The association between cancer and thromboembolic disease is a well-known phenomenon and contributes to the morbidity and mortality of cancer patients. Clinical studies of thrombosis in these patients show that heparins may have beneficial effects on survival. Antithrombotic agents have been shown to exert an anti-tumor effect in various experimental models however the underlying mechanism remains unknown. We show that heparins inhibit in vivo tumor angiogenesis and offer molecular evidence that heparins exert an anti-angiogenic effect by directly sequestering fibroblast growth factor (FGF) from its receptor on tumor derived endothelial cells (TDECs). NIH-3T3 fibroblasts were stably transfected with an expression construct that results in the constitutive excretion of FGF-1 (Clone C). Clone C gives rise to aggressive and highly angiogenic xenograft tumors. Clone C was inoculated into nude mice and therapeutic doses of Low Molecular Weight (LMW) heparins were injected daily beginning on Day 2. Tumors in the control group were grossly angiogenic and highly vascularized. In contrast, the heparin treated tumors were pale and possessed only scant peri-tumoral vessels. In order to assess the biologic mechanism of this, murine TDECs were isolated and cultured as previously published. Unfractionated and LMW heparins inhibit FGF-induced TDEC mitogenesis in a concentration- and time-dependent manner. FGF overcame and rescued heparin-induced inhibition suggesting that an FGF-heparin interaction is responsible. In order to test the hypothesis that heparin strips and sequesters FGF off its receptor on TDECs, we used a FGF protein fused to a hemagglutinin peptide tag at the carboxyl-terminus end (FGF-HA). FGF-HA is biologically identical to wild type FGF, but its detection limit is 10X more sensitive. FGF-HA was allowed to bind to FGFR on TDECs. These cells were subsequently incubated with Heparin covalently linked to Sepharose beads (Heparin-Sepharose) or to Sepharose alone. These beads were removed, and TDEC growth analyzed prospectively. Heparin-Sepharose treatment results in significant TDEC growth inhibition as compared to incubation with Sepharose alone. Western blot analysis shows that FGF was sequestered only on the Heparin-Sepharose beads.
Conclusion: The anti-angiogenic mechanism of heparins resides, in part, in its ability to sequester angiogenic cytokines such as FGF from its receptor on tumor endothelium.