Regulation of cytokine expression in osteoblasts by parathyroid hormone: rapid stimulation of interleukin-6 and leukemia inhibitory factor mRNA

Inflammatory Processes Affecting Bone Health and Repair

PURPOSE OF REVIEW

The purpose of this article is to review the current understanding of inflammatory processes on bone, including direct impacts of inflammatory factors on bone cells, the effect of senescence on inflamed bone, and the critical role of inflammation in bone pain and healing.

RECENT FINDINGS

Advances in osteoimmunology have provided new perspectives on inflammatory bone loss in recent years. Characterization of so-called inflammatory osteoclasts has revealed insights into physiological and pathological bone loss. The identification of inflammation-associated senescent markers in bone cells indicates that therapies that reduce senescent cell burden may reverse bone loss caused by inflammatory processes. Finally, novel studies have refined the role of inflammation in bone healing, including cross talk between nerves and bone cells. Except for the initial stages of fracture healing, inflammation has predominately negative effects on bone and increases fracture risk. Eliminating senescent cells, priming the osteo-immune axis in bone cells, and alleviating pro-inflammatory cytokine burden may ameliorate the negative effects of inflammation on bone.

[Assessment of the prevalence of anemia in patients with primary hyperparathyroidism: a single-center observational study]

BACKGROUND

The combination of primary hyperparathyroidism (PHPT) with anemia was first described in 1931. It remains unclear whether PHPT is the direct cause of anemia, or it develops due to PHPT's complications. The frequency of PHPT--associated anemia in the Russian population is unknown.

AIM

To assess the prevalence of anemia in patients with PHPT admitted to the Department of Parathyroid Glands Pathology in the Endocrinology Research Centre from January 2017 to August 2020.

MATERIALS AND METHODS

The study included patients with PHPT over 18 years old. A single-center observational one-stage one-sample uncontrolled study was carried out. We analyzed laboratory and instrumental data obtained during inpatient examination in accordance with the standards of medical care. Statistical analysis was performed using Statistica 13 (StatSoft, USA) and SPSS (IBM, USA) software packages.

RESULTS

The study included 327 patients with PHPT, 28 (9%) men and 299 (91%) women. The median age was 59 years [51; 66]. 26 patients (8%) with anemia were identified. Statistically significant differences between patients with and without anemia were found only in the GFR. Comparison of patients with and without anemia didn't reveal any significant differences in the incidence of PHPT's complications.Significant differences in serum hemoglobin concentration and average hemoglobin concentration in erythrocytes were revealed between patients with and without vertebrae fractures. In the group of patients without compression fractures these parameters were higher.In the subgroup of patients with total calcium concentration above 3 mmol/L and PTH above 3 normal values, the incidence of anemia reached 21% (95% CI: 10%; 35%). Within this group we revealed tendencies to higher levels of PTH, ionized calcium and osteocalcin in patients with anemia.

CONCLUSION

In general, there was no correlation between hypercalcemia, the degree of PTH elevation and the presence of anemia in patients with PHPT. However, in the subgroup of patients with severe hypercalcemia, there was a relationship between the concentration of PTH, ionized calcium and the presence of anemia. In patients with PHPT and vertebral fractures, significantly lower concentrations of blood hemoglobin and hemoglobin in erythrocytes were observed.

Anemia in primary hyperparathyroidism

Introduction. Anaemia is a complication of primary hyperparathyroidism (PHPT). Pathogenesis of PHPT-induced anaemia involves inhibited erythroid cell proliferation associated with the underlying disease and non-specific factors (blood loss, chronic kidney disease). However, its specific mechanisms remain unclear.

Aim. Clinical description of a PHPT case with multifactorial complicating anaemia.

Main findings. With putative evidence existing on relationships between PHPT and anaemia, no large clinical trials substantiated suitable algorithms for such patients’ management. The genesis of anaemia reported in this case was multifactorial and not decisively excluding PHPT from putative causes.

Relationship between GFR, intact PTH, oxidized PTH, non-oxidized PTH as well as FGF23 in patients with CKD

Fibroblast growth factor 23 (FGF23) and parathyroid hormone (PTH) are regulators of renal phosphate excretion and vitamin D metabolism. In chronic kidney disease (CKD), circulating FGF23 and PTH concentrations progressively increase as renal function declines. Oxidation of PTH at two methionine residues (positions 8 and 18) causes a loss of function. The impact of n-oxPTH and oxPTH on FGF23 synthesis, however, and how n-oxPTH and oxPTH concentrations are affected by CKD, is yet unknown. The effects of oxidized and non-oxidized PTH 1-34 on Fgf23 gene expression were analyzed in UMR106 osteoblast-like cells. Furthermore, we investigated the relationship between n-oxPTH and oxPTH, respectively, with FGF23 in two independent patients' cohorts (620 children with CKD and 600 kidney transplant recipients). While n-oxPTH stimulated Fgf23 mRNA synthesis in vitro, oxidation of PTH in particular at Met8 led to a markedly weaker stimulation of Fgf23. The effect was even stronger when both Met8 and Met18 were oxidized. In both clinical cohorts, n-oxPTH-but not oxPTH-was significantly associated with FGF23 concentrations, independent of known confounding factors. Moreover, with progressive deterioration of kidney function, intact PTH (iPTH) and oxPTH increased substantially, whereas n-oxPTH increased only moderately. In conclusion, n-oxPTH, but not oxPTH, stimulates Fgf23 gene expression. The increase in PTH with decreasing GFR is mainly due to an increase in oxPTH in more advanced stages of CKD.

Inflammatory diseases and bone fragility

Systemic osteoporosis and increased fracture rates have been described in chronic inflammatory diseases such as rheumatoid arthritis, spondyloarthritis, systemic lupus erythematosus, inflammatory bowel diseases, and chronic obstructive pulmonary disease. Most of these patients receive glucocorticoids, which have their own deleterious effects on bone. However, the other main determinant of bone fragility is the inflammation itself, as shown by the interactions between the inflammatory mediators, the actors of the immune system, and the bone remodelling. The inflammatory disease activity is thus on top of the other well-known osteoporotic risk factors in these patients. Optimal control of inflammation is part of the prevention of osteoporosis, and potent anti-inflammatory drugs have positive effects on surrogate markers of bone fragility. More data are needed to assess the anti-fracture efficacy of a tight control of inflammation in patients with a chronic inflammatory disorder. This review aimed at presenting different clinical aspects of inflammatory diseases which illustrate the relationships between inflammation and bone fragility.

Protein kinase inhibitor γ reciprocally regulates osteoblast and adipocyte differentiation by downregulating leukemia inhibitory factor

The protein kinase inhibitor (Pki) gene family inactivates nuclear protein kinase A (PKA) and terminates PKA-induced gene expression. We previously showed that Pkig is the primary family member expressed in osteoblasts and that Pkig knockdown increases the effects of parathyroid hormone and isoproterenol on PKA activation, gene expression, and inhibition of apoptosis. Here, we determined whether endogenous levels of Pkig regulate osteoblast differentiation. Pkig is the primary family member in murine embryonic fibroblasts (MEFs), murine marrow-derived mesenchymal stem cells, and human mesenchymal stem cells. Pkig deletion increased forskolin-dependent nuclear PKA activation and gene expression and Pkig deletion or knockdown increased osteoblast differentiation. PKA signaling is known to stimulate adipogenesis; however, adipogenesis and osteogenesis are often reciprocally regulated. We found that the reciprocal regulation predominates over the direct effects of PKA since adipogenesis was decreased by Pkig deletion or knockdown. Pkig deletion or knockdown also simultaneously increased osteogenesis and decreased adipogenesis in mixed osteogenic/adipogenic medium. Pkig deletion increased PKA-induced expression of leukemia inhibitory factor (Lif) mRNA and LIF protein. LIF neutralizing antibodies inhibited the effects on osteogenesis and adipogenesis of either Pkig deletion in MEFs or PKIγ knockdown in both murine and human mesenchymal stem cells. Collectively, our results show that endogenous levels of Pkig reciprocally regulate osteoblast and adipocyte differentiation and that this reciprocal regulation is mediated in part by LIF. Stem Cells 2013;31:2789-2799.

Decline in calcitonin receptor expression in osteocytes with age

We have previously shown that co-administration of the transient osteoclast inhibitor, salmon calcitonin (sCT), blunts the anabolic effect of parathyroid hormone (PTH) in young rats and increases osteocytic expression of the bone formation inhibitor sclerostin (Sost). To determine whether this also occurs in adult animals, we co-administered sCT with PTH to 6-month-old sham-operated (SHAM) and ovariectomised (OVX) rats. While sCT reduced the stimulatory effect of PTH on serum amino-terminal propeptide of type 1 procollagen levels, in contrast to its influence in young rats, sCT did not reduce the anabolic effect of PTH on femoral bone mineral density, tibial trabecular bone volume or bone formation rate in 6-month-old SHAM or OVX rats. Quantitative real-time PCR analysis of femoral metaphyses collected 1 and 4 h after a single PTH injection confirmed a significant increase in mRNA levels for interleukin 6 (Il6) and ephrinB2 (EfnB2), and a significant reduction in Sost and dentin matrix protein-1 (Dmp1) in response to PTH. However, in contrast to observations in young rats, these effects were not modified by co-administration of sCT, nor did sCT significantly modify Sost, Dmp1, or matrix extracellular phosphoglycoprotein (Mepe) mRNA levels. Furthermore, while CT receptor (CTR) mRNA (Calcr) was readily detected in GFP+ osteocytes isolated from young (3-week-old) DMP1-GFP mice, Calcr levels in osteocytes declined as mice aged, reaching levels that were undetectable in long bone at 49 weeks of age. These data indicate that osteocyte-mediated responses to CT are most likely to be of physiological relevance in young rodents.

Hyperlipidemia affects multiscale structure and strength of murine femur

To improve bone strength prediction beyond limitations of assessment founded solely on the bone mineral component, we investigated the effect of hyperlipidemia, present in more than 40% of osteoporotic patients, on multiscale structure of murine bone. Our overarching purpose is to estimate bone strength accurately, to facilitate mitigating fracture morbidity and mortality in patients. Because (i) orientation of collagen type I affects, independently of degree of mineralization, cortical bone׳s micro-structural strength; and, (ii) hyperlipidemia affects collagen orientation and μCT volumetric tissue mineral density (vTMD) in murine cortical bone, we have constructed the first multiscale finite element (mFE), mouse-specific femoral model to study the effect of collagen orientation and vTMD on strength in Ldlr(-/-), a mouse model of hyperlipidemia, and its control wild type, on either high fat diet or normal diet. Each µCT scan-based mFE model included either element-specific elastic orthotropic properties calculated from collagen orientation and vTMD (collagen-density model) by experimentally validated formulation, or usual element-specific elastic isotropic material properties dependent on vTMD-only (density-only model). We found that collagen orientation, assessed by circularly polarized light and confocal microscopies, and vTMD, differed among groups and that microindentation results strongly correlate with elastic modulus of collagen-density models (r(2)=0.85, p=10(-5)). Collagen-density models yielded (1) larger strains, and therefore lower strength, in simulations of 3-point bending and physiological loading; and (2) higher correlation between mFE-predicted strength and 3-point bending experimental strength, than density-only models. This novel method supports ongoing translational research to achieve the as yet elusive goal of accurate bone strength prediction.

Talking among ourselves: paracrine control of bone formation within the osteoblast lineage

While much research focuses on the range of signals detected by the osteoblast lineage that originate from endocrine influences, or from other cells within the body, there are also multiple interactions that occur within this family of cells. Osteoblasts exist as teams and form extensive communication networks both on, and within, the bone matrix. We provide four snapshots of communication pathways that exist within the osteoblast lineage between different stages of their differentiation, as follows: (1) PTHrP, a factor produced by early osteoblasts that stimulates the activity of more mature bone-forming cells and the most mature osteoblast embedded within the bone matrix, the osteocyte; (2) sclerostin, a secreted factor, released by osteocytes into their extensive communication network to restrict the activity of younger osteoblasts on the bone surface; (3) oncostatin M, a member of the IL-6/gp130 family of cytokines, expressed throughout osteoblast differentiation and acting to stimulate osteoblast activity that works on a different receptor in the mature osteocyte compared to the preosteoblast; and (4) Eph/ephrins, cell-contact-dependent kinases, and the osteoblast-lineage-specific interaction of EphB4 and ephrinB2, which provides a checkpoint for entry to the late stages of osteoblast differentiation and restricts RANKL expression.

EphrinB2/EphB4 inhibition in the osteoblast lineage modifies the anabolic response to parathyroid hormone

Previous reports indicate that ephrinB2 expression by osteoblasts is stimulated by parathyroid hormone (PTH) and its related protein (PTHrP) and that ephrinB2/EphB4 signaling between osteoblasts and osteoclasts stimulates osteoblast differentiation while inhibiting osteoclast differentiation. To determine the role of the ephrinB2/EphB4 interaction in the skeleton, we used a specific inhibitor, soluble EphB4 (sEphB4), in vitro and in vivo. sEphB4 treatment of cultured osteoblasts specifically inhibited EphB4 and ephrinB2 phosphorylation and reduced mRNA levels of late markers of osteoblast/osteocyte differentiation (osteocalcin, dentin matrix protein-1 [DMP-1], sclerostin, matrix-extracellular phosphoglycoprotein [MEPE]), while substantially increasing RANKL. sEphB4 treatment in vivo in the presence and absence of PTH increased osteoblast formation and mRNA levels of early osteoblast markers (Runx2, alkaline phosphatase, Collagen 1α1, and PTH receptor [PTHR1]), but despite a substantial increase in osteoblast numbers, there was no significant change in bone formation rate or in late markers of osteoblast/osteocyte differentiation. Rather, in the presence of PTH, sEphB4 treatment significantly increased osteoclast formation, an effect that prevented the anabolic effect of PTH, causing instead a decrease in trabecular number. This enhancement of osteoclastogenesis by sEphB4 was reproduced in vitro but only in the presence of osteoblasts. These data indicate that ephrinB2/EphB4 signaling within the osteoblast lineage is required for late stages of osteoblast differentiation and, further, restricts the ability of osteoblasts to support osteoclast formation, at least in part by limiting RANKL production. This indicates a key role for the ephrinB2/EphB4 interaction within the osteoblast lineage in osteoblast differentiation and support of osteoclastogenesis.

Parathyroid hormone (1-34) counteracts the suppression of interleukin-11 expression by glucocorticoid in murine osteoblasts: a possible mechanism for stimulating osteoblast differentiation against glucocorticoid excess

Glucocorticoid (GC) excess causes a rapid loss of bone with a reduction in bone formation. Intermittent PTH (1-34) administration stimulates bone formation and counteracts the inhibition of bone formation by GC excess. We have previously demonstrated that mechanical strain enhances interleukin (IL)-11 gene transcription by a rapid induction of ΔFosB expression and protein kinase C (PKC)-δ-mediated phosphorylation of phosphorylated mothers against decapentaplegic (Smad)-1. Because IL-11 suppresses the expression of dickkopf-1 and -2 and stimulates Wnt signaling, IL-11 appears to mediate at least a part of the effect of mechanical strain on osteoblast differentiation and bone formation. The present study was undertaken to examine the effect of PTH(1-34) and GCs on IL-11 expression in murine primary osteoblasts (mPOBs). PTH(1-34) treatment of mPOBs enhanced IL-11 expression in a time- and dose-dependent manner. PTH(1-34) also stimulated ΔFosB expression and Smad1 phosphorylation, which cooperatively stimulated IL-11 gene transcription. PTH(1-34)-induced Smad1 phosphorylation was mediated via PKCδ and was abrogated in mPOBs from PKCδ knockout mice. Dexamethasone suppressed IL-11 gene transcription enhanced by PTH(1-34) without affecting ΔFosB expression or Smad1 phosphorylation, and dexamethasone-GC receptor complex was bound to JunD, which forms heterodimers with ΔFosB. High doses of PTH(1-34) counteracted the effect of dexamethasone on apoptosis of mPOBs, which was blunted by neutralizing anti-IL-11 antibody or IL-11 small interfering RNA. These results demonstrate that PTH(1-34) and GCs interact to regulate IL-11 expression in parallel with osteoblast differentiation and apoptosis and suggest that PTH(1-34) and dexamethasone may regulate osteoblast differentiation and apoptosis via their effect on IL-11 expression.

Metabolism-related cytokine and hormone levels in the serum of patients with myelodysplastic syndromes

BACKGROUND

A number of cytokines secreted from the bone marrow stromal cells and circulating hormones related to bone, adipose tissue and glucose metabolism might be involved in the pathogenesis of myelodysplastic syndromes (MDS).

METHODS

Serum levels of cytokines related to the metabolism of bone tissue [osteocalcin and parathyroid hormone (PTH)], adipose tissue (adiponectin, leptin and ghrelin) and glucose [insulin and insulin-like growth factor-1 (IGF-1)] were determined in 72 patients suffering from MDS, mostly of the low-risk group according to FAB classification, and 41 healthy individuals (controls).

RESULTS

Adiponectin and osteocalcin serum levels were significantly elevated in the MDS patients. Leptin, insulin and IGF-1 serum levels were reduced. No difference was found in the serum levels of PTH and ghrelin. Leptin levels were reversibly associated with patient blast count.

CONCLUSION

Increased serum levels of adiponectin and low levels of IGF-1 in MDS patients may counterbalance the increased rate of apoptosis in the pool of hematopoietic progenitors. Osteocalcin secreted by osteoblasts regulates the renewal and proliferation of hematopoietic stem cells. Hormones and cytokines either secreted by the cells of the bone marrow stroma or transferred by the microcirculation act on hematopoietic progenitors and may regulate their differentiation, apoptosis and proliferation rate in MDS.

Vitamin B6 status and interferon-γ-mediated immune activation in primary hyperparathyroidism

OBJECTIVES

Primary hyperparathyroidism (PHPT) has been associated with low-grade inflammation and elevated risk of cardiovascular disease (CVD). In inflammatory conditions, interferon-γ (IFN-γ) activity is enhanced and a decreased circulating concentration of vitamin B6 is often observed. Such changes in IFN-γ activity or vitamin B6 levels have been associated with increased incidence of CVD. The aim of the study was to investigate systemic markers of IFN-γ-mediated immune activation, such as neopterin, the kynurenine-to-tryptophan ratio (KTR) and kynurenine pathway metabolites, as well as B6 vitamers in patients with PHPT.

DESIGN/SUBJECTS

A total of 57 patients with PHPT and a control group of 20 healthy blood donors were included in this study. PHPT patients who responded positively to parathyroidectomy were followed for 6 months. Forty-three patients participated in the longitudinal study in which blood samples were taken at inclusion and 1, 3 and 6 months after surgery.

RESULTS

Plasma concentrations of the B6 vitamers pyridoxal 5'-phosphate (PLP) (P = 0.007) and pyridoxal (P = 0.013) were significantly lower in the patient group compared to healthy control subjects. An increase in the KTR indicated that the kynurenine pathway of tryptophan metabolism was altered in PHPT patients (P = 0.015). During the initial 6 months after surgery, levels of PLP (P < 0.001) and anthranilic acid (P < 0.001) increased significantly, whereas neopterin decreased (P = 0.018).

CONCLUSIONS

The results of this study demonstrate altered levels of vitamin B6 and the KTR in PHPT patients, both of which may reflect cellular immune activation. These abnormalities should be considered in relation to the increased risk of CVD previously observed in patients with PHPT.

Leukemia inhibitory factor: a paracrine mediator of bone metabolism

Leukemia inhibitory factor (LIF) is a soluble interleukin-6 family cytokine that regulates a number of physiologic functions, including normal skeletal remodeling. LIF signals through the cytokine co-receptor glycoprotein-130 in complex with its cytokine-specific receptor [LIF receptor (LIFR)] to activate signaling cascades in cells of the skeletal system, including stromal cells, chondrocytes, osteoblasts, osteocytes, adipocytes, and synovial fibroblasts. LIF action on skeletal cells is cell-type specific, and frequently dependent on the state of cell differentiation. This review describes the expression patterns of LIF and LIFR in bone, their regulation by physiological and inflammatory agents, as well as cell-specific influences of LIF on osteoblast, osteoclast, chondrocyte, and adipocyte differentiation. The actions of LIF in normal skeletal growth and maintenance, in pathological states (e.g. autocrine tumor cell signaling and growth in bone) and inflammatory conditions (e.g. arthritis) will be discussed, as well as the signaling pathways activated by LIF and their importance in bone formation and resorption.

Sustained RANKL response to parathyroid hormone in oncostatin M receptor-deficient osteoblasts converts anabolic treatment to a catabolic effect in vivo

Parathyroid hormone (PTH) is the only approved anabolic agent for osteoporosis treatment. It acts via osteoblasts to stimulate both osteoclast formation and bone formation, with the balance between these two activities determined by the mode of administration. Oncostatin M (OSM), a gp130-dependent cytokine expressed by osteoblast lineage cells, has similar effects and similar gene targets in the osteoblast lineage. In this study, we investigated whether OSM might participate in anabolic effects of PTH. Microarray analysis and quantitative real-time polymerase chain reaction (qPCR) of PTH-treated murine stromal cells and primary calvarial osteoblasts identified significant regulation of gp130 and gp130-dependent coreceptors and ligands, including a significant increase in OSM receptor (OSMR) expression. To determine whether OSMR signaling is required for PTH anabolic action, 6-week-old male Osmr(-/-) mice and wild-type (WT) littermates were treated with hPTH(1-34) for 3 weeks. In WT mice, PTH increased trabecular bone volume and trabecular thickness. In contrast, the same treatment had a catabolic effect in Osmr(-/-) mice, reducing both trabecular bone volume and trabecular number. This was not explained by any alteration in the increased osteoblast formation and mineral apposition rate in response to PTH in Osmr(-/-) compared with WT mice. Rather, PTH treatment doubled osteoclast surface in Osmr(-/-) mice, an effect not observed in WT mice. Consistent with this finding, when osteoclast precursors were cultured in the presence of osteoblasts, more osteoclasts were formed in response to PTH when Osmr(-/-) osteoblasts were used. Neither PTH1R mRNA levels nor cAMP response to PTH were modified in Osmr(-/-) osteoblasts. However, RANKL induction in PTH-treated Osmr(-/-) osteoblasts was sustained at least until 24 hours after PTH exposure, an effect not observed in WT osteoblasts. These data indicate that the transient RANKL induction by intermittent PTH administration, which is associated with its anabolic action, is changed to a prolonged induction in OSMR-deficient osteoblasts, resulting in bone destruction.

Proteoglycan 4, a novel immunomodulatory factor, regulates parathyroid hormone actions on hematopoietic cells

Proteoglycan 4 (PRG4), a critical protective factor in articular joints, is implicated in hematopoietic progenitor cell expansion and megakaryopoiesis. PRG4 loss-of-function mutations result in camptodactyly-arthropathy-coxa vara-pericarditis (CACP) syndrome, which is characterized primarily by precocious joint failure. PRG4 was identified as a novel parathyroid hormone (PTH) responsiveness gene in osteoblastic cells in bone, and was investigated as a potential mediator of PTH actions on hematopoiesis. Sixteen-week-old Prg4(-/-) mutant and Prg4(+/+) wild-type mice were treated daily with intermittent PTH (residues 1-34) or vehicle for 6 weeks. At 22 weeks of age, Prg4 mutant mice had increased peripheral blood neutrophils and decreased marrow B220(+) (B-lymphocytic) cells, which were normalized by PTH. The PTH-induced increase in marrow Lin(-)Sca-1(+)c-Kit(+) (hematopoietic progenitor) cells was blunted in mutant mice. Basal and PTH-stimulated stromal cell-derived factor-1 (SDF-1) was decreased in mutant mice, suggesting SDF-1 as a candidate regulator of proteoglycan 4 actions on hematopoiesis in vivo. PTH stimulation of IL-6 mRNA was greater in mutant than in wild-type calvaria and bone marrow, suggesting a compensatory mechanism in the PTH-induced increase in marrow hematopoietic progenitor cells. In summary, proteoglycan 4 is a novel PTH-responsive factor regulating immune cells and PTH actions on marrow hematopoietic progenitor cells.

Expression of LIF and LIFR in periodontal tissue during orthodontic tooth movement

OBJECTIVES

To test the hypothesis that leukemia inhibitor factor (LIF) and LIF receptor (LIFR) are expressed in periodontal tissue and that their expression may be upregulated during orthodontic tooth movement.

MATERIALS AND METHODS

Forces of 0.3 N were applied to move the upper left first molars mesially in 24 rats. These forces were kept constant for 3, 7, and 14 days and followed by animal sacrifice. The contralateral molars served as control. The rate of tooth movement was measured by Image J software. Paraffin-embedded sections of the upper jaws were prepared for histological and immunohistochemical analysis to test the LIF and LIFR expression.

RESULTS

Loaded teeth showed a significantly higher rate of tooth movement. The periodontium of the moved teeth experienced tissue remodeling, while there was no obvious change in the contralateral controls. Furthermore, LIF and LIFR were expressed in the periodontal tissue, and there were statistically significant differences between the loaded and unloaded teeth at 3 and 14 days. LIF presented significantly higher expression on the tension side compared with the pressure side at 3 days.

CONCLUSION

Both LIF and LIFR exist in the periodontal tissue, and continuous orthodontic forces induce the upregulation of LIF/LIFR production, suggesting that LIF/LIFR may play important roles in periodontium remodeling.

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.

Vitamin D and cardiometabolic health: a review of the evidence

The cardiometabolic syndrome (MetS) is a clustering of related metabolic abnormalities including abdominal adiposity, insulin resistance, hypertension, dyslipidaemia and increased inflammatory and thrombotic markers, which is linked to increased risk of type 2 diabetes, CVD and overall mortality. Several cross-sectional and prospective studies have shown an association between low vitamin D status, as indicated by concentrations of serum 25-hydroxyvitamin D (s25(OH)D), and increased prevalence of the MetS and individual CVD risk factors. These epidemiological observations are supported by mechanistic studies but experimental data are limited. The available data from intervention studies are largely confounded as most vitamin D supplementation trials were mainly carried out to explore the role of Ca in CVD and include Ca in the treatment arms. Inadequate consideration of seasonal effects on s25(OH)D concentrations is also a common design flaw in most studies. Further complications arise from shared risk factors such as adiposity and ageing, which predispose individuals to exhibit both a more pronounced risk profile and relatively lower s25(OH)D concentrations. In conclusion, while epidemiological associations are promising and a rationale for low vitamin D status as a potentially modifiable risk factor for CVD is supported by mechanistic data, suitable experimental data from appropriately designed trials are just beginning to emerge. As yet, this body of literature is too immature to draw firm conclusions on the role of vitamin D in CVD prevention. Carefully controlled vitamin D trials in well-described population groups using intervention doses that are titrated against target s25(OH)D concentrations could yield potentially valuable outcomes that may have a positive impact on CVD risk modification.

Parathyroid hormone mediates hematopoietic cell expansion through interleukin-6

Parathyroid hormone (PTH) stimulates hematopoietic cells through mechanisms of action that remain elusive. Interleukin-6 (IL-6) is upregulated by PTH and stimulates hematopoiesis. The purpose of this investigation was to identify actions of PTH and IL-6 in hematopoietic cell expansion. Bone marrow cultures from C57B6 mice were treated with fms-like tyrosine kinase-3 ligand (Flt-3L), PTH, Flt-3L plus PTH, or vehicle control. Flt-3L alone increased adherent and non-adherent cells. PTH did not directly impact hematopoietic or osteoclastic cells but acted in concert with Flt-3L to further increase cell numbers. Flt-3L alone stimulated proliferation, while PTH combined with Flt-3L decreased apoptosis. Flt-3L increased blasts early in culture, and later increased CD45⁺ and CD11b⁺ cells. In parallel experiments, IL-6 acted additively with Flt-3L to increase cell numbers and IL-6-deficient bone marrow cultures (compared to wildtype controls) but failed to amplify in response to Flt-3L and PTH, suggesting that IL-6 mediated the PTH effect. In vivo, PTH increased Lin^- Sca-1⁺c-Kit⁺ (LSK) hematopoietic progenitor cells after PTH treatment in wildtype mice, but failed to increase LSKs in IL-6-deficient mice. In conclusion, PTH acts with Flt-3L to maintain hematopoietic cells by limiting apoptosis. IL-6 is a critical mediator of bone marrow cell expansion and is responsible for PTH actions in hematopoietic cell expansion.

PTH and PTHrP signaling in osteoblasts

The striking clinical benefit of PTH in osteoporosis began a new era of skeletal anabolic agents. Several studies have been performed, new studies are emerging out and yet controversies remain on PTH anabolic action in bone. This review focuses on the molecular aspects of PTH and PTHrP signaling in light of old players and recent advances in understanding the control of osteoblast proliferation, differentiation and function.

IL-6 receptor expression and IL-6 effects change during osteoblast differentiation

Studies of the effects of interleukin-6 on osteoblasts have yielded conflicting results. In several earlier in vitro studies it has been stated that IL-6 has no effects on osteoblasts unless soluble IL-6 receptor is added. These results are contradictory to the fact that IL-6 receptors are expressed in osteoblasts in vivo. In this study, MC3T3 preosteoblast cells and rat bone marrow stromal cells were cultured in bone inducing medium containing ascorbic acid, beta-glycerophosphate or dexamethasone. We found that IL-6 receptor expression increased in both types of cells during in vitro differentiation. Furthermore in MC3T3 cells IL-6 decreased proliferation and enhanced expression of two osteoblast-specific differentiation markers, Runx2 and osteocalcin, in proper sequential order. Interestingly, in both cell types IL-6-induced apoptosis only in later culture stages. We also found in MC3T3 cells that IL-6 induced STAT3 activation was significantly higher in later culture stages, i.e. when IL-6 receptor expression was high. The present study shows that IL-6 receptor expression increases during in vitro osteoblast differentiation and that IL-6 functions as a differentiation regulator of preosteoblast cells and an apoptosis initiator in more mature cells.

Osteoimmunology

Osteoimmunology is an interdisciplinary research field combining the exciting fields of osteology and immunology. An observation that contributed enormously to the emergence of osteoimmunology was the accelerated bone loss caused by inflammatory diseases such as rheumatoid arthritis. Receptor activator of nuclear factor kappaB ligand (RANKL), which is the main regulator of osteoclastogenesis, was found to be the primary culprit responsible for the enhanced activation of osteoclasts: activated T cells directly and indirectly increased the expression of RANKL, and thereby promoted osteoclastic activity. Excessive bone loss is not only present in inflammatory diseases but also in autoimmune diseases and cancer. Furthermore, there is accumulating evidence that the very prevalent skeletal disorder osteoporosis is associated with alterations in the immune system. Meanwhile, numerous connections have been discovered in osteoimmunology beyond merely the actions of RANKL. These include the importance of osteoblasts in the maintenance of the hematopoietic stem cell niche and in lymphocyte development as well as the functions of immune cells participating in osteoblast and osteoclast development. Furthermore, research is being done investigating cytokines, chemokines, transcription factors and co-stimulatory molecules which are shared by both systems. Research in osteoimmunology promises the discovery of new strategies and the development of innovative therapeutics to cure or alleviate bone loss in inflammatory and autoimmune diseases as well as in osteoporosis. This review gives an introduction to bone remodeling and the cells governing that process and summarizes the most recent discoveries in the interdisciplinary field of osteoimmunology. Furthermore, an alternative large animal model will be discussed and the pathophysiological alterations of the immune system in osteoporosis will be highlighted.

TNFalpha and PTH utilize distinct mechanisms to induce IL-6 and RANKL expression with markedly different kinetics

Parathyroid hormone (PTH) and tumor necrosis factoralpha (TNFalpha) are bone resorptive agents that upregulate interleukin-6 (IL-6) and RANKL production by osteoblasts. IL-6 mRNA expression induced by PTH is rapid and transient in osteoblasts both in vitro and in vivo. This study found that IL-6 secretion induced by PTH is also rapid and transient. The induction of RANKL mRNA by PTH is also rapid and transient although with an extended time course compared to that of IL-6 mRNA. In contrast, the effects of TNFalpha are biphasic. During the first 2 h of stimulation with TNFalpha, the responses are similar to those induced by PTH. This is followed by a period of relatively low IL-6 and RANKL mRNA levels and little IL-6 secretion. A late phase of increased IL-6 and RANKL mRNA expression occurs 12-24 h after stimulation with TNFalpha leading to a significant increase in IL-6 secretion. A similar biphasic pattern of activation of p38 MAP kinase is induced by TNFalpha. p38alpha/beta activation is required for the increased RANKL mRNA during the early phase of stimulation by TNFalpha but not in the late phase. In contrast, p38alpha/beta activation is not required for increased IL-6 mRNA or IL-6 protein secretion in either the early or late phases of stimulation by TNFalpha. Blocking the increases in IL-6 transcription completely eliminates IL-6 secretion induced during the early phases of stimulation by either PTH or TNFalpha. Consistent with the dependence on transcription, IL-6 mRNA is rapidly degraded with half-lives of 10-14 min following stimulation with either PTH or TNFalpha. In contrast to IL-6, RANKL mRNA is substantially more stable with half-lives of 40-60 min. Taken together, our results show that TNFalpha and PTH utilize distinct mechanisms to induce IL-6 and RANKL expression with markedly different kinetics. The more extensive effect of TNFalpha likely reflects that TNFalpha stimulates IL-6 production and bone resorption in pathological situations. In contrast, the less extensive effect of PTH likely reflects that it acts in physiological situations where it is important to minimize the potential adverse effects of high levels of IL-6 on bone and/or surrounding tissues.

IL-6 is not required for parathyroid hormone stimulation of RANKL expression, osteoclast formation, and bone loss in mice

Continuous elevation of parathyroid hormone (PTH) increases osteoclast precursors, the number of osteoclasts on cancellous bone, and bone turnover. The essential molecular mediators of these effects are controversial, however, and both increased receptor activator of NF-kappaB ligand (RANKL) and IL-6 have been implicated. The goal of these studies was to determine whether continuous elevation of endogenous PTH alters IL-6 gene expression in vivo and whether IL-6 is required for PTH-induced bone loss. To accomplish this, we generated transgenic mice harboring a luciferase reporter gene under the control of IL-6 gene regulatory regions to allow accurate quantification of IL-6 gene activity in vivo. In these mice, induction of secondary hyperparathyroidism using a calcium-deficient diet did not alter IL-6-luciferase transgene expression, whereas RANKL mRNA expression was elevated in bone tissue. Moreover, secondary hyperparathyroidism induced an equivalent amount of bone loss in wild-type and IL-6-deficient mice, and PTH elevated RANKL mRNA and osteoclast formation to the same extent in bone marrow cultures derived from wild-type and IL-6-deficient mice. These results demonstrate that IL-6 is not required for the osteoclast formation and bone loss that accompanies continuous elevation of PTH.

Increased expression of interleukin-6 by vasoactive intestinal peptide is associated with regulation of CREB, AP-1 and C/EBP, but not NF-kappaB, in mouse calvarial osteoblasts

Interleukin-6 (IL-6), and the related cytokines IL-11, leukemia inhibitory factor (LIF) and oncostatin M (OSM), are potent stimulators of osteoclastic bone resorption. In the present study, we have addressed the possibility that the neuropeptide vasoactive intestinal peptide (VIP) may regulate the production of and/or sensitivity to the IL-6 family of cytokines in mouse calvarial osteoblasts. VIP stimulated IL-6 mRNA expression and protein release in a time- and concentration-dependent manner, whereas mRNA expression of the IL-6 receptor, as well as mRNA expressions of IL-11, LIF, OSM and their cognate receptors, were unaffected by VIP. In cells transfected with the IL-6 promoter coupled to luciferase, VIP increased transcriptional activity. The effects of VIP were shared by the related neuropeptide PACAP-38, belonging to the same superfamily of neuropeptides, whereas secretin did not have any effect, indicating that the effects were mediated by VPAC2 receptors. The effects of VIP were potentiated by the cyclic AMP phosphodiesterase inhibitor rolipram and mimicked by forskolin, indicating the involvement of the cyclic AMP/protein kinase A pathway. This was further demonstrated by the facts that the stimulatory effect of VIP on luciferase activity could be reversed by the PKA inhibitors H-89 and KT5720 and was mimicked by cyclic AMP analogues selective for PKA, but not by those selective for Epac. In addition, VIP enhanced the phosphorylation of CREB, as assessed by both immunocytochemical analysis and Western blot. The DNA binding activity of nuclear extracts to C/EBP was increased by VIP, whereas binding to AP-1 was decreased. In contrast, DNA binding to NF-kappaB, as well as nuclear translocation of NF-kappaB and C/EBP, were unaffected by VIP. The mRNA expressions of C/EBPbeta, C/EBPdelta, C/EBPgamma, c-Jun, JunB, c-Fos, Fra-1 and IkappaBalpha and protein level of IkappaBalpha were all unaffected by VIP. These observations, together, demonstrate that VIP stimulates IL-6 production in osteoblasts by a mechanism likely to be mediated by VPAC2 receptors and dependent on cyclic AMP/protein kinase A/CREB activation and also involving the transcription factors C/EBP and AP-1.

Rolipram, a phosphodiesterase 4 inhibitor, stimulates inducible cAMP early repressor expression in osteoblasts

Phosphodiesterase (PDE) 4 inhibitors have been shown to induce the cAMP-mediated signaling pathway by inhibiting cAMP hydrolysis. This study investigated the effect of a PDE4 inhibitor on the expression of the inducible cAMP early repressor (ICER), which is an endogenous inhibitor of CRE- mediated transcription, in osteoblastic cells. RT-PCR analysis revealed that rolipram, a PDE4 inhibitor, stimulates the ICER mRNA in a dose dependent manner. The induction of ICER mRNA expression by rolipram was suppressed by the inhibitors of protein kinase A (PKA) and p38 MAPK, suggesting the involvement of PKA and p38 MAPK activation in ICER expression by rolipram. It was previously shown that rolipram induced the expression of TNF-related activation-induced cytokine (TRANCE, also known as RANKL, ODF, or OPGL) in osteoblasts. This paper provides evidences that a transcriptional repressor like ICER might modulate TRANCE mRNA expression by rolipram in osteoblasts.

Endogenous protein kinase inhibitor gamma terminates immediate-early gene expression induced by cAMP-dependent protein kinase (PKA) signaling: termination depends on PKA inactivation rather than PKA export from the nucleus

Expression of many genes induced by cAMP-dependent protein kinase (PKA) signaling is rapidly terminated. Although many mechanisms contribute to regulation of PKA signaling, members of the endogenous protein kinase inhibitor (PKI) family may be particularly important for terminating nuclear PKA activity and gene expression. Here we used both siRNA and antisense knockdown strategies to examine PKA signaling activated by parathyroid hormone or the beta-adrenergic agonist, isoproterenol. We found that endogenous PKIgamma is required for efficient termination of nuclear PKA activity, transcription factor phosphorylation, and immediate-early genes. Moreover, PKIgamma is required for export of PKA catalytic subunits from the nucleus back to the cytoplasm following activation of PKA signaling because this is also inhibited by PKIgamma knockdown. Leptomycin B blocks PKA nuclear export but has little or no effect on nuclear PKA activity or immediate-early gene expression. Thus, inactivation of PKA activity in the nucleus is sufficient to terminate signaling, and nuclear export is not required. These results were the first in any cell type showing that endogenous levels of PKI regulate PKA signaling.

IL-6, RANKL, TNF-alpha/IL-1: interrelations in bone resorption pathophysiology

All osteogenic cells (osteoclasts, osteoblasts) contribute individually to bone remodeling. Their cellular interactions control their cellular activities and the bone remodeling intensity. These interactions can be established either through a cell-cell contact, involving molecules of the integrin family, or by the release of many polypeptidic factors and/or their soluble receptor chains. These factors can act directly on osteogenic cells and their precursors to control differentiation, formation and functions (matrix formation, mineralization, resorption...). Here, we present the involvement of three groups of cytokines which seem to be of particular importance in bone physiology: interleukin-6 (IL-6), tumor necrosis factor-alpha (TNF-alpha) (TNF-alpha)/IL-1, and the more recently known triad osteoprotegerin (OPG)/receptor activator of NF-kappaB (RANK)/RANK ligand (RANKL). The interactions between these three groups are presented within the framework of bone resorption pathophysiology such as tumor associated osteolysis. The central role of the OPG/RANK/RANKL triad is pointed out.

Significance of primary hyperparathyroidism in the management of osteoporosis

Primary hyperparathyroidism (HPT) has catabolic effects on cortical bone and anabolic effects on cancellous bone with overall deleterious effects on skeleton. Primary HPT is associated with increased fracture risk both at the cancellous bone-enriched spine and the cortical bone-enriched distal one third of the radius. This risk is reversed by parathyroidectomy.

Interleukin-6 production and secretion by human parathyroids

Parathyroid hormone (PTH) stimulates osteoblasts to produce the proinflammatory cytokine interleukin-6 (IL-6), causing bone resorption. In patients with primary hyperparathyroidism, elevated serum levels of IL-6 normalize after resection of parathyroid tumours. Because IL-6 is also expressed in normal parathyroids and in other endocrine cells (adrenal and islet), we hypothesized that parathyroid tumours might contribute directly to the elevated serum IL-6 levels in patients with hyperparathyroidism. Immunohistochemistry identified IL-6, PTH, and chromogranin-A (an endocrine and neuroendocrine tumour marker) in normal, adenomatous and hyperplastic parathyroids. Using immunofluorescence and confocal microscopy, IL-6 co-localized with PTH and with chromogranin-A in parathyroid cells. All cultured parathyroid tumours secreted IL-6 at levels markedly higher than optimally stimulated peripheral blood mononuclear cells. Supernates from cultured parathyroids stimulated proliferation of an IL-6-dependent cell line, and anti-IL-6 MoAb abolished this stimulatory effect. IL-6 mRNA was documented in cultured parathyroid tumours, cultured normal parathyroids, fresh operative parathyroid tumours and fresh operative normal specimens. In conclusion, these data show that parathyroid tumours and normal parathyroids contain, produce and secrete IL-6. Our findings present a novel pathway by which human parathyroids may contribute markedly to IL-6 production and elevation of serum IL-6 levels in patients with hyperparathyroidism. The physiological relevance of IL-6 production by human parathyroids remains to be determined, but IL-6 secretion by parathyroid tumours may contribute to bone loss and to other multi-system complaints observed in these patients.

Osteoclastogenesis in fibrous dysplasia of bone: in situ and in vitro analysis of IL-6 expression

Fibrous dysplasia of bone (FD) is caused by somatic mutations of the GNAS1 gene, which lead to constitutive activation of adenylyl cyclase and overproduction of cAMP in osteogenic cells. Previous in vitro studies using nonclonal, heterogeneous strains of FD-derived cells suggested that IL-6 might play a critical role in promoting excess osteoclastogenesis in FD. In this study, we investigated IL-6 expression in FD in situ and its relationship to the actual patterns of osteoclastogenesis within the abnormal tissue. We found that osteoclastogenesis is not spatially restricted to bone surfaces in FD but occurs to a large extent ectopicly in the fibrous tissue, where stromal cells diffusely express IL-6 mRNA and exhibit a characteristic cell morphology. We also observed specific expression of IL-6 mRNA in a proportion of osteoclasts, suggesting that an autocrine/paracrine loop may contribute to osteoclastogenesis in vivo in FD, as in some other bone diseases, including Paget's disease. We also generated homogeneous, clonally derived strains of wild-type and GNAS1-mutated stromal cells from the same individual, parent FD lesions. In this way, we could show that mutated stromal cells produce IL-6 at a basal magnitude and rate that are significantly higher than in the cognate wild-type cells. Conversely, wild-type cells respond to db-cAMP with a severalfold increase in magnitude and rate of IL-6 production, whereas mutant strains remain essentially unresponsive. Our data establish a direct link between GNAS1 mutations in stromal cells and IL-6 production but also define the complexity of the role of IL-6 in regulating osteoclastogenesis in FD in vivo. Here, patterns of osteoclastogenesis and bone resorption reflect not only the cell-autonomous effects of GNAS1 mutations in osteogenic cells (including IL-6 production) but also the local and systemic context to which non-osteogenic cells, local proportions of wild-type vs mutated cells, and systemic hormones contribute.

Expression of inducible cAMP early repressor is coupled to the cAMP-protein kinase A signaling pathway in osteoblasts

We previously showed that parathyroid hormone (PTH) induces inducible cAMP early repressor (ICER) in osteoblastic cells and mouse calvariae. PTH signaling in osteoblastic cells is transduced by PTH receptor 1, which is coupled to cAMP-protein kinase A (PKA), protein kinase C (PKC), and calcium signaling pathways. In the present study, we examined the role of these pathways in mediating PTH-induced ICER mRNA and protein expression in osteoblastic MC3T3-E1 cells. Using RT-PCR, we found that PTH(1-34), forskolin (FSK), and 8-bromo-cAMP (8Br-cAMP) induced ICER expression, while phorbol myristate acetate (PMA), ionomycin, and PTH(3-34) did not. Similar results were found for the induction of ICER protein. PKA inhibition by H89 markedly reduced PTH- and FSK-induced ICER expression, while PKC depletion by PMA had little effect. We also tested ICER induction by other osteotropic signaling agonists. Other cAMP-PKA pathway activators, such as PTH-related protein (PTHrP), induced ICER expression, while agents that signal through other pathways did not. PTHrP maximally induced ICER mRNA at 2-4 h, which then returned to baseline by 10 h. Finally, PTH, FSK, and PTHrP induced ICER in cultured mouse calvariae and osteoblastic ROS 17/2.8, UMR-106, and Pyla cells. We conclude that ICER expression in osteoblasts requires activation of the cAMP-PKA signaling pathway.

Termination of immediate-early gene expression after stimulation by parathyroid hormone or isoproterenol

cAMP/PKA signaling transiently stimulates mRNA expression of immediate-early genes, including IL-6 and c-fos. We confirmed that these mRNAs are transiently stimulated by parathyroid hormone (PTH) in ROS 17/2.8 osteoblastic cells. Consistent with the role for cAMP/PKA signaling in this response, PTH induces transient cAMP elevation, PKA activation, and cAMP-responsive element-binding protein (CREB) phosphorylation. Our goal was to determine whether termination of immediate-early gene expression is due to receptor desensitization or cAMP degradation. The approaches used were 1) inhibition of PTH receptor desensitization with G protein-coupled receptor kinase 2 (GRK2) antisense oligonucleotides or antisense plasmids, 2) sustained activation of adenyl cyclase with forskolin, and 3) inhibition of cAMP degradation with 3-isobutyl-1-methylxanthine. These experiments show that mechanisms downstream of receptor desensitization and cAMP degradation are primarily responsible for termination of PKA activity, CREB phosphorylation, and immediate-early gene expression. Similar conclusions were also obtained in response to PTH in a second osteoblastic cell line (MC3T3-E1) and in response to isoproterenol in NIH3T3 fibroblasts. This conclusion may therefore reflect a general mechanism for termination of immediate-early gene expression after induction by cAMP/PKA.

Cytokines synergistically induce osteoclast differentiation: support by immortalized or normal calvarial cells

Conditionally immortalized murine calvarial (CIMC) cells that support differentiation of precursors into mature osteoclasts were isolated. All six CIMC cell lines supported osteoclast differentiation in response to 1,25-dihydroxyvitamin D(3) or interleukin (IL)-11. CIMC-4 cells also supported osteoclast differentiation in response to tumor necrosis factor (TNF)-alpha, IL-1beta, or IL-6. The resultant multinucleated cells expressed tartrate-resistant acid phosphatase and formed resorption lacunae on mineralized surfaces. CIMC-4 cells, therefore, establish an osteoclast differentiation assay that is responsive to many cytokines and does not rely on isolation of primary stromal support cells. Low concentrations of the cytokines synergistically stimulated differentiation when osteoclast precursors were cocultured with either CIMC-4 cells or primary calvarial cells. Osteoclast differentiation induced by all stimuli other than TNF-alpha was completely blocked by osteoprotegerin, whether the stimulators were examined alone or in combination. Moreover, study of precursors that lack TNF-alpha receptors showed that TNF-alpha induces osteoclast differentiation primarily through direct actions on osteoclast precursors, which is a distinct mechanism from that used by the other bone-resorptive agents examined in this study.

Can altered production of interleukin-1beta, interleukin-6, transforming growth factor-beta and prostaglandin E(2) by isolated human subchondral osteoblasts identify two subgroups of osteoarthritic patients

OBJECTIVE

To determine the capacity of human subchondral osteoarthritic osteoblasts (Ob) to produce interleukin (IL)-1beta, IL-6, transforming growth factor-beta (TGF-beta) and prostaglandin E(2) (PGE(2)), and determine if a relationship exists between IL-1beta, TGF-beta, PGE(2) and IL-6 production.

METHODS

We measured the abundance of IL-1beta, IL-6, TGF-beta and PGE(2) using very sensitive ELISA in conditioned-media of human primary subchondral Ob from normal individuals and osteoarthritic patients. Selective inhibition of IL-6 or IL-6 receptor signaling was performed to determine its effect on PGE(2) production whereas the inhibiton of PGE(2) production was performed to determine its effect on IL-6 production. The expression of bone cell markers and urokinase plasminogen activator (uPA) activity was also determined.

RESULTS

Osteoarthritic Ob produced all these factors with greater variability than normal cells. Interestingly, the production of IL-6 and PGE(2) by osteoarthritic Ob separated patients into two subgroups, those whose Ob produced levels comparable to normal (low producers) and those whose Ob produced higher levels (high producers). In those cells classified as high osteoarthritic Ob, PGE(2) and IL-6 levels were increased two- to three-fold and five- to six-fold, respectively, compared with normal. In contrast, while using their IL-6 and PGE(2) production to separate osteoarthritic Ob into low and high producers, we found that IL-1beta levels were similar in normal and all osteoarthritic Ob. Using the same criteria, TGF-beta levels were increased in all osteoarthritic Ob compared with normal. Reducing PGE(2) synthesis by Indomethacin [a cyclo-oxygenase (COX) -1 and -2 inhibitor] reduced IL-6 levels in all osteoarthritic Ob, whereas Naproxen (a more selective COX-2 inhbitor) reduced PGE(2) and IL-6 levels only in the high osteoarthritic group. Conversely, PGE(2) addition to osteoarthritic Ob enhanced IL-6 production in both groups. Moreover, the addition of parathyroid hormone also stimulated IL-6 production to similar normal levels in both osteoarthritic groups. In contrast, using an antibody against IL-6 or IL-6 receptors did not reduce PGE(2) levels in either group. The evaluation of alkaline phosphatase activity, osteocalcin release, collagen type I and uPA activity in osteoarthritic Ob failed to show any differences between these cells regardless to which subgroup they were assigned.

CONCLUSIONS

These results indicate that IL-6 and PGE(2) production by subchondral Ob can discriminate two subgroups of osteoarthritic patients that cannot otherwise be separated by their expression of cell markers, and that endogenous PGE(2) levels influence IL-6 synthesis in osteoarthritic Ob.

Induction of cholinergic function in cultured sympathetic neurons by periosteal cells: cellular mechanisms

Periosteum, the connective tissue surrounding bone, alters the transmitter properties of its sympathetic innervation during development in vivo and after transplantation. Initial noradrenergic properties are downregulated and the innervation acquires cholinergic and peptidergic properties. To elucidate the cellular mechanisms responsible, sympathetic neurons were cultured with primary periosteal cells or osteoblast cell lines. Both primary cells and an immature osteoblast cell line, MC3T3-E1, induced choline acetyltransferase (ChAT) activity. In contrast, lines representing marrow stromal cells or mature osteoblasts did not increase ChAT. Growth of periosteal cells with sympathetic neurons in transwell cultures that prevent direct contact between the neurons and periosteal cells or addition of periosteal cell-conditioned medium to neuron cultures induced ChAT, indicating that periosteal cells release a soluble cholinergic inducing factor. Antibodies against LIFRbeta, a receptor subunit shared by neuropoietic cytokines, prevented ChAT induction in periosteal cell/neuron cocultures, suggesting that a member of this family is responsible. ChAT activity was increased in neurons grown with periosteal cells or conditioned medium from mice lacking either leukemia inhibitory factor (LIF) or LIF and ciliary neurotrophic factor (CNTF). These results provide evidence that periosteal cells influence sympathetic neuron phenotype by releasing a soluble cholinergic factor that is neither LIF nor CNTF but signals via LIFRbeta.

Effects of hydroxyapatite particulate debris on the production of cytokines and proteases in human fibroblasts

Cytokines and proteases are secreted by fibroblasts in response to particulate wear debris, and these proteins are felt to play an important role in the development of osteolysis and implant loosening. Although metallic and polyethlyene debris have been studied extensively, little is known about the cellular responses to hydroxyapatite, despite the wide clinical use of these materials. Therefore, the effects of hydroxyapatite (HA) and hydroxyapatite/beta-tricalciumphosphate (HA/TCP) on cellular proliferation, cytokine gene expression and protein secretion, protease synthesis, and gelatinolytic activity were investigated in human fibroblasts. HA and HA/TCP particles were synthesized, and their effects were compared to the responses elicited by titanium and cobalt chromium. Sample characterization by scanning electron microscopy and Coulter Counter demonstrated that the materials had a mean particle size of less than 10 microm, and all of the particles were compared using the same concentration ranges. Aliquots of particle suspensions were added to human fibroblasts maintained in tissue culture, and dose-response and time-course experiments were performed. Effects of the particles on fibroblast proliferation were assessed, and alterations in cytokine levels were determined by specific enzyme linked immunosorbent assays (ELISA). Cytokines that were evaluated included interleukin-1 (IL-1beta), interleukin-6 (IL-6), and tumor necrosis factor-alpha (TNF-alpha), all of which have been demonstrated to enhance bone resorption and are associated with osteolysis and implant loosening. Gene expression was determined using Northern blot analysis with cytokine-specific probes, while secretion of the proteases collagenase and stromelysin was determined by Western blot analysis. Functional gelatinolytic assay was assessed using zymogram gels. The particles were evaluated in a concentration range from 0.000021 to 0.021 vol%. All of the particles produced increases in cellular proliferation up to 0.0021 vol%, with the largest increases being seen at 0.021 vol% with HA/TCP and titanium. At the highest concentration, both cobalt chromium and HA samples decreased cellular proliferation relative to lower doses, possibly representing cytotoxicity. Hydroxyapatite particles yielded a 30-fold increase in interleukin-6 secretion compared to unstimulated controls, which was also greater than three times the levels produced by cobalt chromium, titanium, or HA/TCP. HA particles also tripled the secretion of IL-1beta at 0.00021 vol%, and doubled TNF-alpha secretion at 0.021 vol%. Addition of conditioned media prepared by incubation of the particles in culture medium in the absence of cells did not alter the secretion of any of the cytokines. Northern blot analysis using IL-6 probes also demonstrated strong increases with HA compared to the other materials, suggesting that the action of the HA particles was at the level of transcription. Secretion of the protease collagenase was increased by all of the samples including HA when compared to unstimulated controls. Stromelysin secretion into the culture medium was decreased by cobalt chromium, but increased by titanium, HA, and HA/TCP. All of the particles including HA increased the gelatinolytic activity of the fibroblasts. These findings demonstrate that HA and HA/TCP particles are capable of stimulating the expression and secretion of cytokines and proteases that enhance bone resorption, and suggest that particulate debris from implants using these coatings may also increase osteolysis and loosening.

Stimulation of interleukin-6 promoter by parathyroid hormone, tumor necrosis factor alpha, and interleukin-1beta in UMR-106 osteoblastic cells is inhibited by protein kinase C antagonists

To investigate the level at which protein kinase C (PKC) regulates expression of interleukin-6 (IL-6) in osteoblastic cells, effects of several PKC antagonists and PKC down-regulation by phorbol ester were studied in UMR-106 osteoblastic cells that had been transiently transfected with a -224/+11-base pair (bp) IL-6 promoter coupled to a luciferase reporter. Parathyroid hormone (PTH) elicited a dose-dependent stimulation of the IL-6 promoter expression, with significant increases produced by 5 h of treatment with concentrations of PTH as low as 10(-14) M. The increase in IL-6 promoter expression was inhibited by the PKC antagonists GF109203X, 30 nM to 1 microM, and calphostin C, 250 nM. Prior down-regulation of PKC with 100 nM phorbol-12,13-dibutyrate (PDBU) for 48 h inhibited the PTH effect as well as the smaller stimulatory effects elicited by tumor necrosis factor alpha (TNF-alpha), 10(-9)-10(-8) M, and by IL-1beta, 1-10 ng/ml. In contrast to these findings, the stimulatory effects of PTH, TNF-alpha, and IL-1beta on the IL-6 promoter expression were enhanced by staurosporine. Treatment with GF109203X or down-regulation of PKC with PDBU prevented the stimulatory effects of staurosporine. PKC activity was increased by staurosporine. The findings with staurosporine are consistent with our earlier observations that this agent enhances the calcium signaling and bone resorption elicited by PTH. The studies support the role of PKC in the stimulatory effects of PTH, TNF-alpha, and IL-1beta on IL-6 expression.

Parathyroid hormone induces hepatic production of bioactive interleukin-6 and its soluble receptor

Interleukin-6 (IL-6) is an important mediator of parathyroid hormone (PTH)-induced bone resorption. Serum levels of IL-6 and its soluble receptor (IL-6sR) are regulated in part by PTH. The PTH/PTH-related protein type 1 receptor is highly expressed in the liver, and in the current study we investigated whether the liver produces IL-6 or IL-6sR in response to PTH. Perfusion of the isolated rat liver with PTH-(1-84) stimulated rapid, dose-dependent production of bioactive IL-6 and the IL-6sR. These effects were observed at near physiological concentrations of the hormone such that 1 pM PTH induced hepatic IL-6 production at a rate of approximately 0.6 ng/min. In vitro, hepatocytes, hepatic endothelial cells, and Kupffer cells, but not hepatic stellate cells, were each found to produce both IL-6 and IL-6sR in response to higher (10 nM) concentrations of PTH. Our data suggest that hepatic-derived IL-6 and IL-6sR contribute to the increase in circulating levels of these cytokines induced by PTH in vivo and raise the possibility that PTH-induced, liver-derived IL-6 may exert endocrine effects on tissues such as bone.

gp130 Cytokine family and bone cells

Bone tissue is continually being remodelled according to physiological circumstances. Two main cell populations (osteoblasts and osteoclasts) are involved in this process, and cellular activities (including cell differentiation) are modulated by hormones, cytokines and growth factors. Within the last 20 years, many factors involved in bone tissue metabolism have been found to be closely related to the inflammatory process. More recently, a cytokine family sharing a common signal transducer (gp130) had been identified, which appears to be a key factor in bone remodelling. This family includes interleukin 6, interleukin 11, oncostatin M, leukaemia inhibitory factor, ciliary neurotrophic factor and cardiotrophin-1. This paper provides an exhaustive review of recent knowledge on the involvement of gp130 cytokine family in bone cell (osteoblast, osteoclast, etc.) differentiation/activation and in osteoarticular pathologies.

Local and chronic application of PTH accelerates tooth movement in rats

We previously reported that whereas systemic continuous infusion of parathyroid hormone (PTH) accelerated orthodontic tooth movement, systemic but intermittent injection of PTH did not increase the rate of tooth movement. Analysis of these data suggested that continuous administration of PTH could be applicable for orthodontic therapy. In the present study, we investigated whether local and chronic application of PTH(1-34) would accelerate orthodontic tooth movement. To increase the residence time of PTH in the injected area, we used methylcellulose (MC) gel (2% W/V) for a slow-release formulation of PTH. MC gel containing PTH (PTH-MC) continuously released biologically active PTH into the acceptor medium for more than 72 hrs in vitro. When male rats received a local injection of PTH-MC into the subperiosteum in the mesio-palatal region of the maxillary first molar (M1) every other day, M1 movement, which was mesially drawn by an orthodontic coil spring attached to the maxillary incisors, was accelerated in a dose-dependent manner. PTH-MC injection at 1 microg/400 g body weight caused a 1.6-fold increase in the rate of tooth movement. The acceleration of tooth movement by PTH-MC injection was marked on days 6, 9, and 12. Local injection of PTH dissolved in saline without MC did not significantly accelerate tooth movement on day 6 or later. Histological examination revealed active osteoclastic bone resorption and a widened periodontal space on the compression side of the periodontal tissue in the PTH-MC-injected rats. These results suggest that local injection of PTH in a slow-release formulation is applicable to orthodontic therapy.