FoxO proteins are transcriptional factors acting downstream of the tumor suppressor PTEN. Their activity is negatively regulated by AKT-mediated phosphorylation. Our previous studies showed a mutual suppression between PTEN and the androgen receptor (AR) in regulating growth and apoptosis in prostate cancer (PCa) cells. We hypothesize that nuclear FoxO proteins are involved in mediating this mutual antagonism. In this dissertation, we report that PTEN inhibits AR activity through FoxO1 and provide evidence for the involvement of FoxO factors in the androgen-mediated suppression of PTEN-induced apoptosis. Our studies identify a novel mechanism for AR inhibition by FoxO1 and demonstrated the participation of FoxO1 in AR inhibition by PTEN. Ectopic expression of active FoxO1 decrease the transcriptional activity of the AR as well as androgen-induced cell proliferation and production of prostate-specific antigen in PCa cells. FoxO1 knock down by RNA interference increased the transcriptional activity of the AR in PTEN intact cells and relieved its inhibition by ectopic PTEN in PTEN null cells. Mutational analysis revealed that FoxO1 region 150-655, which contains the fork head box and C-terminal activation domain, was required for AR inhibition. Mammalian two-hybrid assays demonstrated that the inhibition of AR activity by PTEN through FoxO1 involved the interference of androgen-induce interaction of the N- and C- termini of the AR and the recruitment of the p160 coactivators to the AR N-terminus. In addition to the inhibition of AR by FoxO1, we also demonstrated that PTEN-induced apoptosis is mediated through FoxO factors and that AR inhibited FoxO1 activity by yet-to-be identified downstream target gene. Mutation of AR DNA binding domain partially relieved the inhibition of FoxO1 trasnscriptional activity by androgens. Inhibiton of new protein synthesis abolished the AR-mediated decrease in the mRNA level of FoxO1 target gene. Overall, these studies reveal novel mechanisms for the mutual inhibition of AR and FoxO1 activity and establish FoxO proteins as important nuclear factors that mediate the mutual antagonism between AR and PTEN tumor suppressor in PCa cells.