Colocalization of glucocorticoid and mineralocorticoid receptors in human bone

Osteoporosis is a poorly understood but common complication of glucocorticoid therapy. The actions of glucocorticoids are mediated via glucocorticoid receptors (GRs), but in vitro, glucocorticoids also can bind to mineralocorticoid receptors (MRs). It is not known if MR protein is present in human bone 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 in neonatal rib and adult iliac crest biopsy specimens using antibodies specific for MR, GR alpha, and GR alpha beta. Colocalization [MR GR alpha] [MR GR alpha beta] was performed using fluorescent-conjugated secondary antibodies. GR alpha, GR beta, and MR show distinct but overlapping patterns of expression, suggesting important functions for each receptor type. Osteoclasts showed no staining for GR alpha but strong staining for GR alpha beta, indicating expression of GR beta and a specific role in addition to antagonizing the transcriptional activity of GR alpha. MR also was observed in osteoclasts and colocalized with GR alpha beta. Coexpression of MR, GR alpha, and GR alpha beta was seen in osteoblasts. Reverse-transcription-polymerase chain reaction (RT-PCR) of cultured osteoblast RNA confirmed expression of both GR alpha and GR beta. Osteocytes stained with MR, GR alpha, and GR alpha 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 MR as well as GR and that GR alpha, GR beta, and MR synergize to influence corticosteroid metabolism in human bone.