Molecular mechanisms of transcriptional control of bcl-2 and c-myc in follicular and transformed lymphoma.

A synergistic interaction of Bcl-2 and c-Myc plays a role in lymphomagenesis in mice and in some patients as well. Progression of follicular lymphoma to a more aggressive lymphoma is seen in the majority of patients, and approximately 10% of the transformed lymphomas have a translocation of c-myc in addition to the translocation of bcl-2 found in the original follicular lymphoma. We investigated whether transcriptional deregulation of bcl-2 and c-myc could be examined in primary lymphoma cells by in vivo footprinting and in vitro protein-DNA binding studies. A matched pair of follicular and transformed lymphoma samples was examined. The transformed lymphoma had acquired a translocation of c-myc into the immunoglobulin heavy chain locus. High levels of bcl-2 expression were observed in both the follicular and transformed lymphomas, whereas the expression of c-myc was low in the follicular lymphoma and increased in the transformed lymphoma. In vivo footprint analysis revealed that a CRE site and a Cdx site in the bcl-2 promoter were occupied on the translocated alleles but not on the normal alleles in both the follicular and transformed lymphomas. Two nuclear factor kappaB sites were occupied on the translocated c-myc allele in the transformed lymphoma. Gel shift analysis revealed that these proteins bound to their respective sites in the bcl-2 or c-myc promoter. There was no evidence that the presence of one of the translocations in the immunoglobulin heavy chain locus influenced the expression of the other translocated gene.

Negative regulation of bcl-2 expression by p53 in hematopoietic cells.

The p53 protein activates promoters containing p53 binding sites, and it represses other promoters. We examined the effect of p53 on bcl-2 expression in both the DHL-4 B cell line and the K562 erythroleukemia line. Transient transfection analyses revealed that wild-type p53 repressed the bcl-2 full-length promoter. The region of the bcl-2 promoter that was responsive to p53 was mapped to the bcl-2 P2 minimal promoter region, and we showed that p53 and the TATA binding protein bound to the bcl-2 TATA sequence. The TATA binding protein, p53, histone deacetylase-1 and mSin3a could be co-immunoprecipitated from K562 cell nuclear extract. The TATA binding protein and mSin3a could be recovered in a complex at the bcl-2 promoter TATA sequence, however, the formation of this complex was not dependent on the presence of p53. Treatment of K562 cells with the histone deacetylase inhibitor, trichostatin A, resulted in an increase in bcl-2 promoter activity whether p53 was present or not. Therefore, we demonstrated that p53 and the histone deacetylases repress the bcl-2 promoter independently. Similar results were obtained when endogenous bcl-2 mRNA or protein levels were measured in response to either p53 or trichostatin A, and p53 expression resulted in enhanced apoptosis. RNase protection assays demonstrated that transcription from the endogenous 3' bcl-2 promoter was decreased by p53. The regions of p53 that were required for repression of the bcl-2 promoter were defined. We conclude that the TATA sequence in the bcl-2 P2 minimal promoter is the target for repression by p53, and that the interaction between p53 and TBP is most likely responsible for the repression. Mutation of p53 may play a role in the up-regulation of bcl-2 expression in some B cell lymphomas.

A-Myb up-regulates Bcl-2 through a Cdx binding site in t(14;18) lymphoma cells.

In follicular lymphoma, bcl-2 is translocated to the immunoglobulin heavy chain locus leading to deregulation of bcl-2 expression. We examined the role of Myb proteins in the regulation of bcl-2 expression in lymphoma cells. We showed that A-Myb up-regulates bcl-2 promoter activity. Northern and Western analyses demonstrated that A-Myb was expressed in the DHL-4 t(14;18) cell line. In t(14;18) cells and mature B cells, A-Myb up-regulated bcl-2 expression, whereas B- and c-Myb had little effect on bcl-2 gene expression. Deletion analysis of the bcl-2 5'-region identified a region responsive to A-Myb in t(14;18) cells. A potential binding site for the Cdx homeodomain proteins was located in this sequence. Analysis of the A-Myb-responsive region by UV cross-linking experiments revealed that a 32-kDa protein formed a complex with this region, but direct binding by Myb proteins could not be demonstrated. A-Myb could be recovered along with Cdx2 when nuclear extracts were passed over the Cdx site. Mutagenesis of the Cdx binding site abolished binding by the 32-kDa protein and significantly reduced the ability of A-Myb to induce bcl-2 expression. A strong induction of bcl-2 P2 promoter activity was observed in cotransfection studies of DHL-4 cells with the A-Myb and Cdx2 expression vectors, and increased endogenous Bcl-2 protein expression was observed in B cells transfected with A-Myb and/or Cdx2 expression constructs.