Insulin resistance as a membrane microdomain disorder

Interaction between caveolin-1 polymorphism and dietary fat quality indexes on visceral adiposity index (VAI) and body adiposity index (BAI) among overweight and obese women: a cross-sectional study

BACKGROUND AND AIMS

Caveolin-1 (CAV-1) in adipocyte tissue and other body parts possesses numerous biological functions. In the present study, we sought to investigate the interaction between CAV-1 polymorphism and dietary fat quality indexes on visceral adiposity index (VAI) and body adiposity index (BAI) among overweight and obese women.

METHODS

This study was conducted on 386 women aged 18-48 years old. Biochemical measurements were assessed by standard protocols. We used a food frequency questionnaire (FFQ) to calculate the dietary intake and the indexes of dietary fat quality intake. Anthropometric values and body composition were measured by standard methods. Finally, the CAV-1 genotype was measured using the PCR-RFLP method.

RESULTS

We found marginally significant differences between AA and GG genotypes of waist-to-hip ratio (WHR) (P = 0.06) and BAI (P = 0.06) of participants after adjusting for potential confounders. For dietary intakes, after adjusting with the energy intake, mean differences in biotin (P = 0.04) and total fiber (P = 0.06) were significant and marginally significant, respectively. The interaction between two risk alleles (AA) with omega-6 to omega-3 ratio (W6/W3) on BAI, after adjustment for potential confounders (age, physical activity, energy intake, education), was marginally positive (β = 14.08, 95% CI = - 18.65, 46.81, P = 0.07). In comparison to the reference group (GG), there was a positive interaction between the two risk alleles (AA) with W6/W3 ratio on VAI (β = 2.81, 95% CI = 1.20, 8.84, P = 0.06) in the adjusted model.

CONCLUSIONS

We found that there might be an interaction between CAV-1 genotypes with dietary quality fat indexes on VAI and BAI among overweight and obese women.

Effects of apolipoprotein E on regulating insulin sensitivity via regulating insulin receptor signalosome in caveolae

AIMS

Although impaired insulin signaling at a post-receptor level was a well-established key driver of insulin resistance, the role of surface abnormal insulin receptor (INSR) location in insulin resistance pathogenesis tended to be ignored and its molecular mechanisms remained obscure. Herein, this study aimed to investigate hepatic apolipoprotein E (APOE) impaired cellular insulin action via reducing cell surface INSR, especially in caveolae.

KEY FINDINGS

Downregulation of APOE enhanced the caveolae translocation of INSR and glucose transporter 2 (GLUT2), and improved hepatic cells' sensitivity to insulin. Consistently, mice with selective suppression of liver tissue APOE showed lower fasting insulin and fasting glucose levels, better homeostatic model assessment (HOMA) index (HOMA-IS, HOMA-IR, HOMA-β) and quantitative insulin sensitivity check index (QUICKI). Furthermore, the co-localization of INSR and CAV1 in the liver of these mice were more substantial than controls.

SIGNIFICANCE

APOE might adversely set the basal gain of INSR signaling implied that APOE could be a new endogenous INSR regulator.

Sphingolipids in obesity and related complications

Sphingolipids are biologically active lipids ubiquitously produced in all vertebrate cells. Asides from structural components of cell membrane, sphingolipids also function as intracellular and extracellular mediators that regulate many important physiological cellular processes including cell survival, proliferation, apoptosis, differentiation, migration and immune processes. Recent studies have also indicated that disruption of sphingolipid metabolism is strongly associated with different diseases that exhibit diverse neurological and metabolic consequences. Here, we briefly summarize current evidence for understanding of sphingolipid pathways in obesity and associated complications. The regulation of sphingolipids and their enzymes may have a great impact in the development of novel therapeutic modalities for a variety of metabolic diseases.

Plasma sphingolipids are biomarkers of metabolic syndrome in non-human primates maintained on a Western-style diet

BACKGROUND

The intake of a Western diet enriched in animal fat has been shown to be a major risk factor for Type 2 diabetes and obesity. Previous rodent studies have indicated that these conditions may be triggered by the accumulation of the sphingolipid ceramide in insulin-sensitive tissues. However, data are lacking in this regard from both humans and non-human primates.

OBJECTIVE

Here we have investigated the relationship between plasma ceramides and metabolic syndrome in Rhesus macaques fed a high-fat and high-fructose (HFFD) 'western' diet.

METHODS

We investigated this relationship in cohorts of monkeys fed a HFFD for a period of 8 months to 5 years. Animals were classified as control, pre-diabetic or diabetic based on fasting plasma parameters and insulin sensitivity.

RESULTS

HFFD treatment produced significant increases in body weight and body fat and also resulted in a decline in insulin sensitivity. In parallel to the reduction in insulin sensitivity, significant increases in both plasma ceramide and dihydroceramide levels were observed, which further increased as animals progressed to the diabetic state. Plasma levels of the rare sphingolipid C18:0 deoxysphinganine, a marker of increased metabolic flux through serine palmitoyl transferase (SPT), were also elevated in both pre- and diabetic animals. Furthermore, plasma serine levels were significantly elevated in diabetic monkeys, which may indicate a shift in SPT substrate selectivity from serine to alanine or glycine. In contrast, branch chain amino acids were unchanged in pre-diabetic non-human primates, and only plasma valine levels were elevated in diabetic animals.

CONCLUSION

Together, these data indicate that HFFD induces de novo synthesis of ceramides in non-human primates, and that increased production of plasma ceramides is significantly correlated with the decline in insulin sensitivity.

A ceramide-centric view of insulin resistance

The recent implementation of genomic and lipidomic approaches has produced a large body of evidence implicating the sphingolipid ceramide in a diverse range of physiological processes and as a critical modulator of cellular stress. In this review, we discuss from a historical perspective the most important discoveries produced over the last decade supporting a role for ceramide and its metabolites in the pathogenesis of insulin resistance and other obesity-associated metabolic diseases. Moreover, we describe how a ceramide-centric view of insulin resistance might be reconciled in the context of other prominent models of nutrient-induced insulin resistance.

Rimonabant reduces the essential value of food in the genetically obese Zucker rat: an exponential demand analysis

Research on free-food intake suggests that cannabinoids are implicated in the regulation of feeding. Few studies, however, have characterized how environmental factors that affect food procurement interact with cannabinoid drugs that reduce food intake. Demand analysis provides a framework to understand how cannabinoid blockers, such as rimonabant, interact with effort in reducing demand for food. The present study examined the effects rimonabant had on demand for sucrose in obese Zucker rats when effort to obtain food varied and characterized the data using the exponential ("essential value") model of demand. Twenty-nine male (15 lean, 14 obese) Zucker rats lever-pressed under eight fixed ratio (FR) schedules of sucrose reinforcement, in which the number of lever-presses to gain access to a single sucrose pellet varied between 1 and 300. After behavior stabilized under each FR schedule, acute doses of rimonabant (1-10mg/kg) were administered prior to some sessions. The number of food reinforcers and responses in each condition was averaged and the exponential and linear demand equations were fit to the data. These demand equations quantify the value of a reinforcer by its sensitivity to price (FR) increases. Under vehicle conditions, obese Zucker rats consumed more sucrose pellets than leans at smaller fixed ratios; however, they were equally sensitive to price increases with both models of demand. Rimonabant dose-dependently reduced reinforcers and responses for lean and obese rats across all FR schedules. Data from the exponential analysis suggest that rimonabant dose-dependently increased elasticity, i.e., reduced the essential value of sucrose, a finding that is consistent with graphical depictions of normalized demand curves.

Reducing plasma membrane sphingomyelin increases insulin sensitivity

It has been shown that inhibition of de novo sphingolipid synthesis increases insulin sensitivity. For further exploration of the mechanism involved, we utilized two models: heterozygous serine palmitoyltransferase (SPT) subunit 2 (Sptlc2) gene knockout mice and sphingomyelin synthase 2 (Sms2) gene knockout mice. SPT is the key enzyme in sphingolipid biosynthesis, and Sptlc2 is one of its subunits. Homozygous Sptlc2-deficient mice are embryonic lethal. However, heterozygous Sptlc2-deficient mice that were viable and without major developmental defects demonstrated decreased ceramide and sphingomyelin levels in the cell plasma membranes, as well as heightened sensitivity to insulin. Moreover, these mutant mice were protected from high-fat diet-induced obesity and insulin resistance. SMS is the last enzyme for sphingomyelin biosynthesis, and SMS2 is one of its isoforms. Sms2 deficiency increased cell membrane ceramide but decreased SM levels. Sms2 deficiency also increased insulin sensitivity and ameliorated high-fat diet-induced obesity. We have concluded that Sptlc2 heterozygous deficiency- or Sms2 deficiency-mediated reduction of SM in the plasma membranes leads to an improvement in tissue and whole-body insulin sensitivity.

Lipid raft redox signaling: molecular mechanisms in health and disease

Lipid rafts, the sphingolipid and cholesterol-enriched membrane microdomains, are able to form different membrane macrodomains or platforms upon stimulations, including redox signaling platforms, which serve as a critical signaling mechanism to mediate or regulate cellular activities or functions. In particular, this raft platform formation provides an important driving force for the assembling of NADPH oxidase subunits and the recruitment of other related receptors, effectors, and regulatory components, resulting, in turn, in the activation of NADPH oxidase and downstream redox regulation of cell functions. This comprehensive review attempts to summarize all basic and advanced information about the formation, regulation, and functions of lipid raft redox signaling platforms as well as their physiological and pathophysiological relevance. Several molecular mechanisms involving the formation of lipid raft redox signaling platforms and the related therapeutic strategies targeting them are discussed. It is hoped that all information and thoughts included in this review could provide more comprehensive insights into the understanding of lipid raft redox signaling, in particular, of their molecular mechanisms, spatial-temporal regulations, and physiological, pathophysiological relevances to human health and diseases.

Insulin signaling in retinal neurons is regulated within cholesterol-enriched membrane microdomains

Neuronal cell death is an early pathological feature of diabetic retinopathy. We showed previously that insulin receptor signaling is diminished in retinas of animal models of diabetes and that downstream Akt signaling is involved in insulin-mediated retinal neuronal survival. Therefore, further understanding of the mechanisms by which retinal insulin receptor signaling is regulated could have therapeutic implications for neuronal cell death in diabetes. Here, we investigate the role of cholesterol-enriched membrane microdomains to regulate PKC-mediated inhibition of Akt-dependent insulin signaling in R28 retinal neurons. We demonstrate that PKC activation with either a phorbol ester or exogenous application of diacylglycerides impairs insulin-induced Akt activation, whereas PKC inhibition augments insulin-induced Akt activation. To investigate the mechanism by which PKC impairs insulin-stimulated Akt activity, we assessed various upstream mediators of Akt signaling. PKC activation did not alter the tyrosine phosphorylation of the insulin receptor or IRS-2. Additionally, PKC activation did not impair phosphatidylinositol 3-kinase activity, phosphoinositide-dependent kinase phosphorylation, lipid phosphatase (PTEN), or protein phosphatase 2A activities. Thus, we next investigated a biophysical mechanism by which insulin signaling could be disrupted and found that disruption of lipid microdomains via cholesterol depletion blocks insulin-induced Akt activation and reduces insulin receptor tyrosine phosphorylation. We also demonstrated that insulin localizes phosphorylated Akt to lipid microdomains and that PMA reduces phosphorylated Akt. In addition, PMA localizes and recruits PKC isotypes to these cholesterol-enriched microdomains. Taken together, these results demonstrate that both insulin-stimulated Akt signaling and PKC-induced inhibition of Akt signaling depend on cholesterol-enriched membrane microdomains, thus suggesting a putative biophysical mechanism underlying insulin resistance in diabetic retinopathy.

Demand for sucrose in the genetically obese Zucker (fa/fa) rat

Obese Zucker rats (fa/fa) eat more food than lean controls in free-feeding conditions, which strongly influences their phenotypic expression. Few studies, however, characterize their food consumption in environments that are more representative of foraging conditions, e.g., how effort plays a role in food procurement. This study examined the reinforcing efficacy of sucrose in obese Zucker rats by varying the responses required to obtain single sucrose pellets. Male Zucker rats (15 lean, 14 obese) lever-pressed under eight fixed ratio (FR) schedules of sucrose reinforcement, in which the number of lever-presses required to gain access to a single sucrose pellet varied from 1 to 300. Linear and exponential demand equations, which characterize the value of a reinforcer by its sensitivity to price (FR), were fit to the number of food reinforcers and responses made. Free food consumption was also examined. Obese Zuckers, compared to leans, consumed more food under free-feeding conditions. Moreover, they had higher levels of consumption and response output, but only at low FR values. Both groups were equally sensitive to price increases at higher FR values. This suggests that environmental conditions may interact with genes in the expression of food reinforcer efficacy.

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

Background

Obesity is characterized by the accumulation of fat in the liver and other tissues, leading to insulin resistance. We have previously shown that a specific inhibitor of glucosylceramide synthase, which inhibits the initial step in the synthesis of glycosphingolipids (GSLs), improved glucose metabolism and decreased hepatic steatosis in both ob/ob and diet-induced obese (DIO) mice. Here we have determined in the DIO mouse model the efficacy of a related small molecule compound, Genz-112638, which is currently being evaluated clinically for the treatment of Gaucher disease, a lysosomal storage disorder.

Methodology/Principal Findings

DIO mice were treated with the Genz-112638 for 12 to 16 weeks by daily oral gavage. Genz-112638 lowered HbA1c levels and increased glucose tolerance. Whole body adiposity was not affected in normal mice, but decreased in drug-treated obese mice. Drug treatment also significantly lowered liver triglyceride levels and reduced the development of hepatic steatosis. We performed hyperinsulinemic-euglycemic clamps on the DIO mice treated with Genz-112638 and showed that insulin-mediated suppression of hepatic glucose production increased significantly compared to the placebo treated mice, indicating a marked improvement in hepatic insulin sensitivity.

Conclusions/Significance

These results indicate that GSL inhibition in obese mice primarily results in an increase in insulin action in the liver, and suggests that GSLs may have an important role in hepatic insulin resistance in conditions of obesity.

Palmitoylation of prohibitin at cysteine 69 facilitates its membrane translocation and interaction with Eps 15 homology domain protein 2 (EHD2)

Plasma membrane translocation of specific cytosolic proteins plays an important role in cell signaling pathways. We have recently shown that prohibitin (PHB) , a protein present in the plasma membranes of various cell types, interacts with Eps 15 homology domain protein 2 (EHD2), a lipid raft protein. However, the mechanism involved in membrane translocation of PHB is not known.We report that PHB undergoes palmitoylation at cysteine 69 (Cys69), and that this palmitoylation is required for PHB's membrane translocation. Furthermore, we demonstrate that membrane translocation of PHB facilitates tyrosine phosphorylation and its interaction with EHD2. Thus, the palmitoylation and membrane translocation of PHB and its interaction with EHD2 may play a role in cell signaling.

Dual-action lipophilic iminosugar improves glycemic control in obese rodents by reduction of visceral glycosphingolipids and buffering of carbohydrate assimilation

The lipophilic iminosugar N-[5-(adamantan-1-ylmethoxy)pentyl]-1-deoxynojirimycin (2, AMP-DNM) potently controls hyperglycemia in obese rodent models of insulin resistance. The reduction of visceral glycosphingolipids by 2 is thought to underlie its beneficial action. It cannot, however, be excluded that concomitant inhibition of intestinal glycosidases and associated buffering of carbohydrate assimilation add to this. To firmly establish the mode of action of 2, we developed a panel of lipophilic iminosugars varying in configuration at C-4/C-5 and N-substitution of the iminosugar. From these we identified the l-ido derivative of 2, l-ido-AMP-DNM (4), as a selective inhibitor of glycosphingolipid synthesis. Compound 4 lowered visceral glycosphingolipids in ob/ob mice and ZDF rats on a par with 2. In contrast to 2, 4 did not inhibit sucrase activity or sucrose assimilation. Treatment with 4 was significantly less effective in reducing blood glucose and HbA1c. We conclude that the combination of reduction of glycosphingolipids in tissue and buffering of carbohydrate assimilation by 2 produces a superior glucose homeostasis.

The nutritional and clinical significance of lipid rafts

PURPOSE OF REVIEW

Lipid rafts are potentially modifiable by diet, particularly (but not exclusively) by dietary fatty acids. This review examines the potential for dietary modification of raft structure and function in the immune system, brain and retinal tissue, the gut, and in cancer cells.

RECENT FINDINGS

In-vitro and ex-vivo studies suggest that dietary n-3 polyunsaturated fatty acids (PUFAs) may exert immunosuppressive and anticancer effects through changes in lipid raft organization. In addition, gangliosides and cholesterol may modulate lipid raft organization in a number of tissues, and recent work has highlighted sphingolipids in membrane microdomains as potential targets for inhibition of tumor growth. The roles of fatty acids and gangliosides, especially in relation to lipid rafts, in cognitive development, age-related cognitive decline, psychiatric disorders, and Alzheimer's disease are poorly understood and require further investigation. The roles of lipid rafts in cancer, in microbial pathogenesis, and in insulin resistance are starting to emerge, and indicate compelling evidence for the growing importance of membrane microdomains in health and disease.

SUMMARY

In-vitro and animal studies show that n-3 PUFAs, cholesterol, and gangliosides modulate the structure and composition of lipid rafts, potentially influencing a wide range of biological processes, including immune function, neuronal signaling, cancer cell growth, entry of pathogens through the gut barrier, and insulin resistance in metabolic disorders. The physiological, clinical, and nutritional relevance of these observations remains to be determined.

Insulin augments gonadotropin-releasing hormone induction of translation in LbetaT2 cells

The integrated signaling of insulin and gonadotropin-releasing hormone in the pituitary gonadotropes may have a profound bearing on reproductive function, although the cross-receptor signaling mechanisms are unclear. We demonstrate that the insulin receptor is constitutively localized to non-caveolar lipid raft microdomains in the pituitary gonadotrope cell line LbetaT2. The localization to rafts is consistent with similar localization of the GnRH receptor. Insulin receptor phosphorylation occurs in raft domains and activates the downstream signaling targets Insulin Receptor Substrate1 and Akt/Protein Kinase B. Although insulin alone does not strongly activate the extracellular signal-regulated kinase second messenger cascade, co-stimulation potentiates the phosphorylation of the extracellular signal-regulated kinase by gonadotropin-releasing hormone. The co-stimulatory effect of insulin and gonadotropin-releasing hormone is also evident in increased activation of cap-dependent translation. In contrast, co-stimulation attenuates Akt/Protein Kinase B activation. Our results show that both gonadotropin-releasing hormone and insulin are capable of mutually altering their respective regulatory signaling cascades. We suggest that this provides a mechanism to integrate neuropeptide and energy homeostatic signals to modulate reproductive function.

Ganglioside GM3 and its biological functions

Metabolism, topology, and possible mechanisms for regulation of the ganglioside GM3 content in the cell are reviewed. Under consideration are biological functions of GM3, such as involvement in cell differentiation, proliferation, oncogenesis, and apoptosis.

Zebrafish and mouse alpha2,3-sialyltransferases responsible for synthesizing GM4 ganglioside

We have previously reported that fish pathogens causing vibriosis specifically adhere to GM4 on the epithelial cells of fish intestinal tracts (Chisada, S., Horibata, Y., Hama, Y., Inagaki, M., Furuya, N., Okino, N., and Ito, M. (2005) Biochem. Biophys. Res. Commun. 333, 367-373). To identify the gene encoding the enzyme for GM4 synthesis in the fish intestinal tract, a phylogenetic tree of vertebrate ST3GalVs, including Danio rerio and Oryzias latipes, was generated in which two putative subfamilies of fish ST3GalVs were found. Two putative ST3GalVs of zebrafish (zST3GalV-1 and -2), each belonging to different subfamilies, were cloned from the zebrafish cDNA library. Interestingly, zST3GalV-1 synthesized GM3 (NeuAcalpha2-3Galbeta1-4Glcbeta1-1'Cer) but not GM4, whereas zSTGalV-2 synthesized both gangliosides in vitro when expressed in CHO-K1 and RPMI1846 cells. Flow cytometric analysis using anti-GM4 antibody revealed that the transformation of RPMI1846 cells with zST3GalV-2 but not zST3GalV-1 cDNA increased the cell-surface expression of GM4. Whole mount in situ hybridization showed that the zST3GalV-2 transcript was strongly expressed in the gastrointestinal tract, whereas zST3GalV-1 was expressed in the brain and esophagus but not gastrointestinal tract in 3-day post-fertilization embryos. It has long been a matter of controversy which enzyme is responsible for the synthesis of GM4 in mammals. We found that three isoforms of mouse ST3GalV (mST3GalV) having different N-terminal sequences can synthesize GM4 as well as GM3 when expressed in RPMI1846 and CHO-K1 cells. Furthermore, mST3GalV knock-out mice were found to lack GM4 synthase activity and GM4 in contrast to wild-type mice. These results clearly indicate that zST3GalV-2 and mST3GalV are the enzymes responsible for the synthesis of GM4 in zebrafish and mice, respectively.

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.

Inhibition of cholesterol biosynthesis impairs insulin secretion and voltage-gated calcium channel function in pancreatic beta-cells

Insulin secretion from pancreatic beta-cells is mediated by the opening of voltage-gated Ca2+ channels (CaV) and exocytosis of insulin dense core vesicles facilitated by the secretory soluble N-ethylmaleimide-sensitive factor attachment protein receptor protein machinery. We previously observed that beta-cell exocytosis is sensitive to the acute removal of membrane cholesterol. However, less is known about the chronic changes in endogenous cholesterol and its biosynthesis in regulating beta-cell stimulus-secretion coupling. We examined the effects of inhibiting endogenous beta-cell cholesterol biosynthesis by using the squalene epoxidase inhibitor, NB598. The expression of squalene epoxidase in primary and clonal beta-cells was confirmed by RT-PCR. Cholesterol reduction of 36-52% was observed in MIN6 cells, mouse and human pancreatic islets after a 48-h incubation with 10 mum NB598. A similar reduction in cholesterol was observed in the subcellular compartments of MIN6 cells. We found NB598 significantly inhibited both basal and glucose-stimulated insulin secretion from mouse pancreatic islets. CaV channels were markedly inhibited by NB598. Rapid photolytic release of intracellular caged Ca2+ and simultaneous measurements of the changes in membrane capacitance revealed that NB598 also inhibited exocytosis independently from CaV channels. These effects were reversed by cholesterol repletion. Our results indicate that endogenous cholesterol in pancreatic beta-cells plays a critical role in regulating insulin secretion. Moreover, chronic inhibition of cholesterol biosynthesis regulates the functional activity of CaV channels and insulin secretory granule mobilization and membrane fusion. Dysregulation of cellular cholesterol may cause impairment of beta-cell function, a possible pathogenesis leading to the development of type 2 diabetes.

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.

Sphingolipids, insulin resistance, and metabolic disease: new insights from in vivo manipulation of sphingolipid metabolism

Obesity and dyslipidemia are risk factors for metabolic disorders including diabetes and cardiovascular disease. Sphingolipids such as ceramide and glucosylceramides, while being a relatively minor component of the lipid milieu in most tissues, may be among the most pathogenic lipids in the onset of the sequelae associated with excess adiposity. Circulating factors associated with obesity (e.g., saturated fatty acids, inflammatory cytokines) selectively induce enzymes that promote sphingolipid synthesis, and lipidomic profiling reveals relationships between tissue sphingolipid levels and certain metabolic diseases. Moreover, studies in cultured cells and isolated tissues implicate sphingolipids in certain cellular events associated with diabetes and cardiovascular disease, including insulin resistance, pancreatic beta-cell failure, cardiomyopathy, and vascular dysfunction. However, definitive evidence that sphingolipids contribute to insulin resistance, diabetes, and atherosclerosis has come only recently, as researchers have found that pharmacological inhibition or genetic ablation of enzymes controlling sphingolipid synthesis in rodents ameliorates each of these conditions. Herein we will review the role of ceramide and other sphingolipid metabolites in insulin resistance, beta-cell failure, cardiomyopathy, and vascular dysfunction, focusing on these in vivo studies that identify enzymes controlling sphingolipid metabolism as therapeutic targets for combating metabolic disease.

Update on lipid membrane microdomains

PURPOSE OF REVIEW

Lipid membrane microdomains are involved in major types of disease, ranging from vascular and metabolic diseases to neurodegeneration, autoimmunity, infectious and inflammatory diseases, and cancer. This review provides an update of membrane microdomain abnormalities.

RECENT FINDINGS

Lipid membrane microdomains are dynamic assemblies of sphingolipids, cholesterol and proteins that dissociate and associate rapidly and form functional clusters. Membrane microdomain clustering is the key to how membrane microdomains can form lipid-protein platforms in cell membranes, functioning in membrane trafficking, cell polarization and signalling. Clustering of membrane microdomains can be modified, for example by dietary lipids and pharmacological agents.

SUMMARY

Metabolic overload through a cholesterol-rich and fat-rich diet can trigger metabolic learning, which is associated with membrane microdomain persistence, persistent signalling and disturbed vesicular traffic. Detailed characterization of lipid membrane microdomains and dynamics at the molecular level is necessary and will help to identify new dietary and pharmacological therapeutic targets for the treatment and prevention of lipid membrane microdomain related diseases.

Type I Gaucher disease, a glycosphingolipid storage disorder, is associated with insulin resistance

CONTEXT

Complex glycosphingolipids, in majority the ganglioside GM3, surround the insulin receptor in a special membrane compartment (raft) and modulate signaling through this receptor. Increased levels of GM3 in rafts impair insulin signaling, resulting in insulin resistance. Gaucher disease is a lysosomal storage disorder in which impaired breakdown of glucosylceramide leads to its accumulation in macrophages. Secondary to this defect, GM3 concentrations, for which glucosylceramide is the precursor, in plasma and several cell types are elevated.

OBJECTIVE

We studied the influence of glycosphingolipid storage on whole body glucose and fat metabolism by measuring insulin-mediated (IMGU) and noninsulin-mediated glucose uptake (NIMGU) and suppression of free fatty acids by insulin.

DESIGN AND MAIN OUTCOME MEASURES

We studied six Gaucher patients, either naive to treatment or with considerable remaining burden of disease, and six matched healthy control subjects in the basal state, during an euglycemic and a hyperglycemic clamp with somatostatin measuring NIMGU and during an euglycemic hyperinsulinemic clamp measuring IMGU, using stable isotopes.

RESULTS

NIMGU (both during euglycemia and hyperglycemia) did not differ between patients and control subjects. IMGU was lower in Gaucher patients, compared with controls. Suppression of lipolysis by insulin tended to be less effective in Gaucher patients.

CONCLUSION

Gaucher disease, a lysosomal glycosphingolipid storage disorder, is associated with (peripheral) insulin resistance, possibly through the influence of glycosphingolipids on insulin receptor functioning.

Expression of caveolin-1 in human adipose tissue is upregulated in obesity and obesity-associated type 2 diabetes mellitus and related to inflammation

OBJECTIVE

Caveolin-1 (CAV-1) plays important roles in many aspects of cellular biology, including vesicular transport, cholesterol homeostasis and signal transduction. The aim of the present study was to explore gene expression levels of CAV-1 in human adipose tissue in obesity and obesity-associated type 2 diabetes mellitus (T2DM) and to analyse its potential implication in the inflammatory state associated with obesity.

DESIGN AND METHODS

Visceral adipose tissue (VAT) and subcutaneous adipose tissue (SAT) obtained from 15 females were used in the study. Patients were classified as lean (BMI 20.8 +/- 1.0 kg/m(2)) or obese (BMI 50.5 +/- 2.6 kg/m(2)). The obese group was further subclassified as normoglycaemic (NG) or patients with T2DM. Anthropometric measurements as well as circulating metabolites, hormones and adipokines were determined. Real-time polymerase chain reaction (PCR) analyses were performed to quantify transcript levels of CAV-1 and monocyte chemoattractant protein (MCP-1).

RESULTS

The presence of CAV-1 protein was detected in VAT and SAT by immunohistochemistry. Both obese NG and with T2DM patients exhibited significantly higher CAV-1 expression levels in VAT and SAT compared with lean subjects (P < 0.05). No differences between obese NG and T2DM patients were observed in VAT. However, obese T2DM patients were found to have higher CAV-1 expression levels in SAT (P < 0.05) compared with obese NG patients. A significant correlation was found between CAV-1 mRNA expression levels in VAT and different circulating inflammatory markers such as sialic acid (SA) (P < 0.001) and fibrinogen (P < 0.001) as well as with MCP1 mRNA expression (P < 0.05).

CONCLUSION

Our findings show for the first time the upregulation of mRNA CAV-1 expression levels in VAT and SAT of obese NG and obese T2DM patients compared with lean controls, suggesting a role for CAV-1 in obesity and T2DM development. The association with different inflammatory markers further suggests an implication of CAV-1 in the low-grade inflammation accompanying obesity.

Lipid mediators of insulin resistance

Lipid abnormalities such as obesity, increased circulating free fatty acid levels, and excess intramyocellular lipid accumulation are frequently associated with insulin resistance. These observations have prompted investigators to speculate that the accumulation of lipids in tissues not suited for fat storage (e.g., skeletal muscle and liver) is an underlying component of insulin resistance and the metabolic syndrome. We review the metabolic fates of lipids in insulin-responsive tissues and discuss the roles of specific lipid metabolites (e.g., ceramides, GM3 ganglioside, and diacylglycerol) as antagonists of insulin signaling and action.

Lipid rafts are triage centers for multimeric and monomeric thyrotropin receptor regulation

The TSH receptor (TSHR), a heptahelical G protein-coupled receptor on the surface of thyrocytes, is a major autoantigen and physiological regulator of the thyroid gland. Unlike other G protein-coupled receptors, the TSHR undergoes posttranslational cleavage of its ectodomain, leading to the existence of several forms of the receptor on the plasma membrane. We previously hypothesized that to achieve high fidelity and specificity of TSH ligand or TSHR autoantibody signaling, the TSHR may compartmentalize into microdomains within the plasma membrane. In support of this hypothesis we have shown previously that TSHRs reside in GM1 ganglioside-enriched lipid rafts in the plasma membrane of TSHR-expressing cells. In this study, we further explored the different forms of TSHRs that reside in lipid rafts. We studied both TSHR-transfected cells and rat thyrocytes, using both nondetergent biochemical analyses and receptor-lipid raft colocalization. Using the biochemical approach, we observed that monomeric receptors existed in both raft and nonraft fractions of the cell surface in the steady state. We also demonstrated that the multimeric forms of the receptor were preferentially partitioned into the lipid microdomains. Different TSHR forms, including multimers, were dynamically regulated both by receptor-specific and postreceptor-specific modulators. TSH ligand and TSHR antibody of the stimulating variety induced a decrease of multimeric forms in the raft fractions. In addition, multimeric and monomeric forms of the receptor were both associated with Gsalpha within and without the rafts. Although failure to achieve total lipid raft disruption prevented a conclusion regarding the relative power of TSHR signaling within and without the raft domains, these data showed clearly that not only were a significant proportion of TSHRs residing within lipid microdomains but that constitutive multimerization of TSHRs was actually regulated within the lipid rafts.

Role of caveolins in body weight and insulin resistance regulation

In spite of the growing understanding of the fundamental biology of energy homeostasis, it is evident that crucial pathways have yet to be identified. Caveolins, integral plasma membrane proteins present in caveolae with proven scaffolding, transport and signalling capabilities, have emerged as key players in shifting the focus of obesity and insulin resistance development to lipid dynamics and membrane microdomain disorders. Caveolin-1-knockout mice exhibit a lean phenotype with overt resistance to diet-induced obesity. Conversely, caveolin-3-knockout mice show marked insulin resistance together with increased body weight and adiposity despite normal food intake. Here, we discuss the participation of caveolins as novel modulators of adipocyte biology, highlight their role in insulin signalling and outline their potential relevance to human obesity development and treatment.