Enhancement of the tumorigenicity of preneoplastic mammary nodule lines by enzymatic dissociation.

Preneoplastic mammary nodule lines D1, D2, and C4 were enzymatically dissociated with collagenase, hyaluronidase, and pronase or with only collagenase and hyaluronidase to produce high yields of viable single cells; 10(5) cells were injected into the cleared mammary fat pads of syngeneic BALB/cCrgl mice. In 11 experiments involving three different preneoplastic nodule lines with different tumor potentials, all dissociated nodule cell lines showed a marked increase in tumorigenicity as compared to the same tissues transplanted as 1-mm3 pieces. The results could not be explained on the basis of differences between the amounts of cells transplanted in the two procedures. In a second series of experiments, normal mammary cells from virgin, pregnant, or lactating mice were mixed in different ratios with 10(5) nodule cells and injected into the mammary fat pads. The presence of normal cells reversed the marked increase in the tumorigenicity of enzymatically dissociated nodule cells to a level equal to or less than the tumorigenicity of control transplants (1-mm3 pieces). The growth of 10(5) mammary tumor cells was not inhibited when tumor cells were mixed with 3 x 10(5) normal cells and transplanted into the mammary fat pads. These results showed that enzymatic dissociation can lead to an increase in tumor potential of preneoplastic mammary nodule lines, and they supported the hypothesis that nodule cells, but not neoplastic cells, are sensitive to the growth-inhibitory effects of normal mammary cells.