CPEB and two poly(A) polymerases control miR-122 stability and p53 mRNA translation

Control of p53-linked cell senescence Messenger RNAs are capped at their 3′ ends by a non-templated run of adenines — the polyA tail — that enhances their translation. The polyA polymerase Gld2 is guided to 3′ untranslated regions by the sequence-specific CPEB protein. Richter and colleagues have now found that the p53 tumour suppressor is not simply polyadenylated by Gld2. Rather, Gld2 can add a single A to the miR-122 microRNA, thereby stabilizing it. This adenylated miR-122/RISC complex then downregulates CPEB expression by binding target sites in its 3′ untranslated region. If CPEB is not downregulated by miR-122, CPEB binds the p53 3′ untranslated region and recruits a different polyA polymerase, Gld4. The data demonstrate a previously unknown hierarchy of translational control of p53 mRNA leading to cellular senescence. Cytoplasmic polyadenylation-induced translation controls germ cell development 1 , 2 , neuronal synaptic plasticity 3 , 4 , 5 and cellular senescence 6 , 7 , a tumour-suppressor mechanism that limits the replicative lifespan of cells 8 , 9 . The cytoplasmic polyadenylation element binding protein (CPEB) promotes polyadenylation by nucleating a group of factors including defective in germline development 2 (Gld2), a non-canonical poly(A) polymerase 10 , 11 , 12 , on specific messenger RNA (mRNA) 3′ untranslated regions (UTRs). Because CPEB regulation of p53 mRNA polyadenylation/translation is necessary for cellular senescence in primary human diploid fibroblasts 6 , we surmised that Gld2 would be the enzyme responsible for poly(A) addition. Here we show that depletion of Gld2 surprisingly promotes rather than inhibits p53 mRNA polyadenylation/translation, induces premature senescence and enhances the stability of CPEB mRNA. The CPEB 3′ UTR contains two miR-122 binding sites, which when deleted, elevate mRNA translation, as does an antagomir of miR-122. Although miR-122 is thought to be liver specific, it is present in primary fibroblasts and destabilized by Gld2 depletion. Gld4, a second non-canonical poly(A) polymerase, was found to regulate p53 mRNA polyadenylation/translation in a CPEB-dependent manner. Thus, translational regulation of p53 mRNA and cellular senescence is coordinated by Gld2/miR-122/CPEB/Gld4.