In the absence of disease, the vasculature of the mammalian eye is quiescent, in part because of the action of angiogenic inhibitors that prevent vessels from invading the cornea and vitreous. Here, an inhibitor responsible for the avascularity of these ocular compartments is identified as pigment epithelium-derived factor (PEDF), a protein previously shown to have neurotrophic activity. The amount of inhibitory PEDF produced by retinal cells was positively correlated with oxygen concentrations, suggesting that its loss plays a permissive role in ischemia-driven retinal neovascularization. These results suggest that PEDF may be of therapeutic use, especially in retinopathies where pathological neovascularization compromises vision and leads to blindness.

Keratinocyte growth factor (KGF), also called fibroblast growth factor-7, is widely known as a paracrine growth and differentiation factor that is produced by mesenchymal cells and has been thought to act specifically on epithelial cells. Here it is shown to affect a new cell type, the microvascular endothelial cell. At subnanomolar concentrations KGF induced in vivo neovascularization in the rat cornea. In vitro it was not effective against endothelial cells cultured from large vessels, but did act directly on those cultured from small vessels, inducing chemotaxis with an ED50 of 0.02-0.05 ng/ml, stimulating proliferation and activating mitogen activated protein kinase (MAPK). KGF also helped to maintain the barrier function of monolayers of capillary but not aortic endothelial cells, protecting against hydrogen peroxide and vascular endothelial growth factor/vascular permeability factor (VEGF/VPF) induced increases in permeability with an ED50 of 0.2-0.5 ng/ml. These newfound abilities of KGF to induce angiogenesis and to stabilize endothelial barriers suggest that it functions in microvascular tissue as it does in epithelial tissues to protect them against mild insults and to speed their repair after major damage.

No abstract

Post-transcriptional gene regulation plays an important role in the expression of granulocyte-macrophage colony-stimulating factor (GM-CSF). Cytokine secretion by activated lymphocytes or mast cells is preceded by dramatic stabilization of the normally labile GM-CSF mRNA. The 3'-untranslated region of GM-CSF and other labile mRNAs contain the destabilizing motif adenosine-uridine-uridine-uridine-adenosine (AUUUA). We recently identified a cytoplasmic protein denoted the adenosine-uridine binding factor (AUBF) which binds with high affinity and specificity to AUUUA elements in synthetic RNA transcripts. We now demonstrate that AUBF binds specifically to GM-CSF mRNA through the destabilizing AUUUA elements. The formation of AUBF-GM-CSF RNA complexes required calcium or magnesium which were sensitive to EDTA or EGTA. A variety of other divalent metals blocked magnesium-dependent AUBF activity. These observations suggest that AUBF may protect GM-CSF mRNA from rapid degradation and play a crucial role in the expression of cytokine genes.