Identification of novel therapeutic targets in the secretome of ionizing radiation induced senescent tumor cells

Cellular senescence is a state of irreversible growth arrest that can be triggered by multiple mechanisms, including telomere shortening, the epigenetic derepression of the INK4/ARF locus and DNA damage. Senescence has been considered a tumorsuppressing mechanism that permanently arrests cells at risk for malignant transformation. However accumulating evidence shows that senescent cells have deleterious effects on the tissue microenvironment. Some of these effects could be attributed to the senescenceassociated secretory phenotype that has the ability to promote tumor progression. However, secreted proteins from senescent tumor cells and their effects on the tumor microenvironment due to ionizing radiation (IR) exposure have not yet been fully elucidated. In the present study, we analyzed cytokines secreted from IR induced senescent MCF7 cells by using cytokine microarrays and confirmed by western blot analysis that increased secretion of osteoprotegerin (OPG) midkine (MDK) and apolipoprotein E3 (ApoE3) occurs in these cells. Invasive, migratory and wound healing activities were observed in MDA MB 231 and MCF10A cells following treatment with recombinant human OPG, MDK and ApoE3 proteins. Additionally, tube formation activity was assessed in OPG, MDK and ApoE3 treated human umbilical vein endothelial cells (HUVECs). We found that OPG, MDK and ApoE3 affected cell motility and tubeformation activity. Since OPG markedly affected cell motility, we examined the effect of senescent conditioned media containing neutralizing OPG antibodies on migration and woundhealing activity. Our results demonstrated that IRinduced senescent tumor cells influence the tumor microenvironment by increasing the production of cytokines, such as OPG, MDK and ApoE3. Furthermore, these data suggest that OPG is likely a promising target capable of reducing the deleterious effects on the tumor microenvironment during radiation therapy.