Structure and expression of mouse VL30 genes

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.

SV40 large T antigen up-regulates the retrotransposition frequency of viral-like 30 elements

The regulation of non-autonomous retrotransposition is not known. A recombinant bearing a hygromycin gene and a viral-like 30 (VL30) retrotransposon tagged with an enhanced green fluorescent protein (EGFP) gene-based retrotransposition cassette was constructed and used for detection of retrotransposition events. Transfection of this recombinant produced retrotransposition events, detected both by EGFP fluorescence and PCR analysis, in hygromycin-selected clones of two established simian virus 40 (SV40)-transformed mouse NIH3T3 cell lines but not in normal NIH3T3 cells. The retrotransposition potential of this recombinant, as a provirus, was studied in stably transfected NIH3T3 clones. Transfection of these clones with either a wild-type or a mutant LE1135T SV40 large T antigen gene, not expressing small t protein, induced retrotransposition events at high frequencies as measured by fluorescence-activated cell scanning (FACS). In addition, measuring retrotransposition frequencies over a period of nine days following infection with isolated SV40 particles, revealed that the frequency of retrotransposition was time-dependent and induced as early as 24 h, increasing exponentially to high levels (>10(-2) events per cell per generation) up to nine days post-infection. Furthermore, ectopic expression of a cloned MoMLV-reverse transcriptase gene also produced retrotransposition events and suggested that the large T antigen most likely acted through induction of expression of endogenous reverse transcriptase genes. Our results show a direct correlation between SV40-cell transformation and VL30 retrotransposition and provide for the first time strong evidence that SV40 large T antigen up-regulates the retrotransposition of VL30 elements.

Retrotransposons and the evolution of mammalian gene expression

Transposable elements, and retroviral-like elements in particular, are a rich potential source of genetic variation within a host's genome. Many mutations of endogenous genes in phylogenetically diverse organisms are due to insertion of elements that affect gene expression by altering the normal pattern of regulation. While few such associations are known to have been maintained over time, two recently elucidated examples suggest transposable elements may have a significant impact in evolution of gene expression. The first example, concerning the mouse sex-limited protein (Slp), clearly establishes that ancient retroviral enhancer sequences now confer hormonal dependence on the adjacent gene. The second example shows that within the human amylase gene family, salivary specific expression has arisen due to inserted sequences, deriving perhaps from a conjunction of two retrotransposable elements.

Retrotransposon Gene Engineering

We have used a mobile mouse VL30 genetic element together with retroviral helper cells to efficiently transmit and express chimeric foreign gene sequences in murine and human cells. The construct comprised a cDNA copy of retrotransposon NVL3, an internal promoter [rat cytosolic phosphoenolpyruvate carboxykinase (PEPCK, EC 4.1.1.32)] and an expressed bacterial neomycin resistance gene. Thirty to sixty thousand colony forming units/ml (CFU/ml) were recovered from the supernatant of mass cultured psi2 helper cells transfected with the recombinant retrotransposon plasmid DNA. RNA was expressed from both the VL30 long terminal repeat and from the internal PEPCK promoter, resulting in a G418 drug resistance phenotype in recipient cells. Integrated VL30 DNA sequences transduced from psi2 or PA317 retroviral helper cells failed to regenerate detectable replication competent virus. Human and rodent recipient cells transduced by the retrotransposons appeared to bear intact vector sequences after two rounds of transmission by helper cells.

Isolation of virus-like (VL30) elements from the Q10 and D regions of the major histocompatibility complex

Previous studies from our laboratory have described two endogenous provirus-like sequences in a series of cosmids spanning the TL region of the major histocompatibility complex (MHC) of normal C57BL/10 mice. At least one of these viruses shares similarities with VL30 elements. To determine if additional VL30-like retroviral elements are integrated in the MHC, we constructed a cosmid library using DNA from a radiation leukemia virus (RadLV)-transformed cell line derived from C57BL/6 mice. The library was first screened using the H-2III (5') probe, which detects Class I genes of the H-2 complex. In the primary screening 163 H-2III positives were isolated. The H-2III-positive isolates were then hybridized with an AKR-derived virus probe, EcoB/S, which contains sequences from both the pol and the env genes of the virus. Nine virus-positive isolates were detected. Localization of these cosmid isolates containing viral sequences within the H-2 complex was done utilizing low-copy probes and confirmed using previously mapped cosmid isolates from other laboratories. We report here the isolation and characterization of VL30-like elements from the Qa and D regions of the MHC of several inbred mouse strains.

Efficient packaging of a specific VL30 retroelement by psi 2 cells which produce MoMLV recombinant retroviruses

FTO-2B rat hepatoma cells acquired mouse VL30 retrotransposon(s) when infected with Moloney murine leukemia virus (MoMLV) recombinant retroviruses produced from psi 2 cells. The VL30 provirus was integrated into the rat genome, expressed at high levels, and its transcription induced 40-fold by dexamethasone, VL30 RNA was detected in hepatoma cells even without selection for the expression of the amino-3'-glycosyl phosphotransferase (neo) gene, which was co-transferred with a MoMLV retrovirus. However, the extent of transfer of the VL30 RNA was inversely related to the titer of the MoMLV recombinant retrovirus. The restriction map analysis of the transferred VL30 provirus was identical to the mouse VL30s of the NVL subfamily which is known to be a significant fraction of the transcriptionally active VL30 subset. Additionally, the regenerating liver from an adult rat, which was infected with a defective MoMLV-derived retrovirus, expressed VL30 RNA. These results indicate that great care should be given to the transfer of unwanted passengers, like VL30, present in retroviral packaging cell lines like the psi 2 cells, which are currently being used for gene therapy.

Differential screening of murine ascites cDNA libraries by means of in vitro transcripts of cell-cycle-phase-specific cDNA and digital image processing

Cell-cycle-phase-specific cDNA libraries were prepared in the lambda gt10 vector and in the in vitro transcription vector, pBluescript. Plaques of the cDNA libraries prepared in the lambda gt10 vector were differentially screened with (a) in vitro transcripts of the cell-cycle-phase-specific cDNAs cloned in the transcription vector and (b) with first-strand cDNA of mRNA from phase-synchronous cells. The results suggest that first-strand cDNA can be replaced, at least in prescreening experiments, by in vitro transcripts of representative cDNA libraries prepared in in vitro transcription vectors. The fractions of differential clones detected with in vitro transcripts (1.2%) and with first-strand cDNA (1%) were in the same order. Individual clones selected by differential hybridization with in vitro transcripts could be verified by differential hybridization with cell-cycle-phase-specific first-strand cDNA. This indicates that the pattern of stage-specific prevalences of cDNA clones is essentially retained during careful amplifications of large cDNA libraries. The application of in vitro transcripts of stage-specific cDNA for differential screening experiments is of interest in cases where the amount of biological material is either limited or difficult to prepare. It also allows standardization of the probes in repeated screening experiments. Three clones reflecting cell-cycle-phase-specific mRNA prevalences were chosen and analyzed on the sequence level. Two sequences with S-phase prevalences were identified. They code for elongation factor EF1 alpha and for glyceraldehyde-3-phosphate dehydrogenase, respectively. The third sequence reflects the first cDNA of a mRNA with significant prevalence in G2-phase cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Altered levels of endogenous retrovirus-like sequence (VL30) RNA during mouse epidermal cell carcinogenesis

Gene expression during mouse keratinocyte carcinogenesis was examined in a clonal cell model. Tumor cells from three separate initiated cell lineages were compared with their nontumorigenic precursors and with the progenitor cell strain prior to treatment with 7,12-dimethylbenz[a]anthracene (DMBA). The steady-state levels of VL30 RNA in normal and papilloma cells were regulated by extracellular Ca2+ (which controls proliferation and differentiation in normal epidermal keratinocytes) and culture density. In contrast, steady-state levels of VL30 RNA were not regulated by these factors in the squamous cell carcinoma or the anaplastic carcinoma cells. VL30 expression was Ca2+ dependent in the initiated cell precursors within each tumor cell lineage, suggesting that the loss of response to extracellular Ca2+ was associated with the malignant conversion stage of carcinogenesis. No differences between normal and tumor cells were found in the cellular RNA levels of five additional proto-oncogenes. The mouse epidermal cell model should provide a means for direct assessment of a potential functional role of VL30 sequences in cancer development.

Cycloheximide Increases Endogenous Retroviral RNA Levels in Murine Liver and Lung

The expression of moloney murine leukemia virus (Mo-MuLV), intracisternal A particle (IAP), virus-like 30S sequence (VL30), early transposon (ET) endogenous retroviral sequences was analysed in the liver of untreated C3Hf, C57BL/6J and AKR mice, and in the lungs of untreated A/J, BALB/c, C3Hf and C57BL/6J mice. C3Hf mice are genetically susceptible to hepatocarcinogenesis, whereas the other strains are resistant. A/J and BALB/c mice are genetically susceptible to lung carcinogenesis, whereas the other strains are resistant. Both in liver and lung tissues we found differences between murine strains in the pattern of basal retrovirus expression. The effect of inhibition of protein synthesis on the levels of retroviral mRNAs was studied in the same tissues and strains 3 h following in vivo cycloheximide treatment. Cycloheximide treatment increased the liver and lung levels of virus Mo-MuLV, IAP, ET related transcripts in a strain dependent way, whereas VL30 mRNA levels increased in both tissues of all strains examined. These results suggested the existence of labile proteins that regulate the abundance of specific retroviral mRNAs in murine liver and lung in a strain specific fashion. No clear relationships between pattern of retrovirus expression and genetical susceptibility to hepatocarcinogenesis was found. The strains genetically resistant to lung carcinogenesis (C3Hf, C57BL/6J) showed higher lung basal levels and higher cycloheximide inducibility of mRNAs homologous to Mo-MuLV than the susceptible strains (A/J, BALB/c).

The genomic DNA organisation and evolution of a retrovirus-transmissible family of mouse (VL30) genetic elements

The sequence organisation of endogenous VL30 elements in the mouse genome was investigated by using a cloned representative of a retrovirus-transmissible VL30 cDNA. The majority of dispersed VL30 sequences could be assigned to a proviral-like structure 5.2-5.3 kbp long and bounded by long terminal repeats (LTRs). The existence of a hierarchy of evolutionarily conserved elements was rather limited and sequence heterogeneity between different elements was randomly distributed. However, the retrovirus-transmissible class of VL30 element was found to represent a distinct minority subgroup distinguishable by restriction sites and size (4.6-4.9 kbp long). Analysis of sequence conservation showed that VL30 elements display a more rapid turnover than endogenous murine leukaemia virus-related proviral sequences, and that VL30 LTRs show the most limited evolutionary distribution. Although discrete subsets of VL30 unique sequence were conserved in different rodents, the location of conserved regions was found to be variable, arguing against the presence of a functionally conserved protein coding region. These observations support the hypothesis that high frequency recombination, probably occurring during reverse transcription and the accompanying processes of duplicative transposition and amplification, have been a major determinant in the mode of evolution of the VL30 gene family.

Differential expression of mRNA coding for heparin-binding growth factor type 2 in human cells

The proliferation of normal human fibroblasts, keratinocytes, and melanocytes in vitro can be controlled by purified polypeptide growth factors and serum. We have studied the cellular expression of the heparin-binding growth factor type 2/basic fibroblast growth factor (HBGF-2/bFGF) gene to determine whether these cell types synthesize mRNA for this mitogen. Our results indicate that normal human fibroblasts synthesize four distinct mRNAs of 7.0, 3.7, 2.2, and 1.5 kilobases, which hybridize to a specific HBGF-2/bFGF cDNA probe. In fibroblasts, the level of all four of these transcripts increases dramatically (more than tenfold) within 4 hours of treatment of quiescent cells with fresh fetal bovine serum. Of the purified growth factors tested, transforming growth factor type-beta also increased HBGF-2/bFGF mRNA abundance, but not to the levels attained by serum treatment. Treatment of fibroblasts with cycloheximide before and during serum treatment blocked the ability of serum to induce the expression of the HBGF-2/bFGF gene. The gene is expressed at low levels in human fibroblasts rapidly growing in serum-free medium and at higher levels in cells rapidly growing in serum-containing medium. In contrast to fibroblasts, mRNA coding for HBGF-2/bFGF is undetectable in proliferating normal human keratinocytes, melanocytes, or mammary epithelial cells. Because keratinocytes and melanocytes proliferate in response to purified HBGF-2/bFGF, our results suggest that HBGF-2/bFGF may mediate the proliferation of epidermal cells through paracrine mechanisms involving stromal fibroblasts. Moreover, we have shown that a human squamous cell carcinoma cell line (SCC-25) expresses mRNA coding for HBGF-2/bFGF, suggesting that the gene may become activated in some carcinomas.

Temporal and tissue-specific expression of distinct retrovirus-like (VL30) elements during mouse development

We have analyzed the expression of VL30 retroviral RNA transcripts during mouse embryogenesis. VL30 RNA was found in all tissues examined from mid-gestation, but increased dramatically at later times in the extraembryonic amnion and visceral yolk sac, and to a lesser extent in embryonic liver. The overall temporal and tissue-specific pattern of expression was unlike that of other retrotransposon gene families. S1 nuclease mapping experiments which distinguish transcripts from distinct VL30 elements suggest that they are independently regulated in different tissues during mouse development.

Genetic susceptibility to murine hepatocarcinogenesis is associated with high growth rate of NDEA-initiated hepatocytes

The murine hybrids (C57BL/6J X C3Hf)F1 (B6C3) and (C57BL/6J X BALB/c)F1 (B6C), which have a high and low spontaneous and induced incidence of hepatocellular tumors, respectively, were treated with a single dose of NDEA at 1 week of age followed by TCPOBOP, a phenobarbital-like promoter of liver carcinogenesis, or by vehicle, and sacrificed at 30 weeks of age. The frequency per liver of hepatocellular nodules was similar in the two hybrids. However, in male mice the mean volume of nodules was about 10-fold greater in B6C3 than in B6C mice receiving NDEA followed by vehicle, and the treatment with TCPOBOP after NDEA stimulated nodule growth, with a much greater response in B6C3 mice. In female mice no differences in the mean volume of nodules were seen between hybrids after NDEA and vehicle, whereas upon NDEA and TCPOBOP treatment the mean volume of nodules was 25-fold greater in B6C3 than in B6C females. In addition, a few hepatocellular adenomas and carcinomas were observed, mostly in animals treated with NDEA and TCPOBOP, and they were 3-fold more numerous among B6C3 than B6C mice. TCPOBOP alone induced the same biochemical and hyperplastic effects in the liver of both hybrids. Using DNA probes homologous to Moloney murine leukemia virus, intracisternal A particle and virus-like 30S sequences, no correlation was apparent between the expression of any of these endogenous retroviral families and the strain susceptibility to hepatocarcinogenesis. We hypothesize that the different susceptibility to hepatocarcinogenesis between B6C3 and B6C mice is related to a higher growth rate of B6C3 than B6C initiated liver cells.

Unusual priming mechanism of RNA-directed DNA synthesis in copia retrovirus-like particles of Drosophila

Drosophila cells contain virus-like particles (VLPs) containing 5 kilobases (kb) of RNA (VLP H-RNA) homologous to the transposable element copia. The identity between VLP H-RNA and copia DNA has previously been confirmed at the nucleotide sequence level and reverse transcriptase activity is also detected in the VLPs. These results suggest that VLPs and copia are derivatives of viral particle and provirus forms, respectively, of the copia retrovirus-like particle. If the copia retrovirus-like particle replicates by a mechanism similar to the mechanism of vertebrate retroviral replication, a cellular transfer RNA would prime synthesis of the first DNA strand. We show that this is indeed so but that copia retrovirus-like particle has a novel type of priming mechanism; the first DNA extension does not start from the 3' end of a tRNA, but from an internal site (two nucleotides after the anticodon loop) of the Drosophila initiator methionine tRNA (tRNAMeti).

Isolation of a retroviruslike sequence from the TL locus of the C57BL/10 murine major histocompatibility complex

Two retroviruslike sequences have been isolated from the TL locus of the major histocompatibility complex of C57BL/10 mice. One sequence (TLev2) hybridizes only with probes derived from the pol region of the murine leukemia provirus AKR; the other sequence (TLev1) hybridizes with gag, pol, and env AKR region probes. This 9-kilobase endogenous, TL region-associated virus (TLev1) has been further characterized. The TLev1 genome has been shown to contain murine leukemia virus-related sequences bounded by retroviruslike, VL30 long terminal repeats. Hybridization of TLev1-derived probes to mouse genomic digests reveals multiple copies which show distinct patterns compared with those observed with murine leukemia virus probes. The study of TLev1 may prove significant with respect to the interaction of retroviral sequences within the genome, expression of genes within the TL locus, and polymorphisms within the major histocompatibility complex.

Transforming growth factor type beta regulation of actin mRNA

Stimulation of quiescent cultures of AKR-2B cells with transforming growth factor type beta (TGF beta) resulted in a transitory increase in actin cytoplasmic poly(A) + RNA. Levels of actin mRNA peaked approximately 4-8 hours subsequent to TGF beta addition and approached basal levels by 24 hours. The accumulation of actin transcripts was dose dependent and insensitive to inhibitors of protein synthesis; 1-3 ng/ml TGF beta induced maximal actin gene expression. Actin isotype-specific probes demonstrated that both beta- and gamma-cytoplasmic actins are induced by TGF beta.

Expression and transmission of a rodent retrovirus-like (VL30) gene family

A transcriptionally active sub-set of the dispersed mouse VL30 family of proviral genetic elements was shown to be highly transmissible as a murine leukaemia virus pseudotype. Newly acquired VL30 proviruses (present at 1 to 2 copies per cell) were shown to be transcriptionally active. These data substantiate the hypothesis that this process of duplicative transposition may have played a major role in the evolution of the gene family and also demonstrate that VL30 elements would be capable of mediating oncogene activation by a promoter-insertion-type mechanism during leukaemia virus-induced tumourgenesis.

Absence of gross change in primary DNA sequence during aging process of mice

The possibility of age-associated changes in DNA sequence in terms of amplification and rearrangement was examined in mouse spleen, liver and brain using the method of Southern transfer and filter hybridization. The DNA regions studied were at and around nine cloned sequences, most of which are known to move or amplify in certain situations. No detectable age-associated change, however, was observed in all DNA regions studied. These results suggest that widespread DNA sequence rearrangements or amplifications do not occur during the ageing process in mice.