Fenofibrate, a specific agonist of peroxisome proliferator–activated receptor-α (PPARα), displays robust therapeutic effects on diabetic retinopathy (DR) in patients with type 2 diabetes. Our recent studies have shown that PPARα is downregulated in the diabetic retina, which contributes to the pathogenesis of DR. However, the mechanism for diabetes-induced downregulation of PPARα remains unknown. We investigated the role of microRNA-21 (miR-21) in regulating PPARα in DR. miR-21 was overexpressed, while PPARα levels were decreased in the retina of db/db mice, a model of type 2 diabetes. Such alterations were also observed in palmitate-treated retinal endothelial cells. miR-21 targeted PPARα by inhibiting its mRNA translation. Knockout of miR-21 prevented the decrease of PPARα, alleviated microvascular damage, ameliorated inflammation, and reduced cell apoptosis in the retina of db/db mice. Intravitreal injection of miR-21 inhibitor attenuated PPARα downregulation and ameliorated retinal inflammation in db/db mice. Further, retinal miR-21 levels were increased, while PPARα levels were decreased in oxygen-induced retinopathy (OIR). Knockout of miR-21 prevented PPARα downregulation and ameliorated retinal neovascularization and inflammation in OIR retinas. In conclusion, diabetes-induced overexpression of miR-21 in the retina is at least partly responsible for PPARα downregulation in DR. Targeting miR-21 may represent a novel therapeutic strategy for DR.

PurposeThis study was designed to evaluate effects of fenofibric acid (Feno-FA), a peroxisome proliferator–activated receptor-alpha (PPARα) agonist, on ocular neovascularization (NV) in models recapitulating neovascular age-related macular degeneration (AMD), and to explore whether the effects are PPARα dependent.MethodsLaser-induced choroidal NV (CNV) in rats and very low-density lipoprotein receptor knockout (Vldlr−/−) mice received daily intraperitoneal injections of Feno-FA or vehicle. Vascular leakage was examined by fundus fluorescein angiography and permeability assay using Evans blue as tracer. In CNV rats, severity of CNV was evaluated by CNV areas and CNV volume. In Vldlr−/− mice, subretinal NV (SRNV) and intraretinal NV (IRNV) were quantified in choroid flat mount and retina flat mount, respectively. Inflammatory factors were measured using Western blotting and retinal leukostasis assay. Further, Pparα−/− mice and age-matched wild-type (WT) mice were used for laser-induced CNV and treated with Feno-FA to explore the underlying mechanism.ResultsFeno-FA significantly reduced vascular leakage in CNV rats and Vldlr−/− mice, reduced CNV volume in laser-induced CNV rats, and suppressed SRNV and IRNV in Vldlr−/− mice. In addition, Feno-FA downregulated the expression of inflammatory factors, including VEGF, TNF-α, and intercellular cell adhesion molecule-1 (ICAM-1), in the eyecups of CNV rats and decreased adherent retinal leukocytes in Vldlr−/− mice. Furthermore, Pparα−/− mice developed more severe CNV compared with WT mice, and PPARα knockout abolished the beneficial effects of Feno-FA on CNV.ConclusionsFeno-FA has therapeutic effects on ocular NV in models recapitulating neovascular AMD through a PPARα-dependent mechanism.

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BackgroundThe wingless-type MMTV integration site (Wnt) signaling is a group of signal transduction pathways. In canonical Wnt pathway, Wnt ligands bind to low-density lipoprotein receptor-related protein 5 or 6 (LRP5 or LRP6), resulting in phosphorylation and activation of the receptor. We hypothesize that canonical Wnt pathway plays a role in the retinal lesion of age-related macular degeneration (AMD), a leading cause of irreversible central visual loss in elderly.MethodsWe examined LRP6 phosphorylation and Wnt signaling cascade in human retinal sections and plasma kallistatin, an endogenous inhibitor of the Wnt pathway in AMD patients and non-AMD subjects. We also used the Ccl2−/−/Cx3cr1−/−/rd8 and Ccl2−/−/Cx3cr1gfp/gfp mouse models with AMD-like retinal degeneration to further explore the involvement of Wnt signaling activation in the retinal lesions in those models and to preclinically evaluate the role of Wnt signaling suppression as a potential therapeutic option for AMD.ResultsWe found higher levels of LRP6 (a key Wnt signaling receptor) protein phosphorylation and transcripts of the Wnt pathway-targeted genes, as well as higher beta-catenin protein in AMD macula compared to controls. Kallistatin was decreased in the plasma of AMD patients. Retinal non-phosphorylated-β-catenin and phosphorylated-LRP6 were higher in Ccl2−/−/Cx3cr1−/−/rd8 mice than that in wild type. Intravitreal administration of an anti-LRP6 antibody slowed the progression of retinal lesions in Ccl2−/−/Cx3cr1−/−/rd8 and Ccl2−/−/Cx3cr1gfp/gfp mice. Electroretinography of treated eyes exhibited larger amplitudes compared to controls in both mouse models. A2E, a retinoid byproduct associated with AMD was lower in the treated eyes of Ccl2−/−/Cx3cr1−/−/rd8 mice. Anti-LRP6 also suppressed the expression of Tnf-α and Icam-1 in Ccl2−/−/Cx3cr1−/−/rd8 retinas.ConclusionsWnt signaling may be disturbed in AMD patients, which could contribute to the retinal inflammation and increased A2E levels found in AMD. Aberrant activation of canonical Wnt signaling might also contribute to the focal retinal degenerative lesions of mouse models with Ccl2 and Cx3cr1 deficiency, and intravitreal administration of anti-LRP6 antibody could be beneficial by deactivating the canonical Wnt pathway.Electronic supplementary materialThe online version of this article (doi:10.1186/s12967-015-0683-x) contains supplementary material, which is available to authorized users.

Fenofibrate is a peroxisome proliferator-activated receptor α (PPARα) agonist and has been shown to have therapeutic effects on diabetic retinopathy (DR). However, the effects of fenofibrate through systemic administration are not as potent as desired due to inefficient drug delivery to the retina. The present study aimed to explore the sustained therapeutic effects of fenofibrate-loaded biodegradable nanoparticles (NP) on both DR and neovascular age-related macular degeneration (AMD). Fenofibrate was successfully encapsulated into poly(lactic-co-glycolic acid) (PLGA) NP (Feno-NP), and Feno-NP were optimized by varying polymer composition to achieve high drug loading and prolonged drug release. The Feno-NP made of PLGA 34kDa demonstrated a drug content of 6% w/w and a sustained drug release up to 60 days in vitro. Feno-NP (PLGA 34kDa) was selected for following in vivo studies, and one single intravitreal (IVT) injection of Feno-NP into rat eyes with a 30G fine needle maintained sustained fenofibric acid drug level in the eye for more than 60 days. The efficacy of Feno-NP in DR and neovascular AMD was investigated using streptozotocin (STZ)-induced diabetic rats, laser-induced choroidal neovascularization (CNV) rats

Deficiency of endothelial progenitor cells, including endothelial colony-forming cells (ECFCs) and circulating angiogenic cells (CACs), plays an important role in retinal vascular degeneration in diabetic retinopathy (DR). Fenofibrate, an agonist of peroxisome proliferator-activated receptor a (PPARa), has shown therapeutic effects on DR in both patients and diabetic animal models. However, the function of PPARa in ECFC/CACs has not been defined. In this study, we determined the regulation of ECFC/CAC by PPARa. As shown by flow cytometry and Seahorse analysis, ECFC/CAC numbers and mitochondrial function were decreased in the bone marrow, circulation, and retina of db/db mice, correlating with PPARa downregulation. Activation of PPARa by fenofibrate normalized ECFC/CAC numbers and mitochondrial function in diabetes. In contrast, PPARa knockout exacerbated ECFC/CAC number decreases and mitochondrial dysfunction in diabetic mice. Primary ECFCs from PPARa 2/2 mice displayed impaired proliferation, migration, and tube formation. Furthermore, PPARa 2/2 ECFCs showed reduced mitochondrial oxidation and glycolysis compared with wild type, correlating with decreases of Akt phosphorylation and expression of its downstream genes regulating ECFC fate and metabolism. These findings suggest that PPARa is an endogenous regulator of ECFC/ CAC metabolism and cell fate. Diabetes-induced downregulation of PPARa contributes to ECFC/CAC deficiency and retinal vascular degeneration in DR.

Purpose Dickkopf-1 (DKK-1) is a secreted inhibitor of the Wnt/b-catenin signaling pathway, which plays a pathogenic role in diabetic retinopathy (DR). We aimed to investigate whether DKK-1 levels in the plasma and the vitreous are associated with DR in type 2 diabetes mellitus (DM) patients. Methods Case-control study: plasma samples were collected from 125 type 2 DM including 81 DR (29 non-proliferative DR (NPDR) and 52 proliferative DR (PDR)), 44 non-DR patients (NDR), and 100 non-diabetic controls. Undiluted vitreous fluid samples were obtained from 30 PDR and 25 nondiabetic patients. DKK-1 concentrations in samples were determined using enzymelinked immunosorbent assay. Variables were compared with the Kruskal-Wallis H test, Mann-Whitney U-test, and w 2 -test, when appropriate. Results Plasma DKK-1 levels were significantly lower in DR patients (median: 465.77 pg/ml, range: 137.11-1190.31) than in non-diabetic controls (656.83 pg/ml, 171.63-1795.08; Po0.001) and NDR patients (693.04 pg/ml, 305.43-1218.35; Po0.001). Furthermore, DKK-1 levels were lower in PDR patients (425.21 pg/ml, 137.10-1077.32) compared with NPDR patients (594.86 pg/ml, 256.36-1393.27; P ¼ 0.003). Vitreous absolute DKK-1 levels in PDR patients (259.04 pg/ml, 104.44-596.96) were higher than in nondiabetic controls (138.26 pg/ml, 18.69-239.52; Po0.001). After normalizing by total vitreous protein concentrations, however, there was no significant difference between the groups. DKK-1 levels in vitreous were lower than those in plasma in both groups (Po0.001 for controls; P ¼ 0.002 for PDR patients).Conclusions Decreased plasma DKK-1 levels, which may contribute to the Wnt pathway activation, are associated with the presence and progression of DR, and have potential to become a biomarker for DR.

Purpose: Elevated blood levels of C-reactive protein (CRP) are associated with both type 1 and type 2 diabetes and diabetic complications, such as diabetic retinopathy (DR). However, its pathogenic role in DR remains unknown. The present study aims to investigate the potential role of CRP in DR pathogenesis and explore its underlying mechanism. Materials and methods: Human CRP transgenic (hCRP-Tg) rats were employed for streptozotocin (STZ)-induced diabetic and oxygen-induced retinopathy (OIR) models. The retina function was monitored by electroretinography (ERG) and retinal thickness was measured by optical coherence tomography (OCT). TUNEL and cell death ELISA were performed to measure the apoptosis. Oxidative stress was detected by the measurement of reactive oxygen species (ROS) in cells and 3-Nitrotyrosine staining in tissue sections. Results: In non-diabetic condition, hCRP-Tg with elevated hCRP levels in the retinas demonstrated declined ERG responses and decreased retinal thickness. In STZ-induced diabetic condition, overexpression of hCRP deteriorated retinal neurodegeneration as shown by ERG and apoptosis assays. hCRP also exacerbated retinal leukostasis and acellular capillary formation induced by diabetes. In the OIR model, overexpression of hCRP exacerbated retinal neovascularization (NV). In retinal cell lines, hCRP treatment induced cell death and over-production of ROS. Furthermore, hCRP-induced overexpression of pro-inflammatory, pro-oxidative, and pro-angiogenic factors was associated with up-regulation of CD32 and the NF-κB signaling in the retinas. Conclusions: Elevated hCRP levels play a pathogenic role in DR. Targeting the hCRP-CD32-NF-κB pathway may represent a novel therapeutic strategy for DR.