Platelet-activating Factor Inactivation by Local Expression of Platelet-activating Factor Acetyl-Hydrolase Modifies Tumor Vascularization and Growth

Purpose: Platelet-activating factor (PAF), a phospholipid mediator of inflammation, has been recently detected on tumor cells but its effect in tumor development is largely undefined. Experimental Design: To address its potential role in tumor biology, we inhibited intratumor PAF activity by engineering tumor cell lines to express plasma PAF-acetylhydrolase (PAF-AH), the major PAF-inactivating enzyme, and studied their behavior in vitro and in vivo . Results: When transfected with PAF-AH, KS-Imm human Kaposi’s sarcoma cells implanted in SCID mice and B16F10 mouse melanoma cells implanted in syngenic C57Bl/6J mice showed significantly reduced vascularization and growth allowing longer survival compared with control tumors. The amounts of bioactive PAF extracted from PAF-AH-transfected tumors were significantly reduced. In vitro , expression of PAF-AH did not influence cell proliferation, whereas it inhibited PAF-dependent cell motility in Kaposi’s sarcoma cells that express PAF-receptor but not in melanoma cells that did not express it. On the other hand, PAF-induced endothelial tubulogenesis in Matrigel was inhibited by incubation with supernatant from PAF-AH-transfected melanoma cells, indicating that PAF-AH inhibits in vitro neoangiogenesis. Conclusions: We demonstrated that in situ PAF inactivation affects tumor vascularization and growth through inhibition of neoangiogenesis and, in the case of cells expressing PAF receptor, also tumor cell motility.