Persistent ER stress causes GPI anchor deficit to convert a GPI-anchored prion protein into pro-PrP via the ATF6–miR449c-5p–PIGV axis

Abstract

Endoplasmic reticulum (ER) stress and unfolded protein response are cells' survival strategies to thwart disruption of proteostasis. Tumor cells are continuously being challenged by ER stress. The prion protein, PrP, normally a glycosylphosphatidylinositol (GPI)-anchored protein exists as a pro-PrP retaining its GPI-peptide signal sequence in human pancreatic ductal cell adenocarcinoma (PDAC). Higher abundance of pro-PrP indicates poorer prognosis in PDAC patients. The reason why PDAC cells express pro-PrP is unknown. Here, we report that persistent ER stress causes conversion of GPI-anchored PrP to pro-PrP via a conserved ATF6-miRNA449c-5p-PIGV axis. Mouse neurons and AsPC-1, a PDAC cell line, express GPI-anchored PrP. However, continuous culture of these cells with the ER stress inducers thapsigargin or brefeldin A results in the conversion of a GPI-anchored PrP to pro-PrP. Such a conversion is reversible; removal of the inducers allows the cells to re-express a GPI-anchored PrP. Mechanistically, persistent ER stress increases the abundance of an active ATF6, which increases the level of miRNA449c-5p (miR449c-5p). By binding the mRNA of PIGV at its 3'-UTRs, miR449c-5p suppresses the level of PIGV, a mannosyltransferase pivotal in the synthesis of the GPI anchor. Reduction of PIGV leads to disruption of the GPI anchor assembly, causing pro-PrP accumulation and enhancing cancer cell migration and invasion. The importance of ATF6-miR449c-5p-PIGV axis is recapitulated in PDAC biopsies as the higher levels of ATF6 and miR449c-5p and lower levels of PIGV are markers of poorer outcome for patients with PDAC. Drugs targeting this axis may prevent PDAC progression.