Anti-inflammatory properties of pigment epithelium-derived factor

Abstract

Inflammation is part of the complex biological response to harmful stimuli, such as cell damage, pathogens, or irritants. An excessive inflammatory response can lead to a variety of diseases. Pigment epithelium-derived factor (PEDF) is an endogenous glycoprotein that belongs to the superfamily of serine protease inhibitors and has multiple biological activities. Accumulating evidence suggests that PEDF participates in various inflammatory-related diseases, such as diabetic retinopathy, atherosclerosis, nonalcoholic steatohepatitis, and retinal diseases. However, the mechanism is still incompletely understood. In this paper, we review the anti-inflammatory properties of PEDF and discuss the underlying mechanisms. PEDF can exert its anti-inflammatory effects by downregulating the expression of inflammatory factors, promoting the synthesis of anti-inflammatory factors, inhibiting the activation of proinflammatory pathways and activating anti-inflammatory pathways. Examining the function of PEDF in inflammation addresses the need for further investigation and subsequent target-specific strategies for inflammatory disorders.

Introduction

Inflammation refers to the defensive response of living tissue to infection, physical injury, ischaemia, toxicosis, or autoimmune injury. ¹ It is a complex set of interactions among the injury agents and cells that can arise in any tissue. Inflammation plays a protective role by removing injury-related stimuli and initiating the healing process. ² However, if targeted destruction and assisted repair are not properly executed, inflammation can lead to persistent tissue damage and consequently contribute to pathological processes, such as cardiovascular diseases, ³ cancer, ⁴ diabetes mellitus, ⁵ Alzheimer’s disease, ⁶ pulmonary diseases, ⁷ and autoimmune diseases. ⁸

Pigment epithelium-derived factor (PEDF), which is encoded by serpin peptidase inhibitor, clade F, member 1, is a 50 kDa glycoprotein that belongs to the superfamily of serine protease inhibitors. ⁹ PEDF is an endogenously produced protein that is widely expressed throughout the human body and exerts multiple and varied biological effects. ¹⁰ PEDF was originally identified as a growth factor with neuronal differentiation activity and has been shown to be neuroprotective in different toxin-induced models of neurodegeneration. ¹¹ PEDF was also shown to be one of the most potent antagonists of angiogenic factors (such as vascular endothelial growth factor (VEGF)), which provided the impetus to study possible applications in cancer therapy. ¹² Over the past few years, PEDF has been described as a multifaceted protein with neurotrophic, ¹³ anti-angiogenic, ¹⁴ anti-proliferative, ¹⁵ antioxidant, ¹⁶ anti-apoptotic, ¹⁷ and morphogenetic properties. ¹⁸

Increasing evidence suggests that PEDF participates in various inflammation-related diseases through different pathways. Downregulation of PEDF alone is sufficient to induce inflammatory responses, which suggests that PEDF is an endogenous anti-inflammatory factor.^19,20 In this review, we discuss the anti-inflammatory mechanism of PEDF and focus on emerging concepts and the mechanisms of action.

Anti-inflammatory properties of PEDF

Downregulating the expression of inflammatory factors

The anti-inflammatory role of PEDF was discovered based on the fact that it could downregulate the expression of inflammatory factors in neovascular diseases, such as diabetic retinopathy (DR). ²¹ DR is a serious complication of diabetes mellitus and a major cause of blindness in patients aged 20–64 years worldwide. ²² Accumulating evidence suggests that the upregulation of inflammatory factors, such as intercellular adhesion molecule-1 (ICAM-1) and interleukin (IL)-1β, and subsequent leukocyte-endothelial interactions contribute to the progression of DR.^23–25 Miyamoto K et al. demonstrated for the first time that PEDF inhibited retinal leukostasis by suppressing ICAM-1 expression in experimental DR. ²⁶ In addition, PEDF was shown to increase the expression of glutamine synthetase (GS) to protect against IL-1β and act as an anti-inflammatory factor in retinal Müller cells in DR. ²⁷ In addition, PEDF could inhibit macrophage activation induced by lipopolysaccharides (LPS) and induce macrophage apoptosis, which demonstrates the potential of PEDF to attenuate macrophage-mediated inflammatory activities in DR. ²⁸

Pigment epithelium-derived factor can also downregulate the expression of retinal proinflammatory factors, such as monocyte chemoattractant factor-1 (MCP-1). The expression of ICAM-1 and MCP-1 is upregulated under oxidative stress conditions in endothelial cells via transcriptional activation of nuclear factor-kappa B (NF-κB), and this overexpression plays an important role in the early phase of atherosclerosis by initiating monocyte and/or T-cell recruitment to the vessel walls. ²⁹ By blocking the NF-κB pathway, PEDF actively protects against atherosclerosis through its anti-inflammatory activity. ³⁰ Indeed, immune system activation and inflammation are intricately involved in the development and progression of atherosclerosis. Circulating levels of ICAM-1 and MCP-1 are associated with traditional risk factors for cardiovascular disease, and their expression is increased in human atherosclerotic plaques.^31,32 PEDF could prevent the acceleration of atherosclerosis by reducing ICAM-1 and MCP-1 levels and subsequently decreasing atheromatous lesion formation. ³³

Caveolin-1 (Cav) is a major protein component of caveolae, a specialized subset of membrane lipid rafts in endothelial cells that are involved in multiple cellular processes, such as molecular transport, cell adhesion, and signal transduction. ³⁴ Exogenously administered histidine-rCav increases membrane levels of Cav and dose-dependently upregulates mRNA levels of MCP-1, vascular cell adhesion molecule-1 (VCAM-1), and plasminogen activator inhibitor-1 (PAI-1). Matsui T et al. found that PEDF could inhibit Cav-induced inflammatory and thrombogenic reactions by inhibiting MCP-1, VCAM-1, and PAI-1 gene induction in human umbilical vein endothelial cells (HUVECs). ³⁵

Promotion of the synthesis of anti-inflammatory factors

Neuroprotectin D1 (NPD1) is a bioactive mediator derived by the docosahexaenoic acid (DHA) oxygenation pathway and is endogenously synthesized by retinal pigment epithelial (RPE) cells. It has protective bioactivity in RPE cells that are under oxidative stress challenge and is thus considered to be a DHA neuroprotective messenger with potent inhibitory effects on proinflammatory factor (cyclooxygenase-2) gene expression. ³⁶ Moreover, NPD1 and the pro-resolving lipid mediator eicosapentaenoic acid (EPA)-derived resolvin E1 have been shown to promote phagocyte removal during acute inflammation by regulating leukocyte infiltration and increasing macrophage ingestion in vivo and in vitro. ³⁷

Pigment epithelium-derived factor has been shown to activate NPD1 synthesis in ageing retinal pigment epithelial-19 cells in response to oxidative stress. ³⁶ Most synthetic NPD1 is released from the cells, further triggering the anti-inflammatory process. Pigment epithelium-derived factor can prevent Bcl-xl translocation from the cytoplasm to the nucleus during oxidative stress, offering further insight into a new signal for neuroprotection against inflammation. ³⁸ In addition, PEDF plus DHA inhibited corneal expression of IL-18, a proinflammatory cytokine that has been reported to increase herpes simplex virus type-1 (HSV-1) production in infected T cells. ³⁹

Inhibiting the activation of proinflammatory pathways

Wnt/β-catenin

The Wnt/β-catenin signalling pathway is an important extracellular pathway involved in many developmental and physiological processes, such as cell proliferation, migration, ⁴⁰ differentiation, ⁴¹ inflammation, ⁴² apoptosis, ⁴³ stem cell self-renewal ⁴⁴ and the maintenance of tissue homeostasis. ⁴⁵ The Wnt/β-catenin pathway is closely related to inflammation, primarily by a variety of important inflammatory factors. The activation of Wnt/β-catenin can upregulate inflammatory cytokines and enhance monocyte adhesion to endothelial cells, causing endothelial dysfunction. ⁴⁶ Infection with pathogens can lead to uncontrolled inflammation and increase the risk of carcinogenesis by through the overexpression of Wnt/β-catenin. ⁴⁷

Pigment epithelium-derived factor functions as a part of a negative feedback loop to modulate Wnt/β-catenin signalling. Pigment epithelium-derived factor^−/− mice and siRNA silencing of PEDF expression can activate Wnt/β-catenin in vivo and in vitro. ⁴⁸ These results suggest that a reduction in endogenous PEDF can activate Wnt/β-catenin and vice versa. Pigment epithelium-derived factor overexpression can inhibit the Wnt/β-catenin signalling pathway. Pigment epithelium-derived factor inhibits Wnt/β-catenin by binding to the Wnt coreceptor low-density lipoprotein receptor-related protein 6 (LRP6). ⁴⁹ There is evidence suggesting that PEDF may reduce inflammation and oxidative stress by inhibiting Wnt/β-catenin in unilateral ureteral obstruction kidneys. ⁵⁰ Interestingly, suppressing Wnt/β-catenin can also block the production of reactive oxygen species (ROS). ⁵¹

Reactive oxygen species

Various haemodynamic and hyperglycaemia-induced metabolic derangements, including increased formation of advanced glycation end products (AGEs) and the production of ROS, contribute to the characteristic histopathological changes observed in vascular complications associated with diabetes mellitus. ⁵² Mitochondrial-derived ROS could be a molecular target for the antioxidative effect of PEDF on high glucose-induced HUVECs. ⁵³ PEDF could block high glucose-induced inflammatory effects by inhibiting ROS generation and activating NF-κB partly by restoring mitochondrial superoxide dismutase (SOD) expression. ⁵⁴ Normalizing mitochondrial superoxide generation blocks high glucose-induced NF-κB activation, protein kinase C (PKC) induction, and AGE formation. Moreover, PEDF could inhibit inflammation by suppressing angiotensin (Ang) II-induced increase in cellular ROS concentrations in vivo. This effect is achieved by regulating NF-κB and suppressing nicotinamide adenine dinucleotide phosphate (NADPH) oxidase. ⁵⁵

p38/mitogen activated protein kinase

In addition, PEDF exerts its anti-inflammatory effect by inhibiting mitogen-activated protein kinase (MAPK). Phosphorylation of p38/MAPK could result in the release of inflammatory cytokines, including IL-1β, IL-6, and tumour necrosis factor-alpha (TNF-α). ⁵⁶ PEDF exerts its anti-inflammatory effects on human corneal epithelial cells by inhibiting the phosphorylation of p38/MAPK and plays an immunoregulatory role in the pathogenesis of dry eye disease. ⁵⁷ PEDF can also prevent H₂O₂-induced p38 phosphorylation and preserve the barrier function of RPE cells against oxidative stress. ⁵⁸

Activating the anti-inflammatory pathway

Pigment epithelium-derived factor exerts its anti-inflammatory effect by activating peroxisome proliferator-activated receptor gamma (PPAR-γ). PPAR-γ agonists, such as DHA, omega-3 fatty acids and EPA, are known to induce anti-inflammatory activity.^59,60 Some natural and synthetic PPAR-γ agonists can have a chemoprotective effect by targeting inflammatory agents. ⁶¹ PEDF can inhibit oxidative stress generation and apoptosis by activating PPAR-γ in podocytes. ⁶² PPAR-γ activation inhibits the activity of TNF-α, which seems to affect inflammation. ⁶³ In addition, PEDF can inhibit inflammation by inducing macrophage apoptosis and necrosis through PPAR-γ activation. ⁶⁴

Molecular mechanisms of action

The various biological activities of PEDF are thought to depend on its interactions with cell-surface receptors such as PEDF receptor, laminin receptor (LR), F1 ATPase/synthase and LRP6. ⁶⁵

PEDF receptor

Pigment epithelium-derived factor receptor, which is encoded by patatin-like phospholipase domain-containing protein 2 (PNPLA2), is also known as adipose triglyceride lipase (ATGL), a cell surface transmembrane protein with phospholipase activity that is activated by PEDF binding to release bioactive lipids. ⁹ The interaction of PEDF with ATGL has been suggested to mediate its antiangiogenic, antitumorigenic, and neurotrophic activities by stimulating lipid mediators. ⁶⁶ In our previous study, we found that PEDF induced hyperpermeability by binding to the ATGL receptor, which led to the activation of RhoA, actin rearrangement and intercellular junction disruption. ⁹ Adipocyte-derived free fatty acids (FFAs) promote TNF-α and IL-6 release by macrophages in a Toll-like receptor 4 (TLR4)-dependent manner and have been implicated in inflammation and insulin resistance. ⁶⁷ Recombinant PEDF induces a proinflammatory phenotype in macrophages by binding to ATGL. In addition, in the absence of functional ATGL, macrophages develop an anti-inflammatory M2-like phenotype. ⁶⁸

Laminin receptor

LR is expressed in tumoral, muscular, neuronal, epithelial, and endothelial cells and is involved in many biological processes, such as migration, adhesion, and proliferation, in many cell types. ⁶⁹ LR is also implicated in retinal neuronal differentiation and vascular development. ⁷⁰ Recently, LR has been identified as a cell surface receptor that mediates the anti-inflammatory activity of epigallocatechin-3-gallate (EGCG). ⁷¹ PEDF causes antiangiogenic reactions through its interaction with LR. ⁷⁰ Matsui T et al. found that in siLR-transfected myeloma cells, the antiangiogenic, anti-inflammatory, and antithrombogenic effects of PEDF were lost. ⁷² LR antibodies or the LR antagonist mEGF_33–42 alone significantly reduced VEGF, MCP-1, ICAM-1, and PAI-1 mRNA levels in myeloma cells. This finding suggests that LR antibodies or LR antagonists alone could mimic the effects of PEDF partly by interacting with LR. These observations suggest that PEDF could have anti-inflammatory properties through its interaction with LR. However, the anti-inflammatory mechanisms of PEDF are different in different disease models. Therefore, whether the effect is caused by binding to LR or other receptors requires further study.

Conclusions and future research

In recent years, the incidence of inflammation-related diseases has increased. The anti-inflammatory mechanism has become an important research direction for the treatment of these diseases. PEDF, which is an endogenous anti-inflammatory factor, can exert anti-inflammatory effects in these ways. Thus, future research should address target specificity to provide a better understanding of the cellular mechanisms regulated by PEDF in the context of inflammation. Such endeavours should facilitate the development of novel treatments for inflammatory disorders.