Abstract 400: Functional, structural and binding studies of the atypical ER-resident protein FKBP7, a potential target in chemoresistant prostate cancer
Authors: Dreno Luce , Tiraboschi Carolina , Lacoste Sofian , Gomez Sofia Calpe , Garrido Marine F. , Bertrand Matthieu , Tannoury Mariana , Jacquet Eric , Nhiri Naima , Loriot Yohann , Fizazi Karim , Compagno Daniel , Assrir Nadine , Lescop Ewen , Chauchereau Anne 
Published: June 2022

Abstract
Background: Metastatic Castration Resistant Prostate Cancer (mCRPC) is the latest stage of Prostate Cancer (PCa). Despite the use of taxane-based chemotherapies Docetaxel and Cabazitaxel, mCRPC remains lethal due to chemoresistance. We recently identified FKBP7 as a potential therapeutic target of interest in PCa. FKBP7 is overexpressed in taxane-resistant PCa cells and impacts both cell proliferation and Docetaxel efficacy in chemoresistant models. We demonstrated FKBP7 affects translation and binds to the translation initiation complex eIF4F. FKBP7 is an ER-resident FKBP, but its properties remain poorly described.
Methods: FKBP7 expression upon chemotherapy treatment was assessed by Western Blot and correlation between FKBP7 expression and clinical prognosis was examined on public cancer databases. Subcellular localization of FKBP7 was addressed by digitonin-based fractionation. Finally, FKBP7-predicted structure was obtained with structural modeling servers and sequence alignments were performed on AliView. We then produced and purified the recombinant catalytic domain of FKBP7 and collected high quality 15N HSQC NMR spectra. Afterwards, we examined its interaction with well-known FKBP ligands.
Results: Increased FKBP7 expression was observed upon treatment with several cytotoxic chemotherapies in PCa-cell lines, and TCGA data underlined FKBP7 impacts survival in other cancers than PCa. Together, this suggests an enlarged potential involvement of FKBP7 in adaptive resistance. Mechanistically, in taxane-resistant models, FKBP7 overexpression did not result from higher protein stability but rather from a transcriptional regulation. Preliminary results of subcellular fractionation showed FKBP7 is present in the Endoplasmic Reticulum (ER) and seems to localize in the cytosol, independently of proteasomal degradation. We also confirmed FKBP7 is N45-glycosylated and observed that cytosolic-FKBP7 appears mainly glycosylated. This suggests that the ER-resident FKBP7 could be refluxed in the cytosol depending on its glycosylation state to regulate the activity of the cytosolic eIF4F complex. We are currently validating this hypothesis. At last, structural modeling predicted a FKBP-type fold with a potential FKBP7 substrate specificity as its catalytic pocket presents distinct composition for charge and bulkiness compared to other FKBPs. Accordingly, FKBP7 strongly binds Rapamycin and Everolimus, but surprisingly not FK506, confirming a selective binding profile.
Conclusion: Our work depicts the recently identified FKBP7/eIF4F pathway in PCa-resistance and points atypical properties for the ER-resident FKBP7. It also indicates FKBP7 could be a druggable target in adaptive resistance of other solid cancers. Besides, we gathered original data on its structure and specificity for future drug-targeting strategies.
Citation Format: Luce Dreno, Carolina Tiraboschi, Sofian Lacoste, Sofia Calpe Gomez, Marine F. Garrido, Matthieu Bertrand, Mariana Tannoury, Eric Jacquet, Naima Nhiri, Yohann Loriot, Karim Fizazi, Daniel Compagno, Nadine Assrir, Ewen Lescop, Anne Chauchereau. Functional, structural and binding studies of the atypical ER-resident protein FKBP7, a potential target in chemoresistant prostate cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 400.