Regulation of NF-kappa B through the nuclear processing of p105 (NF-kappa B1) in Epstein-Barr virus-immortalized B cell lines

Transcription factors of the NF-kappa-B/Rel family are retained in the cytoplasm as inactive complexes through association with I-kappa-B inhibitory proteins. Several NF-kappa-B activators induce the proteolysis of I-kappa-B proteins, which results in the nuclear translocation and DNA binding of NF-kappa-B complexes. Here, we report a novel mechanism of NF-kappa-B regulation mediated by p105 (NF-kappa-B1) 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-kappa-B activation, mitomycin C induced the processing of p105 to p50 in the nucleus, while it did not affect the steady-state protein levels of I-kappa-B-alpha and p105 in the cytoplasm. Differently, phorbol 12-myristate 13-acetate induced a significant proteolysis of both I-kappa-B-alpha 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-kappa-B activation in response to specific signaling molecules, such as DNA-damaging agents.