Ken-ichi Nakahama

Cellular communications in bone homeostasis and repair.

Cellular communication between the bone component cells osteoblasts, osteocytes and (pre-)osteoclasts is essential for bone remodeling which maintains bone integrity. As in the remodeling of other organs, cell death is a trigger for remodeling of bone. During the systematic process of bone remodeling, direct or indirect cell–cell communication is indispensable. Thus, osteoblasts induce migration and differentiation of preosteoclasts, which is followed by bone resorption (by mature multinuclear osteoclasts). After completion of bone resorption, apoptosis of mature osteoclasts and differentiation of osteoblasts are initiated. At this time, the osteoblasts do not support osteoclast differentiation but do support bone formation. Finally, osteoblasts differentiate to osteocytes in bone or to bone lining cells on bone surfaces. In this way, old bone areas are regenerated as new bone. In this review the role of cell–cell communication in bone remodeling is discussed.

Direct formation of proteo-liposomes by in vitro synthesis and cellular cytosolic delivery with connexin-expressing liposomes

Liposomes are widely utilized in molecular biology and medicine as drug carriers. Here we report a new liposome-cell interaction through connexins. Connexin 43 (Cx43)-containing liposomes were prepared by using cell-free transcription/translation systems with plasmids encoding Cx43 in the presence of liposome. The expressed membrane protein, Cx43, was directly constituted to the liposome membrane upon in vitro synthesis, leading to pure membrane protein-containing liposomes. The hydrophilic dye calcein was efficiently transferred from Cx43-expressing liposomes to cultured cells (Cx43 expressing). The transfer is significantly blocked in the presence of gap junction inhibitor (18beta-glycyrrhetinic acid) and in the case of the other type of connexin (Cx32)-expressing cell. The results show that calcein entered the cell through connexin-mediated pathway. Cx43 liposomes containing a soluble NEMO-binding domain peptide suppressed the intracellular signaling cascade IL-1beta-induced NF-kappaB activation and cyclooxygenase-2 expression in Cx43-expressing cells, confirming effective peptide transfer into the cell. This is a new method for direct cytosolic delivery of hydrophilic molecules.

The effects of polyunsaturated fatty acids and their metabolites on osteoclastogenesis in vitro

Bone homeostasis is maintained by active remodeling through the balance between resorption (by osteoclasts) and synthesis (by osteoblasts). In this study, we examined the effects of polyunsaturated fatty acids (PUFAs) and their metabolites on sRANKL-induced differentiation of bone marrow-derived macrophages (BMMs) into osteoclasts in vitro. Docosahexaenoic acid (DHA) strongly inhibited osteoclastogenesis; however, dihomo-gamma-linolenic acid (DGLA), arachidonic acid (AA) and eicosapentaenoic acid (EPA) enhanced it. The enhancement effect of PUFAs on osteoclastogenesis was mediated predominantly by cyclooxygenase (COX) products, because the effect was inhibited by a COX inhibitor. It was also found that COX products of PUFAs, prostaglandin E(1), E(2), and E(3), clearly increased in osteoclastogenesis. The inhibitory effect of DHA on osteoclastogenesis was reversed by treatment with a lipoxygenase (LOX) inhibitor. Furthermore, resolvin D1, a LOX product of DHA, significantly inhibited osteoclastogenesis. Quantitative analysis of specific mRNA levels revealed that DHA-mediated attenuation of osteoclastogenesis might be due to a decrease in DC-STAMP expression. These results suggested that the effect of DHA on osteoclastogenesis is, at least in part, mediated by lipoxygenase products. This study showed a distinct mechanism of the effect of PUFAs on osteoclastogenesis and will provide evidence for therapeutic treatment with DHA in osteoporotic patients.

The steady-state expression of connexin43 is maintained by the PI3K/Akt in osteoblasts.

The gap junction protein connexin43 (Cx43) plays an important role in bone development and its homeostasis. Therefore, it is important that the Cx43 expression is kept at a high-level in bone tissues under normal conditions. To investigate the mechanism to keep Cx43 expression level, the effects of protein kinase inhibitors on the basal expression of Cx43 were examined. It was found that the specific PI3K inhibitor LY294002 significantly decreased the steady-state expression levels of Cx43 mRNA and protein in osteoblastic cell line, MC3T3-E1 cells. Furthermore, dominant-negative Akt expression reduced both Cx43 expression and gap junction activity. These results suggest an important role of PI3K/Akt in the regulation of basal Cx43 expression. A promoter assay for Cx43 and an actinomycin D chase experiment revealed that PI3K/Akt modulated Cx43 expression post-transcriptionally via mRNA stability. The present findings could provide new insights into the molecular understanding of Cx43 expression.

Implantation of Capillary Structure Engineered by Optical Lithography Improves Hind Limb Ischemia in Mice

We previously reported a novel optical lithographic technique for the construction of a capillary network consisting of endothelial cells. To investigate the feasibility of clinical application in the treatment of ischemic diseases, capillary structures were formed on scaffolds made from amniotic membrane (AM) and implanted into mice. The capillary network remained in place for at least 5 days and blood perfusion through the implanted capillaries was histologically detected in an ear flap model. Moreover, blood was observed flowing through the capillary network implanted in abdominal subcutaneous tissue of mice at 5 days after insertion. Implantation of the AM capillary structure into the ischemic hind limbs of mice significantly increased reperfusion compared with controls (AM only). Blood flow was restored in the ischemic limbs to the level of corresponding nonischemic limbs as early as 9 days after surgical implantation. The treatment reversed ischemic symptoms, and ambulatory impairment was significantly improved. Thus, the implantation of a capillary network engineered ex vivo could have therapeutic potential for ischemic diseases.

Vitamin K2 suppresses malignancy of HuH7 hepatoma cells via inhibition of connexin 43

The anti-cancer potential of vitamin K(2) (VK(2)) in hepatoma has gained considerable attention but the underlying mechanisms are unclear. Treatment of HuH7 hepatoma cells with VK(2) produced a normal liver phenotype. Following treatment of cells with VK(2), there was an increase in gap junctional intercellular communication activity, accompanied by up-regulation of connexin 32 (Cx32), dominantly expressed in normal hepatocyte. In contrast, Cx43 expression was inhibited. Moreover, the effect of VK(2) on Cx32 was abolished by over-expression of Cx43. Taken together, we propose that the anti-tumor effect of VK(2) is at least partly due to a decrease in Cx43 promoter activity.

Cell-printing and transfer technology applications for bone defects in mice

Bone regeneration therapy based on the delivery of osteogenic factors and/or cells has received a lot of attention in recent years since the discovery of pluripotent stem cells. We reported previously that the implantation of capillary networks engineered ex vivo by the use of cell‐printing technology could improve blood perfusion. Here, we developed a new substrate prepared by coating glass with polyethylene glycol (PEG) to create a non‐adhesive surface and subsequent photo‐lithography to finely tune the adhesive property for efficient cell transfer. We examined the cell‐transfer efficiency onto amniotic membrane and bone regenerative efficiency in murine calvarial bone defect. Cell transfer of KUSA‐A1 cells (murine osteoblasts) to amniotic membrane was performed for 1 h using the substrates. Cell transfer using the substrate facilitated cell engraftment onto the amniotic membrane compared to that by direct cell inoculation. KUSA‐A1 cells transferred onto the amniotic membrane were applied to critical‐sized calvarial bone defects in mice. Micro‐computed tomography (micro‐CT) analysis showed rapid and effective bone formation by the cell‐equipped amniotic membrane. These results indicate that the cell‐printing and transfer technology used to create the cell‐equipped amniotic membrane was beneficial for the cell delivery system. Our findings support the development of a biologically stable and effective bone regeneration therapy. Copyright

The reduction in pigment epithelium-derived factor is a sign of malignancy in ovarian cancer expressing low-level of vascular endothelial growth factor

Background.u2003Angiogenesis is a critical factor in the progression of solid tumors and metastasis. The aim of this study was to characterise the roles of angiogenic and anti-angiogenic factors on ovarian cancer. Methods.u2003The expression levels of vascular endothelial growth factor (VEGF, angiogenic factor) and pigment epithelial growth factor (PEDF, anti-angiogenic factor) were measured by real-time polymerase chain reaction and Western blotting in ovarian tumors. Microvessel density (MVD) was evaluated by the total microvessel length in high-power field of tumor tissue preparations. Results.u2003MVD correlated with tumor malignancy. The tissues with the highest expression levels of VEGF (VEGF-H) were malignant tumors. The VEGF expression levels in some malignant tumors (VEGF-L) were as low as that in benign tumors. Therefore, the expression of PEDF was examined. The PEDF expression levels in VEGF-L malignant tumors were significantly lower than those in benign tumors. On the other hand, the PEDF expression levels in VEGF-H malignant tumor tissues were not significantly different from those in benign tumors. Conclusion.u2003The reduction in PEDF expression levels may be, in part, responsible for tumor malignancy in VEGF-L ovarian tumors. Furthermore, PEDF may be a useful marker of malignancy in VEGF-L ovarian tumors.

Potential enhancement of osteoclastogenesis by severe acute respiratory syndrome coronavirus 3a/X1 protein

Severe acute respiratory syndrome coronavirus (SARS-CoV) causes a lung disease with high mortality. In addition, osteonecrosis and bone abnormalities with reduced bone density have been observed in patients following recovery from SARS, which were partly but not entirely explained by the short-term use of steroids. Here, we demonstrate that human monocytes, potential precursors of osteoclasts, partly express angiotensin converting enzyme 2 (ACE2), a cellular receptor of SARS-CoV, and that expression of an accessory protein of SARS-CoV, 3a/X1, in murine macrophage cell line RAW264.7 cells, enhanced NF-κB activity and differentiation into osteoclast-like cells in the presence of receptor activator of NF-κB ligand (RANKL). Furthermore, human epithelial A549 cells expressed ACE2, and expression of 3a/X1 in these cells up-regulated TNF-α, which is known to accelerate osteoclastogenesis. 3a/X1 also enhanced RANKL expression in mouse stromal ST2 cells. These findings indicate that SARS-CoV 3a/X1 might promote osteoclastogenesis by direct and indirect mechanisms.

Amyloid β Enhances Migration of Endothelial Progenitor Cells by Upregulating CX3CR1 in Response to Fractalkine, Which May Be Associated With Development of Choroidal Neovascularization

Objective—Deposits that accumulate beneath retinal pigment epithelium, called drusen, are early signs of age-related macular degeneration (AMD). We have shown that amyloid &bgr; (A&bgr;) is present in drusen, and A&bgr; may be involved in AMD development. We have also shown that endothelial progenitor cells (EPCs) may contribute to the development of choroidal neovascularization (CNV). Thus, the purpose of this study was to investigate the role played by CX3CR1, a chemokine receptor, in EPC migration and CNV formation. Methods and Results—EPCs collected from human umbilical cords were found to express higher levels of CX3CR1 than human umbilical vein endothelial cells, and exposure of EPCs to A&bgr; caused further upregulation of CX3CR1. This upregulation was decreased by blocking fractalkine, a ligand of CX3CR1. Exposure of EPCs to fractalkine increased their migration, but pretreatment with A&bgr; enhanced the migration. The fractalkine-induced EPC migration was more inhibited by EPCs derived from CX3CR1−/− mice than wild-type mice. The area of laser-induced CNV was significantly smaller in wild-type mice that received bone marrow transplantation from CX3CR1−/− mice than in those that received transplantation from wild-type mice. Conclusion—These data suggest that A&bgr; enhances EPC migration through the upregulation of CX3CR1. This upregulation might play a role in development of CNV.