Leukemia inhibitory factor is mitogenic to osteoblasts

Current perspectives on the multiple roles of osteoclasts: Mechanisms of osteoclast-osteoblast communication and potential clinical implications

Bone remodeling is a complex process involving the coordinated actions of osteoblasts and osteoclasts to maintain bone homeostasis. While the influence of osteoblasts on osteoclast differentiation is well established, the reciprocal regulation of osteoblasts by osteoclasts has long remained enigmatic. In the past few years, a fascinating new role for osteoclasts has been unveiled in promoting bone formation and facilitating osteoblast migration to the remodeling sites through a number of different mechanisms, including the release of factors from the bone matrix following bone resorption and direct cell-cell interactions. Additionally, considerable evidence has shown that osteoclasts can secrete coupling factors known as clastokines, emphasizing the crucial role of these cells in maintaining bone homeostasis. Due to their osteoprotective function, clastokines hold great promise as potential therapeutic targets for bone diseases. However, despite long-standing work to uncover new clastokines and their effect in vivo, more substantial efforts are still required to decipher the mechanisms and pathways behind their activity in order to translate them into therapies. This comprehensive review provides insights into our evolving understanding of the osteoclast function, highlights the significance of clastokines in bone remodeling, and explores their potential as treatments for bone diseases suggesting future directions for the field.

Bone remodeling: an operational process ensuring survival and bone mechanical competence

Bone remodeling replaces old and damaged bone with new bone through a sequence of cellular events occurring on the same surface without any change in bone shape. It was initially thought that the basic multicellular unit (BMU) responsible for bone remodeling consists of osteoclasts and osteoblasts functioning through a hierarchical sequence of events organized into distinct stages. However, recent discoveries have indicated that all bone cells participate in BMU formation by interacting both simultaneously and at different differentiation stages with their progenitors, other cells, and bone matrix constituents. Therefore, bone remodeling is currently considered a physiological outcome of continuous cellular operational processes optimized to confer a survival advantage. Bone remodeling defines the primary activities that BMUs need to perform to renew successfully bone structural units. Hence, this review summarizes the current understanding of bone remodeling and future research directions with the aim of providing a clinically relevant biological background with which to identify targets for therapeutic strategies in osteoporosis.

Novel insights into the coupling of osteoclasts and resorption to bone formation

Bone remodeling consists of resorption by osteoclasts (OCs) and formation by osteoblasts (OBs). Precise coordination of these activities is required for the resorbed bone to be replaced with an equal amount of new bone in order to maintain skeletal mass throughout the lifespan. This coordination of remodeling processes is referred to as the "coupling" of resorption to bone formation. In this review, we discuss the essential role for OCs in coupling resorption to bone formation, mechanisms for this coupling, and how coupling becomes less efficient or disrupted in conditions of bone loss. Lastly, we provide perspectives on targeting coupling to treat human bone disease.

Identification of osteoclast-osteoblast coupling factors in humans reveals links between bone and energy metabolism

Bone remodeling consists of resorption by osteoclasts followed by formation by osteoblasts, and osteoclasts are a source of bone formation-stimulating factors. Here we utilize osteoclast ablation by denosumab (DMAb) and RNA-sequencing of bone biopsies from postmenopausal women to identify osteoclast-secreted factors suppressed by DMAb. Based on these analyses, LIF, CREG2, CST3, CCBE1, and DPP4 are likely osteoclast-derived coupling factors in humans. Given the role of Dipeptidyl Peptidase-4 (DPP4) in glucose homeostasis, we further demonstrate that DMAb-treated participants have a significant reduction in circulating DPP4 and increase in Glucagon-like peptide (GLP)-1 levels as compared to the placebo-treated group, and also that type 2 diabetic patients treated with DMAb show significant reductions in HbA1c as compared to patients treated either with bisphosphonates or calcium and vitamin D. Thus, our results identify several coupling factors in humans and uncover osteoclast-derived DPP4 as a potential link between bone remodeling and energy metabolism.

Anti-resorptive bone therapies also inhibit bone formation, as osteoclasts secrete factors that stimulate bone formation by osteoblasts. Here, the authors identify osteoclast-secreted factors that couple bone resorption to bone formation in healthy subjects, and show that osteoclast-derived DPP4 may be a factor coupling bone resorption to energy metabolism.

Osteoclasts Provide Coupling Signals to Osteoblast Lineage Cells Through Multiple Mechanisms

Bone remodeling is essential for the repair and replacement of damaged and old bone. The major principle underlying this process is that osteoclast-mediated resorption of a quantum of bone is followed by osteoblast precursor recruitment; these cells differentiate to matrix-producing osteoblasts, which form new bone to replace what was resorbed. Evidence from osteopetrotic syndromes indicate that osteoclasts not only resorb bone, but also provide signals to promote bone formation. Osteoclasts act upon osteoblast lineage cells throughout their differentiation by facilitating growth factor release from resorbed matrix, producing secreted proteins and microvesicles, and expressing membrane-bound factors. These multiple mechanisms mediate the coupling of bone formation to resorption in remodeling. Additional interactions of osteoclasts with osteoblast lineage cells, including interactions with canopy and reversal cells, are required to achieve coordination between bone formation and resorption during bone remodeling.

A Jak1/2 inhibitor, baricitinib, inhibits osteoclastogenesis by suppressing RANKL expression in osteoblasts in vitro

The Janus kinases (Jaks) are hubs in the signaling process of more than 50 cytokine or hormone receptors. However, the function of Jak in bone metabolism remains to be elucidated. Here, we showed that the inhibition of Jak1 and/or Jak2 in osteoblast-lineage cells led to impaired osteoclastogenesis due to the reduced expression of receptor activator of nuclear factor-κB ligand (RANKL). Murine calvaria-derived osteoblasts induced differentiation of bone marrow cells into osteoclasts in the presence of 1,25-dihydroxyvitamin D₃ (1,25D₃) and prostaglandin E₂ (PGE₂) in vitro. However, treatment with the Jak1/2 inhibitor, baricitinib, markedly inhibited osteoclastogenesis in the co-culture. On the other hand, baricitinib did not inhibit RANKL-induced osteoclast differentiation of bone marrow macrophages. These results indicated that baricitinib acted on osteoblasts, but not on bone marrow macrophages. Baricitinib suppressed 1,25D₃ and PGE₂-induced up-regulation of RANKL in osteoblasts, but not macrophage colony-stimulating factor expression. Moreover, the addition of recombinant RANKL to co-cultures completely rescued baricitinib-induced impairment of osteoclastogenesis. shRNA-mediated knockdown of Jak1 or Jak2 also suppressed RANKL expression in osteoblasts and inhibited osteoclastogenesis. Finally, cytokine array revealed that 1,25D₃ and PGE₂ stimulated secretion of interleukin-6 (IL-6), IL-11, and leukemia inhibitory factor in the co-culture. Hence, Jak1 and Jak2 represent novel therapeutic targets for osteoporosis as well as inflammatory bone diseases including rheumatoid arthritis.

CHMP5 controls bone turnover rates by dampening NF-κB activity in osteoclasts

Physiological bone remodeling requires that bone formation by osteoblasts be tightly coupled to bone resorption by osteoclasts. However, relatively little is understood about how this coupling is regulated. Here, we demonstrate that modulation of NF-κB signaling in osteoclasts via a novel activity of charged multivesicular body protein 5 (CHMP5) is a key determinant of systemic rates of bone turnover. A conditional deletion of CHMP5 in osteoclasts leads to increased bone resorption by osteoclasts coupled with exuberant bone formation by osteoblasts, resembling an early onset, polyostotic form of human Paget's disease of bone (PDB). These phenotypes are reversed by haploinsufficiency for Rank, as well as by antiresorptive treatments, including alendronate, zolendronate, and OPG-Fc. Accordingly, CHMP5-deficient osteoclasts display increased RANKL-induced NF-κB activation and osteoclast differentiation. Biochemical analysis demonstrated that CHMP5 cooperates with the PDB genetic risk factor valosin-containing protein (VCP/p97) to stabilize the inhibitor of NF-κBα (IκBα), down-regulating ubiquitination of IκBα via the deubiquitinating enzyme USP15. Thus, CHMP5 tunes NF-κB signaling downstream of RANK in osteoclasts to dampen osteoclast differentiation, osteoblast coupling and bone turnover rates, and disruption of CHMP5 activity results in a PDB-like skeletal disorder.

Impact of inflammation on the osteoarthritic niche: implications for regenerative medicine

Osteoarthritis (OA) is the most common form of arthritis worldwide and is the sixth leading cause of disability. It costs the UK economy approximately 1% of gross national product per annum. With an aging population, the cost of chronic conditions such as OA continues to rise. Historically, treatments for OA have been limited to painkillers, physiotherapy and joint injections. When these fail, patients are referred for joint replacement surgery. With the advent of tissue engineering strategies aimed at generating new bone and cartilage for repair of osteochondral defects, there has been considerable interest in exploiting these techniques to devise new treatments for OA. To date, little consideration has been given to the OA niche and attendant inflammatory milieu for any regenerative skeletal strategy. This review highlights the importance of understanding the osteoarthritic niche in order to modify existing tissue engineering and regenerative medicine strategies for the future treatment of OA.

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.

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.

Alteration of bone cell function by RANKL and OPG in different in vitro models

BACKGROUND

Receptor activator of nuclear factor-kappaB ligand (RANKL) and osteoprotegerin (OPG) are well-documented potent regulators of osteoclast development. However, their effects in mature bone cells and in organ cultures have not been well studied. It is uncertain whether their activities in different experimental models are comparable.

MATERIALS AND METHODS

RANKL and OPG were evaluated for their activities in mouse calvarial organ cultures, mouse bone marrow cultures, isolated rat mature osteoclast assays and rat primary osteoblast cultures. Results In murine calvarial organ culture, both muRANKL (> or = 10 ng mL(-1)) and rRANKL (> or = 100 ng mL(-1)) significantly stimulated (45)Ca release, while OPG (> or = 50 ng mL(-1)) was an inhibitor of bone resorption. Meanwhile, [(3)H]-thymidine incorporation in this assay was also modulated (indicating proliferation increases in the osteoblast lineage of cells) although these peptides had no direct effect on [(3)H]-thymidine incorporation in isolated osteoblast assays. In mouse bone marrow cultures, muRANKL (> or = 1 ng mL(-1)) and rRANKL (> or = 5 ng mL(-1)) significantly stimulated osteoclastogenesis. The number of nuclei per osteoclast was also significantly increased. OPG strongly inhibited this index, with over 90% suppression at 1 ng mL(-1). Both muRANKL (10 ng mL(-1)) and rRANKL (100 ng mL(-1)) stimulated, while OPG (10 ng mL(-1)) inhibited osteoclast activity in isolated mature osteoclast assays.

CONCLUSION

The current study demonstrated that bone resorption modulated by RANKL and OPG, in murine calvarial organ culture, leads to changes in osteoblast proliferation, suggesting a feedback mechanism from osteoclasts to osteoblasts. In addition, it was found that RANKL and OPG have more potent effects on osteoclastogenesis than on the activity of mature osteoclasts.

Lactoferrin is a potent regulator of bone cell activity and increases bone formation in vivo

Lactoferrin is an iron-binding glycoprotein present in epithelial secretions, such as milk, and in the secondary granules of neutrophils. We found it to be present in fractions of milk protein that stimulated osteoblast growth, so we assessed its effects on bone cell function. Lactoferrin produced large, dose-related increases in thymidine incorporation in primary or cell line cultures of human or rat osteoblast-like cells, at physiological concentrations (1-100 microg/ml). Maximal stimulation was 5-fold above control. Lactoferrin also increased osteoblast differentiation and reduced osteoblast apoptosis by up to 50-70%. Similarly, lactoferrin stimulated proliferation of primary chondrocytes. Purified, recombinant, human, or bovine lactoferrins had similar potencies. In mouse bone marrow cultures, osteoclastogenesis was dose-dependently decreased and was completely arrested by lactoferrin, 100 microg/ml, associated with decreased expression of receptor activator of nuclear factor-kappaB ligand. In contrast, lactoferrin had no effect on bone resorption by isolated mature osteoclasts. Lactoferrin was administered over calvariae of adult mice for 5 d. New bone formation, assessed using fluorochrome labels, was increased 4-fold by a 4-mg dose of lactoferrin. Thus, lactoferrin has powerful anabolic, differentiating, and antiapoptotic effects on osteoblasts and inhibits osteoclastogenesis. Lactoferrin is a potential therapeutic target in bone disorders such as osteoporosis and is possibly an important physiological regulator of bone growth.

Late gestation modulation of fetal glucocorticoid effects requires the receptor for leukemia inhibitory factor: an observational study

BACKGROUND

Ablation of the low-affinity receptor subunit for leukemia inhibitory factor (LIFR) causes multi-systemic defects in the late gestation fetus. Because corticosterone is known to have a broad range of effects and LIF function has been associated with the hypothalamo-pituitary-adrenal axis, this study was designed to determine the role for LIFR in the fetus when exposed to the elevated maternal glucocorticoid levels of late gestation. Uncovering a requirement for LIFR in appropriate glucocorticoid response will further understanding of control of glucocorticoid function.

METHODS

Maternal adrenalectomy or RU486 administration were used to determine the impact of the maternal glucocorticoid surge on fetal development in the absence of LIFR. The mice were analyzed by a variety of histological techniques including immunolabeling and staining techniques (hematoxylin and eosin, Alizarin red S and alcian blue). Plasma corticosterone was assayed using radioimmunoassay.

RESULTS

Maternal adrenalectomy does not improve the prognosis for LIFR null pups and exacerbates the effects of LIFR loss. RU486 noticeably improves many of the tissues affected by LIFR loss: bone density, skeletal muscle integrity and glial cell formation. LIFR null pups exposed during late gestation to RU486 in utero survive natural delivery, unlike LIFR null pups from untreated litters. But RU486 treated LIFR null pups succumb within the first day after birth, presumably due to neural deficit resulting in an inability to suckle.

CONCLUSION

LIFR plays an integral role in modulating the fetal response to elevated maternal glucocorticoids during late gestation. This role is likely to be mediated through the glucocorticoid receptor and has implications for adult homeostasis as a direct tie between immune, neural and hormone function.

Adrenomedullin--a regulator of bone formation

Bone growth and maintenance are highly regulated processes. Throughout life, bone constantly undergoes remodelling, maintaining a balance between bone formation by osteoblasts and bone resorption by osteoclasts. This balance depends on the coordinated activities of many systemic hormones and locally acting factors in the bone microenvironment. Understanding the mechanisms of action of these factors provides a better appreciation of the cellular and molecular basis of bone remodelling. Adrenomedullin has recently been found to stimulate the proliferation of osteoblasts in vitro, and to increase indices of bone formation when administered either locally or systemically in vivo. Adrenomedullin receptors, as well as adrenomedullin itself, are expressed in primary osteoblasts and in osteoblast-like cell lines. In this paper we discuss the anabolic effect of adrenomedullin in bone, and present new evidence for a possible role of adrenomedullin in the regulation of cartilage cells. We show that adrenomedullin stimulates proliferation of primary chondrocytes in culture and that mRNA for adrenomedullin and for adrenomedullin receptors are expressed in these cells. Studies of structure-activity relationships have demonstrated that osteotropic effects of adrenomedullin can be retained in peptide fragments of the molecule which lack the parent molecule's vasodilatory properties. Thus, these small peptides, or their analogues, are attractive candidates as anabolic therapies for osteoporosis.

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.

Age-related bone loss: old bone, new facts

The human skeleton serves several functions for the body: support, locomotion, protection of vital organs, and housing of bone marrow. Bone remodeling is the result of the interactions of multiple elements, including osteoblasts, osteoclasts, hormones, growth factors, and cytokines, the end result being the maintenance of the bone architecture and to maintain systemic calcium homeostasis. In early life, a careful balance exists between bone formation by osteoblasts and bone resorption by osteoclasts. With aging, the process of coupled bone formation is affected by the reduction of osteoblast differentiation, activity, and life span which is further potentiated in the perimenopausal years with hormone deprivation and increased osteoclast activity. Age-related bone loss is thus not only a consequence of hormone deprivation, but also the result of changes in bone formation and cell-cell interactions with a unique pathophysiology. In this review, we describe the cellular and metabolic changes associated with aging bone and present recent evidence regarding cell differentiation within the bone marrow. We also consider the mechanism of programmed cell death, apoptosis, as being an important determinant of aging in bone as well as describe possible future interventions to prolong the life span of osteoblasts.

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.

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.

Leukemia-inhibitory factor-neuroimmune modulator of endocrine function

Leukemia-inhibitory factor (LIF) is a pleiotropic cytokine expressed by multiple tissue types. The LIF receptor shares a common gp130 receptor subunit with the IL-6 cytokine superfamily. LIF signaling is mediated mainly by JAK-STAT (janus-kinase-signal transducer and activator of transcription) pathways and is abrogated by the SOCS (suppressor-of cytokine signaling) and PIAS (protein inhibitors of activated STAT) proteins. In addition to classic hematopoietic and neuronal actions, LIF plays a critical role in several endocrine functions including the utero-placental unit, the hypothalamo-pituitary-adrenal axis, bone cell metabolism, energy homeostasis, and hormonally responsive tumors. This paper reviews recent advances in our understanding of molecular mechanisms regulating LIF expression and action and also provides a systemic overview of LIF-mediated endocrine regulation. Local and systemic LIF serve to integrate multiple developmental and functional cell signals, culminating in maintaining appropriate hormonal and metabolic homeostasis. LIF thus functions as a critical molecular interface between the neuroimmune and endocrine systems.

Osteoblasts and stromal cells isolated from femora in rheumatoid arthritis (RA) and osteoarthritis (OA) patients express IL-11, leukaemia inhibitory factor and oncostatin M

We investigated both in vitro and ex vivo the role of mature osteoblasts (OB) and bone marrow stromal cells (BMSC) in RA and OA by analysing the expression of the following IL-6-type cytokines: IL-11, leukaemia inhibitory factor (LIF), oncostatin M (OSM) and IL-6. OB and BMSC were isolated from femora of RA, OA and post-traumatic (PT) patients, cultured in vitro in the presence or absence of IL-1beta and tumour necrosis factor-alpha (TNF-alpha), and assessed for the production and mRNA expression of IL-6-type cytokines. Trabecular bone biopsies were obtained from the inner portions of femoral heads and used for cytokine in situ immunostaining. Cultured OB and BMSC from different patients constitutively secreted IL-11 and IL-6 but not OSM. LIF was secreted only by BMSC, at very low levels. Interestingly, IL-11 basal production was significantly higher in BMSC than in OB in all three groups tested. IL-1beta and TNF-alpha strongly stimulated IL-6-type cytokine release (except for OSM) by both OB and BMSC. OSM was expressed only at mRNA levels in all groups studied. Cytokine immunostaining on bone biopsies confirmed the data obtained on cultured cells: IL-11, IL-6 and LIF proteins were detected both in mesenchymal (BMSC and OB) and mononuclear cells; OSM was found only in mononuclear cells. These data demonstrate that IL-6-type cytokines are constitutively expressed in the bone compartment in RA, OA and PT patients and can be secreted by bone cells at different stages of differentiation (BMSC and OB). This suggests that these cytokines may be involved in the mechanisms of bone remodelling in OA and RA.

Leukemia inhibitory factor, Interleukin 6, and other cytokines using the GP130 transducing receptor: roles in inflammation and injury

Inflammation refers to a complex set of mechanisms by which tissues respond to injury and infection. Among the many soluble mediators associated with this process, cytokines are known to be crucial in regulating a variety of cellular and molecular events. Leukemia inhibitory factor (LIF), interleukin 6 (IL-6), IL-11, and possibly other members of this cytokine family are key mediators in various inflammatory processes such as the acute-phase reaction, tissue damage, and infection. These cytokines can act in both pro-inflammatory and anti-inflammatory ways, depending on a number of variables. We emphasize here recent work utilizing knockout mice, which has highlighted the roles of LIF and IL-6, particularly in interactions between the immune and nervous systems.