Infection of chick iris epithelium with the Rous sarcoma virus in vitro

The role of the Src homology domains in morphological transformation by v-src

The Src homology (SH2 and SH3) domains of v-Src are required for transformation of Rat-2 cells and for wild-type (morphr) transformation of chicken embryo fibroblasts (CEFs). We report herein that the N-terminal domains of v-Src, when expressed in trans, cannot complement the transformation defect of a deletion mutant lacking the "unique," SH3, and SH2 regions. However, the same regions of Src can promote transformation when translocated to the C terminus of v-Src, although the transformation of CEFs is somewhat slower. We conclude that the SH3 and SH2 domains must be present in cis to the catalytic domain to promote transformation but that transformation is not dependent on the precise intramolecular location of these domains. In CEFSs and in Rat-2 cells, the expression of wild-type v-Src results in tyrosine phosphorylation of proteins that bind to the v-Src SH3 and SH2 domains in vitro; mutations in the SH2 or SH3 and SH2 domains prevent the phosphorylation of these proteins. These findings are most consistent with models in which the SH3 and SH2 domains of v-Src directly or indirectly target the catalytic domain to substrates involved in transformation. However, the N-terminal domains of v-Src can promote tyrosine phosphorylation of certain proteins, in particular p130Cas, even when expressed in the absence of the catalytic domain, indicating that the N-terminal domains of v-Src have effects that are independent of the catalytic domain.

METABOLIC CONTROLS IN CULTURED MAMMALIAN CELLS

A number of apparently unrelated factors are known to have a profound effect on the metabolism of cultured mammalian cells; and some of these may be operative as metabolic controls in the whole animal as well. The more complete exploration of (i) homotypic and heterotypic cellular interactions, (ii) the spontaneous transformations sometimes observed in cultured cells, (iii) the mode of action of cytotoxic agents, (iv) the multiple metabolic effects of viral infection, and (v) the conditions necessary for the maintenance of specialized function in cultured cells, can be expected to throw light on the basic mechanisms underlying such complex processes as differentiation, senescence, and cancer.

The embryonic environment strongly attenuates v-src oncogenesis in mesenchymal and epithelial tissues, but not in endothelia

We demonstrate that the behavior of cells expressing v-src, a tyrosine kinase oncogene, differs profoundly between the embryonic and culture environments. V-src was introduced into avian embryo cells both in culture and in stage-24 embryo limbs, using replication-defective retroviral vectors. These vectors were used as single-hit, cellular markers to determine the environmental influences imposed by normal cells and tissues on clonal cell growth. The marker gene lacZ was coexpressed with v-src in order to locate the descendent cells. In culture, v-src induced rapid morphological transformation and anchorage-independent growth of embryo fibroblasts; the vectors were also tumorigenic in hatchling chickens. In contrast, most of the cell clones expressing v-src in the embryo grew normally without neoplasia. Expression of v-src vectors could be found in a wide range of cell types, demonstrating not only that neoplastic transformation is attenuated in ovo, but also that differentiation commitment in many lineages can be maintained concurrently with oncogene expression. Significantly, the embryonic control of cell growth could be perturbed by v-src under certain conditions. Rare, marked clones showed hyperplasia or dysplasia, and the primitive endothelium could succumb to rapid neoplasia; thus, these embryonic tissues are not inherently deficient in transformation factors. We propose that the environmental conditions imposed on cells in ovo are critical for the attenuation of neoplasia, while cultured cells lose this requisite environment.

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.

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.

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.

Hematopoietic cell transformation by a murine recombinant retrovirus containing the src gene of Rous sarcoma virus

A recombinant murine retrovirus (MRSV) containing the src gene of avian Rous sarcoma virus (RSV) was shown to induce hematopoietic colonies in infected mouse bone marrow. MRSV-induced colony formation followed single-hit kinetics and required mercaptoethanol in the agar medium. Cells from the colonies induced by MRSV could be established as continuous cell lines that demonstrated unrestricted self-renewal in vitro and tumorigenicity in vivo. The transformants, all of which expressed high levels of the Rous sarcoma virus transforming protein, pp60src, appeared to be at an early stage in lymphoid cell differentiation. They lacked Fc receptors and detectable immunoglobulin mu heavy chain synthesis, markers normally associated with committed B cells. The majority of the MRSV-transformed cell lines contained high levels of terminal deoxynucleotidyl transferase, an enzyme present in lymphoid progenitor cells committed to the T-cell lineage. One cell line expressed Thy-1 antigen, but none expressed Lyt-1 and Lyt-2, markers of more differentiated T cells. These findings demonstrate that the src gene is capable of transforming cells of hematopoietic origin.

Transformation of rat cerebral endothelial cells by Rous sarcoma virus

Rat cerebral microvascular endothelial cells were infected with Schmidt-Ruppin Rous sarcoma virus-strain D (SR-RSV-D), an avian retrovirus. A single focus of transformed cells was isolated and the resultant cell line designated RCE-T1. The specificity for SR-RSV-D transformation was determined by virus rescue assay and demonstration of virus-specific antigens. RCE-T1 cells are virogenic when fused with chicken embryo fibroblasts (CEF) and do not produce infectious virus as demonstrated by the absence of detectable virus in culture fluid from these cells alone. Studies using an enzyme-linked immunosorbent assay (ELISA) for avian retrovirus-coded internal proteins show that RSV-transformed endothelial cells contain mainly p27 and react to some extent to p19 and p15 viral antigens. These data demonstrate conclusively that the transformation event was indeed due to SR-RSV-D. In addition, chromosome analysis confirmed these cells to be of rat origin. RSV-transformed endothelial cells express the typical array of transformation-related properties such as anchorage-independent cell growth in soft agar, decreased cell adhesiveness, ability to grow in low serum, and capability of producing tumors in newborn rats. Demonstration of differentiated endothelial characteristics included positive immunofluorescent staining for factor VIII antigen and angiotensin-converting enzyme and histochemical localization of gamma-glutamyl transpeptidase activity. This cell line should provide a useful model to study not only specialized biochemical and other functional characteristics of cerebrovascular endothelium but also the cellular mechanisms that involve the transition from normal to neoplastic expression.

Stimulation of growth rate of chondrocytes by Rous sarcoma virus is not coordinated with other expressions of the src gene phenotype

Infection and transformation of chondrocytes by Rous sarcoma viruses (RSVs) (Schmidt-Ruppin, Prague) stimulated the rate of cell growth. In contrast, several transformation-defective (td) mutants (tdPRA, tdNY105, tdNY106, tdNY107, and tdNY108) retaining various sizes of the src gene did not stimulate cell growth, indicating that the stimulation of growth of chondrocytes is due to the function of the src gene. With the use of various T (transformation)-class temperature-sensitive (ts) mutants of RSV, growth stimulation of chondrocytes by the src gene was examined. It was found that there are two types of T-class ts mutants with regard to the stimulatory effect on the growth of chondrocytes. One type (tsNY68) stimulates cell growth at both permissive (36 degrees C) and nonpermissive (41.5 degrees C) temperature, as does the wild type of RSV. Another type (ts GI201 [clone 9]) stimulates cell growth only at the permissive temperature. Chondrocytes infected with either of these two types of T-class ts mutants showed ts properties in other transformation markers, such as uptake of 2-deoxy-D-glucose, change of cell morphology, and focus formation. These data indicate that the effect of the src gene on cell growth does not occur coordinately with other transformation markers.

Response of hemopoietic cells to avian acute leukemia viruses: effects on the differentiation of the target cells

Chicken bone marrow cells were infected with three avian acute leukemia viruses (ALV)--avian myeloblastosis virus (AMV), myelocytomatosis virus strain MC29 and Mill Hill 2 virus (MH2)--and then cultured in agar in the presence of conditioned medium. Under these conditions, it was found that very few cells served as target cells for these three viruses. Density gradient separation showed that ALV target cells were found primarily in the light density fractions and might be represented by cells committed to the mononuclear phagocyte pathway. Separation of bone marrow cells on the basis of their sedimentation velocity at unit gravity suggested that MC29 and AMV did not share the same target cells. In addition, the analysis of surface receptors and functional markers characteristic of macrophages (Fc and complement receptors, phagocytosis and immune phagocytosis) indicated that the ALV-transformed cells were blocked during their differentiation. These results indicate that the transforming ability of ALV interferes with the differentiation of their target cells.

Morphological and biochemical differentiation in RSV transformed chick embryo myoblasts

Chick embryo myoblasts have been transformed with a temperature sensitive mutant of Rous Sarcoma virus (RSV ts68). At permissive temperature (36 degrees C) it is shown that transformed myoblasts have lost their ability to form myotubes as well as to express the biochemical markers of myogenic differentiation. Upon a shift to the non-permissive temperature (41 degrees C), the normal program of differentiation is restored; myotubes are formed which express muscle specific proteins.

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

Retinal melanoblasts were transformed by a temperature-sensitive mutant of Rous sarcoma virus (ts-RSV). At the permissive temperature for transformation, the cells cease melanin synthesis, degrade their melanosomes and release much of their accumulated melanin into the medium. At the nonpermissive temperature, the cells assume an epithelioid morphology, actively synthesize melanin and become difficult to distinguish from normal uninfected control cultures. Both the transformed phenotype and the differentiated cell phenotype are temperature-dependent. Infected retinal melanoblasts which are incubated at the nonpermissive temperature and which accumulate a large amount of melanin are unable to transform in response to a temperature shift; instead, the cells degenerate and die. Retinal melanoblasts can be infected by subgroups A, B, C and D of RSV; however, their level of susceptibility to infection is about 1/40 compared to fibroblasts. Cultures infected by ts-RSV produce virus at both temperatures, suggesting that cell phenotype does not regulate virus synthesis.

Transformation of chondroblasts by Rous sarcoma virus and synthesis of the sulfated proteoglycan matrix

The presence of the extracellular matrix synthesized by chondroblasts provides a barrier to virus penetration. Chondroblasts can be infected and transformed following treatment with proteolytic enzymes. Using a temperature-sensitive transformation mutant of Rous sarcoma virus and rearing the cells at permissive temperature, we demonstrate that transformed chondroblasts stop synthesizing their cell-unique sulfated proteoglycan. If such transformed chondroblasts are shifted to nonpermissive temperature, the cells reinitiate the synthesis of their cell-unique sulfated proteoglycan.

In vitro transformation of hamster chondrocytes by 4-nitroquinoline 1-oxide

Hamster sternal chondrocytes were transfored morphologically in vitro 29-51 days after exposure to 4-nitroquinoline 1-oxide for 3 h. The transformed chondrocytes were randomly oriented, piled up and grew continuously. These cells produced nodules when transferred to the cheek pouch of a hamster. Histologically, these nodules revealed chondromatous patterns. Untreated cells in the control cultures failed to produce nodules.

Transformation of chick embryo neuroretinal cells by Rous sarcoma virus in vitro: induction of cell proliferation

Neuroretinal cells from 7-day-old chick embryos are transformed and induced to proliferate after infection with Rous sarcoma virus in vitro. Susceptibility of neuroretinal cells to the virus is also dependent on the stage of development since infection of cells from 10-day-old embryos is uneffective.

Control of pigment production in mouse melanoma cells in vitro. Evocation and maintenance

A clonally derived amelanotic melanoma cell line repeatedly has been forced to produce pigment by the inhibitor of DNA synthesis, I-beta-D-arabinofuranosylcytosine (ara-C) at sublethal levels. One ara-C-derived melanotic line has been cloned, and has continued to produce pigment for 2 years on normal medium. The inhibitor is most effective when administered to synchronized cells in four pulses on successive days at 1.8 x 10(-5)M during the S phase of the cell cycle. Colcemid at a sublethal concentration, and growth on medium solidified with agar also evoked pigment production in this line, but a large number of other inhibitors of biosynthetic processes did not, under the conditions tested. The melanotic lines are active producers of tyrosinase (DOPA oxidase), whereas the amelanotic line produces an inhibitor of tyrosinase activity. Both enzyme and inhibitor are labile at 4 degrees C and -20 degrees C, and decay of the inhibitor in homogenates of amelanotic cells reveals a low level of residual DOPA oxidase activity. The mean population doubling time of a cloned melanotic line is 23 hr, and that of a cloned amelanotic line 16.5 hr. A similar decrease in rate of growth is found in other melanotic lines and is believed to be a significant factor in maintaining this differentiated function. Rapid growth may be related to the production of an inhibitor by the amelanotic cells.

An electron microscopic study of the chorioallantoic membrane following infection with Rous sarcoma virus

Infection of the chick chorioallantoic membrane (CAM) with Rous sarcoma virus (RSV) has been thought by earlier workers (12, 20) to result in the transformation of the ectoderm and then the mesoderm of that organ. In the present study, CAM were infected with 10(4) PFU (pock-forming units) of RSV (Bryan high titre strain) and collected for electron microscopy at 2, 4, and 6 days postinfection. Observations of the fine structural changes in the CAM after RSV infection support a singular role of the mesenchyme in the initiation of the tumors. The ectodermal hyperplasia often associated with RSV tumors of the CAM appears to be a secondary response to the alteration of the underlying mesenchyme. These findings are discussed in detail, and an alternate course of RSV transformation of the CAM by way of the vascular bed is suggested.

Viral antigens in differentiating muscle colonies after infection with Rous sarcoma virus in vitro

Images

The susceptibility of differentiating muscle clones to Rous sarcoma virus

Images

Heritability of cellular differentiation: clonal growth and expression of differentiation in retinal pigment cells in vitro

Images