TRIB3 increases cell resistance to arsenite toxicity by limiting the expression of the glutathione-degrading enzyme CHAC1

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.

Genome-wide census of ATF4 binding sites and functional profiling of trait-associated genetic variants overlapping ATF4 binding motifs

Activating Transcription Factor 4 (ATF4) is an important regulator of gene expression in stress responses and developmental processes in many cell types. Here, we catalogued ATF4 binding sites in the human genome and identified overlaps with trait-associated genetic variants. We probed these genetic variants for allelic regulatory activity using a massively parallel reporter assay (MPRA) in HepG2 hepatoma cells exposed to tunicamycin to induce endoplasmic reticulum stress and ATF4 upregulation. The results revealed that in the majority of cases, the MPRA allelic activity of these SNPs was in agreement with the nucleotide preference seen in the ATF4 binding motif from ChIP-Seq. Luciferase and electrophoretic mobility shift assays in additional cellular models further confirmed ATF4-dependent regulatory effects for the SNPs rs532446 (GADD45A intronic; linked to hematological parameters), rs7011846 (LPL upstream; myocardial infarction), rs2718215 (diastolic blood pressure), rs281758 (psychiatric disorders) and rs6491544 (educational attainment). CRISPR-Cas9 disruption and/or deletion of the regulatory elements harboring rs532446 and rs7011846 led to the downregulation of GADD45A and LPL, respectively. Thus, these SNPs could represent examples of GWAS genetic variants that affect gene expression by altering ATF4-mediated transcriptional activation.

CHAC1 exacerbates arsenite cytotoxicity by lowering intracellular glutathione levels

We here examined whether CHAC1 is implicated in arsenite (As(III))-induced cytotoxicity in HaCaT cells. We found that HaCaT cells in which the intracellular GSH levels were elevated by transfection with CHAC1 siRNA showed decreased sensitivity to As(III) compared to the control cells. Treatment with BSO (an inhibitor of GSH biosynthesis) abolished the decrease in sensitivity to As(III), suggesting that an increase in intracellular GSH levels was involved in the decrease in sensitivity to As(III) due to the decrease in the levels of CHAC1 expression. When we examined the expression of CHAC1 after exposure of HaCaT cells to As(III), the levels of CHAC1 were increased. Since CHAC1 is a proapoptotic factor, we examined appearance of apoptotic cells and cleavage of caspase-3 after exposure to As(III) to determine whether As(III)-induced CHAC1 up-regulation was involved in apoptosis induction. The results showed that induction of apoptosis by As(III) exposure was not detected in CHAC1 siRNA-transfected cells. Together, our findings indicate that CHAC1 is involved in the sensitivity of HaCaT cells to As(III) by regulating the intracellular GSH levels, and in particular, CHAC1 is involved in As(III)-induced apoptosis.

Transcriptomic analyses of gastrulation-stage mouse embryos with differential susceptibility to alcohol

Genetics are a known contributor to differences in alcohol sensitivity in humans with fetal alcohol spectrum disorders (FASDs) and in animal models. Our study profiled gene expression in gastrulation-stage embryos from two commonly used, genetically similar mouse substrains, C57BL/6J (6J) and C57BL/6NHsd (6N), that differ in alcohol sensitivity. First, we established normal gene expression patterns at three finely resolved time points during gastrulation and developed a web-based interactive tool. Baseline transcriptional differences across strains were associated with immune signaling. Second, we examined the gene networks impacted by alcohol in each strain. Alcohol caused a more pronounced transcriptional effect in the 6J versus 6N mice, matching the increased susceptibility of the 6J mice. The 6J strain exhibited dysregulation of pathways related to cell death, proliferation, morphogenic signaling and craniofacial defects, while the 6N strain showed enrichment of hypoxia and cellular metabolism pathways. These datasets provide insight into the changing transcriptional landscape across mouse gastrulation, establish a valuable resource that enables the discovery of candidate genes that may modify alcohol susceptibility that can be validated in humans, and identify novel pathogenic mechanisms of alcohol. This article has an associated First Person interview with the first author of the paper.

Pharmacological or TRIB3-Mediated Suppression of ATF4 Transcriptional Activity Promotes Hepatoma Cell Resistance to Proteasome Inhibitor Bortezomib

Simple Summary

Proteasome inhibitors are currently used in the treatment of certain blood cancers, and clinical trials to treat solid tumors, including liver cancer, have also been conducted. However, different malignancies are not equally susceptible to proteasome inhibitors, and resistance to the drug may develop during the therapy. Here, we characterize the molecular mechanisms underlying the resilience of liver cancer cells to the proteasome inhibitor bortezomib. The results demonstrate that the activity of the eIF2α–ATF4 stress response pathway affects the viability of cells treated with bortezomib. We found that the pseudokinase TRIB3, an endogenous regulator of ATF4 and a gene highly expressed in liver cancer, resides predominantly at the same chromatin sites as ATF4 and constrains ATF4 activity. The survival of bortezomib-exposed hepatoma cells proved sensitive to TRIB3 overexpression and inactivation. Thus, TRIB3 is a novel factor contributing to bortezomib resistance of liver cancer cells.

Abstract

The proteasome is an appealing target for anticancer therapy and the proteasome inhibitor bortezomib has been approved for the treatment of several types of malignancies. However, the molecular mechanisms underlying cancer cell resistance to bortezomib remain poorly understood. In the current article, we investigate how modulation of the eIF2α–ATF4 stress pathway affects hepatoma cell response to bortezomib. Transcriptome profiling revealed that many ATF4 transcriptional target genes are among the most upregulated genes in bortezomib-treated HepG2 human hepatoma cells. While pharmacological enhancement of the eIF2α–ATF4 pathway activity results in the elevation of the activities of all branches of the unfolded protein response (UPR) and sensitizes cells to bortezomib toxicity, the suppression of ATF4 induction delays bortezomib-induced cell death. The pseudokinase TRIB3, an inhibitor of ATF4, is expressed at a high basal level in hepatoma cells and is strongly upregulated in response to bortezomib. To map genome-wide chromatin binding loci of TRIB3 protein, we fused a Flag tag to endogenous TRIB3 in HepG2 cells and performed ChIP-Seq. The results demonstrate that TRIB3 predominantly colocalizes with ATF4 on chromatin and binds to genomic regions containing the C/EBP–ATF motif. Bortezomib treatment leads to a robust enrichment of TRIB3 binding near genes induced by bortezomib and involved in the ER stress response and cell death. Disruption of TRIB3 increases C/EBP–ATF-driven transcription, augments ER stress and cell death upon exposure to bortezomib, while TRIB3 overexpression enhances cell survival. Thus, TRIB3, colocalizing with ATF4 and limiting its transcriptional activity, functions as a factor increasing resistance to bortezomib, while pharmacological over-activation of eIF2α–ATF4 can overcome the endogenous restraint mechanisms and sensitize cells to bortezomib.

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.

A human-specific VNTR in the TRIB3 promoter causes gene expression variation between individuals

Tribbles homolog 3 (TRIB3) is pseudokinase involved in intracellular regulatory processes and has been implicated in several diseases. In this article, we report that human TRIB3 promoter contains a 33-bp variable number tandem repeat (VNTR) and characterize the heterogeneity and function of this genetic element. Analysis of human populations around the world uncovered the existence of alleles ranging from 1 to 5 copies of the repeat, with 2-, 3- and 5-copy alleles being the most common but displaying considerable geographical differences in frequency. The repeated sequence overlaps a C/EBP-ATF transcriptional regulatory element and is highly conserved, but not repeated, in various mammalian species, including great apes. The repeat is however evident in Neanderthal and Denisovan genomes. Reporter plasmid experiments in human cell culture reveal that an increased copy number of the TRIB3 promoter 33-bp repeat results in increased transcriptional activity. In line with this, analysis of whole genome sequencing and RNA-Seq data from human cohorts demonstrates that the copy number of TRIB3 promoter 33-bp repeats is positively correlated with TRIB3 mRNA expression level in many tissues throughout the body. Moreover, the copy number of the TRIB3 33-bp repeat appears to be linked to known TRIB3 eQTL SNPs as well as TRIB3 SNPs reported in genetic association studies. Taken together, the results indicate that the promoter 33-bp VNTR constitutes a causal variant for TRIB3 expression variation between individuals and could underlie the results of SNP-based genetic studies.

Impaired AKT signaling and lung tumorigenesis by PIERCE1 ablation in KRAS-mutant non-small cell lung cancer

KRAS-mutant non-small cell lung cancer (NSCLC) is a major lung cancer subtype that leads to many cancer-related deaths worldwide. Although numerous studies on KRAS-mutant type NSCLC have been conducted, new oncogenic or tumor suppressive genes need to be detected because a large proportion of NSCLC patients does not respond to currently used therapeutics. Here, we show the tumor-promoting function of a cell cycle-related protein, PIERCE1, in KRAS-mutant NSCLC. Mechanistically, PIERCE1 depletion inhibits cell growth and AKT phosphorylation (pAKT) at S473, which is particularly observed in KRAS-mutant lung cancers. Analyses of AKT-related genes using microarray, immunoblotting, and real-time quantitative PCR indicated that PIERCE1 negatively regulates the gene expression of the AKT suppressor, TRIB3, through the CHOP pathway, which is a key regulatory pathway for TRIB3 expression. Similarly, in vivo analyses of PIERCE1 depletion in the KRAS mutation-related lung cancer mouse models revealed the suppressive effect of PIERCE1 knockout in urethane- and KRAS^G12D-induced lung tumorigenesis with decreased pAKT levels observed in the tumors. Tissue microarrays of human lung cancers indicated the expression of PIERCE1 in 83% of lung cancers and its correlation with pAKT expression. Thus, we illustrate how PIERCE1 depletion may serve as a therapeutic strategy against KRAS-mutant NSCLC and propose the clinical benefit of PIERCE1.

TRIB3 limits FGF21 induction during in vitro and in vivo nutrient deficiencies by inhibiting C/EBP-ATF response elements in the Fgf21 promoter

Mammals must be able to endure periods of limited food availability, and the liver plays a central role in the adaptation to nutritional stresses. TRIB3 (Tribbles homolog 3) is a cellular stress-inducible gene with a liver-centric expression pattern and it has been implicated in stress response regulation and metabolic control. In the current article, we study the involvement of TRIB3 in responses to nutrient deficiencies, including fasting for up to 48 h in mice. We show that hepatic expression of Trib3 is increased after 48 h of fasting and mice with a targeted deletion of the Trib3 gene present elevated hepatic triglyceride content and liver weight at 48 h, along with an upregulation of lipid utilization genes in the liver. Further, hepatic and serum levels of the metabolic stress hormone FGF21 are considerably increased in 48-h-fasted Trib3 knockout mice compared to wild type. Trib3 deficiency also leads to elevated FGF21 levels in the mouse liver during essential amino acid deficiency and in cultured mouse embryonic fibroblasts during glucose starvation. Reporter assays reveal that TRIB3 regulates FGF21 by inhibiting ATF4-mediated, C/EBP-ATF site-dependent activation of Fgf21 transcription. Based on chromatin immunoprecipitation from mouse liver, the binding of TRIB3 and ATF4, a transcription factor known to physically interact with TRIB3, is significantly increased at the Fgf21 promoter following 48 h of fasting. Thus, under nutrient-limiting conditions that stimulate ATF4 activity, TRIB3 is implicated in the regulation of metabolic adaptation by restraining the transcription of Fgf21.

Glutathione Degradation

SIGNIFICANCE

Glutathione degradation has for long been thought to occur only on noncytosolic pools. This is because there has been only one enzyme known to degrade glutathione (γ-glutamyl transpeptidase) and this localizes to either the plasma membrane (mammals, bacteria) or the vacuolar membrane (yeast, plants) and acts on extracellular or vacuolar pools. The last few years have seen the discovery of several new enzymes of glutathione degradation that function in the cytosol, throwing new light on glutathione degradation. Recent Advances: The new enzymes that have been identified in the last few years that can initiate glutathione degradation include the Dug enzyme found in yeast and fungi, the ChaC1 enzyme found among higher eukaryotes, the ChaC2 enzyme found from bacteria to man, and the RipAY enzyme found in some bacteria. These enzymes play roles ranging from housekeeping functions to stress responses and are involved in processes such as embryonic neural development and pathogenesis.

CRITICAL ISSUES

In addition to delineating the pathways of glutathione degradation in detail, a critical issue is to find how these new enzymes impact cellular physiology and homeostasis.

FUTURE DIRECTIONS

Glutathione degradation plays a far greater role in cellular physiology than previously envisaged. The differential regulation and differential specificities of various enzymes, each acting on distinct pools, can lead to different consequences to the cell. It is likely that the coming years will see these downstream effects being unraveled in greater detail and will lead to a better understanding and appreciation of glutathione degradation. Antioxid. Redox Signal. 27, 1200-1216.