Connective tissue growth factor (CTGF/CCN2) is a member of the CCN family of matricellular proteins and also has been designated Hcs24, FISP12, IGFBP8, IGFBP-rP2, ␤IG-M2, and ecogenin. The other CCN proteins include Cyr61/CCN1, NOV/CCN3, WISP1/CCN4, WISP2/CCN5, and WISP3/CCN6 (5, 26, 38, 39) as well, and they are structurally and functionally related glycoproteins involved in cell differentiation, proliferation, adhesion, migration, and the formation of the extracellular matrix. These matricellular functions of CCNs are involved in physiological processes such as wound healing, angiogenesis, morphogenesis, and embryogenesis as well as in pathological states including fibrotic disorders, cancer, and arthritis.Earlier we showed that CCN2 promotes endochondral ossification by acting on chondrocytes, osteoblasts, and endothelial cells (35,37,46). For example, CCN2 promotes physiological chondrocytic proliferation and extracellular matrix (ECM) formation. We also reported the regeneration of defects in articular cartilage in rat knee joints following treatment with recombinant CCN2 (36). Furthermore, ctgf-null mice were dead on delivery and were characterized by defective angiogenesis, the derangement of endochondral ossification, and dysmorphisms that occurred as a result of impaired chondrocyte proliferation and an abnormal ECM composition within the hypertrophic zone (24).Matrix metalloproteinases (MMPs) are zinc-dependent endopeptidases that are involved in the remodeling and turnover of the ECM in physiological processes such as angiogenesis, wound healing, embryogenesis, and morphogenesis as well as in pathological states including cancers, myocardial infarction, fibrotic disorders, rheumatism, and osteoarthritis (33, 49). Cartilage is a connective tissue that is constructed by chondrocytes embedded within an ECM predominantly composed of collagens and proteoglycans. ECM remodeling is achieved by regulating the production and degradation of specific ECM components. MMPs, which comprise a large family of enzymes with differential abilities to degrade specific ECM components, play a vital role in this process. MMPs also cleave growth factors and their binding proteins, thereby activating or inhibiting specific signaling events (15). Of note, the expression and role of MMP3 have been investigated in the pathological status of articular cartilage, such as in osteoarthritis and rheumatism (1,52).Recent study has demonstrated the existence and functions of intracellular MMPs and tissue inhibitors of metalloproteinases (TIMPs). TIMP-1 accumulates in the cellular nuclei in association with the cell cycle (54). Alternative splicing and promoter usage generate an intracellular MMP11 isoform directly translated as an active MMP (31). MMP2 is found in the nuclei of cardiac myocytes and is capable of cleaving poly-(ADP-ribose) polymerase (PARP) in vitro (28). MMP3 also is detected in the nuclei of hepatocytes and is involved in apop-* Corresponding author. Mailing address:

To examine the role of bone morphogenetic protein (BMP) signaling in chondrocytes during endochondral ossification, the dominant negative (DN) forms of BMP receptors were introduced into immature and mature chondrocytes isolated from lower and upper portions of chick embryo sternum, respectively. We found that control sternal chondrocyte populations expressed type IA, IB, and II BMP receptors as well as BMP-4 and -7. Expression of a DN-type II BMP receptor (termed DN-BMPR-II) in immature lower sternal (LS) chondrocytes led to a loss of differentiated functions; compared with control cells, the DN-BMPR- II–expressing LS chondrocytes proliferated more rapidly, acquired a fibroblastic morphology, showed little expression of type II collagen and aggrecan genes, and upregulated type I collagen gene expression. Expression of DN-BMPR-II in mature hypertrophic upper sternal (US) chondrocytes caused similar effects. In addition, the DN-BMPR-II–expressing US cells exhibited little alkaline phosphatase activity and type X collagen gene expression, while the control US cells produced both alkaline phosphatase and type X collagen. Both DN-BMPR-II–expressing US and LS chondrocytes failed to respond to treatment with BMP-2 . When we examined the effects of DN forms of types IA and IB BMP receptors, we found that DN-BMPR-IA had little effect, while DN-BMPR-IB had similar but weaker effects compared with those of DN-BMPR-II. We conclude that BMP signaling, particularly that mediated by the type II BMP receptor, is required for maintenance of the differentiated phenotype, control of cell proliferation, and expression of hypertrophic phenotype.

Integrins are cell surface receptors involved in cellular processes including adhesion, migration, and matrix assembly. In the present study, we analyzed the possible involvement of ␣5␤1 integrin in the regulation of chondrocyte adhesion, spreading, and proliferation. We found that rabbit growth plate chondrocytes were able to attach to substrates coated with type I collagen, type II collagen, or fibronectin within 24 h of culture. During this time period, attachment to fibronectin appeared to be dependent on ␣5␤1 integrin, whereas adhesion to collagens was not. By day 3 of culture, chondrocytes spread onto all the substrates tested. We found that regardless of the nature of the substrate, cell spreading was reversed by treatment with RGD peptide or antibodies against ␣5␤1 or fibronectin, indicating that cell spreading involved ␣5␤1 and fibronectin endogenously produced and deposited by the chondrocytes themselves. Colony formation by chondrocytes in soft agar was inhibited by treatment with RGD peptides or BIIG2, an antibody that interferes with ␣5␤1 integrin-ligand interactions. Furthermore, DNA content was decreased by treatment with anti-fibronectin antibody in micromass culture of chondrocytes. Immunohistochemical analysis on tissue sections revealed that the ␣5 subunit was particularly abundant in the proliferative and hypertrophic zones of growth plate. The results of the study indicate that ␣5␤1 integrin plays multiple roles in chondrocyte behavior and function and appears to be involved in the regulation of both chondrocyte-matrix interactions and