Abstract
Purpose: :
Proliferative diabetic retinopathy (PDR) is an increasingly common cause of vision loss resulting from retinal neovascularization. Identification of factors associated with retinal angiogenesis may lead to rational anti-angiogenic therapy. We have demonstrated upregulation of the gene apln (coding for apelin, an oxygen-sensitive cytokine required for normal vasculogenesis and for fluid homeostasis in various organ systems) in two rodent models of oxygen-induced retinopathy. This study aimed to determine whether vitreous levels of apelin were elevated in cases of PDR, in order to rationalize, by way of clinical correlation, further study of the apelin pathway in PDR pathogenesis.
Methods: :
Undiluted vitreous and serum samples were obtained from patients undergoing vitreoretinal surgery for PDR and from diabetic and non-diabetic patients undergoing surgery for non-vascular disease. Concentrations of apelin and VEGF were measured in duplicate by ELISA and compared. The angiogenic effect of apelin was assessed by a tube formation assay using primary cultures of human retinal vascular endothelial cells (HRVECs) stimulated with varying concentrations of apelin (2.5 µM - 20 µM) and with 2.5 µM VEGF.
Results: :
Vitreous apelin concentrations were significantly different among the three groups (PDR, non-diabetics, and diabetics without PDR; p=0.007 by ANOVA). Patients with PDR (n=21) had a mean vitreous apelin concentration of 50.3 pg/mL, while non-diabetic patients (n=33) had a mean concentration of 29.3 pg/mL (p=0.014 by Wilcoxon rank-sum). Serum concentrations of apelin did not differ significantly between PDR (367.1 pg/mL) and non-diabetic (358.8 pg/mL) groups (p=0.21). Vitreous VEGF levels differed significantly (p<0.001) between PDR patients (522.1 pg/mL) and non-diabetic patients (89.6 pg/mL). In HRVECs, 2.5 µM of recombinant apelin significantly increased capillary tube formation (total tube length of 71.0 mm vs. 25.8 mm in untreated cells, p=0.01) and to a similar degree as 2.5 µM VEGF (total tube length of 71.3 mm).
Conclusions: :
Vitreous levels of apelin were significantly elevated in patients with PDR and may be released by ischemic retina, rather than diffusing from serum. The increase in vitreous apelin and its in vitro angiogenic potential, independent of VEGF, suggest that the apelin signaling pathway warrants further study as to its involvement in PDR and its potential as an antiangiogenic target.
Keywords: retinal neovascularization • diabetic retinopathy • neovascularization