mediated repression of VCAM1 expression, with significant implications for both EL and PPARs in inflammation and vascular biology.

The inflammatory properties of electronegative low-density lipoprotein from type 1 diabetic patients are related to increased platelet-activating factor acetylhydrolase activity

AIMS/HYPOTHESIS

Chemical and biological characteristics of LDL(-) from type 1 diabetic subjects were analysed. The diabetic patients were studied during poor and optimised glycaemic control.

MATERIALS AND METHODS

Total LDL was subfractionated into electropositive LDL(+) and electronegative LDL(-) by anion exchange chromatography and the lipid and protein composition of the two determined.

RESULTS

LDL(-) differed from LDL(+) in that it had higher triglyceride, non-esterified fatty acids, apoE, apoC-III and platelet-activating factor acetylhydrolase (PAF-AH), as well as lower apoB relative content. No evidence of increased oxidation was observed in LDL(-). LDL(-) increased two-fold the release of interleukin 8 (IL-8) and monocyte chemotactic protein 1 (MCP-1) in endothelial cells, suggesting an inflammatory role. Optimisation of glycaemic control after insulin therapy decreased the proportion of LDL(-), but did not modify the composition of LDL subfractions, except for a decrease in PAF-AH activity in LDL(-). The possibility that LDL(-) could be generated by non-enzymatic glycosylation was studied. Fructosamine and glycated LDL content in LDL subfractions from type 1 diabetic patients was greater than in LDL subfractions isolated from normoglycaemic subjects, and decreased after glycaemic optimisation in both subfractions. However, no difference was observed between LDL(+) and LDL(-) before and after insulin therapy.

CONCLUSIONS/INTERPRETATION

These results provide evidence that LDL(-) is not produced by glycosylation. Nevertheless, LDL(-) from diabetic patients displays inflammatory potential reflected by the induction of chemokine release in endothelial cells. This proatherogenic effect could be related to the high PAF-AH activity in LDL(-).

Relation between insulin resistance and fast-migrating LDL subfraction as characterized by capillary isotachophoresis

The proportion of the electronegative low density lipoprotein [LDL(-)] subfraction, which is atherogenic, is increased in type 2 diabetes but is not reduced by glycemic control. Therefore, we evaluated the ability of a new technique, capillary isotachophoresis (cITP), to quantify charge-based LDL subfractions and examined the relation between insulin resistance and the cITP fast-migrating (f) LDL levels. Seventy-five 10-year-old boys were included. The two cITP LDL subfractions, fLDL and major LDL subfractions, were proportional to the LDL protein content within the range of 0.1-0.8 mg/ml LDL protein. Levels of cITP fLDL were positively correlated with triglyceride (TG) levels and negatively correlated with LDL size. Insulin resistance as assessed by the homeostasis model assessment (HOMA-IR) was positively correlated (P < 0.01) with cITP fLDL levels (r = 0.41). The relation between HOMA-IR and cITP fLDL levels depended on TG levels but was independent of body mass index and LDL size. cITP lipoprotein analysis is an accurate and sensitive method for quantifying charge-based LDL subfractions in human plasma, and insulin resistance is related to cITP fLDL independent of LDL size.

Differential Effects of Lipoprotein Lipase on Tumor Necrosis Factor-α and Interferon-γ-mediated Gene Expression in Human Endothelial Cells

Lipoprotein lipase (LPL) is a key enzyme in the hydrolysis of triglyceride-rich lipoproteins. In vascular diseases, such as atherosclerosis, inflammation plays an important role in the pathogenesis of the disease. We examined the role of LPL in modulating tumor necrosis factor-alpha (TNF-alpha)- and interferon-gamma (IFN-gamma)-mediated inflammatory cytokine signal transduction pathways in human aortic endothelial cells (HAECs). LPL significantly suppressed TNF-alpha-induced gene expression, and this suppression was reversed by tetrahydrolipstatin and heparinase. In contrast, LPL synergistically enhanced IFN-gamma-induced gene expression in HAECs. To elucidate the molecular mechanisms of LPL action, we investigated the role of transcription factors nuclear factor kappa B (NF-kappaB) and signal transducer and activator of transcription factor 1 (Stat1). The anti-inflammatory response of LPL in suppressing TNF-alpha-induced gene expression was a result of its inhibition of NF-kappaB activity by the abrogation of IkappaB-alpha degradation and phosphorylation of the p65 subunit. Although LPL alone had no effect on Stat1 activation, LPL enhanced IFN-gamma-induced phosphorylation of Stat1 on tyrosine 701 and serine 727, as well as Stat1-mediated transactivation. The synergistic effect of LPL on IFN-gamma-induced Stat1 activation was mediated by enhanced activation of the tyrosine kinase JAK2 and was abrogated by LY294002, a specific inhibitor of the phosphatidylinositol 3'-kinase pathway. Our studies indicate that LPL has differential effects on several inflammatory pathways known to be important in atherosclerosis.

Therapeutic roles of peroxisome proliferator-activated receptor agonists

Peroxisome proliferator-activated receptors (PPARs) play key roles in the regulation of energy homeostasis and inflammation, and agonists of PPARalpha and -gamma are currently used therapeutically. Fibrates, first used in the 1970s for their lipid-modifying properties, were later shown to activate PPARalpha. These agents lower plasma triglycerides and VLDL particles and increase HDL cholesterol, effects that are associated with cardiovascular benefit. Thiazolidinediones, acting via PPARgamma, influence free fatty acid flux and thus reduce insulin resistance and blood glucose levels. PPARgamma agonists are therefore used to treat type 2 diabetes. PPARalpha and -gamma agonists also affect inflammation, vascular function, and vascular remodeling. As knowledge of the pleiotropic effects of these agents advances, further potential indications are being revealed, including roles in the management of cardiovascular disease (CVD) and the metabolic syndrome. Dual PPARalpha/gamma agonists (currently in development) look set to combine the properties of thiazolidinediones and fibrates, and they hold considerable promise for improving the management of type 2 diabetes and providing an effective therapeutic option for treating the multifactorial components of CVD and the metabolic syndrome. The functions of a third PPAR isoform, PPARdelta, and its potential as a therapeutic target are currently under investigation.

Epoxide hydrolases: their roles and interactions with lipid metabolism

The epoxide hydrolases (EHs) are enzymes present in all living organisms, which transform epoxide containing lipids by the addition of water. In plants and animals, many of these lipid substrates have potent biologically activities, such as host defenses, control of development, regulation of inflammation and blood pressure. Thus the EHs have important and diverse biological roles with profound effects on the physiological state of the host organisms. Currently, seven distinct epoxide hydrolase sub-types are recognized in higher organisms. These include the plant soluble EHs, the mammalian soluble epoxide hydrolase, the hepoxilin hydrolase, leukotriene A4 hydrolase, the microsomal epoxide hydrolase, and the insect juvenile hormone epoxide hydrolase. While our understanding of these enzymes has progressed at different rates, here we discuss the current state of knowledge for each of these enzymes, along with a distillation of our current understanding of their endogenous roles. By reviewing the entire enzyme class together, both commonalities and discrepancies in our understanding are highlighted and important directions for future research pertaining to these enzymes are indicated.

High lipoprotein lipase activity increases insulin sensitivity in transgenic rabbits

Lipoprotein lipase (LPL) is the rate-limiting enzyme in the hydrolysis of triglyceride-rich lipoproteins and plays an important role in glucose metabolism. To examine the hypothesis that increased LPL activity may alter insulin sensitivity, we investigated glucose metabolism and insulin sensitivity in transgenic (Tg) rabbits expressing the human LPL gene under the control of a I(2) -actin promoter. An intravenous glucose tolerance test showed that the plasma glucose clearance rate was not significantly different between Tg and non-Tg rabbits; however, the area under the curve for insulin and free fatty acids in Tg rabbits was significantly reduced compared with that of non-Tg rabbits (P < .05). Using the intravenous insulin tolerance test, we found that the area of under the curve of glucose of Tg rabbits was also significantly reduced (P < .01). Furthermore, euglycemic-hyperinsulinemic clamp test revealed that the mean glucose infusion rate in Tg rabbits was significantly higher than in non-Tg rabbits (P < .05). These results demonstrate that systemic overexpression of LPL increases whole-body insulin sensitivity and genetic manipulation of LPL genes may be a potential target for the treatment of diabetic patients.

Impaired Binding Affinity of Electronegative Low-Density Lipoprotein (LDL) to the LDL Receptor Is Related to Nonesterified Fatty Acids and Lysophosphatidylcholine Content†

The binding characteristics of electropositive [LDL(+)] and electronegative LDL [LDL(-)] subfractions to the LDL receptor (LDLr) were studied. Saturation kinetic studies in cultured human fibroblasts demonstrated that LDL(-) from normolipemic (NL) and familial hypercholesterolemic (FH) subjects had lower binding affinity than their respective LDL(+) fractions (P < 0.05), as indicated by higher dissociation constant (K(D)) values. FH-LDL(+) also showed lower binding affinity (P < 0.05) than NL-LDL(+) (K(D), sorted from lower to higher affinity: NL-LDL(-), 33.0 +/- 24.4 nM; FH-LDL(-), 24.4 +/- 7.1 nM; FH-LDL(+), 16.6 +/- 7.0 nM; NL-LDL(+), 10.9 +/- 5.7 nM). These results were confirmed by binding displacement studies. The impaired affinity binding of LDL(-) could be attributed to altered secondary and tertiary structure of apolipoprotein B, but circular dichroism (CD) and tryptophan fluorescence (TrpF) studies revealed no structural differences between LDL(+) and LDL(-). To ascertain the role of increased nonesterified fatty acids (NEFA) and lysophosphatidylcholine (LPC) content in LDL(-), LDL(+) was enriched in NEFA or hydrolyzed with secretory phospholipase A(2). Modification of LDL gradually decreased the affinity to LDLr in parallel to the increasing content of NEFA and/or LPC. Modified LDLs with a NEFA content similar to that of LDL(-) displayed similar affinity. ApoB structure studies of modified LDLs by CD and TrpF showed no difference compared to LDL(+) or LDL(-). Our results indicate that NEFA loading or phospholipase A(2) lipolysis of LDL leads to changes that affect the affinity of LDL to LDLr with no major effect on apoB structure. Impaired affinity to the LDLr shown by LDL(-) is related to NEFA and/or LPC content rather than to structural differences in apolipoprotein B.

Peroxisome proliferator-activated receptor alpha induces NADPH oxidase activity in macrophages, leading to the generation of LDL with PPAR-alpha activation properties

Peroxisome proliferator-activated receptors (PPARs) are nuclear receptors controlling lipid and glucose metabolism as well as inflammation. PPARs are expressed in macrophages, cells that also generate reactive oxygen species (ROS). In this study, we investigated whether PPARs regulate ROS production in macrophages. Different PPAR-alpha, but not PPAR-gamma agonists, increased the production of ROS (H2O2 and ) in human and murine macrophages. PPAR-alpha activation did not induce cellular toxicity, but significantly decreased intracellular glutathione levels. The increase in ROS production was not attributable to inherent prooxidant effects of the PPAR-alpha agonists tested, but was mediated by PPAR-alpha, because the effects were lost in bone marrow-derived macrophages from PPAR-alpha-/- mice. The PPAR-alpha-induced increase in ROS was attributable to the induction of NADPH oxidase, because (1) preincubation with the NADPH oxidase inhibitor diphenyleneiodinium prevented the increase in ROS production; (2) PPAR-alpha agonists increased production measured by superoxide dismutase-inhibitable cytochrome c reduction; (3) PPAR-alpha agonists induced mRNA levels of the NADPH oxidase subunits p47(phox), p67phox, and gp91phox and membrane p47phox protein levels; and (4) induction of ROS production was abolished in p47phox-/- and gp91phox-/- macrophages. Finally, induction of NADPH oxidase by PPAR-alpha agonists resulted in the formation of oxidized LDL metabolites that exert PPAR-alpha-independent proinflammatory and PPAR-alpha-dependent decrease of lipopolysaccharide-induced inducible nitric oxide synthase expression in macrophages. These data identify a novel mechanism of autogeneration of endogenous PPAR-alpha ligands via stimulation of NADPH oxidase activity.

Lipoprotein-X reduces LDL atherogenicity in primary biliary cirrhosis by preventing LDL oxidation

Hypercholesterolemic human LDL contains oxidized subfractions that have atherogenic properties. Paradoxically, atherosclerosis incidence is low in patients with primary biliary cirrhosis (PBC), a disease characterized by marked increases in plasma LDL, including the LDL subfraction lipoprotein-X (Lp-X). To investigate the mechanisms underlying this paradox, we first examined the propensity to oxidation of unfractionated LDL isolated from PBC patients. After prolonged incubation with copper, PBC-LDL failed to increase the oxidation index or electrophoretic mobility noted in control LDL. An admixture of PBC-LDL or Lp-X with control LDL prevented oxidation of the latter in a dose-dependent manner. PBC-LDL was also noncompetitive against copper-oxidized LDL (oxLDL) for binding with a murine monoclonal anti-oxLDL antibody in a competitive ELISA. OxLDL exerts its proapoptotic and antiangiogenic effects in part by inhibiting fibroblast growth factor 2 (FGF2) expression. Preincubation of oxLDL with PBC-LDL, but not control LDL, attenuated the inhibitory effects of oxLDL on FGF2 expression in cultured bovine aortic endothelial cells (ECs). The antioxidant and prosurvival properties of PBC-LDL diminished after the patients underwent orthotopic liver transplantation. These results suggest that Lp-X reduces LDL atherogenicity by preventing LDL oxidation to protect EC integrity in the presence of hypercholesterolemia. They also suggest that altering LDL composition may be as important as reducing LDL concentration in preventing or treating atherosclerosis.

Lipolytic PPAR activation: new insights into the intersection of triglycerides and inflammation?

PURPOSE OF REVIEW

To examine connections between triglyceride metabolism and inflammation, especially as they relate to transcriptional regulation through peroxisomal proliferator activated receptors activation.

RECENT FINDINGS

Peroxisomal proliferator activated receptors, members of the steroid hormone nuclear receptor family, have been of particular interest as a mechanism through which different dietary components might control gene expression. Extensive prior work has defined the central role peroxisomal proliferator activated receptors play in many key metabolic responses, including glucose control and lipid metabolism. Emerging evidence suggests peroxisomal proliferator activated receptor activation may limit inflammation and atherosclerosis. The demonstration that certain fatty acids can activate peroxisomal proliferator activated receptors belies the potential link between nutritional components and peroxisomal proliferator activated receptor responses. Interest in this connection had been heightened by recent evidence that lipolysis in certain situations can both generate peroxisomal proliferator activated receptor ligands and limit some known inflammatory responses.

SUMMARY

Lipolytic peroxisomal proliferator activated receptor activation suggests new ways in which to reconsider triglycerides and the distal consequences of their metabolism, including the possible effects on inflammation and atherosclerosis.

The role of the peroxisome proliferator-activated receptor alpha pathway in pathological remodeling of the diabetic heart

PURPOSE OF REVIEW

Cardiovascular disease is exceptionally prevalent in patients with diabetes mellitus, which is an emerging pandemic. Unfortunately, the pathogenesis of diabetic myocardial disease is still poorly understood.

RECENT FINDINGS

Evidence has emerged that perturbed cardiac energy metabolism (excessive dependence on fatty acid utilization and diminished carbohydrate metabolism) plays a role in the development of myocardial dysfunction in diabetes. In support of this, transgenic mice with cardiac-specific activation of the peroxisome proliferator-activated receptor alpha, a nuclear receptor transcription factor that enhances the expression of myriad genes controlling cellular fatty acid utilization, exhibit several classic signatures of the diabetic heart. In particular, the hearts of these mice display: increased import, oxidation, and storage of fatty acids; strong counter-regulatory inhibition of glucose import and oxidation; and cardiomyopathic remodeling and dysfunction. A high-fat diet exacerbates the cardiomyopathic phenotype in peroxisome proliferator-activated receptor alpha transgenic mice in a completely reversible manner. The dysfunctional phenotype and its resolution correlates with levels of several potential toxic mediators, including triglycerides, ceramide, and reactive oxygen species. These findings contrast with several studies demonstrating beneficial effects of peroxisome proliferator-activated receptor alpha ligands in the treatment of type 2 diabetes.

SUMMARY

Such studies provide a rationale for greater emphasis on serum lipid-lowering strategies and the control of dietary fat content in the treatment of diabetic cardiomyopathy. Moreover, the use of peroxisome proliferator-activated receptor alpha activators or related compounds as therapeutic agents will require rigorous evaluation of the effects on cardiac function in the diabetic patient.