MITOGENIC ACTIVITY OF LAMININ ON HUMAN-MELANOMA AND MELANOCYTES - DIFFERENT SIGNAL REQUIREMENTS AND ROLE OF BETA-1 INTEGRINS

The possible mitogenic activity of laminin (LN) on normal and neoplast ic cells of the melanocyte Lineage was tested by culturing growth-arre sted human melanoma cells and neonatal foreskin melanocytes on LN. Ser um-deprived, quiescent melanoma cells proliferated, in serum-free medi um, in a dose-dependent fashion to immobilized LN as determined by [H- 3]thymidine incorporation, cell cycle analysis, and change in cell num ber. The mitogenic activity of LN on melanoma cells was not mediated t hrough autocrine release of growth factors and was observed with prima ry or metastatic melanoma cells and with clones isolated from the same metastasis but only on cells expressing very late antigen (VLA)-3 and VLA-6 laminin receptors. Proliferation of melanoma cells to LN was si gnificantly inhibited by a mAb to the beta 1 subunit of VLA integrins and by a combination of mAbs to the alpha subunits of VLA-3 and VLA-6. By contrast, LN did not act as a mitogen on human melanocytes express ing VLA-3 and VLA-6 and cultured in serum-free medium. However, a cost imulatory activity of immobilized LN for proliferation of melanocytes was observed in the presence of a second signal provided by a set of d ifferent growth factors. The costimulatory activity of LN on melanocyt es could be significantly inhibited by mAbs directed to the alpha and beta chain of VLA-6 but not to VLA-3. These data suggest that LN itsel f, and not growth factors possibly associated with it, can exert a mit ogenic activity on quiescent human melanoma cells and that a change in the signal requirements for response to LN occurs upon neoplastic tra nsformation in the melanocyte lineage. Furthermore, beta 1 integrins a re differentially involved in the response of the normal and the neopl astic cells to LN, since VLA-3 and VLA-6 cooperate in the proliferatio n of neoplastic cells, while VLA-6 is relevant for the response of mel anocytes.