Angiostatic Activity Factor 4 That Lacks Affinity for Heparin and Retains Potent Inhibition of Tumor Growth in Mice by an Analogue of Platelet

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 r l'I 4-241 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, 11(1-116, 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 rl'l 4. Both rl'l 4 and rl'l 4-241 inhibited angiogenesis in the chicken chorioallantoic mem brane. The analogue, however, was inhibitory at lower concentrations than rl'l 4 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 rl'l 4 and rl'l 4-241 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.