Interaction between the human nuclear cap-binding protein complex and hnRNP F

hnRNP F was identified in a screen for proteins that interact with human CBP80 and CBP20, the components of the nuclear cap-binding complex (CBC). In vitro interaction studies showed that hnRNP F can bind to both CBP20 and CBP80 individually. hnRNP F and CBC bind independently to RNA, but hnRNP F binds preferentially to CBC-RNA complexes rather than to naked RNA. The hnRNP H protein, which is 78% identical to hnRNP F and also interacts with both CBP80 and CBP20 in vitro, does not discriminate between naked RNA and CBC-RNA complexes, showing that this effect is specific. Depletion of hnRNP F from HeLa cell nuclear extract decreases the efficiency of pre-mRNA splicing, a defect which can be partially compensated by addition of recombinant hnRNP F. Thus, hnRNP F is required for efficient pre-mRNA splicing in vitro and may participate in the effect of CBC on pre-mRNA splicing.

A protein-arginine methyltransferase binds to the intracytoplasmic domain of the IFNAR1 chain in the type I interferon receptor

The intracytoplasmic domain (IC) of cytokine receptors provides docking sites for proteins which mediate signal transduction. Thus, in interferon-alpha,beta receptors (IFNAR1 and 2), the IC region binds protein-tyrosine and -serine/threonine kinases which phosphorylate the receptor and the associated Stat transcription factors. A two-hybrid screening was carried out to identify additional proteins which could interact with the IC domain of the IFNAR1 chain of the IFN-alpha,beta receptor. Several positive clones representing a protein sequence designated IR1B4 were recovered from a human cDNA library. IR1B4 was identified as the human homolog of PRMT1, a protein-arginine methyltransferase from rat cells. Flag-IR1B4 fusion proteins bind to the isolated IFNAR1 intracytoplasmic domain produced in Escherichia coli, as well as to the intact IFNAR1 chain extracted by detergent from human U266 cell membranes. S-Adenosylmethionine-dependent methyltransferase activity was precipitated by anti-IFNAR1 antibodies from untreated human cells. IR1B4/PRMT1 is involved in IFN action since U266 cells rendered deficient in this methyltransferase by antisense oligonucleotides become more resistant to growth inhibition by IFN. Methylation of proteins by enzymes which can attach to the IC domains of receptors may be a signaling mechanism complementing protein phosphorylation. Among substrates methylated by PRMT1 are RNA-binding heterogeneous nuclear ribonucleoproteins (hnRNPs) which are involved in mRNA processing, splicing and transport into the cytoplasm.

Rapid changes in heteronuclear RNA for corticotrophin-releasing hormone and arginine vasopressin in response to acute stress

The rapid detection of gene activation is important for our understanding of gene regulation. We have therefore studied heteronuclear (i.e. nascent) RNA (hnRNA) by using 35S-labelled corticotrophin-releasing hormone (CRH) riboprobes and arginine vasopressin (AVP) oligo-nucleotide probes directed against intronic and exonic sequences of both CRH and AVP transcripts for in situ hybridization studies of transcriptional changes during acute stress. CRH and AVP intronic signals (found in newly synthesized transcripts) were confined to the nuclei of the parvocellular cells in the paraventricular nucleus (PVN) whilst CRH and AVP exonic signals (found in both newly formed and mature transcripts) were primarily located in the cytoplasm of these cells. AVP hnRNA and mRNA were also present at high level, in the magnocellular PVN. The levels of CRH hnRNA and parvocellular AVP hnRNA in the PVN were significantly increased 1 and 2 h after the onset of restraint. The levels of CRH mRNA on the other hand were not significantly increased until 4 h after the onset of restraint. The number of AVP mRNA-expressing neurons in the medial parvo-cellular cells of the PVN significantly increased at 2 h and peaked at 4 h after the onset of stress. In contrast, densitometric analysis indicated that the increase in AVP mRNA levels in these cells did nor reach significant difference from control until 4 h after the onset of restraint. There were no significant changes on AVP hnRNA or AVP mRNA levels in the magnocellular subdivision of PVN at any time point after the onset of restraint.

Recognition of subsets of the mammalian A/B-type core heterogeneous nuclear ribonucleoprotein polypeptides by novel autoantibodies

The structurally related A/B-type core heterogeneous nuclear ribonucleoprotein (hnRNP) polypeptides of 34-39 kDa (A1, A2, B1 and B2) belong to a family of RNA-binding proteins that are major components of 40 S hnRNP complexes. By two-dimensional gel electrophoresis and peptide mapping analysis we compared each member of the A/B-type core proteins in the human and rat liver cells. This comparison revealed the unique presence in rat cells of major protein species, referred to as mBx polypeptides, that appeared as three charge isoforms at a position corresponding to the minor HeLa B1b protein spot. In addition, clear differences in the ratios of the A1 polypeptide to the A1b isoform were observed. The detection, in sera of patients with rheumatic autoimmune diseases, of two novel autoantibody specificities, one recognizing solely B2 protein and the second both the B2 and mBx polypeptides, helped to identify mBx proteins as new A/B-type hnRNP components, immunologically related to B2 protein. A common immunoreactive V8 protease peptide of approx. 17 kDa has been identified in B2 and mBx hnRNP polypeptides. mBx protein species are identified in cells of murine origin, and have a ubiquitous tissue distribution and developmental appearance.

Autocrine down-regulation of collagenase-3 in rat bone cell cultures by insulin-like growth factors

Insulin-like growth factors (IGF)-I and -II are presumed to act as autocrine regulators of bone formation. Recently, we demonstrated that IGF-I and -II inhibit bone collagen degradation and collagenase-3 synthesis in osteoblast cultures. Therefore, we tested the autocrine role of IGFs in the endogenous expression of collagenase-3 in cultures of osteoblast-enriched cells from 22-day fetal rat calvariae (Ob cells). Steady-state messenger RNA (mRNA) levels were determined by Northern blot analysis and collagenase concentrations in the culture medium were determined by Western immunoblot. Basal level collagenase-3 transcripts decreased in Ob cell cultures, coinciding with an increase in IGF-I and -II protein levels. Removal of the conditioned medium modestly increased collagenase-3 mRNA levels and restored the ability of exogenously added IGF-I to repress collagenase-3 transcripts. IGF neutralizing antibodies and IGF binding proteins-2 and -3 in excess increased and sustained collagenase mRNA, heterogeneous nuclear RNA, and protease levels in Ob cell cultures. In conclusion, IGF-I and -II are autocrine repressors of collagenase-3 synthesis, and this effect may contribute to their actions on the maintenance of a normal bone collagen matrix.

hTAF(II)68, a novel RNA/ssDNA-binding protein with homology to the pro-oncoproteins TLS/FUS and EWS is associated with both TFIID and RNA polymerase II

TFIID is the main sequence-specific DNA-binding component of the RNA polymerase II (Pol II) transcriptional machinery. It is a multiprotein complex composed of the TATA-binding protein (TBP) and TBP-associated factors (TAF(II)s). Here we report the cloning and characterization of a novel human TBP-associated factor, hTAF(II)68. It contains a consensus RNA-binding domain (RNP-CS) and binds not only RNA, but also single stranded (ss) DNA. hTAF(II)68 shares extensive sequence similarity with TLS/FUS and EWS, two human nuclear RNA-binding pro-oncoproteins which are products of genes commonly translocated in human sarcomas. Like hTAF(II)68, TLS/FUS is also associated with a sub-population of TFIID complexes chromatographically separable from those containing hTAF(II)68. Therefore, these RNA and/or ssDNA-binding proteins may play specific roles during transcription initiation at distinct promoters. Moreover, we demonstrate that hTAF(II)68 co-purifies also with the human RNA polymerase II and can enter the preinitiation complex together with Pol II.

Evidence that heteronuclear proteins interact with XIST RNA in vitro

The process of X chromosome inactivation results in the transcriptional silencing of one of the two X chromosomes in mammalian females. A large heterogeneous nuclear RNA that is expressed exclusively from the inactive X chromosome (XIST--X Inactive Specific Transcripts) has been implicated in the inactivation process. The XIST RNA colocalizes with the inactive X chromosome and therefore proteins that interact with the XIST RNA may be involved in the inactivation of the X chromosome. In order to identify such proteins we have used an in vitro UV light cross-linking technique to detect nuclear proteins associating with sections of the XIST RNA. The strongest interaction detected by this technique was between a pair of approximately 40 kDa proteins and a 5' region of the XIST RNA which contains a series of well-conserved tandem repeats. Immunoprecipitation suggested that these proteins may be the heteronuclear proteins hnRNPC1/C2.

The roles of heterogeneous nuclear ribonucleoproteins (hnRNP) in RNA metabolism

In eukaryotic cells, messenger RNAs are formed by extensive post-transcriptional processing of primary transcripts, assembled with a large number of proteins and processing factors in ribonucleoprotein complexes. The protein moiety of these complexes mainly constitutes a class of about 20 major polypeptides called heterogeneous nuclear ribonucleoproteins or hnRNPs. The function and the mechanism of action of hnRNPs is still not fully understood, but the identification of RNA binding domains and RNA binding specificities, and the development of new functional assays, has stimulated interest in them. In contrast to previous models that hypothesised a mere structural (histone-like) function, a more diversified and dynamic role for these proteins is now emerging. In fact, they can be viewed as a subset of the trans-acting pre-mRNA maturation factors. They might actively participate in post-transcriptional events such as regulated splicing and mRNA export. Moreover, recent data suggest an involvement of some of these proteins in molecular diseases. Here we present an overview of the most relevant properties of hnRNPs and discuss some emerging ideas on their roles.

Transcriptional regulation of insulin-like growth factor-binding protein-5 by prostaglandin E2 in osteoblast cells

Insulin-like growth factor (IGF)-binding protein-5 (IGFBP-5) is an autocrine and paracrine factor that modulates the effects of IGFs. We examined the mechanisms that regulate IGFBP-5 synthesis by PGE2 in osteoblast-enriched cells from fetal rat calvaria (Ob cells). PGE2 at 1 microM for 2-8 h increased IGFBP-5 heterogeneous nuclear RNA levels and did not change the half-life of IGFBP-5 messenger RNA in Ob cells, suggesting that PGE2 stimulates IGFBP-5 transcription. To analyze the elements responsible for this effect, regions of the mouse IGFBP-5 promoter from -2695 to +120 bp were ligated into pGL-2-basic and transiently transfected into Ob cells. PGE2 caused a time- and dose-dependent increase in IGFBP-5 promoter activity. Further analysis revealed two potential PGE2-responsive regions in the -2695 to -1470 and the -989 to -332 fragments. The effect of PGE2 on IGFBP-5 messenger RNA and heterogeneous nuclear RNA levels was mimicked by forskolin and inhibited by the PKA inhibitor H-89, suggesting that part of the PGE2 effect was mediated through a cAMP-dependent pathway. H-89 also blocked basal and PGE2-stimulated IGFBP-5 promoter activities. We conclude that PGE2 regulates IGFBP-5 synthesis in Ob cells by transcriptional mechanisms. PKA-dependent pathways account for part of the effect of PGE2 on IGFBP-5 expression. Deletion analysis of the IGFBP-5 promoter suggests the presence of two PGE2-responsive regions.

Heterogeneous nuclear ribonucleoprotein K is a transcription factor

The CT element is a positively acting homopyrimidine tract upstream of the c-myc gene to which the well-characterized transcription factor Spl and heterogeneous nuclear ribonucleoprotein (hnRNP) K, a less well-characterized protein associated with hnRNP complexes, have previously been shown to bind. The present work demonstrates that both of these molecules contribute to CT element-activated transcription in vitro. The pyrimidine-rich strand of the CT element both bound to hnRNP K and competitively inhibited transcription in vitro, suggesting a role for hnRNP K in activating transcription through this single-stranded sequence. Direct addition of recombinant hnRNP K to reaction mixtures programmed with templates bearing single-stranded CT elements increased specific RNA synthesis. If hnRNP K is a transcription factor, then interactions with the RNA polymerase II transcription apparatus are predicted. Affinity columns charged with recombinant hnRNP K specifically bind a component(s) necessary for transcription activation. The depleted factors were biochemically complemented by a crude TFIID phosphocellulose fraction, indicating that hnRNP K might interact with the TATA-binding protein (TBP)-TBP-associated factor complex. Coimmunoprecipitation of a complex formed in vivo between hnRNP K and epitope-tagged TBP as well as binding in vitro between recombinant proteins demonstrated a protein-protein interaction between TBP and hnRNP K. Furthermore, when the two proteins were overexpressed in vivo, transcription from a CT element-dependent reporter was synergistically activated. These data indicate that hnRNP K binds to a specific cis element, interacts with the RNA polymerase II transcription machinery, and stimulates transcription and thus has all of the properties of a transcription factor.

Identification of two RNA-binding proteins in Balbiani ring premessenger ribonucleoprotein granules and presence of these proteins in specific subsets of heterogeneous nuclear ribonucleoprotein particles

Balbiani ring (BR) granules are premessenger ribonucleoprotein particles (RNPs) generated in giant chromosomal puffs, the BRs, in the larval salivary glands of the dipteran chironomus tentans. Monoclonal antibodies were raised against nuclear proteins collected on a single-stranded-DNA-agarose affinity column, and two of them were used to identify RNA-binding proteins in BR granules. First, in Western blots (immunoblots), one of the antibodies recognized a 36-kDa protein and the other recognized a 45-KDa protein. Second, both antibodies bound to the BRs in immunocytological experiments. It was shown in cross-linking experiments that the two proteins are associated with heterogeneous nuclear RNP (hnRNP) complexes extracted from C. tentans nuclei. By immunoelectron microscopy of isolated and partly unfolded BR RNPs, it was specifically demonstrated that the BR granules contain the two proteins and, in addition, that both proteins are distributed frequently along the RNP fiber of the particles. Thus, the 36- and 45-KDa proteins are likely to be abundant, RNA-binding proteins in the BR particles. To elucidate to what extent the two proteins are also present in other hnRNPs, we studied the binding of the antibodies to chromosomal puffs in general. It was observed that many puffs in addition to the BRs harbor the two proteins, but there are also puffs containing only one of the components, either the 36- or the 45-kDa protein. We conclude that the two proteins are not randomly bound to all hnRNPs but that each of them seems to be linked to a specific subset of the particles.

trans-Activation by the hnRNP K protein involves an increase in RNA synthesis from the reporter genes

The function of many of the pre-mRNA-binding proteins in mRNA biogenesis is unclear. We have analyzed the biochemical function of the hnRNP K protein by using a mouse cDNA clone. A previous study indicated that the expression of hnRNP K activates c-myc promoter in transient transfection assays. We show that the expression of hnRNP K results in a trans-activation of a variety of RNA polymerase II promoters. The trans-activation function depends on the sequences of hnRNP K that are also necessary for RNA binding. However, the RNA binding motifs are not sufficient for trans-activation. We could identify a mutant that bound RNA in vitro but was impaired in its ability to trans-activate the reporter genes. The trans-activation was not a result of the stabilization of the reporter mRNA, because hnRNP K increased the steady-state level of the reporter mRNA without altering its decay rate. By doing nuclear run-on assays, we provide evidence that the hnRNP K protein trans-activates the reporter genes by increasing the level of transcription.

Specific binding of polypyrimidine tract binding protein and hnRNP A1 to HIV-1 CRS elements

The human immunodeficiency virus (HIV) Rev and human T-cell leukemia virus (HTLV) Rex proteins regulate viral RNA processing. Both proteins act to overcome the block to viral structural gene expression, at least in part, by reversing the inhibitory effect of intronic RNA sequences, termed cis-acting repressive (CRS) sequences. Using HTLV type II (HTLV-II) as a model, we recently showed that the function of a 5' long terminal repeat (LTR) CRS correlates with in vitro binding by both polypyrimidine tract binding (PTB) protein (also known as hnRNP I) and hnRNP A1 to CRS RNA (1,2). Using radioimmunoprecipitation of proteins ultraviolet (UV) crosslinked to each HIV CRS RNA with monoclonal anti-hnRNP antibodies, we now demonstrate that hnRNP I and hnRNP A1 bind to two different HIV-1 CRS RNAs. In addition, we show that hnRNP I and hnRNP A1 binding to HIV-1 CRS RNAs can be specifically competed by HTLV-II CRS RNAs using electrophoretic mobility shift assay (EMSA)/UV crosslinking assays. Binding by both hnRNP I and hnRNP A1 to HIV-1 and HTLV-II CRS RNAs suggests a role for these proteins in CRS function that may be influenced by the Rev and Rex proteins, respectively.

Heterogeneous nuclear ribonucleoproteins H, H', and F are members of a ubiquitously expressed subfamily of related but distinct proteins encoded by genes mapping to different chromosomes

Molecular cDNA cloning, two-dimensional gel immunoblotting, and amino acid microsequencing identified three sequence-unique and distinct proteins that constitute a subfamily of ubiquitously expressed heterogeneous nuclear ribonucleoproteins corresponding to hnRNPs H, H', and F. These proteins share epitopes and sequence identity with two other proteins, isoelectric focusing sample spot numbers 2222 (37.6 kDa; pI 6.5) and 2326 (39.5 kDa; pI 6.6), indicating that the subfamily may contain additional members. The identity between hnRNPs H and H' is 96%, between H and F 78%, and between H' and F 75%, respectively. The three proteins contain three repeats, which we denote quasi-RRMs (qRRMs) since they have a remote similarity to the RNA recognition motif (RRM). The three qRRMs of hnRNP H, with a few additional NH2-terminal amino acids, were constructed by polymerase chain reaction amplification and used for ribohomopolymer binding studies. Each qRRM repeat bound poly(rG), while only the NH2-terminal qRRM bound poly(rC) and poly(rU). None of the repeats bound detectable amounts of poly(rA). The expression levels of hnRNPs H and F were differentially regulated in pairs of normal and transformed fibroblasts and keratinocytes. In normal human keratinocytes, the expression level of H was unaffected by treatment with several substances tested including two second messengers and seven cytokines. Likewise the expression level of F was independent of these substances, although it was strikingly down-regulated by long term treatment with 4 beta-phorbol 12-myristate 13-acetate, indicating that the protein kinase C signaling pathway regulates its expression. No effect of 4 beta-phorbol 12-myristate 13-acetate was observed on the expression of hnRNP H. The genes coding for hnRNPs H, H', and F were chromosome-mapped to 5q35.3 (HNRPH1), 6q25.3-q26, and/or Xq22 (HNRPH2) and 10q11.21-q11.22 (HNRPF), respectively.

Cyclosporin A and FK506 are potent activators of proopiomelanocortin-derived peptide secretion without affecting corticotrope glucocorticoid receptor function

Unliganded glucocorticoid receptors (GR) are localized in the cytoplasm and are associated with heat shock protein (hsp)90, hsp70, and a member of the immunophilin family, FK506 binding protein 59 (FKBP59). Several members of the cyclophilin and FKBP families have now been shown to associate with unactivated steroid receptors, however the physiological role these immunophilins play in steroid receptor function is questionable. In the present study we have measured GR binding and nuclear translocation of activated receptor in corticotrope cells following treatment with the immunophilin ligands FK506 and cyclospcrin A (CsA). Extensive GR binding studies in AtT20 cells, a mouse corticotrope tumor cell line failed to demonstrate an effect of FK506 or CsA on either the ability of GR to bind ligand, or on nuclear translocation of the liganded receptor at either a saturating or subsaturating dose of dexamethasone (DEX). Consistent with the binding data, functionally, neither CsA nor FK506 altered the glucocorticoid induced decrease in either proopiomelanocortin (POMC) derived peptide secretion or POMC heteronuclear (hn) RNA expression. Despite the fact these drugs did not modulate the actions of glucocorticoids on corticotrope cells, both FK506 and CsA were potent stimulators of basal beta-endorphin secretion (4-6 fold) from rat anterior pituitary cultures and AtT20 cells. In addition, FK506 and CsA potentiated beta-endorphin secretion induced by corticotropin releasing factor (CRF) and phorbol ester, but had no apparent acute (60 min) effect on POMC hnRNA levels. Unlike the acute actions of these immunosuppressant drugs, chronic (24 h) treatment lead to a decrease in cytoplasmic POMC mRNA with no apparent change in the amount of secreted beta-endorphin. Taken together these data suggest that FK506 and CsA do not alter GR activation or function in corticotrope cells, however, they are potent but short lived stimulators of POMC-derived peptide secretion. The observation that CsA and FK506 stimulate POMC-derived peptide secretion, and potentiate both phorbol ester and CRF induced secretion, suggests that these immunosuppressant drugs are acting upon a common point within these intracellular pathways.

Transcriptional repression of insulin-like growth factor I by glucocorticoids in rat bone cells

Insulin-like growth factor I (IGF-I) is an abundant autocrine and paracrine growth factor secreted by osteoblasts. It promotes osteoblast proliferation and expression of their differentiated phenotype. Glucocorticoids decrease IGF-I production by osteoblasts, which may mediate some actions of the steroid on bone in both normal and pathological states. The mechanisms by which the glucocorticoid cortisol down-regulates IGF-I transcripts were explored using cultures of osteoblast-enriched cells derived from fetal rat calvaria (Ob cells). Repression of IGF-I transcripts was apparent after 8 h of treatment, was sustained for at least 24 h, and was not altered by cotreatment with cycloheximide. Cortisol did not alter the stability of IGF-I messenger RNAs in transcriptionally arrested Ob cells. Cortisol decreased IGF-I heterogeneous nuclear RNA and gene transcription, as determined by reverse transcription-linked polymerase chain reaction and nuclear run-on assay, respectively. Transient transfection of Ob cells with constructs containing portions of the rat IGF-I exon 1 promoter and 5'-flanking DNA linked to the reporter gene luciferase were performed to determine glucocorticoid-responsive region of the rat IGF-I exon 1 promoter was localized to 34 to 192 relative to the first start site of transcription. In conclusion, cortisol inhibits the transcription of IGF-I in osteoblasts, an effect that may be relevant to the actions of cortisol in bone.

Inhibition of the intracellular transport of influenza viral RNA by actinomycin D

In primary chicken embryo cells infected with fowl plague virus addition of actinomycin D at defined times during the infection cycle has different consequences on viral replication. If actinomycin D is added immediately after infection with a concentration, which inhibits viral RNA synthesis only partially, it interferes with the nucleo-cytoplasmic transport of all viral RNA species (mRNA and vRNA) so far tested. If actinomycin D is present during infection (adsorption, penetration and uncoating) no viral RNA is synthesized, and the nucleocapsid of the infecting virus does not reach the nucleus, as shown by fluorescent antibodies. Therefore the primary effect of actinomycin D on influenza virus replication is on the transport of the incoming vRNPs from the cytoplasm to the cell nucleus, which is the cell compartment where transcription takes place.

On unequal allelic expression of the neurofibromin gene in neurofibromatosis type 1

The autosomal dominantly inherited disease neurofibromatosis type 1 (NF1) is caused by mutations of a large gene comprising 59 exons, which code for a protein with 2818 amino acids called neurofibromin. Employing an expressed polymorphic site in exon 5 of the neurofibromin gene, the expression of its alleles was analysed quantitatively by scanning radioactive RT-PCR fragments of this exon prepared from the RNA of fibroblast cell cultures from 15 NF1 patients and of white blood cells from one NF1 patient. Thirteen of the RNA preparations yielded unequal amounts of the allelic messages. The deviations of the expression ratios (A2:A1) from 1.0 ranged from -0.9 to +25.8. The allelic messages were equally represented in the RNA preparations from five informative healthy donors. Apart from fibroblasts this phenomenon could also be detected in keratinocytes, melanocytes from normally pigmented skin and melanocytes from a café-au-lait spot of one patient. Only one of three patients affected by stop mutations exhibited unequal allelic expression. When nuclear RNA from 10 of the 13 patients was examined, equal amounts of the primary transcripts were found (average ratio A2/A1: 1.08 +/- 0.07 S.E.M.), indicating that unequal expression on the level of mRNA was not caused by mutations affecting transcriptional regulation. The ratio of the amount of neurofibromin to that of p120 GAP did not seem to be correlated with the extent of unequal allelic expression.

Skeletal growth factors regulate the synthesis of insulin-like growth factor binding protein-5 in bone cell cultures

Skeletal cells secrete insulin-like growth factors (IGFs) I and II and six known IGF binding proteins (IGFBPs). IGFBP-5 stimulates bone formation, and its synthesis correlates with changes in osteoblast cell growth. We tested the effects of basic fibroblast growth factor (bFGF), transforming growth factor beta 1 (TGF beta 1), and platelet-derived growth factor (PDGF) BB on IGFBP-5 expression in cultures of osteoblast-enriched cells from 22-day-old fetal rat calvariae (Ob cells). Treatment of Ob cells with bFGF, TGF beta 1, and PDGF BB caused a time- and dose-dependent decrease in IGFBP-5 mRNA levels and inhibited IGFBP-5 polypeptide levels in the extracellular matrix. The effects of bFGF, TGF beta 1, and PDGF BB on IGFBP-5 transcripts were independent of cell division and were observed in the presence and absence of hydroxyurea. bFGF, TGF beta 1, and PDGF BB did not modify the decay of IGFBP-5 mRNA in transcriptionally arrested Ob cells, and they inhibited IGFBP-5 heterogeneous nuclear RNA and the rate of IGFBP-5 transcription. In conclusion, bFGF, TGF beta 1, and PDGF BB inhibit IGFBP-5 expression in Ob cells independently of their mitogenic activity and through mechanisms that involve decreased transcription.

Insulin-like growth factor (IGF) I and retinoic acid induce the synthesis of IGF-binding protein 5 in rat osteoblastic cells

The insulin-like growth factor (IGF) regulatory system has a major impact on bone physiology. Among the modulators of IGFs, a family of structurally related proteins, the IGF-binding proteins (IGFBPs), have been shown to either potentiate or inhibit IGF actions on bone growth. However, the regulation of IGFBP expression in bone cells is not completely understood. In the present study, the expression of IGFBP-5 was analyzed in primary osteoblastic cells (Ob cells) isolated from 22-day-old fetal rat calvariae. Treatment of Ob cells with either IGF-I or all-trans-retinoic acid (RA) caused a time- and dose-dependent increase in IGFBP-5 messenger RNA (mRNA) levels, as determined by Northern blot analysis. Stimulation of IGFBP-5 mRNA was obtained at 100 nM IGF-I between 6 and 16 h (2- to 2.5-fold) and 100 nM RA between 16 and 24 h (3- to 4-fold). Concomitant treatment of Ob cells with IGF-I and RA revealed an additive effect and a 5- to 7-fold increase in IGFBP-5 mRNA levels after 16-24 h. The effect of IGF-I and RA and their combination on IGFBP-5 transcripts was similar in confluent and subconfluent cultures of Ob cells. IGF-I and RA did not change IGFBP-5 mRNA stability in Ob cells after transcription arrest with the RNA polymerase II inhibitor 5,6-dichloro-1-beta-D-ribofuranosyl benzimidazole. IGF-I and RA at 100 nM elevated the levels of IGFBP-5 heterogenous nuclear RNA, measured by reverse transcription-polymerase chain reaction. The effect was similar to that observed on mRNA levels. IGFBP-5 from rat Ob cells appeared as a single band of 31 kilodaltons in both the conditioned medium and the extracellular matrix as determined by Western immunoblots. IGF-I and RA, both at 100 nM, increased IGFBP-5 by 2- to 3-fold after 24 h. In conclusion, IGF-I and RA modify the synthesis and secretion of IGFBP-5 in rat Ob cells through pathways that may involve increased transcription and elongation and/or altered processing of heterogenous nuclear RNA. Our data suggest that IGFBP-5 may play a role in the osteoblastic-differentiated function regulated by IGF-I and RA.

Stress regulation of mineralocorticoid receptor heteronuclear RNA in rat hippocampus

Localization and regulation of mineralocorticoid receptor (MR) heteronuclear RNA (hnRNA) was assessed in rat hippocampus using an intron-directed in situ hybridization approach. The presence of hnRNA in the cell nucleus reflects recent gene transcriptional events and can be used as an index of neuronal transcriptional activation or inhibition. In the present study, sections incubated with an MR intron probe labeled cells in all regions known to contain MR mRNA. Signal generated by the intron probe was localized specifically to the nuclear compartment, consistent with recognition of hnRNA. Analysis of distribution across hippocampus indicated that MR hnRNA was particularly abundant in dentate gyrus (DG). In contrast, MR mRNA shows similar levels of expression across all hippocampal subfields. Exposure of animals to acute restraint markedly reduced MR hnRNA levels in CA1 and DG 60 min and 120 min following stress exposure, consistent with reduced gene transcription. These data support the hypothesis that stress-induced glucocorticoid secretion can rapidly affect transcription of steroid receptor genes. MR mRNA levels did not decrease over comparable time periods, suggesting either a significant lag-time between transcriptional changes and changes in cytoplasmic mRNA pools or stimulus-driven alterations in post-transcriptional RNA processing.

Epstein-Barr virus lytic replication is controlled by posttranscriptional negative regulation of BZLF1

Regulation of the immediate-early gene BZLF1 is assumed to play a key role in triggering the lytic replication of Epstein-Barr virus (EBV). The expression of BZLF1 is regulated on multiple levels, including control of transcription by several positive and negative cis-acting elements as well as posttranslational modifications and protein-protein interactions. Localization of BZLF1 on one strand of the genome and the latent EBNA1 transcription unit on the complementary strand suggests a regulatory mechanism via hybridization of antisense RNA. With a plasmid encoding a defective BZLF1 RNA, which could not be translated, we were able to induce expression of endogenous BZLF1 gene product Zta and other proteins of the lytic cycle. Our data show for the first time that latent replication is stabilized by negative regulation of an immediate-early gene of the lytic cycle by a posttranscriptional mechanism. This might be a common theme of herpes simplex virus and EBV latency.

Association of the vav proto-oncogene product with poly(rC)-specific RNA-binding proteins

We have used the yeast two-hybrid system to isolate proteins that interact with the carboxy-terminal SH3-SH2-SH3 region of Vav. One of the clones encoded heterogeneous nuclear ribonucleoprotein K (hnRNP K), a poly(rC)-specific RNA-binding protein. The interaction between Vav and hnRNP K involves the binding of the most carboxy-terminal SH3 domain of Vav to two proline-rich sequences present in the central region of hnRNP K. Overexpression of Vav in mouse fibroblasts leads to the formation of a stable complex with the endogenous hnRNP K and to the preferential redistribution of this protein to the cytoplasmic fraction. More importantly, Vav and hnRNP K proteins also interact in hematopoietic cells. In addition, Vav associates in vitro with a second 45-kDa poly(rC)-specific RNA-binding protein via its SH3-SH2-SH3 region. These results suggest that Vav plays a role in the regulation of the late steps of RNA biogenesis by modulating the function of poly(rC)-specific ribonucleoproteins.

The biogenesis of the coiled body during early mouse development

The coiled body is an ubiquitous nuclear organelle that contains essential components of the pre-mRNA splicing machinery as well as the nucleolar protein fibrillarin. Here we have studied the biogenesis of the coiled body in early mouse embryos. The results show that coiled bodies form and concentrate splicing snRNPs as early as in the maternal and paternal pronuclei of 1-cell embryos. This argues that the coiled body is likely to play a basic role in the nucleus of mammalian cells. In order to correlate the appearance of coiled bodies with the onset of transcriptional activity, embryos were incubated with brominated UTP and the incorporated nucleotide was visualized by fluorescence microscopy. In agreement with previous studies, transcriptional activity was first observed during the 2-cell stage. Thus, coiled bodies form before activation of embryonic gene expression. The appearance of coiled bodies in 1-cell embryos was preceded by the formation of morphologically distinct structures that also contain coilin and which we therefore refer to as pre-coiled bodies. At the electron microscopic level pre-coiled bodies have a compact fibrillar structure, whereas coiled bodies resemble a tangle of coiled threads. Although both pre-coiled bodies and coiled bodies contain the nucleolar protein fibrillarin, the assembly of coiled bodies is separated both in time and in space from ribosome synthesis. Our results suggest that the embryonic ‘nucleolus-like body’ is a structural scaffold that nucleates independently the formation of the coiled body and the assembly of the machinery responsible for ribosome biosynthesis.

Effect of enzymic methylation of heterogeneous ribonucleoprotein particle A1 on its nucleic-acid binding and controlled proteolysis

Recombinant unmethylated heterogeneous nuclear ribonucleoprotein particle (hnRNP) protein A1 was enzymatically methylated by nuclear protein/histone protein methylase I [Rajpurohit, Lee, Park, Paik and Kim (1994) J. Biol. Chem. 269, 1057-1082] and the effect of methylation on several physiocochemical properties was studied. The relative binding-affinity of methylated and unmethylated protein A1 to nucleic acid was quite different. This was observed by the elution behaviour of the protein A1 on a single-stranded DNA/cellulose column; the concentration of NaCl required to release the bound protein A1 was 0.59 M for the methylated and 0.63 M for the unmethylated, respectively. Employing isoelectrofocusing, pI values of the methylated and unmethylated proteins were found to be 9.41 and 9.48, respectively. Maximum fluorescence quenching of protein A1 in the presence of coliphage MS2-RNA was found to be 40% with methylated and 45% with unmethylated. When both species of protein A1 were subjected to controlled trypsin digestion, t1/2 of the methylated protein was 1.31 min and the unmethylated, 1.63 min. The difference in their t1/2 values was much greater in the presence of MS2-RNA; 2.4 min for the former and 4.3 min for the latter, indicating that the methylated species was less stabilized by the RNA than the unmethylated. All of the above results consistently suggested that the binding-property of hnRNP protein A1 to single-stranded nucleic acid was significantly reduced subsequent to its arginine-methylation. The biological significance of this observation is discussed.