Increased hepatic insulin action in diet-induced obese mice following inhibition of glucosylceramide synthase

Targeting UGCG Overcomes Resistance to Lysosomal Autophagy Inhibition

Lysosomal autophagy inhibition (LAI) with hydroxychloroquine or DC661 can enhance cancer therapy, but tumor regrowth is common. To elucidate LAI resistance, proteomics and immunoblotting demonstrated that LAI induced lipid metabolism enzymes in multiple cancer cell lines. Lipidomics showed that LAI increased cholesterol, sphingolipids, and glycosphingolipids. These changes were associated with striking levels of GM1+ membrane microdomains (GMM) in plasma membranes and lysosomes. Inhibition of cholesterol/sphingolipid metabolism proteins enhanced LAI cytotoxicity. Targeting UDP-glucose ceramide glucosyltransferase (UGCG) synergistically augmented LAI cytotoxicity. Although UGCG inhibition decreased LAI-induced GMM and augmented cell death, UGCG overexpression led to LAI resistance. Melanoma patients with high UGCG expression had significantly shorter disease-specific survival. The FDA-approved UGCG inhibitor eliglustat combined with LAI significantly inhibited tumor growth and improved survival in syngeneic tumors and a therapy-resistant patient-derived xenograft. These findings nominate UGCG as a new cancer target, and clinical trials testing UGCG inhibition in combination with LAI are warranted.

SIGNIFICANCE

We discovered UGCG-dependent lipid remodeling drives resistance to LAI. Targeting UGCG with a drug approved for a lysosomal storage disorder enhanced LAI antitumor activity without toxicity. LAI and UGCG inhibition could be tested clinically in multiple cancers. This article is highlighted in the In This Issue feature, p. 247.

Inhibitors of Glucosylceramide Synthase

Glucosylceramide synthase can be targeted by high affinity small molecular weight inhibitors for the study of glycosphingolipid metabolism and function or for the treatment of glycosphingolipid storage disorders, including Gaucher and Fabry disease. This work is exemplified by the discovery and development of eliglustat tartrate, the first stand-alone small chemical entity approved for the treatment of Gaucher disease type 1. The development of inhibitors of glucosylceramide synthase that have utility for either research or clinical purposes begins with a testing funnel for screening candidate inhibitors for activity against this enzyme and for activity in lowering the content of glucosylceramide in intact cells. Two common assays for glucosylceramide synthase, one enzyme based and another cell based, are the focus of this chapter.

Eliglustat prevents Shiga toxin 2 cytotoxic effects in human renal tubular epithelial cells

BACKGROUND

Shiga toxin-producing Escherichia coli is responsible for post-diarrheal (D+) hemolytic uremic syndrome (HUS), which is a cause of acute renal failure in children. The glycolipid globotriaosylceramide (Gb3) is the main receptor for Shiga toxin (Stx) in kidney target cells. Eliglustat (EG) is a specific and potent inhibitor of glucosylceramide synthase, first step of glycosphingolipid biosynthesis, actually used for the treatment of Gaucher's disease. The aim of the present work was to evaluate the efficiency of EG in preventing the damage caused by Stx2 in human renal epithelial cells.

METHODS

Human renal tubular epithelial cell (HRTEC) primary cultures were pre-treated with different dilutions of EG followed by co-incubation with EG and Stx2 at different times, and cell viability, proliferation, apoptosis, tubulogenesis, and Gb3 expression were assessed.

RESULTS

In HRTEC, pre-treatments with 50 nmol/L EG for 24 h, or 500 nmol/L EG for 6 h, reduced Gb3 expression and totally prevented the effects of Stx2 on cell viability, proliferation, and apoptosis. EG treatment also allowed the development of tubulogenesis in 3D-HRTEC exposed to Stx2.

CONCLUSIONS

EG could be a potential therapeutic drug for the prevention of acute kidney injury caused by Stx2.

IMPACT

For the first time, we have demonstrated that Eliglustat prevents Shiga toxin 2 cytotoxic effects on human renal epithelia, by reducing the expression of the toxin receptor globotriaosylceramide. The present work also shows that Eliglustat prevents Shiga toxin 2 effects on tubulogenesis of renal epithelial cells. Eliglustat, actually used for the treatment of patients with Gaucher's disease, could be a therapeutic strategy to prevent the renal damage caused by Shiga toxin.

Restriction of Dietary Advanced Glycation End Products Induces a Differential Plasma Metabolome and Lipidome Profile

SCOPE

Diets with low content in advanced glycation end products (AGEs) lead to beneficial properties in highly prevalent age-related diseases. To shed light on the mechanisms behind, the changes induced by a low AGE dietary intervention in the circulating metabolome are analyzed.

METHODS AND RESULTS

To this end, 20 non-diabetic patients undergoing peritoneal dialysis are randomized to continue their usual diet or to one with a low content of AGEs for 1 month. Then, plasmatic metabolome and lipidomes are analyzed by liquid-chromatography coupled to mass spectrometry. The levels of defined AGE structures are also quantified by ELISA and by mass-spectrometry. The results show that the low AGE diet impinged significant changes in circulating metabolomes (166 molecules) and lipidomes (91 lipids). Metabolic targets of low-AGE intake include sphingolipid, ether-lipids, and glycerophospholipid metabolism. Further, it reproduces some of the plasma characteristics of healthy aging.

CONCLUSION

The finding of common pathways induced by low-AGE diets with previous metabolic traits implicated in aging, insulin resistance, and obesity suggest the usefulness of the chosen approach and supports the potential extension of this study to other populations.

Correction of cilia structure and function alleviates multi-organ pathology in Bardet-Biedl syndrome mice

Bardet–Biedl syndrome (BBS) is a pleiotropic autosomal recessive ciliopathy affecting multiple organs. The development of potential disease-modifying therapy for BBS will require concurrent targeting of multi-systemic manifestations. Here, we show for the first time that monosialodihexosylganglioside accumulates in Bbs2^−/− cilia, indicating impairment of glycosphingolipid (GSL) metabolism in BBS. Consequently, we tested whether BBS pathology in Bbs2^−/− mice can be reversed by targeting the underlying ciliary defect via reduction of GSL metabolism. Inhibition of GSL synthesis with the glucosylceramide synthase inhibitor Genz-667161 decreases the obesity, liver disease, retinal degeneration and olfaction defect in Bbs2^−/− mice. These effects are secondary to preservation of ciliary structure and signaling, and stimulation of cellular differentiation. In conclusion, reduction of GSL metabolism resolves the multi-organ pathology of Bbs2^−/− mice by directly preserving ciliary structure and function towards a normal phenotype. Since this approach does not rely on the correction of the underlying genetic mutation, it might translate successfully as a treatment for other ciliopathies.

Dietary Supplementation with Exogenous Sea-Cucumber-Derived Ceramides and Glucosylceramides Alleviates Insulin Resistance in High-Fructose-Diet-Fed Rats by Upregulating the IRS/PI3K/Akt Signaling Pathway

Endogenous ceramide is considered to be associated with the progress of insulin resistance. However, the effects of dietary exogenous glucosylceramides and ceramides on insulin resistance are unclear. A model of fructose-induced male Sprague Dawley rats was used to compare the effects of sea-cucumber-derived glucosylceramides and ceramides on insulin resistance. Both glucosylceramides and ceramides significantly improved glucose tolerance, reduced the concentrations of serum glucose and glycosylated hemoglobin, and alleviated the accompanied hypertension. Ceramides significantly enhanced glycogen levels in skeletal muscle, whereas glucosylceramides significantly increased the hepatic glycogen levels. Moreover, glucosylceramides alleviated insulin resistance by inhibiting gluconeogenesis, promoting glycogen synthesis and insulin signal transduction in the liver; meanwhile, ceramides were mainly due to the promotion of glycogen synthesis and insulin signal transduction in skeletal muscle. Additionally, glucosylceramides and ceramides effectively attenuated inflammation in adipose tissue. These results indicate that glucosylceramides and ceramides have potential value in the prevention and alleviation of insulin resistance.

Beyond adiponectin and leptin: adipose tissue-derived mediators of inter-organ communication

The breakthrough discoveries of leptin and adiponectin more than two decades ago led to a widespread recognition of adipose tissue as an endocrine organ. Many more adipose tissue-secreted signaling mediators (adipokines) have been identified since then, and much has been learned about how adipose tissue communicates with other organs of the body to maintain systemic homeostasis. Beyond proteins, additional factors, such as lipids, metabolites, noncoding RNAs, and extracellular vesicles (EVs), released by adipose tissue participate in this process. Here, we review the diverse signaling mediators and mechanisms adipose tissue utilizes to relay information to other organs. We discuss recently identified adipokines (proteins, lipids, and metabolites) and briefly outline the contributions of noncoding RNAs and EVs to the ever-increasing complexities of adipose tissue inter-organ communication. We conclude by reflecting on central aspects of adipokine biology, namely, the contribution of distinct adipose tissue depots and cell types to adipokine secretion, the phenomenon of adipokine resistance, and the capacity of adipose tissue to act both as a source and sink of signaling mediators.

Variations in hepatic lipid species of age-matched male mice fed a methionine-choline-deficient diet and housed in different animal facilities

Background

Non-alcoholic steatohepatitis (NASH) is a common disease and feeding mice a methionine-choline-deficient (MCD) diet is a frequently used model to study its pathophysiology. Genetic and environmental factors influence NASH development and liver lipid content, which was studied herein using C57BL/6 J mice bred in two different animal facilities.

Methods

Age-matched male C57BL/6 J mice bred in two different animal facilities (later on referred to as WT1 and WT2) at the University Hospital of Regensburg were fed identical MCD or control chows for 2 weeks. Hepatic gene and protein expression and lipid composition were determined.

Results

NASH was associated with increased hepatic triglycerides, which were actually higher in WT1 than WT2 liver in both dietary groups. Cholesterol contributes to hepatic injury but was only elevated in WT2 NASH liver. Ceramides account for insulin resistance and cell death, and ceramide species d18:1/16:0 and d18:1/18:0 were higher in the NASH liver of both groups. Saturated sphingomyelins only declined in WT1 NASH liver. Lysophosphatidylcholine concentrations were quite normal in NASH and only one of the 12 altered phosphatidylcholine species declined in NASH liver of both groups. Very few phosphatidylethanolamine, phosphatidylserine, and phosphatidylinositol species were comparably regulated in NASH liver of both animal groups. Seven of these lipid species declined and two increased in NASH. Notably, hepatic mRNA expression of proinflammatory (F4/80, CD68, IL-6, TNF and chemerin) and profibrotic genes (TGF beta and alpha SMA) was comparable in WT1 and WT2 mice.

Conclusions

Mice housed and bred in different animal facilities had comparable disease severity of NASH whereas liver lipids varied among the groups. Thus, there was no specific lipid signature for NASH in the MCD model.

Electronic supplementary material

The online version of this article (10.1186/s12944-019-1114-4) contains supplementary material, which is available to authorized users.

The Biology of Gangliosides

Gangliosides comprise a varied family of glycosphingolipid structures bearing one or more sialic acid residues. They are found in all mammalian tissues but are most abundant in the brain, where they represent the quantitatively major class of sialoglycans. As prominent molecular determinants on cell surfaces, they function as molecular-recognition partners for diverse glycan-binding proteins ranging from bacterial toxins to endogenous cell-cell adhesion molecules. Gangliosides also regulate the activity of plasma membrane proteins, including protein tyrosine kinases, by lateral association in the same membranes in which they reside. Their roles in molecular recognition and membrane protein regulation implicate gangliosides in human physiology and pathology, including infectious diseases, diabetes, cancer, and neurodegeneration. The varied structures and biosynthetic pathways of gangliosides are presented here, along with representative examples of their biological functions in health and disease.

Mice lacking sialyltransferase ST3Gal-II develop late-onset obesity and insulin resistance

Sialyltransferases are a family of 20 gene products in mice and humans that transfer sialic acid from its activated precursor, CMP-sialic acid, to the terminus of glycoprotein and glycolipid acceptors. ST3Gal-II (coded by the St3gal2 gene) transfers sialic acid preferentially to the three positions of galactose on the Galβ1-3GalNAc terminus of gangliosides GM1 and GD1b to synthesize GD1a and GT1b, respectively. Mice with a targeted disruption of St3gal2 unexpectedly displayed late-onset obesity and insulin resistance. At 3 months of age, St3gal2-null mice were the same weight as their wild type (WT) counterparts, but by 13 months on standard chow they were visibly obese, 22% heavier and with 37% greater fat/lean ratio than WT mice. St3gal2-null mice became hyperglycemic and displayed impaired glucose tolerance by 9 months of age. They had sharply reduced insulin responsiveness despite equivalent pancreatic islet morphology. Analyses of insulin receptor (IR) tyrosine kinase substrate IRS-1 and downstream target Akt revealed decreased insulin-induced phosphorylation in adipose tissue but not liver or skeletal muscle of St3gal2-null mice. Thin-layer chromatography and mass spectrometry revealed altered ganglioside profiles in the adipose tissue of St3gal2-null mice compared to WT littermates. Metabolically, St3gal2-null mice display a reduced respiratory exchange ratio compared to WT mice, indicating a preference for lipid oxidation as an energy source. Despite their altered metabolism, St3gal2-null mice were hyperactive. We conclude that altered ganglioside expression in adipose tissue results in diminished IR sensitivity and late-onset obesity.

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.

siRNA-based spherical nucleic acids reverse impaired wound healing in diabetic mice by ganglioside GM3 synthase knockdown

Spherical nucleic acid (SNA) gold nanoparticle conjugates (13-nm-diameter gold cores functionalized with densely packed and highly oriented nucleic acids) dispersed in Aquaphor have been shown to penetrate the epidermal barrier of both intact mouse and human skin, enter keratinocytes, and efficiently down-regulate gene targets. ganglioside-monosialic acid 3 synthase (GM3S) is a known target that is overexpressed in diabetic mice and responsible for causing insulin resistance and impeding wound healing. GM3S SNAs increase keratinocyte migration and proliferation as well as insulin and insulin-like growth factor-1 (IGF1) receptor activation under both normo- and hyperglycemic conditions. The topical application of GM3S SNAs (50 nM) to splinted 6-mm-diameter full-thickness wounds in diet-induced obese diabetic mice decreases local GM3S expression by >80% at the wound edge through an siRNA pathway and fully heals wounds clinically and histologically within 12 d, whereas control-treated wounds are only 50% closed. Granulation tissue area, vascularity, and IGF1 and EGF receptor phosphorylation are increased in GM3S SNA-treated wounds. These data capitalize on the unique ability of SNAs to naturally penetrate the skin and enter keratinocytes without the need for transfection agents. Moreover, the data further validate GM3 as a mediator of the delayed wound healing in type 2 diabetes and support regional GM3 depletion as a promising therapeutic direction.

New evidence for the role of ceramide in the development of hepatic insulin resistance

Aim

There are few and contradictory data on the role of excessive accumulation of intracellular sphingolipids, particularly ceramides, in the development of hepatic insulin resistance. In our study we assessed accumulated sphingolipid fractions and clarify the mechanisms of hepatic insulin resistance development as well as involvement of fatty acid and ceramide transporters in this process.

Methods

In culture of primary rat hepatocytes, exposed to high concentration of palmitic acid (0.75mM) during short and prolonged incubation, high performance liquid chromatography was used to assess intra- and extracellular sphingolipid fractions content. Degree of palmitate-induced insulin resistance was estimated by measuring changes in phosphorylation of insulin pathway proteins by western blotting as well as changes in expression of different type of transporters.

Results

In our study short and prolonged exposure of primary hepatocytes to palmitic acid resulted in increased intracellular accumulation of ceramide which inhibited insulin signaling pathway. We observed a significant increase in the expression of fatty-acid transport protein (FATP2) and ceramide transfer protein (CERT) what is consistent with enhanced intracellular ceramide content. The content of extracellular ceramide was increased nearly threefold after short and twofold after long incubation period. Expression of microsomal triglyceride transfer protein (MTP) and ATP-binding cassette transporter (ABCA1) was increased significantly mainly after short palmitate incubation.

Conclusion

Our data showed that increase in intarcellular ceramide content contributes to the development of hepatic insulin resistance. We suggest pivotal role of transporters in facilitating fatty acid influx (FATP2), accumulation of ceramides (CERT) and export to the media (MTP and ABCA1).

Adiponectin signaling in the liver

High glucose production contributes to fed and fasted hyperglycemia in Type 1 Diabetes (T1D) and Type 2 Diabetes (T2D). The breakdown of the adiponectin signaling pathway in T1D and the reduction of circulating adiponectin in T2D contribute to this abnormal increase in glucose production. Sufficient amounts of insulin could compensate for the loss of adiponectin signaling in T1D and T2D and reduce hyperglycemia. However, the combination of low adiponectin signaling and high insulin resembles an insulin resistance state associated with cardiovascular disease, fatty liver disease and decreased life expectancy. The future development of "adiponectin sensitizers", medications that correct the deficiency in adiponectin signaling, could restore the metabolic balance in T1D and T2D and reduce the need for insulin. This article reviews the adiponectin signaling pathway in the liver through T-cadherin, AdipoR1, AdipoR2, AMPK, ceramidase activity, APPL1 and the recently discovered Suppressor Of Glucose from Autophagy (SOGA).

Eliglustat tartrate, a prototypic glucosylceramide synthase inhibitor

Eliglustat tartrate is a highly specific inhibitor of glucosylceramide synthase, developed for the treatment glucosylceramide-based glycosphingolipidoses. Eliglustat is in late clinical development for Gaucher disease type 1. Phase II and III clinical trials have demonstrated clinical efficacy for eliglustat as a stand-alone agent for newly diagnosed patients that are naïve to prior therapy and for patients who have been previously treated with enzyme replacement therapy. Importantly, the reported toxicity of eliglustat has been limited. Eliglustat will be submitted for the US FDA and EMA review in late 2013. Several structurally unrelated glucosylceramide synthase inhibitors have been identified and are in various stages of development, some of which cross the blood-brain barrier. Targeting glucosylceramide synthesis is also a promising approach for the treatment of type 2 diabetes mellitus, autosomal dominant polycystic kidney disease and certain cancers.

The role of angiotensin II in nonalcoholic steatohepatitis

Nonalcoholic fatty liver disease (NAFLD) is now considered the most prevalent chronic liver disease, affecting over 30% of the US adult population. NAFLD is strongly linked to insulin resistance and is considered the hepatic manifestation of the metabolic syndrome. Activation of the renin-angiotensin-aldosterone system (RAAS) is known to play a role in the hypertension observed in the metabolic syndrome and also is thought to play a central role in insulin resistance and NAFLD. Angiotensin II (AngII) is considered the primary effector of the physiological outcomes of RAAS signaling, both at the systemic and local tissue level. Herein, we review data describing the potential involvement of AngII-mediated signaling at multiple levels in the development and progression of NAFLD, including increased steatosis, inflammation, insulin resistance, and fibrosis. Additionally, we present recent work on the potential therapeutic benefits of RAAS and angiotensin II signaling inhibition in rodent models and patients with NAFLD.

Cell-specific in vivo functions of glycosphingolipids: lessons from genetic deletions of enzymes involved in glycosphingolipid synthesis

Glycosphingolipids (GSLs) are believed to be involved in many cellular events including trafficking, signaling and cellular interactions. Over the past decade considerable progress was made elucidating the function of GSLs by generating and exploring animal models with GSL-deficiency. Initial studies focused on exploring the role of complex sialic acid containing GSLs (gangliosides) in neuronal tissue. Although complex gangliosides were absent, surprisingly, the phenotype observed was rather mild. In subsequent studies, several mouse models with combinations of gene-deletions encoding GSL-synthesizing enzymes were developed. The results indicated that reduction of GSL-complexity correlated with severity of phenotypes. However, in these mice, accumulation of precursor GSLs or neobiosynthesized GSL-series seemed to partly compensate the loss of GSLs. Thus, UDP-glucose:ceramide glucosyltransferase (Ugcg), catalyzing the basic step of the glucosylceramide-based GSL-biosynthesis, was genetically disrupted. A total systemic deletion of Ugcg caused early embryonic lethality. Therefore, Ugcg was eliminated in a cell-specific manner using the cre/loxP-system. New insights into the cellular function of GSLs were gained. It was demonstrated that neurons require GSLs for differentiation and maintenance. In keratinocytes, preservation of the skin barrier depends on GSL synthesis and in enterocytes of the small intestine GSLs are involved in endocytosis and vesicular transport.

Inhibition of sphingolipid synthesis improves dyslipidemia in the diet-induced hamster model of insulin resistance: evidence for the role of sphingosine and sphinganine in hepatic VLDL-apoB100 overproduction

Sphingolipids have emerged as important bioactive lipid species involved in the pathogenesis of type 2 diabetes and cardiovascular disease. However, little is known of the regulatory role of sphingolipids in dyslipidemia of insulin-resistant states. We employed hamster models of dyslipidemia and insulin resistance to investigate the role of sphingolipids in hepatic VLDL overproduction, induction of insulin resistance, and inflammation. Hamsters were fed either a control chow diet, a high fructose diet, or a diet high in fat, fructose and cholesterol (FFC diet). They were then treated for 2 weeks with vehicle or 0.3 mg/kg myriocin, a potent inhibitor of de novo sphingolipid synthesis. Both fructose and FFC feeding induced significant increases in hepatic sphinganine, which was normalized to chow-fed levels with myriocin (P < 0.05); myriocin also lowered hepatic ceramide content (P < 0.05). Plasma TG and cholesterol as well as VLDL-TG and -apoB100 were similarly reduced with myriocin treatment in all hamsters, regardless of diet. Myriocin treatment also led to improved insulin sensitivity and reduced hepatic SREBP-1c mRNA, though it did not appear to ameliorate the activation of hepatic inflammatory pathways. Importantly, direct treatment of primary hamster hepatocytes ex vivo with C2 ceramide or sphingosine led to an increased secretion of newly synthesized apoB100. Taken together, these data suggest that a) hepatic VLDL-apoB100 overproduction may be stimulated by ceramides and sphingosine and b) inhibition of sphingolipid synthesis can reduce circulating VLDL in hamsters and improve circulating lipids--an effect that is possibly due to improved insulin signaling and reduced lipogenesis but is independent of changes in inflammation.

The design and clinical development of inhibitors of glycosphingolipid synthesis: will invention be the mother of necessity?

The treatment of glycosphingolipid storage diseases by synthesis inhibition was first proposed 40 years ago as an alternative approach to enzyme replacement therapy. We have pursued this strategy through the rational design of potent and selective inhibitors of glucosylceramide synthase, the first step in glycosphingolipid synthesis. Eliglustat tartrate was the result of these efforts and is currently the focus of phase 3 trials for type 1 Gaucher disease. Phase 2 studies showed a reduction in splenomegaly and hepatomegaly and improvements of anemia and thrombocytopenia at levels equivalent to or exceeding the historic response to imiglucerase. Structural analogues of eliglustat have also been designed that lack pgp-1 recognition and cross the blood brain barrier. These may have utility for central nervous system- based sphingolipidoses. Because glycosphingolipids are important regulators of receptor tyrosine kinases, glucosylceramide synthase inhibitors may also be beneficial for disorders such as type 2 diabetes mellitus and polycystic kidney disease.

Correction of liver steatosis by a hydrophobic iminosugar modulating glycosphingolipids metabolism

The iminosugar N-(5′-adamantane-1′-yl-methoxy)-pentyl-1-deoxynoijirimycin (AMP-DNM), an inhibitor of glycosphingolipid (GSL) biosynthesis is known to ameliorate diabetes, insulin sensitivity and to prevent liver steatosis in ob/ob mice. Thus far the effect of GSL synthesis inhibition on pre-existing NASH has not yet been assessed. To investigate it, LDLR(−/−) mice were kept on a western-type diet for 12 weeks to induce NASH. Next, the diet was continued for 6 weeks in presence or not of AMP-DNM in the diet. AMP-DNM treated mice showed less liver steatosis, inflammation and fibrosis. Induction of fatty acid beta-oxydation was observed, as well as a reduction of plasma lipids. Our study demonstrates that AMP-DNM treatment is able to significantly correct pre-existing NASH, suggesting that inhibiting GSL synthesis may represent a novel strategy for the treatment of this pathology.

Lipidomic analysis of the liver identifies changes of major and minor lipid species in adiponectin deficient mice

Adiponectin protects from hepatic fat storage but adiponectin deficient mice (APN-/-) fed a standard chow do not develop liver steatosis. This indicates that other pathways might be activated to compensate for adiponectin deficiency. An unbiased and comprehensive screen was performed to identify hepatic alterations of lipid classes in these mice. APN-/- mice had decreased hepatic cholesteryl esters while active SREBP2 and systemic total cholesterol were not altered. Upregulation of cytochromes for bile acid synthesis suggests enhanced biliary cholesterol excretion. Analysis of 37 individual fatty acid species showed reduced stearate whereas total fatty acids were not altered. Total amount of triglycerides and phospholipids were equally abundant. A selective increase of monounsaturated phosphatidylcholine and phosphatidylethanolamine which positively correlate with hepatic and systemic triglycerides with the latter being elevated in APN-/- mice, was identified. Stearoyl-CoA desaturase 1 (SCD1) is involved in the synthesis of monounsaturated fatty acids and despite higher mRNA expression enzyme activity was not enhanced. Glucosylceramide postulated to contribute to liver damage was decreased. This study demonstrates that adiponectin deficiency is associated with hepatic changes in lipid classes in mice fed a standard chow which may protect from liver steatosis.

Adamantyl glycosphingolipids provide a new approach to the selective regulation of cellular glycosphingolipid metabolism

Mammalian glycosphingolipid (GSL) precursor monohexosylceramides are either glucosyl- or galactosylceramide (GlcCer or GalCer). Most GSLs derive from GlcCer. Substitution of the GSL fatty acid with adamantane generates amphipathic mimics of increased water solubility, retaining receptor function. We have synthesized adamantyl GlcCer (adaGlcCer) and adamantyl GalCer (adaGalCer). AdaGlcCer and adaGalCer partition into cells to alter GSL metabolism. At low dose, adaGlcCer increased cellular GSLs by inhibition of glucocerebrosidase (GCC). Recombinant GCC was inhibited at pH 7 but not pH 5. In contrast, adaGalCer stimulated GCC at pH 5 but not pH 7 and, like adaGlcCer, corrected N370S mutant GCC traffic from the endoplasmic reticulum to lysosomes. AdaGalCer reduced GlcCer levels in normal and lysosomal storage disease (LSD) cells. At 40 μM adaGlcCer, lactosylceramide (LacCer) synthase inhibition depleted LacCer (and more complex GSLs), such that only GlcCer remained. In Vero cell microsomes, 40 μM adaGlcCer was converted to adaLacCer, and LacCer synthesis was inhibited. AdaGlcCer is the first cell LacCer synthase inhibitor. At 40 μM adaGalCer, cell synthesis of only Gb(3) and Gb(4) was significantly reduced, and a novel product, adamantyl digalactosylceramide (adaGb(2)), was generated, indicating substrate competition for Gb(3) synthase. AdaGalCer also inhibited cell sulfatide synthesis. Microsomal Gb(3) synthesis was inhibited by adaGalCer. Metabolic labeling of Gb(3) in Fabry LSD cells was selectively reduced by adaGalCer, and adaGb(2) was produced. AdaGb(2) in cells was 10-fold more effectively shed into the medium than the more polar Gb(3), providing an easily eliminated "safety valve" alternative to Gb(3) accumulation. Adamantyl monohexosyl ceramides thus provide new tools to selectively manipulate normal cellular GSL metabolism and reduce GSL accumulation in cells from LSD patients.

Accumulation of long-chain glycosphingolipids during aging is prevented by caloric restriction

Background

Chronic kidney disease and end-stage renal disease are major causes of morbidity and mortality that are seen far more commonly in the aged population. Interestingly, kidney function declines during aging even in the absence of underlying renal disease. Declining renal function has been associated with age-related cellular damage and dysfunction with reports of increased levels of apoptosis, necrosis, and inflammation in the aged kidney. Bioactive sphingolipids have been shown to regulate these same cellular processes, and have also been suggested to play a role in aging and cellular senescence.

Methodology/Principal Findings

We hypothesized that alterations in kidney sphingolipids play a role in the declining kidney function that occurs during aging. To begin to address this, the sphingolipid profile was measured in young (3 mo), middle aged (9 mo) and old (17 mo) C57BL/6 male mice. Interestingly, while modest changes in ceramides and sphingoid bases were evident in kidneys from older mice, the most dramatic elevations were seen in long-chain hexosylceramides (HexCer) and lactosylceramides (LacCer), with C14- and C16-lactosylceramides elevated as much as 8 and 12-fold, respectively. Increases in long-chain LacCers during aging are not exclusive to the kidney, as they also occur in the liver and brain. Importantly, caloric restriction, previously shown to prevent the declining kidney function seen in aging, inhibits accumulation of long-chain HexCer/LacCers and prevents the age-associated elevation of enzymes involved in their synthesis. Additionally, long-chain LacCers are also significantly elevated in human fibroblasts isolated from elderly individuals.

Conclusion/Significance

This study demonstrates accumulation of the glycosphingolipids HexCer and LacCer in several different organs in rodents and humans during aging. In addition, data demonstrate that HexCer and LacCer metabolism is regulated by caloric restriction. Taken together, data suggest that HexCer/LacCers are important mediators of cellular processes fundamental to mammalian aging.

Adiponectin suppresses gluconeogenic gene expression in mouse hepatocytes independent of LKB1-AMPK signaling

The adipocyte-derived hormone adiponectin signals from the fat storage depot to regulate metabolism in peripheral tissues. Inversely correlated with body fat levels, adiponectin reduction in obese individuals may play a causal role in the symptoms of metabolic syndrome. Adiponectin lowers serum glucose through suppression of hepatic glucose production, an effect attributed to activation of AMPK. Here, we investigated the signaling pathways that mediate the effects of adiponectin by studying mice with inducible hepatic deletion of LKB1, an upstream regulator of AMPK. We found that loss of LKB1 in the liver partially impaired the ability of adiponectin to lower serum glucose, though other actions of the hormone were preserved, including reduction of gluconeogenic gene expression and hepatic glucose production as assessed by euglycemic hyperinsulinemic clamp. Furthermore, in primary mouse hepatocytes, the absence of LKB1, AMPK, or the transcriptional coactivator CRTC2 did not prevent adiponectin from inhibiting glucose output or reducing gluconeogenic gene expression. These results reveal that whereas some of the hormone's actions in vivo may be LKB1 dependent, substantial LKB1-, AMPK-, and CRTC2-independent signaling pathways also mediate effects of adiponectin.

Identification of potent and selective glucosylceramide synthase inhibitors from a library of N-alkylated iminosugars

Glucosylceramide synthase (GCS) is an important target for clinical drug development for the treatment of lysosomal storage disorders and a promising target for combating type 2 diabetes. Iminosugars are useful leads for the development of GCS inhibitors; however, the effective iminosugar type GCS inhibitors reported have some unwanted cross-reactivity toward other glyco-processing enzymes. In particular, iminosugar type GCS inhibitors often also inhibit to some extent human acid glucosylceramidase (GBA1) and the nonlysosomal glucosylceramidase (GBA2), the two enzymes known to process glucosylceramide. Of these, GBA1 itself is a potential drug target for the treatment of the lysosomal storage disorder, Gaucher disease, and selective GBA1 inhibitors are sought after as potential chemical chaperones. The physiological importance of GBA2 in glucosylceramide processing in relation to disease states is less clear, and here, selective inhibitors can be of use as chemical knockout entities. In this communication, we report our identification of a highly potent and selective N-alkylated l-ido-configured iminosugar. In particular, the selectivity of 27 for GCS over GBA1 is striking.