Roles Of NLK In Inflammation And Cellular Antiviral Response And Cancer Development

1. Roles of NLK in TNFa-induced NF-κB signalingStringent negative regulation of the transcription factor NF-κB is essential for maintaining cellular stress responses and homeostasis. However, the tight regulation mechanisms of IKKβ are still not clear. Here, we reported that the nemo-like kinase (NLK) is a suppressor of tumor necrosis factor (TNFα)-induced NF-κB signaling by inhibiting the phosphorylation of IKKβ. Overexpression of NLK largely blocked TNFa-induced NF-κB activation, p65nuclear localization and IκBαdegradation; whereas genetic inactivation of NLK showed opposing results. Mechanistically, we identified that NLK interacted with IκB kinase (IKK)-associated complex, which in turn inhibited the assembly of the TAK1/IKKβ and thereby, diminished the IκB kinase phosphorylation. Our results indicate that NLK functions as a pivotal negative regulator in TNFa-induced activation of NF-κB via disrupting the interaction of TAK1with IKKβ.2. Function of NLK in cellular antiviral responseDetection of viral acid by the RIG-like helicase (RLH) family of intracellular receptors activates the mitochondrial antiviral signaling adaptor MAVS and TRAF3, which leads to the rapid production of type I IFN and initiates innate antiviral immune response. Although TRAF3is vital to antiviral immunity, the molecular mechanisms responsible for its tight regulation are still poorly understood. Here, we show that the Nemo-like Kinase (NLK) inhibits IFN-β induction and antiviral immune response during a viral infection by regulating ubiquitination of TRAF3. Expression of NLK results in the potent inhibition of virus-induced IFN-β promoter activity and antiviral cytokines production. Depletion of NLK using AAV-mediated homologous recombination promotes virus-induced antiviral cytokines production and decreases the viral replication, which potent rescue by reintroduction NLK. Moreover, NLK interacts with TRAF3and phosphorylates it, which in turn leads to inhibition of ubiquitination of TRAF3and subsequent activation of TBK1and IRF3. We further found that mimics phosphorylation of TRAF3has the similar effects for inhibition of virus-induced IFN-β production. These findings identify an unrecognized role for NLK in the regulation of type I interferon signaling and provide direct insight into the molecular mechanisms which NLK regulates ubiquitination of TRAF3for influence on its activation.3. Roles of NLK in the colorectal carcinogenesisThe cell cycle checkpoints are essential for cell proliferation, cell cycle and tumorigenesis in all eukaryotes. Herein, we report that NLK as a pivotal oncongene involves in the tumorigenesis. Completely depletion of NLK using knock-out rather than down-regulation hindered tumorigenesis both in vitro and in vivo, as assessed via colony formation, growth curve, soft agar and xenograft assays. In the NLK-deficient cells, we find the observed cell cycle changes, which have increased G1phase and decreased G2/M phase. In the NLK-deficient cells, the protein level of cyclinD3is potently decreased. We also have observed the reduced phosphorylation of Rb. NLK also associated with HDAC1, which form a complex with Rb to control G1/S cell cycle checkpoint. These results suggest that NLK control the G1/S cell cycle checkpoint by impacting protein level of cyclinD3and Rb complex. Collectively, these results suggest that NLK is a promising drug target for the treatment of colorectal cancer.