Extravascular modified lipoproteins: a role in the propagation of diabetic retinopathy in a mouse model of type 1 diabetes

Associations between lipid abnormalities and diabetic retinopathy across a large United States national database

BACKGROUND/OBJECTIVES

While dyslipidaemia has been suggested as a potential risk factor for diabetic retinopathy (DR), previous studies have reported conflicting findings. This study aimed to better characterize the relationship between abnormal serum levels of various lipid markers and the risk of the development and progression of DR.

SUBJECTS/METHODS

This retrospective cohort study utilized a United States national database of electronic medical records. Adults with a history of type 2 diabetes mellitus without type 1 diabetes mellitus were divided into cohorts based on the presence of abnormal serum levels of various lipid markers. Propensity score matching was performed to match cohorts with abnormal lipid levels to those with normal lipid levels on covariates. The cohorts were then compared to evaluate the hazard ratios (HR) of receiving a new DR diagnosis, pars plana vitrectomy, panretinal photocoagulation, vitreous haemorrhage, proliferative diabetic retinopathy, diabetic macular oedema (DMO), and traction retinal detachment.

RESULTS

The database contained 1,126,231 eligible patients (mean age: 60.8 [14.2] years; 46.0% female). Among patients without prior DR, low HDL (HR = 0.94, CI = 0.90-0.98), total cholesterol (HR = 0.88, CI = 0.85-0.91), and high triglyceride (HR = 0.91, CI = 0.86-0.97) levels were associated with a decreased risk of receiving a DR diagnosis. Among patients with preexisting DR, high LDL levels was associated with an increased risk of DMO (HR = 1.42, CI = 1.15-1.75), whereas low HDL levels was associated with a marginally decreased risk (HR = 0.92, CI = 0.85-0.99).

CONCLUSIONS

Elevated levels of markers of dyslipidaemia are inversely associated with the risk of receiving a DR diagnosis, but this relationship is blunted after the onset of DR.

Lipoprotein (a) as a predictor of diabetic retinopathy in patients with type 2 diabetes: A systematic review

BACKGROUND

Lipoprotein a (LP(a)), an LDL-like lipoprotein, known as a risk factor for cardiovascular diseases, has a controversial association with diabetic retinopathy in patients with type 2 diabetes-the current systematic review aimed to critically assess the association between LP(a) and diabetic retinopathy.

METHODS

A systematic review of relevant studies was conducted after a thorough search in PubMed, Scopus, and Google Scholar electronic databases. We used English observational, case-control, and prospective cohort studies published up to August 2022, including type 2 diabetic patients as the population, diabetic retinopathy as the outcome, and LP(a) as the intervention.

RESULT

17 relevant studies, including 4688 patients with diabetes, were included in this systematic review. While in 13 studies, Lipoprotein(a) was recognized as a risk factor for diabetic retinopathy, only three studies reported no evidence of a relationship between the two. Also, another study showed a mixed outcome of the relationship between LP(a) and diabetic retinopathy.

CONCLUSION

High serum lipoprotein(a) in patients with type 2 diabetes is considered a risk factor for diabetic retinopathy. However, further large-scaled cohort studies are still required to validate this finding.

Role of acyl-coenzyme A: cholesterol transferase 1 (ACAT1) in retinal neovascularization

BACKGROUND

We have investigated the efficacy of a new strategy to limit pathological retinal neovascularization (RNV) during ischemic retinopathy by targeting the cholesterol metabolizing enzyme acyl-coenzyme A: cholesterol transferase 1 (ACAT1). Dyslipidemia and cholesterol accumulation have been strongly implicated in promoting subretinal NV. However, little is known about the role of cholesterol metabolism in RNV. Here, we tested the effects of inhibiting ACAT1 on pathological RNV in the mouse model of oxygen-induced retinopathy (OIR).

METHODS

In vivo studies used knockout mice that lack the receptor for LDL cholesterol (LDLR^-/-) and wild-type mice. The wild-type mice were treated with a specific inhibitor of ACAT1, K604 (10 mg/kg, i.p) or vehicle (PBS) during OIR. In vitro studies used human microglia exposed to oxygen-glucose deprivation (OGD) and treated with the ACAT1 inhibitor (1 μM) or PBS.

RESULTS

Analysis of OIR retinas showed that increased expression of inflammatory mediators and pathological RNV were associated with significant increases in expression of the LDLR, increased accumulation of neutral lipids, and formation of toxic levels of cholesterol ester (CE). Deletion of the LDLR completely blocked OIR-induced RNV and significantly reduced the AVA. The OIR-induced increase in CE formation was accompanied by significant increases in expression of ACAT1, VEGF and inflammatory factors (TREM1 and MCSF) (p < 0.05). ACAT1 was co-localized with TREM1, MCSF, and macrophage/microglia makers (F4/80 and Iba1) in areas of RNV. Treatment with K604 prevented retinal accumulation of neutral lipids and CE formation, inhibited RNV, and decreased the AVA as compared to controls (p < 0.05). The treatment also blocked upregulation of LDLR, ACAT1, TREM1, MCSF, and inflammatory cytokines but did not alter VEGF expression. K604 treatment of microglia cells also blocked the effects of OGD in increasing expression of ACAT1, TREM1, and MCSF without altering VEGF expression.

CONCLUSIONS

OIR-induced RNV is closely associated with increases in lipid accumulation and CE formation along with increased expression of LDLR, ACAT1, TREM1, and MCSF. Inhibiting ACAT1 blocked these effects and limited RNV independently of alterations in VEGF expression. This pathway offers a novel strategy to limit vascular injury during ischemic retinopathy.

White Matter Microstructural Abnormalities of the Visual Pathway in Type 2 Diabetes Mellitus: A Generalized Q-sampling Imaging Study

RATIONALE AND OBJECTIVES

Neurodegeneration is an early event in the pathogenesis of diabetic retinopathy (DR). We assessed the white matter microstructural integrity of the visual pathway in diabetes patients vs. healthy subjects, and investigated the advantages of generalized Q-sampling imaging (GQI) in the assessment of the visual pathway.

MATERIALS AND METHODS

T1-weighted, T2-weighted fluid-attenuated inversion recovery, and simultaneous multislice- diffusion sequences were acquired from 21 DR patients, 29 diabetes patients without DR (NDR group), and 28 age- and gender-matched healthy controls. Diffusion source images were reconstructed to GQI. Region of interest (ROI)-based analysis was utilized to evaluate microstructural alterations in the visual pathway. Multivariate linear regression analysis (forward stepwise method) was performed to investigate associations between clinical data and mean GQI parameters.

RESULTS

ROI-based analyses indicated that the GQI parameters generalized fractional anisotropy, quantitative anisotropy (QA), and normalized QA (NQA) were significantly lower in the NDR group than in the healthy controls, and even lower in the DR group than in the NDR group. Disease duration was significantly and negatively correlated with mean generalized fractional anisotropy and mean NQA.

CONCLUSION

GQI could sensitively and non-invasively evaluate the visual pathway in diabetes patients. The nerve fibers of the visual pathway were damaged before the onset of retinopathy, and this damage was aggravated after retinopathy onset, as a consequence of long exposure to hyperglycemia.

Tumor necrosis factor ligand-related molecule 1A maintains blood-retinal barrier via modulating SHP-1-Src-VE-cadherin signaling in diabetic retinopathy

An impaired blood-retinal barrier (BRB) leads to diabetic macular edema (DME), which is a major complication of Diabetic retinopathy (DR). Mediators such as inflammation cause BRB breakdown. However, the explicit mechanism of its disruption is largely unknown. In this study, we identified tumor necrosis factor ligand-related molecule 1A (TL1A) as a crucial factor which protect retinal endothelial cells integrity in DR. By providing both human and mouse data, we show that TL1A is significantly decreased in the retinas of DME patients and diabetic rodents. We further demonstrate that the loss of TL1A accelerated diabetes-induced retinal barrier breakdown. TL1A supplementation protects the diabetic retina against BRB breakdown. Mechanistically, TL1A stabilize intracellular junctions and protect vascular integrity by blocking SHP1-Src-regulated VE-cadherin phosphorylation. Collectively, our findings reveal that loss of TL1A in the retina leads to increased vascular permeability in DR, and that TL1A treatment is of potential therapeutic interest for the treatment of DME.

MD2 blockade prevents modified LDL-induced retinal injury in diabetes by suppressing NADPH oxidase-4 interaction with Toll-like receptor-4

Modified LDL-induced inflammation and oxidative stress are involved in the pathogenesis of diabetic retinopathy. Recent studies have also shown that modified LDL activates Toll-like receptor 4 (TLR4) to mediate retinal injury. However, the mechanism by which modified LDL activates TLR4 and the potential role of the TLR4 coreceptor myeloid differentiation protein 2 (MD2) are not known. In this study, we inhibited MD2 with the chalcone derivatives L2H17 and L6H21 and showed that MD2 blockade protected retinal Müller cells against highly oxidized glycated-LDL (HOG-LDL)-induced oxidative stress, inflammation, and apoptosis. MD2 inhibition reduced oxidative stress by suppressing NADPH oxidase-4 (NOX4). Importantly, HOG-LDL activated TLR4 and increased the interaction between NOX4 and TLR4. MD2 was required for the activation of these pathways, as inhibiting MD2 prevented the association of NOX4 with TLR4 and reduced NOX4-mediated reactive oxygen species production and TLR4-mediated inflammatory factor production. Furthermore, treatment of diabetic mice with L2H17 significantly reduced LDL extravasation in the retina and prevented retinal dysfunction and apoptosis by suppressing the TLR4/MD2 pathway. Our findings provide evidence that MD2 plays a critical role in mediating modified LDL-induced cell injury in the retina and suggest that targeting MD2 may be a potential therapeutic strategy.

Blocking the activity of a protein involved in triggering inflammation and oxidative stress in the retina may reduce diabetes-induced visual impairment and blindness. Besides hyperglycemia, plasma lipids and lipoproteins have also been proposed as potential risk factors for diabetic retinopathy. However, the precise mechanisms controlling low-density lipoprotein-induced retinal damage are unclear. In experiments on mice, Yi Wang at Wenzhou Medical University in Wenzhou, China, and co-workers found that retinal injury caused by highly oxidized glycated-LDL is mediated by the myeloid differentiation protein 2 (MD2). Blocking MD2 with an inhibitor suppressed two key signaling pathways, reducing the production of reactive oxygen species and inflammatory signaling proteins in the retina. Further investigations are needed, but the team believe MD2 could be a vital therapeutic target for diabetic retinopathy.

Effects of Modified Low-Density Lipoproteins and Fenofibrate on an Outer Blood-Retina Barrier Model: Implications for Diabetic Retinopathy

Purpose: There is a lack of treatment for early diabetic retinopathy (DR), including blood-retina barrier (BRB) breakdown. The robust clinical benefit of fenofibrate in DR provides an opportunity to explore disease mechanisms and therapeutic targets. We have previously found that modified lipoproteins contribute to DR and that fenofibrate protects the inner BRB. We now investigate (1) whether modified lipoproteins elicit outer BRB injury and (2) whether fenofibrate may alleviate such damage.

Methods: Human retinal pigment epithelium ARPE-19 cells were cultured in semipermeable transwells to establish a monolayer barrier and then exposed to heavily oxidized, glycated low-density lipoprotein (HOG-LDL, 25–300 mg/L, up to 24 h) versus native (N)-LDL. Transepithelial electric resistance (TEER) and FITC-dextran permeability were measured. The effects of fenofibrate, its active metabolite fenofibric acid, and other peroxisome proliferator-activated receptor (PPARα) agonists (gemfibrozil, bezafibrate, and WY14643) were evaluated, with and without the PPARα antagonist GW6471 or the adenosine monophosphate-activated protein kinase (AMPK) inhibitor Compound C.

Results: HOG-LDL induced concentration- and time-dependent barrier impairment, decreasing TEER and increasing dextran leakage, effects that were amplified by high glucose. Fenofibric acid, but not fenofibrate, gemfibrozil, bezafibrate, or WY14643, attenuated barrier impairment. This effect was reversed significantly by Compound C, but not by GW6471.

Conclusions: Modified lipoproteins elicited outer BRB injury in an experimental model, which was reduced by fenofibric acid through a PPARα-independent, AMPK-mediated mechanism. These findings suggest a protective role of fenofibric acid on the outer BRB in diabetic retina.

A modified laser-induced choroidal neovascularization animal model with intravitreal oxidized low-density lipoprotein

AIM

To investigate whether intravitreal injection of oxidized low-density lipoprotein (OxLDL) can promote laser-induced choroidal neovascularization (CNV) formation in mice and the mechanism involved, thereby to develop a better animal model.

METHODS

C57BL6/J mice were randomized into three groups. Immediately after CNV induction with 532 nm laser photocoagulation, 1.0 µL of OxLDL [100 µg/mL in phosphate-buffered saline (PBS)] was intravitreally injected, whereas PBS and the same volume low-density lipoprotein (LDL; 100 µg/mL in PBS) were injected into the vitreous as controls. Angiogenic and inflammatory cytokines were measured by quantitative real-time polymerase chain reaction (qRT-PCR) and Western blotting (WB) after 5d, and CNV severity was analyzed by choroid flat mount and immunofluorescence staining after 1wk. In vitro, retinal pigment epithelial (RPE) cell line (ARPE19) were treated with OxLDL (LDL as control) for 8h. Angiogenic and inflammatory cytokine levels were measured. A specific inhibitor of lectin-like oxidized low-density lipoprotein receptor 1 (LOX1) was used to evaluate the role of LOX1 in this process.

RESULTS

At 7d after intravitreal injection of 1 µL (100 µg/mL) OxLDL, T15-labeled OxLDL was mainly deposited around the CNV area, and the F4/80-labeled macrophages, the CD31-labeled vascular endothelial cells number and CNV area were increased. Meanwhile, WB and qRT-PCR results showed that vascular endothelial growth factor (VEGF), CC chemokine receptor 2 (CCR2), interleukin-6 (IL-6), IL-1β, and matrix metalloproteinase 9 (MMP9) expressions were increased, which was supported by in vitro experiments in RPE cells. LOX1 inhibitors significantly reduced expressions of inflammatory factors IL-1β and VEGF.

CONCLUSION

A modified laser-induced CNV animal model is established with intravitreal injection of 1 µL (100 µg/mL) of OxLDL at 7d, which at least partially through LOX1. This animal model can be used as a simple model for studying the role of OxLDL in age-related macular degeneration.

The Multifunctionality of CD36 in Diabetes Mellitus and Its Complications-Update in Pathogenesis, Treatment and Monitoring

CD36 is a multiligand receptor contributing to glucose and lipid metabolism, immune response, inflammation, thrombosis, and fibrosis. A wide range of tissue expression includes cells sensitive to metabolic abnormalities associated with metabolic syndrome and diabetes mellitus (DM), such as monocytes and macrophages, epithelial cells, adipocytes, hepatocytes, skeletal and cardiac myocytes, pancreatic β-cells, kidney glomeruli and tubules cells, pericytes and pigment epithelium cells of the retina, and Schwann cells. These features make CD36 an important component of the pathogenesis of DM and its complications, but also a promising target in the treatment of these disorders. The detrimental effects of CD36 signaling are mediated by the uptake of fatty acids and modified lipoproteins, deposition of lipids and their lipotoxicity, alterations in insulin response and the utilization of energy substrates, oxidative stress, inflammation, apoptosis, and fibrosis leading to the progressive, often irreversible organ dysfunction. This review summarizes the extensive knowledge of the contribution of CD36 to DM and its complications, including nephropathy, retinopathy, peripheral neuropathy, and cardiomyopathy.

Oxidative Stress and Microvascular Alterations in Diabetic Retinopathy: Future Therapies

Diabetes is a disease that can be treated with oral antidiabetic agents and/or insulin. However, patients' metabolic control is inadequate in a high percentage of them and a major cause of chronic diseases like diabetic retinopathy. Approximately 15% of patients have some degree of diabetic retinopathy when diabetes is first diagnosed, and most will have developed this microvascular complication after 20 years. Early diagnosis of the disease is the best tool to prevent or delay vision loss and reduce the involved costs. However, diabetic retinopathy is an asymptomatic disease and its development to advanced stages reduces the effectiveness of treatments. Today, the recommended treatment for severe nonproliferative and proliferative diabetic retinopathy is photocoagulation with an argon laser and intravitreal injections of anti-VEGF associated with, or not, focal laser for diabetic macular oedema. The use of these therapeutic approaches is severely limited, such as uncomfortable administration for patients, long-term side effects, the costs they incur, and the therapeutic effectiveness of the employed management protocols. Hence, diabetic retinopathy is the widespread diabetic eye disease and a leading cause of blindness in adults in developed countries. The growing interest in using polyphenols, e.g., resveratrol, in treatments related to oxidative stress diseases has spread to diabetic retinopathy. This review focuses on analysing the sources and effects of oxidative stress and inflammation on vascular alterations and diabetic retinopathy development. Furthermore, current and antioxidant therapies, together with new molecular targets, are postulated for diabetic retinopathy treatment.

Elevated serum OxLDL is associated with progression of type 2 Diabetes Mellitus to diabetic retinopathy

Hyperlipidemia is associated with the progression of diabetic retinopathy (DR). Paraoxonase 1 (PON1), an esterase is known to prevent systemic LDL oxidation. This study assessed if serum oxLDL is associated with the progression of Type 2 DM to DR. This study is part of a three-year hospital based prospective study where 87 subjects were recruited. This included T2DM without DR (n = 22); Non-Proliferative (NPDR) (n = 21) and Proliferative DR (PDR) (n = 22) along with age/sex matched controls (n = 22). Serum oxLDL-Ab was estimated by ELISA. Serum PON esterase activity and plasma Malondialdehyde (MDA) level were estimated by spectrophotometry and the serum Advanced Glycation End products (AGE) level by spectroflourimetry. The systemic levels of oxLDL, AGE and MDA were increased with the progression of T2DM without DR to DR as seen by ANOVA (P < 0.05). Serum oxLDL-Ab levels showed a positive correlation to total cholesterol (P = 0.04) as evaluated in the DR group. Statin intake was found to lower PON esterase activity (P < 0.05). Based on this pilot study, it is proposed that elevated serum oxLDL should be validated in larger cohort studies to ensure it could be potential risk factor for the progression of T2DM to DR.

Association Between Serum Lipoprotein(a) and Diabetic Retinopathy in Han Chinese Patients With Type 2 Diabetes

CONTEXT

Contrasting observations have been made on the relationship between lipoprotein(a) [Lp(a)] concentration and diabetic retinopathy (DR) in patients with type 2 diabetes (T2D).

OBJECTIVE

To measure serum Lp(a) concentrations in patients with T2D to investigate whether Lp(a) affects risk for DR.

DESIGN AND PATIENTS

Serum Lp(a) was determined in 377 Han Chinese patients with T2D. Demographic and clinical information, including presence of DR and vision-threatening DR (VTDR), were collected on admission. The relationship between serum Lp(a) and DR or VTDR was evaluated using univariate and multivariate regression analyses.

RESULTS

Patients with DR or VTDR had significantly higher serum Lp(a) concentrations on admission (P < 0.001). The distribution across Lp(a) quartiles ranged from 11.7% (DR) and 4.3% (VTDR) in the first quartile to 47.9% (DR) and 19.1% (VTDR) in the fourth quartile (P for trend < 0.001). Multivariate logistic regression analysis adjusted for common DR and VTDR risk factors showed that the third and fourth Lp(a) quartiles were significantly associated with DR and VTDR compared with the first Lp(a) quartile (P < 0.001). The patient group with highest concentrations of both Lp(a) (fourth quartile) and hemoglobin A1c (≥7%) had an odds ratio for DR of 5.15 [95% confidence interval (CI), 2.78 to 9.55; P < 0.001] and for VTDR of 5.32 (95% CI, 2.92 to 10.15; P < 0.001) compared with patients with lower concentrations of both factors.

CONCLUSIONS

Lp(a) concentration was independently associated with DR in patients with T2D. More frequent retinal examinations should be recommended for patients with T2D and high Lp(a) concentrations.

Survival or death: a dual role for autophagy in stress-induced pericyte loss in diabetic retinopathy

AIMS/HYPOTHESIS

Intra-retinal extravasation and modification of LDL have been implicated in diabetic retinopathy: autophagy may mediate these effects.

METHODS

Immunohistochemistry was used to detect autophagy marker LC3B in human and murine diabetic and non-diabetic retinas. Cultured human retinal capillary pericytes (HRCPs) were treated with in vitro-modified heavily-oxidised glycated LDL (HOG-LDL) vs native LDL (N-LDL) with or without autophagy modulators: green fluorescent protein-LC3 transfection; small interfering RNAs against Beclin-1, c-Jun NH(2)-terminal kinase (JNK) and C/EBP-homologous protein (CHOP); autophagy inhibitor 3-MA (5 mmol/l) and/or caspase inhibitor Z-VAD-fmk (100 μmol/l). Autophagy, cell viability, oxidative stress, endoplasmic reticulum stress, JNK activation, apoptosis and CHOP expression were assessed by western blots, CCK-8 assay and TUNEL assay. Finally, HOG-LDL vs N-LDL were injected intravitreally to STZ-induced diabetic vs control rats (yielding 50 and 200 mg protein/l intravitreal concentration) and, after 7 days, retinas were analysed for ER stress, autophagy and apoptosis.

RESULTS

Intra-retinal autophagy (LC3B staining) was increased in diabetic vs non-diabetic humans and mice. In HRCPs, 50 mg/l HOG-LDL elicited autophagy without altering cell viability, and inhibition of autophagy decreased survival. At 100-200 mg/l, HOG-LDL caused significant cell death, and inhibition of either autophagy or apoptosis improved survival. Further, 25-200 mg/l HOG-LDL dose-dependently induced oxidative and ER stress. JNK activation was implicated in autophagy but not in apoptosis. In diabetic rat retina, 50 mg/l intravitreal HOG-LDL elicited autophagy and ER stress but not apoptosis; 200 mg/l elicited greater ER stress and apoptosis.

CONCLUSIONS

Autophagy has a dual role in diabetic retinopathy: under mild stress (50 mg/l HOG-LDL) it is protective; under more severe stress (200 mg/l HOG-LDL) it promotes cell death.