It is well known that long-term
glucocorticoid treatment causes
osteoporosis, but the precise mechanism remains unclear. Recently,
osteoprotegerin (OPG) has been identified as a
cytokine that inhibits osteoclast differentiation. We have previously demonstrated that serum OPG is suppressed by
glucocorticoids. Therefore, the present study was carried out to clarify the interrelationships between OPG and other markers of bone metabolism during
glucocorticoid treatment. Thirteen patients (7 men, 6 women; 44.1 +/- 5.9 years old) with chronic
glomerulonephritis who were to be treated with
glucocorticoids for the first time were chosen for this study. Markers of bone metabolism, including serum OPG,
osteocalcin (OC), bone-specific
alkaline phosphatase activity (bAP),
parathyroid hormone (PTH),
tartrate-resistant acid phosphatase (TRAP), and bone mineral density (BMD), were measured before and during the treatment period.
Glucocorticoids significantly reduced BMD of the lumbar spine in the 6 month treatment period (p < 0.01). Serum OPG was decreased significantly by
glucocorticoids within 2 weeks (p < 0.001), and serum TRAP, a marker of
bone resorption, was markedly increased (p < 0.001). On the other hand, there were no remarkable changes in serum PTH. Serum OC and bAP, markers of bone formation, were transiently reduced during the treatment period (p < 0.01). Furthermore, only serum OPG was positively and independently correlated with percentage BMD of age-matched reference (%AMR). These findings imply that
glucocorticoid-induced bone loss develops rapidly via enhanced
bone resorption and suppressed bone formation. Moreover, the increased
bone resorption caused by
glucocorticoids may be, at least in part, mediated by inhibition of OPG, not increment of PTH.