Expression of RANKL and OPG correlates with age-related bone loss in male C57BL/6 mice

Osteoblasts regulate the recruitment and activity of osteoclasts through expression of RANKL and osteoprotegerin (OPG). To determine whether expression of RANKL and OPG change with age and how these changes relate to the bone loss of aging, we measured bone mass and cancellous volume, and expression of RANKL, OPG, alkaline phosphatase (AP), osteocalcin (OC), and alpha I collagen (COLL) in whole bone and osteoblast-like cells in culture using 6-week- (young), 6-month- (adult), and 24-month-old (old) mice. Cancellous volume decreased by 20% from young to adult and by 52% from adult to old. RANKL mRNA levels in whole bone were 2.1-fold and 4.4-fold higher in adult and old mice, respectively, compared with young mice, whereas OPG mRNA levels decreased with age slightly. RANKL expression was negatively (r = -0.99) and OPG was positively (r = 0.92) correlated with cancellous bone volume. Expression of RANKL was higher and OPG lower in cells from older animals early in culture (day 7). With cell maturation, RANKL mRNA levels in cells from young and adult mice increased, whereas levels in cells from old animals decreased. By 21 and 28 days of culture, no differences were found in RANKL mRNA in osteoblast-like cells among different age groups. We conclude that expression of RANKL and OPG change with age in whole bone and in cultured osteoblast-like cells. These changes favor increased osteoclast over osteoblast activity, and may explain, in part, the imbalance in bone formation and resorption associated with aging.