Nuclear factor I and epithelial cell-specific transcription of human papillomavirus type 16

Transcription factor activities and gene expression during mouse mammary gland involution

Maintenance of mammary epithelial differentiation and milk production during lactation is a consequence of milk removal and the presence of lactogenic hormones, particularly glucocorticoids, insulin and prolactin. After weaning the fall in lactogenic hormones and milk stasis lead to involution, a process that is mainly characterized by three events: (i) downregulation of milk protein gene expression, (ii) loss of epithelial cells by apoptosis and, (iii) tissue remodeling and preparation of the gland for a new pregnancy. Each of these processes is likely to depend on the activity of specific sets of transcription factors in the mammary epithelium and stroma that ensure the timely and spatially coordinated expression of critical gene products such as mediators of apoptosis (e.g., caspase-1 and regulators of tissue remodeling events (e.g., matrix metalloproteinases). Here we describe signal transduction events such as activation of protein kinase A and JNK and changes in the activity of several transcription factors including Stat5, Stat3, NF1, Oct-1, and AP-1 during the early and late phases of mammary gland involution. We discuss their possible role in regulating and coordinating involution with emphasis on the apoptotic process of involution.

A novel YY1-independent silencer represses the activity of the human papillomavirus type 16 enhancer

Regulation of the human papillomavirus type 16 (HPV-16) E6 promoter is a complex process in which transcriptional repression as well as activation plays an important role. Here, we identify a negative regulatory element that in the context of a continuous long control region fragment overcomes the activation of the HPV-16 enhancer. This silencing element, which we have termed a PSM (papillomavirus silencing motif), consists of two copies of the sequence 5'-TAYAATAAT-3' that overlap the origin of replication. Each copy of this 9-bp sequence binds the same unknown cellular factor, which we refer to as PSM-BP (PSM binding protein). Both copies of the binding sequence are required for transcriptional repression, and we provide evidence that suggests that this particular organization results in the stabilization of a PSM-BP dimer. The silencing motif, while functioning in either orientation, showed a positional requirement between the enhancer and the promoter. Experiments with both a heterologous enhancer and a promoter also demonstrated a general ability of this element to function as a transcriptional silencer in non-HPV systems. Our findings provide an important addition to our understanding of HPV-16 gene regulation and an interesting model for the study of transcriptional repression.

Molecular targets for human papillomaviruses: prospects for antiviral therapy

A substantial medical need exists for the development of antiviral medicines for the treatment of diseases associated with infection by human papillomaviruses (HPVs). HPVs are associated with various benign and malignant lesions including benign genital condyloma, common skin warts, laryngeal papillomas and anogenital cancer. Since treatment options are limited and typically not very satisfactory, the development of safe and effective antiviral drugs for HPV could have substantial clinical impact. In the last few years, exciting advances have been made in our understanding of papillomavirus replication and the effects that the virus has on growth of the host cell. Although still somewhat rudimentary, techniques have been developed for limited virion production in vitro offering the promise of more rapid advances in the dissection and understanding of the virus life cycle. Of the 8-10 HPV gene products that are made during infection, only one encodes enzymatic activities, the E1 helicase. Successful antiviral therapies have traditionally targeted viral enzymes such as polymerases, kinases and proteases. In contrast, macromolecular interactions which mediate the functions of E6, E7 and E2 are thought to be more difficult targets for small molecule therapy.

Nuclear factor I (NFI) isoforms differentially activate simple versus complex NFI-responsive promoters

Promoter-specific differences in the function of transcription factors play a central role in the regulation of gene expression. We have measured the maximal transcriptional activation potentials of nuclear factor I (NFI) proteins encoded by each of the four identified NFI genes (NFI-A, -B, -C, and -X) by transient transfection in JEG-3 cells using two model NFI-dependent promoters: 1) a simple chimeric promoter containing a single NFI-binding site upstream of the adenovirus major late promoter (NFI-Ad), and 2) the more complex mouse mammary tumor virus long terminal repeat promoter. The relative activation potentials for the NFI isoforms differed between the two promoters, with NFI-X being the strongest activator of NFI-Ad and NFI-B being the strongest activator of the MMTV promoter. To determine if these promoter-specific differences in activation potential were due to the presence of glucocorticoid response elements (GREs), we added GREs upstream of the NFI-binding site in NFI-Ad. NFI-X remains the strongest activator of the GRE containing simple promoter, indicating that differences in relative activation potential are not due solely to the presence of GREs. Since NFI proteins bind to DNA as dimers, we assessed the activation potentials of NFI heterodimers. Here, we show that NFI heterodimers have intermediate activation potentials compared with homodimers, demonstrating one potential mechanism by which different NFI proteins can regulate gene expression.

Nuclear factor I family members regulate the transcription of surfactant protein-C

Transcription of the surfactant protein-C (SP-C) gene is restricted to Type II epithelial cells in the adult lung. We have shown previously that the 0.32-kilobase pair (kb) mouse SP-C promoter is functional in transient transfection assays of the lung epithelial cell-derived cell line, MLE-15, and that thyroid transcription factor 1 (TTF-1) transactivates promoter activity. The 0.32-kb SP-C promoter can be separated into a proximal promoter region (-230 to +18) and an enhancer region (-318 to -230). Three DNase I footprints were mapped in the promoter region (C1 through C3) and two in the enhancer region (C4 and C5). We now show that nuclear factor I (NFI) family members bind to both individual NFI half-sites in footprints C1, C3, and C5, and to a composite site in footprint C4 by competition gel retardation and antibody supershift analyses. Mutational analysis of the 0.32-kb mouse SP-C promoter and transient transfection of MLE-15 cells demonstrated that the NFI binding sites are required for promoter activity in this cell type. Site-specific mutation of the proximal or distal NFI sites drastically reduced transactivation by a co-transfected NFI-A expression vector in HeLa cells. These data indicate that NFI family member(s), binding to sites in both the promoter and enhancer regions, regulate SP-C gene expression in a process independent of TTF-1.

Transcription activities of human papillomavirus type 11 E6 promoter-proximal elements in raft and submerged cultures of foreskin keratinocytes

Human papillomaviruses (HPVs) replicate only in differentiated squamous epithelia in warts and in epithelial raft cultures grown at the medium-air interface. Virus-encoded and host transcription factors are thought to be responsible for repressing the viral enhancer and promoter located within the upstream regulatory region (URR) in the undifferentiated basal and parabasal cells while up-regulating their activities in the differentiated spinous cells. Using recombinant retroviruses, we acutely transduced neonatal foreskin keratinocytes (PHKs) with a lacZ reporter gene driven by the wild-type URR of the low-risk HPV type 11 or by a URR with individual mutations in seven promoter-proximal elements, some of which have not been characterized previously. Beta-galactosidase activities were detected in the submerged, proliferating PHKs and also in the differentiated spinous cells, but not in the steady-state proliferating basal cells, of stratified raft cultures. In particular, mutation of an Oct1, an Sp1, or a previously unknown promoter-proximal AP1 site severely reduced the reporter activity, whereas mutation of either of two NF1 sites flanking the Oct1 site had no effect. These results demonstrate changes in cellular transcription factor profiles under different culture conditions and begin to characterize the naturally differentiation-dependent activation of the URR. They provide one molecular explanation for the patterns of HPV expression in warts and help validate epithelial raft cultures as an important experimental system for genetic dissection of HPV regulatory elements.

The 5' region of the human papillomavirus type 31 upstream regulatory region acts as an enhancer which augments viral early expression through the action of YY1

Cis-elements which control human papillomavirus early gene expression have previously been localized to sequences in the upstream regulatory region (URR) which are proximal to the E6 open reading frame. These elements include an enhancer element which functions preferentially in keratinocytes as well as promoter elements. The function of the remaining approximate 500-bp region of the URR in regulating viral expression in the high risk papillomaviruses has been largely uncharacterized. In HPV 6, a negative regulator of early expression, or silencer, has been identified in this 5' region of the URR. In this study, we have investigated the role of the 5' portion of the HPV 31 URR in regulating viral expression. Sequences in this region were found to exert a minimal negative effect on homologous or heterologous promoters. In contrast, a 128-bp sequence within this region was found to exhibit enhancer activity on heterologous and homologous promoters. This enhancer also augmented the activity of the previously identified minimum keratinocyte enhancer. The cellular factors, YY1 and TEF-1, were determined by DNase I footprint analysis and electrophoretic mobility shift assays (EMSAs) to bind the 128-bp enhancer. The binding of YY1 to two of these sites was found to be important for enhancer activity.

The epidermis: genes on - genes off

The epidermal keratinocyte stem cell is distinguished by a relatively undifferentiated phenotype and an ability to proliferate. As part of a carefully orchestrated process, the offspring of these stem cells lose the ability to proliferate and begin a process of morphologic and biochemical transformation that results in their conversion into corneocytes. This process requires the coordinated expression of a host of cellular genes. The mechanisms responsible for regulation of these genes is an area of intense interest. In keratinocytes, as in other cell types, the expression of most genes is regulated at the transcriptional level by a class of proteins called transcription factors. Transcription factors are nuclear proteins that regulate transcription by mediating the final steps in the relay of information from the cell surface to the nucleus and the gene. These factors bind to specific DNA sequence elements located within the target gene. In this brief review we summarize evidence implicating activator protein 1 (AP1), AP2, Sp1, POU domain, CCAAT enhancer binding protein, and several other transcription factors as regulators of expression of keratinocyte genes.

The epithelial cell default-phenotype hypothesis and its implications for cancer

The expression of epithelial cell adhesion and cytoskeletal genes is orchestrated by an apparently unique set of rules. No tissue-specific transactivator proteins have been found to drive them; only ubiquitous factors are utilized. In non-epithelial cells, they are actively repressed. Moreover, it was recently found that a single protein (adenovirus E1a) coordinately represses non-epithelial genes while inducing epithelial genes. A simple model is offered to explain how epithelial gene expression is coordinated. Under this model, the epithelial cell gene expression program is a transcriptional 'default'; that is, it occurs in the absence of tissue-specific transactivation. Conversion to this default requires only that mesenchymal transactivators are not expressed, or that central 'integrator' proteins are inactive. In their absence, mesenchymal gene expression cannot occur. Moreover, because the repressors cease to be expressed, the epithelial genes are induced. Oncogenes generally cause the breakdown of the epithelial phenotype--generating carcinomas--so genes such as E1a that cause epithelial conversion may prove useful for both understanding and controlling cancer.

NFI-B3, a novel transcriptional repressor of the nuclear factor I family, is generated by alternative RNA processing

Nuclear factor I (NFI) proteins constitute a family of sequence-specific transcription factors whose functional diversity is generated through transcription from four different genes (NFI-A, NFI-B, NFI-C, and NFI-X), alternative RNA splicing, and protein heterodimerization. Here we describe a naturally truncated isoform, NFI-B3, which is derived from the human NFI-B gene, in addition to characterizing further human NFI-B1 and NFI-B2, two differentially spliced variants previously isolated from hamster and chicken. Although NFI-B1 and NFI-B2 proteins are translated from an 8. 7-kilobase message, the mRNA for NFI-B3 has a size of only 1.8 kilobases. The NFI-B3 message originates from the failure to excise the first intron downstream of the exons encoding the DNA binding domain and subsequent processing of this transcript at an intron-internal polyadenylation signal. The translation product includes the proposed DNA binding and dimerization domain and terminates after translation of two additional "intron" encoded codons. In SL-2 cells, which are void of endogenous NFI, NFI-B3 by itself had no effect on transcriptional regulation and failed to bind DNA. Coexpression of NFI-B3 with other isoforms of the NFI-B, -C, and -X family, however, led to a strong reduction of transcriptional activation compared with the expression of these factors alone. Gel shift analysis indicated that NFI-B3 disrupts the function of other NFI proteins by reducing their DNA binding activity by heterodimer formation. The efficiency of NFI-B3 heterodimers to bind to DNA correlated with the degree of transcriptional repression. The abundance of NFI-B transcripts varied significantly between different human cell lines and tissues, suggesting a potential involvement of these factors in the complex mechanisms that generate cell type specificity.

Expression patterns of the four nuclear factor I genes during mouse embryogenesis indicate a potential role in development

The nuclear factor I (NFI) family of site-specific DNA-binding proteins is required for both the cell-type specific transcription of many viral and cellular genes and for the replication of adenovirus DNA. Although binding sites for NFI proteins within the promoters of several tissue-specific genes have been shown to be essential for their expression, it is unclear which NFI gene products function in specific tissues during development. We have isolated cDNAs from all four murine NFI genes (gene designations Nfia, Nfib, Nfic, and Nfix), assessed the embryonic and postnatal expression patterns of the NFI genes, and determined the ability of specific NFI proteins to activate transcription from the NFI-dependent mouse mammary tumor virus (MMTV) promoter. In adult mice, all four NFI genes are most highly expressed in lung, liver, heart, and other tissues but only weakly expressed in spleen and testis. The embryonic expression patterns of the NFI genes is complex, with NFI-A transcripts appearing earliest-within 9 days postcoitum in the heart and developing brain. The four genes exhibit unique but overlapping patterns of expression during embryonic development, with high level expression of NFI-A, NFI-B, and NFI-X transcripts in neocortex and extensive expression of the four genes in muscle, connective tissue, liver, and other organ systems. The four NFI gene products studied differ in their ability to activate expression of the NFI-dependent MMTV promoter, with the NFI-B protein being most active and the NFI-A protein being least active. These data are discussed in the context of the developmental expression patterns of known NFI-responsive genes. The differential activation of an NFI-dependent promoter, together with the expression patterns observed for the four genes, indicate that the NFI proteins may play an important role in regulating tissue-specific gene expression during mammalian embryogenesis.

Propagation of the human polyomavirus, JCV, in human neuroblastoma cell lines

Susceptibility to infection by the human polyomavirus, JCV, is determined by intracellular mechanisms which control transcription and replication. Originally thought to propagate well only in human cells of oligodendroglial lineage, JCV has recently been shown to infect astrocytes, astrogliomas, and a neuroblastoma cell line. The data reported here describe two cell types that have been subcultured from a human neuroblastoma cell line, SK-N-SH. The SH-SY5Y subclone displays neuronal phenotypes and is not susceptible to JCV infection, while the SH-EP subclone displays glial cell phenotypes and is susceptible to infection. Binding of nuclear proteins from the permissive SH-EP cells to the nuclear factor-1 (NF-1) site in the JCV regulatory DNA sequences results in a gel shift pattern that is different from the nonpermissive SH-SY5Y cell proteins. Northern analysis of mRNA for the four classes of NF-1 proteins showed a predominance of the NF-1/X class in SH-EP cells similar to the highly permissive human fetal glial cells. Very low levels of mRNA for NF-1/X were seen in the nonpermissive SH-SY5Y cells, similar to that seen for the nonpermissive HeLa cells. Several other cell lines tested that were permissive for JCV infection also showed synthesis of the NF-1/X class of proteins. SH-EP cells represent a cell line in a glial cell lineage which is susceptible to JCV multiplication. These cells may be a useful cell culture system for the investigation of DNA binding factors which correlates with viral susceptibility.

Second-site proviral enhancer alterations in lymphomas induced by enhancer mutants of SL3-3 murine leukemia virus: negative effect of nuclear factor 1 binding site

SL3-3 is a highly T-lymphomagenic murine retrovirus. Previously, mutation of binding sites in the U3 repeat region for the AML1 transcription factor family (also known as core binding factor [CBF], polyomavirus enhancer binding protein 2 [PEBP2], and SL3-3 enhancer factor 1 [SEF1]) were found to strongly reduce the pathogenicity of SL3-3 (B. Hallberg, J. Schmidt, A. Luz, F. S. Pedersen, and T. Grundström, J. Virol. 65:4177-4181, 1991). We have now examined the few cases in which tumors developed harboring proviruses that besides the AML1 (core) site mutations carried second-site alterations in their U3 repeat structures. In three distinct cases we observed the same type of alteration which involved deletions of regions known to contain binding sites for nuclear factor 1 (NF1) and the addition of extra enhancer repeat elements. In transient-expression experiments in T-lymphoid cells, these new U3 regions acted as stronger enhancers than the U3 regions of the original viruses. This suggests that the altered proviruses represent more-pathogenic variants selected for in the process of tumor formation. To analyze the proviral alterations, we generated a series of different enhancer-promoter reporter constructs. These constructs showed that the additional repeat elements are not critical for enhancer strength, whereas the NF1 sites down-regulate the level of transcription in T-lymphoid cells whether or not the AML1 (core) sites are functional. We therefore also tested SL3-3 viruses with mutated NF1 sites. These viruses have unimpaired pathogenic properties and thereby distinguish SL3-3 from Moloney murine leukemia virus.

Human papillomaviruses and cervical cancer

Molecular and epidemiological studies conducted over the last 20 years led to the recognition of certain types of human papillomavirus (HPV) as the etiological agents of cervical cancer, a very common neoplasia, particularly in developing countries. More than 70 HPVs have been described, including both cutaneous and mucosal types. About half of the known HPVs, and an even higher number of variants, have been isolated from genital mucosas. The association of certain types primarily with normal tissues and benign lesions, as opposed to cancer-associated types, has led to the concept of low and high oncogenic risk HPVs, respectively. The latter express oncogenic proteins that interfere with cell growth control functions. As a consequence of the continuous expression of these viral genomes, chromosome instability may occur, leading to fully transformed cells. Studies indicate that persistence of high-risk HPVs may determine progression to more severe stages of cervical disease, while the majority of HPV infections are transient and do not seem to be important in cervical carcinogenesis. The risk for disease progression seems also to be associated with viral burden. Prospective epidemiological studies will contribute to the knowledge of the natural history of HPV infections and provide information on the determinants of viral persistence. Data derived from these studies may define the clinical utility of HPV testing and its use in cervical cancer prevention programs.

Expression of a 74-kDa nuclear factor 1 (NF1) protein is induced in mouse mammary gland involution. Involution-enhanced occupation of a twin NF1 binding element in the testosterone-repressed prostate message-2/clusterin promoter

Testosterone repressed prostate message-2 (TRPM-2)/clusterin gene expression is rapidly induced in early involution of the mouse mammary gland, after weaning, and in the rat ventral prostate, after castration. A search for involution-enhanced DNaseI footprints in the proximal mouse TRPM-2/clusterin gene promoter led to the identification and characterization (by DNase I footprinting and EMSA) of a twin nuclear factor 1 (NF1) binding element at -356/-309, relative to the proposed transcription start site; nuclear extracts from 2-day involuting mouse mammary gland showed an enhanced footprint over the proximal NF1 element; extracts from involuting prostate showed enhanced occupancy of both NF1 binding elements. Subsequent EMSA and Western analysis led to the detection of a 74-kDa NF1 protein whose expression is triggered in early involution in the mouse mammary gland; such an induced protein is not found in the involuting rat ventral prostate. This protein was not found in lactation where three other NF1 proteins of 114, 68, and 46 kDa were detected. Reiteration of the epithelial cell apoptosis associated with early mammary gland involution, in vitro, in a primary cell culture system, triggered the appearance of the 74-kDa NF1. Overlaying the cells with laminin-rich extracellular matrix suppressed the apoptosis and the expression of the 74-kDa NF1 and, in the presence of lactogenic hormones, initiated milk protein gene expression and the expression of two of the lactation-associated NF1 proteins (68 and 46 kDa). This study, thus, identifies for the first time the occurrence of a switch in expression of different members of the family of NF1 transcription factors as mammary epithelial cells move from the differentiated to the involution/apoptotic state, and it is likely that the involution-specific 74-kDa NF1 accounts for the enhanced NF1 footprint detected on the TRPM-2/clusterin promoter with extracts of mouse mammary gland.

Differential expression of the monocyte chemoattractant protein-1 gene in human papillomavirus-16-infected squamous intraepithelial lesions and squamous cell carcinomas of the cervix uteri

Monocyte chemoattractant protein (MCP)-1 is an important factor involved in the cross-talk between mononuclear cells and human papilloma-virus (HPV)-infected cervical epithelia. To prove the experimental model of a negative regulatory loop between the expression of the HPV oncogenes E6/E7 and the MCP-1 gene in vivo, we examined HPV-16-infected conization/hysterectomy specimens consisting of 6 low- and 6 high-grade squamous intraepithelial lesions (SILs) and 5 squamous cell carcinomas (SCCs) as well as the adjacent mucosa by isotopic RNA in situ hybridization. Langerhans cells and stromal macrophages were identified by the antibodies S-100 and PG-M1. E6/E7 expression was restricted to dysplastic or neoplastic keratinocytes, whereas MCP-1 transcripts were detectable in normal, dysplastic, and neoplastic epithelia, in endothelia, and in stromal macrophages. Langerhans cells were always negative. MCP-1 expression was predominant at the epithelial-mesenchymal junctions and especially intense when the stromal macrophage reaction was increased. Generally, the synchronous expression of E6/E7 and MCP-1 was very rare in SILs (2 of 12 cases). In high-grade SIL, MCP-1 expression was observed in 1 of 6 cases, whereas all lesions strongly hybridized with E6/E7 probes. In contrast, 4 of 5 SCCs re-expressed MCP-1, and 2 cases revealed marked transcriptional activities for both E6/E7 and MCP-1. Although preliminary, our observations lend support to the suggestion that the experimental model of transcriptional regulation and exclusion of either HPV E6/E7 or MCP-1 expression is especially pertinent to high-grade SIL, whereas in most SCCs, other environmental factors may influence this relationship.

YY1 represses human papillomavirus type 16 transcription by quenching AP-1 activity

YY1 is a multifunctional transcription factor that has been shown to regulate the expression of a number of cellular and viral genes, including the human papillomavirus (HPV) oncogenes E6 and E7. In this study, we have analyzed the YY1-mediated repression of the HPV type 16 (HPV-16) E6-E7 promoter. A systematic analysis to identify YY1 sites present in the HPV-16 long control region showed that of 30 potential YY1 binding motifs, 24 bound purified recombinant YY1 protein, but only 10 of these were able to bind YY1 when nuclear extracts of HeLa cells were used. Of these, only a cluster of five sites, located in the vicinity of an AP-1 motif, were found to be responsible for repressing the HPV-16 P97 promoter. All five sites were required for repression, the mutation of any one site giving rise to a four- to sixfold increase in transcriptional activity. The target for YY1-mediated repression was identified as being a highly conserved AP-1 site, and we propose that AP-1 may represent a common target for YY1 repression. We also provide data demonstrating that YY1 can bind the transcriptional coactivator CREB-binding protein and propose a potentially novel mechanism by which YY1 represses AP-1 activity as a result of this YY1-CREB-binding protein interaction.

The affinity of nuclear factor 1 for its DNA site is drastically reduced by nucleosome organization irrespective of its rotational or translational position

A DNA-bending sequence has been used for in vitro reconstitution of nucleosomes in order to direct a nuclear factor 1 (NF-1) binding site into different nucleosome positions. By this strategy nucleosomes were obtained that had one of two rotational positions of the NF-1 binding site, one oriented toward the periphery and the other toward the histone octamer, translationally positioned 50 and 45 base pairs, respectively, from the nucleosome dyad. The affinity of partially purified NF-1 for these nucleosomal targets was compared with its affinity for free DNA by dimethylsulfate methylation protection and DNase I footprinting assays. The binding affinity of NF-1 to all nucleosomal targets was reduced 100-300-fold compared with its affinity for free DNA. The two rotational settings of the NF-1 site showed the same binding affinity for NF-1 as did other nucleosome constructs in which the NF-1 binding site was translationally positioned from 10 to 40 base pairs from the nucleosome dyad. We conclude that the nucleosomal inhibition of NF-1 binding is an inherent characteristic of NF-1 since another transcription factor, the glucocorticoid receptor, is able to bind to its DNA site in a nucleosome.

Mechanisms identified in the transcriptional control of epithelial gene expression

Epithelium-specific gene expression is fundamental in both embryogenesis and the maintenance of adult tissues, and impairment of epithelial characteristics contributes to diseases such as cancer. We have here analyzed the 5'-region of the epithelial (E-) cadherin gene in order to understand mechanisms of epithelium-specific transcription and loss of expression during epithelial-mesenchymal transitions. The regulatory region of the mouse epithelial cadherin gene is composed of a promoter (from position -94 to the transcription start site) and a 150-base pair enhancer located in the first intron. The 5'-promoter consists of positive regulatory elements (a CCAAT-box and two AP-2 binding sites in a GC-rich region) and the palindromic element E-Pal that activates and represses transcription in epithelial and mesenchymal cells, respectively. The enhancer of the first intron stimulates the activity of heterologous promoters exclusively in epithelial cells. This epithelium-specific enhancer consists of three elements (E I to E III; E II and E III bind AP-2) that are necessary and sufficient for activity. We thus propose two regulatory mechanisms by which epithelial specificity of epithelial cadherin expression is determined: suppression of promoter activity in mesenchymal cells by E-Pal and enhancement of activity in epithelial cells by both E-Pal and the epithelium-specific enhancer.

Regulation of human papillomavirus transcription by the differentiation-dependent epithelial factor Epoc-1/skn-1a

Human papillomavirus (HPV) early gene expression is closely linked to the differentiation status of infected epithelial cells. Typically, HPV type 16 (HPV16) or HPV18 E6 and E7 transcripts are only barely detectable within the undifferentiated basal cell layer, but their levels increase concomitantly with higher degrees of epithelial cell differentiation in suprabasal cells. A similar differentiation-dependent distribution of expression has been reported for the recently cloned epithelial cell specific transcription factor Epoc-1/skn-1a. We therefore examined whether Epoc-1/skn-1a may be directly involved in the activation of HPV E6/E7 transcription. Transient transfection studies showed that Epoc-1/skn-1a specifically stimulated the HPV16 and HPV18 E6/E7 promoters. Moreover, ectopically expressed Epoc-1/skn-1a was sufficient to stimulate HPV transcription also in nonepithelial cells. By deletion analyses, the Epoc-1/skn-1a-responsive element was mapped to the promoter-proximal portion of the HPV18 transcriptional control region. Footprint analyses and gel retardation assays demonstrated direct binding of Epoc-1/skn-1a to a hitherto uncharacterized site within this region. Mutation of the Epoc-1/skn-1a recognition site within the context of the complete HPV18 upstream regulatory region inhibited Epoc-1/skn-1a-mediated transactivation. These results show that Epoc-1/skn-1a can directly activate the E6/E7 promoter by binding to the viral transcriptional control region. Thus, Epoc-1/skn-1a may be involved in the differentiation-dependent regulation of HPV transcription.

Induction of the HPV16 enhancer activity by Jun-B and c-Fos through cooperation of the promoter-proximal AP-1 site and the epithelial cell type--specific regulatory element in fibroblasts

The epithelial cell type-specific enhancer of the human papillomavirus (HPV) type 16 termed the long control region (LCR) carries three AP-1 binding sites. We investigated the roles of the AP-1 sites for transactivation of the LCR by Jun-B that may be a cell type specific-transactivator for the HPVs in human fibroblasts in which expression of the endogenous Jun-B gene is low. Transient expression of Jun-B alone poorly activated transcriptional activity of the LCR. However, when combined with c-Fos, Jun-B activated the LCR. The promoter-proximal AP-1 site was required for transactivation of the LCR by Jun-B:c-Fos, but the site itself was not sufficient for the maximal response. The proposed cell type specific--regulatory element that harbors binding sites for NF1 as well as TEF-1 and PEA3 motifs, but neither other AP-1 sites nor the proximal enhancer region, could augment the transcriptional response of the promoter-proximal AP-1 site to Jun-B:c-Fos.

Regulation of the DNA-binding and transcriptional activities of Xenopus laevis NFI-X by a novel C-terminal domain

The nuclear factor I (NFI) family consists of sequence-specific DNA-binding proteins that activate both transcription and adenovirus DNA replication. We have characterized three new members of the NFI family that belong to the Xenopus laevis NFI-X subtype and differ in their C-termini. We show that these polypeptides can activate transcription in HeLa and Drosophila Schneider line 2 cells, using an activation domain that is subdivided into adjacent variable and subtype-specific domains each having independent activation properties in chimeric proteins. Together, these two domains constitute the full NFI-X transactivation potential. In addition, we find that the X. laevis NFI-X proteins are capable of activating adenovirus DNA replication through their conserved N-terminal DNA-binding domains. Surprisingly, their in vitro DNA-binding activities are specifically inhibited by a novel repressor domain contained within the C-terminal part, while the dimerization and replication functions per se are not affected. However, inhibition of DNA-binding activity in vitro is relieved within the cell, as transcriptional activation occurs irrespective of the presence of the repressor domain. Moreover, the region comprising the repressor domain participates in transactivation. Mechanisms that may allow the relief of DNA-binding inhibition in vivo and trigger transcriptional activation are discussed.

Organization of the alpha-globin promoter and possible role of nuclear factor I in an alpha-globin-inducible and a noninducible cell line

Nuclear factor I (NFI) was suggested to be involved in the expression of the human alpha-globin gene. Two established cell lines, which express alpha-globin differentially, were therefore compared for differences in binding of NFI at the alpha-globin promoter in vivo. HeLa cells, in which alpha-globin is repressed, show a high density promoter occupation with several proteins associated with structurally distorted DNA. Cell line K562, which is inducible for alpha-globin, surprisingly was found to be heterogeneous consisting mainly of cells (approximately 95%) unable to express alpha-globin. However, the promoter of the nonexpressing K562 cells was clearly different from that of HeLa cells, being occupied only at basal transcriptional elements. Therefore, the alpha-globin gene in these K562 cells may not be truly repressed, but in an intermediate state between repression and active transcription. The NFI site of the alpha-globin promoter appeared occupied in HeLa but free of proteins in K562 cells. All cells of both cell lines produce NFI, but the composition and DNA binding affinity of NFI species differ significantly between the two cell lines. Therefore, distinct forms of NFI may repress alpha-globin transcription in HeLa cells. However, NFI is apparently not involved in establishing the latent transcriptional state of the majority of K562 cells.

The enhancer in the long control region of human papillomavirus type 16 is up-regulated by PEF-1 and down-regulated by Oct-1

The minimal enhancer in the long control region of human papillomavirus type 16 regulates cell type and constitutive expression from the promoter P97. This region contains at least four DNase I footprints (fp4e, fp5e, fp6e, and fp7e). We have shown that fp5e is crucial to enhancer function and have described an apparently novel factor (PEF-1) binding fp5e (S. Cuthill, G. J. Sibbet, and M. S. Campo, Mol. Carcinog. 8:9-104, 1993). Further analyses reveal that Oct-1 or an Oct-related factor binds fp5e at a site overlapping that of PEF-1. The binding of Oct-1 to fp5e has been demonstrated by electrophoretic mobility shift assays, by oligonucleotide competition studies, and by using an Oct-1-specific anti-POU serum. The location of the Oct-1 site has been confirmed by a panel of mutants across fp5e. Mutations that block PEF-1 binding to fp5e also block enhancer/promoter activity of the long control region, whereas mutations that block Oct-1 binding significantly increase enhancer/promoter activity. Thus, although both PEF-1 and Oct-1 interact with fp5e, they appear to regulate human papillomavirus expression in opposite ways.

Can a test for E6/E7 transcripts of human papillomavirus type 16 serve as a diagnostic tool for the detection of micrometastasis in cervical cancer?

Tissue from 11 cases of cervical cancer positive for human papillomavirus (HPV) type 16 DNA and 69 pelvic lymph nodes from the same patients were examined for HPV 16 DNA and mRNA from the E6/E7 genes. Five of the tumors were squamous, 3 adeno- and 3 adenosquamous carcinoma. From the primary tumors and the extirpated lymph nodes DNA and RNA or mRNA was subjected to PCR and RT-PCR. Three transcription profiles (only E6*I, E6*I and E6*II or full-length E6-E7 plus both of the spliced transcripts) were found in all of the 11 HPV 16 DNA-positive primary tumors. From the total of 69 lymph nodes analyzed 28 were positive for mRNA. HPV 16 DNA was found in 7 additional samples. Cytokeratin was found in 19 of these lymph nodes, indicating epithelial origin of tumor cells. Only 1 patient had 2 metastases evidenced by histology. These were both positive for HPV DNA and mRNA. The finding of HPV DNA, mRNA and cytokeratin in lymph nodes of patients with cervical cancer should be an indication of lymphogenically driven micrometastases of the tumor. The HPV mRNA assay should offer higher specificity than the DNA test since mRNA can be found in live cells only, while HPV DNA also can originate from dead cell material sequestered in the lymph nodes.

E1a induces the expression of epithelial characteristics

Cells closely resembling epithelia constitute the first specific cell type in a mammalian embryo. Many other cell types emerge via epithelial-mesenchymal differentiation. The transcription factors and signal transduction pathways involved in this differentiation are being elucidated. I have previously reported (Frisch, 1991) that adenovirus E1a is a tumor suppressor gene in certain human cell lines. In the present report, I demonstrate that E1a expression caused diverse human tumor cells (rhabdomyosarcoma, fibrosarcoma, melanoma, osteosarcoma) and fibroblasts to assume at least two of the following epithelial characteristics: (a) epithelioid morphology; (b) epithelial-type intercellular adhesion proteins localized to newly formed junctional complexes; (c) keratin-containing intermediate filaments; and (d) down-regulation of non-epithelial genes. E1a thus appeared to partially convert diverse human tumor cells into an epithelial phenotype. This provides a new system for molecular analysis of epithelial-mesenchymal interconversions. This effect may also contribute to E1a's tumor suppression activity, possibly through sensitization to anoikis (Frisch, S.M., and H. Francis, 1994. J. Cell Biol. 124:619-626).

Cloning and functional analysis of spliced isoforms of human nuclear factor I-X: interference with transcriptional activation by NFI/CTF in a cell-type specific manner

Previous studies of the epithelial specificity of the human papillomavirus type 16 (HPV-16) enhancer pointed out an important role of nuclear factor I (NFI). In epithelial cells, NFI proteins are derived from the NFI-C gene and referred to as NFI/CTF. In contrast, fibroblasts, where the enhancer is inactive, express high levels of NFI from the NFI-X gene. To compare NFI-C and NFI-X derived transcription factors, we cloned and functionally investigated two differentially spliced forms of NFI-X from human fibroblasts. NFI-X1 has 95% homology with a transcript previously identified in hamster liver cells. NFI-X2, a spliced variant, misses 41 amino acids of the proline-rich activation domain. NFI-X expression, examined by Northern blots, shows strong cell-type specific variation in comparison with NFI/CTF. While the transcriptional activation domain of NFI-X2, functionally tested as GAL4-fusion protein in epithelial and fibroblast cells, activates transcription from promoter as well as enhancer position similar to NFI/CTF-1, the activation domain of NFI-X1 fails to activate transcription from enhancer position. In Drosophila cells, void of endogenous NFI proteins, full length NFI/CTF-1 and NFI-X2 activate a reporter construct containing only NFI sites as well as the NFI dependent HPV-16 enhancer. In contrast, NFI-X1 fails to activate the HPV-16 enhancer. Furthermore, overexpression of NFI-X1 in epithelial cells down-regulates the HPV-16 enhancer. Our findings suggest that the family of NFI transcription factors should not be viewed as constitutive activators, but rather, that NFI-C and NFI-X have divergent functions after binding in promoter or enhancer position. This property, combined with the differential expression of NFI-X, can achieve cell-type specificity of NFI dependent promoters and enhancers.

Cellular control of human papillomavirus oncogene transcription

Specific types of human papillomaviruses (HPVs) are closely associated with the development of cervical cancer. The transforming ability of these high-risk HPV types depends on the expression of the viral E6 and E7 oncogenes. It is therefore of particular interest to elucidate the molecular mechanisms that result in the activation of E6/E7 expression during HPV-associated tumorigenesis. Recently, much progress has been made in characterizing the proteins involved in the regulation of HPV oncogene transcription. This review describes the functional significance of cellular factors involved in the transcriptional control of the E6/E7 promoter for the two most common HPV types associated with cervical cancer, HPV16 and HPV18. In addition, we discuss regulatory pathways that may contribute to the epithelial cells specificity of E6/E7 transcription. The definition of the factors that regulate HPV oncogene transcription could provide new insights into the molecular mechanisms activating viral oncogene expression during cervical carcinogenesis and forms an experimental basis for investigating the specific biochemical pathways that contribute to HPV-associated malignant cell transformation.