Identification of Chondromodulin I as a Novel Endothelial Cell Growth Inhibitor

Abstract

Cartilage is unique among tissues of mesenchymal origin in that it is resistant to vascular invasion due to an intrinsic angiogenic inhibitor. During endochondral bone formation, however, calcified cartilage formed in the center of the cartilaginous bone rudiment allows vascular invasion, which initiates the replacement of cartilage by bone. The transition of cartilage from the angioresistant to the angiogenic status thus plays a key role in bone formation. However, the molecular basis of this phenotypic transition of cartilage has been obscure. We report here purification of an endothelial cell growth inhibitor from a guanidine extract of bovine epiphyseal cartilage. The N-terminal amino acid sequence indicated that the inhibitor was identical to chondromodulin I (ChM-I), a cartilage-specific growthmodulating factor. Purified ChM-I inhibited DNA synthesis and proliferation of vascular endothelial cells as well as tube morphogenesis in vitro. Expression of ChM-I cDNA in COS7 cells indicated that mature ChM-I molecules were secreted from the cells after post-translational modifications and cleavage from the transmembrane precursor at the predicted processing signal. Recombinant ChM-I stimulated DNA synthesis and proteoglycan synthesis of cultured growth plate chondrocytes, but inhibited tube morphogenesis of endothelial cells. In situ hybridization and immunohistochemical studies indicated that ChM-I is specifically expressed in the avascular zone of cartilage in developing bone, but not present in calcifying cartilage. These results suggest a regulatory role of ChM-I in vascular invasion during endochondral bone formation.

Post-translational Processing of Bovine Chondromodulin-I

“…The mRNA for ChM-I was shown to be abundant in fetal cartilage and, although present in adult cartilage, was reduced to 5% of the fetal level (8). Immunohistochemistry and in situ hybridization localized ChM-I and its mRNA to the interterritorial matrix of the avascular zones of growth plate cartilage (3). ChM-I stimulates DNA synthesis and proteoglycan biosynthesis in the presence of basic fibroblast growth factor 2 (2).…”

“…The pattern of ChM-I expression and the fact that purified ChM-I inhibited DNA synthesis and proliferation in vascular endothelial cells as well as tube morphogenesis in vitro (3) suggest that one role for ChM-I is as an inhibitor of angiogenesis. However, the abundance of ChM-I in fetal cartilage suggests that it may have other roles in chondrocyte growth modulation.…”

Methylation in the Core-promoter Region of the Chondromodulin-I Gene Determines the Cell-specific Expression by Regulating the Binding of Transcriptional Activator Sp3

“…No expression of the ChM-I gene is observed in bone tissues developing after vascular invasion in the area adjacent to hypertrophic chondrocytes (3). In matured limbs, the expression of the ChM-I gene is limited to cells in the resting, proliferating, and early hypertrophic zone of the growth plate (2)(3)(4). These results suggest that the expression of the ChM-I gene is regulated strictly in a cell-and stage-related manner, although the molecular mechanisms leading to this spatiotemporal expression have not been elucidated.…”

“…Chondromodulin-I (ChM-I) 1 is a 25-kDa glycoprotein originally purified from bovine epiphyseal cartilage on the basis of growth-promoting activity for chondrocytes (1) and subsequently revealed to be a potent vascular endothelial cell growth inhibitor (2). During embryonal development, the expression of the ChM-I gene is first observed in all of the cartilaginous tissues, which are composed of prehypertrophic chondrocytes (3).…”

Co-localization of Chondromodulin-I (ChM-I) and Bone Morphogenetic Protein-6 (BMP-6) in Myoepithelial Cells of Canine Mammary Tumors

“…The major biological functions of ChM-I are to stimulate the proliferation of chondrocytes and inhibit angiogenesis [8][9][10]. Several reports have indicated that, in human salivary pleomorphic adenomas, ChM-I might be expressed in the lacunar cells of the chondroid matrix and in the myoepithelial cells [13,14].…”