3’UTR-directed, kinase proximal mRNA decay inhibits C/EBPβ phosphorylation/activation to suppress senescence in tumor cells

C/EBPβ is a potent regulator of RAS-induced senescence (RIS) and the SASP. C/EBPβ is post- translationally activated in RIS cells by the effector kinases ERK1/2 and CK2, but in tumor cells activation is suppressed by the CEBPB 3’UTR. 3′UTR regulation of protein activity (UPA) requires a G/U-rich element (GRE) and its cognate binding protein, HuR. These components segregate CEBPB transcripts away from a perinuclear compartment harboring ERK1/2 and CK2, restricting C/EBPβ from its activating kinases. We report here that the mRNA decay proteins UPF1 and Staufen1/2 are essential UPA factors enriched within the perinuclear cytoplasm. STAU1/2 colocalize with CK2 on perinuclear signaling endosomes where they promote localized CEBPB mRNA decay. UPF1 or STAU1/2 depletion in tumor cells increased CEBPB transcripts adjacent to CK2 foci, coinciding with C/EBPβ activation and senescence. The GRE and an adjacent STAU binding site act to suppress C/EBPβ-mediated senescence, while a separate 3’UTR region inhibits SASP induction. Accordingly, KrasG12D -driven lung tumors in mice carrying a Cebpb GRE deletion rarely progressed to malignant adenocarcinomas. Our findings identify kinase-proximal mRNA decay as a novel mechanism to inhibit C/EBPβ activating modifications in tumor cells to facilitate senescence bypass. ### Competing Interest Statement The authors have declared no competing interest.