Diabetic retinopathy remains the leading cause of vision loss in working age adults. Previous research has led to the development of anti-VEGF therapy, which is an effective treatment for proliferative diabetic retinopathy and macular edema in some patients, while other patients are unresponsive. For treatment of non-proliferative diabetic retinopathy, few options are available save good glycemic control, which is problematic for many patients. Recent discoveries offer new insights into the molecular mechanisms underlying diabetic retinopathy and suggest that in addition to oxidative stress, increased inflammation may be a major causative factor in diabetes-induced retinal damage. We recently reported that high glucose significantly increased high mobility group box 1 (HMGB1) protein levels, suggesting a potential role for the alarmin system in regulating retinal responses to high glucose. HMGB1 is extensively involved in inflammation; it can serve as a chaperone to regulate transcription in the nucleus, is secreted by immune cells, interacts with p53, and activates cytokine release. As such, it provides a promising target to blunt the inflammatory response in the retina. Due to its multiple mechanisms of activation and roles in various cell types, an improved understanding of the cellular regulation of HMGB1 actions in the retina becomes increasingly important. Our preliminary data demonstrate that insulin-like growth factor binding protein 3 (IGFBP-3) can inhibit high glucose-induced increases in HMGB1 levels in retinal endothelial cells (REC). We have previously reported that IGFBP-3 KO mice have retinal damage similar to rodent models of diabetic retinopathy, despite normal glucose levels. In addition to IGFBP-3, studies have shown that PKA can directly phosphorylate the Box A region of yeast HMGB1 leading to decreased HMGB1 actions. Studies also showed that increased SIRT1 promoted deacetylation of HMGB1, leading to reduced cytoplasmic translocation. Thus, there is scientific rationale to investigate the regulation of HMGB1 by PKA, Epac1, IGFBP-3, and SIRT1. Furthermore, inhibition of HMGB1 activity using an inhibitor (glycyrrhizin) restored retinal thickness and reduced retinal degenerate capillaries in an in vivo model of retinal ischemia/reperfusion injury in mice. These data have led to the hypothesis that inhibition of HMGB1 activity in the retina protects against diabetes-induced damage. Our overall goal is to determine the mechanisms by which the PKA and Epac1 pathways inhibit HMGB1/inflammation-induced retinal injury and serve as protective pathways that may block diabetic retinal damage.
Overall, these studies will test two potential regulatory pathways that hold promise as therapeutic targets for the retinal damage associated with inflammation caused by diabetes and other disorders. Additionally, work on the cellular mechanisms of HMGB1 inhibition can be used to specifically target therapies toward inhibition of only the detrimental actions of HMGB1.
|Liu, Li; Patel, Paragi; Steinle, Jena J (2018) PKA regulates HMGB1 through activation of IGFBP-3 and SIRT1 in human retinal endothelial cells cultured in high glucose. Inflamm Res 67:1013-1019|
|Jiang, Youde; Liu, Li; Steinle, Jena J (2018) Epac1 deacetylates HMGB1 through increased IGFBP-3 and SIRT1 levels in the retinal vasculature. Mol Vis 24:727-732|