RESUMO
B cell-specific coactivator OCA-B, together with Oct-1/2, binds to octamer sites in promoters and enhancers to activate transcription of immunoglobulin (Ig) genes, although the mechanisms underlying their roles in enhancer-promoter communication are unknown. Here, we demonstrate a direct interaction of OCA-B with transcription factor TFII-I, which binds to DICE elements in Igh promoters, that affects transcription at two levels. First, OCA-B relieves HDAC3-mediated Igh promoter repression by competing with HDAC3 for binding to promoter-bound TFII-I. Second, and most importantly, Igh 3' enhancer-bound OCA-B and promoter-bound TFII-I mediate promoter-enhancer interactions, in both cis and trans, that are important for Igh transcription. These and other results reveal an important function for OCA-B in Igh 3' enhancer function in vivo and strongly favor an enhancer mechanism involving looping and facilitated factor recruitment rather than a tracking mechanism.
Assuntos
Elementos Facilitadores Genéticos/genética , Cadeias Pesadas de Imunoglobulinas/genética , Regiões Promotoras Genéticas/genética , Transativadores/metabolismo , Fatores de Transcrição TFII/metabolismo , Transcrição Gênica , Animais , Linfócitos B/metabolismo , Sequência de Bases , Linhagem Celular Tumoral , Imunoprecipitação da Cromatina , Histona Desacetilases/metabolismo , Humanos , Immunoblotting , Camundongos , Modelos Genéticos , Ligação Proteica , Interferência de RNA , Sequências Repetitivas de Ácido Nucleico , Transativadores/genética , Fatores de Transcrição TFII/genéticaRESUMO
OCA-B was originally identified as a nuclear transcriptional coactivator that is essential for antigen-driven immune responses. The later identification of a membrane bound, myristoylated form of OCA-B suggested additional, unique functions in B cell signaling pathways. This study has shown that OCA-B also functions in the pre-B1-to-pre-B2 cell transition and, most surprisingly, that it directly interacts with SYK, a tyrosine kinase critical for pre-BCR and BCR signaling. This unprecedented type of interaction-a transcriptional coactivator with a signaling kinase-occurs in the cytoplasm and directly regulates SYK stability. This study indicates that OCA-B is required for pre-BCR and BCR signaling at multiple stages of B cell development through its nontranscriptional regulation of SYK. Combined with the deregulation of OCA-B target genes, this may help explain the multitude of defects observed in B cell development and immune responses of Oca-b-/- mice.
Assuntos
Linfócitos B/fisiologia , Regulação Enzimológica da Expressão Gênica , Peptídeos e Proteínas de Sinalização Intracelular/metabolismo , Proteínas Tirosina Quinases/metabolismo , Transativadores/metabolismo , Transcrição Gênica , Animais , Linfócitos B/citologia , Células da Medula Óssea , Diferenciação Celular , Células Cultivadas , Estabilidade Enzimática , Feminino , Imunoglobulina M/metabolismo , Imuno-Histoquímica , Interleucina-7/metabolismo , Camundongos , Camundongos Endogâmicos C57BL , Camundongos Knockout , Receptores de Superfície Celular/metabolismo , Transdução de Sinais/fisiologia , Quinase SykRESUMO
Most lymphomas arise by transformation of germinal center (GC) B cells. TCL1, a proto-oncogene first recognized for its role in T-cell transformation, also induces GC B-cell malignancies when dysregulated in pEmu-B29-TCL1 transgenic (TCL1-tg) mice. Clonal B-cell lymphomas develop from polyclonal populations with latencies of 4 months or more, suggesting that secondary genetic events are required for full transformation. The goals of this study were to determine the GC-related effects of TCL1 dysregulation that contribute to tumor initiation and to identify companion genetic alterations in tumors that function in disease progression. We report that compared with wild-type (WT) cells, B cells from TCL1-tg mice activated in a manner resembling a T-dependent GC reaction show enhanced resistance to FAS-mediated apoptosis with CD40 stimulation, independent of a B-cell antigen receptor (BCR) rescue signal. Mitogenic stimulation of TCL1-tg B cells also resulted in increased expression of Aicda. These GC-related enhancements in survival and Aicda expression could underlie B-cell transformation. Supporting this notion, no B-cell lymphomas developed for 20 months when TCL1-tg mice were crossed onto an Oct coactivator from B cell (OCA-B)-deficient background to yield mice incapable of forming GCs. Spectral karyotype analyses showed that GC lymphomas from TCL1-tg mice exhibit recurrent chromosome translocations and trisomy 15, with corresponding MYC overexpression. We conclude that pEmu-B29-TCL1 transgenic B cells primed for transformation must experience the GC environment and, for at least some, develop genome instability to become fully malignant.
Assuntos
Linfócitos B/metabolismo , Transformação Celular Neoplásica/genética , Instabilidade Genômica , Proteínas Proto-Oncogênicas/genética , Proto-Oncogenes , Animais , Apoptose , Linfócitos B/imunologia , Linfócitos B/patologia , Antígenos CD40/metabolismo , Diferenciação Celular , Proliferação de Células , Transformação Celular Neoplásica/patologia , Citidina Desaminase/metabolismo , Proteína Ligante Fas , Feminino , Expressão Gênica , Genes myc , Centro Germinativo/metabolismo , Centro Germinativo/patologia , Linfoma de Células B/etiologia , Linfoma de Células B/genética , Linfoma de Células B/patologia , Masculino , Glicoproteínas de Membrana/metabolismo , Camundongos , Camundongos Knockout , Camundongos Transgênicos , Modelos Biológicos , Fatores de Necrose Tumoral/metabolismoRESUMO
The tissue-specific transcriptional coactivator OCA-B is required for antigen-dependent B cell differentiation events, including germinal center formation. However, the identity of OCA-B target genes involved in this process is unknown. This study has used large-scale cDNA arrays to monitor changes in gene expression patterns that accompany mature B cell differentiation. B cell receptor ligation alone induces many genes involved in B cell expansion, whereas B cell receptor and helper T cell costimulation induce genes associated with B cell effector function. OCA-B expression is induced by both B cell receptor ligation alone and helper T cell costimulation, suggesting that OCA-B is involved in B cell expansion as well as B cell function. Accordingly, several genes involved in cell proliferation and signaling, such as Lck, Kcnn4, Cdc37, cyclin D3, B4galt1, and Ms4a11, have been identified as OCA-B-dependent genes. Further studies on the roles played by these genes in B cells will contribute to an understanding of B cell differentiation.