Regulation of AP-3 enhancer activity during hematopoietic differentiation

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Abstract
Phorbol ester treatment of the human leukemic cell line U937 induces macrophage differentiation over 24–28 hr. This differentiation is mediated by the activation and/or repression of specific gene transcription by proteins, enhancer binding factors, that bind to the DNA upstream of the start site of transcription. We find that differentiation of U937 cells induced by phorbol eters and bryostatin 1, activators of protein kinase C, and the phosphatase inhibitor, okadaic acid, stimulates transcription from an enhancer sequence which contains multimerized AP‐3 binding sequences but not from one that contains multimerized AP‐2 binding motifs. Electrophoretic mobility shift assays (EMSA) demonstrate that AP‐3 DNA binding activity peaks at 24 hr, remains elevated for 24 hr, and then decreases thereafter. Southwestern blotting demonstrates that the AP‐3 enhancer sequence binds to a 48 kDa protein present in these leukemic cells. Because the AP‐3‐oligomer also contains an overlapping NF‐kB‐like site, the role of NF‐kB proteins in regulating transcription from this multimerized oligonucleotide was investigated. Transfection of U937 cells with NF‐kB family members demonstrated activation of AP‐3‐mediated transcription by rel A but little effect induced by NFKB1 and c‐rel. It is unlikely, however, that phorbol ester‐induced transcription from this AP‐3 sequence is solely mediated by this NF‐kB family member since treatment of U937 cells with antisense rel A oligodeoxynucleotides did not block phorbol ester‐mediated transcription from the AP‐3 site. These data demonstrate that AP‐3, but not AP‐2 sequences, functions to activate mRNA transcription during phorbol ester‐induced hematopoietic differentiation and suggests a complex interaction between NF‐kB and AP‐3 proteins in the regulation of this enhancer element.