Transforming growth factor-β1 (TGF-β1) is a potent cytokine with pleiotropic effects, including anti-inflammatory activity. Here we show that the signaling protein Smad6 bound to Pellino-1, an adaptor protein of mammalian interleukin 1 receptor (IL-1R)–associated kinase 1 (IRAK1), and thereby promoted TGF-β-mediated anti-inflammatory effects. Smad6–Pellino-1 interaction abrogated signaling mediated by a complex of IRAK1, Pellino-1 and adaptor protein TRAF6 that formed after stimulation by IL-1β treatment. Blockade of IRAK1–Pellino-1–TRAF6 signaling prevented degradation of the inhibitor IκBα and subsequent nuclear translocation of transcription factor NF-κB and thus expression of proinflammatory genes. 'Knockdown' of endogenous Smad6 expression by RNA interference reduced anti-inflammatory activity mediated by TGF-β1 or the TGF-β family member BMP-4. Thus Smad6 is a critical mediator of the TGF-β–BMP pathway that mediates anti-inflammatory activity and negatively regulates IL-1R–Toll-like receptor signals.
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We thank G. Merlino and A. Hobbie for critical reading of the manuscript. Supported by Korea Research Foundation Grant funded by the Korean Government (KRF-2003-015-C00528 to S.H.P.; laboratory equipment, R08-2003-000-10077-0 to S.H.P.), the intramural research fund of the National Cancer Center of Korea (C.-H.L. and I.-H.K) and the Intramural Research Program of the National Cancer Institute.
The authors declare no competing financial interests.
Smad6 inhibits the expression of the NF-κB-promoter reporter induced by LPS in RAW264.7 cells. (PDF 687 kb)
Schematic representation of Smad6 'knockdown' plasmid (pU6-siSmad6) by siRNA. (PDF 757 kb)
Inhibition of endogenous Pellino-1 by siRNA abrogates NF-κB activation induced by IL-1β treatment. (PDF 711 kb)
Stable expression of Peli1 siRNA decreases NF-κB activation upon treatment of IL-1β. (PDF 186 kb)
LPS induced IRAK1-mediated signaling complexes are disrupted by pre-treatment with TGF-β1. (PDF 201 kb)
IL-1β-induced endogenous TRAF6-mediated signaling complexes are disrupted by pre-treatment with TGF-β1. (PDF 199 kb)
Models for the negative regulation of IL-1R–TLR signaling by Smad6. (PDF 201 kb)
Primer sequences. (PDF 33 kb)
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Choi, K., Lee, Y., Lim, S. et al. Smad6 negatively regulates interleukin 1-receptor–Toll-like receptor signaling through direct interaction with the adaptor Pellino-1. Nat Immunol 7, 1057–1065 (2006). https://doi.org/10.1038/ni1383
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