Autonomous proliferation of mewo human melanoma cell lines in serum‐free medium: Secretion of growth‐stimulating activities

The growth properties of a human melanoma cell line (MeWo) and of a variant (MeWo‐LCI) endowed with higher metastatic potential in nude mice were compared using hormonally‐defined serum‐free media. The two cell lines failed to arrest in Go following serum deprivation, and responded to INS and MSA but not to EGF, PDGF or FGF. Only MeWo‐LCI cells divided persistently in completely serum‐free medium, and formed a high percentage of spheroids in agarose supplemented or not with serum or individual growth factors. The conditioned media of serum‐free cultures of MeWo and MeWo‐LCI cells exhibited mitogenic activities. These were detected without prior concentration, fractionation or acid treatment. They stimulated DNA replication into sparse monolayers of autologous (MeWo, MeWo‐LCI) or homologous (SKMEL28 melanoma) cells and into NRK‐49F normal rat fibroblasts, and acted in synergy with INS in a dose‐dependent manner. Over a period of 5 days in culture, MeWo‐LCI cells produced bioactive material at a 2 to 3‐fold higher rate than MeWo cells, in both the absence and presence of INS. MeWo‐LCI‐conditioned medium promoted or enhanced colony formation of MeWo and NRK‐49F cells plated in serum‐free (± INS) agarose. The two cell lines expressed the same amount of NGF and EGF receptors. On the basis of these results we suggest that: (i) MeWo and MeWo‐LCI melanoma cells have obviated allocrine stimulation of the division process characteristic of normal cells by responding to their own mitogens; (ii) some of these mitogens are akin to TGFs; (iii) the less efficient synthesis of autostimulatory factors by MeWo cells may account for their weaker proliferative capacity in vitro, and possibly in vivo.