Interactions of apolipoprotein E genotype, total cholesterol level, age, and sex in prediction of Alzheimer's disease: a case-control study

APP intracellular domain is increased and soluble Aβ is reduced with diet-induced hypercholesterolemia in a transgenic mouse model of Alzheimer disease

Cholesterol is one of multiple factors, other than familial genetic mutations, that can influence amyloid-beta peptide (Abeta) metabolism and accumulation in Alzheimer disease (AD). The effect of a high-cholesterol diet on amyloid precursor protein (APP) processing in brain has not been thoroughly studied. This study was designed to further investigate the role of cholesterol in the production of Abeta and APP intracellular domain (AICD) in 12-month-old Tg2576 transgenic mice. The mice were maintained on a high-cholesterol diet for 6 weeks. We found that diet-induced hypercholesterolemia increased the APP cytosolic fragment AICD and reduced sAPPalpha in the Tg2576 mice compared to the mice on a control basal diet. In addition, the levels of detergent-extracted Abeta40 were reduced, although no change in guanidine-extracted Abeta levels was observed. Full-length APP, alpha/betaC-terminal fragment (alpha/betaCTF), and beta-secretase (BACE) were not different in the cholesterol-fed mice compared to the control diet-fed mice. This study suggests that a high dietary cholesterol in aged mice may not only influence Abeta metabolism, but also regulate the AICD levels. AICD has a proposed role in signal transduction and apoptosis, hence modulation of AICD production could be an alternative mechanism by which cholesterol contributes to AD pathogenesis.

Iron, atherosclerosis, and neurodegeneration: a key role for cholesterol in promoting iron-dependent oxidative damage?

This article reviews the roles and interactions of iron, atherosclerosis, and neurodegeneration. It highlights the importance of cholesterol in promoting iron-dependent oxidative damage. An intriguing possibility is that hypercholesterolemia can increase brain iron load and both the aggregation of beta-amyloid and the ability of iron on plaques to catalyze oxidative damage. This could explain why hypercholesterolemia is a risk factor for Alzheimer's disease. Further work is necessary to study the mechanism of increased iron transport across the blood brain barrier in atherosclerosis.

Biomarkers of Alzheimer disease in plasma

Plasma and serum biochemical markers proposed for Alzheimer disease (AD) are based on pathophysiologic processes such as amyloid plaque formation [amyloid beta-protein (A beta), A beta autoantibodies, platelet amyloid precursor protein (APP) isoforms], inflammation (cytokines), oxidative stress (vitamin E, isoprostanes), lipid metabolism (apolipoprotein E, 24S-hydroxycholesterol), and vascular disease [homocysteine, lipoprotein (a)]. Most proteins or metabolites evaluated in plasma or serum thus far are, at best, biological correlates of AD: levels are statistically different in AD versus controls in some cohorts, but they lack sensitivity or specificity for diagnosis or for tracking response to therapy. Approaches combining panels of existing biomarkers or surveying the range of proteins in plasma (proteomics) show promise for discovering biomarker profiles that are characteristic of AD, yet distinct from nondemented patients or patients with other forms of dementia.

Metabolite-initiated protein misfolding may trigger Alzheimer's disease

Anfinsen showed that a protein's fold is specified by its sequence. Although it is clear why mutant proteins form amyloid, it is harder to rationalize why a wild-type protein adopts a native conformation in most individuals, but it misfolds in a minority of others, in what should be a common extracellular environment. This discrepancy suggests that another event likely triggers misfolding in sporadic amyloid disease. One possibility is that an abnormal metabolite, generated only in some individuals, covalently modifies the protein or peptide and causes it to misfold, but evidence for this is sparse. Candidate metabolites are suggested by the recently appreciated links between Alzheimer's disease (AD) and atherosclerosis, known chronic inflammatory metabolites, and the newly discovered generation of ozone during inflammation. Here we report detection of cholesterol ozonolysis products in human brains. These products and a related, lipid-derived aldehyde covalently modify Abeta, dramatically accelerating its amyloidogenesis in vitro, providing a possible chemical link between hypercholesterolemia, inflammation, atherosclerosis, and sporadic AD.

Cholesterol modifies classical conditioning of the rabbit (Oryctolagus cuniculus) nictitating membrane response

Cholesterol plays an important role in synapse formation, receptor function, and synaptic plasticity, and animal studies show that modifying cholesterol may improve learning and memory. Other data show that feeding animals cholesterol can induce beta amyloid accumulation. Rabbits (Oryctolagus cuniculus) fed 2% cholesterol for 8 weeks were given trace conditioning of the nictitating membrane response using a 100-ms tone, a 700-ms trace, and periorbital electrical stimulation or airpuff. Rabbits fed cholesterol showed significant facilitation of trace conditioning to airpuff and conditioning-specific reflex modification to periorbital electrical stimulation and airpuff. The cholesterol-fed rabbits had beta amyloid accumulation in the cortex, but little in the hippocampus. The data suggest cholesterol had facilitative effects that outweighed potential amnesic effects of cortical beta amyloid.

Proteomic studies of potential cerebrospinal fluid protein markers for Alzheimer's disease

By comparing the cerebrospinal fluid (CSF) proteome between Alzheimer's disease (AD) patients and controls, it may be possible to identify proteins that play a role in the disease process and thus to study the pathogenesis of AD. Two-dimensional gel electrophoresis (2-DE), SYPRO Ruby staining and mass spectrometry were used for clinical screening of disease-influenced CSF proteins in AD patients compared to controls. In order to increase the detection of CSF proteins and to improve the separation of protein isoforms in a 2-D gel, micro-narrow range IPG strips were used. The levels of eight proteins and their isoforms, including apolipoprotein A1, apolipoprotein E, apolipoprotein J, beta-trace, retinol-binding protein, kininogen, alpha-1 antitrypsin, cell cycle progression 8 protein, and alpha-1beta glycoprotein were significantly altered in CSF of AD patients compared to controls. Furthermore, we also used liquid-phase IEF, as a prefractionation step, prior to 2-DE for comparison of CSF proteins between individual AD patients and controls. The levels of 37 proteins spots were altered in AD patients. One of the identified proteins, alpha-2-HS glycoprotein, has not previously been linked to AD. Our study shows that several glycoproteins are altered in AD.

Alzheimer's disease, genes, and environment: the value of international studies

OBJECTIVE

To describe the construction of a disease model incorporating both genetic an environmental factors in the etiology of Alzheimer's disease (AD), using data generated from the Indianapolis-Ibadan dementia project (I-IDP).

METHOD

The I-IDP is a longitudinal comparative study of the prevalence and incidence o dementia in 2 communities: elderly African Americans living in Indianapolis, Indiana, an Yoruba living in Ibadan, Nigeria.

RESULTS

African Americans are more than twice as likely as Yoruba to develop AD. Possible explanations for this finding include genetic factors: the possession of the apolipoprotein E epsilon4 allele does not increase risk for AD among Yoruba but confers a sligh increase in AD risk for African Americans. As well, environmental factors may play a role: African Americans have a higher risk of vascular risk factors than do Yoruba.

CONCLUSIONS

International comparative studies, particularly those involving population from developing and developed countries, offer a unique opportunity for applying new in formation regarding population genetics to traditional AD risk factor research.

A liver X receptor and retinoid X receptor heterodimer mediates apolipoprotein E expression, secretion and cholesterol homeostasis in astrocytes

Apolipoprotein E (apoE) is an important protein involved in lipoprotein clearance and cholesterol redistribution. ApoE is abundantly expressed in astrocytes in the brain and is closely linked to the pathogenesis of Alzheimer's disease (AD). We report here that small molecule ligands that activate either liver X receptors (LXR) or retinoid X receptor (RXR) lead to a dramatic increase in apoE mRNA and protein expression as well as secretion of apoE in a human astrocytoma cell line (CCF-STTG1 cells). Examination of primary mouse astrocytes also revealed significant induction of apoE mRNA, and protein expression and secretion following incubation with LXR/RXR agonists. Moreover, treatment of mice with a specific synthetic LXR agonist T0901317 resulted in up-regulation of apoE mRNA and protein in both hippocampus and cerebral cortex, indicating that apoE expression in brain can be up-regulated by LXR agonists in vivo. Along with a dramatic induction of ABCA1 cholesterol transporter expression, these ligands effectively mediate cholesterol efflux in both CCF-STTG1 cells and mouse astrocytes in the presence or absence of apolipoprotein AI (apoAI). Our studies provide strong evidence that small molecule LXR/RXR agonists can effectively mediate apoE synthesis and secretion as well as cholesterol homeostasis in astrocytes. LXR/RXR agonists may have significant impact on the pathogenesis of multiple neurological diseases, including AD.

Alzheimer’s Disease and Vascular Dementia

This chapter explores the frequency with which dementing illnesses occur in populations, their distributions by personal characteristics, and what is known about their causes and potential protective factors. The primary focus is on the most common forms of dementia: Alzheimer's disease and vascular dementia. The chapter summarizes clinical and pathologic features of Alzheimer's disease and vascular dementia, and highlights recent theories of how risk factors affect brain reserve. With the evolution of the epidemiology of dementia from case-control studies to prospective cohort studies has come a new set of methodological challenges. These include identification of representative populations, enhancing subject participation and retention in studies, the need to include institutionalized as well as community-dwelling populations, the non-standardized use of cognitive tests to screen for dementia, and the complexities of the diagnostic process itself. Finally, the chapter summarizes risk and protective factors for disease expression, including cardiovascular risk factors, lifestyle factors, and factors that affect brain reserve.

Association of dyslipidemia and effects of statins on nonmacrovascular diseases

BACKGROUND

Statins have mechanisms of action that expand their effects beyond cholesterol lowering and atherosclerotic medical conditions.

OBJECTIVE

This review summarizes clinical evidence for the association of dyslipidemia and the effects of statin use on aortic stenosis, Alzheimer's dementia (AD), osteoporosis, prevention of diabetes mellitus (DM), diabetic retinopathy, age-related macular degeneration, and diabetic/nondiabetic nephropathy.

METHODS

An English-language literature search was conducted using MEDLINE (1966-June 2003). Bibliographies of retrieved articles were reviewed. Search terms included statin, HMG-CoA reductase inhibitors, aortic stenosis, Alzheimer's dementia, osteoporosis, prevention of diabetis, diabetic retinopathy, age-related macular degeneration, diabetic nephropathy, and nondiabetic nephropathy.

RESULTS

Three retrospective cohort trials have shown an association between statin use and the progression of aortic stenosis; one of these trials observed a 45% decrease in aortic valve area in 1 year. In AD, one cross-sectional analysis found 60% to 73% lower AD rates in lovastatin or pravastatin recipients ( P<0.001 ). Of the multiple observational studies on the effect of statins on fracture risk, some have shown a decreased risk, with an odds ratio as low as 0.50 (95% CI, 0.33-0.76); others have demonstrated no association. A post hoc analysis of the West of Scotland Coronary Prevention Study found a 30% reduction in the development of DM ( P=0.042 ), but this was not duplicated in the Anglo-Scandinavian Cardiac Outcomes Trial-Lipid Lowering Arm. A small clinical trial of 6 patients (11 eyes) demonstrated improved retinal hard exudates with pravastatin treatment in patients with diabetic retinopathy. In a cross-sectional analysis, age-related macular degeneration was found to be less common among statin users than nonusers (4% [ 1/27 ] vs 22% [ 76/352 ]; P=0.02. Multiple small clinical trials of 19 to 56 patients with diabetic and nondiabetic nephropathy at various stages generated inconsistent results for an association between statin use and decreased albumin excretion rate and decreased rate of decline in glomerular filtration.

CONCLUSION

Data of variable quantity and quality support the use of statins as adjuncts in the treatment of nonmacrovascular diseases.

The Role of Cholesterol and Statins in Alzheimer's Disease

OBJECTIVE:

To briefly discuss the impact of elevated total and low-density-lipoprotein cholesterol levels, as well as the potential relationship of hydroxymethylglutaryl coenzyme A reductase inhibitor (statin) use, on the development of Alzheimer's disease (AD).

DATA SOURCES:

Biomedical literature was accessed through MEDLINE and International Pharmaceutical Abstracts (1966–June 2003). The authors independently reviewed literature for possible inclusion in this article.

STUDY SELECTION AND DATA EXTRACTION:

Clinical studies were selected and reviewed from the data sources, with special emphasis on those dealing with statin use and AD.

DATA SYNTHESIS:

The impact of AD is significant, as it is rapidly becoming one of our country's most debilitating and costly diseases. Data from epidemiologic trials indicate that statins may have some protective effect against the development of AD. These trials also allude to theories regarding possible mechanisms of action for this use, data implicating possible superiority of one statin over another, and their lack in certain populations, specifically the very old elderly population.

CONCLUSIONS:

Preliminary evidence suggests that statins may offer a protective effect against the development of AD. However, review of the literature does not lend credence to the use of statins in the general nondemented population without hyperlipidemia. Potential confounding variables have not been considered in the majority of trials. Placebo-controlled clinical trials are ongoing and should yield more definitive results.

Multitasking of the 3-hydroxy-3-methylglutaryl coenzyme a reductase inhibitor: Beyond cardiovascular diseases

Statins can profoundly affect cellular metabolism by inhibiting 3-hydroxy-3-methylglutaryl coenzyme A reductase, which is the rate-limiting enzyme responsible for cholesterol synthesis. Many physicians prescribe statins to lower plasma cholesterol levels, which has beneficial effects in both the primary and secondary prevention of coronary artery disease. However, in vitro, in vivo, animal, and clinical studies have all shown that statins may also have important pleiotropic properties. In fact, a number of clinical studies have suggested that statins are involved in modulating diseases such as cancer, osteoporosis, and dementia (including Alzheimer's disease). However, because these studies have been only preliminary and observational in nature, large randomized, placebo-controlled studies are needed to confirm the modulatory role of statins in these important diseases.

A beta, aging, and Alzheimer's disease: a tale, models, and hypotheses

In this paper we explore the potential functional role of the A beta peptides in the context of Alzheimer's disease (AD). We begin by defining the morphology of the amyloid deposits in relation to surrounding glial cells and, more importantly, in relation to the brain vasculature. Amyloid accumulation in the brain's microvasculature causes disturbances in the blood-brain barrier (BBB), and in larger arteries, impairment in control of regional cerebral blood flow due to myocyte degeneration. We postulate that the deposition of vascular amyloid may represent a hydrophobic protein plaster to seal leaks in the BBB, occasionally observed in aging and catastrophically common in AD. The vasoconstrictive activity of A beta may also be related to leaky vessels whereby decreasing the arterial diameter may also help to control breaches in the BBB. The admission of plasma neurotoxic proteins into the brain may be controlled by activation of microglia elicited by soluble A beta peptides creating a subtle, but permanent brain inflammatory reaction. We also delve into the influence that cholesterol metabolism may have in membrane topology and A beta production, and the close correlations that exist between cardiovascular disease and AD. Finally, we speculate about the possibility of a peripheral source of A beta that may, by crossing the BBB, contribute to the vascular and parenchymal deposits of A beta in the AD brain.

Pravastatin at 10 mg/day does not decrease plasma levels of either amyloid-β (Aβ) 40 or Aβ 42 in humans

It has been assumed that statins work as a preventative drug for Alzheimer's disease (AD). Although some epidemiological observations raise doubts to the effectiveness of statins for AD, many in vitro and clinical studies insist on the effectiveness of statins decreasing amyloid-beta (Abeta) levels in medium or blood. To explore the effect of pravastatin on Abeta production, we followed the longitudinal plasma levels of both Abeta 40 and Abeta 42 during the allocation of pravastatin in 46 patients with hyperlipidemia. We found no correlation between plasma cholesterol levels or the decreasing values of total cholesterol and those of Abeta 40 or Abeta 42. Patients having Apolipoprotein E4 (ApoE4) had higher low-density lipoprotein levels and lower Abeta 40 levels in plasma, suggesting ApoE4 seems to influence plasma Abeta levels via cholesterol metabolism.

Treatment of Alzheimer's disease: current status and new perspectives

Alzheimer's disease (AD) is the most common neurodegenerative disorder and the most prevalent cause of dementia with ageing. Pharmacological treatment of AD is based on the use of acetylcholinesterase inhibitors, which have beneficial effects on cognitive, functional, and behavioural symptoms of the disease, but their role in AD pathogenesis is unknown. Other pharmacological therapies are becoming available--including the recently approved drug memantine, an NMDA channel blocker indicated for advanced AD. Here, we review clinical features of the available cholinesterase inhibitors (donepezil, rivastigmine, and galantamine) including their pharmacological properties, the evidence for switching from one agent to another, "head to head" studies, and the emerging evidence for the use of memantine in AD. New therapeutic approaches--including those more closely targeted to the pathogenesis of the disease--will also be reviewed. These potentially disease modifying treatments include amyloid-beta-peptide vaccination, secretase inhibitors, cholesterol-lowering drugs, metal chelators, and anti-inflammatory agents.

Cholesterol: from heart attacks to Alzheimer's disease

The accumulation and aggregation of the amyloid-beta peptide (Abeta) in the brain are important contributing factors to Alzheimer's disease (AD). Consequently, blocking the generation of Abeta is a potentially important treatment strategy. Recent work on the metabolism of Abeta has identified several cellular proteins and proteases that collectively promote or prevent the generation of Abeta. In addition, accumulating in vitro and in vivo evidence suggests a role for cholesterol in modulating the cellular processing of Abeta with the potential to affect AD.

Use of in vivo models to study the role of cholesterol in the etiology of Alzheimer's disease

Cholesterol has been implicated in the pathogenesis of Alzheimer's disease, both through intracellular effects, and through an extracellular effect due to its physical interaction with plaque associated amyloid. Epidemiology studies have implicated high cholesterol as a risk factor for AD, and have shown that the use of cholesterol reducing agents (statins) can be protective against the disease. We, and others have shown that cholesterol levels modulate the processing of the amyloid precursor protein (APP) both in vivo and in vitro, affecting the accumulation of Abeta (Abeta) peptides which may directly impact the risk of AD. This review describes the biology of sterols, and identifies how cholesterol may exacerbate the pathogenesis of AD. Data from in vivo and in vitro studies will then be presented to describe how treatments aimed at modulating lipid levels may be efficacious in treating AD.

Health-related quality of life and long-term therapy with pravastatin and tocopherol (vitamin E) in older adults

INTRODUCTION

Concerns about the effects of HMG-CoA reductase inhibitors ('statins') on health-related quality of life may contribute to their underuse in older adults with and at risk for cardiovascular disease. These concerns also may prevent clinicians from enrolling older patients in clinical trials assessing the efficacy of statins as a preventive therapy for Alzheimer's disease.

OBJECTIVE

To determine the effects of pravastatin and tocopherol (vitamin E), alone and in combination, on health-related quality of life in older adults.

STUDY DESIGN

Double-blind, randomised, placebo-controlled, crossover study.

PARTICIPANTS

Forty-one community-dwelling men and women aged > or = 70 years with low-density lipoprotein-cholesterol (LDL-C) > or = 3.62 mmol/L (140 mg/dl) participated.

METHODS

Subjects received pravastatin for 6 months then pravastatin plus tocopherol for an additional 6 months (group 1), or tocopherol for 6 months then pravastatin plus tocopherol for an additional 6 months (group 2). Dosages were pravastatin 20 mg daily and tocopherol 400 IU daily.

MAIN OUTCOME MEASURES

The following health-related quality-of-life measures were assessed at baseline, after 6 months and after 1 year: health perception, depression, physical function, cognitive function and sleep behaviour. In addition, data on adverse effects and laboratory abnormalities were obtained.

RESULTS

Pravastatin reduced levels of total cholesterol (-21%, p < 0.001) and LDL-C (-29%, p < 0.001). Health-related quality-of-life scores, physical adverse effects, muscle enzyme levels and liver function tests did not change after 12 months of therapy with pravastatin, tocopherol or their combination.

CONCLUSION

Both pravastatin and tocopherol have a good safety profile, are well tolerated and do not adversely affect health-related quality of life in older patients with hypercholesterolaemia. Given the significant beneficial cardiovascular effects of statin therapy in older adults and the potential role of statins in prevention of Alzheimer's disease, concerns about adverse effects on quality of life should not deter use of these medications in this population.

Alzheimer's disease: the cholesterol connection

A hallmark of all forms of Alzheimer's disease (AD) is an abnormal accumulation of the beta-amyloid protein (Abeta) in specific brain regions. Both the generation and clearance of Abeta are regulated by cholesterol. Elevated cholesterol levels increase Abeta in cellular and most animals models of AD, and drugs that inhibit cholesterol synthesis lower Abeta in these models. Recent studies show that not only the total amount, but also the distribution of cholesterol within neurons, impacts Abeta biogenesis. The identification of a variant of the apolipoprotein E (APOE) gene as a major genetic risk factor for AD is also consistent with a role for cholesterol in the pathogenesis of AD. Clinical trials have recently been initiated to test whether lowering plasma and/or neuronal cholesterol levels is a viable strategy for treating and preventing AD. In this review, we describe recent findings concerning the molecular mechanisms underlying the cholesterol-AD connection.

Beta-amyloid and cholinergic neurons

It is generally accepted that the crucial events in the pathogeny of Alzheimer's disease (AD) are the increased accumulation of amyloidogenic peptides derived from amyloid precursor protein and the harmful actions of these peptides on neurons, which bring about neurodegeneration. The enhanced beta-amyloid accumulation is known to be caused by mutations of specific genes in patients who suffer from the familial (hereditary) form of AD but who represent just a minor group within the total population of AD patients. The reasons for beta-amyloid accumulation are not known in the much larger group of patients with the sporadic form of the disease. A biochemical feature common to either form of the disease is the preferential atrophy and degeneration of cholinergic neurons, which is probably responsible for much of the cognitive decline characteristic of the disease. We present an overview of recent investigations on the interactions between beta-amyloid and cholinergic neurons.

Expression of acetoacetyl-CoA synthetase, a novel cytosolic ketone body-utilizing enzyme, in human brain

Acetoacetyl-CoA synthetase (AACS, acetoacetate-CoA ligase, EC 6.2.1.16) is a ketone body-utilizing enzyme, the physiological role of which remains unclear yet in mammals, particularly has never been studied in human. In order to investigate the tissue distribution of AACS in human, cDNA encoding AACS was isolated from HepG2 cells. Amino acid sequence of human AACS deduced from the open reading frame showed high homology (89.3%) with that of rat AACS and much less homology (43.7%) with that of bacterial AACS. The expression level of the AACS mRNA was high in kidney, heart and brain, but low in liver, and the expression profile of AACS in the human brain was quite similar to that of 3-hydroxy-3-methylglutaryl-CoA reductase.

Molecular characterization of the detergent-insoluble cholesterol-rich membrane microdomain (raft) of the central nervous system

Many fundamental neurological issues such as neuronal polarity, the formation and remodeling of synapses, synaptic transmission, and the pathogenesis of the neuronal cell death are closely related to the membrane dynamics. The elucidation of functional roles of a detergent-insoluble cholesterol-rich domain (raft) could therefore provide good clues to the molecular understanding of these important phenomena, for the participation of the raft in the fundamental cell functions, such as signal transduction and selective transport of lipids and proteins, has been elucidated in nonneural cells. Interestingly, the brain is rich in raft and the brain-derived raft differs in its lipid and protein components from other tissue-derived rafts. Since many excellent reviews are written on the membrane lipid dynamics of this microdomain, signal transduction, and neuronal glycolipids, we review on the characterization of the raft proteins recovered in the detergent-insoluble low-density fraction from rat brain. Special focus is addressed on the biochemical characterization of a neuronal enriched protein, NAP-22, for the lipid organizing activity of this protein has become increasingly clear.

Apolipoprotein E and cholesterol metabolism in the pathogenesis and treatment of Alzheimer's disease

There is much evidence suggesting that there is a strong relationship between the deterioration of brain lipid homeostasis, vascular changes and the pathogenesis of Alzheimer's disease (AD). These associations include: (1). recognition that a key cholesterol transporter, apolipoprotein E type 4, acts a major genetic risk factor for both familial and sporadic AD; (2). epidemiological studies linking cardiovascular risk factors, such as hypertension and high plasma cholesterol, to dementia; (3). the discovery that small strokes can precipitate clinical dementia in cognitively normal elderly subjects; (4). the modulation of degradation of the amyloid precursor protein by cholesterol administration in cell culture and in animal models of beta-amyloid overproduction; and (5). the beneficial effect of cholesterol-lowering drugs, such as Probucol and statins, in combating common AD. The recent finding that there is a genetic association between the HMGR gene locus and sporadic AD further suggests that brain cholesterol metabolism is central to AD pathophysiology, and a potential therapeutic target for disease stabilization and primary disease prevention.

Prevention of stroke and dementia with statins: Effects beyond lipid lowering

Stroke is a major cause of mortality and morbidity. The epidemiologic association between elevated serum cholesterol and stroke risk is controversial. However, recent secondary prevention studies with 3-hydroxy-3-methylglutaryl coenzyme A reductase inhibitors (statins) have demonstrated a significant reduction in ischemic stroke without an increase in hemorrhagic stroke. Statins probably reduce stroke by a variety of mechanisms, including modulation of precerebral atherothrombosis in the aorta and the carotid artery, thus preventing plaque disruption and artery-to-artery thromboembolism. Statins also improve endothelial homeostasis by increasing the bioavailability of nitric oxide, which orchestrates the paracrine antiatherosclerotic functions of the endothelium. Studies in experimental models of ischemic stroke show that statin therapy reduces brain infarct size and improves neurologic outcome by directly upregulating brain endothelial nitric oxide synthase. Putative anti-inflammatory actions of statins may also contribute to neuroprotection and stroke prevention. Although the clinical benefit of statins largely depends on lowering low-density lipoprotein cholesterol, accumulating data indicate that many of the pleiotropic effects of statins are attributable to the cellular consequences of depletion of intermediates in the cholesterol biosynthetic pathway (isoprenoids). These molecules play fundamental roles in cell growth, signal transduction, and mitogenesis. In addition to reducing stroke risk, emerging data suggest that statins may reduce dementia. Further studies are needed to fully address the role of statins in the prevention of stroke in patients without established vascular disease and the role of cholesterol modulation in the treatment of dementia.

The role of cholesterol in pathogenesis of Alzheimer's disease: dual metabolic interaction between amyloid beta-protein and cholesterol

The implication that cholesterol plays an essential role in the pathogenesis of Alzheimer's disease (AD) is based on the 1993 finding that the presence of apolipoprotein E (apoE) allele epsilon;4 is a strong risk factor for developing AD. Since apoE is a regulator of lipid metabolism, it is reasonable to assume that lipids such as cholesterol are involved in the pathogenesis of AD. Recent epidemiological and biochemical studies have strengthened this assumption by demonstrating the association between cholesterol and AD, and by proving that the cellular cholesterol level regulates synthesis of amyloid beta-protein (Abeta). Yet several studies have demonstrated that oligomeric Abeta affects the cellular cholesterol level, which in turn has a variety of effects on AD related pathologies, including modulation of tau phosphorylation, synapse formation and maintenance of its function, and the neurodegenerative process. All these findings suggest that the involvement of cholesterol in the pathogenesis of AD is dualistic-it is involved in Abeta generation and in the amyloid cascade, leading to disruption of synaptic plasticity, promotion of tau phosphorylation, and eventual neurodegeneration. This review article describes recent findings that may lead to the development of a strategy for AD prevention by decreasing the cellular cholesterol level, and also focuses on the impact of Abeta on cholesterol metabolism in AD and mild cognitive impairment (MCI), which may result in promotion of the amyloid cascade at later stages of the AD process.

Secretion of apolipoprotein E by brain glia requires protein prenylation and is suppressed by statins

Apolipoprotein E (ApoE) genotype modulates the risk of Alzheimer's disease. ApoE has been shown essential for amyloid beta-peptide fibrillogenesis and deposition, a defining pathological feature of this disease. Because astrocytes and microglia represent the major source of extracellular apoE in brain, we investigated apoE secretion by glia. We determined that protein prenylation is required for apoE release from a continuous microglial cell line, primary mixed glia, and from organotypic hippocampal cultures. Using selective protein prenylation inhibitors, apoE secretion was found to require protein geranylgeranylation. This prenylation involved a protein critical to apoE secretion, not apoE proper. ApoE secretion could also be suppressed by inhibiting synthesis of mevalonate, the precursor to both types of protein prenylation, using hydroxyl-3-methylglutaryl coenzyme A reductase inhibitors (statins). Recent reports have described the beneficial effects of statins on the risk of dementia. Our finding that protein geranylgeranylation is required for apoE secretion in the brain parenchyma provides another contributing mechanism to explain the effective properties of statins against the development of dementia. In this model, statin-mediated inhibition of mevalonate synthesis, an essential reaction in forming geranylgeranyl lipid, would lower extracellular levels of parenchymal apoE. Because apoE has been found necessary for plaque development in transgenic models of Alzheimer's disease, suppressing apoE secretion by statins could reduce plaques and, in turn, improve cognitive function.

Apolipoprotein E alters the processing of the beta-amyloid precursor protein in APP(V717F) transgenic mice

We have recently reported a critical role for apolipoprotein E (apoE) in the process of amyloid deposition and neuritic plaque formation in APP(V717F) transgenic (Tg) mice, an animal model of Alzheimer's disease (AD). In the present study, we have investigated whether the presence or absence of apoE alters the processing of the amyloid precursor protein (APP) to various fragments, including the beta-amyloid peptides (Abeta). Here we show that, in contrast to APP(V717F) Tg mice expressing apoE, APP(V717F) Tg mice deficient in apoE develop anti-Abeta immunoreactive multifocal aggregates, which contain the beta-cleaved C-terminal fragments (beta-CTFs) of APP. Tg mice deficient in apoE also display altered levels of mature full-length APP, increased amounts of beta-CTFs, as well as elevated levels of Abeta(1-40) and Abeta(1-42) in an age- and region-dependent manner when compared to Tg mice expressing apoE. Taken together, these data support a role for apoE in APP processing in vivo.

Cholesterol in Alzheimer's disease and tauopathy

Cholesterol has been implicated in the pathogenesis of amyloid plaques in Alzheimer's disease (AD) and in the formation of neurofibrillary pathology in Niemann-Pick disease. Several epidemiology studies have implicated high cholesterol as a risk factor for AD and have shown that the use of cholesterol-reducing agents (statins) can be protective against the disease. We and others have shown that cholesterol levels modulate the processing of the amyloid precursor protein (APP) both in vivo and in vitro, affecting the accumulation of A-beta (Abeta) peptides that may directly impact the risk of AD. Mutations in the Niemann-Pick C gene (NPC) result in deficient cholesterol transport/storage. Clinically, Niemann-Pick disease causes a severe childhood lipidosis, with neurodegeneration characterized by the presence of AD-type neurofibrillary tangles (NFTs) composed of hyperphosphorylated tau. Studies of mouse models of NPC show that defects in cellular cholesterol trafficking are associated with enhanced generation of Abeta and the hyperphosphorylation of tau, further implicating the cholesterol homeostasis pathway as a risk factor for amyloidosis.

Cholesterol and pathological processes in Alzheimer's disease

Fundamental questions on the pathogenesis of Alzheimer's disease (AD) are how nontoxic, soluble amyloid beta-protein (A beta) is converted to its toxic, aggregated form and how functional tau is hyperphosphorylated to form neurofibrillary tangles. Growing evidence from recent biochemical and cell biological studies suggests that altered cholesterol metabolism in neurons may underlie such pathological processes. The possibility that cholesterol is a risk factor in the development of AD has also been supported by recent epidemiological studies. Based on this line of evidence, it is noteworthy to examine the potency of cholesterol-lowering medicine and/or diet in suppressing the development or the progression of AD.

Hypertension and hypercholesterolaemia as risk factors for Alzheimer's disease: potential for pharmacological intervention

This paper focuses on hypertension and hypercholesterolaemia as risk factors for Alzheimer's disease and, as such, subjects for prevention. The long-term, prospective, population-based studies regarding the relationship between hypertension or hypercholesterolaemia and Alzheimer's disease, and the clinical studies regarding the association between antihypertensive or lipid-lowering medications and the risk of Alzheimer's disease, are reviewed. These studies provide evidence to suggest that elevated blood pressure and cholesterol levels earlier in life may have an important role in the development and expression of late-life Alzheimer's disease. Based on these data, we propose that proper, early interventions aimed at reducing these cardiovascular risk factors may have an impact on the future incidence and prevalence of Alzheimer's disease.

Apolipoprotein E (ApoE) isoform-dependent lipid release from astrocytes prepared from human ApoE3 and ApoE4 knock-in mice

We have reported previously (Michikawa, M., Fan, Q.-W., Isobe, I., and Yanagisawa, K. (2000) J. Neurochem. 74, 1008-1016) that exogenously added recombinant human apolipoprotein E (apoE) promotes cholesterol release in an isoform-dependent manner. However, the molecular mechanism underlying this isoform-dependent promotion of cholesterol release remains undetermined. In this study, we demonstrate that the cholesterol release is mediated by endogenously synthesized and secreted apoE isoforms and clarify the mechanism underlying this apoE isoform-dependent cholesterol release using cultured astrocytes prepared from human apoE3 and apoE4 knock-in mice. Cholesterol and phospholipids were released into the culture media, resulting in the generation of two types of high density lipoprotein (HDL)-like particles; one was associated with apoE and the other with apoJ. The amount of cholesterol released into the culture media from the apoE3-expressing astrocytes was approximately 2.5-fold greater than that from apoE4-expressing astrocytes. In contrast, the amount of apoE3 released in association with the HDL-like particles was similar to that of apoE4, and the sizes of the HDL-like particles released from apoE3- and apoE4-expressing astrocytes were similar. The molar ratios of cholesterol to apoE in the HDL fraction of the culture media of apoE3- and apoE4-expressing astrocytes were 250 +/- 6.0 and 119 +/- 5.1, respectively. These data indicate that apoE3 has an ability to generate similarly sized lipid particles with less number of apoE molecules than apoE4, suggesting that apoE3-expressing astrocytes can supply more cholesterol to neurons than apoE4-expressing astrocytes. These findings provide a new insight into the issue concerning the putative alteration of apoE-related cholesterol metabolism in Alzheimer's disease.

Evidence for parent of origin effect in late-onset Alzheimer disease

Evidence for a parent of origin effect in Alzheimer disease was obtained from a sample of 148 sibships on which affection status of the parents was sought using family history interviews. The parent study recruited families with two or more affected sibs for late onset AD utilizing rigorous diagnostic criteria. In 74 families, there was evidence of an affected parent, 49 maternal and 25 paternal. Genome scan data were analyzed for the sample as a whole and for the maternal and paternal families separately, using Genehunter-ASM. Seven regions with Z(lr) scores >or=2 were identified, four in maternal families (chr. 10,12,19,20) and three in paternal families (chr. 1,7,13). With the exception of the chromosome 10 finding, analysis by parent of origin greatly increased evidence of linkage in areas showing no linkage in the overall analyses. For example, a chr. 12 region reached a LOD = 2.29 among maternal families whereas the same region showed a LOD = 0.3 when all families were analyzed together. The strongest findings among maternal families (chr. 10 and 12) were followed up with fine mapping that resulted in an increase in maximum LOD scores from 2.7-3.2 on chr. 10, and 2.29-2.42 on chr. 12. These analyses highlight the importance of parent of origin effects in late-onset AD families and identify several genomic regions that may include genes linked to late-onset AD specific to disease transmission from the mother and require further investigation.

Should the guidelines for monitoring serum cholesterol levels in the elderly be re-evaluated?

Elevated circulating cholesterol can have profound effects on the health of an individual. Such excess cholesterol can promote coronary artery disease, production and accumulation of beta-amyloid in the brain, and possibly Alzheimer's disease (AD). In a clinical trial evaluating the benefit of a cholesterol-lowering drug in the treatment of AD, mean cholesterol levels at baseline among individuals participating in the trial were found to be relatively high. Based on this observation we suggest that cholesterol levels should be actively monitored in the elderly, as many individuals with AD are over 65 years of age and therefore excluded by currently accepted guidelines.

Statin therapy for Alzheimer's disease: will it work?

Disease-modifying therapies are being developed for Alzheimer's disease (AD). These are expected to slow the clinical progression of the disease or delay its onset. Cerebral accumulation of amyloid beta (A beta) peptides is an early and perhaps necessary event for establishing AD pathology. Consequently therapies aimed at attenuating brain amyloidosis are expected to be disease modifying. Based on the epidemiological evidence pointing to a link between cholesterol metabolism and AD and the numerous laboratory studies implicating cholesterol in the process of A beta production and accumulation, it is now believed that cholesterol-lowering therapies will be of value as disease modifying agents. Several epidemiological studies revealed that statin use for the treatment of coronary arterial disease is associated with a decreased prevalence or a decreased risk of developing AD. These observations require both preclinical and clinical validation. The former involves testing statins in one or more animal models of AD in order to establish which disease features are affected by statin treatment, the relative efficacy with which different statins modify these features and the mechanism(s) by which statins affect AD phenotypes. The latter requires prospective, randomized, placebo controlled trials to evaluate the effect of statin treatment on cognitive and AD biomarker outcomes. We have initiated a study aimed at determining the effects of atorvastatin (Lipitor), a statin with the largest US market share, on brain A beta deposition in the PSAPP transgenic mouse model of Alzheimer's amyloidosis. Our results indicate that Lipitor treatment markedly attenuates A beta deposition in this animal model.

Cholesterol, oxidative stress, and Alzheimer's disease: expanding the horizons of pathogenesis

Recent epidemiological, clinical, and experimental data suggest that cholesterol may play a role in Alzheimer's disease (AD). We have recently shown that cholesterolemia has a profound effect in the development and modulation of amyloid pathology in a transgenic model of AD. This review summarizes recent advancements in our understanding of the potential role of cholesterol and the amyloid beta protein in initiating the generation of free radicals and points out their role in a chain of events that causes damage of essential macromolecules in the central nervous system and culminates in neuronal dysfunction and loss. Experimental data links cholesterol and oxidative stress with some neurodegenerative aspects of AD.

The impact of the use of statins on the prevalence of dementia and the progression of cognitive impairment

BACKGROUND

Previous evidence suggests that treatment with 3-hydroxy-3-methylglutaryl-coenzyme-A reductase inhibitors (statins) has a positive impact on dementia. We decided to investigate the association between the use of statins and the prevalence of dementia and statins' impact on the progression of cognitive impairment.

METHODS

This is a case-control and a retrospective cohort study of a community-based ambulatory primary care geriatric practice. We included a convenience sample of all patients (N = 655, mean age 78.7 +/- 0.3 years, 85% Caucasian, 74% women) with hypercholesterolemia or dementia, or using statins. We compared those using statins with those who do not with respect to the clinical diagnosis of dementia and its subtypes and the progression of cognitive impairment.

RESULTS

At the initial visit, 35% had dementia, and 17% were using statins. After covariate adjustments, patients on statins were less likely to have dementia (odds ratio [OR] for dementia based on composite definition = 0.23; 95% confidence interval [CI] [0.1-0.56], p =.001, OR Alzheimer's disease = 0.37; 95% CI [0.19-0.74], p =.005, OR vascular dementia = 0.25; 95% CI [0.08-0.85], p =.027). At follow-up, patients on statins showed an improvement on their Mini-Mental Status Examination score by 0.7 +/- 0.4 compared to a decline by 0.5 +/- 0.3 in controls, p =.025 (OR for no change or improvement on statins = 2.81; 95% CI [1.02-8.43], p =.045) and scored higher on the Clock Drawing Test (difference of 1.5 +/- 0.1, p =.036).

CONCLUSIONS

The use of statins is associated with a lower prevalence of dementia and has a positive impact on the progression of cognitive impairment.

Lipoprotein(a), apolipoprotein E genotype, and risk of Alzheimer's disease

OBJECTIVES

To explore the possible role of serum lipoprotein(a) (Lp(a)), apolipoprotein E polymorphism, and total cholesterol (TC) serum concentrations in Alzheimer's disease (AD).

METHODS

Lp(a) serum concentrations, apolipoprotein E genotypes, and TC serum concentrations were determined in 61 patients with a diagnosis of probable AD and in 63 healthy unrelated age matched controls. Genomic DNA was obtained and amplified by polymerase chain reaction and apolipoprotein E genotypes were defined following a previously described procedure.

RESULTS

Lp(a) serum concentrations were significantly associated in a non-linear relation with an increased risk for AD, independently of apolipoprotein E genotypes and sex and dependent on age (truth association) and TC serum concentrations (spurious association). The effect of age adjusted for TC on the odds of having AD increased non-linearly with increasing Lp(a) serum concentrations, with a plateau between 70 and 355 mg/l (odds ratio 11.33). For Lp(a) serum concentrations > or = 360 mg/l, the effect of age (> or = 72 years) was associated with a reduction in odds of having AD (odds ratio 0.15).

CONCLUSION

It is suggested that increased Lp(a) serum concentrations, by increasing the risk for cerebrovascular disease, may have a role in determining clinical AD.

Regulation of cholesterol homeostasis by the liver X receptors in the central nervous system

The nuclear oxysterol receptors liver X receptor-alpha [LXRalpha (NR1H3)] and LXRbeta (NR1H2) coordinately regulate genes involved in cholesterol homeostasis. Although both LXR subtypes are expressed in the brain, their roles in this tissue remain largely unexplored. In this report, we show that LXR agonists have marked effects on gene expression in murine brain tissue both in vitro and in vivo. In primary astrocyte cultures, LXR agonists regulated several established LXR target genes, including ATP binding cassette transporter A1, and enhanced cholesterol efflux. In contrast, little or no effect on gene expression or cholesterol efflux was detected in primary neuronal cultures. Treatment of mice with a selective LXR agonist resulted in the induction of several LXR target genes related to cholesterol homeostasis in the cerebellum and hippocampus. These data provide the first evidence that the LXRs regulate cholesterol homeostasis in the central nervous system. Because dysregulation of cholesterol balance is implicated in central nervous system diseases such as Alzheimer's and Niemann-Pick disease, pharmacological manipulation of the LXRs may prove beneficial in the treatment of these disorders.

No association of paraoxonase genotype or atherosclerosis with cerebral amyloid angiopathy

BACKGROUND AND PURPOSE

Both cerebral amyloid angiopathy (CAA) and paraoxonase have been reported to be related to lipid metabolism and atherosclerosis. We investigated whether the paraoxonase gene (PON1) polymorphism and atherosclerosis are associated with risk of CAA.

METHODS

Associations of the PON1 polymorphism and atherosclerosis of the aorta and coronary and cerebral arteries with the severity of CAA were investigated in 154 elderly Japanese individuals, including 47 patients with Alzheimer's disease.

RESULTS

The PON1 polymorphism or severity of atherosclerosis of the arteries was not associated with the severity of CAA.

CONCLUSIONS

The PON1 polymorphism and atherosclerosis would not appear to be associated with risk of CAA in the elderly, although further study with larger samples is necessary for confirmation.

Cholesterol-dependent gamma-secretase activity in buoyant cholesterol-rich membrane microdomains

Buoyant membrane fractions containing presenilin 1 (PS1), an essential component of the gamma-secretase complex, and APP CTFbeta, a gamma-secretase substrate, can be isolated from cultured cells and brain by several different fractionation procedures that are compatible with in vitro gamma-secretase assays. Analysis of these gradients for amyloid beta protein (Abeta) and CTFgamma production indicated that gamma-secretase activity is predominantly localized in these buoyant membrane microdomains. Consistent with this localization, we find that gamma-secretase activity is cholesterol dependent. Depletion of membrane cholesterol completely inhibits gamma-secretase cleavage, which can be restored by cholesterol replacement. Thus, altering cholesterol levels may influence the development of Alzheimer's disease (AD) by influencing production and deposition of Abeta within cholesterol rich membrane microdomains.

Reduction in cholesterol and sialic acid content protects cells from the toxic effects of beta-amyloid peptides

beta-Amyloid (Abeta) is the primary protein component of senile plaques associated with Alzheimer's disease and has been implicated in the neurotoxicity associated with the disease. A variety of evidence points to the importance of Abeta-membrane interactions in the mechanism of Abeta neurotoxicity and indicates that cholesterol and gangliosides are particularly important for Abeta aggregation and binding to membranes. We investigated the effects of cholesterol and sialic acid depletion on Abeta-induced GTPase activity in cells, a step implicated in the mechanism of Abeta toxicity, and Abeta-induced cell toxicity. Cholesterol reduction and depletion of membrane-associated sialic acid residues both significantly reduced the Abeta-induced GTPase activity. In addition, cholesterol and membrane-associated sialic acid residue depletion or inhibition of cholesterol and ganglioside synthesis protected PC12 cells from Abeta-induced toxicity. These results indicate the importance of Abeta-membrane interactions in the mechanism of Abeta toxicity. In addition, these results suggest that control of cellular cholesterol and/or ganglioside content may prove useful in the prevention or treatment of Alzheimer's disease.

Nutritional factors and Alzheimer's disease

Nutritional factors are integrally linked with Alzheimer's disease (AD). Although AD patients have no changes in energy metabolism, fluctuations in weight are fairly common. The potential role of vitamin B(12) and folate, with the production of hyperhomocysteinemia, in the pathophysiology of AD is explored. The role of free-radical damage in AD is discussed. It is stressed that alterations in dietary lipids may play an important role in cognitive defects in AD secondary to their effects on neuronal membrane lipids. More research is needed on the role of nutrition in the ongoing development of cognitive changes in AD.

Cholesterol modulation as an emerging strategy for the treatment of Alzheimer's disease

The basis for therapeutic strategies targeting the amyloid-beta protein (Abeta) has come from studies showing that accumulation and aggregation of the Abeta within the brain is likely to cause Alzheimer's disease (AD). Along with an ever-increasing understanding of Abeta metabolism, many potential therapeutic strategies aimed at altering Abeta metabolism have emerged. Among the more intriguing targets for therapy are enzymes involved in cholesterol homeostasis, because it has been found that altering cholesterol can influence Abeta metabolism in experimental model systems, and that cholesterol-lowering agents, specifically HMG-CoA reductase inhibitors, could reduce the incidence of AD. It is likely that cholesterol influences Abeta metabolism in several ways, including altering Abeta production and perhaps altering Abeta deposition and clearance. Thus, pharmacological modulation of cholesterol levels could provide a relatively safe means to reduce Abeta accumulation in the brain, and thereby prevent or slow the development of AD.

A cholesterol-lowering drug reduces beta-amyloid pathology in a transgenic mouse model of Alzheimer's disease

Clinical, epidemiological, and laboratory studies suggest that cholesterol may play a role in the pathogenesis of Alzheimer's disease (AD). Transgenic mice exhibiting an Alzheimer's beta-amyloid phenotype were treated with the cholesterol-lowering drug BM15.766 and tested for modulation of beta-amyloid levels. BM15.766 treatment reduced plasma cholesterol, brain Abeta peptides, and beta-amyloid load by greater than twofold. A strong, positive correlation between the amount of plasma cholesterol and Abeta was observed. Furthermore, drug treatment reduced the amyloidogenic processing of the amyloid precursor protein, suggesting alterations in processing in response to cholesterol modulation. This study demonstrates that hypocholesterolemia is associated with reduced Abeta accumulation suggesting that lowering cholesterol by pharmacological means may be an effective approach for reducing the risk of developing AD.