Integrating signals from T-cell receptor and serum by T cells enhance translation of tumour necrosis factor-alpha

Mnk kinase pathway: Cellular functions and biological outcomes

The mitogen-activated protein kinase (MAPK) interacting protein kinases 1 and 2 (Mnk1 and Mnk2) play important roles in controlling signals involved in mRNA translation. In addition to the MAPKs (p38 or Erk), multiple studies suggest that the Mnk kinases can be regulated by other known kinases such as Pak2 and/or other unidentified kinases by phosphorylation of residues distinct from the sites phosphorylated by the MAPKs. Several studies have established multiple Mnk protein targets, including PSF, heterogenous nuclear ribonucleoprotein A1, Sprouty 2 and have lead to the identification of distinct biological functions and substrate specificity for the Mnk kinases. In this review we discuss the pathways regulating the Mnk kinases, their known substrates as well as the functional consequences of engagement of pathways controlled by Mnk kinases. These kinases play an important role in mRNA translation via their regulation of eukaryotic initiation factor 4E (eIF4E) and their functions have important implications in tumor biology as well as the regulation of drug resistance to anti-oncogenic therapies. Other studies have identified a role for the Mnk kinases in cap-independent mRNA translation, suggesting that the Mnk kinases can exert important functional effects independently of the phosphorylation of eIF4E. The role of Mnk kinases in inflammation and inflammation-induced malignancies is also discussed.

Posttranscriptional gene expression regulation in CpG-activated macrophages depends on FXR1P RNA-binding protein

An RNA-binding protein (RBP) was recently identified, FXR1P, which regulates tumour necrosis factor (TNF) gene expression at the posttranscriptional level in response to lipopolysaccharide, was recently identified resulting in higher TNF production in macrophages from FXR1 knockout (KO) mice compared with wild-type (WT) macrophages. In this study, the importance of FXR1P in the induction of TNF by toll-like receptor 7 (TLR7) ligand S28463 and TLR9 ligand CpG is evaluated. The results clearly reveal a much higher level of TNF protein expression in FXR1-KO than in WT macrophages following stimulation with CpG but not with S28463. To better understand the molecular mechanism, both the steady-state levels and the stability of TNF mRNA were assessed. It was found that the TNF mRNA steady-state level was more elevated in CpG-stimulated FXR1-KO macrophages, while the stability of TNF mRNA was not affected in CpG-stimulated FXR1-KO macrophages. It was also established that FXR1P is involved in regulating the expression of several other inflammatory cytokines and chemokines. Together, the data clearly demonstrate the importance of FXR1P RBP in the regulation of a wide spectrum of inflammatory genes and suggest an important role of MAP signalling in the response of macrophages to selected TLR ligands, including CpG.

The Mnks are novel components in the control of TNF alpha biosynthesis and phosphorylate and regulate hnRNP A1

Posttranscriptional regulatory mechanisms control TNFalpha expression through AU-rich elements in the 3'UTR of its mRNA. This is mediated through Erk and p38 MAP kinase signaling, although the mechanisms involved remain poorly understood. Here, we show that the MAP kinase signal-integrating kinases (Mnks), which are activated by both these pathways, regulate TNFalpha expression in T cells via the 3'UTR. A selective Mnk inhibitor or siRNA-mediated knockdown of Mnk1 inhibits TNFalpha production in T cells, whereas Mnk1 overexpression enhances expression of a reporter construct containing the TNFalpha 3'UTR. We identify ARE binding proteins that are Mnk substrates, such as hnRNP A1, which they phosphorylate at two sites in vitro. hnRNP A1 is phosphorylated in response to T cell activation, and this is blocked by Mnk inhibition. Moreover, Mnk-mediated phosphorylation decreases binding of hnRNP A1 to TNFalpha-ARE in vitro or TNFalpha-mRNA in vivo. Therefore, Mnks are novel players in cytokine regulation and potential new targets for anti-inflammatory therapy.

The role of the AU-rich elements of mRNAs in controlling translation

Adenosine- and uridine-rich elements (AREs) located in 3'-untranslated regions are the best-known determinants of RNA instability. These elements have also been shown to control translation in certain mRNAs, including mRNAs for prominent pro-inflammatory and tumor growth-related proteins, and physiological anti-inflammatory processes that target ARE-controlled translation of mRNAs coding for pro-inflammatory proteins have been described. A major research effort is now being made to understand the mechanisms by which the translation of these mRNAs is controlled and the signalling pathways involved. This review focuses on the role of ARE-containing gene translation in inflammation, and the disease models that have improved our understanding of ARE-mediated translational control.

Activated T cells mediate direct blood-brain barrier endothelial cell death and dysfunction

Neuro-inflammation is characterized by immune cell infiltration across the blood-brain barrier, a process instrumental in neuronal cell death. In neuro-inflammation the blood-brain barrier is also damaged and the consequences of activated lymphocytes on the integrity of the blood-brain barrier is not well characterized. Utilizing an blood-brain barrier model we demonstrate that endothelial cell viability and barrier integrity are directly altered following lymphocyte exposure. The effect of activated lymphocytes is cell number dependent, mostly mediated by direct contact, and is not associated with the pro-inflammatory cytokine TNF-alpha. For the successful treatment of neuro-inflammatory disease, intervention of this direct effect at the blood-brain barrier is warranted.