Morphologic evidence for retrovirus production by epithelial cells derived from a human testicular tumor metastasis

HERVs and Cancer-A Comprehensive Review of the Relationship of Human Endogenous Retroviruses and Human Cancers

Genomic instability and genetic mutations can lead to exhibition of several cancer hallmarks in affected cells such as sustained proliferative signaling, evasion of growth suppression, activated invasion, deregulation of cellular energetics, and avoidance of immune destruction. Similar biological changes have been observed to be a result of pathogenic viruses and, in some cases, have been linked to virus-induced cancers. Human endogenous retroviruses (HERVs), once external pathogens, now occupy more than 8% of the human genome, representing the merge of genomic and external factors. In this review, we outline all reported effects of HERVs on cancer development and discuss the HERV targets most suitable for cancer treatments as well as ongoing clinical trials for HERV-targeting drugs. We reviewed all currently available reports of the effects of HERVs on human cancers including solid tumors, lymphomas, and leukemias. Our review highlights the central roles of HERV genes, such as gag, env, pol, np9, and rec in immune regulation, checkpoint blockade, cell differentiation, cell fusion, proliferation, metastasis, and cell transformation. In addition, we summarize the involvement of HERV long terminal repeat (LTR) regions in transcriptional regulation, creation of fusion proteins, expression of long non-coding RNAs (lncRNAs), and promotion of genome instability through recombination.

Expression of human endogenous retrovirus K is stimulated by ultraviolet radiation in melanoma

Human endogenous retroviruses (HERVs) represent a cellular reservoir of potentially pathogenic retroviral genes. A growing body of evidence indicates that the activation of endogenous retroviral sequences might be involved in the transformation of melanocytes. In this study, we investigated the effects of ultraviolet radiation (UVR) on the expression of human endogenous retrovirus type K (HERV-K) in melanoma cells and non-melanoma cells in vitro. Solely in melanoma cell lines, irradiation with UVB (200 mJ/cm(2)) resulted in a significant transcriptional activation of the retroviral pol gene as well as in an enhanced expression of the retroviral envelope protein (env). In addition, UVB treatment induced the production of retroviral particles in the supernatants of melanoma cell lines. These data indicate that HERV-K expression can be activated by UVB irradiation and suggest an involvement of HERV-K in UVR-related melanoma pathogenesis.

Identification of a Melanoma Marker Derived from Melanoma-Associated Endogenous Retroviruses

We previously described the expression of melanoma-associated endogenous retrovirus (MERV) proteins and viral particles in human melanomas and metastases. The objective of the present study was to determine whether a humoral immune response to MERV proteins occurs in melanoma. Candidate B-cell epitopes on MERV proteins were predicted using bioinformatic screening. The reactivity of MERV peptides corresponding to the predicted epitopes with antibodies prevalent in sera of melanoma patients was analyzed. An immunodominant peptide located in the env protein of MERV was identified. Subsequent analyzes using 81 samples from stage I to stage IV melanoma patients and 95 sera from healthy subjects revealed statistically significant differences in seroprevalence of antibodies in melanoma sera samples when compared with reference samples from healthy subjects. The prevalence of anti-MERV antibodies in melanoma patient sera was confirmed by immunofluorescence on env-transfected cells. These data indicate the potential of this candidate peptide as target for diagnosis and immunotherapy.

Retroelements and the human genome: new perspectives on an old relation

Retroelements constitute a large portion of our genomes. One class of these elements, the human endogenous retroviruses (HERVs), is comprised of remnants of ancient exogenous retroviruses that have gained access to the germ line. After integration, most proviruses have been the subject of numerous amplifications and have suffered extensive deletions and mutations. Nevertheless, HERV-derived transcripts and proteins have been detected in healthy and diseased human tissues, and HERV-K, the youngest, most conserved family, is able to form virus-like particles. Although it is generally accepted that the integration of retroelements can cause significant harm by disrupting or disregulating essential genes, the role of HERV expression in the etiology of malignancies and autoimmune and neurologic diseases remains controversial. In recent years, striking evidence has accumulated indicating that some proviral sequences and HERV proteins might even serve the needs of the host and are therefore under positive selection. The remarkable progress in the analysis of host genomes has brought to light the significant impact of HERVs and other retroelements on genetic variation, genome evolution, and gene regulation.

Characterization of the antibody response specific for the human endogenous retrovirus HTDV/HERV-K

Differentiated human teratocarcinoma cell lines produce the human teratocarcinoma-derived virus (HTDV) particles encoded by the human endogenous retrovirus sequence HERV-K. We screened almost 2,000 human sera for antibodies against this endogenous human retrovirus, HTDV/HERV-K. Specificity of the immunofluorescence reactions using particle producing teratocarcinoma cells was confirmed by immunoelectron microscopy of ultrathin frozen sections. Immunoblot analyses using lysates of HTDV-producing cells revealed a 80-kDa HERV-K Gag precursor and a 90-kDa putative viral Env protein after incubation with positive sera. No processed Gag protein could be observed. Virus-specific bands were not detected in lysates of nonproducing cells. High antibody titers were found in about 60% of male patients with germ cell tumors. Antibody reactivity declined after tumor removal. In healthy blood donors, anti-HTDV reactivity was found only at low titers in a small percentage (3.9%) of individuals. A slightly elevated but statistically significant percentage of HTDV positivity was also observed for sera of pregnant women, whereas human immunodeficiency virus-positive individuals exhibited no peculiarity compared to normal blood donors. Our results provide evidence that HTDV particles are expressed in vivo and that the immune reaction against HTDV/HERV-K is specific for defined viral proteins.

The viruses in all of us: characteristics and biological significance of human endogenous retrovirus sequences

Human endogenous retroviruses (HERVs) are very likely footprints of ancient germ-cell infections. HERV sequences encompass about 1% of the human genome. HERVs have retained the potential of other retroelements to retrotranspose and thus to change genomic structure and function. The genomes of almost all HERV families are highly defective. Recent progress has allowed the identification of the biologically most active family, HTDV/HERV-K, which codes for viral proteins and particles and is highly expressed in germ-cell tumors. The demonstrable and potential roles of HTDV/HERV-K as well as of other human elements in disease and in maintaining genome plasticity are illustrated.

Human endogenous retroviruses: nature, occurrence, and clinical implications in human disease

Retroviral diagnostics have become standard in human laboratory medicine. While current emphasis is placed on the human exogenous viruses (human immunodeficiency virus and human T-cell leukemia virus), evidence implicating human endogenous retroviruses (HERVs) in various human disease entities continues to mount. Literature on the occurrence of HERVs in human tissues and cells was analyzed. Substantial evidence documents that retrovirus particles were clearly demonstrable in various tissues and cells in both health and disease and were abundant in the placenta and that their occurrence could be implicated in some of the reproductive diseases. The characteristics of HERVs are summarized, mechanisms of replication and regulation are outlined, and the consistent hormonal responsiveness of HERVs is noted. Clear evidence implicating HERV gene products as participants in glomerulonephritis in some cases of systemic lupus erythematosus is adduced. Data implicating HERVs as etiologic factors in reproductive diseases, in some of the autoimmune diseases, in some forms of rheumatoid arthritis and connective tissue disease, in psoriasis, and in some of the inflammatory neurologic diseases are reviewed. The current major needs are to improve methods for HERV detection, to identify the most appropriate HERV prototypes, and to develop diagnostic reagents so that the putative biologic and pathologic roles of HERVs can be better evaluated.

Human teratocarcinomas

Teratocarcinomas are one of the commonest forms of cancer in young adult men. Cell lines derived from these tumors, and particularly the cell lines composed of their embryonal carcinoma (EC) stem cells, may provide useful information concerning the development and subsequent pathology of teratocarcinomas in humans. In addition, it is likely that human EC cells resemble early embryonic cells and can be used as an in vitro counterpart of such cells from the human embryo. Several common properties of human EC cells have been identified, and a human EC cell line, TERA-2, that is capable of extensive somatic differentiation has been cloned. In nude mice, TERA-2 EC cells form tumors containing neural elements and glandular structures that resemble primitive gut. In culture, these EC cells can be induced to differentiate by exposure to retinoic acid and hexamethylenebisacetamide (HMBA). Differentiation is marked by the disappearance of several cell surface antigens characteristic of human EC cells, and the appearance of other antigens on the various subsets of differentiated derivatives. In retinoic acid-induced cultures, these differentiated derivatives include neurons and cells permissive for the replication of cytomegalovirus, a virus that can cause birth defects in humans. On the other hand, HMBA appears to activate an alternative pathway of differentiation for TERA-2 EC cells, although the identity of the resulting cells remains to be elucidated. In addition to providing a tool for analyzing the evolution of teratocarcinomas in human patients, the TERA-2 EC cells may provide us with insights into the mechanisms of cellular differentiation in the human embryo and a model in which to investigate how teratogenic agents such as HCMV can disrupt these processes.

RNA-tumorviruses, oncogenes, and their possible role in human carcinogenesis

The detection and characterization of oncogenes via RNA tumor viruses (or retroviruses) and the recognition of their location at breakpoints of chromosomal translocations which are frequently found in certain human neoplasms has promoted present understanding of molecular mechanisms underlying carcinogenesis. Oncogenes are cellular genes which can be transduced by RNA tumorviruses and induce malignant transformation under experimental conditions in vivo and in vitro. A role of retroviruses in human leukemogenesis is suggested by epidemiological observations and by the isolation of such viruses from several human T-cell leukemias and lymphomas (human T-cell leukemia/lymphoma virus or HTLV) as well as by biochemical association of retroviral markers with human leukemias. A role of HTLV has been suggested also in a human immune deficiency syndrome (AIDS). In view of the well known role of many factors in carcinogenesis the concept of carcinogenesis as a multistep process as well as the concept of cocarcinogenesis and the role of cofactors other than viruses, such as radiation and chemicals, aging, hormones, graft vs host reaction, environmental factors etc., will have to be carefully considered.

Oncogenes in retroviruses and cells

Oncogenes are genes that cause cancer. Retroviruses contain oncogenes and cause cancer in animals and, perhaps, in man. The viruses have appropriated their oncogenes from normal cellular DNA by genetic recombination. Correspondingly, uninfected vertebrate cells contain a family of evolutionary conserved cellular oncogenes. Retrovirus infection, introducing additional viral oncogenes into the cells, as well as carcinogen-mediated activation of cellular oncogenes may both lead to increased synthesis of oncogene encoded transforming proteins which convert normal cells to tumor cells. Unique retroviruses of human origin have recently been identified. They may, on occasion, directly cause tumors in man. However, the general significance of retroviruses may better be illustrated by their remarkable genetic composition which allows them to promote tumor growth by a variety of genetic mechanisms.

Synthesis of retrovirus-like particles in testicular teratocarcinomas

Two lines of evidence led to the investigation of human teratocarcinoma cells in vitro for oncogenic retroviruses: the observation by electron microscopy of retrovirus-like particles budding from the syncytial trophoblasts of human placentas, and the demonstration that teratocarcinoma patients before treatment show a high serum antibody reactivity against envelope proteins of mammalian retroviruses. In all 5 teratocarcinoma cell lines studied so far, retrovirus-like particles have been detected by electron microscopy. The production of these human teratocarcinoma-derived (HTD)-particles is enhanced by induction procedures known to be effective in animal virus model systems. In parallel, virus induction also increases the level of chorionic gonadotropin in the culture supernatant, demonstrating syncytial trophoblast-like cells in the heterogeneous cell population of teratocarcinomas which could be responsible for HTD-particle production. The origin of these virus-like particles--endogenous or exogenous--as well as their role in the pathogenesis of teratocarcinomas is not known so far. The potential value of serum antibody reactivity in teratocarcinoma patients as a diagnostic marker is discussed.