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Plk1 Regulates the Repressor Function of FoxM1b by inhibiting its Interaction with the Retinoblastoma Protein
Nishit K Mukhopadhyay1, Vaibhav Chand1, Akshay Pandey1
1Department of Biochemistry and Molecular Genetics (M/C 669), University of Illinois, College of Medicine, 900 S, USA Ashland Ave., Chicago, IL-60607, USA.
Abstract:
FoxM1b is a cell cycle-regulated transcription factor, whose over-expression is a marker for poor outcome in cancers. Its transcriptional activation function requires phosphorylation by Cdk1 or Cdk2 that primes FoxM1b for phosphorylation by Plk1, which triggers association with the co-activator CBP. FoxM1b also possesses transcriptional repression function. It represses the mammary differentiation gene GATA3 involving DNMT3b and Rb. We investigated what determines the two distinct functions of FoxM1b: activation and repression. We show that Rb binds to the C-terminal activation domain of FoxM1b. Analyses with phospho-defective and phospho-mimetic mutants of FoxM1b identified a critical role of the Plk1 phosphorylation sites in regulating the binding of FoxM1b to Rb and DNMT3b. That is opposite of what was seen for the interaction of FoxM1b with CBP. We show that, in addition to GATA3, FoxM1b also represses the mammary luminal differentiation marker FoxA1 by promoter-methylation, and that is regulated by the Plk1 phosphorylation sites in FoxM1b. Our results show that the Plk1 phosphorylation sites in FoxM1b serve as a regulator for its repressor function, and they provide insights into how FoxM1b inhibits differentiation genes and activates proliferation genes during cancer progression.
Insights
Forkhead box M1b (FoxM1b) transcription factor phosphorylation by Plk1 regulates its dual function. Plk1 phosphorylation sites control FoxM1b
Area of Science:
- Molecular Biology
- Cancer Biology
- Cell Signaling
Background:
- FoxM1b is a cell cycle-regulated transcription factor overexpressed in cancers, correlating with poor prognosis.
- FoxM1b's activation function involves Cdk1/2 and Plk1 phosphorylation, leading to co-activator CBP binding.
- FoxM1b also exhibits transcriptional repression, notably inhibiting the mammary differentiation gene GATA3 via DNMT3b and Rb.
Purpose of the Study:
- To elucidate the regulatory mechanisms determining FoxM1b's distinct activation and repression functions.
- To investigate the role of Plk1 phosphorylation sites in modulating FoxM1b's interactions and functions.
Main Methods:
- Utilized phospho-defective and phospho-mimetic mutants of FoxM1b for functional analyses.
- Investigated the binding of FoxM1b to Rb and DNMT3b.
- Assessed the impact of Plk1 phosphorylation on FoxM1b's repression of differentiation genes like GATA3 and FoxA1.
Main Results:
- Rb binds to the C-terminal activation domain of FoxM1b.
- Plk1 phosphorylation sites critically regulate FoxM1b binding to Rb and DNMT3b, inversely to CBP binding.
- FoxM1b represses the mammary luminal differentiation marker FoxA1 via promoter-methylation, controlled by Plk1 phosphorylation sites.
Conclusions:
- Plk1 phosphorylation sites act as key regulators of FoxM1b's repressor function.
- These findings offer insights into how FoxM1b promotes cancer progression by inhibiting differentiation and activating proliferation genes.