Glucocorticoid Receptors and Bone

Abstract

Glucocorticoid Receptors (GRs) have been identified in all bone cells. The molecular structure of human GR is organized into 3 major functional domains: the N-terminal immunogenic domain, the central DNA-binding domain and the C-terminal ligand-binding domain. Human GR is a product of a gene composed of 10 exons, located in the chromosome 5q31-32. An alternative splicing in exon 9 gives rise to 2 mRNAs encoding the classical hGRα and hGRβ isoforms. Human GRα is present in the cytoplasm of almost all cells, as a multiprotein complex and works as a ligand-dependent transcription factor. In contrast to hGRα, hGRβ is located in the nucleus, does not bind hormone or activate glucocorticoid (GC)-response genes. It works as a dominant negative inhibitor of hGRα. The effects of GCs are - at least in part - mediated via specific GRs (genomic effect), however GCs also have acute non genomic effects. Osteoblasts are the most obvious target of GCs in bone, suppressing their maturation, activity and survival. Osteoblasts stimulate osteoclastic activity through the RANKL-osteoprotegerin-RANK system, but this effect is weaned off rapidly by the incoming suppression of the global osteoblast activity. The direct action of GCs on osteoclasts results almost invariably in a suppression of cell activity. When exposed to high concentrations of GCs, osteocytes undergo a slow process of apoptosis. Osteocytes with their dendritic network sense the skeletal strain and stress of normal daily activities. This continuous stimulus prevents the production of sclerostin and possibly DKK1, which are able to strongly suppress osteoblast formation by interacting with the Wnt system. GCs are thought to stimulate sclerostin secretion from osteocytes.