Transcriptional down-regulation of N-myc expression during B-cell development.
We have used nuclear run-on and DNase I sensitivity analyses to study the activity of the N-myc genes in cell lines that represent different stages of B-cell development. Both transformed pre-B-cell lines and a nontransformed pre-B-cell clone transcribe the N- and c-myc genes at substantial levels; in the nontransformed clone, transcription of these genes is regulated by the pre-B-cell growth factor interleukin-7. In contrast, transformed cell lines that represent the more mature stages of the B-cell pathway and mitogen-stimulated normal splenic B lymphocytes express the c-myc gene but do not express the N-myc gene at detectable levels. Down-regulation of N-myc expression in these cells occurs at the level of transcriptional initiation. Correspondingly, a set of DNase I-hypersensitive sites present in the 5' region of the N-myc promoter of pre-B-cell lines are absent in B-cell lines. To further elucidate this process, we have constructed fusion cell lines between an N-myc-expressing pre-B-cell line and a nonexpressing myeloma line; the hybrid cell lines transcriptionally down-regulate the pre-B copies of the N-myc gene. Lack of N-myc expression in a number of nonlymphoid cell lines also resulted from lack of N-myc transcription. Together, our findings demonstrate that the down-regulation of N-myc expression in the later stages of B-cell development is mediated primarily at the level of transcriptional initiation. They further show that dominant, trans-acting factors present in more mature B-lineage cell lines act to down-regulate the transcription of N-myc.
Smith
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RK
Howard Hughes Medical Institute, College of Physicians and Surgeons of Columbia University, New York, New York 10032.
Zimmerman
K
K
Yancopoulos
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GD
Ma
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Alt
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D000818:Animals; D001402:B-Lymphocytes / Q000378:metabolism*; D002454:Cell Differentiation / Q000235:genetics*; D002467:Cell Nucleus / Q000378:metabolism; D002474:Cell-Free System / Q000378:metabolism; D002478:Cells, Cultured; D002874:Chromosome Mapping; D003850:Deoxyribonuclease I / Q000378:metabolism; D015536:Down-Regulation; D016259:Genes, myc / Q000235:genetics*; D006801:Humans; D006825:Hybridomas / Q000378:metabolism; D008213:Lymphocyte Activation; D051379:Mice; D011401:Promoter Regions, Genetic / Q000235:genetics*; D012322:RNA Precursors / Q000235:genetics / Q000378:metabolism; D012323:RNA Processing, Post-Transcriptional; D014158:Transcription, Genetic
D012322:RNA Precursors; D003850:Deoxyribonuclease I