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Function of the c-Myc antagonist Mad1 during a molecular switch from proliferation to differentiation
C M Cultraro1, T Bino, S Segal
1NCI-Navy Medical Oncology Branch, National Cancer Institute, National Institutes of Health, Bethesda, Maryland 20889-5105, USA.
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.
Insights
Mad proteins actively repress cell growth, promoting differentiation by antagonizing Myc. This mechanism involves Mad-Max complexes switching from Myc-Max, enabling a transition from proliferation to differentiation.
Area of Science:
- Molecular Biology
- Cell Biology
- Cancer Research
Background:
- Mad-Max heterodimers antagonize Myc's transforming activity through complex interactions.
- The precise role of Mad in cellular differentiation remains unclear.
Purpose of the Study:
- To elucidate the mechanism by which Mad proteins regulate cellular differentiation.
- To investigate Mad's role in antagonizing Myc's growth-promoting functions.
Main Methods:
- Utilized murine erythroleukemia cells for differentiation studies.
- Employed co-transfection of c-myc and zinc-inducible mad1.
- Performed co-immunoprecipitation to analyze protein complex formation.
Main Results:
- Exogenously expressed c-Myc inhibits differentiation by preventing the Myc-Max to Mad-Max switch.
- Inducible Mad expression, triggered by zinc, promotes differentiation in co-transfected cells.
- Mad-Max complexes were found to be in excess over Myc-Max complexes, facilitating differentiation.
Conclusions:
- Mad functions via active repression to antagonize Myc and drive cellular differentiation.
- Specific Mad motifs (Sin-binding, basic region, leucine zipper) are crucial for mediating the proliferation-to-differentiation switch.