Delayed translational silencing of ceruloplasmin transcript in gamma interferon-activated U937 monocytic cells: Role of the 3 ' untranslated region

Ceruloplasmin (Cp) is an acute-phase protein with ferroxidase, amine oxidas e, and pro- and antioxidant activities. The primary site of Cp synthesis in human adults is the liver, but it is also synthesized by cells of monocyti c origin. We have shown that gamma interferon (IFN-gamma) induces the synth esis of Cp mRNA and protein in monocytic cells. We now report that the indu ced synthesis of Cp is terminated by a mechanism involving transcript-speci fic translational repression. Cp protein synthesis in U937 cells ceased aft er 16 h even in the presence of abundant Cp mRNA. RNA isolated from cells t reated with IFN-gamma for 24 h exhibited a high in vitro translation rate, suggesting that the transcript was not defective. Ribosomal association of Cp mRNA was examined by sucrose centrifugation, When Cp synthesis was high, i.e., after 8 h of IFN-gamma treatment, Cp mRNA was primarily associated w ith polyribosomes. However, after 24 h, when Cp synthesis was low, Cp mRNA was primarily in the nonpolyribosomal fraction. Cytosolic extracts from cel ls treated with IFN-gamma for 24 h, but not for 8 h, contained a factor whi ch blocked in vitro Cp translation. Inhibitor expression was cell type spec ific and present in extracts of human cells of myeloid origin, but not in s everal nonmyeloid cells. The inhibitory factor bound to the 3' untranslated region (3'-UTR) of Cp mRNA, as shown by restoration of in vitro translatio n by synthetic 3'-UTR added as a "decoy" and detection of a binding complex by RNA gel shift analysis, Deletion mapping of the Cp 3'-UTR indicated an internal 100-nucleotide region of the Cp 3'-UTR that was required for compl ex formation as well as for silencing of translation. Although transcript-s pecific translational control is common during development and differentiat ion and global translational control occurs during responses to cytokines a nd stress, to our knowledge, this is the first report of translational sile ncing of a specific transcript following cytokine activation.