Phenotypic interconversion of B16 melanoma clonal cell populations: relationship between metastasis and tumor growth rate.

Three distinct dissemination-related phenotypes have been recognized in clones of the mouse B16 melanoma based on in vivo behavior: metastatic (spontaneously disseminating to the lungs from solid tumors), colonizing (capable of forming tumor colonies in the lungs following intravenous injection), and null (tumorigenic but non-metastatic and non-colonizing). From a progenitor null clone, G3, subclones that became phenotypically diversified in vitro (metastatic G3.5 and null G3.15) and in vivo (metastatic G3.12 and colonizing G3.26) were derived. During long-term culturing, G3 cells became metastatic and then lost that activity, G3.5 and G3.12 cells gradually lost metastatic activity, and G3.26 cells became slightly metastatic and non-colonizing. Subclone G3.15 became highly metastatic after a single subcutaneous (s.c.) tumor passage. In aged mice, and in young mice injected with incompletely-tumorigenic cell doses, G3 and G3.26 s.c. tumors were metastatic, but cells cultured from those tumors or metastases were non-metastatic when tested in young mice at standard highly-tumorigenic cell doses. The behavior of G3.5 and G3.12 tumors was not altered in aged mice or when tumors were initiated with small cell inocula. Analysis of growth characteristics associated with these phenotypic interconversions indicated that lung-colonizing potential was directly related to the ability of the cells to grow as multicell colonies in 0.3% agar, and that metastatic activity was expressed by tumors that grew at moderate rates. In young mice receiving standard cell doses, G3.5 and G3.12 tumors inherently grew at that rate, whereas G3 and G3.26 tumors grew more rapidly and G3.15 tumors grew more slowly. Regardless of inherent phenotype, all clones were capable of expressing metastatic activity, at least transiently, as tumor growth was altered to moderate rates. Expression of metastatic behavior might, therefore, be regulated to some extent by tumor growth characteristics.