Rosiglitazone reduces glucose-induced oxidative stress mediated by NAD(P)H oxidase via AMPK-dependent mechanism

Activation of AMP-activated protein kinase prevents lipotoxicity in retinal pericytes

PURPOSE

The recent FIELD study demonstrated that the lipid-lowering agent fenofibrate significantly reduces the development and progression of diabetic retinopathy (DR). These results suggest that lipids may play a causal role in DR. They also suggest that AMP-activated protein kinase (AMPK) activation could account for these findings given that fenofibrate is an AMPK activator. The authors previously demonstrated that free fatty acids, in addition to hyperglycemia, can induce apoptosis in retinal pericytes (PCs), the first cells lost in the diabetic retina. Incubation with the saturated fatty acid palmitate, but not the monounsaturated fatty acid oleate, elicited cytotoxicity in a manner dependent on oxidative stress, NF-κB activation, and ceramide accumulation. In this study, the authors explored whether AMPK can downregulate these pathways and, in doing so, protect PCs from apoptosis.

METHODS

PCs were incubated with palmitate or oleate to determine whether the factors previously linked to lipotoxicity were uniquely increased by palmitate. The effects of AMPK activation on these parameters and on apoptosis were concurrently examined.

RESULTS

Only palmitate increased NF-κB activation, ceramide and diacylglycerol mass, and apoptosis. Activation of AMPK with AICAR or, where used, expression of a constitutively active AMPK prevented all these effects. In contrast, both palmitate and oleate markedly increased oxidative stress, and the activation of AMPK did not prevent this.

CONCLUSIONS

AMPK activation prevents the metabolic abnormalities and apoptosis specifically caused by palmitate in cultured PCs. Pharmacologic agents that activate AMPK in the diabetic retina may warrant consideration as a therapeutic option to avert PC apoptosis and to maintain microvascular homeostasis.

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.

The PPAR-RXR Transcriptional Complex in the Vasculature

The peroxisome proliferator-activated receptors (PPARs) and the retinoid X receptors (RXRs) are ligand-activated transcription factors that coordinately regulate gene expression. This PPAR-RXR transcriptional complex plays a critical role in energy balance, including triglyceride metabolism, fatty acid handling and storage, and glucose homeostasis: processes whose dysregulation characterize obesity, diabetes, and atherosclerosis. PPARs and RXRs are also involved directly in inflammatory and vascular responses in endothelial and vascular smooth muscle cells. New insights into fundamental aspects of PPAR and RXR biology, and their actions in the vasculature, continue to appear. Although RXRs are obligate heterodimeric partners for PPAR action, the part that RXRs, and their endogenous retinoid mediators, exert in the vessel wall is less well understood. Biological insights into PPAR-RXRs may help inform interpretation of clinical trials with synthetic PPAR agonists and prospects for future PPAR therapeutics. Importantly, the extensive data establishing a key role for PPARs and RXRs in energy balance, inflammation, and vascular biology stands separately from the clinical experience with any given synthetic PPAR agonist. Both the basic science data and the clinical experience with PPAR agonists identify the need to better understand these important transcriptional regulators.

Hydroxytyrosol reduces intracellular reactive oxygen species levels in vascular endothelial cells by upregulating catalase expression through the AMPK-FOXO3a pathway

Reactive oxygen species are critically involved in the endothelial dysfunction that contributes to atherosclerosis development. Hydroxytyrosol (HT), a main phenolic compound in olive oil and leaves from Olea europaea L., has antiatherogenic properties with powerful antioxidant activity. The present study verifies the antioxidant activity of HT on H2O2-induced intracellular reactive oxygen species in porcine pulmonary artery endothelial cells (VECs) and the involved molecular mechanisms. Incubation of VECs with HT prevented the increase in intracellular reactive oxygen species levels in the presence of H2O2. HT increased catalase mRNA, protein and activity. Catalase siRNA suppressed HT-dependent reduction of intracellular reactive oxygen species. HT increased both cytosolic and nuclear protein levels of forkhead transcription factor 3a (FOXO3a), as well as the phosphorylation of AMP-activated protein kinase (AMPK) that translocates FOXO3a to the nucleus. AMPK siRNA and a specific inhibitor suppressed HT-induced FOXO3a upregulation and catalase expression. Moreover, FOXO3a siRNA blocked HT-dependent increase in catalase expression. Taken together, our findings strongly demonstrate that HT positively regulates the antioxidant defense system in VECs by inducing the phosphorylation of AMPK with subsequent activation of FOXO3a and catalase expression, and provides a molecular basis for the prevention of cardiovascular diseases by HT.

Enhancing AMPK activation during ischemia protects the diabetic heart against reperfusion injury

AMPK activation during ischemia helps the myocardium to cope with the deficit of energy production. As AMPK activity is considered to be impaired in diabetes, we hypothesized that enhancing AMPK activation during ischemia above physiological levels would protect the ischemic diabetic heart through AMPK activation and subsequent inhibition of mitochondrial permeability transition pore (mPTP) opening. Isolated perfused hearts from normoglycemic Wistar or diabetic Goto-Kakizaki (GK) rats (n ≥ 6/group) were subjected to 35 min of ischemia in the presence of 10, 20, and 40 μM of A-769662, a known activator of AMPK, followed by 120 min of reperfusion with normal buffer. Myocardial infarction and AMPK phosphorylation were assessed. The effect of A-769662 on mPTP opening in adult cardiomyocytes isolated from both strains was also determined. A-769662 at 20 μM reduced infarct size in both Wistar (30.5 ± 2.7 vs. 51.8 ± 3.9% vehicle; P < 0.001) and GK hearts (22.7 ± 3.0 vs. 48.5 ± 4.7% vehicle; P < 0.001). This protection was accompanied by a significant increase in AMPK and GSK-3β phosphorylation. In addition, A-769662 significantly inhibited mPTP opening in both Wistar and GK cardiomyocytes subjected to oxidative stress. We demonstrate that AMPK activation during ischemia via A-769662 reduces myocardial infarct size in both the nondiabetic and diabetic rat heart. Furthermore, this cardioprotective effect appears to be mediated through inhibition of mPTP opening. Our findings suggest that improving AMPK activation during ischemia can be another mechanism for protecting the ischemic heart.

2-Deoxy-D-glucose treatment of endothelial cells induces autophagy by reactive oxygen species-mediated activation of the AMP-activated protein kinase

Autophagy is a cellular self-digestion process activated in response to stresses such as energy deprivation and oxidative stress. However, the mechanisms by which energy deprivation and oxidative stress trigger autophagy remain undefined. Here, we report that activation of AMP-activated protein kinase (AMPK) by mitochondria-derived reactive oxygen species (ROS) is required for autophagy in cultured endothelial cells. AMPK activity, ROS levels, and the markers of autophagy were monitored in confluent bovine aortic endothelial cells (BAEC) treated with the glycolysis blocker 2-deoxy-D-glucose (2-DG). Treatment of BAEC with 2-DG (5 mM) for 24 hours or with low concentrations of H(2)O(2) (100 µM) induced autophagy, including increased conversion of microtubule-associated protein light chain 3 (LC3)-I to LC3-II, accumulation of GFP-tagged LC3 positive intracellular vacuoles, and increased fusion of autophagosomes with lysosomes. 2-DG-treatment also induced AMPK phosphorylation, which was blocked by either co-administration of two potent anti-oxidants (Tempol and N-Acetyl-L-cysteine) or overexpression of superoxide dismutase 1 or catalase in BAEC. Further, 2-DG-induced autophagy in BAEC was blocked by overexpressing catalase or siRNA-mediated knockdown of AMPK. Finally, pretreatment of BAEC with 2-DG increased endothelial cell viability after exposure to hypoxic stress. Thus, AMPK is required for ROS-triggered autophagy in endothelial cells, which increases endothelial cell survival in response to cell stress.

Double antioxidant activities of rosiglitazone against high glucose-induced oxidative stress in hepatocyte

Chronic hyperglycemia is the hallmark of diabetes and its complication. High glucose-induced excessive reactive oxygen species (ROS) production has been considered to play an important role in the development of diabetes. However, the influence of high glucose on the liver remains to be clarified. Rosiglitazone (RSG) is a member of thiazolidinediones (TDZs) family, which is the ligand of the of nuclear transcription factor peroxisome proliferator-activated receptor-γ (PPARγ), being used clinically for the treatment of type 2 diabetic patients through their insulin-sensitizing effect. In the present study, we investigated the cytotoxicity of high glucose in QZG hepatocytes and evaluated the protective effect of RSG. The results showed that high glucose significantly reduced cell viability through generation of ROS via activation of PKC, which was inhibited by RSG. On the one hand, RSG notably inhibited the activation of PKC induced by high glucose independent of PPARγ, leading to the decrease of ROS generation. On the other hand, RSG notably increased the expression of key antioxidant transcription factor Nrf2 and antioxidant enzyme HO-1 in a PPARγ-dependent manner, leading to the elimination of excessive ROS. In addition, RSG also inhibited the decrease of COX-2 expression induced by high glucose through activating PPARγ. Furthermore, the activation of Akt and MAPKs was involved in the effect of RSG on Nrf2, HO-1 and COX-2. In summary, our study supports the hypothesis that RSG protect hepatocytes from high glucose-induced toxicity through PPARγ-dependent and PPARγ-independent pathways.

AMPK activation, a preventive therapeutic target in the transition from cardiac injury to heart failure

Heart failure is a progressive muscular disorder leading to a deterioration of the heart characterized by a contractile dysfunction and a chronic energy deficit. As a consequence, the failing heart is unable to meet the normal metabolic and energy needs of the body. The transition between compensated left ventricular hypertrophy and the de-compensated heart is multifactorial, although metabolic disturbances are considered to play a significant role. In this respect, the AMP-activated protein kinase (AMPK) could be a potential target in heart failure development. AMPK senses the energy state of the cell and orchestrates a global metabolic response to energy deprivation. We briefly review here the current knowledge about the chronic energy deficit of the failing heart, as well as the role of AMPK in energy homeostasis and in the control of non-metabolic targets in relation to cardiac hypertrophy and heart failure. The relative importance of energetic and non-metabolic effects in the potential cardioprotective action of AMPK is discussed.

Inhibitory effect of rosiglitazone on the acid-induced intracellular generation of hydrogen peroxide in cultured feline esophageal epithelial cells

Peroxisome proliferator-activated receptor-gamma (PPARγ) agonists have been reported to enhance antioxidant defenses by increasing levels of catalase and copper-zinc superoxide dismutase (Cu/Zn SOD) in oligodendrocyte progenitor cells. In this study, we investigated the effects of the PPARγ agonist, rosiglitazone, on hydrogen peroxide (H(2)O(2)) generation by acidified medium at pH 5.5 (AM5.5), which is in the pH range of duodenogastric refluxates, in primary cultured feline esophageal epithelial cells (EEC). Successful isolation of EEC was identified by immunocytochemistry. AM5.5- and rosiglitazone-induced cell viabilities were determined using 3-(4,5-dimethylthiozol-2-yl)-2,5-diphenyltetrazolium bromide assays. The NAD(P)H oxidase activity was measured, and expression of catalase or SOD protein by AM5.5 in the absence and presence of rosiglitazone was assessed using western blotting analysis. PPARγ protein and mRNA were constitutively expressed in EEC. In the incubation with rosiglitazone alone, cell viability was shown more than 90% at 0-10 μM for 72 h. After exposure to AM5.5 for 8 h, intracellular H(2)O(2) was significantly generated. Treatment with rosiglitazone prior to and during exposure to AM5.5 inhibited the H(2)O(2) generation whereas the specific PPARγ antagonist GW9662 offsets the inhibitory action of rosiglitazone. H(2)O(2) generation was also prevented by a nonspecific ROS scavenger N-acetylcysteine or an inhibitor of NADPH oxidase diphenyleneiodonium. The enhanced AM5.5-induced NAD(P)H oxidase activity was not suppressed by rosiglitazone. Instead, the pretreatment of rosiglitazone enhanced the protein expression of catalase, Cu/Zn SOD, and Mn SOD, which are endogenous antioxidative enzymes. These findings indicate that rosiglitazone inhibits AM5.5-induced intracellular H(2)O(2) production, which occurs via NAD(P)H oxidase activation, by using a PPARγ-dependent pathway, and that the underlying mechanism involves an increase in the expression of catalase and SOD proteins.

AMPK and vasculoprotection

AMP-activated protein kinase (AMPK) is proposed to be a key regulator of cellular and organismal metabolism and has reported vasculoprotective effects. In addition, many therapeutic agents used in the treatment of diabetes and atherosclerosis such as metformin, thiazolidinediones and statins may exert their vasculoprotective effects through activation of AMPK. Activation of AMPK has a number of potentially beneficial anti-atherosclerotic effects including reducing adhesion of inflammatory cells to the blood vessel endothelium, reducing lipid accumulation and the proliferation of inflammatory cells caused by oxidised lipids, stimulation of gene expression responsible for cellular antioxidant defenses and stimulation of enzymes responsible for nitric oxide formation. In humans and animals the AMPK cascade triggers vascular protective mechanisms that have been shown to reduce myocardial ischaemic injury and mutations in AMPK can cause familial hypertrophic cardiomyopathy. Taken together, these data suggest that activation and function of AMPK contributes to cardiovascular health. In this review we propose to focus on the vasculoprotective effects of AMPK, the evidence for AMPK activation with currently used therapeutic agents and the potential for agents which specifically activate AMPK as a treatment for vascular disease.

NADPH oxidase-derived reactive oxygen species: involvement in vascular physiology and pathology

Reactive oxygen species (ROS) are essential mediators of normal cell physiology. However, in the last few decades, it has become evident that ROS overproduction and/or alterations of the antioxidant system associated with inflammation and metabolic dysfunction are key pathological triggers of cardiovascular disorders. NADPH oxidases (Nox) represent a class of hetero-oligomeric enzymes whose primary function is the generation of ROS. In the vasculature, Nox-derived ROS contribute to the maintenance of vascular tone and regulate important processes such as cell growth, proliferation, differentiation, apoptosis, cytoskeletal organization, and cell migration. Under pathological conditions, excessive Nox-dependent ROS formation, which is generally associated with the up-regulation of different Nox subtypes, induces dysregulation of the redox control systems and promotes oxidative injury of the cardiovascular cells. The molecular mechanism of Nox-derived ROS generation and the means by which this class of molecule contributes to vascular damage remain debatable issues. This review focuses on the processes of ROS formation, molecular targets, and neutralization in the vasculature and provides an overview of the novel concepts regarding Nox functions, expression, and regulation in vascular health and disease. Because Nox enzymes are the most important sources of ROS in the vasculature, therapeutic perspectives to counteract Nox-dependent oxidative stress in the cardiovascular system are discussed.

Redox regulation of the AMP-activated protein kinase

Redox state is a critical determinant of cell function, and any major imbalances can cause severe damage or death.

Adiponectin prevents diabetic premature senescence of endothelial progenitor cells and promotes endothelial repair by suppressing the p38 MAP kinase/p16INK4A signaling pathway

OBJECTIVE

A reduced number of circulating endothelial progenitor cells (EPCs) are casually associated with the cardiovascular complication of diabetes. Adiponectin exerts multiple protective effects against cardiovascular disease, independent of its insulin-sensitizing activity. The objective of this study was to investigate whether adiponectin plays a role in modulating the bioavailability of circulating EPCs and endothelial repair.

RESEARCH DESIGN AND METHODS

Adiponectin knockout mice were crossed with db(+/-) mice to produce db/db diabetic mice without adiponectin. Circulating number of EPCs were analyzed by flow cytometry. Reendothelialization was evaluated by staining with Evans blue after wire-induced carotid injury.

RESULTS

In adiponectin knockout mice, the number of circulating EPCs decreased in an age-dependent manner compared with the wild-type controls, and this difference was reversed by the chronic infusion of recombinant adiponectin. In db/db diabetic mice, the lack of adiponectin aggravated the hyperglycemia-induced decrease in circulating EPCs and also diminished the stimulatory effects of the PPARγ agonist rosiglitazone on EPC production and reendothelialization. In EPCs isolated from both human peripheral blood and mouse bone marrow, treatment with adiponectin prevented high glucose-induced premature senescence. At the molecular level, adiponectin decreased high glucose-induced accumulation of intracellular reactive oxygen species and consequently suppressed activation of p38 MAP kinase (MAPK) and expression of the senescence marker p16(INK4A).

CONCLUSIONS

Adiponectin prevents EPC senescence by inhibiting the ROS/p38 MAPK/p16(INK4A) signaling cascade. The protective effects of adiponectin against diabetes vascular complications are attributed in part to its ability to counteract hyperglycemia-mediated decrease in the number of circulating EPCs.

Signaling of reactive oxygen and nitrogen species in Diabetes mellitus

Disorder of physiological signaling functions of reactive oxygen species (ROS) superoxide and hydrogen peroxide and reactive nitrogen species (RNS) nitric oxide and peroxynitrite is an important feature of diabetes mellitus type 1 and type 2. It is now known that hyperglycemic conditions of cells are associated with the enhanced levels of ROS mainly generated by mitochondria and NADPH oxidase. It has been established that ROS stimulate many enzymatic cascades under normal physiological conditions, but hyperglycemia causes ROS overproduction and the deregulation of ROS signaling pathways initiating the development of diabetes mellitus. On the other hand the deregulation of RNS signaling leads basically to a decrease in NO formation with subsequent damaging disorders. In the present work we will consider the pathological changes of ROS and RNS signaling in enzyme/gene regulated processes catalyzed by protein kinases C and B (Akt/B), phosphatidylinositol 3'-kinase (PI3-kinase), extracellular signal-regulated kinase 1/2 (ERK1/2), and some others. Furthermore we will discuss a particularly important role of several ROS-regulated genes and adapter proteins such as the p66shc, FOXO3a, and Sirt2. The effects of low and high ROS levels in diabetes will be also considered. Thus the regulation of damaging ROS levels in diabetes by antioxidants and free radical scavengers must be one of promising treatment of this disease, however, because of the inability of traditional antioxidative vitamin E and C to interact with superoxide and hydrogen peroxide, new free radical scavengers such as flavonoids, quinones and synthetic mimetics of superoxide dismutase (SOD) should be intensively studied.

AMPK in cardiovascular health and disease

AbstractAdenosine Monophosphate-activated Protein Kinase (AMPK), a serine/threonine kinase and a member of the Snf1/AMPK protein kinase family, consists of three protein subunits that together make a functional enzyme. AMPK, which is expressed in a number of tissues, including the liver, brain, and skeletal muscle, is allosterically activated by a rise in the AMP: ATP ratio (ie in a low ATP or energy depleted state). The net effect of AMPK activation is to halt energy consuming (anabolic) pathways but to promote energy conserving (catabolic) cellular pathways. AMPK has therefore often been dubbed the "metabolic master switch". AMPK also plays a critical physiological role in the cardiovascular system. Increasing evidence suggest that AMPK might also function as a sensor by responding to oxidative stress. Mostly importantly, AMPK modulates endogenous antioxidant gene expression and/or suppress the production of oxidants. AMPK promotes cardiovascular homeostasis by ensuring an optimum redox balance on the heart and vascular tissues. Dysfunctional AMPK is thought to underlie several cardiovascular pathologies. Here we review this kinase from its structure and discovery to current knowledge of its adaptive and maladaptive role in the cardiovascular system.

AMPK-Dependent Metabolic Regulation by PPAR Agonists

Comprehensive studies support the notion that the peroxisome proliferator-activated receptors, (PPARs), PPARα, PPARβ/δ, and PPARγ, regulate cell growth, morphogenesis, differentiation, and homeostasis. Agonists of each PPAR subtype exert their effects similarly or distinctly in different tissues such as liver, muscle, fat, and vessels. It is noteworthy that PPARα or PPARγ agonists have pharmacological effects by modulating the activity of AMPK, which is a key cellular energy sensor. However, the role of AMPK in the metabolic effects of PPAR agonists has not been thoroughly focused. Moreover, AMPK activation by PPAR agonists seems to be independent of the receptor activation. This intriguing action of PPAR agonists may account in part for the mechanistic basis of the therapeutics in the treatment of metabolic disease. In this paper, the effects of PPAR agonists on metabolic functions were summarized with particular reference to their AMPK activity regulation.

Potential manipulation of endothelial progenitor cells in diabetes and its complications

Diabetes mellitus increases cardiovascular risk through its negative impact on vascular endothelium. Although glucotoxicity and lipotoxicity account for endothelial cell damage, endothelial repair is also affected by diabetes. Endothelial progenitor cells (EPCs) are involved in the maintenance of endothelial homoeostasis and in the process of new vessel formation. For these reasons, EPCs are thought to have a protective impact within the cardiovascular system. In addition, EPCs appear to modulate the functioning of other organs, providing neurotropic signals and promoting repair of the glomerular endothelium. The exact mechanisms by which EPCs provide cardiovascular protection are unknown and the definition of EPCs is not standardized. Notwithstanding these limitations, the literature consistently indicates that EPCs are altered in type 1 and type 2 diabetes and in virtually all diabetic complications. Moreover, experimental models suggest that EPC-based therapies might help prevent or reverse the features of end-organ complications. This identifies EPCs as having a novel pathogenic role in diabetes and being a potential therapeutic target. Several ways of favourably modulating EPCs have been identified, including lifestyle intervention, commonly used medications and cell-based approaches. Herein, we provide a comprehensive overview of EPC pathophysiology and the potential for EPC modulation in diabetes.

Oral Intake of Rosiglitazone Promotes a Central Antihypertensive Effect Via Upregulation of Peroxisome Proliferator-Activated Receptor-γ and Alleviation of Oxidative Stress in Rostral Ventrolateral Medulla of Spontaneously Hypertensive Rats

Rosiglitazone, a synthetic ligand of transcription factor peroxisome proliferator-activated receptor-γ (PPAR-γ), possesses a blood pressure–lowering effect beyond insulin sensitizing and glucose lowering. Oxidative stress in rostral ventrolateral medulla (RVLM), where sympathetic premotor neurons for the maintenance of neurogenic vasomotor tone are located, contributes to neural mechanisms of hypertension. Activation of PPAR-γ protects against oxidative stress in RVLM by upregulation of mitochondrial uncoupling protein 2 (UCP2). We tested the hypothesis that oral intake of rosiglitazone exerts a central antihypertensive effect by ameliorating oxidative stress in RVLM via transcriptional upregulation of UCP2 after PPAR-γ activation. In adult spontaneously hypertensive rats but not normotensive Wistar-Kyoto rats, oral intake of rosiglitazone for 1 week resulted in vasodepression and a reduction in the vasomotor components of the systemic arterial pressure spectrum, our experimental index for sympathetic vasomotor tone. These antihypertensive effects of rosiglitazone in spontaneously hypertensive rats were abrogated by microinjection bilaterally into RVLM of PPAR-γ small interfering RNA. Oral intake of rosiglitazone also upregulated UCP2 and ameliorated the heightened superoxide anion level in RVLM of spontaneously hypertensive rats. Protection against oxidative stress in RVLM by rosiglitazone was abrogated by PPAR-γ small interfering RNA or by antisense oligonucleotide against ucp2 mRNA. Gene knockdown of ucp2 in RVLM also reversed the antihypertensive effect of rosiglitazone. These results suggest that oral intake of rosiglitazone promotes a central antihypertensive effect by decreasing sympathetic vasomotor activity through a PPAR-γ–dependent protection against oxidative stress in RVLM via transcriptional upregulation of the mitochondrial UCP2.

Involvement of mTOR in globular adiponectin-induced generation of reactive oxygen species

Globular adiponectin (gAd) induces the generation of reactive oxygen species (ROS) and nitric oxide (NO) in the murine macrophage cell line RAW 264. This study investigated the role of the mammalian target of rapamycin (mTOR) in gAd-induced ROS and NO generation. gAd stimulation induced phosphorylation of mTOR, which peaked at 20 min and dissolved rapidly. Inhibition of phosphatidylinositol 3-kinase activity with wortmannin suppressed gAd-induced phosphorylation of Akt and mTOR. Administration of rapamycin partially reduced gAd-induced generation of intracellular and mitochondrial ROS, but not release of NO. To further confirm the role of mTOR in gAd stimulation, the effect of the activators of AMP-activated protein kinase (AMPK) on gAd-induced mTOR phosphorylation was examined. Pre-treatment with three kinds of AMPK activators, AICAR, 2-deoxy-D-glucose and A-769662, suppressed gAd-induced mTOR phosphorylation. Furthermore, these AMPK activators significantly reduced gAd-evoked intracellular and mitochondrial ROS generation and NO release.

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.

The effects of rosiglitazone and high glucose on protein expression in endothelial cells

Rosiglitazone is a thiazolidinedione used to treat insulin resistance in diabetes. Although thiazolidinediones may also exert cardiovascular effects, contrasting results were reported. Favorable effects were shown for pioglitazone, whereas adverse reactions were suspected for rosiglitazone. Therefore, a reassessment of the molecular effects of rosiglitazone on vascular cells is required. We tested the effects of rosiglitazone on the proteome of human endothelial cells grown under either normal or high glucose levels. Protein profiles were analyzed in both membrane and cytosolic fractions. About 150 cytosolic proteins, and approximately 100 membrane proteins, were detected. Two-thirds of the proteins significantly altered by high glucose were also modulated by rosiglitazone in an antagonistic way. Half of these proteins are involved in apoptosis. Using an independent assay of apoptosis based on nucleosome quantification, an approximately 20% stimulation by high versus normal glucose was shown (p < 0.05). Conversely, rosiglitazone reduced apoptosis by approximately 30-50% in cells exposed to either glucose conditions (p < 0.001). In addition, rosiglitazone differently modulated cytoskeleton and energy metabolism-related proteins. Our data show novel, potential sites of action of rosiglitazone through protein expression of endothelial cells. These mechanisms may foster new investigations on the overall vascular effects of this compound, and help to discriminate between desired and adverse effects.

Biochemical effects of SIRT1 activators

SIRT1 is the closest mammalian homologue of enzymes that extend life in lower organisms. Its role in mammals is incompletely understood, but includes modulation of at least 34 distinct targets through its nicotinamide adenine dinucleotide (NAD(+))-dependent deacetylase activity. Recent experiments using small molecule activators and genetically engineered mice have provided new insight into the role of this enzyme in mammalian biology and helped to highlight some of the potentially relevant targets. The most widely employed activator is resveratrol, a small polyphenol that improves insulin sensitivity and vascular function, boosts endurance, inhibits tumor formation, and ameliorates the early mortality associated with obesity in mice. Many of these effects are consistent with modulation of SIRT1 targets, such as PGC1alpha and NFkappaB, however, resveratrol can also activate AMPK, inhibit cyclooxygenases, and influence a variety of other enzymes. A novel activator, SRT1720, as well as various methods to manipulate NAD(+) metabolism, are emerging as alternative methods to increase SIRT1 activity, and in many cases recapitulate effects of resveratrol. At present, further studies are needed to more directly test the role of SIRT1 in mediating beneficial effects of resveratrol, to evaluate other strategies for SIRT1 activation, and to confirm the specific targets of SIRT1 that are relevant in vivo. These efforts are especially important in light of the fact that SIRT1 activators are entering clinical trials in humans, and "nutraceutical" formulations containing resveratrol are already widely available.

Naturally occurring flavonoids attenuate high glucose-induced expression of proinflammatory cytokines in human monocytic THP-1 cells

Activation of circulating monocytes by hyperglycemia is bound to play a role in inflammatory and atherosclerosis. In this study, we examined whether flavonoids (catechin, EGCG, luteolin, quercetin, rutin) - phytochemicals that may possible belong to a new class of advanced glycation end products (AGEs) inhibitors - can attenuate high glucose (15 mmol/L, HG)-induced inflammation in human monocytes. Our results show that all flavonoids significantly inhibited HG-induced expression of proinflammatory genes and proteins, including TNF-alpha, interleukin-1beta (IL-1beta), and cyclooxygenase (COX)-2, at a concentration of 20 microM. Flavonoids also prevented oxidative stress in activated monocytes, as demonstrated by their inhibitory effects on intracellular reactive oxygen species (ROS) and N(epsilon)-(carboxymethyl)lysine formation caused by HG. These inhibitory effects may involve inhibition of nuclear factor-kappaB activation and may be supported by downregulation of the following: i) PKC-dependent NADPH oxidase pathway; ii) phosphorylation of p38 mitogen-activated protein kinase and extracellular signal-regulated protein kinase, and iii) mRNA expression of receptor of AGEs. In addition, we found for the first time that lower levels of Bcl-2 protein under HG conditions could be countered by the action of flavonoids. Our data suggest that, along with their antioxidant activities, flavonoids possess anti-inflammatory properties and might therefore have additional protective effects against glycotoxin-related inflammation.

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.

The PPARgamma agonist pioglitazone ameliorates aging-related progressive renal injury

Peroxisome proliferator-activated receptor-gamma (PPAR-gamma) agonists not only improve metabolic abnormalities of diabetes and consequent diabetic nephropathy, but they also protect against nondiabetic chronic kidney disease in experimental models. Here, we found that the PPAR-gamma agonist pioglitazone protected against renal injury in aging; it reduced proteinuria, improved GFR, decreased sclerosis, and alleviated cell senescence. Increased local expression of PPAR-gamma paralleled these changes. Underlying mechanisms included increased expression of klotho, decreased systemic and renal oxidative stress, and decreased mitochondrial injury. Pioglitazone also regulated p66(Shc) phosphorylation, which integrates many signaling pathways that affect mitochondrial function and longevity, by reducing protein kinase C-beta. These results suggest that PPAR-gamma agonists may benefit aging-related renal injury by improving mitochondrial function.

Treatment of spontaneously hypertensive rats with rosiglitazone ameliorates cardiovascular pathophysiology via antioxidant mechanisms in the vasculature

Oxidative stress contributes to cardiovascular complications of diabetes, in part, by reducing the bioavailability of nitric oxide (NO). We investigated the mechanisms whereby the insulin sensitizer rosiglitazone may ameliorate oxidative stress in the vasculature of spontaneously hypertensive rats (SHR). Nine-week-old SHR were treated by gavage for 7 wk with rosiglitazone (5 mg x kg(-1) x day(-1)) or vehicle control. Treatment of SHR with rosiglitazone lowered systolic blood pressure, reduced fasting plasma insulin and asymmetrical dimethylarginine, and increased insulin sensitivity (when compared with vehicle treatment). In vessel homogenates and serum from rosiglitazone-treated SHR, SOD activity was enhanced, while 8-iso-PGF(2alpha) (lipid peroxidation product) was reduced (when compared with samples from vehicle-treated SHR). Moreover, expression of p22phox (catalytic subunit of NADPH oxidase) as well as nitrotyrosine and superoxide content were all reduced in the aortas of rosiglitazone-treated SHR. In mesenteric vascular beds (MVB) isolated ex vivo from rosiglitazone-treated SHR, NO-dependent vasodilator actions of insulin were improved when compared with MVB from vehicle-treated SHR. Acute pretreatment of MVB from vehicle-treated SHR with apocynin (NADPH oxidase inhibitor) enhanced vasodilator actions of insulin (results comparable to those in MVB from rosiglitazone-treated SHR). In Langendorff heart preparations from rosiglitazone-treated SHR, ischemia/reperfusion injury caused infarcts 40% smaller than in hearts from vehicle-treated SHR. Acute pretreatment of hearts from vehicle-treated SHR with apocynin produced similar results. Finally, rosiglitazone treatment of endothelial cells in primary culture reduced superoxide induced by insulin-resistant conditions. We conclude that rosiglitazone therapy in SHR increases SOD activity and decreases p22phox expression in the vasculature to reduce oxidant stress leading to an improved cardiovascular phenotype.

Resveratrol prevents hyperglycemia-induced endothelial dysfunction via activation of adenosine monophosphate-activated protein kinase

Endothelial dysfunction secondary to persistent hyperglycemia plays a key role in the development of type 2 diabetic vascular disease. The aim of the present study was to examine the protective effect of resveratrol against hyperglycemia-induced endothelial dysfunction. In cultured human umbilical vein endothelial cells (HUVECs), resveratrol (10-100 microM) concentration dependently enhanced phosphorylation of endothelial nitric oxide synthesis (eNOS) at Ser1177 and nitric oxide (NO) production. In addition, resveratrol can increase the phosphorylation of adenosine monophosphate-activated protein kinase (AMPK) at Thr172 and suppress high glucose-induced generation of superoxide anion. In mouse aortic rings, resveratrol (1-100 microM) elicited endothelium-dependent vasodilatations and alleviated high glucose-mediated endothelial dysfunction. All these beneficial effects of resveratrol on the endothelium were abolished by pharmacological antagonism of AMPK by compound C. These results provide new insight into the protective properties of resveratrol against endothelial dysfunction caused by high glucose, which is attributed to the AMPK mediated reduction of superoxide level.

Activation and signaling by the AMP-activated protein kinase in endothelial cells

The AMP-activated protein kinase (AMPK) was initially identified as the kinase that phosphorylates the 3-hydroxy 3-methylglutaryl coenzyme A reductase, the rate-limiting enzyme for cholesterol biosynthesis. As the name suggests, the AMPK is activated by increased intracellular concentrations of AMP, and is generally described as a "metabolite-sensing kinase" and when activated initiates steps to conserve cellular energy. Although there is a strong link between the activity of the AMPK and metabolic control in muscle cells, the activity of the AMPK in endothelial cells can be regulated by stimuli that affect cellular ATP levels, such as hypoxia as well as by fluid shear stress, Ca(2+)-elevating agonists, and hormones such as adiponectin. To date the AMPK in endothelial cells has been implicated in the regulation of fatty acid oxidation, small G protein activity and nitric oxide production as well as inflammation and angiogenesis. Moreover, there is evidence indicating that the activation of the AMPK may help to prevent the vascular complications associated with the metabolic syndrome.

Peroxisome proliferator-activated receptor-gamma agonist improves skeletal muscle insulin signaling in the pregestational intrauterine growth-restricted rat offspring

The effect of early intervention with a peroxisome proliferator-activated receptor-gamma (PPARgamma) agonist on skeletal muscle GLUT4 translocation and insulin signaling was examined in intrauterine (IUGR) and postnatal (PNGR) growth-restricted pregestational female rat offspring. Rosiglitazone [11 mumol/day provided from postnatal day (PN)21 to PN60] improved skeletal muscle insulin sensitivity and GLUT4 translocation in prenatal nutrient restriction [50% calories from embryonic day (e)11 to e21; IUGR] with (IUGR+PNGR) and without (IUGR) postnatal nutrient restriction (50% calories from PN1 to PN21; PNGR) similar to that of control (ad libitum feeds throughout; Con) (n = 6 each). This was accomplished by diminished basal and improved insulin-responsive GLUT4 association with the plasma membrane in IUGR, IUGR+PNGR, and PNGR mimicking that in Con (P < 0.005). While no change in p85-phosphatidylinositol 3-kinase (PI3-K) and phosphatase and tensin homolog deleted on chromosome 10 (PTEN) was observed, a decrease in protein tyrosine phosphatase 1B (PTP1B; P < 0.0002) and SH2-containing protein tyrosine phosphatase 2 (SHP2; P < 0.05) contributing to the rosiglitazone-induced insulin sensitivity was seen only in IUGR+PNGR. In contrast, an increase in phosphorylated 5'-adenosine monophosphate kinase (pAMPK; P < 0.04) and insulin responsiveness of phosphorylated phosphoinositide-dependent protein kinase-1 (pPDK1; P < 0.05), pAkt (P < 0.01), and particularly pPKCzeta (P < 0.0001) and its corresponding enzyme activity (P < 0.005) were observed in all four experimental groups. We conclude that early introduction of PPARgamma agonist improved skeletal muscle activation of AMPK and insulin signaling, resulting in insulin-independent AMPK and insulin-responsive GLUT4 association with plasma membranes in IUGR, IUGR+PNGR, and PNGR adult offspring, similar to that of Con. These findings support a role for insulin sensitizers in preventing the subsequent development of gestational or type 2 diabetes mellitus in intrauterine and postnatal growth-restricted offspring.

Rosiglitazone prevents high glucose-induced vascular endothelial growth factor and collagen IV expression in cultured mesangial cells

Peroxisome proliferator-activated receptor (PPARγ), a ligand-dependent transcription factor, negatively modulates high glucose effects. We postulated that rosiglitazone (RSG), an activator of PPARγ prevents the upregulation of vascular endothelial growth factor (VEGF) and collagen IV by mesangial cells exposed to high glucose. Primary cultured rat mesangial cells were growth-arrested in 5.6 mM (NG) or 25 mM D-glucose (HG) for up to 48 hours. In HG, PPARγ mRNA and protein were reduced within 3 h, and enhanced ROS generation, expression of p22^phox, VEGF and collagen IV, and PKC-ζ membrane association were prevented by RSG. In NG, inhibition of PPARγ caused ROS generation and VEGF expression that were unchanged by RSG. Reduced AMP-activated protein kinase (AMPK) phosphorylation in HG was unchanged with RSG, and VEGF expression was unaffected by AMPK inhibition. Hence, PPARγ is a negative modulator of HG-induced signaling that acts through PKC-ζ but not AMPK and regulates VEGF and collagen IV expression by mesangial cells.

AMP-activated protein kinase pathway: a potential therapeutic target in cardiometabolic disease

AMPK (AMP-activated protein kinase) is a heterotrimetric enzyme that is expressed in many tissues, including the heart and vasculature, and plays a central role in the regulation of energy homoeostasis. It is activated in response to stresses that lead to an increase in the cellular AMP/ATP ratio caused either by inhibition of ATP production (i.e. anoxia or ischaemia) or by accelerating ATP consumption (i.e. muscle contraction or fasting). In the heart, AMPK activity increases during ischaemia and functions to sustain ATP, cardiac function and myocardial viability. There is increasing evidence that AMPK is implicated in the pathophysiology of cardiovascular and metabolic diseases. A principle mode of AMPK activation is phosphorylation by upstream kinases [e.g. LKB1 and CaMK (Ca²⁺/calmodulin-dependent protein kinase], which leads to direct effects on tissues and phosphorylation of various downstream kinases [e.g. eEF2 (eukaryotic elongation factor 2) kinase and p70 S6 kinase]. These upstream and downstream kinases of AMPK have fundamental roles in glucose metabolism, fatty acid oxidation, protein synthesis and tumour suppression; consequently, they have been implicated in cardiac ischaemia, arrhythmias and hypertrophy. Recent mechanistic studies have shown that AMPK has an important role in the mechanism of action of MF (metformin), TDZs (thiazolinediones) and statins. Increased understanding of the beneficial effects of AMPK activation provides the rationale for targeting AMPK in the development of new therapeutic strategies for cardiometabolic disease.

Endocannabinoids and the heart

Endocannabinoids, such as anandamide and 2-arachidonoylglycerol, are synthesized from membrane phospholipids in the heart and other cardiovascular tissues. They activate cannabinoid CB1 and CB2 receptors, transient receptor potential V1 (TRPV1), peroxisome proliferator-activated receptors, and perhaps a novel vascular G-protein-coupled receptor. Inactivation is by cellular uptake and fatty acid amide hydrolase. Endocannabinoids relax coronary and other arteries and decrease cardiac work but seem not to be involved in tonic regulation of cardiovascular function. They act as a stress response system, which is activated, for example, in myocardial infarction and circulatory shock. Endocannabinoids are largely protective; they decrease tissue damage and arrhythmia in myocardial infarction and may reduce progression of atherosclerosis (CB2 receptor stimulation inhibits lesion progression), and fatty acid amide hydrolase knockout mice (which have enhanced endocannabinoid levels) show decreased cardiac dysfunction with age compared with wild types. However, endocannabinoids may mediate doxorubicin-induced cardiac dysfunction. Their signaling pathways are not fully elucidated but they can lead to changed expression of a variety of genes, including those involved in inflammatory responses. There is potential for therapeutic targeting of endocannabinoids and their receptors, but their apparent involvement in both protective and deleterious actions on the heart means that careful risk assessment is needed before any treatment can be introduced.

Berberine suppresses proinflammatory responses through AMPK activation in macrophages

Berberine (BBR) has been shown to improve several metabolic disorders, such as obesity, type 2 diabetes, and dyslipidemia, by stimulating AMP-activated protein kinase (AMPK). However, the effects of BBR on proinflammatory responses in macrophages are poorly understood. Here we show that BBR represses proinflammatory responses through AMPK activation in macrophages. In adipose tissue of obese db/db mice, BBR treatment significantly downregulated the expression of proinflammatory genes such as TNF-alpha, IL-1beta, IL-6, monocyte chemoattractant protein-1 (MCP-1), inducible nitric oxide synthase (iNOS), and cyclooxygenase-2 (COX-2). Consistently, BBR inhibited LPS-induced expression of proinflammatory genes including IL-1beta, IL-6, iNOS, MCP-1, COX-2, and matrix metalloprotease-9 in peritoneal macrophages and RAW 264.7 cells. Upon various proinflammatory signals including LPS, free fatty acids, and hydrogen peroxide, BBR suppressed the phosphorylation of MAPKs, such as p38, ERK, and JNK, and the level of reactive oxygen species in macrophages. Moreover, these inhibitory effects of BBR on proinflammatory responses were abolished by AMPK inhibition via either compound C, an AMPK inhibitor, or dominant-negative AMPK, implying that BBR would downregulate proinflammatory responses in macrophages via AMPK stimulation.

Up-regulation of thioredoxin interacting protein (Txnip) by p38 MAPK and FOXO1 contributes to the impaired thioredoxin activity and increased ROS in glucose-treated endothelial cells

Oxidative stress induced by hyperglycemia is a key factor in the development of cardiovascular diseases in diabetes. Thioredoxin (Trx) system, a major thiol antioxidant system, regulates the reduction of intracellular reactive oxygen species (ROS). In this study, we demonstrated that high glucose significantly increased intracellular ROS levels in human aortic endothelial cells (HAECs). Additionally, high glucose reduced the antioxidant activity of thioredoxin. To investigate the mechanisms involved, we found that glucose enhanced the expression of thioredoxin interacting protein (Txnip), a Trx inhibitory protein, through p38 mitogen-activated protein kinase (MAPK). We also showed that glucose regulated Txnip at transcription level and p38 MAPK and forkhead box O1 transcriptional factor (FOXO1) were involved in the process. Taken together, upregulation of Txnip and subsequent impairment of thioredoxin antioxidative system through p38 MAPK and FOXO1 may represent a novel mechanism for glucose-induced increase in intracellular ROS.

Berberine prevents hyperglycemia-induced endothelial injury and enhances vasodilatation via adenosine monophosphate-activated protein kinase and endothelial nitric oxide synthase

AIMS

Endothelial dysfunction is a key event that links obesity, diabetes, hypertension, and cardiovascular diseases. The aim of the present study was to examine the protective effect of the alkaloid drug berberine against hyperglycemia-induced cellular injury and endothelial dysfunction.

METHODS AND RESULTS

In both cultured endothelial cells and blood vessels isolated from rat aorta, berberine concentration dependently enhanced phosphorylation of endothelial nitric oxide synthase (eNOS) at Ser1177 and promoted the association of eNOS with heat shock protein 90 (HSP90), leading to an increased production of nitric oxide. Furthermore, berberine attenuated high glucose-induced generation of reactive oxygen species, cellular apoptosis, nuclear factor-kappaB activation, and expression of adhesion molecules, thus suppressing monocyte attachment to endothelial cells. In mouse aortic rings, berberine elicited endothelium-dependent vasodilatations and alleviated high glucose-mediated endothelial dysfunction. All these beneficial effects of berberine on the endothelium were abolished by either pharmacological inhibition of adenosine monophosphate-activated protein kinase (AMPK) or adenovirus-mediated overexpression of a dominant negative version of AMPK.

CONCLUSION

Berberine protects against endothelial injury and enhances the endothelium-dependent vasodilatation, which is mediated in part through activation of the AMPK signalling cascade. Berberine or its derivatives may be useful for the treatment and/or prevention of endothelial dysfunction associated with diabetes and cardiovascular disease.

Endothelial dysfunction: causes and consequences in patients with diabetes mellitus

Vascular endothelium plays a major role in maintaining cardiovascular homeostasis. Nitric oxide is the most powerful vasodilating compound. Diabetes disrupts endothelial integrity through an increased oxidative stress. Recent evidence indicates that there is a strong interaction between diabetes, the secretory proteins of adipocytes, called adipokines, and endothelium. Endothelial dysfunction may precede the development of overt DM, and a prolonged and repeated exposure to postprandial hyperglycemia may play an important role in the development of atherosclerosis, even in those who have normal fasting plasma glucose levels.