dTransforming growth factor ␤ (TGF-␤)-activated kinase 1 (TAK1) is a key regulator in the signals transduced by proinflammatory cytokines and Toll-like receptors (TLRs). The regulatory mechanism of TAK1 in response to various tissue types and stimuli remains incompletely understood. Here, we show that ribosomal S6 kinase 1 (S6K1) negatively regulates TLR-mediated signals by inhibiting TAK1 activity. S6K1 overexpression causes a marked reduction in NF-B and AP-1 activity induced by stimulation of TLR2 or TLR4. In contrast, S6K1؊/؊ and S6K1 knockdown cells display enhanced production of inflammatory cytokines. Moreover, S6K1؊/؊ mice exhibit decreased survival in response to challenge with lipopolysaccharide (LPS). We found that S6K1 inhibits TAK1 kinase activity by interfering with the interaction between TAK1 and TAB1, which is a key regulator protein for TAK1 catalytic function. Upon stimulation with TLR ligands, S6K1 deficiency causes a marked increase in TAK1 kinase activity that in turn induces a substantial enhancement of NF-B-dependent gene expression, indicating that S6K1 is negatively involved in the TLR signaling pathway by the inhibition of TAK1 activity. Our findings contribute to understanding the molecular pathogenesis of the impaired immune responses seen in type 2 diabetes, where S6K1 plays a key role both in driving insulin resistance and modulating TLR signaling.T ransforming growth factor ␤-activated kinase 1 (TAK1) is a member of the mitogen-activated protein kinase (MAPK) kinase kinase (MAP3K) family (1). TAK1 is essential for the production of tumor necrosis factor (TNF-␣) and other inflammatory mediators by activating several MAPKs, such as p38␣ MAPK, Jun N-terminal protein kinases 1 and 2 (JNK1 and JNK2), and extracellular signal-regulated kinases 1 and 2 (ERK1/2). TAK1 also plays a key regulatory role in several cytokine-mediated innate immunity signal transduction cascades, including interleukin-1 (IL-1) and the downstream signaling of Toll-like receptors (TLRs) and NOD1/2 (2, 3). In these pathways, various proinflammatory cytokines and TLR agonists trigger TAK1 activity, leading to its autophosphorylation and subsequent recruitment to the IB kinase (IKK) complex, ultimately resulting in the activation of the transcription factor NF-B and the upregulation of genes encoding proinflammatory cytokines, chemokines, adhesion molecules, and proteolytic enzymes.Several binding partners of TAK1, including TAK1-binding protein 1 (TAB1), TAB2, and TAB3, have been implicated in the regulation of TAK1 activity in response to various stimuli (1, 4). Previous reports demonstrated that the native forms of TAK1 comprise a catalytic kinase subunit in complex with the regulatory subunit TAB1 and either of two homologous proteins, TAB2 and TAB3 (2, 5, 6). Importantly, it has been reported that TAB1 might play a key role in the regulation of the TAK1 complex (7,8). Studies with TAB1-deficient mouse embryonic fibroblasts (TAB1 Ϫ/Ϫ MEFs) demonstrated that TAB1 is able to recruit p38␣ MAPK to the TAK1 complex f...

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