Inhibition of STAT1 accelerates bone fracture healing.

Skeletal fracture healing involves a variety of cellular and molecular events; however, the mechanisms behind these processes are not fully understood. In the current study, we investigated the potential involvement of the signal transducer and activator of transcription 1 (STAT1), a critical regulator for both osteoclastogenesis and osteoblast differentiation, in skeletal fracture healing. We used a fracture model and a cortical defect model in mice, and found that fracture callus remodeling and membranous ossification are highly accelerated in STAT1-deficient mice. Additionally, we found that STAT1 suppresses Osterix transcript levels and Osterix promoter activity in vitro, indicating the suppression of Osterix transcription as one of the mechanisms behind the inhibitory effect of STAT1 on osteoblast differentiation. Furthermore, we found that fludarabine, a potent STAT1 inhibitor, significantly increases bone formation in a heterotopic ossification model. These results reveal previously unknown functions of STAT1 in skeletal homeostasis and may have important clinical implications for the treatment of skeletal bone fracture.

Accelerated cartilage resorption by chondroclasts during bone fracture healing in osteoprotegerin-deficient mice.

Receptor activator of nuclear factor-kappaB ligand (RANKL) and osteoprotegerin (OPG), a decoy receptor of RANKL, maintain bone mass by regulating the differentiation of osteoclasts, which are bone-resorbing cells. Endochondral bone ossification and bone fracture healing involve cartilage resorption, a less well-understood process that is needed for replacement of cartilage by bone. Here we describe the role of OPG produced by chondrocytes in chondroclastogenesis. Fracture healing in OPG(-/-) mice showed faster union of the fractured bone, faster resorption of the cartilaginous callus, and an increased number of chondroclasts at the chondroosseous junctions compared with that in wild-type littermates. When a cultured pellet of OPG(-/-) chondrocytes was transplanted beneath the kidney capsule, the pellet recruited many chondroclasts. The pellet showed the ability to induce tartrate-resistant acid phosphatase-positive multinucleated cells from RAW 264.7 cells in vitro. Finally, OPG(-/-) chondrocytes (but not wild-type chondrocytes) cultured with spleen cells induced many tartrate-resistant acid phosphatase-positive multinucleated cells. The expression of RANKL and OPG in chondrocytes was regulated by several osteotropic factors including 1,25-dihydroxyvitamin D(3), PTHrP, IL-1alpha, and TNF-alpha. Thus, local OPG produced by chondrocytes probably controls cartilage resorption as a negative regulator for chondrocyte-dependent chondroclastogenesis.

Fra-1/AP-1 impairs inflammatory responses and chondrogenesis in fracture healing.

Inflammation inevitably follows injury of various tissues, including bone. Transgenic overexpression of Fra-1, a component of the transcription factor activator protein-1 (AP-1), in various tissues progressively and globally enhances bone formation, but little is known about the possible effects of Fra-1/AP-1 on fracture healing. We created a transverse fracture of the mouse tibial diaphysis and examined fracture healing radiologically, histologically, and immunologically. Strikingly, fracture union was delayed even though the bone formation rate in callus was higher in Fra-1 transgenic (Tg) mice. In these mice, chondrogenesis around the fracture site was impaired, resulting in accumulation of fibrous tissue, which interferes with the formation of a bony bridge across the callus. Curiously, immediately after fracture, induction of the inflammatory mediators TNF-alpha, interleukin (IL)-6, and Cox-2 was significantly suppressed in Fra-1 Tg mice followed, by the reduced expression of Sox-9 and BMP-2. Because serum prostaglandin E(2) (PGE(2)) levels were dramatically low in these mice, we administered PGE(2) to the fracture site using a slow-release carrier. The accumulation of fibrous tissue in Fra-1 Tg mice was significantly reduced by PGE(2) administration, and chondrogenesis near the fracture site was partially restored. These data suggest that the Fra-1-containing transcription factor AP-1 inhibits fracture-induced endochondral ossification and bony bridge formation presumably through suppression of inflammation-induced chondrogenesis.

Cementless total hip replacement: past, present, and future.

Cementless total hip replacement (THR) is rapidly being accepted as the surgery for arthritic diseases of the hip joint. The bone-ingrowth rate in porous-type cementless implants was about 90% over 10 years after surgery, showing that biological fixation of cementless THR was well maintained on both the stem and cup sides. As for the stress shielding of the femur operated using a distal fixation-type stem, severe bone resorption was observed. The severe bone resorption group showed continuous progression for more than 10 years after surgery. Stem loosening directly caused by stress shielding has been considered less likely; however, close attention should be paid to bone resorption-associated disorders including femoral fracture. Cementless cups have several specific problems. It is difficult to decide whether a cup should be placed in the physiological position for the case of acetabular dysplasia by bone grafting or at a relatively higher position without bone grafting. The bone-ingrowth rate was lower in the group with en bloc bone grafting, and the reactive line was frequently noted in the bone-grafted region. Although no data indicated that en bloc bone grafting directly led to poor outcomes, such as loosening, cup placement at a higher site without bone grafting is now selected by most operators. The polyethylene liner in a cementless cup is thinned due to the metal cup thickness; however, it has been suggested that the apparent relation between the cup size and the wear rate was absent as long as a cementless cup is used. Comparative study indicated cementless THR was inferior with regard to the yearly polyethylene wear rate and incidence of osteolysis on both the stem and cup sides. Meta-analysis study on the survival rate between cement and cementless THR reported that cemented THR was slightly superior. It should be considered that specific problems for cementless THR, especially with regard to polyethylene wear, do occur.

Cell surface colony-stimulating factor 1 can be cleaved by TNF-alpha converting enzyme or endocytosed in a clathrin-dependent manner.

CSF-1 is a hemopoietic growth factor, which plays an essential role in macrophage and osteoclast development. Alternative splice variants of CSF-1 are synthesized as soluble or membrane-anchored molecules, although membrane CSF-1 (mCSF-1) can be cleaved from the cell membrane to become soluble CSF-1. The activities involved in this proteolytic processing, also referred to as ectodomain shedding, remain poorly characterized. In the present study, we examined the properties of the mCSF-1 sheddase in cell-based assays. Shedding of mCSF-1 was up-regulated by phorbol ester treatment and was inhibited by the metalloprotease inhibitors GM6001 and tissue inhibitor of metalloproteases 3. Moreover, the stimulated shedding of mCSF-1 was abrogated in fibroblasts lacking the TNF-alpha converting enzyme (TACE, also known as a disintegrin and metalloprotease 17) and was rescued by expression of wild-type TACE in these cells, strongly suggesting that the stimulated shedding is TACE dependent. Additionally, we observed that mCSF-1 is predominantly localized to intracellular membrane compartments and is efficiently internalized in a clathrin-dependent manner. These results indicate that the local availability of mCSF-1 is actively regulated by ectodomain shedding and endocytosis. This mechanism may have important implications for the development and survival of monocyte lineage cells.

Localized pigmented villonodular synovitis presenting as a loose body following minor trauma in the knee: a case report.

Localized pigmented villonodular synovitis (LPVS) is widely accepted to frequently develop symptoms resembling internal derangement in the knee, including limitation of motion and episodes of giving way and locking. We report the case of a 31-year-old man with LPVS displaying an unusual presentation. After sustaining a twisting injury to the knee, he suffered constant but subtle knee discomfort, sudden attacks of pain and a feeling of a loose body. Arthroscopic examination 1 month after injury revealed a freely mobile tumor in the supra-patellar pouch that was not pedunculated and displayed no soft tissue attachments to the synovium. Histological findings for the tumor were consistent with a diagnosis of LPVS. This case illustrates that LPVS may present with symptoms of a loose body after trauma to the knee.

Administration of cyclooxygenase-2 inhibitor reduces joint inflammation but exacerbates osteopenia in IL-1 alpha transgenic mice due to GM-CSF overproduction.

IL-1alpha transgenic (Tg) mice exhibit chronic inflammatory arthritis and subsequent osteopenia, with IL-1-induced GM-CSF playing an important role in the pathogenesis. This study analyzed the mechanisms underlying osteopenia in Tg mice, and the therapeutic effects of the cyclooxygenase-2 inhibitor celecoxib on such osteopenia. Inhibited osteoclast formation was observed in RANKL-treated bone marrow cell (BMC) cultures from Tg mice and coculture of Tg-derived BMCs and wild-type-derived primary osteoblasts (POBs). FACS analysis indicated that this inhibition was attributable to a decreased number of osteoclast precursors within Tg-derived BMCs. Moreover, in coculture of Tg-derived POBs and either Tg- or wild-type-derived BMCs, osteoclast formation was markedly inhibited because Tg-derived POBs produced abundant GM-CSF, known as a potent inhibitor of osteoclast differentiation. Histomorphometric analysis of Tg mice revealed that both bone formation and resorption were decreased, with bone formation decreased more prominently. Interestingly, administration of celecoxib resulted in further deterioration of osteopenia where bone formation was markedly suppressed, whereas bone resorption remained unchanged. These results were explained by our in vitro observation that celecoxib dose-dependently and dramatically decreased osteogenesis by Tg mouse-derived POBs in culture, whereas mRNA expressions of GM-CSF and M-CSF remained unchanged. Consequently, blockade of PGE(2) may exert positive effects on excessively enhanced bone resorption observed in inflammatory bone disease, whereas negative effects may occur mainly through reduced bone formation, when bone resorption is constitutively down-regulated as seen in Tg mice.

Impaired bone fracture healing in matrix metalloproteinase-13 deficient mice.

Vascular and cellular invasion into the cartilage is a critical step in the fracture healing. Matrix metalloproteinase-13 (MMP-13) is a member of the zinc-dependent endopeptidase family and plays an important role in remodeling of extracellular matrix. Therefore we investigated the possible involvement of MMP-13 in a murine model of stabilized bone fracture healing. Repair of the fracture in MMP-13 deficient (MMP-13(-/-)) mice was significantly delayed and characterized by a retarded cartilage resorption in the fracture callus. Immunohistochemistry indicated severe defects in vascular penetration and chondroclast recruitment to the fracture callus in MMP-13(-/-) mice. Consistent with the observations, the chondrocyte pellets cultured from the MMP13(-/-) mice exhibited diminished angiogenic activities when the pellets were co-cultured with endothelial cells. These results suggest that MMP-13 is crucial to the process of angiogenesis during healing of fracture, especially in the cartilage resorption process.