Novel functions of Tribbles 1 in macrophages
Tribbles (Trib) protein was first described in Drosophila as a regulator of proliferation, later being implicated as a G2/M modulator. In mammalian systems, three Trib gene family members have been identified, which share a conserved motif similar to the catalytic domain of serine/threonine kinases. However, they lack several conserved residues in the ATP-binding pocket and the core motif of the catalytic domain necessary for catalytic function. Tribbles 1 (Trib1) is involved in inflammation through its ability to regulate MAPK, NF-κB and the CCAAT Enhancer Binding Protein (C/EBP). Moreover, Trib1 is associated with human disease, such as atherosclerosis and acute myeloid leukaemia. In this thesis, I investigated the functional role of Trib1 in Toll-like Receptor (TLR)-induced inflammatory responses together with pro- or anti-inflammatory cytokines. The RAW264.7 myeloid cell line was stimulated with TLR2/9 ligands in the presence or absence of IFN-γ or IL-10. I observed a high level of Trib1 expression in the presence of IFN-γ and TLR2 ligands, but weak Trib1 expression following treatment with IL-10 and TLR9 ligands. In gene knock-down experiments using small interfering RNAs (siRNA) to reduce Trib1 expression, C/EBPβ was up-regulated in both stimulated (by IFN-γ and TLR2 ligands) and resting macrophage populations. TNF-α production was increased following Trib1 knockdown after treatment with IFN-γ and/or TLR2 ligands but IL-6 secretion remained unchanged. Furthermore, ERK1/2 expression was reduced in Trib1 siRNA-treated cells and failed to induce chemokinesis in macrophages. Finally, Trib1 was demonstrated to act as a modulator of cell cycle (G2/M) transition and displays a delayed apoptotic phenotype. The work in this thesis demonstrates that mammalian Trib1 contributes to the pro-inflammatory response and functions as a regulator of the ERK1/2 and C/EBPβ pathways following TLR ligand-mediated activation. Its novel functions include acting as a modulator of G2/M arrest and suppressing macrophage migration.