AMPK, insulin resistance, and the metabolic syndrome

Dietary nitrate improves age-related hypertension and metabolic abnormalities in rats via modulation of angiotensin II receptor signaling and inhibition of superoxide generation

Advanced age is associated with increased risk for cardiovascular disease and type 2 diabetes. A proposed central event is diminished amounts of nitric oxide (NO) due to reduced generation by endothelial NO synthase (eNOS) and increased oxidative stress. In addition, it is widely accepted that increased angiotensin II (ANG II) signaling is also implicated in the pathogenesis of endothelial dysfunction and hypertension by accelerating formation of reactive oxygen species. This study was designed to test the hypothesis that dietary nitrate supplementation could reduce blood pressure and improve glucose tolerance in aged rats, via attenuation of NADPH oxidase activity and ANG II receptor signaling. Dietary nitrate supplementation for two weeks reduced blood pressure (10-15mmHg) and improved glucose clearance in old, but not in young rats. These favorable effects were associated with increased insulin responses, reduced plasma creatinine as well as improved endothelial relaxation to acetylcholine and attenuated contractility to ANG II in resistance arteries. Mechanistically, nitrate reduced NADPH oxidase-mediated oxidative stress in the cardiovascular system and increased cGMP signaling. Finally, nitrate treatment in aged rats normalized the gene expression profile of ANG II receptors (AT_1A, AT₂, AT_1A/AT₂ ratio) in the renal and cardiovascular systems without altering plasma levels of renin or ANG II. Our results show that boosting the nitrate-nitrite-NO pathway can partly compensate for age-related disturbances in endogenous NO generation via inhibition of NADPH oxidase and modulation of ANG II receptor expression. These novel findings may have implications for nutrition-based preventive and therapeutic strategies against cardiovascular and metabolic diseases.

C-peptide protects against hyperglycemic memory and vascular endothelial cell apoptosis

C-peptide exerts protective effects against diabetic complications; however, its role in inhibiting hyperglycemic memory (HGM) has not been elucidated. We investigated the beneficial effect of C-peptide on HGM-induced vascular damage in vitro and in vivo using human umbilical vein endothelial cells and diabetic mice. HGM induced apoptosis by persistent generation of intracellular ROS and sustained formation of ONOO(-) and nitrotyrosine. These HGM-induced intracellular events were normalized by treatment with C-peptide, but not insulin, in endothelial cells. C-peptide also inhibited persistent upregulation of p53 and activation of mitochondrial adaptor p66(shc) after glucose normalization. Further, C-peptide replacement therapy prevented persistent generation of ROS and ONOO(-) in the aorta of diabetic mice whose glucose levels were normalized by the administration of insulin. C-peptide, but not insulin, also prevented HGM-induced endothelial apoptosis in the murine diabetic aorta. This study highlights a promising role for C-peptide in preventing HGM-induced intracellular events and diabetic vascular damage.

Recent progress in the identification of adenosine monophosphate-activated protein kinase (AMPK) activators

Adenosine monophosphate-activated protein kinase (AMPK), a serine/threonine heterotrimeric protein kinase, is a critical regulator of cellular and whole body energy homeostasis. There are twelve known AMPK isoforms that are differentially expressed in tissues and species. Dysregulation of AMPK signaling is associated with a multitude of human pathologies. Hence isoform-selective activators of AMPK are actively being sought for the treatment of cardiovascular and metabolic diseases. The present review summarizes the status of direct AMPK activators from the patent and published literature.

FGF21 activates AMPK signaling: impact on metabolic regulation and the aging process

Fibroblast growth factor 21 (FGF21) has a significant role in the regulation of energy metabolism, e.g., in the control of systemic glucose and lipid metabolism. For instance, FGF21 enhances insulin sensitivity, increases glucose uptake, and thus can decrease serum hyperglycemia, while it also increases lipid oxidation and inhibits lipogenesis. AMP-activated protein kinase (AMPK) is a tissue energy sensor involved in maintaining the energy balance and tissue integrity. It is known that AMPK signaling generates an energy metabolic profile which displays a remarkable overlap with that of FGF21. There is convincing evidence that endocrine FGF21 signaling activates the AMPK pathway, either directly through FGFR1/β-klotho signaling or indirectly by stimulating the secretion of adiponectin and corticosteroids, which consequently can activate AMPK signaling in their target tissues. By activating AMPK, FGF21 can promote a healthy aging process and thus extend mammalian lifespan. We will examine the signaling mechanisms through which FGF21 can activate the AMPK pathway and then discuss the significance of the close connection between FGF21 and AMPK signaling in the control of metabolic disorders and the aging process.

Activation of SIRT1 Attenuates Klotho Deficiency-Induced Arterial Stiffness and Hypertension by Enhancing AMP-Activated Protein Kinase Activity

Arterial stiffness is an independent risk factor for stroke and myocardial infarction. This study was designed to investigate the role of SIRT1, an important deacetylase, and its relationship with Klotho, a kidney-derived aging-suppressor protein, in the pathogenesis of arterial stiffness and hypertension. We found that the serum level of Klotho was decreased by ≈45% in patients with arterial stiffness and hypertension. Interestingly, Klotho haplodeficiency caused arterial stiffening and hypertension, as evidenced by significant increases in pulse wave velocity and blood pressure in Klotho-haplodeficient (KL^+/-) mice. Notably, the expression and activity of SIRT1 were decreased significantly in aortic endothelial and smooth muscle cells in KL^+/- mice, suggesting that Klotho deficiency downregulates SIRT1. Treatment with SRT1720 (15 mg/kg/d, IP), a specific SIRT1 activator, abolished Klotho deficiency-induced arterial stiffness and hypertension in KL^+/- mice. Klotho deficiency was associated with significant decreases in activities of AMP-activated protein kinase α (AMPKα) and endothelial NO synthase (eNOS) in aortas, which were abolished by SRT1720. Furthermore, Klotho deficiency upregulated NADPH oxidase activity and superoxide production, increased collagen expression, and enhanced elastin fragmentation in the media of aortas. These Klotho deficiency-associated changes were blocked by SRT1720. In conclusion, this study provides the first evidence that Klotho deficiency downregulates SIRT1 activity in arterial endothelial and smooth muscle cells. Pharmacological activation of SIRT1 may be an effective therapeutic strategy for arterial stiffness and hypertension.

Mitochondrial uncoupler carbonyl cyanide m-chlorophenylhydrazone induces vasorelaxation without involving KATP channel activation in smooth muscle cells of arteries

BACKGROUND AND PURPOSE

The effects and mechanisms of chemical mitochondrial uncouplers on vascular function have never been identified. Here, we characterized the effects of the typical mitochondrial uncoupler carbonyl cyanide m-chlorophenylhydrazone (CCCP) on vascular function in rat mesenteric arteries and aorta and elucidated the potential mechanisms.

EXPERIMENTAL APPROACH

Isometric tension of mesenteric artery and thoracic aorta was recorded by using a multiwire myograph system. Protein levels were measured by western blot analyses. Cytosolic [Ca²⁺ ]_i , mitochondrial ROS (mitoROS) and mitochondrial membrane potential of smooth muscle cells (A10) were measured by laser scanning confocal microscopy.

KEY RESULTS

Acute treatment with CCCP relaxed phenylephrine (PE)- and high K⁺ (KPSS)-induced constriction of rat mesenteric arteries with intact and denuded endothelium. Pretreatment with CCCP prevented PE- and KPSS-induced constriction of rat mesenteric arteries with intact and denuded endothelium. Similarly, CCCP prevented PE- and KPSS-induced constriction of rat thoracic aorta. CCCP increased the cellular ADP/ATP ratio in vascular smooth muscle cells (A10) and activated AMPK in A10 cells and rat thoracic aorta tissues. CCCP-induced aorta relaxation was attenuated in AMPK α1 knockout (-/-) mice. SERCA inhibitors thapsigargin and cyclopiazonic acid (CPA) but not the K_ATP channel blocker glibenclamide partially inhibited CCCP-induced vasorelaxation in endothelium-denuded rat mesenteric arteries. CCCP increased cytosolic [Ca²⁺ ]_i , mitoROS production and depolarized mitochondrial membrane potential in A10 cells. FCCP, the analogue of CCCP, had similar vasoactivity as CCCP in rat mesenteric arteries.

CONCLUSIONS AND IMPLICATIONS

CCCP induces vasorelaxation by a mechanism that does not involve K_ATP channel activation in smooth muscle cells of arteries.

Liraglutide prevents microvascular insulin resistance and preserves muscle capillary density in high-fat diet-fed rats

Muscle microvasculature critically regulates endothelial exchange surface area to facilitate transendothelial delivery of insulin, nutrients, and oxygen to myocytes. Insulin resistance blunts insulin-mediated microvascular recruitment and decreases muscle capillary density; both contribute to lower microvascular blood volume. Glucagon-like peptide 1 (GLP-1) and its analogs are able to dilate blood vessels and stimulate endothelial cell proliferation. In this study, we aim to determine the effects of sustained stimulation of the GLP-1 receptors on insulin-mediated capillary recruitment and metabolic insulin responses, small arterial endothelial function, and muscle capillary density. Rats were fed a high-fat diet (HFD) for 4 wk with or without simultaneous administration of liraglutide and subjected to a euglycemic hyperinsulinemic clamp for 120 min after an overnight fast. Insulin-mediated muscle microvascular recruitment and muscle oxygenation were determined before and during insulin infusion. Muscle capillary density was determined and distal saphenous artery used for determination of endothelial function and insulin-mediated vasodilation. HFD induced muscle microvascular insulin resistance and small arterial vessel endothelial dysfunction and decreased muscle capillary density. Simultaneous treatment of HFD-fed rats with liraglutide prevented all of these changes and improved insulin-stimulated glucose disposal. These were associated with a significantly increased AMPK phosphorylation and the expressions of VEGF and its receptors. We conclude that GLP-1 receptor agonists may exert their salutary glycemic effect via improving microvascular insulin sensitivity and muscle capillary density during the development of insulin resistance, and early use of GLP-1 receptor agonists may attenuate metabolic insulin resistance as well as prevent cardiovascular complications of diabetes.

The emerging role of AMPK in the regulation of breathing and oxygen supply

Regulation of breathing is critical to our capacity to accommodate deficits in oxygen availability and demand during, for example, sleep and ascent to altitude. It is generally accepted that a fall in arterial oxygen increases afferent discharge from the carotid bodies to the brainstem and thus delivers increased ventilatory drive, which restores oxygen supply and protects against hypoventilation and apnoea. However, the precise molecular mechanisms involved remain unclear. We recently identified as critical to this process the AMP-activated protein kinase (AMPK), which is key to the cell-autonomous regulation of metabolic homoeostasis. This observation is significant for many reasons, not least because recent studies suggest that the gene for the AMPK-α1 catalytic subunit has been subjected to natural selection in high-altitude populations. It would appear, therefore, that evolutionary pressures have led to AMPK being utilized to regulate oxygen delivery and thus energy supply to the body in the short, medium and longer term. Contrary to current consensus, however, our findings suggest that AMPK regulates ventilation at the level of the caudal brainstem, even when afferent input responses from the carotid body are normal. We therefore hypothesize that AMPK integrates local hypoxic stress at defined loci within the brainstem respiratory network with an index of peripheral hypoxic status, namely afferent chemosensory inputs. Allied to this, AMPK is critical to the control of hypoxic pulmonary vasoconstriction and thus ventilation-perfusion matching at the lungs and may also determine oxygen supply to the foetus by, for example, modulating utero-placental blood flow.

Metformin attenuates lung fibrosis development via NOX4 suppression

BACKGROUND

Accumulation of profibrotic myofibroblasts in fibroblastic foci (FF) is a crucial process for development of fibrosis during idiopathic pulmonary fibrosis (IPF) pathogenesis, and transforming growth factor (TGF)-β plays a key regulatory role in myofibroblast differentiation. Reactive oxygen species (ROS) has been proposed to be involved in the mechanism for TGF-β-induced myofibroblast differentiation. Metformin is a biguanide antidiabetic medication and its pharmacological action is mediated through the activation of AMP-activated protein kinase (AMPK), which regulates not only energy homeostasis but also stress responses, including ROS. Therefore, we sought to investigate the inhibitory role of metformin in lung fibrosis development via modulating TGF-β signaling.

METHODS

TGF-β-induced myofibroblast differentiation in lung fibroblasts (LF) was used for in vitro models. The anti-fibrotic role of metfromin was examined in a bleomycin (BLM)-induced lung fibrosis model.

RESULTS

We found that TGF-β-induced myofibroblast differentiation was clearly inhibited by metformin treatment in LF. Metformin-mediated activation of AMPK was responsible for inhibiting TGF-β-induced NOX4 expression. NOX4 knockdown and N-acetylcysteine (NAC) treatment illustrated that NOX4-derived ROS generation was critical for TGF-β-induced SMAD phosphorylation and myofibroblast differentiation. BLM treatment induced development of lung fibrosis with concomitantly enhanced NOX4 expression and SMAD phosphorylation, which was efficiently inhibited by metformin. Increased NOX4 expression levels were also observed in FF of IPF lungs and LF isolated from IPF patients.

CONCLUSIONS

These findings suggest that metformin can be a promising anti-fibrotic modality of treatment for IPF affected by TGF-β.

Liver enzymes and metabolic syndrome: a large-scale case-control study

Previous studies suggested that elevated liver enzymes could be used as potential novel biomarkers of Metabolic syndrome (MetS) and its clinical outcomes, although the results were inconsistent and the conclusions were underpowered. A case-control study with 6,268 MetS subjects and 6,330 frequency-matched healthy controls was conducted to systematically evaluated levels of four liver enzymes (ALT, AST, GGT and ALP), both in overall populations and in subjects with normal liver enzymes, with MetS risk using both quartiles and continuous unit of liver enzymes. We found significant associations were detected for all above analyses. Compared with quartile 1 (Q1), other quartiles have significant higher MetS risk, with ORs ranging from 1.15 to 18.15. The highest effected was detected for GGT, for which the OR value for the highest versus lowest quartile was 18.15 (95% CI: 15.7-20.9). Mutual adjustment proved the independence of the relations for all four liver enzymes. Sensitivity analyses didn’t materially changed the trend. To the best of our knowledge, this study should be the largest, which aimed at evaluating the association between liver enzymes measures and MetS risk. The results can better support that liver enzyme levels could be used as clinical predictors of MetS.

Anti-adipogenic and anti-diabetic effects of cis-3',4'-diisovalerylkhellactone isolated from Peucedanum japonicum Thunb leaves in vitro

Peucedanum japonicum Thunb is a medicinal plant belonging to the family Umbelliferae. This study evaluated the anti-diabetic and anti-obesity effects of cis-3',4'-diisovalerylkhellactone (cDIVK) isolated from Peucedanum japonicum Thunb leaves. cDIVK (30 and 50μM) effectively inhibited adipocyte differentiation and fat accumulation, whereas it stimulated glucose uptake compared with the control in 3T3-L1 cells. cDIVK significantly increased AMPK activation and suppressed protein and mRNA expression of major adipogenic transcriptional factors such as C/EBPα, PPARγ and SREBP-1c in 3T3-L1 cells. In addition, cDIVK had potential α-glucosidase inhibitory activity. These results indicated that cDIVK may act as a natural dual therapeutic agent for diabetes and obesity.

Targeting hepatic glucose metabolism in the treatment of type 2 diabetes

Type 2 diabetes mellitus is characterized by the dysregulation of glucose homeostasis, resulting in hyperglycaemia. Although current diabetes treatments have exhibited some success in lowering blood glucose levels, their effect is not always sustained and their use may be associated with undesirable side effects, such as hypoglycaemia. Novel antidiabetic drugs, which may be used in combination with existing therapies, are therefore needed. The potential of specifically targeting the liver to normalize blood glucose levels has not been fully exploited. Here, we review the molecular mechanisms controlling hepatic gluconeogenesis and glycogen storage, and assess the prospect of therapeutically targeting associated pathways to treat type 2 diabetes.

Stress kinases, endoplasmic reticulum stress, and Alzheimer's disease related markers in peripheral blood mononuclear cells from subjects with increased body weight

We aimed to characterize endoplasmic reticulum stress, inflammation, and Alzheimer’s disease (AD) related markers in peripheral blood mononuclear cells (PBMCs) from males with varied BMI; and to explore whether high glucose and fatty acids (FFAs) might be critical factors for inducing metabolic alterations in PBMCs under obese condition. Approximately 45 middle-aged men were enrolled with varied BMI. At the protein expression level, compared to the lean, the phosphorylation of AMPK, and p-Akt at serine 473 were significantly reduced from the overweight (OW) and/or obese (OB); while the protein expression of p-JNK, cleaved caspase 3, CHOP and p-eIF2α were elevated from the OW and/or OB. At the mRNA expression level, ER stress markers (i.e. GRP78, CHOP and XBP-1), inflammatory markers (i.e.TLR2, TLR4 and CCR2) and AD markers (i.e. APP, PS1 and PS2) were significantly higher in PBMCs from OB compared to lean. In cultured PBMCs, high glucose and FFAs induced GRP78, CHOP and XBP-1 mRNA, and high glucose also induced APP, PS1 and PS2 mRNA. In conclusion, altered markers including AMPK, ER stress and AD related makers under obese condition could be easily obtained from PBMCs. These markers might provide new mechanistic links between obesity and other metabolic complications including AD.

Stimulatory Effect of Balanced Deep-Sea Water Containing Chitosan Oligosaccharides on Glucose Uptake in C2C12 Myotubes

Deep-sea water (DSW) and chitosan oligosaccharides (COS) have recently drawn much attention because of their potential medical and pharmaceutical applications. Balanced DSW (BDSW) was prepared by mixing DSW mineral extracts and desalinated water. This study investigated the effects of BDSW, COS, and BDSW containing COS on glucose uptake and their mode of action in mature C2C12 myotubes. BDSW and COS increased glucose uptake in a dose-dependent manner. BDSW containing COS synergistically increased glucose uptake; this was dependent on the activation of insulin receptor substrate 1 and protein kinase C in insulin-dependent signaling pathways as well as liver kinase B1, AMP-activated protein kinase, and mammalian target of rapamycin in insulin-independent signaling pathways. Quantitative real-time polymerase chain reaction revealed that the expressions of the following genes related to glucose uptake were elevated: glucose transporter 4 (GLUT4), insulin-responsive aminopeptidase, and vesicle-associated membrane protein 2 for abundant proteins of GLUT4 storage vesicles (GSVs); syntaxin 4 and soluble N-ethylmaleimide-sensitive factor attachment protein 23 for trafficking between the plasma membrane and GSVs; and syntaxin 6 and syntaxin 16 for trafficking between GSVs and the trans-Golgi network. Taken together, these results suggest BDSW containing COS has a greater stimulatory effect on glucose uptake than BDSW or COS alone. Moreover, this effect is mediated by the stimulation of diverse signaling pathways via the activation of main signaling molecules related to GSV trafficking.

Irisin ameliorates hepatic glucose/lipid metabolism and enhances cell survival in insulin-resistant human HepG2 cells through adenosine monophosphate-activated protein kinase signaling

Irisin is a newly identified myokine that promotes the browning of white adipose tissue, enhances glucose uptake in skeletal muscle and modulates hepatic metabolism. However, the signaling pathways involved in the effects on hepatic glucose and lipid metabolism have not been resolved. This study aimed to examine the role of irisin in the regulation of hepatic glucose/lipid metabolism and cell survival, and whether adenosine monophosphate-activated protein kinase (AMPK), a master metabolic regulator in the liver, is involved in irisin's actions. Human liver-derived HepG2 cells were cultured in normal glucose-normal insulin (NGNI) or high glucose-high insulin (HGHI/insulin-resistant) condition. Hepatic glucose and lipid metabolism was evaluated by glucose output and glycogen content or triglyceride accumulation assays, respectively. Our results showed that irisin stimulated phosphorylation of AMPK and acetyl-CoA-carboxylase (ACC) via liver kinase B1 (LKB1) rather than Ca(2+)/calmodulin-dependent protein kinase kinase β (CaMKKβ) in HepG2 cells. Irisin ameliorated hepatic insulin resistance induced by HGHI condition. Irisin reduced hepatic triglyceride content and glucose output, but increased glycogen content, with those effects reversed by dorsomorphin, an AMPK inhibitor. Furthermore, irisin also stimulated extracellular signal-regulated kinase (ERK) 1/2 phosphorylation and promoted cell survival in an AMPK-dependent manner. In conclusion, our data indicate that irisin ameliorates dysregulation of hepatic glucose/lipid metabolism and cell death in insulin-resistant states via AMPK activation. These findings reveal a novel irisin-mediated protective mechanism in hepatic metabolism which provides a scientific basis for irisin as a potential therapeutic target for the treatment of insulin resistance and type 2 diabetes mellitus.

Regulation and function of AMPK in physiology and diseases

5′-adenosine monophosphate (AMP)-activated protein kinase (AMPK) is an evolutionarily conserved serine/threonine kinase that was originally identified as the key player in maintaining cellular energy homeostasis. Intensive research over the last decade has identified diverse molecular mechanisms and physiological conditions that regulate the AMPK activity. AMPK regulates diverse metabolic and physiological processes and is dysregulated in major chronic diseases, such as obesity, inflammation, diabetes and cancer. On the basis of its critical roles in physiology and pathology, AMPK is emerging as one of the most promising targets for both the prevention and treatment of these diseases. In this review, we discuss the current understanding of the molecular and physiological regulation of AMPK and its metabolic and physiological functions. In addition, we discuss the mechanisms underlying the versatile roles of AMPK in diabetes and cancer.

Lack of Adipocyte AMPK Exacerbates Insulin Resistance and Hepatic Steatosis through Brown and Beige Adipose Tissue Function

Brown (BAT) and white (WAT) adipose tissues play distinct roles in maintaining whole-body energy homeostasis, and their dysfunction can contribute to non-alcoholic fatty liver disease (NAFLD) and type 2 diabetes. The AMP-activated protein kinase (AMPK) is a cellular energy sensor, but its role in regulating BAT and WAT metabolism is unclear. We generated an inducible model for deletion of the two AMPK β subunits in adipocytes (iβ1β2AKO) and found that iβ1β2AKO mice were cold intolerant and resistant to β-adrenergic activation of BAT and beiging of WAT. BAT from iβ1β2AKO mice had impairments in mitochondrial structure, function, and markers of mitophagy. In response to a high-fat diet, iβ1β2AKO mice more rapidly developed liver steatosis as well as glucose and insulin intolerance. Thus, AMPK in adipocytes is vital for maintaining mitochondrial integrity, responding to pharmacological agents and thermal stress, and protecting against nutrient-overload-induced NAFLD and insulin resistance.

Hypothalamic AMPK: a canonical regulator of whole-body energy balance

AMP-activated protein kinase (AMPK) has a major role in the modulation of energy balance. AMPK is activated in conditions of low energy, increasing energy production and reducing energy consumption. The AMPK pathway is a canonical route regulating energy homeostasis by integrating peripheral signals, such as hormones and metabolites, with neuronal networks. Current evidence has implicated AMPK in the hypothalamus and hindbrain with feeding, brown adipose tissue thermogenesis and browning of white adipose tissue, through modulation of the sympathetic nervous system, as well as glucose homeostasis. Interestingly, several potential antiobesity and/or antidiabetic agents, some of which are currently in clinical use such as metformin and liraglutide, exert some of their actions by acting on AMPK. Furthermore, the orexigenic and weight-gain effects of commonly used antipsychotic drugs are also mediated by hypothalamic AMPK. Overall, this evidence suggests that hypothalamic AMPK signalling is an interesting target for drug development, but is this approach feasible? In this Review we discuss the current understanding of hypothalamic AMPK and its role in the central regulation of energy balance and metabolism.

Tumor necrosis factor receptor-associated factor 5 (Traf5) acts as an essential negative regulator of hepatic steatosis

BACKGROUND & AIMS

Obesity-related metabolic inflammation, insulin resistance (IR), and excessive fat accumulation are linked phenomena that promote the progression of nonalcoholic fatty liver disease (NAFLD). Previous research has indicated that CD40-TRAF5 signaling protects against obesity-related metabolic disorders; however, the precise roles and underlying mechanisms of TRAF5 in obesity-induced pathological processes have not been fully elucidated.

METHODS

TRAF5 expression was evaluated in the livers of NAFLD patients, high-fat diet (HFD)-induced or genetically (ob/ob) induced obese mice, and in palmitate-treated hepatocytes. Gain- or loss-of-function approaches were used to investigate the specific roles and mechanisms of hepatic Traf5 under obesity-related pathological conditions.

RESULTS

TRAF5 expression was decreased in the fatty livers of both NAFLD patients and obese mice, and in palmitate-treated hepatocytes in vitro. Traf5 overexpression significantly suppressed nonalcoholic steatohepatitis (NASH)-like phenotypes in mice after HFD treatment for 24weeks and inhibited the progression of NAFLD in ob/ob mice. Conversely, Traf5 deficiency resulted in the deterioration of metabolic disorders induced by HFD. Investigations of the underlying mechanisms revealed that Traf5 regulates hepatic steatosis by targeting Jnk signaling. Specifically, Jnk1 rather than Jnk2 is responsible for the function of Traf5 in metabolic disorders, as evidenced by the fact that Jnk1 ablation markedly ameliorates the detrimental effects of Traf5 deficiency on obesity, inflammation, IR, hepatic steatosis and fibrosis.

CONCLUSIONS

Traf5 negatively regulates NAFLD/NASH and related metabolic dysfunctions by blocking Jnk1 activity, which represents a potential therapeutic target for obesity-related metabolic disorders.

LAY SUMMARY

Lipid accumulation in the liver induces degradation of Traf5. Increasing Traf5 ameliorates nonalcoholic fatty liver by blocking Jnk1 activity.

Age-related changes in AMPK activation: Role for AMPK phosphatases and inhibitory phosphorylation by upstream signaling pathways

AMP-activated protein kinase (AMPK) is a fundamental regulator of energy metabolism, stress resistance, and cellular proteostasis. AMPK signaling controls an integrated signaling network which is involved in the regulation of healthspan and lifespan e.g. via FoxO, mTOR/ULK1, CRCT-1/CREB, and SIRT1 signaling pathways. Several studies have demonstrated that the activation capacity of AMPK signaling declines with aging, which impairs the maintenance of efficient cellular homeostasis and enhances the aging process. However, it seems that the aging process affects AMPK activation in a context-dependent manner since occasionally, it can also augment AMPK activation, possibly attributable to the type of insult and tissue homeostasis. Three protein phosphatases, PP1, PP2A, and PP2C, inhibit AMPK activation by dephosphorylating the Thr172 residue of AMPKα, required for AMPK activation. In addition, several upstream signaling pathways can phosphorylate Ser/Thr residues in the β/γ interaction domain of the AMPKα subunit that subsequently blocks the activation of AMPK. These inhibitory pathways include the insulin/AKT, cyclic AMP/PKA, and RAS/MEK/ERK pathways. We will examine the evidence whether the efficiency of AMPK responsiveness declines during the aging process. Next, we will review the mechanisms involved in curtailing the activation of AMPK. Finally, we will elucidate the potential age-related changes in the inhibitory regulation of AMPK signaling that might be a part of the aging process.

Therapy with resveratrol attenuates obesity-associated allergic airway inflammation in mice

Obesity and insulin resistance have been associated with deterioration in asthma outcomes. High oxidative stress and deficient activation of AMP-activated protein kinase (AMPK) have emerged as important regulators linking insulin resistance and inflammation. This study aimed to evaluate the effects of resveratrol on obesity-associated allergic pulmonary inflammation. Male C57/Bl6 mice fed with high-fat diet to induce obesity (obese group) or standard-chow diet (lean group) were treated or not with resveratrol (100mg/kg/day, two weeks). Mice were sensitized and challenged with ovalbumin (OVA). At 48h thereafter, bronchoalveolar lavage fluid was performed, and lungs collected for morphological studies and Western blot analysis. Treatment of obese mice with resveratrol significantly reduced hyperglycemia and insulin resistance, as well as the body measures (body mass, fat mass, % fat, and body area). OVA-challenge promoted a higher increase in pulmonary eosinophil infiltration in obese compared with lean mice, which was nearly abrogated by resveratrol treatment. Resveratrol markedly increased the phosphorylated AMPK expression in lung tissues of obese compared with lean mice. Resveratrol reduced the p47phox expression and reactive-oxygen species (ROS) production, and elevated the superoxide dismutase (SOD) levels in lung tissues of obese mice. The increased pulmonary levels of TNF-α and inducible nitric oxide synthase (iNOS) in obese mice were also normalized after resveratrol treatment. In lean mice, resveratrol failed to affect the levels of fasting glucose, p47phox, ROS levels, TNF-α, iNOS and phosphorylated AMPK. Resveratrol exhibits protective effects in obesity-associated lung inflammation that is accompanied by local AMPK activation and antioxidant property.

Podocyte injury: the role of proteinuria, urinary plasminogen, and oxidative stress

Podocytes are the key target for injury in proteinuric glomerular diseases that result in podocyte loss, progressive focal segmental glomerular sclerosis (FSGS), and renal failure. Current evidence suggests that the initiation of podocyte injury and associated proteinuria can be separated from factors that drive and maintain these pathogenic processes leading to FSGS. In nephrotic urine aberrant glomerular filtration of plasminogen (Plg) is activated to the biologically active serine protease plasmin by urokinase-type plasminogen activator (uPA). In vivo inhibition of uPA mitigates Plg activation and development of FSGS in several proteinuric models of renal disease including 5/6 nephrectomy. Here, we show that Plg is markedly increased in the urine in two murine models of proteinuric kidney disease associated with podocyte injury: Tg26 HIV-associated nephropathy and the Cd2ap^-/- model of FSGS. We show that human podocytes express uPA and three Plg receptors: uPAR, tPA, and Plg-RKT. We demonstrate that Plg treatment of podocytes specifically upregulates NADPH oxidase isoforms NOX2/NOX4 and increases production of mitochondrial-dependent superoxide anion (O₂^-) that promotes endothelin-1 synthesis. Plg via O₂^- also promotes expression of the B scavenger receptor CD36 and subsequent increased intracellular cholesterol uptake resulting in podocyte apoptosis. Taken together, our findings suggest that following disruption of the glomerular filtration barrier at the onset of proteinuric disease, podocytes are exposed to Plg resulting in further injury mediated by oxidative stress. We suggest that chronic exposure to Plg could serve as a "second hit" in glomerular disease and that Plg is potentially an attractive target for therapeutic intervention.

Down-regulation of HSP60 Suppresses the Proliferation of Glioblastoma Cells via the ROS/AMPK/mTOR Pathway

Glioblastoma is a fatal and incurable cancer with the hyper-activated mTOR pathway. HSP60, a major chaperone for maintenance of mitochondrial proteostasis, is highly expressed in glioblastoma patients. To understand the effects of HSP60 on glioblastoma tumorigenesis and progression, we characterized the HSP60-knockdowned glioblastoma cells and revealed that HSP60 silencing markedly suppressed cell proliferation and promoted cell to undergo the epithelial-mesenchymal transition (EMT). Proteomic analysis showed that ribosomal proteins were significantly downregulated whereas EMT-associated proteins were up-regulated in HSP60-knockdowned U87 cells as confirmed by a distinct enrichment pattern in newly synthesized proteins with azido-homoalanine labeling. Biochemical analysis revealed that HSP60 knockdown increased reactive oxygen species (ROS) production that led to AMPK activation, similarly to the complex I inhibitor rotenone-induced AMPK activation. Activated AMPK suppressed mTORC1 mediated S6K and 4EBP1 phosphorylation to decrease protein translation, which slowed down cell growth and proliferation. On the other hand, high levels of ROS in HSP60 knockdowned or rotenone-treated U87 cells contributed to EMT. These results indicate that HSP60 silencing deactivates the mTOR pathway to suppress glioblastoma progression, suggesting that HSP60 is a potential therapeutic target for glioblastoma treatment.

Thyroid hormone induction of mitochondrial activity is coupled to mitophagy via ROS-AMPK-ULK1 signaling

Currently, there is limited understanding about hormonal regulation of mitochondrial turnover. Thyroid hormone (T3) increases oxidative phosphorylation (OXPHOS), which generates reactive oxygen species (ROS) that damage mitochondria. However, the mechanism for maintenance of mitochondrial activity and quality control by this hormone is not known. Here, we used both in vitro and in vivo hepatic cell models to demonstrate that induction of mitophagy by T3 is coupled to oxidative phosphorylation and ROS production. We show that T3 induction of ROS activates CAMKK2 (calcium/calmodulin-dependent protein kinase kinase 2, β) mediated phosphorylation of PRKAA1/AMPK (5' AMP-activated protein kinase), which in turn phosphorylates ULK1 (unc-51 like autophagy activating kinase 1) leading to its mitochondrial recruitment and initiation of mitophagy. Furthermore, loss of ULK1 in T3-treated cells impairs both mitophagy as well as OXPHOS without affecting T3 induced general autophagy/lipophagy. These findings demonstrate a novel ROS-AMPK-ULK1 mechanism that couples T3-induced mitochondrial turnover with activity, wherein mitophagy is necessary not only for removing damaged mitochondria but also for sustaining efficient OXPHOS.

Preconditioning mice with activators of AMPK ameliorates ischemic acute kidney injury in vivo

This study had two objectives: 1) to determine whether preconditioning cultured proximal tubular cells (PTCs) with pharmacological activators of AMP-activated protein kinase (AMPK) protects these cells from apoptosis induced by metabolic stress in vitro and 2) to assess the effects of preconditioning mice with these agents on the severity of ischemic acute renal kidney injury (AKI) in vivo. We demonstrate that preconditioning PTCs with 5-aminoimidazole-4-carboxamide-1-β-d-ribofuranoside (AICAR) or A-769662 reduces apoptosis of PTCs induced by subsequent stress. We also show that the reduction in cell death during metabolic stress associated with pretreatment by AMPK activators is associated with an increase in the cytosolic level of ATP, which is mediated by an increase in the rate of glycolysis. In addition, we provide evidence that the effect of AMPK activators on glycolysis is mediated, at least in part, by an increased uptake of glucose, and by the induction of hexokinase II (HK II) expression. Our data also show that the increased in HK II expression associated with preconditioning with AMPK activators is mediated by the activation (phosphorylation) of the cAMP-response element binding protein (CREB). We also provide entirely novel evidence that that A-79662 is substantially more effective than AICAR in mediating these alterations in PTCs in vitro. Finally, we demonstrate that preconditioning mice with AICAR or A-769662 substantially reduces the severity of renal dysfunction and tubular injury in a model of ischemic AKI in vivo and that the efficacy of AICAR and A-768662 in ameliorating ischemic AKI in vivo is comparable.

Olive Leaf Extract from Sicilian Cultivar Reduced Lipid Accumulation by Inducing Thermogenic Pathway during Adipogenesis

Olive leaves contain a wide variety of phenolic compounds belonging to phenolic acids, phenolic alcohols, flavonoids, and secoiridoids, and include also many other pharmacological active compounds. They could play an important role in human diet and health because of their ability to lower blood pressure, increase coronary arteries blood flow and decrease the risk of cardiovascular diseases. The aim of this study was to investigate the effect of olive leaf extract (OLE) from Sicilian cultivar on adipogenic differentiation of human adipose derived mesenchymal stem cells and its impact on lipid metabolism. We showed that OLE treatment during adipogenic differentiation reduces inflammation, lipid accumulation and induces thermogenesis by activation of uncoupling protein uncoupling protein 1, sirtuin 1, peroxisome proliferator-activated receptor alpha, and coactivator 1 alpha. Furthermore, OLE significantly decreases the expression of molecules involved in adipogenesis and upregulates the expression of mediators involved in thermogenesis and lipid metabolism. Taken together, our results suggest that OLE may promote the brown remodeling of white adipose tissue inducing thermogenesis and improving metabolic homeostasis.

12-Lipoxygenase Inhibition on Microalbuminuria in Type-1 and Type-2 Diabetes Is Associated with Changes of Glomerular Angiotensin II Type 1 Receptor Related to Insulin Resistance

(1) BACKGROUND: 12-lipoxygenase (12-LO) is involved in the development of diabetic nephropathy (DN). In the present study, we investigated whether 12-LO inhibition may ameliorate type-2 DN (T2DN) by interfering with insulin resistance (IR); (2) METHODS: Rat glomerular mesangial cells, glomeruli and skeletal muscles were isolated and used in this study. Kidney histological changes were confirmed by periodic-acid Schiff staining; mRNA expression was detected by competitive reverse transcription polymerase chain reaction; and the protein level was determined by Western blot and the enzyme-linked immunosorbent assay, respectively; (3) RESULTS: The inhibition of 12-LO attenuated microalbuminuria (MAU) increases in type-2 diabetic rats, but not in type-1 diabetic rats. Infusion of 12(S)-hydroxyeicosatetraenoic acid (12(S)-HETE) significantly increased the expression of angiotensin II (Ang II) and Ang II type 1 receptor (AT1R), but decreased the expression of AT1R-associated protein (ATRAP) in rat glomeruli, compared to the control. An in vitro study revealed that both 12(S)-HETE and insulin upregulated AT1R expression in rat mesangial cells. In the presence of p38 mitogen-activated protein kinase (MAPK) inhibitor, SB202190, the 12(S)-HETE-induced ATRAP reduction was significantly abolished. Interestingly, 12-LO inhibition did not influence AT1R expression in type-1 diabetic rats, but significantly abolished the increased AT1R and Ang II expression in glomeruli of type-2 diabetic rats. Furthermore, the inhibition of 12-LO significantly corrected impaired insulin sensitivity and fast serum insulin level, as well as the p-AMP-activated protein kinase (AMPK) reduction in skeletal muscle of type-2 diabetic rats; (4) CONCLUSION: The inhibition of 12-LO potentially ameliorated MAU by preventing IR through the downregulation of glomerular AT1R expression in T2DN.

Salidroside improves glucose homeostasis in obese mice by repressing inflammation in white adipose tissues and improving leptin sensitivity in hypothalamus

Salidroside is a functionally versatile natural compound from the perennial flowering plant Rhodiola rosea L. Here, we examined obese mice treated with salidroside at the dosage of 50 mg/kg/day for 48 days. Mice treated with salidroside showed slightly decreased food intake, body weight and hepatic triglyceride content. Importantly, salidroside treatment significantly improved glucose and insulin tolerance. It also increased insulin singling in both liver and epididymal white adipose tissue (eWAT). In addition, salidroside markedly ameliorated hyperglycemia in treated mice, which is likely due to the suppression of gluconeogenesis by salidroside as the protein levels of a gluconeogenic enzyme G6Pase and a co-activator PGC-1α were all markedly decreased. Further analysis revealed that adipogenesis in eWAT was significantly decreased in salidroside treated mice. The infiltration of macrophages in eWAT and the productions of pro-inflammatory cytokines were also markedly suppressed by salidroside. Furthermore, the leptin signal transduction in hypothalamus was improved by salidroside. Taken together, these euglycemic effects of salidroside may due to repression of adipogenesis and inflammation in eWAT and stimulation of leptin signal transduction in hypothalamus. Thus, salidroside might be used as an effective anti-diabetic agent.

Unraveling the actions of AMP-activated protein kinase in metabolic diseases: Systemic to molecular insights

AMP-activated protein kinase (AMPK) plays a critical role both in sensing and regulating cellular energy state. In experimental animals, its activation has been shown to reduce the risk of obesity and diabetes-related co-morbidities such as insulin resistance, the metabolic syndrome and atherosclerotic cardiovascular disease. However, in humans, AMPK activation alone often does not completely resolve these conditions. Thus, an improved understanding of AMPK action and regulation in metabolic and other diseases is needed. Herein, we provide a brief description of the enzymatic regulation of AMPK and review its role in maintaining energy homeostasis. We then discuss tissue-specific actions of AMPK that become distorted during such conditions as obesity, type 2 diabetes and certain cancers. Finally, we explore recent findings regarding the interactions of AMPK with mammalian target of rapamycin complex 1 and the lysosome and discuss how changes in these relationships during overnutrition may lead to AMPK dysfunction. A more thorough understanding of AMPK's molecular interactions during diseases of overnutrition may provide key insights for the development of AMPK-based combinatorial treatments for metabolic disease.

Loss of anti-contractile effect of perivascular adipose tissue in offspring of obese rats

Rationale:

Maternal obesity pre-programmes offspring to develop obesity and associated cardiovascular disease. Perivascular adipose tissue (PVAT) exerts an anti-contractile effect on the vasculature, which is reduced in hypertension and obesity.

Objective:

The objective of this study was to determine whether maternal obesity pre-programmes offspring to develop PVAT dysfunction in later life.

Methods:

Female Sprague–Dawley rats were fed a diet containing 10% (control) or 45% fat (high fat diet, HFD) for 12 weeks prior to mating and during pregnancy and lactation. Male offspring were killed at 12 or 24 weeks of age and tension in PVAT-intact or -denuded mesenteric artery segments was measured isometrically. Concentration–response curves were constructed to U46619 and norepinephrine.

Results:

Only 24-week-old HFD offspring were hypertensive (P<0.0001), although the anti-contractile effect of PVAT was lost in vessels from HFD offspring of each age. Inhibition of nitric oxide (NO) synthase with 100 μMl-NMMA attenuated the anti-contractile effect of PVAT and increased contractility of PVAT-denuded arteries (P<0.05, P<0.0001). The increase in contraction was smaller in PVAT-intact than PVAT-denuded vessels from 12-week-old HFD offspring, suggesting decreased PVAT-derived NO and release of a contractile factor (P<0.07). An additional, NO-independent effect of PVAT was evident only in norepinephrine-contracted vessels. Activation of AMP-activated kinase (with 10 μM A769662) was anti-contractile in PVAT-denuded (P<0.0001) and -intact (P<0.01) vessels and was due solely to NO in controls; the AMPK effect was similar in HFD offspring vessels (P<0.001 and P<0.01, respectively) but was partially NO-independent.

Conclusions:

The diminished anti-contractile effects of PVAT in offspring of HFD dams are primarily due to release of a PVAT-derived contractile factor and reduced NO bioavailability.

Exenatide inhibits fatty acid induced hepatocyte steatosis and inflammation through activating AMPK

AIM: To detect the effect of glucagon-like peptide-1 (GLP-1) agonist exenatide (EXE) on fat deposition in liver cells and explore the underlying mechanism.

METHODS: A HepG2 cell deposition model was induced with palmitic acid (PA). After cells were incubated with different doses of EXE (25-100 nmoL/L) and PA (500 μmoL/L) for 24 h, fatty deposition was assessed by oil red O staining and the level of intracellular triglyceride (TG). Real-time quantitative PCR (qRT-PCR) was used to detect the expression of lipid metabolism related genes, including fatty acid synthase (FAS), tumor necrosis factor α (TNF-α), and interleukin 6 (IL-6). The expression of p-AMPK and AMPK protein was tested by Western blot. An AMPK inhibitor was used to explore the role of AMPK in fat deposition and inflammation.

RESULTS: Compared with the control group, PA significantly elevated TG and oil red O content, as well as the expression of FAS in HepG2 cells (P < 0.05). EXE significantly inhibited PA induced elevation of TG and oil red O content, as well as FAS gene expression in a dose dependent manner (P < 0.05). The expression of TNF-α and IL-6 significantly increased in the PA treated group (P < 0.05). EXE significantly inhibited the expression of TNF-α and IL-6 in PA treated HepG2 cells (P < 0.05). Co-treatment with AMPK inhibitor significantly reduced the effect of EXE on AMPK, and reduced the inhibitory effect of EXE on fatty deposition and PA induced FAS activation (P < 0.05). AMPK inhibitor significantly diminished the inhibitory effect of EXE on TNF-α and IL-6 activation induced by PA (P < 0.05).

CONCLUSION: EXE reduces fatty acid induced fatty deposition and inflammatory response in liver cells through activation of AMPK.

Intermedin Restores Hyperhomocysteinemia-induced Macrophage Polarization and Improves Insulin Resistance in Mice

Hyperhomocysteinemia (HHcy) is a condition characterized by an abnormally high level of homocysteine, an inflammatory factor. This condition has been suggested to promote insulin resistance. To date, the underlying molecular mechanism remains largely unknown, and identifying novel therapeutic targets for HHcy-induced insulin resistance is of high priority. It is well known that intermedin (IMD), a calcitonin family peptide, exerts potent anti-inflammatory effects. In this study, the effects of IMD on HHcy-induced insulin resistance were investigated. Glucose tolerance and insulin tolerance tests were performed on mice treated with IMD by minipump implantation (318 ng/kg/h for 4 weeks) or adipocyte-specific IMD overexpression mice (Adipo-IMD transgenic mice). The expression of genes and proteins related to M1/M2 macrophages and endoplasmic reticulum stress (ERS) was evaluated in adipose tissues or cells. The expression of IMD was identified to be lower in the plasma and adipose tissues of HHcy mice. In both IMD treatment by minipump implantation and Adipo-IMD transgenic mice, IMD reversed HHcy-induced insulin resistance, as revealed by glucose tolerance and insulin tolerance tests. Further mechanistic study revealed that IMD reversed the Hcy-elevated ratio of M1/M2 macrophages by inhibiting AMP-activated protein kinase activity. Adipo-IMD transgenic mice displayed reduced ERS and lower inflammation in adipose tissues with HHcy. Soluble factors from Hcy-treated macrophages induced adipocyte ERS, which was reversed by IMD treatment. These findings revealed that IMD treatment restores the M1/M2 balance, inhibits chronic inflammation in adipose tissues, and improves systemic insulin sensitivity of HHcy mice.

Veratri Nigri Rhizoma et Radix (Veratrum nigrum L.) and Its Constituent Jervine Prevent Adipogenesis via Activation of the LKB1-AMPKα-ACC Axis In Vivo and In Vitro

This study was performed in order to investigate the antiobese effects of the ethanolic extract of Veratri Nigri Rhizoma et Radix (VN), a herb with limited usage, due to its toxicology. An HPLC analysis identified jervine as a constituent of VN. By an Oil Red O assay and a Real-Time RT-PCR assay, VN showed higher antiadipogenic effects than jervine. In high-fat diet- (HFD-) induced obese C57BL/6J mice, VN administration suppressed body weight gain. The levels of peroxisome proliferator-activated receptor gamma (PPARγ), CCAAT-enhancer-binding protein alpha (C/EBPα), adipocyte fatty-acid-binding protein (aP2), adiponectin, resistin, and LIPIN1 were suppressed by VN, while SIRT1 was upregulated. Furthermore, VN activated phosphorylation of the liver kinase B1- (LKB1-) AMP-activated protein kinase alpha- (AMPKα-) acetyl CoA carboxylase (ACC) axis. Further investigation of cotreatment of VN with the AMPK agonist AICAR or AMPK inhibitor Compound C showed that VN can activate the phosphorylation of AMPKα in compensation to the inhibition of Compound C. In conclusion, VN shows antiobesity effects in HFD-induced obese C57BL/6J mice. In 3T3-L1 adipocytes, VN has antiadipogenic features, which is due to activating the LKB1-AMPKα-ACC axis. These results suggest that VN has a potential benefit in preventing obesity.

Pancreatic β-Cell Dysfunction in Diet-Induced Obese Mice: Roles of AMP-Kinase, Protein Kinase Cε, Mitochondrial and Cholesterol Metabolism, and Alterations in Gene Expression

Diet induced obese (DIO) mice can be stratified according to their weight gain in response to high fat diet as low responders (LDR) and high responders (HDR). This allows the study of β-cell failure and the transitions to prediabetes (LDR) and early diabetes (HDR). C57BL/6N mice were fed for 8 weeks with a normal chow diet (ND) or a high fat diet and stratified as LDR and HDR. Freshly isolated islets from ND, LDR and HDR mice were studied ex-vivo for mitochondrial metabolism, AMPK activity and signalling, the expression and activity of key enzymes of energy metabolism, cholesterol synthesis, and mRNA profiling. Severely compromised glucose-induced insulin secretion in HDR islets, as compared to ND and LDR islets, was associated with suppressed AMP-kinase activity. HDR islets also showed reduced acetyl-CoA carboxylase activity and enhanced activity of 3-hydroxy-3-methylglutaryl-CoA reductase, which led respectively to elevated fatty acid oxidation and increased cholesterol biosynthesis. HDR islets also displayed mitochondrial membrane hyperpolarization and reduced ATP turnover in the presence of elevated glucose. Expression of protein kinase Cε, which reduces both lipolysis and production of signals for insulin secretion, was elevated in DIO islets. Genes whose expression increased or decreased by more than 1.2-fold were minor between LDR and ND islets (17 differentially expressed), but were prominent between HDR and ND islets (1508 differentially expressed). In HDR islets, particularly affected genes were related to cell cycle and proliferation, AMPK signaling, mitochondrial metabolism and cholesterol metabolism. In conclusion, chronically reduced AMPK activity, mitochondrial dysfunction, elevated cholesterol biosynthesis in islets, and substantial alterations in gene expression accompany β-cell failure in HDR islets. The β-cell compensation process in the prediabetic state (LDR) is largely independent of transcriptional adaptive changes, whereas the transition to early diabetes (HDR) is associated with major alterations in gene expression.

Biology, Chemistry, and Pharmacology of Sirtuins

Sirtuins are a family of protein deacylases related by amino acid sequence and cellular function to the yeast Saccharomyces cerevisiae protein Sir2 (Silent Information Regulator-2), the first of this class of enzymes to be identified and studied in detail. Based on its initially discovered activity, Sir2 was classified as a histone deacetylase that removes acetyl groups from histones H3 and H4. The acetylation/deacetylation of these particular substrates leads to changes in transcriptional silencing at specific loci in the yeast genome, hence its name. Sirtuins, however, have been shown to regulate a wide variety of cellular processes beyond transcriptional repression in varied subcellular compartments and in different cell types. Mechanistically distinct from Zn(2+)-dependent deacylases, sirtuins use nicotinamide adenine dinucleotide as a cofactor in the removal of acetyl and other acyl groups linking metabolic status and posttranslational modification. Sirtuins' unique position has made them attractive targets for small-molecule drug development. In this chapter, we describe the biological roles, therapeutic areas in which sirtuins may play a role and development of small-molecule inhibitors of sirtuins employing phenotypic screening technologies ranging from assays in yeast, as well as biochemical screens to yield lead drug development candidates targeting a broad spectrum of human diseases.

Metformin revisited: Does this regulator of AMP-activated protein kinase secondarily affect bone metabolism and prevent diabetic osteopathy

Patients with long-term type 1 and type 2 diabetes mellitus (DM) can develop skeletal complications or “diabetic osteopathy”. These include osteopenia, osteoporosis and an increased incidence of low-stress fractures. In this context, it is important to evaluate whether current anti-diabetic treatments can secondarily affect bone metabolism. Adenosine monophosphate-activated protein kinase (AMPK) modulates multiple metabolic pathways and acts as a sensor of the cellular energy status; recent evidence suggests a critical role for AMPK in bone homeostasis. In addition, AMPK activation is believed to mediate most clinical effects of the insulin-sensitizer metformin. Over the past decade, several research groups have investigated the effects of metformin on bone, providing a considerable body of pre-clinical (in vitro, ex vivo and in vivo) as well as clinical evidence for an anabolic action of metformin on bone. However, two caveats should be kept in mind when considering metformin treatment for a patient with type 2 DM at risk for diabetic osteopathy. In the first place, metformin should probably not be considered an anti-osteoporotic drug; it is an insulin sensitizer with proven macrovascular benefits that can secondarily improve bone metabolism in the context of DM. Secondly, we are still awaiting the results of randomized placebo-controlled studies in humans that evaluate the effects of metformin on bone metabolism as a primary endpoint.

CD36 actions in the heart: Lipids, calcium, inflammation, repair and more?

CD36 is a multifunctional immuno-metabolic receptor with many ligands. One of its physiological functions in the heart is the high-affinity uptake of long-chain fatty acids (FAs) from albumin and triglyceride rich lipoproteins. CD36 deletion markedly reduces myocardial FA uptake in rodents and humans. The protein is expressed on endothelial cells and cardiomyocytes and at both sites is likely to contribute to FA uptake by the myocardium. CD36 also transduces intracellular signaling events that influence how the FA is utilized and mediate metabolic effects of FA in the heart. CD36 transduced signaling regulates AMPK activation in a way that adjusts oxidation to FA uptake. It also impacts remodeling of myocardial phospholipids and eicosanoid production, effects exerted via influencing intracellular calcium (iCa(2+)) and the activation of phospholipases. Under excessive FA supply CD36 contributes to lipid accumulation, inflammation and dysfunction. However, it is also important for myocardial repair after injury via its contribution to immune cell clearance of apoptotic cells. This review describes recent progress regarding the multiple actions of CD36 in the heart and highlights those areas requiring future investigation. This article is part of a Special Issue entitled: Heart Lipid Metabolism edited by G.D. Lopaschuk.

Induction of Posttranslational Modifications of Mitochondrial Proteins by ATP Contributes to Negative Regulation of Mitochondrial Function

It is generally accepted that ATP regulates mitochondrial function through the AMPK signaling pathway. However, the AMPK-independent pathway remains largely unknown. In this study, we investigated ATP surplus in the negative regulation of mitochondrial function with a focus on pyruvate dehydrogenase (PDH) phosphorylation and protein acetylation. PDH phosphorylation was induced by a high fat diet in the liver of obese mice, which was associated with ATP elevation. In 1c1c7 hepatoma cells, the phosphorylation was induced by palmitate treatment through induction of ATP production. The phosphorylation was associated with a reduction in mitochondria oxygen consumption after 4 h treatment. The palmitate effect was blocked by etomoxir, which inhibited ATP production through suppression of fatty acid β-oxidation. The PDH phosphorylation was induced by incubation of mitochondrial lysate with ATP in vitro without altering the expression of PDH kinase 2 (PDK2) and 4 (PDK4). In addition, acetylation of multiple mitochondrial proteins was induced by ATP in the same conditions. Acetyl-CoA exhibited a similar activity to ATP in induction of the phosphorylation and acetylation. These data suggest that ATP elevation may inhibit mitochondrial function through induction of the phosphorylation and acetylation of mitochondrial proteins. The results suggest an AMPK-independent mechanism for ATP regulation of mitochondrial function.

Obesity decreases B cell responses in young and elderly individuals

OBJECTIVE

To evaluate the effects of obesity-associated inflammation on influenza vaccine responses.

METHODS

In young and elderly individuals, both lean and with obesity, antibody responses to influenza vaccination were measured.

RESULTS

A decrease in in vivo vaccine responses, circulating switched memory, and transitional B cells and an increase in pro-inflammatory late/exhausted memory B cells were found. In vitro B cell function was measured by activation-induced cytidine deaminase and E47, markers of optimal antibody responses. Moreover, IL-6 production was increased, whereas IL-10 production was decreased in cultures of B cells from individuals with obesity. Markers of immune activation (TNF-α, TLR4, micro-RNAs) in unstimulated B cells were also found increased and were negatively correlated with B cell function. In order to reveal potential mechanisms, we stimulated B cells from lean individuals in vitro with leptin, the adipokine increased in obesity. Leptin increased phospho-STAT3, crucial for TNF-α production, and decreased phospho-AMPK, the energy sensing enzyme upstream of phospho-p38 MAPK and E47. Leptin-induced phospho-STAT3 and phospho-AMPK levels were similar to those in B cells from individuals with obesity.

CONCLUSIONS

These results demonstrate that leptin can be responsible for decreased B cell function in obesity.

Peroxisome proliferator activated receptors at the crossroad of obesity, diabetes, and pancreatic cancer

Pancreatic ductal adenocarcinoma (PDAC) is the fourth cause of cancer death with an overall survival of 5% at five years. The development of PDAC is characteristically associated to the accumulation of distinctive genetic mutations and is preceded by the exposure to several risk factors. Epidemiology has demonstrated that PDAC risk factors may be non-modifiable risks (sex, age, presence of genetic mutations, ethnicity) and modifiable and co-morbidity factors related to the specific habits and lifestyle. Recently it has become evident that obesity and diabetes are two important modifiable risk factors for PDAC. Obesity and diabetes are complex systemic and intertwined diseases and, over the years, experimental evidence indicate that insulin-resistance, alteration of adipokines, especially leptin and adiponectin, oxidative stress and inflammation may play a role in PDAC. Peroxisome proliferator activated receptor-γ (PPARγ) is a nuclear receptor transcription factor that is implicated in the regulation of metabolism, differentiation and inflammation. PPARγ is a key regulator of adipocytes differentiation, regulates insulin and adipokines production and secretion, may modulate inflammation, and it is implicated in PDAC. PPARγ agonists are used in the treatment of diabetes and oxidative stress-associated diseases and have been evaluated for the treatment of PDAC. PPARγ is at the cross-road of diabetes, obesity, and PDAC and it is an interesting target to pharmacologically prevent PDAC in obese and diabetic patients.

Deubiquitination and Activation of AMPK by USP10

The AMP-activated protein kinase (AMPK) is the master regulator of metabolic homeostasis by sensing cellular energy status. When intracellular ATP levels decrease during energy stress, AMPK is initially activated through AMP or ADP binding and phosphorylation of a threonine residue (Thr-172) within the activation loop of its kinase domain. Here we report a key molecular mechanism by which AMPK activation is amplified under energy stress. We found that ubiquitination on AMPKα blocks AMPKα phosphorylation by LKB1. The deubiquitinase USP10 specifically removes ubiquitination on AMPKα to facilitate AMPKα phosphorylation by LKB1. Under energy stress, USP10 activity in turn is enhanced through AMPK-mediated phosphorylation of Ser76 of USP10. Thus, USP10 and AMPK form a key feedforward loop ensuring amplification of AMPK activation in response to fluctuation of cellular energy status. Disruption of this feedforward loop leads to improper AMPK activation and multiple metabolic defects.

Effect of Increasing Doses of Linoleic and α-Linolenic Acids on High-Fructose and High-Fat Diet Induced Metabolic Syndrome in Rats

Doses and ratio of linoleic acid (LA) and α-linolenic acid (ALA) preventing metabolic syndrome (MS) were investigated. SD rats were fed (i) basal diet, (ii) high-fructose and high-fat diet (HFFD), (iii) HFFD with increasing-dose LA (0.75 energy-% ALA + 3, 6, 9, 12, 15, and 30 energy-% LA), and (iv) HFFD with increasing-dose ALA (6 energy-% LA + 0.3, 0.5, 0.75, 1.5, 2.25, and 3.75 energy-% ALA) for 18 weeks. Results showed 6, 12, 15, and 30 energy-% LA significantly ameliorated central obesity, hyperlipidemia, glucose homeostasis, and leptin status; 0.5 and 0.75 energy-% ALA significantly improved insulin sensitivity, adiponectin, and anti-inflammatory status. Moreover, high intakes of ALA (1.5, 2.25, and 3.75 energy-%) presented a pro-oxidant activity. In conclusion, dose instead of ratio determines the prevention of MS. The optimal doses are 6 energy-% LA and 0.75 energy-% ALA; high intakes of ALA may have side effects.

Angiopoietin-like Protein 2 Is a Multistep Regulator of Inflammatory Neovascularization in a Murine Model of Age-related Macular Degeneration

Choroidal neovascularization (CNV) is a pathogenic process of age-related macular degeneration, a vision-threatening disease. The retinal pigment epithelium and macrophages both influence CNV development. However, the underlying mechanisms remain obscure. Here, we focus on Angptl2 (angiopoietin-like protein 2), a cytokine involved in age-related systemic diseases. Angptl2 was originally identified as an adipocytokine and is also expressed in the eye. Using a laser-induced CNV model, we found thatAngptl2KO mice exhibited suppressed CNV development with reduced macrophage recruitment and inflammatory mediator induction. The mediators monocyte chemotactic protein-1, interleukin-1β (Il-1β),Il-6, matrix metalloprotease-9 (Mmp-9), and transforming growth factor-β1 (Tgf-β1) that were up-regulated during CNV development were all suppressed in the retinal pigment epithelium-choroid of CNV models generated in theAngptl2KO mice. Bone marrow transplantation using wild-type and KO mice suggested that both bone marrow-derived and host-derived Angptl2 were responsible for macrophage recruitment and CNV development. Peritoneal macrophages derived fromAngptl2KO mice expressed lower levels of the inflammatory mediators. In the wild-type peritoneal macrophages and RAW264.7 cells, Angptl2 induced the mediators via integrins α4 and β2, followed by the downstream activation of NF-κB and ERK. The activation of NF-κB and ERK by Angptl2 also promoted macrophage migration. Therefore, Angptl2 from focal tissue might trigger macrophage recruitment, and that from recruited macrophages might promote expression of inflammatory mediators including Angptl2 in an autocrine and/or paracrine fashion to facilitate CNV development. Angptl2 might therefore represent a multistep regulator of CNV pathogenesis and serve as a new therapeutic target for age-related macular degeneration.

PKCδ inhibition normalizes the wound-healing capacity of diabetic human fibroblasts

Abnormal fibroblast function underlies poor wound healing in patients with diabetes; however, the mechanisms that impair wound healing are poorly defined. Here, we evaluated fibroblasts from individuals who had type 1 diabetes (T1D) for 50 years or more (Medalists, n = 26) and from age-matched controls (n = 7). Compared with those from controls, Medalist fibroblasts demonstrated a reduced migration response to insulin, lower VEGF expression, and less phosphorylated AKT (p-AKT), but not p-ERK, activation. Medalist fibroblasts were also functionally less effective at wound closure in nude mice. Activation of the δ isoform of protein kinase C (PKCδ) was increased in postmortem fibroblasts from Medalists, fibroblasts from living T1D subjects, biopsies of active wounds of living T1D subjects, and granulation tissues from mice with streptozotocin-induced diabetes. Diabetes-induced PKCD mRNA expression was related to a 2-fold increase in the mRNA half-life. Pharmacologic inhibition and siRNA-mediated knockdown of PKCδ or expression of a dominant-negative isoform restored insulin signaling of p-AKT and VEGF expression in vitro and improved wound healing in vivo. Additionally, increasing PKCδ expression in control fibroblasts produced the same abnormalities as those seen in Medalist fibroblasts. Our results indicate that persistent PKCδ elevation in fibroblasts from diabetic patients inhibits insulin signaling and function to impair wound healing and suggest PKCδ inhibition as a potential therapy to improve wound healing in diabetic patients.

AMPK-Activated Protein Kinase Suppresses Ccr2 Expression by Inhibiting the NF-κB Pathway in RAW264.7 Macrophages

C-C chemokine receptor 2 (Ccr2) is a key pro-inflammatory marker of classic (M1) macrophage activation. Although Ccr2 is known to be expressed both constitutively and inductively, the full regulatory mechanism of its expression remains unclear. AMP-activated protein kinase (AMPK) is not only a master regulator of energy homeostasis but also a central regulator of inflammation. In this study, we sought to assess AMPK's role in regulating RAW264.7 macrophage Ccr2 protein levels in resting (M0) or LPS-induced M1 states. In both M0 and M1 RAW264.7 macrophages, knockdown of the AMPKα1 subunit by siRNA led to increased Ccr2 levels whereas pharmacologic (A769662) activation of AMPK, attenuated LPS-induced increases in Ccr2 expression in an AMPK dependent fashion. The increases in Ccr2 levels by AMPK downregulation were partially reversed by NF-κB inhibition whereas TNF-a inhibition had minimal effects. Our results indicate that AMPK is a negative regulator of Ccr2 expression in RAW264.7 macrophages, and that the mechanism of action of AMPK inhibition of Ccr2 is mediated, in part, through the NF-κB pathway.