Molecular markers for growth hormone sensitivity in man : focus on growth hormone-binding protein and insulin-like growth factor I gene expression

Abstract

Molecular markers for growth hormone sensitivity in man: Focus on growth hormone-binding protein and insulin-like growth factor I gene expression.Ragnar Bjarnason, International Pediatric Growth Research Centre, Department of Pediatrics, and Research Centre for Endocrinology and Metabolism, Department of Medicine, Sahlgrenska University Hospital, 416 85 Göteborg, SwedenAs the indications have widened for GH treatment, the demands for new diagnostic tests have increased. Hitherto, the investigations have been focused on evaluation of GH secretion, and less attention has been payed to variability in GH sensitivity. One possibility for assessment of GH sensitivity might be to measure human GH-binding protein (GHBP), which is identical to the extracellular part of the GH receptor (GHR). GHBP is encoded by the GHR gene, and is probably produced by proteolytic cleavage of the membrane-bound GHR. Most of the effects of GH are believed to be mediated by insulin-like growth factor I (IGF-I) and most of the circulating IGF-I is produced by the liver. However, IGF-I is also produced locally in most tissues. Aim: The general aim of the studies reported in this thesis was to obtain and evaluate tools for studying GH sensitivity in man. Studies were therefore performed to clarify the regulation of GHBP and to evaluate its role as a marker for GH sensitivity. In addition, a method was developed with which changes in IGF-I gene expression can be studied in needle biopsies obtained from different human tissue. Results: The main findings from the five papers that form the basis of this thesis are outlined below.Paper I: Twenty-nine prepubertal children were followed for 1 year, with monthly measurements of height, sitting height and weight. At the same time, blood samples were collected for measurements of GHBP, IGF-I and IGF-binding protein-3 and urinary GH excretion was measured in three consecutive overnight urine samples. There was considerable intra-individual variation in GHBP concentrations during the year, and the the mean GHBP concentration correlated with the mean serum concentrations of IGF-I and IGFBP-3 and with height velocity.Paper II: GHBP and leptin were measured in different groups of prepubertal children: children with normal GH secretion, short children born short for gestational age (SGA) and children diagnosed as having GHD. There was a highly significant correlation between GHBP and leptin levels in children with normal GH secretion and in short children born SGA. In 12 healthy children investigated every month for 1 year, there was a correlation between monthly changes in GHBP and leptin concentrations.Paper III: The influence of GH treatment on GHBP levels in short children was investigated. A 10% decrease in GHBP was found 6-8 hours after the s.c. injection of GH. In children with GHD there was a 30% decrease in GHBP levels after 3 months of GH treatment, and the levels returned to baseline after 1 year. No changes were seen in GHBP levels in children with ISS during GH treatment.Paper IV: Minor changes were seen in GHBP levels, in adults with GHD, after the start of GH treatment. High GHBP levels at the start of GH treatment were negatively correlated with changes in body composition after the start of treatment.Paper V: A new polymerase chain reaction (PCR)-based technique (quantitative reverse-transcriptase PCR [Q-RT-PCR]) was used to measure IGF-I mRNA in small human tissue biopsies. Using this assay, we show that GH prevents the decrease in IGF-I gene expression in skeletal muscle in patients undergoing abdominal surgery. Differential regulation of IGF-I levels in plasma and IGF-I gene expression in skeletal muscle was also demonstrated. Conclusions: There are considerable intra- and interindividual variations in GHBP levels in man. GHBP levels are correlated with markers of body composition. GH itself is not a major regulator of GHBP levels in man. GHBP may be a marker of GH sensitivity in large cohorts, but its role in the evaluation of the individual is limited. A link with nutrition is demonstrated by the correlation between GHBP and leptin. A novel assay for measurement of IGF-I gene expression in man, is reported and its application demonstrated as a new tool to study GH response in specific target tissues.