Angiogenic potential of PAF receptor ligands in irradiated tumors.

Recent evidences suggest participation of the lipid mediator \"Platelet Activating Factor\" (PAF) in tumor growth and resistance to chemotherapy and radiotherapy. PAF acts through its receptor (PAFR), expressed on tumor cells and tumor infiltrating cells. Irradiation leads to the generation of a series of phospholipids that act as PAFR ligands in tumor cells, increasing proliferation and promoting the post-radiotherapy tumor repopulation phenomenon, which is an abnormal proliferation of tumor cells after treatment. The aim of our study was to evaluate PAFR ligands contributions to angiogenesis in irradiated tumor microenvironment. First, we show that PAF receptor positive tumor cells (PAFR&#43), when irradiated or treated with receptor agonist (cPAF), show increased gene expression of angiogenic factors (VEGF, TGF-&#2231, EGF, FGF2 and PDGFB) and increased TGF-&#2231 secretion when compared to the same non-expressing cell (PAFR-). Together with these results we observed increased ERK phosphorylation in PAFR+ tumor cells, one of the most important signaling pathways to angiogenic factors production. In the second part, we show that subcutaneous injection of irradiated PAFR&#43 tumor cells mixed with a small amount of viable tumor cells, simulating in vivo tumor repopulation, generates increased expression of angiogenic factors, larger tumors and higher vascularization than those tumors formed by PAFR- tumor cells. Treatment with cPAF agonist increases angiogenesis in PAFR&#43 tumors, suggesting the participation of PAF as a proangiogenic factor. In the third part, we show that PAFR activation, either by synthetic PAF or PAF-like lipids produced during radiotherapy, induces endothelial cell proliferation (HUVEC) via induction of angiogenic factors produced by tumor cells. In addition, we observed that endothelial cells express PAFR and cell treatment with PAFR antagonist (PCA4288) inhibited endothelial cells proliferation. In the fourth part, we evaluated an in vivo model of irradiated murine carcinoma tumor cells (LLC) in absence of PAFR in the host infiltrate (PAFR-/- animals) resulting in an inhibition of angiogenesis and immunosuppressive factors expression (TGFB1, IL-10, IL-8 and COX1). Thus, our results conclude that PAFR ligands generated during irradiation activates PAFR expressed in tumor cells, increasing proangiogenic factors production and consequent tumor neoangiogenesis. This condition favors post-radiotherapy tumor repopulation. Therefore, we suggest that the use of radiotherapy associated with PAFR antagonists may be a promising therapeutic strategy for solid tumors treatment. (AU)