Expression Profiles of Receptor Activator of Nuclear Factor κB Ligand, Receptor Activator of Nuclear Factor κB, and Osteoprotegerin Messenger RNA in Aged and Ovariectomized Rat Bones

The receptor activator of nuclear factor-kappaB ligand (RANKL; also known as tumor necrosis factor-related activation-induced cytokine [TRANCE], osteoprotegerin ligand [OPGL], and osteoclast differentiation factor [ODF]) is a transmembrane ligand expressed in osteoblasts and bone marrow stromal cells. It binds to RANK, which is expressed in osteoclast progenitor cells, and induces osteoclastogenesis. OPG, a decoy receptor for RANKL, also binds to RANKL, and competitive binding of RANKL with RANK or OPG is thought to regulate bone metabolism. To investigate roles of the RANKL/RANK/OPG system in pathophysiological conditions, the expression of RANKL, RANK, and OPG messenger RNA (mRNA) was analyzed in bones of aged and ovariectomized rats by means of in situ hybridization. In the control 8-week-old male and sham-operated female rat bones, the expression of RANKL mRNA was detected in hypertrophic chondrocytes of the growth plate and some periosteal and endosteal mesenchymal cells. The expression of RANK mRNA was detected in osteoclast-like cells and mononuclear cells in contact with the cortical and trabecular bones. The expression of OPG mRNA was detected in proliferating chondrocytes and osteocytes. In the 2.5-year-old rat bones, the expression of RANKL, RANK, and OPG mRNA tended to decrease except for the endosteal region. In the ovariectomized rat bones, the expression of RANKL, RANK, and OPG mRNA increased, and high expression of OPG mRNA was induced in resting chondrocytes and osteocytes. These results suggest that estrogen deficiency stimulates the RANKL/RANK/OPG system and induces OPG in cells that have been thought to be less important for bone metabolism.