Measuring permeability in human retinal epithelial cells (ARPE-19): implications for the study of diabetic retinopathy

Dependence of Retinal Pigment Epithelium Integrity on the NRF2-Heme Oxygenase-1 Axis

Purpose

Tight junctions (TJs) form the structural basis of retinal pigment epithelium (RPE) barrier functions. Although oxidative stress contributes to age-related macular degeneration, it is unclear how RPE TJ integrity is controlled by redox balance. In this study, we investigated the protective roles of nuclear factor erythroid 2–related factor 2 (NRF2), a transcription factor, and heme oxygenase-1 (HO1), a heme-degrading enzyme encoded by the NRF2 target gene HMOX1.

Methods

ARPE19 cell cultures and mice, including wild-type, Nrf2^−/−, and RPE-specific NRF2-deficient mice, were treated with chemicals that impose oxidative stress or impact heme metabolism. In addition, NRF2 and HO1 expression in ARPE19 cells was knocked down by siRNA. TJ integrity was examined by anti–zonula occludens-1 staining of cultured cells or flatmount RPE tissues from mice. RPE barrier functions were evaluated by transepithelium electrical resistance in ARPE19 cells and immunofluorescence staining for albumin or dextran in eye histological sections.

Results

TJ structures and RPE barrier functions were compromised due to oxidant exposure and NRF2 deficiency but were rescued by HO1 inducer. Furthermore, treatment with HO1 inhibitor or heme precursor is destructive to TJ structures and RPE barrier properties. Interestingly, both NRF2 and HO1 were upregulated under oxidative stress, probably as an adaptive response to mitigate oxidant-inflicted damages.

Conclusions

Our data indicate that the NRF2–HO1 axis protects TJ integrity and RPE barrier functions by driving heme degradation.

Retinal Delivery of the Protein Kinase C-β Inhibitor Ruboxistaurin Using Non-Invasive Nanoparticles of Polyamidoamine Dendrimers

Ruboxistaurin (RBX) is an anti-vascular endothelial growth factor (anti-VEGF) agent that is used in the treatment of diabetic retinopathy and is mainly given intravitreally. To provide a safe and effective method for RBX administration, this study was designed to develop RBX nanoparticles using polyamidoamine (PAMAM) dendrimer generation 5 for the treatment of diabetic retinopathy. Drug loading efficiency, and in vitro release of proposed complexes of RBX: PAMAM dendrimers were determined and the complexation ratio that showed the highest possible loading efficiency was selected. The drug loading efficiency (%) of 1:1, 2.5:1, and 5:1 complexes was 89.2%, 96.4%, and 97.6%, respectively. Loading capacities of 1:1, 2.5:1, and 5:1 complexes were 1.6%, 4.0%, and 7.2% respectively. In comparison, the 5:1 complex showed the best results in the aforementioned measurements. The in vitro release studies showed that in 8 h, the RBX release from 1:1, 2.5:1, and 5:1 complexes was 37.5%, 35.9%, and 77.0%, respectively. In particular, 5:1 complex showed the highest drug release. In addition, particle size measurements showed that the diameter of empty PAMAM dendrimers was 214.9 ± 8.5 nm, whereas the diameters of loaded PAMAM dendrimers in 1:1, 2.5:1, 5:1 complexes were found to be 461.0 ± 6.4, 482.4 ± 12.5, and 420.0 ± 7.1 nm, respectively. Polydispersity index (PDI) showed that there were no significant changes in the PDI between the free and loaded PAMAM dendrimers. The zeta potential measurements showed that the free and loaded nanoparticles possessed neutral charges due to the presence of anionic and cationic terminal structures. Furthermore, the safety of this formulation was apparent on the viability of the MIO-M1 cell lines. This nanoformulation will improve the therapeutic outcomes of anti-VEGF therapy and the bioavailability of RBX to prevent vision loss in patients with diabetic retinopathy.

Glycine-Conjugated Bile Acids Protect RPE Tight Junctions against Oxidative Stress and Inhibit Choroidal Endothelial Cell Angiogenesis In Vitro

We previously demonstrated that the bile acid taurocholic acid (TCA) inhibits features of age-related macular degeneration (AMD) in vitro. The purpose of this study was to determine if the glycine-conjugated bile acids glycocholic acid (GCA), glycodeoxycholic acid (GDCA), and glycoursodeoxycholic acid (GUDCA) can protect retinal pigment epithelial (RPE) cells against oxidative damage and inhibit vascular endothelial growth factor (VEGF)-induced angiogenesis in choroidal endothelial cells (CECs). Paraquat was used to induce oxidative stress and disrupt tight junctions in HRPEpiC primary human RPE cells. Tight junctions were assessed via transepithelial electrical resistance and ZO-1 immunofluorescence. GCA and GUDCA protected RPE tight junctions against oxidative damage at concentrations of 100–500 µM, and GDCA protected tight junctions at 10–500 µM. Angiogenesis was induced with VEGF in RF/6A macaque CECs and evaluated with cell proliferation, cell migration, and tube formation assays. GCA inhibited VEGF-induced CEC migration at 50–500 µM and tube formation at 10–500 µM. GUDCA inhibited VEGF-induced CEC migration at 100–500 µM and tube formation at 50–500 µM. GDCA had no effect on VEGF-induced angiogenesis. None of the three bile acids significantly inhibited VEGF-induced CEC proliferation. These results suggest glycine-conjugated bile acids may be protective against both atrophic and neovascular AMD.

Lipidomics of the brain, retina, and biofluids: from the biological landscape to potential clinical application in schizophrenia

Schizophrenia is a serious neuropsychiatric disorder, yet a clear pathophysiology has not been identified. To date, neither the objective biomarkers for diagnosis nor specific medications for the treatment of schizophrenia are clinically satisfactory. It is well accepted that lipids are essential to maintain the normal structure and function of neurons in the brain and that abnormalities in neuronal lipids are associated with abnormal neurodevelopment in schizophrenia. However, lipids and lipid-like molecules have been largely unexplored in contrast to proteins and their genes in schizophrenia. Compared with the gene- and protein-centric approaches, lipidomics is a recently emerged and rapidly evolving research field with particular importance for the study of neuropsychiatric disorders such as schizophrenia, in which even subtle aberrant alterations in the lipid composition and concentration of the neurons may disrupt brain functioning. In this review, we aimed to highlight the lipidomics of the brain, retina, and biofluids in both human and animal studies, discuss aberrant lipid alterations in correlation with schizophrenia, and propose future directions from the biological landscape towards potential clinical applications in schizophrenia. Recent studies are in support of the concept that aberrations in some lipid species [e.g. phospholipids, polyunsaturated fatty acids (PUFAs)] lead to structural alterations and, in turn, impairments in the biological function of membrane-bound proteins, the disruption of cell signaling molecule accessibility, and the dysfunction of neurotransmitter systems. In addition, abnormal lipidome alterations in biofluids are linked to schizophrenia, and thus they hold promise in the discovery of biomarkers for the diagnosis of schizophrenia.

Glucose-induced microRNA-218 suppresses the proliferation and promotes the apoptosis of human retinal pigment epithelium cells by targeting RUNX2

OBJECTIVE

MicroRNA-218 (miR-218) critical for preventing the progression of numerous diseases, including diseases of the retinal pigment epithelium (RPE). However, the mechanism by which miR-218 regulates the PRE in humans remains largely unknown. Our study investigated the effects of glucose-induced miR-218 expression on human RPE cells (ARPE-19), as well as its targeted regulatory effect.

METHODS

The levels of miR-218 and runt-related transcription factor 2 (RUNX2) expression were investigated by RT-qPCR or Western blot assays. Cell viability and apoptosis were assessed by CCK-8 assays, flow cytometry, and Hoechst staining. A luciferase reporter assay was performed to determine whether Runx2 is a target gene of miR-218.

RESULTS

Our results showed that glucose up-regulated miR-218 expression, suppressed proliferation, and induced the apoptosis of ARPE-19 cells. We verified that miR-218 could inhibit the proliferation and facilitate the apoptosis of ARPE-19 cells, while inhibition of miR-218 expression produced the opposite effects. In terms of mechanism, we demonstrated that RUNX2 was a direct target of miR-218. Functional experiments showed that Runx2 served as a miR-218 target to help inhibit the proliferation and induction of apoptosis in ARPE-19 cells.

CONCLUSION

Our findings suggest the miR-218/Runx2 axis as a potential target for treating diabetic retinopathy (DR).

Effect of Topical Administration of Somatostatin on Retinal Inflammation and Neurodegeneration in an Experimental Model of Diabetes

Somatostatin (SST) is a neuroprotective peptide but little is known regarding the potential role of its anti-inflammatory effects on retinal neuroprotection. In a previous study, we provided the first evidence that topical (eye drops) administration of SST prevents retinal neurodegeneration in streptozotocin (STZ)-induced diabetic rats. However, STZ by itself could cause neurotoxicity, thus acting as a confounding factor. The aims of the present study were: (1) to test the effect of topical administration of SST in the db/db mouse model, a spontaneous model of type 2 diabetes, thus avoiding the confounding effect of STZ on neurodegeneration; (2) to further explore the anti-inflammatory mechanisms of SST in glial cells. This task was performed by using mouse retinal explants and cell cultures. In summary, we confirm that SST topically administered was able to prevent retinal neurodysfunction and neurodegeneration in db/db mice. Furthermore, we found that SST prevented the activation of the classical M1 response of Bv.2 microglial cells upon Lipopolysaccharide (LPS) stimulation as a potent pro-inflammatory trigger. The anti-inflammatory effect of SST in Bv.2 cells was also observed in response to hypoxia. In conclusion, we provide evidence that the neuroprotective effect of SST in diabetic retinas can be largely attributed to anti-inflammatory mechanisms.

Taurocholic acid inhibits features of age-related macular degeneration in vitro

Previous metabolomics studies from our lab found altered plasma levels of bile acids in patients with age-related macular degeneration (AMD) compared to controls. In this study, we investigated the ability of the bile acid taurocholic acid (TCA) to inhibit features of AMD modeled in vitro. Paraquat was used to induce oxidative stress in HRPEpiC primary retinal pigment epithelial (RPE) cells. Cells were treated with 300 μM paraquat alone or with TCA (10, 50, 100, 200, or 500 μM). RPE tight junction integrity was assessed via ZO-1 immunofluorescence and transepithelial electrical resistance (TEER) measurements. RF/6A macaque choroidal endothelial cells were treated with 100 ng/mL vascular endothelial growth factor (VEGF) to induce angiogenesis. The effect of TCA on VEGF-induced angiogenesis was evaluated with cell proliferation, cell migration, and tube formation assays. Addition of TCA at 100 (P = 8.6 × 10^-4), 200 (P = 0.0035), and 500 (P = 2.1 × 10^-4) μM resulted in significant preservation of TEER in paraquat treated cells. In RF/6A cells, TCA did not significantly affect VEGF-induced cell proliferation. VEGF-induced migration of RF/6A cells was significantly inhibited at TCA concentrations of 100 (P = 0.010), 200 (P = 0.023) and 500 (P = 0.0049) μM. VEGF-induced tube formation was significantly inhibited when treated with 200 (P = 0.014) and 500 (P = 7.1 × 10^-4) μM TCA. In vitro, TCA promoted RPE cell integrity and diminished VEGF-induced choroidal endothelial cell migration and tube formation. This suggests that TCA may have protective effects against both degenerative and neovascular AMD.

Elucidating the mechanism of action of alpha-1-antitrypsin using retinal pigment epithelium cells exposed to high glucose. Potential use in diabetic retinopathy

BACKGROUND

Alpha-1-antitrypsin is a protein involved in avoidance of different processes that are seen in diabetic retinopathy pathogenesis. These processes include apoptosis, extracellular matrix remodeling and damage of vessel walls and capillaries. Furthermore, because of its anti-inflammatory effects, alpha-1-antitrypsin has been proposed as a possible therapeutic approach for diabetic retinopathy. Our group tested alpha-1-antitrypsin in a type 1 diabetes mouse model and observed a reduction of inflammation and retinal neurodegeneration. Thus, shedding light on the mechanism of action of alpha-1-antitrypsin at molecular level may explain how it works in the diabetic retinopathy context and show its potential for use in other retinal diseases.

METHODS

In this work, we evaluated alpha-1-antitrypsin in an ARPE-19 human cell line exposed to high glucose. We explored the expression of different mediators on signaling pathways related to pro-inflammatory cytokines production, glucose metabolism, epithelial-mesenchymal transition and other proteins involved in the normal function of retinal pigment epithelium by RT-qPCR and Western Blot.

RESULTS

We obtained different expression patterns for evaluated mediators altered with high glucose exposure and corrected with the use of alpha-1-antitrypsin.

CONCLUSIONS

The expression profile obtained in vitro for the evaluated proteins and mRNA allowed us to explain our previous results obtained on mouse models and to hypothesize how alpha-1-antitrypsin hinder diabetic retinopathy progression on a complex network between different signaling pathways.

GENERAL SIGNIFICANCE

This network helps to understand the way alpha-1-antitrypsin works in diabetic retinopathy and its scope of action.

Fenofibrate prevents the disruption of the outer blood retinal barrier through downregulation of NF-κB activity

AIMS

There is clinical evidence that fenofibrate, a PPARα agonist, arrests the progression of diabetic macular edema (DME). However, the underlying mechanisms of this beneficial effect remain to be elucidated. We previously reported that fenofibric acid (FA), the active metabolite of fenofibrate, prevents the disorganization of tight junction proteins and the hyperpermeability provoked by the diabetic milieu in the retinal pigment epithelium (RPE). The aim of the present study was to evaluate whether this effect is mediated by inhibiting the proinflammatory transcription factor NF-κB, as well as the expression of several proinflammatory cytokines involved in the pathogenesis of DME.

METHODS

Human RPE cells were cultured under standard conditions and under conditions leading to the disruption of the monolayer [IL-1β (10 ng/ml)]. The effect of FA, QNZ (a NF-κB inhibitor), WY14643 (a PPARα agonist), and MK-866 (a PPARα antagonist) in the disruption of the monolayer was determined by dextran permeability and immunohistochemistry analyses. The effect of FA on NF-κB activity was assessed by EMSA and by NF-κB/p65 nuclear translocation analyses. The expression of cytokines (IL-6, IL-8, MCP-1) was measured by RT-PCR.

RESULTS

FA prevented RPE monolayer disruption, and the consequent hyperpermeability induced by IL-1β, through inhibition of NF-κB activity. This effect was due to PPARα activation and was associated with a significant downregulation of the expression of proinflammatory cytokines.

CONCLUSIONS

Our findings suggest that the anti-inflammatory effects of FA through inhibition of NF-κB activity play a key role in the beneficial effect of fenofibrate for treating DME.

geoRge: A Computational Tool To Detect the Presence of Stable Isotope Labeling in LC/MS-Based Untargeted Metabolomics

Studying the flow of chemical moieties through the complex set of metabolic reactions that happen in the cell is essential to understanding the alterations in homeostasis that occur in disease. Recently, LC/MS-based untargeted metabolomics and isotopically labeled metabolites have been used to facilitate the unbiased mapping of labeled moieties through metabolic pathways. However, due to the complexity of the resulting experimental data sets few computational tools are available for data analysis. Here we introduce geoRge, a novel computational approach capable of analyzing untargeted LC/MS data from stable isotope-labeling experiments. geoRge is written in the open language R and runs on the output structure of the XCMS package, which is in widespread use. As opposed to the few existing tools, which use labeled samples to track stable isotopes by iterating over all MS signals using the theoretical mass difference between the light and heavy isotopes, geoRge uses unlabeled and labeled biologically equivalent samples to compare isotopic distributions in the mass spectra. Isotopically enriched compounds change their isotopic distribution as compared to unlabeled compounds. This is directly reflected in a number of new m/z peaks and higher intensity peaks in the mass spectra of labeled samples relative to the unlabeled equivalents. The automated untargeted isotope annotation and relative quantification capabilities of geoRge are demonstrated by the analysis of LC/MS data from a human retinal pigment epithelium cell line (ARPE-19) grown on normal and high glucose concentrations mimicking diabetic retinopathy conditions in vitro. In addition, we compared the results of geoRge with the outcome of X(13)CMS, since both approaches rely entirely on XCMS parameters for feature selection, namely m/z and retention time values. To ensure data traceability and reproducibility, and enabling for comparison with other existing and future approaches, raw LC/MS files have been deposited in MetaboLights (MTBLS213) and geoRge is available as an R script at https://github.com/jcapelladesto/geoRge.

The progress in understanding and treatment of diabetic retinopathy

Diabetic retinopathy is the most frequently occurring complication of diabetes mellitus and remains a leading cause of vision loss globally. Its aetiology and pathology have been extensively studied for half a century, yet there are disappointingly few therapeutic options. Although some new treatments have been introduced for diabetic macular oedema (DMO) (e.g. intravitreal vascular endothelial growth factor inhibitors ('anti-VEGFs') and new steroids), up to 50% of patients fail to respond. Furthermore, for people with proliferative diabetic retinopathy (PDR), laser photocoagulation remains a mainstay therapy, even though it is an inherently destructive procedure. This review summarises the clinical features of diabetic retinopathy and its risk factors. It describes details of retinal pathology and how advances in our understanding of pathogenesis have led to identification of new therapeutic targets. We emphasise that although there have been significant advances, there is still a pressing need for a better understanding basic mechanisms enable development of reliable and robust means to identify patients at highest risk, and to intervene effectively before vision loss occurs.

Paraoxonase enzyme protects retinal pigment epithelium from chlorpyrifos insult

Retinal pigment epithelium (RPE) provides nourishment and protection to the eye. RPE dysfunction due to oxidative stress and inflammation is one of the major reason for many of the retinal disorders. Organophosphorus pesticides are widely used in the agricultural, industrial and household activities in India. However, their effects on the eye in the context of RPE has not been studied. In this study the defense of the ARPE19 cells exposed to Chlorpyrifos (1 nM to 100 µM) in terms of the enzyme paraoxonase (PON) was studied at 24 hr and 9 days of treatment. Chlorpyrifos was found to induce oxidative stress in the ARPE19 cells as seen by significant increase in ROS and decrease in glutathione (GSH) levels without causing cell death. Tissue resident Paraoxonase 2 (PON2) mRNA expression was elevated with chlorpyrifos exposure. The three enzymatic activities of PON namely, paraoxonase (PONase), arylesterase (PON AREase) and thiolactonase (PON HCTLase) were also found to be significantly altered to detoxify and as an antioxidant defense. Among the transcription factors regulating PON2 expression, SP1 was significantly increased with chlorpyrifos exposure. PON2 expression was found to be crucial as ARPE19 cells showed a significant loss in their ability to withstand oxidative stress when the cells were subjected to chlorpyrifos after silencing PON2 expression. Treatment with N-acetyl cysteine positively regulated the PON 2 expression, thus promoting the antioxidant defense put up by the cells in response to chlorpyrifos.

Overexpression of hemopexin in the diabetic eye: a new pathogenic candidate for diabetic macular edema

OBJECTIVE

Hemopexin is a well-recognized permeability factor in the kidney, but its potential role in blood-retinal barrier (BRB) breakdown has not been explored. The main aims of this study were as follows: 1) to determine hemopexin expression in the retina and its content in the vitreous fluid from diabetic patients with diabetic macular edema (DME) and nondiabetic patients, 2) to evaluate the effect of hemopexin on BRB permeability, and 3) to determine whether dexamethasone prevents an eventual hemopexin-induced hyperpermeability.

RESEARCH DESIGN AND METHODS

Biological material included 1) retinas from 10 diabetic donors with nonproliferative retinopathy and from 10 nondiabetic donors and 2) vitreous fluid from 14 patients with DME and 14 nondiabetic patients. Hemopexin and hemopexin receptor mRNA levels were measured by quantitative RT-PCR and hemopexin concentrations by ELISA. The effect of hemopexin on permeability in culture was evaluated in human retinal pigment epithelial (ARPE)-19 cells and bovine retinal endothelial cells. The experiments were repeated in the presence of hemopexin-neutralizing antibodies and dexamethasone.

RESULTS

A higher expression of hemopexin was detected in the retinal pigment epithelium (RPE) from diabetic patients in comparison with nondiabetic control subjects. Intravitreal hemopexin concentration was higher in patients with DME than in nondiabetic subjects. Hemopexin significantly increased permeability in ARPE-19 cells, which was prevented by both hemopexin-neutralizing antibodies and dexamethasone.

CONCLUSIONS

Hemopexin is overexpressed in the RPE of diabetic patients with DME and induces the breakdown of RPE cells in vitro. Dexamethasone was able to prevent hemopexin-induced hyperpermeability. Our results suggest that hemopexin can be considered a new pathogenic candidate for DME.

Effects of vegetable oils on biochemical and biophysical properties of membrane retinal pigment epithelium cells

The aim of this study was to investigate the effect of vegetable oil enrichment of retinal pigment epithelial (RPE) cells on their biochemical and biophysical properties. For this, RPE cells were incubated with 4 different vegetables oils (olive oil, corn oil, argan oil, and camelina oil). The cytotoxicity of these vegetable oils was assessed in vivo on 8-week-old mice and in vitro by using the neutral red and YO-PRO-1 tests. Membrane fluidity was evaluated by fluorescence anisotropy using the fluorescent probe diphenylhexatriene, and membrane fatty acid composition was assessed by gas chromatography. None of the oils tested displayed cytotoxic effects. In vitro, omega-3 rich oils improved membrane fluidity by 47% compared with the control cells. The omega-3 PUFA content within membranes decreased by 38% to 55% when cells were incubated separately with olive oil, corn oil, or argan oil, and increased when cells were incubated with a mixture of those oils, or with camelina oil alone (50% and 103% increase, respectively). Our results show that the fatty acids in vegetable oil incorporate into retinal cells and increase the plasma membrane fluidity.