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 c hain locus leading to deregulation of bcl-2 expression. We examined the rol e 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 Wes tern 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 e xpression, whereas B- and c-Myb had little effect on bcl-8 gene expression. Deletion analysis of the bcl-8 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 reg ion 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 extract s were passed over the Cdx site. Mutagenesis of the Cdx binding site abolis hed 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 a ctivity was observed in cotransfection studies of DHL-4 cells with the A-My b and Cdx2 expression vectors, and increased endogenous Bcl-2 protein expre ssion was observed in B cells transfected with A-Myb and/or Cdx2 expression constructs.