Biological activities of growth hormone and growth hormone antagonists

Bovine growth hormone (bGH) analogs with three amino acids substitutions (bGH-M8: Glu117-Leu, Gly119-Arg, Ala122-Asp) and one amino acid substitution (bGH-M6: Gly119-Arg) have been reported to be functional antagonists of GH using transgenic mice. In this study, the antagonistic effects of bGH-M8, bGH-M6, as well as human (h) equivalent of bGH-M6, hGH-G120R, were analyzed in vitro. Both bGH-M8 and bGH-M6 retained the same binding affinity as bGH. Also, hGH-G120R exhibited the same binding affinity as hGH. GH antagonists did not possess biological effects on insulin-like, lipolytic, diabetogenic, or adipogenic-activities. However, they antagonize the activities of GH in a concentration-dependent manner. Equimolar concentrations of GH antagonists were inhibitory and 10- to 30-fold excess of GH antagonists abolished the effects of GH. The antagonistic effects could be overcome by increasing GH concentrations. The change in bGH of Gly-119 to Arg is sufficient for the antagonistic effect, since bGH-M6 has the same potency as bGH-M8. A wide range of inhibitory actions and apparent competitiveness, together with the results of specific binding assays, suggest that the inhibitory actions of GH antagonists could be exerted at the level of GH-GH receptor association.;In addition, the signalling mechanism of GH was analyzed in 3T3-F442A preadipocytes. GH treatment rapidly produced diacylglycerol (DAG), reaching a peak level in 90 to 120 seconds. However, GH antagonists did not affect DAG levels. The DAG production was not accompanied by phosphoinositide metabolism. Instead, phosphocholine was produced in a time course similar to DAG formation. Intracellular Ca...