Plk1 Regulates the Repressor Function of FoxM1b by inhibiting its Interaction with the Retinoblastoma Protein

Nishit K Mukhopadhyay,Pradip Raychaudhuri,Janai R Carr,Vaibhav Chand,Yi-Ju Chen,Akshay Pandey,Xiubei Liao,Dragana Kopanja

doi:10.1038/srep46017

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.

Highlights

IntroductionIt stimulates expression of genes important for G1 to S progression, as well as mitotic progression
We showed that Rb binds to DNMT3b, and that Rb is critical for the FoxM1-DNMT3b mediated methylation of the CpGs in the GATA3 promoter
The results identify a new molecular link between cell cycle regulators and regulation of differentiation genes that offers insights into how over-expression of FoxM1 might be playing a causal role in maintaining poorly differentiated state of cancer cells

Summary

Introduction

It stimulates expression of genes important for G1 to S progression, as well as mitotic progression. The transcriptional stimulatory function of FoxM1 requires its activation by the Cyclin/Cdk kinase and Plk[142,43]. A phospho-mimetic mutant at the two Plk[1] phosphorylation sites was shown to be constitutively active in stimulation of transcription[42] Because those kinases are cell cycle regulated, the transcriptional stimulatory activity of FoxM1 increases reaching maximal stimulatory activity in G2 and early M phases, where Cdk[1] is maximally active and Plk[1] is expressed at high levels by FoxM142,43. In mammary gland FoxM1 is expressed at high levels in the luminal progenitor cells, and it represses expression of the luminal differentiation factor GATA31. In this study we describe a new function of Plk[1] in regulating the repressor/methylation activities of FoxM1 by controlling its interaction with Rb and DNMT3b

Methods

Results

Conclusion