CCN2 (Connective Tissue Growth Factor) is essential for extracellular matrix production and integrin signaling in chondrocytes

The matricellular protein CCN2 (Connective Tissue Growth Factor; CTGF) is an essential mediator of ECM composition, as revealed through analysis of Ccn2 deficient mice. These die at birth due to complications arising from impaired endochondral ossification. However, the mechanism(s) by which CCN2 mediates its effects in cartilage are unclear. We investigated these mechanisms using Ccn2(-/-) chondrocytes. Expression of type II collagen and aggrecan were decreased in Ccn2(-/-) chondrocytes, confirming a defect in ECM production. Ccn2(-/-) chondrocytes also exhibited impaired DNA synthesis and reduced adhesion to fibronectin. This latter defect is associated with decreased expression of alpha 5 integrin. Moreover, CCN2 can bind to integrin alpha 5 beta 1 in chondrocytes and can stimulate increased expression of integrin alpha 5. Consistent with an essential role for CCN2 as a ligand for integrins, immuno-fluorescence and Western blot analysis revealed that levels of focal adhesion kinase (FAK) and extracellular signal-regulated kinase (ERK)1/2 phosphorylation were reduced in Ccn2(-/-) chondrocytes. These findings argue that CCN2 exerts major effects in chondrocytes through its ability to (1) regulate ECM production and integrin alpha 5 expression, (2) engage integrins and (3) activate integrin-mediated signaling pathways.