Molecular mechanisms of PAF/PAFR axis in tumor repopulation after radiotherapy

Abstract

Tumor resistance to therapeutic treatments such as radiotherapy is an importantclinical problem and is an area of active study. Platelet activating factor (PAF) isa lipid mediator of inflammation, and in recent decades, emerged as an importantfactor in tumor outcome. PAF acts by specific binding to its receptor (PAFR),which is present in both tumor cells and cells that infiltrate tumors. Protumorigeniceffects of PAFR includes promotion of tumor cell proliferation,production of growth factors and immunosuppression. In experimental models,blocking of PAFR reduced tumor growth and increased animal survival. A majordrawback of radiotherapy is the accelerated growth of the surviving tumor cells,a phenomenon known as tumor cell repopulation, where radio-resistant-tumorcells enter an exacerbated proliferation cycle, causing tumor recurrence in a waymore aggressive and resistant to therapeutic resources. Previous results fromour group showed that tumor cells expressing high amounts of PAFR has greaterresistance to radiation-induced cell death than low PAFR expressing tumor cells.This resistance was suppressed by treatment with PAFR antagonist, suggestingthe participation of PAFR-ligands as pro-survival tumor factors. Thus,understanding these signals that promote tumor growth is fundamental for thedevelopment of new therapeutic strategies. In the present project we want tomeasure and identify these different types of PAFR ligands generated afterradiotherapy (via enzymatic or non-enzymatic pathways) and compare with radioresistance profile of tumor cells lacking or not PAFR receptor. This hypothesiswill be investigated in triple receptor negative (TRN) breast cancer cell lines[MDA231 and HCC1806] with CRISPR knockout of PAF receptor in two and threedimensional [tumor spheroid] cultures. We will also investigate the role ofexosomes as potential carriers of metabolically labile PAF agonists betweentumor cells. If a significant role for PAF is found in the in vitro studies, furtherevaluation of the cell lines will be done in vivo xenograft model, where animmunocompetent mouse will be injected with TRN breast cell line EO771, plusor minus a PD-1 inhibitor and stereotactic radiotherapy will be deliveredspecifically to the breast. This will help understand how PAFR pathway activationpromotes tumor cell repopulation after radiotherapy and the potential role ofblocking PAFR as co-adjuvant in cancer therapy.