Growth in an organ microenvironment as a selective process in metastasis

The purpose of these studies was to determine whether the ability of tumor cells to grow in an organ parenchyma selects for cells with enhanced potential to metastasize to this organ. B16-F1 melanoma cells (with low metastatic potential) were cultured in vitro on fragments of mouse lung or kidney suspended in medium supplemented with only 1 per cent fetal bovine serum. Seven days later, the organs were enzymatically dissociated and tumor cells recovered and expanded in monolayer culture. Tumor cells were then harvested and seeded onto fresh organ fragments. This sequence was repeated six times. The cells designated as B16 Lung-6 and B16 Kidney-6 were then injected intravenously into CS7BL/6 mice and the number of experimental metastases counted after 21 days. B16 Lung-6 and, to a lesser degree, B16 Kidney-6 cells produced significantly more lung tumor colonies than B16-F1 cells. Some factor(s) in the organ environment did appear, therefore, to select out cells with greater metastatic potential from the low metastatic B16-F1. Forty-five clones of the B16-F1 melanoma, isolated by limiting dilution, were screened for their ability to grow on expiants of mouse lung in a low-serum medium. Four clones exhibiting least growth and four clones exhibiting most growth, as assessed by examining histological sections of the lung explants, were injected intravenously into syngeneic mice. The eight clonal populations differed in experimental metastatic potential, but this behavior did not correlate with the ability of cells to grow in lung expiants in vitro. The data suggest that selecting for cells with enhanced ability to grow in an organ is a necessary but not sufficient condition for isolating cells with high metastatic potential.