Regenerative protein thymosin beta-4 is a novel regulator of purinergic signaling.

ABSTRACT

By an unknown mechanism, ß-thymosins are extracellular modulators of angiogenesis, inflammation, wound healing, and development. We were interested in identifying ß-thymosin interactors and determining their importance in ß-thymosins signaling in human vein endothelial cells (HUVECs). We performed pulldown experiments with biotinylated thymosin ß-4 (Tß4) in comparison to neutravidin beads alone and used mass spectrometric analysis to identify differentially interacting proteins. By this method, we identified F1-F0 ATP synthase, a known target of antiangiogenic angiostatin. By surface plasmon resonance, we determined for Tß4 binding to the ß subunit of ATP synthase a K(D) of 12 nM. Blocking antibodies and antagonists (oligomycin, IC(50) ∼1.8 µM; piceatannol, IC(50) ∼1.05 µM; and angiostatin, IC(50) ∼2.9 µg/ml) of ATP synthase inhibited the Tß4-induced increase in cell surface ATP levels, as measured by luciferase assay, and the Tß4-induced increase in HUVEC migration, as measured by transwell migration assay. Silencing of the ATP-responsive purinergic receptor P2X4 with siRNA also blocked Tß4-induced HUVEC migration in a transwell assay. Furthermore, in silico we identified common amphiphilic α-helical structural similarities between ß-thymosins and the inhibitory factor 1 (IF1), an inhibitor of ATP synthase hydrolysis. In summary, we have identified an extracellular signaling pathway where Tß4 increases cell surface ATP levels via ATP synthase and have shown further that ATP-responsive P2X4 receptor is required for Tß4-induced HUVEC migration.