BMP‐2 modulates β‐catenin signaling through stimulation of Lrp5 expression and inhibition of β‐TrCP expression in osteoblasts

Canonical BMP and Wnt signaling pathways play critical roles in regulation of osteoblast function and bone formation. Recent studies demonstrate that BMP‐2 acts synergistically with β‐catenin to promote osteoblast differentiation. To determine the molecular mechanisms of the signaling cross‐talk between canonical BMP and Wnt signaling pathways, we have used primary osteoblasts and osteoblast precursor cell lines 2T3 and MC3T3‐E1 cells to investigate the effect of BMP‐2 on β‐catenin signaling. We found that BMP‐2 stimulates Lrp5 expression and inhibits the expression of β‐TrCP, the F‐box E3 ligase responsible for β‐catenin degradation and subsequently increases β‐catenin protein levels in osteoblasts. In vitro deletion of the β‐catenin gene inhibits osteoblast proliferation and alters osteoblast differentiation and reduces the responsiveness of osteoblasts to the BMP‐2 treatment. These findings suggest that BMP‐2 may regulate osteoblast function in part through modulation of the β‐catenin signaling. J. Cell. Biochem. 108: 896–905, 2009. © 2009 Wiley‐Liss, Inc.     

β‐Catenin—A supporting role in the skeleton

In the last 5 years a role for β‐catenin in the skeleton has been cemented. Beginning with mutations in the Lrp5 receptor that control β‐catenin canonical downstream signals, and progressing to transgenic models with bone‐specific alteration of β‐catenin, research has shown that β‐catenin is required for normal bone development. A cell critical to bone in which β‐catenin activity determines function is the marrow‐derived mesenchymal stem cell (MSC), where sustained β‐catenin prevents its distribution into adipogenic lineage. β‐Catenin actions are less well understood in mature osteoblasts: while β‐catenin contributes to control of osteoclastic bone resorption via alteration of the osteoprotegerin/RANKL ratio, a specific regulatory role during osteoblast bone synthesis has not yet been determined. The proven ability of mechanical factors to prevent β‐catenin degradation and induce nuclear translocation through Lrp‐independent mechanisms suggests processes by which exercise might modulate bone mass via control of lineage allocation, in particular, by preventing precursor distribution into the adipocyte pool. Effects resulting from mechanical activation of β‐catenin in mature osteoblasts and osteocytes likely modulate bone resorption, but whether β‐catenin is involved in osteoblast synthetic function remains to be proven for both mechanical and soluble mediators. As β‐catenin appears to support the downstream effects of multiple osteogenic factors, studies clarifying when and where β‐catenin effects occur will be relevant for translational approaches aimed at preventing bone loss and terminal adipogenic conversion. J. Cell. Biochem. 110: 545–553, 2010. © 2010 Wiley‐Liss, Inc.     

Fluid shear stress induces Runx‐2 expression via upregulation of PIEZO1 in MC3T3‐E1 cells

Mechanically induced biological responses in bone cells involve a complex biophysical process. Although various mechanosensors have been identified, the precise mechanotransduction pathway remains poorly understood. PIEZO1 is a newly discovered mechanically activated ion channel in bone cells. This study aimed to explore the involvement of PIEZO1 in mechanical loading (fluid shear stress)‐induced signaling cascades that control osteogenesis. The results showed that fluid shear stress increased PIEZO1 expression in MC3T3‐E1 cells. The fluid shear stress elicited the key osteoblastic gene Runx‐2 expression; however, PIEZO1 silencing using small interference RNA blocked these effects. The AKT/GSK‐3β/β‐catenin pathway was activated in this process. PIEZO1 silencing impaired mechanically induced activation of the AKT/GSK‐3β/β‐catenin pathway. Therefore, the results demonstrated that MC3T3‐E1 osteoblasts required PIEZO1 to adapt to the external mechanical fluid shear stress, thereby inducing osteoblastic Runx‐2 gene expression, partly through the AKT/GSK‐3β/β‐catenin pathway.     

Calcitonin gene‐related peptide (CGRP) inhibits apoptosis in human osteoblasts by β‐catenin stabilization

Transgenic mice over‐expressing calcitonin gene‐related peptide (CGRP) in osteoblasts have increased bone density due to increased bone formation, thus suggesting that CGRP plays a role in bone metabolism. In this study we determined the relationship between CGRP, the canonical Wnt signaling and apoptosis in human osteoblasts (hOBs) in consideration of the well‐documented involvement of this pathway in bone cells. Primary cultures of hOBs were treated with CGRP 10^?8 M. Levels of β‐catenin, which is the cytoplasmic protein mediator of canonical Wnt signaling, and mRNA were determined. CGRP increases both the expression and the levels of cytoplasmic β‐catenin by binding to its receptor, as this effect is blocked by the antagonist CGRP_8–37. This facilitatory action on β‐catenin appears to be mediated by the inhibition of the enzyme GSK‐3β via protein kinase A (PKA) activation. GSK‐3β is a glycogen synthase kinase that, by phosphorylating β‐catenin, promotes its degradation by the proteosomal machinery. Moreover, the peptide is able to inhibit hOBs apoptosis stimulated by dexamethasone or by serum deprivation, possibly through the accumulation of β‐catenin, since the inhibitor of PKA activity H89 partially prevents the antiapoptotic effect of the peptide. In conclusion CGRP, released by nerve fibers, exerts its anabolic action on bone cells by stimulating canonical Wnt signaling and by inhibiting hOBs apoptosis, thus favoring local bone regeneration. J. Cell. Physiol. 225: 701–708, 2010. © 2010 Wiley‐Liss, Inc.     

Imperatorin promotes osteogenesis and suppresses osteoclast by activating AKT/GSK3 β/β‐catenin pathways

Osteoporosis is caused by disturbance in the dynamic balance of bone remodelling, a physiological process, vital for maintenance of healthy bone tissue in adult humans. In this process, a new bone is formed by osteoblasts and the pre‐existing bone matrix is resorbed by osteoclasts. Imperatorin, a widely available and inexpensive plant extract with antioxidative and apoptotic effects, is reported to treat osteoporosis. However, the underlying mechanism and specific effects on bone metabolism have not been elucidated. In this study, we used rat bone marrow‐derived mesenchymal stem cells and found that imperatorin can activate RUNX2, COL1A1 and osteocalcin by promoting the Ser9 phosphorylation of GSK3β and entry of β‐catenin into the nucleus. Imperatorin also enhanced the production of phospho‐AKT (Ser473), an upstream factor that promotes the Ser9 phosphorylation of GSK3β. We used ipatasertib, a pan‐AKT inhibitor, to inhibit the osteogenic effect of imperatorin, and found that imperatorin promotes osteogenesis via AKT/GSK3β/β‐catenin pathway. Next, we used rat bone marrow‐derived monocytes, to check whether imperatorin inhibits osteoclast differentiation via AKT/GSK3β/β‐catenin pathway. Further, we removed the bilateral ovaries of rats to establish an osteoporotic model. Intragastric administration of imperatorin promoted osteogenesis and inhibited osteoclast in vivo. Our experiments showed that imperatorin is a potential drug for osteoporosis treatment.     

Nmp4/CIZ对成骨细胞功能的调节作用

核基质蛋白4(nuclear matrix protein,Nmp4),亦称p130cas结合锌指蛋白(cas-interacting zinc finger protein,CIZ),是一种位于细胞核及粘附斑且具有核质穿梭功能的转录调控因子.体内实验证明,Nmp4/CIZ抑制骨形成活性进而抑制骨密度和骨量增加,而对骨吸收参数无显著影响.Nmp4/CIZ通过特异性结合成骨细胞Ⅰ型胶原α1链和基质金属蛋白酶启动子上游调控序列,调节其转录表达,促进骨转换.此外,Nmp4/CIZ抑制骨形态蛋白和甲状旁腺激素诱导的成骨细胞分化,抑制成骨细胞增殖并促进凋亡发生.Nmp4/CIZ参与调节成骨细胞力学-化学信号转导,其表达沉默可抑制尾悬吊诱导的小鼠骨量减少,并促进流体剪切力诱导的成骨细胞β-catenin信号途径.这些体内外实验证据表明,Nmp4/CIZ主要通过负调控机制发挥作用,提示这是一个潜在的骨丢失治疗药物靶点.     

Suppression of epidermal growth factor receptor‐mediated β‐catenin nuclear accumulation enhances the anti‐tumor activity of phosphoinositide 3‐kinase inhibitor in breast cancer

Phosphoinositide 3‐kinase (PI3K) signaling is frequently deregulated in breast cancer and plays a critical role in tumor progression. However, resistance to PI3K inhibitors in breast cancer has emerged, which is due to the enhanced β‐catenin nuclear accumulation. Until now, the mechanisms underlying PI3K inhibition‐induced β‐catenin nuclear accumulation remains largely unknown. In the present study, we found inhibition of PI3K with LY294002 promoted β‐catenin nuclear accumulation in MCF‐7 and MDA‐MB‐231 breast cancer cells. Combining PI3K inhibitor LY294002 with XAV‐939, an inhibitor against β‐catenin nuclear accumulation, produced an additive anti‐proliferation effect against breast cancer cells. Subsequent experiments suggested β‐catenin nuclear accumulation induced by PI3K inhibition depended on the feedback activation of epidermal growth factor receptor (EGFR) signaling pathway in breast cancer cells. Inhibition of EGFR phosphorylation with Gefitinib enhanced anti‐proliferation effect of PI3K inhibitor LY294002 in MCF‐7 and MDA‐MB‐231 cells. Taken together, our findings may elucidate a possible mechanism explaining the poor outcome of PI3K inhibitors in breast cancer treatment.     

Wnt/β‐catenin signalling induces MLL to create epigenetic changes in salivary gland tumours

We show that activation of Wnt/β‐catenin and attenuation of Bmp signals, by combined gain‐ and loss‐of‐function mutations of β‐catenin and Bmpr1a, respectively, results in rapidly growing, aggressive squamous cell carcinomas (SCC) in the salivary glands of mice. Tumours contain transplantable and hyperproliferative tumour propagating cells, which can be enriched by fluorescence activated cell sorting (FACS). Single mutations stimulate stem cells, but tumours are not formed. We show that β‐catenin, CBP and Mll promote self‐renewal and H3K4 tri‐methylation in tumour propagating cells. Blocking β‐catenin–CBP interaction with the small molecule ICG‐001 and small‐interfering RNAs against β‐catenin, CBP or Mll abrogate hyperproliferation and H3K4 tri‐methylation, and induce differentiation of cultured tumour propagating cells into acini‐like structures. ICG‐001 decreases H3K4me3 at promoters of stem cell‐associated genes in vitro and reduces tumour growth in vivo. Remarkably, high Wnt/β‐catenin and low Bmp signalling also characterize human salivary gland SCC and head and neck SCC in general. Our work defines mechanisms by which β‐catenin signals remodel chromatin and control induction and maintenance of tumour propagating cells. Further, it supports new strategies for the therapy of solid tumours.