Mechanisms of action of insulin-like growth factor binding protein-3 in promoting repair in ischemic retinal vascular mouse injury models

Endothelial progenitor cells (EPCs) are bone-marrow derived cells, which give rise to mature endothelial cells. IGFBP-3 can promote EPC repair and maintenance of blood vessel integrity. This study was conducted to gain insight into IGFBP-3's cellular mechanism of action on EPC-mediated repair in damaged retinal vasculature.;To unveil how IGFBP-3 modulates EPC functional repair, studies were designed to focus on how IGFBP-3 promotes cell migration. For in vivo studies, 2 complementary vascular injury models were utilized: laser occlusion of retinal vessels in adult green fluroscent protein (GFP) chimeric mice and oxygen-induced retinopathy in mouse pups. Intravitreal injection of IGFBP-3 expressing plasmid into lasered adult gfp+ chimeric mouse retinas stimulated homing of EPCs into retinal blood vessels. The gfp+ EPCs also differentiated into various vascular cell types such as pericytes, astrocytes, and endothelial cells. In the OIR model, IGFBP-3 injection prevented cell death of resident vascular endothelial cells, while simultaneously increasing astrocytic ensheathment of retinal blood vessels. For EPCs to orchestrate these cytoprotective and homing effects, they must migrate into ischemic or damaged tissue. Experimental in vitro studies showed their migratory ability is mediated, in part, by endogenous nitric oxide (NO) generation. IGFBP-3 treated EPCs significantly increased NO generation compared to untreated EPCs. IGFBP-3 can signal through the high-density lipoprotein receptor, Scavenger Receptor class B, type 1 (SR-B1), to increase NO production and activity in mature endothelial cells. In EPCs and mature endothelial cells SR-B1 blockade with a neutralizing antibody resulted in a decrease in NO production. Furthermore, when EPCs and mature endothelial cells underwent IGFBP-3 treatment, there was an increase in phospo-endothelial nitric oxide synthase (peNOS) protein expression. IGFBP-3 exposure led to the re-distribution of vasodilator stimulated phosphoprotein (VASP), a NO regulated protein, critical for cell migration of endothelial cells. Lastly, IGFBP-3 effects on vascular permeability were examined. Distinct differences in permeability were found depending on whether the IGFBP-3 was adminstered acutely or there was chronic exposure. IGFBP-3 acutely increased pemeability, while upon longer exposure IGFBP-3 reduced retinal vascular permeability, supportive of its vascular stabilizing ability.;In summay, identification of a new signaling receptor that IGFBP-3 can activate was uncovered. IGFBP-3 can activate the SR-B1 receptor to increase exogenous NO production in EPCs, which leads to increased cell migration of these cells. Due to the positive stimulatory effects of IGFBP-3 on cell migration, it can act as an in vivo homing, resulting in vascular repair and stabilization of injured mouse retinal vasculature. IGFBP-3 has the potential to be used as a therapeutic agent to treat ischemic vascular eye conditions.