Effect of adenoviral transforming growth factor-β (TGF-β) expression on proteoglycan (PG) synthesis and content in cartilage. (a) PG synthesis was measured by 35SO42- incorporation into patellar cartilage 4 days after interleukin-1 (IL-1) injection. (CPM = counts per minute) PG synthesis increased after IL-1 injection (p < 0.005), and this increase was boosted by TGF-β (p < 0.0005 compared with IL-1). (b) PG content of cartilage was measured by Safranin O staining intensity of the cartilage in Safranin O/Fast Green stained sections. The mean PG content of non-treated knee joints was set at 100%. After IL-1 exposure, a clear reduction in PG content was observed (p < 0.05). By adenoviral expression of TGF-β after IL-1-induced damage, the PG content of the cartilage was almost normal. Error bars display standard error.