An inflammation-responsive transcription factor in the pathophysiology of osteoarthritis

Abstract

A number of risk factors including biomechanical stress on the articular cartilage imposed by joint overloading due to obesity, repetitive damage of the joint tissues by injury of the menisci and ligaments, and abnormal joint alignment play a significant role in the onset of osteoarthritis (OA). Genetic predisposition can also lead to the formation of defective cartilage matrix because of abnormal gene expression in the cartilage-specific cells. Another important biochemical event in OA is the consequence of inflammation. It has been shown that synovial inflammation triggers the synthesis of biological stimuli such as cytokines and growth factors which subsequently reach the chondrocyte cells of the articular cartilage activating inflammatory events in the chondrocytes leading to cartilage destruction. In addition to cartilage degradation, hypertrophy of the subchondral bone and osteophyte formation at the joint margins also takes place in OA. Both processes involve abnormal expression of a number of genes including matrix metalloproteinases (MMPs) for cartilage degradation and those associated with bone formation during osteophyte development. To address how diverse groups of genes are activated in OA chondrocyte, we have studied their induction mechanism. We present evidence for abundant expression of an inflammation-responsive transcription factor, SAF-1, in moderate to severely damaged OA cartilage tissues. In contrast, cells in normal cartilage matrix contain very low level of SAF-1 protein. SAF-1 is identified as a major regulator of increased synthesis of MMP-1 and -9 and pro-angiogenic factor, vascular endothelial growth factor (VEGF). While VEGF by stimulating angiogenesis plays a key role in new bone formation in osteophyte, increase of MMP-1 and -9 is instrumental for cartilage erosion in the pathogenesis of OA. Increased expression in degenerated cartilage matrix and in the osteophytes indicate for a key regulatory role of SAF-1 in directing catabolic matrix degrading and anabolic matrix regenerating activities.