Continuous culture of human melanoma cells in protein-free medium: establishment and partial characterization.

Human melanoma cells (MEC) continuously grown in protein-free RPMI-1640 medium were established. These cultured cells, designated "sf-MEC," grow well in suspension and have been successfully subcultured more than 100 passages for 30 months with complete loss of serum requirement for their growth. Although sf-MEC as well as MEC had a moderate number of mature melanosomes in the cytoplasm, the tumorigenicity in nude mice was lower in sf-MEC than in MEC. In the tumor tissue formed by sf-MEC but not by MEC, a few immature melanosomes and an increased C-type-like virus production were detected. Furthermore, conditioned medium of sf-MEC stimulated DNA synthesis in both normal and transformed cells, including MEC and sf-MEC. Addition of the conditioned medium of sf-MEC in its cell growth assay caused a twofold to eighteenfold increase in cell number as compared to that for the control.