Differential roles of CCN family proteins during osteoblast differentiation: Involvement of Smad and MAPK signaling pathways

Compressive force-produced CCN2 induces osteocyte apoptosis through ERK1/2 pathway

Osteocytes produce various factors that mediate the onset of bone formation and resorption and play roles in maintaining bone homeostasis and remodeling in response to mechanical stimuli. One such factor, CCN2, is thought to play a significant role in osteocyte responses to mechanical stimuli, but its function in osteocytes is not well understood. Here, we showed that CCN2 induces apoptosis in osteocytes under compressive force loading. Compressive force increased CCN2 gene expression and production, and induced apoptosis in osteocytes. Application of exogenous CCN2 protein induced apoptosis, and a neutralizing CCN2 antibody blocked loading-induced apoptosis. We further examined how CCN2 induces loaded osteocyte apoptosis. In loaded osteocytes, extracellular signal-regulated kinase 1/2 (ERK1/2) was activated, and an ERK1/2 inhibitor blocked loading-induced apoptosis. Furthermore, application of exogenous CCN2 protein caused ERK1/2 activation, and the neutralizing CCN2 antibody inhibited loading-induced ERK1/2 activation. Therefore, this study demonstrated for the first time to our knowledge that enhanced production of CCN2 in osteocytes under compressive force loading induces apoptosis through activation of ERK1/2 pathway.

CCN2: a master regulator of the genesis of bone and cartilage

CCN family member 2 (CCN2), also known as connective tissue growth factor (CTGF), has been suggested to be an endochondral ossification genetic factor that has been termed “ecogenin”, because in vitro studies revealed that CCN2 promotes the proliferation and differentiation of growth-plate chondrocytes, osteoblasts, and vascular endothelial cells, all of which play important roles in endochondral ossification. In addition to its action toward these three types of cells, CCN2 was recently found to promote the formation of osteoclasts in vitro, which cells play an important role in the replacement of cartilage by bone during endochondral ossification, thus strengthening the “ecogenin” hypothesis. For confirmation of this hypothesis, transgenic mice over-expressing CCN2 in cartilage were generated. The results proved the hypothesis; i.e., the over-expression of CCN2 in cartilage stimulated the proliferation and differentiation of growth-plate chondrocytes, resulting in the promotion of endochondral ossification. In addition to its “ecogenin” action, CCN2 had earlier been shown to promote the differentiation of various cartilage cells including articular cartilage cells. In accordance with these findings, cartilage-specific overexpression of CCN2 in the transgenic mice was shown to protect against the development of osteoarthritic changes in aging articular cartilage. Thus, CCN2 may also play a role as an anti-aging (chondroprotective) factor, stabilizing articular cartilage. CCN2 also had been shown to promote intramembranous ossification, regenerate cartilage and bone, and induce angiogenesis in vivo. For understanding of the molecular mechanism underlying such multifunctional actions, yeast two-hybrid analysis, protein array analysis, solid-phase binding assay, and surface plasmon resonance (SPR) analysis have been used to search for binding partners of CCN2. ECMs such as fibronectin and aggrecan, growth factors including BMPs and FGF2 and their receptors such as FGFR1 and 2 and RANK, as well as CCN family members themselves, were shown to bind to CCN2. Regarding the interaction of CCN2 with some of them, various binding modules in the CCN2 molecule have been identified. Therefore, the numerous biological actions of CCN2 would depend on what kinds of binding partners and what levels of them are present in the microenvironment of different types of cells, as well as on the state of differentiation of these cells. Through this mechanism, CCN2 would orchestrate various signaling pathways, acting as a signal conductor to promote harmonized skeletal growth and regeneration.

CCN3 protein participates in bone regeneration as an inhibitory factor

CCN3, a member of the CCN protein family, inhibits osteoblast differentiation in vitro. However, the role of CCN3 in bone regeneration has not been well elucidated. In this study, we investigated the role of CCN3 in bone regeneration. We identified the Ccn3 gene by microarray analysis as a highly expressed gene at the early phase of bone regeneration in a mouse bone regeneration model. We confirmed the up-regulation of Ccn3 at the early phase of bone regeneration by RT-PCR, Western blot, and immunofluorescence analyses. Ccn3 transgenic mice, in which Ccn3 expression was driven by 2.3-kb Col1a1 promoter, showed osteopenia compared with wild-type mice, but Ccn3 knock-out mice showed no skeletal changes compared with wild-type mice. We analyzed the bone regeneration process in Ccn3 transgenic mice and Ccn3 knock-out mice by microcomputed tomography and histological analyses. Bone regeneration in Ccn3 knock-out mice was accelerated compared with that in wild-type mice. The mRNA expression levels of osteoblast-related genes (Runx2, Sp7, Col1a1, Alpl, and Bglap) in Ccn3 knock-out mice were up-regulated earlier than those in wild-type mice, as demonstrated by RT-PCR. Bone regeneration in Ccn3 transgenic mice showed no significant changes compared with that in wild-type mice. Phosphorylation of Smad1/5 was highly up-regulated at bone regeneration sites in Ccn3 KO mice compared with wild-type mice. These results indicate that CCN3 is up-regulated in the early phase of bone regeneration and acts as a negative regulator for bone regeneration. This study may contribute to the development of new strategies for bone regeneration therapy.

Molecular regulation of CCN2 in the intervertebral disc: lessons learned from other connective tissues

Connective tissue growth factor (CCN2/CTGF) plays an important role in extracellular matrix synthesis, especially in skeletal tissues such as cartilage, bone, and the intervertebral disc. As a result there is a growing interest in examining the function and regulation of this important molecule in the disc. This review discusses the regulation of CCN2 by TGF-β and hypoxia, two critical determinants that characterize the disc microenvironment, and discusses known functions of CCN2 in the disc. The almost ubiquitous regulation of CCN2 by TGF-β, including that seen in the disc, emphasizes the importance of the TGF-β-CCN2 relationship, especially in terms of extracellular matrix synthesis. Likewise, the unique cross-talk between CCN2 and HIF-1 in the disc highlights the tissue and niche specific mode of regulation. Taken together the current literature supports an anabolic role for CCN2 in the disc and its involvement in the maintenance of tissue homeostasis during both health and disease. Further studies of CCN2 in this tissue may reveal valuable targets for the biological therapy of disc degeneration.

OstemiR: a novel panel of microRNA biomarkers in osteoblastic and osteocytic differentiation from mesencymal stem cells

MicroRNAs (miRNAs) are small RNA molecules of 21–25 nucleotides that regulate cell behavior through inhibition of translation from mRNA to protein, promotion of mRNA degradation and control of gene transcription. In this study, we investigated the miRNA expression signatures of cell cultures undergoing osteoblastic and osteocytic differentiation from mesenchymal stem cells (MSC) using mouse MSC line KUSA-A1 and human MSCs. Ninety types of miRNA were quantified during osteoblastic/osteocytic differentiation in KUSA-A1 cells utilizing miRNA PCR arrays. Coincidently with mRNA induction of the osteoblastic and osteocytic markers, the expression levels of several dozen miRNAs including miR-30 family, let-7 family, miR-21, miR-16, miR-155, miR-322 and Snord85 were changed during the differentiation process. These miRNAs were predicted to recognize osteogenic differentiation-, stemness-, epinegetics-, and cell cycle-related mRNAs, and were thus designated OstemiR. Among those OstemiR, the miR-30 family was classified into miR-30b/c and miR-30a/d/e groups on the basis of expression patterns during osteogenesis as well as mature miRNA structures. In silico prediction and subsequent qRT-PCR in stable miR-30d transfectants clarified that context-dependent targeting of miR-30d on known regulators of bone formation including osteopontin/spp1, lifr, ccn2/ctgf, ccn1/cyr61, runx2, sox9 as well as novel key factors including lin28a, hnrnpa3, hspa5/grp78, eed and pcgf5. In addition, knockdown of human OstemiR miR-541 increased Osteopontin/SPP1 expression and calcification in hMSC osteoblastic differentiation, indicating that miR-541 is a negative regulator of osteoblastic differentiation. These observations indicate stage-specific roles of OstemiR especially miR-541 and the miR-30 family on novel targets in osteogenesis.

CCN4 induces vascular cell adhesion molecule-1 expression in human synovial fibroblasts and promotes monocyte adhesion

CCN4 is a cysteine-rich protein that belongs to the Cyr61, CTGF, Nov family of matricellular proteins. Here, we investigated the intracellular signaling pathways involved in CCN4-induced vascular cell adhesion molecule-1 expression in human osteoarthritis synovial fibroblasts. Stimulation of OASFs with CCN4 induced VCAM-1 expression. CCN4-induced VCAM-1 expression was attenuated by αvβ5 or α6β1 integrin antibody, Syk inhibitor, PKCδ inhibitor (rottlerin), JNK inhibitor (SP600125), and AP-1 inhibitors (curcumin and tanshinone). Stimulation of cells with CCN4 increased Syk, PKCδ, and JNK activation. Treatment of OASFs with CCN4 also increased c-Jun phosphorylation, AP-1-luciferase activity, and c-Jun binding to the AP-1 element in the VCAM-1 promoter. Moreover, up-regulation of VCAM-1 increased the adhesion of monocytes to OASF monolayers, and this adhesion was attenuated by transfection with a VCAM-1 siRNA. Our results suggest that CCN4 increases VCAM-1 expression in human OASFs via the Syk, PKCδ, JNK, c-Jun, and AP-1 signaling pathways. The CCN4-induced VCAM-1 expression promoted monocyte adhesion to human OASFs.

Anti-fibrotic effect of CCN3 accompanied by altered gene expression profile of the CCN family

CCN family proteins 2 and 3 (CCN2 and CCN3) belong to the CCN family of proteins, all having a high level of structural similarity. It is widely known that CCN2 is a profibrotic molecule that mediates the development of fibrotic disorders in many different tissues and organs. In contrast, CCN3 has been recently suggested to act as an anti-fibrotic factor in several tissues. This CCN3 action was shown earlier to be exerted by the repression of the CCN2 gene expression in kidney tissue, whereas different findings were obtained for liver cells. Thus, the molecular action of CCN3 yielding its anti-fibrotic effect is still controversial. Here, using a general model of fibrosis, we evaluated the effect of CCN3 overexpression on the gene expression of all of the CCN family members, as well as on that of fibrotic marker genes. As a result, repression of CCN2 gene expression was modest, while type I collagen and α-smooth muscle actin gene expression was prominently repressed. Interestingly, not only CCN2, but also CCN4 gene expression showed a decrease upon CCN3 overexpression. These findings indicate that fibrotic gene induction is under the control of a complex molecular network conducted by CCN family members functioning together.

Roles of heterotypic CCN2/CTGF-CCN3/NOV and homotypic CCN2-CCN2 interactions in expression of the differentiated phenotype of chondrocytes

To identify proteins that regulate CCN2 activity, we carried out GAL4-based yeast two-hybrid screening with a cDNA library derived from a chondrocytic cell line, HCS-2/8. CCN2/CTGF and CCN3/NOV polypeptides were picked up as CCN2-binding proteins, and CCN2–CCN2 and CCN2–CCN3 binding domains were identified. Direct binding between CCN2 and CCN3 was confirmed by coimmunoprecipitation in vitro and in vivo and surface plasmon resonance, and the calculated dissociation constants (K(d)) were 1.17 × 10(-9) m for CCN2 and CCN2, and 1.95 × 10(-9) m for CCN2 and CCN3. Ectopically overexpressed green fluorescent protein–CCN2 and Halo–CCN3 in COS7 cells colocalized, as determined by direct fluorescence analysis. We present evidence that CCN2–CCN3 interactions modulated CCN2 activity such as enhancement of ACAN and col2a1 expression. Curiously, CCN2 enhanced, whereas CCN3 inhibited, the expression of aggrecan and col2a1 mRNA in HCS-2/8 cells, and combined treatment with CCN2 and CCN3 abolished the inhibitory effect of CCN3. These effects were neutralized with an antibody against the von Willebrand factor type C domain of CCN2 (11H3). This antibody diminished the binding between CCN2 and CCN2, but enhanced that between CCN3 and CCN2. Our results suggest that CCN2 could form homotypic and heterotypic dimers with CCN2 and CCN3, respectively. Strengthening the binding between CCN2 and CCN3 with the 11H3 antibody had an enhancing effect on aggrecan expression in chondrocytes, suggesting that CCN2 had an antagonizing effect by binding to CCN3.

Convergent signaling in the regulation of connective tissue growth factor in malignant mesothelioma: TGFβ signaling and defects in the Hippo signaling cascade

Malignant mesothelioma (MM) is a neoplasm that arises from serosal surfaces of the pleural, peritoneal and pericardial cavities with worldwide incidence, much of which is caused by asbestos exposure. Patients suffer from pain and dyspnea due to direct invasion of the chest wall, lungs and vertebral or intercostal nerves by masses of thick fibrotic tumors. Although there has been recent progress in the clinical treatment, current therapeutic approaches do not provide satisfactory results. Therefore, development of a molecularly targeted therapy for MM is urgently required. Our recent studies suggest that normal mesothelial and MM cell growth is promoted by TGFβ, and that TGFβ signaling together with intrinsic disturbances in neurofibromatosis type 2 (NF2) and Hippo signaling cascades in MM cells converges upon further expression of connective tissue growth factor (CTGF). The formation of a YAP-TEAD4-Smad3-p300 complex on the specific CTGF promoter site with an adjacent TEAD and Smad binding motif is a critical and synergistic event caused by the dysregulation of these two distinct cascades. Furthermore, we demonstrated the functional importance of CTGF through the mouse studies and human histological analyses, which may elucidate the clinical features of MM with severe fibrosis in the thoracic cavity.

The many faces of p38 mitogen-activated protein kinase in progenitor/stem cell differentiation

Regulation of stem cells is essential for development and adult tissue homoeostasis. The proper control of stem cell self-renewal and differentiation maintains organ physiology, and disruption of such a balance results in disease. There are many mechanisms that have been established as stem cell regulators, such as Wnt or Notch signals. However, the intracellular mechanisms that mediate and integrate these signals are not well understood. A new intracellular pathway that has been reported to be involved in the regulation of many stem cell types is that of p38 MAPK (mitogen-activated protein kinase). In particular, p38α is essential for the proper differentiation of many haematopoietic, mesenchymal and epithelial stem/progenitor cells. Many reports have shown that disruption of this kinase pathway has pathological consequences in many organs. Understanding the extracellular cues and downstream targets of p38α in stem cell regulation may help to tackle some of the pathologies associated with improper differentiation and regulation of stem cell function. In the present review we present a vision of the current knowledge on the roles of the p38α signal as a regulator of stem/progenitor cells in different tissues in physiology and disease.

Involvement of p38MAPK/NF-κB signaling pathways in osteoblasts differentiation in response to mechanical stretch

Bone morphogenetic proteins (BMPs) are known to be important in osteoblasts' response to mechanical stimuli. BMPs/Smad signaling pathway has been demonstrated to play a regulatory role in the mechanical signal transduction in osteoblasts. However, little is currently known about the Smad independent pathway in osteoblasts differentiation in mechanical loading. In this study, MC3T3-E1 cells were subjected to mechanical stretch of 2000 micro-stain (με) at 0.5 Hz, in order to investigate the involvement of p38MAPK and NF-κB signaling pathways in mechanical response in osteoblasts. We found BMP-2/BMP-4 were up-regulated by mechanical stretch via the earlier activation of p38MAPK and NF-κB signaling pathways, which enhanced osteogenic gene expressions including alkaline phosphatase (ALP), collagen type I (Col I) and osteocalcin (OCN), and the expressions of these osteogenic genes were remarkably decreased with Noggin (an inhibitor for BMPs signals) pretreatment. Furthermore, BMP-2/BMP-4 expressions were suppressed by PDTC, an inhibitor of NF-κB pathway and SB203580, an inhibitor of p38MAPK pathway, respectively, leading to the declined levels of ALP, Col I and OCN. Interestingly, blocking in p38MAPK pathway can also cause the inactivation of NF-κB pathway in mechanical stretch. Collectively, the results indicate during mechanical stretch p38MAPK and NF-κB signaling pathways are activated first, and then up-regulate BMP-2/BMP-4 to enhance osteogenic gene expressions. Moreover, p38MAPK and NF-κB signals have cross-talk in regulation of BMP-2/BMP-4 in mechanical response.

CCN3: a novel anti-fibrotic treatment in end-stage renal disease?

Fibrosis is a major cause of end-stage renal disease (ESRD) a progressive loss in renal function that occurs over a period of months or years, is characterized by a decreased capability of the kidneys to excrete waste products. There is no specific treatment unequivocally shown to slow the worsening of chronic kidney disease. Plasma levels of CCN2, a fibrogenic agent, is a predictor of ESRD and mortality in patients with type 1 diabetic nephropathy. CCN3 has been hypothesized to have antagonistic effects to CCN2 both in vitro and in vivo, including in cultured mesangial cells. In a recent study, van Roeyen and colleagues (Am J Pathol in press, 2012) showed that in vivo overexpression of CCN3 in a model of anti-Thy1.1-induced experimental glomerulonephritis resulted in decreased albuminuria, glomerulosclerosis and reduced cortical collagen type I accumulation. CCN3 enhanced angiogenesis yes suppressed mesangial cell proliferation. Thus CCN3 protein may represent a novel therapeutic approach to help repair glomerular endothelial damage and mesangioproliferative changes and hence prevent renal failure, glomerulosclerosis and tubulointerstitial fibrosis.