The membrane association of the tumour suppressor phosphatase and tensin homologue (PTEN) is required to oppose the phosphatidylinositol-3-kinase / AKT pathway by dephosphorylation of phosphatidylinositol-3,4,5-triphosphate (PIP3). How cytosolic PTEN interacts with its main substrate, PIP3, localized at the inner face of plasma membrane remains unclear. Here we show that PTEN is covalently modifi ed by SUMO1 at both K 266 and K 254 sites in the C2 domain of PTEN. SUMO1 modifi cation at K 266 located in the CBR3 loop, which has a central role in PTEN membrane association, mainly facilitates cooperative binding of PTEN to the plasma membrane by electrostatic interactions. This results in the downregulation of the phosphatidylinositol-3 kinase / AKT pathway and consequently, suppression of anchorageindependent cell proliferation and tumour growth in vivo . Our data demonstrate a molecular mechanism whereby SUMO1 modifi cation is required for PTEN tumour suppressor function by controlling PTEN membrane association and regulation of the phosphatidylinositol-3 kinase / AKT pathway.

RIG-I is a pivotal cytoplasmic sensor that recognizes different species of viral RNAs. This recognition leads to activation of the transcription factors NF-κB and IRF3, which collaborate to induce type I interferons (IFNs) and innate antiviral response. In this study, we identified the TRIM family protein TRIM4 as a positive regulator of RIG-I-mediated IFN induction. Overexpression of TRIM4 potentiated virus-triggered activation of IRF3 and NF-κB, as well as IFN-β induction, whereas knockdown of TRIM4 had opposite effects. Mechanistically, TRIM4 associates with RIG-I and targets it for K63-linked polyubiquitination. Our findings demonstrate that TRIM4 is an important regulator of the virus-induced IFN induction pathways by mediating RIG-I for K63-linked ubiquitination.

The tripartite motif (TRIM)-containing proteins are a family of proteins that have been known to be involved in divergent biological processes, including important roles in immune responses through regulating various signaling pathways. In this study, we identified a member of the TRIM family, TRIM8, as a positive regulator of tumor necrosis factor-α (TNFα) and interleukin-1β (IL-1β)-triggered NF-κB activation. Overexpression of TRIM8 activated NF-κB and potentiated TNFα-and IL-1β-induced activation of NF-κB, whereas knockdown of TRIM8 had opposite effects. Coimmunoprecipitations indicated that TRIM8 interacted with TGFβ activated kinase 1 (TAK1), a serine/threonine kinase essential for TNFα-and IL-β-induced NF-κB activation. Furthermore, we found that TRIM8 mediated K63-linked polyubiquitination of TAK1 triggered by TNFα and IL-1β. Our findings demonstrate that TRIM8 serves as a critical regulator of TNFα-and IL-1β-induced NF-κB activation by mediating K63-linked polyubiquitination of TAK1.T he transcription factor NF-κB plays a pivotal role in many cellular events such as cell proliferation, inhibition of apoptosis, and innate immunity. NF-κB activation requires the signal-induced phosphorylation and degradation of IκB proteins (1). The kinase that phosphorylates IκB, termed IκB kinase (IKK) complex, consists of the catalytic subunits IKKα, IKKβ, and the regulatory subunit NEMO/IKKγ. The IKK complex is activated by a large variety of stimuli, including the proinflammatory cytokines tumor necrosis factor α (TNFα) and interleukin-1β (IL-1β) (2).Binding of TNFα to its receptor TNF-RI results in recruitment of the adaptor protein TRADD. TRADD further recruits TRAF2, TRAF5, and RIP1 to the receptor complex. TRAF2 mediates polyubiquitination of RIP1, mainly in a K63-linked manner. Ubiquitinated RIP further recruits TAB2 and TAK1 and mediates activation of the latter. The activated RIP1-TAK1-TAB2 complex subsequently activates the IKK complex, leading to phosphorylation of IκBα and activation of NF-κB (3). Similarly, binding of IL-1β to its receptor IL-1R leads to the recruitment of adaptor proteins and kinases, including MyD88, IRAK1, and IRAK4. IRAK4 phospholylates and activates IRAK1, which in turn recruits TRAF6. IRAK1 and TRAF6 form a complex that is released from the receptor. TRAF6 possesses an E3 ubiquitin ligase activity that mediates its K63-linked autopolyubiquitination. Ubiquitinated TRAF6 further recruits the TAK1-TAB2-TAB3 complex, resulting in the activation of TAK1. Activated TAK1 eventually phosphorylates and activates IKK, leading to activation of NF-κB (3, 4).TAK1 is a member of the MAPK kinase kinase family that was originally found to function in the transforming growth factor β (TGFβ)-mediated MAPK activation (5). TAK1 has been demonstrated to be essential in TNFα-and IL-1β-mediated activation of NF-κB and JNK (6, 7). In all of these pathways, activation of TAK1 is an important event that transmits the upstream signal from the receptor complex to the downstream signaling molecules (3, 4, 6, 7).S...

No abstract

Signaling pathways responsible for virus-triggered IFN induction have been extensively investigated during the past decade. In limited types of cells, viral RNA is detected by certain membrane-bound Toll-like receptors (TLRs). However, in most cell types, viral RNA is recognized by cytoplasmic pattern recognition receptors (PRRs) RIG-I and MDA5. Both RIG-I and MDA5 contain two CARD modules at their N terminus and a DexD/H-box RNA helicase domain at their C terminus. Upon viral infection, the RNA helicase domains of RIG-I and MDA5 serve as intracellular viral RNA receptors. The recognition of viral RNA by RIG-I and MDA5 leads to their recruitment to the downstream mitochondrion-located CARD-containing adapter protein VISA (also known as MAVS, IPS-1, and Cardif) (6 -10). VISA is constitutively associated with another mitochondrion-associated adapter protein MITA/STING (11,12).Various studies have demonstrated that VISA plays a central role in assembling a complex that activates distinct signaling pathways leading to NF-B and IRF3 activation, respectively. VISA is associated with TRAF2 and TRAF6 through its TRAFinteraction motifs. It has been shown that TRAF2 and TRAF6 facilitate Lys-63-linked polyubiquitination of RIP and NEMO/ IKK␥, respectively, and these processes cause activation of IKK and subsequent NF-B (13-15). VISA is also associated with TRAF3, another member of the TRAF protein family (16). Gene knock-out studies have demonstrated that TRAF3 is essential in virus-triggered IRF3 activation and type I IFN induction (17, 18). However, how TRAF3 is regulated in virus-triggered signaling pathways is still enigmatic.Several studies have suggested that ubiquitination is a central rhythm of regulation of the virus-triggered IFN induction pathways. It has been shown that the E3 ubiquitin ligase TRIM25 catalyzes Lys-63-linked ubiquitination of RIG-I, and this ubiquitination is essential for the interaction of RIG-I with VISA as well as for its ability to signal (19). The Riplet/REUL E3 ubiquitin ligase also targets RIG-I for ubiquitination, which positively regulates RIG-I-mediated signaling (20,21). In contrast, the E3 ubiquitin ligase RNF125 catalyzes Lys-48-linked ubiquitination of RIG-I and negatively regulates RIG-I-mediated signaling (22). MITA and IRF3 are ubiquitinated by RNF5 and RBCK1, respectively, and subsequently degraded by proteasome-dependent processes (23,24). The E3 ubiquitin ligase Nrdp1 catalyzes the ubiquitination of TBK1, leading to its activation (25). cIAP1 and cIAP2 are E3 ubiquitin ligases that were firstly identified as signaling components associated with TRAF1 and TRAF2 and recruited to the TNF receptors TNFR1 and TNFR2 upon ligand stimulation (26,27). Recently, it has been demonstrated that in unstimulated cells, TRAF2/3 and cIAP1/2 form a cytoplasmic complex, which constitutively ubiquitinates NIK and promotes its proteasome-dependent degradation (28,29). Upon stimulation by TNF family members, such as BAFF and CD40L, the TRAF2/3-cIAP1/2 complex is recruited to the receptors, where ...