OPG and RANKL Plasma Level After Administration of Unfractionated Heparin (UFH) and Low-Molecular-Weight Heparin (LMWH) in Hemodialysis

It's well known that treatment with heparin can lead to a reduction in bone density and the development of osteoporosis \[ 1 \]. Until now, it's not clear the mechanism by which heparin produces this side effect, but several studies in animals \[ 2,3\] and in humans \[ 4 \] have shown that LMWH may induce less osteoporosis than UFH. Recently it was observed that heparin interferes with RANK/RANKL/POG system \[5,6\]. RANK, RANKL and OPG are members of TNF alfa receptor superfamily. The pathways involving them in conjunction with various cytokines and calciotrophic hormones play a pivotal role in bone remodelling. In addiction experimental and clinical studies established a consistent relationship between the RANK/RANKL/OPG pathway and both skeletal lesion related to disorders of mineral metabolism \[7,8,9\] and vascular calcification \[7,10\]. OPG exists either as active soluble form or is expressed by osteoblast, stromal and cardiovascular cells, acting as decoy receptor that competes with RANKL for RANK. This interaction inhibits osteoclastic proliferation and differentiation and consequently prevents bone resorption . OPG is also produced by both endothelial cells (EC) and Vascular Smooth Muscle Cells (VSMCs ). EC-derived OPG seems to act as an important autocrine / paracrine factor able to protect against arterial calcification blocking the effects of RANKL that promotes monocytes differentiation in osteoclast -like cells and an osteogenic differentiation program in VSMC. This process leads to the synthesis of bone proteins and matrix calcification within the arterial vessel. OPG levels increase with aging and are higher in ESRD patient \[11,12\]. Recently it was demonstrated in cultures of murine bone marrow that the heparin inhibits osteoprotegerin activity binding OPG competitively and in this way inhibiting the interaction between OPG and RANKL \[5\]. On the other side heparin seems cause the mobilization of OPG into the circulation. It was reported that OPG is co-localized with vWF in Weibel Palade bodies in endothelial cells \[13\] and binds to Glucosaminoglycans (GAGs) at cellular membranes through its highly basic heparin binding domain \[14,15\]. Heparin treatment causes an immediate mobilization of these protein in to the circulation by displacement from the endothelial surface since they have higher affinity for heparins than GAGs at the endothelial surface\[16,17\]. UFH cause a more pronounced vascular mobilization of OPG than LMWH, indicating that UFH have an higher affinity for OPG than LMWH \[6\].