Regulation of 4F2 heavy-chain gene expression during normal human T-cell activation can be mediated by multiple distinct molecular mechanisms

Interleukin-18 up-regulates amino acid transporters and facilitates amino acid-induced mTORC1 activation in natural killer cells

Upon inflammation, natural killer (NK) cells undergo metabolic changes to support their high energy demand for effector function and proliferation. The metabolic changes are usually accompanied by an increase in the expression of nutrient transporters, leading to increased nutrient uptake. Among various cytokines inducing NK cell proliferation, the mechanisms underlying the effect of interleukin (IL)-18 in promoting NK cell proliferation are not completely understood. Here, we demonstrate that IL-18 is a potent cytokine that can enhance the expression of the nutrient transporter CD98/LAT1 for amino acids independently of the mTORC1 pathway and thereby induce a dramatic metabolic change associated with increased proliferation of NK cells. Notably, treatment of IL-18-stimulated NK cells with leucine activates the metabolic sensor mTORC1, indicating that the high expression of amino acid transporters induces amino acid-driven mTORC1 activation. Inhibition of the amino acid transporter CD98/LAT1 abrogated the leucine-driven mTORC1 activation and reduced NK cell effector function. Taken together, our study identified a novel role of IL-18 in up-regulating nutrient transporters on NK cells and thereby inducing metabolic changes, including the mTORC1 activation by amino acids.

Control of amino acid transport coordinates metabolic reprogramming in T-cell malignancy

This study explores the regulation and importance of System L amino acid transport in a murine model of T-cell acute lymphoblastic leukemia (T-ALL) caused by deletion of phosphatase and tensin homolog deleted on chromosome 10 (PTEN). There has been a strong focus on glucose transport in leukemias but the present data show that primary T-ALL cells have increased transport of multiple nutrients. Specifically, increased leucine transport in T-ALL fuels mammalian target of rapamycin complex 1 (mTORC1) activity which then sustains expression of hypoxia inducible factor-1α (HIF1α) and c-Myc; drivers of glucose metabolism in T cells. A key finding is that PTEN deletion and phosphatidylinositol (3,4,5)-trisphosphate (PtdIns(3,4,5)P₃) accumulation is insufficient to initiate leucine uptake, mTORC1 activity, HIF1α or c-Myc expression in T cells and hence cannot drive T-ALL metabolic reprogramming. Instead, a key regulator for leucine transport in T-ALL is identified as NOTCH. Mass spectrometry based proteomics identifies SLC7A5 as the predominant amino acid transporter in primary PTEN^−/− T-ALL cells. Importantly, expression of SLC7A5 is critical for the malignant transformation induced by PTEN deletion. These data reveal the importance of regulated amino acid transport for T-cell malignancies, highlighting how a single amino acid transporter can have a key role.

Ubiquitylation of CD98 limits cell proliferation and clonal expansion

CD98 heavy chain (SLC3A2) facilitates lymphocyte clonal expansion that enables adaptive immunity; however, increased expression of CD98 is also a feature of both lymphomas and leukemias and represents a potential therapeutic target in these diseases. CD98 is transcriptionally regulated and ectopic expression of the membrane-associated RING-CH (MARCH) E3 ubiquitin ligases MARCH1 or MARCH8 leads to ubiquitylation and lysosomal degradation of CD98. Here, we examined the potential role of ubiquitylation in regulating CD98 expression and cell proliferation. We report that blocking ubiquitylation by use of a catalytically inactive MARCH or by creating a ubiquitylation-resistant CD98 mutant, prevents MARCH-induced CD98 downregulation in HeLa cells. March1-null T cells display increased CD98 expression. Similarly, T cells expressing ubiquitylation-resistant CD98 manifest increased proliferation in vitro and clonal expansion in vivo. Thus, ubiquitylation and the resulting downregulation of CD98 can limit cell proliferation and clonal expansion.

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

T lymphocytes must regulate nutrient uptake to meet the metabolic demands of an immune response. Here we show that the intracellular supply of large neutral amino acids (LNAAs) in T cells was regulated by pathogens and the T cell antigen receptor (TCR). T cells responded to antigen by upregulating expression of many amino-acid transporters, but a single System L ('leucine-preferring system') transporter, Slc7a5, mediated uptake of LNAAs in activated T cells. Slc7a5-null T cells were unable to metabolically reprogram in response to antigen and did not undergo clonal expansion or effector differentiation. The metabolic catastrophe caused by loss of Slc7a5 reflected the requirement for sustained uptake of the LNAA leucine for activation of the serine-threonine kinase complex mTORC1 and for expression of the transcription factor c-Myc. Control of expression of the System L transporter by pathogens is thus a critical metabolic checkpoint for T cells.

Homeostatic and innate immune responses: role of the transmembrane glycoprotein CD98

The transmembrane glycoprotein CD98 is a potential regulator of multiple functions, including integrin signaling and amino acid transport. Abnormal expression or function of CD98 and disruption of the interactions between CD98 and its binding partners result in defects in cell homeostasis and immune responses. Indeed, expression of CD98 has been correlated with diseases such as inflammation and tumor metastasis. Modulation of CD98 expression and/or function therefore represents a promising therapeutic strategy for the treatment and prevention of such pathologies. Herein, we review the role of CD98 with focus on its functional importance in homeostasis and immune responses, which could help to better understand the pathogenesis of CD98-associated diseases.

MicroRNA-7 modulates CD98 expression during intestinal epithelial cell differentiation

The transmembrane glycoprotein CD98 regulates multiple cellular functions, including extracellular signaling, epithelial cell adhesion/polarity, amino acid transport, and cell-cell interactions. MicroRNAs post-transcriptionally regulate gene expression, thereby functioning as modulators of numerous cellular processes, such as cell differentiation, proliferation, and apoptosis. Here, we investigated if microRNAs regulate CD98 expression during intestinal epithelial cell differentiation and inflammation. We found that microRNA-7 repressed CD98 expression in Caco2-BBE cells by directly targeting the 3'-untranslated region of human CD98 mRNA. Expression of CD98 was decreased, whereas that of microRNA-7 was increased in well-differentiated Caco2-BBE cells compared with undifferentiated cells. Undifferentiated crypt cells isolated from mouse jejunum showed higher CD98 levels and lower levels of mmu-microRNA-706, a murine original microRNA candidate for CD98, than well-differentiated villus cells. Importantly, microRNA-7 decreased Caco2-BBE cell attachment on laminin-1, and CD98 overexpression recovered this inhibition, suggesting that microRNA-7 modulates epithelial cell adhesion to extracellular matrix, which in turn could affect proliferation and differentiation during the migration of enterocytes across the crypt-villus axis, by regulating CD98 expression. In a pathological context, the pro-inflammatory cytokine interleukin 1-beta increased CD98 expression in Caco2-BBE cells by decreasing microRNA-7 levels. Consistent with the in vitro findings, microRNA-7 levels were decreased in actively inflamed Crohn disease colonic tissues, where CD98 expression was up-regulated, compared with normal tissues. Together, these results reveal a novel mechanism underlying regulation of CD98 expression during patho-physiological states. This study raises microRNAs as a promising target for therapeutic modulations of CD98 expression in intestinal inflammatory disorders.

Notch-induced T cell development requires phosphoinositide-dependent kinase 1

Phosphoinositide-dependent kinase l (PDK1) phosphorylates and activates multiple AGC serine kinases, including protein kinase B (PKB), p70Ribosomal S6 kinase (S6K) and p90Ribosomal S6 kinase (RSK). PDK1 is required for thymocyte differentiation and proliferation, and herein, we explore the molecular basis for these essential functions of PDK1 in T lymphocyte development. A key finding is that PDK1 is required for the expression of key nutrient receptors in T cell progenitors: CD71 the transferrin receptor and CD98 a subunit of L-amino acid transporters. PDK1 is also essential for Notch-mediated trophic and proliferative responses in thymocytes. A PDK1 mutant PDK1 L155E, which supports activation of PKB but no other AGC kinases, can restore CD71 and CD98 expression in pre-T cells and restore thymocyte differentiation. However, PDK1 L155E is insufficient for thymocyte proliferation. The role of PDK1 in thymus development thus extends beyond its ability to regulate PKB. In addition, PDK1 phosphorylation of AGC kinases such as S6K and RSK is also necessary for thymocyte development.

Molecular events involved in up-regulating human Na+-independent neutral amino acid transporter LAT1 during T-cell activation

We investigated the regulation of system-L amino acid transporter (LAT1) during T-cell activation. In quiescent T-cells, L-leucine transport is mediated mainly by the system-L amino acid transport system and is increased significantly during T-cell activation by PMA and ionomycin. In quiescent T-cells, the LAT1 protein was heterocomplexed with 4F2 heavy chain (4F2hc) in the plasma membrane. During T-cell activation, the amounts of 4F2hc and LAT1 heterocomplex were significantly elevated compared with those in quiescent T-cells. In addition, by Northern-blot analysis, these increments were found to be due to elevated levels of LAT1 and 4F2hc mRNA. Transient expression of constructs comprising various LAT1 gene promoter fragments, which contained all three of the GC boxes, was sufficient for promoting luciferase expression in Jurkat T-cells, but the promoter of the LAT1 gene did not respond to PMA and ionomycin. Similar observations were observed in the human 4F2hc gene promoter. In nuclear run-on assay, the LAT1 and 4F2hc genes were actively transcribed even in quiescent T-cells, but the low levels of both transcripts were shown to be the result of a block to transcription elongation within the exon 1 intron 1 regions. These findings indicated that a removal of the block to mRNA elongation stimulates the induction of system-L amino acid transporter gene transcripts (LAT1 and 4F2hc) in activated T-cells.

Expression of the surface antigen 4F2hc affects system-L-like neutral-amino-acid-transport activity in mammalian cells

Mammalian cells possess a variety of amino acid-transport systems with overlapping substrate specificity. System L is one of the major amino acid-transport systems of non-epithelial cells. By expression cloning we have recently demonstrated that the surface antigen 4F2hc (CD98) is a necessary component for expression of system-L-like amino acid-transport activity in C6-BU-1 rat glioma cells [Bröer, Bröer and Hamprecht (1995) Biochem. J. 312, 863-870]. 4F2hc mRNA was detected in CHO cells, COS cells, activated lymphocytes isolated from mouse spleen and primary cultures of astrocytes. In all these cell types, Na+-independent isoleucine transport was mediated by system L. No contribution of system y+L to isoleucine or arginine transport was detected in C6-BU-1 cells. In lymphocytes, both system-L-like amino acid-transport activity and 4F2hc mRNA levels increased after treatment with phorbol ester plus ionomycin. Antisense oligonucleotides caused modest inhibition of Na+-independent isoleucine transport in C6-BU-1 cells and primary cultures of astroglial cells, whereas arginine transport was unaffected. Overexpression of 4F2hc cDNA in CHO cells resulted in an increase in Na+-independent isoleucine transport.

Stimulation of system y(+)-like amino acid transport by the heavy chain of human 4F2 surface antigen in Xenopus laevis oocytes

A kidney cortex cDNA clone (rBAT) has recently been isolated, which upon in vitro transcription and capping complementary RNA (cRNA) and injection into Xenopus laevis oocytes induces a system b0,(+)-like amino acid transport activity. This cDNA encodes a type II membrane glycoprotein that shows significant homology to another type II membrane glycoprotein, the heavy chain of the human and mouse 4F2 surface antigen (4F2hc). Here we demonstrate that injection of human 4F2hc cRNA into oocytes results in the activation of a cation-preferring amino acid transport system that appears to be identical to the y(+)-like transport already present in the oocyte. This is based on the following results: (i) Injection of in vitro transcripts from 4F2hc cDNA (4F2hc cRNA) into oocytes stimulates up to 10-fold the sodium-independent uptake of L-arginine and up to 4.1-fold the sodium-dependent uptake of L-leucine. In contrast, 4F2hc cRNA does not increase the basal sodium-independent uptake of L-leucine. (ii) Basal and 4F2hc cRNA-stimulated sodium-independent uptake of L-arginine is completely inhibited by L-leucine in the presence of sodium. Similarly, the basal and 4F2hc cRNA-stimulated sodium-dependent uptake of L-leucine is entirely inhibited by L-arginine. (iii) The stimulation of sodium-independent uptake of L-arginine and the stimulation of sodium-dependent uptake of L-leucine induced by injection of 4F2hc cRNA are both completely inhibited by dibasic L amino acids and to a lesser extent by D-ornithine. (iv) Both basal and 4F2hc cRNA-stimulated sodium-independent uptake of L-arginine show two additional characteristics of the system y+ transport activity: inhibition of L-arginine uptake by L-homoserine only in the presence of sodium and an increase in the inhibition exerted by L-histidine as the extracellular pH decreased. Our results allow us to propose that an additional family of type II membrane glycoproteins (composed by rBAT and 4F2hc) is involved in amino acid transport, either as specific activators or as components of amino acid transport systems.

Regulation of interleukin 3 gene induction in normal human T cells

The regulation of IL-3 gene induction in human peripheral blood T cells was studied. IL-3 gene expression was inducible by crosslinking of the T cell receptor/CD3 complex using anti-CD3 MAb G19-4. Anti-CD3-induced IL-3 gene expression was found to be limited to the CD28+ T cell subset and could be augmented by costimulating T lymphocytes with antibodies directed against CD28. IL-3 expression could also be induced by costimulation of T cells with both phorbol ester and ionomycin, which are thought to mimic the intracellular effects of T cell receptor-antigen interaction. However, unlike other lymphokines such as IL-2 or granulocyte-macrophage colony-stimulating factor, IL-3 gene expression is not induced by stimulation of cells with phorbol myristate acetate and anti-CD28. We conclude that IL-3 gene regulation is under stringent control since IL-3 gene expression occurs only in the CD28+ subset of T cells, and since IL-3 induction obligately requires increased intracellular calcium.

The first intron of the 4F2 heavy-chain gene contains a transcriptional enhancer element that binds multiple nuclear proteins

We utilized the human 4F2 heavy-chain (4F2HC) gene as a model system to study the regulation of inducible gene expression during normal human T-cell activation. Previous studies have demonstrated that 4F2HC gene expression is induced during normal T-cell activation and that the activity of the gene is regulated, at least in part, by the interaction of a constitutively active 5'-flanking housekeeping promoter and a phorbol ester-responsive transcriptional attenuator element located in the exon 1-intron 1 region of the gene. We now report that 4F2HC intron 1 contains a transcriptional enhancer element which is active on a number of heterologous promoters in a variety of murine and human cells. This enhancer element has been mapped to a 187-base-pair RsaI-AluI fragment from 4F2HC intron 1. DNase I footprinting and gel mobility shift analyses demonstrated that this fragment contains two nuclear protein-binding sites (NF-4FA and NF-4FB) which flank a consensus binding site for the inducible AP-1 transcription factor. Deletion analysis showed that the NF-4FA, NF-4FB, and AP-1 sequences are each necessary for full enhancer activity. Murine 4F2HC intron 1 displayed enhancer activity similar to that of its human counterpart. Comparison of the sequences of human and murine 4F2HC intron 1s demonstrated that the NF-4FA, NF-4FB, and AP-1 sequence motifs have been highly conserved during mammalian evolution.

The first intron of the 4F2 heavy-chain gene contains a transcriptional enhancer element that binds multiple nuclear proteins

We utilized the human 4F2 heavy-chain (4F2HC) gene as a model system to study the regulation of inducible gene expression during normal human T-cell activation. Previous studies have demonstrated that 4F2HC gene expression is induced during normal T-cell activation and that the activity of the gene is regulated, at least in part, by the interaction of a constitutively active 5'-flanking housekeeping promoter and a phorbol ester-responsive transcriptional attenuator element located in the exon 1-intron 1 region of the gene. We now report that 4F2HC intron 1 contains a transcriptional enhancer element which is active on a number of heterologous promoters in a variety of murine and human cells. This enhancer element has been mapped to a 187-base-pair RsaI-AluI fragment from 4F2HC intron 1. DNase I footprinting and gel mobility shift analyses demonstrated that this fragment contains two nuclear protein-binding sites (NF-4FA and NF-4FB) which flank a consensus binding site for the inducible AP-1 transcription factor. Deletion analysis showed that the NF-4FA, NF-4FB, and AP-1 sequences are each necessary for full enhancer activity. Murine 4F2HC intron 1 displayed enhancer activity similar to that of its human counterpart. Comparison of the sequences of human and murine 4F2HC intron 1s demonstrated that the NF-4FA, NF-4FB, and AP-1 sequence motifs have been highly conserved during mammalian evolution.

Isolation and structural characterization of the human 4F2 heavy-chain gene, an inducible gene involved in T-lymphocyte activation

The human 4F2 cell surface antigen is a 120-kilodalton (kDa) disulfide-linked heterodimer which is composed of an 80- to 90-kDa glycosylated heavy chain (4F2HC) and a 35- to 40-kDa nonglycosylated light chain (4F2LC). 4F2 belongs to a family of inducible cell surface molecules which are involved in T-lymphocyte activation and growth. To better understand the molecular mechanism(s) that controls 4F2HC gene expression in both resting and activated T cells, a 4F2HC human genomic clone was isolated and structurally characterized. The 4F2HC gene spans 8 kilobases of chromosome 11 and is composed of nine exons. The 5' upstream region of the gene displays several properties which are characteristic of housekeeping genes. It is G+C rich and hypomethylated in peripheral blood lymphocyte DNA and contains multiple binding sites for the Sp1 transcription factor while lacking TATA or CCAAT sequences. This region of the gene also displays sequence homologies with several other inducible T-cell genes, including the interleukin-2, interleukin-2 receptor alpha chain, dihydrofolate reductase, thymidine kinase, and transferrin receptor genes. A 255-base-pair fragment of the 4F2HC gene which contains 154 base pairs of the 5' flanking sequence was able to efficiently promote expression of the bacterial chloramphenicol acetyltransferase gene in human Jurkat T cells, indicating that it contains promoter or enhancer (or both) sequences. Analyses of chromatin structure in resting and lectin-activated T cells revealed the presence of stable DNase I-hypersensitive sites within both the 5' flanking and intron 1 regions of the 4F2HC gene. Although the 4F2HC gene displayed many of the structural features characteristic of a constitutively expressed gene, lectin-mediated activation of resting peripheral blood T lymphocytes resulted in a dramatic increase in steady-state levels of 4F2HC mRNA.

Structure, expression and regulation of the murine 4F2 heavy chain

The murine 4F2 molecule is a 125 kilodalton disulfide-linked heterodimeric cell-surface glycoprotein which has been shown to be involved in the processes of cellular activation and proliferation (1). To elucidate the structure, expression, and regulation of the 4F2 molecule, a murine 4F2 heavy chain (4F2HC) cDNA has been isolated and structurally characterized. The murine 4F2HC is a 526 amino acid (aa) type II membrane glycoprotein which is composed of a 75 aa N-terminal intracytoplasmic region, a single hydrophobic putative transmembrane domain, and a 428 aa C-terminal extracellular domain. Comparison with the human 4F2HC cDNA reveals the highest degree of sequence identity within the transmembrane and intracytoplasmic domains. Northern blot analyses have demonstrated that the 4F2HC gene is expressed at relatively high levels in adult testis, lung, brain, kidney, and spleen, and at significantly lower levels in adult liver and cardiac and skeletal muscle. Studies designed to elucidate the pattern of regulation of the murine 4F2HC gene have demonstrated that it is induced during the process of cell activation, but is subsequently expressed at constant levels throughout the cell cycle in exponentially growing cells.

CD28 activation pathway regulates the production of multiple T-cell-derived lymphokines/cytokines

CD28 is a 44-kDa glycoprotein expressed as a homodimer on the surface of a major subset of human T cells. Previous studies have demonstrated that the binding of monoclonal antibodies to the CD28 surface antigen can augment the proliferation of purified human T cells stimulated with suboptimal doses of mitogens or anti-T-cell receptor/CD3 complex antibodies. In this report, we show that CD28 stimulation augments T-cell immune responses by specifically inducing a 5- to 50-fold enhancement in the expression and secretion of interleukin 2, tumor necrosis factor type alpha, lymphotoxin, interferon gamma, and granulocyte-macrophage colony-stimulating factor in normal human T cells stimulated to proliferate by crosslinking of the T-cell receptor/CD3 complex. This CD28-mediated induction of lymphokine/cytokine gene expression occurred even in T cells stimulated with optimal concentrations of mitogens or anti-T-cell receptor/CD3 antibodies, although under these conditions CD28 activation failed to enhance the proliferative response. The activation pathway induced by stimulation of CD28 is distinct from other biochemical pathways that induce lymphokines/cytokines because CD28 stimulation can induce lymphokine/cytokine gene expression in the presence of the immunosuppressant cyclosporine. Together these data suggest that the CD28 cell surface molecule is part of a distinct activation pathway that specifically modulates the expression of multiple lymphokine/cytokine genes.

Isolation and structural characterization of the human 4F2 heavy-chain gene, an inducible gene involved in T-lymphocyte activation

The human 4F2 cell surface antigen is a 120-kilodalton (kDa) disulfide-linked heterodimer which is composed of an 80- to 90-kDa glycosylated heavy chain (4F2HC) and a 35- to 40-kDa nonglycosylated light chain (4F2LC). 4F2 belongs to a family of inducible cell surface molecules which are involved in T-lymphocyte activation and growth. To better understand the molecular mechanism(s) that controls 4F2HC gene expression in both resting and activated T cells, a 4F2HC human genomic clone was isolated and structurally characterized. The 4F2HC gene spans 8 kilobases of chromosome 11 and is composed of nine exons. The 5' upstream region of the gene displays several properties which are characteristic of housekeeping genes. It is G+C rich and hypomethylated in peripheral blood lymphocyte DNA and contains multiple binding sites for the Sp1 transcription factor while lacking TATA or CCAAT sequences. This region of the gene also displays sequence homologies with several other inducible T-cell genes, including the interleukin-2, interleukin-2 receptor alpha chain, dihydrofolate reductase, thymidine kinase, and transferrin receptor genes. A 255-base-pair fragment of the 4F2HC gene which contains 154 base pairs of the 5' flanking sequence was able to efficiently promote expression of the bacterial chloramphenicol acetyltransferase gene in human Jurkat T cells, indicating that it contains promoter or enhancer (or both) sequences. Analyses of chromatin structure in resting and lectin-activated T cells revealed the presence of stable DNase I-hypersensitive sites within both the 5' flanking and intron 1 regions of the 4F2HC gene. Although the 4F2HC gene displayed many of the structural features characteristic of a constitutively expressed gene, lectin-mediated activation of resting peripheral blood T lymphocytes resulted in a dramatic increase in steady-state levels of 4F2HC mRNA.