Evidence that the N-terminus of human growth hormone is involved in expression of its growth promoting, diabetogenic, and insulin-like activities.

Abstract

Recent work with various point and deletion mutants of human GH (hGH) has suggested that the proximal N-terminal end of the hormone molecule is important for its growth promoting action. This study was conducted to examine the growth promoting, diabetogenic, and insulin-like activities of two N-terminal mutants of hGH, the deletion mutant Des-7 hGH (met8, ala11), and a chimeric mutant of bovine GH (bGH) and hGH containing the N-terminal 13 amino acids of bGH (met, ala 1-13/14-191, asp11). The CD spectra of these mutants are similar to that of wild-type hGH and they retain lactogenic activity on Nb2 lymphoma cells, whereas their ability to bind to somatogenic receptors on IM-9 lymphocytes and bovine liver membranes is markedly reduced. In this study, growth promoting activity of the mutants was assessed using the 9-day weight gain test in hypophysectomized rats. Des-7 hGH had a potency of 0.03 IU/mg protein in this assay, whereas the potency of the bGH/hGH chimera was 0.71 IU/mg. Diabetogenic activity was tested in the ob/ob mouse, using the elevation of fasting blood glucose and the worsening of glucose tolerance after a 3-day course of treatment as end-points. Both Des-7 hGH and the bGH/hGH chimera had reduced diabetogenic activity compared to that of biosynthetic wild-type hGH, consistent with their reduced growth activity. Insulin-like activity was assessed by testing the in vitro ability of the mutants to stimulate [14C] glucose oxidation by epididymal adipose tissue of hypophysectomized rats. Des-7 hGH had about 1% the activity of wild-type hGH, whereas the chimera was about 20% as active. When Des-7 hGH was added to the incubation medium along with wild-type hGH in ratios of 5, 12.5, or 25:1 (Des-7 hGH:hGH), the insulin-like action of hGH was significantly inhibited, indicating that the mutant is a modest antagonist of the insulin-like action of hGH. When the ability of Des-7 hGH to compete with [125I] hGH for binding to isolated rat adipocytes was tested, the mutant was about 10% as effective as wild-type hGH. Thus, Des-7 hGH appears to be more effective in binding to adipocyte GH receptors than in triggering an insulin-like response, perhaps accounting for its modest antagonistic activity. The results of this study suggest that the proximal N-terminal end of the hGH molecule is involved in the expression of the growth promoting, diabetogenic and insulin-like activities of GH.

• Authors: R Towns, J L Kostyo, T Vogel, E Sakal, A Tchelet, R Maher, A Gertler
• Journal: Endocrinology (Endocrinology) Vol. 130 Issue 3 Pg. 1225-30 (Mar 1992) ISSN: 0013-7227 [Print] United States
• PMID: 1537288 (Publication Type: Journal Article, Research Support, Non-U.S. Gov't, Research Support, U.S. Gov't, Non-P.H.S.)

Chemical References

• Blood Glucose
• Insulin
• Receptors, Somatotropin
• Growth Hormone

Topics

• Adipose Tissue (drug effects, metabolism, ultrastructure)
• Animals
• Blood Glucose (metabolism)
• Female
• Growth Hormone (chemistry, genetics, physiology)
• Insulin (physiology)
• Male
• Mice
• Mice, Mutant Strains
• Mice, Obese
• Mutation (genetics)
• Protein Conformation
• Rats
• Receptors, Somatotropin (drug effects, physiology)
• Weight Gain (drug effects)