This post describes the relationships between oxidative stress and the metabolic syndrome. Evidence suggests that oxidative stress may be involved in the aetiology, pathogenesis, and development of the metabolic syndrome. Additionally an important question remains to be asked: should we focus on antioxidant supplementation for managing the progression of complications, or on earlier steps before the development of complications? We think that it is crucial to interfere in both steps to prevent and treat several diseases, including the metabolic syndrome. Furthermore, antioxidant therapy may not only be too late, but it may also miss a large fraction of the target, as nonoxidative pathways do also contribute to cell damage.
Keywords Adipocyte · Dyslipidaemia · Hypertension · Leptin · Oxidative stress
1 Oxidative Stress and Obesity
Type 2 diabetes, hyperlipidaemia, hypertension, and atherosclerosis have recently been defined as typical lifestyle-related diseases. The common background of these diseases is obesity. Obesity, especially visceral obesity, with a rising prevalence in the last decade, is the main starting point to metabolic syndrome. Although obesity is common among patients with metabolic syndrome, not all obese people have it (Furukawa et al. 2004; Hansel et al. 2004; Ford 2006; Cardona et al. 2008a; Skalicky et al. 2008). On the other hand, lean people can develop this syndrome as well (Sjogren et al. 2005).
We will start by discussing how excess fat affects adipocyte metabolism, and how can oxidative stress be involved in adipocyte dysfunction, to consider then consequences of this on several other tissues.
Obesity can promote the atherosclerotic process through influence on endothelial function as well as through mechanisms of oxidative stress (Furukawa et al. 2004; Skalicky et al. 2008). Adipocytes produce a variety of biologically active molecules, collectively known as adipokines, including TNF-α, resistin, leptin, and adiponectin (Bays et al. 2008). Adipocyte dysfunction provokes a deregulated production of these adipocytokines, which participate in the pathogenesis of obesity-associated metabolic syndrome. As an example, increased production of TNF-α from accumulated fat contribute to the development of insulin resistance in obesity.
It has been well known that oxidative stress occurs more frequently in people with metabolic syndrome phenotype than among those without (Furukawa et al. 2004; Ford 2006; Cardona et al. 2008a; Skalicky et al. 2008), although not all studies have these conclusions (Sjogren et al. 2005). It is important to know if oxidative stress is a cause or a consequence in metabolic syndrome complications, i.e. whether oxidative stress occurs at an early stage, preceding the appearance of complications, or whether it is merely a common con-sequence of the cell damage, reflecting the presence of complications (probably, both). Part of the controversy has to do with what happens first (which remains unclear). What are the causes and what are the effects? As an example, some believe metabolic syndrome (insulin resistance, hypertension or obesity) causes oxidative stress (Furukawa et al. 2004; ), whereas others believe oxidative stress causes insulin resistance, hypertension, atherosclerosis, obesity, and so on, i.e., metabolic syndrome (Baynes and Thorpe 1999; Evans et al. 2003; Urakawa et al. 2003; Evans et al. 2005; Skalicky et al. 2008).
Oxidative stress in adipocyte seems to be responsible for the sub-clinical pro-inflammatory state often observed in visceral obesity (Bastard et al. 2006; de Ferranti and Mozaffarian 2008). In fact, Furukawa et al. (2004) found a good correlation between obesity and systemic oxidative stress. Additionally, they observed a higher expression of NADPH oxidase that is accompanied by a decrease of antioxidant enzymes. The mechanisms involved in elevation of oxidative stress and inflammatory burden seem to include increased production of superoxide anion via the NAD(P)H oxidase pathway. NADPH oxidase activity causes deregulated production of adipocytokines, as plasminogen activator inhibitor-1, IL-6, and monocyte chemotactic protein-1 (Furukawa et al. 2004).
Leptin, a hormone produced by adipocytes, acts on hypothalamic centers to regulate food intake and energy expenditure. Plasma concentrations of this hormone are proportional to the amount of adipose tissue. Leptin has an important role in obesity-induced oxidative stress. This hormone stimulates directly ROS production such as H2O2 and hydroxyl radical. Furthermore, leptin is a proinflammatory factor that stimulates the proliferation of monocytes and macrophages and the production of inflammatory cytokines. Indirectly, leptin stimulates production of inflammatory cytokines such as IL-6 and TNF-α, which increase NADPH oxidase activity and superoxide anion production. Finally, leptin reduces the activity of paraoxonase-1 (PON-1), an enzyme that protects against LDL oxidation (Vincent and Taylor 2006).
It has been clearly demonstrated that chronic imbalance of consumed vs expended calories causes increased storage of the excess energy in the form of adipocyte intracellular triglyceride stores. Interestingly, within several cell types (adipocytes, myocytes, endothelial or β-cells), excess energy substrate in the form of glucose or FFAs enter the citric acid cycle, resulting in the generation of excess mitochondrial NADH, and consequently ROS (Vincent and Taylor 2006; de Ferranti and Mozaffarian 2008; Martyn et al. 2008). When excessive NADH cannot be dissipated by oxidative phosphorylation, the mitochondrial proton gradient increases, and single electrons are transferred to molecular oxygen, forming superoxide anion (Stephens et al. 2008; Cardona et al. 2008b). As further described in the next section, ROS production possibly links obesity with insulin resistance in adipocyte and skeletal muscle cells as well.
In adipocyte, oxidative stress, as a result of several mitochondrial (metabolic substrates: hyperglycaemia, elevated FFAs, uncoupling proteins dysfunction, among others) or extramitochondrial inducers (NADPH oxidase, cytochrome P450, iNOS, among other enzymatic activities), seems to lead to endoplasmic reticulum stress, with UPR (Unfolded Protein Response), which converge in a systemic proinflammatory state (de Ferranti and Mozaffarian 2008; Gregor and Hotamisligil 2007).
There are several possible contributors to oxidative stress in obesity, especially considering a common dietary pattern, with hyperglycaemia, elevated FFAs, and an inadequate antioxidant intake. As a matter of fact, there are studies showing that obese individuals have a lower intake of phytochemical-rich foods, compared with nonobese (Vincent and Taylor 2006). Additionally, it is known that the activities of the major antioxidant enzymes may also be lower in obese individuals (Bełtowski et al. 2000).
Bougoulia et al. (2006) investigated the relationships between cytokines, proinflammatory products and oxidative stress (IL-6, CRP, isoprostane and glutathione peroxidase), as well as their relation to cardiovascular disease risk factor in obese women (with central obesity) and their possible modification by weight reduction. In that work, it was demonstrated that: (1) low values of glutathione peroxidase along with higher levels of isoprostane in obese women indicate defective protection mechanisms against atherosclerosis and oxidative stress; (2) weight reduction ameliorated those parameters. It has been shown that individuals who lose 5-10% of body weight may lose 30% of their visceral fat, which is an important finding in this context.
2 Oxidative Stress and Diabetes
Elevated levels of metabolic substrates (glucose and/or fatty acids) contribute to the diabetic phenotype. Both insulin resistance and decreased insulin secretion are major features of the pathophysiology of type 2 diabetes. Insulin resistance is now clearly considered a major risk factor for developing type 2 diabetes, and most often precedes the onset of this pathology by many years. Initially, insulin resistance seems to be compensated through hyperinsulinemia, and normal glucose tolerance is preserved. When the pancreas has no longer capacity for this increased production of insulin, does hyperglycaemia occur.
In this section, we propose that ROS/RNS and oxidative stress induced by elevations in glucose and possibly FFA levels play a key role in causing insulin resistance and beta-cell dysfunction by their ability to activate stress-sensitive signalling pathways.
2.1 Insulin Resistance (Peripheral Tissues: Adipocyte, Myocyte)
2.1.1 Historical Perspective
Insulin resistance is defined as the condition whereby the body’s cells require more and more insulin to get the same effect on glucose uptake. Liver, brain, and red blood cells do not require insulin action for uptake of plasma glucose. Although insulin is not required by liver cells for glucose uptake, it regulates important functions in liver cells, such as gluconeogenesis. Thus, when we refer to insulin resistance, below, the target insulin-sensitive tissues of the periphery that are being considered mainly encompass adipose tissue and skeletal muscle.
Hyperglycaemia has been demonstrated to constitute a risk factor for develop-ment of diabetic complications (Martyn et al. 2008). There being no consensus in relation to the molecular mechanisms between hyperglycaemia and disease, it has been clearly demonstrated that chronic exposure to elevated glucose concentrations can cause damage in different type of cells (specially, β-cell, adipocyte and myocyte) by different mechanisms involving oxidative stress.
There are several theories about the origin of those complications, including AGE (advanced glycation end products) hypothesis (Vlassara 1997), aldose reductase hypothesis (Hotta 1995), reductive stress (pseudohypoxia) (Ido et al. 1997), true hypoxia (Cameron and Cotter 1997), carbonyl stress (Lyons and Jenkins 1997), oxidative stress (Baynes and Thorpe 1999), altered lipoprotein metabolism (Lyons and Jenkins 1997), increased PKC activity (Ishii et al. 1998) and altered growth factor (Pfeiffer and Schatz 1995) or cytokine (Sharma and Ziyadeh 1997) activities. In this long list it may happen that each hypothesis corresponds to a different point of view of a common pathogenic mechanism. It may also happen that different cells are sensitive to different mechanisms. However, these different theories overlap and intersect with one another. All of those pathways converge in oxidative stress: AGE formation can induce oxidative stress that can accelerate AGE formation, and so on.
The AGE (Advanced Glycation End product) hypothesis proposes that chronic accelerated chemical modification of proteins by reducing sugars in diabetes alters the structure and function of tissue proteins, contributing to pathophysiology. Con-verging with this, the carbonyl stress hypothesis means a generalized increase of reactive carbonyl precursors of AGEs, glycoxidation and lipoxidation products. Mechanistically, carbonyl stress, with consequent increased carbonyls, result from an imbalance between production and detoxification of these reactive groups (Baynes and Thorpe 1999). It must be noticed that this term includes both carbonyls derived from oxidative and nonoxidative pathways. If the carbonyls are derived exclusively from oxidative reactions, then the condition would be described as oxidative stress. The distinction may not be completely academic. The carbonyl stress hypothesis is in fact a mixture of metabolic and chemical hypothesis: altered metabolism and compromised detoxification lead to increase of carbonyl formation, increased chemical modification of proteins, and then to oxidative stress and tis-sue damage, which converge on the development of complications. According to different authors, oxidative stress is a secondary event in the pathogenic process (Baynes and Thorpe 1999). Interestingly enough is that AGEs products originated by nonoxidative pathways may induce oxidative stress and apoptosis in cells, illustrating a possible cause-effect relationship between carbonyl stress and oxidative stress.
Finally, other authors propose that metabolic imbalances in different tissues, resulting from excess of glucose metabolism, induce a state of pseudohypoxia or a reductive stress, rather than oxidative stress, in tissues. Pseudohypoxia is characterized by an increase in the cellular NADH/NAD+ ratio. In diabetes the redox shift is attributed not to oxygen deprivation, but to excessive metabolism of glucose through glycolysis and the polyol pathway or of lipids by β-oxidation (Baynes and Thorpe 1999).
2.1.2 Oxidative Stress as a Link Between Hyperglycaemia and Insulin Resistance
How do elevated glucose and possibly free fatty acids contribute to the pathophysiology of diabetes via oxidative stress?
There is some evidence that oxidative stress caused by hyperglycaemia and/or FFA occurs before complications of diabetes become clinically evident. Hyperglycaemia leads to elevated formation of ROS and/or RNS as a consequence of non-enzymatic protein glycation and glucose autoxidation. Hyperglycaemia does also induce enzymatic production of superoxide anion through activation of NAD(P)H oxidase. For example, a non-enzymatic protein glycation may depend on ROS (superoxide and hydroxyl) formation through metal-catalyzed glucose autoxidation.
Glucose and fatty acid independent sources of oxidative stress include enzymes such as NADPH oxidase and xanthine oxidase, both able to convert molecular oxy-gen into superoxide anion. Mitochondrial superoxide dismutase (SOD) seems to be responsible for oxygen conversion into hydrogen peroxide. In the cytoplasm, cata-lase or glutathione peroxidase then detoxify this further forming water. Therefore, if there is increased NADPH activity, or reduced SOD or glutathione activity, ROS production will be increased. Curiously, detoxifying enzymes seem to be decreased in obese patients (Furukawa et al. 2004).
Intracellular glucose elevations stimulate the polyol pathway in which aldose reductase mediates conversion of glucose to sorbitol. Excess sorbitol causes oxidative damage and activates stress genes, as has been demonstrated in several animal models (Evans et al. 2002). Hyperglycaemia does also increase NADPH oxidase activity, and NADPH produces the superoxide anion. When glucose itself auto-oxidizes, it produces oxidants with reactivity similar to that of hydroxyl radical and superoxide anion.
Oxidative stress may also result from the metabolic impact of intracellular triglycerides. For example, by suppressing the mitochondrial adenine nucleotide transporter, excessive triglycerides may increase superoxide anion production within the mitochondrial chain, and this decreases intramitochondrial ADP levels. electrons then accumulate within electron transport chain and react with adjacent oxygen to form superoxide anion.
Visceral adiposity linked with elevated FFAs and hyperglycaemia does also produce nitroxide radicals via PKC pathway.
Free radicals in cells and ROS-derived lipid peroxides directly damage proteins, lipids and nucleic acids. Since ROS are produced mainly in mitochondria, they are a primary target for ROS-mediated reactions, resulting in mitochondrial damage. It has been assumed that in different pathologies, oxidative stress is a result of mitochondrial dysfunction, in consequence of several different toxic agents.
Mitochondrial dysfunction may contribute to insulin resistance in connection with the role of uncoupling proteins (UCP) as oxidative stress protecting agents.
Metabolic uncoupling refers to a state in which nutrient fuels are oxidized but the resultant energy is not linked to ATP synthesis but dissipated as heat. UCP1 is expressed in brown adipose tissue and shown to be an important thermogenic molecule. UCP2 and UCP3 can modulate cellular metabolism, although with a different tissue localization, suggestive of different physiological roles. Furthermore, according to Kim et al. (2008), overexpression of UCP2 or UCP3lowers ROS pro-duction, stimulates the metabolic rate, and protects against weight gain and insulin resistance.
Molecular Mechanisms for Oxidative Stress-Induced Insulin Resistance
There is strong evidence to indicate that nuclear factor-kB (NF-kB), NH2- terminal Jun kinases (JNK/SAPK), p38 mitogen-activated protein (MAP) kinase, and hexosamine pathways are stress-sensitive signalling systems activated by hyper-glycaemia through oxidative stress. Thus, it has been accepted that activation of these pathways is linked not only to the development of the late complications of diabetes, but also to insulin resistance and P-cell dysfunction (Evans et al. 2003, 2005).
Additionally, strong evidence indicates that elevated FFA levels decrease insulin sensitivity and that pathway could be the link between obesity and diabetes (Randle et al. 1988). Elevated FFA levels have numerous adverse effects on mithocondrial function, as the uncoupling of oxidative phosphorylation, generating ROS. Fur-thermore, FFAs are not only able to induce oxidative stress but also to impair endogenous antioxidant defenses by reducing intracellular glutathione. It is prob-ably through these pathways that FFAs could activate PKC-0 and consequently NK-kB (Slatter et al. 2000; Maassen et al. 2007). Additionally, it is known that fatty acids compete with glucose for metabolism, and fatty acid-derived metabolites, such as acetyl-CoA and citrate, inhibit insulin-stimulated glucose transport. Furthermore, impaired mitochondrial fatty acid oxidation, leading to the accumulation of fatty acid metabolites in muscle, is proposed to be a key factor in the development of insulin resistance in these cells. In good agreement with that, the results obtained by Cardona et al. (2008b) show an increase in oxidative stress after a fat overload, especially in patients with metabolic syndrome.
2.2 β-Cell Dysfunction
Another target for oxidative stress damage is the P-cell. Recent data have implicated that P-cell dysfunction is the result of prolonged exposure to high glucose, elevated FFA levels, or a combination of both.
According with Evans hypothesis (Evans et al. 2003), hyperglycaemia induces deterioration of P-cell function probably through oxidative stress. In fact, there is evidence that oxidative stress leads to tissue damage. It has been described that ROS formation is a direct consequence of hyperglycaemia and of increased FFA levels. In addition to their ability to directly damage macromolecules (DNA, lipids and proteins), ROS can function as signalling molecules to activate a number of cellular stress-sensitive pathways that cause cellular damage and, thus, probably play a key direct role in the pathogenesis of late diabetic complications. These same pathways are linked to insulin resistance and decreased insulin secretion. β-Cells at high risk for oxidative damage have an increased sensitivity for apoptosis. These cells are particularly sensitive to ROS and RNS because they are poor in antioxidant enzymes such as SOD, glutathione peroxidase and catalase (Tiedge et al. 1997; Robertson et al. 2007).
2.2.1 Mitochondrial Dysfunction and Diabetes
Mitochondrial damage, as consequence of oxidative stress, includes decreased mitochondrial ATP synthesis and deregulation of intracellular lipid and calcium homeostasis, with important consequences on cell viability. In the β-cell, the main consequence for UCP2 (higher) activity, and thus for the lower cellular ATP, appears to be impairment of closure of plasma membrane ATP-dependent K+ channels (Chan and Harper 2006; Fridlyand and Philipson 2006; Kim et al. 2008). Such modulation results in reduced glucose-stimulated insulin secretion.
2.2.2 Antioxidant Administration
Although oxidative stress is widely invoked as a pathogenic mechanism for diabetes, there is limited evidence yet that antioxidant vitamin and drug supplements provide protection against progression of this disease, either in human or in animal models (Lee et al. 2004; Steinhubl 2008). In contrast, interventions to decrease substrate (glucose and/or FFAs) concentrations have demonstrated protecting effects on the risk for, and progression of, diabetes. In spite of this, there are studies showing that plasma from diabetic patients present increased levels of end-products of oxidative damage (Nourooz-Zadeh et al. 1995; Borcea et al. 1999; Daviet al. 1999). Further-more, although of short duration, several clinical trials show that vitamin E, vitamin C, or glutathione supplementation improves insulin sensitivity in insulin-resistant patients and/or patients with type 2 diabetes (Paolisso and Giugliano 1996; Evans and Goldfine 2000; Steinhubl 2008).
Altogether, we conclude from results obtained in different epidemiological stud-ies that there seems to be no advantage for antioxidant supplementation.
3 Hypertension, Metabolic Syndrome and Oxidative Stress
Cardiovascular disease is now endemic worldwide and no longer limited to economically developed countries (Ezzati et al. 2002). About 7.6 million deaths (about 13,5% of the total) and 92 million Disability Adjusted Life Years (DALYs) (6,0% of the total) worldwide were attributed to high blood pressure in 2001. High blood pressure was a major health issue in all world regions, and it accounted for more than a third of deaths and almost a fifth of DALYs in Europe and central Asia (Lawes et al. 2008).
High blood pressure (BP) values are one of the main metabolic syndrome components, and metabolic syndrome has been found in about 30–40% of hypertensives. Whether or not the presence of metabolic syndrome increases the hypertension-induced cardiovascular risk is a matter of debate. Moreover, criticisms about the existence of metabolic syndrome have been raised, and there are doubts concerning whether metabolic syndrome itself results in a higher risk than the sum effect of each of the components (Kahn et al. 2005). However, increasing evidence indicates that the clustering of metabolic and hemodynamic abnormalities characterizing the metabolic syndrome is associated with a prevalence of subclinical damage in a variety of organs, such as left ventricular hypertrophy, thickening or atherosclerotic plaques of carotid arteries, microalbuminuria and deranged renal function. This is clinically relevant since these markers of target organ damage are associated with an increased risk of cardiovascular fatal and nonfatal events. The contribution of the metabolic syndrome to target organ damage in hypertensives is presumably responsible for a substantial increase in cardiovascular fatal and nonfatal events (Cuspidi et al. 2008).
Excessive production of reactive oxygen species (ROS), which exceeds antioxidant defence mechanisms, has been implicated in patho-physiological conditions that impact on the cardiovascular system. Hypertension is considered a state of oxidative stress that can contribute to the development of atherosclerosis (Romero 1999). Assessment of antioxidant activities and lipid peroxidation by-products in hypertensives indicates an excessive amount of ROS and a reduction of antioxidant defence activities in blood, as well as in several other cellular systems, including not only vascular wall cells (Orie et al. 1999), but also circulating cells (Yasunari et al. 2002). Antihypertensive treatment attenuated the increase in oxidative stress observed in hypertensive subjects, and extend-ing treatment over time increased the beneficial impact of treatment (Saez et al. 2004).
4 Concluding Remarks
Obesity is the most common disorder in both developed and, also, developing countries and is associated by a reduction in insulin sensitivity. Furthermore, the degree of visceral adiposity conveys an independent prediction of risk beyond body mass index (BMI) for cardiovascular disease.
The molecular mechanisms involved in obesity-related insulin resistance are not well understood. However, it has been well demonstrated that adipocytes are able to synthesize and secrete several cytokines (adipokines), such as leptin, TNFα and IL-6 (Vincent and Taylor 2006) and the hypothesis proposing that these adipokines may be responsible for insulin resistance in obesity (Bastard et al. 2006), in an oxidative stress-dependent way, has emerged.
Taken together, the existence of a link among the hyperglycaemia- and FFA-induced increases in ROS and oxidative stress, activation of stress-sensitive path-ways, and the eventual development of not only the late complications of diabetes, but also insulin resistance and β-cell dysfunction seem highly plausible.
Although the understanding about mechanisms of how hyperglycaemia induces oxidative stress and, consequently, pathology has advanced considerably in last years, effective therapeutic strategies to prevent or delay damage remain limited. Additional research is urgently needed.