METABOLIC CONTROLS IN CULTURED MAMMALIAN CELLS

A unifying hypothesis concerning the nature of malignant growth

It is suggested that the crucial change in a malignant cell is an alteration in the cell surface membrane that results in increased internal concentrations of nutrients that regulate cell growth.

Evidence for reversible inactivation of induced tyrosine aminotransferase in rat liver in vivo

Induction of tyrosine aminotransferase (EC 2.6.1.5) with cortisol is followed by a rapid phase of inactivation of the enzyme, during which administration of puromycin causes a sharp increase in enzyme activity. The increase is not due to puromycin-resistant synthesis of protein, since precipitation of the enzyme with antibodies to tyrosine aminotransferase after pulse labeling inhibited incorporation of radioactive amino acids. Reactivation of the enzyme was specific, since the structurally similar dimethylaminopurine and purine aminonucleoside were ineffective. An action of puromycin other than its capacity to inhibit protein synthesis was required for reactivation, since cycloheximide did not demonstrate such an effect. In vitro studies with free and bound polysomes, isolated during the phase of inactivation, indicated that reactivation by puromycin was not due to release of nascent tyrosine aminotransferase peptides. We conclude that the enzyme is initially reversibly inactivated; therefore, the overall degradation of this intracellular protein is a multistep phenomenon.

Cell culture as a test system for toxicity

Appropriate cell culture systems provide a useful additional method of screening for toxicity, in spite of the obvious problems of relating in vitro effects of test compounds at the cellular level to their effects in the whole animal. It is shown that a wide range of chemically disimilar molecules have a reversible inhibitory activity on the growth of primary cultures of monkey kidney cells. The potency of these compounds correlates with their lipid solubility, suggesting that the cell membranes may be their main site of action. Support for this is obtained by the correlation of inhibitory activity and the ability of the same compounds to stabilize the erythrocyte membrane against hypotonic haemolysis which is known to be a direct effect of interaction with the erythrocyte membrane. It is suggested that the ability of the food additive butylated hydroxytoluene to act as a potent inhibitor of cellular growth may account for its reported ability to prolong the life span of mice.

The effect of alterations in external environment on the dentogingival junction

The effectiveness of the dentogingival junction as a barrier between the oral environment and the periodontal connective tissues was studied in squirrel monkeys. Results indicated that altering the concentration of hyaluronidase and collagenase within the dental plaque can produce a detrimental effect on the integrity of this barrier.

Local stimulation of growth in primary cultures of chick embryo fibroblasts

Confluent, slowly growing primary cultures of chick embryo fibroblasts were stimulated to grow more rapidly by making discontinuities in the monolayers by a "wounding" procedure. Growth was measured by mitotic index and by the fraction of cells synthesizing DNA as determined by autoradiography. Faster growth was confined to free cells at the edges of the discontinuities. Cells enclosed on all sides were not stimulated to renewed DNA synthesis, even when within 0.2 mm of the discontinuities. The time of appearance of increased rate of growth induced by wounding was measured and found to be similar to the time of a transient growth stimulation among enclosed cells induced by a change of medium. The increased rate of growth among free cells was not transient, however, and persisted as long as it was examined. To rule out possible artifacts incurred in wounding, a free edge was created by seeding trypsinized cells as a confluent sheet in a confined area of the tissue culture plate. Here also the growth rate of cells along the edge was much higher than in the confluent areas. It was concluded that interactions at short range between cells in culture can play an important role in regulating their growth.

Retention of differentiation potentialities during prolonged cultivation of myogenic cells

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Incorporation of [5-3H]uridine and attachment of cells to glass during activation of lymphocytes induced by phytohaemagglutinin

1. Shortly after the addition of phytohaemagglutinin to cultures, partially purified pig blood lymphocytes showed increased labelling of RNA by [5-(3)H]uridine and began to attach to the glass of the culture tubes. 2. In the presence of phytohaemagglutinin most cells became attached to glass during the first 5hr. of culture; those first attaching had a low mean RNA/DNA ratio. In the absence of phytohaemagglutinin cell attachment to glass was diminished. 3. During the first 5hr. of culture the rates of increase of radioactivity/unit of DNA of the attached and unattached populations in the presence of phytohaemagglutinin were of the same order and exceeded that of cultures without phytohaemagglutinin. 4. The increase of labelling in cultures to which phytohaemagglutinin was added after cells had sedimented to glass was greater than that occurring in cultures in which cells were resuspended before phytohaemagglutinin was added.

Quantitiative nucleic acid changes during phytohaemagglutinin-induced lymphocyte transformation in vitro. Dependence of the response on phytohaemagglutinin-serum rati

1. Pig and human blood lymphocytes have been grown in culture without replenishment of medium, and stimulated to transform by phytohaemagglutinin. Quantitative nucleic acid changes during this process have been used as an index of transformation. 2. On the first day, cells attach to glass; then they detach and continue transforming. 3. The degree of transformation is dependent on the phytohaemagglutinin/serum ratio, and is independent of cell concentration within the range 0.5x10(6)-2.0x10(6) cells/ml. 4. At low phytohaemagglutinin/serum ratios there is no response; at high phytohaemagglutinin/serum ratios, inhibition appears after the cells have been cultured for a day.