Serum-stimulated release of cell contacts and the initiation of growth in contact-inhibited chick fibroblasts

Correlation between contractility and proliferation in human fibroblasts

The contractile power of human fibroblasts was checked through their life span in vitro, using a plasma clot retraction test. It was found to decline with a pattern analogous to that of the different phases identifiable by the study of the kinetics of proliferation of these cells. The capacity to retract a plasma clot was higher in cells harvested during active growth than in cells harvested in resting phase. The decreased ability to retract during aging becomes apparent when cells are harvested in resting phase. Decreased retractile activity was also observed in postnatal cells as compared with embryonic cells. The results support a correlation between the initiation of DNA synthesis and the turnover of cytoskeletal elements. The data fit our previous results showing that the early proliferative disturbance during cellular senescence consists of a decline in the probability of initiating the division cycle linked to impaired cell attachment and spreading.

Scanning microfluorometric measurement of immunofluorescently labelled microtubules in cultured cells. Dependence of microtubule content on cell density

A method for evaluation of microtubule content in cultured cells has been developed. The method is based on scanning microfluorometric measurement of immunofluorescently labelled microtubules. The method has been applied to the comparison of microtubule content in epithelial XTH-2 cells grown in culture at various cell densities. The results have shown that the microtubule content in the cells is not dependent on their proliferative state rather than it depends on cellular contacts.

Effects of cytoskeletal disrupting agents on replication of bovine endothelium

Colchicine and vinblastine inhibited endothelial cell migration but had no effect on the stimulation of replication seen at wound edges in cultures of endothelium at stationary density. This is in contrast to the effects of cytochalasins which inhibit both migration and replication at wound edges. Moreover, colchicine and vinblastine stimulated cell replication in the unwounded, confluent monolayer. This effect has kinetics similar to the stimulation of replication at a wound edge and is associated with an initial retraction of cell borders, leaving gaps between cells. Cytochalasin D inhibited the growth response to microtubule disrupting agents but did not prevent cell retraction. Stimulation of replication by microtubule disrupting agents was not dependent on serum but was synergistic with serum in cultures rinsed repeatedly with serum-free medium. The replication occurred prior to any cell loss. When, however, cells were allowed to complete mitosis, about one-half of the daughter cells detached from the monolayer so that there was no increase in cell density. We conclude that microtubule disrupting agents are the first agents found to be effective in stimulating growth of vascular endothelium at saturation density. These data further suggest that colchicine and vinblastine stimulate cell growth in a manner similar to wounding, where cell movement is a prerequisite to cell replication.

Vascular wall growth control: the role of the endothelium

The current state of our knowledge of the control of endothelial growth and the role of endothelial injury in the pathogenesis of atherosclerosis can be summarized as follows: 1. Endothelial cells can be grown in plasma-derived serum in the absence of exogenous growth factors. This is quite different from the growth requirements of most other nontransformed cells. These factors may, however, prolong replicative life span and increase the ability of endothelium to grow at sparse density. The relevance of these phenomena to the control of endothelial growth in vivo is unclear. There is no evidence that exogenous growth factors are required for wound edge regeneration. In view of the relative lack of growth factor requirements, it is intriguing to consider the possibility that the critical control factor for endothelial cell growth is cell contact. 2. Endothelial cell regeneration may be dependent on endothelial cell motility. The nature of this relationship may be important in controlling the ability of the endothelium to regenerate itself under different flow conditions around lesions or in different parts of the vessel tree and in determining the ability of the endothelium to respond to changes in the connective tissue overlying lesions. 3. Endothelial cells in vivo are able to regenerate small areas of denudation extremely rapidly. This process may be sufficiently rapid to permit the endothelium to replace dying cells as they are being lost, resulting in desquamation without denudation. 4. We have little evidence for endothelial denudation either spontaneously or in response to atherosclerosis risk factors until after lesion formation has begun. This does not rule out the possibility that small, repeated, transient episodes of denudation occur and play a role in the initiation of atherosclerotic lesions. It is important, however, to begin considering the role of nondenuding injuries in atherosclerosis. 5. The fact that thrombosis occurs in atherosclerosis implies an eventual breakdown of endothelial integrity. The mechanism of that breakdown remains unknown. 6. Finally, there is the question of interactions between smooth muscle cells and endothelial cells at the level of growth control. This includes the evidence that there is a critical amount of endothelium that must be lost before lesion formation is stimulated and the recent evidence that endothelial cells produce substances able to regulate growth of smooth muscle cells.

Serum dependence of expression of the transformed phenotype: experiments with subline of mouse L fibroblasts adapted to growth in serum-free medium

Characteristics of LSF cells grown in serum-containing and serum-free medium were compared. LSF is a subline of the L-strain of mouse transformed fibroblasts adapted to continuous growth is serum-free medium. Proliferation of LSF cells in monolayer on solid substratum was almost similar in serum-containing and in serum-free media. However, several other characters were found to be altered by the addition of serum to the serum-free medium: ability of cells to form colonies in semi-solid medium increased considerably; agglutinability of cells by Concanavalin A increased; uptake of 2-deoxy-D-glucose by the cells increased considerably; ability of cells to metabolize benzo (a) pyrene was inhibited; cell morphology was altered and, in particular, the cells became less spread on the substratum and density of microvilli on the cell surface increased. All these changes induced by serum were reversed by transfer of the cells back into serum-free medium. Thus, addition of serum increased the expression of a number of cellular traits characteristic of transformed phenotype, while in serum-free medium partial phenotypic reversion of transformation was observed. A possible role of serum in the expression of the transformed phenotype is discussed. It is pointed out that cell lines adapted to growth in serum-free medium provide an experimental system convenient for analysis of the effects of different serum components on the cell phenotype.

The pattern of density dependent growth inhibition in murine fibroblasts

Observations on the pattern of nuclear incorporation of 3H-TdR in long term (8-day) and short term (3-day) 3T3 cultures with local cell densities between 0.2 times 10-4 and 6.2 times 10-4 cells/cm2 are reported. Contrary to a number of previous studies our observations indicate that density dependent inhibition is exhibited in relatively sparse cultures, commencing at 0.5 times 10-4 cells/cm2. Various possible mechanisms which could have caused the observed pattern of density-dependent regression in labelling index are discussed.

A diffusible factor restoring contact inhibition of growth to malignant melanocytes

Contact inhibition of growth is the property in vitro whose loss is most closely correlated with tumorigenicity in vivo. A contact-inhibited melanocytic cell line produces a diffusible protein-containing factor capable of restoring contact inhibition of cell division to highly malignant hamster melanocytes. Its addition to subconfluent cultures of the malignant cells is followed by the obligatory acquisition, at confluence, of the contact-inhibited state. Cultures are flat, oriented, and fibroblast-like, and show a 55% decrease in saturation density with no loss of viability. The effect is reversible. Present in conditioned media of cultures of the contact-inhibited melanocytic cell line, isolated by column chromatography on Sephadex G-200, the factor appears to be of high molecular weight. Activity is preserved in aqueous solutions at 4 degrees for at least 8 weeks, but is destroyed by repeated freezethawing or by treatment with Pronase. This newly recognized contact-inhibitory factor may be a prototype for a more general and fundamental mechanism for regulation of normal cell-cell interactions. It is the first cell-elaborated factor to be isolated that is capable of restoring the capacity for contact inhibition of growth to malignant cells.

Contact-inhibited revertant cell lines isolated from SV 40-transformed cells. IV. Microfilament distribution and cell shape in untransformed, transformed, and revertant Balb-c 3T3 cells

A COMPARISON IS MADE OF THE ULTRASTRUCTURE OF THE CELL PERIPHERY IN THREE CLONED CELL LINES: untransformed Balb/c 3T3 cells, SV40-transformed Balb/c 3T3 cells, and revertant cells obtained from the transformed cell line by a selection technique utilizing concanavalin A. Both thin-section and surface replication techniques are used for in situ examination of the cell lines. Microfilaments, 70 A in diameter (called alpha filaments), are abundant in untransformed and revertant cell lines, particularly in the anterior expansions of the cells, which tend to have many microvilli and small pseudopodia. Alpha filaments are diminished in the anterior expansions of transformed cells, which contain large blunt pseudopodia and relatively few microvilli. Surface replicas confirm the impression gained from thin sections that transformed cells have a greater proportion of their cell surface involved in bulging pseudopodia than either untransformed or revertant cells. Since alpha filaments are shown to bind heavy meromyosin and are similar to F-actin, these filaments are thought to be important in cell motility. These observations suggest that a close relationship exists between decreased alpha filaments, bulging pseudopodia, and loss of contact inhibition of movement in transformed cells.

Reversible transformation by urea of contact-inhibited fibroblasts

200 mM urea elicits alterations of cell social behavior in vitro. In the presence of urea, contact inhibition of movement and growth is reduced compared to untreated fibroblastic cell populations. This reduction of contact inhibition is rapidly reversible, but reversal requires some cycloheximide-sensitive cellular process. Cells treated with urea are agglutinable by concanavalin A, suggesting that urea might remove some cell-surface component(s). A nondialyzable factor can, in fact, be detected in the supernatant medium of urea-treated cells. This active constituent appears to be heat-labile and trypsin-sensitive, and can itself restore normal contact-inhibitory behavior to cells maintained in the urea-transformed state by cycloheximide. This system may permit identification and characterization of surface components that are involved in regulating normal cell social behavior.

Turnover of molecules which maintain the normal surfaces of contact-inhibited cells

In confluent cultures normal chick embryo fibroblasts become highty agglutinable by concanavalin A within 6 hours after their synthesis of protein is inhibited by cycloheximide, pactamycin, or emetine. When growing cells are similarly treated, they fail to become more agglutinable. Apparently, molecules which maintain the normal structure of the cell surface must be replaced continually when cell growth is contact inhibited.