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

Cytoplasmic polyadenylation-induced translation controls germ cell development (1,2), neuronal synaptic plasticity (3-5) 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 (10-12), 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 CPEBmRNA. The CPEB3' 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.
Mouse embryo fibroblasts (MEFs) derived from CPEB knockout mice do not senesce as do MEFs derived from wild-type mice, but instead are immortal. Senescence is rescued when ectopic CPEB is [...]