CONFORMATIONAL-CHANGES REQUIRED IN THE HUMAN GROWTH-HORMONE RECEPTOR FOR GROWTH-HORMONE SIGNALING

Growth hormone (GH) plays a significant role in normal growth and deve lopment. Signaling to the cell is believed to require growth hormone r eceptor (GHR) dimerization, which occurs following binding of a single growth hormone molecule to each of two receptors. We have developed,h uman growth hormone receptor-specific monoclonal antibodies, one of wh ich was used here to characterize hormone/receptor interactions. This antibody, GHR05, is directed against the hinge spanning subdomains I a nd II of the receptor's extracellular region. Antibody binding to the cell surface receptor increases upon receptor binding to growth hormon e, but not when it binds a mutant form, hGHG120R, which does not trigg er receptor activation. Growth hormone binding thus appears ta lead to a conformational change in the receptor epitope recognized by GHR05, giving rise to the active dimer configuration, necessary for signal tr ansduction. Using a chimeric receptor-expressing, growth hormone-depen dent murine cell line, we find that GHR05 binds to the receptor in the absence of human GH and delivers a signal leading to cell proliferati on. Finally, GHR05 treatment of IM-9 cells, a human cell line expressi ng a functional human GHR, leads to cell proliferation mediated by the generation of GH-specific signals, including phosphorylation of the J AK2 tyrosine kinase and activation of STAT5.