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Differential effects by Mad and Max on transformation by cellular and viral oncoproteins
Abstract:
c-Myc is an essential component of the regulatory mechanisms controlling cell growth. Max is the obligatory partner of c-Myc for all its biological functions analysed to date. Recently two Max interacting proteins, Mad and Mxi1, have been identified. It has been suggested that these two proteins modulate c-Myc function, in the simplest model by competing with c-Myc for the interaction with Max. We have analysed different aspects of Mad function in comparison to Max. Native Mad/Max heterodimers bound specifically to a c-Myc/Max consensus DNA binding site. Furthermore Mad inhibited efficiently c-Myc, mutant p53, adenovirus E1a, or human papilloma virus type 16 transformation of rat embryo cells in cooperation with activated Ha-Ras. Myc transformed clones showed an increased cell cycle time and a reduced immortalization frequency after cotransfection with either mad or max. In contrast to Mad, Max did not inhibit E1a/Ha-Ras cotransformation but repressed c-Myc/Ha-Ras transformation efficiently. Mad delta N, an N-terminal deletion mutant of Mad, was as efficient in repressing c-Myc/Ha-Ras cotransformation as full length Mad but showed little inhibitory activity when assayed on E1a/Ha-Ras. Unlike wt Mad, Mad delta N had little effect on cell growth. Our data suggest that Mad affects cell growth at least in part by a c-Myc independent mechanism.
Insights
Mad and Max proteins regulate cell growth by interacting with c-Myc. Mad inhibits cell transformation, suggesting a c-Myc-independent role in cell growth regulation.
Area of Science:
- Molecular Biology
- Cell Biology
- Oncogenesis
Background:
- c-Myc is crucial for cell growth regulation and requires Max as a binding partner.
- Mad and Mxi1 are recently identified proteins that interact with Max and may modulate c-Myc function.
- The proposed mechanism involves Mad and Mxi1 competing with c-Myc for Max binding.
Purpose of the Study:
- To investigate the functional relationship between Mad and Max.
- To compare Mad's inhibitory effects with Max's.
- To elucidate Mad's role in cell growth and transformation.
Main Methods:
- Analysis of Mad/Max heterodimer binding to DNA consensus sites.
- Assessing the inhibition of cell transformation by Mad and Max in rat embryo cells.
- Evaluating the effects of Mad and Max on c-Myc-induced transformation and immortalization.
- Investigating the function of a Mad N-terminal deletion mutant (Mad delta N).
Main Results:
- Mad/Max heterodimers bind to c-Myc/Max DNA binding sites.
- Mad efficiently inhibits transformation induced by c-Myc, mutant p53, E1a, or HPV16 in cooperation with Ha-Ras.
- Mad and Max reduce cell cycle time and immortalization frequency in Myc-transformed cells.
- Max inhibits c-Myc/Ha-Ras transformation but not E1a/Ha-Ras transformation.
- Mad delta N represses c-Myc/Ha-Ras transformation similarly to wild-type Mad but has little effect on E1a/Ha-Ras transformation or cell growth.
Conclusions:
- Mad functions distinctly from Max, inhibiting various oncogenic transformations.
- Mad's inhibitory activity on cell transformation is partially independent of c-Myc.
- Mad influences cell growth through mechanisms that may not solely rely on c-Myc interaction.