Circulating osteoprotegerin in women during GnRH-agonist treatment and their relationships with mineral components and biomarkers of bone turnover

A novel cytokine termed osteoprotegerin (OPG) that is also called osteoclastogenesis-inhibitory factor, which inhibits osteoclast maturation and activity, was recently isolated. In order to determine the influence of estrogen deficiency on the levels of circulating OPG in women, we studied the changes in the levels of circulating OPG in 10 Japanese women ages 25–49 (mean ± SD, 34.0 ± 6.9) years with endometriosis receiving gonadotropin-releasing hormone agonists (GnRH-a) therapy. We further analyzed whether the levels of circulating OPG have relations with the levels of the biomarkers of bone turnover or those of circulating mineral components in these patients during GnRH-a treatment. The patients were treated with a monthly injection of 3.75 mg leuprolide acetate depot for 6 months. In all patients, the concentrations of serum estradiol decreased after 6 months of GnRH-a treatment. The bone mineral density of the lumber spines in these patients significantly (P< 0.01) decreased (percentage change: mean ± SD, −5.4 ± 2.1%), while circulating OPG levels significantly (P< 0.01) increased after 6 months of treatment. The values of circulating OPG had significant correlations with those of urinary pyridinoline (r = 0.59, P< 0.01), urinary deoxypylridinoline (Dpd) (r = 0.46, P< 0.05), and serum alkaline phosphatase (r = 0.66, P< 0.01) but not with those of serum carboxy-terminal propeptide of type I procollagen during GnRH-a treatment. The values of circulating OPG also correlated significantly with those of serum calcium (Ca) and phosphorus (P) (r = 0.65 and 0.72, P< 0.01). Further analyses revealed that the percentage change in the value of circulating OPG had a significant correlation with that of urinary Dpd (r = 0.84, P< 0.01). These results suggest that circulating OPG levels rise against the increase in osteoblastic bone resorption and circulating Ca levels in the case of estrogen deficiency, possibly as a compensatory mechanism serving to limit circulating Ca levels and bone density.