Thymosin beta 4 and its N-terminal tetrapeptide, AcSDKP, inhibit proliferation, and induce dysplastic, non-apoptotic nuclei and degranulation of mast cells.

Thymosin beta4 (Tbeta4), a 5 kDa polypeptide, is a member of the beta-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 Tbeta4 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(-17) M with the maximum effect of both at 10(-14) M. Both Tbeta4 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(-14) M Tbeta4 or acSDKP induce mast-cell degranulation. A concentration of 10(-8) M Tbeta4 or acSDKP caused 57 or 89% degranulation, respectively. A number of tryptic fragments of Tbeta4 were assayed beside intact Tbeta4 and the tetrapeptide, and found to be inactive.