Activation of the Wnt Pathway Plays a Pathogenic Role in Diabetic Retinopathy in Humans and Animal Models

Abstract

Although Wnt signaling is known to mediate multiple biological and pathological processes, its association with diabetic retinopathy (DR) has not been established. Here we show that retinal levels and nuclear translocation of ␤-catenin, a key effector in the canonical Wnt pathway, were increased in humans with DR and in three DR models. Retinal levels of lowdensity lipoprotein receptor-related proteins 5 and 6, coreceptors of Wnts , were also elevated in the DR models. The high glucose-induced activation of ␤-catenin was attenuated by aminoguanidine, suggesting that oxidative stress is a direct cause for the Wnt pathway activation in diabetes. Indeed, Dickkopf homolog 1, a specific inhibitor of the Wnt pathway, ameliorated retinal inflammation, vascular leakage, and retinal neovascularization in the DR models. Dickkopf homolog 1 also blocked the generation of reactive oxygen species induced by high glucose, suggesting that Wnt signaling contributes to the oxidative stress in diabetes. These observations indicate that the Wnt pathway plays a pathogenic role in DR and represents a novel therapeutic target. Diabetic retinopathy (DR), the leading cause of blindness in the working age population, represents a common concern in types 1 and 2 of diabetes mellitus (DM). 1 Accumulating evidence suggests that DR is a chronic inflammatory disorder.2 Retinal inflammation is believed to play a causative role in vascular leakage, which can lead to diabetic macular edema, and in retinal neovascularization (NV). It has been shown that levels of soluble intercellular adhesion molecule-1 (ICAM-1) and vascular cell adhesion molecule-1 are significantly higher in the vitreous from patients with proliferative diabetic retinopathy than in nondiabetic vitreous. 3,4 Increased ICAM-1, vascular cell adhesion molecule-1, and e-selectin levels were found in the serum from patients with diabetic microangiopathy. [5][6][7] In diabetic animal models, increased retinal ICAM-1 expression is believed to be responsible for leukocyte adhesion or leukostasis and increased vascular permeability. Leukostasis is believed to contribute to capillary nonperfusion and local ischemia, which subsequently induces the overexpression of vascular endothelial growth factor (VEGF). 8 -11 Increased VEGF levels are responsible for the retinal vascular leakage and retinal NV.12,13 Recent studies have indicated that oxidative stress, induced by hyperglycemia, contributes to retinal inflammation in diabetes.14,15 However, the pathogenic mechanisms by which diabetes and oxidative stress induce inflammation are not certain at the present time.Wnts are a group of secreted, cysteine-rich glycoproteins, which bind to a coreceptor complex of frizzled (Fz) receptors and low-density lipoprotein receptor-related protein 5 or 6 (LRP5/6) and regulate expression of a number of target genes through an intracellular signaling pathway, namely the Wnt pathway. 16 In the absence of Wnt ligands, ␤-catenin, a down-stream effector of the canonical Wnt pathway, is phosphorylated by a pro...

Implication of dysregulation of the canonical wingless-type MMTV integration site (WNT) pathway in diabetic nephropathy

“…Immunohistochemistry was performed as described by Chen et al [11]. Briefly, paraffin-embedded kidney sections were stained with the antibody for β-catenin at a dilution of 1:250 for 1 h. After thorough washes with PBS, immunosignals were developed using an ABC kit (Vectastain ABC; Vector Laboratories) according to the manufacturer's protocol.…”

“…Care, use and treatment of all animals in this study were in strict agreement with the Guidelines for the Care and Use of Laboratory Animals set forth by the University of Oklahoma. Experimental diabetes was induced in the rats as described by Chen et al [11].…”

“…Furthermore, blockage of WNT signalling attenuates retinal inflammation and neovascularisation in diabetic retinopathy models [11]. Documented studies suggest that the presence of either retinopathy or nephropathy may contribute to the development of the other, especially in type 1 diabetic patients [12].…”

“…Our previous studies have shown that the WNT pathway is overactivated in retinas from human patients with diabetic retinopathy and in those from animal models of diabetes [11]. Furthermore, blockage of WNT signalling attenuates retinal inflammation and neovascularisation in diabetic retinopathy models [11].…”

Plasma and vitreous fluid levels of Dickkopf-1 in patients with diabetic retinopathy

“…Recently, great progress has been achieved in identifying soluble factors in the circulation and vitreous fluid, such as TNF-a, VEGF, and sICAM-1, which are associated with the development and progression of DR. [2][3][4][5][6][7][8] A recent study demonstrated that the canonical Wnt signaling pathway is activated in the retina in humans with DR and in DR animal models, and it has a causative role in DR. 9 The Wnt/b-catenin signaling pathway is an important signaling pathway that regulates multiple biological and pathological processes including cell proliferation and differentiation, apoptosis, stem cell maintenance, angiogenesis, inflammation, fibrosis, and carcinogenesis. 10 This signaling pathway is regulated by a number of natural inhibitors including Dickkopf-1 (DKK-1), a well-established specific inhibitor of the Wnt signaling pathway.…”