title: Involvement of ERK-Nrf-2 signaling in ionizing radiation induced cell death in normal and tumor cells.
abstract: Prolonged oxidative stress favors tumorigenic environment and inflammation. Oxidative stress may trigger redox adaptation mechanism(s) in tumor cells but not normal cells. This may increase levels of intracellular antioxidants and establish a new redox homeostasis. Nrf-2, a master regulator of battery of antioxidant genes is constitutively activated in many tumor cells. Here we show that, murine T cell lymphoma EL-4 cells show constitutive and inducible radioresistance via activation of Nrf-2/ERK pathway. EL-4 cells contained lower levels of ROS than their normal counterpart murine splenic lymphocytes. In response to radiation, the thiol redox circuits, GSH and thioredoxin were modified in EL-4 cells. Pharmacological inhibitors of ERK and Nrf-2 significantly enhanced radiosensitivity and reduced clonogenic potential of EL-4 cells. Unirradiated lymphoma cells showed nuclear accumulation of Nrf-2, upregulation of its dependent genes and protein levels. Interestingly, MEK inhibitor abrogated its nuclear translocation suggesting role of ERK in basal and radiation induced Nrf-2 activation in tumor cells. Double knockdown of ERK and Nrf-2 resulted in higher sensitivity to radiation induced cell death as compared to individual knockdown cells. Importantly, NF-kB which is reported to be constitutively active in many tumors was not present at basal levels in EL-4 cells and its inhibition did not influence radiosensitivity of EL-4 cells. Thus our results reveal that, tumor cells which are subjected to heightened oxidative stress employ master regulator cellular redox homeostasis Nrf-2 for prevention of radiation induced cell death. Our study reveals the molecular basis of tumor radioresistance and highlights role of Nrf-2 and ERK.
authors.affiliation: Radiation Biology & Health Sciences Division, Bio-Medical Group, Bhabha Atomic Research Centre, Trombay, Mumbai, India.
authors.forename: Raghavendra S
authors.initials: RS
authors.lastname: Patwardhan
authors.forename: Rahul
authors.initials: R
authors.lastname: Checker
authors.forename: Deepak
authors.initials: D
authors.lastname: Sharma
authors.forename: Santosh K
authors.initials: SK
authors.lastname: Sandur
authors.forename: Krishna B
authors.initials: KB
authors.lastname: Sainis
mesh_terms: D000818:Animals; D015153:Blotting, Western; D016923:Cell Death / Q000187:drug effects* / Q000235:genetics; D002460:Cell Line; D048049:Extracellular Signal-Regulated MAP Kinases / Q000235:genetics / Q000378:metabolism*; D005434:Flow Cytometry; D008297:Male; D051379:Mice; D008810:Mice, Inbred C57BL; D051267:NF-E2-Related Factor 2 / Q000235:genetics / Q000378:metabolism*; D011839:Radiation, Ionizing; D017382:Reactive Oxygen Species / Q000378:metabolism; D060888:Real-Time Polymerase Chain Reaction; D015398:Signal Transduction / Q000528:radiation effects
chemical_list: D051267:NF-E2-Related Factor 2; C495636:Nfe2l2 protein, mouse; D017382:Reactive Oxygen Species; D048049:Extracellular Signal-Regulated MAP Kinases