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 C hanges in gene expression combined with targeted protein degradation dynamically shape the protein landscape. Gene expression is coordinated by transcription factors that specify genes for activation and cofactors that modulate transcription factor activity or alter the local chromatin environment. Posttranslational modifications (PTMs) play a crucial role in transcriptional dynamics. Phosphorylation, acetylation, methylation, and ubiquitylation of histone proteins are well studied and contribute significantly to gene expression dynamics (1). Similarly, posttranslational modification of transcription factors plays an important role in regulating genome output.FoxM1 is an oncogenic, cell cycle-regulated transcription factor that was discovered as both a marker and a key mediator of cell proliferation (2-4). Subsequent work clarified the importance of FoxM1 in proliferation through its role in cell cycle progression (reviewed in reference 5). FoxM1 controls the mitotic transcriptional program, and its depletion significantly impairs normal mitotic entry and progression (6-9). In addition, FoxM1 and its transcriptional network have been associated with numerous cancers (5, 10). Notably, FoxM1 is the key regulator of a proliferative gene expression signature found in high-grade serous ovarian cancer (HGSOC), basal-like breast cancers,

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“…Cyclin A levels were unchanged between DMSO and AKT inhibitor treated cells, indicating that changes in Cyclin F stability were not the result of cell cycle arrest. We performed a similar assay, analyzing endogenous Cyclin F stability in U2OS cells that were first synchronized in S-phase using an thymidine block and release protocol (Wang et al, 2017). Consistently, pharmacological AKT inactivation shortened Cyclin F half-life (Supplemental Figure 3A and B).…”

The Dominant Role of Forkhead Box Proteins in Cancer

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The RepID–CRL4 ubiquitin ligase complex regulates metaphase to anaphase transition via BUB3 degradation

“…4a, input), suggesting that the degradation of BUB3 was mediated by another substrate receptor. To test this hypothesis, we immunoprecipitated CRL4 complexes using an antibody directed against the CRL4 component DDB1 in protein lysates from cells released from mitotic block and analyzed these extracts for the presence of known DCAFs 40,41 . The WD40-containing DCAF RBBP7, but not VprBP and RBBP4, was immunoprecipitated with FLAG-DDB1 concomitant with and immediately after the dissociation of RepID (Fig.…”