Glycosphingolipid Accumulation Inhibits Cholesterol Efflux via the ABCA1/Apolipoprotein A-I Pathway

In situ lipidomics of Staphylococcus aureus osteomyelitis using imaging mass spectrometry

Osteomyelitis occurs when Staphylococcus aureus invades the bone microenvironment, resulting in a bone marrow abscess with a spatially defined architecture of cells and biomolecules. Imaging mass spectrometry and microscopy are invaluable tools that can be employed to interrogate the lipidome of S. aureus-infected murine femurs to reveal metabolic and signaling consequences of infection. Here, nearly 250 lipids were spatially mapped to healthy and infection-associated morphological features throughout the femur, establishing composition profiles for tissue types. Ether lipids and arachidonoyl lipids were significantly altered between cells and tissue structures in abscesses, suggesting their roles in abscess formation and inflammatory signaling. Sterols, triglycerides, bis(monoacylglycero)phosphates, and gangliosides possessed ring-like distributions throughout the abscess, indicating dysregulated lipid metabolism in a subpopulation of leukocytes that cannot be discerned with traditional microscopy. These data provide chemical insight into the signaling function and metabolism of cells in the fibrotic border of abscesses, likely characteristic of lipid-laden macrophages.

Gene therapy approaches for GM1 gangliosidosis: Focus on animal and cellular studies

One of the most important inherited metabolic disorders is GM1 gangliosidosis, which is a progressive neurological disorder. The main cause of this disease is a genetic defect in the enzyme β-galactosidase due to a mutation in the glb1 gene. Lack of this enzyme in cells (especially neurons) leads to the accumulation of ganglioside substrate in nerve tissues, followed by three clinical forms of GM1 disease (neonatal, juvenile, and adult variants). Genetically, many mutations occur in the exons of the glb1 gene, such as exons 2, 6, 15, and 16, so the most common ones reported in scientific studies include missense/nonsense mutations. Therefore, many studies have examined the genotype-phenotype relationships of this disease and subsequently using gene therapy techniques have been able to reduce the complications of the disease and alleviate the signs and symptoms of the disease. In this regard, the present article reviews the general features of GM1 gangliosidosis and its mutations, as well as gene therapy studies and animal and human models of the disease.

Endocrinological, immunological and metabolic features of patients with Fabry disease under therapy

OBJECTIVES

Fabry disease is an X-linked lysosomal disorder caused by decreased or absent alpha galactosidase enzyme. The enzyme deficiency leads to progressive accumulation of globotriaosylceramide (Gb-3) and its deacetylated form lyso-Gb3 in various tissue lysosomes that results in primarily lysosomal deterioration and subsequently mitochondrial, endothelial, and immunologic dysfunctions.

METHODS

The endocrinological, metabolic, immunological and HLA status of 12 patients were evaluated.

RESULTS

A total of 11 patients (91.6 %) had immunologic and/or endocrinologic abnormalities. fT4, anti-TPO, and anti-TG levels were increased in 1, 2, and 2 patients, respectively. Three patients had elevated proinflammatory cytokines. ANA profile, p-ANCA and c-ANCA were positive in 1, 1, and 2 patients, respectively. Tissue transglutaminase antibody was negative in all patients however P5 was diagnosed with Celiac disease at the age of 12 and on gluten free diet. All patients had distinct types of HLA apart from 2 patients with anti-TG and anti-TPO positive and there was no relationship between the HLA types and the autoimmunity biomarkers.

CONCLUSIONS

FD may have impact on endocrinologic and immunologic abnormalities even in the patients under ERT, therefore prevalence of these abnormalities may be higher in ERT naïve patients. However, apparently, they are less likely to cause clinical symptoms. Certain HLA alleles may contribute to the direct impact of immunological pathogenesis in FD by developing abnormal autoimmune biomarkers. To the best of our knowledge, this is the first study investigating HLA status of FD patients; therefore further studies are needed to elucidate the underlying mechanism of action.

Fabry disease: Mechanism and therapeutics strategies

Fabry disease is a monogenic disease characterized by a deficiency or loss of the α-galactosidase A (GLA). The resulting impairment in lysosomal GLA enzymatic activity leads to the pathogenic accumulation of enzymatic substrate and, consequently, the progressive appearance of clinical symptoms in target organs, including the heart, kidney, and brain. However, the mechanisms involved in Fabry disease-mediated organ damage are largely ambiguous and poorly understood, which hinders the development of therapeutic strategies for the treatment of this disorder. Although currently available clinical approaches have shown some efficiency in the treatment of Fabry disease, they all exhibit limitations that need to be overcome. In this review, we first introduce current mechanistic knowledge of Fabry disease and discuss potential therapeutic strategies for its treatment. We then systemically summarize and discuss advances in research on therapeutic approaches, including enzyme replacement therapy (ERT), gene therapy, and chaperone therapy, as well as strategies targeting subcellular compartments, such as lysosomes, the endoplasmic reticulum, and the nucleus. Finally, the future development of potential therapeutic strategies is discussed based on the results of mechanistic studies and the limitations associated with these therapeutic approaches.

Genome-wide CRISPRi/a screens in human neurons link lysosomal failure to ferroptosis

Single-cell transcriptomics provide a systematic map of gene expression in different human cell types. The next challenge is to systematically understand cell-type-specific gene function. The integration of CRISPR-based functional genomics and stem cell technology enables the scalable interrogation of gene function in differentiated human cells. Here we present the first genome-wide CRISPR interference and CRISPR activation screens in human neurons. We uncover pathways controlling neuronal response to chronic oxidative stress, which is implicated in neurodegenerative diseases. Unexpectedly, knockdown of the lysosomal protein prosaposin strongly sensitizes neurons, but not other cell types, to oxidative stress by triggering the formation of lipofuscin, a hallmark of aging, which traps iron, generating reactive oxygen species and triggering ferroptosis. We also determine transcriptomic changes in neurons after perturbation of genes linked to neurodegenerative diseases. To enable the systematic comparison of gene function across different human cell types, we establish a data commons named CRISPRbrain.

Brothers in Arms: ABCA1- and ABCG1-Mediated Cholesterol Efflux as Promising Targets in Cardiovascular Disease Treatment

Atherosclerosis is a leading cause of cardiovascular disease worldwide, and hypercholesterolemia is a major risk factor. Preventive treatments mainly focus on the effective reduction of low-density lipoprotein cholesterol, but their therapeutic value is limited by the inability to completely normalize atherosclerotic risk, probably due to the disease complexity and multifactorial pathogenesis. Consequently, high-density lipoprotein cholesterol gained much interest, as it appeared to be cardioprotective due to its major role in reverse cholesterol transport (RCT). RCT facilitates removal of cholesterol from peripheral tissues, including atherosclerotic plaques, and its subsequent hepatic clearance into bile. Therefore, RCT is expected to limit plaque formation and progression. Cellular cholesterol efflux is initiated and propagated by the ATP-binding cassette (ABC) transporters ABCA1 and ABCG1. Their expression and function are expected to be rate-limiting for cholesterol efflux, which makes them interesting targets to stimulate RCT and lower atherosclerotic risk. This systematic review discusses the molecular mechanisms relevant for RCT and ABCA1 and ABCG1 function, followed by a critical overview of potential pharmacological strategies with small molecules to enhance cellular cholesterol efflux and RCT. These strategies include regulation of ABCA1 and ABCG1 expression, degradation, and mRNA stability. Various small molecules have been demonstrated to increase RCT, but the underlying mechanisms are often not completely understood and are rather unspecific, potentially causing adverse effects. Better understanding of these mechanisms could enable the development of safer drugs to increase RCT and provide more insight into its relation with atherosclerotic risk. SIGNIFICANCE STATEMENT: Hypercholesterolemia is an important risk factor of atherosclerosis, which is a leading pathological mechanism underlying cardiovascular disease. Cholesterol is removed from atherosclerotic plaques and subsequently cleared by the liver into bile. This transport is mediated by high-density lipoprotein particles, to which cholesterol is transferred via ATP-binding cassette transporters ABCA1 and ABCG1. Small-molecule pharmacological strategies stimulating these transporters may provide promising options for cardiovascular disease treatment.

Developments in the treatment of Fabry disease

Enzyme replacement therapy (ERT) with recombinant α-galactosidase A (r-αGAL A) for the treatment of Fabry disease has been available for over 15 years. Long-term treatment may slow down disease progression, but cardiac, renal, and cerebral complications still develop in most patients. In addition, lifelong intravenous treatment is burdensome. Therefore, several new treatment approaches have been explored over the past decade. Chaperone therapy (Migalastat; 1-deoxygalactonojirimycin) is the only other currently approved therapy for Fabry disease. This oral small molecule aims to improve enzyme activity of mutated α-galactosidase A and can only be used in patients with specific mutations. Treatments currently under evaluation in (pre)clinical trials are second generation enzyme replacement therapies (Pegunigalsidase-alfa, Moss-aGal), substrate reduction therapies (Venglustat and Lucerastat), mRNA- and gene-based therapy. This review summarises the knowledge on currently available and potential future options for the treatment of Fabry disease.

microRNAs targeting cellular cholesterol: implications for combating anticancer drug resistance

Over sixty percent of all mammalian protein-coding genes are estimated to be regulated by microRNAs (miRNAs), and unsurprisingly miRNA dysregulation has been linked with cancer. Aberrant miRNA expression in cancer cells has been linked with tumourigenesis and drug resistance. In the past decade, increasing number of studies have demonstrated that cholesterol accumulation fuels tumour growth and contributes to drug resistance, therefore, miRNAs controlling cholesterol metabolism and homeostasis are obvious hypothetical targets for investigating their role in cholesterol-mediated drug resistance in cancer. In this review, we have collated published evidences to consolidate this hypothesis and have scrutinized it by utilizing computational tools to explore the role of miRNAs in cholesterol-mediated drug resistance in breast cancer cells. We found that hsa-miR-128 and hsa-miR-223 regulate genes mediating lipid signalling and cholesterol metabolism, cancer drug resistance and breast cancer genes. The analysis demonstrates that targeting these miRNAs in cancer cells presents an opportunity for developing new strategies to combat anticancer drug resistance.

Mechanistic convergence and shared therapeutic targets in Niemann-Pick disease

Niemann-Pick disease type C (NPC) and Tangier disease are genetically and clinically distinct rare inborn errors of metabolism. NPC is caused by defects in either NPC1 or NPC2; whereas Tangier disease is caused by a defect in ABCA1. Tangier disease is currently without therapy, whereas NPC can be treated with miglustat, a small molecule inhibitor of glycosphingolipid biosynthesis that slows the neurological course of the disease. When a Tangier disease patient was misdiagnosed with NPC and treated with miglustat, her symptoms improved. This prompted us to consider whether there is mechanistic convergence between these two apparently unrelated rare inherited metabolic diseases. In this study, we found that when ABCA1 is defective (Tangier disease) there is secondary inhibition of the NPC disease pathway, linking these two diseases at the level of cellular pathophysiology. In addition, this study further supports the hypothesis that miglustat, as well as other substrate reduction therapies, may be potential therapeutic agents for treating Tangier disease as fibroblasts from multiple Tangier patients were corrected by miglustat treatment.

Tangier disease: update for 2020

PURPOSE OF REVIEW

To appraise recent advances in our knowledge of the severe genetic HDL deficiency disorder, Tangier disease.

RECENT FINDINGS

While Tangier disease can cause premature atherosclerotic cardiovascular disease (ASCVD), new evidence suggests that heterozygous ABCA1 variant carriers are also at increased risk. Advances have been made in the study of the neurological abnormalities observed in Tangier disease, both in their assessment and the identification of potential new therapies.

SUMMARY

Tangier disease is an extremely rare condition and, as such, the published literature around its range of clinical manifestations, including peripheral neuropathy, premature ASCVD and platelet abnormalities is limited. Patient registries may assist in this regard.

Management of metabolic syndrome and reduction in body weight in type II diabetic mice by inhibiting glycosphingolipid synthesis

Metabolic syndrome is defined by hyperlipidemia and cardiovascular complications. We have examined whether inhibition of glycosphingolipid synthesis can interfere with metabolic syndrome in a male mouse model of type II diabetes (db/db). The db/db and control mice (C57/BL6) (n = 6) fed chow for 30 weeks received vehicle (5% Tween-80 in PBS; 100 μl), or a biopolymer-encapsulated D-threo-1-phenyl-2-decanoylamino-3-morpholino-1-propanol (BPD) glycosphingolipid synthesis inhibitor daily via oral gavage for 6 weeks. Echocardiography revealed increased Ao-IMT in db/db mice compared to control. However, BPD decreased Ao-IMT, monohexosylceramide and dihexosylceramide, LDL, triglycerides, glucose, and raised HDL levels in db/db mice. This was due to increased gene expression of HMG-CoA reductase, LDLr, SREBP2, and bile acids: Cy7-a hydroxylase, LXR and FXR, lipoprotein lipase, VLDL receptor and PPAR. Treatment also increased the expression of superoxide dismutase-II to reduce the pro-oxidant status in these mice. We observed that decreased cholesterol levels correlated with decreased cholesterol sensing proteins e.g. NPC1 gene/protein expression and mammalian target of rapamycin (mTORC-1) and reduced body weight. Thus, glycosphingolipid synthesis inhibition is a novel approach to manage metabolic syndrome and reduce body weight in diabetic mice and with potential applications in humans.

Lysosomal storage disorders - challenges, concepts and avenues for therapy: beyond rare diseases

The pivotal role of lysosomes in cellular processes is increasingly appreciated. An understanding of the balanced interplay between the activity of acidic hydrolases, lysosomal membrane proteins and cytosolic proteins is required. Lysosomal storage diseases (LSDs) are characterized by disturbances in this network and by intralysosomal accumulation of substrates, often only in certain cell types. Even though our knowledge of these diseases has increased and therapies have been established, many aspects of the molecular pathology of LSDs remain obscure. This Review aims to discuss how lysosomal storage affects functions linked to lysosomes, such as membrane repair, autophagy, exocytosis, lipid homeostasis, signalling cascades and cell viability. Therapies must aim to correct lysosomal storage not only morphologically, but reverse its (patho)biochemical consequences. As different LSDs have different molecular causes, this requires custom tailoring of therapies. We will discuss the major advantages and drawbacks of current and possible future therapies for LSDs. Study of the pathological molecular mechanisms underlying these 'experiments of nature' often yields information that is relevant for other conditions found in the general population. Therefore, more common diseases may profit from a correction of impaired lysosomal function.

Metabolic Pathway Genes Associated with Susceptibility Genes to Coronary Artery Disease

Coronary artery disease (CAD) is one of the leading threats to global health. Previous research has proven that metabolic pathway disorders, such as high blood lipids and diabetes, are one of the risk factors that mostly cause CAD. However, the crosstalk between metabolic pathways and CAD was mostly studied on physiology processes by analyzing a single gene function. A canonical correlation analysis was used to identify the metabolic pathways, which were integrated as a unit to coexpress with CAD susceptibility genes, and to resolve additional metabolic factors that are related to CAD. Seven pathways, including citrate cycle, ubiquinone, terpenoid quinone biosynthesis, and N-glycan biosynthesis, were identified as an integrated unit coexpressed with CAD genes. These pathways could not be revealed as a coexpressed pathway through traditional methods as each single gene has weak correlation. Furthermore, sets of genes in these pathways were candidate markers for diagnosis and detection from patients' serum.

ABCA7 Mediates Phagocytic Clearance of Amyloid-β in the Brain

Alzheimer's disease (AD) is a neurodegenerative disorder characterized by dementia and abnormal deposits of aggregated amyloid-β in the brain. Recent genome-wide association studies have revealed that ABCA7 is strongly associated with AD. In vitro evidence suggests that the role of ABCA7 is related to phagocytic activity. Deletion of ABCA7 in a mouse model of AD exacerbates cerebral amyloid-β plaque load. However, the biological role of ABCA7 in AD brain pathogenesis is unknown. We show that ABCA7 is highly expressed in microglia and when monocytes are differentiated into macrophages. We hypothesized that ABCA7 plays a protective role in the brain that is related to phagocytic clearance of amyloid-β. We isolated microglia and macrophages from Abca7-/- and wild type mice and tested them for their capacity to phagocytose amyloid-β oligomers. We found that the phagocytic clearance of amyloid-β was substantially reduced in both microglia and macrophages from Abca7-/- mice compared to wild type mice. Consistent with these results, in vivo phagocytic clearance of amyloid-β oligomers in the hippocampus was reduced in Abca7-/- mice. Furthermore, ABCA7 transcription was upregulated in AD brains and in amyloidogenic mouse brains specifically in the hippocampus as a response to the amyloid-β pathogenic state. Together these results indicate that ABCA7 mediates phagocytic clearance of amyloid-β in the brain, and reveal a mechanism by which loss of function of ABCA7 increases the susceptibility to AD.

Lipid profile in adult patients with Fabry disease - Ten-year follow up

BACKGROUND

Fabry disease, an X-linked genetic condition, results from alpha-galactosidase deficiency and increased accumulation of glycosphingolipids in cardiovascular tissues. Clinical manifestation includes vasculature associated complications. Hyperlipidaemia is one of the cardiovascular risk factors however it has never been well defined in Fabry disease. Enzyme Replacement Therapy (ERT) is available but its effect on serum cholesterol is unknown. The aim of this project was to assess the influence of long-term ERT on lipid profile in a large cohort of adult patients with Fabry disease.

METHODS

This was a retrospective analysis of lipid profile results. Patients with Fabry disease were on ERT for 10 years, were not treated with statins and had no severe renal impairment. All patients had lipid profile measured before ERT was commenced and 6, 12, 24, 36, 48, 60, 120 months later. Statistical analysis included ANOVA, Student t-test and descriptive statistics.

RESULTS

Among 72 patients, 40 were females (median age 45; range 29-75), 32 males (median age 46; range 20-69). There was no significant difference in total cholesterol or HDL-cholesterol measured at baseline before ERT was commenced and 6, 12, 24, 36, 48, 60 and 120 months after ERT was commenced in 72 patients (ANOVA; P = 0.673 and P = 0.883, respectively). Female patients on ERT had higher mean HDL-cholesterol as compared to female patients with Fabry disease who were asymptomatic and not treated (P ≥ 0.05). Total cholesterol between treated and non-treated female patients was comparable. Female patients on ERT have higher total cholesterol and HDL-cholesterol when compared to lipid results in male patients on ERT. Total cholesterol/HDL-cholesterol ratio was low in female and male patients on ERT over 10 years.

CONCLUSION

Adult patients with Fabry disease have remarkably elevated HDL-cholesterol and as a result, elevated total cholesterol. It is possible that elevated HDL-cholesterol has a cardioprotective effect in patients with this condition. Long term ERT does not have a significant impact on lipid profile in female and male population with Fabry disease.

Active membrane cholesterol as a physiological effector

Sterols associate preferentially with plasma membrane sphingolipids and saturated phospholipids to form stoichiometric complexes. Cholesterol in molar excess of the capacity of these polar bilayer lipids has a high accessibility and fugacity; we call this fraction active cholesterol. This review first considers how active cholesterol serves as an upstream regulator of cellular sterol homeostasis. The mechanism appears to utilize the redistribution of active cholesterol down its diffusional gradient to the endoplasmic reticulum and mitochondria, where it binds multiple effectors and directs their feedback activity. We have also reviewed a broad literature in search of a role for active cholesterol (as opposed to bulk cholesterol or lipid domains such as rafts) in the activity of diverse membrane proteins. Several systems provide such evidence, implicating, in particular, caveolin-1, various kinds of ABC-type cholesterol transporters, solute transporters, receptors and ion channels. We suggest that this larger role for active cholesterol warrants close attention and can be tested easily.

Inhibition of Rab prenylation by statins induces cellular glycosphingolipid remodeling

Statins, which specifically inhibit HMG Co-A reductase, the rate-limiting step of cholesterol biosynthesis, are widely prescribed to reduce serum cholesterol and cardiac risk, but many other effects are seen. We now show an effect of these drugs to induce profound changes in the step-wise synthesis of glycosphingolipids (GSLs) in the Golgi. Glucosylceramide (GlcCer) was increased several-fold in all cell lines tested, demonstrating a widespread effect. Additionally, de novo or elevated lactotriaosylceramide (Lc3Cer; GlcNAcβ1-3Galβ1-4GlcCer) synthesis was observed in 70%. Western blot showed that GlcCer synthase (GCS) was elevated by statins, and GCS and Lc3Cer synthase (Lc3S) activities were increased; however, transcript was elevated for Lc3S only. Supplementation with the isoprenoid precursor, geranylgeranyl pyrophosphate (GGPP), a downstream product of HMG Co-A reductase, reversed statin-induced glycosyltransferase and GSL elevation. The Rab geranylgeranyl transferase inhibitor 3-PEHPC, but not specific inhibitors of farnesyl transferase, or geranylgeranyl transferase I, was sufficient to replicate statin-induced GlcCer and Lc3Cer synthesis, supporting a Rab prenylation-dependent mechanism. While total cholesterol was unaffected, the trans-Golgi network (TGN) cholesterol pool was dissipated and medial Golgi GCS partially relocated by statins. GSL-dependent vesicular retrograde transport of Verotoxin and cholera toxin to the Golgi/endoplasmic reticulum were blocked after statin or 3-PEHPC treatment, suggesting aberrant, prenylation-dependent vesicular traffic as a basis of glycosyltransferase increase and GSL remodeling. These in vitro studies indicate a previously unreported link between Rab prenylation and regulation of GCS activity and GlcCer metabolism.

A Short Synthetic Peptide Mimetic of Apolipoprotein A1 Mediates Cholesterol and Globotriaosylceramide Efflux from Fabry Fibroblasts

Fabry disease is an X-linked sphingolipid storage disorder caused by a deficiency of the lysosomal enzyme α-galactosidase A (AGA, EC 3.2.1.22) resulting in the intracellular accumulation of globotriaosylceramide (Gb3). We found that Gb3 storage also correlates with accumulation of endosomal-lysosomal cholesterol in Fabry fibroblasts. This cholesterol accumulation may contribute to the phenotypic pathology of Fabry disease by slowing endosomal-lysosomal trafficking. We found that LDL receptor expression is not downregulated in Fabry fibroblasts resulting in accumulation of both cholesterol and Gb3. 5A-Palmitoyl oleoyl-phosphatidylcholine (5AP) is a phospholipid complex containing a short synthetic peptide that mimics apolipoprotein A1, the main protein component of high-density lipoprotein (HDL) that mediates the efflux of cholesterol from cells via the ATP-binding cassette transporter. We used 5AP and HDL to remove cholesterol from Fabry fibroblasts to examine the fate of accumulated cellular Gb3. Using immunostaining techniques, we found that 5AP is highly effective for depleting cholesterol and Gb3 in these cells. 5AP restores the ApoA-1-mediated cholesterol efflux leading to mobilization of cholesterol and reduction of Gb3 in Fabry fibroblasts.

Circulating Lipids Are Associated with Alcoholic Liver Cirrhosis and Represent Potential Biomarkers for Risk Assessment

Liver disease is the greatest cause of death related to alcohol and a major public health problem. While excessive alcohol intake results in hepatosteatosis in most individuals, this can progress in some to more severe forms of liver disease including fibrosis and cirrhosis. An ongoing challenge in the management of alcoholic liver disease is the identification of liver injury early in the disease process such that intervention strategies can prevent serious long term outcomes. Given that excessive alcohol consumption results in dysregulation of lipid metabolism we applied lipid profiling technology to characterise and compare serum lipid profiles from excessive chronic drinkers with no liver disease to those with advanced alcoholic cirrhosis. In a cohort of 59 excessive drinkers (31 with liver cirrhosis and 28 with no evidence of liver disease) we used electrospray ionisation tandem mass spectrometry to measure over 300 individual lipid species in serum, including species of the major phospholipid, sphingolipid, glycerolipid and sterol classes. Six of the 25 lipid classes and subclasses were significantly associated with alcoholic liver cirrhosis; these included dihexosylceramide, trihexosylceramide, alkylphosphatidylcholine, lysoalkylphosphatidylcholine, phosphatidylinositol and free cholesterol. Multivariate classification models created with only clinical characteristics gave an optimal model with an AUC of 0.847 and an accuracy of 79.7%. The addition of lipid measurements to the clinical characteristics resulted in models of improved performance with an AUC of 0.892 and accuracy of 81.8%. The gain in AUC and accuracy of the combined models highlight the potential of serum lipids as markers of liver injury in alcoholic liver disease.

Improved intervention of atherosclerosis and cardiac hypertrophy through biodegradable polymer-encapsulated delivery of glycosphingolipid inhibitor

D-Threo-1-phenyl-2-decanoylamino-3-morpholino-1-propanol (D-PDMP), a glycosphingolipid synthesis inhibitor, holds promise for the treatment of atherosclerosis and cardiac hypertrophy but rapid in vivo clearance has severely hindered translation to the clinic. To overcome this impediment, we used a materials-based delivery strategy wherein D-PDMP was encapsulated within a biodegradable polymer composed of poly ethylene glycol (PEG) and sebacic acid (SA). PEG-SA was formulated into nanoparticles that were doped with (125)I-labeled PEG to allow in vivo bio-distribution and release kinetics of D-PDMP to be determined by using γ-scintigraphy and subsequently, by mass spectrometry. Polymer-encapsulation increased the residence time of D-PDMP in the body of a treated mouse from less than one hour to at least four hours (and up to 48 h or longer). This substantially increased in vivo longevity provided by polymer encapsulation resulted in an order of magnitude gain in efficacy for interfering with atherosclerosis and cardiac hypertrophy in apoE-/- mice fed a high fat and high cholesterol (HFHC) diet. These results establish that D-PDMP encapsulated in a biodegradable polymer provides a superior mode of delivery compared to unconjugated D-PDMP by way of increased gastrointestinal absorption and increased residence time thus providing this otherwise rapidly cleared compound with therapeutic relevance in interfering with atherosclerosis, cardiac hypertrophy, and probably other diseases associated with the deleterious effects of abnormally high glycosphingolipid biosynthesis or deficient catabolism.

Identification of metabolic markers in coronary artery disease using an untargeted LC-MS based metabolomic approach

Coronary artery disease (CAD), a complex metabolic disorder, is one of the largest causes of death worldwide. Both environmental and genetic factors contribute to the etiology of this metabolic disease. The gene-environment interaction could lead to modulation of various metabolic pathways resulting in altered levels of various metabolites. Thus, identifying metabolites could aid in deciphering pathways that could be involved in the pathophysiology of the disease. With the advent of high resolution mass spectrometry based methodologies, it is now possible to screen thousands of metabolites in a single snapshot thus, allowing the identification of potential disease metabolite markers. In this work, using an untargeted metabolomic approach, we attempted to identify metabolites that have altered levels in CAD patients. Using reverse phase and HILIC based chromatography followed by mass spectrometry we identified a total of 32 metabolites (2 fold; p<0.05) in plasma whose levels were significantly altered in CAD samples. Further, we have validated the discriminative ability of these metabolites in an independent set of CAD and control samples using multivariate PLS-DA analysis. Interestingly, Lyso PC (18:0), Cortisol, Lyso PC (P-17:0), and glycerophosphocholine were among the top discriminators for CAD which implies involvement of phosphatidylcholine pathway in the pathogenesis of atherosclerosis.

BIOLOGICAL SIGNIFICANCE

Herein, we report that an unbiased metabolomic study has the potential to identify newer markers which are involved in several important biological pathways like lipid metabolism, phosphatidylcholine pathway etc. which in turn are implicated in CAD. These markers could be of potential clinical importance for screening subjects at risk of CAD. This article is part of a Special Issue entitled: Proteomics in India.

Lysosomal storage diseases and the heat shock response: convergences and therapeutic opportunities

Lysosomes play a vital role in the maintenance of cellular homeostasis through the recycling of cell constituents, a key metabolic function which is highly dependent on the correct function of the lysosomal hydrolases and membrane proteins, as well as correct membrane lipid stoichiometry and composition. The critical role of lysosomal functionality is evident from the severity of the diseases in which the primary lesion is a genetically defined loss-of-function of lysosomal hydrolases or membrane proteins. This group of diseases, known as lysosomal storage diseases (LSDs), number more than 50 and are associated with severe neurodegeneration, systemic disease, and early death, with only a handful of the diseases having a therapeutic option. Another key homeostatic system is the metabolic stress response or heat shock response (HSR), which is induced in response to a number of physiological and pathological stresses, such as protein misfolding and aggregation, endoplasmic reticulum stress, oxidative stress, nutrient deprivation, elevated temperature, viral infections, and various acute traumas. Importantly, the HSR and its cardinal members of the heat shock protein 70 family has been shown to protect against a number of degenerative diseases, including severe diseases of the nervous system. The cytoprotective actions of the HSR also include processes involving the lysosomal system, such as cell death, autophagy, and protection against lysosomal membrane permeabilization, and have shown promise in a number of LSDs. This review seeks to describe the emerging understanding of the interplay between these two essential metabolic systems, the lysosomes and the HSR, with a particular focus on their potential as a therapeutic target for LSDs.

Inhibition of glycosphingolipid synthesis ameliorates atherosclerosis and arterial stiffness in apolipoprotein E-/- mice and rabbits fed a high-fat and -cholesterol diet

BACKGROUND

Glycosphingolipids, integral components of the cell membrane, have been shown to serve as messengers, transducing growth factor-initiated phenotypes. Here, we have examined whether inhibition of glycosphingolipid synthesis could ameliorate atherosclerosis and arterial stiffness in transgenic mice and rabbits.

METHODS AND RESULTS

Apolipoprotein E(-/-) mice (12 weeks of age; n=6) were fed regular chow or a Western diet (1.25% cholesterol, 2% fat). Mice were fed 5 or 10 mg/kg of an inhibitor of glycosphingolipid synthesis, D-threo-1-phenyl-2-decanoylamino-3-morpholino-1-propanol (D-PDMP), solubilized in vehicle (5% Tween-80 in PBS); the placebo group received vehicle only. At 20 and 36 weeks of age, serial echocardiography was performed to measure aortic intima-media thickening. Aortic pulse-wave velocity measured vascular stiffness. Feeding mice a Western diet markedly increased aortic pulse-wave velocity, intima-media thickening, oxidized low-density lipoprotein, Ca(2+) deposits, and glucosylceramide and lactosylceramide synthase activity. These were dose-dependently decreased by feeding D-PDMP. In liver, D-PDMP decreased cholesterol and triglyceride levels by raising the expression of SREBP2, low-density lipoprotein receptor, HMGCo-A reductase, and the cholesterol efflux genes (eg, ABCG5, ABCG8). D-PDMP affected very-low-density lipoprotein catabolism by increasing the gene expression for lipoprotein lipase and very-low-density lipoprotein receptor. Rabbits fed a Western diet for 90 days had extensive atherosclerosis accompanied by a 17.5-fold increase in total cholesterol levels and a 3-fold increase in lactosylceramide levels. This was completely prevented by feeding D-PDMP.

CONCLUSIONS

Inhibition of glycosphingolipid synthesis ameliorates atherosclerosis and arterial stiffness in apolipoprotein E(-/-) mice and rabbits. Thus, inhibition of glycosphingolipid synthesis may be a novel approach to ameliorate atherosclerosis and arterial stiffness.

Normalizing glycosphingolipids restores function in CD4+ T cells from lupus patients

Patients with the autoimmune rheumatic disease systemic lupus erythematosus (SLE) have multiple defects in lymphocyte signaling and function that contribute to disease pathogenesis. Such defects could be attributed to alterations in metabolic processes, including abnormal control of lipid biosynthesis pathways. Here, we reveal that CD4+ T cells from SLE patients displayed an altered profile of lipid raft-associated glycosphingolipids (GSLs) compared with that of healthy controls. In particular, lactosylceramide, globotriaosylceramide (Gb3), and monosialotetrahexosylganglioside (GM1) levels were markedly increased. Elevated GSLs in SLE patients were associated with increased expression of liver X receptor β (LXRβ), a nuclear receptor that controls cellular lipid metabolism and trafficking and influences acquired immune responses. Stimulation of CD4+ T cells isolated from healthy donors with synthetic and endogenous LXR agonists promoted GSL expression, which was blocked by an LXR antagonist. Increased GSL expression in CD4+ T cells was associated with intracellular accumulation and accelerated trafficking of GSL, reminiscent of cells from patients with glycolipid storage diseases. Inhibition of GSL biosynthesis in vitro with a clinically approved inhibitor (N-butyldeoxynojirimycin) normalized GSL metabolism, corrected CD4+ T cell signaling and functional defects, and decreased anti-dsDNA antibody production by autologous B cells in SLE patients. Our data demonstrate that lipid metabolism defects contribute to SLE pathogenesis and suggest that targeting GSL biosynthesis restores T cell function in SLE.

Elevation in sphingomyelin synthase activity is associated with increases in amyloid-beta peptide generation

A pathological hallmark of Alzheimer’s disease (AD) is the presence of amyloid-beta peptide (Aβ) plaques in the brain. Aβ is derived from a sequential proteolysis of the transmenbrane amyloid precursor protein (APP), a process which is dependent on the distribution of lipids present in the plasma membrane. Sphingomyelin is a major membrane lipid, however its role in APP processing is unclear. Here, we assessed the expression of sphingomyelin synthase (SGMS1; the gene responsible for sphingomyelin synthesis) in human brain and found that it was significantly elevated in the hippocampus of AD brains, but not in the cerebellum. Secondly, we assessed the impact of altering SGMS activity on Aβ generation. Inhibition of SGMS activity significantly reduced the level of Aβ in a dose- and time dependent manner. The decrease in Aβ level occurred without changes in APP expression or cell viability. These results when put together indicate that SGMS activity impacts on APP processing to produce Aβ and it could be a contributing factor in Aβ pathology associated with AD.

A multilevel screening strategy defines a molecular fingerprint of proregenerative olfactory ensheathing cells and identifies SCARB2, a protein that improves regenerative sprouting of injured sensory spinal axons

Olfactory ensheathing cells (OECs) have neuro-restorative properties in animal models for spinal cord injury, stroke, and amyotrophic lateral sclerosis. Here we used a multistep screening approach to discover genes specifically contributing to the regeneration-promoting properties of OECs. Microarray screening of the injured olfactory pathway and of cultured OECs identified 102 genes that were subsequently functionally characterized in cocultures of OECs and primary dorsal root ganglion (DRG) neurons. Selective siRNA-mediated knockdown of 16 genes in OECs (ADAMTS1, BM385941, FZD1, GFRA1, LEPRE1, NCAM1, NID2, NRP1, MSLN, RND1, S100A9, SCARB2, SERPINI1, SERPINF1, TGFB2, and VAV1) significantly reduced outgrowth of cocultured DRG neurons, indicating that endogenous expression of these genes in OECs supports neurite extension of DRG neurons. In a gain-of-function screen for 18 genes, six (CX3CL1, FZD1, LEPRE1, S100A9, SCARB2, and SERPINI1) enhanced and one (TIMP2) inhibited neurite growth. The most potent hit in both the loss- and gain-of-function screens was SCARB2, a protein that promotes cholesterol secretion. Transplants of fibroblasts that were genetically modified to overexpress SCARB2 significantly increased the number of regenerating DRG axons that grew toward the center of a spinal cord lesion in rats. We conclude that expression of SCARB2 enhances regenerative sprouting and that SCARB2 contributes to OEC-mediated neuronal repair.

Use of a glycolipid inhibitor to ameliorate renal cancer in a mouse model

In a xenograft model wherein, live renal cancer cells were implanted under the kidney capsule in mice, revealed a 30-fold increase in tumor volume over a period of 26 days and this was accompanied with a 32-fold increase in the level of lactosylceramide (LacCer). Mice fed D- threo-1-phenyl-2-decanoylamino-3-morpholino-1-propanol (D-PDMP), an inhibitor of glucosylceramide synthase and lactosylceramide synthase (LCS: β-1,4-GalT-V), showed marked reduction in tumor volume. This was accompanied by a decrease in the mass of lactosylceramide and an increase in glucosylceramide (GlcCer) level. Mechanistic studies revealed that D-PDMP inhibited cell proliferation and angiogenesis by inhibiting p44MAPK, p-AKT-1 pathway and mammalian target for rapamycin (mTOR). By linking glycosphingolipid synthesis with tumor growth, renal cancer progression and regression can be evaluated. Thus inhibiting glycosphingolipid synthesis can be a bonafide target to prevent the progression of other types of cancer.

The role of sphingolipids in drug metabolism and transport

INTRODUCTION

Sphingolipids represent a diverse class of lipid molecules. In addition to their function as membrane structural components, they serve as signaling molecules involved in various biological processes such as cell metabolism, growth, differentiation, stress and inflammatory responses and apoptosis. Sphingolipids may modulate the activity and/or expression of cytochrome P450s (CYPs) and transporters, which suggests that they may affect drug metabolism and excretion.

AREAS COVERED

In this review, the authors provide an overview of the properties of sphingolipid structures and metabolism. They also describe the effects of sphingolipids on the activity and expression of CYPs and transporters. In addition, the authors discuss the pathologic conditions where the sphingolipid metabolism is dysregulated particularly in association with inflammation and cancer.

EXPERT OPINION

Sphingolipidomic approaches have become accessible with the aid of advances in analytical technology. Sphingolipid profiles are modified by diseases, genetic disorders or certain drug treatment. The consequent changes in sphingolipid contents may alter the activities of detoxifying enzymes and those associated with cell viability. Since CYPs and transporters play roles in xenobiotics metabolism and excretion, sphingolipidomic information may be of use in understanding drug effect and toxicity.

Activation of ganglioside GM3 biosynthesis in human blood mononuclear cells in atherosclerosis

Using blood monocytes and lymphocytes from atherosclerotic patients and healthy subjects we have investigated activity of GM3 synthase, cellular levels of ganglioside GM3 and its role in monocyte adhesion to cultured human umbilical vein endothelial cells (HUVEC). The results showed that activity of GM3 synthase and cellular levels of ganglioside GM3 in blood mononuclear cells from atherosclerotic patients were several-fold higher than those from healthy subjects. In monocytes the activity of GM3 synthase was one an order of magnitude higher than in lymphocytes from both groups studied; this suggests the major contribution of monocytes to enhanced biosynthesis and levels of GM3 in mononuclear cells in atherosclerosis. Enrichment of monocytes from healthy subjects with ganglioside GM3 by incubation in medium containing this ganglioside increased adherence of these monocytes to HUVEC up to the values typical for monocytes from atherosclerotic patients. In addition, an increase in CD11b integrin expression was observed that was comparable to that seen in lipopolysaccharide-activated monocytes. It is suggested that in atherosclerosis the enhanced cellular levels of GM3 in monocytes and lymphocytes may be an important element of cell activation that facilitates their adhesion to endothelial cells and penetration into intima.

Increased dimerization of alpha-synuclein in erythrocytes in Gaucher disease and aging

Gaucher disease (GD) patients and carriers of glucocerebrosidase mutations are at an increased risk for Parkinson's disease (PD). The presynaptic protein alpha-synuclein (AS) is linked to PD. In the current work we examined biochemical properties of AS in GD patients. We generated membrane-enriched lysates from erythrocytes of 27 patients with GD and 32 age- and sex-matched controls and performed Western immunoblotting with antibodies against AS. Levels of monomeric AS did not differ between GD patients and controls and did not change as a function of age. However, the ratio of dimeric to monomeric AS was significantly increased in GD patients, and showed a significant positive correlation with age. Therefore, two major risk factors for PD, aging and GD status, are associated with an increased AS dimer to monomer ratio in erythrocytes. This ratio needs to be validated in further studies as a potential biomarker for PD risk.

Globosides but not isoglobosides can impact the development of invariant NKT cells and their interaction with dendritic cells

Recognition of endogenous lipid Ag(s) on CD1d is required for the development of invariant NKT (iNKT) cells. Isoglobotrihexosylceramide (iGb3) has been implicated as this endogenous selecting ligand and recently suggested to control overstimulation and deletion of iNKT cells in α-galactosidase A-deficient (αGalA(-/-)) mice (human Fabry disease), which accumulate isoglobosides and globosides. However, the presence and function of iGb3 in murine thymus remained controversial. In this study, we generate a globotrihexosylceramide (Gb3)-synthase-deficient (Gb3S(-/-)) mouse and show that in thymi of αGalA(-/-)/Gb3S(-/-) double-knockout mice, which store isoglobosides but no globosides, minute amounts of iGb3 can be detected by HPLC. Furthermore, we demonstrate that iGb3 deficiency does not only fail to impact selection of iNKT cells, in terms of frequency and absolute numbers, but also does not alter the distribution of the TCR CDR 3 of iNKT cells. Analyzing multiple gene-targeted mouse strains, we demonstrate that globoside, rather than iGb3, storage is the major cause for reduced iNKT cell frequencies and defective Ag presentation in αGalA(-/-) mice. Finally, we show that correction of globoside storage in αGalA(-/-) mice by crossing them with Gb3S(-/-) normalizes iNKT cell frequencies and dendritic cell (DC) function. We conclude that, although detectable in murine thymus in αGalA(-/-)/Gb3S(-/-) mice, iGb3 does not influence either the development of iNKT cells or their interaction with peripheral DCs. Moreover, in αGalA(-/-) mice, it is the Gb3 storage that is responsible for the decreased iNKT cell numbers and impeded Ag presentation on DCs.

Dissociation of ERK signalling inhibition from the anti-amyloidogenic action of synthetic ceramide analogues

Inhibition of GSL (glycosphingolipid) synthesis reduces Aβ (amyloid β-peptide) production in vitro. Previous studies indicate that GCS (glucosylceramide synthase) inhibitors modulate phosphorylation of ERK1/2 (extracellular-signal-regulated kinase 1/2) and that the ERK pathway may regulate some aspects of Aβ production. It is not clear whether there is a causative relationship linking GSL synthesis inhibition, ERK phosphorylation and Aβ production. In the present study, we treated CHO cells (Chinese-hamster ovary cells) and SH-SY5Y neuroblastoma cells, that both constitutively express human wild-type APP (amyloid precursor protein) and process this to produce Aβ, with GSL-modulating agents to explore this relationship. We found that three related ceramide analogue GSL inhibitors, based on the PDMP (D-threo-1-phenyl-2-decanoylamino-3-morpholino-1-propanol) structure, reduced cellular Aβ production and in all cases this was correlated with inhibition of pERK (phosphorylated ERK) formation. Importantly, the L-threo enantiomers of these compounds (that are inferior GSL synthesis inhibitors compared with the D-threo-enantiomers) also reduced ERK phosphorylation to a similar extent without altering Aβ production. Inhibition of ERK activation using either PD98059 [2-(2-amino-3-methoxyphenyl)-4H-1-benzopyran-4-one] or U0126 (1,4-diamino-2,3-dicyano-1,4-bis[2-aminophenylthio] butadiene) had no impact on Aβ production, and knockdown of endogenous GCS using small interfering RNA reduced cellular GSL levels without suppressing Aβ production or pERK formation. Our data suggest that the alteration in pERK levels following treatment with these ceramide analogues is not the principal mechanism involved in the inhibition of Aβ generation and that the ERK signalling pathway does not play a crucial role in processing APP through the amyloidogenic pathway.

Filipin recognizes both GM1 and cholesterol in GM1 gangliosidosis mouse brain

Filipin is an antibiotic polyene widely used as a histochemical marker for cholesterol. We previously reported cholesterol/filipin-positive staining in brain of β-galactosidase (β-gal) knockout ((-/-)) mice (GM1 gangliosidosis). The content and distribution of cholesterol and gangliosides was analyzed in plasma membrane (PM) and microsomal (MS) fractions from whole-brain tissue of 15 week-old control (β-gal(+/-)) and GM1 gangliosidosis (β-gal(-/-)) mice. Total ganglioside content (μg sialic acid/mg protein) was 3-fold and 7-fold greater in the PM and MS fractions, respectively, in βgal(-/-) mice than in βgal(+/-) mice. GM1 content was 30-fold and 50-fold greater in the PM and MS fractions, respectively. In contrast, unesterified cholesterol content (μg/mg protein) was similar in the PM and the MS fractions of the βgal(-/-) and βgal(+/-) mice. Filipin is known to bind to various sterol derivatives and phospholipids on thin-layer chromatograms. Biochemical evidence is presented showing that filipin also binds to GM1 with an affinity similar to that for cholesterol, with a corresponding fluorescent reaction. Our data suggest that the GM1 storage seen in the β-gal(-/-) mouse contributes to the filipin ultraviolet fluorescence observed in GM1 gangliosidosis brain. The data indicate that in addition to cholesterol, filipin can also be useful for detecting GM1.

Caseation of human tuberculosis granulomas correlates with elevated host lipid metabolism

The progression of human tuberculosis (TB) to active disease and transmission involves the development of a caseous granuloma that cavitates and releases infectious Mycobacterium tuberculosis bacilli. In the current study, we exploited genome-wide microarray analysis to determine that genes for lipid sequestration and metabolism were highly expressed in caseous TB granulomas. Immunohistological analysis of these granulomas confirmed the disproportionate abundance of the proteins involved in lipid metabolism in cells surrounding the caseum; namely, adipophilin, acyl-CoA synthetase long-chain family member 1 and saposin C. Biochemical analysis of the lipid species within the caseum identified cholesterol, cholesteryl esters, triacylglycerols and lactosylceramide, which implicated low-density lipoprotein-derived lipids as the most likely source. M. tuberculosis infection in vitro induced lipid droplet formation in murine and human macrophages. Furthermore, the M. tuberculosis cell wall lipid, trehalose dimycolate, induced a strong granulomatous response in mice, which was accompanied by foam cell formation. These results provide molecular and biochemical evidence that the development of the human TB granuloma to caseation correlates with pathogen-mediated dysregulation of host lipid metabolism.

Hepatic glycosphingolipid deficiency and liver function in mice

Recent studies have reported that glycosphingolipids (GSLs) might be involved in obesity-induced insulin resistance. Those reports suggested that inhibition of GSL biosynthesis in animals ameliorated insulin resistance accompanied by improved glycemic control and decreased liver steatosis in obese mice. In addition, pharmacologic GSL depletion altered hepatic secretory function. In those studies, ubiquitously acting inhibitors for GSL biosynthesis have been used to inhibit the enzyme Ugcg (UDP-glucose:ceramide glucosyltransferase), catalyzing the first step of the glucosylceramide-based GSL-synthesis pathway. In the present study a genetic approach for selective GSL deletion in hepatocytes was chosen to achieve complete inhibition of GSL synthesis and to avoid possible adverse effects caused by Ugcg inhibitors. Using the Cre/loxP system under control of the albumin promoter, GSL biosynthesis in hepatocytes and their release into the plasma could be effectively blocked. Deletion of GSL in hepatocytes did not change the quantity of bile excretion through the biliary duct. Total bile salt content in bile, feces, and plasma from mutant mice showed no difference as compared to control animals. Cholesterol concentration in liver, bile, feces, and plasma samples remained unaffected. Lipoprotein concentrations in plasma samples in mutant animals reached similar levels as in their control littermates. No alteration in glucose tolerance after intraperitoneal application of glucose and insulin appeared in mutant animals. A preventive effect of GSL deficiency on development of liver steatosis after a high-fat diet was not observed.

CONCLUSION

The data suggest that GSL in hepatocytes are not essential for sterol, glucose, or lipoprotein metabolism and do not prevent high-fat diet-induced liver steatosis, indicating that Ugcg inhibitors exert their effect on hepatocytes either independently of GSL or mediated by other (liver) cell types.

Sterols and sphingolipids: dynamic duo or partners in crime?

One manner in which eukaryotic cells respond to their environments is by optimizing the composition and proportions of sterols and sphingolipids in membranes. The physical association of the planar ring of sterols with the acyl chains of phospholipids, particularly sphingolipids, produces membrane micro-heterogeneity that is exploited to coordinate several crucial pathways. We hypothesize that these lipid molecules play an integrated role in human disease; when one of the partners is mis-regulated, pathology frequently ensues. Sterols and sphingolipid levels are not coordinated by the action of a single master regulator, however the cross-talk between their metabolic pathways is considerable. We describe our perspectives on the key components of synthesis, catabolism and transport of these lipid partners with an emphasis on evolutionarily conserved reactions that produce disease states when defective.

Myriocin-mediated up-regulation of hepatocyte apoA-I synthesis is associated with ERK inhibition

Sphingolipids including sphingomyelin have been implicated as potential atherogenic lipids. Studies in apoE (apolipoprotein E)-null mice have revealed that the serine palmitoyltransferase inhibitor myriocin reduces plasma levels of sphingomyelin, ceramide, sphingosine-1-phosphate and glycosphingolipids and that this is associated with potent inhibition of atherosclerosis. Interestingly, hepatic apoA-I (apolipoprotein A-I) synthesis and plasma HDL (high-density lipoprotein)-cholesterol levels were also increased in apoE-null mice treated with myriocin. Since myriocin is a known inhibitor of ERK (extracellular-signal-related kinase) phosphorylation, we assessed the possibility that myriocin may be acting to increase hepatic apoA-I production via this pathway. To address this, HepG2 cells and primary mouse hepatocytes were treated with 200 muM myriocin for up to 48 h. Myriocin increased apoA-I mRNA and protein levels by approx. 3- and 2-fold respectively. Myriocin also increased apoA-I secretion up to 3.5-fold and decreased ERK phosphorylation by approx. 70%. Similar findings were obtained when primary hepatocytes were isolated from apoE-null mice that were treated with myriocin (intraperitoneal injection at a dose of 0.3 mg/kg body weight). Further experiments revealed that the MEK (mitogen-activated protein kinase/ERK kinase) inhibitor PD98059 potently inhibited ERK phosphorylation, as expected, and increased primary hepatocyte apoA-I production by 3-fold. These results indicate that ERK phosphorylation plays a role in regulating hepatic apoA-I expression and suggest that the anti-atherogenic mechanism of action for myriocin may be linked to this pathway.

Inhibition of glycosphingolipid synthesis induces a profound reduction of plasma cholesterol and inhibits atherosclerosis development in APOE*3 Leiden and low-density lipoprotein receptor-/- mice

OBJECTIVE

The iminosugar N-(5'-adamantane-1'-yl-methoxy)-pentyl-1-deoxynoijirimycin (AMP-DNM), an inhibitor of the enzyme glucosylceramide synthase catalyzing glycosphingolipid (GSL) biosynthesis, ameliorates diabetes and reduces liver steatosis in ob/ob mice. Because an accumulation of sphingolipids, including sphingomyelin and GSLs, has been reported in atherosclerotic lesions in animal models and in humans, the objective of this study was to determine whether AMP-DNM also exerts beneficial effects on the development of atherosclerosis.

METHODS AND RESULTS

APOE*3 Leiden mice, maintained on a high-cholesterol diet, were treated for up to 18 weeks with AMP-DNM. The iminosugar prevented hyperlipidemia, generated a less atherogenic lipid profile, and induced a dramatic reduction in the development of atherosclerotic lesions. At the highest dose, no lesions were detectable. The effect of AMP-DNM was associated with a decrease in liver cholesterol, an increase in bile secretion, and enhanced excretion of cholesterol in the feces. Similar effects of AMP-DNM were observed in mice deficient for the low-density lipoprotein receptor.

CONCLUSION

By lowering plasma cholesterol, the iminosugar AMP-DNM dramatically reduces the development of atherosclerosis in APOE*3 Leiden and low-density lipoprotein receptor -/- mice. Thus, targeting GSL synthesis may be a new treatment modality to prevent cardiovascular disease.

Transport of lipids by ABC proteins: interactions and implications for cellular toxicity, viability and function

Members of the ATP-binding cassette (ABC) family of membrane-bound transporters are involved in multiple aspects of transport and redistribution of various lipids and their conjugates. Most ABC transporters localize to the plasma membrane; some are associated with liquid-ordered cholesterol-/sphingolipid-rich microdomains, and to a lesser extent the membranes of the Golgi and endoplasmic reticulum. Hence, ABC transporters are well placed to regulate plasma membrane lipid composition and the efflux and redistribution of structural phospholipids and sphingolipids during periods of cellular stress and recovery. ABC transporters can also modulate cellular sensitivity to extrinsic pro-apoptotic signals through regulation of sphingomyelin-ceramide biosynthesis and metabolism. The functionality of ABC transporters is, in turn, modulated by the lipid content of the microdomains in which they reside. Cholesterol, a major membrane microdomain component, is not only a substrate of several ABC transporters, but also regulates ABC activity through its effects on microdomain structure. Several important bioactive lipid mediators and toxic lipid metabolites are also effluxed by ABC transporters. In this review, the complex interactions between ABC transporters and their lipid/sterol substrates will be discussed and analyzed in the context of their relevance to cellular function, toxicity and apoptosis.

Reduction of glycosphingolipid biosynthesis stimulates biliary lipid secretion in mice

Recent reports indicate that glycosphingolipids play an important role in regulation of carbohydrate metabolism. We have shown that the iminosugar N-(5'-adamantane-1'-yl-methoxy)-pentyl-1-deoxynojirimycin (AMP-DNM), an inhibitor of the enzyme glucosylceramide synthase, is a potent enhancer of insulin signaling in rodent models for insulin resistance and type 2 diabetes. In this study, we determined whether AMP-DNM also affects lipid homeostasis and, in particular, the reverse cholesterol transport pathway. Treatment of C57BL/6J mice with AMP-DNM for 5 weeks decreased plasma levels of triglycerides and cholesterol by 35%, whereas neutral sterol excretion increased twofold. Secretion of biliary lipid also increased twofold, which resulted in a similar rise in bile flow. This effect was not due to altered expression levels or kinetics of the various export pumps involved in bile formation. However, the bile salt pool size increased and the expression of Cyp7A1 was up-regulated. In vitro experiments using HepG2 hepatoma cell line revealed this to be due to inhibition of fibroblast growth factor-19 (FGF19)-mediated suppression of Cyp7A1 via the FGF receptor.

CONCLUSION

Pharmacological modulation of glycosphingolipid metabolism showed surprising effects on lipid homeostasis in C57BL/6J mice. Upon administration of 100 mg AMP-DNM/kg body weight/day, plasma cholesterol and triglyceride levels decreased, biliary lipid secretion doubled and also the endpoint of reverse cholesterol transport, neutral sterol excretion, doubled.

The glucosylceramide synthase inhibitor N-(5-adamantane-1-yl-methoxy-pentyl)-deoxynojirimycin induces sterol regulatory element-binding protein-regulated gene expression and cholesterol synthesis in HepG2 cells

Recent findings have implicated glycosphingolipids as modulators of insulin receptor activity. Studies with C57BL/6J ob/ob mice have shown that insulin sensitivity is enhanced by the synthetic hydrophobic iminosugar N-(5-adamantane-1-yl-methoxy-pentyl)-deoxynojirimycin (AMP-DNM) that inhibits glucosylceramide synthase. Here, we treated the liver hepatoma cell line HepG2 with AMP-DNM, resulting in a 70% reduction of glycosphingolipids, and we analyzed the effect on gene expression. Using whole human genome 44K oligonucleotide arrays, we identified 89 genes that were significantly (p < 0.01) up- or down-regulated by AMP-DNM treatment. Of the 56 up-regulated genes, 17 were direct target genes for transcription factors sterol regulatory element-binding protein (SREBP) 1 or SREBP2, which activate genes in the sterol biosynthesis pathway. An increase in cholesterol production rate confirmed that the induction of SREBP target genes seen at the mRNA level resulted in activation of the cholesterol biosynthesis pathway. It is interesting to note that the cholesterol content of the cells did not increase. It is noteworthy that no effects were found on expression of genes related to cell receptor signaling pathways, neither on toxicity nor cell growth. Our findings indicate that inhibition of glucosylceramide synthase with AMP-DNM leads to activation of SREBP target genes and synthesis of cholesterol in HepG2 cells.

Reduction of plasma glycosphingolipid levels has no impact on atherosclerosis in apolipoprotein E-null mice

Glycosphingolipids (GSLs) have been implicated as potential atherogenic lipids. Studies in apolipoprotein E-null (apoE(-/-)) mice indicate that exacerbated tissue GSL accumulation resulting from alpha-galactosidase deficiency promotes atherosclerosis, whereas the serine palmitoyl transferase inhibitor myriocin (which reduces plasma and tissue levels of several sphingolipids, including sphingomyelin, ceramide, sphingosine-1-phosphate, and GSLs) inhibits atherosclerosis. It is not clear whether GSL synthesis inhibition per se has an impact on atherosclerosis. To address this issue, apoE(-/-) mice maintained on a high-fat diet were treated with a potent glucosylceramide synthesis inhibitor, d-threo-1-ethylendioxyphenyl-2-palmitoylamino-3-pyrrolidino-propanol (EtDO-P4), 10 mg/kg/day for 94 days, and lesion development was compared in mice that were treated with vehicle only. EtDO-P4 reduced plasma GSL concentration by approximately 50% but did not affect cholesterol or triglyceride levels. Assessment of atherosclerotic lesions at four different sites indicated that EtDO-P4 had no significant impact on lesion area. Thus, despite the previously observed positive correlations between plasma and aortic GSL concentrations and the development of atherosclerosis, and the in vitro evidence implying that GSLs may be pro-atherogenic, our current data indicate that inhibition of GSL synthesis does not inhibit atherosclerosis in vivo.

GM2/GD2 and GM3 gangliosides have no effect on cellular cholesterol pools or turnover in normal or NPC1 mice

These studies investigated the role of gangliosides in governing the steady-state concentration and turnover of unesterified cholesterol in normal tissues and in those of mice carrying the NPC1 mutation. In animals lacking either GM2/GD2 or GM3 synthase, tissue cholesterol concentrations and synthesis rates were normal in nearly all organs, and whole-animal sterol pools and turnover also were not different from control animals. Mice lacking both synthases, however, had small elevations in cholesterol concentrations in several organs, and the whole-animal cholesterol pool was marginally elevated. None of these three groups, however, had changes in any parameter of cholesterol homeostasis in the major regions of the central nervous system. When either the GM2/GD2 or GM3 synthase activity was deleted in mice lacking NPC1 function, the clinical phenotype was not changed, but lifespan was shortened. However, the abnormal cholesterol accumulation seen in the tissues of the NPC1 mouse was unaffected by loss of either synthase, and clinical and molecular markers of hepatic and cerebellar disease also were unchanged. These studies demonstrate that hydrophobic interactions between cholesterol and various gangliosides do not play an important role in determining cellular cholesterol concentrations in the normal animal or in the mouse with the NPC1 mutation.

ATP-binding cassette transporter A7 regulates processing of amyloid precursor protein in vitro

ATP-binding cassette transporter A7 (ABCA7) is expressed in the brain and, like its closest homolog ABCA1, belongs to the ABCA subfamily of full-length ABC transporters. ABCA1 promotes cellular cholesterol efflux to lipid-free apolipoprotein acceptors and also inhibits the production of neurotoxic beta-amyloid (Abeta) peptides in vitro. The potential functions of ABCA7 in the brain are unknown. This study investigated the ability of ABCA7 to regulate cholesterol efflux to extracellular apolipoprotein acceptors and to modulate Abeta production. The transient expression of ABCA7 in human embryonic kidney cells significantly stimulated cholesterol efflux (fourfold) to apolipoprotein E (apoE) discoidal lipid complexes but not to lipid-free apoE or apoA-I. ABCA7 also significantly inhibited Abeta secretion from Chinese hamster ovary cells stably expressing human amyloid precursor protein (APP) or APP containing the Swedish K670M671-->N670L671 mutations when compared with mock-transfected cells. Studies with fluorogenic substrates indicated that ABCA7 had no impact on alpha-, beta-, or gamma-secretase activities. Live cell imaging of Chinese hamster ovary cells expressing APP-GFP indicated an apparent retention of APP in a perinuclear location in ABCA7 co-transfected cells. These studies indicate that ABCA7 has the capacity to stimulate cellular cholesterol efflux to apoE discs and regulate APP processing resulting in an inhibition of Abeta production.