Role of the a$ßiIntegrin Receptor in the Proliferative Response of Quiescent Human Melanoma Cells to Fibronectin1

The possible mitogenic activity of fibronectin (FN) in human primary and metastatic melanoma lines and clones and the involvement of integrins in mediating this effect were evaluated. Quiescent human melanoma cells cultured in serum-free medium proliferated in a doseand timedependent fashion to immobilized FN as indicated by [3H]thymidine incorporation, increment of cell number, and cell cycle analysis. This response to FN was observed with tumor clones isolated from a subcu taneous metastasis and with primary or metastatic melanomas from different patients, but only when tumor cells expressed the as subunit of the FN receptor (i.e., VLA-5). Proliferation to FN by a primary tumor (Me4405) expressing all FN receptors and by a tumor clone (2/60) lacking only the a4 subunit was inhibited by monoclonal antibodies to the <*5and ß, but not by monoclonal antibodies to other subunits of FN receptors. Mapping of FN regions responsible for the proliferative sig nal was performed by stimulating melanoma cells with different FN proteolytic fragments and indicated that a significant mitogenic signal was provided by the M, 120,000 a-chymotrypsin fragment containing the Arg-Gly-Asp sequence. The proliferation of melanoma cells to FN and to FN fragments was also significantly inhibited by peptides con taining the Arg-Gly-Asp sequence. These data indicate that FN can stimulate the proliferation of quiescent melanoma cells and that integrins as HC./Ì, are involved in the response of tumor cells to this extra cellular matrix protein.