Protein Phosphatase 2A Catalytic Subunit α Plays a MyD88-Dependent, Central Role in the Gene-Specific Regulation of Endotoxin Tolerance

Ruyun Du,Yanbao Yu,Debra J Taxman,Cui Liu,Jing Yu,Penggao Dai,Zhen Yan,Leslie V Parise,Harsha P Gunawardena,Li Wang,Jenny P Ting,Ling Xie,Xian Chen,Stephen P Holly,Yisong Y Wan

doi:10.1016/j.celrep.2013.01.029

Ruyun Du, Yanbao Yu + Show 13 more

Open Access

https://doi.org/10.1016/j.celrep.2013.01.029

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Abstract

MyD88, the intracellular adaptor of most TLRs, mediates either proinflammatory or immunosuppressive signaling that contributes to chronic inflammation-associated diseases. Although gene-specific chromatin modifications regulate inflammation, the role of MyD88 signaling in establishing such epigenetic landscapes under different inflammatory states remains elusive. Using quantitative proteomics to enumerate the inflammation-phenotypic constituents of the MyD88 interactome, we found that in endotoxin-tolerant macrophages, protein phosphatase 2A catalytic subunit α (PP2Ac) enhances its association with MyD88 and is constitutively activated. Knockdown of PP2Ac prevents suppression of proinflammatory genes and resistance to apoptosis. Through site-specific dephosphorylation, constitutively active PP2Ac disrupts the signal-promoting TLR4-MyD88 complex and broadly suppresses the activities of multiple proinflammatory/proapoptotic pathways as well, shifting proinflammatory MyD88 signaling to a prosurvival mode. Constitutively active PP2Ac translocated with MyD88 into the nuclei of tolerant macrophages establishes the immunosuppressive pattern of chromatin modifications and represses chromatin remodeling to selectively silence proinflammatory genes, coordinating the MyD88-dependent inflammation control at both signaling and epigenetic levels under endotoxin-tolerant conditions.

Highlights

Toll-like receptors (TLRs) activate the innate immune system by mounting appropriate inflammatory responses to contain infection or repair damaged tissues(Drexler and Foxwell)
By using our AACT-based quantitative proteomic approach(Chen et al, 2000) with modifications for interactome screening (Figure 1A) we dissected the MyD88-interacting complexes assembled in RAW cells under different inflammatory states including (1) no stimulation (N), (2) challenging with a single high LPS dose (LPS-responsive, NL), (3) priming with a low LPS dose (LPS-tolerant, T), and (4) challenging T cells with a high-dose LPS (TL)
Through phenotypic interactome analysis (Figure 1A), we found that MyD88 interacts with different sets of proteins in NL vs TL macrophages: together with many negative immune regulators(Liew et al, 2005) including a negative TLR

Summary

Introduction

Toll-like receptors (TLRs) activate the innate immune system by mounting appropriate inflammatory responses to contain infection or repair damaged tissues(Drexler and Foxwell). By mechanisms yet to be elucidated, the intracellular adaptor protein MyD88 of most TLRs acts as a double-edged sword promoting both protective and harmful inflammation(Huang et al, 2008). Recent work has revealed that inflammation control is achieved by a gene-specific mechanism in which distinct chromatin modifications contribute to selective silencing of TLR4-induced pro-inflammatory or tolerizable (T-class) genes under an endotoxin tolerance (ET)-associated chronic inflammatory state (Foster et al, 2007). Emerging evidence suggests that an effective, unharmful inflammatory response is intrinsically regulated by subtly distinct intracellular protein interactions, i.e., ‘interactomes,’ involved in signal transduction(Liew et al, 2005). Based on our previous identification of multiple, novel signal regulators that interact with MyD88 in a timely and orderly manner to tightly regulate the amplitude and duration of TLR signaling(Dai et al, 2009), we reason that MyD88 may regulate either acute or chronic inflammation via assembling different, inflammationphenotypic interactomes

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