Inhibitory effect of platelet factor 4 on human erythroleukemic cells is dependent on cell surface heparan sulfate.

We have previously reported that platelet factor 4 (PF4) inhibits human erythroleukemic (HEL) cell growth in a dose-dependent fashion in vitro and that PF4 binds to HEL cells in a specific, saturable, and concentration-dependent manner. In this article we demonstrate that the binding of PF4 on HEL cells and its inhibitory effect on HEL cell growth were mediated by heparan sulfate. We found that binding of iodine 125-labeled PF4 to HEL cells was inhibited by heparin, heparan sulfate, and dermatan sulfate and to a smaller extent by chondroitin sulfate. Ninety percent of 125I-labeled PF4 bound to HEL cells was released by cells after exposure to heparin and heparan sulfate. Treatment of cells with heparitinase and heparinase induced a decrease in the binding of 125I-labeled PF4 to cells. Binding of 125I-labeled PF4 was partially inhibited by the presence of increasing concentrations of protamine sulfate and basic fibroblast growth factor. To test whether PF4 bound to cell surface proteoglycans, proteoglycan synthesis was inhibited by using 4-methylumbelliferyl-beta-D-xyloside. The binding of 125I-labeled PF4 on treated cells was decreased, and xyloside treatment of cells abrogated the biologic activity of PF4 in a plasma clot culture system. The inhibitory effect of PF4 was retained in a serum-free agar culture system, which indicates that the binding of PF4 to HEL cells induces cell growth inhibition in a direct fashion. Taken together, these findings suggest that PF4 directly acts on HEL cell growth by fixation on heparan sulfate proteoglycans on the HEL cell surface.