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Rooban B Nahomi, Maneesh Mailankot, Timothy Kern, Jie Tang, Allen Lee, Sruthi Sampathkumar, Ram H Nagaraj; Deletion of IDO prevents diabetes-induced retinal capillary degeneration in mice. Invest. Ophthalmol. Vis. Sci. 2014;55(13):5830.
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Recent studies suggest that inflammatory pathways play a role in diabetic retinopathy. However, the biochemical mechanisms by which inflammatory cytokines damage the diabetic retina are poorly understood. Retinal capillary cell death is an early event in diabetic retinopathy. In this study, we investigated the role of interferon-γ (IFN- γ), an inflammatory cytokine, in human retinal endothelial cell (HREC) apoptosis.
Human eyes were obtained from diabetic and non-diabetic donors (44-70 years old). The eyes were collected 6-10 h after death. HREC were isolated and cultured as previously described. Cells were treated with 1 to 5 units of IFN-γ in the presence or absence 25 mM glucose (HG) for 48 hrs. Indoleamine 2,3-dioxygenase (IDO) expression and activity were assessed by western blotting and by an HPLC method. Reactive oxygen species (ROS) was measured using CM-H2DCFDA and apoptosis was determined by Hoechst staining. 1-methyltryptophan at 20 μM was used to block the IDO activity and Ro61-8048 at 50 µM was used to block kynurenine 3-hydroxylase activity in HREC. Diabetes in wild type (WT) and IDO knockout animals was induced by streptozotocin injection. Retinal capillary degeneration was assessed after 8 months of diabetes.
IDO activity and IFN-γ receptor 2 (IFNGR2) levels were higher in human diabetic retinas than in normal retinas. Immunohistochemical data indicated that IDO is present mainly in retinal capillary endothelial cells. IFN-γ upregulated IDO, the first enzyme of the kynurenine pathway in HRECs. High glucose sensitized HRECs to IFN-γ-mediated signaling by upregulating IDO and IFNGR2. IL-1β and TNF-α (each 5-20 ng/ml) did not appreciably induce IDO. IDO-mediated tryptophan oxidation led to the formation of ROS through 3OH-kynurenine and resulted in apoptosis of HRECs. A blockade of IDO by a targeted inhibitor or inhibition of 3OH-kynurenine formation led to significant inhibition of IFN-γ-mediated apoptosis in HRECs. Capillary degeneration, a common manifestation in diabetic retinopathy in mice, was significantly reduced in diabetic IDO knockout mice when compared with diabetic WT mice.
The results suggest that IDO plays an important role in the inflammatory damage in the diabetic retina and IDO could be a new target for the treatment of diabetic retinopathy.
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