Release of Somatomedin-Like Activity by Cultured WI-38 Human Fibroblasts*

Confluent cultures of normal diploid WI-38 human embryonic lung fibroblasts released somatomedin (SM)-like activity into their incubation medium during culture in serum-free medium. This postculture medium (conditioned medium) stimulated cell division in these same cultured WI-38 fibroblasts and 35SO4 uptake by hypophysectomized rat cartilage in vitro. The conditioned medium also contained immunoreactive SM (IRSM) activity which yielded parallel dose-response curves to human serum in a RIA for SM. The IRSM activity measured in conditioned medium was not the artifactual result of effects of possible SM-binding proteins or proteolytic enzymes in conditioned medium. These studies suggest that cultured WI-38 fibroblasts produce and release SM-like activity which has SM-like biological activity and is immunoreactive with a basic SM purified from human plasma Cohn fraction and having similarity with SM-C and insulin-like growth factor-I. Human GH appears to stimulate production and release of IRSM activity by these cells.