Role of Advanced Glycation End Products in the Progression of Diabetes Mellitus

Diabetes Mellitus (DM) has become a world problem that seriously affected quality of life in concerned population; however, studies concerning its etiology and therapeutics are not so satisfactory. Hyperglycemia and oxidative stress damage are two hallmarks that aggravate the progression of each other. During this process, there will generate amounts of by-products, among which advanced glycation end products (AGEs) have been demonstrated to play a pivotal role in promoting the beginning and progression of DM. AGEs may interact with its receptor named RAGE and induce a series of pathological effects, such as oxidative stress, apoptosis, and infl ammation etc., and form the so-called “hyperglycemia memory”. This article aims to review the pivotal role of AGEs in the progression of DM.. Review Article Role of Advanced Glycation End Products in the Progression of Diabetes Mellitus Hengli Guo1 and Youhua Xu1,2* 1Faculty of Chinese Medicine, Macau University of Science and Technology, Avenida Wai Long, Taipa, Macao. 2State Key Laboratory of Quality Research in Chinese Medicine (Macau University of Science and Technology), Avenida Wai Long, Taipa, Macao. Dates: Received: 08 February, 2017; Accepted: 21 February, 2017; Published: 22 February, 2017 *Corresponding author: Dr. Youhua Xu, Faculty of Chinese Medicine, Macau University of Science and Technology, Avenida Wai Long, Taipa, Macao, Tel: +853-88972452; Fax: +853-28825123; Email:


Introduction
Due to the population aging, urbanization and associated lifestyle changes, the incidence of diabetes worldwide is dramatically increased [1]. It is reported that the number of diabetic population has doubled over the past three decades [2] and this number will increase to 591.9 million by 2035 [3].
Diabetes is mainly composed of two types, namely type 1 and 2 diabetes mellitus (T1DM and T2DM), and T2DM shares more than 90% of diabetic population [1]. Previously, most cases of T2DM were observed in developed rather than developing countries; for instance, the prevalence of T2DM was less than 1% in China in 1980 [4]. However, this trend is completely changed at present. It is reported that 80% cases of diabetes worldwide live in developing countries or areas [5], and Asia has become a "diabetes epicenter" in the world due to its rapid economic development and western food popularization [4]. More importantly, the proportion of young cases with T2DM is higher in developing rather than in developed countries [5], strongly suggesting the association between lifestyle change and the risk of occurring with T2DM. This suggestion is supported by another fact that the global epidemic of T2DM is positively related with overweight and obesity. According to a published report, the global prevalence of overweight or obesity may reach to 57.8% in 2030 [6]. As overweight or obesity are important risk factors of T2DM [7,8], diet-associated factors may signifi cantly exacerbate the prevalence of DM.
A range of metabolic abnormalities, in addition to hyperglycemia, are seen in the diabetes. However, it is obvious from studies in diabetic patients that glucose is the predominant metabolic abnormality and one of the major systemic risk factors in diabetes [9].
Besides hyperglycemia, Oxidative Stress (OS) is widely recognized by investigators as another hallmark and key component in the development of diabetes [10]. It is demonstrated that hyperglycemia itself can contribute to OS by forming glycation products that propagate free radicals and enlarge oxidative damage [10]. Moreover, OS itself can induce free radical damage to the DNA and fi nally promote cell apoptosis [11]; furthermore, OS may directly and indirectly up-regulate infl ammatory proteins expression and exaggerate diabetic infl ammation.
Under hyperglycemia and OS settings, a series of prodiabetic factors will be generated, such as pro-infl ammatory cytokines and advanced glycation end products (AGEs). Within this decade, more and more studies elucidated the pivotal role of AGEs in the occurrence and progression of DM. This article aims to review the role of AGEs in the development of DM.

The generation and accumulation of ages
As have been well demonstrated, level of AGEs in the body is increased during aging and may be accelerated under pathophysiological conditions such as hyperglycemia.
Concerning the generation process of AGEs, there are mainly two sources responsible for the accumulation of AGEs in (an inhibitor of AGE formation) or AGE-albumin, scholars found in streptozotocin-induced diabetic rats that AGEs- Reducing sugars including glucose, fructose, and trioses can react non-enzymatically with the amino groups of proteins to form reversible Schiff bases and then Amadori products. These early glycation products undergo further complex reactions such as rearrangement, dehydration and condensation to become irreversibly cross-linked heterogeneous fl uorescent derivatives termed advanced glycation end products (AGEs) [26], and scholars defi ned this AGEs formation process as "Maillard reaction".

Role of age in DM
In the year 1980s, Brownlee and colleagues [26,42,43] fi rstly described the harmful consequences of AGE formation on the cardiovascular and renal systems in human and diabetic rats. Studies have confi rmed that AGE cross-links accumulate in diabetic patients and animals and will cause a series of diabetic complications [44].
As discussed above, AGEs cross-links are permanent and irreversible complexes formed when glucoses bind to the target proteins. A bad news is that these target proteins are usually housekeeping proteins including collagen and elastin, which play an integral role in the maintenance of tissue integrity However, hyperglycemia-promoted over-generation of ROS will accelerate the progression of diabetic complications [9].
Several lines of evidence demonstrate that oxidative stress (OS) or ROS are involved in the pathogenesis of diabetes [10].
On the other hand, it is found that the antioxidant capacity It is found that Schiffbase products and Amadori products themselves cause ROS production [61], while Nitric Oxide (NO) donors can scavenge free radicals and inhibit AGEs formation [62]. Moreover, there is a signifi cant correlation between the levels of AGEs and antioxidant enzymes, such as glutathione peroxidase and Cu/ Zn-superoxide dismutase, in uremic plasma [63]. Studies found AGE accumulation itself is a source of OS. In hyperglycemic environments, glucose can undergo auto-oxidation and generate OH radicals [61,64]. Besides that, Quagliaro and colleagues [65] observed that AGEs can stimulate production of superoxide within the cells, which may induce free radical damage to the DNA and fi nally cause cell apoptosis.

Oxidative
Stress and Hyperglycemia Memory: Hyperglycemia is a hallmark of DM and hyperglycemia itself can promote formation of reactive oxygen species (ROS), which can interact with both DNA and proteins, and cause damage.
It is found that the mitochondrial DNA may be an especially relevant target of hyperglycemia-promoted damage [66]. Xie et al. [66] found ROS mediated cellular damage is a form of pathologic "memory" in the microvasculature that persists Hyperglycemic memory is fi rstly demonstrated by study in dogs. It is reported in diabetic dogs that euglycemia could not inhibit the progression of diabetic microvascular dysfunction, thus scholars named this phenomenon as "hyperglycemic memory" [70][71][72]. In fact, this phenomenon also exists in human beings [73]. Two possible mechanisms are proposed that responsible for this phenomenon. One is AGEs and the other is OS. As discussed above, once AGEs are formed, they are hardly degraded. So AGEs may continuously damage blood vessels even blood glucose is lowered. On the other hand, the generation of AGEs will cause oxidative stress and oxidative stress will further promote AGEs-induced tissue damage [74][75][76][77]. in ROS production has been shown to cause cell apoptosis [78]. There is a study suggests that NADPH oxidase plays an important role in AGE-RAGE dependent ROS generation and gene activation [79]. The role of RAGE in the progression of DM will be discussed hereinafter this review.

Chronic low grade infl ammation
The cytokine/adipokine profi les of Mexican Americans with diabetes suggest an association between low-grade infl ammation and quality of glucose control [80]. It has been well demonstrated that diabetes as whole was strongly associated with elevated levels of IL-6, leptin, CRP and TNF-, whereas worsening of glucose control was positively and linearly associated with high levels of IL-6, and leptin. Thus, low-grade infl ammation is also believed as a hallmark of diabetes. Previous studies have demonstrated the role of AGEs in driving infl ammatory response in macrophages [81].

AGEs-mediated Infl ammatory Cell Infi ltration:
Clinical studies have found signifi cant increase of AGEs accumulation in vascular tissues [82], which may induce monocyte migration across an endothelial cell monolayer [83] and promote expression of infl ammatory cytokines such as interleukin (IL)- 1 and TNF- mRNA [84]. In fact, scholars have observed AGEs to be present in the plasma and to accumulate in tissues at an accelerated rate in diabetes [26,85].
AGEs also increase permeability and expression of procoagulant activity in cultured endothelium, and induce migration of mononuclear phagocytes, as well as production of platelet-derived growth factor and cytokines [86]. Schmidt et al. [87] observed that levels of AGEs and receptor for AGEs (RAGE) are increased in streptozotocin (STZ)-treated (diabetic) apolipoprotein (apo) E-null mice that have advanced atherosclerosis by 14 weeks of age. Basta and colleagues [88] further found that AGEs located on proteins, in addition to immobilized AGEs on the subendothelium, bound RAGE on the endothelium and induced hyperpermeability in diabetes. These

MCP-1
Both hyperglycemia and the accumulation of AGEs can promote MCP-1 production [91,92]. An additional infl uence that may induce synthesis of MCP-1 is the generation of ROS [93]. Previous reports have demonstrated that NF-B was involved in high glucose-induced production of MCP-1 [94], it can be explained as MCP-1 promoter and enhancer regions contain NF-B binding sequences [95]. Kislinger and colleagues investigated neuronal-associated vessels and found that RAGE is localized with its putative ligand N-epsilon-

ICAM-1
Besides MCP-1 as discussed above, intercellular adhesion molecule (ICAM)-1, a 90-kD cell-surface protein known as CD54 which possess fi ve immunoglobulin-like domains, is also a major molecule involves in promoting leukocyte infi ltration [92,96]. Increased ICAM-1 expression is seen in models of type 1 [97] and type 2 diabetic nephropathy [98] in parallel with disease progression [99]. In patients with diabetes, the soluble form of ICAM-1 (sICAM-1) was observed to be elevated [100], and scholars regarded it as a powerful independent predictor of T2DM and diabetic-associated cardiovascular disease [101].
It is recognized that ICAM-1 may be even more important in promoting nephropathy associated with T2DM because its expression is not only induced by factors common to both types of diabetes, such as hyperglycemia [102], AGEs [103], hyperfi ltration [104], and OS [105], but it can also be increased by additional elements characteristic of T2DM, including hyperlipidemia [106], hyperinsulinemia [107], and elevated levels of circulating TNF- [108], which are associated with obesity [109]. In fact, the up-regulation of ICAM-1 can be observed soon after the induction of diabetes in streptozotocin (STZ)-induced diabetic rats [92,110].
Although it is still controversial whether hyperglycemia (3) AGEs enhance the expression of cell adhesion molecules, including ICAM-1 on vascular endothelial cells [113,114]; (4) Shear stress could also stimulate the induction of ICAM-1 [115].
Reports indicated [97,116] that shear stress, which is increased apoptosis [121]. And another pathway involves PI3K-Akt. Akt is a cell survival factor. Growth factors or extracellular signals lead to the activation of phosphoinositide kinase, which results in Akt phosphorylation and activation leading to cell proliferation. Therefore, Akt activation by phosphorylation is an antiapoptotic signal that may protect cells from programmed cell death and promote survival [122]. Further studies found that Forkhead transcription factors (FOXOs) may participate in Akt-mediated cell survival. FOXOs, including FOXO1, FOXO3, and FOXO4, are a family of proteins that function as sensors of signaling pathways and modulate apoptosis, cell cycle, and metabolism through regulation of gene expression [123]. Akt regulates FOXO activity by inducing a prompt and sustained nuclear exclusion. There is broad consensus on the fact that Akt-dependent phosphorylation is crucial to the regulation of FOXO function. Kops et al. [124] observed activated-Akt phosphorylates and inactivates FOXO proteins. In the absence of Akt inhibition, FOXO is translocated to the nucleus leading to gene activation. FOXO transcription factors activate three major groups of genes: anti-oxidant genes, cell cycle arrest genes, and apoptotic genes [125]. It is reported that AGEs activate FOXO4 leading to apoptosis in podocytes [121] and FOXO1 leading to apoptosis in fi broblasts [78]. Further studies found the activation of JNK also plays a role in in FOXO activity [126,127].

Receptors for age
Mechanisms involved in the pathophysiological role of AGEs in diseases mainly lies in two aspects: (1) oxidative stress damage, which is directly induced by AGEs and has been discussed above; and (2) receptor mediated injury.
The AGE-receptor system can be divided into two arms: one is associated with increased OS, growth, and infl ammatory effects, best represented by RAGE [103]; and the other, involved in the clearance and possible detoxifi cation of AGEs [133], includes transduce cellular signals after engagement by AGEs [133]. In this sense, although several types of AGEs receptors exist, only RAGE is demonstrated to mediate its harmful effects [40].

Structure of RAGE
The receptor for AGEs (RAGE) is the fi rst one that described as a receptor for the products of nonenzymatic glycation and oxidation of proteins. Thereafter, besides AGEs, amounts of natural ligands of RAGE are found, including high mobility group box 1 (HMGB1), S100/calgranulins, Mac-1, phosphatidylserine and amyloid (A) can also serve as RAGE ligands [134,135]. Like AGEs, an increased level of RAGE had been found in cardiac and renal tissue in diabetes [136].
RAGE is a member of the immunoglobulin superfamily [137]. The human RAGE gene is on Chromosome 6 in the major histocompatibility complex between genes for class II and class III. RAGE has a single transmembrane domain followed by a highly charged 43-amino acid cytosolic tail [138].
Specifi cally, it has a 332-amino acid extracellular component, consisting of two "C"-type domains preceded by one "V"-type immunoglobulin-like domain. The V domain in the N-terminus has two Nglycosylation sites and is responsible for most (but not all) extracellular ligand binding [139], and the cytoplasmic tail is believed to be essential for intracellular signaling, possibly binding to diaphanous-1 to mediate cellular migration [140]. During homeostasis, most of the tissues express a basal level of RAGE [136]. As a member of the immunoglobulin (Ig) superfamily protein, RAGE is low expressed in normal tissue and vasculature [64] and this basal level of RAGE expression involves in embryonic growth and development [141], cellular proliferation and survival [142] and the activation of various signaling events [143].
Tissues do not express signifi cant amounts of RAGE under physiological conditions but can be induced to express RAGE in situations where either ligand accumulate and/or various transcription factors regulating RAGE are activated [144]. AGEs activation of RAGE is found in diabetes, neurodegeneration, and aging [145]. Collison and colleagues [146] demonstrated that RAGE will bind to AGE-modifi ed albumin but not nonglycated albumin. In the diabetic vasculature, cells expressing high levels of RAGE are often proximal to or colocalized in areas where AGEs are abundant [147].

Oxidative stress promoted RAGE expression
RAGE expression is found to be elevated in high-OS states including diabetes [147], which implies a sustained ROS and AGEs generation, resulting in RAGE activation, and so on. This cycle could conceivably alter the cell's phenotype, obscuring other receptor properties or depleting cellular antioxidant systems [148]. Furthermore, the activation of RAGE has been demonstrated to engage critical signaling pathways linked to pro-infl ammatory responses, resulting in activation of various infl ammatory genes [149].

Role of RAGE in diabetic-infl ammation
Experimental evidence and observation strongly suggest that RAGE signaling results in profound infl ammation [150].  130,151]. One mechanism underlying this observation probably is the presence of at least two functional binding elements for NF-B in the promoter of the gene encoding RAGE [152,153]. In diabetic apolipoprotein E (apo E) defi cient mice, RAGE signaling mediates prolonged vascular infl ammation, and enhances the expression of vascular cell adhesion molecule (VCAM)-1 and tissue factor, leading to an exacerbation of the infl ammatory state [154]. In addition, RAGE participates in diabetes associated atherosclerosis [155].
The role of RAGE as an infl ammatory mediator is further demonstrated by application of RAGE antibodies or genetic knock-out methods. A report found that soluble RAGE (sRAGE) may play as a direct inhibitor of leukocyte recruitment [156].
By functioning as a co-or counter-receptor of the adhesion molecules, RAGE facilitates the recruitment of leukocytes to the injured vascular tissues. Amounts of reported studies have demonstrated that ligand-activated RAGE serves as an adhesion receptor that interacts with integrins and facilitates the recruitment of proinfl ammatory leukocytes to the sites of infl ammation, and further enhancing the infl ammatory state [157]. Some scholars also recognized RAGE plays as a counterreceptor for leukocyte integrins and directly contributs to the recruitment of infl ammatory cells in vivo and in vitro. Pullerits et al. [158] reported that RAGE acts as an endothelial adhesion receptor that mediates interactions with the 2 integrin Mac-1. Evidence suggests neutrophils and myelomonocytic cells adhere to immobilized RAGE or RAGE-transfected cells, and this interaction is attributed to Mac-1 interactions [159]. Such interactions are augmented by the addition of S100B ligand. Hence, this RAGE-Mac-1-dependent leukocyte recruitment may be involved in the ICAM-1-independent leukocyte transmigration as observed in the ICAM-1 defi cient mice [160]. But some scholars believe that RAGE may serve as an "indirect" promoter in infl ammatory cell recruitment because RAGE mediated cellular activation and upregulation of adhesion molecules and proinfl ammatory factors [156].
Whatever, it seems that RAGE plays a signifi cant role in the process of infl ammatory cell adhesion and infi ltration. Studies demonstrated that RAGE is highly expressed in macrophages, T lymphocytes, and B lymphocytes [161], and this expression may contribute to the infl ammatory mechanisms.
Another impact of RAGE over-expression on diabetic infl ammation is that the activation of RAGE by AGEs increases endothelial permeability to macromolecules [152]. Basta and colleagues [88] found that AGEs located on proteins, in addition to immobilized AGEs on the subendothelium, bind RAGE on the endothelium and induce hyperpermeability in diabetes. This fi nding is supported by another report than administration of sRAGE inhibits vascular leakage in the intestine and skin of streptozotocin-treated rats [34]. Reports have demonstrated that RAGE expression can be up-regulated on AGEs accumulation [64] and limiting RAGE expression can down-regulate expression of pro-infl ammatory cytokines and attenuate vasculitis development [162]. anchored RAGE [163]. As sRAGE blocks AGEs from binding to RAGE, as if sRAGE was a "sponge" soaking up soluble AGEs (sAGEs) [164], it becomes a potential therapeutic tool for the treatment of infl ammatory diseases including diabetes and cardiovascular diseases. Reports indicated that RAGE participates in the formation of diabetes associated atherosclerosis in RAGE/apolipoprotein E double knock mice [155]. Another study observed that blockade of RAGE with anti-RAGE IgG or sRAGE inhibited NF-B activation [34].
Furthermore, study found that the vascular infl ammatory phenotypes such as accelerated expression of VCAM-1, tissue factor, or matrix metalloproteinases in mice are also prevented by sRAGE treatment [165]. Since AGEs may also form adducts with extracellular matrix (ECM), they may affect the structural integrity of the vessel in addition to signaling via RAGE [166].
Thus, sRAGE may also prevent such "non-specifi c" adverse effects of AGEs, especially during aging. Despite of its benefi ts on preventing RAGE ligation, sRAGE cannot be widely applied clinically due to its PROTEIN nature, which may induce immunogenicity in vivo.

Conclusion
The global prevalence of diabetes has seriously infl uenced the quality of life as a whole, and diabetes and diabetic