CCN3-mediated promotion of sulfated proteoglycan synthesis in rat chondrocytes from developing joint heads

Do not overwork: cellular communication network factor 3 for life in cartilage

Cellular communication network factor (CCN) 3, which is one of the founding members of the CCN family, displays diverse functions. However, this protein generally represses the proliferation of a variety of cells. Along with skeletal development, CCN3 is produced in cartilaginous anlagen, growth plate cartilage and epiphysial cartilage. Interestingly, CCN3 is drastically induced in the growth plates of mice lacking CCN2, which promotes endochondral ossification. Notably, chondrocytes in these mutant mice with elevated CCN3 production also suffer from impaired glycolysis and energy metabolism, suggesting a critical role of CCN3 in cartilage metabolism. Recently, CCN3 was found to be strongly induced by impaired glycolysis, and in our study, we located an enhancer that mediated CCN3 regulation via starvation. Subsequent investigations specified regulatory factor binding to the X-box 1 (RFX1) as a transcription factor mediating this CCN3 regulation. Impaired glycolysis is a serious problem, resulting in an energy shortage in cartilage without vasculature. CCN3 produced under such starved conditions restricts energy consumption by repressing cell proliferation, leading chondrocytes to quiescence and survival. This CCN3 regulatory system is indicated to play an important role in articular cartilage maintenance, as well as in skeletal development. Furthermore, CCN3 continues to regulate cartilage metabolism even during the aging process, probably utilizing this regulatory system. Altogether, CCN3 seems to prevent "overwork" by chondrocytes to ensure their sustainable life in cartilage by sensing energy metabolism. Similar roles are suspected to exist in relation to systemic metabolism, since CCN3 is found in the bloodstream.

RFX1-mediated CCN3 induction that may support chondrocyte survival under starved conditions

Cellular communication network factor (CCN) family members are multifunctional matricellular proteins that manipulate and integrate extracellular signals. In our previous studies investigating the role of CCN family members in cellular metabolism, we found three members that might be under the regulation of energy metabolism. In this study, we confirmed that CCN2 and CCN3 are the only members that are tightly regulated by glycolysis in human chondrocytic cells. Interestingly, CCN3 was induced under a variety of impaired glycolytic conditions. This CCN3 induction was also observed in two breast cancer cell lines with a distinct phenotype, suggesting a basic role of CCN3 in cellular metabolism. Reporter gene assays indicated a transcriptional regulation mediated by an enhancer in the proximal promoter region. As a result of analyses in silico, we specified regulatory factor binding to the X-box 1 (RFX1) as a candidate that mediated the transcriptional activation by impaired glycolysis. Indeed, the inhibition of glycolysis induced the expression of RFX1, and RFX1 silencing nullified the CCN3 induction by impaired glycolysis. Subsequent experiments with an anti-CCN3 antibody indicated that CCN3 supported the survival of chondrocytes under impaired glycolysis. Consistent with these findings in vitro, abundant CCN3 production by chondrocytes in the deep zones of developing epiphysial cartilage, which are located far away from the synovial fluid, was confirmed in vivo. Our present study uncovered that RFX1 is the mediator that enables CCN3 induction upon cellular starvation, which may eventually assist chondrocytes in retaining their viability, even when there is an energy supply shortage.

CCN proteins in the musculoskeletal system: current understanding and challenges in physiology and pathology

The acronym for the CCN family was recently revised to represent “cellular communication network”. These six, small, cysteine-enriched and evolutionarily conserved proteins are secreted matricellular proteins, that convey and modulate intercellular communication by interacting with structural proteins, signalling factors and cell surface receptors. Their role in the development and physiology of musculoskeletal system, constituted by connective tissues where cells are interspersed in the cellular matrix, has been broadly studied. Previous research has highlighted a crucial balance of CCN proteins in mesenchymal stem cell commitment and a pivotal role for CCN1, CCN2 and their alter ego CCN3 in chondrogenesis and osteogenesis; CCN4 plays a minor role and the role of CCN5 and CCN6 is still unclear. CCN proteins also participate in osteoclastogenesis and myogenesis. In adult life, CCN proteins serve as mechanosensory proteins in the musculoskeletal system providing a steady response to environmental stimuli and participating in fracture healing. Substantial evidence also supports the involvement of CCN proteins in inflammatory pathologies, such as osteoarthritis and rheumatoid arthritis, as well as in cancers affecting the musculoskeletal system and bone metastasis. These matricellular proteins indeed show involvement in inflammation and cancer, thus representing intriguing therapeutic targets. This review discusses the current understanding of CCN proteins in the musculoskeletal system as well as the controversies and challenges associated with their multiple and complex roles, and it aims to link the dispersed knowledge in an effort to stimulate and guide readers to an area that the writers consider to have significant impact and relevant potentialities.

The Emerging Roles of CCN3 Protein in Immune-Related Diseases

The CCN proteins are a family of extracellular matrix- (ECM-) associated proteins which currently consist of six secreted proteins (CCN1-6). CCN3 protein, also known as nephroblastoma overexpressed protein (NOV), is a member of the CCN family with multiple biological functions, implicated in major cellular processes such as cell growth, migration, and differentiation. Recently, CCN3 has emerged as a critical regulator in a variety of diseases, including immune-related diseases, including rheumatology arthritis, osteoarthritis, and systemic sclerosis. In this review, we will briefly introduce the structure and function of the CCN3 protein and summarize the roles of CCN3 in immune-related diseases, which is essential to understand the functions of the CCN3 in immune-related diseases.

A friend in knee: CCN3 may inhibit osteoarthritis progression

Osteoarthritis (OA) is a major clinical problem among the ageing population, yet no disease-modifying treatments currently exist. This issue arises, in part, due to the complex processes occurring in the microenvironment of articular cartilage that lead to osteoarthritic changes. Gaining a better understanding of these processes is crucial in developing a viable therapy for OA. A recent report in Journal of Bone Mineral Metabolism by Janune et al. (J Bone Miner Metab 35:582–597, 2016) suggests a novel role for CCN3 in maintaining the differentiated phenotype of articular cartilage. This report suggests that CCN3, a member of the CCN family of matricellular proteins, is important for proteoglycan accumulation, as well as expression of type II collagen, tenascin C, and lubricin in vitro. Furthermore, exogenous CCN3 increased tidemark integrity and lubricin protein expression in a rat model of OA. These results implicate the regulation of CCN3 as a potential therapeutic target in patients with OA.

Metabolic regulation of the CCN family genes by glycolysis in chondrocytes

The CCN family consists of 6 genes in the mammalian genome and produces multifunctional proteins involved in a variety of biological processes. Recent reports indicate the profound roles of CCN2 in energy metabolism in chondrocytes, and Ccn2 deficiency is known to alter the expression of 2 other family members including Ccn3. However, almost nothing is known concerning the regulation of the CCN family genes by energy metabolism. In order to gain insight into this critical issue, we initially and comprehensively evaluated the effect of inhibition of glycolysis on the expression of all of the CCN family genes in chondrocytic cells. Upon the inhibition of a glycolytic enzyme, repression of CCN2 expression was observed, whereas CCN3 expression was conversely induced. Similar repression of CCN2 was conferred by the inhibition of aerobic ATP production, which, however, did not induce CCN3 expression. In contrast, glucose starvation significantly enhanced the expression of CCN3 in those cells. The results of a reporter gene assay using a molecular construct containing a CCN3 proximal promoter revealed a dose-dependent induction of the CCN3 promoter activity by the glycolytic inhibitor in chondrocytic cells. These results unveiled a critical role of glycolytic activity in the regulation of CCN2 and CCN3, which activity mediated the mutual regulation of these 2 major CCN family members in chondrocytes.

Targeted mutation of NOV/CCN3 in mice disrupts joint homeostasis and causes osteoarthritis-like disease

OBJECTIVE

The matricellular protein NOV/CCN3, is implicated in osteoarthritis (OA) and targeted mutation of NOV in mice (Nov(del3)) leads to joint abnormalities. This investigation tested whether NOV is required for joint homeostasis and if its disruption causes joint degeneration.

METHOD

NOV expression in the adult mouse joint was characterized by immunohistochemistry. A detailed comparison of the joints of Nov(del3)-/- and Nov(del3)+/+ (wild-type) males and females at 2, 6 and 12 months of age was determined by X-ray, histology and immunohistochemistry.

RESULTS

NOV protein was found in specific cells in articular cartilage, meniscus, synovium and ligament attachment sites in adult knees. Nov(del3)-/- males exhibited severe OA-like pathology at 12 months (OARSI score 5.0 ± 0.5, P < 0.001), affecting all tissues of the joint: erosion of the articular cartilage, meniscal enlargement, osteophytic outgrowths, ligament degeneration and expansion of fibrocartilage. Subchondral sclerosis and changes in extracellular matrix composition consistent with OA, were also seen. The density of articular cartilage cells in Nov(del3)+/+ knee joints is maintained at a constant level from 2 to 12 months of age whereas this is not the case in Nov(del3)-/- mice. Compared with age and sex-matched Nov(del3)+/+ mice, a significant increase in articular cartilage density was seen in Nov(del3)-/- males at 2 months, whereas a significant decrease was seen at 6 and 12 months in both Nov(del3)-/- males and females.

CONCLUSION

NOV is required for the maintenance of articular cartilage and for joint homeostasis, with disruption of NOV in ageing Nov(del3)-/- male mice causing OA-like disease.

CCN3: the-pain-killer inside me

There is increasing evidence that metalloproteinases are involved in neuropathic pain [Dev et al., Expert Opin Investig Drugs 19:455-468 2010] Hence, the identification of molecules that can regulate MMP9 and MMP2 is warranted. In a recent publication, Kular et al. (2012) claim that CCN3 functions to decrease inflammatory pain via the regulation of two metalloproteinases, MMP2 and MMP9, in response to experimentally-induced inflammation. Their conclusion is based on the following observations : i) the expression of CCN3 was reduced following induction of pain by subcutaneous injection of complete Freund's adjuvent in rat's paw, ii) an inhibition of MMP9 decreased CFA-associated mechanical allodynia, iii) inhibition of CCN3 expression by siRNA led to an upregulation of MMP2 in the dorsal horn of the spinal cord (DHSC) and MMP9 in the dorsal root ganglia (DRG), iv) a partial effect of CCN3 on CFA-induced expression of MMP9 and MMP2 in DRG and DHSC following intrathecal injection of CCN3. Unfortunately, the conclusion of this study is weakened by the lack of experimental evidence showing a direct relationship between the expression of CCN3 and MMPs. Furthermore, several results contained in this manuscript only confirm data that were previously established by others. Owing to the wide range of activities which have been attributed to CCN3 (Perbal, Mol Pathol 54:57-79 2001, Brigstock, J Endocrinol 178:169-175 2003, Perbal, Lancet 363(9402):62-64 2004, Perbal, Cell Commun Signal 4:6 2006, Holbourn et al. Trends Biochem Sci. 33:461-473 2008, Leask and Abraham, J Cell Sci 119:4803-4810 2006, Jun and Lau, Nat Rev Drug Discov 10:945-963 2011, Rachfal and Brigstock, Vitam Horm 70:69-103 2005), the mechanisms underlying the potential role of CCN3 in the expression of these MMPs in the context of inflammatory pain must be thoroughly studied before a meaningful conclusion can be reached. Indeed, Kular et al. description of variations in CCN3, MMP9 and MMP2 levels occurring simultaneously is not sufficient to draw a functional relationship between these three proteins. It should be noted that the expression of CCN3 was already reported to repress MMP9 (Benini et al., Oncogene 24:4349-4361 2005, Fukunaga-Kalabis et al., Oncogene 27:2552-2560 2008) and the roles of CCN3 in inflammatory processes has been extensively documented in the past few years (Bleau et al., Front Biosci 10:998-1009 2005, Lin et al., J Biol Chem 280:8229-8237 2005, Perbal, Cell Commun Signal 4:6 2006, Hughes et al., Diabetologia 50:1089-1098 2007, Lin et al., J Cell Commun Signal 4:141-153 2010, Pasmant et al., J Neuropathol Exp Neurol 69:60-69 2010, Shimoyama et al., Thromb Vasc Biol 30:675-682 2010, Lemaire et al., J Invest Dermatol 130:2517 2010, Chen and Lau, J Cell Commun Signal 4:63-69 2010, Le Dréau et al., Glia 58:1510-1521 2010, Rittié et al. J Cell Commun Signal 5:69-80 2011, Janune et al., J Cell Commun Signal 5:167-171 2011). In addition, the expression of CCN3 in the neurons of dorsal root ganglia and dorsal horn of the spinal horn in rat and human has also been documented (Su et al., C R Acad Sci III 321:883-892 1998, Mol Pathol 54:184-191 2001, Kocialkowski et al., Anat Embryol (Berl) 203:417-427 2001). Implication of CCN3 in cognitive functions (Su et al., Sheng Li Xue Bao 52:290-294 2000) and the possible involvement of CCN3 in the regulation of pain was already suggested almost a decade ago (Perbal, Expert Rev Mol Diagn 3:597-604 2003, Perbal et al., Mol Pathol 56:80-85 2003) with the demonstration of cell-specific effects of CCN3 on intracellular calcium stores and inhibition of anionic channels by CCN3 (Li et al., Mol Pathol 55:250-261 2002, Lombet et al., Cell Commun Signal 1:1 2003, Perbal, Expert Rev Mol Diagn 3:597-604 2003, Perbal et al., Mol Pathol 56:80-85 2003). Aside from these general aspects, and in the light of the potential participation of CCN3 in the whole process of pain sensing, the reader would have appreciated the discussion in this manuscript not being essentially a flat summary of the data presented, but a more thorough discussion of the possible role for CCN3 in the regulation of MMPs and its significance in the context of the wide biological functions of CCN3.