title: Sub-lethal irradiation of human colorectal tumor cells imparts enhanced and sustained susceptibility to multiple death receptor signaling pathways.
abstract: BACKGROUND
  Death receptors (DR) of the TNF family function as anti-tumor immune effector molecules. Tumor cells, however, often exhibit DR-signaling resistance. Previous studies indicate that radiation can modify gene expression within tumor cells and increase tumor cell sensitivity to immune attack. The aim of this study is to investigate the synergistic effect of sub-lethal doses of ionizing radiation in sensitizing colorectal carcinoma cells to death receptor-mediated apoptosis.
  
  
  RESULTS
  The ability of radiation to modulate the expression of multiple death receptors (Fas/CD95, TRAILR1/DR4, TRAILR2/DR5, TNF-R1 and LTβR) was examined in colorectal tumor cells. The functional significance of sub-lethal doses of radiation in enhancing tumor cell susceptibility to DR-induced apoptosis was determined by in vitro functional sensitivity assays. The longevity of these changes and the underlying molecular mechanism of irradiation in sensitizing diverse colorectal carcinoma cells to death receptor-mediated apoptosis were also examined. We found that radiation increased surface expression of Fas, DR4 and DR5 but not LTβR or TNF-R1 in these cells. Increased expression of DRs was observed 2 days post-irradiation and remained elevated 7-days post irradiation. Sub-lethal tumor cell irradiation alone exhibited minimal cell death, but effectively sensitized three of three colorectal carcinoma cells to both TRAIL and Fas-induced apoptosis, but not LTβR-induced death. Furthermore, radiation-enhanced Fas and TRAIL-induced cell death lasted as long as 5-days post-irradiation. Specific analysis of intracellular sensitizers to apoptosis indicated that while radiation did reduce Bcl-X(L) and c-FLIP protein expression, this reduction did not correlate with the radiation-enhanced sensitivity to Fas and/or TRAIL mediated apoptosis among the three cell types.
  
  
  CONCLUSIONS
  Irradiation of tumor cells can overcome Fas and TRAIL resistance that is long lasting. Overall, results of these investigations suggest that non-lethal doses of radiation can be used to make human tumors more amenable to attack by anti-tumor effector molecules and cells.
authors.affiliation: Department of Biology, Georgia State University, Atlanta, Georgia, United States of America.
authors.forename: Victoria
authors.initials: V
authors.lastname: Ifeadi
authors.forename: Charlie
authors.initials: C
authors.lastname: Garnett-Benson
mesh_terms: D015153:Blotting, Western; D049109:Cell Proliferation / Q000187:drug effects / Q000528:radiation effects*; D015179:Colorectal Neoplasms / Q000235:genetics / Q000378:metabolism*; D005434:Flow Cytometry; D045325:HCT116 Cells; D006801:Humans; D011839:Radiation, Ionizing; D060888:Real-Time Polymerase Chain Reaction; D053220:Receptors, TNF-Related Apoptosis-Inducing Ligand / Q000235:genetics / Q000378:metabolism*; D015398:Signal Transduction / Q000187:drug effects / Q000528:radiation effects; D053199:TNF Receptor-Associated Death Domain Protein / Q000235:genetics / Q000378:metabolism; D053221:TNF-Related Apoptosis-Inducing Ligand / Q000494:pharmacology; D019014:fas Receptor / Q000235:genetics / Q000378:metabolism
chemical_list: D053220:Receptors, TNF-Related Apoptosis-Inducing Ligand; D053199:TNF Receptor-Associated Death Domain Protein; D053221:TNF-Related Apoptosis-Inducing Ligand; D019014:fas Receptor