TWEAK/Fn14 Pathway is Activated by Hypoxia and is a Novel Mediator of Retinal Neovascularization

Retinal neovascularization (NV) is a major cause of vision loss in vascular eye diseases such as diabetic retinopathy, retinal vein occlusion and retinopathy of permaturity. Tumor necrosis factor (TNF)-like weak inducer of apoptosis (TWEAK) and its receptor, fibroblast growth factor inducible 14 (Fn14), have been implicated in angiogenesis but their role in retinal diseases is largely unknown. The purpose of my study was to evaluate the role of TWEAK/Fn14 pathway in NV. I used mouse model of oxygen-induced retinopathy (OIR), which is commonly used for studying NV. OIR was produced by maintaining C57BL/6 wild type neonatal mice in 75% oxygen from postnatal day (P)7 to P12 and room air from P12 to P17. Control mice were kept in room air (RA). Quantitative PCR showed upregulation of Fn14 mRNA in the OIR retina compared to RA. Next, I wanted to see localization of TWEAK and Fn14 in the normal retina (RA) and whether they change in OIR. Immunostaining of retinal sections displayed localization of both TWEAK and Fn14 in neuronal layers in normal retina. However, in the OIR retina, both TWEAK and Fn14 were predominantly expressed in neovascular tufts. This shift in localization was associated with a dramatic overexpression of Fn14 mRNA in the isolated retinal vessels in OIR compared to RA. Hyperoxia treatment (HT), from P14-P17, has been shown to prevent NV in OIR. Interestingly, HT was associated with normalization of both the TWEAK and Fn14 mRNA to RA levels, further supporting the hypotheses that TWEAK/Fn14 is involved in NV and that hypoxia is an inducer of the TWEAK/Fn14 pathway. To study the mechanism by which hypoxia induces TWEAK/Fn14, I used adenovirus to overexpress hypoxia inducible factor (HIF)-1α in human retinal endothelial (HRE) cells. HIF-1α overexpression was associated with upregulation of both TWEAK and Fn14. Finally, blockade of the TWEAK/Fn14 pathway, by intravitreal injection of Fn14-Fc decoy, was associated in a significant reduction in the extent of NV. Through my experiments, I have demonstrated that the TWEAK/Fn14 pathway is involved in the initiation and progression of NV, and ischemia-induced upregulation of Fn14 is mediated by HIF-1α.