Regulation of tissue insulin-like growth factor-1 production in the post-natal sheep through the growth hormone axis

Abstract

Somatic growth represents the outcome of interactions between the genetic potential of an individual to grow and environmental factors, prominent amongst these is nutrition. GH, the IGFs, and their respective receptors appear to play important and interrelated roles in co-ordinating these interactions, and as such they are considered one hormonal system ‘GH axis’ which controls normal post-natal growth. A large number of external and internal stimuli impinge on the GH axis. Experiments in this thesis were conducted to address whether nutritional status (a powerful external stimulus) or GH (a prominent internal stimulus) treatment differentially alters tissue and plasma IGF-1 immunoreactivity, tissue IGF-1 mRNA and plasma IGFBPs. GH stimulation of IGF-1 in the liver and other tissues and the associated response of circulating IGFBPs was measured with a view to investigating IGF-1 production in both hepatic and extra-hepatic tissue and its regulatory control through the GH axis. This would provide an insight into the relative contribution or importance of local versus hepatic production of IGF-1 in post-natal sheep. To achieve the objectives in this study, methods of IGF-1 tissue extraction were developed and evaluated in the experiments presented in this thesis to provide an efficient extraction of IGF-1 and reliable removal of IGFBP for measurement of IGF-1 by RIA in various tissues in sheep that were fed or fasted, with or without administration of exogenous GH. To monitor changes at transcriptional step, levels of IGF-1 mRNA in tissues were measured using in-situ hybridisation. Responsiveness of circulating IGFBPs to administration of GH was investigated by IGF-1 ligand blot analysis. Administration of exogenous GH significantly elevated IGF-1 levels in liver, kidney, heart, spleen and muscle. The rise in tissue (except muscle) levels of IGF-1 preceded that in plasma, suggesting that IGF-1 derived from multiple tissues contributes to plasma concentrations of the growth factor. GH stimulated IGF-1 mRNA in both hepatic and extra-hepatic tissues. The elevated levels of tissue IGF-1 and GH-dependent IGF-1 mRNA in both hepatic and extra-hepatic tissues indicate production of IGF-1 in the liver and other tissues. The response of muscle IGF-1 to administration of GH followed the rise in plasma IGF-1. This may indicate a late response in local production of IGF-1 in the muscle or may reflect the influence of changes in IGF-1 plasma concentration, implicating a possible endocrine role for IGF-1 in the muscle, although the relative contribution or importance of local versus plasma-origin IGF-1 is not certain in this tissue. Data in this thesis demonstrated that GH not only stimulates IGF-1 production but also plays a pivotal role in regulation of circulating levels and tissue availability of IGF-1 (ie. tissue distribution of IGF-1) by affecting both IGF-1 and its binding proteins either directly or indirectly. Fasting depressed plasma and tissue levels of IGF-1. The degree of the depression in IGF-1 levels varied between tissues. Levels of IGF-1 mRNA in liver were positively correlated with concentrations of liver IGF-1 in animals that were well fed with or without GH treatment, but not in those that were fasted or fasted and treated with GH. This indicates that GH regulation of IGF-1 production in liver is, to a large extent, dependent on nutritional status. Nutrition therefore plays a critical role in the regulation of IGF-1 production in the sheep. The current study has also provided a way to estimate total organ content of IGF-1 and the response of this parameter to GH treatment. The GH-induced increase in total liver IGF-1 is 4-fold more than the corresponding increase in kidney, 13-fold more than that in heart and 30-fold more than that in spleen. These results support the traditional concept that liver is a major contributor of circulating IGF-1, although the data in this study suggested that IGF-1 is synthesised in multiple tissues in the sheep. In conclusion, the results of the present study are compatible with the concept of both local and classic central production of IGF-1 under the influence of GH, and therefore do not exclude either an autocrine/paracrine or an endocrine mechanism of IGF-1 action. The relative importance or contribution of each may be related to a tissue-specific fashion of IGF-1 gene expression which is differentially regulated by GH, nutrition and numerous other factors.