The combination of pamidronate and calcitriol reverses particle- and TNF-α-induced altered functions of bone-marrow-derived stromal cells with osteoblastic phenotype

Polymethylmethacrylate particles inhibit osteoblastic differentiation of bone marrow osteoprogenitor cells

Aseptic implant loosening of total joint replacements often results from particle-mediated bone loss, which may be a combined effect of osteolysis and suppressed bone formation. Bone regeneration in the prosthetic bed depends on the activity of osteoblasts and their differentiation from osteoprogenitors in the bone marrow. This study investigated the effects of polymethylmethacrylate (PMMA) particles on the ability of bone marrow osteoprogenitors to differentiate into osteoblasts in vitro. Murine bone marrow cells challenged with PMMA particles on the first day of differentiation in osteogenic medium showed a dose-dependent decrease in osteoprogenitor proliferation, alkaline phosphatase expression, and mineralization. Undifferentiated bone marrow cells pretreated with PMMA particles in nonosteogenic medium for 5 days also showed a dose-dependent loss in osteogenic potential, which was sustained throughout subsequent growth in particle-free, osteogenic medium. Bone marrow cells challenged with PMMA particles after the fifth day of differentiation in osteogenic medium showed significant reductions in cellular proliferation, but not alkaline phosphatase expression and mineralization, indicating that bone marrow cells were most sensitive to particle treatment during the first 5 days of differentiation. This study demonstrated that PMMA particles inhibit osteoblastic differentiation of bone marrow osteoprogenitor cells, which may contribute to periprosthetic bone loss and implant failure.

Anabolic effects of bisphosphonates on peri-implant bone stock

The long-term durability of total joint replacements is critically dependent on adequate peri-implant bone stock, which can be compromised by wear debris-mediated osteolysis. This study investigated the effects of bisphosphonates on enhancing peri-implant bone in the presence of clinically relevant ultra-high molecular weight polyethylene (UHMWPE) wear debris. Fiber-mesh coated titanium-alloy plugs were implanted bilaterally in the femoral condyles of 36 New Zealand white rabbits. Implants in the left femora were covered with submicron UHMWPE particles during surgery. Rabbits were administered either no drug, subcutaneous alendronate weekly (1.0mg/kg/week) or a single dose of intravenous zoledronate (0.015mg/kg). A total of 6/12 rabbits in each group were sacrificed at 6 weeks and the remainder at 12 weeks postoperatively. Peri-implant bone stock was analyzed radiographically and histomorphometrically. Radiographically, both bisphosphonates significantly increased periprosthetic cortical thickness at 6 weeks (p<0.0001; alendronate: +18%; zoledronate: +11%) and at 12 weeks (p=0.001; alendronate: +17%; zoledronate:+19%). Histomorphometrically, alendronate and zoledronate raised peri-implant bone volume (BV/TV) up to 2-fold after 6 weeks without added wear debris and more than 3-fold when wear debris was present. Furthermore a 6-week bisphosphonate treatment increased osteoid thickness in the absence of wear debris (alendronate: +132%, p=0.007; zoledronate: +67%, p=0.51) and in the presence of wear debris (alendronate: +134%, p=0.023; zoledronate: +138%, p=0.016). In summary, alendronate and zoledronate treatment increased periprosthetic bone stock in a rabbit femoral model, particularly in the presence of UHMWPE wear debris. These new findings suggest that bisphosphonates may more than compensate for the well-documented negative effects of wear debris on peri-implant bone stock. The combined antiresorptive and osteoanabolic effects of bisphosphonates on periprosthetic bone stock may have an important role for critically improving the biological fixation and ultimate durability of total joint arthroplasty.

Effects of a p38 MAP kinase inhibitor on bone ingrowth and tissue differentiation in rabbit chambers

The effects of an oral p38 mitogen-activated protein kinase (MAPK) inhibitor and polyethylene particles separately and together on tissue differentiation in the bone harvest chamber (BHC) in rabbits over a 3-week treatment period were investigated. The harvested tissue was analyzed histomorphometrically for markers of bone formation (percentage of bone area), osteoblasts (alkaline phosphatase staining), and osteoclasts (CD51, the alpha chain of the vitronectin receptor). Polyethylene particles decreased the percentage of bone ingrowth and staining for alkaline phosphatase. The p38 MAPK inhibitor alone decreased alkaline phosphatase staining. When the oral p38 MAPK inhibitor was given and the chamber contained polyethylene particles, there was a suppression of bone ingrowth and alkaline phosphatase staining. In contrast to oral non-steroidal anti-inflammatory drugs (NSAIDs) and local Interleukin-1 receptor antagonist (IL-1ra) administration, the oral p38 MAPK inhibitor alone did not suppress bone formation when given during the initial phase of tissue differentiation. Particle-induced inflammation and the foreign body reaction were not curtailed when the p38 MAPK inhibitor was given simultaneously with particles. Additional experiments are needed to establish the efficacy of p38 MAPK inhibitor administration on mitigating an established inflammatory and foreign body reaction that parallels the clinical situation more closely.

Nonsurgical management of osteolysis: challenges and opportunities

Osteolysis remains a common mode of total hip arthroplasty failure. In vitro and animal models have been used to determine the pathophysiology of osteolysis by carefully dissecting the biochemical pathways leading to particulate wear debris and periprosthetic bone loss. Numerous cytokines and inflammatory mediators, including TNF-alpha and IL-1, are critical participants in this cascade and may represent prime targets for pharmacologic intervention. Osteoclasts, the end effector cells involved in the osteolytic process, also represent potential targets. Cell surface receptors on osteoclast precursors, such as receptor activator of NF-kappaB (RANK) (on osteoclasts) and RANK-ligand (RANKL) (on stromal cells), provide opportunities to arrest osteoclast maturation. Enhancing the naturally occurring osteoprotegerin is another recent attempt at modulating osteoclast behavior and a possible target for pharmacologic therapies. Other nonoperative strategies include intercepting tumor necrosis factor-alpha activity, interfering with the RANK-RANKL interaction necessary for osteoclast development and maturation, bisphosphonate therapy, and using viral vectors to deliver genes. Although each of these approaches has potential benefits, there are substantial challenges to effective implementation. Until there is convincing evidence of efficacy in human clinical trials, we recommend vigilant screening and appropriate surgery with component loosening or substantial likelihood of loosening, periprosthetic fracture, or major bone loss.

The role of fibroblasts and fibroblast-derived factors in periprosthetic osteolysis

OBJECTIVE

This study was undertaken to investigate how fibroblasts respond to stimulation with particulate wear debris and/or conditioned media obtained from pathologic tissue, and whether these activated fibroblasts express compounds that are involved in bone resorption.

METHODS

Conditioned media from explant cultures of synovial tissue, periprosthetic soft tissue (interface membranes), titanium particles, and proinflammatory cytokines were used to stimulate fibroblasts. RNase protection assay was used to measure altered gene expression, and enzyme-linked immunosorbent assay, Western blot hybridization, and flow cytometry were used to determine fibroblast protein expression. Tartrate-resistant acid phosphatase staining was used to identify multinucleated osteoclast-like cells.

RESULTS

The most dominant compounds measured in the conditioned media from interface membranes were tumor necrosis factor alpha (TNFalpha), monocyte chemoattractant protein 1 (MCP-1), interleukin-1beta (IL-1beta), IL-6, IL-8, and vascular endothelial growth factor. Fibroblasts phagocytosed particulate wear debris and responded to cytokine/chemokine stimulation. The most prominent up-regulated genes and proteins secreted by fibroblasts in response to stimulation were matrix metalloproteinase 1, MCP-1, IL-1beta, IL-6, IL-8, cyclooxygenase 1 (COX-1), COX-2, leukemia inhibitory factor 1, transforming growth factor beta1 (TGFbeta1), and TGFbeta receptor type I. In addition, interface membrane fibroblasts expressed RANKL and osteoprotegerin in response to stimulation with conditioned media, TNFalpha, or IL-1beta. Stimulated fibroblasts cocultured with bone marrow cells in the presence of macrophage colony-stimulating factor induced osteoclastogenesis.

CONCLUSION

Interface membrane fibroblasts respond directly to particulate wear debris, possibly via phagocytosis, expressing proinflammatory cytokines and RANKL. Thus, these cells may be actively involved in osteoclastogenesis and pathologic (periprosthetic) bone resorption.

Expression of tumor necrosis factor alpha and its receptors during cellular differentiation

Tumor necrosis factor alpha (TNFalpha) is a potent proinflammatory cytokine also involved in cellular differentiation processes. TNFalpha and both of its receptors (TNFR1 and TNFR2) can be co-expressed on the same cell, allowing for local signaling. This study has examined the expression of all components necessary for autocrine cytokine regulation during human hematopoietic, epithelial, and mesenchymal models of cellular differentiation. Macrophage and dendritic differentiation of human peripheral blood monocytes decreased their TNFalpha and TNFR2 expression while increasing the TNFR1 mRNA. In colon epithelial cell lines (HT-29 and Caco-2) TNFalpha-, TNFR1-, and TNFR2-expression was decreased upon differentiation. No changes, however, were seen during human skin keratinocyte differentiation. TNFR1 expression was unchanged in all three mesenchymal lineages (adipogenesis, chondrogenesis, osteogenesis) tested. Differentiation decreases the TNFalpha message in adipocytes and the TNFR2 mRNA in adipocytes and osteocytes. Our results demonstrate that there is no general principle for TNFalpha signaling during conversion of cells from progenitor to a more differentiated phenotype. Paracrine signaling by TNFalpha to orchestrate different cell types during tissue development and remodeling, therefore, probably overrides the autocrine regulation of differentiation by TNFalpha. Non-signaling TNF-receptors may protect chondrocytes and osteocytes from the anti-differentiation effects of local TNFalpha production.

Chemokine gene activation in human bone marrow-derived osteoblasts following exposure to particulate wear debris

Particulate wear debris induces the expression of pro-inflammatory cytokine and chemokine genes in various cell types of the periprosthetic region. We have previously reported that titanium particles stimulate the selective induction of interleukin-8 (IL-8) and monocyte chemoattractant protein-1 (MCP-1) chemokines in human osteoblast-like osteosarcoma cells. In this study, we characterize the human bone marrow-derived osteoblast chemokine response to titanium particles. We demonstrate that titanium particles result in enhanced IL-8 and MCP-1 protein secretion as well as differential chemokine gene activation. Osteoblast chemokine expression was regulated at the level of gene transcription, with a time-dependent induction of NF-kappaB activation. Inhibition studies with N-acetyl-L-cysteine (Nac) and MG-132 suggest that titanium particle activation of NF-kappaB activity and IL-8 chemokine expression involves oxidant signaling and IkappaBalpha-proteasomal degradation. Activation of the NF-kappaB transcription factor, as well as the IL-8 gene, are redox-regulated. We also demonstrate that while cytochalasin D, a potent inhibitor of phagocytosis, suppressed the titanium particle effect on IL-8 protein release in human bone marrow-derived osteoblasts, the inhibitor had no effect on IL-8 expression in MG-63 osteoblast-like cells. Collectively, these results provide insight into the potential mechanisms responsible for the particulate activation of osteoblast chemokine expression and suggest an important role for the osteoblast in the pathogenesis of periprosthetic osteolysis.

Effects of bisphosphonates on proliferation and osteoblast differentiation of human bone marrow stromal cells

Bisphosphonates are well known potent inhibitors of osteoclast activity and are widely used to treat metabolic bone diseases. Recent evidence from in vitro and in vivo studies indicates that bisphosphonates may additionally promote osteoblastic bone formation. In this study, we evaluated the effects of three FDA-approved and clinically utilized bisphosphonates, on the proliferation and osteogenic differentiation of human bone marrow stromal cells (BMSC). BMSC were obtained from patients undergoing primary total hip arthroplasty for end-stage degenerative joint disease. Cells were treated with or without a bisphosphonate (alendronate, risedronate, or zoledronate) and analyzed over 21 days of culture. Cell proliferation was determined by direct cell counting. Osteogenic differentiation of BMSC was assessed with alkaline phosphatase bioassay and gene expression analyses using conventional RT-PCR as well as real-time quantitative RT-PCR. All bisphosphonates tested enhanced the proliferation of BMSC after 7 and 14 days of culture. Steady-state mRNA levels of key genes involved in osteogenic differentiation such as bone morphogenetic protein-2 (BMP-2), bone sialoprotein-II, core-binding factor alpha subunit 1 (cbfa1) and type 1 collagen, were generally increased by bisphosphonate treatment in a type- and time-dependent manner. Gene expression levels varied among the different donors. Enhancement of osteogenic differentiation was most pronounced after 14 days of culture, particularly following zoledronate treatment (p < 0.05 for BMP-2). In conclusion, using a clinically relevant in vitro model we have demonstrated that bisphosphonates enhance proliferation of BMSC and initiate osteoblastic differentiation. When administered around joint replacements, bisphosphonates may potentially compensate for the deleterious effects of particulate wear debris at the bone-implant interface, by encouraging increased numbers of cells committed to the osteoblastic phenotype, and thus improve the longevity of joint replacements.

Chemokine IL-8 induction by particulate wear debris in osteoblasts is mediated by NF-kappaB

Chemokines, or chemotactic cytokines, are major regulators of the inflammatory response and have been identified as pathogenic factors in the periprosthetic soft tissue. Particulate wear debris induced NF-kappaB activation, the major transcriptional regulator of IL-8 and MCP-1 pro-inflammatory genes and, indeed, both IL-8 and MCP-1 chemokine gene expressions were upregulated in titanium particulate-stimulated human osteoblasts. Here, we demonstrate that phagocytosed particles activate the IL-8 gene promoter via a NF-kappaB-mediated mechanism. Transfection of a dominant negative mutant IkappaBalpha protein that cannot be serine phosphorylated led to suppression of IL-8 promoter activity. The p65/RelA NF-kappaB subunit activity was affected in both a time- and titanium particle concentration-dependent fashion. Titanium particles led to increased ERK, JNK, and p38 activation in MG-63 osteoblast cells, and IL-8 protein release was suppressed by specific inhibitors of the ERK and p38 MAPK pathways. Together, our results suggest that wear debris particles induce chemokine expression in osteoblasts via NF-kappaB-mediated transcriptional activation, which is controlled by the MAPK signal transduction pathway.