Bcl-6 is the most commonly involved oncogene in diffuse large B-cell lymphoma (DLBCL). Bcl-6 is a transcriptional repressor normally required for establishment of germinal centers (GC) by B-cells. Down regulation of Bcl-6 is required for plasmacytic differentiation of B-cells. Bcl-6 has 3 functional domains, an N-terminal BTB/POZ domain, a C-terminal C2H2 zinc finger domain and a middle domain or “RD2” (Repression Domain 2). Our lab designed specific inhibitors of the Bcl-6 BTB domain, which block the association of the SMRT corepressor with Bcl-6. These inhibitors cause apoptosis and growth suppression, but not differentiation of DLBCL cells by triggering direct target genes such as p53, but not differentiation related target genes such as Blimp-1. We hypothesized that other corepressors of Bcl-6 mediate repression of GC differentiation pathways. We examined several candidate corepressors and found that the protein components of the NuRD/MI-2 chromatin-remodeling complex interact with Bcl-6. The RD2 domain and not the BTB domain were required for binding to NuRD as shown by co-immunoprecipitations with Bcl-6 harboring mutations of either the BTB or RD2 domains. The NuRD subunit that binds directly to Bcl-6 is a tissue and stage-specific component, MTA3, which is expressed in GC B-cells but not plasma cells. We found that Bcl-6 positive DLBCL express MTA3 and that the endogenous Bcl-6 and MTA3 proteins interact in such cells. The functional contribution of NuRD to Bcl-6 repression was demonstrated by the fact that MTA3 potently enhances Bcl-6 mediated transcriptional repression. Moreover, NuRD forms a complex with Bcl-6 at its endogenous target genes in B-cell lymphoma cells as shown by chromatin immunoprecipitations. This was confirmed by another group, who also showed that down regulation of MTA3 induced differentiation in GC cells without affecting viability, the opposite effect of that mediated by the Bcl-6 BTB domain. We conclude that Bcl-6 mediates lymphomagenesis through two separate mechanisms - differentiation blockade through the RD2 domain (via MTA3) and suppression of apoptosis and proliferation through the N-terminal domain (via SMRT). Our current studies are geared towards targeting the Bcl-6-NuRD interface for the design of differentiation therapy agents for B-cell lymphomas.
[Proc Amer Assoc Cancer Res, Volume 46, 2005]