Stimulation of anchorage-independent growth of human tumor cells by interleukin 1.

Human peripheral blood monocytes and tumor-associated macrophages release a factor that enhances the clonal growth of a human epithelial tumor cell line (SW-13) in soft agar. We now demonstrate that purified interleukin 1 (IL-1) may account for part of this colony-stimulating activity. Purified IL-1 (0.5 to 8 units/ml) was added to SW-13 cells cultured in soft agar. IL-1 increased colony growth in a dose-dependent manner and did not inhibit colony formation at the highest doses tested. Other purified human monocyte products (alpha-interferon, tumor necrosis factor, transforming growth factor beta, fibronectin) did not stimulate colony growth. Antibody to IL-1 only partially inhibited the ability of monocyte-conditioned medium to stimulate SW-13 colony growth. This antibody did, however, completely inhibit the ability of purified IL-1 to support the growth of SW-13 colonies in soft agar. IL-1 increased growth of quiescent SW-13 cells cultured in monolayers as assessed by tritiated thymidine incorporation assays. The results of this study indicate that IL-1 can enhance clonogenic growth of an epithelial cell line in soft agar. However, other uncharacterized activities in monocyte conditioned medium also promote colony growth. These studies add to an increasing body of evidence indicating that inflammatory products play a role in maintaining the transformed phenotype.