Mixed inocula of mouse mammary tumour cell subpopulations result in changes of organ-specific metastasis

Tumour and metastatic phenotypes, the pattern of mouse mammary tumour virus (MMTV) integration and expression, and the expression of a metastasis associated gene, nm23, were examined in three mammary tumour cell subpopulations, 66, 168 and 4526. Tumour growth, host survival, metastatic aggressiveness, and the distribution of different cell types in metastasis resulting from mixed cell inocula were also analysed. The results of these studies indicated that the cell lines were distinguishable from each other both phenotypically and genotypically. However, a rearrangement of the mammary tumour specific protooncogene, int-1, caused by MMTV was found to be a unique characteristic of the cell line 4526. Therefore, int-1 was used as a stable marker to examine the genotype of the metastatic colonies that developed in mice bearing tumours of mixed cell inocula. Highly metastatic 4526 cells influenced the metastatic range of poorly metastatic 66 cells. Line 66 cells that normally colonize only to lungs were also found to colonize liver when inoculated together with the liver-metastasizing 4526 cells. This acquired metastatic phenotype of 66 cells was transient. On the contrary, mixed cell inocula of 4526 and non-metastatic 168 cells did not produce any colony of 168 cells. The metastatic aggressiveness of 4526 cells was inhibited by both 66 and 168 cells. Furthermore, the metastatic behaviour of mixed inocula differed depending on the relative abundance of the component populations in the mixtures. These findings suggest that interaction between cells of different metastatic phenotypes may result in changes of their metastatic behaviour.