Secreted protein, acidic, cysteine-rich (osteonectin)

PDB rendering based on 1bmo.
Symbols SPARC; ON
External IDs OMIM182120 MGI98373 HomoloGene31132 GeneCards: SPARC Gene
RNA expression pattern
PBB GE SPARC 200665 s at tn.png
PBB GE SPARC 212667 at tn.png
More reference expression data
Species Human Mouse
Entrez 6678 20692
Ensembl ENSG00000113140 ENSMUSG00000018593
UniProt P09486 Q3TXI6
RefSeq (mRNA) NM_003118.2 NM_009242.4
RefSeq (protein) NP_003109.1 NP_033268.1
Location (UCSC) Chr 5:
151.04 – 151.07 Mb
Chr 11:
55.21 – 55.23 Mb
PubMed search [1] [2]

Osteonectin (ON) also known as secreted protein acidic and rich in cysteine (SPARC) or basement-membrane protein 40 (BM-40) is a protein that in humans is encoded by the SPARC gene.

Osteonectin is a glycoprotein in the bone that binds sodium. It is secreted by osteoblasts during bone formation, initiating mineralization and promoting mineral crystal formation. Osteonectin also shows affinity for collagen in addition to bone mineral calcium. A correlation between osteonectin over expression and ampullary cancers and chronic pancreatitis has found.



The human SPARC gene is 26.5 kb long, and contains 10 exons and 9 introns and is located on chromosome 5q31-q33.


Osteonectin is a glycoprotein of 40 kD. Osteonectin is an acidic, cysteine-rich glycoprotein consisting of a single polypeptide chain that can be broken into 4 domains: 1) an Ca++ binding domains near the glutamic acidic-rich region at the amino terminus (domain I) , 2) a cysteine- rich (domain II), 3) a hydrophilic region (domain III) and 4) an EF hand motif at the carboxy terminus region (domain IV).[1]

At least the domains at the amino and carboxy terminus appear to contain calcium-binding regions.


Osteonectin is an acidic, secreted extracellular matrix glycoprotein that plays a vital role in bone mineralization, cell-matrix interactions, and collagen binding. Osteonectin also increases the production and activity of matrix metalloproteinases, a function important to invading cancer cells within bone. Additional functions of osteonectin beneficial to tumor cells include angiogenesis, proliferation and migration. Overexpression of osteonectin is reported in many human cancers such as breast, prostate and colon.[2]

This molecule has been implicated in several biological functions, including mineralization of bone and cartilage, inhibiting mineralization, modulation of cell proliferation, facilitation of acquisition of differentiated phenotype and promotion of cell attachment and spreading.

A number of phosphoproteins and glycoproteins are found in bone. The phosphate is bound to the protein backbone through serine or threonine amino acid residues. The best characterized of these bone protein is osteonectin. It binds collagen and hydroxyapatite through separate areas of its molecule, is found in relatively large amounts in immature bone and promotes mineralization of collagen. Thus it is possible that osteonectin plays a crucial role in mineralization.

Tissue distribution

Fibroblasts, including periodontal fibroblasts, synthesize osteonectin.[3] This protein is synthesized by macrophages at sites of wound repair and platelet degranulation.so it may play a important role in wound healing. SPARC does not support cell attachment and like thrombospondin and tenascin, it antiadhesive and an inhibitor of cell spreading. It distrupt focal adhesion in fibroblasts. Also regulates the proliferation of some cells especially the endothelial cells, and it mediate this effect through its ability to bind to cytokines and growth factors.[4] Osteonectin has also been found decrease DNA synthesis in cultured bone.[5]

High levels of immunodetectable osteonectin are found in active osteoblasts and marrow progenitor cells, odontoblasts, periodontal ligament and gingival cells and some chondrocytes and hypertrophic chondrocytes. Osteonectin is also detectable in osteoid, bone matrix proper and dentin. Osteonectin has been localized in a variety of tissues, it is found in greatest abundance in osseous tissue, tissues characterized by high turnover (such as intestinal epithelium), basement membranes and certain neoplasms. Osteonectin is expressed by a wide variety of cells including chondrocytes, fibroblasts, platelets, endothelial cells, epithelial cells, Leydig cells, Sertoli cells, luteal cells, adrenal cortical cells and numerous neoplastic cell lines (such as SaOS-2 cells from human osteosarcoma).


  1. ^ Villarreal XC, Mann KG, Long GL (July 1989). "Structure of human osteonectin based upon analysis of cDNA and genomic sequences". Biochemistry 28 (15): 6483–91. doi:10.1021/bi00441a049. PMID 2790009. 
  2. ^ Guweidhi A, Kleeff J, Adwan H, Giese NA, Wente MN, Giese T, Büchler MW, Berger MR, Friess H (August 2005). "Osteonectin influences growth and invasion of pancreatic cancer cells". Ann. Surg. 242 (2): 224–234. doi:10.1097/01.sla.0000171866.45848.68. PMC 1357728. PMID 16041213. http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pmcentrez&artid=1357728. 
  3. ^ Wasi S, Otsuka K, Yao KL, Tung PS, Aubin JE, Sodek J, Termine JD (June 1984). "An osteonectinlike protein in porcine periodontal ligament and its synthesis by periodontal ligament fibroblasts". Can. J. Biochem. Cell Biol. 62 (6): 470–8. PMID 6380686. 
  4. ^ Young MF, Kerr JM, Ibaraki K, Heegaard AM, Robey PG (August 1992). "Structure, expression, and regulation of the major noncollagenous matrix proteins of bone". Clin. Orthop. Relat. Res. (281): 275–94. PMID 1499220. 
  5. ^ Lane TF, Sage EH (February 1994). "The biology of SPARC, a protein that modulates cell-matrix interactions". FASEB J. 8 (2): 163–73. PMID 8119487. 

Further reading

External links

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