Thymosin β4 and its N-terminal tetrapeptide, AcSDKP, inhibit proliferation, and induce dysplastic, non-apoptotic nuclei and degranulation of mast cells

Thymosin β 4 (Tβ 4 ), a 5 kDa polypeptide, is a member of the β-thymosin family. It acts as the principal intracellular G-actin sequestering peptide and exhibits extracellular functions in angiogenesis and wound healing. The N-terminus of Tβ 4 contains a bioactive tetrapeptide, acSDKP, a negative regulator of hematopoietic stem-cell proliferation. Here, we show that both peptides inhibit mast-cell proliferation over the concentration range of 10 - 6 to 10 - 1 7 M with the maximum effect of both at 10 - 1 4 M. Both Tβ 4 and acSDKP caused dysplastic mast-cell nuclei that were confirmed by DAPI fluorescent staining. Flow-cytometric analysis of ploidy revealed that the dysplastic nuclei were not multinucleated, but fragmented nuclei in G2 growth arrest. We could further demonstrate that 10 - 8 or 10 - 1 4 M Tβ 4 or acSDKP induce mast-cell degranulation. A concentration of 10 - 8 M Tβ 4 or acSDKP caused 57 or 89% degranulation, respectively. A number of tryptic fragments of Tβ 4 were assayed beside intact Tβ 4 and the tetrapeptide, and found to be inactive.