Date Published: August 10, 2017
Publisher: Public Library of Science
Author(s): Hui He, Rebecca L. Weir, Jordan J. Toutounchian, Jayaprakash Pagadala, Jena J. Steinle, Jerome Baudry, Duane D. Miller, Charles R. Yates, Rajesh Mohanraj.
Retinal microaneurysms, an early disease manifestation of diabetic retinopathy, are associated with retinal endothelial cell (REC) death and macular edema. We previously demonstrated that a quinic acid (QA) analog, KZ-41, promoted REC survival by blunting stress-induced p38 MAPK activation. Herein, we sought to expand our understanding of the pro-survival signal transduction pathways actuated by KZ-41. Using human RECs exposed to high glucose (25 mM, 72 hours), we demonstrated that KZ-41 blocks caspase-3 activation by triggering phosphorylation of the PI3K regulatory subunit (p85; Tyr458) and its downstream target Akt (Ser473). Akt signal transduction was accompanied by autophosphorylation of the receptor tyrosine kinase, insulin growth factor-1 receptor (IGF-1R). IGF-1R knockdown using either the tyrosine kinase inhibitor AG1024 or silencing RNA abolished KZ-41’s pro-survival effect. Under high glucose stress, caspase-3 activation correlated with elevated ERK1/2 phosphorylation and decreased insulin receptor substrate-1 (IRS-1) levels. KZ-41 decreased ERK1/2 phosphorylation and reversed the glucose-dependent reduction in IRS-1. To gain insight into the mechanistic basis for IGF-1R activation by KZ-41, we used molecular modeling and docking simulations to explore a possible protein:ligand interaction between the IGF-1R kinase domain and KZ-41. Computational investigations suggest two possible KZ-41 binding sites within the kinase domain: a region with high homology to the insulin receptor contains one potential allosteric binding site, and another potential site on the other side of the kinase domain, near the hinge domain. These data, together with previous proof-of-concept efficacy studies demonstrating KZ-41 mitigates pathologic retinal neovascularization in the murine oxygen-induced retinopathy model, suggests that QA derivatives may offer therapeutic benefit in ischemic retinopathies.
Diabetic retinopathy (DR), the most frequently occurring microvascular complication of diabetes, is a leading cause of vision loss. Retinal microaneurysms, an early disease manifestation, are associated with retinal endothelial cell (REC) death, capillary dropout, and macular edema . The resultant ischemia triggers hypoxia-induced factor-1 (HIF-1) driven VEGF, eNOS, and ET-1 expression, which are biomarkers of retinal neovascularization (RNV) . Acellular capillary formation in response to hypoxia exacerbates vascular leakage thus propagating a cycle of ischemia and pathological RNV. A better understanding of the mechanisms contributing to glucose-induced REC death may provide novel targets for the development of treatments for DR.
There is an active debate in the literature as to whether or not DR begins with abnormalities in the neuronal and glial cells of the retina or in the retinal blood vessels. Regardless of the initiating event(s), it is clear that microaneurysms in the retina represent the first clinically observable manifestation of disease . Microaneurysms are associated with REC and pericyte loss, which leads to capillary dropout and development of retinal ischemia. Thus, protection of capillary cellular components has been the focus of intensive research in identifying novel targets for treatment of both non-proliferative (NPDR) and proliferative DR (PDR). In the present study, we demonstrate that a quinic acid derivative, KZ-41, rescues RECs from high glucose-induced apoptosis by enhancing pro-survival signaling through PI3K/Akt. Pharmacologic and genomic knockdown experiments identify the IGF-1R as indispensable to KZ-41’s pro-survival mechanism of action (Fig 11).