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About 8% of our genome is composed of sequences from Human Endogenous Retroviruses (HERVs). The HERV-K (HML.2) family, here abbreviated HML.2, is able to produce virus particles that were detected in cell lines, malignant tumors and in autoimmune diseases. Parameters and properties of HML.2 released from teratocarcinoma cell lines GH and Tera-1 were investigated in detail. In most experiments, analyzed viruses were purified by density gradient centrifugation. HML.2 structural proteins, reverse transcriptase (RT) activity, viral RNA (vRNA) and particle morphology were analyzed. The HML.2 markers were predominantly detected in fractions with a buoyant density of 1.16 g/cm 3 . Deglycosylation of TM revealed truncated forms of transmembrane (TM) protein. Free virions and extracellular vesicles (presumably microvesicles-MVs) with HML.2 elements, including budding intermediates, were detected by electron microscopy. Viral elements and assembled virions captured and exported by MVs can boost specific immune responses and trigger immunomodulation in recipient cells. Sequencing of cDNA clones demonstrated exclusive presence of HERV-K108 env in HML.2 from Tera-1 cells. Not counting two recombinant variants, four known env sequences were found in HML.2 from GH cells. Obtained results shed light on parameters and morphology of HML.2. A possible mechanism of HML.2-induced diseases is discussed.

Human endogenous retroviruses are remnants of ancient germline infections that make up approximately 8% of the modern human genome. The HERV-K (HML-2) family is one of the most recent entrants into the human germline, these viruses appear to be transcriptionally active, and HERV-K viral like particles (VLPs) are found in cell lines from a number of human malignancies. HERV-K VLPs were first found to be produced in teratocarcinoma cell lines, and since then teratocarcinoma has been thought of as the classical model for HERV-Ks, with the NCCIT teratocarcinoma cell line particularly known to produce VLPs. Treatment for teratocarcinoma has progressed since its discovery, with improved prognosis for patients. Since the introduction of platinum based therapy, first year survival has greatly improved even with disseminated disease; however, it is estimated that 20% to 30% of patients present with metastatic germ cell tumor relapse following initial treatments. Also, the toxicity associated with the use of chemotherapeutic agents used to treat germ cell tumors is still a major concern. In this study, we show that the depletion of the HERV-K accessory protein Np9 increases the sensitivity of NCCIT teratocarcinoma cells to bleomycin and cisplatin. While decreasing the expression of Np9 had only a modest effect on the baseline viability of the cells, the reduced expression of Np9 increased the sensitivity of the teratocarcinoma cells to environmental (serum starvation) and chemical (chemotherapeutic) stresses. Np9 is also essential to the migration of NCCIT teratocarcinoma cells: in a wound closure assay, reduced expression of Np9 resulted in cells migrating into the wound at a slower rate, whereas reintroduction of Np9 resulted in NCCIT cells migrating back into the wound in a manner similar to the control. These findings support the implication that the HERV-K accessory protein Np9 has oncogenic potential., Competing Interests: The authors have declared that no competing interests exist.

Human endogenous retrovirus (HERV-K (HML-2)) proviruses are among the few endogenous retroviral elements in the human genome that retain coding sequence. HML-2 expression has been widely associated with human disease states, including different types of cancers as well as with HIV-1 infection. Understanding of the potential impact of this expression requires that it be annotated at the proviral level. Here, we utilized the high throughput capabilities of next-generation sequencing to profile HML-2 expression at the level of individual proviruses and secreted virions in the teratocarcinoma cell line Tera-1. We identified well-defined expression patterns, with transcripts emanating primarily from two proviruses located on chromosome 22, only one of which was efficiently packaged. Interestingly, there was a preference for transcripts of recently integrated proviruses, over those from other highly expressed but older elements, to be packaged into virions. We also assessed the promoter competence of the 5' long terminal repeats (LTRs) of expressed proviruses via a luciferase assay following transfection of Tera-1 cells. Consistent with the RNASeq results, we found that the activity of most LTRs corresponded to their transcript levels.

Herpes simplex virus type 1 (HSV-1) immediate-early regulatory protein ICP0 stimulates the initiation of lytic infection and reactivation from quiescence in human fibroblast cells. These functions correlate with its ability to localize to and disrupt centromeres and specific subnuclear structures known as ND10, PML nuclear bodies, or promyelocytic oncogenic domains. Since the natural site of herpesvirus latency is in neurons, we investigated the status of ND10 and centromeres in uninfected and infected human cells with neuronal characteristics. We found that NT2 cells, a neuronally committed human teratocarcinoma cell line, have abnormal ND10 characterized by low expression of the major ND10 component PML and no detectable expression of another major ND10 antigen, Sp100. In addition, PML is less extensively modified by the ubiquitin-like protein SUMO-1 in NT2 cells compared to fibroblasts. After treatment with retinoic acid, NT2 cells differentiate into neuron-like hNT cells which express very high levels of both PML and Sp100. Infection of both NT2 and hNT cells by HSV-1 was poor compared to human fibroblasts, and after low-multiplicity infection yields of virus were reduced by 2 to 3 orders of magnitude. ICP0-deficient mutants were also disabled in the neuron-related cell lines, and cells quiescently infected with an ICP0-null virus could be established. These results correlated with less-efficient disruption of ND10 and centromeres induced by ICP0 in NT2 and hNT cells. Furthermore, the ability of ICP0 to activate gene expression in transfection assays in NT2 cells was poor compared to Vero cells. These results suggest that a contributory factor in the reduced HSV-1 replication in the neuron-related cells is inefficient ICP0 function; it is possible that this is pertinent to the establishment of latent infection in neurons in vivo.

Human endogenous retrovirus (HERV) sequences represent about 0.5% of the human genome. The only HERV known to express virus particles is human teratocarcinoma-derived virus (HTDV), which is now termed HTDV/HERV-K. Between 25 and 50 different copies of HERV-K are present in the human genome, three of which contain full-length genes for viral structural proteins. To determine whether genes of different HERV-K proviruses can be expressed, the morphologies and protein expression patterns of HTDV/HERV-K produced by various human teratocarcinoma cell lines were compared. Three different types of retrovirus-like particles were observed, showing differences in the presence of viral surface proteins and the existence of free mature virions. These distinct morphological features between virion types were in accordance with the results of immunoblotting analyses that revealed differences in the cleavage of a viral Gag protein precursor and the presence of a putative Env protein. These data suggest that different HERV-K proviruses are transcribed in human teratocarcinoma cell lines.

Therapy with the antiepileptic drug valproic acid (2-propylpentanoic acid, VPA) during early pregnancy can cause teratogenic effects (neural tube defects) in humans and in mice. VPA and a teratogenic derivative specifically induce differentiation of F9 teratocarcinoma cells and activate PPARdelta. We have now studied structure-activity relationships of 11 VPA-related compounds by quantitatively comparing their teratogenic potency with their effects in the two novel in vitro systems. Based on the induction of a Rous sarcoma virus (RSV) promoter-driven reporter gene, which is associated with the differentiation of F9 cells, a system suitable for high-throughput and quantitative screening was established. Structure-activity investigations showed that only teratogenic derivatives of VPA induced the response in F9 cells as well as activated the PPARdelta-dependent reporter system in Chinese hamster ovary (CHO) cells. Increases in the length of the side chain in the VPA-related 2-alkyl-pentynoic acid generate more potent inducers in the cell-culture-based assays, which also show higher teratogenicity and embryonic lethality rates. Activation of PPARdelta correlated well with the effects in the F9 cell assay and with teratogenic potency in vivo (p < 0.007). Evaluation of the effects of the presented set of compounds allows the conclusion that the in vitro systems faithfully reflect teratogenicity of VPA-related compounds. Whether the activation of PPARdelta is causally related to the disruption of proper embryonic development or whether it reflects other yet unknown VPA-induced events remains to be established., (Copyright 1999 Academic Press.)

Epithelial ovarian cancer (EOC) remains localized within the peritoneal cavity in a large number of patients, lending itself to i.p. approaches of therapy. In the present study, we investigated the effect of replication-selective herpes simplex virus-1 (HSV-1) used as an oncolytic agent against EOC and the use of human teratocarcinoma PA-1 as carrier cells for i.p. therapy. HSV-1716, a replication-competent attenuated strain lacking ICP34.5, caused a direct dose-dependent oncolytic effect on EOC cells in vitro. A single i.p. administration of 5 x 10(6) plaque-forming units resulted in a significant reduction of tumor volume and tumor spread and an increase in survival in a mouse xenograft model. PA-1 cells supported HSV replication in vitro and bound preferentially to human ovarian carcinoma surfaces compared with mesothelial surfaces in vitro and in vivo. In comparison with the administration of HSV-1716 alone, irradiated PA-1 cells, infected at two multiplicities of infection with HSV-1716 and injected i.p. at 5 x 10(6) cells/animal, led to a significant tumor reduction in the two models tested and the significant prolongation of mean survival in one model. Histological evaluation revealed extensive necrosis in tumor areas infected by HSV-1716. Immunohistochemistry against HSV-1 revealed areas of viral infection within tumor nodules, which persisted for several weeks after treatment. Administration of HSV-infected PA-1 carrier cells resulted in larger areas of tumor infected by the virus. Our results indicate that replication-competent attenuated HSV-1 exerts a potent oncolytic effect on EOC, which may be further enhanced by the utilization of a delivery system with carrier cells, based on amplification of the viral load and possibly on preferential binding of carrier cells to tumor surfaces.

Prompted by the observation of retroviral particle formation in teratocarcinoma cell lines and the consistent finding of antibodies against Gag and Env proteins encoded by human endogenous retrovirus (HERV)-K genomes in the sera of patients with classical seminoma, we studied ovarian and testicular germ cell tumours, their precursor lesions, dysgenetic gonads, and trophoblast lesions for expression of HERV-K sequences by in situ hybridization using radioactive and non-radioactive probes. HERV-K transcripts were detected in all testicular and ovarian germ cell tumours with the exception of teratomas and spermatocytic seminomas. HERV-K expression was also common to testicular carcinoma in situ as well as gonocytes of dysgenetic gonads. Among gestational trophoblastic lesions, HERV-K expression was regularly found in choriocarcinomas, but not in molar lesions. The patterns of HERV-K expression suggest a common molecular pathogenesis of most germ cell tumour entities and malignant gestational trophoblastic disease. They furthermore support the concept of carcinoma in situ as a precursor lesion common to most testicular germ cell neoplasms. The detection of HERV-K gene products in body fluids and tissues may aid diagnosis and monitoring of germ cell tumours and related lesions.

Human papillomaviruses (HPVs) are known to infect human keratinocytes and cause alterations in epithelial differentiation. We showed in this study that expression of the HPV-16 genome was able to interfere with the in vitro differentiation of a human simple-epithelial cell type, the 2102Ep teratocarcinoma cell line. Stable HPV-16 genome-expressing 2102Ep cell lines were generated, and subsequent alterations in differentiation were analyzed in comparison with parental 2102Ep cells. We found that in 2102Ep cells phorbol ester-induced differentiation led to changes in the expression of SSEA antigens, whereas in HPV-transfected cell lines only minor changes were observed.

The human genome contains a wide variety of endogenous retrovirus-like sequences. The human endogenous retrovirus type K (HERV-K) family comprises 30-50 members per haploid genome in humans and is highly conserved in Old World monkeys and apes. Some proviruses are displaying open reading frames (ORF) with coding capacity for viral particles. HERV-K sequences most likely code for the previously described human teratocarcinoma-derived virus (HTDV) and correlated expression of HERV-K Gag has been demonstrated by immunoelectron microscopy studies. Protease, but not yet reverse transcriptase (RT), enzymatic activity was demonstrated for recombinant HERV-K proteins. However, an ultrasensitive RT assay revealed specific polymerase activity associated with the HTDV particles. HERV-K transcription is specifically regulated by viral long terminal repeats and RNA is expressed at low steady-state levels in a variety of human tissues and tumours. In teratocarcinoma cell lines, HERV-K is highly expressed in a complex pattern showing full-length as well as subgenomic envelope (env) and two alternatively spliced small transcripts. The doubly spliced 1.8-kb mRNA codes for cORF protein which resembles Rev of HIV-1 and is located in the nucleolus. In addition, the cORF sequence acts as a leader and is essential for effective expression of glycosylated HERV-K Env protein. Although HERV-K sequences code for all necessary retroviral proteins, infectious particles could not yet be demonstrated. The putative implication of HERV sequences in pathophysiological processes, for example, testicular malignancies, remains to be elucidated.

The expression of transgenes in eukaryotic cells is a powerful approach in cell biology. In most cases, it is based on the activity of strong and constitutive viral cis-acting elements in eukaryotic expression vectors. Here we show that a widely used such element derived from an early gene of human cytomegalovirus is strongly and differentially regulated in mouse cell lines. We analyzed cytomegalovirus promoter-driven expression of stably transfected transgenes in growing, confluent, and differentiating mouse 3T3 fibroblasts, C2C12 myoblasts, and P19 teratocarcinoma cells. In the fibroblasts, transgene expression was strongly downregulated in confluent cultures and was upregulated in growing or confluent cultures by phorbol ester. In contrast, no downregulation by confluency, nor upregulation by phorbol ester, was detected in C2C12 cells. In addition, while marked upregulation was detected in differentiating myotubes, transgene expression was downregulated when differentiating teratocarcinoma cells assumed a neuronal phenotype. These results demonstrate the existence of drastic differences in the regulation of transgene expression in different types of cell lines, indicating that when studying transgene function in cells that are not growing exponentially, viral promoter-driven expression should not be taken for granted.

Northern hybridization with five HERV-K family members, previously cloned from human teratocarcinoma genomic DNA, indicated that two (HERV-(K)27 and -(K)67) of the five clones are expressed in these teratocarcinoma cells. These two clones are closely related (98.49%), however, and Northern blot hybridization lacks the specificity to distinguish between their respective mRNAs. Therefore, PCR analysis with mixed oligonucleotide primers homologous to conserved retroviral pol gene regions was employed to amplify cDNA synthesized from teratocarcinoma cell RNA. This amplification scheme yielded two novel HERV-K family members, HERV-(K)55 and HERV-(K)91. Clone HERV-(K)55 has approximately 98% nucleotide sequence identity to clones HERV-(K)27 and -(K)67. Subsequent RNase protection assays confirmed the expression of HERV-(K)55 and indicated that clones HERV-(K)27 and -(K)67 were not expressed in these cells. One interpretation is that the HERV-(K)27 and -(K)67 probes detected transcripts of clone HERV-(K)55 or other closely-related elements because of their high homologies. In addition, clone HERV-(K)91, which has approximately 81% nucleotide sequence identity to clones HERV-(K)27, -(K)67, and -(K)55, was obtained only from teratocarcinoma 2102E-Pr cells, but the RNase protection assay showed that this clone is also expressed in other human teratocarcinoma cell lines.