Sinomenine (SIN), an alkaloid extracted from the stem of the Chinese medicinal plant sinomenium acutum, has been used for treating rheumatoid arthritis. But little is known whether SIN has a protective effect on osteoarthritis (OA). In this study, we investigated the protective effect of SIN on IL-1beta-induced proteoglycan degradation and apoptosis in rabbit articular cartilage and chondrocytes. Treatment with 10 ng/ml IL-1beta increased the level of glycosaminoglycan (GAG) released into the culture media, and up-regulated the activity and mRNA expression of matrix metalloproteinase 13 (MMP-13) and down-regulated the activity and mRNA expression of tissue inhibitor of metalloproteinase 1 (TIMP-1) in cartilage explants, as confirmed by the methods of GAG quantitation, MMP-13/TIMP-1 enzyme-linked immunosorbent assay (ELISA) and real-time quantitative RT-PCR. Treatment with 10 ng/ml IL-1beta resulted in marked apoptosis in chondrocytes, as demonstrated by decreased cell viability, occurrence of DNA laddering and increased caspase-3 activity and annexin V binding of phosphatidylserine. However, simultaneous treatment with SIN (10, 50 or 250 microM) inhibited the GAG release and the activity and mRNA expression of MMP-13, and enhanced the activity and mRNA expression of TIMP-1 in a dose-dependent manner in cartilage explants. Furthermore, DNA fragment, caspase-3 activity and apoptosis rate were down-regulated, and cell viability was up-regulated dose-dependently in chondrocytes. Thus, SIN has the protective capacity to antagonize cartilage degradation and chondrocyte apoptosis, which suggest that SIN may act as an agent for pharmacological intervention in the progress of OA.