Identification of a novel kinase-like gene induced during neuronal cell death

The endoplasmic reticulum: Homeostasis and crosstalk in retinal health and disease

The endoplasmic reticulum (ER) is the largest intracellular organelle carrying out a broad range of important cellular functions including protein biosynthesis, folding, and trafficking, lipid and sterol biosynthesis, carbohydrate metabolism, and calcium storage and gated release. In addition, the ER makes close contact with multiple intracellular organelles such as mitochondria and the plasma membrane to actively regulate the biogenesis, remodeling, and function of these organelles. Therefore, maintaining a homeostatic and functional ER is critical for the survival and function of cells. This vital process is implemented through well-orchestrated signaling pathways of the unfolded protein response (UPR). The UPR is activated when misfolded or unfolded proteins accumulate in the ER, a condition known as ER stress, and functions to restore ER homeostasis thus promoting cell survival. However, prolonged activation or dysregulation of the UPR can lead to cell death and other detrimental events such as inflammation and oxidative stress; these processes are implicated in the pathogenesis of many human diseases including retinal disorders. In this review manuscript, we discuss the unique features of the ER and ER stress signaling in the retina and retinal neurons and describe recent advances in the research to uncover the role of ER stress signaling in neurodegenerative retinal diseases including age-related macular degeneration, inherited retinal degeneration, achromatopsia and cone diseases, and diabetic retinopathy. In some chapters, we highlight the complex interactions between the ER and other intracellular organelles focusing on mitochondria and illustrate how ER stress signaling regulates common cellular stress pathways such as autophagy. We also touch upon the integrated stress response in retinal degeneration and diabetic retinopathy. Finally, we provide an update on the current development of pharmacological agents targeting the UPR response and discuss some unresolved questions and knowledge gaps to be addressed by future research.

The molecular characteristic analysis of TRIB2 gene and its expressional patterns in Bos grunniens tissue and granulosa cells

Tribbles homolog 2 (TRIB2) plays an important role in the follicular development of female mammals. However, its expression and function in the yak (Bos grunniens) are still unclear. In this study, we predicted the molecular characteristics of TRIB2, and revealed its expression pattern in yak (Bos grunniens) tissues and ovarian granulosa cells. We cloned the full length of the yak TRIB2 gene obtained by RT-PCR was 1368 bp and the coding sequence (CDS) was 624 bp, encoding 207 amino acids (AA). Homology analysis showed that the yak TRIB2 is highly conserved among species. TRIB2 was detected to be extensively expressed in seven tissues of the yak liver, spleen, lung, kidney, ovary, oviduct and uterus by qPCR. The expression of TRIB2 mRNA in the ovary during gestation was significantly lower than that in the non-pregnant (p < 0.05). At each stage of follicle development, the TRIB2 mRNA in granulosa cells showed a significant upward trend with the development of follicles. The expression of TRIB2 gradually decreased with the increase of the culture time of the granulosa cells in vitro. In conclusion, these results suggest that TRIB2 may play an important role in the follicular development of yaks.

The Pseudokinase TRIB1 in Immune Cells and Associated Disorders

Simple Summary

TRIB1 is at the center of major cell signaling pathways. In this review, we describe its role in immune cells and highlight TRIB1 interacting partners which suggests cell-specific functions and that TRIB1 is involved in cellular homeostasis and also in different cancers and immune-related disorders.

Abstract

Research advances in Tribbles homolog (TRIB) genes have established the consensus that this protein family plays roles in diverse biological conditions and regulates intracellular signaling networks and several human diseases. In this review, we focus on one member of the family, TRIB1, and its role at the crossroads of immune signaling. TRIB1 directly interacts with transcription factors such as FOXP3 and C/EBPα, with several signaling molecules such as MEK1 and MALT1 and directly acts on key cell signaling pathways such as the MAPK and NF-κB pathways. Altogether, these interactions emphasize that TRIB1 is at the center of major cell signaling pathways while TRIB1 has cell-specific roles, potentially depending on the expressing cells and binding partners. In this review, we describe its roles in immune cells and highlight the interacting partners explaining these functions which suggests TRIB1 as a precise mediator of cellular homeostasis as well as in different cancers and immune-related disorders.

Inhibition of TRIB3 Protects Against Neurotoxic Injury Induced by Kainic Acid in Rats

Epilepsy refers to a group of neurological disorders of varying etiologies characterized by recurrent seizures, resulting in brain dysfunction. Endoplasmic reticulum (ER) stress is highly activated in the process of epilepsy-related brain injury. However, the mechanisms by which ER stress triggers neuronal apoptosis remain to be fully elucidated. Tribbles pseudokinase 3 (TRIB3) is a pseudokinase that affects a number of cellular functions, and its expression is increased during ER stress. Here, we sought to clarify the role of TRIB3 in neuronal apoptosis mediated by ER stress. In the kainic acid (KA) (10 mg/kg)-induced rat seizure model, we characterized neuronal injury and apoptosis after KA injection. KA induced an ER stress response, as indicated by elevated expression of glucose-regulated protein 78 (GRP78) and C/EBP homologous protein (CHOP). TRIB3 protein was upregulated concomitantly with the downregulation of phosphorylated-protein kinase B (p-AKT) in rats following KA administration. In rat cortical neurons treated with KA, TRIB3 knockdown by siRNA reduced the number of dying neurons, decreased the induction of GRP78 and CHOP and the activation of caspase-3, and blocked the dephosphorylation of AKT after KA treatment. Our findings indicate that TRIB3 is involved in neuronal apoptosis occurring after KA-induced seizure. The knockdown of TRIB3 effectively protects against neuronal apoptosis in vitro, suggesting that TRIB3 may be a potential therapeutic target for the treatment of epilepsy.

TRB3 reverses chemotherapy resistance and mediates crosstalk between endoplasmic reticulum stress and AKT signaling pathways in MHCC97H human hepatocellular carcinoma cells

Tribbles homolog 3 (TRB3), a type of pseudokinase that contains a consensus serine/threonine kinase catalytic core structure, is upregulated in hepatocellular carcinoma. However, the effect of TRB3 expression in hepatocellular carcinoma and the molecular mechanisms underlying TRB3-mediated effects on tumorigenesis in hepatocellular carcinoma have not been fully elucidated. The present study focused on the effect of TRB3 expression in MHCC97H hepatocellular carcinoma cells and investigated the underlying molecular mechanisms in MHCC97H cells. In the present study, it was revealed that TRB3 was significantly overexpressed in the MHCC97H hepatocellular carcinoma cell compared with L-02 normal hepatic cells. Under endoplasmic reticulum (ER) stress induced by thapsigargin and tunicamycin, the levels of TRB3, CCAAT/enhancer binding protein homologous protein (CHOP), protein kinase B (AKT) and phosphorylated (p)AKT expression were upregulated. Furthermore, when the expression of TRB3 was silenced by short hairpin (sh)RNA, the survival of MHCC97H hepatocellular carcinoma cells was increased. Notably, following transduction with lentiviral containing TRB3-shRNA, cell survival also increased after treatment with chemotherapy drug cisplatin. The present study demonstrated that knockdown of CHOP by shRNA was able to reduce TRB3 expression, and the knockdown of TRB3 markedly increased the level of pAKT. TRB3 was overexpressed in MHCC97H hepatocellular carcinoma cells, particularly under endoplasmic reticulum stress. Knockdown of TRB3 was able to increase cell survival. Therefore, TRB3 expression may induce apoptosis and reverse resistance to chemotherapy in MHCC97H hepatic carcinoma cells. The present study suggests that TRB3 is a key molecule that mediates the crosstalk between ER stress and AKT signal pathways. Furthermore, the present study may provide further insight into the cancer biology of hepatocellular carcinoma and the development of anticancer drugs targeting the ER stress and AKT signaling pathways.

Enhanced Expression of Trib3 during the Development of Myelin Breakdown in dmy Myelin Mutant Rats

The demyelination (dmy) rat exhibits hind limb ataxia and severe myelin breakdown in the central nervous system. The causative gene of dmy rats is the MRS2 magnesium transporter gene. Tribbles homolog 3 (Trib3) is a pseudokinase molecule that modifies certain signal pathways, and its expression is increased in response to various stresses. Here we sought to clarify the mechanism of myelin breakdown by focusing Trib3, which is remarkably up-regulated in dmy rats. The expression of Trib3 mRNA was significantly increased at 4, 5, 6, 7 and 8 weeks of age in the dmy rats, prior to the prominent myelin breakdown between 7 and 10 weeks of age. The expression level of Trib3 was increased concurrently with the progression of the clinical and pathological conditions in the dmy rats. Double immunofluorescence demonstrated that TRIB3 was mainly expressed in neurons and oligodendrocytes and localized in the Golgi apparatus. Our findings indicate that Trib3 may be associated with the pathogenic mechanism of dmy rats.

Reduced AKT phosphorylation contributes to endoplasmic reticulum stress-mediated hippocampal neuronal apoptosis in rat recurrent febrile seizure

AIMS

Febrile seizure (FS) is one of the most common types of seizures in childhood. Recurrent FS can result in hippocampus injury and thus impair learning capacity and memory, while the underlying molecular mechanisms are still elusive. Studies indicated that endoplasmic reticulum stress (ERS), involved in many diseases including some neurodegenerative diseases, can increase the expression of tribbles-related protein 3 (TRIB3), which thus inhibits the activity of AKT. The current study assessed whether ERS, TRIB3 and AKT signalling is involved in the hippocampus injury following recurrent FS.

MAIN METHODS

Recurrent FS was induced in Sprague-Dawley (SD) rats by using a heated water-bath. TdT-mediated dUTP nick-end labeling (TUNEL) assay was performed to assess hippocampus apoptosis, and electron microscopy was used to examine ultrastructural changes. Protein expression and localization of TRIB3, glucose-regulated protein 78(GRP78) and CCAAT/enhancer-binding protein homologous protein (CHOP) as well as AKT were examined by using western blot and double immunofluorescence staining. Knockdown of TRIB3 was studied in primary cultured neurons treated with hyperthermia.

KEY FINDINGS

As compared with control, apoptosis of hippocampus was significantly induced in FS group. Abundance of TRIB3, GRP78 and CHOP was remarkably elevated, while phosphor-AKT decreased significantly in hippocampus of rats with recurrent FS. Double immunofluorescence indicated that phosphor-AKT was not detected in cells with induction of TRIB3 in FS rats. Hyperthermia-treated cells showed up-regulates TRIB3 expression and that TRIB3 reduces AKT phosphorylation.

SIGNIFICANCE

These results show that recurrent FS may induce injury of hippocampal cell by interfering with AKT activation through ERS-mediated up-regulation of TRIB3.

Nerve growth factor enhances the CRE-dependent transcriptional activity activated by nobiletin in PC12 cells

Prevention and treatment of Alzheimer disease are urgent problems for elderly people in developed countries. We previously reported that nobiletin, a poly-methoxylated flavone from the citrus peel, improved the symptoms in various types of animal models of memory loss and activated the cAMP responsive element (CRE)-dependent transcription in PC12 cells. Nobiletin activated the cAMP/PKA/MEK/Erk/MAPK signaling pathway without using the TrkA signaling activated by nerve growth factor (NGF). Here, we examined the effect of combination of nobiletin and NGF on the CRE-dependent transcription in PC12 cells. Although NGF alone had little effect on the CRE-dependent transcription, NGF markedly enhanced the CRE-dependent transcription induced by nobiletin. The NGF-induced enhancement was neutralized by a TrkA antagonist, K252a. This effect of NGF was effective on the early signaling event elicited by nobiletin. These results suggested that there was crosstalk between NGF and nobiletin signaling in activating the CRE-dependent transcription in PC12 cells.

Trib3 Is Elevated in Parkinson's Disease and Mediates Death in Parkinson's Disease Models

Parkinson's disease (PD) is characterized by the progressive loss of select neuronal populations, but the prodeath genes mediating the neurodegenerative processes remain to be fully elucidated. Trib3 (tribbles pseudokinase 3) is a stress-induced gene with proapoptotic activity that was previously described as highly activated at the transcriptional level in a 6-hydroxydopamine (6-OHDA) cellular model of PD. Here, we report that Trib3 immunostaining is elevated in dopaminergic neurons of the substantia nigra pars compacta (SNpc) of human PD patients. Trib3 protein is also upregulated in cellular models of PD, including neuronal PC12 cells and rat dopaminergic ventral midbrain neurons treated with 6-OHDA, 1-methyl-4-phenylpyridinium (MPP+), or α-synuclein fibrils (αSYN). In the toxin models, Trib3 induction is substantially mediated by the transcription factors CHOP and ATF4. Trib3 overexpression is sufficient to promote neuronal death; conversely, Trib3 knockdown protects neuronal PC12 cells as well as ventral midbrain dopaminergic neurons from 6-OHDA, MPP+, or αSYN. Mechanism studies revealed that Trib3 physically interacts with Parkin, a prosurvival protein whose loss of function is associated with PD. Elevated Trib3 reduces Parkin expression in cultured cells; and in the SNpc of PD patients, Parkin levels are reduced in a subset of dopaminergic neurons expressing high levels of Trib3. Loss of Parkin at least partially mediates the prodeath actions of Trib3 in that Parkin knockdown in cellular PD models abolishes the protective effect of Trib3 downregulation. Together, these findings identify Trib3 and its regulatory pathways as potential targets to suppress the progression of neuron death and degeneration in PD.

SIGNIFICANCE STATEMENT

Parkinson's disease (PD) is the most common neurodegenerative movement disorder. Current treatments ameliorate symptoms, but not the underlying neuronal death. Understanding the core neurodegenerative processes in PD is a prerequisite for identifying new therapeutic targets and, ultimately, curing this disease. Here, we describe a novel pathway involving the proapoptotic protein Trib3 in neuronal death associated with PD. These findings are supported by data from multiple cellular models of PD and by immunostaining of postmortem PD brains. Upstream, Trib3 is induced by the transcription factors ATF4 and CHOP; and downstream, Trib3 interferes with the PD-associated prosurvival protein Parkin to mediate death. These findings establish this new pathway as a potential and promising therapeutic target for treatment of PD.

Dipeptidyl peptidase-4 inhibitor, vildagliptin, inhibits pancreatic beta cell apoptosis in association with its effects suppressing endoplasmic reticulum stress in db/db mice

AIMS

Vildagliptin promotes beta cell survival by inhibiting cell apoptosis. It has been suggested that chronic ER (endoplasmic reticulum) stress triggers beta cell apoptosis. The objective of the study is to explore whether the pro-survival effect of vildagliptin is associated with attenuation of endoplasmic reticulum stress in islets of db/db mice.

METHODS

Vildagliptin was orally administered to db/db mice for 6 weeks, followed by evaluation of beta cell apoptosis by caspase3 activity and TUNEL staining method. Endoplasmic reticulum stress markers were determined with quantitative RT-PCR, immunohistochemistry and immunoblot analysis.

RESULTS

After 6 weeks of treatment, vildagliptin treatment increased plasma active GLP-1 levels (22.63±1.19 vs. 11.69±0.44, P<0.001), inhibited beta cell apoptosis as demonstrated by lower amounts of TUNEL staining nuclei (0.37±0.03 vs. 0.55±0.03, P<0.01) as well as decreased caspase3 activity (1.48±0.11 vs. 2.67±0.13, P<0.01) in islets of diabetic mice compared with untreated diabetic group. Further, vildagliptin treatment down-regulated several genes related to endoplasmic reticulum stress including TRIB3 (tribbles homolog 3) (15.9±0.4 vs. 33.3±1.7, ×10⁻³, P<0.001), ATF-4(activating transcription factor 4) (0.83±0.06 vs. 1.42±0.02, P<0.001) and CHOP(C/EBP homologous protein) (0.07±0.01 vs. 0.16±0.01, P<0.001).

CONCLUSIONS

Vildagliptin promoted beta cell survival in db/db mice in association with down-regulating markers of endoplasmic reticulum stress including TRIB3, ATF-4 as well as CHOP.

A feed-forward loop involving Trib3, Akt and FoxO mediates death of NGF-deprived neurons

The mechanisms governing neuron death following NGF deprivation are incompletely understood. Here, we show that Trib3, a protein induced by NGF withdrawal, has a key role in such death via a loop involving the survival kinase Akt and FoxO transcription factors. Trib3 overexpression is sufficient to induce neuron death, and silencing of endogenous Trib3 strongly protects from death when NGF is withdrawn. Mechanism studies reveal that Trib3 interferes with phosphorylation/activity of Akt and contributes to Akt inactivation after NGF deprivation. FoxO1a, a direct Akt substrate, is dephosphorylated upon NGF withdrawal and consequently undergoes nuclear translocation and activates pro-apoptotic genes. We find that Trib3 is required for FoxO1a dephosphorylation and nuclear translocation after NGF deprivation. Conversely, Trib3 induction requires FoxO transcription factors, which show enhanced occupancy of the Trib3 promoter region following NGF withdrawal. Collectively, these findings support a mechanism in which NGF deprivation, Akt dephosphorylation/inactivation, FoxO dephosphorylation/activation and Trib3 induction are linked in a self-amplifying feed-forward loop that culminates in neuron death.

Trib3 is developmentally and nutritionally regulated in the brain but is dispensable for spatial memory, fear conditioning and sensing of amino acid-imbalanced diet

Tribbles homolog 3 (TRIB3) is a mammalian pseudokinase that is induced in neuronal cell cultures in response to cell death-inducing stresses, including neurotrophic factor deprivation. TRIB3 is an inhibitor of activating transcription factor 4 (ATF4), the central transcriptional regulator in the eukaryotic translation initiation factor 2α (eIF2α) phosphorylation pathway that is involved in the cellular stress response and behavioral processes. In this article, we study the expression of Trib3 in the mouse brain, characterize the brain morphology of mice with a genetic ablation of Trib3 and investigate whether Trib3 deficiency alters eIF2α-dependent cognitive abilities. Our data show that the consumption of a leucine-deficient diet induces Trib3 expression in the anterior piriform cortex, the brain region responsible for detecting essential amino acid intake imbalance. However, the aversive response to leucine-devoid diet does not differ in Trib3 knockout and wild type mice. Trib3 deletion also does not affect long-term spatial memory and reversal learning in the Morris water maze and auditory or contextual fear conditioning. During embryonic development, Trib3 expression increases in the brain and persists in the early postnatal stadium. Neuroanatomical characterization of mice lacking Trib3 revealed enlarged lateral ventricles. Thus, although the absence of Trib3 does not alter the eIF2α pathway-dependent cognitive functions of several areas of the brain, including the hippocampus, amygdala and anterior piriform cortex, Trib3 may serve a role in other central nervous system processes and molecular pathways.

Trb3 regulates LR axis formation in zebrafish embryos

Tribless family proteins are pseudokinases that lack DFG (Asp-Phe-Gly) motif in the functional kinase domain, regulating Akt and BMP pathways, insulin metabolism, hypoxia, and ubiquitination. This report concerns expression patterns and functional roles of trb3 in zebrafish embryonic development. trb3 is evolutionarily well-conserved and located on zebrafish chromosome 11. Spatiotemporal expression studies show that trb3 transcripts are abundant throughout embryogenesis, but confined to mesendodermal cells during the late blastula phase. Over-expression of trb3 ventralizes the embryos while a knockdown of trb3 using morpholino alters positioning of the heart, liver, and pancreatic buds as well as gut looping. Furthermore, constitutive activation of TGF-signaling with TARAM-A* (TGF-related type I receptor) significantly increases the level of trb3 transcripts during the late blastula phase. Over-expression of trb3 reduces the level of smurf1 transcripts, a member of TGF-signaling. We thus propose that Trb3 governs left-right (LR) axis patterning as a component of TGF-signaling in vertebrate embryonic development.

Mammalian tribbles homologs at the crossroads of endoplasmic reticulum stress and Mammalian target of rapamycin pathways

In 2000, investigators discovered Tribbles, a Drosophila protein that coordinates morphogenesis by inhibiting mitosis. Further work has delineated Xenopus (Xtrb2), Nematode (Nipi-3), and mammalian homologs of Drosophila tribbles, which include TRB1, TRB2, and TRB3. The sequences of tribbles homologs are highly conserved, and despite their protein kinase structure, to date they have not been shown to have kinase activity. TRB family members play a role in the differentiation of macrophages, lymphocytes, muscle cells, adipocytes, and osteoblasts. TRB isoforms also coordinate a number of critical cellular processes including glucose and lipid metabolism, inflammation, cellular stress, survival, apoptosis, and tumorigenesis. TRB family members modulate multiple complex signaling networks including mitogen activated protein kinase cascades, protein kinase B/AKT signaling, mammalian target of rapamycin, and inflammatory pathways. The following review will discuss metazoan homologs of Drosophila tribbles, their structure, expression patterns, and functions. In particular, we will focus on TRB3 function in the kidney in podocytes. This review will also discuss the key signaling pathways with which tribbles proteins interact and provide a rationale for developing novel therapeutics that exploit these interactions to provide better treatment options for both acute and chronic kidney disease.

Human Tribbles 3 protects nuclear DNA from cytidine deamination by APOBEC3A

The human polydeoxynucleotide cytidine deaminases APOBEC3A, APOBEC3C, and APOBEC3H are capable of mutating viral DNA in the nucleus, whereas APOBEC3A alone efficiently edits nuclear DNA. Deamination is rapidly followed by excision of uracil residues and can lead to double-stranded breaks. It is not known to which protein networks these DNA mutators belong. Using a yeast two-hybrid screen, we identified the human homolog of Drosophila Tribbles 3, TRIB3, as an interactor for APOBEC3A and APOBEC3C. The interaction was confirmed by co-affinity purification. Co-transfection of APOBEC3A with a TRIB3 expression vector reduced nuclear DNA editing whereas siRNA knockdown of TRIB3 increased the levels of nuclear DNA editing, indicating that TRIB3 functioned as a repressor of A3A. It also repressed A3A-associated γH2AX positive double-stranded breaks. The interaction results in degradation of A3A in a proteasome-independent manner. TRIB3 has been linked to cancer and via its own interactors and links the A3A DNA mutators to the Rb-BRCA1-ATM network. TRIB3 emerges as an important guardian of genome integrity.

The mammalian tribbles homolog TRIB3, glucose homeostasis, and cardiovascular diseases

Insulin signaling plays a physiological role in traditional insulin target tissues controlling glucose homeostasis as well as in pancreatic β-cells and in the endothelium. Insulin signaling abnormalities may, therefore, be pathogenic for insulin resistance, impaired insulin secretion, endothelial dysfunction, and eventually, type 2 diabetes mellitus (T2DM) and cardiovascular disease. Tribbles homolog 3 (TRIB3) is a 45-kDa pseudokinase binding to and inhibiting Akt, a key mediator of insulin signaling. Akt-mediated effects of TRIB3 in the liver, pancreatic β-cells, and skeletal muscle result in impaired glucose homeostasis. TRIB3 effects are also modulated by its direct interaction with other signaling molecules. In humans, TRIB3 overactivity, due to TRIB3 overexpression or to Q84R genetic polymorphism, with R84 being a gain-of-function variant, may be involved in shaping the risk of insulin resistance, T2DM, and cardiovascular disease. TRIB3 overexpression has been observed in the liver, adipose tissue, skeletal muscle, and pancreatic β-cells of individuals with insulin resistance and/or T2DM. The R84 variant has also proved to be associated with insulin resistance, T2DM, and cardiovascular disease. TRIB3 direct effects on the endothelium might also play a role in increasing the risk of atherosclerosis, as indicated by studies on human endothelial cells carrying the R84 variant that are dysfunctional in terms of Akt activation, NO production, and other proatherogenic changes. In conclusion, studies on TRIB3 have unraveled new molecular mechanisms underlying metabolic and cardiovascular abnormalities. Additional investigations are needed to verify whether such acquired knowledge will be relevant for improving care delivery to patients with metabolic and cardiovascular alterations.

Developmental roles of tribbles protein family members

The gene tribbles (trbl), identified 12 years ago in genetic screens for mutations that control both cell division and cell migration during embryonic Drosophila development, is the founding member of the Tribbles (Trib) family of kinase-like proteins that have diverse roles in cell signaling, tissue homeostasis, and cancer. Trib proteins share three motifs: (1) a divergent kinase region (Trib domain) with undetermined catalytic activity, (2) a COP1 site used to direct key target proteins to the proteosome for degradation, and (3) a MEK1 site that binds and modulates MAPKK kinase activity. The notion that Tribs act as scaffolding proteins to balance signaling levels in multiple pathways retains an attractive simplicity, but given recent data showing that divergent kinases act by means of novel catalytic mechanisms, the enzymatic activity of Tribs remains untested. Here, we focus on the role of Tribs during development. Developmental analysis of Drosophila trbl phenotypes reveals tissue-specific, sometimes contradictory roles. In mammals, multiple Trib isoforms exhibit overlapping and tissue-specific functions. Recent data indicate the mechanism of Trib activity is conserved and requires the Trib domain. Finally, we discuss the connections between Tribs in disease and cancer that have implications for their normal roles during organogenesis.

Global analysis of gene expression in NGF-deprived sympathetic neurons identifies molecular pathways associated with cell death

BACKGROUND

Developing sympathetic neurons depend on nerve growth factor (NGF) for survival and die by apoptosis after NGF withdrawal. This process requires de novo gene expression but only a small number of genes induced by NGF deprivation have been identified so far, either by a candidate gene approach or in mRNA differential display experiments. This is partly because it is difficult to obtain large numbers of sympathetic neurons for in vitro studies. Here, we describe for the first time, how advances in gene microarray technology have allowed us to investigate the expression of all known genes in sympathetic neurons cultured in the presence and absence of NGF.

RESULTS

We have used Affymetrix Exon arrays to study the pattern of expression of all known genes in NGF-deprived sympathetic neurons. We identified 415 up- and 813 down-regulated genes, including most of the genes previously known to be regulated in this system. NGF withdrawal activates the mixed lineage kinase (MLK)-c-Jun N-terminal kinase (JNK)-c-Jun pathway which is required for NGF deprivation-induced death. By including a mixed lineage kinase (MLK) inhibitor, CEP-11004, in our experimental design we identified which of the genes induced after NGF withdrawal are potential targets of the MLK-JNK-c-Jun pathway. A detailed Gene Ontology and functional enrichment analysis also identified genetic pathways that are highly enriched and overrepresented amongst the genes expressed after NGF withdrawal. Five genes not previously studied in sympathetic neurons - trib3, ddit3, txnip, ndrg1 and mxi1 - were validated by real time-PCR. The proteins encoded by these genes also increased in level after NGF withdrawal and this increase was prevented by CEP-11004, suggesting that these genes are potential targets of the MLK-JNK-c-Jun pathway.

CONCLUSIONS

The sympathetic neuron model is one of the best studied models of neuronal apoptosis. Overall, our microarray data gives a comprehensive overview of, and provides new information about, signalling pathways and transcription factors that are regulated by NGF withdrawal.

Tribbles: 'puzzling' regulators of cell signalling

Sensing and interpreting extracellular signals in response to changes in the environment has been a fundamental feature of all life forms from the very beginning of evolution. To fulfil this function, networks of proteins have evolved, forming the intracellular signal transduction machinery. Whereas the appropriate control of these signal transduction systems is essential to homoeostasis, dysregulation of signalling leads to disease and often the death of the organism. The tribbles family of pseudokinases have emerged in recent years as key controllers of signal transduction via their interactions with several key kinases, ubiquitin ligases and transcription factors. In line with their role in regulating fundamentally important signalling pathways, members of the tribbles family have been implicated in the development of a range of human diseases. Whereas our mechanistic understanding of how these proteins contribute to disease is far from complete, the present paper attempts to summarize some of the most important recent developments in this field of research.

Tribbles in disease: Signaling pathways important for cellular function and neoplastic transformation

The tribbles family of genes encodes pseudokinase proteins that are highly conserved in evolution. Instead of direct phosphorylation of target proteins, tribbles act as adaptors in signaling pathways for important cellular processes. These include mitogen-activated protein kinase kinase (MAPKK), CCAAT/enhancer-binding protein (C/EBP), activating transcription factor 4 (ATF4) and C/EBP-homologous protein (CHOP). Trib1 and Trib2 have been identified as myeloid oncogenes, and both may be involved in human leukemia. Tribbles proteins are also involved in a series of non-neoplastic disorders including metabolic and neurological diseases. The RAS/mitogen-activated protein kinase (MAPK) pathway molecules (in particular MAPK/ERK kinase 1 (MEK1) and C/EBP transcription factors) include tribbles-binding proteins that are involved in leukemogenesis, and the role of Trib1 as a linker between MAPK signaling and C/EBP degradation is proposed. Although the molecular function of tribbles is still under investigation, the research on tribbles in cellular processes, homeostasis of organisms and human diseases will provide valuable information for therapy of cancer as well as non-neoplastic disorders.

TRB3 is stimulated in diabetic kidneys, regulated by the ER stress marker CHOP, and is a suppressor of podocyte MCP-1

The prevalence of diabetic nephropathy continues to rise, highlighting the importance of investigating and discovering novel treatment strategies. TRB3 is a kinase-like molecule that modifies cellular survival and metabolism and interferes with signal transduction pathways. Herein, we report that TRB3 expression is increased in the kidneys of type 1 and type 2 diabetic mice. TRB3 is expressed in conditionally immortalized podocytes; however, it is not stimulated by elevated glucose. The diabetic milieu is associated with increased oxidative stress and circulating free fatty acids (FFA). We show that reactive oxygen species (ROS) such as H(2)O(2) and superoxide anion (via the xanthine/xanthine oxidase reaction) as well as the FFA palmitate augment TRB3 expression in podocytes. C/EBP homologous protein (CHOP) is a transcription factor that is associated with the endoplasmic reticulum stress response. CHOP expression increases in diabetic mouse kidneys and in podocytes treated with ROS and FFA. In podocytes, transfection of CHOP increases TRB3 expression, and ROS augment recruitment of CHOP to the proximal TRB3 promoter. MCP-1/CCL2 is a chemokine that contributes to the inflammatory injury associated with diabetic nephropathy. In these studies, we demonstrate that TRB3 can inhibit basal and stimulated podocyte production of MCP-1. In summary, enhanced ROS and/or FFA associated with the diabetic milieu induce podocyte CHOP and TRB3 expression. Because TRB3 inhibits MCP-1, manipulation of TRB3 expression could provide a novel therapeutic approach in diabetic kidney disease.

Mixed lineage kinase-3 stabilizes and functionally cooperates with TRIBBLES-3 to compromise mitochondrial integrity in cytokine-induced death of pancreatic beta cells

Mixed lineage kinases (MLKs) have been implicated in cytokine signaling as well as in cell death pathways. Our studies show that MLK3 is activated in leukocyte-infiltrated islets of non-obese diabetic mice and that MLK3 activation compromises mitochondrial integrity and induces apoptosis of beta cells. Using an ex vivo model of islet-splenocyte co-culture, we show that MLK3 mediates its effects via the pseudokinase TRB3, a mammalian homolog of Drosophila Tribbles. TRB3 expression strongly coincided with conformational change and mitochondrial translocation of BAX. Mechanistically, MLK3 directly interacted with and stabilized TRB3, resulting in inhibition of Akt, a strong suppressor of BAX translocation and mitochondrial membrane permeabilization. Accordingly, attenuation of MLK3 or TRB3 expression each prevented cytokine-induced BAX conformational change and attenuated the progression to apoptosis. We conclude that MLKs compromise mitochondrial integrity and suppress cellular survival mechanisms via TRB3-dependent inhibition of Akt.

The UPR and the anti-oxidant response: relevance to sleep and sleep loss

Oxidative stress has been linked to various physiological and pathological processes such as aging and neurological disorders. Recent evidence has now implicated a role for oxidative stress in sleep and sleep loss. Studies suggest that wakefulness results in an oxidative burden and sleep provides a protective mechanism against these harmful effects. Prolonged wakefulness/sleep deprivation activates an adaptive stress pathway termed the unfolded protein response (UPR), which temporarily guards against the deleterious consequences of reactive oxygen species. The UPR affects the function of the endoplasmic reticulum, which is the site for integral and secretory membrane processing and folding. Several downstream effectors of the UPR operate in an antioxidant capacity to reduce the load of these toxic species; a process that may be important in delaying the progression of neurodegenerative diseases. This review will highlight the molecular components of the UPR that ameliorate the accumulation of oxidative stress and may therefore provide potential therapeutic targets.

Acute exercise reverses TRB3 expression in the skeletal muscle and ameliorates whole body insulin sensitivity in diabetic mice

AIM

TRB3 became of major interest in diabetes research when it was shown to interact with and inhibit the activity of Akt. Conversely, physical exercise has been linked to improved glucose homeostasis. Thus, the current study was designed to investigate the effects of acute exercise on TRB3 expression and whole body insulin sensitivity in obese diabetic mice.

METHODS

Male leptin-deficient (ob/ob) mice swam for two 3-h-long bouts, separated by a 45-min rest period. After the second bout of exercise, food was withdrawn 6 h before antibody analysis. Eight hours after the exercise protocol, the mice were submitted to an insulin tolerance test (ITT). Gastrocnemius muscle samples were evaluated for insulin receptor (IR) and IRS-1 tyrosine phosphorylation, Akt serine phosphorylation, TRB3/Akt association and membrane GLUT4 expression.

RESULTS

Western blot analysis showed that TRB3 expression was reduced in the gastrocnemius of leptin-deficient (ob/ob) mice submitted to exercise when compared with respective ob/ob mice at rest. In parallel, there was an increase in the insulin-signalling pathway in skeletal muscle from leptin-deficient mice after exercise. Furthermore, the GLUT4 membrane expression was increased in the muscle after the exercise protocol. Finally, a single session of exercise improved the glucose disappearance (K(ITT)) rate in ob/ob mice.

CONCLUSION

Our results demonstrate that acute exercise reverses TRB3 expression and insulin signalling restoration in muscle. Thus, these results provide new insights into the mechanism by which physical activity ameliorates whole body insulin sensitivity in type 2 diabetes.

PPARalpha ligands cause lymphocyte depletion and cell cycle block and this is associated with augmented TRB3 and reduced Cyclin B1 expression

PPARalpha ligands are medications used clinically to prevent cardiovascular events, however studies have shown that these agents are also anti-inflammatory. Our previous studies have shown that PPARalpha ligands induce lymphocyte depletion. PPARalpha ligands also potently upregulate TRB3, a protein that has been associated with cell cycle arrest. Therefore the following studies were undertaken to determine the mechanisms associated with lymphocyte depletion. Our studies demonstrate that WY14,643, a PPARalpha ligand, decreases the amount of lymphocytes recovered after stimulation and reduces cellular divisions. Cells treated with WY14,643 also accumulate in the G2/S phase of the cell cycle. TRB3 has been shown to inhibit the phosphorylation of AKT/Protein Kinase B, and reduced activation of AKT has been associated with decreased cellular divisions and survival. However in lymphocytes, TRB3 did not reduce the phosphorylation of AKT, and WY14,643 treatment was associated with enhanced activation of AKT. Drosophila tribbles (TRB3 homolog) causes G2 arrest by decreasing the expression of a Cdc25c homolog. Lymphocytes stimulated and treated with WY14,643 have reduced expression of Cdc25c, however this is not associated with enhanced expression of phosphorylated-Cdc2 which induces G2 arrest. Instead we observed that WY14,643 consistently reduces the protein and mRNA expression of Cyclin B1. Moreover, TRB3 inhibits activation of a Cyclin B1 promoter construct. In summary, we propose that PPARalpha ligands may reduce cellular number by augmenting TRB3 expression, which in turn induces cell cycle arrest by reducing the expression of Cyclin B1. Reduced cellular divisions and cell cycle arrest may be responsible for some of the immunomodulatory effects of these agents that have been consistently observed in human trials.

ATF4 is an oxidative stress-inducible, prodeath transcription factor in neurons in vitro and in vivo

Oxidative stress is pathogenic in neurological diseases, including stroke. The identity of oxidative stress–inducible transcription factors and their role in propagating the death cascade are not well known. In an in vitro model of oxidative stress, the expression of the bZip transcription factor activating transcription factor 4 (ATF4) was induced by glutathione depletion and localized to the promoter of a putative death gene in neurons. Germline deletion of ATF4 resulted in a profound reduction in oxidative stress–induced gene expression and resistance to oxidative death. In neurons, ATF4 modulates an early, upstream event in the death pathway, as resistance to oxidative death by ATF4 deletion was associated with decreased consumption of the antioxidant glutathione. Forced expression of ATF4 was sufficient to promote cell death and loss of glutathione. In ATF4^−/− neurons, restoration of ATF4 protein expression reinstated sensitivity to oxidative death. In addition, ATF4^−/− mice experienced significantly smaller infarcts and improved behavioral recovery as compared with wild-type mice subjected to the same reductions in blood flow in a rodent model of ischemic stroke. Collectively, these findings establish ATF4 as a redox-regulated, prodeath transcriptional activator in the nervous system that propagates death responses to oxidative stress in vitro and to stroke in vivo.

PI3K activates negative and positive signals to regulate TRB3 expression in hepatic cells

TRB3 is a pseudokinase whose expression is regulated during stress response and changing of nutrient status. TRB3 negatively regulates Akt activation and noticeably, TRB3 expression is induced by insulin. Here, we sought to determine the dynamic relationship between TRB3 expression and Akt activation. We find that insulin induces TRB3 expression in cell type dependent manner such that in hepatic cells and adipocytes but not Beta cells and muscle cells. In Fao hepatoma cells, induction of TRB3 expression by insulin restrains Akt activation and renders Akt refractory to further activation. In addition, we have also analyzed the roles of PI3K and its downstream kinases Akt and atypical PKC in TRB3 expression. Induction of TRB3 expression by insulin requires PI3K. However, inactivation of Akt enhances TRB3 expression whereas inhibition of PKCzeta expression impairs TRB3 expression induced by insulin. Our data demonstrated that PI3K conveys both negative and positive signals to TRB3 expression. We suggest that insulin-induced TRB3 expression functions as an indicator how multiple insulin-induced signal transduction pathways are balanced.

E3 ubiquitin ligase SIAH1 mediates ubiquitination and degradation of TRB3

Tribbles 3 homolog (TRB3) is recently identified as a scaffold-like regulator of various signal transducers and has been implicated in several processes including insulin signaling, NF-kappaB signaling, lipid metabolism and BMP signaling. To further understand cellular mechanisms of TRB3 regulation, we performed a yeast two-hybrid screen to identify novel TRB3 interacting proteins and totally obtained ten in-frame fused preys. Candidate interactions were validated by co-immunoprecipitation assays in mammalian cells. We further characterized the identified proteins sorted by Gene Ontology Annotation. Its interaction with the E3 ubiquitin ligase SIAH1 was further investigated. SIAH1 could interact with TRB3 both in vitro and in vivo. Importantly, SIAH1 targeted TRB3 for proteasome-dependent degradation. Cotransfection of SIAH1 could withdraw up-regulation of TGF-beta signaling by TRB3, suggesting SIAH1-induced degradation of TRB3 represents a potential regulatory mechanism for TGF-beta signaling.

TRB3 protects cells against the growth inhibitory and cytotoxic effect of ATF4

Tribbles homolog 3 (TRB3) is a pseudokinase the level of which is increased in response to various stresses. We and other researchers have previously shown that TRB3 interacts with activating transcription factor 4 (ATF4) and may function as a negative feedback regulator of ATF4. In the present study, we investigate the effect of ATF4 and TRB3 on cell growth and viability, using both the enforced expression and silencing of the genes. HEK293 cells overexpressing ATF4 show retarded growth in the complete medium and decreased viability in the glucose-free medium. The enforced expression of ATF4 increases the level of reactive oxygen species (ROS) and the supplementation of the medium with ROS scavenging and reducing compounds supports the growth and survival of cells overexpressing ATF4. The deleterious effects of elevated ATF4 are suppressed by the coexpression of TRB3, which downregulates ATF4 transcriptional activity and results in the decrease of intracellular ROS. Also, the coexpression of TRB3 rescues postmitotic neuronally differentiated PC12 cells from the apoptosis evoked by ATF4 overexpression. The silencing of ATF4 and TRB3 genes by RNA interference reveals that endogenous ATF4 promotes and TRB3 suppresses the death of glucose-deprived SaOS2 cells. Together, the results indicate that TRB3 protects cells against the growth inhibitory and cytotoxic effect of ATF4.

Enhanced TLR-mediated NF-IL6 dependent gene expression by Trib1 deficiency

Toll-like receptors (TLRs) recognize a variety of microbial components and mediate downstream signal transduction pathways that culminate in the activation of nuclear factor κB (NF-κB) and mitogen-activated protein (MAP) kinases. Trib1 is reportedly involved in the regulation of NF-κB and MAP kinases, as well as gene expression in vitro. To clarify the physiological function of Trib1 in TLR-mediated responses, we generated Trib1-deficient mice by gene targeting. Microarray analysis showed that Trib1-deficient macrophages exhibited a dysregulated expression pattern of lipopolysaccharide-inducible genes, whereas TLR-mediated activation of MAP kinases and NF-κB was normal. Trib1 was found to associate with NF-IL6 (also known as CCAAT/enhancer-binding protein β). NF-IL6–deficient cells showed opposite phenotypes to those in Trib1-deficient cells in terms of TLR-mediated responses. Moreover, overexpression of Trib1 inhibited NF-IL6–dependent gene expression by down-regulating NF-IL6 protein expression. In contrast, Trib1-deficient cells exhibited augmented NF-IL6 DNA-binding activities with increased amounts of NF-IL6 proteins. These results demonstrate that Trib1 is a negative regulator of NF-IL6 protein expression and modulates NF-IL6–dependent gene expression in TLR-mediated signaling.

TRB3 blocks adipocyte differentiation through the inhibition of C/EBPbeta transcriptional activity

TRB3 has been implicated in the regulation of several biological processes in mammalian cells through its ability to influence Akt and other signaling pathways. In this study, we investigated the role of TRB3 in regulating adipogenesis and the activity of adipogenic transcription factors. We find that TRB3 is expressed in 3T3-L1 preadipocytes, and this expression is transiently suppressed during the initial days of differentiation concomitant with induction of C/EBPbeta. This event appears to be a prerequisite for adipogenesis. Overexpression of TRB3 blocks differentiation of 3T3-L1 cells at a step downstream of C/EBPbeta. Ectopic expression of TRB3 in mouse fibroblasts also inhibits the C/EBPbeta-dependent induction of PPARgamma2 and blocks their differentiation into adipocytes. This inhibition of preadipocyte differentiation by TRB3 appears to be the result of two complementary effects. First, TRB3 inhibits extracellular signal-regulated kinase activity, which prevents the phosphorylation of regulatory sites on C/EBPbeta. Second, TRB3 directly interacts with the DR1 domain of C/EBPbeta in the nucleus, further inhibiting both its ability to bind its response element and its ability to transactivate the C/EBPalpha and a-FABP promoters. Thus, TRB3 is an important negative regulator of adipogenesis that acts at an early step in the differentiation cascade to block the C/EBPbeta proadipogenic function.

A novel regulatory mechanism of the bone morphogenetic protein (BMP) signaling pathway involving the carboxyl-terminal tail domain of BMP type II receptor

Bone morphogenetic protein (BMP) signaling regulates many different biological processes, including cell growth, differentiation, and embryogenesis. BMPs bind to heterogeneous complexes of transmembrane serine/threonine (Ser/Thr) kinase receptors known as the BMP type I and II receptors (BMPRI and BMPRII). BMPRII phosphorylates and activates the BMPRI kinase, which in turn activates the Smad proteins. The cytoplasmic region of BMPRII contains a "tail" domain (BMPRII-TD) with no enzymatic activity or known regulatory function. The discovery of mutations associated with idiopathic pulmonary artery hypertension mapping to BMPRII-TD underscores its importance. Here, we report that Tribbles-like protein 3 (Trb3) is a novel BMPRII-TD-interacting protein. Upon BMP stimulation, Trb3 dissociates from BMPRII-TD and triggers degradation of Smad ubiquitin regulatory factor 1 (Smurf1), which results in the stabilization of BMP receptor-regulated Smads and potentiation of the Smad pathway. Downregulation of Trb3 inhibits BMP-mediated cellular responses, including osteoblast differentiation of C2C12 cells and maintenance of the smooth muscle phenotype of pulmonary artery smooth muscle cells. Thus, Trb3 is a critical component of a novel mechanism for regulation of the BMP pathway by BMPRII.

Survival factor withdrawal-induced apoptosis of TF-1 cells involves a TRB2-Mcl-1 axis-dependent pathway

Tribbles, an atypical protein kinase superfamily member, coordinates cell proliferation, migration, and morphogenesis during the development of Drosophila and Xenopus embryos. Although Tribbles are highly conserved throughout evolution, the physiological functions of mammalian Tribbles family remain largely unclear. Here we report that human TRB2 is a pro-apoptotic molecule that induces apoptosis of cells mainly of the hematopoietic origin. TRB2 mRNA is selectively induced by removal of granulocyte macrophage colony-stimulating factor (GM-CSF) or interleukin-2 from human erythroleukemia-derived TF-1 cell line or activated primary CD4(+) T cells, respectively. It is, however, not induced by many other treatments that trigger apoptosis of these two cell types. Overexpression of TRB2 activates many apoptotic events observed in GM-CSF-deprived TF-1 cells, including loss of mitochondrial membrane potential, Mcl-1 cleavage/degradation, and activation of Bax and a number of caspases. Specific knockdown of TRB2 significantly suppresses GM-CSF deprivation-induced apoptosis and all apoptotic events mentioned above. Finally, we demonstrate that TRB2-induced cleavage and degradation of Mcl-1 are mediated via a caspase-dependent but proteasome-independent mechanism, and overexpression of Mcl-1 or its upstream activator Akt can markedly overcome the apoptogenic effect of TRB2. Altogether, these results suggest that the TRB2-Mcl-1 axis plays an important role in survival factor withdrawal-induced apoptosis of TF-1 cells.

TRB3 modulates C2C12 differentiation by interfering with Akt activation

TRB3 is a member of TRB protein family characterized by containing a variant kinase domain without enzymatic activity. Interacting with Ser/Thr protein kinases Akt, TRB3 impairs Akt activation induced by growth factors and insulin. In this study we have examined the potential role of TRB3 in muscle differentiation. Our data indicated that the expression of TRB3 is downregulated during C2C12 cells undergoing muscle differentiation and that overexpression of TRB3 inhibits Akt activation during differentiation. Correspondingly, overexpression of TRB3 inhibits, while knockdown TRB3 enhances C2C12 differentiation. Thus, our studies indicated that TRB3 plays a critical role in muscle differentiation.

Tribbles: A family of kinase-like proteins with potent signalling regulatory function

The recent identification of tribbles as regulators of signal processing systems and physiological processes, including development, together with their potential involvement in diabetes and cancer, has generated considerable interest in these proteins. Tribbles have been reported to regulate activation of a number of intracellular signalling pathways with roles extending from mitosis and cell activation to apoptosis and modulation of gene expression. The current review summarises our current understanding of interactions between tribbles and various other proteins. Since our understanding on the molecular basis of tribbles function is far from complete, we also describe a bioinformatic analysis of various segments of tribbles proteins, which has revealed a number of highly conserved peptide motifs with potentially important functional roles.

Glucose, dexamethasone, and the unfolded protein response regulate TRB3 mRNA expression in 3T3-L1 adipocytes and L6 myotubes

Tribbles 3 (TRB3) is a recently recognized atypical inactive kinase that negatively regulates Akt activity in hepatocytes, resulting in insulin resistance. Recent reports link TRB3 to nutrient sensing and regulation of cell survival under stressful conditions. We studied the regulation of TRB3 by glucose, insulin, dexamethasone (Dex), and the unfolded protein response (UPR) in 3T3-L1 adipocytes and in L6 myotubes. In 3T3-L1 adipocytes, incubation in high glucose with insulin did not increase TRB3 mRNA expression. Rather, TRB3 mRNA increased fourfold with glucose deprivation and two- to threefold after incubation with tunicamcyin (an inducer of the UPR). Incubation of cells in no glucose or in tunicamcyin stimulated the expression of CCAAT/enhancer-binding protein homologous protein. In L6 myotubes, absent or low glucose induced TRB3 mRNA expression by six- and twofold, respectively. The addition of Dex to 5 mM glucose increased TRB3 mRNA expression twofold in 3T3-L1 adipocytes but decreased it 16% in L6 cells. In conclusion, TRB3 is not the mediator of high glucose or glucocorticoid-induced insulin resistance in 3T3-L1 adipocytes or L6 myotubes. TRB3 is induced by glucose deprivation in both cell types as a part of the UPR, where it may be involved in regulation of cell survival in response to glucose depletion.

Cannabinoids induce apoptosis of pancreatic tumor cells via endoplasmic reticulum stress-related genes

Pancreatic adenocarcinomas are among the most malignant forms of cancer and, therefore, it is of especial interest to set new strategies aimed at improving the prognostic of this deadly disease. The present study was undertaken to investigate the action of cannabinoids, a new family of potential antitumoral agents, in pancreatic cancer. We show that cannabinoid receptors are expressed in human pancreatic tumor cell lines and tumor biopsies at much higher levels than in normal pancreatic tissue. Studies conducted with MiaPaCa2 and Panc1 cell lines showed that cannabinoid administration (a) induced apoptosis, (b) increased ceramide levels, and (c) up-regulated mRNA levels of the stress protein p8. These effects were prevented by blockade of the CB(2) cannabinoid receptor or by pharmacologic inhibition of ceramide synthesis de novo. Knockdown experiments using selective small interfering RNAs showed the involvement of p8 via its downstream endoplasmic reticulum stress-related targets activating transcription factor 4 (ATF-4) and TRB3 in Delta(9)-tetrahydrocannabinol-induced apoptosis. Cannabinoids also reduced the growth of tumor cells in two animal models of pancreatic cancer. In addition, cannabinoid treatment inhibited the spreading of pancreatic tumor cells. Moreover, cannabinoid administration selectively increased apoptosis and TRB3 expression in pancreatic tumor cells but not in normal tissue. In conclusion, results presented here show that cannabinoids lead to apoptosis of pancreatic tumor cells via a CB(2) receptor and de novo synthesized ceramide-dependent up-regulation of p8 and the endoplasmic reticulum stress-related genes ATF-4 and TRB3. These findings may contribute to set the basis for a new therapeutic approach for the treatment of pancreatic cancer.

Fibrates upregulate TRB3 in lymphocytes independent of PPAR alpha by augmenting CCAAT/enhancer-binding protein beta (C/EBP beta) expression

Fibrates, which function by binding and activating peroxisome proliferator-activated receptor alpha (PPARalpha), have been used successfully to treat hyperlipidemia and atherosclerosis. Increasing evidence suggests that in addition to their lipid lowering activities these medications also function as immunosuppressive agents. Tribbles is a Drosophila protein that slows cell cycle progression, and its mammalian homolog, TRB3 interferes with insulin-induced activation of AKT. In these studies we demonstrate that fibrates upregulate TRB3 expression in mitogen-activated lymphocytes. Interestingly, in lymphocytes fibrates augment TRB3 expression in both PPARalpha wildtype and knockout mice, suggesting that upregulation of this protein occurs in a PPARalpha-independent manner. Fibrates activate a proximal TRB3 promoter construct and mutation or partial deletion of a potential PPAR response element does not alter the ability of fibrates to drive TRB3 expression. Subsequent studies reveal that fibrates upregulate C/EBPbeta and CHOP in lymphocytes and mutation of potential C/EBPbeta and CHOP consensus sequences abrogates the ability of fibrates to upregulate TRB3 promoter activity. Accordingly, fibrates enhance the recruitment of C/EBPbeta and CHOP to the proximal TRB3 promoter. Finally, TRB3 expression in lymphocytes induces G2 cell cycle delay and cellular depletion. These studies outline a novel PPARalpha-independent mechanism of action of fibrates and document for the first time the expression of TRB3 in activated lymphocytes.

Effect of TRB3 on insulin and nutrient-stimulated hepatic p70 S6 kinase activity

Insulin and nutrients activate hepatic p70 S6 kinase (S6K1) to regulate protein synthesis. Paradoxically, activation of S6K1 also leads to the development of insulin resistance. In this study, we investigated the effect of TRB3, which acts as an endogenous inhibitor of Akt, on S6K1 activity in vitro and in vivo. In cultured cells, overexpression of TRB3 completely inhibited insulin-stimulated S6K1 activation by mammalian target of rapamycin, whereas knockdown of endogenous TRB3 increased both basal and insulin-stimulated activity. In C57BL/6 mice, adenoviral overexpression of TRB3 inhibited insulin-stimulated activation of hepatic S6K1. In contrast, overexpression of TRB3 did not inhibit nutrient-stimulated S6K1 activity. We also investigated the effect of starvation, feeding, or insulin treatment on TRB3 levels and S6K1 activity in the liver of C57BL/6 and db/db mice. Both insulin and feeding activate S6K1 in db/db mice, but only insulin activates in the C57BL/6 strain. TRB3 levels were 3.5-fold higher in db/db mice than C57BL/6 mice and were unresponsive to feeding or insulin, whereas both treatments reduced TRB3 in C57BL/6 mice. Akt was activated by insulin alone in the C57BL/6 strain and but not in db/db mice. Both insulin and feeding activated mammalian target of rapamycin similarly in these mice; however, feeding was unable to activate the downstream target S6K1 in C57BL/6 mice. These results suggest that the nutrient excess in the hyperphagic, hyperinsulinemic db/db mouse primes the hepatocyte to respond to nutrients resulting in elevated S6K1 activity. The combination of elevated TRB3 and constitutive S6K1 activity results in decreased insulin signaling via the IRS-1/phosphatidylinositol 3-kinase/Akt pathway.

Functional mapping and identification of novel regulators for the Toll/Interleukin-1 signalling network by transcription expression cloning

Sustained inflammatory responses are central to the development and progression of chronic diseases, including atherosclerosis and rheumatoid arthritis. A large number of stimuli initiate inflammation by acting on Toll-Interleukin-1 related (TIR) domain containing receptors, producing multiple second messengers and thence large scale transcriptional changes. The mechanism by which this activation occurs is complex, and the continuing isolation of novel pathway components, mostly based on sequence similarities and protein-protein interaction studies, suggests that many elements of the TIR-initiated signalling network remain to be identified. Here we use a new technique, allowing identification of components based on function. We report the performance of the screen, our identification of human tribbles as a novel protein family regulating inflammatory signalling networks, and the detection of ten other components with poorly characterized roles in inflammatory signalling pathways. In total, we have identified 28 signalling molecules of diverse molecular mechanism by screening 11% of a cDNA library for the ability to modulation expression of human IL-8, and other molecules remain to be followed up. The results suggest that the number of human genes involved in IL-8 induction pathways exceed 100. The isolation of signalling components by the approach we describe allows detection of new classes of signalling components independent of existing techniques for doing so; it is simple and robust, and constitutes a general method for mapping signal transduction systems controlling gene expression.

Regulation of the Akt kinase by interacting proteins

Ten years ago, it was observed that the Akt kinase is activated by phosphorylation via a phosphoinositide 3-kinase (PI-3K)-dependent process. This discovery generated enormous interest because it provided a link between PI-3K, an enzyme known to play a critical role in cellular physiology, and its downstream targets. Subsequently, it was shown that the activity of the core components of the 'PI-3K/Akt pathway' is modulated by a complex network of regulatory proteins and pathways. Some of the Akt-binding partners modulate its activation by external signals by interacting with different domains of the Akt protein. This review focuses on the Akt interacting proteins and the mechanisms by which they regulate Akt activation.

Lack of evidence for a role of TRB3/NIPK as an inhibitor of PKB-mediated insulin signalling in primary hepatocytes

The protein TRB3 (tribbles 3), also called NIPK (neuronal cell death-inducible protein kinase), was recently identified as a protein-protein interaction partner and an inhibitor of PKB (protein kinase B). To explore the hypothesis that TRB3/NIPK might act as a negative regulator of insulin signalling in the liver, this protein was overexpressed by adenoviral transduction of primary cultures of rat hepatocytes, and various aspects of insulin action were investigated. The insulin-induced phosphorylation of Ser-473 and Thr-308 of PKB was found to be undiminished in transduced hepatocytes with a molar excess of TRB3/NIPK over PKB of more than 25-fold. Consistent with unimpaired insulin activation of PKB, the stimulation of Ser-21 and Ser-9 phosphorylation of glycogen synthase kinase 3-alpha and -beta, and the apparent phosphorylation level of 4E-BP1 (eukaryotic initiation factor 4-binding protein 1), were similar in transduced and control hepatocytes. The induction by insulin of the mRNAs encoding glucokinase and SREBF1 (sterol-regulatory-element-binding factor 1) were also normal in TRB3/NIPK hepatocytes. In contrast, the insulin-dependent induction of these two genes, as well as the activation of PKB, were shown to be suppressed in hepatocytes treated with the lipid ether compound PIA6 (phosphatidylinositol ether lipid analogue 6), a recently discovered specific inhibitor of PKB. Since TRB3/NIPK was reported to be increased in the liver of fasting mice, the effects of glucagon, glucocorticoids and insulin on the level of endogenous TRB3/NIPK mRNA in primary hepatocytes were investigated. No significant change in mRNA level occurred under any of the hormonal treatments. The present study does not support the hypothesis that the physiological role of TRB3/NIPK might be to put a brake on insulin signalling in hepatocytes.

Characterization of human NIPK (TRB3, SKIP3) gene activation in stressful conditions

The neuronal cell death-inducible putative kinase (NIPK) gene is upregulated in several cell types under stressful conditions. In order to understand the molecular control of the human (h) NIPK gene (also known as TRB3 and SKIP3), we mapped the transcriptional start sites of the gene in HepG2 cells treated with thapsigargin, the inhibitor of endoplasmic reticular Ca(2+)-ATPase, and determined the promoter region of the gene which is essential for endoplasmic reticulum and arsenite stress responses. The analysis of cDNA clones revealed the presence of several hNIPK mRNA isoforms, differing in their 5' regions upstream of the hNIPK translation initiation codon as a result of alternative transcription initiation and alternative splicing. The induction of hNIPK gene in response to thapsigargin and arsenite treatments is mediated by a promoter segment consisting of tandemly arranged 33-bp repeats that contain a regulatory element similar to C/EBP-ATF composite site of the Chop gene promoter. ATF4, whose level is upregulated in the cells exposed to thapsigargin or arsenite, is able to bind to the 33-bp repeat and activate the hNIPK promoter. The coexpression of hNIPK inhibits activation of hNIPK promoter in response to the stress-inducing agents and to overexpressed ATF4, and thus NIPK may function as a negative feedback regulator of ATF4.

Analysis of gene expression changes in a cellular model of Parkinson disease

We employed Serial Analysis of Gene Expression to identify transcriptional changes in a cellular model of Parkinson Disease (PD). The model consisted of neuronally differentiated PC12 cells compared before and after 8 hours' exposure to 6-hydroxydopamine. Approximately 1200 transcripts were significantly induced by 6-OHDA and approximately 500 of these are currently matched to known genes. Here, we categorize the regulated genes according to known functional activities and discuss their potential roles in neuron death and survival and in PD. We find induction of multiple death-associated genes as well as many with the capacity for neuroprotection. This suggests that survival or death of individual neurons in PD may reflect an integrated response to both protective and destructive gene changes. Our findings identify a number of regulated genes as candidates for involvement in PD and therefore as potential targets for therapeutic intervention. Such intervention may include both inhibiting the induction/activity of death-promoting genes and enhancing those with neuroprotective activity.