Determination of the effect of acetylation of specific lysine residues in human growth hormone on its affinity for somatogenic receptors by an affinity selection technique

A long-acting, mono-PEGylated human growth hormone analog is a potent stimulator of weight gain and bone growth in hypophysectomized rats

Recombinant human GH is used to treat GH deficiency in children and adults and wasting in AIDS patients. GH has a circulating half-life of only a few hours in humans and must be administered to patients by daily injection for maximum effectiveness. Previous studies showed that longer-acting forms of GH could be created by modification of GH with multiple 5-kDa amine-reactive polyethylene glycols (PEGs). Eight of nine lysine residues and the N-terminal amino acid were modified to varying extents by amine PEGylation of GH. The amine-PEGylated GH product comprised a complex mixture of multiple PEGylated species that differed from one another in mass, in vitro bioactivity, and in vivo potency. In vitro bioactivity of GH was reduced 100- to 1000-fold by extensive amine PEGylation of the protein. Here we describe a homogeneously modified, mono-PEGylated GH protein that possesses near complete in vitro bioactivity, a long half-life, and increased potency in vivo. The mono-PEGylated GH was created by substituting cysteine for threonine-3 (T3C) of GH, followed by modification of the added cysteine residue with a single 20-kDa cysteine-reactive PEG. The PEG-T3C protein has an approximate 8-fold longer half-life than GH after sc administration to rats. Every other day or every third day administration of PEG-T3C stimulates increases in body weight and tibial epiphysis growth comparable with that produced by daily administration of GH in hypophysectomized rats. Long-acting, mono-PEGylated GH analogs such as PEG-T3C are promising candidates for future testing in humans.

Evidence for involvement of the carboxy terminus of helix 1 of growth hormone in receptor binding: use of charge reversal mutagenesis to account for calcium dependence of binding and for design of higher affinity analogues

In this study we have demonstrated that the C-terminus of helix 1 of porcine GH (pGH) is a receptor-interactive region, thus extending the current binding site model of GH. This was achieved by introducing charge reversal mutations into this region of pGH, which influenced receptor affinity and Ca2+ dependence of binding. The first mutant (R34E pGH, conversion of Arg 34 to Glu) introduced a putative Ca2+ binding site which is present in human GH (hGH) [Barnard et al. (1989) J. Theor. Biol. 140, 355-367] and sits opposite E220 of receptor subunit 1. This mutant exhibited increased Ca2+ dependence of receptor binding but even at optimal Ca2+ did not display higher than wild-type affinity. Introduction of a second Ca2+ binding site adjacent to the first by a second charge reversal (K30E R34E pGH) further increased Ca2+ dependence of binding and also increased affinity for the rabbit GH receptor (2.4 +/- 0.4)-fold relative to wild-type pGH at optimal Ca2+. Equilibrium dialysis and Scatchard analysis of binding of 45Ca2+ to pGH and K30E R34E pGH revealed two Ca2+ binding sites on wild-type pGH and an additional two Ca2+ binding sites on the K30E R34E pGH mutant (Kd 0.5-0.8 mM), as predicted. A third partial charge reversal mutant in the fourth helix (H170D) also led to enhanced Ca2+ dependence of binding, supporting our proposal that E34 and D170 are responsible for the Ca2+ dependence of hGH binding to the rabbit GH receptor. Examination of the crystal structure shows that E34 and D170 are in close proximity and would interact repulsively with a cluster of acidic residues on the receptor consisting of E126, E127, and E220 unless neutralized by Ca2+ or an introduced basic residue. Accordingly, charge reversal at the adjacent pGH residue E33 (E33K pGH) led to a Ca2+ independent (3.0 +/- 0.4)-fold increase in affinity of binding. As well as extending the binding site model of GH, these studies provide a mechanistic explanation for the unique Ca2+ dependence of hGH binding to the rabbit GH receptor. They also indicate that charge reversal can be used to design higher affinity GH analogues and could assist in the mapping of interactive regions in ligand-receptor complexes generally.

High-resolution epitope mapping of hGH-receptor interactions by alanine-scanning mutagenesis

A strategy, called alanine-scanning mutagenesis, was used to identify specific side chains in human growth hormone (hGH) that strongly modulate binding to the hGH receptor cloned from human liver. Single alanine mutations (62 in total) were introduced at every residue contained within the three discontinuous segments of hGH (residues 2 to 19, 54 to 74, and 167 to 191) that have been implicated in receptor recognition. The alanine scan revealed a cluster of a dozen large side chains that when mutated to alanine each showed more than a four times lower binding affinity to the hGH receptor. Many of these residues that promote binding to the hGH receptor are altered in homologs of hGH (such as placental lactogens and prolactins) that do not bind tightly to the hGH receptor. The overall folding of these mutant proteins was indistinguishable from that of the wild-type hGH, as determined by strong cross-reactivities with seven different conformationally sensitive monoclonal antibodies. The alanine scan also identified at least one side chain, Glu174, that hindered binding because when it was mutated to alanine the receptor affinity increased by more than a factor of four.

Receptor and antibody epitopes in human growth hormone identified by homolog-scanning mutagenesis

A strategy, termed homolog-scanning mutagenesis, was used to identify the epitopes on human growth hormone (hGH) for binding to its cloned liver receptor and eight different monoclonal antibodies (Mab's). Segments of sequences (7 to 30 residues long) that were derived from homologous hormones known not to bind to the hGH receptor or Mab's, were systematically substituted throughout the hGH gene to produce a set of 17 chimeric hormones. Each Mab or receptor was categorized by a particular subset of mutant hormones was categorized by a particular subset of mutant hormones that disrupted binding. Each subset of the disruptive mutations mapped within close proximity on a three-dimensional model of hGH, even though the residues changed within each subset were usually distant in the primary sequence. The mapping analysis correctly predicted those Mab's which could or could not block binding of the receptor to hGH and further suggested (along with other data) that the folding of these chimeric hormones is like that of HGH. By this analysis, three discontinuous polypeptide determinants in hGH--the loop between residues 54 and 74, the central portion of helix 4 to the carboxyl terminus, and to a lesser extent the amino-terminal region of helix 1--modulate binding to the liver receptor. Homolog-scanning mutagenesis should be of general use in identifying sequences that cause functional variation among homologous proteins.