THYMOSIN-BETA(4) SYNERGIZES WITH HUMAN GRANULOCYTE-MACROPHAGE COLONY-STIMULATING FACTOR IN MAINTAINING BONE-MARROW PROLIFERATION

Recent evidence supports a role for thymosin beta4 (Tbeta4) in the inhibition of murine hematopoietic stem cell proliferation. This supposition results from studies in which the N-terminal tetrapeptide derived from native Tbeta4 was administered to mice and appeared to prevent CFU-S recruitment into DNA synthesis. The importance of this observation was the concomitant ability of the tetrapeptide to prevent cytosine arabinoside (ara-C) toxicity in mice given LD50 doses of this drug. In the present study, we have extended these observations by demonstrating that whole synthetic Tbeta4 is more effective than the N-terminal tetrapeptide in protecting mice from the toxicity of ara-C. This observation supports the hypothesis that Tbeta4 is the biologically important parent molecule for this activity. To determine if inhibition of cell cycle progression also occurs in committed human bone marrow progenitors treated with Tbeta4, we have investigated the effects of synthetic Tbeta4 on proliferating and unstimulated enriched human bone marrow. In short-term liquid cultures studied sequentially over 1-7 days, Tbeta4 failed to inhibit cell proliferation, but maintained the proliferative effect of granulocyte-macrophage colony stimulating factor (GM-CSF) on days following maximum stimulation (days 5-7). No effect was noted before the fifth day in culture, nor did Tbeta4 exert any demonstrable effect in the absence of added GM-CSF. Any observable effect of Tbeta4 required that it be present in the cultures on or before day 3 of GM-CSF stimulation. These results suggest that an additional effect of Tbeta4 is the stimulation of a subpopulation of committed human bone marrow precursor cells to become more sensitive to the growth-promoting activity of GM-CSF, thereby enhancing myelopoiesis. It is of interest that the N-terminal peptide of Tbeta4 is a shared sequence with tumor necrosis factor alpha, which is also known to have a similar stimulatory capacity. We, therefore, postulate that the growth enhancement noted in short-term cultures is mediated by the region containing these shared sequences.