MONOCLONAL-ANTIBODIES TO HUMAN GROWTH-HORMONE MODULATE ITS BIOLOGICALPROPERTIES

Previous results indicated that monoclonal antibodies (mAbs) termed mA b AE5, mAb AC8 and mAb F11, recognizing the human growth hormone (hGH) region left exposed after binding to lactogenic, somatogenic and hGH- specific receptors, produce allosteric changes in the hormone which mo dify its binding properties. To study whether these mAbs could also in fluence hGH biological activity, experiments were carried out with Nb2 cells, a rat lymphoma cell line which proliferates in the presence of lactogenic hormones. Experiments involving previous binding of the ho rmone to receptors before adding I-125-mAbs indicated that the hGH dom ain defined by overlapped epitopes AE5, AC8 and F11 is uncovered in hG H when it is bound to the cell membranes. To reveal any alteration in the hGH molecule induced by the mAbs, preformed I-125-mAb:hGH complexe s were added to the cell membranes. Data showed that I-125-mAb AE5:hGH complexes bound better to the receptors than free hormone. On the con trary, hGH previously bound to I-125-mAb AC8 Or I-125-mAb F11 was poor ly recognized by Nb2 receptors. Furthermore, both mAbs AC8 and F11 str ongly inhibited I-125-hGH binding to Nb2 cell membranes and hGH-induce d Nb2 cell proliferation whereas mAb AE5 enhanced both hormone binding and hGH mitogenic effect. Additionally, since mAb AC8 is directed tow ards an epitope shared by hGH and human placental lactogen (hPL), it w as also shown that this mAb could impair hPL biological activity even though it recognizes the hPL region left exposed in hPL:Nb2 cell recep tor complexes, Data presented in this work suggest that mAbs directed to the hGH or hPL regions unmasked after binding to Nb2 cell receptors produce allosteric alterations in the binding properties of these hor mones leading to either enhancement or decrease of their biological ac tivities.