CONTACT INHIBITION, MACROMOLECULAR SYNTHESIS, AND POLYRIBOSOMES IN CULTURED HUMAN DIPLOID FIBROBLASTS

pH as a determinant of cellular growth and contact inhibition

1) Both the growth rate and the maximum population density of several normal, virus-transformed, and cancer cells were markedly pH-dependent; the optimum varied from pH 6.9 to 7.8. At the optimum pH, some diploid human cells attained population densities comparable to those of cancer or virus-transformed cells. Contact inhibition of growth is facilitated by repeated fluctuations of pH in nonphysiological ranges, and may not be an intrinsic and necessary attribute of diploid cells in culture. 2) At pH 8.3, at which there was little or no cellular multiplication, the protein content per cell increased 2- to 5-fold over a period of 10-16 days, and was slowly reversed to normal concentrations on restoration of pH to the optimal range. 3) Uridine uptake by contact-inhibited human cell cultures was stimulated by refeeding with salt solution, and to the same extent as by complete (serum-supplemented) growth medium; that immediate increase did not involve the reinitiation of cellular growth and multiplication. Contact inhibition was, however, reversed in 2-4 days by an appropriate increase in the serum concentration of the medium.

Tyrosine transaminase induction by dexamethasone in a new rat liver cell line

A cell line derived from normal, adult rat liver has been established; the cells are similar to hepatocytes, as shown by electron microscopy. The addition of dexamethasone to the culture medium induced a three- to sixfold increase in the specific activity of tyrosine alpha-ketoglutarate transaminase; this increase was inhibited by the simultaneous addition of cycloheximide or actinomycin D. The latter, when added to cells given prior treatment with dexamethasone, further enhanced the transaminase activity. Contact-inhibited cells showed a lower response to dexamethasone than exponentially growing cells.

New growth and viruses

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Morphologic changes accompanying senescence of cultured human diploid cells

The lysosomes of serially propagated human fibroblasts gradually transform to residual bodies which increase in number and size, and show progressive degenerative changes. There is an accompanying, and less regular, decrease in the number of cytoplasmic polyribosomes and an increased number of glycogen particles. The onset of these morphologic alterations occurs shortly after culture initiation and precedes any marked decrease in the rate of cellular growth; however, in their extreme form these changes may be related to the ultimate cessation of cellular multiplication ("senescence"). The lysosomal changes were seen only in those cell strains which eventually showed senescence, and were absent or minimal either in cell lines which can be propagated indefinitely ("spontaneous" and viral transformants, cancer cells), or in skin sections from aging subjects.

Transport changes rapidly initiated by serum addition to "contact inhibited" 3T3 cells

Two- to fourfold increases in uridine and phosphate uptake were brought about within 10 to 15 minutes after adding fresh serum to confluent 3T3 cells. The stimulation of transport was specific, since no serum effect was observed for 3-O-methyl-D-glucose or amino acids. The early increase in RNA labeling, previously identified by (14)C-uridine incorporation, could be accounted for by this transport effect. Increased labeling with (32)P-phosphate of at least five phospholipids occurred soon after adding serum. This increase, however, could not be accounted for by increased transport. Both of these results suggest that specific early membrane changes are involved in "contact inhibition."The following results are consistent with this suggestion. Uptake of uridine and phosphate by nonconfluent 3T3 cells was higher than by confluent cells and was only slightly increased by fresh serum. In contrast, uptake of these substrates by Polyoma virus-transformed 3T3 cells, which are not subject to "contact inhibition," was not significantly decreased after the cells became confluent, and serum addition had no effect on transport by either nonconfluent or confluent Polyoma virus-transformed 3T3 cells. Fractionation of serum on Sephadex G-200 demonstrated that the factor which stimulated phosphate transport was different from the previously identified factor which stimulates DNA synthesis by confluent 3T3 cells.

Interferon production by human leukocytes in vitro: some biological characteristics

Interferon was optimally produced in human peripheral leukocyte cultures incubated for approximately 19 hr in the presence of Sendai virus at a multiplicity of 10 to 50 EID(50)/cell. For determining whether deoxyribonucleic acid (DNA) synthesis per se was essential for interferon production, 1-beta-D-arabinofuranosylcytosine (Ara-C), a potent DNA inhibitor was studied for its effect on interferon production in leukocytic and bone marrow cell cultures. These cells showed no impaired capacity to produce interferon when treated with 15 mug of Ara-C per ml. Interferon yields were also determined in leukocyte cultures treated with actinomycin D (0.1 mug/ml) and puromycin (10 mug/ml) at various times before and after virus inoculation. The data suggested that sequential transcriptive and translational events were required for the de novo synthesis of interferon by the infected leukocytes, in a manner similar to other known virus-induced interferon-producing systems. The synthesis of macromolecules and the effects of antimetabolites in leukocytes and bone marrow cell cultures were followed by measuring the incorporation of thymidine-2-(14)C, uridine-5-(3)H, and L-phenylalanine-1-(14)C. The effect of 0.1 mug of actinomycin per ml on the capacity of leukemic leukocytes to produce interferon was also studied. Preliminary data showed that, in contrast to nonleukemic leukocytes, interferon production by leukemic leukocytes was only partially inhibited by actinomycin.

The loss of phenotypic traits by differentiated cells. VI. Behavior of the progeny of a single chondrocyte

A single, functional, mitotically quiescent chondrocyte may be induced to reenter the mitotic cyde, and produce a progeny of over 10(11) cells. Sessile, adherent, polygonal cells deposit matrix, whereas amoeboid, dispersed, flattened fibroblastic cells do not. The prior synthetic history of a cell is of greater importance in determining whether the characteristic chondrogenic phenotype will be expressed, rather than growth in "permissive" or "nonpermissive" medium. Clonal conditions select for stem-like cells, some of whose progeny may become polygonal chondrocytes. The retention of the characteristic chondrogenic phenotype in vitro is favored by pruning the dedifferentiated chondrocytes which arise in these cultures. Dedifferentiated chondrocytes interfere with the deposition and synthesis of chondroitin sulfate by neighboring functional chondrocytes. Possible mechanisms are proposed to explain this type of cell-cell or cell exudate interference. If the progeny of a single, genetically programmed chondrocyte may or may not synthesize chondroitin sulfate, then extragenic sites in the cytoplasm or cell surface must influence the decision as to which cluster of "luxur" molecules the cell will synthesize.

Influence of the rate of cell growth and cell density on interferon action in chick embryo cells

Chick embryo cells became more sensitive to the action of interferon the longer they remained in culture. This phenomenon was found even before confluency had been reached. The relative insensitivity of newly seeded cells was not due to a loss of receptors. Cells synthesizing deoxyribonucleic acid (DNA) at a high rate were less sensitive to interferon action than cells synthesizing DNA at a low rate, but the inhibition of DNA synthesis had no effect on interferon action. An increase in the number of cells used for seeding resulted in an earlier appearance of increased sensitivity to interferon action. These results are discussed in relation to the induction process in animal cells.

Isolation of plasma membrane fragments from HeLa cells

A method for isolating plasma membrane fragments from HeLa cells is described. The procedure starts with the preparation of cell membrane "ghosts," obtained by gentle rupture of hypotonically swollen cells, evacuation of most of the cell contents by repeated washing, and isolation of the ghosts on a discontinuous sucrose density gradient. The ghosts are then treated by minimal sonication (5 sec) at pH 8.6, which causes the ghost membranes to pinch off into small vesicles but leaves any remaining larger intracellular particulates intact and separable by differential centrifugation. The ghost membrane vesicles are then subjected to isopycnic centrifugation on a 20-50% w/w continuous sucrose gradient in tris-magnesium buffer, pH 8.6. A band of morphologically homogeneous smooth vesicles, derived principally from plasma membrane, is recovered at 30-33% (peak density = 1.137). The plasma membrane fraction contained a Na-K-activated ATPase activity of 1.5 micromole Pi/hr per mg, 3% RNA, and 13.8% of the NADH-cytochrome c reductase activity of a heavier fraction from the same gradient which contained mitochondria and rough endoplasmic vesicles. The plasma membranes of viable HeLa cells were marked with (125)I-labeled horse antibody and followed through the isolation procedure. The specific antibody binding of the plasma membrane vesicle fraction was increased 49-fold over that of the original whole cells.

Persistence of messenger RNA through mitosis in HeLa cells

The decrease in protein synthesis which occurs in mammalian cells during cell division is associated with significant disaggregation of polyribosomes. For determining whether messenger RNA survives this disaggregation, the reformation of polyribosomes was investigated in synchronized HeLa cells as they progressed from metaphase into interphase in the presence of 2 microg/ml Actinomycin D. The persistence of messenger during cell division was evidenced by: (1) a progressive increase in the rate of protein synthesis in both treated and untreated cells for 45 min after metaphase; (2) reformation of polyribosomes, as determined by both sucrose gradients and electron microscopy, within 30 min after the addition of Actinomycin D to metaphase cells; (3) the persistence of approximately 50% of the rapidly labeled nonribosomal RNA which had associated with polyribosomes just before metaphase; (4) the resumption of synthesis, following cell division, of 6 selected peptides in Actinomycin-treated cells.

A diffusible factor which sustains contact inhibition of replication

Confluent 3T3 cultures make and release into the medium a diffusible factor which sustains the state of contact inhibition of replication. Evidence is given that the factor is a specific and reversible inhibitor of the RNA synthesis which precedes cellular replication.

Membranes of animal cells. II. The metabolism and turnover of the surface membrane

Turnover studies of the surface membrane and of cell particulate matter of L cells in tissue culture in logarithmic and plateau phase of growth have been made. The rate of incorporation of isotope into these fractions and the rate of fall of specific activities of labeled L-cell fractions have been observed. The following interpretation of the data appears most likely although other interpretations are possible. Growing and nongrowing cells synthesize approximately similar amounts of surface membrane and particulate material. In the growing cell the material is incorporated with net increases in substance. There is relatively little turnover. In the nongrowing cell newly synthesized material is incorporated, but a corresponding amount of material is eliminated so that there is turnover without net increase of substance. Our results suggest that there is no gross differential turnover between the protein, lipid, and carbohydrate of the surface membrane under the conditions of our experiments. Metabolic inhibitors or omission of amino acids in the culture medium lead to a decrease in synthesis of surface membrane and cell particulates and cause an equivalent decrease in the rate of degradation of surface membrane and of particulates; therefore the synthetic and degradative aspects of turnover appear to be coupled. As cultures of nongrowing cells in suspension or on a glass surface age, their synthetic and turnover capacities diminish. Our results suggest that the cell may exist in a nongrowing state with a level of synthesis similar to that of a growing cell. It can exist in this state with a high level of turnover.

Phenotypic expression of transformation: induction in cell culture by a phorbol ester

A biologically potent, tumor-promoting phorbol ester from Croton tiglium L. induces the appearance of transformed clones in a population of contact-inhibited fibroblasts of the mouse cell line 3T3. Mixed populations of cells with 10,000 3T3 cells and 100 virus-transformed cells were exposed to the phorbol ester and, in comparison with untreated cells, showed a marked increase in the numbers of transformed clones that grew. Exposure of 3T3 cultures to the carcinogen 7,12-dimethylbenz(a)anthracene alone or prior to exposure to the phorbol ester did not cause any increase in the number of transformed clones.

Human fibroblasts infected with rubella virus produce a growth inhibitor

A protein that inhibited mitosis of normal human diploid cells was demonstrated in extracts of WI-38 cells that were infected with rubella virus and that had gone into mitotic arrest subsequent to infection. A possible mechanism for the pathogenesis of the rubella syndrome is suggested.

Reversible degradation of polyribosomes in Chang cells cultured in a glutamine-deficient medium

The effects of temporary glutamine deficiency on the protein and nucleic acid metabolism of Chang's liver cells in suspension cultures have been studied. It was observed that cells maintained in a glutamine-free medium showed a reduced incorporation of labeled precursors into protein and RNA. At the same time, the activity of the ribosomes and the proportion of polyribosomal aggregates in cell extracts diminished. These effects were reversed when the glutamine content of the medium was restored. The restoration of a normal rate of amino acid incorporation by intact cells as well as by cell-free systems was time dependent, and took place within a few hours after glutamine addition without preceding increase in the prevailing low rate of RNA synthesis. The addition of actinomycin D at concentrations that strongly inhibited the RNA metabolism of the cells did not prevent the increase in protein synthesis or the reappearance of polyribosomal aggregates. These facts suggest that the restoration of protein synthesis in the cells after glutamine starvation was not dependent on a production of new messenger RNA. The experimental data are consistent with the hypothesis that previously synthesized messenger RNA, preserved in the cells in a stable form, was brought into action in response to the reestablishment of an adequate cellular environment.

Studies of cellular proliferation in human leukemia. I. Estimation of growth rates of leukemic and normal hematopoietic cells in two adults with acute leukemia given single injections of tritiated thymidine

Two adults with rapidly progressive acute myeloblastic and myelomonoblastic leukemia were given single injections of tritiated thymidine, and measurements were made of the growth rates of their leukemic and normal hematopoietic cells by radioautographic methods. Although almost all leukemic blasts in both marrow and blood were metabolically active as shown by their ability to incorporate tritiated uridine and leucine in vitro, only 5.6% and 6.1% of the blasts in the marrow and even fewer in the blood incorporated tritiated thymidine. The mitotic indexes of the marrow blasts were 0.66% and 0.52%; no circulating blasts were dividing. The mean generation times of the actively proliferating blasts were estimated to be 49 and 83 hours. This cannot be equated with the doubling time of the total leukemic population as there is evidence that many blasts fail to continue dividing and die. The mean durations of the phases of the blasts' mitotic cycles were as follows: DNA synthesis (S) = 22 and 19 hours, premitosis (G(2)) = 3 hours, mitosis (M) = 0.47 and 0.62 hour (minimal estimates), and postmitosis (G(1)) = 24 and 61 hours. In both patients the maximal mean transit time of the blasts in the blood was 36 hours, and the minimal numbers of actively dividing blasts present were 1.6 and 2.6 x 10(9) per kg of body weight.Estimates were also made of the rates of proliferation and maturation of the residual normal erythrocytic and granulocytic cells in these two patients. Although total production was markedly diminished because of reduction in the number of normal elements, the relatively few remaining normal cells appeared to be dividing and maturing at rates that are about the same or only slightly slower than those found in normal subjects. We conclude that main reason leukemic blasts displace normal hematopoietic precursors in acute leukemia is that the blasts largely fail to differentiate. Many die but many others persist in the marrow and elsewhere as primitive cells and continue to proliferate. As the blasts accumulate, they gradually displace the normal hematopoietic cells, most of which continue their normal course of differentiation and leave the marrow as nondividing mature cells. It is not known why the over-all production of normal cells is not adequately increased to compensate for the anemia, granulocytopenia, and thrombocytopenia that develop, but apparently the leukemic cells somehow interfere with the proliferation or differentiation or both of normal stem cells.