An ocular view of the IGF-IGFBP system

Abstract

IGFs and their binding proteins have been shown to exhibit both protective and deleterious effects in ocular disease. Recent studies have characterized the expression patterns of different IGFBPs in retinal layers and within the vitreous. IGFBP-3 has roles in vascular protection stimulating proliferation, migration, and differentiation of vascular progenitor cells to sites of injury. IGFBP-3 increases pericyte ensheathment and shows anti-inflammatory effects by reducing microglia activation in diabetes. IGFBP-5 has recently been linked to mediating fibrosis in proliferative vitreoretinopathy but also reduces neovascularization. Thus, the regulatory balance between IGF and IGFBPs can have profound impact on target tissues. This review discusses recent findings of IGF and IGFBP expression in the eye with relevance to different retinopathies.

Figure 1. Retinal vasculature in a GFP⁺ chimeric mouse (WT mouse undergoing bone marrow transplantation) that received within the vitreous liposomes containing a plasmid expressing IGFBP-3 under an endothelial specific promoter

The mouse then was subjected to the retinal branch vein occlusion model. Three weeks after liposome injection, the mouse was sacrificed and a retinal flat mount was prepared. (A) The retinal vessels were labeled with rhodamine agglutinin (red) and imaged using confocal fluorescence microscopy. (B) Enhanced incorporation of GFP⁺ progenitor cells (green) into the retinal vessels is shown. (C) Merged channel. IGFBP-3’s protective and reparative effects on the vasculature may be, in part, the result of its ability to recruit endothelial progenitor cells, to sites of retinal injury.

Figure 2. IGFBP-3-mediated VASP redistribution in human microvascular endothelial cells from the lungs (HMVEC-L)

HMVEC-L, cultured on fibronectin-coated coverslips, were left untreated (A) or were treated with 100ng/ml IGFBP-3 for 15 minutes (B) and Vasodilator-stimulated phosphoprotein (VASP) biodistribution was detected by immunofluorescence. IGFBP-3 treatment caused the rapid redistribution of VASP to the cells’ periphery (A and B). C and D show, at higher magnification, a single cell in A and B, respectively. Note the uniform VASP distribution throughout the cytoplasm along the actin filaments in the untreated sample (C) and the presence of VASP-free areas together with increased VASP immunoreactivity along the plasmamembrane in the IGFBP-3- treated cell, (D). Representative results from three independent experiments are shown. Green: VASP; Blue: DAPI (for nuclear staining). (Scale bar = 25μm)

Figure 3. General mechanism of vascular repair by IGFBP-3

CD34⁺ cells are recruited from the bone marrow by increased IGFBP-3 levels in the vasculature. IGFBP-3 binds to SR-B1, thereby leading to phosphorylation of eNOS and generating NO. IGFBP-3 can also mediate S1P generation by phosphorylating SphK-1, where S1P acts on its receptor to also stimulate phosphorylation of eNOS. NO stimulates a signaling cascade leading to VASP phosphorylation which is redistributed to polar ends of the cell and induces cell migration. Cells migrate to sites of injury to initiate capillary tube formation and vascular remodeling. IGFBP-3 can maintain tight and adherens junctions to maintain endothelium integrity as observed in the BRB.