Control of amino-acid transport by antigen receptors coordinates the metabolic reprogramming essential for T cell differentiation

IL-7 coordinates proliferation, differentiation and Tcra recombination during thymocyte β-selection

Signaling via the pre-T cell antigen receptor (pre-TCR) and the receptor Notch1 induces transient self-renewal (β-selection) of TCRβ(+) CD4(-)CD8(-) double-negative stage 3 (DN3) and DN4 progenitor cells that differentiate into CD4(+)CD8(+) double-positive (DP) thymocytes, which then rearrange the locus encoding the TCR α-chain (Tcra). Interleukin 7 (IL-7) promotes the survival of TCRβ(-) DN thymocytes by inducing expression of the pro-survival molecule Bcl-2, but the functions of IL-7 during β-selection have remained unclear. Here we found that IL-7 signaled TCRβ(+) DN3 and DN4 thymocytes to upregulate genes encoding molecules involved in cell growth and repressed the gene encoding the transcriptional repressor Bcl-6. Accordingly, IL-7-deficient DN4 cells lacked trophic receptors and did not proliferate but rearranged Tcra prematurely and differentiated rapidly. Deletion of Bcl6 partially restored the self-renewal of DN4 cells in the absence of IL-7, but overexpression of BCL2 did not. Thus, IL-7 critically acts cooperatively with signaling via the pre-TCR and Notch1 to coordinate proliferation, differentiation and Tcra recombination during β-selection.

mTOR and metabolic regulation of conventional and regulatory T cells

mTOR signaling links bioenergetic and biosynthetic metabolism to immune responses. mTOR is activated by diverse upstream stimuli, including immune signals, growth factors, and nutrients. Recent studies highlight crucial roles of mTOR signaling in immune functions mediated by conventional T cells and T_regs In this review, we discuss the regulation of mTOR signaling in T cells and the functional impacts of mTOR and metabolic pathways on T cell-mediated immune responses, with a particular focus on the differentiation and function of T_regs.

T-cell metabolism in autoimmune disease

Cancer cells have long been known to fuel their pathogenic growth habits by sustaining a high glycolytic flux, first described almost 90 years ago as the so-called Warburg effect. Immune cells utilize a similar strategy to generate the energy carriers and metabolic intermediates they need to produce biomass and inflammatory mediators. Resting lymphocytes generate energy through oxidative phosphorylation and breakdown of fatty acids, and upon activation rapidly switch to aerobic glycolysis and low tricarboxylic acid flux. T cells in patients with rheumatoid arthritis (RA) and systemic lupus erythematosus (SLE) have a disease-specific metabolic signature that may explain, at least in part, why they are dysfunctional. RA T cells are characterized by low adenosine triphosphate and lactate levels and increased availability of the cellular reductant NADPH. This anti-Warburg effect results from insufficient activity of the glycolytic enzyme phosphofructokinase and differentiates the metabolic status in RA T cells from those in cancer cells. Excess production of reactive oxygen species and a defect in lipid metabolism characterizes metabolic conditions in SLE T cells. Owing to increased production of the glycosphingolipids lactosylceramide, globotriaosylceramide and monosialotetrahexosylganglioside, SLE T cells change membrane raft formation and fail to phosphorylate pERK, yet hyperproliferate. Borrowing from cancer metabolomics, the metabolic modifications occurring in autoimmune disease are probably heterogeneous and context dependent. Variations of glucose, amino acid and lipid metabolism in different disease states may provide opportunities to develop biomarkers and exploit metabolic pathways as therapeutic targets.

Insight into the role of mTOR and metabolism in T cells reveals new potential approaches to preventing graft rejection

PURPOSE OF REVIEW

In this review, we discuss the recent advances with regard to the mammalian target of rapamycin (mTOR) signaling pathway and focus on how this pathway modulates immune responses. Overall, these insights provide important clues in terms of strategically integrating mTOR and metabolic inhibitors into transplantation rejection protocols.

RECENT FINDINGS

mTOR is regulated by environmental cues and activates diverse downstream pathways to guide cell growth and fate. What has emerged from recent studies is that mechanistically mTOR directs T cell differentiation and function in part by regulating metabolic programs. Such findings not only inform us with regard to the metabolic demands of effector and memory T cells but also elucidate metabolic pathways that might be targeted to selectively regulate immune responses.

SUMMARY

Initial studies focused on the ability of the mTOR inhibitor rapamycin to suppress immune responses by inhibiting T cell proliferation. Since then, both pharmacologic and genetic studies have revealed a central role for mTOR in regulating T cell activation, differentiation, and function independent of proliferation. Specifically, it has become clear that mTOR plays an important role in regulating the metabolic machinery necessary for effector, regulatory, and memory T cell generation. As such, direct inhibition of metabolism may emerge as a potent and selective means of preventing graft rejection. This review will discuss new insights regarding the ability of downstream signaling pathways, including mTOR-dependent metabolic pathways in regulating T cell responses. Finally, we will discuss these new insights in the context of developing novel immunoregulatory regimens for transplantation.

mTOR Links Environmental Signals to T Cell Fate Decisions

T cell fate decisions play an integral role in maintaining the health of organisms under homeostatic and inflammatory conditions. The localized microenvironment in which developing and mature T cells reside provides signals that serve essential functions in shaping these fate decisions. These signals are derived from the immune compartment, including antigens, co-stimulation, and cytokines, and other factors, including growth factors and nutrients. The mechanistic target of rapamycin (mTOR), a vital sensor of signals within the immune microenvironment, is a central regulator of T cell biology. In this review, we discuss how various environmental cues tune mTOR activity in T cells, and summarize how mTOR integrates these signals to influence multiple aspects of T cell biology.

Targeting T cell metabolism for therapy

In the past several years a wealth of evidence has emerged illustrating how metabolism supports many aspects of T cell biology, as well as how metabolic changes drive T cell differentiation and fate. We outline developing principles in the regulation of T cell metabolism, and discuss how these processes are affected in settings of inflammation and cancer. In this context we discuss how metabolic pathways might be manipulated for the treatment of human disease, including how metabolism may be targeted to prevent T cell dysfunction in inhospitable microenvironments, to generate more effective adoptive cellular immunotherapies in cancer, and to direct T cell differentiation and function towards non-pathogenic phenotypes in settings of autoimmunity.

mTOR signaling promotes a robust and continuous production of IFN-γ by human memory CD8+ T cells and their proliferation

Human CD8(+) T cells are functionally heterogeneous and can be divided into phenotypically and functionally distinct subsets according to CCR7 and CD45RA expression levels. Among these, CCR7(low) CD45RA(low) effector memory CD8(+) T cells (Tem) and CCR7(low) CD45RA(high) CD8(+) T cells, which are designated as Temra and considered to be terminally differentiated cells, are Ag-experienced T cells but show different functionalities. Here, we show that, while Tem proliferate vigorously and produce IFN-γ persistently and robustly, Temra proliferate poorly and lose the ability to produce IFN-γ over time after TCR stimulation. Temra showed impaired cell growth upon TCR stimulation, which was associated with defective activation of the mammalian target of rapamycin (mTOR) signaling. Furthermore, rapamycin, an inhibitor of mTOR signaling, interfered with the robust and continuous proliferation of and IFN-γ production by Tem at later time points after TCR stimulation. Thus, these data collectively indicate that activation of mTOR signaling is required for the robust functions of Tem cells in humans and suggest that defective mTOR signaling in Temra contributes to their functional impairment.

The energy sensor AMPK regulates T cell metabolic adaptation and effector responses in vivo

Naive T cells undergo metabolic reprogramming to support the increased energetic and biosynthetic demands of effector T cell function. However, how nutrient availability influences T cell metabolism and function remains poorly understood. Here we report plasticity in effector T cell metabolism in response to changing nutrient availability. Activated T cells were found to possess a glucose-sensitive metabolic checkpoint controlled by the energy sensor AMP-activated protein kinase (AMPK) that regulated mRNA translation and glutamine-dependent mitochondrial metabolism to maintain T cell bioenergetics and viability. T cells lacking AMPKα1 displayed reduced mitochondrial bioenergetics and cellular ATP in response to glucose limitation in vitro or pathogenic challenge in vivo. Finally, we demonstrated that AMPKα1 is essential for T helper 1 (Th1) and Th17 cell development and primary T cell responses to viral and bacterial infections in vivo. Our data highlight AMPK-dependent regulation of metabolic homeostasis as a key regulator of T cell-mediated adaptive immunity.

The role of Fatty Acid oxidation in the metabolic reprograming of activated t-cells

Activation represents a significant bioenergetic challenge for T-cells, which must undergo metabolic reprogramming to keep pace with increased energetic demands. This review focuses on the role of fatty acid metabolism, both in vitro and in vivo, following T-cell activation. Based upon previous studies in the literature, as well as accumulating evidence in allogeneic cells, I propose a multi-step model of in vivo metabolic reprogramming. In this model, a primary determinant of metabolic phenotype is the ubiquity and duration of antigen exposure. The implications of this model, as well as the future challenges and opportunities in studying T-cell metabolism, will be discussed.

Impaired T cell function in argininosuccinate synthetase deficiency

ASS1 is a cytosolic enzyme that plays a role in the conversion of citrulline to arginine. In human and mouse tissues, ASS1 protein is found in several components of the immune system, including the thymus and T cells. However, the role of ASS1 in these tissues remains to be defined. Considerable attention has been focused recently on the role of metabolism in T cell differentiation and function. Based on the expression of ASS1 in the immune system, we hypothesized that ASS1 deficiency would result in T cell defects. To evaluate this question, we characterized immune function in hypomorphic fold/fold mice. Analysis of splenic T cells by flow cytometry showed a marked reduction in T cell numbers with normal expression of activation surface markers. Gene therapy correction of liver ASS1 to enhance survival resulted in a partial recovery of splenic T cells for characterization. In vitro and in vivo studies demonstrated the persistence of the ASS1 enzyme defect in T cells and abnormal T cell differentiation and function. Overall, our work suggests that ASS1 plays a role in T cell function, and deficiency produces primary immune dysfunction. In addition, these data suggest that patients with ASS1 deficiency (citrullinemia type I) may have T cell dysfunction.

L-type amino-acid transporter 1 (LAT1): a therapeutic target supporting growth and survival of T-cell lymphoblastic lymphoma/T-cell acute lymphoblastic leukemia

The altered metabolism of cancer cells is a treasure trove to discover new antitumoral strategies. The gene (SLC7A5) encoding system L amino-acid transporter 1 (LAT1) is overexpressed in murine lymphoma cells generated via T-cell deletion of the pten tumor suppressor, and also in human T-cell acute lymphoblastic leukemia (T-ALL)/lymphoma (T-LL) cells. We show here that a potent and LAT1 selective inhibitor (JPH203) decreased leukemic cell viability and proliferation, and induced transient autophagy followed by apoptosis. JPH203 could also alter the in vivo growth of luciferase-expressing-tPTEN-/- cells xenografted into nude mice. In contrast, JPH203 was nontoxic to normal murine thymocytes and human peripheral blood lymphocytes. JPH203 interfered with constitutive activation of mTORC1 and Akt, decreased expression of c-myc and triggered an unfolded protein response mediated by the C/EBP homologous protein (CHOP) transcription factor associated with cell death. A JPH203-resistant tPTEN-/-clone appeared CHOP induction deficient. We also demonstrate that targeting LAT1 may be an efficient broad spectrum adjuvant approach to treat deadly T-cell malignancies as the molecule synergized with rapamycin, dexamethasone, doxorubicin, velcade and l-asparaginase to alter leukemic cell viability.

Serine-threonine kinases in TCR signaling

T lymphocyte proliferation and differentiation are controlled by signaling pathways initiated by the T cell antigen receptor. Here we explore how key serine-threonine kinases and their substrates mediate T cell signaling and coordinate T cell metabolism to meet the metabolic demands of participating in an immune response.

Metabolic programming and PDHK1 control CD4+ T cell subsets and inflammation

Activation of CD4+ T cells results in rapid proliferation and differentiation into effector and regulatory subsets. CD4+ effector T cell (Teff) (Th1 and Th17) and Treg subsets are metabolically distinct, yet the specific metabolic differences that modify T cell populations are uncertain. Here, we evaluated CD4+ T cell populations in murine models and determined that inflammatory Teffs maintain high expression of glycolytic genes and rely on high glycolytic rates, while Tregs are oxidative and require mitochondrial electron transport to proliferate, differentiate, and survive. Metabolic profiling revealed that pyruvate dehydrogenase (PDH) is a key bifurcation point between T cell glycolytic and oxidative metabolism. PDH function is inhibited by PDH kinases (PDHKs). PDHK1 was expressed in Th17 cells, but not Th1 cells, and at low levels in Tregs, and inhibition or knockdown of PDHK1 selectively suppressed Th17 cells and increased Tregs. This alteration in the CD4+ T cell populations was mediated in part through ROS, as N-acetyl cysteine (NAC) treatment restored Th17 cell generation. Moreover, inhibition of PDHK1 modulated immunity and protected animals against experimental autoimmune encephalomyelitis, decreasing Th17 cells and increasing Tregs. Together, these data show that CD4+ subsets utilize and require distinct metabolic programs that can be targeted to control specific T cell populations in autoimmune and inflammatory diseases.

Integrating canonical and metabolic signalling programmes in the regulation of T cell responses

Over the past decade, our understanding of T cell activation, differentiation and function has markedly expanded, providing a greater appreciation of the signals and pathways that regulate these processes. It has become clear that evolutionarily conserved pathways that regulate stress responses, metabolism, autophagy and survival have crucial and specific roles in regulating T cell responses. Recent studies suggest that the metabolic pathways involving MYC, hypoxia-inducible factor 1α (HIF1α), AMP-activated protein kinase (AMPK) and mammalian target of rapamycin (mTOR) are activated upon antigen recognition and that they are required for directing the consequences of T cell receptor engagement. The purpose of this Review is to provide an integrated view of the role of these metabolic pathways and of canonical T cell signalling pathways in regulating the outcome of T cell responses.

Tsc1 promotes the differentiation of memory CD8+ T cells via orchestrating the transcriptional and metabolic programs

Memory CD8(+) T cells are an essential component of protective immunity. Signaling via mechanistic target of rapamycin (mTOR) has been implicated in the regulation of the differentiation of effector and memory T cells. However, little is understood about the mechanisms that control mTOR activity, or the effector pathways regulated by mTOR. We describe here that tuberous sclerosis 1 (Tsc1), a regulator of mTOR signaling, plays a crucial role in promoting the differentiation and function of memory CD8(+) T cells in response to Listeria monocytogenes infection. Mice with specific deletion of Tsc1 in antigen-experienced CD8(+) T cells evoked normal effector responses, but were markedly impaired in the generation of memory T cells and their recall responses to antigen reexposure in a cell-intrinsic manner. Tsc1 deficiency suppressed the generation of memory-precursor effector cells while promoting short-lived effector cell differentiation. Transcriptome analysis indicated that Tsc1 coordinated gene expression programs underlying immune function, transcriptional regulation, and cell metabolism. Furthermore, Tsc1 deletion led to excessive mTORC1 activity and dysregulated glycolytic and oxidative metabolism in response to IL-15 stimulation. These findings establish a Tsc1-mediated checkpoint in linking immune signaling and cell metabolism to orchestrate memory CD8(+) T-cell development and function.

mTORC1-dependent metabolic reprogramming is a prerequisite for NK cell effector function

The mammalian target of rapamycin complex 1 (mTORC1) is a key regulator of cellular metabolism and also has fundamental roles in controlling immune responses. Emerging evidence suggests that these two functions of mTORC1 are integrally linked. However, little is known regarding mTORC1 function in controlling the metabolism and function of NK cells, lymphocytes that play key roles in antiviral and antitumor immunity. This study investigated the hypothesis that mTORC1-controlled metabolism underpins normal NK cell proinflammatory function. We demonstrate that mTORC1 is robustly stimulated in NK cells activated in vivo and in vitro. This mTORC1 activity is required for the production of the key NK cell effector molecules IFN-γ, which is important in delivering antimicrobial and immunoregulatory functions, and granzyme B, a critical component of NK cell cytotoxic granules. The data reveal that NK cells undergo dramatic metabolic reprogramming upon activation, upregulating rates of glucose uptake and glycolysis, and that mTORC1 activity is essential for attaining this elevated glycolytic state. Directly limiting the rate of glycolysis is sufficient to inhibit IFN-γ production and granzyme B expression. This study provides the highly novel insight that mTORC1-mediated metabolic reprogramming of NK cells is a prerequisite for the acquisition of normal effector functions.

Adenosine-mono-phosphate-activated protein kinase-independent effects of metformin in T cells

The anti-diabetic drug metformin regulates T-cell responses to immune activation and is proposed to function by regulating the energy-stress-sensing adenosine-monophosphate-activated protein kinase (AMPK). However, the molecular details of how metformin controls T cell immune responses have not been studied nor is there any direct evidence that metformin acts on T cells via AMPK. Here, we report that metformin regulates cell growth and proliferation of antigen-activated T cells by modulating the metabolic reprogramming that is required for effector T cell differentiation. Metformin thus inhibits the mammalian target of rapamycin complex I signalling pathway and prevents the expression of the transcription factors c-Myc and hypoxia-inducible factor 1 alpha. However, the inhibitory effects of metformin on T cells did not depend on the expression of AMPK in T cells. Accordingly, experiments with metformin inform about the importance of metabolic reprogramming for T cell immune responses but do not inform about the importance of AMPK.

Structure and function of thyroid hormone plasma membrane transporters

Thyroid hormones (TH) cross the plasma membrane with the help of transporter proteins. As charged amino acid derivatives, TH cannot simply diffuse across a lipid bilayer membrane, despite their notorious hydrophobicity. The identification of monocarboxylate transporter 8 (MCT8, SLC16A2) as a specific and very active TH transporter paved the way to the finding that mutations in the MCT8 gene cause a syndrome of psychomotor retardation in humans. The purpose of this review is to introduce the current model of transmembrane transport and highlight the diversity of TH transmembrane transporters. The interactions of TH with plasma transfer proteins, T3 receptors, and deiodinase are summarized. It is shown that proteins may bind TH owing to their hydrophobic character in hydrophobic cavities and/or by specific polar interaction with the phenolic hydroxyl, the aminopropionic acid moiety, and by weak polar interactions with the iodine atoms. These findings are compared with our understanding of how TH transporters interact with substrate. The presumed effects of mutations in MCT8 on protein folding and transport function are explained in light of the available homology model.

Germinal center B cell maintenance and differentiation are controlled by distinct NF-κB transcription factor subunits

Germinal centers (GCs) are the sites where memory B cells and plasma cells producing high-affinity antibodies are generated during T cell-dependent immune responses. The molecular control of GC B cell maintenance and differentiation remains incompletely understood. Activation of the NF-κB signaling pathway has been implicated; however, the distinct roles of the individual NF-κB transcription factor subunits are unknown. We report that GC B cell-specific deletion of the NF-κB subunits c-REL or RELA, which are both activated by the canonical NF-κB pathway, abolished the generation of high-affinity B cells via different mechanisms acting at distinct stages during the GC reaction. c-REL deficiency led to the collapse of established GCs immediately after the formation of dark and light zones at day 7 of the GC reaction and was associated with the failure to activate a metabolic program that promotes cell growth. Conversely, RELA was dispensable for GC maintenance but essential for the development of GC-derived plasma cells due to impaired up-regulation of BLIMP1. These results indicate that activation of the canonical NF-κB pathway in GC B cells controls GC maintenance and differentiation through distinct transcription factor subunits. Our findings have implications for the role of NF-κB in GC lymphomagenesis.

Nutrient Sensing via mTOR in T Cells Maintains a Tolerogenic Microenvironment

We have proposed that tolerance can be maintained through the induction, by Treg cells, of a tolerogenic microenvironment within tolerated tissues that inhibits effector cell activity but which supports the generation of further Treg cells by “infectious tolerance.” Two important components of this tolerogenic microenvironment depend on metabolism and nutrient sensing. The first is due to the up-regulation of multiple enzymes that consume essential amino acids, which are sensed in naïve T cells primarily via inhibition of the mechanistic target of rapamycin (mTOR) pathway, which in turn encourages their further differentiation into FOXP3⁺ Treg cells. The second mechanism is the metabolism of extracellular ATP to adenosine by the ectoenzymes CD39 and CD73. These two enzymes are constitutively co-expressed on Treg cells, but can also be induced on a wide variety of cell types by TGFβ and the adenosine generated can be shown to be a potent inhibitor of T cell proliferation. This review will focus on mechanisms of nutrient sensing in T cells, how these are integrated with TCR and cytokine signals via the mTOR pathway, and what impact this has on intracellular metabolism and subsequently the control of differentiation into different effector or regulatory T cell subsets.

Glucose and glutamine metabolism regulate human hematopoietic stem cell lineage specification

The metabolic state of quiescent hematopoietic stem cells (HSCs) is an important regulator of self-renewal, but it is unclear whether or how metabolic parameters contribute to HSC lineage specification and commitment. Here, we show that the commitment of human and murine HSCs to the erythroid lineage is dependent upon glutamine metabolism. HSCs require the ASCT2 glutamine transporter and active glutamine metabolism for erythroid specification. Blocking this pathway diverts EPO-stimulated HSCs to differentiate into myelomonocytic fates, altering in vivo HSC responses and erythroid commitment under stress conditions such as hemolytic anemia. Mechanistically, erythroid specification of HSCs requires glutamine-dependent de novo nucleotide biosynthesis. Exogenous nucleosides rescue erythroid commitment of human HSCs under conditions of limited glutamine catabolism, and glucose-stimulated nucleotide biosynthesis further enhances erythroid specification. Thus, the availability of glutamine and glucose to provide fuel for nucleotide biosynthesis regulates HSC lineage commitment under conditions of metabolic stress.

Metabolic reprogramming towards aerobic glycolysis correlates with greater proliferative ability and resistance to metabolic inhibition in CD8 versus CD4 T cells

T lymphocytes (T cells) undergo metabolic reprogramming after activation to provide energy and biosynthetic materials for growth, proliferation and differentiation. Distinct T cell subsets, however, adopt metabolic programs specific to support their needs. As CD4 T cells coordinate adaptive immune responses while CD8 T cells become cytotoxic effectors, we compared activation-induced proliferation and metabolic reprogramming of these subsets. Resting CD4 and CD8 T cells were metabolically similar and used a predominantly oxidative metabolism. Following activation CD8 T cells proliferated more rapidly. Stimulation led both CD4 and CD8 T cells to sharply increase glucose metabolism and adopt aerobic glycolysis as a primary metabolic program. Activated CD4 T cells, however, remained more oxidative and had greater maximal respiratory capacity than activated CD8 T cells. CD4 T cells were also associated with greater levels of ROS and increased mitochondrial content, irrespective of the activation context. CD8 cells were better able, however, to oxidize glutamine as an alternative fuel source. The more glycolytic metabolism of activated CD8 T cells correlated with increased capacity for growth and proliferation, along with reduced sensitivity of cell growth to metabolic inhibition. These specific metabolic programs may promote greater growth and proliferation of CD8 T cells and enhance survival in diverse nutrient conditions.

c-Myc-induced transcription factor AP4 is required for host protection mediated by CD8+ T cells

Although c-Myc is essential to establish a metabolically active and proliferative state in T cells after priming, its expression is transient. It remains unknown how T cell activation is maintained after c-Myc down-regulation. Here, we identify AP4 as the transcription factor that is induced by c-Myc and sustains activation of antigen-specific CD8⁺ T cells. Despite normal priming, AP4-deficient CD8⁺ T cells fail to continue transcription of a broad range of c-Myc-dependent targets. Mice lacking AP4 specifically in CD8⁺ T cells showed enhanced susceptibility to West Nile virus infection. Genome-wide analysis suggests that many activation-induced metabolic genes are shared targets of c-Myc and AP4. Thus, AP4 maintains c-Myc-initiated cellular activation programs in CD8⁺ T cells to control microbial infections.

Amino acids fuel T cell-mediated inflammation

Reprogramming cellular metabolism helps support T cell growth and effector function upon activation. In this issue of Immunity, Nakaya et al. (2014) report that the glutamine transporter ASCT2 regulates T cell metabolism and mTOR kinase signaling to shape inflammatory T helper cell responses.

The glucose transporter Glut1 is selectively essential for CD4 T cell activation and effector function

CD4 T cell activation leads to proliferation and differentiation into effector (Teff) or regulatory (Treg) cells that mediate or control immunity. While each subset prefers distinct glycolytic or oxidative metabolic programs in vitro, requirements and mechanisms that control T cell glucose uptake and metabolism in vivo are uncertain. Despite expression of multiple glucose transporters, Glut1 deficiency selectively impaired metabolism and function of thymocytes and Teff. Resting T cells were normal until activated, when Glut1 deficiency prevented increased glucose uptake and glycolysis, growth, proliferation, and decreased Teff survival and differentiation. Importantly, Glut1 deficiency decreased Teff expansion and the ability to induce inflammatory disease in vivo. Treg cells, in contrast, were enriched in vivo and appeared functionally unaffected and able to suppress Teff, irrespective of Glut1 expression. These data show a selective in vivo requirement for Glut1 in metabolic reprogramming of CD4 T cell activation and Teff expansion and survival.

Cytosolic branched chain aminotransferase (BCATc) regulates mTORC1 signaling and glycolytic metabolism in CD4+ T cells

Here we show that expression of the cytosolic branched chain aminotransferase (BCATc) is triggered by the T cell receptor (TCR) of CD4(+) T cells. Induction of BCATc correlates with increased Leu transamination, whereas T cells from the BCATc(-/-) mouse exhibit lower Leu transamination and higher intracellular Leu concentrations than the cells from wild type (WT) mice. Induction of BCATc by TCR in WT cells is prevented by the calcineurin-nuclear factor of activated T cells (NFAT) inhibitor, cyclosporin A (CsA), suggesting that NFAT controls BCATc expression. Leu is a known activator of the mammalian target of rapamycin complex 1 (mTORC1). mTOR is emerging as a critical regulator of T cell activation, differentiation, and metabolism. Activated T cells from BCATc(-/-) mice show increased phosphorylation of mTORC1 downstream targets, S6 and 4EBP-1, indicating higher mTORC1 activation than in T cells from WT mice. Furthermore, T cells from BCATc(-/-) mice display higher rates of glycolysis, glycolytic capacity, and glycolytic reserve when compared with activated WT cells. These findings reveal BCATc as a novel regulator of T cell activation and metabolism and highlight the important role of Leu metabolism in T cells.

Inflammatory T cell responses rely on amino acid transporter ASCT2 facilitation of glutamine uptake and mTORC1 kinase activation

Glutamine has been implicated as an immunomodulatory nutrient, but how glutamine uptake is mediated during T cell activation is poorly understood. We have shown that naive T cell activation is coupled with rapid glutamine uptake, which depended on the amino acid transporter ASCT2. ASCT2 deficiency impaired the induction of T helper 1 (Th1) and Th17 cells and attenuated inflammatory T cell responses in mouse models of immunity and autoimmunity. Mechanistically, ASCT2 was required for T cell receptor (TCR)-stimulated activation of the metabolic kinase mTORC1. We have further shown that TCR-stimulated glutamine uptake and mTORC1 activation also required a TCR signaling complex composed of the scaffold protein CARMA1, the adaptor molecule BCL10, and the paracaspase MALT1. This function was independent of IKK kinase, a major downstream target of the CARMA1 complex. These findings highlight a mechanism of T cell activation involving ASCT2-dependent integration of the TCR signal and a metabolic signaling pathway.

The mTOR pathway and integrating immune regulation

The mammalian target of rapamycin (mTOR) pathway is an important integrator of nutrient-sensing signals in all mammalian cells, and acts to coordinate the cell proliferation with the availability of nutrients such as glucose, amino acids and energy (oxygen and ATP). A large part of the immune response depends on the proliferation and clonal expansion of antigen-specific T cells, which depends on mTOR activation, and the pharmacological inhibition of this pathway by rapamycin is therefore potently immunosuppressive. It is only recently, however, that we have started to understand the more subtle details of how the mTOR pathway is involved in controlling the differentiation of effector versus memory CD8(+) T cells and the decision to generate different CD4(+) helper T-cell subsets. In particular, this review will focus on how nutrient sensing via mTOR controls the expression of the master transcription factor for regulatory T cells in order to maintain the balance between tolerance and inflammation.

The activities of MYC, MNT and the MAX-interactome in lymphocyte proliferation and oncogenesis

The MYC family of proteins plays essential roles in embryonic development and in oncogenesis. Efforts over the past 30 years to define the transcriptional activities of MYC and how MYC functions to promote proliferation have produced evolving models of MYC function. One picture that has emerged of MYC and its partner protein MAX is of a transcription factor complex with a seemingly unique ability to stimulate the transcription of genes that are epigenetically poised for transcription and to amplify the transcription of actively transcribed genes. During lymphocyte activation, MYC is upregulated and stimulates a pro-proliferative program in part through the upregulation of a wide variety of metabolic effector genes that facilitate cell growth and cell cycle progression. MYC upregulation simultaneously sensitizes cells to apoptosis and activated lymphocytes and lymphoma cells have pro-survival attributes that allow MYC-driven proliferation to prevail. For example, the MAX-interacting protein MNT is upregulated in activated lymphocytes and was found to protect lymphocytes from MYC-dependent apoptosis. Here we review the activities of MYC, MNT and other MAX interacting proteins in the setting of T and B cell activation and oncogenesis. This article is part of a Special Issue entitled: Myc proteins in cell biology and pathology.

Cellular metabolism on T-cell development and function

Cell metabolism is closely related to the host immunity in many respects. We herein briefly summarized the recent progress on the roles of cellular metabolism in T-cell development, homeostasis, differentiation and functions. Relatively quiescent naïve T cells only require energy for survival and migration, and they mainly metabolize glucose to carbon dioxide through oxidative phosphorylation. However, activated T cells engage in robust cell proliferation, produce of a range of effector molecules and migrate through peripheral tissues, so they utilizes glycolysis to convert glucose to lactate (termed aerobic glycolysis) to meet the significantly increased metabolic demands. Importantly, the differentiation of T-cell subsets and memory T cells (Tm) was also significantly shaped by distinct cellular metabolic pathways including glucose, amino acids (AA), fatty acids (FA), and others. Understanding the regulatory metabolic networks on immunity may offer new insights into the immune-related disorders and open novel potential therapies to prevent and treat immune diseases.

The catalytic subunit of the system L1 amino acid transporter (slc7a5) facilitates nutrient signalling in mouse skeletal muscle

The System L1-type amino acid transporter mediates transport of large neutral amino acids (LNAA) in many mammalian cell-types. LNAA such as leucine are required for full activation of the mTOR-S6K signalling pathway promoting protein synthesis and cell growth. The SLC7A5 (LAT1) catalytic subunit of high-affinity System L1 functions as a glycoprotein-associated heterodimer with the multifunctional protein SLC3A2 (CD98). We generated a floxed Slc7a5 mouse strain which, when crossed with mice expressing Cre driven by a global promoter, produced Slc7a5 heterozygous knockout (Slc7a5+/−) animals with no overt phenotype, although homozygous global knockout of Slc7a5 was embryonically lethal. Muscle-specific (MCK Cre-mediated) Slc7a5 knockout (MS-Slc7a5-KO) mice were used to study the role of intracellular LNAA delivery by the SLC7A5 transporter for mTOR-S6K pathway activation in skeletal muscle. Activation of muscle mTOR-S6K (Thr389 phosphorylation) in vivo by intraperitoneal leucine injection was blunted in homozygous MS-Slc7a5-KO mice relative to wild-type animals. Dietary intake and growth rate were similar for MS-Slc7a5-KO mice and wild-type littermates fed for 10 weeks (to age 120 days) with diets containing 10%, 20% or 30% of protein. In MS-Slc7a5-KO mice, Leu and Ile concentrations in gastrocnemius muscle were reduced by ∼40% as dietary protein content was reduced from 30 to 10%. These changes were associated with >50% decrease in S6K Thr389 phosphorylation in muscles from MS-Slc7a5-KO mice, indicating reduced mTOR-S6K pathway activation, despite no significant differences in lean tissue mass between groups on the same diet. MS-Slc7a5-KO mice on 30% protein diet exhibited mild insulin resistance (e.g. reduced glucose clearance, larger gonadal adipose depots) relative to control animals. Thus, SLC7A5 modulates LNAA-dependent muscle mTOR-S6K signalling in mice, although it appears non-essential (or is sufficiently compensated by e.g. SLC7A8 (LAT2)) for maintenance of normal muscle mass.

Metabolism of activated T lymphocytes

Activated T cells undergo metabolic reprogramming which promotes glycolytic flux and lactate production as well as elevated production of lipids, proteins, nucleic acids and other carbohydrates (i.e. induction of biomass) even in the presence of oxygen. Activated T cells show induced expression of, among other things, Glucose Transporter 1 and several glycolytic enzymes, including ADP-Dependent Glucokinase and the low affinity isoform Pyruvate Kinase-M2 (which promote glycolytic flux), as well Glutamine Transporters and Glycerol-3-phosphate Dehydrogenase 2 which make available glutamate and glycerol-3-phosphate as mitochondrial energy sources. Intracellular leucine concentrations critically regulate mammalian target of rapamycin (mTOR) signaling to promote Th1, Th2, and Th17 CD4(+) T effector cell differentiation. In contrast, T regulatory (Treg) cells are generated when AMP-Activating Protein Kinase signaling is activated and mTOR activation is suppressed. Unlike effector CD4(+) and CD8(+) T cells, Tregs and memory T cells oxidize fatty acids for fuel. Effector and memory T cells perform different functions and thus show distinct metabolic profiles which are exquisitely controlled by cellular signaling. Upon activation, T cells express the insulin and leptin receptors on their surface and become sensitive to insulin signaling and nutrient availability and show changes in differentiation. Thus, metabolism and nutrient availability influence T cell activation and function.

Itk-mediated integration of T cell receptor and cytokine signaling regulates the balance between Th17 and regulatory T cells

Loss of the Tec family kinase Itk results in a bias to FoxP3⁺ T_reg cell differentiation and reduced TCR-induced phosphorylation of mTOR targets.

A proper balance between Th17 and T regulatory cells (T_reg cells) is critical for generating protective immune responses while minimizing autoimmunity. We show that the Tec family kinase Itk (IL2-inducible T cell kinase), a component of T cell receptor (TCR) signaling pathways, influences this balance by regulating cross talk between TCR and cytokine signaling. Under both Th17 and T_reg cell differentiation conditions, Itk^−/− CD4⁺ T cells develop higher percentages of functional FoxP3⁺ cells, associated with increased sensitivity to IL-2. Itk^−/− CD4⁺ T cells also preferentially develop into T_reg cells in vivo. We find that Itk-deficient T cells exhibit reduced TCR-induced phosphorylation of mammalian target of rapamycin (mTOR) targets, accompanied by downstream metabolic alterations. Surprisingly, Itk^−/− cells also exhibit reduced IL-2–induced mTOR activation, despite increased STAT5 phosphorylation. We demonstrate that in wild-type CD4⁺ T cells, TCR stimulation leads to a dose-dependent repression of Pten. However, at low TCR stimulation or in the absence of Itk, Pten is not effectively repressed, thereby uncoupling STAT5 phosphorylation and phosphoinositide-3-kinase (PI3K) pathways. Moreover, Itk-deficient CD4⁺ T cells show impaired TCR-mediated induction of Myc and miR-19b, known repressors of Pten. Our results demonstrate that Itk helps orchestrate positive feedback loops integrating multiple T cell signaling pathways, suggesting Itk as a potential target for altering the balance between Th17 and T_reg cells.

Integrated analysis of transcript-level regulation of metabolism reveals disease-relevant nodes of the human metabolic network

Metabolic diseases and comorbidities represent an ever-growing epidemic where multiple cell types impact tissue homeostasis. Here, the link between the metabolic and gene regulatory networks was studied through experimental and computational analysis. Integrating gene regulation data with a human metabolic network prompted the establishment of an open-sourced web portal, IDARE (Integrated Data Nodes of Regulation), for visualizing various gene-related data in context of metabolic pathways. Motivated by increasing availability of deep sequencing studies, we obtained ChIP-seq data from widely studied human umbilical vein endothelial cells. Interestingly, we found that association of metabolic genes with multiple transcription factors (TFs) enriched disease-associated genes. To demonstrate further extensions enabled by examining these networks together, constraint-based modeling was applied to data from human preadipocyte differentiation. In parallel, data on gene expression, genome-wide ChIP-seq profiles for peroxisome proliferator-activated receptor (PPAR) γ, CCAAT/enhancer binding protein (CEBP) α, liver X receptor (LXR) and H3K4me3 and microRNA target identification for miR-27a, miR-29a and miR-222 were collected. Disease-relevant key nodes, including mitochondrial glycerol-3-phosphate acyltransferase (GPAM), were exposed from metabolic pathways predicted to change activity by focusing on association with multiple regulators. In both cell types, our analysis reveals the convergence of microRNAs and TFs within the branched chain amino acid (BCAA) metabolic pathway, possibly providing an explanation for its downregulation in obese and diabetic conditions.

Role of amino acid transporters in amino acid sensing

Amino acid (AA) transporters may act as sensors, as well as carriers, of tissue nutrient supplies. This review considers recent advances in our understanding of the AA-sensing functions of AA transporters in both epithelial and nonepithelial cells. These transporters mediate AA exchanges between extracellular and intracellular fluid compartments, delivering substrates to intracellular AA sensors. AA transporters on endosomal (eg, lysosomal) membranes may themselves function as intracellular AA sensors. AA transporters at the cell surface, particularly those for large neutral AAs such as leucine, interact functionally with intracellular nutrient-signaling pathways that regulate metabolism: for example, the mammalian target of rapamycin complex 1 (mTORC1) pathway, which promotes cell growth, and the general control non-derepressible (GCN) pathway, which is activated by AA starvation. Under some circumstances, upregulation of AA transporter expression [notably a leucine transporter, solute carrier 7A5 (SLC7A5)] is required to initiate AA-dependent activation of the mTORC1 pathway. Certain AA transporters may have dual receptor-transporter functions, operating as "transceptors" to sense extracellular (or intracellular) AA availability upstream of intracellular signaling pathways. New opportunities for nutritional therapy may include targeting of AA transporters (or mechanisms that upregulate their expression) to promote protein-anabolic signals for retention or recovery of lean tissue mass.

T cell exit from quiescence and differentiation into Th2 cells depend on Raptor-mTORC1-mediated metabolic reprogramming

Naive T cells respond to antigen stimulation by exiting from quiescence and initiating clonal expansion and functional differentiation, but the control mechanism is elusive. Here we describe that Raptor-mTORC1-dependent metabolic reprogramming is a central determinant of this transitional process. Loss of Raptor abrogated T cell priming and T helper 2 (Th2) cell differentiation, although Raptor function is less important for continuous proliferation of actively cycling cells. mTORC1 coordinated multiple metabolic programs in T cells including glycolysis, lipid synthesis, and oxidative phosphorylation to mediate antigen-triggered exit from quiescence. mTORC1 further linked glucose metabolism to the initiation of Th2 cell differentiation by orchestrating cytokine receptor expression and cytokine responsiveness. Activation of Raptor-mTORC1 integrated T cell receptor and CD28 costimulatory signals in antigen-stimulated T cells. Our studies identify a Raptor-mTORC1-dependent pathway linking signal-dependent metabolic reprogramming to quiescence exit, and this in turn coordinates lymphocyte activation and fate decisions in adaptive immunity.

Fueling immunity: insights into metabolism and lymphocyte function

Lymphocytes face major metabolic challenges upon activation. They must meet the bioenergetic and biosynthetic demands of increased cell proliferation and also adapt to changing environmental conditions, in which nutrients and oxygen may be limiting. An emerging theme in immunology is that metabolic reprogramming and lymphocyte activation are intricately linked. However, why T cells adopt specific metabolic programs and the impact that these programs have on T cell function and, ultimately, immunological outcome remain unclear. Research on tumor cell metabolism has provided valuable insight into metabolic pathways important for cell proliferation and the influence of metabolites themselves on signal transduction and epigenetic programming. In this Review, we highlight emerging concepts regarding metabolic reprogramming in proliferating cells and discuss their potential impact on T cell fate and function.

Targeting amino acid transport in metastatic castration-resistant prostate cancer: effects on cell cycle, cell growth, and tumor development

BACKGROUND

L-type amino acid transporters (LATs) uptake neutral amino acids including L-leucine into cells, stimulating mammalian target of rapamycin complex 1 signaling and protein synthesis. LAT1 and LAT3 are overexpressed at different stages of prostate cancer, and they are responsible for increasing nutrients and stimulating cell growth.

METHODS

We examined LAT3 protein expression in human prostate cancer tissue microarrays. LAT function was inhibited using a leucine analog (BCH) in androgen-dependent and -independent environments, with gene expression analyzed by microarray. A PC-3 xenograft mouse model was used to study the effects of inhibiting LAT1 and LAT3 expression. Results were analyzed with the Mann-Whitney U or Fisher exact tests. All statistical tests were two-sided.

RESULTS

LAT3 protein was expressed at all stages of prostate cancer, with a statistically significant decrease in expression after 4-7 months of neoadjuvant hormone therapy (4-7 month mean = 1.571; 95% confidence interval = 1.155 to 1.987 vs 0 month = 2.098; 95% confidence interval = 1.962 to 2.235; P = .0187). Inhibition of LAT function led to activating transcription factor 4-mediated upregulation of amino acid transporters including ASCT1, ASCT2, and 4F2hc, all of which were also regulated via the androgen receptor. LAT inhibition suppressed M-phase cell cycle genes regulated by E2F family transcription factors including critical castration-resistant prostate cancer regulatory genes UBE2C, CDC20, and CDK1. In silico analysis of BCH-downregulated genes showed that 90.9% are statistically significantly upregulated in metastatic castration-resistant prostate cancer. Finally, LAT1 or LAT3 knockdown in xenografts inhibited tumor growth, cell cycle progression, and spontaneous metastasis in vivo.

CONCLUSION

Inhibition of LAT transporters may provide a novel therapeutic target in metastatic castration-resistant prostate cancer, via suppression of mammalian target of rapamycin complex 1 activity and M-phase cell cycle genes.

Clathrin-independent endocytosis: a cargo-centric view

Clathrin-independent endocytosis occurs in all cells and interest in this mode of cellular entry has grown. Although this form of endocytosis was first described for entry of bacterial toxins, here we focus our attention on the endogenous cell surface "cargo" proteins that enter cells by this mechanism. The cargo proteins entering by this mechanism are varied and include nutrient transporters, ion channels, cell adhesion molecules and proteins associated with the immune system. Despite the apparent lack of selection at the cell surface, we provide some examples of specific sorting of these cargo proteins after entry, leading to distinct itineraries and cellular fates.

Protein phosphatase 2A enables expression of interleukin 17 (IL-17) through chromatin remodeling

Protein phosphatase 2A (PP2A) is a heterotrimeric serine/threonine phosphatase involved in essential cellular functions. T cells from patients with systemic lupus erythematosus (SLE) express high levels of the catalytic subunit of PP2A (PP2Ac). A mouse overexpressing PP2Ac in T cells develops glomerulonephritis in an IL-17-dependent manner. Here, using microarray analyses, we demonstrate that increased expression of PP2Ac grants T cells the capacity to produce an array of proinflammatory effector molecules. Because IL-17 is important in the expression of glomerulonephritis, we studied the mechanism through which PP2Ac dysregulation facilitates its production. We report that PP2Ac is involved in the regulation of the Il17 locus by enhancing histone 3 acetylation through a mechanism that involves activation of interferon regulatory factor 4. Increased histone 3 acetylation of the Il17 locus is shared between T cells of PP2Ac transgenic mice and patients with SLE. We propose that, by promoting the inflammatory capacity of T cells, PP2Ac dysregulation contributes to the pathogenesis of SLE.

mTOR and lymphocyte metabolism

Upon antigen engagement and proper co-stimulation, naïve lymphocytes exit quiescence and undergo clonal expansion and differentiate into functional effector cells, after which they either die through apoptosis or survive as memory cells. Lymphocytes at different activation stages exhibit distinct metabolic signatures. Emerging evidence highlights a central role for the mechanistic target of rapamycin (mTOR) in bridging immune signals and metabolic cues to direct lymphocyte proliferation, differentiation and survival. Here we review recent advances in understanding the functional significance and signal transduction of mTOR in T cell biology, and the interplay between mTOR signaling and metabolic programs.

Metabolic Control of Th17 Cell Generation and CNS Inflammation

Multiple sclerosis (MS), an inflammatory demyelinating disease of the central nervous system (CNS), results from uncontrolled auto reactive T cells that infiltrate the CNS and attack the myelin sheath. Th17 cells play a prominent role in the pathogenesis of MS and experimental autoimmune encephalomyelitis (EAE), a mouse model of MS. Extensive studies have focused on understanding the roles of cytokine signaling and transcriptional network in the differentiation of Th17 cells and their pathogenicity in CNS inflammation. Aside from these events, activated T cells dynamically reprogram their metabolic pathways to fulfill the bioenergic and biosynthetic requirements for proper T cell functions. Emerging evidence indicates that modulation of these metabolic pathways impinges upon the differentiation of Th17 cells and the pathogenesis of EAE. Thus, a better understanding of the functions and mechanisms of T cell metabolism in Th17 cell biology may provide new avenues for therapeutic targeting of MS. In this review, we discuss the recent advances in our understanding of T cell metabolic pathways involved in Th17 cell differentiation and CNS inflammation.