Inhibitory effect of platelet factor 4 (PF4) on the growth of human erythroleukemia cells: proposed mechanism of action of PF4.

The effect of platelet factor 4 (PF4) on the growth of human erythroleukemia cell line (HEL) and the binding characteristics of iodine 125-labeled PF4 to cells were studied to determine the mechanism of action of PF4. HEL cells were cocultured with various doses of PF4 in either a plasma clot system for colony assay or a liquid system for tritiated thymidine incorporation. A significant inhibition of HEL colony growth and tritiated thymidine incorporation was seen at PF4 doses of 1 microgram/ml and 0.5 microgram/ml, respectively. The inhibitory effect of PF4 could be abrogated by the addition of heparin (5 to 10 micrograms/ml). Enzyme-linked immunosorbent assay showed that PF4 had no obvious effect on the expression of platelet glycoprotein IIb/IIIa of HEL cells. Binding of 125I-PF4 to HEL cells reached equilibrium within 20 to 30 minutes with dissociation constant of 1.3 x 10(-10)M and Bmax of 6.3 pmol/10(5) cells and was inhibited by an excess of unlabeled PF4, beta TG, and heparin but was not affected by PMA, IL-3, IL-6, GM-CSF, and interferon-alpha. PF4 did not affect the binding of 125I-IL-3 and 125I-IL-6 to HEL cells. These data demonstrate that PF4 inhibits the growth of HEL cells by specific binding to HEL cells and suggest that the action of PF4 may be associated with the heparin-binding sites of the molecule.