An analogue of human
platelet factor 4 (PF4) lacking affinity for
heparin was specifically designed to evaluate the importance of this property in the antitumor effects of recombinant PF4. The purified
protein, recombinant PF4-241 (rPF4-241), failed to bind
heparin but retained the ability to suppress the growth of
tumors in mice. Daily
intralesional injections of
rPF4-241 significantly inhibited the growth of the B-16
melanoma in syngeneic mice without direct inhibitory effects on B-16 cell growth in vitro. Similar antitumor effects were observed with the human colon
carcinoma, HCT-116, grown in nude mice, indicating that the inhibitory activity was neither
tumor-type specific nor T-cell dependent.
rPF4-241 inhibited endothelial cell proliferation in vitro with dose dependence similar to the native sequence rPF4. Both rPF4 and
rPF4-241 inhibited angiogenesis in the chicken chorioallantoic membrane. The analogue, however, was inhibitory at lower concentrations than rPF4 in the chorioallantoic membrane system and its inhibitory effects were not abrogated by the presence of
heparin. The present findings support the conclusion that both rPF4 and
rPF4-241 inhibit
tumor growth by suppression of
tumor-induced neovascularization. The finding that this activity is independent of
heparin binding may allow the development of PF4-based
angiostatic agents with reduced toxicity and improved bioavailability. These results also suggest that PF4 may play a more specific role in modulation of blood vessel development than previously recognized.