Sorting out of normal and virus-transformed cells in cellular aggregates

Changes in fibroblast contractility, morphology, and adhesion in response to a phorbol ester tumor promoter

We have studied the effects of the phorbol ester tumor promoter 12-O-tetradecanoyl-phorbol-13-acetate (TPA) on the contractility, locomotion, morphology, and adhesion of two mammalian fibroblastic cell lines. Using the silicone rubber substratum technique, we have found that the first observable response to the tumor promoter is a rapid weakening of cell contractility (8-15 min). This is followed by gradual morphological changes, characterized by a hyperextension of the cells' leading lamellae, which stretch out to an unlimited degree, and occasionally even detach from the cell bodies. Treated cells also become able to crawl onto hydrophobic substrata which are insufficiently adhesive to support the spreading of untreated fibroblasts. We suggest that both the hyperextension and the ability to spread on nonadhesive surfaces can be explained as consequences of the reduced contractility, and that this reduced contractility may also help to explain the increased invasiveness and loss of anchorage dependence by transformed cells.

Molecular mechanisms of cell adhesion in normal and transformed cells

Alterations in the adhesive mechanisms of cancer cells are likely to play an important role in determining the invasive or metastatic potential of these cells. An understanding of these alterations at the molecular level is now within reach, due to recent progress in the identification and characterization of several cell adhesion molecules (CAMs). Two of these molecules, the neural cell adhesion molecule N-CAM and the liver cell adhesion molecule L-CAM, are expressed on a variety of cell types from early embryos and throughout adult life, and appear to play several important roles in early inductive events, formation of specific intercellular connections, and maintenance of adult tissues. Two other molecules, the neuron-glia adhesion molecule Ng-CAM and a molecule involved in the specific adhesion of lymphocytes, appear to be more restricted in their developmental expression and function. The molecular characterization of N-CAM made possible for the first time an examination of the effects of transformation on the expression of a defined cell adhesion molecule. In both established cell lines from rat cerebellum and embryonic chick neuroepithelial cells, transformation by Rous sarcoma virus caused a large reduction in expression of N-CAM. In both cases, the N-CAM-mediated adhesion was correspondingly reduced. The neuroepithelial cells also became more highly motile after transformation. The decrease in N-CAM coupled with this increase in cell motility may significantly enhance the invasiveness of these cells. Other surface antigens have also been identified that may be involved in essential steps of invasion and metastasis. Such studies represent the initial step toward a detailed understanding of the role of CAMs in the various steps of metastasis. The accessibility of CAMs on tumor cell surfaces, and the availability of specific antibodies to these components suggests that reagents may become available in the near future that will offer new opportunities for preventing the formation of metastases.

Experimental systems for analysis of the malignant phenotype

Identification of the cellular and subcellular alterations responsible for the metastatic behavior of malignant tumor cells and development of reliable screening programs for detecting new therapeutic agents for improved treatment of metastatic disease both depend crucially on the availability of experimental systems that can serve as relevant models of human cancer. Recent advances in our understanding of the pathogenesis of cancer metastasis have raised serious doubts about the usefulness of many of the experimental approaches that have long been used in the study of metastasis. Recent findings showing that metastases are caused by specific subpopulations of metastatic tumor cells, and that not all cells in a malignant primary tumor possess metastatic properties, are of profound importance for experimental efforts to understand the mechanism of metastatic phenotype among cells from the same tumor means that the traditional, and widely used, approach of analyzing primary tumors and cultured cell lines containing multiple, phenotypically heterogeneous, subpopulations of cells may provide little or no insight into the properties of the metastatic subpopulations, particularly if they represent only a minor fraction of the entire population. Similarly, the practice of screening potential therapeutic modalities for their ability to reduce the mass and/or growth rate of a primary tumor may be inadequate in predicting the responsiveness of metastatic lesions. Solution of these problems requires that new methods must be devised to isolate and characterize the specific subpopulations of tumor cells endowed with metastatic potential. In addition, knowledge of how the extraordinary phenotypic diversity found in tumor cell subpopulations from the same tumor is generated and how subpopulation diversity is regulated during progressive growth of both the primary tumor and its metastases are of fundamental importance if we are to design meaningful experimental systems for studying the metastatic process. This article reviews our current understanding of these complex issues and their implications for the experimental analysis of the malignant phenotype. The merits and shortcomings of different experimental systems are discussed in detail together with the identification of areas in which new experimental strategies and models are now needed.'

Quantitative analysis of intercellular adhesive specificity in freshly explanted and cultured cells

A new method is presented for the quantitative analysis of intercellular adhesive specificity. In this assay, two cell types are mixed, one unlabeled and the other labeled with the fluorescent dye, fluorescamine [4-phenylspiro(feran-2[3H],1'-phthalan)-3,3'-dione]. The resulting aggregates are analyzed by fluorescence microscopy to determine the number of labeled and unlabeled cells per aggregate. Random (nonspecific) aggregation was characterized by a binomial distribution, and adhesive specificity was accordingly quantified by the deviation (as determined by a chi-square test) from the calculated binomial distribution. The labeling procedure was simple and rapid, and experiments with 18 different cell types showed that it did not affect cell viability, morphology, rate and extent of adhesion, plating efficiency, and the capability of myogenic cells to undergo terminal differentiation. Most important, assays with morphologically identifiable cell pairs indicated that the fluorescent label neither induced apparent nor destroyed existing adhesive specificity. The most pronounced adhesive specificities were observed with freshly explanted cells from adult tissues and also with mixtures of simian virus 40-transformed and nontransformed BALB/c 3T3 cells. A glucosamine-6-phosphate N-acetylase-deficient mutant 3T3 line (AD6), however, aggregated randomly with parental 3T3 cells. Lectin-resistant mutant Chinese hamster ovary (CHO) cells displayed marginal adhesive specificity when mixed with normal CHO cells.

Cell cycle parameters of 3T3 cells cultured as aggregates

BALB/c3T3 cells cultured as aggregates were examined by two independent techniques to determine whether or not cells accumulated at a specific point in the cell cycle, and if so to determine the point at which they accumulate. Replating cells onto dishes followed by pulse labeling with [3H]thymidine and autoradiography indicated that aggregate-cultured cells were in the same phase of the cell cycle as cells cultured as confluent monolayers. Flow microfluorometry confirmed that 75% of the aggregate-maintained cells were arrested in G(0) or G(1), with 25% distributed throughout the rest of the cell cycle. Labeling and mitotic indices of cells in aggregates were also consistent with about 20 to 25% of the cells being in S + G(2) = M phases of the cell cycle at any time.

Human lymphoblastoid cell variants defective in cell-cell adhesion

Adhesion mutants were selected from a human lymphoblastoid cell line. Initially, cells were selected on the basis of survival in serum-free medium. Subclones that grow as single cells rather than macroscopic aggregates were selected from the serum-independent variant. The defect in cell-cell adhesion is stable over many generations and is not corrected by growth in serum or the presence of serum in the culture medium. Analysis of mixed cultures composed of adhesive cells and nonadhesive cells indicates that the two cell types do not interact to form mixed aggregations. Furthermore, those results suggest that the adhesion-deficient phenotype does not result from the production of a transferable inhibitor. In a previous study [Whipple, A.P., Dalvin, M. & Millis, A.J.T. (1978) Exp. Cell Res. 116, 457-461], we found that the growth rate in serum-containing medium is identical for the two classes of cells. This suggests that cell-cell adhesion is not a critical factor in the growth of these cells.

Division of BALB/c mouse 3T3 and simian virus 40-transformed 3T3 cells in cellular aggregates

BALB/c mouse 3T3 cells and 3T3 cells transformed by simian virus 40 (SV40) were cultured as aggregates in agitated liquid medium. When maintained with daily medium changes, 3T3 cells incorporated [(3)H]thymidine into acid-insoluble material at a rate (1/4) that of 3T3 cells in logarithmic-phase flat cultures but 16 times that of the same cells in stationary-phase flat cultures. Similarly, SV40-transformed 3T3 cells in aggregates incorporated [(3)H]thymidine at (1/3) the rate of SV40-3T3 cells in logarithimic-phase flat culture and 14 times that of these cells in stationary-phase flat culture. Autoradiographs of aggregates of 3T3 and SV40-3T3 cells incubated for 2 hr in the presence of [(3)H]uridine and [(3)H]thymidine indicated that penetration of the nucleosides into aggregates during this period was limited to the outer four to six cell layers. Aggregates were also incubated in the presence of radiolabeled thymidine and uridine continuously for 4 days. Under these conditions, where nucleoside penetration was not limiting, 100% of the SV40-3T3 cells and 56% of the 3T3 cells incorporated [(3)H]thymidine into acid-insoluble material. The rates of cell division, cell loss, and net cell accumulation in aggregates of 3T3 and SV40-3T3 cells were measured by techniques not influenced by possible alterations in transport, pool size, or penetration. SV40-3T3 cells divided with a doubling time for the total population of 26.0 hr. The total cell number increased more slowly (doubling time of 48.3 hr) because of cell loss, which occurred with a half-time (time for 50% of the cells to be lost) of 53.3 hr. 3T3 cells in aggregates began to divide only after 3 days, then did so with a doubling time for the total population of 76.4 hr. Total cell number decreased (half-time of 26.3 hr) because this rate of cell division was exceeded by the rate of cell loss, which was constant with a half-time of 22.9 hr. The results suggest that 3T3 and SV40-3T3 cells display growth properties in aggregates consistent with their previously reported behavior in conventional flat culture: SV40-transformed 3T3 cells can proliferate under conditions of high cell density to a much greater extent than 3T3 cells can. Both cell lines, however, display an increased capacity to divide in aggregates relative to confluent flat culture, despite conditions of high cell density and absence of anchorage to an artificial solid substrate.

Dibutyryl cyclic AMP treatment of 3T3 and SV40 virus-transformed 3T3 cells in aggregates. Effects on mobility and cell contact ultrastructure

The random cell movement of BALB/c 3T3 and SV40 virus-transformed BALB/c 3T3 cells within homogeneous aggregates was studied by observing the degree of penetration of newly attached [3H]thymidine-labeled cells into the interior of the aggregates. The 3T3 cells penetrated into 3T3 aggregates an average of 0.89 cell diameter in 1.5 days, whereas the SV40-3T3 cells penetrated into SV40-3T3 aggregates an average of 3.20 cell diameters in the same time. Treatment of the aggregates with theophylline, theophylline plus prostaglandin E1, or theophylline plus dibutyryl cyclic AMP all decreased the penetration of the SV40-3T3 cells into SV40-3T3 aggregates (2.36, 1.22, and 0.79 cell diameters, respectively). The same treatments had little effect on 3T3 aggregates. The ultrastructure of 3T3 and SV40-3T3 cells in aggregates was examined by transmission electron microscopy. The 3T3 cells in aggregates were surrounded by microvilli and lamellipodia which were in contact with neighboring cells, whereas SV40-3T3 cells were nearly devoid of microvilli and lamellipodia and made contact at broader, less regular surface undulations. Treatment with theophylline plus dibutyryl cyclic AMP resulted in the appearance of microvilli on SV40-3T3 cells and also appeared to increase the area of intercellular contacts in both 3T3 and SV40-3T3 cells. These observations were supported for the surface cells of the aggregates by scanning electron microscopy.

Decreased adherence to the substrate in Rous sarcoma virus-transformed chicken embryo fibroblasts

Cell-substrate adherence in cultures of chicken embryo fibroblasts was examined by determining the number of cells which could be detached from the culture dish by a stream of medium. Transformed cells were significantly less adherent than their normal counterparts. In cultures infected with a mutant of Rous sarcoma virus which is temperature-conditional for transformation, adherence changed promptly following a temperature shift. This change did not require progression through the cell cycle. The transformation-specific decrease in adherence required new protein synthesis, but the restoration of adherence which occurred following a shift to the restrictive temperature could occur in the absence of new protein synthesis. Inhibitor experiments suggested the importance of microfilaments and perhaps microtubules in the changes in detachability. In addition, there was a positive correlation between levels of surface LETS protein and cell substrate adherence following a temperature shift, although it seems probable that the bulk of the surface LETS is neither necessary nor sufficient for maintenance of normal cell substrate adherence.