Differential expression of cholesterol hydroxylases in Alzheimer's disease

Effect of Cyp27A1 gene dosage on atherosclerosis development in ApoE-knockout mice

In humans, sterol 27-hydroxylase (CYP27A1) deficiency leads to cholesterol deposition in tendons and vasculature. Thus, in addition to its role in bile acid synthesis, where it converts cholesterol to 27-hydroxycholesterol (27-OHC), CYP27A1 may also be atheroprotective. Cyp27A1-deficient (Cyp27A1(-/-)) mice were crossed with apolipoprotein E (apoE)-deficient mice. Cyp27A1(+/+)/apoE(-/-) [ApoE-knockout (KO)], Cyp27A1(+/-)/apoE(-/-) heterozygous (het), and Cyp27A1(-/-)/apoE(-/-) [double-knockout (DKO)] mice were challenged with a Western diet (WD) for 3 and 6 mo. ApoE-KO mice fed a chow diet or a WD were used as the control. The severity of atherosclerosis in DKO mice was reduced 10-fold. Compared with the control, the DKO mice had no 27-OHC, total plasma cholesterol and low-density lipoprotein and very low density lipoprotein (LDL/VLDL) concentrations were reduced 2-fold, and HDL was elevated 2-fold. Expression of hepatic CYP7A1, CYP3A, and CYP8B1 were 5- to 10-fold higher. 3-Hydroxy-3-methyl-glutaryl-CoA reductase (HMGR) activity increased 4-fold. Fecal cholesterol was increased. In contrast, het mice fed a WD developed accelerated atherosclerosis and severe skin lesions, possibly because of reduced reverse cholesterol transport due to diminished 27-OHC production. CYP27A1 activity is involved in the control of cholesterol homeostasis and development of atherosclerosis with a distinct gene dose-dependent effect.

HIV gp120- and methamphetamine-mediated oxidative stress induces astrocyte apoptosis via cytochrome P450 2E1

HIV-1 glycoprotein 120 (gp120) is known to cause neurotoxicity via several mechanisms including production of proinflammatory cytokines/chemokines and oxidative stress. Likewise, drug abuse is thought to have a direct impact on the pathology of HIV-associated neuroinflammation through the induction of proinflammatory cytokines/chemokines and oxidative stress. In the present study, we demonstrate that gp120 and methamphetamine (MA) causes apoptotic cell death by inducing oxidative stress through the cytochrome P450 (CYP) and NADPH oxidase (NOX) pathways. The results showed that both MA and gp120 induced reactive oxygen species (ROS) production in concentration- and time-dependent manners. The combination of gp120 and MA also induced CYP2E1 expression at both mRNA (1.7±0.2- and 2.8±0.3-fold in SVGA and primary astrocytes, respectively) and protein (1.3±0.1-fold in SVGA and 1.4±0.03-fold in primary astrocytes) levels, suggesting the involvement of CYP2E1 in ROS production. This was further confirmed by using a selective inhibitor of CYP2E1, diallylsulfide (DAS), and CYP2E1 knockdown using siRNA, which significantly reduced ROS production (30–60%). As the CYP pathway is known to be coupled with the NOX pathway, including Fenton–Weiss–Haber (FWH) reaction, we examined whether the NOX pathway is also involved in ROS production induced by either gp120 or MA. Our results showed that selective inhibitors of NOX, diphenyleneiodonium (DPI), and FWH reaction, deferoxamine (DFO), also significantly reduced ROS production. These findings were further confirmed using specific siRNAs against NOX2 and NOX4 (NADPH oxidase family). We then showed that gp120 and MA both induced apoptosis (caspase-3 activity and DNA lesion using TUNEL (terminal deoxynucleotidyltransferase-mediated dUTP nick-end labeling) assay) and cell death. Furthermore, we showed that DAS, DPI, and DFO completely abolished apoptosis and cell death, suggesting the involvement of CYP and NOX pathways in ROS-mediated apoptotic cell death. In conclusion, this is the first report on the involvement of CYP and NOX pathways in gp120/MA-induced oxidative stress and apoptotic cell death in astrocytes, which has clinical implications in neurodegenerative diseases, including neuroAIDS.

Mutation in CYP27A1 identified in family with coronary artery disease

Coronary artery disease (CAD) is a leading cause of death worldwide. Myocardial infarction is the most severe outcome of CAD. Despite extensive efforts, the genetics of CAD is poorly understood. We aimed to identify the genetic cause of CAD in a pedigree with several affected individuals. Exome sequencing led to identification of a mutation in CYP27A1 that causes p.Arg225His in the encoded protein sterol 27-hydroxylase as the likely cause of CAD in the pedigree. The enzyme is multifunctional, and several of its functions including its functions in vitamin D metabolism and reverse cholesterol transport (RCT) are relevant to the CAD phenotype. Measurements of vitamin D levels suggested that the mutation does not affect CAD by affecting this parameter. We suggest that the mutation may cause CAD by affecting RCT. Screening of all coding regions of the CYP27A1 in 100 additional patients led to finding four variations (p.Arg14Gly, p.Arg26Lys, p.Ala27Arg, and p.Val86Met) in seven patients that may contribute to their CAD status. CYP27A1 is the known causative gene of cerebrotendinous xanthomatosis, a disorder which is sometimes accompanied by early onset atherosclerosis. This and the observation of potentially harmful variations in unrelated CAD patients provide additional evidence for the suggested causative role of the p.Arg225His mutation in CAD.

Steroids as γ-secretase modulators

Aggregation and accumulation of Aβ42 play an initiating role in Alzheimer's disease (AD); thus, selective lowering of Aβ42 by γ-secretase modulators (GSMs) remains a promising approach to AD therapy. Based on evidence suggesting that steroids may influence Aβ production, we screened 170 steroids at 10 μM for effects on Aβ42 secreted from human APP-overexpressing Chinese hamster ovary cells. Many acidic steroids lowered Aβ42, whereas many nonacidic steroids actually raised Aβ42. Studies on the more potent compounds showed that Aβ42-lowering steroids were bonafide GSMs and Aβ42-raising steroids were inverse GSMs. The most potent steroid GSM identified was 5β-cholanic acid (EC50=5.7 μM; its endogenous analog lithocholic acid was virtually equipotent), and the most potent inverse GSM identified was 4-androsten-3-one-17β-carboxylic acid ethyl ester (EC50=6.25 μM). In addition, we found that both estrogen and progesterone are weak inverse GSMs with further complex effects on APP processing. These data suggest that certain endogenous steroids may have the potential to act as GSMs and add to the evidence that cholesterol, cholesterol metabolites, and other steroids may play a role in modulating Aβ production and thus risk for AD. They also indicate that acidic steroids might serve as potential therapeutic leads for drug optimization/development.

Analysis of oxysterols and cholesterol in prefrontal cortex of suicides

Brain oxysterol levels, which are enzymatic oxidation products of cholesterol (Chl), have been proposed to reflect the dynamic process of physiological synapse maintenance and repair of nerve terminals within the central nervous system (CNS), due to the turnover of membrane Chl. Modifications of oxysterols have important implications in neurological conditions, especially in neurodegenerative and psychiatric disorders in which alterations of synaptic plasticity or cell signalling are implicated, such as depression. Oxysterols can diffuse across the blood-brain barrier and have been hypothesized to provide a mechanism by which the brain can eliminate excess Chl to maintain a steady state. Relations of 24-hydroxycholesterol (24OH) and 27-hydroxycholesterol (27OH) specifically may provide a depiction of CNS Chl homeostasis. Thus, the objective of this study was to integrate oxysterol measures and gene expression measures in an effort to identify how they may relate to depression and suicide. Using post-mortem human prefrontal cortex tissue, quantification of metabolites by GC-MS and gene expression by qRT-PCR were performed with the aim to provide a characterization of enzymatic oxidative Chl homeostasis. Results show a significant increase in 24OH, which suggests a higher turnover of Chl to 24OH in the prefrontal cortex of suicide cases. An increase in 24OH may, in combination with liver-X receptor activation, explain the observed reduction of low central and peripheral Chl in suicide and would have implications for synapse maintenance and loss in the neuropathology of depression and suicide.

Cytochromes p450: roles in diseases

The cytochrome P450 superfamily consists of a large number of heme-containing monooxygenases. Many human P450s metabolize drugs used to treat human diseases. Others are necessary for synthesis of endogenous compounds essential for human physiology. In some instances, alterations in specific P450s affect the biological processes that they mediate and lead to a disease. In this minireview, we describe medically significant human P450s (from families 2, 4, 7, 11, 17, 19, 21, 24, 27, 46, and 51) and the diseases associated with these P450s.

Gastric acid, calcium absorption, and their impact on bone health

Calcium balance is essential for a multitude of physiological processes, ranging from cell signaling to maintenance of bone health. Adequate intestinal absorption of calcium is a major factor for maintaining systemic calcium homeostasis. Recent observations indicate that a reduction of gastric acidity may impair effective calcium uptake through the intestine. This article reviews the physiology of gastric acid secretion, intestinal calcium absorption, and their respective neuroendocrine regulation and explores the physiological basis of a potential link between these individual systems.

Oxysterols in the pathogenesis of major chronic diseases

Pathological accumulation of 27-carbon intermediates or end-products of cholesterol metabolism, named oxysterols, may contribute to the onset and especially to the development of major chronic diseases in which inflammation, but also oxidative damage and to a certain extent cell death, are hallmarks and primary mechanisms of progression. Indeed, certain oxysterols exercise strong pro-oxidant and pro-inflammatory effects at concentrations detectable in the lesions typical of atherosclerosis, neurodegenerative diseases, inflammatory bowel diseases, age-related macular degeneration, and other pathological conditions characterized by altered cholesterol uptake and/or metabolism.

Steady-state levels of retinal 24S-hydroxycholesterol are maintained by glial cells intervention after elevation of intraocular pressure in the rat

PURPOSE

Our previous studies suggested that CYP46A1 and 24S-hydroxycholesterol (24SOH) may be associated with glaucoma. Loss of CYP46A1-expressing retinal ganglion cells is involved in the activation of glia, and therefore possibly in the disbalance of cholesterol. In this context, the purpose of our present work was to emphasize the glial and longitudinal CYP46A1 expression after an interventional glaucoma-related stress triggered by elevated intraocular pressure (IOP).

METHODS

Sprague-Dawley rats were submitted to laser photocoagulation of the trabecular meshwork, limbus and episcleral veins in one eye to induce elevated IOP. Rats were euthanized at days 3, 14, 30 and 60 (n = 10 per time-point), and 24SOH was measured in plasma and retina by gas chromatography-mass spectrometry. CYP46A1 was quantified by Western blotting. Glial activation was assessed by glial fibrillary acidic protein immunoreactivity in Western blots and retinal cryosections.

RESULTS

Sustained high IOP was observed in experimental eyes from day 1 to day 21. Plasma MCP-1 and ICAM-1, quantified using multiplex assay kits, were increased at day 3. Glial activation was observed bilaterally at all time-points, at lower levels in contralateral eyes than in experimental eyes. In experimental retinas, CYP46A1 expression showed a transient increase at day 3 and then returned to baseline. Plasma and retinal 24SOH peaked at day 14 and 30, respectively.

CONCLUSIONS

These data show that CYP46A1 expression was induced early in response to retinal stress but remained constant at late time-points, reinforcing the constitutive role of CYP46A1 in maintaining cholesterol balance in neuronal tissues.

Cholesterol: its regulation and role in central nervous system disorders

Cholesterol is a major constituent of the human brain, and the brain is the most cholesterol-rich organ. Numerous lipoprotein receptors and apolipoproteins are expressed in the brain. Cholesterol is tightly regulated between the major brain cells and is essential for normal brain development. The metabolism of brain cholesterol differs markedly from that of other tissues. Brain cholesterol is primarily derived by de novo synthesis and the blood brain barrier prevents the uptake of lipoprotein cholesterol from the circulation. Defects in cholesterol metabolism lead to structural and functional central nervous system diseases such as Smith-Lemli-Opitz syndrome, Niemann-Pick type C disease, and Alzheimer's disease. These diseases affect different metabolic pathways (cholesterol biosynthesis, lipid transport and lipoprotein assembly, apolipoproteins, lipoprotein receptors, and signaling molecules). We review the metabolic pathways of cholesterol in the CNS and its cell-specific and microdomain-specific interaction with other pathways such as the amyloid precursor protein and discuss potential treatment strategies as well as the effects of the widespread use of LDL cholesterol-lowering drugs on brain functions.

24S-hydroxycholesterol in plasma: a marker of cholesterol turnover in neurodegenerative diseases

Brain cholesterol is mainly involved in the cell membrane structure, in signal transduction, neurotransmitter release, synaptogenesis and membrane trafficking. Impairment of brain cholesterol metabolism was described in neurodegenerative diseases, such as Multiple Sclerosis, Alzheimer and Huntington Diseases. Since the blood-brain barrier efficiently prevents cholesterol uptake from the circulation into the brain, de novo synthesis is responsible for almost all cholesterol present there. Cholesterol is converted into 24S-hydroxycholesterol (24OHC) by cholesterol 24-hydroxylase (CYP46A1) expressed in neural cells. Plasma concentration of 24OHC depends upon the balance between cerebral production and hepatic elimination and is related to the number of metabolically active neurons in the brain. Factors affecting brain cholesterol turnover and liver elimination of oxysterols, together with the metabolism of plasma lipoproteins, genetic background, nutrition and lifestyle habits were found to significantly affect its plasma levels. Either increased or decreased plasma 24OHC concentrations were described in patients with neurodegenerative diseases. A group of evidence suggests that reduced levels of 24OHC are related to the loss of metabolically active cells and the degree of brain atrophy. Inflammation, dysfunction of BBB, increased cholesterol turnover might counteract this tendency resulting in increased levels or, in some cases, in unsignificant changes. The study of plasma 24OHC is likely to offer an insight about brain cholesterol turnover with a limited diagnostic power.

Lipid metabolism and neuroinflammation in Alzheimer's disease: a role for liver X receptors

Liver X receptors (LXR) are nuclear receptors that have emerged as key regulators of lipid metabolism. In addition to their functions as cholesterol sensors, LXR have also been found to regulate inflammatory responses in macrophages. Alzheimer's disease (AD) is a neurodegenerative disease characterized by a progressive cognitive decline associated with inflammation. Evidence indicates that the initiation and progression of AD is linked to aberrant cholesterol metabolism and inflammation. Activation of LXR can regulate neuroinflammation and decrease amyloid-β peptide accumulation. Here, we highlight the role of LXR in orchestrating lipid homeostasis and neuroinflammation in the brain. In addition, diabetes mellitus is also briefly discussed as a significant risk factor for AD because of the appearing beneficial effects of LXR on glucose homeostasis. The ability of LXR to attenuate AD pathology makes them potential therapeutic targets for this neurodegenerative disease.

The link between altered cholesterol metabolism and Alzheimer's disease

Alzheimer's disease (AD), the most common form of dementia, is characterized by the progressive loss of neurons and synapses, and by extracellular deposits of amyloid-β (Aβ) as senile plaques, Aβ deposits in the cerebral blood vessels, and intracellular inclusions of hyperphosphorylated tau in the form of neurofibrillary tangles. Several mechanisms contribute to AD development and progression, and increasing epidemiological and molecular evidence suggests a key role of cholesterol in its initiation and progression. Altered cholesterol metabolism and hypercholesterolemia appear to play fundamental roles in amyloid plaque formation and tau hyperphosphorylation. Over the last decade, growing evidence supports the idea that cholesterol oxidation products, known as oxysterols, may be the missing link between altered brain cholesterol metabolism and AD pathogenesis, as their involvement in neurotoxicity, mainly by interacting with Aβ peptides, is reported.

Regulation of cerebral cholesterol metabolism in Alzheimer disease

Alzheimer disease (AD) is an age-related neurodegenerative disorder that manifests as a progressive loss of memory and deterioration of higher cognitive functions. Alzheimer disease is characterized by accumulation in the brain of the β-amyloid peptide generated by β- and γ-secretase processing of amyloid precursor protein. Epidemiological studies have linked elevated plasma cholesterol and lipoprotein levels in midlife with AD development. Cholesterol-fed animal models exhibit neuropathologic features of AD including accumulation of β-amyloid peptide. Specific isoforms of the cholesterol transporter apolipoprotein E are associated with susceptibility to AD. Although multiple lines of evidence indicate a role for cholesterol in AD, the exact impact and mechanisms involved remain largely unknown. This review summarizes the current state of our knowledge of the influence of cholesterol and lipid pathways in AD pathogenesis in vitro and in vivo.

Induction of mitochondrial changes associated with oxidative stress on very long chain fatty acids (C22:0, C24:0, or C26:0)-treated human neuronal cells (SK-NB-E)

In Alzheimer's disease, lipid alterations point towards peroxisomal dysfunctions. Indeed, a cortical accumulation of saturated very long chain fatty acids (VLCFAs: C22:0, C24:0, C26:0), substrates for peroxisomal β-oxidation, has been found in Alzheimer patients. This study was realized to investigate the effects of VLCFAs at the mitochondrial level since mitochondrial dysfunctions play crucial roles in neurodegeneration. On human neuronal SK-NB-E cells treated with C22:0, C24:0, or C26:0 (0.1-20 μM; 48 h), an inhibition of cell growth and mitochondrial dysfunctions were observed by cell counting with trypan blue, MTT assay, and measurement of mitochondrial transmembrane potential (Δψ(m)) with DiOC(6)(3). A stimulation of oxidative stress was observed with DHE and MitoSOX used to quantify superoxide anion production on whole cells and at the mitochondrial level, respectively. With C24:0 and C26:0, by Western blotting, lower levels of mitochondrial complexes III and IV were detected. After staining with MitoTracker and by transmission electron microscopy used to study mitochondrial topography, mass and morphology, major changes were detected in VLCFAs treated-cells: modification of the cytoplasmic distribution of mitochondria, presence of large mitochondria, enhancement of the mitochondrial mass. Thus, VLCFAs can be potential risk factors contributing to neurodegeneration by inducing neuronal damages via mitochondrial dysfunctions.

Okadaic acid inhibits the trichostatin A-mediated increase of human CYP46A1 neuronal expression in a ERK1/2-Sp3-dependent pathway

The CYP46A1 gene codes for the cholesterol 24-hydroxylase, a cytochrome P450 specifically expressed in neurons and responsible for the majority of cholesterol turnover in the central nervous system. Previously, we have demonstrated the critical participation of Sp transcription factors in the CYP46A1 response to histone deacetylase (HDAC) inhibitors, and in this study we investigated the involvement of intracellular signaling pathways in the trichostatin A (TSA) effect. Our results show that pretreatment of neuroblastoma cells with chemical inhibitors of mitogen-activated kinase kinase (MEK)1 significantly potentiates the TSA-dependent induction of cholesterol 24-hydroxylase, whereas inhibition of protein phosphatases by okadaic acid (OA) or overexpression of MEK1 partially impairs the TSA effect without affecting histone hyperacetylation at the promoter. Immunoblotting revealed that TSA treatment decreases ERK1/2 phosphorylation concomitantly with a decrease in Sp3 binding activity, which are both reversed by pretreatment with OA. Chromatin immunoprecipitation analysis demonstrated that TSA induces the release of p-ERK1/2 from the CYP46A1 proximal promoter, whereas pretreatment with OA restores the co-occupancy of Sp3-ERK1/2 in the same promoter fragments. We demonstrate for the first time the participation of MEK-ERK1/2 signaling pathway in HDAC inhibitor-dependent induction of cytochrome P450 gene expression, underlying the importance of this regulatory signaling mechanism in the control of brain cholesterol elimination.

Interindividual variation in serum cholesterol is associated with regional white matter tissue integrity in older adults

Prior research has demonstrated links among vascular health and the occurrence of stroke, mild cognitive decline, and dementia in older adults. However, little is known about whether normal variation in vascular indicators may be related to changes in neural tissue integrity. Even less is known about how the brain is affected by cholesterol levels in the normal to moderate risk range, leading up to overt disease pathology. This study examined associations between serum lipid levels and DTI indicators of white matter (WM) structural integrity in a sample of 125 generally healthy older adults aged 43-87 years. Whole-brain voxelwise analysis, controlling for age and gender, revealed low density lipoprotein levels (LDL) as the most robust correlate of regional WM structural integrity of the measured lipids. Higher LDL was associated with decreased WM integrity in right frontal and temporal regions, the superior longitudinal fasciculus and internal/external capsules. Increasing LDL was associated with increased radial and axial diffusivity; however, more widespread statistical effects were found for radial diffusivity. These findings suggest that normal interindividual variation in lipid levels is associated with compromised regional WM integrity, even in individuals below clinical thresholds for hyperlipidemia. Given the prevalence of cholesterol-associated sequelae in older adults, and mounting evidence suggesting a vascular role in the etiology of dementia, the current data suggest that understanding the relationship between cholesterol and brain tissue microstructure may have important clinical implications for early detection of vascular-related cognitive disorders and optimal regulation of serum lipids to maintain neural health in older adults.

ABC Transporters and the Alzheimer's Disease Enigma

Alzheimer's disease (AD) is considered the "disease of the twenty-first century." With a 10-fold increase in global incidence over the past 100 years, AD is now reaching epidemic proportions and by all projections, AD patient numbers will continue to rise. Despite intense research efforts, AD remains a mystery and effective therapies are still unavailable. This represents an unmet need resulting in clinical, social, and economic problems. Over the last decade, a new AD research focus has emerged: ATP-binding cassette (ABC) transporters. In this article, we provide an overview of the ABC transporters ABCA1, ABCA2, P-glycoprotein (ABCB1), MRP1 (ABCC1), and BCRP (ABCG2), all of which are expressed in the brain and have been implicated in AD. We summarize recent findings on the role of these five transporters in AD, and discuss their potential to serve as therapeutic targets.

Plasma 24S-hydroxycholesterol levels in elderly subjects with late onset Alzheimer's disease or vascular dementia: a case-control study

BACKGROUND

In central nervous system cholesterol cannot be degraded but is secreted into circulation predominantly in the form of its polar metabolite 24(S)-hydroxycholesterol (24S-OH-Chol). Some studies suggested an association between 24S-OH-Chol metabolism and different neurological diseases including dementia. A possible decrease in 24S-OH-Chol plasma levels has been reported late onset Alzheimer's disease (LOAD) and vascular dementia (VD), but results of previous studies are partially contradictory.

METHODS

By high-speed liquid chromatography/tandem mass spectrometry we evaluated the plasma levels of 24S-OH-Chol in a sample of 160 older individuals: 60 patients with LOAD, 35 patients with VD, 25 subjects affected by cognitive impairment no-dementia (CIND), and 40 (144 for genetics study) cognitively normal Controls. We also investigated the possible association between PPARgamma Pro12Ala polymorphism and dementia or 24S-OH-Chol levels.

RESULTS

Compared with Controls, plasma 24S-OH-Chol levels were higher in LOAD and lower in VD; a slight not-significant increase in CIND was observed (ANOVA p: 0.001). A positive correlation between 24S-OH-Chol/TC ratio and plasma C reactive protein (CRP) levels was found in the whole sample, independent of possible confounders (multiple regression p: 0.04; r2: 0.10). This correlation was strong in LOAD (r: 0.39), still present in CIND (r: 0.20), but was absent in VD patients (r: 0.08). The PPARgamma Pro12Ala polymorphism was not associated with the diagnosis of LOAD, VD, or CIND; no correlation emerged between the Ala allele and 24S-OH-Chol plasma levels.

CONCLUSIONS

Our results suggest that plasma 24S-OH-Chol levels might be increased in the first stages of LOAD, and this phenomenon might be related with systemic inflammation. The finding of lower 24S-OH-Chol concentrations in VD might be related with a more advanced stage of VD compared with LOAD in our sample, and/or to different pathogenetic mechanisms and evolution of these two forms of dementia.

Liver X receptors regulate cholesterol homeostasis in oligodendrocytes

Cholesterol synthesis and transport in oligodendrocytes are essential for optimal myelination and remyelination in pathological conditions such as multiple sclerosis. However, little is known about cholesterol homeostasis in the myelin-forming oligodendrocytes. Liver X receptors (LXRs) are nuclear oxysterol receptors that regulate genes involved in cholesterol homeostasis and may therefore play an important role in de- and remyelination. We investigated whether LXRs regulate cholesterol homeostasis in oligodendrocytes. mRNA expression of genes encoding LXR-α and LXR-β and their target genes (ABCA1, ABCG1, ABCG4, apoE, and LDLR) was detected in oligodendrocytes derived from both neonatal and adult rats using quantitative real-time PCR. The expression of LXR-β and several target genes was increased during oligodendrocyte differentiation. We further demonstrated that treatment of primary neonatal rat oligodendrocytes with the synthetic LXR agonist T0901317 induced the expression of several established LXR target genes, including ABCA1, ABCG1, apoE, and LDLR. Treatment of oligodendrocytes with T0901317 resulted in an enhanced cholesterol efflux in the presence of apolipoprotein A-I or high-density lipoprotein particles. These data show that LXRs are involved in regulating cholesterol homeostasis in oligodendrocytes.

Peroxisome-proliferator-activated receptor gamma coactivator 1 α contributes to dysmyelination in experimental models of Huntington's disease

The peroxisome-proliferator-activated receptor gamma coactivator 1 α (PGC1α) has been implicated in the pathogenesis of several neurodegenerative disorders, including Huntington's disease (HD). Recent data demonstrating white matter abnormalities in PGC1α knock-out (KO) mice prompted us to examine the role of PGC1α in CNS myelination and its relevance to HD pathogenesis. We found deficient postnatal myelination in the striatum of PGC1α KO mice, accompanied by a decrease in myelin basic protein (MBP). In addition, brain cholesterol, its precursors, and the rate-limiting enzymes for cholesterol synthesis, HMG CoA synthase (HMGCS1) and HMG CoA reductase (HMGCR), were also reduced in PGC1α KO mice. Moreover, knockdown of PGC1α in oligodendrocytes by lentiviral shRNA led to a decrease in MBP, HMGCS1, and Hmgcr mRNAs. Chromatin immunoprecipitations revealed the recruitment of PGC1α to MBP promoter in mouse brain, and PGC1α over-expression increased MBP and SREBP-2 promoter activity, suggesting that PGC1α regulates MBP and cholesterol synthesis at the transcriptional level. Importantly, expression of mutant huntingtin (Htt) in primary oligodendrocytes resulted in decreased expression of PGC1α and its targets HmgcS1, Hmgcr, and MBP. Decreased expression of MBP and deficient myelination were found postnatally and in adult R6/2 mouse model of HD. Diffusion tensor imaging detected white matter abnormalities in the corpus callosum of R6/2 mice, and electron microscopy revealed thinner myelin sheaths and increased myelin periodicity in BACHD [bacterial artificial chromosome (BAC)-mediated transgenic model for Huntington's disease] mice expressing full-length mutant Htt. Together, these data suggest that PGC1α plays a role in postnatal myelination and that deficient PGC1α activity in oligodendrocytes may contribute to abnormal myelination in HD.

Relationship between cholesterol metabolism, ApoE and brain volumes in Alzheimer’s disease

APOE genotype, aging and midlife hypercholesterolemia are well-established risk factors for late-onset Alzheimer’s disease (AD). ApoE and cholesterol are involved in the pathogenesis of AD since they influence amyloid-β accumulation and Tau pathology. APOE ε4 carriers were found to present lower levels of amyloid-β1–42, higher tau and phosphorylated tau and a higher degree of brain atrophy at any disease stage. Presence of ApoE4 shifts the onset of the disease towards a younger age and makes progression faster. Hypercholesterolemia together with other major cardiovascular risk factors were found to be involved in the pathogenesis of AD, but reduced plasma cholesterol levels were described in demented patients. Significant correlations were found between cholesterol precursors lathosterol, lanosterol and 24S-hydroxycholesterol (a putative marker of brain cholesterol turnover) in plasma and brain atrophy as quantified by MRI. It is likely that neurodegeneration affects both brain and whole-body cholesterol metabolism in AD.

Dietary lipids and their oxidized products in Alzheimer's disease

Alzheimer's disease (AD) is the commonest form of dementia in the elderly, characterized by memory dysfunction, loss of lexical access, spatial and temporal disorientation, and impaired judgment. A growing body of scientific literature addresses the implication of dietary habits in the pathogenesis of AD. This review reports recent findings concerning the modulation of AD development by dietary lipids, in animals and humans, focusing on the pathogenetic role of lipid oxidation products. Oxidative breakdown products of ω-6 polyunsaturated fatty acids (ω-6 PUFAs), and cholesterol oxidation products (oxysterols), might play a role in favoring β-amyloid deposition, a hallmark of AD's onset and progression. Conversely, ω-3 PUFAs appear to contribute to preventing and treating AD. However, high concentrations of ω-3 PUFAs can also produce oxidized derivatives reacting with important functions of nervous cells. Thus, altered balances between cholesterol and oxysterols, and between ω-3 and ω-6 PUFAs must be considered in AD's pathophysiology. The use of a diet with an appropriate ω-3/ω-6 PUFA ratio, rich in healthy oils, fish and antioxidants, such as flavonoids, but low in cholesterol-containing foods, can be a beneficial component in the clinical strategies of prevention of AD.

Oxysterols in bile acid metabolism

Increasing body of evidence is available indicating that oxysterols are more much than intermediates of metabolic pathways. Oxysterols play a role in the regulation of cholesterol synthesis, transport and efflux. A scavenger effect of cholesterol 27-hydroxylase on elevated serum cholesterol levels is well demonstrated. Bile acid synthesis occurs through two main pathways, the classic and the alternative ones. Since plasma concentrations of 27-hydroxycholesterol were clearly shown to reflect its production rate the alternative pathway of bile acid synthesis can be easily explored. Conversely this was not true for 7α-hydroxycholesterol and also the direct evaluation of the classic pathway by kinetic studies is more difficult since the rate of plasma appearance during continuous infusion of deuterated isotopomers may not exactly measure its production rate. Hepatic cholesterol 7alpha-hydroxylase activity is absent during fetal life in humans and upregulates after birth. Both the classic and alternative pathways become mature after the age of 4 years. It has been clearly demonstrated that in patients with liver disease the classic pathway is impaired while the alternative one is preserved. Conversely, in obese patients, preliminary data suggest an increase of the production rate of 27-hydroxycholesterol, a possible mechanism to counteract the increase of atherosclerotic risk.

Cholesterol metabolism in neurons and astrocytes

Cells in the mammalian body must accurately maintain their content of cholesterol, which is an essential membrane component and precursor for vital signalling molecules. Outside the brain, cholesterol homeostasis is guaranteed by a lipoprotein shuttle between the liver, intestine and other organs via the blood circulation. Cells inside the brain are cut off from this circuit by the blood-brain barrier and must regulate their cholesterol content in a different manner. Here, we review how this is accomplished by neurons and astrocytes, two cell types of the central nervous system, whose cooperation is essential for normal brain development and function. The key observation is a remarkable cell-specific distribution of proteins that mediate different steps of cholesterol metabolism. This form of metabolic compartmentalization identifies astrocytes as net producers of cholesterol and neurons as consumers with unique means to prevent cholesterol overload. The idea that cholesterol turnover in neurons depends on close cooperation with astrocytes raises new questions that need to be addressed by new experimental approaches to monitor and manipulate cholesterol homeostasis in a cell-specific manner. We conclude that an understanding of cholesterol metabolism in the brain and its role in disease requires a close look at individual cell types.

Interaction between 24-hydroxycholesterol, oxidative stress, and amyloid-β in amplifying neuronal damage in Alzheimer's disease: three partners in crime

All three cholesterol oxidation products implicated thus far in the pathogenesis of Alzheimer's disease, 7β-hydroxycholesterol, 24-hydroxycholesterol, and 27-hydroxycholesterol, markedly enhance the binding of amyloid-beta (Aβ) to human differentiated neuronal cell lines (SK-N-BE and NT-2) by up-regulating net expression and synthesis of CD36 and β1-integrin receptors. However, only 24-hydroxycholesterol markedly potentiates the pro-apoptotic and pro-necrogenic effects of Aβ(1-42) peptide on these cells: 7β-hydroxycholesterol and 27-hydroxycholesterol, like unoxidized cholesterol, show no potentiating effect. This peculiar behavior of 24-hydroxycholesterol at physiologic concentrations (1 μm) depends on its strong enhancement of the intracellular generation of NADPH oxidase-dependent reactive oxygen species (ROS), mainly H(2) O(2) , and the consequent impairment of neuronal cell redox equilibrium, measured in terms of the GSSG/GSH ratio. Cell incubation with antioxidants quercetin or genistein prevents 24-hydroxycholesterol's pro-oxidant effect and potentiation of Aβ-induced necrosis and apoptosis. Thus, the presence of 24-hydroxycholesterol in the close vicinity of amyloid plaques appears to enhance the adhesion of large amounts of Aβ to the plasma membrane of neurons and then to amplify the neurotoxic action of Aβ by locally increasing ROS steady-state levels. This report further supports a primary involvement of altered brain cholesterol metabolism in the complex pathogenesis of Alzheimer's disease.

24(S)-hydroxycholesterol induces neuronal cell death through necroptosis, a form of programmed necrosis

24(S)-Hydroxycholesterol (24S-OHC) produced by cholesterol 24-hydroxylase expressed mainly in neurons plays an important physiological role in the brain. Conversely, it has been reported that 24S-OHC possesses potent cytotoxicity. The molecular mechanisms of 24S-OHC-induced cell death have not yet been fully elucidated. In this study, using human neuroblastoma SH-SY5Y cells and primary cortical neuronal cells derived from rat embryo, we characterized the form of cell death induced by 24S-OHC. SH-SY5Y cells treated with 24S-OHC exhibited neither fragmentation of the nucleus nor caspase activation, which are the typical characteristics of apoptosis. 24S-OHC-treated cells showed necrosis-like morphological changes but did not induce ATP depletion, one of the features of necrosis. When cells were treated with necrostatin-1, an inhibitor of receptor-interacting serine/threonine kinase 1 (RIPK1) required for necroptosis, 24S-OHC-induced cell death was significantly suppressed. The knockdown of RIPK1 by transfection of small interfering RNA of RIPK1 effectively attenuated 24S-OHC-induced cell death. It was found that neither SH-SY5Y cells nor primary cortical neuronal cells expressed caspase-8, which was regulated for RIPK1-dependent apoptosis. Collectively, these results suggest that 24S-OHC induces neuronal cell death by necroptosis, a form of programmed necrosis.

Histochemistry and cell biology: the annual review 2010

This review summarizes recent advances in histochemistry and cell biology which complement and extend our knowledge regarding various aspects of protein functions, cell and tissue biology, employing appropriate in vivo model systems in conjunction with established and novel approaches. In this context several non-expected results and discoveries were obtained which paved the way of research into new directions. Once the reader embarks on reading this review, it quickly becomes quite obvious that the studies contribute not only to a better understanding of fundamental biological processes but also provide use-oriented aspects that can be derived therefrom.

Links between ApoE, brain cholesterol metabolism, tau and amyloid beta-peptide in patients with cognitive impairment

Brain neurons remove the excess of cholesterol via conversion into the more polar 24OHC [(24S)-hydroxycholesterol]. 24OHC acts as a signalling molecule inducing ApoE (apolipoprotein E)-mediated cholesterol efflux from astrocytes, by a direct effect on ApoE transcription, protein synthesis and secretion. In CSF (cerebrospinal fluid) collected form from patients with cognitive impairment (Alzheimer's disease and patients with mild cognitive impairment) the levels of ApoE, tau, p-tau (hyperphosphorylated tau) were significantly increased, together with 24OHC, compared with controls. We also found that the levels of tau and p-tau were significantly correlated with ApoE and 24OHC in the same samples. Such a correlation was not found in control patients. Increased levels of cholesterol in membranes and impairment in brain cholesterol metabolism were found to be involved both in APP (amyloid precursor protein) processing and amyloid beta-peptide deposition and, recently, in tau pathology. The CSF tau levels are considered to be related to the neurodegenerative process in Alzheimer's disease. During neurodegeneration, the cholesterol accumulated in neurons is converted into 24OHC. The release of 24OHC from neurons induces ApoE secretion by astrocytes, and both are related to the intensity of the neurodegenerative process and neuronal injury. ApoE can also be involved in the scavenging of tau from neurons. The direct correlations between ApoE, 24OHC and tau suggest that cholesterol metabolism may be involved in generation of both tau and amyloid beta-peptide and that the ApoE is released by astrocytes in order to counteract this ongoing process.

Quercetin activates AMP-activated protein kinase by reducing PP2C expression protecting old mouse brain against high cholesterol-induced neurotoxicity

It is known that a high-cholesterol diet induces oxidative stress, inflammatory response, and beta-amyloid (Abeta) accumulation in mouse brain, resulting in neurodegenerative changes. Quercetin, a naturally occurring flavonoid, has been reported to possess numerous biological activities beneficial to health. Our previous studies have demonstrated that quercetin protects mouse brain against D-galactose-induced oxidative damage. Against this background, we evaluated the effect of quercetin on high-cholesterol-induced neurotoxicity in old mice and explored its potential mechanism. Our results showed that oral administration of quercetin significantly improved the behavioural performance of high-cholesterol-fed old mice in both a step-through test and the Morris water maze task. This is at least in part caused by decreasing ROS and protein carbonyl levels and restoring Cu--Zn superoxide dismutase (Cu, Zn-SOD) activity. Furthermore, quercetin also significantly activated the AMP-activated protein kinase (AMPK) via down-regulation of protein phosphatase 2C (PP2C), which reduced the integral optical density (IOD) of activated microglia cells and CD11b expression, down-regulated iNOS and cyclooxygenase-2 (COX-2) expression, and decreased IL-1beta, IL-6, and TNF-alpha expression in the brains of high-cholesterol-fed old mice through the suppression of NF-kappaB p65 nuclear translocation. Moreover, AMPK activation significantly increased 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase and acetyl-CoA carboxylase (ACC) phosphorylation and reduced fatty acid synthase (FAS) expression in the brains of high-cholesterol-fed old mice, which reduced cholesterol levels, down-regulated cholesterol 24-hydroxylase (CYP46A1) and beta-amyloid converting enzyme 1 (BACE1) expression, decreased eukaryotic translation initiation factor 2alpha (eIF2alpha) phosphorylation, and lowered Abeta deposits. However, the neuroprotective effect of quercetin was weakened by intraperitoneal injection of compound C, an AMPK inhibitor. These results suggest that AMPK activated by quercetin may be a potential target to enhance the resistance of neurons to age-related diseases.

Distribution of cholesterol 24-hydroxylase in the monkey brain

OBJECTIVE

Cholesterol 24-hydroxylase catalyzes the conversion of cholesterol to 24-hydroxycholesterol, which is a major pathway for cholesterol elimination from the brain, since 24-hydroxycholesterol can readily cross the blood brain barrier. The present study aimed to elucidate the distribution of cholesterol 24-hydroxylase in the monkey brain.

METHODS

The distribution of cholesterol 24-hydroxylase in the monkey brain was examined using Western blot and immunohistochemistry methods, and was observed under light microscopy and electron microscopy.

RESULTS

High levels of cholesterol 24-hydroxylase were observed in projection neurons and neuropil in structures derived from telencephalon, including the cerebral neocortex, hippocampus, amygdala, nucleus basalis of Meynert, and striatum. Electron microscopy revealed that the enzyme was localized in the axon terminals. One the other hand, cholesterol 24-hydroxylase was expressed at a lower level in the thalamus, globus pallidus and brainstem.

CONCLUSION

The high level of cholesterol 24-hydroxylase in the telencephalon possibly reflects a high rate of cholesterol turnover in this part of brain.

The ins and outs of cholesterol in the vertebrate retina

The vertebrate retina has multiple demands for utilization of cholesterol and must meet those demands either by synthesizing its own supply of cholesterol or by importing cholesterol from extraretinal sources, or both. Unlike the blood-brain barrier, the blood-retina barrier allows uptake of cholesterol from the circulation via a lipoprotein-based/receptor-mediated mechanism. Under normal conditions, cholesterol homeostasis is tightly regulated; also, cholesterol exists in the neural retina overwhelmingly in unesterified form, and sterol intermediates are present in minimal to negligible quantities. However, under certain pathological conditions, either due to an inborn error in cholesterol biosynthesis or as a consequence of exposure to selective inhibitors of enzymes in the cholesterol pathway, the ratio of sterol intermediates to cholesterol in the retina can rise dramatically and persist, in some cases resulting in progressive degeneration that significantly compromises the structure and function of the retina. Although the relative contributions of de novo synthesis versus extraretinal uptake are not yet known, herein we review what is known about these processes and the dynamics of cholesterol in the vertebrate retina and indicate some future avenues of research in this area.

Genetic connections between neurological disorders and cholesterol metabolism

Cholesterol is an essential component of both the peripheral and central nervous systems of mammals. Over the last decade, evidence has accumulated that disturbances in cholesterol metabolism are associated with the development of various neurological conditions. In addition to genetically defined defects in cholesterol synthesis, which will be covered in another review in this Thematic Series, defects in cholesterol metabolism (cerebrotendinous xanthomatosis) and intracellular transport (Niemann Pick Syndrome) lead to neurological disease. A subform of hereditary spastic paresis (type SPG5) and Huntington's disease are neurological diseases with mutations in genes that are of importance for cholesterol metabolism. Neurodegeneration is generally associated with disturbances in cholesterol metabolism, and presence of the E4 isoform of the cholesterol transporter apolipoprotein E as well as hypercholesterolemia are important risk factors for development of Alzheimer's disease. In the present review, we discuss the links between genetic disturbances in cholesterol metabolism and the above neurological disorders.

Brain injury induces cholesterol 24-hydroxylase (Cyp46) expression in glial cells in a time-dependent manner

Maintaining the cholesterol homeostasis is essential for normal CNS functioning. The enzyme responsible for elimination of cholesterol excess from the brain is cholesterol 24-hydroxylase (Cyp46). Since cholesterol homeostasis is disrupted following brain injury, in this study we examined the effect of right sensorimotor cortex suction ablation on cellular and temporal pattern of Cyp46 expression in the rat brain. Increased expression of Cyp46 at the lesion site at all post injury time points (2, 7, 14, 28 and 45 days post injury, dpi) was detected. Double immunofluorescence staining revealed colocalization of Cyp46 expression with different types of glial cells in time-dependent manner. In ED1(+) microglia/macrophages Cyp46 expression was most prominent at 2 and 7 dpi, whereas Cyp46 immunoreactivity persisted in reactive astrocytes throughout all time points post-injury. However, during the first 2 weeks Cyp46 expression was enhanced in both GFAP(+) and Vim(+) astrocytes, while at 28 and 45 dpi its expression was mostly associated with GFAP(+) cells. Pattern of neuronal Cyp46 expression remained unchanged after the lesion, i.e. Cyp46 immunostaining was detected in dendrites and cell body, but not in axons. The results of this study clearly demonstrate that in pathological conditions, like brain injury, Cyp46 displayed atypical expression, being expressed not only in neuronal cells, but also in microglia and astrocytes. Therefore, injury-induced expression of Cyp46 in microglial and astroglial cells may be involved in the post-injury removal of damaged cell membranes contributing to re-establishment of the brain cholesterol homeostasis.

Neuronal cholesterol esterification by ACAT1 in Alzheimer's disease

Cholesterol has been implicated in various neurodegenerative diseases. Here we review the connection between cholesterol and Alzheimer's disease (AD), focusing on a recent study that links neuronal cholesterol esterification with biosynthesis of 24(S)-hydroxycholesterol and the fate of human amyloid precursor protein in a mouse model of AD. We also briefly evaluate the potential of ACAT1 as a drug target for AD.

Leptin reduces the accumulation of Abeta and phosphorylated tau induced by 27-hydroxycholesterol in rabbit organotypic slices

Accumulation of amyloid-beta (Abeta) peptide and deposition of hyperphosphorylated tau protein are two major pathological hallmarks of Alzheimer's disease (AD). We have shown that cholesterol-enriched diets and its metabolite 27-hydroxycholesterol (27-OHC) increase Abeta and phosphorylated tau levels. However, the mechanisms by which cholesterol and 27-OHC regulate Abeta production and tau phosphorylation remain unclear. Leptin, an adipocytokine involved in cell survival and in learning, has been demonstrated to regulate Abeta production and tau hyperphosphorylation in transgenic mice for AD. However, the involvement of leptin signaling in cholesterol and cholesterol metabolites-induced Abeta accumulation and tau hyperphosphorylation are yet to be examined. In this study, we determined the effect of high cholesterol diet and 27-OHC on leptin expression levels and the extent to which leptin treatment affects 27-OHC-induced AD-like pathology. Our results show that feeding rabbits a 2% cholesterol-enriched diet for 12 weeks reduces the levels of leptin by approximately 80% and incubating organotypic slices from adult rabbit hippocampus with 27-OHC reduced leptin levels by approximately 30%. 27-OHC induces a 1.5-fold increase in Abeta (40) and a 3-fold increase in Abeta (42) and in phosphorylated tau. Treatment with leptin reversed the 27-OHC-induced increase in Abeta and phosphorylated tau by decreasing the levels of BACE-1 and GSK-3beta respectively. Our results suggest that cholesterol-enriched diets and cholesterol metabolites induce AD-like pathology by altering leptin signaling. We propose that leptin administration may prevent the progression of sporadic forms of AD that are related to increased cholesterol and oxidized cholesterol metabolite levels.

The role of ATP-binding cassette transporter A1 in Alzheimer's disease and neurodegeneration

ATP-binding cassette transporter A1 - ABCA1, is the most extensively studied transporter in human pathology. ABCA1 became a primary subject of research in many academic and pharmaceutical laboratories immediately after the discovery that mutations at the gene locus cause severe familial High Density Lipoprotein (HDL) deficiency and, in the homozygous form - Tangier disease. The protein is the major regulator of intracellular cholesterol efflux which is the initial and essential step in the biogenesis and formation of nascent HDL particles. The transcriptional regulation of ABCA1 by nuclear Liver X Receptors (LXR) provided a starting point for drug discovery and development of synthetic LXR ligands/ABCA1 activators for treatment of arteriosclerosis. A series of reports that revealed the role of ABCA1 in Abeta deposition and clearance, as well as the possibility for association of some ABCA1 genetic variants with risk for Alzheimer's disease (AD) brought a new dimension to ABCA1 research. The LXR-ABCA1-APOE regulatory axis is now considered a promising therapeutic target in AD, which includes the only proven risk factor for AD - APOE, at two distinct levels - transcriptional regulation by LXR, and ABCA1 controlled lipidation which can influence Abeta aggregation and amyloid clearance. This review will summarize the results of research on ABCA1, particularly related to AD and neurodegeneration.

Changes in brain cholesterol metabolome after excitotoxicity

Excitotoxicity due to excess stimulation of glutamate receptors in neurons is accompanied by increased Ca(2+) influx, stimulation of Ca(2+)-dependent enzymes, ATP depletion, increase in lipid peroxidation products, and loss of glutathione. These changes resemble neurochemical alterations in acute neuronal injury (stroke, spinal cord injury, and traumatic brain injury) and chronic neurodegenerative diseases such as Alzheimer's disease. Intracerebroventricular injection of the potent glutamate analog kainate in rats results in increased cholesterol concentration in the hippocampus at short to medium time intervals, i.e., 3 days-1 week post-injection, as detected by gas chromatography-mass spectrometry in the lesioned hippocampus. This is accompanied by an early increase in levels of cholesterol biosynthetic precursors and increases in both enzymatically derived oxysterols such as 24-hydroxycholesterol and cholesterol oxidation products (COPs) generated by reactive oxygen species, including cholesterol epoxides and 7-ketocholesterol. In contrast to COPs, no change in concentration of the neurosteroid pregnenolone was found after KA injury. Cholesterol and COPs significantly increase exocytosis in cultured PC12 cells and neurons, and both oxysterols and COPs are able to induce cytotoxic and apoptotic injuries in different cell types, including neurons. Together, the findings suggest that increased cholesterol and COPs after KA excitotoxicity could themselves lead to disturbed neuronal ion homeostasis, increased neurotransmitter release, and propagation of excitotoxicity.

ACAT1 gene ablation increases 24(S)-hydroxycholesterol content in the brain and ameliorates amyloid pathology in mice with AD

Cholesterol metabolism has been implicated in the pathogenesis of several neurodegenerative diseases, including the abnormal accumulation of amyloid-beta, one of the pathological hallmarks of Alzheimer disease (AD). Acyl-CoA:cholesterol acyltransferases (ACAT1 and ACAT2) are two enzymes that convert free cholesterol to cholesteryl esters. ACAT inhibitors have recently emerged as promising drug candidates for AD therapy. However, how ACAT inhibitors act in the brain has so far remained unclear. Here we show that ACAT1 is the major functional isoenzyme in the mouse brain. ACAT1 gene ablation (A1-) in triple transgenic (i.e., 3XTg-AD) mice leads to more than 60% reduction in full-length human APPswe as well as its proteolytic fragments, and ameliorates cognitive deficits. At 4 months of age, A1- causes a 32% content increase in 24-hydroxycholesterol (24SOH), the major oxysterol in the brain. It also causes a 65% protein content decrease in HMG-CoA reductase (HMGR) and a 28% decrease in sterol synthesis rate in AD mouse brains. In hippocampal neurons, A1- causes an increase in the 24SOH synthesis rate; treating hippocampal neuronal cells with 24SOH causes rapid declines in hAPP and in HMGR protein levels. A model is provided to explain our findings: in neurons, A1- causes increases in cholesterol and 24SOH contents in the endoplasmic reticulum, which cause reductions in hAPP and HMGR protein contents and lead to amelioration of amyloid pathology. Our study supports the potential of ACAT1 as a therapeutic target for treating certain forms of AD.

24S-hydroxycholesterol effects on lipid metabolism genes are modeled in traumatic brain injury

Membrane damage during traumatic brain injury (TBI) alters the brain homeostasis of cholesterol and other lipids. Cholesterol 24S-hydroxylase (Cyp46) is a cholesterol metabolic enzyme that is increased after TBI. Here, we systematically examined the effects of the enzymatic product of Cyp46, 24S-hydroxycholesterol, on the cholesterol regulatory genes, SREBP-1 and 2, their posttranslational regulation, and their effects on gene transcription. 24S-hydroxycholesterol increased levels of SREBP-1 mRNA and full-length protein but did not change levels of cleaved SREBP-1, consistent with the role of 24-hydroxycholesterol as an LXR agonist. In contrast, 24S-hydroxycholesterol decreased levels of LXR-independent SREBP-2 mRNA, full-length protein, and SREBP-2 active cleavage product. We examined the downstream effects of changes to these lipid regulatory factors by studying cholesterol and fatty acid synthesis genes. In neuroblastoma cells, 24S-hydroxycholesterol decreased mRNA levels of the cholesterol synthesis genes HMG CoA reductase, squalene synthase, and FPP synthase but did not alter levels of the mRNA of fatty acid synthesis genes acetyl CoA carboxylase or fatty acid synthase. After TBI, as after 24S-hydroxycholesterol treatment in vitro, SREBP-1 mRNA levels were increased while SREBP-2 mRNA levels were decreased. Also similar to the in vitro results with 24S-hydroxycholesterol, HMG CoA reductase and squalene synthase mRNA levels were significantly decreased. Fatty acid synthase mRNA levels were not altered but acetyl CoA carboxylase mRNA levels were significantly decreased. Thus, changes to transcription of cholesterol synthesis genes after TBI were consistent with increases in Cyp46 activity, but changes to fatty acid synthesis genes must be regulated by other mechanisms.