The ADAMs: A novel family of cell surface proteins with adhesive and protease activity

The ADAMs, named for a disintegrin and metalloproteinase structure, are a large family of type I transmembrane proteins with a unique domain structure that contains both adhesive and protease domains. The ADAM adhesive domain shares homology with the snake venom disintegrin proteins and is thought to function predominantly as an integrin ligand in cell-cell adhesion. ADAM-mediated cell adhesion has been demonstrated to play a role in sperm-egg binding, myoblast fusion, and neural crest cell migration. The ADAM protease domain contains a highly conserved Zn-binding domain active site similar to the matrix metal loproteinases, and this domain has been shown to cleave cell surface proteins, which is termed ectodomain shedding. Disruption of ADAM-mediated shedding of diverse cell surface proteins can significantly impair development, cell signaling, and regulation of inflammatory responses. In this thesis, I explored the potential role(s) of ADAM-mediated adhesion and proteolysis in the biology of human endothelial cells by examining two hypotheses. In the first hypothesis, I tested the proposal that ADAM 15, an ADAM containing the RGD integrin-binding sequence, functions as a cell adhesion molecule. We show that overexpression of ADAM 15 in NIH3T3 cells inhibits cell migration and increases cell-cell interaction. Further, when expressed in epithelial cells, ADAM 15 localizes to sites of cell-cell contact. These observations suggest that ADAM 15 may function as a novel component of cell-cell contacts. In the second hypothesis, I examined the possibility that inflammatory adhesion proteins expressed on the surface of activated endothelial cells are substrates of ADAM-mediated ectodomain cleavage, and thus ADAM shedding may contribute to the regulation of endothelial recruitment of leukocytes. We show for the first time that two inflammatory adhesion molecules, fractalkine and vascular cell adhesion molecule-1 (VCAM-1), can be rapidly cleaved from the cell surface, and we identify TACE (ADAM 17) as the responsible sheddase. In contrast, three other adhesion molecules, E-selectin, intercellular adhesion molecule-1 (ICAM-1), and platelet-endothelial cell adhesion molecule-1 (PECAM-1), are not substrates of the ectodomain sheddase machinery. These findings provide the first identified functions for the ADAM family proteins in endothelial cells. The potential relevance of these findings to endothelial cell biology is discussed.