Cartilage oligomeric matrix protein protects cells against death by elevating members of the IAP family of survival proteins

Cartilage oligomeric matrix protein (COMP) is a component of cartilage, synovium, ligament, and tendon, yet its normal function is largely unknown. To identify its function we have expressed it in 293 and HeLa cell lines and in primary human chondrocytes. We find that COMP protects these cells against death, either in the presence or absence of tumor necrosis factor alpha and is able to block activation of caspase 3, a critical effector caspase. This effect appears to be mediated by the IAP (inhibitor of apoptosis protein) family of anti-apoptotic proteins because the levels of XIAP, survivin, cIAP1 and cIAP2 are significantly elevated in the COMP-expressing cells and down-regulation of survivin and XIAP protein levels by small interfering RNAs blocks the ability of COMP to enhance survival. The mRNAs for most of the IAP family members were not increased by COMP, indicating that a translational/post-translational mechanism was involved in their induction. However, in both HeLa cells and chondrocytes, COMP induced survivin mRNA by 5-fold. Thus survivin is the first gene identified to be up-regulated transcriptionally by COMP. The carboxyl-terminal half of the protein comprising the type 3 repeats and the RGD sequence (CaCTD domain) was sufficient to promote survival and to elevate the IAPs. Further, an RGD peptide was able to block the prosurvival effect of COMP and the induction of XIAP and survivin, indicating that survival is likely mediated through integrin signaling. These data point to a new role for COMP in protecting cells against death.