Osteonectin (ON) also known as secreted protein acidic and rich in cysteine (SPARC) or basement-membrane protein 40 (BM-40) is a

protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residue (biochemistry), residues. Proteins perform a vast array of functions within organisms, including Enzyme catalysis, catalysing metab ...

that in humans is encoded by the ''SPARC''

gene In biology, the word gene has two meanings. The Mendelian gene is a basic unit of heredity. The molecular gene is a sequence of nucleotides in DNA that is transcribed to produce a functional RNA. There are two types of molecular genes: protei ...

. Osteonectin is a glycoprotein in the

bone A bone is a rigid organ that constitutes part of the skeleton in most vertebrate animals. Bones protect the various other organs of the body, produce red and white blood cells, store minerals, provide structure and support for the body, ...

that binds calcium. It is secreted by

osteoblast Osteoblasts (from the Greek combining forms for " bone", ὀστέο-, ''osteo-'' and βλαστάνω, ''blastanō'' "germinate") are cells with a single nucleus that synthesize bone. However, in the process of bone formation, osteoblasts fu ...

s during bone formation, initiating mineralization and promoting mineral crystal formation. Osteonectin also shows affinity for

collagen Collagen () is the main structural protein in the extracellular matrix of the connective tissues of many animals. It is the most abundant protein in mammals, making up 25% to 35% of protein content. Amino acids are bound together to form a trip ...

in addition to bone mineral calcium. A correlation between osteonectin over-expression and ampullary cancers and chronic pancreatitis has been found.

Gene

The human SPARC gene is 26.5 kb long, and contains 10

exon An exon is any part of a gene that will form a part of the final mature RNA produced by that gene after introns have been removed by RNA splicing. The term ''exon'' refers to both the DNA sequence within a gene and to the corresponding sequence ...

s and 9

intron An intron is any nucleotide sequence within a gene that is not expressed or operative in the final RNA product. The word ''intron'' is derived from the term ''intragenic region'', i.e., a region inside a gene."The notion of the cistron .e., gen ...

s and is located on chromosome 5q31-q33.

Structure

Osteonectin is a 40 kDa acidic and cysteine-rich glycoprotein consisting of a single polypeptide chain that can be broken into 4 domains: 1) a Ca²⁺ binding domain near the glutamic acid-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).

Function

Osteonectin is an acidic 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, colon and pancreatic. 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 phosphorylated serine or threonine amino acid residues. The best characterized of these bone proteins is osteonectin. It binds collagen and

hydroxyapatite Hydroxyapatite (International Mineralogical Association, IMA name: hydroxylapatite) (Hap, HAp, or HA) is a naturally occurring mineral form of calcium apatite with the Chemical formula, formula , often written to denote that the Crystal struc ...

in separate domains, is found in relatively large amounts in immature bone, and promotes mineralization of collagen.

Tissue distribution

Fibroblasts, including periodontal fibroblasts, synthesize osteonectin. This protein is synthesized by macrophages at sites of wound repair and platelet degranulation, so it may play an important role in wound healing. SPARC does not support cell attachment, and like tenascin, is anti-adhesive and an inhibitor of cell spreading. It disrupts focal adhesions in fibroblasts. It also regulates the proliferation of some cells, especially endothelial cells, mediated by its ability to bind to cytokines and growth factors. Osteonectin has also been found to decrease DNA synthesis in cultured bone. 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, but 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).

References

External links