TYPE-VI COLLAGEN EXPRESSION IS UP-REGULATED IN THE EARLY EVENTS OF CHONDROCYTE DIFFERENTIATION

Dedifferentiated chondrocytes cultured adherent to the substratum proliferate and synthesize large amounts of type I collagen but when transferred to suspension culture they decrease proliferation, resume the chondrogenic phenotype and the synthesis of type II collagen, and continue their maturation to hypertrophic chondrocyte (Castagnola et al., 1986, J.Cell Biol. 102, 2310-2317). In this report, we describe the developmentally regulated expression of type VI collagen in vitro in differentiating avian chondrocytes. Type VI collagen mRNA is barely detectable in dedifferentiated chondrocytes as long as the attachment to the substratum is maintained, but increases very rapidly upon passage of the cells into suspension culture reaching a peak after 48 hours and declining after 5-6 days of suspension culture. The first evidence of a rise in the mRNA steady-state levels is obtained already at 6 hours for the alpha3(VI) chain. Immunoprecipitation of metabolically labeled cells with type VI collagen antibodies reveals that the early mRNA rise is paralleled by an increased secretion of type VI collagen in cell media. Induction of type VI collagen is not the consequence of trypsin treatment of dedifferentiated cells since exposure to the actin-disrupting drug cytochalasin or detachment of the cells by mechanical procedures has similar effects. In 13-day-old chicken embryo tibiae, where the full spectrum of the chondrogenic differentiation process is represented, expression of type VI collagen is restricted to the articular cartilage where chondrocytes developmental stage is comparable to stage I (high levels of type II collagen expression). The steep rise in type VI collagen synthesis suggests that the transition of chondrocytes from a stage characterized by high levels of type I collagen to a later stage with a predominance of type II collagen (stage I chondrocytes) might be subdivided further in an early phase (stage Ia) characterized by a high and transient type VI collagen expression and a later phase (stage Ib) where type II collagen is predominant. These data might suggest a functional relationship between type VI collagen expression and the chondrogenic maturation process.