Free fatty acids inhibit insulin signaling-stimulated endothelial nitric oxide synthase activation through upregulating PTEN or inhibiting Akt kinase

Exendin-4 protects endothelial cells from lipoapoptosis by PKA, PI3K, eNOS, p38 MAPK, and JNK pathways

Experimental studies have indicated that endothelial cells play an important role in maintaining vascular homeostasis. We previously reported that human coronary artery endothelial cells (HCAECs) express the glucagon-like peptide 1 (GLP1) receptor and that the stable GLP1 mimetic exendin-4 is able to activate the receptor, leading to increased cell proliferation. Here, we have studied the effect of exendin-4 and native GLP1 (7-36) on lipoapoptosis and its underlying mechanisms in HCAECs. Apoptosis was assessed by DNA fragmentation and caspase-3 activation, after incubating cells with palmitate. Nitric oxide (NO) and reactive oxidative species (ROS) were analyzed. GLP1 receptor activation, PKA-, PI3K/Akt-, eNOS-, p38 MAPK-, and JNK-dependent pathways, and genetic silencing of transfection of eNOS were also studied. Palmitate-induced apoptosis stimulated cells to release NO and ROS, concomitant with upregulation of eNOS, which required activation of p38 MAPK and JNK. Exendin-4 restored the imbalance between NO and ROS production in which ROS production decreased and NO production was further augmented. Incubation with exendin-4 and GLP1 (7-36) protected HCAECs against lipoapoptosis, an effect that was blocked by PKA, PI3K/Akt, eNOS, p38 MAPK, and JNK inhibitors. Genetic silencing of eNOS also abolished the anti-apoptotic effect afforded by exendin-4. Our results support the notion that GLP1 receptor agonists restore eNOS-induced ROS production due to lipotoxicity and that such agonists protect against lipoapoptosis through PKA-PI3K/Akt-eNOS-p38 MAPK-JNK-dependent pathways via a GLP1 receptor-dependent mechanism.

Free fatty acid induces endoplasmic reticulum stress and apoptosis of β-cells by Ca2+/calpain-2 pathways

Dysfunction of β-cells is a major characteristic in the pathogenesis of type 2 diabetes mellitus (T2DM). The combination of obesity and T2DM is associated with elevated plasma free fatty acids (FFAs). However, molecular mechanisms linking FFAs to β-cell dysfunction remain poorly understood. In the present study, we identified that the major endoplasmic reticulum stress (ERS) marker, Grp78 and ERS-induced apoptotic factor, CHOP, were time-dependently increased by exposure of β-TC3 cells to FFA. The expression of ATF6 and the phosphorylation levels of PERK and IRE1, which trigger ERS signaling, markedly increased after FFA treatments. FFA treatments increased cell apoptosis by inducing ERS in β-TC3 cells. We also found that FFA-induced ERS was mediated by the store-operated Ca²⁺ entry through promoting the association of STIM1 and Orai1. Moreover, calpain-2 was required for FFA-induced expression of CHOP and activation of caspase-12 and caspase-3, thus promoting cell apoptosis in β-TC3 cells. Together, these results reveal pivotal roles for Ca²⁺/calpain-2 pathways in modulating FFA-induced β-TC3 cell ERS and apoptosis.

Free fatty acids inhibit TM-EPCR expression through JNK pathway: an implication for the development of the prothrombotic state in metabolic syndrome

Metabolic syndrome is associated with significant hypercoagulable prothrombotic tendency; however, the mechanism for the prothrombotic state is not completely understood. We hypothesize that higher circulating plasma free fatty acids (FFAs) in metabolic syndrome inhibit the endothelial thrombomodulin (TM)-endothelial protein C receptor (EPCR) pathway, thereby promoting thrombus formation. Human umbilical vein endothelial cells were cultured in media supplemented with various doses of palmitic acid (PA), in the presence or absence of JNK inhibitor, and the expression of TM and EPCR was measured by western blot. The thrombotic state of high fat fed C57BL/6J mice was examined by tail bleeding time and deep venous thrombosis (DVT) model. As a result, PA inhibited the expression of TM and EPCR in endothelial cells, and this effect was blunted by inhibiting JNK signaling. High fat diet fed mice had higher level of circulating FFAs and exhibited prothrombotic state, evidenced by increased tail bleeding time and enlarged thrombotic size in DVT model, compared to the control diet fed mice. Hence, FFAs inhibit TM-EPCR-Protein C system in endothelial cells through activating JNK signaling, which may be a mechanism for the prothrombotic state in metabolic syndrome.

Endoplasmic reticulum stress upregulates protein tyrosine phosphatase 1B and impairs glucose uptake in cultured myotubes

AIMS/HYPOTHESIS

Endoplasmic reticulum (ER) stress has been recognised as a common pathway in the development of obesity-associated insulin resistance. Protein tyrosine phosphatase 1B (PTP1B) is a negative regulator of insulin signalling and is localised on the ER membrane. The aim of the study was to investigate the cross-talk between ER stress and PTP1B.

METHODS

Leptin-deficient obese (ob/ob), Ptp1b (also known as Ptpn1) knockout and C57BL/6J mice were subjected to a high-fat or normal-chow diet for 20 weeks. ER stress was induced in cultured myotubes by treatment with tunicamycin. Immunohistochemistry and western blotting were used to assess proteins involved in the ER stress response. Myotube glucose uptake was determined by measuring 2-deoxy[(3)H]glucose incorporation.

RESULTS

A high-fat diet induced ER stress and PTP1B expression in the muscle tissue of mice and these responses were attenuated by treatment with the ER chaperone tauroursodeoxycholic acid (TUDCA). Cultured myotubes exhibited increased levels of PTP1B in response to tunicamycin treatment. Silencing of Ptp1b with small interfering RNA (siRNA) or overexpression of Ptp1b with adenovirus construct failed to alter the levels of ER stress. Ptp1b knockout mice did not differ from the wild-type mice in the extent of tunicamycin-induced upregulation of glucose-regulated protein-78. However, tunicamycin-induced phosphorylation of eukaryotic initiation factor 2α and c-Jun NH(2)-terminal kinase-2 were significantly attenuated in the Ptp1b knockout mice. Treatment with TUDCA or silencing of PTP1B reversed tunicamycin-induced blunted myotube glucose uptake.

CONCLUSIONS/INTERPRETATION

Our data suggest that PTP1B is activated by ER stress and is required for full-range activation of ER stress pathways in mediating insulin resistance in the skeletal muscle.

Oxidative stress and myocardial injury in the diabetic heart

Reactive oxygen or nitrogen species play an integral role in both myocardial injury and repair. This dichotomy is differentiated at the level of species type, amount and duration of free radical generated. Homeostatic mechanisms designed to prevent free radical generation in the first instance, scavenge, or enzymatically convert them to less toxic forms and water, playing crucial roles in the maintenance of cellular structure and function. The outcome between functional recovery and dysfunction is dependent upon the inherent ability of these homeostatic antioxidant defences to withstand acute free radical generation, in the order of seconds to minutes. Alternatively, pre-existent antioxidant capacity (from intracellular and extracellular sources) may regulate the degree of free radical generation. This converts reactive oxygen and nitrogen species to the role of second messenger involved in cell signalling. The adaptive capacity of the cell is altered by the balance between death or survival signal converging at the level of the mitochondria, with distinct pathophysiological consequences that extends the period of injury from hours to days and weeks. Hyperglycaemia, hyperlipidaemia and insulin resistance enhance oxidative stress in the diabetic myocardium that cannot adapt to ischaemia-reperfusion. Altered glucose flux, mitochondrial derangements and nitric oxide synthase uncoupling in the presence of decreased antioxidant defence and impaired prosurvival cell signalling may render the diabetic myocardium more vulnerable to injury, remodelling and heart failure.

AKT2 confers protection against aortic aneurysms and dissections

RATIONALE

Aortic aneurysm and dissection (AAD) are major diseases of the adult aorta caused by progressive medial degeneration of the aortic wall. Although the overproduction of destructive factors promotes tissue damage and disease progression, the role of protective pathways is unknown.

OBJECTIVE

In this study, we examined the role of AKT2 in protecting the aorta from developing AAD.

METHODS AND RESULTS

AKT2 and phospho-AKT levels were significantly downregulated in human thoracic AAD tissues, especially within the degenerative medial layer. Akt2-deficient mice showed abnormal elastic fibers and reduced medial thickness in the aortic wall. When challenged with angiotensin II, these mice developed aortic aneurysm, dissection, and rupture with features similar to those in humans, in both thoracic and abdominal segments. Aortas from Akt2-deficient mice displayed profound tissue destruction, apoptotic cell death, and inflammatory cell infiltration that were not observed in aortas from wild-type mice. In addition, angiotensin II-infused Akt2-deficient mice showed significantly elevated expression of matrix metalloproteinase-9 (MMP-9) and reduced expression of tissue inhibitor of metalloproteinase-1 (TIMP-1). In cultured human aortic vascular smooth muscle cells, AKT2 inhibited the expression of MMP-9 and stimulated the expression of TIMP-1 by preventing the binding of transcription factor forkhead box protein O1 to the MMP-9 and TIMP-1 promoters.

CONCLUSIONS

Impaired AKT2 signaling may contribute to increased susceptibility to the development of AAD. Our findings provide evidence of a mechanism that underlies the protective effects of AKT2 on the aortic wall and that may serve as a therapeutic target in the prevention of AAD.

Overexpression of steroidogenic acute regulatory protein in rat aortic endothelial cells attenuates palmitic acid-induced inflammation and reduction in nitric oxide bioavailability

BACKGROUND

Endothelial dysfunction is a well documented evidence for the onset of atherosclerosis and other cardiovascular diseases. Lipids disorder is among the main risk factors for endothelial dysfunction in these diseases. Steroidogenic acute regulatory protein (StAR), one of the cholesterol transporters, plays an important role in the maintenance of intracellular lipid homeostasis. However, the effect of StAR on endothelial dysfunction is not well understood. Palmitic acid (PA) has been shown to decrease eNOS activity and induce inflammation, both are the causes of endothelial dysfunction, in an endothelial cell culture model.

METHODS

StAR gene was introduced into primary rat aortic endothelial cells by adenovirus infection. Real-time PCR and Western blotting were performed to determine the relative genes and proteins expression level to elucidate the underlying mechanism. The free fatty acid and cholesterol quantification kits were used to detect total cellular free fatty acid and cholesterol. The levels of inflammatory factors and nitric oxide were determined by ELISA and classic Griess reagent methods respectively.

RESULTS

We successfully overexpressed StAR in primary rat aortic endothelial cells. Following StAR overexpression, mRNA levels of IL-1β, TNFα, IL6 and VCAM-1 and protein levels of IL-1β, , TNFα and IL-6 in culture supernatant were significantly decreased, which duing to blocke NFκB nuclear translocation and activation. Moreover, StAR overexpression attenuated the PA-induced reduction of nitric oxide bioavailability by protecting the bioactivity of pAkt/peNOS/NO pathway. Furthermore, the key genes involved in lipid metabolism were greatly reduced following StAR overexpression. In order to investigate the underlying mechanism, cerulenin and lovastatin, the inhibitor of fatty acid and cholesterol synthase, were added prior to PA treatment. The results showed that both cerulenin and lovastatin had a similar effect as StAR overexpression. On the other hand, the role of StAR was inhibited when siRNA was introduced to reduce StAR expression.

CONCLUSIONS

Our results showed that StAR attenuated lipid synthesis and uptake as well as PA-induced inflammation and reduction in NO bioavailability in aortic endothelial cells. StAR can ameliorate endothelial dysfunction induced by PA via reducing the intracellular lipid levels.

Vascular effects of intravenous intralipid and dextrose infusions in obese subjects

Hyperglycemia and elevated free fatty acids (FFA) are implicated in the development of endothelial dysfunction. Infusion of soy-bean oil-based lipid emulsion (Intralipid®) increases FFA levels and results in elevation of blood pressure (BP) and endothelial dysfunction in obese healthy subjects. The effects of combined hyperglycemia and high FFA on BP, endothelial function and carbohydrate metabolism are not known. Twelve obese healthy subjects received four random, 8-h IV infusions of saline, Intralipid 40 mL/h, Dextrose 10% 40 mL/h, or combined Intralipid and dextrose. Plasma levels of FFA increased by 1.03±0.34 mmol/L (p=0.009) after Intralipid, but FFAs remained unchanged during saline, dextrose, and combined Intralipid and dextrose infusion. Plasma glucose and insulin concentrations significantly increased after dextrose and combined Intralipid and dextrose (all, p<0.05) and were not different from baseline during saline and lipid infusion. Intralipid increased systolic BP by 12±9 mmHg (p<0.001) and diastolic BP by 5±6 mmHg (p=0.022),and decreased flow-mediated dilatation (FMD) from baseline by 3.2%±1.4% (p<0.001). Saline and dextrose infusion had neutral effects on BP and FMD. The co-administration of lipid and dextrose decreased FMD by 2.4%±2.1% (p=0.002) from baseline, but did not significantly increase systolic or diastolic BP. Short-term Intralipid infusion significantly increased FFA and BP; in contrast, FFA and BP were unchanged during combined infusion of Intralipid and dextrose. Combined Intralipid and dextrose infusion resulted in endothelial dysfunction similar to Intralipid alone.

Microvascular blood flow responses to muscle contraction are not altered by high-fat feeding in rats

AIM

Exercise and insulin each increase microvascular blood flow and enhance glucose disposal in skeletal muscle. We have reported that insulin-mediated microvascular recruitment in a diet-induced model of insulin resistance (high-fat feeding for 4 weeks) is markedly impaired; however, the effect of muscle contraction in this model has not been previously explored.

METHODS

We fed rats either normal (ND, 10% calories from fat) or high-fat (HFD, 60% calories from fat) diets ad libitum for 4-8 weeks. Animals were then anaesthetized and one hindlimb electrically stimulated to contract at 0.05, 0.1 and 2 Hz (field stimulation, 30 V, 0.1 ms duration) in 15 min stepwise increments. Femoral artery blood flow (Transonic flow probe), muscle microvascular blood flow (hindleg metabolism of 1-methylxanthine and contrast-enhanced ultrasound) and muscle glucose disposal (uptake of radiolabelled 2-deoxy-d-glucose and hindleg glucose disappearance) were measured.

RESULTS

Both ND and HFD rats received the same voltage across the leg and consequently developed the same muscle tension. Femoral artery blood flow in the contracting leg increased during 2 Hz contraction, but not during the lower frequencies and these effects were similar between ND and HFD rats. Muscle microvascular blood flow significantly increased in a contraction frequency-dependent manner, and preceded increases in total limb blood flow and these effects were similar between ND and HFD rats. Muscle glucose disposal was markedly elevated during 2 Hz contraction and was comparable between ND and HFD rats.

CONCLUSION

Contraction-mediated muscle microvascular recruitment and glucose uptake are not impaired in the HFD insulin resistant rat.

Glu298Asp polymorphism influences the beneficial effects of fish oil fatty acids on postprandial vascular function

Our objective was to determine whether the endothelial nitric oxide synthase (eNOS) Glu298Asp polymorphism influences vascular response to raised NEFA enriched with saturated fatty acids (SFA) or long-chain (LC) n-3 polyunsaturated fatty acids (PUFA). Subjects were prospectively recruited for genotype (Glu298, n = 30 and Asp298, n = 29; balanced for age and gender) consumed SFA on two occasions, with and without the substitution of 0.07 g fat/kg body weight with LC n-3 PUFA, and with heparin infusion to elevate NEFA. Endothelial function was measured before and after NEFA elevation (240 min), with blood samples taken every 30 min. Flow-mediated dilation (FMD) decreased following SFA alone and increased following SFA+LC n-3 PUFA. There were 2-fold differences in the change in FMD response to the different fat loads between the Asp298 and Glu298 genotypes (P = 0.002) and between genders (P < 0.02). Sodium nitroprusside-induced reactivity, measured by laser Doppler imaging with iontophoresis, was significantly greater with SFA+LC n-3 PUFA in all female subjects (P < 0.001) but not in males. Elevated NEFA influences both endothelial-dependent and endothelial-independent vasodilation during the postprandial phase. Effects of fat composition appear to be genotype and gender dependent, with the greatest difference in vasodilatory response to the two fat loads seen in the Asp298 females.

LSDP5 enhances triglyceride storage in hepatocytes by influencing lipolysis and fatty acid β-oxidation of lipid droplets

Lipid storage droplet protein 5 (LSDP5) is a lipid droplet-associated protein of the PAT (perilipin, adipophilin, and TIP47) family that is expressed in the liver in a peroxisome proliferator-activated receptor alpha (PPARα)-dependent manner; however, its exact function has not been elucidated. We noticed that LSDP5 was localized to the surface of lipid droplets in hepatocytes. Overexpression of LSDP5 enhanced lipid accumulation in the hepatic cell line AML12 and in primary hepatocytes. Knock-down of LSDP5 significantly decreased the triglyceride content of lipid droplets, stimulated lipolysis, and modestly increased the mitochondrial content and level of fatty-acid β-oxidation in the mitochondria. The expression of PPARα was increased in LSDP5-deficient cells and required for the increase in the level of fatty acid β-oxidation in LSDP5-deficient cells. Using serial deletions of LSDP5, we determined that the lipid droplet-targeting domain and the domain directing lipid droplet clustering overlapped and were localized to the 188 amino acid residues at the N-terminus of LSDP5. Our findings suggest that LSDP5, a novel lipid droplet protein, may contribute to triglyceride accumulation by negatively regulating lipolysis and fatty acid oxidation in hepatocytes.

PI3K/AKT/PTEN Signaling as a Molecular Target in Leukemia Angiogenesis

PI3K/AKT/PTEN pathway is important in the regulation of angiogenesis mediated by vascular endothelial growth factor in many tumors including leukemia. The signaling pathway is activated in leukemia patients as well as leukemia cell lines together with a decrease in the expression of PTEN gene. The mechanism by which the signaling pathway regulates angiogenesis remains to be further elucidated. However, it has become an attractive target for drug therapy against leukemia, because angiogenesis is a key process in malignant cell growth. In this paper, we will focus on the roles and mechanisms of PI3K/AKT/PTEN pathway in regulating angiogenesis.

p38 mitogen-activated protein kinase (MAPK) promotes cholesterol ester accumulation in macrophages through inhibition of macroautophagy

p38 MAPK has been strongly implicated in the development of atherosclerosis, but its role in cholesterol ester accumulation in macrophages and formation of foam cells, an early step in the development of atherosclerosis, has not been investigated. We addressed this issue and made some brand new observations. First, elevated intracellular cholesterol level induced by the exposure to LDL-activated p38 MAPK and activation of p38 MAPK with anisomycin increased the ratio of cholesterol esters over free cholesterol, whereas inhibition of p38 MAPK with SB203580 or siRNA reduced the LDL loading-induced intracellular accumulation of free cholesterol and cholesterol esters in macrophages. Second, exposure to LDL cholesterol inhibited autophagy in macrophages, and inhibition of autophagy with 3-methyladenine increased intracellular accumulation of cholesterol (free cholesterol and cholesterol esters), whereas activation of autophagy with rapamycin decreased intracellular accumulation of free cholesterol and cholesterol esters induced by the exposure to LDL cholesterol. Third, LDL cholesterol loading-induced inhibition of autophagy was prevented by blockade of p38 MAPK with SB203580 or siRNA. Neutral cholesterol ester hydrolase was co-localized with autophagosomes. Finally, LDL cholesterol loading and p38 activation suppressed expression of the key autophagy gene, ulk1, in macrophages. Together, our results provide brand new insight about cholesterol ester accumulation in macrophages and foam cell formation.

Lipids promote survival, proliferation, and maintenance of differentiation of rat liver sinusoidal endothelial cells in vitro

Primary rat liver sinusoidal endothelial cells (LSEC) are difficult to maintain in a differentiated state in culture for scientific studies or technological applications. Relatively little is known about molecular regulatory processes that affect LSEC differentiation because of this inability to maintain cellular viability and proper phenotypic characteristics for extended times in vitro, given that LSEC typically undergo death and detachment around 48-72 h even when treated with VEGF. We demonstrate that particular lipid supplements added to serum-free, VEGF-containing medium increase primary rat liver LSEC viability and maintain differentiation. Addition of a defined lipid combination, or even oleic acid (OA) alone, promotes LSEC survival beyond 72 h and proliferation to confluency. Moreover, assessment of LSEC cultures for endocytic function, CD32b surface expression, and exhibition of fenestrae showed that these differentiation characteristics were maintained when lipids were included in the medium. With respect to the underlying regulatory pathways, we found lipid supplement-enhanced phosphatidylinositol 3-kinase and MAPK signaling to be critical for ensuring LSEC function in a temporally dependent manner. Inhibition of Akt activity before 72 h prevents growth of SEC, whereas MEK inhibition past 72 h prevents survival and proliferation. Our findings indicate that OA and lipids modulate Akt/PKB signaling early in culture to mediate survival, followed by a switch to a dependence on ERK signaling pathways to maintain viability and induce proliferation after 72 h. We conclude that free fatty acids can support maintenance of liver LSEC cultures in vitro; key regulatory pathways involved include early Akt signaling followed by ERK signaling.

Cell death-inducing DFF45-like effector b (Cideb) is present in pancreatic beta-cells and involved in palmitate induced beta-cell apoptosis

BACKGROUND

Excessive accumulation of long-chain fatty acids in the pancreatic islets is associated with beta cell dysfunction and ultimately contributes to the pathogenesis of type 2 diabetes. It has been well proved that the cell death-inducing DFF45-like effector b (Cideb) is involved in cell apoptosis and lipid metabolism. However, the expression and function of Cideb in endocrine pancreas remain to be investigated.

METHODS

By using reverse transcript polymerase chain reaction, immunohistochemistry and Western blot, we observed the expression of Cideb in pancreas tissues and clonal beta-cell lines. The physiological role of Cideb was examined under the free fatty acid (FFA) administration and Cideb ribonucleic acid interference, and further analysis on apoptosis was measured by terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate nick end labelling assay and caspase-3 activity. Nile red staining and quantitative evaluation of triglyceride were used to detect the lipid accumulation. The changes in esterification of FFA were traced by radiolabelled palmitate.

RESULTS

Cideb was abundantly expressed in pancreas and mainly localized in beta cells. FFAs, especially palmitate, induced an obvious increase of Cideb expression in beta cell lines. Adenoviral-mediated overexpression of Cideb increased the apoptosis, whereas ribonucleic acid interference-based Cideb depletion in beta-TC3 cells had no effect on apoptosis in normal condition. Palmitate supplementation led to beta cell lipoapoptosis, and Cideb silencing exacerbated the apoptosis induced by palmitate, reduced intracellular triglyceride content and aggravated FFA overload in beta cells.

CONCLUSIONS

The present results suggest that increased Cideb expression upon palmitate exposure may be involved in beta cell lipoapoptosis through its influence on conversion of FFAs to lipid esters in lipid droplets.

Garlic-derived S-allylmercaptocysteine is a hepato-protective agent in non-alcoholic fatty liver disease in vivo animal model

PURPOSE

To investigate the hepato-protective properties and underlying mechanisms of SAMC in a non-alcoholic fatty liver disease (NAFLD) rat model.

METHODS

Female rats were fed with a diet comprising highly unsaturated fat diet (30% fish oil) for 8 weeks to develop NAFLD with or without an intraperitoneal injection of 200 mg/kg SAMC three times per week. After euthanasia, blood and liver samples of rats were collected for histological and biochemical analyses.

RESULTS

Co-treatment of SAMC attenuated NAFLD-induced liver injury, fat accumulation, collagen formation and free fatty acids (FFAs). At the molecular level, SAMC decreased the lipogenesis marker and restored the lipolysis marker. SAMC also reduced the expression levels of pro-fibrogenic factors and diminished liver oxidative stress partly through the inhibition in the activity of cytochrome P450 2E1-dependent pathway. NAFLD-induced inflammation was also partially mitigated by SAMC treatment via reduction in the pro-inflammatory mediators, chemokines and suppressor of cytokine signaling. The protective effect of SAMC is also shown partly through the restoration of altered phosphorylation status of FFAs-dependent MAP kinase pathways and diminished in the nuclear transcription factors (NF-κB and AP-1) activity during NAFLD development.

CONCLUSIONS

SAMC is a novel hepato-protective agent against NAFLD caused by abnormal liver functions. Garlic or garlic derivatives could be considered as a potent food supplement in the prevention of fatty liver disease.

The intersection of inflammation, insulin resistance and ageing: implications for the study of molecular signalling pathways in horses

Inflammation-associated insulin resistance contributes to chronic disease in humans and other long-lived species, such as horses. Insulin resistance arises due to an imbalance among molecular signalling mediators in response to pro-inflammatory cytokines in the aged and obese. The mammalian heat shock protein response has received much attention as an avenue for attenuating inflammatory mediator signalling and for contributing to preservation and restoration of insulin signalling in metabolically important tissues. Data on heat shock proteins and inflammatory signalling mediators in untrained and aged horses are lacking, and horses represent an untapped resource for studying the mediator imbalance contributing to insulin resistance in a comparative model.

The link between metabolic abnormalities and endothelial dysfunction in type 2 diabetes: an update

Despite abundant clinical evidence linking metabolic abnormalities to diabetic vasculopathy, the molecular basis of individual susceptibility to diabetic vascular complications is still largely undetermined. Endothelial dysfunction in diabetes-associated vascular complications is considered an early stage of vasculopathy and has attracted considerable research interests. Type 2 diabetes is characterized by metabolic abnormalities, such as hyperglycemia, excess liberation of free fatty acids (FFA), insulin resistance and hyperinsulinemia. These abnormalities exert pathological impact on endothelial function by attenuating endothelium-mediated vasomotor function, enhancing endothelial apoptosis, stimulating endothelium activation/endothelium-monocyte adhesion, promoting an atherogenic response and suppressing barrier function. There are multiple signaling pathways contributing to the adverse effects of glucotoxicity on endothelial function. Insulin maintains the normal balance for release of several factors with vasoactive properties. Abnormal insulin signaling in the endothelium does not affect the whole-body glucose metabolism, but impairs endothelial response to insulin and accelerates atherosclerosis. Excessive level of FFA is implicated in the pathogenesis of insulin resistance. FFA induces endothelial oxidative stress, apoptosis and inflammatory response, and inhibits insulin signaling. Although hyperglycemia, insulin resistance, hyperinsulinemia and dyslipidemia independently contribute to endothelial dysfunction via various distinct mechanisms, the mutual interactions may synergistically accelerate their adverse effects. Oxidative stress and inflammation are predicted to be among the first alterations which may trigger other downstream mediators in diabetes associated with endothelial dysfunction. These mechanisms may provide insights into potential therapeutic targets that can delay or reverse diabetic vasculopathy.

Insulin resistance and liver microcirculation in a rat model of early NAFLD

BACKGROUND & AIMS

Insulin contributes to vascular homeostasis in peripheral circulation, but the effects of insulin in liver microvasculature have never been explored. The aim of this study was to assess the vascular effects of insulin in the healthy and fatty liver.

METHODS

Wistar rats were fed a control or a high fat diet (HFD) for 3days, while treated with a placebo, the insulin-sensitizer metformin, or the iNOS inhibitor 1400W. Vascular responses to insulin were evaluated in the isolated liver perfusion model. Insulin sensitivity at the sinusoidal endothelium was tested by endothelium-dependent vasodilation in response to acetylcholine in the presence or absence of insulin and by the level of liver P-eNOS after an insulin injection.

RESULTS

Rats from the HFD groups developed liver steatosis. Livers from the control group showed a dose-dependent hepatic vasodilation in response to insulin, which was blunted in livers from HFD groups. Metformin restored liver vascular insulin-sensitivity. Pre-treatment with insulin enhanced endothelium-dependent vasodilation of the hepatic vasculature and induced hepatic eNOS phosphorylation in control rats but not in HFD rats. Treatment with metformin or 1400W restored the capacity of insulin to enhance endothelium dependent vasodilation and insulin induced eNOS phosphorylation in HFD rats.

CONCLUSIONS

The administration of a HFD induces insulin resistance in the liver sinusoidal endothelium, which is mediated, at least in part, through iNOS upregulation and can be prevented by the administration of metformin. Insulin resistance at the hepatic vasculature can be detected earlier than inflammation or any other sign of advanced NALFD.

DHA-rich fish oil reverses the detrimental effects of saturated fatty acids on postprandial vascular reactivity

BACKGROUND

Experimental elevation of nonesterified fatty acids (NEFAs) impairs endothelial function, but the effect of NEFA composition is unknown.

OBJECTIVE

The objective was to test the effect of acute elevation of NEFAs enriched with either saturated fatty acids (SFAs) or SFAs with long-chain (LC) n-3 (omega-3) PUFAs on vascular function measured via flow-mediated dilatation (FMD), laser Doppler iontophoresis (LDI), and digital volume pulse (DVP).

DESIGN

In 59 subjects (30 men and 29 women), repeated oral fat feeding of either palm stearin (SFA) or palm stearin with DHA-rich fish oil (SFA + LC n-3 PUFA) was performed on 2 separate occasions with continuous heparin infusion to elevate NEFAs for a duration of 60 to 240 min. Vascular function was measured at baseline and at the end of NEFA elevation; venous blood was collected for measurement of lipids and circulating markers of endothelial function.

RESULTS

NEFA elevation during consumption of the SFA-rich drinks was associated with a marked impairment of FMD, whereas consumption of SFAs + LC n-3 PUFAs improved FMD response, with a mean (±SEM) difference of 2.06 ± 0.29% (P < 0.001). Positive correlations were found with percentage weight of LC n-3 PUFAs in circulating NEFAs and change in FMD response [Spearman's rho (r(s)) = 0.460, P < 0.001]. LDI measures increased during both treatments (P ≤ 0.026), and there was no change in DVP indexes.

CONCLUSIONS

The composition of NEFAs can acutely affect FMD. The beneficial effect of LC n-3 PUFAs on postprandial vascular function warrants further investigation but may be mediated by nitric oxide-independent mechanisms. This trial is registered at clinicaltrials.gov as NCT01351324.

Macrovascular angiopathy in children and adolescents with type 1 diabetes

Diabetes represents one of the most common diseases globally. Worryingly, the worldwide incidence of type 1 diabetes (T1D) is rising by 3% per year. Despite the rapid increase in diabetes incidence, recent advances in diabetes treatment have been successful in decreasing morbidity and mortality from diabetes-related retinopathy, nephropathy, and neuropathy. In contrast, there is clear evidence for the lack of improvement in mortality for cardiovascular diseases (CVDs). This emphasizes the importance of focusing childhood diabetes care strategies for the prevention of CVD in adulthood. Furthermore, although most work on diabetes and macrovascular disease relates to type 2 diabetes, it has been shown that the age-adjusted relative risk of CVD in T1D far exceeds that in type 2 diabetes. As T1D appears predominantly during childhood, those with T1D are at greater risk for coronary events early in life and require lifelong medical attention. Because of the important health effects of CVDs in children and adolescents with T1D, patients, family members, and care providers should understand the interaction of T1D and cardiovascular risk. In addition, optimal cardiac care for the patient with diabetes should focus on aggressive management of traditional cardiovascular risk factors to optimize those well-recognized as well as new specific risk factors which are becoming available. Therefore, a complete characterization of the molecular mechanisms involved in the development and progression of macrovascular angiopathy is needed. Furthermore, as vascular abnormalities begin as early as in childhood, potentially modifiable risk factors should be identified at an early stage of vascular disease development.

Is the beneficial effect of prior exercise on postprandial lipaemia partly due to redistribution of blood flow?

Preprandial aerobic exercise lowers postprandial lipaemia (a risk factor for coronary heart disease); however, the mechanisms responsible are still not clear. The present study investigated whether blood flow to skeletal muscle and/or the liver was increased in the postprandial period after exercise, relative to a control trial, and whether this resulted from increased cardiac output or redistribution of flow. Eight overweight inactive males, aged 49.4±10.5 years (mean±S.D.), acted as their own controls in a counterbalanced design, either walking briskly for 90 min at 60% V̇O2max (maximal oxygen uptake), or resting in the lab, on the evening of day 1. The following morning, a fasting blood sample was collected, participants consumed a high-fat breakfast, and further venous blood samples were drawn hourly for 6 h. Immediately after blood sampling, Doppler ultrasound was used to measure cardiac output and blood flow through both the femoral artery of one leg and the hepatic portal vein, with the ultrasonographer blinded to trial order. The total postprandial triacylglycerol response was 22% lower after exercise (P=0.001). Blood flow through the femoral artery and the hepatic portal vein was increased by 19% (P<0.001) and 16% (P=0.033), respectively, during the 6-h postprandial period following exercise; however, postprandial cardiac output did not differ between trials (P=0.065). Redistribution of blood flow, to both exercised skeletal muscle and the liver, may therefore play a role in reducing the plasma triacylglycerol response to a high-fat meal on the day after an exercise bout.

Palmitate induces C-reactive protein expression in human aortic endothelial cells. Relevance to fatty acid-induced endothelial dysfunction

Circulating levels of free fatty acids are commonly elevated in patients with the metabolic syndrome and exert, through activating proinflammatory pathways, harmful effects of the vascular endothelium. In this study, we examined the effect of palmitate (PA) on endothelial C-reactive protein (CRP) expression and the role of CRP in PA-induced nitric oxide (NO) inhibition. Palmitate increased, in a dose-dependent manner, CRP protein expression and production in human aortic endothelial cells (HAECs). Induction of CRP protein was mimicked by ceramide, whereas bromopalmitate and other common free fatty acids such as oleate or linoleate were ineffective. Palmitate also elicited reactive oxygen species production in HAECs, an effect prevented by protein kinase C (PKC) inhibition and adenosine monophosphate-activated kinase (AMPK) activation. Palmitate-treated HAECs showed increased CRP messenger RNA expression and nuclear factor (NF)-κB activation. Induction of CRP expression by PA was prevented by antioxidants and normalized by PKC and mitogen-activated protein kinase inhibitors. Disrupting NF-κB and Janus kinase/signal transducers and activators of transcription pathways or inducing AMPK activation also suppressed the stimulatory effect of PA on CRP messenger RNA expression. Finally, in HAECs, PA reduced NO release, an effect reversed by anti-CRP antibody. These data demonstrate that PA-induced endothelial CRP expression involves PKC-driven oxidative stress, possibly through AMPK inhibition, and activation of downstream redox-sensitive signaling pathways, including NF-κB. They further support a role for endothelial cell-derived CRP as mediator of the suppressive effect of PA on NO production.

Acute effects of elevated NEFA on vascular function: a comparison of SFA and MUFA

There is emerging evidence to show that high levels of NEFA contribute to endothelial dysfunction and impaired insulin sensitivity. However, the impact of NEFA composition remains unclear. A total of ten healthy men consumed test drinks containing 50 g of palm stearin (rich in SFA) or high-oleic sunflower oil (rich in MUFA) on separate occasions; a third day included no fat as a control. The fats were emulsified into chocolate drinks and given as a bolus (approximately 10 g fat) at baseline followed by smaller amounts (approximately 3 g fat) every 30 min throughout the 6 h study day. An intravenous heparin infusion was initiated 2 h after the bolus, which resulted in a three- to fourfold increase in circulating NEFA level from baseline. Mean arterial stiffness as measured by digital volume pulse was higher during the consumption of SFA (P < 0·001) but not MUFA (P = 0·089) compared with the control. Overall insulin and gastric inhibitory peptide response was greater during the consumption of both fats compared with the control (P < 0·001); there was a second insulin peak in response to MUFA unlike SFA. Consumption of SFA resulted in higher levels of soluble intercellular adhesion molecule-1 (sI-CAM) at 330 min than that of MUFA or control (P ≤ 0·048). There was no effect of the test drinks on glucose, total nitrite, plasminogen activator inhibitor-1 or endothelin-1 concentrations. The present study indicates a potential negative impact of elevated NEFA derived from the consumption of SFA on arterial stiffness and sI-CAM levels. More studies are needed to fully investigate the impact of NEFA composition on risk factors for CVD.

Effects of a highly palatable diet on lipid and glucose parameters, nitric oxide, and ectonucleotidases activity

Obesity has reached epidemic proportions worldwide and is stimulated by the ready availability of food rich in fat and sugar (highly palatable diet). This type of diet increases the risks of obesity-associated pathologies, such as insulin resistance and cardiovascular disease. Nitric oxide, a potent endogenous vasodilator, is decreased in these pathologies, mostly as a result of insulin resistance. Ectonucleotidases are ecto and soluble enzymes that regulate the availability of the nucleotides ATP, ADP, and AMP and the nucleoside adenosine in the vascular system, thereby affecting vasoconstriction, vasodilatation, and platelet aggregation homeostasis. The aim of this study was to evaluate the effects of a highly palatable diet on serum lipid and glucose parameters, nitric oxide, and ectonucleotidase activity. Forty male Wistar rats were fed 1 of 2 diets for either 45 days or 4 months: standard chow (SC, n = 10) or a highly palatable diet enriched with sucrose (HP, n = 10). Body mass, visceral fat mass, glucose tolerance, cholesterol (total, high-density lipoprotein (HDL) and non-HDL), serum triacylglycerol, liver triacylglycerol, and free glycerol were increased in the HP group after 45 days and after 4 months, whereas insulin levels were not different between the groups at either time. Furthermore, levels of nitric oxide metabolites and ATP, ADP, and AMP hydrolysis were significantly lower in the HP group (p < 0.05) after 4 months. In conclusion, the consumption of the HP diet for 4 months induced overall corporal and metabolic changes, and decreased nitric oxide metabolites and ectonucleotidase activity, thereby promoting an appropriate environment for the development of cardiovascular diseases, without apparent changes in insulin levels.

Coronary microvascular dysfunction in diabetes mellitus: A review

The exploration of coronary microcirculatory dysfunction in diabetes has accelerated in recent years. Cardiac function is compromised in diabetes. Diabetic patients manifest accelerated atherosclerosis in coronary arteries. These data are confirmed in diabetic animal models, where lesions of small coronary arteries have been described. These concepts are epitomized in the classic microvascular complications of diabetes, i.e. blindness, kidney failure and distal dry gangrene. Most importantly, accumulating data indicate that insights gained from the link between inflammation and diabetes can yield predictive and prognostic information of considerable clinical utility. This review summarizes the evidence for the predisposing factors and the mechanisms involved in diabetes, and assesses the current state of knowledge regarding the triggers for inflammation in this disease. We evaluate the roles of hyperglycemia, oxidative stress, polyol pathway, protein kinase C, advanced glycation end products, insulin resistance, peroxisome proliferator-activated receptor-γ, inflammation, and diabetic cardiomyopathy as a "stem cell disease". Furthermore, we discuss the mechanisms responsible for impaired coronary arteriole function. Finally, we consider how new insights in diabetes may provide innovative therapeutic strategies.

Free fatty acids induce endothelial dysfunction and activate protein kinase C and nuclear factor-κB pathway in rat aorta

BACKGROUND

Insulin resistance is associated with an inappropriate elevation of plasma free fatty acids (FFAs) and endothelial dysfunction. In this study, we asked if elevated circulating FFA levels led to impaired insulin signaling and endothelial dysfunction in-vivo via activation of PKC-mediated inflammatory pathways.

METHODS

Sprague-Dawley (S-D) rats were infused with 1) 20% intralipid+heparin (FFA group) or 2) saline alone (Control group) for 6h. The intact aorta thoracica and aorta abdominalis were then removed. Aortic rings were isolated and evaluated for endothelial-dependent and non-dependent relaxation in an organ bath. The activities of eNOS and PKC were measured in endothelial homogenates prepared from endothelial cells harvested from the aorta. The expression levels of insulin signaling molecules IRS-1, Akt, eNOS, ERK1/ERK2, PKC-α, NFκB-p65 subunit and IκB-α in rat aortic endothelium were determined by immunohistochemistry and Western blot.

RESULTS

Elevation of FFAs resulted in a 35.9% reduction in the response to acetylcholine (p < 0.01), a 26% decline in plasma NOx levels (p < 0.05), a 53% decrease in eNOS activity and a 34 ± 9% inhibition in IRS-1 tyrosine phosphorylation (p < 0.05). We also found a 46% decrease in Akt phosphorylation and a 36% decrease in eNOS phosphorylation. FFA-induced endothelial insulin resistance was associated with 82% increase in total membrane-associated PKC activity, a 1.7-fold increase in total PKC-α protein, 1.29-fold decrease in IκB-α expression levels and 1.47-fold increase in NF-κB p65 subunit expression in rat aortic endothelium.

CONCLUSION

The molecular mechanisms underlying FFA-induced endothelial insulin resistance and eNOS inhibition may provide an important link implicating the PKC and IκB-α/NF-κB pathways in FFA-mediated inhibition of vascular insulin signaling.

Ginsenoside Rg3 increases nitric oxide production via increases in phosphorylation and expression of endothelial nitric oxide synthase: essential roles of estrogen receptor-dependent PI3-kinase and AMP-activated protein kinase

We previously showed that ginsenosides increase nitric oxide (NO) production in vascular endothelium and that ginsenoside Rg3 (Rg3) is the most active one among ginseng saponins. However, the mechanism for Rg3-mediated nitric oxide production is still uncertain. In this study, we determined whether Rg3 affects phosphorylation and expression of endothelial nitric oxide synthase (eNOS) in ECV 304 human endothelial cells. Rg3 increased both the phosphorylation and the expression of eNOS in a concentration-dependent manner and a maximal effect was found at 10μg/ml of Rg3. The enzyme activities of phosphatidylinositol 3-kinase (PI3-kinase), c-Jun N-terminal kinase (JNK), and p38 kinase were enhanced as were estrogen receptor (ER)- and glucocorticoid receptor (GR)-dependent reporter gene transcriptions in Rg3-treated endothelial cells. Rg3-induced eNOS phosphorylation required the ER-mediated PI3-kinase/Akt pathway. Moreover, Rg3 activates AMP-activated protein kinase (AMPK) through up-regulation of CaM kinase II and Rg3-stimulated eNOS phosphorylation was reversed by AMPK inhibition. The present results provide a mechanism for Rg3-stimulated endothelial NO production.

Metformin reduces intracellular reactive oxygen species levels by upregulating expression of the antioxidant thioredoxin via the AMPK-FOXO3 pathway

BACKGROUND

Oxidative stress induced by free fatty acids plays a critical role in the pathogenesis of endothelial dysfunction and atherosclerosis in patients with metabolic syndrome. Reducing oxidative stress in these patients may prevent cardiovascular complications. The antidiabetic agent metformin has been reported to directly protect the cardiovascular system. In this study, we examined the effect of metformin on the intracellular levels of reactive oxygen species (ROS) induced by palmitic acid (PA) in human aortic endothelial cells and studied the molecular mechanisms involved.

METHODS AND RESULTS

We observed that metformin significantly reduced intracellular ROS levels induced by PA. Additionally, metformin increased the expression of the antioxidant thioredoxin (Trx), which mediated metformin's effects on ROS reduction. Metformin increased Trx expression through the AMP-activated protein kinase (AMPK) pathway. Metformin-regulated Trx at the transcriptional level and forkhead transcription factor 3 (FOXO3) was involved in this process.

CONCLUSION

These results suggest that metformin reduces ROS levels by inducing Trx expression through activation of the AMPK-FOXO3 pathway.

NDRG2 is highly expressed in pancreatic beta cells and involved in protection against lipotoxicity

The N-myc downstream-regulated gene 2 (NDRG2) is involved in cell differentiation and apoptosis, but its function in the pancreas remains to be established. Herein we examine the expression and function of NDRG2 in the endocrine pancreas. NDRG2 immunoreactivity was localized mainly in the cytoplasm of pancreatic beta cells. When beta-TC3 cells were exposed chronically to high levels of free fatty acid (FFA), cell viability was impaired, and Akt and NDRG2 phosphorylation were reduced. NDRG2 is a potential substrate of protein kinase Akt. Overexpression of constitutively active Akt enhanced NDRG2 phosphorylation and abolished the apoptosis induced by FFA in beta-TC3 cells, whereas NDRG2 knock-down attenuated Akt-mediated protection of beta cells against fatty acid-triggered apoptosis. Collectively, these data indicate that NDRG2 acts as a key molecule in pancreatic beta cells and is involved in the Akt-mediated protection of beta cells against lipotoxicity.

An cell-assembly derived physiological 3D model of the metabolic syndrome, based on adipose-derived stromal cells and a gelatin/alginate/fibrinogen matrix

One of the major obstacles in drug discovery is the lack of in vitro three-dimensional (3D) models that can capture more complex features of a disease.Here we established a in vitro physiological model of the metabolic syndrome (MS) using cell-assembly technique (CAT), which can assemble cells into designated places to form complex 3D structures. Adipose-derived stromal (ADS) cells were assembled with gelatin/alginate/fibrinogen. Fibrin was employed as an effective material to regulate ADS cell differentiation and self-organization along with other methods. ADS cells differentiated into adipocytes and endothelial cells, meanwhile, the cells were induced to self-organize into an analogous tissue structure. Pancreatic islets were then deposited at designated locations and constituted the adipoinsular axis with adipocytes. Analysis of the factors involved in energy metabolism showed that this system could capture more pathological features of MS. Drugs known to have effects on MS showed accordant effects in this system, indicating that the model has potential in MS drug discovery. Overall, this study demonstrated that cell differentiation and self-organization can be regulated by techniques combined with CAT. The model presented could result in a better understanding of the pathogenesis of MS and the development of new technologies for drug discovery.

JNK-ATF-2 inhibits thrombomodulin (TM) expression by recruiting histone deacetylase4 (HDAC4) and forming a transcriptional repression complex in the TM promoter

Thrombomodulin (TM) is an important vascular protective molecule that has anticoagulant, anti-inflammatory and anti-apoptotic properties. TM is downregulated in many thrombotic and vascular diseases. However, the mechanisms responsible for TM suppression are not completely understood. In this study, we investigated the mechanism involved in fatty acid-induced suppression of TM expression in human aortic endothelial cells. We found that palmitic acid inhibited TM expression through the JNK and p38 pathways. ATF-2, a JNK and p38 target transcription factor, was involved in the suppression. ATF-2 can bind to the TM promoter, recruit HDAC4 and form a transcriptional repression complex in the promoter, which may lead to chromatin condensation and transcriptional arrest. This study provides novel insight into TM down-regulation by stress signaling pathways.

Metformin reduces lipid accumulation in macrophages by inhibiting FOXO1-mediated transcription of fatty acid-binding protein 4

OBJECTIVE

The accumulation of lipids in macrophages contributes to the development of atherosclerosis. Strategies to reduce lipid accumulation in macrophages may have therapeutic potential for preventing and treating atherosclerosis and cardiovascular complications. The antidiabetic drug metformin has been reported to reduce lipid accumulation in adipocytes. In this study, we examined the effects of metformin on lipid accumulation in macrophages and investigated the mechanisms involved.

METHODS AND RESULTS

We observed that metformin significantly reduced palmitic acid (PA)-induced intracellular lipid accumulation in macrophages. Metformin promoted the expression of carnitine palmitoyltransferase I (CPT-1), while reduced the expression of fatty acid-binding protein 4 (FABP4) which was involved in PA-induced lipid accumulation. Quantitative real-time PCR showed that metformin regulates FABP4 expression at the transcriptional level. We identified forkhead transcription factor FOXO1 as a positive regulator of FABP4 expression. Inhibiting FOXO1 expression with FOXO1 siRNA significantly reduced basal and PA-induced FABP4 expression. Overexpression of wild-type FOXO1 and constitutively active FOXO1 significantly increased FABP4 expression, whereas dominant negative FOXO1 dramatically decreased FABP4 expression. Metformin reduced FABP4 expression by promoting FOXO1 nuclear exclusion and subsequently inhibiting its activity.

CONCLUSIONS

Taken together, these results suggest that metformin reduces lipid accumulation in macrophages by repressing FOXO1-mediated FABP4 transcription. Thus, metformin may have a protective effect against lipid accumulation in macrophages and may serve as a therapeutic agent for preventing and treating atherosclerosis in metabolic syndrome.

High glucose and elevated fatty acids suppress signaling by the endothelium protective ligand angiopoietin-1

Pre-diabetes is characterized by hyperglycemia and dyslipidemia; it is associated with increased cardiovascular disease and endothelial dysfunction. Angiopoietin-1 (Ang1), a ligand for endothelial receptor, is a potent vascular protective factor important in maintaining normal endothelial function. The aim of the study was to examine the influence of elevated glucose and fatty acid concentrations on angiopoietin signaling in human cardiac microvascular endothelial cells. Incubation with 30 mM glucose caused 50% suppression in the ability of Ang1 to activate Tie2-receptor phosphorylation without any decrease in Tie2 expression or increased internalization in microvascular endothelial cells. Examination of downstream signaling revealed inhibition of Ang1-dependent Akt phosphorylation. By contrast, Ang1 activation of Erk1/2 signaling was not affected by hyperglycemia. Similar suppression of Ang1-dependent activation of Akt by hyperglycemia was observed in large vessel human endothelial cells. Incubation of microvascular endothelial cells with 200 microM palmitic acid significantly inhibited Ang1-dependent Akt phosphorylation without affecting phosphorylation of the Tie-2 receptor or of ERK1/2. Therefore, contrary to hyperglycemia, palmitate acted exclusively downstream of the receptor. The present findings suggest a mechanism by which increased glucose or fatty acids may suppress vascular protection by Ang1 and predispose to endothelial dysfunction and vascular disease.

Inhibition of endothelial nitric oxide synthase by the lipid phosphatase PTEN

PTEN (phosphatase and tensin homologue deleted on chromosome 10) is a lipid phosphatase that functions as a negative regulator of the phosphoinositide-3-kinase (PI3K) pathway. The present study sought to examine in depth the interaction between PTEN and eNOS activity. Co-expression of eNOS and PTEN in COS-7 cells significantly decreased NO production compared to eNOS alone, while co-expression of eNOS and the dominant negative mutant PTEN(C124A) significantly increased NO production. Upon examination of the putative eNOS phosphorylation sites, phosphorylation of S116, T497, S617, S635 and S1179 was decreased by PTEN co-expression, while the dominant negative PTEN(C124A) produced an increase in phosphorylation of all sites except S116 and S635. A myristoylation-deficient eNOS construct with little dependence on phosphorylation state (G2AeNOS) was not significantly affected by co-expression with either PTEN or PTEN(C124A). Likewise, an eNOS construct with a triple phospho-null mutation (S617A, S635A and S1179A) was also unaffected by co-expression with either PTEN or PTEN(C124A). Purified PTEN or PTEN(C124A) failed to interact with purified eNOS in vitro, arguing against a direct interaction between PTEN and eNOS. When the PTEN constructs were expressed in human aortic endothelial cells (HAECs), PTEN significantly decreased NO production and PTEN(C124A) increased it, and both S617 and S1179 were altered by co-expression with the PTEN constructs. Increased expression of PTEN in endothelial cells did not influence superoxide production. We conclude that PTEN is a regulator of eNOS function both when expressed in COS-7 cells and in human endothelial cells, and does so via its effects on the PI3K/Akt pathway.

Activation of the AMPK-FOXO3 pathway reduces fatty acid-induced increase in intracellular reactive oxygen species by upregulating thioredoxin

OBJECTIVE

Oxidative stress induced by free fatty acids contributes to the development of cardiovascular diseases in patients with metabolic syndrome. Reducing oxidative stress may attenuate these pathogenic processes. Activation of AMP-activated protein kinase (AMPK) has been reported to reduce intracellular reactive oxygen species (ROS) levels. The thioredoxin (Trx) system is a major antioxidant system. In this study, we investigated the mechanisms involved in the AMPK-mediated regulation of Trx expression and the reduction of intracellular ROS levels.

RESEARCH DESIGN AND METHODS

We observed that activation of AMPK by 5-aminoimidazole-4-carboxamide ribonucleotide (AICAR) significantly reduced ROS levels induced by palmitic acid in human aortic endothelial cells. Activation of AMPK increased expression of the antioxidant Trx, which mediated the ROS reduction. RT-PCR showed that AMPK regulated Trx at the transcriptional level.

RESULTS

Forkhead transcription factor 3 (FOXO3) was identified as the target transcription factor involved in the upregulation of Trx expression. FOXO3 bound to the Trx promoter, recruited the histone acetylase p300 to the Trx promoter, and formed a transcription activator complex, which was enhanced by AICAR treatment. AMPK activated FOXO3 by promoting its nuclear translocation. We further showed that AICAR injection increased the expression of Trx and decreased ROS production in the aortic wall of ApoE-/- mice fed a high-fat diet.

CONCLUSIONS

These results suggest that activation of the AMPK-FOXO3 pathway reduces ROS levels by inducing Trx expression. Thus, the AMPK-FOXO3-Trx axis may be an important defense mechanism against excessive ROS production induced by metabolic stress and could be a therapeutic target in treating cardiovascular diseases in metabolic syndrome.

Contribution of insulin resistance to vascular dysfunction

Insulin is a vascular hormone, able to influence vascular cell responses. In this review, we consider the insulin actions on vascular endothelium and on vascular smooth muscle cells (VSMC) both in physiological conditions and in the presence of insulin resistance. In particular, we focus the relationships between activation of insulin signalling pathways of phosphatidylinositol-3 kinase (PI3-K) and mitogen-activated protein kinase (MAPK) and the different vascular actions of insulin, with a particular attention to the insulin ability to activate the pathway nitric oxide (NO)/cyclic GMP/PKG via PI3-K, owing to the peculiar relevance of NO in vascular biology. We also discuss the insulin actions mediated by the MAPK pathway (such as endothelin-1 synthesis and secretion and VSMC proliferation and migration) and by the interactions between the two pathways, both in insulin-sensitive and in insulin-resistant states. Finally, we consider the influence of free fatty acids, cytokines and endothelin on vascular insulin resistance.

Palmitic acid enhances lectin-like oxidized LDL receptor (LOX-1) expression and promotes uptake of oxidized LDL in macrophage cells

OBJECTIVE

Elevated levels of nonesterified fatty acids (NEFA) in obesity and type 2 diabetes may contribute to the development of atherosclerosis. Therefore, we examined whether NEFA could regulate expression of scavenger receptors responsible for uptake of oxidized LDL (oxLDL) in macrophages, a critical step in atherogenesis.

METHODS AND RESULTS

Expression level of scavenger receptors in NEFA-treated macrophage-like THP-1 and Raw264.7 cells were analyzed by real-time PCR. Palmitic acid showed the greatest enhancement of expression of lectin-like oxidized LDL receptor (LOX-1) among 7 NEFA examined (4 saturated and 3 unsaturated fatty acids). Upregulation of LOX-1 was selective as increases in expression level of other scavenger receptors (CD36, SR-AI, SR-BI, and CD68) were not observed. Western blotting analysis indicated that upregulation of LOX-1 also occurred at the protein level. Uptake of oxLDL by Raw264.7 cells was promoted by palmitic acid, and the enhanced uptake was abrogated when the cells were transfected with siRNA against LOX-1. Downregulation of Toll-like receptor (TLR) 2, TLR4, or IRAK4 with siRNA did not prevent LOX-1 upregulation, whereas inhibitors of p38 MAPK (p38) and reactive oxygen species (ROS) signal inhibited the upregulation of LOX-1 induced by palmitic acid.

CONCLUSIONS

These results suggest that elevated level of palmitic acid may contribute to development of atherosclerosis through enhanced uptake of oxLDL via upregulation of LOX-1 in macrophages. The effects of palmitic acid may be mediated by ROS-p38 pathway rather than TLRs.

Effects of insulin resistance on endothelial progenitor cells and vascular repair

Insulin resistance, a key feature of obesity, the metabolic syndrome and Type 2 diabetes mellitus, results in an array of metabolic and vascular phenomena which ultimately promote the development of atherosclerosis. Endothelial dysfunction is intricately related to insulin resistance through the parallel stimulatory effects of insulin on glucose disposal in metabolic tissues and NO production in the endothelium. Perturbations characteristic of insulin resistance, including dyslipidaemia, inflammation and oxidative stress, may jeopardize the structural or functional integrity of the endothelium. Recent evidence suggests that endothelial damage is mitigated by endogenous reparative processes which mediate endothelial regeneration. EPCs (endothelial progenitor cells) are circulating cells which have been identified as mediators of endothelial repair. Several of the abnormalities associated with insulin resistance, including reduced NO bioavailability, increased production of ROS (reactive oxygen species) and down-regulation of intracellular signalling pathways, have the potential to disrupt EPC function. Improvement in the number and function of EPCs may contribute to the protective actions of evidence-based therapies to reduce cardiometabolic risk. In the present article, we review the putative effects of insulin resistance on EPCs, discuss the underlying mechanisms and highlight potential therapeutic manoeuvres which could improve vascular repair in individuals with insulin resistance.

Thromboxane A2 receptor activates a Rho-associated kinase/LKB1/PTEN pathway to attenuate endothelium insulin signaling

This study was conducted to elucidate the molecular mechanisms of thromboxane A2 receptor (TP)-induced insulin resistance in endothelial cells. Exposure of human umbilical vein endothelial cells (HUVECs) or mouse aortic endothelial cells to either IBOP or U46619, two structurally related thromboxane A(2) mimetics, significantly reduced insulin-stimulated phosphorylation of endothelial nitric-oxide synthase (eNOS) at Ser(1177) and Akt at Ser(473). These effects were abolished by pharmacological or genetic inhibitors of TP. TP-induced suppression of both eNOS and Akt phosphorylation was accompanied by up-regulation of PTEN (phosphatase and tension homolog deleted on chromosome 10), Ser(380)/Thr(382/383) PTEN phosphorylation, and PTEN lipid phosphatase activity. PTEN-specific small interference RNA restored insulin signaling in the face of TP activation. The small GTPase, Rho, was also activated by TP stimulation, and pretreatment of HUVECs with Y27632, a Rho-associated kinase inhibitor, rescued TP-impaired insulin signaling. Consistent with this result, pertussis toxin abrogated IBOP-induced dephosphorylation of both Akt and eNOS, implicating the G(i) family of G proteins in the suppressive effects of TP. In mice, high fat diet-induced diabetes was associated with aortic PTEN up-regulation, PTEN-Ser(380)/Thr(382/383) phosphorylation, and dephosphorylation of both Akt (at Ser(473)) and eNOS (at Ser(1177)). Importantly, administration of TP antagonist blocked these changes. We conclude that TP stimulation impairs insulin signaling in vascular endothelial cells by selectively activating the Rho/Rho-associated kinase/LKB1/PTEN pathway.

Molecular and clinical aspects of endothelial dysfunction in diabetes

Diabetic patients have an increased risk for cardiovascular complications with respect to the general population. Micro- and macrovascular complications such as nephropathy, retinopathy, atherosclerosis, and coronary artery disease are usually preceded by endothelial dysfunction, a condition characterized by impaired vasorelaxation resulting from reduced bioavailability of the endothelial mediator nitric oxide (NO). Nitric oxide is among endothelial mediators released by endothelial cells in response to insulin stimulation. Therefore, metabolic abnormalities such as insulin resistance, dyslipidemia, compensatory hyperinsulinemia and overt hyperglycemia may all contribute to impaired NO bioavailability and abnormal vasodilatation in diabetic patients. Each of these alterations may trigger endothelial dysfunction by multiple intracellular mechanisms including accelerated formation of advanced glycolysis end products, activation of protein kinase C, increased pro-inflammatory signaling, and impaired sensitivity of the PI 3-kinase signaling pathways. This review outlines the most important mechanisms by which insulin takes part in physiological regulation of endothelial function. Abnormal insulin signaling in endothelium under diabetic conditions and patho-physiological consequences on cardiovascular homeostasis will also be discussed.

Association of the insertion/deletion polymorphism of the angiotensin-converting enzyme gene with type 2 diabetes in two ethnic groups of Jerba Island in Tunisia

Introduction.The aim of the current study was to evaluate the role of angiotensin-converting enzyme (ACE) insertion/deletion (I/D) polymorphism on the prediction of type 2 diabetes in two ethnic populations from Jerba Island,Tunisia.

Methods. In this study, we analysed the genotypic and the allelic distributions of the ACE I/D polymorphism and conducted a case/control association study between healthy normoglycaemic controls and diabetic patients in the two studied groups.ACE gene polymorphism was analysed by polymerase chain reaction in 272 individuals consisting of 172 diabetic subjects and 100 controls.

Results.The genotype frequencies for DD, ID and II were 75.50%, 19.60% and 4.89% inArabs and 76.66%, 16.66% and 6.67% in Berbers, respectively, in the case group, and 42.85%, 35.71% and 21.43% inArabs and 57.50%, 22.50% and 20.00% in Berbers, respectively, in the control group.The DD frequency was significantly higher in the case group than in the control group (p<0.001), suggesting that the DD genotype is associated with an increased susceptibility to type 2 diabetes in our study populations.

Conclusions.The current investigation provides new evidence regarding the role of the ACE I/D polymorphism in the pathogenesis of type 2 diabetes in Jerbian populations. Furthermore, it underlines the importance of ethnicity, which should be considered in all studies aiming to test the genetic effects on the susceptibility to type 2 diabetes.