It is now apparent, from the detection of biochemical markers, that activation of the process of endochondral ossification is prominent in articular cartilage in osteoarthrosis (OA). As a consequence, it is believed that in OA the articular surface becomes thinner due to replacement of cartilage by bone, increasing its susceptibility to shear damage. The applicant group and others have demonstrated that, during the process of endochondral ossification, chondrocytes progress through stages of terminal differentiation to finally die by an apoptotic process immediately prior to vascular invasion of the cartilage. It is hypothesized that chondrocyte apoptosis is key to subsequent processes, including blood vessel invasion. To examine this question, a unique protein-free suspension culture system has been developed that allows terminal differentiation and apoptosis of hypertrophic chondrocytes to proceed. The system can also be modulated by the addition of growth factors. The investigators have shown that, in general, induction of the hypertrophic phenotype in immature chondrocytes commits them to apoptosis, while prevention of expression of the hypertrophic phenotype prevents apoptosis. They have demonstrated in preliminary studies that, concomitant with apoptosis, hypertrophic chondrocytes release angiogenic agents, as measured by simulation of endothelial cell chemotaxis. This proposal seeks to extend these observations and to exploit the culture system. It will examine the hypothesis that, by releasing angiogenic factors which lead to vascular invasion and subsequent matrix resorption and bone formation, chondrocyte apoptosis plays an active role in endochondral ossification. The major angiogenic factors released by apoptotic hypertrophic chondrocytes will be isolated and characterized. The applicants will determine if the amount and distribution of angiogenic activity can be altered by altering the onset or duration of chondrocyte hypertrophy, or by inducing premature apoptosis. It is suggested that should angiogenic factors appear only during apoptosis, it would suggest a tight coupling of the production of angiogenic factors with cell death and lead to the conclusion that preventing apoptosis would render growth cartilage stable. If however, angiogenic factors accumulate independently of apoptosis, it would raise the possibility that chondrocytes at all stages of terminal differentiation can release angiogenic factors should the cells die for any reason. Vascular ingrowth and cartilage destruction would be expected to follow. These studies are intended to determine which possibilities exist and provide information that would allow the applicants to rationally design interventions that would abrogate the release or activity of angiogenic agents.
