The global prevalence of diabetic retinopathy continues to rise as more individuals are diagnosed with diabetes each year. With a rising patient population, the need for a cure is greater than ever. The current available treatment strategies are aimed at treating the more advanced stages of the disease, macular edema and neovascularization, and are not efficacious for a large portion of the patient population. In order to develop more effective and targeted therapies, a better understanding of the molecular mechanisms involved in the pathophysiology of diabetic retinopathy are needed. Chronic low-grade inflammation induced by hyperglycemia is commonly understood to play a role in disease progression. Interleukin-6 (IL-6) is a prominent inflammatory cytokine whose role in disease advancement is not well understood. The overall of effects of IL-6 are determined by two diverse signaling pathways: classical IL-6 signaling and IL-6 trans-signaling. The expression of the membrane-bound IL-6 receptor (mIL-6R) and glycoprotein 130 (gp130) are needed for classical IL-6 signaling, while gp130 and the soluble IL-6 receptor (sIL-6R) are needed for IL-6 trans-signaling. Activation of these pathways are hypothesized to promote anti-inflammatory or pro-inflammatory functions, respectively. Therefore, the aim of this study was to determine the IL-6 signaling capabilities of Human Muller and retinal endothelial cells, both of which are crucial for maintaining a healthy retina, and its effect on function and viability. In addition, this dissertation examined the regulation of IL-6 signaling pathways in response to intravitreal treatment with ranibizumab, the most common anti-VEGF therapy in patients with proliferative diabetic retinopathy and diabetic macular edema.First, we determined that human Muller cells (HMC) express both mIL-6R and gp130, which allows them to signal through both classical IL-6 and IL-6 trans-signaling. Interestingly, treatment with IL-6, the agonist of classical IL-6 signaling, protected HMCs from hyperglycemia-induced cell death. Further, these protective effects were mediated by VEGF-A. Surprisingly, treatment with IL-6/sIL-6R, the agonist of IL-6 trans-signaling, had slight protective effects, but were not dependent on VEGF-A. Second, it was established that human retinal endothelial cells (HREC) express gp130, but do not express mIL-6R. Therefore, HRECs were only responsive to IL-6 trans-signaling but not to IL-6-induced classical signaling. Treatment of HRECs with IL-6/sIL-6R (10 ng/mL) decreased cell viability and led to increased release of inflammatory cytokines. Intriguingly, when treated with a higher concentration of IL-6/sIL-6R (50 ng/mL) HRECs began releasing VEGF-A and forming tubular networks. Finally, our clinical study has shown that intravitreal ranibizumab had a strong impact on aqueous humor levels of soluble cytokine receptors, some of which belonging to IL-6 signaling. Interestingly, changes in VEGF family members (A and C) were correlated with changes in IL-6 signaling members. Taken together, all our findings suggest that an IL-6-VEGF signaling axis may be important for the development of diabetic retinopathy.
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