GCN2 eIF2 kinase promotes prostate cancer by maintaining amino acid homeostasis

Potential therapeutic targets of eukaryotic translation initiation factors in tumor therapy

Translation initiation is the first and rate-limiting step in protein synthesis, and its dysregulation is frequently observed in various malignancies. Cap-dependent translation, the predominant form of translation initiation, relies on the coordinated action of eukaryotic translation initiation factors (eIFs), including eIF1, eIF2, eIF4, and others. These factors play critical roles in regulating the efficiency and fidelity of protein synthesis, and their overexpression has been linked to tumor progression, proliferation, and metastasis. Notably, certain eIFs have emerged as potential prognostic markers due to their elevated expression in tumors. Targeting eIFs represents a promising strategy, particularly for cancers characterized by aberrant eIF activity. In this review, we summarize the roles of individual eIFs in cap-dependent translation and discuss their potential as therapeutic targets in cancer treatment. We also highlight recent advances in drug discovery efforts aimed at modulating eIF activity, providing insights into the development of novel anticancer therapies.

An ISR-independent role of GCN2 prevents excessive ribosome biogenesis and mRNA translation

The integrated stress response (ISR) is a corrective physiological programme to restore cellular homeostasis that is based on the attenuation of global protein synthesis and a resource-enhancing transcriptional programme. GCN2 is the oldest of four kinases that are activated by diverse cellular stresses to trigger the ISR and acts as the primary responder to amino acid shortage and ribosome collisions. Here, using a broad multi-omics approach, we uncover an ISR-independent role of GCN2. GCN2 inhibition or depletion in the absence of discernible stress causes excessive protein synthesis and ribosome biogenesis, perturbs the cellular translatome, and results in a dynamic and broad loss of metabolic homeostasis. Cancer cells that rely on GCN2 to keep protein synthesis in check under conditions of full nutrient availability depend on GCN2 for survival and unrestricted tumour growth. Our observations describe an ISR-independent role of GCN2 in regulating the cellular proteome and translatome and suggest new avenues for cancer therapies based on unleashing excessive mRNA translation.

Uncovering somatic genetic drivers in prostate cancer through comprehensive genome-wide analysis

Given that hereditary prostate cancer (PCa) accounts for only a small fraction of PCa phenotypes, there is still a substantial journey ahead in exploring the somatic genetic drivers contributing to sporadic PCa. The expression quantitative trait loci (eQTLs) data were sourced from the GTEx dataset for prostate-specific genes, and the summary statistic information was collected for 5854 genes. Genetic associations with PCa were extracted from three well-established consortiums: the UK Biobank (9131 cases and 173,493 controls), the PRACTICAL study (79,148 cases and 61,106 controls), and the FinnGen cohort (13,216 cases and 119,948 controls). To prioritize potential causal targets, additional analysis, including the protein-protein interaction (PPI), The Cancer Genome Atlas (TCGA) dataset, and the single-cell-type expression analysis, was performed. Generally, a total of 150 common significant genes with the same causal association with PCa were identified. Out of the 150 genes examined, 67.33% (101/150) were found to have protein-coding functions, while only 30.67% (46/150) of these genes had prior mentions in the scientific literature. Notably, the analysis of the TCGA dataset showed that only 44.67% (67/150) of the genes produced consistent results with the Mendelian randomization (MR) analysis. Furthermore, the evaluation of single-cell RNA-seq data and colocalization analysis singled out MSMB as a critical gene associated with the occurrence of PCa. We pinpointed a range of prostate-specific genes that display causal associations with the onset of PCa. Among these, the MSMB gene emerged as a pivotal factor linked to PCa, demonstrating robust consistency across all four assessments, including the MR, TCGA dataset, single-cell RNA-seq data, and colocalization analysis. These findings provided fresh perspectives on the pathogenesis of PCa and presented potential targets for drug development.

Roles and therapeutic potential of the SLC family in prostate cancer-literature review

Prostate cancer (PCa) is one of the most common malignancies in men worldwide. Despite advances in treatment, many patients develop resistance to conventional therapies. Solute carrier (SLC) proteins, as transmembrane transporters, have recently emerged as potential therapeutic targets due to their role in tumor metabolism and progression. This review summarizes the key roles of six SLC proteins in PCa, including their involvement in metabolic reprogramming, regulation of signaling pathways, and effects on the tumor microenvironment. Although targeting of SLC family members in prostate cancer remains an underexplored area, the growing body of evidence suggests that it holds potential for future development.

Hypusinated and unhypusinated isoforms of the translation factor eIF5A exert distinct effects in models of pancreas development and function

Hypusination of eukaryotic translation initiation factor 5A (eIF5A) is essential for its role in translation elongation and termination. Although the function of hypusinated eIF5A (eIF5AHyp) in cellular proliferation is well characterized, the role of its unhypusinated form (eIF5ALys) remains unclear. We hypothesized that eIF5ALys exerts independent and negative effects on cellular replication and metabolism, distinct from the loss of eIF5AHyp. To test this hypothesis, we utilized zebrafish and mouse models with inducible knockdowns of deoxyhypusine synthase (DHPS) and eIF5A to investigate their roles in cellular growth. Gene expression analysis via RNA sequencing and morphometric measurements of pancreas and β-cell mass were performed to assess phenotypic changes and identify affected biological pathways. Loss of DHPS in zebrafish resulted in significant defects in pancreatic growth, accompanied by changes in gene expression related to mRNA translation, neurogenesis, and stress pathways. By contrast, knockdown of eIF5A had minimal impact on pancreas development, suggesting that the effects of DHPS loss are not solely because of the lack of eIF5AHyp. In mice, β-cell-specific deletion of DHPS impaired β-cell mass expansion and glucose tolerance, whereas eIF5A deletion had no statistically significant effects. These findings provide evidence for an independent role for eIF5ALys in regulating developmental and functional responses in pancreas health and disease.

Cellular Signaling of Amino Acid Metabolism in Prostate Cancer

Prostate cancer is one of the most common malignancies affecting men worldwide and a leading cause of cancer-related mortality, necessitating a deeper understanding of its underlying biochemical pathways. Similar to other cancer types, prostate cancer is also characterised by aberrantly activated metabolic pathways that support tumour development, such as amino acid metabolism, which is involved in modulating key physiological and pathological cellular processes during the progression of this disease. The metabolism of several amino acids, such as glutamine and methionine, crucial for tumorigenesis, is dysregulated and commonly discussed in prostate cancer. And the roles of some less studied amino acids, such as histidine and glycine, have also been covered in prostate cancer studies. Aberrant regulation of two major signalling pathways, mechanistic target of rapamycin (mTOR) and general amino acid control non-depressible 2 (GCN2), is a key driver of reshaping the amino acid metabolism landscape in prostate cancer. By summarising our current understanding of how amino acid metabolism is modulated in prostate cancer, here, we provide further insights into certain potential therapeutic targets for managing prostate cancer through metabolic interventions.

High-level L-Gln compromises intestinal amino acid utilization efficiency and inhibits protein synthesis by GCN2/eIF2α/ATF4 signaling pathway in piglets fed low-crude protein diets

Gln, one of the most abundant amino acids (AA) in the body, performs a diverse range of fundamental physiological functions. However, information about the role of dietary Gln on AA levels, transporters, protein synthesis, and underlying mechanisms in vivo is scarce. The present study aimed to explore the effects of low-crude protein diet inclusion with differential doses of L-Gln on intestinal AA levels, transporters, protein synthesis, and potential mechanisms in weaned piglets. A total of 128 healthy weaned piglets (Landrace × Yorkshire) were randomly allocated into four treatments with four replicates. Pigs in the four groups were fed a low-crude protein diet containing 0%, 1%, 2%, or 3% L-Gln for 28 d. L-Gln administration markedly (linear, P < 0.05) increased Ala, Arg, Asn, Asp, Glu, Gln, His, Ile, Lys, Met, Orn, Phe, Ser, Thr, Tyr, and Val levels and promoted trypsin activity in the jejunal content of piglets. Moreover, L-Gln treatment significantly enhanced concentrations of colonic Gln and Trp, and serum Thr (linear, P < 0.01), and quadratically increased serum Lys and Phe levels (P < 0.05), and decreased plasma Glu, Ile, and Leu levels (linear, P < 0.05). Further investigation revealed that L-Gln administration significantly upregulated Atp1a1, Slc1a5, Slc3a2, Slc6a14, Slc7a5, Slc7a7, and Slc38a1 relative expressions in the jejunum (linear, P < 0.05). Additionally, dietary supplementation with L-Gln enhanced protein abundance of general control nonderepressible 2 (GCN2, P = 0.010), phosphorylated eukaryotic initiation factor 2 subunit alpha (eIF2α, P < 0.001), and activating transcription factor 4 (ATF4) in the jejunum of piglets (P = 0.008). These results demonstrated for the first time that a low crude protein diet with high-level L-Gln inclusion exhibited side effects on piglets. Specifically, 2% and 3% L-Gln administration exceeded the intestinal utilization capacity and compromised the jejunal AA utilization efficiency, which is independent of digestive enzyme activities. A high level of L-Gln supplementation would inhibit protein synthesis by GCN2/eIF2α/ATF4 signaling in piglets fed low-protein diets, which, in turn, upregulates certain AA transporters to maintain AA homeostasis.

The causal effect of serum amino acids on the risk of prostate cancer: a two-sample mendelian randomization study

Prostate cancer (PCa) is the second most common malignancy affecting men globally. Recent advances in metabolomics have highlighted significant alterations in specific amino acid (AA) metabolism linked to PCa, indicating their potential utility in diagnosis and therapy. However, no direct causal association between serum AA levels and PCa risk has been established. A total of 35 patients with PCa and 30 individuals with benign prostatic hyperplasia (BPH) were recruited for this study. Targeted metabolomic analysis was performed using ultra-high-performance liquid chromatography-tandem mass spectrometry on serum samples. Two-sample Mendelian randomization (MR) was applied to explore potential causal links between serum AA levels and PCa risk, including mediator effects using dual-phase MR and assessing reverse causality through reverse MR. Results Targeted metabolomic profiling identified six amino acids-glutamate (Glu), Ser, histidine (His), arginine (Arg), aspartic acid (Asp), and glycine (Gly)-that showed significant area under the ROC curve in differentiating between BPH and PCa cases. Notably, Glu demonstrated an inverse association with PCa risk, distinct from the other AAs identified. However, definitive evidence supporting a causal relationship between low Glu levels and increased PCa risk was not observed. Our results suggest a protective role of Glu against PCa development, which may have implications for disease prognosis. Increasing dietary Glu intake may present a potential preventive or therapeutic approach for PCa.

Coordination between the eIF2 kinase GCN2 and p53 signaling supports purine metabolism and the progression of prostate cancer

Cancers invoke various pathways to mitigate external and internal stresses to continue their growth and progression. We previously reported that the eIF2 kinase GCN2 and the integrated stress response are constitutively active in prostate cancer (PCa) and are required to maintain amino acid homeostasis needed to fuel tumor growth. However, although loss of GCN2 function reduces intracellular amino acid availability and PCa growth, there is no appreciable cell death. Here, we discovered that the loss of GCN2 in PCa induces prosenescent p53 signaling. This p53 activation occurred through GCN2 inhibition-dependent reductions in purine nucleotides that impaired ribosome biogenesis and, consequently, induced the impaired ribosome biogenesis checkpoint. p53 signaling induced cell cycle arrest and senescence that promoted the survival of GCN2-deficient PCa cells. Depletion of GCN2 combined with loss of p53 or pharmacological inhibition of de novo purine biosynthesis reduced proliferation and enhanced cell death in PCa cell lines, organoids, and xenograft models. Our findings highlight the coordinated interplay between GCN2 and p53 regulation during nutrient stress and provide insight into how they could be targeted in developing new therapeutic strategies for PCa.

Novel insights into the GCN2 pathway and its targeting. Therapeutic value in cancer and lessons from lung fibrosis development

Defining the mechanisms that allow cells to adapt to environmental stress is critical for understanding the progression of chronic diseases and identifying relevant drug targets. Among these, activation of the pathway controlled by the eIF2-alpha kinase GCN2 is critical for translational and metabolic reprogramming of the cell in response to various metabolic, proteotoxic, and ribosomal stressors. However, its role has frequently been investigated through the lens of a stress pathway signaling via the eIF2α-activating transcription factor 4 (ATF4) downstream axis, while recent advances in the field have revealed that the GCN2 pathway is more complex than previously thought. Indeed, this kinase can be activated through a variety of mechanisms, phosphorylate substrates other than eIF2α, and regulate cell proliferation in a steady state. This review presents recent findings regarding the fundamental mechanisms underlying GCN2 signaling and function, as well as the development of drugs that modulate its activity. Furthermore, by comparing the literature on GCN2's antagonistic roles in two challenging pathologies, cancer and pulmonary diseases, the benefits, and drawbacks of GCN2 targeting, particularly inhibition, are discussed.

Regulatory Role of eIF2αK4 in Amino Acid Transporter Expression in Mouse Brain Capillary Endothelial Cells

PURPOSE

Amino acid transporters are expressed in the brain capillary endothelial cells that form the blood-brain barrier (BBB), and their expression levels change during the neonatal period. This study aimed to investigate the molecular mechanisms regulating amino acid transporter levels in mouse brain capillary endothelial cells.

METHODS

Capillaries were isolated from the brains of neonatal and adult mice. Activation of eukaryotic translation initiation factor 2α kinase 4 (eIF2αK4) was analyzed in MBEC4 (mouse brain capillary endothelial) cells under amino acid-depleted conditions. Protein expression was determined using western blotting and proteomic analyses.

RESULTS

Phosphorylation of eIF2α, a downstream target of eIF2αK4, was induced in the brain capillaries of neonates compared to adults. In vitro experiments using MBEC4 cells revealed that amino acid depletion induced eIF2α phosphorylation and expression of the amino acid transporter, solute carrier (Slc)-7a1. The eIF2αK4 inhibitor, GCN2iB, inhibited these inductions. Proteomic analysis revealed arginine depletion-dependent induction of amino acid transporters Slc1a4, Slc3a2, Slc7a1, Slc7a5, and Slc38a1. These effects were also inhibited by GCN2iB, suggesting the involvement of eIF2αK4 activation. In contrast, the expression of Slc2a1, Slc16a1, Abcb1b, Abcg2, transferrin receptor, insulin receptor, claudin-1, ZO-1, and Jam1 was not suppressed by the GCN2iB treatment.

CONCLUSIONS

Overall, the eIF2αK4 pathway plays a regulatory role in amino acid transporter expression in brain capillary endothelial cells and facilitates the maintenance of amino acid homeostasis in the brain. This study provides new insights into the regulatory mechanisms underlying nutrient transport across the BBB.

Reciprocal Dynamics of Metabolism and mRNA Translation in Tumor Angiogenesis

Angiogenesis, the process of formation of new blood vessels from pre-existing vasculature, is essential for tumor growth and metastasis. Anti-angiogenic treatment targeting vascular endothelial growth factor (VEGF) signaling is a powerful tool to combat tumor growth; however, anti-tumor angiogenesis therapy has shown limited efficacy, with survival benefits ranging from only a few weeks to months. Compensation by upregulation of complementary growth factors and switches to different modes of vascularization have made these types of therapies less effective. Recent evidence suggests that targeting specific players in endothelial metabolism is a valuable therapeutic strategy against tumor angiogenesis. Although it is clear that metabolism can modulate the translational machinery, the reciprocal relationship between metabolism and mRNA translational control during tumor angiogenesis is not fully understood. In this review, we explore emerging examples of how endothelial cell metabolism affects mRNA translation during the formation of blood vessels. A deeper comprehension of these mechanisms could lead to the development of innovative therapeutic strategies for both physiological and pathological angiogenesis.

General control nonderepressible 2 (GCN2) as a therapeutic target in age-related diseases

The function of General Control Nonderepressible 2 (GCN2), an evolutionary-conserved component of the integrated stress response (ISR), has been well-documented across organisms from yeast to mammals. Recently GCN2 has also gained attention for its role in health and disease states. In this review, we provide a brief overview of GCN2, including its structure, activation mechanisms and interacting partners, and explore its potential significance as a therapeutic target in various age-related diseases including neurodegeneration, inflammatory disorders and cancer. Finally, we summarize the barriers to effectively targeting GCN2 for the treatment of disease and to promote a healthier aging process.

Nanodelivery Optimization of IDO1 Inhibitors in Tumor Immunotherapy: Challenges and Strategies

Tryptophan (Trp) metabolism plays a vital role in cancer immunity. Indoleamine 2.3-dioxygenase 1 (IDO1), is a crucial enzyme in the metabolic pathway by which Trp is degraded to kynurenine (Kyn). IDO1-mediated Trp metabolites can inhibit tumor immunity and facilitate immune evasion by cancer cells; thus, targeting IDO1 is a potential tumor immunotherapy strategy. Recently, numerous IDO1 inhibitors have been introduced into clinical trials as immunotherapeutic agents for cancer treatment. However, drawbacks such as low oral bioavailability, slow onset of action, and high toxicity are associated with these drugs. With the continuous development of nanotechnology, medicine is gradually entering an era of precision healthcare. Nanodrugs carried by inorganic, lipid, and polymer nanoparticles (NPs) have shown great potential for tumor therapy, providing new ways to overcome tumor diversity and improve therapeutic efficacy. Compared to traditional drugs, nanomedicines offer numerous significant advantages, including a prolonged half-life, low toxicity, targeted delivery, and responsive release. Moreover, based on the physicochemical properties of these nanomaterials (eg, photothermal, ultrasonic response, and chemocatalytic properties), various combination therapeutic strategies have been developed to synergize the effects of IDO1 inhibitors and enhance their anticancer efficacy. This review is an overview of the mechanism by which the Trp-IDO1-Kyn pathway acts in tumor immune escape. The classification of IDO1 inhibitors, their clinical applications, and barriers for translational development are discussed, the use of IDO1 inhibitor-based nanodrug delivery systems as combination therapy strategies is summarized, and the issues faced in their clinical application are elucidated. We expect that this review will provide guidance for the development of IDO1 inhibitor-based nanoparticle nanomedicines that can overcome the limitations of current treatments, improve the efficacy of cancer immunotherapy, and lead to new breakthroughs in the field of cancer immunotherapy.

Functional validation of EIF2AK4 (GCN2) missense variants associated with pulmonary arterial hypertension

Pulmonary arterial hypertension (PAH) is a disorder with a large genetic component. Biallelic mutations of EIF2AK4, which encodes the kinase GCN2, are causal in two ultra-rare subtypes of PAH, pulmonary veno-occlusive disease and pulmonary capillary haemangiomatosis. EIF2AK4 variants of unknown significance have also been identified in patients with classical PAH, though their relationship to disease remains unclear. To provide patients with diagnostic information and enable family testing, the functional consequences of such rare variants must be determined, but existing computational methods are imperfect. We applied a suite of bioinformatic and experimental approaches to sixteen EIF2AK4 variants that had been identified in patients. By experimentally testing the functional integrity of the integrated stress response (ISR) downstream of GCN2, we determined that existing computational tools have insufficient sensitivity to reliably predict impaired kinase function. We determined experimentally that several EIF2AK4 variants identified in patients with classical PAH had preserved function and are therefore likely to be non-pathogenic. The dysfunctional variants of GCN2 that we identified could be subclassified into three groups: misfolded, kinase-dead, and hypomorphic. Intriguingly, members of the hypomorphic group were amenable to paradoxical activation by a type-1½ GCN2 kinase inhibitor. This experiment approach may aid in the clinical stratification of EIF2AK4 variants and potentially identify hypomorophic alleles receptive to pharmacological activation.

Loss of Gcn2 exacerbates gossypol induced oxidative stress, apoptosis and inflammation in zebrafish

Gossypol, a naturally occurring compound found in cottonseed meal, shows promising therapeutic potential for human diseases. However, within the aquaculture industry, it is considered an antinutritional factor. The incorporation of cottonseed meal into fish feed introduces gossypol, which induces intracellular stresses and hinders overall health of farmed fish. The aim of this study is to determine the role of General control nonderepressible 2 (gcn2), a sensor for intracellular stresses in gossypol-induced stress responses in fish. In the present study, we established two gcn2 knockout zebrafish lines. A feeding trial was conducted to assess the growth-inhibitory effect of gossypol in both wild type and gcn2 knockout zebrafish. The results showed that in the absence of gcn2, zebrafish exhibited increased oxidative stress and apoptosis when exposed to gossypol, resulting in higher mortality rates. In feeding trial, dietary gossypol intensified liver inflammation in gcn2-/- zebrafish, diminishing their growth and feed conversion. Remarkably, administering the antioxidant N-acetylcysteine (NAC) was effective in reversing the gossypol induced oxidative stress and apoptosis, thereby increasing the gossypol tolerance of gcn2-/- zebrafish. Exposure to gossypol induces more severe mitochondrial stress in gcn2-/- zebrafish, thereby inducing metabolic disorders. These results reveal that gcn2 plays a protective role in reducing gossypol-induced oxidative stress and apoptosis, attenuating inflammation responses, and enhancing the survivability of zebrafish in gossypol-challenged conditions. Therefore, maintaining appropriate activation of Gcn2 may be beneficial for fish fed diets containing gossypol.

TFE3-Splicing Factor Fusions Represent Functional Drivers and Druggable Targets in Translocation Renal Cell Carcinoma

TFE3 is a member of the basic helix-loop-helix leucine zipper MiT transcription factor family, and its chimeric proteins are associated with translocation renal cell carcinoma (tRCC). Despite the variety of gene fusions, most TFE3 fusion partner genes are related to spliceosome machinery. Dissecting the function of TFE3 fused to spliceosome machinery factors (TFE3-SF) could direct the development of effective therapies for this lethal disease, which is refractory to standard treatments for kidney cancer. Here, by using a combination of in silico structure prediction, transcriptome profiling, molecular characterization, and high-throughput high-content screening (HTHCS), we interrogated a number of oncogenic mechanisms of TFE3-SF fusions. TFE3-SF fusions drove the transformation of kidney cells and promoted distinct oncogenic phenotypes in a fusion partner-dependent manner, differentially altering the transcriptome and RNA splicing landscape and activating different oncogenic pathways. Inhibiting TFE3-SF dimerization reversed its oncogenic activity and represented a potential target for therapeutic intervention. Screening the FDA-approved drugs library LOPAC and a small-molecule library (Microsource) using HTHCS combined with FRET technology identified compounds that inhibit TFE3-SF dimerization. Hit compounds were validated in 2D and 3D patient-derived xenograft models expressing TFE3-SF. The antihistamine terfenadine decreased cell proliferation and reduced in vivo tumor growth of tRCC. Overall, these results unmask therapeutic strategies to target TFE3-SF dimerization for treating patients with tRCC.

SIGNIFICANCE

TFE3-splicing factor fusions possess both transcription and splicing factor functions that remodel the transcriptome and spliceosome and can be targeted with dimerization inhibitors to suppress the growth of translocation renal cell carcinoma.

Beyond genetics: driving cancer with the tumour microenvironment behind the wheel

Cancer has long been viewed as a genetic disease of cumulative mutations. This notion is fuelled by studies showing that ageing tissues are often riddled with clones of complex oncogenic backgrounds coexisting in seeming harmony with their normal tissue counterparts. Equally puzzling, however, is how cancer cells harbouring high mutational burden contribute to normal, tumour-free mice when allowed to develop within the confines of healthy embryos. Conversely, recent evidence suggests that adult tissue cells expressing only one or a few oncogenes can, in some contexts, generate tumours exhibiting many of the features of a malignant, invasive cancer. These disparate observations are difficult to reconcile without invoking environmental cues triggering epigenetic changes that can either dampen or drive malignant transformation. In this Review, we focus on how certain oncogenes can launch a two-way dialogue of miscommunication between a stem cell and its environment that can rewire downstream events non-genetically and skew the morphogenetic course of the tissue. We review the cells and molecules of and the physical forces acting in the resulting tumour microenvironments that can profoundly affect the behaviours of transformed cells. Finally, we discuss possible explanations for the remarkable diversity in the relative importance of mutational burden versus tumour microenvironment and its clinical relevance.

Integrated Stress Response (ISR) Pathway: Unraveling Its Role in Cellular Senescence

Cellular senescence is a complex process characterized by irreversible cell cycle arrest. Senescent cells accumulate with age, promoting disease development, yet the absence of specific markers hampers the development of selective anti-senescence drugs. The integrated stress response (ISR), an evolutionarily highly conserved signaling network activated in response to stress, globally downregulates protein translation while initiating the translation of specific protein sets including transcription factors. We propose that ISR signaling plays a central role in controlling senescence, given that senescence is considered a form of cellular stress. Exploring the intricate relationship between the ISR pathway and cellular senescence, we emphasize its potential as a regulatory mechanism in senescence and cellular metabolism. The ISR emerges as a master regulator of cellular metabolism during stress, activating autophagy and the mitochondrial unfolded protein response, crucial for maintaining mitochondrial quality and efficiency. Our review comprehensively examines ISR molecular mechanisms, focusing on ATF4-interacting partners, ISR modulators, and their impact on senescence-related conditions. By shedding light on the intricate relationship between ISR and cellular senescence, we aim to inspire future research directions and advance the development of targeted anti-senescence therapies based on ISR modulation.

GCN2 is required to maintain core body temperature in mice during acute cold

Nonshivering thermogenesis in rodents requires macronutrients to fuel the generation of heat during hypothermic conditions. In this study, we examined the role of the nutrient sensing kinase, general control nonderepressible 2 (GCN2) in directing adaptive thermogenesis during acute cold exposure in mice. We hypothesized that GCN2 is required for adaptation to acute cold stress via activation of the integrated stress response (ISR) resulting in liver production of FGF21 and increased amino acid transport to support nonshivering thermogenesis. In alignment with our hypothesis, female and male mice lacking GCN2 failed to adequately increase energy expenditure and veered into torpor. Mice administered a small molecule inhibitor of GCN2 were also profoundly intolerant to acute cold stress. Gcn2 deletion also impeded liver-derived FGF21 but in males only. Within the brown adipose tissue (BAT), acute cold exposure increased ISR activation and its transcriptional execution in males and females. RNA sequencing in BAT identified transcripts that encode actomyosin mechanics and transmembrane transport as requiring GCN2 during cold exposure. These transcripts included class II myosin heavy chain and amino acid transporters, critical for maximal thermogenesis during cold stress. Importantly, Gcn2 deletion corresponded with higher circulating amino acids and lower intracellular amino acids in the BAT during cold stress. In conclusion, we identify a sex-independent role for GCN2 activation to support adaptive thermogenesis via uptake of amino acids into brown adipose.NEW & NOTEWORTHY This paper details the discovery that GCN2 activation is required in both male and female mice to maintain core body temperature during acute cold exposure. The results point to a novel role for GCN2 in supporting adaptive thermogenesis via amino acid transport and actomyosin mechanics in brown adipose tissue.

The human LAT1-4F2hc (SLC7A5-SLC3A2) transporter complex: Physiological and pathophysiological implications

LAT1 and 4F2hc form a heterodimeric membrane protein complex, which functions as one of the best characterized amino acid transporters. Since LAT1-4F2hc is required for the efficient uptake of essential amino acids and hormones, it promotes cellular growth, in part, by stimulating mTORC1 (mechanistic target of rapamycin complex 1) signalling and by repressing the integrated stress response (ISR). Gain or loss of LAT1-4F2hc function is associated with cancer, diabetes, and immunological and neurological diseases. Hence, LAT1-4F2hc represents an attractive drug target for disease treatment. Specific targeting of LAT1-4F2hc will be facilitated by the increasingly detailed understanding of its molecular architecture, which provides important concepts for its function and regulation. Here, we summarize (i) structural insights that help to explain how LAT1 and 4F2hc assemble to transport amino acids across membranes, (ii) the role of LAT1-4F2hc in key metabolic signalling pathways, and (iii) how derailing these processes could contribute to diseases.

Inhibition of GCN2 Reveals Synergy with Cell-Cycle Regulation and Proteostasis

The integrated stress response is a signaling network comprising four branches, each sensing different cellular stressors, converging on the phosphorylation of eIF2α to downregulate global translation and initiate recovery. One of these branches includes GCN2, which senses cellular amino acid insufficiency and participates in maintaining amino acid homeostasis. Previous studies have shown that GCN2 is a viable cancer target when amino acid stress is induced by inhibiting an additional target. In this light, we screened numerous drugs for their potential to synergize with the GCN2 inhibitor TAP20. The drug sensitivity of six cancer cell lines to a panel of 25 compounds was assessed. Each compound was then combined with TAP20 at concentrations below their IC50, and the impact on cell growth was evaluated. The strongly synergistic combinations were further characterized using synergy analyses and matrix-dependent invasion assays. Inhibitors of proteostasis and the MEK-ERK pathway, as well as the pan-CDK inhibitors, flavopiridol, and seliciclib, were potently synergistic with TAP20 in two cell lines. Among their common CDK targets was CDK7, which was more selectively targeted by THZ-1 and synergized with TAP20. Moreover, these combinations were partially synergistic when assessed using matrix-dependent invasion assays. However, TAP20 alone was sufficient to restrict invasion at concentrations well below its growth-inhibitory IC50. We conclude that GCN2 inhibition can be further explored in vivo as a cancer target.

Cellular Stress: Modulator of Regulated Cell Death

Cellular stress response activates a complex program of an adaptive response called integrated stress response (ISR) that can allow a cell to survive in the presence of stressors. ISR reprograms gene expression to increase the transcription and translation of stress response genes while repressing the translation of most proteins to reduce the metabolic burden. In some cases, ISR activation can lead to the assembly of a cytoplasmic membraneless compartment called stress granules (SGs). ISR and SGs can inhibit apoptosis, pyroptosis, and necroptosis, suggesting that they guard against uncontrolled regulated cell death (RCD) to promote organismal homeostasis. However, ISR and SGs also allow cancer cells to survive in stressful environments, including hypoxia and during chemotherapy. Therefore, there is a great need to understand the molecular mechanism of the crosstalk between ISR and RCD. This is an active area of research and is expected to be relevant to a range of human diseases. In this review, we provided an overview of the interplay between different cellular stress responses and RCD pathways and their modulation in health and disease.

The integrated stress response in cancer progression: a force for plasticity and resistance

During their quest for growth, adaptation, and survival, cancer cells create a favorable environment through the manipulation of normal cellular mechanisms. They increase anabolic processes, including protein synthesis, to facilitate uncontrolled proliferation and deplete the tumor microenvironment of resources. As a dynamic adaptation to the self-imposed oncogenic stress, cancer cells promptly hijack translational control to alter gene expression. Rewiring the cellular proteome shifts the phenotypic balance between growth and adaptation to promote therapeutic resistance and cancer cell survival. The integrated stress response (ISR) is a key translational program activated by oncogenic stress that is utilized to fine-tune protein synthesis and adjust to environmental barriers. Here, we focus on the role of ISR signaling for driving cancer progression. We highlight mechanisms of regulation for distinct mRNA translation downstream of the ISR, expand on oncogenic signaling utilizing the ISR in response to environmental stresses, and pinpoint the impact this has for cancer cell plasticity during resistance to therapy. There is an ongoing need for innovative drug targets in cancer treatment, and modulating ISR activity may provide a unique avenue for clinical benefit.

ELAC2 is a functional prostate cancer risk allele

Stentenbach and colleagues have unveiled a functional role of a human germline mutation found in the ribonuclease (RNase) Z enzyme, ELAC2, in prostate cancer. Here, we discuss the importance of these findings in enhancing our understanding of how risk variants enable prostate cancer progression and the post-transcriptional mechanisms underlying oncogenesis.

Non-Essential Amino Acid Availability Influences Proteostasis and Breast Cancer Cell Survival During Proteotoxic Stress

Protein homeostasis (proteostasis) regulates tumor growth and proliferation when cells are exposed to proteotoxic stress, such as during treatment with certain chemotherapeutics. Consequently, cancer cells depend to a greater extent on stress signaling, and require the integrated stress response (ISR), amino acid metabolism, and efficient protein folding and degradation pathways to survive. To define how these interconnected pathways are wired when cancer cells are challenged with proteotoxic stress, we investigated how amino acid abundance influences cell survival when Hsp70, a master proteostasis regulator, is inhibited. We previously demonstrated that cancer cells exposed to a specific Hsp70 inhibitor induce the ISR via the action of two sensors, GCN2 and PERK, in stress-resistant and sensitive cells, respectively. In resistant cells, the induction of GCN2 and autophagy supported resistant cell survival, yet the mechanism by which these events were induced remained unclear. We now report that amino acid availability reconfigures the proteostasis network. Amino acid supplementation, and in particular arginine addition, triggered cancer cell death by blocking autophagy. Consistent with the importance of amino acid availability, which when limited activates GCN2, resistant cancer cells succumbed when challenged with a potentiator for another amino acid sensor, mTORC1, in conjunction with Hsp70 inhibition.

IMPLICATIONS

These data position amino acid abundance, GCN2, mTORC1, and autophagy as integrated therapeutic targets whose coordinated modulation regulates the survival of proteotoxic-resistant breast cancer cells.

Activation of the integrated stress response (ISR) pathways in response to Ref-1 inhibition in human pancreatic cancer and its tumor microenvironment

Pancreatic cancer or pancreatic ductal adenocarcinoma (PDAC) is characterized by a profound inflammatory tumor microenvironment (TME) with high heterogeneity, metastatic propensity, and extreme hypoxia. The integrated stress response (ISR) pathway features a family of protein kinases that phosphorylate eukaryotic initiation factor 2 (eIF2) and regulate translation in response to diverse stress conditions, including hypoxia. We previously demonstrated that eIF2 signaling pathways were profoundly affected in response to Redox factor-1 (Ref-1) knockdown in human PDAC cells. Ref-1 is a dual function enzyme with activities of DNA repair and redox signaling, responds to cellular stress, and regulates survival pathways. The redox function of Ref-1 directly regulates multiple transcription factors including HIF-1α, STAT3, and NF-κB, which are highly active in the PDAC TME. However, the mechanistic details of the crosstalk between Ref-1 redox signaling and activation of ISR pathways are unclear. Following Ref-1 knockdown, induction of ISR was observed under normoxic conditions, while hypoxic conditions were sufficient to activate ISR irrespective of Ref-1 levels. Inhibition of Ref-1 redox activity increased expression of p-eIF2 and ATF4 transcriptional activity in a concentration-dependent manner in multiple human PDAC cell lines, and the effect on eIF2 phosphorylation was PERK-dependent. Treatment with PERK inhibitor, AMG-44 at high concentrations resulted in activation of the alternative ISR kinase, GCN2 and induced levels of p-eIF2 and ATF4 in both tumor cells and cancer-associated fibroblasts (CAFs). Combination treatment with inhibitors of Ref-1 and PERK enhanced cell killing effects in both human pancreatic cancer lines and CAFs in 3D co-culture, but only at high doses of PERK inhibitors. This effect was completely abrogated when Ref-1 inhibitors were used in combination with GCN2 inhibitor, GCN2iB. We demonstrate that targeting of Ref-1 redox signaling activates the ISR in multiple PDAC lines and that this activation of ISR is critical for inhibition of the growth of co-culture spheroids. Combination effects were only observed in physiologically relevant 3D co-cultures, suggesting that the model system utilized can greatly affect the outcome of these targeted agents. Inhibition of Ref-1 signaling induces cell death through ISR signaling pathways, and combination of Ref-1 redox signaling blockade with ISR activation could be a novel therapeutic strategy for PDAC treatment.

Activation of Gcn2 by small molecules designed to be inhibitors

The integrated stress response (ISR) is an important mechanism by which cells confer protection against environmental stresses. Central to the ISR is a collection of related protein kinases that monitor stress conditions, such as Gcn2 (EIF2AK4) that recognizes nutrient limitations, inducing phosphorylation of eukaryotic translation initiation factor 2 (eIF2). Gcn2 phosphorylation of eIF2 lowers bulk protein synthesis, conserving energy and nutrients, coincident with preferential translation of stress-adaptive gene transcripts, such as that encoding the Atf4 transcriptional regulator. While Gcn2 is central for cell protection to nutrient stress and its depletion in humans leads to pulmonary disorders, Gcn2 can also contribute to the progression of cancers and facilitate neurological disorders during chronic stress. Consequently, specific ATP-competitive inhibitors of Gcn2 protein kinase have been developed. In this study, we report that one such Gcn2 inhibitor, Gcn2iB, can activate Gcn2, and we probe the mechanism by which this activation occurs. Low concentrations of Gcn2iB increase Gcn2 phosphorylation of eIF2 and enhance Atf4 expression and activity. Of importance, Gcn2iB can activate Gcn2 mutants devoid of functional regulatory domains or with certain kinase domain substitutions derived from Gcn2-deficient human patients. Other ATP-competitive inhibitors can also activate Gcn2, although there are differences in their mechanisms of activation. These results provide a cautionary note about the pharmacodynamics of eIF2 kinase inhibitors in therapeutic applications. Compounds designed to be kinase inhibitors that instead directly activate Gcn2, even loss of function variants, may provide tools to alleviate deficiencies in Gcn2 and other regulators of the ISR.

Multiple Roles of the Stress Sensor GCN2 in Immune Cells

The serine/threonine-protein kinase general control nonderepressible 2 (GCN2) is a well-known stress sensor that responds to amino acid starvation and other stresses, making it critical to the maintenance of cellular and organismal homeostasis. More than 20 years of research has revealed the molecular structure/complex, inducers/regulators, intracellular signaling pathways and bio-functions of GCN2 in various biological processes, across an organism's lifespan, and in many diseases. Accumulated studies have demonstrated that the GCN2 kinase is also closely involved in the immune system and in various immune-related diseases, such as GCN2 acts as an important regulatory molecule to control macrophage functional polarization and CD4⁺ T cell subset differentiation. Herein, we comprehensively summarize the biological functions of GCN2 and discuss its roles in the immune system, including innate and adaptive immune cells. We also discuss the antagonism of GCN2 and mTOR pathways in immune cells. A better understanding of GCN2's functions and signaling pathways in the immune system under physiological, stressful, and pathological situations will be beneficial to the development of potential therapies for many immune-relevant diseases.

Prognostic Significance of Amino Acid Metabolism-Related Genes in Prostate Cancer Retrieved by Machine Learning

Prostate cancer is among the leading cancers according to both incidence and mortality. Due to the high molecular, morphological and clinical heterogeneity, the course of prostate cancer ranges from slow growth that usually does not require immediate therapeutic intervention to aggressive and fatal disease that spreads quickly. However, currently available biomarkers cannot precisely predict the course of a disease, and novel strategies are needed to guide prostate cancer management. Amino acids serve numerous roles in cancers, among which are energy production, building block reservoirs, maintenance of redox homeostasis, epigenetic regulation, immune system modulation and resistance to therapy. In this article, by using The Cancer Genome Atlas (TCGA) data, we found that the expression of amino acid metabolism-related genes is highly aberrant in prostate cancer, which holds potential to be exploited in biomarker design or in treatment strategies. This change in expression is especially evident for catabolism genes and transporters from the solute carrier family. Furthermore, by using recursive partitioning, we confirmed that the Gleason score is strongly prognostic for progression-free survival. However, the expression of the genes SERINC3 (phosphatidylserine and sphingolipids generation) and CSAD (hypotaurine generation) can refine prognosis for high and low Gleason scores, respectively. Therefore, our results hold potential for novel prostate cancer progression biomarkers.

GCN2 eIF2 kinase promotes prostate cancer by maintaining amino acid homeostasis

A stress adaptation pathway termed the integrated stress response has been suggested to be active in many cancers including prostate cancer (PCa). Here, we demonstrate that the eIF2 kinase GCN2 is required for sustained growth in androgen-sensitive and castration-resistant models of PCa both in vitro and in vivo, and is active in PCa patient samples. Using RNA-seq transcriptome analysis and a CRISPR-based phenotypic screen, GCN2 was shown to regulate expression of over 60 solute-carrier (SLC) genes, including those involved in amino acid transport and loss of GCN2 function reduces amino acid import and levels. Addition of essential amino acids or expression of 4F2 (SLC3A2) partially restored growth following loss of GCN2, suggesting that GCN2 targeting of SLC transporters is required for amino acid homeostasis needed to sustain tumor growth. A small molecule inhibitor of GCN2 showed robust in vivo efficacy in androgen-sensitive and castration-resistant mouse models of PCa, supporting its therapeutic potential for the treatment of PCa.