Hypoxia specific SDF-1 expression by retinal pigment epithelium initiates bone marrow-derived cells to participate in choroidal neovascularization in a laser-induced mouse model

Purpose: Choroidal neovascularization (CNV) is a major cause of vision loss in patients with age-related macular degeneration (AMD). Stromal cell-derived factor-1 (SDF-1)/CXC chemokine receptor 4 (CXCR4) plays a critical role in homing of bone marrow-derived cells (BMCs) to choroidal neovascularization (CNV). In this study, we investigated the contribution of hypoxia specific HIF-1α-induced SDF-1 expression in retinal pigment epithelium (RPE) cells and the potential role of SDF-1 in CNV formation. Materials and Methods: Green fluorescent protein (GFP) chimeric mice were developed by transplanting bone marrow cells of gfp ^+/+ transgenic mice to sublethally irradiated C57BL/6J mice. CNV was induced by laser photocoagulation. Ocular tissue was processed for immunofluorescence to detect HIF-1α and SDF-1 expression, and cell surface markers such as CXCR4, CD34 and CD31 and so on during CNV formation. In vitro, adult human RPE (hRPE) cells were cultured under conditions of chemical hypoxia using CoCl2 administration. And RNAi technique was used to knock down HIF-1α gene to observe the expression of HIF-1α and SDF-1 in hRPE cells. Results: BMCs trafficked around laser lesion adjacent to RPE layer 4h after laser photocoagulation, where SDF-1 expression was relatively higher. With increasing expression of SDF-1, more BMCs were infiltrated into laser lesion to participate in CNV, and both reached peak at 3 d (p<0.05). About 81% BMCs involved in CNV were CXCR4 ⁺. Many of them acquired the surface marker of endothelial precursor cells (CD34 ⁺) and endothelial cells (CD31 ⁺). The constituent ratio of CD34 ⁺ and CD31 ⁺ BMCs increased with SDF-1 expression. In vitro, we proved that hypoxia specific-HIF-1α influenced SDF-1 expression in hRPE cells. Conclusions: These findings suggested that hypoxia-induced SDF-1 expression in RPE might be a critical initiator for recruitment of BMCs in CNV. SDF-1 might be another important factor in BMCs' differentiation into endothelial cells to participate in the CNV.