Autoregulation of MeWo metastatic melanoma cell growth: Characterization of intracellular (FGF, MGSA) and secreted (PDGF) growth factors

Abstract

MeWo melanoma cells (clone LC1) secrete a potent mitogenic activity susceptible to reinitiate DNA replication in quiescent rodent fibroblasts (CCL39, NRK‐49F, NIH‐3T3) but not in BHK‐21 kidney cells. This activity appears to be closely related to platelet‐derived growth factor (PDGF) based on (1) its cationic nature, heat and acid resistance, but sensitivity to reducing agents; (2) its apparent molecular weight (33 kDaltons) as estimated by Biogel filtration, once dissociated from binding proteins by mild acidic treatment; (3) its weak affinity for heparin; and (4) its ability to compete with ¹²⁵I‐PDGF for binding to human and rodent fibroblasts, and to be recognized by anti‐PDGF antibodies. Although MeWo cells coexpress the PDGF‐A and PDGF‐B (c‐sis) chain gene transcripts, the secreted product shows reactivity on CCL39 fibroblasts more compatible with the PDGF‐BB than with the PDGF‐AB isoform. MeWo cell lysates contain activities that bind moderately and strongly to heparin‐Sepharose, being eluted with 1.0 and 2.0 M NaCI, respectively. The latter may correspond to basic fibroblast growth factor (basic FGF), consistent with the expression of basic FGF gene mRNAs. The former has not been fully characterized and is probably not the product of the acidic FGF gene. In addition, MeWo cells react positively with the FB2 AH7 antibody, thus indicating that they elaborate material similar to melanoma growth‐stimulating activity (MGSA). MeWo cells proliferate in serum‐free medium in a cell‐density‐dependent fashion, both in liquid and semisolid cultures. Their division is modestly enhanced by basic FGF and by human and porcine PDGF but not by the factors that they release. However, the absence of demonstrable ¹²⁵I‐PDGF binding sites on MeWo cells, in conjunction with their lack of sensitivity to suramin growth inhibition, suggests that the secreted PDGF does not act as an autocrine factor. Instead, the autonomous proliferation of MeWo melanoma cells may result from the concerted action of basic FGF and MGSA, which are mostly cell‐associated.