EVIDENCE FOR A DIRECT EFFECT OF GROWTH-HORMONE ON OSTEOBLASTS

In order to determine whether growth hormone (GH) exerts a direct effe ct on osteoblasts, in vitro and in vivo immunocytological studies were carried out on new-born rat calvaria and a clonal osteoblast-like cel l line (MC3T3-E1) isolated from newborn mouse calvaria. After exposure to human growth hormone (hGH) or 1,25 dihydroxyvitamin D3 (1,25(OH)2D 3), a significant increase in alkaline phosphatase activity was observ ed in MC3T3-E1 cells. Simultaneous exposure of MC3T3-E1 cells to hGH a nd 10 nM 1,25(OH)2D3 showed a synergistic effect of the two hormones o n this activity. The optimal dose of hGH was 0.1 nM. An immunocytologi cal procedure was performed on ultrathin frozen sections from 7-day-ol d rat calvaria and MC3T3-E1 cells cultured with hGH. GH-like immunorea ctivity was observed in both cases. In calvaria, endogenous GH-like im munoreactivity was localized at the same ultrastructural level (plasma membrane, cytoplasmic and nuclear matrices) as exogenous GH-like immu noreactivity in MC3T3-E1 cells. Following the initial step of binding to the plasma membrane, GH may be internalized in the cytoplasmic matr ix and nucleus. In situ hybridization revealed the presence of mRNA co ding for GH receptor in calvaria cells. The density of these receptors seemed to be lower in osteoblasts than in hepatocytes. In MC3T3-E1 ce lls, hGH induced a dose-dependent secretion of insulin-like growth fac tor 1. In conclusion, these results indicate that GH may act directly on osteoblasts.