EGF regulates a complex pattern of gene expression and represses smooth muscle differentiation during the neurotypic conversion of the neural-crest-derived TC-1S cell line

EGF, known to sustain CNS neuronal progenitors, also promotes a neurotypic response in the thymic neural-crest-derived TC-1S cell line. We report here the use of TC-1S cells as a model to identify the genetic programs regulated during the neurotypic response induced by EGF and to isolate 23 EGF-responsive genes. Among them 5 represent novel cDNAs, while 18 are known genes, whose regulation by EGF is associated with the mitogenic or differentiating effects of the growth factor. The repression of smooth muscle alpha-actin and SM22alpha genes by EGF and their increase by TGFbeta suggest that the TC-1S line includes neural crest multipotent cells whose smooth muscle differentiation is repressed upon EGF treatment and stimulated by TGFbeta. Therefore, we identified a complex pattern of EGF-target genes and propose EGF as a novel signal able to recruit postmigratory neural-crest-derived cells along proliferation and cell lineage choice pathways.

Stromal-epithelial interactions modulate estrogen responsiveness in normal human endometrium

The coculture of endometrial epithelial cells (EEC) with stromal cells (ESC) allows achievement of an improved in vitro system for studying interactions between cells via soluble signals. The purpose of this study was to investigate whether 17beta-estradiol and insulin can induce proliferation of EEC through ESC-secreted factors. No evidence of estrogen-induced EEC proliferation has been reported so far in the conventional culture methods. To this end, we used an in vitro bicameral coculture model where human EEC were grown on extracellular matrix-coated inserts applied in dishes containing ESC. Proliferation was assessed by tritiated thymidine incorporation. Homogeneity of endometrial cell populations was ascertained immunocytochemically. 17beta-estradiol did not induce any proliferative effect on EEC cultured alone. Endometrial epithelial cell proliferation was significantly enhanced in EEC/ESC cocultures; moreover, it was further increased by 17beta-estradiol addition. Insulin increased proliferation in EEC cultured alone, but again the effect was more pronounced in EEC/ESC cocultures. Coincubation of 17beta-estradiol and an antibody against insulin-like growth factor I (IGF I) led to neutralization of ESC-mediated EEC proliferation. This work provides evidence that the effect of 17beta-estradiol on human EEC proliferation may be mediated at least in part through ESC-secreted IGF I. We also showed that insulin effect is also partially due to ESC activation.

Expression of the HMGI(Y) gene products in human neuroblastic tumours correlates with differentiation status

HMGI and HMGY are splicing variants of the HMGI(Y) gene and together with HMGI-C, belong to a family of DNA binding proteins involved in maintaining active chromatin conformation and in the regulation of gene transcription. The expression of the HMGI(Y) gene is maximal during embryonic development, declines in adult differentiated tissues and is reactivated in most transformed cells in vitro and in many human cancers in vivo. The HMGI(Y) genomic locus is frequently rearranged in mesenchymal tumours, suggesting a biological role for HMGI(Y) gene products in tumour biology. HMGIs are both target and modulators of retinoic acid activity. In fact, HMGI(Y) gene expression is differentially regulated by retinoic acid in retinoid-sensitive and -resistant neuroblastoma cells, while HMGI-C participates in conferring retinoic acid resistance in some neuroblastoma cells. In this paper we show that HMGI and HMGY isoforms are equally regulated by retinoic acid in neuroblastoma cell lines at both RNA and protein levels. More importantly our immunohistochemical analysis shows that, although HMGI(Y) is expressed in all neuroblastic tumours, consistently higher levels are observed in less differentiated neuroblastomas compared to more differentiated ganglioneuromas, indicating that HMGI(Y) expression should be evaluated as a potential diagnostic and prognostic marker in neuroblastic tumours.

Expression of metabotropic glutamate receptors in murine thymocytes and thymic stromal cells

RT-PCR combined with immunoblotting showed the expression of group-I (mGlu1 and 5) and group-II (mGlu2 and 3) metabotropic glutamate receptors in whole mouse thymus, isolated thymocytes and TC-1S thymic stromal cell line. Cytofluorimetric analysis showed that mGlu-5 receptors were absent in CD4(-)/CD8(-) but present in more mature CD4(+) CD8(+) and CD4(+)CD8(-) thymocytes. mGlu-1a receptors showed an opposite pattern of expression with respect to mGlu5, whereas mGlu2/3 receptor expression did not differ between double negative and double positive cells. mGlu receptors expressed in both thymic cell components were functional, as indicated by measurements of polyphosphoinositide hydrolysis or cAMP formation. These data suggest a possible role for mGlu receptor signalling in the thymus.

Constitutive activation of NF-kappaB and T-cell leukemia/lymphoma in Notch3 transgenic mice

The multiplicity of Notch receptors raises the question of the contribution of specific isoforms to T-cell development. Notch3 is expressed in CD4(-)8(-) thymocytes and is down-regulated across the CD4(-)8(-) to CD4(+)8(+) transition, controlled by pre-T-cell receptor signaling. To determine the effects of Notch3 on thymocyte development, transgenic mice were generated, expressing lck promoter-driven intracellular Notch3. Thymuses of young transgenics showed an increased number of thymocytes, particularly late CD4(-)8(-) cells, a failure to down-regulate CD25 in post-CD4(-)8(-) subsets and sustained activity of NF-kappaB. Subsequently, aggressive multicentric T-cell lymphomas developed with high penetrance. Tumors sustained characteristics of immature thymocytes, including expression of CD25, pTalpha and activated NF-kappaB via IKKalpha-dependent degradation of IkappaBalpha and enhancement of NF-kappaB-dependent anti-apoptotic and proliferative pathways. Together, these data identify activated Notch3 as a link between signals leading to NF-kappaB activation and T-cell tumorigenesis. The phenotypes of pre-malignant thymocytes and of lymphomas indicate a novel and particular role for Notch3 in co-ordinating growth and differentiation of thymocytes, across the pre-T/T cell transition, consistent with the normal expression pattern of Notch3.

Expression pattern of notch1, 2 and 3 and Jagged1 and 2 in lymphoid and stromal thymus components: distinct ligand-receptor interactions in intrathymic T cell development

The suggested role of Notch1 or its mutants in thymocyte differentiation and T cell tumorigenesis raises the question of how the different members of the Notch family influence distinct steps in T cell development and the role played by Notch ligands in the thymus. We report here that different Notch receptor-ligand partnerships may occur inside the thymus, as we observed differential expression of Notch1, 2 and 3 receptors, their ligands Jagged1 and 2, and downstream intracellular effectors hairy and Enhancer of Split homolog 1 (HES-1) and hairy and Enhancer of Split homolog 5 (HES-5), depending on ontogenetic stage and thymic cell populations. Indeed, while Jagged2 is expressed in both stromal cells and thymocytes, Jagged1 expression is restricted to stromal cells. Moreover, a differential distribution of Notch3, with respect to Notch1, was observed in distinct age-related thymocyte subsets. Finally, Notch3 was preferentially up-regulated in thymocytes, following the induction of their differentiation by interaction with thymic epithelial cells expressing the cognate Jagged1 and 2 ligands, suggesting that, besides Notch1, Notch3 may also be involved in distinct steps of thymocyte development. Our results suggest that the Notch signaling pathway is involved in a complex interplay of T cell developmental stages, as a consequence of the heterogeneity and specific expression of members of the Notch receptor family and their cognate ligands, in distinct thymic cell compartments.

HMGI(Y) and HMGI-C genes are expressed in neuroblastoma cell lines and tumors and affect retinoic acid responsiveness

HMGI-C and HMGI(Y) are architectural DNA-binding proteins that participate in the conformational regulation of active chromatin. Their pattern of expression in embryonal and adult tissues, the analysis of the "pygmy" phenotype induced by the inactivation of the HMGI-C gene, and their frequent qualitative or quantitative alteration in experimental and human tumors indicate their pivotal role in the control of cell growth, differentiation, and tumorigenesis in several tissues representative of the epithelial, mesenchymal, and hematopoietic lineages. In contrast, very little information is available on their expression and function in neural cells. Here, we investigated the expression of the HMGI(Y) and HMGI-C genes in neuroblastoma (NB), a tumor arising from an alteration of the normal differentiation of neural crest-derived cells and in embryonal and adult adrenal tissue. Although HMGI(Y) is constitutively expressed in the embryonal and adult adrenal gland and in all of the NB cell lines and ex vivo tumors examined, its regulation appears to be associated to growth inhibition and differentiation because we observed that HMGI(Y) expression is reduced by retinoic acid (RA) in several NB cell lines that are induced to differentiate into postmitotic neurons, whereas it is up-regulated by RA in cells that fail to differentiate. Furthermore, the decrease of HMGI(Y) expression observed in RA-induced growth arrest and differentiation is abrogated in cells that have been made insensitive to this drug by NMYC overexpression. In contrast, HMGI-C expression is down-regulated during the development of the adrenal gland, completely absent in the adult individual, and only detectable in a subset of ex vivo NB tumors and in RA-resistant NB cell lines. We provide evidence of a causal link between HMGI-C expression and resistance to the growth arrest induced by RA in NB cell lines because exogenous HMGI-C expression in HMGI-C-negative and RA-sensitive cells is sufficient to convert them into RA-resistant cells. Therefore, we suggest that HMGI-C and HMGI(Y) may participate in growth- and differentiation-related tumor progression events of neuroectodermal derivatives.

Thrombospondin-1 is a mediator of the neurotypic differentiation induced by EGF in thymic epithelial cells

Thymic epithelial cell component originates from cranial neural crest as well as from endoderm and ectoderm of the third pharyngeal pouch and branchial cleft. Epidermal growth factor (EGF) has been previously shown to play a crucial role in directing thymic epithelial cells toward a neural-oriented cell fate. To identify genes that are involved in the EGF-induced neurotypic differentiation of the thymic stroma-derived TC-1S cell line, we studied EGF-treated and untreated cells by RNA fingerprinting PCR-based differential screening. We obtained 23 distinct sequences including 18 known genes and 5 sequences previously unreported, which are currently under characterization. Here, we describe the involvement of one of the isolated genes, the thrombospondin-1, as a mediator of the neurotypic differentiation induced by EGF in TC-1S cells. We show that thrombospondin-1 mRNA and protein levels are increased by EGF. Moreover, exogenous thrombospondin-1 is able to enhance the outgrowth of neurite-like processes as well as the expression of neurofilaments and neural cell adhesion molecule in TC-1S cells. These observations suggest that the up-regulation of thrombospondin-1 synthesis induced by EGF contributes to the differentiation choice of thymic epithelial cells toward a neural fate, reminiscent of their neural crest origin.

Modulation of cytokine gene expression by thymic lympho-stromal cell to cell interaction: effect of retinoic acid

We have examined the expression of a panel of cytokines in thymic epithelial cells and CD4-CD8- (DN) thymocytes following cell to cell lymphostromal interaction, in an experimental model which enhances in vitro thymocyte maturation. Since retinoic acid (RA) has been previously shown to be an inhibitor of thymocyte maturation process in this model, we wanted to analyse cytokine expression in DN thymocytes and thymic epithelial cells following the RA-induced impairment of in vitro thymocyte maturation. Cell to cell lymphostromal interaction results in increased IL2 and decreased IL7 expression in thymocytes while the expression of IL1 beta and IL7 increased and decreased, respectively, in thymic epithelial cells. Addition of RA to lympho-stromal cell co-culture results in the enhancement of IL4 and IL7 expression in thymocytes while in thymic epithelial cells IL1 alpha decreased and IL6 and IL7 increased. These data indicate that discrete patterns of cytokine expression are present in thymocyte precursors and in thymic epithelial cells during in vitro T-cell development. They furthermore suggest that specific cytokine modulation might contribute to the RA-induced impairment of thymocyte differentiation.

Interleukin-6 and CAAT/enhancer binding protein beta-deficient mice act as tools to dissect the IL-6 signalling pathway and IL-6 regulation

Interleukin-6 (IL-6) is a pleiotropic cytokine playing important roles in immunity, hemopoiesis and inflammation. IL-6 signalling is known to involve the activation of two independent transcription factors: Stat3 (through phosphorylation by Jak kinases) and C/EBP beta (through activation of the ras pathway). In addition, C/EBP beta is believed to act as a transcriptional activator of the IL-6 gene itself. Making use of IL-6-deficient mice, we have recently demonstrated that IL-6 is essential for the induction of acute phase mRNAs in the liver upon localized tissue damage, but not upon systemically induced inflammation. Here we show that the defective mRNA induction is paralleled by a defective activation of Stat3, thus establishing a direct relationship between IL-6 function, Stat3 activation and acute phase genes induction. On the other hand, making use of C/EBP beta-deficient mice, we show that the induction of IL-6 by a variety of stimuli does not require C/EBP beta activity. In contrast to the predicted activating role of C/EBP beta, IL-6 levels are increased in the C/EBP beta-deficient mice, suggesting that C/EBP beta may act as a down-modulator of the IL-6 gene. Through the generation of C/EBP beta, IL-6 double mutant mice we show that IL-6 hyperproduction is responsible for the development of the Castleman's like lymphoproliferative disease described in the C/EBP beta-deficient mice, since the disorder is completely blocked by inactivating the IL-6 gene.

2-Aminopurine unravels a role for pRB in the regulation of gene expression by transforming growth factor beta

Transforming growth factor type beta (TGFbeta) is a pleiotropic factor that regulates different cellular activities including cell growth, differentiation, and extracellular matrix deposition. All the known effects of TGFbeta appear to be mediated by its interaction with cell surface receptors that possess a serine/threonine kinase activity. However, the intracellular signals that follow receptor activation and lead to the different cellular responses to TGFbeta are still largely unknown. On the basis of the different sensitivity to the protein kinase inhibitor 2-aminopurine and the phosphatase inhibitor okadaic acid, we identified two distinct pathways through which TGFbeta activates a genomic response. Consistently, 2-aminopurine prevented and okadaic acid potentiated the induction of JE by TGFbeta. The induction of PAI-1 and junB was instead potentiated by 2-aminopurine, after a transient inhibition and was unaffected by okadaic acid. The superinducing effect of 2-aminopurine required the presence of a functional RB protein since it was abolished in SV40 large T antigen-transfected cells, absent in the BT549 and Saos-2 RB-defective cell lines, and restored in BT549 and Saos-2 cells after reintroduction of pRB. The effects of 2-aminopurine on the TGFbeta inducible junB expression occur in all the cell lines examined suggesting that junB, and possibly other genes, can be regulated by TGFbeta through a distinct pRB-dependent pathway.

Expression of trKB neurotrophin receptor during T cell development. Role of brain derived neurotrophic factor in immature thymocyte survival

The relationships between the nervous and the immune systems raise the question of whether neurotrophic factors, in addition to the regulation of neural cell ontogeny, may influence lymphocyte development. We report in this work that the pattern of neurotrophin receptor expression depends on the developmental stage of T cells. The presence of nerve growth factor receptor trkA could not be detected in any of the thymocyte subsets, whereas brain-derived neurotrophic factor (BDNF) receptor trkB was expressed in all thymocyte subpopulations. Interestingly, both trkB mRNA and protein expression inversely correlated with the maturation stage and the differentiation potential of thymocytes, being more expressed in CD4-8- immature thymocytes and progressively declining in CD8+ and CD4+ single-positive and CD4+8+ more mature thymocytes. The developmentally regulated expression of trkB is further shown by the inhibition or enhancement of trkB expression induced by signals that either trigger or impair the transition from immature to more differentiated stages of the thymocyte developmental pathway. Signals generated following the interaction of BDNF with trkB receptor resulted in the stimulation of trkB autophosphorylation and in the up-regulation of the expression of the c-fos gene in CD4-8- cells and enhanced thymocyte survival. Finally, BDNF is expressed in thymic stroma and is further up-regulated by signals generated by the thymocyte/stromal cell interaction. These data suggest that BDNF may be a novel survival factor for thymocyte precursors and support the presence of developmentally regulated feedback mechanisms based on autocrine/paracrine neurotrophin/receptor interactions that may be involved in the thymocyte differentiation process.

Expression of trKB neurotrophin receptor during T cell development. Role of brain derived neurotrophic factor in immature thymocyte survival

The relationships between the nervous and the immune systems raise the question of whether neurotrophic factors, in addition to the regulation of neural cell ontogeny, may influence lymphocyte development. We report in this work that the pattern of neurotrophin receptor expression depends on the developmental stage of T cells. The presence of nerve growth factor receptor trkA could not be detected in any of the thymocyte subsets, whereas brain-derived neurotrophic factor (BDNF) receptor trkB was expressed in all thymocyte subpopulations. Interestingly, both trkB mRNA and protein expression inversely correlated with the maturation stage and the differentiation potential of thymocytes, being more expressed in CD4-8- immature thymocytes and progressively declining in CD8+ and CD4+ single-positive and CD4+8+ more mature thymocytes. The developmentally regulated expression of trkB is further shown by the inhibition or enhancement of trkB expression induced by signals that either trigger or impair the transition from immature to more differentiated stages of the thymocyte developmental pathway. Signals generated following the interaction of BDNF with trkB receptor resulted in the stimulation of trkB autophosphorylation and in the up-regulation of the expression of the c-fos gene in CD4-8- cells and enhanced thymocyte survival. Finally, BDNF is expressed in thymic stroma and is further up-regulated by signals generated by the thymocyte/stromal cell interaction. These data suggest that BDNF may be a novel survival factor for thymocyte precursors and support the presence of developmentally regulated feedback mechanisms based on autocrine/paracrine neurotrophin/receptor interactions that may be involved in the thymocyte differentiation process.

Inactivation of the IL-6 gene prevents development of multicentric Castleman's disease in C/EBP beta-deficient mice

Castleman's disease is a lymphoproliferative disorder thought to be related to deregulated production of IL-6. We have previously shown that mice lacking the trans-acting factor C/EBP beta, a transcriptional regulator of IL-6 and a mediator of IL-6 intracellular signaling, develop a pathology nearly identical to multicentric Castleman's disease, together with increasingly high levels of circulating IL-6. We describe here how the simultaneous inactivation of both IL-6 and C/EBP beta genes prevents the development of pathological traits of Castleman's disease observed in C/EBP beta-deficient mice. Histological and phenotypic analysis of lymph nodes and spleen of double mutant mice did not show either the lymphoadenopathy and splenomegaly or the abnormal expansion of myeloid, B and plasma cell compartments observed in C/EBP beta-/- mice, while B cell development, although delayed, was normal. Our data demonstrate that IL-6 is essential for the development of multicentric Castleman's disease in C/EBP beta-/- mice.

Prostaglandin E2 inhibits the interleukin-2 promoter activity through down-regulation of the Oct-dependent transcription of the octamer motif

Prostaglandins, mainly those of the E series (PGE), are modulators of immune responses. Indeed PGE2 inhibits T cell activation and the transcription of the interleukin-2 (IL-2) gene, the major T cell growth factor. We observed that PGE2 inhibits IL-2 promoter transcription activity by interfering with signals activating the (-96 to -66 bp) octamer motif. This motif binds Oct-1 and Oct-2 as well as the phorbol ester and calcium ionophore-inducible jun and fos AP-1 factors. The PGE2-dependent down-modulation is observed in the presence of either the endogenous transacting factor Oct-1 or the exogenously expressed Oct-2. PGE2 does not regulate octamer function by influencing the jun and fos mRNA or Oct-1 protein levels or their DNA-binding abilities. Functional dissection of the octamer motif, through mutations of either the AP-1 or the octamer sites, revealed that the AP-1 site is dispensable for PGE2-dependent inhibition which instead may occur through the interference with the Oct-mediated transactivation of the octamer element. Our data suggest that the Oct-octamer interaction is a novel target of the PGE2-induced down-regulation of the IL-2 promoter.

Human immunodeficiency virus type 1 tat protein modulates fibronectin expression in thymic epithelial cells and impairs in vitro thymocyte development

Infection of both lymphoid and stromal components of the thymus by human immunodeficiency virus type 1 (HIV-1) suggests that impairment of lymphocyte differentiation from early T cells progenitors in the thymus may contribute to the HIV-induced T cell depletion. Cross-talk between immature thymocyte and thymic epithelium through cell-to-cell adhesion mediated by fibronectin/receptor interaction plays a central role in driving T cell development. HIV-1 tat protein, like fibronectin, contains an RGD sequence involved in the interaction with fibronectin receptor. We demonstrated that gene transfer-mediated tat expression in thymic stroma is able to influence the in vitro maturation of T cell progenitors as tat-expressing epithelial cells have a decreased ability to drive the generation of CD4+8+ thymocytes from CD4-8- precursors. Furthermore, tat-expressing cells produce more fibronectin and display upregulation of VLA-5 cell surface receptor levels compared to control cells, while alpha v expression was unchanged. Cellular distribution of fibronectin is also influenced by tat. Fibronectin is distributed in the whole cell surface and along cell processes in control cells whereas it is mainly concentrated in the intracytoplasmic area in tat-expressing cells. Therefore, expression of tat in thymic epithelial cells impairs thymocyte maturation and modulates fibronectin expression: this suggests a crucial role of this viral protein in regulating the T lymphocyte differentiation program through modulation of intrathymic lympho-stromal interactions.

Epidermal growth factor promotes a neural phenotype in thymic epithelial cells and enhances neuropoietic cytokine expression

Neural crest-derived cells populate the thymus, and their coexistence with epithelial cells is required for proper organ development and T cell education function. We show here that epidermal growth factor (EGF), a major epithelial cell growth-enhancing agent, has a morphogenetic action to promote the expression of a neuronal phenotype (e.g., neurofilament expression) in cultured thymic epithelial cells that are characterized by a cytokeratin-positive epithelial cell background. The proliferation of such neurodifferentiated cells is also enhanced by EGF. Furthermore, the growth factor enhances cells that express the genes encoding the preprotachykinin A-generated neuropeptides and bipotential neuropoietic and lymphopoietic cytokines ciliary neurotrophic factor and interleukin-6. These cytokines also enhance the neuronal phenotype of thymic epithelial cells. Therefore, EGF appears to be a composite autocrine/paracrine neuromodulator in thymic stroma. This suggests that EGF may regulate thymus-dependent immune functions by promoting neuronal gene expression in neural crest-derived cells.

Lymphoproliferative disorder and imbalanced T-helper response in C/EBP beta-deficient mice

C/EBP beta is considered a key element of interleukin-6 (IL-6) signalling as well as an important transcriptional regulator of the IL-6 gene itself. We describe here how mice lacking C/EBP beta develop a pathology similar to mice overexpressing IL-6 and nearly identical to multicentric Castleman's disease in human patients, with marked splenomegaly, peripheral lymphadenopathy and enhanced haemopoiesis. Humoral, innate and cellular immunity are also profoundly distorted, as shown by the defective activation of splenic macrophages, the strong impairement of IL-12 production, the increased susceptibility to Candida albicans infection and the altered T-helper function. Our data show that C/EBP beta is crucial for the correct functional regulation and homeostatic control of haemopoietic and lymphoid compartments.

Heavy-metal modulation of the human intercellular adhesion molecule (ICAM-1) gene expression

The intercellular adhesion molecule 1 (ICAM-1) can be induced on many different cell types by a set of various modulators (IL1 beta, TNF, LPS, IFN-gamma), which are released during the inflammatory process. We have investigated the possibility that other factors, related to the stress and biophysical perturbations of the inflammatory response, may also modulate ICAM-1. Here, we report that heavy metals, in particular zinc, can enhance the expression of the ICAM-1 gene on cells actively involved at different levels during inflammation. Kinetic studies of ICAM-1 gene expression shows a maximum level of induction 4 h after treatment with metals, followed by a rapid decrease to basal levels within 12 h. The effect on enhanced gene expression is mostly due to a rapid increase of the transcriptional rate as shown by nuclear run-on experiments. In B lymphoblastoid cells, but not in fibroblasts, the increase in RNA expression seems significantly greater that the subsequent increase in protein expression, suggesting that a further point of post-transcriptional regulation of ICAM-1 occurs and may be linked to the cellular specificity. may be linked to the cellular specificity.

Human immunodeficiency virus type-1 tat enhances interleukin-2 promoter activity through synergism with phorbol ester and calcium-mediated activation of the NF-AT cis-regulatory motif

Interference with T cell activation signals by Human immunodeficiency virus (HIV) gene products is suggested to contribute to the impairment of immune functions observed in AIDS. Interleukin-2 (IL-2) and HIV share common stimulatory signals triggered during T cell activation. The role of HIV tat, which is the main enhancing factor for viral LTR, in the regulation of IL-2 gene transcription has been studied following transient expression of the tat gene in phorbol ester and calcium ionophore-activated Jurkat cells transfected with IL-2 promoter-chloramphenicol acetyltransferase reporter constructs. We observed that tat increased the IL-2 promoter transcriptional activity in response to phorbol ester and ionomycin. This tat-dependent synergism mapped to the (-279 to -263 bp) NFAT motif of the IL-2 enhancer, which was sufficient to be transactivated by tat. Our data suggest that tat links T cell activating signals to the shared IL-2 and HIV regulation. This may play a role in the early phase of HIV infection.

Positive and negative regulation of the composite octamer motif of the interleukin 2 enhancer by AP-1, Oct-2, and retinoic acid receptor

The differentiating agent retinoic acid (RA) has been previously reported to interfere with 12-O-tetradecanoyl-phorbol-13-acetate (TPA)/Ca2+-induced signals for the regulation of the -96 to -66-bp octamer motif found in the enhancer for the interleukin (IL)-2 gene, which encodes a major T lymphocyte growth factor. The IL-2 octamer motif is a composite cis-element which binds Oct-1 and Oct-2 as well as a TPA/Ca2+-inducible nuclear factor, previously termed octamer-associated protein (OAP40). We show here that Oct-2, despite the presence of an active transcriptional activation domain, requires TPA/Ca2+-induced signals to strongly transactivate the IL-2 octamer motif in Jurkat T cells. This Oct-2-dependent transactivation is inhibited by RA. The presence of an intact COOH-terminal domain of Oct-2 contributes to both TPA/Ca2+-induced transactivation and the RA-mediated repression. We also show that both Fos and Jun components of the AP-1 factors participate in the OAP40 complex. Furthermore, transfected c-jun, jun-B, jun-D, c-fos, or Fos-B expression vectors partially substitute for TPA and Ca2+ and cooperate with Oct-2 for the transactivation of the combined OAP/octamer cis-element. Mutations of the genuine octamer-binding site abrogate both the binding of Oct-1 and Oct-2 and the TPA/Ca2+-induced transactivation of the OAP/octamer motif. OAP confers to Oct-2 responsivity to both TPA/Ca2+ and RA, since specific mutations of the AP-1/OAP-binding site significantly reduce the transactivation by Oct-2 in response to TPA and Ca2+ and abolish the inhibition by RA. Furthermore, retinoic acid receptor (RAR) alpha is able to inhibit in vitro the formation of the complex between the nuclear AP-1/OAP and its specific binding site, resulting in the interference with Oct-2-dependent cis-regulatory function of this AP-1 element. Therefore, we propose that the TPA/calcium-activated AP-1/OAP element is the main target of positive or negative regulatory signals influencing the IL-2 octamer motif, through synergism with Oct-2 and antagonism by RAR.

Cell-specific bifunctional role of Jun oncogene family members on glucocorticoid receptor-dependent transcription

Interaction between protein kinase C (PKC)- and glucocorticoid receptor (GR)-mediated signaling is suggested by the ability of the PKC activating phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA) to inhibit GR-dependent transcription of the mouse mammary tumor virus (MMTV) long terminal repeat (LTR). Here we report that this interference is cell specific, as TPA augmented dexamethasone-induced transcriptional activation of the MMTV LTR in several T cell lines but was inhibitory in NIH-3T3 fibroblasts. TPA-GR synergism was determined to have occurred at the GR-responsive element (GRE) level by functional analysis of deletion mutants or synthetic GRE oligonucleotides driving chloramphenicol acetyl-transferase expression. Synergism required an intact GR DNA-binding domain, whereas amino- or carboxyl-terminal domains were dispensable. The effect was abrogated by the PKC inhibitor staurosporine, suggesting a role for PKC. Increased c-jun, jun-B, and jun-D expression above basal levels and increased transcriptional activity of AP-1/TPA responsive elements fused to chloramphenicol acetyl-transferase vectors were observed in T cells treated with TPA alone or in combination with dexamethasone. The ability of Jun proteins to cooperate with GR in T cells has been investigated after transfection of c-jun, jun-B, or jun-D expression vectors, which augmented GR-dependent transcription from either MMTV LTR or GRE. Conversely, c-jun and jun-B transfection blunted GR-dependent transcription in HeLa cells. The presence of c-fos had a negative influence on GR function and correlated with the cell-specific synergistic or antagonistic activity of Jun with respect to GR; high basal expression of c-fos as well as AP-1 DNA binding and transcriptional activity were observed in HeLa cells, but not in T cells. Furthermore overexpression of exogenous c-fos has an inhibitory effect on GR-dependent transcription from GRE in T cells. We propose that Jun plays a bifunctional role on GR-dependent transcriptional activation of GRE, selecting either synergistic or antagonistic activity depending on the cell-specific microenvironment. In this regard, intracellular levels of c-fos appear to be influential.

Neuromodulatory loop mediated by nerve growth factor and interleukin 6 in thymic stromal cell cultures

Neural crest cell derivatives have been suggested to be involved in thymus development. We established nonlymphoid thymic stromal cell cultures capable of supporting T-cell differentiation. In these nonlymphoid cell cultures, we identified cells with phenotypic and biochemical markers specific for neuronal cells. Neurofilament mRNA and 68- and 160-kDa neurofilament proteins, as well as 74-kDa synapsin I isoform, were expressed in many of the cultured cells. For example, neurofilament immunoreactivity was detected in 20-30% of the cells. To see whether thymic neuronal-like cells were involved in a neural differentiation pathway, we investigated the effect of nerve growth factor (NGF) and interleukin 6 (IL-6), two known neurotrophic factors. The expression of the above-described neural markers was enhanced by NGF and IL-6, which we report to be produced in an autocrine way by thymic stromal cell cultures. Finally, we found that IL-6 gene expression in these cell cultures was enhanced by NGF. Evidence is thus offered of a neuromodulatory loop within the thymic stromal cell population supported by local production of NGF and IL-6 and involving neural cell elements. Interestingly, IL-6, which is known to be implicated in thymocyte differentiation, also displays a neuromodulatory activity on thymic stromal cells, suggesting a multivalent role for this cytokine within the thymus.