Aged Callus Skeletal Stem/Progenitor Cells Contain an Inflammatory Osteogenic Population With Increased IRF and NF-κB Pathways and Reduced Osteogenic Potential

Skeletal stem/progenitor cells (SSPCs) are critical for fracture repair by providing osteo-chondro precursors in the callus, which is impaired in aging. However, the molecular signatures of callus SSPCs during aging are not known. Herein, we performed single-cell RNA sequencing on 11,957 CD45-CD31-Ter119- SSPCs isolated from young and aged mouse calluses. Combining unsupervised clustering, putative makers, and DEGs/pathway analyses, major SSPC clusters were annotated as osteogenic, proliferating, and adipogenic populations. The proliferating cluster had a differentiating potential into osteogenic and adipogenic lineages by trajectory analysis. The osteoblastic/adipogenic/proliferating potential of individual clusters was further evidenced by elevated expression of genes related to osteoblasts, adipocytes, or proliferation. The osteogenic cluster was sub-clustered into house-keeping and inflammatory osteogenic populations that were decreased and increased in aged callus, respectively. The majority of master regulators for the inflammatory osteogenic population belong to IRF and NF-κB families, which was confirmed by immunostaining, RT-qPCR, and Western blot analysis. Furthermore, cells in the inflammatory osteogenic sub-cluster had reduced osteoblast differentiation capacity. In conclusion, we identified 3 major clusters in callus SSPCs, confirming their heterogeneity and, importantly, increased IRF/NF-κB-mediated inflammatory osteogenic population with decreased osteogenic potential in aged cells.

The E3 ubiquitin ligase Itch limits the progression of post-traumatic osteoarthritis in mice by inhibiting macrophage polarization

OBJECTIVE

Osteoarthritis (OA) is characterized by articular cartilage loss, associated with synovial inflammation. We recently reported increased pro-inflammatory macrophages in murine post-traumatic OA (PTOA) joints, and blockade of the ubiquitin-proteasome system alleviates PTOA progression. However, the mechanisms whereby protein ubiquitination influences PTOA pathology are not well studied. We hypothesized that loss of the negative regulator of inflammation, E3 ligase Itch, in macrophages contributes to joint OA tissue damage by promoting pro-inflammatory polarization of macrophages.

METHODS

Mice deficient Itch in macrophages (MΔItch) were generated by crossing Itchfl/fl mice with LysM-Cre mice. PTOA surgery was performed on global Itch knockout, Itch-/-, mice and MΔItch mice. Joint tissue damage and synovial macrophages were examined. Itch-/- cells were treated with IL-1 and pro-inflammatory polarization was determined. Expression of Itch protein and mRNA in PTOA synovium were assessed at different time points post PTOA.

RESULTS

Similar to Itch-/- mice, MΔItch mice developed more severe joint damage than control mice following PTOA surgery (mean difference of OARSI score: 1.17 (95% CI 0.31-2.03) between MΔItch and Itchfl/fl mice), accompanied by increased the inflammatory macrophage infiltration in the synovium (mean difference of % F4/80 + CD86 + CD36-inflammatory macrophages: 14.81 (95% CI 8.90-20.73) between MΔItch and Itchfl/fl mice). Itch-/- macrophages exerted pro-inflammatory phenotype in response to IL-1β treatment. Itch protein, but not mRNA levels decreased during PTOA progression.

CONCLUSION

The negative regulator of inflammation, Itch, limits PTOA progression by inhibiting macrophage pro-inflammatory polarization. Itch protein degradation may contribute to PTOA pathology.

An update on the validation of whole slide imaging systems following FDA approval of a system for a routine pathology diagnostic service in the United States

Pathologists have used light microscopes and glass slides to interpret the histologic appearance of normal and diseased tissues for more than 150 years. The quality of both microtomes used to cut tissue sections and microscopes has improved significantly during the past few decades, but the process of rendering diagnoses has changed little. By contrast, major advances in digital technology have occurred since the introduction of hand held electronic devices, including the development of whole slide imaging (WSI) systems with software packages that can convert microscope images into virtual (digital) slides that can be viewed on computer monitors and via the internet. To date, however, these technological developments have had minimal impact on the way pathologists perform their daily work, with the exception of using computers to access electronic medical records and scholarly web sites for pertinent information to assist interpretation of cases. Traditional practice is likely to change significantly during the next decade, especially since the Federal Drug Administration in the USA has approved the first WSI system for routine diagnostic practice. I review here the development and slow acceptance of WSI by pathology departments. I focus on recent advances in validation of WSI systems that is required for routine diagnostic reporting of pathology cases using this technology.

Whole slide imaging: uses and limitations for surgical pathology and teaching

Advances in computer and software technology and in the quality of images produced by digital cameras together with development of robotic devices that can take glass histology slides from a cassette holding many slides and place them in a conventional microscope for electronic scanning have facilitated the development of whole slide imaging (WSI) systems during the past decade. Anatomic pathologists now have opportunities to test the utility of WSI systems for diagnostic, teaching and research purposes and to determine their limitations. Uses include rendering primary diagnoses from scanned hematoxylin and eosin stained tissues on slides, reviewing frozen section or routine slides from remote locations for interpretation or consultation. Also, WSI can replace physical storage of glass slides with digital images, storing images of slides from outside institutions, presenting slides at clinical or research conferences, teaching residents and medical students, and storing fluorescence images without fading or quenching of the fluorescence signal. Limitations include the high costs of the scanners, maintenance contracts and IT support, storage of digital files and pathologists' lack of familiarity with the technology. Costs are falling as more devices and systems are sold and cloud storage costs drop. Pathologist familiarity with the technology will grow as more institutions purchase WSI systems. The technology holds great promise for the future of anatomic pathology.

Advances in the regulation of osteoclasts and osteoclast functions

Osteoclasts are derived from mononuclear hematopoietic myeloid lineage cells, which are formed in the bone marrow and are attracted to the bloodstream by factors, including sphingsine-1 phosphate. These circulating precursors are attracted to bone surfaces undergoing resorption by chemokines and other factors expressed at these sites, where they fuse to form multinucleated bone-resorbing cells. All aspects of osteoclast formation and functions are regulated by macrophage-colony-stimulating factor (M-CSF) and receptor activator of NF-κB ligand (RANKL), cytokines essential for osteoclast formation and expressed by a variety of cell types, including osteoblast lineage cells. Since the discovery of RANKL in the mid-1990s, mouse genetic and molecular studies have revealed numerous signaling pathways activated by RANKL and M-CSF. More recent studies indicate that osteoclasts and their precursors regulate immune responses and osteoblast formation and functions by means of direct cell-cell contact through ligands and receptors, such as ephrins and Ephs, and semaphorins and plexins, and through expression of clastokines. There is also growing recognition that osteoclasts are immune cells with roles in immune responses beyond mediating the bone destruction that can accompany them. This article reviews recent advances in the understanding of the molecular mechanisms regulating osteoclast formation and functions and their interactions with other cells in normal and pathologic states.

Use of a whole-slide imaging system to assess the presence and alteration of lymphatic vessels in joint sections of arthritic mice

We investigated the presence and alteration of lymphatic vessels in joints of arthritic mice using a whole-slide imaging system. Joints and long bone sections were cut from paraffin blocks of two mouse models of arthritis: meniscal-ligamentous injury (MLI)-induced osteoarthritis (OA) and TNF transgene (TNF-Tg)-induced rheumatoid arthritis (RA). MLI-OA mice were fed a high fat diet to accelerate OA development. TNF-Tg mice were treated with lymphatic growth factor VEGF-C virus to stimulate lymphangiogenesis. Sections were double immunofluorescence stained with anti-podoplanin and alpha-smooth muscle actin antibodies. The area and number of lymphatic capillaries and mature lymphatic vessels were determined using a whole-slide imaging system and its associated software. Lymphatic vessels in joints were distributed in soft tissues mainly around the joint capsule, ligaments, fat pads and muscles. In long bones, enriched lymphatic vessels were present in the periosteal areas adjacent to the blood vessels. Occasionally, lymphatic vessels were observed in the cortical bone. Increased lymphatic capillaries, but decreased mature lymphatic vessels, were detected in both OA and RA joints. VEGF-C treatment increased lymphatic capillary and mature vessel formation in RA joints. Our findings suggest that the lymphatic system may play an important role in arthritis pathogenesis and treatment.

Polymicrobial periodontal pathogen transcriptomes in calvarial bone and soft tissue

Porphyromonas gingivalis, Treponema denticola, and Tannerella forsythia are consistently associated with adult periodontitis. This study sought to document the host transcriptome to a P. gingivalis, T. denticola, and T.forsythia challenge as a polymicrobial infection using a murine calvarial model of acute inflammation and bone resorption. Mice were infected with P. gingivalis, T. denticola, and T. forsythia over the calvaria, after which the soft tissues and calvarial bones were excised. A Murine GeneChip(®) array analysis of transcript profiles showed that 6997 genes were differentially expressed in calvarial bones (P < 0.05) and 1544 genes were differentially transcribed in the inflamed tissues after the polymicrobial infection. Of these genes, 4476 and 1035 genes in the infected bone and tissues were differentially expressed by upregulation. Biological pathways significantly impacted by the polymicrobial infection in calvarial bone included leukocyte transendothelial migration (LTM), cell adhesion molecules, adherens junction, major histocompatibility complex antigen, extracellular matrix-receptor interaction, and antigen processing and presentation resulting in inflammatory/cytokine/chemokine transcripts stimulation in bone and soft tissue. Intense inflammation and increased activated osteoclasts were observed in calvarias compared with sham-infected controls. Quantitative real-time RT-PCR analysis confirmed that the mRNA level of selected genes corresponded with the microarray expression. The polymicrobial infection regulated several LTM and extracellular membrane pathway genes in a manner distinct from mono-infection with P. gingivalis, T. denticola, or T. forsythia. To our knowledge, this is the first definition of the polymicrobially induced transcriptome in calvarial bone and soft tissue in response to periodontal pathogens.

The challenge and importance of standardizing pre-analytical variables in surgical pathology specimens for clinical care and translational research

The introduction of targeted cancer therapies into clinical practice, in which patients are selected for novel treatments based on results of companion molecular testing of their tumor specimens, has created significant new challenges for the surgical pathology laboratory. These include standardization of tissue handling and sample preparation with accurate documentation to ensure optimal quality of clinical samples to reduce the risk of errors in molecular biology tests. The assay of tumor tissues for biomarkers that can provide predictive data for prognosis or treatment should enable selection of the most appropriate therapies (Yaziji et al. 2008, Hicks and Kulkarni 2008). Major advances have been made in the ability to profile clinical samples for research at the DNA, RNA and protein levels. To translate this new information into the clinical setting, however, the quality of the starting material, in this case the tumor tissue, determines the accuracy and reliability of companion diagnostic assay results and therefore optimal therapeutic strategies. Inaccurate results owing to compromised tissue quality can lead to false positive or false negative results with therapeutic consequences that can harm patients and affect their eventual outcome.

Tannerella forsythia infection-induced calvarial bone and soft tissue transcriptional profiles

Tannerella forsythia is associated with subgingival biofilms in adult periodontitis, although the molecular mechanisms contributing to chronic inflammation and loss of periodontal bone remain unclear. We examined changes in the host transcriptional profiles during a T. forsythia infection using a murine calvarial model of inflammation and bone resorption. Tannerella forsythia was injected into the subcutaneous soft tissue over calvariae of BALB/c mice for 3 days, after which the soft tissues and calvarial bones were excised. RNA was isolated and Murine GeneChip (Affymetrix, Santa Clara, CA) array analysis of transcript profiles showed that 3226 genes were differentially expressed in the infected soft tissues (P < 0.05) and 2586 genes were differentially transcribed in calvarial bones after infection. Quantitative real-time reverse transcription-polymerase chain reaction analysis of transcription levels of selected genes corresponded well with the microarray results. Biological pathways significantly impacted by T. forsythia infection in calvarial bone and soft tissue included leukocyte transendothelial migration, cell adhesion molecules (immune system), extracellular matrix-receptor interaction, adherens junction, and antigen processing and presentation. Histologic examination revealed intense inflammation and increased osteoclasts in calvariae compared with controls. In conclusion, localized T. forsythia infection differentially induces transcription of a broad array of host genes, and the profiles differ between inflamed soft tissues and calvarial bone.

Molecular characterization of Treponema denticola infection-induced bone and soft tissue transcriptional profiles

Treponema denticola is associated with subgingival biofilms in adult periodontitis and with acute necrotizing ulcerative gingivitis. However, the molecular mechanisms by which T. denticola impacts periodontal inflammation and alveolar bone resorption remain unclear. Here, we examined changes in the host transcriptional profiles during a T. denticola infection using a murine calvarial model of inflammation and bone resorption. T. denticola was injected into the subcutaneous soft tissue over the calvaria of BALB/c mice for 3 days, after which the soft tissues and the calvarial bones were excised. RNA was isolated and analysed for transcript profiling using Murine GeneChip arrays. Following T. denticola infection, 2905 and 1234 genes in the infected calvarial bones and soft tissues, respectively, were differentially expressed (P <or= 0.05). Biological pathways significantly impacted by T. denticola infection in calvarial bone and calvarial tissue included leukocyte transendothelial migration, cell adhesion (immune system) molecules, cell cycle, extracellular matrix-receptor interaction, focal adhesion, B-cell receptor signaling and transforming growth factor-beta signaling pathways resulting in proinflammatory, chemotactic effects, and T-cell stimulation. In conclusion, localized T. denticola infection differentially induces transcription of a broad array of host genes, the profiles of which differed between inflamed calvarial bone and soft tissues.

Porphyromonas gingivalis infection-induced tissue and bone transcriptional profiles

Porphyromonas gingivalis has been associated with subgingival biofilms in adult periodontitis. However, the molecular mechanisms of its contribution to chronic gingival inflammation and loss of periodontal structural integrity remain unclear. This investigation aimed to examine changes in the host transcriptional profiles during a P. gingivalis infection using a murine calvarial model of inflammation and bone resorption. P. gingivalis FDC 381 was injected into the subcutaneous soft tissue over the calvaria of BALB/c mice for 3 days, after which the soft tissues and calvarial bones were excised. RNA was isolated from infected soft tissues and calvarial bones and was analysed for transcript profiles using Murine GeneChip((R)) arrays to provide a molecular profile of the events that occur following infection of these tissues. After P. gingivalis infection, 6452 and 2341 probe sets in the infected soft tissues and calvarial bone, respectively, were differentially expressed (P </= 0.05). Biological pathways significantly impacted by P. gingivalis infection in tissues and calvarial bone included cell adhesion (immune system) molecules, Toll-like receptors, B-cell receptor signaling, transforming growth factor-beta cytokine family receptor signaling, and major histocompatibility complex class II antigen processing pathways resulting in proinflammatory, chemotactic effects, T-cell stimulation, and downregulation of antiviral and T-cell chemotactic effects. P. gingivalis-induced inflammation activated osteoclasts, leading to local bone resorption. This is the first in vivo evidence that localized P. gingivalis infection differentially induces transcription of a broad array of host genes, the profiles of which differed between inflamed soft tissues and calvarial bone.

Tumor necrosis factor prevents alendronate-induced osteoclast apoptosis in vivo by stimulating Bcl-xL expression through Ets-2

OBJECTIVE

To investigate why bisphosphonates are less effective at preventing focal bone loss in rheumatoid arthritis (RA) patients than in those with generalized osteoporosis, and the mechanisms involved.

METHODS

The response of osteoclasts to alendronate (ALN) in tumor necrosis factor-transgenic (TNF-Tg) mice that develop erosive arthritis and in wild-type littermates was studied. TNF-Tg and wild-type mice were given ALN, and the osteoclast numbers in the inflamed joints and in the long bones were compared. The expression levels of Bcl-xL in the osteoclasts of TNF-Tg and wild-type mice were examined by immunostaining. The effect of overexpression of Bcl-xL and Ets-2 proteins on ALN-induced osteoclast apoptosis was determined using an in vitro osteoclast survival assay and retrovirus transfer approach.

RESULTS

ALN reduced osteoclast numbers in the metaphyses by 97%, but by only 46% in the adjacent inflamed joints. Bcl-xL expression was markedly higher in osteoclasts in the joints than in those in the metaphyses of TNF-Tg mice. Bcl-xL or Ets-2 overexpression protected osteoclasts from ALN-induced apoptosis, and TNF stimulated Bcl-xL and Ets-2 expression in osteoclasts. Overexpression of Ets-2 increased Bcl-xL messenger RNA in osteoclasts, while a dominant-negative form of the Ets-2 blocked the protective effect of Bcl-xL or TNF on ALN-induced apoptosis.

CONCLUSION

The reduced efficacy of bisphosphonates to stop bone erosion in the inflamed joints of RA patients may result from local high levels of TNF up-regulating Ets-2 expression in osteoclasts, which in turn stimulates Bcl-xL expression in them and reduces their susceptibility to bisphosphonate-induced apoptosis.

Evidence for a direct role of cyclo-oxygenase 2 in implant wear debris-induced osteolysis

Aseptic loosening is a major complication of prosthetic joint surgery and is manifested as chronic inflammation, pain, and osteolysis at the bone implant interface. The osteolysis is believed to be driven by a host inflammatory response to wear debris generated from the implant. In our current study, we use a selective inhibitor (celecoxib) of cyclo-oxygenase 2 (COX-2) and mice that lack either COX-1 (COX-1-/-) or COX-2 (COX-2-/-) to show that COX-2, but not COX-1, plays an important role in wear debris-induced osteolysis. Titanium (Ti) wear debris was implanted surgically onto the calvaria of the mice. An intense inflammatory reaction and extensive bone resorption, which closely resembles that observed in patients with aseptic loosening, developed within 10 days of implantation in wild-type and COX-1-/- mice. COX-2 and prostaglandin E2 (PGE2) production increased in the calvaria and inflammatory tissue overlying it after Ti implantation. Celecoxib (25 mg/kg per day) significantly reduced the inflammation, the local PGE2 production, and osteolysis. In comparison with wild-type and COX-1-/- mice, COX-2-/- mice implanted with Ti had a significantly reduced calvarial bone resorption response, independent of the inflammatory response, and significantly fewer osteoclasts were formed from cultures of their bone marrow cells. These results provide direct evidence that COX-2 is an important mediator of wear debris-induced osteolysis and suggests that COX-2 inhibitors are potential therapeutic agents for the prevention of wear debris-induced osteolysis.

Genetic evidence for a role for Src family kinases in TNF family receptor signaling and cell survival

Mutant src(-/-) mice have osteopetrosis resulting from defective osteoclasts, the cells that resorb bone. However, signaling pathways involving Src family members in osteoclasts remain unclear. We demonstrate that expression of a truncated Src molecule, Src251, lacking the kinase domain, induces osteopetrosis in wild-type and src(+/-) mice and worsens osteopetrosis in src(-/-) mice by a novel mechanism, increased osteoclast apoptosis. Induction of apoptosis by Src251 requires a functional SH2, but not an SH3, domain and is associated with reduced AKT kinase activity. Expression of Src251 dramatically reduces osteoclast survival in response to RANKL/TRANCE/OPGL, providing evidence that Src family kinases are required in vivo for survival signaling pathways downstream from TNF family receptors.

Decreased c-Src expression enhances osteoblast differentiation and bone formation

c-src deletion in mice leads to osteopetrosis as a result of reduced bone resorption due to an alteration of the osteoclast. We report that deletion/reduction of Src expression enhances osteoblast differentiation and bone formation, contributing to the increase in bone mass. Bone histomorphometry showed that bone formation was increased in Src null compared with wild-type mice. In vitro, alkaline phosphatase (ALP) activity and nodule mineralization were increased in primary calvarial cells and in SV40-immortalized osteoblasts from Src(-/-) relative to Src(+/+) mice. Src-antisense oligodeoxynucleotides (AS-src) reduced Src levels by approximately 60% and caused a similar increase in ALP activity and nodule mineralization in primary osteoblasts in vitro. Reduction in cell proliferation was observed in primary and immortalized Src(-/-) osteoblasts and in normal osteoblasts incubated with the AS-src. Semiquantitative reverse transcriptase-PCR revealed upregulation of ALP, Osf2/Cbfa1 transcription factor, PTH/PTHrP receptor, osteocalcin, and pro-alpha 2(I) collagen in Src-deficient osteoblasts. The expression of the bone matrix protein osteopontin remained unchanged. Based on these results, we conclude that the reduction of Src expression not only inhibits bone resorption, but also stimulates osteoblast differentiation and bone formation, suggesting that the osteogenic cells may contribute to the development of the osteopetrotic phenotype in Src-deficient mice.

A murine model of inflammatory bone disease

We have recently reported the identification of a new recessive mutation on murine chromosome 18 that results in tail kinks and deformity in the lower extremities of mice. Preliminary examination of the bones of these mice showed that there are abnormalities present that resembled chronic recurrent multifocal osteomyelitis. Accordingly, this new mutation was named "CMO." In this report, we describe the histology of bones in CMO mice, as well as the capacity of the bone marrow cells from these animals to form osteoclasts (OCLs). In addition, we tested conditioned media from non-adherent marrow cells and total marrow cells from CMO mice for their capacity to induce OCL formation in normal murine marrow cultures. These studies demonstrated that the bone disease in these animals is inflammatory in nature, and a soluble factor(s) that is not IL-1alpha, IL-6 or TNF-alpha is released by marrow cells from CMO animals and enhances OCL formation in normal murine marrow cultures.

Identification of human asparaginyl endopeptidase (legumain) as an inhibitor of osteoclast formation and bone resorption

We screened a human osteoclast (OCL) cDNA expression library for OCL inhibitory factors and identified a clone that blocked both human and murine OCL formation and bone resorption by more than 60%. This clone was identical to human legumain, a cysteine endopeptidase. Legumain significantly inhibited OCL-like multinucleated cell formation induced by 1,25-dihydroxyvitamin D(3) (1,25-(OH)(2)D(3)) and parathyroid hormone-related protein (PTHrP) in mouse and human bone marrow cultures, and bone resorption in the fetal rat long bone assay in a dose-dependent manner. Legumain was detected in freshly isolated marrow plasma from normal donors and conditioned media from human marrow cultures. Furthermore, treatment of human marrow cultures with an antibody to legumain induced OCL formation to levels that were as high as those induced by 1,25-(OH)(2)D(3). Implantation in nude mice of 293 cells transfected with the legumain cDNA and constitutively expressing high levels of the protein significantly reduced hypercalcemia induced by PTHrP by about 50%, and significantly inhibited the increase in OCL surface and in OCL number expressed per mm(2) bone area and per mm bone surface induced by PTHrP. These results suggest that legumain may be a physiologic local regulator of OCL activity that can negatively modulate OCL formation and activity.

Factors regulating the growth of metastatic cancer in bone

Metastatic tumor cells can interfere directly with the function of bone cells involved in normal bone remodeling or indirectly by influencing the behavior of hematopoietic, stromal and other cells in bone marrow that interact with bone cells. Recent studies of metastatic cancer have revealed that tumor cells interact closely with vascular endothelial cells, basement membrane and bone marrow stromal cells through cell surface proteins or by releasing factors which affect the function of these cells. Bidirectional interaction between marrow cells and tumor cells can give the latter a selective advantage for growth in bone which can lead to the destruction of or to increased production of bone matrix. Understanding of the mechanisms involved in tumor metastasis and growth in bone has increased in recent years, and in this review we shall describe current knowledge of these mechanisms and of the predilection of certain types of cancers to metastasize to bone, their growth in the bone microenvironment and interactions between them and bone cells. Because metastatic breast cancer has been studied more than any other, we shall focus on it as a representative example, although the general principles apply to other types of cancer and to myeloma.

Required and nonessential functions of nuclear factor-kappa B in bone cells

Nuclear factor-kappa B (NF-kappaB) is a set of five polypeptide transcription factors, called p50, p52, p65 (also called Rel A), Rel B, and c-Rel, which regulate the expression of a variety of genes involved in immune and inflammatory responses. They were originally named because they were considered essential regulators of B cell kappa light chain expression. More recent studies indicate that NF-kappaB proteins are involved in the regulation of a variety of other cell functions, including cell proliferation, responses to stress, and apoptosis. NF-kappaB heterodimers reside in the cytoplasm of cells bound to inhibitory proteins, the two commonest of which are IkappaBalpha and IkappaBbeta, which prevent NF-kappaB from entering the nucleus. When cells are stimulated, IkappaB is phosphorylated by specific IkappaB kinases and subsequently is ubiquitinated and degraded in proteosomes. This allows NF-kappaB to translocate to the nucleus to regulate the expression of a growing list of genes, including the proinflammatory cytokines, interleukin-1 (IL-1), IL-6, and tumor necrosis factor. IL-1 and tumor necrosis factor in turn also regulate the expression of NF-kappaB. Thus, once activated, NF-kappaB may be involved in upregulatory loops, which can amplify the effects of the initiating stimulus. Because these proinflammatory cytokines have been implicated in the pathogenesis of estrogen deficiency and inflammation-related bone loss, it is likely that NF-kappaB has a significant role in the increased generation and function of osteoclasts in these circumstances. However, an unexpected and essential role of NF-kappaB in the formation of osteoclasts during development was discovered recently after the generation of knockout mice, which lack the expression of the p50 and p52 subunits. This paper will describe recent studies that reveal an essential role for NF-kappaB signaling in the generation of osteoclasts and that suggest that NF-kappaB may also play a key central role in the activation and survival of osteoclasts in conditions in which osteoclastogenesis is upregulated.

Dental abnormalities associated with failure of tooth eruption in src knockout and op/op mice

c-src knockout and op/op mice develop osteopetrosis as a result of defective osteoclast function and osteoclast formation, respectively. The mutant mice can be distinguished readily from their wild-type littermates around 10-12 days after birth because their incisors do not erupt, but the morphology of their teeth and surrounding bone has not been reported previously in detail. Histologic examination of jaws of src-mutant mice reveals unerupted, abnormal incisors within their bony crypts. The tooth roots are distorted by foci of haphazard proliferation of odontogenic epithelium associated with primitive tooth structures that strongly resemble the tumor-like lesions in humans, known as odontomas. The crowns of the incisors are fused to the adjacent bone, and the developing periodontal ligament is disordered and hypocellular. Osteoclasts are present in the bone surrounding the distorted teeth, but as in other bones in these mice they lack ruffled borders and thus do not resorb effectively. Similar odontogenic proliferation is present around unerupted incisors in op/op mice which form very few osteoclasts, but the amount is significantly less than in src mutant mice. Molars fail to erupt in both types of mutant mice, but they are not accompanied by aberrant odontogenic proliferation. These findings and previous reports of similar abnormalities in jaws from op/op rats suggest that failure of incisor eruption and associated proliferation of odontogenic epithelium in osteopetrotic rodents are a direct result of defective osteoclastic bone resorption.

Systemic administration of acidic fibroblast growth factor (FGF-1) prevents bone loss and increases new bone formation in ovariectomized rats

There are no universally accepted agents that will substantially increase bone mass in osteoporotic patients. A number of peptides important in normal bone formation, such as members of the transforming growth factor-beta superfamily, are not satisfactory for this purpose either because their beneficial effects are predominantly local or there is systemic toxicity associated with their administration. We have examined the effects of exogenous fibroblast growth factor-1 and -2 (FGF-1 and FGF-2) on bone in vivo, since FGFs have been shown recently to be essential for normal skeletal development. FGF-1 was injected daily (0.2 mg/kg intravenously) for 28 days into the tail vein of adult female rats immediately following and 6 months after sham operation or ovariectomy (OVX). In rats treated immediately post-OVX, OVX produced more than a 30% decrease in tibial bone density, which was prevented by FGF-1 and estrogen. However, FGF-1 also had an anabolic effect. In sham-operated rats, FGF-1 increased bone density to 2-fold, whereas estrogen had no effect. In rats 6 months post-OVX, severe bone loss and disruption of trabecular microarchitecture occurred similar to that seen in patients with severe osteoporosis. In these rats, administration of FGF-1 induced extensive new woven bone formation with new trabecular-like structures filling much of the marrow spaces, and bone density in the tibial metaphysis increased 3-fold. FGF-1 and FGF-2 were also administered subcutaneously over the calvaria of mice in doses of 2-2000 microg/day for 3 days and shown to produce substantial increases in bone formation when examined morphologically. Thus, we conclude that both local and systemic FGF-1 increases new bone formation and bone density, and systemic FGF-1 also appears to restore bone microarchitecture and prevent bone loss associated with estrogen-withdrawal.

Ibandronate reduces osteolytic lesions but not tumor burden in a murine model of myeloma bone disease

We determined the effects of the potent bisphosphonate ibandronate in a murine model of human myeloma bone disease. In this model, bone lesions typical of the human disease develop in mice following inoculation of myeloma cells via the tail vein. Treatment with ibandronate (4 micrograms per mouse per day) significantly reduced the occurrence of osteolytic bone lesions in myeloma-bearing mice. However, ibandronate did not prevent the mice from developing hindlimb paralysis and did not produce a detectable effect on survival. There was no significant effect of ibandronate on total myeloma cell burden, as assessed by morphometric measurements of myeloma cells in the bone marrow, liver, and spleen, or by measurement of serum IgG2b levels. These results support clinical findings that bisphosphonates may be useful for the treatment of myeloma-associated bone destruction, but suggest that other therapies are also required to reduce tumor growth.

Bone resorption induced by parathyroid hormone is strikingly diminished in collagenase-resistant mutant mice

Parathyroid hormone (PTH) stimulates bone resorption by acting directly on osteoblasts/stromal cells and then indirectly to increase differentiation and function of osteoclasts. PTH acting on osteoblasts/stromal cells increases collagenase gene transcription and synthesis. To assess the role of collagenase in the bone resorptive actions of PTH, we used mice homozygous (r/r) for a targeted mutation (r) in Col1a1 that are resistant to collagenase cleavage of type I collagen. Human PTH(1-34) was injected subcutaneously over the hemicalvariae in wild-type (+/+) or r/r mice four times daily for three days. Osteoclast numbers, the size of the bone marrow spaces and periosteal proliferation were increased in calvariae from PTH-treated +/+ mice, whereas in r/r mice, PTH-induced bone resorption responses were minimal. The r/r mice were not resistant to other skeletal effects of PTH because abundant interstitial collagenase mRNA was detected in the calvarial periosteum of PTH-treated, but not vehicle-treated, r/r and +/+ mice. Calcemic responses, 0.5-10 hours after intraperitoneal injection of PTH, were blunted in r/r mice versus +/+ mice. Thus, collagenase cleavage of type I collagen is necessary for PTH induction of osteoclastic bone resorption.

Recent advances in bone biology provide insight into the pathogenesis of bone diseases

Bone is modeled during embryonic development by endochondral and membranous ossification and is continuously remodeled thereafter under the influence of local and systemic factors to provide structural support and assist in calcium homeostasis. Recent studies of knockout and transgenic mice have increased understanding of the regulation of bone modeling during development and of remodeling of mature bone and have shed new light on the pathogenesis of a number of bone disorders. For example, fibroblast growth factor receptor-3, parathyroid hormone-related protein, and tartrate-resistant acid phosphatase affect the function of chondrocytes during endochondral ossification (the latter two by regulating their life spans and thus growth plate thickness and bone length). Some ubiquitously expressed genes seem unexpectedly to have unique functions that are largely confined to bone cells: M-CSF, C-Fos, PU.1, and NF-kappaB are required for osteoclast formation, whereas c-Src and Mitf (microphthalmia transcription factor) are required for osteoclast activity after the cells have formed. Knockout of these genes results in osteopetrosis, a disorder characterized by persistence in marrow cavities of unresorbed osteocartilaginous matrix and, as in some affected humans, by increased mortality. Some proteins seem to act as negative regulators of bone cell function, for example osteoprotegerin (a soluble TNF receptor) in osteoclasts; osteocalcin, bone sialoprotein, and 5-lipoxygenase in osteoblasts. Regulation of osteoclast life span may be an important mechanism by which estrogen and bisphosphonates prevent bone loss in conditions characterized by increased bone resorption, such as postmenopausal osteoporosis. The unique requirement of bone cells for certain gene products raises the possibility that these cells may have specific responses to inhibitory or stimulatory agents, and that signaling molecules in these response pathways could be specific targets for novel therapies to treat or prevent common bone diseases.

Interleukin-6-type cytokines stimulate mesenchymal progenitor differentiation toward the osteoblastic lineage

Cytokines that transduce their signals either through glycoprotein 130 (gp130) homodimers or gp 130/leukemia inhibitory factor (LIF) receptor beta heterodimers are potent inducers of osteoclast development in vitro as well as in vivo; and interleukin (IL)-6 has been recognized as an important pathogenic factor in diseases characterized by increased bone remodeling, such as the osteoporosis of sex steroid deficiency. Based on evidence that the same cytokines can also promote committed osteoblast differentiation and stimulate bone formation in vitro and in vivo and that mesenchymal cell differentiation toward the osteoblast lineage may be a prerequisite for osteoclastogenesis, we have investigated whether gp130 activation can affect the differentiation of uncommitted mesenchymal progenitors. Using as our model murine embryonic fibroblasts (EF), we found that IL-6 or IL-11 in combination with their soluble receptors (sIL-6R or sIL-11R) increased dose-dependently the number of alkaline phosphatase (AP)-positive cells in 3-6-day-long cultures. Moreover, EF cells maintained with IL-6/sIL-6R in the presence of ascorbic acid and beta-glycerophosphate expressed osteocalcin messenger RNA (mRNA) by 2 weeks and formed a matrix containing mineralized collagen fibers by 3 weeks. This prodifferentiation effect was specific for the osteoblastic lineage, as we found no evidence for increased differentiation of chondrocytes, adipocytes, or muscle cells. Unlike IL-6/sIL-6R, LIF, oncostatin M (OSM), and ciliary neurotrophic factor (CNTF) did not promote osteoblastic differentiation of EF cells. This pattern of specificity was accounted for by the finding that EF cells express gp130, but not the ligand-binding subunit of the IL-6 receptor (gp80) nor the LIF receptor beta. These observations add credence to the contention that increased production of gp130-utilizing cytokines and their receptors in pathological conditions like sex steroid deficiency is indeed responsible for not only the increased osteoclastogenesis, but also the increased osteoblastogenesis, and thereby for the increased rate of bone remodeling.

Bone resorption caused by three periodontal pathogens in vivo in mice is mediated in part by prostaglandin

Gingival inflammation, bacterial infection, alveolar bone destruction, and subsequent tooth loss are characteristic features of periodontal disease, but the precise mechanisms of bone loss are poorly understood. Most animal models of the disease require injury to gingival tissues or teeth, and the effects of microorganisms are thus complicated by host responses to tissue destruction. To determine whether three putative periodontal pathogens, Porphyromonas gingivalis, Campylobacter rectus, and Fusobacterium nucleatum, could cause localized bone resorption in vivo in the absence of tissue injury, we injected live or heat-killed preparations of these microorganisms into the subcutaneous tissues overlying the calvaria of normal mice once daily for 6 days and then examined the bones histologically. We found that all three microorganisms (both live and heat killed) stimulated bone resorption and that the strain of F. nucleatum used appeared to be the strongest inducer of osteoclast activity. Treatment of the mice concomitantly with indomethacin reduced but did not completely inhibit bone resorption by these microorganisms, suggesting that their effects were mediated, in part, by arachidonic acid metabolites (e.g., prostaglandins). Our findings indicate that these potential pathogens can stimulate bone resorption locally when placed beside a bone surface in vivo in the absence of prior tissue injury and support a role for them in the pathogenesis of bone loss around teeth in periodontitis.

Immortalization of osteoclast precursors by targeting Bcl -XL and Simian virus 40 large T antigen to the osteoclast lineage in transgenic mice

Cellular and molecular characterization of osteoclasts (OCL) has been extremely difficult since OCL are rare cells, and are difficult to isolate in large numbers. We used the tartrate-resistant acid phosphatase promoter to target the bcl-XL and/or Simian Virus 40 large T antigen (Tag) genes to cells in the OCL lineage in transgenic mice as a means of immortalizing OCL precursors. Immunocytochemical studies confirmed that we had targeted Bcl-XL and/or Tag to OCL, and transformed and mitotic OCL were readily apparent in bones from both Tag and bcl-XL/Tag mice. OCL formation in primary bone marrow cultures from bcl-XL, Tag, or bcl-XL/Tag mice was twofold greater compared with that of nontransgenic littermates. Bone marrow cells from bcl-XL/Tag mice, but not from singly transgenic bcl-XL or Tag mice, have survived in continuous culture for more than a year. These cells form high numbers of bone-resorbing OCL when cultured using standard conditions for inducing OCL formation, with approximately 50% of the mononuclear cells incorporated into OCL. The OCL that form express calcitonin receptors and contract in response to calcitonin. Studies examining the proliferative capacity and the resistance of OCL precursors from these transgenic mice to apoptosis demonstrated that the increased numbers of OCL precursors in marrow from bcl-XL/Tag mice was due to their increased survival rather than an increased proliferative capacity compared with Tag, bcl-XL, or normal mice. Histomorphometric studies of bones from bcl-XL/Tag mice also confirmed that there were increased numbers of OCL precursors (TRAP + mononuclear cells) present in vivo. These data demonstrate that by targeting both bcl-XL and Tag to cells in the OCL lineage, we have immortalized OCL precursors that form bone-resorbing OCL with an efficiency that is 300-500 times greater than that of normal marrow.

Requirement for NF-kappaB in osteoclast and B-cell development

NF-kappaB is a family of related, dimeric transcription factors that are readily activated in cells by signals associated with stress or pathogens. These factors are critical to host defense, as demonstrated previously with mice deficient in individual subunits of NF-kappaB. We have generated mice deficient in both the p50 and p52 subunits of NF-kappaB to reveal critical functions that may be shared by these two highly homologous proteins. We now demonstrate that unlike the respective single knockout mice, the p50/p52 double knockout mice fail to generate mature osteoclasts and B cells, apparently because of defects that track with these lineages in adoptive transfer experiments. Furthermore, these mice present markedly impaired thymic and splenic architectures and impaired macrophage functions. The blocks in osteoclast and B-cell maturation were unexpected. Lack of mature osteoclasts caused severe osteopetrosis, a family of diseases characterized by impaired osteoclastic bone resorption. These findings now establish critical roles for NF-kappaB in development and expand its repertoire of roles in the physiology of differentiated hematopoietic cells.

Rescue of osteoclast function by transgenic expression of kinase-deficient Src in src-/- mutant mice

The Src tyrosine kinase has been implicated in a wide variety of signal transduction pathways, yet despite the nearly ubiquitous expression of c-src, src-/- mice show only one major phenotype-osteopetrosis caused by an intrinsic defect in osteoclasts, the cells responsible for resorbing bone. To explore further the role of Src both in osteoclasts and other cell types, we have generated transgenic mice that express the wild-type and mutated versions of the chicken c-src proto-oncogene from the promoter of tartrate resistant acid phosphatase (TRAP), a gene that is expressed highly in osteoclasts. We demonstrate here that expression of a wild-type transgene in only a limited number of tissues can fully rescue the src-/- phenotype. Surprisingly, expression of kinase-defective alleles of c-src also reduces osteopetrosis in src-/- animals and partially rescues a defect in cytoskeletal organization observed in src-/- osteoclasts. These results suggest that there are essential kinase-independent functions for Src in vivo. Biochemical examination of osteoclasts from these mice suggest that Src may function in part by recruiting or activating other tyrosine kinases.

Tumor necrosis factor enhances parathyroid hormone-related protein-induced hypercalcemia and bone resorption without inhibiting bone formation in vivo

Humoral hypercalcemia of malignancy results from the effects of tumor-produced factors on bone, kidney, and intestine that disrupt normal calcium homeostasis. Although parathyroid hormone-related protein (PTHrP) is a major mediator of the syndrome, tumors also produce other hypercalcemic factors, such as tumor necrosis factor (TNF), which may modulate the effects of PTHrP. It has been postulated that TNF may counteract the stimulatory effects of PTHrP on bone formation. To examine the effects of TNF on PTHrP-induced changes in calcium and bone metabolism, a murine tumor model of hypercalcemia was used. Nude mice were inoculated with Chinese hamster ovarian (CHO) cells expressing human TNF (CHO/TNF) or nontransfected CHO cells (CHO/-) and further treated with injections of human PTHrP(1-34) or vehicle. The effects of TNF, PTHrP, and the combination of the two factors on blood ionized calcium, osteoclast recruitment, and bone histomorphometry were evaluated. Mice bearing CHO/TNF tumors that were injected with PTHrP had significantly higher calcium concentrations, increased committed osteoclast progenitors, and mature osteoclasts as well as enhanced bone resorption compared with mice bearing CHO/TNF tumors injected with vehicle or those bearing CHO/- tumors injected with PTHrP or vehicle. A 2-fold increase in new woven bone formed in the calvaria at sites of previous bone resorption was observed in CHO/TNF mice treated with PTHrP. Bone formation rates in the vertebrae were similar in both CHO/- and CHO/TNF mice treated with PTHrP. These data demonstrate that the hypercalcemic effects of PTHrP are enhanced by TNF and that this effect is due to the increased production of committed osteoclast precursors with a subsequent increase in osteoclastic bone resorption. Furthermore, PTHrP caused a coupled increase in osteoclastic bone resorption and new bone formation that was not inhibited by TNF. These findings highlight the complex interactions that may occur between tumor-produced factors on bone that result in malignancy-associated hypercalcemia and suggest that TNF may not be responsible for the decreased bone formation seen in some patients with this condition.

Apoptosis in bone physiology and disease

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Leukotriene B4 stimulates osteoclastic bone resorption both in vitro and in vivo

Upon activation, the enzyme 5-lipoxygenase converts arachidonic acid into principally three products, the peptidoleukotrienes, 5-hydroperoxyeicosatetraenoic acid (5-HETE) or the leukotriene B4. We have shown that the peptido-leukotrienes (known as LTC4, LTD4, or LTE4) and 5-HETE induce osteoclastic bone resorption and that receptors for LTD4 are present on isolated avian osteoclast-like cells. Here, we show the effects of the third metabolic product of the 5-lipoxygenase (5-LO) pathway of arachidonic acid metabolism, the leukotriene LTB4, on osteoclastic bone resorption both in vivo and in vitro. Because LTB4 production is increased in a number of inflammatory conditions, it may be an important contributor to the bone loss which occurs in these disorders. LTB4 increased osteoclastic bone resorption in vivo following local administration over the calvariae of normal mice and in vitro in organ cultures of neonatal mouse calvariae. When LTB4 was injected over the calvaria of mice, there was a significant increase in bone resorption, osteoclast numbers, and eroded surfaces. LTB4 also increased the formation of resorption lacunae by isolated neonatal rat osteoclasts. Greater potency was observed with LTB4 compared with the peptido-leukotriene LTD4. This is in contrast to prostaglandins of the E series, which are reported to inhibit isolated osteoclasts. Experiments using marrow cultures suggest that LTB4 stimulates bone resorption in part by enhancing the formation of osteoclasts.

Estrogen promotes apoptosis of murine osteoclasts mediated by TGF-beta

Postmenopausal osteoporosis, the most common bone disease in the developed world, is associated with estrogen deficiency. This deficiency induces increased generation and activity of osteoclasts, which perforate bone trabeculae, thus reducing their strength and increasing fracture risk. Estrogen replacement prevents these effects, indicating that estrogen negatively regulates osteoclast formation and function, but how it does this is unclear. Because functional osteoclast life span and thus the amount of bone that osteoclasts resorb could also be enhanced following estrogen deficiency, and since sex steroids regulate apoptosis in other target tissues, we investigated whether estrogen may affect osteoclast function by promoting apoptosis. 17 beta-Estradiol promoted apoptosis of murine osteoclasts in vitro and in vivo by two- to threefold. Tamoxifen, which has estrogenic effects on bone resorption, and transforming growth factor-beta 1 (TGF-beta), whose production by osteoblasts is increased by estrogen, had similar effects in vitro. Anti-TGF-beta antibody inhibited TGF-beta-, estrogen- and tamoxifen-induced osteoclast apoptosis, indicating that TGF-beta might mediate this effect. These findings suggest that estrogen may prevent excessive bone loss before and after the menopause by limiting osteoclast life span through promotion of apoptosis. The development of analogues to promote this mechanism specifically could be a useful and novel therapeutic approach to prevent postmenopausal osteoporosis.

Evidence for a causal role of parathyroid hormone-related protein in the pathogenesis of human breast cancer-mediated osteolysis

Breast cancer almost invariably metastasizes to bone in patients with advanced disease and causes local osteolysis. Much of the morbidity of advanced breast cancer is a consequence of this process. Despite the importance of the problem, little is known of the pathophysiology of local osteolysis in the skeleton or its prevention and treatment. Observations in patients with bone metastases suggest that breast cancer cells in bone express parathyroid hormone-related protein (PTHrP) more frequently than in soft tissue sites of metastasis or in the primary tumor. Thus, the role of PTHrP in the causation of breast cancer metastases in bone was examined using human breast cancer cell lines. Four of eight established human breast cancer cell lines expressed PTHrP and one of these cell lines, MDA-MB-231, was studied in detail using an in vivo model of osteolytic metastases. Mice inoculated with MDA-MB-231 cells developed osteolytic bone metastasis without hypercalcemia or increased plasma PTHrP concentrations. PTHrP concentrations in bone marrow plasma from femurs affected with osteolytic lesions were increased 2.5-fold over corresponding plasma PTHrP concentrations. In a separate experiment, mice were treated with either a monoclonal antibody directed against PTHrP(1-34), control IgG, or nothing before tumor inoculation with MDA-MB-231 and twice per week for 26 d. Total area of osteolytic lesions was significantly lower in mice treated with PTHrP antibodies compared with mice receiving control IgG or no treatment. Histomorphometric analysis of bone revealed decreased osteoclast number per millimeter of tumor/bone interface and increased bone area, as well as decreased tumor area, in tumor-bearing animals treated with PTHrP antibodies compared with respective controls. These results indicate that tumor-produced PTHrP can cause local bone destruction in breast cancer metastatic to bone, even in the absence of hypercalcemia or increased circulating plasma concentrations of PTHrP. Thus, PTHrP may have an important pathogenetic role in the establishment of osteolytic bone lesions in breast cancer. Neutralizing antibodies to PTHrP may reduce the development of destructive bone lesions as well as the growth of tumor cells in bone.

Metastatic prostate cancer in a transgenic mouse

We have previously reported the development of a transgenic mouse model for prostate cancer derived from PB-Tag transgenic line 8247, henceforth designated the TRAMP (transgenic adenocarcinoma mouse prostate) model. We now describe the temporal and spatial consequences of transgene expression and report the identification and characterization of metastatic disease in the TRAMP model. TRAMP mice characteristically express the T antigen oncoprotein by 8 weeks of age and develop distinct pathology in the epithelium of the dorsolateral prostate by 10 weeks of age. Distant site metastases can be detected as early as 12 weeks of age. The common sites of metastases are the periaortic lymph nodes and lungs, with occasional metastases to the kidney, adrenal gland, and bone. By 28 weeks of age, 100% harbor metastatic prostate cancer in the lymph nodes or lungs. We have also demonstrated the loss of normal E-cadherin expression, as observed in human prostate cancer, as primary tumors become less differentiated and metastasize. The TRAMP model provides a consistent source of primary and metastatic tumors for histopathobiological and molecular analysis to further define the earliest molecular events involved in the genesis, progression, and metastasis of prostate cancer.

Targeting simian virus 40 T antigen to the osteoclast in transgenic mice causes osteoclast tumors and transformation and apoptosis of osteoclasts

Osteoclasts are terminally differentiated cells that express tartrate-resistant acid phosphatase (TRAP) at a higher level than other normal cells. Therefore, in an attempt to develop immortalized osteoclasts, we produced two lines of transgenic mice in which expression of the simian virus 40 T antigen oncogene was targeted to osteoclasts using the TRAP gene promoter. Osteoclasts were increased in number in bones from both lines. More than 50% of them appeared morphologically transformed, 2-5% were mitotic, but, unexpectedly, 5% were apoptotic. Osteoclast tumors were observed occasionally in one line of mice (line 4), and sheets of TRAP-positive cells (tumorlets) developed in most mice in both lines. Although cells isolated from these tumorlets formed multinucleated TRAP-positive cells that resorbed bone in vitro, to date we have been unable to develop an immortalized osteoclast cell line from them. Osteoclasts from one line (line 5) had reduced ruffled border formation and a higher level of T-antigen expression than osteoclasts in the other line (line 4), and these features were associated with the presence of osteopetrosis. However, osteoclasts from these osteopetrotic mice and from line 4 mice resorbed bone normally when the mice were treated with interleukin-1. These findings indicate that T antigen can be targeted to osteoclasts in transgenic mice and causes osteoclast transformation, tumors, mitosis, and apoptosis. When T antigen is expressed at high levels, functional impairment of osteoclasts can be detected. Furthermore, these results suggest that T antigen is insufficient on its own to immortalize cells in the osteoclast lineage.

Bisphosphonates promote apoptosis in murine osteoclasts in vitro and in vivo

Bisphosphonates inhibit bone resorption and are therapeutically effective in diseases of increased bone turnover, such as Paget's disease and hypercalcemia of malignancy. The mechanisms by which they act remain unclear. Proposed mechanisms include inhibition of osteoclast formation from precursors and inhibitory or toxic effect on mature osteoclasts. We have developed a new in vitro model to study osteoclast survival and in this paper present in vitro and in vivo evidence that may explain both the observed reduction in osteoclast numbers and in bone resorption by mature osteoclasts, namely that bisphosphonates induce programmed cell death (apoptosis). Three bisphosphonates (risedronate, pamidronate, and clodronate) caused a 4- to 24-fold increase in the proportion of osteoclasts showing the characteristic morphology of apoptosis in vitro. This observation was confirmed in vivo in normal mice, in mice with increased bone resorption, and in nude mice with osteolytic cancer metastases, with similar-fold increases to those observed in vitro. Of the three compounds, risedronate, the most potent inhibitor of bone resorption in vivo, was the strongest inducer of osteoclast apoptosis in vitro. Osteoclast apoptosis may therefore be a major mechanism whereby bisphosphonates reduce osteoclast numbers and activity, and induction of apoptosis could be a therapeutic goal for new antiosteoclast drugs.

Bisphosphonate risedronate reduces metastatic human breast cancer burden in bone in nude mice

Human breast cancer frequently metastasizes to the skeleton to cause osteolysis and subsequent pain, pathological fracture, and hypercalcemia. Because bone continuously releases growth factors stored in bone matrix by bone resorption during physiological remodeling and, thus, possibly provides a favorable microenvironment for metastatic breast cancer cells to proliferate, inhibitors of bone resorption used either prophylactically or in patients with established disease, therefore, would seem likely to be useful adjuvant therapy in patients with breast cancer. However, the parameters for monitoring progressive osteolytic bone disease in humans are imprecise. We examined the effects of the third generation bisphosphonate, risedronate, which is a specific inhibitor of osteoclastic bone resorption, in a bone metastasis model in nude mice in which intracardiac injection of the human breast cancer cell line MDA-231 leads to osteolytic bone metastases. Risedronate (4 micrograms/animal/day) was given s.c. to animals (a) after radiologically small but defined osteolytic metastases were observed; (b) simultaneously with MDA-231 cell inoculation through the entire experimental period; or (c) by short-term prophylactic administration before inoculation of MDA-231 cells. In all experiments, risedronate either slowed progression or inhibited the development of bone metastases assessed radiographically. Furthermore, mice treated continuously with risedronate showed significantly longer survival than did control mice. Histomorphometrical analysis revealed that osteoclast numbers were diminished at metastatic tumor sites. Unexpectedly, there was also a marked decrease in tumor burden in bone in risedronate-treated animals. In contrast, the growth of metastatic breast cancer in soft tissues surrounding bones was not affected by risedronate. Moreover, risedronate had no effects on the local growth of s.c. implanted MDA-231 breast cancers in nude mice or on MDA-231 cell proliferation in culture. These data demonstrate that risedronate decreases metastatic MDA-231 breast cancer burden selectively in bone, as well as suppresses progression of established osteolytic lesions and prevents the development of new osteolytic lesions; thus, the data suggest that inhibition of osteoclastic bone resorption may be a useful adjunctive therapy for the treatment of cancers that have colonized in bone.

An in vivo model of human multiple myeloma bone disease

Osteolytic bone destruction and its complications, such as hypercalcemia, pathologic fractures and nerve compression, are the major source of morbidity in patients with multiple myeloma (MM). The bone destruction in MM is due to increased osteoclast activity, but the mechanisms responsible are not entirely clear. We have utilized a human plasma cell leukemia cell line, ARH-77, that has disseminated growth in mice with severe combined immunodeficiency (SCID) and expresses immunoglobulin G kappa (IgG kappa), as a model for human MM. Fifteen SCID mice were irradiated with 400R and 10 of these were injected with 10(6) ARH-77 cells i.v., 24 h after irradiation. Five mice were used as a control group. Development of bone disease was assessed by blood calcium levels, x-rays and histology. Seven out of seven mice that survived irradiation and received ARH-77 cells developed hind limb paralysis 28-35 days after injection. One hundred percent of these mice developed hypercalcemia (1.35-1.46 mmol/l), a mean of five days after becoming paraplegic. Lytic bone lesions were detected by x-ray in all the hypercalcemic mice examined. No lytic lesions or hypercalcemic developed in the controls. Mice were then sacrificed after developing hypercalcemia. Histologic examination of the ARH-77 mice showed infiltration of myeloma cells in the liver and spleen. Marked infiltration by the tumor was found in vertebrae and long bones, with loss of bony trabeculae and increased osteoclast numbers.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of parathyroid hormone (PTH)-related protein and PTH on osteoclasts and osteoclast precursors in vivo

Increased production of PTH-related protein (PTHrP) and PTH is frequently responsible for hypercalcemia and its associated morbidity. However, it is unclear whether these peptides produce identical effects on cells in the osteoclast lineage in vivo. To examine the effects of continuous in vivo exposure to these factors on both the osteoclast precursors and mature osteoclasts, we inoculated Chinese hamster ovarian cells expressing PTH-(1-84), PTHrP-(1-141), or nontransfected Chinese hamster ovarian cells into nude mice. The effects of these tumors on blood ionized calcium, plasma PTH and PTHrP concentrations, and osteoclast formation were then determined. PTH and PTHrP tumor-bearing mice became hypercalcemic (1.90 +/- 0.04 and 1.97 +/- 0.16 mmol/liter, respectively) compared with control mice (1.29 +/- 0.015 mmol/liter). After 4 days of hypercalcemia, mice were killed, and bone marrow cells were harvested to examine cells at three discrete stages of osteoclast development: multipotent osteoclast precursors, the granulocyte/macrophage colony-forming unit; more committed marrow mononuclear osteoclast precursors; and mature osteoclasts. Neither PTH nor PTHrP had an effect on granulocyte/macrophage colony-forming unit, but similarly increased the number of more committed mononuclear osteoclast progenitors as well as mature osteoclasts in the calvaria. No differences were detected between the effects of PTH and PTHrP on cells in the osteoclast lineage in vivo. Thus, PTH and PTHrP appear to affect only more differentiated cells in the osteoclast lineage, and the differences in osteoclastic bone resorption between primary hyperparathyroidism and humoral hypercalcemia of malignancy are probably due to mechanisms other than effects on osteoclast precursor cells in vivo.

Regulation of interleukin-6, osteoclastogenesis, and bone mass by androgens. The role of the androgen receptor

Interleukin-6 is an essential mediator of the bone loss caused by loss of estrogens. Because loss of androgens also causes bone loss, we have examined whether the IL-6 gene is regulated by androgens, and whether IL-6 plays a role in the bone loss caused by androgen deficiency. Both testosterone and dihydrotestosterone inhibited IL-6 production by murine bone marrow-derived stromal cells. In addition, testosterone, dihydrotestosterone, and adrenal androgens inhibited the expression of a chloramphenicol acetyl transferase reporter plasmid driven by the human IL-6 promoter in HeLa cells cotransfected with an androgen receptor expression plasmid; however, these steroids were ineffective when the cells were cotransfected with an estrogen receptor expression plasmid. In accordance with the in vitro findings, orchidectomy in mice caused an increase in the replication of osteoclast progenitors in the bone marrow which could be prevented by androgen replacement or administration of an IL-6 neutralizing antibody. Moreover, bone histomorphometric analysis of trabecular bone revealed that, in contrast to IL-6 sufficient mice which exhibited increased osteoclast numbers and bone loss following orchidectomy, IL-6 deficient mice (generated by targeted gene disruption) did not. This evidence demonstrates that male sex steroids, acting through the androgen-specific receptor, inhibit the expression of the IL-6 gene; and that IL-6 mediates the upregulation of osteoclastogenesis and therefore the bone loss caused by androgen deficiency, as it does in estrogen deficiency.

The role of cadherin in the generation of multinucleated osteoclasts from mononuclear precursors in murine marrow

A critical step in bone resorption is the fusion of mononuclear osteoclast precursors to form multinucleated osteoclasts. However, little is known of the molecular mechanisms that are responsible for this important process. Since the expression of proteins in the cadherin family of homophilic calcium-dependent cell adhesion molecules is involved in the fusion process for certain other cells, we examined their role in osteoclast formation. Immunohistochemical examination of human and mouse bone using monoclonal antibodies to human and mouse E-cadherin clearly demonstrated positive staining in osteoclasts. N- and P-cadherin were not detected. In cultures of murine marrow mononuclear cells in which osteoclasts form by cell fusion, E-cadherin expression determined by Western blotting reached the highest levels as fusion was taking place. Expression of E-cadherin gene fragment was also detected in the marrow cultures by polymerase chain reaction. To study the functional role of E-cadherin expression in osteoclastic differentiation, neutralizing monoclonal antibodies were examined for their effects on osteoclast formation. The antibodies decreased the number of tartrate-resistant acid phosphatase (a marker of murine osteoclast)-positive multinucleated cell (TRAP-positive MNC) by inhibiting the fusion of mononuclear osteoclast precursors, but not proliferation of these cells or their attachment to plastic dish surfaces. This inhibitory effect was reversible. Furthermore, synthetic peptides containing the cell adhesion recognition sequence of cadherins also decreased TRAP-positive MNC formation. The antibodies and peptides inhibited not only osteoclast formation but also bone resorption. Antibodies to other types of cadherins and control rat IgG had no effects in these culture systems. Our findings suggest that E-cadherin expression may be involved in fusion (differentiation) of hemopoietic osteoclast precursors into mature multinucleated osteoclasts.

Interleukin-6 enhances hypercalcemia and bone resorption mediated by parathyroid hormone-related protein in vivo

Tumors frequently induce the multifunctional cytokine IL-6, which has been linked to several paraneoplastic syndromes, most notably cachexia. IL-6 stimulates osteoclast formation, causes mild hypercalcemia, and is produced by bone cells in vitro upon exposure to systemic hormones. Since IL-6 is produced together with parathyroid hormone-related protein (PTH-rP) in some patients with cancer, we tested the hypothesis that production of IL-6 potentiates the effects of PTH-rP on Ca2+ homeostasis and osteoclastic bone resorption and examined potential mechanisms for these interactions in vivo. Chinese hamster ovarian (CHO) cells stably transfected with cDNAs for IL-6 (CHO/IL-6) and PTH-rP sense (CHO/PTH-rP) or antisense (CHO/PTH-rP AS) were inoculated intramuscularly into nude mice. Experimental groups included CHO/IL-6 plus CHO/PTH-rP; CHO/IL-6 plus CHO/PTH-rP AS; CHO/IL-6 alone; and CHO/PTH-rP alone. Blood ionized Ca2+ was measured on days 0, 7, 10, 12, and 13. Three different developmental stages in the osteoclast lineage were examined at day 13: the early multipotential precursor, granulocyte macrophage colony-forming units (CFU-GM); more mature mononuclear osteoclast precursors, assessed by their capacity to form tartrate-resistant acid phosphatase-positive multinucleated cells in marrow cultures; and mature osteoclasts, assessed by histomorphometry. IL-6 increased CFU-GM but not bone resorption or Ca2+. In contrast, PTH-rP induced hypercalcemia and bone resorption and increased multinucleated osteoclasts and more mature precursors cells, but not CFU-GM. However, mice treated with both IL-6 and PTH-rP had very marked hypercalcemia and osteoclastosis as well as an increase in the number of both CFU-GM and mature osteoclast precursors. These data demonstrate that IL-6 enhances PTH-rP-mediated hypercalcemia and bone resorption, most likely by increasing the pool of early osteoclast precursors that in turn can differentiate to mature osteoclasts. We conclude that IL-6 stimulatory effects on osteoclast precursors may enhance the effects of other bone resorption factors that act at later stages in the osteoclast lineage.

Suramin suppresses hypercalcemia and osteoclastic bone resorption in nude mice bearing a human squamous cancer

Suramin is a polyanionic agent which has been found to be an effective antineoplastic agent against various human tumors including adrenal, renal and prostatic cancer, and osteosarcoma. Recently, suramin has been shown to inhibit bone resorption in organ cultures of mouse calvarial bones. In the present study, we examined the effects of suramin on increased osteoclastic bone resorption and hypercalcemia in nude mice bearing a human oral squamous carcinoma. Suramin (1 mg/mouse/injection) was administered i.p. three times a week for the first 2 weeks and then once weekly for the next 6 weeks. Blood ionized calcium levels in the suramin-treated cancer-bearing group were significantly lower than those in the untreated cancer-bearing group. Histological and histomorphometrical examination of bones of these animals showed a significant decrease in osteoclast numbers in the suramin-treated cancer-bearing animals. Suramin at a dose of 0.1 mg/mouse/injection was ineffective and 2 mg/mouse/injection was toxic, confirming its narrow effective dose. Suramin showed no effects on the growth of this squamous cancer. However, suramin markedly inhibited in vivo growth of a rat prostatic adenocarcinoma. In mouse marrow cultures, suramin decreased osteoclast-like cell formation in a dose-dependent manner. Furthermore, suramin also inhibited bone resorption in organ cultures of fetal rat long bones and resorption pit formation by isolated mature rat osteoclasts. These results show that suramin is an effective inhibitor of osteoclastic bone resorption in vitro and in vivo and suggest that suramin may be a useful agent in prevention and treatment of cancer-induced hypercalcemia. However, our results also suggest that for this indication suramin has a confined range of effective dose.

Use of an in vivo model to determine the effects of interleukin-1 on cells at different stages in the osteoclast lineage

In vitro model systems have been used extensively to study factors that affect osteoclast formation and to identify osteoclast precursors. However, in vitro systems do not examine the entire process of osteoclast differentiation simultaneously and lack accessory cells normally present in vivo. Additionally, the role that metabolism of the factor may play on its osteotropic activity in vivo is not addressed by these culture systems. Therefore, we have developed an in vivo model that permits us to examine simultaneously the effects of osteotropic factors on three distinct stages of osteoclast differentiation: (1) multipotent osteoclast precursors, the granulocyte-macrophage colony-forming unit (CFU-GM); (2) more differentiated marrow mononuclear osteoclast precursors; and (3) mature osteoclasts already present on bone surfaces. In the current study, we used interleukin-1 (IL-1) as a prototypic osteotropic factor to test the utility of this system to delineate the cellular mechanisms responsible for enhanced osteoclast activity stimulated by this cytokine. IL-1 induced hypercalcemia and enhanced the growth and differentiation of CFU-GM, increased the number of more committed mononuclear osteoclast precursors, and stimulated mature osteoclasts to resorb bone. These data demonstrate that this simple in vivo model permits the easy delineation of the stages of osteoclast development, in which osteotropic factors act to enhance bone turnover, and may be useful in understanding the mechanism of action of antiresorptive agents.

Extracts of porcine pancreas prevent progression of hypercalcemia and cachexia and prolong survival in nude mice bearing a human squamous carcinoma

Porcine pancreatic extracts (PXs) have previously been shown to decrease blood ionized calcium in BALB/c mice (T. Yoneda, Y. Takaoka, and G. R. Mundy. FEBS Lett., 278: 171-174, 1991). In the present study, we show that the PX is effective in preventing progression of hypercalcemia and decreasing osteoclastic bone resorption associated with a human squamous carcinoma in nude mice. PX inhibited osteoclast-like cell formation in mouse bone marrow cultures and bone resorption in organ cultures of fetal rat long bones which had been stimulated by serum-free culture supernatants of this cancer. In addition, PX increased food intake, decreased weight loss, and prevented development of cachexia. In parallel with these effects, PX prolonged survival of tumor-bearing animals. PX might have therapeutic potential for management of hypercalcemia and cachexia associated with malignancy.