Osteoprotegerin and its correlations with new markers of bone formation and bone resorption in kidney transplant recipients

Osteoprotegerin (OPG), a natural decoy receptor for osteoclast differentiation factor, is produced by osteoblasts in response to PTH. OPG and its ligand RANKL constitute a complex mediator system involved in the regulation of bone resorption, probably playing an important role in the homeostasis of bone turnover. At present, little is known about the effects of OPG on uremic bone. Successful kidney transplantation reverses many abnormalities of bone metabolism; however, the improvement is often incomplete. The aim of the study was to assess OPG and RANKL concentrations in long-term kidney allograft recipients and their correlations with biochemical markers of bone resorption and formation. The present studies on 48 kidney transplant recipients and 25 healthy volunteers included concentrations of parathormone, osteocalcin, bone-specific alkaline phosphatase, serum CrossLaps, calcidiol, calcitriol, ICTP, PICP, tartrate-resistant acid phosphatase, beta2 microglobulin, IGF-1, IFGBP-1, IGFBP-3, OPG, and RANKL using commercially available kits for measurements. Among kidney transplant recipients OPG and RANKL did not differ between transplant patients and healthy volunteers, whereas other markers of bone formation and resorption were significantly higher in the former group. OPD was related to age, time on dialysis prior transplantation, urea, platelet count, CSA dose, azathioprine dose, 25(OH)D-3, TRAP, IGF-1, IGFBP-3, whereas RANKL was related to leukocyte count, CSA concentration and dose, urine DPD, and beta2 microglobulin content. In healthy volunteers OPG correlated only with CrossLaps, whereas RANKL correlated only with osteocalcin and TRAP. Correlations between OPG, IGF system components, and some markers of bone metabolism may indicate the role of OPG/ RANKL system in the pathogenesis of bone metabolism disturbances following renal transplantation.