Myc represses the p21(WAF1/CIP1) promoter and interacts with Sp1/Sp3.
The cyclin-dependent kinase inhibitor p21((WAF1/CIP1)) inhibits proliferation both in vitro and in vivo, and overexpression of p21 in normal and tumor cell lines results in cell cycle arrest. In contrast, ectopic expression of Myc alleviates G(1) cell cycle arrest. Recent studies showed that Myc can repress p21 transcription, thereby overriding a p21-mediated cell cycle checkpoint. We found that activation of a Myc-estrogen receptor fusion protein by 4-hydroxytamoxifen in mouse cells resulted in suppression of endogenous p21 transcription. This effect was observed in the absence of de novo protein synthesis and was independent of histone deacetylase activity. In transient transfection studies, Myc effectively repressed p21 promoter constructs containing only 119 bp of sequence upstream of the transcription start site. This region contains multiple Sp1-binding sites and a potential initiator element, but no canonical Myc DNA-binding sites. Deletion of the potential initiator element does not affect repression of the p21 promoter by c-Myc. Coimmunoprecipitation and glutathione S-transferase pull-down experiments demonstrate that c-Myc may form complexes with Sp1/Sp3. We found that the central region of c-Myc interacts with the zinc finger domain of Sp1. Because Sp1 is required for p21 transcription, it is possible that Myc may down-regulate p21 transcription, at least in part, by sequestering Sp1. Repression of the p21 promoter may contribute to the ability of c-Myc to promote cell proliferation.
Department of Molecular Genetics, University of Illinois College of Medicine, Chicago, IL 60607, USA. agartel@uic.edu
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Gartel
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Ye
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Goufman
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Shianov
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Hay
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Najmabadi
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Tyner
D000818:Animals; D001483:Base Sequence; D002461:Cell Line, Transformed; D050759:Cyclin-Dependent Kinase Inhibitor p21; D016213:Cyclins / Q000235:genetics*; D017931:DNA Primers; D004268:DNA-Binding Proteins / Q000378:metabolism*; D006801:Humans; D051379:Mice; D011401:Promoter Regions, Genetic; D011485:Protein Binding; D016271:Proto-Oncogene Proteins c-myc / Q000378:metabolism / Q000502:physiology*; D020133:Reverse Transcriptase Polymerase Chain Reaction; D016329:Sp1 Transcription Factor / Q000378:metabolism*; D051705:Sp3 Transcription Factor; D014157:Transcription Factors / Q000378:metabolism*
C493956:CDKN1A protein, human; C493957:Cdkn1a protein, mouse; D050759:Cyclin-Dependent Kinase Inhibitor p21; D016213:Cyclins; D017931:DNA Primers; D004268:DNA-Binding Proteins; D016271:Proto-Oncogene Proteins c-myc; C497121:SP3 protein, human; D016329:Sp1 Transcription Factor; C497122:Sp3 protein, mouse; D014157:Transcription Factors; D051705:Sp3 Transcription Factor