Differentiation-inducing factor-1 alters canonical Wnt signaling and suppresses alkaline phosphatase expression in osteoblast-like cell lines

E3 ubiquitin ligase RMND5A maintains the self-renewal state of human neural stem/precursor cells by regulating Wnt and mTOR signaling pathways

During cortical development, neural stem/precursor cells (NS/PCs) sequentially produce neurons, astrocytes, and oligodendrocytes. Before producing these cells, human (h) NS/PCs undergo prolonged self-renewal to form a larger cortex than other mammals, although the mechanisms are mostly unknown. Here, we performed a gene knockout screen using the CRISPR/Cas9 system to search for genes involved in hNS/PC self-renewal. We identified RMND5A, encoding an E3 ubiquitin ligase, among the candidate genes. We further demonstrated that knockdown of RMND5A decreased proliferation and promoted neuronal differentiation of hNS/PCs through the activation and suppression of the Wnt and mTOR signaling pathways, respectively. Taken together, our findings suggest that RMND5A participates in the maintenance of hNS/PC self-renewal by modulating the Wnt and mTOR signaling pathways. Impact statement During cortical development, human neural stem/precursor cells (hNS/PCs) undergo prolonged self-renewal to form a larger cortex than other mammals, although the mechanisms are mostly unknown. We identified RMND5A, an E3 ubiquitin ligase, as essential for maintaining self-renewal of hNS/PCs, providing valuable insights into the evolutionary expansion of the human brain.

Differentiation-inducing factor-1 inhibits EMT by proteasomal degradation of TAZ and YAP in cervical and colon cancer cell lines

We previously reported differentiation-inducing factor-1 (DIF-1) activated glycogen synthase kinase-3 (GSK-3) in various mammalian cells. GSK-3 has been proposed to regulate a number of signaling pathway including TAZ/YAP signaling pathway. To clarify the effect of DIF-1 on TAZ/YAP signaling pathway, we examined whether DIF-1 affect the expression levels of TAZ and YAP. We found that DIF-1-induced proteasomal- and GSK-3-dependent degradation of both TAZ and YAP in human cervical cancer cell line HeLa in a time- and dose-dependent manner. As TAZ/YAP signaling pathway is well known to accelerate the epithelial-mesenchymal transition (EMT) of the cancer cell, we examined the effect of TAZ/YAP signaling pathway on EMT-related proteins. Knockdown of TAZ and YAP proteins by siRNA significantly reduced the expression of fibronectin, vimentin, and Snail. We also found that DIF-1 suppressed the expression levels of TAZ/YAP target gene products and EMT-related protein. Further, overexpression of TAZ and YAP attenuated the inhibitory effects of DIF-1 on these protein expressions. Migration and trans-well invasion assays revealed that DIF-1 significantly inhibited HeLa cell migration and invasion. DIF-1-induced proteasomal- and GSK-3-dependent degradation of TAZ and YAP proteins and inhibition of cell migration and invasion were also observed in human colon cancer cell line HCT-116. These results suggest that DIF-1 inhibits the TAZ/YAP signaling pathway via GSK-3 activation. Further, it has been suggested that the inhibition of EMT induced by DIF-1 is involved with the suppression of TAZ/YAP signaling pathway.

Mammalian target of differentiation-inducing factor-1 is mitochondrial malate dehydrogenase for activation of AMP-activated protein kinase and induction of mitochondrial fission

AIMS

Differentiation-inducing factor-1 (DIF-1) is a polyketide produced by Dictyostelium discoideum that inhibits growth and migration, while promoting the differentiation of Dictyostelium stalk cells through unknown mechanisms. DIF-1 localizes in stalk mitochondria. In addition to its effect on Dictyostelium, DIF-1 also inhibits growth and migration, and induces mitochondrial fission followed by mitophagy in mammalian cells, at least in part by activating AMP-activated protein kinase (AMPK). In a previous study, we found that DIF-1 binds to mitochondrial malate dehydrogenase (MDH2) and inhibits its activity in HeLa cells. In the present study, we investigated whether MDH2 serves as a pharmacological target of DIF-1 in mammalian cells.

MAIN METHODS

To examine the enzymatic activity of MDH, mitochondrial morphology, and molecular mechanisms of DIF-1 action, we conducted an MDH reverse reaction assay, immunofluorescence staining, western blotting, and RNA interference using mammalian cells such as human umbilical vein endothelial cells, human cervical cancer cells, mouse endothelial cells, and mouse breast cancer cells.

KEY FINDINGS

DIF-1 inhibited mitochondrial but not cytoplasmic MDH activity. Similar to DIF-1, LW6, an authentic MDH2 inhibitor, induced phosphorylation of AMPK, resulting in the phosphorylation of acetyl-CoA carboxylase (ACC) and the dephosphorylation of p70 S6 kinase with approximately the same potency. DIF-1 and LW6 induced mitochondrial fission. Furthermore, MDH2 knockdown using siRNA reproduced the DIF-1 action on the AMPK signaling and mitochondrial morphology. Conversely, an AMPK inhibitor prevented DIF-1-induced mitochondrial fission.

SIGNIFICANCE

We propose that MDH2 is a mammalian target of DIF-1 for the activation of AMPK and induction of mitochondrial fission.

17β-Estradiol (E2) Activates Matrix Mineralization through Genomic/Nongenomic Pathways in MC3T3-E1 Cells

Estrogen plays an important role in osteoporosis prevention. We herein report the possible novel signaling pathway of 17β-estradiol (E2) in the matrix mineralization of MC3T3-E1, an osteoblast-like cell line. In the culture media-containing stripped serum, in which small lipophilic molecules such as steroid hormones including E2 were depleted, matrix mineralization was significantly reduced. However, the E2 treatment induced this. The E2 effects were suppressed by ICI182,780, the estrogen receptor (ER)α, and the ERβ antagonist, as well as their mRNA knockdown, whereas Raloxifene, an inhibitor of estrogen-induced transcription, and G15, a G-protein-coupled estrogen receptor (GPER) 1 inhibitor, had little or no effect. Furthermore, the E2-activated matrix mineralization was disrupted by PMA, a PKC activator, and SB202190, a p38 MAPK inhibitor, but not by wortmannin, a PI3K inhibitor. Matrix mineralization was also induced by the culture media from the E2-stimulated cell culture. This effect was hindered by PMA or heat treatment, but not by SB202190. These results indicate that E2 activates the p38 MAPK pathway via ERs independently from actions in the nucleus. Such activation may cause the secretion of certain signaling molecule(s), which inhibit the PKC pathway. Our study provides a novel pathway of E2 action that could be a therapeutic target to activate matrix mineralization under various diseases, including osteoporosis.

Higher-intensity ultrasound accelerates fracture healing via mechanosensitive ion channel Piezo1

Osteoporosis-related fractures are a major public health problem. Mechanobiological stimulation utilizing low-intensity pulsed ultrasound (LIPUS) is the most widely accepted modality for accelerating fracture healing. However, recent evidence has demonstrated the ineffectiveness of LIPUS, and the biophysical mechanisms of ultrasound-induced bone formation also remain elusive. Here, we demonstrate that ultrasound at a higher intensity than LIPUS effectively accelerates fracture healing in a mouse osteoporotic fracture model. Higher-intensity ultrasound promoted chondrogenesis and hypertrophic differentiation of chondrocytes in the fracture callus. Higher-intensity ultrasound also increased osteoblasts and newly formed bone in the callus, resulting in accelerated endochondral ossification during fracture healing. In addition, we found that accelerated fracture healing by ultrasound exposure was attenuated when the mechanosensitive ion channel Piezo1 was inhibited by GsMTx4. Ultrasound-induced new bone formation in the callus was attenuated in fractured mice treated with GsMTx4. Similar results were also confirmed in a 3D osteocyte-osteoblast co-culture system, where osteocytic Piezo1 knockdown attenuated the expression of osteoblastic genes after ultrasound exposure. Together these results demonstrate that higher-intensity ultrasound than clinically used LIPUS can accelerate endochondral ossification after fractures. Furthermore, our results suggest that mechanotransduction via Piezo1 mediates ultrasound-stimulated fracture healing and bone formation.

Tissue-Nonspecific Alkaline Phosphatase, a Possible Mediator of Cell Maturation: Towards a New Paradigm

Alkaline phosphatase (ALP) is a ubiquitous membrane-bound glycoprotein capable of providing inorganic phosphate by catalyzing the hydrolysis of organic phosphate esters, or removing inorganic pyrophosphate that inhibits calcification. In humans, four forms of ALP cDNA have been cloned, among which tissue-nonspecific ALP (TNSALP) (TNSALP) is widely distributed in the liver, bone, and kidney, making it an important marker in clinical and basic research. Interestingly, TNSALP is highly expressed in juvenile cells, such as pluripotent stem cells (i.e., embryonic stem cells and induced pluripotent stem cells (iPSCs)) and somatic stem cells (i.e., neuronal stem cells and bone marrow mesenchymal stem cells). Hypophosphatasia is a genetic disorder causing defects in bone and tooth development as well as neurogenesis. Mutations in the gene coding for TNSALP are thought to be responsible for the abnormalities, suggesting the essential role of TNSALP in these events. Moreover, a reverse-genetics-based study using mice revealed that TNSALP is important in bone and tooth development as well as neurogenesis. However, little is known about the role of TNSALP in the maintenance and differentiation of juvenile cells. Recently, it was reported that cells enriched with TNSALP are more easily reprogrammed into iPSCs than those with less TNSALP. Furthermore, in bone marrow stem cells, ALP could function as a "signal regulator" deciding the fate of these cells. In this review, we summarize the properties of ALP and the background of ALP gene analysis and its manipulation, with a special focus on the potential role of TNSALP in the generation (and possibly maintenance) of juvenile cells.

Differentiation-inducing factor-1 potentiates adipogenic differentiation and attenuates the osteogenic differentiation of bone marrow-derived mesenchymal stem cells

Mesenchymal stem cells (MSCs) are an attractive cell source for tissue regeneration and repair. However, their low differentiation efficacy currently impedes the development of MSC therapy. Therefore, in this study, we investigated the effects of differentiation-inducing factor-1 (DIF-1) on the differentiation efficacy of bone marrow-derived MSCs (BM-MSCs) into adipogenic or osteogenic lineages. BM-MSCs, which were obtained from Sprague-Dawley rats, were positive for the MSC markers (CD29, CD73, and CD90). DIF-1 alone neither affected cell surface antigen expression nor induced adipogenic or osteogenic differentiation. However, DIF-1 significantly enhanced the effects of adipogenic differentiation stimuli, which were evaluated as the number of oil red-O positive cells and the expression of adipocyte differentiation markers (peroxisome proliferator-activated receptor gamma, adipocyte fatty acid-binding protein, and adiponectin). In contrast, DIF-1 significantly attenuated the effects of osteogenic differentiation stimuli, which were evaluated as alizarin red-S positive calcium deposition, and the expression of osteoblast differentiation markers alkaline phosphatase, runt-related transcription factor 2, and osteopontin. We further investigated the mechanism by which DIF-1 affects MSC differentiation efficacy and found that glycogen synthase kinase-3 was the main factor mediating the action of DIF-1 on the adipogenic differentiation of BM-MSCs, whereas it was only partially involved in osteogenic differentiation. These results suggest that DIF-1 supports MSC differentiation toward the desired cell fate by enhancing the differentiation efficacy.

Dental regenerative therapy targeting sphingosine-1-phosphate (S1P) signaling pathway in endodontics

The establishment of regenerative therapy in endodontics targeting the dentin-pulp complex, cementum, periodontal ligament tissue, and alveolar bone will provide valuable information to preserve teeth. It is well known that the application of stem cells such as induced pluripotent stem cells, embryonic stem cells, and somatic stem cells is effective in regenerative medicine. There are many somatic stem cells in teeth and periodontal tissues including dental pulp stem cells (DPSCs), stem cells from the apical papilla, and periodontal ligament stem cells. Particularly, several studies have reported the regeneration of clinical pulp tissue and alveolar bone by DPSCs transplantation. However, further scientific issues for practical implementation remain to be addressed. Sphingosine-1-phosphate (S1P) acts as a bioactive signaling molecule that has multiple biological functions including cellular differentiation, and has been shown to be responsible for bone resorption and formation. Here we discuss a strategy for bone regeneration and a possibility for regenerative endodontics targeting S1P signaling pathway as one of approaches for induction of regeneration by improving the regenerative capacity of endogenous cells.

SCIENTIFIC FIELD OF DENTAL SCIENCE

Endodontology.

Differentiation-inducing factor-1 suppresses cyclin D1-induced cell proliferation of MCF-7 breast cancer cells by inhibiting S6K-mediated signal transducer and activator of transcription 3 synthesis

Differentiation-inducing factor-1 (DIF-1) has been reported to inhibit the proliferation of various mammalian cells by unknown means, although some possible mechanisms of its action have been proposed, including the activation of glycogen synthase kinase-3 (GSK-3). Here, we report an alternative mechanism underlying the action of DIF-1 in human breast cancer cell line MCF-7, on which the effects of DIF-1 have not been examined previously. Intragastric administration of DIF-1 reduced the tumor growth from MCF-7 cells injected into a mammary fat pad of nude mice, without causing adverse effects. In cultured MCF-7, DIF-1 arrested the cell cycle in G₀ /G₁ phase and suppressed cyclin D1 expression, consistent with our previous results obtained in other cell species. However, DIF-1 did not inhibit the phosphorylation of GSK-3. Investigating an alternative mechanism for the reduction of cyclin D1, we found that DIF-1 reduced the protein levels of signal transducer and activator of transcription 3 (STAT3). The STAT3 inhibitor S3I-201 suppressed cyclin D1 expression and cell proliferation and the overexpression of STAT3 enhanced cyclin D1 expression and accelerated proliferation. Differentiation-inducing factor-1 did not reduce STAT3 mRNA or reduce STAT3 protein in the presence of cycloheximide, suggesting that DIF-1 inhibited STAT3 protein synthesis. Seeking its mechanism, we revealed that DIF-1 inhibited the activation of 70 kDa and/or 85 kDa ribosomal protein S6 kinase (p70^S6K /p85^S6K ). Inhibition of p70^S6K /p85^S6K by rapamycin also reduced the expressions of STAT3 and cyclin D1. Therefore, DIF-1 suppresses MCF-7 proliferation by inhibiting p70^S6K /p85^S6K activity and STAT3 protein synthesis followed by reduction of cyclin D1 expression.

BMP9 directly induces rapid GSK3-β phosphorylation in a Wnt-independent manner through class I PI3K-Akt axis in osteoblasts

Bone morphogenetic protein (BMP)9 has been reported to be the most potent BMP to induce bone formation. However, the details of BMP9-transduced intracellular signaling remain ambiguous. Here, we have investigated signal transduction mechanisms of BMP9 in comparison to BMP2, another potent inducer of bone formation, in osteoblasts. In a mouse osteoblast cell line, BMP9 induced higher mRNA levels of alkaline phosphatase (ALP) and runt-related transcription factor 2 (Runx2) than BMP2 within 2 h. Unlike BMP2, BMP9 induced rapid phosphorylation of glycogen synthase kinase 3-β (GSK3-β) and protein kinase B (Akt) and increased the cellular protein content of β-catenin. BMP9 moderately increased mRNA levels of several canonical Wingless-related integration site to lower degrees than BMP2. Furthermore, BMP9-induced GSK3-β phosphorylation was not inhibited by pretreatment with actinomycin D, cycloheximide, or Brefeldin A, indicating it is independent of Wnt protein secretion. BMP9-induced GSK3-β phosphorylation was abrogated by Akt or class I PI3K-specific inhibitors. Moreover, inactivation of GSK3-β by LiCl did not further promote ALP and Runx2 mRNA induction by BMP9 as significantly as that by BMP2. Notably, BMP9-induced GSK3-β phosphorylation was inhibited by small interfering RNA against endoglin and GIPC PDZ domain-containing family, member 1. Taken together, our present findings have indicated that BMP9 directly activates GSK3β-β-catenin signaling pathway through class I PI3K-Akt Axis in osteoblasts, which may be essential for the potent osteoinductive activity of BMP9.-Eiraku, N., Chiba, N., Nakamura, T., Amir, M. S., Seong, C.-H., Ohnishi, T., Kusuyama, J., Noguchi, K., Matsuguchi, T. BMP9 directly induces rapid GSK3-β phosphorylation in a Wnt-independent manner through class I PI3K-Akt axis in osteoblasts.

Dictyostelium: An Important Source of Structural and Functional Diversity in Drug Discovery

The cellular slime mold Dictyostelium discoideum is an excellent model organism for the study of cell and developmental biology because of its simple life cycle and ease of use. Recent findings suggest that Dictyostelium and possibly other genera of cellular slime molds, are potential sources of novel lead compounds for pharmacological and medical research. In this review, we present supporting evidence that cellular slime molds are an untapped source of lead compounds by examining the discovery and functions of polyketide differentiation-inducing factor-1, a compound that was originally isolated as an inducer of stalk-cell differentiation in D. discoideum and, together with its derivatives, is now a promising lead compound for drug discovery in several areas. We also review other novel compounds, including secondary metabolites, that have been isolated from cellular slime molds.

ER stress regulates alkaline phosphatase gene expression in vascular smooth muscle cells via an ATF4-dependent mechanism

OBJECTIVE

Vascular calcification is the deposition of hydroxyapatite crystals in the blood vessel wall. Osteogenic differentiation of vascular smooth muscle cells (VSMCs) plays a key role in this process. Increased expression of alkaline phosphatase (ALP) occurs in some in vitro models of VSMC calcification and is thought to be crucial for mineralization, however, little is known about the transcriptional regulation of ALP in VSMCs. Recently, ALP upregulation was shown to coincide with endoplasmic reticulum (ER) stress-mediated vascular calcification, specifically with expression of the transcription factor ATF4. As no direct links between ALP expression and ER stress have previously been demonstrated in VSMCs, the aim of this study was to investigate whether ATF4 interacts directly with the ALP promoter.

RESULTS

The present study shows that ALP mRNA and activity were significantly increased by ER stress treatment of human primary VSMCs in vitro and that this was ATF4-dependent. Bioinformatics analysis predicted two ATF4 binding sites in ER-stress responsive regions of the ALP promoter (- 3631 to - 2048 bp from the first intron). However, we found that ATF4 does not bind within this fragment of the ALP promoter region.

Aluminum Trichloride Inhibited Osteoblastic Proliferation and Downregulated the Wnt/β-Catenin Pathway

Aluminum (Al) exposure inhibits bone formation. Osteoblastic proliferation promotes bone formation. Therefore, we inferred that Al may inhibit bone formation by the inhibition of osteoblastic proliferation. However, the effects and molecular mechanisms of Al on osteoblastic proliferation are still under investigation. Osteoblastic proliferation can be regulated by Wnt/β-catenin signaling pathway. To investigate the effects of Al on osteoblastic proliferation and whether Wnt/β-catenin signaling pathway is involved in it, osteoblasts from neonatal rats were cultured and exposed to 0, 0.4 mM (1/20 IC₅₀), 0.8 mM (1/10 IC₅₀), and 1.6 mM (1/5 IC₅₀) of aluminum trichloride (AlCl₃) for 24 h, respectively. The osteoblastic proliferation rates; Wnt3a, lipoprotein receptor-related protein 5 (LRP-5), T cell factor 1 (TCF-1), cyclin D1, and c-Myc messenger RNA (mRNA) expressions; and p-glycogen synthase kinase 3β (GSK3β), GSK3β, and β-catenin protein expressions indicated that AlCl₃ inhibited osteoblastic proliferation and downregulated Wnt/β-catenin signaling pathway. In addition, the AlCl₃ concentration was negatively correlated with osteoblastic proliferation rates and the mRNA expressions of Wnt3a, c-Myc, and cyclin D1, while the osteoblastic proliferation rates were positively correlated with mRNA expressions of Wnt3a, c-Myc, and cyclin D1. Taken together, these findings indicated that AlCl₃ inhibits osteoblastic proliferation may be associated with the inactivation of Wnt/β-catenin signaling pathway.

Inhibition of bone formation in rats by aluminum exposure via Wnt/β-catenin pathway

The previous research found that aluminum trichloride (AlCl₃) inhibited rat osteoblastic differentiation through inactivation of Wnt/β-catenin signaling pathway in vitro. On that basis, the experiment in vivo was conducted in this study. Rats were orally exposed to 0 (control group) and 0.4 g/L AlCl₃ (AlCl₃-treated group) for 30, 60, 90 or 120 days, respectively. We found that mRNA expressions of type I collagen and insulin-like growth factor-1, mRNA and protein expressions of Runx2 and survivin, ratio of p-GSK3β/GSK3β and protein expression of β-catenin were all decreased, whereas the mRNA and protein expressions Dkk1 and sFRP1 and the mRNA expressions and activity of Caspase-3 were increased in the AlCl₃-treated group compared with the control group with time prolonged. These results suggest that AlCl₃ inhibits bone formation and induces bone impairment by inhibiting the Wnt/β-catenin signaling pathway in young growing rats.

Celecoxib inhibits osteoblast maturation by suppressing the expression of Wnt target genes

Non-steroidal anti-inflammatory drugs (NSAIDs) have been shown to impair bone healing. We previously reported that in colon cancer cells, celecoxib, a COX-2-selective NSAID, inhibited the canonical Wnt/β-catenin signaling pathway. Since this pathway also plays an important role in osteoblast growth and differentiation, we examined the effect of celecoxib on maturation of osteoblast-like cell line MC3T3-E1. Celecoxib induced degradation of transcription factor 7-like 2, a key transcription factor of the canonical Wnt pathway. Subsequently, we analyzed the effect of celecoxib on two osteoblast differentiation markers; runt-related transcription factor 2 (RUNX2) and alkaline phosphatase (ALP), both of which are the products of the canonical Wnt pathway target genes. Celecoxib inhibited the expression of both RUNX2 and ALP by suppressing their promoter activity. Consistent with these observations, celecoxib also strongly inhibited osteoblast-mediated mineralization. These results suggest that celecoxib inhibits osteoblast maturation by suppressing Wnt target genes, and this could be the mechanism that NSAIDs inhibit bone formation and fracture healing.

Sphingosine-1-phosphate/S1PR2-mediated signaling triggers Smad1/5/8 phosphorylation and thereby induces Runx2 expression in osteoblasts

Sphingosine-1-phosphate (S1P) is a signaling sphingolipid that also plays crucial roles in bone regeneration. Recently, we reported that the S1P receptors S1PR1 and S1PR2 were mainly expressed in osteoblast-like cells, and that the S1P/S1PR1 signaling pathway up-regulated osteoprotegerin and osteoblast differentiation. However, the involvement of S1P/S1PR2 signaling in osteoblast differentiation is not well understood. Here we investigate the role of S1P/S1PR2-mediated signaling in osteoblast differentiation and clarify the underlying signaling mechanisms. We found that an S1P/S1PR2/Gi-independent signaling pathway activated RhoA activity, leading to phosphorylation of Smad1/5/8 in mouse osteoblast-like MC3T3-E1 cells and primary osteoblasts. Furthermore, this signaling pathway promoted nuclear translocation of Smad4, and increased the amount of Smad6/7 protein in the nucleus. S1P also up-regulated runt-related transcription factor 2 (Runx2) expression through S1PR2/RhoA/ROCK/Smad1/5/8 signaling. Moreover, we found that S1P partially triggered S1PR2/RhoA/ROCK pathway leading to bone formation in vivo. These findings suggest that S1P induces RhoA activity, leading to the phosphorylation of Smad1/5/8, thereby promoting Runx2 expression and differentiation in osteoblasts. Our findings describe novel molecular mechanisms in S1P/S1PR2-mediated osteoblast differentiation that could aid future studies of bone regeneration.

Aluminum trichloride inhibits osteoblastic differentiation through inactivation of Wnt/β-catenin signaling pathway in rat osteoblasts

Exposure to aluminum (Al) suppresses bone formation. Osteoblastic differentiation plays a key role in the process of bone formation. However, the effect of Al on osteoblastic differentiation is still controversial, and the mechanism remains unclear. To investigate the effect of Al on osteoblastic differentiation and whether Wnt signaling pathway was involved in it, the primary rat osteoblasts were exposed to 1/40 IC50, 1/20 IC50 and 1/10 IC50 of aluminum trichloride (AlCl3) for 24h, respectively. The activity analysis of alkaline phosphate, qRT-PCR analysis of type I collagen, alkaline phosphate, Wnt3a and Dkk-1, Western blot analysis of p-GSK3β, GSK3β and β-catenin protein and Immunofluorescence staining for β-catenin suggested that AlCl3 inhibited osteoblastic differentiation and Wnt/β-catenin pathway. Moreover, we found exogenous Wnt3a application reversed the inhibitory effect of AlCl3 on osteoblastic differentiation, accompanied by activating the Wnt/β-catenin pathway. Taken together, these findings suggest that AlCl3 inhibites osteoblastic differentiation through inactivation of Wnt/β-catenin pathway in osteoblasts.

Differentiation-inducing factor-3 inhibits intestinal tumor growth in vitro and in vivo

Differentiation-inducing factor-1 (DIF-1) produced by Dictyostelium discoideum strongly inhibits the proliferation of various types of cancer cells by suppression of the Wnt/β-catenin signal transduction pathway. In the present study, we examined the effect of differentiation-inducing factor-3 (DIF-3), a monochlorinated metabolite of DIF-1 that is also produced by D. discoideum, on human colon cancer cell lines HCT-116 and DLD-1. DIF-3 strongly inhibited cell proliferation by arresting the cell cycle at the G0/G1 phase. DIF-3 reduced the expression levels of cyclin D1 and c-Myc by facilitating their degradation via activation of GSK-3β in a time and dose-dependent manner. In addition, DIF-3 suppressed the expression of T-cell factor 7-like 2, a key transcription factor in the Wnt/β-catenin signaling pathway, thereby reducing the mRNA levels of cyclin D1 and c-Myc. Subsequently, we examined the in vivo effects of DIF-3 in Mutyh(-/-) mice with oxidative stress-induced intestinal cancers. Repeated oral administration of DIF-3 markedly reduced the number and size of cancers at a level comparable to that of DIF-1. These data suggest that DIF-3 inhibits intestinal cancer cell proliferation in vitro and in vivo, probably by mechanisms similar to those identified in DIF-1 actions, and that DIF-3 may be a potential novel anti-cancer agent.

Sphingosine-1-phosphate inhibits differentiation of C3H10T1/2 cells into adipocyte

Mesenchymal stem cells (MSCs) can differentiate into a number of cell types, including adipocytes and osteoblasts. MSC differentiation into adipocytes inhibits osteogenic differentiation and vice versa. Therefore, understanding the mechanisms of MSC differentiation at the signaling level can lead to the development of novel therapeutic strategies toward tissue regeneration. Sphingosine-1-phosphate (S1P) is a signaling molecule that regulates many cellular responses, including cellular differentiation. However, the effects of S1P on MSC differentiation are largely unknown. The purpose of study was to investigate whether S1P drives MSCs toward either adipogenic or osteogenic differentiation, and if so, to clarify the underlying signaling mechanisms for such differentiation. We found that S1P inhibited adipogenic differentiation of C3H10T1/2 multipotent stem cells, while promoting their osteogenic differentiation. During adipogenic differentiation, S1P suppressed the cAMP accumulation in a Gi-protein-dependent manner. The Gi-dependent S1P signaling suppressed C/EBPβ expression, which is essential for adipogenic differentiation. Furthermore, S1P did not affect cAMP-independent adipogenic differentiation. These findings suggest that S1P suppresses cAMP accumulation, leading to inhibition of C/EBPβ expression, thereby resulting in decreased adipogenic differentiation of C3H10T1/2 cells. Thus, our findings provide novel molecular mechanisms as regards how S1P inhibits adipogenic differentiation of C3H10T1/2 cells, indicating a potential beneficial role for regeneration and repair of tissues.

DIF-1 inhibits tumor growth in vivo reducing phosphorylation of GSK-3β and expressions of cyclin D1 and TCF7L2 in cancer model mice

We reported that differentiation-inducing factor-1 (DIF-1), synthesized by Dictyostelium discoideum, inhibited proliferation of various tumor cell lines in vitro by suppressing the Wnt/β-catenin signaling pathway. However, it remained unexplored whether DIF-1 also inhibits tumor growth in vivo. In the present study, therefore, we examined in-vivo effects of DIF-1 using three cancer models: Mutyh-deficient mice with oxidative stress-induced intestinal tumors and nude mice xenografted with the human colon cancer cell line HCT-116 and cervical cancer cell line HeLa. In exploration for an appropriate route of administration, we found that orally administered DIF-1 was absorbed through the digestive tract to elevate its blood concentration to levels enough to suppress tumor cell proliferation. Repeated oral administration of DIF-1 markedly reduced the number and size of intestinal tumors that developed in Mutyh-deficient mice, reducing the phosphorylation level of GSK-3β Ser(9) and the expression levels of early growth response-1 (Egr-1), transcription factor 7-like 2 (TCF7L2) and cyclin D1. DIF-1 also inhibited the growth of HCT-116- and HeLa-xenograft tumors together with decreasing phosphorylation level of GSK-3β Ser(9), although it was not statistically significant in HeLa-xenograft tumors. DIF-1 also suppressed the expressions of Egr-1, TCF7L2 and cyclin D1 in HCT-116-xenograft tumors and those of β-catenin, TCF7L2 and cyclin D1 in HeLa-xenograft tumors. This is the first report to show that DIF-1 inhibits tumor growth in vivo, consistent with its in-vitro action, suggesting that this compound may have potential as a novel anti-tumor agent.

Fibroblast growth factor 2 and forskolin induce mineralization-associated genes in two kinds of osteoblast-like cells

Fibroblast growth factor 2 (FGF2) and cyclic AMP (cAMP) play critical roles in controlling the differentiation of osteoblasts and mineralization of bone. We have previously reported that each of FGF2 and forskolin (FSK) alone increase transcription of the bone sialoprotein (BSP) gene, and that together (FGF/FSK) they upregulate BSP gene expression synergistically in rat osteoblast-like ROS 17/2.8 cells. However, other genes that are upregulated after stimulation by FGF2, FSK or FGF/FSK remain unclear. In the present study, we investigated candidate genes associated with mineralization after stimulation by FGF2, FSK and FGF/FSK in two kinds of osteoblast-like cells using microarray and real-time PCR. In ROS17/2.8 cells, FGF2 and FSK each increased the gene expression of c-FOS (7.2-fold and 10.7-fold, respectively). However, FGF/FSK did not induce c-FOS gene expression. FGF2 increased the expression of the dentin matrix protein 1 (DMP1, 129.8-fold) gene. In contrast, FGF/FSK increased the expression of the amphiregulin (AREG, 73-fold) gene maximally. In human osteoblast-like Saos2 cells, FGF2 increased the expression of the osteopontin (SPP1, 16.7-fold), interleukin-8 (IL8, 6.4-fold) and IL11 (4.8-fold) genes. FSK induced the expression of the IL6 (2.6-fold), IL11 (4.0-fold), chemokine ligand 13 (CXCL13, 2.8-fold) and bone morphogenetic protein 2 (BMP2, 2.5-fold) genes. These results suggest that FGF2 and FSK might be crucial regulators of mineralization and bone formation.

Up-regulation of Axin2 by dexamethasone promotes adipocyte differentiation in ROB-C26 mesenchymal progenitor cells

Dexamethasone (Dex) regulates osteoblastic and adipocytic differentiation in mesenchymal progenitor cells through regulation of Wnt/β-catenin signaling. To elucidate the regulatory mechanisms underlying the effects of Dex, we examine the expression of Axin2, which is an intracellular inhibitor of Wnt/β-catenin signaling, in ROB-C26 clonal mesenchymal progenitor cells (C26). We observed the induction of Axin2 mRNA in C26 cells in response to Dex treatment. Treatment with a glucocorticoid receptor (GR) antagonist, mifepristone, showed that Dex-induced up-regulation of Axin2 is mediated by the GR. In the absence of Dex, gene silencing by using Axin2-targeted short hairpin RNA increased the number of alkaline phosphatase (ALP)-positive and nuclear β-catenin-positive cells and ALP activity. In the presence of Dex, Axin2 knockdown resulted in an increased number of ALP-positive and nuclear β-catenin-positive cells. Furthermore, Axin2 knockdown in Dex-treated cells suppressed adipocyte differentiation (as determined by reduced Oil Red O staining), reduced the number of PPARγ-positive and aP2-positive cells and decreased the mRNA expression of PPARγ2 and aP2. These results suggest that Axin2 plays a key role in adipocyte and osteoblastic differentiation by controlling β-catenin expression.

Sphingosine-1-phosphate promotes the nuclear translocation of β-catenin and thereby induces osteoprotegerin gene expression in osteoblast-like cell lines

Sphingosine-1-phosphate (S1P) is a well-known signaling sphingolipid and bioactive lipid mediator. Recently, it was reported that S1P inhibits osteoclast differentiation and bone resorption. On the other hand, S1P effects on osteoblasts and bone formation are little known. In this study, we investigated the effects of S1P on osteoblasts, using two osteoblast-like cell lines, SaOS-2 and MC3T3-E1. S1P activated phosphatidylinositol 3-kinase (PI3K)/Akt signaling, leading to the inhibition of glycogen synthase kinase-3β and the nuclear translocation of β-catenin, followed by the increase of the transcriptional activity by β-catenin/T-cell factor complex formation in both SaOS-2 cells and MC3T3-E1 cells. The inhibitors of PI3K and Akt suppressed S1P-induced nuclear localization of β-catenin. We further investigated the effects of PI3K/Akt signaling on the Wnt/β-catenin signaling pathway, since β-catenin takes a central role in this signaling pathway. Both inhibitors for PI3K and Akt suppressed the nuclear localization of β-catenin and T-cell factor transcriptional activity induced by Wnt-3a. S1P increased the amount of osteoprotegerin at both mRNA and protein levels, and increased the activity of alkaline phosphatase, leading to the mineralization. These findings suggest that S1P activates the PI3K/Akt signaling pathway leading to the promotion of nuclear translocation of β-catenin in osteoblast-like cells, resulting in the upregulation of osteoptotegerin and osteoblast differentiation markers including alkaline phosphatase, probably relating to the inhibition of osteoclast formation and the mineralization, respectively.

Differentiation-inducing factor-1 suppresses the expression of c-Myc in the human cancer cell lines

Differentiation-inducing factor-1 (DIF-1), a morphogen for Dictyostelium discoideum, inhibits the proliferation of human cancer cell lines by suppressing the Wnt/β-catenin signaling pathway. In this study, we examined the effect of DIF-1 on c-Myc, a target gene product of the Wnt/β-catenin signaling pathway, mainly using HCT-116 colon cancer cells. DIF-1 strongly reduced the amount of c-Myc protein in time- and concentration-dependent manners and reduced c-Myc mRNA expression by inhibiting promoter activity through the TCF binding sites. The effect of DIF-1 on c-Myc was also confirmed using the human cervical cell line HeLa. Pretreatment with the proteasome inhibitor MG132 or glycogen synthase kinase-3β (GSK-3β) inhibitors (LiCl and SB216763) attenuated the effect of DIF-1, suggesting that DIF-1 induced c-Myc protein degradation through GSK-3β activation. Furthermore, we examined whether c-Myc was involved in the anti-proliferative effect of DIF-1 using c-Myc-overexpressing cells and found that c-Myc was associated with the anti-proliferative effect of this compound. These results suggest that DIF-1 inhibits c-Myc expression by inhibiting promoter activity and inducing protein degradation via GSK-3β activation, resulting in the inhibition of cell proliferation. Since c-Myc seems to be profoundly involved in accelerated proliferation of various malignant tumors, DIF-1 may have a potential to develop into a novel anti-cancer agent.

Wnt/β-catenin expression does not correlate with serum alkaline phosphatase concentration in canine osteosarcoma patients

Osteosarcoma is an aggressive malignancy of the bone and an increase in serum alkaline phosphatase concentration has clinical prognostic value in both humans and canines. Increased serum alkaline phosphatase concentration at the time of diagnosis has been associated with poorer outcomes for osteosarcoma patients. The biology underlying this negative prognostic factor is poorly understood. Given that activation of the Wnt signaling pathway has been associated with alkaline phosphatase expression in osteoblasts, we hypothesized that the Wnt/β-catenin signaling pathway would be differentially activated in osteosarcoma tissue based on serum ALP status. Archived canine osteosarcoma samples and primary canine osteosarcoma cell lines were used to evaluate the status of Wnt/β-catenin signaling pathway activity through immunohistochemical staining, western immunoblot analyses, quantitative reverse-transcription polymerase chain reaction, and a Wnt-responsive promoter activity assay. We found no significant difference in β-catenin expression or activation between OSA populations differing in serum ALP concentration. Pathway activity was mildly increased in the primary OSA cell line generated from a patient with increased serum ALP compared to the normal serum ALP OSA cell line. Further investigation into the mechanisms underlying differences in serum ALP concentration is necessary to improve our understanding of the biological implications of this negative prognostic indicator.

DIF-1 inhibits the Wnt/β-catenin signaling pathway by inhibiting TCF7L2 expression in colon cancer cell lines

We previously reported that differentiation-inducing factor-1 (DIF-1), a morphogen in Dictyostelium discoideum, inhibits the proliferation of human cancer cell lines by inducing β-catenin degradation and suppressing the Wnt/β-catenin signaling pathway. To determine whether β-catenin degradation is essential for the effect of DIF-1, we examined the effect of DIF-1 on human colon cancer cell lines (HCT-116, SW-620 and DLD-1), in which the Wnt/β-catenin signaling pathway is constitutively active. DIF-1 strongly inhibited cell proliferation and arrested the cell cycle in the G(0)/G(1) phase via the suppression of cyclin D1 expression at mRNA and protein levels without reducing β-catenin protein. TCF-dependent transcriptional activity and cyclin D1 promoter activity were revealed to be inhibited via suppression of transcription factor 7-like 2 (TCF7L2) expression. Luciferase reporter assays and EMSAs using the TCF7L2 promoter fragments indicated that the binding site for the transcription factor early growth response-1 (Egr-1), which is located in the -609 to -601 bp region relative to the start codon in the TCF7L2 promoter, was involved in DIF-1 activity. Moreover, RNAi-mediated depletion of endogenous TCF7L2 resulted in reduced cyclin D1 promoter activity and protein expression, and the overexpression of TCF7L2 overrode the inhibition of the TCF-dependent transcriptional activity and cyclin D1 promoter activity induced by DIF-1. Therefore, DIF-1 seemed to inhibit the Wnt/β-catenin signaling pathway by suppressing TCF7L2 expression via reduced Egr-1-dependent transcriptional activity in these colon cancer cell lines. Our results provide a novel insight into the mechanisms by which DIF-1 inhibits the Wnt/β-catenin signaling pathway.

Lymphocyte enhancer-binding factor 1: an essential factor in odontoblastic differentiation of dental pulp cells enzymatically isolated from rat incisors

Lymphocyte enhancer-binding factor 1 (Lef1), a HMG-domain protein, is thought to play important roles in inductive tissue interaction during tooth development. Lef1 knockdown in mice causes arrest at the bud stage in tooth development. As this gene participates in the regulation of a large and diverse set of peptide growth factors in ectomesenchymal cell differentiation of dental papilla, Lef1 appears to be a key factor in odontoblast differentiation. However, the relationship between Lef1 and odontoblast differentiation is still unclear. To analyze the biological roles of Lef1 in regulating odontoblast differentiation, we transiently overexpressed or suppressed Lef1 in cultured dental pulp cells. Lef1-overexpressing cells expressed higher levels of dentin sialoprotein (DSPP), osteocalcin and alkaline phosphatase (ALP) mRNA and formed larger numbers of mineralized nodules compared to control cells. However, Msx-1 expression or cell proliferation was unaffected by overexpression of Lef1. To further examine the role of Lef1 in dental pulp cells, we knocked down Lef1 expression in dental pulp cells using short interfering RNA (siRNA). Transient expression of siRNA against Lef1 markedly reduced Lef1 mRNA levels, and Lef1-suppressed cells expressed lower levels of DSPP, osteocalcin and ALP mRNA compared to control cells. Furthermore, the formation of mineralized nodules was inhibited by siRNA against Lef1; however, neither Msx-1 expression or cell proliferation was inhibited by siRNA against Lef1. These results outline the role of Lef1 in accelerating odontoblast differentiation by regulating DSP and osteocalcin mRNA expression in dental pulp cells, confirming that Lef1 is a key factor for odontoblast differentiation.

Anti-angiogenic effects of differentiation-inducing factor-1 involving VEGFR-2 expression inhibition independent of the Wnt/β-catenin signaling pathway

BACKGROUND

Differentiation-inducing factor-1 (DIF-1) is a putative morphogen that induces cell differentiation in Dictyostelium discoideum. DIF-1 inhibits proliferation of various mammalian tumor cells by suppressing the canonical Wnt/β-catenin signaling pathway. To assess the potential of a novel cancer chemotherapy based on the pharmacological effect of DIF-1, we investigated whether DIF-1 exhibits anti-angiogenic effects in vitro and in vivo.

RESULTS

DIF-1 not only inhibited the proliferation of human umbilical vein endothelial cells (HUVECs) by restricting cell cycle in the G0/G1 phase and degrading cyclin D1, but also inhibited the ability of HUVECs to form capillaries and migrate. Moreover, DIF-1 suppressed VEGF- and cancer cell-induced neovascularization in Matrigel plugs injected subcutaneously to murine flank. Subsequently, we attempted to identify the mechanism behind the anti-angiogenic effects of DIF-1. We showed that DIF-1 strongly decreased vascular endothelial growth factor receptor-2 (VEGFR-2) expression in HUVECs by inhibiting the promoter activity of human VEGFR-2 gene, though it was not caused by inhibition of the Wnt/β-catenin signaling pathway.

CONCLUSION

These results suggested that DIF-1 inhibits angiogenesis both in vitro and in vivo, and reduction of VEGFR-2 expression is involved in the mechanism. A novel anti-cancer drug that inhibits neovascularization and tumor growth may be developed by successful elucidation of the target molecules for DIF-1 in the future.

Synergism between Wnt3a and heparin enhances osteogenesis via a phosphoinositide 3-kinase/Akt/RUNX2 pathway

A new strategy has emerged to improve healing of bone defects using exogenous glycosaminoglycans by increasing the effectiveness of bone-anabolic growth factors. Wnt ligands play an important role in bone formation. However, their functional interactions with heparan sulfate/heparin have only been investigated in non-osseous tissues. Our study now shows that the osteogenic activity of Wnt3a is cooperatively stimulated through physical interactions with exogenous heparin. N-Sulfation and to a lesser extent O-sulfation of heparin contribute to the physical binding and optimal co-stimulation of Wnt3a. Wnt3a-heparin signaling synergistically increases osteoblast differentiation with minimal effects on cell proliferation. Thus, heparin selectively reduces the effective dose of Wnt3a needed to elicit osteogenic, but not mitogenic responses. Mechanistically, Wnt3a-heparin signaling strongly activates the phosphoinositide 3-kinase/Akt pathway and requires the bone-related transcription factor RUNX2 to stimulate alkaline phosphatase activity, which parallels canonical beta-catenin signaling. Collectively, our findings establish the osteo-inductive potential of a heparin-mediated Wnt3a-phosphoinositide 3-kinase/Akt-RUNX2 signaling network and suggest that heparan sulfate supplementation may selectively reduce the therapeutic doses of peptide factors required to promote bone formation.

Inactive Wnt/beta-catenin pathway in conventional high-grade osteosarcoma

Osteosarcoma is the most common malignant bone tumour, with a peak incidence in children and young adolescents, suggesting a role of rapid bone growth in its pathogenesis. The Wnt/beta-catenin pathway plays a crucial role in skeletal development and is indispensable for osteoblasts' lineage determination. Previous studies suggesting an oncogenic role for the Wnt/beta-catenin pathway in osteosarcoma were based on cytoplasmic staining of beta-catenin or the detection of one component of this pathway. However, those approaches are inappropriate to address whether the Wnt/beta-catenin pathway is functionally active. Therefore, in this study, we examined nuclear beta-catenin expression in 52 human osteosarcoma biopsies, 15 osteoblastomas (benign bone tumours), and four human osteosarcoma cell lines by immunohistochemistry. Furthermore, we modulated Wnt/beta-catenin pathway activity using a GIN (GSK3beta inhibitor) and evaluated its effect on cell growth and osteogenic differentiation. Absence of nuclear beta-catenin staining was found in 90% of the biopsies and all osteosarcoma cell lines, whereas strong nuclear beta-catenin staining was observed in all osteoblastomas. Wnt-luciferase activity was comparable to the negative control in all osteosarcoma cell lines. GIN stimulated the Wnt/beta-catenin pathway, as shown by translocation of beta-catenin into the nucleus and increased Wnt-luciferase activity as well as mRNA expression of AXIN2, a specific downstream target gene. Stimulation of the Wnt/beta-catenin pathway by GIN significantly reduced cell proliferation in the cell lines MG-63 and U-2-OS and enhanced differentiation in the cell lines HOS and SJSA-1, as shown by an increase in alkaline phosphatase (ALP) activity and mineralization. In contrast with the oncogenic role of the Wnt/beta-catenin pathway in osteosarcoma as previous studies suggested, here we demonstrate that this pathway is inactivated in osteosarcoma. Moreover, activation of the Wnt/beta-catenin pathway inhibits cell proliferation or promotes osteogenic differentiation in osteosarcoma cell lines. Our data suggest that loss of Wnt/beta-catenin pathway activity, which is required for osteoblast differentiation, may contribute to osteosarcoma development.

Dictyostelium differentiation-inducing factor-1 binds to mitochondrial malate dehydrogenase and inhibits its activity

We have reported that the differentiation-inducing factors (DIFs) DIF-1 and DIF-3, morphogens secreted from Dictyostelium discoideum, inhibit proliferation of several cancer cells via suppression of the Wnt/beta-catenin signaling pathway. However, the target molecules of DIFs involved in the anti-proliferative effects are still unknown. In the present study, DIF-1-tethered resins were synthesized to explore the target molecules of DIFs, and mitochondrial malate dehydrogenase (mMDH) was identified as one of the target molecules. In the in vitro assay, DIF-1 and other analogs including 2-MIDIF-1, DIF-3, and 6-MIDIF-3 were found to be capable of binding to mMDH but not to cytoplasmic MDH. However, only DIF-1 and 2-MIDIF-1 inhibited the enzymatic activity of mMDH. The effects of DIF analogs on ATP content and cell proliferation were then analyzed using HeLa cells. DIF-1 and 2-MIDIF-1 were found to lower the ATP content and both chemicals inhibited HeLa cell proliferation, suggesting that inhibition of mMDH activity affected cell energy production, probably leading to the inhibition of proliferation. These results suggest that the inhibition of mMDH activity by DIF-1 and 2-MIDIF-1 could be one of the mechanisms to induce anti-proliferative effects, independent of the inhibition of the Wnt/beta-catenin signaling pathway.

SLUG: a new target of lymphoid enhancer factor-1 in human osteoblasts

Background

Lymphoid Enhancer Factor-1 (Lef-1) is a member of a transcription factor family that acts as downstream mediator of the Wnt/β-catenin signalling pathway which plays a critical role in osteoblast proliferation and differentiation. In a search for Lef-1 responsive genes in human osteoblasts, we focused on the transcriptional regulation of the SLUG, a zinc finger transcription factor belonging to the Snail family of developmental proteins. Although the role of SLUG in epithelial-mesenchymal transition and cell motility during embryogenesis is well documented, the functions of this factor in most normal adult human tissues are largely unknown. In this study we investigated SLUG expression in normal human osteoblasts and their mesenchymal precursors, and its possible correlation with Lef-1 and Wnt/β-catenin signalling.

Results

The experiments were performed on normal human primary osteoblasts obtained from bone fragments, cultured in osteogenic conditions in presence of Lef-1 expression vector or GSK-3β inhibitor, SB216763. We demonstrated that the transcription factor SLUG is present in osteoblasts as well as in their mesenchymal precursors obtained from Wharton's Jelly of human umbilical cord and induced to osteoblastic differentiation. We found that SLUG is positively correlated with RUNX2 expression and deposition of mineralized matrix, and is regulated by Lef-1 and β-catenin. Consistently, Chromatin Immunoprecipitation (ChIP) assay, used to detect the direct Lef/Tcf factors that are responsible for the promoter activity of SLUG gene, demonstrated that Lef-1, TCF-1 and TCF4 are recruited to the SLUG gene promoter "in vivo".

Conclusion

These studies provide, for the first time, the evidence that SLUG expression is correlated with osteogenic commitment, and is positively regulated by Lef-1 signal in normal human osteoblasts. These findings will help to further understand the regulation of the human SLUG gene and reveal the biological functions of SLUG in the context of bone tissue.

Secreted frizzled related protein 1 is a target to improve fracture healing

Genetic studies have identified a high bone mass of phenotype in both human and mouse when canonical Wnt signaling is increased. Secreted frizzled related protein 1 (sFRP1) is one of several Wnt antagonists and among the loss-of-function mouse models in which 32-week-old mice exhibit a high bone mass phenotype. Here we show that impact fracture healing is enhanced in this mouse model of increased Wnt signaling at a physiologic level in young (8 weeks) sFRP1(-/-) mice which do not yet exhibit significant increases in BMD. In vivo deletion of sFRP1 function improves fracture repair by promoting early bone union without adverse effects on the quality of bone tissue reflected by increased mechanical strength. We observe a dramatic reduction of the cartilage callous, increased intramembranous bone formation with bone bridging by 14 days, and early bone remodeling during the 28-day fracture repair process in the sFRP1(-/-) mice. Our molecular analyses of gene markers indicate that the effect of sFRP1 loss-of-function during fracture repair is to accelerate bone healing after formation of the initial hematoma by directing mesenchymal stem cells into the osteoblast lineage via the canonical pathway. Further evidence to support this conclusion is the observation of maximal sFRP1 levels in the cartilaginous callus of a WT mouse. Hence sFRP1(-/-) mouse progenitor cells are shifted directly into the osteoblast lineage. Thus, developing an antagonist to specifically inhibit sFRP1 represents a safe target for stimulating fracture repair and bone formation in metabolic bone disorders, osteoporosis and aging.

GSK-3beta regulates cyclin D1 expression: a new target for chemotherapy

Cyclin D1 is known as a proto-oncogene whose gene amplification and protein overexpression are frequently observed in tumor cells. It acts as a mitogenic signal sensor and is expressed as a delayed-early response to many mitogenic signals. Cyclin-dependent kinases (CDKs) 4 and 6 are cyclin D1 binding partners, and activated cyclin D1/CDK4 and cyclin D1/CDK6 complex phosphorylate the retinoblastoma protein to induce the expression of target genes essential for S phase entry, resulting in facilitation of the progression from G1 to S phase. As well as acting as a positive regulator of the cell cycle, cyclin D1 is known to bind and modulate the actions of several transcription factors. Since the protein level of cyclin D1 reflects cell cycle progression, the rates of protein production and degradation are strictly regulated. Glycogen synthase kinase-3beta (GSK-3beta), a serine/threonine protein kinase, has been shown to play an important role in the determination of cyclin D1 expression level by regulating mRNA transcription and protein degradation. This review highlights the regulatory mechanisms of cyclin D1 expression level, with special attention to the involvement of GSK-3beta.

The Wnt/beta-catenin signaling pathway as a target in drug discovery

The cell signaling cascades provoked by Wnt proteins (the Wnt signaling pathways), which are well conserved through evolution, play crucial roles to maintain homeostasis of a variety of tissues such as skin, blood, intestine, and brain, as well as to regulate proliferation, morphology, motility, and fate of cells during embryonic development. Among these pathways, the signal transduction through beta-catenin (the Wnt/beta-catenin signaling pathway) has been most intensively studied because this signal regulates the expression of a number of genes essential for cell proliferation and differentiation and also this pathway is perturbed in a number of diseases such as cancers, bone diseases, and cardiovascular diseases. However, there is no therapeutic agents that can selectively modulate the Wnt/beta-catenin signaling pathway, although some existing drugs (e.g., non-steroidal anti-inflammatory drugs, vitamins, and imatinib mesylate) have been suggested to inhibit this pathway. Here we provide an overview of the Wnt/beta-catenin signaling pathway: its roles in physiology and pathology and the possibility as a target in development of new drugs.

Celecoxib inhibits the expression of survivin via the suppression of promoter activity in human colon cancer cells

We investigated the effect of nonsteroidal anti-inflammatory drugs (NSAIDs) on human colon cancer cell lines to clarify the mechanisms underlying the chemopreventive effect of NSAIDs. Celecoxib, a selective cyclooxygenase-2 (COX-2) inhibitor, induced apoptosis and strongly reduced the expression of an anti-apoptotic protein, survivin, in both protein and mRNA levels in HCT-116 cells. Subsequently, we conducted luciferase reporter assay using a reporter gene driven by the human survivin promoter. A series of analyses using luciferase reporter constructs containing fragments of the survivin promoter and electrophoretic mobility shift assay indicated that the -75/-66 bp region relative to the initiating codon was involved in celecoxib action to suppress survivin promoter activity. Celecoxib also suppressed the activity of TOPflash, T-cell factor reporter plasmid, and the reporter gene driven by the human cyclin D1 promoter, suggesting that this compound inhibited the expression of Wnt/beta-catenin signaling target genes. Further, we found that other NSAIDs including indomethacin, resveratrol, and SC-560 induced apoptosis and suppressed the expression of survivin and the Wnt/beta-catenin signaling pathway in HCT-116 cells, indicating that these effects were likely to be common among NSAIDs. Moreover, NSAIDs (celecoxib, SC-560 and indomethacin) also suppressed the expression of cyclin D1 and survivin on other colon cancer cell lines (DLD-1 and SW-620). Our results suggested that NSAIDs could inhibit proliferation and induce apoptosis in colon cancer cells by inhibition of survivin expression and the Wnt/beta-catenin signaling pathway.