Regulation of Amino Acid Uptake and Deoxyribonucleic Acid Synthesis in Isolated Human Fetal Fibroblasts and Myoblasts: Effect of Human Placental Lactogen, Somatomedin-C, Multiplication-Stimulating Activity, and Insulin*

Abstract

We compared the abilities of human placental lactogen (hPL), somatomedin-C/insulin-like growth factor I (SM-C/IGF-I), multiplication-stimulating activity (MSA), and insulin to induce a rapid anabolic event, the uptake of the nonmetabolizable amino acid [³H[α-aminoisobutyric acid ([³H] AIB) or the more long term action of increasing [³H]thymidine incorporation, as a measure of DNAsynthesis, in isolated human fetal fibroblasts and myoblasts. Myoblasts were derived from skeletal muscle and fibroblasts from skin explants removed from human fetuses delivered between 12 and 19 weeks gestation after prostaglandin-induced abortion. Each of the four peptides caused a dose-dependent increase in [³H]AIB uptake by both fibroblasts and myoblasts, with mean half-maximal concentrations (ED₅₀) ranging from 0.9–1.9 nM. The concentration of each peptide required to stimulate [³H] thymidine uptake was significantly greater, with the exception of insulin, which was inactive. For myoblast cultures, the meanED₅₀ values were: hPL, 7.9 nM; SM-C/IGF-I, 2.0 nM; and MSA, 2.2 nM. For fibroblast cultures, the mean ED₅₀ values were: hPL, 2.3 nM; SM-C/IGF-I, 3.3 nM; and MSA, 4.3 nM. Insulin did not stimulate [³H]thymidine incorporation into either cell type at concentrations up to 6.9 nM. Incubation inthe presence of monoclonal antibody against SM-C/IGF-I abolished the ability of SM-C/IGF-I to stimulate either [³H]thymidine or [³H]AIB uptake into fetal fibroblasts. The antibody substantially inhibited the incorporation of [³H] thymidine by these cells in response to hPL, but was lesseffective in blocking hPL-stimulated [³H]AIB uptake. It did not inhibit the uptake of either radioisotope in response to MSA or [³H]AIB uptake in response to insulin. The actions of SM-C/IGF-I and hPL on thymidine incorporation were additive at submaximal concentrations, but not so at maximal individual concentrations. Their action on AIB uptake were additive at both submaximal and maximal concentrations. The results suggest that hPL as well as the SMs may contribute to the growth stimulus in human fetal connective tissues. Since incubation with SM-C/IGF-I antibod reduced the mitogenic response of fetal cells to hPL, the actions on DNA synthesis may be partially mediated by local release of SM. However, the similar ED₅₀ values with which these peptides stimulated [³H]AIB uptake during a short incubation, and their additive effects atmaximal individual concentrations, suggest that hPL may also have direct actions.