The interaction of angiocidin and tissue transglutaminase

Abstract

Angiogenesis is the formation of new blood vessels from the endothelium of pre-existing vasculature and is the main mechanism of vascularization during physiological processes such as embryonic development, growth, regeneration, and wound healing and pathological process of vascularizing tumors. The major source of these environmental signals is the interactions between the cell and the extracellular matrix (ECM). Cell adhesion molecules are found on the surface of all cells and play a role in cell-cell and cell-extracellular matrix interactions. Tissue transglutaminase is ubiquitously expressed in various tissue types, occurs in both an intracellular and extracellular form, and is highly expressed in endothelial cells and smooth muscle cells in the vasculature. Tissue transglutaminase is secreted from cells externally and binds to the cell surface or extracellular matrix. It has been implicated in the stabilization of the extracellular matrix and in cell-ECM interactions by cross-linking matrix proteins. In 1993, angiocidin was isolated from lung carcinoma extracts by affinity chromatography using a peptide fragment of thrombospondin-1 (TSP-1), a matrix protein that has been implicated in mechanisms of tumor progression. The recombinant protein was expressed and shown to be a potential inhibitor of angiogenesis but the mechanism of action was not characterized. Affinity chromatography showed that tissue transglutaminase binds to recombinant angiocidin. Our studies have shown: Angiocidin binds to tissue transglutaminase in situ and in vitro through different binding studies. Recombinant angiocidin was also a substrate for the enzymatic activity of tissue transglutaminase. Both endogenous and exogenous angiocidin were able to crosslink to themselves. This interaction with angiocidin inhibits transglutaminase function by inhibiting amine incorporation, crosslinking of extracellular matrix proteins, and the promotion of cell-matrix interactions. Additionally, the monomeric and crosslinked angiocidin have different biological activity. Polymeric angiocidin is more adhesive, inhibits, migration, and is resistant to proteolytic degradation. Our studies suggest that angiocidin inhibits angiogenesis by its ability to bind to tissue transglutaminase and crosslink to itself, thereby destabilizing the cell- extracellular matrix interactions through its interaction with tissue transglutaminase. These studies may help to understand the mechanism of recombinant angiocidin in inhibiting tumor vasculature, as well as understanding the significance of these post-translational modifications of endogenous and exogenous proteins in cellular biology.