Wanwisa Promsote

Monomethylfumarate Induces γ-Globin Expression and Fetal Hemoglobin Production in Cultured Human Retinal Pigment Epithelial (RPE) and Erythroid Cells, and in Intact Retina

PURPOSE Sickle retinopathy (SR) is a major cause of vision loss in sickle cell disease (SCD). There are no strategies to prevent SR and treatments are extremely limited. The present study evaluated (1) the retinal pigment epithelial (RPE) cell as a hemoglobin producer and novel cellular target for fetal hemoglobin (HbF) induction, and (2) monomethylfumarate (MMF) as an HbF-inducing therapy and abrogator of oxidative stress and inflammation in SCD retina. METHODS Human globin gene expression was evaluated by RT-quantitative (q)PCR in the human RPE cell line ARPE-19 and in primary RPE cells isolated from Townes humanized SCD mice. γ-Globin promoter activity was monitored in KU812 stable dual luciferase reporter expressing cells treated with 0 to 1000 μM dimethylfumarate, MMF, or hydroxyurea (HU; positive control) by dual luciferase assay. Reverse transcriptase-qPCR, fluorescence-activated cell sorting (FACS), immunofluorescence, and Western blot techniques were used to evaluate γ-globin expression and HbF production in primary human erythroid progenitors, ARPE-19, and normal hemoglobin producing (HbAA) and homozygous β(s) mutation (HbSS) RPE that were treated similarly, and in MMF-injected (1000 μM) HbAA and HbSS retinas. Dihydroethidium labeling and nuclear factor (erythroid-derived 2)-like 2 (Nrf2), IL-1β, and VEGF expression were also analyzed. RESULTS Retinal pigment epithelial cells express globin genes and synthesize adult and fetal hemoglobin MMF stimulated γ-globin expression and HbF production in cultured RPE and erythroid cells, and in HbSS mouse retina where it also reduced oxidative stress and inflammation. CONCLUSIONS The production of hemoglobin by RPE suggests the potential involvement of this cell type in the etiology of SR. Monomethylfumarate influences multiple parameters consistent with improved retinal health in SCD and may therefore be of therapeutic potential in SR treatment.

Iron Overload Accelerates the Progression of Diabetic Retinopathy in Association with Increased Retinal Renin Expression

Diabetic retinopathy (DR) is a leading cause of blindness among working-age adults. Increased iron accumulation is associated with several degenerative diseases. However, there are no reports on the status of retinal iron or its implications in the pathogenesis of DR. In the present study, we found that retinas of type-1 and type-2 mouse models of diabetes have increased iron accumulation compared to non-diabetic retinas. We found similar iron accumulation in postmortem retinal samples from human diabetic patients. Further, we induced diabetes in HFE knockout (KO) mice model of genetic iron overload to understand the role of iron in the pathogenesis of DR. We found increased neuronal cell death, vascular alterations and loss of retinal barrier integrity in diabetic HFE KO mice compared to diabetic wildtype mice. Diabetic HFE KO mouse retinas also exhibited increased expression of inflammation and oxidative stress markers. Severity in the pathogenesis of DR in HFE KO mice was accompanied by increase in retinal renin expression mediated by G-protein-coupled succinate receptor GPR91. In light of previous reports implicating retinal renin-angiotensin system in DR pathogenesis, our results reveal a novel relationship between diabetes, iron and renin-angiotensin system, thereby unraveling new therapeutic targets for the treatment of DR.

Expression of the Niacin Receptor GPR109A in Retina: More than Meets the Eye?

GPR109A was discovered recently as the G-protein coupled receptor for niacin (nicotinic acid), a drug used widely in the treatment of hyperlipidemia. Upon its initial discovery, expression of the receptor was thought to be restricted primarily to adipocytes and immune cells (monocytes/macrophages), a pattern of localization consistent with the known actions of niacin – anti-lipolytic and anti-atherogenic. Of late however, several new reports have arisen detailing expression of the receptor in other cell and tissue types. Interestingly, with the exception of dermal Langerhans cells, the cells responsible for skin flushing, an unwanted side effect of high-dose niacin therapy, the function of the receptor in the additional cell types described is largely anti-inflammatory in nature. The receptor might also have a role in cancer; silencing of the receptor has been reported in colon and breast cancers, and forced expression of the receptor in tumor cells induces apoptosis, thereby suggesting a tumor-suppressive role for the receptor. This supports strongly not only the critical importance of GPR109A expression and activity under normal, basal conditions, but also the strength in impact that therapies capable of augmenting or optimizing its expression and activation may have in thwarting the development and progression of inflammation and cancer. Given the key causative role of inflammation in diabetic retinopathy, and the critical lack of viable strategies for intervening early in this pathology, new therapies, particularly those targeting inflammation, are sorely needed. Herein, we describe preclinical and clinical studies documenting the expression of GPR109A, the pleiotropic effects elicited in response to its activation and the underlying mechanisms to explain these actions. This information we discuss in the context of its relevance to diabetic retina, ultimately providing insight into strategy for future targeting of the receptor and development of new therapies for prevention and treatment of retinopathy in diabetes.