The HERV-K accessory protein Np9 controls viability and migration of teratocarcinoma cells

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.

On the stem cell origin of cancer

In each major theory of the origin of cancer-field theory, chemical carcinogenesis, infection, mutation, or epigenetic change-the tissue stem cell is involved in the generation of cancer. Although the cancer type is identified by the more highly differentiated cells in the cancer cell lineage or hierarchy (transit-amplifying cells), the property of malignancy and the molecular lesion of the cancer exist in the cancer stem cell. In the case of teratocarcinomas, normal germinal stem cells have the potential to become cancers if placed in an environment that allows expression of the cancer phenotype (field theory). In cancers due to chemically induced mutations, viral infections, somatic and inherited mutations, or epigenetic changes, the molecular lesion or infection usually first occurs in the tissue stem cells. Cancer stem cells then give rise to transit-amplifying cells and terminally differentiated cells, similar to what happens in normal tissue renewal. However, the major difference between cancer growth and normal tissue renewal is that whereas normal transit amplifying cells usually differentiate and die, at various levels of differentiation, the cancer transit-amplifying cells fail to differentiate normally and instead accumulate (ie, they undergo maturation arrest), resulting in cancer growth.

Of mice and men: teratomas and teratocarcinomas

Teratomas and teratocarcinomas are tumors containing tissue derivatives of all three germ-layers. They can be induced by transplantation of animal embryos to ectopic microenvironment. Development of malignant teratocarcinomas depends on embryonic stage, species-specificity and immunological competence of the host. In the man, teratomas and teratocarcinomas usually represent a subtype of germ-cell tumors but sacrococcygeal teratomas arise from the remnants of the pluripotent primitive streak. Undifferentiated embryonal carcinoma (EC) cells are responsible for the malignancy of experimental mouse teratocarcinomas. Mouse EC cells injected to the adult give rise to tumors and upon injection to early embryos to differentiated tissues--thus resembling normal mouse embryonic stem cells (mESC). Epigenetic changes rather than mutations are associated with transformation of mESC to EC cells. Human EC and ES cell-lines (hESC) contain chromosomal abnormalities and can form teratocarcinoma after transplantation. ES cells are among those proposed for cell replacement therapy in the man. Suicide gene introduction should be recommended prior to their use in vivo to ablate them in case of malignant transformation.

Teratocarcinomas induced by embryonic stem (ES) cells lacking vimentin: an approach to study the role of vimentin in tumorigenesis

Vimentin is a class III intermediate filament protein widely expressed in the developing embryo and in cells of mesenchymal origin in the adult. Vimentin knock-out mice develop and reproduce without any obvious defect. This is an unexpected finding in view of the high degree of conservation of the vimentin gene among vertebrates. However, it does not exclude the possibility of a role for vimentin in pathological conditions, like tumorigenesis. To address this question directly, we have used a teratocarcinoma model involving the injection of ES cells into syngeneic mice. ES cells lacking vimentin were generated from 129/Sv Vim-/- mice with high efficiency. The absence of vimentin did not affect ES cell morphology, viability or growth rate in vitro. Tumours were induced by subcutaneous injection of either Vim-/- or Vim+/+ ES cells into Vim+/+ and Vim-/- mice, in order to analyse the effect of the absence of vimentin in either the tumorigenic cells or the host mice. No significant differences were found in either tumour incidence, size or vascularization of teratocarcinomas obtained with all possible combinations. Vim-/- ES-derived tumours showed the same cellular composition typical of teratocarcinomas induced by wild-type ES cells together with a very similar apoptotic pattern. Taken together, these results demonstrate that in this model vimentin is not essential for efficient tumour growth and differentiation in vivo.

Cell cycle regulators in testicular cancer: loss of p18INK4C marks progression from carcinoma in situ to invasive germ cell tumours

Cell cycle regulators govern cellular proliferation, modulate differentiation and, when defective, contribute to oncogenesis. Here, we examined expression of cyclins A, B1 and E, and cyclin-dependent kinase (CDK) inhibitors p18INK4C (p18), p21WAF1/Cip1 (p21) and p27KiP1 (p27), in normal human adult testis (n = 5), and 53 testicular tumours, including 23 carcinomas in situ (CIS) and 30 germ cell tumours (GCTs). Immunohistochemical analysis revealed a correlation between proliferation and abundance of the cyclin proteins, and abundant p18 and the lack of p21 and p27 in normal spermatogenesis. Expression of p21 and/or p27 was induced in some differentiated structures seen in teratomas, and was recapitulated in cell culture, using human NTera2/D1 teratocarcinoma cells induced to differentiate into neurons. CIS lesions showed abundant p18, low cyclin E, and moderate p27, in contrast with most invasive seminomas and embryonal carcinomas with very low-to-negative p18, often elevated cyclin E, and, unexpectedly, sustained or increased p27. Our results suggest increased abundance of cyclin E, and particularly downmodulation or loss of p18INK4C as the features that correlate with progression from CIS to invasive germ cell tumours of the human testis.

HIF-1 alpha is required for solid tumor formation and embryonic vascularization

The transcriptional response to lowered oxygen levels is mediated by the hypoxia-inducible transcription factor (HIF-1), a heterodimer consisting of the constitutively expressed aryl hydrocarbon receptor nuclear translocator (ARNT) and the hypoxic response factor HIF-1alpha. To study the role of the transcriptional hypoxic response in vivo we have targeted the murine HIF-1alpha gene. Loss of HIF-1alpha in embryonic stem (ES) cells dramatically retards solid tumor growth; this is correlated with a reduced capacity to release the angiogenic factor vascular endothelial growth factor (VEGF) during hypoxia. HIF-1alpha null mutant embryos exhibit clear morphological differences by embryonic day (E) 8.0, and by E8.5 there is a complete lack of cephalic vascularization, a reduction in the number of somites, abnormal neural fold formation and a greatly increased degree of hypoxia (measured by the nitroimidazole EF5). These data demonstrate the essential role of HIF-1alpha in controlling both embryonic and tumorigenic responses to variations in microenvironmental oxygenation.

Retinoic acid abolishes the calcitonin gene-related peptide autocrine system in F9 teratocarcinoma cells

Calcitonin gene-related peptide (CGRP), expressed predominantly in F9 embryonal carcinoma cells, is both a potent chemotactic agent and an autocrine growth factor for these cells. We analyzed the effect of retinoic acid (RA)-induced differentiation of F9 cells into primitive parietal endoderm-like cells, on CGRP production and the CGRP responsiveness of these cells. Poly(A) RNA extracted from F9 cells and analysed by Northern blotting and hybridization with a CGRP probe showed a specific band of about 1200 bases corresponding to mature CGRP mRNA. This band was not detected in F9 cells treated for 6 days with RA (differentiated primitive parietal endoderm-like cells) or in PYS cells (established parietal endoderm-like cell line). During RA-induced differentiation of F9 cells, CGRP mRNA levels fell within 24 h after treatment and were almost undetectable after 2 days. RA treatment also reduced CGRP secretion by F9 cells; the effect was maximal at 3 days and remained stable thereafter. Similarly, RA rapidly reduced adenylate cyclase responsiveness to chicken CGRP (cCGRP) and human CGRP (hCGRP). An 80% fall in cAMP release into the culture medium in the presence of CGRP was observed after 24 h of RA treatment. These results demonstrate that RA rapidly abolishes the CGRP autocrine system involved in the proliferation of F9 cells, at the same time inducing their differentiation into primitive parietal endoderm. They point to the interaction between retinoic acid and growth factors in the regulation of cell proliferation and differentiation.

Expression of Cdx-2 in the mouse embryo and placenta: possible role in patterning of the extra-embryonic membranes

Three mouse homologues of the Drosophila homeotic gene Caudal (Cad) have been described. They are currently designated Cdx-1, Cdx-2, and Cdx-4. Cdx-1 and 2 are both strongly expressed in the adult mid- and hindgut, while Cdx-1 and 4 have been shown to be activated in the embryonic primitive streak. Using a polyclonal antibody against a fusion protein containing the amino terminal 109 amino acids of murine Cdx-2, we here describe the topographical location of the gene product from early cleavage to 12.5 days of embryonic development. Cdx-2 expression begins at 3.5 days and is confined to the trophectoderm, being absent from the inner cell mass. Subsequently, staining is located in the extra-embryonic ectoderm adjacent to the epiblast, but sparing the more superficially placed polar, as well as the mural trophoblastic cells. Continuing expression in the fetal membranes involves the chorion, the allantoic bud, and, at even later stages, the spongiotrophoblast. From 8.5 days, Cdx-2 begins to be expressed in embryonic tissues, principally (unlike Cdx-1) in the posterior part of the gut from its earliest formation, as well as in the tail bud and in the caudal part of the neural tube. Cdx-2 is, therefore, transcribed well before any other membrane of the Cad homologue group and of the related Hox-C group; its expression in the extra-embryonic membranes and in the hindgut reflects the phylogenetic relationship between the cloaca and the chorio-allantois and suggests the possibility that homeobox genes may be involved in placental development and/or patterning.

Orphan receptor COUP-TF I antagonizes retinoic acid-induced neuronal differentiation

Chicken ovalbumin upstream promoter-transcription factors (COUP-TF) are expressed in the developing nervous system and interact with nuclear hormone receptors to regulate expression of different genes. The role of COUP-TF orphan receptors in neurogenesis is virtually unknown. To study the possible function of COUP-TF I during neuronal differentiation, we generated COUP-TF I overexpressing teratocarcinoma PCC7 cell lines and analyzed retinoic acid (RA)-induced neuronal differentiation of these cells. COUP-TF I overexpression results in the blockade of morphological differentiation after induction to differentiate. COUP-TF I represses expression of microtubule-associated protein 2 (MAP2) gene and delays induction of growth-associated protein 43 (GAP43) gene expression. In contrast, expression of the neurofilament light subunit (NF-L) gene is not affected by COUP-TF I overexpression during neuronal differentiation. Also, cells overexpressing COUP-TF I do not stop proliferating after RA and dBcAMP treatment and possess suppressed transcriptional activation from different RA response elements. These results suggest that COUP-TF I plays an important role in regulating RA-induced neuronal differentiation.

Suramin for germ cell tumors. In vitro growth inhibition and results of a phase II trial

BACKGROUND

Several studies have indicated a role for specific growth factors in the differentiation or deregulated growth of germ cell cancers. One proposed mechanism of antitumor activity for suramin is the inhibition of these tumor-derived growth factors.

METHODS

Based on these data, the effects of suramin were studied on the growth of a monolayer culture of two human teratocarcinoma cell lines, NT2/D1 and N2102ep. These represented one cell line that was sensitive (NT2/D1) and one that was resistant (N2102ep) to the differentiation agent retinoic acid. These studies were followed by a Phase II trial of suramin in patients with germ cell tumors. Patients were treated with a continuous infusion of suramin. Dosing was determined by a nomogram based on weekly serum concentrations.

RESULTS

Suramin inhibited the growth of both cell types in a dose-dependent fashion at a concentration attainable in patient plasma. This provided the rationale for a clinical trial of suramin in patients with cisplatin-refractory germ cell tumors. Fourteen patients were treated, and no complete or partial responses were observed. One patient achieved a prolonged decline in serum alpha-fetoprotein level without change in the size of a bidimensionally measurable retroperitoneal mass for more than 11 months.

CONCLUSIONS

Despite the results of the in vitro studies, a Phase II trial failed to demonstrate significant antitumor activity for suramin in patients with cisplatin-refractory germ cell tumors. Although the dosing of suramin by the nomogram was not associated with neurotoxicity, the time needed to achieve therapeutic levels was lengthy, and the levels, once achieved, were of short duration. Thus, an alternative means of administration, i.e., by intermittent bolus administration, is being investigated in trials for patients with prostatic carcinoma. The lack of definitive efficacy for suramin in this trial has led the authors to consider other agents in Phase II trials for cisplatin-refractory germ cell tumor.