Hedgehog signaling molecules in bone marrow cells at the initial stage of fracture repair

Anabolic Strategies to Augment Bone Fracture Healing

PURPOSE OF REVIEW

The development of therapeutics that target anabolic pathways involved in skeletogenesis is of great importance with regard to disease resulting in bone loss, or in cases of impaired bone repair. This review aims to summarize recent developments in this area.

RECENT FINDINGS

A greater understanding of how drugs that modulate signaling pathways involved in skeletogenesis exert their efficacy, and the molecular mechanisms resulting in bone formation has led to novel pharmacological bone repair strategies. Furthermore, crosstalk between pathways and molecules has suggested signaling synergies that may be exploited for enhanced tissue formation. The sequential pharmacological stimulation of the molecular cascades resulting in tissue repair is a promising strategy for the treatment of bone fractures. It is proposed that a therapeutic strategy which mimics the natural cascade of events observed during fracture repair may be achieved through temporal targeting of tissue repair pathways.

Calvarial Defect Healing Induced by Small Molecule Smoothened Agonist

Hedgehog (Hh) signaling positively regulates both endochondral and intramembranous ossification. Use of small molecules for tissue engineering applications poses several advantages. In this study, we examined whether use of an acellular scaffold treated with the small molecule Smoothened agonist (SAG) could aid in critical-size mouse calvarial defect repair. First, we verified the pro-osteogenic effect of SAG in vitro, using primary neonatal mouse calvarial cells (NMCCs). Next, a 4 mm nonhealing defect was created in the mid-parietal bone of 10-week-old CD-1 mice. The scaffold consisted of a custom-fabricated poly(lactic-co-glycolic acid) disc with hydroxyapatite coating (measuring 4 mm diameter × 0.5 mm thickness). Treatment groups included dimethylsulfoxide control (n = 6), 0.5 mM SAG (n = 7) or 1.0 mM SAG (n = 7). Evaluation was performed at 4 and 8 weeks postoperative, by a combination of high-resolution microcomputed tomography, histology (H & E, Masson's Trichrome), histomorphometry, and immunohistochemistry (BSP, OCN, VEGF). In vivo results showed that SAG treatment induced a significant and dose-dependent increase in calvarial bone healing by all radiographic parameters. Histomorphometric analysis showed an increase in all parameters of bone formation with SAG treatment, but also an increase in blood vessel number and density. In summary, SAG is a pro-osteogenic, provasculogenic stimulus when applied locally in a bone defect environment.

Local administration of a hedgehog agonist accelerates fracture healing in a mouse model

Bone fracture healing is processed through multiple biological stages including the transition from cartilaginous callus to bony callus formation. Because of its specific, temporal and indispensable functions demonstrated by mouse genetic studies, Hedgehog (Hh) signaling is one of the most potent signaling pathways involved in these processes, but the effect of Hh-signaling activation by small compounds on the repair process had not yet been addressed. Here we examined therapeutic effects of local and one shot-administration of the Hh agonist known as smoothened agonist (SAG) on bone fracture healing in a mouse model. A quantitative analysis with three-dimensional micro-computed tomography showed that SAG administration increased the size of both the cartilaginous callus and bony callus at 14 days after the surgery. A histological analysis showed that SAG administration increased the number of cells expressing a proliferation marker and a chondrocyte marker in cartilaginous callus as well as the cells expressing an osteoblast marker in bony callus. These results indicate that the SAG administration resulted in an enhancement of callus formation during bone fracture healing, which is at least in part mediated by an increase in chondrocyte proliferation in cartilaginous callus and the promotion of bone formation in bony callus. Therapeutic strategies with a SAG-mediated protocol may thus be useful for the treatment of bone fractures.

Hedgehog signaling mediates woven bone formation and vascularization during stress fracture healing

Hedgehog (Hh) signaling is critical in developmental osteogenesis, and recent studies suggest it may also play a role in regulating osteogenic gene expression in the post-natal setting. However, there is a void of studies directly assessing the effect of Hh inhibition on post-natal osteogenesis. This study utilized a cyclic loading-induced ulnar stress fracture model to evaluate the hypothesis that Hh signaling contributes to osteogenesis and angiogenesis during stress fracture healing. Immediately prior to loading, adult rats were given GDC-0449 (Vismodegib - a selective Hh pathway inhibitor; 50mg/kg orally twice daily), or vehicle. Hh signaling was upregulated in response to stress fracture at 3 days (Ptch1, Gli1 expression), and was markedly inhibited by GDC-0449 at 1 day and 3 days in the loaded and non-loaded ulnae. GDC-0449 did not affect Hh ligand expression (Shh, Ihh, Dhh) at 1 day, but decreased Shh expression by 37% at 3 days. GDC-0449 decreased woven bone volume (-37%) and mineral density (-17%) at 7 days. Dynamic histomorphometry revealed that the 7 day callus was composed predominantly of woven bone in both groups. The observed reduction in woven bone occurred concomitantly with decreased expression of Alpl and Ibsp, but was not associated with differences in early cellular proliferation (as determined by callus PCNA staining at 3 days), osteoblastic differentiation (Osx expression at 1 day and 3 days), chondrogenic gene expression (Acan, Sox9, and Col2α1 expression at 1 day and 3 days), or bone resorption metrics (callus TRAP staining at 3 days, Rankl and Opg expression at 1 day and 3 days). To evaluate angiogenesis, vWF immunohistochemistry showed that GDC-0449 reduced fracture callus blood vessel density by 55% at 3 days, which was associated with increased Hif1α gene expression (+30%). Dynamic histomorphometric analysis demonstrated that GDC-0449 also inhibited lamellar bone formation. Lamellar bone analysis of the loaded limb (directly adjacent to the woven bone callus) showed that GDC-0449 significantly decreased mineral apposition rate (MAR) and bone formation rate (BFR/BS) (-17% and -20%, respectively). Lamellar BFR/BS in the non-loaded ulna was also significantly decreased (-37%), indicating that Hh signaling was required for normal bone modeling. In conclusion, Hh signaling plays an important role in post-natal osteogenesis in the setting of stress fracture healing, mediating its effects directly through regulation of bone formation and angiogenesis.

The Role of Hedgehog Signaling in Tumor Induced Bone Disease

Despite significant progress in cancer treatments, tumor induced bone disease continues to cause significant morbidities. While tumors show distinct mutations and clinical characteristics, they behave similarly once they establish in bone. Tumors can metastasize to bone from distant sites (breast, prostate, lung), directly invade into bone (head and neck) or originate from the bone (melanoma, chondrosarcoma) where they cause pain, fractures, hypercalcemia, and ultimately, poor prognoses and outcomes. Tumors in bone secrete factors (interleukins and parathyroid hormone-related protein) that induce RANKL expression from osteoblasts, causing an increase in osteoclast mediated bone resorption. While the mechanisms involved varies slightly between tumor types, many tumors display an increase in Hedgehog signaling components that lead to increased tumor growth, therapy failure, and metastasis. The work of multiple laboratories has detailed Hh signaling in several tumor types and revealed that tumor establishment in bone can be controlled by both canonical and non-canonical Hh signaling in a cell type specific manner. This review will explore the role of Hh signaling in the modulation of tumor induced bone disease, and will shed insight into possible therapeutic interventions for blocking Hh signaling in these tumors.

The Hedgehog signalling pathway in bone formation

The Hedgehog (Hh) signalling pathway plays many important roles in development, homeostasis and tumorigenesis. The critical function of Hh signalling in bone formation has been identified in the past two decades. Here, we review the evolutionarily conserved Hh signalling mechanisms with an emphasis on the functions of the Hh signalling pathway in bone development, homeostasis and diseases. In the early stages of embryonic limb development, Sonic Hedgehog (Shh) acts as a major morphogen in patterning the limb buds. Indian Hedgehog (Ihh) has an essential function in endochondral ossification and induces osteoblast differentiation in the perichondrium. Hh signalling is also involved intramembrane ossification. Interactions between Hh and Wnt signalling regulate cartilage development, endochondral bone formation and synovial joint formation. Hh also plays an important role in bone homeostasis, and reducing Hh signalling protects against age-related bone loss. Disruption of Hh signalling regulation leads to multiple bone diseases, such as progressive osseous heteroplasia. Therefore, understanding the signalling mechanisms and functions of Hh signalling in bone development, homeostasis and diseases will provide important insights into bone disease prevention, diagnoses and therapeutics.

Roles for Hedgehog signaling in adult organ homeostasis and repair

The hedgehog (HH) pathway is well known for its mitogenic and morphogenic functions during development, and HH signaling continues in discrete populations of cells within many adult mammalian tissues. Growing evidence indicates that HH regulates diverse quiescent stem cell populations, but the exact roles that HH signaling plays in adult organ homeostasis and regeneration remain poorly understood. Here, we review recently identified functions of HH in modulating the behavior of tissue-specific adult stem and progenitor cells during homeostasis, regeneration and disease. We conclude that HH signaling is a key factor in the regulation of adult tissue homeostasis and repair, acting via multiple different routes to regulate distinct cellular outcomes, including maintenance of plasticity, in a context-dependent manner.

MCP/CCR2 signaling is essential for recruitment of mesenchymal progenitor cells during the early phase of fracture healing

OBJECTIVE

The purpose of this study was to investigate chemokine profiles and their functional roles in the early phase of fracture healing in mouse models.

METHODS

The expression profiles of chemokines were examined during fracture healing in wild-type (WT) mice using a polymerase chain reaction array and histological staining. The functional effect of monocyte chemotactic protein-1 (MCP-1) on primary mouse bone marrow stromal cells (mBMSCs) was evaluated using an in vitro migration assay. MCP-1-/- and C-C chemokine receptor 2 (CCR2)-/- mice were fractured and evaluated by histological staining and micro-computed tomography (micro-CT). RS102895, an antagonist of CCR2, was continuously administered in WT mice before or after rib fracture and evaluated by histological staining and micro-CT. Bone graft exchange models were created in WT and MCP-1-/- mice and were evaluated by histological staining and micro-CT.

RESULTS

MCP-1 and MCP-3 expression in the early phase of fracture healing were up-regulated, and high levels of MCP-1 and MCP-3 protein expression observed in the periosteum and endosteum in the same period. MCP-1, but not MCP-3, increased migration of mBMSCs in a dose-dependent manner. Fracture healing in MCP-1-/- and CCR2-/- mice was delayed compared with WT mice on day 21. Administration of RS102895 in the early, but not in the late phase, caused delayed fracture healing. Transplantation of WT-derived graft into host MCP-1-/- mice significantly increased new bone formation in the bone graft exchange models. Furthermore, marked induction of MCP-1 expression in the periosteum and endosteum was observed around the WT-derived graft in the host MCP-1-/- mouse. Conversely, transplantation of MCP-1-/- mouse-derived grafts into host WT mice markedly decreased new bone formation.

CONCLUSIONS

MCP-1/CCR2 signaling in the periosteum and endosteum is essential for the recruitment of mesenchymal progenitor cells in the early phase of fracture healing.

Activation of hedgehog signaling during fracture repair enhances osteoblastic-dependent matrix formation

Fracture repair is a well orchestrated process involving various cell types and signaling molecules. The hedgehog signaling pathway is activated in chondrocytes during fracture repair and is known to regulate chondrogenesis however, its role in osteoblasts during injury is yet unknown. In this study we observed tibial fracture repair in mice in which hedgehog signaling was modulated through genetic alterations of the pathway activator, smoothened. Levels of the hedgehog target genes Gli1 and Ptch1 in wildtype mice were upregulated in fracture calluses throughout the repair process. Forced activation of the hedgehog pathway in ubiquitous fashion resulted in increased matrix deposition in the fracture callus. Interestingly, inhibition in chondrocytes did not alter the fracture repair phenotype, while activation of hedgehog in osteoblasts was a requirement for normal fracture repair. In vitro, transcript levels of Gli1 and Ptch1 were elevated during osteoblastogenesis. Activation of hedgehog signaling positively affected osteoblastic differentiation and mineralization as detected using alkaline phosphatase and Von Kossa staining and Alp and Col1 expression. Here we show that the hedgehog signaling pathway plays a critical role in osteoblasts during fracture repair: inhibition of the pathway in osteoblasts leads to decreased matrix at the fracture site while activation increased matrix deposition.

Inhibition of miR-92a enhances fracture healing via promoting angiogenesis in a model of stabilized fracture in young mice

MicroRNAs (miRNAs) are endogenous small noncoding RNAs regulating the activities of target mRNAs and cellular processes. Although no miRNA has been reported to play an important role in the regulation of fracture healing, several miRNAs control key elements in tissue repair processes such as inflammation, hypoxia response, angiogenesis, stem cell differentiation, osteogenesis, and chondrogenesis. We compared the plasma concentrations of 134 miRNAs in 4 patients with trochanteric fractures and 4 healthy controls (HCs), and the levels of six miRNAs were dysregulated. Among these miRNAs, miR-92a levels were significantly decreased 24 hours after fracture, compared to HCs. In patients with a trochanteric fracture or a lumbar compression fracture, the plasma concentrations of miR-92a were lower on days 7 and 14, but had recovered on day 21 after the surgery or injury. To determine whether systemic downregulation of miR-92a can modulate fracture healing, we administered antimir-92a, designed using locked nucleic acid technology to inhibit miR-92a, to mice with a femoral fracture. Systemic administration of antimir-92a twice a week increased the callus volume and enhanced fracture healing. Enhancement of fracture healing was also observed after local administration of antimir-92a. Neovascularization was increased in mice treated with antimir-92a. These results suggest that plasma miR-92a plays a crucial role in bone fracture healing in human and that inhibition of miR-92a enhances fracture healing through angiogenesis and has therapeutic potential for bone repair.

Inhibition mechanism exploration of investigational drug TAK-441 as inhibitor against Vismodegib-resistant Smoothened mutant

Hedgehog signaling is a driving force in medulloblastoma and basal cell carcinoma (BCC), making it an attractive therapeutic target. Vismodegib recently received FDA approval for the treatment of inoperable BCC, but a drug-resistant Smoothened (Smo) mutant (D473H) was identified in a clinical study. TAK-441 is a pyrrolo[3,2-c]pyridine-4-one derivative that potently inhibits Hh signal transduction and is currently under investigation in clinical trials. We demonstrated that TAK-441 inhibits reporter activity in D473H-transfected cells with an IC50 of 79nM, while Vismodegib showed an IC50=7100nM. In order to investigate the mode of inhibition, we evaluated the Smo inhibitors with three different binding assays, such as [(3)H]-TAK-441 membrane binding assay, affinity selection-MS detection assay, and bodipy-cylopamine whole cell assay. In three different assays, Vismodegib and cyclopamine showed lower affinity for the D473H mutant in comparison with wild-type Smo. On the other hand, TAK-441 showed almost equal binding affinity for the D473H mutant compared with wild-type Smo in the binding assays, although TAK-441 binds to the same binding site as two other well-known inhibitors. These in vitro findings suggest that TAK-441 has the potential for clinical use in cancers that are dependent on Hedgehog signaling, including wild-type tumors and Vismodegib-resistant D473H mutants.

Sonic hedgehog regulates osteoblast function by focal adhesion kinase signaling in the process of fracture healing

Several biological studies have indicated that hedgehog signaling plays an important role in osteoblast proliferation and differentiation, and sonic hedgehog (SHH) expression is positively correlated with phosphorylated focal adhesion kinase (FAK) Tyr(397). However, the relationship between them and their role in the process of normal fracture repair has not been clarified yet. Immunohistochemical analysis revealed that SHH and pFAK Tyr(397) were expressed in bone marrow cells and that pFAK Tyr(397) was also detected in ALP-positive osteoblasts near the TRAP-positive osteoclasts in the fracture site in the ribs of mice on day 5 after fracture. SHH and pFAK Tyr(397) were detectable in osteoblasts near the hypertrophic chondrocytes on day 14. In vitro analysis showed that SHH up-regulated the expression of FAK mRNA and pFAK Tyr(397) time dependently in osteoblastic MC3T3-E1 cells. Functional analysis revealed that 5 lentivirus encoding short hairpin FAK RNAs (shFAK)-infected MC3T3-E1 cell groups displayed a round morphology and decreased proliferation, adhesion, migration, and differentiation. SHH stimulated the proliferation and differentiation of MC3T3-E1 cells, but had no effect on the shFAK-infected cells. SHH also stimulated osteoclast formation in a co-culture system containing MC3T3-E1 and murine CD11b(+) bone marrow cells, but did not affect the shFAK-infected MC3T3-E1 co-culture group. These data suggest that SHH signaling was activated in osteoblasts at the dynamic remodeling site of a bone fracture and regulated their proliferation and differentiation, as well as osteoclast formation, via FAK signaling.

Overexpression of the Hedgehog signalling pathway in head and neck squamous cell carcinoma

BACKGROUND

Overexpression of the Hedgehog (HH) signalling pathway has been described in several malignancies and is associated with a poor prognosis. HH signalling blockade reduces tumour growth in vitro and in vivo. We aimed to determine whether head and neck squamous cell carcinomas (HNSCCs) express HH proteins in comparison to healthy mucosa.

PATIENTS AND METHODS

Formalin-fixed and paraffin-embedded tissue sections of 10 patients with HNSCC were stained with fluorescence-labelled antibodies for cytokeratin and HH proteins (SHH, PTCH1/2, SMO, Gli1-3) and photographs were taken with a laser scanning microscope. The pixel count and colour intensity were analysed in RGB (red/green/blue) colour mode, and expression levels were compared to healthy mucosa.

RESULTS

Image analysis in RGB mode provided objective evidence for the over-expression of HH signalling components in HNSCC, particularly with regard to the transcription factors Gli1 (10-fold) and SHH (5-fold) in comparison with healthy mucosa. The lowest levels were found for Gli3 in HNSCC.

CONCLUSIONS

We postulate pivotal roles of Gli1 and SHH expression in the carcinogenesis of HNSCC. HH pathway overexpression appears to be involved in the initiation of tumour growth and spread due to its stem cell-modulating properties. Detection of HH pathway components, and especially Gli1 and SHH, in HNSCC might offer a promising target for further anticancer research in HNSCC.

Effect of sonic hedgehog/β-TCP composites on bone healing within the critical-sized rat femoral defect

The creation of entirely synthetically derived bone substitute materials which are as effective as autologous bone grafts is desirable. Osteogenesis involves the concerted action of several proteins within a signaling cascade. Hedgehog proteins act upstream of this cascade, inducing the expression of various bone morphogenetic proteins (BMPs) and promoting physiological bone healing. Therefore, the hypothesis that hedgehog signaling in bone defects improves bone healing more than BMP signaling alone was tested. Recombinant N-terminal sonic hedgehog protein (N-SHh), BMP-2 or a combination of the two was added to β-tricalcium phosphate (β-TCP) and 5-mm femoral midshaft defects in nude rats were filled with these composites. The defects were stabilized with mini-plates. After eight weeks, the animals were sacrificed and the femora were explanted. The radiological evaluation was followed by a three-point bending test and histological examination. BMP-2/β-TCP composites showed a trend of increased stiffness compared with the controls (β-TCP without protein). N-SHh/β-TCP composites had lower stiffness compared with the control group and the N-SHh/BMP-2/β-TCP composites also had lower average stiffness compared with the controls (all not significant). Histomorphometry, however, revealed abundant cartilage and bone core formation in the N-SHh-composite groups. The sum of the new cartilage and bone was highest in the combination group N-SHh/BMP-2 (not significant). The addition of N-SHh to bone substitute materials appears to delay bone healing at the applied concentration and observation time but also showed a trend for higher amounts of ossifying cartilage.

Stromal cell-derived factor 1 regulates the actin organization of chondrocytes and chondrocyte hypertrophy

Stromal cell-derived factor 1 (SDF-1/CXCL12/PBSF) plays important roles in the biological and physiological functions of haematopoietic and mesenchymal stem cells. This chemokine regulates the formation of multiple organ systems during embryogenesis. However, its roles in skeletal development remain unclear. Here we investigated the roles of SDF-1 in chondrocyte differentiation. We demonstrated that SDF-1 protein was expressed at pre-hypertrophic and hypertrophic chondrocytes in the newly formed endochondral callus of rib fracture as well as in the growth plate of normal mouse tibia by immunohistochemical analysis. Using SDF-1(-/-) mouse embryo, we histologically showed that the total length of the whole humeri of SDF-1(-/-) mice was significantly shorter than that of wild-type mice, which was contributed mainly by shorter hypertrophic and calcified zones in SDF-1(-/-) mice. Actin cytoskeleton of hypertrophic chondrocytes in SDF-1(-/-) mouse humeri showed less F-actin and rounder shape than that of wild-type mice. Primary chondrocytes from SDF-1(-/-) mice showed the enhanced formation of philopodia and loss of F-actin. The administration of SDF-1 to primary chondrocytes of wild-type mice and SDF-1(-/-) mice promoted the formation of actin stress fibers. Organ culture of embryonic metatarsals from SDF-1(-/-) mice showed the growth delay, which was recovered by an exogenous administration of SDF-1. mRNA expression of type X collagen in metatarsals and in primary chondrocytes of SDF-1(-/-) mouse embryo was down-regulated while the administration of SDF-1 to metatarsals recovered. These data suggests that SDF-1 regulates the actin organization and stimulates bone growth by mediating chondrocyte hypertrophy.

Sonic Hedgehog-mediated synergistic effects guiding angiogenesis and osteogenesis

Sonic hedgehog (Shh) is a morphogen controlling the skeletal and vascular development in the embryo but is also reactivated during adult repair processes. Thus, this molecule holds great therapeutic potential for biotechnological and biomedical approaches aiming to enhance tissue regeneration or to replace damaged tissues. According to present knowledge, Shh signaling controls the expression of several families of growth factors involved in neovascularization and vessel maturation and acts upstream of the most prominent angiogenic growth factor, vascular endothelial growth factor. In this context, a very interesting feature of Shh is that it controls both angiogenic activity and vessel stabilization by mural cells. In parallel, Shh seems to have a direct effect on endothelial cell tube formation and seems to trigger the differentiation process of mesenchymal stem cells toward the osteogenic lineage. In this chapter, we will therefore shortly summarize the multifaceted potential of Shh for bone repair and vascularization according to the current literature. In addition, we will show how coculture models based on outgrowth endothelial cells and primary osteoblasts can be used to reveal some of the relevant mechanisms by which Shh drives and connects bone regeneration and vascularization.

Novel oxysterols have pro-osteogenic and anti-adipogenic effects in vitro and induce spinal fusion in vivo

Stimulation of bone formation by osteoinductive materials is of great clinical importance in spinal fusion surgery, repair of bone fractures, and in the treatment of osteoporosis. We previously reported that specific naturally occurring oxysterols including 20(S)-hydroxycholesterol (20S) induce the osteogenic differentiation of pluripotent mesenchymal cells, while inhibiting their adipogenic differentiation. Here we report the characterization of two structural analogues of 20S, Oxy34 and Oxy49, which induce the osteogenic and inhibit the adipogenic differentiation of bone marrow stromal cells (MSC) through activation of Hedgehog (Hh) signaling. Treatment of M2-10B4 MSC with Oxy34 or Oxy49 induced the expression of osteogenic differentiation markers Runx2, Osterix (Osx), alkaline phosphatase (ALP), bone sialoprotein (BSP), and osteocalcin (OCN), as well as ALP enzymatic activity and robust mineralization. Treatment with oxysterols together with PPARγ activator, troglitazone (Tro), inhibited mRNA expression for adipogenic genes PPARγ, LPL, and aP2, and inhibited the formation of adipocytes. Efficacy of Oxy34 and Oxy49 in stimulating bone formation in vivo was assessed using the posterolateral intertransverse process rat spinal fusion model. Rats receiving collagen implants with Oxy 34 or Oxy49 showed comparable osteogenic efficacy to BMP2/collagen implants as measured by radiography, MicroCT, and manual inspection. Histological analysis showed trabecular and cortical bone formation by oxysterols and rhBMP2 within the fusion mass, with robust adipogenesis in BMP2-induced bone and significantly less adipocytes in oxysterol-induced bone. These data suggest that Oxy34 and Oxy49 are effective novel osteoinductive molecules and may be suitable candidates for further development and use in orthopedic indications requiring local bone formation.

Hedgehog signaling and osteogenic differentiation in multipotent bone marrow stromal cells are inhibited by oxidative stress

Oxidative stress may play a major role in age-related osteoporosis in part by inhibiting osteoblast generation from osteoprogenitors cells. In the present study, we hypothesized that oxidative stress may inhibit the osteogenic differentiation of bone marrow stromal cells (MSC) in part by inhibiting the Hedgehog (Hh) signaling pathway, which is essential for bone development and maintenance and induces osteogenic differentiation of osteoprogenitor cells. To test this hypothesis, we examined the effects of oxidative stress on Sonic Hh (Shh)-induced osteogenic differentiation and signaling in M2-10B4 (M2) MSC, C3H10T1/2 embryonic fibroblasts, and mouse primary MSC. Treatment of cells with H(2)O(2) inhibited Shh-induced osteogenic differentiation determined by the inhibition of Shh-induced expression of osteogenic differentiation markers alkaline phosphatase (ALP), osterix (OSX), and bone sialoprotein (BSP). Similar effects were found when oxidative stress was induced by xanthine/xanthine oxidase (XXO) or minimally oxidized LDL (MM-LDL). H(2)O(2) , XXO, and MM-LDL treatment inhibited Shh-induced expression of the Hh target genes Gli1 and Patched1 as well as Gli-dependent transcriptional activity in M2 cells. H(2)O(2) treatment also inhibited Hh signaling induced by the direct activation of Smoothened by purmorphamine (PM), but not by Gli1 overexpression. This suggests that oxidative stress may inhibit Hh signaling upstream of Gli activation and Gli-induced gene expression. These findings demonstrate for the first time that oxidative stress inhibits Hh signaling associated with osteogenic differentiation. Inhibition of Hh signaling-mediated osteogenic differentiation of osteoprogenitor cells may in part explain the inhibitory effects of oxidative stress on osteoblast development, differentiation, and maintenance in aging.

Bone regeneration: the stem/progenitor cells point of view

After bone injuries, several molecular mechanisms establish bone repair from stem/progenitor cells. Inflammation factors attract regenerative cells which expand and differentiate in order to build up a bone highly similar to that before injury. Bone marrow (BM) mesenchymal stem cells (MSCs) as skeletal stem cells and endothelial progenitors (EPCs) are at the origin of such reparation mechanisms. However, discrepancies exist about their identities. Although cultured MSCs are extensively described, their in vivo native forms are poorly known. In addition, recent experiments show that several types of EPC exist. We therefore review up-to-date data on the characterization of such stem/progenitor cells and propose a new point of view of their function in bone regeneration.

Evidence for allosteric interactions of antagonist binding to the smoothened receptor

The Smoothened receptor (Smo) mediates hedgehog (Hh) signaling critical for development, cell growth, and migration, as well as stem cell maintenance. Aberrant Hh signaling pathway activation has been implicated in a variety of cancers, and small-molecule antagonists of Smo have entered human clinical trials for the treatment of cancer. Here, we report the biochemical characterization of allosteric interactions of agonists and antagonists for Smo. Binding of two radioligands, [(3)H]3-chloro-N-[trans-4-(methylamino)cyclohexyl]-N-{[3-(4-pyridinyl)-phenyl]methyl}-1-benzothiophene-2-carboxamide (SAG-1.3) (agonist) and [(3)H]cyclopamine (antagonist), was characterized using human Smo expressed in human embryonic kidney 293F membranes. We observed full displacement of [(3)H]cyclopamine by all Smo agonist and antagonist ligands examined. N-[(1E)-(3,5-Dimethyl-1-phenyl-1H-pyrazol-4-yl)methylidene]-4-(phenylmethyl)-1-piperazinamine (SANT-1), an antagonist, did not fully inhibit the binding of [(3)H]SAG-1.3. In a functional cell-based beta-lactamase reporter gene assay, SANT-1 and N-[3-(1H-benzimidazol-2-yl)-4-chlorophenyl]-3,4,5-tris(ethyloxy)-benzamide (SANT-2) fully inhibited 3-chloro-4,7-difluoro-N-[trans-4-(methylamino)cyclohexyl]-N-{[3-(4-pyridinyl)phenyl]methyl}-1-benzothiophene-2-carboxamide (SAG-1.5)-induced Hh pathway activation. Detailed "Schild-type" radioligand binding analysis with [(3)H]SAG-1.3 revealed that two structurally distinct Smoothened receptor antagonists, SANT-1 and SANT-2, bound in a manner consistent with that of allosteric modulation. Our mechanism of action characterization of radioligand binding to Smo combined with functional data provides a better understanding of small-molecule interactions with Smo and their influence on the Hh pathway.

Tenascin-W inhibits proliferation and differentiation of preosteoblasts during endochondral bone formation

We identified a cDNA encoding mouse Tenascin-W (TN-W) upregulated by bone morphogenetic protein (Bmp)2 in ATDC5 osteo-chondroprogenitors. In adult mice, TN-W was markedly expressed in bone. In mouse embryos, during endochondral bone formation TN-W was localized in perichondrium/periosteum, but not in trabecular and cortical bones. During bone fracture repair, cells in the newly formed perichondrium/periosteum surrounding the cartilaginous callus expressed TN-W. Furthermore, TN-W was detectable in perichondrium/periosteum of Runx2-null and Osterix-null embryos, indicating that TN-W is expressed in preosteoblasts. In CFU-F and -O cells, TN-W had no effect on initiation of osteogenesis of bone marrow cells, and in MC3T3-E1 osteoblastic cells TN-W inhibited cell proliferation and Col1a1 expression. In addition, TN-W suppressed canonical Wnt signaling which stimulates osteoblastic differentiation. Our results indicate that TN-W is a novel marker of preosteoblasts in early stage of osteogenesis, and that TN-W inhibits cell proliferation and differentiation of preosteoblasts mediated by canonical Wnt signaling.

Development of a locking femur nail for mice

We herein report on a novel locking intramedullary nail system in a murine closed femur fracture model. The nail system consists of a modified 24-gauge injection needle and a 0.1-mm-diameter tungsten guide wire. Rotation stability was accomplished by flattening the proximal and distal end of the needle. Torsional mechanical testing of the implants in osteotomized cadaveric femora revealed a superiority of the locking nail (3.9+/-1.0 degrees rotation at a torque of 0.9 Nmm, n=10) compared to the unmodified injection needle (conventional nail; 52.4+/-3.2 degrees, n=10, p<0.05). None of the implants, however, achieved the rotation stability of unfractured femora (0.3+/-0.5 degrees, n=10). In a second step, we tested the feasibility of the in vivo application of the locking nail to stabilize a closed femoral midshaft fracture in C57BL/6 mice. Of interest, none of the 10 animals showed a dislocation of the locking nail over a 5-week period, while 3 of 4 animals with conventional nail fracture stabilization showed a significant pin dislocation within the first 3 days (p<0.05). Mechanical testing after 5-weeks stabilization with the locking nail revealed an appropriate bone healing with a torque at failure of 71.6+/-3.4% and a peak rotation before failure of 68.4+/-5.3% relative to the unfractured contralateral femur. With the advantage that closed fractures can be fixed with rotation stability, the herein introduced model may represent an ideal tool to study bone healing in transgenic and knockout mice.

Unique and complimentary activities of the Gli transcription factors in Hedgehog signaling

The Gli family of transcription factors (Gli1, 2 and 3) mediate the Hedgehog morphogenetic signal by regulating the expression of downstream target genes. Aberrations in Hedgehog signaling seriously affect vertebrate development. Postnatally, Hedgehog signaling has been postulated to play a pivotal role in healing and repair processes and inappropriate pathway activation has been implicated in several types of cancers. To better understand both the upstream regulation of the Gli transcription factors, as well as their unique and combinatorial roles in regulating the expression of Hedgehog target genes, we have characterized embryonic fibroblasts (MEFs) from Gli mutant mice. Stimulation of wild-type MEFs by Sonic Hedgehog (Shh) peptide elicited unique profiles of induction of Hedgehog target genes Gli1, Ptc1, and Hip1. Gli2 loss-of-function was associated with diminished Shh-induced target gene expression, while Gli3 loss-of-function was associated with increased basal and Shh-induced target gene expression. The loss of Gli1 alone had no effect on target gene induction but did diminish Shh-induced target gene expression when combined with the loss of Gli2 or Gli3. Additionally, overexpression of Gli1 induced target gene expression in Gli2(-/-)3(-/-) MEFs, while Shh stimulation did not. Using MEFs expressing only Gli2 or Gli3, we found that both cyclopamine and the PKA activator forskolin inhibited target gene induction mediated by Gli2 and Gli3. These results demonstrate that Gli2 and Gli3 share common regulatory mechanisms and modulate Hedgehog target gene expression directly and independently while also regulating Gli1 expression, which in specific contexts, coordinately contributes to target gene activation.

Hedgehog signaling in the normal and diseased pancreas

The hedgehog (Hh) family of genes, sonic hedgehog (Shh), Indian hedgehog (Ihh), and desert hedgehog (Dhh) encode signaling molecules that regulate multiple functions during organ development and in adult tissues. Altered hedgehog signaling has been implicated in disturbed organ development as well as in different degenerative and neoplastic human diseases. Hedgehog signaling plays an important role in determination the fate of the mesoderm of the gut tube, as well as in early pancreatic development, and islet cell function. Recently, it has been shown that deregulation of hedgehog signaling molecules contributes to the pathogenesis and progression of pancreatic cancer and of chronic pancreatitis. Inhibition of hedgehog signaling using hedgehog antagonists reduces pancreatic cancer cell growth in vitro and in vivo, thus holding promise of novel agents in the treatment of this devastating disease. In this review, we discuss the role of hedgehog signaling during pancreatic development, its role in the pathogenesis of both chronic pancreatitis and pancreatic cancer, and lastly, the implications of this newly available information with regards to treatment of pancreatic cancer.

Patterns and localization of gene expression during intramembranous bone regeneration in the rat femoral marrow ablation model

Tissue formation and repair are dependent upon cascades of biological events, but the signals involved and the possible gene coexpression patterns during intramembranous bone repair are only poorly understood. We sought to place this mode of regeneration in context by profiling quantitative gene expression for a panel of 39 genes between days 1 and 14 following rat femoral marrow ablation. In situ hybridization was employed to localize a subset of genes. Additionally, principal components analysis was conducted to identify underlying factors suggestive of coexpression patterns. During inflammation (days 1-5), several genes, including cyclooxygenase-1 and -2, showed downregulation. Other proinflammatory cytokines, tumor necrosis factor-alpha and interleukin-1beta, exhibited increasing levels around day 5. During repair (days 3-10), growth factors, receptors, and inhibitor genes for transforming growth factor- beta; basic fibroblast growth factor; bone morphogenetic proteins 2, 4, and 7; vascular endothelial growth factor; and insulin-like growth factor-I were upregulated. In addition, the gene for core binding factor-alpha1 and markers of osteoblast function such as alkaline phosphatase, collagen type I, osteonectin, osteopontin, and osteocalcin had peak expression at day 5 or 7. The remodeling phase (days 10-14) was characterized by peaks for cytokines associated with osteoclastic activity including receptor activator of nuclear factor-kappaB, receptor activator of nuclear factor-kappaB ligand (RANKL), cathepsin K, tumor necrosis factor-alpha, interleukin-6, and cyclooxygenase-2. In situ hybridization showed that the most common sites of increased signal were within osteoblastic cells on trabecular and endosteal surfaces. Principal components analysis identified eight underlying factors that together explained over 80% of the variance in the data.

Molecular signaling in intervertebral disk development

The purpose of this investigation is to identify and study the expression pattern of pertinent molecular factors involved in the differentiation of the intervertebral disk (IVD). It is likely that hedgehog genes and the BMP inhibitors are key factors involved in spinal joint formation. Radioactive in situ hybridization with mRNA probes for pax-1, SHH, IHH and Noggin gene was performed on mouse embryo and adult tissue. Immunohistochemistry was performed to localize hedgehog receptor, "patched" (ptc). From 14.5 dpc until birth pax-1 mRNA was expressed in the developing anulus fibrosus (AF). During the same developmental period Noggin mRNA is highly expressed throughout the spine, in the developing AF, while ptc protein and SHH mRNA were expressed in the developing nucleus pulposus (NP). IHH mRNA was expressed by condensing chondrocytes of the vertebral bodies and later becomes confined to the vertebral endplate. We show for the first time that pax-1 is expressed in the adult intervertebral disk. Ptc expression in the NP is an indicator of hedgehog protein signaling in the developing IVD. The expression pattern of the BMP inhibitor Noggin appears to be important for the normal formation of the IVD and may prove to play a role in its segmental pattern formation.

Tissue repair and stem cell renewal in carcinogenesis

Cancer is increasingly being viewed as a stem cell disease, both in its propagation by a minority of cells with stem-cell-like properties and in its possible derivation from normal tissue stem cells. But stem cell activity is tightly controlled, raising the question of how normal regulation might be subverted in carcinogenesis. The long-known association between cancer and chronic tissue injury, and the more recently appreciated roles of Hedgehog and Wnt signalling pathways in tissue regeneration, stem cell renewal and cancer growth together suggest that carcinogenesis proceeds by misappropriating homeostatic mechanisms that govern tissue repair and stem cell self-renewal.

Characterization of a closed femur fracture model in mice

OBJECTIVES

The goal of this study was to develop and characterize a closed femur fracture model for mice that can be used for the molecular and genetic analysis of fracture healing.

STUDY DESIGN

Longitudinal time study of species-specific fracture healing.

METHODS

A protocol was developed for creating reproducible, closed femur fractures in mice. Impending fractures were stabilized by retrograde insertion of a 0.01-inch-diameter, stainless steel wire into the intramedullary canal. The intramedullary wire was held in place with a wedge made from the first 2 mm of a 30-gauge needle. Fractures were produced by 3-point bending. Fracture healing was assessed by radiography, histology, and torsional mechanical testing.

RESULTS

The mouse femur fracture technique produced good results with minimal loss of animals. Of the 246 mice used in the study, 22 mice were excluded due to poor fracture quality (8), loss of fracture stabilization (6), or to anesthesia death (8). Radiography showed a consistent pattern of fracture healing between mice with peak fracture callus volume evident at 10 (15 mice) to 14 days (18 mice) after fracture. Fracture bridging was apparent in all 3-week postfracture radiographs (35 mice). Histologic examination of 117 specimens at 9 time points showed chondrocyte differentiation within the fracture callus by 7 days after fracture, endochondral ossification occurring by 10 days after fracture, and bone remodeling evident as early as 3 weeks after fracture. Despite radiologic and histologic evidence of fracture bridging after 3 weeks, torsional mechanical testing of 68 mice at 3, 4, 6, and 12 weeks after fracture (group size of 15 to 18 mice at each time point) indicated that significant increases in structural or material strength did not occur until 6 to 12 weeks after fracture.

CONCLUSIONS

Femur fracture healing in mice follows a typical endochondral ossification pathway with fracture bridging occurring approximately 1 week faster in mice than rats. This fracture model is amenable to the molecular and genetic analysis of fracture healing using different inbred, transgenic, and knockout strains of mice.

A novel role of the hedgehog pathway in lens regeneration

Lens regeneration in the adult newt is a classic example of replacing a lost organ by the process of transdifferentiation. After lens removal, the pigmented epithelial cells of the dorsal iris proliferate and dedifferentiate to form a lens vesicle, which subsequently differentiates to form a new lens. In searching for factors that control this remarkable process, we investigated the expression and role of hedgehog pathway members. These molecules are known to affect retina and pigment epithelium morphogenesis and have been recently shown to be involved in repair processes. Here we show that Shh, Ihh, ptc-1, and ptc-2 are expressed during lens regeneration. The expression of Shh and Ihh is quite unique since these genes have never been detected in lens. Interestingly, both Shh and Ihh are only expressed in the regenerating and developing lens, but not in the intact lens. Interfering with the hedgehog pathway results in considerable inhibition of the process of lens regeneration, including decreased cell proliferation as well as interference with lens fiber differentiation in the regenerating lens vesicle. Down-regulation of ptc-1 was also observed when inhibiting the pathway. These results provide the first evidence of a novel role for the hedgehog pathway in specific regulation of the regenerating lens.

The effect of age on gene expression in adult and juvenile rats following femoral fracture

OBJECTIVE

To compare mRNA gene expression during fracture healing in young and adult rats.

DESIGN

Gene expression was measured at zero, 1, 2, 4, 6, 8 and 10 weeks after fracture (6 rats/age/time point) in rats at 6 and 26 weeks of age at surgery.

SETTING

AAALAC-accredited vivarium of an independent academic medical center.

ANIMALS

Female Sprague-Dawley rats at 6 and 26 weeks of age.

INTERVENTION

An intramedullary rod was placed retrograde in the left femur, and a simple transverse closed middiaphyseal fracture was induced.

MAIN OUTCOME MEASUREMENTS

mRNA gene expression was measured for 34 genes for extracellular matrix, osteoblasts, bone morphogenic protein, inflammation, cytokine, and receptor genes.

RESULTS

The young rats reached radiographic union by 4 weeks after fracture, whereas the adult rats took 8 to 10 weeks to unite. All genes studied increased in mRNA expression with a peak at 1 to 2 weeks after fracture. All genes in the young rats then subsided to baseline by 4 weeks after fracture. However, during the longer period needed for radiographic union in the adult rats, only genes related to bone matrix, osteoblastic markers, angiogenesis, and the fibroblast growth factors remained significantly up-regulated at 4 and 6 weeks after fracture. Genes related to cartilage, Indian hedgehog, the bone morphogenetic proteins, and transforming growth factor-beta came to undetectable baseline values in the adult rats prior to radiographic union.

CONCLUSIONS

Most stimulators of bone healing are not expressed during the later stages of fracture repair in adult rats. Other genes must control bone growth to bridge the fracture gap.

Gene expression in older rats with delayed union of femoral fractures

BACKGROUND

Fracture-healing slows with age. While six-week-old rats regain normal bone biomechanics at four weeks after a fracture, one-year-old rats require more than twenty-six weeks. The objective of this study was to examine the possible role of altered mRNA gene expression in this delayed union.

METHODS

Closed midshaft femoral fractures were created in six-week-old and one-year-old Sprague-Dawley female rats. The animals were killed at zero-time (unfractured) or at 0.4, one, two, three, four, or six weeks after the fracture. mRNA levels were measured by reverse transcription-polymerase chain reaction in the fracture callus for twenty-seven matrix, cytokine, and cytokine-receptor genes for the seven animals per time-point per age-group.

RESULTS

The younger rats healed radiographically by four weeks after the fracture, whereas none of the older rats had healed by the sixth week. Despite the difference in healing rates, the levels of mRNA gene expression, in general, followed the same pattern in both age-groups. The mRNA expression levels increased to a peak at one to two weeks after the fracture and then decreased to very low or undetectable levels at four and six weeks after the fracture for both age-groups. Significantly lower levels of mRNA for Indian hedgehog (Ihh) and bone morphogenetic protein-2 (BMP-2) were detected in the fracture calluses of the older rats (p < 0.01 and p < 0.05, respectively).

CONCLUSIONS

All genes studied were up-regulated by the fracture in both age-groups. Thus, the failure of the older rats to heal promptly was not due to the lack of expression of any of the studied genes. The increase in mRNA for Ihh and BMP-2 in the older rats was smaller than that in the younger rats, which may contribute to slower fracture repair. The return of mRNA gene expression to baseline in the older rats prior to healing may contribute to the delayed union. The slower healing response of the older rats did not stimulate a negative-feedback increase in the mRNA expression of stimulatory cytokines.

Developmental roles and clinical significance of hedgehog signaling

Cell signaling plays a key role in the development of all multicellular organisms. Numerous studies have established the importance of Hedgehog signaling in a wide variety of regulatory functions during the development of vertebrate and invertebrate organisms. Several reviews have discussed the signaling components in this pathway, their various interactions, and some of the general principles that govern Hedgehog signaling mechanisms. This review focuses on the developing systems themselves, providing a comprehensive survey of the role of Hedgehog signaling in each of these. We also discuss the increasing significance of Hedgehog signaling in the clinical setting.

Expression and distribution of transcripts for sonic hedgehog in the early phase of fracture repair

Localization and expression of mRNAs for sonic hedgehog (Shh) at a fracture site in the early phase postfracture were investigated by in situ hybridization and reverse transcription and polymerase chain reaction (RT-PCR). A closed fracture was made in the midshaft of the right tibia of 5-week-old ICR mice, and fractured sites were harvested prefracture (day 0) and on days 2 and 12. In situ hybridization revealed that transcripts for Shh were not detected on day 0, but they were detected in proliferating callus-forming cells in the periosteum and the surrounding tissue, and in the medullary cavity prior to apparent new cartilage and bone formation. Gli 1 (a signaling mediator for Shh) and bone morphogenetic protein-4 transcripts were colocalized with those for Shh transcripts on day 2. The RT-PCR showed that Shh mRNA was detected in the PCR product from day 2, but not from days 0 and 12. These findings are the first description about the activation of Shh gene in the early postfracture reaction.

Determination of variations in gene expression during fracture healing

The genetic make-up and physiological state of a cell or tissue in an organism interact to determine the level at which specific genes are expressed. Identifying genes differentially expressed between 2 genetic or physiological states often gives insight into the molecular mechanisms controlled by the process in question. Various methods have been devised to identify differentially expressed genes and to quantify the expression of differentially regulated genes at the RNA or protein level. These methods are most accurate when the experimental samples are derived from highly controlled and reproducible sources, such as cultured cells. However, no simple in vitro models have been developed to study all biological processes and some are still best studied in the context of the whole organism. Using bone fracture healing as a model, we quantified the expression of 2 housekeeping and 2 regulatory genes during this complex biological process to determine the statistical parameters required to study differential gene expression in tissue samples derived from entire organisms. Our analysis shows that 5 samples in each group are needed to identify genes differentially expressed by a factor of 3 between 2 physiological or genetic states.

Sonic hedgehog is involved in osteoblast differentiation by cooperating with BMP-2

The roles of Sonic hedgehog (Shh) and Bone morphogenetic protein-2 (Bmp-2) in osteoblast differentiation were investigated using in vitro cell systems. Recombinant amino-terminal portion of SHH (rSHH-N) dose dependently stimulated ALP activity in C3H10T1/2 and MC3T3-E1 cells. rSHH-N induced expression of Osteocalcin mRNA in C3H10T1/2 cells. A soluble form of the receptor for type IA BMP receptor antagonized rSHH-N-induced ALP activity in C3H10T1/2 and MC3T3-E1 cells, indicating that BMPs are involved in SHH-induced osteoblast differentiation. Simultaneous supplement with rSHH-N and BMP-2 synergistically induced ALP activity and expression of Osteocalcin mRNA in C3H10T1/2 cells. Pretreatment with rSHH-N for 6 h enhanced the response to BMP-2 by increasing ALP activity in C3H10T1/2 and MC3T3-E1 cells. Stimulatory effects of rSHH-N and additive effects with rSHH-N and BMP-2 on ALP activity were also observed in mouse primary osteoblastic cells. Transplantation of BMP-2 (1 microg) into muscle of mice induced formation of ectopic bone, whereas transplantation of r-SHH-N (1-5 microg) failed to generate it. These results indicate that Shh plays important roles in osteoblast differentiation by cooperating with BMP.

Comparison of hedgehog and patched-1 protein expression in the cranial sutures of craniosynostotic and wild-type rabbits

Craniosynostosis is characterized by premature fusion of the cranial sutures. At the molecular level, mutations in homeobox genes, transcription factors, and growth factor receptors have been implicated in the pathogenesis of this disorder, but the specific etiologic pathways have not yet been elucidated. To further study the molecular biology behind craniosynostosis, perisutural tissues in a unique rabbit model with congenital delayed-onset coronal craniosynostosis were examined for the presence of the hedgehog family of growth factors and their receptor, patched-1. Expression of desert hedgehog, Indian hedgehog, sonic hedgehog, and patched-1 was evaluated in four areas: suture, endosteum, periosteum, and osteocytes, using immuno-histochemistry (n = 8). Protein levels in affected animals were compared with protein levels in wild-type control rabbits (n = 8). Overall, sonic hedgehog, Indian hedgehog, and patched-1 protein levels were greater in affected animals. Specifically, areas of increased staining were seen along the bony interface of the endosteum and periosteum and in the osteocytes of the synostotic rabbits. Interestingly, in the suture, increased levels of Indian hedgehog and sonic hedgehog, but not patched-1, were seen. There was minimal expression of desert hedgehog in both rabbit types. The increased overall presence of hedgehog and patched-1 proteins in synostotic rabbits may be a reactive change to the disorder or part of the pathogenic process. Although the specific cause cannot be determined from the data, it is clear that the molecular milieu of the cranial sutures in synostotic rabbits is markedly different from that of wild-type rabbits.

Accelerated up-regulation of L-Sox5, Sox6, and Sox9 by BMP-2 gene transfer during murine fracture healing

Fracture repair is the best-characterized situation in which activation of chondrogenesis takes place in an adult organism. To better understand the mechanisms that regulate chondrogenic differentiation of mesenchymal progenitor cells during fracture repair, we have investigated the participation of transcription factors L-Sox5, Sox6, and Sox9 in this process. Marked up-regulation of L-Sox5 and Sox9 messenger RNA (mRNA) and smaller changes in Sox6 mRNA levels were observed in RNAse protection assays during early stages of callus formation, followed by up-regulation of type II collagen production. During cartilage expansion, the colocalization of L-Sox5, Sox6, and Sox9 by immunohistochemistry and type II collagen transcripts by in situ hybridization confirmed a close relationship of these transcription factors with the chondrocyte phenotype and cartilage production. On chondrocyte hypertrophy, production of L-Sox5, Sox9 and type II collagen were down-regulated markedly and that of type X collagen was up-regulated. Finally, using adenovirus mediated bone morphogenetic protein 2 (BMP-2) gene transfer into fracture site we showed accelerated up-regulation of the genes for all three Sox proteins and type II collagen in fractures treated with BMP-2 when compared with control fractures. These data suggest that L-Sox5, Sox6, and Sox9 are involved in the activation and maintenance of chondrogenesis during fracture healing and that enhancement of chondrogenesis by BMP-2 is mediated via an L-Sox5/Sox6/Sox9-dependent pathway.

Melanoma cells stimulate osteoclastogenesis, c-Src expression and osteoblast cytokines

Malignant melanomas metastasise to the bone and enhance osteoclast bone resorption. We demonstrated that a 48-h-B16 melanoma cell conditioned media (B16CM) induced osteoclastogenesis in mouse bone marrow cultures, without the requirement of B16 cell-bone marrow cell co-culture. B16 cells transcriptionally expressed detectable levels of TGFbeta1, IL-6, M-CSF, GM-CSF and TNFalpha mRNAs, albeit to a lower extent compared with levels in osteoblasts, and failed to express PTHrP, OPGL, OPG and IL-1beta. Interestingly, B16CM greatly upregulated IL-1beta, IL-6 and GM-CSF, and modestly enhanced TNFalpha and OPGL mRNA expression in osteoblasts, suggesting a potential indirect stimulation of osteoclastogenesis via the osteogenic lineage. B16CM barely upregulated c-Fos, but strongly and time-dependently enhanced c-Src expression in the total bone marrow cultures during osteoclast differentiation. Moreover, c-Src expression was enhanced in differentiated and purified osteoclast preparations to higher levels than in stromal cells. In conclusion, melanoma induces osteoclast generation with a paracrine mechanism independent of cell-cell contact, specifically upregulating c-Src in osteoclasts and cytokine expression in osteoblasts.

Erythroid differentiation in vitro is blocked by cyclopamine, an inhibitor of hedgehog signaling

Adult hematopoietic differentiation is a developmental process that employs many of the same molecular mechanisms as embryogenesis. To explore the possibility that hedgehog signaling is involved in the control of hematopoietic differentiation, we screened a panel of human leukemia cell lines for the expression of Patched1 and Smoothened, the receptor and coreceptor for hedgehog ligands. Expression was found in multiple cell lines, and Patched1 expression was detected in normal marrow. Induction of myeloid differentiation in cell lines downregulated expression of both genes. When normal marrow mononuclear cells were grown in semisolid medium in the presence of 10 microM cyclopamine, development of colonies of granulocytic/monocytic lineage was unaffected in terms of both number and morphology. The number of erythroid colonies, however, was significantly reduced (P < 0.01). Furthermore, hemoglobinization was substantially delayed relative to controls in those erythroid colonies that did form. Incubation of hematopoietic progenitors with Shh-N and GM-CSF resulted in increased granulocyte/monocyte colonies (P < 0.01); the increase was blocked by cyclopamine. Incubation of hematopoietic progenitors with Shh-N and stem cell factor resulted in larger erythroid colonies. These results suggest that elements of the hedgehog signaling pathway are involved in the control of hematopoietic differentiation.

Regulation of osteoblast differentiation mediated by bone morphogenetic proteins, hedgehogs, and Cbfa1

Osteoblasts arise from common progenitors with chondrocytes, muscle and adipocytes, and various hormones and local factors regulate their differentiation. We review here regulation of osteoblast differentiation mediated by the local factors such as bone morphogenetic proteins (BMPs) and hedgehogs and the transcription factor, core-binding factor alpha-1 (Cbfa1). BMPs are the most potent regulators of osteoblast differentiation among the local factors. Sonic and Indian hedgehogs are involved in osteoblast differentiation by interacting with BMPs. Cbfa1, a member of the runt domain gene family, plays a major role in the processes of a determination of osteoblast cell lineage and maturation of osteoblasts. Cbfa1 is an essential transcription factor for osteoblast differentiation and bone formation, because Cbfa1-deficient mice completely lacked bone formation due to maturation arrest ofosteoblasts. Although the regulatory mechanism of Cbfa1 expression has not been fully clarified, BMPs are an important local factor that up-regulates Cbfa1 expression. Thus, the intimate interaction between local factors such as BMPs and hedgehogs and the transcription factor, Cbfa1, is important to osteoblast differentiation and bone formation.