A significant impediment to studies of growth disorders in man has been the lack of an easily accessible target tissue for studies of GH action. As human fibroblasts have been shown to produce a somatomedin-C-like peptide in culture, the aim of this study was to investigate the interaction of GH with cultured fibroblasts. GH binding to human skin fibroblasts and to lung fibroblasts was examined. Specific binding of [125I]iodo human GH [( 125I]iodo hGH) was rapid, reversible, and time and temperature dependent. Maximal binding was achieved within 2 h at 30 C and specifically bound [125I]iodo-hGH was rapidly dissociable at this temperature. A linear relationship between specific binding of [125I]iodo-hGH and cell number was found and negligible degradation of labeled hormone occurred during the binding studies. Half-maximal inhibition of specific binding occurred at 30 ng/ml hGH. Lactogenic and nonprimate somatogenic hormones did not displace [125I]iodo-hGH at the concentrations tested. Scatchard analysis of [125I]iodo-hGH binding to human skin fibroblasts demonstrated a single class of binding sites with an affinity constant of 1.07 +/- 0.07 (SEM, n = 5) X 10(9) M-1 and a capacity of 8355 +/- 1880 site per cell. Similar GH-receptor characteristics were found on each of the fibroblast cell lines examined irrespective of the site of origin (skin or lung) or age of donor. These findings demonstrate for the first time specific GH receptors in cultured human fibroblast cell lines. The demonstration of GH receptors in human fibroblasts should encourage the search for similar receptors in tissues not previously considered to be target tissues for GH action. On the basis of these studies we suggest that cultured skin fibroblasts may be a suitable tissue for the study of GH-receptor status in patients with disorders of growth.

The demonstration that mouse embryo mesenchymal cells produce somatomedin-like immunoreactivity suggests that fibroblasts may be a target tissue for GH action. The aim of this study was to investigate the interaction of GH with mouse fibroblasts. Specific binding sites for GH in mouse fibroblasts (BALB/c and Swiss 3T3) have been characterized. Binding of [125I]iodo-human GH (hGH) was rapid, reversible, and time and temperature dependent. Maximal binding was achieved within 2 h at 30 C and was rapidly dissociable at this temperature with or without excess unlabeled hormone. Specific binding of [125I]iodo-hGH was similar over the pH range 6.6-8.2. Half-maximal inhibition of specific binding was obtained with 10 ng/ml hGH. A linear relationship between specific binding and cell number was found and negligible degradation of [125I]iodo-hGH occurred during the binding studies. Somatogenic hormones from various species, including mouse GH, rat GH, porcine GH, and bovine GH competed for binding with [125I]iodo-hGH. Lactogenic hormones did not displace [125I]iodo-hGH at low concentrations. Scatchard analysis revealed curvilinear plots suggesting that [125I]iodo-hGH was binding to two sites. The affinity constants and capacities of these binding sites on BALB/c 3T3 cells were 7.46 X 10(9) M-1 and 4,000 sites per cell and 0.26 X 10(9) M-1 and 67,000 sites per cell. Using [125I]iodo-bovine GH and [125I]iodo-human placental lactogen as labeled ligands two distinct binding sites were found with affinity constants of 6.1 X 10(9) M-1 and 1.1 X 10(9) M-1, respectively. These data are consistent with the presence of cell surface somatogenic and lactogenic receptors in mouse fibroblasts and suggest GH receptors may be present on many cell types not previously considered to be target tissues for GH action. The mouse fibroblast GH receptor may provide a useful model for somatogenic receptors, particularly if it is coupled to somatomedin production as appears to be the case with human fibroblasts.

The role of pituitary hormones in the pathogenesis of human breast cancer is unclear, although hypophysectomy is of therapeutic benefit in some patients with advanced breast cancer. Agents that lower serum PRL are of little value in the treatment of breast cancer, suggesting that other pituitary hormones may be important in the control of the growth of human breast cancer in vivo. Since human (h) GH is lactogenic in rodents, we investigated the binding of [125I]hGH and [125I]hPRL to the cultured human breast cancer cell lines T-47D and MCF-7. Both [125I]hGH and [125I]hPRL bound to a saturable binding site with high affinity (Ka = 0.94-1.70 X 10(9) M-1) and low capacity (4140-6560 sites/cells) in the two cell types. hGH and hPRL were mutually competitive, indicating that both hormones bound to the same receptor site. After binding of [125I]hGH to cell monolayers, the hormone was rapidly internalized in a time-, temperature-, and energy-dependent fashion. Lysosomotropic agents inhibited degradation of [125I]hGH and enhanced specific binding. Preincubation of MCF-7 cells with either hGH or hPRL resulted in loss of hGH/hPRL-binding sites, although hGH was consistently more potent in inducing down-regulation of the receptor. On the basis of these observations we suggest that hGH is a potent ligand for the lactogenic receptor in human breast cancer cells in vitro and may be important in the pathogenesis, growth, and metastasis of human breast cancer.