Growth hormone-binding protein in normal mice and in transgenic mice expressing bovine growth hormone gene

Abstract

The levels and characteristics of growth hormone (GH)-binding protein (GHBP) and the distribution of GH in peripheral circulation between the free and the bound fractions were studied in three lines of transgenic mice with various degrees of overexpression of bovine (b) GH gene. Two serum fractions bound GH specifically: one with low affinity and high capacity (GHBPI) and one with high affinity and low capacity (GHBPII). The GHBP binding capacity in normal mice (both sexes), transgenic male mice that express the metallothionein-I-hybrid bGH genes, transgenic female mice that express phosphoenolpyruvate carboxykinase (PEPCK)-bGH hybrid genes (PEPCK-bGH-1), and transgenic PEPCK-bGH-5 animals was 1.1 +/- 0.2, 2.0 +/- 0.1, 3.0 +/- 0.1, and 3.9 +/- 0.6 pmol/ml serum, respectively. The amount of GH bound to GHBP in transgenic animals vs. normal siblings was increased 1.8-, 2.5-, and 3.9-fold in these three lines. Consequently, the levels of GH-GHBP complexes in the circulation of PEPCK-bGH-1 transgenic mice were increased approximately 10-fold. Specific GHBP radioimmunoassay confirmed a threefold increase in GHBP in PEPCK-bGH-1 transgenic animals. The levels of GHBP were not significantly correlated to serum GH within or between lines, perhaps due to elevation of serum GH in PEPCK-bGH mice above the level producing maximal response. From these and previous studies, we conclude that life-long exposure to supranormal GH levels leads to major shifts in GH binding in the circulation and in the GH target organs.

Differential regulation of membrane associated-growth hormone binding protein (MA-GHBP) and growth hormone receptor (GHR) expression by growth hormone (GH) in mouse liver

“…BW from each mouse line/BW of normal control siblings) was plotted against mGH equivalents. As previously described [29][30][31][32][33][34][35]38,40,42], the absence of GH was associated with reduced BW while increasing equivalents of mGH in serum produced an increase in BW (Fig. 1).…”

“…PEPCK-bGH, Mt-bGH, PEPCK-hGH and Mt-GHRH transgenic mice were used as models of GH overexpression [31][32][33]35]. Two different lines overexpressing bGH were used because they present different hormone levels, PEPCK-bGH transgenic mice show higher levels of serum bGH compared to Mt-bGH transgenic mice [31,40]. Growth hormone releasing hormone (GHRH) transgenic mice were studied because, as a consequence of GHRH overexpression, these mice exhibit high mouse GH (mGH) levels [35,42].…”

“…mGH, bGH and hGH were determined by RIA as previously described [40,41] using RIA kits obtained through the National Hormone and Pituitary Program, National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK), National Institutes of Health (NIH), Bethesda, MD, USA. Ovine GH (oGH, for bGH RIA), mGH and hGH were labeled using limiting amounts of chloramine T; specific activity ranged from 70 to 120 lCi/lg.…”

“…GHs of human, bovine and mouse origin have different affinities for the mouse GHBP and GHR [11,40,42]. To compare their biological activity it is necessary to consider not only the concentration of the hormones but also their ability to bind and thus activate the GHR, which depends on their different affinity for the GHR and GHBP.…”

Transgenic mice overexpressing GH exhibit hepatic upregulation of GH-signaling mediators involved in cell proliferation

“…PEPCK-bGH transgenic mice used in this work exhibit lifelong elevated bGH levels with a consequent increase in body weight (Sotelo et al 1995, Miquet et al 2004. In accordance with the positive regulation that chronic GH increase exerts over its receptor (McGrane et al 1990, Iida et al 2004, González et al 2007, hepatic levels of GH receptor are increased in transgenic mice overexpressing GH (Aguilar et al 1992, González et al 2001, Miquet et al 2004.…”