Platelet-derived growth factor (PDGF) is one of numerous growth factors that regulate cell growth and division. In particular, PDGF plays a significant role in blood vessel formation, the growth of blood vessels from already-existing blood vessel tissue, mitogenesis, i.e. proliferation, of mesenchymal cells such as fibroblasts, osteoblasts, tenocytes, vascular smooth muscle cells and mesenchymal stem cells as well as chemotaxis, the directed migration, of mesenchymal cells. Platelet-derived growth factor is a dimeric glycoprotein that can be composed of two A subunits (PDGF-AA), two B subunits (PDGF-BB), or one of each (PDGF-AB).
PDGF is a potent mitogen for cells of mesenchymal origin, including fibroblasts, smooth muscle cells and glial cells. In both mouse and human, the PDGF signaling network consists of five ligands, PDGF-AA through -DD (including -AB), and two receptors, PDGFR-alpha and PDGFR-beta. All PDGFs function as secreted, disulphide-linked homodimers, but only PDGFA and B can form functional heterodimers.
Figure 1. Platelet-derived growth factor BB monomer, Human
Members of PDGF Family
Table 1. PDGF family related products
|PDGF Family Ligands||PDGF||PDGFA||PDGFB|
|PDGF Family Receptors||PDGFRA||PDGFRB||PDGFRL|
The PDGF family consists of PDGF-A, PDGF-B, PDGF-C and PDGF -D. The four PDGFs are inactive in their monomeric forms. The PDGFs bind to the protein tyrosine kinase receptors PDGF receptor-α and -β. These two receptor isoforms dimerize upon binding the PDGF dimer, leading to three possible receptor combinations, namely -αα, -ββ and -αβ.
Platelet-derived growth factor subunit A is a protein that in humans is encoded by the PDGFA gene. The protein encoded by this gene is a member of the platelet-derived growth factor family. The four members of this family are mitogenic factors for cells of mesenchymal origin and are characterized by a motif of eight cysteines. This gene product can exist either as a homodimer or as a heterodimer with the platelet-derived growth factor beta polypeptide, where the dimers are connected by disulfide bonds. Studies using knockout mice have shown cellular defects in oligodendrocytes, alveolar smooth muscle cells, and leydig cells in the testis; knockout mice die either as embryos or shortly after birth. Two splice variants have been identified for this gene.
Platelet-derived growth factor subunit B is a protein that in humans is encoded by the PDGFB gene. This gene product can exist either as a homodimer (PDGF-BB) or as a heterodimer with the platelet-derived growth factor alpha (PDGFA) polypeptide (PDGF-AB), where the dimers are connected by disulfide bonds. Mutations in this gene are associated with meningioma. Reciprocal translocations between chromosomes 22 and 17, at sites where this gene and that for COL1A1 are located, are associated with a particular type of skin tumor called dermatofibrosarcoma protuberans resulting from unregulated expression of growth factor. Two splice variants have been identified for this gene.
Platelet-derived growth factor C, also known as PDGF-C, is a 345-amino acid protein that in humans is encoded by the PDGFC gene. Platelet-derived growth factors are important in connective tissue growth, survival and function, and consist of disulphide-linked dimers involving two polypeptide chains, PDGF-A and PDGF-B. PDGF-C is a member of the PDGF/VEGF family of growth factors with a unique two-domain structure and expression pattern. PDGF-C was not previously identified with PDGF-A and PDGF-B, possibly because it may be synthesized and secreted as a latent growth factor, requiring proteolytic removal of the N-terminal CUB domain for receptor binding and activation.
Platelet-derived growth factor D is a protein that in humans is encoded by the PDGFD gene. This gene product only forms homodimers and, therefore, does not dimerize with the other three family members. It differs from alpha and beta members of this family in having an unusual N-terminal domain, the CUB domain. Two splice variants have been identified for this gene.
PDGFs are mitogenic during early developmental stages, driving the proliferation of undifferentiated mesenchyme and some progenitor populations. During later maturation stages, PDGF signaling has been implicated in tissue remodeling and cellular differentiation, and in inductive events involved in patterning and morphogenesis. In addition to driving mesenchymal proliferation, PDGFs have been shown to direct the migration, differentiation and function of a variety of specialized mesenchymal and migratory cell types, both during development and in the adult animal. Other growth factors in this family include vascular endothelial growth factors B and C (VEGF-B, VEGF-C) which are active in angiogenesis and endothelial cell growth, and placenta growth factor (PlGF) which is also active in angiogenesis.
PDGF plays a role in embryonic development, cell proliferation, cell migration, and angiogenesis. Over-expression of PDGF has been linked to several diseases such as atherosclerosis, fibrotic disorders and malignancies. Synthesis occurs due to external stimuli such as thrombin, low oxygen tension, or other cytokines and growth factors.
Role in disease
Recombinant PDGF is used to help heal chronic ulcers and in orthopedic surgery and periodontics to stimulate bone regeneration and repair. PDGF may be beneficial when used by itself or especially in combination with other growth factors to stimulate soft and hard tissue healing.
Like many other growth factors that have been linked to disease, PDGF and its receptors have provided a market for receptor antagonists to treat disease. Such antagonists include (but are not limited to) specific antibodies that target the molecule of interest, which act only in a neutralizing manner.
Age related downregulation of the PDGF receptor on islet beta cells has been demonstrated to prevent islet beta cell proliferation in both animal and human cells and its re-expression triggers beta cell proliferation and corrects glucose regulation via insulin secretion.
A non-viral PDGF "bio patch" can regenerate missing or damaged bone by delivering DNA in a nano-sized particle directly into cells via genes. Repairing bone fractures, fixing craniofacial defects and improving dental implants are among potential uses. The patch employs a collagen platform seeded with particles containing the genes needed for producing bone. In experiments, its new bone fully covered skull wounds in test animals and stimulated growth in human bone marrow stromal cells.
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