Metastatic variant selection

Also the metastatic phenotype is not homogeneous within a tumor population, which is rather composed of a variety of cell subpopulations with widely differing invasive and metastatic capabilities (Hart and Fidler, 1981). By using a modification of the classical fluctuation test of Luria and Delbriick to distinguish between selection and adaptation in the origin of bacterial mutants, metastatic variants have been shown to pre-exist within a tumor... 

In conclusion, it is on the one hand the presence of a heterogeneous population within a tumor, and on the other hand the fact that intrinsic properties of tumor cells, and not adaptive mechanisms, are important for metastatic ability, that renders a tumor (either primary, or a metastasis) amenable to selection of metastatic variants. We will describe next different selection methods for such metastatic variants, which can be performed either in vivo, or in vitro, depending on the type of metastatic ability that is being looked at. 

Selection of metastatic variants with enhanced or decreased metastatic abilities... 

Adhesion to organ-specific structures can also be used to select metastatic variants, that will usually show increased homing to the tissue used for selection. In this way, B16 melanoma cells have been selected on lung cryostat sections (Netland and Zetter, 1981), Lewis lung carcinoma cells on hepatocyte monolayers (Brodt, 1989), and RAW117 large-cell lymphoma cells on hepatic sinusoidal cells (LaBiche et al., 1993). 

The cell lines listed in this table are incomplete, particularly with regard to variants selected from the parental. Since the B16 melanoma is the most commonly used metastatic cell line used, the most popular variants are listed with associated... 

Frost, P., Kerbel, R. S., Hunt, B., Man, S. and Pathak, S. (1987). Selection of metastatic variants with identifiable karyotypic changes from a nonmetastatic murine tumor after treatment with 2 -deoxy-5-azacytidine or hydroxyurea implications for the mechanisms of tumor progression. Cancer Res. 47, 2690-2695. 

Nicolson, G. L., Belloni, P. N., Tressler, R. J., Dulski, K. M., Inoue, T. and Cavanaugh, P. G. (1989). Adhesive, invasive and growth properties of selected metastatic variants of a murine large-cell lymphoma. Invasion Metastasis 9, 102-116. 

Correlation of Certain Proteases with Metastatic Potential in Model Tumor Systems. A variety of different proteases have been found to correlate with metastatic potential in model tumor systems. Many of these early studies were carried out with B16 mouse melanoma cells. Variants of these cells with different metastatic potential have been selected. In separate experiments, total PA activity, CB activity, and collagenase IV activity have all been found to correlate with metastatic potential in these cells (A4, D6). More recently, levels of mRNA for CB have also been found to correlate with metastasis in these melanoma cells (Ql). Correlations also exist between levels of specific proteases and metastatic ability in a number of other model systems see Table 2 (D6). 

Matrix metalloproteases (MMP) are also inhibited by hydroxamic acids and/or thiols. Over 25 variants of these enzymes are known, and some are involved in diseases ranging from inflammation to metastatic cancer (108). MMPs contain a zinc ion in the active site and function through the metallopeptidases catalytic mechanism already discussed. However, subtle differences between enzymes enable selective inhibitors to be developed (109). Fig. 15.25 lists some of the reported MMP inhibitors that use carboxylic acid (52-53), a hydroxamic acid (54-55), or thiol groups (56)as metal chelators. 

Basically, harvest of invasive cells after any invasion assay (see Chapter 4) can be used to select a cell population with increased invasive ability. Poste et al. (1980) have used the chorioallantoic membrane (CAM) assay, a canine blood vessel perfusion-invasion chamber, or retrograde injection of tumor cells in the mouse bladder, to select tumor cell variants with respect to their invasive ability. B16-BL6 melanoma cells, widely used as a prototype of highly invasive metastatic cells have been subjected to six rounds of selection with the bladder system. The application of these three systems for selection is described in detail in the original paper (Poste et al., 1980), and will not be treated further here. 

Bailly, M. and Dor6, J.-F. (1991). Human tumor spontaneous metastasis in im-munosuppressed newborn rats. II. Multiple selections of human melanoma metastatic clones and variants. Inti. J. Cancer 49,750-757. 

Brodt, P. (1989). Selection of a highly metastatic liver-colonizing subpopulation of Lewis lung carcinoma variant H-59 using murine hepatocyte monolayers. Clin. Exp. Metastasis 7,525-539. 

Cavanaugh, P. G. and Nicolson, G. L. (1998). Selection of highly metastatic rat MTLn2 mammary adenocarcinoma cell variants using in vitro growth response to transferrin. J. Cell Physiol. 174, 48-57. 

Kawaguchi, T., Kawaguchi, M., Miner, K. M., Lembo, T. M. and Nicolson, G. L. (1983). Brain meninges tumor formation by in vivo-selected metastatic B16 melanoma variants in mice. Clin. Exp. Metastasis 1, 247-259. 

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