The regulation of the sclerostin gene and the catabolic effects of sclerostin protein on bone.

Abstract

Age and disease-related bone loss is a major health issue. Bone tissue is constantly remodelled throughout life in order to maintain a healthy skeleton and bone loss is caused by an imbalance in the remodelling process. Bone remodelling is a highly coordinated process between osteoclasts, osteoblasts and osteocytes, with bone targeted for renewal being resorbed by osteoclasts and the resorbed bone replaced by the activities of osteoblasts and osteocytes. During the synthesis of new bone organic matrix, osteoblasts become embedded and differentiate into osteocytes. Osteocytes were previously thought to be terminally differentiated, quiescent cells. However, a wealth of recent evidence suggests that osteocytes play important and dynamic roles. Recently, the osteocyte expressed protein, sclerostin, was identified to be a major regulator of bone formation. Various pharmaceutical companies are currently in the process of developing therapies to neutralise sclerostin, in order to reverse its antianabolic effects on bone. In pre-clinical and clinical studies to date, neutralising sclerostin had bone anabolic effects, and although anti-catabolic effects were also observed, these were usually reported as incidental events. Stemming from observations made by our group of pro-catabolic stimuli up-regulating sclerostin expression, it was hypothesised that sclerostin may have a catabolic action in addition to its anti-anabolic actions. Subsequent work identified the pre-osteocyte/osteocyte as cellular targets of sclerostin, and gene microarray analyses of osteocyte-like cells treated with recombinant sclerostin, led to the discovery of two novel mechanisms, by which sclerostin may act in a catabolic manner. As presented in Chapter 2, the work undertaken for this thesis demonstrated that sclerostin promotes osteocyte support of osteoclast formation and activity, consistent with recent reports by other groups that suggest osteocytes play a central role in regulating the formation and activity of osteoclasts. As presented in Chapter 3, sclerostin can also increase the expression by osteocytes of resorption-related molecules, in particular carbonic anhydrase 2. The importance of this observation is that acidification of the extracellular space by osteocytes could promote osteocytic release of mineral and increase of the osteocyte lacunar size, a process termed ‘osteocytic osteolysis’. The results presented in Chapter 3 provide the first mechanistic evidence for this process. As presented in Chapter 4, 1α,25-dihydroxyvitamin D (1,25D) was also identified as a regulator of SOST/sclerostin expression and a putative vitamin D response element (VDRE) was shown to be present in the proximal 6.3 kb SOST promoter. In summary, the novel work presented in this thesis expands our knowledge of the activity and regulation of sclerostin. Together, these findings suggest that a subset of pro-catabolic stimuli may induce sclerostin expression, which in turn may act to promote both osteoclastic and osteocytic removal of bone. This research has implications for the pharmacological inhibition of sclerostin, which is currently being pursued commercially. In embarking on such therapy, it is essential to understand the biology of sclerostin as completely as possible.