BTG1, a member of a new family of antiproliferative genes

Global gene expression analysis identifies molecular pathways distinguishing blastocyst dormancy and activation

Delayed implantation (embryonic diapause) occurs when the embryo at the blastocyst stage achieves a state of suspended animation. During this period, blastocyst growth is very slow, with minimal or no cell division. Nearly 100 mammals in seven different orders undergo delayed implantation, but the underlying molecular mechanisms that direct this process remain largely unknown. In mice, ovariectomy before preimplantation ovarian estrogen secretion on day 4 of pregnancy initiates blastocyst dormancy, which normally lasts for 1-2 weeks by continued progesterone treatment, although blastocyst survival decreases with time. An estrogen injection rapidly activates blastocysts and initiates their implantation in the progesterone-primed uterus. Using this model, here we show that among approximately 20,000 genes examined, only 229 are differentially expressed between dormant and activated blastocysts. The major functional categories of altered genes include the cell cycle, cell signaling, and energy metabolic pathways, particularly highlighting the importance of heparin-binding epidermal growth factor-like signaling in blastocyst-uterine crosstalk in implantation. The results provide evidence that the two different physiological states of the blastocyst, dormancy and activation, are molecularly distinguishable in a global perspective and underscore the importance of specific molecular pathways in these processes. This study has identified candidate genes that provide a scope for in-depth analysis of their functions and an opportunity for examining their relevance to blastocyst dormancy and activation in numerous other species for which microarray analysis is not available or possible due to very limited availability of blastocysts.

Detailed analysis of gene expression during development of T cell lineages in the thymus

The genetic mechanisms that promote lineage commitment and eliminate autoreactive cells in the thymus are not well understood. To better understand this process, we have identified and quantitated transcripts in the two major thymocyte lineages by using serial analysis of gene expression. Approximately 25 genes displayed almost complete segregation to one or the other T cell lineage. Commitment to the CD4 lineage was marked by up-regulation of genes associated with increased survival and chaperone function followed by expression of genes that regulate nucleosome remodeling and T cell receptor signaling. Differentiation within the CD8 lineage, on the other hand, was marked by up-regulation of genes that regulate lymphocyte homing, followed by quenching of genes that inhibit apoptosis. Definition of differential gene expression during development of the two major thymocyte lineages will allow insight into mechanisms of T cell development after positive and negative selection.

Genomic profiling of peripheral T-cell lymphoma, unspecified, and anaplastic large T-cell lymphoma delineates novel recurrent chromosomal alterations

To characterize genetic alterations in peripheral T-cell lymphoma, not otherwise specified (PTCL NOS), and anaplastic large T-cell lymphoma (ALCL), 42 PTCL NOS and 37 ALCL [17 anaplastic large cell kinase (ALK)-negative ALCL, 9 ALK-positive ALCL, 11 cutaneous ALCL] were analyzed by comparative genomic hybridization. Among 36 de novo PTCL NOS, recurrent chromosomal losses were found on chromosomes 13q (minimally overlapping region 13q21, 36% of cases), 6q and 9p (6q21 and 9p21-pter, in 31% of cases each), 10q and 12q (10q23-24 and 12q21-q22, in 28% of cases each), and 5q (5q21, 25% of cases). Recurrent gains were found on chromosome 7q22-qter (31% of cases). In 11 PTCL NOS, high-level amplifications were observed, among them 3 cases with amplification of 12p13 that was restricted to cytotoxic PTCL NOS. Whereas cutaneous ALCL and ALK-positive ALCL showed few recurrent chromosomal imbalances, ALK-negative ALCL displayed recurrent chromosomal gains of 1q (1q41-qter, 46%), and losses of 6q (6q21, 31%) and 13q (13q21-q22, 23%). Losses of chromosomes 5q, 10q, and 12q characterized a group of noncytotoxic nodal CD5+ peripheral T-cell lymphomas. The genetics of PTCL NOS and ALK-negative ALCL differ from other T-NHLs characterized genetically so far, among them enteropathy-type T-cell lymphoma, T-cell prolymphocytic leukemia, and adult T-cell lymphoma/leukemia.

Regulation of subcellular localization of the antiproliferative protein Tob by its nuclear export signal and bipartite nuclear localization signal sequences

Tob, a member of the Tob and BTG antiproliferative protein family, plays an important role in many cellular processes including cell proliferation. In this study, we have addressed molecular mechanisms regulating subcellular localization of Tob. Treatment with leptomycin B, an inhibitor of nuclear export signal (NES) receptor, resulted in a change in subcellular distribution of Tob from its pan-cellular distribution to nuclear accumulation, indicating the existence of NES in Tob. Our results have then identified an N-terminal region (residues 2-14) of Tob as a functional NES. They have also shown that Tob has a functional, bipartite nuclear localization signal (NLS) in residues 18-40. Thus, Tob is shuttling between the nucleus and the cytoplasm by its NES and NLS. To examine a possible relationship between subcellular distribution of Tob and its function, we exogenously added a strong NLS sequence or a strong NES sequence or both to Tob. The obtained results have demonstrated that the strong NLS-added Tob has a much weaker activity to inhibit cell cycle progression from G0/G1 to S phase. These results suggest that cytoplasmic localization or nucleocytoplasmic shuttling is important for the antiproliferative function of Tob.

An anti-proliferative gene BTG1 regulates angiogenesis in vitro

B-cell translocation gene 1 (BTG1) is a member of the anti-proliferative gene family that regulates cell growth and differentiation. To clarify the role of BTG1 in angiogenesis, we examined the regulation of BTG1 expression in cultured endothelial cells and characterized its function in in vitro models of angiogenesis. BTG1 mRNA was abundantly expressed in quiescent endothelial cells. Addition of serum and angiogenic growth factors decreased BTG1 mRNA levels in endothelial cells. In contrast, BTG1 mRNA was up-regulated in tube-forming endothelial cells on Matrigel. This up-regulation was partially blocked by neutralizing antibody against transforming growth factor-beta (TGF-beta), and TGF-beta increased BTG1 mRNA levels. Inhibition of endogenous BTG1 by overexpression of antisense BTG1 resulted in inhibited network formation, and overexpression of sense BTG1 augmented tube formation in these cell lines. BTG1-overexpressing endothelial cells displayed increased cell migration. These findings suggest that BTG1 may play an important role in the process of angiogenesis.

Tob deficiency superenhances osteoblastic activity after ovariectomy to block estrogen deficiency-induced osteoporosis

Tob (transducer of erbB2) is a member of antiproliferative family proteins and acts as a bone morphogenic protein inhibitor as well as a suppressor of proliferation in T cells, which have been implicated in postmenopausal bone loss. To determine the effect of Tob deficiency on estrogen deficiency-induced bone loss, we analyzed bone metabolism after ovariectomy or sham operation in Tob-deficient mice. Ovariectomy in WT mice decreased trabecular bone volume and bone mineral density (BMD) as expected. In Tob-deficient mice, ovariectomy reduced bone volume and BMD. However, even after ovariectomy, both trabecular bone volume and BMD levels in Tob-deficient bone were comparable to those in sham-operated WT bones. Bone formation parameters (mineral apposition rate and bone formation rate) in the ovariectomized Tob-deficient mice were significantly higher than those in the ovariectomized WT mice. In contrast, the ovariectomy-induced increase in the bone resorption parameters, osteoclast surface, and osteoclast number was similar between Tob-deficient mice and WT mice. Furthermore, in ex vivo nodule formation assay, ovariectomy-induced enhancement of nodule formation was significantly higher in the bone marrow cells from Tob-deficient mice than in the bone marrow cells from ovariectomized WT mice. Both Tob and estrogen signalings converge at bone morphogenic protein activation of alkaline phosphatase and GCCG-reporter gene expression in osteoblasts, revealing interaction between the two signals. These data indicate that Tob deficiency prevents ovariectomy-induced bone loss through the superenhancement of osteoblastic activities in bone and that this results in further augmentation in the bone formation rate and the mineral apposition rate after ovariectomy in vivo.

Role of antiproliferative B cell translocation gene-1 as an apoptotic sensitizer in activation-induced cell death of brain microglia

Mouse brain microglial cells undergo apoptosis on exposure to inflammatory stimuli, which is considered as an autoregulatory mechanism to control their own activation. Here, we present evidence that an antiproliferative B cell translocation gene 1 (BTG1) constitutes a novel apoptotic pathway of LPS/IFN-gamma-activated microglia. The expression of BTG1 was synergistically enhanced by LPS and IFN-gamma in BV-2 mouse microglial cells as well as in primary microglia cultures. Levels of BTG1 expression inversely correlated with a proliferative capacity of the microglial cells. Tetracycline-based conditional expression of BTG1 not only suppressed microglial proliferation but also increased the sensitivity of microglial cells to NO-induced apoptosis, suggesting a novel mechanism of cooperation between LPS and IFN-gamma in the induction of microglial apoptosis. An increase in BTG1 expression, however, did not affect microglial production of NO, TNF-alpha, or IL-1beta, indicating that the antiproliferative BTG1 is important in the activation-induced apoptosis of microglia, but not in the activation itself. The synergistic action of LPS and IFN-gamma in the microglial BTG1 induction and apoptosis was dependent on the Janus kinase/STAT1 pathway, but not IFN-regulatory factor-1, as demonstrated by a pharmacological inhibitor of Janus kinase (AG490), STAT1 dominant negative mutant, and IFN-regulatory factor-1-deficient mice. Taken together, antiproliferative BTG1 may participate in the activation-induced cell death of microglia by lowering the threshold for apoptosis; BTG1 increases the sensitivity of microglia to apoptogenic action of autocrine cytotoxic mediator, NO. Our results point out an important link between the proliferative state of microglia and their sensitivity to apoptogenic agents.

Alteration of expression or phosphorylation status of tob, a novel tumor suppressor gene product, is an early event in lung cancer

Tob is a member of the Tob/BTG family, a novel class of anti-proliferative proteins. To investigate the involvement of tob as a tumor suppressor gene in human lung cancer, we analyzed the expression of tob mRNA and protein in lung cancer tissue and adjacent normal lung tissue. Immunohistochemical analysis using anti-Tob antibody showed decreased expression of Tob in 72% (31/43) of lung cancer tissues. Furthermore, 95% (19/20) of squamous cell carcinoma patients showed an apparent decrease in Tob in cancer tissues, associated with smoking status. The phosphorylated form of Tob, an inactive form of Tob, was detected in 76% (16/21) of cancer tissues of adenocarcinoma patients, but not in normal alveolar epithelial cells. Either a decrease in Tob expression or an accumulation of phosphorylated Tob was observed from early clinical stages, even in bronchial dysplasia, a premalignant lesion of squamous cell carcinoma. The above findings suggest that the disruption of anti-proliferative Tob plays a distinct part in the early stage of lung carcinogenesis.

Cooperative signaling between cytokine receptors and the glucocorticoid receptor in the expansion of erythroid progenitors: molecular analysis by expression profiling

Erythroid progenitors undergo renewal (proliferation without apparent differentiation) in response to erythropoietin (Epo), stem cell factor (SCF), and glucocorticoids (dexamethasone) (Dex). SCF and Dex cooperate with Epo to promote proliferation and inhibit differentiation of erythroid progenitors, while Epo alone is required to protect erythroid cells from apoptosis during terminal red cell maturation. To examine the mechanism of the synergistic interactions of Epo, SCF, and Dex, we analyzed gene expression patterns using DNA chip-based large-scale comparative gene profiling using microarrays enriched in hematopoietic transcripts or containing randomly selected genes. Differentially regulated genes were validated by real-time reverse transcription-polymerase chain reaction (RT-PCR). The results reveal cooperative regulation of gene expression by glucocorticoids and Epo/SCF on a number of genes, such as CIS, BTG1, VDUP1, CXCR4, GILZ, and RIKEN29300106B05. While Epo and SCF never showed opposite effects on gene expression, Dex either enhanced or attenuated the effect of Epo and/or SCF. Several glucocorticoid receptor (GR)-target genes were regulated by Dex only in the presence of Epo and/or SCF, suggesting that the GR functions in the context of a larger transactivation complex to regulate these genes. The data also suggest that modulation of cytokine-induced signals by the GR is an important mechanism in erythroid progenitor renewal.

Mice lacking a transcriptional corepressor Tob are predisposed to cancer

tob is a member of antiproliferative family genes. Mice lacking tob are prone to spontaneous formation of tumors. The occurrence rate of diethylnitrosamine-induced liver tumors is higher in tob(-/-) mice than in wild-type mice. tob(-/-)p53(-/-) mice show accelerated tumor formation in comparison with single null mice. Expression of cyclin D1 mRNA is increased in the absence of Tob and is reduced by Tob. Tob acts as a transcriptional corepressor and suppresses the cyclin D1 promoter activity through an interaction with histone deacetylase. Levels of tob mRNA are often decreased in human cancers, implicating tob in cancer development.

Gene array and protein expression profiles suggest post-transcriptional regulation during CD8+ T cell differentiation

Peripheral CD8(+) T cells circulate in a quiescent naive state until they are primed by specific antigen and differentiate into effector cells. In the effector state, CD8(+) T cells acquire cytolytic activity and produce increased levels of cytokines such as interferon-gamma. They also exhibit increased T cell receptor sensitivity, decreased CD28 dependence, and become inhibitable by CTLA-4 and other negative regulatory pathways. We hypothesized that one mechanism by which these two states are regulated is via differential expression of specific genes. To this end, basal gene expression profiles of naive and effector 2C TCR transgenic x RAG2(-/-) CD8(+) T cells were analyzed using Affymetrix arrays representing 11,000 genes. Of the 177 differentially expressed known genes, 68 were expressed at higher levels in effector cells, but 109 were more abundant in naive cells, supporting the notion that the naive state is not passive. Expression of genes related to metabolism, actin cytoskeletal dynamics, and effector function increased with priming, whereas expression of putative anti-proliferative genes decreased. Semiquantitative reverse transcription-PCR was utilized as a secondary validation for selected transcripts, and Western blot analysis was used to examine protein expression for molecules of interest. Surprisingly, for 24 genes examined, 12 showed discordant protein versus mRNA expression. In summary, our study indicates that: 1) not only does the expression of some genes in naive CD8(+) T cells become up-regulated upon priming, but the expression of other genes is down-regulated as well and 2) the complexities of T cell differentiation include regulation at the post-transcriptional level.

Identification of cJun-responsive genes in Rat-1a cells using multiple techniques: increased expression of stathmin is necessary for cJun-mediated anchorage-independent growth

cJun is a major component of the transcription factor AP-1 and mediates a diverse set of biologic properties including proliferation, differentiation, and apoptosis. To identify cJun-responsive genes, we inducibly expressed cJun in Rat-1a cells and observed two distinct phenotypes: changes in cellular morphology with adherent growth and anchorage-independent growth. The biologic effects of cJun were entirely reversible demonstrating that they require the continued presence of cJun. To determine the genes, which mediate the biologic effects of cJun, we employed multiple methods including differential gene analysis, suppression subtractive hybridization, and cDNA microarrays. We identified 38 cJun-responsive genes including three uncharacterized genes under adherent and/or nonadherent conditions. Half of the known 36 genes were cytoskeleton- and adhesion-related genes, suggesting a major role of cJun in the regulation of the genes related to cell morphology. As proof of the principle that this approach could identify genes whose upregulation was necessary for nonadherent growth, we investigated one gene, stathmin whose upregulation by cJun was observed only under these conditions. Although overexpression of stathmin did not result in nonadherent growth, inhibition of stathmin protein expression by antisense oligonucleotides in cJun-induced Rat-1a cells prevented nonadherent growth. These results suggest that stathmin plays an essential role in anchorage-independent growth by cJun and may be a potential target for specific inhibitors for AP-1-dependent processes involved in carcinogenesis.

Tob proteins enhance inhibitory Smad-receptor interactions to repress BMP signaling

Tob inhibits bone morphogenetic protein (BMP) signaling by interacting with receptor-regulated Smads in osteoblasts. Here we provide evidence that Tob also interacts with the inhibitory Smads 6 and 7. A yeast two-hybrid screen identified Smad6 as a protein interacting with Tob. Tob co-localizes with Smad6 at the plasma membrane and enhances the interaction between Smad6 and activated BMP type I receptors. Furthermore, we have isolated Xenopus Tob2, and show that it cooperates with Smad6 in inducing secondary axes when expressed in early Xenopus embryos. Finally, Tob and Tob2 cooperate with Smad6 to inhibit endogenous BMP signaling in Xenopus embryonic explants and in cultured mammalian cells. Our results provide both in vitro and in vivo evidence that Tob inhibits endogenous BMP signaling by facilitating inhibitory Smad functions.

Identification of the Anti-proliferative protein Tob as a MAPK substrate

Mitogen-activated protein kinases (MAPKs) regulate a wide variety of cellular functions by phosphorylating their specific substrates. Here we have identified Tob as a novel substrate of MAPK. Tob, a member of the Tob and B-cell translocation gene anti-proliferative protein family, is shown to negatively regulate the proliferation of osteoblasts and T cells. In this study, our two-hybrid screening has identified Tob as an ERK2-interacting protein. Biochemical analyses have then shown that ERK MAPK (ERK2) and JNK/SAPK (JNK2) bind to and phosphorylate Tob in vitro. ERK catalyzes the phosphorylation more efficiently than JNK. When the ERK pathway is activated in cells, phosphorylation of Tob is induced. An ERK-binding or -docking site locates in the N-terminal portion of Tob, and phosphorylation sites reside in the C-terminal stretch region. The docking is crucial for efficient phosphorylation. Mutant forms of Tob, in which serines are replaced by glutamic acids to mimic phosphorylation, show a much reduced ability to inhibit the cell cycle progression to S phase from G(0)/G(1) phase, as compared with wild-type Tob, indicating that ERK phosphorylation negatively regulates the anti-proliferative function of Tob.

BTG2(TIS21/PC3) induces neuronal differentiation and prevents apoptosis of terminally differentiated PC12 cells

The p53-transcriptional target, BTG2(TIS21/PC3), was previously identified as an antiproliferative gene. However, the precise biological functions of the protein product remain to be elucidated. BTG2(TIS21/PC3) expression is induced in vivo during neurogenesis, and the gene is transiently expressed in vitro in rat pheochromocytoma PC12 cells after induction of neuronal differentiation by addition of nerve growth factor (NGF). These observations suggest that BTG2(TIS21/PC3) is functionally significant during the neuronal differentiation process. To test this hypothesis, a vector that expressed BTG2(TIS21/PC3) under the control of an inducible promoter was introduced into PC12 cells. Growth arrest and differentiation in response to NGF were greatly enhanced by BTG2(TIS21/PC3) overexpression. Furthermore, an antisense oligonucleotide complementary to BTG2(TIS21/PC3) mRNA, which was able to inhibit endogenous BTG2(TIS21/PC3) expression, triggered programmed cell death in differentiated PC12 cells. These observations confirm that BTG2(TIS21/PC3) expression promotes neuronal differentiation and that it is required for survival of terminally differentiated cells.

Antiproliferative proteins of the BTG/Tob family are degraded by the ubiquitin-proteasome system

BTG/Tob family proteins, which are characterized by similarities in their N-terminal BTG/Tob homology domains, control cell growth negatively. Among the BTG/Tob family members, BTG2/TIS21/PC3 proteins have been reported to have short lives and to be degraded by the proteasome. However, the mechanisms regulating the stabilities of other BTG/Tob family proteins have not yet been clarified. Here, we report that BTG1, Tob, and Tob2 proteins, as well as BTG2 protein, are degraded by the ubiquitin-proteasome system; the degradation of Tob protein in HeLa cells and the degradation of BTG1, BTG2, Tob and Tob2 proteins transiently expressed in HEK293 cells were inhibited by treatments with proteasome-specific inhibitors. Co-expression of BTG1, BTG2, Tob, or Tob2 protein with ubiquitin in HEK293 cells revealed specific multiubiquitination of each of the four proteins. Although the full-length and N-terminal truncated forms of BTG1, BTG2, Tob, and Tob2 proteins were unstable, the respective C-terminal truncated forms were found to be almost stable, suggesting that the C-terminal regions control the stabilities of BTG1, BTG2, Tob, and Tob2 proteins. In addition, it was found that the respective C-terminal regions confer instability on green fluorescent protein, a normally stable protein. Thus, it can be concluded that the C-terminal regions are necessary and sufficient to control the stabilities of BTG1, BTG2, Tob, and Tob2 proteins.

Global and specific translational control by rapamycin in T cells uncovered by microarrays and proteomics

Rapamycin has been shown to affect translation. We have utilized two complementary approaches to identify genes that are predominantly affected by rapamycin in Jurkat T cells. One was to compare levels of polysome-bound and total RNA using oligonucleotide microarrays complementary to 6,300 human genes. Another was to determine protein synthesis levels using two-dimensional PAGE. Analysis of expression changes at the polysome-bound RNA levels showed that translation of most of the expressed genes was partially reduced following rapamycin treatment. However, translation of 136 genes (6% of the expressed genes) was totally inhibited. This group included genes encoding RNA-binding proteins and several proteasome subunit members. Translation of a set of 159 genes (7%) was largely unaffected by rapamycin treatment. These genes included transcription factors, kinases, phosphatases, and members of the RAS superfamily. Analysis of [(35)S]methionine-labeled proteins from the same cell populations using two-dimensional PAGE showed that the integrated intensity of 111 of 830 protein spots changed in rapamycin-treated cells by at least 3-fold (70 increased, 41 decreased). We identified 22 affected protein spots representing protein products of 16 genes. The combined microarray and proteomic approach has uncovered novel genes affected by rapamycin that may be involved in its immunosuppressive effect and other genes that are not affected at the level of translation in a context of general inhibition of cap-dependent translation.

Enhancing effect of Tob deficiency on bone formation is specific to bone morphogenetic protein-induced osteogenesis

Tob is a recently reported novel bone morphogenetic protein (BMP) inhibitor, which originally was identified by West-Western procedure using ErbB2 as a probe and contains a nuclear localization signal. To further characterize the effects of Tob deficiency on BMP-induced new bone (NB) formation, we examined microcomputed tomography (microCT) on the cross-section of the bone induced by daily injection with BMP onto the calvariae of newborn mice. The calvariae of the saline-injected Tob-deficient (TD) mice were similar to those of the saline-injected or untreated wild-type (WT) mice. BMP injection locally produced NB on the calvaria in WT mice as known previously. In contrast to WT mice, BMP injection onto the calvariae of TD mice produced a calcified area in the cross-section of NB, which was more than that produced by BMP in the WT calvariae. In addition, the horizontal width and the vertical height of the NB induced by BMP in TD mice were several-fold more than those in WT mice. The effect of Tob deficiency on bone-forming activity was selective to the response to the injection with BMP because the levels of injury-induced NB formation examined by microCT 10 days after bone marrow ablation in the femora were similar between the TD and WT mice. These data indicate that Tob acts as a novel specific antagonist against bone formation induced by BMP treatment in bone.

Phosphorylation of three regulatory serines of Tob by Erk1 and Erk2 is required for Ras-mediated cell proliferation and transformation

tob is a member of an emerging family of genes with antiproliferative function. Tob is rapidly phosphorylated at Ser 152, Ser 154, and Ser 164 by Erk1 and Erk2 upon growth-factor stimulation. Oncogenic Ras-induced transformation and growth-factor-induced cell proliferation are efficiently suppressed by mutant Tob that carries alanines but not glutamates, mimicking phosphoserines, at these sites. Wild-type Tob inhibits cell growth when the three serine residues are not phosphorylated but is less inhibitory when the serines are phosphorylated. Because growth of Rb-deficient cells was not affected by Tob, Tob appears to function upstream of Rb. Intriguingly, cyclin D1 expression is elevated in serum-starved tob(-/-) cells. Reintroduction of wild-type Tob and mutant Tob with serine-to-alanine but not to glutamate mutations on the Erk phosphorylation sites in these cells restores the suppression of cyclin D1 expression. Finally, the S-phase population was significantly increased in serum-starved tob(-/-) cells as compared with that in wild-type cells. Thus, Tob inhibits cell growth by suppressing cyclin D1 expression, which is canceled by Erk1- and Erk2-mediated Tob phosphorylation. We propose that Tob is critically involved in the control of early G(1) progression.

Microarray analysis of altered gene expression in the TM4 Sertoli-like cell line exposed to chromium(III) chloride

Chromium(III) chloride is a common human exposure metal that is a preconceptional carcinogen in mice, although it enters cells poorly, and is non-toxic and non-carcinogenic in most biologic systems. An indirect effect on sperm is postulated, and this effect might be mediated through the testicular Sertoli cells that influence spermatogenesis. To test this possibility, we exposed mouse TM4 Sertoli-like cultured cells to 1mM CrCl(3) x 6H(2)O, a non-toxic dose, for 7 days and then extracted mRNA for microarray analysis. The chromium(III) chloride had modest effects on the expression of many genes, in the range of 1.5-2.3-fold. These effects provided an opportunity for development of statistical approaches for sifting microarray data in a situation where differences were small. Data were winnowed by screening for those ratios that fell outside the 99% confidence limits and/or represented a > or = 50% change in expression in the three comparison pairs. Fifty-two genes/clones were significant after the Bonferroni adjustment for multiple comparisons. The largest average increase was observed for the transcription factor Bach2, and this increase was confirmed by RT-PCR. The results show that Cr(III) has significant effects on gene expression in a Sertoli-like cell line.

Genomic analysis of a mouse model of immunoglobulin A nephropathy reveals an enhanced PDGF-EDG5 cascade

The molecular mechanism of immunoglobulin A nephropathy (IgAN), the most common primary renal glomerular disease worldwide, is unknown. HIGA (high serum IgA) mouse is a valid model of IgAN showing almost all of the pathological features, including mesangial cell proliferation. Here we elucidate a pattern of gene expression associated with IgAN by analyzing the diseased kidneys on cDNA microarrays. In particular, we showed an enhanced expression of several genes regulating the cell cycle and proliferation, including growth factors and their receptors, as well as endothelial differentiation gene-5 (EDG5), a receptor for sphingosine 1-phosphate (SPP). One of the growth factors, platelet-derived growth factor (PDGF) induces a marked upregulation of EDG5 in proliferative mesangial cells, and promotes cell proliferation synergistically with SPP. The genomic approach allows us to identify families of genes involved in a process, and can indicate that enhanced PDGF-EDG5 signaling plays an important role in the progression of IgAN.

cAMP and in vivo hypoxia induce tob, ifr1, and fos expression in erythroid cells of the chick embryo

During avian embryonic development, terminal erythroid differentiation occurs in the circulation. Some of the key events, such as the induction of erythroid 2,3-bisphosphoglycerate (2,3-BPG), carbonic anhydrase (CAII), and pyrimidine 5'-nucleotidase (P5N) synthesis are oxygen dependent (Baumann R, Haller EA, Schöning U, and Weber M, Dev Biol 116: 548-551, 1986; Dragon S and Baumann R, Am J Physiol Regulatory Integrative Comp Physiol 280: R870-R878, 2001; Dragon S, Carey C, Martin K, and Baumann R, J Exp Biol 202: 2787-2795, 1999; Dragon S, Glombitza S, Götz R, and Baumann R, Am J Physiol Regulatory Integrative Comp Physiol 271: R982-R989, 1996; Dragon S, Hille R, Götz R, and Baumann R, Blood 91: 3052-3058, 1998; Million D, Zillner P, and Baumann R, Am J Physiol Regulatory Integrative Comp Physiol 261: R1188-R1196, 1991) in an indirect way: hypoxia stimulates the release of norepinephrine (NE)/adenosine into the circulation (Dragon et al., J Exp Biol 202: 2787-2795, 1999; Dragon et al., Am J Physiol Regulatory Integrative Comp Physiol 271: R982-R989, 1996). This leads via erythroid beta-adrenergic/adenosine A(2) receptor activation to a cAMP signal inducing several proteins in a transcription-dependent manner (Dragon et al., Am J Physiol Regulatory Integrative Comp Physiol 271: R982-R989, 1996; Dragon et al., Blood 91: 3052-3058, 1998; Glombitza S, Dragon S, Berghammer M, Pannermayr M, and Baumann R, Am J Physiol Regulatory Integrative Comp Physiol 271: R973-R981, 1996). To understand how the cAMP-dependent processes are initiated, we screened an erythroid cDNA library for cAMP-regulated genes. We detected three genes that were strongly upregulated (>5-fold) by cAMP in definitive and primitive red blood cells. They are homologous to the mammalian Tob, Ifr1, and Fos proteins. In addition, the genes are induced in the intact embryo during short-term hypoxia. Because the genes are regulators of proliferation and differentiation in other cell types, we suggest that cAMP might promote general differentiating processes in erythroid cells, thereby allowing adaptive modulation of the latest steps of erythroid differentiation during developmental hypoxia.

The human BTG2/TIS21/PC3 gene: genomic structure, transcriptional regulation and evaluation as a candidate tumor suppressor gene

BTG2/TIS21/PC3 protein is involved in the regulation of G1/S transition of the cell cycle by inhibiting pRb function, suggesting that BTG2/TIS21/PC3 regulation is critical for normal cell growth and proliferation. To understand the regulatory mechanisms for the expression of BTG2/TIS21/PC3 we cloned the human gene. Potential binding sites for several transcription factors were identified in the 5'-flanking region of the gene. Transient expression assays with BTG2/TIS21/PC3 promoter deletions and electrophoretic mobility shift analysis identified a major wild-type p53 response element located -74 to -122 relative to the start codon. This genomic fragment was sufficient to constitute a promoter element in the presence of p53. The BTG2/TIS21/PC3 gene is an antiproliferative gene which maps within a chromosomal segment (1q32) frequently altered in breast adenocarcinomas. However, no mutations of BTG2/TIS21/PC3 were detected in breast cancer cells, suggesting that the inactivation of this gene is not a frequent genetic event during breast carcinogenesis.

Interaction of PRMT1 with BTG/TOB proteins in cell signalling: molecular analysis and functional aspects

BACKGROUND

Several recent reports have connected protein methylation with differentiation. Furthermore, the BTG/TOB proteins have also been implicated in such control. BTG1 and 2 have been shown to interact with PRMT1 (predominant cellular arginine N-methyltransferase of type I).

RESULTS

First, we have studied the interaction between PRMT1 and the proteins of the BTG/TOB family. We show that boxC, a sequence present only in BTG1 and BTG2, is essential for this association. Using boxC peptide, we have investigated the importance of PRMT1/BTG protein association during type I protein methylation reactions. Finally, we show that the addition of boxC fused to penetratin interferes with the neuronal differentiation of PC12 cells and ES cell-derived neurones.

CONCLUSIONS

Taken together, these results indicate that PRMT1/BTG proteins could play a key role in the arginine methylation-mediated signalling pathway as well as in neuronal differentiation.

Tob is a negative regulator of activation that is expressed in anergic and quiescent T cells

During a search for genes that maintain T cell quiescence, we determined that Tob, a member of an anti-proliferative gene family, was highly expressed in anergic T cell clones. Tob was also expressed in unstimulated peripheral blood T lymphocytes and down-regulated during activation. Forced expression of Tob inhibited T cell proliferation and transcription of cytokines and cyclins. In contrast, suppression of Tob with an antisense oligonucleotide augmented CD3-mediated responses and abrogated the requirement of costimulation for maximal proliferation and cytokine secretion. Tob associated with Smad2 and Smad4 and enhanced Smad DNA-binding. The inhibitory effect of Tob on interleukin 2 (IL-2) transcription was not mediated by blockade of NFAT, AP-1 or NF-kappaB transactivation but by enhancement of Smad binding on the -105 negative regulatory element of the IL-2 promoter. Thus, T cell quiescence is an actively maintained phenotype that must be suppressed for T cell activation to occur.

Identification and expression of a new sulfhydryl oxidase SOx-3 during the cell cycle and the estrus cycle in uterine cells

Using differential hybridization of a guinea pig endometrial cell cDNA library, a potentially negatively estrogen-regulated gene, SOX-3, was isolated. According to the nucleotide and protein sequence similarities, SOx-3 belonged to the FAD-linked sulfhydryl oxidase family containing the egg white sulfhydryl oxidase, the rat seminal vesicle sulfhydryl oxidase-2 SOx-2, the quiescence-inducible protein hQ6. The SOX-3 transcript in the guinea pig as well as 5 different mRNAs in human tissues appeared differentially expressed in the tissues studied. In secondary endometrial cell culture, the SOX-3 mRNA level increased during a serum depletion-induced quiescence, decreased when cells enter the G1 phase after serum stimulation, and was restored during the S and G2/M phases. Thus, SOX-3 could be implicated in the negative cell cycle control. The SOx-3 protein appeared to be specific of epithelial cells in the uterus. Its expression level varied during the estrus cycle in the guinea pig, suggesting a regulation by steroid hormones.

The arginine-1493 residue in QRRGRTGR1493G motif IV of the hepatitis C virus NS3 helicase domain is essential for NS3 protein methylation by the protein arginine methyltransferase 1

The NS3 protein of hepatitis C virus (HCV) contains protease and RNA helicase activities, both of which are likely to be essential for HCV propagation. An arginine residue present in the arginine-glycine (RG)-rich region of many RNA-binding proteins is posttranslationally methylated by protein arginine methyltransferases (PRMTs). Amino acid sequence analysis revealed that the NS3 protein contains seven RG motifs, including two potential RG motifs in the 1486-QRRGRTGRG-1494 motif IV of the RNA helicase domain, in which arginines are potentially methylated by PRMTs. Indeed, we found that the full-length NS3 protein is arginine methylated in vivo. The full-length NS3 protein and the NS3 RNA helicase domain were methylated by a crude human cell extract. The purified PRMT1 methylated the full-length NS3 and the RNA helicase domain, but not the NS3 protease domain. The NS3 helicase bound specifically and comigrated with PRMT1 in vitro. Mutational analyses indicate that the Arg(1493) in the QRR(1488)GRTGR(1493)G region of the NS3 RNA helicase is essential for NS3 protein methylation and that Arg(1488) is likely methylated. NS3 protein methylation by the PRMT1 was decreased in the presence of homoribopolymers, suggesting that the arginine-rich motif IV is involved in RNA binding. The results suggest that an arginine residue(s) in QRXGRXGR motif IV conserved in the virus-encoded RNA helicases can be posttranslationally methylated by the PRMT1.

In vivo PC3 overexpression by retroviral vector affects cell differentiation of rat cortical precursors

The PC3 gene is a marker of dividing neuroepithelial (NE) cells. We transduced single cortical precursors of the ventricular zone (VZ) with a PC3-carrying retroviral vector at E16 stage, and analysed the effects of transgene expression on their progeny in 3-week-old animals. Unlike control-transduced cells, all viable PC3-transduced cells remained close to the ventricle and displayed a round-shaped, undifferentiated morphology.

Genes modulated by histone acetylation as new effectors of butyrate activity

A wealth of evidence correlates the chemopreventive activity of a fiber-rich diet with the production of butyrate. In order to identify the genes transcriptionally modulated by the molecule, we analyzed the expression profile of butyrate-treated colon cancer cells by means of cDNA expression arrays. Moreover, the effect of trichostatin A, a specific histone deacetylase inhibitor, was studied. A superimposable group of 23 genes out of 588 investigated is modulated by both butyrate and trichostatin A. Among them, a major target was tob-1, a gene involved in the control of cell cycle. tob-1 is also up-regulated by butyrate in a neuroblastoma-derived cell line, and its overexpression in the colon cells caused growth arrest. Our findings represent an extensive analysis of genes modulated by butyrate and identify completely new effectors of its biological activities.

Augmentation in expression of activation-induced genes differentiates memory from naive CD4+ T cells and is a molecular mechanism for enhanced cellular response of memory CD4+ T cells

In an attempt to understand the molecular basis for the immunological memory response, we have used cDNA microarrays to measure gene expression of human memory and naive CD4+ T cells at rest and after activation. Our analysis of 54,768 cDNA clones provides the first glimpse into gene expression patterns of memory and naive CD4+ T cells at the genome-scale and reveals several novel findings. First, memory and naive CD4+ T cells expressed similar numbers of genes at rest and after activation. Second, we have identified 14 cDNA clones that expressed higher levels of transcripts in memory cells than in naive cells. Third, we have identified 135 (130 known genes and 5 expressed sequence tags) up-regulated and 68 (42 known genes and 26 expressed sequence tags) down-regulated cDNA clones in memory CD4+ T after in vitro stimulation with anti-CD3 plus anti-CD28. Interestingly, the increase in mRNA levels of up-regulated genes was greater in memory than in naive CD4+ T cells after in vitro stimulation and was higher with anti-CD3 plus anti-CD28 than with anti-CD3 alone in both memory and naive CD4+ T cells. Finally, the changes in expression of actin and cytokine genes identified by cDNA microarrays were confirmed by Northern and protein analyses. Together, we have identified approximately 200 cDNA clones whose expression levels changed after activation and suggest that the level of expression of up-regulated genes is a molecular mechanism that differentiates the response of memory from naive CD4+ T cells.

Expression of zebrafish btg-b, an anti-proliferative cofactor, during early embryogenesis

BTG/tob family proteins are thought to be a potential tumor suppressor due to their anti-proliferative activity. We cloned zebrafish btg-b, a member of the BTG1/2 subfamily, using in situ hybridization screening. The tissue-specific expression of btg-b is first observed in the organizer region at the early gastrula stage. Later in development, the forebrain, the hindbrain, the polster and the paraxial mesoderm transiently express btg-b. Recently, mouse Btg1 and Btg2 have been shown to be a cofactor for Hoxb9. Double in situ hybridization with zebrafish btg-b and hoxb9a indicates that the expression domains of these two genes overlap in the posterior paraxial mesoderm.

Association of ANA, a member of the antiproliferative Tob family proteins, with a Caf1 component of the CCR4 transcriptional regulatory complex

A 35-kDa protein, ANA, belongs to an emerging family of antiproliferative proteins consisting of Tob, Tob2, ANA / BTG3, PC3B, PC3 / TIS21 / BTG2, and BTG1. All of these, except ANA and PC3B, have been shown to interact with the CCR4 transcription factor-associated protein Caf1. Here we show that ANA also associates with Caf1, ANA being the preferred partner of Caf1 among the Tob family proteins. Although ANA is likely to interact with Caf1 at its amino-terminal half, which is conserved among the family members, our data suggest that the carboxyl-terminal half of ANA plays a role in the interaction. Finally, in situ hybridization experiments revealed that expression of Caf1 overlaps at least in part with that of ANA. Thus, ANA could function through its interaction with Caf1.

In search of a function for the TIS21/PC3/BTG1/TOB family

The Btg family of anti-proliferative gene products includes Pc 3/Tis 21/Btg 2, Btg 1, Tob, Tob2, Ana/Btg3, Pc3k and others. These proteins are characterized by similarities in their amino-terminal region: the Btg1 homology domain. However, the pleiotropic nature of these family proteins has been observed and no common physiological function among family members was suggested from the history of their identification. Recent progress in the search for Btg family functions has come from the analysis of cell regulation and of cell differentiation. It is now emerging that every member of this family has a potential to regulate cell growth. We would like to propose here to use a nomenclature APRO as a new term for the family.

Identification of functional domains involved in BTG1 cell localization

We have previously shown that BTG1 stimulates myoblast differentiation. In addition, this protein displays a major nuclear localization in confluent myoblasts, decreasing during the early steps of differentiation, and is essentially detected in the cytoplasm of mature myotubes. To identify the domains involved in the cellular trafficking of BTG1, we observed the localization of several BTG1 sequences fused to betaGalactosidase. The highly conserved B box among all members of the BTG family induces a significant nuclear localization of the betaGal moiety, enhanced by presence of the BTG1 carboxy-terminal sequence. In addition, a functional Nuclear Export Signal (NES) overlaps the B box. Moreover, presence of the first 43 NH(2)-terminal amino acids reduced the nuclear localization of each chimeric protein tested. Last, the BTG1 amino-terminal domain bears an LxxLL motif favouring nuclear accumulation, and another region encompassing the A box inhibiting nuclear localization. In contrast to a BTG1 mutant exclusively localized in the cytoplasm, transient expression of a mutant displaying a nuclear localization enhanced myoblasts withdrawal from the cell cycle and terminal differentiation, thus mimicking the myogenic influence of BTG1. In conclusion, several regions of BTG1 are implicated in its cellular localization, and BTG1 myogenic activity is induced at the nuclear level.

The gene PC3(TIS21/BTG2), prototype member of the PC3/BTG/TOB family: regulator in control of cell growth, differentiation, and DNA repair?

PC3(TIS21/BTG2) is the founding member of a family of genes endowed with antiproliferative properties, namely BTG1, ANA/BTG3, PC3B, TOB, and TOB2. PC3 was originally isolated as a gene induced by nerve growth factor during neuronal differentiation of rat PC12 cells, or by TPA in NIH3T3 cells (named TIS21), and is a marker for neuronal birth in vivo. This and other findings suggested its implication in the process of neurogenesis as mediator of the growth arrest before differentiation. Remarkably, its human homolog, named BTG2, was shown to be p53-inducible, in conditions of genotoxic damage. PC3(TIS21/BTG2) impairs G(1)-S progression, either by a Rb-dependent pathway through inhibition of cyclin D1 transcription, or in a Rb-independent fashion by cyclin E downregulation. PC3(TIS21/BTG2) might also control the G(2) checkpoint. Furthermore, PC3(TIS21/BTG2) interacts with carbon catabolite repressor protein-associated factor 1 (CAF-1), a molecule that associates to the yeast transcriptional complex CCR4 and might influence cell cycle, with the transcription factor Hoxb9, and with the protein-arginine methyltransferase 1, that might control transcription through histone methylation. Current evidence suggests a physiological role of PC3(TIS21/BTG2) in the control of cell cycle arrest following DNA damage and other types of cellular stress, or before differentiation of the neuron and other cell types. The molecular function of PC3(TIS21/BTG2) is still unknown, but its ability to modulate cyclin D1 transcription, or to synergize with the transcription factor Hoxb9, suggests that it behaves as a transcriptional co-regulator.

Relationships of the antiproliferative proteins BTG1 and BTG2 with CAF1, the human homolog of a component of the yeast CCR4 transcriptional complex: involvement in estrogen receptor alpha signaling pathway

We have reported previously the physical interaction of B-cell translocation gene proteins (BTG)1 and BTG2 with the mouse protein CAF1 (CCR4-associated factor 1) and suggested that these proteins may participate, through their association with CAF1, in transcription regulation. Here we describe the in vitro and in vivo association of these proteins with hPOP2, the human paralog of hCAF1. The physical and functional relationships between the BTG proteins and their partners hCAF1 and hPOP2 were investigated to find out how these interactions affect cellular processes, and in particular transcription regulation. We defined their interaction regions and examined their expression in various human tissues. We also show functional data indicating their involvement in estrogen receptor alpha (ERalpha)-mediated transcription regulation. We found that BTG1 and BTG2, probably through their interaction with CAF1 via a CCR4-like complex, can play both positive or negative roles in regulating the ERalpha function. In addition, our results indicate that two LXXLL motifs, referred to as nuclear receptor boxes, present in both BTG1 and BTG2, are involved in the regulation of ERalpha-mediated activation.

A serine/threonine kinase p90rsk1 phosphorylates the anti-proliferative protein Tob

BACKGROUND

tob is a member of a gene family with anti-proliferative function. Over-expression of Tob in NIH3T3 cells results in the suppression of cell proliferation. The growth suppression is hampered by the presence of activated ErbB2 kinase. The molecular mechanisms by which Tob suppresses cell growth and by which ErbB2 abrogates Tob function remain to be elucidated.

RESULTS

We show that Tob is phosphorylated on serines and threonines, but not tyrosines, by a kinase(s) that associates with Tob in the lysates of various cells, including ErbB2-over-expressed cells. We also show that a 95 kDa kinase associates with Tob in vitro. The autophosphorylation activity of this kinase co-chromatographes with Tob-phosphorylating activity, suggesting that the 95 kDa kinase phosphorylates Tob. Among the known kinases with molecular mass around 95 kDa, p90rsk1 associates with Tob in vitro and in vivo, and phosphorylates Tob at least in vitro. Therefore, it is likely that p90rsk1 represents the 95 kDa kinase and is involved in the regulation of Tob function through phosphorylation.

CONCLUSION

p90rsk1 associates with and phosphorylates Tob. Because p90rsk1 is activated downstream of receptor tyrosine kinases, we propose that Tob function is at least in part under the control of growth factor-stimulated tyrosine kinases through its phosphorylation by p90rsk1.

Negative regulation of BMP/Smad signaling by Tob in osteoblasts

Bone morphogenetic protein (BMP) controls osteoblast proliferation and differentiation through Smad proteins. Here we show that Tob, a member of the emerging family of antiproliferative proteins, is a negative regulator of BMP/Smad signaling in osteoblasts. Mice carrying a targeted deletion of the tob gene have a greater bone mass resulting from increased numbers of osteoblasts. Orthotopic bone formation in response to BMP2 is elevated in tob-deficient mice. Overproduction of Tob represses BMP2-induced, Smad-mediated transcriptional activation. Finally, Tob associates with receptor-regulated Smads (Smad1, 5, and 8) and colocalizes with these Smads in the nuclear bodies upon BMP2 stimulation. The results indicate that Tob negatively regulates osteoblast proliferation and differentiation by suppressing the activity of the receptor-regulated Smad proteins.

Cloning of PC3B, a novel member of the PC3/BTG/TOB family of growth inhibitory genes, highly expressed in the olfactory epithelium

We identified in the EST database murine and human sequences similar, but not identical, to the members of the PC3/BTG/TOB family of cell cycle inhibitors. A conserved domain (aa 50-68) of the PC3 protein, the prototype member of the family, was used as a query. That domain has been shown by us to be necessary for the antiproliferative activity of PC3. A murine EST clone and a highly homologous human EST clone, containing the entire ORF, were chosen for sequencing. Comparison to databases and a phylogenetic tree analysis indicated that these EST clones are the mouse and human homologues of a gene that represents a novel member of the PC3/BTG/TOB family. This gene, named PC3B, is endowed with marked antiproliferative activity, being able to induce G(1) arrest, and is highly expressed in testis, in oocyte, and in preimplantation embryos. Analysis of its expression during murine development indicated a specific localization in the olfactory epithelium at midgestation, suggesting that PC3B might be involved in the differentiation of this neuronal structure. Human PC3B mapped to chromosome 11q23, as indicated by radiation hybrid analysis.

Gene expression networks underlying retinoic acid–induced differentiation of acute promyelocytic leukemia cells

To elucidate the molecular mechanism of all-trans-retinoic acid (ATRA)–induced differentiation of acute promyelocytic leukemia (APL) cells, the gene expression patterns in the APL cell line NB4 before and after ATRA treatment were analyzed using complementary DNA array, suppression-subtractive hybridization, and differential-display–polymerase chain reaction. A total of 169 genes, including 8 novel ones, were modulated by ATRA. The ATRA-induced gene expression profiles were in high accord with the differentiation and proliferation status of the NB4 cells. The time courses of their modulation were interesting. Among the 100 up-regulated genes, the induction of expression occurred most frequently 12-48 hours after ATRA treatment, while 59 of 69 down-regulated genes found their expression suppressed within 8 hours. The transcriptional regulation of 8 induced and 24 repressed genes was not blocked by cycloheximide, which suggests that these genes may be direct targets of the ATRA signaling pathway. A balanced functional network seemed to emerge, and it formed the foundation of decreased cellular proliferation, maintenance of cell viability, increased protein modulation, and promotion of granulocytic maturation. Several cytosolic signaling pathways, including JAKs/STAT and MAPK, may also be implicated in the symphony of differentiation.

Gene expression networks underlying retinoic acid–induced differentiation of acute promyelocytic leukemia cells

To elucidate the molecular mechanism of all-trans-retinoic acid (ATRA)–induced differentiation of acute promyelocytic leukemia (APL) cells, the gene expression patterns in the APL cell line NB4 before and after ATRA treatment were analyzed using complementary DNA array, suppression-subtractive hybridization, and differential-display–polymerase chain reaction. A total of 169 genes, including 8 novel ones, were modulated by ATRA. The ATRA-induced gene expression profiles were in high accord with the differentiation and proliferation status of the NB4 cells. The time courses of their modulation were interesting. Among the 100 up-regulated genes, the induction of expression occurred most frequently 12-48 hours after ATRA treatment, while 59 of 69 down-regulated genes found their expression suppressed within 8 hours. The transcriptional regulation of 8 induced and 24 repressed genes was not blocked by cycloheximide, which suggests that these genes may be direct targets of the ATRA signaling pathway. A balanced functional network seemed to emerge, and it formed the foundation of decreased cellular proliferation, maintenance of cell viability, increased protein modulation, and promotion of granulocytic maturation. Several cytosolic signaling pathways, including JAKs/STAT and MAPK, may also be implicated in the symphony of differentiation.

Identification and molecular analysis of BANP

BTG3 belongs to a family of structurally related genes whose biochemical functions remain elusive. In order to investigate the mechanism underlying BTG3-mediated functions, we tried to identify BTG3 potential partners. The use of the yeast 'two-hybrid system', with BTG3 as bait, enabled us to isolate BANP (BTG3 Associated Nuclear Protein). Other commonly used protein-binding assays did not confirm this yeast interaction. However, BANP had never been described before, and this prompted us to further characterise this gene. In this paper, we present data on its molecular organization in mouse, then we speculate on the nature of this nuclear protein, and finally we localise BANP on the human chromosome 16q24 subregion; we discuss the fact that frequent loss of heterozygosity within this region has been observed in different tumours.

Cloning and characterization of the mouse tob2 gene

Human Tob2 is a member of the Tob/BTG1 anti-proliferative family of proteins. Here, we report the molecular cloning and characterization of the mouse tob2 gene. The tob2 gene contains an open reading frame of 345 amino acids with an 89% identity to its human counterparts. The coding region of mouse tob2 is not interrupted by introns. The tob2 transcript is 4.2kb long, the size being similar to that of the human tob2 transcript, and detected ubiquitously in various tissues of adult mice. In addition, in situ hybridization shows that tob2 is ubiquitously expressed in embryo, the level of expression being especially high in skeletal muscle. Collectively, Tob2 is suggested to play roles both during embryogenesis and in adults.

Protein-arginine methyltransferase I, the predominant protein-arginine methyltransferase in cells, interacts with and is regulated by interleukin enhancer-binding factor 3

Arginine methylation is a common post-translation modification found in many proteins. Protein-arginine methyltransferase I (PRMT1) contributes >90% of type I protein-arginine methyltransferase activity in cells and tissues. To expand our knowledge on the regulation and role of PRMT1 in cells, we used the yeast two-hybrid system to identify proteins that interact with PRMT1. One of the interacting proteins we cloned is interleukin enhancer-binding factor 3 (ILF3), also known as M phase phosphoprotein 4. ILF3 is closely related to nuclear factor 90 (NF90). Using an immunofluorescence analysis, we determined that ILF3 and PRMT1 co-localize in the nucleus. Moreover, PRMT1 and ILF3 co-precipitate in immunoprecipitation assays and can be isolated together in "pull-down" experiments using recombinant fusion proteins. ILF3 is a robust substrate for methylation by PRMT1 and can modulate PRMT1 activity in in vitro methylation assays. Deletion studies demonstrated that the COOH-terminal region of ILF3, which is rich in arginine, glycine, and serine, is responsible for the strong interaction between PRMT1 and ILF3 and is the site of ILF3 methylation by PRMT1. Although ILF3 and NF90 are highly similar, they differ in their carboxyl-terminal regions. Because of this difference, NF90 does not interact with PRMT1, is a much poorer substrate than ILF3 for PRMT1-dependent methylation, and does not modulate PRMT1 enzyme activity.

The RGG domain in hnRNP A2 affects subcellular localization

The heterogeneous nuclear ribonucleoproteins (hnRNP) associate with pre-mRNA in the nucleus and play an important role in RNA processing and splice site selection. In addition, hnRNP A proteins function in the export of mRNA to the cytoplasm. Although the hnRNP A proteins are predominantly nuclear, hnRNP A1 shuttles rapidly between the nucleus and the cytoplasm. HnRNP A2, whose cytoplasmic overexpression has been identified as an early biomarker of lung cancer, has been less well studied. Cytosolic hnRNP A2 overexpression has also been noted in brain tumors, in which it has been correlated with translational repression of Glucose Transporter-1 expression. We now examine the role of arginine methylation on the nucleocytoplasmic localization of hnRNP A2 in the HEK-293 and NIH-3T3 mammalian cell lines. Treatment of either cell line with the methyltransferase inhibitor adenosine dialdehyde dramatically shifts hnRNP A2 localization from the nuclear to the cytoplasmic compartment, as shown both by immunoblotting and by immunocytochemistry. In vitro radiolabeling with [(3)H]AdoMet of GST-tagged hnRNP A2 RGG mutants, using recombinant protein arginine methyltransferase (PRMT1), shows (i) that hnRNP A2 is a substrate for PRMT1 and (ii) that methylated residues are found only in the RGG domain. Deletion of the RGG domain (R191-G253) of hnRNP A2 results in a cytoplasmic localization phenotype, detected both by immunoblotting and by immunocytochemistry. These studies indicate that the RGG domain of hnRNP A2 contains sequences critical for cellular localization of the protein. The data suggest that hnRNP A2 may contain a novel nuclear localization sequence, regulated by arginine methylation, that lies in the R191-G253 region and may function independently of the M9 transportin-1-binding region in hnRNP A2.

A screen for targets of the Xenopus T-box gene Xbra

Brachyury (T), a member of the T-box gene family, is essential for the formation of posterior mesoderm and notochord in vertebrate development. Expression of the Xenopus homologue of Brachyury, Xbra, causes ectopic ventral and lateral mesoderm formation in animal cap explants and co-expression of Xbra with Pintallavis, a forkhead/HNF3beta-related transcription factor, induces notochord. Although eFGF and the Bix genes are thought to be direct targets of Xbra, no other target genes have been identified. Here, we describe the use of hormone-inducible versions of Xbra and Pintallavis to construct cDNA libraries enriched for targets of these transcription factors. Five putative targets were isolated: Xwnt11, the homeobox gene Bix1, the zinc-finger transcription factor Xegr-1, a putative homologue of the antiproliferative gene BTG1 called Xbtg1, and BIG3/1A11, a gene of unknown function. Expression of Xegr-1 and Xbtg1 is controlled by Pintallavis alone as well as by a combination of Xbra and Pintallavis. Overexpression of Xbtg1 perturbed gastrulation and caused defects in posterior tissues and in notochord and muscle formation, a phenotype reminiscent of that observed with a dominant-negative version of Pintallavis called Pintallavis-En(R). The Brachyury-inducible genes we have isolated shed light on the mechanism of Brachyury function during mesoderm formation. Specification of mesodermal cells is regulated by targets including Bix1-4 and eFGF, while gastrulation movements and perhaps cell division are regulated by Xwnt11 and Xbtg1.

PRMT1 is the predominant type I protein arginine methyltransferase in mammalian cells

Type I protein arginine methyltransferases catalyze the formation of asymmetric omega-N(G),N(G)-dimethylarginine residues by transferring methyl groups from S-adenosyl-L-methionine to guanidino groups of arginine residues in a variety of eucaryotic proteins. The predominant type I enzyme activity is found in mammalian cells as a high molecular weight complex (300-400 kDa). In a previous study, this protein arginine methyltransferase activity was identified as an additional activity of 10-formyltetrahydrofolate dehydrogenase (FDH) protein. However, immunodepletion of FDH activity in RAT1 cells and in murine tissue extracts with antibody to FDH does not diminish type I methyltransferase activity toward the methyl-accepting substrates glutathione S-transferase fibrillarin glycine arginine domain fusion protein or heterogeneous nuclear ribonucleoprotein A1. Similarly, immunodepletion with anti-FDH antibody does not remove the endogenous methylating activity for hypomethylated proteins present in extracts from adenosine dialdehyde-treated RAT1 cells. In contrast, anti-PRMT1 antibody can remove PRMT1 activity from RAT1 extracts, murine tissue extracts, and purified rat liver FDH preparations. Tissue extracts from FDH(+/+), FDH(+/-), and FDH(-/-) mice have similar protein arginine methyltransferase activities but high, intermediate, and undetectable FDH activities, respectively. Recombinant glutathione S-transferase-PRMT1, but not purified FDH, can be cross-linked to the methyl-donor substrate S-adenosyl-L-methionine. We conclude that PRMT1 contributes the major type I protein arginine methyltransferase enzyme activity present in mammalian cells and tissues.

Arrest of G(1)-S progression by the p53-inducible gene PC3 is Rb dependent and relies on the inhibition of cyclin D1 transcription

The p53-inducible gene PC3 (TIS21, BTG2) is endowed with antiproliferative activity. Here we report that expression of PC3 in cycling cells induced accumulation of hypophosphorylated, growth-inhibitory forms of pRb and led to G(1) arrest. This latter was not observed in cells with genetic disruption of the Rb gene, indicating that the PC3-mediated G(1) arrest was Rb dependent. Furthermore, (i) the arrest of G(1)-S transition exerted by PC3 was completely rescued by coexpression of cyclin D1 but not by that of cyclin A or E; (ii) expression of PC3 caused a significant down-regulation of cyclin D1 protein levels, also in Rb-defective cells, accompanied by inhibition of CDK4 activity in vivo; and (iii) the removal from the PC3 molecule of residues 50 to 68, a conserved domain of the PC3/BTG/Tob gene family, which we term GR, led to a loss of the inhibition of proliferation as well as of the down-regulation of cyclin D1 levels. These data point to cyclin D1 down-regulation as the main factor responsible for the growth inhibition by PC3. Such an effect was associated with a decrease of cyclin D1 transcript and of cyclin D1 promoter activity, whereas no effect of PC3 was observed on cyclin D1 protein stability. Taken together, these findings indicate that PC3 impairs G(1)-S transition by inhibiting pRb function in consequence of a reduction of cyclin D1 levels and that PC3 acts, either directly or indirectly, as a transcriptional regulator of cyclin D1.

The leukemia-associated protein Btg1 and the p53-regulated protein Btg2 interact with the homeoprotein Hoxb9 and enhance its transcriptional activation

BTG1 and BTG2 belong to a family of functionally related genes involved in the control of the cell cycle. As part of an ongoing attempt to understand their biological functions, we used a yeast two-hybrid screening to look for possible functional partners of Btg1 and Btg2. Here we report the physical and functional association between these proteins and the homeodomain protein Hoxb9. We further show that Btg1 and Btg2 enhance Hoxb9-mediated transcription in transfected cells, and we report the formation of a Hoxb9.Btg2 complex on a Hoxb9-responsive target, and the fact that this interaction facilitates the binding of Hoxb9 to DNA. The transcriptional activity of the Hoxb9.Btg complex is essentially dependent on the activation domain of Hoxb9, located in the N-terminal portion of the protein. Our data indicate that Btg1 and Btg2 act as transcriptional cofactors of the Hoxb9 protein, and suggest that this interaction may mediate their antiproliferative function.