Niacin accelerates intracellular ApoB degradation by inhibiting triacylglycerol synthesis in human hepatoblastoma (HepG2) cells

Phosphatidylinositol acts through mitogen-activated protein kinase to stimulate hepatic apolipoprotein A-I secretion

Phosphatidylinositol (PI) has been shown to stimulate reverse cholesterol transport in animal models and to increase plasma apolipoprotein (apo) A-I levels and high-density lipoprotein cholesterol in human subjects. The objective of this study was to determine the molecular mechanism through which PI stimulates apo A-I secretion in hepatic cells. PI (12 mumol/L) significantly stimulates apo A-I secretion from HepG2 cells over 24 hours. The stimulation in apo A-I secretion is completely blocked by phospholipase C inhibitors (D609 and U73122) and the Ras inhibitor sulindac sulfide. Apolipoprotein A-I secretion is augmented with a protein kinase C agonist (dioctanoyl glycerol) and inhibited by a protein kinase C inhibitor (dioleoyl ethylene glycol). The PI-induced apo A-I secretion is unaffected by PI-3-kinase inhibitors but is sensitive to mitogen-activated protein kinase (MAPK) inhibitors. Whereas the p38MAPK inhibitor SB203580 has no effect on PI-induced apo A-I secretion, the MAPK kinase 1/2 inhibitor U0126 and the c-Jun-N-terminal kinase/stress-activated protein kinase inhibitor SP600125 block PI-induced apo A-I secretion. PI also increased extracellular-regulated protein kinase 1 and 2 phosphorylation in HepG2 cells in a time-dependent manner. PI does not appear to stimulate apo A-I gene transcription, as cellular apo A-I messenger RNA levels remained unchanged over the 24-hour incubation. However, PI significantly decreases apo A-I binding and degradation in HepG2 cells. Collectively, the data suggest that PI acts through MAPK pathways to increase plasma apo A-I levels by protecting it from reuptake and degradation.

[New trends in lipidology: the increasing role of HDL-cholesterol]

Previous epidemiological studies have demonstrated the low level of high-density lipoprotein (HDL) cholesterol as an independent risk factor for cardiovascular diseases, the increase of which is one of the cornerstones of preventive cardiovascular care. In addition to its major role in reverse cholesterol transport, HDL-C has other biological activities that may contribute to its protective effects against atherosclerosis. These include antioxidant, anti-inflammatory, antithrombotic/profibrinolytic and vasoprotective effects. Current guidelines recommend aggressive lifestyle modifications, niacin, fibrate, statin or a combination of these to increase HDL-cholesterol levels. In addition, several novel HDL-based therapeutic strategies have been or are currently being tested. These include newer formulations of nicotinic acid/receptor agonists, CETP inhibitors, cannabinoid-1 receptor antagonists, PPAR agonists, liver X receptor/farnesoid X receptor agonists, and apoA-I and/or phospholipid-derived therapies. In this article previous clinical trials, epidemiological observations, basic science studies and the most important trials of novel agents are reviewed.

Effects of niacin and chromium on the expression of ATP-binding cassette transporter A1 and apolipoprotein A-1 genes in HepG2 cells

The ATP-binding cassette transporter A1 (ABCA1) and apolipoprotein A1 (ApoA-1) are both involved in the regulation of cholesterol efflux from cells. The overexpression of ABCA1 and ApoA-1 genes are associated with increased high-density lipoprotein (HDL) levels. Previous studies have shown that niacin and chromium reduce plasma cholesterol while increasing HDL levels. The aim of the present study was to determine the effects of niacin and chromium on HDL formation by investigating the changes in ABCA1 and ApoA-1 transcription in the human hepatoblastoma cell line (HepG2 cells). Cells were treated with either niacin or chromium, or the combination of both. The expression of ABCA1 and ApoA-1 mRNA was measured by a relative quantitative real-time reverse transcriptase-polymerase chain reaction method. Results showed that niacin at concentrations of 1 and 5 mM significantly increased ABCA1 (1.3-1.7-fold), without affecting ApoA-1 (0.8-1.2-fold), whereas chromium at 3 mM significantly increased both ABCA1 (1.7+/-0.01-fold) and ApoA-1 (1.5+/-0.1-fold) transcription when compared to untreated cells. Niacin and chromium cotreatment significantly induced the expression of peroxisome proliferator-activated receptor-alpha (PPARalpha) mRNA by approximately 1.3-1.8-fold. It was likely that the increases observed for the ABCA1 transcript may be regulated by the increases in PPARalpha transcription. A combination of niacin and chromium chloride did not significantly increase (3+1 mM) but instead reduced (1+3 mM) ABCA1 gene expression. In the case of ApoA gene, the combination of niacin and chromium chloride at concentrations of 1+3 mM significantly elevated expression; however, this effect was not observed at concentrations of 3+1 mM. When cells were treated with the combination at both concentrations, only slight increases in PPARalpha mRNA was observed. Niacin, but not chromium, significantly reduced intracellular cholesterol. We hypothesize that the stimulation of ABCA1 gene expression causes an enhanced cholesterol efflux, perhaps mediated by PPARalpha pathway(s).

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.

Nicotinic acid: pharmacological effects and mechanisms of action

Pharmacological doses of nicotinic acid induce a profound change in the plasma levels of various lipids and lipoproteins. The ability of nicotinic acid to strongly increase the plasma concentration of high-density lipoprotein (HDL) cholesterol has in recent years led to an increased interest in the pharmacological potential of nicotinic acid. There is increasing evidence that nicotinic acid alone or in addition to LDL cholesterol-lowering drugs can reduce the progression of atherosclerosis and reduce the risk of cardiovascular events. The clinical use of nicotinic acid is, however, hindered by harmless but unpleasant side effects, especially by a strong cutaneous vasodilation called flushing. The recent discovery of the G protein-coupled receptor GPR109A (HM74A or PUMA-G) as a receptor for nicotinic acid has allowed for better understanding of the mechanisms underlying the metabolic and vascular effects of nicotinic acid. On the basis of recent progress in understanding the pharmacological effects of nicotinic acid, new strategies are in development to better exploit the pharmacological potential of nicotinic acid. New drugs acting via the nicotinic acid receptor or related receptors, as well as new co-medications that suppress unwanted effects of nicotinic acid, will most likely be introduced as new therapeutic options in the treatment of dyslipidemia and the prevention of cardiovascular diseases.

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.

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.

Nicotinic acid: an old drug with a promising future

Nicotinic acid has been used for decades to treat dyslipidaemic states. In particular its ability to raise the plasma HDL cholesterol concentration has led to an increased interest in its pharmacological potential. The clinical use of nicotinic acid is somewhat limited due to several harmless but unpleasant side effects, most notably a cutaneous flushing phenomenon. With the recent discovery of a nicotinic acid receptor, it has become possible to better understand the mechanisms underlying the metabolic and vascular effects of nicotinic acid. Based on these new insights into the action of nicotinic acid, novel strategies are currently under development to maximize the pharmacological potential of this drug. The generation of both flush-reducing co-medications of nicotinic acid and novel drugs targeting the nicotinic acid receptor will provide future therapeutic options for the treatment of dyslipidaemic disorders.

Do we need a statin-nicotinic acid-aspirin mini-polypill to treat combined hyperlipidaemia?

This review considers the treatment for combined hyperlipidaemia (CH) with a combination formulation of three drugs: a statin, nicotinic acid (NA) and aspirin--a mini-polypill. CH is a highly atherogenic dyslipidaemia manifested either as familial combined hyperlipidaemia or dyslipidaemia related to the metabolic syndrome or Type 2 diabetes mellitus. These types of dyslipidaemia are highly prevalent in the general population. Statin plus extended-release NA is a promising treatment option for the normalisation of these atherogenic lipid alterations, regression of atherosclerosis, as well as for primary or secondary prevention of cardiovascular disease (CVD) events. The addition of aspirin might prove a useful adjunct that might reduce the cutaneous side effects of NA while also acting as an antiplatelet agent in high-CVD-risk patients. However, the effective dose of aspirin may need to be at least 160 mg/day. This triple combination might improve patient compliance when compared with the three drugs administered separately.

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.

Augmented antioxidant status in Tamoxifen treated postmenopausal women with breast cancer on co-administration with Coenzyme Q10, Niacin and Riboflavin

BACKGROUND

Reactive oxygen species (ROS) such as superoxide anion, hydrogen peroxide (H(2)O(2)), hydroxyl radical have been implicated in pathogenesis of various diseases including cancer and metastasis. Tamoxifen (TAM) is a non-steroidal anti-estrogen drug most widely used as an adjuvant hormonal therapy in breast cancer. TAM also has estrogenic activity on liver and endometrium causing severe oxidative stress and hypertriglycerdemia. Coenzyme Q(10) (CoQ(10)), Niacin and Riboflavin are well-known potent antioxidants and protective agents against many diseases including cancer. In this context, this study was undertaken to find if co-administration of CoQ(10), Niacin and Riboflavin along with TAM could augment the antioxidant (AO) status in postmenopausal women with breast cancer.

METHODS

The vitamin supplementation with Tamoxifen was given for a period of 90 days. Blood samples were collected at the base line, 45th and 90th day during the course of treatment. Plasma lipids, lipid peroxides and various circulating enzymatic and non-enzymatic antioxidants were estimated in 78 untreated, sole TAM treated and combinatorial treated group along with 46 age- and sex-matched controls.

RESULTS

Enhanced oxidative stress as evidenced by increased lipids and lipid peroxides with decreased AO levels in untreated breast cancer patients was observed. Adjuvant TAM-treated group had a limited impact on the increased oxidative stress with decreased AO status. Severe hypertriglycerdemia was observed in TAM-treated group when compared to untreated and control subjects. Combinatorial therapy (CT) of CoQ(10), Niacin and Riboflavin along with TAM decreased the oxidative stress and increased the AO status.

CONCLUSION

The antioxidant defense system is compromised in breast cancer patients. There is a shift in the oxidant / antioxidant balance in favor of lipid peroxidation (LPO), which could lead to tumour promotion observed in the disease. CT of CoQ(10), Niacin and Riboflavin along with TAM significantly increased the AO status, while decreasing lipid and lipid peroxides. The results suggest the necessity of therapeutic co-administration of antioxidants along with conventional drug to such patients. However, due to limited number of cases included in this study, more studies may be required to substantiate the results and arrive at a definitive conclusion, in terms of safety and efficacy of adding an AO therapy in treatment of breast cancer.

The Effects of Niacin on Lipoprotein Subclass Distribution

Dyslipidemia is a heterogeneous metabolic condition; high-density lipoprotein (HDL), low-density lipoprotein (LDL), and very-low-density lipoprotein represent families of lipoprotein particles that differ in size and composition and vary in atherogenicity. Lipoprotein subclasses containing apolipoprotein B promote atherosclerosis, of which the most atherogenic appear to be the small, dense LDL and large very-low-density lipoprotein subclasses, while the large HDL2 subclass, which transports esterified cholesterol from the periphery to the liver, is considered the more cardioprotective. Niacin has long been known to improve concentrations of all major lipids and lipoproteins, but it also has consistently favorable effects on subclass distribution. A MEDLINE search was conducted for clinical studies reporting the effects of niacin on lipoprotein subclasses. The niacin-associated elevations in HDL cholesterol likely stem from differential drug effects on subclasses, producing favorable changes in levels of HDL2 and apolipoprotein A-I. Niacin has more moderate LDL cholesterol-lowering efficacy, but this change is associated with an increase in LDL particle size and a shift from small LDL to the less atherogenic, large LDL subclasses. In addition, it also tends to decrease concentrations of the larger very-low-density lipoprotein subclasses. Niacin confers diverse benefits with respect to both the quantity and quality of lipid and lipoprotein particles.

HDL Cholesterol: Physiology, Pathophysiology, and Management

Numerous epidemiological studies have identified high-density lipoprotein cholesterol (HDL) to be an independent risk factor for coronary heart disease (CHD). HDL is an emerging therapeutic target that could rival the impact of 3-hydroxy-3-methyl-glutaryl-CoA (HMG-CoA) reductase inhibitors (statins) on LDL and CHD risk reduction. HDL metabolism, HDL kinetics, the concentration of various HDL subclasses, and other genetic factors affecting HDL functionality may all contribute to the anti-atherogenic properties of HDL; thus, standard plasma measurement may not capture the full range of HDL effects. Algorithms have been suggested to treat low HDL levels in subgroups of patients; however, no formal HDL target goals or treatment guidelines have been implemented as there is a lack of strong clinical evidence to support effective pharmacologic therapy for primary risk reduction. Available therapies have a modest impact on serum HDL levels; however, emerging therapies could have a more significant influence.

Ameliorating effect of coenzyme Q10, riboflavin and niacin in tamoxifen-treated postmenopausal breast cancer patients with special reference to lipids and lipoproteins

OBJECTIVES

Tamoxifen (TAM), a non-steroidal anti-estrogen that is widely used in adjuvant therapy for all stages of breast carcinomas and in chemoprevention of high-risk group. The hepatic estrogenic effect of TAM induces hypertriglyceridemia by reduced activity of lipolytic enzymes (LPL) on triglycerides. Coenzyme Q10 (Co Q10), riboflavin and niacin are proved to be potent antioxidant and protective agents against many diseases including cancer and cardiovascular diseases (CVD). In this context, the objective of the study is to find the effect of the combined modality of Co Q10 (100 mg), riboflavin (10 mg) and niacin (50 mg) with TAM (10 mg twice a day) on serum lipids and lipoprotein levels in postmenopausal women with breast cancer.

DESIGN AND METHODS

The vitamin supplementation with tamoxifen was given for a period of 90 days. Blood samples were collected at the base line, 45th and 90th day during the course of treatment. Plasma total cholesterol (TC), free cholesterol (FC), ester cholesterol (EC), phospholipids (PL), triglycerides (TGL), free fatty acids (FFA), high-density lipoprotein cholesterol (HDL-C), low-density lipoprotein cholesterol (LDL-C) and very low density cholesterol (VLDL-C) were estimated in 78 untreated, only TAM-treated and combinatorialy treated group along with 46 age- and sex-matched controls.

RESULTS

Serum TGL and VLDL-C (p<0.001) were found to be significantly elevated and LDL-C (p<0.01), significantly reduced among TAM-treated patients as compared to the untreated breast cancer subjects. All the lipids and lipoprotein levels were found to be significantly altered in the untreated breast cancer patients when compared to their normal counterparts. All the lipid and lipoprotein abnormalities were reverted back to near normal levels on 90 days of treatment on combinatorial therapy.

CONCLUSION

The study figures the altered lipid and lipoprotein levels in the untreated and TAM-treated breast cancer patients. On combination therapy with Co Q10, riboflavin and niacin, it counteracts the tamoxifen-induced hyperlipidemia to normal levels.

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.

Flushing out the role of GPR109A (HM74A) in the clinical efficacy of nicotinic acid

The recent discovery of the G(i) protein-coupled receptor GPR109A (HM74A in humans; PUMA-G in mice) as a receptor for nicotinic acid has provided the opportunity to gain greater understanding of the underlying biology contributing to the clinical efficacy (increases in HDL, decreases in VLDL, LDL, and triglycerides) and the characteristic side-effect profile of nicotinic acid. GPR109A has been proven to be the molecular target for the actions of nicotinic acid on adipose tissue, and in this issue of the JCI, Benyó et al. have confirmed the involvement of GPR109A in the nicotinic acid-induced flushing response, a common side effect. The involvement of GPR109A in both the desirable and undesirable clinical actions of nicotinic acid raises interesting questions regarding the function of this receptor.

Triglyceride modulation by acifran analogs: activity towards the niacin high and low affinity G protein-coupled receptors HM74A and HM74

Niacin is known to exert profound beneficial effects on cholesterol levels in humans, although its use is somewhat hampered by the gram quantities necessary to exert effects and the prevalence of compliance-limiting skin flushing side effects that occur. Recently, two G protein-coupled receptors (GPCRs) for niacin were identified and characterized as high (HM74A; GPR109A) and low (HM74; GPR109B) affinity receptors based on the binding affinities of niacin. These receptors also bind acifran (AY-25,712), which is known to modulate lipid levels like niacin, with similar affinities. Twelve analogs of acifran were chemically synthesized. One analogue demonstrated a dose-dependent decrease in serum triglycerides in rats within 3h of oral administration. Next, the acifran analogs were assessed for their activity towards the high and low affinity niacin receptors expressed in CHO-K1 cells. Constructs expressing HM74A or HM74 were stably transfected into CHO-K1 cells and shown to elicit phosphorylation of p42 and p44 mitogen-activated protein kinase (ERK1/ERK2) phosphorylation upon addition of niacin or acifran. The presence of functionally coupled GPCRs was further confirmed using Pertussis toxin, which completely inhibited the ability of either niacin or acifran to elicit phospho-ERK1/ERK2. The EC(50) of p-ERK1/ERK2 for niacin for the high and low affinity receptors was 47nM and indeterminate (i.e., >100microM), respectively, while the EC(50) for acifran was 160 and 316nM, respectively. Two chemical analogs of acifran demonstrated robust phosphorylation of ERK1/ERK2. Collectively, these data suggest that the synthesis of acifran analogs may be a suitable path for developing improved HM74A agonists.

Clinical utility of extended-release niacin: update and summary

In the treatment of dyslipidemia and coronary heart disease, niacin extended-release (ER) (Niaspan) uniquely targets the atherogenic lipid abnormalities of the metabolic syndrome. Niacin ER raises high-density lipoprotein cholesterol more effectively than other agents, while reducing triglyceride and lipoprotein(a) levels and increasing low-density lipoprotein particle size. It is formulated to minimize the toxicities and adverse effects associated with other niacin formulations, making niacin ER more tolerable for patients. Previous concerns of hyperglycemia have been addressed by numerous studies demonstrating that morbidity and mortality benefits outweigh potential increases in glucose levels. Niacin ER is an important lipid-lowering agent in preventing fatal and nonfatal coronary events and slowing the progression of atherosclerosis.

New lipid-lowering agents

Lipid-lowering is established as a proven intervention to reduce atherosclerosis and its complications. This article summarises imminent developments in lipid-lowering therapy, including new statins and cholesterol absorption inhibitors currently undergoing investigation for licensing. It also discusses other therapeutic targets such as squalene synthase, microsomal transfer protein (MTP), acyl-cholesterol acyl transferase (ACAT), cholesterol ester transfer protein (CETP), peroxosimal proliferator activating receptors (PPARs) and lipoprotein (a) (LP(a)), for which compounds have been developed and have at least reached trials in animal models. Lipid-lowering drugs are likely to prove a fast-developing area for novel treatments, as possible synergies exist between new and established compounds for the treatment of atherosclerosis.

Lipid-lowering therapies in development

Lipid lowering is established as a proven intervention to reduce atherosclerosis and its complications. This article summarises novel developments in the lipid-altering therapies under development, including combination therapies, squalene synthase inhibitors, microsomal transfer protein inhibitors, acyl-cholesterol acyl transferase inhibitors, cholesterol ester transfer protein antagonists, peroxisome proliferator-activated receptor agonists, high-density lipoprotein-derived peptides and inflammation inhibitors, which have at least reached trials in animal models. Lipid-altering drugs are likely to to be a fast-developing area for novel treatments as possible synergies exist between new and established compounds for the treatment of atherosclerosis. New agents will have to show significant advantage in tolerability or efficacy over existing agents and have the potential to be used in combination therapy as is well established for bile acid sequestrants, nicotinic acid or fibrates and statins. Any new drugs will also have to be assessed in clinical end-point trials against current compounds with proven outcome benefits.

Pharmacologic treatment of type 2 diabetic dyslipidemia

Patients with diabetes mellitus have a higher risk for cardiovascular heart disease (CHD) than does the general population, and once they develop CHD, mortality is higher. Good glycemic control will reduce CHD only modestly in patients with diabetes. Therefore, reduction in all cardiovascular risks such as dyslipidemia, hypertension, and smoking is warranted. The focus of this article is on therapy for dyslipidemia in patients with type 2 diabetes. Patients with the metabolic syndrome (insulin resistance) share similarities with patients with type 2 diabetes and may have a comparable cardiovascular risk profile. Diabetic patients tend to have higher triglyceride, lower high-density lipoprotein cholesterol (HDL), and similar low-density lipoprotein cholesterol (LDL) levels compared with those levels in nondiabetic patients. However, diabetic patients tend to have a higher concentration of small dense LDL particles, which are associated with higher CHD risk. Current recommendations are for an LDL goal of less than 100 mg/dl (an option of < 70 mg/dl in very high-risk patients), an HDL goal greater than 40 mg/dl for men and greater than 50 mg/dl for women, and a triglyceride goal less than 150 mg/dl. Nonpharmacologic interventions (diet and exercise) are first-line therapies and are used with pharmacologic therapy when necessary. Lowering LDL levels is the first priority in treating diabetic dyslipidemia. Statins are the first drug choice, followed by resins or ezetimibe, then fenofibrate or niacin. If a single agent is inadequate to achieve lipid goals, combinations of the preceding Drugs may be used. For elevated triglyceride levels, hyperglycemia must be controlled first. If triglyceride or HDL levels remain uncontrolled, pharmacologic agents should be considered. Fibrates are slightly more effective than niacin in lowering triglyceride levels, but niacin increases HDL levels appreciably more than do fibrates. Unlike gemfibrozil, niacin selectively increases subfraction Lp A-I, a cardioprotective HDL. Niacin is distinct in that it has a broad spectrum of beneficial effects on lipids and atherogenic lipoprotein subfraction levels. Niacin produces additive results when used in combination therapy. Recent data suggest that lower dosages and newer formulations of niacin can be used safely in diabetic patients with good glycemic control. Current evidence and guidelines mandate that diabetic dyslipidemia be treated aggressively, and lipid goals can be achieved in most patients with diabetes when all available products are considered and, if necessary, used in combination.

Niacin therapy in atherosclerosis

PURPOSE OF REVIEW

Well designed, randomized, placebo-controlled studies show that niacin prevents cardiovascular disease and death. Unfortunately, early studies and anecdotal evidence have limited its use by promoting the opinion that niacin is intolerable and contraindicated in diabetes. As evidence mounts that treating multiple lipid risk factors decreases cardiovascular risk, the use of niacin in the treatment of atherosclerosis is experiencing somewhat of a renaissance.

RECENT FINDINGS

Emerging clinical evidence shows that niacin is both safe and effective in diabetes. Niacin beneficially alters lipoprotein subclass distribution and when used in combination with statins, has additional effects on lipoproteins. Niacin selectively and directly inhibits hepatic diacylglycerol acyltransferase 2, but not diacylglycerol acyltransferase 1, thus inhibiting hepatic triglyceride synthesis and very low density lipoprotein secretion. The recent discovery and characterization of a membrane-bound nicotinic acid receptor (HM74) explains niacin's acute inhibition of adipocyte lipolysis, but the role of HM74 in lowering triglycerides is unclear. Niacin possesses antioxidant, antiinflammatory, and other beneficial effects on atherosclerosis unrelated to lipid lowering. Finally, niacin appears to activate nuclear transcription factors such peroxisome proliferator activator receptor gamma, possibly via prostaglandin metabolism.

SUMMARY

New data indicate that niacin alters lipoprotein metabolism in novel ways, and mediates other beneficial nonlipid changes that may be atheroprotective. This information forms the rationale for the use of niacin in combination with agents possessing complementary mechanisms of action (e.g. statins) for cardiovascular risk reduction beyond that observed with monotherapy. Further research into the specific mechanisms of niacin may identify additional targets for future drug development.

Lipid-altering agents: the future

Lipid-lowering is established as proven intervention to reduce atherosclerosis and its complications. This article summarises novel developments in the lipid-altering therapies under development. It also discusses other therapeutic targets, such as squalene synthase, microsomal transfer protein, acyl-cholesterol acyl transferase, cholesterol ester transfer protein, peroxosimal proliferator-activating receptors and lipoprotein (a), for which compounds have been developed and have at least reached trials in animal models. Lipid-altering drugs are likely to prove a fast-developing area for novel treatments, as possible synergies exist between new and established compounds for the treatment of atherosclerosis.

Management of the metabolic syndrome-nicotinic acid

Nicotinic acid effectively treats each of the common lipid abnormalities found in the metabolic syndrome, and much progress has recently been made in understanding its mechanisms of action. Early concern that nicotinic acid can precipitate or worsen diabetes has been eased with recent trials, which demonstrated its safety and effectiveness in insulin-resistant states. Furthermore, nicotinic acid prevents cardiovascular disease and death in persons with a high prevalence of risk factors for the metabolic syndrome. When used by an experienced physician and taken by a motivated patient, nicotinic acid can be safe and effective in treating the dyslipidemia of the metabolic syndrome.

Niacin noncompetitively inhibits DGAT2 but not DGAT1 activity in HepG2 cells

Niacin is a widely used lipid-regulating agent in dyslipidemic patients. Previously, we have shown that niacin inhibits triacylglycerol synthesis. In this report, using HepG2 cells, we have examined the effect of niacin on the mRNA expression and microsomal activity of diacylglycerol acyltransferase 1 and 2 (DGAT1 and DGAT2), the last committed but distinctly different enzymes for triglyceride synthesis. Addition of niacin to the DGAT assay reaction mixture dose-dependently (0-3 mM) inhibited DGAT activity by 35-50%, and the IC(50) was found to be 0.1 mM. Enzyme kinetic studies showed apparent K(m) values of 8.3 microM and 100 microM using [(14)C]oleoyl-CoA and sn-1,2-dioleoylglycerol as substrates, respectively. A decrease in apparent V(max) was observed with niacin, whereas the apparent K(m) remained constant. A Lineweaver-Burk plot of DGAT inhibition by niacin showed a noncompetitive type of inhibition. Niacin selectively inhibited DGAT2 but not DGAT1 activity. Niacin inhibited overt DGAT activity. Niacin had no effect on the expression of DGAT1 and DGAT2 mRNA. These data suggest that niacin directly and noncompetitively inhibits DGAT2 but not DGAT1, resulting in decreased triglyceride synthesis and hepatic atherogenic lipoprotein secretion, thus indicating a major target site for its mechanism of action.

Stimulation of CD36 and the key effector of reverse cholesterol transport ATP-binding cassette A1 in monocytoid cells by niacin

Niacin, the first lipid lowering drug shown to improve survival after myocardial infarction, decreases LDL and increases HDL cholesterol levels. These effects cannot fully be explained by its suspected mechanism of action, inhibition of lipolysis and hepatic VLDL synthesis. Niacin has also been shown to interfere with the cyclic AMP (cAMP)/protein kinase A (PKA) pathway and massively stimulate prostaglandin D2 (PGD2) formation. The major metabolite of PGD2, 15-deoxy-Delta(12,14)-prostaglandin J2 (15d-PGJ2), was recently identified as the most potent endogenous PPARgamma activator. We, therefore, studied the effects of niacin on the PPARgamma- and cAMP-dependent expression of receptors promoting reverse cholesterol transport. The transcription of PPARgamma-, HDL-, LDL- and scavenger-receptors and the sterol exporter ABCA1, were measured by quantitative RT-PCR and cellular cholesterol efflux and PPARgamma activation studied in macrophage and hepatocyte models. Niacin stimulated the translocation of PPARgamma and the transcription of PPARgamma, CD36 and ABCA1 in monocytoid cells, whereas the LDL-receptor (LDL-R) was unchanged. Thereby niacin enhanced HDL-mediated cholesterol efflux from the cells resulting in a reduced cellular cholesterol content. The niacin effect on CD36 but not on ABCA1 was prevented by cyclooxygenase inhibition, whereas the niacin effect on ABCA1 but not on CD36 was prevented by PKA inhibition, suggesting mediation by the 15d-PGJ2/PPARgamma and the cAMP/PKA pathways, respectively. These new actions of niacin on several key effectors of reverse cholesterol transport out of the vessel wall provide a rational to expect regression of atherosclerosis and test the combination of niacin with statins for an overadditive clinical benefit.

Apolipoprotein B and apolipoprotein A-I: risk indicators of coronary heart disease and targets for lipid-modifying therapy

Although LDL cholesterol (LDL-C) is associated with an increased risk of coronary heart disease, other lipoproteins and their constituents, apolipoproteins, may play an important role in atherosclerosis. Elevated levels of apolipoprotein (apo) B, a constituent of atherogenic lipoproteins, and reduced levels of apo A-I, a component of anti-atherogenic HDL, are associated with increased cardiac events. Apo B, apo A-I and the apo B/apo A-I ratio have been reported as better predictors of cardiovascular events than LDL-C and they even retain their predictive power in patients receiving lipid-modifying therapy. Measurement of these apolipoproteins could improve cardiovascular risk prediction.

Niacin, lipids, and heart disease

Niacin is the most effective medication in current clinical use for increasing high-density lipoprotein (HDL) cholesterol. It has the broadest effect on the lipid profile, reducing all atherogenic apolipoprotein (apo) B and increasing all antiatherogenic apo AI-containing lipoproteins, resulting in significant reduction in atherosclerotic complications and total mortality in trials. Recent research indicates novel major target sites of action in the liver to 1) directly inhibit diacylglycerol acyltransferase 2 (DGAT2), explaining its effect on triglycerides and apo B lipoproteins, and 2) inhibit the HDL apo AI catabolism pathway, resulting in higher HDL levels. Such information may lead to new drug discovery and supply the rationale for combination with other lipid regulators that are known to have different mechanisms of action. Trial evidence shows that niacin is not only safe to use in persons with diabetes, but that its combination with 3-hydroxy-3-methyl-glutaryl coenzyme A (HMG CoA) reductase inhibitors (statins) is also safe and effective. Recently, a new formulation of niacin has made it easier to tolerate and administer. Clinical trials are needed to determine whether niacin in combination with other lipid-modulating agents decreases the risk of cardiovascular events beyond the approximately 30% that has been noted with monotherapy.

The lipid and non-lipid effects of statins

The 3-hydroxy-3-methyl glutaryl coenzyme A (HMG-CoA) reductase inhibitors, more commonly known as statins, are a class of drug widely used for the treatment of hypercholesterolaemia in patients with established cardiovascular disease as well as those at high risk of developing atherosclerosis. Their predominant action is to reduce circulating levels of low-density lipoprotein (LDL) cholesterol; to a smaller degree, they also increase high-density lipoprotein (HDL) cholesterol and reduce triglyceride concentrations. In recent years, however, there has been an increasing body of evidence that their effects on lipid profile cannot fully account for their cardiovascular protective actions: their beneficial effects are too rapid to be easily explained by their relatively slow effects on atherogenesis and too large to be accounted for by their relatively small effects on plaque regression. Experimental models have revealed that statins exert a variety of other cardiovascular effects, which would be predicted to be of clinical benefit: they possess anti-inflammatory properties, as evidenced by their ability to reduce the accumulation of inflammatory cells in atherosclerotic plaques; they inhibit vascular smooth muscle cell proliferation, a key event in atherogenesis; they inhibit platelet function, thereby limiting both atherosclerosis and superadded thrombosis; and they improve vascular endothelial function, largely through augmentation of nitric oxide (NO) generation. The relative importance of the lipid- and non-lipid-related effects of the statins in the clinical situation remains the subject of much continuing research.

Niacin and cholesterol: role in cardiovascular disease (review)

Niacin has been widely used as a pharmacologic agent to regulate abnormalities in plasma lipid and lipoprotein metabolism and in the treatment of atherosclerotic cardiovascular disease. Although the use of niacin in the treatment of dyslipidemia has been reported as early as 1955, only recent studies have yielded an understanding about the cellular and molecular mechanism of action of niacin on lipid and lipoprotein metabolism. In brief, the beneficial effect of niacin to reduce triglycerides and apolipoprotein-B containing lipoproteins (e.g., VLDL and LDL) are mainly through: a) decreasing fatty acid mobilization from adipose tissue triglyceride stores, and b) inhibiting hepatocyte diacylglycerol acyltransferase and triglyceride synthesis leading to increased intracellular apo B degradation and subsequent decreased secretion of VLDL and LDL particles. The mechanism of action of niacin to raise HDL is by decreasing the fractional catabolic rate of HDL-apo AI without affecting the synthetic rates. Additionally, niacin selectively increases the plasma levels of Lp-AI (HDL subfraction without apo AII), a cardioprotective subfraction of HDL in patients with low HDL. Using human hepatocytes (Hep G2 cells) as an in vitro model system, recent studies indicate that niacin selectively inhibits the uptake/removal of HDL-apo AI (but not HDL-cholesterol ester) by hepatocytes, thereby increasing the capacity of retained HDL-apo AI to augment cholesterol efflux through reverse cholesterol transport pathway. The studies discussed in this review provide evidence to extend the role of niacin as a lipid-lowering drug beyond its role as a vitamin.

Dyslipidemia treatment: current considerations and unmet needs

Monumental evidence from clinical trials indicates an approximately 30% reduction in atheroslcerotic cardiovascular disease (ASCVD) risk using monotherapy with lipid-regulating drugs in dyslipidemic patients. In order to achieve greater reductions in risk, other approaches are necessary, including improvements in technology designed to assess ASCVD risk. Recent preliminary, but encouraging evidence indicates that by combining drugs that have different mechanisms of action on lipid metabolism yields not only an additive effect on the lipoprotein spectrum, but also reduces ASCVD events. New studies indicate that niacin potently increases high-density lipoproteins (HDL) by inhibiting HDL catabolism and decreases hepatic production of atherogenic very-low- and low-density lipoproteins by inhibiting the key enzyme for triglyceride synthesis (diacylglycerol acyltransferase). Statins, fibrates, bile acid sequestrants and ezetimibe have mechanisms that are different. Combination therapy using statins and niacin not only safely corrects dyslipidemia, but also yields ASCVD risk reduction significantly in excess of the 30% seen with monotherapy. Newer drugs with different mechanisms of action or combinations of new formulations have recently become available. Drug discovery research is likely to yield additional agents. Clinical trials focused on combination therapies to reduce ASCVD risk well beyond 30% need to be conducted to establish the rationale for further reducing the incidence of the primary cause of death today.

Optimal therapy of low levels of high density lipoprotein-cholesterol

Plasma levels of high-density lipoprotein-cholesterol (HDL-C) are a powerful independent cardiovascular risk factor, bearing an inverse relationship with atherosclerotic cardiovascular disease (with risk rising sharply when levels are <1.04 mmol/L). Apart from its protective role in atherosclerosis, HDL-C increases fibrinolysis, is an antioxidant to low density lipoprotein-cholesterol (LDL-C), and decreases platelet aggregability. Up to a third of patients with atherosclerotic cardiovascular disease have 'desirable' plasma levels of total cholesterol but low HDL-C levels. Benefits of treating low plasma HDL-C levels were clearly demonstrated in the Veterans Affairs HDL Intervention Trial (VA-HIT) where gemfibrozil reduced nonfatal infarcts and coronary deaths by 22%. This was achieved by a 6% increase in plasma HDL-C levels, and a 24.5% decrease in plasma levels of triglycerides, without any significant decrease in LDL-C levels. Multivariate analyses revealed the rise in plasma HDL-C levels after treatment, but not decreases in plasma levels of triglycerides or LDL-C, predicted coronary artery disease events. The typical patient under consideration in this article is one with plasma levels of HDL-C <1 mmol/L, LDL-C <3.37 mmol/L [either receiving therapeutic lifestyle changes or or LDL-C-lowering therapy comprising a hydroxymethylglutaryl coenzyme-A (HMG-CoA) reductase inhibitor or bile acid sequestrant] and fasting triglycerides <2.26 mmol/L. We propose this dyslipidemia be classified as Type VI phenotype following the Frederickson and Lees classification. High-risk patients (with >/=2 risk factors for atherosclerotic cardiovascular disease, or 10-year cardiovascular risk >20%), patients with established atherosclerotic cardiovascular disease, or type 2 diabetes mellitus, or metabolic syndrome should receive pharmacotherapy. Plasma HDL-C levels >1.16 mmol/L may be considered optimal and between 1 and 1.16 mmol/L as desirable. Fibric acid derivatives, nicotinic acid, HMG-CoA reductase inhibitors, estrogens, and ethanol (not recommended as therapy) increase plasma HDL-C levels. Nicotinic acid is the most potent agent and recent reports indicate that, in contrast to gemfibrozil, it selectively increases antiatherogenic HDL subfraction, lipoprotein (Lp) AI (without apolipoprotein AII), in patients with low plasma HDL-C levels. An extended-release formulation, administered once daily, has improved the tolerability of nicotinic acid. Recent evidence also indicates that nicotinic acid may effectively correct dyslipidemia in patients with diabetes mellitus without significantly compromising glycemic control. Fibric acid derivatives and estrogen raise plasma HDL-C levels by different mechanisms of action, and these agents may be used with nicotinic acid. Combination therapy (especially HMG-CoA reductase inhibitor and nicotinic acid) should be considered in patients with atherosclerotic cardiovascular disease and low plasma HDL-C levels.

Effect of gemfibrozil on apolipoprotein B secretion and diacylglycerol acyltransferase activity in human hepatoblastoma (HepG2) cells

The mechanism of action of a widely used drug gemfibrozil to reduce triglycerides (TG) and apolipoprotein B (apo B) is incompletely understood. Using human hepatoblastoma (HepG2) cells, we examined the effect of gemfibrozil on apo B secretion and TG synthesis catalyzed by diacylglycerol acyltransferase (DGAT), primary processes associated with the secretion of LDL. Gemfibrozil significantly decreased apo B secretion by HepG2 cells. It decreased oleate-induced stimulation of apo B secretion, suggesting that gemfibrozil-mediated inhibition of apo B secretion may be dependent on the synthesis of TG catalyzed by DGAT. Pre-incubation of HepG2 cells with gemfibrozil (200-400 micromol/l for 48 h) significantly inhibited microsomal DGAT activity. When added directly to the DGAT assay system containing control microsomes, gemfibrozil significantly inhibited the activity of DGAT by 14-25%. Gemfibrozil (200-400 micromol/l) inhibited TG synthesis by 47-50% as measured by the incorporation of 3H-oleic acid into TG. The data indicate that gemfibrozil inhibits DGAT activity resulting in decreased synthesis of TG and its availability for apo B lipidation rendering it susceptible to intracellular apo B degradation leading to the decreased secretion. These in-vitro data suggest a novel additional mechanism by which gemfibrozil lowers plasma TG and atherogenic apo B lipoproteins in dyslipidemic patients.

Long-term safety and efficacy of a once-daily niacin/lovastatin formulation for patients with dyslipidemia

Combination therapy is increasingly recommended for patients with multiple lipid disorders, especially those at high risk for coronary events. We investigated the long-term safety and effectiveness of a new drug formulation containing once-daily extended-release niacin and lovastatin. A total of 814 men and women (mean age 59 years) with dyslipidemia were enrolled in a 52-week multicenter, open-label study. We used 4 escalating doses (niacin/lovastatin in milligrams): 500/10 for the first month, 1,000/20 for the second, 1,500/30 for the third, and 2,000/40 for the fourth month through week 52. Dose-dependent effects were observed for all major lipid parameters. At week 16, mean low-density lipoprotein (LDL) cholesterol and triglycerides were reduced by 47% and 41%, respectively; mean high-density lipoprotein (HDL) cholesterol was increased by 30% (all p <0.001). LDL/HDL cholesterol and total/HDL cholesterol ratios were also decreased by 58% and 48%, respectively. These effects persisted through week 52, except for the mean increase in HDL cholesterol, which had increased to 41% at 1 year. Lipoprotein (a) and C-reactive protein also decreased in a dose-related manner (by 25% and 24%, respectively, on 2,000/40 mg; p <0.01 vs baseline). Treatment was generally well tolerated. The most common adverse event was flushing, which caused 10% of patients to withdraw. Other adverse events included gastrointestinal upset, pruritus, rash, and headache. Drug-induced myopathy did not occur in any patient. The incidence of elevated liver enzymes to >3 times the upper limit of normal was 0.5%. Once-daily niacin/lovastatin exhibits substantial effects on multiple lipid risk factors and represents a significant new treatment option in the management of dyslipidemia.

The evolving role of high-density lipoprotein in reducing cardiovascular risk

In many patients with coronary artery disease, a low level of high-density lipoprotein cholesterol (HDL-C), rather than substantially elevated low-density lipoprotein cholesterol (LDL-C), is often the predominant lipid abnormality. Although the National Cholesterol Education Program treatment guidelines include HDL-C concentration as a major risk factor for primary prevention, the guidelines' emphasis on LDL-C as the primary target of therapy may cause uncertainty as to whether risk reduction strategies should focus on lowering LDL-C or raising HDL-C in high-risk patients with low HDL-C. Recent clinical trial evidence and epidemiologic data suggest that HDL-C should play a more important role in risk assessment, and that the definition of low HDL-C may need adjustment from the current National Cholesterol Education Program definition of <35 mg/dL to perhaps <40 mg/dL in men and <45 mg/dL in women. Patients with low HDL-C should receive aggressive risk factor modification, and more emphasis on increasing HDL-C may be warranted in addition to lowering LDL-C. (c) 2001 by CHF, Inc.

Intracellular mechanisms mediating the inhibition of apoB-containing lipoprotein synthesis and secretion in HepG2 cells by avasimibe (CI-1011), a novel acyl-coenzyme A: cholesterol acyltransferase (ACAT) inhibitor

We have studied the cellular and molecular mechanisms involved in the suppression of apoB secretion from HepG2 cells following incubation with avasimibe (CI-1011), a novel inhibitor of acyl-coenzyme A: cholesterol acyltransferase (ACAT). Cellular lipid analysis revealed that avasimibe significantly decreased the synthesis of cholesterol and cholesteryl ester, and, at higher doses, of triglyceride. Time-course trypsin protection assays revealed that avasimibe induced the accumulation of translocationally arrested apoB intracellularly. Pulse-chase studies showed that the treatment with avasimibe induced a >75% decrease in apoB secretion relative to control, but initially enhanced the protein stability and cellular accumulation of apoB. Subcellular fractionation of microsomes further confirmed the accumulation of secretion-incompetent apoB-lipoproteins in the endoplasmic reticulum (ER) and Golgi compartments of avasimibe-treated HepG2 cells. Although incubation of drug-treated cells with carbobenzoxyl-leucinyl-leucinyl-leucinal (MG132), a potent proteasome inhibitor, increased cellular apoB (70%), it failed to increase apoB secretion. Drug treatment induced an accumulation of secretion-incompetent apoB-containing lipoprotein particles, the majority of which demonstrated a density in a range similar to that of high-density lipoprotein. However, studies in permeabilized cells demonstrated that, at longer chase times, intracellularly accumulated apoB was eventually degraded, indicating that the inhibition of degradation may be transient. Oleate treatment of avasimibe-treated cells partially restored apoB secretion but not to the levels seen in control cells. In summary, we hypothesize that avasimibe acutely blocks the secretion of apoB and its associated lipoproteins from HepG2 cells, transiently enhancing its membrane association and cellular accumulation with eventual intracellular degradation of accumulated apoB.

Hepatitis C virus core protein inhibits microsomal triglyceride transfer protein activity and very low density lipoprotein secretion: a model of viral-related steatosis

Liver steatosis, which involves accumulation of intracytoplasmic lipid droplets, is characteristic of hepatitis C virus (HCV) infection. By use of an in vivo transgenic murine model, we demonstrate that hepatic overexpression of HCV core protein interferes with the hepatic assembly and secretion of triglyceride-rich very low density lipoproteins (VLDL). Core expression led to reduction in microsomal triglyceride transfer protein (MTP) activity and in the particle size of nascent hepatic VLDL without affecting accumulation of MTP and protein disulfide isomerase. Hepatic human apolipoprotein AII (apo AII) expression in double-core/apo AII transgenic mice diminished intrahepatic core protein accumulation and abrogated its effects on VLDL production. Apo AII and HCV core colocalized in human HCV-infected liver biopsies, thus testifying to the relevance of this interaction in productive HCV infection. Our results lead us to propose a new pathophysiological animal model for induction of viral-related steatosis whereby the core protein of HCV targets microsomal triglyceride transfer protein activity and modifies hepatic VLDL assembly and secretion.

Effects of etofibrate upon the metabolism of chylomicron-like emulsions in patients with coronary artery disease

Slow chylomicron intravascular catabolism has been associated with coronary artery disease and screening for drugs that can speed-up this process can be important. In this study, the effects of etofibrate upon chylomicron metabolism was tested by determination of the plasma kinetics of a chylomicron-like emulsion model in 12 patients with coronary artery disease, aged 59+/-11 years, (total cholesterol: 240+/-41 mg/dl; triglycerides: 188+/-42 mg/dl) submitted to a randomized, crossover, double-blind, placebo-controlled study with administration of 1 g per day etofibrate or placebo for 1-month. A 1-month washout period was inserted between the treatment periods. Patients were intravenously injected a chylomicron-like emulsion doubly labeled with 14C-cholesteryl oleate and 3H-triolein at baseline and after treatments. After etofibrate treatment, there was decrease of total cholesterol and triglyceride plasma levels and a trend to increase high-density lipoprotein cholesterol plasma levels. Etofibrate elicited 62% enhancement of post-heparin lipolytic activity and 100% increase of 3H-triglyceride fractional clearance rate compared with placebo treatment. 14C-cholesterol ester fractional clearance rate was 260% greater after etofibrate than after placebo. Therefore, a potent effect of etofibrate on both chylomicron lipolysis and remnant removal was achieved, indicating that this drug can be used to improve this metabolism in future prospective studies.

The benefits of niacin in atherosclerosis

Niacin favorably alters all major lipid subfractions at pharmacologic doses. Alone or in combination, it promotes regression of coronary artery disease, decreases coronary events, stroke, and total mortality. Major recent progress in niacin is in four areas. Firstly, recent data indicate that it increases high-density lipoprotein (HDL) and lowers triglycerides and low-density lipoprotein (LDL) by mechanisms different from statins, fibrates, and bile-sequestrants, giving rationale for combination therapy to achieve synergistic effects for complete lipid goal achievement. Secondly, new data on an extended-release preparation of niacin given once nightly indicates that it is as effective and has greater tolerability than immediate-release niacin. Thirdly, preliminary data with a single tablet formulation extended-release niacin and an HMG CoA reductase inhibitor (lovastatin) shows it to be safe and very effective, especially for raising HDL. Finally, emerging evidence indicates that niacin can be used effectively and safely in patients with type 2 diabetes mellitus, who often have low HDL levels.

Mechanism of action of niacin on lipoprotein metabolism

It is generally accepted that the increased concentrations of apolipoprotein (apo) B containing very low-density lipoproteins (VLDL) and low-density lipoproteins (LDL), and decreased levels of apo AI containing high-density lipoproteins (HDL) are correlated to atherosclerotic cardiovascular disease. Current evidence indicates that the post-translational apo-B degradative processes regulate the hepatic assembly and secretion of VLDL and the subsequent generation of LDL particles. The availability of triglycerides (TG) for the addition to apo B during intracellular processing appears to play a central role in targeting apo B for either intracellular degradation or assembly and secretion as VLDL particles. Based on the availability of TG, the liver secretes either dense TG-poor VLDL2 or large TG-rich VLDL1 particles, and these particles serve as precursors for the formation of more buoyant or small, dense LDL particles by lipid transfer protein- and hepatic lipase-mediated processes. HDLs are a heterogenous class of lipoproteins, and apo AI (the major protein of HDL) participates in reverse cholesterol transport, a process by which excess cholesterol is eliminated. Recent studies indicate that HDL particles containing only apo A-I (LPA-I) are more effective in reverse cholesterol transport and more anti-atherogenic than HDL particles containing both apo A-I and apo A-II (LPA-I + A-II).