Abstract PR14: Defining isoform-specific roles for AKTs in BRAFV600E-driven melanoma

Abstract

Abstract The PI3K/AKT pathway is frequently dysregulated in cutaneous melanoma and impacts both tumor aggression and resistance to BRAFV600E/K inhibitors. While current clinical approaches to AKT inhibition are severely limited by the toxicity of pan-AKT inhibitors, selective inhibition of individual AKT isoforms (AKT1, AKT2, or AKT3) remains an attractive, if yet unattainable, approach. A critical gap in our understanding concerns how the three highly homologous yet functionally distinct AKT isoforms contribute to melanomagenesis and treatment response. To address this question, we are employing RNAi and gene editing approaches to interrogate the effect of selective suppression or CRISPR/Cas9-mediated deletion of each isoform in vitro, as well as the impact of AKT isoform loss on 1) the growth of melanoma xenografts and 2) the development of spontaneous tumors in a melanoma-prone mouse model. In addition, we are interrogating tumor-promoting functions of AKT isoform-selective substrates recently identified in a phospho-proteomic screen of mouse fibroblasts, a system that allows us to identify downstream actionable targets that may mediate the effects of AKT isoforms in tumorigenesis. Broadly, we are focusing on the importance of AKTs in tumor cell growth, metastasis, and response to inhibitors of BRAFV600E/K or CDK4/6. We find that loss of AKT1 impacts growth of BRAF mutant human melanoma cells both in vitro and in vivo. Additionally, AKT1 appears to play an isoform-specific role in response to pharmacologic CDK4/6 inhibition, impacting transcriptional and morphologic changes typically associated with permanent cell cycle arrest. In contrast, loss of AKT2 has a minimal impact on melanoma cell growth or response to CDK4/6 inhibition, but severely limits the development of metastatic disease, potentially through a combination of impaired seeding at the metastatic site and defects in glycolysis. While AKT3 loss does not appreciably impact the above-mentioned cellular phenotypes, the protumorigenic role of AKT3 may involve activation of broadly-acting neutral proteases previously implicated in cell cycle progression, cell migration, and CDK5 regulation. Taken together, we provide evidence for distinct roles for AKT isoforms in several aspects of the tumorigenic process as well as response to current therapies. Future studies will focus on identification and targeting of the relevant downstream mediators of these phenotypes. This abstract is also being presented as Poster A16. Citation Format: Jaymes Farrell, Jodie Pietruska, Siobhan McRee, Philip Tsichlis, Philip Hinds. Defining isoform-specific roles for AKTs in BRAFV600E-driven melanoma [abstract]. In: Proceedings of the AACR Special Conference on Melanoma: From Biology to Target; 2019 Jan 15-18; Houston, TX. Philadelphia (PA): AACR; Cancer Res 2020;80(19 Suppl):Abstract nr PR14.