OVEREXPRESSION OF VASCULAR ENDOTHELIAL GROWTH-FACTOR IN THE AVIAN EMBRYO INDUCES HYPERVASCULARIZATION AND INCREASED VASCULAR-PERMEABILITY WITHOUT ALTERATIONS OF EMBRYONIC PATTERN-FORMATION

Vascular endothelial growth factor (VEGF)-also known as vascular perme ability factor-has been implicated in the regulation of blood vessel f ormation, i.e., vasculogenesis and angiogenesis. High amounts of VEGF mRNA and protein have been detected during embryonic and tumor angioge nesis, but it remained unclear whether the level of VEGF correlated wi th the extent of vascularization in a given organ or tissue. We examin ed the role of VEGF and the high affinity, signal-transducing VEGF rec eptor-2 (flk-1) in the avian embryo. In a gain of function transgene-l ike approach the retroviral expression vector RCAS was used to increas e the level of quail VEGF during critical periods of avian limb bud gr owth and morphogenesis. In contrast to basic fibroblast growth factor, which recently was demonstrated to induce morphogenetic alterations w hen overexpressed in this system, overexpression of VEGF in the limb b ud exclusively resulted in hypervascularization as reflected by an inc rease in vascular density. However, cartilage expressing the construct was not vascularized prematurely. Thus hypervascularization was proba bly due to the augmentation of the VEGF signaling mechanism in a permi ssive environment. In addition to hypervascularization, vascular perme ability was dramatically increased, leading to local and in some cases to general edema. This is the first indication of a link between the functions of VEGF as a vascular growth factor and as a permeability fa ctor. VEGF receptor-2 (flk-1) was found to be upregulated only in thos e areas where VEGF was overexpressed. This implies a positive feedback system of the VEGF receptor on its own synthesis and would provide a basis for a paracrine system in which ligand concentration is critical for the extent of tissue vascularization. Our results show that the V EGF/VEGF-receptor system is specific and sufficient for the formation of new blood vessels. They also have implications for somatic gene the rapy of diseases which are characterized by a lack of blood vessels su ch as chronic ischemic diseases of heart and brain. (C) 1995 Academic Press, Inc.