Inducible and cell type-specific expression of VL30 U3 subgroups correlate with their enhancer design

A somatic role for the histone methyltransferase Setdb1 in endogenous retrovirus silencing

Subsets of endogenous retroviruses (ERVs) are derepressed in mouse embryonic stem cells (mESCs) deficient for Setdb1, which catalyzes histone H3 lysine 9 trimethylation (H3K9me3). Most of those ERVs, including IAPs, remain silent if Setdb1 is deleted in differentiated embryonic cells; however they are derepressed when deficient for Dnmt1, suggesting that Setdb1 is dispensable for ERV silencing in somatic cells. However, H3K9me3 enrichment on ERVs is maintained in differentiated cells and is mostly diminished in mouse embryonic fibroblasts (MEFs) lacking Setdb1. Here we find that distinctive sets of ERVs are reactivated in different types of Setdb1-deficient somatic cells, including the VL30-class of ERVs in MEFs, whose derepression is dependent on cell-type-specific transcription factors (TFs). These data suggest a more general role for Setdb1 in ERV silencing, which provides an additional layer of epigenetic silencing through the H3K9me3 modification.

Previous studies suggest that DNA methylation is the main mechanism to silence endogenous retroviruses (ERVs) in somatic cells. Here the authors provide evidence that distinctive sets of ERVs are silenced by Setdb1 in different types of somatic cells, suggesting a general function in ERV silencing.

Genomic analysis of mouse VL30 retrotransposons

BACKGROUND

Retrotransposons are mobile elements that have a high impact on shaping the mammalian genomes. Since the availability of whole genomes, genomic analyses have provided novel insights into retrotransposon biology. However, many retrotransposon families and their possible genomic impact have not yet been analysed.

RESULTS

Here, we analysed the structural features, the genomic distribution and the evolutionary history of mouse VL30 LTR-retrotransposons. In total, we identified 372 VL30 sequences categorized as 86 full-length and 49 truncated copies as well as 237 solo LTRs, with non-random chromosomal distribution. Full-length VL30s were highly conserved elements with intact retroviral replication signals, but with no protein-coding capacity. Analysis of LTRs revealed a high number of common transcription factor binding sites, possibly explaining the known inducible and tissue-specific expression of individual elements. The overwhelming majority of full-length and truncated elements (82/86 and 40/49, respectively) contained one or two specific motifs required for binding of the VL30 RNA to the poly-pyrimidine tract-binding protein-associated splicing factor (PSF). Phylogenetic analysis revealed three VL30 groups with the oldest emerging ~17.5 Myrs ago, while the other two were characterized mostly by new genomic integrations. Most VL30 sequences were found integrated either near, adjacent or inside transcription start sites, or into introns or at the 3' end of genes. In addition, a significant number of VL30s were found near Krueppel-associated box (KRAB) genes functioning as potent transcriptional repressors.

CONCLUSION

Collectively, our study provides data on VL30s related to their: (a) number and structural features involved in their transcription that play a role in steroidogenesis and oncogenesis; (b) evolutionary history and potential for retrotransposition; and (c) unique genomic distribution and impact on gene expression.

Trim24-repressed VL30 retrotransposons regulate gene expression by producing noncoding RNA

Trim24 (Tif1α) and Trim33 (Tif1γ) interact to form a co-repressor complex that suppresses murine hepatocellular carcinoma. Here we show that Trim24 and Trim33 cooperatively repress retinoic acid receptor-dependent activity of VL30-class endogenous retroviruses (ERVs) in liver. In Trim24-knockout hepatocytes, VL30 derepression leads to accumulation of reverse-transcribed VL30 cDNA in the cytoplasm that correlates with activation of the viral-defense interferon responses mimicking the preneoplastic inflammatory state seen in human liver following exogenous viral infection. Furthermore, upon derepression, VL30 long terminal repeats (LTRs) act as promoter and enhancer elements deregulating expression of neighboring genes and generating enhancer RNAs that are required for LTR enhancer activity in hepatocytes in vivo. These data reinforce the role of the TRIM family of proteins in retroviral restriction and antiviral defense and provide an example of an ERV-derived oncogenic regulatory network.

Epigenetic regulation of a murine retrotransposon by a dual histone modification mark

Large fractions of eukaryotic genomes contain repetitive sequences of which the vast majority is derived from transposable elements (TEs). In order to inactivate those potentially harmful elements, host organisms silence TEs via methylation of transposon DNA and packaging into chromatin associated with repressive histone marks. The contribution of individual histone modifications in this process is not completely resolved. Therefore, we aimed to define the role of reversible histone acetylation, a modification commonly associated with transcriptional activity, in transcriptional regulation of murine TEs. We surveyed histone acetylation patterns and expression levels of ten different murine TEs in mouse fibroblasts with altered histone acetylation levels, which was achieved via chemical HDAC inhibition with trichostatin A (TSA), or genetic inactivation of the major deacetylase HDAC1. We found that one LTR retrotransposon family encompassing virus-like 30S elements (VL30) showed significant histone H3 hyperacetylation and strong transcriptional activation in response to TSA treatment. Analysis of VL30 transcripts revealed that increased VL30 transcription is due to enhanced expression of a limited number of genomic elements, with one locus being particularly responsive to HDAC inhibition. Importantly, transcriptional induction of VL30 was entirely dependent on the activation of MAP kinase pathways, resulting in serine 10 phosphorylation at histone H3. Stimulation of MAP kinase cascades together with HDAC inhibition led to simultaneous phosphorylation and acetylation (phosphoacetylation) of histone H3 at the VL30 regulatory region. The presence of the phosphoacetylation mark at VL30 LTRs was linked with full transcriptional activation of the mobile element. Our data indicate that the activity of different TEs is controlled by distinct chromatin modifications. We show that activation of a specific mobile element is linked to a dual epigenetic mark and propose a model whereby phosphoacetylation of histone H3 is crucial for full transcriptional activation of VL30 elements.

The majority of genomic sequences in higher eukaryotes do not contain protein coding genes. Large fractions are covered by repetitive sequences, many of which are derived from transposable elements (TEs). These selfish genes, only containing sequences necessary for self-propagation, can multiply and change their location within the genome, threatening host genome integrity and provoking mutational bursts. Therefore host organisms have evolved a diverse repertoire of defence mechanisms to counteract and silence these genomic parasites. One way is to package DNA sequences containing TEs into transcriptionally inert heterochromatin, which is partly achieved via chemical modification of the packaging proteins associated with DNA, the histones. To better understand the contribution of histone acetylation in the activation of TEs, we treated mouse fibroblasts with a specific histone deacetylase inhibitor. By monitoring the expression of ten different types of murine mobile elements, we identified a defined subset of VL30 transposons specifically reactivated upon increased histone acetylation. Importantly, phosphorylation of histone H3, a modification that is triggered by stress, is required for acetylation-dependent activation of VL30 elements. We present a model where concomitant histone phosphorylation and acetylation cooperate in the transcriptional induction of VL30 elements.

The regulated retrotransposon transcriptome of mammalian cells

Although repetitive elements pervade mammalian genomes, their overall contribution to transcriptional activity is poorly defined. Here, as part of the FANTOM4 project, we report that 6-30% of cap-selected mouse and human RNA transcripts initiate within repetitive elements. Analysis of approximately 250,000 retrotransposon-derived transcription start sites shows that the associated transcripts are generally tissue specific, coincide with gene-dense regions and form pronounced clusters when aligned to full-length retrotransposon sequences. Retrotransposons located immediately 5' of protein-coding loci frequently function as alternative promoters and/or express noncoding RNAs. More than a quarter of RefSeqs possess a retrotransposon in their 3' UTR, with strong evidence for the reduced expression of these transcripts relative to retrotransposon-free transcripts. Finally, a genome-wide screen identifies 23,000 candidate regulatory regions derived from retrotransposons, in addition to more than 2,000 examples of bidirectional transcription. We conclude that retrotransposon transcription has a key influence upon the transcriptional output of the mammalian genome.

Cerebral ischemia induces neuronal expression of novel VL30 mouse retrotransposons bound to polyribosomes

Mammalian genomes are burdened with a large heterogeneous group of endogenous replication defective retroviruses (retrotransposons). Previously, we identified a transcript resembling a virus-like 30S (VL30) retrotransposon increasing in mouse brain following transient cerebral ischemia. Paradoxically, this non-coding RNA was found bound to polyribosomes. Further analysis revealed that multiple retrotransposon species (BVL-1-like and mVL30-1-like) were bound to polyribosomes and induced by ischemia. These VL30 transcripts remained associated with polyribosomes in the presence of 0.5 M KCl, indicating that VL30 mRNA was tightly associated with ribosomal subunits. Furthermore, the profile of BVL-1 distribution on polyribosomal profiles was distinct from those of translated and translationally repressed mRNA. Consistent with expectations, 5.0 kb VL30 transcripts were detected in ischemic brain with a temporal pattern of expression that was distinct from c-fos. Expression of VL30 was localized in neurons using a combination of in situ hybridization and immunocytochemistry. 3'-RACE-PCR experiments yielded two unique sequences (VL30x-1 and VL30x-2) that were homologous to known VL30 genes. Phylogenetic analysis of VL30 promoter sequence (U3 region) resulted in the identification of two large VL30 subgroups. VL30x-1 and VL30x-2 were closely related and classified in a group that was distinct from the well-characterized VL30 genes BVL-1 and mVL30-1. The promoter regions of VL30x-1 and VL30x-2 did not possess the consensus sequences for either hypoxia or anoxia response elements, suggesting an alternative mechanism for induction. This is the first report that demonstrates ischemia-induced, neuronal expression of unique VL30 retrotransposons in mouse brain.

New regulatory regions of Drosophila 412 retrotransposable element generated by recombination

There are no doubts that transposable elements (TEs) have greatly influenced genomes evolution. They have, however, evolved in different ways throughout mammals, plants, and invertebrates. In mammals they have been shown to be widely present but with low transposition activity; in plants they are responsible for large increases in genome size. In Drosophila, despite their low amount, transposition seems to be higher. Therefore, to understand how these elements have evolved in different genomes and how host genomes have proposed to go around them, are major questions on genome evolution. We analyzed sequences of the retrotransposable elements 412 in natural populations of the Drosophila simulans and D. melanogaster species that greatly differ in their amount of TEs. We identified new subfamilies of this element that were the result of mutation or insertion-deletion process, but also of interfamily recombinations. These new elements were well conserved in the D. simulans natural populations. The new regulatory regions produced by recombination could give rise to new elements able to overcome host control of transposition and, thus, become potential genome invaders.

Human Krüppel-like factor5/KLF5: synergy with NF-kappaB/Rel factors and expression in human skin and hair follicles

In this report we describe the identification of Krüppel-like factor 5 (KLF5/BTEB2) in a yeast one-hybrid screen using a keratinocyte-specific, NF-kappaB binding site as bait. The KLF5 cDNA encodes a larger protein of 457 aa rather than the earlier reported protein of 209 aa. The full-length KLF5 functions as a transactivator in HepG2 cells, and the stimulation of cells with 12-0-tetradecanoylphorbol-13-acetate (TPA) can modulate its transcriptional activity. Overexpression of KLF5 leads to an increase in the TPA response from VLTRE, a TPA-inducible enhancer element that shows keratinocyte specificity with respect to Rel/NF-kappaB binding. The KLF5-mediated transcriptional increase is not observed in the presence of overexpressed NF-kappaB inhibitor, IkappaBalpha. Cotransfection of KLF5 and the p65 subunit of NF-kappaB, results in a synergistic transactivation of the VLTRE-luciferase reporter. The KLF5 mRNA and the protein is expressed in keratinocytes and throughout the adult human epidermis. Its expression is especially strong in the matrix and the inner root sheath cuticle layer of the hair follicle, sebaceous glands and sweat glands. Considering the TPA-responsiveness and expression pattern, we propose that KLF5 like another member of its family KLF4/GKLF may play an important role in skin morphogenesis and carcinogenesis potentially via its interaction with NF-kappaB factors.

Conserved, erythropoietin-responsive VL30 promoters isolated from erythroid progenitor cells

Virus-like 30S (VL30) elements are endogenous retro-elements of the mouse retrotransposon family. These elements are transcriptionally responsive in a temporal and tissue-specific manner due to the U3 promoter region of the elements' long terminal repeat (LTR). We have analyzed VL30 promoters from erythroid progenitor cell lines (MEL 585S and ELM-I-1) that contrasted in their response to erythropoietin (epo). Through RT-PCR-generated cDNAs, VL30 promoters were identified and showed homology to the third and fourth U3 subgroups, with GATA-1, Jak2/STAT5, and B10 RRE sites. One clone (ELM5) showed 97% homology to BVL-1, a putative epo-responsive VL30 element. In addition, a novel U3 promoter (MEL/ELM CONSTIT) showed complete sequence homology between both cell lines. Ribonuclease protection confirmed that epo-induced VL30 promoters were activated in ELM-I-1 cells, whereas the conserved VL30 MEL-ELM CONSTIT VL30 promoter showed no enhanced expression in the epo-unresponsive MEL cells. Identification of these U3 promoters suggests that VL30s are conserved and can be transcriptionally activated in an epo-specific manner.

Comparison of endogenous feline leukemia virus RNA content in feline vaccine and nonvaccine site-associated sarcomas

OBJECTIVE

To determine whether feline vaccine site-associated sarcomas (VSS) contain a higher amount of endogenous FeLV (enFeLV) RNA, compared with feline nonvaccine site-associated sarcomas (non-VSS).

SAMPLE POPULATION

Formalin-fixed paraffin-embedded (FFPE) tissues from 50 VSS and 50 cutaneous non-VSS.

PROCEDURE

RNA was extracted from FFPE sections of each tumor, and regions of the long terminal repeat (LTR) and envelope (env) gene of enFeLV were amplified by use of reverse transcriptase-polymerase chain reaction (RT-PCR). The density of each RT-PCR product band for enFeLV was compared with that of a constitutively expressed gene, glyceraldehyde-3-phosphate dehydrogenase (GAPDH). An integrated density value (IDV) was determined by use of densitometry, and the IDV ratio for enFeLV to GAPDH was calculated for each enFeLV primer set.

RESULTS

The median (interquartile range) of the IDV ratio for the enFeLV LTR primer set was 0.52 (0.26 to 1.17) for the VSS group and 0.84 (0.21 to 1.53) for the non-VSS group. The median (interquartile range) of the IDV ratio for the enFeLV env primer set was 0.60 (0.37 to 0.91) for the VSS group and 0.59 (0.36 to 1.09) for the non-VSS group.

CONCLUSIONS

Because the amount of enFeLV RNA within the LTR and env gene was not significantly different between the VSS and non-VSS groups, enFeLV replication or expression is unlikely to be involved in VSS development.

Enhanced expression of the early retrotransposon in C3H mouse-derived myeloid leukemia cells

Cells of acute myeloid leukemia (AML) from C3H/He mice express an increased amount of RNA for an endogenous retrovirus-like retrotransposon, intracisternal A-particle element. We analyzed the transcription of other mouse retrotransposons in C3H-derived tumor cells and found that all the AML lines from different mice overexpress early-transposon (ETn) RNA. In contrast, only faint levels of ETn were detected in the cells from other tumors, including hepatoma and lymphoma. The polyadenylation sites of the ETn RNA in the AML cells varied. We also determined the binding site for the nuclear extract of the AML cells in the long terminal repeat sequence of ETn. The overexpression of ETn as a common phenotype of AML cells suggests that myeloid cells with this phenotype are the origin of all the AML cells or that the phenotype is acquired during leukemogenesis.

Expression of a truncated retroviral envelope gene enhances expression of normal cellular phenotypes

The envelope gene of Moloney murine leukemia virus (Mo-MLV) and its various functional domains have been studied extensively but not as much in terms of their biological effects on cell growth. In this study, we report the biological characterization of a truncated Mo-MLV envelope gene, LN11, which is devoid of a signal peptide. Its expression in various cell types, as compared to the control, enabled the transduced cells to assume a more normal phenotype, which is defined by an increase in contact inhibition and factor dependence, as well as reduced tumorigenicity. LN11-transduced fibroblasts exhibited a higher degree of contact inhibition, assumed a more flattened morphology and were more adherent compared to the control. In v-abl transformed hematopoietic cells, expression of LN11 resulted in slower cell growth, which was due to an enhanced dependence on exogenous growth factors. Enforced expression of LN11 also resulted in a slower rate of tumor development and a reduced tumor load. Thus, modification of a retroviral genome could have a significant impact on cell growth and development. This is one example where we need to consider the safety issue carefully when constructing retrovirus vectors for gene therapy.

Retinoids induce apoptosis in cultured keratinocytes

BACKGROUND

Apoptosis is a genetically controlled process linked to growth and differentiation, involving specific molecular and cellular events activated as a result of a variety of internal and external stimuli.

OBJECTIVES

To examine the ability of physiological and synthetic retinoids to induce apoptosis in the BALB/MK mouse keratinocyte cell line.

METHODS

Cell viability was assessed by flow cytometry, various staining techniques and the TUNEL method.

RESULTS

When keratinocytes were simultaneously exposed to retinoids and stimulated to differentiate at a high (1.5 mmol L(-1))extracellular Ca(2+) concentration over 48 h, apoptosis was induced. Of the retinoids tested, 3,4-didehydroretinoic acid and 3-methyl-tetrahydro-tetramethyl-naphthylenyl-propenyl benzoic acid were more potent than the others. In this system, the apoptosis induced by retinoids could not be correlated to the expression of tissue transglutaminase or epidermal transglutaminase. Furthermore, expression of antiapoptotic bcl-2 or proapoptotic Bax did not change significantly under the experimental conditions used, indicating that the regulation of apoptosis is complex and may be influenced by different factors.

CONCLUSIONS

The results suggest that retinoids activating either retinoic acid receptors or retinoid X receptors can induce apoptosis in cultured keratinocytes. Moreover, the well-established inhibitory effect of retinoids on keratinocyte differentiation implies that the apoptotic programme represents a distinct biological process.

Repression of transcription by HoxC11 upon phorbol ester stimulation

Hox genes encode transcription factors with a conserved DNA-binding domain and exhibit similar DNA-binding preferences. The in vivo specificity required for their biological function is brought about by combinatorial interactions with other factors. Such interactions also modulate their activation state. Here we show that HoxC11 can either activate or repress transcription in a signal-specific manner. We report the isolation of HoxC11 in a yeast one-hybrid screen for factors binding to a phorbol-ester, 12-O-tetradecanoylphorbol-13-acetate (TPA) response element (VLTRE), which is also a target for TPA-induced binding of Rel factors in gel-shift experiments. Although we detect no binding of in vitro translated HoxC11 to the TPA response element in EMSA, overexpression of HoxC11 in the HepG2 cell line leads to a complete block of TPA-induced transcription from a VLTRE-luciferase reporter. There is, however, no repression of the basal levels. The repression is furthermore not dependent on homeo-domain DNA binding. Our data suggest an interaction of HoxC11 with the basal-transcription machinery. We propose that HoxC11 is capable of mediating transcriptional activation or repression in a signal-specific manner and that its activation of the DNA target sequence in yeast might reflect in vivo recruitment to the promoter complex.

Modular retro-vectors for transgenic and therapeutic use

Retroviruses have been used for many years as vectors for human gene therapy as well as for making transgenic animals. However, the efficient insertion of genes by retroviruses is often complicated by transcriptional inactivation of the retroviral long terminal repeats (LTRs) and by the production of replication-competent retroviruses (RCR). Solutions to these and other difficulties are being found in modular vectors, in which the desirable features of different vector systems are combined. Examples of synergistic vectors include virosomes (liposome/virus delivery), adeno-retro vectors, and MLV/VL30 chimeras. As gene delivery systems become increasingly complex, methodology is also needed for precise assembly of modular vectors. Gene self-assembly (GENSA) technology permits seamless vector construction and simultaneous, multifragment assembly.

Replication of Mus dunni endogenous retrovirus depends on promoter activation followed by enhancer multimerization

Mus dunni endogenous virus (MDEV) is an apparently intact retrovirus that normally lies transcriptionally silent in cultured M. dunni cells, but the provirus can be activated by treatment of the cells with hydrocortisone or 5-iodo-2'-deoxyuridine. Sequence analysis of a molecular clone of the replicating virus revealed a simple retrovirus with a chimeric VL30/GALV-like structure. Interestingly, in the region of the long terminal repeat (LTR) that typically contains the retroviral transcription enhancers, we found over six 80-bp repeats with only a single mismatch, indicating that acquisition of the repeats was a recent event. Here we provide evidence for the following model of MDEV activation and replication. The MDEV provirus in M. dunni cells has a chimeric structure similar to that of the molecular clone but has only 1.15 copies of the 80-bp repeat sequence found in the molecular clone. Activating chemicals directly stimulate transcription from the LTR, allowing a low level of virus replication. Copying errors made during reverse transcription allow multimerization of the 80-bp enhancer region, resulting in viruses with higher transcriptional rates and improved fitness, but increased enhancer copy number is likely balanced by the natural instability of retroviral repeats and constraints imposed by virion packaging limits. The resultant population of replicating MDEV is widely heterogeneous, having from 2.15 to 13.15 enhancer repeats in the LTR. These results reveal a novel mechanism for regulation of transcription and replication of an endogenous retrovirus, in terms of both activation of the virus by the steroid hydrocortisone and the large number and variation in enhancer repeats observed.

A 13-bp cis-regulatory element in the LTR promoter of the tobacco retrotransposon Tto1 is involved in responsiveness to tissue culture, wounding, methyl jasmonate and fungal elicitors

The tobacco Tto1 is one of the few active LTR-retrotransposons of plants, and its transposition is activated by tissue culture and is primarily regulated at the transcriptional level. The expression of Tto1 RNA can also be activated by various stresses, including viral infection, wounding, and treatment with jasmonate, a signal molecule of plant defence responses. It is shown here that the Tto1 LTR promoter is responsible for a high level of expression in cultured tissues of transgenic tobacco plants. We demonstrate that a 13-bp repeated motif (TGGTAGGTGAGAT) in the LTR functions as a cis-regulatory element, which confers the responsiveness to tissue culture, wounding and methyl jasmonate. Fungal elicitors also activate the promoter containing multiple copies of the 13-bp motif. Expression mediated by the 13-bp motif is activated markedly by okadaic acid and moderately by K252a, so that both phosphorylation and dephosphorylation of proteins are possibly involved in the signalling pathways. Interestingly, the 13-bp motif contains a conserved motif, Box L (also called AC-I or H-box like sequence) which has been shown to be involved in the expression of phenylpropanoid synthetic genes. Moreover, extended homologies are found between promoters of Tto1 and an asparagus defence gene, AoPR1, suggesting a possibility that the ancient insertion of an ancestral Tto1-related retrotransposon has provided some of the promoter/regulatory sequences, including the 13-bp motif-related sequence, of the AoPR1 gene. Based on the structural and functional similarity between the two promoters, a possible evolutionary role of the regulatory sequences of LTR-retrotransposons is discussed.

Sequence analysis of Mus dunni endogenous virus reveals a hybrid VL30/gibbon ape leukemia virus-like structure and a distinct envelope

Mus dunni endogenous virus (MDEV) can be activated from M. dunni cells by exposing the cells to hydrocortisone or 5-iodo-2'-deoxyuridine. Interference analysis has revealed that MDEV uses a receptor for cell entry that is different from those used by other murine retroviruses. The entire genome has now been sequenced, revealing a long terminal repeat (LTR)-gag-pol-env-LTR structure typical of simple retroviruses of the murine leukemia virus genus, with no additional open reading frames between env and the 3' LTR. The LTRs and other noncoding regions of MDEV are most closely related to those of VL30 elements, while the majority of the coding sequences are most closely related to those of gibbon ape leukemia virus. MDEV represents the first example of a naturally occurring, replication-competent virus with sequences closely related to VL30 elements. The U3 region of MDEV contains six nearly perfect 80-bp repeats and the beginning of a seventh, and the region expected to contain the packaging sequence contains approximately four imperfect 33-bp repeats. The receptor specificity domains of the envelope are unique among retroviruses and show no apparent similarity to regions of known proteins.

UVB-induced association of tumor necrosis factor (TNF) receptor 1/TNF receptor-associated factor-2 mediates activation of Rel proteins

Exposure of mammalian skin to UV light results in induced gene transcription, playing a role in inflammation, immunosuppression, and tumor promotion. One important group of transcription factors induced by UV radiation is composed of members of the Rel/NF-kappa B family, which are known to play a major role in the transcriptional activation of many genes encoding inflammatory cytokines, adhesion molecules, and viral proteins. However, the upstream events in the transduction of the UVB signal to Rel protein activity are, as yet, unknown. Here, we provide biochemical evidence that exposure of keratinocytes to UVB causes rapid association of tumor necrosis factor (TNF) receptor 1 with its downstream partner TRAF-2. The functional relevance of this association is demonstrated by experiments showing that expression of a dominant negative TNF receptor 1 or TRAF-2 protein inhibits UVB-induced Rel-dependent transcription. Inclusion of a neutralizing antibody toward TNF alpha has no effect on UVB activation of a Rel-responsive reporter gene. Therefore, UVB-induced activation of Rel proteins via TNF receptor 1, independent of ligand activation, is a key component in the UV response in keratinocytes.

Activated Ha-Ras but not TPA induces transcription through binding sites for activating transcription factor 3/Jun and a novel nuclear factor

We report the identification of a 20-base pair sequence mediating induced transcription in response to an activated Ha-ras gene and epidermal growth factor (EGF) but not 12-O-tetradecanoylphorbol-13-acetate stimulation. This signal-specific nuclear target is present in the long terminal repeat of a mouse VL30 retrotransposon expressed in epidermis. Functional studies and in vitro binding analyses using cultured keratinocytes (Balb/MK) reveal that the response element is composed of two cooperating sequence motifs in juxtaposed position, both of which are targets for induced binding activity 1-2 h after EGF stimulation. Of many different activating transcription factor/cAMP-responsive element binding protein/activating protein 1 factors tested, one part of the sequence selectively binds endogenous proteins immunologically related to activating transcription factor 3 (ATF3) and Jun isotypes. The other sequence is a target for a nuclear factor showing binding specificity unrelated to factors known to mediate EGF- or ras-induced transcription as determined by its sequence specificity and by antibody experiments. This component has been characterized and partially purified by gel filtration chromatography and velocity centrifugation revealing a Stokes radius of 43.6 A and a sedimentation coefficient of 9.7 S in solution. Based on these parameters, a molecular mass of 178,000 Da was calculated. The results indicate that the specific binding of ATF3/Jun and a previously uncharacterized factor account for signal-specific transcription in response to EGF or an activated Ha-ras gene in a cell type in which the cooperative action of an activated Ha-ras gene and 12-O-tetradecanoylphorbol-13-acetate cause tumor growth.