Platelet-derived growth factor

Platelet-derived growth factor. It is a potent mitogen and chemoattractant for cells of mesenchymal origin, playing a central role in embryogenesis, wound healing, and angiogenesis. The molecule functions as a dimer that activates specific tyrosine kinase receptors on target cell surfaces, initiating complex intracellular signaling cascades. Its dysregulation is implicated in numerous pathological conditions, including fibrosis, atherosclerosis, and various malignancies.

Structure and isoforms

The active form is a dimer composed of disulfide-linked polypeptide chains, encoded by four distinct genes: PDGFA, PDGFB, PDGFC, and PDGFD. These genes give rise to multiple isoforms, including the classical PDGF-AA, PDGF-BB, and PDGF-AB, as well as the more recently discovered PDGF-CC and PDGF-DD. The three-dimensional structure, elucidated through X-ray crystallography studies at institutions like the MRC Laboratory of Molecular Biology, reveals key receptor-binding domains essential for interaction with the PDGFR. Proteolytic activation, particularly for the PDGF-C and PDGF-D isoforms, involves specific enzymes such as tissue plasminogen activator.

Function and signaling pathways

Upon binding to its cognate tyrosine kinase receptor, primarily PDGFR-α and PDGFR-β, it induces receptor dimerization and autophosphorylation. This recruits adaptor proteins like GRB2 and SOS1, activating the canonical RAS/MAPK pathway to drive cell proliferation. Simultaneously, binding of PI3K to phosphorylated tyrosines stimulates the AKT/PKB pathway, promoting cell survival and motility. Other critical pathways engaged include the PLC-γ pathway, modulating calcium release, and the JAK-STAT pathway, influencing gene expression. These coordinated signals are essential for processes like mesenchymal cell migration during gastrulation.

Role in development and physiology

During embryogenesis, it is crucial for the development of the cardiovascular system, central nervous system, and connective tissues. Targeted gene knockout studies in mice, pioneered by researchers like Rudolf Jaenisch, have demonstrated its necessity for proper vasculogenesis and glomerular mesangial cell formation. In adult physiology, it is a key mediator released by platelets at sites of vascular injury, orchestrating the recruitment and proliferation of fibroblasts and smooth muscle cells for tissue repair. Its role in maintaining the interstitial fluid pressure is also vital for normal kidney and lung function.

Clinical significance and disease associations

Pathological overexpression or constitutive activation of its signaling pathways is a hallmark of several diseases. In oncology, autocrine stimulation is observed in glioblastoma multiforme, sarcoma, and gastrointestinal stromal tumor, often driven by chromosomal translocation events. Its pro-fibrotic activity contributes to disorders like pulmonary fibrosis, liver cirrhosis, and systemic sclerosis. Furthermore, it accelerates atherosclerotic plaque progression by stimulating vascular smooth muscle cell migration within the coronary arteries. Therapeutic strategies, including the tyrosine kinase inhibitor Imatinib developed by Novartis, aim to block this pathway.

History and discovery

The factor was first identified in the 1970s by scientists including Russell Ross and John M. Gauldie as a major mitogenic component in serum derived from clotted blood. Its primary cellular source was traced to the alpha granules of platelets, leading to its name. The cloning of the PDGFB gene, later recognized as the v-sis oncogene of the simian sarcoma virus, was a landmark achievement by the team of Michael D. Waterfield, linking growth factor biology directly to virology and cancer genetics. Subsequent decades of research at institutions like Uppsala University and the Ludwig Institute for Cancer Research have fully characterized its family and receptors.