Up-regulation of estrogen receptors in rabbit osteoarthritic cartilage

The purpose of this study was to investigate the alteration of estrogen receptors in an early stage of estradiol-induced osteoarthritic knees. After 3 weeks of intraarticular injections of estradiol benzoate to both knees in ovariectomized rabbits, the articular cartilages of femoral condyle and tibial plateau were harvested and pooled separately in each rabbit. The cytosolic estradiol receptor was determined by hydroxylapatite-column assay. The receptor levels (fmol/mg protein) were determined by the analysis of specific 3H-labelled estradiol binding data. In the present study, the specific binding to 3H-estradiol in the femoral condyle cartilage of estradiol-treated rabbits was 1.7 fold of that in vehicle-treated rabbits. The difference between them was highly significant (p less than 0.01). On the other hand, estrogen receptors were nondetectable in the tibial plateau of either vehicle- or estradiol-treated rabbits. It appeared that the cytosolic estradiol receptor existed preferentially in the cartilage of the femoral condyle, rather than in that of the tibial plateau. Since a direct link of estrogen to the development of osteoarthritis in rabbit knees was demonstrated as early as 3 weeks, it is suggested that up-regulation of estrogen receptors in cartilage might initiate the osteoarthritic changes.