Paracrine modulation of CXCR4 by IGF-1 and VEGF: implications for choroidal neovascularization.

Choroidal neovascularization (CNV), the hallmark of exudative age-related macular degeneration (AMD), is responsible for approximately 90% of cases of severe vision loss from AMD. Vascular endothelial growth factor (VEGF) plays a key role in the regulation of CNV and the accompanying increase in permeability. Current pharmacologic treatments, such as ranibizumab (Lucentis; Genentech, San Francisco, CA) and bevacizumab (Avastin; Genentech), aggressively target VEGF.1,2 However, despite these therapeutic advances, long-term trials using ranibizumab (Lucentis) indicate that a significant population of AMD patients do not respond to VEGF inhibition.1,2 This is not entirely surprising because, in addition to VEGF, other angiogenic and inflammatory mediators are likely to contribute to CNV lesion development. One such mediator, insulinlike growth factor (IGF)-1, produced in neurons and retinal pigment epithelium, has recently been implicated in CNV progression.3 IGF-1 immunoreactivity was abundantly found in human CNV tissue, and the IGF-1 receptor (IGF-1Rc) was highly expressed on retinal pigment epithelial (RPE) cells.3 Moreover, exposure of human RPE cultures to IGF-1 stimulated VEGF secretion.3 Stromal derived factor (SDF)-1 is a newly implicated cytokine in CNV lesion growth4,5 and in the pathogenesis of proliferative diabetic retinopathy.6 Its actions are not limited to the resident vasculature; rather, SDF-1 is a potent stimulator of endothelial precursor cells (EPCs).5 EPCs are bone marrow–derived cells that enhance new vessel growth both by directly incorporating into newly formed vessels and by secreting paracrine factors. CXCR4, the major receptor for SDF-1, is expressed not only on EPCs but also on mature endothelial cells, neural precursors, and smooth muscle progenitors, and it is critical for the migration of these cells to areas of injury and repair.7 Activation of CXCR4 facilitates EPC differentiation to endothelial cells and EPC survival.8 SDF-1, like VEGF, is regulated by hypoxia. Previously, we demonstrated that elevated vitreous SDF-1 levels strongly correlated with vitreous VEGF levels and paralleled the severity of retinopathy.9 When expressed in epiretinal membranes, SDF-1 is associated with VEGFR-2.10 Circulating EPCs are increased in patients with active CNV, suggesting that these cells may be recruited from bone marrow by factors secreted at the sites of active CNV and that they may play a critical role in CNV severity.11 Blocking SDF-1 prevented the recruitment of EPCs to the retina and choroid after injury to these areas and reduced CNV.5 Despite the clear evidence of cooperation between these factors and cytokines for CNV development, no studies have examined the influence of IGF-1 and VEGF on the in vitro angiogenic effect of SDF-1, nor has the effect of CXCR4 inhibition been completely elucidated in CNV lesion formation. We examined the effects of VEGF and IGF-1 on SDF-1–stimulated proliferation and capillary tube formation in vitro and examined the in vivo effect of highly selective CXCR4 antagonist on the neovascular response after laser rupture of Bruch's membrane.