The tRNA-GCN2-FBXO22-axis-mediated mTOR ubiquitination senses amino acid insufficiency

Structural insights into allosteric inhibition of HRI kinase by heme binding via HDX-MS

Heme-regulated inhibitor (HRI) is one of the four mammalian kinases that phosphorylate eIF2α, facilitating a cellular response to stress through the regulation of mRNA translation. Originally identified as a heme sensor in erythroid progenitor cells, HRI has since emerged as a potential therapeutic target in both cancer and neurodegeneration. Here, we characterise two modes of HRI inhibition using structural mass spectrometry, biochemistry, and biophysics. We examined several competitive ATP-mimetic inhibitors - dabrafenib, encorafenib, and GCN2iB - and compared them with the heme-mimetic allosteric inhibitor, hemin. By combining hydrogen-deuterium exchange mass spectrometry with protein models generated by AlphaFold 3, we investigated the structural basis of inhibition by dabrafenib and hemin. Our analysis revealed that hemin inhibition induces large-scale structural rearrangements in HRI, which are not observed with ATP-mimetic inhibitors. Our results suggest that HRI may be inhibited using two distinctly different modalities, which may guide future drug development.

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.

FBXO22 deficiency defines a pleiotropic syndrome of growth restriction and multi-system anomalies associated with a unique epigenetic signature

FBXO22 encodes an F-box protein, which acts as a substrate-recognition component of the SKP1-CUL1-F-box (SCF) E3 ubiquitin ligase complex. Despite its known roles in the post-translational ubiquitination and degradation of specific substrates, including histone demethylases, the impact of FBXO22 on human development remains unknown. Here, we characterize a pleiotropic syndrome with prominent prenatal onset growth restriction and notable neurodevelopmental delay across 16 cases from 14 families. Through exome and genome sequencing, we identify four distinct homozygous FBXO22 variants with loss-of-function effects segregating with the disease: three predicted to lead to premature translation termination due to frameshift effects and a single-amino-acid-deletion variant, which, we show, impacts protein stability in vitro. We confirm that affected primary fibroblasts with a frameshift mutation are bereft of endogenous FBXO22 and show increased levels of the known substrate histone H3K9 demethylase KDM4B. Accordingly, we delineate a unique epigenetic signature for this disease in peripheral blood via long-read sequencing. Altogether, we identify and demonstrate that FBXO22 deficiency leads to a pleiotropic syndrome in humans, encompassing growth restriction and neurodevelopmental delay, the pathogenesis of which may be explained by broad chromatin alterations.

Methionine is an essential amino acid in doxorubicin-induced cardiotoxicity through modulating mitophagy

Doxorubicin (Dox) is a widely used anticancer drug. However, its time- and dose-dependent side effects, particularly severe cardiotoxicity, limit its clinical use. Understanding the molecular mechanisms underlying Dox-induced cardiotoxicity has become a research focus in recent years. Among these, impaired mitophagy which participated in the process of damaged mitochondria clearance, is considered one of the key mechanisms in Dox-induced cardiomyopathy. Methionine (Met) is an essential amino acid that plays a crucial role in various biological processes. This study aims to investigate the role and mechanism of Met in regulating mitophagy in Dox-induced cardiotoxicity. Met deficiency exacerbated Dox-induced cardiotoxicity, primarily by promoting oxidative stress, affecting mitochondria integrity, disrupting autophagy, and thus leading to cardiomyocyte damage and aggravating heart failure. In addition, Met supplementation alleviated Dox-induced cardiotoxicity, via the general control nonderepessible 2 (GCN2) pathway. This study extends our understanding of the relationship between amino acid metabolism and Dox-induced cardiotoxicity, and indicating the Met-GCN2 axis as a promising therapeutic strategy for Dox-induced cardiotoxicity.

The roles of arginases and arginine in immunity

Arginase activity and arginine metabolism in immune cells have important consequences for health and disease. Their dysregulation is commonly observed in cancer, autoimmune disorders and infectious diseases. Following the initial description of a role for arginase in the dysfunction of T cells mounting an antitumour response, numerous studies have broadened our understanding of the regulation and expression of arginases and their integration with other metabolic pathways. Here, we highlight the differences in arginase compartmentalization and storage between humans and rodents that should be taken into consideration when assessing the effects of arginase activity. We detail the roles of arginases, arginine and its metabolites in immune cells and their effects in the context of cancer, autoimmunity and infectious disease. Finally, we explore potential therapeutic strategies targeting arginases and arginine.

5-Methylcytosine Methylation-Linked Hippo Pathway Molecular Interactions Regulate Lipid Metabolism

5-methylcytosine (5mC) is a common form of DNA methylation, essentially acting as an epigenetic modification that regulates gene expression by affecting the binding of transcription factors to DNA or by recruiting proteins that make it difficult to recognize and transcribe genes. 5mC methylation is present in eukaryotes in a variety of places, such as in CpG islands, within gene bodies, and in regions of repetitive sequences, whereas in prokaryotic organisms, it is mainly present in genomic DNA. The Hippo pathway is a highly conserved signal transduction pathway, which is extremely important in cell proliferation and death, controlling the size of tissues and organs and regulating cell differentiation, in addition to its important regulatory roles in lipid synthesis, transport, and catabolism. Lipid metabolism is an important part of various metabolic pathways in the human body, and problems in lipid metabolism are related to abnormalities in key enzymes, related proteins, epigenetic inheritance, and certain specific amino acids, which are the key factors affecting its proper regulation. In this article, we will introduce the molecular mechanisms of 5mC methylation and the Hippo signaling pathway, and the possibility of their co-regulation of lipid metabolism, with the aim of providing new ideas for further research and novel therapeutic modalities for lipid metabolism and a reference for the development and exploration of related research.

Metabolism Meets Translation: Dietary and Metabolic Influences on tRNA Modifications and Codon Biased Translation

Transfer RNA (tRNA) is not merely a passive carrier of amino acids, but an active regulator of mRNA translation controlling codon bias and optimality. The synthesis of various tRNA modifications is regulated by many "writer" enzymes, which utilize substrates from metabolic pathways or dietary sources. Metabolic and bioenergetic pathways, such as one-carbon (1C) metabolism and the tricarboxylic acid (TCA) cycle produce essential substrates for tRNA modifications synthesis, such as S-Adenosyl methionine (SAM), sulfur species, and α-ketoglutarate (α-KG). The activity of these metabolic pathways can directly impact codon decoding and translation via regulating tRNA modifications levels. In this review, we discuss the complex interactions between diet, metabolism, tRNA modifications, and mRNA translation. We discuss how nutrient availability, bioenergetics, and intermediates of metabolic pathways, modulate the tRNA modification landscape to fine-tune protein synthesis. Moreover, we highlight how dysregulation of these metabolic-tRNA interactions contributes to disease pathogenesis, including cancer, metabolic disorders, and neurodegenerative diseases. We also discuss the new emerging field of GlycoRNA biology drawing parallels from glycobiology and metabolic diseases to guide future directions in this area. Throughout our discussion, we highlight the links between specific modifications, their metabolic/dietary precursors, and various diseases, emphasizing the importance of a metabolism-centric tRNA view in understanding many pathologies. Future research should focus on uncovering the interplay between metabolism and tRNA in specific cellular and disease contexts. Addressing these gaps will guide new research into novel disease interventions.

Anti-Tumor Strategies Targeting Nutritional Deprivation: Challenges and Opportunities

Higher and richer nutrient requirements are typical features that distinguish tumor cells from AU: cells, ensuring adequate substrates and energy sources for tumor cell proliferation and migration. Therefore, nutrient deprivation strategies based on targeted technologies can induce impaired cell viability in tumor cells, which are more sensitive than normal cells. In this review, nutrients that are required by tumor cells and related metabolic pathways are introduced, and anti-tumor strategies developed to target nutrient deprivation are described. In addition to tumor cells, the nutritional and metabolic characteristics of other cells in the tumor microenvironment (including macrophages, neutrophils, natural killer cells, T cells, and cancer-associated fibroblasts) and related new anti-tumor strategies are also summarized. In conclusion, recent advances in anti-tumor strategies targeting nutrient blockade are reviewed, and the challenges and prospects of these anti-tumor strategies are discussed, which are of theoretical significance for optimizing the clinical application of tumor nutrition deprivation strategies.

Harnessing amino acid pathways to influence myeloid cell function in tumor immunity

Amino acids are pivotal regulators of immune cell metabolism, signaling pathways, and gene expression. In myeloid cells, these processes underlie their functional plasticity, enabling shifts between pro-inflammatory, anti-inflammatory, pro-tumor, and anti-tumor activities. Within the tumor microenvironment, amino acid metabolism plays a crucial role in mediating the immunosuppressive functions of myeloid cells, contributing to tumor progression. This review delves into the mechanisms by which specific amino acids-glutamine, serine, arginine, and tryptophan-regulate myeloid cell function and polarization. Furthermore, we explore the therapeutic potential of targeting amino acid metabolism to enhance anti-tumor immunity, offering insights into novel strategies for cancer treatment.

tRNA-Ser-UGA efficiently promotes the rapid release of duck hepatitis A virus from infected enterocytes and its remote dissemination to hepatocytes

Enterocytes are a necessary portal for fecal-oral transmission of viruses, including duck hepatitis A virus (DHAV), that act on the absorption of amino acids (AAs). We note that the rapid death of ducklings caused by DHAV is likely due to its rapid release from enterocytes. However, the underlying mechanism driving the release of DHAV remains poorly understood. Compared to infected fibroblasts, we found that DHAV-infected enterocytes triggered much more rapid viral release and induced swift and diverse remodeling of the mature tRNAome. Surprisingly, we found that tRNA-Ser-UGA in enterocytes was rapidly and specifically upregulated by DHAV infection and was highly correlated with serine decoding of DHAV, which is enriched with UCA codons. Overexpression of tRNA-Ser-UGA in enterocytes promoted rapid DHAV release, whereas overexpression of the cognate tRNA-Ser-GCU in enterocytes or the same tRNA in fibroblasts did not. In enterocytes, inhibition of serine charging of tRNA-Ser-UGA, transfection of a tRNAm-Ala-UGA backbone mutant or a tRNAm-Ser-UGA>CGA anticodon mutant decreased DHAV release. This finding suggests that tRNA-Ser-UGA plays a prominent role in DHAV release in infected enterocytes, which should be supported by efficient protein translation of the virus. Similarly, tRNA-Ser-UGA potently enhances DHAV protein synthesis, and the inhibition of charging of this tRNA or the transfection of the two mutants decreases DHAV protein synthesis. Phenotypically, the overexpression of tRNA-Ser-UGA in enterocytes further accelerates the spread of DHAV to hepatocytes. In conclusion, we revealed a novel tRNA-Ser-UGA that efficiently promotes the rapid release of DHAV by increasing serine decoding in infected enterocytes, thereby promoting remote cell-to-cell dissemination.

Unlocking the potential for optic nerve regeneration over long distances: a multi-therapeutic intervention

Retinal ganglion cells (RGCs) generally fail to regenerate axons, resulting in irreversible vision loss after optic nerve injury. While many studies have shown that modulating specific genes can enhance RGCs survival and promote optic nerve regeneration, inducing long-distance axon regeneration in vivo through single-gene manipulation remains challenging. Nevertheless, combined multi-gene therapies have proven effective in significantly enhancing axonal regeneration. At present, research on promoting optic nerve regeneration remains slow, with most studies unable to achieve axonal growth beyond the optic chiasm or reestablish connections with the brain. Future research priorities include directing axonal growth along correct pathways, facilitating synapse formation and myelination, and modifying the inhibitory microenvironment. These strategies are crucial not only for optic nerve regeneration but also for broader applications in central nervous system repair. In this review, we discuss multifactors therapeutic strategies for optic nerve regeneration, offering insights into advancing nerve regeneration research.

Rap1 and mTOR signaling pathways drive opposing immunotoxic effects of structurally similar aryl-OPFRs, TPHP and TOCP

Aryl organophosphorus flame retardants (aryl-OPFRs), commonly used product additives with close ties to daily life, have been regrettably characterized by multiple well-defined toxicity risks. Triphenyl phosphate (TPHP) and tri-o-cresyl phosphate (TOCP), two structurally similar aryl-OPFRs, were observed in our previous study to exhibit contrasting immunotoxic effects on THP-1 macrophages, yet the underlying mechanisms remain unclear. This study sought to address the knowledge gap by integrating transcriptomic and metabolomic analyses to elucidate the intricate mechanisms. During individual omics analyses, we unfortunately only obtained highly similar results for both TPHP and TOCP, failing to identify the key reasons for their differences. These results revealed comparable disturbances induced by both compounds, including disruptions in nucleic acid synthesis and energy metabolism, blocking ADP to ATP conversion by reducing TCA cycle intermediates, consequently leading to ATP depletion. However, through integrative analysis, specific pathways affected by each compound were successfully identified, shedding light on their unique effects. TPHP reduced GTP levels necessary for Rap1 activation, thereby inhibiting phagocytosis and adhesion of THP-1 macrophages. Conversely, TOCP stimulated the mTOR signaling pathway, enhancing phosphorylation of downstream proteins S6K, RHOA, and PKC, consequently promoting immune responses. This study not only clarified the distinct immunotoxic mechanisms of TPHP and TOCP but also provided critical insights into how structural variations in aryl-OPFRs can lead to markedly different immune responses, thereby informing future risk assessments and regulatory strategies for these compounds.

NUFIP1 integrates amino acid sensing and DNA damage response to maintain the intestinal homeostasis

Nutrient availability strongly affects intestinal homeostasis. Here, we report that low-protein (LP) diets decrease amino acids levels, impair the DNA damage response (DDR), cause DNA damage and exacerbate inflammation in intestinal tissues of male mice with inflammatory bowel disease (IBD). Intriguingly, loss of nuclear fragile X mental retardation-interacting protein 1 (NUFIP1) contributes to the amino acid deficiency-induced impairment of the DDR in vivo and in vitro and induces necroptosis-related spontaneous enteritis. Mechanistically, phosphorylated NUFIP1 binds to replication protein A2 (RPA32) to recruit the ataxia telangiectasia and Rad3-related (ATR)-ATR-interacting protein (ATRIP) complex, triggering the DDR. Consistently, both reintroducing NUFIP1 but not its non-phospho-mutant and inhibition of necroptosis prevent bowel inflammation in male Nufip1 conditional knockout mice. Intestinal inflammation and DNA damage in male mice with IBD can be mitigated by NUFIP1 overexpression. Moreover, NUFIP1 protein levels in the intestine of patients with IBD were found to be significantly decreased. Conclusively, our study uncovers that LP diets contribute to intestinal inflammation by hijacking NUFIP1-DDR signalling and thereby activating necroptosis.

BRAFV600 and ErbB inhibitors directly activate GCN2 in an off-target manner to limit cancer cell proliferation

Targeted kinase inhibitors are well known for their promiscuity and off-target effects. Herein, we define an off-target effect in which several clinical BRAFV600 inhibitors, including the widely used dabrafenib and encorafenib, interact directly with GCN2 to activate the Integrated Stress Response and ATF4. Blocking this off-target effect by co-drugging with a GCN2 inhibitor in A375 melanoma cells causes enhancement rather than suppression of cancer cell outgrowth, suggesting that the off-target activation of GCN2 is detrimental to these cells. This result is mirrored in PC9 lung cancer cells treated with erlotinib, an EGFR inhibitor, that shares the same off-target activation of GCN2. Using an in silico kinase inhibitor screen, we identified dozens of FDA-approved drugs that appear to share this off-target activation of GCN2 and ATF4. Thus, GCN2 activation may modulate the therapeutic efficacy of some kinase inhibitors, depending on the cancer context.

JPH203 alleviates peritoneal fibrosis via inhibition of amino acid-mediated mTORC1 signaling

BACKGROUND AND AIMS

The mesothelial-mesenchymal transition (MMT) of mesothelial cells has been recognized as a critical process during progression of peritoneal fibrosis (PF). Despite its crucial role in amino acid transport and metabolism, the involvement of L-type amino acid transporter 1 (LAT1) and the potential therapeutic role of its inhibitor, JPH203, in fibrotic diseases remain unexplored. Considering the paucity of research on amino acid-mediated mTORC1 activation in PF, our study endeavors to elucidate the protective effects of JPH203 against PF and explore the involvement of amino acid-mediated mTORC1 signaling in this context.

METHODS

We established the transforming growth factor beta 1 (TGF-β1) induced MMT model in primary human mesothelial cells and the peritoneal dialysis fluid (PDF) induced PF model in mice. The therapeutic effects of JPH203 on PF were then examined on these two models by real-time quantitative polymerase chain reaction, western blotting, immunofluorescence staining, Masson's trichrome staining, H&E staining, picro-sirius red staining, and immunohistochemistry. The involvement of amino acid-mediated mTORC1 signaling was screened by RNA sequencing and further verified by western blotting in vitro.

RESULTS

LAT1 was significantly upregulated and JPH203 markedly attenuated fibrotic phenotype both in vitro and in vivo. RNA-seq unveiled a significant enrichment of mTOR signaling pathway in response to JPH203 treatment. Western blotting results indicated that JPH203 alleviates PF by inhibiting amino acid-mediated mTORC1 signaling, which differs from the direct inhibition observed with rapamycin.

CONCLUSION

JPH203 alleviates PF by inhibiting amino acid-mediated mTORC1 signaling.

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.

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.

A nutrigeroscience approach: Dietary macronutrients and cellular senescence

Cellular senescence, a process in which a cell exits the cell cycle in response to stressors, is one of the hallmarks of aging. Senescence and the senescence-associated secretory phenotype (SASP)-a heterogeneous set of secreted factors that disrupt tissue homeostasis and promote the accumulation of senescent cells-reprogram metabolism and can lead to metabolic dysfunction. Dietary interventions have long been studied as methods to combat age-associated metabolic dysfunction, promote health, and increase lifespan. A growing body of literature suggests that senescence is responsive to diet, both to calories and specific dietary macronutrients, and that the metabolic benefits of dietary interventions may arise in part through reducing senescence. Here, we review what is currently known about dietary macronutrients' effect on senescence and the SASP, the nutrient-responsive molecular mechanisms that may mediate these effects, and the potential for these findings to inform the development of a nutrigeroscience approach to healthy aging.

Phosphorylation of GCN2 by mTOR confers adaptation to conditions of hyper-mTOR activation under stress

Adaptation to the shortage in free amino acids (AA) is mediated by 2 pathways, the integrated stress response (ISR) and the mechanistic target of rapamycin (mTOR). In response to reduced levels, primarily of leucine or arginine, mTOR in its complex 1 configuration (mTORC1) is suppressed leading to a decrease in translation initiation and elongation. The eIF2α kinase general control nonderepressible 2 (GCN2) is activated by uncharged tRNAs, leading to induction of the ISR in response to a broader range of AA shortage. ISR confers a reduced translation initiation, while promoting the selective synthesis of stress proteins, such as ATF4. To efficiently adapt to AA starvation, the 2 pathways are cross-regulated at multiple levels. Here we identified a new mechanism of ISR/mTORC1 crosstalk that optimizes survival under AA starvation, when mTORC1 is forced to remain active. mTORC1 activation during acute AA shortage, augmented ATF4 expression in a GCN2-dependent manner. Under these conditions, enhanced GCN2 activity was not dependent on tRNA sensing, inferring a different activation mechanism. We identified a labile physical interaction between GCN2 and mTOR that results in a phosphorylation of GCN2 on serine 230 by mTOR, which promotes GCN2 activity. When examined under prolonged AA starvation, GCN2 phosphorylation by mTOR promoted survival. Our data unveils an adaptive mechanism to AA starvation, when mTORC1 evades inhibition.

The ribotoxic stress response drives UV-mediated cell death

While ultraviolet (UV) radiation damages DNA, eliciting the DNA damage response (DDR), it also damages RNA, triggering transcriptome-wide ribosomal collisions and eliciting a ribotoxic stress response (RSR). However, the relative contributions, timing, and regulation of these pathways in determining cell fate is unclear. Here we use time-resolved phosphoproteomic, chemical-genetic, single-cell imaging, and biochemical approaches to create a chronological atlas of signaling events activated in cells responding to UV damage. We discover that UV-induced apoptosis is mediated by the RSR kinase ZAK and not through the DDR. We identify two negative-feedback modules that regulate ZAK-mediated apoptosis: (1) GCN2 activation limits ribosomal collisions and attenuates ZAK-mediated RSR and (2) ZAK activity leads to phosphodegron autophosphorylation and its subsequent degradation. These events tune ZAK's activity to collision levels to establish regimes of homeostasis, tolerance, and death, revealing its key role as the cellular sentinel for nucleic acid damage.

The high-affinity tryptophan uptake transport system in human cells

The L-tryptophan (Trp) transport system is highly selective for Trp with affinity in the nanomolar range. This transport system is augmented in human interferon (IFN)-γ-treated and indoleamine 2,3-dioxygenase 1 (IDO1)-expressing cells. Up-regulated cellular uptake of Trp causes a reduction in extracellular Trp and initiates immune suppression. Recent studies demonstrate that both IDO1 and tryptophanyl-tRNA synthetase (TrpRS), whose expression levels are up-regulated by IFN-γ, play a pivotal role in high-affinity Trp uptake into human cells. Furthermore, overexpression of tryptophan 2,3-dioxygenase (TDO2) elicits a similar effect as IDO1 on TrpRS-mediated high-affinity Trp uptake. In this review, we summarize recent findings regarding this Trp uptake system and put forward a possible molecular mechanism based on Trp deficiency induced by IDO1 or TDO2 and tryptophanyl-AMP production by TrpRS.

Post-translational regulation of the mTORC1 pathway: A switch that regulates metabolism-related gene expression

The mechanistic target of rapamycin complex 1 (mTORC1) is a kinase complex that plays a crucial role in coordinating cell growth in response to various signals, including amino acids, growth factors, oxygen, and ATP. Activation of mTORC1 promotes cell growth and anabolism, while its suppression leads to catabolism and inhibition of cell growth, enabling cells to withstand nutrient scarcity and stress. Dysregulation of mTORC1 activity is associated with numerous diseases, such as cancer, metabolic disorders, and neurodegenerative conditions. This review focuses on how post-translational modifications, particularly phosphorylation and ubiquitination, modulate mTORC1 signaling pathway and their consequential implications for pathogenesis. Understanding the impact of phosphorylation and ubiquitination on the mTORC1 signaling pathway provides valuable insights into the regulation of cellular growth and potential therapeutic targets for related diseases.

The mTORC2 signaling network: targets and cross-talks

The mechanistic target of rapamycin, mTOR, controls cell metabolism in response to growth signals and stress stimuli. The cellular functions of mTOR are mediated by two distinct protein complexes, mTOR complex 1 (mTORC1) and mTORC2. Rapamycin and its analogs are currently used in the clinic to treat a variety of diseases and have been instrumental in delineating the functions of its direct target, mTORC1. Despite the lack of a specific mTORC2 inhibitor, genetic studies that disrupt mTORC2 expression unravel the functions of this more elusive mTOR complex. Like mTORC1 which responds to growth signals, mTORC2 is also activated by anabolic signals but is additionally triggered by stress. mTORC2 mediates signals from growth factor receptors and G-protein coupled receptors. How stress conditions such as nutrient limitation modulate mTORC2 activation to allow metabolic reprogramming and ensure cell survival remains poorly understood. A variety of downstream effectors of mTORC2 have been identified but the most well-characterized mTORC2 substrates include Akt, PKC, and SGK, which are members of the AGC protein kinase family. Here, we review how mTORC2 is regulated by cellular stimuli including how compartmentalization and modulation of complex components affect mTORC2 signaling. We elaborate on how phosphorylation of its substrates, particularly the AGC kinases, mediates its diverse functions in growth, proliferation, survival, and differentiation. We discuss other signaling and metabolic components that cross-talk with mTORC2 and the cellular output of these signals. Lastly, we consider how to more effectively target the mTORC2 pathway to treat diseases that have deregulated mTOR signaling.

mTOR takes charge: Relaying uncharged tRNA levels by mTOR ubiquitination

Nutrient availability is conveyed to the mechanistic target of rapamycin (mTOR), which couples metabolic processes with cell growth and proliferation. How mTOR itself is modulated by amino acid levels remains poorly understood. Ge and colleagues now demonstrate that broad sensing of uncharged tRNAs by GCN2/FBXO22 inactivates mTOR complex 1 (mTORC1) via mTOR ubiquitination.