ZHU Lin, XU Yun-sheng

1. The First Clinical Medical College, Shandong University of Traditional Chinese Medicine, Jinan 250014,China

2. Department of Endocrinology, the Second Affiliated Hospital of Shandong University of Traditional Chinese Medicine, Jinan 250001, China

Keywords:Diabetic retinopathy Inflammatory mechanism Traditional Chinese medicine

ABSTRACT Diabetic retinopathy (DR), as a common microvascular complication of diabetes, is the result of the interaction of multiple factors and multiple mechanisms. Inflammation is one of the important pathophysiological mechanisms of DR. Leukocytes accumulate and adhere to the surface of endothelial cells, glial cell activation and its dysfunction, and the release of vascular endothelial growth factor, interleukin and other inflammatory factors are directly related to the occurrence and development of DR inflammation. Antiinflammation plays an important role in the treatment of DR. Studies have shown that traditional Chinese medicine has a unique advantage in targeting the inflammatory mechanism of DR. Therefore, the inflammatory cells,inflammatory factors and related Chinese medicines closely related to DR are reviewed, in order to provide references for the early prevention and treatment of DR.

1. Introduction

Diabetes is a common chronic disease caused by relatively insufficient or impaired secretion of insulin. According to statistics,the prevalence of diabetes among adults (20-79 years old) has risen to 8.8%, becoming one of the main burdens of global health and economy[1]. Diabetes has a profound impact on multiple organ systems, but diabetic retinopathy, as one of its pathologies,may cause visual impairment or even blindness to be one of the diseases that has the greatest impact on the quality of life of patients[2]. The pathogenesis of DR is complicated, and the inflammatory mechanism is considered to be an important driving force for the occurrence and development of DR. Exploring the inflammatory pathogenesis of DR, discovering effective traditional Chinese medicine treatment methods and digging out its treatment mechanism has great clinical significance for the treatment of DR.

2. Inflammation and DR

The duration of diabetes and the level of blood sugar control are important risk factors that affect the development of DR.Under continuous high blood sugar environment stimulation, the vascular structure is destroyed, white blood cells can pass through the damaged blood vessel wall, causing capillary occlusion and aggravated retinal ischemia. Glial cells are also activated through oxidative stress and other pathways under the stimulation of hyperglycemia, and the subsequent release of inflammatory factors such as growth factors, interleukins, and tumor necrosis factors further increase the inflammatory response and ultimately lead to the occurrence of DR.

2.1 Inflammatory cells and DR

Leukocyte aggregation and adhesion to the surface of endothelial cells is one of the important pathological mechanisms of inflammation[3]. Intercellular adhesion molecule-1 is one of the components of leukocyte integrin. It is expressed on the surface of leukocytes and can bind to CD11/CD18 of endothelial cells to activate leukocytes and increase the adhesion of leukocytes,resulting in abnormal stasis of leukocytes in blood vessels and retinal circulation. Obstacles, eventually leading to retinal vascular degeneration[4].

Glial cell activation and its dysfunction are an early feature of retinal neurodegeneration caused by diabetes. Müller glial cells(MGCs), as one of the most important glial cells of the retina,participate in the formation of the blood-retina barrier (BRB) and can regulate the metabolism of retinal tissue[5]. Hyperglycemia environment can lead to the activation of MGCs, that is, reactive gliosis. Activated MGCs down-regulate the expression of Kir4.1 channel and release a large amount of VEGF, TNF-α and other inflammatory factors, destroy the integrity of BRB structure,promote the apoptosis of retinal neurons and the formation of new blood vessels, and ultimately lead to vascular leakage, Macular edema[6]. At the same time, MGCs has a high uptake and recovery effect on glutamate (Glu), and Glu plays a role in longitudinal signal transmission and nutritional support neurons in the retina. Under the influence of the environment, Müller cells will undergo functional changes. Glu transporter dysfunction will increase the expression of Glu excessively, trigger Glu excitotoxic damage, and ultimately lead to apoptosis and dysfunction of retinal neurons[7].

As the activity sensor of the neuron microenvironment,microglia are in close contact with neurons to sense the changes of neurons, and respond quickly to very subtle changes in their microenvironment. Their activation is determined by extracellular signals, and Recognized by multiple receptors such as tolllike receptors[8]. Activated microglia increase the level of proinflammatory factors by activating inflammatory signaling pathways,and release cytotoxic substances to directly induce glial dysfunction and nerve cell death, and the nerve fiber layer becomes thinner,which ultimately leads to vision loss[9].

Astrocytes nutritionally support neurons under physiological conditions, and during the development of DR, astrocytes reactively activate, proliferate, and secrete pro-inflammatory mediators such as IL-6, MCP-1, and VEGF And cause retinal damage[10].

2.2 Inflammatory factors and DR

2.2.1 Vascular growth related factors

2.2.1.1 vascular endothelial growth factor(VEGF)

VEGF is an important signaling molecule that regulates cell proliferation and metabolism. Pathways such as inflammation and oxidative stress are activated in a high glucose environment,which increases the expression of VEGF. VEGF combines with VEGFR receptors to up-regulate the release of intercellular adhesion molecule-1 (ICAM-1) and more active substances in the retina,destroy the structure of blood vessel walls, aggravate ischemia and hypoxia in the retina, and promote the formation of new blood vessels. It can also promote the synthesis of plasma plasminogen activator and so on to reduce the connection between cells, resulting in increased endothelial cell permeability and speeding up the process of DR[11]. Imazeki et al. [12] observed the contents of 12 inflammatory factors such as monocyte chemoattractant protein 1,ICAM-1, IL-6 and their correlation in patients with diabetic macular edema (DME) to explore the relationship between inflammation and VEGF treatment response The results showed that the levels of inflammatory factors in the DME group and the control group were significantly different, while the 12 inflammatory factors in DME patients decreased significantly after 1 month after the start of anti-VEGF treatment. The results indicate that anti-VEGF treatment can inhibit inflammatory factors. The release of DR slows down the process of DR.

2.2.1.2 fibroblast growth factor(FGF)

It has been confirmed that FGF is a family composed of a series of structurally similar polypeptides, mainly secreted by the hypothalamus[13]. FGF2 is an important class of multifunctional polypeptides, which promotes the formation of new blood vessels.Research by Yang et al. [14] showed that FGF2 can interact with VEGF, promote angiogenesis, and jointly participate in the occurrence and development of retinal vascular diseases. Liu et al.[15] found that the expression of FGF2 protein in the vitreous and retinal hyperplasia membrane tissues of patients with proliferative diabetic retinopathy increased, suggesting that FGF2 levels are related to the occurrence of proliferative diabetic retinopathy. FGF2 overexpression may promote excessive cell proliferation, and thus Worse. In addition, in a high glucose environment, FGF2 may promote disease progression by activating various pathways. FGF21 is another important regulator of FGFs family, and the increase of its serum concentration is related to the severity of DR. FGF21 can inhibit the occurrence of inflammation by restricting islet proliferation and the conduction of NF-κB signaling pathway [16].Studies have shown that there is a U-shaped relationship between lower and higher serum FGF21 and DR, indicating that the low level of FGF21 may be related to DR, and the relative increase in serum FGF21 level may also be a compensatory increase [17].

2.2.1.3 platelet derived growth factor(PDGF)

DR microangiopathy is closely related to the loss of pericytes.The role of pericytes includes maintaining vascular permeability and normal vascular function. Under stress, pericytes can release a large amount of VEGF and FGF to promote the formation of new blood vessels. In addition, they can release adhesion factors and chemokines to control the migration of white blood cells[18].PDGF-B/PDGFR-β is one of the important signal pathways to maintain the stability of pericytes. PDGF is mainly synthesized and secreted by endothelial cells, etc. In a high glucose environment,the PDGF-B/PDGFR-β signaling pathway is destroyed, triggering apoptosis of pericytes. Pericyte apoptosis leads to damage to the blood vessel wall, resulting in hypoxia and inflammation of the retina[19]. PDGF can also stimulate the migration and proliferation of retinal pigment epithelial cells and the process of epithelialmesenchymal transition through the destroyed BRB, so that retinal pigment epithelial cells can be transformed into myofibroblasts and fibroblasts. The former can shrink the extracellular matrix components and cause retinal traction. Pull away[20].

2.2.1.4 insulin-like growth factor-1(IGF-1)

IGF-1 is a polypeptide protein substance that can inhibit cell apoptosis and promote cell proliferation. It is mainly produced by liver cells. PI3K-AKT and MAPK are the two major pathways through which IGF-1 exerts apoptotic effects. Studies have found that the level of IGF-1 in the serum of diabetic patients is equivalent to that of the healthy control group, but in patients with proliferative retinopathy, IGF-1 is significantly different from the control group and the type 2 diabetes group[21]. In addition, the combination of IGF-1 and IGF-1 receptor can activate VEGF, leading to increased endothelial cell permeability and angiogenesis, and accelerating the occurrence and development of DR[22].

2.2.2 interleukin(IL)

As a multifunctional inflammatory cytokine, IL plays an important role in regulating immunity and inflammatory response. Now 38 interleukins have been officially named according to the chemical molecular structure. IL-1 has two forms, IL-1α and IL-1β, and is mainly released by B cells and macrophages. The release of IL-1β and ROS can both activate NF-κB, and also promote the release of other cytokines; and NF-κB can also regulate the gene expression of IL-1β, and the interaction between them can aggravate inflammatory damage[23]. As another key pleiotropic cytokine, IL-6 mainly acts through IL-6 signal transduction and transcriptional activator 3[24].IL-6 interacts with endothelial cells to regulate the recruitment and expression of leukocytes and inflammatory proteins, and also interacts with TNF-α to stimulate endothelial cell activation[25].

2.2.3 tumor necrosis factor-alpha(TNF-α)

TNF-α is mainly secreted by monocytes and neutrophils, etc., and plays a role in inflammation, angiogenesis and apoptosis[26]. Niu et al. [27] in order to explore the role of TNF-α in mouse retinal damage,they performed 14 consecutive days of light damage to adult mice and found that TNF-α was significantly increased, leading to the proliferation of Müller cells in vitro. It is shown in cells that TNF-α can up-regulate the expression of genes related to cell proliferation and increase the release of IL-6 and other inflammatory factors.Whitmore et al.[28]. found that TNF-α induced an increase in the expression of RUNX1 by activating JNK. RUNX1 is a mediator of abnormal retinal angiogenesis and is closely related to the occurrence and development of proliferative diabetic retinopathy.

2.2.4 transforming growth factor-β(TGF-β)

TGF-β is mainly secreted by retinal pigment epithelial cells and exists in eye tissues such as ciliary body and retinal blood vessels.TGF-β has a two-way regulatory effect. Under pathological conditions, TGF-β can promote the proliferation and differentiation of retinal pigment epithelial cells and fibroblasts, promote the formation of new blood vessels, and stimulate the up-regulation of other cytokines, such as PDGF[29].

2.2.5 interferon(IFN)

IFN can be divided into three types: α, β, and γ according to the different tissues of its source and the difference in three-dimensional structure. Mao Yong et al. [30] found that IFN-γ levels were highly expressed in DR patients, which was significantly higher than that of healthy controls and NDR groups, indicating that IFN-γ can be used as an index to predict DR. IFN-γ can promote the differentiation of neutrophils and immune cells, and play an important role in antagonizing the production of inflammatory factors. Luo Nanping et al. [31] found that the level of IFN-γ and the concentration of serum VEGF have a significant linear relationship. In the early stage of DR,the level of IFN-γ was significantly increased, exerting its effect of antagonizing other factors to promote cell proliferation.

2.2.6 Chemokines

Chemokines are cytokines with similar structures and chemotaxis.It can be divided into four subgroups according to the spatial location distribution. As a member of the CX3C family, CX3CL1 triggers the flow of intracellular calcium ions to chemoattract the aggregation and migration of monocytes and lymphocytes, promote the adhesion between the two and endothelial cells, thereby causing vascular endothelial damage[32]. MCP-1 (monocyte chemokine-1),as a member of the CC-type cell chemokine superfamily, can directly promote NO through signal pathways such as MAPK,phosphatidylinositol 3 kinase (PI3K) and ERK1/2 The expression of VEGF and VEGF can also chemoattract macrophages to attach to inflammation sites, produce cytokines such as TNF-α, PDGF, bFGF and IFN-γ, and promote the formation of new blood vessels[33].

3. Chinese medicine and DR inflammation

3.1 Principles of TCM treatment of DR

According to the characteristics and clinical manifestations of DR,ancient physicians classified it as a category of "visionlessness".Many medical scholars believe that the syndromes of Diabetes eye disease are the combination of deficiency and excess, and the essence of deficiency and excess. Therefore, the treatment is usually based on the syndrome differentiation of "deficiency" and "stasis", and the prescriptions for invigorating qi and nourishing yin, promoting blood circulation and removing blood stasis are often used. Many recent studies have proved that traditional Chinese medicine plays an important role in regulating DR inflammation and can slow down the development of DR.

3.2 Treatment of DR with Chinese herbal extracts

3.2.1 Wolfberry

Lycium barbarum polysaccharide is one of the main components of Lycium barbarum to protect the retina, which can reduce and inhibit inflammation and oxidative stress. Xue et al. [34] confirmed by studying the inflammatory factors in the retina of diabetic mice that the levels of IL-1β and TNF-α in the retinal tissue of mice in the wolfberry polysaccharide group were significantly lower than those in the diabetic group, indicating that wolfberry polysaccharide can inhibit diabetes mellitus. Inflammation of the mouse retina.Yan et al. [35] observed and studied the inflammation of Lycium barbarum polysaccharides on human retinal pigment epithelial(ARPE-19) cells induced by bacterial lipopolysaccharide and its possible signaling pathways. The results showed that Lycium barbarum polysaccharides can inhibit intracellular inflammatory factors. Release and phosphorylation of NF-κB/MAPK pathway related factors, thereby preventing ARPE-19 cells from producing inflammation.

3.2.2 Prunella

Prunella vulgaris is a perennial plant in the Lamiaceae family,which can clear fire, improve eyesight, disperse knots and reduce swelling. Modern pharmacological studies have proved that Prunella vulgaris has a variety of biological effects, including antiinflammatory, anti-oxidant and anti-allergic effects. Kim et al.[36]found that Prunella vulgaris extract inhibits the production of VEGF secretin in RPE cells in a concentration-dependent manner, and inhibits the release of NF-κB signaling pathway and other proinflammatory factors, suggesting that Prunella vulgaris extract induces blue light irradiation There is a protective effect on retinal degeneration and inflammation. Mei et al. [37] also proved that Prunella vulgaris extract can improve BRB leakage in STZ-induced diabetic mice, reduce serum TNF-α content and phosphorylated p65 protein expression, and alleviate inflammatory damage to the retina.

3.2.3 Dendrobium

Dendrobium, as a special medicine for treating diabetes, is divided into 4 types of Dendrobium, namely Dendrobium officinale and Dendrobium nobile. Dendrobium polysaccharide is one of the active ingredients of Dendrobium candidum. The results of Li and others[38]proved that Dendrobium officinale can inhibit the expression of inflammatory factors IL-6, TNF-α and VEGF in retinopathy of diabetic rats. Huang et al.[39] studied the immunomodulatory effect of Dendrobium polysaccharides on macrophages, and the results showed that Dendrobium polysaccharides can increase the phagocytic activity of macrophages, reduce the release level of inflammatory factors, and promote cell polarization classification.Liu [40] investigated the protective mechanism of Dendrobium nobile alkaloids on BV2 cell inflammation induced by lipopolysaccharide,and the results showed that Dendrobium nobile alkaloids can inhibit the activation of NLRP3 inflammasomes, reduce the expression of NF-κB and NF-κB to the nucleus. The metastasis finally inhibited the neuroinflammatory response of BV2 cells. Erianin is the main active component of Dendrobium candidum, which can block the ERK1/2 pathway in RPE and microglia to reduce angiogenesis[41].

3.2.4 Scutellaria

Wogonin is one of the components of Scutellaria baicalensis Georgi. It can increase glucose into cells through the AKT/GLUT4 pathway to relieve high blood sugar. Yan et al.[42] used enzymelinked immunoassay and other methods to detect the mRNA levels of VEGF, bFGF and TGF-β to explore the effect of wogonin on inflammatory factors in diabetic retinopathy. The results showed that VEGF, bFGF and TGF-β in the wogonin group The mRNA level and protein expression of TGF-β are inhibited, suggesting that wogonin can improve the inflammatory response of diabetic retinopathy.Studies have also shown that wogonin can reduce the production of ROS in macrophages and reduce NF-κB-mediated inflammatory factor gene transcription to inhibit the inflammatory response[43].

3.3 Chinese patent medicines and Chinese herbal compound treatments for DR

3.3.1 Shuangdan Mingmu Capsules

Zhao et al.[44] observed the effect of Shuangdanmingmu Capsules on the retinal tissues of DR rats, and randomly divided the diabetic rat models into a blank control group, a model group,and Shuangdanmingmu Capsule medium and high dose groups.The results found that the model group The VEGF level in rats was significantly increased, the retinal tissue arrangement was disordered, and the damage was obvious. The VEGF level in the Shuangdanmingmu capsule group was significantly reversed,effectively inhibiting the formation of new blood vessels, suggesting that it may be related to the up-regulation of retinal Spred-1 expression. Peng et al.[45] also confirmed that Shuangdan Mingmu Capsules can significantly reduce the expression of VEGF family and its receptors in the retina, and have a certain protective effect on the retina of diabetic rats.

3.3.2 Yiqi Huoxue Lishi Decoction

Yiqi Huoxue Lishi Decoction uses the methods of replenishing qi and activating blood, invigorating the spleen and promoting dampness, and has a significant clinical effect on DR. Zhou Yu et al. [46] observed the effect of Yiqi Huoxue Lishi Decoction on inflammatory factors in DR rats and the protective effect of the optic nerve, and found that the serum levels of IL-6, TNF-α, hs-CRP and other inflammatory factors in the treatment group of rats Significantly lower than the model group, higher than the control group, and up-regulate the expression of NF-κB, suggesting that Yiqi Huoxue Lishi Decoction can protect the retina and optic nerve structure of DR rats and inhibit inflammation.

3.3.3 Bushen Huoxue Recipe

Ma et al. [47] observed the effect of Bushen Huoxue Recipe on the secretion of VEGF and PEDF by Müller cells and the mechanism of action under AGEs or hypoxia. The results of Bushen Huoxue Decoction group were reversed and the imbalance between VEGF and PEDF was alleviated, which may be two important mechanisms of Bushen Huoxue Decoction to alleviate DR.

3.3.4 Tongluo Zhujing Wan

Tongluo Zhujing Wan is optimized by Professor Lei Xiaoqin on the basis of Zhujing Wan. Ren et al.[48] Diabetic rat models of SD rats were randomly divided into 4 groups and were treated with different doses of Tongluo Zhujing pills. The results showed that the expression of VEGF and its mRNA in the serum of rats in the Tongluo Zhujing group was inhibited. The inhibition was most obvious in the high-dose group. Another study showed that Tongluo Zhujing Pills can reduce the leakage of Albumin in the retina of diabetic rats and increase the expression of tight junction proteins Occludin, ZO-1 and Occludin mRNA in the retina of diabetic rats[49]. These studies suggest that Tongluo Zhujing Pill can reduce the expression level of VEGF in the serum and retina of diabetic rats,reduce angiogenesis, and protect the blood-retinal barrier.

4. Outlook

Early prevention and treatment is the key to clinical treatment of DR. Controlling blood sugar and actively anti-inflammatory are important entry points for the treatment of DR. Based on modern pharmacology and clinical trials, traditional Chinese medicine has certain advantages in multi-target and multi-channel treatment of DR, but there is still a lot of research space. To understand the pathogenesis of DR, explore the relationship between traditional Chinese medicine and DR from a micro perspective, explore more traditional Chinese medicines or compound prescriptions to prevent and treat DR, and provide references for early prevention and late intervention treatment of DR.

Author's contribution:

This article was written by Zhu Lin, the first author, and revised under the guidance of the corresponding author, Xu Yun-sheng.