Photoreceptor Cells Produce Inflammatory Mediators That Contribute to Endothelial Cell Death in Diabetes

Recent studies suggest that photoreceptor cells regulate local inflammation in the retina in diabetes. The purpose of this study was to determine if photoreceptor cells themselves produce inflammatory proteins in diabetes and if soluble factors released by photoreceptors in elevated glucose induce inflammatory changes in nearby cells. Laser capture microdissection was used to isolate the outer retina (photoreceptors) from the inner retina in nondiabetic and diabetic mice. Diabetes-induced changes in the expression of inflammatory targets were assessed by reverse transcription polymerase chain reaction and immunohistochemistry. Cell culture experiments were carried out to determine if photoreceptors in vitro and ex vivo release soluble mediators that can stimulate nearby cells. Photoreceptor contribution to leukocyte-mediated endothelial cell death was tested using coculture models. Messenger ribonucleic acid and protein expression levels for inflammatory proteins intercellular adhesion molecule 1 (ICAM1), inducible nitric oxide synthase (iNOS), and cyclooxygenase 2 (COX2) were increased in photoreceptors cells in diabetes. In vitro and ex vivo studies show that photoreceptor cells in elevated glucose release mediators that can induce tumor necrosis factor-α in leukocytes and endothelial cells, but not in glia. The soluble mediators released by photoreceptor cells in elevated glucose are regulated by transforming growth factor β-activated kinase 1 and nicotinamide adenine dinucleotide phosphate oxidase (NADPH oxidase) signaling. In contrast to enhanced leukocyte-mediated killing of endothelial cells by leukocytes from wild-type diabetic mice, leukocytes from diabetic mice lacking photoreceptor cells (opsin-/-) did not kill endothelial cells. These data indicate that photoreceptor cells are a source of inflammatory proteins in diabetes, and their release of soluble mediators can contribute to the death of retinal capillaries in diabetes.