Loss of FOXO1 cooperates with TMPRSS2-ERG overexpression to promote prostate tumorigenesis and cell invasion

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.

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