Nephrol Dial Transplant (2001) 16: 479-482
© 2001 European Renal Association-European Dialysis and Transplant Association
Rapid Communication
Glucocorticoid decreases circulating osteoprotegerin (OPG): possible mechanism for glucocorticoid induced osteoporosis
1 Department of Nephrology, Jichi Medical School, Tochigi, 2 Research Institute of Life Sciences, Snow Brand Milk Products Co., Ltd, Tochigi, Japan
Abstract
Background. Osteoporosis is a well known side-effect of long-term treatment with glucocorticoids. However, the precise mechanism of this disorder is unclear. Recently, osteoprotegerin (OPG) [osteoclastogenesis inhibitory factor (OCIF)] has been identified as a novel cytokine, which inhibits differentiation and activation of osteoclast. In the present study, in order to clarify the roles of OPG in the development of glucocorticoid-induced osteoporosis, we measured circulating OPG before and after glucocorticoid therapy.
Methods. The study subjects were 12 patients (five males, seven females, 53.4±4.8 [SE] years) with various renal diseases that required glucocorticoid therapy. All patients received glucocorticoids for the first time. Treatment was initiated at an average dose of 32.5±3.0 mg per day. Serum OPG was measured using enzyme-linked immunosorbent assay (ELISA). Laboratory data, markers of bone metabolism and circulating OPG were compared before and after treatment for 4 weeks.
Results. Serum OPG prior to glucocorticoid therapy was positively and independently correlated with serum creatinine. Serum OPG decreased significantly (P<0.0001) from 1.03±0.14 to 0.77±0.12 ng/ml after short-term administration of glucocorticoids. Furthermore, serum osteocalcin as a marker of bone formation was also reduced markedly (P=0.001). On the other hand, there were no remarkable changes in serum calcium, total alkaline phosphatases, creatinine and intact parathyroid hormone in response to glucocorticoid administration.
Conclusion. These findings indicate that short-term administration of glucocorticoids significantly suppresses serum OPG and osteocalcin. It might participate in the development of glucocorticoid-induced osteoporosis via an enhancement of bone resorption and suppression of bone formation.
Keywords: osteoprotegerin (OPG); osteoclastogenesis inhibitory factor (OCIF); glucocorticoid-induced osteoporosis; bone formation; bone resorption
Introduction
It is well known that long-term glucocorticoid therapy causes osteoporosis [1]. However, the precise mechanism remains unclear. Recently, osteoprotegerin (OPG) [osteoclastogenesis inhibitory factor (OCIF)] has been identified as a novel, secreted cytokine receptor, which plays an important role in the negative regulation of osteoclastic bone resorption [2–4]. Furthermore, recent reports suggested that glucocorticoids promote osteoclastogenesis by inhibiting OPG production in vitro, thereby enhancing bone resorption [5, 6]. These findings may provide a potential mechanism for glucocorticoid-induced osteoporosis.
However, there are only few clinical reports exploring the regulatory functions of OPG. In the present study, in order to clarify the potential role of OPG in the pathogenesis of glucocorticoid-induced osteoporosis, we measured serum OPG and other markers of bone metabolism before and after glucocorticoid therapy in patients with various renal diseases.
Patients and methods
We evaluated the short-term effect of glucocorticoids on circulating OPG and other markers of bone metabolism as well as various routine biochemical parameters. The subjects were 12 patients (five males, seven females) with an average age of 53.4±4.8 years. Original renal diseases and characteristics of the patients are shown in Table 1
. All patients were treated with glucocorticoids for the first time and methylprednisolone pulse therapy (500 mg/dayx3 days) was added in three patients. Glucocorticoid treatment was initiated at an average dose of 32.5±3.0 mg/day, excluding methylprednisolone pulses. Drugs affecting bone metabolism such as oestrogen, progesterone, bisphosphonates and active vitamin D3 were not used. The study periods were approx. 4 weeks in length.
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Laboratory data and serum OPG were compared before and after glucocorticoid therapy. Serum OPG was measured using enzyme-linked immunosorbent assay (ELISA) developed by Yano et al. [7]. In addition, we measured serum intact parathyroid hormone (iPTH) as an indirect marker of bone metabolism and osteocalcin as an independent marker of bone formation.
All data were expressed as mean±SE. Paired Student's t-test was used for statistical analysis and P<0.05 was considered significant.
Results
Using multiple regression analysis for comparison of serum OPG concentrations prior to glucocorticoid therapy with other parameters (age, serum creatinine, calcium, total alkaline phosphatases, iPTH and osteocalcin), serum OPG was positively and independently correlated with serum creatinine. No other parameter significantly entered the model.
Glucocorticoids caused a reduction in serum OPG levels within a few days after starting glucocorticoid therapy. As shown in Fig 1, serum OPG decreased significantly from 1.03±0.14 to 0.77±0.12 ng/ml (P<0.0001) within 27.8±2.4 days of glucocorticoid administration (total dose: 825±118 mg). Although serum OPG levels in the two patients with renal failure due to RPGN were higher than those with normal renal function before treatment, their OPG levels also markedly decreased after administration of glucocorticoids, including methylprednisolone pulse therapy, without significant changes in serum creatinine (Fig 1).
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In addition, we examined other biochemical parameters and markers of bone metabolism before and after glucocorticoid therapy (Table 2
). There were no remarkable changes in serum calcium, phosphorus, total alkaline phosphatases, creatinine and blood urea nitrogen. Furthermore, serum iPTH also did not change significantly. In contrast, serum osteocalcin as an osteoblastic marker was significantly reduced from 5.4±1.1 to 2.5±0.6 ng/ml (P=0.001, Fig 2).
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Discussion
The present study clearly demonstrates for the first time that circulating OPG is reduced by short-term administration of glucocorticoids in patients with various renal diseases. It is assumed that glucocorticoids immediately suppress OPG production, and may result in an acceleration of osteoclastic bone resorption.
Glucocorticoid-induced osteoporosis is a common and serious complication of long-term glucocorticoid treatment. It is generally accepted that glucocorticoids rapidly decrease bone formation and increase bone resorption [1]. However, the precise mechanisms have not been defined.
Recently, OPG has been identified as a novel cytokine, which acts on bone tissues to increase bone mineral density and volume by decreasing the number of active osteoclasts in vitro [2–4]. It is noteworthy that overexpression of OPG in transgenic mice resulted in osteopetrosis [3], while OPG-deficient mice developed severe osteoporosis [7, 8]. In addition, a recent study has shown that glucocorticoids stimulate OPG-ligand and inhibit OPG production in vitro [5, 6]. Therefore, it has been suggested that the inhibitory effect of glucocorticoids on OPG production might be related to glucocorticoid-induced osteoporosis.
In clinical studies, Yano et al. have reported that circulating OPG increased with age and was significantly elevated in postmenopausal women with osteoporosis [9]. This finding has led to the suggestion that circulating OPG levels are regulated by age-related factors and that the increase in serum concentration may be a compensatory response against enhanced osteoclastic bone resorption. Another recent study showed that serum OPG was increased in uraemic patients, independent of serum iPTH [10]. The authors concluded that circulating OPG was an independent factor affecting bone metabolism in uraemic patients.
In the present study, serum OPG prior to glucocorticoid therapy was significantly correlated with serum creatinine, as reported previously [9, 10]. Furthermore, the present data provide the novel finding that circulating OPG was significantly suppressed by a short-term administration of glucocorticoids (P<0.0001), without changes in other biochemical parameters (Fig 1
). This reduction was not due to an improvement of serum creatinine, since renal function remained stable (Table 2). It is therefore likely that the reduction in serum OPG was caused by a direct suppressive effect of glucocorticoids on bone.
It is widely accepted that glucocorticoids decrease intestinal calcium absorption and increase urinary calcium excretion, eventually leading to secondary hyperparathyroidism. Therefore, Lukert et al. suggested that enhanced bone resorption during long-term glucocorticoid therapy might be mainly due to secondary hyperparathyroidism [11]. However, in the present study, serum iPTH as well as calcium and phosphorus did not change significantly. Thus, one can assume that bone resorption has resulted not from an increment of iPTH, but rather from an inhibition of OPG by short-term administration of glucocorticoids.
On the other hand, a decrease of serum osteocalcin was found as expected as shown in Fig 2. Osteocalcin is produced by osteoblasts, and it correlates with the rate of bone mineralization. A recent study documented an increase in osteoblastic apoptosis in patients with glucocorticoid induced osteoporosis [12]. Therefore, it was suggested that short-term administration of glucocorticoids also led to suppression of bone formation.
In conclusion, these observations indicate that glucocorticoid-induced bone loss may be caused by both suppressed bone formation and enhanced bone resorption, and that suppression of OPG may be, at least in part, associated with glucocorticoid-induced osteoporosis, independently of serum iPTH levels. Further long-term studies are needed to elucidate the mechanisms of the glucocorticoid-induced decrease in circulating OPG and its participation in the pathogenesis of osteoporosis.
Notes
Correspondence and offprint requests to: Eiji Kusano MD, Department of Nephrology, Jichi Medical School, Minamikawachi, Tochigi 329-0498, Japan 
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Revision received 29.11.00.
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