RESUMO
Programmed mutagenesis of the immunoglobulin locus of B lymphocytes during class switch recombination (CSR) and somatic hypermutation requires RNA polymerase II (polII) transcription complex-dependent targeting of the DNA mutator activation-induced cytidine deaminase (AID). AID deaminates cytidine residues on substrate sequences in the immunoglobulin (Ig) locus via a transcription-dependent mechanism, and this activity is stimulated by the RNA polII stalling cofactor Spt5 and the 11-subunit cellular noncoding RNA 3'-5' exonucleolytic processing complex RNA exosome. The mechanism by which the RNA exosome recognizes immunoglobulin locus RNA substrates to stimulate AID DNA deamination activity on its in vivo substrate sequences is an important question. Here we report that E3-ubiquitin ligase Nedd4 destabilizes AID-associated RNA polII by a ubiquitination event, leading to generation of 3' end free RNA exosome RNA substrates at the Ig locus and other AID target sequences genome-wide. We found that lack of Nedd4 activity in B cells leads to accumulation of RNA exosome substrates at AID target genes and defective CSR. Taken together, our study links noncoding RNA processing following RNA polII pausing with regulation of the mutator AID protein. Our study also identifies Nedd4 as a regulator of noncoding RNAs that are generated by stalled RNA polII genome-wide.
Assuntos
Linfócitos B/enzimologia , Citidina Desaminase/metabolismo , Complexos Endossomais de Distribuição Requeridos para Transporte/metabolismo , RNA Polimerase II/metabolismo , Ubiquitina-Proteína Ligases/metabolismo , Animais , Linhagem Celular Tumoral , Células Cultivadas , Complexos Endossomais de Distribuição Requeridos para Transporte/genética , Complexo Multienzimático de Ribonucleases do Exossomo/metabolismo , Técnicas de Silenciamento de Genes , Células HEK293 , Humanos , Região de Troca de Imunoglobulinas/genética , Camundongos , Ubiquitina-Proteína Ligases Nedd4 , Proteínas Nucleares/metabolismo , Ligação Proteica , Fatores de Elongação da Transcrição/metabolismo , Ubiquitina-Proteína Ligases/genética , UbiquitinaçãoRESUMO
The V protein of parainfluenza virus 5 (PIV5) plays an important role in the evasion of host immune responses. The V protein blocks interferon (IFN) signaling in human cells by causing degradation of the STAT1 protein, a key component of IFN signaling, and blocks IFN-beta production by preventing nuclear translocation of IRF3, a key transcription factor for activating IFN-beta promoter. Interleukin-6 (IL-6), along with tumor necrosis factor (TNF)-alpha and IL-1beta, is a major proinflammatory cytokine that plays important roles in clearing virus infection through inflammatory responses. Many viruses have developed strategies to block IL-6 expression. Wild-type PIV5 infection induces little, if any, expression of cytokines such as IL-6 or TNF-alpha, whereas infection by a mutant PIV5 lacking the conserved C-terminal cysteine rich domain (rPIV5VDeltaC) induced high levels of IL-6 expression. Examination of mRNA levels of IL-6 indicated that the transcription activation of IL-6 played an important role in the increased IL-6 expression. Co-infection with wild-type PIV5 prevented the activation of IL-6 transcription by rPIV5VDeltaC, and a plasmid encoding the full-length PIV5 V protein prevented the activation of IL-6 promoter-driven reporter gene expression by rPIV5VDeltaC, indicating that the V protein played a role in inhibiting IL-6 transcription. The activation of IL-6 was independent of IFN-beta even though rPIV5VDeltaC-infected cells produced IFN-beta. Using reporter gene assays and chromatin immunoprecipitation (ChIP), it was found that NF-kappaB played an important role in activating expression of IL-6. We have proposed a model of activating and inhibiting IL-6 transcription by PIV5.
