Abstract 92: The role of AKT3 in prostate cancer

Abstract The five-year survival rate for metastatic prostate cancer (mPC) patients remains significantly lower than that of patients with primary prostate cancer (PC). The loss of PTEN, a lipid phosphatase that negatively regulates PI3K/AKT pathway, is one of the most frequent mutations in prostate cancer. Although AKT inhibitors seem promising for mCRPC patients, they often perform poorly in the clinic as monotherapies. We hypothesize that this poor response to AKT inhibitors may be due to a failure to account for the distinct and opposing roles of AKT isoforms. Previously published data from our lab suggests that AKT2 and possibly AKT3, are the mediators of PC progression while AKT1 is responsible for PC development. To further investigate the mechanisms underlying AKT isoform plasticity in the context of PTEN loss, we conducted a phosphoproteomics analysis that identified Akt isoform-specific substrates using T402 (mouse prostate epithelial cells with PTEN-/-/Rb-/-) and T402[PTEN] (T402 cells with PTEN re-expression). We identified both canonical (RXRXXS/T*P) and novel non-canonical phosphorylation motifs. Interestingly, more substrates were associated with Akt3 compared to Akt1 and Akt2, indicating that Akt3 may be an underappreciated driver of PC progression. Furthermore, Akt3 was found to preferentially phosphorylate substrates with non-canonical motifs. However, the direct mechanisms through which AKT3 and its substrates contribute to PC progression and metastasis remain unclear. From our phosphoproteomics analysis, we identified PAK4S181 as a potential Akt3 direct substrate. Knockdown of PAK4 in PTEN deficient PC cells resulted in a greater reduction in chemotaxis compared to PTEN intact PC cells. We hypothesize that AKT3 directly phosphorylates PAK4S181 to promote PC metastatic progression. These studies will provide a functional characterization of the downstream activity of AKT3, through substrate usage, to aid in the development of an AKT3 specific inhibitor. This research highlights the importance of discovering biomarkers to unveil a more targeted approach toward treating advanced stage prostate cancer patients. Citation Format: Dineen Vogler, Seamus Degan, Henry Withers, Irwin Gelman. The role of AKT3 in prostate cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 92.