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-LC1) endowed with higher metastatic potential in nude mice were compared using hormonally-defined serum-free media. The two cell lines failed to arrest in G0 following serum deprivation, and responded to INS and MSA but not to EGF, PDGF or FGF. Only MeWo-LC1 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-LC1 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-LC1) or homologous (SK-MEL28 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-LC1 cells produced bioactive material at a 2 to 3-fold higher rate than MeWo cells, in both the absence and presence of INS. MeWo-LC1-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-LC1 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.