Osteoporosis in Organ Transplant Patients

Abstract

As transplantation osteoporosis results from alterations in the bone remodeling system, a review of normal bone remodeling is helpful to understanding the pathogenesis of this disease. The bone remodeling cycle is an orderly progression of events in which old, microdamaged bone is replaced by new, mechanically stronger bone. The complete remodeling cycle at each remodeling site requires approximately 3-6 months. Bone remodeling occurs by two main processes, resorption [1] followed by formation [2], which take place on both cancellous and cortical bone surfaces. Resorption begins with activation of macrophage precursors to form osteoclasts, giant multinucleated cells that excavate a cavity on the bone surface. Osteoclasts express receptors for receptor activator of receptor activator of nuclear factor-kappaB ligand (RANKL), which is produced by osteoblasts and osteocytes, and for calcitonin, prostaglandins (PGs), calcium, and vitronectin (integrin α 1 β 3 ). Approximately 0.05 mm 3 of bone tissue is resorbed by each osteoclast, leaving small resorption pits on the bone surface called Howship's lacunae in a process that takes approximately 2-3 weeks. A brief rest period called the reversal phase follows. Next, local mesenchymal bone marrow stem cells differentiate into osteoblasts that are attracted to the empty resorption pits where they accumulate as clusters of plump cuboidal cells along the bone surface. Osteoblasts produce the collagenous and noncollagenous proteins that constitute the matrix of the newly formed bone and are responsible for mineralization of the matrix or osteoid after a period of maturation that lasts approximately 20 days. Osteoblasts express receptors for several hormones (thyroid and parathyroid hormones, estrogens, vitamin D 3 ), cell adhesion molecules (integrins), and cytokines. RANKL, receptor activator of nuclear factor-kappaB (RANK), and osteoprotegerin (OPG) are three members of the tumor necrosis factor (TNF) ligand and receptor-signaling family that are final mediators of bone resorption [3,4]. RANKL is expressed in osteoblasts, osteocytes, and bone marrow stromal cells. When sufficient concentrations of macrophage colony stimulating factor (M-CSF) are present, RANKL binds to RANK, which is expressed on the surface of osteoclast lineage cells. This binding results in rapid differentiation of osteoclast precursors in bone marrow to mature osteoclasts, increased osteoclast activity, and reduced apoptosis of mature osteoclasts. RANKL is neutralized by binding to OPG, which is secreted by cells of the osteoblast lineage. Competitive binding of RANKL to either RANK or OPG regulates bone remodeling by increasing (RANK) or decreasing (OPG) osteoclastogenesis. Immunosuppressants exert their effects on bone remodeling by interacting with the RANK/RANKL/ OPG system [5].In healthy adults, declines in bone mass as a result of remodeling imbalance occur, but are small. When bone remodeling becomes "uncoupled," such that the rate of resorption exceeds the rate of formation, bone loss will occur...

Evaluation the Risk Factors that are Associated with Osteoporosis in Post Kidney Transplantation in a Sample of Iraqi Patients

“…Osteoporosisa disease that show reduction in bone mass, micro architectural disruption, and enhanced skeletal fragility, with subsequent low bone strength and high rates of fracture (5) . After renal transplantation, there are changes in the normal bone remodeling system which will lead to more accelerated osteoporotic changes compared to normal individuals, while for transplantation-related bone loss results from both an increase in the rate of resorption and a decrease in the rate of bone formation (6) .The current work aimed to investigate the incidence of osteoporosis in post kidney transplant patients when compared to the general population, and study the relationship between post kidney transplant immunosuppression therapy and osteoporosis and determine some biochemical changes. Also to evaluate the bone mass by using dual X-ray absorptiometry (DEXA) and the possible correlation between demographic data and the development of osteoporosis .…”