GH replacement therapy increases plasma osteoprotegerin levels in GH-deficient adults

Osteoprotegerin (OPG), a glycoprotein belonging to the tumor necrosis factor receptor family, is an endogenous inhibitor of osteoclastogenesis produced by cells of the osteoblast lineage. OPG is a key cytokine involved in the regulation of osteoblast/osteoclast cross-talk. Since GH replacement therapy in GH deficiency (GHD) activates bone remodeling and increases bone mass, we investigated if short-term GH replacement therapy affects plasma OPG levels.

Eighteen adults with GHD, ranging from 17 to 51 Years (nine childhood-onset and nine adult-onset) were enrolled in the study. All subjects were on stable replacement therapy, especially sex hormones. The starting dose of GH replacement therapy was 4 microg/kg per day x 7 days/week, and was progressively increased according to the serum IGF-I values. Biochemical parameters of bone and mineral metabolism were measured before and after 6 Months of GH replacement therapy. Bone mass density (BMD) was monitored at three skeletal sites (lumbar vertebrae, femur, radius) by dual-energy X-ray absorptiometry.

After 6 Months of therapy, ionized calcium, parathyroid hormone and 25-OH vitamin D did not change, whereas total serum calcium and urinary calcium excretion increased significantly (P<0.01). Also osteocalcin and urinary deoxypyridinoline/24 h increased significantly (P<0.02, P<0.05 respectively). Mean basal T-scores of BMD values showed an osteopenic state, which remained unchanged after GH therapy. Plasma OPG increased significantly after 6 Months of therapy (P<0.02) and this increase was significantly correlated with the increase of osteocalcin (r=-0.52; P=0.04) and deoxypyridinoline values (r=-0.64; P=0.011).

Our results suggest that the bone anabolic effect of GH replacement therapy could in part be mediated by a positive bone balance at each remodeling unit due to the inhibitory action of OPG on osteoclastogenesis.