Regulation of NFKB through the nuclear processing of p105 (NFKB1) in Epstein-Barr Virus immortalized B cell lines.

Transcription factors of the NF-κB/Rel family are retained in the cytoplasm as inactive complexes through association with IκB inhibitory proteins. Several NF-κB activators induce the proteolysis of IκB proteins, which results in the nuclear translocation and DNA binding of NF-κB complexes. Here, we report a novel mechanism of NF-κB regulation mediated by p105 (NF-κB1) precursor of p50 directly at the nuclear level. In Epstein- Barr virus-immortalized B cells, p105 was found in the nucleus, where it was complexed with p65. In concomitance with NF-κB activation, mitomycin C induced the processing of p105 to p50 in the nucleus, while it did not affect the steady-state protein levels of IκBα and p105 in the cytoplasm. Differently, phorbol 12-myristate 13-acetate induced a significant proteolysis of both IκBα and p105 in the cytoplasm, while it did not affect the protein level of p105 in the nucleus. These results suggest that in Epstein-Barr virus-positive B cell lines the nuclear processing of p105 can contribute to NF-κB activation in response to specific signaling molecules, such as DNA-damaging agents.