Abstract
Purpose: To test the hypothesis that upregulated expression of the proinflammatory cytokine high-mobility group box-1 (HMGB1) in the eyes from patients with proliferative diabetic retinopathy (PDR) regulates the expression of matrix metalloproteinase-9 (MMP-9) in diabetic retina. Methods: Vitreous samples from 25 PDR and 17 nondiabetic patients, retinas from 1-month diabetic rats and normal rats intravitreally injected with HMGB1 and human retinal microvascular endothelial cells (HRMEC) were studied with the use of enzyme-linked immunosorbent assay, Western blot analysis and RT-PCR. We also studied the effects of HMGB1 inhibitor glycyrrhizin and targeted deletion of the MMP-9 gene on diabetes-induced biochemical changes in the retina. An assay for in vitro cell migration was performed on human retinal microvascular endothelial cells (HRMEC). Results: Levels of HMGB1 and MMP-9 were significantly higher in the vitreous fluid from PDR patients compared with nondiabetic patients (P < 0.001 for both comparisons) and these were significantly correlated (r = 0.5, P = 0.003). Diabetes induction and intravitreal injection of HMGB1 in normal rats induced significant upregulation of MMP-9 and downregulation of tissue inhibitor of metalloproteinase-1 (TIMP-1) mRNA levels. Constant glycyrrhizin intake from onset of diabetes attenuated diabetes-induced upregulation of MMP-9, but did not affect TIMP-1 expression in the retina. Deletion of the MMP-9 gene in mice did not inhibit diabetes-induced upregulation of HMGB1 in the retina. However, the MMP-9 inhibitor inhibited HMGB1-induced MMP-9 upregulation and migration in HRMEC. Conclusions: Our findings suggest that MMP-9 acts downstream of HMGB1 and mediates the effect of HMGB1 in diabetic retinopathy.