Metastatic instability of murine tumor metastases: dependence on tumor type

The cellular basis of tumor progression

Variability in disease presentation and course is a hallmark of cancer. Variability is seen among similarly diagnosed cancers in different patients or animal hosts and in the same cancer at different periods of time. This latter type of variability, termed "tumor progression," was defined by Foulds in a series of six rules that describe the independent behavior of individual cancers and the independent evolution of different cancer characteristics. Tumor progression is believed to result from variability among subpopulations of tumor cells within individual cancers and from selection of these subpopulations by conditions within the cancer environment, such that different subpopulations come to prominence over the course of cancer development and growth. Interactions among subpopulations, however, modulate tumor behavior as well as tumor evolution. The leading hypothesis for the origin of tumor subpopulations is the genetic instability of cancer cells. There are a number of possible mechanisms of genetic instability, some internal to cancer cells (mutation, amplification, mutator phenotypes, DNA repair deficiencies) and some present in the tumor microenvironment (endogenous mutagens). There are also potential epigenetic mechanisms of variability, including alterations in gene regulation, differentiation, adaptation, and cell fusion. Regardless of mechanism, the heterogeneity of tumor subpopulations poses a number of challenges to the practice of cancer research, including the design of reproducible and meaningful experiments. Tumor heterogeneity also has significant consequences for the clinical assessment of tumor prognosis and the development of effective treatment regimens.

The use of quantitative genetics for estimating the non-inherited and inherited contributions to metastasis formation

The contribution of both non-inherited (stochastic, random, environmental, and other non-inherited influences) and inherited factors (genetic and inherited epigenetic factors) to the variability of spontaneous lung metastasis formation in over 100 metastatic lines from each of three murine tumors was measured. The contribution of inherited and genetic sources of variability to metastasis formation was significantly greater than 0 in all cases, but only in the lines of sarcoma SANH was it the major influence on metastatic variability. In the sarcoma SA4020 and hepatocarcinoma HCA-1 lines, non-inherited factors accounted for the majority of the variation in spontaneous lung metastasis formation. A similar situation was also observed in the variability of the tumors with respect to the diameter doubling time. In conclusion, both non-inherited and genetic/inherited factors significantly influenced the formation of spontaneous metastases in the tumors examined. The significance of this finding for the cloning of metastatic genes is discussed.

Human tumor spontaneous metastasis in immunosuppressed newborn rats. II. Multiple selections of human melanoma metastatic clones and variants

Using a recently developed model of human tumor spontaneous metastasis in immunosuppressed newborn rats, we selected variants with different metastatic abilities from the human melanoma cell line M4Be. We used 4 in vivo selection approaches, by direct serial tumor transplantations or by techniques involving in vitro reculture of the cells recovered from s.c. tumors or lung and lymph-node metastases. In addition, 3 series of clones were derived in vitro from the M4Be cell line, either by limiting-dilution or by cloning in semi-solid agar and harvesting small and large colonies. A considerable amount of heterogeneity in tumorigenicity and metastatic ability was demonstrated among variants and clones following in vivo selections and in vitro cloning. Four main malignant phenotypes were identified among those expressed by the selected cells: poorly tumorigenic and poorly metastatic; poorly tumorigenic and highly metastatic; highly tumorigenic and poorly metastatic; and highly tumorigenic and highly metastatic. However, while malignant phenotype (i.e., tumorigenicity and metastatic ability) did not appear to be grossly influenced by in vitro cloning procedure, it appeared greatly influenced both by the in vivo selection procedure (direct transplantations or use of in vitro culture between the in vivo passages) and by the origin of the cells under selection (s.c. tumor or metastases). Our study provided us with a large panel of variants and clones with varying metastatic abilities, which represent a model of human melanoma spontaneous metastasis allowing the study of critical determinants in human tumor metastasis.

Influence of tumor transplantation methods on tumor growth rate and metastatic potential of solitary tumors derived from metastases

This study was performed to determine whether the growth rate and metastatic potential of tumors generated by spontaneous lung metastases is influenced by transplantation methods. Three different tumors syngeneic to C3Hf/Kam mice were studied: the SA-NH and SA-4020 sarcomas and the hepatocarcinoma HCA-I. Solitary tumors in the legs of mice were generated by a single metastatic nodule taken at random from lung metastases, by a single metastatic nodule taken from each mouse with the highest number of metastases, by a mixture of cells from lung metastases taken randomly, or by a mixture of cells from primary leg tumors. These transplantation procedures were repeated for two to four isotransplant generations. Repeated isotransplants of primary tumors showed little if any change in the growth rate and metastatic spread. In contrast, primary tumors derived from spontaneous metastases frequently exhibited a decrease in their growth rate and an increase in metastatic potential. This was particularly frequent when tumors were established from single metastatic nodules taken randomly from the lung, or taken from lungs that contained the largest number of metastatic nodules. The magnitude of this change varied greatly among the three tumors studied. Increased metastatic formation in the lung was also frequently associated with slower growth of the primary tumors. Thus, transplantation methods used for establishing primary tumors have an important influence on the metastatic potential of tumor transplants.

Metastatic abilities of murine sarcomas and carcinomas. II. Relationship to cell volume and DNA index

The cell volume and DNA content were determined in 12 murine tumors and correlated with the ability of these tumors to metastasize spontaneously or to form lung nodules when injected i.v. The cell volume significantly correlated with spontaneous metastatic potential of investigated tumors (r = 0.683; 0.02 less than P less than 0.05) but not with the ability of tumor cells to form artificial metastasis. The DNA index significantly correlated with both spontaneous metastasis (r = 0.594; 0.02 less than P less than 0.05), and lung colonization (r = 0.631; 0.02 less than P less than 0.05). The DNA index barely correlated with the cell volume (r = 0.564; P = 0.10).

Molecular oncogenetics of metastasis

Metastasis is a complex non-stochastic process that is most likely the result of genetic and epigenetic interactions of a wide variety of genes. The search for a single gene which can encompass such a pleiotropic response as to account for the observed phenotypic characteristics of metastatic tumour populations has been unsuccessful. Particular studies involving gene transfection, subtractive hybridisation and cell fusion are beginning to identify specific genes which contribute to metastasis in some cell types. However, such analyses are complicated by the inherent genetic instability and phenotypic heterogeneity present in tumour populations. A more detailed understanding of the metastatic process may require an abandoning of current generalised approaches to metastasis in favour of concentrating on key components of the metastatic cascade such as adhesion and invasion.

Loss of spontaneous metastasizing potential in mouse mammary tumors

Among 25 C3H/He and C3Hf/He spontaneous mammary tumors that all had produced pulmonary metastases in the autochthonous mice, and also produced pulmonary metastases during subsequent sequential syngeneic intramammary passages, three were observed to stop producing metastases. The specific investigation of the observed changes in behavior is described. When the tumors were re-started in serial intramammary passage from early, cryopreserved transplant generations, the loss of metastasizing potential reoccurred in the same, or in nearly the same, transplant generations. In one tumor from which three transplant lines had been established, the metastasizing potential was lost in all transplant lines during closely related transplant generations. It seems that a spontaneous reduction of malignant behavior in mouse mammary tumors may be an occasional neoplastic characteristic, and that the observed losses of metastasizing potential may have been determined by a programmed genetic change.