E26 transformation-specific (ETS) transcription factor ERG is aberrantly overexpressed in approximately 50% of all human PCa due to TMPRSS2-ERG gene rearrangements. However, mice with prostate-specific transgenic expression of PCa-associated ERG alone fail to develop PCa, highlighting that ERG requires other lesions to drive prostate tumorigenesis. Forkhead box (FOXO) transcription factor FOXO1 is a tumor suppressor that is frequently inactivated in human PCa. Here we demonstrate that FOXO1, but not other FOXO proteins (FOXO3 and FOXO4), binds and inhibits the transcriptional activity of PCa-associated ERG independently of FOXO1 transcriptional activity. Knockdown of endogenous FOXO1 increased invasion of TMPRSS2-ERG fusion positive VCaP cells, an effect completely abolished by ERG knockdown. Patient specimen analysis demonstrated that FOXO1 and ERG protein expression inversely correlated in a subset of human PCa. Although human ERG transgene expression or homozygous deletion of Foxo1 alone in the mouse prostate failed to promote tumorigenesis, concomitant ERG transgene expression and Foxo1 deletion resulted in upregulation of ERG target genes, increased cell proliferation and formation of high-grade prostatic intraepithelial neoplasia (HGPIN). Overall, we provide biochemical and genetic evidence that aberrantly activated ERG cooperates with FOXO1 deficiency to promote prostate tumorigenesis and cell invasion. Our findings enhance understanding of PCa etiology and suggest that the FOXO1-ERG signaling axis can be a potential target for treatment of PCa.

Deletions or mutations in and tumor suppressor genes have been linked to lineage plasticity in therapy-resistant prostate cancer. Fusion-driven overexpression of the oncogenic transcription factor is observed in approximately 50% of all prostate cancers, many of which also harbor and alterations. However, the role of ERG in lineage plasticity of/-altered tumors is unclear. Understanding the collective effect of multiple mutations within one tumor is essential to combat plasticity-driven therapy resistance. We generated a -negative/-mutated/-overexpressing mouse model of prostate cancer and integrated RNA-sequencing with ERG chromatin immunoprecipitation-sequencing (ChIP-seq) to identify pathways regulated by ERG in the context of / alteration. We investigated ERG-dependent sensitivity to the antiandrogen enzalutamide and cyclin-dependent kinase 4 and 6 (CDK4/6) inhibitor palbociclib in human prostate cancer cell lines, xenografts, and allografted mouse tumors. Trends were evaluated in TCGA, SU2C, and Beltran 2016 published patient cohorts and a human tissue microarray. Transgenic expression in mice blocked/ alteration-induced decrease of AR expression and downstream luminal epithelial genes. ERG directly suppressed expression of cell cycle-related genes, which induced RB hypophosphorylation and repressed E2F1-mediated expression of mesenchymal lineage regulators, thereby restricting adenocarcinoma plasticity and maintaining antiandrogen sensitivity. In ERG-negative tumors, CDK4/6 inhibition delayed tumor growth. Our studies identify a previously undefined function of ERG to restrict lineage plasticity and maintain antiandrogen sensitivity in /-altered prostate cancer. Our findings suggest ERG fusion as a biomarker to guide treatment of /-altered, -intact prostate cancer..

Prostate cancer (PCa) that becomes resistant to hormone castration and next-generation androgen receptor (AR)-targeted therapies, called castration-resistant prostate cancer (CRPC), poses a significant clinical challenge. A better understanding of PCa progression and key molecular mechanisms could bring novel therapies to light. One potential therapeutic target is ERG, a transcription factor aberrantly up-regulated in PCa due to chromosomal rearrangements between androgen-regulated gene TMPRSS2 and ERG. Here we show that the most common PCa-associated truncated ERG T1–E4 (ERGΔ39), encoded by fusion between TMPRSS2 exon 1 and ERG exon 4, binds to bromodomain-1 (BD1) of bromodomain containing protein 4 (BRD4), a member of the bromodomain and extraterminal domain (BET) family. This interaction is partially abrogated by BET inhibitors JQ1 and iBET762. Meta-analysis of published ERG (T1–E4) and BRD4 chromatin immunoprecipitation-sequencing (ChIP-seq) data demonstrates overlap in a substantial portion of their binding sites. Gene expression profile analysis shows some ERG-BRD4 co-target genes are upregulated in CRPC compared to hormone-naïve counterparts. We provide further evidence that ERG-mediated invasion of PCa cells was significantly enhanced by an acetylation-mimicking mutation in ERG that augments the ERG-BRD4 interaction. Our findings reveal that PCa-associated ERG can interact and co-occupy with BRD4 in the genome, and suggest this druggable interaction is critical for ERG-mediated cell invasion and PCa progression.

Tissues and organs undergo constant physical perturbations and individual cells must respond to mechanical forces to maintain tissue integrity. However, molecular interactions underlying mechano-transduction are not fully defined at cell-cell junctions. This is in part due to weak and transient interactions that are likely prevalent in force-induced protein complexes. Using in situ proximal biotinylation by the promiscuous biotin ligase BirA tagged to α-catenin and a substrate stretch cell chamber, we sought to identify force-dependent molecular interactions surrounding α-catenin, an actin regulator at the sites of cadherin mediated cell-cell adhesion. While E-cadherin, β-catenin, vinculin and actin localize with α-catenin at cell-cell contacts in immuno-fluorescent staining, only β-catenin and plakoglobin were biotinylated, suggesting that this proximal biotinylation is limited to the molecules that are in the immediate vicinity of α-catenin. In mechanically stretched samples, increased biotinylation of non-muscle myosin IIA, but not myosin IIB, suggests close spatial proximity between α-catenin and myosin IIA during substrate stretching. This force-induced biotinylation diminished as myosin II activity was inhibited by blebbistatin. Taken together, this promising technique enables us to identify force sensitive complexes that may be essential for mechano-responses in force bearing cell adhesion.

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