Synthesis of a novel radical trapping and carbonyl group trapping anti-AGE agent: a pyridoxamine analogue for inhibiting advanced glycation (AGE) and lipoxidation (ALE) end products

Protective Effects of Rifampicin and Its Analog Rifampicin Quinone in a Mouse Model of Obesity-Induced Type 2 Diabetes

Advanced glycation end-products (AGEs) form when glucose reacts non-enzymatically with proteins, leading to abnormal protein function, oxidative stress, and inflammation. AGEs are associated with aging and age-related diseases; their formation is aggravated during diabetes. Therefore, drugs preventing AGE formation can potentially treat diabetic complications, positively affecting health. Earlier, we demonstrated that rifampicin and its analogs have potent anti-glycating activities and increase the life span of Caenorhabditis elegans. This study aimed to investigate the effects of rifampicin during hyperglycemia in C. elegans and in a mouse model of obesity-induced type 2 diabetes. The effects of rifampicin were assessed by determining the life span of C. elegans cultured in the presence of glucose and by measuring HbA1c, AGE levels, and glucose excursions in the diabetic mouse model. Our results show that rifampicin protects C. elegans from glucose-induced toxicity and increases life span. In mice, rifampicin reduces HbA1c and AGEs, improves insulin sensitivity, and reduces indications of diabetic nephropathy without inducing hepatotoxicity. Rifampicin quinone, an analog with lower anti-microbial activity, also reduces HbA1c levels, improves glucose homeostasis and insulin sensitivity, and lowers indications of diabetic nephropathy, without adversely affecting the liver of the diabetic mice. Altogether, our results indicate that rifampicin and its analog have protective roles during diabetes without inflicting hepatic damage and may potentially be considered for repositioning to treat hyperglycemia-related complications in patients.

Inhibition of colitis by ring-modified analogues of 6-acetamido-2,4,5-trimethylpyridin-3-ol

6-Aminopyridin-3-ol scaffold has shown an excellent anti-inflammatory bowel disease activity. Various analogues with the scaffold were synthesized in pursuit of the diversity of side chains tethering on the C(6)-position. Structure-activity relationship among the analogues was investigated to understand the effects of the side chains and their linkers on their anti-inflammatory activities. In this study, structural modification moved beyond side chains on the C(6)-position and reached to pyridine ring itself. It expedited us to synthesize diverse ring-modified analogues of a representative pyridine-3-ol, 6-acetamido-2,4,5-trimethylpyridin-3-ol (9). In the evaluation of compounds on their inhibitory actions against TNF-α-induced adhesion of monocytic cells to colonic epithelial cells, an in vitro model mimicking colon inflammation, the effects of compounds 9, 17, and 19 were greater than tofacitinib, an orally available anti-colitis drug, and compound 17 showed the greatest activity. In addition, TNF-α-induced angiogenesis, which permits more inflammatory cell migration into inflamed tissues, was significantly blocked by compounds 17 and 19 in a concentration-dependent manner. In the comparison of in vivo therapeutic effects of compounds 9, 17, and 19 on dextran sulfate sodium (DSS)-induced colitis in mice, compound 17 was the most potent and efficacious, and compound 19 was better than compound 9 which showed a similar degree of inhibitory effect to tofacitinib. Taken together, it seems that either the trimethyl system or the hydroxyl group on the pyridinol ring is essential to the activity. This finding might become a new milestone in the development of pyridinol-based anti-inflammatory bowel disease agents.

Ferulic acid prevents methylglyoxal-induced protein glycation, DNA damage, and apoptosis in pancreatic β-cells

Methylglyoxal (MG) can react with amino acids of proteins to induce protein glycation and consequently the formation of advanced glycation end-products (AGEs). Previous studies reported that ferulic acid (FA) prevented glucose-, fructose-, and ribose-induced protein glycation. In this study, FA (0.1-1 mM) inhibited MG-induced protein glycation and oxidative protein damage in bovine serum albumin (BSA). Furthermore, FA (0.0125-0.2 mM) protected against lysine/MG-mediated oxidative DNA damage, thereby inhibiting superoxide anion and hydroxyl radical generation during lysine and MG reaction. In addition, FA did not have the ability to trap MG. Finally, FA (0.1 mM) pretreatment attenuated MG-induced decrease in cell viability and prevented MG-induced cell apoptosis in pancreatic β-cells. The results suggest that FA is capable of protecting β-cells from MG-induced cell damage during diabetes.

The role of advanced glycation end products in various types of neurodegenerative disease: a therapeutic approach

Protein glycation is initiated by a nucleophilic addition reaction between the free amino group from a protein, lipid or nucleic acid and the carbonyl group of a reducing sugar. This reaction forms a reversible Schiff base, which rearranges over a period of days to produce ketoamine or Amadori products. The Amadori products undergo dehydration and rearrangements and develop a cross-link between adjacent proteins, giving rise to protein aggregation or advanced glycation end products (AGEs). A number of studies have shown that glycation induces the formation of the β-sheet structure in β-amyloid protein, α-synuclein, transthyretin (TTR), copper-zinc superoxide dismutase 1 (Cu, Zn-SOD-1), and prion protein. Aggregation of the β-sheet structure in each case creates fibrillar structures, respectively causing Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis, familial amyloid polyneuropathy, and prion disease. It has been suggested that oligomeric species of glycated α-synuclein and prion are more toxic than fibrils. This review focuses on the pathway of AGE formation, the synthesis of different types of AGE, and the molecular mechanisms by which glycation causes various types of neurodegenerative disease. It discusses several new therapeutic approaches that have been applied to treat these devastating disorders, including the use of various synthetic and naturally occurring inhibitors. Modulation of the AGE-RAGE axis is now considered promising in the prevention of neurodegenerative diseases. Additionally, the review covers several defense enzymes and proteins in the human body that are important anti-glycating systems acting to prevent the development of neurodegenerative diseases.

Vitamin B6: beyond coenzyme functions

Endogenous reactive intermediates such as photoexcited states of tissue chromophores, reactive oxygen species (ROS), reactive carbonyl species (RCS), and transition metal ions are mediators of tissue damage involved in initiation and progression of human pathologies including tumorigenesis, atherosclerosis, diabetes, and neurodegenerative disease. A large body of evidence now suggests that B6 vitamers antagonize the harmful activity of endogenous reactive intermediates fulfilling a very different role than that established as a cofactor for numerous enzymes. In this chapter, the structural basis of vitamin B6 activity as a potent antioxidant, metal chelator, carbonyl scavenger, and photosensitizer is presented and the physiological relevance is discussed.

Naturally occurring inhibitors against the formation of advanced glycation end-products

Advanced glycation end-products (AGEs) are the final products of the non-enzymatic reaction between reducing sugars and amino groups in proteins, lipids and nucleic acids. Recently, the accumulation of AGEs in vivo has been implicated as a major pathogenic process in diabetic complications, atherosclerosis, Alzheimer's disease and normal aging. The early recognition of AGEs can ascend to the late 1960s when a non-enzymatic glycation process was found in human body which is similar to the Maillard reaction. To some extent, AGEs can be regarded as products of the Maillard reaction. This review firstly introduces the Maillard reaction, the formation process of AGEs and harmful effects of AGEs to human health. As AGEs can cause undesirable diseases or disorders, it is necessary to investigate AGE inhibitors to offer a potential therapeutic approach for the prevention of diabetic or other pathogenic complications induced by AGEs. Typical effective AGE inhibitors with different inhibition mechanisms are also reviewed in this paper. Both synthetic compounds and natural products have been evaluated as inhibitors against the formation of AGEs. However, considering toxic or side effects of synthetic molecules present in clinical trials, natural products are more promising to be developed as potent AGE inhibitors.

Preparation and investigation of vitamin B6-derived aminopyridinol antioxidants

3-Pyridinols bearing amine substitution para to the hydroxylic moiety have previously been shown to inhibit lipid peroxidation more effectively than typical phenolic antioxidants, for example, α-tocopherol. We report here high-yielding, large-scale syntheses of mono- and bicyclic aminopyridinols from pyridoxine hydrochloride (i.e., vitamin B(6)). This approach provides straightforward, scaleable access to novel, potent, molecular scaffolds whose antioxidant properties have been investigated in homogeneous solutions and in liposomal vesicles. These molecular aggregates mimic cell membranes that are the targets of oxidative damage in vivo.

Intervention strategies to inhibit protein carbonylation by lipoxidation-derived reactive carbonyls

Protein carbonylation induced by reactive carbonyl species (RCS) generated by peroxidation of polyunsaturated fatty acids plays a significant role in the etiology and/or progression of several human diseases, such as cardiovascular (e.g., atherosclerosis, long-term complications of diabetes) and neurodegenerative diseases (e.g., Alzheimer's disease, Parkinson's disease, and cerebral ischemia). Most of the biological effects of intermediate RCS, mainly alpha,beta-unsaturated aldehydes, di-aldehydes, and keto-aldehydes, are due to their capacity to react with the nucleophilic sites of proteins, forming advanced lipoxidation end-products (ALEs). Because of the emerging deleterious role of RCS/protein adducts in several human diseases, different potential therapeutic strategies have been developed in the last few years. This review sheds focus on fundamental studies on lipid-derived RCS generation, their biological effects, and their reactivity with proteins, with particular emphasis to 4-hydroxy-trans-2-nonenal (HNE)-, acrolein (ACR)-, malondialdehyde (MDA)-, and glyoxal (GO)-modified proteins. It also discusses the recently developed pharmacological approaches for the management of chronic diseases in which oxidative stress and RCS formation are massively involved. Inhibition of ALE formation, based on carbonyl-sequestering agents, seems to be the most promising pharmacological tool and is reviewed in detail.

Pyridoxamine: The Many Virtues of a Maillard Reaction Inhibitor

Pyridoxamine (PM) is one of three natural forms of vitamin B6. It is a critical transient intermediate in catalysis of transamination reactions by vitamin B6-dependent enzymes. The discovery eight years ago that PM can inhibit the Maillard reaction stimulated new interest in this B6 vitamer as a prospective pharmacological agent for treatment of complications of diabetes. PM application in diabetic nephropathy has now progressed to a phase III clinical trial. Investigation of the PM mechanism of action demonstrated that PM inhibits post-Amadori steps of the Maillard reaction by sequestering catalytic metal ions and blocking oxidative degradation of Amadori intermediate. PM also has the capacity to scavenge toxic carbonyl products of sugar and lipid degradation, and to inhibit reactive oxygen species. These multiple activities position PM as a promising drug candidate for treatment of multifactorial chronic conditions in which oxidative reactions and/or carbonyl compounds confer pathogenicity.

Paradoxical impact of antioxidants on post-Amadori glycoxidation: Counterintuitive increase in the yields of pentosidine and Nepsilon-carboxymethyllysine using a novel multifunctional pyridoxamine derivative

The inhibition of post-Amadori advanced glycation end product (AGE) formation by three different classes of AGE inhibitors, carbonyl group traps, chelators, and radical-trapping antioxidants, challenge the current paradigms that: 1) AGE inhibitors will not increase the formation of any AGE product, 2) transition metal ions are required for oxidative formation of AGE, and 3) screening AGE inhibitors only in systems containing transition metal ions represents a valid estimate of potential in vivo mechanisms. This work also introduces a novel multifunctional AGE inhibitor, 6-dimethylaminopyridoxamine (dmaPM), designed to function as a combined carbonyl trap, metal ion chelator, and radical-trapping antioxidant. Other AGE inhibitors including pyridoxamine, aminoguanidine, o-phenylenediamine, dipyridoxylamine, and diethylenetriaminepentaacetic acid were also examined. The results during uninterrupted and interrupted ribose glycations show: 1) an unexpected increase in the yield of pentosidine in the presence of radical-trapping phenolic antioxidants such as Trolox and dmaPM, 2) significant formation of Nepsilon-carboxymethyllysine (CML) in the presence of strong chelators and phenolic antioxidants, which implies that there must be nonradical routes to CML, 3) prevention of intermolecular cross-links with radical-trapping inhibitors, and 4) that dmaPM shows excellent inhibition of AGE. Glucose glycations reveal the expected inhibition of pentosidine and CML with all compounds tested, but in a buffer free of trace metal ions the yield of CML in the presence of radical-trapping antioxidants was between the metal ion-free and metal ion-containing controls. Protein molecular weight analyses support the conclusion that Amadori decomposition pathways are constrained in the presence of metal ion chelators and radical traps.