Modulation of rabbit articular chondrocyte (RAC) proliferation by TGF-β isoforms

We have previously shown that TGF-β1 exerts a bifunctional effect on RAC proliferation. Added to quiescent cultures, it inhibits the entry of G₀/G₁ cells into S phase whereas in S phase synchronized populations, it stimulates the DNA replication rate with a delayed G₂+ M phase and a subsequent transient increase of cell number. As TGF-β2 and β3 isoforms are also expressed in bone and cartilage tissues, it was of interest to study their effect on RAC proliferation, in comparison to that of TGF-β1. Using cell counting and tritiated thymidine incorporation, we found that all the TGF-βs used here induced an increase of RAC proliferation rate occurring between 24 and 48 h of exposure. TGF-β2 appeared as the most efficient form as judged from the maximum of thymidine labelling. However, TGF-β3 induced an increase of cell number slightly higher than both TGF-β1 and TGF-β2 (+30% versus 20% for TGF-β1 and β2). TGF-β2 and β3 were able to stimulate the DNA replication rate as previously demonstrated for TGF-β1. However, the effect occurred later for TGF-β2 and β3 (12 h) than for TGF-β1 (6 h). This was confirmed by flow cytometric analysis of DNA content. In addition, immunodetection by flow cytometry demonstrated that all TGF-β isoforms enhanced endogenous expression of TGF-β-related peptides. The effect was shown to be associated with the cell cycle S phase and was greater for TGF-β3 than for TGF-β1 and β2. These findings suggest that TGF-βs could act on RAC functions via autocrine and paracrine ways. Taken together, these data indicate that TGF-βs may modulate proliferation of articular chondrocytes and therefore could play a role in the activation of these cells in the early stages of osteoarthritis.