A Glyoxalase-1 Knockdown Does Not Have Major Short Term Effects on Energy Expenditure and Atherosclerosis in Mice

Glyoxalase I is a novel target for the prevention of metabolic derangement

Obesity prevalence in the US has nearly tripled since 1975 and a parallel increase in prevalence of type 2 diabetes (T2D). Obesity promotes a myriad of metabolic derangements with insulin resistance (IR) being perhaps the most responsible for the development of T2D and other related diseases such as cardiovascular disease. The precarious nature of IR development is such that it provides a valuable target for the prevention of further disease development. However, the mechanisms driving IR are numerous and complex making the development of viable interventions difficult. The development of metabolic derangement in the context of obesity promotes accumulation of reactive metabolites such as the reactive alpha-dicarbonyl methylglyoxal (MG). MG accumulation has long been appreciated as a marker of disease progression in patients with T2D as well as the development of diabetic complications. However, recent evidence suggests that the accumulation of MG occurs with obesity prior to T2D onset and may be a primary driving factor for the development of IR and T2D. Further, emerging evidence also suggests that this accumulation of MG with obesity may be a result in a loss of MG detoxifying capacity of glyoxalase I. In this review, we will discuss the evidence that posits MG accumulation because of GLO1 attenuation is a novel target mechanism of the development of metabolic derangement. In addition, we will also explore the regulation of GLO1 and the strategies that have been investigated so far to target GLO1 regulation for the prevention and treatment of metabolic derangement.

Glyoxalase 1 knockdown induces age-related β-cell dysfunction and glucose intolerance in mice

Tight control of glycemia is a major treatment goal for type 2 diabetes mellitus (T2DM). Clinical studies indicated that factors other than poor glycemic control may be important in fostering T2DM progression. Increased levels of methylglyoxal (MGO) associate with complications development, but its role in the early steps of T2DM pathogenesis has not been defined. Here, we show that MGO accumulation induces an age-dependent impairment of glucose tolerance and glucose-stimulated insulin secretion in mice knockdown for glyoxalase 1 (Glo1KD). This metabolic alteration associates with the presence of insular inflammatory infiltration (F4/80-positive staining), the islet expression of senescence markers, and higher levels of cytokines (MCP-1 and TNF-α), part of the senescence-activated secretory profile, in the pancreas from 10-month-old Glo1KD mice, compared with their WT littermates. In vitro exposure of INS832/13 β-cells to MGO confirms its casual role on β-cell dysfunction, which can be reverted by senolytic treatment. These data indicate that MGO is capable to induce early phenotypes typical of T2D progression, paving the way for novel prevention approaches to T2DM.

Downregulation of the Glo1 Gene Is Associated with Reduced Adiposity and Ectopic Fat Accumulation in Spontaneously Hypertensive Rats

Methylglyoxal (MG), a potent precursor of advanced glycation end-products (AGE), is increased in metabolic disorders such as diabetes and obesity. MG and other dicarbonyl metabolites are detoxified by the glyoxalase system in which glyoxalase 1, coded by the Glo1 gene, serves as the rate-limiting enzyme. In this study, we analyzed the effects of Glo1 downregulation on glucose and lipid metabolism parameters in spontaneously hypertensive rats (SHR) by targeting the Glo1 gene (SHR-Glo1^+/− heterozygotes). Compared to SHR wild-type animals, SHR-Glo1^+/− rats showed significantly reduced Glo1 expression and lower GLO1 activity in tissues associated with increased MG levels. In contrast to SHR controls, SHR-Glo1^+/− rats exhibited lower relative weight of epididymal fat, reduced ectopic fat accumulation in the liver and heart, and decreased serum triglycerides. In addition, compared to controls, SHR-Glo1^+/− rats showed reduced serum insulin and increased basal and insulin stimulated incorporation of glucose into white adipose tissue lipids (lipogenesis). Reduced ectopic fat accumulation in the heart was associated with significantly increased pAMPK/AMPK ratio and GLUT4 activity. These results provide evidence that Glo1 downregulation in SHR is associated with reduced adiposity and ectopic fat accumulation, most likely mediated by AMPK activation in the heart.

The Glyoxalase System-New Insights into an Ancient Metabolism

The glyoxalase system was discovered over a hundred years ago and since then it has been claimed to provide the role of an indispensable enzyme system in order to protect cells from a toxic byproduct of glycolysis. This review gives a broad overview of what has been postulated in the last 30 years of glyoxalase research, but within this context it also challenges the concept that the glyoxalase system is an exclusive tool of detoxification and that its substrate, methylglyoxal, is solely a detrimental burden for every living cell due to its toxicity. An overview of consequences of a complete loss of the glyoxalase system in various model organisms is presented with an emphasis on the role of alternative detoxification pathways of methylglyoxal. Furthermore, this review focuses on the overlooked posttranslational modification of Glyoxalase 1 and its possible implications for cellular maintenance under various (patho-)physiological conditions. As a final note, an intriguing point of view for the substrate methylglyoxal is offered, the concept of methylglyoxal (MG)-mediated hormesis.

Inhibitors of Advanced Glycation End Product (AGE) Formation and Accumulation

A range of chemically different compounds are known to inhibit the formation and accumulation of advanced glycation end products (AGEs) or disrupt associated signalling pathways. There is evidence that some of these agents can provide end-organ protection in chronic diseases including diabetes. Whilst this group of therapeutics are structurally and functionally different and have a range of mechanisms of action, they ultimately reduce the deleterious actions and the tissue burden of advanced glycation end products. To date it remains unclear if this is due to the reduction in tissue AGE levels per se or the modulation of downstream signal pathways. Some of these agents either stimulate antioxidant defence or reduce the formation of reactive oxygen species (ROS), modify lipid profiles and inhibit inflammation. A number of existing treatments for glucose lowering, hypertension and hyperlipidaemia are also known to reduce AGE formation as a by-product of their action. Targeted AGE formation inhibitors or AGE cross-link breakers have been developed and have shown beneficial effects in animal models of diabetic complications as well as other chronic conditions. However, only a few of these agents have progressed to clinical development. The failure of clinical translation highlights the importance of further investigation of the advanced glycation pathway, the diverse actions of agents which interfere with AGE formation, cross-linking or AGE receptor activation and their effect on the development and progression of chronic diseases including diabetic complications. Advanced glycation end products (AGEs) are (1) proteins or lipids that become glycated as a result of exposure to sugars or (2) non-proteinaceous oxidised lipids. They are implicated in ageing and the development, or worsening, of many degenerative diseases, such as diabetes, atherosclerosis, chronic kidney and Alzheimer's disease. Several antihypertensive and antidiabetic agents and statins also indirectly lower AGEs. Direct AGE inhibitors currently investigated include pyridoxamine and epalrestat, the inhibition of the formation of reactive dicarbonyls such as methylglyoxal as an important precursor of AGEs via increased activation of the detoxifying enzyme Glo-1 and inhibitors of NOX-derived ROS to reduce the AGE/RAGE signalling.

GeneFishing to reconstruct context specific portraits of biological processes

Rapid advances in genomic technologies have led to a wealth of diverse data, from which novel discoveries can be gleaned through the application of robust statistical and computational methods. Here, we describe GeneFishing, a semisupervised computational approach to reconstruct context-specific portraits of biological processes by leveraging gene-gene coexpression information. GeneFishing incorporates multiple high-dimensional statistical ideas, including dimensionality reduction, clustering, subsampling, and results aggregation, to produce robust results. To illustrate the power of our method, we applied it using 21 genes involved in cholesterol metabolism as "bait" to "fish out" (or identify) genes not previously identified as being connected to cholesterol metabolism. Using simulation and real datasets, we found that the results obtained through GeneFishing were more interesting for our study than those provided by related gene prioritization methods. In particular, application of GeneFishing to the GTEx liver RNA sequencing (RNAseq) data not only reidentified many known cholesterol-related genes, but also pointed to glyoxalase I (GLO1) as a gene implicated in cholesterol metabolism. In a follow-up experiment, we found that GLO1 knockdown in human hepatoma cell lines increased levels of cellular cholesterol ester, validating a role for GLO1 in cholesterol metabolism. In addition, we performed pantissue analysis by applying GeneFishing on various tissues and identified many potential tissue-specific cholesterol metabolism-related genes. GeneFishing appears to be a powerful tool for identifying related components of complex biological systems and may be used across a wide range of applications.

Inhibition of Glyoxalase-I Leads to Reduced Proliferation, Migration and Colony Formation, and Enhanced Susceptibility to Sorafenib in Hepatocellular Carcinoma

Background: Glyoxalase-I (Glo-I) is essential for detoxification of methylglyoxal (MGO), a byproduct of glycolysis. Overexpression of Glo-I has been linked to multi-drug resistance in cancer therapy. The aim of this study was to analyze Glo-I in hepatocellular carcinoma (HCC) and the effect of the multi-tyrosine kinase inhibitor sorafenib on Glo-I. Methods: Expression and specific activity of Glo-I was measured in human HCC samples, HCC-cell lines (HepG2, Huh7) and a hepatocyte cell line (AML 12). Cells were either treated with Glo-I inhibitors, ethyl pyruvate (EP, 1-20 mM) and BrBzGSHCp2 (1-10 μM), or sorafenib (2.5-10 μM) and protein expression (Western Blot), proliferation (WST-assay), migration (scratch assay), and colony formation (clonogenic assay) were assessed. Results: High expression of Glo-I was detected in human HCC tissue samples. Huh7 showed highest expression and activity of Glo-I and revealed highest proliferation compared to AML 12 and HepG2. Targeting Glo-I by EP or BrBzGSHCp₂ led to significantly reduced proliferation (20 mM EP 24 h: 57 ± 12%), migration and colony formation. Glo-I inhibition by 20 mM EP resulted in reduced expression of PDGFR-β (18 ± 10%), VEGFR2 (46 ± 11%), VEGF (61 ± 10%), pERK/ERK (62 ± 6%), NF-κB (44 ± 12%) as well as stimulation of Nrf2 (243 ± 36%). Similar results were seen with BrBzGSHCp2. Sorafenib treatment revealed elevation of Glo-I (10 μM: 209 ± 25%) and MGO. Co-treatment of EP and sorafenib led to an additional reduction of proliferation compared to sorafenib alone. Conclusion: Glo-I is positively correlated with HCC proliferation. Inhibition of Glo-I reduced proliferation, migration, and colony formation. In turn, sorafenib increases Glo-I. Co-treatment using Glo-I inhibitors could enhance susceptibility of HCC to sorafenib.

Compensatory mechanisms for methylglyoxal detoxification in experimental & clinical diabetes

OBJECTIVES

The deficit of Glyoxalase I (Glo1) and the subsequent increase in methylglyoxal (MG) has been reported to be one the five mechanisms by which hyperglycemia causes diabetic late complications. Aldo-keto reductases (AKR) have been shown to metabolize MG; however, the relative contribution of this superfamily to the detoxification of MG in vivo, particularly within the diabetic state, remains unknown.

METHODS

CRISPR/Cas9-mediated genome editing was used to generate a Glo1 knock-out (Glo1-/-) mouse line. Streptozotocin was then applied to investigate metabolic changes under hyperglycemic conditions.

RESULTS

Glo1-/- mice were viable and showed no elevated MG or MG-H1 levels under hyperglycemic conditions. It was subsequently found that the enzymatic efficiency of various oxidoreductases in the liver and kidney towards MG were increased in the Glo1-/- mice. The functional relevance of this was supported by the altered distribution of alternative detoxification products. Furthermore, it was shown that MG-dependent AKR activity is a potentially clinical relevant pathway in human patients suffering from diabetes.

CONCLUSIONS

These data suggest that in the absence of GLO1, AKR can effectively compensate to prevent the accumulation of MG. The combination of metabolic, enzymatic, and genetic factors, therefore, may provide a better means of identifying patients who are at risk for the development of late complications caused by elevated levels of MG.

Methylglyoxal-induced dicarbonyl stress in aging and disease: first steps towards glyoxalase 1-based treatments

Dicarbonyl stress is the abnormal accumulation of dicarbonyl metabolites leading to increased protein and DNA modification contributing to cell and tissue dysfunction in aging and disease. It is produced by increased formation and/or decreased metabolism of dicarbonyl metabolites. MG (methylglyoxal) is a dicarbonyl metabolite of relatively high flux of formation and precursor of the most quantitatively and functionally important spontaneous modifications of protein and DNA clinically. Major MG-derived adducts are arginine-derived hydroimidazolones of protein and deoxyguanosine-derived imidazopurinones of DNA. These are formed non-oxidatively. The glyoxalase system provides an efficient and essential basal and stress-response-inducible enzymatic defence against dicarbonyl stress by the reduced glutathione-dependent metabolism of methylglyoxal by glyoxalase 1. The GLO1 gene encoding glyoxalase 1 has low prevalence duplication and high prevalence amplification in some tumours. Dicarbonyl stress contributes to aging, disease and activity of cytotoxic chemotherapeutic agents. It is found at a low, moderate and severe level in obesity, diabetes and renal failure respectively, where it contributes to the development of metabolic and vascular complications. Increased glyoxalase 1 expression confers multidrug resistance to cancer chemotherapy and has relatively high prevalence in liver, lung and breast cancers. Studies of dicarbonyl stress are providing improved understanding of aging and disease and the basis for rational design of novel pharmaceuticals: glyoxalase 1 inducers for obesity, diabetes and cardiovascular disease and glyoxalase 1 inhibitors for multidrug-resistant tumours. The first clinical trial of a glyoxalase 1 inducer in overweight and obese subjects showed improved glycaemic control, insulin resistance and vascular function.

Cellular mechanisms and consequences of glycation in atherosclerosis and obesity

Post-translational modification of proteins imparts diversity to protein functions. The process of glycation represents a complex set of pathways that mediates advanced glycation endproduct (AGE) formation, detoxification, intracellular disposition, extracellular release, and induction of signal transduction. These processes modulate the response to hyperglycemia, obesity, aging, inflammation, and renal failure, in which AGE formation and accumulation is facilitated. It has been shown that endogenous anti-AGE protective mechanisms are thwarted in chronic disease, thereby amplifying accumulation and detrimental cellular actions of these species. Atop these considerations, receptor for advanced glycation endproducts (RAGE)-mediated pathways downregulate expression and activity of the key anti-AGE detoxification enzyme, glyoxalase-1 (GLO1), thereby setting in motion an interminable feed-forward loop in which AGE-mediated cellular perturbation is not readily extinguished. In this review, we consider recent work in the field highlighting roles for glycation in obesity and atherosclerosis and discuss emerging strategies to block the adverse consequences of AGEs. This article is part of a Special Issue entitled: The role of post-translational protein modifications on heart and vascular metabolism edited by Jason R.B. Dyck & Jan F.C. Glatz.