THE GROWTH-HORMONE (GH)-BINDING PROTEIN CLONED FROM HUMAN IM-9 LYMPHOCYTES MODULATES THE DOWN-REGULATION OF GH RECEPTORS BY 22-KILODALTON AND 20-KILODALTON HUMAN GH IN IM-9 LYMPHOCYTES AND THE BIOLOGICAL EFFECTS OF THE HORMONE IN NB-2 LYMPHOMA-CELLS

The cDNA coding for the 246-amino acid long N-terminal extracellular p ortion of the human (h) GH receptor, corresponding to the circulating GH-binding protein (hGHBP), was cloned by polymerase chain reaction fr om human IM-9 lymphocytes. The cDNA sequence was identical to that rep orted for human liver and placenta and demonstrated alternative splici ng of exon 3. The protein with the exon 3-encoded domain was expressed and secreted in glycosylated form from baby hamster kidney (BHK) cell s, purified to homogeneity, and sequenced; the amino acid sequence was identical to that predicted from liver cDNA. The cloned hGHBP compete d in a dose-dependent fashion for binding of I-125-labeled 22-kilodalt on (kDa) hGH, and at higher concentrations for binding of I-125-labele d 20-kDa hGH, to IM-9 lymphocytes. hGHBP decreased the association rat e of [I-125]hGH to the cells without decreasing the dissociation rate. hGHBP blocked the down-regulation of GH receptor in IM-9 cells by bot h 22- and 20-kDa hGH. hGHBP also blocked the binding of [I-125]hGH to PRL receptors on Nb2 lymphoma cells and the effect of the hormone on t hymidine incorporation. Binding of both 22- and 20-kDa hGH to the bind ing protein was demonstrated directly by immunoprecipitation with mono clonal antibody 263. The present work thus establishes the identity of the IM-9 human GHBP from those of liver and placenta, and demonstrate s its ability to bind both 22- and 20-kDa hGH with good affinity and t o block their biological actions mediated though both somatogenic and lactogenic receptors. The modulation of receptor down-regulation by th e BP may be a relevant facet of its physiological role.