Angiocidin, a protein over-expressed in many different solid tumors and tumor capillary endothelial cells inhibits angiogenesis and tumor growth [Zhou, J., et al., 2004. Cloning and characterization of angiocidin, a tumor cell binding protein for thrombospondin-1. J Cell Biochem. 92, 125-146]. Since several splice variants of angiocidin have distinct biochemical functions in membrane transport and protein degradation, we sought to evaluate the function of endogenously expressed angiocidin in human umbilical vein endothelial (HUVE) cells using siRNA. We observed a 90% reduction of the target mRNA levels after 24 h. Endogenous angiocidin protein expression was reduced by 80% after three days, as evaluated by Western blot analysis. We also found that anti-angiocidin siRNA down-regulated 90% of the protein expression of matrix metalloproteinase 2 (MMP-2) and 50% of its gelatinolytic activity. Reduction of endogenous angiocidin completely inhibited endothelial cord formation on Matrigel. Cells expressing low levels of angiocidin grew more slowly, were less invasive and less adhesive than control cells. Consistent with the reported function of one of the angiocidin analogues S5a, we found that the expression of polyubiquitinated proteins was higher in anti-angiocidin siRNA-treated cells as compared to normal and control siRNA-treated cells. These results suggest that endogenous angiocidin and its homologues promote endothelial cell invasion, adhesion, and angiogenesis through mechanisms involving polyubiquitin-dependent protein degradation and MMP-2 expression.