Abstract P4-08-05: Novel tumor suppressor regulating the PI3K/AKT pathway in triple negative breast cancer (TNBC)
Authors: Chan Stefanie , Smith Emma , Monti Stefano , Cardamone M Dafne , Perissi Valentina 
Published: March 2020

The PI3K/AKT signaling pathway is a known oncogenic pathway in breast cancer, involved in essential cellular processes necessary for cancer development and progression, including altered proliferation, metabolism, and cell survival. Mutations in this pathway occur in 20% of all TNBC cases, making it the most deregulated pathway in the only subclass of breast cancer with no targeted therapy. Few clinical trials testing PI3K/AKT pathway inhibition in these breast cancers have shown limited long term efficacy; the heterogeneity of tumors and variable response rates in clinical studies underscore the necessity to better understand the mechanisms governing TNBC and the PI3K/AKT pathway in order to predict patient response to targeted therapies. In particular, appropriate biomarkers are required to identify the tumor subtypes that may be more responsive to PI3K pathway inhibition.

While regulation of AKT activation through a classic phosphorylation signaling cascade is well understood, recent studies have revealed a non-canonical pathway contributing to regulate AKT activation and possibly providing an unexpected mechanism for resistance to PI3K inhibition. Non-canonical regulation of AKT is provided by non-proteolytic ubiquitination of AKT favoring its recruitment to the plasma membrane and leading to hyperactivation, but a full understanding of the mechanism and players involved in this process is missing.

Our lab has recently identified G-Protein Pathway Suppressor 2 (GPS2) as a novel regulator of the insulin signaling pathway through the inhibition of AKT ubiquitination in the adipose tissue. Here, we explore the role of GPS2 as a tumor suppressor in the context of breast cancer. Deletion of GPS2 in aggressive MDA-MB231 TNBC cells results in increased AKT phosphorylation associated with greater proliferation, migration and invasion in vitro, with all phenotypes mitigated in presence of the allosteric AKT inhibitor MK2206.Transcriptomic profiling of wild-type MB231, MB231-GPS2-KO, and myristoylated AKT (MB231-myrAKT) revealed a significant overlap between the MB231-GPS2-KO and myrAKT signatures. Conversely, deletion of GPS2 in MDA-MB468, a TNBC cell line with an activating mutation in the PI3K/AKT pathway, do not show any increase in proliferation, migration or invasion, but remain sensitive to AKT inhibition. Together these results indicate that loss of GPS2-mediated inhibition of AKT ubiquitination is sufficient to drive activation of the PI3K/AKT pathway in absence of other activating mutations. Furthermore, preliminary analysis of an orthotopic model of MB231-GPS2-KO cells in NOD scid gamma (NSG) mice resulted in greater breast tumor sizes when compared to the parental cell line, confirming GPS2 putative tumor suppressor role in vivo. Further studies are required to investigate GPS2 expression levels and mutational status in tumor samples, and to confirm GPS2 relevance as a predictive biomarker for treatment outcome, thus exposing an additional category of tumors that could be responsive to PI3K/AKT class inhibition and/or would benefit from alternative strategies to prevent resistance.

Citation Format: Stefanie Chan, Emma Smith, Stefano Monti, M Dafne Cardamone, Valentina Perissi. Novel tumor suppressor regulating the PI3K/AKT pathway in triple negative breast cancer (TNBC) [abstract]. In: Proceedings of the 2019 San Antonio Breast Cancer Symposium; 2019 Dec 10-14; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2020;80(4 Suppl):Abstract nr P4-08-05.