Transformation of chicken embryo retinal melanoblasts by a temperature-sensitive mutant of Rous sarcoma virus

The rise and fall of central dogmas

Stimulated by Francis Cricks "central dogma of molecular biology" open to criticism and rebuttal, five statements are formulated on current notions of tumor biology, inviting the criticism of the reader.

Cellular processes of v-Src transformation revealed by gene profiling of primary cells--implications for human cancer

Background

Cell transformation by the Src tyrosine kinase is characterized by extensive changes in gene expression. In this study, we took advantage of several strains of the Rous sarcoma virus (RSV) to characterize the patterns of v-Src-dependent gene expression in two different primary cell types, namely chicken embryo fibroblasts (CEF) and chicken neuroretinal (CNR) cells. We identified a common set of v-Src regulated genes and assessed if their expression is associated with disease-free survival using several independent human tumor data sets.

Methods

CEF and CNR cells were infected with transforming, non-transforming, and temperature sensitive mutants of RSV to identify the patterns of gene expression in response to v-Src-transformation. Microarray analysis was used to measure changes in gene expression and to define a common set of v-Src regulated genes (CSR genes) in CEF and CNR cells. A clustering enrichment regime using the CSR genes and two independent breast tumor data-sets was used to identify a 42-gene aggressive tumor gene signature. The aggressive gene signature was tested for its prognostic value by conducting survival analyses on six additional tumor data sets.

Results

The analysis of CEF and CNR cells revealed that cell transformation by v-Src alters the expression of 6% of the protein coding genes of the genome. A common set of 175 v-Src regulated genes (CSR genes) was regulated in both CEF and CNR cells. Within the CSR gene set, a group of 42 v-Src inducible genes was associated with reduced disease- and metastasis-free survival in several independent patient cohorts with breast or lung cancer. Gene classes represented within this group include DNA replication, cell cycle, the DNA damage and stress responses, and blood vessel morphogenesis.

Conclusion

By studying the v-Src-dependent changes in gene expression in two types of primary cells, we identified a set of 42 inducible genes associated with poor prognosis in breast and lung cancer. The identification of these genes provides a set of biomarkers of aggressive tumor behavior and a framework for the study of cancer cells characterized by elevated Src kinase activity.

Developmental control of transcription of a retina-specific gene, QR1, during differentiation: involvement of factors from the POU family

Developmental control of gene expression often results from the coupling of growth arrest with the establishment of differentiation programs. QR1 is a gene specifically expressed in retinas during the late phase of embryogenesis. At this stage neuroectodermal precursors have reached terminal mitosis and are undergoing differentiation into distinct cell types. Transcription of the QR1 gene is tightly regulated during retinal development: this gene is expressed between embryonic day 9 (ED9) and ED17 and is completely repressed at hatching in quail. Moreover, QR1 transcription is downregulated when postmitotic neural retina cells are induced to proliferate by pp60v-src. We studied the stage-dependent transcriptional control of this gene during quail neural retina (QNR) cell development. Transient transfection experiments with QR1/CAT constructs at various stages of development showed that a region located between -935 and -1265 bp upstream of the transcription start site is necessary to promote transcription in retina cells during the late phase of embryonal development (QNR9, corresponding to ED9). By in vivo footprinting assays we identified at least two elements that are occupied by DNA-protein complexes in QNR cells: the A and B boxes. The A box allows formation of several biochemically distinct complexes: C1, C2, C3, and C4. Formation of the C2 complex mainly during early stages (ED7) and of C2, C3, and C4 complexes during postnatal life correlates with repression of QR1 transcription, whereas the C1 complex is strongly induced at ED11 when the QR1 gene is expressed. We previously showed that C1 was involved in downregulation of QR1 transcription by pp60v-src. Several complexes are also formed on the B box. We show that these complexes are exclusively present in neural tissues and that they involve members of the POU family of transcription factors. Mutations of each one of the two regions which abolish the binding of the C1 factor(s) on the A box and of the POU factor(s) on the B box also prevent stimulation of QR1 transcription in QNR9. Therefore, both elements appear to be required for the stage-specific transcription of the QR1 gene. We also show that the regulatory region from position -1265 to position -935 is able to confer stage-specific transcription upon a heterologous promoter (thymidine kinase). Indeed, this region stimulates transcription in differentiating retinas (QNR9) and represses transcription in terminally differentiated retinas (QNR17, corresponding to postnatal life). Our results suggest that cell growth regulation and developmental control are coordinated through the A and B boxes in regulating QR1 transcription during retinal differentiation.

A novel cDNA corresponding to transcripts expressed in retina post-mitotic neurons

The long term objective of this study is to isolate genes specifically expressed at the onset of neuronal cell cycle withdrawal. As an experimental paradigm we have used a quail neuroretinal cell clone (clone K2) immortalized by a thermosensitive mutant of Rous Sarcoma Virus. K2 cells proliferate at 36 degrees C but stop synthesizing DNA after a shift to 41.5 degrees C. We have constructed a cDNA library from K2 cells transferred to 41.5 degrees C and autosubtracted with RNAs from K2 cells maintained at 36 degrees C. This strategy has led to the isolation of cDNAs which recognize mRNAs expressed in quail neuroretina (NR) during development. We report here one of these cDNAs, cDNA QN1, that hybridizes with transcripts expressed in retina neurons, in parallel with their withdrawal from the cell cycle. QN1 ORF codes for a 138 kDa polypeptide corresponding to the protein observed in Western blot analysis. A role of QN1 product(s) on neuronal quiescence is suggested by the positive effect of an antisense oligonucleotide on DNA synthesis of K2 cells.

Transcriptional downregulation of the retina-specific QR1 gene by pp60v-src and identification of a novel v-src-responsive unit

The embryonic avian neuroretina (NR) is part of the central nervous system and is composed of various cell types: photoreceptors and neuronal and Müller (glial) cells. These cells are derived from proliferating neuroectodermal precursors which differentiate after terminal mitosis and become organized in cell strata. Proliferation of differentiating NR cells can be induced by infection with Rous sarcoma virus (RSV) and requires the expression of a functional v-src gene. To understand the mechanisms involved in the regulation of neural cell growth and differentiation, we studied the transcriptional regulation of QR1, a gene specifically expressed in postmitotic NR cells. Transcription of this gene is detected primarily in Müller cells and is strongly downregulated by the v-src gene product. Moreover, QR1 expression takes place only during the late phase of retinal development and is shut off abruptly at hatching. We have isolated a promoter region(s) of the QR1 gene that confers v-src responsiveness. By transfection of QR1-CAT constructs into quail NR cells infected with the temperature-sensitive mutant of RSV, PA101, we have identified a v-src-responsive region located between -1208 and -1161 upstream of the transcription initiation site. This sequence is able to form two DNA-protein complexes, C1 and C2. Formation of complex C2 is specifically induced in cells expressing an active v-src product, while formation of C1 is detected mainly in nonproliferating quail NR cells upon pp60v-src inactivation. C1 is also a target for regulation during development. We have identified the DNA binding site for the C1 complex, a repeated GCTGAC sequence, and shown that mutations in this element abolish binding of this factor as well as transcription of the gene at the nonpermissive temperature. Neither formation of C1 nor that of C2 seems to involve factors known to be targeted in the pp60v-src cascade. Our data suggest that C1 could be a novel target for both developmental control and oncogene-induced cell growth regulation.

Transcriptional downregulation of the retina-specific QR1 gene by pp60v-src and identification of a novel v-src-responsive unit

The embryonic avian neuroretina (NR) is part of the central nervous system and is composed of various cell types: photoreceptors and neuronal and Müller (glial) cells. These cells are derived from proliferating neuroectodermal precursors which differentiate after terminal mitosis and become organized in cell strata. Proliferation of differentiating NR cells can be induced by infection with Rous sarcoma virus (RSV) and requires the expression of a functional v-src gene. To understand the mechanisms involved in the regulation of neural cell growth and differentiation, we studied the transcriptional regulation of QR1, a gene specifically expressed in postmitotic NR cells. Transcription of this gene is detected primarily in Müller cells and is strongly downregulated by the v-src gene product. Moreover, QR1 expression takes place only during the late phase of retinal development and is shut off abruptly at hatching. We have isolated a promoter region(s) of the QR1 gene that confers v-src responsiveness. By transfection of QR1-CAT constructs into quail NR cells infected with the temperature-sensitive mutant of RSV, PA101, we have identified a v-src-responsive region located between -1208 and -1161 upstream of the transcription initiation site. This sequence is able to form two DNA-protein complexes, C1 and C2. Formation of complex C2 is specifically induced in cells expressing an active v-src product, while formation of C1 is detected mainly in nonproliferating quail NR cells upon pp60v-src inactivation. C1 is also a target for regulation during development. We have identified the DNA binding site for the C1 complex, a repeated GCTGAC sequence, and shown that mutations in this element abolish binding of this factor as well as transcription of the gene at the nonpermissive temperature. Neither formation of C1 nor that of C2 seems to involve factors known to be targeted in the pp60v-src cascade. Our data suggest that C1 could be a novel target for both developmental control and oncogene-induced cell growth regulation.

Immortalization of polarized rat retinal pigment epithelium

Rat retinal pigment epithelial (RPE) cells were immortalized by infection with a temperature-sensitive tsA SV40 virus and following cloning and selection for epithelial properties the polarized RPE-J cell line was obtained. At the permissive temperature of 33 degrees C, RPE-J cells behave as an immortalized cell line. When RPE-J cells are grown on nitrocellulose filters coated with a thin layer of Matrigel in the presence of 10(−8) M retinoic acid for 6 days at 33 degrees C and then switched for 33–36 hours to the non-permissive temperature of 40 degrees C, they acquire a differentiated polarized RPE phenotype. Under these growth conditions, RPE-J cells exhibit circumferential staining for the tight-junction protein ZO-1 and acquire a transepithelial resistance of 350 ohms cm2. Morphologically, RPE-J cells exhibit a characteristic RPE morphology with extensive apical microvilli as well as numerous dense bodies including premelanosomes and varied multilamellar structures. Ruthenium red labeling revealed the frequent basal localization of the tight junction. The cells were identified to be of rat RPE origin by their expression of the rat RPE marker RET-PE2 and their ability to phagocytose latex beads. While RPE-J cells are capable of sorting influenza and vesicular stomatitis virus to the apical and basal surfaces, respectively, the Na,K-ATPase is not polarized and the neural cell adhesion molecule, N-CAM, is localized exclusively to the lateral surface. In vivo the apical surface of RPE interacts with the adjacent neural retina and the Na,K-ATPase and N-CAM are both apical; the altered polarity of these two proteins in RPE-J cells may be a consequence of the absence of apical interaction with the neural retina in culture. Previous studies of RPE have been restricted to the use of primary cultures and the RPE-J cell line should prove an excellent model system for the study of the mechanisms determining the characteristic polarity and functions of the retinal pigment epithelium.

Expression of v-src in embryonic neural retina alters cell adhesion, inhibits histogenesis, and prevents induction of glutamine synthetase

Using Rous sarcoma virus as the vector, v-src or c-src genes were introduced into 6-day chicken embryo retina tissue in organ culture and their effects on retina development were investigated. Overexpression of c-src in many of the cells had no noticeable effect on retina development. In contrast, infection with v-src resulted in abnormal histogenesis and inhibition of differentiation. Although only a portion of the cells in infected tissue expressed the oncogene and displayed the transformation phenotype, the other cells were also hindered from becoming normally positioned and organized. Therefore, presence of oncogene-transformed cells within the tissue hindered organization and development of adjacent nontransformed cells. Failure of normal cell relationships impeded induction by cortisol of glutamine synthetase in Muller glia, which requires contact associations of the glia cells with neurons. The transformed cells tended to assemble into chaotic clusters, suggesting that their adhesiveness and contact affinities had become altered. This was confirmed by aggregation experiments with dissociated cells which showed that adhesiveness of transformed cells was greatly reduced and that they had lost the ability to cohere with nontransformed cells. In binary mixtures of transformed and nontransformed cells, the two sorted out into separate aggregates. Transformed cells formed loose clusters devoid of tissue architecture; aggregates of nontransformed cells became organized into retinotypic structures, and glutamine synthetase was inducible. Our findings suggest that the mechanisms of cell adhesion and cell affinities are a key target of v-src activity in infected cells and that modification of the cell surface may be a leading factor in other cellular changes characteristic of the v-src transformation phenotype.

Expression of v-src in embryonic neural retina alters cell adhesion, inhibits histogenesis, and prevents induction of glutamine synthetase

Using Rous sarcoma virus as the vector, v-src or c-src genes were introduced into 6-day chicken embryo retina tissue in organ culture and their effects on retina development were investigated. Overexpression of c-src in many of the cells had no noticeable effect on retina development. In contrast, infection with v-src resulted in abnormal histogenesis and inhibition of differentiation. Although only a portion of the cells in infected tissue expressed the oncogene and displayed the transformation phenotype, the other cells were also hindered from becoming normally positioned and organized. Therefore, presence of oncogene-transformed cells within the tissue hindered organization and development of adjacent nontransformed cells. Failure of normal cell relationships impeded induction by cortisol of glutamine synthetase in Muller glia, which requires contact associations of the glia cells with neurons. The transformed cells tended to assemble into chaotic clusters, suggesting that their adhesiveness and contact affinities had become altered. This was confirmed by aggregation experiments with dissociated cells which showed that adhesiveness of transformed cells was greatly reduced and that they had lost the ability to cohere with nontransformed cells. In binary mixtures of transformed and nontransformed cells, the two sorted out into separate aggregates. Transformed cells formed loose clusters devoid of tissue architecture; aggregates of nontransformed cells became organized into retinotypic structures, and glutamine synthetase was inducible. Our findings suggest that the mechanisms of cell adhesion and cell affinities are a key target of v-src activity in infected cells and that modification of the cell surface may be a leading factor in other cellular changes characteristic of the v-src transformation phenotype.

Transcription of muscle-specific genes is repressed by reactivation of pp60v-src in postmitotic quail myotubes

Quail myogenic cells infected with temperature sensitive (ts) mutants of Rous sarcoma virus (RSV) exhibit a temperature-dependent transformation and block of differentiation. When the cells are allowed to differentiate at the restrictive temperature (41 degrees C) and then shifted back to the permissive temperature (35 degrees C), a sharp reduction in the accumulation of muscle-specific mRNAs is observed, following reactivation of the transforming protein pp60v-src. A kinetic analysis of this down-regulation reveals that the reduction in the accumulation of muscle-specific transcripts occurs fairly rapidly within 6 to 20 h after the shift back, depending on the mRNA analyzed. Studies on transcription of endogenous muscle-specific genes and a transfected chloramphenicol acetyltransferase reporter gene under the control of muscle-specific promoters, at the different temperatures, suggest that the oncogene exerts its control mainly at the transcriptional level. On the contrary, transcription of the CMD1 gene, the avian homolog of the mouse muscle regulatory MyoD gene, is not significantly affected by the oncogene both in proliferating myoblasts and in myotubes shifted back to 35 degrees C. These findings are consistent with the conclusion that v-src blocks myogenesis by controlling transcription of muscle-specific genes independently of cell proliferation. Furthermore, they suggest the existence of an alternative pathway, not requiring the silencing of CMD1 transcription, through which the oncogene exerts its effect.

Transcription of muscle-specific genes is repressed by reactivation of pp60v-src in postmitotic quail myotubes

Quail myogenic cells infected with temperature sensitive (ts) mutants of Rous sarcoma virus (RSV) exhibit a temperature-dependent transformation and block of differentiation. When the cells are allowed to differentiate at the restrictive temperature (41 degrees C) and then shifted back to the permissive temperature (35 degrees C), a sharp reduction in the accumulation of muscle-specific mRNAs is observed, following reactivation of the transforming protein pp60v-src. A kinetic analysis of this down-regulation reveals that the reduction in the accumulation of muscle-specific transcripts occurs fairly rapidly within 6 to 20 h after the shift back, depending on the mRNA analyzed. Studies on transcription of endogenous muscle-specific genes and a transfected chloramphenicol acetyltransferase reporter gene under the control of muscle-specific promoters, at the different temperatures, suggest that the oncogene exerts its control mainly at the transcriptional level. On the contrary, transcription of the CMD1 gene, the avian homolog of the mouse muscle regulatory MyoD gene, is not significantly affected by the oncogene both in proliferating myoblasts and in myotubes shifted back to 35 degrees C. These findings are consistent with the conclusion that v-src blocks myogenesis by controlling transcription of muscle-specific genes independently of cell proliferation. Furthermore, they suggest the existence of an alternative pathway, not requiring the silencing of CMD1 transcription, through which the oncogene exerts its effect.

Histocompatible miniature, boar model: selection of transformed cell lines of B and T lineages producing retrovirus

A lymphoblastoid cell line (B1) was isolated in culture following a brief exposure to 5-azacytidine from peripheral-blood mononuclear cells of a boar previously injected with cells (Shimozuma) producing porcine retrovirus (Tsukuba-1) and suffering a severe non-neoplastic syndrome at autopsy. B1 cell line and 5 of its sublines were propagated for more than 100 generations, retaining doubling times comprised between 16.8 and 27.5 hr and growing readily in agarose or agar (plating efficiency: 5 to 50%). Karyotype analyses showed that 4 sublines were nearly diploid, except for cells of L14, which displayed a monosomy affecting chromosome 18 pair. Two sublines (L35 and L45) were considered as being of T-cell lineage, since MSA, antigen was observed on the surface of approximately 30% of cells. Three sublines (L23, L14 and L52) were considered of B-cell lineage, since membrane immunoglobulins were observed on the cell surface. In addition, sublines L23 and L52 were actively secreting immunoglobulin of mu isotype. Retrovirus particles were evidenced in gradient-purified preparation of 200-fold-concentrated cell culture supernatants of the B1 cell line, L14, L35 and L52 sublines, using both a reverse transcriptase activity assay and electron microscopic observation. These cell lines can be used to select for porcine retrovirus variants with transforming potential for lymphocytes of B and T lineages.

The current state of oncogenes and cancer: experimental approaches for analyzing oncogenetic events in human cancer

The development of cancer is a multistage process. The activation of proto-oncogenes and the inactivation of tumor suppressor genes play a critical role in the induction of tumors. Using human cell model systems of carcinogenesis, we have studied how oncogenes, tumor suppressor genes, and recessive cancer susceptibility genes participate in this multistep process. Normal human cells are resistant to the transforming potential of oncogenes, such as ras oncogenes, which are activated by specific point mutations. Since as many as 40% of some tumor types contain activated ras oncogenes, a preneoplastic transition in multistage carcinogenesis must involve changing from an oncogene-resistant stage to an oncogene-susceptible stage. The analysis of such critical steps in carcinogenesis using rodent systems has usually not represented the human disease with fidelity. In order to study this carcinogenic process, we have developed human cell, in vitro systems that represent some of the genetic changes that occur in cellular genes during human carcinogenesis. Using these systems, we have learned some of the functions of dominant activated-transforming oncogenes, tumor suppressor genes, and cellular immortalization genes and how they influence the carcinogenic process in human cells. Using our understanding of these processes, we are attempting to clone critical genes involved in the etiology of familial cancers. These investigations may help us to develop procedures that allow us to predict, in these cancer families, which individuals are at high risk for developing cancer.

Distinct and different effects of the oncogenes v-myc and v-src on avian sympathetic neurons: retroviral transfer of v-myc stimulates neuronal proliferation whereas v-src transfer enhances neuronal differentiation

Immature avian sympathetic neurons are able to proliferate in culture for a limited number of divisions albeit expressing several neuron-specific properties. The effect of avian retroviral transfer of oncogenes on proliferation and differentiation of sympathetic neurons was investigated. Primary cultures of 6-d-old quail sympathetic ganglia, consisting of 90% neuronal cells, were infected by Myelocytomatosis virus (MC29), which contains the oncogene v-myc, and by the v-src-containing Rous sarcoma virus (RSV). RSV infection, in contrast to findings in other cellular systems, resulted in a reduction of neuronal proliferation as determined by 3H-thymidine incorporation (50% of control 4 d after infection) and in increased morphological differentiation. This is reflected by increased neurite production, cell size, and expression of neurofilament protein. In addition, RSV-infected neurons, unlike uninfected cells, are able to survive in culture for time periods up to 14 d in the absence of added neurotrophic factors. In contrast, retroviral transfer of v-myc stimulated the proliferation of immature sympathetic neurons preserving many properties of uninfected cells. The neuron-specific cell surface antigen Q211 and the adrenergic marker enzyme tyrosine hydroxylase were maintained in MC29-infected cells and in the presence of chick embryo extract the cells could be propagated over several weeks and five passages. Within 7 d after infection, the number of Q211-positive neurons increased approximately 100-fold. These data demonstrate distinct and different effects of v-src and v-myc-containing retroviruses on proliferation and differentiation of sympathetic neurons: v-src transfer results in increased differentiation, whereas v-myc transfer maintains an immature status reflected by proliferation, immature morphology, and complex growth requirements. The possibility of expanding immature neuronal populations by transfer of v-myc will be of considerable importance for the molecular analysis of neuronal proliferation and differentiation.

Transformation of murine melanocytes by basic fibroblast growth factor cDNA and oncogenes and selective suppression of the transformed phenotype in a reconstituted cutaneous environment

Constitutive expression of basic fibroblast growth factor (bFGF), a common characteristic of metastatic melanomas, was reproduced in vitro by infection of normal murine melanocytes with a recombinant retrovirus carrying a cDNA for bFGF. Expression of bFGF in these cells conferred autonomous growth in culture and extinguished differentiated functions, such as the synthesis of melanin and formation of dendrites. Independence from exogenous bFGF and loss of differentiated functions in vitro were induced also by transformation of melanocytes with the oncogenes myc, Ela, ras, and neu, although bFGF was not expressed by the respective transformants. As shown in skin reconstitution experiments onto syngeneic mice and subcutaneous injections into nude mice, the various transformants differed in their behavior in vivo. The bFGF transformants did not form tumors. They reverted to having a normal, melanotic phenotype and restricted growth. Myc and Ela transformants grew as tumors in nude mice but not in syngeneic, immunocompetent animals. Ras-transformed melanocytes were always tumorigenic, whereas the formation of tumors by neu transformants was suppressed by the concomitant grafting of keratinocytes in reconstituted skin of syngeneic mice. These data show that melanocytes genetically manipulated to produce bFGF acquire properties in vitro similar to those of metastatic melanoma cells or those induced by various oncogenes but that constitutive production of bFGF by itself is insufficient to make melanocytes tumorigenic. The experiments also show that melanocytes transformed by the selected oncogenes respond differentially to various environments in vivo.

Changes in c-onc expression during embryonal carcinoma cell differentiation

Protooncogenes expressed in murine embryonal carcinoma (EC) cells or their differentiated daughter cells include more or less ubiquitously expressed protooncogenes such as c-myc, c-K-ras, and c-abl, as well as c-onc genes with a very restricted expression pattern. Examples of the latter are N-myc, c-mos, and int-2. These c-onc genes are transcriptionally active in EC cells, as well as in germ cells and/or early embryonic cells. When EC cells are induced to differentiate some protooncogenes or oncogene-related products undergo changes in expression. Thus, EC cell differentiation has been associated with increased expression of c-src, c-fos, int-1, int-2, and the epidermal growth factor (EGF) receptor, whereas decreased expression has been observed for c-mos, c-K-ras, c-myc, N-myc, and platelet-derived growth factor. The relationships between these changes in expression and EC cell differentiation are not understood. They may be important for the differentiation process or for expression of a differentiated phenotype. They may, however, also be secondary events with no functional significance to EC cell differentiation.

c-src and other proto-oncogenes implicated in neuronal differentiation

The proto-oncogene c-src has been implicated in the development and mature function of the nervous system. pp60c-src, the protein product of the c-src gene, is a tyrosine protein kinase that is highly enriched in fetal neural tissue. pp60c-src appears in two phases of neuronal development. Neuroectodermal cells of gastrulating embryos first express pp60c-src around the time of commitment to neuronal or glial pathways. Later, committed neuroepithelial cells express pp60c-src near the onset of terminal neuronal differentiation. Immunocytochemical analyses of pp60c-src in developing chick retina, telencephalon, and cerebellum show immunoreactivity concentrated in regions rich in growth cones and neurites. Moreover, pp60c-src is concentrated approximately 10-fold in a biochemical fraction from fetal rat brain that is enriched in nerve growth cone membranes. These results point toward a function for pp60c-src in neurite outgrowth. A functional role for other proto-oncogenes in the development of the nervous system was indicated from a study of the expression of a battery of proto-oncogenes during the retinoic acid-induced differentiation of the mouse embryonal carcinoma cell line P19 to a neuronal phenotype. Nuclear runoff transcription of the proto-oncogenes c-src, c-fms, c-sis, N-ras, c-myc, and c-fos was observed in proliferating and retinoic acid-treated cells.

Establishment and characterization of osteogenic cell lines from a spontaneous murine osteosarcoma

Five clonal cell lines were established from a spontaneous BALB/c mouse osteosarcoma, and characterized. Four of these lines showed some similarities in morphology, in vitro growth properties, production of collagenous and noncollagenous extracellular matrix proteins and osteogenic differentiation. The cells formed colonies with characteristic differences in size and morphology in soft agar, and osteogenic sarcomas and metastases in syngeneic mice after transplantation. Ultrastructurally, cells in the transplant tumours showed marked osteogenic features. There were no osteoclast-like cells. The fifth cell line had somewhat different characteristics. All five lines expressed infectious endogenous murine leukemia viruses. Increased c-myc protoon-cogene expression was found in one cell line and c-fos expression at different levels in all lines. There was only very low expression of c-Ha-ras and no expression of c-Ki-ras and c-sis. DNA analysis showed the presence of newly acquired proviral genomes integrated at different sites in the cellular DNA. The results show that distinct osteogenic neoplastic subclones can be obtained from a primary mouse osteosarcoma. Although the clones exhibited an appreciable morphological, functional, and molecular diversity they retained the basic pathogenic properties of the tumour from which they were derived.

Cooperation between the polyomavirus middle-T-antigen gene and the human c-myc oncogene in a rat thyroid epithelial differentiated cell line: model of in vitro progression

Two rat thyroid epithelial differentiated cell lines, PC Cl 3 and PC myc, were infected with the polyoma murine leukemia virus (PyMLV) carrying the Middle-T-antigen gene of polyomavirus. After infection, both cell lines acquired the typical markers of neoplastic transformation; however, the PC myc cells showed a greater malignant phenotype. Furthermore, the thyroid differentiated functions were completely suppressed in PC myc cells transformed by PyMLV, whereas they were, at least partially, retained in PC Cl 3 cells transformed by PyMLV, and in particular, thyroglobulin synthesis and secretion were not affected at all. Since no differences in the expression of the middle-T-antigen gene were observed in the two PyMLV-transformed cell lines, the different properties shown by these two infected cell lines must be ascribed to the expression of the c-myc oncogene.

Inhibition of chicken embryo lens differentiation and lens junction formation in culture by pp60v-src

A culture system was developed which permitted the differentiation of chicken lens epithelial cells to lentoid bodies which contained several cell layers, accumulated high levels of delta-crystallin, and produced extensive gap junctions. This differentiation process was prevented when the cells were infected with a temperature-sensitive src mutant of Rous sarcoma virus and maintained at the permissive temperature. These transformed cells continued to proliferate and also synthesized the major lens gap junction protein, MP28, at near-normal rates. However, this MP28 was not assembled to produce gap junctions. Cultures shifted to the nonpermissive temperature formed lentoid bodies similar to those in uninfected lens cultures, including the establishment of gap junctions containing MP28.

Cooperation between the polyomavirus middle-T-antigen gene and the human c-myc oncogene in a rat thyroid epithelial differentiated cell line: model of in vitro progression

Two rat thyroid epithelial differentiated cell lines, PC Cl 3 and PC myc, were infected with the polyoma murine leukemia virus (PyMLV) carrying the Middle-T-antigen gene of polyomavirus. After infection, both cell lines acquired the typical markers of neoplastic transformation; however, the PC myc cells showed a greater malignant phenotype. Furthermore, the thyroid differentiated functions were completely suppressed in PC myc cells transformed by PyMLV, whereas they were, at least partially, retained in PC Cl 3 cells transformed by PyMLV, and in particular, thyroglobulin synthesis and secretion were not affected at all. Since no differences in the expression of the middle-T-antigen gene were observed in the two PyMLV-transformed cell lines, the different properties shown by these two infected cell lines must be ascribed to the expression of the c-myc oncogene.

Inhibition of chicken embryo lens differentiation and lens junction formation in culture by pp60v-src

A culture system was developed which permitted the differentiation of chicken lens epithelial cells to lentoid bodies which contained several cell layers, accumulated high levels of delta-crystallin, and produced extensive gap junctions. This differentiation process was prevented when the cells were infected with a temperature-sensitive src mutant of Rous sarcoma virus and maintained at the permissive temperature. These transformed cells continued to proliferate and also synthesized the major lens gap junction protein, MP28, at near-normal rates. However, this MP28 was not assembled to produce gap junctions. Cultures shifted to the nonpermissive temperature formed lentoid bodies similar to those in uninfected lens cultures, including the establishment of gap junctions containing MP28.

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.

The approaching era of the tumor suppressor genes

Genes that can inhibit the expression of the tumorigenic phenotype have been detected by the fusion of normal and malignant cells, the phenotypic reversion of in vitro transformants, the induction of terminal differentiation of malignant cell lineages, the loss of "recessive cancer genes," the discovery of regulatory sequences in the immediate vicinity of certain oncogenes, and the inhibition of tumor growth by normal cell products. Such tumor suppressor genes will probably turn out to be as, if not more, diversified as the oncogenes. Consideration of both kinds of genes may reveal common or interrelated functional properties.

Reversibility of differentiation and proliferative capacity in avian myelomonocytic cells transformed by tsE26 leukemia virus

Chicken hematopoietic cells infected with E26 leukemia virus can be transformed into growth factor-dependent, rapidly proliferating cells that exhibit properties of immature myelomonocytic cells. Cells infected with a mutant of E26 that carries a temperature-sensitive lesion, presumably residing in the myb oncogene, differentiate into resting, macrophage-like cells when shifted from 37 degrees to 42 degrees C (Beug et al. 1984). Here we show that differentiated tsE26 cells gradually reacquire an immature phenotype and proliferative capacity when shifted back to 37 degrees C, provided that they are kept at 42 degrees C no longer than 4-8 days. We also show that DNA synthesis inhibitors do not prevent terminal differentiation at 42 degrees C but inhibit the complete reexpression of the immature phenotype in downshift experiments. Our results suggest that the reactivation of the E26 protein function can both induce a "retro-differentiation" and cell proliferation in myelomonocytic target 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.

Crystallin gene expression and lentoid body formation in quail embryo neuroretina cultures transformed by the oncogenic retrovirus Mill Hill 2 or Rous sarcoma virus

The lens-specific proteins alpha and delta crystallins and lentoid bodies, structures that follow a differentiation pathway similar to that of the lens, regularly appear after 4 to 5 weeks in quail embryo neuroretina monolayer cultures. We have investigated the effects of the avian oncogenic retroviruses Mill Hill 2 and Rous sarcoma virus on this process. Quail embryo neuroretina cells transformed by Mill Hill 2 virus were established into permanent cultures that synthesized alpha and delta crystallins and contained stem cells for the production of lentoid bodies. In contrast, transformation with the Rous sarcoma virus mutant tsNY-68 blocked the appearance of mRNA crystallins, but cytoplasmic alpha and delta crystallin mRNA and alpha crystallin appeared 44 h after a shift to the nonpermissive temperature. However, delta crystallins and lentoid bodies were only present after 7 days. The crystallins of transformed quail neuroretina cultures were immunologically indistinguishable from those of quail lenses and of normal quail embryo neuroretina cultures.

Crystallin gene expression and lentoid body formation in quail embryo neuroretina cultures transformed by the oncogenic retrovirus Mill Hill 2 or Rous sarcoma virus

The lens-specific proteins alpha and delta crystallins and lentoid bodies, structures that follow a differentiation pathway similar to that of the lens, regularly appear after 4 to 5 weeks in quail embryo neuroretina monolayer cultures. We have investigated the effects of the avian oncogenic retroviruses Mill Hill 2 and Rous sarcoma virus on this process. Quail embryo neuroretina cells transformed by Mill Hill 2 virus were established into permanent cultures that synthesized alpha and delta crystallins and contained stem cells for the production of lentoid bodies. In contrast, transformation with the Rous sarcoma virus mutant tsNY-68 blocked the appearance of mRNA crystallins, but cytoplasmic alpha and delta crystallin mRNA and alpha crystallin appeared 44 h after a shift to the nonpermissive temperature. However, delta crystallins and lentoid bodies were only present after 7 days. The crystallins of transformed quail neuroretina cultures were immunologically indistinguishable from those of quail lenses and of normal quail embryo neuroretina cultures.

Neurones express high levels of a structurally modified, activated form of pp60c-src

Neural tissues contain high levels of the cellular homologue of the transforming protein of Rous sarcoma virus (RSV), but neither the specific cell types expressing high levels of c-src, nor the function of the cellular src (c-src) protein has been determined. Using primary culture methods, we have found that pure neurones and astrocytes derived from the rat central nervous system (CNS) contain 15- to 20-times higher levels of the c-src protein than fibroblasts. However, the specific activity of the c-src protein from the neuronal cultures is 6- to 12-times higher than that from the astrocyte cultures. In addition, the c-src protein expressed in neuronal cultures contains a structural alteration within the amino-terminal region of the molecule that causes a shift in the mobility of the c-src protein on the SDS-polyacrylamide gels. These results indicate that a structurally distinct form of the cellular src protein that possesses an activated tyrosylkinase activity is expressed at very high levels in post-mitotic CNS neurones.

Control of types I and II collagen and fibronectin gene expression in chondrocytes delineated by viral transformation

We have analyzed the effects of transformation by Rous sarcoma virus on expression of types I and II collagen and fibronectin genes in vertebral chondrocytes and compared them with expression of these genes in skin fibroblasts. Transformed chondrocytes display a dramatically decreased amount of type II collagen RNA, which can account fully for the decreased synthetic rate of this protein. Paradoxically, these cells also display greatly increased amounts of type I collagen RNAs, which are translated efficiently in vitro, but not in the intact cells. We show here that the type I collagen RNAs in transformed chondrocytes are nearly indistinguishable from those found in skin fibroblasts, and that they clearly differ from the type I collagen RNAs found in normal chondrocytes. Transformed chondrocytes also display an increased amount of fibronectin RNAs, which can account fully for the increased synthetic rate of this protein. Thus, the effects of transformation by Rous sarcoma virus on type I collagen and fibronectin RNAs in chondrocytes are the opposite of those observed in fibroblasts, which display decreased amounts of these three RNAs. These data indicate that the effects of transformation on the genes encoding type I collagen and fibronectin must be modulated by host cell-specific factors. They also imply that the types I and II collagen genes may be regulated by different mechanisms, the type I genes being controlled at both transcriptional and posttranscriptional levels, and the type II gene being controlled primarily at the transcriptional level.

Embryonic erythroid cells transformed by avian erythroblastosis virus may proliferate and differentiate

Embryonic chick cells from the primitive streak stage to later stages of the developing embryo were infected with avian erythroblastosis virus (AEV). The data indicate that the greatest number of target cells for AEV was observed in the 12-somite blastoderm and gradually decreased in hemopoietic tissues with the development of the embryo. The target cell for AEV is not in the BFU-E compartment, as it is in the adult bone marrow, but is probably recruited within the CFU-M compartment which precedes the BFU-E compartment. Our studies also show that a significant number of transformed colonies derived from embryonic hemopoietic tissues undergo hemoglobinization in contrast with what is observed in transformed colonies of bone marrow. A complete characterization of the embryonic and adult hemoglobin is at present under study.

Control of types I and II collagen and fibronectin gene expression in chondrocytes delineated by viral transformation

We have analyzed the effects of transformation by Rous sarcoma virus on expression of types I and II collagen and fibronectin genes in vertebral chondrocytes and compared them with expression of these genes in skin fibroblasts. Transformed chondrocytes display a dramatically decreased amount of type II collagen RNA, which can account fully for the decreased synthetic rate of this protein. Paradoxically, these cells also display greatly increased amounts of type I collagen RNAs, which are translated efficiently in vitro, but not in the intact cells. We show here that the type I collagen RNAs in transformed chondrocytes are nearly indistinguishable from those found in skin fibroblasts, and that they clearly differ from the type I collagen RNAs found in normal chondrocytes. Transformed chondrocytes also display an increased amount of fibronectin RNAs, which can account fully for the increased synthetic rate of this protein. Thus, the effects of transformation by Rous sarcoma virus on type I collagen and fibronectin RNAs in chondrocytes are the opposite of those observed in fibroblasts, which display decreased amounts of these three RNAs. These data indicate that the effects of transformation on the genes encoding type I collagen and fibronectin must be modulated by host cell-specific factors. They also imply that the types I and II collagen genes may be regulated by different mechanisms, the type I genes being controlled at both transcriptional and posttranscriptional levels, and the type II gene being controlled primarily at the transcriptional level.

Avian myelocytomatosis virus immortalizes differentiated quail chondrocytes

Quail embryo chondrocytes in culture display two morphological phenotypes: polygonal epithelial-like and floating cells. Both cell populations synthesize cartilage extracellular matrix proteins (type II collagen and specific proteoglycans), whereas type X collagen, which appears to be a marker of later stages of chondrocyte differentiation, is expressed only by the epithelial-like cells. Avian myelocytomatosis virus strain MC29 does not induce morphological transformation in quail embryo chondrocytes but stimulates these cells to proliferate with a progressively reduced doubling time. MC29-infected chondrocytes can be established in culture as a continuous cell line, whereas control (uninfected) cultures only survive a few months. Rapidly dividing MC29-infected chondrocytes still express type II collagen and cartilage proteoglycans but do not synthesize type X collagen.

Two divergent cellular src genes are expressed in Xenopus laevis

Genomic and cDNA clones of the X. laevis src gene have been isolated and characterized by hybridization and DNA sequence analyses. The haploid genome of X. laevis contains two src genes, which can be distinguished from one another by virtue of sequence divergence in the 3' untranslated regions. Both of the genes are functional as indicated by the fact that oocytes contain RNAs transcribed from each of the genes. The two genes each encode an RNA which is 3.3 kb in length, or twice the length required to encode the 60,000 dalton src protein (pp60). Sequence analysis of the cDNA clones revealed that nearly all of the non-coding sequence is located at the 3' end. The availability of sequence data from cDNA clones has also made it possible for the first time to identify with certainty the carboxyl terminal sequence of a cellular pp60 molecule.

Transformation of erythroid cells by Rous sarcoma virus (RSV)

RSV transforms several nonhematopoietic cell types and as reported here also has the capacity to transform hematopoietic cells of the erythroid lineage. In vitro, the three RSV isolates tested induced erythroblast-like colonies in infected bone marrow cells that were distinguishable by size and cell arrangement from those induced by avian erythroblastosis virus (AEV). Also in contrast to AEV-transformed erythroblast cultures, isolated cell colonies induced by RSV required complex growth conditions in liquid medium similar to the in vitro conditions necessary for erythroblasts transformed by the acute leukemia virus E26. Temperature-shift experiments using temperature-sensitive (ts) NY68 RSV revealed that when grown at the nonpermissive temperature (42 degrees), mutant-infected cells became benzidine positive and partially differentiated into erythrocytes. Wild-type (wt) RSV-transformed cells did not undergo similar changes. However, both wt RSV-, and to a greater extent, ts RSV-transformed cultures at the permissive temperature (37 degrees) did contain populations of spontaneously differentiating erythroid cells signifying that the transforming activity of the virus did not fully arrest erythroid maturation. In addition, the RSV-transformed cells did express tyrosine kinase activity. When injected intravenously into birds, RSV induced an erythroblastosis-like disease similar to AEV but also caused fibrosarcomas and leg paralysis. These results show that RSV can alter the pattern of erythroid differentiation in a manner similar to, but distinct from, AEV and indicate that the tyrosine-specific pp60src kinase is involved in erythroid cell transformation. Since the src and erb B proteins share a significant amino acid homology, these data suggest that both may also share a common functional homology.

Expression of the Rous sarcoma virus src gene in avian macrophages fails to elicit transformed cell phenotype

Infection of avian macrophages with Rous sarcoma virus does not induce any changes in the morphology, growth behavior, or expression of macrophage-specific proteins. The absence of cellular transformation does not result from a block in the synthesis of viral proteins, since infectious viruses are released from a majority of cells in the culture. In this report, we examine the synthesis, processing, and functional activity of pp60src in Rous sarcoma virus-infected macrophages to determine whether the absence of transformation is due to an alteration in the functional expression of pp60src. Although the absolute level of pp60src was reduced compared with fibroblasts, the protein exhibited the same phosphorylation pattern and subcellular distribution and was able to phosphorylate immunoglobulin in the immune complex-protein kinase assay. These results imply that the failure of Rous sarcoma virus to transform macrophage may be due to a restriction in the cellular response to a functional src protein, perhaps due to the absence of cellular products which are essential for mediating pp60src-induced transformation.

Distinctive effects of the viral oncogenes myc, erb, fps, and src on the differentiation program of quail myogenic cells

The relationship between susceptibility to transformation in vitro by different oncogenes and terminal differentiation was analyzed in embryonic quail myogenic cells. Infection with Rous sarcoma virus (RSV), Fujinami sarcoma virus (FSV), avian erythroblastosis virus (AEV), and the avian myelocytomatosis virus MC29 led to rapid and massive transformation. Transformed cells had distinctive morphological alterations, increased proliferation rates, and the ability to grow in agar suspension. Furthermore, homogeneously transformed cultures failed to fuse into multinucleated myotubes and to express muscle-specific genes. However, cloned populations of RSV-, FSV-, and AEV-transformed myogenic cells could, under appropriate culture conditions, partially differentiate into atypical "revertant" myotubes. In contrast, competence for terminal differentiation was completely and irreversibly suppressed on transformation by MC29. The specificity of action of a given oncogenic sequence on the inhibition of differentiation was further studied by using conditional and nonconditional transformation mutants. Myogenic cells infected with temperature-sensitive (ts) mutants of RSV and FSV exhibited a fully reversible block of differentiation after shift to restrictive temperature, while cells infected with ts34 AEV were not temperature sensitive for differentiation. Cultures infected with the partially transformation-defective mutant of MC29 td10H were morphologically transformed and acquired anchorage independence for proliferation but maintained a residual competence for terminal differentiation.

Phenotypic changes and loss of N-CAM-mediated adhesion in transformed embryonic chicken retinal cells

Transformation of 6-d-old embryonic chicken retinal cells by Rous sarcoma virus (RSV) was found to cause significant changes in several cellular properties including adhesiveness, motility, and state of differentiation. The alterations in cell adhesivity were analyzed by means of specific antibodies to the calcium-independent neural cell adhesion molecule, N-CAM. In the RSV-transformed cells the amount of N-CAM present at the cell surface was significantly decreased relative to normal cells, as assessed by immunofluorescent staining, specific immunoprecipitation, and immunoblotting experiments. This decrease was reflected in a marked reduction in N-CAM-mediated adhesiveness measured in vitro. A different, calcium-dependent, adhesive system also present on neurons was not detectably altered by RSV transformation and, in contrast with previous studies on normal neurons, this adhesive system was detected without treatment by proteases. In culture, the transformed cells formed fewer and less compact colonies than the normal retinal cells. Observation of the RSV-transformed retinal cells by time-lapse cinematography confirmed the reduction in adhesiveness and also revealed that the transformed cells were more highly motile than their normal counterparts. In addition, RSV transformation appeared to alter the differentiation of the cultured retinal cells. Immunofluorescent staining studies indicated that in contrast to mature neurons, transformed neural retinal cells expressed the 34,000-mol-wt tyrosine kinase substrate and reduced amounts of a neuron-specific ganglioside recognized by monoclonal antibody A2B5. These characteristics are shared by untransformed glial cells. In double immunofluorescent staining experiments, many cells expressed both N-CAM and pp60src shortly after viral infection, which implies that the N-CAM-positive neuroepithelial cells were transformed by RSV. In addition, a highly purified population of N-CAM-positive neural retinal cells, selected using a fluorescence-activated cell sorter, was rapidly and extensively transformed by RSV at rates comparable to those of the unfractionated population. These results established that the transformed cells were largely derived from RSV-infected neuroepithelial cells rather than from a small population of retinal glial cells present in the primary culture. The findings suggest reconsideration of the possible origin of tumors classified by morphological criteria as derived from glia and raise the possibility that the normal homologue of pp60src may play a role in the commitment of neuroepithelial cells to neuronal or glial differentiation pathways.

pp60c-src Kinase is in chick and human embryonic tissues

The normal cellular protein pp60c-src is a tyrosine-specific protein kinase that is homologous to the transforming protein of Rous sarcoma virus (RSV) but its function is unknown. The expression of pp60c-src in chick and human embryonic tissues was monitored by the immune complex protein kinase assay, Western transfer analysis, and immunocytochemical staining at the light microscope level. pp60c-src kinase was expressed in the head and trunk regions of the chick embryo at all stages of development examined; however, expression increased significantly during the major period of organogenesis (Hamburger and Hamilton stages 21 to 32). Western transfer analysis showed that the amount of pp60c-src protein increased in parallel with the increase in kinase activity. Highest levels of pp60c-src kinase were present in the neural tube, brain, and heart of the stage 32 chick embryo. Lower levels of activity were found in eye, limb bud, and liver. Immunocytochemical staining of the neural tube region and heart of the chick confirmed the results of biochemical analysis and showed immunoreactive pp60c-src distributed throughout the neural tube and heart. The distribution of pp60c-src kinase in human fetal tissues was similar to that in the chick embryo; elevated levels of pp60c-src kinase were present in cerebral cortex, spinal cord, and heart, but all other tissues examined expressed some pp60c-src kinase. The results of our studies suggest that pp60c-src plays a fundamental role in an aspect of cellular metabolism that is particularly important in electrogenic tissues.

Expression of the Rous sarcoma virus src gene in avian macrophages fails to elicit transformed cell phenotype

Infection of avian macrophages with Rous sarcoma virus does not induce any changes in the morphology, growth behavior, or expression of macrophage-specific proteins. The absence of cellular transformation does not result from a block in the synthesis of viral proteins, since infectious viruses are released from a majority of cells in the culture. In this report, we examine the synthesis, processing, and functional activity of pp60src in Rous sarcoma virus-infected macrophages to determine whether the absence of transformation is due to an alteration in the functional expression of pp60src. Although the absolute level of pp60src was reduced compared with fibroblasts, the protein exhibited the same phosphorylation pattern and subcellular distribution and was able to phosphorylate immunoglobulin in the immune complex-protein kinase assay. These results imply that the failure of Rous sarcoma virus to transform macrophage may be due to a restriction in the cellular response to a functional src protein, perhaps due to the absence of cellular products which are essential for mediating pp60src-induced transformation.

Differentiation and transformation of neural plate cells

The developmental potential of presumptive neural plate cells of prestreak chick embryos (stage 1) and neural plate cells from definitive streak chick embryos (stage 4) has been examined in cell culture using specific markers that identify the major cell types in the vertebrate central nervous system. The prestreak presumptive neural plate (PSPNP) cells, stage 1, assume an epithelial appearance in vitro and synthesize cellular fibronectin, but do not express markers for the neuronal, astrocytic, melanocytic, or oligodendrocytic lineages. Conversely, definitive-streak neural plate (DSNP) cells contain precursors which express cell-type-specific markers for terminally differentiated neurons, astrocytes, and melanocytes, and synthesize an extracellular matrix of cellular fibronectin. Differentiation of DSNP cells in vitro can be prevented by infection with a temperature-sensitive mutant of Rous sarcoma virus (RSV), tsNY68. Differentiation of DSNP cell transformants can be resumed by a temperature shift to the nonpermissive temperature. The morphological and biochemical changes associated with tsNY68 transformation are accompanied by alterations in pp60src kinase activity in the transformed cells.

Transformation by Rous sarcoma virus prevents acetylcholine receptor clustering on cultured chicken muscle fibers

Acetylcholine receptors aggregate in the membrane of cultured chicken myotubes; the process of receptor clustering can be stimulated by exogenous factors that we, among others, have begun to characterize. Chicken myoblasts transformed by temperature-sensitive mutants of Rous sarcoma virus, such as tsNY68, fuse to form multinucleated myotubes at 42 degrees C, the nonpermissive temperature for transformation. However, tsNY68-infected myotubes do not cluster acetylcholine receptors at 42 degrees C, even in the presence of active clustering agents. This phenomenon is not merely a result of viral infection, since myotubes infected with a transformation-deficient viral mutant, td107A, behave like noninfected myotubes with respect to receptor clustering; thus, the effects of tsNY68 on the clustering process must be mediated by the src gene product. These experiments may provide a method of identifying essential elements of acetylcholine receptor clusters.

Effect of src infection on long-term marrow cultures: increased self-renewal of hemopoietic progenitor cells without leukemia

Long-term marrow cultures prepared from mice have been infected with a molecular recombinant of Rous sarcoma virus and murine amphitropic leukemia virus. This resulted in introduction of the src gene into the cultured cells and expression of its protein kinase function. The infected cultures displayed an altered balance in the accumulation of cells in different compartments of granulocyte differentiation. There was a dramatic increase in the stem cell (CFU-S) compartment and the committed progenitor cell (GM-CFC) compartment and a decrease in mature granulocytes. The altered balance appears to be caused by intrinsic alterations in the CFU-S and GM-CFC themselves, which increase their "self-renewal" capacity at the expense of cell differentiation. Remarkably, unlike its effects in other systems, src did not produce a neoplastic transformation of the hemopoietic cells.