Activation of PP2A-B56α leads to aberrant EGFR signaling and proliferative phenotypes in PDAC

Abstract

Pancreatic ductal adenocarcinoma (PDAC) stands to become the second most deadly cancer by 2030. The small GTPase, KRAS, is mutated in over 90% of PDAC patients and considered the primary driver mutation. Despite being an almost ubiquitous event, KRAS mutations have been difficult to target therapeutically, particularly KRASG12D, the most common mutation in PDAC. In addition to these pharmacological challenges, KRAS mutations have been shown to drive signaling plasticity and therapeutic resistance through phosphorylation cascades in most cancers. Protein phosphatases are master regulators of kinase signaling, however the contribution of phosphatase deregulation to mutant KRAS cancer phenotypes is poorly understood. Protein phosphatase 2A (PP2A) inhibits effectors downstream of KRAS, placing this family of enzymes as key regulators of PDAC oncogenic signaling. However, our previous studies utilizing small molecule activating compounds of PP2A show a heterogeneous response in PDAC, with some cell lines displaying increased oncogenic signaling despite induction of phosphatase activity. Similarly, specific PP2A subunits exhibit both tumor suppressive and oncogenic functions depending on the cellular context. Therefore, understanding the role of PP2A in regulating cancer phenotypes is critical for the future development of therapeutic strategies that leverage this phosphatase. Here, we determined the impact of the specific PP2A subunit, B56, on PDAC phenotypes using both genetic and pharmacological activation strategies in human PDAC cell lines and genetic mouse models. We demonstrate that while PP2A-B56 suppresses specific oncogenic pathways, B56 activation exacerbates PDAC proliferative phenotypes and decreases overall survival in vivo, potentially through increased epidermal growth factor receptor (EGFR) signaling. EGFR is a critical signaling node in PDAC as inhibition or loss of EGFR prevents KRAS-driven tumorigenesis and increased EGFR activity is associated with poor patient outcome. The activation of EGFR by PP2A-B56 is in part mediated through increased expression and processing of EGFR ligands, specifically amphiregulin, heparin-binding EGF-like growth factor (HB-EGF), and epiregulin. Furthermore, pharmacological PP2A activation in combination with EGFR inhibitors mitigates this signaling and increases cell death. Together, these studies implicate a previously undescribed non-canonical role for PP2A-B56 in EGFR signaling that contributes to PDAC progression.