Urolithiasis is prevalent and associated with reduced bone mineral density in β-thalassaemia major

Asymptomatic urolithiasis is common and of mixed composition in patients with β-thalassaemia major. Twenty-seven subjects were imaged using dual-energy computer tomography to determine the presence and composition of urolithiasis. The prevalence of urolithiasis was 59% and affected patients generally had multiple stones, often with more than one component: struvite (33%), calcium oxalate (31%) and cystine (22%). Hypercalciuria was present in 78% of subjects and calcium-containing urolithiasis was associated with reduced femoral neck Z scores.

Melphalan modifies the bone microenvironment by enhancing osteoclast formation

Melphalan is a cytotoxic chemotherapy used to treat patients with multiple myeloma (MM). Bone resorption by osteoclasts, by remodeling the bone surface, can reactivate dormant MM cells held in the endosteal niche to promote tumor development. Dormant MM cells can be reactivated after melphalan treatment; however, it is unclear whether melphalan treatment increases osteoclast formation to modify the endosteal niche.

Melphalan treatment of mice for 14 days decreased bone volume and the endosteal bone surface, and this was associated with increases in osteoclast numbers. Bone marrow cells (BMC) from melphalan-treated mice formed more osteoclasts than BMCs from vehicle-treated mice, suggesting that osteoclast progenitors were increased. Melphalan also increased osteoclast formation in BMCs and RAW264.7 cells in vitro, which was prevented with the cell stress response (CSR) inhibitor KNK437. Melphalan also increased expression of the osteoclast regulator the microphthalmia-associated transcription factor (MITF), but not nuclear factor of activated T cells 1 (NFATc1). Melphalan increased expression of MITF-dependent cell fusion factors, dendritic cell-specific transmembrane protein (Dc-stamp) and osteoclast-stimulatory transmembrane protein (Oc-stamp) and increased cell fusion. Expression of osteoclast stimulator receptor activator of NFκB ligand (RANKL) was unaffected by melphalan treatment.

These data suggest that melphalan stimulates osteoclast formation by increasing osteoclast progenitor recruitment and differentiation in a CSR-dependent manner. Melphalan-induced osteoclast formation is associated with bone loss and reduced endosteal bone surface. As well as affecting bone structure this may contribute to dormant tumor cell activation, which has implications for how melphalan is used to treat patients with MM.

Bone Disease in Thalassemia: A Molecular and Clinical Overview

Thalassemia bone disease is a common and severe complication of thalassemia-an inherited blood disorder due to mutations in the α or β hemoglobin gene. In its more severe form, severe anemia is present, and treatment with frequent red blood cell transfusion is necessary. Because the body has limited capacity to excrete iron, concomitant iron chelation is required to prevent the complications of iron overload. The effects of chronic anemia and iron overload can lead to multiple end-organ complications such as cardiomyopathy, increased risks of blood-borne diseases, and liver, pituitary, and bone disease. However, our understanding of thalassemia bone disease is incomplete and is composed of a complex piecemeal of risk factors that include genetic factors, hormonal deficiency, marrow expansion, skeletal dysmorphism, iron toxicity, chelators, and increased bone turnover. The high prevalence of bone disease in transfusion-dependent thalassemia is seen in both younger and older patients as life expectancy continues to improve. Indeed, hypogonadism and GH deficiency contribute to a failure to achieve peak bone mass in this group. The contribution of kidney dysfunction to bone disease in thalassemia is a new and significant complication. This coincides with studies confirming an increase in kidney stones and associated accelerated bone loss in the thalassemia cohort. However, multiple factors are also associated with reduced bone mineral density and include marrow expansion, iron toxicity, iron chelators, increased bone turnover, GH deficiency, and hypogonadism. Thalassemia bone disease is a composite of not only multiple hormonal deficiencies but also multiorgan diseases. This review will address the molecular mechanisms and clinical risk factors associated with thalassemia bone disease and the clinical implications for monitoring and treating this disorder.

Deferasirox at therapeutic doses is associated with dose-dependent hypercalciuria

Deferasirox is an oral iron chelator used widely in the treatment of thalassemia major and other transfusion-dependent hemoglobinopathies. Whilst initial long-term studies established the renal safety of deferasirox, there are now increasing reports of hypercalciuria and renal tubular dysfunction. In addition, urolithiasis with rapid loss of bone density in patients with β thalassemia major has been reported. We conducted a cross-sectional cohort study enrolling 152 adult patients comprising of β thalassemia major (81.5%), sickle cell disease (8%), thalassemia intermedia (2%), HbH disease (6.5%) and E/β thalassemia (2%). Cases were matched with normal control subjects on age, gender and serum creatinine. Iron chelator use was documented and urine calcium to creatinine ratios measured. At the time of analysis, 88.8% of patients were receiving deferasirox and 11.2% were on deferoxamine. Hypercalciuria was present in 91.9% of subjects on deferasirox in a positive dose-dependent relationship. This was not seen with subjects receiving deferoxamine. At a mean dose of 30.2±8.8mg/kg/day, deferasirox was associated with an almost 4 fold increase in urine calcium to creatinine ratio (UCa/Cr). Hypercalciuria was present at therapeutic doses of deferasirox in a dose-dependent manner and warrants further investigation and vigilance for osteoporosis, urolithiasis and other markers of renal dysfunction.

RARγ is a negative regulator of osteoclastogenesis

Vitamin A is known to influence post-natal bone content, with excess intake being associated with reduced bone mineral density and increased fracture risk. Despite this, the roles retinoids play in regulating osteoclastogenesis, particularly in vivo, remain unresolved. This study therefore aimed to determine the effect of loss of retinoic acid receptors (RAR)α or RARγ on bone mass (analyzed by histomorphometry and dual-energy X-ray absorptiometry) and osteoclastogenesis in mice in vivo. RARγ null mice had significantly less trabecular bone at 8 weeks of age compared to wildtype littermates. In contrast, no change in trabecular bone mass was detected in RARα null mice at this age. Further histomorphometric analysis revealed a significantly greater osteoclast surface in bones from 8-week-old RARγ null male mice. This in vivo effect was cell lineage autonomous, and was associated with increased osteoclastogenesis in vitro from hematopoietic cells obtained from 8-week-old RARγ null male mice. The use of highly selective agonists in RANKL-induced osteoclast differentiation of wild type mouse whole bone marrow cells and RAW264.7 cells in vitro showed a stronger inhibitory effect of RARγ than RARα agonists, suggesting that RARγ is a more potent inhibitor of osteoclastogenesis. Furthermore, NFAT activation was also more strongly inhibited by RARγ than RARα agonists. While RARα and RARγ antagonists did not significantly affect osteoclast numbers in vitro, larger osteoclasts were observed in cultures stimulated with the antagonists, suggesting increased osteoclast fusion. Further investigation into the effect of retinoids in vivo revealed that oral administration of 5mg/kg/day ATRA for 10 days protected against bone loss induced by granulocyte colony-stimulating factor (G-CSF) by inhibiting the pro-osteoclastogenic action of G-CSF. Collectively, our data indicates a physiological role for RARγ as a negative regulator of osteoclastogenesis in vivo and in vitro, and reveals distinct influences of RARα and RARγ in bone structure regulation.

Genetic investigation of MHC-independent missing-self recognition by mouse NK cells using an in vivo bone marrow transplantation model

MHC-I-specific receptors play a vital role in NK cell-mediated "missing-self" recognition, which contributes to NK cell activation. In contrast, MHC-independent NK recognition mechanisms are less well characterized. In this study, we investigated the role of NKR-P1B:Clr-b (Klrb1:Clec2d) interactions in determining the outcome of murine hematopoietic cell transplantation in vivo. Using a competitive transplant assay, we show that Clr-b(-/-) bone marrow (BM) cells were selectively rejected by wild-type B6 recipients, to a similar extent as H-2D(b-/-) MHC-I-deficient BM cells. Selective rejection of Clr-b(-/-) BM cells was mitigated by NK depletion of recipient mice. Competitive rejection of Clr-b(-/-) BM cells also occurred in allogeneic transplant recipients, where it was reversed by selective depletion of NKR-P1B(hi) NK cells, leaving the remaining NKR-P1B(lo) NK subset and MHC-I-dependent missing-self recognition intact. Moreover, competitive rejection of Clr-b(-/-) hematopoietic cells was abrogated in Nkrp1b-deficient recipients, which lack the receptor for Clr-b. Of interest, similar to MHC-I-deficient NK cells, Clr-b(-/-) NK cells were hyporesponsive to both NK1.1 (NKR-P1C)-stimulated and IL-12/18 cytokine-primed IFN-γ production. These findings support a unique and nonredundant role for NKR-P1B:Clr-b interactions in missing-self recognition of normal hematopoietic cells and suggest that optimal BM transplant success relies on MHC-independent tolerance mechanisms. These findings provide a model for human NKR-P1A:LLT1 (KLRB1:CLEC2D) interactions in human hematopoietic cell transplants.

Osteoclast formation elicited by interleukin-33 stimulation is dependent upon the type of osteoclast progenitor

Osteoclasts are bone resorbing multinucleated cells (MNCs) derived from macrophage progenitors. IL-33 has been reported to drive osteoclastogenesis independently of receptor activator of NFκB ligand (RANKL) but this remains controversial as later studies did not confirm this. We found IL-33 clearly elicited functional dentine-resorbing osteoclast formation from human adult monocytes. However, monocytes from only 3 of 12 donors responded this way, while all responded to RANKL. Human cord blood-derived progenitors and murine bone marrow macrophages lacked an osteoclastogenic response to IL-33. In RAW264.7 cells, IL-33 elicited NFκB and p38 responses but not NFATc1 signals (suggesting poor osteoclastogenic responses) and formed only mononuclear tartrate-resistant acid phosphatase positive (TRAP(+)) cells. Since TGFβ boosts osteoclastogenesis in RAW264.7 cells we employed an IL-33/TGFβ co-treatment, which resulted in small numbers of MNCs expressing key osteoclast markers TRAP and calcitonin receptors. Thus, IL-33 possesses weak osteoclastogenic activity suggesting pathological significance and, perhaps, explaining previous conflicting reports.

Thalassemia bone disease: a 19-year longitudinal analysis

Thalassemia is an inherited disorder of alpha or beta globin chain synthesis leading to ineffective erythropoiesis requiring chronic transfusion therapy in its most severe form. This leads to iron overload, marrow expansion, and hormonal complications, which are implicated in bone deformity and loss of bone mineral density (BMD). In this 19-year retrospective longitudinal study, the relationships between BMD (determined by dual-energy X-ray absorptiometry) and risk factors for osteoporosis in 277 subjects with transfusion-dependent thalassemia were examined. The mean age at first review was 23.2 ± 11.9 years and 43.7% were male. Hypogonadism was present in 28.9%. Fractures were confirmed in 11.6% of subjects and were more frequent in males (16.5%) compared with females (7.7%). Lumbar spine (LS), femoral neck (FN), and total body (TB) Z-scores were derived. Patients with transfusion-dependent thalassemia had a significant longitudinal decline in BMD at the FN and TB. In the linear mixed-model analysis of BMD and risk factors for bone loss, FN Z-score was more significantly associated with risk factors compared with the LS and TB. The rate of decline at the FN was 0.02 Z-score per year and was 3.85-fold greater in males. The decline in FN Z-score over the last 5 years (years 15 to 19) was 2.5-fold that of the previous 7 years (years 8 to 14) and coincided with a change in iron chelator therapy from desferrioxamine to deferasirox. Hemoglobin (Hb) levels positively correlated with higher TB and LS Z-scores. In conclusion, the FN is the preferred site for follow-up of BMD. Male patients with β-thalassemia experienced a greater loss of BMD and had a higher prevalence of fractures compared with females. Transfusing patients (particularly males) to a higher Hb target may reduce the decline in BMD. Whether deferasirox is implicated in bone loss warrants further study.

Molecular stress-inducing compounds increase osteoclast formation in a heat shock factor 1 protein-dependent manner

Many anticancer therapeutic agents cause bone loss, which increases the risk of fractures that severely reduce quality of life. Thus, in drug development, it is critical to identify and understand such effects. Anticancer therapeutic and HSP90 inhibitor 17-(allylamino)-17-demethoxygeldanamycin (17-AAG) causes bone loss by increasing osteoclast formation, but the mechanism underlying this is not understood. 17-AAG activates heat shock factor 1 (Hsf1), the master transcriptional regulator of heat shock/cell stress responses, which may be involved in this negative action of 17-AAG upon bone. Using mouse bone marrow and RAW264.7 osteoclast differentiation models we found that HSP90 inhibitors that induced a heat shock response also enhanced osteoclast formation, whereas HSP90 inhibitors that did not (including coumermycin A1 and novobiocin) did not affect osteoclast formation. Pharmacological inhibition or shRNAmir knockdown of Hsf1 in RAW264.7 cells as well as the use of Hsf1 null mouse bone marrow cells demonstrated that 17-AAG-enhanced osteoclast formation was Hsf1-dependent. Moreover, ectopic overexpression of Hsf1 enhanced 17-AAG effects upon osteoclast formation. Consistent with these findings, protein levels of the essential osteoclast transcription factor microphthalmia-associated transcription factor were increased by 17-AAG in an Hsf1-dependent manner. In addition to HSP90 inhibitors, we also identified that other agents that induced cellular stress, such as ethanol, doxorubicin, and methotrexate, also directly increased osteoclast formation, potentially in an Hsf1-dependent manner. These results, therefore, indicate that cellular stress can enhance osteoclast differentiation via Hsf1-dependent mechanisms and may significantly contribute to pathological and therapeutic related bone loss.

The effect of gonadal status on body composition and bone mineral density in transfusion-dependent thalassemia

Patients with transfusion-dependent thalassemia have abnormal growth, hormonal deficits, and increased bone loss. We investigated the relationship between skeletal muscle mass, fat mass, and bone mineral density in adult subjects with transfusion-dependent thalassemia based on their gonadal status. Our findings show that hypogonadism attenuates the strength of the muscle-bone relationship in males but strengthens the positive correlation of skeletal muscle mass and fat mass in female subjects.

INTRODUCTION

Transfusion-dependent thalassemia is associated with a high prevalence of fractures. Multiple hormonal complications, in particular hypogonadism, can lead to changes in body composition and bone mineral density (BMD). We investigated for the first time the relationship between skeletal muscle mass (SMM), fat mass, and BMD in adult subjects with transfusion-dependent thalassemia based on their gonadal status.

METHODS

A retrospective cohort study of 186 adults with transfusion-dependent thalassemia was analyzed. Body composition and BMD were measured using dual energy X-ray absorptiometry. The association between skeletal muscle, fat, and BMD was investigated through uni-, multi-, and stepwise regression analyses after adjusting for multicollinearity. SMM was derived using the formula, SMM = 1.19 × ALST-1.65, where ALST is equivalent to the sum of both arm and leg lean tissue mass.

RESULTS

There were 186 subjects, males (43.5 %) and females (56.5 %), with a median age of 36.5. Hypogonadism was reported in 44.4 % of males and 44.7 % of females. SMM and BMD were positively correlated and strongest in eugonadal males (0.36 ≤ R (2) ≤ 0.59), but the association was attenuated in hypogonadal males. SMM (0.27 ≤ R (2) ≤ 0.69) and total fat mass (0.26 ≤ R (2) ≤ 0.55) were positively correlated with BMD in hypogonadal females, but the correlation was less pronounced in eugonadal females. Leg lean tissue mass and arm lean tissue mass in males and females, respectively, were most highly correlated to BMD in the stepwise regression analysis.

CONCLUSION

Hypogonadism attenuates the strength of the muscle-bone relationship in males but strengthens the positive correlation of skeletal muscle mass and fat mass in female subjects. This study supports the notion that exercise is important for maintaining BMD and the need to optimize treatment of hypogonadism in patients with transfusion-dependent thalassemia.

Prolonged hypocalcemia following denosumab therapy in metastatic hormone refractory prostate cancer

Prostate cancer is a leading cause of cancer death, frequently associated with widespread bone metastases. We report two cases of hypocalcemia following the first dose of denosumab in metastatic hormone refractory prostate cancer, the first case requiring 26 days of intravenous calcium therapy. This is the first report of prolonged hypocalcemia following denosumab in a patient with normal renal function.

Thalassemia bone disease: the association between nephrolithiasis, bone mineral density and fractures

Thalassemia bone disease is well described, but the prevalence of nephrolithiasis has not been characterized. The association between nephrolithiasis, reduced bone density, and increased fractures has been demonstrated through this retrospective study of 166 participants with transfusion-dependent thalassemia. The findings support the need for increased vigilance of kidney and bone disease in this cohort.

INTRODUCTION

Previous studies have revealed that thalassemia is associated with reduced bone mineral density (BMD) and fractures. Many causes are implicated including hypogonadism, growth hormone deficiency, marrow expansion, and iron overload. Nephrolithiasis is associated with reduced BMD and increased fractures in the general population. However, the prevalence of nephrolithiasis and its association with bone density and fractures have not been characterized in thalassemia.

METHODS

We have addressed this question by performing a retrospective cohort study of 166 participants with transfusion-dependent thalassemia who had undergone dual-energy X-ray absorptiometry between 2009 and 2011. Logistic regression modeling was used to adjust for potential confounders.

RESULTS

We found a high prevalence of kidney stones (18.1 %) which was greater in males compared to females (28.7 vs 9.7 %, respectively). Renal stones were associated with reduced femoral neck Z-score and fractures in men after adjusting for potential confounders. These results indicate that nephrolithiasis is highly prevalent in patients with transfusion-dependent thalassemia and is significantly associated with reduced BMD and increased fractures.

CONCLUSIONS

The findings from this study strongly support the need for ongoing surveillance of BMD, fractures, and nephrolithiasis in the management of transfusion-dependent thalassemia.

PTHrP Overexpression Increases Sensitivity of Breast Cancer Cells to Apo2L/TRAIL

Parathyroid hormone-related protein (PTHrP) is a key component in breast development and breast tumour biology. PTHrP has been discovered as a causative agent of hypercalcaemia of malignancy and is also one of the main factors implicated in breast cancer mediated osteolysis. Clinical studies have determined that PTHrP expression by primary breast cancers was an independent predictor of improved prognosis. Furthermore, PTHrP has been demonstrated to cause tumour cell death both in vitro and in vivo. Apo2L/TRAIL is a promising new anti-cancer agent, due to its ability to selectively induce apoptosis in cancer cells whilst sparing most normal cells. However, some cancer cells are resistant to Apo2L/TRAIL-induced apoptosis thus limiting its therapeutic efficacy. The effects of PTHrP on cell death signalling pathways initiated by Apo2L/TRAIL were investigated in breast cancer cells. Expression of PTHrP in Apo2L/TRAIL resistant cell line MCF-7 sensitised these cells to Apo2L/TRAIL-induced apoptosis. The actions of PTHrP resulted from intracellular effects, since exogenous treatment of PTHrP had no effect on Apo2L/TRAIL-induced apoptosis. Apo2L/TRAIL-induced apoptosis in PTHrP expressing cells occurred through the activation of caspase-10 resulting in caspase-9 activation and induction of apoptosis through the effector caspases, caspase-6 and -7. PTHrP increased cell surface expression of Apo2L/TRAIL death receptors, TRAIL-R1 and TRAIL-R2. Antagonistic antibodies against the death receptors demonstrated that Apo2L/TRAIL mediated its apoptotic signals through activation of the TRAIL-R2 in PTHrP expressing breast cancer cells. These studies reveal a novel role for PTHrP with Apo2L/TRAIL that maybe important for future diagnosis and treatment of breast cancer.

Abstract 2933: HSP90 inhibiting anti-cancer therapeutics enhance bone loss by increasing osteoclast formation: Mechanism of action

Breast cancer cells metastasize to bone where they cause bone destruction. Here, cancer cells promote bone loss through the recruitment of osteoclasts, specialised macrophage-like cells that resorb bone. Bone resorption releases bone matrix factors that increase tumor growth, resulting in vicious cycle of bone resorption and tumor progression.

Previously we found 17-AAG, an Hsp90 inhibitor and anti-cancer therapeutic significantly increases MDA-MB-231 breast cancer cell growth in bone following intracardiac inoculation in nude mice. 17-AAG also increases osteoclast formation in vitro and in vivo even though Hsp90 client proteins are important in osteoclast formation. In addition to degradation of Hsp90 client proteins, inhibition by 17-AAG also causes cell stress responses through the activation of Heat Shock Factor 1 (HSF1). Therefore we sought to characterize 17-AAG and new HSP90 inhibitors, to examine the role of HSF1 in their effects and to determine the molecular mechanism of 17-AAG.

17-AAG and new, but structurally unrelated, Hsp90 blockers CCT018159 and NVP-AUY922 dose-dependently enhanced osteoclast formation from mouse bone marrow and RAW264.7 cells. Ablating HSF1 action by shRNA transfection and by pharmacological inhibition using KNK437 inhibited 17-AAG effects upon osteoclast formation. Osteoclast formation from HSF1 knockout mouse bone marrow were unresponsive to 17-AAG, but wild type cells did respond. To determine the mechanism of how 17-AAG is increasing osteoclast formation, the effect of 17-AAG upon critical transcription factors in the osteoclast formation pathway was observed. 17-AAG treatment did not affect transcriptional activity of NFκB, AP-1 or NFATc1. Moreover 17-AAG did not increase NFATc1 or cFos protein expression over time. In contrast, 17-AAG treatment increased protein expression of MITF,in a dose and time dependent manner. RAW264.7 cells were then transiently transfected with a luciferase reporter construct containing the promoter region of a MITF target gene v-ATPase-d2. 17-AAG treatment increased the transcriptional activity of this promoter. RANKL and 17-AAG combination treatment increased reporter activity of the v-ATPase-d2 promoter in an additive manner. The stress induced kinase p38 is known to be important in osteoclast formation. Our results show inhibiting p38 with SB203580 decreases MITF protein expression.

These data indicate that Hsp90 inhibition causes an increase in osteoclast formation in an HSF1-dependent manner. The data provides a mechanism of action, whereby 17-AAG is increasing osteoclast numbers, through the increased protein expression of Mitf.

Citation Format: A. Gabrielle J. van der Kraan, Ryan R. Chai, Michelle M. Kouspou, Ben J. Lang, Preetinder P. Singh, Jiake Xu, Damien Eeles, Matthew T. Gillespie, Julian M. Quinn, John T. Price. HSP90 inhibiting anti-cancer therapeutics enhance bone loss by increasing osteoclast formation: Mechanism of action. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 2933. doi:10.1158/1538-7445.AM2013-2933

Abstract 4886: PTHrP sensitizes breast cancer cells to TRAIL-induced apoptosis

Parathyroid hormone-related protein (PTHrP) is a multifunctional protein that has been implicated in breast tumor biology. Breast cancers commonly express PTHrP, and when such tumors are grown in bone in murine models, PTHrP results in enhanced osteolysis. Whilst clinical studies have determined that PTHrP expression by primary breast cancers was an independent predictor of improved prognosis (Henderson et al., 2006). Thus, the action of PTHrP upon breast cancers directly remains to be elucidated. The aim of this study was to assess the interaction of PTHrP with molecules associated with cell survival and cell death pathways, such as Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL). TRAIL is a member of the tumor necrosis factor (TNF) family of cytokines that induces apoptosis in a wide range of transformed cells but not in normal cells. To understand the role of PTHrP in relation to TRAIL in breast cancer cells, PTHrP was overexpressed in two breast cancer cell lines; MDA-MB-231 (TRAIL-sensitive) and MCF-7 (TRAIL-resistant), and functional assays were performed. Over-expression of PTHrP in both MCF-7 and MDA-MB-231 cells enhanced cell cycle progression and cell growth rate in comparison to parental and vector controls. However, treatment of PTHrP overexpressing breast cancer cells with TRAIL resulted in an enhancement of TRAIL-induced apoptosis in the TRAIL sensitive cell line, MDA-MB-231, and a sensitisation to TRAIL-induced apoptosis in the TRAIL-resistant cell line, MCF-7. Western blot showed decreased levels of Caspase-10, Caspase-9, Caspase-6, Capase-7 precursor proteins and increased PARP cleavage levels in PTHrP overexpressing cells treated with TRAIL when compared to controls. There was no change observed in levels of Caspase-8 percursor protein, suggesting that the apoptosis signal is acting via the intrinsic pathway in PTHrP overexpressing cells. To determine TRAIL sensitivity in MCF-7 cells overexpressing PTHrP, TRAIL receptor mRNA and protein expression was assessed by qRT-PCR and FACS analysis. TRAIL receptors; DR4, DR5, DcR1 and DcR2, mRNA expression was not altered in PTHrP overexpressing cells. However, flow cytometric analysis demonstrated an increase in cell surface expression of both death receptors (DR4 and DR5) in cells overexpressing PTHrP. To determine whether TRAIL-induced apoptosis in PTHrP overexpressing cells was signaling either through DR4, DR5 or a combination of both death receptors, antagonistic antibodies against DR4 or DR5 were used. Pre-incubation of PTHrP overexpressing cells with DR5 antagonistic antibody attenuated TRAIL-induced apoptosis. This demonstrated that TRAIL preferentially signals through DR5 in the PTHrP overexpressing cells to activate the intrinsic apoptotic pathway. In conclusion, these results demonstrate a new function for PTHrP in breast cancer and may provide utility when TRAIL is used as a treatment for breast cancer.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 4886. doi:1538-7445.AM2012-4886

Interleukin-33, a target of parathyroid hormone and oncostatin m, increases osteoblastic matrix mineral deposition and inhibits osteoclast formation in vitro

IL-33 is an important inflammatory mediator in allergy, asthma, and joint inflammation, acting via its receptor, ST2L, to elicit Th₂ cell cytokine secretion. IL-33 is related to IL-1 and IL-18, which both influence bone metabolism, IL-18 in particular inhibiting osteoclast formation and contributing to PTH bone anabolic actions. We found IL-33 immunostaining in osteoblasts in mouse bone and IL-33 mRNA expression in cultured calvarial osteoblasts, which was elevated by treatment with the bone anabolic factors oncostatin M and PTH. IL-33 treatment strongly inhibited osteoclast formation in bone marrow and spleen cell cultures but had no effect on osteoclast formation in receptor activator of nuclear factor-κB ligand/macrophage colony-stimulating factor-treated bone marrow macrophage (BMM) or RAW264.7 cultures, suggesting a lack of direct action on immature osteoclast progenitors. However, osteoclast formation from BMM was inhibited by IL-33 in the presence of osteoblasts, T cells, or mature macrophages, suggesting these cell types may mediate some actions of IL-33. In bone marrow cultures, IL-33 induced mRNA expression of granulocyte macrophage colony-stimulating factor, IL-4, IL-13, and IL-10; osteoclast inhibitory actions of IL-33 were rescued only by combined antibody ablation of these factors. In contrast to osteoclasts, IL-33 promoted matrix mineral deposition by long-term ascorbate treated primary osteoblasts and reduced sclerostin mRNA levels in such cultures after 6 and 24 h of treatment; sclerostin mRNA was also suppressed in IL-33-treated calvarial organ cultures. In summary, IL-33 stimulates osteoblastic function in vitro but inhibits osteoclast formation through at least three separate mechanisms. Autocrine and paracrine actions of osteoblast IL-33 may thus influence bone metabolism.

Zinc finger protein 467 is a novel regulator of osteoblast and adipocyte commitment

Osteoblasts and adipocytes are derived from common mesenchymal progenitor cells. The bone loss of osteoporosis is associated with altered progenitor differentiation from an osteoblastic to an adipocytic lineage. cDNA microarrays and quantitative real-time PCR (Q-PCR) were carried out in a differentiating mouse stromal osteoblastic cell line, Kusa 4b10, to identify gene targets of factors that stimulate osteoblast differentiation including parathyroid hormone (PTH) and gp130-binding cytokines, oncostatin M (OSM) and cardiotrophin-1 (CT-1). Zinc finger protein 467 (Zfp467) was rapidly down-regulated by PTH, OSM, and CT-1. Retroviral overexpression and RNA interference for Zfp467 in mouse stromal cells showed that this factor stimulated adipocyte formation and inhibited osteoblast commitment compared with controls. Regulation of adipocyte markers, including peroxisome proliferator-activated receptor (PPAR) γ, C/EBPα, adiponectin, and resistin, and late osteoblast/osteocyte markers (osteocalcin and sclerostin) by Zfp467 was confirmed by Q-PCR. Intra-tibial injection of calvarial cells transduced with retroviral Zfp467 doubled the number of marrow adipocytes in C57Bl/6 mice compared with vector control-transduced cells, providing in vivo confirmation of a pro-adipogenic role of Zfp467. Furthermore, Zfp467 transactivated a PPAR-response element reporter construct and recruited a histone deacetylase complex. Thus Zfp467 is a novel co-factor that promotes adipocyte differentiation and suppresses osteoblast differentiation. This has relevance to therapeutic interventions in osteoporosis, including PTH-based therapies currently available, and may be of relevance for the use of adipose-derived stem cells for tissue engineering.

Calcitonin impairs the anabolic effect of PTH in young rats and stimulates expression of sclerostin by osteocytes

The therapeutic goal of increasing bone mass by co-treatment of parathyroid hormone (PTH) and an osteoclast inhibitor has been complicated by the undefined contribution of osteoclasts to the anabolic activity of PTH. To determine whether active osteoclasts are required at the time of PTH administration, we administered a low dose of the transient osteoclast inhibitor salmon calcitonin (sCT) to young rats receiving an anabolic PTH regimen. Co-administration of sCT significantly blunted the anabolic effect of PTH as measured by peripheral quantitative computer tomography (pQCT) and histomorphometry in the femur and tibia, respectively. To determine gene targets of sCT, we carried out quantitative real time PCR and microarray analysis of metaphyseal samples 1.5, 4 and 6.5h after administration of a single injection of PTH, sCT or PTH+sCT. Known targets of PTH action, IL-6, ephrinB2 and RANKL, were not modified by co-administration with sCT. Surprisingly, at all time points, we noted a significant upregulation of sclerostin mRNA by sCT treatment, as well as down-regulation of two other osteocyte gene products, MEPE and DMP1. Immunohistochemistry confirmed that sCT administration increased the percentage of osteocytes expressing sclerostin, suggesting a mechanism by which sCT reduced the anabolic effect of PTH. Neither mRNA for CT receptor (Calcr) nor labeled CT binding could be detected in sclerostin-enriched cells differentiated from primary calvarial osteoblasts. In contrast, osteocytes freshly isolated from calvariae expressed a high level of Calcr mRNA. Furthermore immunohistochemistry revealed co-localization of CT receptor (CTR) and sclerostin in some osteocytes in calvarial sections. Taken together these data indicate that co-treatment with sCT can blunt the anabolic effect of PTH and this may involve direct stimulation of sclerostin production by osteocytes. These data directly implicate calcitonin as a negative regulator of bone formation through a previously unsuspected mechanism.

Oncostatin M promotes bone formation independently of resorption when signaling through leukemia inhibitory factor receptor in mice

Effective osteoporosis therapy requires agents that increase the amount and/or quality of bone. Any modification of osteoclast-mediated bone resorption by disease or drug treatment, however, elicits a parallel change in osteoblast-mediated bone formation because the processes are tightly coupled. Anabolic approaches now focus on uncoupling osteoblast action from osteoclast formation, for example, by inhibiting sclerostin, an inhibitor of bone formation that does not influence osteoclast differentiation. Here, we report that oncostatin M (OSM) is produced by osteoblasts and osteocytes in mouse bone and that it has distinct effects when acting through 2 different receptors, OSM receptor (OSMR) and leukemia inhibitory factor receptor (LIFR). Specifically, mouse OSM (mOSM) inhibited sclerostin production in a stromal cell line and in primary murine osteoblast cultures by acting through LIFR. In contrast, when acting through OSMR, mOSM stimulated RANKL production and osteoclast formation. A key role for OSMR in bone turnover was confirmed by the osteopetrotic phenotype of mice lacking OSMR. Furthermore, in contrast to the accepted model, in which mOSM acts only through OSMR, mOSM inhibited sclerostin expression in Osmr-/- osteoblasts and enhanced bone formation in vivo. These data reveal what we believe to be a novel pathway by which bone formation can be stimulated independently of bone resorption and provide new insights into OSMR and LIFR signaling that are relevant to other medical conditions, including cardiovascular and neurodegenerative diseases and cancer.

Germline deletion of AMP-activated protein kinase beta subunits reduces bone mass without altering osteoclast differentiation or function

Since AMP-activated protein kinase (AMPK) plays important roles in modulating metabolism in response to diet and exercise, both of which influence bone mass, we examined the influence of AMPK on bone mass in mice. AMPK is an alphabetagamma heterotrimer where the beta subunit anchors the alpha catalytic and gamma regulatory subunits. Germline deletion of either AMPK beta1 or beta2 subunit isoforms resulted in reduced trabecular bone density and mass, but without effects on osteoclast (OC) or osteoblast (OB) numbers, as compared to wild-type littermate controls. We tested whether activating AMPK in vivo would enhance bone density but found AICA-riboside treatment caused a profound loss of trabecular bone volume (49.5%) and density and associated increased OC numbers. Consistent with this, AICA-riboside strongly stimulated OC differentiation in vitro, in an adenosine kinase-dependent manner. OCs and macrophages (unlike OBs) lacked AMPK beta2 subunit expression, and when generated from AMPK beta1(-/-) mice displayed no detectable AMPK activity. Nevertheless, AICA-riboside was equally effective at stimulating OC differentiation from wild-type or beta1(-/-) progenitors, indicating that AMPK is not essential for OC differentiation or the stimulatory action of AICA-riboside. These results show that AMPK is required to maintain normal bone density, but not through bone cell differentiation, and does not mediate powerful osteolytic effects of AICA-riboside.

The chemokine Cxcl1 is a novel target gene of parathyroid hormone (PTH)/PTH-related protein in committed osteoblasts

The PTH receptor (PTHR1) is expressed on osteoblasts and responds to PTH or PTHrP in an endocrine or autocrine/paracrine manner, respectively. A microarray study carried out on PTHR1-positive osteoblasts (Kusa 4b10 cells) identified the cysteine-X-cysteine (CXC) family chemokine ligand 1 (Cxcl1) as a novel immediate PTH/PTHrP-responsive gene. Cxcl1 is a potent neutrophil chemoattractant with recognized roles in angiogenesis and inflammation, but a role in bone biology has not been described. Cxcl1 mRNA levels were up-regulated 1 h after either PTH or PTHrP treatment of differentiated Kusa 4b10 osteoblasts (15-fold) and mouse calvarial osteoblasts (160-fold) and in rat metaphyseal bone (5-fold) 1 h after a single sc injection of PTH. Furthermore, PTH treatment stimulated a 10-fold increase in secreted Cxcl1 protein by both Kusa 4b10 cells and calvarial osteoblasts. Immunohistochemistry and PCR demonstrated that CXCR2, the receptor for Cxcl1, is highly expressed in osteoclast precursors (hemopoietic cells) but is predominantly undetectable in the osteoblast lineage, suggesting that osteoblast-derived Cxcl1 may act as a chemoattractant for osteoclast precursors. Confirming this hypothesis, recombinant Cxcl1 dose-dependently stimulated migration of osteoclast precursors in cell culture studies, as did conditioned media from Kusa 4b10 cells treated with PTH. These data indicate that local action through the PTHR1 could stimulate cells of the osteoblast lineage to release a chemokine capable of attracting osteoclast precursors to the bone environment.

Structural basis for antibody discrimination between two hormones that recognize the parathyroid hormone receptor

Parathyroid hormone-related protein (PTHrP) plays a vital role in the embryonic development of the skeleton and other tissues. When it is produced in excess by cancers it can cause hypercalcemia, and its local production by breast cancer cells has been implicated in the pathogenesis of bone metastasis formation in that disease. Antibodies have been developed that neutralize the action of PTHrP through its receptor, parathyroid hormone receptor 1, without influencing parathyroid hormone action through the same receptor. Such neutralizing antibodies against PTHrP are therapeutically effective in animal models of the humoral hypercalcemia of malignancy and of bone metastasis formation. We have determined the crystal structure of the complex between PTHrP (residues 1-108) and a neutralizing monoclonal anti-PTHrP antibody that reveals the only point of contact is an alpha-helical structure extending from residues 14-29. Another striking feature is that the same residues that interact with the antibody also interact with parathyroid hormone receptor 1, showing that the antibody and the receptor binding site on the hormone closely overlap. The structure explains how the antibody discriminates between the two hormones and provides information that could be used in the development of novel agonists and antagonists of their common receptor.

Crystallization of the receptor-binding domain of parathyroid hormone-related protein in complex with a neutralizing monoclonal antibody Fab fragment

Parathyroid hormone-related protein (PTHrP) plays an important role in regulating embryonic skeletal development and is abnormally regulated in the pathogenesis of skeletal complications observed with many cancers and osteoporosis. It exerts its action through binding to a G-protein-coupled seven-transmembrane cell-surface receptor (GPCR). Structurally, GPCRs are very difficult to study by X-ray crystallography. In this study, a monoclonal antibody Fab fragment which recognizes the same region of PTHrP as its receptor, PTH1R, was used to aid in the crystallization of PTHrP. The resultant protein complex was crystallized using the hanging-drop vapour-diffusion method with polyethylene glycol as a precipitant. The crystals belonged to the orthorhombic space group P2(1)2(1)2, with unit-cell parameters a = 72.6, b = 96.3, c = 88.5 A, and diffracted to 2.0 A resolution using synchrotron radiation. The crystal structure will shed light on the nature of the key residues of PTHrP that interact with the antibody and will provide insights into how the antibody is able to discriminate between PTHrP and the related molecule parathyroid homone.

Communication between ephrinB2 and EphB4 within the osteoblast lineage

Members of the ephrin and Eph family are local mediators of cell function through largely contact-dependent processes in development and in maturity. Production of ephrinB2 mRNA and protein are increased by PTH and PTHrP in osteoblasts. Both a synthetic peptide antagonist of ephrinB2/EphB4 receptor interaction and recombinant soluble extracellular domain of EphB4 (sEphB4), which is an antagonist of both forward and reverse EphB4 signaling, were able to inhibit mineralization and the expression of several osteoblast genes involved late in osteoblast differentiation. The findings are consistent with ephrinB2/EphB4 signaling within the osteoblast lineage having a paracrine role in osteoblast differentiation, in addition to the proposed role of osteoclast-derived ephrinB2 in coupling of bone formation to resorption. This local regulation might contribute to control of osteoblast differentiation and bone formation at remodeling sites, and perhaps also in modeling.

IL-23 inhibits osteoclastogenesis indirectly through lymphocytes and is required for the maintenance of bone mass in mice

IL-23 stimulates the differentiation and function of the Th17 subset of CD4(+) T cells and plays a critical role in chronic inflammation. The IL-23 receptor-encoding gene is also an inflammatory disease susceptibility gene. IL-23 shares a common subunit with IL-12, a T cell-dependent osteoclast formation inhibitor, and we found that IL-23 also dose-dependently inhibited osteoclastogenesis in a CD4(+) T lymphocyte-dependent manner. When sufficiently enriched, gammadelta T cells also mediated IL-23 inhibition. Like IL-12, IL-23 acted synergistically with IL-18 to block osteoclastogenesis but, unlike IL-12, IL-23 action depended on T cell GM-CSF production. IL-23 did not mediate IL-12 action although IL-12 induced its expression. Male mice lacking IL-23 (IL-23p19(-/-)) had approximately 30% lower bone mineral density and tibial trabecular bone mass (bone volume (BV)/total volume (TV)) than wild-type littermates at 12 wk and 40% lower BV/TV at 26 wk of age; male heterozygotes also had lower bone mass. Female IL-23p19(-/-) mice also had reduced BV/TV. IL-23p19(-/-) mice had no detectable osteoclast defect in trabecular bone but IL-23p19(-/-) had thinner growth plate hypertrophic and primary spongiosa zones (and, in females, less cartilage remnants) compared with wild type. This suggests increased osteoclast action at and below the growth plate, leading to reduced amounts of mature trabecular bone. Thus, IL-23 inhibits osteoclast formation indirectly via T cells in vitro. Under nonpathological conditions (unlike inflammatory conditions), IL-23 favors higher bone mass in long bones by limiting resorption of immature bone forming below the growth plate.

Osteoclast inhibitory lectin, an immune cell product that is required for normal bone physiology in vivo

Osteoclast inhibitory lectin (OCIL or clrb) is a member of the natural killer cell C-type lectins that have a described role mostly in autoimmune cell function. OCIL was originally identified as an osteoblast-derived inhibitor of osteoclast formation in vitro. To determine the physiological function(s) of OCIL, we generated ocil(-/-) mice. These mice appeared healthy and were fertile, with no apparent immune function defect, and phenotypic abnormalities were limited to bone. Histomorphometric analysis revealed a significantly lower tibial trabecular bone volume and trabecular number in the 10- and 16-week-old male ocil(-/-) mice compared with wild type mice. Furthermore, ocil(-/-) mice showed reduced bone formation rate in the 10-week-old females and 16-week-old males while Static markers of bone formation showed no significant changes in male or female ocil(-/-) mice. Examination of bone resorption markers in the long bones of ocil(-/-) mice indicated a transient increase in osteoclast number per unit bone perimeter. Enhanced osteoclast formation was also observed when either bone marrow or splenic cultures were generated in vitro from ocil(-/-) mice relative to wild type control cultures. Loss of ocil therefore resulted in osteopenia in adult mice primarily as a result of increased osteoclast formation and/or decreased bone formation. The enhanced osteoclastic activity led to elevated serum calcium levels, which resulted in the suppression of circulating parathyroid hormone in 10-week-old ocil(-/-) mice compared with wild type control mice. Collectively, our data suggest that OCIL is a physiological negative regulator of bone.

BMP-2 regulation of PTHrP and osteoclastogenic factors during osteoblast differentiation of C2C12 cells

Bone morphogenetic protein-2 (BMP-2) is strongly involved in the induction of osteoblast differentiation from mesenchymal cell precursors, as well as in enhancing bone matrix production by osteoblastic cells. Likewise, the osteoporotic phenotype of PTHrP deficient mice makes clear the importance of this paracrine regulator in bone physiology. Here, we report that BMP-2 rapidly down-regulated PTHrP gene expression through a transcriptional mechanism in pluripotent mesenchymal C2C12 cells, whereas BMP-2 increased expression of PTHrP receptor. PTHrP did not significantly alter the BMP-dependent Smad transcriptional pathway. Similarly, PTHrP did not significantly modify the BMP-regulated expression of RANKL or OPG, cytokines involved in osteoclastogenesis. More importantly, addition of PTHrP, through the PKA signaling pathway, partially prevented the BMP-dependent induction of some osteogenic markers such as Runx2 and Osterix in C2C12 cells. Our data suggest that BMP-2 down-regulation of PTHrP could facilitate terminal differentiation of osteoblasts.

Impact of cytokines and T lymphocytes upon osteoclast differentiation and function

Historically, the osteoblast has been considered the master cell in the control of osteoclast development and, therefore, bone resorption. Now the interactions between cells of the immune system and bone cells have redefined our thinking on the regulation of bone resorption. Moreover, the crosstalk between these cell types has special significance in inflammatory conditions such as rheumatoid arthritis. This report highlights the contribution that T lymphocytes make in regulating osteoclast formation and bone resorption.

Inflammation-induced bone loss: can it be prevented?

Inflammatory synovitis induces profound bone loss and OCLs are the instrument of this destruction. TNF blockers have an established role in the prevention of inflammatory bone loss in RA; however, not all patients respond to anti-TNF therapy and side effects may prevent long-term treatment in others. The B-cell--depleting antibody rituximab and the T-cell costimulation blocker abatacept are emerging as major treatment options for patients who are resistant to anti-TNF [96,97]. Proof-of-concept studies demonstrate that targeting RANK-mediated osteoclastogenesis prevents inflammatory bone loss and clinical application has only just begun. The efficacy of RANKL inhibition has been witnessed in trials of Denosumab, and RANKL-neutralizing antibodies are likely to become the treatment of choice for blocking RANKL in RA [77,78]. A major limitation of RANKL antagonism is that it does not treat synovitis. Therefore, anti-RANKL therapy most likely will be used in the context of MTX therapy. There is uncertainty about the possible extraskeletal adverse effects of long-term effects of long-term RANKL blockade. In particular, anti-RANKL therapy could jeopardize dendritic cell function or survival. The demonstrable role of OCLs in inflammation-induced bone loss also invites a reconsideration of the new BPs for bone protection [98]. Studies of ZA in preclinical models indicate that bone protection is comparable to that afforded by OPG. One possible caveat is that intravenous BPs are linked to jaw osteonecrosis [99], although the incidence is confined mainly to intensive treatment in the oncology setting. Although pulsed PTH stimulated bone formation in arthritic models, it has yet to be proven clinically in the context of powerful OCL inhibition with TNF or RANKL antagonists. With strategies that normalize OCL numbers, clinicians are poised to accomplish effective prevention of inflammation-induced bone loss.

Osteoclast inhibitory lectin (OCIL) inhibits osteoblast differentiation and function in vitro

Osteoclast inhibitory lectin (OCIL) is a type II C-type lectin and binds NK cell-associated receptor Nkrp1d and sulfated glycosaminoglycans. OCIL is expressed by several cell types found in bone and inhibits osteoclast differentiation. To determine whether OCIL may have wider effects on bone metabolism, we examined the effects of recombinant soluble OCIL on cultured osteoblasts and pre-osteoblastic KUSA O cells. Although OCIL did not affect osteoblast proliferation or apoptosis, or the formation of alkaline phosphatase positive colonies in cultured bone marrow, OCIL profoundly inhibited mineralization by primary osteoblasts and KUSA O cells in vitro. Analysis of ascorbate-treated KUSA O cells showed that addition of OCIL reduced bone sialoprotein (BSP), osterix and osteocalcin mRNA expression, as well as alkaline phosphatase activity while, in contrast, expression of markers associated with the earlier stages of osteoblast maturation or the transcription factors Runx2, ATF4 and c-fos were not affected by OCIL treatment. Indeed, osteocalcin expression was strongly inhibited within 3 days in a dose-dependent manner, although after subsequent removal of OCIL, osteocalcin mRNA levels recovered within 4 days. OCIL treatment also reduced osteocalcin expression in BMP-2 stimulated C2C12 cells. In support of a role for OCIL in mineralization, OCIL anti-sense oligonucleotide treatment of KUSA O cells increased mineralization and osteocalcin expression. In addition, insulin-, dexamethasone- and IBMX-stimulated KUSA O cells undergo adipocyte differentiation and OCIL treatment greatly suppressed this process. Consistent with this, OCIL also reduced adiponectin and resistin mRNA expression in these cells. Our data indicate that OCIL reduces osteoblastic function in vitro and this may be due to an inhibitory effect on osteoblast maturation. In addition, the reduction of adipocyte formation in KUSA O cells by OCIL indicates that OCIL may have wider effects on the mesenchymal lineage that may be important for both bone metabolism and other connective tissue functions.

Mechanisms involved in skeletal anabolic therapies

Since parathyroid hormone (PTH) is the only proven anabolic therapy for bone, it becomes the benchmark by which new treatments will be evaluated. The anabolic effect of PTH is dependent upon intermittent administration, but when an elevated PTH level is maintained even for a few hours it initiates processes leading to new osteoclast formation, and the consequent resorption overrides the effects of activating genes that direct bone formation. Identification of PTH-related protein (PTHrP) production by cells early in the osteoblast lineage, and its action through the PTH1R upon more mature osteoblastic cells, together with the observation that PTHrP+/- mice are osteoporotic, all raise the possibility that PTHrP is a crucial paracrine regulator of bone formation. The finding that concurrent treatment with bisphosphonates impairs the anabolic response to PTH, adds to other clues that osteoclast activity is necessary to complement the direct effect that PTH has in promoting differentiation of committed osteoblast precursors. This might involve the generation of a coupling factor from osteoclasts that are transiently activated by receptor activator of nuclear factor-kappaB ligand (RANKL) in response to PTH. New approaches to anabolic therapies may come from the discovery that an activating mutation in the LRP5 gene is responsible for an inherited high bone mass syndrome, and the fact that this can be recapitulated in transgenic mice, whereas inactivating mutations result in severe bone loss. This has focused attention on the Wnt/frizzled/beta-catenin pathway as being important in bone formation, and proof of the concept has been obtained in experimental models.

Osteoprotegerin Overexpression by Breast Cancer Cells Enhances Orthotopic and Osseous Tumor Growth and Contrasts with That Delivered Therapeutically

Osteoprotegerin (OPG) acts as a decoy receptor for receptor activator of NF-kappaB ligand (RANKL), which is a pivotal molecule required for osteoclast formation. In vitro OPG inhibits osteoclast formation and in vivo (administered as Fc-OPG) it reduces hypercalcemia and the establishment of osteolytic lesions in mouse models of tumor cell growth in bone. Osteolysis can be induced by parathyroid hormone-related protein (PTHrP) produced by breast cancer cells that results in an increased osteoblastic RANKL/OPG ratio. We examined the effect of local tumor production of OPG on the ability of breast cancer cells to establish and grow in bone and mammary fat pad. MCF-7 cells or MCF-7 cells overexpressing PTHrP were transfected with full-length OPG and inoculated into the proximal tibiae of athymic nude mice. Mice injected with cells overexpressing PTHrP and OPG showed enhanced tumor growth, increased osteolysis (2-fold compared with MCF-7 cells overexpressing PTHrP), and altered histology that was reflective of a less differentiated (more aggressive) phenotype compared with MCF-7 cells. In contrast, administration of recombinant Fc-OPG reduced tumor growth and limited osteolysis even in mice inoculated with OPG overexpressing cells. Similarly, OPG overexpression by breast cancer cells enhanced tumor growth following orthotopic inoculation. These results indicate that OPG overexpression by breast cancer cells increases tumor growth in vivo and that there are strikingly different responses between therapeutically administered Fc-OPG and full-length OPG produced by tumor cells.

Effects of parathyroid hormone on Wnt signaling pathway in bone

The Wnt signaling pathway has recently been demonstrated to play an important role in bone cell function. In previous studies using DNA microarray analyses, we observed a change in some of the molecular components of the canonical Wnt pathway namely, frizzled-1 (FZD-1) and axil, in response to continuous parathyroid hormone (PTH) treatment in rats. In the present study, we further explored other components of the Wnt signaling pathway in rat distal metaphyseal bone in vivo, and rat osteoblastic osteosarcoma cells (UMR 106) in culture. Several Wnt pathway components, including low-density lipoprotein-receptor-related protein 5 (LRP5), LRP6, FZD-1, Dickkopf-1 (Dkk-1), and Kremen-1 (KRM-1), were expressed in bone in vivo and in osteoblasts in vitro. Continuous exposure to PTH (1-38) both in vivo and in vitro upregulated the mRNA expression of LRP6 and FZD-1 and decreased LRP5 and Dkk-1. These effects in UMR 106 cells were associated with an increase in beta-catenin as measured by Western blots and resulted in functional activation (three to six-fold) of a downstream Wnt responsive TBE6-luciferase (TCF/LEF-binding element) reporter gene. Activation of the TBE6-luciferase reporter gene by PTH (1-38) in UMR 106 cells was inhibited by the protein kinase A (PKA) inhibitor, H89. Activation was mimicked by PTH (1-31), PTH-related protein (1-34), and forskolin, but both PTH (3-34) and (7-34) had no effect. These findings suggest that the effect of PTH on the canonical Wnt signaling pathway occurs at least in part via the cAMP-PKA pathway through the differential regulation of the receptor complex proteins (FZD-1/LRP5 or LRP6) and the antagonist (Dkk-1). Taken together, these results reveal a possible role for the Wnt signaling pathway in PTH actions in bone.

Interleukin-11 receptor signaling is required for normal bone remodeling

IL-6 and -11 regulate bone turnover and have been implicated in estrogen deficiency-related bone loss. In this study, deletion of IL-11 signaling, but not that of IL-6, suppressed osteoclast differentiation, resulting in high trabecular bone volume and reduced bone formation. Furthermore, IL-11 signaling was not required for the effects of estradiol or estrogen deficiency on the mouse skeleton.

INTRODUCTION

Interleukin (IL)-6 and -11 stimulate osteoclastogenesis and bone formation in vitro and have been implicated in bone loss in estrogen deficiency. Because of their common use of the gp130 co-receptor signaling subunit, the roles of these two cytokines are linked, and each may compensate for the absence of the other to maintain trabecular bone volume and bone cell differentiation.

MATERIALS AND METHODS

To determine the interactions in bone between IL-11 and IL-6 in vivo and whether IL-11 is required for normal bone turnover, we examined the bone phenotype of mature male and female IL-11 receptor knockout mice (IL-11Ralpha1-/-) and compared with the bone phenotype of IL-6-/- mice and mice lacking both IL-6 and IL-11Ralpha. To determine whether IL-11 is required for the effects of estrogen on trabecular bone, mature IL-11Ralpha1-/- mice were ovariectomized and treated with estradiol.

RESULTS

In both male and female IL-11Ralpha1-/- mice, trabecular bone volume was significantly higher than that of wildtype controls. This was associated with low bone resorption and low bone formation, and the low osteoclast number generated by IL-11Ralpha1-/- precursors was reproduced in ex vivo cultures, whereas elevated osteoblast generation was not. Neither trabecular bone volume nor bone turnover was altered in IL-6-/- mice, and compound IL-6-/- :IL-11Ralpha1-/- mice showed an identical bone phenotype to IL-11Ralpha1-/- mice. The responses of IL-11Ralpha1-/- mice to ovariectomy and estradiol treatment were the same as those observed in wildtype mice.

CONCLUSIONS

IL-11 signaling is clearly required for normal bone turnover and normal trabecular bone mass, yet not for the effects of estradiol or estrogen deficiency on the skeleton. In the absence of IL-11Ralpha, increased trabecular bone mass seems to result from a cell lineage-autonomous reduction in osteoclast differentiation, suggesting a direct effect of IL-11 on osteoclast precursors. The effects of IL-11Ralpha deletion on the skeleton are not mediated or compensated for by changes in IL-6 signaling.

The Heat Shock Protein 90 Inhibitor, 17-Allylamino-17-demethoxygeldanamycin, Enhances Osteoclast Formation and Potentiates Bone Metastasis of a Human Breast Cancer Cell Line

Breast cancer metastasis to the bone occurs frequently, causing numerous complications including severe pain, fracture, hypercalcemia, and paralysis. Despite its prevalence and severity, few effective therapies exist. To address this, we examined whether the heat shock protein 90 (Hsp90) inhibitor, 17-allylamino-17-demethoxygeldanamycin (17-AAG), would be efficacious in inhibiting breast cancer metastasis to bone. Utilizing the human breast cancer subline, MDA-MB-231SA, previously in vivo selected for its enhanced ability to generate osteolytic bone lesions, we determined that 17-AAG potently inhibited its in vitro proliferation and migration. Moreover, 17-AAG significantly reduced MDA-MB-231SA tumor growth in the mammary-fat pad of nude mice. Despite these findings, 17-AAG enhanced the incidence of bone metastasis and osteolytic lesions following intracardiac inoculation in the nude mouse. Consistent with these findings, 17-AAG enhanced osteoclast formation 2- to 4-fold in mouse bone marrow/osteoblast cocultures, receptor activator of nuclear factor kappaB ligand (RANKL)-stimulated bone marrow, and RAW264.7 cell models of in vitro osteoclastogenesis. Moreover, the drug enhanced osteoclastogenesis in human cord blood progenitor cells, demonstrating that its effects were not limited to mouse models. In addition to 17-AAG, other Hsp90 inhibitors, such as radicicol and herbimycin A, also enhanced osteoclastogenesis. A pro-osteolytic action of 17-AAG independent of tumor presence was also determined in vivo, in which 17-AAG-treated tumor-naive mice had reduced trabecular bone volume with an associated increase in osteoclast number. Thus, HSP90 inhibitors can stimulate osteoclast formation, which may underlie the increased incidence of osteolysis and skeletal tumor incidence caused by 17-AAG in vivo. These data suggest an important contraindication to the Hsp90 targeted cancer therapy currently undergoing clinical trial.

Modulation of osteoclast formation

Osteoclasts are derived following the fusion of precursors of hematopoietic and myelomonocytic origin after appropriate stimulus, such as that afforded by RANKL and M-CSF. Thus the osteoclast can be considered as a specialized type of macrophage, and several of the factors that affect osteoclast formation also have affects upon macrophage differentiation. Inhibitors of osteoclast formation may perturb RANKL or M-CSF signalling or affect other signalling pathways. Several of these inhibitors are discussed with the view of their capacity to influence osteoclast differentiation, but not necessarily their activity.

Secreted frizzled-related protein-1 inhibits RANKL-dependent osteoclast formation

We determined that sFRP-1 mRNA was differentially expressed by osteoblast/stromal cell lines and that sFRP-1 neutralizing antibodies and siRNA complementary to sFRP-1 coding sequence enhanced, while recombinant sFRP-1 inhibited, osteoclast formation. In studying the mechanism of action for sFRP-1, we found that sFRP-1 could bind recombinant RANKL. These results suggest potential cross-talk between Wnt and RANKL pathways.

INTRODUCTION

Osteoclast formation in normal bone remodeling requires the presence of osteoblast lineage cells that express RANKL and macrophage-colony-stimulating factor (M-CSF), which interact with their cognate receptors on the osteoclast precursor. We identified secreted Frizzled-related protein-1 (sFRP-1), which is known to bind to Wnt and inhibit the Wnt signaling pathway, as an osteoblast-derived factor that impinges on osteoclast formation and activity.

MATERIALS AND METHODS

Differential display of mRNA from osteoblast lineage cell lines established sFRP-1 to be highly expressed in an osteoclast supporting cell line. sFRP-1 expression in bone was determined by in situ hybridization, and the effects of sFRP-1 on osteoclast formation were determined using a neutralizing antibody, siRNA, for sFRP-1 and recombinant protein.

RESULTS

In situ hybridization revealed sFRP-1 mRNA expression in osteoblasts and chondrocytes in murine bone. sFRP-1 mRNA expression could be elevated in calvarial primary osteoblasts in response to prostaglandin E2 (PGE2) or interleukin (IL)-11, whereas many other osteotropic agents (e.g., IL-1, IL-6, calcitrol, parathyroid hormone) were without any effect. In vitro assays of osteoclast formation established sFRP-1 to be an inhibitor of osteoclast formation. Neutralizing antibodies against sFRP-1 enhanced TRACP+ mononuclear and multinuclear osteoclast formation (3- and 2-fold, respectively) in co-cultures of murine osteoblasts with spleen cells, whereas siRNA complementary to sFRP-1 coding sequence significantly enhanced osteoclast formation in co-cultures of KUSA O (osteoblast/stromal cell line) and bone marrow cells, cultured in the presence of PGE2 and 1,25(OH)2 vitamin D3. Recombinant sFRP-1 dose-dependently inhibited osteoclast formation in osteoblast/spleen co-cultures, RANKL + M-CSF-treated splenic cultures, and RANKL-treated RAW264.7 cell cultures, indicating a direct action of sFRP-1 on hematopoietic cells. Consistent with this, sFRP-1 was found to bind to RANKL in ELISAs.

CONCLUSION

sFRP-1 is expressed by osteoblasts and inhibits osteoclast formation. While sFRP-1 activity might involve the blocking of endogenous Wnt signaling, our results suggest that, alternatively, it could be because of direct binding to RANKL. This study describes a new mechanism whereby osteoblasts regulate osteoclastogenesis through the expression and release of sFRP-1.

Myeloma cells can directly contribute to the pool of RANKL in bone bypassing the classic stromal and osteoblast pathway of osteoclast stimulation

Summary The ratio of osteoprotegerin [OPG, tumour necrosis factor receptor superfamily, member 11b (TNFRSF11B)] to receptor activator of nuclear factor kappaB ligand [RANKL, tumour necrosis factor (ligand) superfamily, member 11 (TNFSF11)] in bone is critical for the regulation of bone remodelling. Myeloma cells can home to bone, triggering increased RANKL and decreased OPG expression by stromal cells, leading to osteolysis. Whether myeloma cells contribute directly to the pool of RANKL or OPG in bone has been contentious. Here we provide evidence of RANKL expression by reverse transcription polymerase chain reaction and in situ hybridization, demonstrating transcripts encoding both the membrane-bound and secreted forms of RANKL in five human multiple myeloma cell lines (LP-1, NCI-H929, OPM-2, RPMI8226, U266) and myeloma cells purified from bone marrow aspirates of myeloma patients. We demonstrated that RANKL encoding mRNAs are translated to protein by antibody detection of RANKL. In vitro assays showed that myeloma cells induced bone marrow derived mononuclear cells to differentiate into adherent tartrate-resistant acid phosphatase positive multinucleated cells, indicative of the formation of functional osteoclasts. This differentiation could also be achieved with passaged myeloma media alone, implicating secreted products. Finally, we provide evidence that the differentiation observed is at least in part the result of myeloma cell expression of RANKL. We therefore conclude that myeloma cells can directly contribute to the pool of RANKL in bone.

Targeting osteoclasts with zoledronic acid prevents bone destruction in collagen-induced arthritis

OBJECTIVE

To study the effect of zoledronic acid (ZA) on synovial inflammation, structural joint damage, and bone metabolism in rats during the effector phase of collagen-induced arthritis (CIA).

METHODS

CIA was induced in female dark agouti rats. At the clinical onset of CIA, rats were assigned to treatment with vehicle or single subcutaneous doses of ZA (1.0, 10, 50, or 100 microg/kg). Clinical signs in all 4 paws were scored on a daily basis. After 2 weeks, the joints in the hind paws were assessed using plain radiographs, microfocal computed tomography (micro-CT), histologic scoring, and histomorphometry, and the serum levels of type I collagen crosslinks were measured by enzyme-linked immunosorbent assay.

RESULTS

Although ZA mildly exacerbated synovitis, it effectively suppressed structural joint damage. At doses of >/=10 microg/kg, ZA significantly reduced radiographic bone erosions, Larsen scores, and juxtaarticular trabecular bone loss as quantified by micro-CT. ZA prevented increased type I collagen (bone) breakdown in CIA and diminished histologic scores of focal bone erosion by up to 80%. Increases in the percentage of eroded surface, osteoclast surface, and osteoclast numbers associated with CIA were prevented by ZA, even though synovitis scores were unchanged.

CONCLUSION

Single doses (>/=10 microg/kg) of ZA strikingly reduced focal bone erosions and juxtaarticular trabecular bone loss, although synovitis was mildly exacerbated. Targeting osteoclasts with ZA may therefore be an effective strategy for preventing structural joint damage in rheumatoid arthritis.

Osteopenia in Siah1a mutant mice

Siah1a has been implicated in numerous signaling pathways because of its ability to induce ubiquitin-mediated degradation of many protein substrates. Siah1a knockout mice are growth-retarded, exhibit early lethality, and display spermatogenic defects. In this study we identified a striking low bone volume phenotype in these mice (trabecular bone volume was halved compared with wild type mice), linking Siah1a to bone metabolism for the first time. Markers of bone formation, including osteoblast numbers and osteoid volume, were decreased by up to 40%, whereas the number of osteoclasts was more than doubled in Siah1a mutant mice. However, ex vivo osteoclast formation occurs normally and hematopoietic osteoclast progenitor cell types were present in normal numbers in Siah1a mutant mice. Moreover, adoptive transfer of Siah1a mutant bone marrow into wild type mice failed to reproduce the osteopenia or increased osteoclast numbers observed in mutant mice. Although ex vivo osteoblast colony formation was normal in Siah1a mutant mice, mineralization from these cells was elevated in cultures from Siah1a mutant mice, which may explain the reduction in osteoid volume seen in vivo. These findings suggest that although Siah1a is clearly essential for normal bone metabolism, the bone defect in Siah1a mutant mice is not due to cell-autonomous requirements for Siah1a in osteoblast or osteoclast formation. We propose that bone metabolism defects in Siah1a mutant mice are secondary to an alteration in an unidentified systemic, paracrine, or metabolic factor in these mice.

Characterization of sugar binding by osteoclast inhibitory lectin

Osteoclast inhibitory lectin (OCIL) is a membrane-bound C-type lectin that blocks osteoclast differentiation and, via binding to its cognate receptor NKRP1D, inhibits natural killer cell-mediated cytotoxicity. OCIL is a member of the natural killer cell receptor C-type lectin group that includes CD69 and NKRP1D. We investigated carbohydrate binding of soluble recombinant human and mouse OCIL in enzyme-linked immunosorbent assay-based assays. OCIL bound immobilized high molecular weight sulfated glycosaminoglycans, including fucoidan, lambda-carrageenan, and dextran sulfate, but not unsulfated dextran or sialated hyaluronic acid. Carbohydrate binding was Ca(2+)-independent. Binding of immobilized low molecular weight glycosaminoglycans, including chondroitin sulfate (A, B, and C forms) and heparin, was not observed. However, the soluble forms of these low molecular weight glycosaminoglycans competed for OCIL binding of immobilized fucoidan (as did soluble fucoidan, dextran sulfate, and lambda-carrageenan), indicating that OCIL does recognize these carbohydrates. Inhibition constants for chondroitin sulfate A and heparin binding were 380 and 5 nm, respectively. Immobilized and soluble monosaccharides did not bind OCIL. The presence of saturating levels of fucoidan, dextran sulfate, and lambda-carrageenan did not affect OCIL inhibition of osteoclast formation. The fucoidan-binding lectins Ulex europaeus agglutinin I and Anguilla anguilla agglutinin did not block osteoclast formation or affect the inhibitory action of OCIL. Although the osteoclast inhibitory action of OCIL is independent of sugar recognition, we have found that OCIL, a lectin widely distributed, but notably localized in bone, skin, and other connective tissues, binds a range of physiologically important glycosaminoglycans, and this property may modulate OCIL actions upon other cells.

Glycoprotein 130 regulates bone turnover and bone size by distinct downstream signaling pathways

The gp130-dependent cytokines, which signal through at least two intracellular pathways, regulate osteoclast and osteoblast formation. To define their roles in regulating bone mass, we analyzed mice in which gp130 signaling via either the signal transducer and activator of transcription (STAT) 1/3 (gp130(DeltaSTAT/DeltaSTAT)) or SHP2/ras/MAPK (gp130(Y757F/Y757F)) pathway was attenuated. In gp130(DeltaSTAT/DeltaSTAT) mice, trabecular bone volume (BV/TV) and turnover were normal, but bone length was reduced by premature growth plate closure, indicating an essential role for gp130-STAT1/3 signaling in chondrocyte differentiation. In contrast, while bone size was normal in gp130(Y757F/Y757F) mice, BV/TV was reduced due to high bone turnover, indicated by high osteoclast surface/bone surface (OcS/BS) and osteoblast surface/bone surface (ObS/BS). Furthermore, generation of functional osteoclasts from bone marrow of gp130(Y757F/Y757F) mice was elevated, revealing that while gp130 family cytokines stimulate osteoclastogenesis through the osteoblast lineage, gp130, via SHP2/Ras/MAPK, inhibits osteoclastogenesis in a cell lineage-autonomous manner. Genetic ablation of IL-6 in gp130(Y757F/Y757F) mice exacerbated this osteopenia by reducing ObS/BS without affecting OcS/BS. Thus, while IL-6 is critical for high bone formation in gp130(Y757F/Y757F) mice, it is not involved in the increased osteoclastogenesis. In conclusion, gp130 is essential for normal bone growth and trabecular bone mass, with balanced regulation depending on selective activation of STAT1/3 and SHP2/ras/MAPK, respectively. Furthermore, the latter pathway can directly inhibit osteoclastogenesis in vivo.

Generation and analysis of Siah2 mutant mice

Siah proteins function as E3 ubiquitin ligase enzymes to target the degradation of diverse protein substrates. To characterize the physiological roles of Siah2, we have generated and analyzed Siah2 mutant mice. In contrast to Siah1a knockout mice, which are growth retarded and exhibit defects in spermatogenesis, Siah2 mutant mice are fertile and largely phenotypically normal. While previous studies implicate Siah2 in the regulation of TRAF2, Vav1, OBF-1, and DCC, we find that a variety of responses mediated by these proteins are unaffected by loss of Siah2. However, we have identified an expansion of myeloid progenitor cells in the bone marrow of Siah2 mutant mice. Consistent with this, we show that Siah2 mutant bone marrow produces more osteoclasts in vitro than wild-type bone marrow. The observation that combined Siah2 and Siah1a mutation causes embryonic and neonatal lethality demonstrates that the highly homologous Siah proteins have partially overlapping functions in vivo.

Isolation of a human homolog of osteoclast inhibitory lectin that inhibits the formation and function of osteoclasts

Osteoclast inhibitory lectin (OCIL) is a newly recognized inhibitor of osteoclast formation. We identified a human homolog of OCIL and its gene, determined its regulation in human osteoblast cell lines, and established that it can inhibit murine and human osteoclast formation and resorption. OCIL shows promise as a new antiresorptive.

INTRODUCTION

Murine and rat osteoclast inhibitory lectins (mOCIL and rOCIL, respectively) are type II membrane C-type lectins expressed by osteoblasts and other extraskeletal tissues, with the extracellular domain of each, expressed as a recombinant protein, able to inhibit in vitro osteoclast formation.

MATERIALS AND METHODS

We isolated the human homolog of OCIL (hOCIL) from a human fetal cDNA library that predicts a 191 amino acid type II membrane protein, with the 112 amino acid C-type lectin region in the extracellular domain having 53% identity with the C-type lectin sequences of rOCIL and mOCIL. The extracellular domain of hOCIL was expressed as a soluble recombinant protein in E. coli, and its biological effects were determined.

RESULTS AND CONCLUSIONS

The hOCIL gene is 25 kb in length, comprised of five exons, and is a member of a superfamily of natural killer (NK) cell receptors encoded by the NK gene complex located on chromosome 12. Human OCIL mRNA expression is upregulated by interleukin (IL)-1alpha and prostaglandin E2 (PGE2) in a time-dependent manner in human osteogenic sarcoma MG63 cells, but not by dexamethasone or 1,25 dihydroxyvitamin D3. Soluble recombinant hOCIL had biological effects comparable with recombinant mOCIL on human and murine osteoclastogenesis. In addition to its capacity to limit osteoclast formation, OCIL was also able to inhibit bone resorption by mature, giant-cell tumor-derived osteoclasts. Thus, a human homolog of OCIL exists that is highly conserved with mOCIL in its primary amino acid sequence (C-lectin domain), genomic structure, and activity to inhibit osteoclastogenesis.

Differentiation potential of a mouse bone marrow stromal cell line

In order to study osteoblast differentiation we subcloned a cell derived from a mouse a bone marrow stromal cell line, Kusa O, and obtained a number of clones representative of three different phenotypes. One that neither differentiated into osteoblasts nor into adipocytes, a second that differentiated into osteoblasts but not adipocytes, and a third that differentiated into both osteoblasts and adipocytes. Four subclones were selected for further characterization according to their ability to mineralize and/or differentiate into adipocytes. The non-mineralizing clone had no detectable alkaline phosphatase activity although some alkaline phosphatase mRNA was detected after 21 days in osteoblast differentiating medium. Alkaline phosphatase activity and mRNA in the three mineralizing clones were comparable with the parent clones. Osteocalcin mRNA and protein levels in the non-mineralizing clone were low and non-detectable, respectively, while both were elevated in the parent cells and mineralizing subclones after 21 days in differentiating medium. PTH receptor mRNA and activity increased in the four subclones and parent cells with differentiation. mRNA for two other osteoblast phenotypic markers, osteopontin and bone sialoprotein, were similarly expressed in the parent cells and subclones while mRNAs for the transcription factors, Runx2 and osterix, were detectable in both parent and subclone cells. Runx2 was unchanged with differentiation while osterix was increased. Interestingly, PPARgamma mRNA expression did not correlate with cell line potential to differentiate into adipocytes. Indian hedgehog mRNA and its receptor (patched) mRNA levels both increased with differentiation while mRNA levels of the Wnt pathway components beta-catenin and dickkopf also increased with differentiation. Although we have focussed on characterizing these clones from the osteoblast perspective it is clear that they may be useful for studying both osteoblast and adipocyte differentiation as well as their transdifferentiation.

Parathyroid hormone-related protein (PTHrP): a nucleocytoplasmic shuttling protein with distinct paracrine and intracrine roles

Parathyroid hormone-related protein (PTHrP) was first discovered as a circulating factor secreted by certain cancers responsible for the syndrome of humoral hypercalcemia of malignancy. PTHrP possesses distinct paracrine and intracrine signaling roles. The similarity of its N-terminus to that of parathyroid hormone (PTH) enables it to share PTH's paracrine signaling properties, whereas the rest of the molecule possesses other functions, largely relating to an intracrine signaling role in the nucleus/nucleolus in regulating apoptosis and cell proliferation. Recent advances have shown that intracellularly expressed PTHrP is able to shuttle in cell-cycle- and signal-dependent fashion between nucleus and cytoplasm through the action of the distinct intracellular transport receptors importin beta 1 and exportin 1 (Crm1) mediating nuclear import and export of PTHrP, respectively. Together, the import and export pathways constitute an integrated system for PTHrP subcellular localization. Intriguingly, PTHrP nuclear/nucleolar import is dependent on microtubule integrity, transport to the nucleus appearing to occur in vectorial fashion along microtubules, mediated in part by the action of importin beta 1. PTHrP has recently been shown to be able to bind to RNA, meaning that PTHrP's nucleocytoplasmic shuttling ability may relate to a specific role within the nucleus/nucleolus to regulate RNA synthesis and/or transport.

T-cells mediate an inhibitory effect of interleukin-4 on osteoclastogenesis

IL-4 is an important cytokine that can influence bone. We identified two distinct actions of IL-4 to inhibit osteoclast formation: one direct on osteoclast progenitors and the second through the production of a novel T-cell surface-associated molecule(s). These data show a new link between the immune system and bone. The Th2 cytokine interleukin (IL)-4 inhibits osteoclast formation in vitro but also acts on other cell types found in bone, including T-cells and macrophages. Because some osteoclastogenesis inhibitors (e.g., IL-12) act indirectly through T-cells, we investigated IL-4 action on osteoclastogenesis in the presence of T-cells. Osteoclast formation from murine spleen cells treated with RANKL and macrophage colony-stimulating factor (M-CSF) was blocked by IL-4 even when spleen cells were depleted of T-cells (Thy 1.2+) and/or B-cells (B220+). Also, IL-4 inhibited osteoclastogenesis in RANKL/M-CSF-stimulated adherent spleen cells, Rag1 -/- (lymphocyte-deficient) spleen cells, and bone marrow macrophages, indicating an action on myelomonocytic cells to block osteoclastogenesis. In contrast, IL-4 did not inhibit osteoclastogenesis in cells from IL-4 receptor null mice (IL-4R -/-). However, when wildtype T-cells were added to IL-4R -/- spleen cell cultures, IL-4 inhibited osteoclast formation, indicating a T-cell-dependent action. Osteoclast formation in RANKL-stimulated RAW 264.7 cells was not inhibited by IL-4 unless T-cells were added to the culture. Separation of RAW 264.7 cells and T-cells by semipermeable membrane ablated this action of IL-4, suggesting the induction of a membrane-associated osteoclastogenesis inhibitor. However, membrane-bound inhibitors thymic shared antigen-1 (TSA-1) and osteoclast inhibitory lectin (OCIL) were not regulated by IL-4. In summary, at least two mechanisms of IL-4 -mediated osteoclastogenesis inhibition exist, including a direct action on myelomonocytic progenitors (from which osteoclasts derive) and an indirect action through T-cells that may involve novel anti-osteoclastic factors.

Interleukin-18 is a novel mitogen of osteogenic and chondrogenic cells

IL-18 was identified due to its ability to induce interferon-gamma (IFNgamma) production by T cells. It is a pleiotropic factor that shares structural features with IL-1 and functional activities with IL-12. IL-18 has a role in T cell development, where it has been demonstrated to act cooperatively with IL-12 to regulate IFNgamma. In bone, IL-18 is mainly produced by macrophages, but is also expressed by osteoblasts and inhibits osteoclast formation through granulocyte-macrophage colony-stimulating factor (GM-CSF) and not IFNgamma production by T cells. We have investigated the effects of IL-18 on mature osteoclast activity and for potential actions on osteoblasts or chondrocytes. The effects of IL-18 on mature osteoclast activity were determined using two assays: isolated mature osteoclast cell culture and neonatal murine calvarial organ culture. IL-18 did not affect bone resorption in either assay system. The actions of IL-18 on osteogenic cells (primary cell cultures of fetal rat and neonatal mouse osteoblasts, as well as neonatal mouse calvarial organ culture) and primary chondrocytes (canine) were assessed by proliferation assays (quantification of cell numbers and thymidine incorporation). In each assay system, IL-18 acted as a mitogen to the osteogenic and chondrogenic cells. Since IL-18 signal transduction may involve IFNgamma or GM-CSF, we assessed their involvement in the IL-18 response. IL-18 did not induce IFNgamma production by primary osteoblasts, but, of greater significance, IFNgamma had the opposing action to IL-18 in that it inhibited the primary osteoblast cell proliferation. Although IL-18 rapidly induced GM-CSF production by primary osteoblasts, IL-18 was still mitogenic in osteoblast preparations established from GM-CSF-deficient mice. Combined, these studies indicate that IL-18 may have an autocrine/paracrine mitogen role for both osteogenic and chondrogenic cells, independent of the production of IFNgamma or GM-CSF.

Osteoclast inhibitory lectin, a family of new osteoclast inhibitors

We have identified two novel type II membrane-bound C-lectins, designated mOCILrP1 and mOCILrP2, of 218 and 217 amino acids, respectively, that share substantial identity with the murine osteoclast inhibitory lectin (OCIL). The extracellular domains of mOCILrP1 and mOCILrP2 share 83 and 75% identity, respectively, with the extracellular domain of mOCIL. When the extracellular domains were expressed as recombinant proteins, each inhibited osteoclast formation in murine bone marrow cultures treated with M-CSF and RANKL with similar potencies to mOCIL (IC(50) of 0.2 ng/ml). Distinct but highly related genes encoded the three OCIL family members, with mOCIL and mOCILrP2 controlled by an inverted TATA promoter, and mOCILrP1 by a TTAAAA promoter. However only mOCIL was robustly regulated by calciotropic agents, while mOCILrP1 was not expressed, and mOCILrP2 was constitutively expressed in osteoblasts. Immunohistochemistry using antipeptide antibodies to the intracellular domain of mOCILrP1/mOCILrP2 and to mOCIL demonstrated that mOCIL and mOCILrP1/mOCILrP2 were concordantly expressed in osteoblasts, chondrocytes, and in extraskeletal tissues. Further, their cellular distribution was identical to that of RANKL. The identification of three distinct genes that were functionally related implies redundancy for OCIL, and their concordant expression with that of RANKL suggests that the RANKL:OPG axis may be further influenced by OCIL family members.

Molecular basis for the recognition of a nonclassical nuclear localization signal by importin beta

Nuclear import of proteins containing a classical nuclear localization signal (NLS) involves NLS recognition by importin alpha, which associates with importin beta via the IBB domain. Other proteins, including parathyroid hormone-related protein (PTHrP), are imported into the nucleus by direct interaction with importin beta. We solved the crystal structure of a fragment of importin beta-1 (1-485) bound to the nonclassical NLS of PTHrP. The structure reveals a second extended cargo binding site on importin beta distinct from the IBB domain binding site. Using a permeabilized cell import assay we demonstrate that importin beta (1-485) can import PTHrP-coupled cargo in a Ran-dependent manner. We propose that this region contains a prototypical nuclear import receptor domain, which could have evolved into the modern importin beta superfamily.