Modulation of human growth hormone binding to somatogenic and lactogenic receptors by monoclonal antibodies to human growth hormone

The relationship between the structure of human growth hormone (hGH) and the hormone-receptor interactions was investigated by studying the effects of specific monoclonal antibodies (MAbs) to hGH on the binding of [ 125I]hGH to rabbit liver and mouse liver microsomes. Receptor binding assays were carried out using a constant dose (1 ng) of [ 125I]hGH and varying concentrations of MAbs. The assay was carried out in the presence of either excess ovine prolactin for the measurement of somatogenic (SOM) bindings sites, or excess bovine growth hormone for the determination of lactogenic (LAC) binding sites. Anti-hGH MAbs were found to have a whole spectrum of effects on hGH binding, including inhibitory, non-effect and enhancing activities. Enhancement of the binding of [ 125I]hGH to both SOM and LAC receptors was observed in liver membranes of rabbit or mouse. The observed amplified signal of [ 125I]hGH binding to various receptors in the presence of MAb no. 8 may be due to conformational changes which occur following MAb binding to hGH. On the other hand, most of the other MAbs caused inhibition of [ 125I]hGH binding. A negative correlation exists betweed the cross-reaction of various MAbs with the N-terminus truncated forms of hGH (Met 14-hGH or Met 8Leu-hGH) and their respective K D IC 50 values enable the evaluation of the crucial role of the N-terminus region in hGH binding to both LAC and SOME receptors. MAb nos 1 and 19, which are directed towards acid residues 95–134 and the C-terminus, inhibited SOM binding more potently than LAC binding. Thus, it seems that these mid-molecule and C-terminus regions are also important in hGH binding, and that they play a role in the partial overlap of SOM and LAC binding.