Response of cyclin B1 to ionizing radiation: Regulation by NF-κB and mitochondrial antioxidant enzyme MnSOD

Background: To understand the molecular response of tumor cells to therapeutic ionizing radiation (IR), we previously reported that human breast cancer cells derived following chronic exposure to fractionated ionizing radiation (MCF + FIR) showed a transient radioresistance. MCF + FIR cells also demonstrated increased activity of NF-κB, increased expression of the mitochondrial antioxidant enzyme (MnSOD), and increased expression of a cell cycle regulatory protein (Cyclin B1). The present studies were designed to determine the relationship of NF-κB, MnSOD and Cyclin B1 expression in cellular adaptive responses to ionizing radiation. Materials and Methods: The first intron of the cyclin B1 gene with a putative NF-κB element was cloned into the pGL3 luciferase reporter (pGL3CB1EI1). PGL3CB1EI1 and control NF-κB luciferase activities were determined in MCF-7 and MCF + FIR cells treated with a single dose of radiation, over expression of the dominant negative mutant IkB (mIkB) or over expression of the SOD2 gene. Results: MCF + FIR cells derived from fractionated IR demonstrated increased transactivation of the pGL3CB1EI1 and NF-κB controlled reporter activities, relative to the parental cell line. Transfection of dominant negative mutant IkB that inhibits NF-κB nuclear translocation, inhibited pGL3CB1EI1 and NF-κB activity, indicating the NF-κB dependence of pGL3CB1EI1 mediated transcription. In addition, over expression of the human SOD2 gene (MnSOD) inhibited NF-κB and pGL3CB1EI1 activity, indicating that superoxide or some species derived from superoxide may have participated in the up-regulation of reporter activity in response to chronic exposure to fractionated ionizing radiation. These results provide evidence suggesting that a signaling pathway involving NF-κB and Cyclin B1 may contribute to adaptive radioresistance induced by chronic exposure to fractionated IR and support the conclusion that MnSOD appears to be a negative regulator of this pathway.