Abstract
Oncogene-induced senescence (OIS) is an intrinsic tumor suppression mechanism that requires the p53 and RB pathways and post-translational activation of C/EBPβ through the RAS-ERK cascade. We previously reported that in transformed/proliferating cells, C/EBPβ activation is inhibited by G/U-rich elements (GREs) in its 3'UTR. This mechanism, termed "3'UTR regulation of protein activity" (UPA), maintains C/EBPβ in a low-activity state in tumor cells and thus facilitates senescence bypass. Here we show that C/EBPβ UPA is overridden by AMPK signaling. AMPK activators decrease cytoplasmic levels of the GRE binding protein HuR, which is a key UPA component. Reduced cytoplasmic HuR disrupts 3'UTR-mediated trafficking of Cebpb transcripts to the peripheral cytoplasm-a fundamental feature of UPA-thereby stimulating C/EBPβ activation and growth arrest. In primary cells, oncogenic RAS triggers a Ca++-CaMKKβ-AMPKα2-HuR pathway, independent of AMPKα1, that is essential for C/EBPβ activation and OIS. This axis is disrupted in cancer cells through down-regulation of AMPKα2 and CaMKKβ. Thus, CaMKKβ-AMPKα2 signaling constitutes a key tumor suppressor pathway that activates a novel UPA-cancelling mechanism to unmask the cytostatic and pro-senescence functions of C/EBPβ.