Osteoporosis and Inflammation Gregory R. Mundy, MD

Transcription

1 December 2007(II): S147 S151 Osteoporosis and Inflammation Gregory R. Mundy, MD Osteoporosis represents a major healthcare burden, affecting approximately 10 million people aged over 50 years in the United States and with another 30 million or more at risk. One of the major contributing factors to osteoporosis is withdrawal of estrogen during menopause in women. Human and animal experiments have implicated pro-inflammatory cytokines as primary mediators of the accelerated bone loss at menopause including interleukin-1, tumor necrosis factor-, and interleukin-6. Increased production of pro-inflammatory cytokines is associated with osteoclastic bone resorption in a number of disease states including rheumatoid arthritis, periodontitis, and multiple myeloma; estrogen withdrawal is associated with increased production of pro-inflammatory cytokines, and exposure of bone cultures to supernatants from activated leukocytes is associated with increased bone resorption. A major advance has been the discovery of RANKL, its receptor RANK, and the endogenous inhibitor osteoprotegerin. The binding of RANKL to RANK is essential for the differentiation and activation of osteoclasts and mediates the actions of essentially all known stimulators of osteoclastic bone resorption. RANKL expression is heightened in post- compared with pre-menopausal women, and this effect is attenuated by estrogen replacement therapy. RANKL is also a therapeutic target; a human antibody with high specificity and affinity to RANKL is currently under clinical evaluation for the treatment of osteoporosis in post-menopausal women and of metastatic bone disease in cancer patients with bone metastasis. Early data are promising. Dr. Mundy is John A. Oates Chair in Translational Medicine and Director of the Vanderbilt Center for Bone Biology; Professor of Medicine, Pharmacology, Orthopaedics, Cancer Biology, Vanderbilt University Medical Center, Nashville, Tennessee, USA. Please direct correspondence to: Dr. Gregory Mundy, Vanderbilt University Medical Center, 1235 Medical Research Building IV, 2215B Garland Avenue, Nashville, TN , USA. Phone: , Fax: , gregory.r.mundy@ vanderbilt.edu Key words: osteoporosis, inflammation, cytokines, osteoclast activating factor, postmenopausal, RANKL, RANK, osteoprotegerin, bone resorption 2007 International Life Sciences Institute doi: /nr.2007.dec.S147 S151 INTRODUCTION Osteoporosis, low bone mass associated with a destruction of trabecular architecture and loss of connectivity between plates of trabecular bone, affects about 10 million people over the age of 50 in the United States; a further 33.6 million people have low bone mass and are at high risk of developing the condition. 1,2 This places an immense burden on healthcare systems, due to the 1.5 million osteoporosis-related fractures each year, which often lead to hospitalizations, emergency room and physician visits, and nursing home placements. 3 Eighty percent of individuals with osteoporosis are women, largely due to the marked loss in bone density associated with the withdrawal of estrogen that accompanies loss of ovarian function at menopause. 1 Estrogen-containing hormone replacement therapy is very effective in reversing the impact of menopause on bone density, but it is currently out of favor due to unwanted side effects, including increased risk of stroke, venous thrombosis and, possibly, dementia. 4 6 The loss of bone density associated with estrogen withdrawal is a result of a marked increase in osteoclast activity. But this presents a mechanistic puzzle: Estrogen has not been found to have a major direct effect on osteoclast activity, 7 so its withdrawal must also be stimulating osteoclast activity indirectly. One hint of how this might be achieved comes from the social nature of osteoclasts in the bone microenvironment. Activated osteoclasts are usually found in the presence of accessory cells, including stromal cells, cells in the osteoclast lineage, and cells involved in the inflammatory response. 8 These cells possess the ability to express proinflammatory cytokines 9 and there is now strong evidence to suggest that production of pro-inflammatory cytokines in response to estrogen withdrawal at menopause is responsible for the characteristic loss of bone density through their effect on osteoclast activity. S147

2 PRO-INFLAMMATORY CYTOKINES AND BONE RESORPTION In one of the earliest studies providing direct support for cytokines as mediators of osteoclast activity, bone cultures were exposed to the supernatant from human leukocytes activated either by exposure to an antigen to which they had previous exposure, or by exposure to the mitogen phytohemagglutinin. 10 Exposure to the activated leukocyte supernatant was associated with an increase in osteoclastic bone resorption and in the number of osteoclasts present in the organ culture, suggesting that one or more factors, termed osteoclast activating factor, produced by activated leukocytes, induced osteoclast formation as well as activation. 10 Since this early study, a long list of pro-inflammatory cytokines has been shown capable of stimulating osteoclastic bone resorption including interleukin (IL)-1, tumor necrosis factor alpha (TNF- ), IL-6, IL-11, 23,24 IL-15, 25 and IL Pro-inflammatory cytokines have been associated with bone resorption in some inflammatory diseases and malignancies. In the autoimmune disease rheumatoid arthritis (RA), for example, high levels of pro-inflammatory cytokines have been reported in the synovial fluid of affected joints, and are associated with osteoclast-mediated focal bone erosion at the margins of these joints. 27 Periodontal disease, a chronic inflammation of the gums due to persistent bacterial infection, is associated with osteoclast-mediated destruction of the alveolar bone of the jaw. 28 Osteolytic lesions are also a characteristic of multiple myeloma, a neoplasm of B-cell origin, leading to considerable morbidity. 29 In multiple myeloma, osteoclasts accumulate on bone-resorbing surfaces adjacent to myeloma cells, and the myeloma cells have been shown to secrete a number of pro-inflammatory cytokines was associated with estrogen withdrawal, and a concomitant increase in bone resorption. 32 The role of pro-inflammatory cytokines as mediators of bone loss following estrogen withdrawal has also been investigated using a number of animal models. Ovariectomy in rats was accompanied by an increase in bone marrow cell secretion of IL-1 and TNF-, and a decrease in bone density. However, administration of IL-1 receptor antagonist and TNF-binding protein following ovariectomy completely abolished the profound reduction in bone density observed in those not receiving these cytokine inhibitors, as did estrogen replacement (Figure 1). 15 Interestingly, administration of either cytokine inhibitor alone reduced, but did not abolish, the bone loss following ovariectomy. 15 Mice lacking a functional IL-1 type 1 receptor maintained their bone volume following ovariectomy at preoperative levels, while in wild-type controls, as would be expected, it was significantly reduced. 14 Similarly, transgenic mice expressing soluble TNF- type 1 receptor did not experience the profound loss of bone density experienced by their non-transgenic littermates. 33 Mice deficient in IL-6 maintained a higher rate of bone turnover throughout, unlike wild-type controls. Ovariectomy was associated with a 10-fold increase in osteoclast formation in mice, but administration of antibodies to IL-6 or the implantation of estrogen pellets abolished this effect. 21 Furthermore, IL-6-deficient mice maintained their bone mass following ovariectomy and did not experience a change in bone turnover rate, unlike the IL-6-producing wild-type controls. 22 Together, these data suggest estrogen withdrawal is associated with an increase in production of pro-inflammatory cytokines, which in turn increases osteoclast activity resulting in a profound loss of bone. ESTROGEN WITHDRAWAL STIMULATES PRODUCTION OF PRO-INFLAMMATORY CYTOKINES There is also evidence from both human and animal studies to suggest bone loss associated with menopause may be linked to activation of osteoclasts by pro-inflammatory cytokines. In pre-menopausal women, ovariectomy (surgical menopause) was associated with a significant, sustained increase in spontaneous IL-1 and TNF- secretion by peripheral blood mononuclear cells (PBMCs), with a concomitant increase in bone resorption. 32 In women who received estrogen replacement therapy after ovariectomy, IL-1 and TNF- secretion increased transiently, but returned to pre-operative levels by the fourth week of estrogen therapy, indicating that the increased IL-1 and TNF- production by PBMCs Figure 1. Effect (mean SEM) of cytokine blockade using interleukin-1 receptor antagonist and tumor necrosis factorbinding protein on bone mineral density following ovariectomy in rats. Results are expressed as the percent change from baseline (N 10 rats/group). *P 0.05 compared with baseline. Adapted from Kimble et al. 15 (Endocrinology. 1995;136: ). S148

3 FINAL COMMON MEDIATOR OF OSTEOCLAST FUNCTION A breakthrough in our understanding of how all these osteoclast-activating factors interact was realized with the discovery of a final common mediator of osteoclast differentiation and activation, receptor activator of nuclear factor- B (RANK), its functional ligand (RANKL), and the soluble decoy receptor to RANKL, osteoprotegerin (OPG). RANKL, (also known as TRANCE (TNF-related activation induced cytokine), OPG ligand, osteoclast differentiation factor or TNF superfamily 11) is a membrane-bound TNF-related factor expressed by osteoblasts and stromal cells, and in lymphoid tissue RANK is a membrane-bound TNF receptor expressed on osteoblast precursor cells that recognizes RANKL through a direct cell:cell interaction, a process essential for the differentiation of osteoclasts from their precursor cells. 36,38 This highlights the significance of the social nature of osteoclasts mentioned earlier. RANKL also promotes the bone resorbing activity of osteoclasts and prolongs their survival Osteoprotegerin acts as an endogenous inhibitor for RANKL, specifically binding it and blocking its interaction with RANK. 35,36 Every known modulator of osteoclast activity exerts its effect through this system It should be noted, however, that some pro-inflammatory cytokines, including IL-1 and TNF-, also stimulate osteoclast activity by increasing production of macrophage-colony stimulating factor (M-CSF), which binds to its receptor c-fms on precursor cells, thereby modulating the pool of precursor cells available for differentiation via the actions of RANKL (Figure 2). 45 Estrogen, as well as modulating the production of pro-inflammatory cytokines known to stimulate osteoclastogenesis, also exerts effects by down-regulating RANKL-induced JNK activation. 46 The physiological significance of RANK:RANKL interaction in osteoblast development was demonstrated Figure 2. Mechanisms involved in the regulation of osteoclastogenesis. The pool of osteoclast precursor cells is modulated by macrophage-colony stimulating factor (M-CSF). Differentiation of osteoblast precursor cells into activated osteoclasts is stimulated by the interaction between receptor activator of nuclear factor- B (RANK) on their cell surface and RANKL (the RANK ligand) expressed by accessory cells (e.g., stromal cells and osteoblasts). Osteoprotegerin (OPG) acts as an endogenous inhibitor to RANKL, binding to it and thereby blocking its interaction with RANK. in a series of gene ablation experiments. Mice lacking functional RANKL develop a form of osteopetrosis and lack osteoclasts, despite the presence of osteoclast precursors that are capable of maturation in the presence of osteoblast or stromal cells that express RANKL. 47,48 Conversely, OPG knockout mice develop osteopenia with increased numbers of activated osteoclasts present. 49,50 THERAPEUTIC IMPLICATIONS The RANK/RANKL/OPG system also presents an attractive therapeutic target for the treatment of osteoporosis. RANKL expression is significantly higher in postmenopausal compared with pre-menopausal women, and post-menopausal women treated with estrogen. 51 Agents currently under study include OPG, RANK-Fc (RANK receptor), and RANKL antibodies; however, these may lack the specificity required. 52 Another challenge is that since RANKL is involved in the normal remodeling process as well as with pathological bone loss, 44 inhibition of this ligand should be aimed at preventing excessive bone loss while still preserving normal bone turnover. Since an increase in RANKL is detected in peripheral blood cells of patients with osteoporosis undergoing menopause, 51 inhibition of RANKL to premenopausal levels may represent a promising therapeutic option. An inhibitor of RANKL, denosumab, is currently under investigation for the treatment of osteoporosis in postmenopausal women and of metastatic bone disease in cancer patients with bone metastasis (ClinicalTrials. gov number NCT ). Denosumab is a fully human monoclonal antibody shown to block binding of RANKL to RANK. 53 It has a high affinity to human RANKL and does not bind to TNF-, TNF-, TRAIL, or CD-40 ligand. 53 Results of the Phase II trial involving administration of denosumab to postmenopausal women with low bone density every six months for a period of one year, have been published recently. 54 Denosumab administration at doses ranging from 60 mg to 210 mg increased bone mineral density significantly compared with placebo, and by a similar amount to once weekly infusions of the bisphosphonate alendronate at a dose of 70 mg (Figure 3). 54 Discontinuation rates were similar to placebo, and no significant differences were observed between the profiles of adverse events in the denosumab groups and the placebo group or the alendronate group. 54 Further data from its clinical development are awaited eagerly. CONCLUSIONS There seems little doubt that osteoporosis associated with estrogen withdrawal at menopause is modulated by an array of pro-inflammatory cytokines, cytokines also S149

4 Figure 3. Effect of a one-year regimen of denosumab every six months at varying doses, alendronate 70 mg once weekly, and placebo, on lumbar spine bone mineral density. Placebo (N 46); F 14 mg of denosumab (N 53); Π60 mg of denosumab (N 46); 100 mg of denosumab (N 41); 210 mg of denosumab (N 46); 70 mg of alendronate weekly (N 46). Reprinted from McClung et al. 54 (N Engl J Med. 2006;354: ) with permission from the Massachusetts Medical Society. involved in other diseases associated with both generalized and local bone loss such as RA and myeloma. Withdrawal of estrogen is associated with increased production of pro-inflammatory cytokines and profound bone loss; blocking the actions of those cytokines, or hormone replacement, abates this effect. Particularly promising is the discovery of RANKL, representing a final common mediator of osteoclast differentiation and activation through its interaction with the osteoclastic membrane receptor RANK. Denosumab, a human antibody to RANKL that blocks this interaction is currently under clinical investigation, and the data holds promise of a therapeutic solution to what is one of the latest epidemics of modern medicine. ACKNOWLEDGMENT Dr. Mundy is a Scientific Advisory Board member of Interleukin Genetics, and is a Board member of Neosil Inc, and a founder and shareholder of OsteoScreen Inc, and OsteoGenix. His work is supported by two RO1 grants from NIAMS, one RO1 grant from NCI, a PO1 grant from NCI, and a Merit Review grant from the Veterans Administration. REFERENCES 1. Libby P, Ridker PM, Maseri A. Inflammation and atherosclerosis. Circulation. 2002;105: Bone Health and Osteoporosis: A Report of the Surgeon General. Rockville, MD: U.S. Department of Health and Human Services; Gass M, Dawson-Hughes B. Preventing osteoporosis-related fractures: an overview. Am J Med. 2006; 119(suppl 1): Anderson GL, Limacher M, Assaf AR, et al. Effects of conjugated equine estrogen in postmenopausal women with hysterectomy: the Women s Health Initiative randomized controlled trial. JAMA. 2004;291: Curb JD, Prentice RL, Bray PF, et al. Venous thrombosis and conjugated equine estrogen in women without a uterus. Arch Intern Med. 2006;166: Shumaker SA, Legault C, Kuller L, et al. Conjugated equine estrogens and incidence of probable dementia and mild cognitive impairment in postmenopausal women: Women s Health Initiative Memory Study. JAMA. 2004;291: Caputo CB, Meadows D, Raisz LG. Failure of estrogens and androgens to inhibit bone resorption in tissue culture. Endocrinology. 1976;98: Rodan GA, Martin TJ. Role of osteoblasts in hormonal control of bone resorption a hypothesis. Calcif Tissue Int. 1982;34: Reddy SV, Roodman GD. Control of osteoclast differentiation. Crit Rev Eukaryot Gene Expr. 1998;8: Horton JE, Raisz LG, Simmons HA, Oppenheim JJ, Mergenhagen SE. Bone resorbing activity in supernatant fluid from cultured human peripheral blood leukocytes. Science. 1972;177: Boyce BF, Aufdemorte TB, Garrett IR, Yates AJ, Mundy GR. Effects of interleukin-1 on bone turnover in normal mice. Endocrinology. 1989;125: Dewhirst FE, Stashenko PP, Mole JE, Tsurumachi T. Purification and partial sequence of human osteoclast-activating factor: identity with interleukin 1 beta. J Immunol. 1985;135: Gowen M, Wood DD, Ihrie EJ, McGuire MK, Russell RG. An interleukin 1 like factor stimulates bone resorption in vitro. Nature. 1983;306: Lorenzo JA, Naprta A, Rao Y, et al. Mice lacking the type I interleukin-1 receptor do not lose bone mass after ovariectomy. Endocrinology. 1998;139: Kimble RB, Matayoshi AB, Vannice JL, Kung VT, Williams C, Pacifici R. Simultaneous block of interleukin-1 and tumor necrosis factor is required to completely prevent bone loss in the early postovariectomy period. Endocrinology. 1995;136: Konig A, Muhlbauer RC, Fleisch H. Tumor necrosis factor alpha and interleukin-1 stimulate bone resorption in vivo as measured by urinary [3H]tetracycline excretion from prelabeled mice. J Bone Miner Res. 1988;3: Bertolini DR, Nedwin GE, Bringman TS, Smith DD, Mundy GR. Stimulation of bone resorption and inhibition of bone formation in vitro by human tumour necrosis factors. Nature. 1986;319: Thomson BM, Mundy GR, Chambers TJ. Tumor necrosis factors alpha and beta induce osteoblastic cells to stimulate osteoclastic bone resorption. J Immunol. 1987;138: van der Pluijm G, Most W, van der Wee-Pals L, de Groot H, Papapoulos S, Lowik C. Two distinct effects of recombinant human tumor necrosis factoralpha on osteoclast development and subsequent resorption of mineralized matrix. Endocrinology. 1991;129: S150

5 20. Ishimi Y, Miyaura C, Jin CH, et al. IL-6 is produced by osteoblasts and induces bone resorption. J Immunol. 1990;145: Jilka RL, Hangoc G, Girasole G, et al. Increased osteoclast development after estrogen loss: mediation by interleukin-6. Science. 1992;257: Poli V, Balena R, Fattori E, et al. Interleukin-6 deficient mice are protected from bone loss caused by estrogen depletion. Embo J. 1994;13: Girasole G, Passeri G, Jilka RL, Manolagas SC. Interleukin-11: a new cytokine critical for osteoclast development. J Clin Invest. 1994;93: Hill PA, Tumber A, Papaioannou S, Meikle MC. The cellular actions of interleukin-11 on bone resorption in vitro. Endocrinology. 1998;139: Ogata Y, Kukita A, Kukita T, et al. A novel role of IL-15 in the development of osteoclasts: inability to replace its activity with IL-2. J Immunol. 1999;162: Kotake S, Udagawa N, Takahashi N, et al. IL-17 in synovial fluids from patients with rheumatoid arthritis is a potent stimulator of osteoclastogenesis. J Clin Invest. 1999;103: Goldring SR. Pathogenesis of bone erosions in rheumatoid arthritis. Curr Opin Rheumatol. 2002;14: Bartold PM, Marshall RI, Haynes DR. Periodontitis and rheumatoid arthritis: a review. J Periodontol. 2005;76(suppl): Anderson KC, Shaughnessy JD, Jr, Barlogie B, Harousseau JL, Roodman GD. Multiple myeloma. Hematology Am Soc Hematol Educ Program. 2002: Mundy GR, Raisz LG, Cooper RA, Schechter GP, Salmon SE. Evidence for the secretion of an osteoclast stimulating factor in myeloma. N Engl J Med. 1974;291: Yeh HS, Berenson JR. Treatment for myeloma bone disease. Clin Cancer Res. 2006;12(20 Pt 2): Pacifici R, Brown C, Puscheck E, et al. Effect of surgical menopause and estrogen replacement on cytokine release from human blood mononuclear cells. Proc Natl Acad Sci USA. 1991;88: Ammann P, Rizzoli R, Bonjour JP, et al. Transgenic mice expressing soluble tumor necrosis factor-receptor are protected against bone loss caused by estrogen deficiency. J Clin Invest. 1997;99: Wong AH, Gottesman, II, Petronis A. Phenotypic differences in genetically identical organisms: the epigenetic perspective. Hum Mol Genet. 2005; 14(special no. 1):R 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: Lacey DL, Timms E, Tan HL, et al. Osteoprotegerin ligand is a cytokine that regulates osteoclast differentiation and activation. Cell. 1998;93: Anderson DM, Maraskovsky E, Billingsley WL, et al. A homologue of the TNF receptor and its ligand enhance T-cell growth and dendritic-cell function. Nature. 1997;390: Nakagawa N, Kinosaki M, Yamaguchi K, et al. RANK is the essential signaling receptor for osteoclast differentiation factor in osteoclastogenesis. Biochem Biophys Res Commun. 1998;253: Burgess TL, Qian Y, Kaufman S, et al. The ligand for osteoprotegerin (OPGL) directly activates mature osteoclasts. J Cell Biol. 1999;145: Fuller K, Wong B, Fox S, Choi Y, Chambers TJ. TRANCE is necessary and sufficient for osteoblastmediated activation of bone resorption in osteoclasts. J Exp Med. 1998;188: Lacey DL, Tan HL, Lu J, et al. Osteoprotegerin ligand modulates murine osteoclast survival in vitro and in vivo. Am J Pathol. 2000;157: Khosla S. Minireview: the OPG/RANKL/RANK system. Endocrinology. 2001;142: Roux S, Orcel P. Bone loss. Factors that regulate osteoclast differentiation: an update. Arthritis Res. 2000;2: Tanaka S, Nakamura K, Takahasi N, Suda T. Role of RANKL in physiological and pathological bone resorption and therapeutics targeting the RANKL-RANK signaling system. Immunol Rev. 2005;208: Udagawa N, Takahashi N, Akatsu T, et al. Origin of osteoclasts: mature monocytes and macrophages are capable of differentiating into osteoclasts under a suitable microenvironment prepared by bone marrow-derived stromal cells. Proc Natl Acad Sci USA. 1990;87: Srivastava S, Toraldo G, Weitzmann MN, Cenci S, Ross FP, Pacifici R. Estrogen decreases osteoclast formation by down-regulating receptor activator of NF-kappa B ligand (RANKL)-induced JNK activation. J Biol Chem. 2001;276: Kong YY, Yoshida H, Sarosi I, et al. OPGL is a key regulator of osteoclastogenesis, lymphocyte development and lymph-node organogenesis. Nature. 1999;397: Nicklin MJ, Hughes DE, Barton JL, Ure JM, Duff GW. Arterial inflammation in mice lacking the interleukin 1 receptor antagonist gene. J Exp Med. 2000; 191: Bucay N, Sarosi I, Dunstan CR, et al. osteoprotegerin-deficient mice develop early onset osteoporosis and arterial calcification. Genes Dev. 1998;12: Mizuno A, Amizuka N, Irie K, et al. Severe osteoporosis in mice lacking osteoclastogenesis inhibitory factor/osteoprotegerin. Biochem Biophys Res Commun. 1998;247: Eghbali-Fatourechi G, Khosla S, Sanyal A, Boyle WJ, Lacey DL, Riggs BL. Role of RANK ligand in mediating increased bone resorption in early postmenopausal women. J Clin Invest. 2003;111: Chen H, Wilkins LM, Aziz N, et al. Single nucleotide polymorphisms in the human interleukin-1b gene affect transcription according to haplotype context. Hum Mol Genet. 2006;15: Dougall WC, Chaisson M. The RANK/RANKL/OPG triad in cancer-induced bone diseases. Cancer Metastasis Rev. 2006;25: McClung MR, Lewiecki EM, Cohen SB, et al. Denosumab in postmenopausal women with low bone mineral density. N Engl J Med. 2006;354: S151