Abstract
Growth hormone (GH) promotes postnatal longitudinal growth in children and is active throughout an individual’s live in protein, fat and carbohydrate metabolism. GH is an anabolic hormone, inducing a positive nitrogen balance and protein synthesis in muscle. The multiple actions of GH start when GH binds to the growth hormone
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receptor (GHR). The effectiveness of the peptide hormone depends on its presence in the circulation as well as the availability of receptors at the plasma membrane of target cells. The GHR is synthesized in most cells of the human body with high expression levels in liver and adipose tissue. Because of the rapid turnover of the GHR and the pulsatile secretion of GH into the circulation, controlling the number of GHR per cell is a crucial factor in the potential of the system. In this thesis we show that the ubiquitin-proteasome pathway regulates the cell surface expression of the GHR, and thus the availability of the GHR for signalling.
The ubiquitin-proteasome pathway has been implicated in many cellular processes. Ubiquitination is a post-translational modification that can regulate protein levels, protein localization and receptor signalling. GHR ubiquitination occurs at the cell surface and coincides with receptor recruitment to clathrin-coated membrane domains, indicating that the ubiquitin-conjugation system and the classical endocytic machinery may cooperate to regulate the time-span of the GHR at the cell surface. The number of GHR at the cell surface is regulated by endocytosis and by shedding. GHR internalisation requires proteasome action in addition to an active ubiquitin-conjugation system. In the presence of specific proteasome inhibitors, GHR endocytosis and lysosomal degradation are inhibited. By comparing internalisation of the receptor with shedding of the GH-binding protein, we show that ubiquitin system-dependent internalisation followed by lysosomal degradation is the major pathway for receptor degradation.
Plasma membrane proteins are essential for cells to communicate with the outside world, as well as to sense signals and changing conditions. The activity of plasma membrane proteins can be regulated by their number at the cell surface. In this thesis we established a role for the ubiquitin-proteasome pathway in regulating the number of GHR at the cell surface. Nutritional state profoundly affects all levels of the GH/Insulin-like growth factor-1 (IGF-1) axis. Changes due to fasting represents a condition known as GH resistance. High GH and low IGF-1 levels are characteristic for GH resistance, indicating reduced signalling via the GHR. Malnutrition and other catabolic states that have been associated with acquired GH insensitivity are accompanied by reduced GH-binding protein concentrations, increased protein turnover, and a negative nitrogen balance. Protein degradation exceeds synthesis in catabolic diseases and muscle atrophy will occur as seen in starvation, cancer cachexia, sepsis, renal failure, burns and AIDS. The enhanced proteolysis in these various pathological states is primarily due to activation of the ubiquitin-proteasome pathway. From our results it is expected that the upregulation of the ubiquitin-proteasome pathway results in the downregulation of the GHR at the surface of target cells, thereby providing a possible explanation for the decreased protein synthesis in these patients
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