Abstract Background: FoxF1 belongs to the family of forkhead box transcription factors. Many forkhead box proteins are linked to cancer development and progression. However, the regulation and exact function of FoxF1 remains unclear. The aim of this study was to elucidate how the transcriptional activity of FoxF1 is controlled and to assess the role of FoxF1 within prostate cancer. Design: For assessment of FoxF1 functions tumor cell lines were stably transfected with wild type FoxF1 or mutant FoxF1 constructs lacking putative phosphorylation sites. Protein expression and localization of FoxF1 were analyzed by immunprecipitation and Western Blot. Using tissue microarrays of prostate cancer cohorts we analyzed the protein expression and intracellular localization of FoxF1 in tumors compared to benign tissue. Results: Primary tumors and distant metastases exhibited a significantly higher FoxF1 expression compared to benign prostate tissue. In tumors and metastases nuclear localized FoxF1 was more abundant compared to benign tissue. Transfection of tumor cell lines with FoxF1 followed by immunprecipitation and Western Blot showed that nuclear FoxF1 but not cytoplasmic FoxF1 is phosphorylated in tyrosine residues, conceivably tyrosine-39, as suggested by literature data. We further transfected tumor cell lines with different amino acid exchange mutants of putative phosphorylation sites and analyzed the transcription of the known FoxF1 target gene Pecam-1, revealing a complex pattern of regulation, dependent on the site of phosphorylation. Overexpression of FoxF1 in tumor cell lines resulted in epithelial-mesenchymal transition, as observed by loss of E-cadherin expression, gain of Vimentin expression, the up-regulation of Snail and Twist and changes to a spindle-like morphology. Furthermore, overexpression of FoxF1 led to anchorage-independent growth in soft agar as well as increased migration rates in vitro without affecting the proliferation rate. Transfection of tumor cell lines with FoxF1 followed by immunprecipitation and Western Blot showed that nuclear FoxF1 but not cytoplasmic FoxF1 is phosphorylated in tyrosine residues, conceivably tyrosine-39, as suggested by literature data. We further transfected tumor cell lines with different amino acid exchange mutants of putative phosphorylation sites and analyzed the transcription of the known FoxF1 target gene Pecam-1, revealing a complex pattern of regulation. Conclusion: In summary, our results point to a role of FoxF1 as a potential oncogene in prostate cancer whose nuclear localization and activity is regulated by phosphorylation. Citation Format: Carolin Bossmann, Constanze Merz, Yuri Tolkach, Jessica Carlsson, Sven Perner, Ove Andrén, Glen Kristiansen, Michael Nowak. FoxF1 is a potential oncogene in prostate cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 2559. doi:10.1158/1538-7445.AM2017-2559