Our previous works have shown that bone marrow stromal cells secrete thymosin β4 (Tβ4) and AcSDKP. Tβ4 and AcSDKP are existed in the conditioned medium of bone marrow endothelial cells. They exerted inhibitory effects on hematopoietic cells and then had protective effect on the early hematopoietic cells, which were cultured in the presence of hematopoietic stimulators.
Thymosin β4 consists of 43 peptides with a molecular weight of 4963. It contains at its N-terminal end the sequence of the acetylated tetrapeptide Ac-N-Ser-Asp-Lys-Pro (AcSDKP). This study was performed to evaluate the effect of Tβ4 and AcSDKP on the growth of HL-60 cells. It was showed that Tβ4 (10−11–10−7 mol/L) and AcSDKP (10−11–10−7 mol/L) had the dose-dependent inhibitory effect on the proliferation of HL-60 cells. Based on cell morphology and NBT reduction, Tβ4 and AcSDKP induced differentiation of HL-60 cells. Morphologic and DNA fragment analysis proved that Tβ4 and AcSDKP induced apoptosis of HL-60 cells. In order to analyze the mechanism of the effects of Tβ4 and AcSDKP, intracellular free Ca2+ concentration ([Ca2+]i) of HL-60 leukemic cells was tested and Atlas cDNA Expression Array was performed. The results showed that Tβ4 and AcSDKP could increased [Ca2+]i by stimulating the release of Ca2+ from intracellular Ca2+ pool. Moreover, AcSDKP could also elicit a potent extracelluar calcium influx in HL-60 cells. Tβ4 could also change apoptotic-related gene expression in leukemic cells, and resulted in the inhibition of proliferation and induction of differentiation and apoptosis of leukemic cells.