APPL may function as an adapter protein to modulate the phosphatidylinositol 3-kinase (PI3K)/Akt pathway. Although we have previously proven that the PI3K/Akt pathway can suppress androgen receptor (AR) transactivation, the potential linkage from APPL to the AR remains unclear. Here we demonstrated that APPL could suppress AR-mediated transactivation in a dose-dependent manner in LNCaP and PC-3 cells. This suppressive effect could be blocked by either dominant-negative Akt or dominant-negative PI3K or LY294002, suggesting that the APPL-mediated suppression of AR transactivation is dependent on the PI3K/Akt pathway. We also observed that APPL could further enhance the Akt-mediated suppression of AR transactivation and AR target gene using the reporter gene and Northern blot assay. APPL was able to enhance insulin-like growth factor (IGF-1)-mediated Akt activation. The abrogation of IGF-1-mediated Akt activation by the dominant-negative PI3K or LY294002 or antisense APPL suggests that APPL may function as an important adapter protein in controlling the IGF-1 --> Akt signal pathway. Co-immunoprecipitation and glutathione S-transferase pull-down assays suggest that APPL, Akt, and AR may exist in a complex and Akt may serve as an important bridge factor for the association of APPL with AR. Together, our data indicate that APPL may suppress AR transactivation via potentiating Akt activity.