Hypochlorous acid via peroxynitrite activates protein kinase Cθ and insulin resistance in adipocytes

Curcumin improves insulin sensitivity and increases energy expenditure in high-fat-diet-induced obese mice associated with activation of FNDC5/irisin

OBJECTIVE

Curcumin (Cur) has a beneficial role in preventing metabolic dysfunctions; however, the underlying mechanism are not yet fully understood. The aim of this study was to evaluate whether the beneficial metabolic effects of curcumin are associated with the regulation of energy metabolism and activation of fibronectin type 3 domain-containing protein 5 (FNDC5)/irisin.

METHODS

We used cellular and molecular techniques to investigate the effects of Cur on C57 BL/6 mice that were fed either a control diet or a high-fat diet (HFD) with or without 0.2% Cur for 10 wk. Factors involved in energy metabolism, inflammatory responses, and insulin signaling, as well as the involvement of FNDC5/irisin pathway, were assessed.

RESULTS

Cur alleviated adiposity and suppressed inflammatory response in white adipose tissue (WAT) of HFD mice. Meanwhile, Cur administration increased plasma irisin concentration and improved insulin sensitivity of HFD mice. Cur increased the oxygen consumption and heat production and reduced respiratory exchange ratio (RES) in HFD mice, which were accompanied by the enhancement of metabolic activity in brown fat and inguinal WAT. Additionally, the improvement of basal metabolic rate by Cur may be partly regulated by the FNDC5/ p38 mitogen-activated protein kinase (p38 MAPK)/extracellular signal-related kinase (ERK) 1/2 pathway.

CONCLUSIONS

These findings demonstrated that dietary Cur alleviated diet-induced adiposity by improving insulin sensitivity and whole body energy metabolism via the FNDC5/p38 MAPK/ERK pathways.

Hypochlorous acid decreases antioxidant power, inhibits plasma membrane redox system and pathways of glucose metabolism in human red blood cells

Hypochlorous acid (HOCl) is generated at a high concentration by activated neutrophils at sites of inflammation in a myeloperoxidase catalyzed reaction. The increased and sustained production of HOCl at inflammatory sites may lead to tissue injury and this process is believed to play an important role in the progression of several diseases like chronic inflammation, atherosclerosis and some types of cancers. We have examined the effect of HOCl on human red blood cells (RBCs) under in vitro conditions. Treatment of RBC with different concentrations of HOCl (0.05-2.5 mM) at 37°C resulted in decreased activities of major antioxidant enzymes while the antioxidant power of RBC was weakened, as shown by lowered metal-reducing and free radical quenching ability of HOCl treated cells. RBC plasma membrane redox system was also inhibited suggesting membrane damage. The enzymes of glucose metabolism were inhibited indicating deranged energy metabolism. Electron microscopic images showed gross morphological changes in HOCl treated RBC. These results show that HOCl causes major alterations in the cellular antioxidant defense system and inhibition of glycolytic pathways, which increase the susceptibility of RBC to oxidative damage.

The effects of neutrophil-generated hypochlorous acid and other hypohalous acids on host and pathogens

Neutrophils are predominant immune cells that protect the human body against infections by deploying sophisticated antimicrobial strategies including phagocytosis of bacteria and neutrophil extracellular trap (NET) formation. Here, we provide an overview of the mechanisms by which neutrophils kill exogenous pathogens before we focus on one particular weapon in their arsenal: the generation of the oxidizing hypohalous acids HOCl, HOBr and HOSCN during the so-called oxidative burst by the enzyme myeloperoxidase. We look at the effects of these hypohalous acids on biological systems in general and proteins in particular and turn our attention to bacterial strategies to survive HOCl stress. HOCl is a strong inducer of protein aggregation, which bacteria can counteract by chaperone-like holdases that bind unfolding proteins without the need for energy in the form of ATP. These chaperones are activated by HOCl through thiol oxidation (Hsp33) or N-chlorination of basic amino acid side-chains (RidA and CnoX) and contribute to bacterial survival during HOCl stress. However, neutrophil-generated hypohalous acids also affect the host system. Recent studies have shown that plasma proteins act not only as sinks for HOCl, but get actively transformed into modulators of the cellular immune response through N-chlorination. N-chlorinated serum albumin can prevent aggregation of proteins, stimulate immune cells, and act as a pro-survival factor for immune cells in the presence of cytotoxic antigens. Finally, we take a look at the emerging role of HOCl as a potential signaling molecule, particularly its role in neutrophil extracellular trap formation.

Redox regulation in host-pathogen interactions: thiol switches and beyond

Our organism is exposed to pathogens on a daily basis. Owing to this age-old interaction, both pathogen and host evolved strategies to cope with these encounters. Here, we focus on the consequences of the direct encounter of cells of the innate immune system with bacteria. First, we will discuss the bacterial strategies to counteract powerful reactive species. Our emphasis lies on the effects of hypochlorous acid (HOCl), arguably the most powerful oxidant produced inside the phagolysosome of professional phagocytes. We will highlight individual examples of proteins in gram-negative bacteria activated by HOCl via thiol-disulfide switches, methionine sulfoxidation, and N-chlorination of basic amino acid side chains. Second, we will discuss the effects of HOCl on proteins of the host. Recent studies have shown that both host and bacteria address failing protein homeostasis by activation of chaperone-like holdases through N-chlorination. After discussing the role of individual proteins in the HOCl-defense, we will turn our attention to the examination of effects on host and pathogen on a systemic level. Recent studies using genetically encoded redox probes and redox proteomics highlight differences in redox homeostasis in host and pathogen and give first hints at potential cellular HOCl signaling beyond thiol-disulfide switch mechanisms.

Endothelial cell senescence in aging-related vascular dysfunction

Increased cardiovascular disease in aging is partly a consequence of the vascular endothelial cell (EC) senescence and associated vascular dysfunction. In this contest, EC senescence is a pathophysiological process of structural and functional changes including dysregulation of vascular tone, increased endothelium permeability, arterial stiffness, impairment of angiogenesis and vascular repair, and a reduction of EC mitochondrial biogenesis. Dysregulation of cell cycle, oxidative stress, altered calcium signaling, hyperuricemia, and vascular inflammation have been implicated in the development and progression of EC senescence and vascular disease in aging. A number of abnormal molecular pathways are associated with these underlying pathophysiological changes including Sirtuin 1, Klotho, fibroblast growth factor 21, and activation of the renin angiotensin-aldosterone system. However, the molecular mechanisms of EC senescence and associated vascular impairment in aging are not completely understood. This review provides a contemporary update on molecular mechanisms, pathophysiological events, as well functional changes in EC senescence and age-associated cardiovascular disease. This article is part of a Special Issue entitled: Genetic and epigenetic regulation of aging and longevity edited by Jun Ren & Megan Yingmei Zhang.

The Contribution of Singlet Oxygen to Insulin Resistance

Insulin resistance contributes to the development of diabetes and cardiovascular dysfunctions. Recent studies showed that elevated singlet oxygen-mediated lipid peroxidation precedes and predicts diet-induced insulin resistance (IR), and neutrophils were suggested to be responsible for such singlet oxygen production. This review highlights literature suggesting that insulin-responsive cells such as endothelial cells, hepatocytes, adipocytes, and myocytes also produce singlet oxygen, which contributes to insulin resistance, for example, by generating bioactive aldehydes, inducing endoplasmic reticulum (ER) stress, and modifying mitochondrial DNA. In these cells, nutrient overload leads to the activation of Toll-like receptor 4 and other receptors, leading to the production of both peroxynitrite and hydrogen peroxide, which react to produce singlet oxygen. Cytochrome P450 2E1 and cytochrome c also contribute to singlet oxygen formation in the ER and mitochondria, respectively. Endothelial cell-derived singlet oxygen is suggested to mediate the formation of oxidized low-density lipoprotein which perpetuates IR, partly through neutrophil recruitment to adipose tissue. New singlet oxygen-involving pathways for the formation of IR-inducing bioactive aldehydes such as 4-hydroperoxy-(or hydroxy or oxo)-2-nonenal, malondialdehyde, and cholesterol secosterol A are proposed. Strategies against IR should target the singlet oxygen-producing pathways, singlet oxygen quenching, and singlet oxygen-induced cellular responses.

Metformin Suppresses Diabetes-Accelerated Atherosclerosis via the Inhibition of Drp1-Mediated Mitochondrial Fission

Metformin is a widely used antidiabetic drug that exerts cardiovascular protective effects in patients with diabetes. How metformin protects against diabetes-related cardiovascular diseases remains poorly understood. Here, we show that metformin abated the progression of diabetes-accelerated atherosclerosis by inhibiting mitochondrial fission in endothelial cells. Metformin treatments markedly reduced mitochondrial fragmentation, mitigated mitochondrial-derived superoxide release, improved endothelial-dependent vasodilation, inhibited vascular inflammation, and suppressed atherosclerotic lesions in streptozotocin (STZ)-induced diabetic ApoE^-/- mice. In high glucose-exposed endothelial cells, metformin treatment and adenoviral overexpression of constitutively active AMPK downregulated mitochondrial superoxide, lowered levels of dynamin-related protein (Drp1) and its translocation into mitochondria, and prevented mitochondrial fragmentation. In contrast, AMPK-α2 deficiency abolished the effects of metformin on Drp1 expression, oxidative stress, and atherosclerosis in diabetic ApoE^-/-/AMPK-α2^-/- mice, indicating that metformin exerts an antiatherosclerotic action in vivo via the AMPK-mediated blockage of Drp1-mediated mitochondrial fission. Consistently, mitochondrial division inhibitor 1, a potent and selective Drp1 inhibitor, reduced mitochondrial fragmentation, attenuated oxidative stress, ameliorated endothelial dysfunction, inhibited inflammation, and suppressed atherosclerosis in diabetic mice. These findings show that metformin attenuated the development of atherosclerosis by reducing Drp1-mediated mitochondrial fission in an AMPK-dependent manner. Suppression of mitochondrial fission may be a therapeutic approach for treating macrovascular complications in patients with diabetes.

Contribution of Maladaptive Adipose Tissue Expansion to Development of Cardiovascular Disease

The overweight and obesity epidemic has led to an increase in the metabolic syndrome and associated cardiovascular disease (CVD). These abnormalities include insulin resistance, type 2 diabetes mellitus, vascular stiffness, hypertension, stroke, and coronary heart disease. Visceral white adipocyte tissue (WAT) expansion and associated fibrosis/stiffness of WAT promote insulin resistance and CVD through increases in proinflammatory adipokines, oxidative stress, activation of renin-angiotensin-aldosterone system, dysregulation of adipocyte apoptosis and autophagy, dysfunctional immune modulation, and adverse changes in the gut microbiome. The expansion of WAT is partly determined by activation of peroxisome proliferator-activated receptor gamma and mammalian target of rapamycin/ribosomal S6 kinase signaling pathways. Further, the chronic activation of these signaling pathways may not only induce adipocyte hypertrophy and fibrosis, but also contribute to systemic inflammation, and impairment of insulin metabolic signaling in fat, liver, and skeletal muscle tissue. Therefore, the interplay of adipocyte dysfunction, maladaptive immune and inflammatory responses, and associated metabolic disorders often coexist leading to systemic low-grade inflammation and insulin resistance that are associated with increased CVD in obese individuals. © 2017 American Physiological Society. Compr Physiol 7:253-262, 2017.

Peroxynitrite: From interception to signaling

Peroxynitrite is a strong oxidant and nitrating species that mediates certain biological effects of superoxide and nitrogen monoxide. These biological effects include oxidative damage to proteins as well as the formation of 3-nitrotyrosyl moieties in proteins. As a consequence, such proteins may lose their activity, gain altered function, or become prone to proteolytic degradation - resulting in modulation of cellular protein turnover and in the modulation of signaling cascades. In analogy to hydrogen peroxide, peroxynitrite may be scavenged by selenoproteins like glutathione peroxidase-1 (GPx-1) or by selenocompounds with a GPx-like activity, such as ebselen; in further analogy to H2O2, peroxiredoxins have also been established as contributors to peroxynitrite reduction. This review covers three aspects of peroxynitrite biochemistry, (i) the interaction of selenocompounds/-proteins with peroxynitrite, (ii) peroxynitrite-induced modulation of cellular proteolysis, and (iii) peroxynitrite-induced modulation of cellular signaling.

Aspalathin improves glucose and lipid metabolism in 3T3-L1 adipocytes exposed to palmitate

SCOPE

Saturated-free fatty acids, such as palmitate, are associated with insulin resistance. This study aimed to establish if an aspalathin-enriched green rooibos extract (GRE) and, its major flavanoid, aspalathin (ASP) could contribute significantly to the amelioration of experimentally induced insulin resistance in 3T3-L1 adipocytes.

METHODS AND RESULTS

3T3-L1 adipocytes were cultured in DMEM containing 0.75 mM palmitate for 16 h to induce insulin resistance before treatment for 3 h with GRE (10 μg/mL) or ASP (10 μM). GRE and ASP reversed the palmitate-induced insulin resistance. At a protein level GRE and ASP suppressed nuclear factor kappa beta (NF-κB), insulin receptor substrate one (serine 307) (IRS1 (Ser (307) )) and AMP-activated protein kinase phosphorylation and increased serine/threonine kinase AKT (AKT) activation, while only GRE increased glucose transporter four (Glut4) protein expression. Peroxisome proliferator-activated receptor alpha and gamma (PPARα and γ), and carnitine palmitoyltransferase one (CPT1) expression were increased by ASP alone.

CONCLUSION

Together these effects offer a plausible explanation for the ameliorative effect of GRE and ASP on insulin-resistance, an underlying cause for obesity and type 2 diabetes.