Biomedical subjects
C Brou
Publications and source records attributed to C Brou.
Functional interaction between SEL-10, an F-box protein, and the nuclear form of activated Notch1 receptor.
The Notch signaling pathway is essential in many cell fate decisions in invertebrates as well as in vertebrates. After ligand binding, a two-step proteolytic cleavage releases the intracellular part of the receptor which translocates to the nucleus and acts as a transcriptional activator. Although Notch-induced transcription of genes has been reported extensively, its endogenous nuclear form has been seldom visualized. We report that the nuclear intracellular domain of Notch1 is stabilized by proteasome inhibitors and is a substrate for polyubiquitination in vitro. SEL-10, an F-box protein of the Cdc4 family, was isolated in a genetic screen for Lin12/Notch-negative regulators in Caenorhabditis elegans. We isolated human and murine counterparts of SEL-10 and investigated the role of a dominant-negative form of this protein, deleted of the F-box, on Notch1 stability and activity. This molecule could stabilize intracellular Notch1 and enhance its transcriptional activity but had no effect on inactive membrane-anchored forms of the receptor. We then demonstrated that SEL-10 specifically interacts with nuclear forms of Notch1 and that this interaction requires a phosphorylation event. Taken together, these data suggest that SEL-10 is involved in shutting off Notch signaling by ubiquitin-proteasome-mediated degradation of the active transcriptional factor after a nuclear phosphorylation event.
A novel proteolytic cleavage involved in Notch signaling: the role of the disintegrin-metalloprotease TACE.
The Notch1 receptor is presented at the cell membrane as a heterodimer after constitutive processing by a furin-like convertase. Ligand binding induces the proteolytic release of Notch intracellular domain by a gamma-secretase-like activity. This domain translocates to the nucleus and interacts with the DNA-binding protein CSL, resulting in transcriptional activation of target genes. Here we show that an additional processing event occurs in the extracellular part of the receptor, preceding cleavage by the gamma-secretase-like activity. Purification of the activity accounting for this cleavage in vitro shows that it is due to TACE (TNFalpha-converting enzyme), a member of the ADAM (a disintegrin and metalloprotease domain) family of metalloproteases. Furthermore, experiments carried out on TACE-/- bone marrow-derived monocytic precursor cells suggest that this metalloprotease plays a prominent role in the activation of the Notch pathway.
The Notch1 receptor is cleaved constitutively by a furin-like convertase.
The Notch receptor, which is involved in numerous cell fate decisions in invertebrates and vertebrates, is synthesized as a 300-kDa precursor molecule (p300). We show here that proteolytic processing of p300 is an essential step in the formation of the biologically active receptor because only the cleaved fragments are present at the cell surface. Our results confirm and extend recent reports indicating that the Notch receptor exists at the plasma membrane as a heterodimeric molecule, but disagree as to the nature of the protease that is responsible for the cleavage that takes place in the extracellular region. We report here that constitutive processing of murine Notch1 involves a furin-like convertase. We show that the calcium ionophore A23187 and the alpha1-antitrypsin variant, alpha 1-PDX, a known inhibitor of furin-like convertases, inhibit p300 processing. When expressed in the furin-deficient Lovo cell line, p300 is not processed. In vitro digestion of a recombinant Notch-derived substrate with purified furin allowed mapping of the processing site to the carboxyl side of the sequence RQRR (amino acids 1651-1654). Mutation of these four amino acids (and of two secondary dibasic furin sites located nearby) completely abolished processing of the Notch1 receptor.
Delta-1 activation of notch-1 signaling results in HES-1 transactivation.
The Notch receptor is involved in many cell fate determination events in vertebrates and invertebrates. It has been shown in Drosophila melanogaster that Delta-dependent Notch signaling activates the transcription factor Suppressor of Hairless, leading to an increased expression of the Enhancer of Split genes. Genetic evidence has also implicated the kuzbanian gene, which encodes a disintegrin metalloprotease, in the Notch signaling pathway. By using a two-cell coculture assay, we show here that vertebrate Dl-1 activates the Notch-1 cascade. Consistent with previous data obtained with active forms of Notch-1 a HES-1-derived promoter construct is transactivated in cells expressing Notch-1 in response to Dl-1 stimulation. Impairing the proteolytic maturation of the full-length receptor leads to a decrease in HES-1 transactivation, further supporting the hypothesis that only mature processed Notch is expressed at the cell surface and activated by its ligand. Furthermore, we observed that Dl-1-induced HES-1 transactivation was dependent both on Kuzbanian and RBP-J activities, consistent with the involvement of these two proteins in Notch signaling in Drosophila. We also observed that exposure of Notch-1-expressing cells to Dl-1 results in an increased level of endogenous HES-1 mRNA. Finally, coculture of Dl-1-expressing cells with myogenic C2 cells suppresses differentiation of C2 cells into myotubes, as previously demonstrated for Jagged-1 and Jagged-2, and also leads to an increased level of endogenous HES-1 mRNA. Thus, Dl-1 behaves as a functional ligand for Notch-1 and has the same ability to suppress cell differentiation as the Jagged proteins do.
Signalling downstream of activated mammalian Notch.
Notch belongs to a family of transmembrane proteins that are widely conserved from flies to vertebrates and are thought to be involved in cell-fate decisions. In Drosophila, the Suppressor of hairless (Su(H)) gene and genes of the Enhancer of split (E(Spl)) complex, which encode proteins of the basic helix-loop-helix type have been implicated in the Notch signalling pathway. Mammalian homologues of E(Spl), such as the mouse Hairy enhancer of split (HES-1), have been isolated. Both HES-1 and the intracellular domain of murine Notch (mNotch) are able to block MyoD-induced myogenesis. Here we show that activated forms of mNotch associate with the human analogue of Su(H), KBF2/RBP-J kappa (refs 8,9) and act as transcriptional activators through the KBF2-binding sites of the HES-1 promoter.
The human J kappa recombination signal sequence binding protein (RBP-J kappa) targets the Epstein-Barr virus EBNA2 protein to its DNA responsive elements.
The Epstein-Barr virus (EBV) protein EBNA2, which is essential for the immortalization of human primary B cells by EBV, acts as a transcriptional activator of cellular and viral genes. Specific responsive elements have been characterized in several of the promoters activated by EBNA2. They all share the core sequence GTGGGAA. EBNA2 does not, however, bind to these sequences directly, but appears to be targeted to them by a cellular protein. A similar core sequence has recently been identified as a high-affinity binding site for the human recombination signal sequence binding protein RBP-J kappa. Here we provide evidence that RBP-J kappa binds to specific sequences in EBNA2-responsive elements. Our results also demonstrate that RBP-J kappa makes direct physical contact with EBNA2 in solution and recruits EBNA2 to its cognate DNA sequences, suggesting that RBP-J kappa may mediate EBNA2 transactivation of both cellular and viral genes.
Inhibition of the DNA-binding activity of Drosophila suppressor of hairless and of its human homolog, KBF2/RBP-J kappa, by direct protein-protein interaction with Drosophila hairless.
We have purified the sequence-specific DNA-binding protein KBF2 and cloned the corresponding cDNA, which is derived from the previously described RBP-J kappa gene, the human homolog of the Drosophila Suppressor of Hairless [Su(H)] gene. Deletion studies of the RBP-J kappa and Su(H) proteins allowed us to define a DNA-binding domain conserved during evolution. Because Su(H) mutant alleles exhibit dose-sensitive interactions with Hairless (H) loss-of-function mutations, we have investigated whether the RBP-J kappa or Su(H) proteins directly interact with the H protein in vitro. We show here that H can inhibit the DNA binding of both Su(H) and RBP-J kappa through direct protein-protein interactions. Consistent with this in vitro inhibitory effect, transcriptional activation driven by Su(H) in transfected Drosophila S2 cells is inhibited by H. These results support a model in which H acts, at least in part, as a negative regulator of Su(H) activity. This model offers a molecular view to the antagonistic activities encoded by the H and Su(H) genes for the control of sensory organ cell fates in Drosophila. We further propose that a similar mechanism might occur in mammals.
Human TAFII30 is present in a distinct TFIID complex and is required for transcriptional activation by the estrogen receptor.
We showed previously that coactivators mediating stimulation by different activators were associated with the TATA-binding protein (TBP) in distinct TFIID complexes. We have characterized a human TBP-associated factor (TAF), hTAFII30, associated with a subset of TFIID complexes. hTAFII30 interacts with the AF-2-containing region E of the human estrogen receptor (ER), but not with ER AF-1 or VP16. An antibody against hTAFII30 inhibited transcriptional stimulation by the ER AF-2 without affecting basal or VP16-activated transcription and allowed the separation of TFIID complex(es) containing hTAFII30 from complexes mediating the activity of VP16. These results directly demonstrate the existence of functionally distinct TFIID populations that share common TAFIIs but differ in specific TAFIIs.
A cell-specific factor represses stimulation of transcription in vitro by transcriptional enhancer factor 1.
Transcription in HeLa cell extracts in vitro was stimulated 8- to 10-fold by a recombinant chimera, GAL-TEF-1, consisting of the DNA-binding domain of GAL4 and the activation function of the HeLa cell activator TEF-1. In contrast, only a 2- to 3-fold stimulation was obtained with GAL-TEF-1 in extracts from BJA-B lymphoid cells. Stimulation by GAL-TEF-1 in BJA-B extracts was dramatically increased by the addition of immunopurified HeLa cell TFIID, suggesting that BJA-B TFIID lacks or contains lower quantities of a TATA-binding-protein-associated factor(s) required for the activity of the TEF-1 activation function. However, chromatography, immunopurification, and transcriptional reconstitution experiments indicated that BJA-B extracts did not lack the previously identified TATA-binding-protein-associated factors required for TEF-1 activity but rather contained a negatively acting factor(s) which inhibited transactivation by GAL-TEF-1. These results indicate that the relative lack of activity of the TEF-1 activation function in vitro in BJA-B cell extracts does not result from the absence of positively acting factors from the presence of a cell-specific negatively acting factor(s).
Sequence-specific transactivators counteract topoisomerase II-mediated inhibition of in vitro transcription by RNA polymerases I and II.
An inhibitor of RNA polymerase II transcription in vitro has been purified from HeLa cell nuclear extracts. Partial amino acid sequences derived from the purified protein revealed that the inhibitor of transcription corresponded to human topoisomerase II. Order of addition experiments provided evidence indicating that topoisomerase II inhibited transcription by binding over the core promoter and blocking preinitiation complex formation. Topoisomerase II-mediated repression could be relieved by sequence-specific transcriptional activators, having different activating and/or DNA binding domains, but antirepression required a transcriptional activation function in addition to a DNA binding domain. Moreover, transcription by RNA polymerase I was also inhibited by topoisomerase II and this inhibition could be relieved by the RNA polymerase I transactivator UBF. These observations suggest that topoisomerase II may participate in a general repression of transcription which can be counteracted by transcriptional activators.
Different TBP-associated factors are required for mediating the stimulation of transcription in vitro by the acidic transactivator GAL-VP16 and the two nonacidic activation functions of the estrogen receptor.
The estrogen receptor (ER) contains two nonacidic transcriptional activation functions, AF-1 and AF-2 (formerly TAF-1 and TAF-2). In this study we show that AF-1 and AF-2 are able to stimulate transcription in vitro in a HeLa cell system when fused to the DNA binding domain of the yeast activator GAL4. We also demonstrate that a factor(s) required for the function of the ER AFs is chromatographically separable from a factor(s) necessary for the activity of the acidic activation domain of VP16. Moreover, immunoprecipitation experiments using a monoclonal antibody directed against the TATA box binding protein (TBP) indicate, that these different factors are associated with TBP in distinct TFIID complexes.
Distinct TFIID complexes mediate the effect of different transcriptional activators.
Multiple chromatographically separable complexes containing the TATA binding protein (TBP), which exhibit different functional properties, exist in HeLa cells. At least three distinct subpopulations of such complexes can be functionally defined as TFIID since they function with RNA polymerase II. Using a partially reconstituted HeLa cell in vitro transcription system and immunoprecipitation with a monoclonal antibody directed against TBP, we show that stimulation of transcription by the chimeric activators GAL-VP16, GAL-TEF-1 and GAL-ER(EF) requires the presence of factors which are tightly associated with these TFIID complexes. Moreover, the activity of GAL-TEF-1 appears to be mediated by at least two chromatographically distinct populations of TFIID. The factor(s) associated with one of these populations is also required for the activity of GAL-ER (EF) and GAL-VP16, while the factor(s) associated with the other population functions selectively with GAL-TEF-1. These two TFIID populations are composed of both common and unique TBP associated factors (TAFs).
In vitro activity of the transcription activation functions of the progesterone receptor. Evidence for intermediary factors.
The human progesterone receptor (hPR) is a ligand-dependent transcription factor which contains two distinct transcription activation functions (TAFs). The full-length hPR and its individual TAFs were overexpressed in the baculovirus system and tested in a HeLa cell-derived in vitro transcription system. hPR stimulated transcription in a ligand-independent manner. When the two TAFs fused to the DNA-binding domain of GAL4 were tested, only the constitutive TAF-1 was functional in vitro, strongly suggesting that the transcriptional activity of baculovirus-expressed hPR comes solely from TAF-1. The GAL-TAF-1 activator was found to self-squelch without affecting basal transcription. A partially purified fraction relieved this self-squelching and, moreover, stimulated transcriptional activation by GAL-TAF-1, while having no influence on basal transcription. These results strongly suggest that the transcriptional activity of GAL-TAF-1 requires a factor(s) distinct from the general transcription factors.
The acidic transcriptional activator GAL-VP16 acts on preformed template-committed complexes.
The action of the chimeric acidic transcriptional activator GAL-VP16 has been investigated by performing a series of kinetic experiments using the detergent Sarkosyl as well as monoclonal antibodies which specifically inhibit GAL-VP16 DNA binding and transcriptional activation. GAL-VP16 binds to recognition site rapidly, remains bound after transcriptional initiation and is required to maintain stimulated levels of reinitiation. GAL-VP16 action, which appears to result in an increase in the number of preinitiation complexes formed, occurs after the formation of template-committed complexes composed of promoter-bound TFIIA (STF) and a partially purified TFIID fraction conferring GAL-VP16 responsiveness on a reconstituted basal transcription system. This TFIID fraction cannot be replaced by TFIIB or cloned TFIID. Our results suggest that GAL-VP16 activates step(s) in preinitiation complex assembly occurring after TFIID has bound.
Evidence for a factor required for transcriptional stimulation by the chimeric acidic activator GAL-VP16 in HeLa cell extracts.
We provide biochemical evidence for the existence of a transcriptional intermediary factor (TIF) in HeLa whole-cell extracts (WCE) that is distinct from the basic transcription factors and that is required for transcriptional stimulation by the chimeric acidic activator GAL-VP16. We have fractionated HeLa WCE by heparin-agarose chromatography. Of transcriptionally active fractions eluting in a step between 0.24 and 0.6 M KCl, the initial fractions are refractory to GAL-VP16 stimulation, whereas subsequent fractions are strongly stimulated by the activator. Aliquots of GAL-VP16-responsive fractions efficiently complement refractory fractions for transcriptional stimulation. Aliquots of responsive fractions are also far more efficient than those of refractory fractions in overcoming transcriptional inhibition that is brought about by high concentrations of GAL-VP16. Experiments performed with heat-treated WCE support the idea that HeLa cells contain a TIF that is essential for GAL-VP16 stimulation, but that is not required for basal transcription. Addition of recombinant yeast or human transcription factor TFIID (rTFIIDY and rTFIIDH, respectively) to a WCE heated at 48 degrees C for 15 min restores basal transcription, but in neither case is the reconstituted system activated by GAL-VP16. However, a 45 degrees C heat-treated WCE reconstituted with either rTFIIDH or rTFIIDY is stimulated by GAL-VP16, suggesting that a HeLa TIF can be selectively inactivated by heating at 48 degrees C, but not at 45 degrees C. Interestingly, a TFIID fraction partially purified from HeLa cell extracts, but not rTFIIDH, efficiently relieves transcriptional inhibition by GAL-VP16, suggesting that there may be an association between TIF(s) and TFIID and, moreover, that TIF(s) may be the direct target of the acidic domain of GAL-VP16. In summary, our results support the existence of a TIF that is not essential for basal transcription but that is required to mediate the stimulatory activity of the acidic activator GAL-VP16.
The human estrogen receptor has two independent nonacidic transcriptional activation functions.
We have previously reported the presence of a hormone-inducible transcriptional activation function (TAF-2) within the region of the estrogen receptor (ER) that contains the hormone binding domain. We show here that the N-terminal A/B region of the ER contains an independent constitutive activation function (TAF-1) that exhibits cell type specificity since it activates transcription efficiently in chicken embryo fibroblasts, but only poorly in HeLa cells. By analyzing the ability of TAF-1, TAF-2, and the GAL4 and VP16 acidic activating domains (AADs) to homosynergize and heterosynergize with one another and with the factor binding to the upstream element (UE) of the adenovirus 2 major late promoter, we show that the activation properties of TAF-1 and TAF-2 are different and distinct from those of AADs, in agreement with the absence of acidic amino acid stretches in TAF-1 and TAF-2.
Characterization of a bovine acidic FGF cDNA clone and its expression in brain and retina.
A cDNA encoding the acidic eye-derived growth factor (EDGF II) similar to the acidic fibroblast growth factor (aFGF), a potent cell mitogen, has been isolated from a bovine retinal cDNA library. The cDNA, 4.1 kb in size, has a sequence coding for the 155 amino acids of bovine aFGF, and shows similarity with human aFGF (87% identity). The coding sequence is flanked by a 5'-untranslated region of 0.8 kb and a 3'-untranslated end of 3.0 kb. Northern blot analysis of bovine brain and retina poly(A+) RNAs showed the existence of four aFGF mRNA species. Two of these species are 9.9 and 6.0 kb in size, not abundant and could represent premessengers. The other two species, 4.2 and 2.5 kb, are abundant.