Activation of c-myc promoter P1 by immunoglobulin κ gene enhancers in Burkitt lymphoma: functional characterization of the intron enhancer motifs κB, E box 1 and E box 2, and of the 3′ enhancer motif PU

Deregulated expression of the proto-oncogene c-myc in Burkitt lymphoma (BL) cells carrying a t(2;8) translocation is mediated by a synergistic interaction of the translocated immunoglobulin (Ig) κ gene intron (κEi) and 3′ (κE3′) enhancers and characterized by a strong activation of the promoter P1. We have investigated the functional role of distinct κ enhancer sequence motifs in P1 activation on both minichromosomes and reporter gene constructs. Stable and transient transfections of BL cells revealed critical roles of the κEi and κE3′ elements κB and PU, respectively. Joint mutation of κB and PU completely abolished P1 activity, implying that an interaction of κB- and PU-binding factors is essential for the enhancer synergism. Mutation of the E box 1 and E box 2 motifs markedly decreased P1 activity in transient but not in stable transfection experiments. Co-expression of the NF-κB subunit p65(RelA) and Sp1, an essential factor for P1 transcription, in Drosophila melanogaster SL2 cells synergistically enhanced promoter activity. Our results support a model which proposes cross-talk between promoter and enhancer binding factors as the basic mechanism for κ enhancer-mediated c-myc activation in BL cells.