PLATELET FACTOR 4 ACTS AS BOTH INHIBITOR AND PROTECTOR OF MEGAKARYOCYTOPOIESIS THROUGH A REVERSIBLE INHIBITION

Development of megakaryocyte (MK) from CD34+ cord blood (CB) cells in both plasma clot culture and liquid culture was significantly inhibite d by human platelet factor 4 (PF4) and human transforming growth facto r beta 1 (TGF beta 1). Inhibition of cell growth by PF4 was reversible judging from the fact that the CD34+ cells preincubated with PF4 coul d regenerate colonies after washing and replating into the cultures. B y contrast, TGF beta 1-pretreated CD34+ cells gave rise to few colonie s following replating. Moreover, incubation of CD34+ cells with PF4 in liquid culture caused an increase in the number of both stem cell fac tor (SCF)-binding cells and CD34 antigen-bearing cells, and exhibited greater capacity to form MK colonies than control after the treatment of 5-FU. In vivo in mice, twice injections of PF4 at 40 mu g/kg with a n interval of 6 h followed by one injection of 5-FU at 150 mg/kg resul ted in a significant increase in the number of colony-forming cells wi th high proliferative potential (HPP-CFC) and colony-forming unit-mega karyocyte (CFU-MK) in bone marrow. In exponentially growing human eryt hroleukemia cells (HEL), the addition of PF4 prolonged cell cycle prog ression and therefore resulted in an increased cell population in S ph ase, as determined by flow cytometric analysis. Different from PF4, TG F beta 1 blocked more cells in G1 phase. These results demonstrate tha t PF4 and TGF beta 1 inhibit MK development from CD34+ CB cells by dif ferent mechanisms and suggest that PF4, unlike TGF beta 1, exerts its inhibitory effect on cell growth in a reversible and S phase-specific manner by which it protects stem cells and MK progenitor cells from 5- FU cytotoxicity.