Tumor necrosis factor-alpha (TNF) is a potent osteoclastogenic cytokine that has a fundamental role in the pathogenesis of implant particle-induced osteolysis. The nuclear transcription factor NF-kappaB mediates TNF signaling and this transcription complex is necessary for osteoclastogenesis. Because polymethylmethacrylate (PMMA) particles cause osteolysis, we reasoned the PMMA would induce NF-kappaB activation. In fact, we find that exposure of osteoclast precursors, in the form of colony stimulating factor-1 (CSF-1) dependent murine bone marrow macrophages, to PMMA particles prompts nuclear translocation and activation of NF-kappaB. Supershift assays confirm the presence of the p50 and p65 NF-kappaB subunits in the activated transcription factor. Particle-induced NF-kappaB activation is equal in both wild type and LPS- hyporesponsive cells indicating that the phenomenon does not represent endotoxin contamination. A soluble, competitive inhibitor of TNF (huTNFr:Fc) dampens particle-directed NF-kappaB activation and this response is also abrogated in TNF-/- osteoclast precursors. Thus, PMMA particle activation of NF-kappaB is a secondary event resulting from enhanced TNF expression and is independent of LPS contamination.

Osteolysis complicating arthroplasty reflects progressive generation of implant-derived wear particles, which prompt an inflammatory reaction attended by recruitment of osteoclasts to the prosthesis-bone interface. To identify a soluble mediator of periprosthetic osteolysis we first showed that implant particles induce c-src in murine bone marrow macrophages (BMMs), a protein specifically expressed when these cells commit to the osteoclast phenotype. The fact that tumor necrosis factor-alpha (TNF) is a potent osteoclastogenic agent while at the same time is the only soluble moiety known to be c-src inductive suggests that this cytokine may mediate implant particle-induced osteoclastogenesis. Consistent with this hypothesis, prosthesis-derived wear particles, recovered at revision arthroplasty, dose-dependently prompt TNF secretion by BMMs. Similarly, particulate polymemthylmethacrylate, the major component of orthopedic implant cement, induces BMM expression of TNF mRNA and protein in a time- and dose-dependent manner. Furthermore, failure of BMMs derived from mice deleted of both the p55 and p75 TNF receptors to express c-src in response to polymemthyl-methacrylate indicates TNF is an essential mediator of particle induction of this osteoclast specific protein. To test the hypothesis that TNF mediates implant osteolysis, we established an in vivo murine model of this condition that histologically mirrors that of man. Verifying that TNF is essential to development of particle osteolysis, mice failing to express both the p55 and p75 TNF receptors are protected from the profound bone resorption attending polymemthyl-methacrylate particle implantation on calvariae of wild-type animals. Finally, the protective effect of deletion of both TNF receptors is recapitulated in mice lacking only the p55 receptor. Thus, targeting TNF and/or its p55 receptor may arrest wear particle osteolysis.

Osteoclasts are multinucleated cells and the principal resorptive cells of bone. Although osteoclasts are of myeloid origin1, the role of haematopoietic transcription factors in osteoclastogenesis has not been explored. Here we show that messenger RNA for the myeloid- and B-cell-specific transcription factor PU.1 progressively increases as marrow macrophages assume the osteoclast phenotype in vitro. The association between PU.1 and osteoclast differentiation was confirmed by demonstrating that PU.1 expression increased with the induction of osteoclastogenesis by either 1,25-dihydroxyvitamin D3 or dexamethasone. Consistent with the participation of PU.1 in osteoclastogenesis, we found that the development of both osteoclasts and macrophages is arrested in PU.1-deficient mice. Reflecting the absence of osteoclasts, PU.1−/− mice exhibit the classic hallmarks of osteopetrosis, a family of sclerotic bone diseases2. These animals were rescued by marrow transplantation, with complete restoration of osteoclast and macrophage differentiation, verifying that the PU.1 lesion is intrinsic to haematopoietic cells. The absence of both osteoclasts and macrophages in PU.1-mutant animals suggests that the transcription factor regulates the initial stages of myeloid differentiation, and that its absence represents the earliest developmental osteopetrotic mutant yet described.

Interleukin 4 (IL-4) is an immune cytokine that inhibits bone resorption in mice and suppresses osteoclastic cell formation in vitro through an undefined mechanism. In this report, we have established the cellular identity of the IL-4 target cell using a variety of bone marrow/stromal cell coculture methods. Initially, we found that the majority of IL-4's inhibition of osteoclastic cell formation was due to its effect on bone marrow cells, not stromal cells. Consequently, bone marrow macrophages were used as osteoclastic cell progenitors after they had been transiently exposed to IL-4 (48 h), before the addition of stromal cells, 1,25-dihydroxyvitamin D3, and dexamethasone. In this circumstance, IL-4 impaired subsequent osteoclastic cell formation, suggesting that the macrophage may be potentially targeted by many factors known to influence osteoclast formation. Consequently, we discovered that interferon-gamma (IFN gamma), prostaglandin E (PGE), and cell-permeant cAMP analogs also impacted osteoclastic cell formation when used to selectively treat bone marrow macrophages. IFN gamma suppressed osteoclastic cell formation, whereas PGE and cAMP analog treatment led to the formation of significantly enlarged osteoclastic cells. Importantly, PGE antagonized the inhibitory effects of both IL-4 and IFN gamma on the osteoclastic cell-forming potential of bone marrow macrophages. Collectively, these findings establish bone marrow macrophages as osteoclastic cell precursors with the degree of their commitment to the osteoclast pathway sensitive to the effects of soluble mediators, including IL-4, IFN gamma, and PGE.

To determine if interleukin 4's (IL-4) recently discovered skeletal effects could be explained by its effects on osteoblasts, we have examined IL-4's impact on macrophage colony stimulating factor (M-CSF) and interleukin 6 (IL-6) secretion by the murine osteoblastic cell line MC3T3-E1. Interleukin-4 increased colony-forming activity in MC3T3 supernatants two-threefold with colony cytomorphology, cytohistochemistry, and blockade of the effect by anti-M-CSF antibody, indicating that the IL-4-induced activity was M-CSF. MC3T3 M-CSF supernatant activity increased in a time-dependent manner with positive IL-4 effects seen after a 24-hour exposure. The maximal IL-4 effective dose was 100 U/ml where conditioned media from IL-4-treated cells contained twofold more M-CSF than control cells (400 U/ml versus 200 U/ml M-CSF) as detected by a sandwich M-CSF ELISA. Northern blots showed that IL-4 (200 U/ml) rapidly increased steady-state M-CSF mRNA levels with maximal induction observed by 2 hours followed by a decline to near basal levels by 24 hours. IL-4 also dose dependently increased M-CSF mRNA levels with maximal induction (fourfold) seen at 100 U/ml IL-4. In contrast to its impact on MC3T3 M-CSF production, IL-4 (200 U/ml) did not stimulate MC3T3 IL-6 secretion whereas IL-1 (1 pM) stimulated a 500-fold increase in MC3T3 IL-6 release. When utilized to treat newborn calvarial osteoblast-enriched cultures, IL-4 dose dependently augmented M-CSF production, with the maximal effect seen at 200 U/ml where IL-4-treated, osteoblast-conditioned media contained almost 500 U/ml M-CSF, compared with 200 U/ml M-CSF in control-conditioned media. These observations indicate that the range of IL-4 cellular targets in skeletal tissues include osteoblastic cells, and that this cytokine increases osteoblast expression of M-CSF, a hematopoietic cytokine pivotal for monocyte/macrophage differentiation. Furthermore, IL-4's impact on osteoblast-produced M-CSF levels is selective because IL-6 levels were unaltered by IL-4 treatment.

Interleukin-4 (IL-4) is an immune cytokine recently shown to inhibit bone resorption. To determine whether IL-4 directly acts on osteoclasts, we have analyzed its effect on cytosolic calcium concentration [Ca2+](i) and bone resorptive function of murine osteoclastic cells generated from bone marrow/stromal cell co-cultures. IL-4 exposure induced an immediate and sustained increase in [Ca2+](i) that remained elevated for at least 10 min. This IL-4 effect was dose-dependent, with the maximal effect (209 +/- 15% of baseline, n = 16) at 200 units/ml and an apparent ED(0.5) of 60 units/ml. The IL-4-induced [Ca2+](i). rise required extracellular Ca2+ influx, since the response was prevented by LaCl3, and voltage-gated Ca2+ channel blockers, although the IL-4 effect was more sensitive to nicardipine and nifedipine than to diltiazem. Depolarization by high extracellular K+ concentration also raised [Ca2+](i), and, under these conditions, osteoclasts failed to respond to IL-4. On the other hand, when intracellular Ca2+ stores were depleted by thapsigargin, IL-4 still induced an increase in [Ca2+](i), although smaller in amplitude and transient. Calcitonin also produced [Ca2+](i) increases in osteoclasts, yet it only slightly desensitized these cells to IL-4. Furthermore, IL-4 was much less effective on osteoclasts pretreated (5-10 min) with either forskolin or 8-bromo-cAMP. Both IL-4 and calcitonin were effective even when [Ca2+](i) had been increased by exposure to high extracellular Ca2+. Finally, IL-4 dose dependently inhibited the bone resorptive activity of mature osteoclasts. Therefore, IL-4 signal transduction in osteoclasts involves a rapid and sustained elevation of [Ca2+](i) mediated by a voltage dependent Ca2+ influx, in combination with Ca2+ release from intracellular stores. Modulation of osteoclast [Ca2+](i) represents a potential mechanism by which IL-4 inhibits bone resorption.

Type 5 acid phosphatase is a lysosomal enzyme expressed in cells of monocyte/macrophage lineage frequently used as a marker of osteoclastic differentiation. Oligonucleotide primers for DNA amplification were designed following sequence alignment of rat bone and human macrophage type 5 acid phosphatases. DNA (330 bp in length) obtained using these primers and reverse transcribed total cell RNA from in vitro generated murine osteoclastic cells was cloned and sequenced. DNA sequence analysis of two clones demonstrates that the amplified material was 91% and 96% identical to rat bone type 5 acid phosphatase at the nucleotide and amino acid level, respectively. Northern blots of murine tissue RNA show the presence of 1.5‐kb transcripts that are most highly expressed in the long bones. Total cell RNA from the osteoclastic cells contain a marked level of type 5 acid phosphatase mRNA when compared to the levels seen in the tissue samples. Additionally, osteoclastic cell RNA contains two additional transcripts of 2.5 and 5 kb. Bone marrow macrophages grown in the presence of M‐CSF express low levels of the 1.5‐kb transcript with no signal observed for either of the two larger transcripts that were seen in the osteoclastic RNA samples. Importantly, bone marrow macrophage 1.5‐kb type 5 acid phosphatase transcript levels are increased by interleukin 4 treatment in both a time and concentration‐dependent manner. These findings indicate that type 5 acid phosphatase, while a cytochemical marker for osteoclasts, can be induced in macrophages by agents that block in vitro osteoclastic differentiation. Increased type 5 acid phosphatase may play a role in interleukin 4‐stimulated monocyte activities.

Interleukin-4 (IL-4) is an immune cytokine recently shown to inhibit bone resorption. To determine whether IL-4 directly acts on osteoclasts, we have analyzed its effect on cytosolic calcium concentration [Ca2+]i and bone resorptive function of murine osteoclastic cells generated from bone marrow/stromal cell co-cultures. IL-4 exposure induced an immediate and sustained increase in [Ca2+]i that remained elevated for at least 10 min. This IL-4 effect was dose-dependent, with the maximal effect (209 +/- 15% of baseline, n = 16) at 200 units/ml and an apparent ED0.5 of 60 units/ml. The IL-4-induced [Ca2+]i rise required extracellular Ca2+ influx, since the response was prevented by LaCl3, and voltage-gated Ca2+ channel blockers, although the IL-4 effect was more sensitive to nicardipine and nifedipine than to diltiazem. Depolarization by high extracellular K+ concentration also raised [Ca2+]i, and, under these conditions, osteoclasts failed to respond to IL-4. On the other hand, when intracellular Ca2+ stores were depleted by thapsigargin, IL-4 still induced an increase in [Ca2+]i, although smaller in amplitude and transient. Calcitonin also produced [Ca2+]i increases in osteoclasts, yet it only slightly desensitized these cells to IL-4. Furthermore, IL-4 was much less effective on osteoclasts pretreated (5-10 min) with either forskolin or 8-bromo-cAMP. Both IL-4 and calcitonin were effective even when [Ca2+]i had been increased by exposure to high extracellular Ca2+. Finally, IL-4 dose dependently inhibited the bone-resorptive activity of mature osteoclasts. Therefore, IL-4 signal transduction in osteoclasts involves a rapid and sustained elevation of [Ca2+]i mediated by a voltage-dependent Ca2+ influx, in combination with Ca2+ release from intracellular stores. Modulation of osteoclast [Ca2+]i represents a potential mechanism by which IL-4 inhibits bone resorption.

The immune cytokine interleukin 4 has newly recognized effects on skeletal metabolism. While the interaction of many cells ultimately determines bone mass, we have examined the possibility that the osteoblast may be an IL‐4 target in bone by characterizing IL‐4 receptor (IL‐4R) expression by MC3T3‐E1 (MC3T3) murine osteoblastic cells. Based on 125 I‐IL‐4 binding, MC3T3 cells express large numbers of IL‐4 receptors ( 125 I‐IL‐4 Bmax = 3,000–7,500 sites/cell, 125 I‐IL‐4 K = 13–40 pM) with an affinity similar to the IL‐4 receptor expressed by an IL‐4–responsive T cell line. Monoclonal anti–IL‐4R antibodies (M1) blocked specific MC3T3 125 I‐IL‐4 binding and MC3T3 total cell RNA contained full‐length IL‐4R mRNA as detected by reverse transcription DNA amplification utilizing IL‐4R primers and Northern blot analysis. Functionally, IL‐4 treatment of MC3T3 cells resulted in increased cellular proliferation (10–20%) and inhibition of alkaline phosphatase levels (20–40%). While parathyroid hormone (PTH) exposure did not influence IL‐4R levels, vitamin D 3 treatment augmented MC3T3 125 I‐IL‐4 binding, in a time‐dependent manner, up to threefold after a 24 h exposure with a metabolite specificity indicating the involvement of the vitamin D receptor. Equilibrium binding studies showed that the impact of 1,25 (OH) 2 D 3 on MC3T3 125 I‐IL‐4 binding was due to an increased IL‐4R Bmax. Cycloheximide treatment inhibited 1,25 (OH) 2 D 3 ‐‐induced IL‐4R upregulation, suggesting that protein synthesis was required. Furthermore, the steroid increased steady‐state IL‐4R mRNA levels in both a time‐ and concentration‐dependent manner. The IL‐4R message half‐life was not altered by 1,25 (OH) 2 D 3 , suggesting that increased IL‐4R mRNA expression resulted from increased IL‐4R gene transcription. Taken together, these findings raise the possibility that IL‐4′s influence on mineral metabolism could be mediated by osteoblasts and that the effectiveness of this cytokine may be influenced by vitamin D 3 ′s impact on IL‐4R expression.

The biologically active metabolite of vitamin D3, 1,25 (OH)2 D3, exerts important immunoregulatory effects in addition to being a central mediator of calcium/phosphate metabolism. Utilizing an interleukin 1 responsive murine T cell line and I-125-interleukin 1alpha, we show that 1,25 (OH)2 D3 (5,50 nM) enhanced I-125-interleukin 1alpha binding up to almost 2-fold over control. This 1,25 (OH)2 D3 effect occurred in a dose-dependent manner and was detectable after 24 h but not before 7 h of culture. Scatchard analysis of I-125-interleukin 1alpha binding data demonstrated that 1,25 (OH)2 D3 enhanced interleukin 1 receptor number without a significant change in affinity. The biologically less potent metabolite of vitamin D3, 25 (OH) D3, also augmented I-125-interleukin 1alpha binding but at steroid levels 2-3 log orders greater than 1,25 (OH)2 D3. This observation, combined with the presence of high-affinity H-3-1,25 (OH)2D3 receptors (88 sites/cell, K = 0.45 nM) in cytosolic extracts, strongly suggests that the nuclear vitamin D receptor mediates this steroid's effect on interleukin 1 receptor expression. Based on the capacity of an anti-type 1 interleukin 1 receptor monoclonal antibody (35F5) to block 1,25 (OH)2 D3-enhanced I-125-interleukin 1alpha binding, we conclude that this steroid augments type 1 interleukin 1 receptor expression. When combined with interleukin 1, a cytokine that also impacts MD10 interleukin 1 receptor expression, 1,25 (OH)2 D3 enhanced interleukin 1 receptor expression. Northern blots hybridized with a 32p-type 1 interleukin 1 receptor cDNA probe show that 1,25 (OH)2 D3 enhanced type 1 interleukin 1 receptor steady state mRNA levels. Functionally, 1,25 (OH)2 D3 pretreatment augmented the MD10 proliferative response to suboptimal levels of interleukin 1 (<100 fM interleukin 1alpha).These findings further support 1,25 (OH)2 D3'S role as an immunoregulatory molecule and provides a possible mechanism by which this steroid could potentiate certain immune activities.

The combination of IL-1 and IL-4 stimulates the proliferation of certain murine T cell populations. Although this effect has been best characterized for a number of murine type 2 Th cell (Th2) clones, the mechanism(s) by which these cytokines effect this response is unclear. We have examined the effects of IL-1 and IL-4 on IL-1R expression by MD10 cells, and IL-1-responsive murine T cell line. These cells bear specific IL-1R, which bind human and murine IL-1 alpha and -beta. The measured apparent IL-1R dissociation constant ranged from 41 to 255 pM using 125I-HrIL-1 alpha. Cross-linking studies demonstrated two different 125I-HrIL-1 alpha binding complexes having Mr of 70,000 and 130,000 to 156,000. When removed from passage conditions and placed in non-growth factor-supplemented media, MD10 IL-1R expression spontaneously increased two- to fourfold over the first 11 to 12 h of culture followed by a decline. This phenomenon is partially inhibitable by cycloheximide suggesting that protein synthesis is involved. In agreement with other reports, HrIL-1 alpha down-regulated the expression of its own receptor with an ED50 of between 1 and 10 pM HrIL-1 alpha for this effect. In most experiments, low amounts of HrIL-1 alpha (1.0, 0.1 pM) significantly augmented IL-1R expression. Scatchard analysis of data obtained with all HrIL-1 alpha treatment conditions showed that the effects were due to a change in receptor number, not affinity. Significantly, purified murine IL-4 (MpIL-4) augmented MD10 IL-1R expression in both a time- and dose-dependent fashion. In the presence of 50 U/ml MpIL-4, MD10 IL-1R expression increased two- to threefold after 24 h without a change in receptor affinity. When MpIL-4 (50 U/ml) and various amounts of HrIL-1 alpha (.01-1000 pM) were co-added, the down-regulatory effect of high levels of HrIL-1 alpha was significantly antagonized. When added to cultures after 24 h of HrIL-1 alpha (100 pM) treatment, MpIL-4 reversed the IL-1R down-regulatory effect induced by high levels of HrIL-1 alpha. Finally, when combined in MD10 proliferation assays, MpIL-4 synergistically enhanced the proliferation of MD10 cells treated with suboptimal levels of HrIL-1 alpha.(ABSTRACT TRUNCATED AT 400 WORDS)

The binding and mitogenic properties of thrombin have been established in various transformed cell lines. In such systems, thrombin induces cell division in the absence of exogenous growth factors, and the enzyme is considered to act directly as a mitogen. This study explores thrombin's interaction with nontransformed, growth factor-dependent cells. Binding of 125I-alpha-thrombin to colony-stimulating factor (CSF)-1-dependent bone marrow-derived macrophages is saturable, time-dependent, and displaceable by both unlabeled alpha-thrombin, and esterolytically inactive thrombin. Both dissociation studies of pre-bound radio-labeled thrombin and Scatchard analysis assisted by the program Ligand suggest adherence of thrombin-binding data to a multi-site model. There are an estimated 2 x 10(4) high affinity sites (Kd = 7 x 10(-9)M) and 2 x 10(6) low affinity sites (Kd = 9 x 10(-7)M) per cell. Quiescent bone marrow-derived macrophages were cultured with either 10(-8)M thrombin, 1000 units of CSF-1/ml, or both and [3H]thymidine incorporation was determined. Thrombin alone did not induce mitogenesis. CSF-1 induced mitogenesis with peak [3H] thymidine incorporation occurring 24 h after addition of the mitogen. This CSF-1-dependent mitogenic influence was enhanced greater than 2-fold by treatment with thrombin.