Interaction between PKR and PACT mediated by LPS-inducible NF-κB in human gingival cells

The double‐stranded RNA‐dependent protein kinase (PKR) is a serine/threonine kinase expressed constitutively in mammalian cells. PKR is activated upon virus infection by double‐stranded RNA (dsRNA), and plays a critical role in host antiviral defense mechanisms. PKR is also known to regulate various biological responses, including cell differentiation and apoptosis. However, whether PKR is involved in the progress of periodontitis is not clear. The present study explained the phosphorylation of PKR by LPS in the human gingival cell line, Sa3. Expression of genes encoding LPS receptors was detected in Sa3 cells and treatment of cells with 1 µg/mL LPS for 6 h caused PKR phosphorylation. LPS elevated the expression of the protein activator of PKR (PACT) mRNA and protein, followed by the enhanced association between PACT and PKR within 3 h. In addition, LPS treatment induced the translocation of NF‐κB to the nucleus after 30 min, and inhibition of NF‐κB decreased the PACT–PKR interaction induced by LPS. The level of pro‐inflammatory cytokine mRNA, including interleukin‐6 (IL‐6) and tumor necrosis factor alpha (TNFα), appeared within 45 min and reached at the maximal levels by 90 min after the addition of LPS. This induction of pro‐inflammatory cytokines was not affected by RNAi‐mediated silencing of PKR and a pharmacological inhibitor of PKR, whereas the inhibition of NF‐κB decreased it. These results indicated that LPS induces PKR phosphorylation and the PACT–PKR association in Sa3 cells. Our results also suggest that NF‐κB is involved in the PACT–PKR interaction and the production of pro‐inflammatory cytokines in periodontitis. J. Cell. Biochem. 113: 165–173, 2012. © 2011 Wiley Periodicals, Inc.     

Functional domains and the antiviral effect of the double-stranded RNA-dependent protein kinase PKR from Paralichthys olivaceus

The double-stranded RNA (dsRNA)-dependent protein kinase PKR is thought to mediate a conserved antiviral pathway by inhibiting viral protein synthesis via the phosphorylation of the alpha subunit of eukaryotic initiation factor 2 (eIF2alpha). However, little is known about the data related to the lower vertebrates, including fish. Recently, the identification of PKR-like, or PKZ, has addressed the question of whether there is an orthologous PKR in fish. Here, we identify the first fish PKR gene from the Japanese flounder Paralichthys olivaceus (PoPKR). PoPKR encodes a protein that shows a conserved structure that is characteristic of mammalian PKRs, having both the N-terminal region for dsRNA binding and the C-terminal region for the inhibition of protein translation. The catalytic activity of PoPKR is further evidence that it is required for protein translation inhibition in vitro. PoPKR is constitutively transcribed at low levels and is highly induced after virus infection. Strikingly, PoPKR overexpression increases eIF2alpha phosphorylation and inhibits the replication of Scophthalmus maximus rhabdovirus (SMRV) in flounder embryonic cells, whereas phosphorylation and antiviral effects are impaired in transfected cells expressing the catalytically inactive PKR-K421R variant, indicating that PoPKR inhibits virus replication by phosphorylating substrate eIF2alpha. The interaction between PoPKR and eIF2alpha is demonstrated by coimmunoprecipitation assays, and the transfection of PoPKR-specific short interfering RNA further reveals that the enhanced eIF2alpha phosphorylation is catalyzed by PoPKR during SMRV infection. The current data provide significant evidence for the existence of a PKR-mediated antiviral pathway in fish and reveal considerable conservation in the functional domains and the antiviral effect of PKR proteins between fish and mammals.     

Mechanism of macrophage activation induced by β-glucan produced from Paenibacillus polymyxa JB115

β-Glucans are heterogeneous groups of glucose polymers found in the cell walls of fungi, plants and some bacteria. Our previous report showed that a novel β-1,3/1,6-glucan produced from Paenibacillus (P.) polymyxa JB115 can induce nitric oxide (NO) production in RAW264.7 cells. In the present study, the β-glucan significantly increased luciferase activity in cells transfected with NFκB or AP1, but not STAT1, reporter vector DNA, which contain their binding promoter site. All specific NFκB and MAPKs pathway inhibitors (pyrrolidine dithiocarbamate, AG490, U0126, SB203580 and SP600125) remarkably attenuated NO production induced by the β-glucan. Furthermore, Western blot analysis revealed that the stimulation of Raw264.7 cells by β-glucan induced phosphorylation of IκB and the consequent translocation of NFκB into the nucleus. Meanwhile, phosphorylation of ERK1/2, JNK/SAPK and p38 MAPKs in cytoplasm were also confirmed. All these results indicated that β-glucan from P. polymyxa JB115 activates macrophages through MAPKs and NFκB signaling pathway.     

NFκB regulates expression of Polo-like kinase 4

Activation of the NFκB signaling pathway allows the cell to respond to infection and stress and can affect many cellular processes. As a consequence, NFκB activity must be integrated with a wide variety of parallel signaling pathways. One mechanism through which NFκB can exert widespread effects is through controlling the expression of key regulatory kinases. Here we report that NFκB regulates the expression of genes required for centrosome duplication, and that Polo-like kinase 4 (PLK4) is a direct NFκB target gene. RNA interference, chromatin immunoprecipitation, and analysis of the PLK4 promoter in a luciferase reporter assay revealed that all NFκB subunits participate in its regulation. Moreover, we demonstrate that NFκB regulation of PLK4 expression is seen in multiple cell types. Significantly long-term deletion of the NFκB2 (p100/p52) subunit leads to defects in centrosome structure. This data reveals a new component of cell cycle regulation by NFκB and suggests a mechanism through which deregulated NFκB activity in cancer can lead to increased genomic instability and uncontrolled proliferation.     

Heterologous dimerization domains functionally substitute for the double-stranded RNA binding domains of the kinase PKR.

The protein kinase PKR (dsRNA-dependent protein kinase) phosphorylates the eukaryotic translation initiation factor eIF2alpha to downregulate protein synthesis in virus-infected cells. Two double-stranded RNA binding domains (dsRBDs) in the N-terminal half of PKR are thought to bind the activator double-stranded RNA, mediate dimerization of the protein and target PKR to the ribosome. To investigate further the importance of dimerization for PKR activity, fusion proteins were generated linking the PKR kinase domain to heterologous dimerization domains. Whereas the isolated PKR kinase domain (KD) was non-functional in vivo, expression of a glutathione S-transferase-KD fusion, or co-expression of KD fusions containing the heterodimerization domains of the Xlim-1 and Ldb1 proteins, restored PKR activity in yeast cells. Finally, coumermycin-mediated dimerization of a GyrB-KD fusion protein increased eIF2alpha phosphorylation and inhibited reporter gene translation in mammalian cells. These results demonstrate the critical importance of dimerization for PKR activity in vivo, and suggest that a primary function of double-stranded RNA binding to the dsRBDs of native PKR is to promote dimerization and activation of the kinase domain.     

The TGFβ1 pathway is required for NFκB dependent gene expression in mouse keratinocytes

The transforming growth factor-beta 1 (TGFβ1) and NFκB pathways are important regulators of epidermal homeostasis, inflammatory responses and carcinogenesis. Previous studies have shown extensive crosstalk between these pathways that is cell type and context dependent, but this has not been well-characterized in epidermal keratinocytes. Here we show that in primary mouse keratinocytes, TGFβ1 induces NFκB-luciferase reporter activity that is dependent on both NFκB and Smad3. TGFβ1-induced NFκB-luciferase activity was blocked by the IκB inhibitor parthenolide, the IκB super-repressor, a dominant negative TGFβ1-activated kinase 1 (TAK1) and genetic deletion of NFκB1. Coexpression of NFκB p50 or p65 subunits enhanced NFκB-luciferase activity. Similarly, inhibition of the TGFβ1 type I receptor with SB431542 or genetic deletion of Smad3 blocked TGFβ1 induction of NFκB-luciferase. TGFβ1 rapidly induced IKK phosphorylation but did not cause a detectable decrease in cytoplasmic IκB levels or nuclear translocation of NFκB subunits, although EMSA showed rapid NFκB nuclear binding activity that could be blocked by SB431542 treatment. TNFα, a well characterized NFκB target gene was also induced by TGFβ1 and this was blocked in NFκB+/− and −/− keratinocytes and by the IκB super-repressor. To test the effects of the TGFβ1 pathway on a biologically relevant activator of NFκB, we exposed mice and primary keratinocytes in culture to UVB irradiation. In primary keratinocytes UVB caused a detectable increase in levels of Smad2 phosphorylation that was dependent on ALK5, but no significant increase in SBE-dependent gene expression. Inhibition of TGFβ1 signaling in primary keratinocytes with SB431542 or genetic deletion of Tgfb1 or Smad3 suppressed UVB induction of TNFα message. Similarly, UVB induction of TNFα mRNA was blocked in skin of Tgfb1+/− mice. These studies demonstrate that intact TGFβ1 signaling is required for NFκB-dependent gene expression in mouse keratinocytes and skin and suggest that a convergence of these pathways in the nucleus rather than the cytoplasm may be critical for regulation of inflammatory pathways in skin by TGFβ1.     

Regulation of the RNA-dependent protein kinase by triple helix formation

The RNA-dependent protein kinase (PKR) is an interferon-induced, RNA-activated enzyme that phosphorylates the α-subunit of the translation initiation factor eIF-2, inhibiting its function. PKR is activated in vitro by binding to double-stranded RNA (dsRNA) molecules of ~30 bp or longer. Here we show that triple helix forming oligonucleotides (TFOs) inhibit dsRNA binding to the isolated RNA binding domain of PKR. The inhibition is specific to the targeted RNA and dependent on TFO length. Binding to a 30 bp duplex is inhibited by a 28 nt TFO and a 20 nt TFO with an IC₅₀ of 35 ± 2 and 210 ± 22 nM, respectively. An 18 nt TFO partially inhibits binding. The activation of the kinase domain of PKR by a 30 bp RNA duplex is also inhibited by a 28 nt TFO. Inhibition of binding is most effective when the triple helix is formed prior to addition of the protein. These results indicate that triplex formation can be used to prevent the binding of an RNA binding protein with dsRNA-binding motifs.