VprBP/DCAF1 Regulates the Degradation and Nonproteolytic Activation of the Cell Cycle Transcription Factor FoxM1

ABSTRACT

The oncogenic transcription factor FoxM1 plays a vital role in cell cycle progression, is activated in numerous human malignancies, and is linked to chromosome instability. We characterize here a cullin 4-based E3 ubiquitin ligase and its substrate receptor, VprBP/DCAF1 (CRL4^VprBP), which we show regulate FoxM1 ubiquitylation and degradation. Paradoxically, we also found that the substrate receptor VprBP is a potent FoxM1 activator. VprBP depletion reduces expression of FoxM1 target genes and impairs mitotic entry, whereas ectopic VprBP expression strongly activates a FoxM1 transcriptional reporter. VprBP binding to CRL4 is reduced during mitosis, and our data suggest that VprBP activation of FoxM1 is ligase independent. This implies a nonproteolytic activation mechanism that is reminiscent of, yet distinct from, the ubiquitin-dependent transactivation of the oncoprotein Myc by other E3s. Significantly, VprBP protein levels were upregulated in high-grade serous ovarian patient tumors, where the FoxM1 signature is amplified. These data suggest that FoxM1 abundance and activity are controlled by VprBP and highlight the functional repurposing of E3 ligase substrate receptors independent of the ubiquitin system.

KEYWORDS:

• cell cycle
• cullin ring ligase
• FoxM1
• transcriptional regulation
• ubiquitination

Supplemental material for this article may be found at https://doi.org/10.1128/MCB.00609-16.

ACKNOWLEDGMENTS

Thanks go to David Allison and Greg Wang (UNC—Chapel Hill) for assistance with ChIP experiments and the following individuals for reagents (see Materials and Methods for details): Yue Xiong (UNC—Chapel Hill), Lee Graves (UNC—Chapel Hill), Ben Major (UNC—Chapel Hill), Anja Bielinsky (University of Minnesota), and Mike Whitfield (Dartmouth University). We acknowledge the UNC Flow Cytometry Core Facility (supported in part by P30 CA016086 Cancer Center Core Support Grant to the Lineberger Cancer Center).

The Emanuele lab is supported by start-up funds from UNC (University Cancer Research Fund) and grants from the Susan G. Komen Foundation (CCR14298820), the Jimmy-V Foundation, and the National Institutes of Health (R01GM120309).

We declare that we have no competing financial interests related to this work.

Author contributions are as follows. M.J.E. conceived of and designed experiments. X.W. helped design and carried out the vast majority of the cell biological experiments. A.A. performed cell cycle assays and aided in qPCR analysis. C.A.M., J.L.K., and R.C. contributed experimentally to the cell biology aspects of this work. K.B. and A.B. performed in vitro binding assays. V.B.-J. and C.Z. coordinated the procurement of tissue samples.