The Maillard reaction, which is generally termed nonenzymatic browning or glycation, has been implicated in accelerated aging and diabetic complications in vivo. Although the molecular basis of glycation-induced pathogenesis is not well understood, the following have been noted: (1) protein glycation leads to the formation and accumulation of toxic advanced glycation endproducts (AGEs); (2) AGEs can permanently alter the structure and function of body proteins; and (3) the interaction between AGE-modified proteins and AGE-specific receptors (RAGEs) on the cell surface induces the overproduction of reactive oxygen species (ROSs) and inflammatory mediators, which leads to cellular disorders in biological systems. To date, studies that have examined the contribution of protein glycation to disease-states have primarily focused on the deleterious effects and related mechanisms of these glycotoxins. However, it remains unknown whether phytochemicals exert protective effects against glycotoxin-induced damage. Thus, the development and investigation of AGE inhibitors, especially the natural anti-AGE agents without adverse effects, may provide a therapeutic approach for delaying and preventing premature aging and diabetic complications. In this review, we provide an outline of anti-glycation properties of foodstuffs and/or their active components, and discuss their mechanisms of action.

In the process of glycation, methylglyoxal is a reactive dicarbonyl compound physiologically generated as an intermediate of glycolysis, and is found in high levels in blood or tissue of diabetic models. Biological glycation has been commonly implicated in the development of diabetic microvascular complications of neuropathy. Increasing evidence suggests that neuronal cell cycle regulatory failure followed by apoptosis is an important mechanism in the development of diabetic neuropathy complication. Naturally occurring antioxidants, especially phenolic acids have been recommended as the major bioactive compounds to prevent chronic diseases and promote health benefits. The objective of this study was to investigate the inhibitory abilities of phenolic acids (chlorogenic acid, syringic acid and vanillic acid) on methylglyoxal-induced mouse Neuro-2A neuroblastoma (Neuro-2A) cell apoptosis in the progression of diabetic neuropathy. The data indicated that methylglyoxal induced mouse Neuro-2A neuroblastoma (Neuro-2A) cell apoptosis via alternation of mitochondria membrane potential and Bax/Bcl-2 ratio, activation of caspase-3, and cleavage of poly (ADP-ribose) polymerase. Furthermore, the results demonstrated that activation of mitogen-activated protein kinase signal pathways (JNK and p38) participated in the methylglyoxal-induced Neuro-2A cell apoptosis process. Treatment of Neuro-2A cells with phenolic acids markedly suppresses cell apoptosis induced by methylglyoxal, suggesting that phenolic acids possess cytoprotective ability in the prevention of diabetic neuropathy complication.

The current study was designed to evaluate the effects of silymarin (SM) on advanced glycation endproduct (AGE) formation and monocyte activation induced by S100b, a specific ligand of receptor for AGEs. The in vivo verification of antiglycation, antioxidant, and antiinflammatory capacities was examined by 12 weeks of SM administration in streptozotocin-diabetic rats. In vitro glycation assays demonstrated that SM exerted marked inhibition during the late stages of glycation and subsequent crosslinking. Dual action mechanisms, namely, antioxidant and reactive carbonyl trapping activities, may contribute to its antiglycation effect. SM produced a significant decrease in monocytic interleukin-1β and COX-2 levels and prevented oxidant formation caused by S100b, which appeared to be mediated by inhibition of p47phox membrane translocation. Chromatin immunoprecipitation demonstrated that S100b increased the recruitment of nuclear factor-kappaB transcription factor as well as cAMP response element-binding-binding protein and coactivator-associated arginine methyltransferase-1 cofactors to the interleukin-1β promoter, whereas these changes were inhibited with SM treatment. In vivo, SM reduced tissue AGE accumulation, tail collagen crosslinking, and concentrations of plasma glycated albumin. Levels of oxidative and inflammatory biomarkers were also significantly decreased in SM-treated groups compared with the diabetic group. These data suggest that SM supplementation may reduce the burden of AGEs in diabetics and may prevent resulting complications.

Increasing evidence has shown advanced glycation end products (AGEs) receptor ligation (RAGE) to be an important part of complex interactions of the oxidative stress and pro-inflammatory responses. In this study, flavonoids were used to monitor the protective effects against the oxidative damage and inflammation mediated by AGEs in human monocytes. S100B (RAGE ligand) treatment in human THP-1 monocytic cells (THP-1) significantly increased gene expression of the pro-inflammatory cytokines TNF-alpha and IL-1beta; chemokines MCP-1 and IP-10; adhesion factors platelet endothelial cell adhesion molecule (PECAM-1) and beta2-integrin; and pro-inflammatory cyclooxygenase-2 (COX-2). S100B treatment with quercetin and catechin in THP-1 cells had inhibitory effects on the expression of pro-inflammatory genes and protein levels. Quercetin and catechin could regulate S100B-activated oxidant stress-sensitive pathways through blocking p47phox protein expression. Treatment with quercetin and catechin could eliminate reactive oxygen species (ROS) to reduce oxidative stress stimulated by S100B in THP-1 cells. Quercetin and catechin also showed different regulatory abilities on mitogen-activated protein kinase (MAPK) signaling pathways by inhibiting protein expression in S100B-stimulated inflammatory responses in THP-1 cells. This study suggests that quercetin and catechin may be of benefit for diabetic vascular complications due to its antioxidant abilities against AGE-mediated oxidative stress through oxidative stress-sensitive and oxidative stress-responsive signaling pathways, which lead to inflammation in human monocytes.

These findings suggest that downregulation of autophagy may be one of the approaches to control the impairment of energy metabolism leading to neuronal injury in the early development of Alzheimer's disease, and hesperetin or hesperidin may be a potential agent in the preventing of Alzheimer's disease progression.