Serum and substratum‐dependent coupled loss of differentiated and tumorigenic phenotypes in B16‐ conv melanoma cells

Clonal B16 mouse melanoma conv cells are tumorigenic spindle‐shaped cells (S‐type cells) exhibiting tyrosinase activity and melanosomes under usual culture conditions. When the cells passaged on glass substratum were plated for colony formation on plastic substratum in Eagle's minimum essential medium (MEM) with 10% calf bovine serum, most of them converted to fibroblastlike cells (F‐type cells) with the coupled loss of differentiated and tumorigenic phenotypes. However, they continued to be S‐type cells provided that they were plated on glass substratum. The conversion from S‐ to F‐type cell was not induced with high frequency even on plastic substratum when the concentration of calf serum in the medium was low (1–2%). These results indicate that both plastic substratum and serum factor are requisites for converting the phenotypic expression of the conv cells. Partial characterization of the serum factor indicates that it is adsorbable to plastic substratum, inactivated at 70 ° C for 10 min, salted out at 40% of saturated ammonium sulfate; in addition the factor seems to act on cells within 1 day after plating.