Involvement of the AU‐Rich Elements in the Translational Control of the Human IL‐3 mRNA.

Many transiently expressed mRNAs are regulated at the level of stability and translation. Human interleukin‐3 (IL‐3), which has been implicated in cancer, harbors sequences rich in adenosine and uridine (AREs) in the 3′ untranslated region (3′‐UTR) of its mRNA that play a role in post‐transcriptional regulation. To understand how the AREs in the 3′‐UTR of the IL‐3 mRNA regulate its expression, we made a series of firefly luciferase reporter constructs harboring specific regions of the IL‐3 3′‐UTR. Transient transfection assays of HeLa cells using these ARE‐luciferase chimeras showed a dramatic translational repression by the presence of the IL‐3 ARE. We also monitored the effects on translation of these 3′‐UTR sequences in both rabbit reticulocyte lysates (RRLs) and translationally‐competent HeLa cell‐free extracts. IL‐3 AREs can dramatically reduce the luciferase activity of the reporter chimera in both in vitro translation systems. The reduction observed in luciferase levels is not due to an altered stability of the reporter mRNAs. Gel shift assays demonstrated the formation of several complexes in the IL‐3 3′‐UTR regions. TIA‐1 and TIAR proteins, known bona fide translational repressors, are present within these complexes. These results provide the first evidence for ARE‐dependent translational control of the human IL‐3 mRNA. This work is supported by grants from the NIH to C.I.G. (KO1 HL‐04355‐05, GM008102‐3052, U54 CA96297‐03).