title: Induction of transcription of "immediate early genes" by low-dose ionizing radiation.
abstract: The induction of transcription of specific genes after exposure to ionizing radiation has previously been reported after lethal doses of radiation (2-50 Gy). Little attention has been focused on expression of "immediate early genes" after low doses of ionizing radiation, where cell viability remains high. This dose range (0.25-2.0 Gy) is above the diagnostic dose level but at or below the doses typical for a single exposure in fractionated radiotherapy treatment of cancer. In this study, it was observed that doses in the range of 0.25-2.0 Gy induced different amounts of the mRNAs of the proto-oncogenes c-fos, c-jun, c-myc and c-Ha-ras at a given dose and time in Epstein-Barr virus-transformed human lymphoblastoid 244B cells. A maximum response was seen after a dose of 0.5 Gy for all but c-fos, which showed a maximum response after exposure to 0.25 Gy. Time-course studies demonstrated that, for all four proto-oncogenes, the induction was transient, reaching a maximum at 1 h and declining to the constitutive level at 4 h after irradiation. Using second-messenger specific inhibitors, the signaling pathways involved in the induction of these proto-oncogenes was also investigated. The results showed that all four of the proto-oncogenes induced after 0.5 Gy shared a common pathway of tyrosine kinase activation. Other signaling pathways included protein kinase C, reactive oxygen intermediates and calcium-dependent kinases; these were found to be differentially involved in the induction of transcription of the individual proto-oncogenes. In summary, this study suggests that low-dose ionizing radiation (0.25-2.0 Gy) can modulate expression of immediate early genes. Secondly, the activation of immediate early genes after low-dose exposure involves multiple second-messenger signaling pathways. Third, the magnitude of involvement of the different signaling pathways after low-dose radiation is different for each proto-oncogene expressed.
authors.affiliation: Department of Radiology, University of Texas Health Science Center, San Antonio 78284-7800, USA.
authors.forename: A V
authors.initials: AV
authors.lastname: Prasad
authors.forename: N
authors.initials: N
authors.lastname: Mohan
authors.forename: B
authors.initials: B
authors.lastname: Chandrasekar
authors.forename: M L
authors.initials: ML
authors.lastname: Meltz
mesh_terms: D002461:Cell Line, Transformed; D004307:Dose-Response Relationship, Radiation; D005786:Gene Expression Regulation / Q000528:radiation effects; D017781:Genes, Immediate-Early; D016762:Genes, fos; D016758:Genes, jun; D011905:Genes, ras; D006801:Humans; D000090063:Proto-Oncogene Mas; D012333:RNA, Messenger / Q000032:analysis; D015398:Signal Transduction; D014158:Transcription, Genetic / Q000528:radiation effects*
chemical_list: C000715470:MAS1 protein, human; D000090063:Proto-Oncogene Mas; D012333:RNA, Messenger