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F. M. Recchia, L. Xu, J. S. Penn; The Apelin/APJ Pathway in Retinal Angiogenesis. Invest. Ophthalmol. Vis. Sci. 2010;51(13):28.
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© ARVO (1962-2015); The Authors (2016-present)
Our laboratory has recently demonstrated increased retinal gene expression of apelin (apln), an angiogenic cytokine involved in cardiovascular development and fluid homeostasis, in two rodent models of oxygen-induced retinopathy (OIR). The present studies were undertaken to elucidate the role of apelin and its receptor, APJ, in retinal angiogenesis.
Primary cultures of human retinal vascular endothelial cells (HRVECs) were used for in vitro experiments, and the rat model of OIR was used for in vivo experiments. Near-confluent HRVECs were treated for 24 hrs with either 10% FBS or serum-free medium, in the presence or absence of 2.5uM VEGF. In each of the four conditions, expression of the apelin and APJ genes were measured by qRT-PCR and concentration of apelin in the medium was measured by ELISA. Tube formation assays were performed in triplicate with HRVECs following addition of varying concentrations (2.5uM - 20uM) of recombinant apelin-12, and varying concentrations of two apelin inhibitors (apelin-13[F13A], a competitive antagonist, and siRNA against apln). Following induction of OIR by oxygen cycling, eyes of P15 rats were treated with an intravitreal injection of varying concentrations (1uM - 100uM) of apelin-13(F13A) or vehicle. Six days later, retinal flatmounts were stained with ADPase, and retinal vascular growth was assessed.
Expression of apelin and APJ mRNA in HRVECs, and secretion of apelin by HRVECs, were increased ≥ 2-fold following either stress with serum-free media or stimulation with VEGF. As little as 2.5 uM apelin significantly increased total tube formation (71.0 mm vs. 25.8 mm, P=0.01), an effect comparable to equimolar VEGF. Tube formation was reduced by 50% and 80% following treatment with apelin-13(F13A) and siRNA, respectively. In vivo, mean retinal avascular area was decreased (13.8mm2 vs 29.3mm2, p=0.001), and mean peripheral retinal neovascularization was significantly decreased (4.5 clock-hrs vs. 1.2 clock-hrs), in eyes treated with as little as 10 uM of apelin-13(F13A).
The apelin/APJ pathway appears to be involved in retinal angiogenesis in vitro. Its effects may result from autocrine actions by retinal endothelial cells and through both VEGF-dependent and VEGF-independent mechanisms. Apelin inhibition reduces retinal NV in vivo, possibly by reducing peripheral avascularity. The apelin/APJ pathway may merit further study as a rational therapeutic target for anti-angiogenesis.
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