The NF-kB transcription factor plays a central role in the regulation of genes involved in immunity, inflammation, development and cell survival. NF-kB activation by proinflammatory cytokines such as tumor necrosis factor (TNF) or interleukin-1 (IL-1), and bacterial products such as lipopolysaccharide (LPS) is normally transient due to the upregulation of multiple negative feedback inhibitors, including IkBa and A20 (also known as TNFAIP3). A20 is a ubiquitin-editing enzyme that plays an essential role in the termination of NF-kB responses triggered by TNF and LPS by targeting key signaling molecules such as TRAF6 and RIP1 for inactivation. Persistent NF-kB activation may lead to pathologic consequences such as chronic inflammation, autoimmunity or malignancy. The focus of this proposal is centered on a cellular protein TAX1BP1 (also known as T6BP or TXBP151), a known interacting protein of A20, TRAF6 and the HTLV-I Tax oncoprotein. To elucidate the physiological function of TAX1BP1, we have generated knockout mice by a gene trapping strategy. Although homozygous null mice are not viable, murine embryonic fibroblasts (MEFs) from these mice exhibit elevated and persistent NF-kB as well as c-jun N-terminal kinase (JNK) activation in response to TNF, IL-1 and LPS stimulation. TAX1BP1 functions as a negative regulator of RIP1 and TRAF6 ubiquitination and is required for the function and specificity of A20 by acting as an adaptor molecule. Furthermore, we have found that TAX1BP1 recruits the HECT E3 ligase Itch to A20 via two PPXY motifs. Itch-/- MEFs similarly exhibit a defect in the termination of NF-kB and JNK signaling in response to stimulation with proinflammatory cytokines. Thus, both TAX1BP1 and Itch are essential subunits of an A20 ubiquitin- editing protein complex that restricts inflammatory signaling pathways. Finally, we have demonstrated that TAX1BP1 undergoes IKKa-dependent phosphorylation in response to proinflammatory signals. IKKa, but not IKKb or NEMO, is required for TNF-induced interactions among TAX1BP1, Itch, A20 and RIP1. Therefore, IKKa phosphorylation of TAX1BP1 is a key negative feedback mechanism that promotes the assembly and function of the A20 ubiquitin-editing complex. In this proposal, we will further define the mechanisms of TAX1BP1 negative regulatory function in cytokine-induced NF-kB and JNK activation.
The specific aims of this proposal are to determine (1) the molecular mechanisms by which TAX1BP1 inhibits persistent NF-kB activation;(2) the mechanism by which TAX1BP1 inhibits TRAF2 ubiquitination;and (3) the role of phosphorylation in TAX1BP1 function. Completion of the proposed studies will provide mechanistic insight into how NF-kB signaling is negatively regulated, and may lead to novel therapies for autoimmunity and chronic inflammation.
Persistent activation of the transcription factor NF-kB is associated with autoimmunity, chronic inflammation and malignancy. We have found that the cellular proteins TAX1BP1 and Itch are pivotal negative regulators of persistent NF-kB activation by functioning as subunits of an A20 ubiquitin editing enzyme complex. This proposal will provide mechanistic insight into how TAX1BP1, Itch and A20 function together to inactivate key signaling molecules and restrict inflammatory signaling pathways.
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