Mechanism of NF-kappaB activation induced by gamma-irradiation in B lymphoma cells : role of Ras.

Nuclear factor (NF)-kappaB is a ubiquitous transcription factor involved in diverse cellular responses to various stimuli, including growth factors and radiation stress. Recently it was reported that gamma-irradiation (gamma-IR) upregulates allergy-associated adhesion molecule CD23 on B cells and monocytes via NF-kappaB activation. In the present study, the mechanism of NF-kappaB activation by gamma-IR was investigated to understand the signaling pathways involved in IR-induced, NF-kappaB-mediated enhancement of CD23 expression. In human B-cell line Ramos, gamma-IR induced a dose-dependent increase of nuclear translocation and transcriptional activity of NF-kappaB. The gamma-IR-induced NF-kappaB activation in these cells was sensitive to a proteosome inhibitor MG132 and an antioxidant, pyrollidine dithiocarbamate (PDTC), which suggests that gamma-IR-induced NF-kappaB activation proceeds via IkappaB gradation and redox regulation. Since Ras was shown to play a role in NF-kappaB-mediated survival and inflammation of cancer cells against radiation, the role of Ras signaling in the gamma-IR-induced NF-kappaB activation in these transformed B cells was examined. Transfection and overexpression of dominant active Ras produced an increase in NF-kappaB activity as shown by DNA binding and transcriptional activities of the kappaB-dependent reporter gene. gamma-IR, however, did not induce Erk activation, nor the gamma-IR-induced kappaB activity that was suppressed by inhibitors of Ras/Raf interaction or MEK/Erk. Importantly, it was noted that Ras significantly augmented both the gamma-IR-induced NF-kappaB activity and the gamma-IR-induced CD23 expression. Together these results suggest that while gamma-IR and Ras both contribute to the upregulation of CD23 expression via NF-kappaB Raf or Erk is not involved in gamma-IR-induced NF-kappaB activation.