Induction of proliferation or transformation of neuroretina cells by the mil and myc viral oncogenes

Extrachromosomal circular DNA: biogenesis, structure, functions and diseases

Extrachromosomal circular DNA (eccDNA), ranging in size from tens to millions of base pairs, is independent of conventional chromosomes. Recently, eccDNAs have been considered an unanticipated major source of somatic rearrangements, contributing to genomic remodeling through chimeric circularization and reintegration of circular DNA into the linear genome. In addition, the origin of eccDNA is considered to be associated with essential chromatin-related events, including the formation of super-enhancers and DNA repair machineries. Moreover, our understanding of the properties and functions of eccDNA has continuously and greatly expanded. Emerging investigations demonstrate that eccDNAs serve as multifunctional molecules in various organisms during diversified biological processes, such as epigenetic remodeling, telomere trimming, and the regulation of canonical signaling pathways. Importantly, its special distribution potentiates eccDNA as a measurable biomarker in many diseases, especially cancers. The loss of eccDNA homeostasis facilitates tumor initiation, malignant progression, and heterogeneous evolution in many cancers. An in-depth understanding of eccDNA provides novel insights for precision cancer treatment. In this review, we summarized the discovery history of eccDNA, discussed the biogenesis, characteristics, and functions of eccDNA. Moreover, we emphasized the role of eccDNA during tumor pathogenesis and malignant evolution. Therapeutically, we summarized potential clinical applications that target aberrant eccDNA in multiple diseases.

Stable expression of intracellular Notch suppresses v-Src-induced transformation in avian neural cells

Understanding how disruption of differentiation contributes to the cancer cell phenotype is required to identify alterations essential for malignant transformation and provide experimental basis for their correction. We investigated whether primary quail neuroretina cells, transformed by a conditional v-Src mutant (QNR/v-src(ts)), could revert to a normal phenotype, in response to the stable expression of constitutively active Notch1 intracellular domain (ICN). This model system was chosen because Notch signaling plays an instructive role in cell fate determination during NR development, and because the intrinsic capacity of QNR cultures to differentiate is blocked by v-Src. We report that stable ICN expression results in suppression of QNR/v-src(ts) cell transformation in the presence of an active oncoprotein. This phenotypic reversion coincides with a major switch in cell identity, as these undifferentiated cells acquire glial differentiation traits. Both changes appear to be mediated by CBF, a transcription factor that binds to ICN and activates target genes. Cells restored to a normal and differentiated phenotype have undergone changes in the functioning of signaling effectors, essentially regulating cell morphology and cytoskeleton organization. This dominant interference may be partially mediated by an autocrine/paracrine mechanism, as revertant cells secrete a factor(s), which inhibits transformation properties of QNR/v-src(ts) cells.

Negative control of the Myc protein by the stress-responsive kinase Pak2

Pak2 is a serine/threonine kinase that participates in the cellular response to stress. Among the potential substrates for Pak2 is the protein Myc, encoded by the proto-oncogene MYC. Here we demonstrate that Pak2 phosphorylates Myc at three sites (T358, S373, and T400) and affects Myc functions both in vitro and in vivo. Phosphorylation at all three residues reduces the binding of Myc to DNA, either by blocking the requisite dimerization with Max (through phosphorylation at S373 and T400) or by interfering directly with binding to DNA (through phosphorylation at T358). Phosphorylation by Pak2 inhibits the ability of Myc to activate transcription, to sustain cellular proliferation, to transform NIH 3T3 cells in culture, and to elicit apoptosis on serum withdrawal. These results indicate that Pak2 is a negative regulator of Myc, suggest that inhibition of Myc plays a role in the cellular response to stress, and raise the possibility that Pak2 may be the product of a tumor suppressor gene.

Induction of postmitotic neuroretina cell proliferation by distinct Ras downstream signaling pathways

Ras-induced cell transformation is mediated through distinct downstream signaling pathways, including Raf, Ral-GEFs-, and phosphatidylinositol 3-kinase (PI 3-kinase)-dependent pathways. In some cell types, strong activation of the Ras-Raf-MEK-extracellular signal-regulated kinase (ERK) cascade leads to cell cycle arrest rather than cell division. We previously reported that constitutive activation of this pathway induces sustained proliferation of primary cultures of postmitotic chicken neuroretina (NR) cells. We used this model system to investigate the respective contributions of Ras downstream signaling pathways in Ras-induced cell proliferation. Three RasV12 mutants (S35, G37, and C40) which differ by their ability to bind to Ras effectors (Raf, Ral-GEFs, and the p110 subunit of PI 3-kinase, respectively) were able to induce sustained NR cell proliferation, although none of these mutants was reported to transform NIH 3T3 cells. Furthermore, they all repressed the promoter of QR1, a neuroretina growth arrest-specific gene. Overexpression of B-Raf or activated versions of Ras effectors Rlf-CAAX and p110-CAAX also induced NR cell division. The mitogenic effect of the RasC40-PI 3-kinase pathway appears to involve Rac and RhoA GTPases but not the antiapoptotic Akt (protein kinase B) signaling. Division induced by RasG37-Rlf appears to be independent of Ral GTPase activation and presumably requires an unidentified mechanism. Activation of either Ras downstream pathway resulted in ERK activation, and coexpression of a dominant negative MEK mutant or mKsr-1 kinase domain strongly inhibited proliferation induced by the three Ras mutants or by their effectors. Similar effects were observed with dominant negative mutants of Rac and Rho. Thus, both the Raf-MEK-ERK and Rac-Rho pathways are absolutely required for Ras-induced NR cell division. Activation of these two pathways by the three distinct Ras downstream effectors possibly relies on an autocrine or paracrine loop, implicating endogenous Ras, since the mitogenic effect of each Ras effector mutant was inhibited by RasN17.

Ras/MEK signaling suppresses Myc-dependent apoptosis in cells transformed by c-myc and activated ras

Cooperation of myc and activated ras has been suggested to cause malignant cell transformation but the mechanism is still unknown. Here we isolated a transformed cell line in which activation of c-Myc and Ras are independently controllable, and show that after establishment of the transformed state by c-myc and activated ras, removal of activated Ras initiates apoptosis that is dependent on c-Myc activity. Apoptosis is also initiated by an inhibitor of MEK (MAPK/ERK kinase), a kinase downstream of Ras, and apoptosis is blocked by activated Mek1. These results suggest that one of the conditions required for establishment of the transformed state is a block of apoptosis involving MEK activity. We tested the effect of MEK inhibition on cells transformed by various oncogenes. Suppression of apoptosis by MEK is not critical in general, but in cells transformed by c-myc plus a gene that activates the MAPK cascade it is necessary to avoid cell death. Activated Ras/MEK did not suppress c-myc-dependent apoptosis due to serum-limitation. Overexpression of chicken bcl-xL suppressed apoptosis under serum-limiting conditions, but not apoptosis initiated by Ras/MEK inhibition in cells transformed by myc and activated ras. Altogether, these results suggest the existence of a novel regulatory mechanism for myc-dependent apoptosis in certain transformed cells.

Genetic regulatory elements introduced into neural stem and progenitor cell populations

The genetic manipulation of neural cells has advantage in both basic biology and medicine. Its utility has provided a clearer understanding of how the survival, connectivity, and chemical phenotype of neurones is regulated during, and after, embryogenesis. Much of this achievement has come from the recent generation by genetic means of reproducible and representative supplies of precursor cells which can then be analyzed in a variety of paradigms. Furthermore, advances made in the clinical use of transplantation for neurodegenerative disease have created a demand for an abundant, efficacious and safe supply of neural cells for grafting. This review describes how genetic methods, in juxtaposition to epigenetic means, have been used advantageously to achieve this goal. In particular, we detail how gene transfer techniques have been developed to enable cell immortalization, manipulation of cell differentiation and commitment, and the controlled selection of cells for purification or safety purposes. In addition, it is now also possible to genetically modify antigen presentation on cell surfaces. Finally, there is detailed the transfer of therapeutic products to discrete parts of the central nervous system (CNS), using neural cells as elegant and sophisticated delivery vehicles. In conclusion, once the epigenetic and genetic controls over neural cell production, differentiation and death have been more fully determined, providing a mixture of hard-wired elements and more flexibly expressed characteristics becomes feasible. Optimization of the contributions and interactions of these two controlling systems should lead to improved cell supplies for neurotransplantation.

The v-myc oncogene

v-myc is the viral homolog of c-myc transduced by several acute transforming retroviruses, many of which encode this gene as a Gag-Myc fusion protein. The v-myc oncogene can transform several lineages of mammalian and avian cells either alone or in cooperation with other oncogenes. While the Gag portion of the Gag-Myc fusion protein and the nuclear localization signal each appear to be dispensable for transformation, the N- and C-termini of the Myc sequence have been found to be essential for transformation. All v-myc genes contain point mutations which seem to confer a greater potency to v-myc in the process of transformation, proliferation, and apoptosis. In v-myc-transformed myelomonocytic cells, secondary events occur, such as the expression of colony stimulating factor-1 (CSF-1) which play a critical role in immortalization and subsequent tumor progression. Inhibition of the autocrine loop of CSF-1 was found to induce apoptosis in the immortalized cells. While overexpression of v-Myc blocks terminal differentiation of hematopoietic cells, this is not sufficient to block the differentiation of certain neural and skeletal muscle cells. Recent developments on the effects of v-myc on cell growth, transformation, differentiation and apoptosis are discussed in this review.

Modulation of kinase activity and oncogenic properties by alternative splicing reveals a novel regulatory mechanism for B-Raf

Members of the raf oncogene family encode serine/threonine protein kinases, which activate the mitogen-activated protein kinase kinase MEKs (MAPK or ERK kinases) through direct interaction and phosphorylation. Several recent studies have revealed interesting differences between two members of this family, Raf-1 and B-Raf, regarding their activation, regulation, and kinase activity. In particular, B-Raf was shown to display higher MEK kinase activity than Raf-1. By using both two-hybrid analysis and coimmunoprecipitation experiments, we demonstrate here that B-Raf also markedly differs from Raf-1 by a higher affinity for MEK. We previously reported that the B-raf gene encodes multiple protein isoforms resulting from complex alternative splicing of two exons (exons 8b and 10) located upstream of B-Raf kinase domain. In the present study, we show that these naturally occurring modifications within the protein sequence markedly modulate both the biochemical and oncogenic properties of B-Raf. The presence of exon 10 sequences enhances the affinity for MEK, the basal kinase activity, as well as the mitogenic and transforming properties of full-length B-Raf, whereas the presence of exon 8b sequences seems to have opposite effects. Therefore, alternative splicing represents a novel regulatory mechanism for a protein of the Raf family.

Target preference of embryonic retina cells and retinal cell lines is cell-autonomous, position-specific, early determined and heritable

Retinal ganglion cells (RGCs) form the topographic connection between retina and optic tectum in the developing avian embryo. In vitro, neurons with the morphological traits and marker expression of RGCs were found both in single-cell cultures from embryonic day (E) 6 chick retina and in retinal cell lines derived from E3.5 quail retina. Rapid and substantial differentiation of RGC-like cells could be induced in the lines by addition of fibroblast growth factor aFGF or bFGF. RGC-like cells were examined with respect to their target discrimination properties as single cells in the stripe carpet assay. In this assay system, alternating stripes of membrane vesicles prepared from the anterior and posterior tectum are offered to growing axonal processes as a substrate. Temporal RGC-like cells, both primary cells prepared from the temporal retina and immortalized cells of those retinal lines derived from the temporal retina, avoid stripes of membrane vesicles from posterior tectum; they prefer to grow on membrane vesicles from the anterior tectum, which is their in vivo target. Nasal RGC-like cells did not exhibit a target preference, in accordance with previous findings. Together the experiments show that target preference of RGCs is a cell-autonomous and heritable mechanism that is determined early and is position-dependent.

Generation of cell lines from embryonic quail retina capable of mature neuronal differentiation

The avian embryonic retina is widely used as a model system for cellular and molecular studies on central nervous system neurons. We aimed at the generation of cell lines from the early embryonic quail retina by retroviral oncogene transduction. For this, we made use of the retina organ culture system which exhibits both proliferation, necessary for stable oncogene transduction, and initial neuronal differentiation, a prerequisite for the generation of cell lines with mature neuronal properties. The oncogene myc was chosen as it is both proliferation-inducing and differentiation-compatible. A chimeric gene, mycER, containing v-myc and the hormone-binding domain of the estrogen receptor, was used for transduction in order to allow for hormone regulation of myc activity. Transduced organ-cultured cells from temporal and nasal retina were passaged into sparse single cell cultures. From these, colonies of rapidly dividing cells were isolated and the progeny expanded as cell lines. The lines contained cells with features of neuroepithelial cells, showing vimentin and A2B5. They also contained spontaneously differentiated neuronal cells showing neurofilament L and N-CAM180. A subpopulation of the neuronal cells exhibited the morphological characteristics of retinal ganglion cells, i.e., large pear-shaped somata each emitting one long process with a distinct growth cone. In addition, they showed the marker profile of retinal ganglion cells, i.e., expression of Thy-1, G4, DM-GRASP, Nr-CAM, neurofilament H, and tau. Neuronal differentiation could be induced by the addition of db cAMP and retinoic acid. The mature neuronal features of the lines open new possibilities to study properties of retinal neurons, including ganglion cells, in a defined and manipulable experimental system.

Establishment of a human retinal cell line by transfection of SV40 T antigen gene with potential to undergo neuronal differentiation

Recently, a number of laboratories have been interested in developing cell lines of ocular tissues to understand the pathogenesis of ocular diseases. Toward this end, we report here the generation of cell lines of human retina by transfection of simian virus SV40 T antigen gene. Established retinal cells grow as a monolayer and exhibit limited serum dependence. Phase-contrast and electron microscopic studies revealed distinct morphological cell types. Immunofluorescence studies showed that the established retinal cells were positive for neuron-specific enolase, neurofilament protein, glycine receptor, synaptophysin, and secretogranin. Cells were negative for glial fibrillary acidic protein, glutamine synthetase, galactocerebroside, and carbonic anhydrase II. In addition to neuronal features, a small percentage of flat cells were, however, positive for cellular retinaldehyde binding protein, and cells with the phenotype of rod and cone photoreceptor coexpressed opsin and interphotoreceptor retinoid-binding protein. An important feature of this cell line is that addition of phorbol ester and cAMP induced dramatic changes, with 100% of the cells extending long, thin neuritic processes. Thus, the established retinal cells would be useful for studies dealing with differentiation and plasticity of the cells of the nervous system.

Cell cycle-dependent association of Gag-Mil and hsp90

Immunoprecipitated p100Gag-Mil protein kinase from MH2-transformed quail embryo fibroblasts is associated with an 89 kDa protein. The molar ratio between p89 and Gag-Mil in the immunocomplex is 0.72, indicating that the majority of Gag-Mil is complexed with p89. During mitosis part of Gag-Mil is shifted to a form with reduced electrophoretic mobility, p102Gag-Mil. Appearance of p102Gag-Mil leads to a reduced association with p89 indicating that p102 is not associated with p89. Microsequencing of p89 isolated from immunoprecipitates of Gag-Mil identified the protein as the quail homologue of chicken hsp90. Our results show that p100Gag-Mil is associated with hsp90 with a high stoichiometry and that upshifted p102Gag-Mil is released from the complex with hsp90.

In vivo cooperation of two nuclear oncogenic proteins, P135gag-myb-ets and p61/63myc, leads to transformation and immortalization of chicken myelomonocytic cells

To investigate a possible in vivo cooperation between the p61/63myc and P135gag-myb-ets proteins, we used a previously constructed retrovirus, named MHE226, which contains the fused v-myb and v-ets oncogenes of the E26 retrovirus and the v-myc oncogene of MH2. For that purpose, chicken neuroretina cells producing MHE226 and pseudotyped with the Rous associated virus-1 (RAV-1) helper virus were injected in 1-day-old chickens. In control experiments, we also injected chicken neuroretina cells producing E26 (RAV-1), RAV-1 alone, or constructs lacking one of the oncogenes of MHE226. The average life span of MHE226-infected chickens is half that of E26-infected chickens. MHE226-infected chickens harbor tumors scattered in many organs, but compared with E26, MHE226 induced a weak leukemia. Study of integration sites suggests that the majority of the tumors results from clonal or oligoclonal events. Cell cultures were derived from the tumors of MHE226-infected chickens and grown in standard medium without addition of exogenous chicken myelomonocytic growth factor. These cells still divide at high rate after more than 100 passages and can thus be considered immortalized. By using several criteria, these cells were characterized as precursors of the myelomonocytic lineages.

Symmetrical segregation of potassium channels at cytokinesis

To determine how voltage-gated ion channels segregate between sibling cells at cytokinesis, we used a whole-cell patch clamp to measure the electrophysiological phenotypes of siblings within 45 min of division. Recently born siblings in an immortalized line of embryonic retinal cells were identified as pairs of spherical cells adhering to one another. All siblings were electrically coupled when cells were simultaneously voltage clamped, whereas nonsiblings were not coupled. Twelve pairs of siblings were electrically isolated by mechanical separation so that their phenotypes could be measured independently. Cells expressed two principal membrane conductances, delayed rectifier-like (IK) and inward rectifier (IK(IR)) potassium currents. Despite qualitative and quantitative variability in IK and IK(IR) expression within the population, each cell of a given pair expressed similar steady-state current densities between -110 and +50 mV. We estimated IK(IR) slope conductance by blocking the current specifically with 5 mM Cs and calculated IK(IR) ratios in siblings and nonsiblings. Three pairs of siblings expressed IK(IR) ratios of approximately 1.2, while ratios in three pairs of adhered nonsiblings varied between 1.6 and 5.4. When currents were sampled continuously through cytokinesis by using the perforated-patch recording mode, current amplitude showed no net change within 30 min of division. Because channel number did not appear to change in siblings during this interval, parental channels were inherited by each daughter in proportion to the area of membrane received. Heterogeneity therefore arises after siblings reenter interphase and is not due to the asymmetrical segregation of channels at cytokinesis.

Oncogenes in X-ray-transformed C3H 10T1/2 mouse cells and in X-ray-induced mouse fibrosarcoma (RIF-1) cells

In order to better understand the molecular basis of X-ray induced carcinogenesis we have investigated RNA levels of oncogenes in an X-ray transformed C3H 10T1/2 fibroblast line (XTD) and RIF-1 cells isolated from an X-ray-induced fibrosarcoma in a C3H mouse. Steady-state levels of K-ras, H-ras, N-ras, abl, sis, src, and fos were unchanged in the X-ray-transformed cells compared with non-transformed C3H 10T1/2 cells. However, myc and raf mRNA levels were increased dramatically in the transformed cells. Data further suggests a possible alteration in processing of raf RNA in the XTD cells. Southern blot analysis of secondary transfectants induced with XTD DNA indicated that the oncogenic phenotype did not segregate with the myc or raf loci; nor with nine other oncogenes analysed.

Structure of the chicken myelomonocytic growth factor gene and specific activation of its promoter in avian myelomonocytic cells by protein kinases

In chicken myeloid cells but not in erythroid cells, kinase-type oncogenes activate expression of the chicken myelomonocytic growth factor (cMGF). The autocrine loop established this way plays a key role in lineage-specific cooperation of nuclear and kinase-type oncogenes in retrovirally induced myeloid leukemia. In this report, we describe the cloning of the cMGF gene, including its promoter. The structure of the cMGF gene is homologous to those of the granulocyte colony-stimulating factor and interleukin-6 genes. Expression from reporter constructs containing the cMGF promoter is specific to myelomonocytic cells. Kinases activate cMGF at the transcriptional level in macrophages and strongly induce reporter expression in myelomonocytic cells.

Structure of the chicken myelomonocytic growth factor gene and specific activation of its promoter in avian myelomonocytic cells by protein kinases

In chicken myeloid cells but not in erythroid cells, kinase-type oncogenes activate expression of the chicken myelomonocytic growth factor (cMGF). The autocrine loop established this way plays a key role in lineage-specific cooperation of nuclear and kinase-type oncogenes in retrovirally induced myeloid leukemia. In this report, we describe the cloning of the cMGF gene, including its promoter. The structure of the cMGF gene is homologous to those of the granulocyte colony-stimulating factor and interleukin-6 genes. Expression from reporter constructs containing the cMGF promoter is specific to myelomonocytic cells. Kinases activate cMGF at the transcriptional level in macrophages and strongly induce reporter expression in myelomonocytic cells.

Clonal cells from embryonic retinal cell lines express qualitative electrophysiological differences

Cells from the embryonic quail retina were immortalized with the v-mil oncogene and cloned by limiting dilution. Their phenotype was examined using the whole-cell patch clamp method. Three membrane currents, IK(IR), INa and IK, were found at different frequencies within a sample of 170 cells drawn from a large clone. Nearly all combinations of these three markers were found and the frequency of combinations showed that the markers assorted independently. Examination of clones of less than 10 cells showed that heterogeneity originates with a high probability within clones, arguing that chromosomal mutation, for example, is unlikely to account for phenotypic diversity. A possible explanation is that phenotypic differences between cells might reflect the local exchange of instructive signals. If so, then the genes for the three phenotypic markers are controlled independently.

Transcription of a quail gene expressed in embryonic retinal cells is shut off sharply at hatching

The avian neuroretina (NR) is part of the central nervous system and is composed of photoreceptors, neuronal cells, and Müller (glial) cells. These cells are derived from proliferating neuroectodermal precursors that differentiate after terminal mitosis and become organized in cell strata. Genes that are specifically expressed at the various stages of retinal development are presently unknown. We have isolated a quail (Coturnix coturnix japonica) cDNA clone, named QR1, encoding a 676-amino acid protein whose carboxyl-terminal portion shows significant similarity to those of the extracellular glycoprotein osteonectin/SPARC/BM40 and of the recently described SC1 protein. The QR1 cDNA identifies a mRNA detected in NR but not in other embryonic tissues examined. The levels of this mRNA are markedly reduced when nondividing NR cells are induced to proliferate by the v-src oncogene. QR1 expression in NR is limited to the middle portion of the inner nuclear layer, a localization that essentially corresponds to that of Müller cells. Transcription of QR1 takes place only during the late phase of retinal development and is shut off sharply at hatching. Signals that regulate this unique pattern of expression appear to originate within the NR, since the QR1 mRNA is transcribed in cultured NR cells and is shut off also in vitro at a time coinciding with hatching.

myc products induce the expression of catecholaminergic traits in quail neural crest-derived cells

The avian myelocytomatosis virus strain MC29 v-myc oncogene transforms a wide panel of avian cells in vitro and either blocks or maintains differentiation, depending on the cell type. In the present work, we have investigated the effect of this oncogene on the differentiation of early embryonic cells, neural crest cells, grown in vitro. We report that the MC29 v-myc gene product induces a strong cellular proliferation of 2-day quail neural crest with the appearance of catecholaminergic traits. Other v-myc as well as the c-myc gene products also trigger this phenotype. Retroviruses carrying some other oncogenes do not elicit this phenotypic expression, although they activate cell multiplication. Thus, our results indicate that myc gene products induce (directly or indirectly) a differentiated phenotype in a subpopulation of neural crest cells.

Molecular and biological properties of c-mil transducing retroviruses generated during passage of Rous-associated virus type 1 in chicken neuroretina cells

IC1, IC2, and IC3 are novel c-mil transducing retroviruses generated during serial passaging of Rous-associated virus type 1 (RAV-1) in chicken embryo neuroretina cells. They were isolated by their ability to induce proliferation of these nondividing cells. IC2 and IC3 were generated during early passages of RAV-1 in neuroretina cells, whereas IC1 was isolated after six consecutive passages of virus supernatants. We sequenced the transduced genes and the mil-RAV-1 junctions of the three viruses. The 5' RAV-1-mil junction of IC2 and IC3 was formed by a splicing process between the RAV-1 leader sequence and exon 8 of the c-mil gene. The 5' end of IC1 resulted from homologous recombination between gag and mil sequences. Reconstitution experiments showed that serial passaging of IC2 in neuroretina cells also led to the formation of a gag-mil-containing retrovirus. Therefore, constitution of a U5-leader-delta c-mil-delta RAV-1-U3 virus represents early steps in c-mil transduction by RAV-1. This virus further recombined with RAV-1 to generate a gag-mil-containing virus. The three IC viruses transduced the serine/threonine kinase domain of the cellular gene. Hence, amino-terminal truncation is sufficient to activate the mitogenic property of c-mil. Comparison of the transforming properties of IC2 and IC1 showed that the transduced mil gene, expressed as a unique protein independent of gag sequences, was weakly transforming in avian cells. Acquisition of gag sequences by IC1 not only increased the rate of virus replication but also enhanced the transforming capacity of the virus.

Two nuclear oncogenic proteins, P135gag-myb-ets and p61/63myc, cooperate to induce transformation of chicken neuroretina cells

Several studies have shown that full transformation of primary rodent fibroblasts can be achieved in vitro through the cooperation of two oncogenes (usually one nuclear and one cytoplasmic) classified on the basis of different complementation groups. We have shown previously that cooperation between v-mil (cytoplasmic, serine-threonine kinase product), and v-myc (nuclear, DNA-binding product) is required to transform 7-day-old chicken neuroretina cells, which in usual culture medium do not rapidly proliferate. v-mil induces sustained growth of chicken neuroretina cells without transformation; v-myc fails to stimulate the proliferation of chicken neuroretina cells but is required to achieve transformation of the proliferating cells. Here, we present results indicating that the P135gag-myb-ets nuclear protein of avian erythroblastosis virus E26 is able to induce proliferation but not transformation of chicken neuroretina cells. v-myc is required in addition to P135gag-myb-ets to achieve chicken neuroretina cell transformation. In contrast, we found that the P135gag-myb-ets and P100gag-mil proteins are not able to cooperate in this system.

Malignant transformation of human bladder epithelial cells by DNA transfection with the v-raf oncogene

Transfection of the v-raf oncogene into immortalized, nontumorigenic human bladder epithelial cells resulted in the isolation of two tumorigenic transformants. Both were identified as human and of the same origin as the parent cell line by human leukocyte antigen typing and Southern blot analysis. Both the primary tumorigenic transfectants and the cell lines established from the induced tumors expressed v-raf mRNA and v-raf protein. In both tumorigenic transformants the level of c-myc mRNA was enhanced compared with that of the parent cell line.

Oncogenes, protooncogenes, and signal transduction: toward a unified theory?

This paper has reviewed, in a broad sense, the potential involvement of the oncogenes and their progenitors, the protooncogenes, in signal transduction pathways. The membrane-associated oncogene products appear to be connected with the generation and/or regulation of secondary messengers, particularly those associated with Ca2+/phospholipid-dependent activation of the serine/threonine kinase protein kinase C. Activation of transmembrane receptors, either through binding their native ligand or through point mutations that lead to constitutive expression, results in the expression of their intrinsic tyrosine-specific protein kinases. In PDGF-stimulated cells, this results in the increased turnover of phosphatidylinositols and the subsequent release of IP3 (Habenicht et al., 1981; Berridge et al., 1984). This coincides with activation of a PI kinase activity (Kaplan et al., 1987). Likewise, the fms product, which is the receptor for CSF-1, induces a guanine nucleotide-dependent activation of phospholipase C (Jackowski et al., 1986). Receptor functions are potentially regulated through differential binding of ligands (as proposed with PDGF), through interactions with other receptors, and through the "feedback" regulation mediated by protein kinase C. PDGF stimulation leads to modulation of the EGF receptor through protein kinase C (Bowen-Pope et al., 1983; Collins et al., 1983; Davis and Czech, 1985). Similarly, the neu product becomes phosphorylated on tyrosine residues following treatment of cells with EGF, although the neu protein does not bind EGF itself (King et al., 1988; Stern and Kamps, 1988). The tyrosine kinases of the src family are not receptors themselves, although they may mediate specific receptor-generated signals. The clck product is physically and functionally associated with the T-cell receptors CD4 and CD8, and becomes active upon specific stimulation of cells expressing those markers (Veillette et al., 1988a,b). The precise physiological role of the src family products has not been established, but their kinase activity is intrinsic to that function. The v- and c-src products are hyperphosphorylated during mitosis (Chackalaparampil and Shalloway, 1988), which correlates with periods of reduced cell-to-cell adhesion and communication (Warren and Nelson, 1987; Azarnia et al., 1988). Furthermore, pp60c-src is associated with a PI kinase activity when complexed with MTAg of polyoma virus, suggesting a function in stimulating increased turnover of the phosphatidylinositols (Heber and Courtneidge, 1987; Kaplan et al., 1987).(ABSTRACT TRUNCATED AT 400 WORDS)

Activation and transduction of c-mil sequences in chicken neuroretina cells induced to proliferate by infection with avian lymphomatosis virus

We report that nondividing neuroretina cells from chicken embryos can be induced to proliferate following infection with Rous-associated virus type 1 (RAV-1), an avian lymphomatosis retrovirus lacking transforming genes. Multiplication of RAV-1-infected neuroretina cells is observed after a long latency period and takes place initially in a small number of cells. We also show that serial virus passaging onto fresh neuroretina cultures leads to the generation of novel mitogenic viruses containing the mil oncogene. DNA analysis indicated that RAV-1 is the only provirus detected in cells infected at virus passage 1, whereas neuroretina cells infected at subsequent virus passages harbor mil-containing proviruses. Three viruses, designated IC1, IC2, and IC3, were molecularly cloned. Restriction mapping indicated that in each virus, truncated c-mil sequences were inserted within different portions of the RAV-1 genome. In addition, IC1 and IC2 viruses have transduced novel sequences that belong to the 3' noncoding portion of the c-mil locus. All three viruses induce neuroretina cell multiplication and direct the synthesis of mil-specific proteins. Proliferation of neuroretina cells infected at passage 1 of RAV-1 was not associated with any detectable rearrangement of c-mil, when a v-mil probe was used. However, these cells expressed high levels of an aberrant 2.8-kilobase mRNA hybridizing to mil but not to a long terminal repeat probe. Therefore, transcriptional activation of a portion of c-mil could represent the initial events induced by RAV-1 infection and lead to retroviral transduction of activated c-mil sequences.

Mapping by in vitro constructs of the P100gag-mil region, accounting for induction of chicken neuroretina cell proliferation

The v-mil oncogene of the avian retrovirus MH2 is expressed as a fusion protein with viral gag determinants in infected cells. This P100gag-mil protein accounts for the proliferation of chicken embryo neuroretina cells (CNR) induced by MH2 in vitro. We constructed a series of mutants by in-frame deletions in different parts of the gag and mil domains and tested their ability to induce CNR growth. We show that gag sequences, as well as 200-base-pair 5' mil sequences, were not required to induce such a proliferation. However, gag sequences seem to contribute to a full proliferation of growing CNR. In contrast, deletions in the kinase domain abolish this induction. In particular, by deleting only 9 nucleotides localized around the unique SphI site of v-mil, we produced a totally inactive mutant (BalSp). This mutant directs the synthesis of a v-mil protein lacking the dipeptide Tyr-Leu, which is conserved in almost all the members of the large protein kinase family, and a histidine residue highly conserved in Ser-Thr protein kinase members.

Alternative splicing of RNAs transcribed from the chicken c-mil gene

Two distinct c-mil-related cDNA clones have been isolated from a chicken embryo cDNA library. Results presented here show that the single chicken c-mil gene is coding for two c-mil mRNA species, different by at least 60 base pairs and generated by an alternative splicing mechanism. These mRNA molecules can be translated into two distinct proteins of 73 and 71 kilodaltons.

[Cell proliferation and cooperation of v-mil and v-myc oncogenes]

Retroviruses which possess the property to recombine with genetic material from the cell, have cloned and activated some oncogenes and hence are a privileged source for the study of these genes. Cellular oncogene activation can occur following two non mutually exclusive ways: (i) by over-expression of their products; (ii) by modifications of their products through mutations. Retroviruses can combine these two ways of activation leading to the over-expression of a modified product. In this paper, we present results obtained in the study of MH2, a retrovirus containing two oncogenes. We have shown that the two oncogenes of MH2 (v-mil and v-myc) cooperate in vitro to transform neuroretina cells from chicken embryos. These cells which normally do not grow in a defined medium, are induced to proliferate and become transformed upon infection by MH2. Our data enabled us to show that in MH2 v-mil was responsible for the induction of proliferation and v-myc for the transformation of the proliferating cells. Using in vitro constructs we located two regions in the protein encoded by v-mil which are important for its mitogenic property. We have also cloned the cellular counterpart of v-mil and the study of its biological activity on neuroretina cells enabled us to propose a mechanism of activation of the cellular gene by truncation of its 5' part.

Transformed and tumorigenic phenotypes induced by avian retroviruses containing the v-mil oncogene

Avian retrovirus MH2 contains two oncogenes, v-mil and v-myc. We have previously shown that a spontaneous mutant of MH2 (PA200-MH2), expressing only the v-mil oncogene, is able to induce proliferation of quiescent neuroretina cells. In this study, we investigated the transforming and tumorigenic properties of v-mil. PA200 induced fibrosarcomas in about 60% of the injected chickens, whereas inoculation of MH2 resulted mainly in the appearance of kidney carcinomas. Analysis of several parameters of transformation showed that PA200, in contrast to MH2, induced only limited in vitro transformation of fibroblasts and neuroretina cells. These results suggest that v-myc is the major transforming and tumorigenic gene in MH2-infected cells. This low in vitro transforming capacity differentiates v-mil not only from other avian oncogenes, but also from the homologous murine v-raf gene.

Alternative splicing of RNAs transcribed from the chicken c-mil gene

Two distinct c-mil-related cDNA clones have been isolated from a chicken embryo cDNA library. Results presented here show that the single chicken c-mil gene is coding for two c-mil mRNA species, different by at least 60 base pairs and generated by an alternative splicing mechanism. These mRNA molecules can be translated into two distinct proteins of 73 and 71 kilodaltons.

Control of myogenic differentiation by cellular oncogenes

The establishment of a differentiated phenotype in skeletal muscle cells requires withdrawal from the cell cycle and termination of DNA synthesis. Myogenesis can be inhibited by serum components, purified mitogens, and transforming growth factors, but the intracellular signaling pathways utilized by these molecules are unknown. Recent studies have confirmed a role for proteins encoded by cellular proto-oncogenes in transduction of growth factor effects that lead to cell proliferation. To test the contrasting hypothesis that cellular oncogenes might also regulate tissue-specific gene expression in developing muscle cells, myoblasts have been modified by incorporation of the cognate viral oncogenes, the corresponding normal or oncogenic cellular homologs, and chimeric oncogenes, whose expression can be induced reversibly. Regulation of the endogenous cellular oncogenes also has been examined in detail. Down-regulation of c-myc is not obligatory for myogenesis; rather, inhibitory effects of myc on muscle differentiation are contingent on sustained proliferation. In contrast, activated src and ras genes block myocyte differentiation directly, through a mechanism that is independent of DNA synthesis and is rapidly reversible, resembling the effects of inhibitory growth factors. The coordinate regulation of diverse tissue-specific gene products including muscle creatine kinase, nicotinic acetylcholine receptors, sarcomeric proteins, and voltage-gated ion channels, raises the hypothesis that inhibitors such as transforming growth factor-beta and ras proteins might exert their effects through a transacting transcriptional signal shared by multiple muscle-specific genes.

One- and two-step transformations of rat thyroid epithelial cells by retroviral oncogenes

A system of epithelial cells is described in which it is possible to study the number and the nature of genes capable of conferring the malignant phenotype. Two fully differentiated, hormone-responsive cell lines from rat thyroid glands are presented which are susceptible to one-step or two-step transformation upon infection with several murine acute retroviruses. After infection, both cell lines became independent from their thyrotropic hormone requirement for growth. However, complete transformation was achieved with one of the cell lines (FRTL-5 Cl 2), whereas the other cell line (PC Cl 3) failed to grow in agar and to give rise to tumors in vivo. The latter cell line was susceptible to complete transformation upon cooperation of the v-ras-Ha and the human c-myc oncogenes.

Infection of neuroretinal cells in vitro by avian sarcoma viruses UR1 and UR2: transformation, cell growth stimulation, and changes in transducin levels

Infection in vitro of differentiating chick embryo neuroretinal cells with avian sarcoma viruses UR1 and UR2 results in mitogenic stimulation and morphologic conversion of both support neuronal cells. This was shown by the continuous propagation of transformed cells for over 4 months and growth of reaggregated colonies in liquid medium as well as in soft agar. Production of the transforming proteins p 150 gag-fps and p68 gag-ros of UR1 and UR2, respectively, was similar to that of transformed chick embryo fibroblasts, as judged from in vitro kinase activity assays. The two protein subunits, T beta and T gamma, but not T alpha of the GTP binding protein transducin, found in the retina of many animal species, were present in control neuroretinal cells. Infection with Rous sarcoma virus or UR2 resulted in an inhibition of T gamma synthesis and enhancement of T beta-like protein production.

One- and two-step transformations of rat thyroid epithelial cells by retroviral oncogenes

A system of epithelial cells is described in which it is possible to study the number and the nature of genes capable of conferring the malignant phenotype. Two fully differentiated, hormone-responsive cell lines from rat thyroid glands are presented which are susceptible to one-step or two-step transformation upon infection with several murine acute retroviruses. After infection, both cell lines became independent from their thyrotropic hormone requirement for growth. However, complete transformation was achieved with one of the cell lines (FRTL-5 Cl 2), whereas the other cell line (PC Cl 3) failed to grow in agar and to give rise to tumors in vivo. The latter cell line was susceptible to complete transformation upon cooperation of the v-ras-Ha and the human c-myc oncogenes.

Cellular oncogenes in neoplasia

In recent years cellular homologues of many viral oncogenes have been identified. As these genes are partially homologous to viral oncogenes and are activated in some tumour cell lines they are termed "proto-oncogenes". In tumour cell lines proto-oncogenes are activated by either quantitative or qualitative changes in gene structure: activation of these genes was originally thought to be a necessary primary event in carcinogenesis, but activated cellular oncogenes, unlike viral oncogenes, do not transform normal cells in culture. In experimental models cooperation between two oncogenes can induce transformation of early passage cells, and this has become the basis of an hypothesis for multistep carcinogenesis. Proto-oncogene products also show sequence homology to various components in the mitogenic pathway (growth factors, growth factor receptors, signal transducing proteins and nuclear proteins), and it has been postulated that they may cause deregulation of the various components of this pathway. In human tumours single or multiple oncogene activation occurs. The pattern of oncogene activation in common solid malignancies is not consistent within any one class of tumour, nor is it uniform between classes, with three exceptions. In neuroblastoma, breast cancer, and perhaps in lung cancer there is relatively consistent activation of N-myc, neu, and c-myc/N-myc, respectively. Amplification of these genes generally correlates with poor prognosis. The introduction of methods for the direct study of oncogene transcription and their products will undoubtedly broaden our vision of cancer biology in man and, hopefully, add diagnostic and prognostic precision to tumour typing.

Induction of proliferation of neuroretina cells by long terminal repeat activation of the carboxy-terminal part of c-mil

Expression of the P100gag-mil protein of avian retrovirus MH2 in cultured chicken embryo neuroretina cells was previously shown to result in the proliferation of normally quiescent cell populations. We show here that long terminal repeat activation of the carboxy terminus of the c-mil gene is sufficient to induce neuroretina cell proliferation.

Fibroblast transformation parameters induced by the avian v-mil oncogene

An avian retrovirus containing only the v-mil oncogene (PA200-MH2) was analyzed for its ability to induce a transformed phenotype in chicken embryo fibroblasts. Infected cultures exhibited an altered morphology, disarranged actin cable filaments, and a decrease in the amount of cell surface fibronectin. In addition, these cells showed a high level of plasminogen activator protease activity and were also capable of growth in low serum concentrations. In contrast, PA200-MH2 was very inefficient at inducing foci under agar and colonies in semisolid medium relative to the Mill Hill 2 and Rous sarcoma viruses. This inefficiency was further reflected in vivo by the total inability of PA200-MH2 to induce wing tumors in young birds. However, 40% of the birds inoculated in the wing web with PA200-MH2-infected cells did develop slow-growing tumors at the site of injection, with no evidence of hematopoietic involvement. Our results indicate that the v-mil oncogene is transforming both in vitro and in vivo and that each of the oncogenes in the Mill Hill 2 virus, v-mil and v-myc, can independently transform fibroblasts. These data suggest that v-mil is functionally related to its homologous murine counterpart, v-raf, which also transforms fibroblasts.

Induction of proliferation of neuroretina cells by long terminal repeat activation of the carboxy-terminal part of c-mil

Expression of the P100gag-mil protein of avian retrovirus MH2 in cultured chicken embryo neuroretina cells was previously shown to result in the proliferation of normally quiescent cell populations. We show here that long terminal repeat activation of the carboxy terminus of the c-mil gene is sufficient to induce neuroretina cell proliferation.

The v-myc oncogene is sufficient to induce growth transformation of chick neuroretina cells

A number of studies have shown that full transformation of non-established rodent fibroblasts can be efficiently achieved in vitro by the concerted action of two oncogenes belonging to different complementation groups. Extension of the two-genes carcinogenesis model to other differentiated cell types, presumably endowed with different controls of growth, is desirable for a better understanding of questions such as the host cell selectivity of oncogene action. A recent report claimed that cooperation between two oncogenes, v-myc and v-mil, is required to achieve transformation of chicken embryo neuroretina cells, which are characterized by a limited growth capacity in monolayer culture. Here we present evidence that the v-myc oncogene alone is sufficient to induce growth transformation of glial and neuronal precursor cell types from chick neuroretina. We also report that induction of transformation by v-myc is accompanied by faithful preservation of some of the differentiated functions of the chick cells.

Differentiation of retrovirus-infected avian neuroretina cells

The effects of oncogenic retroviruses on the expression of differentiation markers were studied in monolayer cultures of chick and quail embryo neuroretinas. Transformation by Rous sarcoma virus (RSV) did not affect the appearance of synapses, and the expression of glutamic acid decarboxylase was stimulated by pp60v-src, the product of the src gene. Quail embryo neuroretina cells transformed by Mill Hill 2 (which contains the two oncogenes v-mil and v-myc) were induced to proliferate into permanent cultures that synthesized crystallins and produced lentoid bodies. In contrast, transformation with a temperature-sensitive mutant of RSV reversibly blocked the production of crystallins and lentoid bodies. These data show that given cellular genes can respond differently to distinct oncogenes.

Characterization of murine A-raf, a new oncogene related to the v-raf oncogene

A 1.6-kilobase cDNA (A-raf) has been isolated from a murine spleen cDNA library which encodes part of a protein related to the raf oncogene. Its amino acid sequence has 85% homology to raf in a central portion of 100 amino acids. In contrast to raf, A-raf shows a highly restricted tissue distribution of expression, with highest levels observed in epididymis, followed by intestine. When incorporated into a retrovirus, the resulting gag-A-raf fusion gene causes transformation in vitro and induces tumors in newborn mice. Thus, A-raf represents a new proto-oncogene. Transformation by A-raf is independent of ras gene function, as is the case for raf and mos but not other oncogenes.

Embryonal central neuroepithelial tumors: current concepts and future challenges

While the embryonal central neuroepithelial tumors present complex conceptual and clinical problems, advances in cell type identification by special neurohistological, immunohisto- and immunocytochemical techniques have permitted discrimination of distinct cytomorphogenetic entities. These are based in part on their resemblance to the normal phases of neurocytogenesis. Four of these tumors, medulloepithelioma, desmoplastic infantile ganglioglioma, pineoblastoma and medulloblastoma, are designated as multipotential in light of their capacity to undergo divergent differentiation. Cytomorphogenetic, clinical and experimental data implicate fetal neural cell targets for transformation and raise the possibility that aberrant developmental regulatory mechanisms may contribute to the biologic behavior of these tumors. Growth factors and some neuroregulatory neurotransmitters (such as serotonin) are known to act as modulators of normal neuromorphogenesis. They could play a regulatory role in central neuroepithelial tumors on the hypothesis that the aberrant behavior of the embryonal neoplasms could either be modified by functional receptor responses or result from abnormal receptor responses to these substances. Future challenges include the definition of new cytomorphogenetic entities and subgroups of the currently defined forms of embryonal CNS tumors based on the presence of specific growth factors and neuroregulatory neurotransmitters, or their receptors, the characterization of neoplastic receptor responses mediating any modulatory role of the presently known growth factors or neuroregulatory neurotransmitters on the growth and maturation potential of the embryonal central neuroepithelial tumors and the further definition of developmental, stage-specific modulators that might be operative in these tumors.