Inflammation is a pathological hallmark associated with bacterial and viral infections, autoimmune diseases, genetic disorders, obesity and diabetes, as well as environmental stresses including ...physical and chemical trauma. Among numerous proteins regulating proinflammatory signaling, very few such as Protein kinase R (PKR), have been shown to play an all‐pervading role in inflammation induced by varied stimuli. PKR was initially characterized as an interferon‐inducible gene activated by viral double‐stranded RNA with a role in protein translation inhibition. However, it has become increasingly clear that PKR is involved in multiple pathways that promote inflammation in response to stress activation, both dependent on and independent of its cellular protein activator of PKR (PACT). In this review, we discuss the signaling pathways that contribute to the initiation of inflammation, including Toll‐like receptor, interferon, and RIG‐I‐like receptor signaling, as well as inflammasome activation. We go on to discuss the specific roles that PKR and PACT play in such proinflammatory signaling, as well as in metabolic syndrome‐ and environmental stress‐induced inflammation.
Inflammation is the body's protective response against injury, infection, and stress. Several cellular proteins play specific roles in the inflammatory response to particular stimuli. Interestingly, some proteins such as protein kinase R (PKR) and the protein activator of PKR (PACT) play diverse roles in mediating the cellular inflammatory response to various stimuli. This review summarizes the current knowledge on the diverse roles of the proteins PKR and PACT in inflammation and their implications in select diseases.
Plasticity of neoplasia, whereby cancer cells attain stem-cell-like properties, is required for disease progression and represents a major therapeutic challenge. We report that in breast cancer cells ...NANOG, SNAIL and NODAL transcripts manifest multiple isoforms characterized by different 5' Untranslated Regions (5'UTRs), whereby translation of a subset of these isoforms is stimulated under hypoxia. The accumulation of the corresponding proteins induces plasticity and "fate-switching" toward stem cell-like phenotypes. Mechanistically, we observe that mTOR inhibitors and chemotherapeutics induce translational activation of a subset of NANOG, SNAIL and NODAL mRNA isoforms akin to hypoxia, engendering stem-cell-like phenotypes. These effects are overcome with drugs that antagonize translational reprogramming caused by eIF2α phosphorylation (e.g. ISRIB), suggesting that the Integrated Stress Response drives breast cancer plasticity. Collectively, our findings reveal a mechanism of induction of plasticity of breast cancer cells and provide a molecular basis for therapeutic strategies aimed at overcoming drug resistance and abrogating metastasis.
Cancer cells often require glutamine for growth, thereby distinguishing them from most normal cells. Here we show that PIK3CA mutations reprogram glutamine metabolism by upregulating glutamate ...pyruvate transaminase 2 (GPT2) in colorectal cancer (CRC) cells, making them more dependent on glutamine. Compared with isogenic wild-type (WT) cells, PIK3CA mutant CRCs convert substantially more glutamine to α-ketoglutarate to replenish the tricarboxylic acid cycle and generate ATP. Mutant p110α upregulates GPT2 gene expression through an AKT-independent, PDK1-RSK2-ATF4 signalling axis. Moreover, aminooxyacetate, which inhibits the enzymatic activity of aminotransferases including GPT2, suppresses xenograft tumour growth of CRCs with PIK3CA mutations, but not with WT PIK3CA. Together, these data establish oncogenic PIK3CA mutations as a cause of glutamine dependency in CRCs and suggest that targeting glutamine metabolism may be an effective approach to treat CRC patients harbouring PIK3CA mutations.
Certain stress conditions can induce cleavage of tRNAs around the anticodon loop via the use of the ribonuclease angiogenin. The cellular factors that regulate tRNA cleavage are not well known. In ...this study we used normal and eIF2α phosphorylation-deficient mouse embryonic fibroblasts and applied a microarray-based methodology to identify and compare tRNA cleavage patterns in response to hypertonic stress, oxidative stress (arsenite), and treatment with recombinant angiogenin. In all three scenarios mouse embryonic fibroblasts deficient in eIF2α phosphorylation showed a higher accumulation of tRNA fragments including those derived from initiator-tRNAMet. We have shown that tRNA cleavage is regulated by the availability of angiogenin, its substrate (tRNA), the levels of the angiogenin inhibitor RNH1, and the rates of protein synthesis. These conclusions are supported by the following findings: (i) exogenous treatment with angiogenin or knockdown of RNH1 increased tRNA cleavage; (ii) tRNA fragment accumulation was higher during oxidative stress than hypertonic stress, in agreement with a dramatic decrease of RNH1 levels during oxidative stress; and (iii) a positive correlation was observed between angiogenin-mediated tRNA cleavage and global protein synthesis rates. Identification of the stress-specific tRNA cleavage mechanisms and patterns will provide insights into the role of tRNA fragments in signaling pathways and stress-related disorders.
Background: Regulation of stress-induced tRNA cleavage by angiogenin is not well studied.
Results: tRNA fragment accumulation was higher during oxidative than hypertonic stress.
Conclusion: tRNA cleavage is regulated by the availability of angiogenin and tRNA substrate, levels of RNH1, and the rates of protein synthesis.
Significance: Stress-specific tRNA cleavage mechanisms and patterns will provide insights into novel stress signaling pathways.
The sulfhydration of cysteine residues in proteins is an important mechanism involved in diverse biological processes. We have developed a proteomics approach to quantitatively profile the changes of ...sulfhydrated cysteines in biological systems. Bioinformatics analysis revealed that sulfhydrated cysteines are part of a wide range of biological functions. In pancreatic β cells exposed to endoplasmic reticulum (ER) stress, elevated H2S promotes the sulfhydration of enzymes in energy metabolism and stimulates glycolytic flux. We propose that transcriptional and translational reprogramming by the integrated stress response (ISR) in pancreatic β cells is coupled to metabolic alternations triggered by sulfhydration of key enzymes in intermediary metabolism.
Eukaryotic initiation factor 2A (eIF2A) is a 65 kDa protein that functions in minor initiation pathways, which affect the translation of only a subset of messenger ribonucleic acid (mRNAs), such as ...internal ribosome entry site (IRES)‐containing mRNAs and/or mRNAs harboring upstream near cognate/non‐AUG start codons. These non‐canonical initiation events are important for regulation of protein synthesis during cellular development and/or the integrated stress response. Selective eIF2A knockdown in cellular systems significantly inhibits translation of such mRNAs, which rely on alternative initiation mechanisms for their translation. However, there exists a gap in our understanding of how eIF2A functions in mammalian systems in vivo (on the organismal level) and ex vivo (in cells). Here, using an eIF2A‐knockout (KO) mouse model, we present evidence implicating eIF2A in the biology of aging, metabolic syndrome and central tolerance. We discovered that eIF2A‐KO mice have reduced life span and that eIF2A plays an important role in maintenance of lipid homeostasis, the control of glucose tolerance, insulin resistance and also reduces the abundance of B lymphocytes and dendritic cells in the thymic medulla of mice. We also show the eIF2A KO affects male and female mice differently, suggesting that eIF2A may affect sex‐specific pathways. Interestingly, our experiments involving pharmacological induction of endoplasmic reticulum (ER) stress with tunicamycin did not reveal any substantial difference between the response to ER stress in eIF2A‐KO and wild‐type mice. The identification of eIF2A function in the development of metabolic syndrome bears promise for the further identification of specific eIF2A targets responsible for these changes.
Adaptation to changes in extracellular tonicity is essential for cell survival. However, severe or chronic hyperosmotic stress induces apoptosis, which involves cytochrome c (Cyt c) release from ...mitochondria and subsequent apoptosome formation. Here, we show that angiogenin-induced accumulation of tRNA halves (or tiRNAs) is accompanied by increased survival in hyperosmotically stressed mouse embryonic fibroblasts. Treatment of cells with angiogenin inhibits stress-induced formation of the apoptosome and increases the interaction of small RNAs with released Cyt c in a ribonucleoprotein (Cyt c-RNP) complex. Next-generation sequencing of RNA isolated from the Cyt c-RNP complex reveals that 20 tiRNAs are highly enriched in the Cyt c-RNP complex. Preferred components of this complex are 5′ and 3′ tiRNAs of specific isodecoders within a family of isoacceptors. We also demonstrate that Cyt c binds tiRNAs in vitro, and the pool of Cyt c-interacting RNAs binds tighter than individual tiRNAs. Finally, we show that angiogenin treatment of primary cortical neurons exposed to hyperosmotic stress also decreases apoptosis. Our findings reveal a connection between angiogenin-generated tiRNAs and cell survival in response to hyperosmotic stress and suggest a novel cellular complex involving Cyt c and tiRNAs that inhibits apoptosome formation and activity.
Pancreatic β-cells are prone to endoplasmic reticulum (ER) stress due to their role in insulin secretion. They require sustainable and efficient adaptive stress responses to cope with this stress. ...Whether episodes of chronic stress directly compromise β-cell identity is unknown. We show here under reversible, chronic stress conditions β-cells undergo transcriptional and translational reprogramming associated with impaired expression of regulators of β-cell function and identity. Upon recovery from stress, β-cells regain their identity and function, indicating a high degree of adaptive plasticity. Remarkably, while β-cells show resilience to episodic ER stress, when episodes exceed a threshold, β-cell identity is gradually lost. Single cell RNA-sequencing analysis of islets from type 1 diabetes patients indicates severe deregulation of the chronic stress-adaptation program and reveals novel biomarkers of diabetes progression. Our results suggest β-cell adaptive exhaustion contributes to diabetes pathogenesis.
The integrated stress response (ISR) is a homeostatic mechanism induced by endoplasmic reticulum (ER) stress. In acute/transient ER stress, decreased global protein synthesis and increased uORF mRNA ...translation are followed by normalization of protein synthesis. Here, we report a dramatically different response during chronic ER stress. This chronic ISR program is characterized by persistently elevated uORF mRNA translation and concurrent gene expression reprogramming, which permits simultaneous stress sensing and proteostasis. The program includes PERK-dependent switching to an eIF3-dependent translation initiation mechanism, resulting in partial, but not complete, translational recovery, which, together with transcriptional reprogramming, selectively bolsters expression of proteins with ER functions. Coordination of transcriptional and translational reprogramming prevents ER dysfunction and inhibits “foamy cell” development, thus establishing a molecular basis for understanding human diseases associated with ER dysfunction.
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•Protein synthesis during chronic ER stress is independent of eIF2B GEF activity•mRNA translation during chronic ISR is mediated by an eIF3-dependent mechanism•Coordination of transcriptional and translational reprogramming signifies chronic ISR•ER function in chronic ISR relies on PERK-dependent translational reprograming
Guan et al. unravel the mechanism of adaptation to chronic stress that encompasses previously unappreciated remodeling of the translation initiation machinery guided by PERK. These changes in the translation machinery are coordinated with stress-induced transcriptional reprograming and, when disrupted, result in a foamy cell phenotype and cell death.
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