N-terminal-truncated recombinant analogs of bovine placental lactogen: interaction with human and rat growth hormone receptors and insulin-like growth factor-I secretion mediated by somatogenic receptors in rat hepatocytes.

Bovine placental lactogen (bPL) was found to be as potent as human GH (hGH) in its ability to bind to soluble full-size recombinant hGH-binding protein (hGHBP) and to membrane-embedded hGH receptor in intact IM-9 human lymphocytes. bPL was also capable of forming a 1:2 complex with hGHBP, although the structure of this complex was probably more compact than that with hGH. Removal of 13 amino acids from the N-terminus of bPL did not affect its ability to bind to hGHBP or hGH receptors in intact IM-9 cells. Its ability to form a 1:2 complex with hGHBP was, however, impaired, unlike that of a corresponding analog in which an L28F mutation has been simultaneously introduced. Truncation of 17 amino acids decreased its affinity toward both hGHBP and GH receptors on intact IM-9 lymphocytes and in liver rat microsomal fraction and inhibited the formation of 1:2 complexes with hGHBP. Simultaneous L28F mutation did not affect affinity toward hGHBP, but increased affinity toward rat liver GH receptors and restored affinity toward membrane-embedded hGH receptors in IM-9 lymphocytes and the ability to form a 1:2 complex with hGHBP. Truncation of 20 amino acids further decreased affinity toward both hGHBP and receptors in intact IM-9 lymphocytes and completely abolished formation of a 1:2 complex with hGHBP. Both des-13-bPLs and bPL-des-17 (L28F) retained their full ability to stimulate insulin-like growth factor-I secretion by rat hepatocytes compared to that of bPL. The insulin-like growth factor-I stimulatory activities of bPL-des-17 and bPL-des-20, however, were decreased to 1-5%. These results indicate that the stoichiometry of 1:2 complex formation with hGHBP may be preserved despite decreased receptor binding affinity, but the lower affinity of the putative site 1 or site 2 of the analog may account for the decrease in biological activity. Furthermore, the ability or inability of bPL or its truncated analogs to form 1:2 complexes with soluble hGHBP cannot predict their somatogen receptor-mediated biological activity in rat hepatocytes.