Kenjiro Bandow

JNK Activity Is Essential for Atf4 Expression and Late-Stage Osteoblast Differentiation†

Osteoblasts differentiate from mesodermal progenitors and play a pivotal role in bone formation and mineralization. Several transcription factors including runt‐related transcription factor 2 (RUNX2), Osterix (OSX), and activating transcription factor4 (ATF4) are known to be crucial for the process, whereas the upstream signal transduction controlling the osteoblast differentiation sequence is largely unknown. Here, we explored the role of c‐jun N‐terminal kinase (JNK) in osteoblast differentiation using in vitro differentiation models of primary osteoblasts and MC3T3‐E1 cells with ascorbic acid/β‐glycerophosphate treatment. Terminal osteoblast differentiation, represented by matrix mineralization, was significantly inhibited by the inactivation of JNK with its specific inhibitor and exogenous overexpression of MKP‐M (MAP kinase phosphatase isolated from macrophages), which preferentially inactivates JNK. Conversely, enhanced mineral deposition was observed by inducible overexpression of p54JNK2, whereas it was not observed by the overexpression of p46JNK1 or p46JNK2, indicating a distinct enhancing role of p54JNK2 in osteoblast differentiation. Inactivation of JNK significantly inhibited late‐stage molecular events of osteoblast differentiation, including gene expression of osteocalcin (Ocn) and bone sialoprotein (Bsp). In contrast, earlier differentiation events including alkaline phosphatase (ALP) activation and osteopontin (Opn) expression were not inhibited by JNK inactivation. Although the expression levels of two transcription factor genes, Runx2 and Osx, were not significantly affected by JNK inactivation, induction of Atf4 mRNA during osteoblast differentiation was significantly inhibited. Taken together, these data indicate that JNK activity is specifically required for the late‐stage differentiation events of osteoblasts.

Low-intensity pulsed ultrasound (LIPUS) induces RANKL, MCP-1, and MIP-1β expression in osteoblasts through the angiotensin II type 1 receptor

Constant mechanical stress is essential for the maintenance of bone mass and strength, which is achieved through the cooperative functions of osteoblasts and osteoclasts. However, it has not been fully elucidated how these cell types mediate mechanical signals. Low‐intensity pulsed ultrasound (LIPUS) therapy is a recently developed method for application of mechanical stress, and is used clinically to promote bone fracture healing. In the present study, we applied LIPUS to osteoblasts at different stages of maturation and analyzed their chemokine and cytokine expression. In comparison with their immature counterparts, mature osteoblasts expressed significantly higher levels of mRNAs for the receptor activator of nuclear factor kappa B ligand (RANKL), monocyte chemoattractant protein (MCP)‐1, and macrophage‐inflammatory protein (MIP)‐1β after a few hours of LIPUS treatment. Intriguingly, protein and mRNA expression of angiotensin II type 1 receptor (AT1), a known mechanoreceptor in cardiomyocytes, was detected in osteoblasts, and the level of expression increased significantly during cell maturation. Furthermore, LIPUS‐induced extracellular signal‐regulated kinase (ERK) phosphorylation and RANKL/chemokine expression was abrogated by a specific AT1 inhibitor. Thus, AT1 may play one of the essential roles in bone metabolism as a mechanoreceptor of osteoblasts. J. Cell. Physiol. 211: 392–398, 2007.

Molecular mechanisms of the inhibitory effect of lipopolysaccharide (LPS) on osteoblast differentiation.

Osteoblasts express Toll like receptor (TLR) 4 and produce osteoclast-activating cytokines in response to the stimulation by lipopolysaccharide (LPS). It has recently been reported that LPS exerts an inhibitory effect on osteoblast differentiation into osteocytes. However, the molecular mechanisms of this inhibitory effect remain ambiguous. The downstream signals of TLR4 are mediated by adaptor molecules including myeloid differentiation factor 88 (MyD88), leading to the activation of mitogen-activated protein kinases (MAPKs), such as extracellular signal-regulated kinases (ERKs), whose activation by LPS requires the upstream serine/threonine kinase, Cot/Tpl2. To determine the signal molecules responsible for the inhibitory effects of LPS on osteoblast differentiation, we examined the in vitro differentiation of the primary osteoblasts from myd88(-/-) and cot/tpl2(-/-) mice. The matrix mineralization by the wild-type and cot/tpl2(-/-) osteoblasts was significantly inhibited by LPS, whereas that of myd88(-/-) was not affected. During differentiation, LPS suppressed the mRNA expression of runt related transcription factor 2 (Runx2), osterix (Sp7), and activating transcription factor 4 (ATF4) in the wild-type, but not in the myd88(-/-) osteoblasts. The inhibitory effect of LPS on the mRNA expression of these transcription factors was absent in the early phase but partially impaired in the late phase of differentiation in the cot/tpl2(-/-) osteoblasts. Thus, the inhibitory effect of LPS on osteoblast differentiation is Myd88-dependent, whereas the degree of its requirement for Cot/Tpl2 varies depending on the differentiation phase.

Oxidative stress causes alveolar bone loss in metabolic syndrome model mice with type 2 diabetes

BACKGROUND AND OBJECTIVE Alveolar bone loss is caused by a host response to periodontal pathogens, and its progression is often enhanced by systemic conditions such as insulin resistance. Alveolar bone dehiscence has been observed in KK-A(y) mice, which are metabolic syndrome model mice with type 2 diabetes. The aim of this study was to investigate inducements responsible for alveolar bone dehiscence in the KK-A(y) mice. MATERIAL AND METHODS The expression of endothelial nitric oxide synthase in the mandibles of mice was detected using immunohistochemical staining and the reverse transcription-polymerase chain reaction. After administration of N-acetylcysteine, an antioxidant, to KK-A(y) mice, alveolar bone loss and the expression of endothelial nitric oxide synthase protein in gingival keratinocytes and of hydrogen peroxide concentrations in plasma, were analyzed. The effect of hydrogen peroxide on endothelial nitric oxide synthase expression in keratinocytes was examined using cultured keratinocytes. RESULTS The expression of endothelial nitric oxide synthase was decreased in gingival keratinocytes from KK-A(y) mice compared with gingival keratinocytes from control mice. Administration of N-acetylcysteine to the mice restored endothelial nitric oxide synthase expression in the gingival keratinocytes, suppressed the alveolar bone loss and decreased the hydrogen peroxide concentrations in plasma without the improvement of obesity or diabetes. In vitro, stimulation with hydrogen peroxide decreased the expression level of endothelial nitric oxide synthase in cultured keratinocytes, which was restored by the addition of N-acetylcysteine. CONCLUSION Reactive oxygen species, such as hydrogen peroxide, are responsible for the alveolar bone loss accompanied by decreased endothelial nitric oxide synthase expression in KK-A(y) mice. Therefore, we propose a working hypothesis that the generation of oxidative stress is an underlying systemic condition that enhances alveolar bone loss in periodontitis occurring as a complication of diabetes.

Osteoblast differentiation is functionally associated with decreased AMP kinase activity.

Osteoblasts, originating from mesenchymal stem cells, play a pivotal role in bone formation and mineralization. Several transcription factors including runt‐related transcription factor 2 (Runx2) have been reported to be essential for osteoblast differentiation, whereas the cytoplasmic signal transduction pathways controlling the differentiation process have not been fully elucidated. AMP‐activated protein kinase (AMPK) is a serine–threonine kinase generally regarded as a key regulator of cellular energy homeostasis, polarity, and division. Recent lines of evidence have indicated that the activity of the catalytic α subunit of AMPK is regulated through its phosphorylation by upstream AMPK kinases (AMPKKs) including LKB1. Here, we explored the role of AMPK in osteoblast differentiation using in vitro culture models. Phosphorylation of AMPKα was significantly decreased during osteoblastic differentiation in both primary osteoblasts and MC3T3‐E1, a mouse osteoblastic cell line. Conversely, the terminal differentiation of primary osteoblasts and MC3T3‐E1 cells, represented by matrix mineralization, was significantly inhibited by glucose restriction and stimulation with metformin, both of which are known activators of AMPK. Matrix mineralization of MC3T3‐E1 cells was also inhibited by the forced expression of a constitutively active form of AMPKα. Metformin significantly inhibited gene expression of Runx2 along with osteoblast differentiation markers including osteocalcin (Ocn), bone sialo protein (Bsp), and osteopontin (Opn). Thus, our present data indicate that differentiation of osteoblasts is functionally associated with decreased AMPK activity. J. Cell. Physiol. 221: 740–749, 2009.

Low Intensity Pulsed Ultrasound (LIPUS) Influences the Multilineage Differentiation of Mesenchymal Stem and Progenitor Cell Lines through ROCK-Cot/Tpl2-MEK-ERK Signaling Pathway

Background: Low intensity pulsed ultrasound (LIPUS) is a mechanical stimulus clinically used to promote bone fracture healing. Results: LIPUS suppresses adipogenesis and promotes osteogenesis of mesenchyme stem/progenitor cell lines by inhibiting PPARγ2 through ROCK-Cot/Tpl2-MEK-ERK pathway. Conclusion: LIPUS influences multilineage differentiation of mesenchymal stem and progenitor cells. Significance: LIPUS may be a new clinical approach to chronic bone metabolic disorders, including osteoporosis. Mesenchymal stem cells (MSCs) are pluripotent cells that can differentiate into multilineage cell types, including adipocytes and osteoblasts. Mechanical stimulus is one of the crucial factors in regulating MSC differentiation. However, it remains unknown how mechanical stimulus affects the balance between adipogenesis and osteogenesis. Low intensity pulsed ultrasound (LIPUS) therapy is a clinical application of mechanical stimulus and facilitates bone fracture healing. Here, we applied LIPUS to adipogenic progenitor cell and MSC lines to analyze how multilineage cell differentiation was affected. We found that LIPUS suppressed adipogenic differentiation of both cell types, represented by impaired lipid droplet appearance and decreased gene expression of peroxisome proliferator-activated receptor γ2 (Pparg2) and fatty acid-binding protein 4 (Fabp4). LIPUS also down-regulated the phosphorylation level of peroxisome proliferator-activated receptor γ2 protein, inhibiting its transcriptional activity. In contrast, LIPUS promoted osteogenic differentiation of the MSC line, characterized by increased cell calcification as well as inductions of runt-related transcription factor 2 (Runx2) and Osteocalcin mRNAs. LIPUS induced phosphorylation of cancer Osaka thyroid oncogene/tumor progression locus 2 (Cot/Tpl2) kinase, which was essential for the phosphorylation of mitogen-activated kinase kinase 1 (MEK1) and p44/p42 extracellular signal-regulated kinases (ERKs). Notably, effects of LIPUS on both adipogenesis and osteogenesis were prevented by a Cot/Tpl2-specific inhibitor. Furthermore, effects of LIPUS on MSC differentiation as well as Cot/Tpl2 phosphorylation were attenuated by the inhibition of Rho-associated kinase. Taken together, these results indicate that mechanical stimulus with LIPUS suppresses adipogenesis and promotes osteogenesis of MSCs through Rho-associated kinase-Cot/Tpl2-MEK-ERK signaling pathway.

LPS‐induced chemokine expression in both MyD88‐dependent and ‐independent manners is regulated by Cot/Tpl2‐ERK axis in macrophages

LPS signaling is mediated through MyD88‐dependent and ‐independent pathways, activating NF‐κB, MAP kinases and IRF3. Cot/Tpl2 is an essential upstream kinase in LPS‐mediated activation of ERKs. Here we explore the roles of MyD88 and Cot/Tpl2 in LPS‐induced chemokine expression by studying myd88 −/− and cot/tpl2 −/− macrophages. Among the nine LPS‐responsive chemokines examined, mRNA induction of ccl5, cxcl10, and cxcl13 is mediated through the MyD88‐independent pathway. Notably, Cot/Tpl2‐ERK signaling axis exerts negative effects on the expression of these three chemokines. In contrast, LPS‐induced gene expression of ccl2, ccl7, cxcl2, cxcl3, ccl8, and cxcl9 is mediated in the MyD88‐dependent manner. The Cot/Tpl2‐ERK axis promotes the expression of the first four and inhibits the expression of the latter two. Thus, LPS induces expression of multiple chemokines through various signaling pathways in macrophages.

Force-induced IL-8 from Periodontal Ligament Cells Requires IL-1β

During orthodontic tooth movement, mechanical stresses induce inflammatory reactions in the periodontal ligament (PDL). We hypothesized that chemokines released from PDL cells under mechanical stress regulate osteoclastogenesis, and investigated the profiles and mechanisms of chemokine expression by human PDL cells in response to mechanical stress. In vitro, shear stress and pressure force rapidly increased the gene and protein expressions of IL-8/CXCL8 by PDL cells. Consistently, amounts of IL-8 in the gingival crevicular fluid of healthy individuals increased within 2 to 4 days of orthodontic force application. The PDL cells constitutively expressed low levels of IL-1β, which were not further increased by mechanical stress. Interestingly, neutralization of IL-1β abolished IL-8 induction by mechanical stresses, indicating that IL-1β is essential for IL-8 induction, presumably though autocrine or paracrine mechanisms. Finally, experiments with signal-specific inhibitors indicated that MAP kinase activation is essential for IL-8 induction.

Low-intensity pulsed ultrasound (LIPUS) inhibits LPS-induced inflammatory responses of osteoblasts through TLR4–MyD88 dissociation

Previous reports have shown that osteoblasts are mechano-sensitive. Low-intensity pulsed ultrasound (LIPUS) induces osteoblast differentiation and is an established therapy for bone fracture. Here we have examined how LIPUS affects inflammatory responses of osteoblasts to LPS. LPS rapidly induced mRNA expression of several chemokines including CCL2, CXCL1, and CXCL10 in both mouse osteoblast cell line and calvaria-derived osteoblasts. Simultaneous treatment by LIPUS significantly inhibited mRNA induction of CXCL1 and CXCL10 by LPS. LPS-induced phosphorylation of ERKs, p38 kinases, MEK1/2, MKK3/6, IKKs, TBK1, and Akt was decreased in LIPUS-treated osteoblasts. Furthermore, LIPUS inhibited the transcriptional activation of NF-κB responsive element and Interferon-sensitive response element (ISRE) by LPS. In a transient transfection experiment, LIPUS significantly inhibited TLR4-MyD88 complex formation. Thus LIPUS exerts anti-inflammatory effects on LPS-stimulated osteoblasts by inhibiting TLR4 signal transduction.

Hepatocyte growth factor/scatter factor stimulates migration of muscle precursors in developing mouse tongue.

Hepatocyte growth factor (HGF) stimulates the migration of myogenic cells during the development of skeletal muscles. The inactivation of HGF genes or that of its receptor, c‐met, in mice causes hypoplasia of skeletal muscle organs, such as the tongue. Basic fibroblast growth factor (FGF‐2) also induces migration of skeletal myoblasts. A comparison of the functions of HGF and FGF‐2 in myogenesis revealed the crucial effect of HGF in the development of skeletal muscles. Unlike FGF‐2, HGF induced migration of myoblasts from the developing mouse tongue. The differences between the activities of HGF and FGF‐2 were determined by comparing their effects on the expression of matrix metalloproteinase‐9 (MMP‐9) in myoblasts, C2C12 cells, cultured in collagen‐coated dishes. The results showed that HGF, but not FGF‐2, stimulated MMP‐9 expression, and that the stimulation was mediated through the activation of phosphoinositide 3‐kinase (PI3K) which was not associated with FGF‐2 signal transduction. Nevertheless, both growth factors exerted almost the same effect on the reduction of myogenin expression in, and on the proliferation of, C2C12 cells, suggesting that HGF, rather than FGF‐2, plays a crucial role in the generation of skeletal muscles, including the tongue. Moreover, the specific role of HGF through the PI3K signal pathway is the induction of MMP‐9 expression in, and the migration of, myoblasts.