Therapeutic interventions targeted at the augmentation of reverse cholesterol transport

Airway mesenchymal cell death by mevalonate cascade inhibition: integration of autophagy, unfolded protein response and apoptosis focusing on Bcl2 family proteins

HMG-CoA reductase, the proximal rate-limiting enzyme in the mevalonate pathway, is inhibited by statins. Beyond their cholesterol lowering impact, statins have pleiotropic effects and their use is linked to improved lung health. We have shown that mevalonate cascade inhibition induces apoptosis and autophagy in cultured human airway mesenchymal cells. Here, we show that simvastatin also induces endoplasmic reticulum (ER) stress and the unfolded protein response (UPR) in these cells. We tested whether coordination of ER stress, autophagy and apoptosis determines survival or demise of human lung mesenchymal cells exposed to statin. We observed that simvastatin exposure activates UPR (activated transcription factor 4, activated transcription factor 6 and IRE1α) and caspase-4 in primary human airway fibroblasts and smooth muscle cells. Exogenous mevalonate inhibited apoptosis, autophagy and UPR, but exogenous cholesterol was without impact, indicating that sterol intermediates are involved with mechanisms mediating statin effects. Caspase-4 inhibition decreased simvastatin-induced apoptosis, whereas inhibition of autophagy by ATG7 or ATG3 knockdown significantly increased cell death. In BAX(-/-)/BAK(-/-) murine embryonic fibroblasts, simvastatin-triggered apoptotic and UPR events were abrogated, but autophagy flux was increased leading to cell death via necrosis. Our data indicate that mevalonate cascade inhibition, likely associated with depletion of sterol intermediates, can lead to cell death via coordinated apoptosis, autophagy, and ER stress. The interplay between these pathways appears to be principally regulated by autophagy and Bcl-2-family pro-apoptotic proteins. These findings uncover multiple mechanisms of action of statins that could contribute to refining the use of such agent in treatment of lung disease.

Novel therapies for increasing serum levels of HDL

The protectiveness of elevated HDL-C against CHD and its long-term sequelae is a subject of intense investigation throughout the world. HDL has the capacity to modulate a large number of atherogenic mechanisms, such as inflammation, oxidation, thrombosis, and cell proliferation. Among lipoproteins, HDL is also unique, in that it promotes the mobilization and clearance of excess lipid via the series of reactions collectively termed "reverse cholesterol transport." Numerous therapeutic agents are being developed in an attempt to modulate serum levels of HDL-C as well as its functionality. This article discusses the development of newer treatments targeted at raising HDL-C and HDL particle numbers to reduce residual risk in patients at risk for CHD.

When high is low: raising low levels of high-density lipoprotein cholesterol

Low serum levels of high-density lipoprotein cholesterol (HDL-C) are highly prevalent and are recognized as an independent risk factor for cardiovascular morbidity (myocardial infarction, stroke, peripheral arterial disease, and restenosis after coronary stenting) and mortality. HDL plays an important role in modulating atherogenesis, although its functions are varied and complex and the mechanisms for its antiatherogenic effects have not been completely elucidated. The inverse relationship between HDL-C and cardiovascular risk is well established, and epidemiologic studies and clinical trials have provided ample evidence that higher levels of HDL-C are vasculoprotective. Although considerable interest exists in the development of novel approaches to raise serum HDL-C and to augment HDL functionality, this article discusses currently available therapies to raise suboptimal levels of this important lipoprotein.

Statin-dependent suppression of the Akt/mammalian target of rapamycin signaling cascade and programmed cell death 4 up-regulation in renal cell carcinoma

PURPOSE

Statins are pharmacologic inhibitors of the 3-hydroxy-3-methylglutaryl-coenzyme A reductase with potent regulatory effects on cholesterol biosynthesis in vitro and in vivo. There is accumulating evidence that, beyond their cholesterol-lowering properties, statins inhibit cell proliferation and promote apoptosis of malignant cells in vitro, but the mechanisms by which they generate such responses remain to be defined.

EXPERIMENTAL DESIGN

Combinations of experimental approaches were used, including immunoblotting and cell proliferation and apoptosis assays.

RESULTS

We provide evidence that fluvastatin is a potent inducer of apoptosis and suppresses proliferation of renal cell carcinoma (RCC) cells in vitro. Such effects are mediated by direct targeting of the Akt/mammalian target of rapamycin (mTOR) pathway, as evidenced by the suppression of phosphorylation/activation of Akt, resulting in inhibition of its downstream effectors, mTOR and p70 S6 kinase. In addition, fluvastatin blocks the mTOR-dependent phosphorylation/deactivation of the translational repressor eukaryotic initiation factor 4E (eIF4E)-binding protein, leading to the formation of eIF4E-binding protein-eIF4E complexes that suppress initiation of cap-dependent mRNA translation. Importantly, inhibition of p70 S6 kinase activity by fluvastatin results in the up-regulation of expression of programmed cell death 4 (PDCD4), a tumor suppressor protein with inhibitory effects on the translation initiation factor eIF4A, suggesting a mechanism for the generation of antitumor responses.

CONCLUSIONS

Altogether, our findings establish that fluvastatin exhibits potent anti-RCC activities via inhibitory effects on the Akt/mTOR pathway and raise the possibility that combinations of statins and Akt inhibitors may be of future therapeutic value in the treatment of RCC.

High-density lipoprotein metabolism: potential therapeutic targets

It is well recognized that the lowering of low-density lipoprotein (LDL) cholesterol can substantially reduce coronary artery disease (CAD)-related morbidity and mortality. The prevention and management of CAD has chiefly focused on 1 component of the lipid profile: the reduction of LDL cholesterol. Yet, the majority of patients in both the primary and secondary prevention settings continue to experience significant residual risk for acute cardiovascular events even when their LDL cholesterol is lowered aggressively with combinations of lifestyle modification and pharmacologic intervention. As a result, there is increased focus on targeting and treating low serum levels of high-density lipoprotein (HDL) cholesterol in an effort to further reduce risk for cardiovascular events, including myocardial infarction, unstable angina, ischemic stroke, and death. Epidemiologically high serum levels of HDL cholesterol are associated with reduced risk for the development of atherosclerotic disease. HDL particles are believed to be antiatherogenic secondary to their capacity to drive reverse cholesterol transport and antagonize pathways of inflammation, thrombosis, and oxidation. HDL cholesterol can be quite challenging to raise in many individuals because of the large number of polymorphisms in the genes, enzymes, cell surface receptors, and apoproteins that regulate the serum concentrations, functionality, and patterns of metabolism of HDL particles This article reviews HDL metabolism and established as well as emerging therapeutic approaches to raising serum concentrations of this fascinating and complex lipoprotein.

High-Density Lipoprotein Cholesterol: A Potential Therapeutic Target for Prevention of Coronary Artery Disease

High-density lipoprotein cholesterol has an important role in the pathophysiology of coronary artery disease. High-density lipoprotein cholesterol is becoming an increasingly important prognostic and therapeutic target. The purpose of this paper is to review the biochemical pathways involved in reverse cholesterol transport and to discuss potential, clinically based high-density lipoprotein therapies that may contribute to reduction in risk of atherosclerosis.

Reducing cardiovascular risk by targeting high-density lipoprotein cholesterol

Although lowering low-density lipoprotein (LDL) cholesterol with statins can substantially reduce cardiovascular morbidity and mortality, many treated patients retain a residual risk for cardiovascular events. Low levels of high-density lipoprotein (HDL) cholesterol may underpin this residual risk and may represent an additional target for intervention. Several new therapies for substantially increasing HDL cholesterol levels are under investigation, including cholesteryl ester transfer protein (CETP) inhibitors, apolipoprotein A-I mimetics and recombinant HDL, liver X receptor (LXR) agonists, and peroxisome proliferator-activated receptor (PPAR) agonists. Combining new HDL cholesterol-elevating agents with existing LDL cholesterol-lowering agents may improve the cardiovascular risk reductions currently attainable.

A feasibility study quantifying in vivo human alpha-tocopherol metabolism

BACKGROUND

Quantitation of human vitamin E metabolism is incomplete, so we quantified RRR- and all-rac-alpha-tocopherol metabolism in an adult.

OBJECTIVE

The objective of the study was to quantify and interpret in vivo human vitamin E metabolism.

DESIGN

A man was given an oral dose of 0.001821 micromol [5-14CH3]RRR-alpha-tocopheryl acetate (with 101.5 nCi 14C), and its fate in plasma, plasma lipoproteins, urine, and feces was measured over time. Data were analyzed and interpreted by using kinetic modeling. The protocol was repeated later with 0.001667 micromol [5-14CH3]all-rac-alpha-tocopheryl acetate (with 99.98 nCi 14C).

RESULTS

RRR-alpha-tocopheryl acetate and all-rac-alpha-tocopheryl acetate were absorbed equally well (fractional absorption: approximately 0.775). The main route of elimination was urine, and approximately 90% of the absorbed dose was alpha-2(2'-carboxyethyl)-6-hydroxychroman. Whereas 93.8% of RRR-alpha-tocopherol flow to liver kinetic pool B from plasma was returned to plasma, only 80% of the flow of all-rac-alpha-tocopherol returned to plasma; the difference (14%) was degraded and eliminated. Thus, for newly digested alpha-tocopherol, the all-rac form is preferentially degraded and eliminated over the RRR form. Respective residence times in liver kinetic pool A and plasma for RRR-alpha-tocopherol were 1.16 and 2.19 times as long as those for all-rac-alpha-tocopherol. Model-estimated distributions of plasma alpha-tocopherol, extrahepatic tissue alpha-tocopherol, and liver kinetic pool B for RRR-alpha-tocopherol were, respectively, 6.77, 2.71, and 3.91 times as great as those for all-rac-alpha-tocopherol. Of the lipoproteins, HDL had the lowest 14C enrichment. Liver had 2 kinetically distinct alpha-tocopherol pools.

CONCLUSIONS

Both isomers were well absorbed; all-rac-alpha-tocopherol was preferentially degraded and eliminated in urine, the major route. RRR-alpha-tocopherol had a longer residence time and larger distribution than did all-rac-alpha-tocopherol. Liver had 2 distinct alpha-tocopherol pools. The model is a hypothesis, its estimates are model-dependent, and it encourages further testing.

High-density lipoprotein as a therapeutic target: clinical evidence and treatment strategies

The clinical importance of low serum levels of high-density lipoprotein (HDL) cholesterol is often under-recognized and underappreciated as a risk factor for premature atherosclerosis as well as for cardiovascular morbidity and mortality. Low serum levels of HDL are frequently encountered, especially in patients who are obese or have the metabolic syndrome. In prospective epidemiologic studies, every 1-mg/dL increase in HDL is associated with a 2% to 3% decrease in coronary artery disease risk, independent of low-density lipoprotein (LDL) cholesterol and triglyceride (TG) levels. The primary mechanism for this protective effect is believed to be reverse cholesterol transport, but several other anti-inflammatory, antithrombotic, and antiproliferative functions for HDL have also been identified. In recognition of these antiatherogenic effects, recent guidelines have increased the threshold for defining low levels of HDL for both men and women. The first step in achieving these revised targets is therapeutic lifestyle changes. When these measures are inadequate, pharmacotherapy specific to the patient's lipid profile should be instituted. Niacin therapy, currently the most effective means for raising HDL levels, should be initiated in patients with isolated low HDL (HDL <40 mg/dL, LDL and non-HDL at or below National Cholesterol Education Program (NCEP) targets based on global cardiovascular risk evaluation). Patients who have both low HDL and elevated LDL should receive a statin or statin-niacin combination therapy, and patients with concomitant low HDL and elevated TGs should receive a fibrate initially, with a statin, niacin, or ezetimibe added thereafter as needed to help attain NCEP lipoprotein targets.

Cholesteryl esters in malignancy

Cholesteryl esters, formed by the esterification of cholesterol with long-chain fatty acids, on one hand, are the means by which cholesterol is transported through the blood by lipoproteins, on the other, the way cholesterol itself can be accumulated in the cells. Therefore, these important molecules play an active part in metabolic pathways that form the basis of cholesterol trafficking and homeostasis. The role of different regulatory mechanisms in cholesterol homeostasis in physiologic and neoplastic conditions with emphasis on intracellular content of cholesteryl esters is here reviewed. Numerous studies carried out on tumor cell lines, experimental tumors, and human tumors have shown an abnormal cholesterol metabolism that is reflected by an increase in intracellular cholesteryl esters due to an alteration in all the mechanisms that form the basis of regulation, in particular: cholesterol de novo biosynthesis; uptake of exogenous cholesterol LDL receptor mediated; cholesterol esterification mediated by the ACAT activity; cholesterol efflux HDL receptor mediated. The most recent analytic-spectroscopic applications that permit cholesteryl ester determination on tumor lipidic extracts and directly in vivo are also reported. This review gives an overview of cholesterol homeostasis in physiological and pathological conditions where cholesteryl esters are over-expressed.

Apolipoprotein structural organization in high density lipoproteins: belts, bundles, hinges and hairpins

PURPOSE OF REVIEW

To summarize recent advances towards an understanding of the three-dimensional structures of the apolipoprotein components of HDL with a specific focus on high resolution models of apolipoprotein A-I.

RECENT FINDINGS

Since the primary sequence was first reported, various models have been advanced for the structure of apolipoprotein A-I, the major protein constituent of HDL, in its lipid-free and lipid-bound forms. Unfortunately, the generation of experimental data capable of distinguishing among the competing models has lagged far behind. However, recent experimental strategies, including X-ray crystallography, applications of resonance energy transfer and mass spectrometry, have combined with sophisticated theoretical approaches to develop three-dimensional structural models of apolipoprotein A-I with previously unavailable resolution.

SUMMARY

The recent synergy of sophisticated computer modeling techniques with hard experimental data has generated new models for apolipoprotein A-I in certain subclasses of HDL produced in vitro. The challenge now is to adapt and test these models in the more complex forms of HDL isolated directly from human plasma.

Simvastatin plus ezetimibe: combination therapy for the management of dyslipidaemia

Hyperlipidaemia is a pivotal risk factor for the development of atherosclerotic disease. A large number of studies have demonstrated that the treatment of abnormalities in lipoprotein levels reduces the risk for myocardial infarction, peripheral vascular disease, carotid artery disease, stroke, and cardiovascular mortality. Despite the development of multiple drug classes to treat dyslipidaemias and the promulgation of clearly defined guidelines for the management of lipid disorders, dyslipidaemia tends to be undertreated in the majority of patients at risk for cardiovascular disease. A part of the reluctance to treat different lipoprotein fractions to goal levels is attributable to physician- and patient-related concerns over the increasing toxicity of available therapies, as their dosages are increased. The risks of hepatotoxicity, myalgia, and rhabdomyolysis are fairly well characterised in patients receiving statins, fibrates and niacin. Another issue affecting treatment success rates is the fact that many patients with complex dyslipidaemias are inadequately responsive to single-agent therapy. As the epidemics of obesity, metabolic syndrome and diabetes mellitus continue to worsen, physicians will encounter severe, mixed dyslipidaemias more frequently. Many of these patients will require combinations of drugs to address the various metabolic derangements causing changes in multiple lipoprotein fractions. Although the need for combination therapy is well-established in the management of disorders, such as hypertension and diabetes, it is less often used for the treatment of dyslipidaemias. The development of safe, cost-effective, and efficacious combination dyslipidaemic therapy is an important goal in cardiovascular medicine. Simvastatin plus ezetimibe has recently been combined as a fixed dose therapy, which offers clinicians the opportunity to simultaneously inhibit two key pathways in cholesterol metabolism: hepatic cholesterol biosynthesis and the absorption of cholesterol at the level of the proximal jejunum. This dual mechanism of inhibition substantially increases the capacity to decrease serum levels of atherogenic low-density lipoproteins and increase high-density lipoprotein, compared with that observed when either drug is used alone. This combination increases the likelihood of therapeutic success in patients with dyslipidaemia.