Parathyroid hormone (PTH) and growth hormone (GH) are known to decline with ageing and have been associated with the process of apoptosis in the osteoblast and osteocyte cell populations. The presence of viable osteocytes in bone is an important parameter in maintaining bone quality while osteocyte death by apoptosis during ageing has been associated with loss of bone strength. PTH (1-34) administration as part of hormone replacement therapy (HRT) has been shown to exert positive effects via the stimulation of bone formation and improved bone microarchitecture, mainly through the inhibition of osteoblast and osteocyte apoptosis. GH on the other hand has been shown to exert positive effects on bone formation, but due to financial cost not many clinical trials have been perfonned. However in vivo studies have demonstrated the positive effects of GH on osteoblasts and osteocytes with a direct or indirect manner, possibly through the stimulation of growth factors and mainly IGF-I. Although their potential use as anabolic treatments has long been recognised and their effects on osteoblasts and osteocytes have been studied for many years, their effects on the maintenance of the osteocytic population in vivo are less clear. Work in this thesis has investigated whether recombinant human parathyroid hormone (rhPTH) and recombinant human growth hormone (rhGH) can inhibit osteocyte apoptosis, either due to ageing or damage as well as the potential of these hormones to trigger specific gene response on the osteoblast in the ageing rat skeleton. An in vitro study in this thesis has indicated the protective effects of rhPTH and rhGH in osteocyte cultures following physical injury in vitro. These data have provided evidence for the first time of effects of these hormones in the repair of cell membranes following disruption. In this study, physical membrane disruption to MLO-Y4 cells in vitro was taken to represent physical injury-which is one aspect of microdamage to osteocytes in bone. These findings may provide a new understanding of how osteocytes sense and respond to injury and the potential of therapeutical compounds to maintain osteocyte viability in disease and old age. rhPTH and rhGH treatment was shown to reduced osteocyte apoptosis in aged female rat bone, while improved trabecular architecture was observed in response to rhPTH treatment but not to rhGH. These data point to the potential of these hormones to be used as anabolic treatments. However the molecular mechanisms by which rhPTH and rhGH maintain the osteocytic population and lead to bone fonnation are not well understood. The effects of mechanical loading on the maintenance of the osteocytic population were investigated in human cancellous bone explants ex vivo in the presence or absence of rhPTH. Osteocyte viability was increased and a significant reduction in osteocyte apoptosis was observed compared to unloading conditions after 7 days in culture. Furthermore alkaline phosphatase a marker of an osteogenic repsonse was increased by mechanical stimulation in the presence or absence of rhPTH compared to unloading conditions, indicating a stimulatory effect on osteoblasts. Moreover SOST gene expression was decreased in the presence or absence of mechanical stimulation following administration of rhPTH. These data pointed to important anabolic effects induced by mechanical stimulation in the presence or absence of rhPTH. Finally a laser microdisection study using the tibias of old female rats presented in this thesis provides the basis for future studies aimed to identify specific osteoblastic genes regulated by rhPTH and rhGH at different anatomical sites of bone. Such data would provide the necessary infonnation to elucidate the molecular mechanisms that regulate rhPTH and rhGH and their effects on bone. In this study identification of the anti-apoptotic and protective effects of rhPTH and rhGH on osteocytes in vivo, in vitro and ex vivo highlighted important infonnation on their bioactivity in vivo. Moreover an attempt to elucidate the molecular mechanisms behind rhPTH and rhGH effects on osteoblasts in vivo has been presented and provides the basis for future studies that may prove useful in identifying specific drug targets for the treatment of pathological bone.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:726425 |
| Date | January 2009 |
| Creators | Voultsiadou, Antiopi |
| Publisher | University of Edinburgh |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://hdl.handle.net/1842/25277 |
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