Niina Piippo

Decline in cellular clearance systems induces inflammasome signaling in human ARPE-19 cells

Retinal pigment epithelium (RPE) plays a major role in the maintenance of photoreceptors, and degeneration of RPE results in the development of age-related macular degeneration (AMD). Accumulation of intracellular protein aggregates, increased oxidative stress, and chronic inflammation are all factors damaging the functionality of aged RPE cells. Here, we report that inhibition of proteasomal degradation with MG-132 and autophagy with bafilomycin A1 resulted in the release of IL-1β but not that of IL-18 in human ARPE-19 cells. NLRP3 receptor became upregulated, and caspase-1, the functional component of an inflammasome complex, was activated. In addition to accumulating intracellular protein aggregates, inhibition of degradation systems induced oxidative stress which was demonstrated by elevated amounts of intracellular 4-hydroxynonenal (HNE)-protein adducts. Along with IL-1β, exposure to MG-132 and bafilomycin A1 resulted in the secretion of IL-8. A low concentration (1pg/ml) of IL-1β was capable of triggering significant IL-8 production which also became attenuated by treatment with a specific caspase-1 inhibitor. These results suggest that decline in intracellular degradation systems results not only in increased amounts of intracellular protein aggregates and oxidative stress but also in the activation of NLRP3 inflammasomes, arisen as a result of elevated production of biologically active IL-1β.

Fisetin and luteolin protect human retinal pigment epithelial cells from oxidative stress-induced cell death and regulate inflammation

Degeneration of retinal pigment epithelial (RPE) cells is a clinical hallmark of age-related macular degeneration (AMD), the leading cause of blindness among aged people in the Western world. Both inflammation and oxidative stress are known to play vital roles in the development of this disease. Here, we assess the ability of fisetin and luteolin, to protect ARPE-19 cells from oxidative stress-induced cell death and to decrease intracellular inflammation. We also compare the growth and reactivity of human ARPE-19 cells in serum-free and serum-containing conditions. The absence of serum in the culture medium did not prevent ARPE-19 cells from reaching full confluency but caused an increased sensitivity to oxidative stress-induced cell death. Both fisetin and luteolin protected ARPE-19 cells from oxidative stress-induced cell death. They also significantly decreased the release of pro-inflammatory cytokines into the culture medium. The decrease in inflammation was associated with reduced activation of MAPKs and CREB, but was not linked to NF- κB or SIRT1. The ability of fisetin and luteolin to protect and repair stressed RPE cells even after the oxidative insult make them attractive in the search for treatments for AMD.

Quercetin alleviates 4-hydroxynonenal-induced cytotoxicity and inflammation in ARPE-19 cells.

Retinal pigment epithelium (RPE) plays the principal role in age-related macular degeneration (AMD), a progressive eye disease with no cure and limited therapeutical options. In the pathogenesis of AMD, degeneration of RPE cells by multiple factors including increased oxidative stress and chronic inflammation precedes the irreversible loss of photoreceptors and central vision. Here, we report that the plant-derived polyphenol, quercetin, increases viability and decreases inflammation in stressed human ARPE-19 cells after exposure to the lipid peroxidation end product 4-hydroxynonenal (HNE). Several previous studies have been conducted using the direct oxidant H2O2 but we preferred HNE since natural characteristics predispose RPE cells to the type of oxidative damage evoked by lipid peroxidation. Quercetin improved cell membrane integrity and mitochondrial function as assessed in LDH and MTT tests. Decreased production of proinflammatory mediators IL-6, IL-8, and MCP-1 were indicated at the RNA level by qPCR and at the protein level by the ELISA technique. In addition, we probed the signaling behind the effects and observed that p38 and ERK MAPK pathways, and CREB signaling are regulated by quercetin in ARPE-19 cells. In conclusion, our present data suggests that HNE is highly toxic to serum-starved ARPE-19 cells but quercetin is able to reverse these adverse effects even when administered after an oxidative insult.

NLRP3 inflammasome activation is associated with proliferative diabetic retinopathy

Innate immunity and dysregulation of inflammatory processes play a role in vascular diseases like atherosclerosis or diabetes. Nucleotide‐binding domain and Leucine‐rich repeat Receptor containing a Pyrin domain 3 (NLRP3) inflammasomes are pro‐inflammatory signalling complexes that were found in 2002. In addition to pathogens and other extracellular threats, they can be activated by various endogenous danger signals. The purpose of this study was to find out whether NLRP3 activation occurs in patients with sight‐threatening forms of diabetic retinopathy (DR).

Two dietary polyphenols, fisetin and luteolin, reduce inflammation but augment DNA damage-induced toxicity in human RPE cells ☆ ☆☆

Plant-derived polyphenols are known to possess anti-inflammatory and antioxidant effects. In recent years, several studies have investigated their potential benefits for treating chronic diseases associated with prolonged inflammation and excessive oxidative stress, such as age-related macular degeneration (AMD). Previously, two polyphenols, fisetin and luteolin, have been reported to increase the survival of retinal pigment epithelial (RPE) cells suffering from oxidative stress as well as decreasing inflammation but the benefits of polyphenol therapy seem to depend on the model system used. Our aim was to analyze the effects of fisetin and luteolin on inflammation and cellular viability in a model of nonoxidative DNA damage-induced cell death in human RPE (hRPE) cells. Pretreatment of ARPE-19 or primary hRPE cells with the polyphenols augmented etoposide-induced cell death as measured by the lactate dehydrogenase and 3-(4,5-dimethyldiazol-2-yl)-2,5-diphenyltetrazolium bromide assays. However, the treatment was able to reduce the release of two proinflammatory cytokines, IL-6 and IL-8, which were determined by enzyme-linked Immunosorbent assay. Analyses of caspase 3 activity, p53 acetylation and SIRT1 protein levels revealed the apoptotic nature of etoposide-evoked cell death and that fisetin and luteolin augmented the etoposide-induced acetylation of p53 and decreased SIRT1 levels. Taken together, our findings suggest that the cytoprotective effects of fisetin and luteolin depend on the stressor they need to combat, whereas their anti-inflammatory potential is sustained over a variety of model systems. Careful consideration of disease pathways will be necessary before fisetin or luteolin can be recommended as therapeutic agents for inflammatory diseases in general and specifically AMD.

Inhibition of BET bromodomains alleviates inflammation in human RPE cells

Bromodomain-containing proteins are vital for controlling the expression of many pro-inflammatory genes. Consequently, compounds capable of inhibiting specific bromodomain-facilitated protein-protein interactions would be predicted to alleviate inflammation, making them valuable agents in the treatment of diseases caused by dysregulated inflammation, such as age-related macular degeneration. Here, we assessed the ability of known inhibitors JQ-1, PFI-1, and IBET-151 to protect from the inflammation and cell death caused by etoposide exposure in the human retinal pigment epithelial cell line, ARPE-19. The potential anti-inflammatory effects of the bromodomain inhibitors were assessed by ELISA (enzyme-linked immunosorbent assay) profiling. The involvement of NF-κB and SIRT1 in inflammatory signaling was monitored by ELISA and western blotting. Furthermore, SIRT1 was knocked down using a specific siRNA or inhibited by EX-527 to elucidate its role in the inflammatory reaction. The bromodomain inhibitors effectively decreased etoposide-induced release of IL-6 and IL-8. This anti-inflammatory effect was not related to SIRT1 activity, although all bromodomain inhibitors decreased the extent of acetylation of p53 at the SIRT1 deacetylation site. Overall, since bromodomain inhibitors display anti-inflammatory properties in human retinal pigment epithelial cells, these compounds may represent a new way of alleviating the inflammation underlying the onset of age-related macular degeneration.

Lysosomal destabilization activates the NLRP3 inflammasome in human umbilical vein endothelial cells (HUVECs)

Inflammation is a crucial component in the pathogenesis of many vascular diseases, such as atherosclerosis and diabetes. Inflammasomes are intracellular signalling complexes whose activation promotes inflammation. Nucleotide-binding domain and Leucine-rich repeat Receptor containing a Pyrin domain 3 (NLRP3) is a pattern recognition receptor (PRR) forming the best-known inflammasome. Disturbances in NLRP3 have been associated with multiple diseases. The purpose of this study was to explore the lysosomal destabilization-related NLRP3 inflammasome signaling pathway in human endothelial cells. In order to prime and activate NLRP3, human umbilical vein cells (HUVECs) were exposed to TNF-α and the lysosomal destructive agent Leusine-Leusine-O-Methylesther (Leu-Leu-OMe), respectively. A caspase-1 inhibitor was used to block caspase-1’s enzymatic function and an interleukin 1 receptor antagonist (IL-1RA) to prevent any possible secondary effects of IL-1β. Leu-Leu-OMe increased the expression of NLRP3, IL-1β, and IL-18 in HUVECs. Exposure to Leu-Leu-OMe significantly promoted the production of IL-6 and IL-8 in primed HUVECs; this effect was prevented by the pre-treatment of cells with an IL-1RA. Our results suggest that lysosomal destabilization activates the NLRP3 inflammasome pathway that promotes the production of IL-6 and IL-8 in an autocrine manner in HUVEC cells.

Hsp90 inhibition as a means to inhibit activation of the NLRP3 inflammasome

Once activated, the intracellular receptor NLRP3 assembles an inflammasome protein complex that facilitates the caspase-1-mediated maturation of IL-1β and IL-18. Inactive NLRP3 is guarded by a protein complex containing Hsp90. In response to stress stimuli, Hsp90 is released, and NLRP3 can be activated to promote inflammation. In this study, we blocked Hsp90 with geldanamycin and studied the fate of NLRP3 in human retinal pigment epithelial (RPE) cells. RPE cells play a central role in the development of age-related macular degeneration (AMD), a progressive eye disease causing severe vision loss in the elderly. IL-1α-primed ARPE-19 cells, human embryonal stem cell (hESC)-derived RPE cells, and primary human RPE cells were exposed to MG-132 and bafilomycin A to activate NLRP3 via the inhibition of proteasomes and autophagy, respectively. Additionally, RPE cells were treated with geldanamycin at different time points and the levels of NLRP3 and IL-1β were determined. Caspase-1 activity was measured using a commercial assay. Geldanamycin prevented the activation of the inflammasome in human RPE cells. NLRP3 released from its protective complex became degraded by autophagy or secreted from the cells. Controlled destruction of NLRP3 is a potential way to regulate the inflammation associated with chronic diseases, such as AMD.

CB2 receptor activation causes an ERK1/2-dependent inflammatory response in human RPE cells

A chronic low-level inflammation contributes to the pathogenesis of age-related macular degeneration (AMD), the most common cause of blindness in the elderly in Western countries. The loss of central vision results from attenuated maintenance of photoreceptors due to the degeneration of retinal pigment epithelium (RPE) cells beneath the photoreceptor layer. It has been proposed that pathologic inflammation initiated in RPE cells could be regulated by the activation of type 2 cannabinoid receptors (CB2). Here, we have analysed the effect of CB2 activation on cellular survival and inflammation in human RPE cells. RPE cells were treated with the selective CB2 agonist JWH-133 in the presence or absence of the oxidative stressor 4-hydroxynonenal. Thereafter, cellular viability as well as the release of pro-inflammatory cytokines and potential underlying signalling pathways were analysed. Our results show that JWH-133 led to increased intracellular Ca2+ levels, suggesting that RPE cells are capable of responding to a CB2 agonist. JWH-133 could not prevent oxidative stress-induced cell death. Instead, 10 µM JWH-133 increased cell death and the release of proinflammatory cytokines in an ERK1/2-dependent manner. In contrast to previous findings, CB2 activation increased, rather than reduced inflammation in RPE cells.

Oxidative Stress is the Principal Contributor to Inflammasome Activation in Retinal Pigment Epithelium Cells with Defunct Proteasomes and Autophagy

Background/Aims: Previously, we demonstrated that blockade of the intracellular clearance systems in human retinal pigment epithelial (RPE) cells by MG-132 and bafilomycin A1 (BafA) induces NLRP3 inflammasome signaling. Here, we have explored the activation mechanisms behind this process. NLRP3 is an intracellular receptor detecting factors ranging from the endogenous alarmins and adenosine triphosphate (ATP) to ultraviolet radiation and solid particles. Due to the plethora of triggers, the activation of NLRP3 is often indirect and can be mediated through several alternative pathways. Potassium efflux, lysosomal rupture, and oxidative stress are currently the main mechanisms associated with many activators. Methods: NLRP3 inflammasomes were activated in human RPE cells by blocking proteasomes and autophagy using MG-132 and bafilomycin A1 (BafA), respectively. P2X7 inhibitor A740003, potassium chloride (KCl), and glyburide, or N-acetyl-L-cysteine (NAC), ammonium pyrrolidinedithiocarbamate (APDC), diphenyleneiodonium chloride (DPI), and mito-TEMPO were added to cell cultures in order to study the role of potassium efflux and oxidative stress, respectively. IL-1β was measured using the ELISA method. ATP levels and cathepsin B activity were examined using commercial kits, and ROS levels using the fluorescent dye 2´,7´-dichlorodihydrofluorescein diacetate (DCFDA). Results: Elevated extracellular potassium prevented the priming factor IL-1α from inducing the production of reactive oxygen species (ROS). It also prevented IL-1β release after exposure of primed cells to MG-132 and BafA. Inflammasome activation increased extracellular ATP levels, which did not appear to trigger significant potassium efflux. The activity of the lysosomal enzyme, cathepsin B, was reduced by MG-132 and BafA, suggesting that cathepsin B was not playing any role in this phenomenon. Instead, MG-132 triggered ROS production already 30 min after exposure, but treatment with antioxidants blocking NADPH oxidase and mitochondria-derived ROS significantly prevented IL-1β release after this activating signal. Conclusion: Our data suggest that oxidative stress strongly contributes to the NLRP3 inflammasome activation upon dysfunctional cellular clearance. Clarification of inflammasome activation mechanisms provides novel options for alleviating pathological inflammation present in aggregation diseases, such as age-related macular disease (AMD) and Alzheimer’s disease.