The protein phosphatase-2A subunit PR130 is linked to cytotoxic protein aggregate formation in mesenchymal pancreatic ductal adenocarcinoma cells

Abstract

Protein phosphatase-2A (PP2A) is a major source of cellular serine/threonine phosphatase activity. PP2A B-type subunits regulate the intracellular localization and the catalytic activity of PP2A-A/PP2A-C complexes towards individual proteins. There is limited knowledge on how PP2A B-type subunits regulate biologically important functions and if these subunits determine the growth and drug responsiveness of tumor cells. Pancreatic ductal adenocarcinoma (PDAC) is a dismal disease with poor prognosis. Mesenchymal PDAC subtypes are more aggressive and metastasis-prone than epithelial subtypes. We show that mesenchymal PDAC cells express significantly higher levels of the PP2A B-type subunit PR130 and its mRNA Ppp2r3a than epithelial PDAC cells (n=38). Among 17 PP2A B-type subunits, this differential regulation is unique for Ppp2r3a and PR130. The higher levels of PR130 in mesenchymal PDAC cells are linked to their vulnerability to the PP2A inhibitor phendione. Phendione induces apoptosis and an accumulation of cytotoxic protein aggregates in such cells. These processes occur independently of the major tumor suppressor p53, which is frequently mutated in PDAC cells. Proteomic analyses reveal that phendione upregulates the chaperone heat shock protein HSP70 in mesenchymal PDAC cells. Inhibition of HSP70 promotes phendione-induced apoptosis. We additionally disclose that phendione promotes a proteasomal degradation of PR130. Genetic elimination of PR130 sensitizes mesenchymal PDAC cells to phendione-induced apoptosis and protein aggregate formation. These data illustrate pharmacologically amenable, selective dependencies of mesenchymal PDAC cells on PP2A-PR130 and HSP70. PP2A inhibition triggers a harmful accumulation of protein aggregates in neurons. This undesired mechanism might be exploited to kill mesenchymal tumor cells.