Potassium fluxes and ouabain binding in growing,density-inhibited and rous sarcoma virus-transformed chicken embryo cells

Potassium fluxes, ouabain binding, and Na⁺ and K⁺ intracellular concentrations were determined for cultures of growing normal, density-inhibited and Rous sarcoma virus-transformed chicken embryo fibroblasts. No significant differences in K⁺ influx or ouabain binding were detected between growing normal cells and Rous sarcoma virus-transformed cells; however, ouabain binding and ouabain-sensitive K⁺ influx were 1.5- to 1.8-fold lower in density-inhibited cells. Thus, potassium influx in this system can be classified as a growth-related, but not transformation-specific change. As determined by both flame photometry and radioisotopic (⁴²K) equilibration, growing normal and density-inhibited cells had similar potassium contents, whereas transformed cells exhibited 1.4-fold higher potassium levels. Sodium ion levels, as measured by flame photometry, were also 2- to 4.5-fold higher in transformed than normal or density-inhibited cells. Complementary studies of potassium efflux showed a 1.3- to 1.5-fold higher rate (based on the percentage of pool exiting the cell) in growing normal versus density-inhibited or transformed fibroblasts. Because of the larger potassium pool in transformed cells, efflux based on absolute number of potassium ions is similar in normal and transformed chicken embryo fibroblasts.     

Dynamics of the spreading of normal and transformed hamster cells]

Morphological peculiarities of spreading studied by scanning electron microscopy in two lines of transformed hamster fibroblasts (HETR and HEC--40) were compared to the normal hamster embryo fibroblasts (NHG). The surface of spherical not streading cells was of a mixed type microrelief (small blebs and microvilli). In transformed cells, the microvillous component was more developed than in their normal counterparts. During cell spreading distinct differences were observed between normal and transformed cells in their cell surface contact interaction with solid substratum. NHF cells formed a well-developed concentrically disposed thin lamelloplasm, while HEC-40 cells had asimmetrically disposed lamelloplasm in combination with long filopodia and HETR cells had a rather thick lamelloplasm consisting of several fragments (often forming star-like pattern). At the polarization stage of spreading neither HETR nor HEC-40 reached the same degree of spreading and flattening as NHF. Moreover, the dorsal surface of transformed cells had a complex microrelief in contrast to a rather smooth surface of NHF. These results are discussed in connection with earlier results on spreading of normal and transformed mouse fibroblasts.     

Analysis of changes induced by oncogene <Emphasis Type="Italic">N-RAS</Emphasis> expression in pattern and distribution of pseudopodial activity of fibroblasts

It is not known which morphological properties of fibroblasts induced by malignant transformation modulate their migration pattern. We studied the changes in the distribution and dynamics of the leading edge of 10(3) mouse fibroblasts after their transformation by oncogene N-RAS ^asp13 and analyzed the changes in the pattern of cell migration. Transformation proved to increase the leading edge proportion and to considerably redistribute pseudopodial activity along the cell edge. As the result of transformation, small pseudopodia are formed in the stable lateral regions of the cell edge typical of normal fibroblasts, i.e., the lateral edge is no more truly stable. In addition, pseudopodial activity of the leading edge in transformed fibroblasts proved higher compared to normal ones. It is necessary to notice, the leading edge activity is equally high immediately after induction in both normal and transformed fibroblasts; although, it is suppressed with time in normal cells but not in transformed ones where it remains steadily high. These properties promote the random component of malignant cell motility and modify the cell migration pattern after transformation     

Differences in membrane electrical properties between C3H 10T1/2 mouse embryo fibroblasts and their ionizing radiation and chemically transformed counterparts

Membrane electrical properties of mouse embryo fibroblasts and their ionizing radiation and chemically transformed counterparts were investigated using dielectric relaxation measurements in the radio frequency range. This determination is possible because, in the radio frequency range, suspensions of cells in an electrolyte buffer show a conductivity dispersion due to interfacial polarization. An analysis of the experimental data based on a single-shell model showed that conductivity and permittivity of the membranes of both radiation and chemically transformed fibroblasts were lower than in normal cells. In addition, the conductivity of the cytoplasm was higher in both transformed cell types than in the normal mouse fibroblasts. We discuss the significance of these findings in view of the possible structural and functional modifications brought about by the process of neoplastic transformation.     

Intercellular communication and tissue growth

Summary Epithelial cells of normal rat (adult) liver and hamster embryo in tissue culture communicate through membrane junctions: the membrane regions of cell contact are highly ion-permeable. Cancerous counterparts of these cells, cells from Morris' and Reuber's liver tumors and from x-ray-transformed embryo cultures, do not communicate under the same experimental conditions. These cells also fail to communicate with contiguous normal cells. Cancerous fibroblastic cells from a variety of tissues, including cells transformed by virus, x-radiation and chemicals, communicate as well as their normal counterparts; this is so for long- and short-term cell cultures. Communication in some fibroblastic cells is sensitive to components of blood serum: normal and transformed hamster embryo fibroblasts, which communicate when cultured in medium containing fetal calf serum, appear to lose communication in medium containing calf serum; the converse holds for hamster (adult) fibroblasts and 3T3 cells.The preceding papers of this series appeared in the Journal of Cell Biology.Trainee of the National Institutes of Health, National Cancer Institute, Grant CA 05011.     

Agglutination of Normal and Rous Sarcoma Virus-transformed Chick Embryo Cells by Concanavalin A and Wheat Germ Agglutinin

RODENT cells in culture transformed by oncogenic DNA viruses have surface sites that on normal cells are usually present in latent form only^1,2. This difference in surface properties can be detected by plant glycoproteins such as wheat germ agglutinin (WGA) and concanavalin A (Con A), which agglutinate only transformed cells, because they have certain carbohydrate moieties on their neoplastic surfaces^1–4. According to some investigators, normal and neoplastic cells that have been freshly isolated also exhibit this marked difference^3,5; according to others^6,7, there is no such distinction. We have looked for such differences in cells transformed by RNA tumour viruses and in several types of normal and naturally occurring malignant cells and their normal counterparts.     

Synthesis of Cell Coat in Normal and Transformed Cells

THE surface of transformed cells has been a focus of considerable attention recently because some of the properties which distinguish these cells from their precursors, such as decreased cell adhesiveness, altered cell orientation and loss of contact and density dependent inhibition^1–3, may relate to changes on their surface. A common feature of vertebrate cells is the cell coat, a glycoprotein structure surrounding the plasma membrane⁴. Electron microscopy has revealed that transformed cells have a thicker coat than normal cells⁵ and we have now found that coat synthesis in cells transformed by an oncogenic DNA virus and in cells transformed by a chemical carcinogen occurs faster than in normal controls whereas only in the virus-transformed cells is the coat significantly thicker.     

Vimentin-derived proteins : Differences between normal human fibroblasts and transformed human cells

Two-dimensional gel electrophoresis revealed a quantitative difference in a series of polypeptides ranging in MW from 45 000 to 51 000 and of lower isoelectric pH than vimentin, when comparing normal human fibroblasts with a virally transformed subline and with HeLa cells. Re-extraction of purified [³⁵S]vimentin with cold whole cell homogenates and peptide mapping showed that these polypeptides are derivatives of vimentin. They may be natural components of a normal fibroblast's architecture or they may arise from a pool of vimentin that is not structured within intermediate filaments at the time of extraction. Furthermore, we show that vimentin from the two transformed cell types is more resistant to proteolysis by whole cell homogenates than vimentin from normal fibroblasts. Structural alteration of vimentin may play an important role in the expression of transformation.     

Extracellular regulation of fibroblast multiplication: A direct kinetic approach to analysis of role of low molecular weight nutrients and serum growth factors

The principles of enzyme kinetic analysis were applied to quantitate the relationships among serum-derived growth factors, nutrients, and the rate of survival and multiplication of human fibroblasts in culture. The survival or multiplication rate of a population of cells plotted against an increasing concentration of a growth factor or nutrient in the medium exhibited a hyperbolic pattern that is characteristic of a dissociable, saturable interaction between cells and the ligands. Parameters equivalent to the K_m and V_max of enzyme kinetics were assigned to nutrients and growth factors. When all nutrient concentrations were optimized and in steady state, serum factors accelerated the rate of multiplication of a normal cell population. The same set of nutrients that supported a maximal rate of multiplication in the presence of serum factors supported the maintenance of non-proliferating cells in the absence of serum factors. Therefore, under this condition, serum factors are required for cell division and play a purely regulatory iole in multiplication of the cell population. The quantitative requirement for 18 nutrients of 29 that were examined was significantly higher (P < 0.001) for cell multiplication in the presence of serum factors than for cell maintenance in the absence of serum factors. This indicated specific nutrients that may be quantitatively important in cell division processes as well as in cell maintenance. The quantitative requirement for Ca²⁺, Mg²⁺, K⁺, Pi, and 2-oxocarboxylic acid for cell multiplication was modified by serum factors and other purified growth factors. The requirement for over 30 other nutrients could not clearly be related to the level of serum factors in the medium. Serum factors also determined the Ca²⁺, K⁺, and 2-oxocarboxylic acid requirement for maintenance of non-proliferating cells. Therefore, when either Ca²⁺, K⁺, or 2-oxocarboxylic acid concentration was limiting, factors in serum played a role as cell “survival or maintenance” factors in addition to their role in cell division as “growth regulatory” factors. However, with equivalent levels of serum factors in the medium, the requirement for Ca²⁺, K⁺, and 2-oxocarboxylic acids was still much higher for multiplication than for maintenance. Kinetic analysis revealed that the concentrations of individual nutrients modify the quantitative requirement for others for cell multiplication in a specific pattern. Thus, specific quantitative relationships among different nutrients in the medium are important in the control of the multiplication rate of the cell population. When all nutrient concentrations were optimal for multiplication of normal cells, the multiplication response of SV40-virus-transformed cells to serum factors was similar to that of normal cells. When serum factors were held constant, transformed cells required significantly less (P < 0.001) of 12 of the 26 nutrients examined. Therefore, the transformed cells only have a growth advantage when the external concentration of specific nutrients limits the multiplication rate of normal cells. Taken together, the results suggest that the control of cell multiplication is intimately related to external concentrations of nutrients. Specific growth regulatory factors may stimulate cell proliferation by modification of the response of normal cells to nutrients. Transforming agents may confer a selective growth advantage on cells by a constitutive alteration of their response to extracellular nutrients.     

Cholesterol content and metabolism in normal and polyoma virus-transformed hamster embryo fibroblasts

The total cholesterol content of normal hamster embryo fibroblasts and polyoma virus-transformed hamster embryo fibroblasts were found to be similar. However, the free cholesterol: cholesterol ester ratio was 41.5 in normal cells as contrasted to 1.8 in their transformed counterparts. This difference is due in part to an increase in cholesterol esterification and a decrease in the hydrolysis of cholesterol esters in the transformed cell.     

Serum-free growth of normal and transformed fibroblasts in milk: differential requirements for fibronectin

Bovine milk may be used as a supplement for the serum-free growth of certain fibroblastic cells in culture. The growth properties of three representative cell types in milk-supplemented medium were examined; fibroblastic cell strains, fibroblastic cell lines, and transformed fibroblasts. Transformed fibroblasts, which included RNA and DNA tumor virus-transformed cells and carcinogen-transformed cells, grew in milk. Instead of growing attached to the culture dishes, as they normally do in serum, transformed fibroblasts grew in milk as large clusters in suspension. In contrast, nontransformed fibroblastic cell strains and cell lines did not grow in milk-supplemented medium. Fibroblasts transformed by a temperature-sensitive transformation mutant of Rous sarcoma virus were temperature-sensitive for growth in milk. The failure of cells to adhere to the substratum in milk-supplemented medium suggested that milk might be deficient in attachment factors for fibroblasts. When the attachment of fibroblastic cells in milk- supplemented medium was facilitated by pretreating culture dishes with fibronectin, (a) transformed cells grew attached rather than in suspension, (b) normal cell lines attached and grew to confluence, and (c) normal cell strains adhered and survived but did not exhibit appreciable cell proliferation.     

Manganese superoxide dismutase in normal and transformed human embryonic lung fibroblasts

The manganese superoxide dismutase (MnSOD) activity of W138 human embryonic lung fibroblasts and SV40-transformed WI38 cells was measured. Under various growth conditions, the transformed cells always had lower MnSOD activity than their normal cell counterparts. The activity of MnSOD changes greatly with the growth conditions in the WI38 cells, while the MnSOD activity in the tumor cells remained more constant. The amount of immunoreactive MnSOD was measured by Western blotting. In all cases studied, the amount of immunoreactive MnSOD was lower in the transformed cells than in the normal cells.     

Differences in sialic acid contents of low cancer cells,high cancer cells and normal mouse lung counterparts

Sialic acid contents of low cancer (P 4 BIS) high cancer (P 4 BIS T) cells and their normal (PB) mouse lung counterparts have been determined. This content is 5 to 10 fold higher for cells in logarithmic phase growth than for confluent cells, as well for normal cells as for transformed derived cells lines. Growing normal PB cells contain a large amount of sialic acid (21.2 μg/10⁶ cells): it is reported that cellular sialic acid content decreases dramatically with the tumor producing capacities of the cells (3.4 μg/10⁶ P 4 BIS cells; 2.1 μg/10⁶ P 4 BIS T cells).It has been found in conditions which maintain cell viability that transformed neuraminidase treated cells or trypsin treated cells liberate large percentages of sialic acid, or sialoglycoproteins, whereas small percentages are liberated from normal cells, indicating that transformed cell surface glycoproteins may be reached more easy by enzymes that normal cells: in that aspect low cancer cells (P 4 BIS) appear transitory between normal (PB) and high cancer cells (P 4 BIS T) in the same way they are transitory in tumor producing capacities.     

31P NMR spectra of rodent and avian fibroblasts transformed by Rous or Kirsten sarcoma viruses

31P NMR spectra of normal rodent and avian fibroblasts were compared to those of the same cells transformed either by the Rous sarcoma virus (RSV) or by the Kirsten sarcoma virus (Ki-MSV). Under physiological conditions, the spectra of living or perchloric acid extracted chicken embryo fibroblasts, rat cell line FR3T3 and mouse cell line C127 did not differ from those of their counterparts transformed by RSV or Ki-MSV. However, in the case of FR3T3 cells, on shifting from 37 degrees C to 20 degrees C, and particularly if PBS replaced serum growth medium, a different, though transitory, response of the transformed cells was detected. They then showed, within few minutes, a more rapid ATP depletion with accumulation of fructose 1,6-diphosphate (FDP), as compared to normal control cells.     

Membrane lipid dynamics and density dependent growth control in normal and transformed avian cells

Membrane fluidity of normal chick embryo fibroblasts and normal Japanese quail fibroblasts and their Rous sarcoma virus and methylcholanthrene transformed counterparts was investigated using the technique of fluorescence depolarisation of 1,6-diphenylhexatriene incorporated in the whole cells and in their isolated plasma membrane vesicles. Normal cells and isolated plasma membranes of normal cells showed significant changes in fluidity as a function of population density while neither Rous sarcoma virus transformed nor methylcholanthrene tumor cells or their isolated plasma membrane showed this effect. Stimulation of growth by addition of calf serum to cultures of quiescent, density-inhibited normal cells was accompanied by rapid changes in the direction of increased membrane lipid fluidity. Neither sparse normal cells, nor sparse or dense transformed cells showed any significant fluidity change in their membrane lipids upon addition of serum. Enzyme and electron microscopic analysis of the ratios of different membrane types in each cell type showed that this ratio was invariant with respect to cell population density but different between transformed and normal cells. Hence, the fluidity changes observed, measured as the mean rotational correlation time of the fluorescene probe in the membrane lipids, truly reflect organisational differences, occurring as a function of population density in cultures of cells which retain density-dependent growth control.     

Selective inhibition of cell growth and associated changes in glycolipid metabolism induced by monovalent antibodies to glycolipids

Monovalent antibodies directed against N-acetylhematoside are growth inhibitory for BALB/3T3 and NIL hamster fibroblasts but not their transformed counterparts. Within a similar dose range antibodies directed against globoside have no effect on cell growth. Inhibition of 3T3 cell growth by anti-hematoside correlates with a specific change in the metabolism of hematoside within the cell membrane. Following antibody treatment the radiolabeling of hematoside is elevated for cell in logarithmic growth but reduced relative to control at final saturation density. This effect is not observed for 3T3 cells transformed by Kirsten murine sarcoma virus. It is suggested that cell surface glycolipids may play a role in the control of normal cell growth in vitro.     

Effect of proteolytic inhibitors on growth and surface architecture of normal and transformed cells

Protease inhibitors were tested for their effect on the growth of normal and SV40-transformed mouse fibroblasts. The protease inhibitors TAME¹ and EWTI¹, which act competitively on proteases, reduce the growth of transformed cells more than that of untransformed parent cells. However, transformed cells grown in medium containing these drugs do not show contact inhibition of cell division or decreased agglutinability with Concanavalin A. The inhibition of growth is due to an extended duration of all phases of the cell cycle. The protease inhibitor TLCK¹, an active site titrant reacting irreversibly with trypsin, blocks transformed cells in the premitotic stage of the cell cycle. This effect does not occur in the untransformed parent cells. The decrease in agglutinability of transformed cells treated with TLCK is correlated with a partial synchronisation in the G2 stage of the cell cycle. Our results do not support the hypothesis that protease inhibitors induce transformed cells to assume a normal growth pattern and that this is accompanied by a decreased agglutinability with plant lectins.     

Glucose-regulated membrane properties of untransformed and virus-transformed BHK21 cells.

BHK21 fibroblasts transformed by hamster sarcoma virus have a higher rate of uptake of hexoses than their untransformed counterparts, and therefore rapidly exhaust glucose from the culture medium. The effects of culturing normal and transformed BHK cells, both in limiting and in excess glucose, on several membrane properties related to malignant transformation have been studied. The increase in the rate of hexose uptake in transformed cells is partially but not entirely dependent on extracellular glucose concentration. Two transformation-increased membrane proteins of molecular weights 95 000 and 78 000 are shown to be regulated by extracellular glucose concentration in both normal and transformed cells. The loss of LETS-protein, the high density of intramembranous particles, the increase in the amount of a 177K integral plasma membrane protein and the increase in the amount of high molecular weight surface glycopeptides in transformed cells, are not related to glucose depletion of the medium. Beside LETS, another iodinated protein, of molecular weight 160 000, is decreased in transformed cells. The exposure of this protein increased in both normal and transformed cells when arrested in G1 by asparagine deprivation.     

Non-lytic, non-ionic detergent extractions of plasma membrane constituents from normal and transformed fibroblasts

A technique has been developed for the selective extraction of plasma membrane protein constituents from normal and transformed cells employing non-ionic detergents. The extraction procedure does not damage cells as judged by cell viability, ⁵¹Cr release, and trypan blue staining. Lactoperoxidase-catalyzed iodina- tion followed by detergent extraction permits demonstration of a 100 000 dalton protein which is found on the surface of normal but not transformed hamster and mouse fibroblasts.     

Correlation of (Na---K)-ATPase activity with growth of normal and transformed cells

The (Na⁺---K⁺)-stimulated Mg²⁺-dependent ATPase activities of 3T3 and SV40 transformed 3T3 cells were compared as a function of cell population density. For normal cells the enzyme activity remained relatively constant during exponential growth, but decreased sharply coincident with contact inhibition of growth at confluence. This decrease in activity could be reversed by stimulating contact-inhibited cultures to undertake renewed short-term growth either by adding fetal calf serum or changing the medium completely. Transformed cells did not experience a decrease in (Na⁺---K⁺)-ATPase activity upon reaching confluence, but this is consistent with the fact that they were still growing exponentially at this stage. However, non-confluent cultures of both normal and transformed cells incurred a marked decrease in levels of the enzyme when growth was inhibited by serum depletion. The results have been interpreted as indicating that levels of (Na⁺---K⁺)-ATPase in both normal and transformed cells are correlated with growth. Hence the different patterns of ATPase activity displayed by malignant cells and their normal counterparts with increase in cell number appear to be a reflection of their dissimilar growth behaviours rather than of any innate difference between them.