Peroxisome proliferator-activated receptor agonists inhibit interleukin-1beta-mediated nitric oxide production in cultured lacrimal gland acinar cells

The role of sphingolipids in meibomian gland dysfunction and ocular surface inflammation

Inflammation occurs in response to tissue injury and invasion of microorganisms and is carried out by the innate and adaptive immune systems, which are regulated by numerous chemokines, cytokines, and lipid mediators. There are four major families of bioactive lipid mediators that play an integral role in inflammation - eicosanoids, sphingolipids (SPL), specialized pro-resolving mediators (SPM), and endocannabinoids. SPL have been historically recognized as important structural components of cellular membranes; their roles as bioactive lipids and inflammatory mediators are recent additions. Major SPL metabolites, including sphingomyelin, ceramide, ceramide 1-phosphate (C1P), sphingosine, sphingosine 1-phosphate (S1P), and their respective enzymes have been studied extensively, primarily in cell-culture and animal models, for their roles in cellular signaling and regulating inflammation and apoptosis. Less focus has been given to the involvement of SPL in eye diseases. As such, the aim of this review was to examine relationships between the SPL family and ocular surface diseases, focusing on their role in disease pathophysiology and discussing the potential of therapeutics that disrupt SPL pathways.

The role of peroxisome proliferator-activated receptors in healthy and diseased eyes

Peroxisome Proliferator-Activated Receptors (PPARs) are a family of nuclear receptors that play essential roles in modulating cell differentiation, inflammation, and metabolism. Three subtypes of PPARs are known: PPAR-alpha (PPARα), PPAR-gamma (PPARγ), and PPAR-beta/delta (PPARβ/δ). PPARα activation reduces lipid levels and regulates energy homeostasis, activation of PPARγ results in regulation of adipogenesis, and PPARβ/δ activation increases fatty acid metabolism and lipolysis. PPARs are linked to various diseases, including but not limited to diabetes, non-alcoholic fatty liver disease, glaucoma and atherosclerosis. In the past decade, numerous studies have assessed the functional properties of PPARs in the eye and key PPAR mechanisms have been discovered, particularly regarding the retina and cornea. PPARγ and PPARα are well established in their functions in ocular homeostasis regarding neuroprotection, neovascularization, and inflammation, whereas PPARβ/δ isoform function remains understudied. Naturally, studies on PPAR agonists and antagonists, associated with ocular pathology, have also gained traction with the development of PPAR synthetic ligands. Studies on PPARs has significantly influenced novel therapeutics for diabetic eye disease, ocular neuropathy, dry eye, and age-related macular degeneration (AMD). In this review, therapeutic potentials and implications will be highlighted, as well as reported adverse effects. Further investigations are necessary before any of the PPARs ligands can be utilized, in the clinics, to treat eye diseases. Future research on the prominent role of PPARs will help unravel the complex mechanisms involved in order to prevent and treat ocular diseases.

The role of nitric oxide in ocular surface physiology and pathophysiology

Nitric oxide (NO) has a wide array of biological functions including the regulation of vascular tone, neurotransmission, immunomodulation, stimulation of proinflammatory cytokine expression and antimicrobial action. These functions may depend on the type of isoform that is responsible for the synthesis of NO. NO is found in various ocular tissues playing a pivotal role in physiological mechanisms, namely regulating vascular tone in the uvea, retinal blood circulation, aqueous humor dynamics, neurotransmission and phototransduction in retinal layers. Unregulated production of NO in ocular tissues may result in production of toxic superoxide free radicals that participate in ocular diseases such as endotoxin-induced uveitis, ischemic proliferative retinopathy and neurotoxicity of optic nerve head in glaucoma. However, the role of NO on the ocular surface in mediating physiology and pathophysiological processes is not fully understood. Moreover, methods used to measure levels of NO in the biological samples of the ocular surface are not well established due to its rapid oxidation. The purpose of this review is to highlight the role of NO in the physiology and pathophysiology of ocular surface and propose suitable techniques to measure NO levels in ocular surface tissues and tears. This will improve the understanding of NO's role in ocular surface biology and the development of new NO-based therapies to treat various ocular surface diseases. Further, this review summarizes the biochemistry underpinning NO's antimicrobial action.

CannabinEYEds: The Endocannabinoid System as a Regulator of the Ocular Surface Nociception, Inflammatory Response, Neovascularization and Wound Healing

The endocannabinoid system (ECS) is a complex regulatory system, highly conserved among vertebrates. It has been widely described in nearly all human tissues. In the conjunctiva and cornea, the ECS is believed to play a pivotal role in the modulation of the local inflammatory state as well as in the regulation of tissue repair and fibrosis, neo-angiogenesis and pain perception. This review aims to summarize all the available data on ECS expression and its function in ocular surface structures to provide a specific insight concerning its modulation in dry eye disease, and to propose directions for future research.

High-Fat Diet-Induced Functional and Pathologic Changes in Lacrimal Gland

The lacrimal gland is critical for maintaining the homeostasis of the ocular surface microenvironment through secreting aqueous tears in mammals. Many systemic diseases such as Sjögren syndrome, rheumatoid arthritis, and diabetes can alter the lacrimal gland function, eventually resulting in aqueous tear-deficient dry eye. Here, a high-fat diet (HFD) experimental mouse model was used to clarify how hyperlipidemia affects lacrimal gland function. Aqueous tear secretion fell about 50% after 1 month on a HFD. Lipid droplets accumulated in the matrix and acinar cells of the lacrimal gland after this period, along with changes in the lipid metabolism, changes in gene expression levels, and disruption of fatty acid oxidative activity. Immune cell infiltration and rises in the gene expression levels of the inflammation-related cytokines Il1β, Tnfα, Tsg6, Il10, Mmp2, and Mmp9 were found. HFD also induced mitochondrial hypermegasoma, increased apoptosis, and decreased lacrimal gland acinar cell proliferation. Replacement of the HFD with the standard diet partially reversed pathologic changes in the lacrimal gland. Similarly, supplementing the HFD with fenofibrate also partially reversed the inhibited tear secretion and reduced lipid accumulation, inflammation, and oxidative stress levels. The authors conclude that a HFD induces pathophysiological changes and functional decompensation of the lacrimal gland. Therefore, ingestion of a HFD may be a causative factor of dry eye disease.

The roles of PPARγ and its agonists in autoimmune diseases: A comprehensive review

Autoimmune diseases are common diseases of the immune system that are characterized by the loss of self-tolerance and the production of autoantibodies; the breakdown of immune tolerance and the prolonged inflammatory reaction are undisputedly core steps in the initiation and maintenance of autoimmunity. Peroxisome proliferator-activated receptors (PPARs) are ligand-dependent transcription factors that belong to the nuclear hormone receptor family and act as ligand-activated transcription factors. There are three different isotypes of PPARs: PPARα, PPARγ, and PPARβ/δ. PPARγ is an established regulator of glucose homeostasis and lipid metabolism. Recent studies have demonstrated that PPARγ exhibits anti-inflammatory and anti-fibrotic effects in multiple disease models. PPARγ can also modulate the activation and polarization of macrophages, regulate the function of dendritic cells and mediate T cell survival, activation, and differentiation. In this review, we summarize the signaling pathways and biological functions of PPARγ and focus on how PPARγ and its agonists play protective roles in autoimmune diseases, including autoimmune thyroid diseases, multiple sclerosis, rheumatoid arthritis, systemic sclerosis, systemic lupus erythematosus, primary Sjogren syndrome and primary biliary cirrhosis.

PPARγ: the dominant regulator among PPARs in dry eye lacrimal gland and diabetic lacrimal gland

AIM

To investigate the regulatory roles of the members of the peroxisome proliferator-activated receptor (PPAR) family in lacrimal gland dysfunction under conditions of desiccating stress or diabetes.

METHODS

Quantitative polymerase chain reaction (qPCR) was used to examine the expression of PPARs in the cornea, conjunctiva, meibomian gland, and lacrimal gland in adult rats. The rats were divided into 3 groups: a control group, dry eye group, and diabetic group. The phenol red threads test, tear film break-up time (BUT) test and fluorescein staining were carried out to evaluate the development of dry eye. Based on bioinformatics research, qPCR was used to examine the expression level of PPARγ, tumor necrosis factor-α (TNF-α), interleukin-1β (IL-1β), interleukin-6 (IL-6), sirtuin 1 (Sirt1), myeloid differentiation factor 88 (MyD88) and transforming growth factor-β (TGF-β) in the lacrimal glands.

RESULTS

PPARα and PPARβ/δ were mainly expressed in the conjunctiva and the lacrimal gland, respectively. However, PPARγ was expressed in both the conjunctiva and lacrimal gland, at much higher levels than those measured for PPARα and PPARβ/δ. Dry eye rats and diabetic rats both showed decreased tear secretion, shortened BUT, and increased corneal staining. Significant changes in gene expression were observed compared with the control group. In the lacrimal glands of dry eye rats and diabetic rats, expression of PPARγ decreased (P<0.05), expression of Sirt1 also decreased (P<0.01), whereas expression of TNF-α, IL-1β, IL-6, MyD88, and TGF-β increased (P<0.05).

CONCLUSION

Among PPARs, PPARγ might play a dominant role in the regulation of metabolic- and inflammatory-signaling pathways on the ocular surfaces and in lacrimal glands. Down-regulation of PPARγ is highly relevant to lacrimal gland dysfunction under desiccating-stress and diabetic conditions. PPARγ, thus, is a potential therapeutic target in the treatment of environment- or diabetes-induced dry eye diseases.

Association of Peroxisome Proliferator-Activated Receptors (PPARs) with Diabetic Retinopathy in Human and Animal Models: Analysis of the Literature and Genome Browsers

Diabetic retinopathy (DR) is a condition that develops after long-lasting and poorly handled diabetes and is presently the main reason for blindness among elderly and youth. Peroxisome proliferator-activated receptors (PPARs) are nuclear receptors that are involved in carbohydrate and fatty-acid metabolism and have also been associated with DR. Three PPAR isoforms are known: PPARG, PPARA, and PPARD. In the present study, we retrieved articles reporting associations between PPARs and DR from PubMed database and compiled the data in two catalogues, for human and animal models. Extracted data was then complemented with additional relevant genomic information. Seven retrieved articles reported testing an association between PPARs with DR in human. Four of them concluded association of PPARG and PPARA with DR in European and Asian populations, having a protective role on DR development. One study reported pathogenic role of PPARG, while two articles reported no association between PPARG and DR among Indian and Chinese populations. Six retrieved articles reported testing of involvement of PPARG and PPARA in DR in animal models, including mouse and rat. The review includes case-control studies, meta-analysis, expression studies, animal models, and cell line studies. Despite a large number of documented sequence variants of the PPAR genes available in genome browsers, researchers usually focus on a small set of previously reported variants. Data extraction from Ensembl genome browser revealed several sequence variants with predicted deleterious effect on protein function which present candidates for further experimental validation. Results of the present analysis will enable more holistic approach for understanding of PPARs in DR development. Additionally, developed catalogues present a baseline for standardized reporting of PPAR-phenotype association in upcoming studies.

Global analysis of gene expression profiles in the submandibular salivary gland of klotho knockout mice

Salivary dysfunction commonly occurs in many older adults and is considered a physiological phenomenon. However, the genetic changes in salivary glands during aging have not been characterized. The present study analyzed the gene expression profile in salivary glands from accelerated aging klotho deficient mice (klotho−/−, 4 weeks old). Microarray analysis showed that 195 genes were differentially expressed (z‐score > 2 in two independent arrays) in klotho null mice compared to wild‐type mice. Importantly, alpha2‐Na⁺/K⁺‐ATPase (Atp1a2), Ca²⁺‐ATPase (Atp2a1), epidermal growth factor (EGF), and nerve growth factor (NGF), which have been suggested to be regulators of submandibular salivary gland function, were significantly decreased. When a network was constructed from the differentially expressed genes, proliferator‐activated receptor‐γ (PPAR γ), which regulates energy homeostasis and insulin sensitivity, was located at the core of the network. In addition, the expression of genes proposed to regulate various PPAR γ‐related cellular pathways, such as Klk1b26, Egfbp2, Cox8b, Gpx3, Fabp3, EGF, and NGFβ, was altered in the submandibular salivary glands of klotho−/− mice. Our results may provide clues for the identification of novel genes involved in salivary gland dysfunction. Further characterization of these differentially expressed genes will be useful in elucidating the genetic basis of aging‐related changes in the submandibular salivary gland.

Role of Peroxisome Proliferator-Activated Receptor γ in Ocular Diseases

Peroxisome proliferator-activated receptor γ (PPAR γ), a member of the nuclear receptor superfamily, is a ligand-activated transcription factor that plays an important role in the control of a variety of physiological processes. The last decade has witnessed an increasing interest for the role played by the agonists of PPAR γ in antiangiogenesis, antifibrosis, anti-inflammation effects and in controlling oxidative stress response in various organs. As the pathologic mechanisms of major blinding diseases, such as age-related macular degeneration (AMD), diabetic retinopathy (DR), keratitis, and optic neuropathy, often involve neoangiogenesis and inflammation- and oxidative stress-mediated cell death, evidences are accumulating on the potential benefits of PPAR γ to improve or prevent these vision threatening eye diseases. In this paper we describe what is known about the role of PPAR γ in the ocular pathophysiological processes and PPAR γ agonists as novel adjuvants in the treatment of eye diseases.

Decreased PPAR-γ expression in the conjunctiva and increased expression of TNF-α and IL-1β in the conjunctiva and tear fluid of dry eye mice

The aim of this study was to investigate the expression of peroxisome proliferator-activated receptor γ (PPAR-γ), tumor necrosis factor (TNF)-α and interleukin (IL)-1β in the conjunctiva and the association between inflammatory cytokines and PPAR-γ in dry eye mice. Dry eye was induced in 6-week-old female C57 mice. mRNA expression of PPAR-γ, TNF-α and IL-1β were measured. PPAR-γ protein expression in the conjunctiva, and the contents of TNF-α and IL-1β in the conjunctiva and tear-wash fluid were determined. A PPAR-γ agonist, pioglitazone (PIO), was used to treat dry eye mice. Dry eye mice presented with similar manifestations as in humans. The PPAR-γ expression in the conjunctiva of dry eye mice was downregulated, accompanied by increased contents of TNF-α and IL-1β. PIO treatment markedly reduced the contents of TNF-α and IL-1β in tear fluid of dry eye mice. Following PIO treatment, the PPAR-γ expression increased markedly. PIO may activate PPAR-γ to inhibit the expression of the inflammatory cytokines TNF-α and IL-1β in dry eye mice. This suppresses the inflammatory progression, increases the tear fluid production, elevates the tear film stability and reduces the damage to the ocular surface, exerting a therapeutic effect on dry eye.

Peripheral blood gene expression profiling in Sjögren's syndrome

Sjögren’s syndrome (SS) is a common chronic autoimmune disease characterized by lymphocytic infiltration of exocrine glands. Affected cases commonly present with oral and ocular dryness, thought to be the result of inflammatory cell-mediated gland dysfunction. To identify important molecular pathways involved in SS, we used high-density microarrays to define global gene expression profiles in peripheral blood. We first analyzed 21 SS cases and 23 controls and identified a prominent pattern of overexpressed genes that are inducible by interferons (IFNs). These results were confirmed by evaluation of a second independent dataset of 17 SS cases and 22 controls. Additional inflammatory and immune-related pathways with altered expression patterns in SS cases included B and T cell receptor, IGF-1, GM-CSF, PPARα/RXRα, and PI3/AKT signaling. Exploration of these data for relationships to clinical features of disease revealed that expression levels for most IFN-inducible genes were positively correlated with titers of anti-Ro/SSA (P<0.001) and anti-La/SSB (P<0.001) autoantibodies. Diagnostic and therapeutic approaches targeting IFN signaling pathway may prove most effective in the subset of SS cases who produce anti-Ro/SSA and anti-La/SSB autoantibodies. Our results strongly support innate and adaptive immune processes in the pathogenesis of SS and provide numerous candidate disease markers for further study.

Increased expression of inducible nitric oxide synthase (iNOS) in N-nitrosobis(2-oxopropyl)amine-induced hamster pancreatic carcinogenesis and prevention of cancer development by ONO-1714, an iNOS inhibitor

Elevated protein expression of inducible nitric oxide synthase (iNOS) has been observed in human pancreatic cancers and therefore, iNOS may play important roles in pancreatic carcinogenesis. This was examined in the present study, using an experimental model with N-nitrosobis(2-oxopropyl)amine (BOP)-treated hamsters. Reverse transcription-polymerase chain reaction analysis demonstrated iNOS expression in a hamster pancreatic cancer cell line as well as in human pancreatic cancer cell lines. Immunohistochemical analysis revealed increased expression of iNOS protein in atypical hyperplasia and ductal adenocarcinomas of the pancreas in BOP-treated hamsters. In addition, iNOS expression was also observed in macrophages and islet cells in pancreatic tissue surrounding tumors. In order to assess the role of iNOS expression in carcinogenesis in the pancreas, the effects of ONO-1714 [(1S, 5S, 6R, 7R)-7-chloro-3-imino-5-methyl-2-azabicyclo[4.1.0]heptane], an iNOS inhibitor, on hamster pancreatic ductal carcinogenesis were investigated. Female Syrian golden hamsters were treated with BOP at 10 mg/kg body wt, four times for 1 week, and 1 week after the last carcinogen treatment, ONO-1714 was administered at doses of 100 and 200 p.p.m. in the diet for 15 weeks. The incidences and multiplicities of atypical hyperplasia and invasive adenocarcinoma and total adenocarcinomas (non-invasive and invasive adenocarcinomas) in the pancreas were significantly lowered by treatment with 200 p.p.m. ONO-1714. Treatment with 100 p.p.m. ONO-1714 also significantly decreased the multiplicities of invasive and total adenocarcinomas. Moreover, treatment with 200 p.p.m. ONO-1714 reduced the number of BOP-induced cholangiocellular tumors. These results suggest that iNOS plays roles in promoting pancreatic carcinogenesis in both early and late stages in hamsters.

Effects of selected pharmaceutical products on phagocytic activity in Elliptio complanata mussels

Municipal wastewaters are recognized as a major source of pharmaceutical and personal care products to the aquatic environment, thereby exposing biota to unknown chronic effects. This study sought to examine the immunotoxic effects of pharmaceutical and urban waste products on the freshwater mussel Elliptio complanata. Hemolymph samples were collected and treated in vitro with increasing concentrations of various drugs (bezafibrate, carbamazepine, fluoxetine, gemfibrozil, morphine, naproxen, novobiocin, oxytetracycline, sulfamethazole, sulfapyridine and trimethoprim) and urban waste related chemicals (coprostanol, caffeine, cotinine) for 24 h at 15 degrees C. In a parallel experiment, mussels were caged and placed in two final aeration lagoons for the treatment of domestic wastewaters. At the end of the exposure period, hemolymphs were tested for phagocytic activity, intracellular esterase activity, cell adherence and lipid peroxidation (LPO). The products that most increased phagocytosis were bezafibrate, gemfibrozil and trimethoprim, while novobiocin and morphine reduced its activity. Intracellular esterase activity was reduced most strongly with sulfamethazole, novobiocin, gemfibrozil, bezafibrate and carbamazepine. Cell adherence was decreased by oxytetracycline, novobiocin and naproxen, and increased by gemfibrozil, bezafibrate and sulfapyridine. Exposure to these products also modulated LPO in hemocytes. Coprostanol and naproxen were more potent to reduce LPO while novobiocin and sulfapyridine were the most potent to induce LPO. The potential to induce LPO was positively correlated with the number of functional groups on the molecule (i.e., its nucleophilicity). Mussels exposed to domestic wastewater treatment plant aeration lagoons had decreased intracellular esterase and phagocytic activity as well, suggesting immunosuppression. PPCPs (pharmaceuticals and personal care products) that are recognized to disrupt cytokine signalling network by the nitric oxide pathway and cell permeability were generally the most potent ones. The data suggest that PPCPs have the potential to cause adverse effects on the immune system of bivalves.