The role of cytokines in skeletal remodelling: possible consequences for renal osteodystrophy
Esther A. González
Division of Nephrology, Saint Louis University, St Louis, Missouri, USA
Introduction
The pathogenesis of renal bone disease, a common complication encountered in patients with renal insufficiency, has centred around perturbations in the parathyroid hormonevitamin D axis [1]. Thus, excess parathyroid hormone (PTH) can give rise to the high bone turnover state of osteitis fibrosa, and conversely, relatively low levels of PTH are associated with the low bone turnover state of adynamic bone. However, there are many observations which imply that the protean manifestations of renal bone disease cannot be explained simply by abnormalities of PTH or vitamin D metabolites. For example, levels of PTH correlate relatively poorly with various parameters of bone histology, and there is clearly a wide scatter of the data points for any given level of PTH [2,3]. While low levels of calcitriol are clearly important in the pathogenesis of hyperparathyroidism, and the administration of vitamin D metabolites is a key component of therapy, a direct role of vitamin D in bone has recently come into question in that normalization of serum levels of calcium and phosphorus in vitamin D receptor (VDR) knockout mice results in complete normalization of bone histology and bone strength [4].
In recent times, there has been an explosion in our understanding of the complex biology of bone with the demonstration that a multitude of local and circulating growth factors and cytokines play major roles in bone cell biology. Abnormalities in several of these factors may be present during the course of chronic renal failure, and therefore should be considered for their contribution to the abnormalities of bone remodelling in uraemia.
Bone remodelling
In order to maintain mechanical integrity, the skeleton is constantly undergoing remodelling; that is, old bone is being replaced by new bone. The normal remodelling cycle requires that the processes of bone resorption and bone formation take place in a coordinated fashion which in turn depends on the orderly development and activation of osteoclasts and osteoblasts respectively. Activation of the remodelling cycle by a factor such as PTH results in the stimulation of osteoblast and osteoclast differentiation from precursor cells in the bone marrow. Osteoclasts derive from the haematopoietic granulocyte-macrophage colony forming unit whereas osteoblasts originate from the pluripotential mesenchymal stem cells. Bone remodelling is accomplished by bone resorption followed by new bone formation. Once osteoclasts have resorbed a cavity of bone, they detach from the bone surface and are replaced by cells of the osteoblast lineage which in turn initiate the process of bone formation. The osteoblasts fill the resorption cavity by laying down matrix that later becomes mineralized. When the cycle is completed, the amount of bone formed should equal the amount of bone resorbed. The complex series of events that take place during the different processes involved in bone remodelling are under the regulation of a multitude of systemic and local factors, and it is the integrated effects of these factors on cell differentiation, proliferation and activity that are responsible for the maintenance of a healthy skeleton. A detailed description of the different cytokine systems responsible for the regulation of bone remodelling is beyond the scope of this review; however, recent developments in the area of osteoclastogenesis are worth discussing in some detail. The terminology used in this rapidly developing field is somewhat confusing because of the several synonyms used to describe the same mediators. These terms are summarized in Table 1
and a consensus on terminology for this area is eagerly awaited.
The RANK/RANKL/OPG system
It had long been recognized that the development of osteoclasts in vitro required close interaction between osteoclast precursors and osteoblastic stromal cells, and that the latter produced an osteoclast differentiating factor (ODF) in response to a variety of stimuli which was essential for the formation of mature osteoclasts. This factor has recently been identified as RANKL (receptor activator of NF-
B ligand), also known as OPGL, which is a membrane protein expressed on the osteoblastic stromal cell and belongs to the tumour necrosis factor family [5,6]. Osteoclast progenitors, in turn, express on their surface the receptor for RANKL which has been named RANK (receptor activator of NF-
B) [7]. Several groups of investigators have demonstrated that the interaction between RANKL and RANK is essential for the development of the mature osteoclast (Figure 1
) [5,8,9]. Thus, exposure of bone marrow cultures to RANKL in combination with M-CSF stimulates osteoclast development [5]; similarly, osteoclasts can be generated from mouse spleen cells and monocytes as well as from cultured human monocytes in the presence of M-CSF and RANKL [9]. In addition to stimulating osteoclastogenesis, it has been demonstrated that RANKL plays a key role in stimulating osteoclast activity in vivo and in vitro. Thus, injection of OPGL to experimental animals results in hypercalcaemia secondary to increased osteoclastic activity as suggested by the finding of large osteoclasts present in the bone of the animals examined [5]. Furthermore, mice lacking the gene for RANKL are characterized by the presence of severe osteopetrosis and the absence of mature osteoclasts [8]. Furthermore, antibodies to RANKL can inhibit the bone resorptive activity induced by known inducers of bone resorption such as calcitriol, PTH and PGE2 [10].

View larger version (37K):
[in this window]
[in a new window]
|
Fig. 1. Diagrammatic representation of the role of the RANK/RANKL/OPG system in osteoclast development. RANKL is expressed on the surface of the osteoblastic stromal cell and serves as a ligand for RANK, expressed on the osteoclast progenitor cell. The interaction between the osteoblastic stromal cell and the osteoclast progenitor triggers the development of the mature osteoclast. OPG, produced by the osteoblastic stromal cell, serves as a decoy receptor for RANKL and inhibits osteoclastogenesis by preventing the interaction between the osteoblastic stromal cell and the osteoclast progenitor. (Modified from Nakagawa et al. [7] with permission.)
|
|
The actions of RANKL may be influenced by osteoprotegerin (OPG), also known as osteoclastogenesis inhibitory factor (OCIF) or TNF receptor-like molecule 1 (TR1) [1115]. This protein is a tumour necrosis factor receptor-like molecule which is produced and secreted by osteoblastic stromal cells. OPG can interact with RANKL and prevent the binding of RANKL to its receptor on the osteoclast precursor, RANK (Figure 1
). Therefore, the role of OPG as a decoy receptor for RANKL leads to failure of osteoblastic stromal cells to communicate with osteoclast precursors resulting in impaired osteoclastogenesis. Several pieces of evidence have confirmed the ability of OPG to counteract the actions of RANKL. Thus, in vitro, OPG has been shown to inhibit the increase in osteoclast development and activity induced by RANKL, and in experimental animals, OPG inhibits the increase in serum calcium caused by the administration of RANKL [5]. These findings support the notion that OPG acts as a decoy receptor for RANKL, and therefore, it should have a protective effect on bone by decreasing osteoclastogenesis and osteoclastic bone resorption. Indeed, in vitro studies have demonstrated that OPG inhibits osteoclast differentiation from precursor cells, a finding that is consistent with studies in experimental animals showing that the administration of OPG leads to increased bone mineral density, overexpression of OPG in transgenic animals results in osteopetrosis, and targeted disruption of the OPG gene results in osteoporosis [12,16,17]. Although the role of the RANK/RANKL/OPG system in disorders of bone remodelling in humans has not yet been well studied, recent findings indicate that this system is involved in the effects of oestrogen and glucocorticoids on bone, and strongly suggest that abnormalities in any of its components are likely to result in skeletal disorders [18,19]. The actions of many cytokines and hormones on the balance between activators and suppressors of osteoclast number and activity is depicted in Figure 2
.

View larger version (24K):
[in this window]
[in a new window]
|
Fig. 2. Diagrammatic representation of the influence of RANKL and OPG on osteoclast number and activity. RANKL will tip the balance towards increased osteoclast number and activity whereas increases in OPG will oppose this effect. The hormones and cytokines scattered around the ends of the balance beam will tip the balance in the direction indicated by the arrows. (Adapted from Hofbauer et al. [35,36] with permission.)
|
|
Effects of uraemia on skeletal biology
There is accumulating evidence to suggest that the various cytokine systems involved in the regulation of the different stages of the bone remodelling cycle may be altered in patients with chronic renal failure, and therefore, are likely to contribute to the pathogenesis of the remodelling abnormalities of renal osteodystrophy [20,21]. For example, activation of the bone remodelling cycle may be influenced by the high levels of interleukin 1 (IL-1) which have been reported in dialysis patients [22,23]. Similarly, high levels of IL-1 receptor antagonist, which are expected to oppose the effects of IL-1, have also been detected in these patients, and interestingly, Ferreira et al. reported an inverse relationship between the levels of IL-1Ra and osteoblast surface [23,24]. Another important cytokine for the activation of the remodeling cycle is TNF-
, which has also been found to circulate at high levels in uraemic patients [22].
Osteoclastogenesis and osteoclast function may be influenced by abnormalities in the major bone resorptive cytokines such as IL-6 and the RANK/RANKL/OPG system. Elevated levels of IL-6 have been reported in dialysis as well as in pre-dialysis patients, and more recently, Montalban and his group found a correlation between IL-6 levels and bone remodelling markers in patients with renal osteodystrophy [25,26]. High levels of soluble IL-6 receptors (sIL-6R), which modulate the actions of IL-6 have been reported in haemodialysis patients, and studies by Ferreira and his group found an inverse relationship between the sIL-6R : IL-6 ratio and osteoclast surface in uraemic patients [23,27]. Langub and colleagues, using in situ hybridization, examined the expression of IL-6 receptor mRNA in osteoclasts of uraemic patients and found that it paralleled bone resorbing activity [28]. Thus, although the exact contribution of abnormal IL-6 metabolism to the pathogenesis of renal bone disease is not clear at the present time, it is likely to be of importance based on the role played by this cytokine in other states of abnormal bone resorption such as post-menopausal osteoporosis. In view of the now established importance of the RANK/RANKL/OPG system in bone biology, it is necessary to consider how disturbances of this system may impact on renal bone disease. Although this cytokine system has not been well studied in uraemia, alterations in these patients are expected since the system is regulated by calciotropic hormones which are known to be abnormal in the uraemic setting. Preliminary studies have shown high circulating levels of OPG in uraemic patients, and are likely to have an impact on bone remodelling [29].
Alterations in factors known to regulate osteoblast growth, differentiation and activity have also been demonstrated in uraemic patients. For example, increased circulating levels of IGF-I have been reported, and there is some evidence to suggest that bone formation rate correlates with the levels of IGF-I in these patients [30]. Epidermal growth factor is also known to affect osteoblast development, and uraemia may affect the actions of EGF on bone by regulating the expression of EGF receptors [31]. Although abnomalities in TGF-ß have yet to be described in uraemia, the system is clearly under the influence of parathyroid hormone and vice-versa thus introducing another potential site for derangements in bone remodelling in chronic renal failure [32,33]. Interestingly, a member of the TGF-ß family, known as osteogenic protein-1 or bone morphogenetic protein 1 (OP-1 or BMP-1), which is crucial in osteoblast differentiation from mesenchymal stem cells, is normally expressed in the kidney [34]. Therefore, a deficiency of OP-1 in uraemia as renal mass decreases could potentially lead to failure of osteoblast development and contribute to the low turnover bone disease (adynamic bone) which is part of the spectrum of skeletal abnormalities of renal osteodystrophy.
Summary
In addition to the contributions of the well-known calciotropic hormones, normal bone remodelling depends on the integrated effects of a variety of growth factors and cytokine systems. Renal osteodystrophy is a multifactorial disorder of bone remodelling commonly manifested in patients with chronic renal failure which has been traditionally attributed to disordered calciotropic hormone metabolism. However, there is accumulating evidence to suggest that abnormalities of bone acting cytokines and growth factors as well as their receptors and endogenous modulators are also present in uraemia, and therefore, should be considered as potential contributors to the pathogenesis of renal bone disease.
Acknowledgments
This work was supported in part by grant DK-51099 from the National Institutes of Health.
Notes
Correspondence and offprint requests to: Esther A. González, MD, Division of Nephrology, Saint Louis University, 3635 Vista Avenue, St Louis, MO 63110, USA. 
References
-
Slatopolsky E, Delmez JA. Pathogenesis of secondary hyperparathyroidism. Nephrol Dial Transplant1996; 11 [Suppl 3]: 130135
-
Quarles LD, Lobaugh B, Murphy G. Intact parathyroid hormone overestimates the presence and severity of parathyroid-mediated osseous abnormalities in uremia. J Clin Endocrinol Metab1992; 75: 145150[Abstract]
-
Wang M, Hercz G, Sherrard DJ, Maloney NA, Segre GV, Pei Y. Relationship between intact 184 parathyroid hormone and bone histomorphometric parameters in dialysis patients without aluminum toxicity. Am J Kidney Dis1995; 26: 836844[ISI][Medline]
-
Amling M, Priemel M, Holzmann T et al. Rescue of the skeletal phenotype of vitamin D receptor-ablated mice in the setting of normal mineral ion homeostasis: formal histomorphometric and biomechanical analyses. Endocrinology1999; 140: 49824987[Abstract/Free Full Text]
-
Lacey DL, Timms E, Tan HL et al. Osteoprotegerin ligand is a cytokine that regulates osteoclast differentiation and activation. Cell1998; 93: 165176[ISI][Medline]
-
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 USA1998; 95: 35973602[Abstract/Free Full Text]
-
Nakagawa N, Kinosaki M, Yamaguchi K et al. RANK is the essential signaling receptor for osteoclast differentiation factor in osteoclastogenesis. Biochem Biophys Res Commun1998; 253: 395400[ISI][Medline]
-
Kong YY, Yoshida H, Sarosi I et al. OPGL is a key regulator of osteoclastogenesis, lymphocyte development and lymph-node organogenesis. Nature1999; 397: 315323[ISI][Medline]
-
Quinn JM, Elliott J, Gillespie MT, Martin TJ. A combination of osteoclast differentiation factor and macrophage-colony stimulating factor is sufficient for both human and mouse osteoclast formation in vitro. Endocrinology1998; 139: 44244427[Abstract/Free Full Text]
-
Tsukii K, Shima N, Mochizuki S et al. Osteoclast differentiation factor mediates an essential signal for bone resorption induced by 1 alpha,25-dihydroxyvitamin D3, prostaglandin E2, or parathyroid hormone in the microenvironment of bone. Biochem Biophys Res Commun1998; 246: 337341[ISI][Medline]
-
Tan KB, Harrop J, Reddy M et al. Characterization of a novel TNF-like ligand and recently described TNF ligand and TNF receptor superfamily genes and their constitutive and inducible expression in hematopoietic and non-hematopoietic cells. Gene1997; 204: 3546[ISI][Medline]
-
Simonet WS, Lacey DL, Dunstan CR et al. Osteoprotegerin: a novel secreted protein involved in the regulation of bone density [see comments]. Cell1997; 89: 309319[ISI][Medline]
-
Tsuda E, Goto M, Mochizuki S et al. Isolation of a novel cytokine from human fibroblasts that specifically inhibits osteoclastogenesis. Biochem Biophys Res Commun1997; 234: 137142[ISI][Medline]
-
Kwon BS, Wang S, Udagawa N et al. TR1, a new member of the tumor necrosis factor receptor superfamily, induces fibroblast proliferation and inhibits osteoclastogenesis and bone resorption. Faseb J1998; 12: 845854[Abstract/Free Full Text]
-
Emery JG, McDonnell P, Burke MB et al. Osteoprotegerin is a receptor for the cytotoxic ligand TRAIL. J Biol Chem1998; 273: 1436314367[Abstract/Free Full Text]
-
Bucay N, Sarosi I, Dunstan CR et al. osteoprotegerin-deficient mice develop early onset osteoporosis and arterial calcification. Genes Dev1998; 12: 12601268[Abstract/Free Full Text]
-
Mizuno A, Amizuka N, Irie K et al. Severe osteoporosis in mice lacking osteoclastogenesis inhibitory factor/osteoprotegerin. Biochem Biophys Res Commun1998; 247: 610615[ISI][Medline]
-
Hofbauer LC, Khosla S, Dunstan CR, Lacey DL, Spelsberg TC, Riggs BL. Estrogen stimulates gene expression and protein production of osteoprotegerin in human osteoblastic cells. Endocrinology1999; 140: 43674370[Abstract/Free Full Text]
-
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 [see comments]. Endocrinology1999; 140: 43824389[Abstract/Free Full Text]
-
González E, Martin K. Bone cell response in uremia. Seminars Dial1996; 9: 339346[ISI]
-
Hory B, Drüeke TB. The parathyroid-bone axis in uremia: new insights into old questions. Curr Opinion Nephrol Hypertension1997; 6: 4048[ISI][Medline]
-
Herbelin A, Nguyen AT, Zingraff J, Urena P, Descamps-Latscha B. Influence of uremia and hemodialysis on circulating interleukin-1 and tumor necrosis factor alpha. Kidney Int1990; 37: 116125[ISI][Medline]
-
Ferreira A, Simon P, Drueke TB, Deschamps-Latscha B. Potential role of cytokines in renal osteodystrophy [letter]. Nephrol Dial Transplant1996; 11: 399400[ISI][Medline]
-
Moutabarrik A, Nakanishi I, Namiki M, Tsubakihara Y. Interleukin-1 and its naturally occurring antagonist in peritoneal dialysis patients. Clin Nephrol1995; 43: 243248[ISI][Medline]
-
Herbelin A, Ureña P, Nguyen AT, Zingraff J, Descamps-Latscha B. Elevated circulating levels of interleukin-6 in patients with chronic renal failure. Kidney Int1991; 39: 954960[ISI][Medline]
-
Montalbán C, García-Unzueta MT, De Francisco AL, Amado JA. Serum interleukin-6 in renal osteodystrophy: relationship with serum PTH and bone remodeling markers. Hormone Metab Res1999; 31: 1417[ISI][Medline]
-
Le Meur Y, Lorgeot V, Aldigier JC, Wijdenes J, Leroux-Robert C, Praloran V. Whole blood production of monocytic cytokines (IL-1ß, IL-6, TNF-
, sIL-6R, IL-1Ra) in haemodialysed patients. Nephrol Dial Transplant1999; 14: 24202426[Abstract/Free Full Text]
-
Langub MC, Jr, Koszewski NJ, Turner HV, Monier-Faugere MC, Geng Z, Malluche HH. Bone resorption and mRNA expression of IL-6 and IL-6 receptor in patients with renal osteodystrophy. Kidney Int1996; 50: 515520[ISI][Medline]
-
Kazama J, Shigematsu T, Tsuda E, et al. Increased circulating osteoprotegerin/osteoclastogenesis inhibitory factor (OPG/OCIF) in patients with chronic renal failure. J Am Soc Nephrol1999; 10: 599A
-
Andress DL, Pandian MR, Endres DB, Kopp JB. Plasma insulin-like growth factors and bone formation in uremic hyperparathyroidism. Kidney Int1989; 36: 471477[ISI][Medline]
-
Drake MT, Baldassare JJ, McConkey CL, Gonzalez EA, Martin KJ. Parathyroid hormone increases the expression of receptors for epidermal growth factor in UMR 10601 cells. Endocrinology1994; 134: 17331737[Abstract]
-
Pfeilschifter J, Laukhuf F, Müller-Beckmann B, Blum WF, Pfister T, Ziegler R. Parathyroid hormone increases the concentration of insulin-like growth factor-I and transforming growth factor beta 1 in rat bone. J Clin Invest1995; 96: 767774[ISI][Medline]
-
Jongen JW, Willemstein-Van Hove EC, Van der Meer JM et al. Down-regulation of the receptor for parathyroid hormone (PTH) and PTH-related peptide by transforming growth factor-beta in primary fetal rat osteoblasts. Endocrinology1995; 136: 32603266[Abstract]
-
Ozkaynak E, Schnegelsberg PN, Oppermann H. Murine osteogenic protein (OP-1): high levels of mRNA in kidney. Biochem Biophys Res Commun1991; 179: 116123[ISI][Medline]
-
Hofbauer LC. Osteoprotegerin ligand and osteoprotegerin: novel implications for osteoclast biology and bone metabolism. Eur J Endocrinol1999; 141: 195210[ISI][Medline]
-
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 Res2000; 15: 212[ISI][Medline]