Glutathionylated products of lipid peroxidation: A novel mechanism of adipocyte to macrophage signaling

In Search of Differential Inhibitors of Aldose Reductase

Aldose reductase, classified within the aldo-keto reductase family as AKR1B1, is an NADPH dependent enzyme that catalyzes the reduction of hydrophilic as well as hydrophobic aldehydes. AKR1B1 is the first enzyme of the so-called polyol pathway that allows the conversion of glucose into sorbitol, which in turn is oxidized to fructose by sorbitol dehydrogenase. The activation of the polyol pathway in hyperglycemic conditions is generally accepted as the event that is responsible for a series of long-term complications of diabetes such as retinopathy, cataract, nephropathy and neuropathy. The role of AKR1B1 in the onset of diabetic complications has made this enzyme the target for the development of molecules capable of inhibiting its activity. Virtually all synthesized compounds have so far failed as drugs for the treatment of diabetic complications. This failure may be partly due to the ability of AKR1B1 to reduce alkenals and alkanals, produced in oxidative stress conditions, thus acting as a detoxifying agent. In recent years we have proposed an alternative approach to the inhibition of AKR1B1, suggesting the possibility of a differential inhibition of the enzyme through molecules able to preferentially inhibit the reduction of either hydrophilic or hydrophobic substrates. The rationale and examples of this new generation of aldose reductase differential inhibitors (ARDIs) are presented.

Intra-site differential inhibition of multi-specific enzymes

The ability to catalyse a reaction acting on different substrates, known as “broad-specificity” or “multi-specificity”, and to catalyse different reactions at the same active site (“promiscuity”) are common features among the enzymes. These properties appear to go against the concept of extreme specificity of the catalytic action of enzymes and have been re-evaluated in terms of evolution and metabolic adaptation. This paper examines the potential usefulness of a differential inhibitory action in the study of the susceptibility to inhibition of multi-specific or promiscuous enzymes acting on different substrates. Aldose reductase is a multi-specific enzyme that catalyses the reduction of both aldoses and hydrophobic cytotoxic aldehydes and is used here as a concrete case to deal with the differential inhibition approach.

Aldose Reductase Differential Inhibitors in Green Tea

Aldose reductase (AKR1B1), the first enzyme in the polyol pathway, is likely involved in the onset of diabetic complications. Differential inhibition of AKR1B1 has been proposed to counteract the damaging effects linked to the activity of the enzyme while preserving its detoxifying ability. Here, we show that epigallocatechin gallate (EGCG), one of the most representative catechins present in green tea, acts as a differential inhibitor of human recombinant AKR1B1. A kinetic analysis of EGCG, and of its components, gallic acid (GA) and epigallocatechin (EGC) as inhibitors of the reduction of L-idose, 4-hydroxy2,3-nonenal (HNE), and 3-glutathionyl l-4-dihydroxynonanal (GSHNE) revealed for the compounds a different model of inhibition toward the different substrates. While EGCG preferentially inhibited L-idose and GSHNE reduction with respect to HNE, gallic acid, which was still active in inhibiting the reduction of the sugar, was less active in inhibiting HNE and GSHNE reduction. EGC was found to be less efficient as an inhibitor of AKR1B1 and devoid of any differential inhibitory action. A computational study defined different interactive modes for the three substrates on the AKR1B1 active site and suggested a rationale for the observed differential inhibition. A chromatographic fractionation of an alcoholic green tea extract revealed that, besides EGCG and GA, other components may exhibit the differential inhibition of AKR1B1.

Enzymatic and non-enzymatic detoxification of 4-hydroxynonenal: Methodological aspects and biological consequences

4-Hydroxynonenal (HNE), an electrophilic end-product deriving from lipid peroxidation, undergoes a heterogeneous set of biotransformations including enzymatic and non-enzymatic reactions. The former mostly involve red-ox reactions on the HNE oxygenated functions (phase I metabolism) and GSH conjugations (phase II) while the latter are due to the HNE capacity to spontaneously condense with nucleophilic sites within endogenous molecules such as proteins, nucleic acids and phospholipids. The overall metabolic fate of HNE has recently attracted great interest not only because it clearly determines the HNE disposal, but especially because the generated metabolites and adducts are not inactive molecules (as initially believed) but show biological activities even more pronounced than those of the parent compound as exemplified by potent pro-inflammatory stimulus induced by GSH conjugates. Similarly, several studies revealed that the non-enzymatic reactions, initially considered as damaging processes randomly involving all endogenous nucleophilic reactants, are in fact quite selective in terms of both reactivity of the nucleophilic sites and stability of the generated adducts. Even though many formed adducts retain the expected toxic consequences, some adducts exhibit well-defined beneficial roles as documented by the protective effects of sublethal concentrations of HNE against toxic concentrations of HNE. Clearly, future investigations are required to gain a more detailed understanding of the metabolic fate of HNE as well as to identify novel targets involved in the biological activity of the HNE metabolites. These studies are and will be permitted by the continuous progress in the analytical methods for the identification and quantitation of novel HNE metabolites as well as for proteomic analyses able to offer a comprehensive picture of the HNE-induced adducted targets. On these grounds, the present review will focus on the major enzymatic and non-enzymatic HNE biotransformations discussing both the molecular mechanisms involved and the biological effects elicited. The review will also describe the most important analytical enhancements that have permitted the here discussed advancements in our understanding of the HNE metabolic fate and which will permit in a near future an even better knowledge of this enigmatic molecule.

Antioxidants and HNE in redox homeostasis

Under physiological conditions, cells are in a stable state known as redox homeostasis, which is maintained by the balance between continuous ROS/RNS generation and several mechanisms involved in antioxidant activity. ROS overproduction results in alterations in the redox homeostasis that promote oxidative damage to major components of the cell, including the biomembrane phospholipids. Lipid peroxidation subsequently generates a diverse set of products, including α,β-unsaturated aldehydes. Of these products, 4-hydroxy-2-nonenal (HNE) is the most studied aldehyde on the basis of its involvement in cellular physiology and pathology. This review summarizes the current knowledge in the field of HNE generation, metabolism, and detoxification, as well as its interactions with various cellular macromolecules (protein, phospholipid, and nucleic acid). The formation of HNE-protein adducts enables HNE to participate in multi-step regulation of cellular metabolic pathways that include signaling and transcription of antioxidant enzymes, pro-inflammatory factors, and anti-apoptotic proteins. The most widely described roles for HNE in the signaling pathways are associated with its activation of kinases, as well as transcription factors that are responsible for redox homeostasis (Ref-1, Nrf2, p53, NFκB, and Hsf1). Depending on its level, HNE exerts harmful or protective effects associated with the induction of antioxidant defense mechanisms. These effects make HNE a key player in maintaining redox homeostasis, as well as producing imbalances in this system that participate in aging and the development of pathological conditions.

4-Hydroxynonenal metabolites and adducts in pre-carcinogenic conditions and cancer

4-hydroxy-2-nonenal (HNE) is an amazing reactive compound, originating from lipid peroxidation within cells but also in food and considered as a "second messenger" of oxidative stress. Due to its chemical features, HNE is able to make covalent links with DNA, proteins and lipids. The aim of this review is to give a comprehensive summary of the chemical properties of HNE and of the consequences of its reactivity in relation to cancer development. The formation of exocyclic etheno-and propano-adducts and genotoxic effects are addressed. The adduction to cellular proteins and the repercussions on the regulation of cell signaling pathways involved in cancer development are reviewed, notably on the Nrf2/Keap1/ARE pathway. The metabolic pathways leading to the inactivation/elimination or, on the contrary, to the bioactivation of HNE are considered. A special focus is given on the link between HNE and colorectal cancer development, due to its occurrence in foodstuffs and in the digestive lumen, during digestion.

Adipocyte-Specific Deletion of Manganese Superoxide Dismutase Protects From Diet-Induced Obesity Through Increased Mitochondrial Uncoupling and Biogenesis

Obesity and insulin resistance are associated with oxidative stress (OS). The causal role of adipose OS in the pathogenesis of these conditions is unknown. To address this issue, we generated mice with an adipocyte-selective deletion of manganese superoxide dismutase (MnSOD). When fed a high-fat diet (HFD), the AdSod2 knockout (KO) mice exhibited less adiposity, reduced adipocyte hypertrophy, and decreased circulating leptin. The resistance to diet-induced adiposity was the result of an increased metabolic rate and energy expenditure. Furthermore, palmitate oxidation was elevated in the white adipose tissue (WAT) and brown adipose tissue of AdSod2 KO mice fed an HFD, and the expression of key fatty acid oxidation genes was increased. To gain mechanistic insight into the increased fat oxidation in HFD-fed AdSod2 KO mice, we quantified the mitochondrial function and mitochondrial content in WAT and found that MnSOD deletion increased mitochondrial oxygen consumption and induced mitochondrial biogenesis. This effect was preserved in cultured adipocytes from AdSod2 KO mice in vitro. As expected from the enhanced fat oxidation, circulating levels of free fatty acids were reduced in the HFD-fed AdSod2 KO mice. Finally, HFD-fed AdSod2 KO mice were protected from hepatic steatosis, adipose tissue inflammation, and glucose and insulin intolerance. Taken together, these results demonstrate that MnSOD deletion in adipocytes triggered an adaptive stress response that activated mitochondrial biogenesis and enhanced mitochondrial fatty acid oxidation, thereby preventing diet-induced obesity and insulin resistance.

Physiological and therapeutic effects of carnosine on cardiometabolic risk and disease

Obesity, type 2 diabetes (T2DM) and cardiovascular disease (CVD) are the most common preventable causes of morbidity and mortality worldwide. They represent major public health threat to our society. Increasing prevalence of obesity and T2DM contributes to escalating morbidity and mortality from CVD and stroke. Carnosine (β-alanyl-L-histidine) is a dipeptide with anti-inflammatory, antioxidant, anti-glycation, anti-ischaemic and chelating roles and is available as an over-the-counter food supplement. Animal evidence suggests that carnosine may offer many promising therapeutic benefits for multiple chronic diseases due to these properties. Carnosine, traditionally used in exercise physiology to increase exercise performance, has potential preventative and therapeutic benefits in obesity, insulin resistance, T2DM and diabetic microvascular and macrovascular complications (CVD and stroke) as well as number of neurological and mental health conditions. However, relatively little evidence is available in humans. Thus, future studies should focus on well-designed clinical trials to confirm or refute a potential role of carnosine in the prevention and treatment of chronic diseases in humans, in addition to advancing knowledge from the basic science and animal studies.

Loss of multidrug resistance-associated protein 1 potentiates chronic doxorubicin-induced cardiac dysfunction in mice

Doxorubicin (DOX), an effective cancer chemotherapeutic agent, induces dose-dependent cardiotoxicity, in part due to its ability to cause oxidative stress. We investigated the role of multidrug resistance-associated protein 1 (Mrp1/Abcc1) in DOX-induced cardiotoxicity in C57BL wild-type (WT) mice and their Mrp1 null (Mrp1(-/-)) littermates. Male mice were administered intraperitoneal DOX (3 or 2 mg/kg body weight) or saline twice a week for 3 weeks and examined 2 weeks after the last dose (protocol A total dose: 18 mg/kg) or for 5 weeks, and mice were examined 48 hours and 2 weeks after the last dose (protocol B total dose: 20 mg/kg). Chronic DOX induced body weight loss and hemotoxicity, adverse effects significantly exacerbated in Mrp1(-/-) versus WT mice. In the heart, significantly higher basal levels of glutathione (1.41-fold ± 0.27-fold) and glutathione disulfide (1.35-fold ± 0.16-fold) were detected in Mrp1(-/-) versus WT mice, and there were comparable decreases in the glutathione/glutathione disulfide ratio in WT and Mrp1(-/-) mice after DOX administration. Surprisingly, DOX induced comparable increases in 4-hydroxynonenal glutathione conjugate concentration in hearts from WT and Mrp1(-/-) mice. However, more DOX-induced apoptosis was detected in Mrp1(-/-) versus WT hearts (P < 0.05) (protocol A), and cardiac function, assessed by measurement of fractional shortening and ejection fraction with echocardiography, was significantly decreased by DOX in Mrp1(-/-) versus WT mice (P < 0.05; 95% confidence intervals of 20.0%-24.3% versus 23.7%-29.5% for fractional shortening, and 41.5%-48.4% versus 47.7%-56.7% for ejection fraction; protocol B). Together, these data indicate that Mrp1 protects the mouse heart against chronic DOX-induced cardiotoxicity.

On the role of 4-hydroxynonenal in health and disease

Polyunsaturated fatty acids are susceptible to peroxidation and they yield various degradation products, including the main α,β-unsaturated hydroxyalkenal, 4-hydroxy-2,3-trans-nonenal (HNE) in oxidative stress. Due to its high reactivity, HNE interacts with various macromolecules of the cell, and this general toxicity clearly contributes to a wide variety of pathological conditions. In addition, growing evidence suggests a more specific function of HNE in electrophilic signaling as a second messenger of oxidative/electrophilic stress. It can induce antioxidant defense mechanisms to restrain its own production and to enhance the cellular protection against oxidative stress. Moreover, HNE-mediated signaling can largely influence the fate of the cell through modulating major cellular processes, such as autophagy, proliferation and apoptosis. This review focuses on the molecular mechanisms underlying the signaling and regulatory functions of HNE. The role of HNE in the pathophysiology of cancer, cardiovascular and neurodegenerative diseases is also discussed.

Multidrug resistance protein 1 (MRP1, ABCC1), a "multitasking" ATP-binding cassette (ABC) transporter

The multidrug resistance protein 1 (MRP1) encoded by ABCC1 was originally discovered as a cause of multidrug resistance in tumor cells. However, it is now clear that MRP1 serves a broader role than simply mediating the ATP-dependent efflux of drugs from cells. The antioxidant GSH and the pro-inflammatory cysteinyl leukotriene C4 have been identified as key physiological organic anions effluxed by MRP1, and an ever growing body of evidence indicates that additional lipid-derived mediators are also substrates of this transporter. As such, MRP1 is a multitasking transporter that likely influences the etiology and progression of a host of human diseases.