Pancreatic ductal adenocarcinoma (PDAC) has the highest mortality rate of all major cancers with limited treatment options. PDAC has a 5-year survival rate of 10% which has not significantly improved in the past 40 years due to poor screening capabilities, intrinsic and acquired resistance to current therapeutics, metastasis already present at time of diagnosis, and heterogeneities in the tumors. Activating mutant KRAS is dominantly present in >90% of PDAC patients. New therapeutics targeting mutant KRASG12C are now showing some promise in the clinic, however there is already evidence of PDAC developing resistance to some of these and thus improved therapeutic and co-therapeutic approaches are urgently needed. Targeting KRAS downstream effectors has similarly had poor effect due to acquired resistance, feedback mechanisms and toxicities in normal tissues. Therefore, identifying additional KRAS interactors in PDAC may allow for a better understanding of drug resistance, metastasis, and feedback mechanisms and lead to new therapeutics and/or diagnostics. RSKs are serine/threonine kinases with more than 70 known substrates. These kinases are activated by both ERK MAPK and PI3K pathways downstream of mutant KRAS. We have demonstrated that RSK2 regulates cell motility and invasion in some cancers via activation of RhoA by phosphorylating the Guanine nucleotide Exchange Factor (GEF) LARG (ARHGEF12). RSK2 can also activate cell inflammation. Importantly, RSK has been shown to be in complex with KRAS in PDAC and may therefore play a key unknown role in PDAC development and progression (Chen et al, PNAS, 2021). Additionally, from proteomic analysis, we found a novel RSK2 interacting protein called G3BP1 that is a marker of stress granules which is related to cell survival response to environmental stress and viral infections. G3BP1 is highly expressed in PDCA and patients with high level of G3BP1 have significantly worse survival (Human Protein Atlas). We have further mastered culture of PDAC Patient-derived organoids (PDOs) which are a 3-D culture model that allows patient derived primary tumor cells to propagate while maintaining the original tumor heterogeneity and genetics. Using PDAC PDOs, we found RSK inhibitors effectively promoted PDAC cell death. We hypothesize that RSK promotes PDAC invasion and metastasis via regulation of RhoA activity and induces cell survival via regulation of stress granules via G3BP1. We propose the following aims to test this hypothesis: 1) Define how RSK2 differentially regulates Rho and LARG activities and focal adhesion dynamics in PDAC; 2) Define how RSK2 regulates G3BP1 localization and function upon various cellular stresses; 3) Define how RSK2 contributes to PDAC PDO KRAS inhibitor resistance; This work will delineate mechanisms by which RSK2 orchestrates PDAC invasion and resistance to KRAS inhibitors. These studies will provide groundwork to develop new prognostics and new co-therapeutics for PDAC that control invasion and therapeutic resistance in pre-clinical models.
