Hepatic Apolipoprotein M (ApoM) Overexpression Stimulates Formation of Larger ApoM/Sphingosine 1-Phosphate-enriched Plasma High Density Lipoprotein

Apolipoprotein M (apoM), a lipocalin family member, preferentially associates with plasma HDL and binds plasma sphingosine 1-phosphate (S1P), a signaling molecule active in immune homeostasis and endothelial barrier function. ApoM overexpression in ABCA1-expressing HEK293 cells stimulated larger nascent HDL formation, compared with cells that did not express apoM; however, the in vivo role of apoM in HDL metabolism remains poorly understood. To test whether hepatic apoM overexpression increases plasma HDL size, we generated hepatocyte-specific apoM transgenic (APOM Tg) mice, which had an ∼3–5-fold increase in plasma apoM levels compared with wild-type mice. Although HDL cholesterol concentrations were similar to wild-type mice, APOM Tg mice had larger plasma HDLs enriched in apoM, cholesteryl ester, lecithin:cholesterol acyltransferase, and S1P. Despite the presence of larger plasma HDLs in APOM Tg mice, in vivo macrophage reverse cholesterol transport capacity was similar to that in wild-type mice. APOM Tg mice had an ∼5-fold increase in plasma S1P, which was predominantly associated with larger plasma HDLs. Primary hepatocytes from APOM Tg mice generated larger nascent HDLs and displayed increased sphingolipid synthesis and S1P secretion. Inhibition of ceramide synthases in hepatocytes increased cellular S1P levels but not S1P secretion, suggesting that apoM is rate-limiting in the export of hepatocyte S1P. Our data indicate that hepatocyte-specific apoM overexpression generates larger nascent HDLs and larger plasma HDLs, which preferentially bind apoM and S1P, and stimulates S1P biosynthesis for secretion. The unique apoM/S1P-enriched plasma HDL may serve to deliver S1P to extrahepatic tissues for atheroprotection and may have other as yet unidentified functions.     

Magainin-AM2 improves glucose homeostasis and beta cell function in high-fat fed mice

Background

Magainin-AM2, a previously described amphibian host-defense peptide, stimulates insulin- and glucagon-like peptide-1-release in vitro. This study investigated anti-diabetic effects of the peptide in mice with diet-induced obesity and glucose intolerance.

Methods

Male National Institute of Health Swiss mice were maintained on a high-fat diet for 12-weeks prior to the daily treatment with magainin-AM2. Various indices of glucose tolerance were monitored together with insulin secretory responsiveness of islets at conclusion of study.

Results

Following twice daily treatment with magainin-AM2 for 15 days, no significant difference in body weight and food intake was observed compared with saline-treated high fat control animals. However, non-fasting blood glucose was significantly (P < 0.05) decreased while plasma insulin concentrations were significantly (P < 0.05) increased. Oral and intraperitoneal glucose tolerance and insulin secretion following glucose administration via both routes were significantly (P < 0.05) enhanced. The peptide significantly (P < 0.001) improved insulin sensitivity as well as the beta cell responses of islets isolated from treated mice to a range of insulin secretagogues. Oxygen consumption, CO₂ production, respiratory exchange ratio and energy expenditure were not significantly altered by sub-chronic administration of magainin-AM2 but a significant (P < 0.05) reduction in fat deposition was observed.

Conclusion

These results indicate that magainin-AM2 improves glucose tolerance, insulin sensitivity and islet beta cells secretory responsiveness in mice with obesity-diabetes.

General significance

The activity of magainin-AM2 suggests the possibility of exploiting this peptide for treatment of type 2 diabetes.     

载脂蛋白M

载脂蛋白M(apoM)是一类在血液中主要与高密度脂蛋白（HDL）结合的载脂蛋白,呈组织特异性表达且有着众多生物学功能.体内外多种因素可从转录或转录后水平对其表达进行调控:肝细胞核因子-1α,4α（HNF-1α,4α）、肝受体同系物-1（LRH-1）、叉头框转录因子a2（Foxa2）、血小板活化因子（PAF）等可上调其表达;肝X受体（LXR）、维甲酸X受体（RXR）、法尼酯X受体（FXR）、小异源二聚体-1(SHP-1)以及绝大多数细胞因子可下调其表达,具体调节机制复杂. 结构上,apoM含有一个特征性的疏水性信号肽,可结合1 磷酸鞘氨醇（S1P）等小的生物活性脂,以此介导多项生命活动. 功能上,apoM能促进preβ-HDL的生成,并提高其一系列抗动脉粥样硬化的生物活性,如胆固醇逆向转运、抗炎、以及低密度脂蛋白（LDL）的抗氧化等.在一些糖尿病病人体内,apoM的含量也显著降低,而apoM含量的提高可以降低血糖含量,增加胰岛素分泌以及改善胰岛素抵抗,不少学者将其视为该病发生发展的一项预测指标.本文就近年来对apoM的生物学特性,特别是其表达调控机制和功能的研究进展进行综述.     

Novel GPR40 agonist AS2575959 exhibits glucose metabolism improvement and synergistic effect with sitagliptin on insulin and incretin secretion

Aims

GPR40 is a free fatty acid receptor that regulates glucose-dependent insulin secretion at pancreatic β-cells and glucagon-like peptide-1 (GLP-1), one of the major incretins, secretion at the endocrine cells of the gastrointestinal tract. We investigated the synergistic effect of AS2575959, a novel GPR40 agonist, in combination with sitagliptin, a major dipeptidyl peptidase-IV (DPP-IV) inhibitor, on glucose-dependent insulin secretion and GLP-1 secretion. In addition, we investigated the chronic effects of AS2575959 on whole-body glucose metabolism.

Main methods

We evaluated acute glucose metabolism on insulin and GLP-1 secretion using an oral glucose tolerance test (OGTT) as well as assessed the chronic glucose metabolism in diabetic ob/ob mice following the repeated administration of AS2575959.

Key findings

We discovered the novel GPR40 agonist sodium [(3S)-6-({4′-[(3S)-3,4-dihydroxybutoxy]-2,2′,6′-trimethyl[1,1′-biphenyl]-3-yl}methoxy)-3H-spiro[1-benzofuran-2,1′-cyclopropan]-3-yl]acetate (AS2575959) and found that the compound influenced glucose-dependent insulin secretion both in vitro pancreas β-cell-derived cells and in vivo mice OGTT. Further, we observed a synergistic effect of AS2575959 and DPP-IV inhibitor on insulin secretion and plasma GLP-1 level. In addition, we discovered the improvement in glucose metabolism on repeated administration of AS2575959.

Significance

To our knowledge, this study is the first to demonstrate the synergistic effect of a GPR40 agonist and DPP-IV inhibitor on the glucose-dependent insulin secretion and GLP-1 concentration increase. These findings suggest that GPR40 agonists may represent a promising therapeutic strategy for the treatment of type 2 diabetes mellitus, particularly when used in combination with DPP-IV inhibitors.     

The MTMR9 rs2293855 polymorphism is associated with glucose tolerance,insulin secretion,insulin sensitivity and increased risk of prediabetes

Background

Polymorphism of rs2293855 in gene MTMR9 has been associated with obesity and metabolic syndrome. We aim to study the association of rs2293855 with type 2 diabetes mellitus (T2DM) intermediate phenotypes in a Han Chinese population.

Methods

The polymorphism was genotyped in 838 Han Chinese individuals using Matrix-Assisted Laser Desorption/Ionization Time of Flight Mass Spectrometry (MALDI-TOF MS); all participants underwent a 75 g oral glucose tolerance test (OGTT); associations between the polymorphism and glucose tolerance, indices of insulin secretion and indices of insulin sensitivity were analyzed.

Results

The frequency of genotypes and alleles differed significantly between normal glucose tolerance and prediabetes (P = 0.043 and P = 0.009, respectively). The GG homozygous presented higher fasting plasma glucose (P = 0.009), higher 2-hour plasma glucose (P = 0.024) and higher glucose area under the curve (AUC, P = 0.01). Moreover, the G allele of rs2293855 was associated with glucose intolerance (fasting glucose, P = 0.012; glucose AUC, P = 0.006; 2-h glucose, P = 0.024); it is also associated with decreased indices of insulin sensitivity (fasting insulin, P = 0.043; insulin sensitivity index composite, P = 0.009; homeostasis model assessment of insulin resistance, HOMA-IR, P = 0.008) and decreased indices of insulin secretion (HOMA of beta cell function, HOMA-B, P = 0.028; insulinogenic index, P = 0.003). In addition, the minor allele G was also associated with increased risk of prediabetes (OR = 1.463, 95%CI: 1.066–2.009, P = 0.018).

Conclusions

Polymorphism of rs2293855 in MTMR9 is associated with measures of glucose tolerance, indices of insulin secretion and indices of insulin sensitivity. We also suggest that allele G is likely to increase the risk of prediabetes by influencing both insulin secretion and insulin sensitivity.     

Intralipid Decreases Apolipoprotein M Levels and Insulin Sensitivity in Rats

Background

Apolipoprotein M (ApoM) is a constituent of high-density lipoproteins (HDL). It plays a crucial role in HDL-mediated reverse cholesterol transport. Insulin resistance is associated with decreased ApoM levels.

Aims

To assess the effects of increased free fatty acids (FFAs) levels after short-term Intralipid infusion on insulin sensitivity and hepatic ApoM gene expression.

Methods

Adult male Sprague-Dawley (SD) rats infused with 20% Intralipid solution for 6 h. Glucose infusion rates (GIR) were determined by hyperinsulinemic-euglycemic clamp during Intralipid infusion and plasma FFA levels were measured by colorimetry. Rats were sacrificed after Intralipid treatment and livers were sampled. Human embryonic kidney 293T cells were transfected with a lentivirus mediated human apoM overexpression system. Goto-Kakizaki (GK) rats were injected with the lentiviral vector and insulin tolerance was assessed. Gene expression was assessed by real-time RT-PCR and PCR array.

Results

Intralipid increased FFAs by 17.6 folds and GIR was decreased by 27.1% compared to the control group. ApoM gene expression was decreased by 40.4% after Intralipid infusion. PPAR_β/δ expression was not changed by Intralipid. Whereas the mRNA levels of Acaca, Acox1, Akt1, V-raf murine sarcoma 3611 viral oncogene homolog, G6pc, Irs2, Ldlr, Map2k1, pyruvate kinase and RBC were significantly increased in rat liver after Intralipid infusion. The Mitogen-activated protein kinase 8 (MAPK8) was significantly down-regulated in 293T cells overexpressing ApoM. Overexpression of human ApoM in GK rats could enhance the glucose-lowering effect of exogenous insulin.

Conclusion

These results suggest that Intralipid could decrease hepatic ApoM levels. ApoM overexpression may have a potential role in improving insulin resistance in vivo and modulating apoM expression might be a future therapeutic strategy against insulin resistance in type 2 diabetes.     

Apolipoprotein M promotes mobilization of cellular cholesterol in vivo

Objective

The HDL associated apolipoprotein M (apoM) protects against experimental atherosclerosis but the mechanism is unknown. ApoM increases preβ-HDL formation. We explored whether plasma apoM affects mobilization of cholesterol from peripheral cells in mice.

Methods and results

ApoM-enriched HDL from apoM-transgenic mice increased the in vitro efflux of ³H-cholesterol from macrophages by 24 ± 3% (p < 0.05) as compared with HDL from wild type (WT) mice, thus confirming previous findings. However, apoM-free HDL was not poorer than that of WT HDL to mobilize ³H-cholesterol. ³H-cholesterol-labeled foam cells were implanted in the peritoneal cavity of apoM^−/−, WT and apoM-transgenic mice to assess the mobilization of cholesterol from foam cells in vivo and subsequent excretion into feces. The results showed a statistically non-significant trend towards increased mobilization of cellular cholesterol to plasma with increasing plasma apoM. However, the apoM-genotype did not affect the excretion of ³H-cholesterol in feces. Nevertheless, when apoM^−/−, apoM-transgenic and WT mice received a constant intravenous infusion of ¹³C₂-cholesterol/intralipid for 5 h, the rate of enrichment of blood free cholesterol with free ¹³C₂-cholesterol was significantly lower (consistent with an increase in flux of unlabeled free cholesterol into the plasma) in the apoM-transgenic (3.0 ± 0.9‰/h) as compared to WT (5.7 ± 0.9‰/h, p < 0.05) and apoM^−/− (6.5 ± 0.6‰/h, p < 0.01) mice.

Conclusion

The present data indicate that the plasma apoM levels modulate the ability of plasma to mobilize cellular cholesterol, whereas apoM has no major effect on the excretion of cholesterol into feces.     

High-fat/high-cholesterol diet promotes a S1P receptor-mediated antiapoptotic activity for VLDL

Withdrawing growth factors or serum from endothelial cells leads to the activation of effector caspases 3 and 7, resulting in apoptotic cell death. HDL protects against caspase induction through sphingosine-1-phosphate (S1P) receptors. This anti-caspase activity of HDL is antagonized by VLDL from apolipoprotein E4 (apoE4) (genotype, APOE4/4; apolipoprotein, apoE) targeted replacement (TR) mice, but not by VLDL from TR APOE3/3 mice, and requires the binding of apoE4-VLDL to an LDL receptor family member. In the absence of HDL, apoE4-VLDL and apoE3-VLDL from TR mice have limited antiapoptotic activity. In contrast, we show here that a high-fat/high-cholesterol/cholate diet (HFD) radically alters this biological activity of VLDL. On HFD, both apoE3-VLDL and apoE4-VLDL (HFD VLDL) inhibit caspase 3/7 activation initiated by serum withdrawal. This activity of HFD VLDL is independent of an LDL receptor family member but requires the activation of S1P(3) receptors, as shown by the ability of pharmacological block of S1P receptors by VPC 23019 and by small interfering RNA-mediated downregulation of S1P(3) receptors to inhibit HFD VLDL anticaspase activity.     

The plasma concentration of HDL-associated apoM is influenced by LDL receptor-mediated clearance of apoB-containing particles

ApoM is mainly associated with HDL. Nevertheless, we have consistently observed positive correlations of apoM with plasma LDL cholesterol in humans. Moreover, LDL receptor deficiency is associated with increased plasma apoM in mice. Here, we tested the idea that plasma apoM concentrations are affected by the rate of LDL receptor-mediated clearance of apoB-containing particles. We measured apoM in humans each carrying one of three different LDL receptor mutations (n = 9) or the apoB3500 mutation (n = 12). These carriers had increased plasma apoM (1.34 ± 0.13 μM, P = 0.003, and 1.23 ± 0.10 μM, P = 0.02, respectively) as compared with noncarriers (0.93 ± 0.04 μM). When we injected human apoM-containing HDL into Wt (n = 6) or LDL receptor-deficient mice (n = 6), the removal of HDL-associated human apoM was delayed in the LDL receptor-deficient mice. After 2 h, 54 ± 5% versus 90 ± 8% (P < 0.005) of the initial amounts of human apoM remained in the plasma of Wt and LDL receptor-deficient mice, respectively. Finally, we compared the turnover of radio-iodinated LDL and plasma apoM concentrations in 45 normocholesterolemic humans. There was a negative correlation between plasma apoM and the fractional catabolic rate of LDL (r = -0.38, P = 0.009). These data suggest that the plasma clearance of apoM, despite apoM primarily being associated with HDL, is influenced by LDL receptor-mediated clearance of apoB-containing particles.     

High uric acid directly inhibits insulin signalling and induces insulin resistance

Background and aim

Accumulating clinical evidence suggests that hyperuricemia is strongly associated with abnormal glucose metabolism and insulin resistance. However, how high uric acid (HUA) level causes insulin resistance remains unclear. We aimed to determine the direct role of HUA in insulin resistance in vitro and in vivo in mice.

Methods

An acute hyperuricemia mouse model was created by potassium oxonate treatment, and the impact of HUA level on insulin resistance was investigated by glucose tolerance test, insulin tolerance test and insulin signalling, including phosphorylation of insulin receptor substrate 1 (IRS1) and Akt. HepG2 cells were exposed to HUA treatment and N-acetylcysteine (NAC), reactive oxygen species scavenger; IRS1 and Akt phosphorylation was detected by Western blot analysis after insulin treatment.

Results

Hyperuricemic mice showed impaired glucose tolerance with insulin resistance. Hyperuricemia inhibited phospho-Akt (Ser473) response to insulin and increased phosphor-IRS1 (Ser307) in liver, muscle and fat tissues. HUA induced oxidative stress, and the antioxidant NAC blocked HUA-induced IRS1 activation and Akt inhibition in HepG2 cells.

Conclusion

This study supplies the first evidence of HUA directly inducing insulin resistance in vivo and in vitro. Increased uric acid level may inhibit IRS1 and Akt insulin signalling and induce insulin resistance. The reactive oxygen species pathway plays a key role in HUA-induced insulin resistance.     

Mitochondrial dysfunction and complications associated with diabetes

Background

Diabetes is a metabolic syndrome that results in chronically increased blood glucose (hyperglycaemia) due to defects either in insulin secretion consequent to the loss of beta cells in the pancreas (type 1) or to loss of insulin sensitivity in target organs in the presence of normal insulin secretion (type 2). Long term hyperglycaemia can lead to a number of serious health-threatening pathologies, or complications, especially in the kidney, heart, retina and peripheral nervous system.

Scope of review

Here we summarise the current literature on the role of the mitochondria in complications associated with diabetes, and the limitations and potential of rodent models to explore new modalities to limit complication severity.

Major conclusions

Prolonged hyperglycaemia results in perturbation of catabolic pathways and in an over-production of ROS by the mitochondria, which in turn may play a role in the development of diabetic complications. Furthermore, current models don't offer a comprehensive recapitulation of these complications.

General significance

The onset of complications associated with type 1 diabetes can be varied, even with tightly controlled blood glucose levels. The potential role of inherited, mild mitochondrial dysfunction in accelerating diabetic complications, both in type 1 and 2 diabetes, remains unexplored. This article is part of a Special Issue entitled Frontiers of Mitochondrial Research.     

Association of resistin gene polymorphisms with insulin resistance in Egyptian obese patients

Background

Obesity associated insulin resistance is a major risk factor for type 2 diabetes mellitus. Resistin is recently reported to provide a link between obesity, insulin resistance and type 2 diabetes mellitus. We aimed to investigate the possible associations of resistin gene (RETN) polymorphisms with obesity, and to detect whether these polymorphisms are associated with glucose intolerance and type 2 diabetes mellitus in obese patients.

Methods

One hundred and forty-five Egyptian obese patients with or without glucose intolerance and 155 unrelated healthy controls were enrolled in this study. Polymorphisms of RETN + 299G>A and RETN –420 C>G gene were detected by polymerase chain reaction - restriction fragment length polymorphism (PCR-RFLP). Serum resistin was measured by ELISA.

Results

RETN + 299 AA and RETN − 420 GG genotypes were significantly associated with obesity in Egyptian population. Moreover, the mutant alleles or genotypes of both examined polymorphisms were associated with impaired glucose tolerance and diabetes mellitus compared to normal glucose tolerant obese patients. Furthermore, our results revealed elevated waist/hip ratio, BMI, blood pressure, fasting blood glucose level, HOMA-IR, triglycerides, total cholesterol, resistin level, and decreased HDL cholesterol level in homozygote mutant genotypes carriers of both RETN polymorphisms among obese patients.

Conclusion

Resistin gene polymorphisms may play an important role in pathogenesis and susceptibility to obesity, impaired glucose tolerance, and type 2 diabetes mellitus in Egyptian population.     

Uncleaved ApoM Signal Peptide Is Required for Formation of Large ApoM/Sphingosine 1-Phosphate (S1P)-enriched HDL Particles

Apolipoprotein M (apoM), a plasma sphingosine 1-phosphate (S1P) carrier, associates with plasma HDL via its uncleaved signal peptide. Hepatocyte-specific apoM overexpression in mice stimulates formation of both larger nascent HDL in hepatocytes and larger mature apoM/S1P-enriched HDL particles in plasma by enhancing hepatic S1P synthesis and secretion. Mutagenesis of apoM glutamine 22 to alanine (apoM^Q22A) introduces a functional signal peptidase cleavage site. Expression of apoM^Q22A in ABCA1-expressing HEK293 cells resulted in the formation of smaller nascent HDL particles compared with wild type apoM (apoM^WT). When apoM^Q22A was expressed in vivo, using recombinant adenoviruses, smaller plasma HDL particles and decreased plasma S1P and apoM were observed relative to expression of apoM^WT. Hepatocytes isolated from both apoM^WT- and apoM^Q22A-expressing mice displayed an equivalent increase in cellular levels of S1P, relative to LacZ controls; however, relative to apoM^WT, apoM^Q22A hepatocytes displayed more rapid apoM and S1P secretion but minimal apoM^Q22A bound to nascent lipoproteins. Pharmacologic inhibition of ceramide synthesis increased cellular sphingosine and S1P but not medium S1P in both apoM^WT and apoM^Q22A hepatocytes. We conclude that apoM secretion is rate-limiting for hepatocyte S1P secretion and that its uncleaved signal peptide delays apoM trafficking out of the cell, promoting formation of larger nascent apoM- and S1P-enriched HDL particles that are probably precursors of larger apoM/S1P-enriched plasma HDL.     

Association of P213S polymorphism of the L-selectin gene with type 2 diabetes and insulin resistance in Chinese population

Aims

L-selectin belongs to selectin family of adhesion molecule and participates in the generation and development of type 2 diabetes (T2D). In this study, we evaluated the relationship between the P213S polymorphism of L-selectin gene and T2D and insulin resistance in the Chinese population.

Methods

We genotyped P213S polymorphism in 801 patients with T2D and 834 healthy controls in the Chinese population using polymerase chain reaction–ligase detection reaction (PCR–LDR) technique. Plasma glucose, insulin, lipid, blood urea nitrogen, creatinine and uric acid levels were measured by biochemical technique.

Results

The frequency of 213PP genotype and P allele of the L-selectin gene in patients with T2D was significantly higher than that in controls (P = 0.007; P = 0.019, respectively). The relative risk of allele P suffered from T2D was 1.191 times higher than that of allele S. Moreover, the levels of FPG and HOMA-IR of PP and PS genotype carriers were significantly higher than those of SS genotype carriers in the T2D group (P < 0.05).

Conclusion

These findings indicated that the P213S polymorphism of L‐selectin gene may contribute to susceptibility to T2D and insulin resistance in the Chinese population, and P allele appears to be a risk factor for T2D.     

HDL3, but not HDL2, stimulates plasminogen activator inhibitor-1 release from adipocytes: the role of sphingosine-1-phosphate

Sphingosine-1-phosphate (S1P) is a bioactive lysophospholipid that regulates numerous key cardiovascular functions. High-density lipoproteins (HDLs) are the major plasma lipoprotein carriers of S1P. Fibrinolysis is a physiological process that allows fibrin clot dissolution, and decreased fibrinolytic capacity may result from increased circulating levels of plasminogen activator inhibitor-1 (PAI-1). We examined the effect of S1P associated with HDL subfractions on PAI-1 secretion from 3T3 adipocytes. S1P concentration in HDL3 averaged twice that in HDL2. Incubation of adipocytes with increasing concentrations of S1P in HDL3, but not HDL2, or with S1P complexed to albumin stimulated PAI-I secretion in a concentration-dependent manner. Quantitative RT-PCR revealed that S1P_1–3 are expressed in 3T3 adipocytes, with S1P₂ expressed in the greatest amount. Treatment of adipocytes with the S1P₁ and S1P₃ antagonist VPC23019 did not block PAI-1 secretion. Inhibiting S1P₂ with JTE-013 or reducing the expression of the gene coding for S1P₂ using silencing RNA (siRNA) technology blocked PAI-1 secretion, suggesting that the S1P₂ receptor mediates PAI-1 secretion from adipocytes exposed to HDL3 or S1P. Treatment with the phospholipase C (PLC) inhibitor {"type":"entrez-nucleotide","attrs":{"text":"U73122","term_id":"4098075","term_text":"U73122"}}U73122, the protein kinase C (PKC) inhibitor RO-318425, or the Rho-associated protein kinase (ROCK) inhibitor Y27632 all significantly inhibited HDL3- and S1P-mediated PAI-1 release, suggesting that HDL3- and/or S1P-stimulated PAI-1 secretion from 3T3 cells is mediated by activation of multiple, downstream signaling pathways of S1P₂.     

S1P stimulates chemotactic migration and invasion in OVCAR3 ovarian cancer cells

OVCAR3 ovarian cancer cells express three sphingosine 1-phosphate (S1P) receptors, S1P(1), S1P(2), and S1P(3), but not S1P(4). Stimulation of OVCAR3 cells with S1P induced intracellular calcium increases, which were partly inhibited by VPC 23019 (an S1P(1/3) antagonist). S1P-induced calcium increases were mediated by phospholipase C and pertussis toxin (PTX)-sensitive G-proteins in OVCAR3 cells. S1P stimulated extracellular signal-regulated kinase, p38 kinase, and Akt which were inhibited by PTX. S1P-stimulated chemotactic migration of OVCAR3 cells in a PTX-sensitive manner, indicating crucial role of G(i) protein(s) in the process. S1P-induced chemotactic migration of OVCAR3 cells was completely inhibited by LY294002 and SB203580. Pretreatment of VPC 23019 (an S1P(1/3) antagonist) completely inhibited S1P-induced chemotaxis. S1P also induced invasion of OVCAR3 cells, which was also inhibited by VPC 23019. Taken together, this study suggests that S1P stimulate chemotactic migration and cellular invasion, and VPC 23019-sensitive S1P receptor(s) might be involved in the processes.     

HDL-like lipoproteins in cerebrospinal fluid affect neural cell activity through lipoprotein-associated sphingosine 1-phosphate

High-density lipoprotein (HDL)-associated sphingosine 1-phosphate mediates a variety of lipoprotein-induced actions in vascular cell systems. However, it remains unknown whether extracellular S1P is associated with lipoproteins to exert biological actions in central nervous system. Human cerebrospinal fluid (CSF) induced rat astrocyte migration in a manner sensitive to S1P receptor antagonist VPC23019 and the migration activity was recovered in S1P fraction by thin-layer chromatography. Density-gradient separation of CSF revealed that the major S1P activity was detected in the HDL fraction. In conditioned medium of rat astrocytes cultured with sphingosine, the S1P activity was recovered again in the HDL fraction. The HDL fraction also induced migration of astrocytes and process retraction of oligodendrocytes in a manner similar to S1P. We concluded that S1P is accumulated in HDL-like lipoproteins in CSF and mediates some of lipoprotein-induced neural cell functions in central nervous system.     

The mitochondrial isoform of phosphoenolpyruvate carboxykinase (PEPCK-M) and glucose homeostasis: Has it been overlooked?

Background

Plasma glucose levels are tightly regulated within a narrow physiologic range. Insulin-mediated glucose uptake by tissues must be balanced by the appearance of glucose from nutritional sources, glycogen stores, or gluconeogenesis. In this regard, a common pathway regulating both glucose clearance and appearance has not been described. The metabolism of glucose to produce ATP is generally considered to be the primary stimulus for insulin release from beta-cells. Similarly, gluconeogenesis from phosphoenolpyruvate (PEP) is believed to be the primarily pathway via the cytosolic isoform of phosphoenolpyruvate carboxykinase (PEPCK-C). These models cannot adequately explain the regulation of insulin secretion or gluconeogenesis.

Scope of review

A metabolic sensing pathway involving mitochondrial GTP (mtGTP) and PEP synthesis by the mitochondrial isoform of PEPCK (PEPCK-M) is associated with glucose-stimulated insulin secretion from pancreatic beta-cells. Here we examine whether there is evidence for a similar mtGTP-dependent pathway involved in gluconeogenesis. In both islets and the liver, mtGTP is produced at the substrate level by the enzyme succinyl CoA synthetase (SCS-GTP) with a rate proportional to the TCA cycle. In the beta-cell PEPCK-M then hydrolyzes mtGTP in the production of PEP that, unlike mtGTP, can escape the mitochondria to generate a signal for insulin release. Similarly, PEPCK-M and mtGTP might also provide a significant source of PEP in gluconeogenic tissues for the production of glucose. This review will focus on the possibility that PEPCK-M, as a sensor for TCA cycle flux, is a key mechanism to regulate both insulin secretion and gluconeogenesis suggesting conservation of this biochemical mechanism in regulating multiple aspects of glucose homeostasis. Moreover, we propose that this mechanism may be important for regulating insulin secretion and gluconeogenesis compared to canonical nutrient sensing pathways.

Major conclusions

PEPCK-M, initially believed to be absent in islets, carries a substantial metabolic flux in beta-cells. This flux is intimately involved with the coupling of glucose-stimulated insulin secretion. PEPCK-M activity may have been similarly underestimated in glucose producing tissues and could potentially be an unappreciated but important source of gluconeogenesis.

General significance

The generation of PEP via PEPCK-M may occur via a metabolic sensing pathway important for regulating both insulin secretion and gluconeogenesis. This article is part of a Special Issue entitled Frontiers of Mitochondrial Research.     

Chlorella modulates insulin signaling pathway and prevents high-fat diet-induced insulin resistance in mice

Aims

The search for natural agents that minimize obesity-associated disorders is receiving special attention. In this regard, the present study aimed to evaluate the prophylactic effect of Chlorella vulgaris (CV) on body weight, lipid profile, blood glucose and insulin signaling in liver, skeletal muscle and adipose tissue of diet-induced obese mice.

Main methods

Balb/C mice were fed either with standard rodent chow diet or high-fat diet (HFD) and received concomitant treatment with CV for 12 consecutive weeks. Triglyceride, free fatty acid, total cholesterol and fractions of cholesterol were measured using commercial assay. Insulin and leptin levels were determined by enzyme-linked immunosorbent assay (ELISA). Insulin and glucose tolerance tests were performed. The expression and phosphorylation of IRβ, IRS-1 and Akt were determined by Western blot analyses.

Key findings

Herein we demonstrate for the first time in the literature that prevention by CV of high-fat diet-induced insulin resistance in obese mice, as shown by increased glucose and insulin tolerance, is in part due to the improvement in the insulin signaling pathway at its main target tissues, by increasing the phosphorylation levels of proteins such as IR, IRS-1 and Akt. In parallel, the lower phosphorylation levels of IRS-1^ser307 were observed in obese mice. We also found that CV administration prevents high-fat diet-induced dyslipidemia by reducing triglyceride, cholesterol and free fatty acid levels.

Significance

We propose that the modulatory effect of CV treatment preventing the deleterious effects induced by high-fat diet is a good indicator for its use as a prophylactic–therapeutic agent against obesity-related complications.     

Lipoic acid prevents liver metabolic changes induced by administration of a fructose-rich diet

Background

To evaluate whether co-administration of R/S-α-lipoic acid can prevent the development of oxidative stress and metabolic changes induced by a fructose-rich diet (F).

Methods

We assessed glycemia in the fasting state and during an oral glucose tolerance test, triglyceridemia and insulinemia in rats fed with standard diet (control) and fructose without or with R/S-α-lipoic acid. Insulin resistance and hepatic insulin sensitivity were also calculated. In liver, we measured reduced glutathione, protein carbonyl groups, antioxidant capacity by ABTS assay, antioxidant enzymes (catalase and superoxide dismutase 1 and 2), uncoupling protein 2, PPARδ and PPARγ protein expressions, SREBP-1c, fatty acid synthase and glycerol-3-phosphate acyltransferase-1 gene expression, and glucokinase activity.

Results

R/S-α-lipoic acid co-administration to F-fed rats a) prevented hyperinsulinemia, hypertriglyceridemia and insulin resistance, b) improved hepatic insulin sensitivity and glucose tolerance, c) decreased liver oxidative stress and increased antioxidant capacity and antioxidant enzymes expression, d) decreased uncoupling protein 2 and PPARδ protein expression and increased PPARγ levels, e) restored the basal gene expression of PPARδ, SREBP-1c and the lipogenic genes fatty acid synthase and glycerol-3-phosphate acyltransferase, and f) decreased the fructose-mediated enhancement of glucokinase activity.

Conclusions

Our results suggest that fructose-induced oxidative stress is an early phenomenon associated with compensatory hepatic metabolic mechanisms, and that treatment with an antioxidant prevented the development of such changes.

General significance

This knowledge would help to better understand the mechanisms involved in liver adaptation to fructose-induced oxidative stress and to develop effective strategies to prevent and treat, at early stages, obesity and type 2 diabetes mellitus.