Functional mapping of Toll/interleukin-1 signalling networks by expression cloning

Competition between members of the tribbles pseudokinase protein family shapes their interactions with mitogen activated protein kinase pathways

Spatio-temporal regulation of intracellular signalling networks is key to normal cellular physiology; dysregulation of which leads to disease. The family of three mammalian tribbles proteins has emerged as an important controller of signalling via regulating the activity of mitogen activated protein kinases (MAPK), the PI3-kinase induced signalling network and E3 ubiquitin ligases. However, the importance of potential redundancy in the action of tribbles and how the differences in affinities for the various binding partners may influence signalling control is currently unclear. We report that tribbles proteins can bind to an overlapping set of MAPK-kinases (MAPKK) in live cells and dictate the localisation of the complexes. Binding studies in transfected cells reveal common regulatory mechanisms and suggest that tribbles and MAPKs may interact with MAPKKs in a competitive manner. Computational modelling of the impact of tribbles on MAPK activation suggests a high sensitivity of this system to changes in tribbles levels, highlighting that these proteins are ideally placed to control the dynamics and balance of activation of concurrent signalling pathways.

Macrophage migration is controlled by Tribbles 1 through the interaction between C/EBPβ and TNF-α

In mammals, three Tribbles gene family members have been identified, Tribbles 1, 2 and 3 (Trib1, Trib2 and Trib3). All family members are considered to be pseudokinases in that they contain domains homologous to serine/threonine kinase catalytic cores, but they lack several conserved residues in the ATP-binding pocket. Trib1 is implicated in the inflammatory response pathway through its ability to regulate mitogen-activated protein kinase (MAPK), nuclear factor kappa B (NF-κB) and CCAAT Enhancer Binding Protein (C/EBP). However, its role in macrophages function is unknown. Here, we investigated the functional role of Trib1 in Toll-like receptor-mediated inflammatory responses to IFN-γ in RAW264.7 cells. In gene knock-down experiments in macrophages using small interfering RNAs targeted to Trib1, it was observed that TNF-α production was increased following treatment with IFN-γ and/or TLR2 ligands. Finally, Trib1-silenced macrophages failed to show MCP-1 induced chemokinesis and indicating involvement of Trib1 in controlling of macrophage migration. This work demonstrates that Trib1 contributes to the pro-inflammatory response caused by TLR2 ligands and controls macrophage migration as well as being a biomarker in macrophage-related diseases in both human and veterinary medicine.

Identification of 34 novel proinflammatory proteins in a genome-wide macrophage functional screen

Signal transduction pathways activated by Toll-like Receptors and the IL-1 family of cytokines are fundamental to mounting an innate immune response and thus to clearing pathogens and promoting wound healing. Whilst mechanistic understanding of the regulation of innate signalling pathways has advanced considerably in recent years, there are still a number of critical controllers to be discovered. In order to characterise novel regulators of macrophage inflammation, we have carried out an extensive, cDNA-based forward genetic screen and identified 34 novel activators, based on their ability to induce the expression of cxcl2. Many are physiologically expressed in macrophages, although the majority of genes uncovered in our screen have not previously been linked to innate immunity. We show that expression of particular activators has profound but distinct impacts on LPS-induced inflammatory gene expression, including switch-type, amplifier and sensitiser behaviours. Furthermore, the novel genes identified here interact with the canonical inflammatory signalling network via specific mechanisms, as demonstrated by the use of dominant negative forms of IL1/TLR signalling mediators.

Mammalian Tribbles homolog 3 impairs insulin action in skeletal muscle: role in glucose-induced insulin resistance

Tribbles homolog 3 (TRIB3) was found to inhibit insulin-stimulated Akt phosphorylation and modulate gluconeogenesis in rodent liver. Currently, we examined a role for TRIB3 in skeletal muscle insulin resistance. Ten insulin-sensitive, ten insulin-resistant, and ten untreated type 2 diabetic (T2DM) patients were metabolically characterized by hyperinsulinemic euglycemic glucose clamps, and biopsies of vastus lateralis were obtained. Skeletal muscle samples were also collected from rodent models including streptozotocin (STZ)-induced diabetic rats, db/db mice, and Zucker fatty rats. Finally, L6 muscle cells were used to examine regulation of TRIB3 by glucose, and stable cell lines hyperexpressing TRIB3 were generated to identify mechanisms underlying TRIB3-induced insulin resistance. We found that 1) skeletal muscle TRIB3 protein levels are significantly elevated in T2DM patients; 2) muscle TRIB3 protein content is inversely correlated with glucose disposal rates and positively correlated with fasting glucose; 3) skeletal muscle TRIB3 protein levels are increased in STZ-diabetic rats, db/db mice, and Zucker fatty rats; 4) stable TRIB3 hyperexpression in muscle cells blocks insulin-stimulated glucose transport and glucose transporter 4 (GLUT4) translocation and impairs phosphorylation of Akt, ERK, and insulin receptor substrate-1 in insulin signal transduction; and 5) TRIB3 mRNA and protein levels are increased by high glucose concentrations, as well as by glucose deprivation in muscle cells. These data identify TRIB3 induction as a novel molecular mechanism in human insulin resistance and diabetes. TRIB3 acts as a nutrient sensor and could mediate the component of insulin resistance attributable to hyperglycemia (i.e., glucose toxicity) in diabetes.

Human tribbles-1 controls proliferation and chemotaxis of smooth muscle cells via MAPK signaling pathways

Migration and proliferation of smooth muscle cells are key to a number of physiological and pathological processes, including wound healing and the narrowing of the vessel wall. Previous work has shown links between inflammatory stimuli and vascular smooth muscle cell proliferation and migration through mitogen-activated protein kinase (MAPK) activation, although the molecular mechanisms of this process are poorly understood. Here we report that tribbles-1, a recently described modulator of MAPK activation, controls vascular smooth muscle cell proliferation and chemotaxis via the Jun kinase pathway. Our findings demonstrate that this regulation takes place via direct interactions between tribbles-1 and MKK4/SEK1, a Jun activator kinase. The activity of this kinase is dependent on tribbles-1 levels, whereas the activation and the expression of MKK4/SEK1 are not. In addition, tribbles-1 expression is elevated in human atherosclerotic arteries when compared with non-atherosclerotic controls, suggesting that this protein may play a role in disease in vivo. In summary, the data presented here suggest an important regulatory role for trb-1 in vascular smooth muscle cell biology.

Regulation of expression and signalling modulator function of mammalian tribbles is cell-type specific

The constant need to respond to changes in the environment is a common feature for all life forms. During evolution, a number of intracellular signal processing systems have evolved to fulfill this requirement. One of the most ancient such systems is the mitogen activated protein kinase (MAPK) signalling network, shared by all eukaryotes. Activation of MAPKs is key to regulation of mitosis and in cellular responses to stress or hormones, for instance. In addition, activity of this signalling system is essential during embryonic development. However, many aspects of MAPK mediated responses are strongly cell-type specific. A family of proteins, called tribbles have recently been described as novel regulators of MAPK function. Our group has previously shown that alterations in tribbles levels lead to profound changes in the activation of the various MAPKs. However, little is known about the cell-type specific aspects of regulation of tribbles expression. Here, we report that expression of all three members of the human tribbles family is dynamically controlled in response to inflammatory stimulation. This regulation, however, is strongly cell-type dependent. Our observations suggest regulation of tribbles expression may play an important role in the cell-type specific cellular responses, mediated by the MAPK network.