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

Interferon-stimulated response element (ISRE)-binding protein complex DRAF1 is activated in Sindbis virus (HR)-infected cells

To elucidate the host cell defense mechanisms in response to Sindbis viral infection, we have started to characterize interferon (IFN)-stimulated response element (ISRE)-binding proteins activated in infected cells that are involved in the transcriptional induction of IFN type I-inducible genes. Using electromobility shift assays (EMSA), we detected several protein complexes with a human IFN-stimulated gene 15 (ISG15) ISRE in extracts from virus-infected L929 cells that were absent in extracts from uninfected cells. Comigration with Newcastle disease virus-activated ISRE-binding complexes, ISRE-binding specificity, supershift experiments, and conditions of formation indicate that the complexes activated by Sindbis viral infection in L929 cells correspond to DRAF1 and ISG factor 3 (ISGF3). Transfection of L929 cells with poly rI:rC induced only ISGF3. DRAF1 could be detected in Sindbis virus-infected mouse embryo fibroblasts derived from IFNR type I and type II KO mice. Viral RNA synthesis is required for activation of DRAF1.

Molecular biology of HMGA proteins: hubs of nuclear function

Members of the HMGA (a.k.a. HMGI/Y) family of 'high mobility group' (HMG) proteins participate in a wide variety of nuclear processes ranging from chromosome and chromatin mechanics to acting as architectural transcription factors that regulate the expression of numerous genes in vivo. As a consequence, they function in the cell as highly connected 'nodes' of protein-DNA and protein-protein interactions that influence a diverse array of normal biological processes including growth, proliferation, differentiation and death. The HMGA proteins, likewise, participate in pathological processes by, for example, acting as regulators of viral gene transcription and by serving as host-supplied proteins that facilitate retroviral integration. HMGA genes are bona fide proto-oncogenes that promote tumor progression and metastasis when overexpressed in cells. High constitutive HMGA protein levels are among the most consistent feature observed in all types of cancers with increasing concentrations being correlated with increasing malignancy. The intrinsic attributes that endow the HMGA proteins with these remarkable abilities are a combination of structural, biochemical and biological characteristics that are unique to these proteins. HMGA proteins have little, if any, secondary structure while free in solution but undergo disordered-to-ordered structural transitions when bound to substrates such as DNA or other proteins. Each protein contains three copies of a conserved DNA-binding peptide motif called the 'AT-hook' that preferentially binds to the minor groove of stretches of AT-rich sequence. In vivo HMGA proteins specifically interact with a large number of other proteins, most of which are transcription factors. They are also subject to many types of in vivo biochemical modifications that markedly influence their ability to interact with DNA substrates, other proteins and chromatin. And, most importantly, both the transcription of HMGA genes and the biochemical modifications of HMGA proteins are direct downstream targets of numerous signal transduction pathways making them exquisitely responsive to various environmental influences. This review covers recent advances that have contributed to our understanding of how this constellation of structural and biological features allows the HMGA proteins to serve as central 'hubs' of nuclear function.

High mobility group I (Y) proteins bind HIPK2, a serine-threonine kinase protein which inhibits cell growth

The HMGI proteins (HMGI, HMGY and HMGI-C) have an important role in the chromatin organization and interact with different transcriptional factors. The HMGI genes are expressed at very low levels in normal adult tissues, whereas they are very abundant during embryonic development and in several experimental and human tumours. In order to isolate proteins interacting with the HMGI(Y) proteins, a yeast two-hybrid screening was performed using the HMGI(Y) protein as bait. This analysis led to the isolation of homeodomain-interacting protein kinase-2 (HIPK2), a serine/threonine nuclear kinase. HIPK2 co-immunoprecipitates with the HMGI(Y) protein in 293T cells. The interaction between HIPK2 and HMGI(Y) occurs through the PEST domain of HIPK2 and it is direct because in vitro translated HIPK2 binds HMGI(Y). We also show that HIPK2 is able to phosphorylate the HMGI(Y) protein by an in vitro kinase assay. In order to understand a possible role of HIPK2 gene in cell growth we performed a colony assay which showed an impressive HIPK2 inhibitory effect on normal thyroid cells. Flow cytometric analysis would indicate the block of cell growth at the G2/M phase of the cell cycle. Since normal thyroid cells do not express detectable HMGI(Y) protein levels, we assume that the HIPK2 inhibitory effect is independent from the interaction with the HMGI(Y) protein.

Characterization of Sp1, AP-1, CBF and KRC binding sites and minisatellite DNA as functional elements of the metastasis-associated mts1/S100A4 gene intronic enhancer

The mts1/S100A4 gene encodes a small acidic calcium-binding protein that is expressed in a cell-specific manner in development, tumorigenesis and certain tissues of adult mice. A composite enhancer that is active in murine mammary adenocarcinoma cells was previously identified in the first intron of the mts1/S100A4 gene. Here we present a detailed analysis of the structure and function of this enhancer in the Mts1/S100A4-expressing CSML100 and non-expressing CSML0 mouse adenocarcinoma cell lines. In CSML100 cells the enhancer activity is composed of at least six cis-elements interacting with Sp1 and AP-1 family members and CBF/AML/PEBP2 and KRC transcription factors. In addition, a minisatellite-like DNA sequence significantly contributes to the enhancer activity via interaction with abundant proteins, which likely have been described previously under the name minisatellite-binding proteins. Extensive mutational analysis of the mts1/S100A4 enhancer revealed a cooperative function of KRC and the factors binding minisatellite DNA. This is the first example of an enhancer where two nuclear factors earlier implicated in different recombination processes cooperate to activate transcription. In Mts1/S100A4-negative CSML0 cells the strength of the enhancer was 7- to 12.5-fold lower compared to that in CSML100 cells, when referred to the activities of three viral promoters. In CSML0 cells the enhancer could be activated by exogenous AP-1 and CBF transcription factors.

Requirement for high mobility group protein HMGI-C interaction with STAT3 inhibitor PIAS3 in repression of alpha-subunit of epithelial Na+ channel (alpha-ENaC) transcription by Ras activation in salivary epithelial cells

Previously, we have demonstrated that oxidative stress or Ras/ERK activation leads to the transcriptional repression of alpha-subunit of epithelial Na(+) channel (ENaC) in lung and salivary epithelial cells. Here, we further investigated the coordinated molecular mechanisms by which alpha-ENaC expression is regulated. Using both stable and transient transfection assays, we demonstrate that the overexpression of high mobility group protein I-C (HMGI-C), a Ras/ERK-inducible HMG-I family member, represses glucocorticoid receptor (GR)/dexamethasone (Dex)-stimulated alpha-ENaC/reporter activity in salivary epithelial cells. Northern analyses further confirm that the expression of endogenous alpha-ENaC gene in salivary Pa-4 cells is suppressed by an ectopic HMGI-C overexpression. Through yeast two-hybrid screening and co-immunoprecipitation assays from eukaryotic cells, we also discovered the interaction between HMGI-C and PIAS3 (protein inhibitor of activated STAT3 (signal transducer and activator of transcription 3)). A low level of ectopically expressed PIAS3 cooperatively inhibits GR/Dex-dependent alpha-ENaC transcription in the presence of HMGI-C. Reciprocally, HMGI-C expression also coordinately enhances PIAS3-mediated repression of STAT3-dependent transactivation. Moreover, overexpression of antisense HMGI-C construct is capable of reversing the repression mediated by Ras V12 on GR- and STAT3-dependent transcriptional activation. Together, our results demonstrate that Ras/ERK-mediated induction of HMGI-C is required to effectively repress GR/Dex-stimulated transcription of alpha-ENaC gene and STAT3-mediated transactivation. These findings delineate a network of inhibitory signaling pathways that converge on HMGI-C.PIAS3 complex, causally associating Ras/ERK activation with the repression of both GR and STAT3 signaling pathways in salivary epithelial cells.

IRF family of transcription factors as regulators of host defense

Interferon regulatory factors (IRFs) constitute a family of transcription factors that commonly possess a novel helix-turn-helix DNA-binding motif. Following the initial identification of two structurally related members, IRF-1 and IRF-2, seven additional members have now been reported. In addition, virally encoded IRFs, which may interfere with cellular IRFs, have also been identified. Thus far, intensive functional analyses have been done on IRF-1, revealing a remarkable functional diversity of this transcription factor in the regulation of cellular response in host defense. Indeed, IRF-1 selectively modulates different sets of genes, depending on the cell type and/or the nature of cellular stimuli, in order to evoke appropriate responses in each. More recently, much attention has also been focused on other IRF family members. Their functional roles, through interactions with their own or other members of the family of transcription factors, are becoming clearer in the regulation of host defense, such as innate and adaptive immune responses and oncogenesis.

Onset of natural killer cell lymphomas in transgenic mice carrying a truncated HMGI-C gene by the chronic stimulation of the IL-2 and IL-15 pathway

Rearrangements of the high mobility group protein I-C (HMGI-C) gene, consisting in the loss of the carboxyl-terminal tail, have been frequently detected in benign human tumors of mesenchymal origin. We have previously demonstrated that transgenic (TG) mice carrying a truncated HMGI-C construct (HMGI-C/T) exhibit a giant phenotype together with a predominantly abdominal/pelvic lipomatosis. Here, we report that HMGI-C/T TG mice develop natural killer (NK)-T/NK cell lymphomas starting from 12 months of age. We found an increased expression of IL-2 and IL-15 proteins and their receptors in these lymphomas, and we demonstrate that HMGI-C/T protein positively regulates their expression in vitro. Therefore, the HMGI-C/T-mediated chronic stimulation of the IL-2/IL-15 pathway could be responsible for the onset of NK-T/NK cell lymphomas in HMGI-C/T TG mice.

HMG-1 enhances HMG-I/Y binding to an A/T-rich enhancer element from the pea plastocyanin gene

High-mobility-group proteins HMG-1 and HMG-I/Y bind at overlapping sites within the A/T-rich enhancer element of the pea plastocyanin gene. Competition binding experiments revealed that HMG-1 enhanced the binding of HMG-I/Y to a 31-bp region (P31) of the enhancer. Circularization assays showed that HMG-1, but not HMG-I/Y, was able to bend a linear 100-bp DNA containing P31 so that the ends could be ligated. HMG-1, but not HMG-I/Y, showed preferential binding to the circular 100-bp DNA compared with the equivalent linear DNA, indicating that alteration of the conformation of the DNA by HMG-1 was not responsible for enhanced binding of HMG-I/Y. Direct interaction of HMG-I/Y and HMG-1 in the absence of DNA was demonstrated by binding of 35S-labeled proteins to immobilized histidine-tagged proteins, and this was due to an interaction of the N-terminal HMG-box-containing region of HMG-1 and the C-terminal AT-hook region of HMG-I/Y. Kinetic analysis using the IAsys biosensor revealed that HMG-1 had an affinity for immobilized HMG-I/Y (Kd = 28 nM) similar to that for immobilized P31 DNA. HMG-1-enhanced binding of HMG-I/Y to the enhancer element appears to be mediated by the formation of an HMG-1-HMG-I/Y complex, which binds to DNA with the rapid loss of HMG-1.

HMGI/Y proteins: flexible regulators of transcription and chromatin structure

The mammalian HMGI/Y (HMGA) non-histone proteins participate in a wide variety of cellular processes including regulation of inducible gene transcription, integration of retroviruses into chromosomes and the induction of neoplastic transformation and promotion of metastatic progression of cancer cells. Recent advances have contributed greatly to our understanding of how the HMGI/Y proteins participate in the molecular mechanisms underlying these biological events. All members of the HMGI/Y family of 'high mobility group' proteins are characterized by the presence of multiple copies of a conserved DNA-binding peptide motif called the 'AT hook' that preferentially binds to the narrow minor groove of stretches of AT-rich sequence. The mammalian HMGI/Y proteins have little, if any, secondary structure in solution but assume distinct conformations when bound to substrates such as DNA or other proteins. Their intrinsic flexibility allows the HMGI/Y proteins to participate in specific protein-DNA and protein-protein interactions that induce both structural changes in chromatin substrates and the formation of stereospecific complexes called 'enhanceosomes' on the promoter/enhancer regions of genes whose transcription they regulate. The formation of such regulatory complexes is characterized by reciprocal inductions of conformational changes in both the HMGI/Y proteins themselves and in their interacting substrates. It may well be that the inherent flexibility of the HMGI/Y proteins, combined with their ability to undergo reversible disordered-to-ordered structural transitions, has been a significant factor in the evolutionary selection of these proteins for their functional role(s) in cells.

Inhibition of the NF-kappa B transcription factor increases Bax expression in cancer cell lines

The NF-kappa B transcription factor has been shown to inhibit apoptosis in several experimental systems. We therefore investigated whether the expression of the Bax proapoptotic protein could be influenced by NF-kappa B activity. Increased Bax protein expression was detected in HCT116, OVCAR-3 and MCF7 cells stably expressing a mutated unresponsive I kappa B-alpha inhibitory protein that blocks NF-kappa B activity. Northern blots showed that bax mRNA expression was increased as a consequence of mutated I kappa B-alpha expression in HCT116 cells. A careful examination of the human bax gene promoter sequence showed three putative binding sites for NF-kappa B, and the kappa B2 site at position -687 could indeed bind NF-kappa B complexes in vitro. Transient transfection of a bax promoter luciferase construct in HCT116 cells showed that NF-kappa B proteins could partially inhibit the transactivation of the bax promoter by p53. Mutations or deletions of the kappa B sites, including kappa B2, indicated that this NF-kappa B-dependent inhibitory effect did not require NF-kappa B DNA-binding, and was thus an indirect effect. However, cotransfection of expression vectors for several known cofactors failed to identify a competition between p53 and NF-kappa B for a transcription coactivator. Our findings thus demonstrate for the first time that NF-kappa B regulates, through an indirect pathway, the bax gene expression.

Regulation of nuclear factor kappa B transactivation. Implication of phosphatidylinositol 3-kinase and protein kinase C zeta in c-Rel activation by tumor necrosis factor alpha

Transactivation by c-Rel (nuclear factor kappaB) was dependent on phosphorylation of several serines in the transactivation domain, indicating that it is a phosphorylation-dependent Ser-rich domain. By Ser --> Ala mutational and deletion analysis, we have identified two regions in this domain: 1) a C-terminal region (amino acids 540-588), which is required for basal activity; and 2) the 422-540 region, which responds to external stimuli as tumor necrosis factor (TNF) alpha or phorbol myristate acetate plus ionomycin. Ser from 454 to 473 were shown to be required for TNFalpha-induced activation, whereas Ser between 492 and 519 were required for phorbol myristate acetate plus ionomycin activation. Phosphatidylinositol 3-kinase (PI3K) and protein kinase C (PKC) zeta were identified as downstream signaling molecules of TNFalpha-activation of c-Rel transactivating activity. Interestingly, dominant negative forms of PI3K inhibited PKCzeta activation and dominant negative PKCzeta inhibited PI3K-mediated activation of c-Rel transactivating activity, indicating a cross-talk between both enzymes. We have identified the critical role of different Ser for PKCzeta- and PI3K-mediated responses. Interestingly, those c-Rel mutants not only did not respond to TNFalpha but also acted as dominant negative forms of nuclear factor kappaB activation.

Lysophosphatidylcholine Induces Early Growth Response Factor-1 Expression and Activates the Core Promoter of PDGF-A Chain in Vascular Endothelial Cells

Abstract —Lysophosphatidylcholine (lyso-PC), a polar phospholipid that is increased in atherogenic lipoproteins and atherosclerotic lesions, has been shown to transcriptionally induce the expression of endothelial genes relevant to atherogenesis. In cultured bovine aortic endothelial cells (BAECs), we show that lyso-PC induces the expression of early growth response factor (Egr)-1 and thereby activates the proximal promoter of the platelet-derived growth factor (PDGF)-A chain located 55 to 71 bp upstream from the transcription start site, which has been shown to be crucial for PDGF-A chain expression induced by fluid shear stress and fibroblast growth factor-1. Northern blot analyses showed that lyso-PC (10 to 20 μmol/L) transiently (30 minutes to 1 hour) induced expression of Egr-1 mRNA. Induced expression of Egr-1 mRNA, which was associated with increased amounts of Egr-1 protein in nuclei, preceded PDGF-A chain mRNA induction in lyso-PC–activated BAECs. Nuclear runoff assay revealed that lyso-PC stimulates transcription of the Egr-1 gene. Transient transfection of the oligonucleotide corresponding to the proximal promoter of the PDGF-A chain (oligo A) linked to the luciferase reporter gene revealed that lyso-PC can activate the core promoter of the PDGF-A chain by 5-fold. Insertion of a guanine at 3 sites in the oligo A abolished the lyso-PC–induced increases in luciferase activities. Electrophoretic mobility shift assay with use of radiolabeled oligo A showed a lyso-PC–inducible shift band, which was suppressed by excess amounts of unlabeled oligo A or an anti–Egr-1 antibody. In addition, lyso-PC–induced Egr-1 expression was inhibited by PD98059, a specific inhibitor of mitogen-activated protein kinase kinase-1 (MEK1), suggesting that lyso–PC-induced expression of Egr-1 depends on the MEK1/extracellular signal–regulated kinase pathway. Taken together, transcriptional activation of Egr-1–dependent genes by this atherogenic lipid may be a key regulator of atherogenesis.

Critical role of the HMGI(Y) proteins in adipocytic cell growth and differentiation

The high-mobility group I (HMGI) nonhistone chromosomal proteins HMGI(Y) and HMGI-C have been implicated in defining chromatin structure and in regulating the transcription of several genes. These proteins have been implicated in adipocyte homeostasis: a severe deficiency of fat tissue is found in mice with targeted disruption of the HMGI-C locus, and lipomagenesis in humans is frequently associated with somatic mutations of HMGI genes. The aim of this study was to examine the role of HMGI(Y) proteins in adipocytic cell growth and differentiation. First, we found that differentiation of the preadipocytic 3T3-L1 cell line caused early induction of HMGI(Y) gene expression. Suppression of HMGI(Y) expression by antisense technology dramatically increased the growth rate and impaired adipocytic differentiation in these cells. The process of adipogenic differentiation involves the interplay of several transcription factors, among which is the CCAAT/enhancer-binding protein (C/EBP) family of proteins. These factors are required for the transcriptional activation of adipocyte-specific genes. We also tested the hypothesis that HMGI(Y) might participate in transcriptional control of adipocyte-specific promoters. We found that HMGI(Y) proteins bind C/EBPbeta in vivo and in vitro. Furthermore, we show that HMGI(Y) strongly potentiates the capacity of C/EBPbeta to transactivate the leptin promoter, an adipose-specific promoter. Taken together, these results indicate that the HMGI(Y) proteins play a critical role in adipocytic cell growth and differentiation.

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.

PU.1-mediated transcription is enhanced by HMG-I(Y)-dependent structural mechanisms

The ets transcription factor PU.1 is an important regulator of the immunoglobulin heavy chain gene intronic enhancer, or mu enhancer. However, PU.1 is only one component of the large multiprotein complex required for B cell-specific enhancer activation. The transcriptional coactivator HMG-I(Y), a protein demonstrated to physically interact with PU.1, increases PU.1 affinity for the mu enhancer muB element, indicating that HMG-I(Y) may play a role in the transcriptionally active mu enhanceosome. Increased PU.1 affinity is not mediated by HMG-I(Y)-induced changes in DNA structure. Investigation of alternative mechanisms to explain the HMG-I(Y)-mediated increase in PU.1/mu enhancer binding demonstrated, by trypsin and chymotrypsin mapping, that interaction between PU.1 and HMG-I(Y) in solution induces a structural change in PU.1. In the presence of HMG-I(Y) and wild-type mu enhancer DNA, PU.1 becomes more chymotrypsin resistant, suggesting an additional change in PU.1 structure upon HMG-I(Y)-induced PU.1/DNA binding. From these results, we suggest that increased DNA affinity under limiting PU.1 concentrations is mediated by an HMG-I(Y)-induced structural change in PU.1. In functional assays, HMG-I(Y) further augments transcriptional synergy between PU.1 and another member of the ets family, Ets-1, indicating that HMG-I(Y) is a functional component of the active enhancer complex. These studies suggest a new mechanism for HMG-I(Y)-mediated coactivation; HMG-I(Y) forms protein-protein interactions with a transcription factor, which alters the three-dimensional structure of the factor, resulting in enhanced DNA binding and transcriptional activation. This mechanism may be important for transcriptional activation under conditions of limiting transcription factor concentration, such as at the low levels of PU.1 expressed in B cells.

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.

HMGI-C gene expression is not required for in vivo thyroid cell transformation

We have previously demonstrated that HMGI proteins are required for the transformation of rat thyroid cells by v-mos and v-ras-Ki oncogenes. To determine whether HMGI proteins are also required for in vivo thyroid carcinogenesis, mice carrying a disrupted HMGI-C gene (pygmy mice) were either treated with radioactive iodine or crossed with transgenic mice carrying the E7 papilloma virus oncogene under the transcriptional control of thyroglobulin gene promoter. The pygmy mice developed thyroid carcinomas with the same frequency as occurred in wild-type mice without significant macroscopic and microscopic differences. Therefore, these results indicate that HMGI-C gene expression is not required in in vivo thyroid cell malignant transformation.

Transcriptional regulation of human insulin receptor gene by the high-mobility group protein HMGI(Y)

We have previously identified two closely related nuclear binding proteins that specifically interact with two unique functional AT-rich sequences of the 5' regulatory region of the human insulin receptor gene. Expression of these nuclear binding proteins increases during myocyte and adipocyte differentiation, and in other tissues appears to correlate with insulin receptor content. We have hypothesized, therefore, that insulin receptor expression in the insulin target tissues is regulated at least in part by these nuclear proteins. Here we show data on purification and biochemical characterization of these DNA binding proteins. Using a conventional chromatographic purification procedure combined with electrophoresis mobility shift assay and immunoblot analyses, a unique approximately 15 kDa protein, either identical to or highly related to the architectural transcription factor HMGI(Y), has now been identified, suggesting an essential role for HMGI(Y) in regulating insulin receptor gene transcription. Direct evidence of HMGI(Y) insulin receptor promoter interactions is provided by functional analysis with the CAT reporter gene and by hormone binding studies in cells expressing HMGI(Y) antisense RNA. In these experiments, antisense HMGI(Y) specifically inhibits insulin receptor promoter function and insulin receptor protein expression, indicating that HMGI(Y) is required for proper transcription of insulin receptor gene. Moreover, our data consistently support the hypothesis that a putative defect in this nuclear binding protein may cause insulin receptor dysfunction with subsequent impairment of insulin signaling and action.

Distinct organization of DNA complexes of various HMGI/Y family proteins and their modulation upon mitotic phosphorylation

High mobility group (HMG) proteins HMGI, HMGY, HMGI-C, and Chironomus HMGI are DNA-binding proteins thought to modulate the assembly and the function of transcriptional complexes. Each of these proteins contains three DNA-binding domains (DBD), properties of which appear to be regulated by phosphorylation. High levels of these proteins are characteristic for rapidly dividing cells in embryonic tissues and tumors. On the basis of their occurrence, specific functions for each of these proteins have been postulated. In this study we demonstrate differences in the nature of contacts of these proteins with promoter region of the interferon-beta gene. We show that HMGI and HMGY interact with this DNA via three DBDs, whereas HMGI-C and Chironomus HMGI bind to this DNA using only two domains. Phosphorylation of HMGY protein by Cdc2 kinase leads to impairing of contacts between the N-terminally located DBD and a single promoter element. The perturbations in the architecture of the protein.DNA complexes involve changes in the degree of unbending of the intrinsically bent IFNbeta promoter. Our results provide first insights into the molecular basis of functional specificity of proteins of the HMGI/Y family and their regulation by phosphorylation.

2-acetylaminofluorene up-regulates rat mdr1b expression through generating reactive oxygen species that activate NF-kappa B pathway

Overexpression of multidrug resistance genes and their encoded P-glycoproteins is a major mechanism for the development of multidrug resistance in cancer cells. The hepatocarcinogen 2-acetylaminofluorene (2-AAF) efficiently activates rat mdr1b expression. However, the underlying mechanisms are largely unknown. In this study, we demonstrated that a NF-kappa B site on the mdr1b promoter was required for this induction. Overexpression of antisense p65 and I kappa B alpha partially abolished the induction. We then delineated the pathway through which 2-AAF activates NF-kappa B. 2-AAF treatment led to the increase of intracellular reactive oxygen species (ROS) which causes activation of IKK kinases, degradation of I kappa B beta (but not I kappa B alpha), and increase in NF-kappa B DNA binding activity. Consistent with the idea that ROS may participate in mdr1b regulation, antioxidant N-acetylcysteine inhibited the induction of mdr1b by 2-AAF. Overproduction of a physiological antioxidant glutathione (GSH) blocked the activation of IKK kinase complex and NF-kappa B DNA binding. Based on these results, we conclude that 2-AAF up-regulates mdr1b through the generation of ROS, activation of IKK kinase, degradation of I kappa B beta, and subsequent activation of NF-kappa B. This is the first report that reveals the specific cis-elements and signaling pathway responsible for the induction of mdr1b by the chemical carcinogen 2-AAF.

Architectural transcription factor HMGI(Y) promotes tumor progression and mesenchymal transition of human epithelial cells

Numerous studies have demonstrated that overexpression or aberrant expression of the HMGI(Y) family of architectural transcription factors is frequently associated with both neoplastic transformation of cells and metastatic tumor progression. Little is known, however, about the molecular roles played by the HMGI(Y) proteins in these events. Here we report that human breast epithelial cells harboring tetracycline-regulated HMGI(Y) transgenes acquire the ability to form both primary and metastatic tumors in nude mice only when the transgenes are actively expressed. Unexpectedly, the HMG-Y, rather than the HMG-I, isoform of these proteins is the most effective elicitor of both neoplastic transformation and metastatic progression in vivo. Furthermore, expression of either antisense or dominant-negative HMGI(Y) constructs inhibits both the rate of proliferation of tumor cells and their ability to grow anchorage independently in soft agar. Array analysis of transcription profiles demonstrates that the HMG-I and HMG-Y isoform proteins each modulate the expression of distinctive constellations of genes known to be involved in signal transduction, cell proliferation, tumor initiation, invasion, migration, induction of angiogenesis, and colonization. Immunohistochemical analyses of tumors formed in nude mice indicate that many have undergone an epithelial-mesenchymal transition in vivo. Together, these findings demonstrate that overexpression of the HMGI(Y) proteins, more specifically, the HMG-Y isoform protein, is causally associated with both neoplastic transformation and metastatic progression and suggest that induction of integrins and their signaling pathways may play significant molecular roles in these biological events.

Activation of NF-kappaB by double-stranded RNA (dsRNA) in the absence of protein kinase R and RNase L demonstrates the existence of two separate dsRNA-triggered antiviral programs

Double-stranded RNA (dsRNA) of viral origin triggers two programs of the innate immunity in virus-infected cells. One is intended to decrease the rate of host cell protein synthesis and thus to prevent viral replication. This program is mediated by protein kinase R (PKR) and by RNase L and contributes, eventually, to the self-elimination of the infected cell via apoptosis. The second program is responsible for the production of antiviral (type I) interferons and other alarmone cytokines and serves the purpose of preparing naive cells for the viral invasion. This second program requires the survival of the infected cell and depends on the expression of antiapoptotic genes through the activation of the NF-kappaB transcription factor. The second program therefore relies on ongoing transcription and translation. It has been proposed that PKR plays an essential role in the activation of NF-kappaB by dsRNA. Here we present evidence that the dsRNA-induced NF-kappaB activity and the expression of beta interferon and inflammatory cytokines do not require either PKR or RNase L. Our results indicate, therefore, that the two dsRNA-activated programs are separate and can function independently of each other.

Molecular analysis of the prostate-specific antigen upstream gene enhancer

BACKGROUND

Our objective was to identify factors other than androgen receptor that bind to and regulate the prostate-specific antigen (PSA) upstream gene enhancer (PSE).

METHODS

DNAse I footprinting and electromobility shift assays (EMSA) were performed over the PSE using lysates from PSA-producing cell lines, LNCaP and LAPC4, and nonproducing PSA cell lines, PC-3 cells, U937 monocytes, and Namalwa B cells. Mutational analysis and transient transfection assays were used to determine the contributions made by different elements towards the regulation of the enhancer.

RESULTS

Three distinct regions surrounding androgen response elements of the PSE were found to bind unknown ubiquitous and cell type-specific proteins. These regions, when mutated in a PSE reporter construct, were shown to be required for maximal activation in LNCaP cells.

CONCLUSIONS

These results correlate unknown sequence-specific DNA binding proteins with androgen-mediated regulation of a prostate-specific gene, thus providing further insight into the mechanism of PSA gene expression.

Transcriptional activation of the interleukin-2 promoter by hepatitis C virus core protein

Most patients infected with hepatitis C virus (HCV) become chronic carriers. Viruses that efficiently establish persistent infections must have effective ways of evading host defenses. In the case of HCV, little is known about how chronic infections are established or maintained. Besides hepatocytes, several reports suggest that HCV can infect T and B lymphocytes. Since T cells are essential for viral clearance, direct or indirect effects of HCV on T-cell function could influence the outcome of infection. Given that T-cell growth and differentiation require the cytokine interleukin 2 (IL-2), we asked whether HCV might modulate synthesis of IL-2. Portions of the HCV polyprotein were expressed in Jurkat cells under a variety of conditions. We found that the highly conserved HCV core protein, in combination with other stimuli, was able to dramatically activate transcription from the IL-2 promoter. The carboxy-terminal hydrophobic portion of the core protein was required for this activity. Activation was dependent on nuclear factor of activated T cells (NFAT), occurred in cells deficient in the tyrosine kinase p56(lck), and could be blocked by addition of cyclosporin A and by depletion of calcium. These results suggest that the HCV core protein can activate transcription of the IL-2 promoter through the NFAT pathway. This novel activity may have consequences for T-cell development and establishment of persistent infections.

Retroviral cDNA integration: stimulation by HMG I family proteins

To replicate, a retrovirus must synthesize a cDNA copy of the viral RNA genome and integrate that cDNA into a chromosome of the host. We have investigated the role of a host cell cofactor, HMG I(Y) protein, in integration of human immunodeficiency virus type 1 (HIV-1) and Moloney murine leukemia virus (MoMLV) cDNA. Previously we reported that HMG I(Y) cofractionates with HIV-1 preintegration complexes (PICs) isolated from freshly infected cells. PICs depleted of required components by treatment with high concentrations of salt could be reconstituted by addition of purified HMG I(Y) in vitro. Here we report studies using immunoprecipitation that indicate that HMG I(Y) is associated with MoMLV preintegration complexes. In mechanistic studies, we show for both HIV-1 and MoMLV that each HMG I(Y) monomer must contain multiple DNA binding domains to stimulate integration by HMG I(Y)-depleted PICs. We also find that HMG I(Y) can condense model HIV-1 or MoMLV cDNA in vitro as measured by stimulation of intermolecular ligation. This reaction, like reconstitution of integration, depends on the presence of multiple DNA binding domains in each HMG I(Y) monomer. These data suggest that binding of multivalent HMG I(Y) monomers to multiple cDNA sites compacts retroviral cDNA, thereby promoting formation of active integrase-cDNA complexes.

Interleukin 1β inhibits interleukin 6–mediated rat γ fibrinogen gene expression

Interleukin (IL)-1β and IL-6 are the 2 major inducers of a group of hepatic genes during acute inflammation; however, each cytokine uses different intracellular signaling molecules. In most instances, the 2 cytokines interact positively to enhance hepatic gene expression, but in one class of acute-phase reactants, which includes fibrinogen, IL-1β exerts a transient inhibitory effect over the IL-6 stimulatory signal. This study explored the effects of IL-1β/nuclear factor κB (NF-κB) and IL-6/signal transducer and activator of transcription 3 (STAT3) combinatory signaling on the transcriptional regulation of the rat γ fibrinogen gene. Northern blot and functional analyses employing luciferase reporter constructs driven by the rat γ fibrinogen promoter demonstrated that IL-1β inhibited the IL-6-mediated transcription of this gene. Exposing primary rat hepatocytes to IL-1β had no effect on IL-6-mediated STAT3 activation; instead, IL-1β-activated NF-κB associated with 2 IL-6 responsive elements (STAT3 binding site) on the rat γ fibrinogen promoter and blocked STAT3 binding to these regions. The competitive binding of NF-κB and STAT3 on the overlapping binding site provides a mechanism for the inhibition by IL-1β of the IL-6-mediated transactivation of rat γ fibrinogen.

Interleukin 1β inhibits interleukin 6–mediated rat γ fibrinogen gene expression

Interleukin (IL)-1β and IL-6 are the 2 major inducers of a group of hepatic genes during acute inflammation; however, each cytokine uses different intracellular signaling molecules. In most instances, the 2 cytokines interact positively to enhance hepatic gene expression, but in one class of acute-phase reactants, which includes fibrinogen, IL-1β exerts a transient inhibitory effect over the IL-6 stimulatory signal. This study explored the effects of IL-1β/nuclear factor κB (NF-κB) and IL-6/signal transducer and activator of transcription 3 (STAT3) combinatory signaling on the transcriptional regulation of the rat γ fibrinogen gene. Northern blot and functional analyses employing luciferase reporter constructs driven by the rat γ fibrinogen promoter demonstrated that IL-1β inhibited the IL-6-mediated transcription of this gene. Exposing primary rat hepatocytes to IL-1β had no effect on IL-6-mediated STAT3 activation; instead, IL-1β-activated NF-κB associated with 2 IL-6 responsive elements (STAT3 binding site) on the rat γ fibrinogen promoter and blocked STAT3 binding to these regions. The competitive binding of NF-κB and STAT3 on the overlapping binding site provides a mechanism for the inhibition by IL-1β of the IL-6-mediated transactivation of rat γ fibrinogen.

Isolation of a Brassica napus L. cDNA encoding a putative high-mobility-group HMG I/Y protein

A cDNA encoding a high-mobility-group protein has been isolated from a microspore-specific library of Brassica napus. The 930 bp cDNA contains a 612 bp open reading frame encoding a protein of 203 amino acids residues exhibiting significant homology to HMG-I/Y protein from Arabidopsis thaliana (62%). The predicted protein contains four copies of the 'AT-hook' motif which is involved in binding A/T-rich DNA. Southern blotting indicates that the HMG-I/Y gene is a single-copy gene in B. napus. Transcription of the HMG-I/Y gene was detected in all tissues examined, with the highest expression in pollen-derived embryos. In situ localization studies of flower organs indicate the transcript to be preferentially located in petals and sepals. Subcellular localization analysis performed during pollen development showed that the transcript of the HMG-I/Y gene is predominantly associated with polysomes.

High-mobility-group protein I can modulate binding of transcription factors to the U5 region of the human immunodeficiency virus type 1 proviral promoter

HMG I/Y appears to be a multifunctional protein that relies on in its ability to interact with DNA in a structure-specific manner and with DNA, binding transcriptional activators via distinct protein-protein interaction surfaces. To investigate the hypothesis that HMG I/Y may have a role in human immunodeficiency virus type 1 (HIV-1) expression, we have analyzed whether HMG I/Y interacts with the 5' long terminal repeat and whether this interaction can modulate transcription factor binding. Using purified recombinant HMG I, we have identified several high-affinity binding sites which overlap important transcription factor binding sites. One of these HMG I binding sites coincides with an important binding site for AP-1 located downstream of the transcriptional start site, in the 5' untranslated region at the boundary of a positioned nucleosome. HMG I binding to this composite site inhibits the binding of recombinant AP-1. Consistent with this observation, using nuclear extracts prepared from Jurkat T cells, we show that HMG I (but not HMG Y) is strongly induced upon phorbol myristate acetate stimulation and this induced HMG I appears to both selectively inhibit the binding of basal DNA-binding proteins and enhance the binding of an inducible AP-1 transcription factor to this AP-1 binding site. We also report the novel finding that a component present in this inducible AP-1 complex is ATF-3. Taken together, these results argue that HMG I may play a fundamental role in HIV-1 expression by determining the nature of transcription factor-promoter interactions.

NFkappa B interacts with serum amyloid A3 enhancer factor to synergistically activate mouse serum amyloid A3 gene transcription

We had previously identified a distal regulatory element (DRE) in the mouse serum amyloid A3 (SAA3) promoter that functions as a cytokine-inducible transcription enhancer. Within this DRE, three functional elements interact with CCAAT/enhancer-binding protein (C/EBP) and SAA3 enhancer factor (SEF) transcription factors. In this study, we show that cotransfection of the SEF expression plasmid with an SAA3/luciferase reporter resulted in 3-5-fold activation of the SAA3 promoter. When SEF-transfected cells were further stimulated with conditioned medium or interleukin-1, SAA3 promoter activity was dramatically increased, suggesting that SEF may cooperate functionally with other interleukin-1-inducible transcription factors to synergistically up-regulate SAA3 gene transcription. Indeed, cotransfection of SEF and NFkappaBp65 expression DNAs resulted in synergistic activation of the SAA3 promoter. Intriguingly, no consensus NFkappaB-binding site was found in the SAA3 promoter region; rather a putative NFkappaB-binding sequence with 3-base pair mismatches was identified in the DRE. When this sequence was used in an electrophoretic mobility shift assay, it interacted with NFkappaBp50, albeit with binding affinities that were several hundredfold lower than that with the consensus NFkappaB probe. Functional cooperation between SEF and NFkappaB was further strengthened by the finding that SEF and NFkappaB formed stable cytokine-inducible protein-protein complexes. Finally, despite its weak binding, mutation of this NFkappaB-binding site nevertheless dramatically reduced both NFkappaBp65- and cytokine-mediated induction of SAA3 promoter. Therefore, the molecular basis for the functional synergy between SEF and NFkappaB may, in part, be the ability of SEF to recruit NFkappaB through physical interactions that lead to enhancement or stabilization of NFkappaB binding to the SAA3 promoter element.

FR900482 class of anti-tumor drugs cross-links oncoprotein HMG I/Y to DNA in vivo

BACKGROUND

Overexpression of the high-mobility group, HMG I/Y, family of chromatin oncoproteins has been implicated as a clinical diagnostic marker for both neoplastic cellular transformation and increased metastatic potential of several human cancers. These minor groove DNA-binding oncoproteins are thus an attractive target for anti-tumor chemotherapy. FR900482 represents a new class of anti-tumor agents that bind to the minor groove of DNA and exhibit greatly reduced host toxicity compared to the structurally related mitomycin C class of anti-tumor drugs. We report covalent cross-linking of DNA to HMG I/Y by FR900482 in vivo which represents the first example of a covalent DNA-drug-protein cross-link with a minor groove-binding oncoprotein and a potential novel mechanism through which these compounds exert their anti-tumor activity.

RESULTS

Using a modified chromatin immunoprecipitation procedure, fragments of DNA that have been covalently cross-linked by FR900482 to HMG I/Y proteins in vivo were polymerase chain reaction-amplified, isolated and characterized. The nuclear samples from control cells were devoid of DNA fragments whereas the nuclear samples from cells treated with FR900482 contained DNA fragments which were cross-linked by the drug to the minor groove-binding HMG I/Y proteins in vivo. Additional control experiments established that the drug also cross-linked other non-oncogenic minor groove-binding proteins (HMG-1 and HMG-2) but did not cross-link major groove-binding proteins (Elf-1 and NFkappaB) in vivo. Our results are the first demonstration that FR900482 cross-links a number of minor groove-binding proteins in vivo and suggests that the cross-linking of the HMG I/Y oncoproteins may participate in the mode of efficacy as a chemotherapeutic agent.

CONCLUSIONS

We have illustrated that the FR class of anti-tumor antibiotics, represented in this study by FR900482, is able to produce covalent cross-links between the HMG I/Y oncoproteins and DNA in vivo. The ability of this class of compounds to cross-link the HMG I/Y proteins in the minor groove of DNA represents the first demonstration of drug-induced cross-linking of a specific cancer-related protein to DNA in living cells. We have also demonstrated that FR900482 cross-links other minor groove-binding proteins (HMG-1 and HMG-2 in the present study) in vivo; however, since HMG I/Y is the only minor groove-binding oncoprotein presently known, it is possible that these non-histone chromatin proteins are among the important in vivo targets of this family of drugs. These compounds have already been assessed as representing a compelling clinical replacement for mitomycin C due to their greatly reduced host toxicity and superior DNA interstrand cross-linking efficacy. The capacity of FR900482 to cross-link the HMG I/Y oncoprotein with nuclear DNA in vivo potentially represents a significant elucidation of the anti-tumor efficacy of this family of anticancer agents.

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.

Distant enhancers stimulate the albumin promoter through complex proximal binding sites

The albumin-alpha-fetoprotein locus epitomizes the main features of transcriptional regulation of fetal and adult hepatocyte-specific genes: developmentally regulated promoters and strong distant enhancers. Full enhancer activity required only a proximal albumin-promoter region containing the TATA box, hepatic nuclear factor 1 (HNF1), and nuclear factor Y (NF-Y) sites. Deletion of the HNF1 site abrogated enhancer and promoter activity, whereas methylation of the site reduced all activity by about 3-fold. Deletion of the NF-Y site attenuated activity by about half, but much of the activity could be replaced by juxtaposition of an upstream region (designated distal element IV). Gel shift and competition analysis demonstrated that binding of architectural factors overlapped NF-Y binding. Moreover, a mutation that eliminated NF-Y binding but only minimally perturbed the surrounding region did not affect enhancer function. In plasmids with a second promoter, the enhancers simultaneously stimulated both albumin and alpha-fetoprotein promoters with minimal competition, but surprisingly some mutations in the albumin promoter attenuated expression from both promoters, whereas another uncoupled their expression. With single promoters, the function of the proximal promoter region was controlled by three parameters in the following hierarchy: HNF1 binding > local architecture > NF-Y binding, but integrated two-promoter function had a much greater dependence on NF-Y.

The chicken genome contains no HMG1 retropseudogenes but a functional HMG1 gene with long introns

We have cloned the genomic sequence coding for the high mobility group 1 (HMG1) protein in chickens. Multiple sequence alignment shows that the chicken HMG1 gene is highly homologous to the human and the mouse HMG1 genes. The gene structure of chicken HMG1 is similar to that of the mouse and the human HMG1 genes, with the same exon-intron boundaries. However, in contrast to other avian genes that have shorter introns, the chicken HMG1 gene has introns that are twice as long as their mammalian homologues. In addition to the functional, intron-containing HMG1 gene, all mammalian genomes contain more than 50 copies of HMG1 retropseudogenes each, while in the chicken genome there are no HMG1 retropseudogenes. This finding suggests that the HMG1 retropseudogenes arose in mammals after their divergence away from the birds.

Mechanism of kappa B DNA binding by Rel/NF-kappa B dimers

The DNA binding of three different NF-kappaB dimers, the p50 and p65 homodimers and the p50/p65 heterodimer, has been examined using a combination of gel mobility shift and fluorescence anisotropy assays. The NF-kappaB p50/p65 heterodimer is shown here to bind the kappaB DNA target site of the immunoglobulin kappa enhancer (Ig-kappaB) with an affinity of approximately 10 nm. The p50 and p65 homodimers bind to the same site with roughly 5- and 15-fold lower affinity, respectively. The nature of the binding isotherms indicates a cooperative mode of binding for all three dimers to the DNA targets. We have further characterized the role of pH, salt, and temperature on the formation of the p50/p65 heterodimer-Ig-kappaB complex. The heterodimer binds to the Ig-kappaB DNA target in a pH-dependent manner, with the highest affinity between pH 7.0 and 7.5. A strong salt-dependent interaction between Ig-kappaB and the p50/p65 heterodimer is observed, with optimum binding occurring at monovalent salt concentrations below 75 mm, with binding becoming virtually nonspecific at a salt concentration of 200 mm. Binding of the heterodimer to DNA was unchanged across a temperature range between 4 degrees C and 42 degrees C. The sensitivity to ionic environment and insensitivity to temperature indicate that NF-kappaB p50/p65 heterodimers form complexes with specific DNA in an entropically driven manner.

HMG-I/Y, a new c-Myc target gene and potential oncogene

The HMG-I/Y gene encodes the HMG-I and HMG-Y proteins, which function as architectural chromatin binding proteins important in the transcriptional regulation of several genes. Although increased expression of the HMG-I/Y proteins is associated with cellular proliferation, neoplastic transformation, and several human cancers, the role of these proteins in the pathogenesis of malignancy remains unclear. To better understand the role of these proteins in cell growth and transformation, we have been studying the regulation and function of HMG-I/Y. The HMG-I/Y promoter was cloned, sequenced, and subjected to mutagenesis analysis. A c-Myc-Max consensus DNA binding site was identified as an element important in the serum stimulation of HMG-I/Y. The oncoprotein c-Myc and its protein partner Max bind to this site in vitro and activate transcription in transfection experiments. HMG-I/Y expression is stimulated by c-Myc in a Myc-estradiol receptor cell line in the presence of the protein synthesis inhibitor cycloheximide, indicating that HMG-I/Y is a direct c-Myc target gene. HMG-I/Y induction is decreased in Myc-deficient fibroblasts. HMG-I/Y protein expression is also increased in Burkitt's lymphoma cell lines, which are known to have increased c-Myc protein. Like Myc, increased expression of HMG-I protein leads to the neoplastic transformation of both Rat 1a fibroblasts and CB33 cells. In addition, Rat 1a cells overexpressing HMG-I protein form tumors in nude mice. Decreasing HMG-I/Y proteins using an antisense construct abrogates transformation in Burkitt's lymphoma cells. These findings indicate that HMG-I/Y is a c-Myc target gene involved in neoplastic transformation and a member of a new class of potential oncogenes.

Viral activation of interleukin-15 (IL-15): characterization of a virus-inducible element in the IL-15 promoter region

We identified an interferon regulatory factor motif (IRF-E) upstream of an NF-kappaB binding site in the interleukin-15 (IL-15) promoter. Since these two motifs are part of the virus-inducible enhancer region of the beta interferon promoter, we speculated that there might be similar responses of these two genes to stimuli such as viruses. To test this hypothesis, L929 cells were infected with Newcastle disease virus (NDV), which led to the induction of IL-15 mRNA and protein expression. Using IL-15 promoter-reporter deletion constructs, a virus-inducible region, encompassing IRF-E, NF-kappaB, and a 13-nucleotide sequence flanked by these two motifs, was mapped to the -295-to--243 position relative to the transcription initiation site. Using cotransfection studies, it was demonstrated that all three motifs were essential to achieve the maximum promoter activity induced by IRF-1 and NF-kappaB expression plasmids. The presence of a virus-inducible region in the IL-15 promoter suggests a role for IL-15 as a component of host antiviral defense mechanisms.

Assembly of a functional beta interferon enhanceosome is dependent on ATF-2-c-jun heterodimer orientation

Heterodimeric transcription factors, including the basic region-leucine zipper (bZIP) protein ATF-2-c-jun, are well-characterized components of an enhanceosome that mediates virus induction of the human beta interferon (IFN-beta) gene. Here we report that within the IFN-beta enhanceosome the ATF-2-c-jun heterodimer binds in a specific orientation, which is required for assembly of a complex between ATF-2-c-jun and interferon regulatory factor 3 (IRF-3). We demonstrate that correct orientation of the ATF-2-c-jun binding site is required for virus induction of the IFN-beta gene and for IRF-3-dependent activation of a composite ATF-2- c-jun-IRF site in the IFN-beta promoter. We also show that in vitro the DNA-bound ATF-2-c-jun heterodimer adopts a fixed orientation upon the binding of IRF-3 at an adjacent site in the IFN-beta enhancer and that the DNA-binding domain of IRF-3 is sufficient to mediate this effect. In addition, we show that the DNA-binding domain of ATF-2 is necessary and sufficient for selective protein-protein interactions with IRF-3. Strikingly, in vivo chromatin immunoprecipitation experiments with IFN-beta reporter constructs reveal that recruitment of IRF-3 to the IFN-beta promoter upon virus infection is dependent on the orientation of the ATF-2-c-jun heterodimer binding site. These observations demonstrate functional and physical cooperativity between the bZIP and IRF transcription factor families and illustrate the critical role of heterodimeric transcription factors in formation of the IFN-beta enhanceosome.

Mouse embryos do not wait for the MBT: chromatin and RNA polymerase remodeling in genome activation at the onset of development

In Xenopus and Drosophila embryos, activation of the zygotic genome occurs after a series of rapid nuclear divisions in which DNA replication occupies most of the cell cycle. In these organisms, it has been proposed that zygotic transcription does not begin until a threshold nucleocytoplasmic ratio has been obtained in which repressive factors are titrated out and interphase becomes long enough to allow synthesis of transcripts. In mammalian embryos, however, a model of threshold nucleocytoplasmic ratios does not seem to apply, as beginning with the 1-cell stage, there are regulated cell cycles with the expression of zygotic transcripts during the cleavage period. By taking advantage of the slower kinetics at the onset of mouse development, we have characterized changes in chromatin structure and the basal transcription machinery throughout the transition from transcriptional incompetence, to minor activation of the zygotic genome during the 1-cell stage, and through major genome activation at the 2-cell stage. Further maturation of chromatin structure continues through subsequent cleavage cycles as a foundation for the first cellular differentiations in the blastocyst. The epigenetic chromatin modifications that occur during the cleavage period may have long range and inheritable effects and are undoubtedly important in the ability of the mammalian oocyte to remodel previously defined nuclear structures and cell fates.

Biochemical analyses of the AF10 protein: the extended LAP/PHD-finger mediates oligomerisation

Leukaemogenesis correlates with alterations in chromatin structure brought about by the gain or loss of interactive domains from regulatory factors that are disrupted by chromosomal translocations. The gene MLL, a target of such translocation events, forms chimaeric fusion products with a variety of partner genes. While MLL appears to be involved in chromatin-mediated gene regulation, the functions of its partner genes are largely speculative. We report the biochemical analysis of the MLL partner gene AF10 and its possible role in leukaemogenesis. AF10 has been reported to be re-arranged with genes other than MLL leading to the same phenotype, a myeloid leukaemia. We have identified a novel protein-protein interaction motif in the AF10 protein comprising the extended LAP/PHD-finger. This domain mediates homo-oligomerisation of recombinant AF10 and is conserved in several proteins, including MLL itself. AF10 binds cruciform DNA via a specific interaction with an AT-hook motif and is localised to the nucleus by a defined bipartite nuclear localisation signal in the N-terminal region.

Observations on the covalent cross-linking of the binding domain (BD) of the high mobility group I/Y (HMG I/Y) proteins to DNA by FR66979

FR66979, a drug closely related to the mitomycin C class of antitumor antibiotics, is shown to covalently cross-link DNA to the DNA-binding domain of the High Mobility Group I/Y (HMG I/Y) DNA-binding proteins in the minor groove.

Induction of early transcription in one-cell mouse embryos by microinjection of the nonhistone chromosomal protein HMG-I

In the mouse embryo, the onset of zygotic transcription occurs at the end of the first cell cycle, upon completion of DNA replication. We show that the nonhistone chromosomal protein HMG-I, whose translocation into the pronuclei of one-cell embryos is linked to this first round of DNA synthesis, plays a critical role in the activation of zygotic transcription. Indeed, microinjection of purified HMG-I results in a higher nuclear accumulation of the protein and triggers an earlier activation of zygotic transcription, an effect which is abolished by the preincubation of the protein with a specific antibody directed against its AT-hook DNA-binding motifs. Significantly, microinjection of this antibody also prevents the normal onset of transcription in the embryo, suggesting that endogenous HMG-I is similarly involved in this process. Finally, microinjection of the exogenous protein modifies chromatin structure as measured by in situ accessibility to DNase I. We propose that general chromosomal architectural factors such as HMG-I can modulate the accessibility of chromatin to specialized regulatory factors, thereby promoting a transcriptionally competent state.

Transgenic mice expressing a truncated form of the high mobility group I-C protein develop adiposity and an abnormally high prevalence of lipomas

Chromosomal translocations in human lipomas frequently create fusion transcripts encoding high mobility group (HMG) I-C DNA-binding domains and C-terminal sequences from different presumed transcription factors, suggesting a potential role for HMG I-C in the development of lipomas. To evaluate the role of the HMG I-C component, the three DNA-binding domains of HMG I-C have now been expressed in transgenic mice. Despite the ubiquitous expression of the truncated HMG I-C protein, the transgenic mice develop a selective abundance of fat tissue early in life, show marked adipose tissue inflammation, and have an abnormally high incidence of lipomas. These findings demonstrate that the DNA-binding domains of HMG I-C, in the absence of a C-terminal fusion partner, are sufficient to perturb adipogenesis and predispose to lipomas. We provide data supporting the central utility of this animal model as a tool to understand the molecular mechanisms underlying the development of one of the most common kind of human benign tumors.

Transcriptional activation of the rice tungro bacilliform virus gene is critically dependent on an activator element located immediately upstream of the TATA box

To investigate the transcriptional mechanisms of rice tungro bacilliform virus, we have systematically analyzed an activator element located immediately upstream of the TATA box in the rice tungro bacilliform virus promoter and its cognate trans-acting factors. Using electrophoretic mobility shift assays, we showed that rice nuclear proteins bind to the activator element, forming multiple specific DNA-protein complexes via protein-protein interactions. Copper-phenanthroline footprinting and DNA methylation interference analysis indicated that multiple DNA-protein complexes share a common binding site located between positions -60 to -39, and the proteins contact the activator element in the major groove. DNA UV cross-linking assays further showed that two nuclear proteins (36 and 33 kDa), found in rice cell suspension and shoot nuclear extracts, and one (27 kDa), present in root nuclear extracts, bind to this activator element. In protoplasts derived from a rice (Oryza sativa) suspension culture, the activator element is a prerequisite for promoter activity and its function is critically dependent on its position relative to the TATA box. Thus, transcriptional activation may function via interactions with the basal transcriptional machinery, and we propose that this activation is mediated by protein-protein interactions in a position-dependent mechanism.

Genomic structure and expression of the murine Hmgi(y) gene

Mammalian HMGI proteins belong to the high mobility group (HMG) of small non-histone nuclear proteins, and function as architectural factors to mediate structural changes in DNA. The HMGI family consists of three members: HMGI, HMGY and HMGI-C. As pseudogenes have complicated the genomic analysis of murine Hmgi(y), a mouse lambda FIX II genomic library was screened with an intron-specific probe to identify and characterize the authentic Hmgi(y) gene. The murine Hmgi(y) gene is 7.2kb long and contains four protein coding exons and two additional exons encoding part of the 5' untranslated region. Sequencing confirms that an alternative splicing site within exon 3 results in the two protein isoforms: Hmgi and Hmgy. Primer extension experiments revealed that at least three transcription start sites exist in the 5' end of the gene. It has been well established that the expression of both Hmgi-c and Hmgi(y) is readily detectable throughout embryogenesis. Unlike Hmgi-c, whose expression is restricted to embryogenesis, a Northern hybridization analysis showed low-level expression of Hmgi(y) in adult mouse tissues. Similarly, when tissues from newborn animals were examined, Hmgi(y) expression was readily detected at a level of intensity intermediate between that found in embryos and adults. Understanding the gene structure and expression pattern will provide important insights into the in-vivo function of Hmgi(y).

Regulation of the mammalian alcohol dehydrogenase genes

This review focuses on the regulation of the mammalian medium-chain alcohol dehydrogenase (ADH) genes. This family of genes encodes enzymes involved in the reversible oxidation of alcohols to aldehydes. Interest in these enzymes is increased because of their role in the metabolism of beverage alcohol as well as retinol, and their influence on the risk for alcoholism. There are six known classes ADH genes that evolved from a common ancestor. ADH genes differ in their patterns of expression: most are expressed in overlapping tissue-specific patterns, but class III ADH genes are expressed ubiquitously. All have proximal promoters with multiple cis-acting elements. These elements, and the transcription factors that can interact with them, are being defined. Subtle differences in sequence can affect affinity for these factors, and thereby influence the expression of the genes. This provides an interesting system in which to examine the evolution of tissue specificity. Among transcription factors that are important in multiple members of this gene family are the C/EBPs, Sp1,USF, and AP1, HNF-1, CTF/NF-1, glucocorticoid, and retinoic acid receptors, and several as-yet unidentified negative elements, are important in at least one of the genes. There is evidence that cis-acting elements located far from the proximal promoter are necessary for proper expression. Three of the genes have upstream AUGs in the 5' nontranslated regions of their mRNA, unusual for mammalian genes. The upstream AUGs have been shown to significantly affect expression of the human ADH5 gene.

Morphine treatment affects the regulation of high mobility group I-type chromosomal phosphoproteins in C6 glioma cells

High mobility group (HMG) I-type chromosomal phosphoproteins HMG I/Y and HMG I-C were investigated following morphine treatment of C6BU-1 glioma. Cells were labelled with [32P]-orthophosphoric acid. Electrophoretic profiles and autoradiograms of the control cells revealed the presence of HMG I and HMG I-C proteins. HMG Y was not detected. Northern blot analysis showed a single HMG I/Y transcript. Treatment with morphine lowered the [32P]-incorporation in HMG I and HMGI-C proteins and the level of the HMG I/Y transcript. However, it did not change the protein ratios on the Coomassie stained gels. These results suggest that morphine may trigger independent reaction pathways affecting either transcription regulation and/or postsynthetic phosphorylation of the preexisting HMG I-type proteins. In addition, opposing changes in the postsynthetic phosphorylation of HMG 14 and histones H1AB were also noticed.