Tinzaparin inhibits VL30 retrotransposition induced by oxidative stress and/or VEGF in HC11 mouse progenitor mammary cells: Association between inhibition of cancer stem cell proliferation and mammosphere disaggregation

Tinzaparin is an anticoagulant and antiangiogenic drug with inhibitory properties against tumor growth. VEGF stimulates angiogenesis, while an association between reactive oxygen species (ROS) and angiogenesis is involved in tumor progression. The present study aimed to investigate the effect of tinzaparin on VL30 retrotransposition-positive mouse HC11 mammary stem-like epithelial cells, previously reported to be associated with induced mammosphere/cancer stem cell (CSC) generation and tumorigenesis. Under 24 h serum starvation, 15.2% nominal retrotransposition frequency was increased to 29%. Additionally, while treatment with 3–12 ng/ml VEGF further induced retrotransposition frequency in a dose-dependent manner (up to 40.3%), pre-incubation with tinzaparin (2 IU/ml) for 0.5–4 h reduced this frequency to 18.3% in a time-dependent manner, confirmed by analogous results in NIH3T3 fibroblasts. Treatment with 10–40 pg/ml glucose oxidase (GO) for 24 h induced HC11 cell retrotransposition in a dose-dependent manner (up to 82.5%), while a 3 h pre-incubation with tinzaparin (1 or 2 IU/ml) elicited a 13.5 or 25.5% reduction in retrotransposition, respectively. Regarding tumorigenic VL30 retrotransposition-positive HC11 cells, treatment with 2 IU/ml tinzaparin for 5 days reduced proliferation rate in a time-dependent manner (up to ~55%), and after 3 weeks, disaggregated soft agar-formed foci, as well as low-adherent mammospheres, producing single mesenchymal-like cells with a ~50% reduced retrotransposition. With respect to the VL30 retrotransposition mechanism: While 12 ng/ml VEGF increased the level of VL30 and endogenous reverse transcriptase (enRT) transcripts ~1.41- and ~1.16-fold, respectively, subsequent tinzaparin treatment reduced both endogenous/ROS- and VEGF-induced levels 1.15- and 0.40-fold (VL30) and 0.60- and 0.52-fold (enRT), respectively. To the best of our knowledge, these data demonstrate for the first time, the novel inhibition activity of tinzaparin against ROS- and VEGF-induced VL30 retrotransposition, and the proliferation and/or aggregation of mouse HC11 mammosphere/tumor-initiating CSCs, thus contributing to the inhibition of VL30 retrotransposition-induced primary tumor growth.

VL30 retrotransposition is associated with induced EMT, CSC generation and tumorigenesis in HC11 mouse mammary stem‑like epithelial cells

Retrotransposons copy their sequences via an RNA intermediate, followed by reverse transcription into cDNA and random insertion, into a new genomic locus. New retrotransposon copies may lead to cell transformation and/or tumorigenesis through insertional mutagenesis. Methylation is a major defense mechanism against retrotransposon RNA expression and retrotransposition in differentiated cells, whereas stem cells are relatively hypo-methylated. Epithelial-to-mesenchymal transition (EMT), which transforms normal epithelial cells into mesenchymal-like cells, also contributes to tumor progression and tumor metastasis. Cancer stem cells (CSCs), a fraction of undifferentiated tumor-initiating cancer cells, are reciprocally related to EMT. In the present study, the outcome of long terminal repeat (LTR)-Viral-Like 30 (VL30) retrotransposition was examined in mouse mammary stem-like/progenitor HC11 epithelial cells. The transfection of HC11 cells with a VL30 retrotransposon, engineered with an EGFP-based retrotransposition cassette, elicited a higher retrotransposition frequency in comparison to differentiated J3B1A and C127 mouse mammary cells. Fluorescence microscopy and PCR analysis confirmed the specificity of retrotransposition events. The differentiated retrotransposition-positive cells retained their epithelial morphology, while the respective HC11 cells acquired mesenchymal features associated with the loss of E-cadherin, the induction of N-cadherin, and fibronectin and vimentin protein expression, as well as an increased transforming growth factor (TGF)-β1, Slug, Snail-1 and Twist mRNA expression. In addition, they were characterized by cell proliferation in low serum, and the acquisition of CSC-like properties indicated by mammosphere formation under anchorage-independent conditions. Mammospheres exhibited an increased Nanog and Oct4 mRNA expression and a CD44⁺/CD24^−/low antigenic phenotype, as well as self-renewal and differentiation capacity, forming mammary acini-like structures. DNA sequencing analysis of retrotransposition-positive HC11 cells revealed retrotransposed VL30 copies integrated at the vicinity of EMT-, cancer type- and breast cancer-related genes. The inoculation of these cells into Balb/c mice produced cytokeratin-positive tumors containing pancytokeratin-positive cells, indicative of cell invasion features. On the whole, the findings of the present study demonstrate, for the first time, to the best of our knowledge, that stem-like epithelial HC11 cells are amenable to VL30 retrotransposition associated with the induction of EMT and CSC generation, leading to tumorigenesis.

Exogenous retroelement integration in sperm and embryos affects preimplantation development

Retroelement transcripts are present in male and female gametes, where they are typically regulated by methylation, noncoding RNAs and transcription factors. Such transcripts are required for occurrence of retrotransposition events, while failure of retrotransposition control may exert negative effects on cellular function and proliferation. In order to investigate the occurrence of retrotransposition events in mouse epididymal spermatozoa and to address the impact of uncontrolled retroelement RNA expression in early preimplantation embryos, we performed in vitro fertilization experiments using spermatozoa preincubated with plasmid vectors containing the human retroelements LINE-1, HERVK-10 or the mouse retroelement VL30, tagged with an enhanced green fluorescence (EGFP) gene-based cassette. Retrotransposition events in mouse spermatozoa and embryos were detected using PCR, FACS analysis and confocal microscopy. Our findings show that: (i) sperm cell incorporates exogenous retroelements and favors retrotransposition events, (ii) the inhibition of spermatozoa reverse transcriptase can decrease the retrotransposition frequency in sperm cells, (iii) spermatozoa can transfer exogenous human or mouse retroelements to the oocyte during fertilization and (iv) retroelement RNA overexpression affects embryo morphology and impairs preimplantation development. These findings suggest that the integration of exogenous retroelements in the sperm genome, as well as their transfer into the mouse oocyte, could give rise to new retrotransposition events and genetic alterations in mouse spermatozoa and embryos.

Retrotransposon expression and incorporation of cloned human and mouse retroelements in human spermatozoa

OBJECTIVE

To investigate the expression of long interspersed element (LINE) 1, human endogenous retrovirus (HERV) K10, and short interspersed element-VNTR-Alu element (SVA) retrotransposons in ejaculated human spermatozoa by means of reverse-transcription (RT) polymerase chain reaction (PCR) analysis as well as the potential incorporation of cloned human and mouse active retroelements in human sperm cell genome.

DESIGN

Laboratory study.

SETTING

University research laboratories and academic hospital.

PATIENT(S)

Normozoospermic and oligozoospermic white men.

INTERVENTION(S)

RT-PCR analysis was performed to confirm the retrotransposon expression in human spermatozoa. Exogenous retroelements were tagged with a plasmid containing a green fluorescence (EGFP) retrotransposition cassette, and the de novo retrotransposition events were tested with the use of PCR, fluorescence-activated cell sorting analysis, and confocal microscopy.

MAIN OUTCOME MEASURE(S)

Retroelement expression in human spermatozoa, incorporation of cloned human and mouse active retroelements in human sperm genome, and de novo retrotransposition events in human spermatozoa.

RESULT(S)

RT-PCR products of expressed human LINE-1, HERV-K10, and SVA retrotransposons were observed in ejaculated human sperm samples. The incubation of human spermatozoa with either retrotransposition-active human LINE-1 and HERV-K10 or mouse reverse transcriptase-deficient VL30 retrotransposons tagged with an EGFP-based retrotransposition cassette led to EGFP-positive spermatozo; 16.67% of the samples were positive for retrotransposition. The respective retrotransposition frequencies for the LINE-1, HERV-K10, and VL30 retrotransposons in the positive samples were 0.34 ± 0.13%, 0.37 ± 0.17%, and 0.30 ± 0.14% per sample of 10,000 spermatozoa.

CONCLUSION(S)

Our results show that: 1) LINE-1, HERV-K10, and SVA retrotransposons are transcriptionally expressed in human spermatozoa; 2) cloned active retroelements of human and mammalian origin can be incorporated in human sperm genome; 3) active reverse transcriptases exist in human spermatozoa; and 4) de novo retrotransposition events occur in human spermatozoa.

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.

Abnormal DLK1/MEG3 imprinting correlates with decreased HERV-K methylation after assisted reproduction and preimplantation genetic diagnosis

Retrotransposons participate in cellular responses elicited by stress, and DNA methylation plays an important role in retrotransposon silencing and genomic imprinting during mammalian development. Assisted reproduction technologies (ARTs) may be associated with increased stress and risk of epigenetic changes in the conceptus. There are similarities in the nature and regulation of LTR retrotransposons and imprinted genes. Here, we investigated whether the methylation status of Human Endogenous Retroviruses (HERV)-K LTR retrotransposons and the imprinting signatures of the DLK1/MEG3. p57(KIP2) and IGF2/H19 gene loci are linked during early human embryogenesis by examining trophoblast samples from ART pregnancies and preimplantation genetic diagnosis (PGD) cases and matched naturally conceived controls. Methylation analysis revealed that HERV-Ks were totally methylated in the majority of controls while, in contrast, an altered pattern was detected in ART-PGD samples that were characterized by a hemi-methylated status. Importantly, DLK1/MEG3 demonstrated disturbed methylation in ART-PGD samples compared to controls and this was associated with altered HERV-K methylation. No differences were detected in p57(KIP2) and IGF2/H19 methylation patterns between ART-PGD and naturally conceived controls. Using bioinformatics, we found that while the genome surrounding the p57(KIP2) and IGF2/H19 genes differentially methylated regions had low coverage in transposable element (TE) sequences, the respective one of DLK1/MEG3 was characterized by an almost 2-fold higher coverage. Moreover, our analyses revealed the presence of KAP1-binding sites residing within retrotransposon sequences only in the DLK1/MEG3 locus. Our results demonstrate that altered HERV-K methylation in the ART-PGD conceptuses is correlated with abnormal imprinting of the DLK1/MEG3 locus and suggest that TEs may be affecting the establishment of genomic imprinting under stress conditions.

Arsenic induces VL30 retrotransposition: the involvement of oxidative stress and heat-shock protein 70

Arsenic is an environmental contaminant with known cytotoxic and carcinogenic properties, but the cellular mechanisms of its action are not fully known. As retrotransposition consists a potent mutagenic factor affecting genome stability, we investigated the effect of arsenic on retrotransposition of an enhanced green fluorescent protein (EGFP)-tagged nonautonomous long terminal repeat (LTR)-retrotransposon viral-like 30 (VL30) in a mouse NIH3T3 cell culture-retrotransposition assay. Flow cytometry analysis of assay cells treated with 2.5-20μM sodium arsenite revealed induction of retrotransposition events in a dose- and time-dependent manner, which was further confirmed as genomic integrations by PCR analysis and appearance of EGFP-positive cells by UV microscopy. Specifically, 20μM sodium arsenite strongly induced the VL30 retrotransposition frequency, which was ~90,000-fold higher than the natural one and also VL30 RNA expression was ~6.6-fold. Inhibition of the activity of endogenous reverse transcriptases by efavirenz at 15μM or nevirapine at 375μM suppressed the arsenite-induced VL30 retrotransposition by 71.16 or 79.88%, respectively. In addition, the antioxidant N-acetyl-cysteine reduced the level of arsenite-induced retrotransposition, which correlated with the rescue of arsenite-induced G2/M cell cycle arrest and cell toxicity. Treatment of assay cells ectopically overexpressing the human heat-shock protein 70 (Hsp70) with 15μM sodium arsenite resulted in an additional ~4.5-fold induction of retrotransposition compared with normal assay cells, whereas treatment with 20μM produced a massive cell death. Our results show for the first time that arsenic both as an oxidative and heat-shock mimicking agent is a potent inducer of VL30 retrotransposition in mouse cells. The impact of arsenic-induced retrotransposition, as a cellular response, on contribution to or explanation of the arsenic-associated toxicity and carcinogenicity is discussed.

H2O2 signals via iron induction of VL30 retrotransposition correlated with cytotoxicity

The impact of oxidative stress on mobilization of endogenous retroviruses and their effects on cell fate is unknown. We investigated the action of H2O2 on retrotransposition of an EGFP-tagged mouse LTR-retrotransposon, VL30, in an NIH3T3 cell-retrotransposition assay. H2O2 treatment of assay cells caused specific retrotranspositions documented by UV microscopy and PCR analysis. Flow cytometric analysis revealed an unusually high dose- and time-dependent retrotransposition frequency induced, ∼420,000-fold at 40 μM H2O2 compared to the natural frequency, which was reduced by ectopic expression of catalase. Remarkably, H2O2 moderately induced the RNA expression of retrotransposon B2 without affecting the basal expression of VL30s and L1 and significantly induced the expression of various endogenous reverse transcriptase genes. Further, whereas treatment with 50 μM FeCl2 alone was ineffective, cotreatment with 10 μM H2O2 and 50 μM FeCl2 caused a 6-fold higher retrotransposition induction than H2O2 alone, which was associated with cytotoxicity. H2O2- or H2O2/FeCl2-induced retrotransposition was significantly reduced by the iron chelator DFO or the antioxidant NAC, respectively. Furthermore, both H2O2-induced retrotransposition and associated cytotoxicity were inhibited after pretreatment of cells with DFO or the reverse transcriptase inhibitors efavirenz and etravirine. Our data show for the first time that H2O2, acting via iron, is a potent stimulus of retrotransposition contributing to oxidative stress-induced cell damage.

A twisted kiss: in vitro and in vivo evidence of genetic variation and suppressed transcription of the metastasis-suppressor gene KiSS1 in early breast cancer

KiSS-1 is ametastasis suppressor gene, its inactivation linked to advanced tumor stage and dismal prognosis. We studied its mutational status ,transcription and protein expression in human cancer cell lines and patients with early breast cancer.<br />Tumor tissue DNA and messenger RNA (mRNA) of KiSS1 exons III and IV from the human cancer cell lines Hela, Jurkat, A549, W138t, MCF-7 and from formalin-fixed resected breast adenocarcinomas from 50 women were analysed by means of PCR-SSCP, RT-PCR and sequencing. Tumor tissue was stained for KiSS1 protein expression by means of the streptavidin-biotin complex immunoperoxidase assay. Presence of KiSS1 mutation, mRNA levels and protein staining were examined for correlations with patient/tumor characteristics. <br />A transversion in exon IVa replacing cytosine with guanine was identified 242 base pairs from the translation start site (242C>G) in the cell lines MCF-7, A549 and in 5/50 tumors (10%), resulting in substitution of proline by arginine (P81R) and alteration of the protein tertiary structure. As the substitution was present in germ-line DNA in 3/5 breast cancer patients harbouring the polymorphism in their tumor, the incidence of tumour-specific somatic mutation was 4% among the 50 patiens with early breast cancer. Although the P81R substitution was associated with reduced KiSS1 protein immunoreactivity (56% in wild-type tumors versus 20% in KiSS1-variant tumours) and with axillary nodal involvement (55% in wild-type versus 80% in KiSS1-variant tumors), the correlations did not reach statistical significance. KiSS1 mRNA was detected in only 15/48 tumours (31%) and showed no correlation with mutation or protein expression. Twenty-six tumors stained for KiSS1 protein, in contrast to the universal strong staining seen in normal breast parenchyma and placental tissues. At amedian follow-up of 38 months, relapses occurred in 20% of women with non wild-type tumors versus 13% of women with wild-type KiSS1 tumors (p=0.7). Presence of KiSS1 mutation, mRNA levels and protein expression did not have prognostic significance for relapse-free survival.<br />In conclusion, altered nucleotide sequence and repression of transcription are two potential mechanisms of suppression of the anti-metastatic effects of KiSS1 in early breast cancer: Confirmation in larger cohorts and study of functional effects of the 242C>G exon IVa mutation are warranted.

KEYWORDS

KiSS1, metastasis-suppressor gene, breast cancer, mutation, transcription.

Hsp70 translocates to the nuclei and nucleoli, binds to XRCC1 and PARP-1, and protects HeLa cells from single-strand DNA breaks

For many years, there has been uncertainty concerning the reason for Hsp70 translocation to the nucleus and nucleolus. Herein, we propose that Hsp70 translocates to the nucleus and nucleoli in order to participate in pathways related to the protection of the nucleoplasmic DNA or ribosomal DNA from single-strand breaks. The absence of Hsp70 in HeLa cells, via Hsp70 gene silencing (knockdown), indicated the essential role of Hsp70 in DNA integrity. Therefore, HeLa Hsp70 depleted cells were very sensitive in heat treatment and their DNA breaks were multiple compared to that of control HeLa cells. The molecular mechanism with which Hsp70 performs its role at the level of nucleus and nucleolus during stress was examined. Hsp70 co-localizes with PARP1 in the nucleus/nucleoli as was observed in confocal studies and binds to the BCRT domain of PARP1 as was revealed with protein-protein interaction assays. It was also found that Hsp70 binds simultaneously to XRCC1 and PARP-1, indicating that Hsp70 function takes place at the level of DNA repair and possibly at the base excision repair system. Making a hypothetical model, we have suggested that Hsp70 is the molecule that binds and interrelates with PARP1 creating the repair proteins simultaneously, such as XRCC1, at the single-strand DNA breaks. Our data partially clarify a previously unrecognized cellular response to heat stress. Finally, we can speculate that Hsp70 plays a role in the quality and integrity of DNA.

Vanadium-induced apoptosis of HaCaT cells is mediated by c-fos and involves nuclear accumulation of clusterin

Vanadium exerts a variety of biological effects, including antiproliferative responses through activation of the respective signaling pathways and the generation of reactive oxygen species. As epidermal cells are exposed to environmental insults, human keratinocytes (HaCaT) were used to investigate the mechanism of the antiproliferative effects of vanadyl(IV) sulfate (VOSO(4)). Treatment of HaCaT cells with VOSO(4) inhibited proliferation and induced apoptosis in a dose-dependent manner. Inhibition of proliferation was associated with downregulation of cyclins D1 and E, E2F1, and the cyclin-dependent kinase inhibitors p21(Cip1/Waf1) and p27(Kip1). Induction of apoptosis correlated with upregulation of the c-fos oncoprotein, changes in the expression of clusterin (CLU), an altered ratio of antiapoptotic to proapoptotic Bcl-2 protein family members, and poly(ADP-ribose) polymerase-1 cleavage. Forced overexpression of c-fos induced apoptosis in HaCaT cells that correlated with secretory CLU downregulation and upregulation of nuclear CLU (nCLU), a pro-death protein. Overexpression of Bcl-2 protected HaCaT cells from vanadium-induced apoptosis, whereas secretory CLU overexpression offered no cytoprotection. In contrast, nCLU sensitized HaCaT cells to apoptosis. Our data suggest that vanadium-mediated apoptosis was promoted by c-fos, leading to alterations in CLU isoform processing and induction of the pro-death nCLU protein.

Retrotransposon RNA expression and evidence for retrotransposition events in human oocytes

Although human diseases of retrotransposition-derived etiology have been documented, retrotransposon RNA expression and the occurrence of retrotransposition events in the human oocyte are not studied. We investigated the RNA expression of L1 and HERV-K10 retrotransposons in human oocytes by RT-PCR analysis with designed primers. Using denucleated germinal vesicles (GVs), we detected RT-PCR products of expressed L1, HERV-K10 and, unexpectedly, SINE-R, VNTR and Alu (SVA) retrotransposons. Their transcript specificities were identified as such following RNA-FISH and their origin by cloning and sequence alignment analyses. Assessing the expression level in comparison with somatic cells by densitometry analysis, we found that although in normal lymphocytes and transformed HeLa cells their profile was in an order of L1 > HERV-K10 > SVA, remarkably this was reversed in oocytes. To investigate whether de novo retrotransposition events occur and reverse transcriptases are expressed in the human oocyte, we introduced in GVs either a retrotransposition active human L1 or mouse reverse transcriptase deficient-VL30 retrotransposon tagged with an EGFP-based retrotransposition cassette. Interestingly, in both the cases, we observed EGFP-positive oocytes, associated with an abnormal morphology for L1 and granulation for VL30, and the retrotransposition events were confirmed by PCR. Our results: (i) show that L1, HERV-K10 and SVA retrotransposons are transcriptionally expressed and (ii) provide evidence, for the first time, for retrotransposition events occurring in the human oocyte. These findings suggest that both, network of retrotransposon transcripts and controlled retrotranspositions, might serve important functions required for oocyte development and fertilization while the uncontrolled ones might explain the onset of genetic disorders.

Involvement of heat shock protein-70 in the mechanism of hydrogen peroxide-induced DNA damage: the role of lysosomes and iron

Heat shock protein-70 (Hsp70) is the main heat-inducible member of the 70-kDa family of chaperones that assist cells in maintaining proteins functional under stressful conditions. In the present investigation, the role of Hsp70 in the molecular mechanism of hydrogen peroxide-induced DNA damage to HeLa cells in culture was examined. Stably transfected HeLa cell lines, overexpressing or lacking Hsp70, were created by utilizing constitutive expression of plasmids containing the functional hsp70 gene or hsp70-siRNA, respectively. Compared to control cells, the Hsp70-overexpressing ones were significantly resistant to hydrogen peroxide-induced DNA damage, while Hsp70-depleted cells showed an enhanced sensitivity. In addition, the "intracellular calcein-chelatable iron pool" was determined in the presence or absence of Hsp70 and found to be related to the sensitivity of nuclear DNA to H(2)O(2). It seems likely that the main action of Hsp70, at least in this system, is exerted at the lysosomal level, by protecting the membranes of these organelles against oxidative stress-induced destabilization. Apart from shedding additional light on the mechanistic details behind the action of Hsp70 during oxidative stress, our results indicate that modulation of cellular Hsp70 may represent a way to make cancer cells more sensitive to normal host defense mechanisms or chemotherapeutic drug treatment.

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.

Factors influencing the expression of endogenous reverse transcriptases and viral-like 30 elements in mouse NIH3T3 cells

Retroviral reverse transcriptase (RT) plays a definite role in retroviral life cycle and is essential for the process of retrotransposition. We investigated the RNA expression of endogenous reverse transcriptases (enRTs) in the NIH3T3 mouse genome using, as a probe, a mixture of RT-PCR generated reverse transcriptase products potentially detecting a large number of RTs following treatment with different agents. We found that the expression of enRTs is induced approximately 500-fold following 5'-azacytidine-treatment. Amongst steroid hormones used such as estradiol, diethylstilbestrol, progesterone and dexamethasone only the latter was effective in inducing enRTs up to 4-fold at a concentration of 10(-7) M. Expression of a mouse dominant-negative form of p53 protein in cell clones resulted in induction of 20- to 50-fold, whereas C2-ceramide in a 4-fold induction at concentrations of 20-80 micro M. In a parallel analysis, the respective expression of the transposable viral-like 30 elements (VL30s) was also measured. Their expression was induced up to 50-fold by 5'-azacytidine, overexpression of the p53 gene and C2-ceramide at 80 micro M. It was also induced approximately 3- to 5-fold following estradiol, diethylstilbestrol or progesterone treatment and 30-fold by dexamethasone. Collectively, our results suggest that such stimuli inducing enRTs might play a role in the activation of transcription and retrotransposition of VL30.

CYTOTOXIC ACTIVITY OF KAEMPFEROL GLYCOSIDES AGAINST HUMAN LEUKAEMIC CELL LINES IN VITRO

Two kaempferol coumaroyl glycosides (i.e. platanoside and tiliroside) isolated from the methanolic extract of Platanus orientalis L. buds, were examined for their in vitro cytotoxic activity against a panel of human leukaemic cell lines. Platanoside (1) exhibited cytotoxic activity against most of the cell lines tested, while tiliroside (2) was active against two of the nine tested cell lines. Compound 1, was examined for its effect on the uptake of [(3)H]thymidine as a marker of DNA synthesis. Kaempferol was used as a control. 2000 Academic Press@p$hr Copyright 2000 Academic Press.

Cytotoxic activity and antiproliferative effects of a new semi-synthetic derivative of Ent-3 beta-hydroxy-13-epi-manoyl oxide on human leukemic cell lines

Ent-3 beta-hydroxy-13-epi-manoyl oxide (1), was converted to its thiomidazolide derivative (2) which was tested for its cytotoxic activity against a panel of established human leukemic cell lines. Compound 2, exhibited cytotoxic activity against 13 of the cell lines tested. Additionally, compound 2 was examined for its effect on the uptake of [3H]-thymidine as a marker of DNA synthesis and on cell proliferation. The morphology of the cells and the kind of death induced, was investigated. Flow cytometry experiments on a leukemic cell line was also performed. The results show that the semi-synthetic compound, showed a significant antiproliferative effect and kills cells through the process of apoptosis. The appearance of the apoptotic signs was time and dose dependent. From the flow cytometry experiments, a synchronisation through a delay of the cells in G0/1, phase seems to take place.

The effect of sclareol on growth and cell cycle progression of human leukemic cell lines

Sclareol, a labdane-type diterpene, was tested for cytotoxic effect against a panel of established human leukemic cell lines. The compound showed an IC50 lower than 20 microg/ml in most cell lines tested, while it was higher for resting peripheral blood mononuclear leukocytes (PBML). Furthermore, the compound was tested for cytostatic activity against four of the leukemic cell lines used. At a concentration of 20 microg/ml the compound showed a significant cytostatic effect as soon as 4 h after continuous incubation against two from B and two from T lineage cell lines. The morphology and the kind of death induced from sclareol in three cell lines, was also investigated. The effect of sclareol on the cell cycle progression of two cell lines, using flow cytometry, was examined. The results show that sclareol kills cell lines, through the process of apoptosis. The appearance of the apoptotic signs is time and dose dependent. From the flow cytometry experiments, a delay of the cell population on G0/1 seems to take place. This is the first report, that a labdane type diterpene kills tumor cells via a phase specific mechanism which induces apoptosis.

Clone-specific high-frequency retrotransposition of a recombinant virus containing a VL30 promoter in SV40-transformed NIH3T3 cells

A recombinant virus, containing the promoter of a VL30 LTR and tagged with the neomycin gene as a selection and indicator marker, was constructed to investigate transposition events in NIH3T3 cells after SV40 transformation. This retroviral construct was transfected into psi/CRE packaging cells, and pseudovirions were used to infect NIH3T3 cells. Clones resistant to G418 bearing single-copy integrations of the recombinant virus were isolated and transformed by SV40 virus. Transpositions were detected through RFLPs with a neomycin probe and 'retrotransposition' was further confirmed by inverse PCR and DNA sequencing of transposed and parental copies. We found that: (1) retrotransposition of this recombinant virus occurred with a high frequency in a parental clone transformed with SV40 virus suggesting that the frequency of retrotransposition depended on the initial site of provirus integration; (2) the transposition frequency was independent of the transcription level of the recombinant construct; and (3) analysis of transposition-positive transformants showed that the high transposition frequency appeared to be associated with the induction of endogenous reverse transcriptases.

Defective endogenous proviruses are expressed in feline lymphoid cells: evidence for a role in natural resistance to subgroup B feline leukemia viruses

Endogenous feline leukemia virus (FeLV)-related sequences (enFeLV) are a family of proviral elements found in domestic cats and their close relatives. These elements can recombine with exogenous, infectious FeLVs of subgroup A (FeLV-A), giving rise to host range variants of FeLV-B. We found that a subset of defective enFeLV proviruses is highly expressed in lymphoma cell lines and in a variety of primary tissues, including lymphoid tissues from healthy specific-pathogen-free cats. At least two RNA species were detected, a 4.5-kb RNA containing gag, env, and long terminal repeat sequences and a 2-kb RNA containing env and long terminal repeat sequences. Cloning of enFeLV cDNA from two FeLV-free lymphoma cell lines (3201 and MCC) revealed a long open reading frame (ORF) encoding a truncated env gene product corresponding to the N-terminal portion of gp70env. Interestingly, all of three natural FeLV-B isolates include 3' env sequences which are missing from the highly transcribed subset and hence must be derived from other enFeLV elements. The enFeLV env ORF cDNA clones were closely similar to a previously characterized enFeLV provirus, CFE-16, but were polymorphic at a site corresponding to an exogenous FeLV neutralization epitope. Site-specific antiserum raised to a C-terminal 30-amino-acid peptide of the enFeLV env ORF detected an intracellular product of 35 kDa which was also shed from cells in stable form. Expression of the 35-kDa protein correlated with enFeLV RNA levels and was negatively correlated with susceptibility to infection with FeLV-B. Cell culture supernatant containing the 35-kDa protein specifically blocked infection of permissive fibroblast cells with FeLV-B isolates. We suggest that the truncated env protein mediates resistance by receptor blockade and that this form of enFeLV expression mediates the natural resistance of cats to infection with FeLV-B in the absence of FeLV-A.

Molecular cloning and characterization of a defective recombinant feline leukaemia virus associated with myeloid leukaemia

The GM1 strain of feline leukaemia virus (FeLV) was isolated from a naturally occurring case of myeloid leukaemia and induces severe haematopoietic abnormalities, including myeloblastic leukaemia, on inoculation into cats. Molecular clones of FeLV-GM1 proviruses were obtained and studied by restriction enzyme mapping, blot hybridization and partial DNA sequence analysis. Two types of clone were isolated; the first was a replication-competent FeLV of subgroup A, resembling other low or minimally pathogenic FeLV-A isolates; the second was replication-defective with extensive deletions and mutations in gag and pol, although it has an intact env gene of subgroup B phenotype. Large segments of the defective proviruses, from the 5' leader sequence upstream of the gag gene to the 5' half of the env gene, show structural hallmarks of endogenous FeLV-related proviruses. Infectious FeLV-GM1 viruses recovered after transfection were tested for their leukaemogenic potential in newborn cats. Early polyclonal myeloproliferative changes were observed in cats inoculated with FeLV-A/GM1 alone, although these were more pronounced in animals receiving the full FeLV-AB/GM1 complex reconstituted by cotransfection of the defective virus FeLV-B with its FeLV-A helper. Analysis of viruses in the bone marrow showed that replication of the subgroup B component is delayed and restricted to a proportion of cats. Most of the infected cats developed persistent abnormalities of haematopoiesis and one progressed to disseminated myeloid leukaemia. The defective recombinant FeLV-B/GM1 appears to play an indirect but important role in myeloid leukaemogenesis.

Receptor-mediated leukaemogenesis: hypothesis revisited

The discovery of the first example of retroviral transduction of an immunological effector molecule has led us to reconsider the possible importance of cell surface receptors of the immune system in leukaemia development. Antigen receptors on lymphoid cells not only bind external ligands but are crucial in the control of cellular proliferation. The concept of autocrine stimulation in oncogenesis is already well established and we see no reason to exclude the possibility of analogous mechanism operating through antigen receptors. At present, we are investigating the oncogenic function of the retrovirus (FeLV-T17) carrying a T-cell receptor gene (v-tcr). In addressing the general concept of oncogenesis by ligand/receptor interactions in the immune system we face the problem of the diversity and, for T-cell antigen receptors, the complex nature of receptor-ligand interaction. Nevertheless, the implications of the model encourage us to continue to search for new experimental tools and approaches to the question.

The effect of partially purified dexamethasone receptor on RNA synthesis in rat thymus and liver nuclei

DEAE Sephadex-A-50 chromatography of rat liver and thymus cytosol charged with dexamethasone (0.1 microM) and heated at 25 degrees C for 30 min, separates two main hormone--receptor complexes, one in the flow through (DE-1) and one eluting with 150 mM [Cl-] (DE-2). Incubation of these two fractions with homologous nuclei results in a 20% stimulation and a 20-25% inhibition of transcription of liver and thymus nuclei, respectively. No difference was observed between the action of DE-1 and DE-2. If the DE-1 and DE-2 fractions of cytosol charged with the glucocorticoid antagonist, cortexolone, are used under the same experimental conditions, no effect on the transcriptional activity of liver or thymus nuclei can be observed. The fact that from nuclei only DE-1 can be isolated, not DE-2, suggests that DE-1 is the intranuclearly active form and that DE-2, with a still unknown mechanism, is transformed to DE-1. A linear relation exists between the number of GR-complexes (DE-1 or DE-2) translocated and the change in RNA synthesis. Transcriptional effects start when a critical number of acceptor sites are occupied in the nucleus (approximately 200-300 sites/nucleus) for both receptor forms.