Insulin regulates TRB3 and other stress-responsive gene expression through induction of C/EBPbeta

Evaluation of sestrin 2 and tribbles homolog 3 levels in obese and nonobese women with polycystic ovary syndrome

Background/aim

This study was designed to evaluate the relationship of two new biomarkers [tribbles homolog 3 (TRB3) and sestrin 2 levels], which were previously associated with obesity, with metabolic parameters in obese and nonobese women with polycystic ovary syndrome (PCOS).

Materials and methods

This cross-sectional case control study was conducted between September 2017 and August 2019 in the gynecology department of a tertiary referral hospital. The values of the plasma sestrin 2, TRB3, insulin, fasting plasma glucose, lipid profile, and homeostasis model assessment of insulin resistance (HOMA-IR) were compared in 90 obese women with PCOS (BMI > 30), 90 women with nonobese PCOS (BMI < 30), and 90 control patients (BMI < 30).

Results

The mean age of the study group consisting of all PCOS patients (26.11 ± 4.64 years) and the mean age of the control group (26.3 ± 4.4 years) were statistically similar (p = 0.239). The serum sestrin 2 values of the obese PCOS group were found to be statistically significantly lower than the control and non-obese PCOS groups (p = 0.001, p = 0.0001), while the sestrin 2 values of the nonobese PCOS group were found to be statistically significantly lower than the control group (p = 0.0001). The TRB3 values of the control group were found to be statistically significantly lower than the obese and nonobese PCOS groups (p = 0.0001), while the TRB3 values of the nonobese PCOS group were found to be statistically significantly lower than the obese PCOS group (p = 0.0001). A negative correlation was observed between the sestrin 2 level and BMI (r = -0.272 p = 0.0001), insulin (r = -0.261 p = 0.0001), and HOMA-IR levels (r = -0.250 p = 0.0001). A positive correlation was observed between the TRB3 values and TG (r = 0.248 p = 0.0001), and LDL-C values (r = 0.235 p = 0.0001).

Conclusion

According to the findings in this study, low sestrin 2 and high TRB3 levels may be related to impaired metabolic status in the obese PCOS group. Thus, it may be promising for the development of treatment of PCOS and associated metabolic disorder in the future.

Gut microbiota regulate Alzheimer's disease pathologies and cognitive disorders via PUFA-associated neuroinflammation

OBJECTIVE

This study is to investigate the role of gut dysbiosis in triggering inflammation in the brain and its contribution to Alzheimer's disease (AD) pathogenesis.

DESIGN

We analysed the gut microbiota composition of 3×Tg mice in an age-dependent manner. We generated germ-free 3×Tg mice and recolonisation of germ-free 3×Tg mice with fecal samples from both patients with AD and age-matched healthy donors.

RESULTS

Microbial 16S rRNA sequencing revealed Bacteroides enrichment. We found a prominent reduction of cerebral amyloid-β plaques and neurofibrillary tangles pathology in germ-free 3×Tg mice as compared with specific-pathogen-free mice. And hippocampal RNAseq showed that inflammatory pathway and insulin/IGF-1 signalling in 3×Tg mice brain are aberrantly altered in the absence of gut microbiota. Poly-unsaturated fatty acid metabolites identified by metabolomic analysis, and their oxidative enzymes were selectively elevated, corresponding with microglia activation and inflammation. AD patients' gut microbiome exacerbated AD pathologies in 3×Tg mice, associated with C/EBPβ/asparagine endopeptidase pathway activation and cognitive dysfunctions compared with healthy donors' microbiota transplants.

CONCLUSIONS

These findings support that a complex gut microbiome is required for behavioural defects, microglia activation and AD pathologies, the gut microbiome contributes to pathologies in an AD mouse model and that dysbiosis of the human microbiome might be a risk factor for AD.

Increased expression of tribbles homolog 3 predicts poor prognosis and correlates with tumor immunity in clear cell renal cell carcinoma: a bioinformatics study

Tribbles homolog 3 (TRIB3), a pseudokinase that regulates multiple intracellular signaling pathways, has been reported to promote the growth of multiple tumors. However, its role in clear cell renal cell carcinoma (ccRCC) remains unelucidated. We evaluated the role of TRIB3 in ccRCC using publicly available data from The Cancer Genome Atlas and analyzed its relationship with the tumor microenvironment; moreover, we used gene knockout and overexpression techniques to detect the effects of TRIB3 on the biological behavior of ccRCC cells. RT-qPCR and western blotting were used to detect transfection efficiency, and the invasiveness of ccRCC cells was determined by Transwell migration assays. We found that TRIB3 overexpression was significantly associated with increased grade, stage, and distant metastasis, positively correlated with ccRCC invasiveness, and also an independent risk factor for overall survival (OS). In addition, 361 differentially expressed genes (DEGs) related to TRIB3 were identified. Functional enrichment analysis showed that DEGs were mainly enriched in humoral immune responses, collagen-containing extracellular matrix, and serine hydrolase activity. Immune landscape characterization revealed that TRIB3 expression was significantly and negatively associated with CD8⁺ T and hematopoietic stem cells, whereas it was positively associated with NK T and macrophage M1 cells. Single-cell sequencing showed that localization and binding targets of TRIB3 mainly involved monocytes/macrophages and CD4⁺ and CD8⁺ T cells. Overall, our study revealed that elevated TRIB3 expression represents a promising prognostic marker for ccRCC patients and may play a key role in tumor microenvironment modulation.

TRIB3‒GSK-3β interaction promotes lung fibrosis and serves as a potential therapeutic target

Pulmonary fibrosis (PF) is a chronic, progressive, fatal interstitial lung disease with limited available therapeutic strategies. We recently reported that the protein kinase glycogen synthase kinase-3β (GSK-3β) interacts with and inactivates the ubiquitin-editing enzyme A20 to suppress the degradation of the transcription factor CCAAT/enhancer-binding protein beta (C/EBPβ) in alveolar macrophages (AMs), resulting in a profibrotic phenotype of AMs and promoting the development of PF. Here, we showed that chronic lung injury upregulated the stress response protein tribbles homolog 3 (TRIB3), which interacted with GSK-3β and stabilized GSK-3β from ubiquitination and degradation. Elevated GSK-3β expression phosphorylated A20 to inhibit its ubiquitin-editing activity, causing the accumulation of C/EBPβ and the production of several profibrotic factors in AMs and promoting PF development. Activated C/EBPβ, in turn, increased the transcription of TRIB3 and GSK-3β, thereby establishing a positive feedback loop in AMs. The knockdown of TRIB3 expression or the pharmacologic disruption of the TRIB3‒GSK-3β interaction was an effective PF treatment. Our study reveals an intact profibrotic axis of TRIB3‒GSK-3β‒A20‒C/EBPβ in AMs, which represents a target that may provide a promising treatment strategy for PF.

Decreased 11β-Hydroxysteroid Dehydrogenase Type 2 Expression in the Kidney May Contribute to Nicotine/Smoking-Induced Blood Pressure Elevation in Mice

[Figure: see text].

Obesity-induced TRB3 negatively regulates Brown adipose tissue function in mice

Brown adipose tissue (BAT) and stimulating adaptive thermogenesis have been implicated as anti-obese and anti-diabetic tissues due to their ability to dissipate energy as heat by the expression of UCP1. We have recently demonstrated that TRB3 impairs differentiation of brown preadipocytes via inhibiting insulin signaling. However, the roles of the protein in BAT function and thermogenesis in vivo have not yet been established. For this study we tested the hypothesis that TRB3 mediates obesity- and diabetes-induced impairments in BAT differentiation and function, and that inhibition of TRB3 improves BAT function. TRB3 expression was increased in BAT from high-fat fed mice and ob/ob mice, which was associated with decreased UCP1 expression. Incubation of brown adipocytes with palmitate increased TRB3 expression and decreased UCP1. Knockout of TRB3 in mice displayed higher UCP1 expression in BAT and cold resistance. Incubation of brown adipocytes with ER stressors increased TRB3 but decreased UCP1 and ER stress markers were elevated in BAT from high-fat fed mice and ob/ob mice. Finally, high-fat feeding in TRB3KO mice were protected from obesity-induced glucose intolerance and displayed cold resistance and higher expression of BAT-specific markers. These data demonstrate that high-fat feeding and obesity increase TRB3 in BAT, resulting in impaired tissue function.

TRIB3 promotes MYC-associated lymphoma development through suppression of UBE3B-mediated MYC degradation

The transcription factor MYC is deregulated in almost all human cancers, especially in aggressive lymphomas, through chromosomal translocation, amplification, and transcription hyperactivation. Here, we report that high expression of tribbles homologue 3 (TRIB3) positively correlates with elevated MYC expression in lymphoma specimens; TRIB3 deletion attenuates the initiation and progression of MYC-driven lymphoma by reducing MYC expression. Mechanistically, TRIB3 interacts with MYC to suppress E3 ubiquitin ligase UBE3B-mediated MYC ubiquitination and degradation, which enhances MYC transcriptional activity, causing high proliferation and self-renewal of lymphoma cells. Use of a peptide to disturb the TRIB3-MYC interaction together with doxorubicin reduces the tumor burden in Myc^Eμ mice and patient-derived xenografts. The pathophysiological relevance of UBE3B, TRIB3 and MYC is further demonstrated in human lymphoma. Our study highlights a key mechanism for controlling MYC expression and a potential therapeutic option for treating lymphomas with high TRIB3-MYC expression.

c-MYC is often deregulated in human cancers including lymphomas. Here, the authors show that a member of the pseudokinase family, tribbles homologue 3 (TRIB3), interacts with c-MYC to suppress c-MYC ubiquitination and degradation, leading to increased proliferation and self-renewal of lymphoma cells.

Notch Signaling and the Breast Cancer Microenvironment

Notch promotes breast cancer progression through tumor initiating cell maintenance, tumor cell fate specification, proliferation, survival, and motility. In addition, Notch is recognized as a decisive mechanism in regulating various juxtacrine and paracrine communications in the tumor microenvironment (TME). In this chapter, we review recent studies on stress-mediated Notch activation within the TME and sequelae such as angiogenesis, extracellular matrix remodeling, changes in the innate and adaptive immunophenotype, and therapeutic perspectives.

PML-RARα interaction with TRIB3 impedes PPARγ/RXR function and triggers dyslipidemia in acute promyelocytic leukemia

Although dyslipidemia commonly occurs in patients with acute promyelocytic leukemia (APL) in response to anti-APL therapy, the underlying mechanism and the lipid statuses of patients with newly diagnosed APL remain to be addressed.

Methods: We conducted a retrospective study to investigate the lipid profiles of APL patients. PML-RARα transgenic mice and APL cells-transplanted mice were used to assess the effects of APL cells on the blood/liver lipid levels. Subsequently, gene set enrichment analysis, western blot and dual luciferase reporter assay were performed to examine the role and mechanism of PML-RARα and TRIB3 in lipid metabolism regulation in APL patients at pretreatment and after induction therapy.

Results: APL patients exhibited a higher prevalence of dyslipidemia before anti-APL therapy based on a retrospective study. Furthermore, APL cells caused secretion of triglycerides, cholesterol, and PCSK9 from hepatocytes and degradation of low-density lipoprotein receptors in hepatocytes, which elevated the lipid levels in APL cell-transplanted mice and Pml-Rarα transgenic mice. Mechanistically, pseudokinase TRIB3 interacted with PML-RARα to inhibit PPARγ activity by interfering with the interaction of PPARγ and RXR and promoting PPARγ degradation. Thus, reduced PPARγ activity in APL cells decreased leptin but increased resistin expression, causing lipid metabolism disorder in hepatocytes and subsequent dyslipidemia in mice. Although arsenic/ATRA therapy degraded PML-RARα and restored PPARγ expression, it exacerbated dyslipidemia in APL patients. The elevated TRIB3 expression in response to arsenic/ATRA therapy suppressed PPARγ activity by disrupting the PPARγ/RXR dimer, which resulted in dyslipidemia in APL patients undergoing therapy. Indeed, the PPAR activator not only enhanced the anti-APL effects of arsenic/ATRA by suppressing TRIB3 expression but also reduced therapy-induced dyslipidemia in APL patients.

Conclusion: Our work reveals the critical role of the PML-RARα/PPARγ/TRIB3 axis in the development of dyslipidemia in APL patients, potentially conferring a rationale for combining ATRA/arsenic with the PPAR activator for APL treatment.

Endoplasmic reticulum stress and development of insulin resistance in adipose, skeletal, liver, and foetoplacental tissue in diabesity

Diabesity is an abnormal metabolic condition shown by patients with obesity that develop type 2 diabetes mellitus. Patients with diabesity present with insulin resistance, reduced vascular response to insulin, and vascular endothelial dysfunction. Along with the several well-described mechanisms of insulin resistance, a state of endoplasmic reticulum (ER) stress, where the primary human targets are the adipose tissue, liver, skeletal muscle, and the foetoplacental vasculature, is apparent. ER stress characterises by the activation of the unfolded protein response via three canonical ER stress sensors, i.e., the protein kinase RNA-like endoplasmic reticulum kinase (PERK), inositol-requiring enzyme 1α (IRE1α), and activating transcription factor 6. Slightly different cell signalling mechanisms preferentially enable in diabesity in the ER stress-associated insulin resistance for adipose tissue (IRE1α/X-box binding protein 1 mRNA splicing/c-jun N-terminal kinase 1 activation), skeletal muscle (tribbles-like protein 3 (TRB3)/proinflammatory cytokines activation), and liver (PERK/activating transcription factor 4/TRB3 activation). There is no information in human subjects with diabesity in the foetoplacental vasculature. However, the available literature shows that pregnant women with pre-pregnancy obesity or overweight that develop gestational diabetes mellitus (GDM) and their newborn show insulin resistance. ER stress is recently reported to be triggered in endothelial cells from the human umbilical vein from mothers with pre-pregnancy obesity. However, whether a different metabolic alteration to obesity in pregnancy or GDM is present in women with pre-pregnancy obesity that develop GDM, is unknown. In this review, we summarised the findings on diabesity-associated mechanisms of insulin resistance with emphasis in the primary targets adipose, skeletal muscle, liver, and foetoplacental tissues. We also give evidence on the possibility of a new GDM-associated metabolic condition triggered in pregnancy by maternal obesity, i.e. gestational diabesity, leading to ER stress-associated insulin resistance in the human foetoplacental vasculature.

TRIB3 Promotes APL Progression through Stabilization of the Oncoprotein PML-RARα and Inhibition of p53-Mediated Senescence

Acute promyelocytic leukemia (APL) is driven by the oncoprotein PML-RARα, which antagonizes myeloid differentiation and promotes APL-initiating cell self-renewal. Combined all-trans retinoic acid (ATRA) with arsenic trioxide (As₂O₃) or chemotherapy dramatically improves the prognosis of APL patients. Here we report that expression of pseudokinase Tribble 3 (TRIB3) associates positively with APL progression and therapeutic resistance. The elevated TRIB3 expression promotes APL by interacting with PML-RARα and suppressing its sumoylation, ubiquitylation, and degradation. This represses PML nuclear body assembly, p53-mediated senescence, and cell differentiation, and supports cellular self-renewal. Genetically inhibiting TRIB3 expression or combination of a peptide disturbing TRIB3/PML-RARα interaction with ATRA/As₂O₃ eradicates APL by accelerating PML-RARα degradation. Our study provides insight into APL pathogenesis and a potential therapeutic option against APL.

Governing roles for Trib3 pseudokinase during stress erythropoiesis

In response to anemia, the heightened production of erythropoietin (EPO) can sharply promote erythroid progenitor cell (EPC) formation. Specific mediators of such EPO- accelerated erythropoiesis, however, are not well understood. Presently, we first report that the expression of Trib3 in adult bone marrow EPCs in vivo is nominal at steady state, but strongly activated on EPO challenge. In a knockout mouse model, Trib3 disruption modestly increased steady-state erythrocyte numbers and decreased mean corpuscular volume. Following 5-fluorouracil myeloablation, however, rebound red blood cell production and hemoglobin levels were substantially (and selectively) compromised in Trib3^-/- mice versus Trib3^+/+ congenic controls. Erythrocytes from 5-fluorouracil-treated Trib3^-/- mice additionally were more prone to lysis and exhibited elevated peroxide-induced reactive oxygen species. Ex vivo, the development of CD71^posTer119^pos erythroblasts from Trib3^-/- bone marrow progenitors was attenuated, and this was associated with heightened EPO-dependent Erk1/2 activation and moderately increased Akt activation. For developmentally staged EPCs, gene profiling provided further initial insight into candidate mediators of EPO-induced Trib3 gene expression, including Cebp-beta, Atf4, Egr-1, and Nab1. Overall, Trib3 is indicated to act as a novel EPC-intrinsic governor of stress erythropoiesis.

Cellular stress induces TRB3/USP9x-dependent Notch activation in cancer

Expression of the Notch ligand JAG1 and Notch pathway activation promote poor prognosis, basal-like breast cancer. We have recently shown that the pseudokinase Tribbles homolog 3 (TRB3) regulates JAG1 expression in this malignancy. TRB3 is a stress and metabolic sensor, and here we show that nutrient deprivation or endoplasmic reticulum stress markedly upregulate TRB3, which serves as a scaffold for the deubiquitinase USP9x. USP9x in turn stimulates JAG1 activity through two mechanisms: (1) through TRB3 deubiquitination and stabilization, and (2) through deubiquitination and activation of Mind Bomb 1, an E3 ligase required for JAG1 ubiquitination-mediated endocytosis and Notch activation. These USP9x activities are confined to the signal-sending cell of a cell pair undergoing Notch signaling. We demonstrate that USP9x is required for TRB3 upregulation and Notch activation in response to cellular stress in basal-like breast cancer cells. These data suggest that TRB3 functions as a sensor of tumor microenvironmental stress and together with USP9x induces the cell survival and tumor-promoting activities of Notch. These findings identify a novel mechanism by which cancer cells survive in their hostile environment and provide potential therapeutic targets in breast cancer.

Prolonged insulin treatment sensitizes apoptosis pathways in pancreatic β cells

Insulin resistance results from impaired insulin signaling in target tissues that leads to increased levels of insulin required to control plasma glucose levels. The cycle of hyperglycemia and hyperinsulinemia eventually leads to pancreatic cell deterioration and death by a mechanism that is yet unclear. Insulin induces ROS formation in several cell types. Furthermore, death of pancreatic cells induced by oxidative stress could be potentiated by insulin. Here, we investigated the mechanism underlying this phenomenon. Experiments were done on pancreatic cell lines (Min-6, RINm, INS-1), isolated mouse and human islets, and on cell lines derived from nonpancreatic sources. Insulin (100nM) for 24h selectively increased the production of ROS in pancreatic cells and isolated pancreatic islets, but only slightly affected the expression of antioxidant enzymes. This was accompanied by a time- and dose-dependent decrease in cellular reducing power of pancreatic cells induced by insulin and altered expression of several ER stress response elements including a significant increase in Trb3 and a slight increase in iNos The effect on iNos did not increase NO levels. Insulin also potentiated the decrease in cellular reducing power induced by H2O2 but not cytokines. Insulin decreased the expression of MCL-1, an antiapoptotic protein of the BCL family, and induced a modest yet significant increase in caspase 3/7 activity. In accord with these findings, inhibition of caspase activity eliminated the ability of insulin to increase cell death. We conclude that prolonged elevated levels of insulin may prime apoptosis and cell death-inducing mechanisms as a result of oxidative stress in pancreatic cells.

The TRIB3 Q84R polymorphism, insulin resistance and related metabolic alterations

Insulin resistance is pathogenic for many prevalent disorders including type 2 diabetes mellitus (T2DM), cardiovascular disease (CVD), polycystic ovary syndrome, non-alcoholic fatty liver disease, Alzheimer's and Parkinson's diseases and several cancers. Unravelling molecular abnormalities of insulin resistance may therefore pave the way for tackling such heavy weight on healthcare systems. This review will be focused on studies addressing the role of genetic variability of TRIB3, an inhibitor of insulin signalling at the AKT level on insulin resistance and several related abnormalities. Studies carried out in several cultured cells clearly report that the TRIB3 Q84R missense polymorphism, is a gain-of-function amino acid substitution, with the Arg(84) variant being a stronger inhibitor of insulin-mediated AKT activation as compared with the more frequent Gln(84) variant. Given the key role of AKT in modulating not only insulin signalling but also insulin secretion, it was not surprising that β-cells and human pancreatic islets carrying the Arg(84) variant showed also impaired insulin secretion. Also, of note is that in human vein endothelial cells carrying the Arg(84) variant showed a reduced insulin-induced nitric oxide release, an established early atherosclerotic step. Accordingly with in vitro studies, in vivo studies indicate that TRIB3 Arg(84) is associated with insulin resistance, T2DM and several aspects of atherosclerosis, including overt CVD. In all, several data indicate that the TRIB3 Arg(84) variant plays a role on several aspects of glucose homoeostasis and atherosclerotic processes, thus unravelling new molecular pathogenic mechanisms of highly prevalent disorders such as T2DM and CVD.

Positive Regulation of Interleukin-2 Expression by a Pseudokinase, Tribbles 1, in Activated T Cells

Tribbles 1 (TRB1), a member of the Tribbles family, is a pseudokinase that is conserved among species and implicated in various human diseases including leukemia, cardiovascular diseases, and metabolic disorders. However, the role of TRB1 in the immune response is not understood. To evaluate this role, we examined regulation of TRB1 expression and the function of TRB1 in interleukin-2 (IL-2) induction in Jurkat cells, a human acute T cell leukemia cell line. We found that TRB1 was strongly induced by phorbol 12-myristate 13-acetate (PMA) and ionomycin in these cells. IL-2 expression was induced in Jurkat cells activated by PMA and ionomycin; however, knockdown of TRB1 resulted in decreased induction of IL-2. TRB1 null Jurkat cells established using the CRISPR/Cas9 system also showed reduction of IL-2 expression on PMA/ionomycin stimulation. TRB1 knockdown also markedly inhibited IL-2 promoter activation. To determine the mechanism of the stimulatory effect on IL-2 induction, we focused on histone deacetylases (HDACs), and found that HDAC1 preferentially interacts with TRB1. TRB1 suppressed the interaction of HDAC1 with nuclear factor of activated T cells 2 (NFAT2), which is a crucial transcription factor for IL-2 induction. These results indicate that TRB1 is a positive regulator of IL-2 induction in activated T cells.

TRB3 links insulin/IGF to tumour promotion by interacting with p62 and impeding autophagic/proteasomal degradations

High insulin/IGF is a biologic link between diabetes and cancers, but the underlying molecular mechanism remains unclear. Here we report a previously unrecognized tumour-promoting mechanism for stress protein TRB3, which mediates a reciprocal antagonism between autophagic and proteasomal degradation systems and connects insulin/IGF to malignant promotion. We find that several human cancers express higher TRB3 and phosphorylated insulin receptor substrate 1, which correlates negatively with patient's prognosis. TRB3 depletion protects against tumour-promoting actions of insulin/IGF and attenuates tumour initiation, growth and metastasis in mice. TRB3 interacts with autophagic receptor p62 and hinders p62 binding to LC3 and ubiquitinated substrates, which causes p62 deposition and suppresses autophagic/proteasomal degradation. Several tumour-promoting factors accumulate in cancer cells to support tumour metabolism, proliferation, invasion and metastasis. Interrupting TRB3/p62 interaction produces potent antitumour efficacies against tumour growth and metastasis. Our study opens possibility of targeting this interaction as a potential novel strategy against cancers with diabetes.

Drosophila tribbles antagonizes insulin signaling-mediated growth and metabolism via interactions with Akt kinase

Drosophila Tribbles (Trbl) is the founding member of the Trib family of kinase-like docking proteins that modulate cell signaling during proliferation, migration and growth. In a wing misexpression screen for Trbl interacting proteins, we identified the Ser/Thr protein kinase Akt1. Given the central role of Akt1 in insulin signaling, we tested the function of Trbl in larval fat body, a tissue where rapid increases in size are exquisitely sensitive to insulin/insulin-like growth factor levels. Consistent with a role in antagonizing insulin-mediated growth, trbl RNAi knockdown in the fat body increased cell size, advanced the timing of pupation and increased levels of circulating triglyceride. Complementarily, overexpression of Trbl reduced fat body cell size, decreased overall larval size, delayed maturation and lowered levels of triglycerides, while circulating glucose levels increased. The conserved Trbl kinase domain is required for function in vivo and for interaction with Akt in a yeast two-hybrid assay. Consistent with direct regulation of Akt, overexpression of Trbl in the fat body decreased levels of activated Akt (pSer505-Akt) while misexpression of trbl RNAi increased phospho-Akt levels, and neither treatment affected total Akt levels. Trbl misexpression effectively suppressed Akt-mediated wing and muscle cell size increases and reduced phosphorylation of the Akt target FoxO (pSer256-FoxO). Taken together, these data show that Drosophila Trbl has a conserved role to bind Akt and block Akt-mediated insulin signaling, and implicate Trib proteins as novel sites of signaling pathway integration that link nutrient availability with cell growth and proliferation.

Overexpression of TRB3 in muscle alters muscle fiber type and improves exercise capacity in mice

Increasing evidence suggests that TRB3, a mammalian homolog of Drosophila tribbles, plays an important role in cell growth, differentiation, and metabolism. In the liver, TRB3 binds and inhibits Akt activity, whereas in adipocytes, TRB3 upregulates fatty acid oxidation. In cultured muscle cells, TRB3 has been identified as a potential regulator of insulin signaling. However, little is known about the function and regulation of TRB3 in skeletal muscle in vivo. In the current study, we found that 4 wk of voluntary wheel running (6.6 ± 0.4 km/day) increased TRB3 mRNA by 1.6-fold and protein by 2.5-fold in the triceps muscle. Consistent with this finding, muscle-specific transgenic mice that overexpress TRB3 (TG) had a pronounced increase in exercise capacity compared with wild-type (WT) littermates (TG: 1,535 ± 283; WT: 644 ± 67 joules). The increase in exercise capacity in TRB3 TG mice was not associated with changes in glucose uptake or glycogen levels; however, these mice displayed a dramatic shift toward a more oxidative/fatigue-resistant (type I/IIA) muscle fiber type, including threefold more type I fibers in soleus muscles. Skeletal muscle from TRB3 TG mice had significantly decreased PPARα expression, twofold higher levels of miR208b and miR499, and corresponding increases in the myosin heavy chain isoforms Myh7 and Myb7b, which encode these microRNAs. These findings suggest that TRB3 regulates muscle fiber type via a peroxisome proliferator-activated receptor-α (PPAR-α)-regulated miR499/miR208b pathway, revealing a novel function for TRB3 in the regulation of skeletal muscle fiber type and exercise capacity.

Tribbles 3 mediates endoplasmic reticulum stress-induced insulin resistance in skeletal muscle

Endoplasmic Reticulum (ER) stress has been linked to insulin resistance in multiple tissues but the role of ER stress in skeletal muscle has not been explored. ER stress has also been reported to increase tribbles 3 (TRB3) expression in multiple cell lines. Here, we report that high fat feeding in mice, and obesity and type 2 diabetes in humans significantly increases TRB3 and ER stress markers in skeletal muscle. Overexpression of TRB3 in C2C12 myotubes and mouse tibialis anterior muscles significantly impairs insulin signaling. Incubation of C2C12 cells and mouse skeletal muscle with ER stressors thapsigargin and tunicamycin increases TRB3 and impairs insulin signaling and glucose uptake, effects reversed in cells overexpressing RNAi for TRB3 and in muscles from TRB3 knockout mice. Furthermore, TRB3 knockout mice are protected from high fat diet-induced insulin resistance in skeletal muscle. These data demonstrate that TRB3 mediates ER stress-induced insulin resistance in skeletal muscle.

Interaction of constitutive photomorphogenesis 1 protein with protein-tyrosine phosphatase 1B suppresses protein-tyrosine phosphatase 1B activity and enhances insulin signaling

Recent studies reveal that COP1 suppresses the expression of gluconeogenetic genes and prohibits hepatic glucose production. To get more insight into COP1 in hepatic cells, we examined the impact of COP1 on insulin-responsive genes and insulin signaling. We found that COP1 increased the responsiveness of insulin-modulated genes to insulin in that it promoted the expression of insulin-induced genes and inhibited that of insulin-suppressed genes and that COP1 enhanced insulin signaling as it promoted phosphorylation of Akt and ERK as well as tyrosine phosphorylation of IRβ induced by insulin. To delineate the mechanism under which COP1 modulates insulin signaling, we examined the possibility that COP1 modulates the activity of PTP1B, the major insulin receptor tyrosine phosphatase. The results indicated that COP1 physically interacted with PTP1B and suppressed PTP1B phosphatase activity as well as the association of PTP1B with IRβ. We suggest that COP1 is a positive regulator of hepatic insulin signaling.

Role of TRIB3 in regulation of insulin sensitivity and nutrient metabolism during short-term fasting and nutrient excess

We have suggested previously that Tribbles homolog 3 (TRIB3), a negative regulator of Akt activity in insulin-sensitive tissues, could mediate glucose-induced insulin resistance in muscle under conditions of chronic hyperglycemia (Liu J, Wu X, Franklin JL, Messina JL, Hill HS, Moellering DR, Walton RG, Martin M, Garvey WT. Am J Physiol Endocrinol Metab 298: E565-E576, 2010). In the current study, we have assessed short-term physiological regulation of TRIB3 in skeletal muscle and adipose tissues by nutrient excess and fasting as well as TRIB3's ability to modulate glucose transport and mitochondrial oxidation. In Sprague-Dawley rats, we found that short-term fasting enhanced insulin sensitivity concomitantly with decrements in TRIB3 mRNA (66%, P < 0.05) and protein (81%, P < 0.05) in muscle and increments in TRIB3 mRNA (96%, P < 0.05) and protein (~10-fold, P < 0.05) in adipose tissue compared with nonfasted controls. On the other hand, rats fed a Western diet for 7 days became insulin resistant concomitantly with increments in TRIB3 mRNA (155%, P < 0.05) and protein (69%, P = 0.0567) in muscle and a decrease in the mRNA (76%, P < 0.05) and protein (70%, P < 0.05) in adipose. In glucose transport and mitochondria oxidation studies using skeletal muscle cells, we found that stable TRIB3 overexpression impaired insulin-stimulated glucose uptake without affecting basal glucose transport and increased both basal glucose oxidation and the maximal uncoupled oxygen consumption rate. With stable knockdown of TRIB3, basal and insulin-stimulated glucose transport rates were increased, whereas basal glucose oxidation and the maximal uncoupled oxygen consumption rate were decreased. In conclusion, TRIB3 impacts glucose uptake and oxidation oppositely in muscle and fat according to levels of nutrient availability. The above data for the first time implicate TRIB3 as a potent physiological regulator of insulin sensitivity and mitochondrial glucose oxidation under conditions of nutrient deprivation and excess.

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.

The contribution of endoplasmic reticulum stress to liver diseases

The unfolded protein response (UPR) is an evolutionarily conserved cell signaling pathway that is activated to regulate protein synthesis and restore homeostatic equilibrium when the cell is stressed from increased client protein load or the accumulation of unfolded or malfolded proteins. Once activated, this signaling pathway can either result in the recovery of homeostasis or can activate a cascade of events that ultimately result in cell death. The UPR/endoplasmic reticulum (ER) stress response spectrum and its interplay with other cellular organelles play an important role in the pathogenesis of disease in secretory cells rich in ER, such as hepatocytes. Over the past 2 decades, the contribution of ER stress to various forms of liver diseases has been examined. Robust support for a contributing, as opposed to a secondary role, for ER stress response is seen in the nonalcoholic steatohepatitis, alcoholic liver disease, ischemia/reperfusion injury, and cholestatic models of liver disease. The exact direction of the cause and effect relationship between modes of cell injury and ER stress remains elusive. It is apparent that a complex interplay exists between ER stress response, conditions that promote it, and those that result from it. A vicious cycle in which ER stress promotes inflammation, cell injury, and steatosis and in which steatogenesis, inflammation, and cell injury aggravate ER stress seems to be at play. It is perhaps the nature of such a vicious cycle that is the key pathophysiologic concept. Therapeutic approaches aimed at interrupting the cycle may dampen the stress response and the ensuing injury.

Knockdown of the gene encoding Drosophila tribbles homologue 3 (Trib3) improves insulin sensitivity through peroxisome proliferator-activated receptor-γ (PPAR-γ) activation in a rat model of insulin resistance

AIMS/HYPOTHESIS

Insulin action is purportedly modulated by Drosophila tribbles homologue 3 (TRIB3), which in vitro prevents thymoma viral proto-oncogene (AKT) and peroxisome proliferator-activated receptor-γ (PPAR-γ) activation. However, the physiological impact of TRIB3 action in vivo remains controversial.

METHODS

We investigated the role of TRIB3 in rats treated with either a control or Trib3 antisense oligonucleotide (ASO). Tissue-specific insulin sensitivity was assessed in vivo using a euglycaemic-hyperinsulinaemic clamp. A separate group was treated with the PPAR-γ antagonist bisphenol-A-diglycidyl ether (BADGE) to assess the role of PPAR-γ in mediating the response to Trib3 ASO.

RESULTS

Trib3 ASO treatment specifically reduced Trib3 expression by 70% to 80% in liver and white adipose tissue. Fasting plasma glucose, insulin concentrations and basal rate of endogenous glucose production were unchanged. However, Trib3 ASO increased insulin-stimulated whole-body glucose uptake by ~50% during the euglycaemic-hyperinsulinaemic clamp. This was attributable to improved skeletal muscle glucose uptake. Despite the reduction of Trib3 expression, AKT2 activity was not increased. Trib3 ASO increased white adipose tissue mass by 70% and expression of Ppar-γ and its key target genes, raising the possibility that Trib3 ASO improves insulin sensitivity primarily in a PPAR-γ-dependent manner. Co-treatment with BADGE blunted the expansion of white adipose tissue and abrogated the insulin-sensitising effects of Trib3 ASO. Finally, Trib3 ASO also increased plasma HDL-cholesterol, a change that persisted with BADGE co-treatment.

CONCLUSIONS/INTERPRETATION

These data suggest that TRIB3 inhibition improves insulin sensitivity in vivo primarily in a PPAR-γ-dependent manner and without any change in AKT2 activity.

The anti-inflammatory and immunosuppressive effects of glucocorticoids, recent developments and mechanistic insights

Since the discovery of glucocorticoids in the 1940s and the recognition of their anti-inflammatory effects, they have been amongst the most widely used and effective treatments to control inflammatory and autoimmune diseases. However, their clinical efficacy is compromised by the metabolic effects of long-term treatment, which include osteoporosis, hypertension, dyslipidaemia and insulin resistance/type 2 diabetes mellitus. In recent years, a great deal of effort has been invested in identifying compounds that separate the beneficial anti-inflammatory effects from the adverse metabolic effects of glucocorticoids, with limited effect. It is clear that for these efforts to be effective, a greater understanding is required of the mechanisms by which glucocorticoids exert their anti-inflammatory and immunosuppressive actions. Recent research is shedding new light on some of these mechanisms and has produced some surprising new findings. Some of these recent developments are reviewed here.

Amino acid availability controls TRB3 transcription in liver through the GCN2/eIF2α/ATF4 pathway

In mammals, plasma amino acid concentrations are markedly affected by dietary or pathological conditions. It has been well established that amino acids are involved in the control of gene expression. Up to now, all the information concerning the molecular mechanisms involved in the regulation of gene transcription by amino acid availability has been obtained in cultured cell lines. The present study aims to investigate the mechanisms involved in transcriptional activation of the TRB3 gene following amino acid limitation in mice liver. The results show that TRB3 is up-regulated in the liver of mice fed a leucine-deficient diet and that this induction is quickly reversible. Using transient transfection and chromatin immunoprecipitation approaches in hepatoma cells, we report the characterization of a functional Amino Acid Response Element (AARE) in the TRB3 promoter and the binding of ATF4, ATF2 and C/EBPβ to this AARE sequence. We also provide evidence that only the binding of ATF4 to the AARE plays a crucial role in the amino acid-regulated transcription of TRB3. In mouse liver, we demonstrate that the GCN2/eIF2α/ATF4 pathway is essential for the induction of the TRB3 gene transcription in response to a leucine-deficient diet. Therefore, this work establishes for the first time that the molecular mechanisms involved in the regulation of gene transcription by amino acid availability are functional in mouse liver.

Control of adipose tissue inflammation through TRB1

OBJECTIVE

Based on its role as an energy storage compartment and endocrine organ, white adipose tissue (WAT) fulfills a critical function in the maintenance of whole-body energy homeostasis. Indeed, WAT dysfunction is connected to obesity-related type 2 diabetes triggered at least partly by an inflammatory response in adipocytes. The pseudokinase tribbles (TRB) 3 has been identified by us and others as a critical regulator of hepatic glucose homeostasis in type 2 diabetes and WAT lipid homeostasis. Therefore, this study aimed to test the hypothesis that the TRB gene family fulfills broader functions in the integration of metabolic and inflammatory pathways in various tissues.

RESEARCH DESIGN AND METHODS

To determine the role of TRB family members for WAT function, we profiled the expression patterns of TRB13 under healthy and metabolic stress conditions. The differentially expressed TRB1 was functionally characterized in loss-of-function animal and primary adipocyte models.

RESULTS

Here, we show that the expression of TRB1 was specifically upregulated during acute and chronic inflammation in WAT of mice. Deficiency of TRB1 was found to impair cytokine gene expression in white adipocytes and to protect against high-fat diet-induced obesity. In adipocytes, TRB1 served as a nuclear transcriptional coactivator for the nuclear factor kappaB subunit RelA, thereby promoting the induction of proinflammatory cytokines in these cells.

CONCLUSIONS

As inflammation is typically seen in sepsis, insulin resistance, and obesity-related type 2 diabetes, the dual role of TRB1 as both a target and a (co) activator of inflammatory signaling might provide a molecular rationale for the amplification of proinflammatory responses in WAT in these subjects.