While both wt and shFKBP5 xenograft mice were able to benefit from combination therapy with gemcitabine and the Akt inhibitor, triciribine, shFKBP5 mice showed a greater effect after combination treatment. We recently reported that FKBP5 is a scaffolding protein that can enhance PHLPP-Akt interaction. The functional consequence of this interaction results in negative regulation of Akt activity. Down regulation of FKBP5 results in decreased PHLPP-Akt interaction and increased Akt phosphorylation at the Ser473 site, suggesting that FKBP5 may function as a tumor suppressor, an important fact contributing to chemoresistance. Based on our previous findings with FKBP5 and its role in chemoresistance, we tested this hypothesis in vivo using a xenograft mice model. We found that tumors in shFKBP5 mice were more resistant to gemcitabine treatment and also exhibited a faster tumor growth rate. This phenomenon appeared to involve the regulation of Akt activation, as determined by phosphorylated Akt and downstream signaling molecules. Since Akt is activated when FKBP5 is Selumetinib knocked down,we hypothesized that the addition of inhibitors targeting this pathway might reverse the drug resistance phenotype. The PI3K-Akt pathway has multiple drugable targets, so we tested a series of inhibitors targeting PI3K, Akt and mTOR. In addition to the role of FKBP5 in chemoresistance, based on our xenograft models it could also function as a tumor suppressor through negative regulation of the Akt pathway. As shown in Figures 3 and 5A, activity of the Akt pathway is significantly higher in FKBP5 knockdown SU86 xenografts than that in wild type SU86 xenografts and these observations correlated with higher tumor growth rates in shFKBP5 mice. Therefore, probably because of the higher basal levels of Akt activity, shFKBP5 xenografts responded better to combination treatment, which was seen as enhanced inhibition of tumor growth. This phenomenon was also reflected by decreased Akt 473 phosphorylation levels after gemcitabine and TCN treatment. The shFKBP5 xenografts showed a more dramatic decrease in Akt 473 phosphorylation levels wt xenografts. Our in vivo results further confirmed findings observed using the cell lines. Those studies demonstrated that lack of expression of FKBP5 led to increased Akt phosphorylation at the regulatory S473 amino acid residue as well as for downstream genes in the Akt pathway such as phosphorylated FOXO1 and GSK3b. Therefore, FKBP5 could be a tumor suppressor in pancreatic cancer and it could also be a biomarker for response to chemotherapy, especially gemcitabine therapy, a first line treatment for pancreatic cancer. Our findings that a specific Akt inhibitor can reverse resistance to gemcitabine in FKBP5 knockdown cells and xenografts indicate that FKBP5 levels might be used to stratify patients into different treatment arms, such as gemcitabine or gemcitabine plus an Akt inhibitor. Future clinical studies will be needed to test this hypothesis. In addition, the mechanisms underlying differences between the effects of PI3K inhibition, mTOR inhibition and Akt inhibition in combination with gemcitabine need to be explored further. PI3K activation causes phosphatidylinositol-3,4,5-triphosphate dependent membrane localization of Akt and PDK1, in which the latter can phosphorylate Akt 308. Therefore, the inhibition of PI3K might have less effect on 473 phosphorylation. Rapamycin can potentially LY294002 in vivo activate Akt 473 phosphorylation in an mTOR-2 dependent manner due to relief of feedback inhibition of IGF-1R signaling. That may explain why treatment with rapamycin plus gemcitabine failed to show a significant reduction of Akt 473 phosphorylation.