Abstract Pancreatic cancer (PC) is one of the most lethal cancers in the U.S. with a 5-year survival rate of 7%, which has essentially unchanged for the past 40 years. Despite extensive research, there are no effective targeted therapies for this disease, suggesting an insufficient understanding of oncogenic signaling networks in PC. While almost all of the signaling networks implicated in cancer rely on the cooperation of kinases and phosphatases to transduce signals promoting cancer, most efforts have only been put on the study of protein kinases in cancer while leaving protein phosphatases as a blind spot. PP2A is a family of heterotrimers that serve as the main source of Ser/Thr phosphatase activities in human cells, with each consisting of a scaffold subunit (Aα/β), a catalytic subunit (Cα/β), and one of many non-conserved regulatory (B) subunits (>27) that control the substrate specificities of PP2A complexes. Our preliminary studies identified PR55α, a regulatory subunit of PP2A, as an important new player in promoting malignant PC. In particular, our studies revealed that PR55 inhibits the MOB1/LATS auto-activation loop of the Hippo tumor suppression pathway, whose function is to induce the phosphorylation of YAP causing its cytoplasmic retention and proteasomal degradation. In line with the essential role of YAP signaling in Kras-driven PC development and progression, knockdown of PR55α impeded the anchorage-independent growth, tumorigenicity, and metastasis of PC cells. Our preliminary data also revealed that the stability of PR55α protein is negatively regulated by the tumor suppressor p53 and its target gene FBXL20, a substrate adaptor for the SCF (SKP1-CUL1-F-box protein) E3 ligase. Consistently, we observed that PR55α is elevated in PC specimens, where its expression correlates with poor patient survival. Based on these findings, we hypothesize that loss of p53 function results in the stabilization of PR55α protein, which, in turn, activates the YAP oncogenic signaling required for Kras-driving PC initiation and progression. We will test the hypothesis by three specific aims: Aim 1 to define the mechanism by which PR55α/PP2A promotes YAP activation and the significance of this cascade in oncogenesis and malignant potential of PC, Aim 2 to elucidate how p53 suppresses the level of PR55α, and Aim 3 to delineate the role of the PR55/YAP axis in pancreatic cancer development and progression in the KPC spontaneous mouse model. The work is innovative, as it will illuminate a critical pathway that promotes PC initiation and progression through PR55/PP2A activated YAP signaling, which is novel to both the PP2A field and the PC field. Successful completion of the proposed studies will yield an in-depth mechanistic understanding of the critical contribution of PR55/PP2A complex to the observed cooperation between Kras activation and the loss of the p53 tumor suppressor during PC development and progression. The acquired knowledge not onl...