Colocalization of glucocorticoid and mineralocorticoid receptors in human bone

Osteoporosis is a poorly understood but common complication of glucocortico id therapy. The actions of glucocorticoids are mediated via glucocorticoid receptors (GRs), but in vitro, glucocorticoids also can bind to mineralocor ticoid receptors (MRs). It is not known if MR protein is present in human b one and little is known of GR isoform expression (GR alpha and GR beta). GR and MR protein expression and possible sites of action were investigated i n neonatal rib and adult iliac crest biopsy specimens using antibodies spec ific for MR, GR alpha, and GR alpha beta. Colocalization [MR GR alpha] [MR GR alpha beta] was performed using fluorescent-conjugated secondary antibod ies. GR alpha, GR beta, and MR show distinct but overlapping patterns of ex pression, suggesting important functions for each receptor type. Osteoclast s showed no staining for GR alpha but strong staining for GR alpha beta, in dicating expression of GR beta and a specific role in addition to antagoniz ing the transcriptional activity of GR alpha. MR also was observed in osteo clasts and colocalized with GR alpha beta. Coexpression of MR, GR alpha, an d GR alpha beta was seen in osteoblasts. Reverse-transcription-polymerase c hain reaction (RT-PCR) of cultured osteoblast RNA confirmed expression of b oth GR alpha and GR beta. Osteocytes stained with MR, GR alpha, and GR alph a beta antibodies but to a lesser degree than osteoblasts. In the neonatal rib cartilage, staining for GR alpha, GR alpha beta, and MR was present in approximately one-half of the resting and hypertrophic chondrocytes and in most of proliferating chondrocytes and chondrocytes within the mineralizing matrix. Identification of MR raises the possibility that the physiological and pharmacologic effects of glucocorticoids on bone may be mediated via M R as well as GR and that GR alpha, GR beta, and MR synergize to influence c orticosteroid metabolism in human bone.