BAFF-R Receptor Functions in Transcription Regulation in Genes Critical to Survival and Proliferation in Normal and Neoplastic Human B Lymphocytes

Abstract B-lymphocyte stimulator (BLyS also called BAFF), is a potent cell survival factor expressed in many hematopoietic cells. However, many aspects of BLyS signaling pathway remain unclear. In order to investigate BLyS signaling pathway, we studied the function of BAFF-R (also called BR3), a major BLyS receptor, in normal and neoplastic B cell survival and proliferation. In our initial study, we found that BAFF-R was also present in nucleus, in addition to its presence in the plasma membrane of B cells. A candidate nuclear localization sequence was identified in the BAFF-R protein sequence. We also found BAFF-R mediated transcriptional activity, that could be increased through over expression of a NF-κB family member c-rel. Further study showed that BAFF-R co-localized with, c-rel in the nucleus and bound to the NF-κB binding site on the promoters of NF-κB target genes such as BLyS, CD154, Bcl-xL, Bfl-1/A1 and IL-8. The IκB kinase (IKK) protein complex is critical for regulating NF-κB pathway activation. The IKK complex includes three important subunits: the catalytic subunits IKKα and IKKβ (also known as IKK1 and IKK2) and the regulatory subunit IKKγ (also known as NEMO). In the cytoplasm, activation of the IKK complex induces processing of precursors p105 and p100 into p50 and p52 respectively, resulting in NF-κB subunit dimeric partners that migrate from the cytoplasm into the nucleus. In recent studies, IKKα has also been identified in the cell nucleus, functioning in histone H3 phosphorylation. Although IKKβ was also previously observed in the cell nucleus, its nuclear function has been obscure. Besides functioning as a transcriptional co-factor with c-rel, we also found BAFF-R interacted with IKKβ in the nucleus of normal and neoplastic (lymphoma) B cells, enhancing histone H3 phosphorylation through IKKβ by immuno precipitation experiments and in vitro kinase assays. Inhibition of BAFF-R entry into the nucleus by BAFF-R NLS mutant transfection, decreased the level of phosphorylated histone H3 compared to the controls in NHL-B cells. These findings not only demonstrate a novel nuclear function of IKKβ, but also determine a new mechanism of how BAFF-R promotes survival and proliferation of normal B cells and NHL-B cells. In addition to activating NF-κB pathways in the plasma membrane, BAFF-R also functions as a transcriptional regulator binding to NF-κB targeted gene promoters possibly through a chromatin remodeling mechanism(s).