Distinct mechanisms for rescue from apoptosis in Ramos human B cells by signaling through CD40 and interleukin-4 receptor: role for inhibition of an early response gene, Berg36

CARD11 is a novel target of miR-181b that is upregulated in chronic lymphocytic leukemia

Aim: To investigate the targets of miR-181b in patients with chronic lymphocytic leukemia (CLL). Materials & methods: The bioinformatic softwares were used to indicate the key target genes associated with miR-181b, and the results were verified in CLL patient samples and 293T cells. Results: CARD11 is a potential target gene of miR-181b, an inverse relationship was revealed between the expression of CARD11 and miR-181b in 104 CLL patients, and it was confirmed in vitro with luciferase assays and western blotting. Kaplan-Meier analysis showed that CLL patients with high CARD11 expression demonstrated poor survival. Conclusion: CARD11 is a novel target of miR-181b that is upregulated, which could be a poor prognostic indicator for CLL patients.

Post-transcriptional regulation of BCL2 mRNA by the RNA-binding protein ZFP36L1 in malignant B cells

The human ZFP36 zinc finger protein family consists of ZFP36, ZFP36L1, and ZFP36L2. These proteins regulate various cellular processes, including cell apoptosis, by binding to adenine uridine rich elements in the 3' untranslated regions of sets of target mRNAs to promote their degradation. The pro-apoptotic and other functions of ZFP36 family members have been implicated in the pathogenesis of lymphoid malignancies. To identify candidate mRNAs that are targeted in the pro-apoptotic response by ZFP36L1, we reverse-engineered a gene regulatory network for all three ZFP36 family members using the 'maximum information coefficient' (MIC) for target gene inference on a large microarray gene expression dataset representing cells of diverse histological origin. Of the three inferred ZFP36L1 mRNA targets that were identified, we focussed on experimental validation of mRNA for the pro-survival protein, BCL2, as a target for ZFP36L1. RNA electrophoretic mobility shift assay experiments revealed that ZFP36L1 interacted with the BCL2 adenine uridine rich element. In murine BCL1 leukemia cells stably transduced with a ZFP36L1 ShRNA lentiviral construct, BCL2 mRNA degradation was significantly delayed compared to control lentiviral expressing cells and ZFP36L1 knockdown in different cell types (BCL1, ACHN, Ramos), resulted in increased levels of BCL2 mRNA levels compared to control cells. 3' untranslated region luciferase reporter assays in HEK293T cells showed that wild type but not zinc finger mutant ZFP36L1 protein was able to downregulate a BCL2 construct containing the BCL2 adenine uridine rich element and removal of the adenine uridine rich core from the BCL2 3' untranslated region in the reporter construct significantly reduced the ability of ZFP36L1 to mediate this effect. Taken together, our data are consistent with ZFP36L1 interacting with and mediating degradation of BCL2 mRNA as an important target through which ZFP36L1 mediates its pro-apoptotic effects in malignant B-cells.

ZFP36L1 negatively regulates plasmacytoid differentiation of BCL1 cells by targeting BLIMP1 mRNA

The ZFP36/Tis11 family of zinc-finger proteins regulate cellular processes by binding to adenine uridine rich elements in the 3′ untranslated regions of various mRNAs and promoting their degradation. We show here that ZFP36L1 expression is largely extinguished during the transition from B cells to plasma cells, in a reciprocal pattern to that of ZFP36 and the plasma cell transcription factor, BLIMP1. Enforced expression of ZFP36L1 in the mouse BCL1 cell line blocked cytokine-induced differentiation while shRNA-mediated knock-down enhanced differentiation. Reconstruction of regulatory networks from microarray gene expression data using the ARACNe algorithm identified candidate mRNA targets for ZFP36L1 including BLIMP1. Genes that displayed down-regulation in plasma cells were significantly over-represented (P = <0.0001) in a set of previously validated ZFP36 targets suggesting that ZFP36L1 and ZFP36 target distinct sets of mRNAs during plasmacytoid differentiation. ShRNA-mediated knock-down of ZFP36L1 in BCL1 cells led to an increase in levels of BLIMP1 mRNA and protein, but not for mRNAs of other transcription factors that regulate plasmacytoid differentiation (xbp1, irf4, bcl6). Finally, ZFP36L1 significantly reduced the activity of a BLIMP1 3′ untranslated region-driven luciferase reporter. Taken together, these findings suggest that ZFP36L1 negatively regulates plasmacytoid differentiation, at least in part, by targeting the expression of BLIMP1.

An emerging role of RNA-binding proteins as multifunctional regulators of lymphocyte development and function

Sequence-specific RNA-binding proteins (RBP) and the regulation of RNA decay have long been recognized as important regulators of the inflammatory response. RBP influence gene expression throughout the lifespan of the mRNA by regulating splicing, polyadenylation, cellular localization, translation, and decay. Increasing evidence now indicates that these proteins, together with the RNA decay machinery that they recruit, also regulate the development and activation of lymphocytes. The activity of RBP is regulated by the same signal transduction pathways that govern lymphocyte development and differentiation in response to antigen and cytokine receptor engagement. Roles for these proteins in regulating the diverse functions of lymphocytes are becoming increasingly apparent.

Is transcription the dominant force during dynamic changes in gene expression?

Dynamic changes in gene expression punctuate lymphocyte development and are a characteristic of lymphocyte activation. A prevailing view has been that these changes are driven by DNA transcription factors, which are the dominant force in gene expression. Accumulating evidence is challenging this DNA centric view and has highlighted the prevalence and dynamic nature of RNA handling mechanisms. Alternative splicing and differential polyadenylation appear to be more widespread than first thought. Changes in mRNA decay rates also affect the abundance of transcripts and this mechanism may contribute significantly to gene expression. Additional RNA handling mechanisms that control the intracellular localization of mRNA and association with translating ribosomes are also important. Thus, gene expression is regulated through the coordination of transcriptional and post-transcriptional mechanisms. Developing a more "RNA centric" view of gene expression will allow a more systematic understanding of how gene expression and cell function are integrated.

TIS11 family proteins and their roles in posttranscriptional gene regulation

Posttranscriptional regulation of gene expression of mRNAs containing adenine-uridine rich elements (AREs) in their 3′ untranslated regions is mediated by a number of different proteins that interact with these elements to either stabilise or destabilise them. The present review concerns the TPA-inducible sequence 11 (TIS11) protein family, a small family of proteins, that appears to interact with ARE-containing mRNAs and promote their degradation. This family of proteins has been extensively studied in the past decade. Studies have focussed on determining their biochemical functions, identifying their target mRNAs, and determining their roles in cell functions and diseases.

A unique C-terminal repeat domain maintains the cytosolic localization of the placenta-specific tristetraprolin family member ZFP36L3

Members of the tristetraprolin family of CCCH tandem zinc finger proteins bind to AU-rich elements in certain cellular mRNAs, leading to their deadenylation and destabilization. Studies in knock-out mice demonstrated roles for three of the family members, tristetraprolin, ZFP36L1, and ZFP36L2, in inflammation, chorioallantoic fusion, and early embryonic development, respectively. However, little is known about a recently discovered placenta-specific tristetraprolin family member, ZFP36L3. Tristetraprolin, ZFP36L1, and ZFP36L2 have been shown to shuttle between the nucleus and cytoplasm, using typical hydrophobic amino acid-rich nuclear export sequences, and nuclear localization sequences located within the tandem zinc finger domain. In contrast, we previously showed that green fluorescent protein-labeled ZFP36L3, expressed in HEK 293 cells, remained cytosolic, even in the presence of the nuclear export blocker leptomycin B. We show here that the conserved tandem zinc finger domain contains an active nuclear localization signal. However, the sequence corresponding to the nuclear export signal in the other family members was nonfunctional, and thus did not contribute to the cytosolic localization. The unique C-terminal repeat domain could override the activity of the nuclear localization sequence, preventing the import of ZFP36L3 into the nucleus. Immunostaining of mouse placenta demonstrated that ZFP36L3 was located only in the cytoplasm of trophoblast cells. Thus, in contrast to the other mammalian members of this protein family, ZFP36L3 is a "full-time" cytosolic protein, rather than a nucleocytoplasmic shuttling protein. The significance of this difference in subcellular localization to the physiology of placental trophoblast cells, where ZFP36L3 is selectively expressed, remains to be determined.

The RNA binding proteinZfp36l1is required for normal vascularisation and post-transcriptionally regulates VEGF expression

The Zfp36l1 gene encodes a zinc finger-containing mRNA binding protein implicated in the posttranscriptional control of gene expression. Mouse embryos homozygous for a targeted mutation in the Zfp36l1 locus died mid-gestation and exhibited extraembryonic and intraembryonic vascular abnormalities and heart defects. In the developing placenta, there was a failure of the extraembryonic mesoderm to invaginate the trophoblast layer. The phenotype was associated with an elevated expression of vascular endothelial growth factor (VEGF)-A in the embryos and in embryonic fibroblasts cultured under conditions of both normoxia and hypoxia. VEGF-A overproduction by embryonic fibroblasts was not a consequence of changes in Vegf-a mRNA stability; instead, we observed enhanced association with polyribosomes, suggesting Zfp36l1 influences translational regulation. These data implicate Zfp36l1as a negative regulator of Vegf-a gene activity during development.

Identification of genes involved in Ca2+ ionophore A23187-mediated apoptosis and demonstration of a high susceptibility for transcriptional repression of cell cycle genes in B lymphoblasts from a patient with Scott syndrome

Background

In contrast to other agents able to induce apoptosis of cultured cells, Ca²⁺ ionophore A23187 was shown to elicit direct activation of intracellular signal(s). The phenotype of the cells derived from patients having the hemorrhagic disease Scott syndrome, is associated with an abnormally high proportion of apoptotic cells, both in basal culture medium and upon addition of low ionophore concentrations in long-term cultures. These features are presumably related to the mutation also responsible for the defective procoagulant plasma membrane remodeling. We analyzed the specific transcriptional re-programming induced by A23187 to get insights into the effect of this agent on gene expression and a defective gene regulation in Scott cells.

Results

The changes in gene expression upon 48 hours treatment with 200 nM A23187 were measured in Scott B lymphoblasts compared to B lymphoblasts derived from the patient's daughter or unrelated individuals using Affymetrix microarrays. In a similar manner in all of the B cell lines, results showed up-regulation of 55 genes, out of 12,000 represented sequences, involved in various pathways of the cell metabolism. In contrast, a group of 54 down-regulated genes, coding for histones and proteins involved in the cell cycle progression, was more significantly repressed in Scott B lymphoblasts than in the other cell lines. These data correlated with the alterations of the cell cycle phases in treated cells and suggested that the potent effect of A23187 in Scott B lymphoblasts may be the consequence of the underlying molecular defect.

Conclusion

The data illustrate that the ionophore A23187 exerts its pro-apoptotic effect by promoting a complex pattern of genetic changes. These results also suggest that a subset of genes participating in various steps of the cell cycle progress can be transcriptionally regulated in a coordinated fashion. Furthermore, this research brings a new insight into the defect in cultured Scott B lymphoblasts, leading to hypothesize that a mutated gene plays a role not only in membrane remodeling but also in signal transduction pathway(s) leading to altered transcriptional regulation of cell cycle genes.

Butyrate response factor 1 enhances cisplatin sensitivity in human head and neck squamous cell carcinoma cell lines

Cisplatin is a widely used chemotherapeutic agent in head and neck squamous cell carcinoma (HNSCC). Resistance to cisplatin is a common feature of HNSCC. To identify genes that may regulate cisplatin sensitivity, we carried out a cDNA microarray analysis of gene expression in cisplatin-sensitive and cisplatin-resistant HNSCC-derived cell lines. Among genes differentially expressed by cisplatin treatment, we have confirmed the elevated expression of butyrate responsive factor 1 (BRF1) in cisplatin-sensitive HNSCC cells and have demonstrated that the expression level of BRF1 is associated with cisplatin-sensitivity. Specific inhibition of BRF1 expression using an antisense oligodeoxynucleotide (ODN) decreased the cisplatin-sensitivity and, on the contrary, overexpression of BRF1 increased cisplatin-sensitivity in HNSCC cells. Elevated expression of BRF1 decreased the level of the human inhibitor of apoptosis protein-2 (cIAP2) and increased the caspase-3 activity in HNSCC cells. In addition, elevated expression of BRF1 decreased the expression level of enhanced green fluorescent protein (EGFP) linked to a 3' terminal AU-rich element (ARE) of cIAP2 mRNA. These findings demonstrate that BRF1 expression enhanced cisplatin sensitivity in HNSCC cells by reducing the levels of cIAP2 mRNA.

Zfp36l3, a Rodent X Chromosome Gene Encoding a Placenta-Specific Member of the Tristetraprolin Family of CCCH Tandem Zinc Finger Proteins

Members of the tristetraprolin (TTP) family of CCCH tandem zinc finger (TZF) proteins can bind directly to AU-rich elements (ARE) in mRNA, causing deadenylation and destabilization of the transcripts to which they bind. We describe here a novel fourth mammalian member of the TTP protein family, designated ZFP36L3, which could also bind directly to ARE-containing RNAs and could promote the deadenylation and degradation of ARE-containing target RNAs. Zfp36l3 transcript expression was detected only in placenta and extraembryonic tissues in the mouse. It was expressed throughout development in the placenta and was particularly highly expressed in the cells of the labyrinthine layer of the trophoblastic placenta. Unlike the other family members, the expression of a ZFP36L3-green fluorescent protein fusion protein was entirely cytoplasmic when expressed in 293 cells, even in the presence of the CRM1-dependent nuclear export inhibitor leptomycin B. Zfp36l3 was located on the mouse X chromosome; a similar predicted gene was present on the rat X chromosome, but there was no evidence for a similar gene in humans. ZFP36L3 may thus be a rodent-specific or even murine-specific member of the TTP protein family. Its presumed role in placental physiology may be unique to rodents or murine rodents, but this role may be subsumed by other family members in nonrodents.

Recruitment of mRNA-destabilizing protein TIS11 to stress granules is mediated by its zinc finger domain

TIS11, a member of the CCCH zinc finger protein family, was found to be distributed throughout cells with a preferential cytoplasmic localization when transiently expressed in COS-7 cells. Upon treatment with heat shock, TIS11 became localized in discrete particles in the cytoplasm of the transfectants. We showed the TIS11-positive particles to be stress granules (SGs), which are known to be formed in the cytoplasm of eukaryotic cells in response to environmental stresses. By deletion studies using the green fluorescent protein fusion system, we mapped a functional stress granule (SG) localization signal to a region containing two tandem repeats of the zinc finger motif of TIS11. Site-directed mutations of Tyr105/Tyr113, Gly109/Gly 114, and Phe119 in the first zinc finger motif diminished the ability of this TIS11 domain to direct SG localization. Importantly, when the zinc-chelating Cys residues in either the first or second zinc finger were mutated to Ala residues, the recruitment of the TIS11 zinc finger region to SG was significantly inhibited by the mutation and was completely abolished by the mutation in both zinc fingers. These results suggest that recruitment of TIS11 to heat shock-induced SG is governed by the tandem zinc finger domains of this protein.

Chorioallantoic fusion defects and embryonic lethality resulting from disruption of Zfp36L1, a gene encoding a CCCH tandem zinc finger protein of the Tristetraprolin family

The mouse gene Zfp36L1 encodes zinc finger protein 36-like 1 (Zfp36L1), a member of the tristetraprolin (TTP) family of tandem CCCH finger proteins. TTP can bind to AU-rich elements within the 3'-untranslated regions of the mRNAs encoding tumor necrosis factor (TNF) and granulocyte-macrophage colony-stimulating factor (GM-CSF), leading to accelerated mRNA degradation. TTP knockout mice exhibit an inflammatory phenotype that is largely due to increased TNF secretion. Zfp36L1 has activities similar to those of TTP in cellular RNA destabilization assays and in cell-free RNA binding and deadenylation assays, suggesting that it may play roles similar to those of TTP in mammalian physiology. To address this question we disrupted Zfp36L1 in mice. All knockout embryos died in utero, most by approximately embryonic day 11 (E11). Failure of chorioallantoic fusion occurred in about two-thirds of cases. Even when fusion occurred, by E10.5 the affected placentas exhibited decreased cell division and relative atrophy of the trophoblast layers. Although knockout embryos exhibited neural tube abnormalities and increased apoptosis within the neural tube and also generalized runting, these and other findings may have been due to deficient placental function. Embryonic expression of Zfp36L1 at E8.0 was greatest in the allantois, consistent with a potential role in chorioallantoic fusion. Fibroblasts derived from knockout embryos had apparently normal levels of fully polyadenylated compared to deadenylated GM-CSF mRNA and normal rates of turnover of this mRNA species, both sensitive markers of TTP deficiency in cells. We postulate that lack of Zfp36L1 expression during mid-gestation results in the abnormal stabilization of one or more mRNAs whose encoded proteins lead directly or indirectly to abnormal placentation and fetal death.

The CD40-CD154 interaction in B cell-T cell liaisons

The CD40 receptor is expressed constitutively on B lymphocytes, for which it provides important signals regulating clonal expansion, antibody production and isotype switching, as well as the development of humoral memory. The major source of CD154, the ligand for CD40, is activated T lymphocytes. Interactions between CD40 and CD154 provide a number of signals that play important roles in regulating the complex and multifactorial interactions between these two major cell types of the adaptive immune response. Understanding both the biological effects of this receptor-ligand interaction, as well as how CD40 signaling pathways are controlled, adds to our detailed picture of the complex interplay between B and T cells.

Polymorphisms in the genes encoding members of the tristetraprolin family of human tandem CCCH zinc finger proteins

The three known mammalian CCCH tandem zinc finger proteins of the tristetraprolin (TTP) class have recently been demonstrated to be mRNA-binding proteins. The prototype, TTP, functions in normal physiology to promote the instability of the tumor necrosis factor alpha (TNFalpha) and granulocyte-macrophage colony-stimulating factor mRNAs. Conversely, these mRNAs are stabilized in TTP-deficient mice, leading to an inflammatory phenotype characterized by overproduction of these cytokines. To explore sequence variations in TTP and its two related proteins, we sequenced genomic DNA encoding the TTP protein (ZFP36) and those of its two known mammalian relatives, ZFP36L1 and ZFP36L2, from 72 to 92 anonymous human subjects from various geographical and ethnic backgrounds. We also sequenced ZFP36 in genomic DNA from 92 subjects exhibiting evidence of excessive TNFalpha action. The resequencing strategy identified 13 polymorphisms in the protein-coding regions of these three genes, of which six would result in amino acid changes; other putative polymorphisms were identified by EST searches. One mutation in ZFP36L1 was a dinucleotide substitution that would prevent splicing of the single intron. This mutation was identified in only one allele of the original 144 sequenced from an adult female Aka Pygmy from the Central African Republic; a second individual with the same variant allele was found by genotyping 58 additional Aka DNA samples. Analysis of mRNA from one of these subject's lymphoblasts confirmed that ZFP36L1 mRNA levels were approximately 50% of those in a comparable sample without the mutation. The functional significance of this and the other polymorphisms identified remains to be determined by both biochemical and population linkage studies.

Functional cloning of BRF1, a regulator of ARE-dependent mRNA turnover

To identify regulators of AU-rich element (ARE)-dependent mRNA turnover we have followed a genetic approach using a mutagenized cell line (slowC) that fails to degrade cytokine mRNA. Accordingly, a GFP reporter construct whose mRNA is under control of the ARE from interleukin-3 gives an increased fluorescence signal in slowC. Here we describe rescue of slowC by a retroviral cDNA library. Flow cytometry allowed us to isolate revertants with reconstituted rapid mRNA decay. The cDNA was identified as butyrate response factor-1 (BRF1), encoding a zinc finger protein homologous to tristetraprolin. Mutant slowC carries frame-shift mutations in both BRF1 alleles, whereas slowB with intermediate decay kinetics is heterozygous. By use of small interfering (si)RNA, independent evidence for an active role of BRF1 in mRNA degradation was obtained. In transiently transfected NIH 3T3 cells, BRF1 accelerated mRNA decay and antagonized the stabilizing effect of PI3-kinase, while mutation of the zinc fingers abolished both function and ARE-binding activity. This approach, which identified BRF1 as an essential regulator of ARE-dependent mRNA decay, should also be applicable to other cis-elements of mRNA turnover.

Identification of nuclear import and export signals within the structure of the zinc finger protein TIS11

TIS11, a member of the CCCH zinc finger protein family, functions as a positive transcriptional regulator. TIS11 was localized in both the cytoplasm and nucleus when transiently expressed in COS-7 cells. Upon treatment with leptomycin B, a specific inhibitor of the nuclear export receptor CRM1, a marked nuclear accumulation of TIS11 was observed, indicating that TIS11 shuttles between the nucleus and the cytoplasm. By deletion studies using a green fluorescent protein fusion system, we have mapped a functional nuclear localization signal (NLS) to a region containing two tandem repeats of the zinc finger motif of TIS11. A site-directed mutagenesis analysis of TIS11 NLS has revealed the critical importance of two arginine residues (Arg127 and Arg131 in the rat TIS11). Furthermore, we demonstrated that the N-terminal Leu-rich region of TIS11 serves as an LMB-sensitive nuclear export signal (NES), indicating that TIS11 follows a CRM1-mediated export pathway. These results suggest that TIS11 is subject to constant nucleocytoplasmic shuttling due to its NLS and NES.

Members of the tristetraprolin family of tandem CCCH zinc finger proteins exhibit CRM1-dependent nucleocytoplasmic shuttling

Members of the tristetraprolin (TTP) family of CCCH tandem zinc finger proteins can bind directly to certain types of AU-rich elements (AREs) in mRNA. Experiments in TTP-deficient mice have shown that TTP is involved in the physiological destabilization of at least two cytokine mRNAs, those encoding tumor necrosis factor alpha and granulocyte-macrophage colony-stimulating factor. The two other known mammalian members of the TTP family, CMG1 and TIS11D, also contain ARE-binding CCCH tandem zinc finger domains and can also destabilize ARE-containing mRNAs. To investigate the effects of primary sequence on the subcellular localization of these proteins, we constructed green fluorescent protein fusions with TTP, CMG1, and TIS11D; these were predominantly cytoplasmic when expressed in 293 or HeLa cells. Deletion and mutation analyses revealed functional nuclear export signals in the amino terminus of TTP and in the carboxyl termini of CMG1 and TIS11D. This type of leucine-rich nuclear export signal interacts with the nuclear export receptor CRM1; abrogation of CRM1 activity resulted in nuclear accumulation of TTP, CMG1, and TIS11D. These proteins are thus nucleocytoplasmic shuttling proteins and rely on CRM1 for their export from the nucleus. Although TTP, CMG1, and TIS11D lack known nuclear import sequences, mapping experiments revealed that their nuclear accumulation required an intact tandem zinc finger domain but did not require RNA binding ability. These findings suggest possible roles for nuclear import and export in the regulation of cellular TTP, CMG1, and TIS11D activity.

A factor of inducing IgE from a filarial parasite is an agonist of human CD40

Immune responses to parasitic helminth are usually characterized by quite mysterious phenomena: dominance of Th2-like immunity and antigen-nonspecific IgE secretion. We previously purified a factor from Dirofilaria immitis that induces antigen-nonspecific IgE in rats and named it DiAg. In the presence of IL-4, DiAg induces mouse B cells to secrete IgE, which is antigen-nonspecific polyclonal antibody. We investigated the biochemical characteristics of DiAg as a factor of inducing IgE in this study. Recombinant DiAg (rDiAg) with interleukin (IL)-4 induced IgE synthesis in highly purified human normal B cells in vitro cell culture systems. The addition of recombinant human soluble CD40 IgG fusion protein (rsCD40-Ig) inhibited induction of IgE synthesis by rDiAg with IL-4. Monocyte cells were stimulated with rDiAg and recombinant human soluble CD40L (rsCD40L); IL-12 and TNF-alpha were induced. The addition of rsCD40-Ig to THP-1 cells activated with rDiAg and rsCD40L inhibited the production of IL-12. rDiAg bound to the monocyte cell membrane fraction and recombinant human soluble CD40; this binding of rDiAg was competitively inhibited by addition of rsCD40L. Moreover, in CD40-deficient mice, IgE production and MLN-B cell proliferation by rDiAg were completely absent. Based on these results, we concluded that DiAg is an agonist of CD40.

Transcriptional activation function of zinc finger protein TIS11 and its negative regulation by phorbol ester

TIS11, a CCCH zinc finger protein, is one of the typical growth factor-inducible nuclear proteins. We found that TIS11 possesses the potential to activate transcription when fused to the GAL4 DNA binding domain and transiently cotransfected into rat pheochromocytoma PC12 cells along with a GAL4-responsive luciferase reporter gene. The study with deletion mutants of TIS11 revealed that the major transactivation region is located at the N-terminal 101 amino acid residues and that the remaining central and C-terminal region had a moderate transactivational activity. In addition, the transactivational activity of TIS11 was found to be significantly reduced by treating the transfectants with phorbol 12-myristate 13-acetate (PMA). PMA-induced inactivation of TIS11 was blocked by calphostin C, a protein kinase C inhibitor, and PD98059, a mitogen-activated protein (MAP) kinase kinase inhibitor. These results suggested that TIS11 functions as a positive transcriptional regulator and that the protein kinase C/MAP kinase signaling cascade is involved in negative regulation of TIS11 by PMA.

Cytokine-mediated protection of human dendritic cells from prostate cancer-induced apoptosis is regulated by the Bcl-2 family of proteins

Prostate cancer is the most common cancer in men in the United States, and second in cancer-induced mortality. It is likely that tumour-induced immunosuppression is one of the reasons for low treatment efficacy in patients with advanced prostate cancer. It has been recently demonstrated that prostate cancer tissue is almost devoid of dendritic cells (DC), the major antigen-presenting cells responsible for the induction of specific antitumour immune responses. In this study, we have tested the hypothesis that prostate cancer induces progressive suppression of the DC system. We found that co-incubation of human DC with three prostate cancer cell lines led to the high levels of premature apoptosis of DC, which were significantly higher than in DC cultures co-incubated with normal prostate cells or blood leucocytes. Stimulation of DC for 24 hours with CD40 ligand (CD154), IL-12 or IL-15 prior to their co-incubation with prostate cancer cells resulted in a significant increase in DC survival in the tumour microenvironment. Furthermore, activation of DC with these cytokines was also accompanied by increased expression of the anti-apoptotic protein Bcl-x(L) in DC, suggesting a possible mechanism involved in DC protection from apoptotic death. In summary, our data demonstrate that prostate cancer induces active elimination of DC in the tumour microenvironment. Stimulation of DC by CD154, IL-12 or IL-15 leads to an increased expression of the anti-apoptotic protein Bcl-x(L) and increased resistance of DC to prostate cancer-induced apoptosis. These results suggest a new mechanism of tumour escape from immune recognition and demonstrate the cytokine-based approaches which might significantly increase the efficacy of DC-based therapies for cancer.

Interactions of CCCH zinc finger proteins with mRNA. Binding of tristetraprolin-related zinc finger proteins to Au-rich elements and destabilization of mRNA

Macrophages derived from tristetraprolin (TTP)-deficient mice exhibited increased tumor necrosis factor alpha (TNFalpha) release as a consequence of increased stability of TNFalpha mRNA. TTP was then shown to destabilize TNFalpha mRNA after binding directly to the AU-rich region (ARE) of the 3'-untranslated region of the TNFalpha mRNA. In mammals and in Xenopus, TTP is the prototype of a small family of three known zinc finger proteins containing two CCCH zinc fingers spaced 18 amino acids apart; a fourth more distantly related family member has been identified in Xenopus and fish. We show here that representatives of all four family members were able to bind to the TNFalpha ARE in a cell-free system and, in most cases, promote the breakdown of TNFalpha mRNA in intact cells. Because the primary sequences of these CCCH proteins are most closely related in their tandem zinc finger domains, we tested whether various fragments of TTP that contained both zinc fingers resembled the intact protein in these assays. We found that amino- and carboxyl-terminal truncated forms of TTP, as well as a 77 amino acid fragment that contained both zinc fingers, could bind to the TNFalpha ARE in cell-free cross-linking and gel shift assays. In addition, these truncated forms of TTP could also stimulate the apparent deadenylation and/or breakdown of TNFalpha mRNA in intact cells. Alignments of the tandem zinc finger domains from all four groups of homologous proteins have identified invariant residues as well as group-specific signature amino acids that presumably contribute to ARE binding and protein-specific activities, respectively.

Lineage-specific modulation of interleukin 4 signaling by interferon regulatory factor 4

Interleukin (IL)-4 is an immunoregulatory cytokine that exerts distinct biological activities on different cell types. Our studies indicate that interferon regulatory factor (IRF)-4 is both a target and a modulator of the IL-4 signaling cascade. IRF-4 expression is strongly upregulated upon costimulation of B cells with CD40 and IL-4. Furthermore, we find that IRF-4 can interact with signal transducer and activator of transcription (Stat)6 and drive the expression of IL-4-inducible genes. The transactivating ability of IRF-4 is blocked by the repressor factor BCL-6. Since expression of IRF-4 is mostly confined to lymphoid cells, these data provide a potential mechanism by which IL-4-inducible genes can be regulated in a lineage-specific manner.

Inhibition of B Cell Receptor-Mediated Apoptosis by IFN

IFNs are a family of cytokines that are involved in the regulation of immune and inflammatory responses. Clinical use of IFN-α/β encompasses treatment for a variety of diseases; however, prolonged exposure to IFN-α/β results in elevated levels of autoreactive Abs. In this study, we investigated the potential of IFNs to modulate apoptotic signals in B cells. We demonstrate that IFN-α or IFN-β inhibit Ag receptor-mediated apoptosis in a dose-dependent manner. Inhibition of phosphatidylinositol 3′ (PI3)-kinase did not abolish the effect of IFN, indicating that the antiapoptotic mechanism is PI3-kinase- and protein kinase B/Akt-independent. Instead, IFN-α and IFN-β, but not IFN-γ, significantly increase the levels of the survival protein Bcl-2, and to a lesser extent, Bcl-xL expression. Thus, IFN-α/β-mediated inhibition of B cell Ag receptor-triggered apoptosis may offer a model for the process that leads to the escape of self-reactive B cells from negative selection and consequently results in autoantibody production.

Establishment and characterization of a novel ALL-L3 cell line (BALM-18): induction of apoptosis by anti-IgM and inhibition of apoptosis by bone marrow stroma cells

A human acute lymphoblastic leukemia (ALL) cell line, BALM-18, was established from the peripheral blood specimen of a patient with B cell ALL L3 type (ALL-L3) at diagnosis using bone marrow stroma cells (BST) as feeder cells. The primary leukemia cells did not grow without feeder cells. As with the primary leukemia cells, BALM-18 showed an immunophenotype of Burkitt's lymphoma group I [CD10+, CD20+, CD23-, CD39-, CD77+] and carried the t(8;14)(q24;q32) chromosomal abnormality which is highly associated with ALL-L3 and Burkitt's lymphoma. It also revealed a significantly low level of bcl-2 protein. Strikingly, anti-human IgM antibody did induce apoptosis in induction experiments. However, it was reversed by the addition of anti-CD40 antibody or BST cells, whereas the culture supernatant of the stroma cells did not show any effect on the inhibition of apoptosis. BALM-18 may be useful for analyzing both the mechanisms of anti-IgM induced apoptosis and signaling during the inhibition of apoptosis by CD40 or BST cells.

Identification of four CCCH zinc finger proteins in Xenopus, including a novel vertebrate protein with four zinc fingers and severely restricted expression

Tristetraprolin (TTP), the prototype of a class of CCCH zinc finger proteins, is a phosphoprotein that is rapidly and transiently induced by growth factors and serum in fibroblasts. Recent evidence suggests that a physiological function of TTP is to inhibit tumor necrosis factor alpha secretion from macrophages by binding to and destabilizing its mRNA (Carballo, E., Lai, W.S., Blackshear, P.J., 1998. Science, 281, 1001-1005). To investigate possible functions of CCCH proteins in early development of Xenopus, we isolated four Xenopus cDNAs encoding members of this class. Based on 49% overall amino acid identity and 84% amino acid identity within the double zinc finger domain, one of the Xenopus proteins (XC3H-1) appears to be the homologue of TTP. By similar analyses, XC3H-2 and XC3H-3 are homologues of ERF-1 (cMG1, TIS11B) and ERF-2 (TIS11D). A fourth protein, XC3H-4, is a previously unidentified member of the CCCH class of vertebrate zinc finger proteins; it contains four Cx8Cx5Cx3H repeats, two of which are YKTEL Cx8Cx5Cx3H repeats that are closely related to sequences found in the other CCCH proteins. Whereas XC3H-1, XC3H-2, and XC3H-3 were widely expressed in adult tissues, XC3H-4 mRNA was not detected in any of the adult tissues studied except for the ovary. Its expression appeared to be limited to the ovary, oocyte, egg and the early embryonic stages leading up to the mid-blastula transition. Its mRNA was highly expressed in oocytes of all ages, and was enriched in the animal pole cytosol of mature oocytes. Maternal expression was also seen with the other three messages, suggesting the possibility that these proteins are involved in regulating mRNA stability in oocyte maturation and/or early embryogenesis.

Ectopic expression of Bcl-2, but not Bcl-xL rescues Ramos B cells from Fas-mediated apoptosis

The human Burkitt lymphoma Ramos B cell line can be induced to undergo apoptosis in response to a variety of different agents, including calcium ionophores, anti-immunoglobulin (Ig) and macromolecular synthesis inhibitors. In addition, following up-regulation of the Fas (CD95) surface receptor by CD40 ligation, these cells also become susceptible to apoptosis induction by Fas ligation. We have previously shown that protection from calcium ionophore- and macromolecular synthesis inhibitor-induced apoptosis by CD40 ligation is associated with a rapid up-regulation of Bcl-xL followed by a more moderate and delayed up-regulation of Bcl-2. We show here that overexpression of Bcl-xL, like Bcl-2, protects Ramos cells from apoptosis induction in response to calcium ionophore, anti-Ig and macromolecular synthesis inhibition. However, in contrast to Bcl-2, ectopic overexpression of Bcl-xL does not rescue from Fas-mediated apoptosis. Thus, in Ramos B cells, the Fas apoptotic pathway exhibits differential sensitivity to inhibition by Bcl-2 family members. These findings also suggest that CD40 signaling provides a switch which renders the cells susceptible to Fas-ligand mediated apoptosis through up-regulation of Fas whilst affording protection from anti-Ig-induced apoptosis through up-regulation of Bcl-xL.