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 (PPARß/δ). PPARα activation reduces lipid levels and regulates energy homeostasis, activation of PPARγ results in regulation of adipogenesis, and PPARß/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 PPARß/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.

PPARα is essential for retinal lipid metabolism and neuronal survival.

BACKGROUND: Peroxisome proliferator activated receptor-alpha (PPARα) is a ubiquitously expressed nuclear receptor. The role of endogenous PPARα in retinal neuronal homeostasis is unknown. Retinal photoreceptors are the highest energy-consuming cells in the body, requiring abundant energy substrates. PPARα is a known regulator of lipid metabolism, and we hypothesized that it may regulate lipid use for oxidative phosphorylation in energetically demanding retinal neurons. RESULTS: We found that endogenous PPARα is essential for the maintenance and survival of retinal neurons, with Pparα -/- mice developing retinal degeneration first detected at 8 weeks of age. Using extracellular flux analysis, we identified that PPARα mediates retinal utilization of lipids as an energy substrate, and that ablation of PPARα ultimately results in retinal bioenergetic deficiency and neurodegeneration. This may be due to PPARα regulation of lipid transporters, which facilitate the internalization of fatty acids into cell membranes and mitochondria for oxidation and ATP production. CONCLUSION: We identify an endogenous role for PPARα in retinal neuronal survival and lipid metabolism, and furthermore underscore the importance of fatty acid oxidation in photoreceptor survival. We also suggest PPARα as a putative therapeutic target for age-related macular degeneration, which may be due in part to decreased mitochondrial efficiency and subsequent energetic deficits.

Noninvasive Vagus Nerve Stimulation in Postural Tachycardia Syndrome: A Randomized Clinical Trial.

BACKGROUND: Low-level transcutaneous stimulation of the auricular branch of the vagus nerve at the tragus is antiarrhythmic and anti-inflammatory in animals and humans. Preliminary studies show that transcutaneous vagus nerve stimulation (tVNS) is beneficial in animal models of postural tachycardia syndrome (POTS). OBJECTIVES: In this study the authors conducted a sham-controlled, double-blind, randomized clinical trial to examine the effect of tVNS on POTS over a 2-month period relative to sham stimulation. METHODS: tVNS (20 Hz, 1 mA below discomfort threshold) was delivered using an ear clip attached to either the tragus (active; n = 12) or the ear lobe (sham; n = 14) for 1 hour daily over a 2-month period. Postural tachycardia was assessed during the baseline and 2-month visit. Heart rate variability based on 5-minute electrocardiogram, serum cytokines, and antiautonomic autoantibodies were measured at the respective time points. RESULTS: Mean age was 34 ± 11 years (100% female; 81% Caucasian). Adherence to daily stimulation was 83% in the active arm and 86% in the sham arm (P > 0.05). Postural tachycardia was significantly less in the active arm compared with the sham arm at 2 months (mean postural increase in heart rate 17.6 ± 9.9 beats/min vs 31.7 ± 14.4 beats/min; P = 0.01). Antiadrenergic autoantibodies and inflammatory cytokines were lower in the active arm compared with the sham arm at 2 months (P < 0.05). Heart rate variability was better in the active arm. No device-related side effects were observed. CONCLUSIONS: Our results support the emerging paradigm of noninvasive neuromodulation to treat POTS. Mechanistically, this effect appears to be related to reduction of antiautonomic autoantibodies and inflammatory cytokines, and improvement in autonomic tone. Further studies are warranted. (Autoimmune Basis for Postural Tachycardia Syndrome; NCT05043051).

Hydroxybenzamide derivatives protect pancreatic ß cell by suppressing unfolded protein response activation.

Endoplasmic reticulum (ER) stress-induced Pancreatic ß-cell dysfunction and death plays important roles in the development of diabetes. The 1,2,3-triazole derivative 1 is one of only a few structures that have thus far been identified that protect ß cells against ER stress, but it is limited for its narrow activity range. In this study, we designed and synthesized a series of hydroxybenzamide (HBA) derivatives in which the triazole pharmacophore was substituted with an amide linker. Structure-activity relationship studies identified WO3i (3-hydroxy-N-(4-[trifluoromethyl]benzyl)benzamide) that possesses ß-cell protective activity against ER stress at a 100% maximal activity with EC50 at 0.19 µM). We showed that WO3i suppresses the expression of CHOP, a key mediator of ER stress-induced apoptosis, and the activation of apoptotic genes. Mechanistically, we further showed that WO3i suppresses the ER stress-induced activation of all three pathways of unfolded protein response-ATF6, IRE1α, and PERK. Identification of this novel ß-cell-protective scaffold thus provides a new promising modality for the potential for drug development for the treatment of diabetes.

Pathogenic Role of PPARα Downregulation in Corneal Nerve Degeneration and Impaired Corneal Sensitivity in Diabetes.

The purpose of this study was to investigate the protective role of peroxisome proliferator-activated receptor α (PPARα) against diabetic keratopathy and corneal neuropathy. Corneal samples were obtained from human donors with and without diabetes. Streptozotocin-induced diabetic rats and mice were orally treated with PPARα agonist fenofibrate. As shown by immunohistochemistry and Western blotting, PPARα was downregulated in the corneas of humans with diabetes and diabetic rats. Immunostaining of ß-III tubulin demonstrated that corneal nerve fiber metrics were decreased significantly in diabetic rats and mice, which were partially prevented by fenofibrate treatment. As evaluated using a Cochet-Bonnet aesthesiometer, corneal sensitivity was significantly decreased in diabetic mice, which was prevented by fenofibrate. PPARα -/- mice displayed progressive decreases in the corneal nerve fiber density. Consistently, corneal sensitivity was decreased in PPARα -/- mice relative to wild-type mice by 21 months of age. Diabetic mice showed increased incidence of spontaneous corneal epithelial lesion, which was prevented by fenofibrate while exacerbated by PPARα knockout. Western blot analysis revealed significantly altered neurotrophic factor levels in diabetic rat corneas, which were partially restored by fenofibrate treatment. These results indicate that PPARα protects the corneal nerve from degeneration induced by diabetes, and PPARα agonists have therapeutic potential in the treatment of diabetic keratopathy.

Pathogenic Role of microRNA-21 in Diabetic Retinopathy Through Downregulation of PPARα.

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.