The plating of tumor components on the subcutaneous expanses of young mice. Findings with benign and malignant epidermal growths and with mammary carcinomas

Cell-cycle fate-monitoring distinguishes individual chemosensitive and chemoresistant cancer cells in drug-treated heterogeneous populations demonstrated by real-time FUCCI imaging

Essentially every population of cancer cells within a tumor is heterogeneous, especially with regard to chemosensitivity and resistance. In the present study, we utilized the fluorescence ubiquitination-based cell cycle indicator (FUCCI) imaging system to investigate the correlation between cell-cycle behavior and apoptosis after treatment of cancer cells with chemotherapeutic drugs. HeLa cells expressing FUCCI were treated with doxorubicin (DOX) (5 μM) or cisplatinum (CDDP) (5 μM) for 3 h. Cell-cycle progression and apoptosis were monitored by time-lapse FUCCI imaging for 72 h. Time-lapse FUCCI imaging demonstrated that both DOX and CDDP could induce cell cycle arrest in S/G2/M in almost all the cells, but a subpopulation of the cells could escape the block and undergo mitosis. The subpopulation which went through mitosis subsequently underwent apoptosis, while the cells arrested in S/G2/M survived. The present results demonstrate that chemoresistant cells can be readily identified in a heterogeneous population of cancer cells by S/G2/M arrest, which can serve in future studies as a visible target for novel agents that kill cell-cycle-arrested cells.

Evolution of the cancer stem cell model

Genetic analyses have shaped much of our understanding of cancer. However, it is becoming increasingly clear that cancer cells display features of normal tissue organization, where cancer stem cells (CSCs) can drive tumor growth. Although often considered as mutually exclusive models to describe tumor heterogeneity, we propose that the genetic and CSC models of cancer can be harmonized by considering the role of genetic diversity and nongenetic influences in contributing to tumor heterogeneity. We offer an approach to integrating CSCs and cancer genetic data that will guide the field in interpreting past observations and designing future studies.

Studies on clonal heterogeneity in two spontaneously metastasizing mammary carcinomas of recent origin

We have studied the clonal heterogeneity of 2 spontaneously metastasizing mammary carcinomas which recently arose spontaneously in C3H/He female retired breeders. Cells of early (2nd to 5th) transplant generations of these tumors were cloned by a combination of semi-solid agarose colony formation and limiting dilution techniques. Growth characteristics of the various clones in vitro and their tumorigenicity in vivo were evaluated. Subsequently, the role of host immunity and of interclonal interactions in regulating growth of the different clones in vivo were analyzed. We found that, whereas all 16 clones isolated from one tumor (T-58) grew rapidly in vivo and in vitro, 10 clones isolated from the second tumor (MT-2) showed a wide disparity in their growth rates in vivo. Taken together, these clones could generally be divided into 3 categories: (1) rapidly growing lines which grew in vivo at rates similar to or higher than those of the parental line; (2) slow-growing lines which grew more slowly than the parental line; and (3) non-growers which failed to produce tumors in vivo with doses of up to 5 X 10(6) cells injected either s.c. or i.v. but grew in vitro at rates comparable to the parental line. No correlation could be established between the various growth potentials exhibited by these tumor lines and tumor cell morphology in vitro and in vivo, as determined by light and electron microscopy. Sublethal irradiation (550-650 R) of young animals prior to tumor inoculation, or before inoculation of tumor cells into old, low NK syngeneic mice, failed to modify the growth of slow-or non-growing lines in vivo, indicating that host cellular defense mechanisms against the clones, if existent, were not mediated by NK or radiosensitive B or T cells. When clonal interactions were studied by the simultaneous injection of different clones in vivo at different s.c. sites, we found that a slow-growing line failed to modify the growth rate of a rapidly growing line, but accelerated the growth of a second slow-growing line injected simultaneously on the contralateral side, and that this enhancement of tumor growth was radioresistant. A mixture of these 2 lines also grew more rapidly than the individual lines alone. Our findings suggest that phenotypic variations in tumorigenicity can be found in clonal lines derived from spontaneous primary tumors and that these variations are not related to cell cycle properties as measured in vitro.(ABSTRACT TRUNCATED AT 400 WORDS)

The propagation of cancer, a process of tissue remodeling

Effective study of the malignant phenotype at the tissue level requires model systems that are intelligible both to cell biologists and to pathologists, and that also observe the spatial imperatives intrinsic to tissues in nature. Malignant cells commonly appear in multicellular units, and growth of tumor tissue is seen as an increase in the number of cells and multicellular units. The supporting stroma frequently has an abnormal appearance, and this component of the tissue also increases in mass as the tumor enlarges and spreads. The direction of invasion is influenced by the direction of available metabolites. Histophysiologic gradient culture complies with nature's spatial rationale, since at the substrate-parenchymal interface three functions coincide. These are anchorage, initiation of epithelial renewal, and complete exchange of metabolites. Our model system provides a setting for reconstructing and manipulating many features of the malignant phenotype seen in cancer tissues in nature, such as abnormalities in the sequence of maturation of stratified epithelium, hyperplasias, dysplasias, interaction between different types of epithelium, aggregate formation, tumor angiogenesis, and neoplastic blockade.

The selective nature of metastasis

The issue of whether metastases result from the random survival of cells released from a primary tumor or from the selective growth of specialized tumor subpopulations endowed with metastatic properties is important to our understanding of the metastatic process and to the development of therapeutic modalities against metastatic disease. We have found that the tumor cells populating spontaneous metastases are more metastatic than the cells populating the parent neoplasm, clearly indicating that metastasis is selective and not random. The selective nature of metastasis is a consistent observation, however, only when tumor cells are obtained from spontaneous metastases from mice bearing heterogenous, poorly metastatic tumors. Tumor cells from spontaneous metastases from mice bearing tumors that have been selected for metastatic potential or that are homogeneous (cloned) do not differ significantly in metastatic potential from tumor cells populating the parent tumor. Thus, under some conditions the process of metastasis can appear random. Although tumor cells from different individual metastases may be homogeneous with regard to a metastatic phenotype, they may be heterogeneous with regard to their sensitivity to chemotherapeutic agents. Thus, although metastasis selects for metastatic variants, resulting in the population of metastatic foci with tumor cells endowed with metastatic properties, it does not appear to select for phenotypes irrelevant to the process of metastasis such as sensitivity to therapeutic agents.

Biological diversity in metastatic neoplasms: origins and implications

Whether neoplasms are unicellular or multicellular in their origin, the process of tumor evolution and progression can rapidly generate biological diversity. Metastases result from the survival and proliferation of specialized subpopulations of cells within the parent tumor. Metastases may have a clonal origin and different metastases may develop from different progenitor cells. However, as with the primary tumor, the origin of metastases is unimportant since the process of tumor evolution and progression can generate biological diversity within and among different metastatic foci.

Evidence for the clonal origin of spontaneous metastases

A cultured cell line of the K-1735 melanoma was x-irradiated to induce chromosome breakage and rearrangements and then was implanted into the footpads of syngenic C3H mice. Spontaneous lung metastases were isolated from different animals, established in culture as individual lines, and then karyotyped. Within certain metastases, the same chromosomal abnormality (or abnormalities) (recombinant chromosomes) was found in all the cells examined. Most metastases differed from one another in that they exhibited characteristic combinations of chromosomal markers. These findings indicated that the metastases were clonal and that they probably originated from different progenitor cells.

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.'

Heterogeneity of tumorigenicity phenotype in murine tumors. I. Characterization of regressor and progressor clones isolated from a nonmutagenized ultraviolet regressor tumor

We have shown that both regressor and progressor clones can be isolated from a UV regressor tumor, RD-1024. Although the daughter clones are characterized by differences in tumorigenic potential in normal transplant hosts, they nevertheless seem to express the same major tumor rejection antigens, because immunization with either the regressor parent tumor, RD-1024, or with regressor Cl 8 protects against subsequent challenge with progressor C1 4 or Cl 9. Consistent with the in vivo-generated data is the evidence that draining lymph node cells with functional specificity for regressor Cl 8 are capable of cross-reactive cytotoxicity in an in vitro chromium release assay. We have demonstrated an indirect interaction occurring in vivo between regressor and progressor cells, in that Cl 8 cells have the ability to influence the outcome of simultaneous or sequential challenge with Cl 4 or Cl 9 cells. Because 500 rad of gamma irradiation has been shown to compromise the ability of mice to respond to a primary challenge with tumor, an immunological mechanism is implicated in the ultimate rejection of progressor tumor in a doubly challenged host. The importance of these results lies in the knowledge that these interacting subpopulations have been isolated directly from a tumor growing in vivo and that no selection pressure has been exerted on the cells greater than the short in vitro culture period necessary for the isolation and expansion of individual clones. The apparent immunoregulatory potential in a tumor-bearing animal is thus seen to be modified in accordance with the phenotypic heterogeneity of the cells within that tumor.

Cellular heterogeneity in an ethylnitrosourea-induced glioma: malignancy, karyology and other properties of tumour cell types

Two types of tumour cell have been obtained from a glioma transplacentally induced by ethylnitrosourea in a BD-IX rat. These were distinguished in culture by their different morphologies, responses to dibutyryl cyclic adenosine monophosphate, inducibility of glycerol phosphate dehydrogenase and growth in soft agar. They had different karyotypes, with distinctive numbers and arrangements of chromosomes. One cell type had an apparently normal diploid set of 42 whilst the other had 43 chromosomes. An additional chromosome No 4 was identified in the latter by Giemsa banding. Translocations and other abnormalities involving this chromosome were consistently observed. Both cell types produced malignant, astrocytic tumours when injected into newborn syngeneic rats, but with different latent periods and morphological features.

Interference by cortisone with endotoxin's adjuvator action on transplantation of a mouse tumour

Observations on the in vivo plating of mouse mammary tumour are extended by making counts of tumours at a significantly earlier phase of development than in previously reported work. In the experiments now described, most of the growth of the tumours has been without benefit of stroma. The noteworthy economy of the experimental method is discussed. The persistence of endotoxin's adjuvator effect on such tumour counts is tested in the face of gamma irradiation and cortisone. Cortisone, it is found, offsets endotoxin's adjuvator action; irradiation does not. Antagonism between endotoxin and cortisone, in this system with tumour cells plated in vivo, seems to indicate that endotoxin's enhancing effect depends more on inflammatory than on immunological factors.

Adjuvators to the propagation of mouse mammary tumor cells on expanses of subcutaneous tissue

The cells of a mouse mammary cancer were obtained with enzymes in suspensions which could be filtered to exclude all which were not single and most which were already dead. Heavy suspensions of these individual cells were plated over the dorsal subcutaneous expanses of female weanling mice where they implanted and grew to form coalescent tumors covering the back more or less entirely. Sparser suspensions, similarly plated (a) gave rise to fewer tumors, and (b) gave rise to tumors reaching measurable size later. These two consequences of sparser plating left room for the testing of adjuvators to transplantation. Adjuvator effects were obtained by splitting the subcutaneous expanses beforehand and by injecting liver along with the plated cells. Through 28 plated generations over 4 yr the tumor maintained completely stable, morphological heterogeneity. These findings with a complex tumor indicate strongly that its heterogeneity comes not from repeated cellular mutation late in its development, but from diverse potentiality to give rise to cells of specific and differing character, inherent in its individual cells at the time of its multicellular origin.