Niacin therapy in atherosclerosis

Mechanism of action of niacin

Nicotinic acid (niacin) has long been used for the treatment of lipid disorders and cardiovascular disease. Niacin favorably affects apolipoprotein (apo) B-containing lipoproteins (eg, very-low-density lipoprotein [VLDL], low-density lipoprotein [LDL], lipoprotein[a]) and increases apo A-I-containing lipoproteins (high-density lipoprotein [HDL]). Recently, new discoveries have enlarged our understanding of the mechanism of action of niacin and challenged older concepts. There are new data on (1) how niacin affects triglycerides (TGs) and apo B-containing lipoprotein metabolism in the liver, (2) how it affects apo A-I and HDL metabolism, (3) how it affects vascular anti-inflammatory events, (4) a specific niacin receptor in adipocytes and immune cells, (5) how niacin causes flushing, and (6) the characterization of a niacin transport system in liver and intestinal cells. New findings indicate that niacin directly and noncompetitively inhibits hepatocyte diacylglycerol acyltransferase-2, a key enzyme for TG synthesis. The inhibition of TG synthesis by niacin results in accelerated intracellular hepatic apo B degradation and the decreased secretion of VLDL and LDL particles. Previous kinetic studies in humans and recent in vitro cell culture findings indicate that niacin retards mainly the hepatic catabolism of apo A-I (vs apo A-II) but not scavenger receptor BI-mediated cholesterol esters. Decreased HDL-apo A-I catabolism by niacin explains the increases in HDL half-life and concentrations of lipoprotein A-I HDL subfractions, which augment reverse cholesterol transport. Initial data suggest that niacin, by inhibiting the hepatocyte surface expression of beta-chain adenosine triphosphate synthase (a recently reported HDL-apo A-I holoparticle receptor), inhibits the removal of HDL-apo A-I. Recent studies indicate that niacin increases vascular endothelial cell redox state, resulting in the inhibition of oxidative stress and vascular inflammatory genes, key cytokines involved in atherosclerosis. The niacin flush results from the stimulation of prostaglandins D(2) and E(2) by subcutaneous Langerhans cells via the G protein-coupled receptor 109A niacin receptor. Although decreased free fatty acid mobilization from adipose tissue via the G protein-coupled receptor 109A niacin receptor has been a widely suggested mechanism of niacin to decrease TGs, physiologically and clinically, this pathway may be only a minor factor in explaining the lipid effects of niacin.

HDL-cholesterol modulation and its impact on the management of cardiovascular risk

There is strong epidemiological evidence that HDL-cholesterol (HDL-C) is an independent cardiovascular risk factor. A low HDL-C is a common finding in the general population, and is a feature of metabolic syndrome, the prevalence of which is increasing globally. The importance of HDL-C as a coronary risk factor is generally accepted, and has been incorporated into most of the commonly applied risk engines. There are several causes of a low HDL-C, including rare genetic abnormalities, and more common conditions associated with lifestyle and drug therapy. Not all of these are associated with an increased risk of coronary disease. A better understanding of HDL metabolism and the mechanisms by which it may exert its protective effects has revealed why this may be the case. New therapeutic targets have also been identified that may allow HDL-C to be raised in patients with low levels. However, good clinical endpoint studies are still required to enable the setting of absolute target values for HDL-C in patients at high cardiovascular risk.

Is raising HDL a futile strategy for atheroprotection?

The dramatic failure of clinical trials evaluating the cholesterol ester transfer protein inhibitor torcetrapib has led to considerable doubt about the value of raising high-density lipoprotein cholesterol (HDL-C) as a treatment for cardiovascular disease. These results have underscored the intricacy of HDL metabolism, with functional quality perhaps being a more important consideration than the circulating quantity of HDL. As a result, HDL-based therapeutics that maintain or enhance HDL functionality warrant closer investigation. In this article, we review the complexity of HDL metabolism, discuss clinical-trial data for HDL-raising agents, including possible reasons for the failure of torcetrapib, and consider the potential for future HDL-based therapies.

Niacin for stroke prevention: evidence and rationale

Low levels of high-density lipoprotein (HDL) cholesterol are associated with increased atherothrombotic events, including stroke. Niacin is a safe and effective means of raising HDL, yet its role in stroke prevention is not well characterized. The purpose of the study is to determine the role of niacin in stroke prevention. A search of the PUBMED database using the keywords niacin, stroke, atherosclerosis, and/or carotid artery was undertaken to identify studies for review. National guidelines from the American Heart Association and National Cholesterol Education Program were reviewed. Treatment of low serum HDL (<40 mg/dL) is an identified goal of dyslipidemic therapy. Niacin is effective in raising HDL levels and reducing cardiovascular events in individuals with high vascular risk and can be used for treatment of stroke patients with low serum HDL. Niacin can be used safely in combination with statins, the first-line dyslipidemic treatment for secondary stroke risk reduction, with increased efficacy. Studies are needed to better define the role for niacin in secondary stroke prevention. Treatment of stroke patients with extended-release (ER) of niacin, alone or in combination with statins, should be considered in stroke patients with atherosclerotic mechanisms with low serum HDL-C levels.

[Pharmacological bases of coffee nutrients for diabetes prevention]

With an increasing number of studies describing the negative correlation of coffee consumption and the risk for type 2 diabetes mellitus, we were compelled to elucidate the nutrients which bring pharmacological effects on risk reduction for diabetes. In this review, the author's interest is focused on chlorogenic and caffeic acids derived from lightly roasted coffee beans, as well as nicotinic acid, volatile Maillard reaction products (vMRPs), and another structurally unknown compound contained in heavily roasted beans. Caffeine is a common compound in both lightly and heavily roasted beans and its anti-inflammatory effects on degenerative diseases such as diabetes mellitus has been reevaluated recently. The prophylactic effects of coffee on diabetes involve pleiotropy of plural components in accordance to the degree of the roasting. A new concept of nutritional blended coffee may be important to optimize the prophylactic effects of coffee on lowering the risk factors of diabetes and delaying the progress of diabetes complications as well.

Nicotinic acid (niacin) receptor agonists: will they be useful therapeutic agents?

Nicotinic acid (niacin) favorably affects very-low-density lipoprotein (VLDL), low-density lipoprotein (LDL), and lipoprotein (a) (LP[a]) and increases high-density lipoprotein (HDL). Emerging data indicates vascular anti-inflammatory properties to additionally account for niacin's proven effects in cardiovascular disease. Recent evidence indicates that niacin acts on GPR109A and GPR109B (HM74A and HM74, respectively), receptors expressed in adipocytes and immune cells. In adipocytes, GPR109A activation reduces triglyceride (TG) lipolysis, resulting in decreased free fatty acid (FFA) mobilization to the liver. In humans, this mechanism has yet to be confirmed because the plasma FFA decrease is transient and is followed by a rebound increase in FFA levels. New evidence indicates niacin directly inhibits diacylglycerol acyltransferase 2 (DGAT2) isolated from human hepatocytes, resulting in accelerated hepatic apolipoprotein (apo)B degradation and decreased apoB secretion, thus explaining reductions in VLDL and LDL. This raises important questions as to whether stimulation of GPR109A in adipocytes or inhibition of DGAT2 in liver by niacin best explain the reduction in VLDL and LDL in dyslipidemic patients. Kinetic and in vitro studies indicate that niacin retards the hepatic catabolism of apoA-I but not liver scavenger receptor B1-mediated cholesterol esters, suggesting that niacin inhibits hepatic holoparticle HDL removal. Indeed, recent preliminary evidence suggests that niacin decreases surface expression of hepatic beta-chain of adenosine triphosphate synthase, which has been implicated in apoA-I/HDL holoparticle catabolism. GPR109A-mediated production of prostaglandin D2 in macrophages and Langerhan cells causes skin capillary vasodilation and explains, in part, niacin's effect on flushing. Development of niacin receptor agonists would, theoretically, result in adipocyte TG accumulation (and clinical adiposity) and increased flushing. This raises questions about niacin receptor agonists as therapeutic agents. Several niacin receptor agonists have been developed and patented, but their clinical effects have not been described. Future research is needed to determine whether niacin receptor agonists will demonstrate all the beneficial properties of nicotinic acid on atherosclerosis and without significant adverse effects.

Mechanism of nicotinic acid transport in human liver cells: experiments with HepG2 cells and primary hepatocytes

This study reports on the functional expression of a specific, high-affinity carrier-mediated mechanism for the transport of niacin (nicotinic acid) in human liver cells. Both human-derived liver HepG2 cells and human primary hepatocytes were used as models in these investigations. The initial rate of transport of nicotinic acid into HepG2 cells was found to be acidic pH, temperature, and energy dependent; it was, however, Na(+) independent in nature. Evidence for the existence of a carrier-mediated system that is specific for [(3)H]nicotinic acid transport was found and included the following: 1) saturability as a function of concentration with an apparent K(m) of 0.73 +/- 0.16 microM and V(max) of 25.02 +/- 1.45 pmol.mg protein(-1).3 min(-1), 2) cis-inhibition by unlabeled nicotinic acid and nicotinamide but not by unrelated organic anions (lactate, acetate, butyrate, succinate, citrate, and valproate), and 3) trans-stimulation of [(3)H]nicotinic acid efflux by unlabeled nicotinic acid. Transport of the vitamin into human primary hepatocytes occurs similarly via an acidic pH-dependent and specific carrier-mediated process. Inhibitors of the Ca(2+)-calmodulin-mediated pathway (but not modulators of the PKC-, PKA-, and protein tyrosine kinase-mediated pathways) inhibited nicotinic acid transport into both HepG2 cells and human primary hepatocytes. Maintenance of HepG2 cells (for 48 h) in growth medium oversupplemented with nicotinic acid (or nicotinamide) did not affect the subsequent transport of [(3)H]nicotinic acid into HepG2 cells. These results show, for the first time, the existence of a specific and regulated membrane carrier-mediated system for nicotinic acid transport in human liver cells.

Niacin in cardiovascular prevention: mechanisms, efficacy, and safety

PURPOSE OF REVIEW

This review describes niacin's mechanism of action, efficacy in cardiovascular prevention, and safety.

RECENT FINDINGS

A G-protein-coupled receptor [GPR109A/HM74A, mouse PUMA-G (protein upregulated in macrophages by interferon-gamma)] was found to mediate the antilipolytic effect of niacin via inhibition of adenylyl cyclase in adipocytes. The same receptor in skin Langerhans cells mediates the common flushing side effect. The endogenous ligand for the receptor may be beta-hydroxybutyrate. Among nine controlled clinical trials using niacin, mostly combined with other drugs, statistically significant positive impact on clinical or anatomic cardiovascular end-points was found in seven, which represents a remarkably consistent record of benefit. Although niacin induces insulin resistance, deterioration of glycemic control in diabetes is usually minor, and there is no evidence of increased incidence of new onset diabetes. Hepatic toxicity is common with higher doses of sustained-release niacin but rare with immediate-release and extended-release niacin at doses up to 2000 mg/day. Extended-release and immediate-release niacin do not substantially potentiate myopathic effects when given in combination with statins.

SUMMARY

Recently developed understanding of the mechanisms, efficacy, and safety of niacin, along with progress in reducing the chief side effect of flushing, should enhance the use of this valuable agent for cardiovascular prevention.

Cardiovascular metabolic syndrome - an interplay of, obesity, inflammation, diabetes and coronary heart disease

Cardiovascular disease is currently one of the biggest causes of morbidity and mortality facing humanity. Such a paradigm shift of disease pattern over the last century has only worsened due to the alarming global prevalence of obesity and type 2 diabetes. In recent years there is increasing focus on inflammation as one of the key players in the patho-physiology of these disorders. In addition to these overt risk factors new research is unraveling the significance of a constellation of early metabolic abnormalities that include weight gain, insulin resistance, prehypertension and a specific pattern of dyslipidaemia. There exists a complex interrelationship of these various metabolic disorders and their effect on cardiovascular system. Simplified explanation can be that inflammation increases insulin resistance, which in turn leads to obesity while perpetuating diabetes, high blood pressure, prothrombotic state and dyslipidaemia. While inflammation and insulin resistance have direct adverse effects on cardiac muscle, these metabolic abnormalities as a whole cause causes cardiovascular complications; warranting a multi pronged therapeutic and preventive approach for the 'Cardiovascular Metabolic Syndrome' as an entity.

High-Density Lipoprotein Cholesterol

Background and Purpose— This review characterizes the current state of knowledge of high-density lipoprotein (HDL) in relation to stroke.

Summary of Review— HDL has anti-atherosclerotic and anti-inflammatory properties and is an important component in atherosclerosis. Serum HDL-cholesterol levels are inversely related to heart disease and stroke risk. There are various established and experimental treatments which can raise serum HDL cholesterol and improve its function.

Conclusion— HDL is an emerging target for atherosclerotic stroke treatment with the potential to dramatically impact the care of stroke patients.

Molecular regulation of HDL metabolism and function: implications for novel therapies

HDL metabolism represents a major target for the development of therapies intended to reduce the risk of atherosclerotic cardiovascular disease. HDL metabolism is complex and involves dissociation of HDL apolipoprotein and HDL cholesterol metabolism. Advances in our understanding of the molecular regulation of HDL metabolism, macrophage cholesterol efflux, and HDL function will lead to a variety of novel therapeutics.

Anti-inflammatory effect is an important property of niacin on atherosclerosis beyond its lipid-altering effects

Niacin has been used for decades to lower the plasma concentrations of cholesterol, free fatty acids, and triglycerides in humans, and in addition it raises more than any other drug the levels of the protective high density lipoprotein. These effects have been used to treat dyslipidemic states. Trials have shown that treatment with niacin reduces progression of atherosclerosis, and clinical events and mortality from coronary heart disease. The beneficial clinical efficacy of niacin appropriately emphasizes the prominent role of its lipid-altering effects; however, high expression of niacin receptor in a variety of immune cell types, lowering of inflammatory markers, and beneficial impact on adipokines expression could provide rational to the hypothesis that anti-inflammatory effect is also an important property of niacin on atherosclerosis beyond its lipid-altering effects.

Preventing another event: role of the hospitalist in discharge stroke prevention

Only 1 of 8 stroke patients is managed exclusively by a neurologist. Furthermore, many stroke patients harbor other vascular comorbidities and are also at risk for developing general medical complications that can lead to death following stroke. With the growing hospitalist system, it is quite clear that hospitalists are, and will increasingly be, an integral part of the care team for many hospitalized stroke patients. Because prevention remains the mainstay of treatment for ischemic stroke and TIA, it would be useful for practicing hospitalists to know the scientific evidence behind recommended therapeutic approaches to reducing vascular risk following stroke, as well as strategies for bridging the prevailing evidence-practice gap for hospitalized stroke patients, which this review article presents.

Dietetic guidelines on food and nutrition in the secondary prevention of cardiovascular disease ? evidence from systematic reviews of randomized controlled trials (second update, January 2006)

AIM

To update dietetic guidelines based on systematic review evidence on dietary advice to prevent further events in people with existing cardiovascular disease (CVD) (secondary prevention).

METHODS

The Cochrane Library, MEDLINE and EMBASE were comprehensively searched to January 2005 for systematic reviews on aspects of diet and heart health. Reviews were included if they searched systematically for randomized controlled trials relating to diet and secondary prevention of CVD. Each review was critically appraised by at least two members of the UK Heart Health and Thoracic Dietitians Group. The quality and results of each review were discussed and summarized at a group meeting.

RESULTS

Evidence-based strategies that reduce cardiovascular events in those with CVD include reduction in saturated fat and substitution with unsaturated fats. Individuals who have suffered a myocardial infarction may also benefit from adopting a Mediterranean type diet and increasing intake of omega 3 fats, but it is not clear whether they are beneficial for all patients with CVD. There is no systematic review evidence to support the use of antioxidant vitamins supplements, low glycaemic index diets, or homocysteine lowering therapies in this group.

CONCLUSION

There remains good evidence that reducing saturated fat reduces morbidity in patients with CVD. This advice is consistent for most manifestations of CVD, with the addition of Mediterranean dietary advice and increased omega 3 fats for those who have had a myocardial infarction.

Nicotinic acid induces secretion of prostaglandin D2 in human macrophages: an in vitro model of the niacin flush

Nicotinic acid is a safe, broad-spectrum lipid agent shown to prevent cardiovascular disease, yet its widespread use is limited by the prostaglandin D2 (PGD2) mediated niacin flush. Previous research suggests that nicotinic acid-induced PGD2 secretion is mediated by the skin, but the exact cell type remains unclear. We hypothesized that macrophages are a source of nicotinic acid-induced PGD2 secretion and performed a series of experiments to confirm this. Nicotinic acid (0.1-3 mM) induced PGD2 secretion in cultured human macrophages, but not monocytes or endothelial cells. The PGD2 secretion was dependent on the concentration of nicotinic acid and the time of exposure. Nicotinuric acid, but not nicotinamide, also induced PGD2 secretion. Pre-incubation of the cells with aspirin (100 microM) entirely prevented the nicotinic acid effects on PGD2 secretion. The PGD2 secreting effects of nicotinic acid were additive to the effects of the calcium ionophore A23187 (6 microM), but were independent of extra cellular calcium. These findings, combined with recent in vivo work, provide evidence that macrophages play a significant role in mediating the niacin flush and may lead to better strategies to eliminate this limiting side effect.

Functionally defective high-density lipoprotein: a new therapeutic target at the crossroads of dyslipidemia, inflammation, and atherosclerosis

High-density lipoproteins (HDL) possess key atheroprotective biological properties, including cellular cholesterol efflux capacity, and anti-oxidative and anti-inflammatory activities. Plasma HDL particles are highly heterogeneous in physicochemical properties, metabolism, and biological activity. Within the circulating HDL particle population, small, dense HDL particles display elevated cellular cholesterol efflux capacity, afford potent protection of atherogenic low-density lipoprotein against oxidative stress and attenuate inflammation. The antiatherogenic properties of HDL can, however be compromised in metabolic diseases associated with accelerated atherosclerosis. Indeed, metabolic syndrome and type 2 diabetes are characterized not only by elevated cardiovascular risk and by low HDL-cholesterol (HDL-C) levels but also by defective HDL function. Functional HDL deficiency is intimately associated with alterations in intravascular HDL metabolism and structure. Indeed, formation of HDL particles with attenuated antiatherogenic activity is mechanistically related to core lipid enrichment in triglycerides and cholesteryl ester depletion, altered apolipoprotein A-I (apoA-I) conformation, replacement of apoA-I by serum amyloid A, and covalent modification of HDL protein components by oxidation and glycation. Deficient HDL function and subnormal HDL-C levels may act synergistically to accelerate atherosclerosis in metabolic disease. Therapeutic normalization of attenuated antiatherogenic HDL function in terms of both particle number and quality of HDL particles is the target of innovative pharmacological approaches to HDL raising, including inhibition of cholesteryl ester transfer protein, enhanced lipidation of apoA-I with nicotinic acid and infusion of reconstituted HDL or apoA-I mimetics. A preferential increase in circulating concentrations of HDL particles possessing normalized antiatherogenic activity is therefore a promising therapeutic strategy for the treatment of common metabolic diseases featuring dyslipidemia, inflammation, and premature atherosclerosis.

Antiatherogenic small, dense HDL--guardian angel of the arterial wall?

Our understanding of the relationship between the atheroprotective activities of HDL and heterogeneity of HDL particles has advanced greatly. HDL particles are highly heterogeneous in structure, intravascular metabolism and antiatherogenic activity. In this review, we discuss new findings on the antiatherogenic properties of HDL particles. Small, dense HDL possesses potent antioxidative activity but this is compromised under conditions of atherogenic dyslipidemia. HDL functional deficiency frequently coincides with reductions in HDL-cholesterol concentration and alterations in HDL metabolism and structure. Formation of small, dense HDL particles with attenuated antiatherogenic activity can be mechanistically related to HDL enrichment in triglycerides and in serum amyloid A, depletion of cholesteryl esters, covalent modification of HDL apolipoproteins and attenuated antiatherogenic function of apolipoprotein AI. Low circulating levels of HDL cholesterol might, therefore, be associated with the defective functionality of small HDL particles of abnormal structure and composition. In common metabolic diseases, such as type 2 diabetes and metabolic syndrome, deficiency of HDL particle number and function favor accelerated atherosclerosis. Therapeutic normalization of the quantity, quality and biological activities of HDL particles thus represents a novel approach to attenuating atherosclerosis in dyslipidemic individuals with metabolic disease. Cholesteryl ester transfer protein inhibitors, nicotinic acid, reconstituted HDL and other HDL-raising agents are being investigated. Induction of selective increase in the circulating concentrations of small, dense HDL3 particles with increased antiatherogenic activity seems especially promising, particularly for therapy of atherogenic dyslipidemia.

Enhancement of arachidonic acid signaling pathway by nicotinic acid receptor HM74A

HM74A is a G protein-coupled receptor for nicotinic acid (niacin), which has been used clinically to treat dyslipidemia for decades. The molecular mechanisms whereby niacin exerts its pleiotropic effects on lipid metabolism remain largely unknown. In addition, the most common side effect in niacin therapy is skin flushing that is caused by prostaglandin release, suggesting that the phospholipase A(2) (PLA(2))/arachidonic acid (AA) pathway is involved. Various eicosanoids have been shown to activate peroxisome-proliferator activated receptors (PPAR) that play a diverse array of roles in lipid metabolism. To further elucidate the potential roles of HM74A in mediating the therapeutic effects and/or side effects of niacin, we sought to explore the signaling events upon HM74A activation. Here we demonstrated that HM74A synergistically enhanced UTP- and bradykinin-mediated AA release in a pertussis toxin-sensitive manner in A431 cells. Activation of HM74A also led to Ca(2+)-mobilization and enhanced bradykinin-promoted Ca(2+)-mobilization through Gi protein. While HM74A increased ERK1/2 activation by the bradykinin receptor, it had no effects on UTP-promoted ERK1/2 activation.Furthermore, UTP- and bradykinin-mediated AA release was significantly decreased in the presence of both MAPK kinase inhibitor PD 098059 and PKC inhibitor GF 109203X. However, the synergistic effects of HM74A were not dramatically affected by co-treatment with both inhibitors, indicating the cross-talk occurred at the receptor level. Finally, stimulation of A431 cells transiently transfected with PPRE-luciferase with AA significantly induced luciferase activity, mimicking the effects of PPARgamma agonist rosiglitazone, suggesting that alteration of AA signaling pathway can regulate gene expression via endogenous PPARs.

High-density lipoprotein cholesterol therapies: the next frontier in lipid management

Current cholesterol treatment guidelines target low-density lipoprotein cholesterol as the primary goal of therapy and recommend statins as first line therapy. However, despite aggressive treatment and success at reaching the recommended goals, coronary heart disease is still a leading cause of morbidity and mortality. Thus, other lipoproteins, such as high-density lipoprotein, are now being looked to as the next promising targets of therapy to help reduce the burden of coronary heart disease and atherosclerosis. This review details currently available strategies to raise high-density lipoprotein cholesterol, and then turns to several new compounds in development that target the varying components of the complex metabolism of high-density lipoprotein.

Pharmacologic augmentation of high-density lipoproteins: mechanisms of currently available and emerging therapies

PURPOSE OF REVIEW

With the limited effects of low-density lipoprotein-based lipid intervention, more attention is being paid to drugs that augment or mimic high-density lipoprotein's beneficial effects. A thorough understanding of the anti-atherogenic effects of high-density lipoprotein, and the mechanisms of existing or emerging high-density lipoprotein-based therapies, is essential for rational strategy for the prevention of cardiovascular disease.

RECENT FINDINGS

High-density lipoprotein mediates its beneficial effects through reverse cholesterol transport and direct anti-inflammatory effects of apolipoprotein AI and other component parts. Currently available drugs increase high-density lipoprotein-C through increasing apoAI synthesis (statins, fibrates) and decreasing apolipoprotein AI catabolism (niacin). Cholesteryl ester transfer protein inhibitors dramatically raise high-density lipoprotein-C, but clinical data are still required to verify their cardioprotective effects. Novel therapies such as apolipoprotein AImilano, apolipoprotein AI mimetic peptide, and exogenous phospholipids show tremendous promise as treatments for atherosclerosis.

SUMMARY

High-density lipoprotein and its defining functional protein apoAI prevent atherosclerosis through reverse cholesterol transport and other direct effects. Research has led to the development of novel therapies that increase high-density lipoprotein-C or that mimic direct anti-atherogenic effects of apolipoprotein AI. As these emerging therapies find a place in clinical medicine, we can anticipate preventing a much higher degree of cardiovascular events.

Hypertriglyceridemia—why, when and how should it be treated?

The relationship between serum triglyceride levels and cardiovascular disease has remained enigmatic despite four decades of research. The majority of the available evidence tends to support the role of hypertriglyceridemia as an independent risk factor for cardiovascular disease. However, there are no guidelines recommending a target triglyceride level for prevention of cardiovascular disease. The focus of lipid lowering therapy still remains the reduction of global cardiovascular risk by optimizing LDL cholesterol levels. Therapeutic options for triglyceride-lowering include lifestyle modification and pharmacological agents, such as fibrates, omega 3 fatty acids and niacin. Post-hoc analyses of the Helsinki Heart Study, Veterans Affairs High-Density Lipoprotein Cholesterol Intervention Trial and Bezafibrate Infarction Prevention Study suggest a beneficial effect of the treatment of hypertriglyceridemia with fibrates, mainly in obese subjects with insulin resistance. However, in order to establish the actual clinical relevance of lowering triglyceride levels, prospective trials need to be conducted with the specific purpose of studying the effects of triglyceride reduction on clinical end points, i. e. coronary events and stroke.

Functional characteristics of H+ -dependent nicotinate transport in primary cultures of astrocytes from rat cerebral cortex

In the present study, we report the characteristics of H(+)-coupled nicotinate transport in primary cultures of astrocytes from rat cerebral cortex. The [(3)H]nicotinate transport in rat astrocytes increased up to a pH 5.5. The nicotinic acid uptake at pH 6.0 was both energy-dependent and saturable with a Michaelis constant (K(t)) of 2.8+/-0.4 mM and the maximal uptake rate (V(max)) of 31+/-3.2 nmol/mg protein/10 min. This process was reduced by a protonophore, carbonylcyanide p-trifluoromethoxyphenylhydrazone, and a typical monocarboxylate transporter (MCT) inhibitor, alpha-cyano-4-hydroxycinnamic acid, suggesting that nicotinate uptake by rat astrocytes is mediated by H(+)-coupled monocarboxylate transport system. [(3)H]Nicotinate transport in rat astrocytes was significantly inhibited by various monocarboxylic acids such as l-lactic acid and pyruvic acid with a relatively low affinity (K(i)>10 mM). On the other hand, the uptake process of l-lactic acid was also saturable with a high-affinity component (K(t)=0.27 mM) and a low-affinity component (K(t)=35.9 mM). Reverse transcription-PCR and Western blot analyses revealed that three MCT subtypes, MCT1/Slc16a1, MCT2/Slc16a7, and MCT4/Slc16a3, were expressed in these cells. Because l-lactate reduced to 67% of the nicotinate uptake even at 10mM, it is unlikely that nicotinate uptake in rat astrocytes is mediated by MCT1 and/or MCT2. These results provide biochemical evidence of a H(+)-coupled and saturable transport system, presumed to be a low-affinity monocarboxylate transporter MCT4 or other unknown H(+)-coupled monocarboxylate transport system, for nicotinate in rat cerebrocortical astrocytes.

Transcriptional regulatory networks in lipid metabolism control ABCA1 expression

The ATP-binding cassette transporters, ABCA1 and ABCG1, are major players in mediating cellular efflux of phospholipids and cholesterol to apoA-I containing lipoproteins including prebeta-HDL and alphaHDL and thereby exert important antiatherogenic properties. Although the exact mechanisms how ABC transporters mediate lipid transport are not completely resolved, recent evidence from several laboratories including ours suggests that vesicular transport processes involving different interactive proteins like beta2-syntrophin, alpha1-syntrophin, Lin7, and cdc42 are critically involved in cellular lipid homeostasis controlled by ABCA1 and ABCG1. Besides sterols and fatty acids as known physiological modulators of the LXR/RXR and SREBP pathways, a growing list of natural and synthetic substances and metabolic regulators such as retinoids, PPAR-ligands, hormones, cytokines, and drugs are particularly effective in modulating ABCA1 and ABCG1 gene expression. Although ABCA1 protein amounts are regulated at the level of stability, the majority of potent activating and repressing mechanisms on ABCA1 function directly act on the ABCA1 gene promoter. Among the inducing factors, liver-X-receptors (LXR), retinoic acid receptors (RAR) and peroxisome proliferator-activated receptors (PPARs) along with their coactivators provide an amplification loop for ABCA1 and ABCG1 expression. The ABCA1 promoter is further stimulated by the ubiquitous factor Sp1 and the hypoxia-induced factor 1 (HIF1), which bind to GC-boxes and the E-box, respectively. Shutdown of ABCA1 expression in the absence of sterols or in certain tissues is mediated by corepressor complexes involving unliganded LXR, sterol-regulatory element binding protein 2 (SREBP2), Sp3, and the SCAN-domain protein ZNF202, which also impacts nuclear receptor signaling. Thus, a highly sophisticated transcriptional network controls the balanced expression of ABCA1.

Drugs in development: targeting high-density lipoprotein metabolism and reverse cholesterol transport

PURPOSE OF REVIEW

This review summarizes currently available therapies for raising high-density lipoprotein cholesterol (HDL-C) and expands on therapies currently in development that target high-density lipoprotein cholesterol.

RECENT FINDINGS

In the realm of new high-density lipoprotein-raising therapies, there is a strong focus on high-density lipoprotein metabolism and the reverse cholesterol transport pathway. Several infusions of recombinant apoA-I Milano/phospholipid complexes appeared to reduce atheroma volume as measured by intravascular ultrasound. Both intravenous and oral apoA-I mimetic peptides are in early clinical trials. Next generation PPAR-alpha agonists are more potent at high-density lipoprotein-raising than currently available fibrates, and dual PPAR-alpha/PPAR-gamma agonists are under investigation to help correct atherogenic dyslipidemia seen in many diabetics. Two small molecule inhibitors of the cholesteryl ester transfer protein have shown promise in clinical trials at substantially raising high-density lipoprotein cholesterol.

SUMMARY

Larger scale clinical trials, including those with additional surrogate outcome measures as well as cardiovascular event outcomes are needed to further assess the benefit of newer high-density lipoprotein-raising therapies. Additional therapeutics are currently in development that target other parts of the reverse cholesterol transport pathway and, in addition to providing new potential pharmaceuticals, will help to further elucidate the atheroprotective mechanisms of high-density lipoprotein cholesterol.

Nicotinic acid in the management of dyslipidaemia associated with diabetes and metabolic syndrome: a position paper developed by a European Consensus Panel

Individuals with type 2 diabetes and metabolic syndrome are at markedly increased risk of cardiovascular morbidity and mortality. The increasing prevalence of both conditions poses a major challenge for clinicians in the 21st century. Both diabetes and metabolic syndrome are associated with a clustering of cardiovascular risk factors. In particular, dyslipidaemia characterised by low plasma levels of high-density lipoprotein cholesterol (HDL-C), elevated triglycerides and an increase in small, dense low-density lipoprotein (LDL) particles (the lipid triad), has been established as the most important modifiable risk factor for coronary heart disease (CHD). Current treatment guidelines recognise the increased CHD risk associated with diabetes and metabolic syndrome and focus on LDL-C lowering with statin treatment, in addition to dietary and lifestyle modification, as the primary lipid-modifying therapy. However, while there is no doubt that statin therapy significantly reduces CHD risk in these patients, their residual absolute risk remains higher than in individuals without diabetes or metabolic syndrome. Thus, there is a clear need to target other aspects of lipoprotein metabolism, notably low HDL-C and hypertriglyceridaemia, to further reduce CHD risk. Combining statin therapy (targeting LDL-C) with interventions that also modify low HDL-C and elevated triglycerides could be a useful strategy to optimise CHD risk reduction. Cautious combination of a fibrate or nicotinic acid with a statin is useful for the management of combined dyslipidaemia. Nicotinic acid is the more potent agent for raising HDL-C (by up to 29% at clinically recommended doses). It also substantially reduces triglycerides and LDL-C, and promotes a shift from small, dense LDL to larger, more buoyant LDL particles. Preliminary clinical data suggest that combining nicotinic acid with a statin will produce a greater reduction in cardiovascular risk in patients with diabetes and metabolic syndrome than statin monotherapy alone. Nicotinic acid is also safe for use in patients with diabetes, with no evidence of clinically relevant deterioration in glycaemic control at recommended doses (< or = 2 g/day). On review of the available evidence, this European Consensus Panel recommends the combination of nicotinic acid and a statin, together with lifestyle modification, as a useful strategy to lower CHD risk in patients with diabetes and metabolic syndrome. Prolonged-release nicotinic acid with improved tolerability compared with previous formulations may have obvious advantages for use in this setting.

Mechanism and regulation of human intestinal niacin uptake

The mechanism of uptake of dietary niacin (nicotinic acid) by intestinal epithelial cells is not well understood, and nothing is known about regulation of the uptake process. In this investigation, we used human-derived intestinal epithelial Caco-2 cells and purified intestinal brush-border membrane vesicles (BBMVs) isolated from human organ donors to assess niacin uptake. Our findings show niacin uptake by Caco-2 cells to be 1) temperature and energy dependent; 2) Na+ independent, but highly dependent on extracellular acidic pH; 3) saturable as a function of concentration, with an apparent K(m) of 0.53 +/- 0.08 microM; 4) severely inhibited by the membrane-impermeable sulfhydryl group of reagents; and 5) highly specific for niacin but not affected by monocarboxylic acids. A marked trans stimulation in [3H]niacin efflux from preloaded Caco-2 cells by unlabeled niacin in the incubation buffer was also observed. These findings suggest the involvement of a specialized, pH-dependent, carrier-mediated mechanism for human intestinal niacin uptake. This suggestion was confirmed in studies with native human intestinal BBMVs. We also examined possible regulation of niacin uptake by Caco-2 cells via specific intracellular regulatory pathways. The results show that while the PKA-, PKC-, and Ca2+/calmodulin-mediated regulatory pathways play no role in regulating niacin uptake, a role for a protein tyrosine kinase (PTK)-mediated pathway is apparent. The results of these studies show for the first time the existence of a specialized, acidic pH-dependent, carrier-mediated system of niacin uptake by human intestinal epithelial cells that operates at the micromolar (physiological) range of niacin. The results also suggest the possible involvement of a PTK-mediated pathway in the regulation of niacin uptake.