CARTILAGE HOMEOPROTEIN-1, A HOMEOPROTEIN SELECTIVELY EXPRESSED IN CHONDROCYTES

We identified a rat cDNA that encodes cartilage homeoprotein 1 (Cart-1 ). The deduced amino scid sequence of Cart-1 contains a paired-type ho meodomain. Northern blot hybridization and RNase protection assay reve aled that Cart-1 RNA was present at high levels in a well-differentiat ed rat chondrosarcoma tumor and in a cell line derived from this tumor . Cart-1 RNA was detected in primary mouse and rat chondrocytes but no t in various fibroblasts including mouse 10T1/2 cells, NIH 3T3 cells, BALB 3T3 cells, and rat skin fibroblasts. It was also undetectable in mouse C2 myoblasts, S194 myeloma cells, and embryonic stem cells. Cart -1 RNA was present at a very low level in testes but was not detected in other soft tissues of 8-week-old rats. In situ hybridization of rat embryos between 14.5 and 16.5 days post coitum revealed relatively hi gh levels of Cart-1 RNA in condensed prechondrocytic mesenchymal cells and in early chondrocytes of cartilage primordia. The levels of Cart- 1 RNA were lower in mature chondrocytes. No hybridization was observed in brain, spinal cord, heart, spleen, gastrointestinal tract, liver, and muscle. We speculate that Cart-1 has a role in chondrocyte differe ntiation.