RIKEN IMS AnnualReport 2020

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-The inflammatory response is critical for immune cells to fight invading 36Inflammatory responsesNF-B p65NF-B p65PDLIM2PDZToll–like receptornucleusUbNF-B p65LIMLIMPDLIM7PDZUbactininInflammatory responsesdendritic cellssequestrationPDLIM1PDZPDZPDLIM7UbNF-B p65cytoplasmActin stressfibersPML nuclearbodiesp62PDLIM2proteasomedegradationLIMLIMUbp62microbial pathogens. On the other hand, excessive inflammation causes massive damage to the host, indicating that regulatory mechanisms to promptly terminate inflammatory responses are important to prevent immunopathology. Our research goal is to identify a series of key negative regulators of inflamma-tion and to clarify the complete picture of the molecular mechanisms for regu-lating inflammatory responses.We previously identified PDLIM2 (PDZ and LIM domain-containing pro-tein-2), a nuclear protein that belongs to a large family of LIM proteins, as a key factor negatively regulating inflammatory responses. PDLIM2 is a ubiquitin E3 ligase for the p65 subunit of NF-κB in dendritic cells that negatively regulates NF-κB-mediated inflammation. We have recently shown that PDLIM7, another member of the LIM protein family, is also a ubiquitin E3 ligase that inhibits NF-κB-mediated inflammatory responses. PDLIM7 polyubiquitinates p65 and promotes its proteasomal degradation. Moreover, PDLIM7 heterodimerizes with PDLIM2 and promotes synergistic PDLIM2-mediated degradation of p65. Mechanistically, PDLIM7 induces K63-linked ubiquitination of PDLIM2 and then the proteasome/autophagosome cargo protein p62/Sqstm1 binds to both polyubiquitinated PDLIM2 and the proteasome, thereby facilitating the delivery of the NF-κB-PDLIM2 complex to the proteasome and subsequent p65 deg-radation. Consistently, double knockdown of PDLIM7 and either PDLIM2 or p62/Sqstm1 results in augmented proinflammatory cytokine production com-pared to control cells or single knockdown cells. These data delineate a new role for PDLIM7 and p62/Sqstm1 in the regulation of NF-κB signaling by bridging a ubiquitin E3 ligase and the proteasome.We have also reported that PDLIM1, another LIM protein, binds to and se-questers p65 in the cytoplasm, thereby inhibiting NF-κB activation in dendritic cells, whereas PDLIM4 suppresses STAT3 signaling by recruiting PTPBL, a protein tyrosine phosphatase, and facilitating dephosphorylation of STAT3 in CD4+T cells. We propose that LIM proteins constitute a new family negatively regulating inflammatory responses through different mechanisms.Figure: LIM proteins constitute a new family of negative regulators of NF-κB signalingPDLIM2 and PDLIM7, ubiquitin E3 ligases for the p65 subunit of NF-κB, form heterodimers and cooperatively promote the delivery of p65 to the proteasome via p62/Sqstm1 for subsequent p65 degradation. By contrast, PDLIM1 sequestrates p65 in the cytoplasm and inhibits its nuclear translocation, thereby suppressing NF-κB sig-naling. Thus, the LIM protein family negatively regulates inflammatory responses, but individual family members do so by different mechanisms.Recent Major PublicationsSugimoto-I A, Harada M, Tanaka M, Terooatea T, Adachi Y, Takahashi Y, Tanaka T, Burrows PD, Hikidda M, Takemori T. Bim establishes the B cell repertories from early to late in the immune response. Int Immunol 33, 79-90 (2020)Jodo A, Shibazaki A, Onuma A, Kaishi T, Tanaka T. PDLIM7 synergizes with PDLIM2 and p62/Sqstm1 to inhibit in-flammatory signaling by promoting degradation of the p65 subunit of NF-κB. Front Immunol 11, 1559 (2020)Miyazaki R, Saiga H, Kato T, Bakoshi T, Senba R, Shintani A, Suzuki M, Takao K, Sasaki I, Iizuka A, Sugiyama M, Iwami N, Fukuda-O Y, Hemmi H, Tanaka T, Miyake M, Kaisho T, Hoshino K. The mechanism of action of Spi-B in the transcriptional activation of the interferon-α4 gene. Biochem Biophys Res Commu 525, 477-482 (2020)Laboratory for Inflammatory RegulationTeam Leader: Takashi Tanaka