The effect of bone matrix extract on bone cell activity

Bone remodelling is a complex process, which involves the coupling of bone formation to completed foci of bone resorption, the balance between these 2 processes determines if bone is lost or gained at a particular site. During bone resorption osteoclasts release growth factors sequestered in bone matrix, which are thought to initiate new bone formation. On the other hand, osteoblasts can regulate osteoclast activity through the expression of the counter-acting cytokines, RANKL and OPG. The aim of this project was to determine if factors released during bone resorption impact on the RANKL/OPG system or on osteoclasts directly to regulate bone remodelling. OPG secretion was characterized in a number of osteoblast-like cells and the osteosarcoma cell line MG-63 was chosen as a model for osteoblastic cell behaviour in vitro. EDTA bone extracts prepared from normal human cortical bone powder were used to treat MG-63 cells in vitro. The response to the extract was dependent on the purification procedure used. OPG production was inhibited by partially purified extracts prepared using hydrophobic interaction chromatography, C18 SPE. In comparison extracts prepared using size exclusion centrifugal filters stimulated OPG secretion in confluent MG-63 cells. Therefore bone matrix constituents were able to influence osteoclast activity directly and indirectly through the osteoblastic cells to produce the same response. The simplest mechanism for this co-ordinated response would be the presence of one factor in the extract that is able to influence both osteoblasts and osteoclasts. The identity of the factor responsible for the opposing effects seen in the bone matrix extracts is at the moment unknown. The work presented in this thesis clearly demonstrated that unknown growth factors present in bone matrix influence bone remodelling