Positive and negative thymic selection in T cell receptor-transgenic mice correlate with Nur77 mRNA expression

Retinoids induce Nur77-dependent apoptosis in mouse thymocytes

Nur77 is a transcription factor, which plays a determinant role in mediating T cell receptor-induced cell death of thymocytes. In addition to regulation of transcription, Nur77 contributes to apoptosis induction by targeting mitochondria, where it can convert Bcl-2, an anti-apoptotic protein into a proapoptotic molecule. Previous studies have demonstrated that retinoids are actively produced in the mouse thymus and can induce a transcription-dependent apoptosis in mouse thymocytes. Here we show that retinoic acids induce the expression of Nur77, and retinoid-induced apoptosis is completely dependent on Nur77, as retinoids were unable to induce apoptosis in Nur77 null thymocytes. In wild-type thymocytes retinoids induced enhanced expression of the apoptosis-related genes FasL, TRAIL, NDG-1, Gpr65 and Bid, all of them in a Nur77-dependent manner. The combined action of these proteins led to Caspase 8-dependent Bid cleavage in the mitochondria. In addition, we could demonstrate the Nur77-dependent induction of STAT1 leading to enhanced Bim expression, and the mitochondrial translocation of Nur77 leading to the exposure of the Bcl-2/BH3 domain. The retinoid-induced apoptosis was dependent on both Caspase 8 and STAT1. Our data together indicate that retinoids induce a Nur77-dependent cell death program in thymocytes activating the mitochondrial pathway of apoptosis.

Macrophages engulfing apoptotic thymocytes produce retinoids to promote selection, differentiation, removal and replacement of double positive thymocytes

The thymus provides the microenvironment in which thymocytes develop into mature T-cells, and interactions with thymic stromal cells are thought to provide the necessary signals for thymocyte maturation. Recognition of self-MHC by T-cells is a basic requirement for mature T-cell functions, and those thymocytes that do not recognize or respond too strongly to the peptide-loaded self-MHC molecules found in the thymus undergo apoptosis. As a result, 95% of the thymocytes produced will die and be subsequently cleared by macrophages. This review describes a complex crosstalk between developing thymocytes and engulfing macrophages which is mediated by retinoids produced by engulfing macrophages. The interaction results in the harmonization of the rate of cell death of dying double positive cells with their clearance and replacement, and in promotion of the differentiation of the selected cells in the thymus.

Retinoids enhance glucocorticoid-induced apoptosis of T cells by facilitating glucocorticoid receptor-mediated transcription

Glucocorticoid-induced apoptosis of thymocytes is one of the first recognized forms of programmed cell death. It was shown to require gene activation induced by the glucocorticoid receptor (GR) translocated into the nucleus following ligand binding. In addition, the necessity of the glucocorticoid-induced, but transcription-independent phosphorylation of phosphatidylinositol-specific phospholipase C (PI-PLC) has also been shown. Here we report that retinoic acids, physiological ligands for the nuclear retinoid receptors, enhance glucocorticoid-induced death of mouse thymocytes both in vitro and in vivo. The effect is mediated by retinoic acid receptor (RAR) alpha/retinoid X receptor (RXR) heterodimers, and occurs when both RARα and RXR are ligated by retinoic acids. We show that the ligated RARα/RXR interacts with the ligated GR, resulting in an enhanced transcriptional activity of the GR. The mechanism through which this interaction promotes GR-mediated transcription does not require DNA binding of the retinoid receptors and does not alter the phosphorylation status of Ser232, known to regulate the transcriptional activity of GR. Phosphorylation of PI-PLC was not affected. Besides thymocytes, retinoids also promoted glucocorticoid-induced apoptosis of various T-cell lines, suggesting that they could be used in the therapy of glucocorticoid-sensitive T-cell malignancies.

Induction of apoptosis in PA-1 ovarian cancer cells by vitamin K2 is associated with an increase in the level of TR3/Nur77 and its accumulation in mitochondria and nuclei

PURPOSE

We examined the growth-inhibitory and apoptosis-inducing effects of vitamin K(2) (VK(2); menaquinone-4) on various lines of human ovarian cancer cells to study the mechanism of induction of apoptosis by VK(2).

METHODS

Cell proliferation was determined by XTT method, and apoptotic cells were detected by Hoechst staining. TR3, also known as Nur77 and NGFI-B, was detected by immunoblotting and immunofluorescence analysis. Role of TR3 on induction of apoptosis was examined by a siRNA experiment.

RESULTS AND CONCLUSIONS

We found that PA-1 cells were the most sensitive to VK(2) (IC(50) = 5.0 +/- 0.7 microM), while SK-OV-3 cells were resistant to VK(2). Immunoblotting and immunofluorescence analyses indicated that levels of TR3 were elevated in cell lysates 48 h after the start of treatment with 30 microM VK(2). In the VK(2)-treated cells, TR3 accumulated at significant levels in mitochondria, as well as in the nuclei of PA-1 cells. No similar changes were observed in SK-OV-3 cells under the same conditions. Treatment of PA-1 cells with small interfering RNA (siRNA) directed against TR3, and with cycloheximide or SP600125 (an inhibitor of c-jun N-terminal kinase; JNK), separately, inhibited the VK(2)-induced synthesis of TR3 and apoptosis. From these results, we can conclude that an increase in the synthesis of TR3 and the accumulation of TR3 in mitochondria and in nuclei might be involved in the induction of apoptosis by VK(2) and that the synthesis of TR3 might be regulated through a JNK signaling pathway.

Generalized resistance to thymic deletion in the NOD mouse; a polygenic trait characterized by defective induction of Bim

The cause of common polygenic autoimmune diseases is not understood because of genetic and cellular complexity. Here, we pinpoint the action of a subset of autoimmune susceptibility loci in the NOD mouse strain linked to D1mit181, D2mit490, D7mit101, and D15mit229, which cause a generalized resistance to thymic deletion in vivo that applies equally to Aire-induced organ-specific gene products in the thymic medulla and to systemic antigens expressed at high levels throughout the thymus and affects CD4(+), CD4(+)8(+), and CD4(+)25(+) thymocytes. Resistance to thymic deletion does not reflect a general deficit in TCR signaling to calcineurin- or ERK-induced genes, imbalance in constitutive regulators of apoptosis, nor excessive signaling to prosurvival genes but is distinguished by failure to induce the proapoptotic gene and protein, Bim, during in vivo encounter with high-avidity autoantigen. These findings establish defects in thymic deletion and Bim induction as a key mechanism in the pathogenesis of autoimmunity.

Distinct transcriptional programs in thymocytes responding to T cell receptor, Notch, and positive selection signals

T cell antigen receptor (TCR) signaling is necessary but not sufficient to promote the positive selection of CD4+CD8+ thymocytes into CD4+ or CD8+ mature T cells. Notch signaling has also been implicated as a potential regulator of both CD4/CD8 T cell development and TCR signaling. However, the relationship between positive selection, TCR signaling, and Notch remains unclear. Here we use DNA microarray analysis to compare gene expression changes in CD4+CD8+ double-positive thymocytes undergoing positive selection, TCR stimulation, and Notch activation. We find that the genes induced during positive selection can be resolved into two distinct sets. One set, which we term "TCR-induced," is also induced by in vitro TCR stimulation and contains a large proportion of transcription factors. A second set, which we term "positive-selection-induced," is not induced by in vitro TCR simulation and contains a large proportion of genes involved in signal transduction pathways. Genes induced by Notch activity overlap substantially with genes induced during positive selection. We also find that Notch activity potentiates the effects of TCR stimulation on gene expression. These results help to identify TCR- and positive-selection-specific transcriptional events and help to clarify the relationship between positive selection and Notch.

Ternary complex factor SAP-1 is required for Erk-mediated thymocyte positive selection

Thymocyte selection and differentiation requires extracellular signal-regulated kinase (Erk) signaling, but transcription factor substrates of Erk in thymocytes are unknown. We have characterized the function of SAP-1 (Elk4), an Erk-regulated transcription factor, in thymocyte development. Early thymocyte development was normal, but single-positive thymocyte and peripheral T cell numbers were reduced, reflecting a T cell-autonomous defect. T cell receptor-induced activation of SAP-1 target genes such as Egr1 was substantially impaired in double-positive thymocytes, although Erk activation was normal. Analysis of T cell receptor transgenes showed that positive selection was reduced by 80-90% in SAP-1-deficient mice; heterozygous mice showed a moderate defect. Negative selection was unimpaired. SAP-1 thus directly links Erk signaling to the transcriptional events required for thymocyte positive selection.

Mitogenic effect of orphan receptor TR3 and its regulation by MEKK1 in lung cancer cells

TR3, also known as NGFI-B or nur77, is an immediate-early response gene and an orphan member of the steroid/thyroid/retinoid receptor superfamily. We previously reported that TR3 expression was induced by apoptotic stimuli and was required for their apoptotic effect in lung cancer cells. Here, we present evidence that TR3 was also induced by epidermal growth factor (EGF) and serum and was required for their mitogenic effect in lung cancer cells. Ectopic expression of TR3 in both H460 and Calu-6 lung cancer cell lines promoted their cell cycle progression and BrdU incorporation, while inhibition of TR3 expression by the small interfering RNA approach suppressed the mitogenic effect of EGF and serum. Analysis of TR3 mutants showed that both TR3 DNA binding and transactivation were required for its mitogenic effect. In contrast, they were dispensable for its apoptotic activity. Furthermore, confocal microscopy analysis demonstrated that TR3 functioned in the nucleus to induce cell proliferation, whereas it acted on mitochondria to induce apoptosis. In examining the signaling that regulates the mitogenic function of TR3, we observed that coexpression of constitutive-active MEKK1 inhibited TR3 transcriptional activity and TR3-induced proliferation. The inhibitory effect of MEKK1 was mediated through activation of Jun N-terminal kinase, which efficiently phosphorylated TR3, resulting in loss of its DNA binding. Together, our results demonstrate that TR3 is capable of inducing both proliferation and apoptosis in the same cells depending on the stimuli and its cellular localization.

Targeting mature T cell leukemia: new understanding of molecular pathways

The best studied T cell leukemia/lymphoma from a genetic and biochemical point of view is T-cell chronic lymphocytic/prolymphocytic leukemia (T-CLL/T-PLL). This neoplasia commonly shows chromosomal rearrangements at 14q32.1 including translocations [t(14;14)(q11;q32), t(7;14)(q35;q32)], and inversions [inv(14)(q11;q32)]. The investigation of the locus in question at 14q32.1 resulted in the identification of two related genes named T cell leukemia/lymphoma 1 (TCL1) and TCL1b. Both genes are activated in T-CLL/T-PLL by the chromosomal aberrations mentioned above. Mice from a transgenic mouse strain expressing the TCL1 gene under the thymocyte specific lck promoter developed a mature T cell leukemia late in life, thereby demonstrating that over-expression of TCL1 induces the neoplastic transformation of T cells. Biochemically, Tcl1 protein works as a co-factor of the Akt kinase, a key regulator of antiapoptotic and proliferative signals. Tcl1 interacts physically with Akt, increases its kinase activity and facilitates its transport to the nucleus. The pathogenesis of T-CLL/T-PLL may also involve Nur77, a T cell transcription factor required for T cell receptor-mediated apoptosis. Akt phosphorylates Nur77, thereby blocking its DNA-binding ability and rendering the transcription factor inactive. The recently emerged insights into the molecular mechanisms of T cell leukemogenesis will allow for the development of specific pharmacological tools for the treatment of these hematopoietic malignancies.

Cutting edge: identification of the targets of clonal deletion in an unmanipulated thymus

Autoreactive thymocytes can be eliminated by clonal deletion during their development in the thymus. The precise developmental stage(s) at which clonal deletion occurs in a normal thymus has been difficult to assess, in large part because of the absence of a specific marker for TCR-mediated apoptosis. In this report, we reveal that Nur77 expression can be used as a specific marker of clonal deletion in an unmanipulated thymus and directly identify TCRintCD4+CD8+ and semimature CD4+CD8- thymocytes as the principal targets of deletion. These data indicate that clonal deletion normally occurs at a relatively late stage of development, as cells mature from CD4+CD8+ thymocytes to single-positive T cells.

Diffuse large B-cell lymphoma outcome prediction by gene-expression profiling and supervised machine learning

Diffuse large B-cell lymphoma (DLBCL), the most common lymphoid malignancy in adults, is curable in less than 50% of patients. Prognostic models based on pre-treatment characteristics, such as the International Prognostic Index (IPI), are currently used to predict outcome in DLBCL. However, clinical outcome models identify neither the molecular basis of clinical heterogeneity, nor specific therapeutic targets. We analyzed the expression of 6,817 genes in diagnostic tumor specimens from DLBCL patients who received cyclophosphamide, adriamycin, vincristine and prednisone (CHOP)-based chemotherapy, and applied a supervised learning prediction method to identify cured versus fatal or refractory disease. The algorithm classified two categories of patients with very different five-year overall survival rates (70% versus 12%). The model also effectively delineated patients within specific IPI risk categories who were likely to be cured or to die of their disease. Genes implicated in DLBCL outcome included some that regulate responses to B-cell-receptor signaling, critical serine/threonine phosphorylation pathways and apoptosis. Our data indicate that supervised learning classification techniques can predict outcome in DLBCL and identify rational targets for intervention.

Glucocorticoid resistance in thymocytes from mice expressing a T cell receptor transgene

A majority of thymocytes undergo apoptosis during differentiation due to lack of survival signals provided by T cell receptor (TCR) activation. As glucocorticoids (GC) have been suggested to be involved in this process, we have investigated the GC sensitivity in thymocytes from mice expressing a transgenic selecting TCR. We now report that immature CD4(+)CD8(+) double-positive thymocytes from these mice are comparatively more resistant to corticosterone-induced apoptosis. This is associated with reduced glucocorticoid receptor (GR) expression, increased levels of membrane CD28, increased NF-kappaB DNA binding activity, and increased binding to the CD28 response element in the interleukin-2 gene promoter. Analysis of NF-kappaB/Rel proteins from nuclear extracts demonstrated altered levels of some of these proteins. Our results suggest that TCR recognition of self major histocompatibility antigens generates intracellular signals which alter the thymocyte GC sensitivity and thereby protect them against apoptosis induced by endogenous GC.