Tbx5 navigates through the labyrinth of adult cardiac regeneration

Background

Heart failure is the major cause of death and morbidity in industrialized countries with an estimated 23 million people affected per year, representing 30% of all global deaths. Injury to the adult mammalian cardiac muscle, often leads to a heart attack due to irreversible loss of a large number of cardiomyocytes (CM) and other cardiac interstitial cells, creating an unmet need for identifying a cardiac progenitor cell (CPC) population for cardiac replenishment. In contrast, amphibians and neonatal rodents possess the ability to regenerate their heart upon injury. It has been suggested recently that idle cardiac regenerative mechanisms may be present in adult mammals, inhibited by exogenous cues, or lack of.

Murine and human CPCs can be isolated through the expression of Pdgfra, Kdr, and our novel surface marker, Gfra2. In addition, the expression of the embryonic transcription factor TBX5, is paramount for differentiation towards a cardiomyocyte fate. Therefore, Tbx5-expressing CPCs could be an effective target for proof-of-concept studies in the heart repair field, inclined to pharmacological modulation in patients with ischemic heart disease.

Purpose

To characterise an adult Tbx5-expressing CPC population in the injured heart. Using a developmental approach to two adult heart injury murine models, we show that Tbx5-expressing CPC exist in the injured adult mammalian heart, with a molecular signature that strongly correlates with that of embryonic and neonatal CPCs.

Methods

A well-defined tamoxifen-induced Tbx5-Cre; Rosa26R-eYFP/eYFP transgenic mouse model was employed, where myocardial infarction (MI) was induced through reperfusion/ischemia or chemical injury.

Cardiac cells expressing YFP+ cells were collected from the adult injured hearts five to seven days post-injury. Flow-cytometric and single-cell mRNA-seq analysis was performed in order to collect and compare those YFP+ cells to control adult uninjured cardiac interstitial cells, and early neonatal-derived CPCs.

Results

Immunohistochemical analysis indicated that YFP+ interstitial cells were mostly present in the injury site, but also in distal cardiac areas.

Flow cytometric analysis of live cells pinpoint these YFP+ cells are part of a CPC-like population.

Single-cell mRNA transcriptomic analysis revealed that YFP+ cells resemble early postnatal CPC.

Yet, YFP+ cells never expressed CM markers in vivo, but they did differentiate into CM-like cells, in vitro.

Conclusions

Upon MI, the adult heart possess an interstitial cell population that transiently re-activates the pioneer cardiac embryonic transcription factor Tbx5.

The Tbx5-expressing cell population transcriptomically resembles that of CPC, which could promote CM regeneration upon neonatal injury.

We show that Tbx5 lies in the centre of a novel adult CPC population. The adult heart microenvironment may hinder mammalian CM regeneration through regulation of the Tbx5-induced cardiac gene program.

Funding Acknowledgement

Type of funding sources: Public grant(s) – National budget only. Main funding source(s): Hellenic Foundation for Research & Innovation

Nucleosome sliding via TBP DNA binding in vivo

Here, we show that a nucleosome obstructing transcription from the IFN-beta promoter slides in vivo in response to virus infection, thus exposing the previously masked TATA box and the initiation site, a requirement for transcriptional activation. Our experiments also revealed that this mode of chromatin remodeling is a two-step reaction. First, the enhanceosome recruits the SWI/SNF chromatin-remodeling complex that modifies the nucleosome to allow binding of TBP. Second, DNA bending is induced by TBP binding, and the nucleosome slides to a new position. Experiments with other DNA binding proteins demonstrated a strong correlation between the ability to bend DNA and nucleosome sliding, suggesting that the sliding is induced by the bend.

Coordination of a transcriptional switch by HMGI(Y) acetylation

Dynamic control of interferon-beta (IFN-beta) gene expression requires the regulated assembly and disassembly of the enhanceosome, a higher-order nucleoprotein complex formed in response to virus infection. The enhanceosome activates transcription by recruiting the histone acetyltransferase proteins CREB binding protein (CBP) and p300/CBP-associated factors (PCAF)/GCN5, which, in addition to modifying histones, acetylate HMGI(Y), the architectural component required for enhanceosome assembly. We show that the accurate execution of the IFN-beta transcriptional switch depends on the ordered acetylation of the high-mobility group I protein HMGI(Y) by PCAF/GCN5 and CBP, which acetylate HMGI(Y) at distinct lysine residues on endogenous promoters. Whereas acetylation of HMGI(Y) by CBP at lysine-65 destabilizes the enhanceosome, acetylation of HMGI(Y) by PCAF/GCN5 at lysine-71 potentiates transcription by stabilizing the enhanceosome and preventing acetylation by CBP.

Overexpression of proteins HMGA1 induces cell cycle deregulation and apoptosis in normal rat thyroid cells

The high mobility group (HMG) proteins (HMGA1a, HMGA1b, and HMGA2) bind to DNA and interact with various transcriptional factors. Therefore, they play an important role in chromatin organization. HMGA protein expression is low in normal adult tissues, but abundant during embryonic development and in several experimental and human tumors. Blockage of HMGA expression inhibits the transformation of rat thyroid PC Cl 3 cells treated with oncogene-carrying retroviruses, thus implicating HMGA in rat thyroid transformation. To better understand the role of HMGA and to establish whether its up-regulated expression is sufficient to induce the transformed phenotype, we generated PC Cl 3 cells that overexpress the protein. We demonstrate that HMGA1b protein overexpression does not transform normal rat thyroid PC Cl 3 cells, but it deregulates their cell cycle: cells enter S-phase earlier and the G(2)-M transition is delayed. HMGA1-overexpressing cells undergo apoptosis through a pathway involving caspase-3 activation, probably consequent to the conflict between mitogenic pressure and the inability to proceed through the cell cycle. Using various HMGA1b gene mutations, we found that the third AT-hook domain and the acetylation site K60 are the protein regions required for induction of apoptosis in PC Cl 3 cells. In conclusion, although HMGA1 protein overexpression is associated with the malignant phenotype of rat and human thyroid cells, it does not transform normal thyroid cells in culture but leads them to programmed cell death.

Enhanceosomes

Gene-specific transcriptional regulation in higher eukaryotes is mediated by complex cis-acting control elements that specify the location, timing and magnitude of the response. During the past five years, an argument has been made that in several cases specificity in gene transcription is achieved by the assembly of higher-order three-dimensional transcription factor/enhancer DNA complexes, termed enhanceosomes. The inherent co-operativity in enhanceosome assembly and the embedded synergy in transcription ensure that a specific gene would be selected for activation only if all the enhanceosome components are present in the same nucleus. Enhanceosomes activate transcription by recruiting chromatin-modifying activities and basal transcription factors to the nearby promoters.

Ordered recruitment of chromatin modifying and general transcription factors to the IFN-beta promoter

Here, we show that the IFN-beta enhanceosome activates transcription by directing the ordered recruitment of chromatin modifying and general transcription factors to the IFN-beta promoter. The enhanceosome is assembled in the nucleosome-free enhancer region of the IFN-beta gene, leading to the modification and remodeling of a strategically positioned nucleosome that masks the TATA box and the start site of transcription. Initially, the GCN5 complex is recruited, which acetylates the nucleosome, and this is followed by recruitment of the CBP-PolII holoenzyme complex. Nucleosome acetylation in turn facilitates SWI/SNF recruitment by CBP, resulting in chromatin remodeling. This program of recruitment culminates in the binding of TFIID to the promoter and the activation of transcription.

The architectural transcription factor high mobility group I(Y) participates in photoreceptor-specific gene expression

The nonhistone chromosomal proteins high mobility group I(Y) [HMG I(Y)] have been shown to function as architectural transcription factors facilitating enhanceosome formation on a variety of mammalian promoters. Specifically, they have been shown to act as a "molecular glue" mediating protein-protein and protein-DNA contacts within the enhanceosome complex. HMG I(Y) proteins are expressed at high levels in embryonic and transformed cells and have been implicated in transcriptional regulation in these cells. Terminally differentiated cells, however, have been reported to express only minimal, if any, HMG I(Y). In contrast to these observations, we show here that adult mouse retinal photoreceptors, which are terminally differentiated cells, express high levels of these proteins. Using retinoblastoma cells as an approximate model, we further demonstrate in transiently transfected cells that inhibition of HMG I(Y) expression and mutation of HMG I(Y) binding sites significantly reduce rhodopsin promoter activity. DNase I footprint analysis indicates that HMG I protein interacts with a discrete site within the rhodopsin proximal promoter. This site overlaps with the binding site for Crx, a paired-like homeodomain transcription factor that is essential for photoreceptor functioning and that when mutated causes several forms of human photoreceptor degeneration. Both biochemical and functional experiments demonstrate that HMG I(Y) physically associate with Crx and that their interaction with DNA is required for high-level transcription of the rhodopsin gene. These data provide the first demonstration that HMG I(Y) can be important for gene activation in terminally differentiated cells.

Gene repression by coactivator repulsion

We show that the IRF-2 oncoprotein represses virus-induced IFN-beta gene transcription via a novel mechanism. Virus infection induces recruitment of IRF-2 to some of the endogenous IFN-beta enhancers as part of the enhanceosome. Enhanceosomes bearing IRF-2 cannot activate transcription, due to the presence of a domain in IRF-2 that prevents enhanceosome-dependent recruitment of the CBP-Pol II holoenzyme complex. As a consequence, IRF-2 incorporation into enhanceosomes restricts the number of IFN-beta promoters directing transcription. Remarkably, deletion of the IRF-2 gene increases IFN-beta expression by expanding the number of cells capable of inducing IFN-beta gene transcription in response to virus infection.

An ikaros-containing chromatin-remodeling complex in adult-type erythroid cells

We have previously described a SWI/SNF-related protein complex (PYR complex) that is restricted to definitive (adult-type) hematopoietic cells and that specifically binds DNA sequences containing long stretches of pyrimidines. Deletion of an intergenic DNA-binding site for this complex from a human beta-globin locus construct results in delayed human gamma- to beta-globin switching in transgenic mice, suggesting that the PYR complex acts to facilitate the switch. We now show that PYR complex DNA-binding activity also copurifies with subunits of a second type of chromatin-remodeling complex, nucleosome-remodeling deacetylase (NuRD), that has been shown to have both nucleosome-remodeling and histone deacetylase activities. Gel supershift assays using antibodies to the ATPase-helicase subunit of the NuRD complex, Mi-2 (CHD4), confirm that Mi-2 is a component of the PYR complex. In addition, we show that the hematopoietic cell-restricted zinc finger protein Ikaros copurifies with PYR complex DNA-binding activity and that antibodies to Ikaros also supershift the complex. We also show that NuRD and SWI/SNF components coimmunopurify with each other as well as with Ikaros. Competition gel shift experiments using partially purified PYR complex and recombinant Ikaros protein indicate that Ikaros functions as a DNA-binding subunit of the PYR complex. Our results suggest that Ikaros targets two types of chromatin-remodeling factors-activators (SWI/SNF) and repressors (NuRD)-in a single complex (PYR complex) to the beta-globin locus in adult erythroid cells. At the time of the switch from fetal to adult globin production, the PYR complex is assembled and may function to repress gamma-globin gene expression and facilitate gamma- to beta-globin switching.

Stimulus-specific assembly of enhancer complexes on the tumor necrosis factor alpha gene promoter

The human tumor necrosis factor alpha (TNF-alpha) gene is rapidly activated in response to multiple signals of stress and inflammation. We have identified transcription factors present in the TNF-alpha enhancer complex in vivo following ionophore stimulation (ATF-2/Jun and NFAT) and virus infection (ATF-2/Jun, NFAT, and Sp1), demonstrating a novel role for NFAT and Sp1 in virus induction of gene expression. We show that virus infection results in calcium flux and calcineurin-dependent NFAT dephosphorylation; however, relatively lower levels of NFAT are present in the nucleus following virus infection as compared to ionophore stimulation. Strikingly, Sp1 functionally synergizes with NFAT and ATF-2/c-jun in the activation of TNF-alpha gene transcription and selectively associates with the TNF-alpha promoter upon virus infection but not upon ionophore stimulation in vivo. We conclude that the specificity of TNF-alpha transcriptional activation is achieved through the assembly of stimulus-specific enhancer complexes and through synergistic interactions among the distinct activators within these enhancer complexes.

Mechanism by which the IFN-beta enhanceosome activates transcription

We demonstrate that in contrast to previous findings by using simple synthetic promoters or activators, the natural IFN-beta enhanceosome activates transcription by causing a dramatic increase of the rate by which preinitiation complexes assemble at the promoter. This effect totally depends on the recruitment of the CBP-PolII holoenzyme by the enhanceosome, because its depletion from the extract decelerates the rate of transcription. However, addition of the CBP-PolII holoenzyme back to these extracts fully restores the speed by which the enhanceosome activates transcription. Strikingly, preincubation of the enhanceosome with the CBP-RNA PolII holoenzyme complex results in instant assembly of preinitiation complexes. In contrast, individual IFN-beta gene activators function solely by increasing the number of functional preinitiation complexes and not the rate of their assembly. Thus, fast recruitment of the CBP-RNA PolII holoenzyme complex is critical for the rapid activation of IFN-beta gene expression by virus infection.

Transcriptional activation by NF-kappaB requires multiple coactivators

Nuclear factor-kappaB (NF-kappaB) plays a role in the transcriptional regulation of genes involved in inflammation and cell survival. In this report we demonstrate that NF-kappaB recruits a coactivator complex that has striking similarities to that recruited by nuclear receptors. Inactivation of either cyclic AMP response element binding protein (CREB)-binding protein (CBP), members of the p160 family of coactivators, or the CBP-associated factor (p/CAF) by nuclear antibody microinjection prevents NF-kappaB-dependent transactivation. Like nuclear receptor-dependent gene expression, NF-kappaB-dependent gene expression requires specific LXXLL motifs in one of the p160 family members, and enhancement of NF-kappaB activity requires the histone acetyltransferase (HAT) activity of p/CAF but not that of CBP. This coactivator complex is differentially recruited by members of the Rel family. The p50 homodimer fails to recruit coactivators, although the p50-p65 heterodimeric form of the transcription factor assembles the integrator complex. These findings provide new mechanistic insights into how this family of dimeric transcription factors has a differential effect on gene expression.

The role of HMG I(Y) in the assembly and function of the IFN-beta enhanceosome

Transcriptional activation of the virus inducible enhancer of the human interferon-beta (IFN-beta) gene in response to virus infection requires the assembly of an enhanceosome, consisting of the transcriptional activators NF-kappaB, ATF-2/c-Jun, IRFs and the architectural protein of the mammalian high mobility group I(Y) [HMG I(Y)]. Here, we demonstrate that the first step in enhanceosome assembly, i.e. HMG I(Y)-dependent recruitment of NF-kappaB and ATF-2/c-Jun to the enhancer, is facilitated by discrete regions of HMG I and is mediated by allosteric changes induced in the DNA by HMG I(Y) and not by protein-protein interactions between HMG I(Y) and these proteins. However, we show that completion of the enhanceosome assembly process requires protein-protein interactions between HMG I(Y) and the activators. Finally, we demonstrate that once assembled, the IFN-beta enhanceosome is an unusually stable nucleoprotein structure that can activate transcription at high levels by promoting multiple rounds of reinitiation of transcription.

Mechanisms by which IkappaB proteins control NF-kappaB activity

The biological activity of the transcription factor NF-kappaB is differentially controlled by three IkappaB proteins, Ikappa Balpha, Ikappa Bbeta, and Ikappa Bepsilon. We have examined the molecular basis for the differential inhibitory strengths of IkappaB proteins by constructing hybrid IkappaBs and found that the first ankyrin repeat of Ikappa Balpha is responsible for its strong inhibitory effect. Swapping a putative beta-turn within the first ankyrin repeat between the strong Ikappa Balpha and the weak IkappaBbeta inhibitors switches their in vivo inhibitory activity on NF-kappaB. The qualitatively distinct contacts made by this beta-turn in Ikappa Balpha and Ikappa Bbeta with NF-kappaB determine the efficiency by which IkappaBs sequester NF-kappaB to the cytoplasm, thus explaining their distinct effects on gene activity.

Nuclear integration of glucocorticoid receptor and nuclear factor-kappaB signaling by CREB-binding protein and steroid receptor coactivator-1

The p65 (RelA) component of nuclear factor-kappaB (NF-kappaB) and the glucocorticoid receptor (GR) mutually repress each other's ability to activate transcription. Both of these transcriptional activators depend upon the coactivators CREB-binding protein (CBP) and steroid receptor coactivator-1 (SRC-1) for maximal activity. Here we show that increased levels of CBP relieves the inhibition of glucocorticoid-mediated repression of NF-kappaB activity and the NF-kappaB-mediated repression of GR activity. SRC-1 can relieve the NF-kappaB-mediated repression of GR activity. We propose that cross-talk between the p65 component of NF-kappaB and glucocorticoid receptors is due, at least in part, to nuclear competition for limiting amounts of the coactivators CBP and SRC-1, thus providing a novel mechanism for decreasing expression of genes involved in the inflammatory response.

High mobility group I/Y protein functions as a specific cofactor for Oct-2A: mapping of interaction domains

The octamer motif (ATTTGCAT) present in several eukaryotic promoters and enhancers is now known to influence the transcription of several genes by interacting with members of a broad family of homeodomain proteins. The promoter of the human class II MHC gene HLA-DRA contains a conserved octamer element that can bind (among other proteins) the transcription factor Oct-2A and the high mobility group proteins (HMG) I/Y. We have previously determined that HMG I(Y) and Oct-2A cooperatively activate HLA-DRA gene expression, most likely due to the ability of HMG I(Y) to selectively recruit Oct-2A to the octamer motif. In this report, we present results of our investigations of the mechanisms of cooperative transactivation of HLA-DRA transcription by Oct-2A and HMG I(Y). We show that both the amino- and the carboxy-terminal domains of Oct-2A are required for HLA-DRA transactivation. Experiments using domain-swap chimeras of the Oct-1 and Oct-2A polypeptides indicate that cooperative activation of the DRA gene by HMG I(Y) and Oct-2A requires the carboxy-terminal domain (CTD) of Oct-2A. However, HMG I(Y) physically interacts with the conserved POU domains of both Oct-1 and Oct-2A. We therefore postulate that the nature of the CTD attached to the POU homeodomain influences the outcome of interaction with HMG I(Y). These studies support the view that HMG I(Y) is an important cofactor for HLA-DRA gene activation by Oct-2A and provide insights into its mechanism of action.

Acetylation of HMG I(Y) by CBP turns off IFN beta expression by disrupting the enhanceosome

The transcriptional coactivators CBP and P/CAF are required for activation of transcription from the IFN beta enhanceosome. We show that CBP and P/CAF acetylate HMG I(Y), the essential architectural component required for enhanceosome assembly, at distinct lysine residues, causing distinct effects on transcription. Thus, in the context of the enhanceosome, acetylation of HMG I by CBP, but not by P/CAF, leads to enhanceosome destabilization and disassembly. We demonstrate that acetylation of HMG I(Y) by CBP is essential for turning off IFN beta gene expression. Finally, we show that the acetyltransferase activities of CBP and P/CAF modulate both the strength of the transcriptional response and the kinetics of virus-dependent activation of the IFN beta gene.

Recruitment of CBP/p300 by the IFN beta enhanceosome is required for synergistic activation of transcription

Transcriptional activation of the IFN beta gene in response to virus infection requires the assembly of an enhanceosome, consisting of the transcriptional activators NF-kappa B, IRF1, ATF2/c-Jun, and the architectural protein HMG I(Y). The level of transcription generated by all of these activators is greater than the sum of the levels generated by individual factors, a phenomenon designated transcriptional synergy. We demonstrate that this synergy, in the context of the enhanceosome, requires a new protein-protein interaction domain in the p65 subunit of NF-kappa B. Transcriptional synergy requires recruitment of the CBP/p300 coactivator to the enhanceosome, via a new activating surface assembled from the novel p65 domain and the activation domains of all of the activators. Deletion, substitution, or rearrangement of any one of the activation domains in the context of the enhanceosome decreases both recruitment of CBP and transcriptional synergy.

Structure of IRF-1 with bound DNA reveals determinants of interferon regulation

The family of interferon regulatory factor (IRF) transcription factors is important in the regulation of interferons in response to infection by virus and in the regulation of interferon-inducible genes. The IRF family is characterized by a unique 'tryptophan cluster' DNA-binding region. Here we report the crystal structure of the IRF-1 region bound to the natural positive regulatory domain I (PRD I) DNA element from the interferon-beta promoter. The structure provides the first three-dimensional view of a member of the growing IRF family, revealing a new helix-turn-helix motif that latches onto DNA through three of the five conserved tryptophans. The motif selects a short GAAA core sequence through an obliquely angled recognition helix, with an accompanying bending of the DNA axis in the direction of the protein. Together, these features suggest a basis for the occurrence of GAAA repeats within IRF response elements and provide clues to the assembly of the higher-order interferon-beta enhancesome.

Cloning and functional characterization of mouse IkappaBepsilon

The biological activity of the transcription factor NF-kappaB is mainly controlled by the IkappaB proteins IkappaBalpha and IkappaBbeta, which restrict NF-kappaB in the cytoplasm and enter the nucleus where they terminate NF-kappaB-dependent transcription. In this paper we describe the cloning and functional characterization of mouse IkappaBepsilon. Mouse IkappaBepsilon contains 6 ankyrin repeats required for its interaction with the Rel proteins and is expressed in different cell types where we found that it is up-regulated by NF-kappaB inducers, as is the case for IkappaBalpha and human IkappaBepsilon. IkappaBepsilon functions as a bona fide IkappaB protein by restricting Rel proteins in the cytoplasm and inhibiting their in vitro DNA binding activity. Surprisingly, IkappaBepsilon did not inhibit transcription of genes regulated by the p50/p65 heterodimer efficiently, such as the human interferon-beta gene. However, IkappaBepsilon was a strong inhibitor of interleukin-8 expression, a gene known to be regulated by p65 homodimers. In addition, IkappaBepsilon appears to function predominantly in the cytoplasm to sequester p65 homodimers, in contrast with the other two members of the family, IkappaBalpha and IkappaBbeta, which also function in the nucleus to terminate NF-kappaB-dependent transcriptional activation.

cDNA cloning of the translocon associated protein beta-subunit in the chick cerebellum

The 'translocon associated protein' is a tetrameric complex residing in the translocation sites, in which nascent polypeptides pass through the endoplasmic reticulum membrane. The beta subunit of this complex is a single spanning membrane protein, as deduced from cDNAs deriving from canine or human epithelial tissues. We have isolated and analysed a cDNA clone of the beta subunit from chick nervous tissue, namely cerebellum. Its deduced protein sequence is 91% homologous to both the canine and the human protein sequences, showing that the molecule is highly conserved.

Expression, purification, and co-crystallization of IRF-I bound to the interferon-beta element PRDI

Interferon regulatory factor 1 (IRF-1) is an essential factor involved in the regulation of type I interferon (IFN) and IFN-inducible genes. The protein consists of 329 amino acids that are highly conserved from mouse to human. Similar to other transcription factors, the protein is modular in nature with a basic N-terminal region involved in DNA binding and an acidic C-terminal region required for activation. We report here the expression, purification and co-crystallization of the minimal N-terminal region of IRF-1 involved in DNA binding (amino acids 1-113) with a 13 bp DNA fragment from the IFN-beta promoter. The crystals diffract to at least 3.0 A in resolution and belong to space group R3 with unit cell parameters of a = b = 84.8 A, c = 203.7 A.

Distinct functional properties of IkappaB alpha and IkappaB beta

The biological activity of the transcription factor NF-kappaB is controlled mainly by the IkappaB alpha and IkappaB beta proteins, which restrict NF-kappaB to the cytoplasm and inhibit its DNA binding activity. Here, we carried out experiments to determine and compare the mechanisms by which IkappaB alpha and IkappaB beta inhibit NF-kappaB-dependent transcriptional activation. First, we found that in vivo IkappaB alpha is a stronger inhibitor of NF-kappaB than is IkappaB beta. This difference is directly correlated with their abilities to inhibit NF-kappaB binding to DNA in vitro and in vivo. Moreover, IkappaB alpha, but not IkappaB beta, can remove NF-kappaB from functional preinitiation complexes in in vitro transcription experiments. Second, we showed that both IkappaBs function in vivo not only in the cytoplasm but also in the nucleus, where they inhibit NF-kappaB binding to DNA. Third, the inhibitory activity of IkappaB beta, but not that of IkappaB alpha, is facilitated by phosphorylation of the C-terminal PEST sequence by casein kinase II and/or by the interaction of NF-kappaB with high-mobility group protein I (HMG I) on selected promoters. The unphosphorylated form of IkappaB beta forms stable ternary complexes with NF-kappaB on the DNA either in vitro or in vivo. These experiments suggest that IkappaB alpha works as a postinduction repressor of NF-kappaB independently of HMG I, whereas IkappaB beta functions preferentially in promoters regulated by the NF-kappaB/HMG I complexes.

An NF-kappaB-like transcription factor in axoplasm is rapidly inactivated after nerve injury in Aplysia

We found a protein in Aplysia neurons that has many characteristics of the transcription factor NF-kappaB. Thus, the protein recognized a radiolabeled probe containing the kappaB sequence from the human interferon-beta gene enhancer element (PRDII), and the binding was not affected by PRDIV, an ATF-2 enhancer sequence from the same gene. Binding was efficiently inhibited, however, by nonradioactive oligonucleotides containing H2, the kappaB site from the major histocompatibility complex I gene promotor. In addition, recombinant mammalian IkappaB-alpha, which associates specifically with the P65 subunit of NF-kappaB, inhibited the binding to the PRDII probe in a dose-dependent manner. The nuclear form of the Aplysia protein was constitutively active. Axoplasm, however, contained the constitutively active form as well as a latent form. The latter was activated by treatment with deoxycholate under the same conditions as mammalian NF-kappaB. Based on these findings, we believe the protein to be a homolog of NF-kappaB. To investigate the role of apNF-kappaB in the axon, we crushed the peripheral nerves to the body wall. Surprisingly, there was a rapid loss of apNF-kappaB binding at the crush site and, within 15 min, as far as 2.5 cm along the axon. In contrast, exposing either the intact animal or the nervous system in situ to levels of 5-HT that induce synaptic facilitation did not affect apNF-kappaB activity.

Intra- and intermolecular cooperative binding of high-mobility-group protein I(Y) to the beta-interferon promoter

The mammalian high-mobility-group protein I(Y) [HMG I(Y)], while not a typical transcriptional activator, is required for the expression of many eukaryotic genes. HMG I(Y) appears to recruit and stabilize complexes of transcriptional activators through protein-DNA and protein-protein interactions. The protein binds to the minor groove of DNA via three short basic repeats, preferring tracts of adenines and thymines arranged on the same face of the DNA helix. However, the mode by which these three basic repeats function together to recognize HMG I(Y) binding sites has remained unclear. Here, using deletion mutants of HMG I(Y), DNase I footprinting, methylation interference, and in vivo transcriptional assays, we have characterized the binding of HMG I(Y) to the model beta-interferon enhancer. We show that two molecules of HMG I(Y) bind to the enhancer in a highly cooperative fashion, each molecule using a distinct pair of basic repeats to recognize the tandem AT-rich regions of the binding sites. We have also characterized the function of each basic repeat, showing that only the central repeat accounts for specific DNA binding and that the presence of a second repeat bound to an adjacent AT-rich region results in intramolecular cooperativity in binding. Surprisingly, the carboxyl-terminal acidic tail of HMG I(Y) is also important for specific binding in the context of the full-length protein. Our results present a detailed examination of HMG I(Y) binding in an important biological context, which can be extended not only to HMG I(Y) binding in other systems but also to the binding mode of many other proteins containing homologous basic repeats, which have been conserved from bacteria to humans.

Cell-type-specific regulation of the human tumor necrosis factor alpha gene in B cells and T cells by NFATp and ATF-2/JUN

The human tumor necrosis factor alpha (TNF-alpha) gene is one of the earliest genes transcribed after the stimulation of a B cell through its antigen receptor or via the CD-40 pathway. In both cases, induction of TNF-alpha gene transcription can be blocked by the immunosuppressants cyclosporin A and FK506, which suggested a role for the NFAT family of proteins in the regulation of the gene in B cells. Furthermore, in T cells, two molecules of NFATp bind to the TNF-alpha promoter element kappa 3 in association with ATF-2 and Jun proteins bound to an immediately adjacent cyclic AMP response element (CRE) site. Here, using the murine B-cell lymphoma cell line A20, we show that the TNF-alpha gene is regulated in a cell-type-specific manner. In A20 B cells, the TNF-alpha gene is not regulated by NFATp bound to the kappa 3 element. Instead, ATF-2 and Jun proteins bind to the composite kappa 3/CRE site and NFATp binds to a newly identified second NFAT site centered at -76 nucleotides relative to the TNF-alpha transcription start site. This new site plays a critical role in the calcium-mediated, cyclosporin A-sensitive induction of TNF-alpha in both A20 B cells and Ar-5 cells. Consistent with these results, quantitative DNase footprinting of the TNF-alpha promoter using increasing amounts of recombinant NFATp demonstrated that the -76 site binds to NFATp with a higher affinity than the kappa 3 site. Two other previously unrecognized NFATp-binding sites in the proximal TNF-alpha promoter were also identified by this analysis. Thus, through the differential use of the same promoter element, the composite kappa 3/CRE site, the TNF-alpha gene is regulated in a cell-type-specific manner in response to the same extracellular signal.

Mechanisms of transcriptional synergism of eukaryotic genes. The interferon-beta paradigm

The virus-inducible enhancer of the human interferon-beta gene has served as an excellent example for the mechanisms controlling the activation and repression of transcription. This enhancer is activated by three different transcription factors that, with the help of the high mobility group protein HMG I(Y), assemble in a unique nucleoprotein complex that interacts as a unit with the basal transcriptional machinery. The assembly of unique enhancer complexes from similar sets of transcription factors may provide the specificity required for regulation of complex patterns of gene expression in higher eukaryotes.

Reversal of intrinsic DNA bends in the IFN beta gene enhancer by transcription factors and the architectural protein HMG I(Y)

In this paper, we investigate DNA bending induced by proteins required for virus induction of the human interferon-beta (IFN beta) gene. We show that NF-kappa B-DNA complexes that are functionally distinct in the context of the IFN beta enhancer are also conformationally distinct and that two sites in the enhancer contain in-phase bends that are counteracted or reversed by the binding of NF-kappa B, ATF-2/c-Jun, and HMG I(Y). Strikingly, this modulation of intrinsic enhancer architecture results in an orientation that favors predicted protein-protein interactions in a functional nucleoprotein complex, the enhanceosome. Furthermore, the subtle modulation of DNA structure by HMG I(Y) in this process distinguishes it from other architectural factors.

Virus induction of human IFN beta gene expression requires the assembly of an enhanceosome

We present evidence that transcriptional activation of the human interferon-beta (IFN beta) gene requires the assembly of a higher order transcription enhancer complex (enhanceosome). This multicomponent complex includes at least three distinct transcription factors and the high mobility group protein HMG I(Y). Both the in vitro assembly and in vivo transcriptional activity of this complex require a precise helical relationship between individual transcription factor-binding sites. In addition, HMG I(Y), which binds specifically to three sites within the enhancer, promotes cooperative binding of transcriptional factors in vitro and is required for transcriptional synergy between these factors in vivo. Thus, HMG I(Y) plays an essential role in the assembly and function of the IFN beta gene enhanceosome.

Inhibition of NF-AT-dependent transcription by NF-kappa B: implications for differential gene expression in T helper cell subsets

Activation of individual CD4+ T cells results in differential lymphokine expression: interleukin 2 (IL-2) is preferentially produced by T helper type 1 (TH1) cells, which are involved in cell-mediated immune responses, whereas IL-4 is synthesized by TH2 cells, which are essential for humoral immunity. The Ca(2+)-dependent factor NF-ATp plays a key role in the inducible transcription of both these lymphokine genes. However, while IL2 expression requires the contribution of Ca(2+)- and protein kinase C-dependent signals, we report that activation of human IL4 transcription through the Ca(2+)-dependent pathway is diminished by protein kinase C stimulation in Jurkat T cells. This phenomenon is due to mutually exclusive binding of NF-ATp and NF-kappa B to the P sequence, an element located 69 bp upstream of the IL4 transcription initiation site. Human IL4 promoter-mediated transcription is downregulated in Jurkat cells stimulated with the NF-kappa B-activating cytokine tumor necrosis factor alpha and suppressed in RelA-overexpressing cells. In contrast, protein kinase C stimulation or RelA overexpression does not affect the activity of a human IL4 promoter containing a mouse P sequence, which is a higher-affinity site for NF-ATp and a lower-affinity site for RelA. Thus, competition between two general transcriptional activators, RelA and NF-ATp, mediates the inhibitory effect of protein kinase C stimulation on IL4 expression and may contribute to differential gene expression in TH cells.

Functional roles of the transcription factor Oct-2A and the high mobility group protein I/Y in HLA-DRA gene expression

The class II major histocompatibility complex gene HLA-DRA is expressed in B cells, activated T lymphocytes, and in antigen-presenting cells. In addition, HLA-DRA gene expression is inducible in a variety of cell types by interferon-gamma (IFN-gamma). Here we show that the lymphoid-specific transcription factor Oct-2A plays a critical role in HLA-DRA gene expression in class II-positive B cell lines, and that the high mobility group protein (HMG) I/Y binds to multiple sites within the DRA promoter, including the Oct-2A binding site. Coexpression of HMG I/Y and Oct-2 in cell lines lacking Oct-2 results in high levels of HLA-DRA gene expression, and in vitro DNA-binding studies reveal that HMG I/Y stimulates Oct-2A binding to the HLA-DRA promoter. Thus, Oct-2A and HMG I/Y may synergize to activate HLA-DRA expression in B cells. By contrast, Oct-2A is not involved in the IFN-gamma induction of the HLA-DRA gene in HeLa cells, but antisense HMG I/Y dramatically decreases the level of induction. We conclude that distinct sets of transcription factors are involved in the two modes of HLA-DRA expression, and that HMG I/Y may be important for B cell-specific expression, and is essential for IFN-gamma induction.

Transcriptional regulation of endothelial cell adhesion molecules: NF-kappa B and cytokine-inducible enhancers

Transcription of endothelial-leukocyte adhesion molecule-1 (E-selectin or ELAM-1), vascular cell adhesion molecule-1 (VCAM-1), and intercellular adhesion molecule-1 (ICAM-1) is induced by the inflammatory cytokines interleukin-1 beta (IL-1 beta) and tumor necrosis factor-alpha (TNF alpha). The positive regulatory domains required for maximal levels of cytokine induction have been defined in the promoters of all three genes. DNA binding studies reveal a requirement for nuclear factor-kappa B (NF-kappa B) and a small group of other transcriptional activators. The organization of the cytokine-inducible element in the E-selectin promoter is remarkably similar to that of the virus-inducible promoter of the human interferon-beta gene in that both promoters require NF-kappa B, activating transcription factor-2 (ATF-2), and high mobility group protein I(Y) for induction. Based on this structural similarity, a model has been proposed for the cytokine-induced E-selectin enhancer that is similar to the stereospecific complex proposed for the interferon-beta gene promoter. In these models, multiple DNA bending proteins facilitate the assembly of higher order complexes of transcriptional activators that interact as a unit with the basal transcriptional machinery. The assembly of unique enhancer complexes from similar sets of transcriptional factors may provide the specificity required to regulate complex patterns of gene expression and correlate with the distinct patterns of expression of the leukocyte adhesion molecules.

Endothelial interferon regulatory factor 1 cooperates with NF-kappa B as a transcriptional activator of vascular cell adhesion molecule 1

Transcription of the vascular cell adhesion molecule 1 (VCAM-1) gene in endothelial cells is induced by lipopolysaccharide and the inflammatory cytokines interleukin-1 beta and tumor necrosis factor alpha (TNF-alpha). Previous studies have demonstrated that tandem binding sites for the inducible transcription factor NF-kappa B are necessary but not sufficient for full cytokine-mediated transcriptional activation. Herein, we demonstrate that full cytokine-induced accumulation of VCAM1 transcript requires protein synthesis. We report the definition of a functional regulatory element in the VCAM1 promoter interacting with the transcriptional activator interferon regulatory factor 1 (IRF-1). DNA-protein binding studies with endothelial nuclear extracts revealed that IRF-1 is cytokine inducible and binds specifically to a consensus sequence motif located 3' of the TATA element. We have identified heterodimeric p65 and p50 as the NF-kappa B species binding to the VCAM1 promoter in TNF-alpha-activated endothelial cells. Experiments with recombinant proteins showed that p50/p65 and high-mobility-group I(Y) protein cooperatively facilitated the binding of IRF-1 to the VCAM1 IRF binding site and that IRF-1 physically interacted with p50 and with high-mobility-group I(Y) protein. Transient transfection assay in endothelial cells showed that overexpressed IRF-1 resulted in superinduction of TNF-alpha-stimulated transcription. Site-directed mutations in the IRF binding element decreased TNF-alpha-induced activity and totally abolished superinduction. Cotransfection assays in P19 embryonal carcinoma cells revealed that IRF-1 synergized with p50/p65 NF-kappa B to activate the VCAM1 promoter or heterologous promoter constructs bearing isolated VCAM1 NF-kappa B and IRF binding motifs. Cytokine inducibility of VCAM1 in endothelial cells utilizes the interaction of heterodimeric p50/p65 proteins with IRF-1.

Structure of the NF-kappa B transcription factor: a holistic interaction with DNA

An unexpected mode of binding to DNA is revealed in two crystal structures of a transcription factor that is essential for many signalling pathways in eukaryotic cells.

Identification of the rel family members required for virus induction of the human beta interferon gene

We have carried out experiments to determine which members of the rel family of transcription factors are involved in virus induction of the beta interferon (IFN-beta) gene. First, we examined the inducibility of artificial DNA binding sites that preferentially interact with different homo- or heterodimeric combinations of rel proteins in vitro. We found that only those sites capable of binding the p50/p65 heterodimer are virus inducible. Second, we analyzed a series of mutant rel DNA-binding sites in the context of the intact IFN-beta promoter. We found a correlation between (i) sites capable of binding both the p50/p65 heterodimer and the high-mobility-group protein HMG I(Y) and (ii) virus inducibility. Third, cotransfection of the IFN-beta gene enhancer/promoter with plasmids capable of expressing several different rel proteins revealed that only the combination of p50 and p65 efficiently activated transcription. Finally, we have used antibodies directed against different rel proteins to show that virus-inducible protein-DNA complexes assembled on the IFN-beta enhancer in vitro contain both p50 and p65. We conclude that the p50/p65 heterodimer is responsible for the NF-kappa B-dependent activation of the IFN-beta gene promoter in response to virus infection.

A novel cysteine-rich sequence-specific DNA-binding protein interacts with the conserved X-box motif of the human major histocompatibility complex class II genes via a repeated Cys-His domain and functions as a transcriptional repressor

The class II major histocompatibility complex (MHC) molecules function in the presentation of processed peptides to helper T cells. As most mammalian cells can endocytose and process foreign antigen, the critical determinant of an antigen-presenting cell is its ability to express class II MHC molecules. Expression of these molecules is usually restricted to cells of the immune system and dysregulated expression is hypothesized to contribute to the pathogenesis of a severe combined immunodeficiency syndrome and certain autoimmune diseases. Human complementary DNA clones encoding a newly identified, cysteine-rich transcription factor, NF-X1, which binds to the conserved X-box motif of class II MHC genes, were obtained, and the primary amino acid sequence deduced. The major open reading frame encodes a polypeptide of 1,104 amino acids with a symmetrical organization. A central cysteine-rich portion encodes the DNA-binding domain, and is subdivided into seven repeated motifs. This motif is similar to but distinct from the LIM domain and the RING finger family, and is reminiscent of known metal-binding regions. The unique arrangement of cysteines indicates that the consensus sequence CX3CXL-XCGX1-5HXCX3CHXGXC represents a novel cysteine-rich motif. Two lines of evidence indicate that the polypeptide encodes a potent and biologically relevant repressor of HLA-DRA transcription: (a) overexpression of NF-X1 from a retroviral construct strongly decreases transcription from the HLA-DRA promoter; and (b) the NF-X1 transcript is markedly induced late after induction with interferon gamma (IFN-gamma), coinciding with postinduction attenuation of HLA-DRA transcription. The NF-X1 protein may therefore play an important role in regulating the duration of an inflammatory response by limiting the period in which class II MHC molecules are induced by IFN-gamma.

A striking similarity in the organization of the E-selectin and beta interferon gene promoters

Transcription of the endothelial leukocyte adhesion molecule 1 (E-selectin or ELAM-1) gene is induced by the inflammatory cytokines interleukin-1 beta and tumor necrosis factor alpha (TNF-alpha). In this report, we identify four positive regulatory domains (PDI to PDIV) in the E-selectin promoter that are required for maximal levels of TNF-alpha induction in endothelial cells. In vitro DNA binding studies reveal that two of the domains contain novel adjacent binding sites for the transcription factor NF-kappa B (PDIII and PDIV), a third corresponds to a recently described CRE/ATF site (PDII), and a fourth is a consensus NF-kappa B site (PDI). Mutations that decrease the binding of NF-kappa B to any one of the NF-kappa B binding sites in vitro abolished cytokine-induced E-selectin gene expression in vivo. Previous studies demonstrated a similar correlation between ATF binding to PDII and E-selectin gene expression. Here we show that the high-mobility-group protein I(Y) [HMG I(Y)] also binds specifically to the E-selectin promoter and thereby enhances the binding of both ATF-2 and NF-kappa B to the E-selectin promoter in vitro. Moreover, mutations that interfere with HMG I(Y) binding decrease the level of cytokine-induced E-selectin expression. The organization of the TNF-alpha-inducible element of the E-selectin promoter is remarkably similar to that of the virus-inducible promoter of the human beta interferon gene in that both promoters require NF-kappa B, ATF-2, and HMG I(Y). We propose that HMG I(Y) functions as a key architectural component in the assembly of inducible transcription activation complexes on both promoters.

An HMG-like protein that can switch a transcriptional activator to a repressor

One protein can activate some genes and repress others in the same cell. The Drosophila protein Dorsal (which, like the human protein NF-kappa B3, is a member of the Rel family of transcriptional activators) activates the twist gene and represses the zen gene in the ventral region of early embryos. Here we describe a Drosophila HMG1 protein, called DSP1 (dorsal switch protein), that converts Dorsal and NF-kappa B from transcriptional activators to repressors. This effect requires a sequence termed a negative regulatory element (NRE), found adjacent to Dorsal-binding sites in the zen promoter and adjacent to the NF-kappa B-binding site in the human interferon-beta (IFN-beta) enhancer. Previous studies have shown that another type of HMG protein, HMG I(Y), can stimulate NF-kappa B activity. Thus, the HMG-like proteins DSP1 and HMG I(Y) can determine whether a specific regulator functions as an activator or a repressor of transcription.

The MHC class II E beta promoter: a complex arrangement of positive and negative elements determines B cell and interferon-gamma (IFN-gamma) regulated expression

The 5' proximal region of the E beta gene was studied with respect to B lymphoid expression and responsiveness to cytokines, revealing a complex array of general and cell type specific cis-elements and factors. Full lymphoid activity and response to interferon-gamma (IFN-gamma) is generated by the concerted action of the MHC boxes (H, X and Y) and additional elements. Combinatorial interactions between elements and their cognate factors are indicated by several lines of evidence. Thus, mutations within the X box in the promoter context are strongly deleterious to both B lymphoid activity and IFN-gamma regulation. However, the X box alone has minimal lymphoid activity upon heterologous promoters. Data from deletion, insertion and site directed mutagenesis demonstrate that sequences extending approximately 35 bp 5' of the X box (designated as Cytokine Response Sequence--CRS) have a dual role: they are required for cytokine-regulated expression as well as serving as an enhancer element for cell-specific constitutive expression. A region that carries X and CRS permits both lymphoid activity and IFN-gamma response. In contrast, sequences that include X and the downstream Y box are constitutively active in all cell types tested. Combination of the sequences both upstream and downstream of the X box results in a tissue-specific and cytokine-regulated enhancer of full strength. In vivo competition studies show that titratable trans-acting factors, shared by Class I and Class II promoters, mediate the CRS-dependent IFN-gamma response. We report here the identification of novel nuclear complexes that bind to the CRS and recognize sites which correlate with its negative or positive elements. One of these complexes is present in B lymphoid cells only. Three other CRS complexes that are upregulated by either IFN-alpha and IFN-gamma are competed by a non-Class II, IFN-alpha stimulated response element (ISRE), providing evidence for the functional interconnection of these cytokines.

Mechanisms of transcriptional synergism between distinct virus-inducible enhancer elements

The high mobility group protein HMG I(Y) and the transcription factor NF-kappa B are required for the activity of positive regulatory domain II (PRDII), a virus-inducible regulatory element of the human interferon-beta gene promoter. In this paper we provide evidence that HMG I(Y) is also required for the activity of PRDIV, a regulatory element that synergizes with PRDII. In this case, HMG I(Y) stimulates binding of activating transcription factor 2 (ATF-2) and the assembly of inducible complexes containing ATF-2 and c-Jun. Remarkably, HMG I(Y) also specifically interacts with the leucine zipper/basic region of ATF-2, and ATF-2 in turn interacts with NF-kappa B. We therefore propose that the HMG I(Y) plays a critical structural role in establishing transcriptional synergy between PRDII and PRDIV by promoting the activities and/or binding of NF-kappa B and ATF-2 and by facilitating their interaction.

The high mobility group protein HMG I(Y) is required for NF-kappa B-dependent virus induction of the human IFN-beta gene

In this paper, we show that both NF-kappa B and the high mobility group protein I(Y) (HMG I(Y)) are required for virus induction of the human interferon-beta (IFN-beta) gene. NF-kappa B binds to the terminal regions of a 10 bp regulatory sequence through contacts in the major groove. while HMG I(Y) recognizes the central region of the same sequence through contacts in the minor groove. Mutations that interfere with binding of either protein decrease the level of virus induction, and activation of the gene can be blocked by either NF-kappa B or HMG I(Y) antisense RNA. HMG I(Y) stimulates the binding of NF-kappa B to the IFN-beta promoter, and it may also function as a promoter-specific accessory factor for NF-kappa B transcriptional activity.

Prothymosin alpha enhances human and murine MHC class II surface antigen expression and messenger RNA accumulation

Prothymosin alpha (ProT alpha) is an acidic polypeptide with potentiating effects on HLA-DR-restricted in vitro cellular immune response systems such as T cell proliferative responses to soluble proteins and cellular auto- or alloantigens. Experiments were performed to investigate the effect of ProT alpha on MHC class II Ag expression in human monocytes, murine splenocytes, and tumor cell lines at both protein and molecular levels. RIA and immunofluorescence analysis revealed that ProT alpha enhances HLA-DR surface Ag expression whereas Northern blot analysis demonstrated that ProT alpha causes significant accumulation of MHC class II mRNA. The enhancing effect of ProT alpha was demonstrated convincingly using precultured human peripheral monocytes, which are known to express decreased amounts of surface HLA-DR Ag, and HLA-DR-positive human cell lines. Moreover, ProT alpha was shown to induce HLA-DR Ag expression in a priori HLA-DR-negative tumor cells. Furthermore, ProT alpha was shown to be active in vivo. Splenocytes from mice pretreated with ProT alpha expressed more surface Ilpha Ag and contained more I alpha-specific mRNA. These findings suggest that ProT alpha may be important in the regulation of the immune response by enhancing MHC class II Ag expression in APC.

Prothymosin alpha enhances human and murine MHC class II surface antigen expression and messenger RNA accumulation

Prothymosin alpha (ProT alpha) is an acidic polypeptide with potentiating effects on HLA-DR-restricted in vitro cellular immune response systems such as T cell proliferative responses to soluble proteins and cellular auto- or alloantigens. Experiments were performed to investigate the effect of ProT alpha on MHC class II Ag expression in human monocytes, murine splenocytes, and tumor cell lines at both protein and molecular levels. RIA and immunofluorescence analysis revealed that ProT alpha enhances HLA-DR surface Ag expression whereas Northern blot analysis demonstrated that ProT alpha causes significant accumulation of MHC class II mRNA. The enhancing effect of ProT alpha was demonstrated convincingly using precultured human peripheral monocytes, which are known to express decreased amounts of surface HLA-DR Ag, and HLA-DR-positive human cell lines. Moreover, ProT alpha was shown to induce HLA-DR Ag expression in a priori HLA-DR-negative tumor cells. Furthermore, ProT alpha was shown to be active in vivo. Splenocytes from mice pretreated with ProT alpha expressed more surface Ilpha Ag and contained more I alpha-specific mRNA. These findings suggest that ProT alpha may be important in the regulation of the immune response by enhancing MHC class II Ag expression in APC.

Intracytoplasmic detection of the Tac (p55) chain of interleukin-2 receptor in pre-B leukemic cells associated with a constitutive expression of Tac mRNA

The pre-B Reh-6 leukemic cells do not express membrane interleukin-2 (IL-2)-R alpha (Tac or p55) chain; however, their incubation with PMA induces the expression of both high and low affinity IL-2-R. Northern analysis of nonstimulated Reh-6 as well as leukemic cells from patients with acute B cell precursor lymphoblastic leukemia displayed a constitutive expression of p55 mRNA transcripts, which could be enhanced by PMA. Both actinomycin-D and cycloheximide could abrogate PMA-induced p55 membrane expression, suggesting the need for de novo mRNA and protein synthesis. The increased PMA-induced p55 mRNA accumulation was an early event (4 hr) and could be enhanced, specifically, by rIL-2 because anti-Tac moAb inhibited this rIL-2-mediated effect. Immunofluorescence and cross-linking studies using 125I-rIL-2 failed to reveal membrane-associated p55 protein on both Reh-6 and patients' leukemic cells. Conversely, immunogold staining and electron microscopy studies, revealed p55 immunoreactive molecules in the cytoplasm but not in the nucleus of all Reh-6 cells. Using a sensitive EIA, p55 molecules could be detected in cell lysates but not the culture supernatants of Reh-6 cells, suggesting that p55 was not released into the culture medium. These results indicate that constitutively expressed p55 mRNA on pre-B leukemic cells is translated into a relative immunoreactive protein that cannot be expressed on cell surface for unknown, yet, reasons.