Proposal for a unified CCN nomenclature

CCN proteins: A centralized communication network

The CCN family of proteins includes six members presently known as CCN1, CCN2, CCN3, CCN4, CCN5 and CCN6. These proteins were originally designated CYR61, CTGF, NOV, and WISP-1, WISP-2, WISP-3. Although these proteins share a significant amount of structural features and a partial identity with other large families of regulatory proteins, they exhibit different biological functions. A critical examination of the progress made over the past two decades, since the first CCN proteins were discovered brings me to the conclusion that most of our present knowledge regarding the functions of these proteins was predicted very early after their discovery. In an effort to point out some of the gaps that prevent us to reach a comprehensive view of the functional interactions between CCN proteins, it is necessary to reconsider carefully data that was already published and put aside, either because the scientific community was not ready to accept them, or because they were not fitting with the « consensus » when they were published. This review article points to avenues that were not attracting the attention that they deserved. However, it is quite obvious that the six members of this unique family of tetra-modular proteins must act in concert, either simultaneously or sequentially, on the same sites or at different times in the life of living organisms. A better understanding of the spatio-temporal regulation of CCN proteins expression requires considering the family as such, not as a set of single proteins related only by their name. As proposed in this review, there is enough convincing pieces of evidence, at the present time, in favor of these proteins playing a role in the coordination of multiple signaling pathways, and constituting a Centralized Communication Network. Deciphering the hierarchy of regulatory circuits involved in this complex system is an important challenge for the near future. In this article, I would like to briefly review the concept of a CCN family of proteins and critically examine the progress made over the past 10 years in the understanding of their biological functions and involvement in both normal and pathological processes.

The myofibroblast matrix: implications for tissue repair and fibrosis

Myofibroblasts, and the extracellular matrix (ECM) in which they reside, are critical components of wound healing and fibrosis. The ECM, traditionally viewed as the structural elements within which cells reside, is actually a functional tissue whose components possess not only scaffolding characteristics, but also growth factor, mitogenic, and other bioactive properties. Although it has been suggested that tissue fibrosis simply reflects an 'exuberant' wound-healing response, examination of the ECM and the roles of myofibroblasts during fibrogenesis instead suggest that the organism may be attempting to recapitulate developmental programmes designed to regenerate functional tissue. Evidence of this is provided by the temporospatial re-emergence of embryonic ECM proteins by fibroblasts and myofibroblasts that induce cellular programmatic responses intended to produce a functional tissue. In the setting of wound healing (or physiological fibrosis), this occurs in a highly regulated and exquisitely choreographed fashion which results in cessation of haemorrhage, restoration of barrier integrity, and re-establishment of tissue function. However, pathological tissue fibrosis, which oftentimes causes organ dysfunction and significant morbidity or mortality, likely results from dysregulation of normal wound-healing processes or abnormalities of the process itself. This review will focus on the myofibroblast ECM and its role in both physiological and pathological fibrosis, and will discuss the potential for therapeutically targeting ECM proteins for treatment of fibrotic disorders.

Molecular control of vascular development by the matricellular proteins CCN1 (Cyr61) and CCN2 (CTGF)

The circulatory system is the first hierarchically ordered network to form during the development of vertebrates as it is an indispensable means of adequate oxygen and nutrient delivery to developing organs. During the initial phase of vascular development, endothelial lineage-committed cells differentiate, migrate, and coalesce to form the central large axial vessels and their branches. The subsequent phase of vessel expansion (i.e., angiogenesis) involves a cascade of events including endothelial cell migration, proliferation, formation of an immature capillary structure, recruitment of mural cells and deposition of a basement membrane to yield a functional vasculature. These series of events are tightly regulated by the coordinated expression of several angiogenic, morphogenic and guidance factors. The extracellular matrix (ECM) is synthesized and secreted by embryonic cells at the earliest stages of development and forms a pericellular network of bioactive stimulatory and inhibitory angiogenesis regulatory factors. Here we describe the role of a subset of inducible immediate-early gene-encoded, ECM-associated integrin- and heparin-binding proteins referred to as CCN1 (or Cyr61) and CCN2 (or CTGF) and their function in the development of the vascular system. Gene-targeting experiments in mice have identified CCN1 and CCN2 as critical rate-limiting determinants of endothelial cell differentiation and quiescence, mural cell recruitment and basement membrane formation during embryonic vascular development. Emphasis will be placed on the regulation and function of these molecules and their contextual mode of action during vascular development. Further understanding of the mechanisms of CCN1- and CCN2-mediated blood vessel expansion and remodeling would enhance the prospects that these molecules provide for the development of new treatments for vascular diseases.

The skeletal site-specific role of connective tissue growth factor in prenatal osteogenesis

BACKGROUND

Connective tissue growth factor (CTGF/CCN2) is a matricellular protein that is highly expressed during bone development. Mice with global CTGF ablation (knockout, KO) have multiple skeletal dysmorphisms and perinatal lethality. A quantitative analysis of the bone phenotype has not been conducted.

RESULTS

We demonstrated skeletal site-specific changes in growth plate organization, bone microarchitecture, and shape and gene expression levels in CTGF KO compared with wild-type mice. Growth plate malformations included reduced proliferation zone and increased hypertrophic zone lengths. Appendicular skeletal sites demonstrated decreased metaphyseal trabecular bone, while having increased mid-diaphyseal bone and osteogenic expression markers. Axial skeletal analysis showed decreased bone in caudal vertebral bodies, mandibles, and parietal bones in CTGF KO mice, with decreased expression of osteogenic markers. Analysis of skull phenotypes demonstrated global and regional differences in CTGF KO skull shape resulting from allometric (size-based) and nonallometric shape changes. Localized differences in skull morphology included increased skull width and decreased skull length. Dysregulation of the transforming growth factor-β-CTGF axis coupled with unique morphologic traits provides a potential mechanistic explanation for the skull phenotype.

CONCLUSIONS

We present novel data on a skeletal phenotype in CTGF KO mice, in which ablation of CTGF causes site-specific aberrations in bone formation.

Glucocorticoids induce CCN5/WISP-2 expression and attenuate invasion in oestrogen receptor-negative human breast cancer cells

CCN5 (cysteine-rich 61/connective tissue growth factor/nephroblastoma overexpressed 5)/WISP-2 [WNT1 (wingless-type MMTV integration site family, member 1)-inducible signalling pathway protein 2] is an oestrogen-regulated member of the CCN family. CCN5 is a transcriptional repressor of genes associated with the EMT (epithelial-mesenchymal transition) and plays an important role in maintenance of the differentiated phenotype in ER (oestrogen receptor)-positive breast cancer cells. In contrast, CCN5 is undetectable in more aggressive ER-negative breast cancer cells. We now report that CCN5 is induced in ER-negative breast cancer cells such as MDA-MB-231 following glucocorticoid exposure, due to interaction of the endogenous glucocorticoid receptor with a functional glucocorticoid-response element in the CCN5 gene promoter. Glucocorticoid treatment of MDA-MB-231 cells is accompanied by morphological alterations, decreased invasiveness and attenuated expression of mesenchymal markers, including vimentin, cadherin 11 and ZEB1 (zinc finger E-box binding homeobox 1). Interestingly, glucocorticoid exposure did not increase CCN5 expression in ER-positive breast cancer cells, but rather down-regulated ER expression, thereby attenuating oestrogen pathway signalling. Taken together, our results indicate that glucocorticoid treatment of ER-negative breast cancer cells induces high levels of CCN5 expression and is accompanied by the appearance of a more differentiated and less invasive epithelial phenotype. These findings propose a novel therapeutic strategy for high-risk breast cancer patients.

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.

Regulation of pancreatic function by connective tissue growth factor (CTGF, CCN2)

Connective tissue growth factor (CTGF/CCN2) is a cysteine-rich matricellular secreted protein that regulates diverse cell functions including adhesion, migration, proliferation, differentiation, survival, senescence and apoptosis. In the pancreas, CTGF/CCN2 regulates critical functions including β cell replication during embryogenesis, stimulation of fibrogenic pathways in pancreatic stellate cells during pancreatitis, and regulation of the epithelial and stromal components in pancreatic ductal adenocarcinoma. This article reviews the evidence establishing CTGF/CCN2 as an important player in pancreatic physiology and pathology, highlighting the specific cell types that are involved in each process and the importance of CTGF/CCN2 as a component of autocrine or paracrine signaling within or between these various cells. Translational applications, including the potential for CTGF/CCN2-based therapies in diabetes, fibrosis, or cancer, are discussed.

Antiproliferative factor regulates connective tissue growth factor (CTGF/CCN2) expression in T24 bladder carcinoma cells

Connective tissue growth factor (CTGF/CNN2) is a novel APF target gene. A novel mechanism is described by which the APF cellular receptor, cytoskeleton-associated protein 4 (CKAP4), mediates APF-induced CTGF transcription.

Antiproliferative factor (APF) is a sialoglycopeptide elevated in the urine of patients with interstitial cystitis (IC)—a chronic, painful bladder disease of unknown etiology. APF inhibits the proliferation of normal bladder epithelial and T24 bladder carcinoma cells in vitro by binding to cytoskeleton-associated protein 4 (CKAP4) and altering the transcription of genes involved in proliferation, cellular adhesion, and tumorigenesis; however, specific molecular mechanisms and effector genes that control APF's antiproliferative effects are unknown. In this study, we found that there was a 7.5-fold up-regulation of connective tissue growth factor (CTGF/CCN2) expression in T24 bladder carcinoma cells treated with APF. Western blot revealed a dose-dependent increase in CCN2 protein levels, with secretion into the culture medium after APF treatment. CCN2 overexpression enhanced APF's antiproliferative activity, whereas CCN2 knockdown diminished APF-induced p53 expression. Using a luciferase reporter construct, we found that APF treatment resulted in fivefold activation of the CCN2 proximal promoter and, of importance, that small interfering RNA–mediated knockdown of CKAP4 inhibited CCN2 upregulation. In addition, we demonstrate that CKAP4 translocates to the nucleus and binds to the CCN2 proximal promoter in an APF-dependent manner, providing evidence that CCN2 regulation by APF involves CKAP4 nuclear translocation and binding to the CCN2 promoter.

CCN3 modulates bone turnover and is a novel regulator of skeletal metastasis

The CCN family of proteins is composed of six secreted proteins (CCN1-6), which are grouped together based on their structural similarity. These matricellular proteins are involved in a large spectrum of biological processes, ranging from development to disease. In this review, we focus on CCN3, a founding member of this family, and its role in regulating cells within the bone microenvironment. CCN3 impairs normal osteoblast differentiation through multiple mechanisms, which include the neutralization of pro-osteoblastogenic stimuli such as BMP and Wnt family signals or the activation of pathways that suppress osteoblastogenesis, such as Notch. In contrast, CCN3 is known to promote chondrocyte differentiation. Given these functions, it is not surprising that CCN3 has been implicated in the progression of primary bone cancers such as osteosarcoma, Ewing’s sarcoma and chondrosarcoma. More recently, emerging evidence suggests that CCN3 may also influence the ability of metastatic cancers to colonize and grow in bone.

Osteopontin in systemic sclerosis and its role in dermal fibrosis

Osteopontin (OPN) is a matricellular protein with proinflammatory and profibrotic properties. Previous reports demonstrate a role for OPN in wound healing and pulmonary fibrosis. Here, we determined whether OPN levels are increased in a large cohort of patients with systemic sclerosis (SSc) and whether OPN contributes to the development of dermal fibrosis. The plasma OPN levels were increased in SSc patients, including patients with limited and diffuse disease, compared with healthy controls. Immunohistology demonstrated OPN on fibroblast-like and inflammatory cells in SSc skin and lesional skin from mice in the bleomycin (bleo)-induced dermal fibrosis model. OPN-deficient (OPN(-/-)) mice developed less dermal fibrosis compared with wild-type (WT) mice in the bleo-induced dermal fibrosis model. Additional in vivo studies have demonstrated that lesional skin from OPN(-/-)mice had fewer Mac-3-positive cells, fewer myofibroblasts, decreased transforming growth factor (TGF)-β and genes in the TGF-β pathway, and decreased numbers of cells expressing phosphorylated SMAD2 (pSMAD) and extracellular signal-regulated kinase. In vitro, OPN(-/-) dermal fibroblasts had decreased migratory capacity but similar phosphorylation of SMAD2 by TGF-β. Finally, TGF-β production by OPN-deficient macrophages was reduced compared with WT. These data demonstrate an important role for OPN in the development of dermal fibrosis and suggest that it may be a new therapeutic target in SSc.

The role of connective tissue growth factor (CTGF/CCN2) in skeletogenesis

Connective tissue growth factor (CTGF) is a 38 kDa, cysteine rich, extracellular matrix protein composed of 4 domains or modules. CTGF has been shown to regulate a diverse array of cellular functions and has been implicated in more complex biological processes such as angiogenesis, chondrogenesis, and osteogenesis. A role for CTGF in the development and maintenance of skeletal tissues first came to light in studies demonstrating its expression in cartilage and bone cells, which was dramatically increased during skeletal repair or regeneration. The physiological significance of CTGF in skeletogenesis was confirmed in CTGF-null mice, which exhibited multiple skeletal dysmorphisms as a result of impaired growth plate chondrogenesis, angiogenesis, and bone formation/mineralization. Given the emerging importance of CTGF in osteogenesis and chondrogenesis, this review will focus on its expression in skeletal tissues, its effects on osteoblast and chondrocyte differentiation and function, and the skeletal implications of ablation or over-expression of CTGF in knockout or transgenic mouse models, respectively. In addition, this review will examine the role of integrin-mediated signaling and the regulation of CTGF expression as it relates to skeletogenesis. We will emphasize CTGF studies in bone or bone cells, and will identify opportunities for future investigations concerning CTGF and chondrogenesis/osteogenesis.

CCN6 as a profibrotic mediator that stimulates the proliferation of lung fibroblasts via the integrin β1/focal adhesion kinase pathway

Idiopathic pulmonary fibrosis is a progressive and lethal disease of the lung that is characterized by the proliferation of fibroblasts and increased deposition of the extracellular matrix. The CCN6/WISP-3 is a member of the CCN family of matricellular proteins, which consists of six members that are involved in many vital biological functions. However, the regulation of lung fibroblasts mediated by CCN6 protein has not been fully elucidated. Here, we demonstrated that CCN6 induced the proliferation of lung fibroblasts by binding to integrin β1, leading to the phosphorylation of FAK(Y397). Furthermore, CCN6 showed a weak, but significant, ability to stimulate the expression of fibronectin. CCN6 was highly expressed in the lung tissues of mice treated with bleomycin. Our results suggest that CCN6 plays a role in the fibrogenesis of the lungs mainly by stimulating the growth of lung fibroblasts and is a potential target for the treatment of pulmonary fibrosis.

CCN3 impairs osteoblast and stimulates osteoclast differentiation to favor breast cancer metastasis to bone

Bone is a preferred site for breast cancer metastasis, causing pain, fractures, spinal cord compressions, and hypercalcemia, all of which can significantly diminish the patient's quality of life. We identified CCN3 as a novel factor that is highly expressed in bone metastatic breast cancer cells from a xenograft mouse model and in bone metastatic lesions from patients with breast cancer. We demonstrate that CCN3 overexpression enhances the ability of weakly bone metastatic breast cancer cells to colonize and grow in the bone without altering their growth in the mammary fat pad. We further demonstrated that human recombinant CCN3 inhibits osteoblast differentiation from primary bone marrow cultures, leading to a higher receptor activator of NF-κB ligand (RANKL)/osteoprotegerin (OPG) ratio. In conjunction with its ability to impair osteoblast differentiation, we uncovered a novel role for CCN3 in promoting osteoclast differentiation from RANKL-primed monocyte precursors. CCN3 exerts its pro-osteoclastogenic effects by promoting calcium oscillations and nuclear factor of activated T cells c1 (NFATc1) nuclear translocation. Together, these results demonstrate that CCN3 regulates the differentiation of bone resident cells to create a resorptive environment that promotes the formation of osteolytic breast cancer metastases.

Plasma CCN2 (connective tissue growth factor; CTGF) is a potential biomarker in idiopathic pulmonary fibrosis (IPF)

BACKGROUND

Idiopathic pulmonary fibrosis (IPF) is a chronic, progressive and fatal pulmonary fibrotic disease and useful biomarkers are required to diagnose and predict disease activity. CCN2 (connective tissue growth factor; CTGF) has been reported as one of the key profibrotic factors associated with transforming growth factor-β (TGF-β), and its assay has potential as a non-invasive measure in various fibrotic diseases. Recently, we developed a new subtraction method for determination of plasma CCN2 levels. We examined the utility of plasma CCN2 levels as a surrogate marker in IPF.

METHODS

Plasma CCN2 levels were calculated in 33 patients with IPF, 14 patients with non-IPF idiopathic interstitial pneumonias (IIPs) and 101 healthy volunteers by sandwich enzyme-linked immunosorbent assay (ELISA) using specific monoclonal antibodies for two distinct epitopes of human CCN2. We evaluated the utility of plasma CCN2 levels by comparison with clinical parameters.

RESULTS

Plasma CCN2 levels were significantly higher in patients with IPF than in those with non-IPF IIPs and healthy volunteers. Importantly, plasma CCN2 levels showed significantly negative correlation with 6-month change of forced vital capacity (FVC) in patients with IPF.

CONCLUSIONS

Plasma CCN2 is a potential biomarker for IPF.

CCN1/CYR61: the very model of a modern matricellular protein

CCN1 (CYR61) is a dynamically expressed, multifunctional matricellular protein that plays essential roles in cardiovascular development during embryogenesis, and regulates inflammation, wound healing and fibrogenesis in the adult. Aberrant CCN1 expression is associated with myriad pathologies, including various cancers and diseases associated with chronic inflammation. CCN1 promotes diverse and sometimes opposing cellular responses, which can be ascribed, as least in part, to disparate activities mediated through its direct binding to distinct integrins in different cell types and contexts. Accordingly, CCN1 promotes cell proliferation, survival and angiogenesis by binding to integrin α(v)β(3), and induces apoptosis and senescence through integrin α(6)β(1) and heparan sulfate proteoglycans. The ability of CCN1 to trigger the accumulation of a robust and sustained level of reactive oxygen species underlies some of its unique activities as a matrix cell-adhesion molecule. Emerging studies suggest that CCN1 might be useful as a biomarker or therapeutic target in certain diseases.

Expression of connective tissue growth factor (CTGF/CCN2) in breast cancer cells is associated with increased migration and angiogenesis

Connective tissue growth factor (CTGF/CCN2) belongs to the CCN family of matricellular proteins, comprising Cyr61, CTGF, NovH and WISP1-3. The CCN proteins contain an N-terminal signal peptide followed by four conserved domains sharing sequence similarities with the insulin-like growth factor binding proteins, von Willebrand factor type C repeat, thrombospondin type 1 repeat, and a C-terminal growth factor cysteine knot domain. To investigate the role of CCN2 in breast cancer, we transfected MCF-7 cells with full-length CCN2, and with four mutant constructs in which one of the domains had been deleted. MCF-7 cells stably expressing full-length CCN2 demonstrated reduced cell proliferation, increased migration in Boyden chamber assays and promoted angiogenesis in chorioallantoic membrane assays compared to control cells. Deletion of the C-terminal cysteine knot domain, but not of any other domain-deleted mutants, abolished activities mediated by full-length CCN2. We have dissected the role of CCN2 in breast tumorigenesis on a structural basis.

First structural glimpse of CCN3 and CCN5 multifunctional signaling regulators elucidated by small angle x-ray scattering

The CCN (cyr61, ctgf, nov) proteins (CCN1-6) are an important family of matricellular regulatory factors involved in internal and external cell signaling. They are central to essential biological processes such as adhesion, proliferation, angiogenesis, tumorigenesis, wound healing, and modulation of the extracellular matrix. They possess a highly conserved modular structure with four distinct modules that interact with a wide range of regulatory proteins and ligands. However, at the structural level, little is known although their biological function(s) seems to require cooperation between individual modules. Here we present for the first time structural determinants of two of the CCN family members, CCN3 and CCN5 (expressed in Escherichia coli), using small angle x-ray scattering. The results provide a description of the overall molecular shape and possible general three-dimensional modular arrangement for CCN proteins. These data unequivocally provide insight of the nature of CCN protein(s) in solution and thus important insight into their structure-function relationships.

Nephroblastoma overexpressed (Nov) induces gremlin in ST-2 stromal cell lines by post-transcriptional mechanisms

Nephroblastoma overexpressed (Nov) inhibits osteoblastogenesis in part because it binds bone morphogenetic protein (BMP)-2. In the present study, we investigated whether Nov regulated the expression of the BMP antagonist gremlin. Overexpression of Nov increased gremlin mRNA levels in ST-2 cells, and its downregulation by RNA interference decreased gremlin mRNA. Nov did not affect Grem1 transcription, but prolonged the half-life of gremlin mRNA in ST-2 cells, demonstrating that Nov acts by post-transcriptional mechanisms. This was confirmed by demonstrating that downregulation of Nov destabilizes gremlin transcripts. To assess whether the 3'-untranslated region (UTR) of gremlin mRNA mediated the effect of Nov, the decay of a chimeric cfos gremlin 3'-UTR construct was compared to that of cfos in ST-2 cells. The presence of the gremlin 3'-UTR prolonged the half-life of cfos and was responsible for the effect of Nov. To examine the binding of the gremlin 3'-UTR to ribonucleoproteins, radiolabeled gremlin RNA fragments were incubated with cytosolic extracts from Nov overexpressing and control cells. RNA electrophoretic mobility analysis revealed that Nov enhanced the binding of cytosolic proteins to the fragments spanning the 3'-UTR of gremlin between bases 1,358-1,557 and 1,158-1,357 from the transcriptional start. Mutations of AU-rich elements in these two RNA fragments prevented the formation of RNA-protein complexes induced by Nov. Nov did not alter the binding of cytosolic extracts to sequences present in the 5'-UTR or coding region of gremlin. In conclusion, Nov stabilizes gremlin transcripts, and this effect is possibly mediated by AU-rich elements present in the 3'-UTR of gremlin.

CCN6 (WISP3) decreases ZEB1-mediated EMT and invasion by attenuation of IGF-1 receptor signaling in breast cancer

During progression of breast cancer, CCN6 protein exerts tumor inhibitory functions. CCN6 is a secreted protein that modulates the insulin-like growth factor-1 (IGF-1) signaling pathway. Knockdown of CCN6 in benign mammary epithelial cells triggers an epithelial to mesenchymal transition (EMT), with upregulation of the transcription factor ZEB1/δEF1. How CCN6 regulates ZEB1 expression is unknown. We hypothesized that CCN6 might regulate ZEB1, EMT and breast cancer invasion by modulating IGF-1 signaling. Exogenously added human recombinant CCN6 protein was sufficient to downregulate ZEB1 mRNA and protein levels in CCN6-deficient (CCN6 KD) HME cells and MDA-MB-231 breast cancer cells. Recombinant CCN6 protein decreased invasion of CCN6 KD cells compared with controls. We discovered that knockdown of CCN6 induced IGF-1 secretion in HME cells cultivated in serum-free medium to higher concentrations than found in MDA-MB-231 cells. Treatment with recombinant CCN6 protein was sufficient to decrease IGF-1 protein and mRNA to control levels, rescuing the effect of CCN6 knockdown. Specific inhibition of IGF-1 receptors using the pharmacological inhibitor NVP-AE541 or short hairpin shRNAs revealed that ZEB1 upregulation due to knockdown of CCN6 requires activation of IGF-1 receptor signaling. Recombinant CCN6 blunted IGF-1-induced ZEB1 upregulation in MDA-MB-231 cells. Our data define a pathway in which CCN6 attenuates IGF-1 signaling to decrease ZEB1 expression and invasion in breast cancer. These results suggest that CCN6 could be a target to prevent or halt breast cancer invasion.

A coding RNA segment that enhances the ribosomal recruitment of chicken ccn1 mRNA

CCN1, a member of the CCN family of proteins, plays important physiological or pathological roles in a variety of tissues. In the present study, we initially found a highly guanine-cytosine (GC)-rich region of approximately 200 bp near the 5'-end of the open reading frame, which was always truncated by amplification of the corresponding cDNA region through the conventional polymerase chain reaction. An RNA in vitro folding assay and selective ribonuclease digestion of the corresponding segment of the ccn1 mRNA confirmed the involvement of a stable secondary structure. Subsequent RNA electromobility-shift assays demonstrated the specific binding of some cytoplasmic factor(s) in chicken embryo fibroblasts to the RNA segment. Moreover, the corresponding cDNA fragment strongly enhanced the expression of the reporter gene in cis at the 5'-end, but did not do so at the 3'-end. According to the results of a ribosomal assembly test, the effect of the mRNA segment can predominantly be ascribed to the enhancement of transport and/or entry of the mRNA into the ribosome. Finally, the minimal GC-rich mRNA segment that was predicted and demonstrated to form a secondary structure was confirmed to be a functional regulatory element. Thus, we here uncover a novel dual-functionality of the mRNA segment in the ccn1 open reading frame, which segment acts as a cis-element that mediates posttranscriptional gene regulation, while retaining the information for the amino acid sequence of the resultant protein.

The role of CCN2 in cartilage and bone development

CCN2, a classical member of the CCN family of matricellular proteins, is a key molecule that conducts cartilage development in a harmonized manner through novel molecular actions. During vertebrate development, all cartilage is primarily formed by a process of mesenchymal condensation, while CCN2 is induced to promote this process. Afterwards, cartilage develops into several subtypes with different fates and missions, in which CCN2 plays its proper roles according to the corresponding microenvironments. The history of CCN2 in cartilage and bone began with its re-discovery in the growth cartilage in long bones, which determines the skeletal size through the process of endochondral ossification. CCN2 promotes physiological developmental processes not only in the growth cartilage but also in the other types of cartilages, i.e., Meckel's cartilage representing temporary cartilage without autocalcification, articular cartilage representing hyaline cartilage with physical stiffness, and auricular cartilage representing elastic cartilage. Together with its significant role in intramembranous ossification, CCN2 is regarded as a conductor of skeletogenesis. During cartilage development, the CCN2 gene is dynamically regulated to yield stage-specific production of CCN2 proteins at both transcriptional and post-transcriptional levels. New functional aspects of known biomolecules have been uncovered during the course of investigating these regulatory systems in chondrocytes. Since CCN2 promotes integrated regeneration as well as generation (=development) of these tissues, its utility in regenerative therapy targeting chondrocytes and osteoblasts is indicated, as has already been supported by experimental evidence obtained in vivo.

CCN5, a novel transcriptional repressor of the transforming growth factor β signaling pathway

CCN5 is a member of the CCN (connective tissue growth factor/cysteine-rich 61/nephroblastoma overexpressed) family and was identified as an estrogen-inducible gene in estrogen receptor-positive cell lines. However, the role of CCN5 in breast carcinogenesis remains unclear. We report here that the CCN5 protein is localized mostly in the cytoplasm and in part in the nucleus of human tumor breast tissue. Using a heterologous transcription assay, we demonstrate that CCN5 can act as a transcriptional repressor presumably through association with histone deacetylase 1 (HDAC1). Microarray gene expression analysis showed that CCN5 represses expression of genes associated with epithelial-mesenchymal transition (EMT) as well as expression of key components of the transforming growth factor β (TGF-β) signaling pathway, prominent among them TGF-βRII receptor. We show that CCN5 is recruited to the TGF-βRII promoter, thereby providing a mechanism by which CCN5 restricts transcription of the TGF-βRII gene. Consistent with this finding, CCN5, we found, functions to suppress TGF-β-induced transcriptional responses and invasion that is concomitant with EMT. Thus, our data uncovered CCN5 as a novel transcriptional repressor that plays an important role in regulating tumor progression functioning, at least in part, by inhibiting the expression of genes involved in the TGF-β signaling cascade that is known to promote EMT.

WNT1 inducible signaling pathway protein 1 (WISP1): a novel mediator linking development and disease

WISP1 is a secreted, matricellular protein allocated to the CCN protein family. The CCN protein family consists of six, modular structured, secreted proteins. WISP1 is mainly expressed during organ development and under diseased conditions, such as fibrosis or cancer. Its expression is associated with proliferation, cytoprotection, as well as extracellular matrix production, thereby representing a highly attractive therapeutical target for future applications.

Temporal and spatial expression of CCN genes in zebrafish

The six mammalian CCN genes (Cyr61, CTGF, Nov, WISP1, WISP2, WISP3) encode a family of secreted, cysteine-rich, multimodular proteins having roles in cell proliferation, adhesion, migration, and differentiation during embryogenesis, wound healing, and angiogenesis. We used bioinformatics to identify 9 CCN genes in zebrafish (zCCNs), 6 of which have not been previously described. When compared with mammalian CCN family members, 3 were paralogs of Cyr61, 2 of CTGF, 2 of WISP1, 1 of WISP2, and 1 of WISP3. No paralog of Nov was found. In situ hybridization was performed to characterize the sites of expression of the zCCNs during early zebrafish development. zCCNs demonstrated both unique and overlapping patterns of expression, suggesting potential division of labor between orthologous genes and providing an alternate approach to gene function studies that will complement studies in mammalian models. Developmental Dynamics 239:1755–1767, 2010. © 2010 Wiley-Liss, Inc.

Connective tissue growth factor (CTGF) transactivates nuclear factor of activated T-cells (NFAT) in cells of the osteoblastic lineage

Connective tissue growth factor (CTGF), a member of the Cyr 61, CTGF, Nov (CCN) family of proteins, regulates multiple cellular functions. Overexpression of CTGF in vivo causes osteopenia, but in vitro CTGF can induce osteoblastogenesis. To investigate mechanisms involved in the effects of CTGF on osteoblastic cell differentiation, we examined whether CTGF modifies the activity of nuclear factor of activated T-cells (NFATc) 1, a transcription factor that cooperates with osterix in the formation of new bone. CTGF increased the transactivation of a transiently transfected reporter construct, where 9 NFAT binding sites direct the expression of luciferase (9xNFAT-Luc) and the activity of the Regulators of calcineurin 1 exon 4 (Rcan1.4) promoter, an NFAT target gene. We postulated that CTGF could modify the phosphorylation of NFAT by regulating glycogen synthase kinase 3beta (GSK3beta). CTGF increased the mRNA levels of Protein kinase cyclic guanosine monophosphate (cGMP) dependent type II (Prkg2), the gene encoding for cGMP dependent protein kinase II (cGKII) which phosphorylates GSK3beta. Accordingly, CTGF induced GSK3beta phosphorylation and decreased the active pool of GSK3beta, a kinase that phosphorylates NFAT and leads to its nuclear export. As a consequence, CTGF favored the nuclear localization of NFATc1. Downregulation of PRKG2 by RNA interference reversed the effect of CTGF on the transactivation of the 9xNFAT reporter construct and the Rcan 1.4 promoter, confirming the role of cGKII in the activation of NFAT by CTGF. In conclusion, CTGF enhances NFAT signaling through the induction of cGKII and the phosphorylation of GSK3beta.

When there's smoke there's.....CCN2

Smoking causes oral fibrosis. In a recent report, Takeuchi and colleagues (J Dent Res 89:34-9, 2010) evaluate whether nicotine can directly elevate collagen production in gingival fibroblasts. They show that CCN2 (connective tissue growth factor, CTGF) is elevated in response to nicotine and that a neutralizing CCN2 antibody reduces the ability of nicotine to promote collagen production. These data suggest that nicotine from smoking may promote periodontal fibrosis via CCN2. This commentary summarizes these findings.

Connective tissue growth factor is required for skeletal development and postnatal skeletal homeostasis in male mice

Connective tissue growth factor (CTGF), a member of the cysteine-rich 61 (Cyr 61), CTGF, nephroblastoma overexpressed (NOV) (CCN) family of proteins, is synthesized by osteoblasts, and its overexpression inhibits osteoblastogenesis and causes osteopenia. The global inactivation of Ctgf leads to defective endochondral bone formation and perinatal lethality; therefore, the consequences of Ctgf inactivation on the postnatal skeleton are not known. To study the function of CTGF, we generated Ctgf(+/LacZ) heterozygous null mice and tissue-specific null Ctgf mice by mating Ctgf conditional mice, where Ctgf is flanked by lox sequences with mice expressing the Cre recombinase under the control of the paired-related homeobox gene 1 (Prx1) enhancer (Prx1-Cre) or the osteocalcin promoter (Oc-Cre). Ctgf(+/LacZ) heterozygous mice exhibited transient osteopenia at 1 month of age secondary to decreased trabecular number. A similar osteopenic phenotype was observed in 1-month-old Ctgf conditional null male mice generated with Prx1-Cre, suggesting that the decreased trabecular number was secondary to impaired endochondral bone formation. In contrast, when the conditional deletion of Ctgf was achieved by Oc-Cre, an osteopenic phenotype was observed only in 6-month-old male mice. Osteoblast and osteoclast number, bone formation, and eroded surface were not affected in Ctgf heterozygous or conditional null mice. In conclusion, CTGF is necessary for normal skeletal development but to a lesser extent for postnatal skeletal homeostasis.

Expression patterns of connective tissue growth factor and of TGF-beta isoforms during glomerular injury recapitulate glomerulogenesis

Transforming growth factor (TGF)-beta(1), -beta(2), and -beta(3) are involved in control of wound repair and development of fibrosis. Connective tissue growth factor (CTGF) expression is stimulated by all TGF-beta isoforms and is abundant in glomerulosclerosis and other fibrotic disorders. CTGF is hypothesized to mediate profibrotic effects of TGF-beta(1) or to facilitate interaction of TGF-beta(1) with its receptor, but its interactions with TGF-beta isoforms in nonpathological conditions are unexplored so far. Tissue repair and remodeling may recapitulate gene transcription at play in organogenesis. To further delineate the relationship between CTGF and TGF-beta, we compared expression patterns of CTGF and TGF-beta isoforms in rat and human glomerulogenesis and in various human glomerulopathies. CTGF mRNA was present in the immediate precursors of glomerular visceral and parietal epithelial cells in the comma- and S-shaped stages, but not in earlier stages of nephron development. During the capillary loop and maturing glomerular stages and simultaneous with the presence of TGF-beta(1), -beta(2), and -beta(3) protein, CTGF mRNA expression was maximal and present only in differentiating glomerular epithelial cells. CTGF protein was also present on precursors of mesangium and glomerular endothelium, suggesting possible paracrine interaction. Concomitant with the presence of TGF-beta(2) and -beta(3) protein, and in the absence of TGF-beta(1), CTGF mRNA and protein expression was restricted to podocytes in normal adult glomeruli. However, TGF-beta(1) and CTGF were again coexpressed, often with TGF-beta(2) and -beta(3), in particular in podocytes in proliferative glomerulonephritis and also in mesangial cells in diabetic nephropathy and IgA nephropathy (IgA NP). Coordinated expression of TGF-beta isoforms and of CTGF may be involved in normal glomerulogenesis and possibly in maintenance of glomerular structure and function at adult age. Prolonged overexpression of TGF-beta(1) and CTGF is associated with development of severe glomerulonephritis and glomerulosclerosis.

The CCN proteins: important signaling mediators in stem cell differentiation and tumorigenesis

The CCN proteins contain six members, namely CCN1 to CCN6, which are small secreted cysteine-rich proteins. The CCN proteins are modular proteins, containing up to four functional domains. Many of the CCN members are induced by growth factors, cytokines, or cellular stress. The CCNs show a wide and highly variable expression pattern in adult and in embryonic tissues. The CCN proteins can integrate and modulate the signals of integrins, BMPs, VEGF, Wnts, and Notch. The involvement of integrins in mediating CCN signaling may provide diverse context-dependent responses in distinct cell types. CCN1 and CCN2 play an important role in development, angiogenesis and cell adhesion, whereas CCN3 is critical to skeletal and cardiac development. CCN4, CCN5 and CCN6 usually inhibit cell growth. Mutations of Ccn6 are associated with the progressive pseudorheumatoid dysplasia and spondyloepiphyseal dysplasia tarda. In stem cell differentiation, CCN1, CCN2, and CCN3 play a principal role in osteogenesis, chondrogenesis, and angiogenesis. Elevated expression of CCN1 is associated with more aggressive phenotypes of human cancer, while the roles of CCN2 and CCN3 in tumorigenesis are tumor type-dependent. CCN4, CCN5 and CCN6 function as tumor suppressors. Although CCN proteins may play important roles in fine-tuning other major signaling pathways, the precise function and mechanism of action of these proteins remain undefined. Understanding of the biological functions of the CCN proteins would not only provide insight into their roles in numerous cellular processes but also offer opportunities for developing therapeutics by targeting CCN functions.

Ultraviolet irradiation induces CYR61/CCN1, a mediator of collagen homeostasis, through activation of transcription factor AP-1 in human skin fibroblasts

UV irradiation from the sun elevates the production of collagen-degrading matrix metalloproteinases (MMPs) and reduces the production of new collagen. This imbalance of collagen homeostasis impairs the structure and function of the dermal collagenous extracellular matrix (ECM), thereby promoting premature skin aging (photoaging). We report here that aberrant dermal collagen homeostasis in UV-irradiated human skin is mediated in part by a CCN-family member, cysteine-rich protein-61 (CYR61/CCN1). CYR61 is significantly elevated in acutely UV-irradiated human skin in vivo, and UV-irradiated human skin fibroblasts. Knockdown of CYR61 significantly attenuates UV irradiation-induced inhibition of type-I procollagen and upregulation of MMP-1. Determination of CYR61 mRNA and protein indicates that the primary mechanism of CYR61 induction by UV irradiation is transcriptional. Analysis of CYR61 proximal promoter showed that a sequence conforming to the consensus binding site for transcription factor activator protein-1 (AP-1) is required for promoter activity. UV irradiation increased the binding of AP-1-family members c-Jun and c-Fos to this AP-1 site. Furthermore, functional blockade of c-Jun or knockdown of c-Jun significantly reduced the UV irradiation-induced activation of CYR61 promoter and CYR61 gene expression. These data show that CYR61 is transcriptionally regulated by UV irradiation through transcription factor AP-1, and mediates altered collagen homeostasis that occurs in response to UV irradiation in human skin fibroblasts.

CCN3/CCN2 regulation and the fibrosis of diabetic renal disease

Prior work in the CCN field, including our own, suggested to us that there might be co-regulatory activity and function as part of the actions of this family of cysteine rich cytokines. CCN2 is now regarded as a major pro-fibrotic molecule acting both down-stream and independent of TGF-beta1, and appears causal in the disease afflicting multiple organs. Since diabetic renal fibrosis is a common complication of diabetes, and a major cause of end stage renal disease (ESRD), we examined the possibility that CCN3 (NOV), might act as an endogenous negative regulator of CCN2 with the capacity to limit the overproduction of extracellular matrix (ECM), and thus prevent, or ameliorate fibrosis. We demonstrate, using an in vitro model of diabetic renal fibrosis, that both exogenous treatment with CCN3 and transfection with the over-expression of the CCN3 gene in mesangial cells markedly down-regulates CCN2 activity and blocks ECM over-accumulation stimulated by TGF-beta1. Conversely, TGF-beta1 treatment reduces endogenous CCN3 expression and increases CCN2 activity and matrix accumulation, indicating an important, novel yin/yang effect. Using the db/db mouse model of diabetic nephropathy, we confirm the expression of CCN3 in the kidney, with temporal localization that supports these in vitro findings. In summary, the results corroborate our hypothesis that one function of CCN3 is to regulate CCN2 activity and at the concentrations and conditions used down-regulates the effects of TGF-beta1, acting to limit ECM turnover and fibrosis in vivo. The findings suggest opportunities for novel endogenous-based therapy either by the administration, or the upregulation of CCN3.

CCN3 inhibits neointimal hyperplasia through modulation of smooth muscle cell growth and migration

OBJECTIVE

CCN3 belongs to the CCN family, which constitutes multifunctional secreted proteins that act as matrix cellular regulators. We investigated the pathophysiological roles of CCN3 in the vessels.

METHODS AND RESULTS

We examined the effects of CCN3 on the proliferation and migration of rat vascular smooth muscle cells (VSMC). CCN3 knockout mice were created, and vascular phenotypes and neointimal hyperplasia induced by photochemically induced thrombosis were investigated. CCN3 suppressed the VSMC proliferation induced by fetal bovine serum. The neutralizing antibody for transforming growth factor-beta did not affect the growth inhibitory effect of CCN3. Moreover, CCN3 enhanced the mRNA expression of cyclin-dependent kinase inhibitors, p21 and p15. Gamma secretase inhibitor, an inhibitor of Notch signaling, partially inhibited the enhanced expression of p21 induced by CCN3. CCN3 also inhibited the VSMC migration. Finally, the histopathologic evaluation of the arteries 21 days after the endothelial injury revealed a 6-fold enhancement of neointimal thickening in the null mice compared with the wild-type mice.

CONCLUSIONS

CCN3 suppresses neointimal thickening through the inhibition of VSMC migration and proliferation. Our findings indicate the involvement of CCN3 in vascular homeostasis, especially on injury, and the potential usefulness of this molecule in the modulation of atherosclerotic vascular disease.

Nephroblastoma overexpressed (Nov) inactivation sensitizes osteoblasts to bone morphogenetic protein-2, but nov is dispensable for skeletal homeostasis

Overexpression of nephroblastoma overexpressed (Nov), a member of the Cyr 61, connective tissue growth factor, Nov family of proteins, inhibits osteoblastogenesis and causes osteopenia. The consequences of Nov inactivation on osteoblastogenesis and the postnatal skeleton are not known. To study the function of Nov, we inactivated Nov by homologous recombination. Nov null mice were maintained in a C57BL/6 genetic background after the removal of the neomycin selection cassette and compared with wild-type controls of identical genetic composition. Nov null mice were identified by genotyping and absent Nov mRNA in calvarial extracts and osteoblast cultures. Nov null mice did not exhibit developmental skeletal abnormalities or postnatal changes in weight, femoral length, body fat, or bone mineral density and appeared normal. Bone volume and trabecular number were decreased only in 1-month-old female mice. In older mice, after 7 months of age, osteoblast surface and bone formation were increased in females, and osteoclast and eroded surfaces were increased in male Nov null mice. Calvarial osteoblasts from Nov null mice displayed enhanced alkaline phosphatase activity, alkaline phosphatase mRNA, and transactivation of a bone morphogenetic protein (BMP)/phosphorylated mothers against decapentaplegic reporter construct in response to BMP-2. Similar results were obtained after the down-regulation of Nov by RNA interference in ST-2 stromal and MC3T3 cells. Osteoclast number was increased in marrow stromal cell cultures from Nov null mice. Surface plasmon resonance demonstrated direct interactions between Nov and BMP-2. In conclusion, Nov sensitizes osteoblasts to BMP-2, but Nov is dispensable for the maintenance of bone mass.

Nicotine-induced CCN2: from Smoking to Periodontal Fibrosis

Since fibrosis is observed in smokers’ gingiva, it was hypothesized that fibrosis was caused by nicotine in the periodontium. Therefore, in this study, we investigated the effects of nicotine on the induction of a profibrotic molecule, connective tissue growth factor (CCN2/CTGF), in human gingival fibroblasts (HGFs) and periodontal ligament (PDL) cells. With 1 μg/mL nicotine, vacuolization and attenuated proliferation were observed. Interestingly, 1 μg/mL nicotine increased the production of CCN2/CTGF protein in both cells without increasing mRNA expression. Furthermore, type I collagen mRNA and protein were also increased and were significantly blocked by a CCN2/CTGF neutralizing antibody. This is the first report to describe a relationship between nicotine and CCN2/CTGF in periodontal tissue cells. Analysis of our data also indicated that nicotine was cytotoxic, while it increased CCN2/CTGF and, eventually, type I collagen production. These findings suggest that periodontal fibrosis can be promoted by nicotine from smoking via effects on CCN2/CTGF.

Thrombopoietic-mesenchymal interaction that may facilitate both endochondral ossification and platelet maturation via CCN2

CCN2 plays a central role in the development and growth of mesenchymal tissue and promotes the regeneration of bone and cartilage in vivo. Of note, abundant CCN2 is contained in platelets, which is thought to play an important role in the tissue regeneration process. In this study, we initially pursued the possible origin of the CCN2 in platelets. First, we examined if the CCN2 in platelets was produced by megakaryocyte progenitors during differentiation. Unexpectedly, neither megakaryocytic CMK cells nor megakaryocytes that had differentiated from human haemopoietic stem cells in culture showed any detectable CCN2 gene expression or protein production. Together with the fact that no appreciable CCN2 was detected in megakaryocytes in vivo, these results suggest that megakaryocytes themselves do not produce CCN2. Next, we suspected that mesenchymal cells situated around megakaryocytes in the bone marrow were stimulated by the latter to produce CCN2, which was then taken up by platelets. To evaluate this hypothesis, we cultured human chondrocytic HCS-2/8 cells with medium conditioned by differentiating megakaryocyte cultures, and then monitored the production of CCN2 by the cells. As suspected, CCN2 production by HCS-2/8 was significantly enhanced by the conditioned medium. We further confirmed that human platelets were able to absorb/uptake exogenous CCN2 in vitro. These findings indicate that megakaryocytes secrete some unknown soluble factor(s) during differentiation, which factor stimulates the mesenchymal cells to produce CCN2 for uptake by the platelets. We also consider that, during bone growth, such thrombopoietic-mesenchymal interaction may contribute to the hypertrophic chondrocyte-specific accumulation of CCN2 that conducts endochondral ossification.

CCN3 and bone marrow cells

CCN3 expression was observed in a broad variety of tissues from the early stage of development. However, a kind of loss of function in mice (CCN3 del VWC domain -/-) demonstrated mild abnormality, which indicates that CCN3 may not be critical for the normal embryogenesis as a single gene. The importance of CCN3 in bone marrow environment becomes to be recognized by the studies of hematopoietic stem cells and Chronic Myeloid Leukemia cells. CCN3 expression in bone marrow has been denied by several investigations, but we found CCN3 positive stromal and hematopoietic cells at bone extremities with a new antibody although they are a very few populations. We investigated the expression pattern of CCN3 in the cultured bone marrow derived mesenchymal stem cells and found its preference for osteogenic differentiation. From the analyses of in vitro experiment using an osteogenic mesenchymal stem cell line, Kusa-A1, we found that CCN3 downregulates osteogenesis by two different pathways; suppression of BMP and stimulation of Notch. Secreted CCN3 from Kusa cells inhibited the differentiation of osteoblasts in separate culture, which indicates the paracrine manner of CCN3 activity. CCN3 may also affect the extracellular environment of the niche for hematopoietic stem cells.

Proteins on the catwalk: modelling the structural domains of the CCN family of proteins

The CCN family of proteins (CCN1, CCN2, CCN3, CCN4, CCN5 and CCN6) are multifunctional mosaic proteins that play keys roles in crucial areas of physiology such as angiogenesis, skeletal development tumourigenesis, cell proliferation, adhesion and survival. This expansive repertoire of functions comes through a modular structure of 4 discrete domains that act both independently and in concert. How these interactions with ligands and with neighbouring domains lead to the biological effects is still to be explored but the molecular structure of the domains is likely to play an important role in this. In this review we have highlighted some of the key features of the individual domains of CCN family of proteins based on their biological effects using a homology modelling approach.

Osteogenic differentiation of mesenchymal stem cells promoted by overexpression of connective tissue growth factor

OBJECTIVE

Large segmental bone defect repair remains a clinical and scientific challenge with increasing interest focusing on combining gene transfection with tissue engineering techniques. The aim of this study is to investigate the effect of connective tissue growth factor (CTGF) on the proliferation and osteogenic differentiation of the bone marrow mesenchymal stem cells (MSCs).

METHODS

A CTGF-expressing plasmid (pCTGF) was constructed and transfected into MSCs. Then expressions of bone morphogenesis-related genes, proliferation rate, alkaline phosphatase activity, and mineralization were examined to evaluate the osteogenic potential of the CTGF gene-modified MSCs.

RESULTS

Overexpression of CTGF was confirmed in pCTGF-MSCs. pCTGF transfection significantly enhanced the proliferation rates of pCTGF-MSCs (P<0.05). CTGF induced a 7.5-fold increase in cell migration over control (P<0.05). pCTGF transfection enhanced the expression of bone matrix proteins, such as bone sialoprotein, osteocalcin, and collagen type I in MSCs. The levels of alkaline phosphatase (ALP) activities of pCTGF-MSCs at the 1st and 2nd weeks were 4.0- and 3.0-fold higher than those of MSCs cultured in OS-medium, significantly higher than those of mock-MSCs and normal control MSCs (P<0.05). Overexpression of CTGF in MSCs enhanced the capability to form mineralized nodules.

CONCLUSION

Overexpression of CTGF could improve the osteogenic differentiation ability of MSCs, and the CTGF gene-modified MSCs are potential as novel cell resources of bone tissue engineering.

CCN3 (NOV) is a negative regulator of CCN2 (CTGF) and a novel endogenous inhibitor of the fibrotic pathway in an in vitro model of renal disease

Fibrosis is a major cause of end-stage renal disease, and although initiation factors have been elucidated, uncertainty concerning the downstream pathways has hampered the development of anti-fibrotic therapies. CCN2 (CTGF) functions downstream of transforming growth factor (TGF)-beta, driving increased extracellular matrix (ECM) accumulation and fibrosis. We examined the possibility that CCN3 (NOV), another CCN family member with reported biological activities that differ from CCN2, might act as an endogenous negative regulator of ECM and fibrosis. We show that cultured rat mesangial cells express CCN3 mRNA and protein, and that TGF-beta treatment reduced CCN3 expression levels while increasing CCN2 and collagen type I activities. Conversely, either the addition of CCN3 or CCN3 overexpression produced a marked down-regulation of CCN2 followed by virtual blockade of both collagen type I transcription and its accumulation. This finding occurred in both growth-arrested and CCN3-transfected cells under normal growth conditions after TGF-beta treatment. These effects were not attributable to altered cellular proliferation as determined by cell cycle analysis, nor were they attributable to interference of Smad signaling as shown by analysis of phosphorylated Smad3 levels. In conclusion, both CCN2 and CCN3 appear to act in a yin/yang manner to regulate ECM metabolism. CCN3, acting downstream of TGF-beta to block CCN2 and the up-regulation of ECM, may therefore serve to naturally limit fibrosis in vivo and provide opportunities for novel, endogenous-based therapeutic treatments.

Expression of CCN family of genes in human skin in vivo and alterations by solar-simulated ultraviolet irradiation

The CCN family of proteins is involved in diverse biological functions such as cell growth, adhesion, migration, angiogenesis, and regulation of extracellular matrix. We have investigated expression of CCN family genes and alternations induced by solar-simulated ultraviolet irradiation in human skin in vivo. Transcripts of all six CCN genes were expressed in human skin in vivo. CCN5 was most abundantly expressed followed by CCN2>CCN3>CCN1>CCN4>CCN6. Solar-simulated ultraviolet irradiation increased mRNA expression of CCN1 and CCN2. In contrast, mRNA levels of CCN3, CCN4, CCN5, and CCN6, were reduced. Knowledge gained from this study provides the foundation to explore the functional roles of CCN gene products in cutaneous biology and responses to solar ultraviolet irradiation.

Promotion of bone regeneration by CCN2 incorporated into gelatin hydrogel

CCN family protein 2/connective tissue growth factor (CCN2/CTGF) is a unique molecule that promotes the entire endochondral ossification process and regeneration of damaged articular cartilage. Also, CCN2 has been shown to enhance the adhesion and migration of bone marrow stromal cells as well as the growth and differentiation of osteoblasts; hence, its utility in bone regeneration has been suggested. Here, we evaluated the effect of CCN2 on the regeneration of an intractable bone defect in a rat model. First, we prepared two recombinant CCN2s of different origins, and the one showing the stronger effect on osteoblasts in vitro was selected for further evaluation, based on the result of an in vitro bioassay. Next, to obtain a sustained effect, the recombinant CCN2 was incorporated into gelatin hydrogel that enabled the gradual release of the factor. Evaluation in vivo indicated that CCN2 continued to be released at least for up to 14 days after its incorporation. Application of the gelatin hydrogel-CCN2 complex, together with a collagen scaffold to the bone defect prepared in a rat femur resulted in remarkable induction of osteoblastic mineralization markers within 2 weeks. Finally, distinct enhancement of bone regeneration was observed 3 weeks after the application of the complex. These results confirm the utility of CCN2 in the regeneration of intractable bone defects in vivo when the factor is incorporated into gelatin hydrogel.

Role of CCN, a vertebrate specific gene family, in development

The CCN family of genes constitutes six members of small secreted cysteine rich proteins, which exists only in vertebrates. The major members of CCN are CCN1 (Cyr61), CCN2 (CTGF), and CCN3 (Nov). CCN4, CCN5, and CCN6 were formerly reported to be in the Wisp family, but they are now integrated into CCN due to the resemblance of their four principal modules: insulin like growth factor binding protein, von Willebrand factor type C, thrombospondin type 1, and carboxy-terminal domain. CCNs show a wide and highly variable expression pattern in adult and in embryonic tissues, but most studies have focused on their principal role in osteo/chondrogenesis and vasculo/angiogenesis from the aspect of migration, growth, and differentiation of mesenchymal cells. CCN proteins simultaneously integrate and modulate the signals of integrins, bone morphogenetic protein, vascular endothelial growth factor, Wnt, and Notch by direct binding. However, the priority in the use of the signals is different depending on the cell status. Even the equivalent counterparts show a difference in signal usage among species. It may be that the evolution of the CCN family continues to keep pace with vertebrate evolution itself.

The CCN family of proteins: structure-function relationships

The CCN proteins are key signalling and regulatory molecules involved in many vital biological functions, including cell proliferation, angiogenesis, tumourigenesis and wound healing. How these proteins influence such a range of functions remains incompletely understood but is probably related to their discrete modular nature and a complex array of intra- and inter-molecular interactions with a variety of regulatory proteins and ligands. Although certain aspects of their biology can be attributed to the four individual modules that constitute the CCN proteins, recent results suggest that some of their biological functions require cooperation between modules. Indeed, the modular structure of CCN proteins provides important insight into their structure-function relationships.

The CCN family of proteins: structure-function relationships

The CCN proteins are key signalling and regulatory molecules involved in many vital biological functions, including cell proliferation, angiogenesis, tumourigenesis and wound healing. How these proteins influence such a range of functions remains incompletely understood but is probably related to their discrete modular nature and a complex array of intra- and inter-molecular interactions with a variety of regulatory proteins and ligands. Although certain aspects of their biology can be attributed to the four individual modules that constitute the CCN proteins, recent results suggest that some of their biological functions require cooperation between modules. Indeed, the modular structure of CCN proteins provides important insight into their structure-function relationships.

Skeletal overexpression of connective tissue growth factor impairs bone formation and causes osteopenia

Connective tissue growth factor (CTGF), a member of the CCN family of proteins, is expressed in skeletal cells, and the ctgf null mutation leads to neonatal lethality due to defects in skeletal development. To define the function of CTGF in the postnatal skeleton, we created transgenic mice overexpressing CTGF under the control of the human osteocalcin promoter. CTGF transgenic female and male mice exhibited a significant decrease in bone mineral density, compared with wild-type littermate controls. Bone histomorphometry revealed that CTGF overexpression caused decreased trabecular bone volume due to impaired osteoblastic activity because mineral apposition and bone formation rates were decreased. Osteoblast and osteoclast number and bone resorption were not altered. Calvarial osteoblasts and stromal cells from CTGF transgenics displayed decreased alkaline phosphatase and osteocalcin mRNA levels and reduced bone morphogenetic protein (BMP) signaling mothers against decapentaplegic, Wnt/beta-catenin, and IGF-I/Akt signaling. In conclusion, CTGF overexpression in vivo causes osteopenia, secondary to decreased bone formation, possibly by antagonizing BMP, Wnt, and IGF-I signaling and activity.

Identification of phylogenetically conserved enhancer elements implicated in pancreas development in the WISP1 and CTGF orthologs

WISP1 and CTGF are members of the CCN family of growth factors encoding extracellular matrix proteins participating in several developmental and tumorigenic processes. Both are induced by the WNT signaling pathway, and microarray data suggest that expression of WISP1 and CTGF is repressed by Neurogenin3 (Ngn3 (NEUROG3)), a transcription factor directing specification of the endocrine pancreas. Single-cell reverse transcription polymerase chain reaction analysis suggested that this was a cell autonomous effect. To identify possible common regulatory networks involved in WISP1 and CTGF gene expression, their genomic regions were searched for common transcription factor motifs using a combination of in silico approaches and documented knowledge concerning pancreas development. This analysis revealed the presence of a conserved enhancer in both CTGF and WISP1 regulatory regions in 10 species covering a wide evolutionary distance. This enhancer contains binding sites for Ngn1/3 (NEUROG1/3) and transcription factors that are critically involved in pancreas development. Furthermore, it contained binding sites for three additional transcription factor families, which may indicate novel players are involved in this process.

Functions and mechanisms of action of CCN matricellular proteins

Members of the CCN (CYR61/CTGF/NOV) family have emerged as dynamically expressed, extracellular matrix-associated proteins that play critical roles in cardiovascular and skeletal development, injury repair, fibrotic diseases and cancer. The synthesis of CCN proteins is highly inducible by serum growth factors, cytokines, and environmental stresses such as hypoxia, UV exposure, and mechanical stretch. Consisting of six secreted proteins in vertebrate species, CCNs are typically comprised of four conserved cysteine-rich modular domains. They function primarily through direct binding to specific integrin receptors and heparan sulfate proteoglycans, thereby triggering signal transduction events that culminate in the regulation of cell adhesion, migration, proliferation, gene expression, differentiation, and survival. CCN proteins can also modulate the activities of several growth factors and cytokines, including TGF-beta, TNFalpha, VEGF, BMPs, and Wnt proteins, and may thereby regulate a broad array of biological processes. Recent studies have uncovered novel CCN activities unexpected for matricellular proteins, including their ability to induce apoptosis as cell adhesion substrates, to dictate the cytotoxicity of inflammatory cytokines such as TNFalpha, and to promote hematopoietic stem cell self-renewal. As potent regulators of angiogenesis and chondrogenesis, CCNs are essential for successful cardiovascular and skeletal development during embryogenesis. In the adult, the expression of CCN proteins is associated with injury repair and inflammation, and has been proposed as diagnostic or prognostic markers for diabetic nephropathy, hepatic fibrosis, systemic sclerosis, and several types of cancer. Targeting CCN signaling pathways may hold promise as a strategy of rational therapeutic design.