Serum Requirement for in vitro Invasion by Tumor Cells

The effect of fetal calf serum (FCS) on in vitro invasion by rat ascites hepatoma cells (AH130) was studied by using the in vitro invasion assay. Although the coculture of the highly invasive clone (MM1) of AH130 cells and the mesothelial cell layer or endothelial cell layer in modified minimum essential medium supplemented with 10% PCS resulted in extensive penetration of the layer by the tumor cells, the omission of PCS resulted in an almost complete elimination of the in vitro invasion. The in vitro invasiveness by human small cell lung cancer cells (OCIO) was also remarkably reduced by the omission of PCS from the assay medium, suggesting a requirement of serum for the in vitro tumor cell invasion. When 10% PCS was added to the medium 2 h after the tumor cell seeding in FCS‐free invasion assay system, penetration by MM1 cells was observed within an hour. This rate of penetration was almost the same as that when 10% PCS was added at the time of tumor cell seeding. PCS was also required for the penetration of a mesothelial cell monolayer by MM1 cells in a defined growth medium (SFM‐101), in which MM1 cells were well maintained. The invasion‐inducing activity appears to be independent of the growth‐stimulating activity in serum.