Perlecan is a component of cartilage matrix and promotes chondrocyte attachment

Aggrecan, a chondroitin/keratan sulfate-containing proteoglycan, is a major component of cartilaginous tissues. Immunolocalization studies, using antibodies directed to perlecan, a heparan sulfate proteoglycan first detected in basement membranes, and laminin (another major component of basement membranes), indicate that perlecan and laminin are also present in the matrices of hyaline cartilage in the nasal septum, the articular surface of the bone and the growth plate of the developing bone. Consequently, we used antibodies to both aggrecan and perlecan to characterize their synthesis and secretion by primary cultures of chondrocytes derived from the rat chondrosarcoma. Chondrocytes were pulsed for 20 minutes with [35S]methionine and then chased for up to six hours. The radiolabeled perlecan and aggrecan were immunoprecipitated and analyzed by SDS-PAGE. The results show that chondrocytes synthesize precursor proteins to both proteoglycans, but that only the aggrecan precursor protein is secreted as a proteoglycan. Perlecan was also secreted but with less posttranslational modifications than aggrecan. Northern blot analyses of the RNAs from immortalized rat chondrocytes indicated that the major mRNA encoding for perlecan was approximately 13 kb in length, similar in size to that expressed by other cell types, which synthesize 400 kDa core protein perlecan. Analyses of the proteoglycan fractions from the extracts of bovine articular surface indicated that perlecan in this tissue contains both chondroitin and heparan sulfate side-chains. Purified perlecan and laminin were found to promote attachment of immortalized rat chondrocytes in vitro. These studies indicated that perlecan, once thought to be a unique component of the basement membranes, is more widely distributed and is an important component of the cartilage matrix, where it may provide for cell adhesion to the matrix.