Diane J. Rodi – Principal Investigator

Project Members:
Suneeta Mandava, Wen Zhang, David Glesne, Lee Makowski, Frank Collart, Ed Uberbacher (ORNL)

Vascular development, both normal and aberrant, is critically dependent upon the activities of endothelial cells (ECs). These cells, which line the lumen of all the blood vessels of the body, are the ‘ringmaster’ of a process called angiogenesis - the generation of new capillary blood vessels. Angiogenesis is a complex process involving extensive interplay between cells, soluble factors, and extracellular matrix components. In the adult, the proliferation rate of endothelial cells is very low compared with that of many other cell types in the body. ECs are one of the few cell types who migrate (albeit under carefully regulated conditions) in the adult organism under normal healthy conditions. A recent upsurge in EC research is due to the realization that unregulated angiogenesis is at the heart of numerous pathologies including rheumatoid arthritis, diabetic retinopathy, psoriasis, juvenile hemangiomas, and the maturation and metastasis of most human tumors. Therefore, the delineation of protein-protein interaction networks both within and proximally external to the EC has the potential to impact upon a huge number of human disorders.

The major driving force for new blood vessel production is tissue hypoxia. This low-oxygen state, the result of new tissue growth (either normal as in wound healing or endometrium build-up; or aberrant as in tumor growth or macular  degeneration), stimulates the production of EC growth factors which causes these cells to 1. lower their cell-cell adhesion properties (via angiopoietin production), 2. gain motility, 3. upregulate protease production including matrix metalloproteases) in order to degrade the underlying basement membrane and invade into the stroma of neighboring hypoxic tissue, and 4. synthesize specialized extracellular matrix (ECM) components. Following proteolytic degradation of the ECM , ‘leader’ ECs start to migrate through the degraded matrix. These are followed by proliferating ECs, whose growth has been stimulated in part by soluble factors released by either hypoxic tissue or tumor cells and partly by cryptic growth factors released from the degraded ECM.  The localized release of factors which downregulate angiogenesis keeps the process from getting out of control. Normal angiogenesis results in the formation of a well-formed capillary, whose ECs have successfully recruited both smooth muscle cells (SMCs) and pericytes, the latter of which have been shown to regulate and stabilize EC structure and function. Aberrent, or pathological angiogenesis frequently results in a pericyte and SMC-deficient immature vessel, whose properties include EC hyperplasia, increased capillary diameter, abnormal EC shape and structure and increased transendothelial permeability, the latter of which accounts for the frequently ‘bloody’ appearance of numerous tumors due to erythrocyte leakage into the tissue.

Although over 20 naturally occurring angiogenic growth factors have been identified to date, the vascular endothelial growth factor(VEGF)/VEGF receptor system has been shown to play a central regulatory role in both physiological and pathological angiogenesis. The VEGF/VEGF receptor system has been shown to be critical during embryogenesis, tissue remodeling in the adult organism (including compensatory formation of blood vessels post-MI and post-CVA) and a variety of pathological conditions. Recent clinical studies have shown serum levels of VEGF to be a prognostic indicator for successful chemotherapy during cancer treatment. In spite of the importance of this receptor-ligand complex in numerous pathological conditions, the categorization of VEGF-induced changes in the proteome of the EC has been somewhat anecdotal. It is known that VEGF induces the expression of uPA, PAI-1, uPAR and MMP-1 in cultured ECs in vitro, and one report came out last year of an attempt to catalog transcriptome differences between normal and tumor-associated ECs within colon tissue in vivo. However, to date there has been no attempt to systematically delineate the changes within ECs which result from any pro- or anti-angiogenic stimulation as shown in the literature.