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FKBP51, a protein with a relevant role in developmental stages in mammals and other organisms, is highly expressed in melanoma and correlates with tumor aggressiveness and resistance to therapy. We found that FKBP51 positively regulates melanoma cancer stem cell (CSC) phenotype at the transcriptional level. We also found that melanoma CSCs are endowed with epithelial to mesenchimal transition (EMT) traits. The immunochemistry study of 10 primitive cutaneous and 20 brain melanoma metastases showed that melanocytic cells capable of extravasation and subverting the architecture of metastatic tissue display intense FKBP51 nuclear signal and cytoplasmic positivity for the CSC marker nestin. These findings support the conclusion that the genetic programs of cancer stemness and invasiveness overlap in melanoma, and that FKBP51 plays a pivotal role in sustaining such a program. To confirm the role of FKBP51 in melanoma invasion, we utilized an experimental metastasis model. Animal studies were performed in accordance with NIH recommendations and the approval of the local institutional animal research committee. Injecting SAN melanoma cells into the tail vein of IL2γ-NOD-SCID mice produced prominent multifocal lesions in lungs and liver. We systemically administered two separate doses of FKBP51 siRNA to mice after the injection of melanoma cells. Organ colonization was evaluated both in live animals and post mortem. Qualitative PET/CT image analysis demonstrated low FDG uptake limited to small areas in lungs and liver. By contrast, untreated mice had diffuse metastatic liver and lung lesions and high FDG uptake in the same organs. Quantitative analysis performed on PET/CT images revealed that mean standardized uptake values (SUV) in treated mice were significantly lower than in untreated mice, both for lungs and liver. High Frequency Ultrasound examination revealed few or no liver metastases in treated mice versus untreated mice. Post-mortem organ examination showed a dramatic difference between the morphologically preserved organs dissected from treated mice, and severely compromised organs from untreated mice. The histological examination confirmed these findings. Most interestingly, invading cells co-expressed FKBP51 and nestin. Our study suggests a prominent role for FKBP51 in the pathogenesis of metastatic melanoma; we envision that targeting this protein for melanoma metastasis prevention may produce a successful outcome. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 5626. doi:1538-7445.AM2012-5626

Background. FKBP51 is a cochaperone with peptidyl-prolyl isomerase activity that regulates several biological processes in the cell, through protein-protein interaction. There is increasing evidence that FKBP51 hyperexpression is associated with cancer and this protein has a relevant role in sustaining cell growth, malignancy, and resistance to therapy. Very recently, we have found that FKBP51 is a protein associated with malignant melanoma. Its expression correlates with tumor aggressiveness and is maximal in metastatic lesions. Moreover, we have found that melanoma cancer stem cells express FKBP51 at levels higher than the other tumor cells and observed a 4 to 40 fold increase in the number of cancer stem cells, in cultures of melanoma cells stably transfected with FKBP51. Mitotic index, stemness features and angiogenesis are the main signs of aggressive melanoma. To better understand the role of FKBP51 in the biology and progression of melanoma, we attempted to investigate if this protein regulated the expression of molecules associated with tumor progression, namely the Chromatin Assembly Factor-1 (CAF-1) p60; the ATP-Binding Cassette family members ABCG2 and ABCB5; and the pro-angiogenic factor, endoglin or TβRIII. Methods. The melanoma cell line SAN was used for these experiments. FKBP51 knock down was obtained with siRNA, FKBP51 hyperexpression was obtained by stably transfecting SAN with a vector containing the FKBP51 human gene under a CMV promoter. Protein expression was measured by western blot and flow cytometry; mRNA was quantified by Realtime PCR. Results. We found that ABCG2 and CAF1 p60 mRNA levels were reduced by 5 and 2.5 folds, respectively, in FKBP51-silenced cells, in comparison with non-silenced cells. Reduced ABCB5 and CAF-1 p60 protein levels were detected by Western blot. By contrast, a clear increase of ABCB5 and CAF-1 p60 protein levels was observed in FKBP51 hyperexpressing cells. Flow cytometry showed that 39% of melanoma cells expressed endoglin. Such percentage was decreased to 5% by FKBP51 silencing, whereas it increased up to 78% in FKBP51 hyperexpressing cells. Finally, we found that CAF-1 and endoglin were co-expressed in more than 80% of ABCG2+ cells, whereas their expression was Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 4999. doi:10.1158/1538-7445.AM2011-4999