Contribution of Human Somatomedin Activity to the Serum Growth Requirement of Human Skin Fibroblasts and Chick Embryo Fibroblasts in Culture*

Although purified human somatomedins and related peptides have been shown to stimulate growth-related processes in cultured human skin fibroblasts, it is unknown whether the serum macromolecules required for the routine growth of human fibroblasts in culture include the somatomedins. To evaluate this question we have obtained sera from five GH-deficient patients before and after GH treatment and have compared these pre- and post-GH sera for their ability to stimulate 3Hthymidine incorporation into DNA and cell multiplication in the patients' own fibroblasts in culture. Subconfluent human fibroblasts demonstrated a dose-dependent increase in thymidine incorporation and cell number when exposed to human sera. Pre- and post-GH sera were equipotent. Partial purification of somatomedin activity by boiling sera at pH 5.5 diminished the level of thymidine incorporation to 10-20% of the level achieved with unboiled sera but did not unmask a difference between pre- and post-GH sera. In contrast to the results in human fibroblasts, boiled post-GH sera were 2- to 5-fold more potent than pre-GH sera in stimulating thymidine incorporation in chick embryo fibroblasts. Mixing experiments failed to demonstrate inhibition of thymidine incorporation by boiled pre-GH sera. Boiled post-GH sera also were twice as active as boiled pre-GH sera in stimulating multiplication of chick embryo fibroblasts and confirmed that thymidine incorporation reflected DNA synthesis. GH added directly to chick embryo fibroblasts did not stimulate thymidine incorporation. We conclude that despite the presence of specific receptors for somatomedin-like peptides on human skin fibroblasts, somatomedins are not a major component of the serum macromolecules required for the routine growth of human fibroblasts in culture. In contrast, in chick embryo fibroblasts, somatomedins do appear to constitute an important part of the serum macromolecules supporting cell growth.