The RNA-binding protein CUGBP1 regulates translation of proteins in a variety of biological processes. In this study, we show that aging liver increases CUGBP1 translational activities by induction of a high molecular weight protein-protein complex of CUGBP1. The complex contains CUGBP1, subunits α, β, and γ of the initiation translation factor eIF2, and four proteins of the endoplasmic reticulum, eR90, CRT, eR60, and Grp78. The induction of the CUGBP1-eIF2 complex in old livers is associated with the elevation of protein levels of CUGBP1 and with the hyper-phosphorylation of CUGBP1 by a cyclin D3-cdk4 kinase, activity of which is increased with age. We have examined the role of the elevation of CUGBP1 and the role of cyclin D3-cdk4-mediated phosphorylation of CUGBP1 in the formation of the CUGBP1-eIF2 complex by using CUGBP1 transgenic mice and young animals expressing high levels of cyclin D3 after injection with cyclin D3 plasmid. These studies showed that both the increased levels of CUGBP1 and cdk4-mediated hyper-phosphorylation of CUGBP1 are involved in the age-associated induction of the CUGBP1-eIF2 complex. The CUGBP1-eIF2 complex is bound to C/EBPβ mRNA in the liver of old animals, and this binding correlates with the increased amounts of liverenriched activator protein and liver-enriched inhibitory protein. Consistent with these observations, the purified CUGBP1-eIF2 complex binds to the 5′ region of C/EBPβ mRNA and significantly increases translation of the three isoforms of C/EBPβ in a cell-free translation system, in cultured cells, and in the liver. Thus, these studies demonstrated that age-mediated induction of the CUGBP1-eIF2 complex changes translation of C/EBPβ in old livers.
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