Comparative effects of somatomedin C and insulin on the metabolism and growth of cultured human fibroblasts

At concentrations of 25 ng/ml in serum‐free medium, somatomedin C (SM‐C) and insulin stimulated ³H‐thymidine incorporation in adult human fibroblasts 4‐ and 1.5‐fold, respectively. The presence of 0.25% human hypopituitary serum (HHS), which by itself had little effect, enhanced the mitogenicity of both SM‐C and insulin. Furthermore, 10⁻⁷ M dexamethasone dramatically potentiated SM‐C stimulation (70‐fold) and insulin stimulation (28‐fold) of ³H‐thymidine incorporation. With dexamethasone and 0.25% HHS, significant stimulation of DNA synthesis was seen at 2.5 ng/ml for both SM‐C and insulin. The effects of SM‐C and insulin on ³H‐thymidine incorporation were additive. These ³H‐thymidine incorporation results were clearly supported by cell replication studies. On the other hand, SM‐C and insulin had equivalent, non‐additive effects on RNA and protein synthesis and protein degradation. Half‐maximal effects were seen for both peptides on all three metabolic processes at 2–5 ng/ml. In contrast to their synergism with SM‐C in the stimulation of DNA synthesis and cell replication, HHS and dexamethasone did not enhance SM‐C stimulation of RNA or protein synthesis or protein degradation. These data indicate that SM‐C and insulin stimulate DNA, RNA, and protein synthesis, protein degradation, and cell replication in adult human fibroblasts at nanomolar concentrations, suggesting that each peptide is capable of acting through its own receptor. Both SM‐C and insulin are also capable of synergism with low concentrations of serum and dexamethasone in the stimulation of DNA synthesis and cell replication. It is proposed that SM‐C and insulin both participate in the regulation of cell growth and metabolism in vivo.