The actin binding site on thymosin β4 promotes angiogenesis

Thymosin beta(4) is a ubiquitous 43 amino acid, 5 kDa polypeptide that is an important mediator of cell proliferation, migration, and differentiation. It is the most abundant member of the beta-thymosin family in mammalian tissue and is regarded as the main G-actin sequestering peptide. Thymosin beta(4) is angiogenic and can promote endothelial cell migration and adhesion, tubule formation, aortic ring sprouting, and angiogenesis. It also accelerates wound healing and reduces inflammation when applied in dermal wound-healing assays. Using naturally occurring thymosin beta(4), proteolytic fragments, and synthetic peptides, we find that a seven amino acid actin binding motif of thymosin beta(4) is essential for its angiogenic activity. Migration assays with human umbilical vein endothelial cells and vessel sprouting assays using chick aortic arches show that thymosin beta(4) and the actin-binding motif of the peptide display near-identical activity at similar to50 nM, whereas peptides lacking any portion of the actin motif were inactive. Furthermore, adhesion to thymosin beta(4) was blocked by this seven amino acid peptide demonstrating it as the major thymosin beta(4) cell binding site on the molecule. The adhesion and sprouting activity of thymosin beta(4) was inhibited with the addition of 5-50 nM soluble actin. These results demonstrate that the actin binding motif of thymosin beta(4) is an essential site for its angiogenic activity.