Osteoprotegerin-receptor activator of nuclear factor-[kappa]B ligand ratio: a new approach to osteoporosis treatment?

Abstract: Osteoporosis, the most commonly occurring bone disease, is characterized by enhanced bone fragility and increased risk of fracture. Bone remodeling is the process in which bone is broken down by osteoclasts and then built back again by osteoblasts. In healthy adult bone, these two processes are balanced and a constant level of bone mass is maintained. Some of the proteins involved in the interaction between osteoblasts and osteoclasts have recently been identified. Receptor activator of nuclear factor-[kappa]B (RANK) ligand is produced by osteoblasts and exerts its effects through binding to its receptor (RANK) on osteoclast precursor cells. Binding results in activation of osteoclasts. Osteoblasts also produce osteoprotegerin (OPG), a potent inhibitor of osteoclast formation and a decoy receptor for RANK. The relative ratio of OPG and RANK ligand in the bone marrow microenvironment may determine the number of active osteoclasts, bone resorption rate, and bone mass. OPG is currently under investigation for osteoporosis treatment.

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Osteoporosis, the most commonly occurring bone disease, is characterized by low bone mass together with microarchitectural deterioration. This leads to a consequent increase in bone fragility and susceptibility to fractures. Bones most likely to break are in the wrist, spine, and hip. Osteoporosis is the leading cause of serious morbidity and functional loss in the elderly. (1) The prevalence of low bone mass increases with increasing age in both men and women. It is higher for women than men because accelerated bone loss is associated with estrogen decline in the immediate postmenopausal period. (2) In the United States, approximately 1.5 million osteoporotic fractures occur per year. One in two women and one in eight men over 50 will have an osteoporotic fracture. (3)

The mature skeleton is a metabolically active organ that undergoes continuous remodeling by a process that replaces old bone with new bone. Remodeling is necessary to maintain the structural integrity of the skeleton and to serve its metabolic functions as a storehouse of calcium and phosphorus. This dual function often comes into conflict under conditions of changing mechanical forces or of nutritional and metabolic stress. (4) Cells responsible for bone formation, the osteoblasts originate from precursor cells that then differentiate into mature osteoblasts. Osteoblast precursors originate from bone marrow stromal cells. Osteoclasts responsible for bone resorption originate from hematopoietic stem cells known as monocytes. (4)

The remodeling cycle is finely regulated by a variety of systemic and local factors, for example, estrogen (E2), parathyroid hormone (PTH), vitamin D, growth factors, and cytokines. (4-6) Bone formation and resorption are usually balanced and a constant level of bone mass is maintained. An imbalance between bone formation and resorption causes metabolic diseases such as osteoporosis and osteopetrosis (a family of diseases characterized by increased bone mass due to decreased bone resorption). (4)

Mechanism of Action between Osteoblasts and Osteoclasts

Cell-to-cell contact between osteoblasts (or bone marrow stroma cells) and osteoclast precursors is required for osteoclast formation. (7) Recently, various research groups have identified some of the proteins involved in the interaction between cells of osteoblastic and osteoclastic lineage. These proteins belong to the families of tumor necrosis factors (TNFs) and receptors. Receptor activator of nuclear factor-[kappa]B ligand (RANKL), a protein expressed on the osteoblast cell membrane, binds to receptor activator of nuclear factor-[kappa]B (RANK), a receptor located on the osteoclast membrane. This cell-to-cell interaction initiates a cascade of events resulting in activation and differentiation of osteoclasts. (8)

Osteoprotegerin (OPG), a secreted member of the TNF receptor family, is produced by cells of the osteoblast lineage and by other cells in the marrow. Binding of RANKL and RANK can be prevented by OPG binding to RANKL. If the binding between RANK and RANKL is interrupted by OPG, the osteoclast precursor cannot differentiate and fuse to form mature resorbing osteoclasts. OPG thus acts as a decoy receptor in the RANK-RANKL signaling system inhibiting osteoclast formation (9) (Fig. 1). Apart from inhibiting osteoclast formation, it has also been reported that OPG is involved in suppressing osteoclast survival. (10,11) The presence of OPG in the bone microenvironment thus limits the number of mature osteoclasts and could therefore have a determining effect on resorption rate and bone mass. To limit confusion, the American Society for Bone and Mineral Research President's Committee on Nomenclature proposed standard nomenclature for the newly discovered TNF members (Table 1). (12)

A number of in vivo animal studies support the importance of the OPG-RANKL ratio for maintenance of healthy bone. Mice injected daily with recombinant OPG for 7 days had a threefold increase in trabecular bone mass at the proximal tibial metaphysis. This finding shows that already within a short period of time OPG treatment had an effect on bone formation. (9) Ovariectomized animals are often used as a model for postmenopausal bone loss. Administration of recombinant OPG to ovariectomized rats protected the animals from bone loss, suggesting that OPG prevented estrogen-induced bone loss. (9) It should be noted that the preserving effect of OPG on bone is caused by decreased bone resorption as opposed to increased formation.

[FIGURE 1 OMITTED]

Investigating bone metabolism in genetically altered animals presented some interesting findings. Overexpression of OPG in transgenic mice results in severe osteopetrosis with a large increase of mineralized trabecular bone, as bone remodeling is favored toward bone formation and the number of osteoclasts is decreased. (9) In contrast, adult OPG knockout mice lacking OPG exhibit severe osteoporosis due to increased osteoclast formation and activation and therefore bone resorption. (13) From these studies, it is evident that OPG has an important regulating effect on the number of mature osteoclasts, resorption rate, and bone structure.

Absence of either RANKL or RANK (as shown in RANKL and RANK knockout mice) interrupts RANKL-RANK signaling between osteoblasts and immature osteoclasts. (14,15) This causes a lack of functioning mature osteoclasts and eventually causes osteopetrosis. The OPG-RANKL ratio is of central importance in bone remodeling and could present a new therapeutic approach for osteoporosis treatment.

Regulators of the OPG-RANKL Ratio: In Vitro Studies and Animal Studies

The OPG-RANKL ratio is influenced by a variety of substances including hormones and cytokines. Treatment of calvarial osteoblasts with stimulators of osteoclast formation (e.g., vitamin D, PTH, prostaglandin [E.sub.2] [PG[E.sub.2]], and interleukin-11) up-regulate expression of RANKL mRNA. (16) Bone morphogenetic protein-2, interleukin-1[beta], and TNF-[alpha] increase OPG mRNA and protein in fetal osteoblasts. (17) It is beyond the scope of this article to discuss all of the above-mentioned regulators of the OPG-RANKL ratio; therefore, only a few are discussed in more detail.

Estrogen

The main cause of bone loss in postmenopausal osteoporosis is estrogen deficiency. Estrogen deficiency results in increased osteoclast formation, leading to an imbalance between bone formation and resorption. Estrogen treatment has long been known to inhibit bone loss in postmenopausal women.

The molecular mechanisms of estrogen action on bone are not completely understood. The principal effect on bone is a decrease in bone resorption. Estrogen acts directly on osteoblasts through high-affinity receptors located on the nuclear membrane. (6)

Recent reports suggest that OPG is involved in the paracrine mediation of estrogen (17 [beta]-estradiol) effects on bone. Estrogen treatment dose and time dependently stimulate OPG secretion by human osteoblasts in vitro. OPG secretion was highest in osteoblasts containing the largest number of estrogen receptors. (18) In another study, up-regulation of OPG expression by mouse bone marrow stromal cells (precursors of osteoblasts) after estrogen treatment was also reported. Estrogen withdrawal after a 5-day pretreatment, mimicking the event occurring in vivo at menopause, dramatically downregulates the expression of OPG by these cells. (19)

Estrogen's protective effect on bone could well be explained by its stimulatory effect on OPG synthesis. OPG is known to be a potent inhibitor of osteoclast formation and activation. A local increase in OPG in the bone microenvironment may be an important mechanism by which estrogen reduces bone resorption. OPG specifically blocks RANKL-RANK interaction and therefore inhibits osteoclast differentiation and function. (9) Down-regulation of OPG expression on estrogen withdrawal could therefore increase osteoclastic bone resorption. Riggs (20) postulated that the antiresorptive effect of estrogen could be ascribed to various additive effects on the RANK-RANKL system.

PTH

PTH has dual activities in bone. Continuous PTH administration results in enhanced bone resorption. Intermittent PTH therapy, although having a net anabolic effect on bone, stimulates both bone formation and bone resorption. (5)

Depending on the model used, disparate results were reported on the effects of PTH on the OPG-RANKL ratio. In murine bone marrow cultures, PTH stimulates RANKL and inhibits OPG expression, thus adversely affecting the OPG-RANKL ratio. Although these cells were exposed to PTH for several days, the reported effect on the OPG-RANKL ratio was most evident after only a few hours. (21,22) In a similar study using murine osteoblasts, no stimulatory effect of PTH on RANKL expression could be demonstrated. As PTH also had no effect on OPG expression in these cells, it did not affect the OPG-RANKL ratio in this model. (22)

In a rat model, Onyia et al (23) reported decreased OPG expression in bones of rats treated with human PTH. Subcutaneous administration of a single injection of PTH induced a rapid and transient decrease in OPG mRNA expression in both metaphyseal and diaphyseal bone. Decreased OPG expression was evident by 1 hour and mRNA levels returned to baseline within 24 hours.

The reviewed results suggest that PTH administration rapidly and transiently inhibits the level of OPG mRNA in bone cells both in vivo and in vitro. Regulation, however, is complex and depends on the differentiation state of the cells and the interval after stimulation when they were examined.

PG[E.sub.2]

Osteoblasts produce prostaglandins from fatty acid precursors. Prostaglandins, especially PG[E.sub.2] derived from its precursor arachidonic acid, have pronounced effects on bone. Depending on the concentration and experimental model, both antiresorptive and proresorptive effects of prostaglandins were reported. (24)

PG[E.sub.2] stimulates osteoclast formation in bone marrow cultures, (25) increases expression of mRNA for RANKL, (26) and down-regulates OPG in cultures of primary human bone marrow stromal cells. (27) These reported effects of PG[E.sub.2] on OPG and RANKL will ultimately have a detrimental effect on the OPG-RANKL ratio in the bone microenvironment and could ultimately lead to a decrease in bone mass as previously described.

Inflammatory conditions are associated with increased PG[E.sub.2] levels. (28) One could thus speculate that the effects of PG[E.sub.2] on OPG (down-regulation) and RANKL expression (up-regulation) might be the cause of increased bone loss adjacent to inflammatory tissues, as is observed in rheumatoid arthritis and other diseases. Nonsteroidal anti-inflammatory drugs (e.g., indomethacin) have been shown to inhibit bone loss in vivo and bone resorption in vitro, and this is associated with a loss of osteoclasts from the bone surface. (29) In a recent study, increased OPG secretion was reported after PG[E.sub.2] inhibition by indomethacin in mouse calvaria in vitro. (30) Anti-inflammatory drugs aimed at reducing PG[E.sub.2] production could thus also be advantageous to the maintenance of bone integrity.

Glucocorticoids 

Osteoporosis is a serious complication of systemic glucocorticoid use. Dexamethasone concurrently inhibits OPG and stimulates RANKL production by osteoblasts in vitro. (31) Decreased OPG mRNA levels were detected within 2 hours after hydrocortisone treatment in several osteoblast-like human cell lines. (32)

The finding that OPG levels are decreased by glucocorticoids indicates that reduced production of OPG by osteoblasts could, in part, explain glucocorticoid-induced resorption. Strategies aimed at increasing the OPG-RANKL ratio during glucocorticoid treatment may therefore be useful in preventing osteoporosis.

Hofbauer et al (33) proposed a "convergence hypothesis" for the regulation of osteoclast functions by cytokines. The regulation of RANKL and OPG by various proresorptive and antiresorptive agents suggests that their effects may converge at the level of RANKL and OPG, which then functions as the final effector system to modulate differentiation and activation of osteoclasts. For example, the stimulation of RANKL by PTH and PG[E.sub.2], (21, 26) and the inhibition of OPG by these same agents, (22, 27) may mediate the proresorptive effects of these agents (Fig. 2).

Regulators of the OPG-RANKL Ratio: In Vivo Human Studies

Measuring OPG in serum has shown contradictory data with regard to bone mass. Yano et al (34) reported a threefold increase in serum OPG levels with age in postmenopausal women. Serum OPG concentrations were also significantly increased in women with a high rate of bone turnover as determined by specific bone markers. It was suggested that age-related factors regulate circulating OPG levels. These "age-related" factors were not identified. It was speculated that increased serum OPG concentrations might reflect compensative responses to enhanced osteoclastic bone resorption and resultant bone loss. In another study, however, Browner et al (35) failed to show association of serum OPG levels with bone mineral density.

Analyzing the results from the above-mentioned studies, it is still questionable what relevance serum OPG levels have. Because tissues other than bone (e.g., lung and kidney) also produce OPG, (9) the serum OPG level does not necessarily reflect the actual OPG concentration in the bone microenvironment. More studies will have to be conducted to determine whether measuring serum OPG levels could be applied for diagnostic purposes.

[FIGURE 2 OMITTED]

Measuring OPG and RANKL in bone samples produced conflicting results. Although the mRNA OPG-RANKL ratio correlated with bone modeling indices in normal human bone, no significant differences were found between premenopausal and postmenopausal bone. More trials are indicated before final conclusions can be drawn. Also, evaluation of OPG and RANKL expression in bone samples of subjects with increased circulating PTH levels did not confirm results obtained from in vitro cell culture studies. These findings once again confirm the complexity of evaluating in vivo results. (36, 37)

Potential Therapeutic Promise of OPG

A number of animal studies suggest the possible advantage of OPG treatment in various clinical conditions. Systemic administration of OPG inhibits ovariectomy-associated bone loss in rats. (9) Treatment of ovariectomized rats with OPG or PTH increases bone mineral density. A significantly greater and more rapid increase in bone mineral density was observed with OPG-PTH cotreatment. These data suggest that OPG combined with PTH could represent a potentially useful therapeutic option for patients with severe osteoporosis. OPG inhibited much of the hypercalcemic effects of PTH treatment. (38) Administration of OPG to rats with established hypercalcemia decreased serum calcium rapidly and dramatically. OPG did not increase urinary calcium excretion, suggesting that OPG affects the function and/or survival of mature osteoclasts in vivo. (39)

In postmenopausal women, Bekker et al (40) recently showed that a single subcutaneous dose of OPG causes rapid and sustained inhibition of bone resorption as indicated by bone resorption marker changes. OPG injections were well tolerated.

It is evident from these studies that OPG could potentially have clinical applications as a new antiresorptive agent in treating osteoporosis. OPG, however, is a fairly large protein, which can be difficult to administer in ways other than by injection. A significantly greater and more rapid increase in bone mineral density was observed when ovariectomized rats were cotreated with OPG and PTH rather than administering these agents separately. (38) As low-dosage intermittent PTH treatment is already being tested for a possible therapeutic treatment for osteoporosis, a combination of PTH and OPG could therefore be even more advantageous. A further advantage of OPG-PTH cotreatment is the possible protection it provides against increased serum calcium levels.

One should, however, bear in mind that OPG might induce immune responses and act in organs other than bone. More research is indicated before OPG can be considered a safe therapeutic option for osteoporosis treatment.

Conclusion

The discovery of the OPG-RANKL-RANK system has revealed a completely new perspective in bone biology, and many cytokines and hormones affecting bone remodeling have been shown to act by means of this system. OPG is likely to be a major regulator of bone metabolism; as cells of the osteoblast lineage produce it, its production is regulated by many of the major calcitropic hormones and cytokines, and it is a potent inhibitor of osteoclast formation and activation. The use of OPG as a therapeutic agent in osteoporosis is currently under investigation. In animal models, OPG has a potent effect in preventing bone loss. (9) A study on postmenopausal women has shown OPG to cause a rapid reduction in bone resorption. (40) More clinical studies, including data on bone mineral density and fracture incidence, are required to determine whether the application of OPG in the treatment of bone disease, such as osteoporosis, is worthwhile in the long run.

A large number of stimulators and inhibitors of osteoclast formation converge on the RANKL/RANK/OPG pathway, making this an appropriate target for therapeutic intervention. A logical therapeutic approach would be to search for ways to either increase OPG formation or decrease RANKL formation by bone cells, tilting the scale toward decreased osteoclast formation and less bone resorption, thus protecting bone. To identify new potential therapeutic agents for treatment of osteoporosis, more work needs to be done.

Table 1. Listing of terms used for the three tumor necrosis factor
family molecules involved in regulation of osteoclast formation and
function (a)

Acronym                       Synonym

Ligand
  RANKL (b)     RANK ligand
  ODF           Osteoclast differentiation factor
  OPGL          Osteoprotegerin ligand
  TRANCE        TNF-related activation-induced cytokine
  SOFA          Stromal osteoclast-forming activity
  TNFSF-11      TNF superfamily 11
Receptor
  RANK (b)      Receptor activator of NF-[kappa]B
  ODF           Osteoclast differentiation factor
  ODAR          Osteoclast differentiation and activation receptor
  TNFRSF-11A    TNF superfamily receptor 11A
Decoy receptor
  OPG (b)       Osteoprotegerin
  OCIF          Osteoclastogenesis inhibitory factor
  TR-1          TNF receptor-like molecule 1
  FDCR-1        Follicular dendrite receptor 1
  TNFRSF-11B    TNF superfamily receptor 11B

(a) TNF, tumor necrosis factor; NF, nuclear factor; RANK, receptor
activator of NF-[kappa]B.
(b) Terms recommended by the American Society for Bone and Mineral
Research President's Committee on Nomenclature. (12)

Accepted April 10, 2003.

Copyright [c] 2004 by The Southern Medical Association

0038-4348/04/9705-0506

References

1. Inzerillo AM, Zaidi M. Osteoporosis: Trends and intervention. Mt Sinai J Med 2002;69:220-231.

2. Looker AC, Orwoll ES, Johnston CC, et al. Prevalence of low femoral bone density in older U.S. adults from NHANES III. J Bone Miner Res 1997;12:1761-1768.

3. Consensus Development Conference, Hong Kong, April 1-2, 1993. Am J Med 1993;95:1S-78S.

4. Raisz LG. Physiology and pathophysiology of bone remodeling. Clin Chem 1999;45:1353-1358.

5. Dempster DW, Cosman F, Parisien M, et al. Anabolic actions of parathyroid hormone on bone. Endocr Rev 1993;14:690-709.

6. Turner RT, Riggs BL, Spelsberg TC. Skeletal effects of estrogen. Endocr Rev 1994;15:275-300.

7. Jimi E, Nakamura I, Amano H, et al. Osteoclast function is activated by osteoblastic cells through a mechanism involving cell-to-cell contact. Endocrinology 1996;137:2187-2190.

8. Yasuda H, Shima N, Nakagawa N, et al. Osteoclast differentiation factor is a ligand for osteoprotegerin/osteoclastogenesis-inhibitory factor and is identical to TRANCE/RANKL. Proc Natl Acad Sci USA 1998;95:3597-3602.

9. Simonet WS, Lacey DL, Dunstan CR, et al. Osteoprotegerin: A novel secreted protein involved in the regulation of bone density. Cell 1997;89:309-319.

10. Akatsu T, Murakami T, Nishikawa M, et al. Osteoclastogenesis inhibitory factor suppresses osteoclast survival by interfering in the interaction of stromal cells with osteoclast. Biochem Biophys Res Commun 1998;250:229-234.

11. Emery JG, McDonnell P, Burke MB, et al. Osteoprotegerin is a receptor for the cytotoxic ligand TRAIL. J Biol Chem 1998;273:14363-14367.

12. The American Society for Bone and Mineral Research President's Committee on Nomenclature. Proposed standard nomenclature for new tumor necrosis factor members involved in the regulation of bone resorption. Bone 2000;27:761-764.

13. Mizuno A, Amizuka N, Irie K, et al. Severe osteoporosis in mice lacking osteoclastogenesis inhibitory factor/osteoprotegerin. Biochem Biophys Res Commun 1998;247:610-615.

14. Kong YY, Yoshida H, Sarosi I, et al. OPGL is a key regulator of osteoclastogenesis, lymphocyte development and lymph-node organogenesis. Nature 1999;397:315-323.

15. Li J, Sarosi I, Yan XQ, et al. RANK is the intrinsic hematopoietic cell surface receptor that controls osteoclastogenesis and regulation of bone mass and calcium metabolism. Proc Natl Acad Sci U S A 2000;97:1566-1571.

16. Suda T, Takahashi N, Udagawa N, et al. Modulation of osteoclast differentiation and function by the new members of the tumor necrosis factor receptor and ligand families. Endocr Rev 1999;20:345-357.

17. Hofbauer LC, Dunstan CR, Spelsberg TC, et al. Osteoprotegerin production by human osteoblast lineage cells is stimulated by vitamin D, bone morphogenic protein-2, and cytokines. Biochem Biophys Res Commun 1998;250:776-781.

18. Hofbauer LC, Khosla S, Dunstan CR, et al. Estrogen stimulates gene expression and protein production of osteoprotegerin in human osteoblastic cells. Endocrinology 1999;140:4367-4370.

19. Saika M, Inoue D, Kido S, et al. 17[beta]-Estradiol stimulates expression of osteoprotegerin by a mouse stromal cell line, ST-2, via estrogen receptor-[alpha]. Endocrinology 2001;142:2205-2212.

20. Riggs BL. The mechanisms of estrogen regulation of bone resorption. J Clin Invest 2000;106:1203-1204.

21. Horwood NJ, Elliott J, Martin TJ, et al. Osteotropic agents regulate the expression of osteoclast differentiation factor and osteoprotegerin in osteoblastic stromal cells. Endocrinology 1998;139:4743-4746.

22. Lee SK, Lorenzo JA. Parathyroid hormone stimulates TRANCE and inhibits osteoprotegerin messenger ribonucleic acid expression in murine bone marrow cultures: Correlation with osteoclast-like cell formation. Endocrinology 1999;140:3552-3561.

23. Onyia JE, Miles RR, Yang X, et al. In vivo demonstration that human parathyroid hormone 1-38 inhibits the expression of osteoprotegerin in bone with the kinetics of an immediate early gene. J Bone Miner Res 2000;15:863-871.

24. Kawaguchi H, Pilbeam CC, Harrison JR, et al. The role of prostaglandins in the regulation of bone metabolism. Clin Orthop 1995;313:36-46.

25. Akatsu T, Takahashi N, Debari K, et al. Prostaglandins promote osteoclastlike cell formation by a mechanism involving cyclic adenosine 3', 5'-monophosphate in mouse bone marrow cell cultures. J Bone Miner Res 1989;4:29-35.

26. Nakashima T, Kobayashi Y, Yamasaki S, et al. Protein expression and functional difference of membrane-bound and soluble receptor activator of NF-kB ligand: Modulation of the expression by osteotropic factors and cytokines. Biochem Biophys Res Commun 2000;275:768-775.

27. Brandstrom H, Bjorkman T, Ljunggren O. Regulation of osteoprotegerin secretion from primary cultures of human bone marrow stromal cells. Biochem Biophys Res Commun 2001;280:831-835.

28. Masferrer JL, Zweifel BS, Manning PT, et al. Selective inhibition of inducible cyclooxygenase 2 in vivo is antiinflammatory and nonulcerogenic. Proc Natl Acad Sci U S A 1994;91:3228-3232.

29. Marshall MJ, Holt I, Davie MW. Inhibition of prostaglandin synthesis leads to a change in adherence of mouse osteoclasts from bone to periosteum. Calcif Tissue Int 1996;59:207-213.

30. O'Brien EA, Williams JH, Marshall MJ. Osteoprotegerin is produced when prostaglandin synthesis is inhibited causing osteoclasts to detach from the surface of mouse parietal bone and attach to the endocranial membrane. Bone 2001;28:208-214.

31. Hofbauer LC, Gori F, Riggs BL, et al. Stimulation of osteoprotegerin ligand and inhibition of osteoprotegerin production by glucocorticoids in human osteoblastic lineage cells: Potential paracrine mechanisms of glucocorticoid-induced osteoporosis. Endocrinology 1999;140:4382-4389.

32. Vidal NO, Brandstrom H, Jonsson KB, et al. Osteoprotegerin mRNA is expressed in primary human osteoblast-like cells: Down-regulation by glucocorticoids. J Endocrinol 1998;159:191-195.

33. Hofbauer LC, Khosla S, Dunstan CR, et al. The roles of osteoprotegerin and osteoprotegerin ligand in the paracrine regulation of bone resorption. J Bone Miner Res 2000;15:2-12.

34. Yano K, Tsuda E, Washida N, et al. Immunological characterization of circulating osteoprotegerin/osteoclastogenesis inhibitory factor: Increased serum concentrations in postmenopausal women with osteoporosis. J Bone Miner Res 1999;14:518-527.

35. Browner WS, Lui LY, Cummings SR. Associations of serum osteoprotegerin levels with diabetes, stroke, bone density, fractures, and mortality in elderly women. J Clin Endocrinol Metab 2001;86:631-637.

36. Fazzalari NL, Kuliwaba JS, Atkins GJ, et al. The ratio of messenger RNA levels of receptor activator of nuclear factor kB ligand to osteoprotegerin correlates with bone remodeling indices in normal human cancellous bone but not in osteoarthritis. J Bone Miner Res 2001;16:1015-1027.

37. Seck T, Diel I, Bismar H, et al. Serum parathyroid hormone, but not menopausal status, is associated with the expression of osteoprotegerin and RANKL mRNA in human bone samples. Eur J Endocrinol 2001;145:199-205.

38. Kostenuik PJ, Capparelli C, Morony S, et al. OPG and PTH-(1-34) have additive effects on bone density and mechanical strength in osteopenic ovariectomized rats. Endocrinology 2001;142:4295-4304.

39. Yamamoto M, Murakami T, Nishikawa M, et al. Hypocalcemic effect of osteoclastogenesis inhibitory factor/osteoprotegerin in the thyroparathyroidectomized rat. Endocrinology 1998;139:4012-4015.

40. Bekker PJ, Holloway D, Nakanishi A, et al. The effect of a single dose of osteoprotegerin in postmenopausal women. J Bone Miner Res 2001;16:348-360.

RELATED ARTICLE: Key Points

* Osteoporosis results in weak, fragile bones that can break easily.

* Mature bone is constantly remodeled by osteoblasts that form bone and osteoclasts that destroy it.

* In osteoporosis, there is more osteoclast than osteoblast activity.

* Osteoprotegerin and receptor activator of nuclear factor-[kappa]B ligand, unique agents influencing bone remodeling, are currently under investigation in osteoporosis treatment.

Magdalena Coetzee, MSC, and Marlena C. Kruger, PHD

From the Department of Physiology, University of Pretoria, Pretoria, Republic of South Africa; and the Institute of Food, Nutrition and Human Health, Massey University, Palmerston North, New Zealand.

Reprint requests to Magdalena Coetzee, MSc, Department of Physiology, University of Pretoria, P.O. Box 2034, Pretoria 0001, Republic of South Africa. Email: [e-mail]