An oxidative stress paradox: time for a conceptual change?
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Oxidative stress has long been considered a key driving factor of many obesity-related health problems. However, recent work by Merry, Tran et al (Diabetologia DOI 10.1007/s00125-016-4084-3) challenges this idea with an interesting study using a hepatocyte-specific Gpx1-knockout (HGKO) mouse. GPX1 is an important detoxification enzyme that converts H2O2 to water. The authors found that high-fat diet-fed HGKO mice were more insulin sensitive than wildtype controls, despite elevated hepatic levels of H2O2 and evidence of increased systemic oxidative stress. When challenged with a non-alcoholic steatohepatitis (NASH)-inducing diet, HGKO mice were also protected, displaying reduced levels of inflammation and fibrosis with similar levels of steatosis compared with controls. These findings call into question the role of reactive oxygen species in NASH pathogenesis and highlight a potential paradox whereby increased H2O2 may be beneficial in some contexts.
KeywordsGlutathione peroxidase Insulin resistance Liver Non-alcoholic fatty liver disease Non-alcoholic steatohepatitis Oxidative stress Reactive oxygen species
Non-alcoholic fatty liver disease
Reactive oxygen species
Oxidative stress has long been considered an important factor driving obesity-related insulin resistance and its pathophysiological consequences. In this issue of Diabetologia, a study from Merry, Tran, et al  refutes this over-simplistic idea and highlights the beneficial effects of reduced antioxidant capacity through the hepatic loss of glutathione peroxidase (GPX)1. The authors show that despite elevated H2O2 levels, hepatocyte-specific Gpx1-knockout (HGKO) mice were more insulin sensitive and had better whole body glucose metabolism. These effects on hepatic insulin signalling were also present when mice were challenged with a high-fat diet. Though insulin sensitivity was not specifically assessed, HGKO mice on the choline-deficient amino-acid-defined (CDAA) diet (which induces a non-alcoholic steatohepatitis [NASH]-like pathology), displayed reduced severity of inflammation and fibrosis, without influencing steatosis. These findings run contrary to the prevailing dogma that H2O2 overproduction is an obligate precursor to insulin resistance, NASH and other pathologies.
Reactive oxygen species (ROS) are produced as part of normal cellular function. Superoxide anion (O2•−), the most potent ROS compound, has several cellular sources. It is a natural byproduct of the electron transport chain in mitochondria, as part of glucose or fatty acid oxidation, and is also produced by membrane-associated oxidases, such as NADPH oxidase, as well as by cytosolic endoplasmic reticulum and peroxisomal oxidases. Normally, superoxide is converted to the less reactive H2O2 by the superoxide dismutase (SOD) enzymes. H2O2 can then be converted to water by a number of enzymes, including GPXs. These enzymes use glutathione to convert H2O2 to water, thereby producing oxidised glutathione (GSSG). H2O2 is also generated as part of the normal signal transduction of many hormones, notably insulin itself , as well as cytokines such as IL-6, TNFα and others . It is currently unknown whether there are specific mechanisms for handling metabolic vs signalling-induced ROS, or how these pathways might interact, but it can be postulated that the origin of ROS may differentially modulate physiological vs pathological pathways.
In diseased states, the combination of continuous oxidation of abundant energy substrates and dysregulation of many signalling pathways is thought to overwhelm the antioxidant system, resulting in oxidative stress . Excessive ROS can drive cellular dysfunction through direct (i.e. signalling) and indirect (e.g. protein, lipid or DNA modification) mechanisms. Evidence in support of this hypothesis comes from numerous association studies in obese humans, demonstrating positive correlations between systemic markers of oxidative stress and BMI . Moreover, ROS production has been demonstrated to play a key role in mediating TNFα and glucocorticoid-induced insulin resistance in adipocytes . Evidence of oxidative stress is also present in more advanced stages of metabolic disease, for example in NASH-affected livers  and in the vascular wall in atherosclerosis . It is not surprising, then, that the theory of oxidative stress-induced metabolic deterioration has gained wide attention.
One central question related to the H2O2-metabolising GPXs is whether their function is more important in normal physiological states or in the protection against oxidative stress in disease. GPX1 and GPX4 are the major isoforms in liver, with GPX1 accounting for nearly all cytosolic and mitochondrial GPX activity . While Gpx4-knockout mice are not viable, global Gpx1-knockout mice are viable and are less susceptible to high-fat diet-induced insulin resistance . This effect was attributed to enhanced insulin-stimulated glucose uptake in the muscle of knockout animals. However, the nature of the global knockout makes it difficult to determine the various organ level effects, and how localised oxidative stress may affect systemic metabolism.
To address this question, the authors generated an HGKO mouse and assessed variables of hepatic and systemic glucose metabolism. Under normal physiological conditions, HGKO mice showed no evidence of systemic or hepatic oxidative stress and were moderately more insulin sensitive. In clamp studies, HGKO mice had a higher glucose infusion rate and a trend towards higher disappearance rate, together suggesting enhanced peripheral insulin sensitivity and glucose uptake. In conjunction with reduced hepatic gluconeogenic gene expression, this led to reduced fasting glucose and insulin levels in the HGKO mice, compared with controls. The authors went on to demonstrate that insulin-induced insulin receptor and Akt phosphorylation were increased and that these increases were potentiated for a longer period of time in livers of HGKO mice, compared with controls. When challenged with a high-fat diet, HGKO showed elevated levels of systemic oxidative stress as well as increased hepatic H2O2. Despite this, high-fat diet-fed HGKO mice displayed many of these same features of enhanced insulin sensitivity when compared with high-fat-fed controls. Overall, these findings suggest that hepatic GPX1 is more important in the modulation of insulin signalling, rather than in the response to metabolic stress.
How can we begin to reconcile these apparently paradoxical findings? Data from both global and liver specific knockout models suggest that GPX1 acts primarily to attenuate insulin signalling. Indeed, the authors show that the absence of GPX1 leads to enhanced oxidation (and thus inactivation) of protein phosphatases that would otherwise suppress the insulin (and likely other) signalling cascades. There is precedent for such a mechanism. Studies performed in adipose tissue have also demonstrated beneficial (and necessary) effects of enhanced H2O2 production . It is also important to consider that compared with O2•− and HO•−, H2O2 is much less oxidative. The present study suggests that the more active ROS may be more relevant in disease-associated oxidative stress.
What is next in the debate on the role of oxidative stress in driving metabolic disease? In the present study, the authors have explored relatively short-term interventions. It will be interesting to evaluate the long-term effects of GPX1-deficiency on ageing processes, where prolonged exposure to H2O2 may become detrimental. Indeed, diseases like type 2 diabetes, atherosclerosis and NASH evolve over the course of years or decades in humans. Additionally, the role of oxidative stress in driving NASH pathology (especially fibrosis) is much less proven in the CDAA model. By contrast, the strongly fibrogenic CCl4 model drives fibrogenesis as a result of ROS production during its metabolism by cytochrome P450 2E1 (CYP2E1). Future studies are thus needed to determine whether the protection of HGKO mice from advanced NASH pathology indeed arises from their enhanced insulin sensitivity; this would be an important advance in our understanding of the connection between type 2 diabetes and NAFLD. In the meantime, this elegant study again warns against over-simplistic models of the role of oxidative stress in metabolic disease and provides a potential explanation for the failure of non-specific antioxidant strategies against metabolic diseases.
B. Staels is a member of the Institut Universitaire de France.
This work was supported by grants from the European Genomic Institute for Diabetes (ANR-10-LABX-46) and the European Commission (RESOLVE, Contract FP7-305707). JTH is supported by an EMBO Long Term Fellowship.
Duality of Interest
The authors declare that there is no duality of interest associated with this manuscript.
JTH and BS wrote and edited this manuscript and accompanying figure and are the sole contributors to this paper.