Growth hormone (GH) and estradiol regulation of membrane-associated GH binding protein and GH receptors in GH releasing hormone transgenic mice

eGrowth hormone (GH) releasing hormone (GHRH) transgenic mice were used to examine the influence of GH on GH receptor (GHR) and membrane-associated GH binding protein (MA-GHBP) levels by means of specific radioimmunoassays an d Western blot analysis, since MA-GHBP was described as the major constitue nt of somatogenic binding to liver membranes in mice. In transgenic animals , a 10-fold increment over normal values was found for hepatic somatogenic binding that could be accounted for by a 3-4-fold increase in GHR and a-fol d augmentation of MA-GHBP levels. The apparent molecular weight of MA-GHBP was smaller than that of serum GHBP, a difference that was partially abolis hed by endoglycosidase F digestion. In vivo treatment of female mice with 1 7 beta -estradiol led to an unexpected down-regulation of MA-GHBP and GHR b y 60-75% only in transgenic animals. MA-GHBP and GHR levels are strongly up -regulated by GH, although MA-GHBP up-regulation is much more important tha n that of GHR. (C) 2001 Harcourt Publishers Ltd.