Abstract
Post-transcriptional mechanisms play an important role in the control of inflammatory gene expression. The heterogeneous nuclear ribonucleoprotein K homology (KH)-type splicing regulatory protein (KSRP) triggers rapid degradation of mRNAs for various cytokines, chemokines, and other inflammation-related proteins by interacting with AU-rich elements (AREs) in the 3'-untranslated mRNA regions. In addition to destabilizing mRNAs, AU-rich elements can restrict their translation. Evidence that KSRP also participates in translational silencing was obtained in a screen comparing the polysome profiles of cells with siRNA-mediated depletion of KSRP with that of control cells. Among the group of mRNAs showing increased polysome association upon KSRP depletion are those of interleukin (IL)-6 and IL-1α as well as other ARE-containing transcripts. Redistribution of IL-6 mRNA to polysomes was associated with increased IL-6 protein secretion by the KSRP-depleted cells. Silencing of IL-6 and IL-1α mRNAs depended on their 3'-untranslated regions. The sequence essential for translational control of IL-6 mRNA and its interaction with KSRP was located to an ARE. KSRP-dependent silencing was reversed by IL-1, a strong inducer of IL-6 mRNA and protein expression. The results identify KSRP as a protein involved in ARE-mediated translational silencing. They suggest that KSRP restricts inflammatory gene expression not only by enhancing degradation of mRNAs but also by inhibiting translation, both functions that are counteracted by the proinflammatory cytokine IL-1.
Highlights
With and mediate the effects of AU-rich elements (AREs) have been identified
Translation of Individual mRNAs Is Suppressed by KSRP—As reported previously [27], IL-1 induces an increase in polysome association of several mRNAs, including that of IL-6
KSRP expression was suppressed in HeLa cells by specific siRNAs (Fig. 1A). mRNAs from cytoplasmic extracts of IL-1-treated cells were separated according to their ribosome occupancy by density gradient centrifugation (Fig. 1B), and the distribution profile of IL-6 mRNA was analyzed by Northern blotting (Fig. 1, C and D)
Summary
Plasmids—The complete cDNA of human IL-6 was cloned into the BamHI site of expression vector pUHD10-3 (kindly provided by Hermann Bujard) to obtain pUHD10/IL6-IL6-IL6. PUHD10/B-IL6-IL6 was cloned by inserting a PCR-amplified fragment spanning the coding region and the 3Ј-UTR of IL-6 into the BamHI site of pUHD10-3. Plasmids with different regions of the IL-6 3Ј-UTR were generated with overlap extension PCR cloning as described [28] (see primer sequences in the supplemental data). The IL-1␣ mRNA fragment spanning its 5Ј-UTR and coding region was amplified by RT-PCR and cloned into the BamHI site of pUHD10-3. Purification of the labeled RNA probes, incubation with cytoplasmic extracts or purified Strep-tagged proteins, and separation of the samples on non-denaturing gels were performed as described [15]. Pulldown Assay—Cells expressing Strep-tagged KSRP or GFP were lysed in buffer (20 mM -glycerophosphate (pH 7.4), 150 mM KCl, 5 mM MgCl2, 0.25 mM DTT, 1 mM sodium orthovanadate, 0.5% (v/v) Nonidet P-40, protease inhibitor mixture (Roche Applied Science)), and the cytoplasmic extract was adsorbed to Strep-Tactin agarose beads (IBA) essentially as described [15]. Co-purified mRNAs eluted from the beads and mRNAs in the cytoplasmic extracts before adsorption (input, 10% of the sample) were quantified by RT-qPCR
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