Basal levels of eIF2alpha phosphorylation determine cellular antioxidant status by regulating ATF4 and xCT expression

Regulation of placental amino acid transport in health and disease

Abnormal fetal growth, i.e., intrauterine growth restriction (IUGR) or fetal growth restriction (FGR) and fetal overgrowth, is associated with increased perinatal morbidity and mortality and is strongly linked to the development of metabolic and cardiovascular disease in childhood and later in life. Emerging evidence suggests that changes in placental amino acid transport may contribute to abnormal fetal growth. This review is focused on amino acid transport in the human placenta, however, relevant animal models will be discussed to add mechanistic insights. At least 25 distinct amino acid transporters with different characteristics and substrate preferences have been identified in the human placenta. Of these, System A, transporting neutral nonessential amino acids, and System L, mediating the transport of essential amino acids, have been studied in some detail. Importantly, decreased placental Systems A and L transporter activity is strongly associated with IUGR and increased placental activity of these two amino acid transporters has been linked to fetal overgrowth in human pregnancy. An array of factors in the maternal circulation, including insulin, IGF-1, and adiponectin, and placental signaling pathways such as mTOR, have been identified as key regulators of placental Systems A and L. Studies using trophoblast-specific gene targeting in mice have provided compelling evidence that changes in placental Systems A and L are mechanistically linked to altered fetal growth. It is possible that targeting specific placental amino acid transporters or their upstream regulators represents a novel intervention to alleviate the short- and long-term consequences of abnormal fetal growth in the future.

Sensing of H2O2-induced oxidative stress by the UPF factor complex is crucial for activation of catalase-3 expression in Neurospora

UPF-1-UPF-2-UPF-3 complex-orchestrated nonsense-mediated mRNA decay (NMD) is a well-characterized eukaryotic cellular surveillance mechanism that not only degrades aberrant transcripts to protect the integrity of the transcriptome but also eliminates normal transcripts to facilitate appropriate cellular responses to physiological and environmental changes. Here, we describe the multifaceted regulatory roles of the Neurospora crassa UPF complex in catalase-3 (cat-3) gene expression, which is essential for scavenging H2O2-induced oxidative stress. First, losing UPF proteins markedly slowed down the decay rate of cat-3 mRNA. Second, UPF proteins indirectly attenuated the transcriptional activity of cat-3 gene by boosting the decay of cpc-1 and ngf-1 mRNAs, which encode a well-studied transcription factor and a histone acetyltransferase, respectively. Further study showed that under oxidative stress condition, UPF proteins were degraded, followed by increased CPC-1 and NGF-1 activity, finally activating cat-3 expression to resist oxidative stress. Together, our data illustrate a sophisticated regulatory network of the cat-3 gene mediated by the UPF complex under physiological and H2O2-induced oxidative stress conditions.

GADD34 suppresses eIF2α phosphorylation and improves cognitive function in Alzheimer's disease-model mice

Alzheimer's disease (AD) causes neurodegeneration, leading to cognitive impairment and memory loss. Our previous studies have demonstrated that the induction of growth arrest and DNA damage-inducible gene 34 (GADD34) by quercetin can affect eukaryotic translation initiation factor 2α (eIF2α) phosphorylation-activated transcription factor 4 (ATF4) signaling. However, the relationship between GADD34 expression and cognitive function has not been clarified. In this study, we determined the direct effect of GADD34 on memory. To achieve this, truncated GADD34 (GADD34.5) was injected into the mouse brain to suppress eIF2α phosphorylation and evaluate memory. The injection of GADD34.5 into the hippocampus in AD-model mice did not improve novel object recognition but improved novel object location. The injection of GADD34.5 into the amygdala also resulted in the maintenance of contextual fear memory based on the fear condition test. These results suggest that GADD34 is effective in improving memory for spatial cognition and contextual fear conditioning in AD by inhibiting eIF2α phosphorylation. In summary, GADD34 suppresses eIF2α phosphorylation in the brain and prevents memory loss. As quercetin feeding increases GADD34 expression, it might be used in preventative applications for AD.

Protein transport from pre- and postsynapse to the nucleus: Mechanisms and functional implications

The extreme length of neuronal processes poses a challenge for synapse-to-nucleus communication. In response to this challenge several different mechanisms have evolved in neurons to couple synaptic activity to the regulation of gene expression. One of these mechanisms concerns the long-distance transport of proteins from pre- and postsynaptic sites to the nucleus. In this review we summarize current evidence on mechanisms of transport and consequences of nuclear import of these proteins for gene transcription. In addition, we discuss how information from pre- and postsynaptic sites might be relayed to the nucleus by this type of long-distance signaling. When applicable, we highlight how long-distance protein transport from synapse-to-nucleus can provide insight into the pathophysiology of disease or reveal new opportunities for therapeutic intervention.

Cooperation effects of radiation and ferroptosis on tumor suppression and radiation injury

Ferroptosis is a kind of oxidative stress-dependent cell death characterized by iron accumulation and lipid peroxidation. It can work in conjunction with radiation to increase reactive oxygen species (ROS) generation and disrupt the antioxidant system, suppressing tumor progression. Radiation can induce ferroptosis by creating ROS, depleting glutathione, activating genes linked to DNA damage and increasing the expression of acyl-CoA synthetase long-chain family member 4 (ACSL4) in tumor cells. Furthermore, ferroptosis can enhance radiosensitivity by causing an iron overload, destruction of the antioxidant system, and lipid peroxidation. Radiation can also cause ferroptosis in normal cells, resulting in radiation injury. The role of ferroptosis in radiation-induced lung, intestinal, skin, and hematological injuries have been studied. In this review, we summarize the potential mechanisms linking ferroptosis, oxidative stress and radiation; analyze the function of ferroptosis in tumor suppression and radiation injury; and discuss the potential of ferroptosis regulation to improve radiotherapy efficacy and reduce adverse effects.

Neuron-specific translational control shift ensures proteostatic resilience during ER stress

Proteostasis is essential for cellular survival and particularly important for highly specialised post‐mitotic cells such as neurons. Transient reduction in protein synthesis by protein kinase R‐like endoplasmic reticulum (ER) kinase (PERK)‐mediated phosphorylation of eukaryotic translation initiation factor 2α (p‐eIF2α) is a major proteostatic survival response during ER stress. Paradoxically, neurons are remarkably tolerant to PERK dysfunction, which suggests the existence of cell type‐specific mechanisms that secure proteostatic stress resilience. Here, we demonstrate that PERK‐deficient neurons, unlike other cell types, fully retain the capacity to control translation during ER stress. We observe rescaling of the ATF4 response, while the reduction in protein synthesis is fully retained. We identify two molecular pathways that jointly drive translational control in PERK‐deficient neurons. Haem‐regulated inhibitor (HRI) mediates p‐eIF2α and the ATF4 response and is complemented by the tRNA cleaving RNase angiogenin (ANG) to reduce protein synthesis. Overall, our study elucidates an intricate back‐up mechanism to ascertain translational control during ER stress in neurons that provides a mechanistic explanation for the thus far unresolved observation of neuronal resilience to proteostatic stress.

The Role of SLC7A11 in Cancer: Friend or Foe?

SLC7A11 controls the uptake of extracellular cystine in exchange for glutamate at a ratio of 1:1, and it is overexpressed in a variety of tumours. Accumulating evidence has shown that the expression of SLC7A11 is fine-tuned at multiple levels, and plays diverse functional and pharmacological roles in tumours, such as cellular redox homeostasis, cell growth and death, and cell metabolism. Many reports have suggested that the inhibition of SLC7A11 expression and activity is favourable for tumour therapy; thus, SLC7A11 is regarded as a potential therapeutic target. However, emerging evidence also suggests that on some occasions, the inhibition of SLC7A11 is beneficial to the survival of cancer cells, and confers the development of drug resistance. In this review, we first briefly introduce the biological properties of SLC7A11, including its structure and physiological functions, and further summarise its regulatory network and potential regulators. Then, focusing on its role in cancer, we describe the relationships of SLC7A11 with tumourigenesis, survival, proliferation, metastasis, and therapeutic resistance in more detail. Finally, since SLC7A11 has been linked to cancer through multiple approaches, we propose that its contribution and regulatory mechanism require further elucidation. Thus, more personalised therapeutic strategies should be adapted when targeting SLC7A11.

Non-coding RNAs and ferroptosis: potential implications for cancer therapy

Ferroptosis is a recently defined form of regulated cell death, which is biochemically and morphologically distinct from traditional forms of programmed cell death such as apoptosis or necrosis. It is driven by iron, reactive oxygen species, and phospholipids that are oxidatively damaged, ultimately resulting in mitochondrial damage and breakdown of membrane integrity. Numerous cellular signaling pathways and molecules are involved in the regulation of ferroptosis, including enzymes that control the cellular redox status. Alterations in the ferroptosis-regulating network can contribute to the development of various diseases, including cancer. Evidence suggests that ferroptosis is commonly suppressed in cancer cells, allowing them to survive and progress. However, cancer cells which are resistant to common chemotherapeutic drugs seem to be highly susceptible to ferroptosis inducers, highlighting the great potential of pharmacologic modulation of ferroptosis for cancer treatment. Non-coding RNAs (ncRNAs) are considered master regulators of various cellular processes, particularly in cancer where they have been implicated in all hallmarks of cancer. Recent work also demonstrated their involvement in the molecular control of ferroptosis. Hence, ncRNA-based therapeutics represent an exciting alternative to modulate ferroptosis for cancer therapy. This review summarizes the ncRNAs implicated in the regulation of ferroptosis in cancer and highlights their underlying molecular mechanisms in the light of potential therapeutic applications.

Regulation and function of elF2B in neurological and metabolic disorders

Eukaryotic initiation factor 2B, eIF2B is a guanine nucleotide exchange, factor with a central role in coordinating the initiation of translation. During stress and disease, the activity of eIF2B is inhibited via the phosphorylation of its substrate eIF2 (p-eIF2α). A number of different kinases respond to various stresses leading to the phosphorylation of the alpha subunit of eIF2, and collectively this regulation is known as the integrated stress response, ISR. This targeting of eIF2B allows the cell to regulate protein synthesis and reprogramme gene expression to restore homeostasis. Advances within structural biology have furthered our understanding of how eIF2B interacts with eIF2 in both the productive GEF active form and the non-productive eIF2α phosphorylated form. Here, current knowledge of the role of eIF2B in the ISR is discussed within the context of normal and disease states focusing particularly on diseases such as vanishing white matter disease (VWMD) and permanent neonatal diabetes mellitus (PNDM), which are directly linked to mutations in eIF2B. The role of eIF2B in synaptic plasticity and memory formation is also discussed. In addition, the cellular localisation of eIF2B is reviewed and considered along with the role of additional in vivo eIF2B binding factors and protein modifications that may play a role in modulating eIF2B activity during health and disease.

Subchronic Acrylamide Exposure Activates PERK-eIF2α Signaling Pathway and Induces Synaptic Impairment in Rat Hippocampus

Acrylamide (ACR), a well-documented neurotoxicant to humans, is extensively found in starchy foods. More than 30% of the typical daily calorie intake comes from ACR-containing foods. Epidemiological and toxicological studies have found that ACR exposure is associated with mild cognitive change in men and experimental animals. However, there is limited information on the mechanisms by which ACR exposure induces memory deficits. The aberrant activation of the PKR-like ER kinase (PERK)-eukaryotic initiation factor 2α (eIF2α) signaling pathway is emerging as a major common theme in cognitive decline. The present study is designed to explore the effect of subchronic ACR exposure on the PERK signaling and the synaptic impairment to elucidate the potential mechanism of ACR-induced cognitive dysfunction in rat. ACR exposure at 5 and 10 (mg/kg)/day by gavage for 14 weeks results in gait abnormality and cognitive impairment in rats, which were accompanied by neuronal loss, glial cell proliferation, and synaptic ultrastructure damage in the hippocampus. ACR reduced the expression of phosphorylated cAMP response element-binding protein (P-CREB), brain-derived neurotrophic factor (BDNF), and synaptic vesicle proteins synapsin-1 and synaptophysin synthesis. ACR also excessively activates the PERK-eIF2α signaling, resulting in overexpression of C/EBP homologous protein (CHOP) and activating transcription factor 4 (ATF4). This work helps to propose a possible mechanism of subchronic exposure of ACR-induced neurotoxicity.

Upregulation of tRNA-Ser-AGA-2-1 Promotes Malignant Behavior in Normal Bronchial Cells

Serine tRNAs (tRNA^Ser) are frequently overexpressed in tumors and associated with poor prognosis and increased risk of recurrence in breast cancer. Impairment of tRNA biogenesis and abundance also impacts proteome homeostasis, and activates protein quality control systems. Herein, we aimed at testing whether increasing tRNA^Ser abundance could foster tumor establishment through activation of the UPR. In order to do so, firstly we confirmed that the expression of tRNA-Ser-AGA-2-1 [hereafter tRNA^Ser(AGA)] was upregulated by 1.79-fold in Stage I NSCLC tumors when compared to normal adjacent tissue. To study the impact of tRNA^Ser(AGA) in early stage tumorigenesis, we induced its upregulation in a non-tumoral bronchial cell line, BEAS-2B. Upregulation of this tRNA increased cellular proliferation and protein synthesis rate, driven by eIF2α dephosphorylation and ATF4 activation downstream of PERK signaling. Futhermore, tRNA^Ser(AGA) enhanced transformation potential in vitro, and promoted the establishment of slow growing tumors with aggressive features in nude mice. Our work highlights the importance of studying tRNA deregulation on early stage tumorigenesis, as they may be potential malignancy and aggressiveness biomarkers.

Astrocytic Glutamatergic Transmission and Its Implications in Neurodegenerative Disorders

Several neurodegenerative disorders involve impaired neurotransmission, and glutamatergic neurotransmission sets a prototypical example. Glutamate is a predominant excitatory neurotransmitter where the astrocytes play a pivotal role in maintaining the extracellular levels through release and uptake mechanisms. Astrocytes modulate calcium-mediated excitability and release several neurotransmitters and neuromodulators, including glutamate, and significantly modulate neurotransmission. Accumulating evidence supports the concept of excitotoxicity caused by astrocytic glutamatergic release in pathological conditions. Thus, the current review highlights different vesicular and non-vesicular mechanisms of astrocytic glutamate release and their implication in neurodegenerative diseases. As in presynaptic neurons, the vesicular release of astrocytic glutamate is also primarily meditated by calcium-mediated exocytosis. V-ATPase is crucial in the acidification and maintenance of the gradient that facilitates the vesicular storage of glutamate. Along with these, several other components, such as cystine/glutamate antiporter, hemichannels, BEST-1, TREK-1, purinergic receptors and so forth, also contribute to glutamate release under physiological and pathological conditions. Events of hampered glutamate uptake could promote inflamed astrocytes to trigger repetitive release of glutamate. This could be favorable towards the development and worsening of neurodegenerative diseases. Therefore, across neurodegenerative diseases, we review the relations between defective glutamatergic signaling and astrocytic vesicular and non-vesicular events in glutamate homeostasis. The optimum regulation of astrocytic glutamatergic transmission could pave the way for the management of these diseases and add to their therapeutic value.

Inhibition of the ISR abrogates mGluR5-dependent long-term depression and spatial memory deficits in a rat model of Alzheimer's disease

Soluble amyloid-β-protein (Aβ) oligomers, a major hallmark of AD, trigger the integrated stress response (ISR) via multiple pathologies including neuronal hyperactivation, microvascular hypoxia, and neuroinflammation. Increasing eIF2α phosphorylation, the core event of ISR, facilitates metabotropic glutamate receptor (mGluR)-dependent long-term depression (LTD), and suppressing its phosphorylation has the opposite effect. Having found the facilitation of mGluR5-LTD by Aβ in live rats, we wondered if suppressing eIF2α phosphorylation cascade would protect against the synaptic plasticity and cognitive disrupting effects of Aβ. We demonstrate here that the facilitation of mGluR5-LTD in a delayed rat model by single i.c.v. injection of synthetic Aβ_1-42. Systemic administration of the small-molecule inhibitor of the ISR called ISRIB (trans-isomer) prevents Aβ-facilitated LTD and abrogates spatial learning and memory deficits in the hippocampus in exogenous synthetic Aβ-injected rats. Moreover, ex vivo evidence indicates that ISRIB normalizes protein synthesis in the hippocampus. Targeting the ISR by suppressing the eIF2α phosphorylation cascade with the eIF2B activator ISRIB may provide protective effects against the synaptic and cognitive disruptive effects of Aβ which likely mediate the early stage of sporadic AD.

Curcumin Attenuates the PERK-eIF2α Signaling to Relieve Acrylamide-Induced Neurotoxicity in SH‑SY5Y Neuroblastoma Cells

Curcumin is a natural polyphenolic compound with neuroprotective and antioxidant properties. Acrylamide (ACR) is a by-product of food processing that produces neurotoxicity in humans and animals. The pancreatic endoplasmic reticulum kinase (PERK)-eukaryotic initiation factor-2α (eIF2α) signaling is involved in the occurrence of neurotoxicities. This study is aimed to investigate the protective effect of curcumin on ACR-induced cytotoxicity and explore the role of PERK-eIF2α signaling in this process. ACR exposure at 2.5 mM for 24 h caused oxidative stress as revealed by the distinct increase in cellular reactive oxygen species (ROS) and malondialdehyde (MDA) level, and a significant decrease in glutathione (GSH) content. ACR induced phosphorylated tau aggregation, phosphorylated cAMP response elements binding protein (CREB) reduction, and Bax/Bcl-2 ratio up-regulation in SH-SY5Y cells. ACR also activated the PERK-eIF2α signaling in SH-SY5Y cells and triggered the activation of glycogen synthase kinase-3β (GSK-3β), up-regulated activating transcription factor 4 (ATF4) and C/EBP homologous protein (CHOP). Curcumin pretreatment significantly attenuated ACR-induced neuronal toxicity as revealed by the ameliorated cell viability, mitigated intracellular ROS and MDA level, and elevated GSH content. Moreover, curcumin pretreatment inhibited PERK-dependent eIF2α phosphorylation, further suppressed GSK-3β and ATF4 function, and abolished abnormal tau phosphorylation, P-CREB reduction, and CHOP-induced apoptosis in SH-SY5Y cells. These results provided empirical evidence between curcumin and PERK-eIF2α signaling in ACR-induced neurotoxicity.

S-Adenosylmethionine Alleviates Amyloid-β-Induced Neural Injury by Enhancing Trans-Sulfuration Pathway Activity in Astrocytes

BACKGROUND

Glutathione (GSH) is an important endogenous antioxidant protecting cells from oxidative injury. Cysteine (Cys), the substrate limiting the production of GSH, is mainly generated from the trans-sulfuration pathway. S-adenosylmethionine (SAM) is a critical molecule produced in the methionine cycle and can be utilized by the trans-sulfuration pathway. Reductions in GSH and SAM as well as dysfunction in the trans-sulfuration pathway have been documented in the brains of Alzheimer's disease (AD) patients. Our previous in vivo study revealed that SAM administration attenuated oxidative stress induced by amyloid-β (Aβ) through the enhancement of GSH.

OBJECTIVE

To investigate the effect of Aβ-induced oxidative stress on the trans-sulfuration pathway in astrocytes and neurons, respectively, and the protective effect of SAM on neurons.

METHODS

APP/PS1 transgenic mice and the primary cultured astrocytes, neurons, and HT22 cells were used in the current study.

RESULTS

SAM could rescue the low trans-sulfuration pathway activity induced by Aβ only in astrocytes, accompanying with increasing levels of Cys and GSH. The decrease of cellular viability of neurons caused by Aβ was greatly reversed when co-cultured with astrocytes with SAM intervention. Meanwhile, SAM improved cognitive performance in APP/PS1 mice.

CONCLUSION

In terms of astrocyte protection from oxidative stress, SAM might be a potent antioxidant in the therapy of AD patients.

Crosstalk between Oxidative Stress and Ferroptosis/Oxytosis in Ischemic Stroke: Possible Targets and Molecular Mechanisms

Oxidative stress is a key cause of ischemic stroke and an initiator of neuronal dysfunction and death, mainly through the overproduction of peroxides and the depletion of antioxidants. Ferroptosis/oxytosis is a unique, oxidative stress-induced cell death pathway characterized by lipid peroxidation and glutathione depletion. Both oxidative stress and ferroptosis/oxytosis have common molecular pathways. This review summarizes the possible targets and the mechanisms underlying the crosstalk between oxidative stress and ferroptosis/oxytosis in ischemic stroke. This knowledge might help to further understand the pathophysiology of ischemic stroke and open new perspectives for the treatment of ischemic stroke.

Activating transcription factor-4 promotes neuronal death induced by Parkinson's disease neurotoxins and α-synuclein aggregates

Parkinson's disease (PD) is a neurodegenerative disease characterized by the loss of dopaminergic neurons in the substantia nigra resulting in severe and progressive motor impairments. However, the mechanisms underlying this neuronal loss remain largely unknown. Oxidative stress and ER stress have been implicated in PD and these factors are known to activate the integrated stress response (ISR). Activating transcription factor 4 (ATF4), a key mediator of the ISR, and has been reported to induce the expression of genes involved in cellular homeostasis. However, during prolonged activation ATF4 can also induce the expression of pro-death target genes. Therefore, in the present study, we investigated the role of ATF4 in neuronal cell death in models of PD. We demonstrate that PD neurotoxins (MPP+ and 6-OHDA) and α-synuclein aggregation induced by pre-formed human alpha-synuclein fibrils (PFFs) cause sustained upregulation of ATF4 expression in mouse cortical and mesencephalic dopaminergic neurons. Furthermore, we demonstrate that PD neurotoxins induce the expression of the pro-apoptotic factors Chop, Trb3, and Puma in dopaminergic neurons in an ATF4-dependent manner. Importantly, we have determined that PD neurotoxin and α-synuclein PFF induced neuronal death is attenuated in ATF4-deficient dopaminergic neurons. Furthermore, ectopic expression of ATF4 but not transcriptionally defective ATF4ΔRK restores sensitivity of ATF4-deficient neurons to PD neurotoxins. Finally, we demonstrate that the eIF2α kinase inhibitor C16 suppresses MPP+ and 6-OHDA induced ATF4 activation and protects against PD neurotoxin induced dopaminergic neuronal death. Taken together these results indicate that ATF4 promotes dopaminergic cell death induced by PD neurotoxins and pathogenic α-synuclein aggregates and highlight the ISR factor ATF4 as a potential therapeutic target in PD.

Activation of the eIF2α/ATF4 axis drives triple-negative breast cancer radioresistance by promoting glutathione biosynthesis

Triple-negative breast cancer (TNBC) is the most aggressive breast cancer subtype. Radiotherapy is an effective option for the treatment of TNBC; however, acquired radioresistance is a major challenge to the modality. In this study, we show that the integrated stress response (ISR) is the most activated signaling pathway in radioresistant TNBC cells. The constitutive phosphorylation of eIF2α in radioresistant TNBC cells promotes the activation of ATF4 and elicits the transcription of genes implicated in glutathione biosynthesis, including GCLC, SLC7A11, and CTH, which increases the intracellular level of reduced glutathione (GSH) and the scavenging of reactive oxygen species (ROS) after irradiation (IR), leading to a radioresistant phenotype. The cascade is significantly up-regulated in human TNBC tissues and is associated with unfavorable survival in patients. Dephosphorylation of eIF2α increases IR-induced ROS accumulation in radioresistant TNBC cells by disrupting ATF4-mediated GSH biosynthesis and sensitizes them to IR in vitro and in vivo. These findings reveal ISR as a vital mechanism underlying TNBC radioresistance and propose the eIF2α/ATF4 axis as a novel therapeutic target for TNBC treatment.

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The eIF2α/ATF4 axis is constitutively activated in radioresistant TNBC.

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Phosphorylated eIF2α increases the expression of ATF4 and GCLC at the translational level in TNBC.

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The eIF2α/ATF4 axis activation causes radioresistance in TNBC by promoting GSH biosynthesis and ROS scavenging.

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ATF4 promotes GSH biosynthesis in radioresistant TNBC by triggering the expression of GCLC, CTH, and SLC7A11.

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Inhibition of the eIF2α/ATF4 axis can improve the sensitivity of TNBC to radiotherapy in vitro and in vivo.

Crosstalk between endoplasmic reticulum stress and oxidative stress: a dynamic duo in multiple myeloma

Under physiological and pathological conditions, cells activate the unfolded protein response (UPR) to deal with the accumulation of unfolded or misfolded proteins in the endoplasmic reticulum. Multiple myeloma (MM) is a hematological malignancy arising from immunoglobulin-secreting plasma cells. MM cells are subject to continual ER stress and highly dependent on the UPR signaling activation due to overproduction of paraproteins. Mounting evidence suggests the close linkage between ER stress and oxidative stress, demonstrated by overlapping signaling pathways and inter-organelle communication pivotal to cell fate decision. Imbalance of intracellular homeostasis can lead to deranged control of cellular functions and engage apoptosis due to mutual activation between ER stress and reactive oxygen species generation through a self-perpetuating cycle. Here, we present accumulating evidence showing the interactive roles of redox homeostasis and proteostasis in MM pathogenesis and drug resistance, which would be helpful in elucidating the still underdefined molecular pathways linking ER stress and oxidative stress in MM. Lastly, we highlight future research directions in the development of anti-myeloma therapy, focusing particularly on targeting redox signaling and ER stress responses.

Heteromeric Solute Carriers: Function, Structure, Pathology and Pharmacology

Solute carriers form one of three major superfamilies of membrane transporters in humans, and include uniporters, exchangers and symporters. Following several decades of molecular characterisation, multiple solute carriers that form obligatory heteromers with unrelated subunits are emerging as a distinctive principle of membrane transporter assembly. Here we comprehensively review experimentally established heteromeric solute carriers: SLC3-SLC7 amino acid exchangers, SLC16 monocarboxylate/H⁺ symporters and basigin/embigin, SLC4A1 (AE1) and glycophorin A exchanger, SLC51 heteromer Ost α-Ost β uniporter, and SLC6 heteromeric symporters. The review covers the history of the heteromer discovery, transporter physiology, structure, disease associations and pharmacology - all with a focus on the heteromeric assembly. The cellular locations, requirements for complex formation, and the functional role of dimerization are extensively detailed, including analysis of the first complete heteromer structures, the SLC7-SLC3 family transporters LAT1-4F2hc, b^0,+AT-rBAT and the SLC6 family heteromer B⁰AT1-ACE2. We present a systematic analysis of the structural and functional aspects of heteromeric solute carriers and conclude with common principles of their functional roles and structural architecture.

Functional role of TRPC6 and STIM2 in cytosolic and endoplasmic reticulum Ca2+ content in resting estrogen receptor-positive breast cancer cells

TRPC6 forms non-selective cation channels activated by a variety of stimuli that are involved in a wide number of cellular functions. In estrogen receptor-positive (ER+) breast cancer cells, the store-operated Ca2+ entry has been reported to be dependent on STIM1, STIM2 and Orai3, with TRPC6 playing a key role in the activation of store-operated Ca2+ entry as well as in proliferation, migration and viability of breast cancer cells. We have used a combination of biotinylation, Ca2+ imaging as well as protein knockdown and overexpression of a dominant-negative TRPC6 mutant (TRPC6dn) to show that TRPC6 and STIM2 are required for the maintenance of cytosolic and endoplasmic reticulum Ca2+ content under resting conditions in ER+ breast cancer MCF7 cells. These cells exhibit a greater plasma membrane expression of TRPC6 under resting conditions than non-tumoral breast epithelial cells. Attenuation of STIM2, TRPC6 and Orai3, alone or in combination, results in impairment of resting cytosolic and endoplasmic reticulum Ca2+ homeostasis. Similar results were observed when cells were transfected with expression plasmid for TRPC6dn. TRPC6 co-immunoprecipitates with STIM2 in resting MCF7 cells, a process that is impaired by rises in cytosolic Ca2+ concentration. Impairment of TRPC6 function leads to abnormal Ca2+ homeostasis and endoplasmic reticulum stress, thus, suggesting that TRPC6 might be a potential target for the development of anti-tumoral therapies.

Heme Causes Pain in Sickle Mice via Toll-Like Receptor 4-Mediated Reactive Oxygen Species- and Endoplasmic Reticulum Stress-Induced Glial Activation

Aims: Lifelong pain is a hallmark feature of sickle cell disease (SCD). How sickle pathobiology evokes pain remains unknown. We hypothesize that increased cell-free heme due to ongoing hemolysis activates toll-like receptor 4 (TLR4), leading to the formation of reactive oxygen species (ROS) and endoplasmic reticulum (ER) stress. Together, these processes lead to spinal microglial activation and neuroinflammation, culminating in acute and chronic pain. Results: Spinal heme levels, TLR4 transcripts, oxidative stress, and ER stress were significantly higher in sickle mice than controls. In vitro, TLR4 inhibition in spinal cord microglial cells attenuated heme-induced ROS and ER stress. Heme treatment led to a time-dependent increase in the characteristic features of sickle pain (mechanical and thermal hyperalgesia) in both sickle and control mice; this effect was absent in TLR4-knockout sickle and control mice. TLR4 deletion in sickle mice attenuated chronic and hypoxia/reoxygenation (H/R)-evoked acute hyperalgesia. Sickle mice treated with the TLR4 inhibitor resatorvid; selective small-molecule inhibitor of TLR4 (TAK242) had significantly reduced chronic hyperalgesia and had less severe H/R-evoked acute pain with quicker recovery. Notably, reducing ER stress with salubrinal ameliorated chronic hyperalgesia in sickle mice. Innovation: Our findings demonstrate the causal role of free heme in the genesis of acute and chronic sickle pain and suggest that TLR4 and/or ER stress are novel therapeutic targets for treating pain in SCD. Conclusion: Heme-induced microglial activation via TLR4 in the central nervous system contributes to the initiation and maintenance of sickle pain via ER stress in SCD. Antioxid. Redox Signal. 34, 279-293.

The Harmonious Interplay of Amino Acid and Monocarboxylate Transporters Induces the Robustness of Cancer Cells

There is a growing body of evidence that metabolic reprogramming contributes to the acquisition and maintenance of robustness associated with malignancy. The fine regulation of expression levels of amino acid and monocarboxylate transporters enables cancer cells to exhibit the metabolic reprogramming that is responsible for therapeutic resistance. Amino acid transporters characterized by xCT (SLC7A11), ASCT2 (SLC1A5), and LAT1 (SLC7A5) function in the uptake and export of amino acids such as cystine and glutamine, thereby regulating glutathione synthesis, autophagy, and glutaminolysis. CD44 variant, a cancer stem-like cell marker, stabilizes the xCT antiporter at the cellular membrane, and tumor cells positive for xCT and/or ASCT2 are susceptible to sulfasalazine, a system Xc(-) inhibitor. Inhibiting the interaction between LAT1 and CD98 heavy chain prevents activation of the mammalian target of rapamycin (mTOR) complex 1 by glutamine and leucine. mTOR signaling regulated by LAT1 is a sensor of dynamic alterations in the nutrient tumor microenvironment. LAT1 is overexpressed in various malignancies and positively correlated with poor clinical outcome. Metabolic reprogramming of glutamine occurs often in cancer cells and manifests as ASCT2-mediated glutamine addiction. Monocarboxylate transporters (MCTs) mediate metabolic symbiosis, by which lactate in cancer cells under hypoxia is exported through MCT4 and imported by MCT1 in less hypoxic regions, where it is used as an oxidative metabolite. Differential expression patterns of transporters cause functional intratumoral heterogeneity leading to the therapeutic resistance. Therefore, metabolic reprogramming based on these transporters may be a promising therapeutic target. This review highlights the pathological function and therapeutic targets of transporters including xCT, ASCT2, LAT1, and MCT.

Activating transcription factor 4-dependent lactate dehydrogenase activation as a protective response to amyloid beta toxicity

Accumulation of amyloid beta peptides is thought to initiate the pathogenesis of Alzheimer's disease. However, the precise mechanisms mediating their neurotoxicity are unclear. Our microarray analyses show that, in Drosophila models of amyloid beta 42 toxicity, genes involved in the unfolded protein response and metabolic processes are upregulated in brain. Comparison with the brain transcriptome of early-stage Alzheimer's patients revealed a common transcriptional signature, but with generally opposing directions of gene expression changes between flies and humans. Among these differentially regulated genes, lactate dehydrogenase (Ldh) was up-regulated by the greatest degree in amyloid beta 42 flies and the human orthologues (LDHA and LDHB) were down-regulated in patients. Functional analyses revealed that either over-expression or inhibition of Ldh by RNA interference (RNAi) slightly exacerbated climbing defects in both healthy and amyloid beta 42-induced Drosophila. This suggests that metabolic responses to lactate dehydrogenase must be finely-tuned, and that its observed upregulation following amyloid beta 42 production could potentially represent a compensatory protection to maintain pathway homeostasis in this model, with further manipulation leading to detrimental effects. The increased Ldh expression in amyloid beta 42 flies was regulated partially by unfolded protein response signalling, as ATF4 RNAi diminished the transcriptional response and enhanced amyloid beta 42-induced climbing phenotypes. Further functional studies are required to determine whether Ldh upregulation provides compensatory neuroprotection against amyloid beta 42-induced loss of activating transcription factor 4 activity and endoplasmatic reticulum stress. Our study thus reveals dysregulation of lactate dehydrogenase signalling in Drosophila models and patients with Alzheimer's disease, which may lead to a detrimental loss of metabolic homeostasis. Importantly, we observed that down-regulation of ATF4-dependent endoplasmic reticulum-stress signalling in this context appears to prevent Ldh compensation and to exacerbate amyloid beta 42-dependent neuronal toxicity. Our findings, therefore, suggest caution in the use of therapeutic strategies focussed on down-regulation of this pathway for the treatment of Alzheimer's disease, since its natural response to the toxic peptide may induce beneficial neuroprotective effects.

Predicting the Possible Physiological/Biological Utility of the Hydropersulfide Functional Group Based on Its Chemistry: Similarities Between Hydropersulfides and Selenols

Significance: Hydropersulfides (RSSH) and related polysulfide species (RS_nR, n > 2, R = alkyl, H) are highly biologically prevalent with likely important physiological functions. Due to their prevalence, many labs have begun to investigate their possible roles, especially with regards to their protective, redox, and signaling properties. Recent Advances: A significant amount of work has been performed while delineating the chemical reactivity/chemical properties of hydropersulfides, and it is clear that their overall chemistry is distinct from all other biologically relevant sulfur species (e.g., thiols, disulfides, sulfenic acids, etc.). Critical Issues: One way to predict and ultimately understand the biological functions of hydropersulfides is to focus on their unique chemistry, which should provide the rationale for why this unique functionality is present. Interestingly, some of the chemical properties of RSSH are strikingly similar to those of selenols (RSeH). Therefore, it may be important to consider the known functions of selenoproteins when speculating about the possible functions of RSSH species. Future Directions: Currently, many of the inherent chemical differences between hydropersulfides and other biological sulfur species have been established. It remains to be determined, however, whether and how these differences are utilized to accomplish specific biochemical/physiological goals. A significant aspect of elucidating the biological utility of hydropersulfides will be to determine the mechanisms of regulation of their formation and/or biosynthesis, that is, based on whether it can be determined under what cellular conditions hydropersulfides are made, more meaningful speculation regarding their functions/roles can be developed.

Using the Oxytosis/Ferroptosis Pathway to Understand and Treat Age-Associated Neurodegenerative Diseases

Oxytosis was first described over 30 years ago in nerve cells as a non-excitotoxic pathway for glutamate-induced cell death. The key steps of oxytosis, including glutathione depletion, lipoxygenase activation, reactive oxygen species accumulation, and calcium influx, were identified using a combination of chemical and genetic tools. A pathway with the same characteristics as oxytosis was identified in transformed fibroblasts in 2012 and named ferroptosis. Importantly, the pathophysiological changes seen in oxytosis and ferroptosis are also observed in multiple neurodegenerative diseases as well as in the aging brain. This led to the hypothesis that this pathway could be used as a screening tool to identify novel drug candidates for the treatment of multiple age-associated neurological disorders, including Alzheimer's disease (AD). Using this approach, we have identified several AD drug candidates, one of which is now in clinical trials, as well as new target pathways for AD.

Astrocytes release glutamate via cystine/glutamate antiporter upregulated in response to increased oxidative stress related to sporadic amyotrophic lateral sclerosis

A vast body of evidence implicates increased oxidative stress and extracellular glutamate accumulation in the pathomechanism of sporadic amyotrophic lateral sclerosis (ALS). Cystine/glutamate antiporter (xCT) carries extracellular cystine uptake and intracellular glutamate release (cystine/glutamate exchange) in the presence of oxidative stress. The aim of the present study was to determine the involvement of xCT in ALS. Immunohistochemical observations in the spinal cord sections demonstrated that xCT was mainly expressed in astrocytes, with staining more intense in 12 sporadic ALS patients as compared to 12 age-matched control individuals. Western blot and densitometric analyses of the spinal cord samples revealed that the relative value of xCT/β-actin optical density ratio was significantly higher in the ALS group as compared to the control group. Next, we conducted cell culture experiments using a human astrocytoma-derived cell line (1321N1) and a mouse motor neuron/neuroblastoma hybrid cell line (NSC34). In 1321N1 cells, the normalized xCT expression levels in cell lysates were significantly increased by H₂ O₂ treatment. Glutamate concentrations in 1321 N1 cell culture-conditioned media were significantly elevated by H₂ O₂ treatment, and the H₂ O₂ -driven elevations were completely canceled by the xCT inhibitor erastin pretreatment. In motor neuron-differentiated NSC34 cells (NSC34d cells), both the normalized xCT expression levels in the cell lysates and glutamate concentrations in the cell-conditioned media were constant with or without H₂ O₂ treatment. The present results provide in vivo and in vitro evidence that astrocytes upregulate xCT expression to release glutamate in response to increased oxidative stress associated with ALS, contributing to extracellular glutamate accumulation.

Ubiquitination and the proteasome rather than caspase-3-mediated C-terminal cleavage are involved in the EAAT2 degradation by staurosporine-induced cellular stress

Diminished glutamate (Glu) uptake via the excitatory amino acid transporter EAAT2, which normally accounts for ~90% of total forebrain EAAT activity, may contribute to neurodegeneration via Glu-mediated excitotoxicity. C-terminal cleavage by caspase-3 (C3) was reported to mediate EAAT2 inactivation and down-regulation in the context of neurodegeneration. For a detailed analysis of C3-dependent EAAT2 degradation, we employed A172 glioblastoma as well as hippocampal HT22 cells and murine astrocytes over-expressing VSV-G-tagged EAAT2 constructs. C3 activation was induced by staurosporine (STR). In HT22 cells, STR-induced C3 activation-induced rapid EAAT2 protein degradation. The mutation of asparagine 504 to aspartate (D504N), which should inactivate the putative C3 cleavage site, increased EAAT2 activity in A172 cells. In contrast, the D504N mutation did not protect EAAT2 protein against STR-induced degradation in HT22 cells, whereas inhibition of caspases, ubiquitination and the proteasome did. Similar results were obtained in astrocytes. Phylogenetic analysis showed that C-terminal ubiquitin acceptor sites-but not the putative C3 cleavage site-exhibit a high degree of conservation. Moreover, C-terminal truncation mimicking C3 cleavage increased rather than decreased EAAT2 activity and stability as well as protected EAAT2 against STR-induced ubiquitination-dependent degradation. We conclude that cellular stress associated with endogenous C3 activation degrades EAAT2 via a pathway involving ubiquitination and the proteasome but not direct C3-mediated cleavage. In addition, C3 cleavage of EAAT2, described to occur in other models, is unlikely to inactivate EAAT2. However, mutation of the highly conserved D504 within the putative C3 cleavage site increases EAAT2 activity via an unknown mechanism.

Pathogenesis and promising therapeutics of Alzheimer disease through eIF2α pathway and correspondent kinases

Eukaryotic initiation factor 2 (eIF2α) pathway is overactivated in Alzheimer disease and is probably associated with synaptic and memory deficiencies. EIF2α protein is principally in charge of the regulation of protein synthesis in eukaryotic cells. Four kinases responsible for eIF2α phosphorylation at ser-51 are: General control non-derepressible-2 kinase (GCN2), double-stranded RNA-activated protein kinase (PKR), PKR-like endoplasmic reticulum kinase (PERK), and heme-regulated inhibitor kinase (HRI) are the four kinases. They lead to reduced levels of general translation and paradoxical increase of stress-responsive mRNAs expression including the B-secretase (BACE1) and the transcriptional modulator activating transcription factor 4 (ATF4), which in turn accelerates the beta-amyloidogenesis, tau phosphorylation, proapoptotic pathway induction and autophagy elements formation leading to the main pathological hallmarks of AD. Findings suggest that genetic or pharmacological inhibition of correspondent kinases can restore memory and prevent neurodegeneration. This implies that inhibition of eIF2α phosphorylation through respondent kinases is indeed a feasible prospect of clinical application. This review discusses recent therapeutic approaches targeting eIF2α pathway and provides an overview of the links between correspondent kinases overactivation with neurodegeneration in AD.

Functional analysis of CD44 variants and xCT in canine tumours

The cell surface glycoprotein CD44 has various types of splicing variants, which contribute to its multiple distinct cellular functions. Recently, it was reported that the CD44v8‐10 isoform interacts with the system Xc(‐) transporter‐related protein (xCT), and inhibits the accumulation of reactive oxygen species by promoting the synthesis of the antioxidant glutathione in human tumour cells. In this study, we investigated the expression and function of CD44 variants and xCT in canine tumours. From semi‐quantitative reverse transcription polymerase chain reaction analysis, the mRNA expression of the CD44v8‐10 isoform was observed in canine tumour tissues as well as human cases. The overexpression of CD44v8‐10 may promote the synthesis of glutathione and enhance the resistance to radiation of canine breast tumour cells. Furthermore, canine xCT mRNA expression was significantly upregulated in the canine breast tumour tissues as compared to the normal tissues surrounding the tumours. To investigate the function of canine xCT, we treated canine tumour cells with the xCT inhibitor sulfasalazine. Consequently, the sulfasalazine‐treated cells were more sensitive to oxidative stress than the non‐treated cells. Taken together, these results suggested that CD44v8‐10 and xCT play important roles in the therapy resistance of canine tumours as well as human tumours.

SLC7A11/xCT in cancer: biological functions and therapeutic implications

Amino acid transporters mediate substrates across cellular membranes and their fine-tuned regulations are critical to cellular metabolism, growth, and death. As the functional component of system Xc-, which imports extracellular cystine with intracellular glutamate release at a ratio of 1:1, SLC7A11 has diverse functional roles in regulating many pathophysiological processes such as cellular redox homeostasis, ferroptosis, and drug resistance in cancer. Notably, accumulated evidence demonstrated that SLC7A11 is overexpressed in many types of cancers and is associated with patients' poor prognosis. As a result, SLC7A11 becomes a new potential target for cancer therapy. In this review, we first briefly introduce the structure and function of SLC7A11, then discuss its pathological role in cancer. We next summarize current available data of how SLC7A11 is subjected to fine regulations at multiple levels. We further describe the potential inhibitors of the SLC7A11 and their roles in human cancer cells. Finally, we propose novel insights for future perspectives on the modulation of SLC7A11, as well as possible targeted strategies for SLC7A11-based anti-cancer therapies.

Cystine-glutamate antiporter xCT as a therapeutic target for cancer

Cystine/glutamic acid reverse transporter (System X_c ^- ), a member of the amino acid transporter family, consists of two subunits, light chain xCT and heavy chain 4F2hc. xCT is the cystine/glutamate antiporter solute carrier family 7 member 11 (SLC7A11), which promotes cystine uptake and glutathione biosynthetic, thus protecting against oxidative stress and ferroptosis. Studies have confirmed that xCT is highly expressed in a variety of tumour and is associated with tumour proliferation, invasion, metastasis, drug resistance and ferroptosis, and can be used as a potential target for tumour treatment. This review provides insights into the biological effects of xCT and contribute to the development of new xCT-based strategies.

The Integrated Stress Response and Phosphorylated Eukaryotic Initiation Factor 2α in Neurodegeneration

The proposed molecular mechanisms underlying neurodegenerative pathogenesis are varied, precluding the development of effective therapies for these increasingly prevalent disorders. One of the most consistent observations across neurodegenerative diseases is the phosphorylation of eukaryotic initiation factor 2α (eIF2α). eIF2α is a translation initiation factor, involved in cap-dependent protein translation, which when phosphorylated causes global translation attenuation. eIF2α phosphorylation is mediated by 4 kinases, which, together with their downstream signaling cascades, constitute the integrated stress response (ISR). While the ISR is activated by stresses commonly observed in neurodegeneration, such as oxidative stress, endoplasmic reticulum stress, and inflammation, it is a canonically adaptive signaling cascade. However, chronic activation of the ISR can contribute to neurodegenerative phenotypes such as neuronal death, memory impairments, and protein aggregation via apoptotic induction and other maladaptive outcomes downstream of phospho-eIF2α-mediated translation inhibition, including neuroinflammation and altered amyloidogenic processing, plausibly in a feed-forward manner. This review examines evidence that dysregulated eIF2a phosphorylation acts as a driver of neurodegeneration, including a survey of observations of ISR signaling in human disease, inspection of the overlap between ISR signaling and neurodegenerative phenomenon, and assessment of recent encouraging findings ameliorating neurodegeneration using developing pharmacological agents which target the ISR. In doing so, gaps in the field, including crosstalk of the ISR kinases and consideration of ISR signaling in nonneuronal central nervous system cell types, are highlighted.

Roles of the Translation Initiation Factor eIF2α Phosphorylation in Cell Structure and Function

It is often assumed that α-subunit phosphorylation of the eukaryotic translation initiation factor 2 (eIF2) complex is just a mechanism to control protein synthesis. However, eIF2α phosphorylation induced by multiple kinases can recognize various intracellular and extracellular stress conditions, and it is involved in various other cellular processes beyond protein synthesis. This review introduces the roles of eIF2α phosphorylation in translational regulation, the generation of reactive oxygen species, changes in mitochondria structure and shape, and mitochondrial retrograde signaling pathways in response to diverse stress conditions.Key words: eIF2α phosphorylation, Translation, Unfolded Protein Response, Reactive Oxygen Species, Mitochondria.

Role of the mitochondrial stress response in human cancer progression

IMPACT STATEMENT

Dysregulated mitochondria often occurred in cancers. Mitochondrial dysfunction might contribute to cancer progression. We reviewed several mitochondrial stresses in cancers. Mitochondrial stress responses might contribute to cancer progression. Several mitochondrion-derived molecules (ROS, Ca2+, oncometabolites, exported mtDNA, mitochondrial double-stranded RNA, humanin, and MOTS-c), integrated stress response, and mitochondrial unfolded protein response act as retrograde signaling pathways and might be critical in the development and progression of cancer. Targeting these mitochondrial stress responses may be an important strategy for cancer treatment.

Fibroblast growth factor 21 and autophagy: A complex interplay in Parkinson disease

Parkinson disease (PD) is the second common neurodegenerative disorder after Alzheimer's disease (AD). The predominant pathological hallmark is progressive loss of dopaminergic (DA) neurones in the substantia nigra (SN) complicated by aggregation of misfolded forms of alpha-synuclein (α-syn). α-syn is a cytosolic synaptic protein localized in the presynaptic neuron under normal circumstances. What drives misfolding of this protein is largely unknown. However, recent studies suggest that autophagy might be an important risk factor for contributing towards PD. Autophagy is an evolutionarily conserved mechanism that causes the clearance or degradation of misfolded, mutated and damaged proteins, organelles etc. However, in an aging individual this process might deteriorate which could possibly lead to the accumulation of damaged proteins. Hence, autophagy modulation might provide some interesting cues for the treatment of PD. Additionally, Fibroblast growth factor 21 (FGF21) which is known for its role as a potent regulator of glucose and energy metabolism has also proved to be neuroprotective in various neurodegenerative conditions possibly via mediation of autophagy.

The Chemical Biology of Ferroptosis in the Central Nervous System

Over the past five decades, thanatology has come to include the study of how individual cells in our bodies die appropriately and inappropriately in response to physiological and pathological stimuli. Morphological and biochemical criteria have been painstakingly established to create clarity around definitions of distinct types of cell death and mechanisms for their activation. Among these, ferroptosis has emerged as a unique, oxidative stress-induced cell death pathway with implications for diseases as diverse as traumatic brain injury, hemorrhagic stroke, Alzheimer's disease, cancer, renal ischemia, and heat stress in plants. In this review, I highlight some of the formative studies that fostered its recognition in the nervous system and describe how chemical biological tools have been essential in defining events necessary for its execution. Finally, I discuss emerging opportunities for antiferroptotic agents as therapeutic agents in neurological diseases.

Reduced EGFR Level in eIF2α PhosphorylationDeficient Hepatocytes Is Responsible for Susceptibility to Oxidative Stress

Reactive oxygen species (ROS) play a significant role in intracellular signaling and regulation, particularly when they are maintained at physiologic levels. However, excess ROS can cause cell damage and induce cell death. We recently reported that eIF2α phosphorylation protects hepatocytes from oxidative stress and liver fibrosis induced by fructose metabolism. Here, we found that hepatocyte-specific eIF2α phosphorylation-deficient mice have significantly reduced expression of the epidermal growth factor receptor (EGFR) and altered EGFR-mediated signaling pathways. EGFR-mediated signaling pathways are important for cell proliferation, differentiation, and survival in many tissues and cell types. Therefore, we studied whether the reduced amount of EGFR is responsible for the eIF2α phosphorylationdeficient hepatocytes' vulnerability to oxidative stress. ROS such as hydrogen peroxide and superoxides induce both EGFR tyrosine phosphorylation and eIF2α phosphorylation. eIF2α phosphorylation-deficient primary hepatocytes, or EGFR knockdown cells, have decreased ROS scavenging ability compared to normal cells. Therefore, these cells are particularly susceptible to oxidative stress. However, overexpression of EGFR in these eIF2α phosphorylationdeficient primary hepatocytes increased ROS scavenging ability and alleviated ROS-mediated cell death. Therefore, we hypothesize that the reduced EGFR level in eIF2α phosphorylation-deficient hepatocytes is one of critical factors responsible for their susceptibility to oxidative stress.

Cysteine becomes conditionally essential during hypobaric hypoxia and regulates adaptive neuro-physiological responses through CBS/H2S pathway

Brain is well known for its disproportionate oxygen consumption and high energy-budget for optimal functioning. The decrease in oxygen supply to brain, thus, necessitates rapid activation of adaptive pathways - the absence of which manifest into vivid pathological conditions. Amongst these, oxygen sensing in glio-vascular milieu and H₂S-dependent compensatory increase in cerebral blood flow (CBF) is a major adaptive response. We had recently demonstrated that the levels of H₂S were significantly decreased during chronic hypobaric hypoxia (HH)-induced neuro-pathological effects. The mechanistic basis of this phenomenon, however, remained to be deciphered. We, here, describe experimental evidence for marked limitation of cysteine during HH - both in animal model as well as human volunteers ascending to high altitude. We show that the preservation of brain cysteine level, employing cysteine pro-drug (N-acetyl-_L-cysteine, NAC), markedly curtailed effects of HH - not only on endogenous H₂S levels but also, impairment of spatial reference memory in our animal model. We, further, present multiple lines of experimental evidence that the limitation of cysteine was causally governed by physiological propensity of brain to utilize cysteine, in cystathionine beta synthase (CBS)-dependent manner, past its endogenous replenishment potential. Notably, decrease in the levels of brain cysteine manifested despite positive effect (up-regulation) of HH on endogenous cysteine maintenance pathways and thus, qualifying cysteine as a conditionally essential nutrient (CEN) during HH. In brief, our data supports an adaptive, physiological role of CBS-mediated cysteine-utilization pathway - activated to increase endogenous levels of H₂S - for optimal responses of brain to hypobaric hypoxia.

Iron dyshomeostasis, lipid peroxidation and perturbed expression of cystine/glutamate antiporter in Alzheimer's disease: Evidence of ferroptosis

Iron dyshomeostasis is implicated in Alzheimer’s disease (AD) alongside β-amyloid and tau pathologies. Despite the recent discovery of ferroptosis, an iron-dependent form cell death, hitherto, in vivo evidence of ferroptosis in AD is lacking. The present study uniquely adopts an integrated multi-disciplinary approach, combining protein (Western blot) and elemental analysis (total reflection X-ray fluorescence) with metabolomics (¹H nuclear magnetic resonance spectroscopy) to identify iron dyshomeostasis and ferroptosis, and possible novel interactions with metabolic dysfunction in age-matched male cognitively normal (CN) and AD post-mortem brain tissue (n = 7/group). Statistical analysis was used to compute differences between CN and AD, and to examine associations between proteins, elements and/or metabolites. Iron dyshomeostasis with elevated levels of ferritin, in the absence of increased elemental iron, was observed in AD. Moreover, AD was characterised by enhanced expression of the light-chain subunit of the cystine/glutamate transporter (xCT) and lipid peroxidation, reminiscent of ferroptosis, alongside an augmented excitatory glutamate to inhibitory GABA ratio. Protein, element and metabolite associations also greatly differed between CN and AD suggesting widespread metabolic dysregulation in AD. We demonstrate iron dyshomeostasis, upregulated xCT (impaired glutathione metabolism) and lipid peroxidation in AD, suggesting anti-ferroptotic therapies may be efficacious in AD.

Metabolic regulation of cell growth and proliferation

Cellular metabolism is at the foundation of all biological activities. The catabolic processes that support cellular bioenergetics and survival have been well studied. By contrast, how cells alter their metabolism to support anabolic biomass accumulation is less well understood. During the commitment to cell proliferation, extensive metabolic rewiring must occur in order for cells to acquire sufficient nutrients such as glucose, amino acids, lipids and nucleotides, which are necessary to support cell growth and to deal with the redox challenges that arise from the increased metabolic activity associated with anabolic processes. Defining the mechanisms of this metabolic adaptation for cell growth and proliferation is now a major focus of research. Understanding the principles that guide anabolic metabolism may ultimately enhance ways to treat diseases that involve deregulated cell growth and proliferation, such as cancer.

EIF2A promotes cell survival during paclitaxel treatment in vitro and in vivo

The integrated stress response (ISR) is critical for cancer cell survival during stress stimuli and has been implicated in the resistance to cancer therapeutics, in which the mechanism, however, is poorly understood. Here, we showed that paclitaxel, the major chemotherapy drug for breast cancer, induced ISR and phosphorylated ser51 residue of EIF2S1 by EIF2AK3 and EIF2AK4. When exposed to paclitaxel, cancer cells activated the EIF2AK3/EIF2AK4-pEIF2S1-ATF4 axis and maintained redox homoeostasis by inducing expression of the major antioxidant enzymes HMOX1, SHMT2 and SLC7A11. Paclitaxel-mediated cell death was significantly increased following loss of ISR or ATF4 expression. This sensitizing effect could be partially rescued by Trolox, a ROS scavenger. We demonstrated that the alternative initiation factor EIF2A was essential for cancer cell survival after paclitaxel-mediated ISR both in vitro and in vivo. Moreover, patients with breast cancer exhibited higher ISR after chemotherapy, and the elevated mRNA levels of HMOX1, SHMT2 and EIF2A were correlated with poor prognosis. Collectively, our findings reveal a novel mechanism for paclitaxel resistance and suggest that targeting EIF2A combined with ISR agonist may be a potential treatment regimen to overcome drug resistance for breast cancer.

Endoplasmic Reticulum Stress and Unfolded Protein Response Pathways Involved in the Health-Promoting Effects of Allicin on the Jejunum

Intestinal endoplasmic reticulum stress (ERS) triggered by adverse factors disturbs the normal function of the intestine. Allicin has been reported to promote intestinal health and development. In the present study, we established in vivo (35-day-old weaned piglets, 4-week-old mice) and in vitro (IPEC-J2 cell line) ERS models to explore the possible mechanisms by which allicin may benefit intestinal health. This study revealed the following: (1) allicin supplementation improved intestinal morphological indices and ameliorated mild ERS in the jejunum of the weaned piglets; (2) allicin supplementation decreased cellular reactive oxygen species and upregulated the XBP-1s signaling pathways in IPEC-J2 cells; (3) allicin supplementation reduced the prolonged ERS-mediated apoptosis of IPEC-J2 cells and in the jejunal tissues of the KM mice; (4) allicin supplementation enhanced the intercellular junction protein levels of jejunal cells by alleviating the prolonged ERS. These novel findings suggest that eating garlic could alleviate some intestinal malfunctions associated with ERS.

The Non-Essential Amino Acid Cysteine Becomes Essential for Tumor Proliferation and Survival

The non-essential amino acid cysteine is used within cells for multiple processes that rely on the chemistry of its thiol group. Under physiological conditions, many non-transformed tissues rely on glutathione, circulating cysteine, and the de novo cysteine synthesis (transsulfuration) pathway as sources of intracellular cysteine to support cellular processes. In contrast, many cancers require exogeneous cystine for proliferation and viability. Herein, we review how the cystine transporter, xCT, and exogenous cystine fuel cancer cell proliferation and the mechanisms that regulate xCT expression and activity. Further, we discuss the potential contribution of additional sources of cysteine to the cysteine pool and what is known about the essentiality of these processes in cancer cells. Finally, we discuss whether cyst(e)ine dependency and associated metabolic alterations represent therapeutically targetable metabolic vulnerabilities.

Mitochondrial Dysfunction and Stress Responses in Alzheimer's Disease

Alzheimer's disease (AD) patients display widespread mitochondrial defects. Brain hypometabolism occurs alongside mitochondrial defects, and correlates well with cognitive decline. Numerous theories attempt to explain AD mitochondrial dysfunction. Groups propose AD mitochondrial defects stem from: (1) mitochondrial-nuclear DNA interactions/variations; (2) amyloid and neurofibrillary tangle interactions with mitochondria, and (3) mitochondrial quality control defects and oxidative damage. Cells respond to mitochondrial dysfunction through numerous retrograde responses including the Integrated Stress Response (ISR) involving eukaryotic initiation factor 2α (eIF2α), activating transcription factor 4 (ATF4) and C/EBP homologous protein (CHOP). AD brains activate the ISR and we hypothesize mitochondrial defects may contribute to ISR activation. Here we review current recognized contributions of the mitochondria to AD, with an emphasis on their potential contribution to brain stress responses.

Cellular eIF2B subunit localization: implications for the integrated stress response and its control by small molecule drugs

Eukaryotic initiation factor 2 (eIF2) is a G protein critical for translation. It is tightly regulated in the integrated stress response (ISR) via phosphorylation of eIF2α and the subsequent control of eukaryotic initiation factor 2B (eIF2B), a multisubunit guanine nucleotide exchange factor. Through studying the localization of eIF2B subunits, we identified cytoplasmic eIF2B bodies in mammalian cells. We highlight a relationship between body size and the eIF2B subunits localizing to them; larger bodies contain all subunits and smaller bodies contain predominantly catalytic subunits. eIF2 localizes to eIF2B bodies and shuttles within these bodies in a manner that correlates with eIF2B activity. On stress, eIF2α-P localizes predominately to larger bodies and results in a decreased shuttling of eIF2. Interestingly, drugs that inhibit the ISR can rescue eIF2 shuttling in a manner correlating to levels of eIF2α-P. In contrast, smaller bodies show increased eIF2 shuttling in response to stress, which is accompanied by the localization of eIF2Bδ to these bodies, suggesting the formation of a novel trimeric complex of eIF2B. This response is mimicked by ISR-inhibiting drugs, providing insight into their potential mechanism of action. This study provides evidence that the composition and function of mammalian eIF2B bodies are regulated by the ISR and the drugs that control it.

Factors that May Protect the Native Hibernator Syrian Hamster Renal Tubular Epithelial Cells from Ferroptosis Due to Warm Anoxia-Reoxygenation

Warm anoxia-reoxygenation induces ferroptotic cell death in mouse proximal renal tubular epithelial cells (RPTECs), whereas RPTECs of the native hibernator Syrian hamster resist cell death. Clarifying how hamster cells escape ferroptosis may reveal new molecular targets for preventing or ameliorating ischemia-reperfusion-induced human diseases or expanding the survival of organ transplants. Mouse or hamster RPTECs were subjected to anoxia and subsequent reoxygenation. Cell death was assessed with the lactated dehydrogenase (LDH) release assay and lipid peroxidation by measuring cellular malondialdehyde (MDA) fluorometrically. The effect of the ferroptosis inhibitor α-tocopherol on cell survival was assessed by the 2,3-bis (2-methoxy-4-nitro-5-sulfophenyl)-5-[(phenylamino) carbonyl]-2H-tetrazolium hydroxide (XTT) assay. The expression of the critical ferroptotic elements cystine-glutamate antiporter (xCT), ferritin, and glutathione peroxidase 4 (GPX4) was assessed by Western blot. Contrary to mouse RPTECs, hamster RPTECs resisted anoxia-reoxygenation-induced cell death and lipid peroxidation. In mouse RPTECs, α-tocopherol increased cell survival. Anoxia increased the levels of xCT, ferritin, and GPX4 in both cell types. During reoxygenation, at which reactive oxygen species overproduction occurs, xCT and ferritin decreased, whereas GPX4 increased in mouse RPTECs. In hamster RPTECs, reoxygenation raised xCT and ferritin, but lowered GPX4. Hamster RPTECs resist lipid peroxidation-induced cell death. From the three main evaluated components of the ferroptotic pathway, the increased expression of xCT and ferritin may contribute to the resistance of the hamster RPTECs to warm anoxia-reoxygenation.

Mast Cells Induce Blood Brain Barrier Damage in SCD by Causing Endoplasmic Reticulum Stress in the Endothelium

Endothelial dysfunction underlies the pathobiology of cerebrovascular disease. Mast cells are located in close proximity to the vasculature, and vasoactive mediators released upon their activation can promote endothelial activation leading to blood brain barrier (BBB) dysfunction. We examined the mechanism of mast cell-induced endothelial activation via endoplasmic reticulum (ER) stress mediated P-selectin expression in a transgenic mouse model of sickle cell disease (SCD), which shows BBB dysfunction. We used mouse brain endothelial cells (mBECs) and mast cells-derived from skin of control and sickle mice to examine the mechanisms involved. Compared to control mouse mast cell conditioned medium (MCCM), mBECs incubated with sickle mouse MCCM showed increased, structural disorganization and swelling of the ER and Golgi, aggregation of ribosomes, ER stress marker proteins, accumulation of galactose-1-phosphate uridyl transferase, mitochondrial dysfunction, reactive oxygen species (ROS) production, P-selectin expression and mBEC permeability. These effects of sickle-MCCM on mBEC were inhibited by Salubrinal, a reducer of ER stress. Histamine levels in the plasma, skin releasate and in mast cells of sickle mice were higher compared to control mice. Compared to control BBB permeability was increased in sickle mice. Treatment of mice with imatinib, Salubrinal, or P-selectin blocking antibody reduced BBB permeability in sickle mice. Mast cells induce endothelial dysfunction via ER stress-mediated P-selectin expression. Mast cell activation contributes to ER stress mediated endothelial P-selectin expression leading to increased endothelial permeability and impairment of BBB. Targeting mast cells and/or ER stress has the potential to ameliorate endothelial dysfunction in SCD and other pathobiologies.

Old age-associated phenotypic screening for Alzheimer's disease drug candidates identifies sterubin as a potent neuroprotective compound from Yerba santa

Alzheimer's disease (AD) is the most frequent age-associated dementia with no treatments that can prevent or slow its progression. Since age is by far the major risk factor for AD, there is a strong rationale for an alternative approach to drug discovery based upon the biology of aging. Phenotypic screening assays that reflect multiple, age-associated neurotoxicity pathways rather than single molecular targets can identify compounds that have therapeutic efficacy by targeting aspects of aging that contribute to AD pathology. And, while the suitability of any single assay can be questioned, a combination of assays can make reliable predictions about the neuroprotective effects of compounds in vivo. Therefore, our lab has developed a combination of phenotypic screening assays that are ideally suited not only to identify novel neuroprotective compounds for the treatment of AD but also their target pathways, thereby potentially providing new therapeutic targets for disease treatment. Using these assays, we screened a large library of extracts from plants with identified pharmacological uses. Analysis of one of these extracts from the plant Yerba santa (Eriodictyon californicum) identified the flavanone sterubin as the active component and further studies showed it to be a potent neuroprotective and anti-inflammatory compound.

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Phenotypic screening of a curated library of plant extracts identifies Yerba santa.

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The flavonoid sterubin is the main active component of Yerba santa.

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Sterubin is very neuroprotective against multiple toxicities of the aging brain.

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Sterubin has potent anti-inflammatory activity that is dependent on Nrf2 induction.

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Sterubin is also an iron chelator which could enhance its neuroprotective activity.