Stimulation of human metastatic melanoma colony-forming cells by an acid-sensitive factor in human platelet sonicate.

A human platelet sonicate was evaluated for its effects on the growth of human metastatic melanoma colony-forming cells in soft agar from cells in culture and from biopsies. The addition of platelet sonicate increased both cloning efficiency and proliferative capacity in that more and larger colonies were formed. In more detailed studies under growth-limiting conditions, melanoma cellular responses to known growth factors were compared to the activity found in the platelet sonicate. None of the growth factors tested either alone or in combination, including platelet-derived growth factor, epidermal growth factor, alpha-type transforming growth factor, and beta-type transforming growth factor, were capable of inducing melanoma colony formation to the 12-fold stimulation observed with the platelet sonicate. Treatment of platelet sonicate with dithiothreitol, trypsin, or acid resulted in loss of activity for human melanoma. Our results suggest that human platelets contain an acid-sensitive protein which can support the expression of the transformed phenotype of human melanoma, and this factor is distinct from acid-stable activities previously characterized from human platelets.