Regulation of translation initiation upon B-cell receptor activation

Abstract

Eukaryotic initiation factor 4A (eIF4A) promotes the translation initiation of genes harboring 5' untranslated regions (UTRs) with complex secondary structures. eIF4A activity is modulated by its inhibitor, programmed cell death 4 (PDCD4), which is regulated by the AKT pathway in B-cells. The release and degradation of PDCD4 frees eIF4A to bind to eIF4G, which together with the m7GTP cap-binding protein, eIF4E, forms the eIF4F cap-binding complex. Human diffuse large B-cell lymphomas (DLBCLs) are derived from antigen experienced B-cells and often display high expression of oncogenes that contain complex secondary structures in their 5' UTRs. We hypothesized that activated B-cells such as GC B-cells require enhanced eIF4A activity through B-cell receptor (BCR) stimulation. We found that eIF4A cap-binding activity is in fact greatly enhanced upon in vitro activation of human splenic B-cells after treatment with anti-BCR, while the pharmacological inhibitor of eIF4A, Silvestrol, potently inhibited BCR-induced protein translation. Upon antigen-induced activation, naïve, IgM+ B-cells differentiate within the germinal centers (GC) and undergo isotype switching to IgG. We found that IgG expressing human splenic B-cells displayed more robust eIF4A cap-binding activity and protein translation than IgM expressing B-cells after isotype specific BCR-stimulation. We also performed translational profiling to determine which mRNAs are preferentially loaded onto the polysome upon BCR activation and depleted upon pharmacological inhibition of eIF4A. Among the top differentially modulated genes was caspase recruitment domain family member 11 (CARD11). We found that CARD11, along with the B-cell CLL/lymphoma 10 (BCL10) and mucosa associated lymphoid tissue lymphoma translocation gene 1 (MALT1) harbor complex 5'UTRs unlike the majority of other components in the BCR signaling cascade. Importantly, these proto-oncogenes form the CARD11-BCL10-MALT1 (CBM) signalosome, which mediates BCR activation of nuclear factor of kappa light chain gene enhancer in B-cells (NF-κB) signaling. Both constitutive activation of NF-κB and the aberrant expression of CBM components are associated with B-cell lymphomas in humans. Collectively, these data indicate that eIF4A-mediated control of oncogene translation may be a critical component for B-cell lymphoma progression and suggest that pharmacological targeting of eIF4A may be an attractive therapeutic approach in the management of human B-cell lymphomas.