FrzB-2: a human secreted frizzled-related protein with a potential role inchondrocyte apoptosis

Objective: To characterize a novel secreted frizzled-related protein (SFRP) and determine its tissue distribution at the mRNA and protein level. Methods: The FrzB-2 gene was identified by expressed sequence tag (EST) ana lysis of human tissue-derived libraries. Tissue distribution of FrzB-2 mRNA was determined by Northern blot analysis and in situ hybridization. FrzB-2 protein reactivity was localized in human OA articular cartilage by immuno cytochemistry, using a polyclonal antibody against a peptide sequence uniqu e to FrzB-2. Apoptosis was detected in articular cartilage sections using T unel staining. Results: ESTs corresponding to FrzB-2 were found in osteoblast, chondrosarc oma, osteosarcoma, osteoclastoma and synovial fibroblast libraries. FrzB-2 mRNA is expressed in a number of tissues and cell types including bone-rela ted cells and tissues such as primary human osteoblasts and osteoclastoma. In situ hybridization studies showed strong FrzB-2 mRNA expression in human chondrocytes in human osteoarthritic (OA) cartilage but negligible levels in normal cartilage chondrocytes. The FrzB-2 cDNA encodes a secreted 40 kDa protein consisting of 346 amino acids. FrzB-2 is 92.5% identical to the ra t orthologue, DDC-4, which has been shown to be associated with physiologic al apoptosis. FrzB-2 protein was selectively detected in human OA articular cartilage by immunocytochemistry, using a polyclonal antibody. Consistent with its potential role in apoptosis, positive FrzB-2 staining and Tunel po sitive nuclei staining were detected in chondrocyte clones in sections of h uman OA cartilage. Conclusion: These data suggest that FrzB-2 may play a role in apoptosis and that the expression of this protein may be important in the pathogenesis o f human OA. (C) 2000 OsteoArthritis Research Society international.