Enhancement of cell adhesion and spreading by a cartilage-specific noncollagenous protein, cartilage matrix protein (CMP/matrilin-1), via integrin

Cartilage matrix protein (CMP; also known as matrilin-1), one of the major noncollagenous proteins in most cartilages, binds to aggrecan and type II collagen. We examined the effect of CMP on the adhesion of chondrocytes and fibroblasts using CMP-coated dishes. The CMP coating at 10-20 μg/ml enhanced the adhesion and spreading of rabbit growth plate, resting and articular chondrocytes, and fibroblasts and human epiphyseal chondrocytes and MRC5 fibroblasts. The effect of CMP on the spreading of chondrocytes was synergistically increased by native, but not heated, type II collagen (gelatin). The monoclonal antibody to integrin α₁ or β₁ abolished CMP- induced cell adhesion and spreading, whereas the antibody to integrin α₂, α₃, α₅, β₂, α₅β₁, or α(v)β₅ had little effect on cell adhesion or spreading. The antibody to integrin α₁, but not to other subunits, coprecipitated ¹²⁵I-CMP that was added to MRC5 cell lysates, indicating the association of CMP with the integrin α₁ subunit. Unlabeled CMP competed for the binding to integrin α₁ with ¹²⁵I-CMP. These findings suggest that CMP is a potent adhesion factor for chondrocytes, particularly in the presence of type II collagen, and that integrin α₁β₁ is involved in CMP- mediated cell adhesion and spreading. Since CMP is expressed almost exclusively in cartilage, this adhesion factor, unlike fibronectin or laminin, may play a special role in the development and remodeling of cartilage.