Translation control of TAK1 mRNA by hnRNP K modulates LPS-induced macrophage activation

Macrophage activation by bacterial lipopolysaccharides (LPS) is induced through Toll-like receptor 4 (TLR4). The synthesis and activity of TLR4 downstream signalling molecules modulates the expression of pro- and anti-inflammatory cytokines. To address the impact of post-transcriptional regulation on that process, we performed RIP-Chip analysis. Differential association of mRNAs with heterogeneous ribonucleoprotein K (hnRNP K), an mRNA-specific translational regulator in differentiating haematopoietic cells, was studied in non-induced and LPS-activated macrophages. Analysis of interactions affected by LPS revealed an enrichment of mRNAs encoding TLR4 downstream kinases and their modulators. We focused on transforming growth factor activated kinase-1 (TAK1), a central player in TLR4 signalling. HnRNP K interacts specifically with a sequence in the TAK1 mRNA 3' UTR in vitro. Silencing of hnRNP K does not affect TAK1 mRNA synthesis and stability, but enhances TAK1 mRNA translation, resulting in elevated TNF-alpha, IL-1beta and IL-10 mRNA expression. Our data suggest that the hnRNP K-3' UTR complex inhibits TAK1 mRNA translation in non-induced macrophages. LPS-dependent TLR4 activation abrogates translational repression and newly synthesised TAK1 initiates the inflammatory response of macrophages.

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Liepelt A, Mossanen JC, Denecke B, Heymann F, De Santis R, Tacke F, Marx G, Ostareck DH, Ostareck-Lederer A