Temperature-sensitive expression of differentiation in transformed myoblasts

Differential effects of transforming avian RNA tumor viruses on avian macrophages

Functionally differentiated chicken macrophages were derived by in vitro differentiation of embryonic yolk sac cells and were characterized by several macrophage-specific cell markers. Uniform, infected, virus-producing cultures were obtained after exposure of these macrophages to avian myoblastosis virus (AMV), avian myelocytomatosis virus (MC29), myeloblastosis-associated virus (MAV-2), and Prague strain of Rous sarcoma virus (PR-B RSV). Both AMV and MC29 induced morphological transformation typical of the in vivo leukemias induced by these virus strains. Analysis of the expression of macrophage-specific markers in these two transformed cell types demonstrated that different markers of the mature macrophage were suppressed by each virus, even though the parental cell immediately preceding the transformation event was a mature macrophage in both cases. Cells infected with PR-B RSV and MAV-2 showed no observable difference from uninfected macrophages in terms of morphological characteristics, growth rate, or expression of the differentiated functions of macrophages. Ths system provides demonstrations of a cell type that produces infectious, transforming RSV but fails to respond by functional alterations induced by the transforming gene, src.

The expression of glial fibrillary acidic protein in a rat cerebellar cell line

A rat cerebellar cell line, WC5, derived by transformation with Rous sarcoma virus, which is temperature-sensitive for transformation (ts-RSV), can be induced to express glial fibrillary acidic protein (GFAP). Immunofluorescence, radioimmune assay, and electron microscopy studies show that GFAP is expressed in WC5 cells grown at the nonpermissive temperature (NPT), but not at the permissive temperature (PT) for transformation. GFAP is first detectable about 3 days after incubating cells at the NPT, and reaches an apparent plateau by the seventh or eighth day. The expression of GFAP is reversible; shifting cells from the NPT to the PT causes a dramatic decrease in GFAP after 96 hr. In order to determine if the expression of GFAP is linked to the temperature-sensitive transforming activity of the viral src gene product, phenotype revertants of WC5 were established. By the criteria of morphology and growth in agar, the revertant lines, in contrast to the parent cell line WC5, were shown to exhibit a transformed phenotype at both the NPT and PT. Immunofluorescence studies on several of the revertant cell lines show that they do not express GFAP at either the PT or NPT. These findings suggest that the expression of GFAP in WC5 is linked to the expression of the src gene product. The advantage of using ts-RSV to derive neural cell lines which exhibit differentiated properties is discussed.

Cell-surface glycoprotein synthesis during differentiation of chicken erythroblasts transformed by temperature-sensitive avian erythroblastosis virus

Chicken erythroblasts transformed by a temperature-sensitive mutant of avian erythroblastosis virus (ts34 AEV) have a greatly increased haemoglobin content (Graf, T., N. Ade and H. Beug: Nature 275, 496-501 (1978)) if allowed to grow for 3-5 days at the non-permissive temperature (41 degrees C), instead of the permissive temperature (35 degrees C) of the virus. Cell-surface molecular changes during this differentiation were investigated by examining the glycoproteins synthesized by a ts34-transformed erythroblast cell line. These cells synthesized a greatly increased amount of a 94,000 molecular weight erythrocyte cell-surface glycoprotein beginning 2-6 h after a shift in growth temperature from 35 degrees to 41 degrees C, consistent with the proposal that such a shift releases these transformed cells from a differentiation block.

Properties of two temperature-sensitive Rous sarcoma virus transformed cerebellar cell lines

Cells from the cerebellum of 3-day-old BD-IX rats were obtained as permanent lines by transforming them with temperature-sensitive Rous sarcoma virus. The presence or absence of veratridine-stimulated Na+-uptake (voltage-dependent channels) was used to operationally classify them as neuronal or glial. When incubated at 34 degrees C, the permissive temperature for transformation, the cerebellar cells exhibit a transformed phenotype determined by anchorage independence, rounded morphology, high growth rate and absence of density-dependent inhibition of growth. In contrast, when the transformed cerebellar cell lines are kept at a temperature (38 degrees C) non-permissive for transformation, they exhibit a normal cellular phenotype with respect to the above properties. Moreover, changes toward neuronal morphology, increase in veratridine-stimulated Na+-uptake, decreased growth rate and the expression of the astrocyte specific protein, glial fibrillary acidic protein, suggest that a degree of differentiation is expressed at the non-permissive temperature.

Tissue-specific expression of onco-fetal antigens during embryogenesis

It has become evident from recent literature that especially in tumor virus systems, cell transformation leads to an arrest of differentiation or to a retrodifferentiation. This may be reflected by the expression of embryonic antigens and it is therefore particularly important to characterize such antigens according to their specificity as well as to their specificity during embryogenesis. We have demonstrated the expression of embryonic antigens which are cross-reactive in avian fibroblasts transformed either by Rous sarcoma virus or by methylcholanthrene. This paper is intended to demonstrate that these embryonic antigens are detected only at a certain period of embryogenesis and particularly in muscle cells. They are detected only occasionally or not at all in cells of other tissues such as brain, liver, lung, and the digestive organs. These antigens are absent from the target cells before transformation and are consequently induced by the transforming agent, either viral or chemical. Therefore, these results suggest that by transformation mechanism, cells become specifically reverted to an earlier stage of differentiation (retrodifferentiation).

The effect of avian retroviruses on limb bud chondrogenesis in vitro

Mesenchymal cells isolated from stage 24 embryonic chicken limb buds were infected with the temperature-sensitive transformation mutants of Rous sarcoma virus tsNY68, tsNY10 and tsLA25 at the nonpermissive temperature for transformation (41 degrees C). Virus infection greatly inhibited subsequent limb bud chondrogenesis under nontransforming conditions, as indicated by a reduction in the rate of 35SO4 incorporation into cell-associated proteoglycans. The inhibition of chondrogenesis was directly related to the percentage of cells infected with tsNY68 at 41 degrees C. The observed inhibition of chondrogenesis was independent of src gene expression since this effect was also caused by many viruses which lack the src gene, including the leukosis viruses RAV-1, RAV-2 and MAV-2(0); the src deletion mutant RSVtd107; and the reticuloendotheliosis viruses REV-T and SNV. Infection of mesenchymal cells with tsNY68 under nontransforming conditions did not cause changes in parameters such as the rate of thymidine incorporation, total cell DNA and total cell protein. Infection with tsNY68 at 41 degrees C resulted in altered kinetics of 35SO4 incorporation into cell-associated proteoglycans and a corresponding reduction in 35SO4-labeled proteoglycans extracted from the cell layer. There were no apparent quantitative effects on the rate of accumulation of proteoglycans in the culture medium. The proteoglycans extracted from the cells and the collected medium of tsNY68-infected cultures were smaller than those of uninfected cultures, as shown by agarose gel chromatography.

Tumor promoters induce changes in the chick embryo fibroblast cytoskeleton

We have examined the effect of the tumor promoter, 12-0-tetradecanoyl phorbol-13-acetate (TPA), on the actin-containing elements of the cytoskeleton of chick embryo fibroblasts (CEF). TPA at concentrations as low as 7.3 times 10-10M indices a reversible change in the cytoskeleton as visualized by indirect immunofluorescence using anti-actin antibodies. Cells incubated with TPA lose the ordered actin-containing structures found in normal cells and resemble Rous sarcoma virus-transformed cells in that the immunofluorescent actin pattern is diffuse. The TPA effects are both dose-and time-dependent. Analogs of TPA which are inactive as tumor promoters do not induce cytoskeletal changes at the concentrations tested, while a second tumor promoter, PDD, is also able to cause alterations in actin-containing structures. The action of TPA requires de novo synthesis of both RNA and protein. The direct cytoskeletal changes are neither plasmin-dependent nor subject to inhibition by incubating the cells with high levels of protease inhibitors during the exposure to TPA. However, plasminogen does increase the sensitivity of cells to TPA.

Manipulation of myogenesis in vitro: reversible inhibition by DMSO

A system has been developed for the detailed analysis of the transition from proliferative myoblast to differentiated muscle cell. Dimethylsulfoxide (DMSO) prevents the terminal differentiation of L8 myoblasts in vitro, and its effect is reversible. DMSO (2%) inhibits the fusion of myoblasts to form multinucleate myotubes, the normal increases in activity of creatine phosphokinase (CPK) and acetylcholinesterase, and the synthesis of alpha-actin and acetylcholine receptor protein. Upon removal of DMSO from the medium, a lag precedes the onset of differentiation. The potential to inhibit muscle differentiation reversibly is not specific to DMSO, but is shared by a number of compounds, including dimethylformamide, hexamethylbisacetamide and butyric acid, all potent inducers of gene expression in Friend erythroleukemia cells. L8 cells routinely cease DNA synthesis and initiate fusion and muscle protein synthesis once they are confluent. In the presence of DMSO, however, nearly all cells continue DNA synthesis, even several days after reaching confluence. Protein synthetic patterns of DMSO-inhibited cells are almost indistinguishable from those of untreated myoblasts and distinct from differentiated myotubes. It appears that cells exposed to DMSO are locked indefinitely in a proliferative myoblast stage of development and are unable to enter the Go phase of the cell cycle necessary for initiation of differentiation. DMSO coordinately inhibits all the differentiative parameters measured. In contrast, cytochalasin B uncouples normally linked differentiative events so that fusion is inhibited while muscle-specific protein synthesis proceeds. DMSO has similar effects on both cytochalasin B-treated and fusing control cultures, suggesting that its primary effect is exerted not at the level of fusion but earlier in the differentiative time-table. Once fusion and the synthesis of muscle-specific proteins are well under way, the addition of DMSO is ineffective and differentiation continues in its presence. The potential to manipulate muscle gene expression in vitro makes this system particularly useful for the detailed analysis of the processes involved in the transition to the differentiated state and for determining the linkage of developmental events.

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.

Neural crest cells: temperature-dependent transformation by Rous sarcoma virus

Cranial neural crest cells from chick embryos, when cultured under appropriate conditions, differentiate after approx. 1 week into pigmented cells. Neurol crest cells were infested with a mutant (RSV-BH-Ta) of the Bryan 'high titer' strain of Rous sarcoma virus on the second day of culture before the cells were morphologically differentiated, or later after they became pigmented. Cells infected and maintained at the temperature permissive for transformation (37 degrees C) proliferated rapidly compared to uninfected cells are produced extensive cytoplasmic vacuoles in a fashion similar to other types of cells transformed with RSV-BH-Ta at 37 degrees C. Cells infected and maintained at the non-permissive temperature for transformation (41 degrees C) also proliferated rapidly but did not become morphologically transformed. Transformation occurred reversibly following a shift of temperature. Infection of morphologically undifferentiated neural crest cells at either temperature prevented their differentiation into pigment cells, and infection of pigmented neural crest cells at either temperature led to a gradual loss of pigmentation. These results suggest that even at the non-permissive temperature the virus may regulate the state of differentiation of certain types of cells.

Plasminogen activator in chick embryo muscle cells: induction of enzyme by RSV, PMA and retinoic acid

To explore the generality of the effects of sarcoma viruses, tumor-promoting phorbol esters and retinoic acid, we have studied plasminogen activator production in differentiating chick myogenic cultures. Although slightly higher than in chick fibroblast cultures, the level of spontaneously synthesized enzyme is low; it reaches a peak shortly after maximum cell fusion has been completed and then declines. Rous sarcoma virus (RSV) transformation of differentiating myotubes was accomplished by infecting myoblasts with a temperature-sensitive mutant, maintaining cultures at the nonpermissive temperature until completion of fusion and shifting to permissive temperatures at selected times thereafter. RSV transformation, phorbol myristate acetate (PMA) and retinoic acid all induced high levels of plasminogen activator production by differentiating myotubes in the absence of DNA synthesis. In comparison with fibroblasts, virus-induced enzyme synthesis by myogenic cultures proceeded more slowly but ultimately reached comparably high levels. Whereas cAMP strongly repressed RSV- and PMA-induced plasminogen activator production by chick fibroblasts, it weakly stimulated enzyme synthesis by myotubes. This suggests that enzyme induction by RSV and PMA is not mediated primarily through effects on cAMP metabolism.

Metabolism of acetylcholine receptor in chick embryo muscle cells: effects of RSV and PMA

We have investigated some aspects of the metabolism of the integral membrane protein acetylcholine receptor (AChR) in normal and transformed cultures of chick embryo muscle cells. Turnover of AChR in control muscle cell cultures was compared with turnover in cultures infected and transformed by a temperature-sensitive mutant of Rous sarcoma virus (RSV) and with cultures treated with the tumor promoter phorbol myristate acetate (PMA). The parameters of AChR metabolism were estimated using 125I-alpha-bungarotoxin as a stoichiometric high affinity ligand for the AChR. We found that both RSV transformation and PMA increased the rate of degradation and decreased the rate of synthesis of AChR. The consequent reduction in steady state receptor levels suggests that oncogenic transformation and tumor promoter significantly alter the metabolism of cell surface membranes. We also observed that parameters of AChR metabolism in control cultures changed systematically in a pattern which depended upon the age of the culture as well as the use of embryo extract or fetal bovine serum as medium supplements. The muscle cell system allows quantitative measurement of an integral membrane protein and its metabolism, and may serve as a more general model for alterations in membrane and surface receptor metabolism associated with the transformed state.

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.

Quantitative studies on in vitro transformation of hamster chondrocytes

Hamster chondrocytes could be transformed in a quantitative assay system which used X-irradiated feeder layer cells. Morphological transformation occurred on addition of, 4NQO, but not in control cultures. Differentiation was classified into 3 types (good, poor and none); normal and transformed colonies contained similar proportions of the 3 types.

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.