title: Function of the c-Myc antagonist Mad1 during a molecular switch from proliferation to differentiation.
abstract: Mad-Max heterodimers have been shown to antagonize Myc transforming activity by a mechanism requiring multiple protein-protein and protein-DNA interactions. However, the mechanism by which Mad functions in differentiation is unknown. Here, we present evidence that Mad functions by an active repression mechanism to antagonize the growth-promoting function(s) of Myc and bring about a transition from cellular proliferation to differentiation. We demonstrate that exogenously expressed c-Myc blocks inducer-mediated differentiation of murine erythroleukemia cells without disrupting the induction of endogenous Mad; rather, high levels of c-Myc prevent a heterocomplex switch from growth-promoting Myc-Max to growth-inhibitory Mad-Max. Cotransfection of a constitutive c-myc with a zinc-inducible mad1 results in clones expressing both genes, whereby a switch from proliferation to differentiation can be modulated. Whereas cells grown in N'N'-hexamethylene bisacetamide in the absence of zinc fail to differentiate, addition of zinc up-regulates Mad expression by severalfold and differentiation proceeds normally. Coimmunoprecipitation analysis reveals that Mad-Max complexes are in excess of Myc-Max in these cotransfectants. Moreover, we show that the Sin-binding, basic region, and leucine zipper motifs are required for Mad to function during a molecular switch from proliferation to differentiation.
authors.affiliation: NCI-Navy Medical Oncology Branch, National Cancer Institute, National Institutes of Health, Bethesda, Maryland 20889-5105, USA.
authors.forename: C M
authors.initials: CM
authors.lastname: Cultraro
authors.forename: T
authors.initials: T
authors.lastname: Bino
authors.forename: S
authors.initials: S
authors.lastname: Segal
mesh_terms: D000081:Acetamides / Q000494:pharmacology; D000970:Antineoplastic Agents / Q000494:pharmacology; D002352:Carrier Proteins; D018797:Cell Cycle Proteins; D002454:Cell Differentiation; D002455:Cell Division; D002472:Cell Transformation, Viral; D004247:DNA / Q000378:metabolism; D005656:Fungal Proteins / Q000378:metabolism*; D005786:Gene Expression Regulation; D006655:Histone Deacetylases; D006801:Humans; D009687:Nuclear Proteins / Q000378:metabolism*; D027383:Papillomaviridae; D010750:Phosphoproteins / Q000378:metabolism*; D016271:Proto-Oncogene Proteins c-myc / Q000037:antagonists & inhibitors*; D012097:Repressor Proteins / Q000378:metabolism; D029701:Saccharomyces cerevisiae Proteins; D014157:Transcription Factors / Q000378:metabolism; D014407:Tumor Cells, Cultured; D015854:Up-Regulation / Q000187:drug effects
chemical_list: D000081:Acetamides; D000970:Antineoplastic Agents; D002352:Carrier Proteins; D018797:Cell Cycle Proteins; D005656:Fungal Proteins; C501382:MAD1 protein, S cerevisiae; C501384:MAD1L1 protein, human; D009687:Nuclear Proteins; D010750:Phosphoproteins; D016271:Proto-Oncogene Proteins c-myc; D012097:Repressor Proteins; C071731:SIN3 protein, S cerevisiae; D029701:Saccharomyces cerevisiae Proteins; D014157:Transcription Factors; D004247:DNA; D006655:Histone Deacetylases; C014026:hexamethylene bisacetamide